Involved in the glyoxylate assimilation cycle used to regenerate acetyl-CoA and produce pyruvate as universal precursor for biosynthesis. Catalyzes the reversible dehydration of beta-methylmalyl-CoA ((2R,3S)-beta-methylmalyl-CoA) to yield mesaconyl-CoA (2-methylfumaryl-CoA). (2R,3S)-beta-methylmalyl-CoA = 2-methylfumaryl-CoA + H2O Optimum pH is 7.5. Homodimer. Under autotrophic growth conditions.