UCK2

Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005737 cytoplasm	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation correctly places UCK2 in the cytoplasm. Multiple lines of evidence confirm cytoplasmic localization, including Reactome annotation as cytosolic enzyme and experimental data showing cytosol localization in transfected cells. Reason: Consistent with experimental evidence from multiple sources confirming cytoplasmic/cytosolic localization. UniProt CC line states function occurs in cytoplasm, Reactome describes enzyme as cytosolic, and direct fluorescence microscopy shows cytosol localization. Supporting Evidence: PMID:27239701 Subcellular localization studies showed that UCK1-GFP and UCK2-GFP were localized in the cell nucleus and cytosol, respectively. Reactome:R-HSA-109903 Cytosolic uridine-cytidine kinase 2 (UCK2) catalyzes the reactions of cytidine or uridine with ATP to form CMP or UMP and ADP file:human/UCK2/UCK2-deep-research-perplexity-lite.md See deep research file for comprehensive analysis file:human/UCK2/UCK2-deep-research-falcon.md cytosolic pyrimidine nucleoside salvage
GO:0004849 uridine kinase activity	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for uridine kinase activity is well-supported. UCK2 catalyzes phosphorylation of uridine to UMP using ATP. This is one of the two core catalytic activities of UCK2. Reason: This represents a core molecular function of UCK2, supported by direct experimental evidence and multiple independent studies. UniProt catalytic activity annotation confirms this function with EC 2.7.1.48. Supporting Evidence: PMID:11306702 The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides. UCK1 mRNA was detected as two isoforms... The enzymes phosphorylated several of the analogs, such as 6-azauridine, 5-fluorouridine PMID:27239701 Uridine-cytidine kinase (UCK) catalyzes the phosphorylation of uridine and cytidine file:human/UCK2/UCK2-deep-research-falcon.md Human UCK2 (UniProt Q9BZX2; EC 2.7.1.48) catalyzes ATP-dependent phosphorylation of the pyrimidine ribonucleosides uridine and cytidine to UMP and CMP, respectively, constituting the first/rate-limiting step of pyrimidine ribonucleoside salvage in many settings.
GO:0009224 CMP biosynthetic process	IEA GO_REF:0000108	MODIFY	Summary: IEA annotation for CMP biosynthetic process based on logical inference from cytidine kinase activity. While technically correct (UCK2 does synthesize CMP from cytidine), this term is overly general as it does not distinguish between de novo and salvage pathways. The more specific term GO:0044211 (CTP salvage) better captures UCK2's specific role in the salvage pathway. Reason: The term is too general and does not accurately reflect that UCK2 functions specifically in the salvage pathway, not de novo biosynthesis. UCK2 catalyzes cytidine + ATP to CMP + ADP, which is the first step of CTP salvage. The annotation should use GO:0044211 (CTP salvage) which is already present with IDA evidence. Proposed replacements: CTP salvage Supporting Evidence: PMID:11306702 shown to catalyze the phosphorylation of Urd and Cyd file:human/UCK2/UCK2-uniprot.txt 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 a very general molecular function term. While UCK2 does bind nucleotides (ATP/GTP as phosphate donors, and uridine/cytidine substrates), this annotation is too non-specific to be informative. The more specific ATP binding term (GO:0005524) is already present. Reason: This term is too general and does not provide meaningful information about UCK2 specific function. The GO:0005524 (ATP binding) annotation is more informative and specific. Nucleotide binding is implied by the kinase activity annotations. Supporting Evidence: PMID:11306702 named UCK1 and UCK2, were expressed in Escherichia coli and shown to catalyze the phosphorylation of Urd and Cyd
GO:0004849 uridine kinase activity	IEA GO_REF:0000120	ACCEPT	Summary: Duplicate annotation of uridine kinase activity with different evidence code. This IEA annotation is redundant with the IBA and IDA annotations for the same term. Reason: While redundant, computational annotations can coexist with experimental ones. The annotation is correct and represents a core function of UCK2. Supporting Evidence: PMID:11306702 The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides... catalyze the phosphorylation of Urd and Cyd
