Sub-Ångstrom resolution x-ray structure details aquaporin-water interactions

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/arginine selectivity filter (SF). Here we present the crystal structure of a yeast aquaporin at 0.88 Å resolution. We visualize the H-bond donor interactions of the dual NPA motif 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.