101
|
Affiliation(s)
- Ian R Booth
- Department of Molecular and Cell Biology, Institute of Medical Sciences, Foresterhill, Aberdeen AB25 2ZD, UK.
| | | | | |
Collapse
|
102
|
Traverso S, Elia L, Pusch M. Gating competence of constitutively open CLC-0 mutants revealed by the interaction with a small organic Inhibitor. J Gen Physiol 2003; 122:295-306. [PMID: 12913089 PMCID: PMC2234481 DOI: 10.1085/jgp.200308784] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
Abstract
Opening of CLC chloride channels is coupled to the translocation of the permeant anion. From the recent structure determination of bacterial CLC proteins in the closed and open configuration, a glutamate residue was hypothesized to form part of the Cl--sensitive gate. The negatively charged side-chain of the glutamate was suggested to occlude the permeation pathway in the closed state, while opening of a single protopore of the double-pore channel would reflect mainly a movement of this side-chain toward the extracellular pore vestibule, with little rearrangement of the rest of the channel. Here we show that mutating this critical residue (Glu166) in the prototype Torpedo CLC-0 to alanine, serine, or lysine leads to constitutively open channels, whereas a mutation to aspartate strongly slowed down opening. Furthermore, we investigated the interaction of the small organic channel blocker p-chlorophenoxy-acetic acid (CPA) with the mutants E166A and E166S. Both mutants were strongly inhibited by CPA at negative voltages with a >200-fold larger affinity than for wild-type CLC-0 (apparent KD at -140 mV approximately 4 micro M). A three-state linear model with an open state, a low-affinity and a high-affinity CPA-bound state can quantitatively describe steady-state and kinetic properties of the CPA block. The parameters of the model and additional mutagenesis suggest that the high-affinity CPA-bound state is similar to the closed configuration of the protopore gate of wild-type CLC-0. In the E166A mutant the glutamate side chain that occludes the permeation pathway is absent. Thus, if gating consists only in movement of this side-chain the mutant E166A should not be able to assume a closed conformation. It may thus be that fast gating in CLC-0 is more complex than anticipated from the bacterial structures.
Collapse
Affiliation(s)
- Sonia Traverso
- Istituto di Biofisica, Sezione di Genova, CNR, via de Marini, 6, I-16149 Genova, Italy
| | | | | |
Collapse
|
103
|
Abstract
Chloride is an abundant anion on earth but studies analyzing a possible function of chloride in prokaryotes are scarce. To address the question, we have tested 44 different Gram-negative and Gram-positive bacteria for a chloride dependence or chloride stimulation of growth. None required chloride for growth at their optimal growth (salt) conditions. However, in hyperosmotic media containing high concentrations of Na+, 11 bacteria (Aeromonas hydrophila, Bacillus megaterium, Bacillus subtilis, Corynebacterium glutamicum, Escherichia coli, Paracoccus denitrificans, Proteus mirabilis, Proteus vulgaris, Staphylococcus aureus, Thermus thermophilus, and Vibrio fischeri) had a strict chloride dependence for growth or were significantly stimulated by chloride. These data show that chloride is essential for growth at high salt (Na+) concentrations in various species of the domain Bacteria.
Collapse
Affiliation(s)
- Markus Roessler
- Section Microbiology, Department Biology I, Ludwig-Maximilians-Universität München, Maria-Ward-Str. 1a, 80686 Munich, Germany
| | | | | |
Collapse
|
104
|
Affiliation(s)
- Christopher Miller
- Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, MA 02454, USA
| |
Collapse
|
105
|
Ding Y, Waldor MK. Deletion of a Vibrio cholerae ClC channel results in acid sensitivity and enhanced intestinal colonization. Infect Immun 2003; 71:4197-200. [PMID: 12819118 PMCID: PMC161967 DOI: 10.1128/iai.71.7.4197-4200.2003] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
ClC chloride channels are found in all three kingdoms of life though little is known about their functions in prokaryotes. Here we investigated the role of a Vibrio cholerae ClC channel in acid resistance and intestinal colonization. The putative V. cholerae ClC channel was found to confer mild resistance to acid when pH was adjusted with HCl, but not with other acids. Surprisingly, a ClC channel deletion mutant exhibited enhanced intestinal colonization in infant mice.
