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Cui M, Xu K, Gada K, Shalomov B, Ban M, Eptaminitaki GC, Kawano T, Plant LD, Dascal N, Logothetis DE. A novel small molecule selective activator of homomeric GIRK4 channels. J Biol Chem 2022; 298:102009. [PMID: 35525275 PMCID: PMC9194863 DOI: 10.1016/j.jbc.2022.102009] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2021] [Revised: 04/27/2022] [Accepted: 05/02/2022] [Indexed: 11/26/2022] Open
Abstract
G protein–sensitive inwardly rectifying potassium (GIRK) channels are important pharmaceutical targets for neuronal, cardiac, and endocrine diseases. Although a number of GIRK channel modulators have been discovered in recent years, most lack selectivity. GIRK channels function as either homomeric (i.e., GIRK2 and GIRK4) or heteromeric (e.g., GIRK1/2, GIRK1/4, and GIRK2/3) tetramers. Activators, such as ML297, ivermectin, and GAT1508, have been shown to activate heteromeric GIRK1/2 channels better than GIRK1/4 channels with varying degrees of selectivity but not homomeric GIRK2 and GIRK4 channels. In addition, VU0529331 was discovered as the first homomeric GIRK channel activator, but it shows weak selectivity for GIRK2 over GIRK4 (or G4) homomeric channels. Here, we report the first highly selective small-molecule activator targeting GIRK4 homomeric channels, 3hi2one-G4 (3-[2-(3,4-dimethoxyphenyl)-2-oxoethyl]-3-hydroxy-1-(1-naphthylmethyl)-1,3-dihydro-2H-indol-2-one). We show that 3hi2one-G4 does not activate GIRK2, GIRK1/2, or GIRK1/4 channels. Using molecular modeling, mutagenesis, and electrophysiology, we analyzed the binding site of 3hi2one-G4 formed by the transmembrane 1, transmembrane 2, and slide helix regions of the GIRK4 channel, near the phosphatidylinositol-4,5-bisphosphate binding site, and show that it causes channel activation by strengthening channel–phosphatidylinositol-4,5-bisphosphate interactions. We also identify slide helix residue L77 in GIRK4, corresponding to residue I82 in GIRK2, as a major determinant of isoform-specific selectivity. We propose that 3hi2one-G4 could serve as a useful pharmaceutical probe in studying GIRK4 channel function and may also be pursued in drug optimization studies to tackle GIRK4-related diseases such as primary aldosteronism and late-onset obesity.
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Affiliation(s)
- Meng Cui
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, 02115, USA; Center for Drug Discovery, Northeastern University, Boston, MA 02115, USA.
| | - Keman Xu
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, 02115, USA
| | - Kirin Gada
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, 02115, USA
| | - Boris Shalomov
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, School of Medicine, Tel Aviv University, Tel Aviv, 69978, Israel
| | - Michelle Ban
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, 02115, USA
| | - Giasemi C Eptaminitaki
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, 02115, USA
| | - Takeharu Kawano
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, 02115, USA
| | - Leigh D Plant
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, 02115, USA; Center for Drug Discovery, Northeastern University, Boston, MA 02115, USA
| | - Nathan Dascal
- Department of Physiology and Pharmacology and Sagol School of Neuroscience, School of Medicine, Tel Aviv University, Tel Aviv, 69978, Israel
| | - Diomedes E Logothetis
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, 02115, USA; Chemistry and Chemical Biology, College of Science, Northeastern University, Boston, MA 02115, USA; Center for Drug Discovery, Northeastern University, Boston, MA 02115, USA.
