TOR2 is required for organization of the actin cytoskeleton in yeast

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.

Loss of ypk1 function causes rapamycin sensitivity, inhibition of translation initiation and synthetic lethality in 14-3-3-deficient yeast

14-3-3 proteins bind to phosphorylated proteins and regulate a variety of cellular activities as effectors of serine/threonine phosphorylation. To define processes requiring 14-3-3 function in yeast, mutants with increased sensitivity to reduced 14-3-3 protein levels were identified by synthetic lethal screening. One mutation was found to be allelic to YPK1, which encodes a Ser/Thr protein kinase. Loss of Ypk function causes hypersensitivity to rapamycin, similar to 14-3-3 mutations and other mutations affecting the TOR signaling pathway in yeast. Similar to treatment with rapamycin, loss of Ypk function disrupted translation, at least in part by causing depletion of eIF4G, a central adaptor protein required for cap-dependent mRNA translation initiation. In addition, Ypk1 as well as eIF4G protein levels were rapidly depleted upon nitrogen starvation, but not during glucose starvation, even though both conditions inhibit translation initiation. These results suggest that Ypk regulates translation initiation in response to nutrient signals, either through the TOR pathway or in a functionally related pathway parallel to TOR.

Yeast protein kinases and the RHO1 exchange factor TUS1 are novel components of the cell integrity pathway in yeast

The PKC1-associated mitogen-activated protein (MAP) kinase pathway of Saccharomyces cerevisiae regulates cell integrity by controlling the actin cytoskeleton and cell wall synthesis. Activation of PKC1 occurs via the GTPase RHO1 and the kinase pair PKH1 and PKH2. Here we report that YPK1 and YPK2, an essential pair of homologous kinases and proposed downstream effectors of PKH and sphingolipids, are also regulators of the PKC1-controlled MAP kinase cascade. ypk mutants display random distribution of the actin cytoskeleton and severely reduced activation of the MAP kinase MPK1. Upregulation of the RHO1 GTPase switch or the PKC1 effector MAP kinase pathway suppresses the growth and actin defects of ypk cells. ypk lethality is also suppressed by overexpression of an uncharacterized gene termed TUS1. TUS1 is a novel RHO1 exchange factor that contributes to cell wall integrity-mediated modulation of RHO1 activity. Thus, TUS1 and the YPKs add to the growing complexity of RHO1 and PKC1 regulation in the cell integrity signaling pathway. Furthermore, our findings suggest that the YPKs are a missing link between sphingolipid signaling and the cell integrity pathway.

The TOR signal transduction cascade controls cellular differentiation in response to nutrients

Rapamycin binds and inhibits the Tor protein kinases, which function in a nutrient-sensing signal transduction pathway that has been conserved from the yeast Saccharomyces cerevisiae to humans. In yeast cells, the Tor pathway has been implicated in regulating cellular responses to nutrients, including proliferation, translation, transcription, autophagy, and ribosome biogenesis. We report here that rapamycin inhibits pseudohyphal filamentous differentiation of S. cerevisiae in response to nitrogen limitation. Overexpression of Tap42, a protein phosphatase regulatory subunit, restored pseudohyphal growth in cells exposed to rapamycin. The tap42-11 mutation compromised pseudohyphal differentiation and rendered it resistant to rapamycin. Cells lacking the Tap42-regulated protein phosphatase Sit4 exhibited a pseudohyphal growth defect and were markedly hypersensitive to rapamycin. Mutations in other Tap42-regulated phosphatases had no effect on pseudohyphal differentiation. Our findings support a model in which pseudohyphal differentiation is controlled by a nutrient-sensing pathway involving the Tor protein kinases and the Tap42-Sit4 protein phosphatase. Activation of the MAP kinase or cAMP pathways, or mutation of the Sok2 repressor, restored filamentation in rapamycin treated cells, supporting models in which the Tor pathway acts in parallel with these known pathways. Filamentous differentiation of diverse fungi was also blocked by rapamycin, demonstrating that the Tor signaling cascade plays a conserved role in regulating filamentous differentiation in response to nutrients.

The TOR kinases link nutrient sensing to cell growth

Rapamycin is an immunosuppressive natural product that inhibits the proliferation of T-cells in response to nutrients and growth factors. Rapamycin binds to the peptidyl-prolyl isomerase FKBP12 and forms protein-drug complexes that inhibit signal transduction by the TOR kinases. The FKBP12 and TOR proteins are conserved from fungi to humans, and in both organisms the TOR signaling pathway plays a role in nutrient sensing. In response to nitrogen sources or amino acids, TOR regulates both transcription and translation, enabling cells to appropriately respond to growth-promoting signals. Rapamycin is having a profound impact on clinical medicine and was approved as an immunosuppressant for transplant recipients in 1999. Ongoing clinical studies address new clinical applications for rapamycin as an antiproliferative drug for chemotherapy and invasive cardiology.

The fission yeast TOR homolog, tor1+, is required for the response to starvation and other stresses via a conserved serine

Targets of rapamycin (TORs) are conserved phosphatidylinositol kinase-related kinases that are involved in the coordination between nutritional or mitogenic signals and cell growth. Here we report the initial characterization of two Schizosaccharomyces pombe TOR homologs, tor1(+) and tor2(+). tor2(+) is an essential gene, whereas tor1(+) is required only under starvation and other stress conditions. Specifically, Deltator1 cells fail to enter stationary phase or undergo sexual development and are sensitive to cold, osmotic stress, and oxidative stress. In complex with the prolyl isomerase FKBP12, the drug rapamycin binds a conserved domain in TORs, FRB, thus inhibiting some of the functions of TORs. Mutations at a conserved serine within the FRB domain of Saccharomyces cerevisiae TOR proteins led to rapamycin resistance but did not otherwise affect the functions of the proteins. The S. pombe tor1(+) exhibits different features; substitution of the conserved serine residue, Ser(1834), with arginine compromises its functions and has no effect on the inhibition that rapamycin exerts on sexual development in S. pombe.

