A novel checkpoint mechanism regulating the G1/S transition

ecl family genes: Factors linking starvation and lifespan extension in Schizosaccharomyces pombe

In the fission yeast Schizosaccharomyces pombe, the duration of survival in the stationary phase, termed the chronological lifespan (CLS), is affected by various environmental factors and the corresponding gene activities. The ecl family genes were identified in the genomic region encoding non-coding RNA as positive regulators of CLS in S. pombe, and subsequently shown to encode relatively short proteins. Several studies revealed that ecl family genes respond to various nutritional starvation conditions via different mechanisms, and they are additionally involved in stress resistance, autophagy, sexual differentiation, and cell cycle control. Recent studies reported that Ecl family proteins strongly suppress target of rapamycin complex 1, which is a conserved eukaryotic nutrient-sensing kinase complex that also regulates longevity in a variety of organisms. In this review, we introduce the regulatory mechanisms of Ecl family proteins and discuss their emerging findings.

Nanotoxicity of multifunctional stoichiometric cobalt oxide nanoparticles (SCoONPs) with repercussions toward apoptosis, necrosis, and cancer necrosis factor (TNF-α) at nano-biointerfaces

Introduction

Apoptosis, necrosis, and cancer necrosis factor (TNF-a) are all impacted by the nanotoxicity of multifunctional stoichiometric cobalt oxide nanoparticles (SCoONPs) at nano-biointerfaces. The creation of multi-functional nanoparticles has had a considerable impact on the transport of drugs and genes, nanotheranostics (in-vivo imaging, concurrent diagnostics), interventions for external healing, the creation of nano-bio interfaces, and the instigation of desired changes in nanotherapeutics.

Objectives

The quantitative structure-activity relationships, chemical transformations, biological interactions as well as toxicological analyses are considered as main objectives. Discrete dimensions of SCoNPs-cell interaction interfaces, their characteristic physical features (size, shape, shell structure, and surface chemistry), impact on cell proliferation and differentiation are the key factors responsible for nanotoxicity.

Methods

The development of multi-functional nanoparticles has been significant in drug/gene delivery, nanotheranostics (in-vivo imaging, coinciding diagnostics), and external healing interventions, designing a nano-bio interface, as well as inciting desired alterations in nanotherapeutics. Every so often, the cellular uptake of multi-functional cobalt [Co, CoO, Co2(CO)8 and Co3O4] nanoparticles (SCoONPs) influences cellular mechanics and initiates numerous repercussions (oxidative stress, DNA damage, cytogenotoxicity, and chromosomal damage) in pathways, including the generation of dysregulating factors involved in biochemical transformations.

Results

The concerns and influences of multifunctional SCoNPs on different cell mechanisms (mitochondria impermeability, hydrolysis of ATP, the concentration of Ca2+, impaired calcium clearance, defective autophagy, apoptosis, and necrosis), and interlinked properties (adhesion, motility, and internalization dynamics, role in toxicity, surface hydrophilic and hydrophobicity, biokinetics and biomimetic behaviors of biochemical reactions) have also been summarized. SCoONPs have received a lot of interest among the nanocarriers family because of its advantageous qualities such as biodegradability, biocompatibility, nontoxicity, and nonimmunogenicity.

Conclusion

Various applications, such as bio-imaging, cell labeling, gene delivery, enhanced chemical stability, and increased biocompatibility, concerning apoptosis, necrosis, and nano-bio interfaces, along with suitable examples. In this analysis, the multi-functional cobalt [Co, CoO, Co2(CO)8 and Co3O4] nanoparticles (SCoNPs) intricacies (cytogenotoxicity, clastogenicity, and immunomodulatory), nanotoxicity, and associated repercussions have been highlighted and explained.

Cross Talk between eIF2α and eEF2 Phosphorylation Pathways Optimizes Translational Arrest in Response to Oxidative Stress

The cellular stress response triggers a cascade of events leading to transcriptional reprogramming and a transient inhibition of global protein synthesis, which is thought to be mediated by phosphorylation of eukaryotic initiation factor-2α (eIF2α). Using mouse embryonic fibroblasts (MEFs) and the fission yeast S. pombe, we report that rapid translational arrest and cell survival in response to hydrogen peroxide-induced oxidative stress do not rely on eIF2α kinases and eIF2α phosphorylation. Rather, H₂O₂ induces a block in elongation through phosphorylation of eukaryotic elongation factor 2 (eEF2). Kinetic and dose-response analyses uncovered cross talk between the eIF2α and eEF2 phosphorylation pathways, indicating that, in MEFs, eEF2 phosphorylation initiates the acute shutdown in translation, which is maintained by eIF2α phosphorylation. Our results challenge the common conception that eIF2α phosphorylation is the primary trigger of translational arrest in response to oxidative stress and point to integrated control that may facilitate the survival of cancer cells.

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Oxidative stress-induced translation arrest is independent of eIF2α phosphorylation

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Oxidative stress blocks translation elongation

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Oxidative stress triggers eEF2 kinase activation

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eEF2K KO cells are hypersensitive to oxidative stress

eIF2α phosphorylation and the regulation of translation

We discuss novel insight into the role and consequences of the phosphorylation of the translation initiation factor eIF2α in the context of stress responses and cell-cycle regulation. eIF2α is centrally located to regulate translation and its phosphorylation in response to different environmental challenges is one of the best characterized stress-response pathways. In addition to its role in stress management, eIF2α phosphorylation is also linked to cell-cycle progression and memory consolidation in the nervous system. The best known consequences of eIF2α phosphorylation are downregulation of global translation and stimulation of translation of some mRNAs. However, recent evidence shows that (i) eIF2α phosphorylation is not always required for the downregulation of global translation after exposure to stress and (ii) eIF2α phosphorylation does not necessarily lead to the downregulation of global translation. These results suggest that the textbook view of eIF2α phosphorylation needs to be revised and that there must be additional regulatory mechanisms at play.

β-N-methylamino-L-alanine (BMAA) suppresses cell cycle progression of non-neuronal cells

β-N-methylamino-L-alanine (BMAA), a natural non-proteinaceous amino acid, is a neurotoxin produced by a wide range of cyanobacteria living in various environments. BMAA is a candidate environmental risk factor for neurodegenerative diseases such as amyotrophic lateral sclerosis and Parkinson-dementia complex. Although BMAA is known to exhibit weak neuronal excitotoxicity via glutamate receptors, the underlying mechanism of toxicity has yet to be fully elucidated. To examine the glutamate receptor-independent toxicity of BMAA, we investigated the effects of BMAA in non-neuronal cell lines. BMAA potently suppressed the cell cycle progression of NIH3T3 cells at the G1/S checkpoint without inducing plasma membrane damage, apoptosis, or overproduction of reactive oxygen species, which were previously reported for neurons and neuroblastoma cells treated with BMAA. We found no evidence that activation of glutamate receptors was involved in the suppression of the G1/S transition by BMAA. Our results indicate that BMAA affects cellular functions, such as the division of non-neuronal cells, through glutamate receptor-independent mechanisms.

