{"gene":"ACO1","run_date":"2026-06-09T22:02:39","timeline":{"discoveries":[{"year":1991,"finding":"IRE-BP (IRP1) shares extensive amino acid sequence homology with aconitase and isopropylmalate isomerase, suggesting a structural and functional relationship between this RNA-binding regulatory protein and known isomerases involved in intermediary metabolism.","method":"Computational sequence homology analysis","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 4 / Strong — computational prediction replicated across subsequent experimental work confirming the aconitase/IRP1 dual function","pmids":["1903202"],"is_preprint":false},{"year":1998,"finding":"IRP1 bound to the cap-proximal IRE of ferritin mRNA allows assembly of the cap-binding complex eIF4F but prevents bridging interactions between eIF4F and the small (43S) ribosomal subunit, thereby blocking translation initiation at a step downstream of cap recognition.","method":"Novel in vitro translation initiation factor assembly assay; gel retardation; sucrose gradient analysis","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with mechanistic resolution of the specific translation initiation step blocked, single rigorous study with multiple orthogonal methods","pmids":["9774976"],"is_preprint":false},{"year":1995,"finding":"IRP1 RNA-binding activity can be inhibited in vitro by alkylation of free sulfhydryl groups (N-ethylmaleimide) or oxidation with diamide; in vivo iron regulation of IRP1 activity is predominantly a post-translational process, not dependent on de novo protein synthesis (unlike IRP2).","method":"In vitro RNA-binding inhibition assays; translation inhibitor experiments; immunoblot analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro biochemical assays combined with cell-based pharmacological dissection, replicated across IRP1 and IRP2 comparisons","pmids":["7544791"],"is_preprint":false},{"year":1995,"finding":"Purified recombinant IRP2 inhibits ferritin mRNA translation with molar efficacy equal to that of recombinant IRP1; translational repressor function correlates quantitatively with IRE-binding affinity measured by gel retardation; IRP1 (but not IRP2) is inactivated for RNA binding by N-ethylmaleimide alkylation of cysteines.","method":"In vitro translation assay with purified recombinant proteins; gel retardation (EMSA)","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted in vitro system with purified proteins, two orthogonal methods (EMSA and translation assay), single lab","pmids":["7890603"],"is_preprint":false},{"year":1997,"finding":"IRP1 is activated by extracellular H2O2 even without detectable increases in intracellular H2O2; intracellular H2O2 elevation alone (via catalase inhibition) is not sufficient for IRP1 activation; antimycin A-mediated mitochondrial oxidative stress activates IRP1 with kinetics lagging behind intracellular H2O2 rise, suggesting a direct attack on the 4Fe-4S cluster is an unlikely primary mechanism.","method":"Cellular H2O2 generation systems; FACS-based redox probe (DCFH-DA); EMSA for IRP1 activity","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal experimental conditions (extracellular vs. intracellular H2O2 sources), quantitative dose-response, negative finding mechanistically informative","pmids":["9092514"],"is_preprint":false},{"year":1996,"finding":"Nitric oxide (NO) increases IRP1 IRE-binding activity and simultaneously decreases IRP1 aconitase activity in rat hepatoma cells; NO has no effect on IRP2 IRE-binding activity in the same cells; the IRP1 activation by NO coincides with translational repression of ferritin synthesis.","method":"SNAP-mediated NO generation; cytokine/LPS induction; EMSA; aconitase activity assay; metabolic labeling of ferritin","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (EMSA, enzymatic assay, translational readout) in single lab","pmids":["8639920"],"is_preprint":false},{"year":1998,"finding":"In immunological stimulated (IFN-γ/LPS) murine RAW 264.7 macrophages, IRP1 IRE-binding activity increases 4-fold while IRP2 activity decreases 2-fold; IRP2 decrease is not due to NO production and does not require de novo synthesis; NO released from activated macrophages acts as an intercellular signal to increase IRP1 activity in adjacent cells within 1 hour without requiring de novo protein synthesis, indicating direct action of NO on IRP1.","method":"EMSA; macrophage/target cell co-culture system; pharmacological inhibitors","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal comparisons of IRP1 vs IRP2 with mechanistic dissection using pharmacological tools, two orthogonal approaches (cell-autonomous and intercellular NO signaling)","pmids":["9545264"],"is_preprint":false},{"year":2001,"finding":"H2O2 generated extracellularly by glucose/glucose oxidase perfusion of intact rat liver rapidly activates IRP1 IRE-binding activity as assessed by mobility shift assays with IRP1-specific probes; IRP2 does not respond to H2O2 in this intact organ model, demonstrating IRP1-selective regulation by oxidative stress at the organ level.","method":"Rat liver perfusion model; EMSA with IRP1- and IRP2-specific probes","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — physiologically relevant organ-level model, isoform-specific probes, single lab with rigorous controls","pmids":["11297549"],"is_preprint":false},{"year":2004,"finding":"Endogenous IRP1 is nitrated on tyrosine residues in NO-producing macrophages that also generate superoxide; nitration is accompanied by both aconitase inhibition and loss of IRE-binding activity; inclusion of cis-aconitate to stabilize the [4Fe-4S] cluster prevents nitration, indicating that cluster loss and conformational change are prerequisites for tyrosine nitration. Myeloperoxidase inhibitors reduce IRP1 nitration, implicating the nitrite/H2O2/peroxidase pathway.","method":"Immunoprecipitation of endogenous IRP1; anti-nitrotyrosine Western blot; aconitase activity assay; EMSA; pharmacological inhibitors","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — endogenous protein study with multiple orthogonal methods, mechanistic dissection with cluster-stabilizing and enzyme-inhibiting conditions","pmids":["15258160"],"is_preprint":false},{"year":2005,"finding":"Ser-711 is a phosphorylation site on IRP1 in HEK-293 cells treated with PMA; the S711E phosphomimetic mutant displays negligible IRE-binding and aconitase activities when expressed in mammalian cells, and impairs the first step of the aconitase reaction (citrate→cis-aconitate) more severely than the second step; sequence conservation of Ser-711 across all species is noted.","method":"Site-directed mutagenesis; recombinant protein purification; aconitase activity assay; EMSA; mammalian cell expression","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis combined with in vitro enzymatic assays and cellular validation, single lab with multiple orthogonal methods","pmids":["15636585"],"is_preprint":false},{"year":2007,"finding":"IRP1 assembles a cubane [4Fe-4S] iron-sulfur cluster (ISC) in iron-replete cells, converting it to cytosolic aconitase and inhibiting RNA binding; apo-IRP1 (unable to form ISC, e.g., C437S mutant) is degraded by the ubiquitin-proteasome pathway in an iron-dependent manner requiring ongoing transcription and translation; siRNA knockdown of the cysteine desulfurase Nfs1 (ISC assembly) sensitizes endogenous IRP1 for iron-dependent proteasomal degradation.","method":"Tetracycline-inducible expression of IRP1(C437S) mutant; pulse-chase half-life assay; proteasome inhibitors (MG132, lactacystin); ubiquitination assay; siRNA knockdown of Nfs1","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (pulse-chase, ubiquitination, proteasome inhibition, siRNA), endogenous and overexpressed protein validated, single lab but rigorous","pmids":["17242182"],"is_preprint":false},{"year":2010,"finding":"Extramitochondrial (mature) frataxin directly interacts with cytosolic aconitase/IRP1; this interaction is ISC-dependent; cytosolic aconitase defect and consequent IRP1 