BOLA1

BOLA1 is a mitochondrial protein that forms a [2Fe-2S] cluster-bridged heterodimer with the monothiol glutaredoxin GLRX5, coordinating a reduced, Rieske-type [2Fe-2S]1+ cluster via a conserved histidine residue; this complex exhibits high cluster binding affinity and is preferentially formed over the BOLA3–GLRX5 complex (PMID:28483642, PMID:25012657). The holo BOLA1–GLRX5 heterodimer can receive its [2Fe-2S] cluster from ISCU or [2Fe-2S](GS)4 but cannot donate it to downstream apo-protein acceptors, arguing against a cluster trafficking function and instead supporting a redox-regulatory role (PMID:32542995). Consistent with this, BOLA1 knockdown increases oxidation of mitochondrial thiol groups, while its overexpression prevents glutathione-depletion-induced mitochondrial morphology changes, establishing BOLA1 as a regulator of mitochondrial thiol redox homeostasis (PMID:22746225).

1. 2012 Medium

   Establishing that BOLA1 functions in mitochondria and partners with GLRX5 resolved its subcellular context and revealed its role in maintaining mitochondrial thiol redox balance, previously uncharacterized for any human BolA-family member.

   Evidence GFP-fusion imaging, Co-IP of BOLA1–GLRX5, RNAi knockdown and overexpression with redox and morphology phenotype readouts in human cells

   PMID:22746225

   Open questions at the time

   • Single-lab study; independent replication of the redox phenotype is lacking
   • Molecular basis of the BOLA1–GLRX5 interaction and any cluster coordination not yet determined
   • Downstream targets or pathways affected by mitochondrial thiol oxidation upon BOLA1 loss are undefined
2. 2014 High

   Solving BolA crystal structures and demonstrating histidine-mediated Rieske-type [2Fe-2S] cluster coordination in the BolA1–Grx heterodimer provided the first atomic-level explanation for how BolA1 class proteins assemble iron-sulfur clusters distinctly from BolA3 class members.

   Evidence X-ray crystallography of three BolA structures, NMR interaction mapping of BolA–Grx interface, CD/EPR spectroscopy on holo-heterodimer (using plant/bacterial orthologs with relevance to human BOLA1)

   PMID:25012657

   Open questions at the time

   • Structures are from non-human orthologs; human BOLA1–GLRX5 complex structure not yet solved
   • Functional significance of cluster redox state difference (Rieske vs. ferredoxin-like) in vivo is unknown
3. 2017 High

   Spectroscopic characterization of the human BOLA1–GLRX5 complex confirmed it coordinates a reduced Rieske-type [2Fe-2S]1+ cluster with higher affinity than the BOLA3–GLRX5 complex, establishing functional divergence between the two paralog complexes.

   Evidence UV/vis, CD, EPR, and NMR spectroscopy with computational docking on recombinant human proteins reconstituted in vitro

   PMID:28483642

   Open questions at the time

   • In vivo evidence for preferential BOLA1–GLRX5 vs. BOLA3–GLRX5 complex formation is absent
   • Biological consequences of the distinct cluster redox states remain untested in cells
4. 2020 High

   Kinetic cluster transfer experiments showed the BOLA1–GLRX5 heterodimer receives its cluster from ISCU but cannot donate it to downstream acceptors, ruling out a trafficking role and supporting the model that BOLA1 functions as a redox sensor or regulator rather than an iron-sulfur cluster relay.

   Evidence CD spectroscopy-based kinetic assays with defined donor and acceptor proteins in vitro

   PMID:32542995

   Open questions at the time

   • The precise redox-sensing or signaling output of the stable holo-heterodimer is unknown
   • The functional role of the BOLA1 homodimer, which does show cluster exchange, is uncharacterized in vivo
   • No in vivo validation of cluster transfer directionality has been performed

• The specific downstream targets or signaling pathways through which the BOLA1–GLRX5 complex exerts its redox-regulatory function in mitochondria remain to be identified, and no in vivo structural data for the human complex exist.
• No in vivo structure of the human BOLA1–GLRX5 holo-complex
• Downstream effectors or redox targets of BOLA1 function are unidentified
• Physiological phenotypes of BOLA1 loss in animal models have not been reported