Purification, reconstitution, and subunit composition of a voltage-gated chloride channel from Torpedo electroplax

Biophysical and Pharmacological Insights to CLC Chloride Channels

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.

An Update On Proficiency of Voltage-gated Ion Channel Blockers in the Treatment of Inflammation-associated Diseases

Inflammation is the body's mechanism to trigger the immune system, thereby preventing bacteria and viruses from manifesting their toxic effect. Inflammation plays a vital role in regulating inflammatory mediator levels to initiate the wound healing process depending on the nature of the stimuli. This process occurs due to chemical release from white blood cells by elevating blood flow to the site of action, leading to redness and increased body temperature. Currently, there are numerous Non-steroidal anti-inflammatory drugs (NSAIDs) available, but these drugs are reported with adverse effects such as gastric bleeding, progressive kidney damage, and increased risk of heart attacks when prolonged use. For such instances, alternative options need to be adopted. The introduction of voltage-gated ion channel blockers can be a substantial alternative to mask the side effects of these currently available drugs. Chronic inflammatory disorders such as rheumatoid and osteoarthritis, cancer and migraine, etc., can cause dreadful pain, which is often debilitating for the patient. The underlying mechanism for both acute and chronic inflammation involves various complex receptors, different types of cells, receptors, and proteins. The working of voltage-gated sodium and calcium channels is closely linked to both inflammatory and neuropathic pain. Certain drugs such as carbamazepine and gabapentin, which are ion channel blockers, have greater pharmacotherapeutic activity for sodium and calcium channel blockers for the treatment of chronic inflammatory pain states. This review intends to provide brief information on the mechanism of action, latest clinical trials, and applications of these blockers in treating inflammatory conditions.

Toward a Multipathway Perspective: pH-Dependent Kinetic Selection of Competing Pathways and the Role of the Internal Glutamate in Cl-/H+ Antiporters

Canonical descriptions of multistep biomolecular transformations generally follow a single-pathway viewpoint, with a series of transitions through intermediates converting reactants to products or repeating a conformational cycle. However, mounting evidence suggests that more complexity and pathway heterogeneity are mechanistically relevant due to the statistical distribution of multiple interconnected rate processes. Making sense of such pathway complexity remains a significant challenge. To better understand the role and relevance of pathway heterogeneity, we herein probe the chemical reaction network of a Cl-/H+ antiporter, ClC-ec1, and analyze reaction pathways using multiscale kinetic modeling (MKM). This approach allows us to describe the nature of the competing pathways and how they change as a function of pH. We reveal that although pH-dependent Cl-/H+ transport rates are largely regulated by the charge state of amino acid E148, the charge state of E203 determines relative contributions from coexisting pathways and can shift the flux pH-dependence. The selection of pathways via E203 explains how ionizable mutations (D/H/K/R) would impact the ClC-ec1 bioactivity from a kinetic perspective and lends further support to the indispensability of an internal glutamate in ClC antiporters. Our results demonstrate how quantifying the kinetic selection of competing pathways under varying conditions leads to a deeper understanding of the Cl-/H+ exchange mechanism and can suggest new approaches for mechanistic control.

A quantitative flux assay for the study of reconstituted Cl- channels and transporters

The recent deluge of high-resolution structural information on membrane proteins has not been accompanied by a comparable increase in our ability to functionally interrogate these proteins. Current functional assays often are not quantitative or are performed in conditions that significantly differ from those used in structural experiments, thus limiting the mechanistic correspondence between structural and functional experiments. A flux assay to determine quantitatively the functional properties of purified and reconstituted Cl^- channels and transporters in membranes of defined lipid compositions is described. An ion-sensitive electrode is used to measure the rate of Cl^- efflux from proteoliposomes reconstituted with the desired protein and the fraction of vesicles containing at least one active protein. These measurements enable the quantitative determination of key molecular parameters such as the unitary transport rate, the fraction of proteins that are active, and the molecular mass of the transport protein complex. The approach is illustrated using CLC-ec1, a CLC-type H⁺/Cl^- exchanger as an example. The assay enables the quantitative study of a wide range of Cl^- transporting molecules and proteins whose activity is modulated by ligands, voltage, and membrane composition as well as the investigation of the effects of compounds that directly inhibit or activate the reconstituted transport systems. The present assay is readily adapted to the study of transport systems with diverse substrate specificities and molecular characteristics, and the necessary modifications needed are discussed.

Proton-dependent inhibition, inverted voltage activation, and slow gating of CLC-0 Chloride Channel

CLC-0, a prototype Cl^- channel in the CLC family, employs two gating mechanisms that control its ion-permeation pore: fast gating and slow gating. The negatively-charged sidechain of a pore glutamate residue, E166, is known to be the fast gate, and the swinging of this sidechain opens or closes the pore of CLC-0 on the millisecond time scale. The other gating mechanism, slow gating, operates with much slower kinetics in the range of seconds to tens or even hundreds of seconds, and it is thought to involve still-unknown conformational rearrangements. Here, we find that low intracellular pH (pH_i) facilitates the closure of the CLC-0’s slow gate, thus generating current inhibition. The rate of low pH_i-induced current inhibition increases with intracellular H⁺ concentration ([H⁺]_i)—the time constants of current inhibition by low pH_i = 4.5, 5.5 and 6 are roughly 0.1, 1 and 10 sec, respectively, at room temperature. In comparison, the time constant of the slow gate closure at pH_i = 7.4 at room temperature is hundreds of seconds. The inhibition by low pH_i is significantly less prominent in mutants favoring the slow-gate open state (such as C212S and Y512A), further supporting the fact that intracellular H⁺ enhances the slow-gate closure in CLC-0. A fast inhibition by low pH_i causes an apparent inverted voltage-dependent activation in the wild-type CLC-0, a behavior similar to those in some channel mutants such as V490W in which only membrane hyperpolarization can open the channel. Interestingly, when V490W mutation is constructed in the background of C212S or Y512A mutation, the inverted voltage-dependent activation disappears. We propose that the slow kinetics of CLC-0’s slow-gate closure may be due to low [H⁺]_i rather than due to the proposed large conformational change of the channel protein. Our results also suggest that the inverted voltage-dependent opening observed in some mutant channels may result from fast closure of the slow gate by the mutations.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

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.

