CheB is a response regulator protein in the bacterial chemotaxis two-component signal transduction pathway. Methylesterase CheB functions together with methyltransferase CheR to modulate the level of glutamate methylation in transmembrane chemoreceptors in response to environmental stimuli. The level of glutamate methylation in turn indirectly controls the direction of flagellar rotation. Like most two-component response regulators, CheB is activated in vivo by phosphorylation of a single aspartate, Asp 56, in its regulatory domain. Extensive biochemical and crystallographic studies have been completed on the inactive, unphosphorylated form of CheB. Because of the inherent lability of aspartyl phosphate bonds and the intrinsic phosphatase activity of CheB, the activated, phosphorylated form of CheB cannot be isolated for further characterization. We present a synthetic scheme to prepare an analogue of phosphorylated CheB using site-specific mutagenesis and chemical modification strategies. Initially, the two native cysteines found in CheB were substituted by serines and a cysteine was substituted for Asp 56 to yield D56C/C207S/C309S CheB. The unique cysteine in the substituted form of CheB was modified by sodium thiophosphate, Na3SPO3, using two sequential disulfide bond exchange reactions. The analogue, D56C/C207S/C309S CheB-SPO3, contained a thiophosphate group covalently bonded to the protein through a disulfide linkage at residue 56. Mass spectrometry showed that the protein was singly modified. Reverse phase chromatography showed that greater than 95% of the protein was modified under optimized conditions and that the analogue had a half-life of 28 days. In in vitro methylesterase assays in the presence of Mg2+, the analogue exhibited activity equivalent to that of fully phosphorylated C207S/C309S CheB. Thus, D56C/C207S/C309S CheB-SPO3 is a stable analogue that may be useful for characterization of the active form of CheB.
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