UCK1 (Uridine-cytidine kinase 1) is a nucleoside kinase that catalyzes the phosphorylation of uridine and cytidine to their respective monophosphates (UMP and CMP), functioning in the pyrimidine nucleotide salvage pathway. The enzyme can use ATP or GTP as phosphate donors and shows specificity for pyrimidine ribonucleosides, not phosphorylating deoxyribonucleosides or purine ribonucleosides. UCK1 is also capable of phosphorylating various nucleoside analogs used in cancer chemotherapy and antiviral therapy.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Cytoplasm is a broad, pathway-compatible location for UCK1, but the localization evidence is mixed. Reactome describes cytosolic UCK1 activity and the IBA row supports cytoplasm, while PMID:27239701 reports UCK1-GFP predominantly in the nucleus and UCK2-GFP in the cytosol. Reason: Accept this broad cytoplasm row only as a non-exclusive location supported by GOA/Reactome-style pathway context. It should not be read as overriding the later GFP evidence for a nuclear UCK1 pool, and a direct nucleus annotation may be warranted after curator review. Supporting Evidence: Reactome:R-HSA-73599 Cytosolic uridine-cytidine kinase 1 (UCK1) catalyzes the reactions of cytidine or uridine with ATP to form CMP or UMP and ADP file:human/UCK1/UCK1-deep-research-perplexity-lite.md See deep research file for comprehensive analysis PMID:27239701 Subcellular localization studies showed that UCK1-GFP and UCK2-GFP were localized in the cell nucleus and cytosol, respectively. |
| GO:0004849 uridine kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Uridine kinase activity is a core function of UCK1, directly demonstrated by experimental evidence showing phosphorylation of uridine to UMP. This IBA annotation is consistent with direct experimental evidence (IDA) from PMID:11306702. The enzyme is highly specific for pyrimidine ribonucleosides. Reason: This represents one of the two core molecular functions of UCK1. The enzyme catalyzes the phosphorylation of uridine to UMP using ATP or GTP as phosphate donor, as demonstrated experimentally in the original characterization study and confirmed through phylogenetic inference. The substrate specificity is well-established - UCK1 does not phosphorylate deoxyribonucleosides or purine ribonucleosides. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides. |
| GO:0009224 CMP biosynthetic process | IEA GO_REF:0000108 | MODIFY | Summary: UCK1 does generate CMP from cytidine, but this GO term is a broad biosynthetic-process parent that does not distinguish the salvage pathway from de novo CMP biosynthesis. The more informative and already annotated term is GO:0044211 CTP salvage. Reason: The evidence supports cytidine phosphorylation as the first step of CTP salvage, not a generic CMP biosynthetic process. Replace with the specific salvage-pathway term already supported by direct experimental evidence and UniProt pathway annotation. Proposed replacements: CTP salvage Supporting Evidence: file:human/UCK1/UCK1-uniprot.txt PATHWAY: Pyrimidine metabolism; CTP biosynthesis via salvage pathway; CTP from cytidine: step 1/3. |
| GO:0000166 nucleotide binding | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: This is an overly general parent term that doesn't provide meaningful information about UCK1's specific function. While technically correct that UCK1 binds nucleotides (ATP/GTP as phosphate donors, and uridine/cytidine as substrates), this broad term lacks functional specificity. The more specific child term GO:0005524 (ATP binding) is already annotated. Reason: This term is too vague and uninformative for curation purposes. It was automatically assigned based on UniProtKB keywords. UCK1 specifically binds ATP (and can also use GTP) as phosphate donors for its kinase activity. The more specific GO:0005524 (ATP binding) provides much more useful functional information and is already annotated. This overly broad parent term adds little value to the annotation set. Supporting Evidence: PMID:11306702 Uridine-cytidine kinases (UCK) have important roles for the phosphorylation of nucleoside analogs that are being investigated for possible use in chemotherapy of cancer. |
| GO:0004849 uridine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: This is a duplicate annotation of GO:0004849 with computational evidence (IEA), while the same term is also annotated with both IBA and IDA evidence. The IEA annotation is redundant but not incorrect. Reason: Although this is a duplicate annotation with weaker evidence than the IBA and IDA annotations for the same term, it is still accurate and represents a core function of UCK1. Duplicate annotations with different evidence codes are acceptable in GO. Supporting Evidence: PMID:11306702 UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. |
| GO:0005524 ATP binding | IEA GO_REF:0000120 | ACCEPT | Summary: UCK1 binds ATP as a phosphate donor for its kinase activity. The enzyme can also use GTP, but ATP is the primary physiological phosphate donor. This IEA annotation is supported by structural data (PDB 2JEO, 2UVQ) showing ATP/ADP binding sites and by the biochemical characterization demonstrating ATP usage. Reason: ATP binding is well-supported by multiple lines of evidence. The enzyme uses ATP (or GTP) as a phosphate donor in the reaction uridine/cytidine + ATP → UMP/CMP + ADP + H+. Crystallographic structures (PDB 2JEO, 2UVQ) directly demonstrate ATP/ADP binding. This is a functionally relevant molecular function term that is more informative than the parent term GO:0000166 (nucleotide binding). The annotation was correctly inferred from InterPro domains and UniProtKB keywords. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. |
