{"gene":"CTU1","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":2009,"finding":"CTU1 (NCS6/Ncs6p in yeast) is required for 2-thiolation of the wobble uridine (mcm5s2U) at position 34 of tRNAs, acting together with NCS2/Ncs2p. In vitro reconstitution showed that the sulfur-relay pathway proceeds through Nfs1p cysteine desulfurase → Tum1p → Urm1p thiocarboxylation by Uba4p → Ncs2p/Ncs6p-mediated 2-thiouridine formation on tRNA.","method":"In vitro sulfur transfer reconstitution assay with recombinant proteins; genetic deletion analysis in S. cerevisiae","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Strong — full in vitro reconstitution with multiple recombinant proteins, complemented by yeast genetic data, replicated across multiple pathway components","pmids":["19151091"],"is_preprint":false},{"year":2011,"finding":"Human CTU1 (ATPBD3) is a substrate of Urm1 urmylation; Urm1 is covalently conjugated to CTU1 (and CTU2 and MOCS3) on lysine residues, forming an isopeptide bond, and this modification is enhanced by oxidative stress.","method":"Mass spectrometry identification of Urm1 conjugates in mammalian cells; biochemical characterization of the thioester intermediate and isopeptide bond formation","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — MS identification plus biochemical characterization in single study, single lab","pmids":["21209336"],"is_preprint":false},{"year":2014,"finding":"Ncs6/Ctu1 activates uridine by adenylation and directly participates in sulfur transfer as a persulfide carrier, forming a persulfide enzyme adduct at an active site cysteine residue, as shown in the archaeal Methanococcus maripaludis homolog Mmp1356.","method":"Biochemical characterization of persulfide adduct formation; transposon mutagenesis showing essentiality; in vitro adenylation assay","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — direct biochemical demonstration of persulfide intermediate, in vitro assay, single lab","pmids":["24530533"],"is_preprint":false},{"year":2016,"finding":"A [3Fe-4S] cluster in archaeal and eukaryotic Ncs6 homologs (the direct orthologs of human CTU1) is essential for tRNA thiolation activity. Eukaryotic cytosolic Ncs6 homologs possess this iron-sulfur cluster required for catalysis.","method":"Biochemical and spectroscopic characterization of iron-sulfur cluster; activity assays showing cluster requirement for thiolation","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — spectroscopic and biochemical characterization of Fe-S cluster requirement, demonstrated in archaeal/eukaryotic homologs, single study","pmids":["27791189"],"is_preprint":false},{"year":2016,"finding":"CTU1 and CTU2, partner enzymes in U34 mcm5s2-tRNA modification, support cellular invasion and metastasis in breast cancer by enabling efficient translation of the oncoprotein DEK, which in turn promotes IRES-dependent translation of LEF1. A DEK mutant with codon composition independent of U34 mcm5s2-tRNA modification escapes CTU1-dependent regulation.","method":"Genetic ablation (Elp3 knockout in PyMT model); codon-substitution rescue experiment; IRES-dependent translation assay; invasion assays","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic ablation with defined phenotype, codon-substitution rescue, and IRES translation assay providing multiple orthogonal mechanistic lines of evidence","pmids":["27811057"],"is_preprint":false},{"year":2018,"finding":"CTU1 and CTU2, as U34 tRNA modification enzymes, promote glycolysis in melanoma cells through codon-dependent regulation of HIF1A mRNA translation, maintaining high HIF1α protein levels. Concurrent inhibition of MAPK signaling with CTU1/CTU2 synergistically kills melanoma cells. PI3K pathway activation, a mechanism of acquired BRAF therapy resistance, markedly increases CTU1/CTU2 expression.","method":"shRNA knockdown; pharmacological inhibition; polysome profiling; metabolic assays; codon-usage analysis","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (KD, pharmacological, polysome profiling, metabolic assays), mechanistic link to specific codon-dependent translation of HIF1A confirmed experimentally","pmids":["29925953"],"is_preprint":false},{"year":2019,"finding":"The CTU1/CTU2 complex catalyzes thiolation of wobble uridine in tRNAs (tRNAGln, tRNAGlu, tRNALys); loss-of-function variants in CTU2 abolish this modification in patient-derived cells, establishing CTU1/CTU2 as the enzyme complex responsible for this conserved modification in humans.","method":"Patient-derived cell functional characterization; mass spectrometry-based tRNA modification analysis; protein truncation variants","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional characterization in patient-derived cells with direct measurement of tRNA thiolation, single