Spatially Distinct Pools of TORC1 Balance Protein Homeostasis.

The eukaryotic TORC1 kinase is a homeostatic controller of growth that integrates nutritional cues and mediates signals primarily from the surface of lysosomes or vacuoles. Amino acids activate TORC1 via the Rag GTPases that combine into structurally conserved multi-protein complexes such as the EGO complex (EGOC) in yeast. Here we show that Ego1, which mediates membrane-anchoring of EGOC via lipid modifications that it acquires while traveling through the trans-Golgi network, is separately sorted to vacuoles and perivacuolar endosomes. At both surfaces, it assembles EGOCs, which regulate spatially distinct pools of TORC1 that impinge on functionally divergent effectors: vacuolar TORC1 predominantly targets Sch9 to promote protein synthesis, whereas endosomal TORC1 phosphorylates Atg13 and Vps27 to inhibit macroautophagy and ESCRT-driven microautophagy, respectively. Thus, the coordination of three key regulatory nodes in protein synthesis and degradation critically relies on a division of labor between spatially sequestered populations of TORC1.

News about amino acid metabolism in plant-microbe interactions.

Plants constantly come into contact with a diverse mix of pathogenic and beneficial microbes. The ability to distinguish between them and to respond appropriately is essential for plant health. Here we review recent progress in understanding the role of amino acid sensing, signaling, transport, and metabolism during plant-microbe interactions. Biochemical pathways converting individual amino acids into active compounds have recently been elucidated, and comprehensive large-scale approaches have brought amino acid sensors and transporters into focus. These findings show that plant central amino acid metabolism is closely interwoven with stress signaling and defense responses at various levels. The individual biochemical mechanisms and the interconnections between the different processes are just beginning to emerge and might serve as a foundation for new plant protection strategies.

GATOR2 complex-mediated amino acid signaling regulates brain myelination.

Amino acids are essential for cell growth and metabolism. Amino acid and growth factor signaling pathways coordinately regulate the mechanistic target of rapamycin complex 1 (mTORC1) kinase in cell growth and organ development. While major components of amino acid signaling mechanisms have been identified, their biological functions in organ development are unclear. We aimed to understand the functions of the critically positioned amino acid signaling complex GAP activity towards Rags 2 (GATOR2) in brain development. GATOR2 mediates amino acid signaling to mTORC1 by directly linking the amino acid sensors for arginine and leucine to downstream signaling complexes. Now, we report a role of GATOR2 in oligodendrocyte myelination in postnatal brain development. We show that the disruption of GATOR2 complex by genetic deletion of meiosis regulator for oocyte development (Mios, encoding a component of GATOR2) selectively impairs the formation of myelinating oligodendrocytes, thus brain myelination, without apparent effects on the formation of neurons and astrocytes. The loss of Mios impairs cell cycle progression of oligodendrocyte precursor cells, leading to their reduced proliferation and differentiation. Mios deletion manifests a cell type-dependent effect on mTORC1 in the brain, with oligodendroglial mTORC1 selectively affected. However, the role of Mios/GATOR2 in oligodendrocyte formation and myelination involves mTORC1-independent function. This study suggests that GATOR2 coordinates amino acid and growth factor signaling to regulate oligodendrocyte myelination.

Ssy1 functions at the plasma membrane as a receptor of extracellular amino acids independent of plasma membrane-endoplasmic reticulum junctions.

Evidence from multiple laboratories has implicated Ssy1, a nontransporting amino acid permease, as the receptor component of the yeast plasma membrane (PM)-localized SPS (Ssy1-Ptr3-Ssy5)-sensor. Upon binding external amino acids, Ssy1 is thought to initiate signaling events leading to the induction of amino acid permease gene expression. In striking contrast, Kralt et al (2015) (Traffic 16:135-147) have questioned the role of Ssy1 in amino acid sensing and reported that Ssy1 is a component of the endoplasmic reticulum (ER), where it reportedly participates in the formation of ER-PM junctions. Here, we have re-examined the intracellular location of Ssy1 and tested the role of ER-PM junctions in SPS sensor signaling. We show that the C-terminal of Ssy1 carries a functional ER-export motif required for proper localization of Ssy1 to the PM. Furthermore, ER-PM junctions are dispensable for PM-localization and function of Ssy1; Ssy1 localizes to the PM in a Δtether strain lacking ER-PM junctions (ist2Δ scs2Δ scs22Δ tcb1Δ tcb2Δ tcb3Δ), and this strain retains the ability to initiate signals induced by extracellular amino acids. The data demonstrate that Ssy1 functions as the primary amino acid receptor and that it carries out this function at the PM.

