The iscU gene in bacteria is located in a gene cluster encoding proteins implicated in iron-sulfur cluster assembly and an hsc70-type (heat shock cognate) molecular chaperone system, iscSUAhscBA. To investigate possible interactions between these systems, we have overproduced and purified the IscU protein from Escherichia coli and have studied its interactions with the hscA and hscB gene products Hsc66 and Hsc20. IscU and its iron-sulfur complex (IscU-Fe͞S) stimulated the basal steady-state ATPase activity of Hsc66 weakly in the absence of Hsc20 but, in the presence of Hsc20, increased the ATPase activity up to 480-fold. Hsc20 also decreased the apparent Km for IscU stimulation of Hsc66 ATPase activity, and surface plasmon resonance studies revealed that Hsc20 enhances binding of IscU to Hsc66. Surface plasmon resonance and isothermal titration calorimetry further showed that IscU and Hsc20 form a complex, and Hsc20 may thereby aid in the targeting of IscU to Hsc66. These results establish a direct and specific role for the Hsc66͞Hsc20 chaperone system in functioning with isc gene components for the assembly of iron-sulfur cluster proteins. P roteins containing iron-sulfur clusters possess important redox, catalytic, and regulatory functions, but the mechanism by which their clusters are formed or repaired is not known (see refs. 1-3). Much of the limited information on this subject has been provided by studies on the nif operon of nitrogen-fixing bacteria, which encodes proteins necessary for the assembly of the Fe͞S centers present in the nitrogenase system. Recently, homologs of nif genes have been found in both nitrogen-fixing and non-nitrogen-fixing bacteria, and these coding regions have been designated isc genes for their putative roles in iron sulfur cluster formation (4). These genes include iscS, homologous to nifS, which encodes a cysteine desulfurase shown to provide sulfide for Fe͞S cluster formation (5, 6); iscU, homologous to nifU, which encodes a protein implicated in Fe͞S binding (7); and iscA, homologous to nif ORF 6, whose function is not known (8).In bacteria, the isc genes are found in a highly conserved gene cluster, iscSUA-hscBA-fdx, which also encodes an hsp70-type molecular chaperone designated Hsc66 (hscA; refs. 9-11), a J type cochaperone designated Hsc20 (hscB; refs. 10 and 11), and a [2Fe-2S] ferredoxin ( fdx; ref. 12). The exact cellular roles of the chaperones and ferredoxin are not known, but the conserved association of their genes with the isc genes suggests a possible function in the assembly and͞or repair of iron-sulfur cluster proteins. Recent studies in which overexpression of the iscSUAhscBA-fdx gene region in Escherichia coli yields increased production of recombinant iron sulfur proteins (13, 14) support such a function. It also seems likely that similar proteins are involved in iron-sulfur cluster assembly in eukaryotes. Earlier studies showed that bacteria and yeast are able to assemble the ironsulfur center of a human [2Fe-2S] protein, suggesting a conserved mechanism ...
