Real-Time Kinetic Probes Support Monothiol Glutaredoxins As Intermediate Carriers in Fe–S Cluster Biosynthetic Pathways

Iron–sulfur (Fe–S) clusters are protein cofactors that are required for many essential cellular functions. Fe–S clusters are synthesized and inserted into target proteins by an elaborate biosynthetic process. The insensitivity of most Fe–S assembly and transfer assays requires high concentrations for components and places major limits on reaction complexity. Recently, fluorophore labels were shown to be effective at reporting cluster content for Fe–S proteins. Here, the incorporation of this labeling approach allowed the design and interrogation of complex Fe–S cluster biosynthetic reactions that mimic in vivo conditions. A bacterial Fe–S assembly complex, composed of the cysteine desulfurase IscS and scaffold protein IscU, was used to generate [2Fe–2S] clusters for transfer to mixtures of putative intermediate carrier and acceptor proteins. The focus of this study was to test whether the monothiol glutaredoxin, Grx4, functions as an obligate [2Fe–2S] carrier protein in the Fe–S cluster distribution network. Interestingly, [2Fe–2S] clusters generated by the IscS–IscU complex transferred to Grx4 at rates comparable to previous assays using uncomplexed IscU as a cluster source in chaperone-assisted transfer reactions. Further, we provide evidence that [2Fe–2S]–Grx4 delivers clusters to multiple classes of Fe–S targets via direct ligand exchange in a process that is both dynamic and reversible. Global fits of cluster transfer kinetics support a model in which Grx4 outcompetes terminal target proteins for IscU-bound [2Fe–2S] clusters and functions as an intermediate cluster carrier. Overall, these studies demonstrate the power of chemically conjugated fluorophore reporters for unraveling mechanistic details of biological metal cofactor assembly and distribution networks.