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

A close-up of regulatory networks and signaling pathways of MKK5 in biotic and abiotic stresses

Mitogen-activated protein Kinase Kinase 5 (MKK5) is a central hub in the complex phosphorylation chain reaction of the Mitogen-activated protein kinases (MAPK) cascade, regulating plant responses to biotic and abiotic stresses. This review manuscript aims to provide a comprehensive analysis of the regulatory mechanism of the MKK5 involved in stress adaptation. This review will delve into the intricate post-transcriptional and post-translational modifications of the MKK5, discussing how they affect its expression, activity, and subcellular localization in response to stress signals. We also discuss the integration of the MKK5 into complex signaling pathways, orchestrating plant immunity against pathogens and its modulating role in regulating abiotic stresses, such as: drought, cold, heat, and salinity, through the phytohormonal signaling pathways. Furthermore, we highlight potential applications of the MKK5 for engineering stress-resilient crops and provide future perspectives that may pave the way for future studies. This review manuscript aims to provide valuable insights into the mechanisms underlying MKK5 regulation, bridge the gap from numerous previous findings, and offer a firm base in the knowledge of MKK5, its regulating roles, and its involvement in environmental stress regulation.

Stress contingent changes in Hog1 pathway architecture and regulation in Candida albicans

The Hog1 stress-activated protein kinase (SAPK) is a key mediator of stress resistance and virulence in Candida albicans. Hog1 activation via phosphorylation of the canonical TGY motif is mediated by the Pbs2 MAPKK, which itself is activated by the Ssk2 MAPKKK. Although this three-tiered SAPK signalling module is well characterised, it is unclear how Hog1 activation is regulated in response to different stresses. Functioning upstream of the Ssk2 MAPKKK is a two-component related signal transduction system comprising three sensor histidine kinases, a phosphotransfer protein Ypd1, and a response regulator Ssk1. Here, we report that Ssk1 is a master regulator of the Hog1 SAPK that promotes stress resistance and Hog1 phosphorylation in response to diverse stresses, except high osmotic stress. Notably, we find Ssk1 regulates Hog1 in a two-component independent manner by functioning to promote interactions between the Ssk2 and Pbs2 kinases. We propose this function of Ssk1 is important to maintain a basal level of Hog1 phosphorylation which is necessary for oxidative stress, but not osmotic stress, mediated Hog1 activation. We find that osmotic stress triggers robust Pbs2 phosphorylation which drives its dissociation from Ssk2. In contrast, Pbs2 is not robustly phosphorylated following oxidative stress and the Ssk1-mediated Ssk2-Pbs2 interaction remains intact. Instead, oxidative stress-stimulated increases in phosphorylated Hog1 is dependent on the inhibition of protein tyrosine phosphatases that negatively regulate Hog1 coupled with the Ssk1-mediated promotion of basal Hog1 activity. Furthermore, we find that inhibition of protein tyrosine phosphatases is linked to the hydrogen peroxide induced oxidation of these negative regulators in a mechanism that is partly dependent on thioredoxin. Taken together these data reveal stress contingent changes in Hog1 pathway architecture and regulation and uncover a novel mode of action of the Ssk1 response regulator in SAPK regulation.

Antioxidant Systems of Plant Pathogenic Fungi: Functions in Oxidative Stress Response and Their Regulatory Mechanisms

During the infection process, plant pathogenic fungi encounter plant-derived oxidative stress, and an appropriate response to this stress is crucial to their survival and establishment of the disease. Plant pathogenic fungi have evolved several mechanisms to eliminate oxidants from the external environment and maintain cellular redox homeostasis. When oxidative stress is perceived, various signaling transduction pathways are triggered and activate the downstream genes responsible for the oxidative stress response. Despite extensive research on antioxidant systems and their regulatory mechanisms in plant pathogenic fungi, the specific functions of individual antioxidants and their impacts on pathogenicity have not recently been systematically summarized. Therefore, our objective is to consolidate previous research on the antioxidant systems of plant pathogenic fungi. In this review, we explore the plant immune responses during fungal infection, with a focus on the generation and function of reactive oxygen species. Furthermore, we delve into the three antioxidant systems, summarizing their functions and regulatory mechanisms involved in oxidative stress response. This comprehensive review provides an integrated overview of the antioxidant mechanisms within plant pathogenic fungi, revealing how the oxidative stress response contributes to their pathogenicity.

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.

