BOLA2

BOLA2 is a cytosolic [2Fe-2S] cluster chaperone that operates in the cytosolic iron-sulfur protein assembly pathway by partnering with the monothiol glutaredoxin GRX3/Glrx3 (PMID:27519415, PMID:28615454). Human BOLA2 forms [2Fe-2S]-bridged complexes with each glutaredoxin-like domain of Glrx3, assembling a heterotrimer of two BOLA2 molecules and one GRX3 molecule that coordinates two [2Fe-2S] clusters, each bridged between a BOLA2 subunit and a glutaredoxin domain (PMID:22309771, PMID:26613676). This complex acquires iron upstream from an iron-loaded, glutathione-bound form of PCBP1, which docks onto BOLA2 through a bridging iron ligand to form a PCBP1-Fe-GSH-BolA2 intermediate required for [2Fe-2S] cluster assembly on the BOLA2-Glrx3 complex (PMID:31406370, PMID:34161287). The mature holo-heterotrimer then transfers both [2Fe-2S] clusters to apo-anamorsin/Ciapin1, maturing it as a downstream client, and cellular GRX3·BOLA2 complexes increase several-fold in response to iron (PMID:27519415, PMID:26613676). Loss of Bola2 in mice produces iron-deficiency anemia with reduced hemoglobin, low plasma iron, and microcytosis, and reduced BOLA2 copy number in human 16p11.2 deletion carriers is associated with increased iron-deficiency anemia (PMID:31668704). BOLA2 arose from a human-specific gene duplication whose copy number scales with RNA and protein levels (PMID:27487209), and in breast cancer cells its transcription is activated directly by STAT5 downstream of β1 integrin signaling, with BOLA2 overexpression conferring resistance to doxorubicin-induced apoptosis (PMID:41465298).

1. 2012 High

   Whether human BOLA2 and Glrx3 physically assemble into a defined iron-sulfur cluster complex was unknown; this work established the coordination geometry of a [2Fe-2S]-bridged BOLA2-Glrx3 assembly.

   Evidence Multiple orthogonal spectroscopies (UV-vis/CD, resonance Raman, EPR) on reconstituted recombinant proteins

   PMID:22309771

   Open questions at the time

   • Did not establish the cellular function or downstream clients of the complex
   • Stoichiometry of the in-cell heterotrimer not yet resolved
2. 2015 High

   The atomic architecture and catalytic output of the assembly were unresolved; NMR defined a two-BOLA2/one-GRX3 heterotrimer carrying two [2Fe-2S] clusters and showed it transfers them to apo-anamorsin to produce the holo protein.

   Evidence Atomic-level NMR structural characterization, in vitro reconstitution, and cluster transfer assays to apo-anamorsin/Ciapin1

   PMID:26613676

   Open questions at the time

   • Iron source feeding the complex not identified
   • Transfer kinetics and directionality regulation not defined
3. 2016 High

   Whether the GRX3·BOLA2 complex acts in cells as a chaperone and responds to iron was open; quantitative IP and BioID showed iron-dependent complex formation and functional [2Fe-2S] transfer to Ciapin1 in human cells.

   Evidence Quantitative immunoprecipitation, live-cell BioID proximity labeling, iron titration, and functional Fe-S transfer assays in human cells

   PMID:27519415

   Open questions at the time

   • Mechanism of iron-responsive complex induction not defined
   • Full client repertoire beyond Ciapin1 unknown
4. 2016 Medium

   The evolutionary origin of BOLA2 was unknown; comparative genomics showed it is a human-specific duplication whose copy number scales with RNA and protein expression.

   Evidence Comparative genomic sequencing, copy-number versus RNA/protein quantification, and evolutionary timing analysis

   PMID:27487209

   Open questions at the time

   • Functional consequence of the expansion not tested in this study
   • Role of the novel fusion transcript uncharacterized
5. 2017 Medium

   This synthesis consolidated GRX3 and BOLA2 as a core [2Fe-2S] chaperone module of the cytosolic iron cofactor distribution system.

   Evidence Review integrating fractionation, Co-IP, and in vitro reconstitution evidence

   PMID:28615454

   Open questions at the time

   • No new primary data
   • In vivo physiological role not addressed
6. 2019 High

   How iron is delivered to the BOLA2-Glrx3 complex was unknown; this work identified an iron-loaded, glutathione-bound PCBP1 that docks onto BolA2 through a bridging iron to form the intermediate required for [2Fe-2S] assembly.

   Evidence Proteomics/Co-IP, in vitro reconstitution, and mutagenesis of PCBP1 iron-coordinating residues with cell-based assays

   PMID:31406370

   Open questions at the time

   • Structural detail of the iron hand-off step not resolved
   • Kinetics of intermediate turnover not defined
7. 2019 Medium

   Whether BOLA2 has a physiological role in iron homeostasis was untested; Bola2 knockout mice and human CNV cohort analysis linked BOLA2 loss to iron-deficiency anemia.

   Evidence Bola2 knockout mice with hematological phenotyping plus human 16p11.2 CNV carrier cohort stratified by BOLA2 copy number

   PMID:31668704

   Open questions at the time

   • In vivo molecular pathway connecting BOLA2 loss to anemia not fully resolved
   • Tissue-specific contributions not delineated
8. 2021 High

   Whether PCBP1 iron coordination is functionally required for its interaction with BolA2 was unclear; mutagenesis showed iron binding is necessary for BolA2 (and ferritin) binding and for cell-cycle/DNA-damage control, separable from nucleic-acid-binding activity.

   Evidence Site-directed mutagenesis of PCBP1 iron-coordinating residues, Co-IP, and functional rescue in PCBP1-depleted cells and mouse tissues

   PMID:34161287

   Open questions at the time

   • Direct contribution of BOLA2 to the cell-cycle/DNA-damage phenotype not isolated
   • Quantitative flux through the PCBP1-BolA2 route in vivo unknown
9. 2025 Medium

   How BOLA2 transcription is controlled was unknown; this work placed BOLA2 downstream of β1 integrin–STAT5 signaling, with STAT5 directly activating the BOLA2 promoter and BOLA2 overexpression conferring apoptosis resistance.

   Evidence RNA-seq, luciferase promoter assay, EMSA for STAT5 promoter binding, BOLA2 overexpression rescue of apoptosis, and β1 integrin re-expression epistasis in breast cancer cells

   PMID:41465298

   Open questions at the time

   • Whether the apoptosis resistance depends on BOLA2 Fe-S chaperone activity not tested
   • Generalizability beyond breast cancer cells unknown

• It remains unknown how the iron-responsive induction of the GRX3·BOLA2 complex is mechanistically regulated and how broadly its chaperone activity supplies cytosolic Fe-S clients beyond Ciapin1/anamorsin.
• Full Fe-S client repertoire undefined
• Link between transcriptional control and cluster-transfer flux unresolved
• In vivo iron-homeostasis mechanism not fully mapped