Uncoupled phosphorylation and activation in bacterial chemotaxis. The 2.1-A structure of a threonine to isoleucine mutant at position 87 of CheY.

Position 87 of the chemotaxis regulatory protein CheY is a highly conserved threonine/serine residue in the response regulator superfamily. A threonine 87 to isoleucine mutant in CheY, identified by its in vivo non-chemotactic phenotype, was also found to be phosphorylatable in vitro. These properties indicate that this mutant does not undergo activation upon phosphorylation. The x-ray crystallographic structure of the threonine to isoleucine CheY mutant has been solved and refined at 2.1-A resolution, to an R factor of 15.6%. Comparison with the wild-type, Mg(2+)-free CheY structure shows that the active site structure is retained, but there are significant localized differences in the backbone conformation distal from the substitution. The presence of the isoleucine side chain also restricts the rotational conformation of another conserved residue in the molecule, tyrosine at position 106. These results provide further evidence for a signaling surface remote from the phosphorylation site of the CheY molecule and implicate threonine 87 and other residues in the post-phosphorylation signaling events.