Aquaporin: A water channel. Aquaporins form pores in the membranes of cells and selectively conduct water molecules through the membrane, while preventing the passage of ions (such as sodium and potassium) and other small molecules.
Aquaporins are typically composed of identical subunit proteins. Water molecules traverse the narrowest portion of the water channel in single file. The presence of water channels increases the permeability of membranes to water by as much as ten-fold. Aquaporins prevent us from dying of dehydration by reabsorbing 99% of the water in the kidney.
The existence of molecular water channels had been long suspected. However, their molecular identity remained unknown until the serendipitous discovery of aquaporin-1 in 1988. The protein known as AQP1 was first purified from red blood cell membranes.
A number of different aquaporins have now been identified. They have been designated aquaporin-0, aquaporin-1, aquaporin-2, and so on while the corresponding aquaporin genes are symbolized AQP0, AQP1, and AQP-2. The results of mutations in the AQP genes differ. Mutation in AQP-0 causes cataracts. Mutations in AQP1 have been found in normal people. Mutations in the aquaporin-2 gene AQP2 cause an autosomal dominant form of nephrogenic diabetes insipidus.
The 2003 Nobel Prize in Chemistry was shared by Peter Agre from Johns Hopkins University "for the discovery of water channels." The other half of the prize went to Roderick MacKinnon from Rockefeller University "for structural and mechanistic studies of ion channels."