The Yeast DNA Damage Checkpoint Kinase Rad53 Targets the Exoribonuclease, Xrn1

The highly conserved DNA damage response (DDR) pathway monitors the genomic integrity of the cell and protects against genotoxic stresses. The apical kinases, Mec1 and Tel1 (ATR and ATM in human, respectively), initiate the DNA damage signaling cascade through the effector kinases, Rad53 and Chk1, to regulate a variety of cellular processes including cell cycle progression, DNA damage repair, chromatin remodeling, and transcription. The DDR also regulates other cellular pathways, but direct substrates and mechanisms are still lacking. Using a mass spectrometry-based phosphoproteomic screen in Saccharomyces cerevisiae, we identified novel targets of Rad53, many of which are proteins that are involved in RNA metabolism. Of the 33 novel substrates identified, we verified that 12 are directly phosphorylated by Rad53 in vitro: Xrn1, Gcd11, Rps7b, Ded1, Cho2, Pus1, Hst1, Srv2, Set3, Snu23, Alb1, and Scp160. We further characterized Xrn1, a highly conserved 5' exoribonuclease that functions in RNA degradation and the most enriched in our phosphoproteomics screen. Phosphorylation of Xrn1 by Rad53 does not appear to affect Xrn1's intrinsic nuclease activity in vitro, but may affect its activity or specificity in vivo.

A checkpoint-independent mechanism delays entry into mitosis after UV irradiation

When cells are exposed to stress they delay entry into mitosis. The most extensively studied mechanism behind this delay is the DNA-damage-induced G2/M checkpoint. Here, we show the existence of an additional stress-response pathway in Schizosaccharomyces pombe that is independent of the classic ATR/Rad3-dependent checkpoint. This novel mechanism delays entry mitosis independently of the spindle assembly checkpoint and the mitotic kinases Fin1, Ark1 and Plo1. The pathway delays activation of the mitotic cyclin-dependent kinase (CDK) Cdc2 after UV irradiation. Furthermore, we demonstrate that translation of the mitotic cyclin Cdc13 is selectively downregulated after UV irradiation, and we propose that this downregulation of Cdc13 contributes to the delayed activation of Cdc2 and the delayed mitosis.

Activation of Gcn2 in response to different stresses

All organisms have evolved pathways to respond to different forms of cellular stress. The Gcn2 kinase is best known as a regulator of translation initiation in response to starvation for amino acids. Work in budding yeast has showed that the molecular mechanism of GCN2 activation involves the binding of uncharged tRNAs, which results in a conformational change and GCN2 activation. This pathway requires GCN1, which ensures delivery of the uncharged tRNA onto GCN2. However, Gcn2 is activated by a number of other stresses which do not obviously involve accumulation of uncharged tRNAs, raising the question how Gcn2 is activated under these conditions. Here we investigate the requirement for ongoing translation and tRNA binding for Gcn2 activation after different stresses in fission yeast. We find that mutating the tRNA-binding site on Gcn2 or deleting Gcn1 abolishes Gcn2 activation under all the investigated conditions. These results suggest that tRNA binding to Gcn2 is required for Gcn2 activation not only in response to starvation but also after UV irradiation and oxidative stress.

Eukaryotic translation initiation factors and cancer

Recent technological advancements have shown tremendous mechanistic accomplishments in our understanding of the mechanism of messenger RNA translation in eukaryotic cells. Eukaryotic messenger RNA translation is very complex process that includes four phases (initiation, elongation, termination, and ribosome recycling) and diverse mechanisms involving protein and non-protein molecules. Translation regulation is principally achieved during initiation step of translation, which is organized by multiple eukaryotic translation initiation factors. Eukaryotic translation initiation factor proteins help in stabilizing the formation of the functional ribosome around the start codon and provide regulatory mechanisms in translation initiation. Dysregulated messenger RNA translation is a common feature of tumorigenesis. Various oncogenic and tumor suppressive genes affect/are affected by the translation machinery, making the components of the translation apparatus promising therapeutic targets for the novel anticancer drug. This review provides details on the role of eukaryotic translation initiation factors in messenger RNA translation initiation, their contribution to onset and progression of tumor, and how dysregulated eukaryotic translation initiation factors can be used as a target to treat carcinogenesis.

Chatty Mitochondria: Keeping Balance in Cellular Protein Homeostasis

Mitochondria are multifunctional cellular organelles that host many biochemical pathways including oxidative phosphorylation (OXPHOS). Defective mitochondria pose a threat to cellular homeostasis and compensatory responses exist to curtail the source of stress and/or its consequences. The mitochondrial proteome comprises proteins encoded by the nuclear and mitochondrial genomes. Disturbances in protein homeostasis may originate from mistargeting of nuclear encoded mitochondrial proteins. Defective protein import and accumulation of mistargeted proteins leads to stress that triggers translation alterations and proteasomal activation. These cytosolic pathways are complementary to the mitochondrial unfolded protein response (UPRmt) that aims to increase the capacity of protein quality control mechanisms inside mitochondria. They constitute putative targets for interventions aimed at increasing the fitness, stress resistance, and longevity of cells and organisms.

Stress-induced inhibition of translation independently of eIF2α phosphorylation

Exposure of fission yeast cells to ultraviolet (UV) light leads to inhibition of translation and phosphorylation of the eukaryotic initiation factor-2α (eIF2α). This phosphorylation is a common response to stress in all eukaryotes. It leads to inhibition of translation at the initiation stage and is thought to be the main reason why stressed cells dramatically reduce protein synthesis. Phosphorylation of eIF2α has been taken as a readout for downregulation of translation, but the role of eIF2α phosphorylation in the downregulation of general translation has not been much investigated. We show here that UV-induced global inhibition of translation in fission yeast cells is independent of eIF2α phosphorylation and the eIF2α kinase general control nonderepressible-2 protein (Gcn2). Also, in budding yeast and mammalian cells, the UV-induced translational depression is largely independent of GCN2 and eIF2α phosphorylation. Furthermore, exposure of fission yeast cells to oxidative stress generated by hydrogen peroxide induced an inhibition of translation that is also independent of Gcn2 and of eIF2α phosphorylation. Our findings show that stress-induced translational inhibition occurs through an unknown mechanism that is likely to be conserved through evolution.

Summary: In contrast to textbook knowledge, the phosphorylation of translation initiation factor eIF2α is not required for UV-induced inhibition of protein synthesis, which we show in three different cell types.

