{"gene":"ISCA1","run_date":"2026-04-28T18:06:54","timeline":{"discoveries":[{"year":2000,"finding":"Yeast Isa1p (ISCA1 ortholog) is targeted to the mitochondrial matrix, and its three invariant cysteine residues are essential for function, likely involved in iron binding; loss of Isa1p causes reduced aconitase and succinate dehydrogenase activity and elevated mitochondrial iron accumulation.","method":"Genetic deletion, subcellular fractionation, site-directed mutagenesis of cysteine residues, enzyme activity assays","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods in foundational study, replicated by subsequent work","pmids":["10805735"],"is_preprint":false},{"year":2002,"finding":"Fission yeast Isa1 (ISCA1 ortholog) is a multimeric protein carrying [2Fe-2S]2+ clusters; it forms a complex with a redox-active ferredoxin (identified by crosslinking), and cysteine ligands are required for cluster coordination as shown by site-directed mutagenesis.","method":"Mössbauer spectroscopy, UV-visible and EPR spectroscopy, chemical crosslinking, site-directed mutagenesis","journal":"Journal of biological inorganic chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro biochemical reconstitution with mutagenesis and multiple spectroscopic methods","pmids":["11941510"],"is_preprint":false},{"year":2004,"finding":"Human ISCA1 (hIscA) is localized to mitochondria and functionally complements yeast isa1Δ mutants, establishing functional conservation; the protein contains the HESB structural domain.","method":"Yeast complementation assay, subcellular localization (mitochondrial targeting), immunological detection","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2 — complementation in yeast with localization, single lab","pmids":["15262227"],"is_preprint":false},{"year":2007,"finding":"Yeast Isa1 and Isa2 are required for the in vivo catalytic function of biotin synthase (Bio2), a [4Fe-4S] enzyme, but not for the de novo synthesis of its Fe/S clusters (which depends on Isu1/Isu2); Isa proteins are specifically needed for functional activity of [4Fe-4S] apoproteins.","method":"Genetic epistasis, enzyme activity assays, protein depletion, overexpression rescue","journal":"Eukaryotic cell","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with multiple mutant combinations and biochemical assays, clear mechanistic distinction","pmids":["17259550"],"is_preprint":false},{"year":2009,"finding":"Human ISCA1 is present in both cytosolic and mitochondrial fractions, and interacts with IOP1/NARFL (a cytosolic CIA pathway factor); siRNA knockdown of ISCA1 in HeLa cells reduces activity of both mitochondrial (aconitase, succinate dehydrogenase) and cytosolic (cytosolic aconitase) Fe/S enzymes.","method":"Subcellular fractionation, Co-immunoprecipitation, siRNA knockdown, enzyme activity assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP with functional knockdown data, single lab; cytosolic localization later disputed","pmids":["19864422"],"is_preprint":false},{"year":2010,"finding":"Human ISCA1 (hIscA1) binds mononuclear iron with an association constant of ~2×10^19 M^-1, and iron-bound hIscA1 can donate iron for Fe/S cluster assembly on the E. coli scaffold protein IscU in vitro, supporting an iron chaperone role.","method":"UV-visible absorption, EPR spectroscopy, in vitro iron transfer assay to IscU, iron content modulation by growth medium","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with spectroscopic characterization and functional iron transfer assay","pmids":["20302570"],"is_preprint":false},{"year":2010,"finding":"In fission yeast, Grx5 physically interacts in vivo with Isa1 and Isa2 in mitochondria (bimolecular fluorescence complementation); overexpression of isa1+ or isa2+ suppresses growth defects of Δgrx5 mutant, placing Grx5 upstream or in the same pathway as Isa1/Isa2 in Fe/S assembly.","method":"Bimolecular fluorescence complementation (BiFC), genetic suppressor screen, multi-copy suppression, Fe/S enzyme activity assays","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2-3 — direct protein interaction shown by BiFC with functional epistasis, single lab","pmids":["20085751"],"is_preprint":false},{"year":2011,"finding":"Yeast Isa1 and Isa2 form a complex specifically required for maturation of mitochondrial [4Fe-4S] proteins (aconitase, homoaconitase) but dispensable for [2Fe-2S] proteins and cytosolic [4Fe-4S] proteins; Isa1/Isa2 bind iron in vivo, and this iron is proposed as the direct donor for de novo [4Fe-4S] cluster synthesis in a reaction assisted by Iba57.","method":"In vivo genetic analysis, iron-binding assay, targeted mitochondrial [2Fe-2S]/[4Fe-4S] ferredoxin reporters, Iba57 depletion, enzyme activity assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — comprehensive in vivo analysis with multiple orthogonal approaches, replicated across labs","pmids":["21987576"],"is_preprint":false},{"year":2012,"finding":"Human ISCA1, ISCA2, and IBA57 are specifically required for maturation of mitochondrial [4Fe-4S] proteins (aconitase, respiratory complex I, lipoic acid synthase) but not [2Fe-2S] proteins (ferrochelatase); ISCA1/ISCA2 do not function in the cytosol for Fe/S protein maturation; their depletion causes massive mitochondrial swelling and loss of cristae.","method":"RNAi knockdown in HeLa cells, enzyme activity assays, heme quantification, mitochondrial morphology by electron microscopy, subcellular fractionation","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods, highly cited, directly tests cytosolic localization claim","pmids":["22323289"],"is_preprint":false},{"year":2018,"finding":"A homozygous missense mutation p.(V10G) in the ISCA1 presequence severely impairs mitochondrial import and stability of ISCA1 protein, leading to defective [4Fe-4S] protein maturation (respiratory complexes, lipoic acid synthesis); RNAi knockdown rescued by wild-type but not mutant ISCA1.","method":"Patient fibroblast biochemical analysis, mitochondrial import assay, RNAi complementation, targeted MitoExome sequencing","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — complementation rescue experiment with mutant vs. wild-type, multiple biochemical readouts","pmids":["29767723"],"is_preprint":false},{"year":2020,"finding":"Mitochondrial [4Fe-4S] cluster assembly on ISCA1-ISCA2 proceeds by reductive fusion of two [2Fe-2S] clusters donated from GLRX5; this reaction requires FDX2 (ferredoxin 2) as the specific electron donor and its reductase FDXR, as well as IBA57, reconstituted in vitro without artificial reductants. FDX1 and other reducing systems cannot substitute for FDX2.","method":"In vitro reconstitution of [4Fe-4S] aconitase maturation with purified components, spectroscopy, functional