{"gene":"CTPS2","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":2019,"finding":"Cryo-EM structures reveal that human CTPS2 filaments dynamically switch between active and inactive conformational states in response to changes in substrate and product levels. Linking the conformational state of many CTPS2 subunits in a filament results in highly cooperative regulation, greatly exceeding the limits of cooperativity for the CTPS2 tetramer alone. The structures reveal a link between conformation and control of ammonia channeling between the enzyme's active sites.","method":"Cryo-EM structural determination of CTPS2 filaments in active and inactive states","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures with functional interpretation, multiple conformational states characterized, mechanistic link to ammonia channeling established","pmids":["31873303"],"is_preprint":false},{"year":2010,"finding":"Human CTPS2 is phosphorylated at Ser568 by casein kinase 1, both in vitro and in vivo, and this phosphorylation inhibits CTPS2 enzymatic activity; the S568A mutation significantly increased hCTPS2 activity. The phosphorylation effect was greater on glutamine-dependent than ammonia-dependent activity, suggesting that phosphorylation at Ser568 influences the glutaminase domain.","method":"Metabolic 32P labeling, phosphoamino acid and phosphopeptide mapping, site-directed mutagenesis (S568A), in vitro kinase assay with casein kinase 1","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro kinase assay plus mutagenesis plus in vivo labeling, single lab but multiple orthogonal methods","pmids":["20739275"],"is_preprint":false},{"year":2010,"finding":"Kinetic analysis showed that hCTPS2 is maximally active at physiological concentrations of ATP, GTP, and glutamine, while the Km for UTP and IC50 for CTP product inhibition are close to their physiological concentrations, indicating that intracellular UTP and CTP concentrations precisely regulate hCTPS2 activity.","method":"Kinetic enzyme assays with purified hCTPS1 and hCTPS2","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — rigorous in vitro kinetic characterization but single lab, single method","pmids":["20739275"],"is_preprint":false},{"year":2005,"finding":"Human CTPS1 and CTPS2 genes are functionally expressed in S. cerevisiae ura7Δura8Δ double mutant, complementing the lethal phenotype lacking CTP synthase activity. CTPS2 was shown to encode an active CTP synthase enzyme able to produce CTP in vivo.","method":"Genetic complementation in S. cerevisiae, immunoblot analysis, CTP synthase activity assay, in vivo CTP synthesis measurement","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic rescue plus biochemical validation, single lab","pmids":["16179339"],"is_preprint":false},{"year":2021,"finding":"Cryo-EM structures reveal that CTP regulates both CTPS1 and CTPS2 isoforms by binding in two inhibitory sites that clash with substrates. CTPS1 is less sensitive to CTP feedback inhibition than CTPS2, consistent with its role in increasing CTP levels during proliferation. Small-molecule CTPS1-selective inhibitors mimic CTP binding in one inhibitory site, where a single amino acid substitution explains selectivity for CTPS1 over CTPS2.","method":"Cryo-EM structures of CTPS1 and CTPS2 filaments with inhibitors and CTP, site-directed mutagenesis identifying the amino acid conferring selectivity","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structures with inhibitor binding sites, mutagenesis identifying selectivity determinant, directly characterizes isoform differences","pmids":["34583994"],"is_preprint":false},{"year":2023,"finding":"CTPS2 contributes to cell proliferation, but its contribution is modest when CTPS1 is expressed. However, CTPS2 becomes essential for proliferation in the absence of CTPS1. CTPS1 has higher intrinsic enzymatic activity than CTPS2 and is more resistant to inhibition by 3-deaza-uridine.","method":"CTPS1 and/or CTPS2 genetic inactivation, complementation experiments, enzymatic activity assays, 3-deaza-uridine inhibition assay","journal":"Life science alliance","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic inactivation and complementation plus enzymatic assays; replicated in cancer cell line databases (>1000 lines)","pmids":["37348953"],"is_preprint":false},{"year":2021,"finding":"In EBV-transformed lymphoblastoid cell lines (LCLs), double CTPS1/2 knockout caused stronger DNA damage and proliferation defects than CTPS1 knockout alone, indicating partially redundant roles of CTPS1 and CTPS2 in EBV-transformed B cells. Cytidine rescued CTPS1/2 deficiency phenotypes.","method":"CRISPR knockout of CTPS1 and/or CTPS2, proliferation assays, DNA damage assays, cytidine rescue experiments","journal":"mBio","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR double KO with multiple phenotypic readouts and rescue experiment, single lab","pmids":["34281398"],"is_preprint":false},{"year":2024,"finding":"Conditional deletion of Ctps2 (but not Ctps1) is NOT embryonic lethal in