The fission yeast mitotic regulator win1+ encodes an MAP kinase kinase kinase that phosphorylates and activates Wis1 MAP kinase kinase in response to high osmolarity

The absence of the ribosomal protein Rpl2702 elicits the MAPK-mTOR signaling to modulate mitochondrial morphology and functions

Ribosomes mediate protein synthesis, which is one of the most energy-demanding activities within the cell, and mitochondria are one of the main sources generating energy. How mitochondrial morphology and functions are adjusted to cope with ribosomal defects, which can impair protein synthesis and affect cell viability, is poorly understood. Here, we used the fission yeast Schizosaccharomyces Pombe as a model organism to investigate the interplay between ribosome and mitochondria. We found that a ribosomal insult, caused by the absence of Rpl2702, activates a signaling pathway involving Sty1/MAPK and mTOR to modulate mitochondrial morphology and functions. Specifically, we demonstrated that Sty1/MAPK induces mitochondrial fragmentation in a mTOR-independent manner while both Sty1/MAPK and mTOR increases the levels of mitochondrial membrane potential and mitochondrial reactive oxygen species (mROS). Moreover, we demonstrated that Sty1/MAPK acts upstream of Tor1/TORC2 and Tor1/TORC2 and is required to activate Tor2/TORC1. The enhancements of mitochondrial membrane potential and mROS function to promote proliferation of cells bearing ribosomal defects. Hence, our study reveals a previously uncharacterized Sty1/MAPK-mTOR signaling axis that regulates mitochondrial morphology and functions in response to ribosomal insults and provides new insights into the molecular and physiological adaptations of cells to impaired protein synthesis.

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.

Nitrogen starvation reveals the mitotic potential of mutants in the S/MAPK pathways

The genetics of quiescence is an emerging field compared to that of growth, yet both states generate spontaneous mutations and genetic diversity fueling evolution. Reconciling mutation rates in dividing conditions and mutation accumulation as a function of time in non-dividing situations remains a challenge. Nitrogen-starved fission yeast cells reversibly arrest proliferation, are metabolically active and highly resistant to a variety of stresses. Here, we show that mutations in stress- and mitogen-activated protein kinase (S/MAPK) signaling pathways are enriched in aging cultures. Targeted resequencing and competition experiments indicate that these mutants arise in the first month of quiescence and expand clonally during the second month at the expense of the parental population. Reconstitution experiments show that S/MAPK modules mediate the sacrifice of many cells for the benefit of some mutants. These findings suggest that non-dividing conditions promote genetic diversity to generate a social cellular environment prone to kin selection.

Nitrogen-starved fission yeast cells survive for weeks without dividing. Here, the authors show that some of these surviving cells accumulate mutations in the stress- and mitogen-activated protein kinase pathways and outcompete their parental cells, which provide nutrients for the mutant cells.

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.

Azoles activate Atf1-mediated transcription through MAP kinase pathway for antifungal effects in fission yeast

Azole antifungals directly inhibit enzymes for ergosterol biosynthesis, and this direct action is thought to underlie antifungal actions of these drugs. Recent studies showed that azoles alter expression of genes for various cellular functions. However, transcription factors regulated by azoles and their roles in antifungal actions remain poorly characterized. Using luciferase assay, we found that miconazole increased luciferase activity under the promoter containing the cAMP response element (CRE) motif. This azole-induced activation of CRE reporter was abolished in Atf1-deficient cells, suggesting that azoles induce Atf1 activation. As Atf1 is activated by stress-activated MAP kinase Sty1 upon various stressors, we examined its involvement. Azoles increased phosphorylation of Sty1 for its activation, and Sty1 deletion impaired azole-induced CRE reporter activation. In contrast, deletion of Pyp1, a tyrosine phosphatase which negatively regulates Sty1, increased CRE reporter activation. In addition, cells deficient in Atf1 and stress-activated MAP kinase pathway showed resistance to azoles, whereas cells lacking Pyp1 increased azole susceptibility, suggesting a critical role for azole-induced activation of MAP kinase-Atf1 pathway in antifungal actions of azoles. Collectively, these results suggest that azoles activate stress-activated MAP kinase pathway, thereby facilitating Atf1-mediated transcription for antifungal effects.

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.

A measurable activation of the bZIP transcription factor Atf1 in a fission yeast strain devoid of stress-activated and cell integrity mitogen-activated protein kinase (MAPK) activities

In Schizosaccharomyces pombe, the stress-activated Sty1 MAPK pathway is essential for cell survival under stress conditions. The Sty1 MAPK regulates Atf1 transcription factor to elicit stress responses in extreme conditions of osmolarity and reactive oxygen species-generating agents such as hydrogen peroxide, heat, low glucose, and heavy metal. Herein, using a newly developed Renilla luciferase reporter assay with enhanced detection sensitivity and accuracy, we show that distinct signaling pathways respond to cadmium and other reactive oxygen species-generating agents for the activation of Atf1. Also, surprisingly, a measurable activation of Atf1 transcription factor was still observed devoid of Sty1 MAPK activity. Further genetic and biological analyses revealed that the residual activation is caused by the activation of the cell wall integrity Pmk1 MAPK pathway and a redox-mediated activation of Atf1.

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.

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.

The selective elimination of messenger RNA underlies the mitosis-meiosis switch in fission yeast

The cellular programs for meiosis and mitosis must be strictly distinguished but the mechanisms controlling the entry to meiosis remain largely elusive in higher organisms. In contrast, recent analyses in yeast have shed new light on the mechanisms underlying the mitosis-meiosis switch. In this review, the current understanding of these mechanisms in the fission yeast Schizosaccharomyces pombe is discussed. Meiosis-inducing signals in this microbe emanating from environmental conditions including the nutrient status converge on the activity of an RRM-type RNA-binding protein, Mei2. This protein plays pivotal roles in both the induction and progression of meiosis and has now been found to govern the meiotic program in a quite unexpected manner. Fission yeast contains an RNA degradation system that selectively eliminates meiosis-specific mRNAs during the mitotic cell cycle. Mmi1, a novel RNA-binding protein of the YTH-family, is essential for this process. Mei2 tethers Mmi1 and thereby stabilizes the transcripts necessary for the progression of meiosis.

