Rheb Activation Research Articles

Tuberous Sclerosis Tumor Suppressor Complex-like Complexes Act as GTPase-activating Proteins for Ral GTPases

The small GTPases RalA and RalB are multifunctional proteins regulating a variety of cellular processes. Like other GTPases, the activity of Ral is regulated by the opposing effects of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Although several RalGEFs have been identified and characterized, the molecular identity of RalGAP remains unknown. Here, we report the first molecular identification of RalGAPs, which we have named RalGAP1 and RalGAP2. They are large heterodimeric complexes, each consisting of a catalytic alpha1 or alpha2 subunit and a common beta subunit. These RalGAP complexes share structural and catalytic similarities with the tuberous sclerosis tumor suppressor complex, which acts as a GAP for Rheb. In vitro GTPase assays revealed that recombinant RalGAP1 accelerates the GTP hydrolysis rate of RalA by 280,000-fold. Heterodimerization was required for this GAP activity. In PC12 cells, knockdown of the beta subunit led to sustained Ral activation upon epidermal growth factor stimulation, indicating that the RalGAPs identified here are critical for efficient termination of Ral activation induced by extracellular stimuli. Our identification of RalGAPs will enable further understanding of Ral signaling in many biological and pathological processes.

Rheb controls misfolded protein metabolism by inhibiting aggresome formation and autophagy

Perinuclear aggresome formation is a key mechanism to dispose of misfolded proteins that exceed the degradative capacity of ubiquitin-proteasome and autophagy-lysosome systems. Functional blockade of either degradative system leads to an enhanced aggresome formation. The tuberous sclerosis complex-Ras homologue enriched in brain-mammalian target of rapamycin (TSC-Rheb-mTOR) pathway is known to play a central role in modulating protein synthesis and autophagy. However, in spite of the constitutive activation of mTOR and the abrogated autophagy activity in TSC1- or TSC2-deficient cells, the TSC mutant cells are defective in aggresome formation and undergo apoptosis upon misfolded protein accumulation both in vitro and in vivo. High Rheb activity in TSC mutant cells inhibits aggresome formation and sensitizes cell death in response to misfolded proteins. Surprisingly, this previously unrecognized function of Rheb is independent of TOR complex 1. Active Rheb disrupts the interaction between dynein and misfolded protein cargos, and therefore blocks aggresome formation by inhibiting dynein-dependent transportation of misfolded proteins. This study reveals a function of Rheb in controlling misfolded protein metabolism by modulating aggresome formation.

The Switch I Region of Rheb Is Critical for Its Interaction with FKBP38

The Ras-like small GTPase Rheb is an upstream activator of the mammalian target of rapamycin (mTOR). It has recently been shown that Rheb activates mTOR by binding to its endogenous inhibitor FKBP38 and preventing it from association with mTOR. The interaction of Rheb with FKBP38 is controlled by its guanine nucleotide binding states, which are responsive to growth factor and amino acid conditions. In this study, we show that Rheb interacts with FKBP38 through a section within its switch I region that is equivalent to the effector domain of other Ras-like small GTPases. We find that the ability for Rheb to interact with FKBP38 correlates with its activity for mTOR activation. Our findings suggest that FKBP38 is a bona fide effector of Rheb and that the ability to interact with FKBP38 is important for Rheb as an activator of mTOR.