Collapse
Affiliation(s)
- Yanpeng Ding
- Division of Geographic Medicine and Infectious Diseases, Tufts-New England Medical Center and Howard Hughes Medical Institute, Boston, Massachusetts 02111, USA
| | | |
Collapse
|
106
|
Chen TY. Coupling gating with ion permeation in ClC channels. SCIENCE'S STKE : SIGNAL TRANSDUCTION KNOWLEDGE ENVIRONMENT 2003; 2003:pe23. [PMID: 12824475 DOI: 10.1126/stke.2003.188.pe23] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
In ClC chloride (Cl(-)) channels, unlike cation-selective ion channels, ion permeation is intimately coupled to fast gating. Recent research comparing the crystallographic structure of a bacterial ClC channel with functional studies of a Torpedo ClC channel suggests that gating depends on the negatively charged carboxyl group on a glutamate residue, which blocks the channel pore. In this model, the permeating Cl(-) competes with the carboxyl group for an anion-binding site in the channel pore. This model of Cl(-) competition with a glutamate gate helps explain the effect of intracellular Cl(-) on channel gating; the mechanism underlying the effects of extracellular Cl(-), however, remains to be determined, as does the nature of the Cl(-) channel slow gate.
Collapse
Affiliation(s)
- Tsung-Yu Chen
- Center for Neuroscience and Department of Neurology, University of California, Davis, CA 95616, USA.
| |
Collapse
|
107
|
Affiliation(s)
- Ian R Booth
- Department of Molecular & Cell Biology, University of Aberdeen, Institute of Medical Sciences, Foresterhill, Aberdeen, AB25 2ZD, UK
| |
Collapse
|
108
|
Devesa F, Chams V, Dinadayala P, Stella A, Ragas A, Auboiroux H, Stegmann T, Poquet Y. Functional reconstitution of the HIV receptors CCR5 and CD4 in liposomes. EUROPEAN JOURNAL OF BIOCHEMISTRY 2002; 269:5163-74. [PMID: 12392548 DOI: 10.1046/j.1432-1033.2002.03213.x] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Reconstitution of membrane proteins allows their study in a membrane environment that can be manipulated at will. Because membrane proteins have diverse biophysical properties, reconstitution methods have so far been developed for individual proteins on an ad hoc basis. We developed a postinsertion reconstitution method for CCR5, a G protein coupled receptor, with seven transmembrane alpha helices and small ecto- and endodomains. A His6-tagged version of CCR5 was expressed in mammalian cells, purified using the detergent N-dodecyl-beta-d-maltoside (DDM) and reconstituted into preformed liposomal membranes saturated with DDM, removing the detergent with hydrophobic polystyrene beads. We then attempted to incorporate CD4, a protein with a single transmembrane helix and a large hydrophilic ectodomain into liposomal membranes, together with CCR5. Surprisingly, reconstitution of this protein was also achieved by the method. Both proteins were found to be present together in individual liposomes. The reconstituted CCR5 was recognized by several monoclonal antibodies, recognized its natural ligand, and CD4 bound a soluble form of gp120, a subunit of the HIV fusion protein that uses CD4 as a receptor. Moreover, cells expressing the entire fusion protein of HIV bound to the liposomes, indicating that the proteins were intact and that most of them were oriented right side out. Thus, functional coreconstitution of two widely different proteins can be achieved by this method, suggesting that it might be useful for other proteins.
Collapse
Affiliation(s)
- François Devesa
- Institut de Pharmacologie et de Biologie Structurale; CNRS UMR 5089, Toulouse, France
| | | | | | | | | | | | | | | |
Collapse
|
109
|
Iyer R, Iverson TM, Accardi A, Miller C. A biological role for prokaryotic ClC chloride channels. Nature 2002; 419:715-8. [PMID: 12384697 DOI: 10.1038/nature01000] [Citation(s) in RCA: 183] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2002] [Accepted: 06/27/2002] [Indexed: 01/17/2023]
Abstract
An unexpected finding emerging from large-scale genome analyses is that prokaryotes express ion channels belonging to molecular families long studied in neurons. Bacteria and archaea are now known to carry genes for potassium channels of the voltage-gated, inward rectifier and calcium-activated classes, ClC-type chloride channels, an ionotropic glutamate receptor and a sodium channel. For two potassium channels and a chloride channel, these homologues have provided a means to direct structure determination. And yet the purposes of these ion channels in bacteria are unknown. Strong conservation of functionally important sequences from bacteria to vertebrates, and of structure itself, suggests that prokaryotes use ion channels in roles more adaptive than providing high-quality protein to structural biologists. Here we show that Escherichia coli uses chloride channels of the widespread ClC family in the extreme acid resistance response. We propose that the channels function as an electrical shunt for an outwardly directed virtual proton pump that is linked to amino acid decarboxylation.
