Expression, purification and characterization of an iron-sulfur cluster assembly protein, IscU, from Acidithiobacillus ferrooxidans

Characterization of iron-sulfur cluster assembly protein IscA from Acidithiobacillus ferrooxidans

IscA is a key member of the iron-sulfur cluster assembly machinery found in bacteria and eukaryotes, but the mechanism of its function in the biogenesis of iron-sulfur cluster remains elusive. In this paper, we demonstrate that Acidithiobacillus ferrooxidans IscA is a [4Fe-4S] cluster binding protein, and it can bind iron in the presence of DTT with an apparent iron association constant of 4·10(20) M(-1). The iron binding in IscA can be promoted by oxygen through oxidizing ferrous iron to ferric iron. Furthermore, we show that the iron bound form of IscA can be converted to iron-sulfur cluster bound form in the presence of IscS and L-cysteine in vitro. Substitution of the invariant cysteine residues Cys35, Cys99, or Cys101 in IscA abolishes the iron binding activity of the protein; the IscA mutants that fail to bind iron are unable to assemble the iron-sulfur clusters. Further studies indicate that the iron-loaded IscA could act as an iron donor for the assembly of iron-sulfur clusters in the scaffold protein IscU in vitro. Taken together, these findings suggest that A. ferrooxidans IscA is not only an iron-sulfur protein, but also an iron binding protein that can act as an iron donor for biogenesis of iron-sulfur clusters.

Expression, purification, and characterization of an iron chaperon protein CyaY from Acidithiobacillus ferrooxidans

CyaY is the bacterial homolog of frataxin, proposed to be involved in the assembly of iron-sulfur clusters. While, the physiological iron donor for the iron-sulfur clusters assembly remains controversial. In this study, the gene of CyaY from Acidithiobacillus ferrooxidans was cloned and expressed in Escherichia coli, the protein was purified by one-step affinity chromatography to homogeneity. The CyaY protein can bind ferric iron and serve as an iron donor for the biogenesis of iron-sulfur clusters on the scaffold protein IscU in the presence of IscS and L-cysteine in vitro.

Structural, Mechanistic and Coordination Chemistry of Relevance to the Biosynthesis of Iron-Sulfur and Related Iron Cofactors

Iron-sulfur clusters are an important class of protein-bound prosthetic center that find wide utility in nature. Roles include electron transfer, enzyme catalysis, protein structure stabilization, and regulation of gene expression as transcriptional and translational sensors. In eukaryotes their biosynthesis requires a complex molecular machinery that is located within the mitochondrion, while bacteria exhibit up to three independent cluster assembly pathways. All of these paths share common themes. This review summarizes some key structural and functional properties of three central proteins dedicated to the Fe-S cluster assembly process: namely, the sulfide donor (cysteine desulfurase); iron donor (frataxin), and the iron-sulfur cluster scaffold protein (IscU/ISU).