specificity tests with FDX1 vs FDX2","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — fully reconstituted in vitro with purified physiological components and rigorous specificity controls","pmids":["32817474"],"is_preprint":false},{"year":2020,"finding":"ISCA1 directly interacts with both ISCU2 and NFU1, donating [2Fe-2S] clusters to NFU1 (along with ISCU2) for reductive formation of a bridging [4Fe-4S] cluster; the interaction site maps to a conserved hydrophobic patch on the C-terminal alpha-helix of NFU1, and mutagenesis of this site abolishes cluster acquisition.","method":"Co-immunoprecipitation, biochemical reconstitution, site-directed mutagenesis of NFU1 interaction surface, functional assays of downstream targets (lipoic acid synthase, pyruvate dehydrogenase lipoylation)","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 1-2 — reconstitution plus mutagenesis defining interaction site, with functional downstream readouts","pmids":["32776106"],"is_preprint":false},{"year":2020,"finding":"A novel pathogenic ISCA1 variant p.(Tyr101Cys) decreases stability of the [2Fe-2S] cluster bound to the protein, establishing that ISCA1 itself carries a [2Fe-2S] cluster whose integrity is essential for its function in [4Fe-4S] protein maturation.","method":"Expression and purification of recombinant mutant ISCA1, spectroscopic characterization of [2Fe-2S] cluster stability, patient fibroblast biochemistry","journal":"Mitochondrion","confidence":"Medium","confidence_rationale":"Tier 1-2 — direct biochemical characterization of purified protein, single lab and case","pmids":["32092383"],"is_preprint":false},{"year":2021,"finding":"ISCA1 acts as the central organizer of the late-acting [4Fe-4S] assembly complex: it interacts with both ISCA2 and NFU1 individually but ISCA2 and NFU1 do not interact with each other; ISCA1 promotes formation of a transient ISCA1-ISCA2-NFU1 ternary complex and drives [4Fe-4S] cluster transfer from the ISCA1-ISCA2 assembly site to a cluster-binding site formed by ISCA1 and NFU1.","method":"NMR spectroscopy, protein interaction mapping, ternary complex characterization, cluster transfer monitoring","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 — NMR-based structural and interaction study with detailed molecular model of cluster transfer mechanism","pmids":["33711344"],"is_preprint":false},{"year":2019,"finding":"Knockout of ISCA1 in rats causes early embryonic lethality (death by embryonic day 8.5), with decreased NDUFA9 (complex I subunit) protein levels and increased aconitase 2 protein in embryos, demonstrating an essential in vivo role in mitochondrial respiratory chain assembly.","method":"CRISPR-Cas9 knockout rats, Western blot of mitochondrial proteins, embryonic phenotype analysis","journal":"Animal models and experimental medicine","confidence":"Medium","confidence_rationale":"Tier 2 — clean KO with specific molecular phenotype readouts, single study","pmids":["31016283"],"is_preprint":false},{"year":2023,"finding":"Human ISCA1, ISCA2, and ISCU proteins have strong copper-binding activity, and excess copper inhibits Fe/S cluster assembly by competing at these proteins, providing a mechanism for copper cytotoxicity in Wilson's disease.","method":"Copper-binding assay with purified proteins, Fe/S enzyme activity assays in cells and mouse model of Wilson's disease (ATP7B knockdown, ATP7A-/- lymphocytes), in vivo and in vitro pharmacological analysis","journal":"Free radical biology & medicine","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding assay with functional cellular and in vivo readouts, single lab","pmids":["37225108"],"is_preprint":false},{"year":2023,"finding":"Neuron-specific Isca1 knockout rats develop MMDS5-like disease with epilepsy, memory impairment, neuronal death, mitochondrial fragmentation and cristae rupture, reduced respiratory chain complex proteins, and decreased ATP production, demonstrating that ISCA1 is required for neuronal mitochondrial integrity.","method":"Conditional CRISPR-Cas9 knockout (Isca1flox/flox-NeuN-Cre), MRI, behavioral testing, TEM, Western blot, ATP assay, immunofluorescence","journal":"Animal models and experimental medicine","confidence":"Medium","confidence_rationale":"Tier 2 — conditional KO with multiple mechanistic readouts, single lab","pmids":["37140997"],"is_preprint":false}],"current_model":"ISCA1 is a mitochondrial iron-sulfur cluster assembly protein that, together with ISCA2 and IBA57, functions as a late-acting specialized subsystem for [4Fe-4S] cluster biogenesis: it receives [2Fe-2S] clusters from GLRX5, uses electrons from ferredoxin FDX2 to catalyze reductive fusion into [4Fe-4S] clusters on the ISCA1-ISCA2 heterodimer, and then—acting as the central organizer—drives transfer of the assembled [4Fe-4S] cluster through a transient ISCA1-ISCA2-NFU1 ternary complex to NFU1 and ultimately into target mitochondrial [4Fe-4S] apoproteins such as aconitase, respiratory complex I, and lipoic acid synthase."},"narrative":{"teleology":[{"year":2000,"claim":"Initial characterization established that the ISCA1 ortholog (yeast Isa1) localizes to the mitochondrial matrix, requires three conserved cysteines for function, and is needed for activity of [4Fe-4S]-containing enzymes aconitase and succinate dehydrogenase — linking the protein to mitochondrial iron-sulfur metabolism.","evidence":"Genetic deletion, site-directed mutagenesis, subcellular fractionation, and enzyme activity assays in S. cerevisiae","pmids":["10805735"],"confidence":"High","gaps":["Iron-sulfur cluster type bound by Isa1 not yet determined","Relationship to other known Fe/S assembly factors unclear","Mammalian function not tested"]},{"year":2002,"claim":"Spectroscopic reconstitution showed that Isa1 itself carries a [2Fe-2S] cluster and physically associates with ferredoxin, providing the first direct evidence that the protein is an Fe/S-binding factor rather than simply an iron donor.","evidence":"Mössbauer, UV-visible, and EPR spectroscopy with chemical crosslinking and mutagenesis on fission yeast Isa1","pmids":["11941510"],"confidence":"High","gaps":["Role of the ferredoxin interaction in cluster chemistry unknown","Whether the protein can also bind mononuclear iron unclear"]},{"year":2004,"claim":"Human ISCA1 was shown to localize to mitochondria and functionally replace yeast Isa1, establishing evolutionary conservation and validating the yeast model for human studies.","evidence":"Yeast complementation and subcellular localization of human ISCA1","pmids":["15262227"],"confidence":"Medium","gaps":["Human-specific interaction partners not identified","No human loss-of-function data"]},{"year":2007,"claim":"Genetic epistasis revealed that Isa1/Isa2 are specifically required for functional maturation of mitochondrial [4Fe-4S] proteins but not for de novo Fe/S cluster synthesis on the scaffold protein Isu1/Isu2, placing the ISA complex downstream in a dedicated [4Fe-4S] branch.","evidence":"Genetic epistasis with Isu1/Isu2 and Isa1/Isa2 depletions, biotin synthase activity as [4Fe-4S] reporter in yeast","pmids":["17259550"],"confidence":"High","gaps":["Biochemical mechanism of [4Fe-4S] cluster formation not reconstituted","Upstream [2Fe-2S] donor to Isa not identified"]},{"year":2010,"claim":"The upstream [2Fe-2S] donor GLRX5 was linked to ISCA1/ISCA2, and ISCA1 was shown to bind mononuclear iron with very high affinity and donate it for Fe/S assembly, raising the question of whether its primary role is as an iron chaperone or cluster relay.","evidence":"BiFC interaction assay and genetic suppression in fission yeast (GLRX5); iron-binding and iron-transfer assays with purified human ISCA1 in vitro","pmids":["20085751","20302570"],"confidence":"High","gaps":["Whether iron or [2Fe-2S] cluster donation is the physiological function debated","Full reconstitution of [4Fe-4S] formation from physiological components not achieved"]},{"year":2011,"claim":"Comprehensive in vivo analysis confirmed that the Isa1–Isa2 complex is selectively required for mitochondrial [4Fe-4S] but not [2Fe-2S] proteins, and that the partner IBA57 is needed for the same step, defining the three-component late-acting [4Fe-4S] assembly system.","evidence":"In vivo reporters for [2Fe-2S] and [4Fe-4S] protein function, iron-binding assays, and IBA57 depletion in yeast","pmids":["21987576"],"confidence":"High","gaps":["Mechanism of IBA57 contribution unknown","How [2Fe-2S] clusters convert to [4Fe-4S] not resolved"]},{"year":2012,"claim":"Studies in human cells confirmed that ISCA1, ISCA2, and IBA57 are specifically required for mitochondrial [4Fe-4S] proteins including respiratory complex I and lipoic acid synthase, refuting an earlier claim of cytosolic function and showing that their loss causes severe mitochondrial structural damage.","evidence":"RNAi knockdown in HeLa cells with enzyme assays, electron microscopy, and subcellular fractionation","pmids":["22323289"],"confidence":"High","gaps":["Direct protein–protein interactions among the three factors not mapped","Downstream transfer route to target apoproteins unknown"]},{"year":2018,"claim":"A pathogenic ISCA1 missense mutation (p.V10G) was shown to impair mitochondrial import and protein stability, directly linking ISCA1 loss of function to human disease (MMDS5) with defective [4Fe-4S] protein maturation.","evidence":"Patient fibroblast biochemistry, mitochondrial import assay, RNAi complementation with wild-type vs. mutant ISCA1","pmids":["29767723"],"confidence":"High","gaps":["Structural basis for import defect not determined","Genotype-phenotype correlations across ISCA1 variants not established"]},{"year":2019,"claim":"Complete and neuron-specific knockout of ISCA1 in rats established that the gene is essential for embryonic viability and neuronal mitochondrial integrity, with neuron-specific loss recapitulating MMDS5 features including epilepsy and respiratory chain failure.","evidence":"CRISPR-Cas9 whole-body and conditional (NeuN-Cre) knockout rats with phenotypic, ultrastructural, and biochemical analysis","pmids":["31016283","37140997"],"confidence":"Medium","gaps":["Cell-type-specific vulnerability beyond neurons not tested","Whether compensatory pathways exist in other tissues unknown"]},{"year":2020,"claim":"Full in vitro reconstitution resolved the complete [4Fe-4S] assembly mechanism: GLRX5 donates [2Fe-2S] clusters to ISCA1–ISCA2, FDX2 (not FDX1) provides the electrons for reductive [2Fe-2S]-to-[4Fe-4S] fusion, and ISCA1 then transfers the [4Fe-4S] cluster to NFU1 via a direct interaction mapped to NFU1's C-terminal hydrophobic patch.","evidence":"Reconstitution of [4Fe-4S] aconitase maturation with purified human proteins, spectroscopy, Co-IP, and NFU1 mutagenesis","pmids":["32817474","32776106"],"confidence":"High","gaps":["Structural model of the ISCA1–ISCA2 heterodimer with bound [4Fe-4S] cluster lacking","How IBA57 participates mechanistically in the fusion reaction unresolved"]},{"year":2021,"claim":"NMR-based interaction mapping established ISCA1 as the central organizer: it bridges ISCA2 and NFU1 (which do not interact directly) in a transient ternary complex, driving directional [4Fe-4S] cluster transfer from the ISCA1–ISCA2 assembly site to an ISCA1–NFU1 delivery site.","evidence":"NMR spectroscopy, protein interaction mapping, ternary complex characterization, cluster transfer monitoring with purified human proteins","pmids":["33711344"],"confidence":"High","gaps":["Atomic-resolution structure of the ternary complex not available","Kinetics and regulation of cluster transfer in vivo unknown","How target apoprotein specificity is achieved downstream of NFU1 not determined"]},{"year":2023,"claim":"ISCA1 was found to bind copper with high affinity, and copper excess inhibits Fe/S cluster assembly by competing at ISCA1/ISCA2/ISCU, providing a molecular basis for copper cytotoxicity in Wilson's disease.","evidence":"Copper-binding assays with purified proteins, Fe/S enzyme activity in ATP7B-knockdown cells and ATP7A-deficient lymphocytes, Wilson's disease mouse model","pmids":["37225108"],"confidence":"Medium","gaps":["Structural basis of copper vs. iron competition not determined","In vivo contribution of ISCA1 copper binding relative to other targets unclear"]},{"year":null,"claim":"Key unresolved questions include the atomic structure of the ISCA1–ISCA2 heterodimer with bound [4Fe-4S] cluster, the precise catalytic role of IBA57 in the reductive fusion reaction, how the assembled [4Fe-4S] clusters are directed from NFU1 to specific target apoproteins, and the regulatory mechanisms controlling ISCA1 activity in different tissues.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of the ISCA1–ISCA2–[4Fe-4S] complex","IBA57 mechanism of action unresolved","Target apoprotein selectivity mechanism unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[5,10,11,13]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[10,11,13]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[0,2,8,9]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,7,8,10,11]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[9,16]}],"complexes":["ISCA1-ISCA2 heterodimer","ISCA1-ISCA2-NFU1 ternary complex","ISCA1-ISCA2-IBA57 complex"],"partners":["ISCA2","NFU1","IBA57","GLRX5","FDX2","ISCU2"],"other_free_text":[]},"mechanistic_narrative":"ISCA1 is a mitochondrial iron-sulfur cluster assembly protein that functions as the central organizer of the late-acting [4Fe-4S] biogenesis machinery. It receives [2Fe-2S] clusters from GLRX5 and, together with ISCA2 and IBA57, catalyzes their FDX2-dependent reductive fusion into [4Fe-4S] clusters on the ISCA1–ISCA2 heterodimer; ISCA1 then bridges ISCA2 and NFU1 in a transient ternary complex to drive [4Fe-4S] cluster transfer to NFU1 and ultimately to target apoproteins including aconitase, respiratory complex I, and lipoic acid synthase [PMID:32817474, PMID:33711344, PMID:22323289]. Three conserved cysteine residues coordinate its [2Fe-2S] cluster, whose integrity is essential for function, and pathogenic variants that destabilize this cluster or impair mitochondrial import cause multiple mitochondrial dysfunctions syndrome type 5 (MMDS5) [PMID:10805735, PMID:32092383, PMID:29767723]. Complete loss of ISCA1 in rats is embryonically lethal, and neuron-specific deletion recapitulates MMDS5 with epilepsy, mitochondrial fragmentation, and respiratory chain failure [PMID:31016283, PMID:37140997]."},"prefetch_data":{"uniprot":{"accession":"Q9BUE6","full_name":"Iron-sulfur cluster assembly 1 homolog, mitochondrial","aliases":["HESB-like domain-containing protein 2","Iron-sulfur assembly protein IscA","hIscA"],"length_aa":129,"mass_kda":14.2,"function":"Involved in the maturation of mitochondrial 4Fe-4S proteins functioning late in the iron-sulfur cluster assembly pathway. Probably involved in the binding of an intermediate of Fe/S cluster assembly","subcellular_location":"Mitochondrion","url":"https://www.uniprot.org/uniprotkb/Q9BUE6/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ISCA1","classification":"Not Classified","n_dependent_lines":306,"n_total_lines":1208,"dependency_fraction":0.2533112582781457},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ISCA1","total_profiled":1310},"omim":[{"mim_id":"617613","title":"MULTIPLE MITOCHONDRIAL DYSFUNCTIONS SYNDROME 5; MMDS5","url":"https://www.omim.org/entry/617613"},{"mim_id":"616370","title":"MULTIPLE MITOCHONDRIAL DYSFUNCTIONS SYNDROME 4; MMDS4","url":"https://www.omim.org/entry/616370"},{"mim_id":"615317","title":"IRON-SULFUR CLUSTER ASSEMBLY 2; ISCA2","url":"https://www.omim.org/entry/615317"},{"mim_id":"615316","title":"IRON-SULFUR CLUSTER ASSEMBLY FACTOR IBA57; IBA57","url":"https://www.omim.org/entry/615316"},{"mim_id":"611118","title":"NUCLEAR PRELAMIN A RECOGNITION FACTOR-LIKE; NARFL","url":"https://www.omim.org/entry/611118"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/ISCA1"},"hgnc":{"alias_symbol":["MGC4276","ISA1","hIscA","hIscA1"],"prev_symbol":["HBLD2"]},"alphafold":{"accession":"Q9BUE6","domains":[{"cath_id":"2.60.300.12","chopping":"24-127","consensus_level":"high","plddt":93.6289,"start":24,"end":127}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BUE6","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BUE6-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BUE6-F1-predicted_aligned_error_v6.png","plddt_mean":89.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ISCA1","jax_strain_url":"https://www.jax.org/strain/search?query=ISCA1"},"sequence":{"accession":"Q9BUE6","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BUE6.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BUE6/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BUE6"}},"corpus_meta":[{"pmid":"22323289","id":"PMC_22323289","title":"The human mitochondrial ISCA1, ISCA2, and IBA57 proteins are required for [4Fe-4S] protein maturation.","date":"2012","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/22323289","citation_count":171,"is_preprint":false},{"pmid":"10805735","id":"PMC_10805735","title":"Role of Saccharomyces cerevisiae ISA1 and ISA2 in iron homeostasis.","date":"2000","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/10805735","citation_count":148,"is_preprint":false},{"pmid":"21987576","id":"PMC_21987576","title":"Specialized function of yeast Isa1 and Isa2 proteins in the maturation of mitochondrial [4Fe-4S] proteins.","date":"2011","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21987576","citation_count":135,"is_preprint":false},{"pmid":"21436381","id":"PMC_21436381","title":"Functional diversity of isoamylase oligomers: the ISA1 homo-oligomer is essential for amylopectin biosynthesis in rice endosperm.","date":"2011","source":"Plant physiology","url":"https://pubmed.ncbi.nlm.nih.gov/21436381","citation_count":73,"is_preprint":false},{"pmid":"32817474","id":"PMC_32817474","title":"Mitochondrial [4Fe-4S] protein assembly involves reductive [2Fe-2S] cluster fusion on ISCA1-ISCA2 by electron flow from ferredoxin FDX2.","date":"2020","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/32817474","citation_count":69,"is_preprint":false},{"pmid":"11941510","id":"PMC_11941510","title":"Iron-sulfur cluster biosynthesis: characterization of Schizosaccharomyces pombe Isa1.","date":"2002","source":"Journal of biological inorganic chemistry : JBIC : a publication of the Society of Biological Inorganic Chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11941510","citation_count":68,"is_preprint":false},{"pmid":"29775500","id":"PMC_29775500","title":"Endosperm sugar accumulation caused by mutation of PHS8/ISA1 leads to pre-harvest sprouting in rice.","date":"2018","source":"The Plant journal : for cell and molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/29775500","citation_count":62,"is_preprint":false},{"pmid":"20085751","id":"PMC_20085751","title":"Monothiol glutaredoxin Grx5 interacts with Fe-S scaffold proteins Isa1 and Isa2 and supports Fe-S assembly and DNA integrity in mitochondria of fission yeast.","date":"2010","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/20085751","citation_count":53,"is_preprint":false},{"pmid":"28356563","id":"PMC_28356563","title":"Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome.","date":"2017","source":"Journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/28356563","citation_count":52,"is_preprint":false},{"pmid":"17259550","id":"PMC_17259550","title":"The ISC [corrected] proteins Isa1 and Isa2 are required for the function but not for the de novo synthesis of the Fe/S clusters of biotin synthase in Saccharomyces cerevisiae.","date":"2007","source":"Eukaryotic cell","url":"https://pubmed.ncbi.nlm.nih.gov/17259550","citation_count":47,"is_preprint":false},{"pmid":"24993830","id":"PMC_24993830","title":"Crystal structure of the Chlamydomonas starch debranching enzyme isoamylase ISA1 reveals insights into the mechanism of branch trimming and complex assembly.","date":"2014","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/24993830","citation_count":46,"is_preprint":false},{"pmid":"19864422","id":"PMC_19864422","title":"Human ISCA1 interacts with IOP1/NARFL and functions in both cytosolic and mitochondrial iron-sulfur protein biogenesis.","date":"2009","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/19864422","citation_count":41,"is_preprint":false},{"pmid":"21790804","id":"PMC_21790804","title":"Stage-specific requirement for Isa1 and Isa2 proteins in the mitochondrion of Trypanosoma brucei and heterologous rescue by human and Blastocystis orthologues.","date":"2011","source":"Molecular microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/21790804","citation_count":33,"is_preprint":false},{"pmid":"20302570","id":"PMC_20302570","title":"Iron-binding activity of human iron-sulfur cluster assembly protein hIscA1.","date":"2010","source":"The Biochemical journal","url":"https://pubmed.ncbi.nlm.nih.gov/20302570","citation_count":32,"is_preprint":false},{"pmid":"37225108","id":"PMC_37225108","title":"Copper exerts cytotoxicity through inhibition of iron-sulfur cluster biogenesis on ISCA1/ISCA2/ISCU assembly proteins.","date":"2023","source":"Free radical biology & medicine","url":"https://pubmed.ncbi.nlm.nih.gov/37225108","citation_count":31,"is_preprint":false},{"pmid":"15262227","id":"PMC_15262227","title":"hIscA: a protein implicated in the biogenesis of iron-sulfur clusters.","date":"2004","source":"Biochimica et 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phytologist","url":"https://pubmed.ncbi.nlm.nih.gov/23952574","citation_count":20,"is_preprint":false},{"pmid":"32776106","id":"PMC_32776106","title":"Assembly of the [4Fe-4S] cluster of NFU1 requires the coordinated donation of two [2Fe-2S] clusters from the scaffold proteins, ISCU2 and ISCA1.","date":"2020","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/32776106","citation_count":20,"is_preprint":false},{"pmid":"28708852","id":"PMC_28708852","title":"Simultaneous silencing of isoamylases ISA1, ISA2 and ISA3 by multi-target RNAi in potato tubers leads to decreased starch content and an early sprouting phenotype.","date":"2017","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/28708852","citation_count":19,"is_preprint":false},{"pmid":"31016283","id":"PMC_31016283","title":"Knockout of ISCA1 causes early embryonic death in rats.","date":"2019","source":"Animal models and experimental 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biology","url":"https://pubmed.ncbi.nlm.nih.gov/35083581","citation_count":10,"is_preprint":false},{"pmid":"29063672","id":"PMC_29063672","title":"Molecular characterization, spatial-temporal expression and magnetic response patterns of iron-sulfur cluster assembly1 (IscA1) in the rice planthopper, Nilaparvata lugens.","date":"2017","source":"Insect science","url":"https://pubmed.ncbi.nlm.nih.gov/29063672","citation_count":8,"is_preprint":false},{"pmid":"30105122","id":"PMC_30105122","title":"Report of the Third Family with Multiple Mitochondrial Dysfunctions Syndrome 5 Caused by the Founder Variant p.(Glu87Lys) in ISCA1.","date":"2018","source":"Journal of pediatric genetics","url":"https://pubmed.ncbi.nlm.nih.gov/30105122","citation_count":8,"is_preprint":false},{"pmid":"37140997","id":"PMC_37140997","title":"A neuron-specific Isca1 knockout rat developments multiple mitochondrial dysfunction syndromes.","date":"2023","source":"Animal models and experimental 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Carcinoma.","date":"2024","source":"Genes","url":"https://pubmed.ncbi.nlm.nih.gov/39766805","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.11.17.688664","title":"A scalable screening platform for discovering predominant miRNA target genes and druggable siRNA sequences","date":"2025-11-17","source":"bioRxiv","url":"https://doi.org/10.1101/2025.11.17.688664","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2025.06.23.661224","title":"LIKE EARLY STARVATION is involved in the regulation of starch initiation in potato ( <i>Solanum tuberosum</i> cv. Désirée) tubers","date":"2025-06-24","source":"bioRxiv","url":"https://doi.org/10.1101/2025.06.23.661224","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":18530,"output_tokens":4064,"usd":0.058275},"stage2":{"model":"claude-opus-4-6","input_tokens":7514,"output_tokens":3248,"usd":0.178155},"total_usd":0.23643,"stage1_batch_id":"msgbatch_01DzrsgVgYngY1hvM3DU1yF2","stage2_batch_id":"msgbatch_01KA9b7iRzKFdhoBUT7vCvSe","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2000,\n      \"finding\": \"Yeast Isa1p (ISCA1 ortholog) is targeted to the mitochondrial matrix, and its three invariant cysteine residues are essential for function, likely involved in iron binding; loss of Isa1p causes reduced aconitase and succinate dehydrogenase activity and elevated mitochondrial iron accumulation.\",\n      \"method\": \"Genetic deletion, subcellular fractionation, site-directed mutagenesis of cysteine residues, enzyme activity assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods in foundational study, replicated by subsequent work\",\n      \"pmids\": [\"10805735\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Fission yeast Isa1 (ISCA1 ortholog) is a multimeric protein carrying [2Fe-2S]2+ clusters; it forms a complex with a redox-active ferredoxin (identified by crosslinking), and cysteine ligands are required for cluster coordination as shown by site-directed mutagenesis.\",\n      \"method\": \"Mössbauer spectroscopy, UV-visible and EPR spectroscopy, chemical crosslinking, site-directed mutagenesis\",\n      \"journal\": \"Journal of biological inorganic chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro biochemical reconstitution with mutagenesis and multiple spectroscopic methods\",\n      \"pmids\": [\"11941510\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Human ISCA1 (hIscA) is localized to mitochondria and functionally complements yeast isa1Δ mutants, establishing functional conservation; the protein contains the HESB structural domain.\",\n      \"method\": \"Yeast complementation assay, subcellular localization (mitochondrial targeting), immunological detection\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — complementation in yeast with localization, single lab\",\n      \"pmids\": [\"15262227\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Yeast Isa1 and Isa2 are required for the in vivo catalytic function of biotin synthase (Bio2), a [4Fe-4S] enzyme, but not for the de novo synthesis of its Fe/S clusters (which depends on Isu1/Isu2); Isa proteins are specifically needed for functional activity of [4Fe-4S] apoproteins.