IBA57

IBA57 is a mitochondrial iron-sulfur cluster (ISC) assembly factor that acts at a late step of the pathway to specifically mature [4Fe-4S] proteins—including respiratory complexes I and II, aconitase, and lipoic acid synthase—while being dispensable for [2Fe-2S] protein maturation (PMID:22323289, PMID:25609768). It functions together with ISCA1 and ISCA2 (PMID:22323289), and forms a heterodimeric complex with ISCA2 that bridges a [2Fe-2S] cluster, with the conserved IBA57 family cysteine and three conserved ISCA2 cysteines serving as cluster ligands; cluster binding is required for complex formation, and the assembled [2Fe-2S]-ISCA2-IBA57 complex—generated via a GLRX5 → ISCA2 → IBA57 transfer pathway—can reactivate apo-aconitase in vitro (PMID:30269484). Structurally the complex adopts a dimer-of-dimers organization with ISCA2 providing the homodimerization core and the cluster bridging the ISCA2-IBA57 interface (PMID:31831856). Despite structural homology to tetrahydrofolate-dependent enzymes, IBA57 does not use folate as a cofactor: its THF-binding pocket is constricted, folate-deficient and THF-catalytic-residue mutants retain function, whereas mutation of the invariant surface-exposed cysteine impairs [4Fe-4S] assembly (PMID:36075292). Loss-of-function mutations in IBA57 cause multiple mitochondrial dysfunctions syndrome (MMDS3), acting through protein destabilization or disruption of interface contacts that reduce [4Fe-4S] enzyme activities and protein lipoylation (PMID:23462291, PMID:25971455, PMID:39408793).

1. 2012 High

   Established the central question of where IBA57 acts—defining it as a late-acting, [4Fe-4S]-specific maturation factor rather than a general ISC component.

   Evidence RNAi depletion in HeLa cells with [4Fe-4S] and [2Fe-2S] enzyme activity readouts

   PMID:22323289

   Open questions at the time

   • Did not define the molecular partners or the biochemical mechanism of cluster transfer
   • Did not resolve how specificity for [4Fe-4S] over [2Fe-2S] targets is achieved
2. 2013 Medium

   Demonstrated that a patient missense allele acts by triggering proteolytic loss of IBA57 below a critical threshold, linking protein abundance to ISC function in disease.

   Evidence Complementation in patient fibroblasts/HeLa, immunoblotting, protease inhibitor treatment, enzyme assays

   PMID:23462291

   Open questions at the time

   • Did not identify the protease responsible for degradation
   • Single-lab cell-model study
3. 2015 Medium

   Confirmed in independent human disease contexts that IBA57 loss selectively impairs [4Fe-4S] (complex I/II) maturation and lipoylation while sparing [2Fe-2S] proteins, and that R146W is a null-equivalent allele.

   Evidence Splice analysis and complementation in patient lymphoblasts and IBA57-depleted HeLa cells with enzyme/lipoylation readouts

   PMID:25609768 PMID:25971455

   Open questions at the time

   • Did not map why specific residues are required at the molecular level
   • Patient-cell biochemistry without reconstitution
4. 2017 Low

   Began addressing how IBA57 connects to other ISC factors by showing IBA57 deficiency lowers NFU1 and downstream SDH/LIAS.

   Evidence Immunoblotting and SDH activity staining in patient myoblasts/fibroblasts with rescue

   PMID:28913435

   Open questions at the time

   • No direct mechanism for how IBA57 controls NFU1 levels
   • Correlative immunoblot data in patient cells only, single lab
5. 2018 High

   Resolved the core biochemical mechanism—reconstituting the [2Fe-2S]-bridged ISCA2-IBA57 heterodimer, identifying the cluster ligands, the GLRX5 → ISCA2 → IBA57 transfer route, and showing functional apo-aconitase reactivation.

   Evidence In vitro reconstitution, UV-vis spectroscopy, Fe/S quantification, cysteine mutagenesis, aconitase reactivation, Co-IP

   PMID:30269484

   Open questions at the time

   • Did not provide an atomic-resolution structure of the complex
   • Mechanism of cluster donation to specific [4Fe-4S] apo-targets not detailed
6. 2019 Medium

   Provided a structural framework for the complex, defining a dimer-of-dimers architecture with the cluster at the ISCA2-IBA57 interface and rationalizing the R146W pathogenic mutation.

   Evidence SAXS, bioinformatics-driven docking, biochemical validation

   PMID:31831856

   Open questions at the time

   • Low-resolution model, not crystal/cryo-EM
   • Interface contacts inferred from docking rather than directly resolved
7. 2022 High

   Settled whether IBA57's THF-enzyme homology reflects a folate-dependent mechanism—showing it does not, and pinpointing the invariant surface cysteine as the functionally essential residue.

   Evidence Crystal structure of fungal Iba57, yeast folate/active-site mutant genetics, [4Fe-4S] enzyme assays

   PMID:36075292

   Open questions at the time

   • Did not establish the catalytic role, if any, of the surface cysteine beyond cluster ligation
   • Structure is of a fungal ortholog
8. 2024 Medium

   Extended the structure-function picture by showing a pathogenic variant (G104C) destabilizes IBA57 protein without blocking heterodimer assembly, separating folding/stability defects from complex-formation defects.

   Evidence SEC-MALS, NMR, CD, fluorescence spectroscopy, in vitro reconstitution

   PMID:39408793

   Open questions at the time

   • No in vivo functional validation of the variant
   • Single-lab biophysical study

• How the [2Fe-2S]-ISCA2-IBA57 complex selectively converts to and donates a [4Fe-4S] cluster to specific apo-target proteins, and the atomic-resolution structure of the cluster-bound complex, remain unresolved.
• No high-resolution structure of the cluster-bound human complex
• Mechanism of reductive [2Fe-2S]→[4Fe-4S] conversion and target selection undefined
• Direct mechanism linking IBA57 to NFU1 regulation unknown