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2
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Nam YW, Cui M, Salem El-Sayed N, Orfali R, Nguyen M, Yang G, Rahman MA, Lee J, Zhang M. Subtype-selective positive modulation of K Ca 2 channels depends on the HA/HB helices. Br J Pharmacol 2022; 179:460-472. [PMID: 34458981 PMCID: PMC8799485 DOI: 10.1111/bph.15676] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/04/2021] [Revised: 06/29/2021] [Accepted: 07/29/2021] [Indexed: 02/03/2023] Open
Abstract
BACKGROUND AND PURPOSE In the activated state of small-conductance Ca2+ -activated potassium (KCa 2) channels, calmodulin interacts with the HA/HB helices and the S4-S5 linker. CyPPA potentiates KCa 2.2a and KCa 2.3 channel activity but not the KCa 2.1 and KCa 3.1 subtypes. EXPERIMENTAL APPROACH Site-directed mutagenesis, patch-clamp recordings and in silico modelling were utilised to explore the structural determinants for the subtype-selective modulation of KCa 2 channels by CyPPA. KEY RESULTS Mutating residues in the HA (V420) and HB (K467) helices of KCa 2.2a channels to their equivalent residues in KCa 3.1 channels diminished the potency of CyPPA. CyPPA elicited prominent responses on mutant KCa 3.1 channels with an arginine residue in the HB helix substituted for its equivalent lysine residue in the KCa 2.2a channels (R355K). KCa 2.1 channels harbouring a three-amino-acid insertion upstream of the cognate R438 residues in the HB helix showed no response to CyPPA, whereas the deletion mutant (KCa 2.1_ΔA434/Q435/K436) became sensitive to CyPPA. In molecular dynamics simulations, CyPPA docked between calmodulin C-lobe and the HA/HB helices widens the cytoplasmic gate of KCa 2.2a channels. CONCLUSION AND IMPLICATIONS Selectivity of CyPPA among KCa 2 and KCa 3.1 channel subtypes relies on the HA/HB helices.
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Affiliation(s)
- Young-Woo Nam
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, California 92618, USA
| | - Meng Cui
- Department of Pharmaceutical Sciences, Northeastern University School of Pharmacy, Boston, Massachusetts, 02115, USA
| | - Naglaa Salem El-Sayed
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, California 92618, USA
| | - Razan Orfali
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, California 92618, USA
| | - Misa Nguyen
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, California 92618, USA
| | - Grace Yang
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, California 92618, USA
| | - Mohammad Asikur Rahman
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, California 92618, USA
| | - Judy Lee
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, California 92618, USA
| | - Miao Zhang
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, California 92618, USA
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3
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Cui M, Alhamshari Y, Cantwell L, Ei-Haou S, Eptaminitaki GC, Chang M, Abou-Assali O, Tan H, Xu K, Masotti M, Plant LD, Thakur GA, Noujaim SF, Milnes J, Logothetis DE. A benzopyran with antiarrhythmic activity is an inhibitor of Kir3.1-containing potassium channels. J Biol Chem 2021; 296:100535. [PMID: 33713702 PMCID: PMC8086025 DOI: 10.1016/j.jbc.2021.100535] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2021] [Revised: 03/03/2021] [Accepted: 03/09/2021] [Indexed: 01/20/2023] Open
Abstract
Atrial fibrillation (AF) is the most commonly diagnosed cardiac arrhythmia and is associated with increased morbidity and mortality. Currently approved AF antiarrhythmic drugs have limited efficacy and/or carry the risk of ventricular proarrhythmia. The cardiac acetylcholine activated inwardly rectifying K+ current (IKACh), composed of Kir3.1/Kir3.4 heterotetrameric and Kir3.4 homotetrameric channel subunits, is one of the best validated atrial-specific ion channels. Previous research pointed to a series of benzopyran derivatives with potential for treatment of arrhythmias, but their mechanism of action was not defined. Here, we characterize one of these compounds termed Benzopyran-G1 (BP-G1) and report that it selectively inhibits the Kir3.1 (GIRK1 or G1) subunit of the KACh channel. Homology modeling, molecular docking, and molecular dynamics simulations predicted that BP-G1 inhibits the IKACh channel by blocking the central cavity pore. We identified the unique F137 residue of Kir3.1 as the critical determinant for the IKACh-selective response to BP-G1. The compound interacts with Kir3.1 residues E141 and D173 through hydrogen bonds that proved critical for its inhibitory activity. BP-G1 effectively blocked the IKACh channel response to carbachol in an in vivo rodent model and displayed good selectivity and pharmacokinetic properties. Thus, BP-G1 is a potent and selective small-molecule inhibitor targeting Kir3.1-containing channels and is a useful tool for investigating the role of Kir3.1 heteromeric channels in vivo. The mechanism reported here could provide the molecular basis for future discovery of novel, selective IKACh channel blockers to treat atrial fibrillation with minimal side effects.
