CRYSTAL-STRUCTURE OF THE CATALYTIC DOMAIN OF THE CHEMOTAXIS RECEPTOR METHYLESTERASE, CHEB

Signaling activity of bacterial chemotaxis transmembrane receptors is modulated by reversible covalent modification of specific receptor glu tamate residues. The level of receptor methylation results from the ac tivities of a specific S-adenosylmethionine-dependent methyltransferas e, CheR, and the CheB methylesterase, which catalyzes hydrolysis of re ceptor glutamine or methylglutamate side-chains to glutamic acid. The CheB methylesterase belongs to a large family of response regulator pr oteins in which N-terminal regulatory domains control the activities o f C-terminal effector domains. The crystal structure of the catalytic domain of the Salmonella typhimurium CheB methylesterase has been dete rmined at 1.75 Angstrom,resolution. The domain has a modified, doubly wound alpha/beta fold in which one of the helices is replaced by an an ti-parallel beta-hairpin. Previous biochemical and mutagenesis data su ggest that the methylester hydrolysis catalyzed by CheB proceeds throu gh a mechanism involving a serine nucleophile. The methylesterase acti ve site is tentatively identified as a cleft at the C-terminal edge of the beta-sheet containing residues Ser164, His190 and Asp286. The thr ee-dimensional fold, and the arrangement of residues within the cataly tic triad distinguishes the CheB methylesterase from any previously de scribed serine protease or serine hydrolase.