β-Alanine Betaine Synthesis in the Plumbaginaceae. Purification and Characterization of a Trifunctional,S-Adenosyl-l-Methionine-DependentN-Methyltransferase from Limonium latifoliumLeaves

beta-Alanine (beta-Ala) betaine is an osmoprotective compound accumulated by most members of the highly stress-tolerant family Plumbaginaceae. Its potential role in plant tolerance to salinity and hypoxia makes its synthetic pathway an interesting target for metabolic engineering. In the Plumbaginaceae, beta-Ala betaine is synthesized by S-adenosyl-L-methionine-dependent N-methylation of beta-Ala via N-methyl beta-Ala and N,N-dimethyl beta-Ala. It was not known how many N-methyltransferases (NMTases) participate in the three N-methylations of beta-Ala. An NMTase was purified about 1,890-fold, from Limonium latifolium leaves, using a protocol consisting of polyethylene glycol precipitation, heat treatment, anion-exchange chromatography, gel filtration, native polyacrylamide gel electrophoresis, and two substrate affinity chromatography steps. The purified NMTase was trifunctional, methylating beta-Ala, N-methyl beta-Ala, and N,N-dimethyl beta-Ala. Gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses indicated that the native NMTase is a dimer of 43-kD subunits. The NMTase had an apparent K(m) of 45 microM S-adenosyl-l-methionine and substrate inhibition was observed above 200 microM. The apparent K(m) values for the methyl acceptor substrates were 5.3, 5.7, and 5.9 mM for beta-Ala, N-methyl beta-Ala, and N,N-dimethyl beta-Ala, respectively. The NMTase had an isoelectric point of 5.15 and was reversibly inhibited by the thiol reagent p-hydroxymercuribenzoic acid.