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Aquaporins: The Molecular Basis of Facilitated Water Movement Through Living Plant Cells?

Osmotic effects observed with living cells indicate that their plasma membranes are freely permeable to water while essentially creating a barrier to other molecules. The hydraulic conductivity of biological membranes is sometimes still ascribed to the simple diffusion of water molecules through the lipid bilayer. However, more than 30 years ago the idea was advanced that hydraulic water movement through living cells occurs by bulk flow of water through pores in the membrane (Sidel and Solomon, 1957). Certain membranes of animal cells are unusually permeable to water and there is now a substantial body of evidence for the existence of water transport channels in such membranes (reviewed by Finkelstein, 1987; Verkman, 1992). For example, membranes from red blood cells and renal proximal tubules are exceptionally permeable to water; the membranes of the convoluted dista1 tubules of the kidney are also highly water permeable, and water permeability can be modulated by hormones. The hydraulic conductivity of plant cells has been thoroughly investigated and numerous reviews on this subject have been published (for a recent review, see Steudle, 1992). Many observations on plants and animals support the recent discovery of proteins that form water channels (Preston et al., 1992; Fushimi et al., 1993; Maurel et al., 1993). We proposed to cal1 such proteins "aquaporins" (Agre et al., 1993). These water channel proteins belong to the MIP family, an ancient family of membrane proteins (see Reizer et al., 1993, for review). Aquaporins form water-selective channels, allowing water to pass freely while excluding ions and metabolites. In plants, aquaporins have been demonstrated in the tonoplast (vacuolar membrane) (Hofte et al., 1992), but they also may be present in the plasma membrane (Kammerloher and Schaffner, 1993). In animal cells, aquaporins are found in the plasma membranes of specific cell types. It is important to note that such channels permit or facilitate the movement of water through membranes and do not act as pumps. The driving forces behind water movement are hydraulic or osmotic in nature. .