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Affiliation(s)
- Meng Cui
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA.
| | - Yaser Alhamshari
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA
| | - Lucas Cantwell
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA
| | - Said Ei-Haou
- Department of Cardiac Biology, Xention Ltd, Cambridge, UK
| | - Giasemi C Eptaminitaki
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA
| | - Mengmeng Chang
- Department of Molecular Pharmacology & Physiology, Morsani College of Medicine, University of South Florida, Tampa, Florida, USA
| | - Obada Abou-Assali
- Department of Molecular Pharmacology & Physiology, Morsani College of Medicine, University of South Florida, Tampa, Florida, USA
| | - Haozhou Tan
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA
| | - Keman Xu
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA
| | - Meghan Masotti
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA
| | - Leigh D Plant
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA; Center for Drug Discovery, Northeastern University, Boston, Massachusetts, USA
| | - Ganesh A Thakur
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA
| | - Sami F Noujaim
- Department of Molecular Pharmacology & Physiology, Morsani College of Medicine, University of South Florida, Tampa, Florida, USA
| | - James Milnes
- Department of Cardiac Biology, Xention Ltd, Cambridge, UK
| | - Diomedes E Logothetis
- Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, Massachusetts, USA; Department of Chemistry and Chemical Biology, College of Science, Northeastern University, Boston, Massachusetts, USA; Center for Drug Discovery, Northeastern University, Boston, Massachusetts, USA.
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4
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Nam YW, Cui M, Orfali R, Viegas A, Nguyen M, Mohammed EHM, Zoghebi KA, Rahighi S, Parang K, Zhang M. Hydrophobic interactions between the HA helix and S4-S5 linker modulate apparent Ca 2+ sensitivity of SK2 channels. Acta Physiol (Oxf) 2021; 231:e13552. [PMID: 32865319 PMCID: PMC7736289 DOI: 10.1111/apha.13552] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2020] [Revised: 08/09/2020] [Accepted: 08/20/2020] [Indexed: 12/11/2022]
Abstract
AIM Small-conductance Ca2+ -activated potassium (SK) channels are activated exclusively by increases in intracellular Ca2+ that binds to calmodulin constitutively associated with the channel. Wild-type SK2 channels are activated by Ca2+ with an EC50 value of ~0.3 μmol/L. Here, we investigate hydrophobic interactions between the HA helix and the S4-S5 linker as a major determinant of channel apparent Ca2+ sensitivity. METHODS Site-directed mutagenesis, electrophysiological recordings and molecular dynamic (MD) simulations were utilized. RESULTS Mutations that decrease hydrophobicity at the HA-S4-S5 interface lead to Ca2+ hyposensitivity of SK2 channels. Mutations that increase hydrophobicity result in hypersensitivity to Ca2+ . The Ca2+ hypersensitivity of the V407F mutant relies on the interaction of the cognate phenylalanine with the S4-S5 linker in the SK2 channel. Replacing the S4-S5 linker of the SK2 channel with the S4-S5 linker of the SK4 channel results in loss of the hypersensitivity caused by V407F. This difference between the S4-S5 linkers of SK2 and SK4 channels can be partially attributed to I295 equivalent to a valine in the SK4 channel. A N293A mutation in the S4-S5 linker also increases hydrophobicity at the HA-S4-S5 interface and elevates the channel apparent Ca2+ sensitivity. The double N293A/V407F mutations generate a highly Ca2+ sensitive channel, with an EC50 of 0.02 μmol/L. The MD simulations of this double-mutant channel revealed a larger channel cytoplasmic gate. CONCLUSION The electrophysiological data and MD simulations collectively suggest a crucial role of the interactions between the HA helix and S4-S5 linker in the apparent Ca2+ sensitivity of SK2 channels.