HEAT repeats mediate plasma membrane localization of Tor2p in yeast

The subcellular distribution of Tor1p and Tor2p, two phosphatidylinositol kinase homologs and targets of the immunosuppressive drug rapamycin in Saccharomyces cerevisiae, was analyzed. We found that Tor protein is peripherally associated with membranes. Subcellular fractionation and immunofluorescence studies showed that Tor1p and Tor2p associate with the plasma membrane and a second fraction that is distinct from Golgi, vacuoles, mitochondria, and nucleus and may represent vesicular structures. Pulse-chase experiments showed that association of Tor protein with plasma membrane and the second compartment is fast, does not appear to involve components of endocytic, secretory, or Golgi to vacuole transport pathways, and is not affected by the immunosuppressive drug rapamycin. Deletion analysis reveals that two domains within Tor2p independently mediate localization to both compartments. These domains are composed of HEAT repeats that are thought to act as protein-protein interaction surfaces. Our studies therefore place Tor proteins at the site of action of their known downstream effectors and suggest that they may be part of a multiprotein complex.

Regulation of cellular growth by the Drosophila target of rapamycin dTOR

The TOR protein kinases (TOR1 and TOR2 in yeast; mTOR/FRAP/RAFT1 in mammals) promote cellular proliferation in response to nutrients and growth factors, but their role in development is poorly understood. Here, we show that the Drosophila TOR homolog dTOR is required cell autonomously for normal growth and proliferation during larval development, and for increases in cellular growth caused by activation of the phosphoinositide 3-kinase (PI3K) signaling pathway. As in mammalian cells, the kinase activity of dTOR is required for growth factor-dependent phosphorylation of p70 S6 kinase (p70(S6K)) in vitro, and we demonstrate that overexpression of p70(S6K) in vivo can rescue dTOR mutant animals to viability. Loss of dTOR also results in cellular phenotypes characteristic of amino acid deprivation, including reduced nucleolar size, lipid vesicle aggregation in the larval fat body, and a cell type-specific pattern of cell cycle arrest that can be bypassed by overexpression of the S-phase regulator cyclin E. Our results suggest that dTOR regulates growth during animal development by coupling growth factor signaling to nutrient availability.

TOR, a central controller of cell growth

Cell growth (increase in cell mass) and cell proliferation (increase in cell number) are distinct yet coupled processes that go hand-in-hand to give rise to an organ, organism, or tumor. Cyclin-dependent kinase(s) is the central regulator of cell proliferation. Is there an equivalent regulator for cell growth? Recent findings reveal that the target of rapamycin TOR controls an unusually abundant and diverse set of readouts all of which are important for cell growth, suggesting that this conserved kinase is such a central regulator.

FAP1, a homologue of human transcription factor NF-X1, competes with rapamycin for binding to FKBP12 in yeast

The immunosuppressive drug rapamycin binds to the peptidyl-prolyl cis-trans isomerase FKBP12, and this complex arrests growth of yeast cells and activated T lymphocytes in the G1 phase of the cell cycle. In yeast, loss-of-function mutations in FPR1, the gene encoding FKBP12, or dominant gain-of-function mutations in TOR1 and TOR2, the genes encoding the physical targets of the FKBP12-rapamycin complex, confer rapamycin resistance. Here, we report the cloning and characterization of a novel gene, termed FAP1, which confers resistance to rapamycin by competing with the drug for binding to FKBP12. FAP1 encodes a member of an evolutionarily conserved family of putative transcription factors that includes human NF-X1, Drosophila melanogaster shuttle craft and previously undescribed homologues in Caenorhabditis elegans, Arabidopsis thaliana and Schizosaccharomyces pombe. We provide genetic and biochemical evidence that FAP1 interacts physically with FKBP12 in vivo and in vitro, and that it competes with rapamycin for interaction. Furthermore, mutations in the FKBP12 drug binding/active site or surface residues abolish binding to FAP1. Our results suggest that FAP1 is a physiological ligand for FKBP12 that is highly conserved from yeast to man. Furthermore, prolyl isomerases may commonly bind and regulate transcription factors.

Signaling from Ras to Rac and beyond: not just a matter of GEFs

Members of a family of intracellular molecular switches, the small GTPases, sense modifications of the extracellular environment and transduce them into a variety of homeostatic signals. Among small GTPases, Ras and the Rho family of proteins hierarchically and/or coordinately regulate signaling pathways leading to phenotypes as important as proliferation, differentiation and apoptosis. Ras and Rho-GTPases are organized in a complex network of functional interactions, whose molecular mechanisms are being elucidated. Starting from the simple concept of linear cascades of events (GTPase-->activator--> GTPase), the work of several laboratories is uncovering an increasingly complex scenario in which upstream regulators of GTPases also function as downstream effectors and influence the precise biological outcome. Furthermore, small GTPases assemble into macromolecular machineries that include upstream activators, downstream effectors, regulators and perhaps even final biochemical targets. We are starting to understand how these macromolecular complexes work and how they are regulated and targeted to their proper subcellular localization. Ultimately, the acquisition of a cogent picture of the various levels of integration and regulation in small GTPase-mediated signaling should define the physiology of early signal transduction events and the pathological implication of its subversion.

GAP activity of the Yersinia YopE cytotoxin specifically targets the Rho pathway: a mechanism for disruption of actin microfilament structure

The YopE cytotoxin of Yersinia pseudotuberculosis is an essential virulence determinant that is injected into the eukaryotic target cell via a plasmid-encoded type III secretion system. Injection of YopE into eukaryotic cells induces depolymerization of actin stress fibres. Here, we show that YopE exhibits a GTPase-activating protein (GAP) activity and that the presence of YopE stimulates downregulation of Rho, Rac and Cdc42 activity. YopE has an arginine finger motif showing homology with those found in other GAP proteins. Exchange of arginine 144 with alanine, located in this arginine finger motif, results in an inactive form of YopE that can no longer stimulate GTP hydrolysis by the GTPase. Furthermore, a yopE(R144A) mutant is unable to induce cytotoxicity on cultured HeLa cells in contrast to the corresponding wild-type strain. Expression of wild-type YopE in cells of Saccharomyces cerevisiae inhibits growth, while in contrast, expression of the inactive form of YopE, YopE(R144A), does not affect the yeast cells. Co-expression of proteins belonging to the Rho1 pathway of yeast, Rho1, Rom2p, Bck1 and Ste20, suppressed the growth phenotype of YopE in yeast cells. These results provide evidence that YopE exhibits a GAP activity to inactivate RhoGTPases, leading to depolymerization of the actin stress fibres in eukaryotic cells and growth inhibition in yeast.