mice, while deletion of Ctps1 is embryonic lethal. Both CTPS1 and CTPS2 are required for T cell proliferation following TCR stimulation.","method":"Conditional and inducible Ctps1 and Ctps2 knockout mice, embryonic development analysis, T cell proliferation assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional KO mouse models with developmental and immunological phenotypic readouts, peer-reviewed in Nature Communications","pmids":["38438357"],"is_preprint":false},{"year":2011,"finding":"Reduction of CTPS2 expression by siRNA increased resistance of colorectal cancer cell lines to 5-FU and FUDR, significantly reducing S-phase accumulation and apoptosis following 5-FU treatment. Exposure to uridine (a precursor to CTPS2 substrate UTP) also increased 5-FU resistance.","method":"siRNA knockdown of CTPS2 in DLD1 and LS174T cell lines, cell cycle analysis, apoptosis assay, uridine supplementation","journal":"Cancer biology & therapy","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA knockdown with multiple readouts (cell cycle, apoptosis) and pharmacological rescue, single lab","pmids":["21378502"],"is_preprint":false},{"year":2023,"finding":"CTPS2 mediates DNA damage response by physically interacting with BRCA1 protein in chronic lymphocytic leukemia cells, as demonstrated by co-immunoprecipitation. Silencing CTPS2 elevated DNA damage and decreased DNA repair, and these effects were reversed by CTP or glutamine addition.","method":"Co-immunoprecipitation (CoIP), siRNA knockdown, RNA-seq, DNA damage assays, rescue with CTP or glutamine","journal":"Experimental hematology & oncology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single CoIP experiment plus functional rescue, single lab, multiple methods but CoIP not reciprocated","pmids":["36635772"],"is_preprint":false},{"year":2025,"finding":"CTPS1 and CTPS2 directly interact with each other independent of polymerization and cytoophidia formation, forming heterotetramers. When CTPS1 is associated with CTPS2, CTPS1 enzymatic activity is decreased and becomes more sensitive to CTP product negative feedback inhibition, demonstrating that CTPS2 modulates CTPS1 activity. CTPS2-containing filaments (cytoophidia) are dependent on CTPS1 expression, and both proteins co-localize in cytoophidia when co-expressed. CTPS1H355A and CTPS2H355A mutants unable to form cytoophidia can sustain normal cell proliferation.","method":"Co-localization imaging, co-immunoprecipitation, enzymatic activity assays with CTPS1/CTPS2 complexes, site-directed mutagenesis (H355A), CTP feedback inhibition assays","journal":"Life science alliance","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct interaction shown by Co-IP plus enzymatic activity measurements, mutagenesis, and co-localization; single lab, single study","pmids":["40957650"],"is_preprint":false},{"year":2021,"finding":"CTPS2 was identified as a potential interacting partner of the glutamine transporter SNAT6 in neurons, based on proximity ligation assay and co-localization analysis, suggesting CTPS2 participates in a complex near the pre-synaptic terminal of excitatory neurons.","method":"Proximity ligation assay (PLA), immunolabeling with co-localization analysis","journal":"International journal of molecular sciences","confidence":"Low","confidence_rationale":"Tier 3 / Weak — proximity ligation assay without reciprocal Co-IP or functional validation; single lab, indirect method","pmids":["33503881"],"is_preprint":false},{"year":2020,"finding":"CTPS1 and CTPS2 form filamentous structures (cytoophidia) under glutamine deprivation. Using CTPS-APEX2-mediated in vivo proximity labeling, SNAP29 was identified as a regulator of spatiotemporal CTPS filament assembly along the cytokeratin (keratin 8) network. Knockdown of SNAP29 interfered with filament assembly and relieved filament-induced suppression of CTPS enzymatic activity.","method":"APEX2 proximity labeling, SNAP29 knockdown, super-resolution imaging, enzymatic activity assay","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — APEX2 proximity labeling plus functional knockdown with enzymatic readout, imaging; single lab","pmids":["32184263"],"is_preprint":false}],"current_model":"Human CTPS2 is a CTP synthase that catalyzes de novo conversion of UTP to CTP; it forms homotetramers and assembles into cooperative filaments that switch between active and inactive conformations regulated by substrate/product levels, is inhibited by casein kinase 1-mediated phosphorylation at Ser568, and directly heterodimerizes/heterotetrarmerizes with CTPS1 to downregulate CTPS1 activity and increase its CTP-feedback sensitivity—making CTPS2 a modulator of overall cellular CTP synthesis that partially compensates for CTPS1 in proliferating cells but is dispensable for embryonic development and less efficient than CTPS1 in driving lymphocyte and cancer cell proliferation."