Four-helical-bundle structure of the cytoplasmic domain of a serine chemotaxis receptor

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Abstract

The bacterial chemotaxis receptors are transmembrane receptors with a simple signalling pathway which has elements relevant to the general understanding of signal recognition and transduction across membranes, how signals are relayed between molecules in a pathway, and how adaptation to a persistent signal is achieved1. In contrast to many mammalian receptors which signal by oligomerizing upon ligand binding2, the chemotaxis receptors are dimeric even in the absence of their ligands, and their signalling does not depend on a monomer–dimer equilibrium3. Bacterial chemotaxis receptors are composed of a ligand-binding domain, a transmembrane domain consisting of two helices TM1 and TM2, and a cytoplasmic domain. All known bacterial chemotaxis receptors have a highly conserved cytoplasmic domain, which unites signals from different ligand domains into a single signalling pathway to flagella motors. Here we report the crystal structure of the cytoplasmic domain of a serine chemotaxis receptor of Escherichia coli, which reveals a 200 å-long coiled-coil of two antiparallel helices connected by a ‘U-turn’. Two of these domains form a long, supercoiled, four-helical bundle in the cytoplasmic portion of the receptor.

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Figure 1: The transmembrane serine chemotaxis receptor of E.coli.
Figure 2: Two views of the cTsrQ dimer structure related by a 90-degree rotation around the non-crystallographic two-fold axis along the length of the molecules.
Figure 3: Typical regions of the solvent-flattened experimental electron density map (contoured at a 12σ level) superposed with the final structural model of cTsrQ.
Figure 4: Trimer of cTsr dimers.
Figure 5: Model of an intact Ecoli Tsr receptor dimer.

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Acknowledgements

We thank R. Sweet, P. Kuhn and H. Bellamy for data collection; D. King for performing the electrospray mass spectrometry; C. Park for plasmid HB915; K. Kamata for help with sample preparation; Z. Zhang and E. Berry for preparing some of the figures; and J. Falke and S. Parkinson for discussion. This work was supported by grants from the Office of Biological and Environmental Research, Office of Science, DOE and the NIH.

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Correspondence to Sung-Hou Kim.

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