activation observed in Friedreich Ataxia cells are reversed by extramitochondrial frataxin action.","method":"Co-immunoprecipitation; frataxin knockdown/overexpression; aconitase activity assay; IRP1 IRE-binding assay in FRDA cells","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct protein-protein interaction by Co-IP, functional rescue experiments, disease cell model, single lab","pmids":["20053667"],"is_preprint":false},{"year":2010,"finding":"IRP1 binds ATP and ADP (approximately 2 molecules per protein, with positive co-operativity); ATP/ADP binding induces a conformational change (altered electrophoretic mobility, shifted CD spectrum) that suppresses IRE-binding activity; IRP1 has ATPase activity; the C437S mutant lacking aconitase activity shows only one ATP-binding site and lacks co-operativity, with increased IRE-binding and lower ATPase activity.","method":"Radiolabeled [α-32P]ATP/ADP binding assays; agarose gel electrophoresis; CD spectroscopy; ATPase activity assay; C437S mutant analysis","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct binding assays with purified recombinant protein, multiple orthogonal biophysical methods, mutagenesis, single lab","pmids":["20569198"],"is_preprint":false},{"year":2002,"finding":"Expression of constitutively active IRP1(C437S) in H1299 and MCF7 cells stabilizes transferrin receptor mRNA and inhibits ferritin mRNA translation via IRE binding; at high cell density, ferritin mRNA translation escapes IRP1(C437S)-mediated repression through a mechanism involving impaired global protein synthesis (increased underphosphorylated 4E-BP1) without altering ferritin mRNA levels or IRE position.","method":"Tetracycline-inducible IRP1(C437S) expression; Northern blot; metabolic labeling; polysome analysis; 4E-BP1 phosphorylation status","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — inducible expression system, multiple molecular readouts, discovery of conditional de-repression mechanism, single lab","pmids":["12052872"],"is_preprint":false},{"year":2004,"finding":"Native PAGE blotting reveals that in most cell types the iron-bound aconitase form of IRP1 is by far the predominant form; cellular iron manipulation causes a shift primarily between free (aconitase) and RNA-bound IRP1 rather than changes in total IRP1 levels; standard EMSA tends to under-evaluate total IRP1 and over-evaluate actual RNA-binding activity.","method":"Non-denaturing PAGE; specific antibody blotting; biochemical fractionation; cell treatments with iron salts, chelators, H2O2","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — novel blotting method validated with recombinant protein and cell/tissue samples, single lab with multiple cell types and conditions","pmids":["14705945"],"is_preprint":false},{"year":2013,"finding":"IRP1 (but not IRP2) is the principal regulator of HIF-2α mRNA translation in vivo; IRP1-null mice display polycythemia due to de-repression of HIF-2α mRNA translation specifically in kidneys, leading to increased renal EPO and elevated serum EPO; IRP1 deficiency also enhances expression of iron transport genes (DCytb, Dmt1, ferroportin) and other HIF-2α targets in duodenum.","method":"Irp1-/- and Irp2-/- mouse models; polysome/ribosome profiling; quantitative mRNA analysis; serum EPO measurement; tissue-specific gene expression","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo genetic model with isoform-specific comparison, ribosome profiling confirming translational regulation, replicated in companion study (PMID:23777768)","pmids":["23395174"],"is_preprint":false},{"year":2013,"finding":"IRP1 deficiency (but not IRP2 deficiency) in mice causes HIF-2α accumulation, elevated EPO, reticulocytosis, polycythemia, suppressed hepatic hepcidin, hyperferremia, and iron depletion in splenic macrophages due to unrestricted ferroportin expression; this demonstrates that translational control of HIF-2α by IRP1 dominates over PHD/VHL-mediated protein stability regulation in young mice.","method":"Irp1-/- and Irp2-/- mouse models; Western blot; qPCR; hematological analysis; serum EPO","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo genetic model, isoform-specific comparison, multiple physiological and molecular readouts, replicated by companion study (PMID:23395174)","pmids":["23777768"],"is_preprint":false},{"year":2017,"finding":"IRP1 RNA-binding inactivation by iron primarily involves [4Fe-4S] cluster insertion by the cytosolic iron-sulfur cluster assembly (CIA) system; FBXL5-mediated proteasomal degradation provides a secondary, synergistic mechanism; suppression of FBXL5 combined with impaired CIA (IRP1(3C>3S) mutant or knockdown of NUBP2/FAM96A) reduces cell viability rescued by iron supplementation; IRP1 phosphorylation at Ser-138 is increased when CIA is inhibited and is required for iron rescue; IRP1 expression induces FBXL5 protein level, forming a negative feedback loop.","method":"siRNA knockdown; inducible mutant IRP1 expression; polyubiquitination assay; phosphorylation analysis; aconitase activity assay; cell viability assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple genetic perturbations and biochemical assays, feedback loop demonstrated, single lab with orthogonal methods","pmids":["28768766"],"is_preprint":false},{"year":2017,"finding":"IRP1 binding to ferritin IRE-RNA and ACO2 IRE-RNA is enthalpy-driven and entropy-favorable; temperature increases IRP1 binding 3-4 fold; Mn2+ (50 µM) increases binding affinity 6-12 fold with altered thermodynamic contributions; different IRE-RNAs (FRT vs. ACO2) show distinct activation energies and thermodynamic profiles for IRP1 binding, demonstrating that conserved sequence differences among IRE-mRNAs selectively influence IRP1 interaction.","method":"Isothermal titration calorimetry; stopped-flow kinetics; fluorescence spectroscopy","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — rigorous thermodynamic and kinetic analyses in vitro, multiple IRE substrates compared, single lab","pmids":["28819260"],"is_preprint":false},{"year":2019,"finding":"The pseudotriloop conformation of the IRE apical loop (C1-G5 pairing) is required for IRP1 (and IRP2) binding; G5-to-A mutation reduces IRP1/2 binding while compensatory C1-to-U mutation restores and even enhances IRP1 binding; deletion of bulged U6 does not significantly affect IRP1 binding but C substitution enhances it; IRP1 shows stronger binding than IRP2 to certain IRE variants (e.g., FTL C1U-G5A); HIF-2α (EPAS1) IRE shows slightly weaker binding than FTL IRE to IRPs.","method":"Yeast three-hybrid (Y3H) system; site-directed mutagenesis of IRE variants","journal":"Gene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo yeast binding system with systematic mutagenesis, multiple IRE variants tested, single lab","pmids":["31200088"],"is_preprint":false},{"year":2020,"finding":"IRP1 is required for iron-starvation-induced FUNDC1-dependent mitophagy; IRP1 binds to a newly characterized IRE in the 5' UTR of Bcl-xL mRNA and suppresses its translation; reduced Bcl-xL releases PGAM5 phosphatase activity, promoting FUNDC1 dephosphorylation and mitophagy activation, coupling ISC biogenesis with selective mitophagy.","method":"IRP1 knockdown; IRE identification in Bcl-xL 5' UTR; luciferase reporter assay; FUNDC1 dephosphorylation assay; mitophagy flux analysis","journal":"Redox biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — identification of novel IRP1 target mRNA (Bcl-xL), functional epistasis linking IRP1 to mitophagy pathway, single lab","pmids":["32795936"],"is_preprint":false},{"year":2022,"finding":"Human IRP1 translocates to the nucleus in a cell-specific and iron-dependent manner: nuclear IRP1 is detected in iron-replete human Huh7 and HepG2 hepatoma cells and human liver sections but not in HeLa cells, mouse embryonic fibroblasts, or primary mouse hepatocytes; pharmacological iron chelation abolishes nuclear IRP1.","method":"Western blotting of subcellular fractions; immunofluorescence; immunohistochemistry; Irp1-/- mouse controls; iron chelation (deferoxamine)","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical fractionation and imaging in multiple cell types/tissues with knockout controls, single lab","pmids":["36142654"],"is_preprint":false},{"year":2025,"finding":"IRP1 binds the HIF-2α IRE with higher affinity than IRP2; this differential binding arises from a bulge uridine in the upper stem of the HIF-2α IRE that specifically impairs IRP2 binding; IRP2 deficiency reduces HIF-2α and EPO expression, compromising stress erythropoiesis, in contrast to IRP1 deficiency which elevates EPO via de-repressed HIF-2α mRNA translation.","method":"IRP1 and IRP2 binding affinity measurements; mutagenesis of HIF-2α IRE bulge uridine; Irp1-/- and Irp2-/- mouse models; EPO and HIF-2α expression analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 / Strong — mechanistic explanation of IRE selectivity by mutagenesis combined with in vivo genetic mouse models, explains divergent phenotypes of IRP1 vs IRP2 deficiency","pmids":["39316647"],"is_preprint":false},{"year":2021,"finding":"ISC synthesis suppression activates IRP2 binding to target mRNAs independent of IRP1, FBXL5, and changes in IRP2 protein level at tissue-level O2 concentrations; deletion of both IRP1 and IRP2 abolishes the iron-starvation response, confirming redundancy; IRP1 binding to target mRNAs is competitively regulated by ISC occupancy.","method":"ISC synthesis inhibition; siRNA knockdown of IRP1, IRP2, FBXL5; IRE-binding assays; cell viability/ferroptosis assays at physiological O2","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 2 / Strong — systematic genetic dissection of IRP1 and IRP2 regulatory mechanisms, double-knockout epistasis, physiological O2 conditions, single lab with rigorous controls","pmids":["34039609"],"is_preprint":false},{"year":2026,"finding":"MUSTN1 directly binds to ACO1 (IRP1), enhancing its interaction with the 3' UTR of TFRC mRNA, thereby promoting TFRC expression and inhibiting SLC39A14, which ultimately alleviates iron accumulation and lipid peroxidation; this identifies ACO1 as a binding partner of MUSTN1 in the context of ferroptosis regulation in skeletal muscle satellite cells.","method":"Co-immunoprecipitation; functional overexpression and knockdown experiments; TFRC 3'-UTR binding assay","journal":"International journal of biological macromolecules","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP identifying direct protein-protein interaction, functional validation, single lab","pmids":["41547500"],"is_preprint":false},{"year":2026,"finding":"IRP1 deficiency in mice causes fasting hypoglycemia and protection against high-fat diet-induced hyperglycemia and hepatic steatosis; Irp1-/- hepatocytes and myotubes show impaired mitochondrial respiratory capacity with a shift from oxidative phosphorylation to glycolytic ATP production; this metabolic rewiring is associated with enhanced insulin sensitivity and increased glucose uptake in skeletal muscle, identifying IRP1 as a regulator of systemic energy homeostasis.","method":"Irp1-/- mouse model; high-fat diet challenge; Seahorse flux analysis; proteomics; metabolomics","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo genetic mouse model with multiple orthogonal metabolic assays (Seahorse, proteomics, metabolomics), clear loss-of-function phenotype","pmids":["41493805"],"is_preprint":false}],"current_model":"ACO1/IRP1 is a bifunctional cytosolic protein that switches between a [4Fe-4S] cluster-containing cytosolic aconitase (enzymatically active, RNA-binding inactive) and an apo-form that binds iron-responsive elements (IREs) in target mRNAs to block translation initiation (by preventing 43S ribosomal subunit recruitment downstream of eIF4F assembly) or stabilize mRNA; the switch is governed by iron-dependent ISC assembly via the CIA pathway, oxidative stress (H2O2, NO, superoxide via cluster disassembly), phosphorylation at Ser-711 (inhibitory) and Ser-138, ATP/ADP binding (suppresses IRE binding), and iron-dependent ubiquitin-proteasomal degradation of the apo-form; IRP1 selectively regulates HIF-2α mRNA translation (due to a bulge-U in the HIF-2α IRE that impairs IRP2 binding), thereby controlling erythropoiesis and systemic iron homeostasis, and also binds a novel IRE in Bcl-xL mRNA to couple iron sensing with mitophagy; nuclear translocation of IRP1 occurs in an iron-replete, cell-specific manner in mammalian hepatoma cells."},"narrative":{"mechanistic_narrative":"ACO1/IRP1 is a bifunctional cytosolic protein that couples cellular iron status to post-transcriptional gene regulation, switching between a [4Fe-4S]-cluster-bearing cytosolic aconitase and an apo-form that binds iron-responsive elements (IREs) in target mRNAs [PMID:1903202, PMID:17242182]. In iron-replete cells the cytosolic iron-sulfur cluster assembly (CIA) machinery inserts a cubane [4Fe-4S] cluster that confers aconitase activity and abolishes RNA binding, whereas iron limitation, oxidative attack, and disruption of cluster assembly drive the apo, IRE-binding state [PMID:17242182, PMID:28768766, PMID:34039609]. When bound to a cap-proximal IRE, IRP1 permits eIF4F assembly but blocks bridging to the 43S ribosomal subunit, repressing translation initiation downstream of cap recognition; binding to other IREs stabilizes target mRNAs such as transferrin receptor mRNA [PMID:9774976, PMID:12052872]. IRP1 activity is tuned by multiple inputs: extracellular H2O2 and nitric oxide activate IRE binding while inactivating aconitase, tyrosine nitration follows cluster loss, ATP/ADP binding induces a conformational change that suppresses IRE binding, and phosphorylation at Ser-711 inhibits both activities while Ser-138 phosphorylation supports iron rescue under CIA stress [PMID:9092514, PMID:8639920, PMID:15258160, PMID:20569198, PMID:28768766]. Apo-IRP1 that cannot form a cluster is degraded by FBXL5-directed ubiquitin-proteasomal turnover, providing a synergistic second layer of control [PMID:17242182, PMID:28768766]. Physiologically, IRP1 is the dominant translational regulator of HIF-2α mRNA—owing to a bulge uridine in the HIF-2α IRE that impairs IRP2 binding—and its loss de-represses HIF-2α to elevate renal EPO, causing polycythemia and systemic iron-handling defects, establishing IRP1 as a master regulator of erythropoiesis and iron homeostasis [PMID:23395174, PMID:23777768, PMID:39316647]. IRP1 also binds a novel IRE in Bcl-xL mRNA to link iron sensing to FUNDC1-dependent mitophagy and regulates systemic energy homeostasis, with deficiency rewiring hepatocyte and muscle metabolism toward glycolysis [PMID:32795936, PMID:41493805]. Frataxin and MUSTN1 are characterized physical partners modulating IRP1 cluster status and its mRNA-binding output [PMID:20053667, PMID:41547500].","teleology":[{"year":1991,"claim":"Establishing that the IRE-binding regulatory protein is structurally an aconitase family member set up the central hypothesis that one protein interconverts between enzyme and RNA-binding regulator.","evidence":"Computational sequence homology analysis comparing IRE-BP with aconitase and isopropylmalate isomerase","pmids":["1903202"],"confidence":"Medium","gaps":["Homology alone did not demonstrate the iron-dependent functional switch","No structural basis for the dual function established"]},{"year":1995,"claim":"Defining iron regulation of IRP1 as post-translational and dissecting cysteine-dependent RNA-binding inactivation distinguished IRP1's regulatory logic from the synthesis-dependent IRP2 and tied RNA binding to free sulfhydryl/cluster status.","evidence":"In vitro RNA-binding inhibition with NEM/diamide, translation-inhibitor experiments, and recombinant IRP1/IRP2 translation-repression comparisons","pmids":["7544791","7890603"],"confidence":"High","gaps":["Did not resolve the cluster-assembly machinery responsible in vivo","Cysteine residues mediating the effect not individually mapped here"]},{"year":1998,"claim":"Resolving the precise translation step blocked by IRP1 showed it acts after cap recognition by preventing eIF4F-43S bridging, defining