1.A.13 The Epithelial Chloride Channel (E-ClC) Family
Mammals have multiple isoforms (at least 6 different gene products plus splice variants (Evans et al., 2004)) of epithelial chloride channel proteins. The first member of this family to be characterized was a respiratory epithelium, Ca2+-regulated, chloride channel protein isolated from bovine tracheal apical membranes. It was biochemically characterized as a 140 kDa complex. The purified complex when reconstituted in a planar lipid bilayer behaved as an anion-selective channel. It was regulated by Ca2+ via a calmodulin kinase II-dependent mechanism. When the cRNA was injected into Xenopus oocytes, an outward rectifying, DIDS-sensitive anion conductance was measured. A related gene, Lu-ECAM, was cloned from the bovine aortic endothelial cell line, BAEC. It is expressed in the lung and spleen but not in the trachea. Homologues are found in several mammals, and at least four paralogues (hCaCC-1-4) are present in humans, each with different tissue distributions.
The bovine EClC protein has 903 amino acids and four putative TMSs at residue positions 7-27, 331-351, 617-337 and 883-903. Distant (partial) homologues may be present in plants, ciliates and bacteria, Synechocystis (Sll0103; 420 aas) and E. coli (YfbK; 575 aas), so at least some domains within E-ClC family proteins have an ancient origin. E-ClC proteins show significant sequence similarity with CCA-α2δ family members (8.A.18).
Gibson et al. (2005) have shown that the human calcium-activated chloride channel, hCLCA1, is a secreted, non-integral membrane protein, and therefore suggest that this protein and its homologues are not ion channels at all. They also point out that the protein may not be sufficiently hydrophobic to insert into the membrane. Moreover, the proteins in subfamily 1.A.13.2 do not have α-TMSs, but hydrophilic domains in these proteins show sequence similarity to hydrophilic regions in proteins of family 8.A.18. The proteins without α-TMSs may not be channels. Thus, while some members of the E-ClC family appear to be able to form anion channels, others may not have this capacity.
The generalized reaction catalyzed by EClC family members is:
Cl- (in) Cl- (out)