GO:0005524 ATP binding	IEA GO_REF:0000120	ACCEPT	Summary: UCK2 uses ATP (or GTP) as a phosphate donor in its kinase reactions. Crystal structures show ATP binding sites. This is an essential component of UCK2 catalytic mechanism. Reason: ATP binding is a core molecular function required for UCK2 kinase activity. Supported by structural data showing ATP binding sites and biochemical evidence that ATP serves as phosphate donor. Supporting Evidence: Reactome:R-HSA-109903 cytidine or uridine with ATP to form CMP or UMP and ADP file:human/UCK2/UCK2-deep-research-falcon.md with ATP serving as the phosphate donor.
GO:0016301 kinase activity	IEA GO_REF:0000120	MARK AS OVER ANNOTATED	Summary: General kinase activity term that is less specific than the actual uridine kinase and cytidine kinase activities already annotated. While technically correct, it provides little additional information. Reason: This is a parent term of the more specific uridine kinase and cytidine kinase activities. The specific terms (GO:0004849, GO:0043771) provide more informative annotations. General parent terms like this are often automatically inferred but do not add functional specificity. Supporting Evidence: PMID:11306702 shown to catalyze the phosphorylation of Urd and Cyd
GO:0016740 transferase activity	IEA GO_REF:0000043	MARK AS OVER ANNOTATED	Summary: Extremely general molecular function term. Kinases are indeed transferases (transferring phosphate groups), but this term is far too broad to be informative about UCK2 specific function. Reason: This is a very high-level parent term that provides minimal functional information. The specific kinase activity terms are far more informative. Such general terms are typically auto-generated but do not contribute meaningful biological insight. Supporting Evidence: PMID:11306702 catalyze the phosphorylation of Urd and Cyd
GO:0043771 cytidine kinase activity	IEA GO_REF:0000120	ACCEPT	Summary: Cytidine kinase activity is one of the two core catalytic activities of UCK2, converting cytidine to CMP. This is well-documented experimentally. Reason: This represents a core molecular function of UCK2, equally important as its uridine kinase activity. Both activities are confirmed by direct biochemical assays and structural studies. Supporting Evidence: PMID:11306702 catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides file:human/UCK2/UCK2-deep-research-falcon.md UCK2 is selective for pyrimidine ribonucleosides and does not phosphorylate purine ribonucleosides or 2′-deoxyribonucleosides
GO:0007631 feeding behavior	IEA GO_REF:0000107	REMOVE	Summary: This annotation appears to be a computational transfer from orthologs and is likely spurious for UCK2. There is no literature evidence connecting UCK2 to feeding behavior, and this seems implausible for a cytosolic nucleoside kinase involved in nucleotide metabolism. Reason: No supporting evidence for UCK2 involvement in feeding behavior. This appears to be an incorrect orthology-based inference. The deep research and all publications focus on nucleotide metabolism, cancer biology, and drug activation, with no mention of feeding behavior. The falcon deep research, which surveyed foundational enzymology through 2024 cancer/antiviral literature, likewise describes UCK2 exclusively as a pyrimidine ribonucleoside salvage kinase with no feeding-behavior role, reinforcing that this is a false positive from automated annotation transfer. Supporting Evidence: file:human/UCK2/UCK2-deep-research-falcon.md Human UCK2 (UniProt Q9BZX2; EC 2.7.1.48) catalyzes ATP-dependent phosphorylation of the pyrimidine ribonucleosides uridine and cytidine to UMP and CMP, respectively, constituting the first/rate-limiting step of pyrimidine ribonucleoside salvage in many settings.
GO:0048678 response to axon injury	IEA GO_REF:0000107	UNDECIDED	Summary: This annotation is based on automated ortholog transfer. While UCK2 is expressed in neural tissues and has been studied in neuroblastoma, there is no direct evidence linking it to axon injury response. This may be an overly specific inference from expression data. Reason: No direct experimental evidence in the available literature for UCK2 role in axon injury response. The neuroblastoma study focuses on UCK2 role in pyrimidine metabolism and drug sensitivity, not axon injury. Would need access to the original ortholog study (GO_REF:0000107) to evaluate this claim properly.
