ClC chloride channels viewed through a transporter lens

Ion selectivity and gating mechanisms of FNT channels

The phospholipid bilayer has evolved to be a protective and selective barrier by which the cell maintains high concentrations of life sustaining organic and inorganic material. As gatekeepers responsible for an immense amount of bidirectional chemical traffic between the cytoplasm and extracellular milieu, ion channels have been studied in detail since their postulated existence nearly three-quarters of a century ago. Over the past fifteen years, we have begun to understand how selective permeability can be achieved for both cationic and anionic ions. Our mechanistic knowledge has expanded recently with studies of a large family of anion channels, the Formate Nitrite Transport (FNT) family. This family has proven amenable to structural studies at a resolution high enough to reveal intimate details of ion selectivity and gating. With five representative members having yielded a total of 15 crystal structures, this family represents one of the richest sources of structural information for anion channels.

A designed inhibitor of a CLC antiporter blocks function through a unique binding mode

The lack of small-molecule inhibitors for anion-selective transporters and channels has impeded our understanding of the complex mechanisms that underlie ion passage. The ubiquitous CLC "Chloride Channel" family represents a unique target for biophysical and biochemical studies because its distinctive protein fold supports both passive chloride channels and secondary-active chloride-proton transporters. Here, we describe the synthesis and characterization of a specific small-molecule inhibitor directed against a CLC antiporter (ClC-ec1). This compound, 4,4'-octanamidostilbene-2,2'-disulfonate (OADS), inhibits ClC-ec1 with low micromolar affinity and has no specific effect on a CLC channel (ClC-1). Inhibition of ClC-ec1 occurs by binding to two distinct intracellular sites. The location of these sites and the lipid dependence of inhibition suggest potential mechanisms of action. This compound will empower research to elucidate differences between antiporter and channel mechanisms and to develop treatments for CLC-mediated disorders.

Functional characterization of a ClC transporter by solid-supported membrane electrophysiology

EcClC, a prokaryotic member of the ClC family of chloride channels and transporters, works as coupled H⁺/Cl⁻ exchanger. With a known structure and the possibility of investigating its behavior with different biochemical and biophysical techniques, the protein has become an important model system for the family. Although many aspects of its function have been previously characterized, it was difficult to measure transport on the same sample under different environmental conditions. To overcome this experimental limitation, we have studied EcClC by solid-supported membrane electrophysiology. The large transport-related transient currents and a simple way of relating transport rates to the measured signal have allowed a thorough investigation of ion selectivity, inhibition, and the dependence of transport on changes in ion concentration and pH. Our results confirm that the protein transports larger anions with about similar rates, whereas the smaller fluoride is not a substrate. We also show that 4,4′-diisothiocyano-2,2’-stilbenedisulfonic acid (DIDS), a known inhibitor of other anion transport protein, irreversibly inhibits EcClC from the intracellular side. The chloride dependence shows an apparent saturation at millimolar concentrations that resembles a similar behavior in eukaryotic ClC channels. Our experiments have also allowed us to quantify the pH dependence of transport. EcClC shows a strong activation at low pH with an apparent pKa of 4.6. The pronounced pH dependence is lost by the mutation of a conserved glutamate facing the extracellular solution that was previously shown to be an acceptor for transported protons, whereas it is largely retained by the mutation of an equivalent residue at the intracellular side. Our results have provided a quantitative basis for the transport behavior of EcClC, and they will serve as a reference for future investigations of novel electrogenic transporters with still-uncharacterized properties.

Energization of vacuolar transport in plant cells and its significance under stress

The plant vacuole is of prime importance in buffering environmental perturbations and in coping with abiotic stress caused by, for example, drought, salinity, cold, or UV. The large volume, the efficient integration in anterograde and retrograde vesicular trafficking, and the dynamic equipment with tonoplast transporters enable the vacuole to fulfill indispensible functions in cell biology, for example, transient and permanent storage, detoxification, recycling, pH and redox homeostasis, cell expansion, biotic defence, and cell death. This review first focuses on endomembrane dynamics and then summarizes the functions, assembly, and regulation of secretory and vacuolar proton pumps: (i) the vacuolar H(+)-ATPase (V-ATPase) which represents a multimeric complex of approximately 800 kDa, (ii) the vacuolar H(+)-pyrophosphatase, and (iii) the plasma membrane H(+)-ATPase. These primary proton pumps regulate the cytosolic pH and provide the driving force for secondary active transport. Carriers and ion channels modulate the proton motif force and catalyze uptake and vacuolar compartmentation of solutes and deposition of xenobiotics or secondary compounds such as flavonoids. ABC-type transporters directly energized by MgATP complement the transport portfolio that realizes the multiple functions in stress tolerance of plants.

On the mechanism of gating charge movement of ClC-5, a human Cl(-)/H(+) antiporter

ClC-5 is a Cl(-)/H(+) antiporter that functions in endosomes and is important for endocytosis in the proximal tubule. The mechanism of transport coupling and voltage dependence in ClC-5 is unclear. Recently, a transport-deficient ClC-5 mutant (E268A) was shown to exhibit transient capacitive currents. Here, we studied the external and internal Cl(-) and pH dependence of the currents of E268A. Transient currents were almost completely independent of the intracellular pH. Even though the transient currents are modulated by extracellular pH, we could exclude that they are generated by proton-binding/unbinding reactions. In contrast, the charge movement showed a nontrivial dependence on external chloride, strongly supporting a model in which the movement of an intrinsic gating charge is followed by the voltage-dependent low-affinity binding of extracellular chloride ions. Mutation of the external Glu-211 (a residue implicated in the coupling of Cl(-) and proton transport) to aspartate abolished steady-state transport, but revealed transient currents that were shifted by ~150 mV to negative voltages compared to E268A. This identifies Glu(ext) as a major component of the gating charge underlying the transient currents of the electrogenic ClC-5 transporter. The molecular events underlying the transient currents of ClC-5 emerging from these results can be explained by an inward movement of the side chain of Glu(ext), followed by the binding of extracellular Cl(-) ions.