the molecular mechanism of IRE-mediated translational repression.","evidence":"In vitro translation initiation factor assembly assay, gel retardation, and sucrose gradient analysis on ferritin IRE","pmids":["9774976"],"confidence":"High","gaps":["Mechanism shown for cap-proximal IREs; 3'-UTR mRNA-stabilization mechanism not addressed","Bridging factor identity not pinned down"]},{"year":2001,"claim":"Demonstrating that oxidative stimuli (H2O2, NO, superoxide-driven nitration) selectively activate IRP1 but not IRP2 across cells, macrophages, and intact liver established IRP1 as a redox sensor and argued against simple direct cluster oxidation as the sole activation route.","evidence":"Cellular and intercellular NO/H2O2 systems, rat-liver perfusion with isoform-specific probes, anti-nitrotyrosine IP/Western, aconitase and EMSA assays","pmids":["9092514","8639920","9545264","11297549","15258160"],"confidence":"High","gaps":["The signaling intermediate linking extracellular H2O2 to cluster loss not identified","Quantitative contribution of nitration vs. cluster disassembly in vivo unclear"]},{"year":2005,"claim":"Identifying Ser-711 phosphorylation as inhibitory to both aconitase and IRE-binding activities added a covalent, signal-driven regulatory layer beyond iron and redox control.","evidence":"Site-directed mutagenesis (S711E phosphomimetic), recombinant protein aconitase assays, EMSA, and mammalian cell expression","pmids":["15636585"],"confidence":"High","gaps":["The kinase phosphorylating Ser-711 in vivo not identified","Physiological stimulus driving phosphorylation not defined"]},{"year":2007,"claim":"Showing that apo-IRP1 unable to form a [4Fe-4S] cluster is targeted for iron-dependent ubiquitin-proteasomal degradation, requiring the cysteine desulfurase Nfs1, established degradation as a regulatory fate complementary to the cluster switch.","evidence":"Inducible IRP1(C437S) expression, pulse-chase, proteasome inhibitors, ubiquitination assay, and Nfs1 siRNA","pmids":["17242182"],"confidence":"High","gaps":["The E3 ligase was not identified in this study","Relative weighting of cluster switch vs. degradation in normal physiology unresolved"]},{"year":2010,"claim":"Defining frataxin as a direct, cluster-dependent IRP1 partner and ATP/ADP binding as a conformational suppressor of IRE binding revealed additional protein and nucleotide inputs governing the enzyme/regulator equilibrium.","evidence":"Co-IP and functional rescue in FRDA cells; radiolabeled ATP/ADP binding, CD spectroscopy, and ATPase assays with C437S mutant","pmids":["20053667","20569198"],"confidence":"High","gaps":["Structural basis of ATP/ADP-induced conformational change not solved","In vivo relevance of IRP1 ATPase activity unclear"]},{"year":2013,"claim":"In vivo genetics established IRP1 as the dominant translational regulator of HIF-2α, with loss causing polycythemia, elevated EPO, and disrupted systemic iron handling—placing IRP1 at the apex of erythropoietic and iron-homeostatic control.","evidence":"Irp1-/- and Irp2-/- mice with ribosome profiling, serum EPO measurement, hematological analysis, and tissue gene expression","pmids":["23395174","23777768"],"confidence":"High","gaps":["Did not yet explain the molecular basis of IRP1 vs IRP2 selectivity for the HIF-2α IRE","Age/condition dependence of dominance over PHD/VHL control only partly mapped"]},{"year":2017,"claim":"Integrating CIA-mediated cluster insertion as the primary RNA-binding off-switch with FBXL5-driven degradation as a synergistic, feedback-coupled secondary mechanism, and adding Ser-138 phosphorylation, unified the control logic of IRP1.","evidence":"siRNA of CIA components, inducible IRP1(3C>3S), polyubiquitination and phosphorylation analysis, viability and aconitase assays","pmids":["28768766"],"confidence":"High","gaps":["Kinase for Ser-138 not identified","Quantitative kinetics of cluster insertion vs. degradation in vivo unresolved"]},{"year":2021,"claim":"Genetic dissection at physiological O2 showed ISC occupancy competitively governs IRP1 binding, that IRP2 acts redundantly and independently of IRP1/FBXL5, and that double deletion abolishes the iron-starvation response, defining the division of labor between the two IRPs.","evidence":"ISC synthesis inhibition, IRP1/IRP2/FBXL5 siRNA, IRE-binding and ferroptosis assays at tissue-level O2","pmids":["34039609"],"confidence":"High","gaps":["Tissue-specific apportioning of IRP1 vs IRP2 function in vivo not fully resolved","How O2 tension is sensed mechanistically not detailed"]},{"year":2025,"claim":"Pinpointing a bulge uridine in the HIF-2α IRE that impairs IRP2 binding provided the structural explanation for IRP1's selective control of HIF-2α and the opposite EPO phenotypes of IRP1 vs IRP2 loss.","evidence":"Affinity measurements with both IRPs, mutagenesis of the HIF-2α IRE bulge uridine, and Irp1-/- vs Irp2-/- mouse EPO/HIF-2α analysis","pmids":["39316647"],"confidence":"High","gaps":["Co-structure of IRP1 with the HIF-2α IRE not reported","Whether other IREs use similar bulge-based discrimination not generalized"]},{"year":2026,"claim":"Expanding IRP1's physiological reach, deficiency was shown to rewire hepatic and muscle metabolism toward glycolysis and alter systemic glucose/insulin handling, and MUSTN1 was identified as a partner enhancing IRP1-TFRC mRNA binding in ferroptosis control.","evidence":"Irp1-/- mice with Seahorse flux, proteomics, metabolomics; Co-IP and TFRC 3'-UTR binding assays for MUSTN1","pmids":["41493805","41547500"],"confidence":"High","gaps":["Direct mRNA targets driving the metabolic phenotype not fully enumerated","MUSTN1 interaction is a single-lab Co-IP without structural detail"]},{"year":null,"claim":"How IRP1's nuclear translocation, identified as cell-specific and iron-dependent, contributes mechanistically to gene regulation remains unresolved.","evidence":"Open question following subcellular fractionation/imaging detection of nuclear IRP1 in hepatoma cells and human liver","pmids":[],"confidence":"Medium","gaps":["No nuclear function or nuclear target identified","Import/export machinery for IRP1 unknown","Significance restricted to specific cell types not generalized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[1,3,13,18,19,20,24]},{"term_id":"GO:0016853","term_label":"isomerase activity","supporting_discovery_ids":[0,5,9,10]},{"term_id":"GO:0045182","term_label":"translation regulator activity","supporting_discovery_ids":[1,3,13,15]},{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[4,10,17,23]},{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[12]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[10,14]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[21]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[1,13]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[25]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[4,8,17,23]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[20]}],"complexes":[],"partners":["IRP2","FBXL5","FXN","NFS1","MUSTN1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P21399","full_name":"Cytoplasmic aconitate hydratase","aliases":["Citrate hydro-lyase","Ferritin repressor protein","Iron regulatory protein 1","IRP1","Iron-responsive element-binding protein 1","IRE-BP 1"],"length_aa":889,"mass_kda":98.4,"function":"Bifunctional iron sensor that switches between 2 activities depending on iron availability (PubMed:1281544, PubMed:1946430, PubMed:8041788). Iron deprivation, promotes its mRNA binding activity through which it regulates the expression of genes involved in iron uptake, sequestration and utilization (PubMed:1281544, PubMed:1946430, PubMed:23891004, PubMed:8041788). 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analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 4 / Strong — computational prediction replicated across subsequent experimental work confirming the aconitase/IRP1 dual function\",\n      \"pmids\": [\"1903202\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"IRP1 bound to the cap-proximal IRE of ferritin mRNA allows assembly of the cap-binding complex eIF4F but prevents bridging interactions between eIF4F and the small (43S) ribosomal subunit, thereby blocking translation initiation at a step downstream of cap recognition.