Abstract
Aquaporins/Aquaglyceroporins (PIPs, TIPs, NIPs, SIPs) are channels that facilitate the diffusion of water and small, uncharged solutes. We initially observed that TIP aquaporin homologs from Arabidopsis transport ammonia and urea in addition to water. In order to investigate if this is also a property of other members of this protein family, a strategy based on molecular simulation techniques, mutational and functional analyses was applied. Based on homology to the three-dimensional structures of several aquaporins solved by X-ray crystallography, models of PIP, TIP and NIPs were constructed. Conduction of various substrates was theoretically assayed using molecular dynamics simulations on equilibrated aquaporin monomers with a constant force vector applied to the substrates. The results suggested a major importance of the putative "selectivity filter" region, and this was experimentally confirmed by exchange of corresponding residues and experimental test of solute transport. In addition, further key residues outside the selectivity filter region determine selectivity.