Significance of TMEM16A expression in colorectal carcinoma

AIM: To investigate the expression of transmembrane protein 16A (TMEM16A) in colorectal carcinoma.

METHODS: The expression of TMEM16A was detected by immunohistochemistry in 67 surgical colorectal carcinoma specimens and matched tumor-adjacent colorectal specimens.

RESULTS: TMEM16A was expressed in both cytoplasm and cell membrane. Among 67 colorectal carcinoma specimens, TMEM16A expression was negative in 4 cases (5.97%), weakly positive in 11 cases (16.42%), positive in 20 cases (29.85%), and strongly positive in 32 cases (47.76%). Among 67 tumor-adjacent healthy tissue specimens, TMEM16A expression was negative in 27 cases (40.30%), weakly positive in 35 cases (52.24%), positive in 3 cases (4.48%), and strongly positive in 2 cases (2.99%). The positive ("positive" plus "strongly positive") rate of TMEM16A expression was significantly higher in colorectal carcinoma tissue than in tumor adjacent healthy tissue (77.61% vs 7.46%, P < 0.005).

CONCLUSION: Aberrant expression of TMEM16A is a frequent feature in colorectal carcinoma. TMEM16A can be used as a new candidate target for diagnosis and treatment of colorectal carcinoma.

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.

Structural and functional characterization of the nitrite channel NirC from Salmonella typhimurium

Nitrite (NO(2)(-)) is a central intermediate in the nitrogen metabolism of microorganisms and plants, and is used as a cytotoxin by macrophages as part of the innate immune response. The bacterial membrane protein NirC acts as a specific channel to facilitate the transport of nitrite anions across lipid bilayers for cytoplasmic detoxification. Despite NirC's importance in nitrogen metabolism and in the pathogenicity of enteric bacteria, available biochemical data are scarce. Here we present a functional and structural characterization of NirC from Salmonella typhimurium by lipid bilayer electrophysiology and X-ray crystallography. NirC is a pentameric member of the formate/nitrite transporter family of membrane proteins that operates as a channel with high conductance. Single-channel measurements reveal fast and slow gating events but, in contrast to the related FocA formate channel, no pH-dependent gating. A 2.4Å crystal structure of NirC at pH 5 shows similarity to FocA and aquaporins, but lacks the structural asymmetry observed in the formate channel at similarly low pH. Resolved water molecules in the protomers suggest a transport mechanism that also permits a facultative NO(2)(-)/H(+) symport.

Fluoride resistance and transport by riboswitch-controlled CLC antiporters

A subclass of bacterial CLC anion-transporting proteins, phylogenetically distant from long-studied CLCs, was recently shown to be specifically up-regulated by F(-). We establish here that a set of randomly selected representatives from this "CLC(F)" clade protect Escherichia coli from F(-) toxicity, and that the purified proteins catalyze transport of F(-) in liposomes. Sequence alignments and membrane transport experiments using (19)F NMR, osmotic response assays, and planar lipid bilayer recordings reveal four mechanistic traits that set CLC(F) proteins apart from all other known CLCs. First, CLC(F)s lack conserved residues that form the anion binding site in canonical CLCs. Second, CLC(F)s exhibit high anion selectivity for F(-) over Cl(-). Third, at a residue thought to distinguish CLC channels and transporters, CLC(F)s bear a channel-like valine rather than a transporter-like glutamate, and yet are F(-)/H(+) antiporters. Finally, F(-)/H(+) exchange occurs with 1:1 stoichiometry, in contrast to the usual value of 2:1.

The formate channel FocA exports the products of mixed-acid fermentation

Formate is a major metabolite in the anaerobic fermentation of glucose by many enterobacteria. It is translocated across cellular membranes by the pentameric ion channel/transporter FocA that, together with the nitrite channel NirC, forms the formate/nitrite transporter (FNT) family of membrane transport proteins. Here we have carried out an electrophysiological analysis of FocA from Salmonella typhimurium to characterize the channel properties and assess its specificity toward formate and other possible permeating ions. Single-channel currents for formate, hypophosphite and nitrite revealed two mechanistically distinct modes of gating that reflect different types of structural rearrangements in the transport channel of each FocA protomer. Moreover, FocA did not conduct cations or divalent anions, but the chloride anion was identified as further transported species, along with acetate, lactate and pyruvate. Formate, acetate and lactate are major end products of anaerobic mixed-acid fermentation, the pathway where FocA is predominantly required, so that this channel is ideally adapted to act as a multifunctional export protein to prevent their intracellular accumulation. Because of the high degree of conservation in the residues forming the transport channel among FNT family members, the flexibility in conducting multiple molecules is most likely a general feature of these proteins.