GO:0071453 cellular response to oxygen levels	IEA GO_REF:0000107	UNDECIDED	Summary: This annotation comes from automated ortholog transfer. There is no direct evidence in the available literature connecting UCK2 to oxygen level response. While UCK2 is overexpressed in various cancers which may involve hypoxic conditions, this is not the same as having a specific role in oxygen level response. Reason: No direct experimental evidence in available publications for UCK2 role in cellular response to oxygen levels. Would need to access the original reference and any supporting ortholog data to properly evaluate this annotation. The cancer association does not necessarily imply a direct oxygen response function.
GO:0042802 identical protein binding	IPI PMID:32296183 A reference map of the human binary protein interactome.	ACCEPT	Summary: This IPI annotation comes from the HuRI human reference interactome study, a large-scale yeast two-hybrid screen that detected UCK2 self-interaction. This is fully consistent with crystallographic data showing UCK2 functions as a homotetramer in its active form. Reason: UCK2 forms homotetramers, which necessarily involves identical protein binding. This is strongly supported by X-ray crystallographic structures (PDB entries 1UDW, 1UEI, 1UEJ, 1UFQ, etc.) showing the tetrameric assembly and the HuRI binary interaction data confirming self-interaction. The functional oligomerization state is well-established as essential for enzymatic activity. Supporting Evidence: PMID:32296183 (#)Contributed equally Global insights into cellular organization and genome function require comprehensive understanding of the interactome networks that mediate genotype-phenotype relationships1,2 file:human/UCK2/UCK2-uniprot.txt SUBUNIT: Homotetramer file:human/UCK2/UCK2-deep-research-falcon.md UCK2 is a ~29 kDa, 261-aa enzyme in the NMP kinase-fold family that forms a homotetramer
GO:0044206 UMP salvage	IEA GO_REF:0000120	ACCEPT	Summary: UMP salvage is a core biological process function of UCK2. The enzyme catalyzes the first and only step in converting uridine to UMP in the salvage pathway. Reason: This is a core biological process for UCK2, converting uridine to UMP in the salvage pathway. Well-supported by experimental data and pathway annotations. This is redundant with IDA annotations below but represents accurate function. Supporting Evidence: PMID:11306702 catalyze the phosphorylation of Urd and Cyd Reactome:R-HSA-109903 cytidine or uridine with ATP to form CMP or UMP and ADP file:human/UCK2/UCK2-deep-research-falcon.md UCK2 catalyzes the first committed phosphorylation step
GO:0044211 CTP salvage	IEA GO_REF:0000120	ACCEPT	Summary: CTP salvage is a core biological process function of UCK2. UCK2 catalyzes the first step (cytidine to CMP) in the three-step salvage pathway converting cytidine to CTP. Reason: This is a core biological process for UCK2. The enzyme performs the first step in the salvage pathway that recycles cytidine into CTP. Well-supported by experimental evidence and pathway databases. Redundant with IDA annotation below but accurate. Supporting Evidence: PMID:11306702 catalyze the phosphorylation of Urd and Cyd Reactome:R-HSA-109903 cytidine or uridine with ATP to form CMP or UMP and ADP file:human/UCK2/UCK2-deep-research-falcon.md UCK2 catalyzes the first committed phosphorylation step
GO:0004849 uridine kinase activity	IDA PMID:27239701 The pivotal role of uridine-cytidine kinases in pyrimidine m...	ACCEPT	Summary: Direct experimental evidence for uridine kinase activity from neuroblastoma study. This IDA annotation provides strong experimental support for this core function. Reason: Excellent experimental support from direct assay. This is a core molecular function of UCK2. The IDA evidence code indicates direct biochemical demonstration of this activity. Supporting Evidence: PMID:27239701 Uridine-cytidine kinase (UCK) catalyzes the phosphorylation of uridine and cytidine as well as the pharmacological activation of several cytotoxic pyrimidine ribonucleoside analogues