\",\n      \"method\": \"Genetic epistasis, enzyme activity assays, protein depletion, overexpression rescue\",\n      \"journal\": \"Eukaryotic cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with multiple mutant combinations and biochemical assays, clear mechanistic distinction\",\n      \"pmids\": [\"17259550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Human ISCA1 is present in both cytosolic and mitochondrial fractions, and interacts with IOP1/NARFL (a cytosolic CIA pathway factor); siRNA knockdown of ISCA1 in HeLa cells reduces activity of both mitochondrial (aconitase, succinate dehydrogenase) and cytosolic (cytosolic aconitase) Fe/S enzymes.\",\n      \"method\": \"Subcellular fractionation, Co-immunoprecipitation, siRNA knockdown, enzyme activity assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP with functional knockdown data, single lab; cytosolic localization later disputed\",\n      \"pmids\": [\"19864422\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Human ISCA1 (hIscA1) binds mononuclear iron with an association constant of ~2×10^19 M^-1, and iron-bound hIscA1 can donate iron for Fe/S cluster assembly on the E. coli scaffold protein IscU in vitro, supporting an iron chaperone role.\",\n      \"method\": \"UV-visible absorption, EPR spectroscopy, in vitro iron transfer assay to IscU, iron content modulation by growth medium\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with spectroscopic characterization and functional iron transfer assay\",\n      \"pmids\": [\"20302570\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"In fission yeast, Grx5 physically interacts in vivo with Isa1 and Isa2 in mitochondria (bimolecular fluorescence complementation); overexpression of isa1+ or isa2+ suppresses growth defects of Δgrx5 mutant, placing Grx5 upstream or in the same pathway as Isa1/Isa2 in Fe/S assembly.\",\n      \"method\": \"Bimolecular fluorescence complementation (BiFC), genetic suppressor screen, multi-copy suppression, Fe/S enzyme activity assays\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — direct protein interaction shown by BiFC with functional epistasis, single lab\",\n      \"pmids\": [\"20085751\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Yeast Isa1 and Isa2 form a complex specifically required for maturation of mitochondrial [4Fe-4S] proteins (aconitase, homoaconitase) but dispensable for [2Fe-2S] proteins and cytosolic [4Fe-4S] proteins; Isa1/Isa2 bind iron in vivo, and this iron is proposed as the direct donor for de novo [4Fe-4S] cluster synthesis in a reaction assisted by Iba57.\",\n      \"method\": \"In vivo genetic analysis, iron-binding assay, targeted mitochondrial [2Fe-2S]/[4Fe-4S] ferredoxin reporters, Iba57 depletion, enzyme activity assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — comprehensive in vivo analysis with multiple orthogonal approaches, replicated across labs\",\n      \"pmids\": [\"21987576\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Human ISCA1, ISCA2, and IBA57 are specifically required for maturation of mitochondrial [4Fe-4S] proteins (aconitase, respiratory complex I, lipoic acid synthase) but not [2Fe-2S] proteins (ferrochelatase); ISCA1/ISCA2 do not function in the cytosol for Fe/S protein maturation; their depletion causes massive mitochondrial swelling and loss of cristae.\",\n      \"method\": \"RNAi knockdown in HeLa cells, enzyme activity assays, heme quantification, mitochondrial morphology by electron microscopy, subcellular fractionation\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods, highly cited, directly tests cytosolic localization claim\",\n      \"pmids\": [\"22323289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"A homozygous missense mutation p.(V10G) in the ISCA1 presequence severely impairs mitochondrial import and stability of ISCA1 protein, leading to defective [4Fe-4S] protein maturation (respiratory complexes, lipoic acid synthesis); RNAi knockdown rescued by wild-type but not mutant ISCA1.\",\n      \"method\": \"Patient fibroblast biochemical analysis, mitochondrial import assay, RNAi complementation, targeted MitoExome sequencing\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — complementation rescue experiment with mutant vs. wild-type, multiple biochemical readouts\",\n      \"pmids\": [\"29767723\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Mitochondrial [4Fe-4S] cluster assembly on ISCA1-ISCA2 proceeds by reductive fusion of two [2Fe-2S] clusters donated from GLRX5; this reaction requires FDX2 (ferredoxin 2) as the specific electron donor and its reductase FDXR, as well as IBA57, reconstituted in vitro without artificial reductants. FDX1 and other reducing systems cannot substitute for FDX2.\",\n      \"method\": \"In vitro reconstitution of [4Fe-4S] aconitase maturation with purified components, spectroscopy, functional specificity tests with FDX1 vs FDX2\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — fully reconstituted in vitro with purified physiological components and rigorous specificity controls\",\n      \"pmids\": [\"32817474\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ISCA1 directly interacts with both ISCU2 and NFU1, donating [2Fe-2S] clusters to NFU1 (along with ISCU2) for reductive formation of a bridging [4Fe-4S] cluster; the interaction site maps to a conserved hydrophobic patch on the C-terminal alpha-helix of NFU1, and mutagenesis of this site abolishes cluster acquisition.\",\n      \"method\": \"Co-immunoprecipitation, biochemical reconstitution, site-directed mutagenesis of NFU1 interaction surface, functional assays of downstream targets (lipoic acid synthase, pyruvate dehydrogenase lipoylation)\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstitution plus mutagenesis defining interaction site, with functional downstream readouts\",\n      \"pmids\": [\"32776106\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"A novel pathogenic ISCA1 variant p.(Tyr101Cys) decreases stability of the [2Fe-2S] cluster bound to the protein, establishing that ISCA1 itself carries a [2Fe-2S] cluster whose integrity is essential for its function in [4Fe-4S] protein maturation.\",\n      \"method\": \"Expression and purification of recombinant mutant ISCA1, spectroscopic characterization of [2Fe-2S] cluster stability, patient fibroblast biochemistry\",\n      \"journal\": \"Mitochondrion\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — direct biochemical characterization of purified protein, single lab and case\",\n      \"pmids\": [\"32092383\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ISCA1 acts as the central organizer of the late-acting [4Fe-4S] assembly complex: it interacts with both ISCA2 and NFU1 individually but ISCA2 and NFU1 do not interact with each other; ISCA1 promotes formation of a transient ISCA1-ISCA2-NFU1 ternary complex and drives [4Fe-4S] cluster transfer from the ISCA1-ISCA2 assembly site to a cluster-binding site formed by ISCA1 and NFU1.