1
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Fortea E, Lee S, Chadda R, Argyros Y, Sandal P, Mahoney-Kruszka R, Ciftci HD, Falzone ME, Huysmans G, Robertson JL, Boudker O, Accardi A. Structural basis of pH-dependent activation in a CLC transporter. Nat Struct Mol Biol 2024; 31:644-656. [PMID: 38279055 PMCID: PMC11262703 DOI: 10.1038/s41594-023-01210-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2023] [Accepted: 12/22/2023] [Indexed: 01/28/2024]
Abstract
CLCs are dimeric chloride channels and anion/proton exchangers that regulate processes such as muscle contraction and endo-lysosome acidification. Common gating controls their activity; its closure simultaneously silences both protomers, and its opening allows them to independently transport ions. Mutations affecting common gating in human CLCs cause dominant genetic disorders. The structural rearrangements underlying common gating are unknown. Here, using single-particle cryo-electron microscopy, we show that the prototypical Escherichia coli CLC-ec1 undergoes large-scale rearrangements in activating conditions. The slow, pH-dependent remodeling of the dimer interface leads to the concerted opening of the intracellular H+ pathways and is required for transport. The more frequent formation of short water wires in the open H+ pathway enables Cl- pore openings. Mutations at disease-causing sites favor CLC-ec1 activation and accelerate common gate opening in the human CLC-7 exchanger. We suggest that the pH activation mechanism of CLC-ec1 is related to the common gating of CLC-7.
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Affiliation(s)
- Eva Fortea
- Department of Physiology and Biophysics, Weill Cornell Medical School, New York, NY, USA
- Department of Anesthesiology, Weill Cornell Medical School, New York, NY, USA
| | - Sangyun Lee
- Department of Anesthesiology, Weill Cornell Medical School, New York, NY, USA
| | - Rahul Chadda
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, MO, USA
| | - Yiorgos Argyros
- Department of Anesthesiology, Weill Cornell Medical School, New York, NY, USA
- Department of Biochemistry, Weill Cornell Medical School, New York, NY, USA
| | - Priyanka Sandal
- Department of Molecular Physiology and Biophysics, The University of Iowa, Iowa City, IA, USA
| | - Robyn Mahoney-Kruszka
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, MO, USA
| | - Hatice Didar Ciftci
- Department of Physiology and Biophysics, Weill Cornell Medical School, New York, NY, USA
- Tri-Institutional Training Program in Chemical Biology, New York, NY, USA
| | - Maria E Falzone
- Department of Anesthesiology, Weill Cornell Medical School, New York, NY, USA
- Department of Biochemistry, Weill Cornell Medical School, New York, NY, USA
| | - Gerard Huysmans
- Department of Physiology and Biophysics, Weill Cornell Medical School, New York, NY, USA
- Erasmus University, Jette, Belgium
| | - Janice L Robertson
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, MO, USA
| | - Olga Boudker
- Department of Physiology and Biophysics, Weill Cornell Medical School, New York, NY, USA.
- Howard Hughes Medical Institute, Chevy Chase, MD, USA.
| | - Alessio Accardi
- Department of Physiology and Biophysics, Weill Cornell Medical School, New York, NY, USA.
- Department of Anesthesiology, Weill Cornell Medical School, New York, NY, USA.
- Department of Biochemistry, Weill Cornell Medical School, New York, NY, USA.
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2
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Levring J, Chen J. Structural identification of a selectivity filter in CFTR. Proc Natl Acad Sci U S A 2024; 121:e2316673121. [PMID: 38381791 PMCID: PMC10907310 DOI: 10.1073/pnas.2316673121] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2023] [Accepted: 01/19/2024] [Indexed: 02/23/2024] Open
Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel that regulates transepithelial salt and fluid homeostasis. CFTR dysfunction leads to reduced chloride secretion into the mucosal lining of epithelial tissues, thereby causing the inherited disease cystic fibrosis. Although several structures of CFTR are available, our understanding of the ion-conduction pathway is incomplete. In particular, the route that connects the cytosolic vestibule with the extracellular space has not been clearly defined, and the structure of the open pore remains elusive. Furthermore, although many residues have been implicated in altering the selectivity of CFTR, the structure of the "selectivity filter" has yet to be determined. In this study, we identify a chloride-binding site at the extracellular ends of transmembrane helices 1, 6, and 8, where a dehydrated chloride is coordinated by residues G103, R334, F337, T338, and Y914. Alterations to this site, consistent with its function as a selectivity filter, affect ion selectivity, conductance, and open channel block. This selectivity filter is accessible from the cytosol through a large inner vestibule and opens to the extracellular solvent through a narrow portal. The identification of a chloride-binding site at the intra- and extracellular bridging point leads us to propose a complete conductance path that permits dehydrated chloride ions to traverse the lipid bilayer.
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Affiliation(s)
- Jesper Levring
- Laboratory of Membrane Biology and Biophysics, The Rockefeller University, New York, NY10065
| | - Jue Chen
- Laboratory of Membrane Biology and Biophysics, The Rockefeller University, New York, NY10065
- HHMI, The Rockefeller University, New York, NY10065
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3
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Kwon HC, Fairclough RH, Chen TY. Biophysical and Pharmacological Insights to CLC Chloride Channels. Handb Exp Pharmacol 2024; 283:1-34. [PMID: 35768555 DOI: 10.1007/164_2022_594] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
The CLC family encompasses two functional categories of transmembrane proteins: chloride conducting channels and proton-chloride antiporters. All members in this chloride channel/transporter family consist of two identical protein subunits, and each subunit forms an independent ion-transport pathway, a structural architecture known as "double barrel." These CLC proteins serve biological functions ranging from membrane excitability and cell volume regulation to acidification of endosomes. Despite their ubiquitous expression, physiological significance, and resolved molecular structures of some of the family members, the mechanisms governing these molecules' biophysical functions are still not completely settled. However, a series of functional and structural studies have brought insights into interesting questions related to these proteins. This chapter explores the functional peculiarities underlying CLC channels aided by information observed from the chloride-proton antiporters in the CLC family. The overall structural features of these CLC proteins will be presented, and the biophysical functions will be addressed. Finally, the mechanism of pharmacological agents that interact with CLC channels will also be discussed.
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Affiliation(s)
- Hwoi Chan Kwon
- Center for Neuroscience and Biophysics Graduate Group, University of California, Davis, CA, USA
| | - Robert H Fairclough
- Department of Neurology and the Biophysics Graduate Group, University of California, Davis, CA, USA
| | - Tsung-Yu Chen
- Center for Neuroscience, Department of Neurology, and Biophysics Graduate Group, University of California, Davis, CA, USA.
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4
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Pusch M, Zifarelli G. Large transient capacitive currents in wild-type lysosomal Cl-/H+ antiporter ClC-7 and residual transport activity in the proton glutamate mutant E312A. J Gen Physiol 2020; 153:211547. [PMID: 33211806 PMCID: PMC7681918 DOI: 10.1085/jgp.202012583] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2020] [Revised: 09/28/2020] [Accepted: 10/28/2020] [Indexed: 12/18/2022] Open
Abstract
ClC-7 is a lysosomal 2 Cl−/1 H+ antiporter of the CLC protein family, which comprises Cl− channels and other Cl−/H+ antiporters. Mutations in ClC-7 and its associated β subunit Ostm1 lead to osteopetrosis and lysosomal storage disease in humans and mice. Previous studies on other mammalian CLC transporters showed that mutations of a conserved, intracellularly located glutamate residue, the so-called proton glutamate, abolish steady-state transport activity but increase transient capacitive currents associated with partial reactions of the transport cycle. In contrast, we observed large, transient capacitive currents for the wild-type ClC-7, which depend on external pH and internal, but not external, Cl−. Very similar transient currents were observed for the E312A mutant of the proton glutamate. Interestingly, and unlike in other mammalian CLC transporters investigated so far, the E312A mutation strongly reduces, but does not abolish, stationary transport currents, potentially explaining the intermediate phenotype observed in the E312A mouse line.
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Affiliation(s)
- Michael Pusch
- Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Genoa, Italy
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5
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McKiernan KA, Koster AK, Maduke M, Pande VS. Dynamical model of the CLC-2 ion channel reveals conformational changes associated with selectivity-filter gating. PLoS Comput Biol 2020; 16:e1007530. [PMID: 32226009 PMCID: PMC7145265 DOI: 10.1371/journal.pcbi.1007530] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2018] [Revised: 04/09/2020] [Accepted: 11/05/2019] [Indexed: 12/18/2022] Open
Abstract
This work reports a dynamical Markov state model of CLC-2 "fast" (pore) gating, based on 600 microseconds of molecular dynamics (MD) simulation. In the starting conformation of our CLC-2 model, both outer and inner channel gates are closed. The first conformational change in our dataset involves rotation of the inner-gate backbone along residues S168-G169-I170. This change is strikingly similar to that observed in the cryo-EM structure of the bovine CLC-K channel, though the volume of the intracellular (inner) region of the ion conduction pathway is further expanded in our model. From this state (inner gate open and outer gate closed), two additional states are observed, each involving a unique rotameric flip of the outer-gate residue GLUex. Both additional states involve conformational changes that orient GLUex away from the extracellular (outer) region of the ion conduction pathway. In the first additional state, the rotameric flip of GLUex results in an open, or near-open, channel pore. The equilibrium population of this state is low (∼1%), consistent with the low open probability of CLC-2 observed experimentally in the absence of a membrane potential stimulus (0 mV). In the second additional state, GLUex rotates to occlude the channel pore. This state, which has a low equilibrium population (∼1%), is only accessible when GLUex is protonated. Together, these pathways model the opening of both an inner and outer gate within the CLC-2 selectivity filter, as a function of GLUex protonation. Collectively, our findings are consistent with published experimental analyses of CLC-2 gating and provide a high-resolution structural model to guide future investigations.
