Heat stress activates fission yeast Spc1/StyI MAPK by a MEKK-independent mechanism

Negative regulation of APC/C activation by MAPK-mediated attenuation of Cdc20Slp1 under stress

Mitotic anaphase onset is a key cellular process tightly regulated by multiple kinases. The involvement of mitogen-activated protein kinases (MAPKs) in this process has been established in Xenopus egg extracts. However, the detailed regulatory cascade remains elusive, and it is also unknown whether the MAPK-dependent mitotic regulation is evolutionarily conserved in the single-cell eukaryotic organisms such as fission yeast (Schizosaccharomyces pombe). Here, we show that two MAPKs in S. pombe indeed act in concert to restrain anaphase-promoting complex/cyclosome (APC/C) activity upon activation of the spindle assembly checkpoint (SAC). One MAPK, Pmk1, binds to and phosphorylates Slp1Cdc20, the co-activator of APC/C. Phosphorylation of Slp1Cdc20 by Pmk1, but not by Cdk1, promotes its subsequent ubiquitylation and degradation. Intriguingly, Pmk1-mediated phosphorylation event is also required to sustain SAC under environmental stress. Thus, our study establishes a new underlying molecular mechanism of negative regulation of APC/C by MAPK upon stress stimuli, and provides a previously unappreciated framework for regulation of anaphase entry in eukaryotic cells.

Heat stress-induced activation of MAPK pathway attenuates Atf1-dependent epigenetic inheritance of heterochromatin in fission yeast

Eukaryotic cells are constantly exposed to various environmental stimuli. It remains largely unexplored how environmental cues bring about epigenetic fluctuations and affect heterochromatin stability. In the fission yeast Schizosaccharomyces pombe, heterochromatic silencing is quite stable at pericentromeres but unstable at the mating-type (mat) locus under chronic heat stress, although both loci are within the major constitutive heterochromatin regions. Here, we found that the compromised gene silencing at the mat locus at elevated temperature is linked to the phosphorylation status of Atf1, a member of the ATF/CREB superfamily. Constitutive activation of mitogen-activated protein kinase (MAPK) signaling disrupts epigenetic maintenance of heterochromatin at the mat locus even under normal temperature. Mechanistically, phosphorylation of Atf1 impairs its interaction with heterochromatin protein Swi6HP1, resulting in lower site-specific Swi6HP1 enrichment. Expression of non-phosphorylatable Atf1, tethering Swi6HP1 to the mat3M-flanking site or absence of the anti-silencing factor Epe1 can largely or partially rescue heat stress-induced defective heterochromatic maintenance at the mat locus.

A peroxiredoxin-P38 MAPK scaffold increases MAPK activity by MAP3K-independent mechanisms

Peroxiredoxins (Prdxs) utilize reversibly oxidized cysteine residues to reduce peroxides and promote H2O2 signal transduction, including H2O2-induced activation of P38 MAPK. Prdxs form H2O2-induced disulfide complexes with many proteins, including multiple kinases involved in P38 MAPK signaling. Here, we show that a genetically encoded fusion between a Prdx and P38 MAPK is sufficient to hyperactivate the kinase in yeast and human cells by a mechanism that does not require the H2O2-sensing cysteine of the Prdx. We demonstrate that a P38-Prdx fusion protein compensates for loss of the yeast scaffold protein Mcs4 and MAP3K activity, driving yeast into mitosis. Based on our findings, we propose that the H2O2-induced formation of Prdx-MAPK disulfide complexes provides an alternative scaffold and signaling platform for MAPKK-MAPK signaling. The demonstration that formation of a complex with a Prdx is sufficient to modify the activity of a kinase has broad implications for peroxide-based signal transduction in eukaryotes.

Fission yeast Cdc14-like phosphatase Flp1/Clp1 modulates the transcriptional response to oxidative stress

Reactive oxygen species (ROS) are an important source of cellular damage. When ROS intracellular levels increase, oxidative stress takes place affecting DNA stability and metabolic functions. To prevent these effects, stress-activated protein kinases (SAPKs) delay cell cycle progression and induce a transcriptional response that activates antioxidant mechanisms ensuring cell adaptation and survival. Fission yeast Cdc14-like phosphatase Flp1 (also known as Clp1) has a well-established role in cell cycle regulation. Moreover, Flp1 contributes to checkpoint activation during replication stress. Here, we show that Flp1 has a role in fine-tuning the cellular oxidative stress response. Phosphorylation-dependent nucleolar release of Flp1 in response to oxidative stress conditions plays a role in the cellular transcriptional response. Thus, Flp1 ablation increases the transcriptional response to oxidative stress, in both intensity and duration, upregulating both Atf1/Pcr1- and Pap1-dependent stress induced genes. Remarkably, we found that Flp1 interacts with the Atf1/Pcr1 complex with Pcr1 acting as a direct substrate. Our results provide evidence that Flp1 modulates the oxidative stress response by limiting the Atf1/Pcr1-mediated transcription.

A new negative feedback mechanism for MAPK pathway inactivation through Srk1 MAPKAP kinase

The fission yeast mitogen-activated kinase (MAPK) Sty1 is essential for cell survival in response to different environmental insults. In unstimulated cells, Sty1 forms an inactive ternary cytoplasmatic complex with the MAPKK Wis1 and the MAPKAP kinase Srk1. Wis1 phosphorylates and activates Sty1, inducing the nuclear translocation of the complex. Once in the nucleus, Sty1 phosphorylates and activates Srk1, which in turns inhibits Cdc25 and cell cycle progression, before being degraded in a proteasome-dependent manner. In parallel, active nuclear Sty1 activates the transcription factor Atf1, which results in the expression of stress response genes including pyp2 (a MAPK phosphatase) and srk1. Despite its essentiality in response to stress, persistent activation of the MAPK pathway can be deleterious and induces cell death. Thus, timely pathway inactivation is essential to ensure an appropriate response and cell viability. Here, uncover a role for the MAPKAP kinase Srk1 as an essential component of a negative feedback loop regulating the Sty1 pathway through phosphorylation and inhibition of the Wis1 MAPKK. This feedback regulation by a downstream kinase in the pathway highlights an additional mechanism for fine-tuning of MAPK signaling. Thus, our results indicate that Srk1 not only facilitates the adaptation to stress conditions by preventing cell cycle progression, but also plays an instrumental role regulating the upstream kinases in the stress MAPK pathway.

Stress-dependent inhibition of polarized cell growth through unbalancing the GEF/GAP regulation of Cdc42

Cdc42 GTPase rules cell polarity and growth in fission yeast. It is negatively and positively regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs), respectively. Active Cdc42-GTP localizes to the poles, where it associates with numerous proteins constituting the polarity module. However, little is known about its downregulation. We describe here that oxidative stress causes Sty1-kinase-dependent Cdc42 inactivation at cell poles. Both the amount of active Cdc42 at tips and cell length inversely correlate with Sty1 activity, explaining the elongated morphology of Δsty1 cells. We have created stress-blinded cell poles either by eliminating two Cdc42 GAPs or through the constitutive tethering of Gef1 to cell tips, and we biochemically demonstrate that the GAPs Rga3/6 and the GEF Gef1 are direct substrates of Sty1. We propose that phosphorylation of Rga3/6 and Gef1 mediates the Sty1-dependent inhibition of Cdc42 at cell tips, halting polarized growth during stress adaptation.