Intrinsically disordered linker and plasma membrane-binding motif sort Ist2 and Ssy1 to junctions.

Membrane junctions or contact sites are close associations of lipid bilayers of heterologous organelles. Ist2 is an endoplasmic reticulum (ER)-resident transmembrane protein that mediates associations between the plasma membrane (PM) and the cortical ER (cER) in baker's yeast. We asked the question what structure in Ist2 bridges the up to 30 nm distance between the PM and the cER and we noted that the region spacing the transmembrane domain from the cortical sorting signal interacting with the PM is predicted to be intrinsically disordered (ID). In Ssy1, a protein that was not previously described to reside at membrane junctions, we recognized a domain organization similar to that in Ist2. We found that the localization of both Ist2 and Ssy1 at the cell periphery depends on the presence of a PM-binding domain, an ID linker region of sufficient length and a transmembrane domain that most probably resides in the ER. We show for the first time that an ID amino acid domain bridges adjacent heterologous membranes. The length and flexibility of ID domains make them uniquely eligible for spanning large distances, and we suggest that this domain structure occurs more frequently in proteins that mediate the formation of membrane contact sites.

Structure-activity relations of leucine derivatives reveal critical moieties for cellular uptake and activation of mTORC1-mediated signaling.

Among amino acids, leucine is a potential signaling molecule to regulate cell growth and metabolism by activating mechanistic target of rapamycin complex 1 (mTORC1). To reveal the critical structures of leucine molecule to activate mTORC1, we examined the structure-activity relationships of leucine derivatives in HeLa S3 cells for cellular uptake and for the induction of phosphorylation of p70 ribosomal S6 kinase 1 (p70S6K), a downstream effector of mTORC1. The activation of mTORC1 by leucine and its derivatives was the consequence of two successive events: the cellular uptake by L-type amino acid transporter 1 (LAT1) responsible for leucine uptake in HeLa S3 cells and the activation of mTORC1 following the transport. The structural requirement for the recognition by LAT1 was to have carbonyl oxygen, alkoxy oxygen of carboxyl group, amino group and hydrophobic side chain. In contrast, the requirement for mTORC1 activation was more rigorous. It additionally required fixed distance between carbonyl oxygen and alkoxy oxygen of carboxyl group, and amino group positioned at α-carbon. L-Configuration in chirality and appropriate length of side chain with a terminal isopropyl group were also important. This confirmed that LAT1 itself is not a leucine sensor. Some specialized leucine sensing mechanism with rigorous requirement for agonistic structures should exist inside the cells because leucine derivatives not transported by LAT1 did not activate mTORC1. Because LAT1-mTOR axis is involved in the regulation of cell growth and cancer progression, the results from this study may provide a new insight into therapeutics targeting both LAT1 and leucine sensor.

Intracellular Ca(2+) oscillations generated via the extracellular Ca(2+)-sensing receptor (CaSR) in response to extracellular Ca(2+) or L-phenylalanine: Impact of the highly conservative mutation Ser170Thr.

The extracellular Ca(2+)-sensing receptor (CaSR) is an allosteric protein that responds to changes in the extracellular concentration of Ca(2+) ([Ca(2+)]e) and aromatic amino acids with the production of different patterns of oscillations in intracellular Ca(2+) concentration ([Ca(2+)]i). An increase in [Ca(2+)]e stimulates sinusoidal oscillations in [Ca(2+)]i whereas aromatic amino acid-induced CaR activation in the presence of a threshold [Ca(2+)]e promotes transient oscillations in [Ca(2+)]i. Here, we examined spontaneous and ligand-evoked [Ca(2+)]i oscillations in single HEK-293 cells transfected with the wild type CaSR or with a mutant CaSR in which Ser170 was converted to Thr (CaSRS170T). Our analysis demonstrates that cells expressing CaSRS170T display [Ca(2+)]i oscillations in the presence of low concentrations of extracellular Ca(2+) and respond to L-Phe with robust transient [Ca(2+)]i oscillations. Our results indicate that the S170T mutation induces a marked increase in CaSR sensitivity to [Ca(2+)]e and imply that the allosteric regulation of the CaSR by aromatic amino acids is not only mediated by an heterotropic positive effect on Ca(2+) binding cooperativity but, as biased agonists, aromatic amino acids stabilize a CaSR conformation that couples to a different signaling pathway leading to transient [Ca(2+)]i oscillations.

