Recent advances in water transport.

Complex organisms regulate the osmolalities of their compartments by limiting water flow across some membranes and promoting rapid water flow across others via proteins called aquaporins. Barrier epithelia limit water flow by reducing the mobilities of fatty acid chains in their apical membranes, especially in the outer leaflets of these membranes. Aquaporins are 28 to 30 kDa, 6 membrane-spanning proteins that are expressed in a wide variety of organisms from bacteria to plants to mammals. The structural and biophysical data are summarized to develop our best understanding of water pore function. In addition, the regulation of trafficking of AQP 2 into and out of the apical membranes of collecting duct principal cells is described.