Tumorigenic activity and therapeutic inhibition of Rheb GTPase

The AKT-mTOR pathway harbors several known and putative oncogenes and tumor suppressors. In a phenotypic screen for lymphomagenesis, we tested candidate genes acting upstream of and downstream from mTOR in vivo. We find that Rheb, a proximal activator of mTORC1, can produce rapid development of aggressive and drug-resistant lymphomas. Rheb causes mTORC1-dependent effects on apoptosis, senescence, and treatment responses that resemble those of Akt. Moreover, Rheb activity toward mTORC1 requires farnesylation and is readily blocked by a pharmacological inhibitor of farnesyltransferase (FTI). In Pten-deficient tumor cells, inhibition of Rheb by FTI is responsible for the drug's anti-tumor effects, such that a farnesylation-independent mutant of Rheb renders these tumors resistant to FTI therapy. Notably, RHEB is highly expressed in some human lymphomas, resulting in mTORC1 activation and increased sensitivity to rapamycin and FTI. Downstream from mTOR, we examined translation initiation factors that have been implicated in transformation in vitro. Of these, only eIF4E was able to enhance lymphomagenesis in vivo. In summary, the Rheb GTPase is an oncogenic activity upstream of mTORC1 and eIF4E and a direct therapeutic target of farnesyltransferase inhibitors in cancer.

The Mammalian Target of Rapamycin Complex 1 Regulates Leptin Biosynthesis in Adipocytes at the Level of Translation: The Role of the 5′-Untranslated Region in the Expression of Leptin Messenger Ribonucleic Acid

Leptin production by adipose cells in vivo is increased after feeding and decreased by food deprivation. However, molecular mechanisms that control leptin expression in response to food intake remain unknown. Here, we test the hypothesis that leptin expression in adipose cells is regulated by nutrient- and insulin-sensitive mammalian target of rapamycin complex 1 (mTORC1)-mediated pathway. The activity of mTORC1 in 3T3-L1 adipocytes was up-regulated by stable expression of either constitutively active Rheb or dominant-negative AMP-activated protein kinase. In both cases, expression of endogenous leptin was significantly elevated at the level of translation. To investigate the role of leptin 5'-untranslated region (UTR) in the regulation of protein expression, we created bicistronic reporter constructs with and without the 5'-UTR. We found that the presence of leptin 5'-UTR renders mRNA resistant to regulation by mTORC1. It appears, therefore, that mTORC1 controls translation of leptin mRNA via a novel mechanism that does not require the presence of either the 5'-terminal oligopyrimidine tract or the 5'-UTR.

A Small Molecule Inhibitor of Isoprenylcysteine Carboxymethyltransferase Induces Autophagic Cell Death in PC3 Prostate Cancer Cells

A number of proteins involved in cell growth control, including members of the Ras family of GTPases, are modified at their C terminus by a three-step posttranslational process termed prenylation. The enzyme isoprenylcysteine carboxylmethyl-transferase (Icmt) catalyzes the last step in this process, and genetic and pharmacological suppression of Icmt activity significantly impacts on cell growth and oncogenesis. Screening of a diverse chemical library led to the identification of a specific small molecule inhibitor of Icmt, cysmethynil, that inhibited growth factor signaling and tumorigenesis in an in vitro cancer cell model (Winter-Vann, A. M., Baron, R. A., Wong, W., dela Cruz, J., York, J. D., Gooden, D. M., Bergo, M. O., Young, S. G., Toone, E. J., and Casey, P. J. (2005) Proc. Natl. Acad. Sci. U. S. A. 102, 4336-4341). To further evaluate the mechanisms through which this Icmt inhibitor impacts on cancer cells, we developed both in vitro and in vivo models utilizing PC3 prostate cancer cells. Treatment of these cells with cysmethynil resulted in both an accumulation of cells in the G(1) phase and cell death. Treatment of mice harboring PC3 cell-derived xenograft tumors with cysmethynil resulted in markedly reduced tumor size. Analysis of cell death pathways unexpectedly showed minimal impact of cysmethynil treatment on apoptosis; rather, drug treatment significantly enhanced autophagy and autophagic cell death. Cysmethynil-treated cells displayed reduced mammalian target of rapamycin (mTOR) signaling, providing a potential mechanism for the excessive autophagy as well as G(1) cell cycle arrest observed. These results identify a novel mechanism for the antitumor activity of Icmt inhibition. Further, the dual effects of cell death and cell cycle arrest by cysmethynil treatment strengthen the rationale for targeting Icmt in cancer chemotherapy.