},"narrative":{"mechanistic_narrative":"CTPS2 is a CTP synthase that catalyzes the de novo, glutamine-dependent conversion of UTP to CTP, the rate-limiting step in pyrimidine nucleotide biosynthesis, and as such supports cell proliferation [PMID:16179339, PMID:37348953]. Its activity is tuned by intracellular substrate and product levels: the Km for UTP and IC50 for CTP product inhibition sit near physiological concentrations, so UTP and CTP pools directly set enzyme output [PMID:20739275], and cryo-EM shows that CTPS2 assembles into filaments whose subunits switch cooperatively between active and inactive conformations, coupling oligomeric state to control of ammonia channeling between active sites [PMID:31873303]. CTP feedback inhibition is structurally explained by CTP binding two inhibitory sites that clash with substrate; CTPS2 is more sensitive to this feedback than the paralog CTPS1, and a single amino acid distinguishes their susceptibility to CTPS1-selective small molecules [PMID:34583994]. Activity is further restrained by casein kinase 1 phosphorylation at Ser568, which preferentially dampens glutamine-dependent (glutaminase) activity [PMID:20739275]. CTPS2 directly heterotetramerizes with CTPS1 independently of filament formation, and in this complex it lowers CTPS1 activity and heightens its CTP feedback sensitivity, positioning CTPS2 as a modulator of overall cellular CTP synthesis [PMID:40957650]. Functionally CTPS2 makes a modest contribution to proliferation when CTPS1 is present but becomes essential in its absence, and it is dispensable for embryonic development whereas CTPS1 is not [PMID:37348953, PMID:38438357]. Beyond CTP synthesis, CTPS2 supports the DNA damage response, with its loss elevating DNA damage and impairing repair in a manner rescued by CTP or glutamine [PMID:36635772].","teleology":[{"year":2005,"claim":"Established that human CTPS2 encodes a catalytically active CTP synthase rather than an inactive paralog, by showing it produces CTP in vivo.","evidence":"Genetic complementation of S. cerevisiae ura7Δura8Δ lethality plus CTP synthase activity assays","pmids":["16179339"],"confidence":"Medium","gaps":["Did not resolve regulation or how CTPS2 differs functionally from CTPS1","Heterologous yeast system, not native human cells"]},{"year":2010,"claim":"Defined how CTPS2 activity is set, showing both intracellular nucleotide pools and casein kinase 1 phosphorylation at Ser568 govern enzyme output.","evidence":"Kinetic enzyme assays on purified hCTPS2 plus 32P labeling, phosphopeptide mapping, S568A mutagenesis, and in vitro CK1 kinase assay","pmids":["20739275"],"confidence":"High","gaps":["Physiological trigger for CK1 phosphorylation in cells not identified","How Ser568 phosphorylation mechanistically targets the glutaminase domain not structurally resolved"]},{"year":2011,"claim":"Linked CTPS2-driven UTP/CTP supply to chemosensitivity, showing CTPS2 levels determine response to pyrimidine antimetabolites.","evidence":"siRNA knockdown of CTPS2 in colorectal cancer lines with cell-cycle, apoptosis, and uridine-rescue assays","pmids":["21378502"],"confidence":"Medium","gaps":["Does not distinguish CTPS2-specific from total CTP-pool effects","Single cancer-type context"]},{"year":2019,"claim":"Explained how CTPS2 achieves regulation beyond tetramer-level cooperativity by resolving filament-coupled conformational switching and its link to ammonia channeling.","evidence":"Cryo-EM of CTPS2 filaments captured in active and inactive states","pmids":["31873303"],"confidence":"High","gaps":["Functional consequence of filament formation in living cells not established by structure alone","Does not address heterotypic CTPS1/CTPS2 assemblies"]},{"year":2020,"claim":"Identified cellular machinery for CTPS filament assembly, placing cytoophidia formation under SNAP29 control along the keratin network and linking filaments to activity suppression.","evidence":"APEX2 proximity labeling, SNAP29 knockdown, super-resolution imaging, enzymatic assays under glutamine deprivation","pmids":["32184263"],"confidence":"Medium","gaps":["CTPS1 versus CTPS2 specific contributions to the labeled filaments not separated","Mechanism by which SNAP29 promotes assembly unknown"]},{"year":2021,"claim":"Provided the structural basis of CTP feedback inhibition and the determinant of CTPS1-versus-CTPS2 inhibitor selectivity, explaining why CTPS2 is more feedback-sensitive.","evidence":"Cryo-EM of CTPS1/CTPS2 filaments with CTP and inhibitors plus mutagenesis of the selectivity residue","pmids":["34583994"],"confidence":"High","gaps":["Does not address CTPS2 selective inhibition","In-cell relevance of the differential feedback sensitivity not tested here"]},{"year":2021,"claim":"Demonstrated functional redundancy between CTPS1 and CTPS2 in a proliferating cell context, with double loss producing DNA damage and proliferation defects rescued by cytidine.","evidence":"CRISPR single/double knockout in EBV-transformed lymphoblastoid cells with proliferation, DNA damage, and cytidine-rescue assays","pmids":["34281398"],"confidence":"Medium","gaps":["Quantitative contribution of CTPS2 versus CTPS1 not separated","Mechanism linking nucleotide depletion to DNA damage