Methyl-accepting chemotaxis proteins of Escherichia coli: methylated at three sites in a single tryptic fragment.

The location of the sites of methylation of the methyl-accepting chemotaxis proteins (MCP) of Escherichia coli has been investigated by trypsin digestion and mapping of the peptides. The two principal MCPs, MCP I and MCP II, were found to have very similar methyl-labeled peptide patterns. Each produced three major species of methylated peptide. Both MCPs migrate on high-resolution sodium dodecyl sulfate--polyacrylamide slab gels a a cluster of discrete protein bands. Each band produced one of the three major methylated peptides as the unique or dominant species. Progressive demethylation of the least acidic peptide produced, sequentially, the intermediate and most acidic peptides before complete loss of the methyl label. This is consistent with a single peptide backbone carrying from one to three methyl groups. The possible significance of three methyl-accepting sites in a single tryptic peptide is discussed.