Phospholipase D1 is an effector of Rheb in the mTOR pathway

The mammalian target of rapamycin (mTOR) assembles a signaling network essential for the regulation of cell growth, which has emerged as a major target of anticancer therapies. The tuberous sclerosis complex 1 and 2 (TSC1/2) proteins and their target, the small GTPase Rheb, constitute a key regulatory pathway upstream of mTOR. Phospholipase D (PLD) and its product phosphatidic acid are also upstream regulators of the mitogenic mTOR signaling. However, how the TSC/Rheb and PLD pathways interact or integrate in the rapamycin-sensitive signaling network has not been examined before. Here, we find that PLD1, but not PLD2, is required for Rheb activation of the mTOR pathway, as demonstrated by the effects of RNAi. The overexpression of Rheb activates PLD1 in cells in the absence of mitogenic stimulation, and the knockdown of Rheb impairs serum stimulation of PLD activation. Furthermore, the overexpression of TSC2 suppresses PLD1 activation, whereas the knockdown or deletion of TSC2 leads to elevated basal activity of PLD. Consistent with a TSC-Rheb-PLD signaling cascade, AMPK and PI3K, both established regulators of TSC2, appear to lie upstream of PLD as revealed by the effects of pharmacological inhibitors, and serum activation of PLD is also dependent on amino acid sufficiency. Finally, Rheb binds and activates PLD1 in vitro in a GTP-dependent manner, strongly suggesting that PLD1 is a bona fide effector for Rheb. Hence, our findings reveal an unexpected interaction between two cascades in the mTOR signaling pathways and open up additional possibilities for targeting this important growth-regulating network for the development of anticancer drugs.

Constitutively active Rheb induces oncogenic transformation

Rheb (Ras-homolog enriched in brain) is a component of the phosphatidylinositol 3-kinase (PI3K) target of rapamycin (TOR) signaling pathway, functioning as a positive regulator of TOR. Constitutively active mutants of Rheb induce oncogenic transformation in cell culture. The transformed cells are larger and contain more protein than their normal counterparts. They show constitutive phosphorylation of the ribosomal protein S6 kinase and the eukaryotic initiation factor 4E-binding protein 1, two downstream targets of TOR. The TOR-specific inhibitor rapamycin strongly interferes with transformation induced by constitutively active Rheb, suggesting that TOR activity is essential for the oncogenic effects of mutant Rheb. Rheb-induced transformation is also dependent on a C-terminal farnesylation signal that mediates localization to a cellular membrane. An engineered N-terminal myristylation signal can substitute for the farnesylation. Immunofluorescence localizes wild-type and mutant Rheb to vesicular structures in the cytoplasm, overlapping with the endoplasmic reticulum.

The Rag GTPases Bind Raptor and Mediate Amino Acid Signaling to mTORC1

The multiprotein mTORC1 protein kinase complex is the central component of a pathway that promotes growth in response to insulin, energy levels, and amino acids and is deregulated in common cancers. We find that the Rag proteins--a family of four related small guanosine triphosphatases (GTPases)--interact with mTORC1 in an amino acid-sensitive manner and are necessary for the activation of the mTORC1 pathway by amino acids. A Rag mutant that is constitutively bound to guanosine triphosphate interacted strongly with mTORC1, and its expression within cells made the mTORC1 pathway resistant to amino acid deprivation. Conversely, expression of a guanosine diphosphate-bound Rag mutant prevented stimulation of mTORC1 by amino acids. The Rag proteins do not directly stimulate the kinase activity of mTORC1, but, like amino acids, promote the intracellular localization of mTOR to a compartment that also contains its activator Rheb.

