{"gene":"MOCS3","run_date":"2026-06-10T02:59:50","timeline":{"discoveries":[{"year":2005,"finding":"The MOCS3 rhodanese-like domain (MOCS3-RLD) forms a persulfide group exclusively on catalytic cysteine C412 (within its six amino acid active loop), as demonstrated by ESI-MS/MS. Mutagenesis of the remaining three cysteines showed none are involved in sulfur transfer; a disulfide bridge was identified between C316 and C324.","method":"ESI-MS/MS, site-directed mutagenesis, in vitro sulfurtransferase assay","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct mass spectrometry identification of persulfide on specific residue, combined with mutagenesis of all four cysteines, replicated in multiple subsequent studies","pmids":["15910006"],"is_preprint":false},{"year":2008,"finding":"Human MOCS3 catalyzes both adenylation of MOCS2A (via its N-terminal MoeB-like domain) and subsequent thiocarboxylation of the C-terminus of MOCS2A (via its C-terminal rhodanese-like domain, RLD). The RLD shows low activity with thiosulfate, suggesting thiosulfate is not the physiological sulfur donor in eukaryotes.","method":"In vitro enzymatic assays, domain-specific mutagenesis, kinetic characterization","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with mutagenesis, consistent with prior mass spectrometry findings and replicated across multiple labs","pmids":["18650437"],"is_preprint":false},{"year":2008,"finding":"Human NFS1 (cytoplasmic L-cysteine desulfurase, purified with Isd11) directly interacts with MOCS3-RLD and transfers sulfur from L-cysteine to MOCS3-RLD via an NFS1-bound persulfide intermediate, establishing NFS1 as the physiological sulfur donor for MOCS3 in the cytosol.","method":"Protein-protein interaction assay, in vitro sulfur transfer assay, kinetic characterization, fractionation showing cytosolic localization of MOCS3","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct interaction and functional sulfur transfer demonstrated in vitro with kinetic parameters, subcellular localization confirmed, consistent with pathway logic","pmids":["18650437"],"is_preprint":false},{"year":2008,"finding":"Yeast Uba4 (the MOCS3 homologue) copurifies as stable heterotetrameric complexes with both human Urm1 and MOCS2A; its N-terminal domain adenylates either MOCS2A or Urm1, and its persulfurated C-terminal rhodanese domain forms a thiocarboxylate at the C-terminal glycine of either substrate. No thioester intermediate between Uba4 and Urm1/MOCS2A was detected in this study.","method":"Protein copurification, in vitro adenylation and thiocarboxylation assays, rhodanese activity assay","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with multiple substrates, biochemical characterization of both domains, single lab but multiple orthogonal methods","pmids":["18491921"],"is_preprint":false},{"year":2012,"finding":"MOCS3 activates both MOCS2A (for molybdenum cofactor biosynthesis) and URM1 (for tRNA thiolation) by adenylation and sulfur transfer to form thiocarboxylate groups; MOCS3 is thus a dual-function protein shared between both pathways. Deletion of the C-terminal glycine of MOCS2A or URM1 abolishes interaction with MOCS3. Extension of the C-terminus of MOCS2A or URM1 with an additional glycine alters MOCS3 localization from cytosol to nucleus. MOCS3 localizes to the cytosol under normal conditions.","method":"FRET (ECFP/EYFP fusions, donor lifetime measurement), cellular localization imaging, co-interaction assays in human cells, site-directed mutagenesis of substrate C-termini","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (FRET, localization imaging, mutagenesis) in human cell context, single lab","pmids":["22453920"],"is_preprint":false},{"year":2015,"finding":"Human MOCS3 (hUBA4) is functionally interchangeable with yeast Uba4 in S. cerevisiae: it supports urmylation of peroxiredoxin Ahp1 and tRNA thiolation, confirming conservation of dual-function urmylation and tRNA thiolation activities from yeast to humans.","method":"Gene shuffle complementation assay in yeast, biochemical urmylation and tRNA thiolation assays","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cross-species complementation with functional biochemical readouts, single lab","pmids":["25747390"],"is_preprint":false},{"year":2018,"finding":"In yeast Uba4 (MOCS3 homologue), a critical thioester linkage forms between Urm1 and Uba4 residue Cys225; this thioester is indispensable for intramolecular transfer of Urm1 between the two Uba4 domains and is essential for tRNA thiolation in vivo. (Note: this contrasts with the 2008 finding that no thioester was detected, with the 2018 study using in vitro thiocarboxylation assay plus structure-function and chemical profiling resolving the mechanism.)","method":"In