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Subangstrom Resolution X-Ray Structure Details Aquaporin-Water Interactions

Choosing Water Aquaporins are proteins that facilitate transport of water across biological membranes. They must be selective for water, without binding it so tightly as to impede transport, and they must prevent proton transfer by rapid exchange between hydrogen-bonded water molecules. Kosinska Eriksson et al. (p. 1346; see the Perspective by Abramson and Vartanian) describe the subangstrom resolution structure of yeast aquaporin, which allows assignment of side-chain conformations and shows that the H-bond geometry of water molecules prevents proton conductance without compromising water transport. A really, really close-up view of an aquaporin hints at how water passes through but protons do not. [Also see Perspective by Abramson and Vartanian] Aquaporins are membrane channels that facilitate the flow of water across biological membranes. Two conserved regions are central for selective function: the dual asparagine-proline-alanine (NPA) aquaporin signature motif and the aromatic and arginine selectivity filter (SF). Here, we present the crystal structure of a yeast aquaporin at 0.88 angstrom resolution. We visualize the H-bond donor interactions of the NPA motif’s asparagine residues to passing water molecules; observe a polarized water-water H-bond configuration within the channel; assign the tautomeric states of the SF histidine and arginine residues; and observe four SF water positions too closely spaced to be simultaneously occupied. Strongly correlated movements break the connectivity of SF waters to other water molecules within the channel and prevent proton transport via a Grotthuss mechanism.