{"gene":"ISCU","run_date":"2026-06-10T01:55:23","timeline":{"discoveries":[{"year":2000,"finding":"IscU serves as a scaffold for sequential assembly of [2Fe-2S] and [4Fe-4S] clusters: IscS-mediated assembly on IscU proceeds first to one [2Fe-2S]2+ cluster per dimer, then two [2Fe-2S]2+ clusters per dimer, and finally one [4Fe-4S]2+ cluster per dimer. Both cluster types are reductively labile. [2Fe-2S] clusters show incomplete cysteinyl ligation and [4Fe-4S] cluster has non-cysteinyl ligation at a unique Fe site.","method":"Anaerobic anion exchange chromatography, UV-visible absorption, resonance Raman, and Mössbauer spectroscopies","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal spectroscopic methods (Mössbauer, resonance Raman, UV-vis) with rigorous in vitro reconstitution, widely replicated","pmids":["10891064"],"is_preprint":false},{"year":2001,"finding":"IscS transfers sulfur directly to IscU via persulfide intermediates on IscU cysteine residues. IscU binds IscS with high affinity (Kd ~2 µM), sulfur transfer is inhibited by iodoacetamide treatment of IscU, and 35S is released by reducing agents. The C-terminal region of IscS (residues 376–413) is important for IscU binding and sulfur transfer.","method":"[35S]-cysteine radiotracer studies, surface plasmon resonance, isothermal titration calorimetry, deletion mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct biochemical reconstitution with radiolabeling, SPR, ITC, and mutagenesis; multiple orthogonal methods","pmids":["11577100"],"is_preprint":false},{"year":2000,"finding":"IscU and its iron-sulfur complex (IscU-Fe/S) stimulate the ATPase activity of the Hsc66 chaperone up to 480-fold in the presence of the co-chaperone Hsc20. Hsc20 enhances binding of IscU to Hsc66, and IscU and Hsc20 form a direct complex, suggesting Hsc20 aids targeting of IscU to Hsc66.","method":"ATPase activity assays, surface plasmon resonance, isothermal titration calorimetry","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal biochemical methods (ATPase assay, SPR, ITC) in a single rigorous study","pmids":["10869428"],"is_preprint":false},{"year":2000,"finding":"IscU behaves as a substrate for the Hsc66 chaperone, interacting with the substrate-binding domain: IscU inhibited Hsc66 suppression of rhodanese and citrate synthase aggregation, and ATP destabilizes Hsc66·IscU complexes in a manner expected for Hsp70-substrate complexes. IscU binds the ATPase + beta-sandwich subdomain but not the isolated ATPase domain of Hsc66.","method":"Chaperone aggregation suppression assay, calorimetry, surface plasmon resonance, truncation mutant binding studies","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal biochemical methods; direct mechanistic characterization of IscU-Hsc66 interaction","pmids":["11053447"],"is_preprint":false},{"year":2002,"finding":"Hsc66 interacts specifically with a region of IscU (residues 99–103, LPPVK) identified by phage display and cellulose-bound peptide array. A synthetic peptide ELPPVKIHC (residues 98–106) stimulates Hsc66 ATPase with similar affinity as native IscU but is not bound by Hsc20 and does not synergistically stimulate Hsc66 with Hsc20, indicating Hsc66 and Hsc20 recognize distinct regions of IscU.","method":"Phage display, peptide array, ATPase stimulation assay, peptide-binding domain competition assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal methods (phage display, peptide array, functional ATPase assays) identifying a specific binding motif","pmids":["11994302"],"is_preprint":false},{"year":2004,"finding":"Crystal structure of HscA substrate-binding domain complexed with IscU-derived peptide ELPPVKIHC at 1.95 Å resolution. The peptide binds in an extended conformation in a hydrophobic cleft in the beta-subdomain, in reverse orientation relative to DnaK–peptide complexes. The alpha-helical subdomain is shifted up to 10 Å relative to DnaK.","method":"X-ray crystallography, molecular replacement","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure at 1.95 Å with full refinement statistics; direct structural evidence of IscU peptide binding mode","pmids":["15351650"],"is_preprint":false},{"year":2004,"finding":"NMR solution structure of monomeric H. influenzae IscU with zinc bound at the Fe-S cluster assembly site by three cysteines and one histidine. The structure has an alpha-beta sandwich architecture with a three-stranded antiparallel beta-sheet and four alpha-helices. Removal of zinc by chelation results in widespread loss of structure in the apo form.","method":"NMR solution structure determination, zinc chelation experiments","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure with functional validation (zinc removal-induced unfolding); single rigorous study with multiple NMR methods","pmids":["15522305"],"is_preprint":false},{"year":2006,"finding":"HscA/HscB co-chaperone system stimulates [2Fe-2S] cluster transfer from IscU to apo-ferredoxin more than 20-fold in an ATP-dependent reaction requiring both HscB and MgATP. No stimulation was observed without HscB or MgATP, establishing that cluster transfer from IscU is an ATP-dependent process.","method":"CD and EPR spectrometry to monitor cluster transfer kinetics, phosphate production assay","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with multiple spectroscopic methods and rigorous controls showing ATP-dependence","pmids":["16964969"],"is_preprint":false},{"year":2007,"finding":"Reductive coupling of two [2Fe-2S]2+ clusters on the IscU homodimer forms a single [4Fe-4S]2+ cluster, demonstrated by UV-visible absorption, Mössbauer, and resonance Raman spectroscopies using dithionite. Isc ferredoxin can also drive partial reductive coupling, suggesting it may be the physiological reductant.","method":"UV-visible absorption, Mössbauer spectroscopy, resonance Raman spectroscopy, EPR spectroscopy","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal spectroscopic methods in vitro demonstrating the reductive coupling mechanism","pmids":["17506525"],"is_preprint":false},{"year":2007,"finding":"The [4Fe-4S] cluster-loaded form of IscU, but not the [2Fe-2S] cluster-loaded form, transfers its cluster intact to apo-aconitase A in vitro to activate the enzyme. An Asp39Ala substitution in IscU distinguishes the two cluster-loaded conformers and acts as a dominant-negative inhibitor of wild-type IscU-directed apo-aconitase activation.","method":"In vitro aconitase activation assay, mutant IscU inhibition studies","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with direct enzyme activity readout and mutant analysis; orthogonal to spectroscopic cluster characterization","pmids":["17506526"],"is_preprint":false},{"year":2008,"finding":"Crystal structure of [2Fe-2S]-bound IscU from Aquifex aeolicus at 2.3 Å reveals a novel asymmetric trimer with only one [2Fe-2S] cluster coordinated by Cys36, His106, Cys63, and Cys107 from a single protomer; the cluster is buried at the inter-protomer interface. The three protomers adopt distinct conformations with substantial rearrangement of loops carrying cluster-ligand residues.","method":"X-ray crystallography, multiwavelength anomalous diffraction","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure at 2.3 Å with MAD phasing; first direct structural observation of holo-IscU cluster coordination","pmids":["18723024"],"is_preprint":false},{"year":2008,"finding":"HscA-mediated acceleration of [2Fe-2S] cluster transfer from IscU to apoferredoxin requires both HscB and ATP hydrolysis. A mutant HscA(T212V) lacking ATPase activity cannot accelerate transfer. ATP addition to the HscA/HscB/IscU complex causes a transient distortion of the IscU-bound cluster detectable by CD, linked to HscA conformational change during T→R transition accompanying ATP hydrolysis.","method":"Circular dichroism, ATPase activity assay, limiting reagent experiments with HscA T212V mutant","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with mutagenesis and CD spectroscopy; mechanistically defines ATP hydrolysis requirement","pmids":["18986169"],"is_preprint":false},{"year":2004,"finding":"IscA acts as an iron donor for Fe-S cluster assembly in IscU under conditions where free iron is limiting (chelated by sodium citrate). IscA binds iron with an association constant of ~3×10^19 M^-1 via invariant Cys-99 and Cys-101 residues. IscA mutants that cannot bind iron fail to deliver iron for IscU cluster assembly.","method":"In vitro Fe-S cluster assembly assay under iron-limiting conditions, site-directed mutagenesis of IscA cysteine residues","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with mutagenesis demonstrating iron delivery function; multiple controls","pmids":["15247288"],"is_preprint":false},{"year":2006,"finding":"Cytosolic human ISCU forms a complex in vitro with cytosolic human ISCS (cysteine desulfurase), and together they facilitate efficient de novo [4Fe-4S] cluster formation on IRP1 when incubated with iron and cysteine, demonstrating that de novo Fe-S cluster biogenesis can occur in the cytosol.","method":"In vitro cluster assembly assay on IRP1, 35S radiotracer studies for ISCS desulfurase activity, Pichia pastoris overexpression","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct in vitro reconstitution showing cytosolic ISCU/ISCS-dependent cluster assembly on a physiological acceptor","pmids":["16527810"],"is_preprint":false},{"year":2006,"finding":"Suppression of human ISCU by RNAi inactivates mitochondrial and cytosolic aconitases in a compartment-specific manner, inappropriately activates iron regulatory proteins (IRP1/IRP2), and disrupts intracellular iron homeostasis, including increased iron uptake. Endogenous ISCU levels are also suppressed by iron deprivation.","method":"RNAi knockdown, aconitase enzyme activity assay, IRP RNA-binding assay, iron homeostasis measurements","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean RNAi knockdown with defined compartment-specific enzymatic phenotypes and iron homeostasis readouts in mammalian cells","pmids":["16517407"],"is_preprint":false},{"year":2009,"finding":"NMR studies of E. coli IscU show that apo-IscU exists as two distinct slowly-interconverting conformations: one largely disordered (D) and one largely ordered (S) except for metal-binding residues. The S-state is stabilized by zinc or by the D39A substitution. HscB binds preferentially to the S-state of IscU and interacts most with the two N-terminal beta-strands and C-terminal alpha-helix.","method":"NMR spectroscopy (chemical shift perturbation), zinc addition experiments, mutant analysis","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structural characterization identifying two conformational states; mutant analysis; functional binding studies","pmids":["19492851"],"is_preprint":false},{"year":2011,"finding":"The disordered (D) conformational state of IscU is the preferred substrate for binding to cysteine desulfurase IscS for Fe-S cluster assembly. IscS addition shifts the IscU equilibrium toward the D-state, increasing H/D exchange. IscU variants favoring the S-state show a lag in cluster assembly; those favoring D-state assemble less stable clusters. This establishes functional importance of the S↔D conformational equilibrium.","method":"NMR spectroscopy (H/D exchange, chemical shift perturbation), in vitro Fe-S cluster assembly assays with S-state and D-state biased IscU variants","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR and in vitro functional assays with multiple mutants; directly demonstrates conformational state selectivity for IscS binding","pmids":["22203963"],"is_preprint":false},{"year":2012,"finding":"HscA (Hsp70 chaperone) binds preferentially to the disordered (D) conformational state of IscU, while the J-protein HscB binds preferentially to the structured (S) state. HscA-ADP stabilizes the D-state; IscU is released when HscA binds ATP. This defines a chaperone cycle in which cluster transfer is coupled to ATP hydrolysis and IscU D-state stabilization, followed by HscB release.","method":"NMR spectroscopy with S-state and D-state biased IscU variants, chaperone interaction studies","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR with multiple conformationally biased variants; mechanistically defines chaperone state selectivity","pmids":["22782893"],"is_preprint":false},{"year":2012,"finding":"The S↔D conformational interconversion of metamorphic IscU involves concerted trans-to-cis isomerization of two conserved peptidyl-prolyl peptide bonds: N13-P14 and P100-P101. The D-state contains an ordered domain stabilizing two cis peptide bonds while P35 and P100 are trans in the S-state.","method":"NMR spectroscopy (chemical shift analysis, NOE measurements for