Cell-cycle checkpoints that ensure coordination between nuclear and cytoplasmic events in Saccharomyces cerevisiae

A continuous-time stochastic Boolean model provides a quantitative description of the budding yeast cell cycle

The cell division cycle is regulated by a complex network of interacting genes and proteins. The control system has been modeled in many ways, from qualitative Boolean switching-networks to quantitative differential equations and highly detailed stochastic simulations. Here we develop a continuous-time stochastic model using seven Boolean variables to represent the activities of major regulators of the budding yeast cell cycle plus one continuous variable representing cell growth. The Boolean variables are updated asynchronously by logical rules based on known biochemistry of the cell-cycle control system using Gillespie's stochastic simulation algorithm. Time and cell size are updated continuously. By simulating a population of yeast cells, we calculate statistical properties of cell cycle progression that can be compared directly to experimental measurements. Perturbations of the normal sequence of events indicate that the cell cycle is 91% robust to random 'flips' of the Boolean variables, but 9% of the perturbations induce lethal mistakes in cell cycle progression. This simple, hybrid Boolean model gives a good account of the growth and division of budding yeast cells, suggesting that this modeling approach may be as accurate as detailed reaction-kinetic modeling with considerably less demands on estimating rate constants.

Persistent actin depolarization caused by ethanol induces the formation of multiple small cortical septin rings in yeast

Short-term exposure to severe ethanol stress has adverse effects on yeast cells. However, limited information is available on the effects of long-term exposure to severe ethanol stress. In this study, we examined the effects of a long-term treatment with a high ethanol concentration [10% (v/v)] on yeast morphology. We found that long-term severe ethanol stress induced the continuous depolarization of the actin cytoskeleton and hypertrophy in yeast cells, accompanied by the aberrant localization of septins, which formed multiple small cortical rings (MSCRs). The formation of MSCRs was also induced by the continuous depolarization of the actin cytoskeleton caused by a treatment with latrunculin-A, an effective inhibitor of actin polymerization. Unlike the formation of conventional septin rings, the formation of MSCRs did not require Cdc42 and its effectors, Gic1, Gic2 and Cla4. These results provide novel insights into the effects of persistent actin depolarization caused by long-term exposure to severe ethanol stress on yeast cytomorphology.

High levels of histones promote whole-genome-duplications and trigger a Swe1WEE1-dependent phosphorylation of Cdc28CDK1

Whole-genome duplications (WGDs) have played a central role in the evolution of genomes and constitute an important source of genome instability in cancer. Here, we show in Saccharomyces cerevisiae that abnormal accumulations of histones are sufficient to induce WGDs. Our results link these WGDs to a reduced incorporation of the histone variant H2A.Z to chromatin. Moreover, we show that high levels of histones promote Swe1^WEE1 stabilisation thereby triggering the phosphorylation and inhibition of Cdc28^CDK1 through a mechanism different of the canonical DNA damage response. Our results link high levels of histones to a specific type of genome instability that is quite frequently observed in cancer and uncovers a new mechanism that might be able to respond to high levels of histones.

The interplay between transcription and mRNA degradation in Saccharomyces cerevisiae

The cellular transcriptome is shaped by both the rates of mRNA synthesis in the nucleus and mRNA degradation in the cytoplasm under a specified condition. The last decade witnessed an exciting development in the field of post-transcriptional regulation of gene expression which underscored a strong functional coupling between the transcription and mRNA degradation. The functional integration is principally mediated by a group of specialized promoters and transcription factors that govern the stability of their cognate transcripts by “marking” them with a specific factor termed “coordinator.” The “mark” carried by the message is later decoded in the cytoplasm which involves the stimulation of one or more mRNA-decay factors, either directly by the “coordinator” itself or in an indirect manner. Activation of the decay factor(s), in turn, leads to the alteration of the stability of the marked message in a selective fashion. Thus, the integration between mRNA synthesis and decay plays a potentially significant role to shape appropriate gene expression profiles during cell cycle progression, cell division, cellular differentiation and proliferation, stress, immune and inflammatory responses, and may enhance the rate of biological evolution.

Structure and inhibition analysis of the mouse SAD-B C-terminal fragment

The SAD (synapses of amphids defective) kinases, including SAD-A and SAD-B, play important roles in the regulation of neuronal development, cell cycle, and energy metabolism. Our recent study of mouse SAD-A identified a unique autoinhibitory sequence (AIS), which binds at the junction of the kinase domain (KD) and the ubiquitin-associated (UBA) domain and exerts autoregulation in cooperation with UBA. Here, we report the crystal structure of the mouse SAD-B C-terminal fragment including the AIS and the kinase-associated domain 1 (KA1) at 2.8 Å resolution. The KA1 domain is structurally conserved, while the isolated AIS sequence is highly flexible and solvent-accessible. Our biochemical studies indicated that the SAD-B AIS exerts the same autoinhibitory role as that in SAD-A. We believe that the flexible isolated AIS sequence is readily available for interaction with KD-UBA and thus inhibits SAD-B activity.

Structural insight into the mechanism of synergistic autoinhibition of SAD kinases

The SAD/BRSK kinases participate in various important life processes, including neural development, cell cycle and energy metabolism. Like other members of the AMPK family, SAD contains an N-terminal kinase domain followed by the characteristic UBA and KA1 domains. Here we identify a unique autoinhibitory sequence (AIS) in SAD kinases, which exerts autoregulation in cooperation with UBA. Structural studies of mouse SAD-A revealed that UBA binds to the kinase domain in a distinct mode and, more importantly, AIS nestles specifically into the KD-UBA junction. The cooperative action of AIS and UBA results in an ‘αC-out' inactive kinase, which is conserved across species and essential for presynaptic vesicle clustering in C. elegans. In addition, the AIS, along with the KA1 domain, is indispensable for phospholipid binding. Taken together, these data suggest a model for synergistic autoinhibition and membrane activation of SAD kinases.

The SAD kinases contain a UBA domain that binds to the kinase domain and has a role in autoinhibition and allosteric activation of the AMPK homoenzyme. Here, the authors identify an autoinhibitory sequence in SAD and show that the UBA domain synergistically functions as an autoinhibitory domain.