Identification and Characterization of a Bacterial Homolog of Chloride Intracellular Channel (CLIC) Protein

Chloride intracellular channels (CLIC) are non-classical ion channels lacking a signal sequence for membrane targeting. In eukaryotes, they are implicated in cell volume regulation, acidification, and cell cycle. CLICs resemble the omega class of Glutathione S-transferases (GST), yet differ from them in their ability to form ion channels. They are ubiquitously found in eukaryotes but no prokaryotic homolog has been characterized. We found that indanyloxyacetic acid-94 (IAA-94), a blocker of CLICs, delays the growth of Escherichia coli. In silico analysis showed that the E. coli stringent starvation protein A (SspA) shares sequence and structural homology with CLICs. Similar to CLICs, SspA lacks a signal sequence but contains an omega GST fold. Electrophysiological analysis revealed that SspA auto-inserts into lipid bilayers and forms IAA-94-sensitive ion channels. Substituting the ubiquitously conserved residue leucine 29 to alanine in the pore-forming region increased its single-channel conductance. SspA is essential for cell survival during acid-induced stress, and we found that acidic pH increases the open probability of SspA. Further, IAA-94 delayed the growth of wild-type but not sspA null mutant E. coli. Our results for the first time show that CLIC-like proteins exist in bacteria in the form of SspA, forming functional ion channels.

Modulation of the slow/common gating of CLC channels by intracellular cadmium

Cd²⁺ binding to the CLC channel dimer interface inhibits slow gating by altering subunit interactions.

Members of the CLC family of Cl⁻ channels and transporters are homodimeric integral membrane proteins. Two gating mechanisms control the opening and closing of Cl⁻ channels in this family: fast gating, which regulates opening and closing of the individual pores in each subunit, and slow (or common) gating, which simultaneously controls gating of both subunits. Here, we found that intracellularly applied Cd²⁺ reduces the current of CLC-0 because of its inhibition on the slow gating. We identified CLC-0 residues C229 and H231, located at the intracellular end of the transmembrane domain near the dimer interface, as the Cd²⁺-coordinating residues. The inhibition of the current of CLC-0 by Cd²⁺ was greatly enhanced by mutation of I225W and V490W at the dimer interface. Biochemical experiments revealed that formation of a disulfide bond within this Cd²⁺-binding site is also affected by mutation of I225W and V490W, indicating that these two mutations alter the structure of the Cd²⁺-binding site. Kinetic studies showed that Cd²⁺ inhibition appears to be state dependent, suggesting that structural rearrangements may occur in the CLC dimer interface during Cd²⁺ modulation. Mutations of I290 and I556 of CLC-1, which correspond to I225 and V490 of CLC-0, respectively, have been shown previously to cause malfunction of CLC-1 Cl⁻ channel by altering the common gating. Our experimental results suggest that mutations of the corresponding residues in CLC-0 change the subunit interaction and alter the slow gating of CLC-0. The effect of these mutations on modulations of slow gating of CLC channels by intracellular Cd²⁺ likely depends on their alteration of subunit interactions.

REVIEW ■ : Chloride Channel Myotonias

Myotonia (muscle stiffness) is a symptom of several inherited diseases in humans and also in animals. It is due to muscle membrane hyperexcitability, which, in turn, can be caused by mutations in plasma membrane ion channels. The skeletal muscle chloride channel CLC-1 provides the major part of muscle membrane conductance and is important for keeping this membrane close to its resting voltage. Mutations in CLC-1 can cause both recessive (Becker) and dominant (Thomsen) forms of myotonia. Some of these mutations have been introduced into the functional cDNA and analyzed in the Xenopus oocyte expression system. From these studies, it was concluded that CLC-1 functions as a homooligomer with probably four subunits. Dominant mutant subunits are assumed to associate with wild-type ones, leading to their inactivation. The principle disease-causing mechanism of dominant mutations is a drastic alteration in the voltage dependence of CLC-1 gating. Some mutations in CLC-1 can be inherited either recessively or dominantly, probably depending on the genetic background. These studies point to the important role of CLC-1 in muscle physiology and provide interesting insights into the structure and function of this gene family of voltage-gated chloride channels. NEUROSCIENTIST 2:225-232, 1996

Functional Monomerization of a ClC-Type Fluoride Transporter

Anion channels and antiporters of the ClC superfamily have been found to be exclusively dimeric in nature, even though each individual monomer contains the complete transport pathway. Here, we describe the destabilization through mutagenesis of the dimer interface of a bacterial F(-)/H(+) antiporter, ClC(F)-eca. Several mutations that produce monomer/dimer equilibrium of the normally dimeric transporter were found, simply by shortening a hydrophobic side chain in some cases. One mutation, L376W, leads to a wholly monomeric variant that shows full activity. Furthermore, we discovered a naturally destabilized homologue, ClC(F)-rla, which undergoes partial monomerization in detergent without additional mutations. These results, in combination with the previous functional monomerization of the distant relative ClC-ec1, demonstrate that the monomer alone is the functional unit for several clades of the ClC superfamily.

Discovery of CLC transport proteins: cloning, structure, function and pathophysiology

After providing a personal description of the convoluted path leading 25 years ago to the molecular identification of the Torpedo Cl(-) channel ClC-0 and the discovery of the CLC gene family, I succinctly describe the general structural and functional features of these ion transporters before giving a short overview of mammalian CLCs. These can be categorized into plasma membrane Cl(-) channels and vesicular Cl(-) /H(+) -exchangers. They are involved in the regulation of membrane excitability, transepithelial transport, extracellular ion homeostasis, endocytosis and lysosomal function. Diseases caused by CLC dysfunction include myotonia, neurodegeneration, deafness, blindness, leukodystrophy, male infertility, renal salt loss, kidney stones and osteopetrosis, revealing a surprisingly broad spectrum of biological roles for chloride transport that was unsuspected when I set out to clone the first voltage-gated chloride channel.

In the beginning: a personal reminiscence on the origin and legacy of ClC-0, the 'Torpedo Cl(-) channel'

This unapologetically subjective essay recalls the Torpedo Cl(-) channel in the years when it had neither a molecular identity nor proper name (ClC-0), and membership in a large superfamily. I discuss the circumstances surrounding its discovery and subsequent research through the 1980s that revealed its unusual molecular architecture and other strange mechanistic characteristics.

