Open Access

Nar1 is an essential eukaryotic protein proposed to function as an iron-sulfur (Fe/S) cluster trafficking factor in the cytosolic iron-sulfur assembly (CIA) machinery. However, such a role has remained unclear due to difficulties in purifying adequate amounts of cofactor-bound protein. The [FeFe]-hydrogenase-like protein has two conserved binding sites for [4Fe-4S] clusters, one of which is predicted to be a labile site for cluster transfer to downstream targets. Here, we report a new preparation procedure for Nar1 that facilitated studies by UV-Vis, EPR, and Mössbauer spectroscopies, along with native mass spectrometry. Nar1 recombinantly produced in E. coli contained a [4Fe-4S] cluster, bound presumably at site 1, along with an unexpected [2Fe-2S] cluster bound at an unknown site. Fe/S reconstitution reactions installed a second [4Fe-4S] cluster at site 2, leading to protein with three Fe/S cofactors. Strikingly, one [4Fe-4S] cluster was rapidly destroyed by molecular oxygen, potentially linking Nar1 oxygen sensitivity to phenotypes observed previously in vivo. These advances now allow for the pursuit of in vitro Fe/S cluster transfer assays, which will shed light on Fe/S trafficking by CIA components and how they may facilitate the insertion of [4Fe-4S] and potentially [2Fe-2S] clusters into target proteins in the cytosol.