| GO:0016301 kinase activity | IEA GO_REF:0000120 | MARK AS OVER ANNOTATED | Summary: This is a high-level parent term in the molecular function hierarchy. While UCK1 is indeed a kinase (specifically a nucleoside kinase), this term is overly general. The more specific and informative terms GO:0004849 (uridine kinase activity) and GO:0043771 (cytidine kinase activity) are already annotated with both IBA and IDA evidence. Reason: This broad parent term was automatically inferred from InterPro domains and UniProtKB keywords. While technically accurate, it provides minimal functional information compared to the specific child terms (uridine kinase activity, cytidine kinase activity) that are already annotated with experimental evidence. In curation, such overly general parent terms are typically considered over-annotations when more specific child terms adequately describe the function. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. |
| GO:0016740 transferase activity | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: This is an extremely high-level parent term in the molecular function ontology. UCK1 is indeed a transferase (kinases transfer phosphate groups from ATP to substrates), but this root-level term provides virtually no functional specificity. Multiple more informative descendant terms are already annotated (kinase activity, uridine kinase activity, cytidine kinase activity). Reason: This is a classic example of over-annotation with an overly broad parent term. It was automatically assigned based on UniProtKB keywords. While technically correct, GO:0016740 is so general that it encompasses a vast array of different enzyme activities. The more specific child terms (GO:0016301 kinase activity, GO:0004849 uridine kinase activity, GO:0043771 cytidine kinase activity) provide far more useful functional information. Annotating to such high-level parent terms when specific child terms exist is generally discouraged in manual curation. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. |
| GO:0043771 cytidine kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: This is a duplicate annotation of GO:0043771 with computational evidence (IEA), while the same term is also annotated with experimental evidence (IDA from PMID:11306702). Cytidine kinase activity is one of the two core molecular functions of UCK1. The IEA annotation was correctly inferred from RHEA reactions and EC number 2.7.1.48. Reason: Although this is a duplicate annotation with weaker evidence than the IDA annotation for the same term, it is still accurate and represents a core function of UCK1. The enzyme catalyzes the reaction cytidine + ATP = CMP + ADP + H+. Duplicate annotations with different evidence codes are acceptable in GO and can arise from independent annotation pipelines (manual curation vs automated inference). Both contribute to the overall evidence for this function. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. |
| GO:0044206 UMP salvage | IEA GO_REF:0000120 | ACCEPT | Summary: This is a duplicate annotation of GO:0044206 with computational evidence (IEA), while the same term is also annotated with experimental evidence (IDA from PMID:11306702). UMP salvage is a core biological process for UCK1, representing the salvage pathway alternative to de novo UMP biosynthesis. The IEA was correctly inferred from UniPathway UPA00574. Reason: Although this is a duplicate annotation with weaker evidence than the IDA annotation, it accurately represents UCK1's role in the pyrimidine salvage pathway. UCK1 catalyzes the sole step in UMP biosynthesis via the salvage pathway (uridine + ATP → UMP + ADP + H+). This is distinct from de novo pyrimidine synthesis. Duplicate annotations with different evidence codes are acceptable and can arise from independent curation and automated inference pipelines. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. |
| GO:0044211 CTP salvage | IEA GO_REF:0000120 | ACCEPT | Summary: This is a duplicate annotation of GO:0044211 with computational evidence (IEA), while the same term is also annotated with experimental evidence (IDA from PMID:11306702). CTP salvage is a core biological process for UCK1, representing the salvage pathway alternative to de novo CTP biosynthesis. The IEA was correctly inferred from UniPathway UPA00579. Reason: Although this is a duplicate annotation with weaker evidence than the IDA annotation, it accurately represents UCK1's role in the pyrimidine salvage pathway. UCK1 catalyzes the first step of three in CTP biosynthesis via the salvage pathway (cytidine + ATP → CMP + ADP + H+, followed by CMP → CDP → CTP by other enzymes). This is distinct from de novo pyrimidine synthesis. Duplicate annotations with different evidence codes are acceptable and can arise from independent curation and automated inference pipelines. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. |
| GO:0004849 uridine kinase activity | IDA PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b... | ACCEPT | Summary: Uridine kinase activity is a core molecular function of UCK1, directly demonstrated by experimental evidence (IDA). This annotation is based on the original biochemical characterization study by Van Rompay et al. (2001) showing that recombinant UCK1 phosphorylates uridine to UMP. The enzyme exhibits strict substrate specificity, not phosphorylating deoxyribonucleosides or purines. Reason: This is the gold standard annotation with direct experimental evidence (IDA) from the original characterization paper PMID:11306702. The study cloned and expressed recombinant UCK1 in E. coli and directly demonstrated its ability to catalyze uridine phosphorylation using biochemical assays. This represents one of the two core enzymatic activities of UCK1 (the other being cytidine kinase activity). The annotation is well-supported, specific, and represents a primary molecular function. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides. |
