The mTOR kinase is the central component of a signaling pathway that controls mass accumulation and metabolism in response to the nutritional state of organisms. The pathway is deregulated in many common human diseases, including cancer, epilepsy, and diabetes, and is also well established to modulate the aging process. Pharmacological or genetic suppression of mTOR is amongst the best- validated approaches for increasing the lifespan of diverse organisms. The mTOR protein kinase is the target of the drug rapamycin and the catalytic subunit of two large protein complexes, mTOR Complex 1 (mTORC1) and 2 (mTORC2), that control separate branches of the pathway and preferentially respond to different stimuli. mTORC1 responds to diverse signals, including many types of growth factors, nutrients, and stresses, and regulates the balance between major anabolic and catabolic processes, including protein, nucleotide, and lipid synthesis as well as autophagy, respectively. Recently, we discovered many of the components through which mTORC1 senses nutrients and we are just starting to understand the role of the nutrient-sensing pathway in vivo. Our preliminary data show that the appropriate regulation of mTORC1 by nutrients is essential for mice to adapt to diets low in the essential amino acid leucine. Moreover, we have evidence that mTORC1 is spatially controlled in unexpected ways in tissues in vivo and that novel in vivo regulatory mechanisms remain to be discovered. The goals of our proposed work are to understand why the capacity of mTORC1 to sense leucine deprivation is important for mice to adapt to a leucine-free diet (Aim 1) and the role of compartmentalized nutrient sensing in the control of tissue physiology and metabolism (Aim 2). In addition, we will exploit in vivo proteomics and genetics to identify novel mTORC1 regulators in the liver (Aim 3). We will accomplish our goals with a multi-disciplinary approach that exploits biochemistry, metabolomics, proteomics, molecular biology, and mouse engineering. Our results should increase our understanding of a central growth regulator in vivo and reveal novel regulatory mechanisms that may have value as therapeutic targets.