Improved tools for efficient mapping of fission yeast genes: identification of microtubule nucleation modifier mod22-1 as an allele of chromatin- remodelling factor gene swr1

Fission yeast genes identified in genetic screens are usually cloned by transformation of mutants with plasmid libraries. However, for some genes this can be difficult, and positional cloning approaches are required. The mutation swi5-39 reduces recombination frequency in homozygous crosses and has been used as a tool in mapping gene position (Schmidt, 1993). However, strain construction in swi5-39-based mapping is significantly more laborious than is desirable. Here we describe a set of strains designed to make swi5-based mapping more efficient and more powerful. The first improvement is the use of a swi5Δ strain marked with kanamycin (G418) resistance, which greatly facilitates identification of swi5 mutants. The second improvement, which follows directly from the first, is the introduction of a large number of auxotrophic markers into mapping strains, increasing the likelihood of finding close linkage between a marker and the mutation of interest. We combine these new mapping strains with a rec12Δ-based approach for initial mapping of a mutation to an individual chromosome. Together, the two methods allow an approximate determination of map position in only a small number of crosses. We used these to determine that mod22-1, a modifier of microtubule nucleation phenotypes, encodes a truncation allele of Swr1, a chromatin-remodelling factor involved in nucleosomal deposition of H2A.Z histone variant Pht1. Expression microarray analysis of mod22-1, swr1Δ and pht1Δ cells suggests that the modifier phenotype of mod22-1 mutants may be due to small changes in expression of one or more genes involved in tubulin function. Copyright © 2009 John Wiley & Sons, Ltd.

Glycolytic enzyme GAPDH promotes peroxide stress signaling through multistep phosphorelay to a MAPK cascade

Phosphorelay signaling of environmental stimuli by two-component systems is prevailing in bacteria and also utilized by fungi and plants. In the fission yeast Schizosaccharomyces pombe, peroxide stress signals are transmitted from the Mak2/3 sensor kinases to the Mpr1 histidine-containing phosphotransfer (HPt) protein and finally to the Mcs4 response regulator, which activates a MAP kinase cascade. Here we show that, unexpectedly, the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) physically associates with the Mcs4 response regulator and stress-responsive MAP kinase kinase kinases (MAPKKKs). In response to H2O2 stress, Cys-152 of the Tdh1 GAPDH is transiently oxidized, which enhances the association of Tdh1 with Mcs4. Furthermore, Tdh1 is essential for the interaction between the Mpr1 HPt protein and the Mcs4 response regulator and thus for phosphorelay signaling. These results demonstrate that the glycolytic enzyme GAPDH plays an essential role in the phosphorelay signaling, where its redox-sensitive cysteine residue may provide additional input signals.

Comparative genomics of the environmental stress response in ascomycete fungi

Unicellular fungi thrive in diverse niches around the world, and many of these niches present unique and stressful challenges that must be contended with by their inhabitants. Numerous studies have investigated the genomic expression responses to environmental stress in 'model' ascomycete fungi, including Saccharomyces cerevisiae, Candida albicans and Schizosaccharomyces pombe. This review presents a comparative-genomics perspective on the environmental stress response, a common response to diverse stresses. Implications for the role of this response, based on its presence or absence in fungi from disparate ecological niches, are discussed.

Molecular mechanisms underlying the mitosis-meiosis decision

Most eukaryotic cells possess genetic potential to perform meiosis, but the vast majority of them never initiate it. The entry to meiosis is strictly regulated by developmental and environmental conditions, which vary significantly from species to species. Molecular mechanisms underlying the mitosis-meiosis decision are unclear in most organisms, except for a few model systems including fission yeast Schizosaccharomyces pombe. Nutrient limitation is a cue to the entry into meiosis in this microbe. Signals from nutrients converge on the activity of Mei2 protein, which plays pivotal roles in both induction and progression of meiosis. Here we outline the current knowledge of how a set of environmental stimuli eventually activates Mei2, and discuss how Mei2 governs the meiotic program molecularly, especially focusing on a recent finding that Mei2 antagonizes selective elimination of meiotic messenger RNAs.

An isoquinolinium derivative selectively inhibits MAPK Spc1 of the stress-activated MAPK cascade of Schizosaccharomyces pombe

We have extended the search for selective inhibitors of the kinases of MAPK cascades by screening a derivative library of one of the isoquinoline rings of the protoberberine backbone. HWY 5069 inhibited the proliferation of wild-type and all mutants of Schizosaccharomyces pombe examined, except spc1Delta, at a minimal inhibitory concentration (MIC) of 3.76 microM. HWY 5069 also completely inhibited Spc1 kinase activity in vitro with an IC(50) of 16.4 microM as a competitive inhibitor of substrate binding. It was highly selective for Spc1 and did not affect the activity of other kinases in the MAPK cascades of fission yeast and mammals, including functional homologs of Spc1.

Cip1 and Cip2 are novel RNA-recognition-motif proteins that counteract Csx1 function during oxidative stress

Eukaryotic cells reprogram their global patterns of gene expression in response to stress. Recent studies in Schizosaccharomyces pombe showed that the RNA-binding protein Csx1 plays a central role in controlling gene expression during oxidative stress. It does so by stabilizing atf1(+) mRNA, which encodes a subunit of a bZIP transcription factor required for gene expression during oxidative stress. Here, we describe two related proteins, Cip1 and Cip2, that were identified by multidimensional protein identification technology (MudPIT) as proteins that coprecipitate with Csx1. Cip1 and Cip2 are cytoplasmic proteins that have RNA recognition motifs (RRMs). Neither protein is essential for viability, but a cip1Delta cip2Delta strain grows poorly and has altered cellular morphology. Genetic epistasis studies and whole genome expression profiling show that Cip1 and Cip2 exert posttranscriptional control of gene expression in a manner that is counteracted by Csx1. Notably, the sensitivity of csx1Delta cells to oxidative stress and their inability to induce expression of Atf1-dependent genes are partially rescued by cip1Delta and cip2Delta mutations. This study emphasizes the importance of a modulated mRNA stability in the eukaryotic stress response pathways and adds new information to the role of RNA-binding proteins in the oxidative stress response.