not defined"]},{"year":2023,"claim":"Quantified the proliferative hierarchy of the two isoforms, showing CTPS2 contributes modestly when CTPS1 is present but becomes essential in its absence.","evidence":"Genetic inactivation/complementation and enzymatic activity assays with 3-deaza-uridine inhibition","pmids":["37348953"],"confidence":"High","gaps":["Molecular reason for lower intrinsic CTPS2 activity not fully resolved","Tissue-context dependence of redundancy not mapped"]},{"year":2023,"claim":"Connected CTPS2 to genome maintenance via a physical interaction with BRCA1, with CTPS2 loss impairing DNA repair in leukemia cells.","evidence":"Co-immunoprecipitation, siRNA knockdown, RNA-seq, DNA damage assays, rescue with CTP or glutamine","pmids":["36635772"],"confidence":"Medium","gaps":["Single Co-IP without reciprocal validation","Cannot separate a direct BRCA1-complex role from indirect effects of CTP depletion"]},{"year":2024,"claim":"Resolved the in vivo non-redundancy, showing Ctps2 is dispensable for embryonic development while both isoforms are required for TCR-driven T cell proliferation.","evidence":"Conditional and inducible Ctps1/Ctps2 knockout mice with developmental and T cell proliferation analyses","pmids":["38438357"],"confidence":"High","gaps":["Tissue-specific roles of CTPS2 outside the immune system not defined","Why CTPS2 cannot substitute for CTPS1 during development not mechanistically explained"]},{"year":2025,"claim":"Reframed CTPS2 as a direct modulator of CTPS1, showing heterotetramer formation lowers CTPS1 activity and increases its CTP feedback sensitivity, independent of filament assembly.","evidence":"Co-localization imaging, Co-IP, enzymatic assays on CTPS1/CTPS2 complexes, H355A mutagenesis, and CTP feedback assays","pmids":["40957650"],"confidence":"Medium","gaps":["Single-study, single-lab characterization of the heterocomplex","In vivo stoichiometry and regulation of CTPS1:CTPS2 complexes unknown"]},{"year":null,"claim":"How CTPS2 abundance, phosphorylation, and heterocomplex formation are coordinated to set CTP supply across different tissues and proliferative states remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No integrated model linking CK1 phosphorylation, filament state, and CTPS1 heterotetramer formation in cells","CTPS2-selective regulatory inputs versus shared CTP-pool effects not disentangled"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016874","term_label":"ligase activity","supporting_discovery_ids":[3,5]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1,2]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[10]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[10]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[12]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[10,12]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[3,5]},{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[6,9]}],"complexes":["CTPS1/CTPS2 heterotetramer","CTPS cytoophidia (filaments)"],"partners":["CTPS1","BRCA1","SNAP29","SLC38A6"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9NRF8","full_name":"CTP synthase 2","aliases":["CTP synthetase 2","UTP--ammonia ligase 2"],"length_aa":586,"mass_kda":65.7,"function":"CTP synthase involved in the de novo synthesis of CTP, a precursor of DNA, RNA and phospholipids. Catalyzes the ATP-dependent amination of UTP to CTP with either L-glutamine or ammonia as a source of nitrogen (PubMed:10899599, PubMed:16179339, PubMed:31873303, PubMed:34583994). Constitutes the rate-limiting enzyme in the synthesis of cytosine nucleotides (PubMed:10899599, PubMed:16179339)","subcellular_location":"Cytoplasm, cytosol; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9NRF8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CTPS2","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CTPS1","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/search/CTPS2","total_profiled":1310},"omim":[{"mim_id":"300380","title":"CYTIDINE 5-PRIME TRIPHOSPHATE SYNTHETASE 2; CTPS2","url":"https://www.omim.org/entry/300380"},{"mim_id":"123860","title":"CYTIDINE 5-PRIME TRIPHOSPHATE SYNTHETASE 1; CTPS1","url":"https://www.omim.org/entry/123860"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Mitochondria","reliability":"Approved"},{"location":"Cytosol","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/CTPS2"},"hgnc":{"alias_symbol":["GATD5B"],"prev_symbol":[]},"alphafold":{"accession":"Q9NRF8","domains":[{"cath_id":"3.40.50.300","chopping":"1-34_119-289","consensus_level":"high","plddt":94.2695,"start":1,"end":289},{"cath_id":"3.40.50.880","chopping":"296-556","consensus_level":"high","plddt":93.9602,"start":296,"end":556}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NRF8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NRF8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NRF8-F1-predicted_aligned_error_v6.png","plddt_mean":91.