Scaffold protein for the de novo synthesis of iron-sulfur (Fe-S) clusters within mitochondria, which is required for maturation of both mitochondrial and cytoplasmic [2Fe-2S] and [4Fe-4S] proteins (PubMed:15507320, PubMed:17417719). First, a [2Fe-2S] cluster is transiently assembled on the scaffold protein ISCU (ISU1, ISU2 or ISU3). In a second step, the cluster is released from ISCU, transferred to a glutaredoxin, followed by the formation of mitochondrial [2Fe-2S] proteins, the synthesis of [4Fe-4S] clusters and their target-specific insertion into the recipient apoproteins. Cluster assembly on ISCU 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 (By similarity). Interacts with HSCB (PubMed:19865480). Localizes to the cytosol when interacting with HSCB. Expressed in roots, stems, leaves, flowers, pollen and siliques. Belongs to the NifU family. Truncated N-terminus.