Summary
De novo purine synthesis (DPS) is up-regulated under conditions of high purine demands to ensure the mass production of genetic materials and chemical energy, thereby supporting cell proliferation. However, the mechanisms by which DPS is activated remain unclear. We herein demonstrated that PRPP amidotransferase (PPAT), the rate-limiting enzyme in DPS, condensated into granules in Saccharomyces cerevisiae cells under purine-depleted conditions. The assembly of granules required phase separation driven by target of rapamycin complex 1 (TORC1)-induced ribosome biosynthesis. A biochemical analysis revealed that the self-assembly of PPAT molecules accounted for granule formation and submillimolar levels of purine nucleotides inhibit self-assembly. Cells unable to form PPAT granules showed growth defects, indicating the physiological importance of granules for the promotion of DPS. These results suggest that the condensation of PPAT by phase transition is a mechanism that regulates DPS sensitively responding to both TORC1 activity and cellular purine demands.
Highlights
Budding yeast PRPP amidotransferase (PPAT) forms intracellular dynamic granules in response to the deprivation of environmental purine bases.
The assembly of PPAT granules is driven by ribosome crowding in the cytoplasm induced by TORC1.
Pure PPAT proteins condensate into granule-sized particles in vitro under molecular crowding conditions.
The interaction of PPAT with purine nucleotides prevents granule assembly.
The condensation of PPAT in granules permits the intermolecular channeling of intermediates, thereby facilitating de novo purine synthesis.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Text revised; Addition of supplementary tables.