Communication between Cyclin-dependent kinase Cdc2 and the Wis1-Spc1 MAPK pathway determines mitotic timing in Schizosaccharomyces pombe

Checkpoint activation and gene expression modulation represent key determinants of cellular survival in adverse conditions. The former is regulated by cyclin-dependent kinases (CDKs) while the latter can be controlled by mitogen-activated protein kinases (MAPKs). Association between cell-cycle progression and MAPK-dependent gene expression exists in cells growing in optimal environments. While MAPK-mediated regulation of the cell cycle is well characterised, the reciprocal influence of mitotic CDK on stress response is not well studied. We present evidence that CDK activity can regulate the extent of MAPK activation in Schizosaccharomyces pombe cells. We show that increasing or decreasing mitotic CDK (Cdc2) activity in S. pombe cells can affect the activation of stress responsive MAPK (Spc1) even in the absence of stress stimuli. Our results indicate that the strong correlation between Cdc2 activity and Spc1 MAPK-activity in S. pombe is important in regulating mitotic timing.This article has an associated First Person interview with the first author of the paper.

A Redox-Sensitive Thiol in Wis1 Modulates the Fission Yeast Mitogen-Activated Protein Kinase Response to H2O2 and Is the Target of a Small Molecule

Oxidation of a highly conserved cysteine (Cys) residue located in the kinase activation loop of mitogen-activated protein kinase kinases (MAPKK) inactivates mammalian MKK6. This residue is conserved in the fission yeast Schizosaccharomyces pombe MAPKK Wis1, which belongs to the H2O2-responsive MAPK Sty1 pathway. Here, we show that H2O2 reversibly inactivates Wis1 through this residue (C458) in vitro We found that C458 is oxidized in vivo and that serine replacement of this residue significantly enhances Wis1 activation upon addition of H2O2 The allosteric MAPKK inhibitor INR119, which binds in a pocket next to the activation loop and C458, prevented the inhibition of Wis1 by H2O2in vitro and significantly increased Wis1 activation by low levels of H2O2in vivo We propose that oxidation of C458 inhibits Wis1 and that INR119 cancels out this inhibitory effect by binding close to this residue. Kinase inhibition through the oxidation of a conserved Cys residue in MKK6 (C196) is thus conserved in the S. pombe MAPKK Wis1.

RNA-Binding Protein Rnc1 Regulates Cell Length at Division and Acute Stress Response in Fission Yeast through Negative Feedback Modulation of the Stress-Activated Mitogen-Activated Protein Kinase Pathway

Control of mRNA localization, stability, turnover, and translation by RNA-binding proteins (RBPs) influences essential processes in all eukaryotes, including signaling by mitogen-activated protein kinase (MAPK) pathways. We describe that in the fission yeast Schizosaccharomyces pombe the RBP Rnc1 negatively regulates cell length at division during unperturbed growth and recovery after acute stress by reducing the activity of the MAPK Sty1, which regulates cell growth and differentiation during environmental cues. This mechanism relies on Rnc1 binding to specific mRNAs encoding both enhancers and negative regulators of Sty1 activity. Remarkably, multiple phosphorylation of Rnc1 by Sty1 favors RBP binding and destabilization of the above mRNAs. Thus, posttranscriptional modulation of MAP kinase signaling by RNA-binding proteins emerges as a major regulatory mechanism that dictates the growth cycle and cellular adaptation in response to the changing environment in eukaryotic organisms.

RNA-binding proteins (RBPs) play a major role during control of mRNA localization, stability, and translation and are central to most cellular processes. In the fission yeast Schizosaccharomyces pombe, the multiple K homology (KH) domain RBP Rnc1 downregulates the activity of the cell integrity pathway (CIP) via stabilization of pmp1⁺ mRNA, which encodes the Pmp1 phosphatase that inactivates Pmk1, the mitogen-activated protein kinase (MAPK) component of this signaling cascade. However, Rnc1 likely regulates the half-life/stability of additional mRNAs. We show that Rnc1 downregulates the activity of Sty1, the MAPK of the stress-activated MAPK pathway (SAPK), during control of cell length at division and recovery in response to acute stress. Importantly, this control strictly depends on Rnc1’s ability to bind mRNAs encoding activators (Wak1 MAPKKK, Wis1 MAPKK) and downregulators (Atf1 transcription factor, Pyp1 and Pyp2 phosphatases) of Sty1 phosphorylation through its KH domains. Moreover, Sty1 is responsible for Rnc1 phosphorylation in vivo at multiple phosphosites during growth and stress, and these modifications trigger Rnc1 for proper binding and destabilization of the above mRNA targets. Phosphorylation by Sty1 prompts Rnc1-dependent mRNA destabilization to negatively control SAPK signaling, thus revealing an additional feedback mechanism that allows precise tuning of MAPK activity during unperturbed cell growth and stress.

Modulation of TOR complex 2 signaling by the stress-activated MAPK pathway in fission yeast

Sin1 is a substrate-binding subunit of target of rapamycin complex 2 (TORC2), an evolutionarily conserved protein kinase complex. In fission yeast, Sin1 has also been identified as a protein that interacts with Spc1 (also known as Sty1) in the stress-activated protein kinase (SAPK) pathway. Therefore, this study examined the relationship between TORC2 and Spc1 signaling. We found that the common docking (CD) domain of Spc1 interacts with a cluster of basic amino acid residues in Sin1. Although diminished TORC2 activity in the absence of the functional Spc1 cascade suggests positive regulation of TORC2 by Spc1, such regulation appears to be independent of the Sin1-Spc1 interaction. Hyperosmotic stress transiently inhibits TORC2, and its swift recovery is dependent on Spc1, the transcription factor Atf1, and the glycelrol-3-phosphate dehydrogenase Gpd1, whose expression is induced upon osmostress by the Spc1-Atf1 pathway. Thus, cellular adaptation to osmostress seems important for TORC2 reactivation, though Spc1 and Atf1 contribute to TORC2 activation also in the absence of osmostress. These results indicate coordinated actions of the SAPK and TORC2 pathways, both of which are essential for fission yeast cells to survive environmental stress.