The Gcn2 Regulator Yih1 Interacts with the Cyclin Dependent Kinase Cdc28 and Promotes Cell Cycle Progression through G2/M in Budding Yeast

The Saccharomyces cerevisiae protein Yih1, when overexpressed, inhibits the eIF2 alpha kinase Gcn2 by competing for Gcn1 binding. However, deletion of YIH1 has no detectable effect on Gcn2 activity, suggesting that Yih1 is not a general inhibitor of Gcn2, and has no phenotypic defect identified so far. Thus, its physiological role is largely unknown. Here, we show that Yih1 is involved in the cell cycle. Yeast lacking Yih1 displays morphological patterns and DNA content indicative of a delay in the G2/M phases of the cell cycle, and this phenotype is independent of Gcn1 and Gcn2. Accordingly, the levels of phosphorylated eIF2α, which show a cell cycle-dependent fluctuation, are not altered in cells devoid of Yih1. We present several lines of evidence indicating that Yih1 is in a complex with Cdc28. Yih1 pulls down endogenous Cdc28 in vivo and this interaction is enhanced when Cdc28 is active, suggesting that Yih1 modulates the function of Cdc28 in specific stages of the cell cycle. We also demonstrate, by Bimolecular Fluorescence Complementation, that endogenous Yih1 and Cdc28 interact with each other, confirming Yih1 as a bona fide Cdc28 binding partner. Amino acid substitutions within helix H2 of the RWD domain of Yih1 enhance Yih1-Cdc28 association. Overexpression of this mutant, but not of wild type Yih1, leads to a phenotype similar to that of YIH1 deletion, supporting the view that Yih1 is involved through Cdc28 in the regulation of the cell cycle. We further show that IMPACT, the mammalian homologue of Yih1, interacts with CDK1, the mammalian counterpart of Cdc28, indicating that the involvement with the cell cycle is conserved. Together, these data provide insights into the cellular function of Yih1/IMPACT, and provide the basis for future studies on the role of this protein in the cell cycle.

Translational Upregulation of an Individual p21Cip1 Transcript Variant by GCN2 Regulates Cell Proliferation and Survival under Nutrient Stress

Multiple transcripts encode for the cell cycle inhibitor p21^Cip1. These transcripts produce identical proteins but differ in their 5’ untranslated regions (UTRs). Although several stresses that induce p21 have been characterized, the mechanisms regulating the individual transcript variants and their functional significance are unknown. Here we demonstrate through ³⁵S labeling, luciferase reporter assays, and polysome transcript profiling that activation of the Integrated Stress Response (ISR) kinase GCN2 selectively upregulates the translation of a p21 transcript variant containing 5’ upstream open reading frames (uORFs) through phosphorylation of the eukaryotic translation initiation factor eIF2α. Mutational analysis reveals that the uORFs suppress translation under basal conditions, but promote translation under stress. Functionally, ablation of p21 ameliorates G₁/S arrest and reduces cell survival in response to GCN2 activation. These findings uncover a novel mechanism of p21 post-transcriptional regulation, offer functional significance for the existence of multiple p21 transcripts, and support a key role for GCN2 in regulating the cell cycle under stress.

Cells sense nutrient levels in their environment in order to determine if conditions are favorable to divide. GCN2 is a protein that senses amino acids and responds to amino acid deficiency by suppressing protein synthesis and increasing the expression of genes involved in recovery from nutrient stress. Although GCN2’s role in amino acid sensing is well-characterized, it is not known how it links nutrient availability with the cell cycle. Here, we show that GCN2 induces the cell cycle inhibitor p21^Cip1 at the level of protein translation. The induction of p21 is limited to a specific messenger RNA variant that contains upstream open reading frames, and these upstream open reading frames are required for its enhanced translation under stress. Previously, the functional significance of these different p21 variants was unknown. Upregulation of p21 allows cells to halt division and survive under conditions of nutrient stress. Collectively, this work demonstrates a new mechanism of p21 regulation and the connection between GCN2 and the cell cycle.

Sirtuin 7 promotes cellular survival following genomic stress by attenuation of DNA damage, SAPK activation and p53 response

Maintaining the genomic integrity is a constant challenge in proliferating cells. Amongst various proteins involved in this process, Sirtuins play a key role in DNA damage repair mechanisms in yeast as well as mammals. In the present work we report the role of one of the least explored Sirtuin viz., SIRT7, under conditions of genomic stress when treated with doxorubicin. Knockdown of SIRT7 sensitized osteosarcoma (U2OS) cells to DNA damage induced cell death by doxorubicin. SIRT7 overexpression in NIH3T3 delayed cell cycle progression by causing delay in G1 to S transition. SIRT7 overexpressing cells when treated with low dose of doxorubicin (0.25 µM) showed delayed onset of senescence, lesser accumulation of DNA damage marker γH2AX and lowered levels of growth arrest markers viz., p53 and p21 when compared to doxorubicin treated control GFP expressing cells. Resistance to DNA damage following SIRT7 overexpression was also evident by EdU incorporation studies where cellular growth arrest was significantly delayed. When treated with higher dose of doxorubicin (>1 µM), SIRT7 conferred resistance to apoptosis by attenuating stress activated kinases (SAPK viz., p38 and JNK) and p53 response thereby shifting the cellular fate towards senescence. Interestingly, relocalization of SIRT7 from nucleolus to nucleoplasm together with its co-localization with SAPK was an important feature associated with DNA damage. SIRT7 mediated resistance to doxorubicin induced apoptosis and senescence was lost when p53 level was restored by nutlin treatment. Overall, we propose SIRT7 attenuates DNA damage, SAPK activation and p53 response thereby promoting cellular survival under conditions of genomic stress.

GCN2, an old dog with new tricks

Gcn2 was first described in budding yeast as a serine/threonine protein kinase involved in the response to amino acid starvation and this is its best characterized role to date. Recent work has revealed new and exciting roles for Gcn2, which affect many aspects of cellular physiology in response to a number of stresses in addition to starvation. Furthermore, the Gcn2 pathway has been implicated in diseases such as cancer and Alzheimer's disease, and therefore elucidating the new roles of Gcn2 seems ever more important.