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Affiliation(s)
- Keri A. McKiernan
- Department of Chemistry, Stanford University, Stanford, California, United States of America
| | - Anna K. Koster
- Department of Chemistry, Stanford University, Stanford, California, United States of America
- Department of Molecular & Cellular Physiology, Stanford University, Stanford, California, United States of America
| | - Merritt Maduke
- Department of Molecular & Cellular Physiology, Stanford University, Stanford, California, United States of America
| | - Vijay S. Pande
- Department of Bioengineering, Stanford University, Stanford, California, United States of America
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6
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Lagostena L, Zifarelli G, Picollo A. New Insights into the Mechanism of NO 3 - Selectivity in the Human Kidney Chloride Channel ClC-Ka and the CLC Protein Family. J Am Soc Nephrol 2019; 30:293-302. [PMID: 30635372 DOI: 10.1681/asn.2018060593] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2018] [Accepted: 12/01/2018] [Indexed: 11/03/2022] Open
Abstract
BACKGROUND The mechanism of anion selectivity in the human kidney chloride channels ClC-Ka and ClC-Kb is unknown. However, it has been thought to be very similar to that of other channels and antiporters of the CLC protein family, and to rely on anions interacting with a conserved Ser residue (Sercen) at the center of three anion binding sites in the permeation pathway Scen. In both CLC channels and antiporters, mutations of Sercen alter the anion selectivity. Structurally, the side chain of Sercen of CLC channels and antiporters typically projects into the pore and coordinates the anion bound at Scen. METHODS To investigate the role of several residues in anion selectivity of ClC-Ka, we created mutations that resulted in amino acid substitutions in these residues. We also used electrophysiologic techniques to assess the properties of the mutants. RESULTS Mutations in ClC-Ka that change Sercen to Gly, Pro, or Thr have only minor effects on anion selectivity, whereas the mutations in residues Y425A, F519A, and Y520A increase the NO3 -/Cl- permeability ratio, with Y425A having a particularly strong effect. CONCLUSION s ClC-Ka's mechanism of anion selectivity is largely independent of Sercen, and it is therefore unique in the CLC protein family. We identified the residue Y425 in ClC-Ka-and the corresponding residue (A417) in the chloride channel ClC-0-as residues that contribute to NO3 - discrimination in these channels. This work provides important and timely insight into the relationship between structure and function for the kidney chloride channels ClC-Ka and ClC-Kb, and for CLC proteins in general.
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Affiliation(s)
- Laura Lagostena
- Dulbecco Telethon Laboratory, Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy; and
| | - Giovanni Zifarelli
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford, UK
| | - Alessandra Picollo
- Dulbecco Telethon Laboratory, Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy; and
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7
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Jentsch TJ, Pusch M. CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease. Physiol Rev 2018; 98:1493-1590. [DOI: 10.1152/physrev.00047.2017] [Citation(s) in RCA: 214] [Impact Index Per Article: 35.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.
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Affiliation(s)
- Thomas J. Jentsch
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) and Max-Delbrück-Centrum für Molekulare Medizin (MDC), Berlin, Germany; and Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy
| | - Michael Pusch
- Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) and Max-Delbrück-Centrum für Molekulare Medizin (MDC), Berlin, Germany; and Istituto di Biofisica, Consiglio Nazionale delle Ricerche, Genova, Italy
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8
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Vindas-Smith R, Fiore M, Vásquez M, Cuenca P, del Valle G, Lagostena L, Gaitán-Peñas H, Estevez R, Pusch M, Morales F. Identification and Functional Characterization ofCLCN1Mutations Found in Nondystrophic Myotonia Patients. Hum Mutat 2015; 37:74-83. [DOI: 10.1002/humu.22916] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2015] [Accepted: 09/25/2015] [Indexed: 10/22/2022]
Affiliation(s)
- Rebeca Vindas-Smith
- Instituto de Investigaciones en Salud (INISA); Universidad de Costa Rica; San José Costa Rica
| | - Michele Fiore
- Istituto di Biofisica; CNR; Via De Marini 6 Genova Italy
| | - Melissa Vásquez
- Instituto de Investigaciones en Salud (INISA); Universidad de Costa Rica; San José Costa Rica
- Centro de Investigación en Neurociencias (CIN); Universidad de Costa Rica; San José Costa Rica
| | - Patricia Cuenca
- Instituto de Investigaciones en Salud (INISA); Universidad de Costa Rica; San José Costa Rica
- Centro de Investigación en Neurociencias (CIN); Universidad de Costa Rica; San José Costa Rica
- Escuela de Medicina; Universidad de Costa Rica; Curridabat San José Costa Rica
| | - Gerardo del Valle
- Laboratorio de Neurofisiología (Neurolab); Curridabat San José Costa Rica
| | | | - Héctor Gaitán-Peñas
- Departament de Ciències Fisiològiques II; Unitat de Fisiologia; Universitat de Barcelona; Carrer Feixa Llarga s/n, L'Hospitalet de Llobregat Barcelona Spain
- U-750, Centro de Investigación en red de enfermedades raras (CIBERER); ISCIII; Barcelona Spain
| | - Raúl Estevez
- Departament de Ciències Fisiològiques II; Unitat de Fisiologia; Universitat de Barcelona; Carrer Feixa Llarga s/n, L'Hospitalet de Llobregat Barcelona Spain
- U-750, Centro de Investigación en red de enfermedades raras (CIBERER); ISCIII; Barcelona Spain
| | - Michael Pusch
- Istituto di Biofisica; CNR; Via De Marini 6 Genova Italy
| | - Fernando Morales
- Instituto de Investigaciones en Salud (INISA); Universidad de Costa Rica; San José Costa Rica
- Centro de Investigación en Neurociencias (CIN); Universidad de Costa Rica; San José Costa Rica
- Escuela de Medicina; Universidad de Costa Rica; Curridabat San José Costa Rica
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9
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Jeworutzki E, Lagostena L, Elorza-Vidal X, López-Hernández T, Estévez R, Pusch M. GlialCAM, a CLC-2 Cl(-) channel subunit, activates the slow gate of CLC chloride channels. Biophys J 2015; 107:1105-1116. [PMID: 25185546 PMCID: PMC4156679 DOI: 10.1016/j.bpj.2014.07.040] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2014] [Revised: 07/18/2014] [Accepted: 07/25/2014] [Indexed: 01/17/2023] Open
Abstract
GlialCAM, a glial cell adhesion molecule mutated in megalencephalic leukoencephalopathy with subcortical cysts, targets the CLC-2 Cl(-) channel to cell contacts in glia and activates CLC-2 currents in vitro and in vivo. We found that GlialCAM clusters all CLC channels at cell contacts in vitro and thus studied GlialCAM interaction with CLC channels to investigate the mechanism of functional activation. GlialCAM slowed deactivation kinetics of CLC-Ka/barttin channels and increased CLC-0 currents opening the common gate and slowing its deactivation. No functional effect was seen for common gate deficient CLC-0 mutants. Similarly, GlialCAM targets the common gate deficient CLC-2 mutant E211V/H816A to cell contacts, without altering its function. Thus, GlialCAM is able to interact with all CLC channels tested, targeting them to cell junctions and activating them by stabilizing the open configuration of the common gate. These results are important to better understand the physiological role of GlialCAM/CLC-2 interaction.
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Affiliation(s)
- Elena Jeworutzki
- Istituto di Biofisica, Consiglio Nazionale delle Ricerche, 16149 Genoa, Italy; Departments of Anesthesia and Biomedizin, ZLF Lab 408, Universitätsspital Basel, Switzerland
| | - Laura Lagostena
- Istituto di Biofisica, Consiglio Nazionale delle Ricerche, 16149 Genoa, Italy
| | - Xabier Elorza-Vidal
- Physiology Section, Department of Physiological Sciences II, School of Medicine, Barcelona, Spain; U-750, Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), ISCIII, Barcelona, Spain
| | - Tania López-Hernández
- Physiology Section, Department of Physiological Sciences II, School of Medicine, Barcelona, Spain; U-750, Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), ISCIII, Barcelona, Spain; Department of Molecular Pharmacology and Cell Biology, FMP, Berlin, Germany
| | - Raúl Estévez
- Physiology Section, Department of Physiological Sciences II, School of Medicine, Barcelona, Spain; U-750, Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERER), ISCIII, Barcelona, Spain
| | - Michael Pusch
- Istituto di Biofisica, Consiglio Nazionale delle Ricerche, 16149 Genoa, Italy.
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10
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Accardi A. Structure and gating of CLC channels and exchangers. J Physiol 2015; 593:4129-38. [PMID: 26148215 DOI: 10.1113/jp270575] [Citation(s) in RCA: 53] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2015] [Accepted: 06/28/2015] [Indexed: 11/08/2022] Open
Abstract
Since their serendipitous discovery the CLC family of Cl(-) transporting proteins has been a never ending source of surprises. From their double-barrelled architecture to their complex structure and divergence as channels and transporters, the CLCs never cease to amaze biophysicists, biochemists and physiologists alike. These unusual functional properties allow the CLCs to fill diverse physiological niches, regulating processes that range from muscle contraction to acidification of intracellular organelles, nutrient accumulation and survival of bacteria to environmental stresses. Over the last 15 years, the availability of atomic-level information on the structure of the CLCs, coupled to the discovery that the family is divided into passive channels and secondary active transporters, has revolutionized our understanding of their function. These breakthroughs led to the identification of the key structural elements regulating gating, transport, selectivity and regulation by ligands. Unexpectedly, many lines of evidence indicate that the CLC exchangers function according to a non-conventional transport mechanism that defies the fundamental tenets of the alternating-access paradigm for exchange transport, paving the way for future unexpected insights into the principles underlying active transport and channel gating.
