Intracellular iron homeostasis is a critical cellular process that ensures intracellular iron concentrations are sufficient to perform essential iron-dependent functions in aerobic respiration, DNA replication and repair, and multiple biosynthetic pathways (amino acid, nucleotide, and lipid) while avoiding the toxicity associated with excess iron. We previously identified the E3 ubiquitin ligase FBXL5 as a master regulator of iron homeostasis that was responsible for sensing intracellular iron levels through an N-terminal iron-binding domain and coupling changes in iron concentration to its ability to degrade Iron Regulatory Proteins (IRPs) - RNA binding proteins that the post-transcriptional expression of genes involved iron utilization, transport, and storage. In the proposed work, we will examine the hypothesis that FBLX5 is a key signaling hub that coordinates IRP- mediated gene regulation with variety of other iron metabolic pathways in order to generate an integrated cellular response to iron deficiency.
Specific aim 1 will explore the role of the Fe-S cluster assembly pathways in regulating iron homeostasis by characterizing a novel interaction identified in our laboratory between FBXL5 and the CIA targeting complex, a protein complex required for Fe-S protein biogenesis.
In specific aim 2, we will elucidate the cellular mechanism by which FBXL5 is degraded in iron-depleted cells by defining the roles of the E3 ubiquitin ligase HERC2 and the kinase SPAK in this proteolytic pathway. Finally, specific aim 3 will focus on examining a role for FBXL5 in regulating ferritin degradation via its association with the autophagy adaptor NCOA4. Investigation of these three aims will uncover the molecular mechanisms that govern how FBXL5 integrates and interprets signals transduced through multiple iron-regulated signaling pathways in order to dictate the multi-faceted cellular response to iron availability.
The attachment of the small protein ubiquitin to specific targets in the cell is an important signal for cellular communication and used in a variety of physiological pathways. This proposal focuses on understanding how the ubiquitin system regulates different facets of iron metabolism. Considering defects in ion metabolism have been linked to many diseases including anemias, hemochromatosis, and cancer, elucidating how iron metabolism is regulated by ubiquitin will have important implications for understanding the molecular basis of these disorders.
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