Diversity of amino acid signaling pathways on autophagy regulation: a novel pathway for arginine.

Autophagy is the intracellular bulk degradation process to eliminate damaged cellular machinery and to recycle building blocks, and is crucial for cell survival and cell death. Amino acids modulate autophagy in response to nutrient starvation and oxidative stress. We investigated the relevance of reactive oxygen species (ROS) production on the regulation of autophagy using amino acids, both as a mixture and individually, in rat hepatoma H4-II-E cells. Nutrient starvation elevated ROS production and stimulated autophagy. Treatment with complete (CAA), regulatory (RegAA) and non-regulatory (NonRegAA) amino acid mixtures showed significant suppression of ROS production, whereas only CAA and RegAA exhibited significant suppression of autophagy, suggesting a dissociation of the two responses. The effects of individual amino acids were examined. Leucine from RegAA decreased ROS production and suppressed autophagy. However, methionine and proline from RegAA and arginine, cystine and glutamic acid from NonRegAA suppressed autophagy with an opposite increase in ROS production. Other amino acids from the NonRegAA group showed stimulating effects on ROS production without an autophagic response. Arginine's effect on autophagy suppression was not blocked by rapamycin, indicating an mTOR-independent pathway. Inhibitor studies on arginine-regulated autophagy may indicate the involvement of NO pathway, which is independent from ROS and mTOR pathways.

BmSLC7A5 is essential for silk protein synthesis and larval development in Bombyx mori.

Insects produce silk to form cocoons, nests, and webs, which are important for their survival and reproduction. However, little is known about the molecular mechanism of silk protein synthesis at the translation level. The solute carrier family 7 (SLC7) genes are involved in activating the target of rapamycin complex 1 (TORC1) signaling pathway and protein translation process, but the physiological roles of SLC7 genes in silk-producing insects have not been reported. Here, we found that amino acid signaling regulates silk protein synthesis and larval development via the L-type amino acid transporter 1 (LAT1; also known as SLC7A5) in Bombyx mori. A total of 12 SLC7 homologs were identified in the silkworm genome, among which BmSLC7A5 was found to be a silk gland-enriched gene and may be involved in leucine transport. Bioinformatics analysis indicated that SLC7A5 displays high homology and a close phylogenetic relationship in silk-producing insects. Subsequently, we found that leucine treatment significantly increased silk protein synthesis by improving the transcription and protein levels of silk genes. Furthermore, systemic and silk gland-specific knockout of BmSLC7A5 led to decreased silk protein synthesis by inhibiting TORC1 signaling, and somatic mutation also resulted in arrested development from the 5th instar to the early pupal stage. Altogether, our study reveals that BmSLC7A5 is involved in regulating silk protein synthesis and larval development by affecting the TORC1 signaling pathway, which provides a new strategy and target for improving silk yield.

Tissue-specific expression differences in Ras-related GTP-binding proteins in male rats.

The protein kinase Mechanistic Target of Rapamycin (mTOR) in Complex 1 (mTORC1) is regulated in part by the Ras-related GTP-binding proteins (Rag GTPases). Rag GTPases form a heterodimeric complex consisting of either RagA or RagB associated with either RagC or RagD and act to localize mTORC1 to the lysosomal membrane. Until recently, RagA and RagB were thought to be functionally redundant, as were RagC and RagD. However, recent research suggests that the various isoforms differentially activate mTORC1. Here, the mRNA expression and protein abundance of the Rag GTPases was compared across male rat skeletal muscle, heart, liver, kidney, and brain. Whereas mRNA expression of RagA was higher than RagB in nearly all tissues studied, RagB protein abundance was higher than RagA in all tissues besides skeletal muscle. RagC mRNA expression was more abundant or equal to RagD mRNA, and RagD protein was more abundant than RagC protein in all tissues. Moreover, the proportion of RagB in the short isoform was greater than the long in liver, whereas the opposite was true in brain. These results serve to further elucidate Rag GTPase expression and offer potential explanations for the differential responses to amino acids that are observed in different tissues.

