Cross-regulation of [2Fe–2S] cluster synthesis by ferredoxin-2 and frataxin

Iron-sulfur (Fe-S) clusters are essential metallocofactors that perform a multitude of biological functions¹⁻⁷. Their synthesis is tightly regulated and defects in this process lead to severe diseases⁸⁻¹⁰, such as Friedreich's ataxia, which is caused by defective expression of frataxin (FXN)¹¹. However, the underlying mechanisms that regulate this process remain unclear. Here we show that efficient Fe-S cluster assembly requires a fine-tuned balance in the ratio of FXN and ferredoxin-2 (FDX2). Fe-S clusters are assembled on the scaffold protein ISCU2; sulfur is provided as a persulfide by NFS1, and the persulfide is cleaved into sulfide by FDX2 (refs. ¹²,¹³). FXN stimulates the whole process by accelerating the transfer of persulfide to ISCU2 (refs. ¹²,¹⁴,¹⁵). Using an in-vitro-reconstituted human system, we show that any deviation from a close-to-equal amount of FXN and FDX2 downregulates Fe-S cluster synthesis. Structure-function investigation reveals that this is due to competition between FXN and FDX2 and their similar affinities for the same binding site on the NFS1-ISCU2 complex, with higher levels of FXN impairing the persulfide reductase activity of FDX2 and higher levels of FDX2 slowing the FXN-accelerated transfer of persulfide to ISCU2. We also find that FDX2 directly hinders persulfide generation and transfer to ISCU2 by interacting with the persulfide-carrying mobile loop of NFS1. We further show that knocking down the expression of FDX2 increases fly lifespan in a Drosophila model of Friedreich's ataxia. Together, 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 Friedreich's ataxia might constitute a novel therapeutic axis.