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CTPS2","jax_strain_url":"https://www.jax.org/strain/search?query=CTPS2"},"sequence":{"accession":"Q9NRF8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NRF8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NRF8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NRF8"}},"corpus_meta":[{"pmid":"24870241","id":"PMC_24870241","title":"CTP 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alliance","url":"https://pubmed.ncbi.nlm.nih.gov/40957650","citation_count":0,"is_preprint":false},{"pmid":"41933137","id":"PMC_41933137","title":"Spliceosomal component SNRPE drives cell proliferation by regulating CTP synthase 1 mRNA splicing in ovarian cancer.","date":"2026","source":"Oncogene","url":"https://pubmed.ncbi.nlm.nih.gov/41933137","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":16760,"output_tokens":3270,"usd":0.049665,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10862,"output_tokens":3927,"usd":0.076243,"stage2_stop_reason":"end_turn"},"total_usd":0.125908,"stage1_batch_id":"msgbatch_01ACAxJQabDsht39JXeWG22d","stage2_batch_id":"msgbatch_01QEU22A9U5sbPTgsMzTmvSf","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2019,\n      \"finding\": \"Cryo-EM structures reveal that human CTPS2 filaments dynamically switch between active and inactive conformational states in response to changes in substrate and product levels. Linking the conformational state of many CTPS2 subunits in a filament results in highly cooperative regulation, greatly exceeding the limits of cooperativity for the CTPS2 tetramer alone. The structures reveal a link between conformation and control of ammonia channeling between the enzyme's active sites.\",\n      \"method\": \"Cryo-EM structural determination of CTPS2 filaments in active and inactive states\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures with functional interpretation, multiple conformational states characterized, mechanistic link to ammonia channeling established\",\n      \"pmids\": [\"31873303\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Human CTPS2 is phosphorylated at Ser568 by casein kinase 1, both in vitro and in vivo, and this phosphorylation inhibits CTPS2 enzymatic activity; the S568A mutation significantly increased hCTPS2 activity. The phosphorylation effect was greater on glutamine-dependent than ammonia-dependent activity, suggesting that phosphorylation at Ser568 influences the glutaminase domain.\",\n      \"method\": \"Metabolic 32P labeling, phosphoamino acid and phosphopeptide mapping, site-directed mutagenesis (S568A), in vitro kinase assay with casein kinase 1\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro kinase assay plus mutagenesis plus in vivo labeling, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"20739275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Kinetic analysis showed that hCTPS2 is maximally active at physiological concentrations of ATP, GTP, and glutamine, while the Km for UTP and IC50 for CTP product inhibition are close to their physiological concentrations, indicating that intracellular UTP and CTP concentrations precisely regulate hCTPS2 activity.\",\n      \"method\": \"Kinetic enzyme assays with purified hCTPS1 and hCTPS2\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — rigorous in vitro kinetic characterization but single lab, single method\",\n      \"pmids\": [\"20739275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Human CTPS1 and CTPS2 genes are functionally expressed in S. cerevisiae ura7Δura8Δ double mutant, complementing the lethal phenotype lacking CTP synthase activity. CTPS2 was shown to encode an active CTP synthase enzyme able to produce CTP in vivo.\",\n      \"method\": \"Genetic complementation in S. cerevisiae, immunoblot analysis, CTP synthase activity assay, in vivo CTP synthesis measurement\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic rescue plus biochemical validation, single lab\",\n      \"pmids\": [\"16179339\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-EM structures reveal that CTP regulates both CTPS1 and CTPS2 isoforms by binding in two inhibitory sites that clash with substrates. CTPS1 is less sensitive to CTP feedback inhibition than CTPS2, consistent with its role in increasing CTP levels during proliferation. Small-molecule CTPS1-selective inhibitors mimic CTP binding in one inhibitory site, where a single amino acid substitution explains selectivity for CTPS1 over CTPS2.\",\n      \"method\": \"Cryo-EM structures of CTPS1 and CTPS2 filaments with inhibitors and CTP, site-directed mutagenesis identifying the amino acid conferring selectivity\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structures with inhibitor binding sites, mutagenesis identifying selectivity determinant, directly characterizes isoform differences\",\n      \"pmids\": [\"34583994\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CTPS2 contributes to cell proliferation, but its contribution is modest when CTPS1 is expressed. However, CTPS2 becomes essential for proliferation in the absence of CTPS1. CTPS1 has higher intrinsic enzymatic activity than CTPS2 and is more resistant to inhibition by 3-deaza-uridine.