THE SHORT-FORM OF CHEA COUPLES CHEMORECEPTION TO CHEA PHOSPHORYLATION

Escherichia coli cells express two forms of the chemotaxis-associated CheA protein, CheA(L) and CheA(S), as the result of translational init iation at two distinct in-frame initiation sites in the gene cheA. The long form, CheA(L), plays a crucial role in chemotactic signal transd uction. As a histidine protein kinase, it first autophosphorylates at amino acid His-48; then, it phosphorylates two other chemotaxis protei ns, CheY and CheB. The short form, CheA(S), lacks the amino-terminal 9 7 amino acids of CheA(L) and, therefore, does not contain the site of autophosphorylation. However, it does retain a functional kinase domai n. As a consequence, CheA(S) can mediate transphosphorylation of kinas e-deficient CheA(L) variants, Here we demonstrate in vitro that CheA(S ) also can mediate transphosphorylation of a CheA(L) variant that lack s the C-terminal segment, a portion of the protein which is thought to interact with CheW and the chemoreceptors. The presence of CheW and t he chemoreceptor Tsr enhances this activity and results in modulation of the transphosphorylation rate in response to the Tsr ligand, L-seri ne. Because CheA(S) can mediate this activity, it can restore chemotac tic ability to Escherichia coli cells that express this truncated CheA (L) variant.