prolyl-peptide bond configuration)","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR with rigorous chemical shift and NOE analysis directly identifying the prolyl isomerization mechanism","pmids":["23110687"],"is_preprint":false},{"year":2013,"finding":"Human mtHSP70 (DnaK-type) and NFS1 (cysteine desulfurase) bind preferentially to the disordered (D) state of ISCU, while HSC20 (DnaJ-type) binds preferentially to the structured (S) state. NFS1-ISD11 complex also preferentially binds the D-state. ISD11 does not interact directly with ISCU. [2Fe-2S] and [4Fe-4S] clusters are assembled on ISCU catalyzed by NFS1, at a higher rate with the NFS1-ISD11 complex.","method":"NMR spectroscopy with ISCU state-biased variants, ATPase activity assay, in vitro Fe-S cluster assembly assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR with multiple variants; in vitro assembly and ATPase assays; multiple orthogonal approaches in human system","pmids":["23940031"],"is_preprint":false},{"year":2011,"finding":"Mammalian frataxin interacts with a preformed ISCU/NFS1/ISD11 core complex (rather than individual components) to form a quaternary complex of ~190 kDa, with ISCU, NFS1, and ISD11 being the main endogenous frataxin interactors. This interaction defines the essential function of frataxin in Fe-S cluster biosynthesis.","method":"Co-immunoprecipitation with recombinant frataxin in mammalian cells, heterologous expression of complex components, size-exclusion chromatography","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 2 / Moderate — co-IP in both directions combined with biochemical reconstitution of the quaternary complex; single lab with two orthogonal approaches","pmids":["21298097"],"is_preprint":false},{"year":2015,"finding":"Frataxin enhances the rate of sulfur transfer from NFS1 persulfide to ISCU cysteine C104, leading to persulfide accumulation on ISCU. Frataxin also enhances sulfur transfer to small thiols (DTT, L-cysteine, GSH) from NFS1 persulfide. This reveals frataxin functions as an enhancer of sulfur transfer within the NFS1-ISD11-ISCU complex.","method":"Maleimide-peptide probing of cysteine-persulfide combined with mass spectrometry","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — novel chemical probing method (maleimide-peptide + MS) directly measuring persulfide transfer kinetics; multiple substrate conditions tested","pmids":["25597503"],"is_preprint":false},{"year":2018,"finding":"FXN (frataxin) tightly binds a single Fe2+ but not Fe3+. FXN does not bind ISCU directly, but both interact with the cysteine desulfurase complex (NIA)2. FXN binds (NIA)2 more strongly in the presence of ISCU. Upon addition of L-cysteine and a reductant (reduced FDX2 or DTT), Fe2+ is released from FXN consistent with Fe2+-FXN being the proximal iron source for Fe-S cluster assembly on ISCU.","method":"Isothermal titration calorimetry, multinuclear NMR spectroscopy","journal":"Journal of inorganic biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — ITC and NMR together directly measuring binding affinities and iron release under physiological conditions","pmids":["29576242"],"is_preprint":false},{"year":2013,"finding":"mTORC1 associates with ISCU and phosphorylates ISCU at serine 14, stabilizing the ISCU protein. mTORC1 inhibition reduces ISCU levels and prevents Fe-S cluster assembly.","method":"Co-immunoprecipitation, in vitro kinase assay, mTORC1 inhibitor treatment, Fe-S cluster assembly readout","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — co-IP and kinase assay identifying a PTM site, but single lab with limited mechanistic follow-up on the phosphorylation consequences","pmids":["23508953"],"is_preprint":false},{"year":2012,"finding":"HscA/HscB co-chaperone system is required for efficient ATP-dependent [2Fe-2S] cluster transfer from IscU to apo-Grx5 (monothiol glutaredoxin), providing a 700-fold enhancement in transfer rate. This establishes a route for [2Fe-2S] clusters assembled on IscU to be stored and transported by monothiol glutaredoxins.","method":"Circular dichroism spectroscopy monitoring cluster transfer, rate measurements ±HscA/HscB/Mg-ATP","journal":"Journal of the American Chemical Society","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with quantitative kinetics; directly demonstrates chaperone-dependent transfer to Grx5","pmids":["22963613"],"is_preprint":false},{"year":2012,"finding":"Three-dimensional NMR structures of E. coli apo-IscU (both wild-type predominantly in S-state and D39A variant >95% S-state) were determined. Hydrophobic residues at position 39 stabilize the S-state by reducing loop flexibility around conserved cysteines. Alanine substitutions at positions 90, 107, and 111 stabilize the protein without affecting loop dynamics.","method":"NMR spectroscopy (3D structure determination), systematic mutagenesis, stability measurements","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR structure determination combined with systematic mutagenesis; defines structural determinants of conformational states","pmids":["22734684"],"is_preprint":false},{"year":2014,"finding":"ISCU G50E mutation (found in compound heterozygous myopathy patients) results in compromised interaction with sulfur donor NFS1 and the J-protein HSCB, impairing Fe-S cluster synthesis rate. This leads to defects in electron transport chain complexes, loss of cellular respiration, increased mitochondrial iron, and reactive oxygen species.","method":"Biochemical interaction assays (NFS1-ISCU, HSCB-ISCU binding), Fe-S cluster synthesis assay, ETC complex activity assays, ROS measurements in human cell lines and yeast models","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple biochemical assays demonstrating direct mechanistic link between mutation, protein interactions, and cellular phenotype; validated in both human cells and yeast","pmids":["24573684"],"is_preprint":false},{"year":2020,"finding":"ISCU interacts directly with NFU1, with Kd = 1.1 µM (by ITC). The structured conformer of ISCU binds NFU1 via its cluster-binding region interacting with two alpha-helices in the C-terminal domain of NFU1. ISCU[4Fe-4S] transfers its Fe-S cluster to apo-NFU1 in a chaperone-independent manner, while ISCU[2Fe-2S] does not transfer its cluster to NFU1.","method":"NMR spectroscopy, small-angle X-ray scattering, isothermal titration calorimetry, size exclusion chromatography, in vitro cluster transfer assay","journal":"Journal of structural biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal biophysical methods (NMR, SAXS, ITC) combined with functional cluster transfer assay; cluster-type specificity demonstrated","pmids":["32151725"],"is_preprint":false},{"year":2021,"finding":"Crystal structure of archaeal IscU (Methanothrix thermoacetophila) dimer at atomic resolution shows two [2Fe-2S] clusters facing each other at the dimer interface, with Asp40 serving as a fourth ligand in addition to three Cys residues. His106 is required for reductive coupling of two [2Fe-2S] to one [4Fe-4S] cluster (EPR-confirmed), without structural alteration by H106A substitution, indicating His106's functional role in cluster conversion.","method":"X-ray crystallography, EPR spectroscopy, site-directed mutagenesis","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure combined with EPR and mutagenesis directly demonstrating dimer architecture and His106 role in [4Fe-4S] formation","pmids":["33938220"],"is_preprint":false},{"year":2022,"finding":"Iron binds to the assembly site of prokaryotic and eukaryotic ISCU via cysteine residues (1-Cys and 2-Cys forms). The 2-Cys form predominates at pH 8 and correlates with Fe-S cluster assembly activity. X-ray absorption, Mössbauer, NMR, CD, and EPR characterization of the 2-Cys form shows iron is coordinated by four conserved residues (Cys35, Asp37, Cys61, His103) in tetrahedral geometry. An auxiliary non-cysteine iron-binding site in ISCU does not exist.","method":"SEC, circular dichroism, Mössbauer, X-ray absorption, NMR, EPR, site-directed mutagenesis","journal":"Journal of the American Chemical Society","confidence":"High","confidence_rationale":"Tier 1 / Strong — comprehensive multi-spectroscopic characterization with mutagenesis; establishes iron insertion mechanism at atomic level","pmids":["36121382"],"is_preprint":false},{"year":2015,"finding":"p53 transcriptionally activates ISCU expression by binding to an intronic p53-binding site. Knockdown of ISCU enhances IRP1 binding to ferritin heavy chain mRNA's iron-responsive element, reducing ferritin translation and causing iron accumulation. p53-mediated ISCU induction is part of a cellular iron homeostasis response to DNA damage.","method":"ChIP for p53 binding, siRNA knockdown of ISCU, IRP1-IRE RNA-binding assay, ferritin translation assay, iron measurement","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP, knockdown with defined molecular phenotypes (IRP1 binding, ferritin translation), and in vivo mouse data; single lab","pmids":["26560363"],"is_preprint":false},{"year":2014,"finding":"In the IscU·IscS complex, IscU retains a folded (structured) conformation as shown by NMR, contradicting proposals that IscS preferentially binds the unstructured form of IscU. Advanced solution NMR directly observes IscU's folded state within the 110 kDa complex.","method":"Solution NMR spectroscopy of the IscS-IscU complex","journal":"Chembiochem","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — NMR structural data on complex is direct and rigorous but single study contradicting other reports; moderate confidence given conflicting literature","pmids":["25044349"],"is_preprint":false},{"year":2018,"finding":"ISCU variants M108I and D39V populate only the structured (S) conformational state. These variants fail to form cysteine desulfurase complexes containing both frataxin (FXN) and ferredoxin (FDX2) simultaneously; when ISCU(M108I) is present, rdFDX2 addition displaces FXN from the [NIA-ISCU-FXN]2 complex, explaining why FXN cannot stimulate cluster assembly on these fully structured ISCU variants with physiological reductant.","method":"NMR titration experiments, in vitro Fe-S cluster assembly assay with DTT vs. reduced FDX2, cysteine desulfurase activity assay","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR and functional assays revealing molecular mechanism of FXN bypass; direct demonstration of FDX2-FXN competition on the complex","pmids":["29406711"],"is_preprint":false},{"year":2008,"finding":"A single intronic mutation in ISCU (IVS5+382G>C) strengthens a weak splice acceptor site, causing retention of intronic sequence in ISCU mRNA. This leads to marked reduction of mitochondrial ISCU protein in skeletal muscle, decreased IRP1 activity, and intracellular iron overload in skeletal muscle, establishing a mechanistic link between ISCU scaffold protein deficiency, Fe-S cluster biogenesis failure, and iron dysregulation.","method":"Genetic mapping, mRNA splice analysis, western blot for ISCU protein, IRP1 activity assay, iron content measurements in patient muscle","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple biochemical measurements (protein levels, enzyme activities, iron content) in patient tissues with defined molecular mechanism; replicated by multiple independent groups","pmids":["18304497"],"is_preprint":false},{"year":2008,"finding":"An intron mutation in ISCU activates cryptic splice sites leading to aberrant mRNA with 100 bp of intron 5, causing a frameshift and premature stop codon. Severe protein deficiency of mitochondrial ISCU is found in homozygous patient skeletal muscle. The phenotype is confirmed as hereditary myopathy with lactic acidosis (HML) mapped to chromosome 12q23.3-24.11.","method":"Genetic mapping (lod score 5.26), RT-PCR splice analysis, western blot","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic mapping combined with splice and protein analysis; independently confirmed by at least two separate studies","pmids":["18296749"],"is_preprint":false},{"year":2010,"finding":"Tissue-specific splicing of the ISCU intron mutation accounts for the skeletal muscle-restricted phenotype in hereditary myopathy with lactic acidosis: the highest level of incorrectly spliced ISCU mRNA is in skeletal muscle while normal splicing predominates in heart. Functional loss of Fe-S cluster-carrying enzymes and iron accumulation occur in muscle but not other tissues. Complete homozygous knockout of ISCU in mice causes early embryonic death.","method":"RT-PCR splice analysis across tissues, enzyme activity assays, iron measurement, mouse knockout","journal":"Human genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — multi-tissue molecular analysis with functional enzyme readouts; mouse KO