A scaffold on which IscS assembles Fe-S clusters. Subsequently gives the nascent cluster to other proteins. Binds 1 2Fe-2S cluster per subunit in a crystal formed under reducing conditions; this subunit provides 3 ligands while IscS2 provides the other ligand. It is likely that Fe-S cluster coordination is flexible as the role of this complex is to build and then hand off Fe-S clusters. Forms a heterotetramer with IscS2. Belongs to the NifU family.