Existing Annotations Review

Core Functions

ISCU is the primary scaffold for de novo [2Fe-2S] cluster assembly in the mitochondrial matrix. Recent structural studies (PMID:34824239, PMID:38627381) demonstrate that ISCU homodimerizes via Tyr-35 and receives persulfide sulfur from NFS1 Cys381 to Cys138. FDX2 provides reducing equivalents to complete cluster assembly.

Molecular Function:

2 iron, 2 sulfur cluster binding

Directly Involved In:

[2Fe-2S] cluster assembly

Cellular Locations:

mitochondrial matrix

In Complex:

mitochondrial [2Fe-2S] assembly complex

ISCU functions as a scaffold/chaperone for Fe-S cluster assembly and subsequent transfer to recipient proteins via the HSPA9-HSCB chaperone system (PMID:38627381, PMID:23940031).

Molecular Function:

iron-sulfur cluster chaperone activity

Directly Involved In:

iron-sulfur cluster assembly

Cellular Locations:

mitochondrial matrix

ISCU binds Fe2+ as part of the cluster assembly process. Mossbauer spectroscopy demonstrates iron coordination during assembly (PMID:38627381).

Molecular Function:

ferrous iron binding

Directly Involved In:

[2Fe-2S] cluster assembly

Cellular Locations:

mitochondrial matrix

Tyr-35-mediated homodimerization is essential for [2Fe-2S] cluster synthesis. Mutation of Tyr-35 abolishes cluster assembly (PMID:34824239).

Molecular Function:

protein homodimerization activity

Directly Involved In:

[2Fe-2S] cluster assembly

Cellular Locations:

mitochondrial matrix

ISCU functions as a molecular scaffold/adaptor in cytoplasmic Fe-S cluster biogenesis via isoform 2 (ISCU1), coordinating with cytosolic NFS1 for cluster assembly.

Molecular Function:

molecular adaptor activity

Directly Involved In:

iron-sulfur cluster assembly

Cellular Locations:

cytosol

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000043

Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping

GO_REF:0000052

Gene Ontology annotation based on curation of immunofluorescence data

GO_REF:0000054

Gene Ontology annotation based on curation of intracellular localizations of expressed fusion proteins

GO_REF:0000107

Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

PMID:11060020

Distinct iron-sulfur cluster assembly complexes exist in the cytosol and mitochondria of human cells.

Identified ISCU as the human homolog of bacterial/yeast IscU scaffold proteins
Demonstrated both mitochondrial and cytoplasmic ISC assembly complexes
Showed ISCU interaction with NFS1

PMID:16517407

Functions of mitochondrial ISCU and cytosolic ISCU in mammalian iron-sulfur cluster biogenesis and iron homeostasis.

Characterized distinct functions of mitochondrial and cytosolic ISCU isoforms
Demonstrated role in cellular iron homeostasis regulation

PMID:16527810

Roles of the mammalian cytosolic cysteine desulfurase, ISCS, and scaffold protein, ISCU, in iron-sulfur cluster assembly.

Characterized cytosolic ISC assembly machinery
Demonstrated ISCU-NFS1 interaction in cytoplasm

PMID:20668094

Characterization of the human HSC20, an unusual DnaJ type III protein, involved in iron-sulfur cluster biogenesis.

Characterized HSCB (HSC20) as J-domain co-chaperone for ISCU
Demonstrated HSCB-ISCU-HSPA9 interactions

PMID:21298097

Mammalian frataxin - an essential function for cellular viability through an interaction with a preformed ISCU/NFS1/ISD11 iron-sulfur assembly complex.

Demonstrated FXN interaction with preformed ISCU/NFS1/ISD11 complex
Established architecture of core ISC assembly complex

PMID:23508953

Mammalian target of rapamycin complex 1 (mTORC1)-mediated phosphorylation stabilizes ISCU protein - implications for iron metabolism.

Identified Ser-14 phosphorylation by mTOR
Linked ISCU stability to iron metabolism

PMID:23940031

Human mitochondrial chaperone (mtHSP70) and cysteine desulfurase (NFS1) bind preferentially to the disordered conformation, whereas co-chaperone (HSC20) binds to the structured conformation of the iron-sulfur cluster scaffold protein (ISCU).

Characterized D (disordered) and S (structured) states of ISCU
Showed differential binding of partners to D vs S states
Demonstrated Fe-S cluster assembly on ISCU in vitro

PMID:24606901

Cochaperone binding to LYR motifs confers specificity of iron sulfur cluster delivery.

Characterized LYR motif recognition by HSCB
Demonstrated cluster delivery pathway to SDHB

PMID:24971490

Human frataxin activates Fe-S cluster biosynthesis by facilitating sulfur transfer chemistry.

Demonstrated FXN role in activating sulfur transfer to ISCU
Characterized sulfhydration of Cys138
Identified critical cysteine residues for cluster assembly

PMID:26702583

Mitochondrial Hspa9/Mortalin regulates erythroid differentiation via iron-sulfur cluster assembly.

Demonstrated HSPA9-ISCU interaction
Linked Fe-S biogenesis to erythroid differentiation

PMID:27519411

Architecture of the Human Mitochondrial Iron-Sulfur Cluster Assembly Machinery.

Low-resolution structure of human ISC assembly complex

PMID:29097656

Structure and functional dynamics of the mitochondrial Fe/S cluster synthesis complex.

Crystal structure of ISCU in complex with NFS1-LYRM4
Revealed ISCU homodimerization
Characterized zinc binding at active site

PMID:30031876

Zinc(II) binding on human wild-type ISCU and Met140 variants modulates NFS1 desulfurase activity.

Characterized Zn2+ binding to ISCU
Demonstrated modulation of NFS1 activity by Zn-bound ISCU

PMID:31101807

Structure of the human frataxin-bound iron-sulfur cluster assembly complex provides insight into its activation mechanism.

Cryo-EM structure of human ISC complex with FXN
Revealed FXN binding mode and activation mechanism

PMID:34800366

Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context.

Confirmed mitochondrial localization of ISCU by quantitative proteomics

PMID:34824239

N-terminal tyrosine of ISCU2 triggers [2Fe-2S] cluster synthesis by ISCU2 dimerization.

Demonstrated Tyr-35-mediated dimerization essential for cluster assembly
Clarified that only [2Fe-2S] clusters form on ISCU (not [4Fe-4S])
High-resolution structures of ISCU in various states

PMID:38627381

Mechanism and structural dynamics of sulfur transfer during de novo [2Fe-2S] cluster assembly on ISCU2.

2024 cryo-EM snapshots of sulfur transfer mechanism
Captured persulfide transfer from NFS1 Cys381 to ISCU Cys138
Mossbauer spectroscopy of Fe coordination states

PMID:20436681

MicroRNA-210 regulates mitochondrial free radical response to hypoxia and krebs cycle in cancer cells by targeting iron sulfur cluster protein ISCU.

Identified ISCU as miR-210 target
Demonstrated effects on mitochondrial function and iron import

PMID:15778465

Targeted proteomic analysis of 14-3-3 sigma, a p53 effector commonly silenced in cancer.

PMID:17353931

Large-scale mapping of human protein-protein interactions by mass spectrometry.

PMID:25416956

A proteome-scale map of the human interactome network.

PMID:26749241

Disease-Causing SDHAF1 Mutations Impair Transfer of Fe-S Clusters to SDHB.

PMID:31515488

Extensive disruption of protein interactions by genetic variants across the allele frequency spectrum in human populations.

PMID:32296183

A reference map of the human binary protein interactome.

PMID:35271311

OpenCell: Endogenous tagging for the cartography of human cellular organization.

PMID:36931259

A central chaperone-like role for 14-3-3 proteins in human cells.

PMID:40205054

Multimodal cell maps as a foundation for structural and functional genomics.

Reactome:R-HSA-1362408

FXN:NFS1:ISD11:ISCU assembles 2Fe-2S iron-sulfur cluster

Reactome:R-HSA-1362416

Frataxin binds iron

Reactome:R-HSA-9771686

nsp12 acquires Fe-S cluster cofactors

Reactome:R-HSA-9854984

Transfer of Fe-S clusters to SDHB

Reactome:R-HSA-9866272

2Fe-2S is inserted in UQCRFS1

file:human/ISCU/ISCU-deep-research-falcon.md

Deep research report on ISCU

📚 Additional Documentation

Deep Research Falcon

(ISCU-deep-research-falcon.md)

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gene_info: Name=ISCU {ECO:0000312|HGNC:HGNC:29882}; Synonyms=NIFUN;
organism_full: Homo sapiens (Human).
protein_family: Belongs to the NifU family. .
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Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

UniProt Accession: Q9H1K1
Protein Description: RecName: Full=Iron-sulfur cluster assembly enzyme ISCU {ECO:0000305}; AltName: Full=NifU-like N-terminal domain-containing protein; AltName: Full=NifU-like protein; Flags: Precursor;
Gene Information: Name=ISCU {ECO:0000312|HGNC:HGNC:29882}; Synonyms=NIFUN;
Organism (full): Homo sapiens (Human).
Protein Family: Belongs to the NifU family. .
Key Domains: ISCU. (IPR011339); NIF_FeS_clus_asmbl_NifU_N. (IPR002871); NifU_N (PF01592)

MANDATORY VERIFICATION STEPS:

Check if the gene symbol "ISCU" matches the protein description above
Verify the organism is correct: Homo sapiens (Human).
Check if protein family/domains align with what you find in literature
If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'ISCU' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene ISCU (gene ID: ISCU, UniProt: Q9H1K1) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

UniProt Accession: Q9H1K1
Protein Description: RecName: Full=Iron-sulfur cluster assembly enzyme ISCU {ECO:0000305}; AltName: Full=NifU-like N-terminal domain-containing protein; AltName: Full=NifU-like protein; Flags: Precursor;
Gene Information: Name=ISCU {ECO:0000312|HGNC:HGNC:29882}; Synonyms=NIFUN;
Organism (full): Homo sapiens (Human).
Protein Family: Belongs to the NifU family. .
Key Domains: ISCU. (IPR011339); NIF_FeS_clus_asmbl_NifU_N. (IPR002871); NifU_N (PF01592)

MANDATORY VERIFICATION STEPS:

Check if the gene symbol "ISCU" matches the protein description above
Verify the organism is correct: Homo sapiens (Human).
Check if protein family/domains align with what you find in literature
If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

Please provide a comprehensive research report on the gene ISCU (gene ID: ISCU, UniProt: Q9H1K1) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Research plan and verification
- Target verification: The query targets human ISCU, UniProt Q9H1K1. Evidence-based sources identify ISCU as the canonical human mitochondrial iron–sulfur (Fe–S) cluster assembly scaffold protein (NifU family) that supports de novo [2Fe–2S] biosynthesis in the mitochondrial matrix, consistent with the UniProt description and NifU-family domain architecture (human) (maio2024ciao1lossof pages 15-16, jain2020assemblyofthe pages 15-16). The core complex membership and mechanistic studies below confirm alignment with the ISCU/ISCU2 scaffold and NifU-like domain features in the human ISC pathway (schulz2024mechanismandstructural pages 1-3, steinhilper2024twostagebindingof pages 1-2).

1) Key concepts and definitions
- Definition: ISCU (human; UniProt Q9H1K1) is the scaffold protein of the mitochondrial iron–sulfur cluster (ISC) assembly machinery upon which a nascent [2Fe–2S] cluster is built from ferrous iron and cysteine-derived sulfur. In mammals, ISCU is the primary platform for de novo [2Fe–2S] assembly in mitochondria (jain2020assemblyofthe pages 15-16, maio2024ciao1lossof pages 15-16). After assembly, the cluster is transferred to recipient apoproteins directly or via intermediate carriers (jain2020assemblyofthe pages 15-16, jain2020assemblyofthe pages 3-4).
- Current understanding: The mitochondrial core ISC complex minimally comprises the NFS1–ISD11(LYRM4)–ACP (NDUFAB1) cysteine-desulfurase subcomplex, the ISCU scaffold, frataxin (FXN) as an allosteric activator, and mitochondrial ferredoxin-2 (FDX2) as the electron donor. High-resolution cryo-EM and biophysical studies have refined the mechanistic steps of persulfide transfer and [2Fe–2S] assembly on ISCU (2024) (schulz2024mechanismandstructural pages 1-3, steinhilper2024twostagebindingof pages 1-2). Release of the [2Fe–2S] cluster for trafficking requires the mitochondrial Hsp70 system HSPA9 with the J-domain co-chaperone HSC20 (heffner2024tipofthe pages 2-4, jain2020assemblyofthe pages 15-16).

2) Recent developments and latest research (priority 2023–2024)
- Persulfide transfer mechanism visualized: 2024 cryo-EM snapshots captured sulfur transfer from Cys381 of NFS1 to the conserved Cys138 of ISCU, demonstrating that Cys138ISCU “rapidly receives the persulfide without a detectable intermediate,” and clarifying frataxin’s role in positioning assembly-site residues to accelerate sulfur transfer. Mössbauer spectroscopy revealed frataxin shifts the equilibrium between pre- and post-sulfur-transfer Fe coordination states during assembly (Nature Communications, 2024; URL: https://doi.org/10.1038/s41467-024-47310-8) (schulz2024mechanismandstructural pages 1-3).
- FDX2’s two-stage binding to the core complex: 2024 structural work showed FDX2 binds the human core ISC complex in two conformations—distal and proximal—that compete with frataxin for overlapping sites. In the proximal conformation, FDX2’s cluster approaches the ISCU assembly site to deliver electrons rapidly; FDX2 is reduced by FDXR/NADPH (Nature Communications, 2024; URL: https://doi.org/10.1038/s41467-024-54585-4) (steinhilper2024twostagebindingof pages 1-2).
- Chaperone-mediated release and delivery: Recent reviews highlight HSC20 recognition of LYR-like motifs in client proteins and its functional interaction with HSPA9 to catalyze ATP-dependent cluster release from holo-ISCU. HSC20 may also connect mitochondrial biogenesis with cytosolic CIA delivery complexes (Inorganics, 2024; URL: https://doi.org/10.20944/preprints202312.1414.v1) (heffner2024tipofthe pages 2-4).

3) Primary function, substrates, and mechanism
- Enzymatic context: ISCU is not a classical enzyme; it is a scaffold that accepts sulfur and iron to assemble a [2Fe–2S] cluster. NFS1 (PLP-dependent) mobilizes sulfur from L-cysteine, forming a persulfide on NFS1 Cys381, which is transferred to ISCU Cys138. FXN optimizes the assembly site for efficient transfer; FDX2 donates electrons to reduce persulfide sulfur to sulfide as [2Fe–2S] forms. Structural data suggest ISCU dimerization assists assembly, potentially by combining two [1Fe–1S] equivalents (Nature Communications, 2024; URLs above) (schulz2024mechanismandstructural pages 1-3, steinhilper2024twostagebindingof pages 1-2). After assembly on ISCU, [2Fe–2S] can be reductively coupled to [4Fe–4S] on downstream carriers (jain2020assemblyofthe pages 3-4, jain2020assemblyofthe pages 15-16).

4) Subcellular localization and isoforms
- Localization: ISCU operates in the mitochondrial matrix as the primary de novo Fe–S assembly scaffold. In human studies, the mitochondrial ISCU/“ISCU2” scaffold is emphasized for de novo synthesis (J Clin Invest, 2024; URL: https://doi.org/10.1172/jci179559; Hum Mol Genet, 2020; URL: https://doi.org/10.1093/hmg/ddaa172) (maio2024ciao1lossof pages 15-16, jain2020assemblyofthe pages 15-16).

5) Pathways and downstream delivery
- Cluster release and delivery to clients: The HSPA9/HSC20 chaperone system recognizes ISCU-bound clusters and LYR-like motifs in recipients, mediating transfer from ISCU to client proteins or to intermediate carriers (Inorganics, 2024; URL above) (heffner2024tipofthe pages 2-4, jain2020assemblyofthe pages 15-16).
- Delivery nodes and maturation logic: Multiple pathways exist. GLRX5 can heterodimerize with BOLA3 holding a bridged [2Fe–2S]; ISCA1/ISCA2 with IBA57 can convert [2Fe–2S] to [4Fe–4S]; NFU1 acts as a late [4Fe–4S] carrier. For NFU1, human genetic/biochemical analysis shows that its [4Fe–4S] forms by coordinated donation of two [2Fe–2S] clusters from ISCU2 and ISCA1, with FDX2 supplying reducing equivalents; NFU1 then donates to targets such as lipoic acid synthase (LIAS) that supports lipoylation of PDH, α-KGDH, and the glycine cleavage complex (Hum Mol Genet, 2020; URL: https://doi.org/10.1093/hmg/ddaa172) (jain2020assemblyofthe pages 3-4, jain2020assemblyofthe pages 15-16, jain2020assemblyofthe pages 2-2, jain2020assemblyofthe pages 9-10, jain2020assemblyofthe pages 19-20).

6) Disease relevance and 2023–2024 clinical context
- ISCU deficiency: A recurrent ISCU splice mutation produces a skeletal muscle–predominant hereditary myopathy with lactic acidosis and exercise intolerance; complete loss of ISCU is embryonic lethal in mice, underscoring its essentiality. Clinical and mechanistic summaries of ISCU deficiency and broader ISC pathway defects were highlighted in 2024 work describing CIAO1 loss-of-function disease where the ISC pathway components (NFS1, LYRM4/ISD11, ACP/NDUFAB1, ISCU, FXN) are central to mitochondrial and nucleocytoplasmic Fe–S biogenesis (J Clin Invest, 2024; URL: https://doi.org/10.1172/jci179559) (maio2024ciao1lossof pages 15-16). Downstream genes (NFU1, ISCA1/2, IBA57, BOLA3) cause multiple mitochondrial dysfunctions syndromes (MMDS) when mutated, reflecting the critical role of post-ISCU delivery nodes (jain2020assemblyofthe pages 3-4, jain2020assemblyofthe pages 15-16).

7) Regulatory aspects
- Chaperone gating: Cluster release from ISCU depends on HSPA9/HSC20, which recognize LYR-like motifs and stimulate transfer by coupling to ATP hydrolysis (Inorganics, 2024; URL above) (heffner2024tipofthe pages 2-4, jain2020assemblyofthe pages 15-16).
- Emerging regulation: Reviews note miRNA and post-translational control as emerging regulatory layers for ISC components in human cells, though detailed, gene-specific regulatory maps for ISCU require further primary evidence beyond the core biogenesis and delivery literature summarized here (heffner2024tipofthe pages 2-4). (Note: specific miRNA–ISCU interactions have been reported in individual disease contexts outside the core mitochondrial literature; confirm gene- and tissue-specificity with primary disease studies before translational use.)

8) Current applications and implementations
- Diagnostics and functional genomics: ISCU’s central role enables genetic diagnosis of hereditary myopathy associated with ISCU splice and other variants. Functional assays tracing Fe–S client activities (e.g., LIAS-dependent lipoylation; SDHB stability) and interaction mapping between ISCU and downstream carriers (NFU1/ISCA1) have been used to validate pathogenic mechanisms and guide interpretation of variants of uncertain significance (Hum Mol Genet, 2020; URL above) (jain2020assemblyofthe pages 3-4, jain2020assemblyofthe pages 15-16, jain2020assemblyofthe pages 2-2, jain2020assemblyofthe pages 9-10, jain2020assemblyofthe pages 19-20).
- Structural biochemistry: 2024 cryo-EM structures of the human core ISC complex provide atomic-level targets and mechanistic points for intervention (e.g., FXN/FDX2 competition at overlapping binding sites; electron delivery geometry), facilitating structure-guided hypotheses in disease and potentially in drug discovery for ISC pathway modulation (Nature Communications, 2024; URLs above) (schulz2024mechanismandstructural pages 1-3, steinhilper2024twostagebindingof pages 1-2).

9) Expert opinions and authoritative synthesis
- The JCI 2024 clinical-genetics study (CIAO1 deficiency) synthesizes the human ISC and CIA systems in disease, explicitly listing ISCU as the initial scaffold central to mitochondrial Fe–S biogenesis. It integrates clinical phenotypes with pathway biology, highlighting the essentiality and tissue dependencies of Fe–S maturation (J Clin Invest, 2024; URL above) (maio2024ciao1lossof pages 15-16).
- The 2024 mechanistic Nature Communications papers (Schulz et al., Steinhilper et al.) provide a consensus on sulfur transfer and electron delivery steps and the roles of FXN and FDX2, reflecting an emerging structural-mechanistic standard for the human ISC field (schulz2024mechanismandstructural pages 1-3, steinhilper2024twostagebindingof pages 1-2).
- Reviews summarizing HSC20/HSPA9-mediated cluster transfer and downstream delivery nodes emphasize the conserved logic of biogenesis and the modularity of handoff to carriers such as GLRX5/BOLA3 and ISCA/NFU1 (Inorganics, 2024; URL above) (heffner2024tipofthe pages 2-4).

10) Quantitative/statistical data points
- Mechanistic kinetics and structural snapshots: Cryo-EM and spectroscopy show sulfur transfer from NFS1 Cys381 to ISCU Cys138 occurs rapidly with no detectable intermediate; FXN shifts Fe coordination equilibria toward post-transfer states (Nature Communications, 2024; URL above) (schulz2024mechanismandstructural pages 1-3).
- Binding dynamics and distances: FDX2 competes with FXN for overlapping sites and binds in distal and proximal conformations; in the proximal state, FDX2 positions its cluster close to the ISCU assembly site to enable rapid electron transfer (Nature Communications, 2024; URL above) (steinhilper2024twostagebindingof pages 1-2).
- Downstream functional readouts: In human cell models, loss of proper downstream delivery (e.g., due to NFU1 variants) decreases LIAS-dependent lipoylation and SDHB abundance, providing quantitative biomarkers of pathway integrity in assays (Hum Mol Genet, 2020; URL above) (jain2020assemblyofthe pages 15-16, jain2020assemblyofthe pages 2-2).

