Metal ions are essential to life as they augment amino acid protein chemistry and thereby catalyze many difficult biological reactions. As free metal ions are toxic and indiscriminately reactive, critical protein systems have evolved to sequester, chaperone, and regulate metal ion concentrations. Defects in these systems lead to metal ion metabolic disease and result in cellular, tissue, and systemic pathology. The iron-sulfur cluster assembly pathway contains a conserved set of metallochaperone proteins that recognize and insert Fe-S clusters into apo metalloproteins. Our working model is that frataxin (Fxn) binds to a Nfs1/Isd11/Isu2 complex and activates the complex for Fe-S cluster biosynthesis. In this proposal, we aim to build on these results by further developing our in vitro system and providing structural properties for the assembly complexes, mechanistic details for Fe-S cluster synthesis, and determine the basis for the compromised function in Friedreich's ataxia clinical variants. This fundamental research will establish a framework for emerging genetic results and discoveries and provide a basis for understanding defects in iron-sulfur cluster metabolism relevant to human health and disease.