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Structural mechanism of plant aquaporin gating

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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Figure 1: Diagram illustrating the structural mechanism of aquaporin gating in plant plasma membranes.
Figure 2: Structures of the closed and open conformations of SoPIP2;1.
Figure 3: Characterizing the SoPIP2;1 channel.
Figure 4: Results of molecular dynamics simulations of the non-phosphorylated, phosphorylated, and induced open systems.
Figure 5: Electron density at the sites of regulation by phosphorylation and pH for SoPIP2;1 in its closed conformation.
Figure 6: Regulation of the cytoplasmic entrance into the SoPIP2;1 channel.

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Acknowledgements

We thank C. Larsson for useful discussions; the Pittsburgh Supercomputer Center and the National Center for Supercomputing Applications for providing computer time; and the European Synchrotron Radiation Facility and the Swiss Light Source for access to synchrotron radiation. Financial support was provided by Formas, the Research School of Pharmaceutical Sciences (FLÄK), Swegene, the Swedish Research Council (VR), the Swedish Strategic Research Foundation (SSF), the European Commission Integrated Projects EMEP and SPINE, the Chalmers Bioscience Programme and the NIH.

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Correspondence to Emad Tajkhorshid, Richard Neutze or Per Kjellbom.

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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. Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Notes

This file contains the Supplementary Methods, Supplementary Tables and Supplementary Figures 1–8. (DOC 9094 kb)

Supplementary Data 1

A pdb file for the non-phosphorylated molecular dynamics trajectory. (TXT 143 kb)

Supplementary Data 2

A pdb file for the phosphorylated molecular dynamics trajectory. (TXT 143 kb)

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Törnroth-Horsefield, S., Wang, Y., Hedfalk, K. et al. Structural mechanism of plant aquaporin gating. Nature 439, 688–694 (2006). https://doi.org/10.1038/nature04316

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