1
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Beltran JL, McGrath LG, Caruso S, Bain RK, Hendrix CE, Kamran H, Johnston HG, Collings RM, Henry MCN, Abera TAL, Donoso VA, Carriker EC, Thurtle-Schmidt BH. Borate Transporters and SLC4 Bicarbonate Transporters Share Key Functional Properties. MEMBRANES 2023; 13:membranes13020235. [PMID: 36837738 PMCID: PMC9959716 DOI: 10.3390/membranes13020235] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/27/2023] [Revised: 02/10/2023] [Accepted: 02/13/2023] [Indexed: 06/03/2023]
Abstract
Borate transporters are membrane transport proteins that regulate intracellular borate levels. In plants, borate is a micronutrient essential for growth but is toxic in excess, while in yeast, borate is unnecessary for growth and borate export confers tolerance. Borate transporters share structural homology with human bicarbonate transporters in the SLC4 family despite low sequence identity and differences in transported solutes. Here, we characterize the S. cerevisiae borate transporter Bor1p and examine whether key biochemical features of SLC4 transporters extend to borate transporters. We show that borate transporters and SLC4 transporters share multiple properties, including lipid-promoted dimerization, sensitivity to stilbene disulfonate-derived inhibitors, and a requirement for an acidic residue at the solute binding site. We also identify several amino acids critical for Bor1p function and show that disease-causing mutations in human SLC4A1 will eliminate in vivo function when their homologous mutations are introduced in Bor1p. Our data help elucidate mechanistic features of Bor1p and reveal significant functional properties shared between borate transporters and SLC4 transporters.
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2
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Structure, dynamics and assembly of the ankyrin complex on human red blood cell membrane. Nat Struct Mol Biol 2022; 29:698-705. [PMID: 35655099 DOI: 10.1038/s41594-022-00779-7] [Citation(s) in RCA: 15] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2021] [Accepted: 04/14/2022] [Indexed: 12/20/2022]
Abstract
The cytoskeleton of a red blood cell (RBC) is anchored to the cell membrane by the ankyrin complex. This complex is assembled during RBC genesis and comprises primarily band 3, protein 4.2 and ankyrin, whose mutations contribute to numerous human inherited diseases. High-resolution structures of the ankyrin complex have been long sought-after to understand its assembly and disease-causing mutations. Here, we analyzed native complexes on the human RBC membrane by stepwise fractionation. Cryo-electron microscopy structures of nine band-3-associated complexes reveal that protein 4.2 stabilizes the cytoplasmic domain of band 3 dimer. In turn, the superhelix-shaped ankyrin binds to this protein 4.2 via ankyrin repeats (ARs) 6-13 and to another band 3 dimer via ARs 17-20, bridging two band 3 dimers in the ankyrin complex. Integration of these structures with both prior data and our biochemical data supports a model of ankyrin complex assembly during erythropoiesis and identifies interactions essential for the mechanical stability of RBC.
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3
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Schmitz M, Schultze A, Vanags R, Voigt K, Di Ventura B, Öztürk MA. patcHwork: a user-friendly pH sensitivity analysis web server for protein sequences and structures. Nucleic Acids Res 2022; 50:W560-W567. [PMID: 35438792 PMCID: PMC9252814 DOI: 10.1093/nar/gkac252] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2022] [Revised: 03/11/2022] [Accepted: 04/12/2022] [Indexed: 11/30/2022] Open
Abstract
pH regulates protein function and interactions by altering the charge of individual residues causing loss or gain of intramolecular noncovalent bonds, which may lead to structural rearrangements. While tools to analyze residue-specific charge distribution of proteins at a given pH exist, currently no tool is available to investigate noncovalent bond changes at two different pH values. To make protein pH sensitivity analysis more accessible, we developed patcHwork, a web server that combines the identification of amino acids undergoing a charge shift with the determination of affected noncovalent bonds at two user-defined pH values. At the sequence-only level, patcHwork applies the Henderson–Hasselbalch equation to determine pH-sensitive residues. When the 3D protein structure is available, patcHwork can be employed to gain mechanistic understanding of the effect of pH. This is achieved using the PDB2PQR and PROPKA tools and noncovalent bond determination algorithms. A user-friendly interface allows visualizing pH-sensitive residues, affected salt bridges, hydrogen bonds and aromatic (pi–pi and cation–pi) interactions. patcHwork can be used to identify patches, a new concept we propose of pH-sensitive residues in close proximity on the protein, which may have a major impact on function. We demonstrate the attractiveness of patcHwork studying experimentally investigated pH-sensitive proteins (https://patchwork.biologie.uni-freiburg.de/).
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Affiliation(s)
- Mirko Schmitz
- Signaling Research Centres BIOSS and CIBSS, University of Freiburg, Schänzlestr. 18, 79104 Freiburg, Germany.,Institute of Biology II, University of Freiburg, Schänzlestr. 1, 79104 Freiburg, Germany
| | - Anne Schultze
- Signaling Research Centres BIOSS and CIBSS, University of Freiburg, Schänzlestr. 18, 79104 Freiburg, Germany.,Institute of Biology II, University of Freiburg, Schänzlestr. 1, 79104 Freiburg, Germany
| | - Raimonds Vanags
- Institute of Biology III, University of Freiburg, Schänzlestr. 1, 79104 Freiburg, Germany
| | - Karsten Voigt
- Institute of Biology III, University of Freiburg, Schänzlestr. 1, 79104 Freiburg, Germany
| | - Barbara Di Ventura
- Signaling Research Centres BIOSS and CIBSS, University of Freiburg, Schänzlestr. 18, 79104 Freiburg, Germany.,Institute of Biology II, University of Freiburg, Schänzlestr. 1, 79104 Freiburg, Germany
| | - Mehmet Ali Öztürk
- Signaling Research Centres BIOSS and CIBSS, University of Freiburg, Schänzlestr. 18, 79104 Freiburg, Germany.,Institute of Biology II, University of Freiburg, Schänzlestr. 1, 79104 Freiburg, Germany
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4
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Jennings ML. Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1. Am J Physiol Cell Physiol 2021; 321:C1028-C1059. [PMID: 34669510 PMCID: PMC8714990 DOI: 10.1152/ajpcell.00275.2021] [Citation(s) in RCA: 24] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl-/HCO3- exchange, one of the steps in CO2 excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO3-, Cl-, O2, CO2, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.
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Affiliation(s)
- Michael L Jennings
- Department of Physiology and Cell Biology, University of Arkansas for Medical Sciences, Little Rock, Arkansas, United States
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5
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Ye. M. Makogonenko RYM, Hrabovskyi OO, Bereznytskyj GK, Pyrogova LV, Gogolinskaya GK, Makogonenko YM. Chlorine-binding structures: role and organization in different proteins. UKRAINIAN BIOCHEMICAL JOURNAL 2021. [DOI: 10.15407/ubj93.04.005] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
The review focuses on chloride-binding structures in the proteins of bacteria, plants, viruses and animals. The structure and amino acid composition of the chloride-binding site and its role in the functioning of structural, regulatory, transport, receptor, channel proteins, transcription factors and enzymes are considered. Data on the important role of chloride-binding structures and chloride anions in the polymerization of fibrin are presented.
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6
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Identification of multiple substrate binding sites in SLC4 transporters in the outward-facing conformation: Insights into the transport mechanism. J Biol Chem 2021; 296:100724. [PMID: 33932403 PMCID: PMC8191340 DOI: 10.1016/j.jbc.2021.100724] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/20/2020] [Revised: 04/21/2021] [Accepted: 04/27/2021] [Indexed: 01/17/2023] Open
Abstract
Solute carrier family 4 (SLC4) transporters mediate the transmembrane transport of HCO3-, CO32-, and Cl- necessary for pH regulation, transepithelial H+/base transport, and ion homeostasis. Substrate transport with varying stoichiometry and specificity is achieved through an exchange mechanism and/or through coupling of the uptake of anionic substrates to typically co-transported Na+. Recently solved outward-facing structures of two SLC4 members (human anion exchanger 1 [hAE1] and human electrogenic sodium bicarbonate cotransporter 1 [hNBCe1]) with different transport modes (Cl-/HCO3- exchange versus Na+-CO32- symport) revealed highly conserved three-dimensional organization of their transmembrane domains. However, the exact location of the ion binding sites and their protein-ion coordination motifs are still unclear. In the present work, we combined site identification by ligand competitive saturation mapping and extensive molecular dynamics sampling with functional mutagenesis studies which led to the identification of two substrate binding sites (entry and central) in the outward-facing states of hAE1 and hNBCe1. Mutation of residues in the identified binding sites led to impaired transport in both proteins. We also showed that R730 in hAE1 is crucial for anion binding in both entry and central sites, whereas in hNBCe1, a Na+ acts as an anchor for CO32- binding to the central site. Additionally, protonation of the central acidic residues (E681 in hAE1 and D754 in hNBCe1) alters the ion dynamics in the permeation cavity and may contribute to the transport mode differences in SLC4 proteins. These results provide a basis for understanding the functional differences between hAE1 and hNBCe1 and may facilitate potential drug development for diseases such as proximal and distal renal tubular acidosis.
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7
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The kidney anion exchanger 1 affects tight junction properties via claudin-4. Sci Rep 2019; 9:3099. [PMID: 30816203 PMCID: PMC6395713 DOI: 10.1038/s41598-019-39430-9] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2018] [Accepted: 01/24/2019] [Indexed: 12/14/2022] Open
Abstract
In the renal collecting duct, intercalated cells regulate acid-base balance by effluxing protons through the v-H+-ATPase, and bicarbonate via apical pendrin or the basolateral kidney anion exchanger 1 (kAE1). Additionally, collecting duct cells play an essential role in transepithelial absorption of sodium and chloride. Expression of kAE1 in polarized MDCK I cells was previously shown to decrease trans-epithelial electrical resistance (TEER), suggesting a novel role for kAE1 in paracellular permeability. In our study, we not only confirmed that inducible expression of kAE1 in mIMCD3 cells decreased TEER but we also observed (i) increased epithelial absolute permeability to both sodium and chloride, and (ii) that this effect was dependent on kAE1 activity. Further, kAE1 regulated tight junction properties through the tight junction protein claudin-4, a protein with which it physically interacts and colocalizes. These findings unveil a novel interaction between the junctional protein claudin-4 and the kidney anion exchanger, which may be relevant to ion and/or pH homeostasis.
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8
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Jennings ML. Carriers, exchangers, and cotransporters in the first 100 years of the Journal of General Physiology. J Gen Physiol 2018; 150:1063-1080. [PMID: 30030301 PMCID: PMC6080889 DOI: 10.1085/jgp.201812078] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Jennings reviews the many contributions of JGP articles to our current understanding of solute transporter mechanisms. Transporters, pumps, and channels are proteins that catalyze the movement of solutes across membranes. The single-solute carriers, coupled exchangers, and coupled cotransporters that are collectively known as transporters are distinct from conductive ion channels, water channels, and ATP-hydrolyzing pumps. The main conceptual framework for studying transporter mechanisms is the alternating access model, which comprises substrate binding and release events on each side of the permeability barrier and translocation events involving conformational changes between inward-facing and outward-facing conformational states. In 1948, the Journal of General Physiology began to publish work that focused on the erythrocyte glucose transporter—the first transporter to be characterized kinetically—followed by articles on the rates, stoichiometries, asymmetries, voltage dependences, and regulation of coupled exchangers and cotransporters beginning in the 1960s. After the dawn of cDNA cloning and sequencing in the 1980s, heterologous expression systems and site-directed mutagenesis allowed identification of the functional roles of specific amino acid residues. In the past two decades, structures of transport proteins have made it possible to propose specific models for transporter function at the molecular level. Here, we review the contribution of JGP articles to our current understanding of solute transporter mechanisms. Whether the topic has been kinetics, energetics, regulation, mutagenesis, or structure-based modeling, a common feature of these articles has been a quantitative, mechanistic approach, leading to lasting insights into the functions of transporters.
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Affiliation(s)
- Michael L Jennings
- Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, AR
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9
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Sedlyarov V, Eichner R, Girardi E, Essletzbichler P, Goldmann U, Nunes-Hasler P, Srndic I, Moskovskich A, Heinz LX, Kartnig F, Bigenzahn JW, Rebsamen M, Kovarik P, Demaurex N, Superti-Furga G. The Bicarbonate Transporter SLC4A7 Plays a Key Role in Macrophage Phagosome Acidification. Cell Host Microbe 2018; 23:766-774.e5. [PMID: 29779931 PMCID: PMC6002608 DOI: 10.1016/j.chom.2018.04.013] [Citation(s) in RCA: 53] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2017] [Revised: 03/14/2018] [Accepted: 04/19/2018] [Indexed: 12/11/2022]
Abstract
Macrophages represent the first line of immune defense against pathogens, and phagosome acidification is a necessary step in pathogen clearance. Here, we identified the bicarbonate transporter SLC4A7, which is strongly induced upon macrophage differentiation, as critical for phagosome acidification. Loss of SLC4A7 reduced acidification of phagocytosed beads or bacteria and impaired the intracellular microbicidal capacity in human macrophage cell lines. The phenotype was rescued by wild-type SLC4A7, but not by SLC4A7 mutants, affecting transport capacity or cell surface localization. Loss of SLC4A7 resulted in increased cytoplasmic acidification during phagocytosis, suggesting that SLC4A7-mediated, bicarbonate-driven maintenance of cytoplasmic pH is necessary for phagosome acidification. Altogether, we identify SLC4A7 and bicarbonate-driven cytoplasmic pH homeostasis as an important element of phagocytosis and the associated microbicidal functions in macrophages. CRISPR screen identifies SLC4A7 to be required for phagosome acidification Intracellular bacterial killing by macrophages is impaired in absence of SLC4A7 Bicarbonate transport activity of SLC4A7 is required for phagosome acidification SLC4A7 regulates phagosome acidification via homeostatic control of cytoplasmic pH
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Affiliation(s)
- Vitaly Sedlyarov
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Ruth Eichner
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Enrico Girardi
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Patrick Essletzbichler
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Ulrich Goldmann
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Paula Nunes-Hasler
- Department of Cell Physiology and Metabolism, University of Geneva, Geneva 1211, Switzerland
| | - Ismet Srndic
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Anna Moskovskich
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Leonhard X Heinz
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Felix Kartnig
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Johannes W Bigenzahn
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Manuele Rebsamen
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria
| | - Pavel Kovarik
- Max F. Perutz Laboratories, University of Vienna, Vienna Biocenter (VBC), Vienna 1030, Austria
| | - Nicolas Demaurex
- Department of Cell Physiology and Metabolism, University of Geneva, Geneva 1211, Switzerland
| | - Giulio Superti-Furga
- CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, Vienna 1090, Austria; Center for Physiology and Pharmacology, Medical University of Vienna, Vienna 1090, Austria.
