Aquaporins are a class of membrane channel proteins found in both prokaryotes and eukaryotes which allow the permeation of certain small molecules (typically water and glycerol), while excluding other small molecules and charged species (such as protons). They therefore allow the electrochemical gradient across the membrane to be maintained while permitting the rapid movement of small molecules. The presence of aquaporins explains the properties of membranes such as those found in the mammalian kidney and red blood cells, where the rate of movement of water is far higher than could be explained by simple diffusion across a lipid bilayer. Since their discovery in red blood cells, aquaporins have been found in many different mammalian tissues, and - along with the functionally distinct subgroup, the aquaglyceroporins - they are involved in many different physiological processes, and have been implicated in the pathophysiology of a wide range of clinical disorders.
Over 10 mammalian aquaporin homologs are known, in addition to many in plants, bacteria, yeast, and other organisms. These have been found in locations as diverse as-
- the lens of the eye (AQP0 or MIP), where mutations lead to cataract formation
- the brain and cerebrospinal fluid filled cavities (AQP4)
- the salivary glands (AQP5)
- the kidneys (AQP2, 3 and 4)
- the rare Co blood group antigen, which is caused by a non-functional mutant form of AQP1.
The current structural model for aquaporins consists of 6 transmembrane alpha helices arranged in a bundle with two hemipore regions which only half span the bilayer. For more details, a quick search through the PDB should turn up something. Interesting structure related features include selectivity for glycerol or water, and kinking of the water channel to prevent protons being leaked through the channel by the transfer of hydrogen bonds.