provides essential function evidence","pmids":["21165651"],"is_preprint":false},{"year":2011,"finding":"PTBP1 represses defective splicing of ISCU in myopathy patients, dramatically reducing mutant transcripts. IGF2BP1 (which preferentially binds the mutant sequence) and RBM39 shift splicing toward the incorrect form. These splicing factors directly interact with the sequence harboring the ISCU intron mutation.","method":"RNA pulldown to identify interacting nuclear factors, splicing reporter assays with siRNA knockdown of individual factors","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct RNA binding and functional splicing assays; single lab but multiple factors analyzed with appropriate controls","pmids":["22125086"],"is_preprint":false},{"year":2012,"finding":"MyoD-driven muscle differentiation enhances ISCU mRNA mis-splicing in ISCU myopathy patient myoblasts. Oxidative stress (H2O2 treatment or elevated oxygen) further reduces ISCU protein levels in patient cells, an effect preventable by ascorbate pretreatment, indicating that oxidative stress destabilizes the small amounts of correctly spliced ISCU protein produced in patient skeletal muscle.","method":"MyoD expression in patient myoblasts, H2O2 and oxygen treatment with western blot for ISCU protein, antioxidant rescue experiments","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-based mechanistic experiments with defined transcription factor (MyoD) and chemical stress conditions; single lab","pmids":["23035118"],"is_preprint":false},{"year":2023,"finding":"Excess copper inhibits Fe-S cluster biogenesis through direct binding to ISCU (as well as ISCA1 and ISCA2) proteins, thereby reducing activity of Fe-S cluster-dependent enzymes and disrupting mitochondrial function. ISCU, ISCA1, and ISCA2 were shown to have strong copper-binding activity.","method":"In vitro copper-binding assays with recombinant ISCU, cellular Fe-S enzyme activity assays in Wilson's disease model cells (ATP7A-/-, ATP7B KD), mouse model of Wilson's disease","journal":"Free radical biology & medicine","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro binding assay combined with cellular and in vivo models; single lab but multi-system validation","pmids":["37225108"],"is_preprint":false},{"year":2015,"finding":"Ferredoxin, in conjunction with NADPH and ferredoxin-NADP reductase, transfers electrons to the IscS/IscU complex to support Fe-S cluster assembly in vitro. This trio of electron-transfer partners is sufficient for the reaction, albeit slower than DTT-mediated assembly. Ferredoxin does not interfere with CyaY's inhibitory activity despite overlapping binding sites on IscS.","method":"In vitro Fe-S cluster assembly assay using NADPH/FNR/Fdx as physiological reductant","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro reconstitution with physiological electron donors; single lab, limited mechanistic follow-up","pmids":["25688831"],"is_preprint":false},{"year":2019,"finding":"IscU residue Y3 (strictly conserved) is essential for in vivo Fe-S cluster assembly function. Substitutions at Y3 abolish function by impairing functional interaction with IscS and sulfur transfer between IscS and IscU. A second-site suppressor mutation in IscS (A349V) rescues Y3 substitutions, linking Y3 function to the conformational dynamics of the flexible loop of IscS required for sulfur transfer.","method":"In vivo genetic complementation, biochemical sulfur transfer assays, second-site suppressor screen","journal":"Molecular microbiology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic epistasis (suppressor screen) combined with direct biochemical sulfur transfer assays; multiple lines of evidence","pmids":["31532036"],"is_preprint":false},{"year":2020,"finding":"In vivo conformational interconversion between D-state and S-state of IscU is required for Fe-S cluster assembly and transfer. IscU single amino acid substitutions that bypass the need for HscA/HscB show enhanced D↔S interconversion in CD spectroscopic studies. IscU variants locked perpetually in either state cannot perform their in vivo role even with the chaperone system present.","method":"In vivo genetic bypass assay of chaperone requirement, CD spectroscopy of IscU variants, Zn2+-induced S-state stabilization","journal":"Molecular microbiology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic and biochemical evidence from multiple IscU variants directly linking conformational dynamics to in vivo function; replicated across variant classes","pmids":["33202070"],"is_preprint":false},{"year":2014,"finding":"Drosophila p38 pathway substrate MK2 kinase phosphorylates IscU in both Drosophila and mammalian cells, linking the p38 stress response pathway to mitochondrial Fe-S cluster metabolism.","method":"In vitro kinase assay identifying IscU as MK2 substrate in Drosophila cells and mammalian cells","journal":"The Journal of biological chemistry","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single in vitro kinase assay; no phosphorylation site identified; limited functional follow-up on Fe-S cluster consequences","pmids":["25204651"],"is_preprint":false},{"year":2025,"finding":"ISCU sequestrates p53 in the cytoplasm in macrophages, reducing p53 nuclear localization and relieving transcriptional repression of xCT and Arg1, promoting M2 macrophage polarization. Inhibition of ISCU expression repolarizes macrophages and enhances CD8+ T cell cytotoxicity.","method":"Co-immunoprecipitation (ISCU-p53), subcellular fractionation, gene expression analysis, macrophage polarization assays, ISCU knockdown in vivo and in vitro","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — co-IP demonstrating ISCU-p53 interaction with functional macrophage polarization readout; single lab, novel function distinct from canonical Fe-S role","pmids":["40541964"],"is_preprint":false}],"current_model":"ISCU is a universally conserved scaffold protein that serves as the central platform for iron-sulfur (Fe-S) cluster biogenesis: it exists as a metamorphic protein toggling between a structured (S) and disordered (D) conformational state via cis-trans prolyl isomerization, where the D-state is preferentially bound by the cysteine desulfurase NFS1/IscS (for sulfur donation as a persulfide) and by the Hsp70 chaperone (mtHSP70/HscA), while the S-state is preferentially bound by the DnaJ co-chaperone (HSC20/HscB); iron is inserted into a tetrahedral assembly site formed by conserved Cys, Asp, and His residues, sulfur is transferred from NFS1 persulfide to ISCU (stimulated by frataxin which also provides Fe2+), [2Fe-2S] clusters assemble first and can be reductively coupled to [4Fe-4S] clusters on IscU dimers, and completed clusters are transferred ATP-dependently to acceptor apo-proteins (ferredoxin, aconitase, NFU1, IRP1) through the HscA/HscB chaperone cycle; loss of ISCU function impairs aconitase and respiratory chain complexes, activates IRP1, and disrupts iron homeostasis, explaining the skeletal muscle myopathy in patients with ISCU splice mutations."},"narrative":{"mechanistic_narrative":"ISCU is the central, universally conserved scaffold on which mitochondrial and cytosolic iron-sulfur (Fe-S) clusters are assembled de novo and from which they are delivered to apo-acceptor proteins [PMID:10891064, PMID:16517407]. Cluster building proceeds sequentially on the ISCU dimer: the cysteine desulfurase NFS1/IscS binds ISCU with high affinity and transfers sulfur from its persulfide to ISCU cysteines [PMID:11577100], iron is inserted at a tetrahedral assembly site coordinated by conserved Cys, Asp, and His residues (Cys35, Asp37, Cys61, His103) [PMID:36121382, PMID:15522305], and two transient [2Fe-2S] clusters are reductively coupled — a step requiring His106 — into a single [4Fe-4S] cluster per dimer [PMID:10891064, PMID:17506525, PMID:33938220]. Frataxin (FXN) acts within the NFS1-ISD11-ISCU complex as a sulfur-transfer enhancer and as the proximal Fe2+ donor, binding the desulfurase complex more tightly in the presence of ISCU and releasing its iron upon reductant addition [PMID:25597503, PMID:29576242], with ferredoxin (FDX2)/NADPH/FNR supplying the electrons for assembly and reductive coupling [PMID:25688831, PMID:17506525]. ISCU is a metamorphic protein that interconverts between a largely structured (S) and a largely disordered (D) state through concerted cis-trans isomerization of conserved prolyl bonds; NFS1 and the Hsp70 chaperone (HscA/mtHSP70) bind the D-state while the J-protein co-chaperone (HscB/HSC20) binds the S-state, and this S↔D cycling is obligatory in vivo for both assembly and chaperone-driven cluster release [PMID:22203963, PMID:22782893, PMID:23110687, PMID:33202070]. Completed clusters are delivered ATP-dependently through the HscA/HscB chaperone cycle to acceptors including ferredoxin and the monothiol glutaredoxin Grx5, with cluster-type and conformer specificity dictating fate: [4Fe-4S]-ISCU activates apo-aconitase and transfers chaperone-independently to NFU1, whereas [2Fe-2S]-ISCU does not [PMID:16964969, PMID:22963613, PMID:17506526, PMID:32151725]. Loss of ISCU function inactivates aconitases and respiratory chain complexes, inappropriately activates iron regulatory proteins (IRP1/IRP2), and disrupts iron homeostasis [PMID:16517407]. Intronic ISCU splice mutations that strengthen a cryptic splice acceptor cause tissue-specific ISCU deficiency in skeletal muscle, defining hereditary myopathy with lactic acidosis [PMID:18304497, PMID:18296749, PMID:21165651].","teleology":[{"year":2000,"claim":"Establishing that IscU is a scaffold that builds Fe-S clusters in a defined sequence answered how nascent clusters are assembled before delivery to apo-enzymes.","evidence":"anaerobic chromatography with Mössbauer, resonance Raman and UV-vis spectroscopy of reconstituted IscU","pmids":["10891064"],"confidence":"High","gaps":["Did not define the iron source in vivo","Coordination ligands of each cluster intermediate not yet resolved structurally"]},{"year":2001,"claim":"Demonstrating direct persulfide-based sulfur transfer from IscS to IscU cysteines defined the chemical mechanism of sulfur donation.","evidence":"[35S]-cysteine radiotracer, SPR, ITC and IscS deletion mutagenesis","pmids":["11577100"],"confidence":"High","gaps":["Which specific IscU cysteine receives the persulfide not pinpointed here","Role of conformational state in binding not addressed"]},{"year":2000,"claim":"Identifying IscU as a substrate that stimulates the Hsc66/Hsc20 chaperone ATPase established that a dedicated Hsp70/J-protein system services Fe-S scaffold function.","evidence":"ATPase assays, SPR, ITC and chaperone aggregation-suppression assays","pmids":["10869428","11053447"],"confidence":"High","gaps":["The functional output of the ATPase cycle (cluster transfer) not yet shown","Binding determinants on IscU undefined"]},{"year":2002,"claim":"Mapping the LPPVK (residues 99–103) motif as the Hsc66 recognition site and showing Hsc20 binds elsewhere defined how the two chaperones engage distinct IscU regions.","evidence":"phage display, peptide arrays and ATPase stimulation assays, later confirmed by a 1.95 Å HscA-peptide crystal structure","pmids":["11994302","15351650"],"confidence":"High","gaps":["Peptide binding mode may differ from full-length IscU engagement","How Hsc20 recognizes the structured conformer not addressed at this stage"]},{"year":2004,"claim":"Solving the IscU fold with a metal at the assembly site and showing apo-IscU unfolds on metal removal revealed the intrinsic conformational lability central to its function.","evidence":"NMR solution structure with zinc, plus zinc chelation experiments; IscA shown separately to act as an iron donor under iron-limiting conditions","pmids":["15522305","15247288"],"confidence":"High","gaps":["Physiological identity of the iron donor remained contested","Functional meaning of the order-disorder transition not yet defined"]},{"year":2006,"claim":"Showing that cluster transfer from IscU to apo-ferredoxin requires HscB and ATP hydrolysis established that the chaperone cycle drives cluster handoff, not just substrate binding.","evidence":"CD/EPR transfer kinetics and phosphate assays; parallel work showed cytosolic human ISCU/ISCS assemble clusters on IRP1, and ISCU RNAi inactivates aconitases and dysregulates iron","pmids":["16964969","16527810","16517407"],"confidence":"High","gaps":["Mechanism coupling ATP hydrolysis to cluster release not yet structural","Cytosolic vs mitochondrial division of labor unresolved"]},{"year":2007,"claim":"Demonstrating reductive coupling of two [2Fe-2S] into one [4Fe-4S] and cluster-type-specific delivery to aconitase explained how ISCU produces and routes different cluster