SakA Regulates Morphological Development, Ochratoxin A Biosynthesis and Pathogenicity of Aspergillus westerdijkiae and the Response to Different Environmental Stresses

Ochratoxin A (OTA), as a common mycotoxin, has seriously harmful effects on agricultural products, livestock and humans. There are reports on the regulation of SakA in the MAPK pathway, which regulates the production of mycotoxins. However, the role of SakA in the regulation of Aspergillus westerdijkiae and OTA production is not clear. In this study, a SakA deletion mutant (ΔAwSakA) was constructed. The effects of different concentrations of D-sorbitol, NaCl, Congo red and H2O2 on the mycelia growth, conidia production and biosynthesis of OTA were investigated in A. westerdijkiae WT and ΔAwSakA. The results showed that 100 g/L NaCl and 3.6 M D-sorbitol significantly inhibited mycelium growth and that a concentration of 0.1% Congo red was sufficient to inhibit the mycelium growth. A reduction in mycelium development was observed in ΔAwSakA, especially in high concentrations of osmotic stress. A lack of AwSakA dramatically reduced OTA production by downregulating the expression of the biosynthetic genes otaA, otaY, otaB and otaD. However, otaC and the transcription factor otaR1 were slightly upregulated by 80 g/L NaCl and 2.4 M D-sorbitol, whereas they were downregulated by 0.1% Congo red and 2 mM H2O2. Furthermore, ΔAwSakA showed degenerative infection ability toward pears and grapes. These results suggest that AwSakA is involved in the regulation of fungal growth, OTA biosynthesis and the pathogenicity of A. westerdijkiae and could be influenced by specific environmental stresses.

Oxidative stress response pathways in fungi

Fungal response to any stress is intricate, specific, and multilayered, though it employs only a few evolutionarily conserved regulators. This comes with the assumption that one regulator operates more than one stress-specific response. Although the assumption holds true, the current understanding of molecular mechanisms that drive response specificity and adequacy remains rudimentary. Deciphering the response of fungi to oxidative stress may help fill those knowledge gaps since it is one of the most encountered stress types in any kind of fungal niche. Data have been accumulating on the roles of the HOG pathway and Yap1- and Skn7-related pathways in mounting distinct and robust responses in fungi upon exposure to oxidative stress. Herein, we review recent and most relevant studies reporting the contribution of each of these pathways in response to oxidative stress in pathogenic and opportunistic fungi after giving a paralleled overview in two divergent models, the budding and fission yeasts. With the concept of stress-specific response and the importance of reactive oxygen species in fungal development, we first present a preface on the expanding domain of redox biology and oxidative stress.

The Fission Yeast Cell Integrity Pathway: A Functional Hub for Cell Survival upon Stress and Beyond

The survival of eukaryotic organisms during environmental changes is largely dependent on the adaptive responses elicited by signal transduction cascades, including those regulated by the Mitogen-Activated Protein Kinase (MAPK) pathways. The Cell Integrity Pathway (CIP), one of the three MAPK pathways found in the simple eukaryote fission of yeast Schizosaccharomyces pombe, shows strong homology with mammalian Extracellular signal-Regulated Kinases (ERKs). Remarkably, studies over the last few decades have gradually positioned the CIP as a multi-faceted pathway that impacts multiple functional aspects of the fission yeast life cycle during unperturbed growth and in response to stress. They include the control of mRNA-stability through RNA binding proteins, regulation of calcium homeostasis, and modulation of cell wall integrity and cytokinesis. Moreover, distinct evidence has disclosed the existence of sophisticated interplay between the CIP and other environmentally regulated pathways, including Stress-Activated MAP Kinase signaling (SAPK) and the Target of Rapamycin (TOR). In this review we present a current overview of the organization and underlying regulatory mechanisms of the CIP in S. pombe, describe its most prominent functions, and discuss possible targets of and roles for this pathway. The evolutionary conservation of CIP signaling in the dimorphic fission yeast S. japonicus will also be addressed.

Downregulation of the ypdA Gene Encoding an Intermediate of His-Asp Phosphorelay Signaling in Aspergillus nidulans Induces the Same Cellular Effects as the Phenylpyrrole Fungicide Fludioxonil

Many eukaryotic histidine-to-aspartate (His-Asp) phosphorelay systems consist of three types of signal transducers: a His-kinase (HK), a response regulator (RR), and a histidine-containing phosphotransfer intermediate (HPt). In general, the HPt acts as an intermediate between the HK and the RR and is indispensable for inducing appropriate responses to environmental stresses. In a previous study, we attempted but were unable to obtain deletion mutants of the ypdA gene in order to characterize its function in the filamentous fungus Aspergillus nidulans. In the present study, we constructed the CypdA strain in which ypdA expression is conditionally regulated by the A. nidulans alcA promoter. We constructed CypdA strains with RR gene disruptions (CypdA-sskAΔ, CypdA-srrAΔ, and CypdA-sskAΔsrrAΔ). Suppression of YpdA induced by ypdA downregulation activated the downstream HogA mitogen-activated protein kinase cascade. YpdA suppression caused severe growth defects and abnormal hyphae, with features such as enhanced septation, a decrease in number of nuclei, nuclear fragmentation, and hypertrophy of vacuoles, both regulated in an SskA-dependent manner. Fludioxonil treatment caused the same cellular responses as ypdA suppression. The growth-inhibitory effects of fludioxonil and the lethality caused by ypdA downregulation may be caused by the same or similar mechanisms and to be dependent on both the SskA and SrrA pathways.

