Family 1.A.11 - The Chloride Channel Family        

Family ID: 52600

The ClC family is a large family consisting of dozens of sequenced proteins derived from Gram-negative and Gram-positive bacteria, cyanobacteria, archaea, yeast, plants and animals. These proteins are essentially ubiquitous, although they are not encoded within genomes of Haemophilus influenzae, Mycoplasma genitalium, and Mycoplasma pneumoniae. Sequenced proteins vary in size from 395 amino acyl residues (M. jannaschii) to 988 residues (man). Several organisms contain multiple ClC family paralogues. For example, Synechocystis has two paralogues, one of 451 residues in length and the other of 899 residues. Arabidopsis thaliana has at least four sequenced paralogues, (775-792 residues), humans also have at least five paralogues (820-988 residues), and C. elegans also has at least five (810-950 residues). There are nine known members in mammals, and mutations in three of the corresponding genes cause human diseases. E. coli, Methanococcus jannaschii and Saccharomyces cerevisiae only have one ClC family member each. With the exception of the larger Synechocystis paralogue, all bacterial proteins are small (395-492 residues) while all eukaryotic proteins are larger (687-988 residues). These proteins exhibit 10-12 putative transmembrane a-helical spanners (TMSs) and appear to be present in the membrane as homodimers. While one member of the family, Torpedo ClC-O, has been reported to have two channels, one per subunit, others are believed to have just one.

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., ClC4 and ClC5 share a NO3- > Cl- > Br- > I- conductance sequence, while ClC3 has an I- > Cl- selectivity. The ClC4 and ClC5 channels and others exhibit outward rectifying currents with currents only at voltages more positive than +20mV.