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Affiliation(s)
- Alessio Accardi
- Departments of Anaesthesiology, Physiology & Biophysics, and Biochemistry, Weill Cornell Medical College, 1300 York Avenue, New York, NY, 10021, USA
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11
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Zifarelli G. A tale of two CLCs: biophysical insights toward understanding ClC-5 and ClC-7 function in endosomes and lysosomes. J Physiol 2015; 593:4139-50. [PMID: 26036722 DOI: 10.1113/jp270604] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2015] [Accepted: 05/22/2015] [Indexed: 11/08/2022] Open
Abstract
The CLC protein family comprises both Cl(-) channels and H(+) -coupled anion transporters. The understanding of the critical role of CLC proteins in a number of physiological functions has greatly contributed to a revision of the classical paradigm that attributed to Cl(-) ions only a marginal role in human physiology. The endosomal ClC-5 and the lysosomal ClC-7 are the best characterized human CLC transporters. Their dysfunction causes Dent's disease and osteopetrosis, respectively. It had been originally proposed that they would provide a Cl(-) shunt conductance allowing efficient acidification of intracellular compartments. However, this model seems to conflict with the transport properties of these proteins and with recent physiological evidence. Currently, there is no consensus on their specific physiological role. CLC proteins present also a number of peculiar biophysical properties, such as the dimeric architecture, the co-existence of intrinsically different thermodynamic modes of transport based on similar structural principles, and the gating mechanism recently emerging for the transporters, just to name a few. This review focuses on the biophysical properties and physiological roles of ClC-5 and ClC-7.
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Affiliation(s)
- Giovanni Zifarelli
- Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, UK
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12
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Imbrici P, Altamura C, Pessia M, Mantegazza R, Desaphy JF, Camerino DC. ClC-1 chloride channels: state-of-the-art research and future challenges. Front Cell Neurosci 2015; 9:156. [PMID: 25964741 PMCID: PMC4410605 DOI: 10.3389/fncel.2015.00156] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2015] [Accepted: 04/08/2015] [Indexed: 01/06/2023] Open
Abstract
The voltage-dependent ClC-1 chloride channel belongs to the CLC channel/transporter family. It is a homodimer comprising two individual pores which can operate independently or simultaneously according to two gating modes, the fast and the slow gate of the channel. ClC-1 is preferentially expressed in the skeletal muscle fibers where the presence of an efficient Cl(-) homeostasis is crucial for the correct membrane repolarization and propagation of action potential. As a consequence, mutations in the CLCN1 gene cause dominant and recessive forms of myotonia congenita (MC), a rare skeletal muscle channelopathy caused by abnormal membrane excitation, and clinically characterized by muscle stiffness and various degrees of transitory weakness. Elucidation of the mechanistic link between the genetic defects and the disease pathogenesis is still incomplete and, at this time, there is no specific treatment for MC. Still controversial is the subcellular localization pattern of ClC-1 channels in skeletal muscle as well as its modulation by some intracellular factors. The expression of ClC-1 in other tissues such as in brain and heart and the possible assembly of ClC-1/ClC-2 heterodimers further expand the physiological properties of ClC-1 and its involvement in diseases. A recent de novo CLCN1 truncation mutation in a patient with generalized epilepsy indeed postulates an unexpected role of this channel in the control of neuronal network excitability. This review summarizes the most relevant and state-of-the-art research on ClC-1 chloride channels physiology and associated diseases.
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Affiliation(s)
- Paola Imbrici
- Department of Pharmacy - Drug Sciences, University of Bari “Aldo Moro”,Bari, Italy
| | - Concetta Altamura
- Department of Pharmacy - Drug Sciences, University of Bari “Aldo Moro”,Bari, Italy
| | - Mauro Pessia
- Department of Pharmacy - Drug Sciences, University of Bari “Aldo Moro”,Bari, Italy
| | - Renato Mantegazza
- Department of Pharmacy - Drug Sciences, University of Bari “Aldo Moro”,Bari, Italy
| | | | - Diana Conte Camerino
- Department of Pharmacy - Drug Sciences, University of Bari “Aldo Moro”,Bari, Italy
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13
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Gradogna A, Imbrici P, Zifarelli G, Liantonio A, Camerino DC, Pusch M. I-J loop involvement in the pharmacological profile of CLC-K channels expressed in Xenopus oocytes. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2014; 1838:2745-56. [PMID: 25073071 PMCID: PMC4331650 DOI: 10.1016/j.bbamem.2014.07.021] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 03/20/2014] [Revised: 07/13/2014] [Accepted: 07/21/2014] [Indexed: 12/19/2022]
Abstract
CLC-K chloride channels and their subunit, barttin, are crucial for renal NaCl reabsorption and for inner ear endolymph production. Mutations in CLC-Kb and barttin cause Bartter syndrome. Here, we identified two adjacent residues, F256 and N257, that when mutated hugely alter in Xenopus oocytes CLC-Ka's biphasic response to niflumic acid, a drug belonging to the fenamate class, with F256A being potentiated 37-fold and N257A being potently blocked with a KD~1μM. These residues are localized in the same extracellular I-J loop which harbors a regulatory Ca(2+) binding site. This loop thus can represent an ideal and CLC-K specific target for extracellular ligands able to modulate channel activity. Furthermore, we demonstrated the involvement of the barttin subunit in the NFA potentiation. Indeed the F256A mutation confers onto CLC-K1 a transient potentiation induced by NFA which is found only when CLC-K1/F256A is co-expressed with barttin. Thus, in addition to the role of barttin in targeting and gating, the subunit participates in the pharmacological modulation of CLC-K channels and thus represents a further target for potential drugs.
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Affiliation(s)
| | - Paola Imbrici
- Dipartimento di Farmacia-Scienze del farmaco, Università degli Studi di Bari, Via Orabona 4, 70125 Bari, Italy
| | | | - Antonella Liantonio
- Dipartimento di Farmacia-Scienze del farmaco, Università degli Studi di Bari, Via Orabona 4, 70125 Bari, Italy
| | - Diana Conte Camerino
- Dipartimento di Farmacia-Scienze del farmaco, Università degli Studi di Bari, Via Orabona 4, 70125 Bari, Italy
| | - Michael Pusch
- Istituto di Biofisica, CNR, Via De Marini 6, 16149 Genoa, Italy.
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14
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Conformational changes required for H(+)/Cl(-) exchange mediated by a CLC transporter. Nat Struct Mol Biol 2014; 21:456-63. [PMID: 24747941 PMCID: PMC4040230 DOI: 10.1038/nsmb.2814] [Citation(s) in RCA: 56] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2013] [Accepted: 03/26/2014] [Indexed: 11/08/2022]
Abstract
CLC-type exchangers mediate transmembrane Cl(-) transport. Mutations altering their gating properties cause numerous genetic disorders. However, their transport mechanism remains poorly understood. In conventional models, two gates alternatively expose substrates to the intra- or extracellular solutions. A glutamate was identified as the only gate in the CLCs, suggesting that CLCs function by a nonconventional mechanism. Here we show that transport in CLC-ec1, a prokaryotic homolog, is inhibited by cross-links constraining movement of helix O far from the transport pathway. Cross-linked CLC-ec1 adopts a wild-type-like structure, indicating stabilization of a native conformation. Movements of helix O are transduced to the ion pathway via a direct contact between its C terminus and a tyrosine that is a constitutive element of the second gate of CLC transporters. Therefore, the CLC exchangers have two gates that are coupled through conformational rearrangements outside the ion pathway.
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15
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Gradogna A, Pusch M. Alkaline pH block of CLC-K kidney chloride channels mediated by a pore lysine residue. Biophys J 2014; 105:80-90. [PMID: 23823226 DOI: 10.1016/j.bpj.2013.05.044] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2013] [Revised: 05/09/2013] [Accepted: 05/24/2013] [Indexed: 10/26/2022] Open
Abstract
CLC-K chloride channels are expressed in the kidney and the inner ear, where they are involved in NaCl reabsorption and endolymph production, respectively. These channels require the beta subunit barttin for proper function. Mutations in ClC-Kb and barttin, lead to Bartter's syndrome. Block of CLC-K channels by acid pH was described in a previous work, and we had identified His-497 as being responsible for the acidic block of CLC-K channels. Here, we show that ClC-K currents are blocked also by alkaline pH with an apparent pK value of ∼8.7 for ClC-K1. Using noise analysis, we demonstrate that alkaline block is mediated by an allosteric reduction of the open probability. By an extensive mutagenic screen we identified K165, a highly conserved residue in the extracellular vestibule of the channel, as the major element responsible for the alkaline pH modulation. Deprotonation of K165 underlies the alkaline block. However, MTS modification of the K165C mutant demonstrated that not only the charge but also the chemical and sterical properties of lysine 165 are determinants of CLC-K gating.
