Receptor conformational changes enhance methylesterase activity during chemotaxis by Bacillus subtilis

Addition and removal of the attractant asparagine causes methanol formation as a consequence of methylation and demethylation of conserved glutamate residues in the Bacillus subtilis chemotaxis receptor McpB C-terminal domain. We found that methanol was released on both addition and removal of asparagine even when the response regulator domain of CheB was removed (to produce CheB(141-357)). Thus, in undergoing the transition from unbound receptor to ligand-bound adapted receptor, the receptor must pass through a state of heightened susceptibility to demethylation by CheB that is independent of phosphorylation. The same result occurred when the aspartate phosphorylation site of CheB, Asp54, had been mutated to an asparagine residue, provided the enzyme was sufficiently induced. However, no methanol release was observed for an active site point mutant, cheB(S173C), in response to addition or removal of asparagine even when induced. Finally, methanol release was observed only for attractant addition in a mutant background lacking the coupling proteins, CheW and CheV, provided CheB(141-357) was present. Thus, on attractant addition, methanol must arise from a transient conformation of the receptor C-terminal domain that is an intrinsic property of the receptor; on attractant removal, however, methanol must arise from a different transient conformation, one dependent on the presence of coupling proteins.