CLCN1 mutations in Czech patients with myotonia congenita, in silico analysis of novel and known mutations in the human dimeric skeletal muscle chloride channel

Myotonia congenita (MC) is a genetic disease caused by mutations in the skeletal muscle chloride channel gene (CLCN1) encoding the skeletal muscle chloride channel (ClC-1). Mutations of CLCN1 result in either autosomal dominant MC (Thomsen disease) or autosomal recessive MC (Becker disease). The ClC-1 protein is a homodimer with a separate ion pore within each monomer. Mutations causing recessive myotonia most likely affect properties of only the mutant monomer in the heterodimer, leaving the wild type monomer unaffected, while mutations causing dominant myotonia affect properties of both subunits in the heterodimer. Our study addresses two points: 1) molecular genetic diagnostics of MC by analysis of the CLCN1 gene and 2) structural analysis of mutations in the homology model of the human dimeric ClC-1 protein. In the first part, 34 different types of CLCN1 mutations were identified in 51 MC probands (14 mutations were new). In the second part, on the basis of the homology model we identified the amino acids which forming the dimer interface and those which form the Cl(-) ion pathway. In the literature, we searched for mutations of these amino acids for which functional analyses were performed to assess the correlation between localisation of a mutation and occurrence of a dominant-negative effect (corresponding to dominant MC). This revealed that both types of mutations, with and without a dominant-negative effect, are localised at the dimer interface while solely mutations without a dominant-negative effect occur inside the chloride channel. This work is complemented by structural analysis of the homology model which provides elucidation of the effects of mutations, including a description of impacts of newly detected missense mutations.

A family of fluoride-specific ion channels with dual-topology architecture

Fluoride ion, ubiquitous in soil, water, and marine environments, is a chronic threat to microorganisms. Many prokaryotes, archea, unicellular eukaryotes, and plants use a recently discovered family of F⁻ exporter proteins to lower cytoplasmic F⁻ levels to counteract the anion’s toxicity. We show here that these ‘Fluc’ proteins, purified and reconstituted in liposomes and planar phospholipid bilayers, form constitutively open anion channels with extreme selectivity for F⁻ over Cl⁻. The active channel is a dimer of identical or homologous subunits arranged in antiparallel transmembrane orientation. This dual-topology assembly has not previously been seen in ion channels but is known in multidrug transporters of the SMR family, and is suggestive of an evolutionary antecedent of the inverted repeats found within the subunits of many membrane transport proteins.

DOI:http://dx.doi.org/10.7554/eLife.01084.001

Fluorine is the thirteenth-most abundant element in the Earth’s crust, and fluoride ions are found in both soil and water, where they accumulate through the weathering of rocks or from industrial pollution. However, high levels of fluoride ions can inhibit two processes essential to life: the production of energy by glycolysis and the synthesis of DNA and RNA bases. In polluted areas, organisms such as bacteria, algae and plants must remove fluoride ions from their cells in order to survive.

Since ions cannot freely cross lipid membranes, organisms use proteins called channels or carriers to move ions into and out of their cells. Channel proteins form a pore, or channel, in the cell membrane, through which ions can quickly move from areas of high concentration to areas of low concentration. In contrast, carrier proteins can transport ions in both directions—that is, to and from areas of high concentration—but they are slower than channel proteins.

A family of proteins that export fluoride from microbe and plant cells, thus allowing them to grow in the presence of this toxic ion, was discovered recently, but it was not clear if these proteins function as channels or as carrier proteins. Now, Stockbridge et al. find that these proteins, called Fluc proteins, are fluoride channels with an unusual architecture.

Fluc proteins are found in many species of bacteria, and Stockbridge et al. show that a number of these, when purified and inserted into a lipid membrane, are channel proteins. Additionally, they do not transport related ions such as chloride, which means that they are unusually selective for ion channels. Two Fluc polypeptides associate to form a channel in the cell membrane, and Stockbridge et al. show that these two subunits are arranged in an antiparallel formation. Although this architecture is unprecedented among ion channels, it has been observed in carrier proteins in a range of organisms, and may indicate that Fluc proteins offer an evolutionary model for many carrier proteins.

DOI:http://dx.doi.org/10.7554/eLife.01084.002

Intracellular proton access in a Cl(-)/H(+) antiporter

Mutagenesis, functional analysis, and crystal structures identify a watery tunnel through which protons enter the interior of a Cl⁻/H⁺ antiport protein involved in acid resistance of enteric bacteria.

Chloride-transporting membrane proteins of the CLC family appear in two distinct mechanistic flavors: H⁺-gated Cl⁻ channels and Cl⁻/H⁺ antiporters. Transmembrane H⁺ movement is an essential feature of both types of CLC. X-ray crystal structures of CLC antiporters show the Cl⁻ ion pathway through these proteins, but the H⁺ pathway is known only inferentially by two conserved glutamate residues that act as way-stations for H⁺ in its path through the protein. The extracellular-facing H⁺ transfer glutamate becomes directly exposed to aqueous solution during the transport cycle, but the intracellular glutamate E203, Glu_in, is buried within the protein. Two regions, denoted “polar” and “interfacial,” at the intracellular surface of the bacterial antiporter CLC-ec1 are examined here as possible pathways by which intracellular aqueous protons gain access to Glu_in. Mutations at multiple residues of the polar region have little effect on antiport rates. In contrast, mutation of E202, a conserved glutamate at the protein–water boundary of the interfacial region, leads to severe slowing of the Cl⁻/H⁺ antiport rate. An X-ray crystal structure of E202Y, the most strongly inhibited of these substitutions, shows an aqueous portal leading to Glu_in physically blocked by cross-subunit interactions; moreover, this mutation has only minimal effect on a monomeric CLC variant, which necessarily lacks such interactions. The several lines of experiments presented argue that E202 acts as a water-organizer that creates a proton conduit connecting intracellular solvent with Glu_in.

Chloride-proton antiport proteins of the “CLC” superfamily are transmembrane proteins that form homodimers and are used for myriad physiological purposes, all requiring the coordinated movements of Cl⁻ anions and H⁺ cations in opposite directions across biological membranes. While the pathway for Cl⁻ ions through CLC antiporters is known, we currently have only indirect glimpses of how protons navigate their way through these membrane-embedded proteins. By combining mechanistic and structural approaches, we identify a proton-access pathway in a bacterial Cl⁻/H⁺ antiporter that allows intracellular protons to enter the protein interior and engage in the coupled antiport mechanism. We conclude that E202, a highly conserved glutamate residue, serves to organize water molecules and guide protons to the adjacent glutamate E203 (known as “Glu_in”), a critical residue for the antiport mechanism.

