2 BINDING MODES REVEAL FLEXIBILITY IN KINASE RESPONSE REGULATOR INTERACTIONS IN THE BACTERIAL CHEMOTAXIS PATHWAY/

The crystal structure at 2.0 Angstrom resolution of the complex of the Escherichia coli chemotaxis response regulator CheY and the phosphoac ceptor-binding domain (P2) of the kinase CheA is presented. The bindin g interface involves the fourth and fifth helices and fifth beta-stran d of CheY and both helices of P2. Surprisingly, the two heterodimers i n the asymmetric unit have two different binding modes involving the s ame interface, suggesting some flexibility in the binding regions. Sig nificant conformational changes have occurred in CheY compared with pr eviously determined unbound structures, The active site of CheY is exp osed by the binding of the kinase domain, possibly to enhance phosphot ransfer from CheA to CheY, The conformational changes upon complex for mation as well as the observation that there are two different binding modes suggest that the plasticity of CheY is an essential feature of response regulator function.