Proline responding1 Plays a Critical Role in Regulating General Protein Synthesis and the Cell Cycle in Maize

Proline, an important amino acid, accumulates in many plant species. Besides its role in plant cell responses to environmental stresses, the potential biological functions of proline in growth and development are unclear. Here, we report cloning and functional characterization of the maize (Zea mays) classic mutant proline responding1 (pro1) gene. This gene encodes a Δ1-pyrroline-5- carboxylate synthetase that catalyzes the biosynthesis of proline from glutamic acid. Loss of function of Pro1 significantly inhibits proline biosynthesis and decreases its accumulation in the pro1 mutant. Proline deficiency results in an increased level of uncharged tRNApro AGG accumulation and triggers the phosphorylation of eukaryotic initiation factor 2α (eIF2α) in the pro1 mutant, leading to a general reduction in protein synthesis in this mutant. Proline deficiency also downregulates major cyclin genes at the transcriptional level, causing cell cycle arrest and suppression of cell proliferation. These processes are reversible when external proline is supplied to the mutant, suggesting that proline plays a regulatory role in the cell cycle transition. Together, the results demonstrate that proline plays an important role in the regulation of general protein synthesis and the cell cycle transition in plants.

Keeping the eIF2 alpha kinase Gcn2 in check

The protein kinase Gcn2 is present in virtually all eukaryotes and is of increasing interest due to its involvement in a large array of crucial biological processes. Some of these are universally conserved from yeast to humans, such as coping with nutrient starvation and oxidative stress. In mammals, Gcn2 is important for e.g. long-term memory formation, feeding behaviour and immune system regulation. Gcn2 has been also implicated in diseases such as cancer and Alzheimer's disease. Studies on Gcn2 have been conducted most extensively in Saccharomyces cerevisiae, where the mechanism of its activation by amino acid starvation has been revealed in most detail. Uncharged tRNAs stimulate Gcn2 which subsequently phosphorylates its substrate, eIF2α, leading to reduced global protein synthesis and simultaneously to increased translation of specific mRNAs, e.g. those coding for Gcn4 in yeast and ATF4 in mammals. Both proteins are transcription factors that regulate the expression of a myriad of genes, thereby enabling the cell to initiate a survival response to the initial activating cue. Given that Gcn2 participates in many diverse processes, Gcn2 itself must be tightly controlled. Indeed, Gcn2 is regulated by a vast network of proteins and RNAs, the list of which is still growing. Deciphering molecular mechanisms underlying Gcn2 regulation by effectors and inhibitors is fundamental for understanding how the cell keeps Gcn2 in check ensuring normal organismal function, and how Gcn2-associated diseases may develop or may be treated. This review provides a critical evaluation of the current knowledge on mechanisms controlling Gcn2 activation or activity.

Cell-cycle analyses using thymidine analogues in fission yeast

Thymidine analogues are powerful tools when studying DNA synthesis including DNA replication, repair and recombination. However, these analogues have been reported to have severe effects on cell-cycle progression and growth, the very processes being investigated in most of these studies. Here, we have analyzed the effects of 5-ethynyl-2′-deoxyuridine (EdU) and 5-Chloro-2′-deoxyuridine (CldU) using fission yeast cells and optimized the labelling procedure. We find that both analogues affect the cell cycle, but that the effects can be mitigated by using the appropriate analogue, short pulses of labelling and low concentrations. In addition, we report sequential labelling of two consecutive S phases using EdU and 5-bromo-2′-deoxyuridine (BrdU). Furthermore, we show that detection of replicative DNA synthesis is much more sensitive than DNA-measurements by flow cytometry.

Crosstalk between the Tor and Gcn2 pathways in response to different stresses

Regulating growth and the cell cycle in response to environmental fluctuations is important for all organisms in order to maintain viability. Two major pathways for translational regulation are found in higher eukaryotes: the Tor signaling pathway and those operating through the eIF2α kinases. Studies from several organisms indicate that the two pathways are interlinked, in that Tor complex 1 (TORC1) negatively regulates the Gcn2 kinase. Furthermore, inactivation of TORC1 may be required for activation of Gcn2 in response to stress. Here, we use the model organism Schizosaccharomyces pombe to investigate this crosstalk further. We find that the relationship is more complex than previously thought. First, in response to UV irradiation and oxidative stress, Gcn2 is fully activated in the presence of TORC1 signaling. Second, during amino-acid starvation, activation of Gcn2 is dependent on Tor2 activity, and Gcn2 is required for timely inactivation of the Tor pathway. Our data show that the crosstalk between the two pathways varies with the actual stress applied.

New roles of the fission yeast eIF2α kinases Hri1 and Gcn2 in response to nutritional stress

In fission yeast, three distinct eukaryotic initiation factor 2α (eIF2α) kinases (Hri1, Hri2 and Gcn2), regulate protein synthesis in response to various environmental stresses. Thus, Gcn2 is activated early after exposure to hydrogen peroxide (H2O2) and methyl methanesulfonate (MMS), whereas Hri2 is the primary activated eIF2α kinase in response to heat shock. The function of Hri1 is still not completely understood. It is also known that the mitogen-activated protein kinase Sty1 negatively regulates Gcn2 and Hri2 activities under oxidative stress. In this study, we demonstrate that Hri1 is mainly activated, and its expression upregulated, during transition from exponential growth to the stationary phase in response to nutritional limitation. Accordingly, both Hri1 and Gcn2, but not Hri2, are activated upon nitrogen source deprivation. In contrast, Hri2 is stimulated early during glucose starvation. We also found that Gcn2 is implicated in nitrogen starvation-induced growth arrest in the cell cycle G1 phase as well as in the non-selective protein degradation process caused upon this particular cellular stress. Moreover, Gcn2, but not Hri1 or Hri2, is essential for survival of cells growing in minimal medium, upon oxidative stress or glucose limitation. We further show that eIF2α phosphorylation at serine 52 by the eIF2α kinases is necessary for efficient cell cycle arrest in the G1 phase, for the consequent protein degradation and for sexual differentiation, under nitrogen starvation. Therefore, the eIF2α kinase signalling pathway modulates G1 phase cell cycle arrest, cell survival and mating under nutritional stress in the fission yeast Schizosaccharomyces pombe.

Downregulation of Cdc6 and pre-replication complexes in response to methionine stress in breast cancer cells

Methionine and homocysteine are metabolites in the transmethylation pathway leading to synthesis of the methyl-donor S-adenosylmethionine (SAM). Most cancer cells stop proliferating during methionine stress conditions, when methionine is replaced in the growth media by its immediate metabolic precursor homocysteine (Met-Hcy+). Non-transformed cells proliferate in Met-Hcy+ media, making the methionine metabolic requirement of cancer cells an attractive target for therapy, yet there is relatively little known about the molecular mechanisms governing the methionine stress response in cancer cells. To study this phenomenon in breast cancer cells, we selected methionine-independent-resistant cell lines derived from MDAMB468 breast cancer cells. Resistant cells grew normally in Met-Hcy+ media, whereas their parental MDAMB468 cells rapidly arrest in the G 1 phase. Remarkably, supplementing Met-Hcy+ growth media with S-adenosylmethionine suppressed the cell proliferation defects, indicating that methionine stress is a consequence of SAM limitation rather than low amino acid concentrations. Accordingly, mTORC1 activity, the primary effector responding to amino acid limitation, remained high. However, we found that levels of the replication factor Cdc6 decreased and pre-replication complexes were destabilized in methionine-stressed MDAMB468 but not resistant cells. Our study characterizes metabolite requirements and cell cycle responses that occur during methionine stress in breast cancer cells and helps explain the metabolic uniqueness of cancer cells.

