Dataset Information

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

ABSTRACT: 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.

SUBMITTER: Orhan G

PROVIDER: S-EPMC3043216 | biostudies-literature | 2011 Mar

REPOSITORIES: biostudies-literature

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Publications

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

Orhan Gökce G Fahlke Christoph C Alekov Alexi K AK

Biophysical journal 20110301 5

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 transport ...[more]

PMID: 21354396

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Project description: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 (pHi) facilitates the closure of the CLC-0's slow gate, thus generating current inhibition. The rate of low pHi-induced current inhibition increases with intracellular H+ concentration ([H+]i)-the time constants of current inhibition by low pHi = 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 pHi = 7.4 at room temperature is hundreds of seconds. The inhibition by low pHi 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 pHi 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.

Project description:ClC-3, ClC-4, and ClC-5 are electrogenic chloride/proton exchangers that can be found in endosomal compartments of mammalian cells. Although the association with genetic diseases and the severe phenotype of knock-out animals illustrate their physiological importance, the cellular functions of these proteins have remained insufficiently understood. We here study the role of two Clcn3 splice variants, ClC-3b and ClC-3c, in granular exocytosis and catecholamine accumulation of adrenal chromaffin cells using a combination of high-resolution capacitance measurements, amperometry, protein expression/gene knock out/down, rescue experiments, and confocal microscopy. We demonstrate that ClC-3c resides in immature as well as in mature secretory granules, where it regulates catecholamine accumulation and contributes to the establishment of the readily releasable pool of secretory vesicles. The lysosomal splice variant ClC-3b contributes to vesicle priming only with low efficiency and leaves the vesicular catecholamine content unaltered. The related Cl-/H+ antiporter ClC-5 undergoes age-dependent downregulation in wild-type conditions. Its upregulation in Clcn3-/- cells partially rescues the exocytotic mutant defect. Our study demonstrates how different CLC transporters with similar transport functions, but distinct localizations can contribute to vesicle functions in the regulated secretory pathway of granule secretion in chromaffin cells.SIGNIFICANCE STATEMENT Cl-/H+ exchangers are expressed along the endosomal/lysosomal system of mammalian cells; however, their exact subcellular functions have remained insufficiently understood. We used chromaffin cells, a system extensively used to understand presynaptic mechanisms of synaptic transmission, to define the role of CLC exchangers in neurosecretion. Disruption of ClC-3 impairs catecholamine accumulation and secretory vesicle priming. There are multiple ClC-3 splice variants, and only expression of one, ClC-3c, in double Cl-/H+ exchanger-deficient cells fully rescues the WT phenotype. Another splice variant, ClC-3b, is present in lysosomes and is not necessary for catecholamine secretion. The distinct functions of ClC-3c and ClC-3b illustrate the impact of expressing multiple CLC transporters with similar transport functions and separate localizations in different endosomal compartments.

Dataset Information

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

Publications

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

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