Involved in the methylaspartate cycle. Catalyzes the reversible hydration of mesaconyl-CoA (2-methylfumaryl-CoA) to yield beta-methylmalyl-CoA ((2R,3S)-beta-methylmalyl-CoA). (2R,3S)-beta-methylmalyl-CoA = 2-methylfumaryl-CoA + H2O Belongs to the enoyl-CoA hydratase/isomerase family. Extended N-terminus.