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Affiliation(s)
- Young-Woo Nam
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, CA, USA
| | - Meng Cui
- Department of Pharmaceutical Sciences, Northeastern University School of Pharmacy, Boston, MA, USA
| | - Razan Orfali
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, CA, USA
| | - Adam Viegas
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, CA, USA
| | - Misa Nguyen
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, CA, USA
| | - Eman H M Mohammed
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, CA, USA
| | - Khalid A Zoghebi
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, CA, USA
| | - Simin Rahighi
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, CA, USA
| | - Keykavous Parang
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, CA, USA
| | - Miao Zhang
- Department of Biomedical and Pharmaceutical Sciences, Chapman University School of Pharmacy, Irvine, CA, USA
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5
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Assur Sanghai Z, Liu Q, Clarke OB, Belcher-Dufrisne M, Wiriyasermkul P, Giese MH, Leal-Pinto E, Kloss B, Tabuso S, Love J, Punta M, Banerjee S, Rajashankar KR, Rost B, Logothetis D, Quick M, Hendrickson WA, Mancia F. Structure-based analysis of CysZ-mediated cellular uptake of sulfate. eLife 2018; 7:27829. [PMID: 29792261 PMCID: PMC5967866 DOI: 10.7554/elife.27829] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2017] [Accepted: 04/11/2018] [Indexed: 01/25/2023] Open
Abstract
Sulfur, most abundantly found in the environment as sulfate (SO42-), is an essential element in metabolites required by all living cells, including amino acids, co-factors and vitamins. However, current understanding of the cellular delivery of SO42- at the molecular level is limited. CysZ has been described as a SO42- permease, but its sequence family is without known structural precedent. Based on crystallographic structure information, SO42- binding and flux experiments, we provide insight into the molecular mechanism of CysZ-mediated translocation of SO42- across membranes. CysZ structures from three different bacterial species display a hitherto unknown fold and have subunits organized with inverted transmembrane topology. CysZ from Pseudomonas denitrificans assembles as a trimer of antiparallel dimers and the CysZ structures from two other species recapitulate dimers from this assembly. Mutational studies highlight the functional relevance of conserved CysZ residues.
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Affiliation(s)
- Zahra Assur Sanghai
- Department of Physiology and Cellular Biophysics, Columbia University, New York, United States.,Department of Biochemistry and Molecular Biophysics, Columbia University, New York, United States
| | - Qun Liu
- Biology Department, Brookhaven National Laboratory, Upton, United States
| | - Oliver B Clarke
- Department of Biochemistry and Molecular Biophysics, Columbia University, New York, United States
| | - Meagan Belcher-Dufrisne
- Department of Physiology and Cellular Biophysics, Columbia University, New York, United States
| | - Pattama Wiriyasermkul
- Center for Molecular Recognition, Department of Psychiatry, Columbia University, New York, United States
| | - M Hunter Giese
- Department of Physiology and Cellular Biophysics, Columbia University, New York, United States.,Department of Biochemistry and Molecular Biophysics, Columbia University, New York, United States
| | - Edgar Leal-Pinto
- Department of Physiology and Biophysics, Virginia Commonwealth University School of Medicine, Richmond, United States.,Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, United States
| | - Brian Kloss
- New York Structural Biology Center, New York, United States
| | | | - James Love
- New York Structural Biology Center, New York, United States
| | - Marco Punta
- Centre for Evolution and Cancer, The Institute of Cancer Research, London, United Kingdom
| | - Surajit Banerjee
- Department of Chemistry and Chemical Biology, Cornell University, NE-CAT, Argonne, United States
| | | | - Burkhard Rost
- Department of Informatics, Technical University of Munich, Munich, Germany
| | - Diomedes Logothetis
- Department of Physiology and Biophysics, Virginia Commonwealth University School of Medicine, Richmond, United States.,Department of Pharmaceutical Sciences, School of Pharmacy, Bouvé College of Health Sciences, Northeastern University, Boston, United States