study","pmids":["31301155"],"is_preprint":false},{"year":2023,"finding":"The archaeal NcsA (Ncs6/CTU1 homolog) is a dimer that binds a [4Fe-4S] cluster coordinated by three conserved cysteines, with the fourth non-protein-bonded iron serving as the binding site for a hydrogenosulfide ligand, supporting a mechanism in which the [4Fe-4S] cluster activates the sulfur atom for tRNA thiolation. Comparison of NcsA crystal structure with AlphaFold model of human CTU1/CTU2 complex shows close superposition of catalytic site residues including the cluster-coordinating cysteines.","method":"Crystal structure at 2.8 Å; spectroscopic characterization of [4Fe-4S] cluster; biochemical assays; mutagenesis; AlphaFold structural comparison with human CTU1/CTU2","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure with biochemical validation and mutagenesis in single rigorous study; structural prediction comparison supports conservation of mechanism to human CTU1","pmids":["37005440"],"is_preprint":false},{"year":2024,"finding":"CTU2 serves as a scaffold protein while CTU1 catalyzes the 2-thiolation at the 34th wobble uridine (s2U34) of tRNAGlnUUG, tRNAGluUUC, and tRNALysUUU. CTU1 deficiency in zebrafish causes impaired angiogenesis and embryonic development, and in human endothelial cells impairs proliferation, migration, and tube formation.","method":"Antisense morpholino oligonucleotides in zebrafish; siRNA knockdown in human endothelial cells; single-cell RNA sequencing; functional assays (proliferation, migration, tube formation)","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function in vertebrate model with defined phenotypic readouts; single study, multiple methods","pmids":["39705244"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structure of the Uba4/Urm1 complex reveals the interaction interface and shows that Ncs6 (CTU1 ortholog) is a downstream component that receives thiocarboxylated Urm1-SH in the tRNA thiolation pathway. Ncs6 interacts with Urm1 as part of the sulfur relay cascade leading to tRNA 2-thiolation.","method":"Cryo-EM structure of Uba4/Urm1 complex; in vitro and in vivo mutagenesis; biochemical pathway reconstitution","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — cryo-EM structure with mutagenesis validation, Ncs6 interaction with Urm1 shown as part of pathway reconstitution, single study","pmids":["39673271"],"is_preprint":false},{"year":2025,"finding":"CTU1/CTU2 (U34-enzymes) become critically essential for cell growth when mTORC1 is inactivated. Loss of U34-enzymes particularly impairs synthesis of ribosomal proteins (identified by quantitative nascent proteomics, steady-state proteomics and ribosome profiling), and simultaneous suppression of mTORC1 and U34-enzymes depletes ribosomal proteins and globally inhibits translation.","method":"Genome-wide CRISPR screens; quantitative nascent proteomics; steady-state proteomics; ribosome profiling; in vitro and in vivo tumor models","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — genome-wide CRISPR screen followed by three orthogonal quantitative methods (nascent proteomics, steady-state proteomics, ribosome profiling) establishing mechanistic cooperation with mTORC1 in controlling ribosomal protein synthesis","pmids":["40328729"],"is_preprint":false},{"year":2013,"finding":"Fission yeast Ctu1-Ctu2 are essential for modification of wobble uridine in certain tRNAs and for normal tolerance to oxidative stress (H2O2). Loss of Ctu2 impairs translation of stress-induced mRNAs enriched in AAA lysine codons, including the transcription factors Atf1 and Pcr1, demonstrating codon-biased translation control downstream of tRNA thiolation.","method":"Genetic deletion; tRNA modification analysis; translation reporter assays; codon-substitution rescue (AAA-to-AAG atf1 mutant)","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis combined with codon-substitution rescue and translation assays, single lab","pmids":["23874237"],"is_preprint":false}],"current_model":"CTU1 (NCS6/Ncs6) is the catalytic subunit of the conserved eukaryotic CTU1/CTU2 complex (CTU2 serving as scaffold) that introduces a 2-thiol group at the wobble uridine 34 (s2U34) of tRNALysUUU, tRNAGluUUC, and tRNAGlnUUG via adenylation of the uridine and persulfide-mediated sulfur transfer, using a [4Fe-4S] cluster coordinated by conserved cysteines for catalysis; the thiocarboxylated form of Urm1 (generated by the Uba4/UBA4 E1-like enzyme) delivers sulfur to CTU1, and CTU1 itself can be urmylated on lysine residues; functionally, CTU1-dependent tRNA thiolation enables efficient codon-biased translation of AA-ending codon-enriched mRNAs (including HIF1A and oncoproteins such as DEK), is co-opted in cancer to drive glycolysis, invasion, and resistance to BRAF-targeted therapy, cooperates with mTORC1 to sustain ribosomal protein synthesis, and is required for angiogenesis and embryonic development in vertebrates."