Amino acid regulation of mTORC1

Abstract

Mammalian target of rapamycin complex I (mTORC1) is an atypical Ser/Thr kinase that regulates cellular and organismal growth. Accordingly, mTORC1 has substantial roles in regulating insulin sensitivity and lifespan, and when deregulated, it is implicated in the pathogenesis of common cancers. mTORC1 responds to a diverse set of stimuli, including growth factors, oxygen availability, energy and amino acid levels in order to control essential anabolic and catabolic processes. Amino acids promote mTORC1 shuttling to the lysosomal surface, its site of activation. This translocation is mediated by a family of heterodimeric GTPases known as the Rags that reside on the lysosomal surface. Unique among the small GTPases, the Rags are obligate heterodimers: the highly related RagA and RagB are functionally redundant and bind to RagC or RagD, which are also very similar to each other. Amino acids regulate the binding of nucleotides to RagB, such that amino acid stimulation increases its GTP loading, leading to the recruitment and binding of mTORC1. In the work described here, we identify two Rag interacting complexes termed 'Ragulator' and 'GATOR' that form a lysosome based signaling platform that controls the activity of the Rags. We find that Ragulator is a pentameric complex that is both necessary and sufficient to determine the intracellular localization of Rags. Moreover, we find that Ragulator functions as a guanine nucleotide exchange factor (GEF) for RagA and RagB stimulating GTP-loading, a key event in the amino-acid dependent activation of mTORC1. Additionally, we describe the function of GATOR, an octomeric complex that is defined by two distinct subcomplexes termed GATOR1 and GATOR2. We find that GATOR2 functions as a positive regulator of mTORC1 whereas GATOR1 negatively controls this pathway. Epistasis analysis reveals GATOR2 functions upstream of GATOR1, which inhibits the Rags through its GTPase activating protein (GAP) activity towards RagA and RagB. GATOR1 components are mutated in gliolbastoma and ovarian tumors and GATOR1 deficient cancer cells are hypersensitive to the mTORC1 inhibitor rapamycin. Thus, we define the molecular mechanisms regulating the function of Rags and propose a model for the activation of the mTORC1 pathway by amino acids.