and CLCNKB E-ClC family Members of Epithelial Chloride Channel (E-ClC) Family (TC# 1.A.13) catalyze bidirectional transport of chloride ions. Mammals have multiple isoforms (at least 6 different gene products plus splice variants) of epithelial chloride channel proteins, catalogued into the Chloride channel accessory (CLCA) family. 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 bovine EClC protein has 903 amino acids and four putative transmembrane segments. 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. Distant homologues may be present in plants, ciliates and bacteria, Synechocystis and Escherichia coli, so at least some domains within E-ClC family proteins have an ancient origin. Genes CLCA1, CLCA2, CLCA3, CLCA4 CLIC family The Chloride Intracellular Ion Channel (CLIC) Family (TC# 1.A.12) consists of six conserved proteins in humans (CLIC1, CLIC2, CLIC3, CLIC4, CLIC5, CLIC6). Members exist as both monomeric soluble proteins and integral membrane proteins where they function as chloride-selective ion channels. These proteins are thought to function in the regulation of the membrane potential and in transepithelial ion absorption and secretion in the kidney. They are a member of the glutathione S-transferase (GST) superfamily. Structure They possess one or two putative transmembrane α-helical segments (TMSs). The bovine p64 protein is 437 amino acyl residues in length and has the two putative TMSs at positions 223-239 and 367-385. The N- and C-termini are cytoplasmic, and the large central luminal loop may be glycosylated. The human nuclear protein (CLIC1 or NCC27) is much smaller (241 residues) and has only one putative TMS at positions 30-36. It is homologous to the second half of p64. Structural studies showed that in the soluble form, CLIC proteins adopt a GST fold with an active site exhibiting a conserved glutaredoxin monothiol motif, similar to the omega class GSTs. Al Khamici et al. demonstrated that CLIC proteins have glutaredoxin-like glutathione-dependent oxidoreductase enzymatic activity. CLICs 1, 2 and 4 demonstrate typical glutaredoxin-like activity using 2-hydroxyethyl disulfide as a substrate. This activity may regulate CLIC ion channel function. Transport reaction The generalized transport reaction believed to be catalyzed chloride channels is: Cl− (cytoplasm) → Cl− (intraorganellar space) CFTR CFTR is a chloride channel belonging to the superfamily of ABC transporters.