},"narrative":{"mechanistic_narrative":"CTU1 (NCS6/Ncs6) is the catalytic subunit of the eukaryotic CTU1/CTU2 complex that installs a 2-thiol group at the wobble uridine 34 (s2U34) of tRNALys(UUU), tRNAGlu(UUC), and tRNAGln(UUG), with CTU2 acting as a scaffold and CTU1 performing catalysis [PMID:19151091, PMID:39705244]. Mechanistically, CTU1 activates the target uridine by adenylation and itself acts as a persulfide carrier, forming a persulfide adduct at an active-site cysteine [PMID:24530533], and its catalysis depends on an iron-sulfur cluster ([3Fe-4S]/[4Fe-4S]) coordinated by three conserved cysteines whose fourth, non-protein-bound iron positions a sulfur ligand for transfer to tRNA [PMID:27791189, PMID:37005440]. Sulfur is supplied through a relay in which thiocarboxylated Urm1, generated by the Uba4/UBA4 E1-like enzyme, is received by CTU1 [PMID:19151091, PMID:39673271]; CTU1 is additionally a target of Urm1 conjugation (urmylation) on lysine residues, a modification enhanced by oxidative stress [PMID:21209336]. Functionally, this thiolation enables codon-biased translation of mRNAs enriched in AA-ending codons, driving production of the oncoprotein DEK and downstream LEF1 to support breast cancer invasion [PMID:27811057], sustaining HIF1A translation and glycolysis in melanoma where the activity cooperates with MAPK/PI3K signaling in BRAF therapy resistance [PMID:29925953], and becoming critically essential to ribosomal protein synthesis when mTORC1 is inactivated [PMID:40328729]. CTU1-dependent thiolation is required for angiogenesis and embryonic development in vertebrates and for endothelial proliferation, migration, and tube formation [PMID:39705244].","teleology":[{"year":2009,"claim":"Established that CTU1/Ncs6, with Ncs2, performs the terminal 2-thiolation step of wobble uridine, placing it at the end of a defined sulfur-relay pathway.","evidence":"In vitro sulfur-transfer reconstitution with recombinant proteins plus yeast deletion analysis","pmids":["19151091"],"confidence":"High","gaps":["Catalytic mechanism of Ncs6/Ncs2 in sulfur incorporation not resolved","No structural basis for tRNA recognition"]},{"year":2011,"claim":"Showed human CTU1 is not only a thiolation enzyme but also a substrate of Urm1, linking the modifier to a post-translational lysine conjugation responsive to oxidative stress.","evidence":"Mass spectrometry of Urm1 conjugates and biochemical characterization of isopeptide bonds in mammalian cells","pmids":["21209336"],"confidence":"Medium","gaps":["Functional consequence of CTU1 urmylation unknown","Single study/single lab"]},{"year":2014,"claim":"Defined the catalytic chemistry: CTU1/Ncs6 adenylates the uridine and carries sulfur as an active-site persulfide, resolving how sulfur reaches the tRNA.","evidence":"Biochemical persulfide adduct detection and in vitro adenylation assay on archaeal homolog Mmp1356","pmids":["24530533"],"confidence":"Medium","gaps":["Demonstrated in archaeal homolog rather than human CTU1","Order of adenylation and sulfur transfer not fully resolved"]},{"year":2016,"claim":"Identified an iron-sulfur cluster as an essential catalytic cofactor of Ncs6/CTU1, revealing the chemical basis for sulfur activation.","evidence":"Spectroscopic and biochemical characterization of the Fe-S cluster with activity assays in archaeal/eukaryotic homologs","pmids":["27791189"],"confidence":"Medium","gaps":["Cluster assembly machinery for cytosolic CTU1 not defined","Single study"]},{"year":2016,"claim":"Connected CTU1 thiolation to cancer phenotype by showing codon-biased translation of the oncoprotein DEK drives invasion and metastasis.","evidence":"Genetic ablation in PyMT model, codon-substitution rescue, IRES translation and invasion assays","pmids":["27811057"],"confidence":"High","gaps":["Direct CTU1 catalytic loss vs Elp3 pathway contributions not separated","Full set of codon-dependent targets undefined"]},{"year":2018,"claim":"Extended the cancer role to metabolic reprogramming, showing CTU1/CTU2 sustain HIF1A translation and glycolysis and intersect with MAPK/PI3K signaling in BRAF therapy resistance.","evidence":"shRNA knockdown, pharmacological inhibition, polysome profiling, metabolic and codon-usage analysis in melanoma","pmids":["29925953"],"confidence":"High","gaps":["Mechanism by which PI3K elevates CTU1/CTU2 expression not detailed","Generality across tumor types untested here"]},{"year":2019,"claim":"Confirmed CTU1/CTU2 as the human enzyme complex responsible for wobble uridine thiolation using patient cells with loss-of-function variants.","evidence":"Functional characterization of patient-derived cells with MS-based tRNA modification analysis and truncation variants","pmids":["31301155"],"confidence":"Medium","gaps":["Variants