UMPS

UMPS encodes the cytoplasmic bifunctional enzyme that catalyzes the final two steps of de novo pyrimidine biosynthesis, converting orotic acid to UMP, a role established through its C. elegans ortholog UMPS-1 in intestinal cells (PMID:20148972). Loss of UMPS-1 causes accumulation of orotic acid that is channeled by the ABCG transporter WHT-2 into lysosome-related gut granules, driving their osmotic enlargement; this phenotype is suppressed by loss of the upstream de novo pyrimidine enzyme PYR-1, placing UMPS within a defined metabolic pathway whose flux controls organelle size (PMID:20148972). Mechanistically, the orotate phosphoribosyltransferase (OPRT) activity proceeds by sequentially ordered substrate binding in which orotate binds first, followed by the Mg2+-PRPP complex as the true co-substrate (PMID:29533816). UMPS activity is negatively regulated by tyrosine phosphorylation, which lowers enzymatic output in hepatic pyrimidine metabolism, and its protein expression is repressed post-transcriptionally by miR-185-5p, an effect relieved by the competing endogenous RNA SNORD3A and linked to 5-FU sensitivity in breast cancer cells (PMID:32382150).

1. 2010 High

   Established that UMPS converts orotic acid to UMP in vivo and that this de novo pyrimidine flux is coupled to organelle homeostasis, answering what cellular consequences follow from loss of the enzyme.

   Evidence Genetic mutant screen, epistasis, and UMPS-1::GFP imaging in C. elegans intestinal cells

   PMID:20148972

   Open questions at the time

   • WHT-2-mediated orotic acid transport into granules is proposed but not directly demonstrated biochemically
   • Relevance of the gut-granule phenotype to mammalian UMPS function unaddressed
   • No structural characterization of the enzyme
2. 2018 Medium

   Resolved the order of substrate binding in the OPRT reaction, establishing that orotate binds before the Mg2+-PRPP complex and that Mg2+-PRPP, not PRPP alone, is the catalytically competent co-substrate.

   Evidence QM/MM computational analysis of S. cerevisiae OPRT, including comparison across divalent metal ions

   PMID:29533816 PMID:29567489

   Open questions at the time

   • Purely computational, no experimental kinetic validation in this work
   • Based on the yeast ortholog rather than human UMPS
   • Co2+ inhibition mechanism is a computational rationalization of prior data
3. 2020 Medium

   Identified a post-transcriptional control circuit for UMPS, showing miR-185-5p directly represses UMPS protein and that lncRNA SNORD3A de-represses it by sponging the miRNA, linking UMPS dosage to chemosensitivity.

   Evidence Luciferase ceRNA/sponge assays, gain/loss-of-function in vitro and xenografts, UMPS immunoblot in breast cancer cells

   PMID:32382150

   Open questions at the time

   • Single lab without independent replication
   • Mechanism of 5-FU sensitization not dissected at the enzymatic level
   • Whether the axis operates outside breast cancer is untested
4. 2024 Medium

   Demonstrated that tyrosine phosphorylation of UMPS is an inhibitory post-translational switch on its enzymatic activity, connecting UMPS regulation to metabolic state.

   Evidence CRISPRi-rescue with phospho-dead/phospho-mimetic mutants and stable isotope tracing in a high-fat diet mouse model (preprint)

   Open questions at the time

   • Preprint, not peer-reviewed
   • Responsible kinase/phosphatase not identified
   • Structural basis for activity inhibition unknown

• How enzymatic regulation (phosphorylation), transcript-level control (miRNA/lncRNA), and metabolic flux integrate to set UMPS output in human tissues remains unresolved.
• No unified model connecting the regulatory layers
• No experimental structure of human bifunctional UMPS
• Upstream signals controlling phosphorylation undefined