In bacteria, archaea, fungi and plants the Trk, Ktr and HKT ion transporters are key components of osmotic regulation, pH homeostasis and resistance to drought and high salinity. These ion transporters are functionally diverse: they can function as Na+ or K+ channels and possibly as cation/K+ symporters. They are closely related to potassium channels both at the level of the membrane protein and at the level of the cytosolic regulatory domains. Here we describe the crystal structure of a Ktr K+ transporter, the KtrAB complex from Bacillus subtilis. The structure shows the dimeric membrane protein KtrB assembled with a cytosolic octameric KtrA ring bound to ATP, an activating ligand. A comparison between the structure of KtrAB–ATP and the structures of the isolated full-length KtrA protein with ATP or ADP reveals a ligand-dependent conformational change in the octameric ring, raising new ideas about the mechanism of activation in these transporters.
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We are grateful for access to ID14-1/ID14-4/ID-29 at ESRF (through the Portuguese BAG), PXII at SLS, XRD1 at ELETTRA and PROXIMA1 at SOLEIL and thank the respective support staff. A.S. was supported by FEBS (Long term fellowship). This work was funded by EMBO (Installation grant), by FEDER funds through the Operational Competitiveness Program–COMPETE and by National Funds through FCT–Fundação para a Ciência e a Tecnologia under the projects FCOMP-01-0124-FEDER-022718 (PEst-C/SAU/LA0002/2011), FCOMP-01-0124-FEDER-009028 (PTDC/BIA-PRO/099861/2008) and FCOMP-01-0124-FEDER-010781 (PTDC/QUI-BIQ/105342/2008). We also thank G. Gabant and M. Cadene at the ‘Plateforme de Spectrometrie de Masse’ at CBM, CNRS, Orleans for mass spectrometry analysis, and C. Harley for critical reading of the manuscript.
The authors declare no competing financial interests.
This file contains Supplementary Figures 1-10, which illustrates different aspects of the structural and biochemical characterization of the KtrAB potassium transporter, Supplementary Table 1 showing statistics for diffraction data and crystallographic refinement for the KtrAB, KtrA-ATP and KtrA-ADP structures, and a Supplementary Discussion containing detailed information about the contact regions established between the KtrB homodimer and KtrA ring, as well as discussion of the activation models presented in the main text. (PDF 27413 kb)
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Vieira-Pires, R., Szollosi, A. & Morais-Cabral, J. The structure of the KtrAB potassium transporter. Nature 496, 323–328 (2013). https://doi.org/10.1038/nature12055
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