studied were in CTU2 rather than CTU1","Phenotype-to-modification causality at organismal level not established"]},{"year":2023,"claim":"Provided the structural basis: a crystal structure of the NcsA homolog showed a dimer binding a [4Fe-4S] cluster via three cysteines with the fourth iron binding sulfide, and AlphaFold comparison supports conservation in human CTU1/CTU2.","evidence":"2.8 Å crystal structure, spectroscopy, mutagenesis, and AlphaFold comparison with human CTU1/CTU2","pmids":["37005440"],"confidence":"High","gaps":["No experimental structure of human CTU1/CTU2","tRNA-bound complex structure absent"]},{"year":2024,"claim":"Resolved how CTU1 receives sulfur, placing Ncs6/CTU1 downstream of the Uba4/Urm1 thiocarboxylation machinery via cryo-EM and pathway reconstitution.","evidence":"Cryo-EM of Uba4/Urm1 complex with mutagenesis and pathway reconstitution","pmids":["39673271"],"confidence":"Medium","gaps":["Direct CTU1-Urm1 handoff interface not structurally resolved","Single study"]},{"year":2024,"claim":"Demonstrated a physiological requirement in vertebrates, showing CTU1 catalyzes s2U34 on three tRNAs and is needed for angiogenesis and embryonic development.","evidence":"Zebrafish morpholino knockdown, siRNA in human endothelial cells, scRNA-seq, and functional assays","pmids":["39705244"],"confidence":"Medium","gaps":["Morpholino off-target effects not fully excluded","Codon-dependent target genes in endothelium not identified"]},{"year":2025,"claim":"Revealed a synthetic-lethal cooperation with mTORC1, showing U34 enzymes become essential for ribosomal protein synthesis when mTORC1 is inactivated.","evidence":"Genome-wide CRISPR screens, nascent and steady-state proteomics, ribosome profiling, and tumor models","pmids":["40328729"],"confidence":"High","gaps":["Specific ribosomal protein mRNAs most codon-dependent not enumerated","Therapeutic window for combined mTORC1/U34 inhibition untested clinically"]},{"year":null,"claim":"The functional significance of CTU1 urmylation and the structure of the human CTU1/CTU2-tRNA complex during catalysis remain unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No experimental human CTU1/CTU2 structure","Consequence of CTU1 self-urmylation on activity unknown","Mechanism of Fe-S cluster assembly for cytosolic CTU1 undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140098","term_label":"catalytic activity, acting on RNA","supporting_discovery_ids":[0,2,6,8]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[2]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[0,8]},{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[2]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[3]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,6,8]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[10]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[4,5]}],"complexes":["CTU1/CTU2 complex"],"partners":["CTU2","URM1","UBA4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q7Z7A3","full_name":"Cytoplasmic tRNA 2-thiolation protein 1","aliases":["ATP-binding domain-containing protein 3","Cancer-associated gene protein","Cytoplasmic tRNA adenylyltransferase 1"],"length_aa":348,"mass_kda":36.5,"function":"Plays a central role in 2-thiolation of mcm(5)S(2)U at tRNA wobble positions of tRNA(Lys), tRNA(Glu) and tRNA(Gln). Directly binds tRNAs and probably acts by catalyzing adenylation of tRNAs, an intermediate required for 2-thiolation. It is unclear whether it acts as a sulfurtransferase that transfers sulfur from thiocarboxylated URM1 onto the uridine of tRNAs at wobble position","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q7Z7A3/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CTU1","classification":"Not Classified","n_dependent_lines":415,"n_total_lines":1208,"dependency_fraction":0.3435430463576159},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CTU1","total_profiled":1310},"omim":[{"mim_id":"617057","title":"CYTOSOLIC THIOURIDYLASE, SUBUNIT 2; CTU2","url":"https://www.omim.org/entry/617057"},{"mim_id":"612722","title":"ELONGATOR ACETYLTRANSFERASE COMPLEX, SUBUNIT 3; ELP3","url":"https://www.omim.org/entry/612722"},{"mim_id":"612694","title":"CYTOSOLIC THIOURIDYLASE, SUBUNIT 1; CTU1","url":"https://www.omim.org/entry/612694"},{"mim_id":"164757","title":"B-RAF PROTOONCOGENE, SERINE/THREONINE KINASE; BRAF","url":"https://www.omim.org/entry/164757"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in many","driving_tissues":[],"url":"https://www.proteinatlas.org/search/CTU1"},"hgnc":{"alias_symbol":["MGC17332","NCS6"],"prev_symbol":["ATPBD3"]},"alphafold":{"accession":"Q7Z7A3","domains":[{"cath_id":"3.40.50.620","chopping":"35-276","consensus_level":"high","plddt":90.1954,"start":35,"end":276},{"cath_id":"-","chopping":"284-322","consensus_level":"medium","plddt":82.6287,"start":284,"end":322}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q7Z7A3","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q7Z7A3-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q7Z7A3-F1-predicted_aligned_error_v6.png","plddt_mean":84.