vitro Urm1 thiocarboxylation assay, structure-function analysis, chemical profiling, site-directed mutagenesis, in vivo tRNA thiolation assay in yeast","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution combined with mutagenesis and in vivo validation, single lab but multiple orthogonal methods","pmids":["29718331"],"is_preprint":false},{"year":2019,"finding":"CRISPR/Cas9 knockout of MOCS3 in HEK293T cells abolishes sulfite oxidase activity (due to loss of molybdenum cofactor) and eliminates mcm5s2U thio-modified tRNAs, confirming MOCS3's dual in vivo role. NFS1 localizes to the centrosome independently of MOCS3.","method":"CRISPR/Cas9 knockout, enzymatic activity assay (sulfite oxidase), tRNA modification analysis, subcellular localization by multiple methods","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — clean knockout with two distinct biochemical readouts plus localization, single lab but multiple orthogonal methods","pmids":["30817134"],"is_preprint":false},{"year":2020,"finding":"Crystal structures of full-length yeast Uba4 and its heterodimeric complex with Urm1 reveal how the two domains (adenylation and rhodanese) orchestrate recognition, binding, and thiocarboxylation of Urm1's C-terminus, and identify a mechanism by which Uba4 protects itself against self-conjugation with activated Urm1-COSH.","method":"X-ray crystallography (full-length Uba4 and Uba4-Urm1 complex), structure-function analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures of full-length protein and complex with substrate, mechanistic validation, published in high-impact peer-reviewed journal","pmids":["32901956"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structure of the Uba4/Urm1 complex reveals the position of rhodanese domains after Urm1 binding. Conserved cysteine residues of Uba4 are required for Uba4-Urm1 thioester formation and Urm1 thiocarboxylation. The thioester intermediate prevents unwanted side-reactions of the adenylate. The Urm1-SH product release mechanism and Urm1 interactions with upstream (Tum1) and downstream (Ncs6) pathway components were characterized.","method":"Cryo-EM structure determination, in vitro mutagenesis, in vivo functional assays","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure combined with mutagenesis and in vitro/in vivo validation, single lab but multiple orthogonal methods","pmids":["39673271"],"is_preprint":false}],"current_model":"MOCS3 (human UBA4) is a dual-function cytosolic E1-like enzyme with an N-terminal adenylation domain and a C-terminal rhodanese-like domain (RLD): it adenylates and thiocarboxylates MOCS2A to provide sulfur for molybdenum cofactor biosynthesis, and also adenylates and thiocarboxylates URM1 to support tRNA thiolation (mcm5s2U wobble modification) and protein urmylation; the physiological sulfur donor is cytoplasmic NFS1 (acting via a persulfide intermediate on RLD catalytic cysteine C412), and substrate engagement requires the C-terminal glycine of MOCS2A/URM1, with a critical thioester intermediate between Uba4 Cys225 and Urm1 mediating intramolecular domain transfer prior to thiocarboxylation."},"narrative":{"mechanistic_narrative":"MOCS3 (human UBA4) is a cytosolic E1-like enzyme that activates ubiquitin-related sulfur-carrier proteins for two parallel sulfur-delivery pathways: molybdenum cofactor biosynthesis and tRNA wobble-uridine thiolation [PMID:22453920, PMID:30817134]. It is a two-domain enzyme, using an N-terminal MoeB-like adenylation domain to adenylate the C-terminus of MOCS2A and URM1, and a C-terminal rhodanese-like domain (RLD) to thiocarboxylate that activated C-terminus [PMID:18650437, PMID:18491921]. The RLD acquires sulfur as a persulfide formed exclusively on the catalytic cysteine C412 within a six-residue active loop, with the physiological sulfur supplied by the cytosolic cysteine desulfurase NFS1 acting through an NFS1-bound persulfide intermediate rather than by thiosulfate [PMID:15910006, PMID:18650437]. Substrate selection requires the C-terminal glycine of MOCS2A or URM1, and extending that C-terminus redirects MOCS3 from the cytosol to the nucleus [PMID:22453920]. Catalysis proceeds through a thioester intermediate between the enzyme cysteine (Uba4 Cys225) and Urm1 that licenses intramolecular transfer of the substrate between the adenylation and rhodanese domains and protects the enzyme from self-conjugation by the activated thiocarboxylate [PMID:29718331, PMID:32901956, PMID:39673271]. CRISPR knockout confirms that loss of MOCS3 abolishes both sulfite oxidase activity (via loss of molybdenum cofactor) and mcm5s2U thio-modified tRNAs, establishing its non-redundant dual in vivo role [PMID:30817134].","teleology":[{"year":2005,"claim":"Established which residue carries the