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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 wor