Molecular insights into amino acid and growth factor control of mTOR signaling [electronic resource]
Keywords: Chemical and Systems Biology.

All cells and organisms must coordinate their metabolic activity with changes in their nutrient environment. Unicellular organisms are directly exposed to environmental fluctuations of nutrients and sense both intracellular and environmental nutrient levels. For multicellular organisms, the ability to sense and respond to fluctuations of both nutrients and systemic growth signals (such as hormones and growth factors) are requisite for life because these signals regulate anabolic and catabolic pathways. They are fundamental processes in our body. Therefore, dysregulation of the sensing mechanisms are reported in human metabolic diseases. The center of such sensing pathways is mTORC1. This evolutionally conserved mega Dalton kinase complex senses a wide variety of both extracellular and intracellular signals such as amino acids, growth factors, oxygen level, and other stresses. mTORC1 integrates these signals to regulates critical cellular processes such as metabolism, growth, proliferation and differentiation. Amino acids are a dominant factor in mTORC1 activation. Amino acids regulate localization of mTORC1 to lysosomes, which is an indispensible step for kinase activation. RagGTPases are a direct factor of mTORC1 lysosome translocation as they physically interact and recruit the kinase to the lysosome surface. Although a number of proteins involved in mTORC1 activation have been discovered, the molecular nature of the amino acid sensing mechanisms is still largely mystery. In particular, how a cell senses amino acid availability and recruits mTORC1 to lysosomal surface is a prominent area of study in this field. From a human genome-wide RNAi study, the Meyer lab identified mTORC1 regulators including a novel lysosomal-seven transmembrane protein, MORTOR1. We showed that MORTOR1 mediates amino acid signal to RagGTPases and regulates mTORC1 lysosome translocation. Depletion of MORTOR1 reduces lysosomal mTORC1 as well as kinase activity while an increase of MORTOR1 is sufficient to promote mTORC1 translocation and activation. Biochemically, MORTOR1 interacts with RagA and promotes physical association with mTORC1. We further showed that RagA and Rag-modulator interactions are regulated by MORTOR1. Based on these data, we speculate that MORTOR1 coordinates the interaction between RagA and the tested modulators and therefore promotes RagA-mTORC1 interaction and mTORC1 translocation. Since MORTOR1 has a unique GPCR-like structure, which is believe to be a druggable structure, an understanding of MORTOR1 may contribute to the development of anti-mTORC1 therapeutics. Although amino signals are crucial for mTORC1 regulation, full activation of the kinase requires additional inputs. Recent studies demonstrated that an inhibitor of mTORC1, the TSC complex, is also recruited to lysosome. In Chapter 3, we measured spatial and temporal dynamics of mTORC1 and TSC and investigated whether their lysosome localization correlated with mTORC1 kinase activity. Interestingly, mTORC1 lysosomal translocation is rapid and transient and mTORC1 substrate phosphorylations are first detected when mTORC1 begins to dissociate from lysosomes. TSC dissociation seems to anti-correlate with initial mTORC1 translocation but not with dissociation. Finally, we demonstrated that mTORC1 kinase inhibition suppresses the dissociation.