| GO:0043771 cytidine kinase activity | IDA PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b... | ACCEPT | Summary: Cytidine kinase activity is a core molecular function of UCK1, directly demonstrated by experimental evidence (IDA). This annotation is based on the original biochemical characterization study by Van Rompay et al. (2001) showing that recombinant UCK1 phosphorylates cytidine to CMP. The enzyme exhibits strict substrate specificity, not phosphorylating deoxyribonucleosides or purines. Reason: This is the gold standard annotation with direct experimental evidence (IDA) from the original characterization paper PMID:11306702. The study cloned and expressed recombinant UCK1 in E. coli and directly demonstrated its ability to catalyze cytidine phosphorylation using biochemical assays. This represents one of the two core enzymatic activities of UCK1 (the other being uridine kinase activity). The annotation is well-supported, specific, and represents a primary molecular function. UCK1's dual substrate specificity for both uridine and cytidine distinguishes it from other nucleoside kinases. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides. |
| GO:0044206 UMP salvage | IDA PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b... | ACCEPT | Summary: UMP salvage is a core biological process for UCK1, directly supported by experimental evidence (IDA). UCK1 catalyzes the conversion of uridine to UMP (uridine + ATP → UMP + ADP + H+), which is the sole enzymatic step in UMP biosynthesis via the salvage pathway. This salvage pathway provides an energy-efficient alternative to de novo pyrimidine synthesis. Reason: This is an accurate and well-supported annotation with direct experimental evidence (IDA) from PMID:11306702. UCK1 performs the complete UMP salvage pathway in a single step, converting free uridine (from nucleotide turnover or dietary sources) back into UMP for nucleotide pool maintenance. This is a core biological process function. The UniProt entry explicitly states this is "step 1/1" in UMP biosynthesis via salvage pathway. The salvage pathway is particularly important in tissues with high nucleotide turnover and for activating nucleoside analog drugs. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. |
| GO:0044211 CTP salvage | IDA PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b... | ACCEPT | Summary: CTP salvage is a core biological process for UCK1, directly supported by experimental evidence (IDA). UCK1 catalyzes the first step in CTP biosynthesis via the salvage pathway by converting cytidine to CMP (cytidine + ATP → CMP + ADP + H+). CMP is subsequently phosphorylated to CDP and then CTP by other enzymes (CMP kinase and nucleoside diphosphate kinase). This salvage pathway provides an energy-efficient alternative to de novo pyrimidine synthesis. Reason: This is an accurate and well-supported annotation with direct experimental evidence (IDA) from PMID:11306702. UCK1 performs the rate-limiting first step (step 1 of 3) in the CTP salvage pathway, converting free cytidine (from nucleotide turnover or dietary sources) to CMP. The complete pathway is cytidine → CMP → CDP → CTP. This is a core biological process function. The UniProt entry explicitly states this is "step 1/3" in CTP biosynthesis via salvage pathway. The salvage pathway is particularly important in rapidly dividing cells and for activating cytidine analog drugs used in cancer chemotherapy. Supporting Evidence: PMID:11306702 The approximately 30-kDa proteins, named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-73599 | ACCEPT | Summary: Reactome explicitly describes UCK1 as cytosolic in the cytidine/uridine phosphorylation reaction, but this conflicts with later UCK1-GFP evidence reporting predominant nuclear localization. The cytosol row is therefore accepted as Reactome pathway-supported but not as the sole or best localization for UCK1. Reason: Keep the cytosol annotation because it is directly supported by the Reactome TAS source for the metabolic reaction. However, this annotation should be interpreted alongside PMID:27239701, which reports nuclear UCK1-GFP localization and UCK1-dependent relocalization of UCK2. The review flags nucleus as a suggested follow-up annotation rather than treating cytosol as exclusive. Supporting Evidence: Reactome:R-HSA-73599 Cytosolic uridine-cytidine kinase 1 (UCK1) catalyzes the reactions of cytidine or uridine with ATP to form CMP or UMP and ADP PMID:27239701 Subcellular localization studies showed that UCK1-GFP and UCK2-GFP were localized in the cell nucleus and cytosol, respectively. |
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Download this section (compressed HTML)Q: Should UCK1 receive a direct nucleus cellular-component annotation based on the GFP localization evidence from PMID:27239701?
Suggested experts: GO cellular component curators, nucleotide metabolism specialists
Q: Which UCK1 isoforms or cell states determine nuclear versus cytosolic pyrimidine-salvage activity?
Suggested experts: cell biologists, metabolic enzyme specialists
Experiment: Use tagged knock-in or validated antibody staining to compare endogenous UCK1 localization with UCK1-GFP localization across proliferating and differentiated cells.
Hypothesis: UCK1 has a substantial nuclear pool that is not captured by current cytosol/cytoplasm pathway annotations.
Type: Endogenous localization
Experiment: Express individual UCK1 isoforms in UCK1/UCK2-deficient cells and measure uridine, cytidine, and nucleoside-analog phosphorylation.
Hypothesis: UCK1 isoforms differ in localization and in their contribution to pyrimidine salvage and analog activation.
Type: Isoform rescue biochemistry
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