transferable sulfur, defining the catalytic chemistry of the rhodanese-like domain.","evidence":"ESI-MS/MS and site-directed mutagenesis of all four RLD cysteines with in vitro sulfurtransferase assay","pmids":["15910006"],"confidence":"High","gaps":["Did not identify the physiological sulfur donor feeding C412","Did not address how the persulfide is transferred to a substrate"]},{"year":2008,"claim":"Resolved the two-step, two-domain enzymatic logic — adenylation by the N-terminal domain followed by thiocarboxylation by the RLD — and ruled out thiosulfate as the eukaryotic donor.","evidence":"In vitro enzymatic and kinetic assays with domain-specific mutagenesis on human MOCS3 and MOCS2A","pmids":["18650437"],"confidence":"High","gaps":["Did not establish the in vivo sulfur source at this point in the same assay set","Mechanism of substrate hand-off between domains not addressed"]},{"year":2008,"claim":"Identified NFS1 as the physiological cytosolic sulfur donor to MOCS3, linking cysteine desulfuration to molybdopterin sulfur delivery.","evidence":"Protein-protein interaction and in vitro sulfur-transfer assays with NFS1/Isd11, plus fractionation showing cytosolic MOCS3","pmids":["18650437"],"confidence":"High","gaps":["Quantitative contribution of NFS1 versus other donors in vivo not measured","Regulation of the NFS1-MOCS3 handoff unknown"]},{"year":2008,"claim":"Demonstrated that the enzyme acts on two distinct sulfur-carrier substrates, forming stable complexes and thiocarboxylating the C-terminal glycine of both MOCS2A and Urm1.","evidence":"Copurification of yeast Uba4 with Urm1 and MOCS2A plus in vitro adenylation, thiocarboxylation, and rhodanese assays","pmids":["18491921"],"confidence":"High","gaps":["No thioester intermediate detected in this study, leaving the transfer mechanism unresolved","Structural basis of substrate discrimination not addressed"]},{"year":2012,"claim":"Defined the substrate-recognition determinant (C-terminal glycine) and showed that C-terminal alteration relocalizes the enzyme, establishing MOCS3 as the shared node between Moco biosynthesis and tRNA thiolation.","evidence":"FRET, localization imaging, and C-terminal mutagenesis of MOCS2A/URM1 in human cells","pmids":["22453920"],"confidence":"High","gaps":["Functional consequence of the cytosol-to-nucleus relocalization not established","Did not test endogenous substrate competition between the two pathways"]},{"year":2015,"claim":"Confirmed evolutionary conservation of the dual urmylation and tRNA-thiolation activities by showing human MOCS3 substitutes for yeast Uba4.","evidence":"Gene-shuffle complementation in S. cerevisiae with urmylation (Ahp1) and tRNA thiolation readouts","pmids":["25747390"],"confidence":"Medium","gaps":["Single-lab complementation does not quantify activity differences between human and yeast enzymes","Urmylation substrate range in human cells not defined here"]},{"year":2018,"claim":"Identified a covalent thioester intermediate (Uba4 Cys225-Urm1) required for intramolecular substrate transfer, resolving the mechanism left open by the 2008 reconstitution.","evidence":"In vitro thiocarboxylation, chemical profiling, mutagenesis, and in vivo tRNA thiolation in yeast","pmids":["29718331"],"confidence":"High","gaps":["Structural geometry of the thioester-mediated domain transfer not visualized","Whether MOCS2A uses an identical thioester step not directly shown"]},{"year":2019,"claim":"Established the non-redundant dual in vivo role by knockout, showing loss of both molybdenum cofactor-dependent sulfite oxidase activity and thio-modified tRNAs.","evidence":"CRISPR/Cas9 knockout in HEK293T with sulfite oxidase assay, tRNA modification analysis, and subcellular localization","pmids":["30817134"],"confidence":"High","gaps":["Downstream physiological phenotypes of dual deficiency not characterized","Functional meaning of MOCS3-independent NFS1 centrosomal localization unclear"]},{"year":2024,"claim":"Provided crystal and cryo-EM structures explaining domain orchestration, substrate C-terminus recognition, self-conjugation protection, and product release across the full thiocarboxylation cycle.","evidence":"X-ray and cryo-EM structures of Uba4 and Uba4-Urm1 complexes with mutagenesis and in vitro/in vivo validation, including Tum1 and Ncs6 connections","pmids":["32901956","39673271"],"confidence":"High","gaps":["Structures are of yeast Uba4; human MOCS3 structural details not directly resolved","Dynamics of NFS1-to-RLD sulfur loading not captured in the structures"]},{"year":null,"claim":"How MOCS3 partitions sulfur between the molybdenum cofactor and tRNA-thiolation pathways, and whether this partitioning is regulated, remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No mechanism