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 (PubMed:18311129). 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 (By similarity). Binds 1 [2Fe-2S] cluster per subunit. Cofactor biosynthesis; iron-sulfur cluster biosynthesis. Component of the core Fe-S cluster (ISC) assembly machinery. It is unknown how the different localizations of several ISC components to mitosomes and cytoplasm can be reconciled with the usually tightly coordinated function of these components. Belongs to the NifU family.