types.","evidence":"Mössbauer/resonance Raman/EPR with dithionite or Isc-ferredoxin reductant, and aconitase activation assays with the D39A discriminating mutant","pmids":["17506525","17506526"],"confidence":"High","gaps":["Physiological reductant for coupling not definitively assigned","Structural basis for [4Fe-4S]-specific recognition by acceptors unknown"]},{"year":2008,"claim":"Crystallizing holo-IscU with a buried [2Fe-2S] cluster and defining the ATP-hydrolysis requirement of HscA-accelerated transfer provided the structural and energetic basis of cluster handoff.","evidence":"MAD-phased crystal structure of [2Fe-2S]-IscU and CD/ATPase analysis with the ATPase-dead HscA T212V mutant","pmids":["18723024","18986169"],"confidence":"High","gaps":["Asymmetric oligomeric state may be crystallization-dependent","How cluster distortion drives release into acceptors not fully resolved"]},{"year":2008,"claim":"Identifying intronic ISCU splice mutations that deplete mitochondrial ISCU in skeletal muscle linked Fe-S scaffold deficiency to hereditary myopathy with lactic acidosis.","evidence":"genetic mapping, splice and protein analysis, IRP1 activity and iron measurements in patient muscle","pmids":["18304497","18296749"],"confidence":"High","gaps":["Why the phenotype is muscle-restricted not yet explained at this stage","Therapeutic correction of splicing not addressed"]},{"year":2009,"claim":"Defining apo-IscU as a metamorphic protein interconverting between disordered (D) and structured (S) states, with HscB binding the S-state, reframed IscU function around conformational dynamics.","evidence":"NMR chemical shift perturbation, zinc stabilization, and D39A mutant analysis","pmids":["19492851"],"confidence":"High","gaps":["Functional partner preference of the D-state not yet tested","Atomic basis of the conformational switch unknown"]},{"year":2012,"claim":"Assigning the D-state to NFS1/IscS binding and assembly, the S-state to HscB, and the D-state to HscA defined a conformation-coupled chaperone cycle, with prolyl isomerization as the molecular switch.","evidence":"NMR with conformationally biased variants, H/D exchange, prolyl-bond NOE analysis, and structures of S-state apo-IscU; transfer to Grx5 also shown to be HscA/HscB/ATP-dependent","pmids":["22203963","22782893","23110687","22734684","22963613"],"confidence":"High","gaps":["Whether a prolyl isomerase catalyzes the switch in vivo not established","One NMR study (idx 31) reported IscU stays folded within the IscS complex, conflicting with D-state preference"]},{"year":2013,"claim":"Extending the D/S conformer-chaperone model to the human NFS1-ISD11-ISCU-mtHSP70-HSC20 system, and showing mTORC1 phosphorylates and stabilizes ISCU, connected conserved mechanism to mammalian regulation.","evidence":"NMR with human ISCU variants, ATPase and assembly assays; co-IP and in vitro kinase assay for mTORC1-ISCU(S14)","pmids":["23940031","23508953"],"confidence":"High","gaps":["Functional consequence of S14 phosphorylation beyond stability not defined (Medium-confidence for the kinase link)","ISD11 role clarified only as not binding ISCU directly"]},{"year":2015,"claim":"Defining frataxin as a sulfur-transfer enhancer that accumulates persulfide on ISCU C104, and later as the proximal Fe2+ donor, resolved frataxin's biochemical role in cluster assembly.","evidence":"maleimide-peptide persulfide probing with MS; ITC/NMR iron-binding and release experiments; FDX2/NADPH/FNR shown sufficient as physiological reductant","pmids":["25597503","29576242","21298097","25688831"],"confidence":"High","gaps":["Whether iron and sulfur are delivered in a fixed order not settled","Quantitative flux through frataxin vs alternative iron sources in vivo unknown"]},{"year":2018,"claim":"Showing that fully S-state-locked ISCU variants (M108I, D39V) cannot form a productive FXN/FDX2 complex with physiological reductant explained why conformational dynamics are mechanistically essential.","evidence":"NMR titrations and assembly assays comparing DTT vs reduced FDX2; FDX2 displaces FXN on locked variants","pmids":["29406711"],"confidence":"High","gaps":["In vivo phenotypes of these specific variants not characterized here","Structural arrangement of the FXN/FDX2-containing complex unresolved"]},{"year":2020,"claim":"Establishing direct, conformer- and cluster-type-specific transfer from [4Fe-4S]-ISCU to NFU1, and proving in vivo that D↔S interconversion is required for function, completed the link between conformational cycling and cluster delivery.","evidence":"NMR/SAXS/ITC and cluster-transfer assays for NFU1; in vivo genetic bypass and CD spectroscopy of conformationally locked IscU variants","pmids":["32151725","33202070"],"confidence":"High","gaps":["Determinants selecting chaperone-independent vs chaperone-dependent transfer routes unresolved","Full acceptor specificity hierarchy not mapped"]},{"year":2022,"claim":"Atomic-resolution definition of the dimeric assembly site, His106's role in reductive coupling, and the tetrahedral 2-Cys iron-binding mode finalized the structural model of iron insertion and cluster conversion.","evidence":"archaeal IscU crystal structure with EPR/mutagenesis; multi-spectroscopic (XAS, Mössbauer, NMR, CD, EPR) characterization of the iron-loaded site ruling out an auxiliary non-Cys site","pmids":["33938220","36121382"],"confidence":"High","gaps":["Dynamics of iron delivery into the assembly site in real time not captured","How the eukaryotic complex coordinates these steps spatially not fully resolved"]},{"year":2014,"claim":"Mechanistically connecting a patient G50E mutation to impaired NFS1/HSCB interaction and downstream respiratory and ROS defects linked ISCU biochemistry to disease pathophysiology.","evidence":"interaction and assembly assays, ETC activity and ROS measurements in human cells and yeast; tissue-specific splicing and oxidative-stress destabilization explaining muscle restriction studied separately","pmids":["24573684","21165651","22125086","23035118"],"confidence":"High","gaps":["Why splicing factors (PTBP1, IGF2BP1, RBM39) regulation is muscle-specific not fully explained (Medium-confidence)","Therapeutic splice correction not demonstrated in patients"]},{"year":2023,"claim":"Reports that copper directly inhibits ISCU and that ISCU sequesters p53 in macrophages to drive M2 polarization extend ISCU into metal-toxicity and immune-regulatory contexts beyond canonical assembly.","evidence":"in vitro copper-binding and cellular Fe-S enzyme assays in Wilson's disease models; co-IP, fractionation and macrophage polarization assays for ISCU-p53","pmids":["37225108","40541964"],"confidence":"Medium","gaps":["Both are single-lab findings awaiting independent confirmation","Whether the p53-sequestration role is separable from Fe-S scaffold function not established","Physiological copper concentrations achieving inhibition not defined"]},{"year":null,"claim":"How conformational interconversion, iron and sulfur delivery, and chaperone-driven release are coordinated in time and space within the intact eukaryotic assembly complex, and how the conflicting reports of ISCU's folded vs disordered state during desulfurase engagement are reconciled, remain open.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No integrated structure of the full assembly/transfer cycle","Conflicting NMR observations on IscU conformation in the IscS complex (idx 31 vs idx 16) unresolved","In vivo regulation of ISCU phosphorylation and conformational switching poorly defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[9,24,27]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,10,28]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[21,29]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[14,33,34]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[13,14]}],"pathway":[{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[0,7,14]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[9,14]}],"complexes":["NFS1-ISD11-ISCU cysteine desulfurase complex","ISCU-NFS1-ISD11-frataxin quaternary complex","HscA/HscB-IscU chaperone complex"],"partners":["NFS1","FXN","FDX2","HSCB","HSPA9","NFU1","ISCA1","ISD11"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9H1K1","full_name":"Iron-sulfur cluster assembly enzyme ISCU","aliases":["NifU-like N-terminal domain-containing protein","NifU-like protein"],"length_aa":167,"mass_kda":18.0,"function":"Mitochondrial scaffold protein, of the core iron-sulfur cluster (ISC) assembly complex, that provides the structural architecture on which the [2Fe-2S] clusters are assembled (PubMed:34824239). The core iron-sulfur cluster (ISC) assembly complex is involved in the de novo synthesis of a [2Fe-2S] cluster, the first step of the mitochondrial iron-sulfur protein biogenesis. This process is initiated by the cysteine desulfurase complex (NFS1:LYRM4:NDUFAB1) that produces persulfide which is delivered on the scaffold protein ISCU in a FXN-dependent manner. Then this complex is stabilized by FDX2 which provides reducing equivalents to accomplish the [2Fe-2S] cluster assembly. Finally, the [2Fe-2S] cluster is transferred from ISCU to chaperone proteins, including HSCB, HSPA9 and GLRX5 (Probable) (PubMed:24971490, PubMed:29576242, PubMed:30031876, PubMed:34824239). Exists as two slow interchanging conformational states, a structured (S) and disordered (D) form (PubMed:23940031). May modulate NFS1 desulfurase activity in a zinc-dependent manner (PubMed:30031876). Modulates the interaction between FXN and the cysteine desulfurase complex (PubMed:29576242) Cytoplasmic scaffold protein, of the cytoplasmic core iron-sulfur cluster (ISC) assembly complex that provides the structural architecture on which the Fe-S clusters are assembled and may be involved in the cytoplasmic iron-sulfur protein biogenesis","subcellular_location":"Cytoplasm; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9H1K1/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/ISCU","classification":"Common Essential","n_dependent_lines":1207,"n_total_lines":1208,"dependency_fraction":0.9991721854304636},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/ISCU","total_profiled":1310},"omim":[{"mim_id":"613311","title":"LYR MOTIF-CONTAINING PROTEIN 4; LYRM4","url":"https://www.omim.org/entry/613311"},{"mim_id":"611911","title":"IRON-SULFUR CLUSTER ASSEMBLY ENZYME; ISCU","url":"https://www.omim.org/entry/611911"},{"mim_id":"609588","title":"GLUTAREDOXIN 5; GLRX5","url":"https://www.omim.org/entry/609588"},{"mim_id":"608142","title":"HSCB MITOCHONDRIAL IRON-SULFUR CLUSTER COCHAPERONE; HSCB","url":"https://www.omim.org/entry/608142"},{"mim_id":"608100","title":"NFU1 IRON-SULFUR CLUSTER SCAFFOLD; NFU1","url":"https://www.omim.org/entry/608100"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Mitochondria","reliability":"Supported"},{"location":"Cytosol","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/ISCU"},"hgnc":{"alias_symbol":["ISU2","hnifU"],"prev_symbol":["NIFUN"]},"alphafold":{"accession":"Q9H1K1","domains":[{"cath_id":"3.90.1010.10","chopping":"38-156","consensus_level":"high","plddt":94.6586,"start":38,"end":156}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H1K1","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H1K1-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H1K1-F1-predicted_aligned_error_v6.png","plddt_mean":85.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ISCU","jax_strain_url":"https://www.jax.org/strain/search?query=ISCU"},"sequence":{"accession":"Q9H1K1","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9H1K1.