Amino acid signalling upstream of mTOR

Abstract

Mammalian target of rapamycin (mTOR) is a conserved Ser/Thr kinase that is part of mTOR complex 1 (mTORC1), a master regulator that couples amino acid availability to cell growth and autophagy. Multiple cues modulate mTORC1 activity, such as growth factors, stress, energy status and amino acids. Although amino acids are key environmental stimuli, exactly how they are sensed and how they activate mTORC1 is not fully understood. Recently, a model has emerged whereby mTORC1 activation occurs at the lysosome and is mediated through an amino acid sensing cascade involving RAG GTPases, Ragulator and vacuolar H(+)-ATPase (v-ATPase).

Figure 1. The mTORC1 signalling pathway

Through a multiple-step process, growth factors stimulate AKT, which in turn phosphorylates and inhibits the tuberous sclerosis complex (TSC; comprising TSC1 and TSC2). TSC acts as a GTPase-activating protein (GAP) for the small GTPase RAS homologue enriched in brain (RHEB), promoting hydrolysis of its bound GTP and thus inhibiting RHEB. Upon inhibition of TSC, GTP-bound RHEB levels increase and can potently activate mammalian target of rapamycin complex 1 (mTORC1), presumably at the lysosome. In addition to growth factors, mTORC1 can be regulated by stress, energy status and amino acids. Stress and energy status regulate mTORC1 through TSC. Moreover, AMPK (AMP-activated protein kinase) is activated in response to a low energy status and phosphorylates RAPTOR (regulatory- associated protein of mTOR) and TSC2. This results in the inhibition of mTORC1. Amino acids regulate mTORC1 in a TSC-independent pathway. Collectively multiple stimuli modulate mTORC1 to control cell growth and autophagy. DEPTOR, DEP domain-containing mTOR-interacting protein; mLST8, mammalian lethal with SEC13 protein 8; PRAS40, 40 kDa Pro-rich AKT substrate.

Figure 2. Amino acid-induced mTORC1 activation in mammals and yeast

Amino acids promote the formation of the active configuration of the RAG GTPase complex in mammals at the lysosome (left) and of Gtr1–Gtr2 at the vacuole in yeast (right). Under amino acid starvation conditions in mammals, inactive mammalian target of rapamycin complex 1 (mTORC1) is diffuse throughout the cytosol. The RAG GTPase complex is inactive, with RAGA or RAGB loaded with GDP (RAGA/B·GDP) and RAGC or RAGD loaded with GTP (RAGC/D·GTP) (left, top). During amino acid deficiency in yeast, inactive TORC1 and Gtr1–Gtr2 remain localized at the vacuolar membrane but do not physically interact. Similarly to RAG GTPases in mammals under amino acid-deficient conditions, Gtr1 is bound to GDP and Gtr2 is bound to GTP, which results in an inactive complex (right, top). Amino acid stimulation signals to vacuolar H⁺-ATPase (v-ATPase), which is required to induce the guanine exchange factor (GEF) activity of Ragulator (right, bottom). Ragulator acts as a GEF for RAGA/B, promoting the conversion of RAGA/B·GDP to RAGA/B·GTP. Amino acids also facilitate the formation of RAGC/D·GDP, giving rise to the active RAG complex, RAGA/B·GTP–RAGC/D·GDP. The mechanisms involved in switching the guanine nucleotide state of RAGC/D are not clear. mTORC1 binds to the RAG complex and is recruited to the lysosome through an unknown mechanism, where it becomes activated. Leucyl-tRNA synthetase (LeuRS) may act as a direct sensor for the amino acid Leu in the cytoplasm and might be involved in the activation of mTORC1. In yeast under amino acid sufficiency, TORC1, already bound to the vacuole, is activated when Gtr1 is loaded with GTP and Gtr2 is loaded with GDP (left, bottom). LeuRS has also been reported to have a role in TORC1 activation in yeast.

Figure 3. mTORC1 activation at the lysosome

Amino acids are thought to accumulate within the lysosomal lumen and to signal to vacuolar H⁺-ATPase (v-ATPase) through an ‘inside–out’ mechanism. v-ATPase controls RAG GTPase–Ragulator binding, and therefore Ragulator guanine exchange factor (GEF) activity and RAGA and RAGB guanine nucleotide loading (RAGA/B·GTP). The active RAG complex (RAGA/B·GTP–RAGC/D·GDP) binds to mammalian target of rapamycin complex 1 (mTORC1) and recruits it to the lysosome, through an unknown mechanism, possibly in close proximity to RHEB (RAS homologue enriched in brain). Downstream of growth factor signalling, GTP-bound RHEB potently activates mTORC1.