Noriko Oshiro

Raptor, a Binding Partner of Target of Rapamycin (TOR), Mediates TOR Action

mTOR controls cell growth, in part by regulating p70 S6 kinase alpha (p70alpha) and eukaryotic initiation factor 4E binding protein 1 (4EBP1). Raptor is a 150 kDa mTOR binding protein that also binds 4EBP1 and p70alpha. The binding of raptor to mTOR is necessary for the mTOR-catalyzed phosphorylation of 4EBP1 in vitro, and it strongly enhances the mTOR kinase activity toward p70alpha. Rapamycin or amino acid withdrawal increases, whereas insulin strongly inhibits, the recovery of 4EBP1 and raptor on 7-methyl-GTP Sepharose. Partial inhibition of raptor expression by RNA interference (RNAi) reduces mTOR-catalyzed 4EBP1 phosphorylation in vitro. RNAi of C. elegans raptor yields an array of phenotypes that closely resemble those produced by inactivation of Ce-TOR. Thus, raptor is an essential scaffold for the mTOR-catalyzed phosphorylation of 4EBP1 and mediates TOR action in vivo.

Nutrient-dependent mTORC1 Association with the ULK1–Atg13–FIP200 Complex Required for Autophagy

Autophagy is an intracellular degradation system, by which cytoplasmic contents are degraded in lysosomes. Autophagy is dynamically induced by nutrient depletion to provide necessary amino acids within cells, thus helping them adapt to starvation. Although it has been suggested that mTOR is a major negative regulator of autophagy, how it controls autophagy has not yet been determined. Here, we report a novel mammalian autophagy factor, Atg13, which forms a stable approximately 3-MDa protein complex with ULK1 and FIP200. Atg13 localizes on the autophagic isolation membrane and is essential for autophagosome formation. In contrast to yeast counterparts, formation of the ULK1-Atg13-FIP200 complex is not altered by nutrient conditions. Importantly, mTORC1 is incorporated into the ULK1-Atg13-FIP200 complex through ULK1 in a nutrient-dependent manner and mTOR phosphorylates ULK1 and Atg13. ULK1 is dephosphorylated by rapamycin treatment or starvation. These data suggest that mTORC1 suppresses autophagy through direct regulation of the approximately 3-MDa ULK1-Atg13-FIP200 complex.

mTOR Is Essential for Growth and Proliferation in Early Mouse Embryos and Embryonic Stem Cells

ABSTRACT TOR is a serine-threonine kinase that was originally identified as a target of rapamycin in Saccharomyces cerevisiae and then found to be highly conserved among eukaryotes. In Drosophila melanogaster, inactivation of TOR or its substrate, S6 kinase, results in reduced cell size and embryonic lethality, indicating a critical role for the TOR pathway in cell growth control. However, the in vivo functions of mammalian TOR (mTOR) remain unclear. In this study, we disrupted the kinase domain of mouse mTOR by homologous recombination. While heterozygous mutant mice were normal and fertile, homozygous mutant embryos died shortly after implantation due to impaired cell proliferation in both embryonic and extraembryonic compartments. Homozygous blastocysts looked normal, but their inner cell mass and trophoblast failed to proliferate in vitro. Deletion of the C-terminal six amino acids of mTOR, which are essential for kinase activity, resulted in reduced cell size and proliferation arrest in embryonic stem cells. These data show that mTOR controls both cell size and proliferation in early mouse embryos and embryonic stem cells.

Tor Directly Controls the Atg1 Kinase Complex To Regulate Autophagy

ABSTRACT Autophagy is a bulk proteolytic process that is indispensable for cell survival during starvation. Autophagy is induced by nutrient deprivation via inactivation of the rapamycin-sensitive Tor complex1 (TORC1), a protein kinase complex regulating cell growth in response to nutrient conditions. However, the mechanism by which TORC1 controls autophagy and the direct target of TORC1 activity remain unclear. Atg13 is an essential regulatory component of autophagy upstream of the Atg1 kinase complex, and here we show that yeast TORC1 directly phosphorylates Atg13 at multiple Ser residues. Additionally, expression of an unphosphorylatable Atg13 mutant bypasses the TORC1 pathway to induce autophagy through activation of Atg1 in cells growing under nutrient-rich conditions. Our findings suggest that the direct control of the Atg1 complex by TORC1 induces autophagy.

The proline-rich Akt substrate of 40 kDa (PRAS40) is a physiological substrate of mammalian target of rapamycin complex 1.

