The asymmetric architecture of 2Fe–2S IscU, a scaffold protein for iron–sulfur cluster biosynthesis

Abstract

Iron-sulfur (Fe-S) clusters are ubiquitous prosthetic groups that are required to maintain fundamental life processes. The assembly of Fe-S clusters in several bacteria as well as eukaryotic mitochondria is achieved by a multicomponet system, called ISC machinery. In this machinery, IscU serves as a scaffold for assembly of a nascent Fe-S cluster, prior to its delivery to an apo-protein. The Hsp70-type molecular chaperon HscA and the J-type cochaperone HscB comprise a specialized chaperone system that selectively binds IscU and facilitate the cluster transfer process. We have determined the crystal structure of holo-IscU from the hyperthermophilic bacterium Aquifex aeolicus at 2.3 A resolution, using multi-wavelength anomalous diffraction of the [2Fe-2S] cluster. IscU formed an asymmetric homotrimeric structure harboring only one [2Fe-2S] cluster that was coordinated by three cysteines and one histidine (Cys36, Cys63, His106, and Cys107) on the surface of just one of the protomers. The cluster was buried inside the homotrimer by the neighboring protomers. The three protomers were conformationally distinct from each other, and associated around a non-crystallographic pseudo-three-fold axis. This unique trimeric holo-IscU architecture was clearly distinct from other known monomeric apo-IscU/SufU structures, indicating that asymmetric trimer organization would be involved in the scaffolding function of IscU. This oligomeric holoIscU structure provides mechanistic implications concerning the initial event in the assembly of Fe-S cluster, as well as the event in the transfer to target apo-proteins.