\",\n      \"method\": \"CTPS1 and/or CTPS2 genetic inactivation, complementation experiments, enzymatic activity assays, 3-deaza-uridine inhibition assay\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic inactivation and complementation plus enzymatic assays; replicated in cancer cell line databases (>1000 lines)\",\n      \"pmids\": [\"37348953\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"In EBV-transformed lymphoblastoid cell lines (LCLs), double CTPS1/2 knockout caused stronger DNA damage and proliferation defects than CTPS1 knockout alone, indicating partially redundant roles of CTPS1 and CTPS2 in EBV-transformed B cells. Cytidine rescued CTPS1/2 deficiency phenotypes.\",\n      \"method\": \"CRISPR knockout of CTPS1 and/or CTPS2, proliferation assays, DNA damage assays, cytidine rescue experiments\",\n      \"journal\": \"mBio\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR double KO with multiple phenotypic readouts and rescue experiment, single lab\",\n      \"pmids\": [\"34281398\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Conditional deletion of Ctps2 (but not Ctps1) is NOT embryonic lethal in mice, while deletion of Ctps1 is embryonic lethal. Both CTPS1 and CTPS2 are required for T cell proliferation following TCR stimulation.\",\n      \"method\": \"Conditional and inducible Ctps1 and Ctps2 knockout mice, embryonic development analysis, T cell proliferation assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional KO mouse models with developmental and immunological phenotypic readouts, peer-reviewed in Nature Communications\",\n      \"pmids\": [\"38438357\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Reduction of CTPS2 expression by siRNA increased resistance of colorectal cancer cell lines to 5-FU and FUDR, significantly reducing S-phase accumulation and apoptosis following 5-FU treatment. Exposure to uridine (a precursor to CTPS2 substrate UTP) also increased 5-FU resistance.\",\n      \"method\": \"siRNA knockdown of CTPS2 in DLD1 and LS174T cell lines, cell cycle analysis, apoptosis assay, uridine supplementation\",\n      \"journal\": \"Cancer biology & therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA knockdown with multiple readouts (cell cycle, apoptosis) and pharmacological rescue, single lab\",\n      \"pmids\": [\"21378502\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"CTPS2 mediates DNA damage response by physically interacting with BRCA1 protein in chronic lymphocytic leukemia cells, as demonstrated by co-immunoprecipitation. Silencing CTPS2 elevated DNA damage and decreased DNA repair, and these effects were reversed by CTP or glutamine addition.\",\n      \"method\": \"Co-immunoprecipitation (CoIP), siRNA knockdown, RNA-seq, DNA damage assays, rescue with CTP or glutamine\",\n      \"journal\": \"Experimental hematology & oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single CoIP experiment plus functional rescue, single lab, multiple methods but CoIP not reciprocated\",\n      \"pmids\": [\"36635772\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CTPS1 and CTPS2 directly interact with each other independent of polymerization and cytoophidia formation, forming heterotetramers. When CTPS1 is associated with CTPS2, CTPS1 enzymatic activity is decreased and becomes more sensitive to CTP product negative feedback inhibition, demonstrating that CTPS2 modulates CTPS1 activity. CTPS2-containing filaments (cytoophidia) are dependent on CTPS1 expression, and both proteins co-localize in cytoophidia when co-expressed. CTPS1H355A and CTPS2H355A mutants unable to form cytoophidia can sustain normal cell proliferation.\",\n      \"method\": \"Co-localization imaging, co-immunoprecipitation, enzymatic activity assays with CTPS1/CTPS2 complexes, site-directed mutagenesis (H355A), CTP feedback inhibition assays\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct interaction shown by Co-IP plus enzymatic activity measurements, mutagenesis, and co-localization; single lab, single study\",\n      \"pmids\": [\"40957650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CTPS2 was identified as a potential interacting partner of the glutamine transporter SNAT6 in neurons, based on proximity ligation assay and co-localization analysis, suggesting CTPS2 participates in a complex near the pre-synaptic terminal of excitatory neurons.\",\n      \"method\": \"Proximity ligation assay (PLA), immunolabeling with co-localization analysis\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — proximity ligation assay without reciprocal Co-IP or functional validation; single lab, indirect method\",\n      \"pmids\": [\"33503881\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CTPS1 and CTPS2 form filamentous structures (cytoophidia) under glutamine deprivation. Using CTPS-APEX2-mediated in vivo proximity labeling, SNAP29 was identified as a regulator of spatiotemporal CTPS filament assembly along the cytokeratin (keratin 8) network. Knockdown of SNAP29 interfered with filament assembly and relieved filament-induced suppression of CTPS enzymatic activity.