Whole transcriptome analysis of the silicon response of the diatom Thalassiosira pseudonana

BACKGROUND

Silicon plays important biological roles, but the mechanisms of cellular responses to silicon are poorly understood. We report the first analysis of cell cycle arrest and recovery from silicon starvation in the diatom Thalassiosira pseudonana using whole genome microarrays.

RESULTS

Three known responses to silicon were examined, 1) silicified cell wall synthesis, 2) recovery from silicon starvation, and 3) co-regulation with silicon transporter (SIT) genes. In terms of diatom cell wall formation, thus far only cell surface proteins and proteins tightly associated with silica have been characterized. Our analysis has identified new genes potentially involved in silica formation, and other genes potentially involved in signaling, trafficking, protein degradation, glycosylation and transport, which provides a larger-scale picture of the processes involved. During silicon starvation, an overrepresentation of transcription and translation related genes were up-regulated, indicating that T. pseudonana is poised to rapidly recover from silicon starvation and resume cell cycle progression upon silicon replenishment. This is in contrast to other types of limitation, and provides the first molecular data explaining the well-established environmental response of diatoms to grow as blooms and to out-compete other classes of microalgae for growth. Comparison of our data with a previous diatom cell cycle analysis indicates that assignment of the cell cycle specific stage of particular cyclins and cyclin dependent kinases should be re-evaluated. Finally, genes co-varying in expression with the SITs enabled identification of a new class of diatom-specific proteins containing a unique domain, and a putative silicon efflux protein.

CONCLUSIONS

Analysis of the T. pseudonana microarray data has provided a wealth of new genes to investigate previously uncharacterized cellular phenomenon related to silicon metabolism, silicon's interaction with cellular components, and environmental responses to silicon.

Hpz1 modulates the G1-S transition in fission yeast

Here we characterize a novel protein in S. pombe. It has a high degree of homology with the Zn-finger domain of the human Poly(ADP-ribose) polymerase (PARP). Surprisingly, the gene for this protein is, in many fungi, fused with and in the same reading frame as that encoding Rad3, the homologue of the human ATR checkpoint protein. We name the protein Hpz1 (Homologue of PARP-type Zn-finger). Hpz1 does not possess PARP activity, but is important for resistance to ultraviolet light in the G1 phase and to treatment with hydroxyurea, a drug that arrests DNA replication forks in the S phase. However, we find no evidence of a checkpoint function of Hpz1. Furthermore, absence of Hpz1 results in an advancement of S-phase entry after a G1 arrest as well as earlier recovery from a hydroxyurea block. The hpz1 gene is expressed mainly in the G1 phase and Hpz1 is localized to the nucleus. We conclude that Hpz1 regulates the initiation of the S phase and may cooperate with Rad3 in this function.

Induction of a G1-S checkpoint in fission yeast

Entry into S phase is carefully regulated and, in most organisms, under the control of a G(1)-S checkpoint. We have previously described a G(1)-S checkpoint in fission yeast that delays formation of the prereplicative complex at chromosomal replication origins after exposure to UV light (UVC). This checkpoint absolutely depends on the Gcn2 kinase. Here, we explore the signal for activation of the Gcn2-dependent G(1)-S checkpoint in fission yeast. If some form of DNA damage can activate the checkpoint, deficient DNA repair should affect the length of the checkpoint-induced delay. We find that the cell-cycle delay differs in repair-deficient mutants from that in wild-type cells. However, the duration of the delay depends not only on the repair capacity of the cells, but also on the nature of the repair deficiency. First, the delay is abolished in cells that are deficient in the early steps of repair. Second, the delay is prolonged in repair mutants that fail to complete repair after the incision stage. We conclude that the G(1)-S delay depends on damage to the DNA and that the activating signal derives not from the initial DNA damage, but from a repair intermediate(s). Surprisingly, we find that activation of Gcn2 does not depend on the processing of DNA damage and that activated Gcn2 alone is not sufficient to delay entry into S phase in UVC-irradiated cells. Thus, the G(1)-S delay depends on at least two different inputs.

Role of the RNA-binding protein Nrd1 in stress granule formation and its implication in the stress response in fission yeast

We have previously identified the RNA recognition motif (RRM)-type RNA-binding protein Nrd1 as an important regulator of the posttranscriptional expression of myosin in fission yeast. Pmk1 MAPK-dependent phosphorylation negatively regulates the RNA-binding activity of Nrd1. Here, we report the role of Nrd1 in stress-induced RNA granules. Nrd1 can localize to poly(A)-binding protein (Pabp)-positive RNA granules in response to various stress stimuli, including heat shock, arsenite treatment, and oxidative stress. Interestingly, compared with the unphosphorylatable Nrd1, Nrd1^DD (phosphorylation-mimic version of Nrd1) translocates more quickly from the cytoplasm to the stress granules in response to various stimuli; this suggests that the phosphorylation of Nrd1 by MAPK enhances its localization to stress-induced cytoplasmic granules. Nrd1 binds to Cpc2 (fission yeast RACK) in a phosphorylation-dependent manner and deletion of Cpc2 affects the formation of Nrd1-positive granules upon arsenite treatment. Moreover, the depletion of Nrd1 leads to a delay in Pabp-positive RNA granule formation, and overexpression of Nrd1 results in an increased size and number of Pabp-positive granules. Interestingly, Nrd1 deletion induced resistance to sustained stresses and enhanced sensitivity to transient stresses. In conclusion, our results indicate that Nrd1 plays a role in stress-induced granule formation, which affects stress resistance in fission yeast.

Between Scylla and Charibdis: eIF2α kinases as targets for cancer chemotherapy

The eIF2α kinases integrate translation initiation rates with nutrient availability, thus allowing cells to adapt to nutrient scarcity. Recent evidence has uncovered new functions of these kinases in tumour cell biology, ranging from regulation of cell cycle progression, maintenance of genome stability, control of apoptosis, and cell survival under nutrient stress and hypoxia. Accordingly, active eIF2α kinases modulate the antineoplasic activity of several antitumour drugs, either by exacerbating their cytotoxic effect or by promoting chemoresistance. Understanding of eIF2α kinases molecular roles may provide mechanistic insights into how tumour cells sense and adapt to nutrient restriction, thus helping to implement more effective approaches for cancer chemotherapy.