Phosphatidylinositol 4,5-bisphosphate regulates two steps of homotypic vacuole fusion

Yeast vacuoles undergo cycles of fragmentation and fusion as part of their transmission to the daughter cell and in response to changes of nutrients and the environment. Vacuole fusion can be reconstituted in a cell free system. We now show that the vacuoles synthesize phosphoinositides during in vitro fusion. Of these phosphoinositides, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) are important for fusion. Monoclonal antibodies to PI(4,5)P(2), neomycin (a phosphoinositide ligand), and phosphatidylinositol-specific phospholipase C interfere with the reaction. Readdition of PI(4, 5)P(2) restores fusion in each case. Phosphatidylinositol 3-phosphate and PI(3,5)P(2) synthesis are not required. PI(4,5)P(2) is necessary for priming, i.e., for the Sec18p (NSF)-driven release of Sec17p (alpha-SNAP), which activates the vacuoles for subsequent tethering and docking. Therefore, it represents the kinetically earliest requirement identified for vacuole fusion so far. Furthermore, PI(4,5)P(2) is required at a step that can only occur after docking but before the BAPTA sensitive step in the latest stage of the reaction. We hence propose that PI(4,5)P(2) controls two steps of vacuole fusion.

PDK1 homologs activate the Pkc1-mitogen-activated protein kinase pathway in yeast

PDK1 (phosphoinositide-dependent kinase 1) is a mammalian growth factor-regulated serine/threonine kinase. Using a genetic selection based on a mutant form of the yeast MAP kinase kinase Ste7, we isolated a gene, PKH2, encoding a structurally and functionally conserved yeast homolog of PDK1. Yeast cells lacking both PKH2 and PKH1, encoding another PDK1 homolog, were nonviable, indicating that Pkh1 and Pkh2 share an essential function. A temperature-sensitive mutant, pkh1(D398G) pkh2, was phenotypically similar to mutants defective in the Pkc1-mitogen-activated protein kinase (MAPK) pathway. Genetic epistasis analyses, the phosphorylation of Pkc1 by Pkh2 in vitro, and reduced Pkc1 activity in the pkh1(D398G) pkh2 mutant indicate that Pkh functions upstream of Pkc1. The Pkh2 phosphorylation site in Pkc1 (Thr-983) is part of a conserved PDK1 target motif and essential for Pkc1 function. Thus, the yeast PDK1 homologs activate Pkc1 and the Pkc1-effector MAPK pathway.

PAK-family kinases regulate cell and actin polarization throughout the cell cycle of Saccharomyces cerevisiae

During the cell cycle of the yeast Saccharomyces cerevisiae, the actin cytoskeleton and cell surface growth are polarized, mediating bud emergence, bud growth, and cytokinesis. We have determined whether p21-activated kinase (PAK)-family kinases regulate cell and actin polarization at one or several points during the yeast cell cycle. Inactivation of the PAK homologues Ste20 and Cla4 at various points in the cell cycle resulted in loss of cell and actin cytoskeletal polarity, but not in depolymerization of F-actin. Loss of PAK function in G1 depolarized the cortical actin cytoskeleton and blocked bud emergence, but allowed isotropic growth and led to defects in septin assembly, indicating that PAKs are effectors of the Rho-guanosine triphosphatase Cdc42. PAK inactivation in S/G2 resulted in depolarized growth of the mother and bud and a loss of actin polarity. Loss of PAK function in mitosis caused a defect in cytokinesis and a failure to polarize the cortical actin cytoskeleton to the mother-bud neck. Cla4-green fluorescent protein localized to sites where the cortical actin cytoskeleton and cell surface growth are polarized, independently of an intact actin cytoskeleton. Thus, PAK family kinases are primary regulators of cell and actin cytoskeletal polarity throughout most or all of the yeast cell cycle. PAK-family kinases in higher organisms may have similar functions.

Antifungal activities of antineoplastic agents: Saccharomyces cerevisiae as a model system to study drug action

Recent evolutionary studies reveal that microorganisms including yeasts and fungi are more closely related to mammals than was previously appreciated. Possibly as a consequence, many natural-product toxins that have antimicrobial activity are also toxic to mammalian cells. While this makes it difficult to discover antifungal agents without toxic side effects, it also has enabled detailed studies of drug action in simple genetic model systems. We review here studies on the antifungal actions of antineoplasmic agents. Topics covered include the mechanisms of action of inhibitors of topoisomerases I and II; the immunosuppressants rapamycin, cyclosporin A, and FK506; the phosphatidylinositol 3-kinase inhibitor wortmannin; the angiogenesis inhibitors fumagillin and ovalicin; the HSP90 inhibitor geldanamycin; and agents that inhibit sphingolipid metabolism. In general, these natural products inhibit target proteins conserved from microorganisms to humans. These studies highlight the potential of microorganisms as screening tools to elucidate the mechanisms of action of novel pharmacological agents with unique effects against specific mammalian cell types, including neoplastic cells. In addition, this analysis suggests that antineoplastic agents and derivatives might find novel indications in the treatment of fungal infections, for which few agents are presently available, toxicity remains a serious concern, and drug resistance is emerging.

Starvation induces vacuolar targeting and degradation of the tryptophan permease in yeast

In Saccharomyces cerevisiae, amino acid permeases are divided into two classes. One class, represented by the general amino acid permease GAP1, contains permeases regulated in response to the nitrogen source. The other class, including the high affinity tryptophan permease, TAT2, consists of the so-called constitutive permeases. We show that TAT2 is regulated at the level of protein stability. In exponentially growing cells, TAT2 is in the plasma membrane and also accumulates in internal compartments of the secretory pathway. Upon nutrient deprivation or rapamycin treatment, TAT2 is transported to and degraded in the vacuole. The ubiquitination machinery and lysine residues within the NH(2)-terminal 31 amino acids of TAT2 mediate ubiquitination and degradation of the permease. Starvation-induced degradation of internal TAT2 is blocked in sec18, sec23, pep12, and vps27 mutants, but not in sec4, end4, and apg1 mutants, suggesting that, upon nutrient limitation, internal TAT2 is diverted from the late secretory pathway to the vacuolar pathway. Furthermore, our results suggest that TAT2 stability and sorting are controlled by the TOR signaling pathway, and regulated inversely to that of GAP1.

TOR kinase homologs function in a signal transduction pathway that is conserved from yeast to mammals

Rapamycin is a natural product with potent antifungal and immunosuppressive activities. Rapamycin binds to the FKBP12 prolyl isomerase, and the resulting protein-drug complex inhibits the TOR kinase homologs. Both the FKBP12 and the TOR proteins are highly conserved from yeast to man, and genetic and biochemical studies reveal that these proteins are the targets of rapamycin in vivo. Treatment of yeast or mammalian cells with rapamycin inhibits translational initiation of a subset of mRNAs and dramatically represses ribosomal mRNA and tRNA transcription. Furthermore, rapamycin exposure blocks cell cycle progression in the early G1 phase of the cell cycle, driving cells into a G0 state and, ultimately, triggering autophagy. Recent findings reveal that the upstream factors regulating the TOR signaling cascade are involved in detecting amino acids, nutrients, or growth factors. These findings indicate that the TOR proteins function in a signal transduction pathway that coordinates nutritional and mitogenic signals to control protein biosynthesis and degradation.