Piezo1: properties of a cation selective mechanical channel

Piezo ion channels have been found to be essential for mechanical responses in cells. These channels were first shown to exist in Neuro2A cells, and the gene was identified by siRNAs that diminished the mechanical response. Piezo channels are approximately 2500 amino acids long, have between 24-32 transmembrane regions, and appear to assemble into tetramers and require no other proteins for activity. They have a reversal potential around 0 mV and show voltage dependent inactivation. The channel is constitutively active in liposomes, indicating that no cytoskeletal elements are required. Heterologous expression of the Piezo protein can create mechanical sensitivity in otherwise insensitive cells.   Piezo1 currents in outside-out patches were blocked by the extracellular MSC inhibitor peptide GsMTx4. Both enantiomeric forms of GsMTx4 inhibited channel activity in a manner similar to endogenous mechanical channels. Piezo1 can adopt a tonic (non-inactivating) form with repeated stimulation. The transition to the non-inactivating form generally occurs in large groups of channels, indicating that the channels exist in domains, and once the domain is compromised, the members simultaneously adopt new properties. Piezo proteins are associated with physiological responses in cells, such as the reaction to noxious stimulus of Drosophila larvae. Recent work measuring cell crowding, shows that Piezo1 is essential for the removal of extra cells without apoptosis. Piezo1 mutations have also been linked to the pathological response of red blood cells in a genetic disease called Xerocytosis. These finding suggest that Piezo1 is a key player in cells' responses to mechanical stimuli.

Identification and characterization of a bacterial hydrosulphide ion channel

The hydrosulphide ion (HS(-)) and its undissociated form, hydrogen sulphide (H(2)S), which are believed to have been critical to the origin of life on Earth, remain important in physiology and cellular signalling. As a major metabolite in anaerobic bacterial growth, hydrogen sulphide is a product of both assimilatory and dissimilatory sulphate reduction. These pathways can reduce various oxidized sulphur compounds including sulphate, sulphite and thiosulphate. The dissimilatory sulphate reduction pathway uses this molecule as the terminal electron acceptor for anaerobic respiration, in which process it produces excess amounts of H(2)S (ref. 4). The reduction of sulphite is a key intermediate step in all sulphate reduction pathways. In Clostridium and Salmonella, an inducible sulphite reductase is directly linked to the regeneration of NAD(+), which has been suggested to have a role in energy production and growth, as well as in the detoxification of sulphite. Above a certain concentration threshold, both H(2)S and HS(-) inhibit cell growth by binding the metal centres of enzymes and cytochrome oxidase, necessitating a release mechanism for the export of this toxic metabolite from the cell. Here we report the identification of a hydrosulphide ion channel in the pathogen Clostridium difficile through a combination of genetic, biochemical and functional approaches. The HS(-) channel is a member of the formate/nitrite transport family, in which about 50 hydrosulphide ion channels form a third subfamily alongside those for formate (FocA) and for nitrite (NirC). The hydrosulphide ion channel is permeable to formate and nitrite as well as to HS(-) ions. Such polyspecificity can be explained by the conserved ion selectivity filter observed in the channel's crystal structure. The channel has a low open probability and is tightly regulated, to avoid decoupling of the membrane proton gradient.

TMEM16A(a)/anoctamin-1 shares a homodimeric architecture with CLC chloride channels

TMEM16A/anoctamin-1 has been identified as a protein with the classic properties of a Ca(2+)-activated chloride channel. Here, we used blue native polyacrylamide gel electrophoresis (BN-PAGE) and chemical cross-linking to assess the quaternary structure of the mouse TMEM16A(a) and TMEM16A(ac) splice variants as well as a genetically concatenated TMEM16A(a) homodimer. The constructs carried hexahistidyl (His) tags to allow for their purification using a nondenaturing metal affinity resin. Neither His-tagging nor head-to-tail concatenation of two copies of TMEM16A(a) noticeably affected Ca(2+)-induced measured macroscopic Cl(-) currents compared with the wild-type TMEM16A(a) channel. The digitonin-solubilized, nondenatured TMEM16A(a) protein migrated in the BN-PAGE gel as a homodimer, as judged by comparison with the concatenated TMEM16A(a) homodimer and channel proteins of known oligomeric structures (e.g. the voltage-gated Cl(-) channel CLC-1). Cross-linking with glutaraldehyde corroborated the homodimeric structure of TMEM16A(a). The TMEM16A(a) homodimer detected in Xenopus laevis oocytes and HEK 293 cells dissociated into monomers following denaturation with SDS, and reducing versus nonreducing SDS-PAGE provided no evidence for the presence of intersubunit disulfide bonds. Together, our data demonstrate that the Ca(2+)-activated chloride channel member TMEM16A shares an obligate homodimeric architecture with the hCLC-1 channel.

Structure and mechanism of a pentameric formate channel

Formate transport across the inner membrane is a critical step in anaerobic bacterial respiration. Members of the formate/nitrite transport protein family function to shuttle substrate across the cytoplasmic membrane. In bacterial pathogens, the nitrite transport protein is involved in protecting bacteria from peroxynitrite released by host macrophages. We have determined the 2.13-A structure of the formate channel FocA from Vibrio cholerae, which reveals a pentamer in which each monomer possesses its own substrate translocation pore. Unexpectedly, the fold of the FocA monomer resembles that found in water and glycerol channels. The selectivity filter in FocA consists of a cytoplasmic slit and a central constriction ring. A 2.5-A high-formate structure shows two formate ions bound to the cytoplasmic slit via both hydrogen bonding and van der Waals interactions, providing a structural basis for the substrate selectivity of the channel.

A provisional transport mechanism for a chloride channel-type Cl-/H+ exchanger

Chloride channel (CLC)-type Cl-/H+ exchangers are widespread throughout the biological world, and one of these, CLC-ec1 from Escherichia coli, has been extensively studied. The structure of this protein is known, and several of its mechanistic hot spots have been identified, but a mechanism for Cl-/H+ exchange has not previously been offered. We herein confirm by direct measurements of Cl- and H+ fluxes a Cl--to-H+ exchange stoichiometry of 2, and summarize experimental facts pertinent to the exchange mechanism. While the mechanism must involve a conformational cycle of alternating exposure of substrate-binding sites to the two sides of the membrane, CLC transporters do not adhere to a familiar ping-pong scheme in which the two ions bind in a mutually exclusive fashion. Instead, Cl- and H+ occupy the ion-binding region simultaneously. A conformational cycle is proposed that accounts for the exchange stoichiometry, several key mutants and the tendency of the protein to become uncoupled and allow 'slippage' of Cl-.

