The Cfd1-Nbp35 complex acts as a scaffold for iron-sulfur protein assembly in the yeast cytosol

Biogenesis of iron-sulfur (Fe-S]) proteins in eukaryotes requires the function of complex proteinaceous machineries in both mitochondria and cytosol. In contrast to the mitochondrial pathway, little is known about Fe-S] protein assembly in the cytosol. So far, four highly conserved proteins (Cfd1, Nbp35, Nar1 and Cia1) have been identified as members of the cytosolic Fe-S] protein assembly machinery, but their molecular function is unresolved. Using in vivo and in vitro approaches, we found that the soluble P-loop NTPases Cfd1 and Nbp35 form a complex and bind up to three 4Fe-4S] clusters, one at the N terminus of Nbp35 and one each at a new C-terminal cysteine-rich motif present in both proteins. These labile Fe-S] clusters can be rapidly transferred and incorporated into target Fe-S] apoproteins in a Nar1- and Cia1-dependent fashion. Our data suggest that the Cfd1-Nbp35 complex functions as a novel scaffold for Fe-S] cluster assembly in the eukaryotic cytosol.