The proline-rich Akt substrate of 40 kilodaltons (PRAS40) was identified as a raptor-binding protein that is phosphorylated directly by mammalian target of rapamycin (mTOR) complex 1 (mTORC1) but not mTORC2 in vitro, predominantly at PRAS40 (Ser183). The binding of S6K1 and 4E-BP1 to raptor requires a TOR signaling (TOS) motif, which contains an essential Phe followed by four alternating acidic and small hydrophobic amino acids. PRAS40 binding to raptor was severely inhibited by mutation of PRAS40 (Phe129 to Ala). Immediately carboxyl-terminal to Phe129 are two small hydrophobic amino acid followed by two acidic residues. PRAS40 binding to raptor was also abolished by mutation of the major mTORC1 phosphorylation site, Ser183, to Asp. PRAS40 (Ser183) was phosphorylated in intact cells; this phosphorylation was inhibited by rapamycin, by 2-deoxyglucose, and by overexpression of the tuberous sclerosis complex heterodimer. PRAS40 (Ser183) phosphorylation was also inhibited reversibly by withdrawal of all or of only the branched chain amino acids; this inhibition was reversed by overexpression of the Rheb GTPase. Overexpressed PRAS40 suppressed the phosphorylation of S6K1 and 4E-BP1 at their rapamycin-sensitive phosphorylation sites, and reciprocally, overexpression of S6K1 or 4E-BP1 suppressed phosphorylation of PRAS40 (Ser183) and its binding to raptor. RNA interference-induced depletion of PRAS40 enhanced the amino acid-stimulated phosphorylation of both S6K1 and 4E-BP1. These results establish PRAS40 as a physiological mTORC1 substrate that contains a variant TOS motif. Moreover, they indicate that the ability of raptor to bind endogenous substrates is limiting for the activity of mTORC1 in vivo and is therefore a potential locus of regulation.

Dissociation of raptor from mTOR is a mechanism of rapamycin-induced inhibition of mTOR function.

The mammalian target of rapamycin (mTOR) is a Ser/Thr protein kinase that plays a crucial role in a nutrient‐sensitive signalling pathway that regulates cell growth. TOR signalling is potently inhibited by rapamycin, through the direct binding of a FK506‐binding protein 12 (FKBP12)/rapamycin complex to the TOR FRB domain, a segment amino terminal to the kinase catalytic domain. The molecular basis for the inhibitory action of FKBP12/rapamycin remains uncertain. Raptor (regulatory associated protein of mTOR) is a recently identified mTOR binding partner that is essential for mTOR signalling in vivo, and whose binding to mTOR is critical for mTOR‐catalysed substrate phosphorylation in vitro. Here we investigated the stability of endogenous mTOR/raptor complex in response to rapamycin in vivo, and to the direct addition of a FKBP12/rapamycin complex in vitro. Rapamycin diminished the recovery of endogenous raptor with endogenous or recombinant mTOR in vivo; this inhibition required the ability of mTOR to bind the FKBP12/rapamycin complex, but was independent of mTOR kinase activity. Rapamycin, in the presence of FKBP12, inhibited the association of raptor with mTOR directly in vitro, and concomitantly reduced the mTOR‐catalysed phosphorylation of raptor‐dependent, but not raptor‐independent substrates; mTOR autophosphorylation was unaltered. These observations indicate that rapamycin inhibits mTOR function, at least in part, by inhibiting the interaction of raptor with mTOR; this action uncouples mTOR from its substrates, and inhibits mTOR signalling without altering mTORs intrinsic catalytic activity.

Tti1 and Tel2 Are Critical Factors in Mammalian Target of Rapamycin Complex Assembly

Mammalian target of rapamycin (mTOR) is a member of the phosphatidylinositol 3-kinase-related kinase (PIKK) family and is a major regulator of translation, cell growth, and autophagy. mTOR exists in two distinct complexes, mTORC1 and mTORC2, that differ in their subunit composition. In this study, we identified KIAA0406 as a novel mTOR-interacting protein. Because it has sequence homology with Schizosaccharomyces pombe Tti1, we named it mammalian Tti1. Tti1 constitutively interacts with mTOR in both mTORC1 and mTORC2. Knockdown of Tti1 suppresses phosphorylation of both mTORC1 substrates (S6K1 and 4E-BP1) and an mTORC2 substrate (Akt) and also induces autophagy. S. pombe Tti1 binds to Tel2, a protein whose mammalian homolog was recently reported to regulate the stability of PIKKs. We confirmed that Tti1 binds to Tel2 also in mammalian cells, and Tti1 interacts with and stabilizes all six members of the PIKK family of proteins (mTOR, ATM, ATR, DNA-PKcs, SMG-1, and TRRAP). Furthermore, using immunoprecipitation and size-exclusion chromatography analyses, we found that knockdown of either Tti1 or Tel2 causes disassembly of mTORC1 and mTORC2. These results indicate that Tti1 and Tel2 are important not only for mTOR stability but also for assembly of the mTOR complexes to maintain their activities.