Shedding light on autophagy coordinating with cell wall integrity signaling to govern pathogenicity of Magnaporthe oryzae

Cells are faced with various stresses during their growth and development, and autophagy is a degradative process in which cells can break down their own components to recycle macromolecules and provide energy under these stresses. For pathogenic fungi that utilize cell wall as the first barrier against external stress, the cell wall integrity (CWI) pathway also provides an essential role in responding to these stresses. However, the specific connection between autophagy and CWI remains elusive in either the model fungi including budding yeast Saccharomyces cerevisiae or the rice blast fungus Magnaporthe oryzae. Here, we provided evidence that the endoplasmic reticulum (ER) stress is highly induced during M. oryzae infection and that CWI MAP kinase kinase MoMkk1 (S. cerevisiae Mkk1/2 homolog) was subject to phosphorylation regulation by MoAtg1, the only identified kinase in the core autophagy machinery. We also identified MoMkk1 serine 115 as the MoAtg1-dependent phosphorylation site and this phosphorylation could activate CWI, similar to that by the conserved MAP kinase kinase kinase MoMck1 (S. cerevisiae Bck1 homolog). Together with the first report of MoMkk1 subjects to phosphorylation regulation by MoAtg1, we revealed a new mechanism by which autophagy coordinates with CWI signaling under ER stress, and this MoAtg1-dependent MoMkk1 phosphorylation is essential for the pathogenicity of M. oryzae.Abbreviations: A/Ala: alanine; Atg: autophagy-related; Bck1: bypass of C kinase 1; co-IP: co-immunoprecipitation; CWI: cell wall integrity;DTT: dithiothreitol; ER: endoplasmic reticulum; GFP: green fluorescent protein; Mo: Magnaporthe oryzae; MAPK: mitogen-activated protein kinase; Mkk1: mitogen-activated protein kinase-kinase 1; MS: mass spectrometry; PAS: phagophore assembly site; RFP: red fluorescent protein; RT: room temperature; S/Ser: serine; Slt2: suppressor of the lytic phenotype 2; T/Thr: threonine; UPR: unfolded protein response; Y2H: yeast two-hybrid screen.

Cmk2 kinase is essential for survival in arsenite by modulating translation together with RACK1 orthologue Cpc2 in Schizosaccharomyces pombe

Different studies have demonstrated multiple effects of arsenite on human physiology. However, there are many open questions concerning the mechanism of response to arsenite. Schizosaccharomyces pombe activates the Sty1 MAPK pathway as a common response to several stress conditions. The specificity of the response is due to the activation of different transcription factors and specific targets such the Cmk2 MAPKAP kinase. We have previously shown that Cmk2 is phosphorylated and activated by the MAPK Sty1 in response to oxidative stress. Here, we report that Cmk2 kinase is specifically necessary to overcome the stress caused by metalloid agents, in particular arsenite. Deletion of cmk2 increases the protein level of various components of the MAPK pathway. Moreover, Cmk2 negatively regulates translation through the Cpc2 kinase: the RACK1 orthologue in fission yeast. RACK1 is a receptor for activated C-kinase. Interestingly, RACK1 is a constituent of the eukaryotic ribosome specifically localized in the head region of the 40 S subunit. Cmk2 controls arsenite response through Cpc2 and it does so through Cpc2 ribosomal function, as observed in genetic analysis using a Cpc2 mutant unable to bind to ribosome. These findings suggest a role for Cmk2 in regulating translation and facilitating adaptation to arsenite stress in the ribosome.

The fission yeast MAPK Spc1 senses perturbations in Cdc25 and Wee1 activities and targets Rad24 to restore this balance

Mitogen-activated protein kinases (MAPKs) play vital roles in multiple cellular processes and represent prominently pursued targets for development of therapeutic regimes. The MAPK Spc1 (p38 homologue) is known to be very important for both mitotic promotion and delay in Schizosaccharomyces pombe. However, the mechanism responsible for mitotic inhibition has remained elusive. Cdc25 (Cdc2 activator) and Wee1 (Cdc2 inhibtor) are important determinants of mitotic timing in all eukaryotes. Our results show that Spc1 can sense the perturbations in the balance of Cdc25 and Wee1 activities in S. pombe and that its function as a mitotic inhibitor is very important for controlling the same. An Spc1-Srk1-Rad24-dependent pathway for mitotic inhibition has been reported earlier.Here we report the presence of an alternative mechanism wherein Spc1 targets the 14-3-3 protein, Rad24, independently of Srk1, leading to relocalization of Cdc25 and mitotic inhibition. Our observations suggest that this pathway can serve as a backup mechanism for Cdc2 inactivation in the absence of Wee1.

Dynamic regulation of Cdr1 kinase localization and phosphorylation during osmotic stress

Environmental conditions modulate cell cycle progression in many cell types. A key component of the eukaryotic cell cycle is the protein kinase Wee1, which inhibits the cyclin-dependent kinase Cdk1 in yeast through human cells. In the fission yeast Schizosaccharomyces pombe, the protein kinase Cdr1 is a mitotic inducer that promotes mitotic entry by phosphorylating and inhibiting Wee1. Cdr1 and Wee1 both localize to punctate structures, termed nodes, on the medial cortex, but it has been unknown whether node localization can be altered by physiological signals. Here we investigated how environmental conditions regulate Cdr1 signaling for cell division. Osmotic stress induced hyperphosphorylation of the mitotic inducer Cdr1 for several hours, and cells delayed division for the same time period. This stress-induced hyperphosphorylation required both Cdr1 autophosphorylation and the stress-activated protein kinase Sty1. During osmotic stress, Cdr1 exited cortical nodes and localized in the cytoplasm. Using a series of truncation mutants, we mapped a C-terminal domain that is necessary and sufficient for Cdr1 node localization and found that Sty1 directly phosphorylates this domain in vitro Sty1 was not required for Cdr1 exit from nodes, indicating the existence of additional regulatory signals. Both Cdr1 phosphorylation and node localization returned to basal levels when cells adapted to osmotic conditions and resumed cell cycle progression. In summary, we identified a mechanism that prevents Cdr1 colocalization with its inhibitory target Wee1 during osmotic stress. Dynamic regulation of protein localization to cortical nodes might represent a strategy to modulate entry into mitosis under differing environmental conditions.

Remodeling of the Fission Yeast Cdc42 Cell-Polarity Module via the Sty1 p38 Stress-Activated Protein Kinase Pathway

The Rho family GTPase Cdc42 is a key regulator of eukaryotic cellular organization and cell polarity [1]. In the fission yeast Schizosaccharomyces pombe, active Cdc42 and associated effectors and regulators (the "Cdc42 polarity module") coordinate polarized growth at cell tips by controlling the actin cytoskeleton and exocytosis [2-4]. Localization of the Cdc42 polarity module to cell tips is thus critical for its function. Here we show that the fission yeast stress-activated protein kinase Sty1, a homolog of mammalian p38 MAP kinase, regulates localization of the Cdc42 polarity module. In wild-type cells, treatment with latrunculin A, a drug that leads to actin depolymerization, induces dispersal of the Cdc42 module from cell tips and cessation of polarized growth [5, 6]. We show that latrunculin A treatment also activates the Sty1 MAP kinase pathway and, strikingly, we find that loss of Sty1 MAP kinase signaling prevents latrunculin A-induced dispersal of the Cdc42 module, allowing polarized growth even in complete absence of the actin cytoskeleton. Regulation of the Cdc42 module by Sty1 is independent of Sty1's role in stress-induced gene expression. We also describe a system for activation of Sty1 kinase "on demand" in the absence of any external stress, and use this to show that Sty1 activation alone is sufficient to disperse the Cdc42 module from cell tips in otherwise unperturbed cells. During nitrogen-starvation-induced quiescence, inhibition of Sty1 converts non-growing, depolarized cells into growing, polarized cells. Our results place MAP kinase Sty1 as an important physiological regulator of the Cdc42 polarity module.