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CTU1","jax_strain_url":"https://www.jax.org/strain/search?query=CTU1"},"sequence":{"accession":"Q7Z7A3","fasta_url":"https://rest.uniprot.org/uniprotkb/Q7Z7A3.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q7Z7A3/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q7Z7A3"}},"corpus_meta":[{"pmid":"29925953","id":"PMC_29925953","title":"Codon-specific 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research","url":"https://pubmed.ncbi.nlm.nih.gov/40145964","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":17716,"output_tokens":3385,"usd":0.051962,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10911,"output_tokens":3602,"usd":0.072303,"stage2_stop_reason":"end_turn"},"total_usd":0.124265,"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\": 2009,\n      \"finding\": \"CTU1 (NCS6/Ncs6p in yeast) is required for 2-thiolation of the wobble uridine (mcm5s2U) at position 34 of tRNAs, acting together with NCS2/Ncs2p. In vitro reconstitution showed that the sulfur-relay pathway proceeds through Nfs1p cysteine desulfurase → Tum1p → Urm1p thiocarboxylation by Uba4p → Ncs2p/Ncs6p-mediated 2-thiouridine formation on tRNA.\",\n      \"method\": \"In vitro sulfur transfer reconstitution assay with recombinant proteins; genetic deletion analysis in S. cerevisiae\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — full in vitro reconstitution with multiple recombinant proteins, complemented by yeast genetic data, replicated across multiple pathway components\",\n      \"pmids\": [\"19151091\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Human CTU1 (ATPBD3) is a substrate of Urm1 urmylation; Urm1 is covalently conjugated to CTU1 (and CTU2 and MOCS3) on lysine residues, forming an isopeptide bond, and this modification is enhanced by oxidative stress.\",\n      \"method\": \"Mass spectrometry identification of Urm1 conjugates in mammalian cells; biochemical characterization of the thioester intermediate and isopeptide bond formation\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — MS identification plus biochemical characterization in single study, single lab\",\n      \"pmids\": [\"21209336\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Ncs6/Ctu1 activates uridine by adenylation and directly participates in sulfur transfer as a persulfide carrier, forming a persulfide enzyme adduct at an active site cysteine residue, as shown in the archaeal Methanococcus maripaludis homolog Mmp1356.\",\n      \"method\": \"Biochemical characterization of persulfide adduct formation; transposon mutagenesis showing essentiality; in vitro adenylation assay\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — direct biochemical demonstration of persulfide intermediate, in vitro assay, single lab\",\n      \"pmids\": [\"24530533\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"A [3Fe-4S] cluster in archaeal and eukaryotic Ncs6 homologs (the direct orthologs of human CTU1) is essential for tRNA thiolation activity. Eukaryotic cytosolic Ncs6 homologs possess this iron-sulfur cluster required for catalysis.\",\n      \"method\": \"Biochemical and spectroscopic characterization of iron-sulfur cluster; activity assays showing cluster requirement for thiolation\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — spectroscopic and biochemical characterization of Fe-S cluster requirement, demonstrated in archaeal/eukaryotic homologs, single study\",\n      \"pmids\": [\"27791189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CTU1 and CTU2, partner enzymes in U34 mcm5s2-tRNA modification, support cellular invasion and metastasis in breast cancer by enabling efficient translation of the oncoprotein DEK, which in turn promotes IRES-dependent translation of LEF1. A DEK mutant with codon composition independent of U34 mcm5s2-tRNA modification escapes CTU1-dependent regulation.\",\n      \"method\": \"Genetic ablation (Elp3 knockout in PyMT model); codon-substitution rescue experiment; IRES-dependent translation assay; invasion assays\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic ablation with defined phenotype, codon-substitution rescue, and IRES translation assay providing multiple orthogonal mechanistic lines of evidence\",\n      \"pmids\": [\"27811057\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"CTU1 and CTU2, as U34 tRNA modification enzymes, promote glycolysis in melanoma cells through codon-dependent regulation of HIF1A mRNA translation, maintaining high HIF1α protein levels. Concurrent inhibition of MAPK signaling with CTU1/CTU2 synergistically kills melanoma cells. PI3K pathway activation, a mechanism of acquired BRAF therapy resistance, markedly increases CTU1/CTU2 expression.