\",\n      \"method\": \"NMR spectroscopy, protein interaction mapping, ternary complex characterization, cluster transfer monitoring\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — NMR-based structural and interaction study with detailed molecular model of cluster transfer mechanism\",\n      \"pmids\": [\"33711344\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Knockout of ISCA1 in rats causes early embryonic lethality (death by embryonic day 8.5), with decreased NDUFA9 (complex I subunit) protein levels and increased aconitase 2 protein in embryos, demonstrating an essential in vivo role in mitochondrial respiratory chain assembly.\",\n      \"method\": \"CRISPR-Cas9 knockout rats, Western blot of mitochondrial proteins, embryonic phenotype analysis\",\n      \"journal\": \"Animal models and experimental medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with specific molecular phenotype readouts, single study\",\n      \"pmids\": [\"31016283\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Human ISCA1, ISCA2, and ISCU proteins have strong copper-binding activity, and excess copper inhibits Fe/S cluster assembly by competing at these proteins, providing a mechanism for copper cytotoxicity in Wilson's disease.\",\n      \"method\": \"Copper-binding assay with purified proteins, Fe/S enzyme activity assays in cells and mouse model of Wilson's disease (ATP7B knockdown, ATP7A-/- lymphocytes), in vivo and in vitro pharmacological analysis\",\n      \"journal\": \"Free radical biology & medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding assay with functional cellular and in vivo readouts, single lab\",\n      \"pmids\": [\"37225108\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Neuron-specific Isca1 knockout rats develop MMDS5-like disease with epilepsy, memory impairment, neuronal death, mitochondrial fragmentation and cristae rupture, reduced respiratory chain complex proteins, and decreased ATP production, demonstrating that ISCA1 is required for neuronal mitochondrial integrity.\",\n      \"method\": \"Conditional CRISPR-Cas9 knockout (Isca1flox/flox-NeuN-Cre), MRI, behavioral testing, TEM, Western blot, ATP assay, immunofluorescence\",\n      \"journal\": \"Animal models and experimental medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with multiple mechanistic readouts, single lab\",\n      \"pmids\": [\"37140997\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ISCA1 is a mitochondrial iron-sulfur cluster assembly protein that, together with ISCA2 and IBA57, functions as a late-acting specialized subsystem for [4Fe-4S] cluster biogenesis: it receives [2Fe-2S] clusters from GLRX5, uses electrons from ferredoxin FDX2 to catalyze reductive fusion into [4Fe-4S] clusters on the ISCA1-ISCA2 heterodimer, and then—acting as the central organizer—drives transfer of the assembled [4Fe-4S] cluster through a transient ISCA1-ISCA2-NFU1 ternary complex to NFU1 and ultimately into target mitochondrial [4Fe-4S] apoproteins such as aconitase, respiratory complex I, and lipoic acid synthase.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"ISCA1 is a mitochondrial iron-sulfur cluster assembly protein that functions as the central organizer of the late-acting [4Fe-4S] biogenesis machinery. It receives [2Fe-2S] clusters from GLRX5 and, together with ISCA2 and IBA57, catalyzes their FDX2-dependent reductive fusion into [4Fe-4S] clusters on the ISCA1–ISCA2 heterodimer; ISCA1 then bridges ISCA2 and NFU1 in a transient ternary complex to drive [4Fe-4S] cluster transfer to NFU1 and ultimately to target apoproteins including aconitase, respiratory complex I, and lipoic acid synthase [PMID:32817474, PMID:33711344, PMID:22323289]. Three conserved cysteine residues coordinate its [2Fe-2S] cluster, whose integrity is essential for function, and pathogenic variants that destabilize this cluster or impair mitochondrial import cause multiple mitochondrial dysfunctions syndrome type 5 (MMDS5) [PMID:10805735, PMID:32092383, PMID:29767723]. Complete loss of ISCA1 in rats is embryonically lethal, and neuron-specific deletion recapitulates MMDS5 with epilepsy, mitochondrial fragmentation, and respiratory chain failure [PMID:31016283, PMID:37140997].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Initial characterization established that the ISCA1 ortholog (yeast Isa1) localizes to the mitochondrial matrix, requires three conserved cysteines for function, and is needed for activity of [4Fe-4S]-containing enzymes aconitase and succinate dehydrogenase — linking the protein to mitochondrial iron-sulfur metabolism.\",\n      \"evidence\": \"Genetic deletion, site-directed mutagenesis, subcellular fractionation, and enzyme activity assays in S. cerevisiae\",\n      \"pmids\": [\"10805735\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Iron-sulfur cluster type bound by Isa1 not yet determined\", \"Relationship to other known Fe/S assembly factors unclear\", \"Mammalian function not tested\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Spectroscopic reconstitution showed that Isa1 itself carries a [2Fe-2S] cluster and physically associates with ferredoxin, providing the first direct evidence that the protein is an Fe/S-binding factor rather than simply an iron donor.\",\n      \"evidence\": \"Mössbauer, UV-visible, and EPR spectroscopy with chemical crosslinking and mutagenesis on fission yeast Isa1\",\n      \"pmids\": [\"11941510\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Role of the ferredoxin interaction in cluster chemistry unknown\", \"Whether the protein can also bind mononuclear iron unclear\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Human ISCA1 was shown to localize to mitochondria and functionally replace yeast Isa1, establishing evolutionary conservation and validating the yeast model for human studies.\",\n      \"evidence\": \"Yeast complementation and subcellular localization of human ISCA1\",\n      \"pmids\": [\"15262227\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Human-specific interaction partners not identified\", \"No human loss-of-function data\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Genetic epistasis revealed that Isa1/Isa2 are specifically required for functional maturation of mitochondrial [4Fe-4S] proteins but not for de novo Fe/S cluster synthesis on the scaffold protein Isu1/Isu2, placing the ISA complex downstream in a dedicated [4Fe-4S] branch.\",\n      \"evidence\": \"Genetic epistasis with Isu1/Isu2 and Isa1/Isa2 depletions, biotin synthase activity as [4Fe-4S] reporter in yeast\",\n      \"pmids\": [\"17259550\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Biochemical mechanism of [4Fe-4S] cluster formation not reconstituted\", \"Upstream [2Fe-2S] donor to Isa not identified\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The upstream [2Fe-2S] donor GLRX5 was linked to ISCA1/ISCA2, and ISCA1 was shown to bind mononuclear iron with very high affinity and donate it for Fe/S assembly, raising the question of whether its primary role is as an iron chaperone or cluster relay.\",\n      \"evidence\": \"BiFC interaction assay and genetic suppression in fission yeast (GLRX5); iron-binding and iron-transfer assays with purified human ISCA1 in vitro\",\n      \"pmids\": [\"20085751\", \"20302570\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether iron or [2Fe-2S] cluster donation is the physiological function debated\", \"Full reconstitution of [4Fe-4S] formation from physiological components not achieved\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Comprehensive in vivo analysis confirmed that the Isa1–Isa2 complex is selectively required for mitochondrial [4Fe-4S] but not [2Fe-2S] proteins, and that the partner IBA57 is needed for the same step, defining the three-component late-acting [4Fe-4S] assembly system.\",\n      \"evidence\": \"In vivo reporters for [2Fe-2S] and [4Fe-4S] protein function, iron-binding assays, and IBA57 depletion in yeast\",\n      \"pmids\": [\"21987576\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of IBA57 contribution unknown\", \"How [2Fe-2S] clusters convert to [4Fe-4S] not resolved\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Studies in human cells confirmed that ISCA1, ISCA2, and IBA57 are specifically required for mitochondrial [4Fe-4S] proteins including respiratory complex I and lipoic acid synthase, refuting an earlier claim of cytosolic function and showing that their loss causes severe mitochondrial structural damage.\",\n      \"evidence\": \"RNAi knockdown in HeLa cells with enzyme assays, electron microscopy, and subcellular fractionation\",\n      \"pmids\": [\"22323289\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct protein–protein interactions among the three factors not mapped\", \"Downstream transfer route to target apoproteins unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"A pathogenic ISCA1 missense mutation (p.V10G) was shown to impair mitochondrial import and protein stability, directly linking ISCA1 loss of function to human disease (MMDS5) with defective [4Fe-4S] protein maturation.\",\n      \"evidence\": \"Patient fibroblast biochemistry, mitochondrial import assay, RNAi complementation with wild-type vs. mutant ISCA1\",\n      \"pmids\": [\"29767723\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for import defect not determined\", \"Genotype-phenotype correlations across ISCA1 variants not established\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Complete and neuron-specific knockout of ISCA1 in rats established that the gene is essential for embryonic viability and neuronal mitochondrial integrity, with neuron-specific loss recapitulating MMDS5 features including epilepsy and respiratory chain failure.\",\n      \"evidence\": \"CRISPR-Cas9 whole-body and conditional (NeuN-Cre) knockout rats with phenotypic, ultrastructural, and biochemical analysis\",\n      \"pmids\": [\"31016283\", \"37140997\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cell-type-specific vulnerability beyond neurons not tested\", \"Whether compensatory pathways exist in other tissues unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Full in vitro reconstitution resolved the complete [4Fe-4S] assembly mechanism: GLRX5 donates [2Fe-2S] clusters to ISCA1–ISCA2, FDX2 (not FDX1) provides the electrons for reductive [2Fe-2S]-to-[4Fe-4S] fusion, and ISCA1 then transfers the [4Fe-4S] cluster to NFU1 via a direct interaction mapped to NFU1's C-terminal hydrophobic patch.\",\n      \"evidence\": \"Reconstitution of [4Fe-4S] aconitase maturation with purified human proteins, spectroscopy, Co-IP, and NFU1 mutagenesis\",\n      \"pmids\": [\"32817474\", \"32776106\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural model of the ISCA1–ISCA2 heterodimer with bound [4Fe-4S] cluster lacking\", \"How IBA57 participates mechanistically in the fusion reaction unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"NMR-based interaction mapping established ISCA1 as the central organizer: it bridges ISCA2 and NFU1 (which do not interact directly) in a transient ternary complex, driving directional [4Fe-4S] cluster transfer from the ISCA1–ISCA2 assembly site to an ISCA1–NFU1 delivery site.\",\n      \"evidence\": \"NMR spectroscopy, protein interaction mapping, ternary complex characterization, cluster transfer monitoring with purified human proteins\",\n      \"pmids\": [\"33711344\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of the ternary complex not available\", \"Kinetics and regulation of cluster transfer in vivo unknown\", \"How target apoprotein specificity is achieved downstream of NFU1 not determined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"ISCA1 was found to bind copper with high affinity, and copper excess inhibits Fe/S cluster assembly by competing at ISCA1/ISCA2/ISCU, providing a molecular basis for copper cytotoxicity in Wilson's disease.\",\n      \"evidence\": \"Copper-binding assays with purified proteins, Fe/S enzyme activity in ATP7B-knockdown cells and ATP7A-deficient lymphocytes, Wilson's disease mouse model\",\n      \"pmids\": [\"37225108\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis of copper vs. iron competition not determined\", \"In vivo contribution of ISCA1 copper binding relative to other targets unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the atomic structure of the ISCA1–ISCA2 heterodimer with bound [4Fe-4S] cluster, the precise catalytic role of IBA57 in the reductive fusion reaction, how the assembled [4Fe-4S] clusters are directed from NFU1 to specific target apoproteins, and the regulatory mechanisms controlling ISCA1 activity in different tissues.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No high-resolution structure of the ISCA1–ISCA2–[4Fe-4S] complex\", \"IBA57 mechanism of action unresolved\", \"Target apoprotein selectivity mechanism unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [5, 10, 11, 13]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [10, 11, 13]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [0, 2, 8, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 7, 8, 10, 11]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [9, 16]}\n    ],\n    \"complexes\": [\n      \"ISCA1-ISCA2 heterodimer\",\n      \"ISCA1-ISCA2-NFU1 ternary complex\",\n      \"ISCA1-ISCA2-IBA57 complex\"\n    ],\n    \"partners\": [\n      \"ISCA2\",\n      \"NFU1\",\n      \"IBA57\",\n      \"GLRX5\",\n      \"FDX2\",\n      \"ISCU2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}