H2O2 Induces Major Phosphorylation Changes in Critical Regulators of Signal Transduction, Gene Expression, Metabolism and Developmental Networks in Aspergillus nidulans

Reactive oxygen species (ROS) regulate several aspects of cell physiology in filamentous fungi including the antioxidant response and development. However, little is known about the signaling pathways involved in these processes. Here, we report Aspergillus nidulans global phosphoproteome during mycelial growth and show that under these conditions, H2O2 induces major changes in protein phosphorylation. Among the 1964 phosphoproteins we identified, H2O2 induced the phosphorylation of 131 proteins at one or more sites as well as the dephosphorylation of a larger set of proteins. A detailed analysis of these phosphoproteins shows that H2O2 affected the phosphorylation of critical regulatory nodes of phosphoinositide, MAPK, and TOR signaling as well as the phosphorylation of multiple proteins involved in the regulation of gene expression, primary and secondary metabolism, and development. Our results provide a novel and extensive protein phosphorylation landscape in A. nidulans, indicating that H2O2 induces a shift in general metabolism from anabolic to catabolic, and the activation of multiple stress survival pathways. Our results expand the significance of H2O2 in eukaryotic cell signaling.

Dense Transposon Integration Reveals Essential Cleavage and Polyadenylation Factors Promote Heterochromatin Formation

Heterochromatin functions as a scaffold for factors responsible for gene silencing and chromosome segregation. Heterochromatin can be assembled by multiple pathways, including RNAi and RNA surveillance. We identified factors that form heterochromatin using dense profiles of transposable element integration in Schizosaccharomyces pombe. The candidates include a large number of essential proteins such as four canonical mRNA cleavage and polyadenylation factors. We find that Iss1, a subunit of the poly(A) polymerase module, plays a role in forming heterochromatin in centromere repeats that is independent of RNAi. Genome-wide maps reveal that Iss1 accumulates at genes regulated by RNA surveillance. Iss1 interacts with RNA surveillance factors Mmi1 and Rrp6, and importantly, Iss1 contributes to RNA elimination that forms heterochromatin at meiosis genes. Our profile of transposable element integration supports the model that a network of mRNA cleavage and polyadenylation factors coordinates RNA surveillance, including the mechanism that forms heterochromatin at meiotic genes.

Lee et al. use dense profiles of transposon integration to identify genes important for the formation of heterochromatin. Among many candidates, Iss1 is a canonical mRNA cleavage and polyadenylation factor found to be important for heterochromatin at meiotic genes by recruiting the nuclear exosome.

Breakpoint: Cell Wall and Glycoproteins and their Crucial Role in the Phytopathogenic Fungi Infection

The cell wall that surrounds fungal cells is essential for their survival, provides protection against physical and chemical stresses, and plays relevant roles during infection. In general, the fungal cell wall is composed of an outer layer of glycoprotein and an inner skeletal layer of β-glucans or α- glucans and chitin. Chitin synthase genes have been shown to be important for septum formation, cell division and virulence. In the same way, chitin can act as a potent elicitor to activate defense response in several plant species; however, the fungi can convert chitin to chitosan during plant infection to evade plant defense mechanisms. Moreover, α-1,3-Glucan, a non-degradable polysaccharide in plants, represents a key feature in fungal cell walls formed in plants and plays a protective role for this fungus against plant lytic enzymes. A similar case is with β-1,3- and β-1,6-glucan which are essential for infection, structure rigidity and pathogenicity during fungal infection. Cell wall glycoproteins are also vital to fungi. They have been associated with conidial separation, the increase of chitin in conidial cell walls, germination, appressorium formation, as well as osmotic and cell wall stress and virulence; however, the specific roles of glycoproteins in filamentous fungi remain unknown. Fungi that can respond to environmental stimuli distinguish these signals and relay them through intracellular signaling pathways to change the cell wall composition. They play a crucial role in appressorium formation and penetration, and release cell wall degrading enzymes, which determine the outcome of the interaction with the host. In this review, we highlight the interaction of phypatophogen cell wall and signaling pathways with its host and their contribution to fungal pathogenesis.

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.