Aspergillus nidulans HOG pathway is activated only by two-component signalling pathway in response to osmotic stress

Genome sequencing analyses revealed that Aspergillus nidulans has orthologous genes to all those of the high-osmolarity glycerol (HOG) response mitogen-activated protein kinase (MAPK) pathway of Saccharomyces cerevisiae. A. nidulans mutant strains lacking sskA, sskB, pbsB, or hogA, encoding proteins orthologous to the yeast Ssk1p response regulator, Ssk2p/Ssk22p MAPKKKs, Pbs2p MAPKK and Hog1p MAPK, respectively, showed growth inhibition under high osmolarity, and HogA MAPK in these mutants was not phosphorylated under osmotic or oxidative stress. Thus, activation of the A. nidulans HOG (AnHOG) pathway depends solely on the two-component signalling system, and MAPKK activation mechanisms in the AnHOG pathway differ from those in the yeast HOG pathway, where Pbs2p is activated by two branches, Sln1p and Sho1p. Expression of pbsB complemented the high-osmolarity sensitivity of yeast pbs2Delta, and the complementation depended on Ssk2p/Ssk22p, but not on Sho1p. Pbs2p requires its Pro-rich motif for binding to the Src-homology3 (SH3) domain of Sho1p, but PbsB lacks a typical Pro-rich motif. However, a PbsB mutant (PbsB(Pro)) with the yeast Pro-rich motif was activated by the Sho1p branch in yeast. In contrast, HogA in sskADelta expressing PbsB(Pro) was not phosphorylated under osmotic stress, suggesting that A. nidulans ShoA, orthologous to yeast Sho1p, is not involved in osmoresponsive activation of the AnHOG pathway. We also found that besides HogA, PbsB can activate another Hog1p MAPK orthologue, MpkC, in A. nidulans, although mpkC is dispensable in osmoadaptation. In this study, we discuss the differences between the AnHOG and the yeast HOG pathways.

Systematic deletion analysis of fission yeast protein kinases

Eukaryotic protein kinases are key molecules mediating signal transduction that play a pivotal role in the regulation of various biological processes, including cell cycle progression, cellular morphogenesis, development, and cellular response to environmental changes. A total of 106 eukaryotic protein kinase catalytic-domain-containing proteins have been found in the entire fission yeast genome, 44% (or 64%) of which possess orthologues (or nearest homologues) in humans, based on sequence similarity within catalytic domains. Systematic deletion analysis of all putative protein kinase-encoding genes have revealed that 17 out of 106 were essential for viability, including three previously uncharacterized putative protein kinases. Although the remaining 89 protein kinase mutants were able to form colonies under optimal growth conditions, 46% of the mutants exhibited hypersensitivity to at least 1 of the 17 different stress factors tested. Phenotypic assessment of these mutants allowed us to arrange kinases into functional groups. Based on the results of this assay, we propose also the existence of four major signaling pathways that are involved in the response to 17 stresses tested. Microarray analysis demonstrated a significant correlation between the expression signature and growth phenotype of kinase mutants tested. Our complete microarray data sets are available at http://giscompute.gis.a-star.edu.sg/~gisljh/kinome.

Response of fission yeast to toxic cations involves cooperative action of the stress-activated protein kinase Spc1/Sty1 and the Hal4 protein kinase

Stress-activated protein kinases (SAPKs), members of a mitogen-activated protein kinase (MAPK) subfamily, are highly conserved among eukaryotes. Studies of yeasts demonstrated that SAPKs play pivotal roles in survival responses to high osmolarity, oxidative stress, and heat shock. Here we report a novel physiological role of the fission yeast Spc1 SAPK in cellular resistance to certain cations, such as Na(+), Li(+), and Ca(2+). Strains lacking Spc1 or its activator, Wis1 MAPK kinase, are hypersensitive to these cations. Spc1 positively regulates expression of sod2(+) encoding a Na(+)/H(+) antiporter through Atf1 and other transcription factors. In addition, we have identified a novel Spc1-interacting protein, Hal4, which is highly homologous to the budding yeast Sat4/Hal4 protein kinase. Like its budding yeast counterpart, the fission yeast Hal4 kinase is essential for cellular resistance to Na(+), Li(+), and Ca(2+). The hal4-null phenotype is complemented by overexpression of the Trk1 potassium transporter or increased K(+) in the growth medium, suggesting that Hal4 promotes K(+) uptake, which consequently increases cellular resistance to other cations. Interestingly, the Spc1-Hal4 interaction appears to be required for cellular resistance to Ca(2+) but not Na(+) and Li(+). We propose that Spc1 SAPK and Hal4 kinase cooperatively function to protect cells from the toxic cations.

Wsh3/Tea4 Is a Novel Cell-End Factor Essential for Bipolar Distribution of Tea1 and Protects Cell Polarity under Environmental Stress in S. pombe

BACKGROUND

The fission yeast Schizosaccharomyces pombe has a cylindrical cell shape, for which growth is strictly limited to both ends, and serves as an excellent model system for genetic analysis of cell-polarity determination. Previous studies identified a cell-end marker protein, Tea1, that is transported by cytoplasmic microtubules to cell tips and recruits other cell-end factors, including the Dyrk-family Pom1 kinase. The deltatea1 mutant cells cannot grow in a bipolar fashion and show T-shaped morphology after heat shock.

