# Protein–Protein Interactions Modulate a Key Branch Point in Monoterpene Indole Alkaloid Biosynthesis

## TL;DR

This study explores how protein interactions control a key step in the production of medicinal plant compounds.

## Contribution

The study identifies new protein–protein interactions and surface residues that regulate a branch point in monoterpene indole alkaloid biosynthesis.

## Key findings

- Protein–protein interactions between reductase-cyclase pairs and homologous proteins were verified and newly discovered.
- Surface residues on cyclases were found to mediate interactions with upstream reductases.
- In vitro assays showed these residues influence the distribution of downstream alkaloid products.

## Abstract

Biosynthetic
pathways of specialized metabolites utilize
protein–protein
interactions (PPIs) to facilitate metabolic flux and sequester reactive
intermediates. The monoterpene indole alkaloid pathway of Catharanthus roseus contains several metabolic branch points
that may be mediated via transient PPIs. We investigated one branch
point of this pathway that is responsible for the conversion of the
intermediate dehydrosecodine into three possible cyclized alkaloid
scaffolds, which act as intermediates en route to medicinally important
alkaloids, such as vinblastine. We verified previously observed PPIs
between reductase-cyclase pairs and additionally uncovered PPIs between
evolutionarily related protein homologues. Through structural analysis
of dehydrosecodine cyclases, we identified surface residues that appear
to mediate interaction with the upstream reductase. We then demonstrated,
via in vitro competition assays, that these residues
impact the distribution of downstream products. These results highlight
the significance of transient PPIs in the control and regulation of
specialized metabolite pathways.