Morphogenesis checkpoint kinase Swe1 is the executor of lipolysis-dependent cell-cycle progression

Cell growth and division requires the precise duplication of cellular DNA content but also of membranes and organelles. Knowledge about the cell-cycle-dependent regulation of membrane and storage lipid homeostasis is only rudimentary. Previous work from our laboratory has shown that the breakdown of triacylglycerols (TGs) is regulated in a cell-cycle-dependent manner, by activation of the Tgl4 lipase by the major cyclin-dependent kinase Cdc28. The lipases Tgl3 and Tgl4 are required for efficient cell-cycle progression during the G1/S (Gap1/replication phase) transition, at the onset of bud formation, and their absence leads to a cell-cycle delay. We now show that defective lipolysis activates the Swe1 morphogenesis checkpoint kinase that halts cell-cycle progression by phosphorylation of Cdc28 at tyrosine residue 19. Saturated long-chain fatty acids and phytosphingosine supplementation rescue the cell-cycle delay in the Tgl3/Tgl4 lipase-deficient strain, suggesting that Swe1 activity responds to imbalanced sphingolipid metabolism, in the absence of TG degradation. We propose a model by which TG-derived sphingolipids are required to activate the protein phosphatase 2A (PP2A(Cdc55)) to attenuate Swe1 phosphorylation and its inhibitory effect on Cdc28 at the G1/S transition of the cell cycle.

Sol-Generating Chemical Vapor into Liquid (SG-CViL) Deposition- A Facile Method for Encapsulation of Diverse Cell Types in Silica Matrices

In nature, cells perform a variety of complex functions such as sensing, catalysis, and energy conversion which hold great potential for biotechnological device construction. However, cellular sensitivity to ex-vivo environments necessitates development of bio-nano interfaces which allow integration of cells into devices and maintain their desired functionality. In order to develop such an interface, the use of a novel Sol Generating Chemical Vapor into Liquid (SG-CViL) deposition process for whole cell encapsulation in silica was explored. In SG-CViL, the high vapor pressure of tetramethyl orthosilicate (TMOS) is utilized to deliver silica into an aqueous medium, creating a silica sol. Cells are then mixed with the resulting silica sol, facilitating encapsulation of cells in silica while minimizing cell contact with the cytotoxic products of silica generating reactions (i.e. methanol), and reduce exposure of cells to compressive stresses induced from silica condensation reactions. Using SG-CVIL, Saccharomyces cerevisiae (S. cerevisiae) engineered with an inducible beta galactosidase system were encapsulated in silica solids and remained both viable and responsive 29 days post encapsulation. By tuning SG-CViL parameters thin layer silica deposition on mammalian HeLa and U87 human cancer cells was also achieved. The ability to encapsulate various cell types in either a multi cell (S. cerevisiae) or a thin layer (HeLa and U87 cells) fashion shows the promise of SG-CViL as an encapsulation strategy for generating cell-silica constructs with diverse functions for incorporation into devices for sensing, bioelectronics, biocatalysis, and biofuel applications.

A lysine deacetylase Hos3 is targeted to the bud neck and involved in the spindle position checkpoint

The Saccharomyces cerevisiae lysine deacetylase Hos3 is asymmetrically targeted to the daughter side of the neck, dependent on the morphogenesis checkpoint member Hsl7, and to the daughter spindle pole body (SPB). In the presence of spindle misalignment, Hos3 at the SPBs functions as a brake component to inhibit mitotic exit.

An increasing number of cellular activities can be regulated by reversible lysine acetylation. Targeting the enzymes responsible for such posttranslational modifications is instrumental in defining their substrates and functions in vivo. Here we show that a Saccharomyces cerevisiae lysine deacetylase, Hos3, is asymmetrically targeted to the daughter side of the bud neck and to the daughter spindle pole body (SPB). The morphogenesis checkpoint member Hsl7 recruits Hos3 to the neck region. Cells with a defect in spindle orientation trigger Hos3 to load onto both SPBs. When associated symmetrically with both SPBs, Hos3 functions as a spindle position checkpoint (SPOC) component to inhibit mitotic exit. Neck localization of Hos3 is essential for its symmetric association with SPBs in cells with misaligned spindles. Our data suggest that Hos3 facilitates cross-talk between the morphogenesis checkpoint and the SPOC as a component of the intricate monitoring of spindle orientation after mitotic entry and before commitment to mitotic exit.

Effect of Ethanol on Cell Growth of Budding Yeast: Genes That Are Important for Cell Growth in the Presence of Ethanol

The budding yeast Saccharomyces cerevisiae has been used in the fermentation of various kinds of alcoholic beverages. But the effect of ethanol on the cell growth of this yeast is poorly understood. This study shows that the addition of ethanol causes a cell-cycle delay associated with a transient dispersion of F-actin cytoskeleton, resulting in an increase in cell size. We found that the tyrosine kinase Swe1, the negative regulator of Cdc28-Clb kinase, is related to the regulation of cell growth in the presence of ethanol. Indeed, the increase in cell size due to ethanol was partially abolished in the SWE1-deleted cells, and the amount of Swe1 protein increased transiently in the presence of ethanol. These results indicated that Swe1 is involved in cell size control in the presence of ethanol, and that a signal produced by ethanol causes a transient up-regulation of Swe1. Further we investigated comprehensively the ethanol-sensitive strains in the complete set of 4847 non-essential gene deletions and identified at least 256 genes that are important for cell growth in the presence of ethanol.

Tropomodulin3-null mice are embryonic lethal with anemia due to impaired erythroid terminal differentiation in the fetal liver

Tropomodulin (Tmod) is a protein that binds and caps the pointed ends of actin filaments in erythroid and nonerythoid cell types. Targeted deletion of mouse tropomodulin3 (Tmod3) leads to embryonic lethality at E14.5-E18.5, with anemia due to defects in definitive erythropoiesis in the fetal liver. Erythroid burst-forming unit and colony-forming unit numbers are greatly reduced, indicating defects in progenitor populations. Flow cytometry of fetal liver erythroblasts shows that late-stage populations are also decreased, including reduced percentages of enucleated cells. Annexin V staining indicates increased apoptosis of Tmod3(-/-) erythroblasts, and cell-cycle analysis reveals that there are more Ter119(hi) cells in S-phase in Tmod3(-/-) embryos. Notably, enucleating Tmod3(-/-) erythroblasts are still in the process of proliferation, suggesting impaired cell-cycle exit during terminal differentiation. Tmod3(-/-) late erythroblasts often exhibit multilobular nuclear morphologies and aberrant F-actin assembly during enucleation. Furthermore, native erythroblastic island formation was impaired in Tmod3(-/-) fetal livers, with Tmod3 required in both erythroblasts and macrophages. In conclusion, disruption of Tmod3 leads to impaired definitive erythropoiesis due to reduced progenitors, impaired erythroblastic island formation, and defective erythroblast cell-cycle progression and enucleation. Tmod3-mediated actin remodeling may be required for erythroblast-macrophage adhesion, coordination of cell cycle with differentiation, and F-actin assembly and remodeling during erythroblast enucleation.

