1. Department of Chemistry and Bioscience, Chalmers University of Technology, P O Box 462, SE-40530 Göteborg, Sweden
2. Theoretical and Computational Biophysics Group, Beckman Institute, University of Illinois at Urbana-Champaign, 405 N. Mathews Avenue, Urbana, Illinois 61801, USA
3. Department of Plant Biochemistry, Lund University, P O Box 124, SE-22100 Lund, Sweden

Correspondence to: Emad Tajkhorshid²Richard Neutze¹Per Kjellbom³ Correspondence and requests for materials should be addressed to R.N. (Email: richard.neutze@chembio.chalmers.se), P.K. (Email: per.kjellbom@plantbio.lu.se) or E.T. (Email: emad@ks.uiuc.edu). The coordinates and structure factor amplitudes for the closed and open structures have been deposited in the Protein Data Bank under the accession codes 1Z98 and 2B5F, respectively.

Abstract

Plants counteract fluctuations in water supply by regulating all aquaporins in the cell plasma membrane. Channel closure results either from the dephosphorylation of two conserved serine residues under conditions of drought stress, or from the protonation of a conserved histidine residue following a drop in cytoplasmic pH due to anoxia during flooding. Here we report the X-ray structure of the spinach plasma membrane aquaporin SoPIP2;1 in its closed conformation at 2.1 Å resolution and in its open conformation at 3.9 Å resolution, and molecular dynamics simulations of the initial events governing gating. In the closed conformation loop D caps the channel from the cytoplasm and thereby occludes the pore. In the open conformation loop D is displaced up to 16 Å and this movement opens a hydrophobic gate blocking the channel entrance from the cytoplasm. These results reveal a molecular gating mechanism which appears conserved throughout all plant plasma membrane aquaporins.

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