Water Channels


Water can cross plasma membranes by diffusion through the lipid bilayer or by channel‐mediated water movement. Aquaporins are transmembrane proteins that are selectively permeable to water. These proteins are the first identified molecular water channels.

Keywords: permeability; oedema; water transport; membrane protein; kidney; lung

Figure 1.

The topology of aquaporin‐1. Shown schematically are the six bilayer‐spanning domains, with intracellular amino and carboxy termini. Three extracellular loops (A, C and E) and two intracellular loops (B and D) are formed; loops B and E fold into the lipid bilayer. The NPA motif is present in both the amino and carboxy terminal halves of the molecule. The front half (repeat‐1) and back half (repeat‐2) of the molecule are highly similar, but are oriented on opposite sides of the membrane. Also shown are the sites for glycosylation, mercury inhibition and PKA phosphorylation (proposed) for AQP1 or other aquaporins.

Figure 2.

The hourglass model of aquaporin‐1. A linear representation of the six bilayer‐spanning domains is shown in (a), with loops B and E folded into the membrane. The amino and carboxy termini fold together to generate the functional monomer; the aqueous pore is formed by the two NPA motifs (b).



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Further Reading

Chou CL, Ma T, Yang B, Knepper MA and Verkman AS (1998) Fourfold reduction of water permeability in inner medullary collecting duct of aquaporin‐4 knockout mice. American Journal of Physiology 274: C549–554.

Denker BM, Smith BL, Kuhajda FP and Agre P (1988) Identification, purification, and characterization of a novel Mr 28,000 integral membrane protein from erythrocytes and renal tubules. Journal of Biological Chemistry 263: 15634–15642.

Jung JS, Jung JS, Preston GM, Smith BL, Guggino WB and Agre P (1994) Molecular structure of the water channel through Aquaporin CHIP: the hourglass model. Journal of Biological Chemistry 269: 14648–14654.

Kaldenhoff R, Grote K, Zhu J‐J and Zimmerman U (1998) Significance of plasmalemma aquaporins for water transport in Arabidopsis thaliana. The Plant Journal 14: 121–128.

Matthay MA, Folkesson HG and Verkman AS (1996) Salt and water transport across the alveolar and distal airway epithelium in the adult lung. American Journal of Physiology 270: L487–503.

Nielsen S, King LS, Christensen BM and Agre P (1997) Aquaporins in complex tissues: II. Cellular and subcellular distribution in respiratory tract and glands of rat. American Journal of Physiology 273: C1549–1561.

Nielsen S, Chou CL, Marples D, Christensen EI, Kishore BK and Knepper MA (1995) Vasopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin‐CD water channels to plasma membrane. Proceedings of the National Academy of Sciences of the USA 92: 1013–1017.

Shiels A and Bassnett S (1996) Mutations in the founder of the MIP gene family underlie cataract development in the mouse. Nature Genetics 12: 212–215.

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King, Landon S, and Agre, Peter(Apr 2001) Water Channels. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0000655]