RESULTS

We identified Wsh3/Tea4 as a novel protein that interacts with Win1 MAP kinase kinase kinase (MAPKKK) of the stress-activated MAP kinase cascade. Wsh3 forms a complex with Tea1 and is transported to cell tips by growing microtubules. The deltawsh3 mutant shows monopolar growth with abnormal Tea1 aggregate at the non-growing cell end; this abnormal aggregate fails to recruit Pom1 kinase. Consistent with the observed interaction between Win1 and Wsh3, cells lacking Wsh3 or Tea1 show more severe cell-polarity defects under osmolarity and heat-stress stimuli that are known to activate the stress MAPK cascade. Furthermore, mutants of the stress MAPK also exhibit cell-polarity defects when exposed to the same stress.

CONCLUSIONS

Wsh3/Tea4 is an essential component of the Tea1 cell-end complex. In addition to its role in bipolar growth during the normal cell cycle, the Wsh3-Tea1 complex, together with the stress-signaling MAPK cascade, contributes to cell-polarity maintenance under stress conditions.

A novel gene, msa1, inhibits sexual differentiation in Schizosaccharomyces pombe

Sexual differentiation in the fission yeast Schizosaccharomyces pombe is triggered by nutrient starvation or by the presence of mating pheromones. We identified a novel gene, msa1, which encodes a 533-aa putative RNA-binding protein that inhibits sexual differentiation. Disruption of the msa1 gene caused cells to hypersporulate. Intracellular levels of msa1 RNA and Msa1 protein diminished after several hours of nitrogen starvation. Genetic analysis suggested that the function of msa1 is independent of the cAMP pathway and stress-responsive pathway. Deletion of the ras1 gene in diploid cells inhibited sporulation and in haploid cells decreased expression of mating-pheromone-induced genes such as mei2, mam2, ste11, and rep1; simultaneous deletion of msa1 reversed both phenotypes. Overexpression of msa1 decreased activated Ras1(Val17)-induced expression of mam2. Phenotypic hypersporulation was similar between cells with deletion of only rad24 and both msa1 and rad24, but simultaneous deletion of msa1 and msa2/nrd1 additively increased hypersporulation. Therefore, we suggest that the primary function of Msa1 is to negatively regulate sexual differentiation by controlling the expression of Ste11-regulated genes, possibly through the pheromone-signaling pathway.

Yeast signaling pathways in the oxidative stress response

Oxidative stress that generates the reactive oxygen species (ROS) is one of the major causes of DNA damage and mutations. The "DNA damage checkpoint" that arrests cell cycle and repairs damaged DNA has been a focus of recent studies, and the genetically amenable model systems provided by yeasts have been playing a leading role in the eukaryotic checkpoint research. However, means to eliminate ROS are likely to be as important as the DNA repair mechanisms in order to suppress mutations in the chromosomal DNA, and yeasts also serve as excellent models to understand how eukaryotes combat oxidative stress. In this article, we present an overview of the signaling pathways that sense oxidative stress and induce expression of various anti-oxidant genes in the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe and the pathogenic yeast Candida albicans. Three conserved signaling modules have been identified in the oxidative stress response of these diverse yeast species: the stress-responsive MAP kinase cascade, the multistep phosphorelay and the AP-1-like transcription factor. The structure and function of these signaling modules are discussed.

Activation of the redox sensor Pap1 by hydrogen peroxide requires modulation of the intracellular oxidant concentration

The transcription factor Pap1 and the MAP kinase Sty1 are key regulators of hydrogen peroxide-induced responses in Schizosaccharomyces pombe. Pap1 can be activated quickly at low, but not high, hydrogen peroxide concentrations. The MAP kinase Sty1 has been reported to participate in Pap1 activation by the oxidant. Here, we provide biochemical and genetic evidence for the in vivo formation of a hydrogen peroxide-induced disulphide bond in Pap1, which precedes the rapid and reversible nuclear accumulation of the transcription factor. We show that activation of the Sty1 cascade before the oxidative insult, or overexpression of the Sty1-regulated genes ctt1 (encoding catalase) or gpx1 (encoding glutathione peroxidase), can accelerate Pap1 entry even at high doses of hydrogen peroxide. In fact, the lack of Sty1 impedes Pap1 nuclear localization, but only at high doses of the oxidant. We propose that, whereas low doses of hydrogen peroxide lead directly to Pap1 oxidation-activation, high concentrations of the oxidant initially activate the Sty1 pathway, with the consequent increase in scavenging enzymes, which in turn helps to decompose the excess of hydrogen peroxide and achieve an appropriate concentration for the subsequent activation of Pap1. Our results also suggest that activation of Sty1 at high doses of hydrogen peroxide may also be required to trigger other antioxidant activities such as those reverting the overoxidation of cysteine residues at the Pap1 pathway.

A 2-Cys Peroxiredoxin Regulates Peroxide-Induced Oxidation and Activation of a Stress-Activated MAP Kinase

Oxidative stress-induced cell damage is an important component of many diseases and ageing. In eukaryotes, activation of JNK/p38 stress-activated protein kinase (SAPK) signaling pathways is critical for the cellular response to stress. 2-Cys peroxiredoxins (2-Cys Prx) are highly conserved, extremely abundant antioxidant enzymes that catalyze the breakdown of peroxides to protect cells from oxidative stress. Here we reveal that Tpx1, the single 2-Cys Prx in Schizosaccharomyces pombe, is required for the peroxide-induced activation of the p38/JNK homolog, Sty1. Tpx1 activates Sty1, downstream of previously identified redox sensors, by a mechanism that involves formation of a peroxide-induced disulphide complex between Tpx1 and Sty1. We have identified conserved cysteines in Tpx1 and Sty1 that are essential for normal peroxide-induced Tpx1-Sty1 disulphide formation and Tpx1-dependent regulation of peroxide-induced Sty1 activation. Thus we provide new insight into the response of SAPKs to diverse stimuli by revealing a mechanism for SAPK activation specifically by oxidative stress.