Analysis of the Schizosaccharomyces pombe Cell Cycle

Schizosaccharomyces pombe cells are rod shaped, and they grow by tip elongation. Growth ceases during mitosis and cell division; therefore, the length of a septated cell is a direct measure of the timing of mitotic commitment, and the length of a wild-type cell is an indicator of its position in the cell cycle. A large number of documented stage-specific changes can be used as landmarks to characterize cell cycle progression under specific experimental conditions. Conditional mutations can permanently or transiently block the cell cycle at almost any stage. Large, synchronously dividing cell populations, essential for the biochemical analysis of cell cycle events, can be generated by induction synchrony (arrest-release of a cell cycle mutant) or selection synchrony (centrifugal elutriation or lactose-gradient centrifugation). Schizosaccharomyces pombe cell cycle studies routinely combine particular markers, mutants, and synchronization procedures to manipulate the cycle. We describe these techniques and list key landmarks in the fission yeast mitotic cell division cycle.

Growth and the Environment of Schizosaccharomyces pombe

Here, we summarize the composition and uses of Schizosaccharomyces pombe media and discuss key issues for consideration in the generation of S. pombe cultures. We discuss the concept of "culture memory," in which the growth state and stress experienced by a strain during storage, propagation, and starter culture preparation can alter experimental outcomes at later stages. We also describe the triggers that are widely used to manipulate signaling through the environment sensing pathways.

Direct interaction of Ste11 and Mkk1/2 through Nst1 integrates high-osmolarity glycerol and pheromone pathways to the cell wall integrity MAPK pathway

Coordination and cross talks of MAPK pathways are critical for signaling efficiency, but their mechanisms are not well understood. Slt2, the MAP kinase of cell wall integrity pathway (CWI), is activated by heat stress even in the absence of upstream components of this pathway, suggesting a supplementary input for Slt2 activation. Here, we identify a new interaction of Ste11 and Mkk1, mediated by Nst1 that connects the high-osmolarity glycerol and pheromone pathways directly to CWI pathway in response to heat and pheromone. We suggest that Ser(407) and Thr(411) are novel residues of Mkk1 activated by these MAPK pathways.

Genome wide transcription profiling of the effects of overexpression of Spc1 and its kinase dead mutant in Schizosaccharomyces pombe

The Mitogen Activated Protein Kinase Spc1 (p38 homolog) is a major player in stress responses of the unicellular fission yeast Schizosaccharomyces pombe. This pathway is therefore also known as the SAPK or Stress Activated Protein Kinase pathway. Spc1 is a known activator of transcription factors that control gene expression in response to extracellular stimuli and is also known to interact with the translation machinery [1], [2], [3], [4], [5], [6], [7], [8]. Spc1 has also been implicated in cell cycle regulation and meiosis in S. pombe[1], [2], [9], [10]. Given its documented role in modulating gene expression, we performed a microarray based identification of genes whose expression in unperturbed cells (absence of stress stimuli) is dependent on Spc1. For this we overexpressed Spc1 in S. pombe. Additionally we also overexpressed Spc1K49R (a kinase dead mutant of Spc1) to understand the contribution of Spc1's kinase activity towards the observed gene expression changes. The microarray data are available at NCBI's Gene Expression Omnibus (GEO) Series (accession number GSE73618). Here we report the annotation of the genes whose expression get altered by Spc1/Spc1K49R overexpression and also provide details related to sample processing and statistical analysis of our microarray data.

PKA antagonizes CLASP-dependent microtubule stabilization to re-localize Pom1 and buffer cell size upon glucose limitation

Cells couple growth with division and regulate size in response to nutrient availability. In rod-shaped fission yeast, cell-size control occurs at mitotic commitment. An important regulator is the DYRK-family kinase Pom1, which forms gradients from cell poles and inhibits the mitotic activator Cdr2, itself localized at the medial cortex. Where and when Pom1 modulates Cdr2 activity is unclear as Pom1 medial cortical levels remain constant during cell elongation. Here we show that Pom1 re-localizes to cell sides upon environmental glucose limitation, where it strongly delays mitosis. This re-localization is caused by severe microtubule destabilization upon glucose starvation, with microtubules undergoing catastrophe and depositing the Pom1 gradient nucleator Tea4 at cell sides. Microtubule destabilization requires PKA/Pka1 activity, which negatively regulates the microtubule rescue factor CLASP/Cls1/Peg1, reducing CLASP's ability to stabilize microtubules. Thus, PKA signalling tunes CLASP's activity to promote Pom1 cell side localization and buffer cell size upon glucose starvation.

Mechanics and morphogenesis of fission yeast cells

The integration of biochemical and biomechanical elements is at the heart of morphogenesis. While animal cells are relatively soft objects which shape and mechanics is mostly regulated by cytoskeletal networks, walled cells including those of plants, fungi and bacteria are encased in a rigid cell wall which resist high internal turgor pressure. How these particular mechanical properties may influence basic cellular processes, such as growth, shape and division remains poorly understood. Recent work using the model fungal cell fission yeast, Schizosaccharomyces pombe, highlights important contribution of cell mechanics to various morphogenesis processes. We envision this genetically tractable system to serve as a novel standard for the mechanobiology of walled cell.

A global profile of replicative polymerase usage

Three eukaryotic DNA polymerases are essential for genome replication. Polymerase (Pol) α-primase initiates each synthesis event and is rapidly replaced by processive DNA polymerases: Polɛ replicates the leading strand, whereas Polδ performs lagging-strand synthesis. However, it is not known whether this division of labor is maintained across the whole genome or how uniform it is within single replicons. Using Schizosaccharomyces pombe, we have developed a polymerase usage sequencing (Pu-seq) strategy to map polymerase usage genome wide. Pu-seq provides direct replication-origin location and efficiency data and indirect estimates of replication timing. We confirm that the division of labor is broadly maintained across an entire genome. However, our data suggest a subtle variability in the usage of the two polymerases within individual replicons. We propose that this results from occasional leading-strand initiation by Polδ followed by exchange for Polɛ.

Protective roles of osmotic stress-resistant Hos3 against oxidative, nitrosative and nutritional stresses in Schizosaccharomyces pombe

Hos3 is involved in cellular growth under osmotic stress in Schizosaccharomyces pombe. The recombinant plasmid pYFHos3 harboring the structural gene encoding Hos3 was constructed. The S. pombe cells harboring pYFHos3 contained the increased hos3 (+) mRNA content and exhibited an enhanced growth in high osmotic conditions, such as 1.5 M KCl and 2.5 M D-glucose, compared with the vector control cells. In the presence of hydrogen peroxide (H2O2), superoxide anion-generating menadione (MD) and nitric oxide (NO)-generating sodium nitroprusside (SNP), they could grow better than the vector control cells. In the presence of H2O2, MD and SNP and in the absence of a nitrogen source, the S. pombe cells harboring pYFHos3 contained less elevated NO and reactive oxygen species (ROS) levels than the vector control cells. Collectively, the S. pombe Hos3 also participate in the cellular defense against oxidative, nitrosative and nutritional stresses through down-regulating ROS and NO levels.

