Catalyzes the S-adenosylmethionine monomethyl esterification of trans-aconitate at high affinity and of cis-aconitate, isocitrate, and citrate at lower velocities and affinities. S-adenosyl-L-methionine + trans-aconitate = (E)-3-(methoxycarbonyl)pent-2-enedioate + S-adenosyl-L-homocysteine Monomer. Belongs to the methyltransferase superfamily. Tam family.