Methylation of the Escherichia coli chemotaxis receptors: intra- and interdimer mechanisms.

The mechanism(s) of methylation of the Escherichia coli chemotaxis receptors was analyzed by experiments involving the construction of a series of aspartate receptor variants. Truncation of five or more residues from the C-terminal end of the aspartate receptor, which prevents the methyltransferase from binding to the receptor, resulted in very low rates of methylation, indicating that the methyltransferase is activated by binding to the receptor. Coexpression of a receptor variant that is unmethylatable but able to C-terminally bind the methyltransferase resulted in much higher methylation rates for all of the truncated receptors. By preventing the possibility of subunit exchange between receptor variants, we showed that the truncated receptors were methylated via an interdimer mechanism. The interdimer methylation rates of the truncated receptors were found to be 3-fold lower than the methylation rate of the unaltered receptor, suggesting that intradimer methylation as well as interdimer methylation accounts for the methylation of the unaltered receptor. In addition, the presence of the cytoplasmic signaling proteins, which have been shown to cause receptor clustering, did not influence the rates of methylation.