A plant virus movement protein regulates the Gcn2p kinase in budding yeast

Virus life cycle heavily depends on their ability to command the host machinery in order to translate their genomes. Animal viruses have been shown to interfere with host translation machinery by expressing viral proteins that either maintain or inhibit eIF2α function by phosphorylation. However, this interference mechanism has not been described for any plant virus yet. Prunnus necrotic ringspot virus (PNRSV) is a serious pathogen of cultivated stone fruit trees. The movement protein (MP) of PNRSV is necessary for the cell-to-cell movement of the virus. By using a yeast-based approach we have found that over-expression of the PNRSV MP caused a severe growth defect in yeast cells. cDNA microarrays analysis carried out to characterise at the molecular level the growth interference phenotype reported the induction of genes related to amino acid deprivation suggesting that expression of MP activates the GCN pathway in yeast cells. Accordingly, PNRSV MP triggered activation of the Gcn2p kinase, as judged by increased eIF2α phosphorylation. Activation of Gcn2p by MP expression required a functional Tor1p kinase, since rapamycin treatment alleviated the yeast cell growth defect and blocked eIF2α phosphorylation triggered by MP expression. Overall, these findings uncover a previously uncharacterised function for PNRSV MP viral protein, and point out at Tor1p and Gcn2p kinases as candidate susceptibility factors for plant viral infections.

Clustering in cell cycle dynamics with general response/signaling feedback

Motivated by experimental and theoretical work on autonomous oscillations in yeast, we analyze ordinary differential equations models of large populations of cells with cell-cycle dependent feedback. We assume a particular type of feedback that we call responsive/signaling (RS), but do not specify a functional form of the feedback. We study the dynamics and emergent behavior of solutions, particularly temporal clustering and stability of clustered solutions. We establish the existence of certain periodic clustered solutions as well as "uniform" solutions and add to the evidence that cell-cycle dependent feedback robustly leads to cell-cycle clustering. We highlight the fundamental differences in dynamics between systems with negative and positive feedback. For positive feedback systems the most important mechanism seems to be the stability of individual isolated clusters. On the other hand we find that in negative feedback systems, clusters must interact with each other to reinforce coherence. We conclude from various details of the mathematical analysis that negative feedback is most consistent with observations in yeast experiments.

Zim17/Tim15 links mitochondrial iron-sulfur cluster biosynthesis to nuclear genome stability

Genomic instability is related to a wide-range of human diseases. Here, we show that mitochondrial iron–sulfur cluster biosynthesis is important for the maintenance of nuclear genome stability in Saccharomyces cerevisiae. Cells lacking the mitochondrial chaperone Zim17 (Tim15/Hep1), a component of the iron–sulfur biosynthesis machinery, have limited respiration activity, mimic the metabolic response to iron starvation and suffer a dramatic increase in nuclear genome recombination. Increased oxidative damage or deficient DNA repair do not account for the observed genomic hyperrecombination. Impaired cell-cycle progression and genetic interactions of ZIM17 with components of the RFC-like complex involved in mitotic checkpoints indicate that replicative stress causes hyperrecombination in zim17Δ mutants. Furthermore, nuclear accumulation of pre-ribosomal particles in zim17Δ mutants reinforces the importance of iron–sulfur clusters in normal ribosome biosynthesis. We propose that compromised ribosome biosynthesis and cell-cycle progression are interconnected, together contributing to replicative stress and nuclear genome instability in zim17Δ mutants.

Cdt1 proteolysis is promoted by dual PIP degrons and is modulated by PCNA ubiquitylation

Cdt1 plays a critical role in DNA replication regulation by controlling licensing. In Metazoa, Cdt1 is regulated by CRL4^Cdt2-mediated ubiquitylation, which is triggered by DNA binding of proliferating cell nuclear antigen (PCNA). We show here that fission yeast Cdt1 interacts with PCNA in vivo and that DNA loading of PCNA is needed for Cdt1 proteolysis after DNA damage and in S phase. Activation of this pathway by ultraviolet (UV)-induced DNA damage requires upstream involvement of nucleotide excision repair or UVDE repair enzymes. Unexpectedly, two non-canonical PCNA-interacting peptide (PIP) motifs, which both have basic residues downstream, function redundantly in Cdt1 proteolysis. Finally, we show that poly-ubiquitylation of PCNA, which occurs after DNA damage, reduces Cdt1 proteolysis. This provides a mechanism for fine-tuning the activity of the CRL4^Cdt2 pathway towards Cdt1, allowing Cdt1 proteolysis to be more efficient in S phase than after DNA damage.

Cell-cycle analysis of fission yeast cells by flow cytometry

The cell cycle of the fission yeast, Schizosaccharomyces pombe, does not easily lend itself to analysis by flow cytometry, mainly because cells in G(1) and G(2) phase contain the same amount of DNA. This occurs because fission yeast cells under standard growth conditions do not complete cytokinesis until after G(1) phase. We have devised a flow cytometric method exploiting the fact that cells in G(1) phase contain two nuclei, whereas cells in G(2) are mononuclear. Measurements of the width as well as the total area of the DNA-associated fluorescence signal allows the discrimination between cells in G(1) and in G(2) phase and the cell-cycle progression of fission yeast can be followed in detail by flow cytometry. Furthermore, we show how this method can be used to monitor the timing of cell entry into anaphase. Fission yeast cells tend to form multimers, which represents another problem of flow cytometry-based cell-cycle analysis. Here we present a method employing light-scatter measurements to enable the exclusion of cell doublets, thereby further improving the analysis of fission yeast cells by flow cytometry.

The roles of stress-activated Sty1 and Gcn2 kinases and of the protooncoprotein homologue Int6/eIF3e in responses to endogenous oxidative stress during histidine starvation

In fission yeast, Sty1 and Gcn2 are important protein kinases that regulate gene expression in response to amino acid starvation. The translation factor subunit Int6/eIF3e promotes Sty1-dependent response by increasing the abundance of Atf1, a transcription factor targeted by Sty1. While Gcn2 promotes expression of amino acid biosynthesis enzymes, the mechanism and function of Sty1 activation and Int6/eIF3e involvement during this nutrient stress are not understood. Here we show that mutants lacking sty1(+) or gcn2(+) display reduced viabilities during histidine depletion stress in a manner suppressible by the antioxidant N-acetyl cysteine, suggesting that these protein kinases function to alleviate endogenous oxidative damage generated during nutrient starvation. Int6/eIF3e also promotes cell viability by a mechanism involving the stimulation of Sty1 response to oxidative damage. In further support of these observations, microarray data suggest that, during histidine starvation, int6Δ increases the duration of Sty1-activated gene expression linked to oxidative stress due to the initial attenuation of Sty1-dependent transcription. Moreover, loss of gcn2 induces the expression of a new set of genes not activated in wild-type cells starved for histidine. These genes encode heatshock proteins, redox enzymes, and proteins involved in mitochondrial maintenance, in agreement with the idea that oxidative stress is imposed on gcn2Δ cells. Furthermore, early Sty1 activation promotes rapid Gcn2 activation on histidine starvation. These results suggest that Gcn2, Sty1, and Int6/eIF3e are functionally integrated and cooperate to respond to oxidative stress generated during histidine starvation.

