Metamorphic protein IscU alternates conformations in the course of its role as the scaffold protein for iron–sulfur cluster biosynthesis and delivery

Abstract

IscU from Escherichia coli, the scaffold protein for iron-sulfur cluster biosynthesis and delivery, populates a complex energy landscape. IscU exists as two slowly interconverting species: one (S) is largely structured with all four peptidyl–prolyl bonds trans; the other (D) is partly disordered but contains an ordered domain that stabilizes two cis peptidyl–prolyl peptide bonds. At pH 8.0, the S-state is maximally populated at 25 °C, but its population decreases at higher or lower temperatures or at lower pH. The D-state binds preferentially to the cysteine desulfurase (IscS), which generates and transfers sulfur to IscU cysteine residues to form persulfides. The S-state is stabilized by Fe–S cluster binding and interacts preferentially with the DnaJ-type co-chaperone (HscB), which targets the holo-IscU:HscB complex to the DnaK-type chaperone (HscA) in its ATP-bound from. HscA is involved in delivery of Fe–S clusters to acceptor proteins by a mechanism dependent on ATP hydrolysis. Upon conversion of ATP to ADP, HscA binds the D-state of IscU ensuring release of the cluster and HscB. These findings have led to a more complete model for cluster biosynthesis and delivery.

The Diabetes Drug Target MitoNEET Governs a Novel Trafficking Pathway to Rebuild an Fe-S Cluster into Cytosolic Aconitase/Iron Regulatory Protein 1

“…It is worth mentioning that the metamorphic scaffold protein IscU from E. coli also alternates between two conformations: a disordered state and a largely structured state that is stabilized by Fe-S cluster binding (50). Meanwhile, we also observed that mNT requires sustained activities of the mitochondrial ISC and export machineries with regard to the Fe-S supply to escape long lasting unstructured and proteasomal degradation.…”

[2Fe-2S] cluster transfer in iron–sulfur protein biogenesis

“…The dimeric state of holo hGRX5 also prevents the [2Fe-2S] cluster from being released in the presence of physiological concentrations of GSH, suggesting that holo hGRX5 works as a metallochaperone that specifically transfers the [2Fe-2S] cluster to partner proteins. In such a way, the [2Fe-2S] cluster, once de novo-synthesized on ISCU (33,34), can be safely transferred from one protein to another, up to its final target protein. The mode of cluster transfer based on a switch between two conformational states of holo hGRX5 described here could be a general mechanism used by monothiol glutaredoxins in Fe/S protein assembly pathways.…”

Architecture of the Yeast Mitochondrial Iron-Sulfur Cluster Assembly Machinery

“…IscU-type scaffolds are also known to exist in monomeric (32,33) and homo-oligomeric states in the absence of protein partners (34,35). In particular, the yeast orthologue, Isu1, has been isolated as a primarily monomeric (10) or dimeric species (8, 36) upon overexpression in E. coli.…”

“…In particular, the yeast orthologue, Isu1, has been isolated as a primarily monomeric (10) or dimeric species (8, 36) upon overexpression in E. coli. Available structures of mono-meric IscU proteins from E. coli and Haemophilus influenzae show a flexible N-terminal region connected to a globular core formed by four ␣-helices packed against three antiparallel ␤-strands (32,33), with the Fe-S cluster binding site residing in a solvent-accessible region at one end of the globular core (37,38). Interestingly, the structure of an asymmetric IscU trimer from A. aeolicus revealed one [2Fe-2S] cluster bound on the surface of one subunit but buried inside the interface formed by all three subunits (34).…”