CheY is a response regulator in bacterial chemotaxis. E. coli CheY mutants T87I and T87I/Y106W CheY are phosphorylatable on Asp57 but unable to generate clockwise rotation of the flagella. To understand this phenotype in terms of structure, stable analogs of the two CheY-P mutants were synthesized: T87I phosphono-CheY and T87I phosphono-CheY. Dissociation constants for peptides derived from flagellar motor protein FliM and phosphatase CheZ were determined for phosphonoCheY and the two mutants. The peptides bind phosphono-CheY almost as strongly as CheY-P; however, they do not bind T87I phosphono-CheY or T87I/Y106W phosphono-CheY, implying that the mutant proteins cannot bind FliM or CheZ tightly in vivo. The structures of T87I phosphonoCheY and T87I/Y106W phosphono-CheY were solved to resolutions of 1.8 Å and 2.4 Å, respectively. The increased bulk of I87 forces the side chain of Y106 or W106, into a more solvent-accessible conformation, which occludes the peptide-binding site.

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