Prolactin and dexamethasone regulate second messenger-stimulated cl(-) secretion in mammary epithelia

Mammary gland ion transport is essential for lactation and is regulated by prolactin and glucocorticoids. This study delineates the roles of prolactin receptors (PRLR) and long-term prolactin and dexamethasone (P-D)-mediation of [Ca²⁺]_i and Cl⁻ transport in HC-11 cells. P-D (24 h) suppressed ATP-induced [Ca²⁺]_i. This may be due to decreased Ca²⁺ entry since P-D decreased transient receptor potential channel 3 (TRPC3) but not secretory pathway Ca²⁺-ATPase 2 (SPCA2) mRNA. ATP increased Cl⁻ transport, measured by iodide (I⁻) efflux, in control and P-D-treated cells. P-D enhanced I⁻ efflux response to cAMP secretagogues without altering Cl⁻ channels or NKCC cotransporter expression. HC-11 cells contain only the long form of PRLR (PRLR-L). Since the short isoform, PRLR-S, is mammopoietic, we determined if transfecting PRLR-S (rs) altered PRLR-L-mediated Ca²⁺ and Cl⁻ transport. Untreated rs cells showed an attenuated [Ca²⁺]_i response to ATP with no further response to P-D, in contrast to vector-transfected (vtc) controls. P-D inhibited TRPC3 in rs and vtc cells but increased SPCA2 only in rs cells. As in wild-type, cAMP-stimulated Cl⁻ transport, in P-D-treated vtc and rs cells. In summary, 24 h P-D acts via PRLR-L to attenuate ATP-induced [Ca²⁺]_i and increase cAMP-activated Cl⁻ transport. PRLR-S fine-tunes these responses underscoring its mammopoietic action.

Molecular mechanism of proton transport in CLC Cl-/H+ exchange transporters

CLC proteins underlie muscle, kidney, bone, and other organ system function by catalyzing the transport of Cl(-) ions across cell and organellar membranes. Some CLC proteins are ion channels while others are pumps that exchange Cl(-) for H(+). The pathway through which Cl(-) ions cross the membrane has been characterized, but the transport of H(+) and the principle by which their movement is coupled to Cl(-) movement is not well understood. Here we show that H(+) transport depends not only on the presence of a specific glutamate residue but also the presence of Cl(-) ions. H(+) transport, however, can be isolated and analyzed in the absence of Cl(-) by mutating the glutamate to alanine and adding carboxylate-containing molecules to solution, consistent with the notion that H(+) transfer is mediated through the entry of a carboxylate group into the anion pathway. Cl(-) ions and carboxylate interact with each other strongly. These data support a mechanism in which the glutamate carboxylate functions as a surrogate Cl(-) ion, but it can accept a H(+) and transfer it between the external solution and the central Cl(-) binding site, coupled to the movement of 2 Cl(-) ions.

Sequential interaction of chloride and proton ions with the fast gate steer the voltage-dependent gating in ClC-2 chloride channels

The interaction of either H(+) or Cl(-) ions with the fast gate is the major source of voltage (V(m)) dependence in ClC Cl(-) channels. However, the mechanism by which these ions confer V(m) dependence to the ClC-2 Cl(-) channel remains unclear. By determining the V(m) dependence of normalized conductance (G(norm)(V(m))), an index of open probability, ClC-2 gating was studied at different [H(+)](i), [H(+)](o) and [Cl(-)](i). Changing [H(+)](i) by five orders of magnitude whilst [Cl(-)](i)/[Cl(-)](o) = 140/140 or 10/140 mm slightly shifted G(norm)(V(m)) to negative V(m) without altering the onset kinetics; however, channel closing was slower at acidic pH(i). A similar change in [H(+)](o) with [Cl(-)](i)/[Cl(-)](o) = 140/140 mm enhanced G(norm) in a bell-shaped manner and shifted G(norm)(V(m)) curves to positive V(m). Importantly, G(norm) was >0 with [H(+)](o) = 10(-10) m but channel closing was slower when [H(+)](o) or [Cl(-)](i) increased implying that ClC-2 was opened without protonation and that external H(+) and/or internal Cl(-) ions stabilized the open conformation. The analysis of kinetics and steady-state properties at different [H(+)](o) and [Cl(-)](i) was carried out using a gating Scheme coupled to Cl(-) permeation. Unlike previous results showing V(m)-dependent protonation, our analysis revealed that fast gate protonation was V(m) and Cl(-) independent and the equilibrium constant for closed–open transition of unprotonated channels was facilitated by elevated [Cl(-)](i) in a V(m)-dependent manner. Hence a V(m) dependence of pore occupancy by Cl(-) induces a conformational change in unprotonated closed channels, before the pore opens, and the open conformation is stabilized by Cl(-) occupancy and V(m)-independent protonation.

Hydrogen sulfide in the mammalian cardiovascular system

For more than a century, hydrogen sulfide (H(2)S) has been regarded as a toxic gas. This review surveys the growing recognition of the role of H(2)S as an endogenous signaling molecule in mammals, with emphasis on its physiological and pathological pathways in the cardiovascular system. In biological fluids, H(2)S gas is a weak acid that exists as about 15% H(2)S, 85% HS(-), and a trace of S(2-). Here, we use "H(2)S" to refer to this mixture. H(2)S has been found to influence heart contractile functions and may serve as a cardioprotectant for treating ischemic heart diseases and heart failure. Alterations of the endogenous H(2)S level have been found in animal models with various pathological conditions such as myocardial ischemia, spontaneous hypertension, and hypoxic pulmonary hypertension. In the vascular system, H(2)S exerts biphasic regulation of a vascular tone with varying effects based on its concentration and in the presence of nitric oxide. Over the past decade, several H(2)S-releasing compounds (NaHS, Na(2)S, GYY4137, etc.) have been utilized to test the effect of exogenous H(2)S under different physiological and pathological situations in vivo and in vitro. H(2)S has been found to promote angiogenesis and to protect against atherosclerosis and hypertension, while excess H(2)S may promote inflammation in septic or hemorrhagic shock. H(2)S-releasing compounds and inhibitors of H(2)S synthesis hold promise in alleviating specific disease conditions. This comprehensive review covers in detail the effects of H(2)S on the cardiovascular system, especially in disease situations, and also the various underlying mechanisms.