Merging Signaling with Structure: Functions and Mechanisms of Plant Glutamate Receptor Ion Channels.

Plant glutamate receptor-like (GLR) genes encode ion channels with demonstrated roles in electrical and calcium (Ca2+) signaling. The expansion of the GLR family along the lineage of land plants, culminating in the appearance of a multiclade system among flowering plants, has been a topic of interest since their discovery nearly 25 years ago. GLRs are involved in many physiological processes, from wound signaling to transcriptional regulation to sexual reproduction. Emerging evidence supports the notion that their fundamental functions are conserved among different groups of plants as well. In this review, we update the physiological and genetic evidence for GLRs, establishing their role in signaling and cell-cell communication. Special emphasis is given to the recent discussion of GLRs' atomic structures. Along with functional assays, a structural view of GLRs' molecular organization presents a window for novel hypotheses regarding the molecular mechanisms underpinning signaling associated with the ionic fluxes that GLRs regulate. Newly uncovered transcriptional regulations associated with GLRs-which propose the involvement of genes from all clades ofArabidopsis thaliana in ways not previously observed-are discussed in the context of the broader impacts of GLR activity. We posit that the functions of GLRs in plant biology are probably much broader than anticipated, but describing their widespread involvement will only be possible with (a) a comprehensive understanding of the channel's properties at the molecular and structural levels, including protein-protein interactions, and (b) the design of new genetic approaches to explore stress and pathogen responses where precise transcriptional control may result in more precise testable hypotheses to overcome their apparent functional redundancies.

Amino Acid Signaling for TOR in Eukaryotes: Sensors, Transducers, and a Sustainable Agricultural fuTORe.

Eukaryotic cells monitor and regulate metabolism through the atypical protein kinase target of rapamycin (TOR) regulatory hub. TOR is activated by amino acids in animals and fungi through molecular signaling pathways that have been extensively defined in the past ten years. Very recently, several studies revealed that TOR is also acutely responsive to amino acid metabolism in plants, but the mechanisms of amino acid sensing are not yet established. In this review, we summarize these discoveries, emphasizing the diversity of amino acid sensors in human cells and highlighting pathways that are indirectly sensitive to amino acids, i.e., how TOR monitors changes in amino acid availability without a bona fide amino acid sensor. We then discuss the relevance of these model discoveries to plant biology. As plants can synthesize all proteinogenic amino acids from inorganic precursors, we focus on the possibility that TOR senses both organic metabolites and inorganic nutrients. We conclude that an evolutionary perspective on nutrient sensing by TOR benefits both agricultural and biomedical science, contributing to ongoing efforts to generate crops for a sustainable agricultural future.

Amino acid transporter LAT1 (SLC7A5) as a molecular target for cancer diagnosis and therapeutics.

Cancer cells require a massive supply of nutrients, including sugars and amino acids-the upregulation of transporters for each nutrient contributes to meet the demand. Distinct from glucose transporters, amino acid transporters include ones whose expression is specific to cancer cells. For example, LAT1 (SLC7A5) displays protein expression mostly limited to the plasma membrane of cancer cells. The exceptions are the placental barrier and the blood-brain barrier, where immunohistochemical and mass spectrometric studies have shown LAT1 expression, although their levels are supposed to be lower than those in cancers. The expression of LAT1 has been reported in cancers from various tissue origins, where high LAT1 expression is related to the poor prognosis of patients. LAT1 is essential for cancer cell growth because the pharmacologic inhibition and knockdown/knockout of LAT1 suppress the proliferation of cancer cells and the growth of xenograft tumors. The inhibition of LAT1 suppresses protein synthesis by downregulating the mTORC1 signaling pathway and mobilizing the general amino acid control (GAAC) pathway in cancer cells. LAT1 is, thus, a candidate molecular target for the diagnosis and therapeutics of cancers. 18F-labeled 3-fluoro-l-α-methyl-tyrosine (FAMT) is used as a LAT1-specific PET probe for cancer detection due to the LAT1 specificity of α-methyl aromatic amino acids. FAMT accumulation is cancer-specific and avoids non-cancer lesions, including inflammation, confirming the cancer-specific expression of LAT1 in humans. Due to the cancer-specific nature, LAT1 can also be used for cancer-specific delivery of anti-tumor agents such as l-para-boronophenylalanine used for boron neutron capture therapy and α-emitting nuclide-labeled LAT1 substrates developed for nuclear medicine treatment. Based on the importance of LAT1 in cancer progression, high-affinity LAT1-specific inhibitors have been developed for anti-tumor drugs. JPH203 (KYT0353) is such a compound designed based on the structure-activity relationship of LAT1 ligands. It is one of the highest-affinity inhibitors with less affecting other transporters. It suppresses tumor growth in vivo without significant toxicity in preclinical studies at doses enough to suppress tumor growth. In the phase-I clinical trial, JPH203 appeared to provide promising activity. Because the mechanisms of action of LAT1 inhibitors are novel, with or without combination with other anti-tumor drugs, they could contribute to the treatment of cancers that do not respond to current therapy. The LAT1-specific PET probe could also be used as companion diagnostics of the LAT1-targeting therapies to select patients to whom therapeutic benefits could be expected. Recently, the cryo-EM structure of LAT1 has been solved, which would facilitate the understanding of the mechanisms of the dynamic interaction of ligands and the binding site, and further designing new compounds with higher activity.

