Regulation and subcellular distribution of a protein methyltransferase and its damaged aspartyl substrate sites in developing Xenopus oocytes

A protein methyltransferase which recognizes racemized and isomerized aspartyl residues in proteins has been identified in both the cytoplasm and nucleus of Xenopus laevis oocytes by enzymatic and immunochemical assays. The methyltransferase activity is maintained at a constant concentration of approximately 0.2 microM throughout vitellogenesis. Two forms of the enzyme can be identified on immunoblots by their cross-reactivity with an antibody prepared against the purified enzyme from bovine brain. Although both forms, with molecular weights of 27,000 and 34,000, are present in the cytoplasm, only the smaller form is found in the oocyte nucleus. A heterogeneous group of endogenous methyl-accepting proteins has been identified following the addition of S-adenosyl-L-[methyl-3H]methionine to oocyte extracts. The subcellular distribution of these methyl-accepting proteins, i.e. those proteins with unmodified or unmetabolized D- and L-isoaspartyl residues, is complementary to that of the methyltransferase. Very low levels of methyl-accepting activity are associated with nuclear proteins, which are actively methylated by the methyltransferase in vivo (O'Connor, C. M., and Germain, B. J. (1987) J. Biol. Chem. 262, 10404-10411). Yolk platelet proteins, which are inaccessible to the methyltransferase in vivo, are readily methylated by the enzyme in vitro. The specific methyl-accepting activity of the yolk proteins increases severalfold during the months required for the development of an early-to-late vitellogenic oocyte, suggesting that derivatized aspartyl residues accumulate with time in proteins which are inaccessible to the methyltransferase. The results support the hypothesis that the methyltransferase initiates either the repair or metabolism of cellular proteins which have been damaged by spontaneous racemization and deamidation processes (Clarke, S. (1985) Annu. Rev. Biochem. 54, 479-506).