The role of molecular chaperones in mediating cluster transfer from [2Fe-2S] cluster-bound IscU
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In bacteria, the ISC (Iron Sulfur Cluster) system assembles Fe-S clusters. In the ISC system, IscU acts as a scaffold protein and molecular chaperones HscA and HscB specifically interact with IscU and are proposed to facilitate ATP-driven cluster transfer. In this work, cluster transfer from [2Fe-2S]-IscU to apo-Grx5 with and without chaperones was monitored by CD spectroscopy. Grx5 is a monothiol glutaredoxin and proposed cluster carrier/storage protein. Our results indicate a ~700 fold enhancement in the rate of [2Fe-2S] cluster transfer in the presence of chaperones, yielding a rate constant of 20000 M-1 min-1 and >90% completion within 3 min. Thus, HscA and HscB are required for efficient ATP-dependent cluster transfer from [2Fe-2S]-IscU. This illustrates the severe limitations of interpreting in vitro cluster transfer studies involving [2Fe-2S]-IscU in the absence of the dedicated HscA/HscB co-chaperone system. Results also support the proposed role of monothiol glutaredoxins as [2Fe-2S] cluster carriers/storage proteins.
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