Collapse
Affiliation(s)
- Ramkumar Iyer
- Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, Massachusetts 02454, USA
| | | | | | | |
Collapse
|
110
|
Abstract
CLC chloride channels form a large gene family that is found in bacteria, archae and eukaryotes. Previous mutagenesis studies on CLC chloride channels, combined with electrophysiology, strongly supported the theory that these channels form a homodimeric structure with one pore per subunit (a'double-barrelled' channel), and also provided clues about gating and permeation. Recently, the crystal structures of two bacterial CLC proteins have been obtained by X-ray diffraction analysis. They confirm the double-barrelled architecture, and reveal a surprisingly complex and unprecedented channel structure. At its binding site in the pore, chloride interacts with the ends of four helices that come from both sides of the membrane. A glutamate residue that protrudes into the pore is proposed to participate in gating. The structure confirms several previous conclusions from mutagenesis studies and provides an excellent framework for their interpretation.
Collapse
Affiliation(s)
- Raúl Estévez
- Zentrum für Molekulare Neurobiologie, Universität Hamburg, Falkenried 94, Germany
| | | |
Collapse
|
111
|
Jentsch TJ, Stein V, Weinreich F, Zdebik AA. Molecular structure and physiological function of chloride channels. Physiol Rev 2002; 82:503-68. [PMID: 11917096 DOI: 10.1152/physrev.00029.2001] [Citation(s) in RCA: 934] [Impact Index Per Article: 42.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.
Collapse
Affiliation(s)
- Thomas J Jentsch
- Zentrum für Molekulare Neurobiologie Hamburg, Universität Hamburg, Hamburg, Germany.
| | | | | | | |
Collapse
|
112
|
Mancia F, Shapiro L. Chloride channel function: partial charges and dipolar rods. Structure 2002; 10:283-4. [PMID: 12005426 DOI: 10.1016/s0969-2126(02)00735-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
The regulated flow of ions across biological membranes is a process fundamental to all living organisms. The crystal structures of representative chloride channels recently published in Nature, together with the previously determined structures of a potassium channel, provide a solid basis for understanding the chemical principles that govern selective ion flow.
Collapse
Affiliation(s)
- Filippo Mancia
- Department of Biochemistry and Molecular Biophysics, Howard Hughes Medical Institute, Columbia University, New York, New York 10032, USA
| | | |
Collapse
|
113
|
Dutzler R, Campbell EB, Cadene M, Chait BT, MacKinnon R. X-ray structure of a ClC chloride channel at 3.0 A reveals the molecular basis of anion selectivity. Nature 2002; 415:287-94. [PMID: 11796999 DOI: 10.1038/415287a] [Citation(s) in RCA: 1181] [Impact Index Per Article: 53.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The ClC chloride channels catalyse the selective flow of Cl- ions across cell membranes, thereby regulating electrical excitation in skeletal muscle and the flow of salt and water across epithelial barriers. Genetic defects in ClC Cl- channels underlie several familial muscle and kidney diseases. Here we present the X-ray structures of two prokaryotic ClC Cl- channels from Salmonella enterica serovar typhimurium and Escherichia coli at 3.0 and 3.5 A, respectively. Both structures reveal two identical pores, each pore being formed by a separate subunit contained within a homodimeric membrane protein. Individual subunits are composed of two roughly repeated halves that span the membrane with opposite orientations. This antiparallel architecture defines a selectivity filter in which a Cl- ion is stabilized by electrostatic interactions with alpha-helix dipoles and by chemical coordination with nitrogen atoms and hydroxyl groups. These findings provide a structural basis for further understanding the function of ClC Cl- channels, and establish the physical and chemical basis of their anion selectivity.
