A water channel of the nematode C-elegans and its implications for channelselectivity of MIP proteins

A genome project focusing on the nematode Caenorhabditis elegans has demons trated the presence of eight cDNAs belonging to the major intrinsic protein superfamily. We functionally characterized one of these cDNAs named C01G6. 1. Injection of C01G6.1 cRNA increased the osmotic water permeability (P-f) of Xenopus oocytes 11-fold and the urea permeability 4.5-fold but failed t o increase the glycerol permeability. It has been speculated that the MIP f amily may be separated into two large subfamilies based on the presence or absence of two segments of extra amino acid residues (similar to 15 amino a cids) at the second and third extracellular loops. Because C01G6.1 (designa ted AQP-CE1), AQP3, and glycerol facilitator (GlpF) all have these two segm ents, we replaced the segments of AQP-CE1 with those of AQP3 and GlpF to id entify their roles. The functional characteristics of these mutants were pr incipally Similar to that of wild-type AQP-CE1, although the values of Pc a nd urea permeability were decreased by 39-74% and 28-65%, respectively Thes e results suggest that the two segments of extra amino acid residues may no t contribute to channel selectivity or formation of the route for small sol utes.