Ligand-gated ion channels (LGICs) mediate excitatory and inhibitory transmission in the nervous system. Among them, the pentameric or ‘Cys-loop’ receptors (pLGICs) compose a family that until recently was found in only eukaryotes. Yet a recent genome search identified putative homologues of these proteins in several bacterial species1. Here we report the cloning, expression and functional identification of one of these putative homologues from the cyanobacterium Gloeobacter violaceus. It was expressed as a homo-oligomer in HEK 293 cells and Xenopus oocytes, generating a transmembrane cationic channel that is opened by extracellular protons and shows slow kinetics of activation, no desensitization and a single channel conductance of 8 pS. Electron microscopy and cross-linking experiments of the protein fused to the maltose-binding protein and expressed in Escherichia coli are consistent with a homo-pentameric organization. Sequence comparison shows that it possesses a compact structure, with the absence of the amino-terminal helix, the canonical disulphide bridge and the large cytoplasmic domain found in eukaryotic pLGICs. Therefore it embodies a minimal structure required for signal transduction. These data establish the prokaryotic origin of the family. Because Gloeobacter violaceus carries out photosynthesis and proton transport at the cytoplasmic membrane2, this new proton-gated ion channel might contribute to adaptation to pH change.
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We wish to thank N. Le Novère, J. L. Popot, P. Delepelaire and C. Beloin for useful assistance, and S. Edelstein for critical reading. This work was supported by the Région Ile de France, the Association Française contre les Myopathies, the Collège de France, the Commission of the European Communities (CEC) and the Association pour la Recherche sur le Cancer.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
This file contains Supplementary Methods, and two Supplementary Figures analysing the ionic permeability of the Glvi ion channel, by whole-cell patch clamp recording of Glvi-transfected HEK cells. The Supplementary Figures present the reversal potential of pH5-elicited currents in different external NaCl and proton concentrations. (PDF 268 kb)
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Bocquet, N., Prado de Carvalho, L., Cartaud, J. et al. A prokaryotic proton-gated ion channel from the nicotinic acetylcholine receptor family. Nature 445, 116–119 (2007). https://doi.org/10.1038/nature05371
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