Protein kinase activity and identification of a toxic effector domain of the target of rapamycin TOR proteins in yeast

In complex with FKBP12, the immunosuppressant rapamycin binds to and inhibits the yeast TOR1 and TOR2 proteins and the mammalian homologue mTOR/FRAP/RAFT1. The TOR proteins promote cell cycle progression in yeast and human cells by regulating translation and polarization of the actin cytoskeleton. A C-terminal domain of the TOR proteins shares identity with protein and lipid kinases, but only one substrate (PHAS-I), and no regulators of the TOR-signaling cascade have been identified. We report here that yeast TOR1 has an intrinsic protein kinase activity capable of phosphorylating PHAS-1, and this activity is abolished by an active site mutation and inhibited by FKBP12-rapamycin or wortmannin. We find that an intact TOR1 kinase domain is essential for TOR1 functions in yeast. Overexpression of a TOR1 kinase-inactive mutant, or of a central region of the TOR proteins distinct from the FRB and kinase domains, was toxic in yeast, and overexpression of wild-type TOR1 suppressed this toxic effect. Expression of the TOR-toxic domain leads to a G1 cell cycle arrest, consistent with an inhibition of TOR function in translation. Overexpression of the PLC1 gene, which encodes the yeast phospholipase C homologue, suppressed growth inhibition by the TOR-toxic domains. In conclusion, our findings identify a toxic effector domain of the TOR proteins that may interact with substrates or regulators of the TOR kinase cascade and that shares sequence identity with other PIK family members, including ATR, Rad3, Mei-41, and ATM.

Rapamycin antifungal action is mediated via conserved complexes with FKBP12 and TOR kinase homologs in Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that causes meningitis in patients immunocompromised by AIDS, chemotherapy, organ transplantation, or high-dose steroids. Current antifungal drug therapies are limited and suffer from toxic side effects and drug resistance. Here, we defined the targets and mechanisms of antifungal action of the immunosuppressant rapamycin in C. neoformans. In the yeast Saccharomyces cerevisiae and in T cells, rapamycin forms complexes with the FKBP12 prolyl isomerase that block cell cycle progression by inhibiting the TOR kinases. We identified the gene encoding a C. neoformans TOR1 homolog. Using a novel two-hybrid screen for rapamycin-dependent TOR-binding proteins, we identified the C. neoformans FKBP12 homolog, encoded by the FRR1 gene. Disruption of the FKBP12 gene conferred rapamycin and FK506 resistance but had no effect on growth, differentiation, or virulence of C. neoformans. Two spontaneous mutations that confer rapamycin resistance alter conserved residues on TOR1 or FKBP12 that are required for FKBP12-rapamycin-TOR1 interactions or FKBP12 stability. Two other spontaneous mutations result from insertion of novel DNA sequences into the FKBP12 gene. Our observations reveal that the antifungal activities of rapamycin and FK506 are mediated via FKBP12 and TOR homologs and that a high proportion of spontaneous mutants in C. neoformans result from insertion of novel DNA sequences, and they suggest that nonimmunosuppressive rapamycin analogs have potential as antifungal agents.

Tor proteins and protein phosphatase 2A reciprocally regulate Tap42 in controlling cell growth in yeast

Tor proteins, homologous to DNA-dependent protein kinases, participate in a signal transduction pathway in yeast that regulates protein synthesis and cell wall expansion in response to nutrient availability. The anti-inflammatory drug rapamycin inhibits yeast cell growth by inhibiting Tor protein signaling. This leads to diminished association of a protein, Tap42, with two different protein phosphatase catalytic subunits; one encoded redundantly by PPH21 and PPH22, and one encoded by SIT4. We show that inactivation of either Cdc55 or Tpd3, which regulate Pph21/22 activity, results in rapamycin resistance and that this resistance correlates with an increased association of Tap42 with Pph21/22. Furthermore, we show Tor-dependent phosphorylation of Tap42 both in vivo and in vitro and that this phosphorylation is rapamycin sensitive. Inactivation of Cdc55 or Tpd3 enhances in vivo phosphorylation of Tap42. We conclude that Tor phosphorylates Tap42 and that phosphorylated Tap42 effectively competes with Cdc55/Tpd3 for binding to the phosphatase 2A catalytic subunit. Furthermore, Cdc55 and Tpd3 promote dephosphorylation of Tap42. Thus, Tor stimulates growth-promoting association of Tap42 with Pph21/22 and Sit4, while Cdc55 and Tpd3 inhibit this association both by direct competition and by dephosphorylation of Tap42. These results establish Tap42 as a target of Tor and add further refinement to the Tor signaling pathway.

Heme initiates changes in the expression of a wide array of genes during the early erythroid differentiation stage

Heme is central to oxygen sensing and utilization in all living organisms. It directly regulates numerous molecular and cellular processes for systems that sense or use oxygen. In mammals, heme plays an indispensable role in erythroid cell differentiation. To investigate heme regulatory functions, we identified, by differential display, and confirmed, by quantitative RT-PCR and Northern blotting analysis, the genes whose expression is altered by heme during the early stage of K562 cell differentiation. These include genes encoding a GAP-associated p62 protein, histone H2A.Z, a subunit of the small nuclear ribonucleoprotein complex, and the chaperonin Tcp20, and a cellular immediate-early-response gene. The results suggest that heme initiates changes in key factors that control a wide array of processes ranging from cell cycle and Ras signaling to chromatin structure, splicing and protein folding. These key factors might act together to mediate heme action, which is critical for erythroid cell differentiation.