Mechanism of the prokaryotic transmembrane disulfide reduction pathway and its in vitro reconstitution from purified components

Making your (Dsb) connection: the redox pathway bringing reducing equivalents from bacterial cytoplasm, across the inner membrane, to the three reductive Dsb pathways in the otherwise oxidizing periplasm (see scheme; TR=thioredoxin reductase, Trx=thioredoxin) is reconstituted from purified components. Transfer of reducing equivalents across the membrane is demonstrated and underlying mechanistic details are revealed.

The Synthesis of 1,2,3-Triazole Derivatives of Deoxycholic Acid for Molecular Tweezer Metal Ion Recognition

A series of molecular tweezer metal receptors based on [1,2,3-triazolo-acetyl]esters of methyl deoxycholate have been synthesised via the 3,12-azidoester using a click chemistry method. Their structures were characterized by 1H NMR, IR, MS spectra and elemental analysis. Their binding properties were examined by UV-Vis spectra titration. The preliminary results indicate that this type of molecular tweezers have good recognition for metal ions.

Influences of mutations on the electrostatic binding free energies of chloride ions in Escherichia coli ClC

Mutations in ClC channel proteins may cause serious functional changes and even diseases. The function of ClC proteins mainly manifests as Cl(-) transport, which is related to the binding free energies of chloride ions. Therefore, the influence of a mutation on ClC function can be studied by investigating the mutational effect on the binding free energies of chloride ions. The present study provides quantitative and systematic investigations on the influences of residue mutations on the electrostatic binding free energies in Escherichia coli ClC (EcClC) proteins, using all-atom molecular dynamics simulations. It was found that the change of the electrostatic binding free energy decreases linearly with the increase of the residue-chloride ion distance for a mutation. This work reveals how changes in the charge of a mutated residue and in the distance between the mutated residue and the binding site govern the variations in the electrostatic binding free energies and therefore influence the transport of chloride ions and conduction in EcClC. This work would facilitate our understanding of the mutational effects on transport of chloride ions and functions of ClC proteins and provide a guideline to estimate which residue mutations will have great influences on ClC functions.

Biophysical properties of the voltage gated proton channel H(V)1

The biophysical properties of the voltage gated proton channel (H(V)1) are the key elements of its physiological function. The voltage gated proton channel is a unique molecule that in contrast to all other ion channels is exclusively selective for protons. Alone among proton channels, it has voltage and time dependent gating like other "classical" ion channels. H(V)1 is furthermore a sensor for the pH in the cell and the surrounding media. Its voltage dependence is strictly coupled to the pH gradient across the membrane. This regulation restricts opening of the channel to specific voltages at any given pH gradient, therefore allowing H(V)1 to perform its physiological task in the tissue it is expressed in. For H(V)1 there is no known blocker. The most potent channel inhibitor is zinc (Zn(2+)) which prevents channel opening. An additional characteristic of H(V)1 is its strong temperature dependence of both gating and conductance. In contrast to single-file water filled pores like the gramicidin channel, H(V)1 exhibits pronounced deuterium effects and temperature effects on conduction, consistent with a different conduction mechanism than other ion channels. These properties may be explained by the recent identification of an aspartate in the pore of H(V)1 that is essential to its proton selectivity.

Voltage-gated proton channels

Voltage-gated proton channels, HV1, have vaulted from the realm of the esoteric into the forefront of a central question facing ion channel biophysicists, namely, the mechanism by which voltage-dependent gating occurs. This transformation is the result of several factors. Identification of the gene in 2006 revealed that proton channels are homologues of the voltage-sensing domain of most other voltage-gated ion channels. Unique, or at least eccentric, properties of proton channels include dimeric architecture with dual conduction pathways, perfect proton selectivity, a single-channel conductance approximately 10(3) times smaller than most ion channels, voltage-dependent gating that is strongly modulated by the pH gradient, ΔpH, and potent inhibition by Zn(2+) (in many species) but an absence of other potent inhibitors. The recent identification of HV1 in three unicellular marine plankton species has dramatically expanded the phylogenetic family tree. Interest in proton channels in their own right has increased as important physiological roles have been identified in many cells. Proton channels trigger the bioluminescent flash of dinoflagellates, facilitate calcification by coccolithophores, regulate pH-dependent processes in eggs and sperm during fertilization, secrete acid to control the pH of airway fluids, facilitate histamine secretion by basophils, and play a signaling role in facilitating B-cell receptor mediated responses in B-lymphocytes. The most elaborate and best-established functions occur in phagocytes, where proton channels optimize the activity of NADPH oxidase, an important producer of reactive oxygen species. Proton efflux mediated by HV1 balances the charge translocated across the membrane by electrons through NADPH oxidase, minimizes changes in cytoplasmic and phagosomal pH, limits osmotic swelling of the phagosome, and provides substrate H(+) for the production of H2O2 and HOCl, reactive oxygen species crucial to killing pathogens.

Molecular dynamics investigation of Cl- and water transport through a eukaryotic CLC transporter

Early crystal structures of prokaryotic CLC proteins identified three Cl(-) binding sites: internal (S(int)), central (S(cen)), and external (S(ext)). A conserved external GLU (GLU(ex)) residue acts as a gate competing for S(ext). Recently, the first crystal structure of a eukaryotic transporter, CmCLC, revealed that in this transporter GLU(ex) competes instead for S(cen). Here, we use molecular dynamics simulations to investigate Cl(-) transport through CmCLC. The gating and Cl(-)/H(+) transport cycle are inferred through comparative molecular dynamics simulations with protonated and deprotonated GLU(ex) in the presence/absence of external potentials. Adaptive biasing force calculations are employed to estimate the potential of mean force profiles associated with transport of a Cl(-) ion from S(ext) to S(int), depending on the Cl(-) occupancy of other sites. Our simulations demonstrate that protonation of GLU(ex) is essential for Cl(-) transport from S(ext) to S(cen). The S(cen) site may be occupied by two Cl(-) ions simultaneously due to a high energy barrier (∼8 Kcal/mol) for a single Cl(-) ion to translocate from S(cen) to S(int). Binding two Cl(-) ions to S(cen) induces a continuous water wire from S(cen) to the extracellular solution through the side chain of the GLU(ex) gate. This may initiate deprotonation of GLU(ex), which then drives the two Cl(-) ions out of S(cen) toward the intracellular side via two putative Cl(-) transport paths. Finally, a conformational cycle is proposed that would account for the exchange stoichiometry.

