Site-specific and synergistic stimulation of methylation on the bacterial chemotaxis receptor Tsr by serine and the binding protein CheW
The reversible methylation of certain residues in the transmembrane receptors mediates adaptation to changes in attractant concentration. In Tsr, four glutamic acid residues (E297, E304, E311, and E493) have been identified as major methylation sites. However, no studies have been conducted on these sites to examine any differences in their rates of methylation. In this study, the methylation rates of these sites were measured by using engineered receptors, in which each has only one major site available for modification. Site four was found to be the most rapid site to get methylated followed by site one, two, and three. This distribution was compared to the one determined for the aspartate receptor and site four was found to be the major difference between these two receptors.
Serine and the adaptor protein, CheW, produced similar site-dependent increases in the rate of methylation of all sites. At saturating concentration of serine or CheW, the rate increases were largest at site one (a slow site in the absence of ligand), and least at site four (the fastest site in the absence of ligand). The effects of serine and CheW were also synergistic, except for site four. In the presence of both CheW and serine, the individually. Altogether, the results of this study imply that the major sites of methylation in Tsr (except site four) are involved in the ligand-specific adaptation process. Moreover, CheW appears to play an important role in determining the kinetics of adaptation in the bacterial cell.