Accessibility of the CLC-0 pore to charged methanethiosulfonate reagents

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.

Electrostatic tuning of the pre- and post-hydrolytic open states in CFTR

Gating of the CFTR channel is coupled to ATP hydrolysis such that two open states can be identified under certain conditions. Zhang and Hwang find that pore-lining mutations differentially affect the permeation properties of these open states and suggest that the internal vestibule expands upon ATP hydrolysis.

Cystic fibrosis transmembrane conductance regulator (CFTR) is an ion channel that couples adenosine triphosphate (ATP) hydrolysis at its nucleotide-binding domains to gating transitions in its transmembrane domains. We previously reported that the charge-neutralized mutant R352C shows two distinct open states, O₁ and O₂. The two states could be distinguished by their single-channel current amplitudes: O₁ having a smaller amplitude (representing a prehydrolytic open state) and O₂ having a larger amplitude (representing a post-hydrolytic open state). In this study, a similar phenotype is described for two mutations of another pore-lining residue, N306D and N306E, suggesting that alterations of the net charge within CFTR’s pore confer this unique conductance aberration. Because moving either of the two endogenous charges, R303 and R352, to positions further along TM5 and TM6, respectively, also results in this O₁O₂ phenotype, we conclude that the position of the charged residue in the internal vestibule affects hydrolysis-dependent conductance changes. Furthermore, our data show that the buffer and CFTR blocker morpholino propane sulfonic acid (MOPS⁻) occludes the O₁ state more than it does the O₂ state when the net charge of the internal vestibule is unchanged or increased. In contrast, when the net charge in the internal vestibule is decreased, the differential sensitivity to MOPS⁻ block is diminished. We propose a three-state blocking mechanism to explain the charge-dependent sensitivity of prehydrolytic and post-hydrolytic open states to MOPS⁻ block. We further posit that the internal vestibule expands during the O₁ to O₂ transition so that mutation-induced electrostatic perturbations within the pore are amplified by the smaller internal vestibule of the O₁ state and thus result in the O₁O₂ phenotype and the charge-dependent sensitivity of the two open states to MOPS⁻ block. Our study not only relates the O₁O₂ phenotype to the charge distribution in CFTR’s internal vestibule but also provides a toolbox for mechanistic studies of CFTR gating by ATP hydrolysis.

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.