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9H1K1/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H1K1"}},"corpus_meta":[{"pmid":"10891064","id":"PMC_10891064","title":"IscU as a scaffold for iron-sulfur cluster biosynthesis: sequential assembly of [2Fe-2S] and [4Fe-4S] clusters in IscU.","date":"2000","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10891064","citation_count":369,"is_preprint":false},{"pmid":"20498629","id":"PMC_20498629","title":"Hypoxia-regulated microRNA-210 modulates mitochondrial function and decreases ISCU and COX10 expression.","date":"2010","source":"Oncogene","url":"https://pubmed.ncbi.nlm.nih.gov/20498629","citation_count":339,"is_preprint":false},{"pmid":"20436681","id":"PMC_20436681","title":"MicroRNA-210 regulates mitochondrial free radical response to hypoxia and krebs cycle in cancer cells by targeting iron sulfur cluster protein ISCU.","date":"2010","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/20436681","citation_count":269,"is_preprint":false},{"pmid":"16517407","id":"PMC_16517407","title":"Functions of mitochondrial ISCU and cytosolic ISCU in mammalian iron-sulfur cluster biogenesis and iron 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the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/10869428","citation_count":193,"is_preprint":false},{"pmid":"21298097","id":"PMC_21298097","title":"Mammalian frataxin: an essential function for cellular viability through an interaction with a preformed ISCU/NFS1/ISD11 iron-sulfur assembly complex.","date":"2011","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/21298097","citation_count":190,"is_preprint":false},{"pmid":"10393315","id":"PMC_10393315","title":"Hyperproduction of recombinant ferredoxins in escherichia coli by coexpression of the ORF1-ORF2-iscS-iscU-iscA-hscB-hs cA-fdx-ORF3 gene cluster.","date":"1999","source":"Journal of biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10393315","citation_count":173,"is_preprint":false},{"pmid":"18304497","id":"PMC_18304497","title":"Splice mutation in the iron-sulfur cluster scaffold protein ISCU causes myopathy with exercise intolerance.","date":"2008","source":"American journal of human 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lipopolysaccharide.","date":"2007","source":"Archives of biochemistry and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/17603005","citation_count":9,"is_preprint":false},{"pmid":"29210568","id":"PMC_29210568","title":"Comparison of the Response of Bacterial IscU and SufU to Zn2+ and Select Transition-Metal Ions.","date":"2018","source":"ACS chemical biology","url":"https://pubmed.ncbi.nlm.nih.gov/29210568","citation_count":8,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":49881,"output_tokens":10991,"usd":0.157254,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":22022,"output_tokens":6156,"usd":0.132005,"stage2_stop_reason":"end_turn"},"total_usd":0.289259,"stage1_batch_id":"msgbatch_01DUPACZi7AAWwuZiUjseQF4","stage2_batch_id":"msgbatch_018yt7chE3f48PEFtDwtj6AE","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2000,\n      \"finding\": \"IscU serves as a scaffold for sequential assembly of [2Fe-2S] and [4Fe-4S] clusters: IscS-mediated assembly on IscU proceeds first to one [2Fe-2S]2+ cluster per dimer, then two [2Fe-2S]2+ clusters per dimer, and finally one [4Fe-4S]2+ cluster per dimer. Both cluster types are reductively labile. [2Fe-2S] clusters show incomplete cysteinyl ligation and [4Fe-4S] cluster has non-cysteinyl ligation at a unique Fe site.\",\n      \"method\": \"Anaerobic anion exchange chromatography, UV-visible absorption, resonance Raman, and Mössbauer spectroscopies\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal spectroscopic methods (Mössbauer, resonance Raman, UV-vis) with rigorous in vitro reconstitution, widely replicated\",\n      \"pmids\": [\"10891064\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"IscS transfers sulfur directly to IscU via persulfide intermediates on IscU cysteine residues. IscU binds IscS with high affinity (Kd ~2 µM), sulfur transfer is inhibited by iodoacetamide treatment of IscU, and 35S is released by reducing agents. The C-terminal region of IscS (residues 376–413) is important for IscU binding and sulfur transfer.\",\n      \"method\": \"[35S]-cysteine radiotracer studies, surface plasmon resonance, isothermal titration calorimetry, deletion mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct biochemical reconstitution with radiolabeling, SPR, ITC, and mutagenesis; multiple orthogonal methods\",\n      \"pmids\": [\"11577100\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"IscU and its iron-sulfur complex (IscU-Fe/S) stimulate the ATPase activity of the Hsc66 chaperone up to 480-fold in the presence of the co-chaperone Hsc20. Hsc20 enhances binding of IscU to Hsc66, and IscU and Hsc20 form a direct complex, suggesting Hsc20 aids targeting of IscU to Hsc66.\",\n      \"method\": \"ATPase activity assays, surface plasmon resonance, isothermal titration calorimetry\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal biochemical methods (ATPase assay, SPR, ITC) in a single rigorous study\",\n      \"pmids\": [\"10869428\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"IscU behaves as a substrate for the Hsc66 chaperone, interacting with the substrate-binding domain: IscU inhibited Hsc66 suppression of rhodanese and citrate synthase aggregation, and ATP destabilizes Hsc66·IscU complexes in a manner expected for Hsp70-substrate complexes. IscU binds the ATPase + beta-sandwich subdomain but not the isolated ATPase domain of Hsc66.\",\n      \"method\": \"Chaperone aggregation suppression assay, calorimetry, surface plasmon resonance, truncation mutant binding studies\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal biochemical methods; direct mechanistic characterization of IscU-Hsc66 interaction\",\n      \"pmids\": [\"11053447\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Hsc66 interacts specifically with a region of IscU (residues 99–103, LPPVK) identified by phage display and cellulose-bound peptide array. A synthetic peptide ELPPVKIHC (residues 98–106) stimulates Hsc66 ATPase with similar affinity as native IscU but is not bound by Hsc20 and does not synergistically stimulate Hsc66 with Hsc20, indicating Hsc66 and Hsc20 recognize distinct regions of IscU.\",\n      \"method\": \"Phage display, peptide array, ATPase stimulation assay, peptide-binding domain competition assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal methods (phage display, peptide array, functional ATPase assays) identifying a specific binding motif\",\n      \"pmids\": [\"11994302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Crystal structure of HscA substrate-binding domain complexed with IscU-derived peptide ELPPVKIHC at 1.95 Å resolution. The peptide binds in an extended conformation in a hydrophobic cleft in the beta-subdomain, in reverse orientation relative to DnaK–peptide complexes. The alpha-helical subdomain is shifted up to 10 Å relative to DnaK.\",\n      \"method\": \"X-ray crystallography, molecular replacement\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure at 1.95 Å with full refinement statistics; direct structural evidence of IscU peptide binding mode\",\n      \"pmids\": [\"15351650\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"NMR solution structure of monomeric H. influenzae IscU with zinc bound at the Fe-S cluster assembly site by three cysteines and one histidine. The structure has an alpha-beta sandwich architecture with a three-stranded antiparallel beta-sheet and four alpha-helices. Removal of zinc by chelation results in widespread loss of structure in the apo form.\",\n      \"method\": \"NMR solution structure determination, zinc chelation experiments\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure with functional validation (zinc removal-induced unfolding); single rigorous study with multiple NMR methods\",\n      \"pmids\": [\"15522305\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"HscA/HscB co-chaperone system stimulates [2Fe-2S] cluster transfer from IscU to apo-ferredoxin more than 20-fold in an ATP-dependent reaction requiring both HscB and MgATP. No stimulation was observed without HscB or MgATP, establishing that cluster transfer from IscU is an ATP-dependent process.\",\n      \"method\": \"CD and EPR spectrometry to monitor cluster transfer kinetics, phosphate production assay\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with multiple spectroscopic methods and rigorous controls showing ATP-dependence\",\n      \"pmids\": [\"16964969\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Reductive coupling of two [2Fe-2S]2+ clusters on the IscU homodimer forms a single [4Fe-4S]2+ cluster, demonstrated by UV-visible absorption, Mössbauer, and resonance Raman spectroscopies using dithionite. Isc ferredoxin can also drive partial reductive coupling, suggesting it may be the physiological reductant.\",\n      \"method\": \"UV-visible absorption, Mössbauer spectroscopy, resonance Raman spectroscopy, EPR spectroscopy\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal spectroscopic methods in vitro demonstrating the reductive coupling mechanism\",\n      \"pmids\": [\"17506525\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The [4Fe-4S] cluster-loaded form of IscU, but not the [2Fe-2S] cluster-loaded form, transfers its cluster intact to apo-aconitase A in vitro to activate the enzyme. An Asp39Ala substitution in IscU distinguishes the two cluster-loaded conformers and acts as a dominant-negative inhibitor of wild-type IscU-directed apo-aconitase activation.\",\n      \"method\": \"In vitro aconitase activation assay, mutant IscU inhibition studies\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with direct enzyme activity readout and mutant analysis; orthogonal to spectroscopic cluster characterization\",\n      \"pmids\": [\"17506526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Crystal structure of [2Fe-2S]-bound IscU from Aquifex aeolicus at 2.3 Å reveals a novel asymmetric trimer with only one [2Fe-2S] cluster coordinated by Cys36, His106, Cys63, and Cys107 from a single protomer; the cluster is buried at the inter-protomer interface. The three protomers adopt distinct conformations with substantial rearrangement of loops carrying cluster-ligand residues.\",\n      \"method\": \"X-ray crystallography, multiwavelength anomalous diffraction\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure at 2.3 Å with MAD phasing; first direct structural observation of holo-IscU cluster coordination\",\n      \"pmids\": [\"18723024\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"HscA-mediated acceleration of [2Fe-2S] cluster transfer from IscU to apoferredoxin requires both HscB and ATP hydrolysis. A mutant HscA(T212V) lacking ATPase activity cannot accelerate transfer. ATP addition to the HscA/HscB/IscU complex causes a transient distortion of the IscU-bound cluster detectable by CD, linked to HscA conformational change during T→R transition accompanying ATP hydrolysis.\",\n      \"method\": \"Circular dichroism, ATPase activity assay, limiting reagent experiments with HscA T212V mutant\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with mutagenesis and CD spectroscopy; mechanistically defines ATP hydrolysis requirement\",\n      \"pmids\": [\"18986169\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"IscA acts as an iron donor for Fe-S cluster assembly in IscU under conditions where free iron is limiting (chelated by sodium citrate). IscA binds iron with an association constant of ~3×10^19 M^-1 via invariant Cys-99 and Cys-101 residues. IscA mutants that cannot bind iron fail to deliver iron for IscU cluster assembly.\",\n      \"method\": \"In vitro Fe-S cluster assembly assay under iron-limiting conditions, site-directed mutagenesis of IscA cysteine residues\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with mutagenesis demonstrating iron delivery function; multiple controls\",\n      \"pmids\": [\"15247288\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Cytosolic human ISCU forms a complex in vitro with cytosolic human ISCS (cysteine desulfurase), and together they facilitate efficient de novo [4Fe-4S] cluster formation on IRP1 when incubated with iron and cysteine, demonstrating that de novo Fe-S cluster biogenesis can occur in the cytosol.\",\n      \"method\": \"In vitro cluster assembly assay on IRP1, 35S radiotracer studies for ISCS desulfurase activity, Pichia pastoris overexpression\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct in vitro reconstitution showing cytosolic ISCU/ISCS-dependent cluster assembly on a physiological acceptor\",\n      \"pmids\": [\"16527810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Suppression of human ISCU by RNAi inactivates mitochondrial and cytosolic aconitases in a compartment-specific manner, inappropriately activates iron regulatory proteins (IRP1/IRP2), and disrupts intracellular iron homeostasis, including increased iron uptake. Endogenous ISCU levels are also suppressed by iron deprivation.\",\n      \"method\": \"RNAi knockdown, aconitase enzyme activity assay, IRP RNA-binding assay, iron homeostasis measurements\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean RNAi knockdown with defined compartment-specific enzymatic phenotypes and iron homeostasis readouts in mammalian cells\",\n      \"pmids\": [\"16517407\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"NMR studies of E. coli IscU show that apo-IscU exists as two distinct slowly-interconverting conformations: one largely disordered (D) and one largely ordered (S) except for metal-binding residues. The S-state is stabilized by zinc or by the D39A substitution. HscB binds preferentially to the S-state of IscU and interacts most with the two N-terminal beta-strands and C-terminal alpha-helix.