Ecl1 is activated by the transcription factor Atf1 in response to H2O2 stress in Schizosaccharomyces pombe

The Ecl1 family genes extend the lifespan of fission yeast when overexpressed. They also cause resistance against H(2)O(2) stress. In this study, we found that the bZip transcription factor Atf1 is a direct activator of the induction of extender of chronological lifespan (ecl1 (+)) by H(2)O(2) stress. Based on ChIP analysis, we identified that Atf1 binds to the upstream DNA region of ecl1(+). Previously, we reported that overexpression of ecl1(+) increased the expression of the catalase-encoding ctt1(+). This ecl1(+)-dependent increase of ctt1(+) expression occurred in ∆atf1 mutant. On the other hand, the activation of ctt1 (+) caused by the ∆pyp1 mutation, which enhances Sty1-Atf1 activity, could occur in ∆ecl1 mutant. Based on these results, we propose that Atf1 can regulate ctt1(+) in both an Ecl1-dependent and an Ecl1-independent manner.

Genetic and physical interaction of Ssp1 CaMKK and Rad24 14-3-3 during low pH and osmotic stress in fission yeast

The Ssp1 calmodulin kinase kinase (CaMKK) is necessary for stress-induced re-organization of the actin cytoskeleton and initiation of growth at the new cell end following division in Schizosaccharomyces pombe. In addition, it regulates AMP-activated kinase and functions in low glucose tolerance. ssp1⁻ cells undergo mitotic delay at elevated temperatures and G₂ arrest in the presence of additional stressors. Following hyperosmotic stress, Ssp1-GFP forms transient foci which accumulate at the cell membrane and form a band around the cell circumference, but not co-localizing with actin patches. Hyperosmolarity-induced localization to the cell membrane occurs concomitantly with a reduction of its interaction with the 14-3-3 protein Rad24, but not Rad25 which remains bound to Ssp1. The loss of rad24 in ssp1⁻ cells reduces the severity of hyperosmotic stress response and relieves mitotic delay. Conversely, overexpression of rad24 exacerbates stress response and concomitant cell elongation. rad24⁻ does not impair stress-induced localization of Ssp1 to the cell membrane, however this response is almost completely absent in cells overexpressing rad24.

Hsp70-Hsp40 chaperone complex functions in controlling polarized growth by repressing Hsf1-driven heat stress-associated transcription

How the molecular mechanisms of stress response are integrated at the cellular level remains obscure. Here we show that the cellular polarity machinery in the fission yeast Schizosaccharomyces pombe undergoes dynamic adaptation to thermal stress resulting in a period of decreased Cdc42 activity and altered, monopolar growth. Cells where the heat stress-associated transcription was genetically upregulated exhibit similar growth patterning in the absence of temperature insults. We identify the Ssa2-Mas5/Hsp70-Hsp40 chaperone complex as repressor of the heat shock transcription factor Hsf1. Cells lacking this chaperone activity constitutively activate the heat-stress-associated transcriptional program. Interestingly, they also exhibit intermittent monopolar growth within a physiological temperature range and are unable to adapt to heat stress. We propose that by negatively regulating the heat stress-associated transcription, the Ssa2-Mas5 chaperone system could optimize cellular growth under different temperature regiments.

Heat stress, caused by fluctuations in ambient temperature, occurs frequently in nature. How organisms adapt and maintain regular patterns of growth over a range of environmental conditions remain poorly understood. Our work in the simple unicellular yeast Schizosaccharomyces pombe suggests that the heat stress-associated transcription must be repressed by the evolutionary conserved Hsp70-Hsp40 chaperone complex to allow cells to adapt the polarized growth machinery to elevated temperature.

Fission yeast nucleolar protein Dnt1 regulates G2/M transition and cytokinesis by downregulating Wee1 kinase

Cytokinesis involves temporally and spatially coordinated action of the cell cycle, cytoskeletal and membrane systems to achieve separation of daughter cells. The septation initiation network (SIN) and mitotic exit network (MEN) signaling pathways regulate cytokinesis and mitotic exit in the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae, respectively. Previously, we have shown that in fission yeast, the nucleolar protein Dnt1 negatively regulates the SIN pathway in a manner that is independent of the Cdc14-family phosphatase Clp1/Flp1, but how Dnt1 modulates this pathway has remained elusive. By contrast, it is clear that its budding yeast relative, Net1/Cfi1, regulates the homologous MEN signaling pathway by sequestering Cdc14 phosphatase in the nucleolus before mitotic exit. In this study, we show that dnt1(+) positively regulates G2/M transition during the cell cycle. By conducting epistasis analyses to measure cell length at septation in double mutant (for dnt1 and genes involved in G2/M control) cells, we found a link between dnt1(+) and wee1(+). Furthermore, we showed that elevated protein levels of the mitotic inhibitor Wee1 kinase and the corresponding attenuation in Cdk1 activity is responsible for the rescuing effect of dnt1Δ on SIN mutants. Finally, our data also suggest that Dnt1 modulates Wee1 activity in parallel with SCF-mediated Wee1 degradation. Therefore, this study reveals an unexpected missing link between the nucleolar protein Dnt1 and the SIN signaling pathway, which is mediated by the Cdk1 regulator Wee1 kinase. Our findings also define a novel mode of regulation of Wee1 and Cdk1, which is important for integration of the signals controlling the SIN pathway in fission yeast.

Response regulator-mediated MAPKKK heteromer promotes stress signaling to the Spc1 MAPK in fission yeast

The Spc1 mitogen-activated protein kinase (MAPK) cascade in fission yeast is activated by two MAPK kinase kinase (MAPKKK) paralogues, Wis4 and Win1, in response to multiple forms of environmental stress. Previous studies identified Mcs4, a "response regulator" protein that associates with the MAPKKKs and receives peroxide stress signals by phosphorelay from the Mak2/Mak3 sensor histidine kinases. Here we show that Mcs4 has an unexpected, phosphorelay-independent function in promoting heteromer association between the Wis4 and Win1 MAPKKKs. Only one of the MAPKKKs in the heteromer complex needs to be catalytically active, but disturbing the integrity of the complex by mutations to Mcs4, Wis4, or Win1 results in reduced MAPKKK-MAPKK interaction and, consequently, compromised MAPK activation. The physical interaction among Mcs4, Wis4, and Win1 is constitutive and not responsive to stress stimuli. Therefore the Mcs4-MAPKKK heteromer complex might serve as a stable platform/scaffold for signaling proteins that convey input and output of different stress signals. The Wis4-Win1 complex discovered in fission yeast demonstrates that heteromer-mediated mechanisms are not limited to mammalian MAPKKKs.

