Department of Microbiology and Immunology, University of Illinois, Chicago 60680.
We have demonstrated that a complex of the proteins CheA (CheAL and CheAS) and CheW can be isolated and constitutes a functional unit that responds to the signaling state of the chemoreceptors. The autophosphorylation rate of CheAL is much greater when CheAL and CheAS are complexed with CheW. Moreover, the presence of mutant chemoreceptors that cause cells to tumble increases this rate. At wild-type levels of expression, the isolated CheAL/CheAS/CheW complex accounts for about 10% of the total number of CheAL, CheAS, and CheW molecules and exists in a 1:1:1 stoichiometry. This complex is also required for CheAL/CheAS and CheW binding to the phosphorylation substrate, CheY. A separate interaction between CheY and another chemotaxis component, CheZ, was also detected. The CheY-CheZ interaction does not require participation of the CheAL/CheAS/CheW complex.