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The mechanistic target of rapamycin (mTOR) kinase controls growth and metabolism, and its deregulation underlies the pathogenesis of many diseases, including cancer, neurodegeneration, and diabetes. mTOR complex 1 (mTORC1) integrates signals arising from nutrients, energy, and growth factors, but how exactly these signals are propagated await to be fully understood. Recent findings have placed the lysosome, a key mediator of cellular catabolism, at the core of mTORC1 regulation by amino acids. A multiprotein complex that includes the Rag GTPases, Ragulator, and the v-ATPase forms an amino acid-sensing machinery on the lysosomal surface that affects the decision between cell growth and catabolism at multiple levels. The involvement of a catabolic organelle in growth signaling may have important implications for our understanding of mTORC1-related pathologies.