Syn. water channels.

Water crosses cell membranes by two routes: by diffusion through the lipid bilayer and through water channels calledaquaporins. Functional characterization of the first aquaporin was reported in 1992, but water channels were suspectedto exist well before that time, because the osmotic permeability of some types of epithelial cells was much too large tobe accounted for by simple diffusion through the plasma membrane.

A single human aquaporin-1 channel facilitates water transport at a rate of roughly 3 billion water molecules per second.Such transport appears to be bidirectional, in accordance with the prevailing osmotic gradient.

The classical aquaporins transport solute-free water across cell membranes; they appear to be exclusive water channelsand do not permeate membranes to ions or other small molecules. Some aquaporins - known as aquaglyceroporins -transport water plus glycerol and a few other small molecules.

The Aquaporin Family

More than 10 different mammalian aquaporins have been identified to date, and additional members are suspected toexist. Closely related water channel proteins have been isolated from plants, insects and bacteria. Aquaporin-1 fromhuman red blood cells was the first to be discovered and is probably the best studied.

Hydrophobicity plots of their amino acid sequences predict that the aquaporins have six membrane-spanning segments,as depicted in the model of aquaporin-1 to the right.

Based on studies with aquaporin-1, it appears that aquaporins exist in the plasma membrane as homotetramers. Eachaquaporin monomer contains two hemi-pores, which fold together to form a water channel.