Key facts summary table
| Key point | Summary | Sources (IDs, URLs) |
|---|---|---|
| Identity verification | ISCU (UniProt Q9H1K1) is the human mitochondrial iron–sulfur (Fe–S) cluster scaffold (NifU family) required for de novo [2Fe-2S] assembly. | (maio2024ciao1lossof pages 15-16) https://doi.org/10.1172/jci179559, (solaveraUnknownyearfunctionalcharacterizationof pages 38-40) https://doi.org/10.1093/hmg/ddaa172 |
| Core ISC complex components | Core mitochondrial ISC: NFS1 (cysteine desulfurase)–ISD11/LYRM4–ACP (NDUFAB1) subcomplex, ISCU (scaffold/ISCU2 isoform), frataxin (FXN), and ferredoxin-2 (FDX2). | (schulz2024mechanismandstructural pages 1-3) https://doi.org/10.1038/s41467-024-47310-8, (steinhilper2024twostagebindingof pages 1-2) https://doi.org/10.1038/s41467-024-54585-4 |
| Mechanistic persulfide transfer & [2Fe-2S] assembly | NFS1 (Cys381) forms a persulfide from L-cysteine and transfers sulfur to ISCU (Cys138); ISCU dimerization and delivery of Fe + electrons (via FDX2/FDXR) yields a [2Fe-2S] cluster on ISCU. FXN accelerates/optimizes persulfide transfer and assembly-site geometry. | (schulz2024mechanismandstructural pages 1-3) https://doi.org/10.1038/s41467-024-47310-8, (steinhilper2024twostagebindingof pages 1-2) https://doi.org/10.1038/s41467-024-54585-4 |
| FDX2 two-stage binding & electron donor role | FDX2 binds the core ISC in two conformations (distal/proximal); the proximal state positions FDX2’s cluster/electrons for rapid reduction of persulfide-derived sulfur during [2Fe-2S] formation; FDXR/NADPH reduce FDX2. | (steinhilper2024twostagebindingof pages 1-2) https://doi.org/10.1038/s41467-024-54585-4, (schulz2024mechanismandstructural pages 1-3) https://doi.org/10.1038/s41467-024-47310-8 |
| Frataxin (FXN) role | FXN transiently binds between NFS1 and ISCU to enhance sulfur transfer rates and to modulate Fe coordination dynamics during assembly; dysfunction of FXN is linked to Friedreich ataxia. | (schulz2024mechanismandstructural pages 1-3) https://doi.org/10.1038/s41467-024-47310-8, (heffner2024tipofthe pages 2-4) https://doi.org/10.20944/preprints202312.1414.v1 |
| Release & chaperone-mediated transfer (HSPA9/HSC20) | After assembly, HSPA9 (mito Hsp70) with J-type co-chaperone HSC20 recognizes ISCU and catalyzes cluster release/transfer to recipients or to intermediate carriers; HSC20 recognizes LYR-like motifs in recipients. | (heffner2024tipofthe pages 2-4) https://doi.org/10.20944/preprints202312.1414.v1, (jain2020assemblyofthe pages 15-16) https://doi.org/10.1093/hmg/ddaa172 |
| Downstream delivery nodes | ISCU → GLRX5/BOLA3 (bridged [2Fe-2S] units), ISCA1/ISCA2–IBA57 (assembly of [4Fe-4S]), NFU1 (late [4Fe-4S] carrier); coordinated handoff enables maturation of lipoyl synthase, respiratory complexes, and many mitochondrial clients. | (jain2020assemblyofthe pages 3-4) https://doi.org/10.1093/hmg/ddaa172, (jain2020assemblyofthe pages 15-16) https://doi.org/10.1093/hmg/ddaa172 |
| Subcellular localization / isoforms | Human ISCU functions in the mitochondrial matrix (commonly referenced as mitochondrial ISCU/ISCU2 isoform); evidence supports mitochondrial-localized scaffold as primary site for de novo Fe–S assembly. | (maio2024ciao1lossof pages 15-16) https://doi.org/10.1172/jci179559, (jain2020assemblyofthe pages 15-16) https://doi.org/10.1093/hmg/ddaa172 |
| Regulatory controls (miRNA, PTM) | miRNA-mediated repression (e.g., miR-210 reported to downregulate ISCU in cancer contexts) and post-translational modulation have been reported in the literature as regulatory layers; phosphorylation/degradation control has emerging evidence. (See primary refs for experimental details.) | (solaveraUnknownyearfunctionalcharacterizationof pages 38-40) https://doi.org/10.1093/hmg/ddaa172, (heffner2024tipofthe pages 2-4) https://doi.org/10.20944/preprints202312.1414.v1 |
| Disease connections & variants | Recessive ISCU splice mutation causes hereditary ISCU myopathy (exercise intolerance, lactic acidosis); heterozygous/dominant ISCU variants also reported; defects in downstream carriers (NFU1, ISCA1/2, BOLA3) cause multiple mitochondrial dysfunction syndromes (MMDS). | (maio2024ciao1lossof pages 15-16) https://doi.org/10.1172/jci179559, (jain2020assemblyofthe pages 15-16) https://doi.org/10.1093/hmg/ddaa172 |
| Key 2023–2024 findings | High-resolution cryo-EM and biochemical studies (2024) defined persulfide transfer snapshots (Cys381NFS1 → Cys138ISCU), clarified FXN’s accelerating role, and described FDX2 two-stage binding dynamics; recent clinical genetics reports continue to expand ISCU variant spectrum. | (schulz2024mechanismandstructural pages 1-3) https://doi.org/10.1038/s41467-024-47310-8, (steinhilper2024twostagebindingof pages 1-2) https://doi.org/10.1038/s41467-024-54585-4, (maio2024ciao1lossof pages 15-16) https://doi.org/10.1172/jci179559 |

Table: Concise reference table summarizing identity, mechanism, partners, localization, regulation, and disease relevance of human ISCU (UniProt Q9H1K1) with source IDs and URLs for the cited evidence.

Conclusions
Human ISCU (Q9H1K1) is the mitochondrial NifU-family scaffold that initiates Fe–S biogenesis by accepting sulfur from NFS1 and assembling [2Fe–2S] with FXN and FDX2. 2024 structural studies now depict sulfur handover to ISCU Cys138 and the two-stage binding of FDX2 that positions electron transfer for cluster formation. Cluster release from ISCU requires HSPA9/HSC20 for transfer to clients and carriers. Delivery nodes—including GLRX5/BOLA3, ISCA1/ISCA2–IBA57, and NFU1—complete maturation to [4Fe–4S] clusters and insertion into specific client enzymes. ISCU deficiency causes a characteristic hereditary myopathy, and defects in downstream nodes cause MMDS, underscoring the centrality of ISCU in human mitochondrial proteostasis (schulz2024mechanismandstructural pages 1-3, steinhilper2024twostagebindingof pages 1-2, jain2020assemblyofthe pages 15-16, jain2020assemblyofthe pages 3-4, maio2024ciao1lossof pages 15-16, heffner2024tipofthe pages 2-4).

References (with URLs and dates)
- Schulz V et al., 2024, Nature Communications. Mechanism and structural dynamics of sulfur transfer during de novo [2Fe–2S] cluster assembly on ISCU2. https://doi.org/10.1038/s41467-024-47310-8 (Apr 2024) (schulz2024mechanismandstructural pages 1-3).
- Steinhilper R et al., 2024, Nature Communications. Two-stage binding of mitochondrial ferredoxin-2 to the core iron–sulfur cluster assembly complex. https://doi.org/10.1038/s41467-024-54585-4 (Dec 2024) (steinhilper2024twostagebindingof pages 1-2).
- Jain A et al., 2020, Human Molecular Genetics. Assembly of the [4Fe–4S] cluster of NFU1 requires the coordinated donation of two [2Fe–2S] clusters from ISCU2 and ISCA1. https://doi.org/10.1093/hmg/ddaa172 (Aug 2020) (jain2020assemblyofthe pages 3-4, jain2020assemblyofthe pages 15-16, jain2020assemblyofthe pages 2-2, jain2020assemblyofthe pages 9-10, jain2020assemblyofthe pages 19-20).
- Maio N et al., 2024, Journal of Clinical Investigation. CIAO1 loss of function causes a neuromuscular disorder with compromise of nucleocytoplasmic Fe–S enzymes. https://doi.org/10.1172/jci179559 (Jun 2024) (maio2024ciao1lossof pages 15-16).
- Heffner AL, Maio N., 2024, Inorganics. Tip of the Iceberg: A New Wave of Fe–S Proteins Found in Viruses (review with host Fe–S biogenesis overview). https://doi.org/10.20944/preprints202312.1414.v1 (Jan 2024) (heffner2024tipofthe pages 2-4).

References

(maio2024ciao1lossof pages 15-16): Nunziata Maio, Rotem Orbach, Irina T. Zaharieva, Ana Töpf, Sandra Donkervoort, Pinki Munot, Juliane Mueller, Tracey Willis, Sumit Verma, Stojan Peric, Deepa Krishnakumar, Sniya Sudhakar, A. Reghan Foley, Sarah Silverstein, Ganka Douglas, Lynn Pais, Stephanie DiTroia, Christopher Grunseich, Ying Hu, Caroline Sewry, Anna Sarkozy, Volker Straub, Francesco Muntoni, Tracey A. Rouault, and Carsten G. Bönnemann. Ciao1 loss of function causes a neuromuscular disorder with compromise of nucleocytoplasmic fe-s enzymes. The Journal of Clinical Investigation, Jun 2024. URL: https://doi.org/10.1172/jci179559, doi:10.1172/jci179559. This article has 13 citations.
(jain2020assemblyofthe pages 15-16): Anshika Jain, Anamika Singh, Nunziata Maio, and Tracey A Rouault. Assembly of the [4fe–4s] cluster of nfu1 requires the coordinated donation of two [2fe–2s] clusters from the scaffold proteins, iscu2 and isca1. Human Molecular Genetics, 29:3165-3182, Aug 2020. URL: https://doi.org/10.1093/hmg/ddaa172, doi:10.1093/hmg/ddaa172. This article has 27 citations and is from a domain leading peer-reviewed journal.
(schulz2024mechanismandstructural pages 1-3): Vinzent Schulz, Ralf Steinhilper, Jonathan Oltmanns, S. Freibert, Nils Krapoth, Uwe Linne, Sonja Welsch, M. H. Hoock, Volker Schünemann, Bonnie J. Murphy, and Roland Lill. Mechanism and structural dynamics of sulfur transfer during de novo [2fe-2s] cluster assembly on iscu2. Nature Communications, Apr 2024. URL: https://doi.org/10.1038/s41467-024-47310-8, doi:10.1038/s41467-024-47310-8. This article has 21 citations and is from a highest quality peer-reviewed journal.
(steinhilper2024twostagebindingof pages 1-2): Ralf Steinhilper, Linda Boß, Sven-A. Freibert, Vinzent Schulz, Nils Krapoth, Susann Kaltwasser, Roland Lill, and Bonnie J. Murphy. Two-stage binding of mitochondrial ferredoxin-2 to the core iron-sulfur cluster assembly complex. Nature Communications, Dec 2024. URL: https://doi.org/10.1038/s41467-024-54585-4, doi:10.1038/s41467-024-54585-4. This article has 21 citations and is from a highest quality peer-reviewed journal.
(jain2020assemblyofthe pages 3-4): Anshika Jain, Anamika Singh, Nunziata Maio, and Tracey A Rouault. Assembly of the [4fe–4s] cluster of nfu1 requires the coordinated donation of two [2fe–2s] clusters from the scaffold proteins, iscu2 and isca1. Human Molecular Genetics, 29:3165-3182, Aug 2020. URL: https://doi.org/10.1093/hmg/ddaa172, doi:10.1093/hmg/ddaa172. This article has 27 citations and is from a domain leading peer-reviewed journal.
(heffner2024tipofthe pages 2-4): Audrey L. Heffner and Nunziata Maio. Tip of the iceberg: a new wave of iron–sulfur cluster proteins found in viruses. Inorganics, 12:34, Jan 2024. URL: https://doi.org/10.20944/preprints202312.1414.v1, doi:10.20944/preprints202312.1414.v1. This article has 4 citations and is from a poor quality or predatory journal.
(jain2020assemblyofthe pages 2-2): Anshika Jain, Anamika Singh, Nunziata Maio, and Tracey A Rouault. Assembly of the [4fe–4s] cluster of nfu1 requires the coordinated donation of two [2fe–2s] clusters from the scaffold proteins, iscu2 and isca1. Human Molecular Genetics, 29:3165-3182, Aug 2020. URL: https://doi.org/10.1093/hmg/ddaa172, doi:10.1093/hmg/ddaa172. This article has 27 citations and is from a domain leading peer-reviewed journal.
(jain2020assemblyofthe pages 9-10): Anshika Jain, Anamika Singh, Nunziata Maio, and Tracey A Rouault. Assembly of the [4fe–4s] cluster of nfu1 requires the coordinated donation of two [2fe–2s] clusters from the scaffold proteins, iscu2 and isca1. Human Molecular Genetics, 29:3165-3182, Aug 2020. URL: https://doi.org/10.1093/hmg/ddaa172, doi:10.1093/hmg/ddaa172. This article has 27 citations and is from a domain leading peer-reviewed journal.
(jain2020assemblyofthe pages 19-20): Anshika Jain, Anamika Singh, Nunziata Maio, and Tracey A Rouault. Assembly of the [4fe–4s] cluster of nfu1 requires the coordinated donation of two [2fe–2s] clusters from the scaffold proteins, iscu2 and isca1. Human Molecular Genetics, 29:3165-3182, Aug 2020. URL: https://doi.org/10.1093/hmg/ddaa172, doi:10.1093/hmg/ddaa172. This article has 27 citations and is from a domain leading peer-reviewed journal.
(solaveraUnknownyearfunctionalcharacterizationof pages 38-40): D Jové Solavera. Functional characterization of novel mutations in x-linked sideroblastic anaemia (xlsa) and the identification of transferrin receptor as a causative gene for a …. Unknown journal, Unknown year.

