Lens-specific aquaporin-0 (AQP0) functions as a specific water pore and forms the thin junctions between fibre cells. Here we describe a 1.9 Å resolution structure of junctional AQP0, determined by electron crystallography of double-layered two-dimensional crystals. Comparison of junctional and non-junctional AQP0 structures shows that junction formation depends on a conformational switch in an extracellular loop, which may result from cleavage of the cytoplasmic amino and carboxy termini. In the centre of the water pathway, the closed pore in junctional AQP0 retains only three water molecules, which are too widely spaced to form hydrogen bonds with each other. Packing interactions between AQP0 tetramers in the crystalline array are mediated by lipid molecules, which assume preferred conformations. We were therefore able to build an atomic model for the lipid bilayer surrounding the AQP0 tetramers, and we describe lipid–protein interactions.
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This work was supported by NIH funding (to T.W.) and a Grant-in Aid for Specially Promoted Research (to Y.F.).
Coordinates and structure factors for junctional and non-junctional AQP0 have been deposited in the Protein Data Bank (accession codes 2B6O and 2B6P, respectively). Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Lipid-protein and lipid-lipid interactions in the AQP0 junction, together with legends for supplementary figures. (DOC 26 kb)
Electron diffraction pattern of a double-layered AQP0 2D crystal tilted to 60°. (PDF 465 kb)
Protein packing in 3D and 2D crystals of AQP0. (PDF 226 kb)
Residues in loop A of AQP0 involved in junction formation. (PDF 392 kb)
The constricted water pore in junctional AQP0. (PDF 653 kb)
The water pore in AQP0. (PDF 95 kb)
Stereo view of the nine lipids surrounding an AQP0 monomer in the 2D crystals. (PDF 207 kb)
Lipids surrounding the AQP0 tetramer mediate the crystal contacts. (PDF 521 kb)
Crystallographic statistics of non-junctional AQP0 (X-ray). (DOC 20 kb)
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Gonen, T., Cheng, Y., Sliz, P. et al. Lipid–protein interactions in double-layered two-dimensional AQP0 crystals. Nature 438, 633–638 (2005). https://doi.org/10.1038/nature04321
Nature Methods (2022)
Journal of Biomolecular NMR (2020)