title: Multiple turnover transfer of [2Fe2S] clusters by the iron-sulfur cluster assembly scaffold proteins IscU and IscA.
abstract: IscU/Isu and IscA/Isa (and related NifU and SufA proteins) have been proposed to serve as molecular scaffolds for preassembly of [FeS] clusters to be used in the biogenesis of iron-sulfur proteins. In vitro studies demonstrating transfer of preformed scaffold-[FeS] complexes to apoprotein acceptors have provided experimental support for this hypothesis, but investigations to date have yielded only single-cluster transfer events. We describe an in vitro assay system that allows for real-time monitoring of [FeS] cluster formation using circular dichroism spectroscopy and use this to investigate de novo [FeS] cluster formation and transfer from Escherichia coli IscU and IscA to apo-ferredoxin. Both IscU and IscA were found to be capable of multiple cycles of [2Fe2S] cluster formation and transfer suggesting that these scaffold proteins are capable of acting "catalytically." Kinetic studies further showed that cluster transfer exhibits Michaelis-Menten behavior indicative of complex formation of holo-IscU and holo-IscA with apoferredoxin and consistent with a direct [FeS] cluster transfer mechanism. Analysis of the dependence of the rate of cluster transfer, however, revealed enhanced efficiency at low ratios of scaffold to acceptor protein suggesting participation of a transient, labile scaffold-[FeS] species in the transfer process.
authors.affiliation: Section of Biochemistry, Dipartimento di Sciencze Molecolari Agroalimentari, University of Milan, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Bonomi
authors.forename: Stefania
authors.initials: S
authors.lastname: Iametti
authors.forename: Dennis
authors.initials: D
authors.lastname: Ta
authors.forename: Larry E
authors.initials: LE
authors.lastname: Vickery
mesh_terms: D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D002384:Catalysis; D002942:Circular Dichroism; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism*; D005288:Ferredoxins / Q000378:metabolism; D007506:Iron-Sulfur Proteins / Q000737:chemistry / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D005288:Ferredoxins; D007506:Iron-Sulfur Proteins; C487901:IscA protein, E coli; C411665:IscU protein, E coli; C026518:apoferredoxin