Evidence for Pentapeptide-Dependent and Independent CheB Methylesterases.

Many bacteria possess multiple chemosensory pathways that are composed of homologous signaling proteins. These pathways appear to be functionally insulated from each other, but little information is available on the corresponding molecular basis. We report here a novel mechanism that contributes to pathway insulation. We show that, of the four CheB paralogs of PAO1, only CheB recognizes a pentapeptide at the C-terminal extension of the McpB (Aer2) chemoreceptor ( = 93 µM). McpB is the sole chemoreceptor that stimulates the Che2 pathway, and CheB is the methylesterase of this pathway. SCRI1043 has a single CheB, CheB_Pec, and 19 of its 36 chemoreceptors contain a C-terminal pentapeptide. The deletion of abolished chemotaxis, but, surprisingly, none of the pentapeptides bound to CheB_Pec. To determine the corresponding structural basis, we solved the 3D structure of CheB_Pec. Its structure aligned well with that of the pentapeptide-dependent enzyme from . However, no electron density was observed in the CheB_Pec region corresponding to the pentapeptide-binding site in the CheB. We hypothesize that this structural disorder is associated with the failure to bind pentapeptides. Combined data show that CheB methylesterases can be divided into pentapeptide-dependent and independent enzymes.