ISCA2

ISCA2 is a mitochondrial iron-sulfur cluster assembly factor that functions late in the ISC pathway, specifically dedicated to the maturation of [4Fe-4S] proteins including aconitase, respiratory complexes, lipoyl synthase, and biotin synthase, while dispensable for [2Fe-2S] protein biogenesis (PMID:22323289, PMID:21987576). ISCA2 forms a heterodimer with ISCA1 that receives [2Fe-2S] clusters from GLRX5 and catalyzes their reductive fusion into [4Fe-4S] clusters via FDX2/FDXR-dependent electron transfer, with IBA57 required for this process; ISCA2 also forms a [2Fe-2S]-bridged complex with IBA57 through its three conserved cysteine residues (PMID:32817474, PMID:30269484). ISCA1 orchestrates transfer of the assembled [4Fe-4S] cluster from the ISCA1–ISCA2 complex to downstream acceptors such as NFU1 through a transient ternary complex (PMID:33711344). Biallelic loss-of-function mutations in ISCA2 cause a mitochondrial neurodegenerative white matter disease characterized by mitochondrial depletion, loss of respiratory chain activities, and impaired [4Fe-4S] enzyme function (PMID:25539947, PMID:29297947).

1. 2000 High

   Establishing the subcellular location and essential residues of the ISCA2 ortholog: yeast Isa2p was shown to localize to mitochondria via a bipartite leader sequence, with three invariant cysteines required for function, providing the first molecular handle on this protein family.

   Evidence Deletion mutagenesis, subcellular fractionation, and genetic complementation in S. cerevisiae

   PMID:10805735

   Open questions at the time

   • Precise sub-mitochondrial localization of the human ortholog was not resolved
   • Iron-binding capacity not yet demonstrated
   • Molecular partners unknown
2. 2007 Medium

   Yeast Isa2 was shown to be required for the catalytic function of the [4Fe-4S] enzyme biotin synthase (Bio2), acting downstream of initial cluster assembly, hinting that the Isa proteins serve a late cluster-maturation or delivery role rather than de novo scaffold function.

   Evidence Genetic epistasis, enzyme activity assays, and overexpression studies in S. cerevisiae

   PMID:17259550

   Open questions at the time

   • Whether Isa2 directly contacts Bio2 was not tested
   • Mechanism of action on Bio2 catalysis versus stability unresolved
3. 2010 Medium

   The physical interaction between the ISCA2 ortholog and the [2Fe-2S] donor GLRX5 was demonstrated in vivo, connecting the late ISC assembly system to the upstream cluster scaffold pathway.

   Evidence Bimolecular fluorescence complementation and genetic suppression in fission yeast

   PMID:20085751

   Open questions at the time

   • Direction of cluster transfer not established
   • Interaction stoichiometry unknown
   • Human ortholog interaction not validated
4. 2011 High

   The yeast Isa1–Isa2 complex was shown to bind iron in vivo and to function specifically in de novo mitochondrial [4Fe-4S] cluster synthesis, not [2Fe-2S] protein maturation or cytosolic Fe-S assembly, establishing the dedicated role of this complex.

   Evidence In vivo iron-binding assays, comprehensive deletion analysis, and enzyme activity measurements in S. cerevisiae

   PMID:21987576

   Open questions at the time

   • Whether iron binding is direct or cluster-mediated was unclear
   • Biochemical reconstitution of the assembly reaction not yet achieved
5. 2012 High

   Human ISCA2 (with ISCA1 and IBA57) was confirmed as specifically required for mitochondrial [4Fe-4S] protein maturation in mammalian cells, with depletion causing dramatic mitochondrial morphological collapse, translating the yeast findings to human biology.

   Evidence RNAi knockdown in HeLa cells with enzyme activity assays and electron microscopy

   PMID:22323289

   Open questions at the time

   • Individual contributions of ISCA1, ISCA2, and IBA57 were not separated
   • Mechanism of mitochondrial morphology disruption unknown
6. 2014 Medium

   A causative link between ISCA2 deficiency and human disease was established: a founder missense mutation caused mitochondrial neurodegenerative white matter disease with reduced complex I activity and decreased expression of the entire late ISC machinery (ISCA1, ISCA2, IBA57).

   Evidence Exome sequencing, patient fibroblast biochemistry, and immunohistochemistry

   PMID:25539947

   Open questions at the time

   • Rescue experiment not performed
   • Whether ISCA2 mutation destabilizes the entire late ISC complex or triggers secondary transcriptional effects unknown
7. 2018 High

   The molecular basis of ISCA2–IBA57 interaction was revealed: the two proteins form a heterodimer bridged by a [2Fe-2S] cluster coordinated by ISCA2's three conserved cysteines and IBA57's conserved cysteine, and this complex can reactivate apo-aconitase, defining a cluster-bridged intermediate in the assembly pathway.

