1. Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, Massachusetts 02115, USA
2. Howard Hughes Medical Institute and Children's Hospital Laboratory of Molecular Medicine, 320 Longwood Avenue, and Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, 240 Longwood Avenue, Boston, Massachusetts 02115, USA
3. School of Biological Sciences, University of Auckland, Auckland PO Box 92019, New Zealand

Correspondence to: Thomas Walz¹ Correspondence and requests for materials should be addressed to T.W. (Email: twalz@hms.harvard.edu).

The sequence for sheep AQP0 has been deposited in GenBank (accession number AY573927). Coordinates and structure factors have been deposited in the Protein Data Bank (accession code 1SOR).

Abstract

The lens-specific water pore aquaporin-0 (AQP0) is the only aquaporin known to form membrane junctions in vivo¹. We show here that AQP0 from the lens core, containing some carboxy-terminally cleaved AQP0^{2, 3}, forms double-layered crystals that recapitulate in vivo junctions. We present the structure of the AQP0 membrane junction as determined by electron crystallography. The junction is formed by three localized interactions between AQP0 molecules in adjoining membranes, mainly mediated by proline residues conserved in AQP0s from different species but not present in most other aquaporins. Whereas all previously determined aquaporin structures show the pore in an open conformation^{4, 5, 6, 7, 8, 9}, the water pore is closed in AQP0 junctions. The water pathway in AQP0 also contains an additional pore constriction, not seen in other known aquaporin structures^{4, 5, 6, 7, 8, 9}, which may be responsible for pore gating.

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