UCK1

UCK1 is a ~30-kDa pyrimidine ribonucleoside kinase of the salvage pathway that uses ATP to phosphorylate uridine and cytidine to their 5'-monophosphates, with no activity toward deoxyribonucleosides or purine ribonucleosides (PMID:11306702). It accepts a broad panel of pyrimidine ribonucleoside analogs, including 5-fluorouridine, 5-fluorocytidine, and other modified uridine/cytidine derivatives (PMID:11306702), as well as the antiviral ribavirin (PMID:18080217) and N4-hydroxycytidine (NHC, the active form of molnupiravir), which it phosphorylates with ~9-fold lower catalytic efficiency than its isoform UCK2 (PMID:42213735); the substrate-bound UCK1-NHC-AMPPNP co-crystal structure defines the structural basis of this catalysis (PMID:42213735). Its specificity is restricted to ribonucleosides: UCK1 does not phosphorylate the 2'-deoxy/2'-fluoro/2'-methyl analogs PSI-6130 or 2'-C-methyl-cytidine (PMID:17101674, PMID:19883694). Beyond direct catalysis, UCK1 functions as a negative regulator of pyrimidine salvage by localizing to the nucleus and physically binding cytosolic UCK2, driving UCK2 nuclear relocalization, impairing salvage flux, and protecting cells from cytidine-analog toxicity (PMID:27239701). UCK1 abundance is controlled by ubiquitin-dependent turnover: the E3 ligase KLHL2 polyubiquitinates UCK1 at K81 to promote its degradation, opposed by the deubiquitinase USP28, with ATM signaling shifting this balance toward UCK1 destabilization (PMID:31938050). Functionally, UCK1 activity is required for activation of the hypomethylating agent azacitidine in leukemia cells (PMID:24192812).

1. 2001 High

   Established the core biochemical identity of UCK1 as a pyrimidine ribonucleoside monophosphate kinase, answering what reaction the gene product catalyzes and on which substrates.

   Evidence Recombinant expression in E. coli and in vitro activity assays against a 28-analog panel

   PMID:11306702

   Open questions at the time

   • Kinetic parameters for natural substrates not quantified
   • Physiological vs. analog substrate preference in cells not resolved
2. 2006 High

   Defined the boundary of UCK1 substrate specificity by showing it cannot phosphorylate 2'-modified deoxy analogs, distinguishing it from dCK and clarifying which antiviral prodrugs it can activate.

   Evidence Coupled spectrophotometric kinase assay with purified recombinant UCK1 on PSI-6130; extended to 2'-C-methyl-cytidine in 2009

   PMID:17101674 PMID:19883694

   Open questions at the time

   • Structural reason for exclusion of 2'-modified sugars not addressed at the time
3. 2007 Medium

   Extended the analog substrate repertoire by identifying ribavirin as a UCK1 substrate, relevant to antiviral prodrug activation.

   Evidence In vitro phosphorylation assay with human UCK1

   PMID:18080217

   Open questions at the time

   • Single method, single lab
   • Relative contribution of UCK1 vs other kinases to ribavirin activation in cells unknown
4. 2016 Medium

   Revealed a non-catalytic regulatory role: UCK1 localizes to the nucleus and binds UCK2 to relocalize and inhibit it, recasting UCK1 as a dominant-negative modulator of salvage rather than purely a catalyst.

   Evidence GFP localization microscopy and his-tag pull-down; UCK1 overexpression with metabolic and cytotoxicity phenotypes in neuroblastoma cells

   PMID:27239701

   Open questions at the time

   • Reciprocal/endogenous interaction validation limited
   • Structural basis of UCK1-UCK2 association not defined
   • Generality beyond neuroblastoma cells untested
5. 2020 High

   Defined how UCK1 protein levels are set, identifying a writer (KLHL2), eraser (USP28), specific ubiquitination site (K81), and ATM-linked signaling control of its stability.

   Evidence Mass spectrometry, Co-IP, K81 site mutagenesis, and AML cell/murine models

   PMID:31938050

   Open questions at the time

   • Downstream metabolic consequence of UCK1 turnover not quantified
   • How ATM-driven destabilization affects salvage flux in vivo unclear
6. 2025 Medium

   Placed UCK1 within a nutrient-sensing regulatory axis by showing it modulates mTORC1-/CTLH-WDR26-dependent turnover of UCK2 through control of UCK2 localization.

   Evidence mTORC1 inhibition with protein stability and localization assays

   PMID:39808525

   Open questions at the time

   • UCK1 role described as modulatory, not mechanistically dissected
   • Single lab
   • Direct effect of UCK1 on CTLH complex not shown
7. 2026 High

   Provided atomic-resolution understanding of catalysis and quantified UCK1's contribution to molnupiravir activation relative to UCK2.

   Evidence Enzyme kinetics with recombinant UCK1/UCK2, UCK1-NHC-AMPPNP co-crystal structure, and siRNA knockdown in SARS-CoV-2-infected cells

   PMID:42213735

   Open questions at the time

   • In vivo relevance of UCK1 vs UCK2 to molnupiravir efficacy not established
   • Structure of UCK1 bound to natural substrates not reported

• How UCK1's catalytic and dominant-negative regulatory functions are balanced under physiological and stress conditions, and the mechanistic basis of its reported role in EGFR signaling, remain unresolved.
• EGFR-signaling requirement rests on a single siRNA screen with no mechanistic follow-up
• Integration of nuclear sequestration and direct catalysis in normal physiology unclear