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10
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A Ser725Arg mutation in Band 3 abolishes transport function and leads to anemia and renal tubular acidosis. Blood 2018; 131:1759-1763. [PMID: 29483102 DOI: 10.1182/blood-2018-01-827725] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
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11
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Ficici E, Faraldo-Gómez JD, Jennings ML, Forrest LR. Asymmetry of inverted-topology repeats in the AE1 anion exchanger suggests an elevator-like mechanism. J Gen Physiol 2017; 149:1149-1164. [PMID: 29167180 PMCID: PMC5715908 DOI: 10.1085/jgp.201711836] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2017] [Accepted: 10/10/2017] [Indexed: 12/02/2022] Open
Abstract
Anion exchanger 1 catalyzes the transmembrane antiport of chloride and bicarbonate ions through a mechanism that has remained unclear. By modeling its inward-facing state and comparing it with the known outward-facing form, Ficici et al. hypothesize that this transporter features an elevator-like mechanism. The membrane transporter anion exchanger 1 (AE1), or band 3, is a key component in the processes of carbon-dioxide transport in the blood and urinary acidification in the renal collecting duct. In both erythrocytes and the basolateral membrane of the collecting-duct α-intercalated cells, the role of AE1 is to catalyze a one-for-one exchange of chloride for bicarbonate. After decades of biochemical and functional studies, the structure of the transmembrane region of AE1, which catalyzes the anion-exchange reaction, has finally been determined. Each protomer of the AE1 dimer comprises two repeats with inverted transmembrane topologies, but the structures of these repeats differ. This asymmetry causes the putative substrate-binding site to be exposed only to the extracellular space, consistent with the expectation that anion exchange occurs via an alternating-access mechanism. Here, we hypothesize that the unknown, inward-facing conformation results from inversion of this asymmetry, and we propose a model of this state constructed using repeat-swap homology modeling. By comparing this inward-facing model with the outward-facing experimental structure, we predict that the mechanism of AE1 involves an elevator-like motion of the substrate-binding domain relative to the nearly stationary dimerization domain and to the membrane plane. This hypothesis is in qualitative agreement with a wide range of biochemical and functional data, which we review in detail, and suggests new avenues of experimentation.
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Affiliation(s)
- Emel Ficici
- Theoretical Molecular Biophysics Laboratory, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD
| | - José D Faraldo-Gómez
- Theoretical Molecular Biophysics Laboratory, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD
| | - Michael L Jennings
- Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, AR
| | - Lucy R Forrest
- Computational Structural Biology Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD
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12
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Rapp C, Bai X, Reithmeier RAF. Molecular analysis of human solute carrier SLC26 anion transporter disease-causing mutations using 3-dimensional homology modeling. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2017; 1859:2420-2434. [PMID: 28941661 DOI: 10.1016/j.bbamem.2017.09.016] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/25/2016] [Revised: 06/08/2017] [Accepted: 09/14/2017] [Indexed: 12/22/2022]
Abstract
The availability of the first crystal structure of a bacterial member (SLC26Dg) of the solute carrier SLC26 family of anion transporters has allowed us to create 3-dimensional models of all 10 human members (SLC26A1-A11, A10 being a pseudogene) of these membrane proteins using the Phyre2 bioinformatic tool. The homology modeling predicted that the SLC26 human proteins, like the SLC26Dg template, all consist of 14 transmembrane segments (TM) arranged in a 7+7 inverted topology with the amino-termini of two half-helices (TM3 and 10) facing each other in the centre of the protein to create the anion-binding site, linked to a C-terminal cytosolic sulfate transporter anti-sigma factor antagonist (STAS) domain. A plethora of human diseases are associated with mutations in the genes encoding human SLC26 transporters, including chondrodysplasias with varying severity in SLC26A2 (~50 mutations, 27 point mutations), congenital chloride-losing diarrhea in SLC26A3 (~70 mutations, 31 point mutations) and Pendred Syndrome or deafness autosomal recessive type 4 in SLC26A4 (~500 mutations, 203 point mutations). We have localized all of these point mutations in the 3-dimensional structures of the respective SLC26A2, A3 and A4 proteins and systematically analyzed their effect on protein structure. While most disease-causing mutations may cause folding defects resulting in impaired trafficking of these membrane glycoproteins from the endoplasmic reticulum to the cell surface - as demonstrated in a number of functional expression studies - the modeling also revealed that a number of pathogenic mutations are localized to the anion-binding site, which may directly affect transport function.
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Affiliation(s)
- Chloe Rapp
- Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada
| | - Xiaoyun Bai
- Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada
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13
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Structure of Bor1 supports an elevator transport mechanism for SLC4 anion exchangers. Proc Natl Acad Sci U S A 2016; 113:10542-6. [PMID: 27601653 DOI: 10.1073/pnas.1612603113] [Citation(s) in RCA: 69] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Boron is essential for plant growth because of its incorporation into plant cell walls; however, in excess it is toxic to plants. Boron transport and homeostasis in plants is regulated in part by the borate efflux transporter Bor1, a member of the solute carrier (SLC) 4 transporter family with homology to the human bicarbonate transporter Band 3. Here, we present the 4.1-Å resolution crystal structure of Arabidopsis thaliana Bor1. The structure displays a dimeric architecture in which dimerization is mediated by centralized Gate domains. Comparisons with a structure of Band 3 in an outward-open state reveal that the Core domains of Bor1 have rotated inwards to achieve an occluded state. Further structural comparisons with UapA, a xanthine transporter from the nucleobase-ascorbate transporter family, show that the downward pivoting of the Core domains relative to the Gate domains may access an inward-open state. These results suggest that the SLC4, SLC26, and nucleobase-ascorbate transporter families all share an elevator transport mechanism in which alternating access is provided by Core domains that carry substrates across a membrane.
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14
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Reithmeier RAF, Casey JR, Kalli AC, Sansom MSP, Alguel Y, Iwata S. Band 3, the human red cell chloride/bicarbonate anion exchanger (AE1, SLC4A1), in a structural context. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2016; 1858:1507-32. [PMID: 27058983 DOI: 10.1016/j.bbamem.2016.03.030] [Citation(s) in RCA: 133] [Impact Index Per Article: 16.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/14/2015] [Revised: 03/21/2016] [Accepted: 03/29/2016] [Indexed: 02/03/2023]
Abstract
The crystal structure of the dimeric membrane domain of human Band 3(1), the red cell chloride/bicarbonate anion exchanger 1 (AE1, SLC4A1), provides a structural context for over four decades of studies into this historic and important membrane glycoprotein. In this review, we highlight the key structural features responsible for anion binding and translocation and have integrated the following topological markers within the Band 3 structure: blood group antigens, N-glycosylation site, protease cleavage sites, inhibitor and chemical labeling sites, and the results of scanning cysteine and N-glycosylation mutagenesis. Locations of mutations linked to human disease, including those responsible for Southeast Asian ovalocytosis, hereditary stomatocytosis, hereditary spherocytosis, and distal renal tubular acidosis, provide molecular insights into their effect on Band 3 folding. Finally, molecular dynamics simulations of phosphatidylcholine self-assembled around Band 3 provide a view of this membrane protein within a lipid bilayer.
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Affiliation(s)
- Reinhart A F Reithmeier
- Department of Biochemistry, 1 King's College Circle, University of Toronto, Toronto M5S 1A8, Canada.
| | - Joseph R Casey
- Department of Biochemistry, Membrane Protein Disease Research Group, University of Alberta, Edmonton, Alberta T6G 2H7, Canada
| | - Antreas C Kalli
- Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK
| | - Mark S P Sansom
- Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK
| | - Yilmaz Alguel
- Division of Molecular Biosciences, Imperial College London, London, SW7 2AZ, UK
| | - So Iwata
- Division of Molecular Biosciences, Imperial College London, London, SW7 2AZ, UK
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15
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Arakawa T, Kobayashi-Yurugi T, Alguel Y, Iwanari H, Hatae H, Iwata M, Abe Y, Hino T, Ikeda-Suno C, Kuma H, Kang D, Murata T, Hamakubo T, Cameron AD, Kobayashi T, Hamasaki N, Iwata S. Crystal structure of the anion exchanger domain of human erythrocyte band 3. Science 2015; 350:680-4. [PMID: 26542571 DOI: 10.1126/science.aaa4335] [Citation(s) in RCA: 168] [Impact Index Per Article: 18.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Anion exchanger 1 (AE1), also known as band 3 or SLC4A1, plays a key role in the removal of carbon dioxide from tissues by facilitating the exchange of chloride and bicarbonate across the plasma membrane of erythrocytes. An isoform of AE1 is also present in the kidney. Specific mutations in human AE1 cause several types of hereditary hemolytic anemias and/or distal renal tubular acidosis. Here we report the crystal structure of the band 3 anion exchanger domain (AE1(CTD)) at 3.5 angstroms. The structure is locked in an outward-facing open conformation by an inhibitor. Comparing this structure with a substrate-bound structure of the uracil transporter UraA in an inward-facing conformation allowed us to identify the anion-binding position in the AE1(CTD), and to propose a possible transport mechanism that could explain why selected mutations lead to disease.
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Affiliation(s)
- Takatoshi Arakawa
- Japan Science and Technology Agency (JST), Exploratory Research for Advanced Technology (ERATO) Human Receptor Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. JST, Research Acceleration Program, Membrane Protein Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Department of Cell Biology, Kyoto University Faculty of Medicine, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
| | - Takami Kobayashi-Yurugi
- Japan Science and Technology Agency (JST), Exploratory Research for Advanced Technology (ERATO) Human Receptor Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Department of Cell Biology, Kyoto University Faculty of Medicine, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
| | - Yilmaz Alguel
- Division of Molecular Biosciences, Membrane Protein Crystallography group, Imperial College London, London SW7 2AZ, UK. Membrane Protein Laboratory, Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Chilton, Oxfordshire OX11 0DE, UK. Research Complex at Harwell Rutherford, Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0FA, UK
| | - Hiroko Iwanari
- Department of Quantitative Biology and Medicine, Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan
| | - Hinako Hatae
- Faculty of Pharmaceutical Sciences, Nagasaki International University, 2825-7 Huis Ten Bosch-cho, Sasebo, Nagasaki 859-3298, Japan
| | - Momi Iwata
- Division of Molecular Biosciences, Membrane Protein Crystallography group, Imperial College London, London SW7 2AZ, UK. Membrane Protein Laboratory, Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Chilton, Oxfordshire OX11 0DE, UK
| | - Yoshito Abe
- Department of Protein Structure, Function and Design, Graduate School of Pharmaceutical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan
| | - Tomoya Hino
- Japan Science and Technology Agency (JST), Exploratory Research for Advanced Technology (ERATO) Human Receptor Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Department of Cell Biology, Kyoto University Faculty of Medicine, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
| | - Chiyo Ikeda-Suno
- Japan Science and Technology Agency (JST), Exploratory Research for Advanced Technology (ERATO) Human Receptor Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. JST, Research Acceleration Program, Membrane Protein Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Department of Cell Biology, Kyoto University Faculty of Medicine, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
| | - Hiroyuki Kuma
- Faculty of Pharmaceutical Sciences, Nagasaki International University, 2825-7 Huis Ten Bosch-cho, Sasebo, Nagasaki 859-3298, Japan
| | - Dongchon Kang
- Department of Clinical Chemistry and Laboratory Medicine, Graduate School of Medical Sciences, Kyushu University, Fukuoka 812-8582, Japan
| | - Takeshi Murata
- Japan Science and Technology Agency (JST), Exploratory Research for Advanced Technology (ERATO) Human Receptor Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Department of Cell Biology, Kyoto University Faculty of Medicine, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Department of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522, Japan
| | - Takao Hamakubo
- Department of Quantitative Biology and Medicine, Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan
| | - Alexander D Cameron
- Japan Science and Technology Agency (JST), Exploratory Research for Advanced Technology (ERATO) Human Receptor Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Division of Molecular Biosciences, Membrane Protein Crystallography group, Imperial College London, London SW7 2AZ, UK. Membrane Protein Laboratory, Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Chilton, Oxfordshire OX11 0DE, UK. Research Complex at Harwell Rutherford, Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0FA, UK. School of Life Sciences, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
| | - Takuya Kobayashi
- Japan Science and Technology Agency (JST), Exploratory Research for Advanced Technology (ERATO) Human Receptor Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. JST, Research Acceleration Program, Membrane Protein Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Department of Cell Biology, Kyoto University Faculty of Medicine, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Platform for Drug Discovery, Informatics, and Structural Life Science, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
| | - Naotaka Hamasaki
- Faculty of Pharmaceutical Sciences, Nagasaki International University, 2825-7 Huis Ten Bosch-cho, Sasebo, Nagasaki 859-3298, Japan
| | - So Iwata
- Japan Science and Technology Agency (JST), Exploratory Research for Advanced Technology (ERATO) Human Receptor Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. JST, Research Acceleration Program, Membrane Protein Crystallography Project, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Department of Cell Biology, Kyoto University Faculty of Medicine, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Division of Molecular Biosciences, Membrane Protein Crystallography group, Imperial College London, London SW7 2AZ, UK. Membrane Protein Laboratory, Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Chilton, Oxfordshire OX11 0DE, UK. Research Complex at Harwell Rutherford, Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0FA, UK. Platform for Drug Discovery, Informatics, and Structural Life Science, Konoe-cho, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
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16
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Sodium is not required for chloride efflux via chloride/bicarbonate exchanger from rat thymic lymphocytes. BIOMED RESEARCH INTERNATIONAL 2014; 2014:569650. [PMID: 25003116 PMCID: PMC4070514 DOI: 10.1155/2014/569650] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 05/08/2014] [Accepted: 05/20/2014] [Indexed: 12/04/2022]
Abstract
Sodium-dependent Cl−/HCO3− exchanger acts as a chloride (Cl−) efflux in lymphocytes. Its functional characterization had been described when Cl− efflux was measured upon substituting extracellular sodium (Na+) by N-methyl-D-glucamine (NMDG). For Na+ and Cl− substitution, we have used D-mannitol or NMDG. Thymocytes of male Wistar rats aged 7–9 weeks were used and intracellular Cl− was measured by spectrofluorimetry using MQAE dye in bicarbonate buffers. Chloride efflux was measured in a Cl−-free buffer (Cl− substituted with isethionate acid) and in Na+ and Cl−-free buffer with D-mannitol or with NMDG. The data have shown that Cl− efflux is mediated in the absence of Na+ in a solution containing D-mannitol and is inhibited by H2DIDS. Mathematical modelling has shown that Cl− efflux mathematical model parameters (relative membrane permeability, relative rate of exchanger transition, and exchanger efficacy) were the same in control and in the medium in which Na+ had been substituted by D-mannitol. The net Cl− efflux was completely blocked in the NMDG buffer. The same blockage of Cl− efflux was caused by H2DIDS. The study results allow concluding that Na+ is not required for Cl− efflux via Cl−/HCO3− exchanger. NMDG in buffers cannot be used for substituting Na+ because NMDG inhibits the exchanger.