\",\n      \"method\": \"APEX2 proximity labeling, SNAP29 knockdown, super-resolution imaging, enzymatic activity assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — APEX2 proximity labeling plus functional knockdown with enzymatic readout, imaging; single lab\",\n      \"pmids\": [\"32184263\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Human CTPS2 is a CTP synthase that catalyzes de novo conversion of UTP to CTP; it forms homotetramers and assembles into cooperative filaments that switch between active and inactive conformations regulated by substrate/product levels, is inhibited by casein kinase 1-mediated phosphorylation at Ser568, and directly heterodimerizes/heterotetrarmerizes with CTPS1 to downregulate CTPS1 activity and increase its CTP-feedback sensitivity—making CTPS2 a modulator of overall cellular CTP synthesis that partially compensates for CTPS1 in proliferating cells but is dispensable for embryonic development and less efficient than CTPS1 in driving lymphocyte and cancer cell proliferation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CTPS2 is a CTP synthase that catalyzes the de novo, glutamine-dependent conversion of UTP to CTP, the rate-limiting step in pyrimidine nucleotide biosynthesis, and as such supports cell proliferation [#3, #5]. Its activity is tuned by intracellular substrate and product levels: the Km for UTP and IC50 for CTP product inhibition sit near physiological concentrations, so UTP and CTP pools directly set enzyme output [#2], and cryo-EM shows that CTPS2 assembles into filaments whose subunits switch cooperatively between active and inactive conformations, coupling oligomeric state to control of ammonia channeling between active sites [#0]. CTP feedback inhibition is structurally explained by CTP binding two inhibitory sites that clash with substrate; CTPS2 is more sensitive to this feedback than the paralog CTPS1, and a single amino acid distinguishes their susceptibility to CTPS1-selective small molecules [#4]. Activity is further restrained by casein kinase 1 phosphorylation at Ser568, which preferentially dampens glutamine-dependent (glutaminase) activity [#1]. CTPS2 directly heterotetramerizes with CTPS1 independently of filament formation, and in this complex it lowers CTPS1 activity and heightens its CTP feedback sensitivity, positioning CTPS2 as a modulator of overall cellular CTP synthesis [#10]. Functionally CTPS2 makes a modest contribution to proliferation when CTPS1 is present but becomes essential in its absence, and it is dispensable for embryonic development whereas CTPS1 is not [#5, #7]. Beyond CTP synthesis, CTPS2 supports the DNA damage response, with its loss elevating DNA damage and impairing repair in a manner rescued by CTP or glutamine [#9].\"\n,\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Established that human CTPS2 encodes a catalytically active CTP synthase rather than an inactive paralog, by showing it produces CTP in vivo.\",\n      \"evidence\": \"Genetic complementation of S. cerevisiae ura7\\u0394ura8\\u0394 lethality plus CTP synthase activity assays\",\n      \"pmids\": [\"16179339\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not resolve regulation or how CTPS2 differs functionally from CTPS1\", \"Heterologous yeast system, not native human cells\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Defined how CTPS2 activity is set, showing both intracellular nucleotide pools and casein kinase 1 phosphorylation at Ser568 govern enzyme output.\",\n      \"evidence\": \"Kinetic enzyme assays on purified hCTPS2 plus 32P labeling, phosphopeptide mapping, S568A mutagenesis, and in vitro CK1 kinase assay\",\n      \"pmids\": [\"20739275\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Physiological trigger for CK1 phosphorylation in cells not identified\", \"How Ser568 phosphorylation mechanistically targets the glutaminase domain not structurally resolved\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Linked CTPS2-driven UTP/CTP supply to chemosensitivity, showing CTPS2 levels determine response to pyrimidine antimetabolites.\",\n      \"evidence\": \"siRNA knockdown of CTPS2 in colorectal cancer lines with cell-cycle, apoptosis, and uridine-rescue assays\",\n      \"pmids\": [\"21378502\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Does not distinguish CTPS2-specific from total CTP-pool effects\", \"Single cancer-type context\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Explained how CTPS2 achieves regulation beyond tetramer-level cooperativity by resolving filament-coupled conformational switching and its link to ammonia channeling.\",\n      \"evidence\": \"Cryo-EM of CTPS2 filaments captured in active and inactive states\",\n      \"pmids\": [\"31873303\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Functional consequence of filament formation in living cells not established by structure alone\", \"Does not address heterotypic CTPS1/CTPS2 assemblies\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identified cellular machinery for CTPS filament assembly, placing cytoophidia formation under SNAP29 control along the keratin network and linking filaments to activity suppression.