Catalyzes the three sequential steps of the methylation pathway for the biosynthesis of phosphatidylcholine, a critical and essential component for membrane structure (PubMed:12431977, PubMed:15927961). Uses S-adenosylmethionine (S-adenosyl-L-methionine, SAM or AdoMet) as the methyl group donor for the methylation of phosphatidylethanolamine (1,2-diacyl-sn-glycero-3-phosphoethanolamine, PE) to phosphatidylmonomethylethanolamine (1,2-diacyl-sn-glycero-3-phospho-N-methylethanolamine, PMME), PMME to phosphatidyldimethylethanolamine (1,2-diacyl-sn-glycero-3-phospho-N,N-dimethylethanolamine, PDME), and PDME to phosphatidylcholine (1,2-diacyl-sn-glycero-3-phosphocholine, PC), producing S-adenosyl-L-homocysteine in each step (PubMed:12431977, PubMed:15927961). Responsible for approximately 30% of hepatic PC with the CDP-choline pathway accounting for the other 70% (Probable). ; [Isoform 1]: Catalyzes the three sequential steps of the methylation of 1,2-diacyl-sn-glycero-3-phospho-N-methylethanolamine (PMME) to 1,2-diacyl-sn-glycero-3-phospho-N,N-dimethylethanolamine (PDME) more efficiently than isoform 2 (PubMed:20860552). Induces increase in PC species with longer polyunsaturated chains than isoform 2 (PubMed:20860552). ; [Isoform 2]: Produces a higher increase in the level of PC species containing long chains with three double bonds than isoform 1.
