An auxiliary protein tunes reductase activity in alkaloid biosynthesis

Monoterpene indole alkaloids encompass a diverse class of plant natural products. Many of these valuable compounds, including the anticancer medicines vinblastine and vincristine (Catharanthus roseus) and the antiaddiction treatment ibogaine (Tabernanthe iboga) utilize a common biosynthetic intermediate, 19E-geissoschizine, which is produced by the medium-chain dehydrogenase/reductase geissoschizine synthase (GS). Herein we report the discovery of a non-catalytic auxiliary protein (facilitator of geissoschizine synthase, FoGS) that improves formation of 19E-geissoschizine by acting in combination with GS. The discovery of a FoGS orthologue that works with GS to produce 19Z-geissoschizine shows how this protein also controls the stereochemistry of the product. We demonstrate that FoGS and GS interact to form a stable heterodimer, and show through mutagenesis that FoGS likely modulates the structure of the GS active site. Microscopy studies suggest that FoGS also changes the subcellular localization of GS. The discovery of FoGS will enable efforts to heterologously reconstitute commercially important geissoschizine-derived monoterpene indole alkaloids, and moreover, highlights how metabolic pathways utilize proteins that are not essential for production of the products, but nevertheless enhance the efficiency and specificity of the catalytic biosynthetic pathway components.