\",\n      \"method\": \"Novel in vitro translation initiation factor assembly assay; gel retardation; sucrose gradient analysis\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with mechanistic resolution of the specific translation initiation step blocked, single rigorous study with multiple orthogonal methods\",\n      \"pmids\": [\"9774976\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"IRP1 RNA-binding activity can be inhibited in vitro by alkylation of free sulfhydryl groups (N-ethylmaleimide) or oxidation with diamide; in vivo iron regulation of IRP1 activity is predominantly a post-translational process, not dependent on de novo protein synthesis (unlike IRP2).\",\n      \"method\": \"In vitro RNA-binding inhibition assays; translation inhibitor experiments; immunoblot analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro biochemical assays combined with cell-based pharmacological dissection, replicated across IRP1 and IRP2 comparisons\",\n      \"pmids\": [\"7544791\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Purified recombinant IRP2 inhibits ferritin mRNA translation with molar efficacy equal to that of recombinant IRP1; translational repressor function correlates quantitatively with IRE-binding affinity measured by gel retardation; IRP1 (but not IRP2) is inactivated for RNA binding by N-ethylmaleimide alkylation of cysteines.\",\n      \"method\": \"In vitro translation assay with purified recombinant proteins; gel retardation (EMSA)\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted in vitro system with purified proteins, two orthogonal methods (EMSA and translation assay), single lab\",\n      \"pmids\": [\"7890603\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"IRP1 is activated by extracellular H2O2 even without detectable increases in intracellular H2O2; intracellular H2O2 elevation alone (via catalase inhibition) is not sufficient for IRP1 activation; antimycin A-mediated mitochondrial oxidative stress activates IRP1 with kinetics lagging behind intracellular H2O2 rise, suggesting a direct attack on the 4Fe-4S cluster is an unlikely primary mechanism.\",\n      \"method\": \"Cellular H2O2 generation systems; FACS-based redox probe (DCFH-DA); EMSA for IRP1 activity\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal experimental conditions (extracellular vs. intracellular H2O2 sources), quantitative dose-response, negative finding mechanistically informative\",\n      \"pmids\": [\"9092514\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Nitric oxide (NO) increases IRP1 IRE-binding activity and simultaneously decreases IRP1 aconitase activity in rat hepatoma cells; NO has no effect on IRP2 IRE-binding activity in the same cells; the IRP1 activation by NO coincides with translational repression of ferritin synthesis.\",\n      \"method\": \"SNAP-mediated NO generation; cytokine/LPS induction; EMSA; aconitase activity assay; metabolic labeling of ferritin\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (EMSA, enzymatic assay, translational readout) in single lab\",\n      \"pmids\": [\"8639920\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"In immunological stimulated (IFN-γ/LPS) murine RAW 264.7 macrophages, IRP1 IRE-binding activity increases 4-fold while IRP2 activity decreases 2-fold; IRP2 decrease is not due to NO production and does not require de novo synthesis; NO released from activated macrophages acts as an intercellular signal to increase IRP1 activity in adjacent cells within 1 hour without requiring de novo protein synthesis, indicating direct action of NO on IRP1.\",\n      \"method\": \"EMSA; macrophage/target cell co-culture system; pharmacological inhibitors\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal comparisons of IRP1 vs IRP2 with mechanistic dissection using pharmacological tools, two orthogonal approaches (cell-autonomous and intercellular NO signaling)\",\n      \"pmids\": [\"9545264\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"H2O2 generated extracellularly by glucose/glucose oxidase perfusion of intact rat liver rapidly activates IRP1 IRE-binding activity as assessed by mobility shift assays with IRP1-specific probes; IRP2 does not respond to H2O2 in this intact organ model, demonstrating IRP1-selective regulation by oxidative stress at the organ level.\",\n      \"method\": \"Rat liver perfusion model; EMSA with IRP1- and IRP2-specific probes\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — physiologically relevant organ-level model, isoform-specific probes, single lab with rigorous controls\",\n      \"pmids\": [\"11297549\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Endogenous IRP1 is nitrated on tyrosine residues in NO-producing macrophages that also generate superoxide; nitration is accompanied by both aconitase inhibition and loss of IRE-binding activity; inclusion of cis-aconitate to stabilize the [4Fe-4S] cluster prevents nitration, indicating that cluster loss and conformational change are prerequisites for tyrosine nitration. Myeloperoxidase inhibitors reduce IRP1 nitration, implicating the nitrite/H2O2/peroxidase pathway.\",\n      \"method\": \"Immunoprecipitation of endogenous IRP1; anti-nitrotyrosine Western blot; aconitase activity assay; EMSA; pharmacological inhibitors\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — endogenous protein study with multiple orthogonal methods, mechanistic dissection with cluster-stabilizing and enzyme-inhibiting conditions\",\n      \"pmids\": [\"15258160\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Ser-711 is a phosphorylation site on IRP1 in HEK-293 cells treated with PMA; the S711E phosphomimetic mutant displays negligible IRE-binding and aconitase activities when expressed in mammalian cells, and impairs the first step of the aconitase reaction (citrate→cis-aconitate) more severely than the second step; sequence conservation of Ser-711 across all species is noted.\",\n      \"method\": \"Site-directed mutagenesis; recombinant protein purification; aconitase activity assay; EMSA; mammalian cell expression\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis combined with in vitro enzymatic assays and cellular validation, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"15636585\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"IRP1 assembles a cubane [4Fe-4S] iron-sulfur cluster (ISC) in iron-replete cells, converting it to cytosolic aconitase and inhibiting RNA binding; apo-IRP1 (unable to form ISC, e.g., C437S mutant) is degraded by the ubiquitin-proteasome pathway in an iron-dependent manner requiring ongoing transcription and translation; siRNA knockdown of the cysteine desulfurase Nfs1 (ISC assembly) sensitizes endogenous IRP1 for iron-dependent proteasomal degradation.\",\n      \"method\": \"Tetracycline-inducible expression of IRP1(C437S) mutant; pulse-chase half-life assay; proteasome inhibitors (MG132, lactacystin); ubiquitination assay; siRNA knockdown of Nfs1\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (pulse-chase, ubiquitination, proteasome inhibition, siRNA), endogenous and overexpressed protein validated, single lab but rigorous\",\n      \"pmids\": [\"17242182\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Extramitochondrial (mature) frataxin directly interacts with cytosolic aconitase/IRP1; this interaction is ISC-dependent; cytosolic aconitase defect and consequent IRP1 activation observed in Friedreich Ataxia cells are reversed by extramitochondrial frataxin action.