Catalyzes the reversible isomerization of (R)-3-methylitaconate to 2,3-dimethylmaleate. Has very low isomerase activity with itaconate. 2-methylene-3-methylsuccinate = dimethylmaleate Inhibited by oxidized glutathione, p-chloromercuriphenylsulfonic acid and iodoacetic acid. Not inhibited by the chelating agent alpha,alpha-dipyridyl. Activity is slightly increased by EDTA. Not activated by Fe(2+), Mg(2+), Mn(2+) or Ca(2+). Unaffected by K(+), Na(+), NH4(+), Rb(+) or Li(+). Optimum pH is 6.8-8.2. Cofactor degradation; nicotinate degradation; propanoate and pyruvate from 6-hydroxynicotinate: step 6/8. Homotetramer. By nicotinate. Belongs to the PrpF family.