1.A.11 The Chloride Channel (ClC) Family

Methanococcus jannaschii and Saccharomyces cerevisiae only have one ClC family member each. With the exception of the large Synechocystis paralogue, bacterial proteins are usually small (395-492 residues) while eukaryotic proteins are usually larger (687-988 residues). These proteins exhibit 12 putative transmembrane α-helical spanners (TMSs) and appear to be present in the membrane as homodimers. A 12 TMS topology with the N- and C-termini in the cytoplasm was suggested. The structure of the E. coli EriC (TC #1.A.11.5.1) ClC family member has been reported at 3.0 Å resolution (Dutzler et al., 2002; Mindell et al., 2001). Two identical water-filled pores, each within a single subunit of the dimeric channel complex were revealed. Each subunit consists of two roughly repeated halves that span the membrane with 5 TMSs each and opposite orientations in the membrane. This antiparallel architecture defines a selectivity filter in which Cl^- is stabilized by electrostatic interactions with α-helix dipoles and chemical coordination with nitrogen and hydroxyl groups in the protein. This protein has been shown to mediate the extreme acid resistance response (Iyer et al., 2002). Thus, E. coli is proposed to use either one of its two ClC channels as electrical shunts for an outwardly directed virtual pump that is linked to amino acid decarboxylation (Iyer et al., 2002).

Recently, the E. coli EriC (also called ClC-ecl) has been studied leading to the conclusion that it is not a simple channel, but instead catalyzes Cl^-:H⁺ antiport with a probable stoichiometry of 2.1. It can thus function as a carrier (Accardi and Miller, 2004). The authors note that the eukaryotic ClC-0, ClC-1 and ClC-2 are unambiguously Cl^--selective channels which display proton-dependent gating but show no indication of H⁺ permeability. Moreover, they note that the 3-D structure published by Dutzler et al. (2002, 2003) does not actually show a transmembrane pore. They further note that a conserved glutamate, when mutated in ClC-0 or ClC-1 eliminates the normal pH-dependency of the Cl^- flux, while in EriC (ClC-ecl), this glutamate E148 may provide the pathway for the proton. E148A or E148Q mutants do not transport H⁺ but do transport Cl^- in an uncoupled process.

All eukaryotic ClC channels so far examined contain two C-terminal CBS domains, each of 50 residue. They are of known structure but unknown function. CBS domains are found in various globular proteins. While the Torpedo ClC-0 and the E. coli YadQ have been reported to have two channels and function by a double barrelled mechanism, one per subunit, others are believed to have just one. Strong evidence suggests that for ClC-0, ClC-1 and ClC-2, each subunit bears a single channel, and the association of the subunits of these channel proteins to form homo- or heterodimers does not alter their conductance properties.

All functionally characterized members of the ClC family transport chloride, some in a voltage-regulated process. These channels serve a variety physiological functions (cell volume regulation; membrane potential stabilization; signal transduction; transepithelial transport, etc.). Different homologues in humans exhibit differing anion selectivities, i.e., ClC-4 and ClC-5 share a NO₃^- > Cl^- > Br^- > I^- conductance sequence, while ClC-3 has an I^- > Cl^- selectivity.

The ClC-4 and ClC-5 channels and others exhibit outward rectifying currents with currents only at voltages more positive than +20mV. Some but not other ClC channels are permeable to the low conductance blockers, I^- and SCN^-. ClCα-1 has been studied in detail. ClO₄^- and SCN^- are more permeant than Br^-, NO₃^- or ClO₃^-, and the hydrophobic anions, benzoate and hexanoates, are more permeable than smaller anions such as BrO₃^-. It is clear that ClCs are general anion channels.

A genetic defect of ClC-5 in humans is the cause of Dent's disease. This protein is expressed in endosomes of the proximal tubule. Disruption of the corresponding clcn5 gene in mice causes proteinuria by reducing apical proximal tubular endocytosis. This delays internalization of the apical transporters NaPi-2 and NHE3 (Piwon et al., 2000). It has been suggested that it plays a role in acidification of both endocytic and exocytic vesicles involved in protein trafficking.

The generalized transport reaction catalyzed by ClC family channels is:

Anion^- (out) Anion^- (in).

References:

Examples: