Activation of mTORC2 by association with the ribosome.

The target of rapamycin (TOR) is a highly conserved protein kinase and a central controller of growth. Mammalian TOR complex 2 (mTORC2) regulates AGC kinase family members and is implicated in various disorders, including cancer and diabetes. Here, we investigated the upstream regulation of mTORC2. A genetic screen in yeast and subsequent studies in mammalian cells revealed that ribosomes, but not protein synthesis, are required for mTORC2 signaling. Active mTORC2 was physically associated with the ribosome, and insulin-stimulated PI3K signaling promoted mTORC2-ribosome binding, suggesting that ribosomes activate mTORC2 directly. Findings with melanoma and colon cancer cells suggest that mTORC2-ribosome association is important in oncogenic PI3K signaling. Thus, TORC2-ribosome interaction is a likely conserved mechanism of TORC2 activation that is physiologically relevant in both normal and cancer cells. As ribosome content determines growth capacity of a cell, this mechanism of TORC2 regulation ensures that TORC2 is active only in growing cells.

Glutaminolysis activates Rag-mTORC1 signaling.

Amino acids control cell growth via activation of the highly conserved kinase TORC1. Glutamine is a particularly important amino acid in cell growth control and metabolism. However, the role of glutamine in TORC1 activation remains poorly defined. Glutamine is metabolized through glutaminolysis to produce α-ketoglutarate. We demonstrate that glutamine in combination with leucine activates mammalian TORC1 (mTORC1) by enhancing glutaminolysis and α-ketoglutarate production. Inhibition of glutaminolysis prevented GTP loading of RagB and lysosomal translocation and subsequent activation of mTORC1. Constitutively active Rag heterodimer activated mTORC1 in the absence of glutaminolysis. Conversely, enhanced glutaminolysis or a cell-permeable α-ketoglutarate analog stimulated lysosomal translocation and activation of mTORC1. Finally, cell growth and autophagy, two processes controlled by mTORC1, were regulated by glutaminolysis. Thus, mTORC1 senses and is activated by glutamine and leucine via glutaminolysis and α-ketoglutarate production upstream of Rag. This may provide an explanation for glutamine addiction in cancer cells.

Hepatic mTORC1 controls locomotor activity, body temperature, and lipid metabolism through FGF21.

The liver is a key metabolic organ that controls whole-body physiology in response to nutrient availability. Mammalian target of rapamycin (mTOR) is a nutrient-activated kinase and central controller of growth and metabolism that is negatively regulated by the tumor suppressor tuberous sclerosis complex 1 (TSC1). To investigate the role of hepatic mTOR complex 1 (mTORC1) in whole-body physiology, we generated liver-specific Tsc1 (L-Tsc1 KO) knockout mice. L-Tsc1 KO mice displayed reduced locomotor activity, body temperature, and hepatic triglyceride content in a rapamycin-sensitive manner. Ectopic activation of mTORC1 also caused depletion of hepatic and plasma glutamine, leading to peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α)-dependent fibroblast growth factor 21 (FGF21) expression in the liver. Injection of glutamine or knockdown of PGC-1α or FGF21 in the liver suppressed the behavioral and metabolic defects due to mTORC1 activation. Thus, mTORC1 in the liver controls whole-body physiology through PGC-1α and FGF21. Finally, mTORC1 signaling correlated with FGF21 expression in human liver tumors, suggesting that treatment of glutamine-addicted cancers with mTOR inhibitors might have beneficial effects at both the tumor and whole-body level.

TORC1-regulated protein kinase Npr1 phosphorylates Orm to stimulate complex sphingolipid synthesis.

The evolutionarily conserved Orm1 and Orm2 proteins mediate sphingolipid homeostasis. However, the homologous Orm proteins and the signaling pathways modulating their phosphorylation and function are incompletely characterized. Here we demonstrate that inhibition of nutrient-sensitive target of rapamycin complex 1 (TORC1) stimulates Orm phosphorylation and synthesis of complex sphingolipids in Saccharomyces cerevisiae. TORC1 inhibition activates the kinase Npr1 that directly phosphorylates and activates the Orm proteins. Npr1-phosphorylated Orm1 and Orm2 stimulate de novo synthesis of complex sphingolipids downstream of serine palmitoyltransferase. Complex sphingolipids in turn stimulate plasma membrane localization and activity of the nutrient scavenging general amino acid permease 1. Thus activation of Orm and complex sphingolipid synthesis upon TORC1 inhibition is a physiological response to starvation.

Selective ATP-competitive inhibitors of TOR suppress rapamycin-insensitive function of TORC2 in Saccharomyces cerevisiae.