\",\n      \"method\": \"Co-immunoprecipitation; frataxin knockdown/overexpression; aconitase activity assay; IRP1 IRE-binding assay in FRDA cells\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct protein-protein interaction by Co-IP, functional rescue experiments, disease cell model, single lab\",\n      \"pmids\": [\"20053667\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"IRP1 binds ATP and ADP (approximately 2 molecules per protein, with positive co-operativity); ATP/ADP binding induces a conformational change (altered electrophoretic mobility, shifted CD spectrum) that suppresses IRE-binding activity; IRP1 has ATPase activity; the C437S mutant lacking aconitase activity shows only one ATP-binding site and lacks co-operativity, with increased IRE-binding and lower ATPase activity.\",\n      \"method\": \"Radiolabeled [α-32P]ATP/ADP binding assays; agarose gel electrophoresis; CD spectroscopy; ATPase activity assay; C437S mutant analysis\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct binding assays with purified recombinant protein, multiple orthogonal biophysical methods, mutagenesis, single lab\",\n      \"pmids\": [\"20569198\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Expression of constitutively active IRP1(C437S) in H1299 and MCF7 cells stabilizes transferrin receptor mRNA and inhibits ferritin mRNA translation via IRE binding; at high cell density, ferritin mRNA translation escapes IRP1(C437S)-mediated repression through a mechanism involving impaired global protein synthesis (increased underphosphorylated 4E-BP1) without altering ferritin mRNA levels or IRE position.\",\n      \"method\": \"Tetracycline-inducible IRP1(C437S) expression; Northern blot; metabolic labeling; polysome analysis; 4E-BP1 phosphorylation status\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — inducible expression system, multiple molecular readouts, discovery of conditional de-repression mechanism, single lab\",\n      \"pmids\": [\"12052872\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Native PAGE blotting reveals that in most cell types the iron-bound aconitase form of IRP1 is by far the predominant form; cellular iron manipulation causes a shift primarily between free (aconitase) and RNA-bound IRP1 rather than changes in total IRP1 levels; standard EMSA tends to under-evaluate total IRP1 and over-evaluate actual RNA-binding activity.\",\n      \"method\": \"Non-denaturing PAGE; specific antibody blotting; biochemical fractionation; cell treatments with iron salts, chelators, H2O2\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — novel blotting method validated with recombinant protein and cell/tissue samples, single lab with multiple cell types and conditions\",\n      \"pmids\": [\"14705945\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"IRP1 (but not IRP2) is the principal regulator of HIF-2α mRNA translation in vivo; IRP1-null mice display polycythemia due to de-repression of HIF-2α mRNA translation specifically in kidneys, leading to increased renal EPO and elevated serum EPO; IRP1 deficiency also enhances expression of iron transport genes (DCytb, Dmt1, ferroportin) and other HIF-2α targets in duodenum.\",\n      \"method\": \"Irp1-/- and Irp2-/- mouse models; polysome/ribosome profiling; quantitative mRNA analysis; serum EPO measurement; tissue-specific gene expression\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo genetic model with isoform-specific comparison, ribosome profiling confirming translational regulation, replicated in companion study (PMID:23777768)\",\n      \"pmids\": [\"23395174\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"IRP1 deficiency (but not IRP2 deficiency) in mice causes HIF-2α accumulation, elevated EPO, reticulocytosis, polycythemia, suppressed hepatic hepcidin, hyperferremia, and iron depletion in splenic macrophages due to unrestricted ferroportin expression; this demonstrates that translational control of HIF-2α by IRP1 dominates over PHD/VHL-mediated protein stability regulation in young mice.\",\n      \"method\": \"Irp1-/- and Irp2-/- mouse models; Western blot; qPCR; hematological analysis; serum EPO\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo genetic model, isoform-specific comparison, multiple physiological and molecular readouts, replicated by companion study (PMID:23395174)\",\n      \"pmids\": [\"23777768\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"IRP1 RNA-binding inactivation by iron primarily involves [4Fe-4S] cluster insertion by the cytosolic iron-sulfur cluster assembly (CIA) system; FBXL5-mediated proteasomal degradation provides a secondary, synergistic mechanism; suppression of FBXL5 combined with impaired CIA (IRP1(3C>3S) mutant or knockdown of NUBP2/FAM96A) reduces cell viability rescued by iron supplementation; IRP1 phosphorylation at Ser-138 is increased when CIA is inhibited and is required for iron rescue; IRP1 expression induces FBXL5 protein level, forming a negative feedback loop.\",\n      \"method\": \"siRNA knockdown; inducible mutant IRP1 expression; polyubiquitination assay; phosphorylation analysis; aconitase activity assay; cell viability assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple genetic perturbations and biochemical assays, feedback loop demonstrated, single lab with orthogonal methods\",\n      \"pmids\": [\"28768766\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"IRP1 binding to ferritin IRE-RNA and ACO2 IRE-RNA is enthalpy-driven and entropy-favorable; temperature increases IRP1 binding 3-4 fold; Mn2+ (50 µM) increases binding affinity 6-12 fold with altered thermodynamic contributions; different IRE-RNAs (FRT vs. ACO2) show distinct activation energies and thermodynamic profiles for IRP1 binding, demonstrating that conserved sequence differences among IRE-mRNAs selectively influence IRP1 interaction.\",\n      \"method\": \"Isothermal titration calorimetry; stopped-flow kinetics; fluorescence spectroscopy\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — rigorous thermodynamic and kinetic analyses in vitro, multiple IRE substrates compared, single lab\",\n      \"pmids\": [\"28819260\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The pseudotriloop conformation of the IRE apical loop (C1-G5 pairing) is required for IRP1 (and IRP2) binding; G5-to-A mutation reduces IRP1/2 binding while compensatory C1-to-U mutation restores and even enhances IRP1 binding; deletion of bulged U6 does not significantly affect IRP1 binding but C substitution enhances it; IRP1 shows stronger binding than IRP2 to certain IRE variants (e.g., FTL C1U-G5A); HIF-2α (EPAS1) IRE shows slightly weaker binding than FTL IRE to IRPs.\",\n      \"method\": \"Yeast three-hybrid (Y3H) system; site-directed mutagenesis of IRE variants\",\n      \"journal\": \"Gene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo yeast binding system with systematic mutagenesis, multiple IRE variants tested, single lab\",\n      \"pmids\": [\"31200088\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"IRP1 is required for iron-starvation-induced FUNDC1-dependent mitophagy; IRP1 binds to a newly characterized IRE in the 5' UTR of Bcl-xL mRNA and suppresses its translation; reduced Bcl-xL releases PGAM5 phosphatase activity, promoting FUNDC1 dephosphorylation and mitophagy activation, coupling ISC biogenesis with selective mitophagy.\",\n      \"method\": \"IRP1 knockdown; IRE identification in Bcl-xL 5' UTR; luciferase reporter assay; FUNDC1 dephosphorylation assay; mitophagy flux analysis\",\n      \"journal\": \"Redox biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — identification of novel IRP1 target mRNA (Bcl-xL), functional epistasis linking IRP1 to mitophagy pathway, single lab\",\n      \"pmids\": [\"32795936\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Human IRP1 translocates to the nucleus in a cell-specific and iron-dependent manner: nuclear IRP1 is detected in iron-replete human Huh7 and HepG2 hepatoma cells and human liver sections but not in HeLa cells, mouse embryonic fibroblasts, or primary mouse hepatocytes; pharmacological iron chelation abolishes nuclear IRP1.