From START to FINISH: the influence of osmotic stress on the cell cycle

The cell cycle is a sequence of biochemical events that are controlled by complex but robust molecular machinery. This enables cells to achieve accurate self-reproduction under a broad range of different conditions. Environmental changes are transmitted by molecular signalling networks, which coordinate their action with the cell cycle. The cell cycle process and its responses to environmental stresses arise from intertwined nonlinear interactions among large numbers of simpler components. Yet, understanding of how these pieces fit together into a coherent whole requires a systems biology approach. Here, we present a novel mathematical model that describes the influence of osmotic stress on the entire cell cycle of S. cerevisiae for the first time. Our model incorporates all recently known and several proposed interactions between the osmotic stress response pathway and the cell cycle. This model unveils the mechanisms that emerge as a consequence of the interaction between the cell cycle and stress response networks. Furthermore, it characterises the role of individual components. Moreover, it predicts different phenotypical responses for cells depending on the phase of cells at the onset of the stress. The key predictions of the model are: (i) exposure of cells to osmotic stress during the late S and the early G2/M phase can induce DNA re-replication before cell division occurs, (ii) cells stressed at the late G2/M phase display accelerated exit from mitosis and arrest in the next cell cycle, (iii) osmotic stress delays the G1-to-S and G2-to-M transitions in a dose dependent manner, whereas it accelerates the M-to-G1 transition independently of the stress dose and (iv) the Hog MAPK network compensates the role of the MEN network during cell division of MEN mutant cells. These model predictions are supported by independent experiments in S. cerevisiae and, moreover, have recently been observed in other eukaryotes.

Cell-cycle perturbations suppress the slow-growth defect of spt10Δ mutants in Saccharomyces cerevisiae

Spt10 is a putative acetyltransferase of Saccharomyces cerevisiae that directly activates the transcription of histone genes. Deletion of SPT10 causes a severe slow growth phenotype, showing that Spt10 is critical for normal cell division. To gain insight into the function of Spt10, we identified mutations that impair or improve the growth of spt10 null (spt10Δ) mutants. Mutations that cause lethality in combination with spt10Δ include particular components of the SAGA complex as well as asf1Δ and hir1Δ. Partial suppressors of the spt10Δ growth defect include mutations that perturb cell-cycle progression through the G1/S transition, S phase, and G2/M. Consistent with these results, slowing of cell-cycle progression by treatment with hydroxyurea or growth on medium containing glycerol as the carbon source also partially suppresses the spt10Δ slow-growth defect. In addition, mutations that impair the Lsm1-7-Pat1 complex, which regulates decapping of polyadenylated mRNAs, also partially suppress the spt10Δ growth defect. Interestingly, suppression of the spt10Δ growth defect is not accompanied by a restoration of normal histone mRNA levels. These findings suggest that Spt10 has multiple roles during cell division.

The fate of the messenger is pre-determined: a new model for regulation of gene expression

Recent years have seen a rise in publications demonstrating coupling between transcription and mRNA decay. This coupling most often accompanies cellular processes that involve transitions in gene expression patterns, for example during mitotic division and cellular differentiation and in response to cellular stress. Transcription can affect the mRNA fate by multiple mechanisms. The most novel finding is the process of co-transcriptional imprinting of mRNAs with proteins, which in turn regulate cytoplasmic mRNA stability. Transcription therefore is not only a catalyst of mRNA synthesis but also provides a platform that enables imprinting, which coordinates between transcription and mRNA decay. Here we present an overview of the literature, which provides the evidence of coupling between transcription and decay, review the mechanisms and regulators by which the two processes are coupled, discuss why such coupling is beneficial and present a new model for regulation of gene expression. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Impaired manganese metabolism causes mitotic misregulation

Manganese is an essential trace element, whose intracellular levels need to be carefully regulated. Mn(2+) acts as a cofactor for many enzymes and excess of Mn(2+) is toxic. Alterations in Mn(2+) homeostasis affect metabolic functions and mutations in the human Mn(2+)/Ca(2+) transporter ATP2C1 have been linked to Hailey-Hailey disease. By deletion of the yeast orthologue PMR1 we have studied the impact of Mn(2+) on cell cycle progression and show that an excess of cytosolic Mn(2+) alters S-phase transit, induces transcriptional up-regulation of cell cycle regulators, bypasses the need for S-phase cell cycle checkpoints and predisposes to genomic instability. On the other hand, we find that depletion of the Golgi Mn(2+) pool requires a functional morphology checkpoint to avoid the formation of polyploid cells.

Hydroquinone, a benzene metabolite, induces Hog1-dependent stress response signaling and causes aneuploidy in Saccharomyces cerevisiae

Previously, we have shown that phenyl hydroquinone, a hepatic metabolite of the Ames test-negative carcinogen o-phenylphenol, efficiently induced aneuploidy in Saccharomyces cerevisiae by arresting the cell cycle at the G2/M transition as a result of the activation of the Hog1 (p38 MAPK homolog)-Swe1 (Wee1 homolog) pathway. In this experiment, we examined the aneuploidy forming effects of hydroquinone, a benzene metabolite, since both phenyl hydroquinone and hydroquinone are Ames-test negative carcinogens and share similar molecular structures. As was seen in phenyl hydroquinone, hydroquinone induced aneuploidy in yeast by delaying the cell cycle at the G2/M transition. Deficiencies in SWE1 and HOG1 abolished the hydroquinone-induced delay at the G2/M transition and aneuploidy formation. Furthermore, Hog1 was phosphorylated by hydroquinone, which may stabilize Swe1. These data indicate that the hydroquinone-induced G2/M transition checkpoint, which is activated by the Hog1-Swe1 pathway, plays a role in the formation of aneuploidy.

SAD kinase keeps centrosomes lonely

Centrosome duplication is under strict control such that it occurs only once per cell cycle. New insights into the molecular mechanisms that control centrosome number come from the discovery of a role for SADB kinase in centrosome biogenesis.