Phosphorylation of ERM proteins at filopodia induced by Cdc42.

ERM (ezrin, radixin, and moesin) proteins function as membrane‐cytoskeletal linkers, and are known to be localized at filopodia and microvilli‐like structures. We have shown that Rho‐associated kinase (Rho‐kinase)/ROKα/ROCK II phosphorylates moesin at Thr‐558 at the lower stream of Rho, and the phosphorylation is crucial to the formation of microvilli‐like structures (Oshiro, N., Fukata, Y. & Kaibuchi, K. (1998) Phosphorylation of moesin by Rho‐associated kinase (Rho‐kinase) plays a crucial role in the formation of microvilli‐like structures. J. Biol. Chem. 273, 34663– 34666). However, the role of ERM proteins in the formation of filopodia is less well characterized.

Molecular Identification and Characterization of Xenopus Egg Uroplakin III, an Egg Raft-associated Transmembrane Protein That Is Tyrosine-phosphorylated upon Fertilization

Here we describe mass spectrometric identification, molecular cloning, and biochemical characterization of a lipid/membrane raft-associated protein that is tyrosine-phosphorylated upon Xenopus egg fertilization. This protein is homologous to mammalian uroplakin III, a member of the uroplakin family proteins (UPs) that constitute asymmetric unit membranes in the mammalian urothelial tissues, thus termed Xenopus uroplakin III (xUPIII). xUPIII contains N-linked sugars and is highly expressed in Xenopus eggs, ovary, urinary tract, and kidney. In unfertilized eggs, xUPIII is predominantly localized to the lipid/membrane rafts and exposed on the cell surface, as judged by surface biotinylation experiments and indirect immunofluorescent studies. After fertilization or hydrogen peroxide-induced egg activation, xUPIII becomes rapidly phosphorylated on tyrosine residue-249, which locates in the carboxyl-terminal cytoplasmic tail of the molecule. Raft localization and tyrosine phosphorylation of xUPIII can be reconstituted in HEK293 cells by coexpression of xUPIII, and Xenopus c-Src, a tyrosine kinase whose fertilization-induced activation in egg rafts is required for initiation of development. In mammals, UPIII is forming a complex with a tetraspanin molecule uroplakin Ib. As another tetraspanin, CD9, is known to be a critical component for sperm-egg fusion in the mouse, we have assumed that xUPIII is involved in sperm-egg interaction. An antibody against the extracellular domain of xUPIII blocks sperm-egg interaction, as judged by the occurrence of egg activation and first cell cleavage. Thus, xUPIII represents an egg raft-associated protein that is likely involved in sperm-egg interaction as well as subsequent Src-dependent intracellular events of egg activation in Xenopus.

Identification of Tau and MAP2 as novel substrates of Rho-kinase and myosin phosphatase.

Rho‐kinase and myosin phosphatase are implicated in the phosphorylation‐state of myosin light chain downstream of Rho, which is thought to induce smooth muscle contraction and stress fibre formation in non‐muscle cells. Here, we found that microtubule‐associated proteins, Tau and MAP2, interacted with the myosin‐binding subunit (MBS) of myosin phosphatase, and were the possible substrates of both Rho‐kinase and myosin phosphatase. We determined the phosphorylation sites of Tau (Thr245, Thr377, Ser409) and MAP2 (Ser1796) by Rho‐kinase. We also found that Rho‐kinase phosphorylated Tau at Ser262 to some extent. Phosphorylation by Rho‐kinase decreased the activity of Tau to promote microtubule assembly in vitro. Substitutions of Ala for Ser/Thr at the phosphorylation sites of Tau (Tau‐AAA) did not affect the activity to promote microtubule assembly, while substitutions of Asp for Ser/Thr (Tau‐DDD), which are expected to mimic the phosphorylation‐state of Tau, slightly reduced the activity. When Tau, or mutated forms of Tau, were expressed in PC12 cells, followed by treatment with cytochalasin D, they promoted extension of the cell process in a cytochalasin‐dependent manner. However, Tau‐DDD showed the weaker activity in this capacity than wild‐type Tau or Tau‐AAA. These results suggest that the phosphorylation‐state of these residues of Tau affects its activity both in vitro and in vivo. Thus, it is likely that the Rho‐kinase/MBS pathway regulates not only the actin‐myosin system but also microtubule dynamics.