Discovering regulators of the amino acid sensing pathway of mTORC1

Abstract

The mechanistic target of rapamycin complex I (mTORC1) protein kinase functions as a master regulator of growth, and its deregulation is common in human disease, including cancer and diabetes. mTORC1 integrates multiple environmental cues to control anabolic and catabolic processes. A key input is amino acids, which function to promote the translocation of mTORC1 to the lysosomal surface, its site of activation. Necessary for this recruitment are the Rag GTPases and several distinct factors that modulate their nucleotide state in response to amino acid availability. Despite these advances, several key questions remain. The components that mediate mTORC1 inhibition upon amino acid deprivation and the identities of the amino acid sensors upstream of mTORC1 are both unknown. To provide insight into these questions, we undertook an unbiased proteomics approach to discover novel mTORC1 regulators. Here, we describe the identification of GATOR2 as a pentameric complex that positively regulates mTORC1 and functions upstream of or in parallel to GATOR1, a GTPase activating protein complex for the Rags and a negative regulator of the mTORC1 pathway. KICSTOR, a four-membered protein complex, is necessary to localize GATOR1 to the lysosome to enable it to suppress mTORC1 activity. GATOR1 components are mutated in cancer and may identify tumors that respond to clinically approved mTORC1 inhibitors. Furthermore, we describe the identification of Sestrin2 and CASTOR1 as GATOR2-interacting proteins that function as leucine and arginine sensors, respectively, for the mTORCI pathway. Both sensors are required to signal the absence of leucine and arginine to mTORC1, and the amino acid-binding capacity of both sensors is necessary for amino acids to activate mTORC1. Altogether, the identification of these mTORC1 regulators furthers our understanding of the mechanisms by which amino acid availability controls cellular growth.