Regulation of ribosome biogenesis by the rapamycin-sensitive TOR-signaling pathway in Saccharomyces cerevisiae

The TOR (target of rapamycin) signal transduction pathway is an important mechanism by which cell growth is controlled in all eucaryotic cells. Specifically, TOR signaling adjusts the protein biosynthetic capacity of cells according to nutrient availability. In mammalian cells, one branch of this pathway controls general translational initiation, whereas a separate branch specifically regulates the translation of ribosomal protein (r-protein) mRNAs. In Saccharomyces cerevisiae, the TOR pathway similarly regulates general translational initiation, but its specific role in the synthesis of ribosomal components is not well understood. Here we demonstrate that in yeast control of ribosome biosynthesis by the TOR pathway is surprisingly complex. In addition to general effects on translational initiation, TOR exerts drastic control over r-protein gene transcription as well as the synthesis and subsequent processing of 35S precursor rRNA. We also find that TOR signaling is a prerequisite for the induction of r-protein gene transcription that occurs in response to improved nutrient conditions. This induction has been shown previously to involve both the Ras-adenylate cyclase as well as the fermentable growth medium-induced pathways, and our results therefore suggest that these three pathways may be intimately linked.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.

The FKBP12-rapamycin-binding domain is required for FKBP12-rapamycin-associated protein kinase activity and G1 progression

The immunosuppressant rapamycin, in complex with its cellular receptor FKBP12, targets the cellular protein FKBP12-rapamycin-associated protein/mammalian target of rapamycin/rapamycin and FKBP12 target 1 (FRAP/mTOR/RAFT1) and inhibits/delays G1 cell cycle progression in mammalian cells. As a member of the novel phosphatidylinositol kinase-related kinase family, FRAP's kinase activity is essential for its signaling function. The FKBP12-rapamycin binding (FRB) domain in FRAP is also speculated to play an important role in FRAP function and signaling. However, the biochemical and physiological functions of FRB, as well as the mechanism for rapamycin inhibition, have been unclear. The present study focuses on investigation of FRB's role and the functional relationship between FRB domain and kinase domain in FRAP. Microinjection of purified FRB protein into human osteosarcoma MG63 cells results in a drastic blockage of the G1 to S cell cycle progression; such a dominant negative effect is reversed by a point mutation (Trp2027 --> Phe). The same mutation also abolishes kinase activity of FRAP without affecting ATP binding, and truncation studies suggest that upstream sequences including FRB are required for kinase activity in vitro. Given these data, we propose a model for FRAP function, in which the FRB domain is required for activation of the kinase domain, possibly through the interaction with an upstream activator. In addition, our observations provide direct evidence linking FRAP function to G1 cell cycle progression.

Target of rapamycin (TOR): balancing the opposing forces of protein synthesis and degradation

Mitogenic and nutritional signals must be integrated for a cell to grow. The target of rapamycin (TOR) is emerging as an effector for signals which indicate to the cell whether the external environment is conducive for growth. Use of the immunosuppressant rapamycin, a bacterial macrolide, has been instructive in identifying potential signaling components downstream of TOR, leading to the observation that both protein synthesis and turnover are under TOR control. The central issues concerning TOR are the identification of the proliferative and anti-proliferative signals which mediate its function and the mechanisms by which these signals are transduced to downstream molecules.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

The TOR nutrient signalling pathway phosphorylates NPR1 and inhibits turnover of the tryptophan permease

The Saccharomyces cerevisiae targets of rapamycin, TOR1 and TOR2, signal activation of cell growth in response to nutrient availability. Loss of TOR or rapamycin treatment causes yeast cells to arrest growth in early G1 and to express several other physiological properties of starved (G0) cells. As part of this starvation response, high affinity amino acid permeases such as the tryptophan permease TAT2 are targeted to the vacuole and degraded. Here we show that the TOR signalling pathway phosphorylates the Ser/Thr kinase NPR1 and thereby inhibits the starvation-induced turnover of TAT2. Overexpression of NPR1 inhibits growth and induces the degradation of TAT2, whereas loss of NPR1 confers resistance to rapamycin and to FK506, an inhibitor of amino acid import. NPR1 is controlled by TOR and the type 2A phosphatase-associated protein TAP42. First, overexpression of NPR1 is toxic only when TOR function is reduced. Secondly, NPR1 is rapidly dephosphorylated in the absence of TOR. Thirdly, NPR1 dephosphorylation does not occur in a rapamycin-resistant tap42 mutant. Thus, the TOR nutrient signalling pathway also controls growth by inhibiting a stationary phase (G0) programme. The control of NPR1 by TOR is analogous to the control of p70 s6 kinase and 4E-BP1 by mTOR in mammalian cells.

Phosphoinositide-specific phospholipase C interacts with phosphatidylinositol kinase homolog TOR2

The Saccharomyces cerevisiae PLC1 gene encodes a homolog of the delta isoform of mammalian phosphoinositide-specific phospholipase C. Cells deleted for PLC1 gene (plc1Delta) are viable but display several phenotypes, including temperature and osmotic sensitivity and defects in utilization of carbon sources other than glucose. We have used the two hybrid screen to identify Plc1p-interacting proteins. One of the identified proteins was Tor2p, a putative phosphatidylinositol (PtdIns) kinase involved in regulation of protein synthesis, cell cycle progression and organization of the actin cytoskeleton. This interaction was confirmed biochemically by coprecipitation of Plc1p and Tor2p. The results suggest that Tor2p, as a PtdIns kinase, produces phosphorylated PtdIns, which is then hydrolyzed by the associated Plc1p. The proximity of Tor2p to Plc1p may therefore result in a regulated spatial and temporal coupling of synthesis and hydrolysis of phosphorylated forms of PtdIns.

The Rho1 effector Pkc1, but not Bni1, mediates signalling from Tor2 to the actin cytoskeleton

In Saccharomyces cerevisiae, the phosphatidylinositol kinase homologue Tor2 controls the cell-cycle-dependent organisation of the actin cytoskeleton by activating the small GTPase Rho1 via the exchange factor Rom2 [1,2]. Four Rho1 effectors are known, protein kinase C 1 (Pkc1), the formin-family protein Bni1, the glucan synthase Fks and the signalling protein Skn7 [2,3]. Rho1 has been suggested to signal to the actin cytoskeleton via Bni1 and Pkc1; rho1 mutants have never been shown to have defects in actin organisation, however [2,4]. We have further investigated the role of Rho1 in controlling actin organisation and have analysed which of the Rho1 effectors mediates Tor2 signalling to the actin cytoskeleton. We show that some, but not all, rho1 temperature-sensitive (rho1ts) mutants arrest growth with a disorganised actin cytoskeleton. Both the growth defect and the actin organisation defect of the rho1-2ts mutant were suppressed by upregulation of Pkc1 but not by upregulation of Bni1, Fks or Skn7. Overexpression of Pkc1, but not overexpression of Bni1, Fks or Skn7, also rescued a tor2ts mutant, and deletion of BNI1 or SKN7 did not prevent the suppression of the tor2ts mutation by overexpressed Rom2. Furthermore, overexpression of the Pkc1-controlled mitogen-activated protein (MAP) kinase Mpk1 suppressed the actin defect of tor2ts and rho1-2ts mutants. Thus, Tor2 signals to the actin cytoskeleton via Rho1, Pkc1 and the cell integrity MAP kinase cascade.