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16
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Yamada T, Bhate MP, Strange K. Regulatory phosphorylation induces extracellular conformational changes in a CLC anion channel. Biophys J 2013; 104:1893-904. [PMID: 23663832 DOI: 10.1016/j.bpj.2013.03.026] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2012] [Revised: 03/15/2013] [Accepted: 03/19/2013] [Indexed: 10/26/2022] Open
Abstract
CLH-3b is a CLC-1/2/Ka/Kb channel homolog activated by meiotic cell cycle progression and cell swelling. Channel inhibition occurs by GCK-3 kinase-mediated phosphorylation of serine residues on the cytoplasmic C-terminus linker connecting CBS1 and CBS2. Two conserved aromatic amino acid residues located on the intracellular loop connecting membrane helices H and I and α1 of CBS2 are required for transducing phosphorylation changes into changes in channel activity. Helices H and I form part of the interface between the two subunits that comprise functional CLC channels. Using a cysteine-less CLH-3b mutant, we demonstrate that the sulfhydryl reagent reactivity of substituted cysteines at the subunit interface changes dramatically during GCK-3-mediated channel inhibition and that these changes are prevented by mutation of the H-I loop/CBS2 α1 signal transduction domain. We also show that GCK-3 modifies Zn(2+) inhibition, which is thought to be mediated by the common gating process. These and other results suggest that phosphorylation of the cytoplasmic C-terminus inhibits CLH-3b by inducing subunit interface conformation changes that activate the common gate. Our findings have important implications for understanding CLC regulation by diverse signaling mechanisms and for understanding the structure/function relationships that mediate intraprotein communication in this important family of Cl(-) transport proteins.
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Affiliation(s)
- Toshiki Yamada
- Boylan Center for Cellular and Molecular Physiology, Mount Desert Island Biological Laboratory, Salisbury Cove, Maine, USA
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17
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De Stefano S, Pusch M, Zifarelli G. A single point mutation reveals gating of the human ClC-5 Cl-/H+ antiporter. J Physiol 2013; 591:5879-93. [PMID: 24099800 PMCID: PMC3872759 DOI: 10.1113/jphysiol.2013.260240] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
ClC-5 is a 2Cl−/1H+ antiporter highly expressed in endosomes of proximal tubule cells. It is essential for endocytosis and mutations in ClC-5 cause Dent's disease, potentially leading to renal failure. However, the physiological role of ClC-5 is still unclear. One of the main issues is whether the strong rectification of ClC-5 currents observed in heterologous systems, with currents elicited only at positive voltages, is preserved in vivo and what is the origin of this rectification. In this work we identified a ClC-5 mutation, D76H, which, besides the typical outward currents of the wild-type (WT), shows inward tail currents at negative potentials that allow the estimation of the reversal of ClC-5 currents for the first time. A detailed analysis of the dependence of these inward tail currents on internal and external pH and [Cl−] shows that they are generated by a coupled transport of Cl− and H+ with a 2 : 1 stoichiometry. From this result we conclude that the inward tail currents are caused by a gating mechanism that regulates ClC-5 transport activity and not by a major alteration of the transport mechanism itself. This implies that the strong rectification of the currents of WT ClC-5 is at least in part caused by a gating mechanism that activates the transporter at positive potentials. These results elucidate the biophysical properties of ClC-5 and contribute to the understanding of its physiological role.
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Affiliation(s)
- Silvia De Stefano
- G. Zifarelli: Istituto di Biofisica, CNR, Via De Marini 6, I-16149 Genova, Italy.
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18
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Molecular determinants of common gating of a ClC chloride channel. Nat Commun 2013; 4:2507. [DOI: 10.1038/ncomms3507] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2013] [Accepted: 08/27/2013] [Indexed: 11/08/2022] Open
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Gradogna A, Fenollar-Ferrer C, Forrest LR, Pusch M. Dissecting a regulatory calcium-binding site of CLC-K kidney chloride channels. ACTA ACUST UNITED AC 2012; 140:681-96. [PMID: 23148261 PMCID: PMC3514729 DOI: 10.1085/jgp.201210878] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
The kidney and inner ear CLC-K chloride channels, which are involved in salt absorption and endolymph production, are regulated by extracellular Ca2+ in the millimolar concentration range. Recently, Gradogna et al. (2010. J. Gen. Physiol.http://dx.doi.org/10.1085/jgp.201010455) identified a pair of acidic residues (E261 and D278) located in the loop between helices I and J as forming a putative intersubunit Ca2+-binding site in hClC-Ka. In this study, we sought to explore the properties of the binding site in more detail. First, we verified that the site is conserved in hClC-Kb and rClC-K1. In addition, we could confer Ca2+ sensitivity to the Torpedo marmorata ClC-0 channel by exchanging its I–J loop with that from ClC-Ka, demonstrating a direct role of the loop in Ca2+ binding. Based on a structure of a bacterial CLC and a new sequence alignment, we built homology models of ClC-Ka. The models suggested additional amino acids involved in Ca2+ binding. Testing mutants of these residues, we could restrict the range of plausible models and positively identify two more residues (E259 and E281) involved in Ca2+ coordination. To investigate cation specificity, we applied extracellular Zn2+, Mg2+, Ba2+, Sr2+, and Mn2+. Zn2+ blocks ClC-Ka as well as its Ca2+-insensitive mutant, suggesting that Zn2+ binds to a different site. Mg2+ does not activate CLC-Ks, but the channels are activated by Ba2+, Sr2+, and Mn2+ with a rank order of potency of Ca2+ > Ba2+ > Sr2+ = Mn2+ for the human CLC-Ks. Dose–response analysis indicates that the less potent Ba2+ has a lower affinity rather than a lower efficacy. Interestingly, rClC-K1 shows an altered rank order (Ca2+ > Sr2+ >> Ba2+), but homology models suggest that residues outside the I–J loop are responsible for this difference. Our detailed characterization of the regulatory Ca2+-binding site provides a solid basis for the understanding of the physiological modulation of CLC-K channel function in the kidney and inner ear.
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Affiliation(s)
- Antonella Gradogna
- Istituto di Biofisica, Consiglio Nazionale delle Ricerche, 16149 Genoa, Italy
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20
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De Stefano S, Pusch M, Zifarelli G. Extracellular determinants of anion discrimination of the Cl-/H+ antiporter protein CLC-5. J Biol Chem 2011; 286:44134-44144. [PMID: 21921031 PMCID: PMC3243520 DOI: 10.1074/jbc.m111.272815] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
Mammalian CLC proteins comprise both Cl− channels and Cl−/H+ antiporters that carry out fundamental physiological tasks by transporting Cl− across plasma membrane and intracellular compartments. The NO3− over Cl− preference of a plant CLC transporter has been pinpointed to a conserved serine residue located at Scen and it is generally assumed that the other two binding sites of CLCs, Sext and Sin, do not substantially contribute to anion selectivity. Here we show for the Cl−/H+ antiporter CLC-5 that the conserved and extracellularly exposed Lys210 residue is critical to determine the anion specificity for transport activity. In particular, mutations that neutralize or invert the charge at this position reverse the NO3− over Cl− preference of WT CLC-5 at a concentration of 100 mm, but do not modify the coupling stoichiometry with H+. The importance of the electrical charge is shown by chemical modification of K210C with positively charged cysteine-reactive compounds that reintroduce the WT preference for Cl−. At saturating extracellular anion concentrations, neutralization of Lys210 is of little impact on the anion preference, suggesting an important role of Lys210 on the association rate of extracellular anions to Sext.
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Affiliation(s)
| | - Michael Pusch
- Istituto di Biofisica, CNR, Via De Marini 6, I-16149 Genova, Italy
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21
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Strange K. Putting the pieces together: a crystal clear window into CLC anion channel regulation. Channels (Austin) 2011; 5:101-5. [PMID: 21317557 DOI: 10.4161/chan.5.2.14694] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
CLC anion transport proteins function as Cl (-) channels and Cl (-) /H (+) exchangers and are found in all major groups of life including archaebacteria. Early electrophysiological studies suggested that CLC anion channels have two pores that are opened and closed independently by a "fast" gating process operating on a millisecond timescale, and a "common" or "slow" gate that opens and closes both pores simultaneously with a timescale of seconds (Figure 1A). Subsequent biochemical and molecular experiments suggested that CLC channels/transporters are homodomeric proteins ( 1-3) .
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Affiliation(s)
- Kevin Strange
- Boylan Center for Cellular and Molecular Physiology, Mount Desert Island Biological Laboratory; Salisbury Cove, ME, USA.
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22
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Gradogna A, Babini E, Picollo A, Pusch M. A regulatory calcium-binding site at the subunit interface of CLC-K kidney chloride channels. ACTA ACUST UNITED AC 2010; 136:311-23. [PMID: 20805576 PMCID: PMC2931146 DOI: 10.1085/jgp.201010455] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
The two human CLC Cl− channels, ClC-Ka and ClC-Kb, are almost exclusively expressed in kidney and inner ear epithelia. Mutations in the genes coding for ClC-Kb and barttin, an essential CLC-K channel β subunit, lead to Bartter syndrome. We performed a biophysical analysis of the modulatory effect of extracellular Ca2+ and H+ on ClC-Ka and ClC-Kb in Xenopus oocytes. Currents increased with increasing [Ca2+]ext without full saturation up to 50 mM. However, in the absence of Ca2+, ClC-Ka currents were still 20% of currents in 10 mM [Ca2+]ext, demonstrating that Ca2+ is not strictly essential for opening. Vice versa, ClC-Ka and ClC-Kb were blocked by increasing [H+]ext with a practically complete block at pH 6. Ca2+ and H+ act as gating modifiers without changing the single-channel conductance. Dose–response analysis suggested that two protons are necessary to induce block with an apparent pK of ∼7.1. A simple four-state allosteric model described the modulation by Ca2+ assuming a 13-fold higher Ca2+ affinity of the open state compared with the closed state. The quantitative analysis suggested separate binding sites for Ca2+ and H+. A mutagenic screen of a large number of extracellularly accessible amino acids identified a pair of acidic residues (E261 and D278 on the loop connecting helices I and J), which are close to each other but positioned on different subunits of the channel, as a likely candidate for forming an intersubunit Ca2+-binding site. Single mutants E261Q and D278N greatly diminished and the double mutant E261Q/D278N completely abolished modulation by Ca2+. Several mutations of a histidine residue (H497) that is homologous to a histidine that is responsible for H+ block in ClC-2 did not yield functional channels. However, the triple mutant E261Q/D278N/H497M completely eliminated H+ -induced current block. We have thus identified a protein region that is involved in binding these physiologically important ligands and that is likely undergoing conformational changes underlying the complex gating of CLC-K channels.