SEA and GATOR 10 Years Later.

The SEA complex was described for the first time in yeast Saccharomyces cerevisiae ten years ago, and its human homologue GATOR complex two years later. During the past decade, many advances on the SEA/GATOR biology in different organisms have been made that allowed its role as an essential upstream regulator of the mTORC1 pathway to be defined. In this review, we describe these advances in relation to the identification of multiple functions of the SEA/GATOR complex in nutrient response and beyond and highlight the consequence of GATOR mutations in cancer and neurodegenerative diseases.

Aminoacyl-tRNA synthetases and amino acid signaling.

Aminoacyl-tRNA synthetases (ARSs) are a family of evolutionarily conserved housekeeping enzymes used for protein synthesis that have pivotal roles in the ligation of tRNA with their cognate amino acids. Recent advances in the structural and functional studies of ARSs have revealed many previously unknown biological functions beyond the classical catalytic roles. Sensing the sufficiency of intracellular nutrients such as amino acids, ATP, and fatty acids is a crucial aspect for every living organism, and it is closely connected to the regulation of diverse cellular physiologies. Notably, among ARSs, leucyl-tRNA synthetase 1 (LARS1) has been identified to perform specifically as a leucine sensor upstream of the amino acid-sensing pathway and thus participates in the coordinated control of protein synthesis and autophagy for cell growth. In addition to LARS1, other types of ARSs are also likely involved in the sensing and signaling of their cognate amino acids inside cells. Collectively, this review focuses on the mechanisms of ARSs interacting within amino acid signaling and proposes the possible role of ARSs as general intracellular amino acid sensors.

A GC-MS/Single-Cell Method to Evaluate Membrane Transporter Substrate Specificity and Signaling.

Amino acid transporters play a vital role in metabolism and nutrient signaling pathways. Typically, transport activity is investigated using single substrates and competing amounts of other amino acids. We used GC-MS and LC-MS for metabolic screening of Xenopus laevis oocytes expressing various human amino acid transporters incubated in complex media to establish their comprehensive substrate profiles. For most transporters, amino acid selectivity matched reported substrate profiles. However, we could not detect substantial accumulation of cationic amino acids by SNAT4 and ATB0,+ in contrast to previous reports. In addition, comparative substrate profiles of two related sodium neutral amino acid transporters known as SNAT1 and SNAT2, revealed the latter as a significant leucine accumulator. As a consequence, SNAT2, but not SNAT1, was shown to be an effective activator of the eukaryotic cellular growth regulator mTORC1. We propose, that metabolomic profiling of membrane transporters in Xe nopus laevis oocytes can be used to test their substrate specificity and role in intracellular signaling pathways.

The Rag GTPase Regulates the Dynamic Behavior of TSC Downstream of Both Amino Acid and Growth Factor Restriction.

The dysregulation of the metabolic regulator TOR complex I (TORC1) contributes to a wide array of human pathologies. Tuberous sclerosis complex (TSC) is a potent inhibitor of TORC1. Here, we demonstrate that the Rag GTPase acts in both the amino-acid-sensing and growth factor signaling pathways to control TORC1 activity through the regulation of TSC dynamics in HeLa cells and Drosophila. We find that TSC lysosomal-cytosolic exchange increases in response to both amino acid and growth factor restriction. Moreover, the rate of exchange mirrors TSC function, with depletions of the Rag GTPase blocking TSC lysosomal mobility and rescuing TORC1 activity. Finally, we show that the GATOR2 complex controls the phosphorylation of TSC2, which is essential for TSC exchange. Our data support the model that the amino acid and growth factor signaling pathways converge on the Rag GTPase to inhibit TORC1 activity through the regulation of TSC dynamics.

