Structure of Papaver somniferum O-Methyltransferase 1 Reveals Initiation of Noscapine Biosynthesis with Implications for Plant Natural Product Methylation

The opium poppy, Papaver somniferum, has been a source of medicinal alkaloids since the earliest civilizations, ca. 3400 B.C. The benzylisoquinoline alkaloid noscapine is produced commercially in P. somniferum for use as a cough suppressant, and it also has potential as an anticancer compound. The first committed step in the recently elucidated noscapine biosynthetic pathway involves the conversion of scoulerine to tetrahydrocolumbamine by 9-O-methylation, catalyzed by O-methyltransferase 1 (PSMT1). We demonstrate, through protein structures (obtained through rational crystal engineering at resolutions from 1.5 to 1.2 A for the engineered variants) across the reaction coordinate, how domain closure allows specific methyl transfer to generate the product. SAM-dependent methyl transfer is central to myriad natural products in plants; analysis of amino acid sequence, now taking the three-dimensional structure of PSMT1 and low identity homologues into account, begins to shed light on the structural features t...