described for prioritizing MOCS2A versus URM1 in vivo","Regulatory inputs controlling MOCS3 activity or localization not identified"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[1,3]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[1,3,4]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[0,2]},{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[1,3]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2,4,7]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[4,7]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[1,7]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[4,5]}],"complexes":["Uba4-Urm1 complex","MOCS3-MOCS2A complex"],"partners":["MOCS2A","URM1","NFS1","ISD11"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O95396","full_name":"Adenylyltransferase and sulfurtransferase MOCS3","aliases":["Molybdenum cofactor synthesis protein 3","Molybdopterin synthase sulfurylase","MPT synthase sulfurylase"],"length_aa":460,"mass_kda":49.7,"function":"Plays a central role in 2-thiolation of mcm(5)S(2)U at tRNA wobble positions of cytosolic tRNA(Lys), tRNA(Glu) and tRNA(Gln) (PubMed:19017811, PubMed:22453920, PubMed:30817134). Also essential during biosynthesis of the molybdenum cofactor (PubMed:15073332, PubMed:22453920, PubMed:30817134). Acts by mediating the C-terminal thiocarboxylation of sulfur carriers URM1 and MOCS2A (PubMed:15073332, PubMed:19017811, PubMed:22453920). Its N-terminus first activates URM1 and MOCS2A as acyl-adenylates (-COAMP), then the persulfide sulfur on the catalytic cysteine is transferred to URM1 and MOCS2A to form thiocarboxylation (-COSH) of their C-terminus (PubMed:19017811, PubMed:22453920). The reaction probably involves hydrogen sulfide that is generated from the persulfide intermediate and that acts as a nucleophile towards URM1 and MOCS2A (PubMed:15073332, PubMed:22453920). Subsequently, a transient disulfide bond is formed (PubMed:15073332, PubMed:22453920). Does not use thiosulfate as sulfur donor; NFS1 acting as a sulfur donor for thiocarboxylation reactions (PubMed:18650437, PubMed:22453920)","subcellular_location":"Cytoplasm, cytosol","url":"https://www.uniprot.org/uniprotkb/O95396/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/MOCS3","classification":"Common Essential","n_dependent_lines":1093,"n_total_lines":1208,"dependency_fraction":0.9048013245033113},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/MOCS3","total_profiled":1310},"omim":[{"mim_id":"621373","title":"MOLYBDENUM COFACTOR DEFICIENCY, TYPE B2; MOCODB2","url":"https://www.omim.org/entry/621373"},{"mim_id":"609277","title":"MOLYBDENUM COFACTOR SYNTHESIS 3; MOCS3","url":"https://www.omim.org/entry/609277"},{"mim_id":"252150","title":"MOLYBDENUM COFACTOR DEFICIENCY, TYPE A; MOCODA","url":"https://www.omim.org/entry/252150"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in many","driving_tissues":[],"url":"https://www.proteinatlas.org/search/MOCS3"},"hgnc":{"alias_symbol":["UBA4","dJ914P20.3"],"prev_symbol":[]},"alphafold":{"accession":"O95396","domains":[{"cath_id":"3.40.50.720","chopping":"44-317","consensus_level":"high","plddt":94.7899,"start":44,"end":317},{"cath_id":"3.40.250.10","chopping":"333-454","consensus_level":"high","plddt":92.3495,"start":333,"end":454}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O95396","model_url":"https://alphafold.ebi.ac.uk/files/AF-O95396-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O95396-F1-predicted_aligned_error_v6.png","plddt_mean":91.56},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MOCS3","jax_strain_url":"https://www.jax.org/strain/search?query=MOCS3"},"sequence":{"accession":"O95396","fasta_url":"https://rest.uniprot.org/uniprotkb/O95396.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O95396/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O95396"}},"corpus_meta":[{"pmid":"18650437","id":"PMC_18650437","title":"A novel role for human Nfs1 in the cytoplasm: Nfs1 acts as a sulfur donor for MOCS3, a protein involved in molybdenum cofactor biosynthesis.","date":"2008","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/18650437","citation_count":101,"is_preprint":false},{"pmid":"18491921","id":"PMC_18491921","title":"The sulfurtransferase activity of Uba4 presents a link between ubiquitin-like protein conjugation and activation of sulfur carrier proteins.","date":"2008","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/18491921","citation_count":78,"is_preprint":false},{"pmid":"15910006","id":"PMC_15910006","title":"Molybdenum cofactor biosynthesis in humans: identification of a persulfide group in the rhodanese-like domain of MOCS3 by mass spectrometry.","date":"2005","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/15910006","citation_count":57,"is_preprint":false},{"pmid":"22453920","id":"PMC_22453920","title":"Dual