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Affiliation(s)
- Antonella Gradogna
- Istituto di Biofisica, Consiglio Nazionale delle Ricerche, 16149 Genoa, Italy
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23
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Zifarelli G, Liantonio A, Gradogna A, Picollo A, Gramegna G, De Bellis M, Murgia AR, Babini E, Conte Camerino D, Pusch M. Identification of sites responsible for the potentiating effect of niflumic acid on ClC-Ka kidney chloride channels. Br J Pharmacol 2010; 160:1652-61. [PMID: 20649569 PMCID: PMC2936838 DOI: 10.1111/j.1476-5381.2010.00822.x] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2009] [Revised: 03/02/2010] [Accepted: 03/09/2010] [Indexed: 01/14/2023] Open
Abstract
BACKGROUND AND PURPOSE ClC-K kidney Cl(-) channels are important for renal and inner ear transepithelial Cl(-) transport, and are potentially interesting pharmacological targets. They are modulated by niflumic acid (NFA), a non-steroidal anti-inflammatory drug, in a biphasic way: NFA activates ClC-Ka at low concentrations, but blocks the channel above approximately 1 mM. We attempted to identify the amino acids involved in the activation of ClC-Ka by NFA. EXPERIMENTAL APPROACH We used site-directed mutagenesis and two-electrode voltage clamp analysis of wild-type and mutant channels expressed in Xenopus oocytes. Guided by the crystal structure of a bacterial CLC homolog, we screened 97 ClC-Ka mutations for alterations of NFA effects. KEY RESULTS Mutations of five residues significantly reduced the potentiating effect of NFA. Two of these (G167A and F213A) drastically altered general gating properties and are unlikely to be involved in NFA binding. The three remaining mutants (L155A, G345S and A349E) severely impaired or abolished NFA potentiation. CONCLUSIONS AND IMPLICATIONS The three key residues identified (L155, G345, A349) are localized in two different protein regions that, based on the crystal structure of bacterial CLC homologs, are expected to be exposed to the extracellular side of the channel, relatively close to each other, and are thus good candidates for being part of the potentiating NFA binding site. Alternatively, the protein region identified mediates conformational changes following NFA binding. Our results are an important step towards the development of ClC-Ka activators for treating Bartter syndrome types III and IV with residual channel activity.
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Affiliation(s)
- G Zifarelli
- Istituto di Biofisica, Consiglio Nazionale delle RicercheGenova, Italy
| | - A Liantonio
- Istituto di Biofisica, Consiglio Nazionale delle RicercheGenova, Italy
- Sezione di Farmacologia, Dipartimento Farmacobiologico, Via Orabona 4, Università di BariBari, Italy
| | - A Gradogna
- Istituto di Biofisica, Consiglio Nazionale delle RicercheGenova, Italy
| | - A Picollo
- Istituto di Biofisica, Consiglio Nazionale delle RicercheGenova, Italy
| | - G Gramegna
- Istituto di Biofisica, Consiglio Nazionale delle RicercheGenova, Italy
- Sezione di Farmacologia, Dipartimento Farmacobiologico, Via Orabona 4, Università di BariBari, Italy
| | - M De Bellis
- Istituto di Biofisica, Consiglio Nazionale delle RicercheGenova, Italy
- Sezione di Farmacologia, Dipartimento Farmacobiologico, Via Orabona 4, Università di BariBari, Italy
| | - AR Murgia
- Istituto di Biofisica, Consiglio Nazionale delle RicercheGenova, Italy
| | - E Babini
- Istituto di Biofisica, Consiglio Nazionale delle RicercheGenova, Italy
| | - D Conte Camerino
- Sezione di Farmacologia, Dipartimento Farmacobiologico, Via Orabona 4, Università di BariBari, Italy
| | - M Pusch
- Istituto di Biofisica, Consiglio Nazionale delle RicercheGenova, Italy
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Dave S, Sheehan JH, Meiler J, Strange K. Unique gating properties of C. elegans ClC anion channel splice variants are determined by altered CBS domain conformation and the R-helix linker. Channels (Austin) 2010; 4:289-301. [PMID: 20581474 DOI: 10.4161/chan.4.4.12445] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
All eukaryotic and some prokaryotic ClC anion transport proteins have extensive cytoplasmic C-termini containing two cystathionine-β-synthase (CBS) domains. CBS domain secondary structure is highly conserved and consists of two α-helices and three β-strands arranged as β1-α1-β2-β3-α2. ClC CBS domain mutations cause muscle and bone disease and alter ClC gating. However, the precise functional roles of CBS domains and the structural bases by which they regulate ClC function are poorly understood. CLH-3a and CLH-3b are C. elegans ClC anion channel splice variants with strikingly different biophysical properties. Splice variation occurs at cytoplasmic N- and C-termini and includes several amino acids that form α2 of the second CBS domain (CBS2). We demonstrate that interchanging α2 between CLH-3a and CLH-3b interchanges their gating properties. The "R-helix" of ClC proteins forms part of the ion-conducting pore and selectivity filter and is connected to the cytoplasmic C-terminus via a short stretch of cytoplasmic amino acids termed the "R-helix linker". C-terminus conformation changes could cause R-helix structural rearrangements via this linker. X-ray structures of three ClC protein cytoplasmic C-termini suggest that α2 of CBS2 and the R-helix linker could be closely apposed and may therefore interact. We found that mutating apposing amino acids in α2 and the R-helix linker of CLH-3b was sufficient to give rise to CLH-3a-like gating. We postulate that the R-helix linker interacts with CBS2 α2, and that this putative interaction provides a pathway by which cytoplasmic C-terminus conformational changes induce conformational changes in membrane domains that in turn modulate ClC function.
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Affiliation(s)
- Sonya Dave
- Boylan Center for Cellular and Molecular Physiology, Mount Desert Island Biological Laboratory, Salisbury Cove, ME, USA
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25
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Accardi A, Picollo A. CLC channels and transporters: proteins with borderline personalities. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2010; 1798:1457-64. [PMID: 20188062 DOI: 10.1016/j.bbamem.2010.02.022] [Citation(s) in RCA: 79] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/08/2009] [Revised: 02/12/2010] [Accepted: 02/18/2010] [Indexed: 11/19/2022]
Abstract
Controlled chloride movement across membranes is essential for a variety of physiological processes ranging from salt homeostasis in the kidneys to acidification of cellular compartments. The CLC family is formed by two, not so distinct, sub-classes of membrane transport proteins: Cl(-) channels and H(+)/Cl(-) exchangers. All CLC's are homodimers with each monomer forming an individual Cl- permeation pathway which appears to be largely unaltered in the two CLC sub-classes. Key residues for ion binding and selectivity are also highly conserved. Most CLC's have large cytosolic carboxy-terminal domains containing two cystathionine beta-synthetase (CBS) domains. The C-termini are critical regulators of protein trafficking and directly modulate Cl- by binding intracellular ATP, H+ or oxidizing compounds. This review focuses on the recent mechanistic insights on the how the structural similarities between CLC channels and transporters translate in unexpected mechanistic analogies between these two sub-classes.
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Affiliation(s)
- Alessio Accardi
- Department of Molecular Physiology and Biophysics, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA.
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26
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Thompson CH, Olivetti PR, Fuller MD, Freeman CS, McMaster D, French RJ, Pohl J, Kubanek J, McCarty NA. Isolation and characterization of a high affinity peptide inhibitor of ClC-2 chloride channels. J Biol Chem 2009; 284:26051-62. [PMID: 19574231 DOI: 10.1074/jbc.m109.031724] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The ClC protein family includes voltage-gated chloride channels and chloride/proton exchangers. In eukaryotes, ClC proteins regulate membrane potential of excitable cells, contribute to epithelial transport, and aid in lysosomal acidification. Although structure/function studies of ClC proteins have been aided greatly by the available crystal structures of a bacterial ClC chloride/proton exchanger, the availability of useful pharmacological tools, such as peptide toxin inhibitors, has lagged far behind that of their cation channel counterparts. Here we report the isolation, from Leiurus quinquestriatus hebraeus venom, of a peptide toxin inhibitor of the ClC-2 chloride channel. This toxin, GaTx2, inhibits ClC-2 channels with a voltage-dependent apparent K(D) of approximately 20 pm, making it the highest affinity inhibitor of any chloride channel. GaTx2 slows ClC-2 activation by increasing the latency to first opening by nearly 8-fold but is unable to inhibit open channels, suggesting that this toxin inhibits channel activation gating. Finally, GaTx2 specifically inhibits ClC-2 channels, showing no inhibitory effect on a battery of other major classes of chloride channels and voltage-gated potassium channels. GaTx2 is the first peptide toxin inhibitor of any ClC protein. The high affinity and specificity displayed by this toxin will make it a very powerful pharmacological tool to probe ClC-2 structure/function.