GO:0044206 UMP salvage	IDA PMID:27239701 The pivotal role of uridine-cytidine kinases in pyrimidine m...	ACCEPT	Summary: Direct experimental evidence for UMP salvage from neuroblastoma study showing UCK2 metabolizes uridine. Strong experimental support for this core biological process. Reason: Excellent experimental support from direct functional assays in neuroblastoma cells. The study directly measured uridine metabolism via UCK2, demonstrating its role in UMP salvage. This is a core biological process for UCK2. Supporting Evidence: PMID:27239701 Transient and stable overexpression of UCK2 in neuroblastoma cells increased the metabolism of uridine and cytidine
GO:0004849 uridine kinase activity	IDA PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b...	ACCEPT	Summary: Direct experimental evidence from the original cloning and characterization study. This is the foundational paper demonstrating UCK2 uridine kinase activity. Reason: This is the primary experimental characterization of UCK2 uridine kinase activity from the original cloning study. Direct biochemical demonstration of this core molecular function with recombinant enzyme. 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: Direct experimental evidence from the original characterization study. This established cytidine kinase as one of the two core activities of UCK2. Reason: Foundational experimental demonstration of UCK2 cytidine kinase activity from the original cloning and characterization study. Direct biochemical evidence with recombinant enzyme. This is a core molecular function. Supporting Evidence: PMID:11306702 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: Direct experimental evidence for UMP salvage function from the original characterization showing uridine phosphorylation. This demonstrated UCK2 role in the salvage pathway. Reason: Excellent experimental support from the foundational study. Direct demonstration that UCK2 phosphorylates uridine to UMP, which is the salvage pathway for UMP biosynthesis. This is a core biological process. Supporting Evidence: PMID:11306702 catalyze the phosphorylation of Urd and Cyd. The enzymes did not phosphorylate deoxyribonucleosides or purine ribonucleosides
GO:0044211 CTP salvage	IDA PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b...	ACCEPT	Summary: Direct experimental evidence for CTP salvage from the original study showing cytidine phosphorylation. UCK2 performs the first step in the CTP salvage pathway. Reason: Strong experimental support from the foundational characterization study. Direct demonstration that UCK2 phosphorylates cytidine to CMP, the first step in CTP salvage pathway. This is a core biological process. Supporting Evidence: PMID:11306702 catalyze the phosphorylation of Urd and Cyd
GO:0005829 cytosol	TAS Reactome:R-HSA-109903	ACCEPT	Summary: TAS (Traceable Author Statement) annotation based on Reactome pathway annotation. Reactome describes UCK2 as cytosolic. This is consistent with experimental localization data. Reason: Cytosol localization is well-supported by Reactome annotation and experimental evidence. While cytosol is a more specific term than cytoplasm (GO:0005737), both are appropriate. The TAS evidence from Reactome is reliable for subcellular localization. Supporting Evidence: Reactome:R-HSA-109903 Cytosolic uridine-cytidine kinase 2 (UCK2) 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
GO:0005525 GTP binding	IDA PMID:11306702 Phosphorylation of uridine and cytidine nucleoside analogs b...	NEW	Summary: UCK2 can use GTP as an alternative phosphate donor in addition to ATP. This is documented in the original characterization study and in UniProt. While ATP is the primary phosphate donor, GTP binding represents an additional molecular function that provides metabolic flexibility. Reason: This represents a documented molecular function that is not currently annotated. The ability to use GTP as a phosphate donor is explicitly stated in UniProt and the original characterization paper. While less prominent than ATP binding, it represents a genuine alternative cofactor usage that should be annotated. Supporting Evidence: file:human/UCK2/UCK2-uniprot.txt Can use ATP or GTP as a phosphate donor PMID:11306702 shown to catalyze the phosphorylation of Urd and Cyd

Gene Description

Existing Annotations Review

Core Functions

ATP-dependent phosphorylation of uridine to UMP in pyrimidine salvage pathway

ATP-dependent phosphorylation of cytidine to CMP in pyrimidine salvage pathway

References

Deep Research

Falcon

Perplexity

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