Cellular and molecular biology of the aquaporin water channels

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 othe r small, uncharged molecules or charged solutes. AQP1 is a tetramer with ea ch subunit containing an aqueous pore likened to an hourglass formed by obv ersely arranged tandem repeats. Cryoelectron microscopy of reconstituted AQ P1 membrane crystals has revealed the three-dimensional structure at 3-6 An gstrom. AQP1 is distributed in apical and basolateral membranes of renal pr oximal tubules and descending thin limbs as well as capillary endothelia. T en mammalian aquaporins have been identified in water-permeable tissues and fall into two groupings. Orthodox aquaporins are water-selective and inclu de 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. Aq uaporins are being defined in numerous other species including amphibia, in sects, plants, and microbials. Members of the aquaporin family are implicat ed in numerous physiological processes as well as the pathophysiology of a wide range of clinical disorders.