role of the molybdenum cofactor biosynthesis protein MOCS3 in tRNA thiolation and molybdenum cofactor biosynthesis in humans.","date":"2012","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/22453920","citation_count":42,"is_preprint":false},{"pmid":"28357324","id":"PMC_28357324","title":"Sulfur transfer and activation by ubiquitin-like modifier system Uba4•Urm1 link protein urmylation and tRNA thiolation in yeast.","date":"2016","source":"Microbial cell (Graz, Austria)","url":"https://pubmed.ncbi.nlm.nih.gov/28357324","citation_count":38,"is_preprint":false},{"pmid":"28544736","id":"PMC_28544736","title":"Molybdenum cofactor deficiency: Identification of a patient with homozygote mutation in the MOCS3 gene.","date":"2017","source":"American journal of medical genetics. Part A","url":"https://pubmed.ncbi.nlm.nih.gov/28544736","citation_count":29,"is_preprint":false},{"pmid":"29718331","id":"PMC_29718331","title":"The Uba4 domain interplay is mediated via a thioester that is critical for tRNA thiolation through Urm1 thiocarboxylation.","date":"2018","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/29718331","citation_count":28,"is_preprint":false},{"pmid":"25747390","id":"PMC_25747390","title":"Urmylation and tRNA thiolation functions of ubiquitin-like Uba4·Urm1 systems are conserved from yeast to man.","date":"2015","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/25747390","citation_count":26,"is_preprint":false},{"pmid":"32901956","id":"PMC_32901956","title":"Molecular basis for the bifunctional Uba4-Urm1 sulfur-relay system in tRNA thiolation and ubiquitin-like conjugation.","date":"2020","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/32901956","citation_count":26,"is_preprint":false},{"pmid":"30817134","id":"PMC_30817134","title":"Analysis of the Cellular Roles of MOCS3 Identifies a MOCS3-Independent Localization of NFS1 at the Tips of the Centrosome.","date":"2019","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/30817134","citation_count":10,"is_preprint":false},{"pmid":"39673271","id":"PMC_39673271","title":"Molecular basis for thiocarboxylation and release of Urm1 by its E1-activating enzyme Uba4.","date":"2024","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/39673271","citation_count":7,"is_preprint":false},{"pmid":"33897766","id":"PMC_33897766","title":"Case Report: Compound Heterozygous Variants in MOCS3 Identified in a Chinese Infant With Molybdenum Cofactor Deficiency.","date":"2021","source":"Frontiers in genetics","url":"https://pubmed.ncbi.nlm.nih.gov/33897766","citation_count":7,"is_preprint":false},{"pmid":"41700452","id":"PMC_41700452","title":"Structural insights into the Urm1-Uba4 pathway and its biological roles.","date":"2025","source":"Essays in biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/41700452","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8520,"output_tokens":2775,"usd":0.033592,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9966,"output_tokens":3303,"usd":0.066202,"stage2_stop_reason":"end_turn"},"total_usd":0.099794,"stage1_batch_id":"msgbatch_01BbTCMyrRPyhtEq6b1MdwCN","stage2_batch_id":"msgbatch_011dEed5bbxYvTAaQWKJevxu","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2005,\n      \"finding\": \"The MOCS3 rhodanese-like domain (MOCS3-RLD) forms a persulfide group exclusively on catalytic cysteine C412 (within its six amino acid active loop), as demonstrated by ESI-MS/MS. Mutagenesis of the remaining three cysteines showed none are involved in sulfur transfer; a disulfide bridge was identified between C316 and C324.\",\n      \"method\": \"ESI-MS/MS, site-directed mutagenesis, in vitro sulfurtransferase assay\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct mass spectrometry identification of persulfide on specific residue, combined with mutagenesis of all four cysteines, replicated in multiple subsequent studies\",\n      \"pmids\": [\"15910006\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Human MOCS3 catalyzes both adenylation of MOCS2A (via its N-terminal MoeB-like domain) and subsequent thiocarboxylation of the C-terminus of MOCS2A (via its C-terminal rhodanese-like domain, RLD). The RLD shows low activity with thiosulfate, suggesting thiosulfate is not the physiological sulfur donor in eukaryotes.\",\n      \"method\": \"In vitro enzymatic assays, domain-specific mutagenesis, kinetic characterization\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with mutagenesis, consistent with prior mass spectrometry findings and replicated across multiple labs\",\n      \"pmids\": [\"18650437\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Human NFS1 (cytoplasmic L-cysteine desulfurase, purified with Isd11) directly interacts with MOCS3-RLD and transfers sulfur from L-cysteine to MOCS3-RLD via an NFS1-bound persulfide intermediate, establishing NFS1 as the physiological sulfur donor for MOCS3 in the cytosol.