Regulating the Regulators

Growth factors and nutrients, particularly amino acids, regulate cell growth and proliferation in part through the serine-threonine protein kinase mTOR (mammalian target of rapamycin). Activity of mTOR is regulated through the small guanosine triphosphatase Rheb (Ras homolog enriched in brain). Bai et al . (see the Perspective by Proud) reveal a mechanism by which Rheb could control activity of mTOR. The protein FKBP38 binds to mTOR and inhibits its kinase activity. Active GTP-bound Rheb can interact with FKBP38 and thereby free mTOR from inhibition. This process may represent a critical control mechanism whose altered function is associated with human diseases, including cancer. X. Bai, D. Ma, A. Liu, X. Shen, Q. J. Wang, Y. Liu, Y. Jiang, Rheb activates mTOR by antagonizing its endogenous inhibitor, FKBP38. Science 318 , 977-980 (2007). [Abstract] [Full Text] C. G. Proud, mTOR, unleashed. Science 318 , 926-927 (2007). [Summary] [Full Text]

PRAS40 Is an Insulin-Regulated Inhibitor of the mTORC1 Protein Kinase

The heterotrimeric mTORC1 protein kinase nucleates a signaling network that promotes cell growth in response to insulin and becomes constitutively active in cells missing the TSC1 or TSC2 tumor suppressors. Insulin stimulates the phosphorylation of S6K1, an mTORC1 substrate, but it is not known how mTORC1 kinase activity is regulated. We identify PRAS40 as a raptor-interacting protein that binds to mTORC1 in insulin-deprived cells and whose in vitro interaction with mTORC1 is disrupted by high salt concentrations. PRAS40 inhibits cell growth, S6K1 phosphorylation, and rheb-induced activation of the mTORC1 pathway, and in vitro it prevents the great increase in mTORC1 kinase activity induced by rheb1-GTP. Insulin stimulates Akt/PKB-mediated phosphorylation of PRAS40, which prevents its inhibition of mTORC1 in cells and in vitro. We propose that the relative strengths of the rheb- and PRAS40-mediated inputs to mTORC1 set overall pathway activity and that insulin activates mTORC1 through the coordinated regulation of both.

Polycystic kidney disease: genes, proteins, animal models, disease mechanisms and therapeutic opportunities

An increased understanding of the genetic, molecular and cellular mechanisms responsible for the development of polycystic kidney disease has laid out the foundation for the development of rational therapies. Many animal models where these therapies can be tested are currently available. This review summarizes the rationale for these treatments, the results of preclinical trials and the prospects for clinical trials, some already in early phases of implementation.

Activation of the mTOR Signaling Pathway in Renal Clear Cell Carcinoma

We determined the frequency of mTOR/p70S6 kinase signaling pathway activation in clear cell renal cell carcinoma. Phospho-S6 (Ser235/236) and phospho-mTOR (Ser2448) staining was performed on renal tumor tissue microarrays containing 29 clear cell renal cell carcinomas. Mutational analysis of Rheb and RhebL1 was performed on DNA from phospho-mTOR/phospho-S6 positive clear cell renal cell carcinoma. The 3 clear cell renal cell carcinoma derived cell lines A498, 786-O and Caki1 were also assessed for mTOR activation and the effect of the mTOR inhibitor rapamycin (Biomol) on proliferation. Moderate or strong phospho-S6 immunoreactivity was found in 17 of 29 clear cell carcinomas (59%), of which 14 were also moderately/strongly positive for phospho-mTOR (Ser2448). We hypothesized that this activation of the mTOR signaling pathway in clear cell renal cell carcinoma could reflect mutational activation of Rheb or RhebL1, which are Ras family members that directly activate mTOR. However, no mutations in exons 3 and 4 (homologous sites of Ras activating mutations) in Rheb or RhebL1 were identified. Two of 3 renal clear cell carcinoma derived cell lines also showed inappropriate S6 hyperphosphorylation. Treatment of all 3 cell lines with rapamycin significantly decreased S6 phosphorylation and proliferation. The mTOR/p70S6 kinase signaling pathway is activated in most clear cell renal cell carcinomas. Moreover, the growth of renal clear cell carcinoma derived cell lines is inhibited by rapamycin. This is especially significant in light of new agents such as CCI-779, an ester of rapamycin and inhibitor of mTOR, which has shown promise in the treatment of renal carcinoma.

