UCKL1 is a 548-amino-acid uridine-kinase-family enzyme that phosphorylates uridine and cytidine with ATP to generate UMP and CMP, respectively, thereby supporting pyrimidine nucleoside salvage. The protein is predominantly cytoplasmic under basal conditions, while the full-length form can accumulate in the nucleus after binding Epstein-Barr virus EBNA3. RNF19B/NKLAM binds, ubiquitinates, and promotes degradation of UCKL1. UCKL1 abundance influences proliferation, apoptosis, and susceptibility to natural-killer-cell-mediated killing in tumor models. A recent study also reports a kinase-independent, nuclear UCK2-UCKL1 scaffold that promotes TGF-beta/SMAD3-dependent cardiac fibroblast activation after myocardial infarction. The C-terminal half is structurally UPRT-like, but uracil phosphoribosyltransferase activity has not been demonstrated.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004849 uridine kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: The IBA assignment matches direct biochemical evidence that purified human UCKL1 phosphorylates uridine with ATP. Reason: Uridine kinase activity is one of UCKL1's two directly demonstrated core molecular activities and is phylogenetically consistent with its uridine-kinase-family membership. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Cytoplasm is the best-supported default location: full-length GFP-UCKL1 was cytoplasmic before EBNA3 expression and endogenous protein was cytoplasmic in EBV-negative lymphoma cells. Reason: This broad location is supported by phylogenetic inference, direct imaging, UniProt curation, and cytosolic Reactome pathway placement. Nuclear localization is conditional and does not invalidate a cytoplasmic pool. Supporting Evidence: PMID:12199906 The predominantly cytoplasmic GFP-F538 translocates to the nucleus when EBNA-3 is expressed. |
| GO:0004849 uridine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: The automated uridine kinase assignment agrees with the direct 2022 biochemical characterization. Reason: Although this row duplicates stronger IBA and IDA evidence, it is accurate; independent annotations with different evidence codes are valid. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | ACCEPT | Summary: UCKL1 contains a predicted ATP-binding P-loop and purified UCKL1 uses ATP as the phosphate donor in its kinase reactions. Reason: ATP binding is mechanistically integral to the directly demonstrated nucleoside-kinase reaction and the InterPro mapping is consistent with experimental donor usage. Supporting Evidence: PMID:35583288 We analyzed the phosphorylation of both uridine and cytidine with ATP as the phosphate donor. |
| GO:0005634 nucleus | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Nuclear UCKL1 is experimentally observed, but the best-established 2002 localization is EBNA3-dependent and differs between full-length and C-terminally truncated forms. Reason: Retain nucleus as a real contextual location rather than a default core location. Full-length UCKL1 accumulates in the nucleus with EBNA3, while the truncated isoform remains cytoplasmic; a 2025 cardiac-fibroblast study also reports a nuclear UCK2-UCKL1 pool. Supporting Evidence: PMID:12199906 The predominantly cytoplasmic GFP-F538 translocates to the nucleus when EBNA-3 is expressed. PMID:12199906 GFP-F538ΞC is not targeted to the nucleus by EBNA-3. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping accurately captures the predominant basal localization observed for full-length and truncated UCKL1. Reason: Direct fluorescence and antibody staining support a cytoplasmic pool, and this location is compatible with cytosolic pyrimidine salvage. Supporting Evidence: PMID:12199906 The predominantly cytoplasmic GFP-F538 translocates to the nucleus when EBNA-3 is expressed. |
| GO:0008655 pyrimidine-containing compound salvage | IEA GO_REF:0000117 | ACCEPT | Summary: UCKL1 generates UMP and CMP from pyrimidine nucleosides, placing it directly in pyrimidine-containing compound salvage. Reason: The ARBA-derived process is broad but biologically central and is supported by direct enzymology plus independent genetic evidence for uridine salvage. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). PMID:39592843 We therefore generated a cell line also lacking the predicted UCK, UCKL1 (UCK tKO), and observed that uridine was unable to rescue the effects of de novo pyrimidine synthesis inhibition on proliferation and Chk1 phosphorylation, confirming the absence of UCK activity (Fig. 3c and Extended Data Fig. 4d). |
| GO:0009224 CMP biosynthetic process | IEA GO_REF:0000108 | MODIFY | Summary: UCKL1 does produce CMP from cytidine, but this broad logical-inference term obscures that the reaction is specifically the first step of CTP salvage. Reason: Replace the generic CMP biosynthetic-process assignment with the pathway-specific CTP salvage term already supported by the same cytidine-kinase reaction and UniPathway annotation. Proposed replacements: CTP salvage Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0016301 kinase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: UCKL1 is a kinase, but this parent term is much less informative than the directly demonstrated uridine kinase and cytidine kinase activities. Reason: The InterPro mapping is technically correct but redundant and overly broad now that substrate-specific molecular-function terms are available. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0043771 cytidine kinase activity | IEA GO_REF:0000116 | ACCEPT | Summary: Rhea mapping correctly assigns cytidine kinase activity, which purified UCKL1 directly demonstrates. Reason: Cytidine kinase activity is a core UCKL1 molecular function; this IEA row agrees with the IDA annotation and the curated Rhea reaction. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0055086 nucleobase-containing small molecule metabolic process | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: UCKL1 participates in this very broad metabolic parent through pyrimidine nucleoside salvage, but the term does not identify its actual pathway. Reason: More specific accepted terms for pyrimidine nucleoside, UMP, and CTP salvage capture the supported biology without the loss of information inherent in this ARBA parent. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:1901137 carbohydrate derivative biosynthetic process | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: Nucleotide synthesis falls under this extremely broad parent, but the term is not informative about UCKL1's direct salvage reactions. Reason: The ARBA inference is not false, but UMP and CTP salvage terms describe the supported process at the appropriate specificity. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0005515 protein binding | IPI PMID:12199906 Epstein-Barr virus encoded nuclear protein EBNA-3 binds a no... | MARK AS OVER ANNOTATED | Summary: UCKL1/F538 specifically binds EBV EBNA3, and the interaction drives conditional nuclear relocalization, but bare protein binding is uninformative as a molecular function. Reason: The interaction is experimentally real and is retained in the review, but GO:0005515 does not communicate the host-virus context or functional consequence and should not be treated as a core UCKL1 activity. Supporting Evidence: PMID:12199906 We have shown that F538 interacts specifically with EBNA-3 in the yeast two-hybrid system and in GST pull down assays. |
| GO:0005515 protein binding | IPI PMID:16709802 NK lytic-associated molecule, involved in NK cytotoxic funct... | MODIFY | Summary: RNF19B/NKLAM binds UCKL1 as an E3-ligase substrate and promotes its ubiquitination and reduced abundance. Reason: The interaction is explicit in the abstract-only source and is more informatively represented by ubiquitin protein ligase binding than by generic protein binding. Proposed replacements: ubiquitin protein ligase binding Supporting Evidence: PMID:16709802 We confirmed that NKLAM and URKL-1 interact in mammalian cells by using both immunoprecipitation and confocal microscopy. PMID:16709802 We demonstrated decreased protein expression and enhanced ubiquitination of URKL-1 in the presence of NKLAM. |
| GO:0043097 pyrimidine nucleoside salvage | TAS Reactome:R-HSA-73614 | ACCEPT | Summary: Reactome places UCKL1 in pyrimidine nucleoside salvage, consistent with direct uridine and cytidine phosphorylation. Reason: This is the appropriate shared process for the two core kinase activities and is independently supported by direct biochemical and genetic evidence. Supporting Evidence: Reactome:R-HSA-73614 In pyrimidine salvage reactions, nucleosides and free bases generated by DNA and RNA breakdown are converted back to nucleotide monophosphates, allowing them to re-enter the pathways of pyrimidine biosynthesis and interconversion. |
| GO:0004849 uridine kinase activity | IDA PMID:35583288 Characterization of uridine-cytidine kinase like-1 nucleosid... | ACCEPT | Summary: Purified recombinant UCKL1 directly phosphorylated uridine with ATP and kinetic parameters were measured. Reason: This is direct biochemical evidence for a core molecular function and is the strongest evidence supporting the uridine-kinase assignments. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0043771 cytidine kinase activity | IDA PMID:35583288 Characterization of uridine-cytidine kinase like-1 nucleosid... | ACCEPT | Summary: Purified recombinant UCKL1 directly phosphorylated cytidine with ATP and kinetic parameters were measured. Reason: This is direct biochemical evidence for UCKL1's second core molecular function and supports its placement in CTP salvage. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0044206 UMP salvage | IEA GO_REF:0000041 | ACCEPT | Summary: UCKL1 converts uridine to UMP, the defining reaction of UMP salvage. Reason: The UniPathway mapping is mechanistically supported by direct uridine kinase activity and independent genetic evidence for UCK-dependent uridine rescue. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). PMID:39592843 We therefore generated a cell line also lacking the predicted UCK, UCKL1 (UCK tKO), and observed that uridine was unable to rescue the effects of de novo pyrimidine synthesis inhibition on proliferation and Chk1 phosphorylation, confirming the absence of UCK activity (Fig. 3c and Extended Data Fig. 4d). |