Collapse
Affiliation(s)
- Raimund Dutzler
- Howard Hughes Medical Institute, Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, 1230 York Avenue, New York, New York 10021, USA
| | | | | | | | | |
Collapse
|
114
|
Abstract
Due to the relative ease of obtaining their crystal structures, bacterial ion channels provide a unique opportunity to analyse structure and function of their eukaryotic homologues. This review describes prokaryotic channels whose structures have been determined. These channels are KcsA, a bacterial homologue of eukaryotic potassium channels, MscL, a bacterial mechanosensitive ion channel and ClC0, a prokaryotic homologue of the eukaryotic ClC family of anion-selective channels. General features of their structure and function are described with a special emphasis on the advantages that these channels offer for understanding the properties of their eukaryotic homologues. We present amino-acid sequences of eukaryotic proteins related in their primary sequences to bacterial mechanosensitive channels. The usefulness of bacterial mechanosensitive channels for the studies on general principles of mechanosensation is discussed.
Collapse
Affiliation(s)
- P Koprowski
- Department of Cell Biology, Nencki Institute of Experimental Biology, Warsaw, Poland
| | | |
Collapse
|
115
|
Christensen M, Strange K. Developmental regulation of a novel outwardly rectifying mechanosensitive anion channel in Caenorhabditis elegans. J Biol Chem 2001; 276:45024-30. [PMID: 11568185 DOI: 10.1074/jbc.m107652200] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The nematode Caenorhabditis elegans offers unique experimental advantages for defining the molecular basis of anion channel function and regulation. However, the relative inaccessibility of somatic cells in adult animals greatly limits direct electrophysiological studies of channel activity. We developed methods to routinely isolate and patch clamp C. elegans embryo cells and oocytes and to culture and patch clamp neurons and muscle cells. Dissociated embryonic cells express a robust outwardly rectifying anion current that is activated by membrane stretch and depolarization. This current, termed I(Cl,mec), is inhibited by anion and mechanosensitive channel inhibitors. I(Cl,mec) has broad anion selectivity and the channel has a unitary conductance of 5-7 picosiemens. I(Cl,mec) is not detectable in whole-cell or isolated patch recordings from oocytes, cultured muscle cells, and cultured neurons but is expressed in single cell and later embryos. Channel density is high, and the current is observed in >80% of membrane patches. Macroscopic currents of 40-120 pA at +100 mV are typically observed in inside-out membrane patches formed using low resistance patch pipettes. Isolated membrane patches of early embryonic cells therefore contain 60-200 I(Cl,mec) channels. The apparent activation of I(Cl,mec) shortly after fertilization and its down-regulation in terminally differentiated cells suggests that the channel may play important roles in embryogenesis and/or cytokinesis.
Collapse
Affiliation(s)
- M Christensen
- Department of Anesthesiology, Vanderbilt University Medical Center, Nashville, Tennessee 37232, USA
| | | |
Collapse
|
116
|
Maduke M, Miller C, Mindell JA. A decade of CLC chloride channels: structure, mechanism, and many unsettled questions. ANNUAL REVIEW OF BIOPHYSICS AND BIOMOLECULAR STRUCTURE 2001; 29:411-38. [PMID: 10940254 DOI: 10.1146/annurev.biophys.29.1.411] [Citation(s) in RCA: 143] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
ClC-type chloride channels are ubiquitous throughout the biological world. Expressed in nearly every cell type, these proteins have a host of biological functions. With nine distinct homologues known in eukaryotes, the ClCs represent the only molecularly defined family of chloride channels. ClC channels exhibit features of molecular architecture and gating mechanisms unprecedented in other types of ion channels. They form two-pore homodimers, and their voltage-dependence arises not from charged residues in the protein, but rather via coupling of gating to the movement of chloride ions within the pore. Because the functional characteristics of only a few ClC channels have been studied in detail, we are still learning which properties are general to the whole family. New approaches, including structural analyses, will be crucial to an understanding of ClC architecture and function.
Collapse
Affiliation(s)
- M Maduke
- Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, Massachusetts 02454, USA
| | | | | |
Collapse
|
117
|
Bianchi L, Miller DM, George AL. Expression of a CIC chloride channel in Caenorhabditis elegans gamma-aminobutyric acid-ergic neurons. Neurosci Lett 2001; 299:177-80. [PMID: 11165764 DOI: 10.1016/s0304-3940(01)01525-7] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
In this study, we report the complete cDNA sequence, genomic organization and expression pattern of the Caenorhabditis elegans ClC chloride channel gene, clh-6. Two different types of reporter gene fusions suggest that clh-6 expression is restricted to two gamma-aminobutyric acid-ergic RME neurons. These results are in striking contrast with the wide tissue distribution of messenger RNA for the related mammalian isoforms, CIC-6 and CIC-7. The restricted expression pattern of clh-6 provides a unique opportunity to study the biological function of a neuronal CIC chloride channel.
