Homologies among bacterial signal transduction proteins suggest that a common mechanism mediates processes such as chemotaxis, osmoregulation, sporulation, virulence, and responses to nitrogen, phosphorous and oxygen deprivation1-3. A common kinase-mediated phosphotransfer reaction has recently been identified in chemotaxis, nitrogen regulation, and osmoregulation4-10. In chemotaxis, the CheA kinase passes a phosphoryl group to the cytoplasmic protein CheY, which functions as a phosphorylation-activated switch that interacts with flagellar components to regulate motility. We report here the X-ray crystal structure of the Salmonella typhimurium CheY protein. The determination of the structure was facilitated by the use of site-specific mutagenesis to engineer heavy-atom binding sites. CheY is a single-domain protein composed of a doubly wound five-stranded parallel β-sheet. The phosphoacceptor site in CheY is probably a cluster of aspartic-acid side chains near the C-terminal edge of the β-sheet. The pattern of sequence similarity of CheY with components of other regulatory systems can be interpreted in the light of the CheY structure and supports the view that this family of proteins have a common structural motif and active site.
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