Stress-Activated Protein Kinases in Human Fungal Pathogens

The ability of fungal pathogens to survive hostile environments within the host depends on rapid and robust stress responses. Stress-activated protein kinase (SAPK) pathways are conserved MAPK signaling modules that promote stress adaptation in all eukaryotic cells, including pathogenic fungi. Activation of the SAPK occurs via the dual phosphorylation of conserved threonine and tyrosine residues within a TGY motif located in the catalytic domain. This induces the activation and nuclear accumulation of the kinase and the phosphorylation of diverse substrates, thus eliciting appropriate cellular responses. The Hog1 SAPK has been extensively characterized in the model yeast Saccharomyces cerevisiae. Here, we use this a platform from which to compare SAPK signaling mechanisms in three major fungal pathogens of humans, Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans. Despite the conservation of SAPK pathways within these pathogenic fungi, evidence is emerging that their role and regulation has significantly diverged. However, consistent with stress adaptation being a common virulence trait, SAPK pathways are important pathogenicity determinants in all these major human pathogens. Thus, the development of drugs which target fungal SAPKs has the exciting potential to generate broad-acting antifungal treatments.

Tracing the origin and evolution of pseudokinases across the tree of life

Protein phosphorylation by eukaryotic protein kinases (ePKs) is a fundamental mechanism of cell signaling in all organisms. In model vertebrates, ~10% of ePKs are classified as pseudokinases, which have amino acid changes within the catalytic machinery of the kinase domain that distinguish them from their canonical kinase counterparts. However, pseudokinases still regulate various signaling pathways, usually doing so in the absence of their own catalytic output. To investigate the prevalence, evolutionary relationships, and biological diversity of these pseudoenzymes, we performed a comprehensive analysis of putative pseudokinase sequences in available eukaryotic, bacterial, and archaeal proteomes. We found that pseudokinases are present across all domains of life, and we classified nearly 30,000 eukaryotic, 1500 bacterial, and 20 archaeal pseudokinase sequences into 86 pseudokinase families, including ~30 families that were previously unknown. We uncovered a rich variety of pseudokinases with notable expansions not only in animals but also in plants, fungi, and bacteria, where pseudokinases have previously received cursory attention. These expansions are accompanied by domain shuffling, which suggests roles for pseudokinases in plant innate immunity, plant-fungal interactions, and bacterial signaling. Mechanistically, the ancestral kinase fold has diverged in many distinct ways through the enrichment of unique sequence motifs to generate new families of pseudokinases in which the kinase domain is repurposed for noncanonical nucleotide binding or to stabilize unique, inactive kinase conformations. We further provide a collection of annotated pseudokinase sequences in the Protein Kinase Ontology (ProKinO) as a new mineable resource for the signaling community.

Functional interaction between Cdc42 and the stress MAPK signaling pathway during the regulation of fission yeast polarized growth

Cell polarization can be defined as the generation and maintenance of directional cellular organization. The spatial distribution and protein or lipid composition of the cell are not symmetric but organized in specialized domains which allow cells to grow and acquire a certain shape that is closely linked to their physiological function. The establishment and maintenance of polarized growth requires the coordination of diverse processes including cytoskeletal dynamics, membrane trafficking, and signaling cascade regulation. Some of the major players involved in the selection and maintenance of sites for polarized growth are Rho GTPases, which recognize the polarization site and transmit the signal to regulatory proteins of the cytoskeleton. Additionally, cytoskeletal organization, polarized secretion, and endocytosis are controlled by signaling pathways including those mediated by mitogen-activated protein kinases (MAPKs). Rho GTPases and the MAPK signaling pathways are strongly conserved from yeast to mammals, suggesting that the basic mechanisms of polarized growth have been maintained throughout evolution. For this reason, the study of how polarized growth is established and regulated in simple organisms such as the fission yeast Schizosaccharomyces pombe has contributed to broaden our knowledge about these processes in multicellular organisms. We review here the function of the Cdc42 GTPase and the stress activated MAPK (SAPK) signaling pathways during fission yeast polarized growth, and discuss the relevance of the crosstalk between both pathways.

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.

Influence of ylHog1 MAPK kinase on Yarrowia lipolytica stress response and erythritol production

Erythritol production is a unique response to hyperosmotic stress that is observed in a small group of yeasts, including Yarrowia lipolytica. This study investigated whether this unusual mechanism is regulated by the HOG pathway, well described in Saccharomyces cerevisiae. The gene YALI0E25135g was identified as the Y. lipolytica homologue of HOG1 and was found to be phosphorylated in response to hyperosmotic shock. Deletion of the gene caused a significant decrease in resistance to hyperosmotic stress and negatively affected erythritol production. Interestingly, the deletion strain yl-hog1Δ displayed significant morphological defects, with the cells growing in a filamentous form. Moreover, yl-hog1Δ cells were also resistant to the cell wall damaging agents Congo red and calcofluor white. Collectively, these results indicate that yl-Hog1 is crucial for the cellular response to hyperosmotic stress, plays a role in the induction of erythritol production, and potentially prevents cross-talk with different MAPK signalling pathways in the cell.