Rheb Inhibits C-Raf Activity and B-Raf/C-Raf Heterodimerization

The Ras-Raf-MEK signaling cascade is critical for normal development and is activated in many forms of cancer. We have recently shown that B-Raf kinase interacts with and is inhibited by Rheb, the target of the GTPase-activating domain of the tuberous sclerosis complex 2 gene product tuberin. Here, we demonstrate for the first time that activation of Rheb is associated with decreased B-Raf and C-Raf phosphorylation at residues Ser-446 and Ser-338, respectively, concomitant with a decrease in the activities of both kinases and decreased heterodimerization of B-Raf and C-Raf. Importantly, the impact of Rheb on B-Raf/C-Raf heterodimerization and kinase activity are rapamycin-insensitive, indicating that they are independent of Rheb activation of the mammalian target of rapamycin-Raptor complex. In addition, we found that Rheb inhibits the association of B-Raf with H-Ras. Taken together, these results support a central role of Rheb in the regulation of the Ras/B-Raf/C-Raf/MEK signaling network.

Frequent of ribosomal protein S6 hyperphosphorylation in lymphangioleiomyomatosis-associated angiomyolipomas

Lymphangioleiomyomatosis is a progressive lung disease characterized by a diffuse proliferation of pulmonary smooth muscle cells and cystic degeneration. Lymphangioleiomyomatosis can occur either independently of other disease or in association with tuberous sclerosis complex, a tumor-suppressor gene syndrome caused by mutations that inactivate either TSC1 or TSC2. TSC2 mutations and loss of heterozygosity have been identified in sporadic lymphangioleiomyomatosis-associated angiomyolipomas, thus implicating the TSC/Ras homolog-enriched in brain (Rheb)/mammalian target of Rapamycin (mTOR)/p70 S6 kinase signaling pathway in their pathogenesis. This study was undertaken to determine whether the mTOR/p70 S6 kinase signaling pathway is activated in lymphangioleiomyomatosis-associated angiomyolipomas lacking TSC1/TSC2 loss of heterozygosity. Phospho-ribosomal protein S6 (Ser235/236) immunohistochemistry was performed on five lymphangioleiomyomatosis-associated angiomyolipomas, two matched lymphangioleiomyomatosis pulmonary samples, and three sporadic angiomyolipomas. TSC1/TSC2 loss of heterozygosity was previously excluded in these angiomyolipomas. Moderate or strong phospho-ribosomal protein S6 immunoreactivity was found in all lymphangioleiomyomatosis-associated and sporadic angiomyolipomas, suggesting a high incidence of mTOR/p70 S6 kinase signaling pathway activation despite a lack of TSC1/TSC2 loss of heterozygosity. Focally positive phospho-S6 staining was also evident in both lymphangioleiomyomatosis pulmonary samples. We hypothesized that this S6 hyperphosphorylation could reflect mutational activation of Rheb or Rheb-like protein (RhebL1), Ras family members which directly activate mTOR. Mutational analysis performed on DNA from these eight angiomyolipomas plus five additional sporadic angiomyolipomas did not reveal mutations in exons 3 and 4 (homologous sites of Ras activating mutations) of either Rheb or RhebL1. These data suggest that activation of the Rheb/mTOR/p70 S6 kinase pathway is related to the pathogenesis of lymphangioleiomyomatosis-associated and sporadic angiomyolipomas lacking TSC1/TSC2 loss of heterozygosity. This high incidence of mTOR signaling pathway activation suggests that treatment with mTOR inhibitors, such as Rapamycin, may benefit patients with angiomyolipomas independent of the detection of TSC1/TSC2 loss of heterozygosity.

