Type 3 carboxyl methyltransferases

Originally isolated and characterized from the petals of Clarkia breweri, salicylic acid carboxyl methyltransferase (SAMT), converts salicylic acid to methylsalicylate.14 Methysalicylate, a volatile ester, has a characteristic wintergreen scent Apart from its putative role in defense, methylsalicylate is an important chemoattractant for moth-pollinated flowers and one of the main volatiles released from Clarkia breweri flowers.44,55 SAMT is a member of the type 3 family of plant MTs. Similarly to the previously discussed families of methyltransferases, type 3 MTs are homodimeric proteins. 

Because the three genes encoding the canonical members of this family -SAMT, BAMT, and JMT have only recently been characterized, the full extent of this family is not known. Database searches using the BLAST 2.0 program reveal a number of unknown gene products and putative SAMT -like enzymes as well as caffeine synthases belonging to the type 3 family of MTs. Thus, the family includes not only carboxyl methyltransferases but also some alkaloid N-methyltransferases. 

A third group of OMTs are the protein carboxyl MTs, whose molecular mass is approximately 27 kDa 43 These proteins are found in almost all organisms, and they serve to repair damaged proteins. In plants, they have been shown to be important during stress and in seed viability.14. Their sequences are related to the animal glycine N-methyltransferases (NMTs) and die RNA/DNA MTs. Furthermore, the plant protein carboxyl MTs share no similarity to the other plant MTs involved in specialized metabolism. The enzyme that methylates a carboxyl functionality of Mg-protoporphyrin IX to produce a methylester constitutes a fourth type of plant OMT. This chloroplastic protein, whose mature form has a molecular mass of 31 kDa, is related to MTs with similar functions from photosynthetic bacteria, but is not closely related to any other plant OMTs.6... 

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