Structural basis of human PRPS2 filaments

PRPP synthase (PRPS) transfers the pyrophosphate groups from ATP to ribose-5-phosphate to produce 5-phosphate ribose-1-pyrophosphate (PRPP), a key intermediate in the biosynthesis of several metabolites including nucleotides, dinucleotides and some amino acids. There are three PRPS isoforms encoded in human genome. While hPRPS1 and hPRPS2 are expressed in most tissues, hPRPS3 is exclusively expressed in testis. Although hPRPS1 and hPRPS2 share 95% sequence identity, hPRPS2 has been shown to be less sensitive to allosteric inhibition and specifically upregulated in certain cancers in the translational level. Recent studies demonstrate that PRPS can form a subcellular compartment termed the cytoophidium in multiple organisms across prokaryotes and eukaryotes. Forming the cytoophidium is considered as a distinctive mechanism involving the polymerization of the protein. In order to investigate the function and molecular mechanism of hPRPS2 polymerization, we solve the polymer structure of hPRPS2 at 3.08[A] resolution using cryo-Electron Microscopy (cryo-EM). hPRPS2 hexamers stack into polymers in the conditions with the allosteric/competitive inhibitor ADP. The binding modes of ADP at the canonical allosteric site and at the catalytic active site are clearly determined. A point mutation disrupting the inter-hexamer interaction prevents hPRPS2 polymerization and results in significantly reduced catalytic activity. Our findings suggest that the regulation of hPRPS2 polymer is distinct from E. coli PRPS polymer and provide new insights to the regulation of hPRPS2 with structural basis.