Regulation of Mih1/Cdc25 by protein phosphatase 2A and casein kinase 1

The Cdc25 phosphatase promotes entry into mitosis by removing cyclin-dependent kinase 1 (Cdk1) inhibitory phosphorylation. Previous work suggested that Cdc25 is activated by Cdk1 in a positive feedback loop promoting entry into mitosis; however, it has remained unclear how the feedback loop is initiated. To learn more about the mechanisms that regulate entry into mitosis, we have characterized the function and regulation of Mih1, the budding yeast homologue of Cdc25. We found that Mih1 is hyperphosphorylated early in the cell cycle and is dephosphorylated as cells enter mitosis. Casein kinase 1 is responsible for most of the hyperphosphorylation of Mih1, whereas protein phosphatase 2A associated with Cdc55 dephosphorylates Mih1. Cdk1 appears to directly phosphorylate Mih1 and is required for initiation of Mih1 dephosphorylation as cells enter mitosis. Collectively, these observations suggest that Mih1 regulation is achieved by a balance of opposing kinase and phosphatase activities. Because casein kinase 1 is associated with sites of polar growth, it may regulate Mih1 as part of a signaling mechanism that links successful completion of growth-related events to cell cycle progression.

Increase in Fru-2,6-P(2) levels results in altered cell division in Schizosaccharomyces pombe

Mitogenic response to growth factors is concomitant with the modulation they exert on the levels of Fructose 2,6-bisphosphate (Fru-2,6-P2), an essential activator of the glycolytic flux. In mammalian cells, decreased Fru-2,6-P2 concentration causes cell cycle delay, whereas high levels of Fru-2,6-P2 sensitize cells to apoptosis. In order to analyze the cell cycle consequences due to changes in Fru-2,6-P2 levels, the bisphosphatase-dead mutant (H258A) of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase enzyme was over-expressed in Schizosaccharomyces pombe cells and the variation in cell phenotype was studied. The results obtained demonstrate that the increase in Fru-2,6-P2 levels results in a defective division of S. pombe, as revealed by an altered multisepted phenotype. The H258A-expressing cells showed impairment of cytokinesis, but normal nuclear division. In order to identify cellular mediators responsible for this effect, we transformed different S. pombe strains and observed that the cytokinetic defect was absent in cells defective for Wee1 kinase function. Therefore, in S. pombe, Wee1 integrates the metabolic signal emerging from changes in Fru-2,6-P2 content, thus coupling metabolism with cell proliferation. As the key regulators of the cell cycle checkpoints are conserved throughout evolution, these results may help to understand the experimental evidences obtained by manipulation of Fru-2,6-P2 levels in mammalian cells.

8-Chloro-adenosine inhibits growth at least partly by interfering with actin polymerization in cultured human lung cancer cells

A key feature of actin is its ability to bind and hydrolyze ATP. 8-Chloro-adenosine (8-Cl-Ado), which can be phosphorylated to the moiety of 8-Cl-ATP in living cells, inhibits tumor cell proliferation. Therefore we tested the hypothesis that 8-Cl-Ado can interfere with the dynamic state of actin polymerization. We found that 8-Cl-Ado inhibited the growth of human lung cancer cell line A549 and H1299 in culture, and arrested the target cells in G2/M phase evidenced by fluorescence-activated cell sorting (FACS). Immunocytochemistry showed that the normal organization of microfilaments was disrupted in 8-Cl-Ado-exposed cells, which is accompanied by the decrease of cell size and the alteration of cell shape, and by aberrant mitosis and apoptosis in targeted cells. Furthermore, in vitro light scattering assays revealed that 8-Cl-ATP could directly inhibit the transition of G-actin to F-actin. DNase I inhibition assays showed that the G/F-actin ratio, a surrogate marker of actin polymerization status in living cells, was significantly increased in 8-Cl-Ado-exposed A549 and H1299 cells, compared to the G/F-actin ratio in unexposed cells. Taken together, these results indicate that 8-Cl-Ado exposure can alter the dynamic properties of actin polymerization, disrupt the dynamic instability or the rearrangement ability of actin filaments. Therefore, our data suggest that 8-Cl-Ado may exert its cytotoxicity at least partly by interfering with the dynamic instability of microfilaments, which may correlate with its inhibitory effects on cell proliferation and cell death.

Pathocycles: Ustilago maydis as a model to study the relationships between cell cycle and virulence in pathogenic fungi

Activation of virulence in pathogenic fungi often involves differentiation processes that need the reset of the cell cycle and induction of a new morphogenetic program. Therefore, the fungal capability to modify its cell cycle constitutes an important determinant in carrying out a successful infection. The dimorphic fungus Ustilago maydis is the causative agent of corn smut disease and has lately become a highly attractive model in addressing fundamental questions about development in pathogenic fungi. The different morphological and genetic changes of U. maydis cells during the pathogenic process advocate an accurate control of the cell cycle in these transitions. This is why this model pathogen deserves attention as a powerful tool in analyzing the relationships between cell cycle, morphogenesis, and pathogenicity. The aim of this review is to summarize recent advances in the unveiling of cell cycle regulation in U. maydis. We also discuss the connection between cell cycle and virulence and how cell cycle control is an important downstream target in the fungus-plant interaction.

Regulation of the cell cycle by protein phosphatase 2A in Saccharomyces cerevisiae

Protein phosphatase 2A (PP2A) has long been implicated in cell cycle regulation in many different organisms. In the yeast Saccharomyces cerevisiae, PP2A controls cell cycle progression mainly through modulation of cyclin-dependent kinase (CDK) at the G(2)/M transition. However, CDK does not appear to be a direct target of PP2A. PP2A affects CDK activity through its roles in checkpoint controls. Inactivation of PP2A downregulates CDK by activating the morphogenesis checkpoint and, consequently, delays mitotic entry. Defects in PP2A also compromise the spindle checkpoint and predispose the cell to an error-prone mitotic exit. In addition, PP2A is involved in controlling the G(1)/S transition and cytokinesis. These findings suggest that PP2A functions in many stages of the cell cycle and its effect on cell cycle progression is pleiotropic.

