A scaffold on which IscS assembles Fe-S clusters. Cluster assembly on IscU homodimers proceeds sequentially from 1 2Fe-2S per dimer, to 2 2Fe-2S per dimer and finally 1 4Fe-4S per dimer. Subsequently IscU transfers the Fe-S cluster to other acceptor proteins. 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. The clusters are very sensitive to O(2). Homodimer. Forms a heterotetramer with IscS; each subunit of the IscS dimer contacts an IscU monomer. Belongs to the NifU family.