Fission yeast receptor of activated C kinase (RACK1) ortholog Cpc2 regulates mitotic commitment through Wee1 kinase

In the fission yeast Schizosaccharomyces pombe, Wee1-dependent inhibitory phosphorylation of the highly conserved Cdc2/Cdk1 kinase determines the mitotic onset when cells have reached a defined size. The receptor of activated C kinase (RACK1) is a scaffolding protein strongly conserved among eukaryotes which binds to other proteins to regulate multiple processes in mammalian cells, including the modulation of cell cycle progression during G(1)/S transition. We have recently described that Cpc2, the fission yeast ortholog to RACK1, controls from the ribosome the activation of MAPK cascades and the cellular defense against oxidative stress by positively regulating the translation of specific genes whose products participate in the above processes. Intriguingly, mutants lacking Cpc2 display an increased cell size at division, suggesting the existence of a specific cell cycle defect at the G(2)/M transition. In this work we show that protein levels of Wee1 mitotic inhibitor are increased in cells devoid of Cpc2, whereas the levels of Cdr2, a Wee1 inhibitor, are down-regulated in the above mutant. On the contrary, the kinetics of G(1)/S transition was virtually identical both in control and Cpc2-less strains. Thus, our results suggest that in fission yeast Cpc2/RACK1 positively regulates from the ribosome the mitotic onset by modulating both the protein levels and the activity of Wee1. This novel mechanism of translational control of cell cycle progression might be conserved in higher eukaryotes.

The roles of nitric oxide synthase and eIF2alpha kinases in regulation of cell cycle upon UVB-irradiation

In response to ultraviolet light (UV)-induced damage, cells initiate cellular recovery mechanisms including activation of repair genes and redistribution of cell cycle phases. While most studies have focused on DNA damage-inducible transcriptional regulation of cell cycle checkpoints, translational regulation also plays an important role in control of cell cycle progression upon UV-irradiation. UV-irradiation activates two kinases, PERK and GCN2, which phosphorylate the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) and subsequently inhibit protein synthesis. We recently identified an upstream regulator, nitric oxide synthase (NOS), which controls the activation of both PERK and GCN2 upon UVB-irradiation. Our data suggested that UVB induces NOS activation and NO(.) production, which reacts with superoxide (O(2)(*-)) to form peroxynitrite (ONOO(-)) and activate PERK. The NO(*) production also leads to L-Arg depletion and GCN2 activation. The elevation of nitric oxide and activation of PERK/GCN2 have been shown to play roles in regulation of cell cycle upon UVB irradiation. In the present study, we show that the cell cycle phases were redistributed by inhibition of NOS activation or reduction of oxidative stress upon UVB irradiation, indicating the roles of NO(*) and its oxidative products in regulation of cell cycle. We also demonstrate that both PERK and GCN2 were involved in regulation of cell cycle upon UVB-irradiation, but the regulation is independent of eIF2alpha phosphorylation. While the mechanism for UVB-induced cell cycle control is yet to be unraveled, we here discuss the differential roles of NOS, PERK and GCN2 in regulation of cell cycle upon UVB-irradiation.

Global transcriptional response after exposure of fission yeast cells to ultraviolet light

Background

In many cell types, including the fission yeast Schizosaccharomyces pombe, a set of checkpoints are induced by perturbations of the cell cycle or by DNA damage. Many of the checkpoint responses include a substantial change of the transcriptional pattern. As part of characterising a novel G1/S checkpoint in fission yeast we have investigated whether a transcriptional response is induced after irradiation with ultraviolet light.

Results

Microarray analyses were used to measure the global transcription levels of all open reading frames of fission yeast after 254 nm ultraviolet irradiation, which is known to induce a G1/S checkpoint. We discovered a surprisingly weak transcriptional response, which is quite unlike the marked changes detected after some other types of treatment and in several other checkpoints. Interestingly, the alterations in gene expression after ultraviolet irradiation were not similar to those observed after ionising radiation or oxidative stress. Pathway analysis suggests that there is little systematic transcriptional response to the irradiation by ultraviolet light, but a marked, coordinated transcriptional response was noted on progression of the cells from G1 to S phase.

Conclusion

There is little response in fission yeast to ultraviolet light at the transcriptional level. Amongst the genes induced or repressed after ultraviolet irradiation we found none that are likely to be involved in the G1/S checkpoint mechanism, suggesting that the checkpoint is not dependent upon transcriptional regulation.

Smc5-Smc6-dependent removal of cohesin from mitotic chromosomes

The function of the essential cohesin-related Smc5-Smc6 complex has remained elusive, though hypomorphic mutants have defects late in recombination, in checkpoint maintenance, and in chromosome segregation. Recombination and checkpoints are not essential for viability, and Smc5-Smc6-null mutants die in lethal mitoses. This suggests that the chromosome segregation defects may be the source of lethality in irradiated Smc5-Smc6 hypomorphs. We show that in smc6 mutants, following DNA damage in interphase, chromosome arm segregation fails due to an aberrant persistence of cohesin, which is normally removed by the Separase-independent pathway. This postanaphase persistence of cohesin is not dependent on DNA damage, since the synthetic lethality of smc6 hypomorphs with a topoisomerase II mutant, defective in mitotic chromosome structure, is also due to the retention of cohesin on undamaged chromosome arms. In both cases, Separase overexpression bypasses the defect and restores cell viability, showing that defective cohesin removal is a major determinant of the mitotic lethality of Smc5-Smc6 mutants.

