β-Alanine betaine synthesis in the Plumbaginaceae.: Purification and characterization of a trifunctional, S-adenosyl-L-methionine-dependent N-methyltransferase from Limonium latifolium leaves

beta -Alanine (P-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. Ln 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 muM S-adenosyl-l-methionine and substrate inhibition was observed above 200 muM. 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.