The Fe/S assembly protein IscU behaves as a substrate for the molecular chaperone Hsc66 from Escherichia coli

IscU, a NifU-like Fe/S-escort protein, binds to and stimulates the ATPase a ctivity of Hsc66, a hsp70-type molecular chaperone, We present evidence tha t stimulation arises from interactions of IscU with the substrate-binding s ite of Hsc66, IscU inhibited the ability of Hsc66 to suppress the aggregati on of the denatured model substrate proteins rhodanese and citrate synthase , and calorimetric and surface plasmon resonance measurements showed that A TP destabilizes HscBB IscU complexes in a manner expected for hsp70-substra te complexes. Studies on the interaction of IscU with Hsc66 truncation muta nts further showed that IscU does not bind the isolated ATPase domain of Hs c66 but does bind and stimulate a mutant containing the ATPase domain and s ubstrate binding beta -sandwich subdomain, These results support a role for IscU as a substrate for Hsc66 and suggest a specialized function for Hsc66 in the assembly, stabilization, or transfer of Fe/S clusters formed on Isc U.