Differential regulation of a CLC anion channel by SPAK kinase ortholog-mediated multisite phosphorylation

Shrinkage-induced inhibition of the Caenorhabditis elegans cell volume and cell cycle-dependent CLC anion channel CLH-3b occurs by concomitant phosphorylation of S742 and S747, which are located on a 175 amino acid linker domain between cystathionine-β-synthase 1 (CBS1) and CBS2. Phosphorylation is mediated by the SPAK kinase homolog GCK-3 and is mimicked by substituting serine residues with glutamate. Type 1 serine/threonine protein phosphatases mediate swelling-induced channel dephosphorylation. S742E/S747E double mutant channels are constitutively inactive and cannot be activated by cell swelling. S742E and S747E mutant channels were fully active in the absence of GCK-3 and were inactive when coexpressed with the kinase. Both channels responded to cell volume changes. However, the S747E mutant channel activated and inactivated in response to cell swelling and shrinkage, respectively, much more slowly than either wild-type or S742E mutant channels. Slower activation and inactivation of S747E was not due to altered rates of dephosphorylation or dephosphorylation-dependent conformational changes. GCK-3 binds to the 175 amino acid inter-CBS linker domain. Coexpression of wild-type CLH-3b and GCK-3 with either wild-type or S742E linkers gave rise to similar channel activity and regulation. In contrast, coexpression with the S747E linker greatly enhanced basal channel activity and increased the rate of shrinkage-induced channel inactivation. Our findings suggest the intriguing possibility that the phosphorylation state of S742 in S747E mutant channels modulates GCK-3/channel interaction and hence channel phosphorylation. These results provide a foundation for further detailed studies of the role of multisite phosphorylation in regulating CLH-3b and GCK-3 activity.

Mechanism of proton/substrate coupling in the heptahelical lysosomal transporter cystinosin

Secondary active transporters use electrochemical gradients provided by primary ion pumps to translocate metabolites or drugs "uphill" across membranes. Here we report the ion-coupling mechanism of cystinosin, an unusual eukaryotic, proton-driven transporter distantly related to the proton pump bacteriorhodopsin. In humans, cystinosin exports the proteolysis-derived dimeric amino acid cystine from lysosomes and is impaired in cystinosis. Using voltage-dependence analysis of steady-state and transient currents elicited by cystine and neutralization-scanning mutagenesis of conserved protonatable residues, we show that cystine binding is coupled to protonation of a clinically relevant aspartate buried in the membrane. Deuterium isotope substitution experiments are consistent with an access of this aspartate from the lysosomal lumen through a deep proton channel. This aspartate lies in one of the two PQ-loop motifs shared by cystinosin with a set of eukaryotic membrane proteins of unknown function and is conserved in about half of them, thus suggesting that other PQ-loop proteins may translocate protons.

Conformational change opening the CFTR chloride channel pore coupled to ATP-dependent gating

Opening and closing of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel are controlled by ATP binding and hydrolysis by its nucleotide binding domains (NBDs). This is presumed to control opening of a single "gate" within the permeation pathway, however, the location of such a gate has not been described. We used patch clamp recording to monitor access of cytosolic cysteine reactive reagents to cysteines introduced into different transmembrane (TM) regions in a cysteine-less form of CFTR. The rate of modification of Q98C (TM1) and I344C (TM6) by both [2-sulfonatoethyl] methanethiosulfonate (MTSES) and permeant Au(CN)(2)(-) ions was reduced when ATP concentration was reduced from 1mM to 10μM, and modification by MTSES was accelerated when 2mM pyrophosphate was applied to prevent channel closure. Modification of K95C (TM1) and V345C (TM6) was not affected by these manoeuvres. We also manipulated gating by introducing the mutations K464A (in NBD1) and E1371Q (in NBD2). The rate of modification of Q98C and I344C by both MTSES and Au(CN)(2)(-) was decreased by K464A and increased by E1371Q, whereas modification of K95C and V345C was not affected. These results suggest that access from the cytoplasm to K95 and V345 is similar in open and closed channels. In contrast, modifying ATP-dependent channel gating alters access to Q98 and I344, located further into the pore. We propose that ATP-dependent gating of CFTR is associated with the opening and closing of a gate within the permeation pathway at the level of these pore-lining amino acids.

Physiology and pathophysiology of CLC-1: mechanisms of a chloride channel disease, myotonia

The CLC-1 chloride channel, a member of the CLC-channel/transporter family, plays important roles for the physiological functions of skeletal muscles. The opening of this chloride channel is voltage dependent and is also regulated by protons and chloride ions. Mutations of the gene encoding CLC-1 result in a genetic disease, myotonia congenita, which can be inherited as an autosmal dominant (Thomsen type) or an autosomal recessive (Becker type) pattern. These mutations are scattered throughout the entire protein sequence, and no clear relationship exists between the inheritance pattern of the mutation and the location of the mutation in the channel protein. The inheritance pattern of some but not all myotonia mutants can be explained by a working hypothesis that these mutations may exert a “dominant negative” effect on the gating function of the channel. However, other mutations may be due to different pathophysiological mechanisms, such as the defect of protein trafficking to membranes. Thus, the underlying mechanisms of myotonia are likely to be quite diverse, and elucidating the pathophysiology of myotonia mutations will require the understanding of multiple molecular/cellular mechanisms of CLC-1 channels in skeletal muscles, including molecular operation, protein synthesis, and membrane trafficking mechanisms.