CLC-0 and CFTR: Chloride Channels Evolved From Transporters

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.

Proton pathways and H+/Cl- stoichiometry in bacterial chloride transporters

H+/Cl- antiport behavior has recently been observed in bacterial chloride channel homologs and eukaryotic CLC-family proteins. The detailed molecular-level mechanism driving the stoichiometric exchange is unknown. In the bacterial structure, experiments and modeling studies have identified two acidic residues, E148 and E203, as key sites along the proton pathway. The E148 residue is a major component of the fast gate, and it occupies a site crucial for both H+ and Cl- transport. E203 is located on the intracellular side of the protein; it is vital for H+, but not Cl-, transport. This suggests two independent ion transit pathways for H+ and Cl- on the intracellular side of the transporter. Previously, we utilized a new pore-searching algorithm, TransPath, to predict Cl- and H+ ion pathways in the bacterial ClC channel homolog, focusing on proton access from the extracellular solution. Here we employ the TransPath method and molecular dynamics simulations to explore H+ pathways linking E148 and E203 in the presence of Cl- ions located at the experimentally observed binding sites in the pore. A conclusion is that Cl- ions are required at both the intracellular (S(int)) and central (S(cen)) binding sites in order to create an electrostatically favorable H+ pathway linking E148 and E203; this electrostatic coupling is likely related to the observed 1H+/2Cl- stoichiometry of the antiporter. In addition, we suggest that a tyrosine residue side chain (Y445), located near the Cl- ion binding site at S(cen), is involved in proton transport between E148 and E203.

CLC Cl /H+ transporters constrained by covalent cross-linking

CLC Cl(-)/H(+) exchangers are homodimers with Cl(-)-binding and H(+)-coupling residues contained within each subunit. It is not known whether the transport mechanism requires conformational rearrangement between subunits or whether each subunit operates as a separate exchanger. We designed various cysteine substitution mutants on a cysteine-less background of CLC-ec1, a bacterial CLC exchanger of known structure, with the aim of covalently linking the subunits. The constructs were cross-linked in air or with exogenous oxidant, and the cross-linked proteins were reconstituted to assess their function. In addition to conventional disulfides, a cysteine-lysine cross-bridge was formed with I(2) as an oxidant. The constructs, all of which contained one, two, or four cross-bridges, were functionally active and kinetically competent with respect to Cl(-) turnover rate, Cl(-)/H(+) exchange stoichiometry, and H(+) pumping driven by a Cl(-) gradient. These results imply that large quaternary rearrangements, such as those known to occur for "common gating" in CLC channels, are not necessary for the ion transport cycle and that it is therefore likely that the transport mechanism is carried out by the subunits working individually, as with "fast gating" of the CLC channels.

Uncoupling and turnover in a Cl-/H+ exchange transporter

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.

A bacterial arginine-agmatine exchange transporter involved in extreme acid resistance

The arginine-dependent extreme acid resistance response of Escherichia coli operates by decarboxylating arginine. AdiC, a membrane antiporter, catalyzes arginine influx coupled to efflux of the decarboxylation product agmatine, effectively exporting a proton in each turnover. Using the adiC coding sequence under control of a tetracycline promoter in an E. coli vector, we expressed and purified the transport-protein with a yield of approximately 10 mg/liter bacterial culture. Glutaraldehyde cross-linking experiments indicate that the protein is a homodimer in detergent micelles and lipid membranes. Purified AdiC reconstituted into liposomes exchanges arginine and agmatine in a strictly coupled, electrogenic fashion. Kinetic analysis yields K(m) approximately 80 microm for Arg, in the same range as its dissociation constant determined by isothermal titration calorimetry.

The Intracellular Region of ClC-3 Chloride Channel Is in a Partially Folded State and a Monomer

In contrast to bacterial ClC chloride channels, all eukaryotic ClC chloride channels have a conserved long intracellular region that makes up of the carboxyl terminus of the protein and is necessary for channel functions as a channel gate. Little is known, however, about the molecular structure of the intracellular region of ClC chloride channels so far. Here, for the first time, we have expressed and purified the intracellular region of the rat ClC-3 chloride channel (C-ClC-3) as a water-soluble protein under physiological conditions, and investigated its structural characteristics and assembly behavior by means of circular dichroism (CD) spectroscopy, differential scanning calorimetry (DSC), size exclusion chromatography and analytical ultracentrifugation. The far-UV CD spectra of C-ClC-3 in the native state and in the presence of urea clearly show that the protein has a significantly folded secondary structure consisting of alpha-helices and beta-sheets, while the near-UV CD spectra and DSC experiments indicate the protein is deficient in well-defined tertiary packing. Its Stokes radius is larger than its expected size as a folded globular protein, as determined on size exclusion chromatography. Furthermore, the DisEMBL program, a useful computational tool for the prediction of disordered/unstructured regions within a protein sequence, predicts that the protein is in a partially folded state. Based on these results, we conclude that C-ClC-3 is partially folded. On the other hand, both size exclusion chromatography and sedimentation equilibrium analysis show that C-ClC-3 exists as a monomer in solution, not a dimer like the whole ClC-3 molecule.

ClC chloride channels viewed through a transporter lens

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.

Role of intramolecular and intermolecular interactions in ClC channel and transporter function

The ClC family of chloride channels and transporters includes several members in which mutations have been associated with human disease. Clearly, an understanding of the structure-function relationships of these proteins will be critical in defining the molecular mechanisms underlying disease pathogenesis. The X-ray crystal structure of prokaryotic ClC proteins provides an exquisite template with which to model molecular aspects of eukaryotic ClC protein function. The dimeric structure of these proteins highlights the pivotal importance of intermolecular interactions in the modulation of channel/transporter activity, while mutagenesis studies implicate a crucial role for intrinsic interdomain interactions in regulated function. In this review, we will initially focus on the channel forming members of this family and discuss interactions within homodimeric channel complexes important for gating. Finally, with regard to both channel and transporter family members, we will discuss the multiple heteromeric interactions which occur with cytosolic proteins, and the putative functional impact of these interactions.