Collapse
Affiliation(s)
- L Bianchi
- Department of Medicine, Division of Genetic Medicine, Vanderbilt University School of Medicine, 451 Preston Research Building, Nashville TN 37232-6304, USA
| | | | | |
Collapse
|
118
|
Weinreich F, Jentsch TJ. Pores formed by single subunits in mixed dimers of different CLC chloride channels. J Biol Chem 2001; 276:2347-53. [PMID: 11035003 DOI: 10.1074/jbc.m005733200] [Citation(s) in RCA: 111] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
CLC chloride channels comprise a gene family with nine mammalian members. Probably all CLC channels form homodimers, and some CLC proteins may also associate to heterodimers. ClC-0 and ClC-1, the only CLC channels investigated at the single-channel level, display two conductances of equal size which are thought to result from two separate pores, formed individually by the two monomers. We generated concatemeric channels containing one subunit of ClC-0 together with one subunit of ClC-1 or ClC-2. They should display two different conductances if one monomer were sufficient to form one pore. Indeed, we found a 8-picosiemens (pS) conductance (corresponding to ClC-0) that was associated with either a 1.8-pS (ClC-1) or a 2.8-pS (ClC-2) conductance. These conductances retained their typical gating, but the slow gating of ClC-0 that affects both pores simultaneously was lost. ClC-2 and ClC-0 current components were modified by point mutations in the corresponding subunit. The ClC-2 single pore of the mixed dimer was compared with the pores in the ClC-2 homodimer and found to be unaltered. We conclude that each monomer individually forms a gated pore. CLC dimers in general must be imagined as having two pores, as shown previously for ClC-0.
Collapse
Affiliation(s)
- F Weinreich
- Zentrum für Molekulare Neurobiologie Hamburg, ZMNH, Hamburg University, Martinistrasse 85, D-20246 Hamburg, Germany
| | | |
Collapse
|
119
|
Mindell JA, Maduke M, Miller C, Grigorieff N. Projection structure of a ClC-type chloride channel at 6.5 A resolution. Nature 2001; 409:219-23. [PMID: 11196649 DOI: 10.1038/35051631] [Citation(s) in RCA: 100] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Virtually all cells in all eukaryotic organisms express ion channels of the ClC type, the only known molecular family of chloride-ion-selective channels. The diversity of ClC channels highlights the multitude and range of functions served by gated chloride-ion conduction in biological membranes, such as controlling electrical excitability in skeletal muscle, maintaining systemic blood pressure, acidifying endosomal compartments, and regulating electrical responses of GABA (gamma-aminobutyric acid)-containing interneurons in the central nervous system. Previously, we expressed and purified a prokaryotic ClC channel homologue. Here we report the formation of two-dimensional crystals of this ClC channel protein reconstituted into phospholipid bilayer membranes. Cryo-electron microscopic analysis of these crystals yields a projection structure at 6.5 A resolution, which shows off-axis water-filled pores within the dimeric channel complex.
Collapse
Affiliation(s)
- J A Mindell
- Department of Biochemistry, Howard Hughes Medical Institute, Basic Medical Science Research Center, Brandeis University, Waltham, Massachusetts 02454, USA.
| | | | | | | |
Collapse
|
120
|
Abstract
SUMMARY Chloride-conducting ion channels of the ClC family are emerging as critical contributors to a host of biological processes. These polytopic membrane proteins form aqueous pathways through which anions are selectively allowed to pass down their concentration gradients. The ClCs are found in nearly all organisms, with members in every mammalian tissue, yet relatively little is known about their mechanism or regulation. It is clear, however, that they are fundamentally different in molecular construction and mechanism from the well-known potassium-, sodium-, and calcium-selective channels. The medical importance of ClC channels - four inherited diseases have been blamed on familial ClC dysfunction to date - highlights their diverse physiological functions and provides strong motivation for further study.