Cryptococcus neoformans Ca(2+) homeostasis requires a chloride channel/antiporter Clc1 in JEC21, but not in H99

CLC-type chloride/proton antiporters are required for copper/iron homeostasis in fungi. A relationship between CLCs and Ca(2+) homeostasis has not been found before. Here we demonstrate the requirement of the antiporter CLC1 for Ca(2+) homeostasis/signaling in Cryptococcus neoformans. The deletion of CLC1 in JEC21 resulted in a mutant hypersensitive to cyclosporine A, an inhibitor of calcineurin. Intracellular Ca(2+) deficiency in the mutant Tx1 was confirmed with Fluo-3 staining epi-fluorescence microscopy. Tx1 failed to grow at elevated temperature and in SDS and displayed defects in cell wall integrity and cell separation. This defective phenotype is because of Ca(2+) deficiency that was restorable by exogenous Ca(2+) . In contrast, H99 CLC1 was dispensable for Ca(2+) homeostasis and had no comparable defective consequences if deleted, suggesting divergent roles of CLCs in Ca(2+) homeostasis. Distinct Ca(2+) homeostasis mechanisms may contribute the virulence difference between the two strains. This work reveals a novel action of CLC antiporters in fungi and may provide information as to the evolution of pathogenicity among cryptococcal strains.

Band 3 missense mutations and stomatocytosis: insight into the molecular mechanism responsible for monovalent cation leak

Missense mutations in the erythroid band 3 protein (Anion Exchanger 1) have been associated with hereditary stomatocytosis. Features of cation leaky red cells combined with functional expression of the mutated protein led to the conclusion that the AE1 point mutations were responsible for Na(+) and K(+) leak through a conductive mechanism. A molecular mechanism explaining mutated AE1-linked stomatocytosis involves changes in AE1 transport properties that become leaky to Na(+) and K(+). However, another explanation suggests that point-mutated AE1 could regulate a cation leak through other transporters. This short paper intends to discuss these two alternatives.

A synthetic prestin reveals protein domains and molecular operation of outer hair cell piezoelectricity

Prestin, a transporter-like protein of the SLC26A family, acts as a piezoelectric transducer that mediates the fast electromotility of outer hair cells required for cochlear amplification and auditory acuity in mammals. Non-mammalian prestin orthologues are anion transporters without piezoelectric activity. Here, we generated synthetic prestin (SynPres), a chimera of mammalian and non-mammalian prestin exhibiting both, piezoelectric properties and anion transport. SynPres delineates two distinct domains in the protein's transmembrane core that are necessary and sufficient for generating electromotility and associated non-linear charge movement (NLC). Functional analysis of SynPres showed that the amplitude of NLC and hence electromotility are determined by the transport of monovalent anions. Thus, prestin-mediated electromotility is a dual-step process: transport of anions by an alternate access cycle, followed by an anion-dependent transition generating electromotility. The findings define structural and functional determinants of prestin's piezoelectric activity and indicate that the electromechanical process evolved from the ancestral transport mechanism.

Determinants of coupled transport and uncoupled current by the electrogenic SLC26 transporters

Members of the SLC26 family of anion transporters mediate the transport of diverse molecules ranging from halides to carboxylic acids and can function as coupled transporters or as channels. A unique feature of the two members of the family, Slc26a3 and Slc26a6, is that they can function as both obligate coupled and mediate an uncoupled current, in a channel-like mode, depending on the transported anion. To identify potential features that control the two modes of transport, we performed in silico modeling of Slc26a6, which suggested that the closest potential fold similarity of the Slc26a6 transmembrane domains is to the CLC transporters, despite their minimal sequence identity. Examining the predicted Slc26a6 fold identified a highly conserved glutamate (Glu⁻; Slc26a6(E357)) with the predicted spatial orientation similar to that of the CLC-ec1 E148, which determines coupled or uncoupled transport by CLC-ec1. This raised the question of whether the conserved Glu⁻ in Slc26a6(E357) and Slc26a3(E367) have a role in the unique transport modes by these transporters. Reversing the Glu⁻ charge in Slc26a3 and Slc26a6 resulted in the inhibition of all modes of transport. However, most notably, neutralizing the charge in Slc26a6(E357A) eliminated all forms of coupled transport without affecting the uncoupled current. The Slc26a3(E367A) mutation markedly reduced the coupled transport and converted the stoichiometry of the residual exchange from 2Cl⁻/1HCO₃⁻ to 1Cl⁻/1HCO₃⁻, while completely sparing the current. These findings suggest the possibility that similar structural motif may determine multiple functional modes of these transporters.

Discrete-state representation of ion permeation coupled to fast gating in a model of CLC-chloride channels: analytic estimation of the state-to-state rate constants

Analytical estimation of state-to-state rate constants is carried out for a recently developed discrete state model of chloride ion motion in a CLC chloride channel (Coalson and Cheng, J. Phys. Chem. B 2010, 114, 1424). In the original presentation of this model, the same rate constants were evaluated via three-dimensional Brownian dynamics simulations. The underlying dynamical theory is an appropriate single- or multiparticle three-dimensional Smoluchowski equation. Taking advantage of approximate geometric symmetries (based on the details of the model channel geometry), well-known formulas for state-to-state transition rates are appealed to herein and adapted as necessary to the problem at hand. Rates of ionic influx from a bulk electrolyte reservoir to the nearest binding site within the channel pore are particularly challenging to compute analytically because they reflect multi-ion interactions (as opposed to single-ion dynamics). A simple empirical correction factor is added to the single-ion rate constant formula in this case to account for the saturation of influx rate constants with increasing bulk Cl(-) concentration. Overall, the agreement between all analytically estimated rate constants is within a factor of 2 of those computed via three-dimensional Brownian dynamics simulations, and often better than this. Current-concentration curves obtained using rate constants derived from these two different computational approaches agree to within 25%.