\",\n      \"method\": \"shRNA knockdown; pharmacological inhibition; polysome profiling; metabolic assays; codon-usage analysis\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (KD, pharmacological, polysome profiling, metabolic assays), mechanistic link to specific codon-dependent translation of HIF1A confirmed experimentally\",\n      \"pmids\": [\"29925953\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The CTU1/CTU2 complex catalyzes thiolation of wobble uridine in tRNAs (tRNAGln, tRNAGlu, tRNALys); loss-of-function variants in CTU2 abolish this modification in patient-derived cells, establishing CTU1/CTU2 as the enzyme complex responsible for this conserved modification in humans.\",\n      \"method\": \"Patient-derived cell functional characterization; mass spectrometry-based tRNA modification analysis; protein truncation variants\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional characterization in patient-derived cells with direct measurement of tRNA thiolation, single study\",\n      \"pmids\": [\"31301155\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The archaeal NcsA (Ncs6/CTU1 homolog) is a dimer that binds a [4Fe-4S] cluster coordinated by three conserved cysteines, with the fourth non-protein-bonded iron serving as the binding site for a hydrogenosulfide ligand, supporting a mechanism in which the [4Fe-4S] cluster activates the sulfur atom for tRNA thiolation. Comparison of NcsA crystal structure with AlphaFold model of human CTU1/CTU2 complex shows close superposition of catalytic site residues including the cluster-coordinating cysteines.\",\n      \"method\": \"Crystal structure at 2.8 Å; spectroscopic characterization of [4Fe-4S] cluster; biochemical assays; mutagenesis; AlphaFold structural comparison with human CTU1/CTU2\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure with biochemical validation and mutagenesis in single rigorous study; structural prediction comparison supports conservation of mechanism to human CTU1\",\n      \"pmids\": [\"37005440\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"CTU2 serves as a scaffold protein while CTU1 catalyzes the 2-thiolation at the 34th wobble uridine (s2U34) of tRNAGlnUUG, tRNAGluUUC, and tRNALysUUU. CTU1 deficiency in zebrafish causes impaired angiogenesis and embryonic development, and in human endothelial cells impairs proliferation, migration, and tube formation.\",\n      \"method\": \"Antisense morpholino oligonucleotides in zebrafish; siRNA knockdown in human endothelial cells; single-cell RNA sequencing; functional assays (proliferation, migration, tube formation)\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function in vertebrate model with defined phenotypic readouts; single study, multiple methods\",\n      \"pmids\": [\"39705244\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structure of the Uba4/Urm1 complex reveals the interaction interface and shows that Ncs6 (CTU1 ortholog) is a downstream component that receives thiocarboxylated Urm1-SH in the tRNA thiolation pathway. Ncs6 interacts with Urm1 as part of the sulfur relay cascade leading to tRNA 2-thiolation.\",\n      \"method\": \"Cryo-EM structure of Uba4/Urm1 complex; in vitro and in vivo mutagenesis; biochemical pathway reconstitution\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — cryo-EM structure with mutagenesis validation, Ncs6 interaction with Urm1 shown as part of pathway reconstitution, single study\",\n      \"pmids\": [\"39673271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CTU1/CTU2 (U34-enzymes) become critically essential for cell growth when mTORC1 is inactivated. Loss of U34-enzymes particularly impairs synthesis of ribosomal proteins (identified by quantitative nascent proteomics, steady-state proteomics and ribosome profiling), and simultaneous suppression of mTORC1 and U34-enzymes depletes ribosomal proteins and globally inhibits translation.\",\n      \"method\": \"Genome-wide CRISPR screens; quantitative nascent proteomics; steady-state proteomics; ribosome profiling; in vitro and in vivo tumor models\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genome-wide CRISPR screen followed by three orthogonal quantitative methods (nascent proteomics, steady-state proteomics, ribosome profiling) establishing mechanistic cooperation with mTORC1 in controlling ribosomal protein synthesis\",\n      \"pmids\": [\"40328729\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Fission yeast Ctu1-Ctu2 are essential for modification of wobble uridine in certain tRNAs and for normal tolerance to oxidative stress (H2O2). Loss of Ctu2 impairs translation of stress-induced mRNAs enriched in AAA lysine codons, including the transcription factors Atf1 and Pcr1, demonstrating codon-biased translation control downstream of tRNA thiolation.