Chemotactic signaling by chemoreceptor teams in Escherichia coli

AMES P, REISER RH, STUDDERT CA AND PARKINSON JS

Salt Lake City, USA

Congreso; 102nd General Meeting of the American Association of Microbiology; 2002

American Society of Microbiology

Chemoreceptors of the MCP (methyl-accepting chemotaxis protein) family form clusters, typically at the cell pole(s), in a variety of bacteria and archaea. To elucidate the signaling role of receptor clustering, we looked for genetic and physical interactions between the serine (Tsr) and aspartate (Tar) chemoreceptors in E. coli. We first constructed a series of mutations at each of the six hydrophobic and five polar residues previously implicated in Tsr "trimer of dimers" formation. Each mutant Tsr protein was expressed in strains lacking other MCPs and tested for expression level and stability, Tsr function, effect on steady-state flagellar rotation, and ability to cluster. Mutant receptors were then tested for ability to interfere with signaling by wild-type Tar (epistasis). Proline (P) replacement mutants showed no receptor clustering or CW flagellar rotation, consistent with null defects in the helical trimer contact segments. In contrast, receptors with alanine (A) or tryptophan (W) replacements formed clusters and many also exhibited CW flagellar rotation, although they no longer mediated serine chemotaxis. Four of the A mutants recovered Tsr function in the presence of wild-type Tar receptors, whereas all of the W mutants blocked Tar function. These rescuable and epistatic phenotypes are consistent with functional interaction of Tsr and Tar dimers in higher order signaling teams. The bulky side chain in W mutants evidently spoils team function, whereas the small side chain in A mutants does not. Two predictions of this team model were tested and confirmed. (i) We found that the epistatic effects of W mutants could be alleviated by compensatory mutations in the trimer contact segments of Tar. The Tar suppressors acted in allele-specific fashion, consistent with a conformational correction mechanism. (ii) We found that Tsr molecules could be chemically crosslinked to Tar molecules in cells expressing the wild-type forms of each receptor. Crosslinking was abolished in a P mutant of Tsr, but not in W mutants. These findings indicate that bacterial chemoreceptors signal in teams that can contain members of different MCP types