A Candida albicans chaperonin subunit (CaCct8p) as a suppressor of morphogenesis and Ras phenotypes in C. albicans and Saccharomyces cerevisiae

Saccharomyces cerevisiae and the pathogen Candida albicans can be induced to undergo morphogenesis from a yeast to a filamentous form. A C. albicans gene (CaCCT8) was identified encoding a subunit of the Cct chaperonin complex, whose expression prevents filament formation in both fungi without interfering with growth of the yeast form. In S. cerevisiae, pseudohyphal growth induced by Ras2Val19, by overproduction of Phd1p or by expression of the C. albicans EFG1 gene, was blocked by CaCct8p and its N-terminally deleted derivative CaCct8-delta1p; in contrast, pseudohyphal induction by other components (Cph1p, Cdc42p) could not be suppressed, indicating that morphogenesis per se is not inhibited. CaCCT8 expression also interfered with other Ras2pVal19 phenotypes, including heat sensitivity, lack of glycogen accumulation and lack of sporulation. In C. albicans, overproduction of CaCct8p effectively blocked hyphal morphogenesis induced by starvation conditions and by serum. The results suggest that the activity of a component in the Ras2p signal transduction pathway is suppressed by excess chaperonin subunits. This component may be a novel folding target for the Cct complex. In agreement with this hypothesis, disruption of one of the two CaCCT8 alleles in C. albicans led to defective hyphal morphogenesis.

Overexpression of MID2 suppresses the profilin-deficient phenotype of yeast cells

Profilin-deficient Saccharomyces cerevisiae cells show abnormal growth, actin localization, chitin deposition, bud formation and cytokinesis. Previous studies have also revealed a synthetic lethality between pfy1 and late secretory mutants, suggesting a role for profilin in intracellular transport. In this work, we document further the secretion defect associated with the pfy1delta mutant. Electron microscopic observations reveal an accumulation of glycoproteins in the bud and in the mother cell. The MATa, pfy1delta cells mate as well as wild-type cells, while the mating efficiency of MAT alpha, pfy1delta cells is reduced. Pulse-chase experiments demonstrate an accumulation of the 19 kDa alpha-factor precursor and delayed secretion of the mature alpha-factor. The TGN protein Kex2p is the principal enzyme responsible for the endoproteolytic cleavage of the alpha-factor precursor. An immunofluorescence detection of Kex2p shows an altered localization in pfy1delta cells. Instead of a discrete punctate distribution, the enzyme is dispersed throughout the cytoplasm. A high-copy-number plasmid containing MID2, which encodes a potential transmembrane protein involved in cell cycle control, suppresses the abnormal growth, actin distribution, alpha-factor maturation and the accumulation of intracellular membranous structures in pfy1delta cells.

MSS4, a phosphatidylinositol-4-phosphate 5-kinase required for organization of the actin cytoskeleton in Saccharomyces cerevisiae

The Saccharomyces cerevisiae protein MSS4 is essential and homologous to mammalian phosphatidylinositol-4-phosphate (PI(4)P) 5-kinases. Here, we demonstrate that MSS4 is a lipid kinase. MSS4 has dual substrate specificity in vitro, converting PI(4)P to PI(4, 5)P2 and to a lesser extent PI(3)P to PI(3,4)P2; no activity was detected with PI or PI(5)P as a substrate. Cells overexpressing MSS4 contain an elevated level specifically of PI(4,5)P2, whereas mss4 mutant cells have only approximately 10% of the normal amount of this phosphorylated phosphoinositide. Furthermore, cells lacking MSS4 are unable to form actin cables and to properly localize their actin cytoskeleton during polarized cell growth. Overexpression of RHO2, encoding a Rho-type GTPase involved in regulation of the actin cytoskeleton, restores growth and polarized distribution of actin in an mss4 mutant. These results suggest that MSS4 is the major PI(4)P 5-kinase in yeast and provide a link between phosphoinositide metabolism and organization of the actin cytoskeleton in vivo.

Inhibition by platelet-activating factor of Src- and hepatocyte growth factor-dependent invasiveness of intestinal and kidney epithelial cells. Phosphatidylinositol 3'-kinase is a critical mediator of tumor invasion

This study was designed to characterize platelet-activating factor receptor (PAF-R) expression and function in normal and cancerous human colonic epithelial cells. PAF-R gene transcripts were analyzed by reverse transcription-polymerase chain reaction and Southern blot, using three sets of primers corresponding either to the coding region of the human PAF-R sequence (polymerase chain reaction product: 682 base pairs (bp)) or to the leukocyte- and tissue-type transcripts of 166 and 252 bp, respectively. An elongated splice variant was identified in the 5'-untranslated region of the tissue-type PAF-R transcript (334 bp) in colonic epithelial crypts and tumors. In human colonic PCmsrc cells transformed by c-src oncogene, the hepatocyte growth factor (HGF)-dependent invasiveness of collagen gels was abolished by 0.1 microM PAF and restored by the PAF-R antagonists WEB2086 and SR27417. PAF blocked HGF-induced tyrosine phosphorylation of p125 focal adhesion kinase. The phosphatidylinositol 3'-kinase (PI3'-K) inhibitors wortmannin and LY294002 totally blocked the HGF-induced invasion. Similar effects were observed in ts-srcMDCK kidney epithelial cells transformed by a v-Src temperature-sensitive mutant: (i) PAF and wortmannin exerted additive inhibitory effects on Src-induced invasion and (ii) activated and dominant negative forms of p110alpha PI3'-K, respectively, amplified and abrogated the Src- and HGF-dependent invasiveness of parental and ts-srcMDCK cells. We also provided the first evidence for the contribution of rapamycin-insensitive, pertussis toxin-dependent G-protein pathways to the integration of the signals emerging from activated Met and PAF receptors. These results indicate that PI3'-K is a critical transducer of invasiveness and strongly suggest that PAF exerts a negative control on invasion by inhibiting this signaling pathway. A possible beneficial role of PAF analogs on tumor invasion is therefore proposed.