Kinome Expansion in the Fusarium oxysporum Species Complex Driven by Accessory Chromosomes

Isolates of Fusarium oxysporum are adapted to survive a wide range of host and nonhost conditions. In addition, F. oxysporum was recently recognized as the top emerging opportunistic fungal pathogen infecting immunocompromised humans. The sensory and response networks of these fungi undoubtedly play a fundamental role in establishing the adaptability of this group. We have examined the kinomes of 12 F. oxysporum isolates and highlighted kinase families that distinguish F. oxysporum from other fungi, as well as different isolates from one another. The amplification of kinases involved in environmental signal relay and regulating downstream cellular responses clearly sets Fusarium apart from other Ascomycetes. Although the functions of many of these kinases are still unclear, their specific proliferation highlights them as a result of the evolutionary forces that have shaped this species complex and clearly marks them as targets for exploitation in order to combat disease.

The Fusarium oxysporum species complex (FOSC) is a group of soilborne pathogens causing severe disease in more than 100 plant hosts, while individual strains exhibit strong host specificity. Both chromosome transfer and comparative genomics experiments have demonstrated that lineage-specific (LS) chromosomes contribute to the host-specific pathogenicity. However, little is known about the functional importance of genes encoded in these LS chromosomes. Focusing on signaling transduction, this study compared the kinomes of 12 F. oxysporum isolates, including both plant and human pathogens and 1 nonpathogenic biocontrol strain, with 7 additional publicly available ascomycete genomes. Overall, F. oxysporum kinomes are the largest, facilitated in part by the acquisitions of the LS chromosomes. The comparative study identified 99 kinases that are present in almost all examined fungal genomes, forming the core signaling network of ascomycete fungi. Compared to the conserved ascomycete kinome, the expansion of the F. oxysporum kinome occurs in several kinase families such as histidine kinases that are involved in environmental signal sensing and target of rapamycin (TOR) kinase that mediates cellular responses. Comparative kinome analysis suggests a convergent evolution that shapes individual F. oxysporum isolates with an enhanced and unique capacity for environmental perception and associated downstream responses.

IMPORTANCE Isolates of Fusarium oxysporum are adapted to survive a wide range of host and nonhost conditions. In addition, F. oxysporum was recently recognized as the top emerging opportunistic fungal pathogen infecting immunocompromised humans. The sensory and response networks of these fungi undoubtedly play a fundamental role in establishing the adaptability of this group. We have examined the kinomes of 12 F. oxysporum isolates and highlighted kinase families that distinguish F. oxysporum from other fungi, as well as different isolates from one another. The amplification of kinases involved in environmental signal relay and regulating downstream cellular responses clearly sets Fusarium apart from other Ascomycetes. Although the functions of many of these kinases are still unclear, their specific proliferation highlights them as a result of the evolutionary forces that have shaped this species complex and clearly marks them as targets for exploitation in order to combat disease.

Stress Adaptation

Fungal species display an extraordinarily diverse range of lifestyles. Nevertheless, the survival of each species depends on its ability to sense and respond to changes in its natural environment. Environmental changes such as fluctuations in temperature, water balance or pH, or exposure to chemical insults such as reactive oxygen and nitrogen species exert stresses that perturb cellular homeostasis and cause molecular damage to the fungal cell. Consequently, fungi have evolved mechanisms to repair this damage, detoxify chemical insults, and restore cellular homeostasis. Most stresses are fundamental in nature, and consequently, there has been significant evolutionary conservation in the nature of the resultant responses across the fungal kingdom and beyond. For example, heat shock generally induces the synthesis of chaperones that promote protein refolding, antioxidants are generally synthesized in response to an oxidative stress, and osmolyte levels are generally increased following a hyperosmotic shock. In this article we summarize the current understanding of these and other stress responses as well as the signaling pathways that regulate them in the fungi. Model yeasts such as Saccharomyces cerevisiae are compared with filamentous fungi, as well as with pathogens of plants and humans. We also discuss current challenges associated with defining the dynamics of stress responses and with the elaboration of fungal stress adaptation under conditions that reflect natural environments in which fungal cells may be exposed to different types of stresses, either sequentially or simultaneously.

The yeasts phosphorelay systems: a comparative view

Cells contain signal transduction pathways that mediate communication between the extracellular environment and the cell interior. These pathways control transcriptional programs and posttranscriptional processes that modify cell metabolism in order to maintain homeostasis. One type of these signal transduction systems are the so-called Two Component Systems (TCS), which conduct the transfer of phosphate groups between specific and conserved histidine and aspartate residues present in at least two proteins; the first protein is a sensor kinase which autophosphorylates a histidine residue in response to a stimulus, this phosphate is then transferred to an aspartic residue located in a response regulator protein. There are classical and hybrid TCS, whose difference consists in the number of proteins and functional domains involved in the phosphorelay. The TCS are widespread in bacteria where the sensor and its response regulator are mostly specific for a given stimulus. In eukaryotic organisms such as fungi, slime molds, and plants, TCS are present as hybrid multistep phosphorelays, with a variety of arrangements (Stock et al. in Annu Rev Biochem 69:183-215, 2000; Wuichet et al. in Curr Opin Microbiol 292:1039-1050, 2010). In these multistep phosphorelay systems, several phosphotransfer events take place between different histidine and aspartate residues localized in specific domains present in more than two proteins (Thomason and Kay, in J Cell Sci 113:3141-3150, 2000; Robinson et al. in Nat Struct Biol 7:626-633, 2000). This review presents a brief and succinct description of the Two-component systems of model yeasts, Saccharomyces cerevisiae, Schizosaccharomyces pombe, Candida albicans, Cryptococcus neoformans and Kluyveromyces lactis. We have focused on the comparison of domain organization and functions of each component present in these phosphorelay systems.