A spatially and functionally distinct pool of TORC1 defines signaling endosomes in yeast.

The evolutionarily conserved target of rapamycin complex 1 (TORC1) regulates cell growth in a homeostatic manner by tuning anabolic and catabolic processes in response to nutritional and hormonal cues. Interestingly, rather than being localized at the plasma membrane as perhaps expected for an integrator of extracellular signals, TORC1 mainly localizes at vacuolar (in yeast) and lysosomal (in more complex eukaryotes) membranes where it seems optimally placed to sense both the nutrient status within the cytoplasm and the vacuolar/lysosomal compartment. How TORC1 controls downstream targets that are distant from the vacuole/lysosome, is currently poorly understood. In this context, we recently identified and characterized 2 spatially and functionally distinct pools of TORC1 in the budding yeast Saccharomyces cerevisiae: one at the vacuole that promotes protein synthesis, and another one at endosomes that inhibits protein degradation. Thus, our findings highlight the presence of spatially separated pools of TORC1 that are commissioned with functionally specific tasks within cells. In addition, they pinpoint the existence of signaling endosomes in yeast, which raises numerous new questions that are warranted to direct future research in this area.

Autophagy contributes to sulfonylurea herbicide tolerance via GCN2-independent regulation of amino acid homeostasis.

Sulfonylurea (SU) herbicides inhibit branched-chain amino acid (BCAA) biosynthesis by targeting acetolactate synthase. Plants have evolved target-site resistance and metabolic tolerance to SU herbicides; the GCN2 (general control non-repressible 2) pathway is also involved in SU tolerance. Here, we report a novel SU tolerance mechanism, autophagy, which we call 'homeostatic tolerance,' is involved in amino acid signaling in Arabidopsis. The activation and reversion of autophagy and GCN2 by the SU herbicide tribenuron-methyl (TM) and exogenous BCAA, respectively, confirmed that TM-induced BCAA starvation is responsible for the activation of autophagy and GCN2. Genetic and biochemical analyses revealed a lower proportion of free BCAA and more sensitive phenotypes in atg5, atg7, and gcn2 single mutants than in wild-type seedlings after TM treatment; the lowest proportion of free BCAA and the most sensitive phenotypes were found in atg5 gcn2 and atg7 gcn2 double mutants. Immunoblotting and microscopy revealed that TM-induced activation of autophagy and GCN2 signaling do not depend on the presence of each other, and these 2 pathways may serve as mutually compensatory mechanisms against TM. TM inhibited the TOR (target of rapamycin), and activated autophagy in an estradiol-induced TOR RNAi line, suggesting that TM-induced BCAA starvation activates autophagy, probably via TOR inactivation. Autophagy and GCN2 were also activated, and independently contributed to TM tolerance in plants conferring metabolic tolerance. Together, these data suggest that autophagy is a proteolytic process for amino acid recycling and contributes to GCN2-independent SU tolerance, probably by its ability to replenish fresh BCAA.

Feedback Inhibition of the Rag GTPase GAP Complex Lst4-Lst7 Safeguards TORC1 from Hyperactivation by Amino Acid Signals.

Amino acids stimulate the eukaryotic target of rapamycin complex 1 (TORC1), and hence growth, through the Rag GTPases and their regulators. Among these, the yeast Lst4-Lst7 Rag GTPase GAP complex clusters, as we previously reported, at the vacuolar membrane upon amino acid starvation. In response to amino acid refeeding, it activates the Rag GTPase-TORC1 branch and is then dispersed from the vacuolar surface. Here, we show that the latter effect is driven by TORC1 itself, which directly phosphorylates several residues within the intra-DENN loop of Lst4 that, only in its non-phosphorylated state, tethers the Lst4-Lst7 complex to the vacuolar membrane. An Lst4 variant disrupting this feedback inhibition mechanism causes TORC1 hyperactivation and proliferation defects in cells grown on poor nitrogen sources. Thus, we identify Lst4 as a TORC1 target and key node of a homeostatic mechanism that adjusts TORC1 activity to the availability of amino acids.