The fission yeast TOR proteins and the rapamycin response: an unexpected tale

The TOR proteins are known as key regulators of cell growth in response to nutritional and mitogenic signals and as targets for the immunosuppressive and anti-cancerous drug rapamycin. The fission yeast Schizosaccharomyces pombe has two TOR homologues, tor1+ and tor2+. Despite their structural similarity, these genes have distinct functions: tor1+ is required under starvation, extreme temperatures, and osmotic or oxidative stress conditions, whereas tor2+ is required under normal growth conditions. Surprisingly, rapamycin does not seem to inhibit the S. pombe TOR-related functions. Rapamycin specifically inhibits sexual development in S. pombe, and this seems to stem from direct inhibition of the S. pombe FKBP12 homologue. Why S. pombe cells are resistant to rapamycin during the growth phase is as yet unclear and awaits further analysis of the TOR-dependent signaling pathways.

LAMMER kinase homolog, Lkh1, is involved in oxidative-stress response of fission yeast

Previously, we reported that the LAMMER kinase homolog, Lkh1, is a negative regulator of filamentous growth and asexual flocculation in the fission yeast, Schizosaccharomyces pombe. Here, we report that the lkh1(+) null mutant is sensitive to oxidative stress because of a reduction in the expression of genes for antioxidant enzymes such as catalase (ctt1(+)) and Cu,Zn-superoxide dismutase (sod1(+)). Furthermore, the lkh1(+) null mutant shows increased levels of intracellular peroxides under conditions of oxidative stress compared with wild-type cells. Interestingly, expression of the gene for the transcription factor Atf1 is reduced in the lkh1(+) null mutant under oxidative stress, whereas expression of the transcription factor Pap1 is not. We report the novel finding that Lkh1 is involved in the oxidative-stress response of the fission yeast, S. pombe, and regulates the expression of antioxidant enzymes via the transcription factor Atf1.

CZK3, a MAP kinase kinase kinase homolog in Cercospora zeae-maydis, regulates cercosporin biosynthesis, fungal development, and pathogenesis

The fungus Cercospora zeae-maydis causes gray leaf spot of maize and produces cercosporin, a photosensitizing perylenequinone with toxic activity against a broad spectrum of organisms. However, little is known about the biosynthetic pathway or factors that regulate cercosporin production. Analysis of a cDNA subtraction library comprised of genes that are up-regulated during cercosporin synthesis revealed a sequence highly similar to mitogen-activated protein (MAP) kinases in other fungi. Sequencing and conceptual translation of the full-length genomic sequence indicated that the gene, which we designated CZK3, contains a 4,119-bp open reading frame devoid of introns and encodes a 1,373-amino acid sequence that is highly similar to Wis4, a MAP kinase kinase kinase in Schizosaccharomyces pombe. Targeted disruption of CZK3 suppressed expression of genes predicted to participate in cercosporin biosynthesis and abolished cercosporin production. The disrupted mutants grew faster on agar media than the wild type but were deficient in conidiation and elicited only small chlorotic spots on inoculated maize leaves compared with rectangular necrotic lesions incited by the wild type. Complementation of disruptants with the CZK3 open reading frame and flanking sequences restored wild-type levels of conidiation, growth rate, and virulence as well as the ability to produce cercosporin. The results suggest that cercosporin is a virulence factor in C. zeae-maydis during maize pathogenesis, but the pleiotropic effects of CZK3 disruption precluded definitive conclusions.

The Srk1 protein kinase is a target for the Sty1 stress-activated MAPK in fission yeast

The fission yeast stress-activated Sty1/Spc1 MAPK pathway responds to a similar range of stresses as do the mammalian p38 and SAPK/JNK MAPK pathways. In addition, sty1(-) cells are sterile and exhibit a G(2) cell cycle delay, indicating additional roles of Sty1 in meiosis and cell cycle progression. To identify novel proteins involved in stress responses, a microarray analysis of the Schizosaccharomyces pombe genome was performed to find genes that are up-regulated following exposure to stress in a Sty1-dependent manner. One such gene identified, srk1(+) (Sty1-regulated kinase 1), encodes a putative serine/threonine kinase homologous to mammalian calmodulin kinases. At the C terminus of Srk1 is a putative MAPK binding motif similar to that in the p38 substrates, MAPK-activated protein kinases 2 and 3. Indeed, we find that Srk1 is present in a complex with the Sty1 MAPK and is directly phosphorylated by Sty1. Furthermore, upon stress, Srk1 translocates from the cytoplasm to the nucleus in a process that is dependent on the Sty1 MAPK. Finally, we show that Srk1 has a role in regulating meiosis in fission yeast; following nitrogen limitation, srk1(-) cells enter meiosis significantly faster than wild-type cells and overexpression of srk1(+) inhibits the nitrogen starvation-induced arrest in G(1).

Cytoplasmic localization of Wis1 MAPKK by nuclear export signal is important for nuclear targeting of Spc1/Sty1 MAPK in fission yeast

Mitogen-activated protein kinase (MAPK) cascade is a ubiquitous signaling module that transmits extracellular stimuli through the cytoplasm to the nucleus; in response to activating stimuli, MAPKs translocate into the nucleus. Mammalian MEK MAPK kinases (MAPKKs) have in their N termini an MAPK-docking site and a nuclear export signal (NES) sequence, which are known to play critical roles in maintaining ERK MAPKs in the cytoplasm of unstimulated cells. Herein, we show that the Wis1 MAPKK of the stress-activated Spc1 MAPK cascade in fission yeast also has a MAPK-docking site and an NES sequence in its N-terminal domain. Unexpectedly, an inactivating mutation to the NES of chromosomal wis1(+) does not affect the subcellular localization of Spc1 MAPK, whereas this NES mutation disturbs the cytoplasmic localization of Wis1. However, when Wis1 is targeted to the nucleus by fusing to a nuclear localization signal sequence, stress-induced nuclear translocation of Spc1 is abrogated, indicating that cytoplasmic Wis1 is required for nuclear transport of Spc1 upon stress. Moreover, we have observed that a fraction of Wis1 translocates into the nucleus in response to stress. These results suggest that cytoplasmic localization of Wis1 MAPKK by its NES is important for stress signaling to the nucleus.

