Structural insights into phosphatidylethanolamine <i>N</i> -methyltransferase PmtA mediating bacterial phosphatidylcholine synthesis

Phosphatidylethanolamine <i>N</i>-methyltransferase (PmtA) catalyzes the biosynthesis of phosphatidylcholine (PC) from phosphatidylethanolamine (PE). Although PC is one of the major phospholipids constituting bilayer membranes in eukaryotes, certain bacterial species encode PmtA, a membrane-associated methyltransferase, to produce PC, which is correlated with cellular stress responses, adaptability to environmental changes, and symbiosis or virulence with eukaryotic hosts. Depending on the organism, multiple PmtAs may be required for producing monomethyl- and dimethyl-PE derivatives along with PC, whereas in organisms such as <i>Rubellimicrobium thermophilum</i>, a single enzyme is sufficient to direct all three methylation steps. In this study, we present the x-ray crystal structures of PmtA from <i>R. thermophilum</i> in complex with dimethyl-PE and <i>S</i>-adenosyl-l-homocysteine, as well as in its lipid-free form. Moreover, we demonstrate that the enzyme associates with the cellular membrane via electrostatic interactions facilitated by a group of critical basic residues and can successively methylate PE and its methylated derivatives, culminating in the production of PC.