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 GGPPSR235C, 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 GGPPSR235C 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, GGPPSY246D. 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.