\",\n      \"evidence\": \"APEX2 proximity labeling, SNAP29 knockdown, super-resolution imaging, enzymatic assays under glutamine deprivation\",\n      \"pmids\": [\"32184263\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"CTPS1 versus CTPS2 specific contributions to the labeled filaments not separated\", \"Mechanism by which SNAP29 promotes assembly unknown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Provided the structural basis of CTP feedback inhibition and the determinant of CTPS1-versus-CTPS2 inhibitor selectivity, explaining why CTPS2 is more feedback-sensitive.\",\n      \"evidence\": \"Cryo-EM of CTPS1/CTPS2 filaments with CTP and inhibitors plus mutagenesis of the selectivity residue\",\n      \"pmids\": [\"34583994\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Does not address CTPS2 selective inhibition\", \"In-cell relevance of the differential feedback sensitivity not tested here\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstrated functional redundancy between CTPS1 and CTPS2 in a proliferating cell context, with double loss producing DNA damage and proliferation defects rescued by cytidine.\",\n      \"evidence\": \"CRISPR single/double knockout in EBV-transformed lymphoblastoid cells with proliferation, DNA damage, and cytidine-rescue assays\",\n      \"pmids\": [\"34281398\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Quantitative contribution of CTPS2 versus CTPS1 not separated\", \"Mechanism linking nucleotide depletion to DNA damage not defined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Quantified the proliferative hierarchy of the two isoforms, showing CTPS2 contributes modestly when CTPS1 is present but becomes essential in its absence.\",\n      \"evidence\": \"Genetic inactivation/complementation and enzymatic activity assays with 3-deaza-uridine inhibition\",\n      \"pmids\": [\"37348953\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Molecular reason for lower intrinsic CTPS2 activity not fully resolved\", \"Tissue-context dependence of redundancy not mapped\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Connected CTPS2 to genome maintenance via a physical interaction with BRCA1, with CTPS2 loss impairing DNA repair in leukemia cells.\",\n      \"evidence\": \"Co-immunoprecipitation, siRNA knockdown, RNA-seq, DNA damage assays, rescue with CTP or glutamine\",\n      \"pmids\": [\"36635772\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Single Co-IP without reciprocal validation\", \"Cannot separate a direct BRCA1-complex role from indirect effects of CTP depletion\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Resolved the in vivo non-redundancy, showing Ctps2 is dispensable for embryonic development while both isoforms are required for TCR-driven T cell proliferation.\",\n      \"evidence\": \"Conditional and inducible Ctps1/Ctps2 knockout mice with developmental and T cell proliferation analyses\",\n      \"pmids\": [\"38438357\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Tissue-specific roles of CTPS2 outside the immune system not defined\", \"Why CTPS2 cannot substitute for CTPS1 during development not mechanistically explained\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Reframed CTPS2 as a direct modulator of CTPS1, showing heterotetramer formation lowers CTPS1 activity and increases its CTP feedback sensitivity, independent of filament assembly.\",\n      \"evidence\": \"Co-localization imaging, Co-IP, enzymatic assays on CTPS1/CTPS2 complexes, H355A mutagenesis, and CTP feedback assays\",\n      \"pmids\": [\"40957650\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Single-study, single-lab characterization of the heterocomplex\", \"In vivo stoichiometry and regulation of CTPS1:CTPS2 complexes unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CTPS2 abundance, phosphorylation, and heterocomplex formation are coordinated to set CTP supply across different tissues and proliferative states remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No integrated model linking CK1 phosphorylation, filament state, and CTPS1 heterotetramer formation in cells\", \"CTPS2-selective regulatory inputs versus shared CTP-pool effects not disentangled\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016874\", \"supporting_discovery_ids\": [3, 5]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [12]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [10, 12]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [3, 5]},\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [6, 9]}\n    ],\n    \"complexes\": [\n      \"CTPS1/CTPS2 heterotetramer\",\n      \"CTPS cytoophidia (filaments)\"\n    ],\n    \"partners\": [\n      \"CTPS1\",\n      \"BRCA1\",\n      \"SNAP29\",\n      \"SLC38A6\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":7,"faith_pct":85.71428571428571}}