Polar clustering of the chemoreceptor complex in Escherichia coli occurs in the absence of complete CheA function

Bacterial chemotaxis requires a phosphorelay system initiated by the intera ction of a ligand with its chemoreceptor and culminating in a change in the directional bias of flagellar rotation. Chemoreceptor-CheA-CheW ternary co mplexes mediate transduction of the chemotactic signal. In vivo, these comp lexes cluster predominantly in large groups at the cell poles. The function of chemoreceptor clustering is currently unknown. To gain insight into the relationship between signaling and chemoreceptor clustering, we examined t hese properties in several Escherichia coil mutant strains that produce Che A variants altered in their ability to mediate chemotaxis, autophosphorylat e, or bind ATP. We show here that polar clustering of chemoreceptor complex es does not require functional CheA protein, although maximal clustering oc curred only in chemotactically competent cells. Surprisingly, in cells cont aining a minimum of 13 gold particles at the cell pole, a significant level of clustering was observed in the absence of CheA, demonstrating that CheA is not absolutely essential for chemoreceptor clustering. Nonchemotactic c ells expressing only CheA(S), a C-terminal CheA deletion, or CheA bearing a mutation in the ATP-binding site mediated slightly less than maximal chemo receptor clustering. Cells expressing only full-length CheA (CheA(L)) from either a chromosomal or a plasmid-encoded allele displayed a methyl-accepti ng chemotaxis protein localization pattern indistinguishable from that of s trains carrying both CheA(L) and CheA(S), demonstrating that CheA(L) alone can mediate polar clustering.