Citations

schulz2024mechanismandstructural pages 1-3
steinhilper2024twostagebindingof pages 1-2
heffner2024tipofthe pages 2-4
jain2020assemblyofthe pages 15-16
jain2020assemblyofthe pages 3-4
jain2020assemblyofthe pages 2-2
jain2020assemblyofthe pages 9-10
jain2020assemblyofthe pages 19-20
2Fe–2S
1Fe–1S
4Fe–4S
2Fe-2S
4Fe-4S
4fe–4s
2fe–2s
2fe-2s
https://doi.org/10.1038/s41467-024-47310-8
https://doi.org/10.1038/s41467-024-54585-4
https://doi.org/10.20944/preprints202312.1414.v1
https://doi.org/10.1172/jci179559;
https://doi.org/10.1093/hmg/ddaa172
https://doi.org/10.1172/jci179559
https://doi.org/10.1172/jci179559,
https://doi.org/10.1038/s41467-024-47310-8,
https://doi.org/10.1038/s41467-024-54585-4,
https://doi.org/10.20944/preprints202312.1414.v1,
https://doi.org/10.1093/hmg/ddaa172,

📄 View Raw YAML

id: Q9H1K1
gene_symbol: ISCU
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: ISCU is the primary scaffold protein of the mitochondrial 
  iron-sulfur cluster (ISC) assembly machinery in human cells. It provides the 
  structural platform upon which nascent [2Fe-2S] clusters are assembled de novo
  before transfer to recipient apoproteins. The mature mitochondrial form 
  (isoform 1/ISCU2) functions in the mitochondrial matrix as part of a core ISC 
  complex that includes the cysteine desulfurase NFS1-LYRM4-NDUFAB1 subcomplex, 
  frataxin (FXN) as an allosteric activator, and ferredoxin-2 (FDX2) as the 
  electron donor. During cluster assembly, persulfide sulfur is transferred from
  NFS1 Cys381 to ISCU Cys138, and FDX2 provides reducing equivalents to reduce 
  persulfide to sulfide for [2Fe-2S] formation. ISCU dimerization, mediated by 
  Tyr-35, is critical for de novo cluster synthesis. Following cluster assembly,
  the HSPA9-HSCB chaperone system catalyzes ATP-dependent cluster release from 
  ISCU for transfer to downstream carriers (GLRX5, NFU1, ISCA1/ISCA2) and 
  ultimately to recipient Fe-S proteins. A cytosolic isoform (isoform 2/ISCU1) 
  participates in cytoplasmic Fe-S biogenesis. ISCU deficiency causes hereditary
  myopathy with exercise intolerance and lactic acidosis.
existing_annotations:
  - term:
      id: GO:0005737
      label: cytoplasm
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: IBA annotation for cytoplasm localization based on phylogenetic 
        inference. ISCU has two isoforms - isoform 2 (ISCU1) lacks the 
        mitochondrial targeting sequence and localizes to the cytoplasm where it
        participates in cytoplasmic Fe-S cluster biogenesis (PMID:11060020, 
        PMID:16527810). The IBA is phylogenetically consistent with yeast and 
        other orthologs.
      action: ACCEPT
      reason: The cytoplasmic localization is well-supported for isoform 2 
        (ISCU1). UniProt confirms cytoplasmic localization for this isoform, and
        functional studies demonstrate a cytoplasmic ISC assembly complex exists
        (PMID:11060020, PMID:16527810, PMID:29309586).
      supported_by:
        - reference_id: PMID:11060020
          supporting_text: Distinct iron-sulfur cluster assembly complexes exist
            in the cytosol and mitochondria of human cells.
        - reference_id: PMID:16527810
          supporting_text: Roles of the mammalian cytosolic cysteine 
            desulfurase, ISCS, and scaffold protein, ISCU, in iron-sulfur 
            cluster assembly.
        - reference_id: file:human/ISCU/ISCU-deep-research-falcon.md
          supporting_text: 'model: Edison Scientific Literature'
  - term:
      id: GO:0008198
      label: ferrous iron binding
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: IBA annotation for ferrous iron binding. ISCU binds Fe2+ as part 
        of the [2Fe-2S] cluster assembly process. Structural and spectroscopic 
        studies confirm iron coordination at the assembly site through conserved
        cysteine residues (Cys69, Cys95, Cys138) and His137 (PMID:38627381, 
        PMID:24971490).
      action: ACCEPT
      reason: Ferrous iron binding is a core function of ISCU as the scaffold 
        for [2Fe-2S] cluster assembly. Recent 2024 cryo-EM and Mossbauer 
        spectroscopy studies demonstrate Fe coordination during cluster assembly
        (Schulz et al. 2024, Nat Commun).
      supported_by:
        - reference_id: PMID:38627381
          supporting_text: Mechanism and structural dynamics of sulfur transfer 
            during de novo [2Fe-2S] cluster assembly on ISCU2
  - term:
      id: GO:0006879
      label: intracellular iron ion homeostasis
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: IBA annotation for iron ion homeostasis. ISCU's role in Fe-S 
        cluster biogenesis is essential for maintaining cellular iron 
        homeostasis, as Fe-S clusters regulate iron uptake and storage pathways.
        Studies show that ISCU knockdown affects cellular iron regulation 
        (PMID:16517407, PMID:23508953).
      action: ACCEPT
      reason: While ISCU's primary function is Fe-S cluster assembly, this 
        directly impacts iron homeostasis. The IBA is phylogenetically 
        appropriate. Studies demonstrate ISCU's role in iron regulation 
        (PMID:16517407, PMID:23508953).
      supported_by:
        - reference_id: PMID:16517407
          supporting_text: Functions of mitochondrial ISCU and cytosolic ISCU in
            mammalian iron-sulfur cluster biogenesis and iron homeostasis.
        - reference_id: PMID:23508953
          supporting_text: 'Mammalian target of rapamycin complex 1 (mTORC1)-mediated
            phosphorylation stabilizes ISCU protein: implications for iron metabolism.'
  - term:
      id: GO:0005759
      label: mitochondrial matrix
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: IBA annotation for mitochondrial matrix localization. ISCU 
        isoform 1 (ISCU2) contains a mitochondrial targeting sequence (residues 
        1-34) and functions in the mitochondrial matrix as the primary site for 
        de novo Fe-S cluster assembly (PMID:11060020, PMID:26702583).
      action: ACCEPT
      reason: Mitochondrial matrix localization is the primary site for ISCU 
        function. UniProt confirms the N-terminal transit peptide (aa 1-34) and 
        experimental evidence supports matrix localization (PMID:11060020, 
        PMID:26702583).
      supported_by:
        - reference_id: PMID:11060020
          supporting_text: Distinct iron-sulfur cluster assembly complexes exist
            in the cytosol and mitochondria of human cells.
        - reference_id: PMID:26702583
          supporting_text: Mitochondrial Hspa9/Mortalin regulates erythroid 
            differentiation via iron-sulfur cluster assembly.
  - term:
      id: GO:0051537
      label: 2 iron, 2 sulfur cluster binding
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: IBA annotation for [2Fe-2S] cluster binding. This is a core 
        molecular function of ISCU as the scaffold protein for [2Fe-2S] 
        assembly. The assembled cluster is coordinated by conserved cysteine 
        residues (Cys69, Cys95, Cys138) before transfer to recipient proteins.
      action: ACCEPT
      reason: '[2Fe-2S] cluster binding is the defining molecular function of ISCU.
        Recent structural studies (PMID:34824239, PMID:31101807) and biochemical work
        (PMID:24971490) demonstrate this function directly.'
      supported_by:
        - reference_id: PMID:34824239
          supporting_text: N-terminal tyrosine of ISCU2 triggers [2Fe-2S] 
            cluster synthesis by ISCU2 dimerization.
        - reference_id: PMID:24971490
          supporting_text: Human frataxin activates Fe-S cluster biosynthesis by
            facilitating sulfur transfer chemistry.
  - term:
      id: GO:0005506
      label: iron ion binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000002
    review:
      summary: IEA annotation from InterPro domain mapping. While accurate, this
        is a more general term than ferrous iron binding (GO:0008198) which is 
        already annotated with IBA evidence.
      action: ACCEPT
      reason: The IEA is correct based on domain analysis. It is more general 
        than the IBA annotation for ferrous iron binding, but both can coexist 
        as the IEA provides independent computational evidence.
  - term:
      id: GO:0005634
      label: nucleus
    evidence_type: IEA
    original_reference_id: GO_REF:0000044
    review:
      summary: IEA from UniProt subcellular location mapping. Some nuclear 
        localization has been reported for isoform 2 (cytoplasmic isoform) based
        on TAS evidence (PMID:16527810).
      action: ACCEPT
      reason: Nuclear localization is supported by literature for the cytosolic 
        isoform (PMID:16527810). This is a minor localization and not the 
        primary functional site.
  - term:
      id: GO:0005737
      label: cytoplasm
    evidence_type: IEA
    original_reference_id: GO_REF:0000044
    review:
      summary: IEA from UniProt subcellular location mapping for cytoplasm. 
        Duplicate of the IBA annotation above but from a different evidence 
        source.
      action: ACCEPT
      reason: Cytoplasmic localization is well-supported for isoform 2 (ISCU1). 
        Multiple evidence sources (IBA and IEA) appropriately capture this 
        localization.
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: IEA
    original_reference_id: GO_REF:0000120
    review:
      summary: IEA from automated annotation combining multiple sources. ISCU 
        isoform 1 localizes to mitochondria where it functions in Fe-S cluster 
        assembly.
      action: ACCEPT
      reason: Mitochondrial localization is the primary site for ISCU function 
        (isoform 1). This is well-established and supported by multiple 
        experimental studies.
  - term:
      id: GO:0016226
      label: iron-sulfur cluster assembly
    evidence_type: IEA
    original_reference_id: GO_REF:0000002
    review:
      summary: IEA from InterPro domain mapping for iron-sulfur cluster 
        assembly. This is the core biological process that ISCU is involved in.
      action: ACCEPT
      reason: Iron-sulfur cluster assembly is the primary biological process for
        ISCU. This is appropriate as the parent term for more specific [2Fe-2S] 
        cluster assembly.
  - term:
      id: GO:0046872
      label: metal ion binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000043
    review:
      summary: IEA from UniProt keyword mapping. Very general term - ISCU binds 
        iron (for cluster assembly) and can also bind zinc (which modulates NFS1
        activity).
      action: ACCEPT
      reason: This general term is appropriate given ISCU's metal binding 
        properties. More specific annotations (ferrous iron binding, zinc ion 
        binding) provide greater detail.
  - term:
      id: GO:0051536
      label: iron-sulfur cluster binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000002
    review:
      summary: IEA from InterPro for iron-sulfur cluster binding. This is the 
        parent term of [2Fe-2S] cluster binding (GO:0051537) which is annotated 
        with IBA.
      action: ACCEPT
      reason: General term for Fe-S cluster binding is appropriate as it 
        encompasses the more specific [2Fe-2S] cluster binding function. Domain 
        analysis supports this annotation.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:15778465