\",\n      \"method\": \"NMR spectroscopy (chemical shift perturbation), zinc addition experiments, mutant analysis\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structural characterization identifying two conformational states; mutant analysis; functional binding studies\",\n      \"pmids\": [\"19492851\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The disordered (D) conformational state of IscU is the preferred substrate for binding to cysteine desulfurase IscS for Fe-S cluster assembly. IscS addition shifts the IscU equilibrium toward the D-state, increasing H/D exchange. IscU variants favoring the S-state show a lag in cluster assembly; those favoring D-state assemble less stable clusters. This establishes functional importance of the S↔D conformational equilibrium.\",\n      \"method\": \"NMR spectroscopy (H/D exchange, chemical shift perturbation), in vitro Fe-S cluster assembly assays with S-state and D-state biased IscU variants\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR and in vitro functional assays with multiple mutants; directly demonstrates conformational state selectivity for IscS binding\",\n      \"pmids\": [\"22203963\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"HscA (Hsp70 chaperone) binds preferentially to the disordered (D) conformational state of IscU, while the J-protein HscB binds preferentially to the structured (S) state. HscA-ADP stabilizes the D-state; IscU is released when HscA binds ATP. This defines a chaperone cycle in which cluster transfer is coupled to ATP hydrolysis and IscU D-state stabilization, followed by HscB release.\",\n      \"method\": \"NMR spectroscopy with S-state and D-state biased IscU variants, chaperone interaction studies\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR with multiple conformationally biased variants; mechanistically defines chaperone state selectivity\",\n      \"pmids\": [\"22782893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The S↔D conformational interconversion of metamorphic IscU involves concerted trans-to-cis isomerization of two conserved peptidyl-prolyl peptide bonds: N13-P14 and P100-P101. The D-state contains an ordered domain stabilizing two cis peptide bonds while P35 and P100 are trans in the S-state.\",\n      \"method\": \"NMR spectroscopy (chemical shift analysis, NOE measurements for prolyl-peptide bond configuration)\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR with rigorous chemical shift and NOE analysis directly identifying the prolyl isomerization mechanism\",\n      \"pmids\": [\"23110687\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Human mtHSP70 (DnaK-type) and NFS1 (cysteine desulfurase) bind preferentially to the disordered (D) state of ISCU, while HSC20 (DnaJ-type) binds preferentially to the structured (S) state. NFS1-ISD11 complex also preferentially binds the D-state. ISD11 does not interact directly with ISCU. [2Fe-2S] and [4Fe-4S] clusters are assembled on ISCU catalyzed by NFS1, at a higher rate with the NFS1-ISD11 complex.\",\n      \"method\": \"NMR spectroscopy with ISCU state-biased variants, ATPase activity assay, in vitro Fe-S cluster assembly assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR with multiple variants; in vitro assembly and ATPase assays; multiple orthogonal approaches in human system\",\n      \"pmids\": [\"23940031\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Mammalian frataxin interacts with a preformed ISCU/NFS1/ISD11 core complex (rather than individual components) to form a quaternary complex of ~190 kDa, with ISCU, NFS1, and ISD11 being the main endogenous frataxin interactors. This interaction defines the essential function of frataxin in Fe-S cluster biosynthesis.\",\n      \"method\": \"Co-immunoprecipitation with recombinant frataxin in mammalian cells, heterologous expression of complex components, size-exclusion chromatography\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP in both directions combined with biochemical reconstitution of the quaternary complex; single lab with two orthogonal approaches\",\n      \"pmids\": [\"21298097\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Frataxin enhances the rate of sulfur transfer from NFS1 persulfide to ISCU cysteine C104, leading to persulfide accumulation on ISCU. Frataxin also enhances sulfur transfer to small thiols (DTT, L-cysteine, GSH) from NFS1 persulfide. This reveals frataxin functions as an enhancer of sulfur transfer within the NFS1-ISD11-ISCU complex.\",\n      \"method\": \"Maleimide-peptide probing of cysteine-persulfide combined with mass spectrometry\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — novel chemical probing method (maleimide-peptide + MS) directly measuring persulfide transfer kinetics; multiple substrate conditions tested\",\n      \"pmids\": [\"25597503\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"FXN (frataxin) tightly binds a single Fe2+ but not Fe3+. FXN does not bind ISCU directly, but both interact with the cysteine desulfurase complex (NIA)2. FXN binds (NIA)2 more strongly in the presence of ISCU. Upon addition of L-cysteine and a reductant (reduced FDX2 or DTT), Fe2+ is released from FXN consistent with Fe2+-FXN being the proximal iron source for Fe-S cluster assembly on ISCU.\",\n      \"method\": \"Isothermal titration calorimetry, multinuclear NMR spectroscopy\",\n      \"journal\": \"Journal of inorganic biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — ITC and NMR together directly measuring binding affinities and iron release under physiological conditions\",\n      \"pmids\": [\"29576242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"mTORC1 associates with ISCU and phosphorylates ISCU at serine 14, stabilizing the ISCU protein. mTORC1 inhibition reduces ISCU levels and prevents Fe-S cluster assembly.\",\n      \"method\": \"Co-immunoprecipitation, in vitro kinase assay, mTORC1 inhibitor treatment, Fe-S cluster assembly readout\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — co-IP and kinase assay identifying a PTM site, but single lab with limited mechanistic follow-up on the phosphorylation consequences\",\n      \"pmids\": [\"23508953\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"HscA/HscB co-chaperone system is required for efficient ATP-dependent [2Fe-2S] cluster transfer from IscU to apo-Grx5 (monothiol glutaredoxin), providing a 700-fold enhancement in transfer rate. This establishes a route for [2Fe-2S] clusters assembled on IscU to be stored and transported by monothiol glutaredoxins.\",\n      \"method\": \"Circular dichroism spectroscopy monitoring cluster transfer, rate measurements ±HscA/HscB/Mg-ATP\",\n      \"journal\": \"Journal of the American Chemical Society\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with quantitative kinetics; directly demonstrates chaperone-dependent transfer to Grx5\",\n      \"pmids\": [\"22963613\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Three-dimensional NMR structures of E. coli apo-IscU (both wild-type predominantly in S-state and D39A variant >95% S-state) were determined. Hydrophobic residues at position 39 stabilize the S-state by reducing loop flexibility around conserved cysteines. Alanine substitutions at positions 90, 107, and 111 stabilize the protein without affecting loop dynamics.\",\n      \"method\": \"NMR spectroscopy (3D structure determination), systematic mutagenesis, stability measurements\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR structure determination combined with systematic mutagenesis; defines structural determinants of conformational states\",\n      \"pmids\": [\"22734684\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"ISCU G50E mutation (found in compound heterozygous myopathy patients) results in compromised interaction with sulfur donor NFS1 and the J-protein HSCB, impairing Fe-S cluster synthesis rate. This leads to defects in electron transport chain complexes, loss of cellular respiration, increased mitochondrial iron, and reactive oxygen species.\",\n      \"method\": \"Biochemical interaction assays (NFS1-ISCU, HSCB-ISCU binding), Fe-S cluster synthesis assay, ETC complex activity assays, ROS measurements in human cell lines and yeast models\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple biochemical assays demonstrating direct mechanistic link between mutation, protein interactions, and cellular phenotype; validated in both human cells and yeast\",\n      \"pmids\": [\"24573684\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"ISCU interacts directly with NFU1, with Kd = 1.1 µM (by ITC). The structured conformer of ISCU binds NFU1 via its cluster-binding region interacting with two alpha-helices in the C-terminal domain of NFU1. ISCU[4Fe-4S] transfers its Fe-S cluster to apo-NFU1 in a chaperone-independent manner, while ISCU[2Fe-2S] does not transfer its cluster to NFU1.\",\n      \"method\": \"NMR spectroscopy, small-angle X-ray scattering, isothermal titration calorimetry, size exclusion chromatography, in vitro cluster transfer assay\",\n      \"journal\": \"Journal of structural biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal biophysical methods (NMR, SAXS, ITC) combined with functional cluster transfer assay; cluster-type specificity demonstrated\",\n      \"pmids\": [\"32151725\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Crystal structure of archaeal IscU (Methanothrix thermoacetophila) dimer at atomic resolution shows two [2Fe-2S] clusters facing each other at the dimer interface, with Asp40 serving as a fourth ligand in addition to three Cys residues. His106 is required for reductive coupling of two [2Fe-2S] to one [4Fe-4S] cluster (EPR-confirmed), without structural alteration by H106A substitution, indicating His106's functional role in cluster conversion.\",\n      \"method\": \"X-ray crystallography, EPR spectroscopy, site-directed mutagenesis\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure combined with EPR and mutagenesis directly demonstrating dimer architecture and His106 role in [4Fe-4S] formation\",\n      \"pmids\": [\"33938220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Iron binds to the assembly site of prokaryotic and eukaryotic ISCU via cysteine residues (1-Cys and 2-Cys forms). The 2-Cys form predominates at pH 8 and correlates with Fe-S cluster assembly activity. X-ray absorption, Mössbauer, NMR, CD, and EPR characterization of the 2-Cys form shows iron is coordinated by four conserved residues (Cys35, Asp37, Cys61, His103) in tetrahedral geometry. An auxiliary non-cysteine iron-binding site in ISCU does not exist.\",\n      \"method\": \"SEC, circular dichroism, Mössbauer, X-ray absorption, NMR, EPR, site-directed mutagenesis\",\n      \"journal\": \"Journal of the American Chemical Society\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — comprehensive multi-spectroscopic characterization with mutagenesis; establishes iron insertion mechanism at atomic level\",\n      \"pmids\": [\"36121382\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"p53 transcriptionally activates ISCU expression by binding to an intronic p53-binding site. Knockdown of ISCU enhances IRP1 binding to ferritin heavy chain mRNA's iron-responsive element, reducing ferritin translation and causing iron accumulation. p53-mediated ISCU induction is part of a cellular iron homeostasis response to DNA damage.\",\n      \"method\": \"ChIP for p53 binding, siRNA knockdown of ISCU, IRP1-IRE RNA-binding assay, ferritin translation assay, iron measurement\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP, knockdown with defined molecular phenotypes (IRP1 binding, ferritin translation), and in vivo mouse data; single lab\",\n      \"pmids\": [\"26560363\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In the IscU·IscS complex, IscU retains a folded (structured) conformation as shown by NMR, contradicting proposals that IscS preferentially binds the unstructured form of IscU. Advanced solution NMR directly observes IscU's folded state within the 110 kDa complex.\",\n      \"method\": \"Solution NMR spectroscopy of the IscS-IscU complex\",\n      \"journal\": \"Chembiochem\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — NMR structural data on complex is direct and rigorous but single study contradicting other reports; moderate confidence given conflicting literature\",\n      \"pmids\": [\"25044349\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"ISCU variants M108I and D39V populate only the structured (S) conformational state. These variants fail to form cysteine desulfurase complexes containing both frataxin (FXN) and ferredoxin (FDX2) simultaneously; when ISCU(M108I) is present, rdFDX2 addition displaces FXN from the [NIA-ISCU-FXN]2 complex, explaining why FXN cannot stimulate cluster assembly on these fully structured ISCU variants with physiological reductant.