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Lin IW, Sosso D, Chen LQ, Gase K, Kim SG, Kessler D, Klinkenberg PM, Gorder MK, Hou BH, Qu XQ, Carter CJ, Baldwin IT, Frommer WB. Nectar secretion requires sucrose phosphate synthases and the sugar transporter SWEET9. Nature 2014; 508:546-9. [PMID: 24670640 DOI: 10.1038/nature13082] [Citation(s) in RCA: 241] [Impact Index Per Article: 24.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2013] [Accepted: 01/27/2014] [Indexed: 01/09/2023]
Abstract
Angiosperms developed floral nectaries that reward pollinating insects. Although nectar function and composition have been characterized, the mechanism of nectar secretion has remained unclear. Here we identify SWEET9 as a nectary-specific sugar transporter in three eudicot species: Arabidopsis thaliana, Brassica rapa (extrastaminal nectaries) and Nicotiana attenuata (gynoecial nectaries). We show that SWEET9 is essential for nectar production and can function as an efflux transporter. We also show that sucrose phosphate synthase genes, encoding key enzymes for sucrose biosynthesis, are highly expressed in nectaries and that their expression is also essential for nectar secretion. Together these data are consistent with a model in which sucrose is synthesized in the nectary parenchyma and subsequently secreted into the extracellular space via SWEET9, where sucrose is hydrolysed by an apoplasmic invertase to produce a mixture of sucrose, glucose and fructose. The recruitment of SWEET9 for sucrose export may have been a key innovation, and could have coincided with the evolution of core eudicots and contributed to the evolution of nectar secretion to reward pollinators.
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Affiliation(s)
- I Winnie Lin
- 1] Department of Biology, Stanford University, Stanford, California 94305, USA [2] Carnegie Institution for Science, 260 Panama Street, Stanford, California 94305, USA
| | - Davide Sosso
- 1] Department of Biology, Stanford University, Stanford, California 94305, USA [2] Carnegie Institution for Science, 260 Panama Street, Stanford, California 94305, USA
| | - Li-Qing Chen
- Carnegie Institution for Science, 260 Panama Street, Stanford, California 94305, USA
| | - Klaus Gase
- Max Planck Institute for Chemical Ecology, Jena D-07745, Germany
| | - Sang-Gyu Kim
- Max Planck Institute for Chemical Ecology, Jena D-07745, Germany
| | - Danny Kessler
- Max Planck Institute for Chemical Ecology, Jena D-07745, Germany
| | - Peter M Klinkenberg
- 1] Department of Biology, University of Minnesota Duluth, Duluth, Minnesota 55812, USA [2] Department of Plant Biology, University of Minnesota, St Paul, Minnesota 55108, USA
| | - Molly K Gorder
- 1] Department of Biology, University of Minnesota Duluth, Duluth, Minnesota 55812, USA [2] Department of Plant Biology, University of Minnesota, St Paul, Minnesota 55108, USA
| | - Bi-Huei Hou
- Carnegie Institution for Science, 260 Panama Street, Stanford, California 94305, USA
| | - Xiao-Qing Qu
- 1] Carnegie Institution for Science, 260 Panama Street, Stanford, California 94305, USA [2] Key Laboratory of Plant and Soil Interactions, College of Resources and Environmental Sciences, China Agricultural University, 100193 Beijing, China
| | - Clay J Carter
- 1] Department of Biology, University of Minnesota Duluth, Duluth, Minnesota 55812, USA [2] Department of Plant Biology, University of Minnesota, St Paul, Minnesota 55108, USA
| | - Ian T Baldwin
- Max Planck Institute for Chemical Ecology, Jena D-07745, Germany
| | - Wolf B Frommer
- 1] Department of Biology, Stanford University, Stanford, California 94305, USA [2] Carnegie Institution for Science, 260 Panama Street, Stanford, California 94305, USA
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18
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Shnitsar V, Li J, Li X, Calmettes C, Basu A, Casey JR, Moraes TF, Reithmeier RAF. A substrate access tunnel in the cytosolic domain is not an essential feature of the solute carrier 4 (SLC4) family of bicarbonate transporters. J Biol Chem 2013; 288:33848-33860. [PMID: 24121512 DOI: 10.1074/jbc.m113.511865] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
Anion exchanger 1 (AE1; Band 3; SLC4A1) is the founding member of the solute carrier 4 (SLC4) family of bicarbonate transporters that includes chloride/bicarbonate AEs and Na(+)-bicarbonate co-transporters (NBCs). These membrane proteins consist of an amino-terminal cytosolic domain involved in protein interactions and a carboxyl-terminal membrane domain that carries out the transport function. Mutation of a conserved arginine residue (R298S) in the cytosolic domain of NBCe1 (SLC4A4) is linked to proximal renal tubular acidosis and results in impaired transport function, suggesting that the cytosolic domain plays a role in substrate permeation. Introduction of single and double mutations at the equivalent arginine (Arg(283)) and at an interacting glutamate (Glu(85)) in the cytosolic domain of human AE1 (cdAE1) had no effect on the cell surface expression or the transport activity of AE1 expressed in HEK-293 cells. In addition, the membrane domain of AE1 (mdAE1) efficiently mediated anion transport. A 2.1-Å resolution crystal structure of cdΔ54AE1 (residues 55-356 of cdAE1) lacking the amino-terminal and carboxyl-terminal disordered regions, produced at physiological pH, revealed an extensive hydrogen-bonded network involving Arg(283) and Glu(85). Mutations at these residues affected the pH-dependent conformational changes and stability of cdΔ54AE1. As these structural alterations did not impair functional expression of AE1, the cytosolic and membrane domains operate independently. A substrate access tunnel within the cytosolic domain is not present in AE1 and therefore is not an essential feature of the SLC4 family of bicarbonate transporters.
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Affiliation(s)
- Volodymyr Shnitsar
- Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada
| | - Jing Li
- Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada
| | - Xuyao Li
- Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada
| | - Charles Calmettes
- Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada
| | - Arghya Basu
- Department of Biochemistry and Membrane Protein Disease Research Group, University of Alberta, Edmonton, Alberta T6G 2H7, Canada
| | - Joseph R Casey
- Department of Biochemistry and Membrane Protein Disease Research Group, University of Alberta, Edmonton, Alberta T6G 2H7, Canada
| | - Trevor F Moraes
- Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada
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19
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Bonar P, Schneider HP, Becker HM, Deitmer JW, Casey JR. Three-dimensional model for the human Cl-/HCO3- exchanger, AE1, by homology to the E. coli ClC protein. J Mol Biol 2013; 425:2591-608. [PMID: 23583773 DOI: 10.1016/j.jmb.2013.04.005] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2013] [Revised: 03/20/2013] [Accepted: 04/03/2013] [Indexed: 01/19/2023]
Abstract
AE1 mediates electroneutral 1:1 exchange of bicarbonate for chloride across the plasma membrane of erythrocytes and type A cells of the renal collecting duct. No high-resolution structure is available for the AE1 membrane domain, which alone is required for its transport activity. A recent electron microscopy structure of the AE1 membrane domain was proposed to have a similar protein fold to ClC chloride channels. We developed a three-dimensional homology model of the AE1 membrane domain, using the Escherichia coli ClC channel structure as a template. This model agrees well with a long list of biochemically established spatial constraints for AE1. To investigate the AE1 transport mechanism, we created point mutations in regions corresponding to E. coli ClC transport mechanism residues. When expressed in HEK293 cells, several mutants had Cl(-)/HCO3(-) exchange rates significantly different from that of wild-type AE1. When further assessed in Xenopus laevis oocytes, there were significant changes in the transport activity of several AE1 point mutants as assessed by changes in pH. None of the mutants, however, added an electrogenic component to AE1 transport activity. This indicates that the AE1 point mutants altered the transport activity of AE1, without changing its electrogenicity and stoichiometry. The homology model successfully identified residues in AE1 that are critical to AE1 transport activity. Thus, we conclude that AE1 has a similar protein fold to ClC chloride channels.
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Affiliation(s)
- Pamela Bonar
- Membrane Protein Disease Research Group, Department of Biochemistry, University of Alberta, Edmonton, Canada T6G 2H7
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20
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Reimold FR, Stewart AK, Stolpe K, Heneghan JF, Shmukler BE, Alper SL. Substitution of transmembrane domain Cys residues alters pH(o)-sensitive anion transport by AE2/SLC4A2 anion exchanger. Pflugers Arch 2012; 465:839-51. [PMID: 23271450 DOI: 10.1007/s00424-012-1196-6] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2012] [Revised: 11/19/2012] [Accepted: 11/30/2012] [Indexed: 10/27/2022]
Abstract
AE2/SLC4A2 is the most widely expressed of the Na(+)-independent SLC4 Cl(-)/HCO3 (-) exchangers and is essential for postnatal survival, but its structure remains unknown. We have generated and expressed a mouse AE2 construct devoid of transmembrane domain cysteine (Cys) residues, mAE2Cys-less, to enhance the utility of Cys-substitution mutagenesis for structural and structure-function studies of mAE2. mAE2Cys-less expressed in Xenopus oocytes exhibited partial reduction of stilbene disulfonate-sensitive anion exchange activity. This activity was independent of the mAE2 N-terminal cytosolic domain and was accompanied by near-normal surface expression, without change in K 1/2 for extracellular Cl(-). mAE2Cys-less exhibited wildtype activation of anion exchange by hypertonicity and by NH4Cl, and wildtype inhibition of anion exchange by acidic intracellular pH (pHi) in the absence of NH4 (+). However, inhibition of anion exchange by extracellular pH (pHo) exhibited an alkaline shifted pHo(50) value of at least 0.6-0.7 pH units. Although SO4 (2-) transport by mAE2Cys-less resembled wildtype mAE2 in its stimulation by acidic pHo, the absence of transmembrane domain Cys residues abrogated activation of oxalate transport by acidic pHo. The contrasting enhancement of SO4 (2-) transport by alkaline pHo in the mAE1 anion translocation pathway mutant E699Q (Am J Physiol Cell Physiol 295: C302) was phenocopied by the corresponding mutant E1007Q in both AE2 and AE2Cys-less. However, the absence of transmembrane domain Cys residues exacerbated the reduced basal anion transport function exhibited by this and other missense substitutions at AE2 residue E1007. AE2Cys-less will be a valuable experimental tool for structure-function studies of the SLC4 gene family, but its utility for studies of AE2 regulation by extracellular pH must be evaluated in the context of its alkaline-shifted pHo sensitivity, resembling that of AE2 gastric parietal cell variant AE2c1.
