Structural basis for phosphatidylcholine synthesis by bacterial phospholipid N-methyltransferases

In phosphatidylcholine (PC)-containing bacteria, PC is synthesized by phospholipid N-methyltransferases (Pmts) and plays an important role in the interactions between symbiotic and pathogenic bacteria and their eukaryotic host cells. Pmts catalyze the SAM dependent three methylation reactions of the head group of phosphatidylethanolamine (PE) to form PC through monomethyl PE and dimethyl PE. However, the precise molecular mechanisms underlying PC biosynthesis by PmtA remain largely unclear, owing to the lack of structural information. Here, we determined the crystal structures of Agrobacterium tumefaciens Pmt (AtPmtA) in complex with SAH or 5'-methylthioadenosine. Crystal structures and NMR analysis revealed the binding mode of AtPmtA to SAH in solution. Structure-based mutational analyses showed that a conserved tyrosine residue in the substrate-binding groove is involved in methylation. Furthermore, we showed that differences in substrate specificity among Pmt homologs were determined by whether the amino acid residues comprising the substrate-binding groove were isoleucine or phenylalanine. These findings provide a structural basis for understanding the mechanisms underlying Pmts-mediated PC biosynthesis.