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27
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Niemeyer MI, Cid LP, Yusef YR, Briones R, Sepúlveda FV. Voltage-dependent and -independent titration of specific residues accounts for complex gating of a ClC chloride channel by extracellular protons. J Physiol 2009; 587:1387-400. [PMID: 19153159 DOI: 10.1113/jphysiol.2008.167353] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
The ClC transport protein family comprises both Cl(-) ion channel and H(+)/Cl(-) and H(+)/NO(3)(-) exchanger members. Structural studies on a bacterial ClC transporter reveal a pore obstructed at its external opening by a glutamate side-chain which acts as a gate for Cl(-) passage and in addition serves as a staging post for H(+) exchange. This same conserved glutamate acts as a gate to regulate Cl(-) flow in ClC channels. The activity of ClC-2, a genuine Cl(-) channel, has a biphasic response to extracellular pH with activation by moderate acidification followed by abrupt channel closure at pH values lower than approximately 7. We have now investigated the molecular basis of this complex gating behaviour. First, we identify a sensor that couples extracellular acidification to complete closure of the channel. This is extracellularly-facing histidine 532 at the N-terminus of transmembrane helix Q whose neutralisation leads to channel closure in a cooperative manner. We go on to show that acidification-dependent activation of ClC-2 is voltage dependent and probably mediated by protonation of pore gate glutamate 207. Intracellular Cl(-) acts as a voltage-independent modulator, as though regulating the pK(a) of the protonatable residue. Our results suggest that voltage dependence of ClC-2 is given by hyperpolarisation-dependent penetration of protons from the extracellular side to neutralise the glutamate gate deep within the channel, which allows Cl(-) efflux. This is reminiscent of a partial exchanger cycle, suggesting that the ClC-2 channel evolved from its transporter counterparts.
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28
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The role of protons in fast and slow gating of the Torpedo chloride channel ClC-0. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2009; 39:869-75. [DOI: 10.1007/s00249-008-0393-x] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/24/2008] [Revised: 12/02/2008] [Accepted: 12/08/2008] [Indexed: 11/26/2022]
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Abstract
Intracellularly applied amphiphilic molecules, such as p-chlorophenoxy acetate (CPA) and octanoate, block various pore-open mutants of CLC-0. The voltage-dependent block of a particular pore-open mutant, E166G, was found to be multiphasic. In symmetrical 140 mM Cl(-), the apparent affinity of the blocker in this mutant increased with a negative membrane potential but, paradoxically, decreased when the negative membrane potential was greater than -80 mV, a phenomenon similar to the blocker "punch-through" shown in many blocker studies of cation channels. To provide further evidence of the punch-through of CPA and octanoate, we studied the dissociation rate of the blocker from the pore by measuring the time constant of relief from the block under various voltage and ionic conditions. Consistent with the voltage dependence of the effect on the steady-state current, the rate of CPA dissociation from the E166G pore reached a minimum at -80 mV in symmetrical 140 mM Cl(-), and the direction of current recovery suggested that the bound CPA in the pore can dissociate into both intracellular and extracellular solutions. Moreover, the CPA dissociation depends upon the Cl(-) reversal potential with a minimal dissociation rate at a voltage 80 mV more negative than the Cl(-) reversal potential. That the shift of the CPA-dissociation rate follows the Cl(-) gradient across the membrane argues that these blockers can indeed punch through the channel pore. Furthermore, a minimal CPA-dissociation rate at a voltage 80 mV more negative than the Cl(-) reversal potential suggests that the outward blocker movement through the CLC-0 pore is more difficult than the inward movement.
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Affiliation(s)
- Xiao-Dong Zhang
- Center for Neuroscience and Department of Neurology, University of California, Davis, CA 95618, USA
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30
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Zhang XD, Tseng PY, Yu WP, Chen TY. Blocking pore-open mutants of CLC-0 by amphiphilic blockers. ACTA ACUST UNITED AC 2008; 133:43-58. [PMID: 19088381 PMCID: PMC2606940 DOI: 10.1085/jgp.200810004] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The blockade of CLC-0 chloride channels by p-chlorophenoxy acetate (CPA) has been thought to be state dependent; the conformational change of the channel pore during the “fast gating” alters the CPA binding affinity. Here, we examine the mechanism of CPA blocking in pore-open mutants of CLC-0 in which the residue E166 was replaced by various amino acids. We find that the CPA-blocking affinities depend upon the volume and the hydrophobicity of the side chain of the introduced residue; CPA affinity can vary by three orders of magnitude in these mutants. On the other hand, mutations at the intracellular pore entrance, although affecting the association and dissociation rates of the CPA block, generate only a modest effect on the steady-state blocking affinity. In addition, various amphiphilic compounds, including fatty acids and alkyl sulfonates, can also block the pore-open mutants of CLC-0 through a similar mechanism. The blocking affinity of fatty acids and alkyl sulfonates increases with the length of these amphiphilic blockers, a phenomenon similar to the block of the Shaker K+ channel by long-chain quaternary ammonium (QA) ions. These observations lead us to propose that the CPA block of the open pore of CLC-0 is similar to the blockade of voltage-gated K+ channels by long-chain QAs or by the inactivation ball peptide: the blocker first uses the hydrophilic end to “dock” at the pore entrance, and the hydrophobic part of the blocker then enters the pore to interact with a more hydrophobic region of the pore. This blocking mechanism appears to be very general because the block does not require a precise structural fit between the blocker and the pore, and the blocking mechanism applies to the cation and anion channels with unrelated pore architectures.
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Affiliation(s)
- Xiao-Dong Zhang
- Center for Neuroscience and Department of Neurology, University of California, Davis, CA 95618, USA
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31
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Arreola J, De Santiago-Castillo JA, Sánchez JE, Nieto PG. Gating and trafficking of ClC-2 chloride channel without cystathionine beta-synthase domains. J Physiol 2008; 586:5289. [PMID: 19011132 PMCID: PMC2655376 DOI: 10.1113/jphysiol.2008.164103] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 08/31/2023] Open
Affiliation(s)
- Jorge Arreola
- Physics Institute, Universidad Autónoma de San Luís Potosí, Ave. Dr M Nava 6, San Luís Potosí, SLP, México, 78290.
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32
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Abstract
Some CLC proteins function as passive Cl− ion channels whereas others are secondary active chloride/proton antiporters. Voltage-dependent gating of the model Torpedo channel ClC-0 is modulated by intracellular and extracellular pH, possibly reflecting a mechanistic relationship with the chloride/proton coupling of CLC antiporters. We used inside-out patch clamp measurements and mutagenesis to explore the dependence of the fast gating mechanism of ClC-0 on intracellular pH and to identify the putative intracellular proton acceptor(s). Among the tested residues (S123, K129, R133, K149, E166, F214L, S224, E226, V227, C229, R305, R312, C415, H472, F418, V419, P420, and Y512) only mutants of E166, F214, and F418 qualitatively changed the pHint dependence. No tested amino acid emerged as a valid candidate for being a pH sensor. A detailed kinetic analysis of the dependence of fast gate relaxations on pHint and [Cl−]int provided quantitative constraints on possible mechanistic models of gating. In one particular model, a proton is generated by the dissociation of a water molecule in an intrapore chloride ion binding site. The proton is delivered to the side chain of E166 leading to the opening of the channel, while the hydroxyl ion is stabilized in the internal/central anion binding site. Deuterium isotope effects confirm that proton transfer is rate limiting for fast gate opening and that channel closure depends mostly on the concentration of OH− ions. The gating model is in natural agreement with the finding that only the closing rate constant, but not the opening rate constant, depends on pHint and [Cl−]int.
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Affiliation(s)
- Giovanni Zifarelli
- Istituto di Biofisica, Consiglio Nazionale delle Ricerche, I-16149 Genova, Italy
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33
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Kuang Z, Liu A, Beck TL. TransPath: a computational method for locating ion transit pathways through membrane proteins. Proteins 2008; 71:1349-59. [PMID: 18058905 DOI: 10.1002/prot.21822] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The finely tuned structures of membrane channel proteins allow selective passage of ions through the available aqueous pores. To understand channel function, it is crucial to locate the pores and study their physical and chemical properties. Here, we propose a new pore-searching algorithm (TransPath), which uses the Configurational Bias Monte Carlo (CBMC) method to generate transmembrane trajectories driven by both geometric and electrostatic features. The trajectories are binned into groups determined by a vector distance criterion. From each group, a representative trajectory is selected based on the Rosenbluth weight, and the geometrically optimal path is obtained by simulated annealing. Candidate ion pathways can then be determined by analysis of the radius and potential profiles. The proposed method and its implementation are illustrated using the bacterial KcsA potassium channel as an example. The procedure is then applied to the more complex structures of the bacterial E. coli chloride channel homolog and a homology model of the ClC-0 channel.
