This goal of this proposal is to uncover the molecular mechanisms for sensing and regulating intracellular iron in the model eukaryote S. cerevisiae. To maintain optimal intracellular iron levels, iron transport and storage is tightly regulated in al eukaryotic cells ranging from yeast to humans. However, there are significant gaps in our understanding of iron regulation mechanisms at the cellular and molecular level. We will address these gaps by teasing out the molecular details of iron regulation in yeast and defining the roles of each component in the iron signaling pathway. In yeast, the monothiol glutaredoxins Grx3 and Grx4, the BolA- like protein Fra2, and the aminopeptidase P-like protein Fra1 function together in an iron-responsive signaling pathway that controls nucleocytoplasmic shuttling of the iron-responsive transcription factor Aft1. Under iron replete conditions, this pathway induces dimerization of Aft1 (and presumably its paralog Aft2), favoring their localization to the cytosol. We have demonstrated that Fra2 forms [2Fe-2S]2+-bridged heterodimers with Grx3 or Grx4 and characterized the Fe-S coordination chemistry of these complexes. In addition, we have strong evidence that [2Fe-2S] Fra2-Grx3 transfers a [2Fe-2S] cluster to Aft2, facilitating Aft2 dimerization. Aft1/2 dimerization, in turn, is proposed to inhibit activation of the iron regulon. Despite our significant progress in defining the molecular interactions between several components in this signaling pathway, some key aspects of the iron sensing and regulation mechanism remain unresolved and will be addressed in this proposal. We will uncover the mechanistic details of Fe-S transfer from Fra2-Grx3/4 to Aft1 and Aft2 and determine the impact of Fra1 on this process by using mutagenesis, biochemical analysis, and biophysical spectroscopy to examine the kinetics and efficiency of cluster transfer to Aft1 and Aft2 and identify residues in Grx3/Grx4/Fra1/Fra2/Aft1/Aft2 that are critical for both donor-target recognition and Fe-S transfer (Aim 1). We will test whether the Fra-Grx complex transfers an Fe-S cluster to Aft1/2 and induces dimerization in vivo by determining how mutations in Fra2, Grx3/4, or Fra1 affect protein-protein interactions within the iron signaling pathway, Fe binding to Aft1/2, and Aft1/2 subcellular localization and dimerization in vivo (Aim 2). Finally, we will elucidate the mechanism by which Fra-Grx mediated Aft1/2 dimerization inhibits activation of the iron regulon by testing if Fra-Grx-mediated dimerization of Aft1/2 disrupts movement of Aft1/2 to the nucleus, binding of Aft1/2 to its DNA targets, or recruitment of transcriptional co-activators using both in vivo and in vitro protein-protein and protein-DNA interaction assays (Aim 3). Since several key proteins in this pathway are conserved in humans and essential for viability, exploiting the yeast system to define their functional and physical interactions will provide a fundamental understanding of their roles in human iron metabolism.