    review:
      summary: IPI for protein binding with SFN (14-3-3 sigma). This is a 
        high-throughput proteomic study identifying 14-3-3 interactions.
      action: MARK_AS_OVER_ANNOTATED
      reason: Protein binding is too vague. ISCU binds specific partners (NFS1, 
        FXN, HSCB, HSPA9, GLRX5) as part of its Fe-S cluster assembly function. 
        The 14-3-3 interaction may be related to phosphorylation-dependent 
        regulation (Ser-14 phosphorylation) but is not well characterized 
        functionally.
      supported_by:
        - reference_id: PMID:15778465
          supporting_text: Epub 2005 Mar 18. Targeted proteomic analysis of 
            14-3-3 sigma, a p53 effector commonly silenced in cancer.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:17353931
    review:
      summary: IPI for protein binding from large-scale protein-protein 
        interaction mapping study. High-throughput data with limited functional 
        validation.
      action: MARK_AS_OVER_ANNOTATED
      reason: Generic protein binding annotation from high-throughput study. 
        More informative MF terms should be used to describe ISCU's specific 
        protein interactions.
      supported_by:
        - reference_id: PMID:17353931
          supporting_text: Large-scale mapping of human protein-protein 
            interactions by mass spectrometry.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:24606901
    review:
      summary: IPI for protein binding with SDHB. This study characterized 
        cochaperone binding to LYR motifs and Fe-S cluster delivery to SDHB.
      action: KEEP_AS_NON_CORE
      reason: This interaction is functionally relevant - ISCU participates in 
        Fe-S cluster delivery to SDHB. However, protein binding is a vague term.
        The interaction represents a cluster transfer event rather than a core 
        binding function.
      additional_reference_ids:
        - PMID:24606901
      supported_by:
        - reference_id: PMID:24606901
          supporting_text: Cochaperone binding to LYR motifs confers specificity
            of iron sulfur cluster delivery.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:25416956
    review:
      summary: IPI from proteome-scale interactome study. High-throughput Y2H 
        data.
      action: MARK_AS_OVER_ANNOTATED
      reason: High-throughput interaction data with many interactions reported. 
        Protein binding is too general to be informative for GO annotation 
        purposes.
      supported_by:
        - reference_id: PMID:25416956
          supporting_text: A proteome-scale map of the human interactome 
            network.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:26749241
    review:
      summary: IPI for protein binding with SDHB, related to SDHAF1 mutations 
        affecting Fe-S cluster transfer.
      action: KEEP_AS_NON_CORE
      reason: This study shows ISCU participates in Fe-S cluster delivery 
        pathway to SDHB. The interaction is functionally meaningful but protein 
        binding is vague.
      supported_by:
        - reference_id: PMID:26749241
          supporting_text: 2015 Dec 31. Disease-Causing SDHAF1 Mutations Impair 
            Transfer of Fe-S Clusters to SDHB.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:31515488
    review:
      summary: IPI from study of genetic variant effects on protein-protein 
        interactions.
      action: MARK_AS_OVER_ANNOTATED
      reason: High-throughput study. Protein binding is too generic for GO 
        annotation.
      supported_by:
        - reference_id: PMID:31515488
          supporting_text: Extensive disruption of protein interactions by 
            genetic variants across the allele frequency spectrum in human 
            populations.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:32296183
    review:
      summary: IPI from human binary protein interactome reference map.
      action: MARK_AS_OVER_ANNOTATED
      reason: High-throughput interactome study. Protein binding is too generic.
      supported_by:
        - reference_id: PMID:32296183
          supporting_text: Apr 8. A reference map of the human binary protein 
            interactome.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:35271311
    review:
      summary: IPI from OpenCell endogenous tagging study for cellular 
        organization mapping.
      action: MARK_AS_OVER_ANNOTATED
      reason: Large-scale study. Protein binding annotation is not informative.
      supported_by:
        - reference_id: PMID:35271311
          supporting_text: '2022 Mar 11. OpenCell: Endogenous tagging for the cartography
            of human cellular organization.'
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:36931259
    review:
      summary: IPI for 14-3-3 protein interactions. Study on chaperone-like role
        of 14-3-3 proteins.
      action: MARK_AS_OVER_ANNOTATED
      reason: 14-3-3 binding may relate to Ser-14 phosphorylation-dependent 
        regulation of ISCU stability. However, protein binding is too generic 
        and the functional significance is not well established.
      supported_by:
        - reference_id: PMID:36931259
          supporting_text: A central chaperone-like role for 14-3-3 proteins in 
            human cells.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:40205054
    review:
      summary: IPI from multimodal cell maps study for structural and functional
        genomics.
      action: MARK_AS_OVER_ANNOTATED
      reason: High-throughput study. Protein binding is too generic.
      supported_by:
        - reference_id: PMID:40205054
          supporting_text: Apr 9. Multimodal cell maps as a foundation for 
            structural and functional genomics.
  - term:
      id: GO:0042803
      label: protein homodimerization activity
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: IEA from ortholog transfer (mouse). ISCU dimerization is 
        essential for [2Fe-2S] cluster assembly, mediated by Tyr-35.
      action: ACCEPT
      reason: ISCU homodimerization is well-established and critical for its 
        function. This is supported by experimental evidence (IDA) from 
        PMID:29097656 and PMID:34824239.
      supported_by:
        - reference_id: PMID:34824239
          supporting_text: N-terminal tyrosine of ISCU2 triggers [2Fe-2S] 
            cluster synthesis by ISCU2 dimerization
  - term:
      id: GO:0044572
      label: '[4Fe-4S] cluster assembly'
    evidence_type: IEA
    original_reference_id: GO_REF:0000120
    review:
      summary: IEA for [4Fe-4S] cluster assembly. Recent evidence suggests ISCU 
        primarily assembles [2Fe-2S] clusters, which are then converted to 
        [4Fe-4S] by downstream carriers (ISCA1, ISCA2, IBA57, NFU1).
      action: MODIFY
      reason: 'Recent studies (PMID:34824239) clarify that ISCU assembles [2Fe-2S]
        clusters, not [4Fe-4S] directly. The [4Fe-4S] clusters are assembled by downstream
        carriers. UniProt CAUTION states: "Recent reports confirm that only [2Fe-2S]
        clusters are formed by the core ISC assembly complex."'
      proposed_replacement_terms:
        - id: GO:0044571
          label: '[2Fe-2S] cluster assembly'
      supported_by:
        - reference_id: PMID:34824239
          supporting_text: N-terminal tyrosine of ISCU2 triggers [2Fe-2S] 
            cluster synthesis by ISCU2 dimerization
  - term:
      id: GO:0099128
      label: mitochondrial [2Fe-2S] assembly complex
    evidence_type: IEA
    original_reference_id: GO_REF:0000120
    review:
      summary: IEA for membership in mitochondrial [2Fe-2S] assembly complex. 
        ISCU is a core component of this complex along with NFS1, LYRM4, 
        NDUFAB1, FXN, and FDX2.
      action: ACCEPT
      reason: ISCU is a core component of the mitochondrial [2Fe-2S] assembly 
        complex. This is well-established by structural studies (PMID:31101807, 
        PMID:29097656).
      supported_by:
        - reference_id: PMID:31101807
          supporting_text: Structure of the human frataxin-bound iron-sulfur 
            cluster assembly complex provides insight into its activation 
            mechanism.
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: IDA
    original_reference_id: GO_REF:0000052
    review:
      summary: IDA from immunofluorescence data curation (Human Protein Atlas). 
        Confirms mitochondrial localization.
      action: ACCEPT
      reason: Mitochondrial localization is the primary site for ISCU isoform 1.
        Well-supported by multiple studies and imaging data.
  - term:
      id: GO:0005829
      label: cytosol
    evidence_type: IDA
    original_reference_id: GO_REF:0000052
    review:
      summary: IDA from immunofluorescence data curation (Human Protein Atlas). 
        Cytosolic localization likely reflects isoform 2 (ISCU1).
      action: ACCEPT
      reason: Cytosolic localization is appropriate for isoform 2 which lacks 
        the mitochondrial targeting sequence.
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: NAS
    original_reference_id: PMID:27519411
    review:
      summary: NAS from ComplexPortal for mitochondrial localization. Study 
        describes architecture of human mitochondrial Fe-S cluster assembly 
        machinery.
      action: ACCEPT
      reason: Mitochondrial localization is well-established for the functional 
        ISCU isoform.
      supported_by:
        - reference_id: PMID:27519411
          supporting_text: Architecture of the Human Mitochondrial Iron-Sulfur 
            Cluster Assembly Machinery.
  - term:
      id: GO:0016226
      label: iron-sulfur cluster assembly
    evidence_type: NAS
    original_reference_id: PMID:29097656
    review:
      summary: NAS from ComplexPortal for iron-sulfur cluster assembly process. 
        This study characterized the structure and dynamics of the mitochondrial
        Fe/S cluster synthesis complex.
      action: ACCEPT
      reason: Iron-sulfur cluster assembly is the core biological process for 
        ISCU.
      supported_by:
        - reference_id: PMID:29097656
          supporting_text: Structure and functional dynamics of the 
            mitochondrial Fe/S cluster synthesis complex.
  - term:
      id: GO:0008198
      label: ferrous iron binding
    evidence_type: IDA
    original_reference_id: PMID:38627381
    review:
      summary: IDA from 2024 structural study showing mechanism of sulfur 
        transfer and Fe coordination during [2Fe-2S] cluster assembly on ISCU2.
      action: ACCEPT
      reason: Direct experimental evidence for ferrous iron binding during 
        cluster assembly. This is a core molecular function.
      supported_by:
        - reference_id: PMID:38627381
          supporting_text: Mechanism and structural dynamics of sulfur transfer 
            during de novo [2Fe-2S] cluster assembly on ISCU2.
  - term:
      id: GO:0044571
      label: '[2Fe-2S] cluster assembly'
    evidence_type: IDA
    original_reference_id: PMID:38627381
    review:
      summary: IDA from 2024 structural study on [2Fe-2S] cluster assembly 
        mechanism. This is the defining biological process for ISCU.
      action: ACCEPT
      reason: Direct experimental evidence for [2Fe-2S] cluster assembly as 
        ISCU's core function.
      supported_by:
        - reference_id: PMID:38627381
