The α-proteobacterium Sinorhizobium meliloti can live freely in the soil or engage in a symbiosis with its legume host. S. meliloti facilitates nitrogen fixation in root nodules, thus providing pivotal, utilizable nitrogen to the host. The organism has eight chemoreceptors, namely McpT to McpZ and IcpA that facilitate chemotaxis. McpX is the first known bacterial sensor of quaternary ammonium compounds (QACs) such as choline and betaines. Because QACs are exuded at chemotaxis-relevant concentrations by germinating alfalfa seeds, McpX has been proposed to contribute to host-specific chemotaxis. We have determined the crystal structure of the McpX periplasmic region (McpXPR) in complex with the proline betaine at 2.7 Å resolution. In the crystal, the protein forms a symmetric dimer with one proline betaine molecule bound to each monomer of McpXPR within membrane-distal CACHE module. The ligand is bound through cation–πinteractions with four aromatic amino acid residues. Mutational analysis in conjunction with binding studies revealed that a conserved aspartate residue is pivotal for ligand binding. We discovered that, in a striking example of convergent evolution, the ligand-binding site of McpXPR resembles that of a group of structurally unrelated betaine-binding proteins including ProX and OpuAC. Through this comparison and docking studies, we rationalized the specificity of McpXPR for this specific group of ligands. Collectively, our structural, biochemical, and molecular docking data have revealed the molecular determinants in McpX that are crucial for its rare ligand specificity for QACs.
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December 2018
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Cover Image
In this issue, Arnal et al. investigate the molecular basis of substrate regiospecificity and processivity of an exemplar bacterial Glycoside Hydrolase Family 74 endo-xyloglucanase. The cover image shows a protein structure with a uniquely large oligosaccharide complex. The enzyme was taken from a wheat rhizome bacterium. The image was supplied by Harry Brumer and Peter Stogios.
Research Article|
December 14 2018
Structure of the sensory domain of McpX from Sinorhizobium meliloti, the first known bacterial chemotactic sensor for quaternary ammonium compounds
Manisha Shrestha;
Manisha Shrestha
1Department of Biological Sciences, Virginia Tech, Derring Hall, Blacksburg, VA 24061, U.S.A.
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Karl K. Compton;
Karl K. Compton
1Department of Biological Sciences, Virginia Tech, Derring Hall, Blacksburg, VA 24061, U.S.A.
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Jordan M. Mancl;
Jordan M. Mancl
1Department of Biological Sciences, Virginia Tech, Derring Hall, Blacksburg, VA 24061, U.S.A.
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Benjamin A. Webb;
Benjamin A. Webb
1Department of Biological Sciences, Virginia Tech, Derring Hall, Blacksburg, VA 24061, U.S.A.
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Anne M. Brown;
Anne M. Brown
2Department of Biochemistry, Virginia Tech, Engel Hall, Blacksburg, VA 24061, U.S.A.
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Birgit E. Scharf;
1Department of Biological Sciences, Virginia Tech, Derring Hall, Blacksburg, VA 24061, U.S.A.
Correspondence: Birgit E. Scharf (bscharf@vt.edu) or Florian D. Schubot (fschubot@vt.edu)
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Florian D. Schubot
1Department of Biological Sciences, Virginia Tech, Derring Hall, Blacksburg, VA 24061, U.S.A.
Correspondence: Birgit E. Scharf (bscharf@vt.edu) or Florian D. Schubot (fschubot@vt.edu)
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Publisher: Portland Press Ltd
Received:
September 21 2018
Revision Received:
November 07 2018
Accepted:
November 14 2018
Accepted Manuscript online:
November 15 2018
Online ISSN: 1470-8728
Print ISSN: 0264-6021
© 2018 The Author(s). Published by Portland Press Limited on behalf of the Biochemical Society
2018
Biochem J (2018) 475 (24): 3949–3962.
Article history
Received:
September 21 2018
Revision Received:
November 07 2018
Accepted:
November 14 2018
Accepted Manuscript online:
November 15 2018
Citation
Manisha Shrestha, Karl K. Compton, Jordan M. Mancl, Benjamin A. Webb, Anne M. Brown, Birgit E. Scharf, Florian D. Schubot; Structure of the sensory domain of McpX from Sinorhizobium meliloti, the first known bacterial chemotactic sensor for quaternary ammonium compounds. Biochem J 21 December 2018; 475 (24): 3949–3962. doi: https://doi.org/10.1042/BCJ20180769
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