The target of rapamycin (TOR) is a critical regulator of growth, survival, and energy metabolism. The allosteric TORC1 inhibitor rapamycin has been used extensively to elucidate the TOR related signal pathway but is limited by its inability to inhibit TORC2. We used an unbiased cell proliferation assay of a kinase inhibitor library to discover QL-IX-55 as a potent inhibitor of S. cerevisiae growth. The functional target of QL-IX-55 is the ATP-binding site of TOR2 as evidenced by the discovery of resistant alleles of TOR2 through rational design and unbiased selection strategies. QL-IX-55 is capable of potently inhibiting both TOR complex 1 and 2 (TORC1 and TORC2) as demonstrated by biochemical IP kinase assays (IC(50) <50 nM) and cellular assays for inhibition of substrate YPK1 phosphorylation. In contrast to rapamycin, QL-IX-55 is capable of inhibiting TORC2-dependent transcription, which suggests that this compound will be a powerful probe to dissect the Tor2/TORC2-related signaling pathway in yeast.

Molecular organization of target of rapamycin complex 2.

The target of rapamycin (TOR), a highly conserved serine/threonine kinase, plays a central role in the control of eukaryotic cell growth. TOR exists in two functionally and structurally distinct complexes, TOR complex 1 (TORC1) and TOR complex 2 (TORC2). TORC1 controls cell growth via a rapamycin-sensitive signaling branch regulating translation, transcription, nutrient uptake, ribosome biogenesis, and autophagy. TORC2 controls the organization of the actin cytoskeleton through a rapamycin-insensitive signaling branch and in yeast consists of the six proteins AVO1, AVO2, AVO3, BIT61, LST8, and TOR2. Here we have focused on the characterization of TORC2. Our studies suggest that TORC2 is oligomeric, likely a TORC2-TORC2 dimer. AVO1 and AVO3 bind cooperatively to the N-terminal HEAT repeat region in TOR2 and are required for TORC2 integrity. AVO2 is a nonessential peripheral protein associated with AVO1 and AVO3. LST8 binds separately to the C-terminal kinase domain region in TOR2 and appears to modulate both the integrity and kinase activity of TORC2. TORC2 autophosphorylates sites in AVO1 and AVO3, but TORC2 kinase activity is not required for TORC2 integrity. We have demonstrated that mammalian TOR is also oligomeric. The architecture of TORC2 is discussed in the context of TORC2 assembly and regulation.

The Tim9p-Tim10p complex binds to the transmembrane domains of the ADP/ATP carrier.

The soluble Tim9p-Tim10p (Tim, translocase of inner membrane) complex of the mitochondrial intermembrane space mediates the import of the carrier proteins and is a component of the TIM22 import system. The mechanism by which the Tim9p-Tim10p complex assembles and binds the carriers is not well understood, but previous studies have proposed that the conserved cysteine residues in the 'twin CX3C' motif coordinate zinc and potentially generate a zinc-finger-like structure that binds to the matrix loops of the carrier proteins. Here we have purified the native and recombinant Tim9p-Tim10p complex, and show that both complexes resemble each other and consist of three Tim9p and three Tim10p. Results from inductively coupled plasma--mass spectrometry studies failed to detect zinc in the Tim9p-Tim10p complex. Instead, the cysteine residues seemingly formed disulfide linkages. The Tim9p-Tim10p complex bound specifically to the transmembrane domains of the ADP/ATP carrier, but had no affinity for Tim23p, an inner membrane protein that is inserted via the TIM22 complex. The chaperone-like Tim9p-Tim10p complex thus may prevent aggregation of the unfolded carrier proteins in the aqueous intermembrane space.

Two TOR complexes, only one of which is rapamycin sensitive, have distinct roles in cell growth control.

The target of rapamycin (TOR) proteins in Saccharomyces cerevisiae, TOR1 and TOR2, redundantly regulate growth in a rapamycin-sensitive manner. TOR2 additionally regulates polarization of the actin cytoskeleton in a rapamycin-insensitive manner. We describe two functionally distinct TOR complexes. TOR Complex 1 (TORC1) contains TOR1 or TOR2, KOG1 (YHR186c), and LST8. TORC2 contains TOR2, AVO1 (YOL078w), AVO2 (YMR068w), AVO3 (YER093c), and LST8. FKBP-rapamycin binds TORC1, and TORC1 disruption mimics rapamycin treatment, suggesting that TORC1 mediates the rapamycin-sensitive, TOR-shared pathway. FKBP-rapamycin fails to bind TORC2, and TORC2 disruption causes an actin defect, suggesting that TORC2 mediates the rapamycin-insensitive, TOR2-unique pathway. Thus, the distinct TOR complexes account for the diversity, specificity, and selective rapamycin inhibition of TOR signaling. TORC1 and possibly TORC2 are conserved from yeast to man.