How many aquaporins have been discovered

Water crosses cell membranes by two routes: by diffusion through the lipid bilayer and through water channels called aquaporins.

Functional characterization of the first aquaporin was reported in , but water channels were suspected to exist well before that time, because the osmotic permeability of some types of epithelial cells was much too large to be 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 channels and 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. More than 10 different mammalian aquaporins have been identified to date, and additional members are suspected to exist. Closely related water channel proteins have been isolated from plants, insects and bacteria. Aquaporin-1 from human 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.

Each aquaporin monomer contains two hemi-pores, which fold together to form a water channel. Different aquaporins have different patterns of glycosylation. Such water movement not only regulates the activity of individual cells but also is responsible for the functioning of many organ systems and for maintaining whole body water balance.

It had long been suspected that water movement across biological cell membranes was in some way enhanced or facilitated by pores or channels, but the search to identify these channels was long and tedious. As is often the case in science, the secret of the water channel was eventually discovered by chance in by Peter Agre and his colleagues at the Johns Hopkins University in Baltimore, who were working on red blood cell membrane proteins.

This "first" water channel was originally named CHIP28 and is now known as aquaporin 1. Agre received the Nobel Prize in Chemistry in for this discovery. There are currently 13 known aquaporins in mammals, distributed in most tissues, but many more have been identified in lower organisms and in the plant kingdom.