| | - Matthias Quick
- Center for Molecular Recognition, Department of Psychiatry, Columbia University, New York, United States.,Division of Molecular Therapeutics, New York State Psychiatric Institute, New York, United States
| | - Wayne A Hendrickson
- Department of Physiology and Cellular Biophysics, Columbia University, New York, United States.,Department of Biochemistry and Molecular Biophysics, Columbia University, New York, United States.,New York Structural Biology Center, New York, United States
| | - Filippo Mancia
- Department of Physiology and Cellular Biophysics, Columbia University, New York, United States
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6
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Leal-Pinto E, Gómez-Llorente Y, Sundaram S, Tang QY, Ivanova-Nikolova T, Mahajan R, Baki L, Zhang Z, Chavez J, Ubarretxena-Belandia I, Logothetis DE. Gating of a G protein-sensitive mammalian Kir3.1 prokaryotic Kir channel chimera in planar lipid bilayers. J Biol Chem 2010; 285:39790-800. [PMID: 20937804 DOI: 10.1074/jbc.m110.151373] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Kir3 channels control heart rate and neuronal excitability through GTP-binding (G) protein and phosphoinositide signaling pathways. These channels were the first characterized effectors of the βγ subunits of G proteins. Because we currently lack structures of complexes between G proteins and Kir3 channels, their interactions leading to modulation of channel function are not well understood. The recent crystal structure of a chimera between the cytosolic domain of a mammalian Kir3.1 and the transmembrane region of a prokaryotic KirBac1.3 (Kir3.1 chimera) has provided invaluable structural insight. However, it was not known whether this chimera could form functional K(+) channels. Here, we achieved the functional reconstitution of purified Kir3.1 chimera in planar lipid bilayers. The chimera behaved like a bona fide Kir channel displaying an absolute requirement for PIP(2) and Mg(2+)-dependent inward rectification. The channel could also be blocked by external tertiapin Q. The three-dimensional reconstruction of the chimera by single particle electron microscopy revealed a structure consistent with the crystal structure. Channel activity could be stimulated by ethanol and activated G proteins. Remarkably, the presence of both activated Gα and Gβγ subunits was required for gating of the channel. These results confirm the Kir3.1 chimera as a valid structural and functional model of Kir3 channels.
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Affiliation(s)
- Edgar Leal-Pinto
- Department of Physiology and Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, Virginia 23298, USA
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7
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Leal-Pinto E, Cohen BE, Lipkowitz MS, Abramson RG. Functional analysis and molecular model of the human urate transporter/channel, hUAT. Am J Physiol Renal Physiol 2002; 283:F150-63. [PMID: 12060597 DOI: 10.1152/ajprenal.00333.2001] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Recombinant protein, designated hUAT, the human homologue of the rat urate transporter/channel (UAT), functions as a highly selective urate channel in lipid bilayers. Functional analysis indicates that hUAT activity, like UAT, is selectively blocked by oxonate from its cytosolic side, whereas pyrazinoate and adenosine selectively block from the channel's extracellular face. Importantly, hUAT is a galectin, a protein with two beta-galactoside binding domains that bind lactose. Lactose significantly increased hUAT open probability but only when added to the channel's extracellular side. This effect on open probability was mimicked by glucose, but not ribose, suggesting a role for extracellular glucose in regulating hUAT channel activity. These functional observations support a four-transmembrane-domain structural model of hUAT, as previously predicted from the primary structure of UAT. hUAT and UAT, however, are not functionally identical: hUAT has a significantly lower single-channel conductance and open probability is voltage independent. These differences suggest that evolutionary changes in specific amino acids in these highly homologous proteins are functionally relevant in defining these biophysical properties.
