IBA57

IBA57 is a mitochondrial iron-sulfur cluster assembly factor that functions late in the ISC pathway to specifically mature [4Fe-4S] proteins—including aconitase, respiratory complexes I and II, and lipoic acid synthase—without affecting [2Fe-2S] protein biogenesis (PMID:22323289). It forms a [2Fe-2S]-bridged heterodimeric complex with ISCA2, receiving the cluster via a GLRX5→ISCA2→IBA57 transfer pathway, with its conserved surface cysteine serving as a cluster ligand essential for function (PMID:30269484, PMID:36075292). IBA57 acts upstream of NFU1 in the mitochondrial [4Fe-4S] assembly hierarchy, and despite structural homology to tetrahydrofolate-dependent GcvT enzymes, it does not require folate for its function (PMID:28913435, PMID:36075292). Biallelic loss-of-function mutations in IBA57 cause multiple mitochondrial dysfunctions syndrome 3 (MMDS3), characterized by impaired [4Fe-4S] enzyme activities and reduced protein lipoylation (PMID:23462291, PMID:39408793).

1. 2012 High

   Establishing that IBA57 (with ISCA1/ISCA2) is specifically required for mitochondrial [4Fe-4S] but not [2Fe-2S] protein maturation resolved its position as a late-acting, substrate-specific factor in the ISC pathway.

   Evidence RNAi depletion in HeLa cells with measurement of multiple [4Fe-4S] and [2Fe-2S] enzyme activities

   PMID:22323289

   Open questions at the time

   • Biochemical mechanism of IBA57 action (cluster transfer vs. scaffold) was not determined
   • Physical interactions among ISCA1, ISCA2, and IBA57 were not characterized
   • Relationship to downstream factors such as NFU1 was unknown
2. 2013 Medium

   Demonstrating that the pathogenic Q314P mutation destabilizes IBA57 protein and that the resulting deficiency impairs [4Fe-4S] enzyme activities and protein lipoylation linked IBA57 loss to human disease (MMDS3) and confirmed lipoic acid synthase as a downstream target.

   Evidence Patient cell studies, HeLa RNAi complementation, protease inhibitor rescue, enzyme activity assays

   PMID:23462291

   Open questions at the time

   • Only a single pathogenic variant was studied; generalizability of proteolytic degradation mechanism to other variants was unclear
   • Molecular basis of IBA57 instability caused by Q314P was not structurally resolved
3. 2017 Medium

   Placing IBA57 upstream of NFU1 in the [4Fe-4S] assembly hierarchy established a linear epistatic order (IBA57→NFU1→target enzymes) for mitochondrial Fe-S cluster delivery.

   Evidence Immunoblotting and SDH activity measurements in IBA57-deficient patient myoblasts/fibroblasts with rescue

   PMID:28913435

   Open questions at the time

   • Direct physical interaction between IBA57 and NFU1 was not demonstrated
   • Whether IBA57 transfers clusters directly to NFU1 or acts indirectly remained unknown
4. 2018 High

   Reconstituting the [2Fe-2S]-bridged ISCA2-IBA57 heterodimer and mapping the GLRX5→ISCA2→IBA57 cluster transfer pathway defined the molecular mechanism by which IBA57 receives its iron-sulfur cluster and reactivates apo-[4Fe-4S] targets.

   Evidence In vitro reconstitution with NMR, UV-visible spectroscopy, cysteine mutagenesis, and aconitase reactivation assay

   PMID:30269484

   Open questions at the time

   • Conversion of the [2Fe-2S] cluster to a [4Fe-4S] cluster on target proteins was not mechanistically resolved
   • Role of ISCA1 relative to the ISCA2-IBA57 complex remained unclear
   • High-resolution structure of the complex was not obtained
5. 2019 Medium

   SAXS-based structural modeling revealed a dimer-of-dimers architecture for the ISCA2-IBA57 complex and showed that the pathogenic R146W mutation disrupts the heterodimer interface, linking structural organization to disease.

   Evidence Small-angle X-ray scattering with bioinformatics-driven docking and structural modeling

   PMID:31831856

   Open questions at the time

   • Low-resolution SAXS envelope lacks atomic detail; no crystal or cryo-EM structure of the complex exists
   • Functional consequences of R146W on cluster transfer activity were not directly tested
6. 2022 High

   Solving the crystal structure of Iba57 established that despite GcvT-family fold homology, the THF-binding pocket is constricted and folate is dispensable for function, while the invariant surface cysteine is essential—clarifying the catalytic residues versus the inherited structural scaffold.

   Evidence X-ray crystallography of Chaetomium thermophilum Iba57, site-directed mutagenesis, yeast genetic complementation including folate auxotrophs

   PMID:36075292

   Open questions at the time

   • Structure of human IBA57 or its complex with ISCA2 at atomic resolution has not been determined
   • How the surface cysteine participates in [2Fe-2S] to [4Fe-4S] cluster conversion is not mechanistically resolved
7. 2024 Medium

   Biophysical characterization of the G104C pathogenic variant showed that it destabilizes IBA57 without preventing ISCA2 complex formation, establishing protein stability as a disease-determining parameter distinct from cluster-binding competence.

   Evidence SEC-MALS, NMR, circular dichroism, and fluorescence spectroscopy on recombinant wild-type and G104C IBA57

   PMID:39408793

   Open questions at the time

   • Single variant analyzed; unclear whether stability loss is a general mechanism across MMDS3 mutations
   • In vivo consequences of the G104C stability defect on [4Fe-4S] target maturation were not measured

• The mechanism by which the [2Fe-2S]-bridged ISCA2-IBA57 complex converts to or delivers a [4Fe-4S] cluster to target apoproteins, and whether ISCA1 participates in this step, remains unresolved.
• No atomic-resolution structure of the human ISCA2-IBA57 complex exists
• The [2Fe-2S] to [4Fe-4S] cluster conversion mechanism is not reconstituted
• Direct physical handoff from IBA57 complex to NFU1 or target apoproteins has not been demonstrated