A Parallel Path to TOR

The mammalian target of rapamycin (mTOR) signaling pathway, which regulates protein synthesis and thereby cell growth and proliferation, is modulated both by amino acid availability and by insulin. Insulin regulates mTOR activity through a pathway that involves activation of class 1 phosphatidylinositol 3-kinase (PI3K) and the subsequent activation of PKB (protein kinase B), which phosphorylates and inactivates the tuberous sclerosis complex proteins 1 and 2 complex (TSC1/2), enabling the small GTPase (guanosine triphosphatase) Rheb to stimulate TOR signaling. It has not been clear, however, how information about nutrient supply regulates mTOR signaling. Nobukuni et al. found that, whereas amino acids, unlike insulin, had no effect on TSC2 phosphorylation on serine 939 (TSC2 S939), TSC2 abundance, or PKB activation, they stimulated phosphorylation of S6K1 (S6 kinase 1, a substrate downstream of mTOR) T389. Further, amino acid depletion inhibited S6K1 phosphorylation even in TSC2-deficient cells with enhanced Rheb activation. Although the PI3K inhibitor wortmannin inhibited amino acid-stimulated S6K1 phosphorylation, as did rapamycin, siRNA against class 1 PI3Kα did not. Amino acids stimulated the in vitro activity of immunoprecipitated class 3 PI3K hVps34 and the abundance in mouse embryo fibroblasts of its product, phosphatidylinositol 3-phosphate (PI3P). HEK293 cells transfected with GST-epitope-tagged hVps34 showed a dose-dependent increase in amino acid-mediated S6K1 phosphorylation, whereas hVps34 knockdown decreased this response, as did knockdown of hVps15 (which associates with and stimulates hVps34) or expression of a construct that competes for PI3P docking sites. Thus, the authors conclude that amino acids regulate TOR signaling through a pathway involving hVps34 that is parallel to that activated by insulin. T. Nobukuni, M. Joaquin, M. Roccio, S. G. Dann, S. Y. Kim, P. Gulati, M. P. Byfield, J. M. Backer, F. Natt, J. L. Bos, F. J. T. Zwartkruis, G. Thomas, Amino acids mediate mTOR/raptor signaling through activation of class 3 phosphatidylinositol 3OH-kinase. Proc. Natl. Acad. Sci. U.S.A. 102 , 14238-14243 (2005). [Abstract] [Full Text]

Regulation of the small GTPase Rheb by amino acids

The mTOR/S6K/4E-BP1 pathway integrates extracellular signals derived from growth factors, and intracellular signals, determined by the availability of nutrients like amino acids and glucose. Activation of this pathway requires inhibition of the tumor suppressor complex TSC1/2. TSC2 is a GTPase-activating protein for the small GTPase Ras homologue enriched in brain (Rheb), GTP loading of which activates mTOR by a yet unidentified mechanism. The level at which this pathway senses the availability of amino acids is unknown but is suggested to be at the level of TSC2. Here, we show that amino-acid depletion completely blocks insulin- and TPA-induced Rheb activation. This indicates that amino-acid sensing occurs upstream of Rheb. Despite this, amino-acid depletion can still inhibit mTOR/S6 kinase signaling in TSC2-/- fibroblasts. Since under these conditions Rheb-GTP levels remain high, a second level of amino-acid sensing exists, affecting mTOR activity in a Rheb-independent fashion.

HVps34 Is a Nutrient-regulated Lipid Kinase Required for Activation of p70 S6 Kinase

Mammalian cells respond to nutrient deprivation by inhibiting energy consuming processes, such as proliferation and protein synthesis, and by stimulating catabolic processes, such as autophagy. p70 S6 kinase (S6K1) plays a central role during nutritional regulation of translation. S6K1 is activated by growth factors such as insulin, and by mammalian target of rapamycin (mTOR), which is itself regulated by amino acids. The Class IA phosphatidylinositol (PI) 3-kinase plays a well recognized role in the regulation of S6K1. We now present evidence that the Class III PI 3-kinase, hVps34, also regulates S6K1, and is a critical component of the nutrient sensing apparatus. Overexpression of hVps34 or the associated hVps15 kinase activates S6K1, and insulin stimulation of S6K1 is blocked by microinjection of inhibitory anti-hVps34 antibodies, overexpression of a FYVE domain construct that sequesters the hVps34 product PI3P, or small interfering RNA-mediated knock-down of hVps34. hVps34 is not part of the insulin input to S6K1, as it is not stimulated by insulin, and inhibition of hVps34 has no effect on phosphorylation of Akt or TSC2 in insulin-stimulated cells. However, hVps34 is inhibited by amino acid or glucose starvation, suggesting that it lies on the nutrient-regulated pathway to S6K1. Consistent with this, hVps34 is also inhibited by activation of the AMP-activated kinase, which inhibits mTOR/S6K1 in glucose-starved cells. hVps34 appears to lie upstream of mTOR, as small interfering RNA knock-down of hVps34 inhibits the phosphorylation of another mTOR substrate, eIF4E-binding protein-1 (4EBP1). Our data suggest that hVps34 is a nutrient-regulated lipid kinase that integrates amino acid and glucose inputs to mTOR and S6K1.