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Affiliation(s)
- Fabian R Reimold
- Renal Division and Molecular and Vascular Medicine Division, Beth Israel Deaconess Medical Center, 99 Brookline Avenue, RN-380F, Boston, MA 02215, USA
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21
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Abstract
SLC4A gene family proteins include bicarbonate transporters that move HCO(3)(-) across the plasma membrane and regulate intracellular pH and transepithelial movement of acid-base equivalents. These transporters are Cl/HCO(3) exchangers, electrogenic Na/HCO(3) cotransporters, electroneutral Na/HCO(3) cotransporters, and Na(+)-driven Cl/HCO(3) exchanger. Studies of the bicarbonate transporters in vitro and in vivo have demonstrated their physiological importance for acid-base homeostasis at the cellular and systemic levels. Recent advances in structure/function analysis have also provided valuable information on domains or motifs critical for regulation, ion translocation, and protein topology. This chapter focuses on the molecular mechanisms of ion transport along with associated structural aspects from mutagenesis of particular residues and from chimeric constructs. Structure/function studies have helped to understand the mechanism by which ion substrates are moved via the transporters. This chapter also describes some insights into the structure of SLC4A1 (AE1) and SLC4A4 (NBCe1) transporters. Finally, as some SLC4A transporters exist in concert with other proteins in the cells, the structural features associated with protein-protein interactions are briefly discussed.
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Affiliation(s)
- Inyeong Choi
- Department of Physiology, Emory University, Atlanta, Georgia, USA.
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22
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Chen LQ, Qu XQ, Hou BH, Sosso D, Osorio S, Fernie AR, Frommer WB. Sucrose efflux mediated by SWEET proteins as a key step for phloem transport. Science 2011; 335:207-11. [PMID: 22157085 DOI: 10.1126/science.1213351] [Citation(s) in RCA: 773] [Impact Index Per Article: 59.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Plants transport fixed carbon predominantly as sucrose, which is produced in mesophyll cells and imported into phloem cells for translocation throughout the plant. It is not known how sucrose migrates from sites of synthesis in the mesophyll to the phloem, or which cells mediate efflux into the apoplasm as a prerequisite for phloem loading by the SUT sucrose-H(+) (proton) cotransporters. Using optical sucrose sensors, we identified a subfamily of SWEET sucrose efflux transporters. AtSWEET11 and 12 localize to the plasma membrane of the phloem. Mutant plants carrying insertions in AtSWEET11 and 12 are defective in phloem loading, thus revealing a two-step mechanism of SWEET-mediated export from parenchyma cells feeding H(+)-coupled import into the sieve element-companion cell complex. We discuss how restriction of intercellular transport to the interface of adjacent phloem cells may be an effective mechanism to limit the availability of photosynthetic carbon in the leaf apoplasm in order to prevent pathogen infections.
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Affiliation(s)
- Li-Qing Chen
- Carnegie Institution for Science, 260 Panama Street, Stanford, CA 94305, USA
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23
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Reimold FR, Heneghan JF, Stewart AK, Zelikovic I, Vandorpe DH, Shmukler BE, Alper SL. Pendrin function and regulation in Xenopus oocytes. Cell Physiol Biochem 2011; 28:435-50. [PMID: 22116357 DOI: 10.1159/000335106] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/19/2011] [Indexed: 01/24/2023] Open
Abstract
SLC26A4/PDS mutations cause Pendred Syndrome and non-syndromic deafness. but some aspects of function and regulation of the SLC26A4 polypeptide gene product, pendrin, remain controversial or incompletely understood. We have therefore extended the functional analysis of wildtype and mutant pendrin in Xenopus oocytes, with studies of isotopic flux, electrophysiology, and protein localization. Pendrin mediated electroneutral, pH-insensitive, DIDS-insensitive anion exchange, with extracellular K((1/2)) (in mM) of 1.9 (Cl(-)), 1.8 (I(-)), and 0.9 (Br(-)). The unusual phenotype of Pendred Syndrome mutation E303Q (loss-of-function with normal surface expression) prompted systematic mutagenesis at position 303. Only mutant E303K exhibited loss-of-function unrescued by forced overexpression. Mutant E303C was insensitive to charge modification by methanethiosulfonates. The corresponding mutants SLC26A2 E336Q, SLC26A3 E293Q, and SLC26A6 E298Q exhibited similar loss-of-function phenotypes, with wildtype surface expression also documented for SLC26A2 E336Q. The strong inhibition of wildtype SLC26A2, SLC26A3, and SLC26A6 by phorbol ester contrasts with its modest inhibition of pendrin. Phorbol ester inhibition of SLC26A2, SLC26A3, and SLC26A6 was blocked by coexpressed kinase-dead PKCδ but was without effect on pendrin. Mutation of SLC26A2 serine residues conserved in PKCδ -sensitive SLC26 proteins but absent from pendrin failed to reduce PKCδ sensitivity of SLC26A2 (190).
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Affiliation(s)
- Fabian R Reimold
- Renal Division and Molecular and Vascular Medicine Unit, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA
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24
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Stewart AK, Shmukler BE, Vandorpe DH, Reimold F, Heneghan JF, Nakakuki M, Akhavein A, Ko S, Ishiguro H, Alper SL. SLC26 anion exchangers of guinea pig pancreatic duct: molecular cloning and functional characterization. Am J Physiol Cell Physiol 2011; 301:C289-303. [PMID: 21593449 PMCID: PMC3154555 DOI: 10.1152/ajpcell.00089.2011] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2011] [Accepted: 05/17/2011] [Indexed: 01/02/2023]
Abstract
The secretin-stimulated human pancreatic duct secretes HCO(3)(-)-rich fluid essential for normal digestion. Optimal stimulation of pancreatic HCO(3)(-) secretion likely requires coupled activities of the cystic fibrosis transmembrane regulator (CFTR) anion channel and apical SLC26 Cl(-)/HCO(3)(-) exchangers. However, whereas stimulated human and guinea pig pancreatic ducts secrete ∼140 mM HCO(3)(-) or more, mouse and rat ducts secrete ∼40-70 mM HCO(3)(-). Moreover, the axial distribution and physiological roles of SLC26 anion exchangers in pancreatic duct secretory processes remain controversial and may vary among mammalian species. Thus the property of high HCO(3)(-) secretion shared by human and guinea pig pancreatic ducts prompted us to clone from guinea pig pancreatic duct cDNAs encoding Slc26a3, Slc26a6, and Slc26a11 polypeptides. We then functionally characterized these anion transporters in Xenopus oocytes and human embryonic kidney (HEK) 293 cells. In Xenopus oocytes, gpSlc26a3 mediated only Cl(-)/Cl(-) exchange and electroneutral Cl(-)/HCO(3)(-) exchange. gpSlc26a6 in Xenopus oocytes mediated Cl(-)/Cl(-) exchange and bidirectional exchange of Cl(-) for oxalate and sulfate, but Cl(-)/HCO(3)(-) exchange was detected only in HEK 293 cells. gpSlc26a11 in Xenopus oocytes exhibited pH-dependent Cl(-), oxalate, and sulfate transport but no detectable Cl(-)/HCO(3)(-) exchange. The three gpSlc26 anion transporters exhibited distinct pharmacological profiles of (36)Cl(-) influx, including partial sensitivity to CFTR inhibitors Inh-172 and GlyH101, but only Slc26a11 was inhibited by PPQ-102. This first molecular and functional assessment of recombinant SLC26 anion transporters from guinea pig pancreatic duct enhances our understanding of pancreatic HCO(3)(-) secretion in species that share a high HCO(3)(-) secretory output.
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Affiliation(s)
- Andrew K Stewart
- Renal Division and Vascular Biology Center, Beth Israel Deaconess Medical Center, Department of Medicine, Harvard Medical School, Boston, MA 02215, USA
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25
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Stewart AK, Kedar PS, Shmukler BE, Vandorpe DH, Hsu A, Glader B, Rivera A, Brugnara C, Alper SL. Functional characterization and modified rescue of novel AE1 mutation R730C associated with overhydrated cation leak stomatocytosis. Am J Physiol Cell Physiol 2011; 300:C1034-46. [PMID: 21209359 PMCID: PMC3093938 DOI: 10.1152/ajpcell.00447.2010] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2010] [Accepted: 12/23/2010] [Indexed: 01/20/2023]
Abstract
We report the novel, heterozygous AE1 mutation R730C associated with dominant, overhydrated, cation leak stomatocytosis and well-compensated anemia. Parallel elevations of red blood cell cation leak and ouabain-sensitive Na(+) efflux (pump activity) were apparently unaccompanied by increased erythroid cation channel-like activity, and defined ouabain-insensitive Na(+) efflux pathways of nystatin-treated cells were reduced. Epitope-tagged AE1 R730C at the Xenopus laevis oocyte surface exhibited severely reduced Cl(-) transport insensitive to rescue by glycophorin A (GPA) coexpression or by methanethiosulfonate (MTS) treatment. AE1 mutant R730K preserved Cl(-) transport activity, but R730 substitution with I, E, or H inactivated Cl(-) transport. AE1 R730C expression substantially increased endogenous oocyte Na(+)-K(+)-ATPase-mediated (86)Rb(+) influx, but ouabain-insensitive flux was minimally increased and GPA-insensitive. The reduced AE1 R730C-mediated sulfate influx did not exhibit the wild-type pattern of stimulation by acidic extracellular pH (pH(o)) and, unexpectedly, was partially rescued by exposure to sodium 2-sulfonatoethyl methanethiosulfonate (MTSES) but not to 2-aminoethyl methanethiosulfonate hydrobromide (MTSEA) or 2-(trimethylammonium)ethyl methanethiosulfonate bromide (MTSET). AE1 R730E correspondingly exhibited acid pH(o)-stimulated sulfate uptake at rates exceeding those of wild-type AE1 and AE1 R730K, whereas mutants R730I and R730H were inactive and pH(o) insensitive. MTSES-treated oocytes expressing AE1 R730C and untreated oocytes expressing AE1 R730E also exhibited unprecedented stimulation of Cl(-) influx by acid pH(o). Thus recombinant cation-leak stomatocytosis mutant AE1 R730C exhibits severely reduced anion transport unaccompanied by increased Rb(+) and Li(+) influxes. Selective rescue of acid pH(o)-stimulated sulfate uptake and conferral of acid pH(o)-stimulated Cl(-) influx, by AE1 R730E and MTSES-treated R730C, define residue R730 as critical to selectivity and regulation of anion transport by AE1.
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Affiliation(s)
- Andrew K Stewart
- Division of Nephrology, Beth Israel Deaconess Medical Center, 330 Brookline Ave., Boston, MA 02215, USA
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26
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Sugar transporters for intercellular exchange and nutrition of pathogens. Nature 2010; 468:527-32. [PMID: 21107422 DOI: 10.1038/nature09606] [Citation(s) in RCA: 930] [Impact Index Per Article: 66.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2010] [Accepted: 10/19/2010] [Indexed: 02/07/2023]
Abstract
Sugar efflux transporters are essential for the maintenance of animal blood glucose levels, plant nectar production, and plant seed and pollen development. Despite broad biological importance, the identity of sugar efflux transporters has remained elusive. Using optical glucose sensors, we identified a new class of sugar transporters, named SWEETs, and show that at least six out of seventeen Arabidopsis, two out of over twenty rice and two out of seven homologues in Caenorhabditis elegans, and the single copy human protein, mediate glucose transport. Arabidopsis SWEET8 is essential for pollen viability, and the rice homologues SWEET11 and SWEET14 are specifically exploited by bacterial pathogens for virulence by means of direct binding of a bacterial effector to the SWEET promoter. Bacterial symbionts and fungal and bacterial pathogens induce the expression of different SWEET genes, indicating that the sugar efflux function of SWEET transporters is probably targeted by pathogens and symbionts for nutritional gain. The metazoan homologues may be involved in sugar efflux from intestinal, liver, epididymis and mammary cells.
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27
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Chen LQ, Hou BH, Lalonde S, Takanaga H, Hartung ML, Qu XQ, Guo WJ, Kim JG, Underwood W, Chaudhuri B, Chermak D, Antony G, White FF, Somerville SC, Mudgett MB, Frommer WB. Sugar transporters for intercellular exchange and nutrition of pathogens. Nature 2010. [PMID: 21107422 DOI: 10.1038/nature09606.sugar] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
Sugar efflux transporters are essential for the maintenance of animal blood glucose levels, plant nectar production, and plant seed and pollen development. Despite broad biological importance, the identity of sugar efflux transporters has remained elusive. Using optical glucose sensors, we identified a new class of sugar transporters, named SWEETs, and show that at least six out of seventeen Arabidopsis, two out of over twenty rice and two out of seven homologues in Caenorhabditis elegans, and the single copy human protein, mediate glucose transport. Arabidopsis SWEET8 is essential for pollen viability, and the rice homologues SWEET11 and SWEET14 are specifically exploited by bacterial pathogens for virulence by means of direct binding of a bacterial effector to the SWEET promoter. Bacterial symbionts and fungal and bacterial pathogens induce the expression of different SWEET genes, indicating that the sugar efflux function of SWEET transporters is probably targeted by pathogens and symbionts for nutritional gain. The metazoan homologues may be involved in sugar efflux from intestinal, liver, epididymis and mammary cells.
