InterPro domain: IPR034294

Aquaporins are water channels, present in both higher and lower organisms, that belong to the major intrinsic protein family. Most aquaporins are highly selective for water, though some also facilitate the movement of small uncharged molecules such as glycerol [ 1 ]. In higher eukaryotes these proteins play diverse roles in the maintenance of water homeostasis, indicating that membrane water permeability can be regulated independently of solute permeability. In microorganisms however, many of which do not contain aquaporins, they do not appear to play such a broad role. Instead, they assist specific microbial lifestyles within the environment, e.g. they confer protection against freeze-thaw stress and may help maintain water permeability at low temperatures [ 2 ]. The regulation of aquaporins is complex, including transcriptional, post-translational, protein-trafficking and channel-gating mechanisms that are frequently distinct for each family member.

Structural studies show that aquaporins are present in the membrane as tetramers, though each monomer contains its own channel [ 3 , 4 , 5 ]. The monomer has an overall "hourglass" structure made up of three structural elements: an external vestibule, an internal vestibule, and an extended pore which connects the two vestibules. Substrate selectivity is conferred by two mechanisms. Firstly, the diameter of the pore physically limits the size of molecules that can pass through the channel. Secondly, specific amino acids within the molecule regulate the preference for hydrophobic or hydrophilic substrates.

Aquaporins are classified into two subgroups: the aquaporins (also known as orthodox aquaporins), which transport only water, and the aquaglyceroporins, which transport glycerol, urea, and other small solutes in addition to water [ 6 , 7 ].

This entry represents the orthodox aquaporins.

1. From structure to disease: the evolving tale of aquaporin biology. Nat. Rev. Mol. Cell Biol. 5, 687-98
2. Why do microorganisms have aquaporins? Trends Microbiol. 14, 78-85
3. Structural basis of water-specific transport through the AQP1 water channel. Nature 414, 872-8
4. The channel architecture of aquaporin 0 at a 2.2-A resolution. Proc. Natl. Acad. Sci. U.S.A. 101, 14045-50
5. Architecture and selectivity in aquaporins: 2.5 a X-ray structure of aquaporin Z. PLoS Biol. 1, E72
6. Cellular and molecular biology of the aquaporin water channels. Annu. Rev. Biochem. 68, 425-58
7. Aquaporin-11: a channel protein lacking apparent transport function expressed in brain. BMC Biochem. 7, 14