Involved in the methylaspartate cycle. Catalyzes the transfer of the CoA moiety from succinyl-CoA to mesaconate to generate mesaconyl-CoA (2-methylfumaryl-CoA) and succinate (PubMed:28932214). Shows also high activity with methylsuccinate as CoA-acceptor, and only low activity with glutarate, acrylate and itaconate (PubMed:28932214). Cannot use other CoA donors like acetyl-CoA, propionyl-CoA, butyryl-CoA or acetoacetyl-CoA (PubMed:28932214). mesaconate + succinyl-CoA = 2-methylfumaryl-CoA + succinate Shows highest activity at 4 M KCl. Does not require divalent ions for activity. Homodimer. Belongs to the CoA-transferase III family.