Catalyzes methyl transfer from S-methylmethionine (SMM) to adenosyl-L-homocysteine (AdoMet). SMM degradation (by HMT-1, HMT-2 and HMT-3) and biosynthesis (by MMT1) constitute the SMM cycle in plants, which is probably required to achieve short term control of AdoMet level. L-homocysteine + S-methyl-L-methionine = H(+) + 2 L-methionine Strongly inhibited by methionine. Monomer. A number of isoforms are produced. According to EST sequences. Expressed predominantly in roots. Expressed in rosette leaves, cauline leaves and developing seeds.