NapA Mediates a Redox Regulation of the Antioxidant Response, Carbon Utilization and Development in Aspergillus nidulans

The redox-regulated transcription factors (TFs) of the bZIP AP1 family, such as yeast Yap1 and fission yeast Pap1, are activated by peroxiredoxin proteins (Prxs) to regulate the antioxidant response. Previously, Aspergillus nidulans mutants lacking the Yap1 ortholog NapA have been characterized as sensitive to H₂O₂ and menadione. Here we study NapA roles in relation to TFs SrrA and AtfA, also involved in oxidant detoxification, showing that these TFs play different roles in oxidative stress resistance, catalase gene regulation and development, during A. nidulans life cycle. We also uncover novel NapA roles in repression of sexual development, normal conidiation, conidial mRNA accumulation, and carbon utilization. The phenotypic characterization of ΔgpxA, ΔtpxA, and ΔtpxB single, double and triple peroxiredoxin mutants in wild type or ΔnapA backgrounds shows that none of these Prxs is required for NapA function in H₂O₂ and menadione resistance. However, these Prxs participate in a minor NapA-independent H₂O₂ resistance pathway and NapA and TpxA appear to regulate conidiation along the same route. Using transcriptomic analysis we show that during conidial development NapA-dependent gene expression pattern is different from canonical oxidative stress patterns. In the course of conidiation, NapA is required for regulation of at least 214 genes, including ethanol utilization genes alcR, alcA and aldA, and large sets of genes encoding proteins involved in transcriptional regulation, drug detoxification, carbohydrate utilization and secondary metabolism, comprising multiple oxidoreductases, membrane transporters and hydrolases. In agreement with this, ΔnapA mutants fail to grow or grow very poorly in ethanol, arabinose or fructose as sole carbon sources. Moreover, we show that NapA nuclear localization is induced not only by oxidative stress but also by growth in ethanol and by carbon starvation. Together with our previous work, these results show that SakA-AtfA, SrrA and NapA oxidative stress-sensing pathways regulate essential aspects of spore physiology (i.e., cell cycle arrest, dormancy, drug production and detoxification, and carbohydrate utilization).

Histone Methyltransferase aflrmtA gene is involved in the morphogenesis, mycotoxin biosynthesis, and pathogenicity of Aspergillus flavus

Arginine methyltransferases catalyze the posttranslational methylation of arginine, which is involved in a range of important biological processes. aflrmtA gene, an arginine methyltransferase was deleted from Aspergillus flavus in this study by homologous recombination. In morphogenesis assay, aflrmtA was found to down-regulate conidiation by regulating the activity of brlA and abaA genes. It was also found to increase sclerotia formation by up-regulating the expression of nsdC and nsdD genes. In mycotoxin biosynthesis, aflrmtA gene was found to significantly up-regulate the biosynthesis of AFB1 in PDA and PDB media by improving the expression of aflR, aflC and aflK, but it was of no effect in YES medium. aflrmtA was further found to be an important regulator of response to plasma membrane lesion, osmotic, and H₂O₂ - induced oxidative stresses. In pathogenicity analysis, aflrmtA was found to repress conidiation and up-regulate the AFB1 biosynthesis of A. flavus on peanut and corn seeds and also the activities of protease and lipase, but the activity of amylase was down-regulated. It was concluded that aflrmtA gene played important roles in the morphogenesis, mycotoxin biosynthesis and pathogenicity of A. flavus, and it could be a potential target in the prevention and control of crop contamination by A. flavus.

Oxidative stress response pathways: Fission yeast as archetype

Schizosaccharomyces pombe is a popular model eukaryotic organism to study diverse aspects of mammalian biology, including responses to cellular stress triggered by redox imbalances within its compartments. The review considers the current knowledge on the signaling pathways that govern the transcriptional response of fission yeast cells to elevated levels of hydrogen peroxide. Particular attention is paid to the mechanisms that yeast cells employ to promote cell survival in conditions of intermediate and acute oxidative stress. The role of the Sty1/Spc1/Phh1 mitogen-activated protein kinase in regulating gene expression at multiple levels is discussed in detail.

