Biogenesis and Redox Transition of Iron-sulfur Clusters in Proteins

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Iron-sulfur proteins comprise one of the most ubiquitous and conserved classes of proteins in biology. The diverse functions of iron-sulfur clusters in proteins range from electron transfer to redox signaling. The assembly of iron-sulfur clusters in cells requires a complex protein system and acquisition of sulfur and iron. While the sulfur source for iron-sulfur cluster assembly is well-established, the iron source remains elusive. The first part of this work provides new evidence for the hypothesis that the conserved iron-sulfur cluster assembly protein IscA may act as the iron donor for iron-sulfur cluster assembly. Iron-sulfur clusters in proteins are also vulnerable to reactive oxygen and nitrogen species. Indeed, iron-sulfur clusters in proteins can be readily destroyed by nitric oxide (NO) forming protein-bound dinitrosyl iron complexes (DNICs). The second part of this work demonstrates that iron-sulfur proteins are the major source of protein-bound DNICs found in NO-exposed cells. The results reveal new aspects of the molecular mechanism underlying NO cytotoxicity. As dysfunction of iron-sulfur clusters has been implicated in several human diseases, two human iron-sulfur proteins have been chosen for functional investigation of their iron-sulfur clusters. The first example is Rtel1, a DNA helicase that regulates telomere length. Rtel1 was predicted to contain an iron-sulfur cluster, but this was not demonstrated. The third part of this work shows that the N-terminal domain of Rtel1 indeed contains a redox active [4Fe-4S] cluster. The second example is the mitochondrial outer membrane protein mitoNEET, a recently identified target of the type II diabetes drug pioglitazone. The studies show that mitoNEET [2Fe-2S] clusters can be readily reduced by biological thiols and human glutathione reductase, and is reversibly oxidized by hydrogen peroxide, suggesting that mitoNEET may undergo redox transitions to regulate mitochondrial energy metabolism in response to oxidative stress. In summary, the research presented in this dissertation advances our understanding of how iron-sulfur clusters may be assembled and how NO modification of iron-sulfur proteins may contribute to NO cytotoxicity. The human iron-sulfur proteins Rtel1 and mitoNEET further illustrate how iron-sulfur clusters may modulate protein functions via redox transition of their iron-sulfur clusters in response to oxidative signals.