Lipid bilayers, such as those of the plasma membrane of plant and animal cells, have limited water permeability. Aquaporins — water-selective, membrane-spanning channel proteins — are highly specific for water and must reject both solutes and proton flow. The aquaporin water channel family is found throughout nature and this particular aquaporin, AQP1, from human red cells is the first aquaporin for which a 3D structure has been published.

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Membrane helices: H1 (residues 8-36), purple; H2 (49-66), violet; H3 (95-115); H4 (137-155), green; H5 (167-183), yellow; H6 (208-228), red. Pore helices: HB (77-84), blue; HE (193-200), orange. Hydrophobic residues on helices H1, H3, and H4 embed the protein into the lipid membrane.

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Monomers are likely held together tightly by hydrogen bonding. (Hydrogen positions unresolved by x-ray techniques.)

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The central axis of the tetramer is lined with hydrophobic residues and is likely filled with a hydrophobic molecule instead of conducting water. Instead, each monomer contains a pore down its length, composed of loops B and E, H2 and H5, along with parts of H1 and H4. The pore size at the narrowest point is 3 Å in diameter, a close fit to the 2.8 Å diameter of water (approximately equal to the size of oxygen as drawn here.) Water permeability is ~3×109 molecules per monomer per second.

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We, the curators, note with pleasure that on 8 October 2003, The Royal Swedish Academy of Sciences announced award of the 2003 Nobel Prize in Chemistry to Peter Agre, author of the paper below describing the first 3D structure of aquaporin and represented here in the Virtual Museum of Minerals and Molecules since 2000, “for discoveries concerning channels in cell membranes”, particularly “for the discovery of water channels” Sharing this prize is Roderick MacKinnon “for structural and mechanistic studies of ion channels” specifically, Kcsa K transport channel.

K Murata, K Mitsuoka, T Hirai, T Walz, P Agre, JB Heymann, A Engel, Y Fujiyoshi. 2000. Structural determinants of water permeation through aquaporin-1. Nature 407:599-605. [quaternary structure obtained from European Bioinformatics Institute based on the authors’ symmetry operations stated in the file header of PDB ID no. 1FQY]