A Pap1-Oxs1 signaling pathway for disulfide stress in Schizosaccharomyces pombe

We describe a Pap1–Oxs1 pathway for diamide-induced disulfide stress in Schizosaccharomyces pombe, where the nucleocytoplasmic HMG protein Oxs1 acts cooperatively with Pap1 to regulate transcription. Oxs1 and Pap1 form a complex when cells are exposed to diamide or Cd that causes disulfide stress. When examined for promoters up-regulated by diamide, effective Pap1 binding to these targets requires Oxs1, and vice versa. With some genes, each protein alone enhances transcription, but the presence of both exerts an additive positive effect. In other genes, although transcription is induced by diamide, Oxs1 or Pap1 plays a negative role with full de-repression requiring loss of both proteins. In a third class of genes, Oxs1 positively regulates expression, but in its absence, Pap1 plays a negative role. The Oxs1–Pap1 regulatory interaction appears evolutionarily conserved, as heterologous (human, mouse and Arabidopsis) Oxs1 and Pap1-homologues can bind interchangeably with each other in vitro, and at least in the fission yeast, heterologous Oxs1 and Pap1-homologues can substitute for S. pombe Oxs1 and Pap1 to enhance stress tolerance.

Stress sensitivity of a fission yeast strain lacking histidine kinases is rescued by the ectopic expression of Chk1 from Candida albicans

The development of new drugs against the pathogenic yeast Candida albicans is compelling and the evolution of relevant bioassays is important to achieve this goal. Promising drug targets are proteins that lack human counterparts which are true for the His-to-Asp phosphorelay signal transduction systems, important for stress sensing in bacteria, fungi, and plants. In the pathogenic yeast, Candida albicans, the CaChk1 histidine kinase is a trigger of the pathway that leads to a switch from yeast to hyphal growth necessary for invasion. Intriguingly, the model yeast Schizosaccharomyces pombe has a similar phosphorelay system, with three histidine kinases named Mak1, Mak2, and Mak3, which are important for the prevention of aberrant mating and sporulation on rich media. This study uncovered distinct functions for the three histidine kinases; Mak1 alone or Mak2 and Mak3 together were sufficient for the repression of the meiotic cycle when nutrients were available. Moreover, strains lacking histidine kinase genes were sensitive to various types of stress conditions in an auxotrophic strain background, while the stress sensitivity was lost in prototrophic strains. Finally, the stress sensitivity of a S. pombe strain that lacks endogenous histidine kinases could be complemented by the ectopic expression of the CaChk1 histidine kinase from C. albicans. This finding opens up for the possibility to perform a drug screen with a biological read-out in S. pombe to find inhibitors of CaChk1.

Ineffective Phosphorylation of Mitogen-Activated Protein Kinase Hog1p in Response to High Osmotic Stress in the Yeast Kluyveromyces lactis

When treated with a hyperosmotic stimulus, Kluyveromyces lactis cells respond by activating the mitogen-activated protein kinase (MAPK) K. lactis Hog1 (KlHog1) protein via two conserved branches, SLN1 and SHO1. Mutants affected in only one branch can cope with external hyperosmolarity by activating KlHog1p by phosphorylation, except for single ΔKlste11 and ΔKlste50 mutants, which showed high sensitivity to osmotic stress, even though the other branch (SLN1) was intact. Inactivation of both branches by deletion of KlSHO1 and KlSSK2 also produced sensitivity to high salt. Interestingly, we have observed that in ΔKlste11 and ΔKlsho1 ΔKlssk2 mutants, which exhibit sensitivity to hyperosmotic stress, and contrary to what would be expected, KlHog1p becomes phosphorylated. Additionally, in mutants lacking both MAPK kinase kinases (MAPKKKs) present in K. lactis (KlSte11p and KlSsk2p), the hyperosmotic stress induced the phosphorylation and nuclear internalization of KlHog1p, but it failed to induce the transcriptional expression of KlSTL1 and the cell was unable to grow in high-osmolarity medium. KlHog1p phosphorylation via the canonical HOG pathway or in mutants where the SHO1 and SLN1 branches have been inactivated requires not only the presence of KlPbs2p but also its kinase activity. This indicates that when the SHO1 and SLN1 branches are inactivated, high-osmotic-stress conditions activate an independent input that yields active KlPbs2p, which, in turn, renders KlHog1p phosphorylation ineffective. Finally, we found that KlSte11p can alleviate the sensitivity to hyperosmotic stress displayed by a ΔKlsho1 ΔKlssk2 mutant when it is anchored to the plasma membrane by adding the KlSho1p transmembrane segments, indicating that this chimeric protein can substitute for KlSho1p and KlSsk2p.

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.