Heat induction of a novel Rad9 variant from a cryptic translation initiation site reduces mitotic commitment

Exposure of human cells to heat switches the activating signal of the DNA damage checkpoint from genotoxic to temperature stress. This change reduces mitotic commitment at the expense of DNA break repair. The thermal alterations behind this switch remain elusive despite the successful use of heat to sensitise cancer cells to DNA breaks. Rad9 is a highly conserved subunit of the Rad9-Rad1-Hus1 (9-1-1) checkpoint-clamp that is loaded by Rad17 onto damaged chromatin. At the DNA, Rad9 activates the checkpoint kinases Rad3(ATR) and Chk1 to arrest cells in G2. Using Schizosaccharomyces pombe as a model eukaryote, we discovered a new variant of Rad9, Rad9-M50, whose expression is specifically induced by heat. High temperatures promote alternative translation from a cryptic initiation codon at methionine-50. This process is restricted to cycling cells and is independent of the temperature-sensing mitogen-activated protein kinase (MAPK) pathway. While full-length Rad9 delays mitosis in the presence of DNA lesions, Rad9-M50 functions in a remodelled checkpoint pathway to reduce mitotic commitment at elevated temperatures. This remodelled pathway still relies on Rad1 and Hus1, but acts independently of Rad17. Heat-induction of Rad9-M50 ensures that the kinase Chk1 remains in a hypo-phosphorylated state. Elevated temperatures specifically reverse the DNA-damage-induced modification of Chk1 in a manner dependent on Rad9-M50. Taken together, heat reprogrammes the DNA damage checkpoint at the level of Chk1 by inducing a Rad9 variant that can act outside of the canonical 9-1-1 complex.

HOG-MAPK signaling regulates the adaptive responses of Aspergillus fumigatus to thermal stress and other related stress

Aspergillus fumigatus is naturally exposed to a highly variable environment and subjected to various kinds of stresses. High-osmolarity glycerol mitogen-activated protein kinase (HOG-MAPK) pathway plays a crucial role in regulating cellular homeostasis in response to environmental changes. Here, we explored the contribution of HOG-MAPK pathway to the adaptive responses to thermal stress and other related stresses in A. fumigatus. We observed the phenotype features of wild-type strains and their derived mutants at 37 and 48 °C, and the results suggested that tcsB participates in response to high temperature. Furthermore, susceptibility test for antifungal drugs showed that SHO1 branch is probably involved in the susceptibility of A. fumigatus to itraconazole at high temperature. Although sakA expression at mRNA level appeared unchanged in wild-type AF293 subjected to thermal stress, phosphorylated SakAp level increased significantly in the strains exposed to cold stress, 250 mmol/L nystatin or 10 % dimethyl sulfoxide in a manner dependent on the SLN1 branch and independent on the SHO1 branch. Taken together, these results indicate that HOG-MAPK pathway, especially the SLN1 branch, plays an important role in the adaptations of A. fumigatus to thermal stress and other related stresses.

A systematic screen reveals new elements acting at the G2/M cell cycle control

BACKGROUND

The major cell cycle control acting at the G2 to mitosis transition is triggered in all eukaryotes by cyclin-dependent kinases (CDKs). In the fission yeast Schizosaccharomyces pombe the activation of the G2/M CDK is regulated primarily by dephosphorylation of the conserved residue Tyr15 in response to the stress-nutritional response and cell geometry sensing pathways. To obtain a more complete view of the G2/M control we have screened systematically for gene deletions that advance cells prematurely into mitosis.

RESULTS

A screen of 82% of fission yeast non-essential genes, comprising approximately 3,000 gene deletion mutants, identified 18 genes that act negatively at mitotic entry, 7 of which have not been previously described as cell cycle regulators. Eleven of the 18 genes function through the stress response and cell geometry sensing pathways, both of which act through CDK Tyr15 phosphorylation, and 4 of the remaining genes regulate the G2/M transition by inputs from hitherto unknown pathways. Three genes act independently of CDK Tyr15 phosphorylation and define additional uncharacterized molecular control mechanisms.

CONCLUSIONS

Despite extensive investigation of the G2/M control, our work has revealed new components of characterized pathways that regulate CDK Tyr15 phosphorylation and new components of novel mechanisms controlling mitotic entry.

A non-Mendelian MAPK-generated hereditary unit controlled by a second MAPK pathway in Podospora anserina

The Podospora anserina PaMpk1 MAP kinase (MAPK) signaling pathway can generate a cytoplasmic and infectious element resembling prions. When present in the cells, this C element causes the crippled growth (CG) cell degeneration. CG results from the inappropriate autocatalytic activation of the PaMpk1 MAPK pathway during growth, whereas this cascade normally signals stationary phase. Little is known about the control of such prion-like hereditary units involved in regulatory inheritance. Here, we show that another MAPK pathway, PaMpk2, is crucial at every stage of the fungus life cycle, in particular those controlled by PaMpk1 during stationary phase, which includes the generation of C. Inactivation of the third P. anserina MAPK pathway, PaMpk3, has no effect on the development of the fungus. Mutants of MAPK, MAPK kinase, and MAPK kinase kinase of the PaMpk2 pathway are unable to present CG. This inability likely relies upon an incorrect activation of PaMpk1, although this MAPK is normally phosphorylated in the mutants. In PaMpk2 null mutants, hyphae are abnormal and PaMpk1 is mislocalized. Correspondingly, stationary phase differentiations controlled by PaMpk1 are defective in the mutants of the PaMpk2 cascade. Constitutive activation of the PaMpk2 pathway mimics in many ways its inactivation, including an effect on PaMpk1 localization. Analysis of double and triple mutants inactivated for two or all three MAPK genes undercover new growth and differentiation phenotypes, suggesting overlapping roles. Our data underscore the complex regulation of a prion-like element in a model organism.

H₂O₂ stress-specific regulation of S. pombe MAPK Sty1 by mitochondrial protein phosphatase Ptc4

In fission yeast, the stress-activated MAP kinase, Sty1, is activated via phosphorylation upon exposure to stress and orchestrates an appropriate response. Its activity is attenuated by either serine/threonine PP2C or tyrosine phosphatases. Here, we found that the PP2C phosphatase, Ptc4, plays an important role in inactivating Sty1 specifically upon oxidative stress. Sty1 activity remains high in a ptc4 deletion mutant upon H(2)O(2) but not under other types of stress. Surprisingly, Ptc4 localizes to the mitochondria and is targeted there by an N-terminal mitochondrial targeting sequence (MTS), which is cleaved upon import. A fraction of Sty1 also localizes to the mitochondria suggesting that Ptc4 attenuates the activity of a mitochondrial pool of this MAPK. Cleavage of the Ptc4 MTS is greatly reduced specifically upon H(2)O(2), resulting in the full-length form of the phosphatase; this displays a stronger interaction with Sty1, thus suggesting a novel mechanism by which the negative regulation of MAPK signalling is controlled and providing an explanation for the oxidative stress-specific nature of the regulation of Sty1 by Ptc4.

