Mapping water’s path through a pore protein

When our bodies need to move water fast—say, to produce tears or to process liquid waste—they deploy aquaporins, special proteins that straddle a cell membrane and form water-permeable pores. In the two decades since aquaporins were discovered, x-ray crystallography and molecular dynamics simulations have laid bare much of the proteins’ inner workings; the numerous variants found in plants, animals, and bacteria are similarly tailored to block the passage of large solutes and ions. Because protons can hop freely along a chain of hydrogen-bonded water molecules, a water-filled pore should conduct protons in much the same way that a wire conducts electrons—but that doesn’t happen. Researchers suspect that an aquaporin prevents such transport by ensuring that no hydrogen-bonded network spans the entire pore. Now a collaboration led by Richard Neutze (University of Gothenburg, Sweden) and Emad Tajkhorshid (University of Illinois at Urbana-Champaign) has produced evidence in support of that argument. Using x-ray crystallography, the team determined the structure of yeast aquaporin with subangstrom resolution—precise enough to identify not only the pore’s atomic structure but also the hydrogen bonds formed by the water molecules inside. The crystallographic structure reveals how certain amino acids lining the pore manage to fracture water’s hydrogen-bonding network: They entice transiting water molecules (orange in the figure) to form hydrogen bonds with the pore wall instead of with one another. (U. K. Eriksson et al., Science 340, 1346, 2013 .)—Ashley G. Smart