A [4Fe-4S] cluster is assembled on a scaffold composed of the P-loop NTPases NUBP2 (CFD1 in yeast) and NUBP1 (Nbp35 in yeast) in a nucleotide-dependent fashion (Stehling et al. 2008). The two proteins form a heterotetramer which transiently binds the [4Fe-4S] cluster in a bridged form between two subunits of the complex. The Fe/S cluster is bound to two highly conserved Cys residues in the C-termini of each of these proteins. NUBP1 contains an additional, stably associated [4Fe-4S] cluster at its N-terminus which is essential for function.
Mitochondria play a crucial role in cytosolic and nuclear Fe/S protein biogenesis. They export via a mitochondrial ABC transporter (yeast ATM1, human ABCB7) a still unknown, sulfur-containing compound which is essential for Fe/S cluster assembly in the cytosol. As inferred from the yeast homolog Atm1p, the compound may be a 2Fe-2S cluster complexed with glutathione (Li and Cowan 2015).
The general cytosolic iron donor, the multi-domain monothiol glutaredoxin (human GRX3 or PICOT, yeast Grx3-Grx4) plays a crucial role in cytosolic-nuclear Fe/S protein biogenesis. The precise molecular function of the glutaredoxin is still unclear.