Osmotic stress signaling and osmoadaptation in yeasts

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

The serine/threonine kinase Cmk2 is required for oxidative stress response in fission yeast

Cmk2, a fission yeast Ser/Thr protein kinase homologous to mammalian calmodulin kinases, is essential for oxidative stress response. Cells lacking cmk2 gene were specifically sensitive to oxidative stress conditions. Upon stress, Cmk2 was phosphorylated in vivo, and this phosphorylation was dependent on the stress-activated MAPK Sty1/Spc1. Co-precipitation assays demonstrated that Cmk2 binds Sty1. Furthermore, in vivo or in vitro activated Sty1 was able to phosphorylate Cmk2, and the phosphorylation occurred at the C-terminal regulatory domain at Thr-411. Cell lethality caused by overexpression of Wis1 MAPK kinase was abolished by deletion of cmk2 or by mutation of Thr-411 of Cmk2. Taken together, our data suggest that Cmk2 acts downstream of Sty1 and is an essential kinase for oxidative stress responses.

Selective inhibition of MAPKK Wis1 in the stress-activated MAPK cascade of Schizosaccharomyces pombe by novel berberine derivatives

Intracellular molecular targets of novel berberine derivatives, HWY 289 and HWY 336, were identified by a screen of a variety of mutants in fission yeast Schizosaccharomyces pombe. HWY 289 and HWY 336 completely inhibited the proliferation of wild type as well as various mutant fission yeast cells (minimal inhibitory concentrations were 29.52 microm for HWY 289 and 11.83 microm for HWY 336), but did not affect the proliferation of Wis1 mitogen-activated protein kinase kinase (MAPKK) deletion mutants. In addition, HWY 289 with an IC(50) value of 7.3 microm or HWY 336 with IC(50) of 5.7 microm specifically inhibited in vitro kinase activities of purified Wis1, whereas either compound did not affect the activities of other kinases in the mitogen-activated protein kinase (MAPK) cascades of fission yeast. These genetic and biochemical results demonstrate the high degree of specificity of HWY 289 and HWY 336 to MAPKK Wis1 and suggest that the cytotoxicity of these compounds is not simply due to the inhibition of Wis1 kinase activity. High salt wash experiments have shown that strong noncovalent binding occurs between Wis1 and either HWY 289 or HWY 336. The preincubation of Wis1 kinase with ATP did not affect the inhibition of Wis1 by HWY 289 and HWY 336, but when Wis1 was preincubated with MBP, a protein substrate, Wis1 kinase activity was no longer inhibited. These observations demonstrate that HWY 289/HWY 336 do inhibit Wis1 kinase, not by binding to the ATP-binding site but by disturbing the binding of substrate to the kinase. Target validation of the complex of HWY 289/HWY 336 and Wis1 kinase will provide important clues for the mechanism of specific cytotoxicity of these compounds in S. pombe. On a broader aspect, it would create an initiative to further modify and develop compounds that selectively inhibit kinases and cause cytotoxicity in various MAPK cascades including those of mammals.

Distinct regulatory proteins control the graded transcriptional response to increasing H(2)O(2) levels in fission yeast Schizosaccharomyces pombe

The signaling pathways that sense adverse stimuli and communicate with the nucleus to initiate appropriate changes in gene expression are central to the cellular stress response. Herein, we have characterized the role of the Sty1 (Spc1) stress-activated mitogen-activated protein kinase pathway, and the Pap1 and Atf1 transcription factors, in regulating the response to H(2)O(2) in the fission yeast Schizosaccharomyces pombe. We find that H(2)O(2) activates the Sty1 pathway in a dose-dependent manner via at least two sensing mechanisms. At relatively low levels of H(2)O(2), a two component-signaling pathway, which feeds into either of the two stress-activated mitogen-activated protein kinase kinase kinases Wak1 or Win1, regulates Sty1 phosphorylation. In contrast, at high levels of H(2)O(2), Sty1 activation is controlled predominantly by a two-component independent mechanism and requires the function of both Wak1 and Win1. Individual transcription factors were also found to function within a limited range of H(2)O(2) concentrations. Pap1 activates target genes primarily in response to low levels of H(2)O(2), whereas Atf1 primarily controls the transcriptional response to high concentrations of H(2)O(2). Our results demonstrate that S. pombe uses a combination of stress-responsive regulatory proteins to gauge and effect the appropriate transcriptional response to increasing concentrations of H(2)O(2).

A PBS2 homologue from Debaryomyces hansenii shows a differential effect on calcofluor and polymyxin B sensitivity in Saccharomyces cerevisiae

The PBS2 gene encodes a MAP kinase kinase that plays a pivotal role in osmosensing signal-transduction pathway in the yeast Saccharomyces cerevisiae. Mutation in the PBS2 gene has a pleotropic effect. Besides being osmosensitive, pbs2 mutants show altered sensitivity to polymyxin B and calcofluor. Recent studies revealed that Pbs2p plays a different role in osmoadaptation and calcofluor sensitivity. We have isolated a gene homologous to PBS2 from the highly salt-tolerant yeast Debaryomyces hansenii by phenotypic complementation. DNA sequencing of the clone revealed that the gene encoded a protein of 683 amino acid residues. Like Pbs2p, this protein also has a proline-rich motif. Further characterization revealed that this gene could complement polymyxin B sensitivity but did not affect calcofluor sensitivity. Thus, it appeared that Pbs2p also has an independent role in these two physiological processes. The GenBank Accession No. of this sequence is AF371315.

