The receptor docking segment and S-adenosyl-L-homocysteine bind independently to the methyltransferase of bacterial chemotaxis

To mediate adaptation to stimuli, the methyltransferase (CheR) catalyzes methyl group transfer from S-adenosyl-L-methionine (SAM) to glutamyl residues in the transmembrane receptors of the bacterial chemosensory signaling pathway. The interaction between receptors and CheR occurs at two sites: a methylation site–active site interaction, and a ‘docking’ site interaction that is separated both from the methylation sites and the CheR active site. It is not certain if the docking site interaction functions merely to localize the transferase in close proximity to the methylation sites, or if it also increases CheR catalytic activity. Isothermal titration calorimetry experiments are conducted to test for allosteric interactions between the docking and active sites on CheR, which are expected to be present if docking activates CheR. The binding parameters (ΔG, ΔH, ΔS) of a substrate analog of SAM, S-adenosyl-L-homocysteine (SAH), are measured both in the absence and presence of saturating concentrations of a pentapeptide (NWETF) that defines the docking receptor docking segment. SAH binding is unaffected by the presence of saturating NWETF, providing evidence that an allosteric activation of CheR does not take place upon docking, and thus supports the idea that the CheR–NWETF interaction merely functions to localize CheR near the sites of methylation.