GTP drives myosin light chain 1 interaction with the class V myosin Myo2 IQ motifs via a Sec2 RabGEF-mediated pathway

The yeast myosin light chain 1 (Mlc1p) belongs to a branch of the calmodulin superfamily and is essential for vesicle delivery at the mother-bud neck during cytokinesis due to is ability to bind to the IQ motifs of the class V myosin Myo2p. While calcium binding to calmodulin promotes binding/release from the MyoV IQ motifs, Mlc1p is unable to bind calcium and the mechanism of its interaction with target motifs has not been clarified. The presence of Mlc1p in a complex with the Rab/Ypt Sec4p and with Myo2p suggests a role for Mlc1p in regulating Myo2p cargo binding/release by responding to the activation of Rab/Ypt proteins. Here we show that GTP or GTPgammaS potently stimulate Mlc1p interaction with Myo2p IQ motifs. The C-terminus of the Rab/Ypt GEF Sec2p, but not Sec4p activation, is essential for this interaction. Interestingly, overexpression of constitutively activated Ypt32p, a Rab/Ypt protein that acts upstream of Sec4p, stimulates Mlc1p/Myo2p interaction similarly to GTP although a block of Ypt32 GTP binding does not completely abolish the GTP-mediated Mlc1p/Myo2p interaction. We propose that Mlc1p/Myo2p interaction is stimulated by a signal that requires Sec2p and activation of Ypt32p.

The Pyst2-L phosphatase is involved in cell-crowding

The dual-specificity phosphatase Pyst2-L was found to be over expressed in leukocytes derived from AML and ALL patients as well as in certain other solid tumors and lymphoblastoid cell lines. Pyst2-L, binds and dephosphorylates both pERKs and pJNKs proteins, and thus, plays a role in regulating the MAP kinase signaling pathway. In the present study, a comparative genomic application was used and sequence analysis of multi-organisms databases were searched in order to identify genes homologous to Pyst2-L. The Xenopus laevis MAP kinase phosphatase X17c gene and the Yeast nitrogen starvation-induced protein phosphatase Yvh1p gene were revealed to be highly homologous with Pyst2-L. Both X17c and Yvh1p genes play a role in cell cycle regulation. A down regulated expression of the Yvh1p gene occurred in Saccharomyces cerevisiae that were synchronized to the G2-phase of the cell cycle by alpha-factor. In conformity with this result, a reduction in Pyst2-L expression levels was observed in G2-phase-synchronized Human K562 cells. Finally, we were able to show that cells in highly crowded cultures express high levels of the Pyst2-L phosphatase. These observations may indicate that low levels of the Pyst2-L phosphatase are essential for the G2-phase of the cell cycle and that this phosphatase might play a role in signaling cascades induced by cellular crowding.

Inhibitory phosphorylation of a mitotic cyclin-dependent kinase regulates the morphogenesis, cell size and virulence of the smut fungusUstilago maydis

The regulation of cyclin-dependent kinase (CDK) activity through inhibitory phosphorylation seems to play an important role in the eukaryotic cell cycle. We have investigated the influence that inhibitory phosphorylation of the catalytic subunit of mitotic CDK has on cell growth and pathogenicity of the corn smut fungus Ustilago maydis. This model pathogen is worthy of attention since it is well suited to analyze the relationships between the cell cycle, morphogenesis and pathogenicity. We set out to study these relationships by producing a cdk1 mutant allele that was refractory to inhibitory phosphorylation. The expression of this mutant in U. maydis cells dramatically altered their morphology. Since this kind of mutation makes the CDK catalytic subunit resistant to regulation by Wee1-related kinases in other organisms, we characterized the orthologous Wee1 kinase from U. maydis. We found that Wee1 is essential in U. maydis. Overexpression of wee1 produces cell cycle arrest in G2, the target of Wee1 apparently being the Cdk1/Clb2 complex, which is required specifically for the onset of mitosis. Given the connection between the cell cycle control and pathogenesis in U. maydis, we also analyzed whether cells with impaired inhibitory phosphorylation of Cdk1 were able to infect plants. We found that inhibitory phosphorylation was required for mating, a prerequisite to initiate pathogenic development. By examining plant-specific expression of the constitutively unphosphorylated cdk1AF allele, we also found that appropriate levels of inhibitory phosphorylation were required at stages of infection subsequent to penetration by the fungus. These data reinforces the connections between cell cycle, morphogenesis and virulence in this smut fungus.

The genetics of Pak

p21-activated kinases (Paks) are a highly conserved family of enzymes that bind to and are activated by small GTPases of the Cdc42 and Rac families. With the notable exception of plants, nearly all eukaryotes encode one or more Pak genes, indicating an ancient origin and important function for this family of enzymes. Genetic approaches in many different experimental systems, ranging from yeast to mice, are beginning to decipher the different functions of Paks. Although some of these functions are unique to a given organism, certain common themes have emerged, such as the activation of mitogen-activated protein kinase (MAPK) cascades and the regulation of cytoskeletal structure through effects on the actin and tubulin cytoskeletons.

Dissection of septin actin interactions using actin overexpression in Saccharomyces cerevisiae

Although many proteins can be overexpressed several fold without much effect on cell viability and morphology, some become toxic upon a slight increase in their intracellular level. This is particularly true for cytoskeletal proteins and has proven useful in the past for studying the cytoskeleton. In yeast, actin and tubulin are examples of proteins that cannot be overexpressed without affecting cell viability. Here, we have analysed the effect of actin overexpression in Saccharomyces cerevisiae. We show that actin overexpression interferes differently with distinct aspects of actin function. For example, two- to fourfold overexpression of actin did not affect the establishment of actin polarity, whereas it abrogated its maintenance. Also, actin structures that are barely visible in wild-type cells could be observed upon actin overexpression. This allowed us to identify a new ring-like actin structure genetically distinguishable from the actomyosin contractile ring. Formation of this actin structure upon actin overexpression was dependent on the septin cytoskeleton, the poorly understood cytokinetic protein Hof1 and the Arp2/3 complex. In contrast to the actomyosin ring, the ring formed upon actin overexpression required neither Myo1 nor formins for assembly. Therefore, we propose that Hof1 acts as a linker between actin and septins. Furthermore, we found that, in the absence of actin overexpression, a novel, Hof1-dependent actin belt is formed at the bud neck of anaphase cells. The physiological role of this belt might be related to that of the similar structure observed in dividing fission yeast.