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Affiliation(s)
- Li-Qing Chen
- Department of Plant Biology, Carnegie Institution for Science, 260 Panama St, Stanford, California 94305, USA
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28
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Alper SL, Stewart AK, Vandorpe DH, Clark JS, Horack RZ, Simpson JE, Walker NM, Clarke LL. Native and recombinant Slc26a3 (downregulated in adenoma, Dra) do not exhibit properties of 2Cl-/1HCO3- exchange. Am J Physiol Cell Physiol 2010; 300:C276-86. [PMID: 21068358 DOI: 10.1152/ajpcell.00366.2010] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
The recent proposal that Dra/Slc26a3 mediates electrogenic 2Cl(-)/1HCO(3)(-) exchange suggests a required revision of classical concepts of electroneutral Cl(-) transport across epithelia such as the intestine. We investigated 1) the effect of endogenous Dra Cl(-)/HCO(3)(-) activity on apical membrane potential (V(a)) of the cecal surface epithelium using wild-type (WT) and knockout (KO) mice; and 2) the electrical properties of Cl(-)/(OH(-))HCO(3)(-) exchange by mouse and human orthologs of Dra expressed in Xenopus oocytes. Ex vivo (36)Cl(-) fluxes and microfluorometry revealed that cecal Cl(-)/HCO(3)(-) exchange was abolished in the Dra KO without concordant changes in short-circuit current. In microelectrode studies, baseline V(a) of Dra KO surface epithelium was slightly hyperpolarized relative to WT but depolarized to the same extent as WT during luminal Cl(-) substitution. Subsequent studies indicated that Cl(-)-dependent V(a) depolarization requires the anion channel Cftr. Oocyte studies demonstrated that Dra-mediated exchange of intracellular Cl(-) for extracellular HCO(3)(-) is accompanied by slow hyperpolarization and a modest outward current, but that the steady-state current-voltage relationship is unaffected by Cl(-) removal or pharmacological blockade. Further, Dra-dependent (36)Cl(-) efflux was voltage-insensitive in oocytes coexpressing the cation channels ENaC or ROMK. We conclude that 1) endogenous Dra and recombinant human/mouse Dra orthologs do not exhibit electrogenic 2Cl(-)/1HCO(3)(-) exchange; and 2) acute induction of Dra Cl(-)/HCO(3)(-) exchange is associated with secondary membrane potential changes representing homeostatic responses. Thus, participation of Dra in coupled NaCl absorption and in uncoupled HCO(3)(-) secretion remains compatible with electroneutrality of these processes, and with the utility of electroneutral transport models for predicting epithelial responses in health and disease.
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Affiliation(s)
- Seth L Alper
- Department of Medicine, Harvard Medical School, Boston, Massachusetts, USA
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29
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Stewart AK, Vandorpe DH, Heneghan JF, Chebib F, Stolpe K, Akhavein A, Edelman EJ, Maksimova Y, Gallagher PG, Alper SL. The GPA-dependent, spherostomatocytosis mutant AE1 E758K induces GPA-independent, endogenous cation transport in amphibian oocytes. Am J Physiol Cell Physiol 2009; 298:C283-97. [PMID: 19907019 DOI: 10.1152/ajpcell.00444.2009] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The previously undescribed heterozygous missense mutation E758K was discovered in the human AE1/SLC4A1/band 3 gene in two unrelated patients with well-compensated hereditary spherostomatocytic anemia (HSt). Oocyte surface expression of AE1 E758K, in contrast to that of wild-type AE1, required coexpressed glycophorin A (GPA). The mutant polypeptide exhibited, in parallel, strong GPA dependence of DIDS-sensitive (36)Cl(-) influx, trans-anion-dependent (36)Cl(-) efflux, and Cl(-)/HCO(3)(-) exchange activities at near wild-type levels. AE1 E758K expression was also associated with GPA-dependent increases of DIDS-sensitive pH-independent SO(4)(2-) uptake and oxalate uptake with altered pH dependence. In marked contrast, the bumetanide- and ouabain-insensitive (86)Rb(+) influx associated with AE1 E758K expression was largely GPA-independent in Xenopus oocytes and completely GPA-independent in Ambystoma oocytes. AE1 E758K-associated currents in Xenopus oocytes also exhibited little or no GPA dependence. (86)Rb(+) influx was higher but inward cation current was lower in oocytes expressing AE1 E758K than previously reported in oocytes expressing the AE1 HSt mutants S731P and H734R. The pharmacological inhibition profile of AE1 E758K-associated (36)Cl(-) influx differed from that of AE1 E758K-associated (86)Rb(+) influx, as well as from that of wild-type AE1-mediated Cl(-) transport. Thus AE1 E758K-expressing oocytes displayed GPA-dependent surface polypeptide expression and anion transport, accompanied by substantially GPA-independent, pharmacologically distinct Rb(+) flux and by small, GPA-independent currents. The data strongly suggest that most of the increased cation transport associated with the novel HSt mutant AE1 E758K reflects activation of endogenous oocyte cation permeability pathways, rather than cation translocation through the mutant polypeptide.
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Affiliation(s)
- Andrew K Stewart
- Division of Nephrology, Beth Israel Deaconess Medical Center, Boston, Massachusetts 02215, USA
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30
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Dai P, Stewart AK, Chebib F, Hsu A, Rozenfeld J, Huang D, Kang D, Lip V, Fang H, Shao H, Liu X, Yu F, Yuan H, Kenna M, Miller DT, Shen Y, Yang W, Zelikovic I, Platt OS, Han D, Alper SL, Wu BL. Distinct and novel SLC26A4/Pendrin mutations in Chinese and U.S. patients with nonsyndromic hearing loss. Physiol Genomics 2009; 38:281-90. [PMID: 19509082 DOI: 10.1152/physiolgenomics.00047.2009] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Mutations of the human SLC26A4/PDS gene constitute the most common cause of syndromic and nonsyndromic hearing loss. Definition of the SLC26A4 mutation spectrum among different populations with sensorineural hearing loss is important for development of optimal genetic screening services for congenital hearing impairment. We screened for SLC26A4 mutations among Chinese and U.S. subjects with hearing loss, using denaturing HPLC (DHPLC) and direct DNA sequencing. Fifty-two of 55 Chinese subjects with deafness accompanied by enlargement of the vestibular aqueduct (EVA) exhibited at least one mutant SLC26A4 allele, whereas SLC26A4 mutations were found in only 2 of 116 deaf Chinese patients without EVA. The spectrum of SLC26A4 mutations differed among Chinese and U.S. subjects and included 10 previously unreported SLC26A4 variants: 4 in the Chinese population (p.E303Q, p.X329, p.X467, p.X573) and 6 in the U.S. population (p.V250A, p.D266N, p.F354S, p.D697A, p.K715N, p.E737D). Among the seven novel in-frame missense mutations, five encoded SLC26A4 proteins with substantially reduced Cl(-)/anion exchange activity as expressed and measured in Xenopus oocytes, but four of these were sufficiently active to allow study of anion selectivity. The only mutant polypeptide exhibiting complete loss of anion exchange function, p.E303Q, was expressed at or near the oocyte surface at near-wild-type levels. Two variants, p.F354S and p.E737D, displayed selective reduction in relative rate of Cl(-)/HCO(3)(-) exchange compared with similarly measured rates of Cl(-)/Cl(-) and Cl(-)/I(-) exchange. Our data show that mutation analysis of the SLC26A4 gene is of high diagnostic yield among subjects with deafness and bilateral EVA in both China and the U.S. However, the pathogenicity of monoallelic SLC26A4 gene variants in patients with hearing loss remains unclear in many instances.
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Affiliation(s)
- Pu Dai
- Department of Otorhinolaryngology Head and Neck Surgery and Genetic Testing Center for Deafness, PLA General Hospital, Beijing, People's Republic of China
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31
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Alper SL. Molecular physiology and genetics of Na+-independent SLC4 anion exchangers. J Exp Biol 2009; 212:1672-83. [PMID: 19448077 PMCID: PMC2683012 DOI: 10.1242/jeb.029454] [Citation(s) in RCA: 170] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 03/12/2009] [Indexed: 01/12/2023]
Abstract
Plasmalemmal Cl(-)/HCO(3)(-) exchangers are encoded by the SLC4 and SLC26 gene superfamilies, and function to regulate intracellular pH, [Cl(-)] and cell volume. The Cl(-)/HCO(3)(-) exchangers of polarized epithelial cells also contribute to transepithelial secretion and reabsorption of acid-base equivalents and Cl(-). This review focuses on Na(+)-independent electroneutral Cl(-)/HCO(3)(-) exchangers of the SLC4 family. Human SLC4A1/AE1 mutations cause the familial erythroid disorders of spherocytic anemia, stomatocytic anemia and ovalocytosis. A largely discrete set of AE1 mutations causes familial distal renal tubular acidosis. The Slc4a2/Ae2(-/-) mouse dies before weaning with achlorhydria and osteopetrosis. A hypomorphic Ae2(-/-) mouse survives to exhibit male infertility with defective spermatogenesis and a syndrome resembling primary biliary cirrhosis. A human SLC4A3/AE3 polymorphism is associated with seizure disorder, and the Ae3(-/-) mouse has increased seizure susceptibility. The transport mechanism of mammalian SLC4/AE polypeptides is that of electroneutral Cl(-)/anion exchange, but trout erythroid Ae1 also mediates Cl(-) conductance. Erythroid Ae1 may mediate the DIDS-sensitive Cl(-) conductance of mammalian erythrocytes, and, with a single missense mutation, can mediate electrogenic SO(4)(2-)/Cl(-) exchange. AE1 trafficking in polarized cells is regulated by phosphorylation and by interaction with other proteins. AE2 exhibits isoform-specific patterns of acute inhibition by acidic intracellular pH and independently by acidic extracellular pH. In contrast, AE2 is activated by hypertonicity and, in a pH-independent manner, by ammonium and by hypertonicity. A growing body of structure-function and interaction data, together with emerging information about physiological function and structure, is advancing our understanding of SLC4 anion exchangers.
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Affiliation(s)
- Seth L Alper
- Renal Division and Molecular and Vascular Medicine Unit, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA.
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32
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Chernova MN, Stewart AK, Barry PN, Jennings ML, Alper SL. Mouse Ae1 E699Q mediates SO42-i/anion-o exchange with [SO42-]i-dependent reversal of wild-type pHo sensitivity. Am J Physiol Cell Physiol 2008; 295:C302-12. [PMID: 18480299 DOI: 10.1152/ajpcell.00109.2008] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The SLC4A1/AE1 gene encodes the electroneutral Cl(-)/HCO(3)(-) exchanger of erythrocytes and renal type A intercalated cells. AE1 mutations cause familial spherocytic and stomatocytic anemias, ovalocytosis, and distal renal tubular acidosis. The mutant mouse Ae1 polypeptide E699Q expressed in Xenopus oocytes cannot mediate Cl(-)/HCO(3)(-) exchange or (36)Cl(-) efflux but exhibits enhanced dual sulfate efflux mechanisms: electroneutral exchange of intracellular sulfate for extracellular sulfate (SO(4)(2-)(i)/SO(4)(2-)(o) exchange), and electrogenic exchange of intracellular sulfate for extracellular chloride (SO(4)(2-)(i)/Cl(-)(o) exchange). Whereas wild-type AE1 mediates 1:1 H(+)/SO(4)(2-) cotransport in exchange for either Cl(-) or for the H(+)/SO(4)(2-) ion pair, mutant Ae1 E699Q transports sulfate without cotransport of protons, similar to human erythrocyte AE1 in which the corresponding E681 carboxylate has been chemically converted to the alcohol (hAE1 E681OH). We now show that in contrast to the normal cis-stimulation by protons of wild-type AE1-mediated SO(4)(2-) transport, both SO(4)(2-)(i)/Cl(-)(o) exchange and SO(4)(2-)(i)/SO(4)(2-)(o) exchange mediated by mutant Ae1 E699Q are inhibited by acidic pH(o) and activated by alkaline pH(o). hAE1 E681OH displays a similarly altered pH(o) dependence of SO(4)(2-)(i)/Cl(-)(o) exchange. Elevated [SO(4)(2-)](i) increases the K(1/2) of Ae1 E699Q for both extracellular Cl(-) and SO(4)(2-), while reducing inhibition of both exchange mechanisms by acid pH(o). The E699Q mutation also leads to increased potency of self-inhibition by extracellular SO(4)(2-). Study of the Ae1 E699Q mutation has revealed the existence of a novel pH-regulatory site of the Ae1 polypeptide and should continue to provide valuable paths toward understanding substrate selectivity and self-inhibition in SLC4 anion transporters.
