Project Summary/Abstract Lipid signaling is crucial for regulating organismal physiology and metabolic expenditure, and disturbances in lipid homeostasis can deleteriously impact health. Cells tightly control the absorption, synthesis, and breakdown of lipids to accommodate energetic demands and ensure energetic reserves later in life. We recently reported a mechanism by which cells sense metabolic demand resulting from lipid depletion and respond by increasing expression of genes involved in membrane trafficking as well as nutrient absorption and catabolism. This is accomplished by the release of the starvation-responsive nuclear hormone receptor, NHR-49, from endocytic vesicles and its subsequent nuclear translocation and transcriptional activation. Yet the mechanisms and participating co-factors facilitating NHR-49 activation and attenuation remain unclear and are critical for our fundamental understanding of lipid sensing and metabolism. Our examination has uncovered a mechanism in which loss of antagonistic ligand binding releases NHR-49 from endocytic vesicles. Next, NHR- 49 sequentially engages a series of cofactor, which through direct interactions and post-translational modifications, facilitate its nucleocytoplasmic shuttling, transcriptional activation, and proteolytic degradation. Through the proposed five-year research period, we aim to understand how cells activation and attenuation this lipid surveillance response by defining the molecular details underlying the lifecycle of NHR-49 including its nuclear import, intranuclear retention, and transcriptional regulation followed by its phosphorylation, nuclear export and degradation. Our preliminary data has identified a series of co-factors including karopherins involved in its nucleocytoplasmic shuttling, nuclear pore complexes involved in its nuclear retention and transcriptional activation, kinases involved in its nuclear export, and a ubiquitin ligase involved in its proteasomal degradation. Through a more detailed description of NHR-49 co-factor interactions and their associated post-translational modifications, we can gain a better understanding of intracellular lipid sensing and metabolic homeostasis.