\",\n      \"method\": \"Protein-protein interaction assay, in vitro sulfur transfer assay, kinetic characterization, fractionation showing cytosolic localization of MOCS3\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct interaction and functional sulfur transfer demonstrated in vitro with kinetic parameters, subcellular localization confirmed, consistent with pathway logic\",\n      \"pmids\": [\"18650437\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Yeast Uba4 (the MOCS3 homologue) copurifies as stable heterotetrameric complexes with both human Urm1 and MOCS2A; its N-terminal domain adenylates either MOCS2A or Urm1, and its persulfurated C-terminal rhodanese domain forms a thiocarboxylate at the C-terminal glycine of either substrate. No thioester intermediate between Uba4 and Urm1/MOCS2A was detected in this study.\",\n      \"method\": \"Protein copurification, in vitro adenylation and thiocarboxylation assays, rhodanese activity assay\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with multiple substrates, biochemical characterization of both domains, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"18491921\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"MOCS3 activates both MOCS2A (for molybdenum cofactor biosynthesis) and URM1 (for tRNA thiolation) by adenylation and sulfur transfer to form thiocarboxylate groups; MOCS3 is thus a dual-function protein shared between both pathways. Deletion of the C-terminal glycine of MOCS2A or URM1 abolishes interaction with MOCS3. Extension of the C-terminus of MOCS2A or URM1 with an additional glycine alters MOCS3 localization from cytosol to nucleus. MOCS3 localizes to the cytosol under normal conditions.\",\n      \"method\": \"FRET (ECFP/EYFP fusions, donor lifetime measurement), cellular localization imaging, co-interaction assays in human cells, site-directed mutagenesis of substrate C-termini\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (FRET, localization imaging, mutagenesis) in human cell context, single lab\",\n      \"pmids\": [\"22453920\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Human MOCS3 (hUBA4) is functionally interchangeable with yeast Uba4 in S. cerevisiae: it supports urmylation of peroxiredoxin Ahp1 and tRNA thiolation, confirming conservation of dual-function urmylation and tRNA thiolation activities from yeast to humans.\",\n      \"method\": \"Gene shuffle complementation assay in yeast, biochemical urmylation and tRNA thiolation assays\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cross-species complementation with functional biochemical readouts, single lab\",\n      \"pmids\": [\"25747390\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In yeast Uba4 (MOCS3 homologue), a critical thioester linkage forms between Urm1 and Uba4 residue Cys225; this thioester is indispensable for intramolecular transfer of Urm1 between the two Uba4 domains and is essential for tRNA thiolation in vivo. (Note: this contrasts with the 2008 finding that no thioester was detected, with the 2018 study using in vitro thiocarboxylation assay plus structure-function and chemical profiling resolving the mechanism.)\",\n      \"method\": \"In vitro Urm1 thiocarboxylation assay, structure-function analysis, chemical profiling, site-directed mutagenesis, in vivo tRNA thiolation assay in yeast\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution combined with mutagenesis and in vivo validation, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"29718331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"CRISPR/Cas9 knockout of MOCS3 in HEK293T cells abolishes sulfite oxidase activity (due to loss of molybdenum cofactor) and eliminates mcm5s2U thio-modified tRNAs, confirming MOCS3's dual in vivo role. NFS1 localizes to the centrosome independently of MOCS3.\",\n      \"method\": \"CRISPR/Cas9 knockout, enzymatic activity assay (sulfite oxidase), tRNA modification analysis, subcellular localization by multiple methods\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean knockout with two distinct biochemical readouts plus localization, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"30817134\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Crystal structures of full-length yeast Uba4 and its heterodimeric complex with Urm1 reveal how the two domains (adenylation and rhodanese) orchestrate recognition, binding, and thiocarboxylation of Urm1's C-terminus, and identify a mechanism by which Uba4 protects itself against self-conjugation with activated Urm1-COSH.