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Affiliation(s)
- Marina N Chernova
- Molecular and Vascular Medicine Unit, Beth Israel Deaconess Med. Ctr., 330 Brookline Ave., Boston, MA 02215, USA
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33
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Stewart AK, Kurschat CE, Vaughan-Jones RD, Shmukler BE, Alper SL. Acute regulation of mouse AE2 anion exchanger requires isoform-specific amino acid residues from most of the transmembrane domain. J Physiol 2007; 584:59-73. [PMID: 17690150 PMCID: PMC2277056 DOI: 10.1113/jphysiol.2007.136119] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
The widely expressed anion exchanger polypeptide AE2/SLC4A2 is acutely inhibited by acidic intracellular (pH(i)), by acidic extracellular pH (pH(o)), and by the calmodulin inhibitor, calmidazolium, whereas it is acutely activated by NH(4)(+). The homologous erythroid/kidney AE1/SLC4A1 polypeptide is insensitive to these regulators. Each of these AE2 regulatory responses requires the presence of AE2's C-terminal transmembrane domain (TMD). We have now measured (36)Cl(-) efflux from Xenopus oocytes expressing bi- or tripartite AE2-AE1 chimeras to define TMD subregions in which AE2-specific sequences contribute to acute regulation. The chimeric AE polypeptides were all functional at pH(o) 7.4, with the sole exception of AE2((1-920))/AE1((613-811))/AE2((1120-1237)). Reciprocal exchanges of the large third extracellular loops were without effect. AE2 regulation by pH(i), pH(o) and NH(4)(+) was retained after substitution of C-terminal AE2 amino acids 1120-1237 (including the putative second re-entrant loop, two TM spans and the cytoplasmic tail) with the corresponding AE1 sequence. In contrast, the presence of this AE2 C-terminal sequence was both necessary and sufficient for inhibition by calmidazolium. All other tested TMD substitutions abolished AE2 pH(i) sensitivity, abolished or severely attenuated sensitivity to pH(o) and removed sensitivity to NH(4)(+). Loss of AE2 pH(i) sensitivity was not rescued by co-expression of a complementary AE2 sequence within separate full-length chimeras or AE2 subdomains. Thus, normal regulation of AE2 by pH and other ligands requires AE2-specific sequence from most regions of the AE2 TMD, with the exceptions of the third extracellular loop and a short C-terminal sequence. We conclude that the individual TMD amino acid residues previously identified as influencing acute regulation of AE2 exert that influence within a regulatory structure requiring essential contributions from multiple regions of the AE2 TMD.
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Affiliation(s)
- A K Stewart
- Department of Medicine, Harvard Medical School, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA
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34
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Martial S, Guizouarn H, Gabillat N, Pellissier B, Borgese F. Consequences of point mutations in trout anion exchanger 1 (tAE1) transmembrane domains: evidence that tAE1 can behave as a chloride channel. J Cell Physiol 2006; 207:829-35. [PMID: 16508976 DOI: 10.1002/jcp.20631] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
In this study, we have shown that, when expressed in Xenopus oocytes, trout anion exchanger 1 (tAE1) was able to act as a bifunctional protein, either an anion exchanger or a chloride conductance. Point mutations of tAE1 were carried out and their effect on Cl- conductance and Cl- unidirectional flux were studied. We have shown that mutations made in transmembrane domain 7 had dramatic effects on tAE1 function. Indeed, when these residues were mutated, either individually or together (mutants E632K, D633G, and ED/KG), Cl- conductance was reduced to 28-44% that of wild-type tAE1. Moreover, ion substitution experiments showed that anion selectivity was altered. However, the exchanger function was unchanged, as evidenced by the fact that Cl- influx and K(m) were identical for each of these mutants and similar to the wild-type protein parameters. By contrast, mutations made in the C-terminal domains of the protein (R819M, Q829K) affected both transport functions. Cl- conductance was increased by approximately 200% with respect to tAE1 and anion selectivity was impaired. Likewise, Cl- influx was increased by approximately 260% and was no longer saturable. These and other mutations carried out in transmembrane domains 7, 8, 12-14 of tAE1 allow us to demonstrate without doubt that, in addition to its anion exchanger activity, tAE1 can also function as a chloride channel. Above all, this work led us to identify amino acids involved in this double function organization.
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Affiliation(s)
- Sonia Martial
- Laboratoire de Physiologie des Membranes Cellulaires, FRE 2721, CNRS-Université de Nice, Bâtiment de Sciences Naturelles, Parc Valrose, 28 av Valrose Nice Cedex 2, France
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35
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Pushkin A, Kurtz I. SLC4 base (HCO3 -, CO3 2-) transporters: classification, function, structure, genetic diseases, and knockout models. Am J Physiol Renal Physiol 2006; 290:F580-99. [PMID: 16461757 DOI: 10.1152/ajprenal.00252.2005] [Citation(s) in RCA: 116] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
In prokaryotic and eukaryotic organisms, biochemical and physiological processes are sensitive to changes in H(+) activity. For these processes to function optimally, a variety of proteins have evolved that transport H(+)/base equivalents across cell and organelle membranes, thereby maintaining the pH of various intracellular and extracellular compartments within specific limits. The SLC4 family of base (HCO(3)(-), CO(3)(2(-))) transport proteins plays an essential role in mediating Na(+)- and/or Cl(-)-dependent base transport in various tissues and cell types in mammals. In addition to pH regulation, specific members of this family also contribute to vectorial transepithelial base transport in several organ systems including the kidney, pancreas, and eye. The importance of these transporters in mammalian cell biology is highlighted by the phenotypic abnormalities resulting from spontaneous SLC4 mutations in humans and targeted deletions in murine knockout models. This review focuses on recent advances in our understanding of the molecular organization and functional properties of SLC4 transporters and their role in disease.
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Affiliation(s)
- Alexander Pushkin
- Division of Nephrology, David Geffen School of Medicine at UCLA, University of California-Los Angeles, 10833 Le Conte Avenue, Rm. 7-155 Factor Bldg., Los Angeles, CA 90095, USA
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36
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Chernova MN, Vandorpe DH, Clark JS, Williams JI, Zasloff MA, Jiang L, Alper SL. Apparent receptor-mediated activation of Ca2+-dependent conductive Cl−transport by shark-derived polyaminosterols. Am J Physiol Regul Integr Comp Physiol 2005; 289:R1644-58. [PMID: 16109810 DOI: 10.1152/ajpregu.00098.2005] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The shark liver antimicrobial polyaminosterol squalamine is an angiogenesis inhibitor under clinical investigation as an anti-cancer agent and as a treatment for the choroidal neovascularization associated with macular degeneration of the retina. The related polyaminosterol MSI-1436 is an appetite suppressant that decreases systemic insulin resistance. However, the mechanisms of action of these polyaminosterols are unknown. We report effects of MSI-1436 on Xenopus oocytes consistent with the existence of a receptor for polyaminosterols. MSI-1436 activates bidirectional, trans-chloride-independent Cl-flux in Xenopus oocytes. At least part of this DIDS-sensitive Cl−flux is conductive, as measured using two-electrode voltage-clamp and on-cell patch-clamp techniques. MSI-1436 also elevates cytosolic Ca2+concentration ([Ca2+]) and increases bidirectional45Ca2+flux. Activation of Cl−flux and elevation of cytosolic [Ca2+] by MSI-1436 both are accelerated by lowering bath Ca2+and are not acutely inhibited by extracellular EGTA. Elevation of cytosolic [Ca2+] by MSI-1436 requires heparin-sensitive intracellular Ca2+stores. Although injected EGTA abolishes the increased conductive Cl−flux, that Cl−flux is not dependent on heparin-sensitive stores. In low-bath Ca2+conditions, several structurally related polyaminosterols act as strong agonists or weak agonists of conductive Cl−flux in oocytes. Weak agonist polyaminosterols antagonize the strong agonist, MSI-1436, but upon addition of the conductive Cl−transport inhibitor DIDS, they are converted into strong agonists. Together, these properties operationally define a polyaminosterol receptor at or near the surface of the Xenopus oocyte, provide an initial description of receptor signaling, and suggest routes toward further understanding of a novel class of appetite suppressants and angiogenesis inhibitors.
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Affiliation(s)
- Marina N Chernova
- Molecular and Vascular Medicine Unit, Beth Israel Deaconess Medical Center, Department of Medicine, Harvard Medical School, Boston, MA 02215, USA
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37
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Abstract
Plasmalemmal Cl- -HCO3- exchangers regulate intracellular pH and [Cl-] and cell volume. In polarized epithelial cells, they contribute also to transepithelial secretion and reabsorption of acid-base equivalents and of Cl-. Members of both the SLC4 and SLC26 mammalian gene families encode Na+-independent Cl- -HCO3- exchangers. Human SLC4A1/AE1 mutations cause either the erythroid disorders spherocytic haemolytic anaemia or ovalocytosis, or distal renal tubular acidosis. SLC4A2/AE2 knockout mice die at weaning. Human SLC4A3/AE3 polymorphisms have been associated with seizure disorder. Although mammalian SLC4/AE polypeptides mediate only electroneutral Cl- -anion exchange, trout erythroid AE1 also promotes osmolyte transport and increased anion conductance. Mouse AE1 is required for DIDS-sensitive erythroid Cl- conductance, but definitive evidence for mediation of Cl- conductance is lacking. However, a single missense mutation allows AE1 to mediate both electrogenic SO4(2-) -Cl- exchange or electroneutral, H+-independent SO4(2)- -SO4(2-) exchange. In the Xenopus oocyte, the AE1 C-terminal cytoplasmic tail residues reported to bind carbonic anhydrase II are dispensable for Cl- -Cl- exchange, but required for Cl- -HCO3- exchange. AE2 is acutely and independently inhibited by intracellular and extracellular H+, and this regulation requires integrity of the most highly conserved sequence of the AE2 N-terminal cytoplasmic domain. Individual missense mutations within this and adjacent regions identify additional residues which acid-shift pHo sensitivity. These regions together are modelled to form contiguous surface patches on the AE2 cytoplasmic domain. In contrast, the N-terminal variant AE2c polypeptide exhibits an alkaline-shifted pHo sensitivity, as do certain transmembrane domain His mutants. AE2-mediated anion exchange is also stimulated by ammonium and by hypertonicity by a mechanism sensitive to inhibition by chelation of intracellular Ca2+ and by calmidazolium. This growing body of structure-function data, together with increased structural information, will advance mechanistic understanding of SLC4 anion exchangers.
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MESH Headings
- Acidosis, Renal Tubular/genetics
- Acidosis, Renal Tubular/metabolism
- Amino Acid Sequence
- Animals
- Anion Exchange Protein 1, Erythrocyte/chemistry
- Anion Exchange Protein 1, Erythrocyte/genetics
- Anion Exchange Protein 1, Erythrocyte/metabolism
- Anion Transport Proteins/chemistry
- Anion Transport Proteins/genetics
- Anion Transport Proteins/metabolism
- Antiporters/chemistry
- Antiporters/genetics
- Antiporters/metabolism
- Carbonic Anhydrases/metabolism
- Chloride-Bicarbonate Antiporters/chemistry
- Chloride-Bicarbonate Antiporters/genetics
- Chloride-Bicarbonate Antiporters/metabolism
- Elliptocytosis, Hereditary/genetics
- Elliptocytosis, Hereditary/metabolism
- Humans
- Hydrogen-Ion Concentration
- Molecular Sequence Data
- Multigene Family
- Mutation
- Polymorphism, Genetic
- Protein Conformation
- SLC4A Proteins
- Spherocytosis, Hereditary/genetics
- Spherocytosis, Hereditary/metabolism
- Structure-Activity Relationship
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Affiliation(s)
- Seth L Alper
- Molecular and Vascular Medicine Unit, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA.
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38
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Shmukler BE, Kurschat CE, Ackermann GE, Jiang L, Zhou Y, Barut B, Stuart-Tilley AK, Zhao J, Zon LI, Drummond IA, Vandorpe DH, Paw BH, Alper SL. Zebrafish slc4a2/ae2 anion exchanger: cDNA cloning, mapping, functional characterization, and localization. Am J Physiol Renal Physiol 2005; 289:F835-49. [PMID: 15914778 DOI: 10.1152/ajprenal.00122.2005] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Although the zebrafish has been used increasingly for the study of pronephric kidney development, studies of renal ion transporters and channels of the zebrafish remain few. We report the cDNA cloning and characterization of the AE2 anion exchanger ortholog from zebrafish kidney, slc4a2/ae2. The ae2 gene in linkage group 2 encodes a polypeptide of 1,228 aa exhibiting 64% aa identity with mouse AE2a. The exon-intron boundaries of the zebrafish ae2 gene are nearly identical to those of the rodent and human genes. Whole-mount in situ hybridization detects ae2 mRNA in prospective midbrain as early as the five-somite stage, then later in the pronephric primordia and the forming pronephric duct, where it persists through 72 h postfertilization (hpf). Zebrafish Ae2 expressed in Xenopus laevis oocytes mediates Na(+)-independent, electroneutral (36)Cl(-)/Cl(-) exchange moderately sensitive to inhibition by DIDS, is inhibited by acidic intracellular pH and by acidic extracellular pH, but activated by (acidifying) ammonium and by hypertonicity. Zebrafish Ae2 also mediates Cl(-)/HCO(3)(-) exchange in X. laevis oocytes and accumulates in or near the plasma membrane in transfected HEK-293 cells. In 24-48 hpf zebrafish embryos, the predominant but not exclusive localization of Ae2 polypeptide is the apical membrane of pronephric duct epithelial cells. Thus Ae2 resembles its mammalian orthologs in function, mechanism, and acute regulation but differs in its preferentially apical expression in kidney. These results will inform tests of the role of Ae2 in zebrafish kidney development and function.