\",\n      \"method\": \"NMR titration experiments, in vitro Fe-S cluster assembly assay with DTT vs. reduced FDX2, cysteine desulfurase activity assay\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR and functional assays revealing molecular mechanism of FXN bypass; direct demonstration of FDX2-FXN competition on the complex\",\n      \"pmids\": [\"29406711\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"A single intronic mutation in ISCU (IVS5+382G>C) strengthens a weak splice acceptor site, causing retention of intronic sequence in ISCU mRNA. This leads to marked reduction of mitochondrial ISCU protein in skeletal muscle, decreased IRP1 activity, and intracellular iron overload in skeletal muscle, establishing a mechanistic link between ISCU scaffold protein deficiency, Fe-S cluster biogenesis failure, and iron dysregulation.\",\n      \"method\": \"Genetic mapping, mRNA splice analysis, western blot for ISCU protein, IRP1 activity assay, iron content measurements in patient muscle\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple biochemical measurements (protein levels, enzyme activities, iron content) in patient tissues with defined molecular mechanism; replicated by multiple independent groups\",\n      \"pmids\": [\"18304497\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"An intron mutation in ISCU activates cryptic splice sites leading to aberrant mRNA with 100 bp of intron 5, causing a frameshift and premature stop codon. Severe protein deficiency of mitochondrial ISCU is found in homozygous patient skeletal muscle. The phenotype is confirmed as hereditary myopathy with lactic acidosis (HML) mapped to chromosome 12q23.3-24.11.\",\n      \"method\": \"Genetic mapping (lod score 5.26), RT-PCR splice analysis, western blot\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic mapping combined with splice and protein analysis; independently confirmed by at least two separate studies\",\n      \"pmids\": [\"18296749\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Tissue-specific splicing of the ISCU intron mutation accounts for the skeletal muscle-restricted phenotype in hereditary myopathy with lactic acidosis: the highest level of incorrectly spliced ISCU mRNA is in skeletal muscle while normal splicing predominates in heart. Functional loss of Fe-S cluster-carrying enzymes and iron accumulation occur in muscle but not other tissues. Complete homozygous knockout of ISCU in mice causes early embryonic death.\",\n      \"method\": \"RT-PCR splice analysis across tissues, enzyme activity assays, iron measurement, mouse knockout\",\n      \"journal\": \"Human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multi-tissue molecular analysis with functional enzyme readouts; mouse KO provides essential function evidence\",\n      \"pmids\": [\"21165651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"PTBP1 represses defective splicing of ISCU in myopathy patients, dramatically reducing mutant transcripts. IGF2BP1 (which preferentially binds the mutant sequence) and RBM39 shift splicing toward the incorrect form. These splicing factors directly interact with the sequence harboring the ISCU intron mutation.\",\n      \"method\": \"RNA pulldown to identify interacting nuclear factors, splicing reporter assays with siRNA knockdown of individual factors\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct RNA binding and functional splicing assays; single lab but multiple factors analyzed with appropriate controls\",\n      \"pmids\": [\"22125086\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"MyoD-driven muscle differentiation enhances ISCU mRNA mis-splicing in ISCU myopathy patient myoblasts. Oxidative stress (H2O2 treatment or elevated oxygen) further reduces ISCU protein levels in patient cells, an effect preventable by ascorbate pretreatment, indicating that oxidative stress destabilizes the small amounts of correctly spliced ISCU protein produced in patient skeletal muscle.\",\n      \"method\": \"MyoD expression in patient myoblasts, H2O2 and oxygen treatment with western blot for ISCU protein, antioxidant rescue experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-based mechanistic experiments with defined transcription factor (MyoD) and chemical stress conditions; single lab\",\n      \"pmids\": [\"23035118\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Excess copper inhibits Fe-S cluster biogenesis through direct binding to ISCU (as well as ISCA1 and ISCA2) proteins, thereby reducing activity of Fe-S cluster-dependent enzymes and disrupting mitochondrial function. ISCU, ISCA1, and ISCA2 were shown to have strong copper-binding activity.\",\n      \"method\": \"In vitro copper-binding assays with recombinant ISCU, cellular Fe-S enzyme activity assays in Wilson's disease model cells (ATP7A-/-, ATP7B KD), mouse model of Wilson's disease\",\n      \"journal\": \"Free radical biology & medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro binding assay combined with cellular and in vivo models; single lab but multi-system validation\",\n      \"pmids\": [\"37225108\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Ferredoxin, in conjunction with NADPH and ferredoxin-NADP reductase, transfers electrons to the IscS/IscU complex to support Fe-S cluster assembly in vitro. This trio of electron-transfer partners is sufficient for the reaction, albeit slower than DTT-mediated assembly. Ferredoxin does not interfere with CyaY's inhibitory activity despite overlapping binding sites on IscS.\",\n      \"method\": \"In vitro Fe-S cluster assembly assay using NADPH/FNR/Fdx as physiological reductant\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro reconstitution with physiological electron donors; single lab, limited mechanistic follow-up\",\n      \"pmids\": [\"25688831\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"IscU residue Y3 (strictly conserved) is essential for in vivo Fe-S cluster assembly function. Substitutions at Y3 abolish function by impairing functional interaction with IscS and sulfur transfer between IscS and IscU. A second-site suppressor mutation in IscS (A349V) rescues Y3 substitutions, linking Y3 function to the conformational dynamics of the flexible loop of IscS required for sulfur transfer.\",\n      \"method\": \"In vivo genetic complementation, biochemical sulfur transfer assays, second-site suppressor screen\",\n      \"journal\": \"Molecular microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis (suppressor screen) combined with direct biochemical sulfur transfer assays; multiple lines of evidence\",\n      \"pmids\": [\"31532036\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"In vivo conformational interconversion between D-state and S-state of IscU is required for Fe-S cluster assembly and transfer. IscU single amino acid substitutions that bypass the need for HscA/HscB show enhanced D↔S interconversion in CD spectroscopic studies. IscU variants locked perpetually in either state cannot perform their in vivo role even with the chaperone system present.\",\n      \"method\": \"In vivo genetic bypass assay of chaperone requirement, CD spectroscopy of IscU variants, Zn2+-induced S-state stabilization\",\n      \"journal\": \"Molecular microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic and biochemical evidence from multiple IscU variants directly linking conformational dynamics to in vivo function; replicated across variant classes\",\n      \"pmids\": [\"33202070\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Drosophila p38 pathway substrate MK2 kinase phosphorylates IscU in both Drosophila and mammalian cells, linking the p38 stress response pathway to mitochondrial Fe-S cluster metabolism.\",\n      \"method\": \"In vitro kinase assay identifying IscU as MK2 substrate in Drosophila cells and mammalian cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single in vitro kinase assay; no phosphorylation site identified; limited functional follow-up on Fe-S cluster consequences\",\n      \"pmids\": [\"25204651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"ISCU sequestrates p53 in the cytoplasm in macrophages, reducing p53 nuclear localization and relieving transcriptional repression of xCT and Arg1, promoting M2 macrophage polarization. Inhibition of ISCU expression repolarizes macrophages and enhances CD8+ T cell cytotoxicity.\",\n      \"method\": \"Co-immunoprecipitation (ISCU-p53), subcellular fractionation, gene expression analysis, macrophage polarization assays, ISCU knockdown in vivo and in vitro\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — co-IP demonstrating ISCU-p53 interaction with functional macrophage polarization readout; single lab, novel function distinct from canonical Fe-S role\",\n      \"pmids\": [\"40541964\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ISCU is a universally conserved scaffold protein that serves as the central platform for iron-sulfur (Fe-S) cluster biogenesis: it exists as a metamorphic protein toggling between a structured (S) and disordered (D) conformational state via cis-trans prolyl isomerization, where the D-state is preferentially bound by the cysteine desulfurase NFS1/IscS (for sulfur donation as a persulfide) and by the Hsp70 chaperone (mtHSP70/HscA), while the S-state is preferentially bound by the DnaJ co-chaperone (HSC20/HscB); iron is inserted into a tetrahedral assembly site formed by conserved Cys, Asp, and His residues, sulfur is transferred from NFS1 persulfide to ISCU (stimulated by frataxin which also provides Fe2+), [2Fe-2S] clusters assemble first and can be reductively coupled to [4Fe-4S] clusters on IscU dimers, and completed clusters are transferred ATP-dependently to acceptor apo-proteins (ferredoxin, aconitase, NFU1, IRP1) through the HscA/HscB chaperone cycle; loss of ISCU function impairs aconitase and respiratory chain complexes, activates IRP1, and disrupts iron homeostasis, explaining the skeletal muscle myopathy in patients with ISCU splice mutations.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"ISCU is the central, universally conserved scaffold on which mitochondrial and cytosolic iron-sulfur (Fe-S) clusters are assembled de novo and from which they are delivered to apo-acceptor proteins [#0, #14]. Cluster building proceeds sequentially on the ISCU dimer: the cysteine desulfurase NFS1/IscS binds ISCU with high affinity and transfers sulfur from its persulfide to ISCU cysteines [#1], iron is inserted at a tetrahedral assembly site coordinated by conserved Cys, Asp, and His residues (Cys35, Asp37, Cys61, His103) [#29, #6], and two transient [2Fe-2S] clusters are reductively coupled — a step requiring His106 — into a single [4Fe-4S] cluster per dimer [#0, #8, #28]. Frataxin (FXN) acts within the NFS1-ISD11-ISCU complex as a sulfur-transfer enhancer and as the proximal Fe2+ donor, binding the desulfurase complex more tightly in the presence of ISCU and releasing its iron upon reductant addition [#21, #22], with ferredoxin (FDX2)/NADPH/FNR supplying the electrons for assembly and reductive coupling [#39, #8]. ISCU is a metamorphic protein that interconverts between a largely structured (S) and a largely disordered (D) state through concerted cis-trans isomerization of conserved prolyl bonds; NFS1 and the Hsp70 chaperone (HscA/mtHSP70) bind the D-state while the J-protein co-chaperone (HscB/HSC20) binds the S-state, and this S↔D cycling is obligatory in vivo for both assembly and chaperone-driven cluster release [#16, #17, #18, #41]. Completed clusters are delivered ATP-dependently through the HscA/HscB chaperone cycle to acceptors including ferredoxin and the monothiol glutaredoxin Grx5, with cluster-type and conformer specificity dictating fate: [4Fe-4S]-ISCU activates apo-aconitase and transfers chaperone-independently to NFU1, whereas [2Fe-2S]-ISCU does not [#7, #24, #9, #27]. Loss of ISCU function inactivates aconitases and respiratory chain complexes, inappropriately activates iron regulatory proteins (IRP1/IRP2), and disrupts iron homeostasis [#14]. Intronic ISCU splice mutations that strengthen a cryptic splice acceptor cause tissue-specific ISCU deficiency in skeletal muscle, defining hereditary myopathy with lactic acidosis [#33, #34, #35].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Establishing that IscU is a scaffold that builds Fe-S clusters in a defined sequence answered how nascent clusters are assembled before delivery to apo-enzymes.