Anion- and proton-dependent gating of ClC-4 anion/proton transporter under uncoupling conditions

ClC-4 is a secondary active transporter that exchanges Cl(-) ions and H(+) with a 2:1 stoichiometry. In external SCN(-), ClC-4 becomes uncoupled and transports anions with high unitary transport rate. Upon voltage steps, the number of active transporters varies in a time-dependent manner, resembling voltage-dependent gating of ion channels. We here investigated modification of the voltage dependence of uncoupled ClC-4 by protons and anions to quantify association of substrates with the transporter. External acidification shifts voltage dependence of ClC-4 transport to more positive potentials and leads to reduced transport currents. Internal pH changes had less pronounced effects. Uncoupled ClC-4 transport is facilitated by elevated external [SCN(-)] but impaired by internal Cl(-) and I(-). Block by internal anions indicates the existence of an internal anion-binding site with high affinity that is not present in ClC channels. The voltage dependence of ClC-4 coupled transport is modulated by external protons and internal Cl(-) in a manner similar to what is observed under uncoupling conditions. Our data illustrate functional differences but also similarities between ClC channels and transporters.

Dual transport properties of anion exchanger 1: the same transmembrane segment is involved in anion exchange and in a cation leak

Previous results suggested that specific point mutations in human anion exchanger 1 (AE1) convert the electroneutral anion exchanger into a monovalent cation conductance. In the present study, the transport site for anion exchange and for the cation leak has been studied by cysteine scanning mutagenesis and sulfhydryl reagent chemistry. Moreover, the role of some highly conserved amino acids within members of the SLC4 family to which AE1 belongs has been assessed in AE1 transport properties. The results suggest that the same transport site within the AE1 spanning domain is involved in anion exchange or in cation transport. A functioning mechanism for this transport site is proposed according to transport properties of the different studied point mutations of AE1.

Putting the pieces together: a crystal clear window into CLC anion channel regulation

CLC anion transport proteins function as Cl (-) channels and Cl (-) /H (+) exchangers and are found in all major groups of life including archaebacteria. Early electrophysiological studies suggested that CLC anion channels have two pores that are opened and closed independently by a "fast" gating process operating on a millisecond timescale, and a "common" or "slow" gate that opens and closes both pores simultaneously with a timescale of seconds (Figure 1A). Subsequent biochemical and molecular experiments suggested that CLC channels/transporters are homodomeric proteins ( 1-3) .

Charge transport in the ClC-type chloride-proton anti-porter from Escherichia coli

The first chloride transporter identified in the superfamily of ClC chloride channels was from Escherichia coli (EClC) (Accardi, A., and Miller, C. (2004) Nature 427, 803-807). Pathways, energetics, and mechanism of proton and chloride translocation and their coupling are up to now unclear. To bridge the hydrophobic gap of proton transport, we modeled four stable buried waters into both subunits of the WT EClC structure. Together they form a "water wire" connecting Glu-203 with the chloride at the central site, which in turn connects to Glu-148, the hypothetical proton exit site. Assuming the transient production of hydrochloride in the central chloride binding site of EClC, the water wire could establish a transmembrane proton transport pathway starting from Glu-203 all the way downstream onto Glu-148. We demonstrated by electrostatic and quantum chemical computations that protonation of the central chloride is energetically feasible. We characterized all chloride occupancies and protonation states possibly relevant for the proton-chloride transport cycle in EClC and constructed a working model. Accordingly, EClC evolves through states involving up to two excess protons and between one and three chlorides, which was required to fulfill the experimentally observed 2:1 stoichiometry. We show that the Y445F and E203H mutants of EClC can operate similarly, thus explaining why they exhibit almost WT activity levels. The proposed mechanism of coupled chloride-proton transport in EClC is consistent with available experimental data and allows predictions on the importance of specific amino acids, which may be probed by mutation experiments.

Sodium transport in plants: a critical review

Sodium (Na) toxicity is one of the most formidable challenges for crop production world-wide. Nevertheless, despite decades of intensive research, the pathways of Na(+) entry into the roots of plants under high salinity are still not definitively known. Here, we review critically the current paradigms in this field. In particular, we explore the evidence supporting the role of nonselective cation channels, potassium transporters, and transporters from the HKT family in primary sodium influx into plant roots, and their possible roles elsewhere. We furthermore discuss the evidence for the roles of transporters from the NHX and SOS families in intracellular Na(+) partitioning and removal from the cytosol of root cells. We also review the literature on the physiology of Na(+) fluxes and cytosolic Na(+) concentrations in roots and invite critical interpretation of seminal published data in these areas. The main focus of the review is Na(+) transport in glycophytes, but reference is made to literature on halophytes where it is essential to the analysis.

A pH-dependent conformational ensemble mediates proton transport through the influenza A/M2 protein

The influenza A/M2 protein exhibits inwardly rectifying, pH-activated proton transport that saturates at low pH. A comparison of high-resolution structures of the transmembrane domain at high and low pH suggests that pH-dependent conformational changes may facilitate proton conduction by alternately changing the accessibility of the N-terminal and C-terminal regions of the channel as a proton transits through the transmembrane domain. Here, we show that M2 functionally reconstituted in liposomes populates at least three different conformational states over a physiologically relevant pH range, with transition midpoints that are consistent with previously reported His37 pK(a) values. We then develop and test two similar, quantitative mechanistic models of proton transport, where protonation shifts the equilibrium between structural states having different proton affinities and solvent accessibilities. The models account well for a collection of experimental data sets over a wide range of pH values and voltages and require only a small number of adjustable parameters to accurately describe the data. While the kinetic models do not require any specific conformation for the protein, they nevertheless are consistent with a large body of structural information based on high-resolution nuclear magnetic resonance and crystallographic structures, optical spectroscopy, and molecular dynamics calculations.