Zinc inhibits human ClC-1 muscle chloride channel by interacting with its common gating mechanism

Transition metals block the muscle Cl- channel ClC-1, which belongs to a large family of double-barreled Cl- channels and transporters. In the Torpedo Cl- channel ClC-0, Zn2+ block is closely related to the common gating mechanism that opens and closes both pores of the channel simultaneously, and the mutation C212S, which locks the common gate open, also eliminates the block. In ClC-1, however, previous results suggested that Zn2+ block is independent of gating, and that the cysteine residues involved in Zn2+ binding are in different positions to those that confer Zn2+ sensitivity on ClC-0. In this work, we show that Zn2+ block of ClC-1 is faster at hyperpolarized potentials where the channel is more likely to be in the closed state. Mutation C277S, equivalent to C212S in ClC-0, which locks the common gate in ClC-1 open, virtually eliminates Zn2+ block. A mutation, V321A, which reduces open probability of the common gate, facilitated Zn2+ block. These results demonstrate that Zn2+ block is state dependent, acting on the common gate. The extent of the block, however, is not a simple function of the open probability of the common gate. The Q10 of approximately 13 of the time course of Zn2+ block, which is significantly higher than the Q10 of common gating transitions in WT ClC-1, suggests that Zn2+ binds to a very high temperature-dependent low-probability closed substate of the common gate, which has not yet been characterized in this channel.

Physiological functions of CLC Cl- channels gleaned from human genetic disease and mouse models

The CLC gene family encodes nine different Cl() channels in mammals. These channels perform their functions in the plasma membrane or in intracellular organelles such as vesicles of the endosomal/lysosomal pathway or in synaptic vesicles. The elucidation of their cellular roles and their importance for the organism were greatly facilitated by mouse models and by human diseases caused by mutations in their respective genes. Human mutations in CLC channels are known to cause diseases as diverse as myotonia (muscle stiffness), Bartter syndrome (renal salt loss) with or without deafness, Dent's disease (proteinuria and kidney stones), osteopetrosis and neurodegeneration, and possibly epilepsy. Mouse models revealed blindness and infertility as further consequences of CLC gene disruptions. These phenotypes firmly established the roles CLC channels play in stabilizing the plasma membrane voltage in muscle and possibly in neurons, in the transport of salt and fluid across epithelia, in the acidification of endosomes and synaptic vesicles, and in the degradation of bone by osteoclasts.

Structure and function of clc channels

The CLC family comprises a group of integral membrane proteins whose major action is to translocate chloride (Cl-) ions across the cell membranes. Recently, the structures of CLC orthologues from two bacterial species, Salmonella typhimurium and Escherichia coli, were solved, providing the first framework for understanding the operating mechanisms of these molecules. However, most of the previous mechanistic understanding of CLC channels came from electrophysiological studies of a branch of the channel family, the muscle-type CLC channels in vertebrate species. These vertebrate CLC channels were predicted to contain two identical but independent pores, and this hypothesis was confirmed by the solved bacterial CLC structures. The opening and closing of the vertebrate CLC channels are also known to couple to the permeant ions via their binding sites in the ion-permeation pathway. The bacterial CLC structures can probably serve as a structural model to explain the gating-permeation coupling mechanism. However, the CLC-ec1 protein in E. coli was most recently shown to be a Cl- -H+ antiporter, but not an ion channel. The molecular basis to explain the difference between vertebrate and bacterial CLCs, especially the distinction between an ion channel and a transporter, remains a challenge in the structure/function studies for the CLC family.

Electrostatic control and chloride regulation of the fast gating of ClC-0 chloride channels

The opening and closing of chloride (Cl⁻) channels in the ClC family are thought to tightly couple to ion permeation through the channel pore. In the prototype channel of the family, the ClC-0 channel from the Torpedo electric organ, the opening-closing of the pore in the millisecond time range known as “fast gating” is regulated by both external and internal Cl⁻ ions. Although the external Cl⁻ effect on the fast-gate opening has been extensively studied at a quantitative level, the internal Cl⁻ regulation remains to be characterized. In this study, we examine the internal Cl⁻ effects and the electrostatic controls of the fast-gating mechanism. While having little effect on the opening rate, raising [Cl⁻]_i reduces the closing rate (or increases the open time) of the fast gate, with an apparent affinity of >1 M, a value very different from the one observed in the external Cl⁻ regulation on the opening rate. Mutating charged residues in the pore also changes the fast-gating properties—the effects are more prominent on the closing rate than on the opening rate, a phenomenon similar to the effect of [Cl⁻]_i on the fast gating. Thus, the alteration of fast-gate closing by charge mutations may come from a combination of two effects: a direct electrostatic interaction between the manipulated charge and the negatively charged glutamate gate and a repulsive force on the gate mediated by the permeant ion. Likewise, the regulations of internal Cl⁻ on the fast gating may also be due to the competition of Cl⁻ with the glutamate gate as well as the overall more negative potential brought to the pore by the binding of Cl⁻. In contrast, the opening rate of the fast gate is only minimally affected by manipulations of [Cl⁻]_i and charges in the inner pore region. The very different nature of external and internal Cl⁻ regulations on the fast gating thus may suggest that the opening and the closing of the fast gate are not microscopically reversible processes, but form a nonequilibrium cycle in the ClC-0 fast-gating mechanism.

Functional and structural conservation of CBS domains from CLC chloride channels

All eukaryotic CLC Cl(-) channel subunits possess a long cytoplasmic carboxy-terminus that contains two so-called CBS (cystathionine beta-synthase) domains. These domains are found in various unrelated proteins from all phylae. The crystal structure of the CBS domains of inosine monophosphate dehydrogenase (IMPDH) is known, but it is not known whether this structure is conserved in CLC channels. Working primarily with ClC-1, we used deletion scanning mutagenesis, coimmunoprecipitation and electrophysiology to demonstrate that its CBS domains interact. The replacement of CBS domains of ClC-1 with the corresponding CBS domains from other CLC channels and even human IMPDH yielded functional channels, indicating a high degree of structural conservation. Based on a homology model of the pair of CBS domains of CLC channels, we identified some residues that, when mutated, affected the common gate which acts on both pores of the dimeric channel. Thus, we propose that the structure of CBS domains from CLC channels is highly conserved and that they play a functional role in the common gate.

