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.