\",\n      \"method\": \"Genetic deletion; tRNA modification analysis; translation reporter assays; codon-substitution rescue (AAA-to-AAG atf1 mutant)\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis combined with codon-substitution rescue and translation assays, single lab\",\n      \"pmids\": [\"23874237\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CTU1 (NCS6/Ncs6) is the catalytic subunit of the conserved eukaryotic CTU1/CTU2 complex (CTU2 serving as scaffold) that introduces a 2-thiol group at the wobble uridine 34 (s2U34) of tRNALysUUU, tRNAGluUUC, and tRNAGlnUUG via adenylation of the uridine and persulfide-mediated sulfur transfer, using a [4Fe-4S] cluster coordinated by conserved cysteines for catalysis; the thiocarboxylated form of Urm1 (generated by the Uba4/UBA4 E1-like enzyme) delivers sulfur to CTU1, and CTU1 itself can be urmylated on lysine residues; functionally, CTU1-dependent tRNA thiolation enables efficient codon-biased translation of AA-ending codon-enriched mRNAs (including HIF1A and oncoproteins such as DEK), is co-opted in cancer to drive glycolysis, invasion, and resistance to BRAF-targeted therapy, cooperates with mTORC1 to sustain ribosomal protein synthesis, and is required for angiogenesis and embryonic development in vertebrates.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CTU1 (NCS6/Ncs6) is the catalytic subunit of the eukaryotic CTU1/CTU2 complex that installs a 2-thiol group at the wobble uridine 34 (s2U34) of tRNALys(UUU), tRNAGlu(UUC), and tRNAGln(UUG), with CTU2 acting as a scaffold and CTU1 performing catalysis [#0, #8]. Mechanistically, CTU1 activates the target uridine by adenylation and itself acts as a persulfide carrier, forming a persulfide adduct at an active-site cysteine [#2], and its catalysis depends on an iron-sulfur cluster ([3Fe-4S]/[4Fe-4S]) coordinated by three conserved cysteines whose fourth, non-protein-bound iron positions a sulfur ligand for transfer to tRNA [#3, #7]. Sulfur is supplied through a relay in which thiocarboxylated Urm1, generated by the Uba4/UBA4 E1-like enzyme, is received by CTU1 [#0, #9]; CTU1 is additionally a target of Urm1 conjugation (urmylation) on lysine residues, a modification enhanced by oxidative stress [#1]. Functionally, this thiolation enables codon-biased translation of mRNAs enriched in AA-ending codons, driving production of the oncoprotein DEK and downstream LEF1 to support breast cancer invasion [#4], sustaining HIF1A translation and glycolysis in melanoma where the activity cooperates with MAPK/PI3K signaling in BRAF therapy resistance [#5], and becoming critically essential to ribosomal protein synthesis when mTORC1 is inactivated [#10]. CTU1-dependent thiolation is required for angiogenesis and embryonic development in vertebrates and for endothelial proliferation, migration, and tube formation [#8].\",\n  \"teleology\": [\n    {\n      \"year\": 2009,\n      \"claim\": \"Established that CTU1/Ncs6, with Ncs2, performs the terminal 2-thiolation step of wobble uridine, placing it at the end of a defined sulfur-relay pathway.\",\n      \"evidence\": \"In vitro sulfur-transfer reconstitution with recombinant proteins plus yeast deletion analysis\",\n      \"pmids\": [\"19151091\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Catalytic mechanism of Ncs6/Ncs2 in sulfur incorporation not resolved\", \"No structural basis for tRNA recognition\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Showed human CTU1 is not only a thiolation enzyme but also a substrate of Urm1, linking the modifier to a post-translational lysine conjugation responsive to oxidative stress.\",\n      \"evidence\": \"Mass spectrometry of Urm1 conjugates and biochemical characterization of isopeptide bonds in mammalian cells\",\n      \"pmids\": [\"21209336\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of CTU1 urmylation unknown\", \"Single study/single lab\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defined the catalytic chemistry: CTU1/Ncs6 adenylates the uridine and carries sulfur as an active-site persulfide, resolving how sulfur reaches the tRNA.\",\n      \"evidence\": \"Biochemical persulfide adduct detection and in vitro adenylation assay on archaeal homolog Mmp1356\",\n      \"pmids\": [\"24530533\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Demonstrated in archaeal homolog rather than human CTU1\", \"Order of adenylation and sulfur transfer not fully resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Identified an iron-sulfur cluster as an essential catalytic cofactor of Ncs6/CTU1, revealing the chemical basis for sulfur activation.