\",\n      \"method\": \"Western blotting of subcellular fractions; immunofluorescence; immunohistochemistry; Irp1-/- mouse controls; iron chelation (deferoxamine)\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical fractionation and imaging in multiple cell types/tissues with knockout controls, single lab\",\n      \"pmids\": [\"36142654\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"IRP1 binds the HIF-2α IRE with higher affinity than IRP2; this differential binding arises from a bulge uridine in the upper stem of the HIF-2α IRE that specifically impairs IRP2 binding; IRP2 deficiency reduces HIF-2α and EPO expression, compromising stress erythropoiesis, in contrast to IRP1 deficiency which elevates EPO via de-repressed HIF-2α mRNA translation.\",\n      \"method\": \"IRP1 and IRP2 binding affinity measurements; mutagenesis of HIF-2α IRE bulge uridine; Irp1-/- and Irp2-/- mouse models; EPO and HIF-2α expression analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mechanistic explanation of IRE selectivity by mutagenesis combined with in vivo genetic mouse models, explains divergent phenotypes of IRP1 vs IRP2 deficiency\",\n      \"pmids\": [\"39316647\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"ISC synthesis suppression activates IRP2 binding to target mRNAs independent of IRP1, FBXL5, and changes in IRP2 protein level at tissue-level O2 concentrations; deletion of both IRP1 and IRP2 abolishes the iron-starvation response, confirming redundancy; IRP1 binding to target mRNAs is competitively regulated by ISC occupancy.\",\n      \"method\": \"ISC synthesis inhibition; siRNA knockdown of IRP1, IRP2, FBXL5; IRE-binding assays; cell viability/ferroptosis assays at physiological O2\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — systematic genetic dissection of IRP1 and IRP2 regulatory mechanisms, double-knockout epistasis, physiological O2 conditions, single lab with rigorous controls\",\n      \"pmids\": [\"34039609\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"MUSTN1 directly binds to ACO1 (IRP1), enhancing its interaction with the 3' UTR of TFRC mRNA, thereby promoting TFRC expression and inhibiting SLC39A14, which ultimately alleviates iron accumulation and lipid peroxidation; this identifies ACO1 as a binding partner of MUSTN1 in the context of ferroptosis regulation in skeletal muscle satellite cells.\",\n      \"method\": \"Co-immunoprecipitation; functional overexpression and knockdown experiments; TFRC 3'-UTR binding assay\",\n      \"journal\": \"International journal of biological macromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP identifying direct protein-protein interaction, functional validation, single lab\",\n      \"pmids\": [\"41547500\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"IRP1 deficiency in mice causes fasting hypoglycemia and protection against high-fat diet-induced hyperglycemia and hepatic steatosis; Irp1-/- hepatocytes and myotubes show impaired mitochondrial respiratory capacity with a shift from oxidative phosphorylation to glycolytic ATP production; this metabolic rewiring is associated with enhanced insulin sensitivity and increased glucose uptake in skeletal muscle, identifying IRP1 as a regulator of systemic energy homeostasis.\",\n      \"method\": \"Irp1-/- mouse model; high-fat diet challenge; Seahorse flux analysis; proteomics; metabolomics\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo genetic mouse model with multiple orthogonal metabolic assays (Seahorse, proteomics, metabolomics), clear loss-of-function phenotype\",\n      \"pmids\": [\"41493805\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ACO1/IRP1 is a bifunctional cytosolic protein that switches between a [4Fe-4S] cluster-containing cytosolic aconitase (enzymatically active, RNA-binding inactive) and an apo-form that binds iron-responsive elements (IREs) in target mRNAs to block translation initiation (by preventing 43S ribosomal subunit recruitment downstream of eIF4F assembly) or stabilize mRNA; the switch is governed by iron-dependent ISC assembly via the CIA pathway, oxidative stress (H2O2, NO, superoxide via cluster disassembly), phosphorylation at Ser-711 (inhibitory) and Ser-138, ATP/ADP binding (suppresses IRE binding), and iron-dependent ubiquitin-proteasomal degradation of the apo-form; IRP1 selectively regulates HIF-2α mRNA translation (due to a bulge-U in the HIF-2α IRE that impairs IRP2 binding), thereby controlling erythropoiesis and systemic iron homeostasis, and also binds a novel IRE in Bcl-xL mRNA to couple iron sensing with mitophagy; nuclear translocation of IRP1 occurs in an iron-replete, cell-specific manner in mammalian hepatoma cells.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"ACO1/IRP1 is a bifunctional cytosolic protein that couples cellular iron status to post-transcriptional gene regulation, switching between a [4Fe-4S]-cluster-bearing cytosolic aconitase and an apo-form that binds iron-responsive elements (IREs) in target mRNAs [#0, #10]. In iron-replete cells the cytosolic iron-sulfur cluster assembly (CIA) machinery inserts a cubane [4Fe-4S] cluster that confers aconitase activity and abolishes RNA binding, whereas iron limitation, oxidative attack, and disruption of cluster assembly drive the apo, IRE-binding state [#10, #17, #23]. When bound to a cap-proximal IRE, IRP1 permits eIF4F assembly but blocks bridging to the 43S ribosomal subunit, repressing translation initiation downstream of cap recognition; binding to other IREs stabilizes target mRNAs such as transferrin receptor mRNA [#1, #13]. IRP1 activity is tuned by multiple inputs: extracellular H2O2 and nitric oxide activate IRE binding while inactivating aconitase, tyrosine nitration follows cluster loss, ATP/ADP binding induces a conformational change that suppresses IRE binding, and phosphorylation at Ser-711 inhibits both activities while Ser-138 phosphorylation supports iron rescue under CIA stress [#4, #5, #8, #12, #17]. Apo-IRP1 that cannot form a cluster is degraded by FBXL5-directed ubiquitin-proteasomal turnover, providing a synergistic second layer of control [#10, #17]. Physiologically, IRP1 is the dominant translational regulator of HIF-2\\u03b1 mRNA\\u2014owing to a bulge uridine in the HIF-2\\u03b1 IRE that impairs IRP2 binding\\u2014and its loss de-represses HIF-2\\u03b1 to elevate renal EPO, causing polycythemia and systemic iron-handling defects, establishing IRP1 as a master regulator of erythropoiesis and iron homeostasis [#15, #16, #22]. IRP1 also binds a novel IRE in Bcl-xL mRNA to link iron sensing to FUNDC1-dependent mitophagy and regulates systemic energy homeostasis, with deficiency rewiring hepatocyte and muscle metabolism toward glycolysis [#20, #25]. Frataxin and MUSTN1 are characterized physical partners modulating IRP1 cluster status and its mRNA-binding output [#11, #24].\",\n  \"teleology\": [\n    {\n      \"year\": 1991,\n      \"claim\": \"Establishing that the IRE-binding regulatory protein is structurally an aconitase family member set up the central hypothesis that one protein interconverts between enzyme and RNA-binding regulator.\",\n      \"evidence\": \"Computational sequence homology analysis comparing IRE-BP with aconitase and isopropylmalate isomerase\",\n      \"pmids\": [\"1903202\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Homology alone did not demonstrate the iron-dependent functional switch\", \"No structural basis for the dual function established\"]\n    },\n    {\n      \"year\": 1995,\n      \"claim\": \"Defining iron regulation of IRP1 as post-translational and dissecting cysteine-dependent RNA-binding inactivation distinguished IRP1's regulatory logic from the synthesis-dependent IRP2 and tied RNA binding to free sulfhydryl/cluster status.\",\n      \"evidence\": \"In vitro RNA-binding inhibition with NEM/diamide, translation-inhibitor experiments, and recombinant IRP1/IRP2 translation-repression comparisons\",\n      \"pmids\": [\"7544791\", \"7890603\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not resolve the cluster-assembly machinery responsible in vivo\", \"Cysteine residues mediating the effect not individually mapped here\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Resolving the precise translation step blocked by IRP1 showed it acts after cap recognition by preventing eIF4F-43S bridging, defining the molecular mechanism of IRE-mediated translational repression.