The fission yeast stress-responsive MAPK pathway promotes meiosis via the phosphorylation of Pol II CTD in response to environmental and feedback cues

The RRM-type RNA-binding protein Mei2 is a master regulator of meiosis in fission yeast, in which it stabilizes meiosis-specific mRNAs by blocking their destruction. Artificial activation of Mei2 can provoke the entire meiotic process, and it is suspected that Mei2 may do more than the stabilization of meiosis-specific mRNAs. In our current study using a new screening system, we show that Mei2 genetically interacts with subunits of CTDK-I, which phosphorylates serine-2 residues on the C-terminal domain of RNA polymerase II (Pol II CTD). Phosphorylation of CTD Ser-2 is essential to enable the robust transcription of ste11, which encodes an HMG-type transcription factor that regulates the expression of mei2 and other genes necessary for sexual development. CTD Ser-2 phosphorylation increases under nitrogen starvation, and the stress-responsive MAP kinase pathway, mediated by Wis1 MAPKK and Sty1 MAPK, is critical for this stress response. Sty1 phosphorylates Lsk1, the catalytic subunit of CTDK-I. Furthermore, a feedback loop stemming from activated Mei2 to Win1 and Wis4 MAPKKKs operates in this pathway and eventually enhances CTD Ser-2 phosphorylation and ste11 transcription. Hence, in addition to starting meiosis, Mei2 functions to reinforce the commitment to it, once cells have entered this process. This study also demonstrates clearly that the stress-responsive MAP kinase pathway can modulates gene expression through phosphorylation of Pol II CTD.

Hundreds of genes are newly expressed during meiosis, a process to form gametes, and the control of meiosis-specific gene expression is not simple. The master regulator of meiosis in fission yeast, Mei2, blocks an RNA destruction system that selectively degrades meiosis-specific mRNAs, highlighting the importance of post-transcriptional control in meiotic gene expression. Here we present another example of unforeseen regulation for meiosis. Ste11 is a key transcription factor responsible for the early meiotic gene expression in fission yeast. The ste11 gene is transcribed robustly only when serine-2 residues on the C-terminal domain (CTD Ser-2) of RNA polymerase II are phosphorylated. We show that the stress-responsive MAP kinase cascade transmits the environmental signal to stimulate CTD Ser-2 phosphorylation. Sty1 MAP kinase appears to phosphorylate and activate the catalytic subunit of CTDK-I, which in turn phosphorylates CTD Ser-2. We demonstrate further that Mei2, expression of which depends on Ste11, can activate the MAP kinase cascade, forming a feedback loop. Thus, we clarify here three important issues in cellular development: the physiological role of CTD Ser-2 phosphorylation, the molecular function of the stress-responsive MAP kinase pathway, and the presence of positive feedback that reinforces the commitment to meiosis.

MAPKKK-independent regulation of the Hog1 stress-activated protein kinase in Candida albicans

The Hog1 stress-activated protein kinase regulates both stress responses and morphogenesis in Candida albicans and is essential for the virulence of this major human pathogen. Stress-induced Hog1 phosphorylation is regulated by the upstream MAPKK, Pbs2, which in turn is regulated by the MAPKKK, Ssk2. Here, we have investigated the role of phosphorylation of Hog1 and Pbs2 in Hog1-mediated processes in C. albicans. Mutation of the consensus regulatory phosphorylation sites of Hog1 (Thr-174/Tyr-176) and Pbs2 (Ser-355/Thr-359), to nonphosphorylatable residues, resulted in strains that phenocopied hog1Δ and pbs2Δ cells. Consistent with this, stress-induced phosphorylation of Hog1 was abolished in cells expressing nonphosphorylatable Pbs2 (Pbs2^AA). However, mutation of the consensus sites of Pbs2 to phosphomimetic residues (Pbs2^DD) failed to constitutively activate Hog1. Furthermore, Ssk2-independent stress-induced Hog1 activation was observed in Pbs2^DD cells. Collectively, these data reveal a previously uncharacterized MAPKKK-independent mechanism of Hog1 activation in response to stress. Although Pbs2^DD cells did not exhibit high basal levels of Hog1 phosphorylation, overexpression of an N-terminal truncated form of Ssk2 did result in constitutive Hog1 activation, which was further increased upon stress. Significantly, both Pbs2^AA and Pbs2^DD cells displayed impaired stress resistance and attenuated virulence in a mouse model of disease, whereas only Pbs2^AA cells exhibited the morphological defects associated with loss of Hog1 function. This indicates that Hog1 mediates C. albicans virulence by conferring stress resistance rather than regulating morphogenesis.

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.

Influence of sorbitol on protein production and glycosylation and cell wall formation in Trichoderma reesei

Sorbitol is often used at 1 mol/liter as an osmotic stabilizer for cultivation of fungi with a fragile cell wall phenotype. On the other hand, at this concentration sorbitol causes an osmotic stress in fungal cells resulting in intensive production of intracellular glycerol. The highly increased consumption of glucose for glycerol synthesis may lead to changes in processes requiring carbohydrate residues. This study provides new information on the consequences of osmotic stress to the cell wall composition, protein production and glycosylation, and cell morphology of Trichoderma reesei. We observed that high osmolarity conditions enhanced biomass production and strongly limited synthesis of cell wall glucans and chitin. Moreover, in these conditions the amount of secreted protein decreased nearly ten-fold and expression of cbh1 and cbh2 genes coding for cellobiohydrolase I and cellobiohydrolase II, the main secretory proteins in T. reesei, was inhibited resulting in a lack of the proteins in the cell and cultivation medium. The activity of DPM synthase, enzyme engaged in both N- and O-glycosylation pathways, was reduced two-fold, suggesting an overall inhibition of protein glycosylation. However, the two modes of glycosylation were affected divergently: O-glycosylation of secreted proteins decreased in the early stages of growth while N-glycosylation significantly increased in the stationary phase.

Oxidative stress in yeast

The mechanisms of production and elimination of reactive oxygen species in the cells of the budding yeast Saccharomyces cerevisiae are analyzed. Coordinative role of special regulatory proteins including Yap1p, Msn2/4p, and Skn7p (Pos9p) in regulation of defense mechanisms in S. cerevisiae is described. A special section is devoted to two other well-studied species from the point of view of oxidative stress -- Schizosaccharomyces pombe and Candida albicans. Some examples demonstrating the use of yeast for investigation of apoptosis, aging, and some human diseases are given in the conclusion part.

Activation of the cell wall integrity pathway promotes escape from G2 in the fungus Ustilago maydis

It is widely accepted that MAPK activation in budding and fission yeasts is often associated with negative effects on cell cycle progression, resulting in delay or arrest at a specific stage in the cell cycle, thereby enabling cells to adapt to changing environmental conditions. For instance, activation of the Cell Wall Integrity (CWI) pathway in the budding yeast Saccharomyces cerevisiae signals an increase in CDK inhibitory phosphorylation, which leads cells to remain in the G2 phase. Here we characterized the CWI pathway of Ustilago maydis, a fungus evolutionarily distant from budding and fission yeasts, and show that activation of the CWI pathway forces cells to escape from G2 phase. In spite of these disparate cell cycle responses in S. cerevisiae and U. maydis, the CWI pathway in both organisms appears to respond to the same class cell wall stressors. To understand the basis of such a difference, we studied the mechanism behind the U. maydis response. We found that activation of CWI pathway in U. maydis results in a decrease in CDK inhibitory phosphorylation, which depends on the mitotic phosphatase Cdc25. Moreover, in response to activation of the CWI pathway, Cdc25 accumulates in the nucleus, providing a likely explanation for the increase in the unphosphorylated form of CDK. We also found that the extended N-terminal domain of Cdc25, which is dispensable under normal growth conditions, is required for this G2 escape as well as for resistance to cell wall stressors. We propose that the process of cell cycle adaptation to cell stress evolved differently in these two divergent organisms so that each can move towards a cell cycle phase most appropriate for responding to the environmental signals encountered.

