A somatic multiple myeloma mutation unravels a mechanism of oligomerization-mediated product inhibition in GGPPS

Protein prenylation regulates the cellular localization of small GTPases and is pivotal for multiple myeloma (MM) pathology. Geranylgeranyl diphosphate synthase (GGPPS), synthesizing a prenylation moiety, exhibits dimeric or hexameric stoichiometry in different species. However, the functional significance of this divergence remains elusive. Focusing on the hexameric human paralog, formed by trimer-of-dimers, we uncover that GGPPS ^R235C , expressed in an MM cell line, localizes to the active site lid region at the inter-dimeric interface. Using crystallography and mass spectrometry (MS), we show that GGPPS ^R235C retains its hexameric stoichiometry but exhibits destabilized inter-dimer interactions. Unexpectedly, this results in increased apparent substrate affinity and product release kinetics. These functional effects are further enhanced in a dimeric mutant, GGPPS ^Y246D . Combining MS and fluorescence spectroscopy, we exposed that reduced lid dynamics and increased active site occupancy by the product are intertwined. Together, our results expose product inhibition as a regulatory mechanism in GGPPS, driven by hexamerization.