\",\n      \"evidence\": \"In vitro translation initiation factor assembly assay, gel retardation, and sucrose gradient analysis on ferritin IRE\",\n      \"pmids\": [\"9774976\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Mechanism shown for cap-proximal IREs; 3'-UTR mRNA-stabilization mechanism not addressed\", \"Bridging factor identity not pinned down\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Demonstrating that oxidative stimuli (H2O2, NO, superoxide-driven nitration) selectively activate IRP1 but not IRP2 across cells, macrophages, and intact liver established IRP1 as a redox sensor and argued against simple direct cluster oxidation as the sole activation route.\",\n      \"evidence\": \"Cellular and intercellular NO/H2O2 systems, rat-liver perfusion with isoform-specific probes, anti-nitrotyrosine IP/Western, aconitase and EMSA assays\",\n      \"pmids\": [\"9092514\", \"8639920\", \"9545264\", \"11297549\", \"15258160\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"The signaling intermediate linking extracellular H2O2 to cluster loss not identified\", \"Quantitative contribution of nitration vs. cluster disassembly in vivo unclear\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identifying Ser-711 phosphorylation as inhibitory to both aconitase and IRE-binding activities added a covalent, signal-driven regulatory layer beyond iron and redox control.\",\n      \"evidence\": \"Site-directed mutagenesis (S711E phosphomimetic), recombinant protein aconitase assays, EMSA, and mammalian cell expression\",\n      \"pmids\": [\"15636585\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"The kinase phosphorylating Ser-711 in vivo not identified\", \"Physiological stimulus driving phosphorylation not defined\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Showing that apo-IRP1 unable to form a [4Fe-4S] cluster is targeted for iron-dependent ubiquitin-proteasomal degradation, requiring the cysteine desulfurase Nfs1, established degradation as a regulatory fate complementary to the cluster switch.\",\n      \"evidence\": \"Inducible IRP1(C437S) expression, pulse-chase, proteasome inhibitors, ubiquitination assay, and Nfs1 siRNA\",\n      \"pmids\": [\"17242182\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"The E3 ligase was not identified in this study\", \"Relative weighting of cluster switch vs. degradation in normal physiology unresolved\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Defining frataxin as a direct, cluster-dependent IRP1 partner and ATP/ADP binding as a conformational suppressor of IRE binding revealed additional protein and nucleotide inputs governing the enzyme/regulator equilibrium.\",\n      \"evidence\": \"Co-IP and functional rescue in FRDA cells; radiolabeled ATP/ADP binding, CD spectroscopy, and ATPase assays with C437S mutant\",\n      \"pmids\": [\"20053667\", \"20569198\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Structural basis of ATP/ADP-induced conformational change not solved\", \"In vivo relevance of IRP1 ATPase activity unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"In vivo genetics established IRP1 as the dominant translational regulator of HIF-2\\u03b1, with loss causing polycythemia, elevated EPO, and disrupted systemic iron handling\\u2014placing IRP1 at the apex of erythropoietic and iron-homeostatic control.\",\n      \"evidence\": \"Irp1-/- and Irp2-/- mice with ribosome profiling, serum EPO measurement, hematological analysis, and tissue gene expression\",\n      \"pmids\": [\"23395174\", \"23777768\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not yet explain the molecular basis of IRP1 vs IRP2 selectivity for the HIF-2\\u03b1 IRE\", \"Age/condition dependence of dominance over PHD/VHL control only partly mapped\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Integrating CIA-mediated cluster insertion as the primary RNA-binding off-switch with FBXL5-driven degradation as a synergistic, feedback-coupled secondary mechanism, and adding Ser-138 phosphorylation, unified the control logic of IRP1.\",\n      \"evidence\": \"siRNA of CIA components, inducible IRP1(3C>3S), polyubiquitination and phosphorylation analysis, viability and aconitase assays\",\n      \"pmids\": [\"28768766\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Kinase for Ser-138 not identified\", \"Quantitative kinetics of cluster insertion vs. degradation in vivo unresolved\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Genetic dissection at physiological O2 showed ISC occupancy competitively governs IRP1 binding, that IRP2 acts redundantly and independently of IRP1/FBXL5, and that double deletion abolishes the iron-starvation response, defining the division of labor between the two IRPs.\",\n      \"evidence\": \"ISC synthesis inhibition, IRP1/IRP2/FBXL5 siRNA, IRE-binding and ferroptosis assays at tissue-level O2\",\n      \"pmids\": [\"34039609\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Tissue-specific apportioning of IRP1 vs IRP2 function in vivo not fully resolved\", \"How O2 tension is sensed mechanistically not detailed\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Pinpointing a bulge uridine in the HIF-2\\u03b1 IRE that impairs IRP2 binding provided the structural explanation for IRP1's selective control of HIF-2\\u03b1 and the opposite EPO phenotypes of IRP1 vs IRP2 loss.\",\n      \"evidence\": \"Affinity measurements with both IRPs, mutagenesis of the HIF-2\\u03b1 IRE bulge uridine, and Irp1-/- vs Irp2-/- mouse EPO/HIF-2\\u03b1 analysis\",\n      \"pmids\": [\"39316647\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Co-structure of IRP1 with the HIF-2\\u03b1 IRE not reported\", \"Whether other IREs use similar bulge-based discrimination not generalized\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Expanding IRP1's physiological reach, deficiency was shown to rewire hepatic and muscle metabolism toward glycolysis and alter systemic glucose/insulin handling, and MUSTN1 was identified as a partner enhancing IRP1-TFRC mRNA binding in ferroptosis control.\",\n      \"evidence\": \"Irp1-/- mice with Seahorse flux, proteomics, metabolomics; Co-IP and TFRC 3'-UTR binding assays for MUSTN1\",\n      \"pmids\": [\"41493805\", \"41547500\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Direct mRNA targets driving the metabolic phenotype not fully enumerated\", \"MUSTN1 interaction is a single-lab Co-IP without structural detail\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How IRP1's nuclear translocation, identified as cell-specific and iron-dependent, contributes mechanistically to gene regulation remains unresolved.\",\n      \"evidence\": \"Open question following subcellular fractionation/imaging detection of nuclear IRP1 in hepatoma cells and human liver\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No nuclear function or nuclear target identified\", \"Import/export machinery for IRP1 unknown\", \"Significance restricted to specific cell types not generalized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [1, 3, 13, 18, 19, 20, 24]},\n      {\"term_id\": \"GO:0016853\", \"supporting_discovery_ids\": [0, 5, 9, 10]},\n      {\"term_id\": \"GO:0045182\", \"supporting_discovery_ids\": [1, 3, 13, 15]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [4, 10, 17, 23]},\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [12]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [10, 14]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [21]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [1, 13]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [25]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [4, 8, 17, 23]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [20]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"IRP2\", \"FBXL5\", \"FXN\", \"NFS1\", \"MUSTN1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}