UNCOUPLED PHOSPHORYLATION AND ACTIVATION IN BACTERIAL CHEMOTAXIS - THE 2.1-ANGSTROM STRUCTURE OF A THREONINE TO ISOLEUCINE MUTANT AT POSITION-87 OF CHEY

Position 87 of the chemotaxis regulatory protein CheY is a highly cons erved threonine/serirle residue in the response regulator superfamily. A threonine 87 to isoleucine mutant in CheY, identified by its in viv o non-chemotactic phenotype, was also found to be phosphorylatable in vitro. These properties indicate that this mutant does not undergo act ivation upon phosphorylation. The x-ray crystallographic structure of the threonine to isoleucine CheY mutant has been solved and refined at 2.1-Angstrom resolution, to an R factor of 15.6%. Comparison with the wild-type, Mg2+-free CheY structure shows that the active site struct ure is retained, but there are significant localized differences in th e backbone conformation distal from the substitution. The presence of the isoleucine side chain also restricts the rotational conformation o f 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 thre onine 87 and other residues in the post-phosphorylation signaling even ts.