Characterization of a Saccharomyces cerevisiae thermosensitive lytic mutant leads to the identification of a new allele of the NUD1 gene

To improve our understanding of the factors involved in the osmotic stability of yeast cells, a search for novel conditional Saccharomyces cerevisiae cell lysis mutants was performed. Ten temperature-sensitive (ts) mutant strains of S. cerevisiae were isolated that lyse at the restrictive temperature on hypotonic, but not on osmotically supported medium. The ten mutants fell into four complementation groups: ts1 to ts4. To clone the wild-type gene corresponding to the ts4 mutation, a strategy aimed at complementing the thermosensitive phenotype-using low-copy and high-copy DNA libraries--was followed, but only two extragenic suppressors were identified. Another approach, in which classic genetic methods were combined with the use of yeast artificial chromosomes and traditional cloning procedures, allowed the identification of the NUD1 gene--which codes for a component of the spindle-pole body-as the wild-type gene corresponding to the ts4 mutation. Cloning and sequencing of the defective allele from the chromosome of the mutant cells resulted in the identification of a point mutation that produces a single amino acid change in the protein: a Gly-to-Glu change at position 585 (the nud1-G585E allele). Further analysis revealed that cells carrying this allele show a thermosensitive growth defect. At the restrictive temperature, the cells arrest with large buds, elongated spindles, and duplicated nuclei. In addition, with longer incubation times they are unable to maintain cellular integrity and lyse. Our results have allowed the identification of the first single amino acid mutation in NUD1, and suggest a link between cell cycle progression and cellular integrity.

Molecular characterization of a WEE1 gene homologue in tomato (Lycopersicon esculentum Mill.)

Early fruit development in tomato (Lycopersicon esculentum Mill.) proceeds in two distinct phases of growth that comprise cell division and cell expansion, respectively. In pericarp and the jelly like locular tissue of tomato fruit, the transition between cell division to cell expansion is characterized by the arrest of mitotic activity, numerous rounds of nuclear DNA endoreduplication and the inhibition of Cyclin-Dependent Kinase A (CDKA) activity. To investigate whether the WEE1 kinase may play a role during the endoreduplication process, we isolated and characterized the tomato homologue for WEE1. The LeWEE1 gene consisted of 10 exons with a predicted 510 amino acid-long protein. The accumulation of the corresponding transcripts was associated with mitotically active organs: developing fruits, seeds and roots. Interestingly, LeWEE1was expressed in the jelly like locular tissue concomitant with endoreduplication during fruit development. Using tobacco BY-2 synchronized cells, we showed that the WEE1 gene expression is cell-cycle regulated with a maximum transcript accumulation at S phase. Our data indicate the putative dual contribution of LeWEE1 in the classical cell cycle and the endocycle.

The Ras/protein kinase A pathway acts in parallel with the Mob2/Cbk1 pathway to effect cell cycle progression and proper bud site selection

In Saccharomyces cerevisiae, Ras proteins connect nutrient availability to cell growth through regulation of protein kinase A (PKA) activity. Ras proteins also have PKA-independent functions in mitosis and actin repolarization. We have found that mutations in MOB2 or CBK1 confer a slow-growth phenotype in a ras2Delta background. The slow-growth phenotype of mob2Delta ras2Delta cells results from a G1 delay that is accompanied by an increase in size, suggesting a G1/S role for Ras not previously described. In addition, mob2Delta strains have imprecise bud site selection, a defect exacerbated by deletion of RAS2. Mob2 and Cbk1 act to properly localize Ace2, a transcription factor that directs daughter cell-specific transcription of several genes. The growth and budding phenotypes of the double-deletion strains are Ace2 independent but are suppressed by overexpression of the PKA catalytic subunit, Tpk1. From these observations, we conclude that the PKA pathway and Mob2/Cbk1 act in parallel to determine bud site selection and promote cell cycle progression.

Hym1p affects cell cycle progression in Saccharomyces cerevisiae

The Saccharomyces cerevisiae HYM1 gene is conserved among eukaryotes. The mammalian orthologue (called MO25) mediates signaling through the AMP-activated protein kinase and other related kinases, implicated in cell proliferation. In yeast, Hym1p plays a role in cellular morphogenesis and also promotes the daughter cell-specific localization of the Ace2p transcription factor. Here, we report that increased dosage of HYM1 apparently shortens the G1 phase of the cell cycle. In the absence of HYM1 or ACE2, mother and daughter cells divide with the same generation times. Genetic analysis of HYM1, ACE2 and CLN3 mutants suggests that these genes together contribute to the establishment of asynchronous mother-daughter cell divisions, but probably not in a linear pathway. Our overall data suggest that Hym1p has a regulatory role in cell cycle progression.

"Sleeping beauty": quiescence in Saccharomyces cerevisiae

The cells of organisms as diverse as bacteria and humans can enter stable, nonproliferating quiescent states. Quiescent cells of eukaryotic and prokaryotic microorganisms can survive for long periods without nutrients. This alternative state of cells is still poorly understood, yet much benefit is to be gained by understanding it both scientifically and with reference to human health. Here, we review our knowledge of one "model" quiescent cell population, in cultures of yeast grown to stationary phase in rich media. We outline the importance of understanding quiescence, summarize the properties of quiescent yeast cells, and clarify some definitions of the state. We propose that the processes by which a cell enters into, maintains viability in, and exits from quiescence are best viewed as an environmentally triggered cycle: the cell quiescence cycle. We synthesize what is known about the mechanisms by which yeast cells enter into quiescence, including the possible roles of the protein kinase A, TOR, protein kinase C, and Snf1p pathways. We also discuss selected mechanisms by which quiescent cells maintain viability, including metabolism, protein modification, and redox homeostasis. Finally, we outline what is known about the process by which cells exit from quiescence when nutrients again become available.

Modeling regulatory networks at Virginia Tech

The life of a cell is governed by the physicochemical properties of a complex network of interacting macromolecules (primarily genes and proteins). Hence, a full scientific understanding of and rational engineering approach to cell physiology require accurate mathematical models of the spatial and temporal dynamics of these macromolecular assemblies, especially the networks involved in integrating signals and regulating cellular responses. The Virginia Tech Consortium is involved in three specific goals of DARPA's computational biology program (Bio-COMP): to create effective software tools for modeling gene-protein-metabolite networks, to employ these tools in creating a new generation of realistic models, and to test and refine these models by well-conceived experimental studies. The special emphasis of this group is to understand the mechanisms of cell cycle control in eukaryotes (yeast cells and frog eggs). The software tools developed at Virginia Tech are designed to meet general requirements of modeling regulatory networks and are collected in a problem-solving environment called JigCell.