Mammalian target of rapamycin: immunosuppressive drugs uncover a novel pathway of cytokine receptor signaling

Recent findings have significantly advanced our understanding of the mechanism by which the potent immunosuppressive drug rapamycin inhibits cytokine-dependent lymphocyte proliferation. The protein targeted by the immunophilin-rapamycin complex is a member of a newly defined family of phosphoinositide-3-kinase-related kinases. The rapamycin target protein functions as a protein kinase in a signal transduction pathway that regulates the synthesis of proteins required for cell-cycle progression in both lymphoid and nonlymphoid cells.

Cell wall integrity modulates RHO1 activity via the exchange factor ROM2

The essential phosphatidylinositol kinase homologue TOR2 of Saccharomyces cerevisiae controls the actin cytoskeleton by activating a GTPase switch consisting of RHO1 (GTPase), ROM2 (GEF) and SAC7 (GAP). We have identified two mutations, rot1-1 and rot2-1, that suppress the loss of TOR2 and are synthetic-lethal. The wild-type ROT1 and ROT2 genes and a multicopy suppressor, BIG1, were isolated by their ability to rescue the rot1-1 rot2-1 double mutant. ROT2 encodes glucosidase II, and ROT1 and BIG1 encode novel proteins. We present evidence that cell wall defects activate RHO1. First, rot1, rot2, big1, cwh41, gas1 and fks1 mutations all confer cell wall defects and suppress tor2(ts). Second, destabilizing the cell wall by supplementing the growth medium with 0.005% SDS also suppresses a tor2(ts) mutation. Third, disturbing the cell wall with SDS or a rot1, rot2, big1, cwh41, gas1 or fks1 mutation increases GDP/GTP exchange activity toward RHO1. These results suggest that cell wall defects suppress a tor2 mutation by activating RHO1 independently of TOR2, thereby inducing TOR2-independent polarization of the actin cytoskeleton and cell wall synthesis. Activation of RHO1, a subunit of the cell wall synthesis enzyme glucan synthase, by a cell wall alteration would ensure that cell wall synthesis occurs only when and where needed. The mechanism of RHO1 activation by a cell wall alteration is via the exchange factor ROM2 and could be analogous to signalling by integrin receptors in mammalian cells.

Tor, a phosphatidylinositol kinase homologue, controls autophagy in yeast

Autophagy is a bulk protein degradation process that is induced by starvation. The control mechanism for induction of autophagy is not well understood. We found that Tor, a phosphatidylinositol kinase homologue, is involved in the control of autophagy in the yeast, Saccharomyces cerevisiae. When rapamycin, an inhibitor of Tor function, is added, autophagy is induced even in cells growing in nutrient-rich medium. A temperature-sensitive tor mutant also leads to induction of autophagy at a nonpermissive temperature. These results indicate that Tor negatively regulates the induction of autophagy. Tor is the first molecule that is identified as a pivotal player in the starvation-signaling pathway of autophagy. Furthermore, we found that a high concentration of cAMP is inhibitory for induction of autophagy. APG gene products are involved in autophagy induced by starvation. Autophagy was not induced in apg mutants in the presence of rapamycin, indicating that the site of action of Tor is upstream of those of Apg proteins. In nutrient-rich medium, Apg proteins are involved also in the transport of aminopeptidase I from the cytosol to the vacuole. Tor may act to switch Apg function between autophagy and transport of aminopeptidase I.

Control of reorganization of the actin cytoskeleton by Rho family small GTP-binding proteins in yeast

Accumulating evidence indicates that Rho family small GTP-binding proteins regulate reorganization of the actin cytoskeleton. There are members of the Rho family in the budding yeast Saccharomyces cerevisiae, in which powerful molecular genetical approaches are applicable. Recent identification of regulators and targets of the Rho family members has enhanced our understanding of the regulation and modes of action of Rho family members in reorganization of the actin cytoskeleton.

Overexpression of a kinase-inactive ATR protein causes sensitivity to DNA-damaging agents and defects in cell cycle checkpoints

ATR, a phosphatidylinositol kinase-related protein homologous to ataxia telangiectasia mutated (ATM), is important for the survival of human cells following many forms of DNA damage. Expression of a kinase-inactive allele of ATR (ATRkd) in human fibroblasts causes increased sensitivity to ionizing radiation (IR), cis-platinum and methyl methanesulfonate, but only slight UV radiation sensitivity. ATRkd overexpression abrogates the G2/M arrest after exposure to IR, and overexpression of wild-type ATR complements the radioresistant DNA synthesis phenotype of cells lacking ATM, suggesting a potential functional overlap between these proteins. ATRkd overexpression also causes increased sensitivity to hydroxyurea that is associated with microtubule-mediated nuclear abnormalities. These observations are consistent with uncoupling of certain mitotic events from the completion of S-phase. Thus, ATR is an important component of multiple DNA damage response pathways and may be involved in the DNA replication (S/M) checkpoint.

TOR2 is part of two related signaling pathways coordinating cell growth in Saccharomyces cerevisiae

The Saccharomyces cerevisiae genes TOR1 and TOR2 encode phosphatidylinositol kinase homologs. TOR2 has two essential functions. One function overlaps with TOR1 and mediates protein synthesis and cell cycle progression. The second essential function of TOR2 is unique to TOR2 and mediates the cell-cycle-dependent organization of the actin cytoskeleton. We have isolated temperature-sensitive mutants that are defective for either one or both of the two TOR2 functions. The three classes of mutants were as follows. Class A mutants, lacking only the TOR2-unique function, are defective in actin cytoskeleton organization and arrest within two to three generations as small-budded cells in the G2/M phase of the cell cycle. Class B mutants, lacking only the TOR-shared function, and class C mutants, lacking both functions, exhibit a rapid loss of protein synthesis and a G1 arrest within one generation. To define further the two functions of TOR2, we isolated multicopy suppressors that rescue the class A or B mutants. Overexpression of MSS4, PKC1, PLC1, RHO2, ROM2, or SUR1 suppressed the growth defect of a class A mutant. Surprisingly, overexpression of PLC1 and MSS4 also suppressed the growth defect of a class B mutant. These genes encode proteins that are involved in phosphoinositide metabolism and signaling. Thus, the two functions (readouts) of TOR2 appear to involve two related signaling pathways controlling cell growth.

