Iron-sulfur cluster biosynthesis : Molucular mechanism of the assembly of intermediate cluster and its transfer to apo proteins.

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Iron-sulfur (Fe-S) proteins are present in almost all living organisms and exhibit diverse functions, which include electron transport, redox and non-redox catalysis, and sensing for regulatory processes. They contain clusters of iron and sulfur atoms with variable complexity, such as [2Fe-2S], [3Fe-4S], and [4Fe-4S] clusters. We have recently found two independent systems, called ISC and SUF, essential for the biosynthesis of Fe-S clusters, although still very little is known about the mechanistic details. In this study we have focused on IcsU, a central key player of the ISC machinery working as a scaffold for a nascent Fe-S cluster, and undertaken genetic, biochemical and structural approaches to elucidate the complex mechanism by which the Fe-S clusters are assembled and transferred to target proteins.1. We have isolated and analyzed 14 psuedorevertants generated from a mutant strain lacking HscA and HscB of the ISC machinery, and found that the responsible suppressor mutations are all located in IscU. The mutated IscUs were conformationally altered from WT IscU at the secondary structure level, suggesting that, in wild-type cells, HscA and HscB induce the structural changes of IscU thereby facilitating the cluster transfer from IscU to target proteins.2. In contrast to the notoriously unstable Fe-S cluster intermediate in the scaffold proteins, we found that the IscU homolog from the hyperthermophilic bacterium Aquifex aeolicus (AaIscU) carries a stable Fe-S cluster. AaIscU was purified, characterized and crystallized for the first time under anaerobic conditions. The crystal structure has revealed that the holo-AaIscU is an asymmetric trimer carrying only one [2Fe-2S] cluster at the interface between two structurally altered protomers. The structure of the asymmetric trimer provides valuable insight into the molecular event: the assembly, maintenance and transfer of the nascent Fe-S cluster is tightly coupled to the conversion of the oligomeric state of IscU.