Dimeric P‐loop ATPase CFD1 and Related Proteins

Abstract The assembly of iron–sulfur (Fe/S) proteins in living cells requires complex machineries. The initial synthesis of a Fe/S cluster occurs on specialized scaffold proteins rather than on target apoproteins. The scaffold needs to support both facile assembly and ready transfer of clusters to Fe/S trafficking proteins that assist cluster delivery to final recipients. In the cytosolic iron–sulfur protein assembly (CIA) machinery of eukaryotes, the hetero‐oligomeric CFD1‐NBP35 complex is performing a scaffold function. Both proteins belong to the metal binding P‐loop NTPase family whose members typically are involved in metal center assembly. In vivo , Fe/S cluster assembly on CFD1‐NBP35 depends on functional nucleotide binding sites in both proteins, a sulfur compound exported from mitochondria, and the CIA electron transfer chain TAH18‐DRE2. In vitro , CFD1‐NBP35 bind a bridging [4Fe–4S] cluster at two conserved cysteine residues of each protein. The crystal structure of homodimeric CFD1, as a model of the heterodimer, locates these clustercoordinating cysteine residues on exposed loops. The bridging cluster coordination appears to be ideal for both facile cluster assembly and rapid transfer to downstream CIA trafficking proteins. The crucial cellular function of CFD1‐NBP35 is indicated by their essential in vivo requirement for assembly of most cytosolic and nuclear Fe/S proteins.