Aquaporins are a family of channel proteins that are embedded in cell lipid membranes and found in all kingdoms of life, from mammals to plants and bacteria. Aquaporins allow water molecules to pass across lipid membranes at a faster rate than would be possible by diffusion alone, while excluding most other molecules. The protein consists of four subunits, each with an hourglass-shaped channel lined with charged chemical groups that interact with polar water molecules so that they can pass through. There are several different kinds of aquaporins, and the specific chemical groups lining the channel can affect levels of permeability to water and potentially other molecules (for instance, some aquaporins are also permeable to glycerol).
For a particular aquaporin, water permeability can also be affected by hormones, pH, and other chemical signals within and outside the cell. For example, aquaporins that play a role in water balance in mammalian kidneys are regulated by the hormone vasopressin.Edit Summary
“The high water permeability characteristic of mammalian red cell membranes is now known to be caused by the protein AQP1. This channel freely permits movement of water across the cell membrane, but it is not permeated by other small, uncharged molecules or charged solutes. AQP1 is a tetramer with each subunit containing an aqueous pore likened to an hourglass formed by obversely arranged tandem repeats. Cryoelectron microscopy of reconstituted AQP1 membrane crystals has revealed the three-dimensional structure at 3–6 Å. AQP1 is distributed in apical and basolateral membranes of renal proximal tubules and descending thin limbs as well as capillary endothelia. Ten mammalian aquaporins have been identified in water-permeable tissues and fall into two groupings. Orthodox aquaporins are water selective and include AQP2, a vasopressin-regulated water channel in renal collecting duct, in addition to AQP0, AQP4, and AQP5. Multifunctional aquaglyceroporins AQP3, AQP7, and AQP9 are permeated by water, glycerol, and some other solutes. Aquaporins are being defined in numerous other species including amphibia, insects, plants, and microbials. Members of the aquaporin family are implicated in numerous physiological processes as well as the pathophysiology of a wide range of clinical disorders.” (Borgnia et al. 1999:425)
“Because water is the major component of all living cells, the ability to absorb and release water must be considered a fundamental property of life. Cell membranes are exquisitely selective barriers that control the solute composition of the enclosed compartments by regulating the entry of ions; small, uncharged solutes; and water into cells. In addition, cells organized in epithelial tissues have apical and basolateral membranes, which constitute serial barriers that regulate the transepithelial movement of solutes and water, thereby contributing to the homeostasis of multicellular organisms…The recognized characteristics of membrane water channels led to chance identification of the first known water channel.” (Borgnia et al. 1999:426)