A scaffold on which IscS assembles Fe-S clusters. Subsequently gives the nascent cluster to other 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 (Probable). Homotrimer upon overexpression in E.coli and in crystal structure. The trimer binds Fe-S clusters, upon exposure to air it loses the clusters and concomitantly dissociates into monomers and dimers. Belongs to the NifU family.