Probing the pore of ClC-0 by substituted cysteine accessibility method using methane thiosulfonate reagents

ClC channels are a family of protein molecules containing two ion-permeation pores. Although these transmembrane proteins are important for a variety of physiological functions, their molecular operations are only superficially understood. High-resolution X-ray crystallography techniques have recently revealed the structures of two bacterial ClC channels, but whether vertebrate ClC channel pores are similar to those of bacterial homologues is not clear. To study the pore architecture of the Torpedo ClC-0 channel, we employed the substituted-cysteine-accessibility method (SCAM) and used charged methane thiosulfonate (MTS) compounds to modify the introduced cysteine. Several conclusions were derived from this approach. First, the MTS modification pattern from Y512C to E526C in ClC-0, which corresponds to residues forming helix R in bacterial ClC channels, is indeed consistent with the suggested helical structure. Second, the ClC-0 pore is more accessible to the negatively charged than to the positively charged MTS compound, a pore property that is regulated by the intrinsic electrostatic potential in the pore. Finally, attempts to modify the introduced cysteine at positions intracellular to the selectivity filter did not result in larger MTS modification rates for the open-state channel, suggesting that the fast gate of ClC-0 cannot be located at a position intracellular to the Cl- selectivity filter. Thus, the proposal that the glutamate side chain is the fast gate of the channel is applicable to ClC-0, revealing a structural and functional conservation of ClC channels between bacterial and vertebrate species.

Side-chain charge effects and conductance determinants in the pore of ClC-0 chloride channels

The charge on the side chain of the internal pore residue lysine 519 (K519) of the Torpedo ClC-0 chloride (Cl-) channel affects channel conductance. Experiments that replace wild-type (WT) lysine with neutral or negatively charged residues or that modify the K519C mutant with various methane thiosulfonate (MTS) reagents show that the conductance of the channel decreases when the charge at position 519 is made more negative. This charge effect on the channel conductance diminishes in the presence of a high intracellular Cl- concentration ([Cl-]i). However, the application of high concentrations of nonpermeant ions, such as glutamate or sulfate (SO42-), does not change the conductance, suggesting that the electrostatic effects created by the charge at position 519 are unlikely due to a surface charge mechanism. Another pore residue, glutamate 127 (E127), plays an even more critical role in controlling channel conductance. This negatively charged residue, based on the structures of the homologous bacterial ClC channels, lies 4-5 A from K519. Altering the charge of this residue can influence the apparent Cl- affinity as well as the saturated pore conductance in the conductance-Cl- activity curve. Amino acid residues at the selectivity filter also control the pore conductance but mutating these residues mainly affects the maximal pore conductance. These results suggest at least two different conductance determinants in the pore of ClC-0, consistent with the most recent crystal structure of the bacterial ClC channel solved to 2.5 A, in which multiple Cl--binding sites were identified in the pore. Thus, we suggest that the occupancy of the internal Cl--binding site is directly controlled by the charged residues located at the inner pore mouth. On the other hand, the Cl--binding site at the selectivity filter controls the exit rate of Cl- and therefore determines the maximal channel conductance.

Involvement of helices at the dimer interface in ClC-1 common gating

ClC-1 is a dimeric, double-pored chloride channel that is present in skeletal muscle. Mutations of this channel can result in the condition myotonia, a muscle disorder involving increased muscle stiffness. It has been shown that the dominant form of myotonia often results from mutations that affect the so-called slow, or common, gating process of the ClC-1 channel. Mutations causing dominant myotonia are seen to cluster at the interface of the ClC-1 channel monomers. This study has investigated the role of the H, I, P, and Q helices, which lie on this interface, as well as the G helix, which is situated immediately behind the H and I helices, on ClC-1 gating. 11 mutant ClC-1 channels (T268M, C277S, C278S, S289A, T310M, S312A, V321S, T539A, S541A, M559T, and S572V) were produced using site-directed mutagenesis, and gating properties of these channels were investigated using electrophysiological techniques. Six of the seven mutations in G, H, and I, and two of the four mutations in P and Q, caused shifts of the ClC-1 open probability. In the majority of cases this was due to alterations in the common gating process, with only three of the mutants displaying any change in fast gating. Many of the mutant channels also showed alterations in the kinetics of the common gating process, particularly at positive potentials. The changes observed in common gating were caused by changes in the opening rate (e.g. T310M), the closing rate (e.g. C277S), or both rates. These results indicate that mutations in the helices forming the dimer interface are able to alter the ClC-1 common gating process by changing the energy of the open and/or closed channel states, and hence altering transition rates between these states.

A novel alteration of muscle chloride channel gating in myotonia levior

Mutations in the voltage-dependent skeletal muscle chloride channel, ClC-1, result in dominant or recessive myotonia congenita. The Q552R mutation causes a variant of dominant myotonia with a milder phenotype, myotonia levior. To characterise the functional properties of this mutation, homodimeric mutant and heterodimeric wild-type (WT) mutant channels were expressed in tsA201 cells and studied using the whole-cell recording technique. Q552R ClC-1 mutants formed functional channels with normal ion conduction but altered gating properties. Mutant channels were activated by membrane depolarisation, with a voltage dependence of activation that was shifted by more than +90 mV compared to WT channels. Q552R channels were also activated by hyperpolarisation, and this process was dependent upon the intracellular chloride concentration ([Cl(-)](i)). Together, these alterations resulted in a substantial reduction in the open probability at -85 mV at a physiological [Cl(-)](i). Heterodimeric WT-Q552R channels did not exhibit hyperpolarisation-activated gating transitions. As was the case for WT channels, activation occurred upon depolarisation, but the activation curve was shifted by 28 mV to more positive potentials. The functional properties of heterodimeric channels suggest a weakly dominant effect, a finding that correlates with the inheritance pattern and symptom profile of myotonia levior.

Kidney-specific chloride channel, OmClC-K, predominantly expressed in the diluting segment of freshwater-adapted tilapia kidney

The kidney plays an important role in osmoregulation in freshwater teleosts, which are exposed to the danger of osmotic loss of Na(+) and Cl(-). However, ion-transport mechanisms in the kidney are poorly understood, and ion transporters of the fish nephron have not been identified thus far. From Mozambique tilapia, Oreochromis mossambicus, we have cloned a chloride channel, which is a homologue of the mammalian kidney-specific chloride channel, ClC-K. The cDNA of the channel, named OmClC-K, encodes a protein whose amino acid sequence has high homology to Xenopus and mammalian ClC-K (Xenopus ClC-K, 41.8%; rat ClC-K2, 40.9%; rat ClC-K1, 40.1%). The mRNA of OmClC-K was expressed exclusively in the kidney, and the expression level of mRNA was increased more in freshwater-adapted fish than seawater-adapted fish. The immunohistochemical study using a specific antibody showed that OmClC-K-positive cells were specifically located in the distal nephron segments. Immunoelectron microscopy further showed that immunoreaction of OmClC-K was recognizable on the structure of basolateral membrane infoldings in the distal tubule cells. The localization of OmClC-K and its induction in hypoosmotic media suggest that OmClC-K is involved in Cl(-) reabsorption in the distal tubule of freshwater-adapted tilapia.