| GO:0044211 CTP salvage | IEA GO_REF:0000041 | ACCEPT | Summary: UCKL1 converts cytidine to CMP, the first step in salvage of cytidine to CTP. Reason: The UniPathway mapping follows directly from experimentally demonstrated cytidine kinase activity and is the most informative process term for that reaction. Supporting Evidence: PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0004849 uridine kinase activity | TAS Reactome:R-HSA-8954327 | ACCEPT | Summary: Reactome's uridine-phosphorylation event predates direct UCKL1 enzymology but is now confirmed by the 2022 biochemical study. Reason: The event accurately captures a core activity. Its older sequence-based UPRT speculation should not be extended into a UPRT annotation. Supporting Evidence: Reactome:R-HSA-8954327 Based on sequence similarity, cytosolic uridine-cytidine kinase-like 1 (UCKL1 aka F538) is thought to function as a novel human uridine kinase/uracil phosphoribosyltransferase (Kashuba et al. 2002). PMID:35583288 Both enzymes efficiently phosphorylated uridine and cytidine (Figure 3). |
| GO:0005634 nucleus | EXP PMID:12199906 Epstein-Barr virus encoded nuclear protein EBNA-3 binds a no... | KEEP AS NON CORE | Summary: Full-length UCKL1/F538 accumulates in the nucleus when EBNA3 is expressed; endogenous protein is also predominantly nuclear in EBV-positive cells. Reason: Accept the experimental observation while classifying it as a host-virus-dependent, isoform-sensitive location rather than UCKL1's default functional compartment. Supporting Evidence: PMID:12199906 The predominantly cytoplasmic GFP-F538 translocates to the nucleus when EBNA-3 is expressed. PMID:12199906 GFP-F538ΞC is not targeted to the nucleus by EBNA-3. |
| GO:0005737 cytoplasm | EXP PMID:12199906 Epstein-Barr virus encoded nuclear protein EBNA-3 binds a no... | ACCEPT | Summary: Direct fluorescence and antibody staining show cytoplasmic UCKL1 in the absence of EBNA3. Reason: This is direct experimental support for UCKL1's predominant basal location and is compatible with its pyrimidine-salvage activity. Supporting Evidence: PMID:12199906 The predominantly cytoplasmic GFP-F538 translocates to the nucleus when EBNA-3 is expressed. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-8954327 | ACCEPT | Summary: Reactome models the UCKL1 phosphorylation event in the cytosol, a more specific location compatible with the experimentally observed cytoplasmic pool. Reason: Retain the TAS location because it is pathway-compatible and nested within the directly supported cytoplasm annotation, while recognizing that nuclear pools occur in specific contexts. Supporting Evidence: Reactome:R-HSA-8954327 Based on sequence similarity, cytosolic uridine-cytidine kinase-like 1 (UCKL1 aka F538) is thought to function as a novel human uridine kinase/uracil phosphoribosyltransferase (Kashuba et al. 2002). |
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Download this section (compressed HTML)Q: Does the C-terminal phosphoribosyltransferase-like domain of human UCKL1 catalyze PRPP-dependent uracil phosphoribosyltransferase activity under any physiological condition?
Suggested experts: Elise C. Matchett, Elise C. Ambrose, Jacki Kornbluth, Elena Kashuba
Q: Are the tumor-cell survival, NF-kappaB, and nucleoside-analogue phenotypes caused by UCKL1 kinase activity, by a kinase-independent scaffold function, or by both?
Suggested experts: Elise C. Ambrose, Jacki Kornbluth, Richard G. Hoover
Q: Is the reported kinase-independent nuclear UCK2-UCKL1-TRIM21 scaffold a reproducible endogenous mechanism in human post-infarction cardiac fibroblasts, and which UCKL1 interface is required?
Suggested experts: Xiao Zhou, Yu Zhang, Yongyi Wang
Q: Which UCKL1 splice isoforms retain nucleoside kinase activity, and how do their C-terminal differences control EBNA3-, RNF19B-, or TGF-beta-dependent localization and turnover?
Suggested experts: Elena Kashuba, Jacki Kornbluth
Experiment: Assay full-length UCKL1 and isolated or active-site-mutated C-terminal domains for conversion of uracil plus PRPP to UMP using orthogonal LC-MS-based product detection and appropriate active UPRT controls.
Hypothesis: The UPRT-like C-terminal domain is either catalytically active only under specific cofactor or oligomerization conditions, or is a catalytically inactive structural domain.
Type: Purified-enzyme biochemistry
Experiment: Create UCKL1-null human cells and rescue them at endogenous abundance with wild-type or kinase-dead UCKL1, then quantify labeled uridine and cytidine flux into UMP/CMP/UTP/CTP together with proliferation, apoptosis, NF-kappaB activity, and NK-cell killing.
Hypothesis: Pyrimidine-salvage flux requires UCKL1 kinase activity, whereas some survival or signaling phenotypes persist with catalytically inactive UCKL1.
Type: Catalytic-dead endogenous rescue and isotope tracing
Experiment: Tag individual endogenous UCKL1 isoforms and follow their localization, interaction, ubiquitination, and half-life after EBNA3 expression, RNF19B induction, or TGF-beta activation without overexpression.
Hypothesis: The C-terminal region controls partner-specific nuclear trafficking and degradation independently of the shared N-terminal nucleoside-kinase domain.
Type: Isoform-resolved endogenous localization
Experiment: Reproduce the UCK2-UCKL1-TRIM21-Smurf2/SMAD3 mechanism with orthogonal endogenous proximity labeling, separation-of-function interface mutants, kinase-dead knock-in rescue, and fibroblast-specific loss and rescue in a second myocardial-infarction model.
Hypothesis: UCKL1 has a genuine kinase-independent scaffold activity in activated cardiac fibroblasts that is separable from pyrimidine salvage.
Type: Independent cardiac-fibroblast mechanism replication
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