Role for RACK1 orthologue Cpc2 in the modulation of stress response in fission yeast

The receptor of activated C kinase (RACK1) is a protein highly conserved among eukaryotes. In mammalian cells, RACK1 functions as an adaptor to favor protein kinase C (PKC)-mediated phosphorylation and subsequent activation of c-Jun NH(2)-terminal kinase mitogen-activated protein kinase. Cpc2, the RACK1 orthologue in the fission yeast Schizosaccharomyces pombe, is involved in the control of G2/M transition and interacts with Pck2, a PKC-type protein member of the cell integrity Pmk1 mitogen-activated protein kinase (MAPK) pathway. Both RACK1 and Cpc2 are structural components of the 40S ribosomal subunit, and recent data suggest that they might be involved in the control of translation. In this work, we present data supporting that Cpc2 negatively regulates the cell integrity transduction pathway by favoring translation of the tyrosine-phosphatases Pyp1 and Pyp2 that deactivate Pmk1. In addition, Cpc2 positively regulates the synthesis of the stress-responsive transcription factor Atf1 and the cytoplasmic catalase, a detoxificant enzyme induced by treatment with hydrogen peroxide. These results provide for the first time strong evidence that the RACK1-type Cpc2 protein controls from the ribosome the extent of the activation of MAPK cascades, the cellular defense against oxidative stress, and the progression of the cell cycle by regulating positively the translation of specific gene products involved in key biological processes.

Transcriptomic comparison of Aspergillus niger growing on two different sugars reveals coordinated regulation of the secretory pathway

Background

The filamentous fungus, Aspergillus niger, responds to nutrient availability by modulating secretion of various substrate degrading hydrolases. This ability has made it an important organism in industrial production of secreted glycoproteins. The recent publication of the A. niger genome sequence and availability of microarrays allow high resolution studies of transcriptional regulation of basal cellular processes, like those of glycoprotein synthesis and secretion. It is known that the activities of certain secretory pathway enzymes involved N-glycosylation are elevated in response to carbon source induced secretion of the glycoprotein glucoamylase. We have investigated whether carbon source dependent enhancement of protein secretion can lead to upregulation of secretory pathway elements extending beyond those involved in N-glycosylation.

Results

This study compares the physiology and transcriptome of A. niger growing at the same specific growth rate (0.16 h^-1) on xylose or maltose in carbon-limited chemostat cultures. Transcription profiles were obtained using Affymetrix GeneChip analysis of six replicate cultures for each of the two growth-limiting carbon sources. The production rate of extracellular proteins per gram dry mycelium was about three times higher on maltose compared to xylose. The defined culture conditions resulted in high reproducibility, discriminating even low-fold differences in transcription, which is characteristic of genes encoding basal cellular functions. This included elements in the secretory pathway and central metabolic pathways. Increased protein secretion on maltose was accompanied by induced transcription of > 90 genes related to protein secretion. The upregulated genes encode key elements in protein translocation to the endoplasmic reticulum (ER), folding, N-glycosylation, quality control, and vesicle packaging and transport between ER and Golgi. The induction effect of maltose resembles the unfolded protein response (UPR), which results from ER-stress and has previously been defined by treatment with chemicals interfering with folding of glycoproteins or by expression of heterologous proteins.

Conclusion

We show that upregulation of secretory pathway genes also occurs in conditions inducing secretion of endogenous glycoproteins – representing a more normal physiological state. Transcriptional regulation of protein synthesis and secretory pathway genes may thus reflect a general mechanism for modulation of secretion capacity in response to the conditional need for extracellular enzymes.

Checkpoint regulation of DNA replication

We discuss the mechanisms regulating entry into and progression through S phase in eukaryotic cells. Methods to study the G1/S transition are briefly reviewed and an overview of G1/S-checkpoints is given, with particular emphasis on fission yeast. Thereafter we discuss different aspects of the intra-S checkpoint and introduce the main molecular players and mechanisms.

The G1-S checkpoint in fission yeast is not a general DNA damage checkpoint

Inhibitory mechanisms called checkpoints regulate progression of the cell cycle in the presence of DNA damage or when a previous cell-cycle event is not finished. In fission yeast exposed to ultraviolet light the G1-S transition is regulated by a novel checkpoint that depends on the Gcn2 kinase. The molecular mechanisms involved in checkpoint induction and maintenance are not known. Here we characterise the checkpoint further by exposing the cells to a variety of DNA-damaging agents. Exposure to methyl methane sulphonate and hydrogen peroxide induce phosphorylation of eIF2alpha, a known Gcn2 target, and an arrest in G1 phase. By contrast, exposure to psoralen plus long-wavelength ultraviolet light, inducing DNA adducts and crosslinks, or to ionizing radiation induce neither eIF2alpha phosphorylation nor a cell-cycle delay. We conclude that the G1-S checkpoint is not a general DNA-damage checkpoint, in contrast to the one operating at the G2-M transition. The tight correlation between eIF2alpha phosphorylation and the presence of a G1-phase delay suggests that eIF2alpha phosphorylation is required for checkpoint induction. The implications for checkpoint signalling are discussed.

Phosphoproteome analysis of fission yeast

Phosphorylation is a key regulator of many events in eukaryotic cells. The acquisition of large-scale phosphorylation data sets from model organisms can pinpoint conserved regulatory inputs and reveal kinase-substrate relationships. Here, we provide the first large-scale phosphorylation analysis of the fission yeast, Schizosaccharomyces pombe. Protein from thiabendazole-treated cells was separated by preparative SDS-PAGE and digested with trypsin. The resulting peptides were subjected to either IMAC or TiO2 phosphopeptide enrichment methods and then analyzed by LC-MS/MS using an LTQ-Orbitrap mass spectrometer. In total, 2887 distinct phosphorylation sites were identified from 1194 proteins with an estimated false-discovery rate of <0.5% at the peptide level. A comparison of the two different enrichment methods is presented, supporting the finding that they are complementary. Finally, phosphorylation sites were examined for phosphorylation-specific motifs and evolutionary conservation. These analyses revealed both motifs and specific phosphorylation events identified in S. pombe were conserved and predicted novel phosphorylation in mammals.

TOR signalling regulates mitotic commitment through the stress MAP kinase pathway and the Polo and Cdc2 kinases

The coupling of growth to cell cycle progression allows eukaryotic cells to divide at particular sizes depending on nutrient availability. In fission yeast, this coupling involves the Spc1/Sty1 mitogen-activated protein kinase (MAPK) pathway working through Polo kinase recruitment to the spindle pole bodies (SPBs). Here we report that changes in nutrients influence TOR signalling, which modulates Spc1/Sty1 activity. Rapamycin-induced inhibition of TOR signalling advanced mitotic onset, mimicking the reduction in cell size at division seen after shifts to poor nitrogen sources. Gcn2, an effector of TOR signalling and modulator of translation, regulates the Pyp2 phosphatase that in turn modulates Spc1/Sty1 activity. Rapamycin- or nutrient-induced stimulation of Spc1/Sty1 activity promotes Polo kinase SPB recruitment and Cdc2 activation to advance mitotic onset. This advanced mitotic onset is abolished in cells depleted of Gcn2, Pyp2, or Spc1/Sty1 or on blockage of Spc1/Sty1-dependent Polo SPB recruitment. Therefore, TOR signalling modulates mitotic onset through the stress MAPK pathway via the Pyp2 phosphatase.