Sequential assignment and secondary structure of the 14 kDa chemotactic protein CheY2 from Sinorhizobium meliloti.

Motile bacteria are able to direct their swimming movement towards the most favourable chemical environment. This ability, known as chemotaxis, is mediated by a signal transduction pathway involving a set of cytoplasmic proteins and extracellular rotating helical flagella. CheA, an autokinase, activates a response regulator, CheY, by phosphorylation. CheY propagates the signal, which is sensed by the transmembrane chemoreceptors, to the flagellar motor. In response, Escherichia coli flagella reverse the direction of rotation, resulting in a tumbling and thus a change in the direction of movement. Dephosphorylation of CheY-P resets the signal and is accelerated by a phosphatase, CheZ. This is different in Sinorhizobium meliloti, where two response regulators, CheY1 and CheY2, are phosphorylated, with CheY2 being the chief regulator (Sourjik and Schmitt, 1998). The phosphorylated states of CheY1 and CheY2 are short-lived due to autophosphatase activity and in case of CheY2-P through a retrophosphorylation mechanism involving CheA and CheY1. Up to now, this retrophosphorylation as a new mechanism of adaptation is not fully understood. The striking differences in the two dephosphorylation reactions of the two response regulators, CheY (E. coli) and CheY2 (S. meliloti), respectively, and the fact that X-ray and NMR structures of the former have been determined (Stock et al., 1989; Santoro et al., 1995), instigated the present study of the molecular structure of CheY2 ultimately aimed at an understanding of interactions with other protein components of the system. We expect