Ensemble cryo-EM structures demonstrate human IMPDH2 filament assembly tunes allosteric regulation

Summary Inosine monophosphate dehydrogenase (IMPDH) mediates the first committed step in guanine nucleotide biosynthesis and plays important roles in cellular proliferation and the immune response. The enzyme is heavily regulated to maintain balance between guanine and adenine nucleotide pools. IMPDH reversibly polymerizes in cells and tissues in response to changes in metabolic demand, providing an additional layer of regulatory control associated with increased flux through the guanine synthesis pathway. Here, we report a series of human IMPDH2 cryo-EM structures in active and inactive conformations, and show that the filament resists inhibition by guanine nucleotides. The structures define the mechanism of filament assembly, and reveal how assembly interactions tune the response to guanine inhibition. Filament-dependent allosteric regulation of IMPDH2 makes the enzyme less sensitive to feedback inhibition, explaining why assembly occurs under physiological conditions, like stem cell proliferation and T-cell activation, that require expansion of guanine nucleotide pools.