Allosteric transitions of Torpedo acetylcholine receptor in lipids, detergent and amphipols: molecular interactions vs. physical constraints

The binding of a fluorescent agonist to the acetycholine receptor from Torpedo electric organ has been studied by time-resolved spectroscopy in three different environments: in native membrane fragments, in the detergent CHAPS, and after complexation by amphipathic polymers ('amphipols'). Binding kinetics was similar in the membrane and in amphipols, demonstrating that the receptor can display unaltered allosteric transitions outside its natural lipid environment. In contrast, allosteric equilibria were strongly shifted towards the desensitized state in CHAPS. Therefore, the effect of CHAPS likely results from molecular interactions rather than from the loss of bulk physical properties of the membrane environment.

CLC chloride channels: correlating structure with function

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.

Molecular structure and physiological function of chloride channels

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.

Different fast-gate regulation by external Cl(-) and H(+) of the muscle-type ClC chloride channels

The fast gate of the muscle-type ClC channels (ClC-0 and ClC-1) opens in response to the change of membrane potential (V). This gating process is intimately associated with the binding of external Cl(-) to the channel pore in a way that the occupancy of Cl(-) on the binding site increases the channel's open probability (P(o)). External H(+) also enhances the fast-gate opening in these channels, prompting a hypothesis that protonation of the binding site may increase the Cl(-) binding affinity, and this is possibly the underlying mechanism for the H(+) modulation. However, Cl(-) and H(+), modulate the fast-gate P(o)-V curve in different ways. Varying the external Cl(-) concentrations ([Cl(-)](o)) shifts the P(o)-V curve in parallel along the voltage axis, whereas reducing external pH mainly increases the minimal P(o) of the curve. Furthermore, H(+) modulations at saturating and nonsaturating [Cl(-)](o) are similar. Thus, the H(+) effect on the fast gating appears not to be a consequence of an increase in the Cl(-) binding affinity. We previously found that a hyperpolarization-favored opening process is important to determine the fast-gate P(o) of ClC-0 at very negative voltages. This [Cl(-)](o)-independent mechanism attracted little attention, but it appears to be the opening process that is modulated by external H(+).

Identification and characterization of membrane-associated polypeptides in Torpedo nicotinic acetylcholine receptor-rich membranes by hydrophobic photolabeling

To identify membrane-associated polypeptides present in Torpedo nicotinic acetylcholine receptor (AChR)-rich membranes, we used hydrophobic photolabeling with [(3)H]diazofluorene ([(3)H]DAF) and 1-azidopyrene (1-AP) to tag the membrane proteins which were then identified by amino-terminal sequence analysis of labeled fragments isolated from proteolytic digests by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by reverse-phase high-performance liquid chromatography. In addition to AChR subunits, identified polypeptides include the 95 kDa alpha-subunit of the (Na(+)+K(+))-ATPase, the 89 kDa voltage-gated chloride channel (CLC-0), the 105 kDa SITS-binding protein, and 32 and 34 kDa polypeptides identified as Torpedo homologues of the mitochondrial membrane ATP/ADP carrier protein and the voltage-dependent anion channel (VDAC), respectively. Further, individual amino acids that reacted with [(3)H]DAF and therefore likely to be in contact with lipid were identified in the transmembrane segment M3 of the alpha-subunit of the (Na(+)+K(+))-ATPase and in a putative transmembrane beta-strand in VDAC. Collectively these results demonstrate that [(3)H]DAF/1-AP photolabeling provides an effective method for tagging the membrane-associated segments of polypeptides in a way that makes it easy to isolate the labeled polypeptide or polypeptide fragments by fluorescence and then to identify amino acids at the lipid-protein interface by (3)H release.

Chloride channels regulate HIT cell volume but cannot fully account for swelling-induced insulin secretion

Insulin-secreting pancreatic islet beta-cells possess anion-permeable Cl- channels (I(Cl,islet)) that are swelling-activated, but the role of these channels in the cells is unclear. The Cl- channel blockers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and niflumic acid were evaluated for their ability to inhibit I(Cl,islet) in clonal beta-cells (HIT cells). Both drugs blocked the channel, but the blockade due to niflumic acid was less voltage-dependent than the blockade due to DIDS. HIT cell volume initially increased in hypotonic solution and was followed by a regulatory volume decrease (RVD). The addition of niflumic acid and, to a lesser extent, DIDS to the hypotonic solution potentiated swelling and blocked the RVD. In isotonic solution, niflumic acid produced swelling, suggesting that islet Cl- channels are activated under basal conditions. The channel blockers glyburide, gadolinium, or tetraethylammonium-Cl did not alter hypotonic-induced swelling or volume regulation. The Na/K/2Cl transport blocker furosemide produced cell shrinkage in isotonic solution and blocked cell swelling normally induced by hypotonic solution. Perifused HIT cells secreted insulin when challenged with hypotonic solutions. However, this could not be completely attributed to I(Cl,islet)-mediated depolarization, because secretion persisted even when Cl- channels were fully blocked. To test whether blocker-resistant secretion occurred via a distal pathway, distal secretion was isolated using 50 mmol/l potassium and diazoxide. Under these conditions, glucose-dependent secretion was blunted, but hypotonically induced secretion persisted, even with Cl- channel blockers present. These results suggest that beta-cell swelling stimulates insulin secretion primarily via a distal I(Cl,islet)-independent mechanism, as has been proposed for K(ATP)-independent glucose- and sulfonylurea-stimulated insulin secretion. Reverse transcriptase-polymerase chain reaction of HIT cell mRNA identified a CLC-3 transcript in HIT cells. In other systems, CLC-3 is believed to mediate swelling-induced outwardly rectifying Cl- channels. This suggests that the proximal effects of swelling to regulate cell volume may be mediated by CLC-3 or a closely related Cl- channel.

A decade of CLC chloride channels: structure, mechanism, and many unsettled questions

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.

Pores formed by single subunits in mixed dimers of different CLC chloride channels

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.

Projection structure of a ClC-type chloride channel at 6.5 A resolution

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.