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

Abstract

The location of the sites of methylation of the methyl-accepting chemotaxis proteins (MCP) of E. coli was investigated by trypsin digestion and mapping of the peptides. The 2 principal MCP, MCP I and MCP II have very similar methyl-labeled peptide patterns. Each produced 3 major species of methylated peptide. Both MCP migrate on high-resolution sodium dodecyl sulfate-polyacrylamide slab gels as a cluster of discrete protein bands. Each band produced one of the 3 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 1-3 methyl groups. The possible significance of 3 methyl-accepting sites in a single tryptic peptide is discussed.