Psilocybin, the natural hallucinogen in magic mushrooms, is produced in a final biosynthetic step where the enzyme PsiM adds two methyl groups to norbaeocystin. Atomic-resolution crystal structures (0.9 Å) of PsiM at different reaction stages reveal its detailed methylation mechanism. Structural and phylogenetic evidence indicates PsiM evolved from METTL16-family RNA methyltransferases, and its bound substrates mimic RNA. Limitations inherited from its ancestral scaffold prevent efficient psilocybin assembly and block trimethylation to aeruginascin. These insights will aid bioengineering efforts to create psilocybin variants with improved therapeutic properties.
The enzyme PsiM from the mushroom Psilocybe cubensis catalyzes the final step in psilocybin biosynthesis, adding two methyl groups to the substrate norbaeocystin. A single amino acid change, M247N, allowed this enzyme to evolve from ancestral monomethylating RNA methyltransferases into a dimethylating enzyme. Mutating this asparagine back to methionine (N247M) or alanine (N247A) eliminated the ability to perform the second methylation. High-resolution crystal structures and kinetic measurements show that Asn247 provides necessary space in the active site for multiple methylations and stabilizes nearby secondary structures through hydrogen bonds, enabling efficient substrate binding and catalysis.