Aquaporins.

Aquaporins (AQP) are members of the major intrinsic protein superfamily of integral membrane proteins, which function as specialized water channels to facilitate the passage of water through the cell membrane in animals, plants and bacterias. Ten AQP homologues named from 0 to 9 have been clones so far in mammals. They are widely distributed and more than one AQP could be present in the same cell. Most of the AQPs are only permeable to water and impermeable to small organic and inorganic molecules, except for AQP 3, 7 and 9 which are also permeable to urea and glycerol. From the hydrophobicity profile all AQPs seem to have six transmembrane domains with five connecting loops and with the amino and carboxyl termini in the cytoplasm. They are synthesized as monomers, but there is evidence suggesting that AQPs are formed in the membrane as tetrameric units, each of which has four water pores. The primary amino acid sequence contains putative phosphorylation sites for protein kinasses A and/or C or casein kinase II, and the expression and membrane protein abundance of some AQPs are known to be under hormonal regulation. The human genes for several AQPs have been cloned and an increasing number of disturbances associated to abnormal functioning of these proteins had been identified.