Collapse
Affiliation(s)
- Joe Mindell
- Howard Hughes Medical Institute and Department of Biochemistry, Brandeis University, Waltham, MA02454, USA. E-mail:
| | - Merritt Maduke
- Howard Hughes Medical Institute and Department of Biochemistry, Brandeis University, Waltham, MA02454, USA. E-mail:
| |
Collapse
|
121
|
Nehrke K, Begenisich T, Pilato J, Melvin JE. Into ion channel and transporter function. Caenorhabditis elegans ClC-type chloride channels: novel variants and functional expression. Am J Physiol Cell Physiol 2000; 279:C2052-66. [PMID: 11078724 DOI: 10.1152/ajpcell.2000.279.6.c2052] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Six ClC-type chloride channel genes have been identified in Caenorhabditis elegans, termed clh-1 through clh-6. cDNA sequences from these genes suggest that clh-2, clh-3, and clh-4 may code for multiple channel variants, bringing the total to at least nine channel types in this nematode. Promoter-driven green fluorescent protein (GFP) expression in transgenic animals indicates that the protein CLH-5 is expressed ubiquitously, CLH-6 is expressed mainly in nonneuronal cells, and the remaining isoforms vary from those restricted to a single cell to those expressed in over a dozen cells of the nematode. In an Sf9 cell expression system, recombinant CLH-2b, CLH-4b, and CLH-5 did not form functional plasma membrane channels. In contrast, both CLH-1 and CLH-3b produced strong, inward-rectifying chloride currents similar to those arising from mammalian ClC2, but which operate over different voltage ranges. Our demonstration of multiple CLH protein variants and comparison of expression patterns among the clh gene family provides a framework, in combination with the electrical properties of the recombinant channels, to further examine the physiology and cell-specific role each isoform plays in this simple model system.
Collapse
Affiliation(s)
- K Nehrke
- Center for Oral Biology, Aab Institute of Biomedical Sciences, University of Rochester Medical Center, Rochester, New York 14642, USA
| | | | | | | |
Collapse
|
122
|
Ramjeesingh M, Li C, Huan LJ, Garami E, Wang Y, Bear CE. Quaternary structure of the chloride channel ClC-2. Biochemistry 2000; 39:13838-47. [PMID: 11076524 DOI: 10.1021/bi001282i] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The chloride channel ClC-2 is thought to be essential for chloride homeostasis in neurons and critical for chloride secretion by the developing respiratory tract. In the present work, we investigated the quaternary structure of ClC-2 required to mediate chloride conduction. We found using chemical cross-linking and a novel PAGE system that tagged ClC-2 expressed in Sf9 cells exists as oligomers. Fusion of membranes from Sf9 cells expressing this protein confers double-barreled channel activity, with each pore exhibiting a unitary conductance of 32 pS. Polyhistidine-tagged ClC-2 from Sf9 cells can be purified as monomers, dimers, and tetramers. Purified, reconstituted ClC-2 monomers do not possess channel function whereas both purified ClC-2 dimers and tetramers do mediate chloride flux. In planar bilayers, reconstitution of dimeric ClC-2 leads to the appearance of a single, anion selective 32 pS pore, and tetrameric ClC-2 confers double-barreled channel activity similar to that observed in Sf9 membranes. These reconstitution studies suggest that a ClC-2 dimer is the minimum functional structure and that ClC-2 tetramers likely mediate double-barreled channel function.
Collapse
Affiliation(s)
- M Ramjeesingh
- Research Institute in the Hospital for Sick Children, Toronto, Canada
| | | | | | | | | | | |
Collapse
|
123
|
Evans GJ, Ferguson GP, Booth IR, Vuilleumier S. Growth inhibition of Escherichia coli by dichloromethane in cells expressing dichloromethane dehalogenase/glutathione S-transferase. MICROBIOLOGY (READING, ENGLAND) 2000; 146 ( Pt 11):2967-2975. [PMID: 11065375 DOI: 10.1099/00221287-146-11-2967] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Dichloromethane (DCM) dehalogenase converts DCM to formaldehyde via the formation of glutathione metabolites and generates 2 mol HCl per mol DCM metabolized. Growth of Escherichia coli expressing DCM dehalogenase was immediately and severely inhibited during conversion of 0.3 mM DCM. Intracellular pH (pH(i)) rapidly decreased and chloride ions were steadily released into the medium. Bacterial growth resumed after completion of DCM conversion and cell viability was unaffected. At 0.6 mM DCM there was no recovery from growth inhibition in liquid culture due to the build-up of inhibitory concentrations of formaldehyde. DCM turnover stimulated potassium efflux from cells, which was suppressed by glucose. The potassium efflux, therefore, did not contribute to growth inhibition. It was concluded that initial growth inhibition results from lowering of the cytoplasmic pH, but severity of growth inhibition was greater than expected for the change in pH(i). Possible contributors to growth inhibition are discussed.
