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:11060020). First, a [2Fe-2S] cluster is transiently assembled on the scaffold protein ISCU. In a second step, the cluster is released from ISCU, transferred to a glutaredoxin GLRX5, 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-LYRM4/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). Functions as a cytoplasmic scaffold protein for the de novo synthesis of iron-sulfur clusters in the cytoplasm. Binds NFS1 (PubMed:11060020). Interacts with HSCB (PubMed:20668094). Interacts with GLRX5 (PubMed:26100117). Interacts with HSPA9 (PubMed:26702583). Interacts with HSC20 (PubMed:29309586). Detected in heart, liver, skeletal muscle, brain, pancreas, kidney, lung and placenta. The disease is caused by variants affecting the gene represented in this entry. Belongs to the NifU family.