Two kidney water channels have been identified: CHIP28 in proximal tubule and thin descending limb, and WCH-CD in collecting duct apical membrane. An homologous cDNA (WCH3) was obtained from rat kidney and found to encode a 276 amino acid, 29 kDa protein with 39% amino acid identity to rat CHIP28, 50% to WCH-CD and 49% to MIP26. The WCH3 transcript of 2.5 kb was expressed exclusively in kidney and was upregulated in dehydrated rats. Cell-free translation produced an approximately 28 kDa protein. Analysis of the predicted amino acid sequence indicated a hydrophobic protein with 4-6 membrane-spanning domains, with one N-linked glycosylation site, two conserved NPA boxes common to MIP26 family proteins, and conserved residue C189 common to water channels. WCH3 is a new member of the MIP26 family of channel-forming proteins in mammalian kidney