Rheb Binding to Mammalian Target of Rapamycin (mTOR) Is Regulated by Amino Acid Sufficiency

The removal of extracellular amino acids or leucine alone inhibits the ability of the mammalian target of rapamycin (mTOR) to signal to the raptor-dependent substrates, p70 S6 kinase and 4E-BP. This inhibition can be overcome by overexpression of the Rheb GTPase. Rheb binds directly to the amino-terminal lobe of the mTOR catalytic domain, and activates mTOR kinase in a GTP-dependent manner. Herein we show that the binding of Rheb to endogenous and recombinant mTOR is reversibly inhibited by withdrawal of all extracellular amino acids or just leucine. The effect of amino acid withdrawal is not attributable to changes in Rheb-GTP charging; amino acid withdrawal does not alter the GTP charging of recombinant Rheb. Moreover, the binding of mTOR to Rheb mutants that are unable to bind guanyl nucleotide in vivo is also inhibited by amino withdrawal. The inhibitory effect of amino acid withdrawal is exerted through an action on mTOR, at a site largely distinct from that responsible for the binding of Rheb; deletion of the larger, carboxyl-terminal lobe of the mTOR catalytic domain eliminates the inhibitory effect of amino acid withdrawal on Rheb binding, without altering Rheb binding per se. The lesser ability of the mTOR catalytic domain to bind Rheb after amino acid withdrawal does not persist after extraction and purification of the mTOR polypeptide. Amino acid withdrawal may generate an inhibitor of the Rheb-mTOR interaction that interferes with the signaling function of TOR complex 1.

Biochemical and functional characterizations of small GTPase Rheb and TSC2 GAP activity.

Tuberous sclerosis complex (TSC) is a genetic disease caused by a mutation in either the tsc1 or tsc2 tumor suppressor gene. Recent studies have demonstrated that TSC2 displays GAP (GTPase-activating protein) activity specifically towards the small G protein Rheb and inhibits its ability to stimulate the mTOR signaling pathway. Rheb and TSC2 comprise a unique pair of GTPase and GAP, because Rheb has high basal GTP levels and TSC2 does not have the catalytic arginine finger found in Ras-GAP. To investigate the function of TSC2 and Rheb in mTOR signaling, we analyzed the TSC2-stimulated Rheb GTPase activity. We found that Arg15, a residue equivalent to Gly12 in Ras, is important for Rheb to function as a substrate for TSC2 GAP. In addition, we identified asparagine residues essential for TSC2 GAP activity. We demonstrated a novel catalytic mechanism of the TSC2 GAP and Rheb that TSC2 uses a catalytic "asparagine thumb" instead of the arginine finger found in Ras-GAP. Furthermore, we discovered that farnesylation and membrane localization of Rheb is not essential for Rheb to stimulate S6 kinase (S6K) phosphorylation. Analysis of TSC1 binding defective mutants of TSC2 shows that TSC1 is not required for the TSC2 GAP activity but may function as a regulatory component in the TSC1/TSC2 complex. Our data further demonstrate that GAP activity is essential for the cellular function of TSC2 to inhibit S6K phosphorylation.