Featured Article - April 2013
Short description: The scaffold protein IscU alternates between structured and disordered conformations during iron-sulfur cluster assembly and delivery.
Iron has a unique biological role because of its redox properties and participation in diverse cellular processes, including catalysis and electron transport. Iron-sulfur (Fe-S) clusters are ancient and universally conserved prosthetic groups that require specialized machinery for assembly and delivery to target proteins.
Markley and colleagues (Mitochondrial Protein Partnership with NESG) have employed NMR spectroscopy to investigate the functional cycle of IscU, the Fe-S cluster scaffold protein from Escherichia coli. Initial analysis indicated that apo-IscU (without the Fe-S cluster) exists in a conformational equilibrium, slowly exchanging between structured (S) and disordered (D) states. This equilibrium can be modulated by pH or temperature conditions, or by mutations in IscU.
The authors solved the NMR structure of the S-state (PDB 2L4X) under conditions in which it was maximally occupied (pH 8.0, 25°C, 80% of the population). The structure was very similar to that of a D39A mutant (∼100% in the S-state, PDB 2KQK), except that the loops ligating the Fe-S cluster are more ordered in D39A and resemble those of the IscU-[2Fe-2S] complex.
Next, the authors sought to determine how the conformational equilibrium contributes to IscU function. Reasoning that the slow interconversion rate could be due to proline cis-trans isomerization, they found that all four peptidyl-prolyl peptide bonds are trans in the S-state, but two are cis in the D-state. The D-state is favored at low pH, and features two protonated histidine residues (His10 and His105), which are deprotonated in the S-state. His105 is involved in cluster binding and its deprotonation is required for iron binding, suggesting a switch to the S-state in the cluster complex.
Finally, the authors examined the interactions of IscU with partner proteins during cluster assembly and delivery. The cysteine desulfurase IscS was found to bind preferentially to the D-state, consistent with transfer of sulfur to IscU cysteine residues before conversion to the S-state upon cluster assembly. The co-chaperone HscB binds selectively to the S-state, which the protein adopts in the IscU-[2Fe-2S] complex. The HscB- IscU-[2Fe-2S] complex then binds to the Hsp70-type chaperone HscA in its ATP-bound form. Upon conversion of ATP to ADP, HscA binds selectively to the D-state of IscU, leading to release of the cluster as required for its transfer to an acceptor protein. The combined data allowed the authors to propose a model for IscU function that will guide future experiments.
J. L. Markley et al. Metamorphic protein IscU alternates conformations in the course of its role as the scaffold protein for iron-sulfur cluster biosynthesis and delivery.
FEBS Lett. (16 January 2013). doi:10.1016/j.febslet.2013.01.003