Function of IscA in biogenesis of Iron-sulfur clusters and repair of NO-modified iron-sulfur proteins

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Iron-sulfur (Fe-S) clusters are ubiquitous prosthetic groups that function in diverse fundamental life processes. However, the biogenesis of iron-sulfur clusters in vivo is not a spontaneous process. Previous studies indicated that Fe-S clusters maybe synthesized by three major systems: the Nif, the ISC, and the SUF systems. Among these three systems, IscU, NifU are the scaffold proteins for the Fe-S clusters assembly. IscA and its paralog, SufA are proposed as an alternative iron-sulfur cluster assembly scaffold proteins. The cysteine desulfurases: IscS, NifS and SufS catalyze desulfurization of the L-cysteine and provide sulfide for Fe-S clusters assembly. However, the iron donor for the Fe-S clusters assembly remains poorly understood. In this research, we reported that IscA is a strong iron binding protein. Under physiological conditions, if only iron is available, iron will bind to IscA. The addition of L-cysteine to this iron-bound IscA mobilizes the iron center in IscA and transfer iron to IscU for the Fe-S cluster assembly. However, if both iron and sulfide are available, Fe-S clusters are preferred to be assembled in IscU. Under oxidative stress conditions, IscA fails to bind ferrous iron due to the oxidation of its iron binding thiolate groups. CyaY, an E. coli homology of Frataxin is able to bind iron under oxidative stress conditions and effectively alleviate the production of the deleterious hydroxyl free radicals. Nevertheless, unlike IscA, CyaY cannot function as an efficient iron donor for the Fe-S clusters assemlby due to its weak iron binding property. We also investigated the repair mechanism for the NO-modified aconitase B [4Fe-4S] clusters. We found that E. coli [4Fe-4S] aconitase B is readily converted to the protein-bound DNICs by NO in vitro and in vivo. L-cysteine and oxygen are required for decomposition of the protein-bound DNICs. We further demonstrated that a complete repair of the NO-modified aconitase B requires two sequential steps: decomposition of the protein-bound DNICs requires both L-cysteine and oxygen, and the reassembly of Fe-S clusters, which requires Fe-S clusters assembly machinery.