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Affiliation(s)
- Edgar Leal-Pinto
- Division of Nephrology, Department of Medicine, Mount Sinai School of Medicine, New York, New York, 10029, USA
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8
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Abstract
Membrane channels provide routes for the rapid, passive movement of solutes across plasma and intracellular membranes. It is generally assumed that the major physiological role of membrane channels is to transport inorganic ions for processes such as transepithelial salt absorption and secretion, cell volume regulation, signal transduction, and control of membrane electrical properties. Increasing evidence indicates, however, that channels play an important role in organic solute transport in a wide variety of cell types and organisms. Some of the major physiological roles of organic solute channels include uptake of nutrients, excretion of metabolic waste products, volume-regulatory organic osmolyte transport, and control of mitochondrial metabolism. This article reviews the functions and characteristics of channels that participate in the transport and regulation of both charged and electroneutral organic solutes.
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Affiliation(s)
- K Kirk
- Division of Biochemistry and Molecular Biology, Australian National University, Canberra, Australia.
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9
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Hanss B, Leal-Pinto E, Bruggeman LA, Copeland TD, Klotman PE. Identification and characterization of a cell membrane nucleic acid channel. Proc Natl Acad Sci U S A 1998; 95:1921-6. [PMID: 9465118 PMCID: PMC19214 DOI: 10.1073/pnas.95.4.1921] [Citation(s) in RCA: 110] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
We have identified a 45-kDa protein purified from rat renal brush border membrane that binds short single-stranded nucleic acid sequences. This activity was purified, reconstituted in proteoliposomes, and then fused with model planar lipid bilayers. In voltage-clamp experiments, the reconstituted 45-kDa protein functioned as a gated channel that allows the passage of nucleic acids. Channel activity was observed immediately after addition of oligonucleotide. Channel activity was not observed in the absence of purified protein or of oligonucleotide or when protein was heat-inactivated prior to forming proteoliposomes. In the presence of symmetrical buffered solution and oligonucleotide, current passed linearly over the range of holding potentials tested. Conductance was 10.4 +/- 0.4 picosiemens (pS) and reversal potential was 0.2 +/- 1.7 mV. There was no difference in channel conductance or reversal potential between phosphodiester and phosphorothioate oligonucleotides. Ion-substitution experiments documented a shift in reversal potential only when a concentration gradient for oligonucleotide was established, indicating that movement of oligonucleotide alone was responsible for current. Movement of oligonucleotide across the bilayer was confirmed by using 32P-labeled oligonucleotides. Channel open probability decreased significantly in the presence of heparan sulfate. These studies provide evidence for a cell surface channel that conducts nucleic acids.
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Affiliation(s)
- B Hanss
- Division of Nephrology, Box 1243, Mt. Sinai School of Medicine, 1 Gustave L. Levy Place, New York, NY 10029, USA.
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10
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Leal-Pinto E, Tao W, Rappaport J, Richardson M, Knorr BA, Abramson RG. Molecular cloning and functional reconstitution of a urate transporter/channel. J Biol Chem 1997; 272:617-25. [PMID: 8995305 DOI: 10.1074/jbc.272.1.617] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Maintenance of urate homeostasis requires urate efflux from urate-producing cells with subsequent renal and gastrointestinal excretion. The molecular basis for urate transport, however, has not been identified. A novel full-length cDNA encoding a 322-amino acid protein, designated UAT (urate transporter), has been cloned from a rat renal cDNA library by antibody screening. UAT mRNA transcripts that approximate 1.55 kilobases are present, but differentially expressed in various rat tissues. Recombinant UAT protein that was expressed from the cloned cDNA in Escherichia coli and purified via immobilized metal affinity chromatography has been functionally reconstituted as a highly selective urate transporter/channel in planar lipid bilayers. The IgG fraction of the polyclonal antibody that was used to select the UAT clone from the cDNA library, but not nonimmune IgG, blocked urate channel activity. Based on the wide tissue distribution of the mRNA for UAT we propose that UAT provides the molecular basis for urate flux across cell membranes, allowing urate that is formed during purine metabolism to efflux from cells and serving as an electrogenic transporter that plays an important role in renal and gastrointestinal urate excretion.
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Affiliation(s)
- E Leal-Pinto
- Division of Nephrology, Department of Medicine, Mount Sinai School of Medicine, New York, New York 10029, USA
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