Initiates the repair of damaged proteins by catalyzing methyl esterification of L-isoaspartyl and D-aspartyl residues produced by spontaneous isomerization and racemization of L-aspartyl and L- asparaginyl residues in aging peptides and proteins (PubMed:3167043, PubMed:6469980). Acts on EIF4EBP2, microtubule-associated protein 2, calreticulin, clathrin light chains a and b, Ubiquitin C-terminal hydrolase isozyme L1, phosphatidylethanolamine-binding protein 1, stathmin, beta-synuclein and alpha-synuclein (By similarity). {ECO:0000250|UniProtKB:P23506, ECO:0000269|PubMed:3167043, ECO:0000269|PubMed:6469980}. This is the function of ENSG00000120265 (PCMT1, protein-L-isoaspartate (D-aspartate) O-methyltransferase).