Structural evidence for two-stage binding of mitochondrial ferredoxin 2 to the core iron-sulfur cluster assembly complex

Iron-sulfur (FeS) clusters are ubiquitous metallocofactors that are essential for life. In eukaryotes, FeS cluster biosynthesis begins with the de novo assembly of a [2Fe-2S] cluster by the core iron-sulfur cluster assembly (ISC) complex in the mitochondrial matrix. This complex comprises the scaffold protein ISCU2, the cysteine desulfurase subcomplex NFS1-ISD11-ACP1, the allosteric activator frataxin (FXN) and the electron donor ferredoxin 2 (FDX2). The interaction of FDX2 with the complex remains unclear. Here, we present cryo-EM structures of the FDX2-bound core ISC complex and show that FDX2 and FXN compete for overlapping binding sites during [2Fe-2S] cluster biosynthesis. FDX2 binds in two conformations; in the ‘distal’ conformation, helix F of FDX2 shows loose electrostatic interaction with an arginine patch of NFS1, while in the ‘proximal’ conformation this interaction tightens and the FDX2-specific C terminus forms contacts with NFS1; in this conformation, the [2Fe-2S] cluster of FDX2 is close enough to the ISCU2 FeS cluster assembly site for rapid electron transfer.