Functional characterization of phospholipid N-methyltransferases from Arabidopsis and soybean

Phospholipid N-methyltransferase (PLMT) enzymes catalyze the S-adenosylmethionine-dependent methylation of ethanolamine-containing phospholipids to produce the abundant membrane lipid phosphatidylcholine (PtdCho). In mammals and yeast, PLMT activities are required for the de novo synthesis of the choline headgroup found in PtdCho. PLMT enzyme activities have also been reported in plants, yet their roles in PtdCho biosynthesis are less clear because most plants can produce the choline headgroup entirely via soluble substrates, initiated by the methylation of free ethanolamine-phosphate. To gain further insights into the function of PLMT enzymes in plants, we isolated PLMT cDNAs from Arabidopsis and soybean (Glycine max) based upon primary amino acid sequence homology to the rat PLMT, phosphatidylethanolamine N-methyltransferase. Using a heterologous yeast expression system, it was shown that plant PLMTs methylate phosphatidylmonomethylethanolamine and phosphatidyldimethylethanolamine but cannot utilize phosphatidylethanolamine as a substrate. Identification of an Arabidopsis line containing a knock-out dissociator transposon insertion within the single copy AtPLMT gene allowed us to investigate the consequences of loss of PLMT function. Although the accumulation of the PLMT substrates phosphatidylmonomethylethanolamine and phosphatidyldimethylethanolamine was considerably elevated in the atplmt knock-out line, PtdCho levels remained normal, and no obvious differences were observed in plant morphology or development under standard growth conditions. However, because the metabolic routes through which PtdCho is synthesized in plants vary greatly among differing species, it is predicted that the degree with which PtdCho synthesis is dependent upon PLMT activities will also vary widely throughout the plant kingdom.