\",\n      \"method\": \"X-ray crystallography (full-length Uba4 and Uba4-Urm1 complex), structure-function analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures of full-length protein and complex with substrate, mechanistic validation, published in high-impact peer-reviewed journal\",\n      \"pmids\": [\"32901956\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structure of the Uba4/Urm1 complex reveals the position of rhodanese domains after Urm1 binding. Conserved cysteine residues of Uba4 are required for Uba4-Urm1 thioester formation and Urm1 thiocarboxylation. The thioester intermediate prevents unwanted side-reactions of the adenylate. The Urm1-SH product release mechanism and Urm1 interactions with upstream (Tum1) and downstream (Ncs6) pathway components were characterized.\",\n      \"method\": \"Cryo-EM structure determination, in vitro mutagenesis, in vivo functional assays\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure combined with mutagenesis and in vitro/in vivo validation, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"39673271\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MOCS3 (human UBA4) is a dual-function cytosolic E1-like enzyme with an N-terminal adenylation domain and a C-terminal rhodanese-like domain (RLD): it adenylates and thiocarboxylates MOCS2A to provide sulfur for molybdenum cofactor biosynthesis, and also adenylates and thiocarboxylates URM1 to support tRNA thiolation (mcm5s2U wobble modification) and protein urmylation; the physiological sulfur donor is cytoplasmic NFS1 (acting via a persulfide intermediate on RLD catalytic cysteine C412), and substrate engagement requires the C-terminal glycine of MOCS2A/URM1, with a critical thioester intermediate between Uba4 Cys225 and Urm1 mediating intramolecular domain transfer prior to thiocarboxylation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"MOCS3 (human UBA4) is a cytosolic E1-like enzyme that activates ubiquitin-related sulfur-carrier proteins for two parallel sulfur-delivery pathways: molybdenum cofactor biosynthesis and tRNA wobble-uridine thiolation [#4, #7]. It is a two-domain enzyme, using an N-terminal MoeB-like adenylation domain to adenylate the C-terminus of MOCS2A and URM1, and a C-terminal rhodanese-like domain (RLD) to thiocarboxylate that activated C-terminus [#1, #3]. The RLD acquires sulfur as a persulfide formed exclusively on the catalytic cysteine C412 within a six-residue active loop, with the physiological sulfur supplied by the cytosolic cysteine desulfurase NFS1 acting through an NFS1-bound persulfide intermediate rather than by thiosulfate [#0, #1, #2]. Substrate selection requires the C-terminal glycine of MOCS2A or URM1, and extending that C-terminus redirects MOCS3 from the cytosol to the nucleus [#4]. Catalysis proceeds through a thioester intermediate between the enzyme cysteine (Uba4 Cys225) and Urm1 that licenses intramolecular transfer of the substrate between the adenylation and rhodanese domains and protects the enzyme from self-conjugation by the activated thiocarboxylate [#6, #8, #9]. CRISPR knockout confirms that loss of MOCS3 abolishes both sulfite oxidase activity (via loss of molybdenum cofactor) and mcm5s2U thio-modified tRNAs, establishing its non-redundant dual in vivo role [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Established which residue carries the transferable sulfur, defining the catalytic chemistry of the rhodanese-like domain.\",\n      \"evidence\": \"ESI-MS/MS and site-directed mutagenesis of all four RLD cysteines with in vitro sulfurtransferase assay\",\n      \"pmids\": [\"15910006\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify the physiological sulfur donor feeding C412\", \"Did not address how the persulfide is transferred to a substrate\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Resolved the two-step, two-domain enzymatic logic — adenylation by the N-terminal domain followed by thiocarboxylation by the RLD — and ruled out thiosulfate as the eukaryotic donor.\",\n      \"evidence\": \"In vitro enzymatic and kinetic assays with domain-specific mutagenesis on human MOCS3 and MOCS2A\",\n      \"pmids\": [\"18650437\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish the in vivo sulfur source at this point in the same assay set\", \"Mechanism of substrate hand-off between domains not addressed\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identified NFS1 as the physiological cytosolic sulfur donor to MOCS3, linking cysteine desulfuration to molybdopterin sulfur delivery.