Protein Methylation at Abnormal Aspartyl Residues

Abstract

Enzymes that catalyze the formation of protein methyl esters have been found in all tissues examined to date. It had been reasonably assumed that all of these enzymes would function as components of reversible covalent modification systems to regulate the activity of various methyl-accepting proteins (Gagnon and Heisler, 1979; Paik and Kim, 1980; O’Dea et al., 1981). Although this does appear to be the case for a class of bacterial methyltransferases which catalyze the formation of L-glutamyl γ-methyl esters on membrane chemoreceptors (Clarke et al., 1980; Terwilliger & Koshland, 1984; Kehry et al., 1983), this assumption does not seem to hold for a second potentially widespread class of enzymes, found both in bacteria and higher cells (Clarke, 1985a). These enzymes appear to catalyze the formation of D-aspartyl β-methyl esters and L-isoaspartyl α-methyl esters on proteins containing these abnormal amino acids (McFadden and Clarke, 1982; Aswad, 1984; Murray & Clarke, 1984; O’Connor et al., 1984).