Correlated switch finding and signaling in bacterial chemotaxis

In Escherichia coli, swimming behavior is mediated by the phosphorylation s tate of the response regulator CheY, In its active, phosphorylated form, Ch eY exhibits enhanced binding to a switch component, FliM, at the flagellar motor, which induces a change from counterclockwise to clockwise flagellar rotation. When Ile(95) of CheY is replaced by a valine, increased clockwise rotation correlates with enhanced binding to FliM. A possible explanation for the hyperactivity of this mutant is that residue 95 affects the conform ation of nearby residues that potentially interact with FliM, In order to a ssess this possibility directly, the crystal structure of CheY95IV was dete rmined. We found that CheY95IV is structurally almost indistinguishable fro m wild-type CheY. Several other mutants with substitutions at position 95 w ere characterized to establish the structural requirements for switch bindi ng and clockwise signaling at this position and to investigate a general re lationship between the two properties. The various rotational phenotypes of these mutants can be explained solely by the amount of phosphorylated CheY bound to the switch, which was inferred from the phosphorylation propertie s of the mutant CheY proteins and their binding affinities to FliM, Combine d genetic, biochemical, and crystallographic results suggest that residue 9 5 itself is critical in mediating the surface complementarity between CheY and FliM.