Methyl transfer in psilocybin biosynthesis

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.

Study at a glance

Abstract

Abstract Psilocybin, the natural hallucinogen produced by Psilocybe (“magic”) mushrooms, holds great promise for the treatment of depression and several other mental health conditions. The final step in the psilocybin biosynthetic pathway, dimethylation of the tryptophan-derived intermediate norbaeocystin, is catalysed by PsiM. Here we present atomic resolution (0.9 Å) crystal structures of PsiM trapped at various stages of its reaction cycle, providing detailed insight into the SAM-dependent methylation mechanism. Structural and phylogenetic analyses suggest that PsiM derives from epitranscriptomic N 6 -methyladenosine writers of the METTL16 family, which is further supported by the observation that bound substrates physicochemically mimic RNA. Inherent limitations of the ancestral monomethyltransferase scaffold hamper the efficiency of psilocybin assembly and leave PsiM incapable of catalysing trimethylation to aeruginascin. The results of our study will support bioengineering efforts aiming to create novel variants of psilocybin with improved therapeutic properties.