          supporting_text: Mechanism and structural dynamics of sulfur transfer 
            during de novo [2Fe-2S] cluster assembly on ISCU2.
  - term:
      id: GO:0140132
      label: iron-sulfur cluster chaperone activity
    evidence_type: IDA
    original_reference_id: PMID:38627381
    review:
      summary: IDA for iron-sulfur cluster chaperone activity from 2024 
        structural study.
      action: ACCEPT
      reason: ISCU functions as a scaffold/chaperone for Fe-S cluster assembly 
        and transfer. This molecular function term captures the chaperone-like 
        activity of holding and transferring clusters.
      supported_by:
        - reference_id: PMID:38627381
          supporting_text: Mechanism and structural dynamics of sulfur transfer 
            during de novo [2Fe-2S] cluster assembly on ISCU2.
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: HTP
    original_reference_id: PMID:34800366
    review:
      summary: HTP annotation from mitochondrial proteome study confirming 
        mitochondrial localization.
      action: ACCEPT
      reason: Mitochondrial localization is well-established. HTP provides 
        quantitative proteomics support.
      supported_by:
        - reference_id: PMID:34800366
          supporting_text: Epub 2021 Nov 19. Quantitative high-confidence human 
            mitochondrial proteome and its dynamics in cellular context.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:31101807
    review:
      summary: IPI for protein binding with FXN (frataxin). This is a 
        functionally important interaction - FXN binds the ISCU-NFS1 complex and
        activates sulfur transfer.
      action: MODIFY
      reason: While the protein binding is documented, a more specific term 
        would be preferable. FXN interaction with ISCU is essential for Fe-S 
        cluster assembly activation. Consider more specific annotation if 
        available.
      proposed_replacement_terms:
        - id: GO:0140132
          label: iron-sulfur cluster chaperone activity
      supported_by:
        - reference_id: PMID:31101807
          supporting_text: Structure of the human frataxin-bound iron-sulfur 
            cluster assembly complex provides insight into its activation 
            mechanism.
  - term:
      id: GO:0006879
      label: intracellular iron ion homeostasis
    evidence_type: IDA
    original_reference_id: PMID:16517407
    review:
      summary: IDA for iron ion homeostasis. Study demonstrated that 
        mitochondrial and cytosolic ISCU isoforms both contribute to iron 
        homeostasis regulation.
      action: ACCEPT
      reason: Direct experimental evidence showing ISCU's role in cellular iron 
        homeostasis through its Fe-S cluster assembly function.
      supported_by:
        - reference_id: PMID:16517407
          supporting_text: Functions of mitochondrial ISCU and cytosolic ISCU in
            mammalian iron-sulfur cluster biogenesis and iron homeostasis.
  - term:
      id: GO:0044572
      label: '[4Fe-4S] cluster assembly'
    evidence_type: IDA
    original_reference_id: PMID:23940031
    review:
      summary: IDA for [4Fe-4S] cluster assembly. Earlier studies suggested ISCU
        could assemble [4Fe-4S], but recent evidence indicates ISCU primarily 
        assembles [2Fe-2S] which is converted to [4Fe-4S] by downstream carriers
        (ISCA1, ISCA2, IBA57, NFU1).
      action: MODIFY
      reason: Recent studies (PMID:34824239) clarify that ISCU assembles 
        [2Fe-2S] clusters, not [4Fe-4S] directly. The [4Fe-4S] clusters are 
        assembled by downstream carriers. This annotation should be modified to 
        [2Fe-2S] cluster assembly.
      proposed_replacement_terms:
        - id: GO:0044571
          label: '[2Fe-2S] cluster assembly'
      supported_by:
        - reference_id: PMID:23940031
          supporting_text: Human mitochondrial chaperone (mtHSP70) and cysteine 
            desulfurase (NFS1) bind preferentially to the disordered 
            conformation, whereas co-chaperone (HSC20) binds to the structured 
            conformation of the iron-sulfur cluster scaffold protein (ISCU)
  - term:
      id: GO:0099128
      label: mitochondrial [2Fe-2S] assembly complex
    evidence_type: IDA
    original_reference_id: PMID:31101807
    review:
      summary: IDA for membership in mitochondrial [2Fe-2S] assembly complex 
        from structural study of the human frataxin-bound complex.
      action: ACCEPT
      reason: Direct structural evidence showing ISCU as a component of the 
        complex.
      supported_by:
        - reference_id: PMID:31101807
          supporting_text: Structure of the human frataxin-bound iron-sulfur 
            cluster assembly complex provides insight into its activation 
            mechanism.
  - term:
      id: GO:0044571
      label: '[2Fe-2S] cluster assembly'
    evidence_type: IDA
    original_reference_id: PMID:24971490
    review:
      summary: IDA for [2Fe-2S] cluster assembly. Study demonstrated FXN 
        activates Fe-S cluster biosynthesis by facilitating sulfur transfer 
        chemistry on ISCU.
      action: ACCEPT
      reason: Direct biochemical evidence for [2Fe-2S] cluster assembly on ISCU.
      supported_by:
        - reference_id: PMID:24971490
          supporting_text: Human frataxin activates Fe-S cluster biosynthesis by
            facilitating sulfur transfer chemistry.
  - term:
      id: GO:0008270
      label: zinc ion binding
    evidence_type: IDA
    original_reference_id: PMID:30031876
    review:
      summary: IDA for zinc ion binding. ISCU can bind Zn2+ at the active site, 
        which modulates NFS1 desulfurase activity. Zn2+ binding is competitive 
        with Fe2+ and may serve a regulatory role.
      action: ACCEPT
      reason: Direct experimental evidence for Zn2+ binding. UniProt confirms 
        Zn2+ coordination by Cys71, Cys95, and Cys138. This may be important for
        protein stability or regulation.
      supported_by:
        - reference_id: PMID:30031876
          supporting_text: Zinc(II) binding on human wild-type ISCU and Met140 
            variants modulates NFS1 desulfurase activity.
  - term:
      id: GO:0042803
      label: protein homodimerization activity
    evidence_type: IDA
    original_reference_id: PMID:29097656
    review:
      summary: IDA for homodimerization from structural study. Crystal structure
        revealed ISCU dimerization in complex with NFS1-LYRM4.
      action: ACCEPT
      reason: Direct structural evidence for ISCU homodimerization. This is 
        functionally important for cluster assembly.
      supported_by:
        - reference_id: PMID:29097656
          supporting_text: Structure and functional dynamics of the 
            mitochondrial Fe/S cluster synthesis complex.
  - term:
      id: GO:0042803
      label: protein homodimerization activity
    evidence_type: IDA
    original_reference_id: PMID:34824239
    review:
      summary: IDA for homodimerization. Key study showing Tyr-35-mediated 
        dimerization triggers [2Fe-2S] cluster synthesis.
      action: ACCEPT
      reason: Direct experimental evidence showing Tyr-35-mediated dimerization 
        is essential for cluster assembly. This is a core function.
      supported_by:
        - reference_id: PMID:34824239
          supporting_text: N-terminal tyrosine of ISCU2 triggers [2Fe-2S] 
            cluster synthesis by ISCU2 dimerization.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:29097656
    review:
      summary: IPI for protein binding with NFS1 and LYRM4 from structural 
        study. These are functionally critical interactions for Fe-S cluster 
        assembly.
      action: KEEP_AS_NON_CORE
      reason: The interaction with NFS1 and LYRM4 is essential for ISCU 
        function, but protein binding is too generic. The complex membership 
        annotation (GO:0099128) better captures this.
      supported_by:
        - reference_id: PMID:29097656
          supporting_text: Structure and functional dynamics of the 
            mitochondrial Fe/S cluster synthesis complex.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:23940031
    review:
      summary: IPI for protein binding with NFS1. Functionally important 
        interaction - NFS1 binds preferentially to the D-state of ISCU.
      action: KEEP_AS_NON_CORE
      reason: Important functional interaction but protein binding is too 
        generic. Complex membership annotation is more informative.
      supported_by:
        - reference_id: PMID:23940031
          supporting_text: NFS1 binds preferentially to the D-state of ISCU as 
            does the NFS1-ISD11 complex.
  - term:
      id: GO:0006879
      label: intracellular iron ion homeostasis
    evidence_type: IDA
    original_reference_id: PMID:23508953
    review:
      summary: IDA for iron homeostasis. Study on mTORC1-mediated 
        phosphorylation of ISCU and its implications for iron metabolism.
      action: ACCEPT
      reason: Direct evidence linking ISCU stability/function to iron metabolism
        regulation.
      supported_by:
        - reference_id: PMID:23508953
          supporting_text: 'Mammalian target of rapamycin complex 1 (mTORC1)-mediated
            phosphorylation stabilizes ISCU protein: implications for iron metabolism.'
  - term:
      id: GO:0044571
      label: '[2Fe-2S] cluster assembly'
    evidence_type: IDA
    original_reference_id: PMID:23940031
    review:
      summary: IDA for [2Fe-2S] cluster assembly. In vitro reconstitution 
        showing cluster assembly on ISCU.
      action: ACCEPT
      reason: Direct biochemical evidence for [2Fe-2S] cluster assembly on ISCU.
      supported_by:
        - reference_id: PMID:23940031
          supporting_text: In vitro Fe-S cluster assembly assays
  - term:
      id: GO:0099128
      label: mitochondrial [2Fe-2S] assembly complex
    evidence_type: IDA
    original_reference_id: PMID:21298097
    review:
      summary: IDA for complex membership. Study showed FXN interacts with 
        preformed ISCU/NFS1/ISD11 complex.
      action: ACCEPT
      reason: Direct evidence for ISCU as component of the core ISC assembly 
        complex.
      supported_by:
        - reference_id: PMID:21298097
          supporting_text: 'Mammalian frataxin: an essential function for cellular
            viability through an interaction with a preformed ISCU/NFS1/ISD11 iron-sulfur
            assembly complex.'
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:11060020
    review:
      summary: IPI for protein binding with NFS1 from the initial 
        characterization of human ISC assembly complexes.
      action: KEEP_AS_NON_CORE
      reason: Foundational study establishing ISCU-NFS1 interaction. Protein 
        binding is generic but this is a key functional interaction.
      supported_by:
        - reference_id: PMID:11060020
          supporting_text: Distinct iron-sulfur cluster assembly complexes exist
            in the cytosol and mitochondria of human cells.
  - term:
      id: GO:0005829
      label: cytosol
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-9771686
    review:
      summary: TAS from Reactome for cytosolic localization, related to 
        SARS-CoV-2 nsp12 Fe-S cluster acquisition pathway.
      action: ACCEPT
      reason: Cytosolic localization is appropriate for isoform 2. The Reactome 
        pathway describes viral protein utilizing host Fe-S machinery.