A three-state multi-ion kinetic model for conduction properties of ClC-0 chloride channel

A three-state, multiion kinetic model was proposed to enable the conduction properties of the mammalian channel ClC-0 to be well characterized. Using this rate-theory based model, the current-voltage and conductance-concentration relations were obtained. The five parameters needed were determined by fitting the data of conduction experiments of the wild-type ClC-0 and its K519C mutant. The model was then tested against available calculation and simulation data, and the energy differences between distinct chloride-occupancy states computed agreed with an independent calculation on the binding free energies solved by using the Poisson-Boltzmann equation. The average ion number of conduction and the ion passing duration calculated closely resembled the values obtained from Brownian dynamics simulations. According to the model, the decrease of conductance caused by mutating residue K519 to C519 can be attributed to the effect of K519C mutation on translocation rate constants. Our study sets up a theoretical model for ion permeation and conductance in ClC-0. It provides a starting point for experimentalists to test the three-state model, and would help in understanding the conduction mechanism of ClC-0.

The structural basis of secondary active transport mechanisms

Secondary active transporters couple the free energy of the electrochemical potential of one solute to the transmembrane movement of another. As a basic mechanistic explanation for their transport function the model of alternating access was put forward more than 40 years ago, and has been supported by numerous kinetic, biochemical and biophysical studies. According to this model, the transporter exposes its substrate binding site(s) to one side of the membrane or the other during transport catalysis, requiring a substantial conformational change of the carrier protein. In the light of recent structural data for a number of secondary transport proteins, we analyze the model of alternating access in more detail, and correlate it with specific structural and chemical properties of the transporters, such as their assignment to different functional states in the catalytic cycle of the respective transporter, the definition of substrate binding sites, the type of movement of the central part of the carrier harboring the substrate binding site, as well as the impact of symmetry on fold-specific conformational changes. Besides mediating the transmembrane movement of solutes, the mechanism of secondary carriers inherently involves a mechanistic coupling of substrate flux to the electrochemical potential of co-substrate ions or solutes. Mainly because of limitations in resolution of available transporter structures, this important aspect of secondary transport cannot yet be substantiated by structural data to the same extent as the conformational change aspect. We summarize the concepts of coupling in secondary transport and discuss them in the context of the available evidence for ion binding to specific sites and the impact of the ions on the conformational state of the carrier protein, which together lead to mechanistic models for coupling.

The proline 160 in the selectivity filter of the Arabidopsis NO(3)(-)/H(+) exchanger AtCLCa is essential for nitrate accumulation in planta

Nitrate, the major nitrogen source for plants, can be accumulated in the vacuole. Its transport across the vacuolar membrane is mediated by AtCLCa, an antiporter of the chloride channel (CLC) protein family. In contrast to other CLC family members, AtCLCa transports nitrate coupled to protons. Recently, the different behaviour towards nitrate of CLC proteins has been linked to the presence of a serine or proline in the selectivity filter motif GXGIP. By monitoring AtCLCa activity in its native environment, we show that if proline 160 in AtCLCa is changed to a serine (AtCLCa(P160S) ), the transporter loses its nitrate selectivity, but the anion proton exchange mechanism is unaffected. We also performed in vivo analyses in yeast and Arabidopsis. In contrast to native AtCLCa, expression of AtCLCa(P160S) does not complement either the ΔScCLC yeast mutant grown on nitrate or the nitrate under-accumulation phenotype of clca knockout plants. Our results confirm the significance of this amino acid in the conserved selectivity filter of CLC proteins and highlight the importance of the proline in AtCLCa for nitrate metabolism in Arabidopsis.

Changes in accessibility of cytoplasmic substances to the pore associated with activation of the cystic fibrosis transmembrane conductance regulator chloride channel

Opening of the cystic fibrosis transmembrane conductance regulator Cl(-) channel is dependent both on phosphorylation and on ATP binding and hydrolysis. However, the mechanisms by which these cytoplasmic regulatory factors open the Cl(-) channel pore are not known. We have used patch clamp recording to investigate the accessibility of cytoplasmically applied cysteine-reactive reagents to cysteines introduced along the length of the pore-lining sixth transmembrane region (TM6) of a cysteine-less variant of cystic fibrosis transmembrane conductance regulator. We find that methanethiosulfonate (MTS) reagents modify irreversibly cysteines substituted for TM6 residues Phe-337, Thr-338, Ser-341, Ile-344, Val-345, Met-348, Ala-349, Arg-352, and Gln-353 when applied to the cytoplasmic side of open channels. However, the apparent rate of modification by internal [2-sulfonatoethyl] methanethiosulfonate (MTSES), a negatively charged MTS reagent, is dependent on the activation state of the channels. In particular, cysteines introduced far along the axis of TM6 from the inside (T338C, S341C, I344C) showed no evidence of significant modification even after prolonged pretreatment of non-activated channels with internal MTSES. In contrast, cysteines introduced closer to the inside of TM6 (V345C, M348C) were readily modified in both activated and non-activated channels. Access of a permeant anion, Au(CN)(2)(-), to T338C was similarly dependent upon channel activation state. The pattern of MTS modification we observe allows us to designate different pore-lining amino acid side chains to distinct functional regions of the channel pore. One logical interpretation of these findings is that cytoplasmic access to residues at the narrowest region of the pore changes concomitant with activation.