Role of small G proteins in yeast cell polarization and wall biosynthesis

In the vegetative (mitotic) cycle and during sexual conjugation, yeast cells display polarized growth, giving rise to a bud or to a mating projection, respectively. In both cases one can distinguish three steps in these processes: choice of a growth site, organization of the growth site, and actual growth and morphogenesis. In all three steps, small GTP-binding proteins (G proteins) and their regulators play essential signaling functions. For the choice of a bud site, Bud1, a small G protein, Bud2, a negative regulator of Bud1, and Bud5, an activator, are all required. If any of them is defective, the cell loses its ability to select a proper bud position and buds randomly. In the organization of the bud site or of the site in which a mating projection appears, Cdc42, its activator Cdc24, and its negative regulators play a fundamental role. In the absence of Cdc42 or Cdc24, the actin cytoskeleton does not become organized and budding does not take place. Finally, another small G protein, Rho1, is required for activity of beta (1-->3)glucan synthase, the enzyme that catalyzes the synthesis of the major structural component of the yeast cell wall. In all of the above processes, G proteins can work as molecular switches because of their ability to shift between an active GTP-bound state and an inactive GDP-bound state.

Analysis of mutationally altered forms of the Cct6 subunit of the chaperonin from Saccharomyces cerevisiae

The Cct double-ring chaperonin complex of Saccharomyces cerevisiae is comprised of eight essential subunits, Cct1p-Cct8p, and assists the folding of substrates such as actins and tubulins. Single and multiple amino acid replacements of Cct6p were constructed by oligonucleotide-directed mutagenesis, including changes of charged to alanine residues and uncharged to charged residues. The replacements were targeted, in part, to residues corresponding to functionally critical regions identified in the published crystal structure of the Escherichia coli chaperonin, GroEL. Here, we report the critical hydrophobic residues and clusters of hydrophilic residues in regions corresponding to those from the apical domain of GroEL implicated in peptide binding and peptide release, and certain residues in the putative equatorial domain implicated in subunit-to-subunit interaction. In contrast to their homologous counterparts in Cct2p and Cct1p, the highly conserved putative ATP binding motifs of Cct6p were relatively amenable to mutations. Our data suggest that the entire Cct6p molecule might be essential for assembly of Cct complex and might participate in binding substrates. However, there appeared to exist a functional hierarchy in ATP binding/hydrolysis among Cct subunits, as suggested by the high tolerance of Cct6p to mutations within the putative ATP binding pocket.

TOR signalling and control of cell growth

TOR, phosphatidylinositol 3-kinase, p70s6k, and 4E-BP1 have recently emerged as components of a major signalling pathway that is dedicated to protein translation and thus to cell growth. This pathway appears to be conserved, at least in part, in yeast, slime molds, plants, flies, and mammals. TOR and phosphatidylinositol 3-kinase control p70s6k and 4E-BP1, which, in turn, directly control the translation initiation machinery.

Characterization of Saccharomyces cerevisiae dna2 mutants suggests a role for the helicase late in S phase

The TOR proteins, originally identified as targets of the immunosuppressant rapamycin, contain an ATM-like "lipid kinase" domain and are required for early G1 progression in eukaryotes. Using a screen to identify Saccharomyces cerevisiae mutants requiring overexpression of Tor1p for viability, we have isolated mutations in a gene we call ROT1 (requires overexpression of Tor1p). This gene is identical to DNA2, encoding a helicase required for DNA replication. As with its role in cell cycle progression, both the N-terminal and C-terminal regions, as well as the kinase domain of Tor1p, are required for rescue of dna2 mutants. Dna2 mutants are also rescued by Tor2p and show synthetic lethality with tor1 deletion mutants under specific conditions. Temperature-sensitive (Ts) dna2 mutants arrest irreversibly at G2/M in a RAD9- and MEC1-dependent manner, suggesting that Dna2p has a role in S phase. Frequencies of mitotic recombination and chromosome loss are elevated in dna2 mutants, also supporting a role for the protein in DNA synthesis. Temperature-shift experiments indicate that Dna2p functions during late S phase, although dna2 mutants are not deficient in bulk DNA synthesis. These data suggest that Dna2p is not required for replication fork progression but may be needed for a later event such as Okazaki fragment maturation.

The Rho-GEF Rom2p localizes to sites of polarized cell growth and participates in cytoskeletal functions in Saccharomyces cerevisiae

Rom2p is a GDP/GTP exchange factor for Rho1p and Rho2p GTPases; Rho proteins have been implicated in control of actin cytoskeletal rearrangements. ROM2 and RHO2 were identified in a screen for high-copy number suppressors of cik1 delta, a mutant defective in microtubule-based processes in Saccharomyces cerevisiae. A Rom2p::3XHA fusion protein localizes to sites of polarized cell growth, including incipient bud sites, tips of small buds, and tips of mating projections. Disruption of ROM2 results in temperature-sensitive growth defects at 11 degrees C and 37 degrees C. rom2 delta cells exhibit morphological defects. At permissive temperatures, rom2 delta cells often form elongated buds and fail to form normal mating projections after exposure to pheromone; at the restrictive temperature, small budded cells accumulate. High-copy number plasmids containing either ROM2 or RHO2 suppress the temperature-sensitive growth defects of cik1 delta and kar3 delta strains. KAR3 encodes a kinesin-related protein that interacts with Cik1p. Furthermore, rom2 delta strains exhibit increased sensitivity to the microtubule depolymerizing drug benomyl. These results suggest a role for Rom2p in both polarized morphogenesis and functions of the microtubule cytoskeleton.

The unique hetero-oligomeric nature of the subunits in the catalytic cooperativity of the yeast Cct chaperonin complex

The structural and functional organization of the Cct complex was addressed by genetic analyses of subunit interactions and catalytic cooperativity among five of the eight different essential subunits, Cct1p-Cct8p, in the yeast Saccharomyces cerevisiae. The cct1-1, cct2-3, and cct3-1 alleles, containing mutations at the conserved putative ATP-binding motif, GDGTT, are cold-sensitive, whereas single and multiple replacements of the corresponding motif in Cct6p are well tolerated by the cell. We demonstrated herein that cct6-3 (L19S), but not the parolog cct1-5 (R26I), specifically suppresses the cct1-1, cct2-3, and cct3-1 alleles, and that this suppression can be modulated by mutations in a putative phosphorylation motif, RXS, and the putative ATP-binding pocket of Cct6p. Our results suggest that the Cct ring is comprised of a single hetero-oligomer containing eight subunits of differential functional hierarchy, in which catalytic cooperativity of ATP-binding/hydrolysis takes place in a sequential manner different from the concerted cooperativity proposed for GroEL.