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Affiliation(s)
- Zhifeng Kuang
- AFRL/MLBP, 2941 Hobson Way, Wright Patterson Air Force Base, Ohio 45433-7750, USA
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34
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Ion permeation through a Cl--selective channel designed from a CLC Cl-/H+ exchanger. Proc Natl Acad Sci U S A 2008; 105:11194-9. [PMID: 18678918 DOI: 10.1073/pnas.0804503105] [Citation(s) in RCA: 89] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The CLC family of Cl(-)-transporting proteins includes both Cl(-) channels and Cl(-)/H(+) exchange transporters. CLC-ec1, a structurally known bacterial homolog of the transporter subclass, exchanges two Cl(-) ions per proton with strict, obligatory stoichiometry. Point mutations at two residues, Glu(148) and Tyr(445), are known to impair H(+) movement while preserving Cl(-) transport. In the x-ray crystal structure of CLC-ec1, these residues form putative "gates" flanking an ion-binding region. In mutants with both of the gate-forming side chains reduced in size, H(+) transport is abolished, and unitary Cl(-) transport rates are greatly increased, well above values expected for transporter mechanisms. Cl(-) transport rates increase as side-chain volume at these positions is decreased. The crystal structure of a doubly ungated mutant shows a narrow conduit traversing the entire protein transmembrane width. These characteristics suggest that Cl(-) flux through uncoupled, ungated CLC-ec1 occurs via a channel-like electrodiffusion mechanism rather than an alternating-exposure conformational cycle that has been rendered proton-independent by the gate mutations.
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35
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Jentsch TJ. CLC chloride channels and transporters: from genes to protein structure, pathology and physiology. Crit Rev Biochem Mol Biol 2008; 43:3-36. [PMID: 18307107 DOI: 10.1080/10409230701829110] [Citation(s) in RCA: 277] [Impact Index Per Article: 17.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
CLC genes are expressed in species from bacteria to human and encode Cl(-)-channels or Cl(-)/H(+)-exchangers. CLC proteins assemble to dimers, with each monomer containing an ion translocation pathway. Some mammalian isoforms need essential beta -subunits (barttin and Ostm1). Crystal structures of bacterial CLC Cl(-)/H(+)-exchangers, combined with transport analysis of mammalian and bacterial CLCs, yielded surprising insights into their structure and function. The large cytosolic carboxy-termini of eukaryotic CLCs contain CBS domains, which may modulate transport activity. Some of these have been crystallized. Mammals express nine CLC isoforms that differ in tissue distribution and subcellular localization. Some of these are plasma membrane Cl(-) channels, which play important roles in transepithelial transport and in dampening muscle excitability. Other CLC proteins localize mainly to the endosomal-lysosomal system where they may facilitate luminal acidification or regulate luminal chloride concentration. All vesicular CLCs may be Cl(-)/H(+)-exchangers, as shown for the endosomal ClC-4 and -5 proteins. Human diseases and knockout mouse models have yielded important insights into their physiology and pathology. Phenotypes and diseases include myotonia, renal salt wasting, kidney stones, deafness, blindness, male infertility, leukodystrophy, osteopetrosis, lysosomal storage disease and defective endocytosis, demonstrating the broad physiological role of CLC-mediated anion transport.
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Affiliation(s)
- Thomas J Jentsch
- Leibniz-Institut für Molekulare Pharmakologie (FMP) and Max-Delbrück-Centrum für Molekulare Medizin (MDC), Berlin, Germany.
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37
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Abstract
CLC-0 and cystic fibrosis transmembrane conductance regulator (CFTR) Cl−channels play important roles in Cl−transport across cell membranes. These two proteins belong to, respectively, the CLC and ABC transport protein families whose members encompass both ion channels and transporters. Defective function of members in these two protein families causes various hereditary human diseases. Ion channels and transporters were traditionally viewed as distinct entities in membrane transport physiology, but recent discoveries have blurred the line between these two classes of membrane transport proteins. CLC-0 and CFTR can be considered operationally as ligand-gated channels, though binding of the activating ligands appears to be coupled to an irreversible gating cycle driven by an input of free energy. High-resolution crystallographic structures of bacterial CLC proteins and ABC transporters have led us to a better understanding of the gating properties for CLC and CFTR Cl−channels. Furthermore, the joined force between structural and functional studies of these two protein families has offered a unique opportunity to peek into the evolutionary link between ion channels and transporters. A promising byproduct of this exercise is a deeper mechanistic insight into how different transport proteins work at a fundamental level.
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38
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Camerino DC, Desaphy JF, Tricarico D, Pierno S, Liantonio A. Therapeutic Approaches to Ion Channel Diseases. ADVANCES IN GENETICS 2008; 64:81-145. [DOI: 10.1016/s0065-2660(08)00804-3] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
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39
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Wijesinghe R, Coorey N, Kuyucak S. Charge state of the fast gate in chloride channels: Insights from electrostatic calculations in a schematic model. J Chem Phys 2007; 127:195102. [DOI: 10.1063/1.2804419] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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40
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Abstract
Members of the CLC 'chloride channel' family play vital roles in a wide variety of physiological settings. Research on prokaryotic CLC homologues provided long-anticipated high-resolution structures as well as the unexpected discovery that some CLCs are not chloride channels, but rather are proton-chloride antiporters. Hence, CLCs encompass two functional classes of transport proteins once thought to be fundamentally different from one another. In this review, we discuss the structural features and molecular mechanisms of CLC channels and antiporters. We focus on ClC-0, the most thoroughly studied CLC channel, and ClC-ec1, the prokaryotic antiporter of known structure. We highlight some striking similarities between these CLCs and discuss compelling questions that remain to be addressed. Prokaryotic CLCs will undoubtedly continue to shed light upon this understudied family of proteins.
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Affiliation(s)
- Kimberly Matulef
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, California 94305, USA
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41
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Engh AM, Faraldo-Gómez JD, Maduke M. The role of a conserved lysine in chloride- and voltage-dependent ClC-0 fast gating. ACTA ACUST UNITED AC 2007; 130:351-63. [PMID: 17846165 PMCID: PMC2151651 DOI: 10.1085/jgp.200709760] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
ClC-0 is a chloride channel whose gating is sensitive to voltage, chloride, and pH. In a previous publication, we showed that the K149C mutation causes a +70-mV shift in the voltage dependence of ClC-0 fast gating. In this paper we analyze the effects of a series of mutations at K149 on the voltage and chloride dependence of gating. By fitting our data to the previously proposed four-state model for ClC-0 fast gating, we show which steps in fast-gate opening are likely to be affected by these mutations. Computational analysis of mutant ClC-0 homology models show electrostatic contributions to chloride binding that may partially account for the effects of K149 on gating. The analysis of gating kinetics in combination with the available structural information suggests some of the structural changes likely to underpin fast-gate opening.
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Affiliation(s)
- Anita M Engh
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA 94305, USA
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42
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Abstract
ClC-0 is a chloride channel whose gating is sensitive to both voltage and chloride. Based on analysis of gating kinetics using single-channel recordings, a five-state model was proposed to describe the dependence of ClC-0 fast-gate opening on voltage and external chloride (Chen, T.-Y., and C. Miller. 1996. J. Gen. Physiol. 108:237–250). We aimed to use this five-state model as a starting point for understanding the structural changes that occur during gating. Using macroscopic patch recordings, we were able to reproduce the effects of voltage and chloride that were reported by Chen and Miller and to fit our opening rate constant data to the five-state model. Upon further analysis of both our data and those of Chen and Miller, we learned that in contrast to their conclusions, (a) the features in the data are not adequate to rule out a simpler four-state model, and (b) the chloride-binding step is voltage dependent. In order to be able to evaluate the effects of mutants on gating (described in the companion paper, see Engh et al. on p. 351 of this issue), we developed a method for determining the error on gating model parameters, and evaluated the sources of this error. To begin to mesh the kinetic model(s) with the known CLC structures, a model of ClC-0 was generated computationally based on the X-ray crystal structure of the prokaryotic homolog ClC-ec1. Analysis of pore electrostatics in this homology model suggests that at least two of the conclusions derived from the gating kinetics analysis are consistent with the known CLC structures: (1) chloride binding is necessary for channel opening, and (2) chloride binding to any of the three known chloride-binding sites must be voltage dependent.
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Affiliation(s)
- Anita M Engh
- Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA 94305, USA
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43
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Walden M, Accardi A, Wu F, Xu C, Williams C, Miller C. Uncoupling and turnover in a Cl-/H+ exchange transporter. ACTA ACUST UNITED AC 2007; 129:317-29. [PMID: 17389248 PMCID: PMC2151619 DOI: 10.1085/jgp.200709756] [Citation(s) in RCA: 113] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
Abstract
The CLC-family protein CLC-ec1, a bacterial homologue of known structure, stoichiometrically exchanges two Cl− for one H+ via an unknown membrane transport mechanism. This study examines mutations at a conserved tyrosine residue, Y445, that directly coordinates a Cl− ion located near the center of the membrane. Mutations at this position lead to “uncoupling,” such that the H+/Cl− transport ratio decreases roughly with the volume of the substituted side chain. The uncoupled proteins are still able to pump protons uphill when driven by a Cl− gradient, but the extent and rate of this H+ pumping is weaker in the more uncoupled variants. Uncoupling is accompanied by conductive Cl− transport that is not linked to counter-movement of H+, i.e., a “leak.” The unitary Cl− transport rate, measured in reconstituted liposomes by both a conventional initial-velocity method and a novel Poisson dilution approach, is ∼4,000 s−1 for wild-type protein, and the uncoupled mutants transport Cl− at similar rates.