Wee1-dependent mechanisms required for coordination of cell growth and cell division

Wee1-related kinases function in a highly conserved mechanism that controls the timing of entry into mitosis. Loss of Wee1 function causes fission yeast and budding yeast cells to enter mitosis before sufficient growth has occurred, leading to formation of daughter cells that are smaller than normal. Early work in fission yeast suggested that Wee1 is part of a cell-size checkpoint that prevents entry into mitosis before cells have reached a critical size. Recent experiments in fission yeast and budding yeast have provided new support for this idea. In addition, studies in budding yeast have revealed the existence of highly intricate signaling networks that are required for regulation of Swe1, the budding yeast homolog of Wee1. Further understanding of these signaling networks may provide important clues to how cell growth and cell division are coordinated.

The spindle assembly and spindle position checkpoints

The mitotic spindle segregates chromosomes to opposite ends of the cell in preparation for cell division. Chromosome attachment to the spindle is monitored by the spindle assembly checkpoint, and at least in yeast cells, penetration of one spindle pole into the bud is monitored by the spindle position checkpoint. We review the historical origins of these checkpoints and recent progress in understanding their surveillance pathways. We also highlight fascinating but as yet unresolved questions, and examine crosstalk between the checkpoints.

Mathematical model of the morphogenesis checkpoint in budding yeast

The morphogenesis checkpoint in budding yeast delays progression through the cell cycle in response to stimuli that prevent bud formation. Central to the checkpoint mechanism is Swe1 kinase: normally inactive, its activation halts cell cycle progression in G2. We propose a molecular network for Swe1 control, based on published observations of budding yeast and analogous control signals in fission yeast. The proposed Swe1 network is merged with a model of cyclin-dependent kinase regulation, converted into a set of differential equations and studied by numerical simulation. The simulations accurately reproduce the phenotypes of a dozen checkpoint mutants. Among other predictions, the model attributes a new role to Hsl1, a kinase known to play a role in Swe1 degradation: Hsl1 must also be indirectly responsible for potent inhibition of Swe1 activity. The model supports the idea that the morphogenesis checkpoint, like other checkpoints, raises the cell size threshold for progression from one phase of the cell cycle to the next.

Morphogenesis during Xenopus gastrulation requires Wee1-mediated inhibition of cell proliferation

Major developmental events in early Xenopus embryogenesis coincide with changes in the length and composition of the cell cycle. These changes are mediated in part through the regulation of CyclinB/Cdc2 and they occur at the first mitotic cell cycle, the mid-blastula transition (MBT) and at gastrulation. In this report, we investigate the contribution of maternal Wee1, a kinase inhibitor of CyclinB/Cdc2, to these crucial developmental transitions. By depleting Wee1 protein levels using antisense morpholino oligonucleotides, we show that Wee1 regulates M-phase entry and Cdc2 tyrosine phosphorylation in early gastrula embryos. Moreover, we find that Wee1 is required for key morphogenetic movements involved in gastrulation, but is not needed for the induction of zygotic transcription. In addition, Wee1 is positively regulated by tyrosine autophosphorylation in early gastrula embryos and this upregulation of Wee1 activity is required for normal gastrulation. We also show that overexpression of Cdc25C, a phosphatase that activates the CyclinB/Cdc2 complex, induces gastrulation defects that can be rescued by Wee1, providing additional evidence that cell cycle inhibition is crucial for the gastrulation process. Together, these findings further elucidate the developmental function of Wee1 and demonstrate the importance of cell cycle regulation in vertebrate morphogenesis.

Securin and B-cyclin/CDK are the only essential targets of the APC

The anaphase-promoting complex/cyclosome (APC) is a highly conserved ubiquitin ligase that controls passage through the cell cycle by targeting many proteins for proteolysis. The complex is composed of at least thirteen core subunits, eight of which are essential, and two activating subunits, Cdc20 (essential) and Cdh1/Hct1 (non-essential). Previously, it was not known which APC targets are sufficient to explain the essential nature of the complex. Here, we show that each of the eight normally essential APC subunits is rendered non-essential ('bypass-suppressed') by the simultaneous removal/inhibition of the APC substrates securin (Pds1) and B-type cyclin/CDK (Clb/CDK). In strains lacking the APC, levels of Clb2 and Clb3 remain constant, but Clb/CDK activity oscillates as cells cycle. This suggests that in the absence of B-type cyclin destruction, oscillation of the Clb/CDK-inhibitor Sic1 is sufficient to trigger the feedback loops necessary for the bi-stable nature of Clb/CDK activity. These results strongly suggest that securin and B-type cyclin/CDK activity are the only obligatory targets of the APC in Saccharomyces cerevisiae.

Genetic interactions amongZDS1,2,CDC37, and protein kinase CK2 inSaccharomyces cerevisiae

We report here the identification of the homologous gene pair ZDS1,2 as multicopy suppressors of a temperature-sensitive allele (cka2-13(ts)) of the CKA2 gene encoding the alpha' catalytic subunit of protein kinase CK2. Overexpression of ZDS1,2 suppressed the temperature sensitivity, geldanamycin (GA) sensitivity, slow growth, and flocculation of multiple cka2 alleles and enhanced CK2 activity in vivo toward a known physiological substrate, Fpr3. Consistent with the existence of a recently described positive feedback loop between CK2 and Cdc37, overexpression of ZDS1,2 also suppressed the temperature sensitivity, abnormal morphology, and GA sensitivity of a CK2 phosphorylation-deficient mutant of CDC37, cdc37-S14A, as well as the GA sensitivity of a cdc37-1 allele. A likely basis for all of these effects is our observation that ZDS1,2 overexpression enhances Cdc37 protein levels. Activation of the positive feedback loop between CK2 and Cdc37 likely contributes to the pleiotropic nature of ZDS1,2, as both CK2 and Cdc37 regulate diverse cellular functions.

A monitor for bud emergence in the yeast morphogenesis checkpoint

Cell cycle transitions are subject to regulation by both external signals and internal checkpoints that monitor satisfactory progression of key cell cycle events. In budding yeast, the morphogenesis checkpoint arrests the cell cycle in response to perturbations that affect the actin cytoskeleton and bud formation. Herein, we identify a step in this checkpoint pathway that seems to be directly responsive to bud emergence. Activation of the kinase Hsl1p is dependent upon its recruitment to a cortical domain organized by the septins, a family of conserved filament-forming proteins. Under conditions that delayed or blocked bud emergence, Hsl1p recruitment to the septin cortex still took place, but hyperphosphorylation of Hsl1p and recruitment of the Hsl1p-binding protein Hsl7p to the septin cortex only occurred after bud emergence. At this time, the septin cortex spread to form a collar between mother and bud, and Hsl1p and Hsl7p were restricted to the bud side of the septin collar. We discuss models for translating cellular geometry (in this case, the emergence of a bud) into biochemical signals regulating cell proliferation.