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Affiliation(s)
- Boris E Shmukler
- Molecular Medicine and Renal Units, Beth Israel Deaconess Med. Ctr. E/RW763, 330 Brookline Ave., Boston, MA 02215, USA
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39
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Abuladze N, Azimov R, Newman D, Sassani P, Liu W, Tatishchev S, Pushkin A, Kurtz I. Critical amino acid residues involved in the electrogenic sodium-bicarbonate cotransporter kNBC1-mediated transport. J Physiol 2005; 565:717-30. [PMID: 15817634 PMCID: PMC1464572 DOI: 10.1113/jphysiol.2005.084988] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
We have previously reported a topological model of the electrogenic Na(+)-HCO(3)(-) cotransporter (NBC1) in which the cotransporter spans the plasma membrane 10 times with N- and C-termini localized intracellularly. An analysis of conserved amino acid residues among members of the SLC4 superfamily in both the transmembrane segments (TMs) and intracellular/extracellular loops (ILs/ELs) provided the basis for the mutagenesis approach taken in the present study to determine amino acids involved in NBC1-mediated ion transport. Using large-scale mutagenesis, acidic and basic amino acids putatively involved in ion transport mediated by the predominant variant of NBC1 expressed in the kidney (kNBC1) were mutated to neutral and/or oppositely charged amino acids. All mutant kNBC1 cotransporters were expressed in HEK-293T cells and the Na(+)-dependent base flux of the mutants was determined using intracellular pH measurements with 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Critical glutamate, aspartate, lysine, arginine and histidine residues in ILs/ELs and TMs were detected that were essential for kNBC1-mediated Na(+)-dependent base transport. In addition, critical phenylalanine, serine, tyrosine, threonine and alanine residues in TMs and ILs/ELs were detected. Furthermore, several amino acid residues in ILs/ELs and TMs were shown to be essential for membrane targeting. The data demonstrate asymmetry of distribution of kNBC1 charged amino acids involved in ion recognition in putative outward-facing and inward-facing conformations. A model summarizing key amino acid residues involved in kNBC1-mediated ion transport is presented.
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Affiliation(s)
- Natalia Abuladze
- Division of Nephrology, David Geffen School of Medicine at UCLA, 10833 Le Conte Avenue, Room 7-155 Factor Building, Los Angeles, CA 90095, USA
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40
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Sherman T, Chernova MN, Clark JS, Jiang L, Alper SL, Nehrke K. The abts and sulp families of anion transporters from Caenorhabditis elegans. Am J Physiol Cell Physiol 2005; 289:C341-51. [PMID: 15814591 DOI: 10.1152/ajpcell.00071.2005] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The slc4 and slc26 gene families encode two distinct groups of gene products that transport HCO(3)(-) and other anions in mammalian cells. The SLC4 and SLC26 proteins are important contributors to transepithelial movement of fluids and electrolytes and to cellular pH and volume regulation. Herein we describe the cDNA cloning from the nematode Caenorhabditis elegans of four anion bicarbonate transporter (abts) homologs of slc4 cDNA and eight sulfate permease (sulp) homologs of slc26 cDNA. Analysis of transgenic nematode strains carrying promoter::GFP fusions suggests relatively restricted expression patterns for many of these genes. At least three genes are expressed primarily in the intestine, three are expressed primarily in the excretory cell, and one is expressed in both of these polarized cell types. One of the genes is also expressed exclusively in the myoepithelium-like cells of the pharynx. Many of the sulp gene products localize to the basolateral membrane rather than to the apical membrane. Several ABTS and SULP proteins exhibited anion transport function in Xenopus oocytes. The strongest Cl(-) transporter among these also mediated Cl(-)/HCO(3)(-) exchange. These findings encourage exploitation of the genetic strengths of the nematode model system in the study of the physiological roles of anion transport by the proteins of these two highly conserved gene families.
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Affiliation(s)
- Teresa Sherman
- Department of Medicine (Nephrology Unit), University of Rochester Medical Center, Rochester, New York 14642, USA
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41
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Jennings ML. Evidence for a second binding/transport site for chloride in erythrocyte anion transporter AE1 modified at glutamate 681. Biophys J 2005; 88:2681-91. [PMID: 15653731 PMCID: PMC1305364 DOI: 10.1529/biophysj.104.056812] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Transport kinetics have been examined in erythrocyte anion transporter AE1 that has been chemically modified to convert glutamate 681 to an alcohol (E681OH AE1). Outward conductive Cl(-) flux in E681OH AE1 is inhibited by removal of extracellular Cl(-); this effect is the opposite of that in native AE1 and is consistent with coupled electrogenic 2:1 Cl(-)/Cl(-) exchange. A second Cl(-) binding/transport site is also suggested by the characteristics of (35)SO(4)(2-) flux in E681OH AE1: bilateral and cis Cl(-), which are normally inhibitory, accelerate (35)SO(4)(2-) flux. These effects would be expected if Cl(-) binds to a second transport site on SO(4)(2-)-loaded E681OH AE1, thereby allowing Cl(-)/SO(4)(2-) cotransport. Alternatively, the data can be explained without proposing Cl(-)/SO(4)(2-) cotransport if the rate-limiting event for (35)SO(4)(2-)/SO(4)(2-) exchange is external SO(4)(2-) release, and the binding of external Cl(-) accelerates SO(4)(2-) release. With either interpretation, these data indicate that E681OH AE1 has a binding/transport site for Cl(-) that is distinct from the main transport site. The effects of graded modification of E681 or inhibition by H(2)DIDS are consistent with the idea that the new Cl(-) binding site is on the same E681OH-modified subunit of the AE1 dimer as the normal transport site.
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Affiliation(s)
- Michael L Jennings
- Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA.
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42
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Chernova MN, Jiang L, Friedman DJ, Darman RB, Lohi H, Kere J, Vandorpe DH, Alper SL. Functional comparison of mouse slc26a6 anion exchanger with human SLC26A6 polypeptide variants: differences in anion selectivity, regulation, and electrogenicity. J Biol Chem 2004; 280:8564-80. [PMID: 15548529 DOI: 10.1074/jbc.m411703200] [Citation(s) in RCA: 125] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023] Open
Abstract
The unusually low 78% amino acid identity between the orthologous human SLC26A6 and mouse slc26a6 polypeptides prompted systematic comparison of their anion transport functions in Xenopus oocytes. Multiple human SLC26A6 variant polypeptides were also functionally compared. Transport was studied as unidirectional fluxes of (36)Cl(-), [(14)C]oxalate, and [(35)S]sulfate; as net fluxes of HCO(3)(-) by fluorescence ratio measurement of intracellular pH; as current by two-electrode voltage clamp; and as net Cl(-) flux by fluorescence intensity measurement of relative changes in extracellular and intracellular [Cl(-)]. Four human SLC26A6 polypeptide variants each exhibited rates of bidirectional [(14)C]oxalate flux, Cl(-)/HCO(3)(-) exchange, and Cl(-)/OH(-) exchange nearly equivalent to those of mouse slc26a6. Cl(-)/HCO(3)(-) exchange by both orthologs was cAMP-sensitive, further enhanced by coexpressed wild type cystic fibrosis transmembrane regulator but inhibited by cystic fibrosis transmembrane regulator DeltaF508. However, the very low rates of (36)Cl(-) and [(35)S]sulfate transport by all active human SLC26A6 isoforms contrasted with the high rates of the mouse ortholog. Human and mouse orthologs also differed in patterns of acute regulation. Studies of human-mouse chimeras revealed cosegregation of the high (36)Cl(-) transport phenotype with the transmembrane domain of mouse slc26a6. Mouse slc26a6 and human SLC26A6 each mediated electroneutral Cl(-)/HCO(3)(-) and Cl(-)/OH(-) exchange. In contrast, whereas Cl(-)/oxalate exchange by mouse slc26a6 was electrogenic, that mediated by human SLC26A6 appeared electroneutral. The increased currents observed in oocytes expressing either mouse or human ortholog were pharmacologically distinct from the accompanying monovalent anion exchange activities. The human SLC26A6 polypeptide variants SLC26A6c and SLC26A6d were inactive as transporters of oxalate, sulfate, and chloride. Thus, the orthologous mouse and human SLC26A6 proteins differ in anion selectivity, transport mechanism, and acute regulation, but both mediate electroneutral Cl(-)/HCO(3)(-) exchange.
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Affiliation(s)
- Marina N Chernova
- Molecular and Vascular Medicine Unit and Renal Unit, Beth Israel Deaconess Medical Center and Department of Medicine, Harvard Medical School, Boston, Massachusetts 02215, USA
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43
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Mount DB, Romero MF. The SLC26 gene family of multifunctional anion exchangers. Pflugers Arch 2004; 447:710-21. [PMID: 12759755 DOI: 10.1007/s00424-003-1090-3] [Citation(s) in RCA: 372] [Impact Index Per Article: 18.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2003] [Accepted: 04/03/2003] [Indexed: 12/18/2022]
Abstract
The ten-member SLC26 gene family encodes anion exchangers capable of transporting a wide variety of monovalent and divalent anions. The physiological role(s) of individual paralogs is evidently due to variation in both anion specificity and expression pattern. Three members of the gene family are involved in genetic disease; SLC26A2 in chondrodysplasias, SLC26A3 in chloride-losing diarrhea, and SLC26A4 in Pendred syndrome and hereditary deafness (DFNB4). The analysis of Slc26a4-null mice has significantly enhanced the understanding of the roles of this gene in both health and disease. Targeted deletion of Slc26a5 has in turn revealed that this paralog is essential for electromotor activity of cochlear outer hair cells and thus for cochlear amplification. Anions transported by the SLC26 family, with variable specificity, include the chloride, sulfate, bicarbonate, formate, oxalate and hydroxyl ions. The functional versatility of SLC26A6 identifies it as the primary candidate for the apical Cl(-)-formate/oxalate and Cl(-)-base exchanger of brush border membranes in the renal proximal tubule, with a central role in the reabsorption of Na(+)-Cl(-) from the glomerular ultrafiltrate. At least three of the SLC26 exchangers mediate electrogenic Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange; the stoichiometry of Cl(-)-HCO(3)(-) exchange appears to differ between SLC26 paralogs, such that SLC26A3 transports >/=2 Cl(-) ions per HCO(3)(-) ion, whereas SLC26A6 transports >/=2 HCO(3)(-) ions per Cl(-) ion. SLC26 Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange is activated by the cystic fibrosis transmembrane regulator (CFTR), implicating defective regulation of these exchangers in the reduced HCO(3)(-) transport seen in cystic fibrosis and related disorders; CFTR-independent activation of these exchangers is thus an important and novel goal for the future therapy of cystic fibrosis.
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Affiliation(s)
- David B Mount
- Renal Divisions, VA Boston Healthcare System and Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115, USA.
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44
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Dahl NK, Jiang L, Chernova MN, Stuart-Tilley AK, Shmukler BE, Alper SL. Deficient HCO3- transport in an AE1 mutant with normal Cl- transport can be rescued by carbonic anhydrase II presented on an adjacent AE1 protomer. J Biol Chem 2003; 278:44949-58. [PMID: 12933803 DOI: 10.1074/jbc.m308660200] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Cl-/HCO3- exchange activity mediated by the AE1 anion exchanger is reduced by carbonic anhydrase II (CA2) inhibition or by prevention of CA2 binding to the AE1 C-terminal cytoplasmic tail. This type of AE1 inhibition is thought to represent reduced metabolic channeling of HCO3- to the intracellular HCO3- binding site of AE1. To test the hypothesis that CA2 binding might itself allosterically activate AE1 in Xenopus oocytes, we compared Cl-/Cl- and Cl-/HCO3- exchange activities of AE1 polypeptides with truncation and missense mutations in the C-terminal tail. The distal renal tubular acidosis-associated AE1 901X mutant exhibited both Cl-/Cl- and Cl-/HCO3- exchange activities. In contrast, AE1 896X, 891X, and AE1 missense mutants in the CA2 binding site were inactive as Cl-/HCO3- exchangers despite exhibiting normal Cl-/Cl- exchange activities. Co-expression of CA2 enhanced wild-type AE1-mediated Cl-/HCO3- exchange, but not Cl-/Cl- exchange. CA2 co-expression could not rescue Cl-/HCO3- exchange activity in AE1 mutants selectively impaired in Cl-/HCO3- exchange. However, co-expression of transport-incompetent AE1 mutants with intact CA2 binding sites completely rescued Cl-/HCO3- exchange by an AE1 missense mutant devoid of CA2 binding, with activity further enhanced by CA2 co-expression. The same transport-incompetent AE1 mutants failed to rescue Cl-/HCO3- exchange by the AE1 truncation mutant 896X, despite preservation of the latter's core CA2 binding site. These data increase the minimal extent of a functionally defined CA2 binding site in AE1. The inter-protomeric rescue of HCO3- transport within the AE1 dimer shows functional proximity of the C-terminal cytoplasmic tail of one protomer to the anion translocation pathway in the adjacent protomer within the AE1 heterodimer. The data strongly support the hypothesis that an intact transbilayer anion translocation pathway is completely contained within an AE1 monomer.