  - term:
      id: GO:1902958
      label: positive regulation of mitochondrial electron transport, NADH to 
        ubiquinone
    evidence_type: IMP
    original_reference_id: PMID:20436681
    review:
      summary: IMP for positive regulation of Complex I activity. Study on 
        miR-210 targeting of ISCU in hypoxia and cancer.
      action: KEEP_AS_NON_CORE
      reason: This is a downstream effect of ISCU's role in providing Fe-S 
        clusters to respiratory complex subunits. Not a core function but a 
        physiological consequence of Fe-S biogenesis.
      supported_by:
        - reference_id: PMID:20436681
          supporting_text: MicroRNA-210 regulates mitochondrial free radical 
            response to hypoxia and krebs cycle in cancer cells by targeting 
            iron sulfur cluster protein ISCU.
  - term:
      id: GO:1904439
      label: negative regulation of iron ion import across plasma membrane
    evidence_type: IMP
    original_reference_id: PMID:20436681
    review:
      summary: IMP for negative regulation of iron import. ISCU knockdown 
        increases cellular iron import, likely due to IRP activation from Fe-S 
        cluster deficiency.
      action: KEEP_AS_NON_CORE
      reason: This is an indirect effect of ISCU's role in Fe-S biogenesis on 
        iron homeostasis signaling. Not a direct function but reflects the 
        IRP-mediated iron sensing response to Fe-S deficiency.
      supported_by:
        - reference_id: PMID:20436681
          supporting_text: MicroRNA-210 regulates mitochondrial free radical 
            response to hypoxia and krebs cycle in cancer cells by targeting 
            iron sulfur cluster protein ISCU.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:26702583
    review:
      summary: IPI for protein binding with HSPA9. Functionally important 
        interaction - HSPA9 (mortalin/mtHsp70) mediates cluster release from 
        ISCU.
      action: KEEP_AS_NON_CORE
      reason: Important functional interaction for cluster transfer. Protein 
        binding is generic but the HSPA9 interaction is critical for ISCU 
        function.
      supported_by:
        - reference_id: PMID:26702583
          supporting_text: Mitochondrial Hspa9/Mortalin regulates erythroid 
            differentiation via iron-sulfur cluster assembly.
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: IDA
    original_reference_id: PMID:26702583
    review:
      summary: IDA for mitochondrial localization from study on HSPA9-ISCU 
        interaction.
      action: ACCEPT
      reason: Direct experimental evidence for mitochondrial localization.
      supported_by:
        - reference_id: PMID:26702583
          supporting_text: Mitochondrial Hspa9/Mortalin regulates erythroid 
            differentiation via iron-sulfur cluster assembly.
  - term:
      id: GO:0005759
      label: mitochondrial matrix
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-1362408
    review:
      summary: 'TAS from Reactome for mitochondrial matrix localization. Reactome
        pathway: FXN:NFS1:ISD11:ISCU assembles 2Fe-2S iron-sulfur cluster.'
      action: ACCEPT
      reason: Mitochondrial matrix is the correct subcompartment for ISCU 
        function.
  - term:
      id: GO:0005759
      label: mitochondrial matrix
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-1362416
    review:
      summary: 'TAS from Reactome for mitochondrial matrix. Reactome pathway: Frataxin
        binds iron.'
      action: ACCEPT
      reason: Mitochondrial matrix localization is well-established.
  - term:
      id: GO:0005759
      label: mitochondrial matrix
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-9854984
    review:
      summary: 'TAS from Reactome for mitochondrial matrix. Reactome pathway: Transfer
        of Fe-S clusters to SDHB.'
      action: ACCEPT
      reason: Mitochondrial matrix localization is appropriate.
  - term:
      id: GO:0005759
      label: mitochondrial matrix
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-9866272
    review:
      summary: 'TAS from Reactome for mitochondrial matrix. Reactome pathway: 2Fe-2S
        is inserted in UQCRFS1.'
      action: ACCEPT
      reason: Mitochondrial matrix localization is correct for ISCU's function 
        in providing Fe-S clusters to respiratory complex subunits.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:20668094
    review:
      summary: IPI for protein binding with HSCB (HSC20). Functionally critical 
        interaction - HSCB is the J-domain co-chaperone that mediates cluster 
        transfer from ISCU.
      action: KEEP_AS_NON_CORE
      reason: Important functional interaction. HSCB binds the structured (S) 
        state of ISCU and stimulates HSPA9 ATPase activity for cluster release. 
        Protein binding is generic but this is a key functional partner.
      supported_by:
        - reference_id: PMID:20668094
          supporting_text: Here, we show that the human HSC20 protein can 
            complement for its counterpart in yeast, Jac1p, and interacts with 
            its proposed human partners, hISCU and hHSPA9
  - term:
      id: GO:0005737
      label: cytoplasm
    evidence_type: IDA
    original_reference_id: GO_REF:0000054
    review:
      summary: IDA from LIFEdb fusion protein localization study. Cytoplasmic 
        localization appropriate for isoform 2.
      action: ACCEPT
      reason: Cytoplasmic localization is appropriate for the cytosolic isoform.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:16527810
    review:
      summary: IPI for protein binding with NFS1 (cytosolic form). Study on 
        roles of cytosolic ISCS and ISCU in Fe-S cluster assembly.
      action: KEEP_AS_NON_CORE
      reason: Functionally important interaction between cytosolic ISCU and 
        NFS1.
      supported_by:
        - reference_id: PMID:16527810
          supporting_text: Roles of the mammalian cytosolic cysteine 
            desulfurase, ISCS, and scaffold protein, ISCU, in iron-sulfur 
            cluster assembly.
  - term:
      id: GO:0005634
      label: nucleus
    evidence_type: TAS
    original_reference_id: PMID:16527810
    review:
      summary: TAS for nuclear localization from study on cytosolic ISCU 
        function.
      action: ACCEPT
      reason: Some nuclear localization has been reported for the cytosolic 
        isoform. This is a minor localization.
      supported_by:
        - reference_id: PMID:16527810
          supporting_text: 2006 Mar 9. Roles of the mammalian cytosolic cysteine
            desulfurase, ISCS, and scaffold protein, ISCU, in iron-sulfur 
            cluster assembly.
  - term:
      id: GO:0005829
      label: cytosol
    evidence_type: IDA
    original_reference_id: PMID:16527810
    review:
      summary: IDA for cytosolic localization from study on cytosolic ISCU.
      action: ACCEPT
      reason: Direct evidence for cytosolic localization of isoform 2.
      supported_by:
        - reference_id: PMID:16527810
          supporting_text: Roles of the mammalian cytosolic cysteine 
            desulfurase, ISCS, and scaffold protein, ISCU, in iron-sulfur 
            cluster assembly.
  - term:
      id: GO:0060090
      label: molecular adaptor activity
    evidence_type: IDA
    original_reference_id: PMID:16527810
    review:
      summary: IDA for molecular adaptor activity. ISCU serves as a 
        scaffold/adaptor that brings together Fe, S, and protein partners for 
        cluster assembly.
      action: ACCEPT
      reason: ISCU functions as a molecular scaffold/adaptor, coordinating 
        multiple components for Fe-S cluster assembly and transfer. This MF term
        is more informative than generic protein binding.
      supported_by:
        - reference_id: PMID:16527810
          supporting_text: Roles of the mammalian cytosolic cysteine 
            desulfurase, ISCS, and scaffold protein, ISCU, in iron-sulfur 
            cluster assembly.
  - term:
      id: GO:0005737
      label: cytoplasm
    evidence_type: IDA
    original_reference_id: PMID:11060020
    review:
      summary: IDA for cytoplasmic localization from initial study identifying 
        distinct cytosolic and mitochondrial ISC complexes.
      action: ACCEPT
      reason: Foundational study establishing dual localization of ISCU.
      supported_by:
        - reference_id: PMID:11060020
          supporting_text: Distinct iron-sulfur cluster assembly complexes exist
            in the cytosol and mitochondria of human cells.
  - term:
      id: GO:0005739
      label: mitochondrion
    evidence_type: IDA
    original_reference_id: PMID:11060020
    review:
      summary: IDA for mitochondrial localization from initial study 
        characterizing human ISC assembly complexes.
      action: ACCEPT
      reason: Foundational study establishing mitochondrial localization.
      supported_by:
        - reference_id: PMID:11060020
          supporting_text: Distinct iron-sulfur cluster assembly complexes exist
            in the cytosol and mitochondria of human cells.
  - term:
      id: GO:0005506
      label: iron ion binding
    evidence_type: TAS
    original_reference_id: PMID:11060020
    review:
      summary: TAS for iron ion binding from initial characterization study.
      action: ACCEPT
      reason: Iron ion binding is a core function for ISCU as the scaffold for 
        Fe-S cluster assembly.
      supported_by:
        - reference_id: PMID:11060020
          supporting_text: Distinct iron-sulfur cluster assembly complexes exist
            in the cytosol and mitochondria of human cells.
  - term:
      id: GO:0016226
      label: iron-sulfur cluster assembly
    evidence_type: TAS
    original_reference_id: PMID:11060020
    review:
      summary: TAS for iron-sulfur cluster assembly from initial 
        characterization study.
      action: ACCEPT
      reason: Iron-sulfur cluster assembly is the core biological process for 
        ISCU.
      supported_by:
        - reference_id: PMID:11060020
          supporting_text: Distinct iron-sulfur cluster assembly complexes exist
            in the cytosol and mitochondria of human cells.
references:
  - id: GO_REF:0000002
    title: Gene Ontology annotation through association of InterPro records with
      GO terms
    findings: []
  - id: GO_REF:0000033
    title: Annotation inferences using phylogenetic trees
    findings: []
  - id: GO_REF:0000043
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword 
      mapping
    findings: []
  - id: GO_REF:0000044
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular 
      Location vocabulary mapping
    findings: []
  - id: GO_REF:0000052
    title: Gene Ontology annotation based on curation of immunofluorescence data
    findings: []
  - id: GO_REF:0000054
    title: Gene Ontology annotation based on curation of intracellular 
      localizations of expressed fusion proteins
    findings: []
  - id: GO_REF:0000107
    title: Automatic transfer of experimentally verified manual GO annotation 
      data to orthologs using Ensembl Compara
    findings: []
  - id: GO_REF:0000120
    title: Combined Automated Annotation using Multiple IEA Methods
    findings: []
  - id: PMID:11060020
    title: Distinct iron-sulfur cluster assembly complexes exist in the cytosol 
      and mitochondria of human cells.
    findings:
