STRUCTURAL BASIS FOR METHYLESTERASE CHEB REGULATION BY A PHOSPHORYLATION-ACTIVATED DOMAIN

We report the x-ray crystal structure of the methylesterase CheB, a ph osphorylation-activated response regulator involved in reversible modi fication of bacterial chemotaxis receptors, Methylesterase CheB and me thyltransferase CheR modulate signaling output of the chemotaxis recep tors by controlling the level of receptor methylation, The structure o f CheB, which consists of an N-terminal regulatory domain and a C-term inal catalytic domain joined by a linker, was solved by molecular repl acement methods using independent search models for the two domains, I n unphosphorylated CheB, the N-terminal domain packs against the activ e site of the C-terminal domain and thus inhibits methylesterase activ ity by directly restricting access to the active site, We propose that phosphorylation of CheB induces a conformational change in the regula tory domain that disrupts tile domain interface, resulting in a reposi tioning of the domains and allowing access to the active site, Structu ral similarity between the two companion receptor modification enzymes , CheB and CheR, suggests an evolutionary and/or functional relationsh ip, Specifically, the phosphorylated N-terminal domain of CheB may fac ilitate interaction with the receptors, similar to the postulated role of the N-terminal domain of CheR. Examination of surfaces in the N-te rminal regulatory domain of CheB suggests that despite a common fold t hroughout the response regulator family, surfaces used for protein-pro tein interactions differ significantly, Comparison between CheB and ot her response regulators indicates that analogous surfaces are used for different functions and conversely, similar functions are mediated by different molecular surfaces.