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Affiliation(s)
- Michael Walden
- Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, MA 02454, USA
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44
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Zifarelli G, Pusch M. CLC chloride channels and transporters: a biophysical and physiological perspective. Rev Physiol Biochem Pharmacol 2007; 158:23-76. [PMID: 17729441 DOI: 10.1007/112_2006_0605] [Citation(s) in RCA: 87] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Chloride-transporting proteins play fundamental roles in many tissues in the plasma membrane as well as in intracellular membranes. They have received increasing attention in the last years because crucial, and often unexpected and novel, physiological functions have been disclosed with gene-targeting approaches, X-ray crystallography, and biophysical analysis. CLC proteins form a gene family that comprises nine members in mammals, at least four of which are involved in human genetic diseases. The X-ray structure of the bacterial CLC homolog, ClC-ec1, revealed a complex fold and confirmed the anticipated homodimeric double-barreled architecture of CLC-proteins with two separate Cl-ion transport pathways, one in each subunit. Four of the mammalian CLC proteins, ClC-1, ClC-2, ClC-Ka, and ClC-Kb, are chloride ion channels that fulfill their functional roles-stabilization of the membrane potential, transepithelial salt transport, and ion homeostasisin the plasma membrane. The other five CLC proteins are predominantly expressed in intracellular organelles like endosomes and lysosomes, where they are probably important for a proper luminal acidification, in concert with the V-type H+-ATPase. Surprisingly, ClC-4, ClC-5, and probably also ClC-3, are not Cl- ion channels but exhibit significant Cl-/H+ antiporter activity, as does the bacterial homolog ClC-ec1 and the plant homolog AtCLCa. The physiological significance of the Cl-/H+ antiport activity remains to be established.
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Affiliation(s)
- G Zifarelli
- CNR, Istituto di Biofisica, Via De Marini 6, 16149 Genova, Italy
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45
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Accardi A, Lobet S, Williams C, Miller C, Dutzler R. Synergism between halide binding and proton transport in a CLC-type exchanger. J Mol Biol 2006; 362:691-9. [PMID: 16949616 DOI: 10.1016/j.jmb.2006.07.081] [Citation(s) in RCA: 92] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2006] [Revised: 07/27/2006] [Accepted: 07/27/2006] [Indexed: 11/25/2022]
Abstract
The Cl-/H+ exchange-transporter CLC-ec1 mediates stoichiometric transmembrane exchange of two Cl- ions for one proton. A conserved tyrosine residue, Y445, coordinates one of the bound Cl- ions visible in the structure of this protein and is located near the intersection of the Cl- and H+ pathways. Mutants of this tyrosine were scrutinized for effects on the coupled transport of Cl- and H+ determined electrophysiologically and on protein structure determined crystallographically. Despite the strong conservation of Y445 in the CLC family, substitution of F or W at this position preserves wild-type transport behavior. Substitution by A, E, or H, however, produces uncoupled proteins with robust Cl- transport but greatly impaired movement of H+. The obligatory 2 Cl-/1 H+ stoichiometry is thus lost in these mutants. The structures of all the mutants are essentially identical to wild-type, but apparent anion occupancy in the Cl- binding region correlates with functional H+ coupling. In particular, as determined by anomalous diffraction in crystals grown in Br-, an electrophysiologically competent Cl- analogue, the well-coupled transporters show strong Br- electron density at the "inner" and "central" Cl- binding sites. However, in the uncoupled mutants, Br- density is absent at the central site, while still present at the inner site. An additional mutant, Y445L, is intermediate in both functional and structural features. This mutant clearly exchanges H+ for Cl-, but at a reduced H+-to-Cl- ratio; likewise, both the central and inner sites are occupied by Br-, but the central site shows lower Br- density than in wild-type (or in Y445F,W). The correlation between proton coupling and central-site occupancy argues that halide binding to the central transport site somehow facilitates movement of H+, a synergism that is not readily understood in terms of alternating-site antiport schemes.
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Affiliation(s)
- Alessio Accardi
- Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, MA 02454-9110, USA
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46
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Dutzler R. The ClC family of chloride channels and transporters. Curr Opin Struct Biol 2006; 16:439-46. [PMID: 16814540 DOI: 10.1016/j.sbi.2006.06.002] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2006] [Revised: 06/07/2006] [Accepted: 06/12/2006] [Indexed: 11/19/2022]
Abstract
The ClC proteins are members of a large family of chloride transport proteins, which are involved in a variety of physiological processes. All family members share a conserved molecular architecture consisting of a complex transmembrane transport domain and a soluble regulatory domain. To date, representative structures for the two parts are available, the transmembrane domain from the structure of a bacterial homologue, the soluble domain from a eukaryotic family member. Despite the strong conservation of the structural framework, the family members show an unusually broad variety of functional behaviors, as some members work as gated chloride channels and others as secondary chloride transporters. The conservation in the structure and the functional resemblance in gating and transport mechanism suggest a strong mechanistic relationship between seemingly contradictory transport modes.
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Affiliation(s)
- Raimund Dutzler
- Department of Biochemistry, University of Zurich, Winterthurer Strasse 190, CH-8057 Zürich, Switzerland.
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47
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Abstract
Since its discovery, the ClC family of chloride channels has presented biophysicists with unexpected behaviours and unusual surprises. The latest of these is the realization that not only does the family feature genuine chloride channels, it also includes proton-coupled chloride transporters, which move chloride ions and protons across the membrane in opposite directions. The crystal structure of such a transporter serves as a useful platform for understanding ClC channels, and features of chloride/proton exchange-transport may provide a key for comprehending voltage-dependent gating of the channels.
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Affiliation(s)
- Christopher Miller
- Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, Massachusetts 02454, USA.
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48
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Abstract
CLC Cl− channels are homodimers in which each subunit has a proper pore and a (fast) gate. An additional slow gate acts on both pores. A conserved glutamate (E166 in CLC-0) is a major determinant of gating in CLC-0 and is crucially involved in Cl−/H+ antiport of CLC-ec1, a CLC of known structure. We constructed tandem dimers with one wild-type (WT) and one mutant subunit (E166A or E166D) to show that these mutations of E166 specifically alter the fast gate of the pore to which they belong without effect on the fast gate of the neighboring pore. In addition both mutations activate the common slow gate. E166A pores have a large, voltage-independent open probability of the fast gate (popen), whereas popen of E166D pores is dramatically reduced. Similar to WT, popen of E166D was increased by lowering pHint. At negative voltages, E166D presents a persistent inward current that is blocked by p-chlorophenoxy-acetic acid (CPA) and increased at low pHext. The pHext dependence of the persistent current is analogous to a similar steady inward current in WT CLC-0. Surprisingly, however, the underlying unitary conductance of the persistent current in E166D is about an order of magnitude smaller than that of the transient deactivating inward Cl− current. Collectively, our data support the possibility that the mutated CLC-0 channel E166D can assume two distinct open states. Voltage-independent protonation of D166 from the outside favors a low conductance state, whereas protonation from the inside favors the high conductance state.
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Affiliation(s)
- Sonia Traverso
- Istituto di Biofisica, Consiglio Nazionale della Ricerche, Genova, Italy
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49
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Yusef YR, Zúñiga L, Catalán M, Niemeyer MI, Cid LP, Sepúlveda FV. Removal of gating in voltage-dependent ClC-2 chloride channel by point mutations affecting the pore and C-terminus CBS-2 domain. J Physiol 2006; 572:173-81. [PMID: 16469788 PMCID: PMC1779660 DOI: 10.1113/jphysiol.2005.102392] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Functional and structural studies demonstrate that Cl(-) channels of the ClC family have a dimeric double-barrelled structure, with each monomer contributing an identical pore. Studies with ClC-0, the prototype ClC channel, show the presence of independent mechanisms gating the individual pores or both pores simultaneously. A single-point mutation in the CBS-2 domain of ClC-0 has been shown to abolish slow gating. We have taken advantage of the high conservation of CBS domains in ClC channels to test for the presence of a slow gate in ClC-2 by reproducing this mutation (H811A). ClC-2-H811A showed faster opening kinetics and opened at more positive potentials than ClC-2. There was no difference in [Cl(-)](i) dependence. Additional neutralization of a putative pore gate glutamate side chain (E207V) abolished all gating. Resolving slow and fast gating relaxations, however, revealed that the H811A mutation affected both fast and slow gating processes in ClC-2. This suggests that slow and fast gating in ClC-2 are coupled, perhaps with slow gating contributing to the operation of the pore E207 as a protopore gate.
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Affiliation(s)
- Yamil R Yusef
- Centro de Estudios Cientificos (CECS), Av. Arturo Prat 514, Valdivia, Chile
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50
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Abstract
It was recently shown that the putative bacterial Cl- channel, ClC-ec1, is in reality a Cl--H+ antiporter. Our group has now shown that this is also the case for two human CLCs, ClC-4 and ClC-5. We found that the flux of Cl- in one direction is stoichiometrically coupled to the movement of protons in the opposite direction, unveiling a behaviour that is typical of a transporter rather than a channel. This discovery will surely stimulate further research to elucidate the molecular elements responsible for the behaviour as a transporter. On the physiological level, the antiport activity of ClC-4/ClC-5 must lead to a review of the role of CLC proteins in intracellular compartments. Small organic molecules have been extremely useful tools for studying ion channels and many commercial drugs target specific ion channel proteins. Several blockers have been found to inhibit the plasma membrane-localized CLC channels ClC-0, ClC-1 and ClC-Ka. These compounds include 9-anthracene-carboxylic acid (9-AC), p-chlorophenoxy-propionic acid (CPP) and its derivatives, and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS). Two different binding sites have been identified, one extracellular and one intracellular. However, high-affinity ligands for most CLC proteins are still missing. Apart from being useful biophysical tools, such drugs may provide a way to modulate protein function in vivo. With these tasks to be accomplished, it is definitely an exciting time in the chloride transport field.
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Affiliation(s)
- M Pusch
- Istituto di biofisica, CNR, Via De Marini 6, I-16149 Genova, Italy.
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