\",\n      \"evidence\": \"Protein-protein interaction and in vitro sulfur-transfer assays with NFS1/Isd11, plus fractionation showing cytosolic MOCS3\",\n      \"pmids\": [\"18650437\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Quantitative contribution of NFS1 versus other donors in vivo not measured\", \"Regulation of the NFS1-MOCS3 handoff unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Demonstrated that the enzyme acts on two distinct sulfur-carrier substrates, forming stable complexes and thiocarboxylating the C-terminal glycine of both MOCS2A and Urm1.\",\n      \"evidence\": \"Copurification of yeast Uba4 with Urm1 and MOCS2A plus in vitro adenylation, thiocarboxylation, and rhodanese assays\",\n      \"pmids\": [\"18491921\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No thioester intermediate detected in this study, leaving the transfer mechanism unresolved\", \"Structural basis of substrate discrimination not addressed\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defined the substrate-recognition determinant (C-terminal glycine) and showed that C-terminal alteration relocalizes the enzyme, establishing MOCS3 as the shared node between Moco biosynthesis and tRNA thiolation.\",\n      \"evidence\": \"FRET, localization imaging, and C-terminal mutagenesis of MOCS2A/URM1 in human cells\",\n      \"pmids\": [\"22453920\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of the cytosol-to-nucleus relocalization not established\", \"Did not test endogenous substrate competition between the two pathways\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Confirmed evolutionary conservation of the dual urmylation and tRNA-thiolation activities by showing human MOCS3 substitutes for yeast Uba4.\",\n      \"evidence\": \"Gene-shuffle complementation in S. cerevisiae with urmylation (Ahp1) and tRNA thiolation readouts\",\n      \"pmids\": [\"25747390\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab complementation does not quantify activity differences between human and yeast enzymes\", \"Urmylation substrate range in human cells not defined here\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identified a covalent thioester intermediate (Uba4 Cys225-Urm1) required for intramolecular substrate transfer, resolving the mechanism left open by the 2008 reconstitution.\",\n      \"evidence\": \"In vitro thiocarboxylation, chemical profiling, mutagenesis, and in vivo tRNA thiolation in yeast\",\n      \"pmids\": [\"29718331\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural geometry of the thioester-mediated domain transfer not visualized\", \"Whether MOCS2A uses an identical thioester step not directly shown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Established the non-redundant dual in vivo role by knockout, showing loss of both molybdenum cofactor-dependent sulfite oxidase activity and thio-modified tRNAs.\",\n      \"evidence\": \"CRISPR/Cas9 knockout in HEK293T with sulfite oxidase assay, tRNA modification analysis, and subcellular localization\",\n      \"pmids\": [\"30817134\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream physiological phenotypes of dual deficiency not characterized\", \"Functional meaning of MOCS3-independent NFS1 centrosomal localization unclear\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Provided crystal and cryo-EM structures explaining domain orchestration, substrate C-terminus recognition, self-conjugation protection, and product release across the full thiocarboxylation cycle.\",\n      \"evidence\": \"X-ray and cryo-EM structures of Uba4 and Uba4-Urm1 complexes with mutagenesis and in vitro/in vivo validation, including Tum1 and Ncs6 connections\",\n      \"pmids\": [\"32901956\", \"39673271\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structures are of yeast Uba4; human MOCS3 structural details not directly resolved\", \"Dynamics of NFS1-to-RLD sulfur loading not captured in the structures\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How MOCS3 partitions sulfur between the molybdenum cofactor and tRNA-thiolation pathways, and whether this partitioning is regulated, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No mechanism described for prioritizing MOCS2A versus URM1 in vivo\", \"Regulatory inputs controlling MOCS3 activity or localization not identified\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [1, 3]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [1, 3, 4]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [1, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2, 4, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [4, 7]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [1, 7]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [4, 5]}\n    ],\n    \"complexes\": [\n      \"Uba4-Urm1 complex\",\n      \"MOCS3-MOCS2A complex\"\n    ],\n    \"partners\": [\n      \"MOCS2A\",\n      \"URM1\",\n      \"NFS1\",\n      \"ISD11\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}