[Journal Club] Antagonistic cross-regulation of Fe-S cluster biosynthesis by Frataxin and Ferredoxin-2 with implications for the Friedreich’s ataxia disease
Iron-sulfur (Fe-S) clusters are ubiquitous metallocofactors constituting the active site of a multitude of enzymes involved in essential biochemical functions such as electron transfer, catalysis, protein and DNA synthesis, sulfur donation and signalling. They are assembled de novo by multi-proteins machineries in the form of [2Fe-2S] clusters which serve as building blocks to generate [4Fe-4S] clusters. Their synthesis is tightly regulated to meet cellular needs. Any defect in the synthesis of [2Fe-2S] clusters and their regulation leads to severe diseases such as Friedreich’s ataxia (FRDA), caused by defective expression of frataxin (FXN), a regulatory protein. The Biochemistry of Metalloproteins and Associated Disease team from Institute for Integrative Biology of the Cell (I2BC, CNRS-CEA-Paris-Saclay University, Gif-Sur-Yvette) has uncovered a new regulatory circuit involving an antagonistic interplay between FXN and ferredoxin-2 (FDX2), a key enzyme of the assembly process, allowing a fine-tuning of [2Fe-2S] clusters production in mitochondria.
[2Fe-2S] clusters are assembled on the scaffold protein ISCU2 in mitochondria. The assembly process is initiated by binding of a ferrous iron (Fe2+) to ISCU2. Then sulfur is provided to ISCU2 as a cysteine-bound persulfide (Cys-SSH) by the cysteine desulfurase NFS1 ; the persulfide is cleaved into sulfide by FDX2, which generates [1Fe-1S] precursors that are fused by dimerization of ISCU2 to generate [2Fe-2S] clusters. FXN stimulates the whole process by accelerating persulfide transfer between NFS1 and ISCU2. Using an in vitro reconstituted human system, the BMAD team showed that any deviation from a close-to-equal amount of FXN or FDX2 downregulates Fe-S cluster synthesis. A structure-function analysis using protein-protein interactions studies by Isothermal Calorimetry (ITC) and Flow Induced Dispersion Analysis (FIDA) combined with structural investigations by Alphafold modelling and site-directed mutagenesis revealed that this is due to competition between FXN and FDX2 for the same binding-site on the NFS1-ISCU2 complex. The analysis of the kinetics of persulfide transfer by Alkylation Reduction Band Shift assays showed that this competition impacts the reaction controlled by FXN and FDX2, with higher levels of FXN impairing the persulfide-reductase activity of FDX2 and higher levels of FDX2 slowing down the FXN-accelerated persulfide transfer to ISCU2. This analysis also revealed that FDX2 directly hinders persulfide generation and transfer to ISCU2 by interacting with the persulfide-carrying mobile loop of NFS1. These data indicate that a balanced level of FXN and FDX2 is required to ensure optimal [2Fe-2S] clusters synthesis. Thereby, decreasing the level of FDX2 in a context of FXN deficiency as in FRDA may improve Fe-S cluster biogenesis. Indeed, in collaboration with a team from Paris-Cité University, they found that knocking-down FDX2 expression in a FRDA drosophila model, increases fly lifespan, thus indicating that decreasing the amount of FDX2 under conditions of low FXN level could be beneficial for FRDA.
This work highlights a direct regulation of Fe-S cluster biosynthesis through antagonistic binding of FXN and FDX2 and suggests that decreasing FDX2 in the context of FXN deficiency in FRDA might constitute a novel therapeutic axis.
Article: Cross-regulation of [2Fe–2S] cluster synthesis by ferredoxin-2 and frataxin. K. Want, H. Gorny, E. Turki, M. Noiray, B. Monfort, R. Mor-Gautier, T. Tubiana, E. Jullian, V. Monnier & B. D’Autréaux*. Nature (2025), doi: s41586-025-09822-1. https://www.nature.com/articles/s41586-025-09822-1