Scaffold protein for the de novo synthesis of iron-sulfur (Fe-S) clusters within mitosomes, which is required for maturation of both [2Fe-2S] and [4Fe-4S] proteins. First, a [2Fe-2S] cluster is transiently assembled on the scaffold protein ISU1. In a second step, the cluster is released from ISU1, transferred to a glutaredoxin, followed by the formation of [2Fe-2S] proteins, the synthesis of [4Fe-4S] clusters and their target-specific insertion into the recipient apoproteins. Cluster assembly on ISU1 depends on the function of the cysteine desulfurase complex NFS1-ISD11, which serves as the sulfur donor for cluster synthesis, the iron-binding protein frataxin as the putative iron donor, and the electron transfer chain comprised of ferredoxin reductase and ferredoxin, which receive their electrons from NADH 