Control of cell polarity in fission yeast by association of Orb6p kinase with the highly conserved protein methyltransferase Skb1p

In the fission yeast Schizosaccharomyces pombe, proper establishment and maintenance of cell polarity require Orb6p, a highly conserved serine/threonine kinase involved in regulating both cell morphogenesis and cell cycle control. Orb6p localizes to the cell tips during interphase and to the cell septum during mitosis. To investigate the mechanisms involved in Orb6p function, we conducted a two-hybrid screen to identify proteins that interact with Orb6p. Using this approach, we identified Skb1p, a highly conserved protein methyltransferase that has been implicated previously in cell cycle control, in the coordination of cell cycle progression with morphological changes, and in hyperosmotic stress response. We found that Skb1p associates with Orb6p in S. pombe cells and that the two proteins interact directly in vitro. Loss of Skb1p exacerbates the phenotype of orb6 mutants, suggesting that Skb1p and Orb6p functionally interact in S. pombe cells. Our results suggest that Skb1p affects the intracellular localization of Orb6p and that loss of Skb1p leads to a redistribution of the Orb6p kinase away from the cell tips. Furthermore, we found that Orb6p kinase activity is strongly increased following exposure to salt shock, suggesting that Orb6p has a role in cell response to hyperosmotic stress. Previous studies have shown that Skb1p interacts with the fission yeast p21-activated kinase homologue Pak1p/Shk1p to regulate cell polarity and cell cycle progression. Our findings identify Orb6p as an additional target for Skb1p and suggest a novel function for Skb1p in the control of cell polarity by regulating the subcellular localization of Orb6p.

The CCT chaperonin promotes activation of the anaphase-promoting complex through the generation of functional Cdc20

The WD repeat protein Cdc20 is essential for progression through mitosis because it is required to activate ubiquitin ligation by the anaphase-promoting complex (APC/C). Here we show in yeast that Cdc20 binds to the CCT chaperonin, which is known as a folding machine for actin and tubulin. The CCT is required for Cdc20's ability to bind and activate the APC/C. In vivo, CCT is essential for Cdc20-dependent cell cycle events such as sister chromatid separation and exit from mitosis. The chaperonin is also required for the function of the Cdc20-related protein Cdh1, which activates the APC/C during G1. We propose that folding of the Cdc20 family of APC/C activators is an essential and evolutionary conserved function of the CCT chaperonin.

Candida albicans septin mutants are defective for invasive growth and virulence

Hyphal growth of Candida albicans is implicated as an important virulence factor for this opportunistic human pathogen. Septin proteins, a family of cytoskeletal elements that regulate membrane events and are important for proper morphogenesis of C. albicans, were examined for their role in tissue invasion and virulence in the mouse model of systemic infection. In vitro, septin mutants are only mildly defective for hyphal growth in liquid culture but display pronounced defects for invasive growth into agar. In vivo, the septin mutants were found to exhibit attenuated virulence. However, mice infected with the mutants displayed high fungal burdens in their kidneys without obvious symptoms of disease. Histological examination of infected kidneys revealed defects in organ invasion for the cdc10 Delta and cdc11 Delta deletion mutants, which displayed both reduced tissue penetration and noninvasive fungal masses. Thus, the septin proteins are necessary for invasive growth, which appears to be more important to the successful pathogenesis of C. albicans than hyphal growth alone.

Asynchronous cell cycle and asymmetric vacuolar inheritance in true hyphae of Candida albicans

Candida albicans forms unconstricted hyphae in serum-containing medium that are divided into discrete compartments. Time-lapse photomicroscopy, flow cytometry, and a novel three-dimensional imaging system were used to demonstrate that the kinetics and cell cycle events accompanying hyphal development were correlated with dynamic changes in vacuole morphology and the pattern of vacuole inheritance. Apical cells of hyphae underwent continuous extension before and after the first cytokinesis event. However, the resulting mother cell and sub-apical compartments did not immediately reenter the cell cycle and instead underwent cell cycle arrest before reentering the cycle. Vacuole was inherited asymmetrically at cytokinesis so that the distal, arrested compartments inherited most vacuole and the growing apical cell inherited most cytoplasm. Hydroxyurea release experiments demonstrated that the arrested, vacuolated hyphal compartments were in the G(1) phase of the cycle. The period of cell cycle arrest was decreased by the provision of assimilatable forms of nitrogen, suggesting that the hyphal cell cycle is regulated by nitrogen limitation that results in sup-apical cell cycle arrest. This pattern of growth is distinct from that of the synchronous, symmetrical development of pseudohyphae of C. albicans and other yeast species. These observations suggest that the cellular vacuole space correlates with alterations in the cell cycles of different cell types and that the total organelle space may influence size-regulated functions and hence the timing of the eukaryotic cell cycle.

The Tap42-protein phosphatase type 2A catalytic subunit complex is required for cell cycle-dependent distribution of actin in yeast

In Saccharomyces cerevisiae, the Tor proteins mediate a wide spectrum of growth-related cellular processes in response to nutrients. The pleiotropic role of the Tor proteins is mediated, at least in part, by type 2A protein phosphatases (PP2A) and 2A-like protein phosphatases. Tor-mediated signaling activity promotes the interaction of phosphatase-interacting protein Tap42 with PP2A and 2A-like protein phosphatases. The distinct complexes formed between Tap42 and different phosphatases mediate various cellular events and modulate phosphorylation levels of many downstream factors in the Tor pathway in a Tor-dependent and rapamycin-sensitive manner. In this study, we demonstrate that the interaction between Tap42 and the catalytic subunits of PP2A (PP2Ac) is required for cell cycle-dependent distribution of actin. We show that mutations in PP2Ac and Tap42 that perturb the interaction cause random distribution of actin during the cell cycle and that overexpression of the Rho2 GTPase suppresses the actin defects associated with the mutants. Our findings suggest that the Tap42-PP2Ac complex regulates the actin cytoskeleton via a Rho GTPase-dependent mechanism. In addition, we provide evidence that PP2A activity plays a negative role in controlling the actin cytoskeleton and, possibly, in regulation of the G(2)/M transition of the cell cycle.