Fungal histidine kinases

Eukaryotic cells predominantly use serine, threonine, and tyrosine phosphorylation in various intracellular signal transduction pathways. In contrast, prokaryotic organisms employ numerous "two-component" systems, in which signaling is achieved by transferring a phosphoryl group from phosphohistidine in the "sensor kinase" component to aspartate in the "response regulator" component. In the last several years, genetic screens and genome projects have identified sensor kinases and response regulators in lower eukaryotes and plants, revealing that eukaryotic organisms also make use of His-Asp phosphotransfer in a limited number of signaling pathways. Extensive studies in yeasts have demonstrated that a variation of the two-component system, a multistep "phosphorelay," is the prevailing mechanism among distantly related yeast species. In the budding yeast Saccharomyces cerevisiae, a His-Asp-His-Asp phosphorelay transmits osmotic stress signals to a mitogen-activated protein kinase (MAPK) cascade to induce adaptive responses. A phosphorelay in the fission yeast Schizosaccharomyces pombe, analogous to the S. cerevisiae phosphorelay, is responsible for MAPK activation in response to peroxide stress. Mammalian cells do not have any two-component or phosphorelay systems, although protein histidine kinases unrelated to the sensor kinase may be involved in cellular signaling. Because some phosphorelay proteins are essential for virulence of microbial pathogens, including the yeast fungus Candida albicans, novel antibiotics targeted to phosphorelays may be effective against eukaryotic pathogens without causing host cell damage.

Bob1, a Gim5/MM-1/Pfd5 homolog, interacts with the MAP kinase kinase Byr1 to regulate sexual differentiation in the fission yeast, Schizosaccharomyces pombe

The MAPKK Byr1 is an essential component of a Ras-dependent MAPK module required for sexual differentiation in the fission yeast, Schizosaccharomyces pombe. Here we describe the genetic and molecular characterization of a highly conserved protein, Bob1, which was identified from a two-hybrid screen for Byr1-interacting proteins. Byrl and Bobl proteins coprecipitate from S. pombe cell lysates, and both proteins localize to the tips and septa of S. pombe cells. S. pombe bob1 null (bob1delta) mutants lack obvious growth defects but exhibit a significant mating deficiency, which can be suppressed by overexpression of Byrl. Overexpression of Bob1 also leads to inhibition of mating in S. pombe, and this defect is likewise suppressed by Byrl overexpression. Bob1 is highly homologous in structure to the mammalian MM-1/Pfd5 and budding yeast Gim5/Pfd5-Sc proteins, which have been implicated as regulators of actin and tubulins. Similar to budding yeast gim5/pfd5-Sc mutants, S. pombe bob1delta cells have cytoskeletal defects, as judged by hypersensitivity to cytoskeletal disrupting drugs. byr1delta mutants do not share this characteristic with bob1delta mutants, and byr1delta bob1delta mutants are not significantly more sensitive to cytoskeletal disrupting drugs than cells carrying only the bob1delta mutation. Taken together, our results suggest that Bob1 has Byr1-related function(s) required for proper mating response of S. pombe cells and Byrl-independent function(s) required for normal cytoskeletal control. We show that the human MM-1/Pfd5 protein can substitute for its counterpart in fission yeast, providing evidence that the functions of Bob1-related proteins have been highly conserved through evolution. Our results lead us to propose that Bob1-related proteins may play diverse roles in eukaryotic organisms.

Redox control of AP-1-like factors in yeast and beyond

Cells have evolved complex and efficient strategies for dealing with variable and often-harsh environments. A key aspect of these stress responses is the transcriptional activation of genes encoding defense and repair proteins. In yeast members of the AP-1 family of proteins are required for the transcriptional response to oxidative stress. This sub-family of AP-1 (called yAP-1) proteins are sensors of the redox-state of the cell and are activated directly by oxidative stress conditions. yAP-1 proteins are bZIP-containing factors that share homology to the mammalian AP-1 factor complex and bind to very similar DNA sequence sites. The generation of reactive oxygen species and the resulting potential for oxidative stress is common to all aerobically growing organisms. Furthermore, many of the features of this response appear to be evolutionarily conserved and consequently the study of model organisms, such as yeast, will have widespread utility. The important structural features of these factors, signaling pathways controlling their activity and the nature of the target genes they control will be discussed.

Hyperthermotolerant fission yeast mutations, sow1 and sow2, suppress the cell cycle defect and stress sensitivity of MAP kinase kinase wis1Delta

Wis1 is a mitogen-activated protein kinase kinase (MAPKK) that regulates mitosis and mediates stress responses in the fission yeast, Schizosaccharomyces pombe. wis1Delta strains are viable but stress-sensitive and show a mitotic delay. At high temperatures, wis1Delta cells cease division but cellular growth continues. Mutations that suppress the heat sensitivity of a wis1Delta strain were isolated and map to two apparently novel loci, sow1 (for suppressor of wis1Delta) and sow2. In addition to suppressing wis1Delta heat sensitivity, sow1 and sow2 can suppress wis1Delta osmosensitivity and cell cycle defects. sow1 and sow2 mutants in a wis1+ background were able to grow at higher temperatures than wild-type and sow1 showed a mitotic advance. The sow genes may therefore define a novel connection between stress tolerance and cell cycle control.

Peroxide sensors for the fission yeast stress-activated mitogen-activated protein kinase pathway

The Schizosaccharomyces pombe stress-activated Sty1p/Spc1p mitogen-activated protein (MAP) kinase regulates gene expression through the Atf1p and Pap1p transcription factors, homologs of human ATF2 and c-Jun, respectively. Mcs4p, a response regulator protein, acts upstream of Sty1p by binding the Wak1p/Wis4p MAP kinase kinase kinase. We show that phosphorylation of Mcs4p on a conserved aspartic acid residue is required for activation of Sty1p only in response to peroxide stress. Mcs4p acts in a conserved phospho-relay system initiated by two PAS/PAC domain-containing histidine kinases, Mak2p and Mak3p. In the absence of Mak2p or Mak3p, Sty1p fails to phosphorylate the Atf1p transcription factor or induce Atf1p-dependent gene expression. As a consequence, cells lacking Mak2p and Mak3p are sensitive to peroxide attack in the absence of Prr1p, a distinct response regulator protein that functions in association with Pap1p. The Mak1p histidine kinase, which also contains PAS/PAC repeats, does not regulate Sty1p or Atf1p but is partially required for Pap1p- and Prr1p-dependent transcription. We conclude that the transcriptional response to free radical attack is initiated by at least two distinct phospho-relay pathways in fission yeast.

