Catalyzes the methyl esterification of L-isoaspartyl residues in peptides and proteins that result from spontaneous decomposition of normal L-aspartyl and L-asparaginyl residues. It plays a role in the repair and/or degradation of damaged proteins. Contributes to seed longevity and germination vigor by limiting the abnormal accumulation of the L-isoaspartyl residues in seed proteins. [protein]-L-isoaspartate + S-adenosyl-L-methionine = [protein]-L-isoaspartate alpha-methyl ester + S-adenosyl-L-homocysteine Optimum pH is 7.0. Monomer. Expressed in roots, rosette leaves, stems, cauline leaves, flowers and developing seeds. By abscisic acid (ABA). Seeds over-expressing PIMT1 have reduced accumulation of L-isoaspartyl residues in seed proteins and increased seed longevity and germination vigor. Conversely, reduced PIMT1 expression is associated with an increase in the accumulation of L-isoaspartyl residues in proteins, leading to increased sensitivity to aging treatments and loss of seed vigor under stressful germination conditions (PubMed:19011119). Belongs to the methyltransferase superfamily. L-isoaspartyl/D-aspartyl protein methyltransferase family.