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). [protein]-L-isoaspartate + S-adenosyl-L-methionine = [protein]-L-isoaspartate alpha-methyl ester + S-adenosyl-L-homocysteine Monomer. The allele frequencies for the polymorphism at codon 120 differ between ethnic groups; in the Caucasian population Ile-120 is present at a frequency of 0.45, while it is found at a frequency of 0.88 and 0.81 in the Asian and the African populations respectively. Val-120 is found at a frequency of 0.55 in the Caucasians, 0.12 and 0.19 in the Asian and African populations respectively. The Ile-120 variant has higher specific activity and thermostability than the Val-120 variant. The Val-120 variant has a higher affinity for protein substrates. Belongs to the methyltransferase superfamily. L-isoaspartyl/D-aspartyl protein methyltransferase family.