Protein kinase A and mitogen-activated protein kinase pathways antagonistically regulate fission yeast fbp1 transcription by employing different modes of action at two upstream activation sites

A significant challenge to our understanding of eukaryotic transcriptional regulation is to determine how multiple signal transduction pathways converge on a single promoter to regulate transcription in divergent fashions. To study this, we have investigated the transcriptional regulation of the Schizosaccharomyces pombe fbp1 gene that is repressed by a cyclic AMP (cAMP)-dependent protein kinase A (PKA) pathway and is activated by a stress-activated mitogen-activated protein kinase (MAPK) pathway. In this study, we identified and characterized two cis-acting elements in the fbp1 promoter required for activation of fbp1 transcription. Upstream activation site 1 (UAS1), located approximately 900 bp from the transcriptional start site, resembles a cAMP response element (CRE) that is the binding site for the atf1-pcr1 heterodimeric transcriptional activator. Binding of this activator to UAS1 is positively regulated by the MAPK pathway and negatively regulated by PKA. UAS2, located approximately 250 bp from the transcriptional start site, resembles a Saccharomyces cerevisiae stress response element. UAS2 is bound by transcriptional activators and repressors regulated by both the PKA and MAPK pathways, although atf1 itself is not present in these complexes. Transcriptional regulation of fbp1 promoter constructs containing only UAS1 or UAS2 confirms that the PKA and MAPK regulation is targeted to both sites. We conclude that the PKA and MAPK signal transduction pathways regulate fbp1 transcription at UAS1 and UAS2, but that the antagonistic interactions between these pathways involve different mechanisms at each site.

Multistep phosphorelay proteins transmit oxidative stress signals to the fission yeast stress-activated protein kinase

In response to oxidative stress, eukaryotic cells induce transcription of genes required for detoxification of oxidants. Here we present evidence that oxidative stress stimuli are transmitted by a multistep phosphorelay system to the Spc1/Sty1 stress-activated protein kinase in the fission yeast Schizosaccharomyces pombe. The fission yeast mpr1(+) gene encodes a novel protein with a histidine-containing phosphotransfer domain homologous to the budding yeast Ypd1. Spc1 activation upon oxidative stress is severely impaired in the Deltampr1 mutant as well as in the mpr1HQ strain, in which the putative phosphorylation site Mpr1-His221 is substituted with glutamine. In response to oxidative stress, Mpr1 binds to the Mcs4 response regulator that functions upstream of the Spc1 cascade, suggesting that Mcs4 is a cognate response regulator for Mpr1. Unexpectedly, when exposed to hydrogen peroxide, Deltampr1 cells can induce the catalase gene ctt1(+), one of the transcriptional targets of the Spc1 pathway, and survive oxidative stress in the absence of significant Spc1 activation. We have found that Pap1, a bZIP transcription factor homologous to human c-Jun, can mediate induction of ctt1(+) expression upon oxidative stress independently of the Spc1 stress-activated protein kinase. These studies show that oxidative stress stimuli are transmitted by multiple pathways to induce specific gene expression.

Skh1, the MEK component of the mkh1 signaling pathway in Schizosaccharomyces pombe

We previously reported the identification of Mkh1, a MEK kinase in Schizosaccharomyces pombe that is required for cell wall integrity, and we presented genetic evidence that Pmk1/Spm1, a MAP kinase, functions downstream from Mkh1 in the same pathway. Here, we report the identification of Skh1, a MEK (MAP kinase kinase) in S. pombe. The sequence of Skh1 is nearly identical to that of the recently reported Pek1 sequence. We present biochemical and genetic evidence that Skh1 is the MEK component of the Mkh1-Spm1 MAP kinase cascade. Our yeast two-hybrid results indicate that Mkh1, Skh1, and Spm1 physically interact to form a ternary complex. Deletion of mkh1, skh1 or spm1 results in identical phenotypes, including sensitivity to (beta)-glucanase treatment, growth inhibition on media containing KCl, and filamentous growth on medium containing caffeine. Double mutant strains exhibit phenotypes that are identical to the single mutant strains. Furthermore, expression of an activated HA-Skh1(DD)protein suppressed these defects in mkh1(delta) cells, and overexpression of Spm1 suppressed these defects in skh1(delta) cells. We also show that HA-Spm1 is hyper-phosphorylated on tyrosine residues in cells co-expressing the activated HA-Skh1(DD) protein. Furthermore, we found the phosphorylated/activated form of GFP-HA-Spm1 at detectable levels in wild-type cells, but not at appreciable levels in mkh1(delta) or skh1(delta) cells expressing this fusion protein. Together, our results indicate that Mkh1, Skh1 and Spm1 constitute a MAPK cascade in fission yeast.

MAP kinases in pollen

The male gametophyte of flowering plants has a highly regulated developmental programme to ensure efficient fertilization of the ovule and the faithful transmission of genetic material to the offspring. Cell cycle control mechanisms dictate the formation of the vegetative and generative (sperm) cells, while an increase in transcriptional/translational activity and the accumulation of stored proteins and mRNA is followed by a quiescent state at maturation. A switch to a new developmental programme occurs after the pollen tube lands on the stigma with the formation of the pollen tube, growth through the style, and subsequent fertilization. Apart from the internal control mechanisms involved in this developmental programme, pollen grains must cope with physical changes during development within the anther (desiccation) and subsequently during germination on the stigma (rehydration). The metabolic and structural changes that occur throughout these processes should require signaling mechanisms to co-ordinate the appropriate response, and recent data demonstrate the presence in pollen of an array of molecules belonging to diverse signalling pathways, including mitogen-activated protein (MAP) kinases. The role of MAP kinases in pollen is discussed in the context of the various developmental and physical changes that occur throughout pollen maturation and germination.