[protein]-L-isoaspartate + S-adenosyl-L-methionine = [protein]-L-isoaspartate alpha-methyl ester + S-adenosyl-L-homocysteine
[protein]-l-isoaspartate + s-adenosyl-l-methionine = [protein]-l-isoaspartate alpha-methyl ester + s-adenosyl-l-homocysteine