A Nucleotide-Dependent Conformational Switch Controls the Polymerization of Human IMP Dehydrogenases to Modulate their Catalytic Activity.

Abstract Inosine 5′-monophosphate dehydrogenase (IMPDH) catalyzes the rate-limiting step in the de novo GTP biosynthetic pathway and plays essential roles in cell proliferation. As a clinical target, IMPDH has been studied for decades, but it has only been within the last years that we are starting to understand the complexity of the mechanisms of its physiological regulation. Here, we report structural and functional insights into how adenine and guanine nucleotides control a conformational switch that modulates the assembly of the two human IMPDH enzymes into cytoophidia and allosterically regulates their catalytic activity. In vitro reconstituted micron-length cytoophidia-like structures show catalytic activity comparable to unassembled IMPDH but, in turn, are more resistant to GTP/GDP allosteric inhibition. Therefore, IMPDH cytoophidia formation facilitates the accumulation of high levels of guanine nucleotides when the cell requires it. Finally, we demonstrate that most of the IMPDH retinopathy-associated mutations abrogate GTP/GDP-induced allosteric inhibition and alter cytoophidia dynamics. Graphical Abstract Unlabelled Image Highlights • Human IMPDH enzymes polymerize into mesoscale polymers both in cells and in vitro. • Purine nucleotides control a conformational switch that modulates activity. • The switch also controls the assembly into cytoophidia-like structures. • Retinopathy mutations alter the switch and the dynamics of the assembly. • Cytoophidia-like structures show resistance to allosteric inhibition in vitro. [ABSTRACT FROM AUTHOR]