   Evidence In vitro reconstitution with spectroscopic characterization and cysteine mutagenesis

   PMID:30269484

   Open questions at the time

   • Relationship of this [2Fe-2S] intermediate to the final [4Fe-4S] product not resolved
   • Whether this complex forms in vivo not confirmed
8. 2018 Medium

   Further patient and knockdown studies confirmed ISCA2 loss specifically impairs [4Fe-4S] but not [2Fe-2S] proteins, and additionally causes mtDNA depletion and reduced mitochondrial membrane potential, broadening the phenotypic consequences.

   Evidence Patient fibroblast analysis, siRNA knockdown, respiratory complex activity assays

   PMID:29297947

   Open questions at the time

   • Mechanism linking [4Fe-4S] cluster deficiency to mtDNA depletion not established
9. 2019 Medium

   Low-resolution structural characterization revealed that the ISCA2–IBA57 complex adopts a dimer-of-dimers quaternary structure with ISCA2 providing the homodimerization core, and the pathogenic IBA57 R146W mutation was mapped to this interface.

   Evidence SAXS and computational docking

   PMID:31831856

   Open questions at the time

   • No high-resolution crystal or cryo-EM structure available
   • Dimer-of-dimers stoichiometry not validated by independent method
10. 2020 High

    The complete [4Fe-4S] cluster assembly mechanism was reconstituted in vitro: GLRX5 donates two [2Fe-2S] clusters to the ISCA1–ISCA2 complex, and FDX2 (not FDX1) coupled to FDXR provides electrons for reductive fusion to form [4Fe-4S], with IBA57 required; this was the first artificial-reductant-free reconstitution of mitochondrial [4Fe-4S] maturation.

    Evidence Complete in vitro reconstitution with defined human ISC components and spectroscopic monitoring

    PMID:32817474

    Open questions at the time

    • Kinetic parameters and rate-limiting steps not determined
    • Structural mechanism of reductive fusion not resolved
11. 2021 High

    The cluster handoff mechanism was elucidated: ISCA1 acts as the central orchestrator, interacting with both ISCA2 and NFU1 to form a transient ternary complex through which [4Fe-4S] clusters are transferred to downstream acceptors; ISCA2 and NFU1 do not interact directly.

    Evidence NMR-based interaction mapping and cluster transfer assays with purified human proteins

    PMID:33711344

    Open questions at the time

    • Whether ISCA1 also transfers clusters to targets other than NFU1 not addressed
    • Ternary complex structure not determined
12. 2022 Medium

    Downstream consequences of ISCA2 deficiency were connected to iron homeostasis: ISCA2 knockdown impairs cytoplasmic aconitase/IRP1, which then represses ALAS2 translation and heme synthesis in erythroid cells, and separately decreases HIF-2α translation via IRE-dependent mechanisms in renal carcinoma, with ISCA2 inhibition also triggering ferroptosis.

    Evidence siRNA knockdown in K562 and renal carcinoma cells, IRP1 activity assays, HIF-2α protein measurements, xenograft models

    PMID:35714932 PMID:36097192

    Open questions at the time

    • Whether ferroptosis is a direct consequence of [4Fe-4S] loss or secondary iron dysregulation is unclear
    • In vivo relevance in normal erythropoiesis not validated
13. 2023 Medium

    ISCA2 was identified as a direct target of copper toxicity: high-affinity copper binding to ISCA2 inhibits iron-sulfur cluster assembly, providing a molecular explanation for Fe-S deficits in copper overload conditions such as Wilson's disease.

    Evidence In vitro copper-binding assays, cellular Fe-S activity measurements, and Wilson's disease mouse model

    PMID:37225108

    Open questions at the time

    • Structural basis of copper versus iron discrimination unknown
    • Whether copper binding is reversible in vivo not tested
    • Relative contribution of ISCA2 versus ISCA1/ISCU copper inhibition not separated

• Key unresolved questions include the high-resolution structure of the ISCA1–ISCA2 heterodimer and its [4Fe-4S]-bound state, the structural mechanism of reductive [2Fe-2S]-to-[4Fe-4S] cluster fusion, the full range of [4Fe-4S] client proteins receiving clusters through this pathway, and the mechanism by which ISCA2 deficiency leads to mtDNA depletion and mitochondrial morphological collapse.
• No high-resolution structure of ISCA1–ISCA2 complex
• Reductive fusion mechanism structurally unresolved
• Complete client protein inventory unknown
• mtDNA depletion mechanism not established