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Bacterial chemoreceptors undergo conformational changes in response to variations in the concentration of extracellular ligands. These changes in chemoreceptor structure initiate a series of signaling events that ultimately result in regulation of rotation of the flagellar motor. Here we have used cryo-electron tomography combined with 3D averaging to determine the in situ structure of chemoreceptor assemblies in Escherichia coli cells that have been engineered to overproduce the serine chemoreceptor Tsr. We demonstrate that chemoreceptors are organized as trimers of receptor dimers and display two distinct conformations that differ principally in arrangement of the HAMP domains within each trimer. Ligand binding and methylation alter the distribution of chemoreceptors between the two conformations, with serine binding favoring the "expanded" conformation and chemoreceptor methylation favoring the "compact" conformation. The distinct positions of chemoreceptor HAMP domains within the context of a trimeric unit are thus likely to represent important aspects of chemoreceptor structural changes relevant to chemotaxis signaling. Based on these results, we propose that the compact and expanded conformations represent the "kinase-on" and "kinase-off" states of chemoreceptor trimers, respectively.

Original publication

DOI

10.1073/pnas.0806401105

Type

Journal article

Journal

Proc Natl Acad Sci U S A

Publication Date

28/10/2008

Volume

105

Pages

16555 - 16560

Keywords

Bacterial Proteins, Chemoreceptor Cells, Chemotaxis, Escherichia coli, Escherichia coli Proteins, Ligands, Membrane Proteins, Methyl-Accepting Chemotaxis Proteins, Methylation, Multiprotein Complexes, Phosphotransferases, Protein Binding, Protein Structure, Tertiary, Signal Transduction