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Affiliation(s)
- Neera K Dahl
- Molecular Medicine and Renal Units, Beth Israel Deaconess Medical Center, Department of Medicine, Harvard Medical School, Boston, Massachusetts 02215, USA
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45
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Chernova MN, Jiang L, Shmukler BE, Schweinfest CW, Blanco P, Freedman SD, Stewart AK, Alper SL. Acute regulation of the SLC26A3 congenital chloride diarrhoea anion exchanger (DRA) expressed in Xenopus oocytes. J Physiol 2003; 549:3-19. [PMID: 12651923 PMCID: PMC2342915 DOI: 10.1113/jphysiol.2003.039818] [Citation(s) in RCA: 136] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Mutations in the human SLC26A3 gene, also known as down-regulated in adenoma (hDRA), cause autosomal recessive congenital chloride-losing diarrhoea (CLD). hDRA expressed in Xenopus oocytes mediated bidirectional Cl--Cl- and Cl--HCO3- exchange. In contrast, transport of oxalate was low, and transport of sulfate and of butyrate was undetectable. Two CLD missense disease mutants of hDRA were nonfunctional in oocytes. Truncation of up to 44 C-terminal amino acids from the putatively cytoplasmic C-terminal hydrophilic domain left transport function unimpaired, but deletion of the adjacent STAS (sulfate transporter anti-sigma factor antagonist) domain abolished function. hDRA-mediated Cl- transport was insensitive to changing extracellular pH, but was inhibited by intracellular acidification and activated by NH4+ at acidifying concentrations. These regulatory responses did not require the presence of either hDRA's N-terminal cytoplasmic tail or its 44 C-terminal amino acids, but they did require more proximate residues of the C-terminal cytoplasmic domain. Although only weakly sensitive to inhibition by stilbenes, hDRA was inhibited with two orders of magnitude greater potency by the anti-inflammatory drugs niflumate and tenidap. cAMP-insensitive Cl--HCO3- exchange mediated by hDRA gained modest cAMP sensitivity when co-expressed with cystic fibrosis transmembrane conductance regulator (CFTR). Despite the absence of hDRA transcripts in human cell lines derived from CFTR patients, DRA mRNA was present at wild-type levels in proximal colon and nearly so in the distal ileum of CFTR(-/-) mice. Thus, pharmacological modulation of DRA might be a useful adjunct treatment of cystic fibrosis.
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Affiliation(s)
- Marina N Chernova
- Molecular Medicine, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA
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46
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Guizouarn H, Musch MW, Goldstein L. Evidence for the presence of three different anion exchangers in a red cell. Functional expression studies in Xenopus oocytes. J Membr Biol 2003; 193:109-20. [PMID: 12879159 DOI: 10.1007/s00232-002-2012-6] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2002] [Indexed: 10/26/2022]
Abstract
Anion exchangers (AE) are transmembrane proteins catalyzing electroneutral exchange of Cl(-) for HCO3-. To date, three different genes coding for this protein, AE1, AE2 and AE3, have been identified in many species. AE1 is considered to be the unique anion exchanger expressed in erythrocytes. In this paper we propose the presence of three different AEs in skate erythrocytes, a skAE1, a skAE2 and a skAE3, cloned by RT-PCR (reverse-transcriptase polymerase chain reaction). These three skAE have a similar predicted secondary structure. All three skAE are divided in two main domains: a hydrophilic cytoplasmic N-terminal domain and a C-terminal domain crossing the lipid bilayer at least 12 times. The greatest similarity is found in the membrane-spanning domain of the three skAE. The size as well as the amino-acid sequence of the cytoplasmic domain differ significantly among three anion exchangers. Functional expression studies in Xenopus oocytes led to the conclusion that skAE-1 and -2 share some functional features (Cl-dependence and DIDS sensitivity). The skAE3 could not be expressed in Xenopus oocytes. These data are in agreement with expression data obtained with AEs of different species utilizing the oocyte system. It is highly probable that these three new AE sequences come from three different genes, thus suggesting for the first time the presence of the three AE genes in Chondrichthyes.
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Affiliation(s)
- H Guizouarn
- Laboratoire de Physiologie des Membranes Cellulaires, UMR 6078, CNRS-Université de Nice, Bâtiment Jean Maetz, 284 chemin du Lazaret, 06230 Villefranche/Mer, France.
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47
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Paw BH, Davidson AJ, Zhou Y, Li R, Pratt SJ, Lee C, Trede NS, Brownlie A, Donovan A, Liao EC, Ziai JM, Drejer AH, Guo W, Kim CH, Gwynn B, Peters LL, Chernova MN, Alper SL, Zapata A, Wickramasinghe SN, Lee MJ, Lux SE, Fritz A, Postlethwait JH, Zon LI. Cell-specific mitotic defect and dyserythropoiesis associated with erythroid band 3 deficiency. Nat Genet 2003; 34:59-64. [PMID: 12669066 DOI: 10.1038/ng1137] [Citation(s) in RCA: 111] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2002] [Accepted: 03/07/2003] [Indexed: 11/09/2022]
Abstract
Most eukaryotic cell types use a common program to regulate the process of cell division. During mitosis, successful partitioning of the genetic material depends on spatially coordinated chromosome movement and cell cleavage. Here we characterize a zebrafish mutant, retsina (ret), that exhibits an erythroid-specific defect in cell division with marked dyserythropoiesis similar to human congenital dyserythropoietic anemia. Erythroblasts from ret fish show binuclearity and undergo apoptosis due to a failure in the completion of chromosome segregation and cytokinesis. Through positional cloning, we show that the ret mutation is in a gene (slc4a1) encoding the anion exchanger 1 (also called band 3 and AE1), an erythroid-specific cytoskeletal protein. We further show an association between deficiency in Slc4a1 and mitotic defects in the mouse. Rescue experiments in ret zebrafish embryos expressing transgenic slc4a1 with a variety of mutations show that the requirement for band 3 in normal erythroid mitosis is mediated through its protein 4.1R-binding domains. Our report establishes an evolutionarily conserved role for band 3 in erythroid-specific cell division and illustrates the concept of cell-specific adaptation for mitosis.
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Affiliation(s)
- Barry H Paw
- Department of Medicine, Division of Hematology-Oncology, Children's Hospital, Boston, Massachusetts, USA
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Chernova MN, Stewart AK, Jiang L, Friedman DJ, Kunes YZ, Alper SL. Structure-function relationships of AE2 regulation by Ca(i)(2+)-sensitive stimulators NH(4+) and hypertonicity. Am J Physiol Cell Physiol 2003; 284:C1235-46. [PMID: 12529246 DOI: 10.1152/ajpcell.00522.2002] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We showed previously that the nonerythroid anion exchanger AE2 and the erythroid anion exchanger AE1 differ greatly in their regulation by acute changes in intracellular pH (pH(i)) and extracellular pH (pH(o)). We have now examined how AE2, but not AE1, is activated by two stimuli with opposing effects on oocyte pH(i): an alkalinizing stimulus, hypertonicity, and an acidifying stimulus, NH(4)(+). We find that both NH(2)-terminal cytoplasmic and COOH-terminal transmembrane domains of AE2 are required for activation by either stimulus. Directed by initial deletion mutagenesis studies of the NH(2)-terminal cytoplasmic domain, an alanine scan of AE2 amino acids 336-347 identified residues whose individual mutation abolished or severely attenuated sensitivity to both or only one activating stimulus. Chelation of cytoplasmic Ca(2+) (Ca(i)(2+)) diminished or abolished AE2 stimulation by NH(4)(+) and by hypertonicity. Calmidazolium inhibited AE2 activity, but not that of AE1. AE2 was insensitive to many other modifiers of Ca(2+) signaling. Unlike AE2 stimulation by NH(4)(+) and by hypertonicity, AE2 inhibition by calmidazolium required only AE2's COOH-terminal transmembrane domain.
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Affiliation(s)
- Marina N Chernova
- Molecular Medicine and Renal Units, Beth Israel Deaconess Medical Center, Department of Medicine, Harvard Medical School, Boston, Massachusetts 02215, USA
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Salhany JM, Sloan RL, Cordes KS. The carboxyl side chain of glutamate 681 interacts with a chloride binding modifier site that allosterically modulates the dimeric conformational state of band 3 (AE1). Implications for the mechanism of anion/proton cotransport. Biochemistry 2003; 42:1589-602. [PMID: 12578372 DOI: 10.1021/bi0205294] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Glutamate 681 is thought to be located within the transport channel of band 3 (AE1, the chloride/bicarbonate exchanger), where it acts as a proton donor for the anion/proton cotransport function. Here we show that neutralization of the negative charge on glutamate 681 by chemically modifying band 3 with Woodward's reagent K plus sodium borohydride (i.e., the modification process) exposes a cryptic, conformationally active chloride-binding site which functions to modulate allosterically the conformational state of the band 3 dimer. Chloride binding was determined by measuring the effect of increasing chloride concentration on the rate of DBDS (4,4'-dibenzamido-2,2'-stilbenedisulfonate) release from band 3 using a stopped-flow fluorescence kinetic inhibitor replacement assay with DIDS (4,4'-diisothiocyanato-2,2'-stilbenedisulfonate) as the replacing inhibitor. The time course for DBDS release from unmodified, control band 3 was monophasic and exponential. Chloride binding to the transport site accelerated the rate of DBDS release, with the observed rate constant showing a hyperbolic dependence on chloride concentration, while the total change in reaction fluorescence remained constant. After modification of glutamate 681, DBDS release was monophasic in the absence of chloride, but the rapid addition of chloride at constant ionic strength induced a doubling in the fluorescence quantum yield for the bound DBDS molecules. This was associated with the development of 50:50 biphasic kinetics for DBDS release. Such changes were independent of the degree of modification of the band 3 subunit population between the 66% and 91% levels. Titration of the increase in total reaction fluorescence gave an apparent chloride binding K(d) of between 7 and 10 mM, which is 25-40-fold higher in affinity than chloride binding to the transport site. The dependence of the kinetic constants for both phases of the DBDS release reaction on chloride concentration was nonhyperbolic, which contrasts with unmodified band 3, and is indicative of the presence of two classes of chloride-binding sites on the modified transporter. We have also found that the fraction of subunits capable of binding DBDS reversibly, or DIDS covalently, decreased nonlinearly in the absence of chloride as the level of modification of the band 3 subunit population increased. In contrast, the same DBDS binding correlation plot showed a maximum in the presence of saturating chloride. The observation of such nonlinear correlation plots is consistent with a noncooperative dimer model for the modification process, where each dimeric species must possess different properties with respect to stilbenedisulfonate binding capacity and with respect to the spectral-kinetic response of bound stilbenedisulfonate molecules to the addition of chloride. Within the context of this model, the fractions of the three molecular dimeric species (i.e., the unmodified dimer, the dimer with one subunit modified, and the fully modified band 3 dimer) are calculated as a function of the level of modification of the band 3 subunit population. Nonlinear correlation plots are generated by then assigning the following specific properties to each dimeric species. The unmodified dimer binds DBDS but does not change its fluorescence quantum yield upon addition of chloride. The half-modified dimer binds DBDS on both modified and unmodified subunits, and both of those DBDS molecules increase their fluorescence quantum yield by 2-fold when chloride is added, and the system develops 50:50 biphasic DBDS release kinetics. Finally, the model requires that the fully modified dimer does not bind DBDS or DIDS. This model generates theoretical correlation plots that can represent the data presented in this study. We propose that neutralization of glutamate 681 on the half-modified band 3 dimer exposes an allosteric, chloride-binding modifier site which functions to facilitate the anion/proton cotransport process (a) by blocking the "redocking" of the carboxyl side chain of glutamate (thus raising its pK) and (b) by inducing amate (thus raising its pK) and (b) by inducing a conformational change in the band 3 dimer from a symmetrical to an asymmetrical state.
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Affiliation(s)
- James M Salhany
- Veterans Administration Medical Center and Department of Internal Medicine, University of Nebraska Medical Center, Omaha, Nebraska 68198-5290, USA.
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50
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Stewart AK, Chernova MN, Shmukler BE, Wilhelm S, Alper SL. Regulation of AE2-mediated Cl- transport by intracellular or by extracellular pH requires highly conserved amino acid residues of the AE2 NH2-terminal cytoplasmic domain. J Gen Physiol 2002; 120:707-22. [PMID: 12407081 PMCID: PMC2229549 DOI: 10.1085/jgp.20028641] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022] Open
Abstract
We reported recently that regulation by intracellular pH (pH(i)) of the murine Cl-/HCO(3)(-) exchanger AE2 requires amino acid residues 310-347 of the polypeptide's NH(2)-terminal cytoplasmic domain. We have now identified individual amino acid residues within this region whose integrity is required for regulation of AE2 by pH. 36Cl- efflux from AE2-expressing Xenopus oocytes was monitored during variation of extracellular pH (pH(o)) with unclamped or clamped pH(i), or during variation of pH(i) at constant pH(o). Wild-type AE2-mediated 36Cl- efflux was profoundly inhibited by acid pH(o), with a value of pH(o50) = 6.87 +/- 0.05, and was stimulated up to 10-fold by the intracellular alkalinization produced by bath removal of the preequilibrated weak acid, butyrate. Systematic hexa-alanine [(A)6]bloc substitutions between aa 312-347 identified the greatest acid shift in pH(o(50)) value, approximately 0.8 pH units in the mutant (A)6 342-347, but only a modest acid-shift in the mutant (A)6 336-341. Two of the six (A)6 mutants retained normal pH(i) sensitivity of 36Cl- efflux, whereas the (A)6 mutants 318-323, 336-341, and 342-347 were not stimulated by intracellular alkalinization. We further evaluated the highly conserved region between aa 336-347 by alanine scan and other mutagenesis of single residues. Significant changes in AE2 sensitivity to pH(o) and to pH(i) were found independently and in concert. The E346A mutation acid-shifted the pH(o(0) value to the same extent whether pH(i) was unclamped or held constant during variation of pH(o). Alanine substitution of the corresponding glutamate residues in the cytoplasmic domains of related AE anion exchanger polypeptides confirmed the general importance of these residues in regulation of anion exchange by pH. Conserved, individual amino acid residues of the AE2 cytoplasmic domain contribute to independent regulation of anion exchange activity by pH(o) as well as pH(i).
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Affiliation(s)
- A K Stewart
- Department of Medicine, Harvard Medical School, Molecular Medicine and Renal Units, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA
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