Affinage

MOCS3

Adenylyltransferase and sulfurtransferase MOCS3 · UniProt O95396

Length
460 aa
Mass
49.7 kDa
Annotated
2026-06-10
13 papers in source corpus 9 papers cited in narrative 10 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

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).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 2005 High

    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

    PMID:15910006

    Open questions at the time
    • Did not identify the physiological sulfur donor feeding C412
    • Did not address how the persulfide is transferred to a substrate
  2. 2008 High

    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

    PMID:18650437

    Open questions at the time
    • 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
  3. 2008 High

    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

    PMID:18650437

    Open questions at the time
    • Quantitative contribution of NFS1 versus other donors in vivo not measured
    • Regulation of the NFS1-MOCS3 handoff unknown
  4. 2008 High

    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

    PMID:18491921

    Open questions at the time
    • No thioester intermediate detected in this study, leaving the transfer mechanism unresolved
    • Structural basis of substrate discrimination not addressed
  5. 2012 High

    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

    PMID:22453920

    Open questions at the time
    • Functional consequence of the cytosol-to-nucleus relocalization not established
    • Did not test endogenous substrate competition between the two pathways
  6. 2015 Medium

    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

    PMID:25747390

    Open questions at the time
    • Single-lab complementation does not quantify activity differences between human and yeast enzymes
    • Urmylation substrate range in human cells not defined here
  7. 2018 High

    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

    PMID:29718331

    Open questions at the time
    • Structural geometry of the thioester-mediated domain transfer not visualized
    • Whether MOCS2A uses an identical thioester step not directly shown
  8. 2019 High

    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

    PMID:30817134

    Open questions at the time
    • Downstream physiological phenotypes of dual deficiency not characterized
    • Functional meaning of MOCS3-independent NFS1 centrosomal localization unclear
  9. 2024 High

    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

    PMID:32901956 PMID:39673271

    Open questions at the time
    • Structures are of yeast Uba4; human MOCS3 structural details not directly resolved
    • Dynamics of NFS1-to-RLD sulfur loading not captured in the structures

Open questions

Synthesis pass · forward-looking unresolved questions
  • How MOCS3 partitions sulfur between the molybdenum cofactor and tRNA-thiolation pathways, and whether this partitioning is regulated, remains unresolved.
  • No mechanism described for prioritizing MOCS2A versus URM1 in vivo
  • Regulatory inputs controlling MOCS3 activity or localization not identified

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 3 GO:0016740 transferase activity 2 GO:0016787 hydrolase activity 2 GO:0140657 ATP-dependent activity 2
Localization
GO:0005829 cytosol 3
Pathway
R-HSA-1430728 Metabolism 2 R-HSA-392499 Metabolism of proteins 2 R-HSA-8953854 Metabolism of RNA 2
Partners
ISD11MOCS2ANFS1URM1
Complex memberships
MOCS3-MOCS2A complexUba4-Urm1 complex

Evidence

Reading pass · 10 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2005 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. ESI-MS/MS, site-directed mutagenesis, in vitro sulfurtransferase assay Biochemistry High 15910006
2008 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. In vitro enzymatic assays, domain-specific mutagenesis, kinetic characterization The Journal of biological chemistry High 18650437
2008 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. Protein-protein interaction assay, in vitro sulfur transfer assay, kinetic characterization, fractionation showing cytosolic localization of MOCS3 The Journal of biological chemistry High 18650437
2008 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. Protein copurification, in vitro adenylation and thiocarboxylation assays, rhodanese activity assay Biochemistry High 18491921
2012 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. FRET (ECFP/EYFP fusions, donor lifetime measurement), cellular localization imaging, co-interaction assays in human cells, site-directed mutagenesis of substrate C-termini The Journal of biological chemistry High 22453920
2015 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. Gene shuffle complementation assay in yeast, biochemical urmylation and tRNA thiolation assays FEBS letters Medium 25747390
2018 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.) In vitro Urm1 thiocarboxylation assay, structure-function analysis, chemical profiling, site-directed mutagenesis, in vivo tRNA thiolation assay in yeast Nucleic acids research High 29718331
2019 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. CRISPR/Cas9 knockout, enzymatic activity assay (sulfite oxidase), tRNA modification analysis, subcellular localization by multiple methods Biochemistry High 30817134
2020 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. X-ray crystallography (full-length Uba4 and Uba4-Urm1 complex), structure-function analysis The EMBO journal High 32901956
2024 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. Cryo-EM structure determination, in vitro mutagenesis, in vivo functional assays Nucleic acids research High 39673271

Source papers

Stage 0 corpus · 13 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2008 A novel role for human Nfs1 in the cytoplasm: Nfs1 acts as a sulfur donor for MOCS3, a protein involved in molybdenum cofactor biosynthesis. The Journal of biological chemistry 101 18650437
2008 The sulfurtransferase activity of Uba4 presents a link between ubiquitin-like protein conjugation and activation of sulfur carrier proteins. Biochemistry 78 18491921
2005 Molybdenum cofactor biosynthesis in humans: identification of a persulfide group in the rhodanese-like domain of MOCS3 by mass spectrometry. Biochemistry 57 15910006
2012 Dual role of the molybdenum cofactor biosynthesis protein MOCS3 in tRNA thiolation and molybdenum cofactor biosynthesis in humans. The Journal of biological chemistry 42 22453920
2016 Sulfur transfer and activation by ubiquitin-like modifier system Uba4•Urm1 link protein urmylation and tRNA thiolation in yeast. Microbial cell (Graz, Austria) 38 28357324
2017 Molybdenum cofactor deficiency: Identification of a patient with homozygote mutation in the MOCS3 gene. American journal of medical genetics. Part A 29 28544736
2018 The Uba4 domain interplay is mediated via a thioester that is critical for tRNA thiolation through Urm1 thiocarboxylation. Nucleic acids research 28 29718331
2020 Molecular basis for the bifunctional Uba4-Urm1 sulfur-relay system in tRNA thiolation and ubiquitin-like conjugation. The EMBO journal 26 32901956
2015 Urmylation and tRNA thiolation functions of ubiquitin-like Uba4·Urm1 systems are conserved from yeast to man. FEBS letters 26 25747390
2019 Analysis of the Cellular Roles of MOCS3 Identifies a MOCS3-Independent Localization of NFS1 at the Tips of the Centrosome. Biochemistry 10 30817134
2024 Molecular basis for thiocarboxylation and release of Urm1 by its E1-activating enzyme Uba4. Nucleic acids research 7 39673271
2021 Case Report: Compound Heterozygous Variants in MOCS3 Identified in a Chinese Infant With Molybdenum Cofactor Deficiency. Frontiers in genetics 7 33897766
2025 Structural insights into the Urm1-Uba4 pathway and its biological roles. Essays in biochemistry 0 41700452

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