Structural basis of methylation-triggered terpene cyclization

Terpene synthases usually generate carbocations by either pyrophosphate (PP) elimination (class I) or substrate protonation (class II) to initiate cyclization cascades. By contrast, recent discoveries describe methyltransferases (MTs) that act on farnesyl PP (FPP) and trigger terpene cyclization through methyl transfer. Here, we present high-resolution structures of four terpene-cyclizing FPP-MTs in complex with cofactor and substrate. The captured open and closed states reveal a dynamic MT-PP sensor that coordinates Mg 2+ and PP, thereby enforcing methylation as the initial step within the catalytic chamber. Despite divergent products, these MTs share conserved active sites, with specificities determined by PP anchoring and substrate orientation. Systematic mutagenesis identified carbocation-stabilizing residues, mapped shunt pathways, and showed that subtle substitutions can redirect the cascade toward distinct terpene products. From an evolutionary perspective, the data suggest that selective pressure acted on the coordinated interplay of residues rather than on single positions, thereby stabilizing defined pathways.