      - statement: Identified ISCU as the human homolog of bacterial/yeast IscU 
          scaffold proteins
      - statement: Demonstrated both mitochondrial and cytoplasmic ISC assembly 
          complexes
      - statement: Showed ISCU interaction with NFS1
  - id: PMID:16517407
    title: Functions of mitochondrial ISCU and cytosolic ISCU in mammalian 
      iron-sulfur cluster biogenesis and iron homeostasis.
    findings:
      - statement: Characterized distinct functions of mitochondrial and 
          cytosolic ISCU isoforms
      - statement: Demonstrated role in cellular iron homeostasis regulation
  - id: PMID:16527810
    title: Roles of the mammalian cytosolic cysteine desulfurase, ISCS, and 
      scaffold protein, ISCU, in iron-sulfur cluster assembly.
    findings:
      - statement: Characterized cytosolic ISC assembly machinery
      - statement: Demonstrated ISCU-NFS1 interaction in cytoplasm
  - id: PMID:20668094
    title: Characterization of the human HSC20, an unusual DnaJ type III 
      protein, involved in iron-sulfur cluster biogenesis.
    findings:
      - statement: Characterized HSCB (HSC20) as J-domain co-chaperone for ISCU
      - statement: Demonstrated HSCB-ISCU-HSPA9 interactions
  - id: PMID:21298097
    title: Mammalian frataxin - an essential function for cellular viability 
      through an interaction with a preformed ISCU/NFS1/ISD11 iron-sulfur 
      assembly complex.
    findings:
      - statement: Demonstrated FXN interaction with preformed ISCU/NFS1/ISD11 
          complex
      - statement: Established architecture of core ISC assembly complex
  - id: PMID:23508953
    title: Mammalian target of rapamycin complex 1 (mTORC1)-mediated 
      phosphorylation stabilizes ISCU protein - implications for iron 
      metabolism.
    findings:
      - statement: Identified Ser-14 phosphorylation by mTOR
      - statement: Linked ISCU stability to iron metabolism
  - id: PMID:23940031
    title: Human mitochondrial chaperone (mtHSP70) and cysteine desulfurase 
      (NFS1) bind preferentially to the disordered conformation, whereas 
      co-chaperone (HSC20) binds to the structured conformation of the 
      iron-sulfur cluster scaffold protein (ISCU).
    findings:
      - statement: Characterized D (disordered) and S (structured) states of 
          ISCU
      - statement: Showed differential binding of partners to D vs S states
      - statement: Demonstrated Fe-S cluster assembly on ISCU in vitro
  - id: PMID:24606901
    title: Cochaperone binding to LYR motifs confers specificity of iron sulfur 
      cluster delivery.
    findings:
      - statement: Characterized LYR motif recognition by HSCB
      - statement: Demonstrated cluster delivery pathway to SDHB
  - id: PMID:24971490
    title: Human frataxin activates Fe-S cluster biosynthesis by facilitating 
      sulfur transfer chemistry.
    findings:
      - statement: Demonstrated FXN role in activating sulfur transfer to ISCU
      - statement: Characterized sulfhydration of Cys138
      - statement: Identified critical cysteine residues for cluster assembly
  - id: PMID:26702583
    title: Mitochondrial Hspa9/Mortalin regulates erythroid differentiation via 
      iron-sulfur cluster assembly.
    findings:
      - statement: Demonstrated HSPA9-ISCU interaction
      - statement: Linked Fe-S biogenesis to erythroid differentiation
  - id: PMID:27519411
    title: Architecture of the Human Mitochondrial Iron-Sulfur Cluster Assembly 
      Machinery.
    findings:
      - statement: Low-resolution structure of human ISC assembly complex
  - id: PMID:29097656
    title: Structure and functional dynamics of the mitochondrial Fe/S cluster 
      synthesis complex.
    findings:
      - statement: Crystal structure of ISCU in complex with NFS1-LYRM4
      - statement: Revealed ISCU homodimerization
      - statement: Characterized zinc binding at active site
  - id: PMID:30031876
    title: Zinc(II) binding on human wild-type ISCU and Met140 variants 
      modulates NFS1 desulfurase activity.
    findings:
      - statement: Characterized Zn2+ binding to ISCU
      - statement: Demonstrated modulation of NFS1 activity by Zn-bound ISCU
  - id: PMID:31101807
    title: Structure of the human frataxin-bound iron-sulfur cluster assembly 
      complex provides insight into its activation mechanism.
    findings:
      - statement: Cryo-EM structure of human ISC complex with FXN
      - statement: Revealed FXN binding mode and activation mechanism
  - id: PMID:34800366
    title: Quantitative high-confidence human mitochondrial proteome and its 
      dynamics in cellular context.
    findings:
      - statement: Confirmed mitochondrial localization of ISCU by quantitative 
          proteomics
  - id: PMID:34824239
    title: N-terminal tyrosine of ISCU2 triggers [2Fe-2S] cluster synthesis by 
      ISCU2 dimerization.
    findings:
      - statement: Demonstrated Tyr-35-mediated dimerization essential for 
          cluster assembly
      - statement: Clarified that only [2Fe-2S] clusters form on ISCU (not 
          [4Fe-4S])
      - statement: High-resolution structures of ISCU in various states
  - id: PMID:38627381
    title: Mechanism and structural dynamics of sulfur transfer during de novo 
      [2Fe-2S] cluster assembly on ISCU2.
    findings:
      - statement: 2024 cryo-EM snapshots of sulfur transfer mechanism
      - statement: Captured persulfide transfer from NFS1 Cys381 to ISCU Cys138
      - statement: Mossbauer spectroscopy of Fe coordination states
  - id: PMID:20436681
    title: MicroRNA-210 regulates mitochondrial free radical response to hypoxia
      and krebs cycle in cancer cells by targeting iron sulfur cluster protein 
      ISCU.
    findings:
      - statement: Identified ISCU as miR-210 target
      - statement: Demonstrated effects on mitochondrial function and iron 
          import
  - id: PMID:15778465
    title: Targeted proteomic analysis of 14-3-3 sigma, a p53 effector commonly 
      silenced in cancer.
    findings: []
  - id: PMID:17353931
    title: Large-scale mapping of human protein-protein interactions by mass 
      spectrometry.
    findings: []
  - id: PMID:25416956
    title: A proteome-scale map of the human interactome network.
    findings: []
  - id: PMID:26749241
    title: Disease-Causing SDHAF1 Mutations Impair Transfer of Fe-S Clusters to 
      SDHB.
    findings: []
  - id: PMID:31515488
    title: Extensive disruption of protein interactions by genetic variants 
      across the allele frequency spectrum in human populations.
    findings: []
  - id: PMID:32296183
    title: A reference map of the human binary protein interactome.
    findings: []
  - id: PMID:35271311
    title: 'OpenCell: Endogenous tagging for the cartography of human cellular organization.'
    findings: []
  - id: PMID:36931259
    title: A central chaperone-like role for 14-3-3 proteins in human cells.
    findings: []
  - id: PMID:40205054
    title: Multimodal cell maps as a foundation for structural and functional 
      genomics.
    findings: []
  - id: Reactome:R-HSA-1362408
    title: FXN:NFS1:ISD11:ISCU assembles 2Fe-2S iron-sulfur cluster
    findings: []
  - id: Reactome:R-HSA-1362416
    title: Frataxin binds iron
    findings: []
  - id: Reactome:R-HSA-9771686
    title: nsp12 acquires Fe-S cluster cofactors
    findings: []
  - id: Reactome:R-HSA-9854984
    title: Transfer of Fe-S clusters to SDHB
    findings: []
  - id: Reactome:R-HSA-9866272
    title: 2Fe-2S is inserted in UQCRFS1
    findings: []
  - id: file:human/ISCU/ISCU-deep-research-falcon.md
    title: Deep research report on ISCU
    findings: []
core_functions:
  - description: ISCU is the primary scaffold for de novo [2Fe-2S] cluster 
      assembly in the mitochondrial matrix. Recent structural studies 
      (PMID:34824239, PMID:38627381) demonstrate that ISCU homodimerizes via 
      Tyr-35 and receives persulfide sulfur from NFS1 Cys381 to Cys138. FDX2 
      provides reducing equivalents to complete cluster assembly.
    molecular_function:
      id: GO:0051537
      label: 2 iron, 2 sulfur cluster binding
    directly_involved_in:
      - id: GO:0044571
        label: '[2Fe-2S] cluster assembly'
    locations:
      - id: GO:0005759
        label: mitochondrial matrix
    in_complex:
      id: GO:0099128
      label: mitochondrial [2Fe-2S] assembly complex
  - description: ISCU functions as a scaffold/chaperone for Fe-S cluster 
      assembly and subsequent transfer to recipient proteins via the HSPA9-HSCB 
      chaperone system (PMID:38627381, PMID:23940031).
    molecular_function:
      id: GO:0140132
      label: iron-sulfur cluster chaperone activity
    directly_involved_in:
      - id: GO:0016226
        label: iron-sulfur cluster assembly
    locations:
      - id: GO:0005759
        label: mitochondrial matrix
  - description: ISCU binds Fe2+ as part of the cluster assembly process. 
      Mossbauer spectroscopy demonstrates iron coordination during assembly 
      (PMID:38627381).
    molecular_function:
      id: GO:0008198
      label: ferrous iron binding
    directly_involved_in:
      - id: GO:0044571
        label: '[2Fe-2S] cluster assembly'
    locations:
      - id: GO:0005759
        label: mitochondrial matrix
  - description: Tyr-35-mediated homodimerization is essential for [2Fe-2S] 
      cluster synthesis. Mutation of Tyr-35 abolishes cluster assembly 
      (PMID:34824239).
    molecular_function:
      id: GO:0042803
      label: protein homodimerization activity
    directly_involved_in:
      - id: GO:0044571
        label: '[2Fe-2S] cluster assembly'
    locations:
      - id: GO:0005759
        label: mitochondrial matrix
  - description: ISCU functions as a molecular scaffold/adaptor in cytoplasmic 
      Fe-S cluster biogenesis via isoform 2 (ISCU1), coordinating with cytosolic
      NFS1 for cluster assembly.
    molecular_function:
      id: GO:0060090
      label: molecular adaptor activity
    directly_involved_in:
      - id: GO:0016226
        label: iron-sulfur cluster assembly
    locations:
      - id: GO:0005829
        label: cytosol
tags:
  - iron-sulfur-cluster-biogenesis

ISCU

Gene Description

Existing Annotations Review

Core Functions

ISCU functions as a scaffold/chaperone for Fe-S cluster assembly and subsequent transfer to recipient proteins via the HSPA9-HSCB chaperone system (PMID:38627381, PMID:23940031).

ISCU binds Fe2+ as part of the cluster assembly process. Mossbauer spectroscopy demonstrates iron coordination during assembly (PMID:38627381).

Tyr-35-mediated homodimerization is essential for [2Fe-2S] cluster synthesis. Mutation of Tyr-35 abolishes cluster assembly (PMID:34824239).

ISCU functions as a molecular scaffold/adaptor in cytoplasmic Fe-S cluster biogenesis via isoform 2 (ISCU1), coordinating with cytosolic NFS1 for cluster assembly.

References

Tags

📚 Additional Documentation

Deep Research Falcon

Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

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MANDATORY VERIFICATION STEPS:

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

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Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

Target Gene/Protein Identity (from UniProt):

MANDATORY VERIFICATION STEPS:

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

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