Collapse
Affiliation(s)
- Gareth J Evans
- Department of Molecular and Cell Biology, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, UK1
| | - Gail P Ferguson
- Department of Molecular and Cell Biology, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, UK1
| | - Ian R Booth
- Department of Molecular and Cell Biology, Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, UK1
| | - Stéphane Vuilleumier
- Institut für Mikrobiologie, ETH Zürich, ETH-Zentrum/LFV, CH-8092 Zürich, Switzerland2
| |
Collapse
|
124
|
Abstract
Gating of the muscle chloride channel CLC-1 involves at least two processes evidenced by double-exponential current relaxations when stepping the voltage to negative values. However, there is little information about the gating of CLC-1 at positive voltages. Here, we analyzed macroscopic gating of CLC-1 over a large voltage range (from -160 to +200 mV). Activation was fast at positive voltages but could be easily followed using envelope protocols that employed a tail pulse to -140 mV after stepping the voltage to a certain test potential for increasing durations. Activation was biexponential, demonstrating the presence of two gating processes. Both time constants became exponentially faster at positive voltages. A similar voltage dependence was also seen for the fast gate time constant of CLC-0. The voltage dependence of the time constant of the fast process of CLC-1, tau(f), was steeper than that of the slow one, tau(s) (apparent activation valences were z(f) approximately -0. 79 and z(s) approximately -0.42) such that at +200 mV the two processes became kinetically distinct by almost two orders of magnitude (tau(f) approximately 16 micros, tau(s) approximately 1 ms). This voltage dependence is inconsistent with a previously published gating model for CLC-1 (Fahlke, C., A. Rosenbohm, N. Mitrovic, A.L. George, and R. Rüdel. 1996. Biophys. J. 71:695-706). The kinetic difference at 200 mV allowed us to separate the steady state open probabilities of the two processes assuming that they reflect two parallel (not necessarily independent) gates that have to be open simultaneously to allow ion conduction. Both open probabilities could be described by Boltzmann functions with gating valences around one and with nonzero "offsets" at negative voltages, indicating that the two "gates" never close completely. For comparison with single channel data and to correlate the two gating processes with the two gates of CLC-0, we characterized their voltage, pH(int), and [Cl](ext) dependence, and the dominant myotonia inducing mutation, I290M. Assuming a double-barreled structure of CLC-1, our results are consistent with the identification of the fast and slow gating processes with the single-pore and the common-pore gate, respectively.
Collapse
Affiliation(s)
- A Accardi
- Istituto di Cibernetica e Biofisica, Consiglio Nazionale delle Ricerche, Via de Marini 6, I-16149 Genova, Italy
| | | |
Collapse
|
125
|
Purdy MD, Wiener MC. Expression, purification, and initial structural characterization of YadQ, a bacterial homolog of mammalian ClC chloride channel proteins. FEBS Lett 2000; 466:26-8. [PMID: 10648805 DOI: 10.1016/s0014-5793(99)01740-8] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
YadQ of Escherichia coli is a homolog of the mammalian chloride channels of the ClC family. The yadQ gene was cloned as a fusion protein with a hexahistidine tag and tobacco etch virus protease site for the removal of the tag. The protein was expressed in the membrane of E. coli and extracted with decylmaltoside. Purification was achieved by metal affinity chromatography followed by cation exchange. Circular dichroism revealed a high alpha-helical content. Size exclusion chromatography suggests that YadQ forms dimers. The similarity in primary, secondary, and quaternary structure and the ability to recombinantly express YadQ in the cell membrane make the protein a good candidate for the structural study of ClC chloride channels.
Collapse
Affiliation(s)
- M D Purdy
- Biophysics Program, University of Virginia, 1300 Jefferson Park Ave, Charlottesville, VA 22906-0011, USA
| | | |
Collapse
|
126
|
Affiliation(s)
- C Miller
- Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, Massachusetts 02454, USA.
| |
Collapse
|