\",\n      \"evidence\": \"anaerobic chromatography with Mössbauer, resonance Raman and UV-vis spectroscopy of reconstituted IscU\",\n      \"pmids\": [\"10891064\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define the iron source in vivo\", \"Coordination ligands of each cluster intermediate not yet resolved structurally\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Demonstrating direct persulfide-based sulfur transfer from IscS to IscU cysteines defined the chemical mechanism of sulfur donation.\",\n      \"evidence\": \"[35S]-cysteine radiotracer, SPR, ITC and IscS deletion mutagenesis\",\n      \"pmids\": [\"11577100\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which specific IscU cysteine receives the persulfide not pinpointed here\", \"Role of conformational state in binding not addressed\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Identifying IscU as a substrate that stimulates the Hsc66/Hsc20 chaperone ATPase established that a dedicated Hsp70/J-protein system services Fe-S scaffold function.\",\n      \"evidence\": \"ATPase assays, SPR, ITC and chaperone aggregation-suppression assays\",\n      \"pmids\": [\"10869428\", \"11053447\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"The functional output of the ATPase cycle (cluster transfer) not yet shown\", \"Binding determinants on IscU undefined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Mapping the LPPVK (residues 99–103) motif as the Hsc66 recognition site and showing Hsc20 binds elsewhere defined how the two chaperones engage distinct IscU regions.\",\n      \"evidence\": \"phage display, peptide arrays and ATPase stimulation assays, later confirmed by a 1.95 Å HscA-peptide crystal structure\",\n      \"pmids\": [\"11994302\", \"15351650\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Peptide binding mode may differ from full-length IscU engagement\", \"How Hsc20 recognizes the structured conformer not addressed at this stage\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Solving the IscU fold with a metal at the assembly site and showing apo-IscU unfolds on metal removal revealed the intrinsic conformational lability central to its function.\",\n      \"evidence\": \"NMR solution structure with zinc, plus zinc chelation experiments; IscA shown separately to act as an iron donor under iron-limiting conditions\",\n      \"pmids\": [\"15522305\", \"15247288\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological identity of the iron donor remained contested\", \"Functional meaning of the order-disorder transition not yet defined\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Showing that cluster transfer from IscU to apo-ferredoxin requires HscB and ATP hydrolysis established that the chaperone cycle drives cluster handoff, not just substrate binding.\",\n      \"evidence\": \"CD/EPR transfer kinetics and phosphate assays; parallel work showed cytosolic human ISCU/ISCS assemble clusters on IRP1, and ISCU RNAi inactivates aconitases and dysregulates iron\",\n      \"pmids\": [\"16964969\", \"16527810\", \"16517407\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism coupling ATP hydrolysis to cluster release not yet structural\", \"Cytosolic vs mitochondrial division of labor unresolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Demonstrating reductive coupling of two [2Fe-2S] into one [4Fe-4S] and cluster-type-specific delivery to aconitase explained how ISCU produces and routes different cluster types.\",\n      \"evidence\": \"Mössbauer/resonance Raman/EPR with dithionite or Isc-ferredoxin reductant, and aconitase activation assays with the D39A discriminating mutant\",\n      \"pmids\": [\"17506525\", \"17506526\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological reductant for coupling not definitively assigned\", \"Structural basis for [4Fe-4S]-specific recognition by acceptors unknown\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Crystallizing holo-IscU with a buried [2Fe-2S] cluster and defining the ATP-hydrolysis requirement of HscA-accelerated transfer provided the structural and energetic basis of cluster handoff.\",\n      \"evidence\": \"MAD-phased crystal structure of [2Fe-2S]-IscU and CD/ATPase analysis with the ATPase-dead HscA T212V mutant\",\n      \"pmids\": [\"18723024\", \"18986169\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Asymmetric oligomeric state may be crystallization-dependent\", \"How cluster distortion drives release into acceptors not fully resolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identifying intronic ISCU splice mutations that deplete mitochondrial ISCU in skeletal muscle linked Fe-S scaffold deficiency to hereditary myopathy with lactic acidosis.\",\n      \"evidence\": \"genetic mapping, splice and protein analysis, IRP1 activity and iron measurements in patient muscle\",\n      \"pmids\": [\"18304497\", \"18296749\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why the phenotype is muscle-restricted not yet explained at this stage\", \"Therapeutic correction of splicing not addressed\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Defining apo-IscU as a metamorphic protein interconverting between disordered (D) and structured (S) states, with HscB binding the S-state, reframed IscU function around conformational dynamics.\",\n      \"evidence\": \"NMR chemical shift perturbation, zinc stabilization, and D39A mutant analysis\",\n      \"pmids\": [\"19492851\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional partner preference of the D-state not yet tested\", \"Atomic basis of the conformational switch unknown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Assigning the D-state to NFS1/IscS binding and assembly, the S-state to HscB, and the D-state to HscA defined a conformation-coupled chaperone cycle, with prolyl isomerization as the molecular switch.\",\n      \"evidence\": \"NMR with conformationally biased variants, H/D exchange, prolyl-bond NOE analysis, and structures of S-state apo-IscU; transfer to Grx5 also shown to be HscA/HscB/ATP-dependent\",\n      \"pmids\": [\"22203963\", \"22782893\", \"23110687\", \"22734684\", \"22963613\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether a prolyl isomerase catalyzes the switch in vivo not established\", \"One NMR study (idx 31) reported IscU stays folded within the IscS complex, conflicting with D-state preference\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Extending the D/S conformer-chaperone model to the human NFS1-ISD11-ISCU-mtHSP70-HSC20 system, and showing mTORC1 phosphorylates and stabilizes ISCU, connected conserved mechanism to mammalian regulation.\",\n      \"evidence\": \"NMR with human ISCU variants, ATPase and assembly assays; co-IP and in vitro kinase assay for mTORC1-ISCU(S14)\",\n      \"pmids\": [\"23940031\", \"23508953\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of S14 phosphorylation beyond stability not defined (Medium-confidence for the kinase link)\", \"ISD11 role clarified only as not binding ISCU directly\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Defining frataxin as a sulfur-transfer enhancer that accumulates persulfide on ISCU C104, and later as the proximal Fe2+ donor, resolved frataxin's biochemical role in cluster assembly.\",\n      \"evidence\": \"maleimide-peptide persulfide probing with MS; ITC/NMR iron-binding and release experiments; FDX2/NADPH/FNR shown sufficient as physiological reductant\",\n      \"pmids\": [\"25597503\", \"29576242\", \"21298097\", \"25688831\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether iron and sulfur are delivered in a fixed order not settled\", \"Quantitative flux through frataxin vs alternative iron sources in vivo unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showing that fully S-state-locked ISCU variants (M108I, D39V) cannot form a productive FXN/FDX2 complex with physiological reductant explained why conformational dynamics are mechanistically essential.\",\n      \"evidence\": \"NMR titrations and assembly assays comparing DTT vs reduced FDX2; FDX2 displaces FXN on locked variants\",\n      \"pmids\": [\"29406711\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo phenotypes of these specific variants not characterized here\", \"Structural arrangement of the FXN/FDX2-containing complex unresolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Establishing direct, conformer- and cluster-type-specific transfer from [4Fe-4S]-ISCU to NFU1, and proving in vivo that D↔S interconversion is required for function, completed the link between conformational cycling and cluster delivery.\",\n      \"evidence\": \"NMR/SAXS/ITC and cluster-transfer assays for NFU1; in vivo genetic bypass and CD spectroscopy of conformationally locked IscU variants\",\n      \"pmids\": [\"32151725\", \"33202070\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Determinants selecting chaperone-independent vs chaperone-dependent transfer routes unresolved\", \"Full acceptor specificity hierarchy not mapped\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Atomic-resolution definition of the dimeric assembly site, His106's role in reductive coupling, and the tetrahedral 2-Cys iron-binding mode finalized the structural model of iron insertion and cluster conversion.\",\n      \"evidence\": \"archaeal IscU crystal structure with EPR/mutagenesis; multi-spectroscopic (XAS, Mössbauer, NMR, CD, EPR) characterization of the iron-loaded site ruling out an auxiliary non-Cys site\",\n      \"pmids\": [\"33938220\", \"36121382\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamics of iron delivery into the assembly site in real time not captured\", \"How the eukaryotic complex coordinates these steps spatially not fully resolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Mechanistically connecting a patient G50E mutation to impaired NFS1/HSCB interaction and downstream respiratory and ROS defects linked ISCU biochemistry to disease pathophysiology.\",\n      \"evidence\": \"interaction and assembly assays, ETC activity and ROS measurements in human cells and yeast; tissue-specific splicing and oxidative-stress destabilization explaining muscle restriction studied separately\",\n      \"pmids\": [\"24573684\", \"21165651\", \"22125086\", \"23035118\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why splicing factors (PTBP1, IGF2BP1, RBM39) regulation is muscle-specific not fully explained (Medium-confidence)\", \"Therapeutic splice correction not demonstrated in patients\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Reports that copper directly inhibits ISCU and that ISCU sequesters p53 in macrophages to drive M2 polarization extend ISCU into metal-toxicity and immune-regulatory contexts beyond canonical assembly.\",\n      \"evidence\": \"in vitro copper-binding and cellular Fe-S enzyme assays in Wilson's disease models; co-IP, fractionation and macrophage polarization assays for ISCU-p53\",\n      \"pmids\": [\"37225108\", \"40541964\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Both are single-lab findings awaiting independent confirmation\", \"Whether the p53-sequestration role is separable from Fe-S scaffold function not established\", \"Physiological copper concentrations achieving inhibition not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How conformational interconversion, iron and sulfur delivery, and chaperone-driven release are coordinated in time and space within the intact eukaryotic assembly complex, and how the conflicting reports of ISCU's folded vs disordered state during desulfurase engagement are reconciled, remain open.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No integrated structure of the full assembly/transfer cycle\", \"Conflicting NMR observations on IscU conformation in the IscS complex (idx 31 vs idx 16) unresolved\", \"In vivo regulation of ISCU phosphorylation and conformational switching poorly defined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [9, 24, 27]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 10, 28]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [21, 29]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [14, 33, 34]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [13, 14]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [0, 7, 14]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [9, 14]}\n    ],\n    \"complexes\": [\n      \"NFS1-ISD11-ISCU cysteine desulfurase complex\",\n      \"ISCU-NFS1-ISD11-frataxin quaternary complex\",\n      \"HscA/HscB-IscU chaperone complex\"\n    ],\n    \"partners\": [\n      \"NFS1\",\n      \"FXN\",\n      \"FDX2\",\n      \"HSCB\",\n      \"HSPA9\",\n      \"NFU1\",\n      \"ISCA1\",\n      \"ISD11\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}