Stress signalling to fungal stress-activated protein kinase pathways

The ability of microorganisms to survive and thrive within hostile environments depends on rapid and robust stress responses. Stress-activated protein kinase (SAPK) pathways are important stress-signalling modules found in all eukaryotes, including eukaryotic microorganisms such as fungi. These pathways consist of a SAPK that is activated by phosphorylation through a kinase cascade, and once activated, the SAPK phosphorylates a range of cytoplasmic and nuclear target substrates, which determine the appropriate response. However, despite their conservation in fungi, mechanisms that have evolved to relay stress signals to the SAPK module in different fungi have diverged significantly. Here, we present an overview of the diverse strategies used in the model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, and the pathogenic fungus Candida albicans, to sense and transduce stress signals to their respective SAPKs.

Lifespan extension by calorie restriction relies on the Sty1 MAP kinase stress pathway

Either calorie restriction, loss-of-function of the nutrient-dependent PKA or TOR/SCH9 pathways, or activation of stress defences improves longevity in different eukaryotes. However, the molecular links between glucose depletion, nutrient-dependent pathways and stress responses are unknown. Here, we show that either calorie restriction or inactivation of nutrient-dependent pathways induces lifespan extension in fission yeast, and that such effect is dependent on the activation of the stress-dependent Sty1 mitogen-activated protein (MAP) kinase. During transition to stationary phase in glucose-limiting conditions, Sty1 becomes activated and triggers a transcriptional stress programme, whereas such activation does not occur under glucose-rich conditions. Deletion of the genes coding for the SCH9-homologue, Sck2 or the Pka1 kinases, or mutations leading to constitutive activation of the Sty1 stress pathway increase lifespan under glucose-rich conditions, and importantly such beneficial effects depend ultimately on Sty1. Furthermore, cells lacking Pka1 display enhanced oxygen consumption and Sty1 activation under glucose-rich conditions. We conclude that calorie restriction favours oxidative metabolism, reactive oxygen species production and Sty1 MAP kinase activation, and this stress pathway favours lifespan extension.

Stress-induced phosphorylation of S. pombe Atf1 abrogates its interaction with F box protein Fbh1

The Atf1 transcription factor is critical for directing stress-induced gene expression in fission yeast [1]. Upon exposure to stress, Atf1 is hyperphosphorylated by the mitogen-activated protein kinase (MAPK) Sty1 [2, 3], which results in its stabilization [4]. The resulting increase in Atf1 is vital for a robust response to certain stresses [4]. Here we investigated the mechanism by which phosphorylation stabilizes Atf1. We show that Atf1 is a target for the ubiquitin-proteasome system and that its degradation is dependent upon an SCF E3 ligase containing the F box protein Fbh1. Turnover of Atf1 requires an intact F box, but not DNA helicase activity of Fbh1. Accordingly, disruption of Fbh1 F box function suppresses phenotypes associated with loss of Atf1 phosphorylation. Atf1 and Fbh1 interact under basal conditions, but this binding is lost upon stress. In contrast, a version of Atf1 lacking all intact MAPK sites still interacts with Fbh1 upon stress, indicating that the association between the F box protein and substrate is disrupted by stress-induced phosphorylation. Most F box protein-substrate interactions described to date are mediated positively by phosphorylation [5]. Thus, our findings represent a novel means of regulating the interaction between an F box protein and its substrate. Moreover, Atf1 is the first target described in any organism for the Fbh1 F box protein.

Nutrition-minded cell cycle

For decades, the fission yeast Schizosaccharomyces pombe has been used as an excellent model with which to explore how cellular growth is coordinated with the division cycle, a yet-unanswered question in biology. New studies in this organism show that TOR (target of rapamycin) kinase and stress-responsive MAPK (mitogen-activated protein kinase) form a signaling pathway that readjusts the timing of mitotic onset in response to poor nutrient conditions. Nutritional environment appears to be translated into graded activity of the protein kinases that influence the activation of Cdc2, a cyclin-dependent kinase driving cell-cycle progression.

Genome-wide screen of genes required for caffeine tolerance in fission yeast

Background

An excess of caffeine is cytotoxic to all eukaryotic cell types. We aim to study how cells become tolerant to a toxic dose of this drug, and the relationship between caffeine and oxidative stress pathways.

Methodology/Principal Findings

We searched for Schizosaccharomyces pombe mutants with inhibited growth on caffeine-containing plates. We screened a collection of 2,700 haploid mutant cells, of which 98 were sensitive to caffeine. The genes mutated in these sensitive clones were involved in a number of cellular roles including the H₂O₂-induced Pap1 and Sty1 stress pathways, the integrity and calcineurin pathways, cell morphology and chromatin remodeling. We have investigated the role of the oxidative stress pathways in sensing and promoting survival to caffeine. The Pap1 and the Sty1 pathways are both required for normal tolerance to caffeine, but only the Sty1 pathway is activated by the drug. Cells lacking Pap1 are sensitive to caffeine due to the decreased expression of the efflux pump Hba2. Indeed, ?hba2 cells are sensitive to caffeine, and constitutive activation of the Pap1 pathway enhances resistance to caffeine in an Hba2-dependent manner.

Conclusions/Significance

With our caffeine-sensitive, genome-wide screen of an S. pombe deletion collection, we have demonstrated the importance of some oxidative stress pathway components on wild-type tolerance to the drug.

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.

Fission yeast Tor1 functions as part of TORC1 to control mitotic entry through the stress MAPK pathway following nutrient stress

TOR signalling coordinates growth and division to control cell size. Inhibition of Schizosaccharomyces pombe Tor1, in response to a reduction in the quality of the nitrogen source (nutrient stress), promotes mitotic onset through activation of the mitogen-activated protein kinase (MAPK) Sty1 (also known as Spc1). Here we show that ;nutrient starvation' (complete withdrawal of nitrogen or leucine) blocks mitotic commitment by altering Sty1 signalling and that different degrees of Sty1 activation determine these differences in mitotic commitment decisions. Mammals contain one TOR kinase, whereas yeasts contain two. In each case, they comprise two distinct complexes: TORC1 and TORC2. We find that nutrient-stress-induced control of mitotic onset, through Tor1, is regulated through changes in TORC1 signalling. In minimal medium, Tor1 interacts with the TORC1 component Mip1 (raptor), and overexpression of tor1+ generates growth defects reminiscent of TORC1 mutants. Strains lacking the TORC2-specific components Sin1 and Ste20 (rictor) still advance mitotic onset in response to nutrient stress. By contrast, Mip1 and the downstream effector Gad8 (a S6K kinase homologue), like Tor1, are essential for nutrient stress to advance mitotic onset. We conclude that S. pombe Tor1 and Tor2 can both act in TORC1. However, it is the inhibition of Tor1 as part of TORC1 that promotes mitosis following nutrient stress.