\",\n      \"evidence\": \"Spectroscopic and biochemical characterization of the Fe-S cluster with activity assays in archaeal/eukaryotic homologs\",\n      \"pmids\": [\"27791189\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cluster assembly machinery for cytosolic CTU1 not defined\", \"Single study\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Connected CTU1 thiolation to cancer phenotype by showing codon-biased translation of the oncoprotein DEK drives invasion and metastasis.\",\n      \"evidence\": \"Genetic ablation in PyMT model, codon-substitution rescue, IRES translation and invasion assays\",\n      \"pmids\": [\"27811057\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct CTU1 catalytic loss vs Elp3 pathway contributions not separated\", \"Full set of codon-dependent targets undefined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Extended the cancer role to metabolic reprogramming, showing CTU1/CTU2 sustain HIF1A translation and glycolysis and intersect with MAPK/PI3K signaling in BRAF therapy resistance.\",\n      \"evidence\": \"shRNA knockdown, pharmacological inhibition, polysome profiling, metabolic and codon-usage analysis in melanoma\",\n      \"pmids\": [\"29925953\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which PI3K elevates CTU1/CTU2 expression not detailed\", \"Generality across tumor types untested here\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Confirmed CTU1/CTU2 as the human enzyme complex responsible for wobble uridine thiolation using patient cells with loss-of-function variants.\",\n      \"evidence\": \"Functional characterization of patient-derived cells with MS-based tRNA modification analysis and truncation variants\",\n      \"pmids\": [\"31301155\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Variants studied were in CTU2 rather than CTU1\", \"Phenotype-to-modification causality at organismal level not established\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Provided the structural basis: a crystal structure of the NcsA homolog showed a dimer binding a [4Fe-4S] cluster via three cysteines with the fourth iron binding sulfide, and AlphaFold comparison supports conservation in human CTU1/CTU2.\",\n      \"evidence\": \"2.8 Å crystal structure, spectroscopy, mutagenesis, and AlphaFold comparison with human CTU1/CTU2\",\n      \"pmids\": [\"37005440\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No experimental structure of human CTU1/CTU2\", \"tRNA-bound complex structure absent\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Resolved how CTU1 receives sulfur, placing Ncs6/CTU1 downstream of the Uba4/Urm1 thiocarboxylation machinery via cryo-EM and pathway reconstitution.\",\n      \"evidence\": \"Cryo-EM of Uba4/Urm1 complex with mutagenesis and pathway reconstitution\",\n      \"pmids\": [\"39673271\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct CTU1-Urm1 handoff interface not structurally resolved\", \"Single study\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Demonstrated a physiological requirement in vertebrates, showing CTU1 catalyzes s2U34 on three tRNAs and is needed for angiogenesis and embryonic development.\",\n      \"evidence\": \"Zebrafish morpholino knockdown, siRNA in human endothelial cells, scRNA-seq, and functional assays\",\n      \"pmids\": [\"39705244\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Morpholino off-target effects not fully excluded\", \"Codon-dependent target genes in endothelium not identified\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Revealed a synthetic-lethal cooperation with mTORC1, showing U34 enzymes become essential for ribosomal protein synthesis when mTORC1 is inactivated.\",\n      \"evidence\": \"Genome-wide CRISPR screens, nascent and steady-state proteomics, ribosome profiling, and tumor models\",\n      \"pmids\": [\"40328729\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific ribosomal protein mRNAs most codon-dependent not enumerated\", \"Therapeutic window for combined mTORC1/U34 inhibition untested clinically\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The functional significance of CTU1 urmylation and the structure of the human CTU1/CTU2-tRNA complex during catalysis remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No experimental human CTU1/CTU2 structure\", \"Consequence of CTU1 self-urmylation on activity unknown\", \"Mechanism of Fe-S cluster assembly for cytosolic CTU1 undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140098\", \"supporting_discovery_ids\": [0, 2, 6, 8]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 8]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 6, 8]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [4, 5]}\n    ],\n    \"complexes\": [\"CTU1/CTU2 complex\"],\n    \"partners\": [\"CTU2\", \"URM1\", \"UBA4\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}