Sites of methyl esterification and deamination on the aspartate receptor involved in chemotaxis

The receptors involved in bacterial chemotaxis are post-translationally modified by specific enzymes which catalyze the deamination of glutaminyl residues and the methyl esterification and demethylation of glutamyl residues. In this work we identify the sites of these covalent modifications on the aspartate receptor from Salmonella typhimurium. These were identified using the properties of the Staphylococcus aureus V8 protease which cleaves peptide bonds following glutamyl but not glutaminyl residues. We show here that bonds following methyl-esterified glutamyl residues are also resistant to the protease. A comparison of the fragments obtained after V8 protease cleavage of methyl-esterified (or deaminated) peptides with the fragments from the corresponding unmodified peptides immediately yields the sites of modification. Three of the four methyl-esterified glutamyl residues are located near the middle of the receptor amino acid sequence; one of these is synthesized as a glutaminyl residue and is deaminated by the esterase to form a glutamyl residue. The fourth site of methyl esterification is located near the carboxyl terminus. All four sites occupy analogous positions in a well-conserved arrangement of residues which may form a binding site for the esterase and the methyltransferase.