Figure 1

Amino acid-dependent target of rapamycin complex 1 (TORC1) activation by key modulators of the Rag GTPases in yeast and mammals. The activation of TORC1 by amino acids occurs via the Rag GTPases, which are regulated by their guanine nucleotide-loading status. In yeast, TORC1 is constitutively found on the vacuolar membrane and is stimulated when the Rag heterodimer is in its active Gtr1^GTP-Gtr2^GDP conformation (left). In mammals, mTORC1 is localized diffusely throughout the cytoplasm and is recruited to lysosomes by the active RagA/B^GTP-RagC/D^GDP heterodimer, where it can be activated by GTP-loaded Rheb within an MCRS1-stabilized complex (right). In amino acid-starved cells, the Rag heterodimer is maintained in its inactive form by the GTPase activating protein (GAP) activity of either (SEACIT toward Gtr1 in yeast or GATOR1 toward RagA/B in mammals. The GATOR1–RagA/B interaction is driven by RNF152-dependent ubiquitination of RagA/B. Sestrins also contribute to the inactivation of the Rag heterodimer by inhibiting GATOR2 and/or preventing the release of GDP from RagA/B. Furthermore, the inactive Rag heterodimer contributes to the inactivation of Rheb (and thus mTORC1) through the recruitment of the tuberous sclerosis complex (TSC) complex that exerts GAP activity toward Rheb. Upon amino acid stimulation, positive regulators of the Rag GTPases stimulate the formation of the active form of the Rag heterodimer. In yeast, the activity of LeuRS and Vam6 on Gtr1 and Lst4-Lst7 on Gtr2 promote the formation of Gtr1^GTP-Gtr2^GDP, while SEACAT interferes with the inhibitory effect of the Gtr1 GAP, SEACIT. In mammalian cells, v-ATPase stimulates the guanine nucleotide exchange factor (GEF) activity of the Ragulator to promote GTP loading of RagA/B, while the FNIP1/2-FLCN complex, and possibly LeuRS, exert GAP activity toward RagC/D, resulting in GDP-loaded RagC/D. GATOR2 also impedes the activity of the RagA/B GAP, GATOR1. Hyperactivation of mTORC1 after prolonged amino acid stimulation may be prevented by Skp2-mediated ubiquitination of RagA/B, which drives the recruitment of GATOR1 to the Rag heterodimer (indicated with a dashed line). The focal points where amino acids directly feed into the Rag GTPase network currently include (i) LeuRS that mediates balanced levels of the branched-chain amino acids leucine, isoleucine and valine to Gtr1 in yeast or leucine levels to RagC/D in mammals, (ii) lysosomal amino permeases that transport and, via still poorly defined mechanisms, signal arginine and/or glutamine (SLC38A9), small unbranched amino acids (SLC36A1/PAT1 (proton-assisted amino acid transporter 1)), and possibly histidine (SLC15A4) levels to the Rag GTPases, (iii) mammalian Sestrin1 and Sestrin2 that dissociate from GATOR2 when directly bound to leucine or to the lower-affinity substrates methionine and isoleucine and (iv) yeast S-adenosylmethionine synthetase and Ppm1 methyltransferase that channel methionine (tandem arrows) into the methylation of the catalytic PP2A subunit (PP2Ac), which antagonizes SEACIT assembly. Whether the v-ATPase or any of the vacuolar membrane-resident amino permeases mediate amino acid signals toward the Rag GTPases in yeast is currently not known (which is why they are depicted in grey in the yeast model on the left). How amino acids impinge on Lst4-Lst7 and FNIP1/2-FLCN is currently not known. Arrows and bars impinging on Rag GTPases denote mechanisms that favor and antagonize, respectively, the indicated nucleotide-binding state. For all other proteins or protein complexes, arrows and bars denote positive and negative interactions, respectively. AA, amino acids; Arg, arginine; Gln; glutamine; His, histidine; Ile, isoleucine; Leu, leucine; Met, methionine; Val, valine. For further details, see text.