The SrkA Kinase Is Part of the SakA Mitogen-Activated Protein Kinase Interactome and Regulates Stress Responses and Development in Aspergillus nidulans

Fungi and many other eukaryotes use specialized mitogen-activated protein kinases (MAPK) of the Hog1/p38 family to transduce environmental stress signals. In Aspergillus nidulans, the MAPK SakA and the transcription factor AtfA are components of a central multiple stress-signaling pathway that also regulates development. Here we characterize SrkA, a putative MAPK-activated protein kinase, as a novel component of this pathway. ΔsrkA and ΔsakA mutants share a derepressed sexual development phenotype. However, ΔsrkA mutants are not sensitive to oxidative stress, and in fact, srkA inactivation partially suppresses the sensitivity of ΔsakA mutant conidia to H2O2, tert-butyl-hydroperoxide (t-BOOH), and menadione. In the absence of stress, SrkA shows physical interaction with nonphosphorylated SakA in the cytosol. We show that H2O2 induces a drastic change in mitochondrial morphology consistent with a fission process and the relocalization of SrkA to nuclei and mitochondria, depending on the presence of SakA. SakA-SrkA nuclear interaction is also observed during normal asexual development in dormant spores. Using SakA and SrkA S-tag pulldown and purification studies coupled to mass spectrometry, we found that SakA interacts with SrkA, the stress MAPK MpkC, the PPT1-type phosphatase AN6892, and other proteins involved in cell cycle regulation, DNA damage response, mRNA stability and protein synthesis, mitochondrial function, and other stress-related responses. We propose that oxidative stress induces DNA damage and mitochondrial fission and that SakA and SrkA mediate cell cycle arrest and regulate mitochondrial function during stress. Our results provide new insights into the mechanisms by which SakA and SrkA regulate the remodelling of cell physiology during oxidative stress and development.

Oxidative stress responses in the human fungal pathogen, Candida albicans

Candida albicans is a major fungal pathogen of humans, causing approximately 400,000 life-threatening systemic infections world-wide each year in severely immunocompromised patients. An important fungicidal mechanism employed by innate immune cells involves the generation of toxic reactive oxygen species (ROS), such as superoxide and hydrogen peroxide. Consequently, there is much interest in the strategies employed by C. albicans to evade the oxidative killing by macrophages and neutrophils. Our understanding of how C. albicans senses and responds to ROS has significantly increased in recent years. Key findings include the observations that hydrogen peroxide triggers the filamentation of this polymorphic fungus and that a superoxide dismutase enzyme with a novel mode of action is expressed at the cell surface of C. albicans. Furthermore, recent studies have indicated that combinations of the chemical stresses generated by phagocytes can actively prevent C. albicans oxidative stress responses through a mechanism termed the stress pathway interference. In this review, we present an up-date of our current understanding of the role and regulation of oxidative stress responses in this important human fungal pathogen.

The fission yeast Schizosaccharomyces pombe as a model to understand how peroxiredoxins influence cell responses to hydrogen peroxide

As a more selectively reactive oxygen species, H2O2 (hydrogen peroxide) has been co-opted as a signalling molecule, but high levels can still lead to lethal amounts of cell damage. 2-Cys Prxs (peroxiredoxins) are ubiquitous thioredoxin peroxidases which utilize reversibly oxidized catalytic cysteine residues to reduce peroxides. As such, Prxs potentially make an important contribution to the repertoire of cell defences against oxidative damage. Although the abundance of eukaryotic 2-Cys Prxs suggests an important role in maintaining cell redox, the surprising sensitivity of their thioredoxin peroxidase activity to inactivation by H2O2 has raised questions as to their role as an oxidative stress defence. Indeed, work in model yeast has led the way in revealing that Prxs do much more than simply remove peroxides and have even uncovered circumstances where their thioredoxin peroxidase activity is detrimental. In the present paper, we focus on what we have learned from studies in the fission yeast Schizosaccharomyces pombe about the different roles of 2-Cys Prxs in responses to H2O2 and discuss the general implications of these findings for other systems.

Protein kinases in plant-pathogenic fungi: conserved regulators of infection

Phytopathogenic fungi have evolved an amazing diversity of infection modes and nutritional strategies, yet the signaling pathways that govern pathogenicity are remarkably conserved. Protein kinases (PKs) catalyze the reversible phosphorylation of proteins, regulating a variety of cellular processes. Here, we present an overview of our current understanding of the different classes of PKs that contribute to fungal pathogenicity on plants and of the mechanisms that regulate and coordinate PK activity during infection-related development. In addition to the well-studied PK modules, such as MAPK (mitogen-activated protein kinase) and cAMP (cyclic adenosine monophosphate)-PKA (protein kinase A) cascades, we also discuss new PK pathways that have emerged in recent years as key players of pathogenic development and disease. Understanding how conserved PK signaling networks have been recruited during the evolution of fungal pathogenicity not only advances our knowledge of the highly elaborate infection process but may also lead to the development of novel strategies for the control of plant disease.

The ATF/CREB transcription factor Atf1 is essential for full virulence, deoxynivalenol production, and stress tolerance in the cereal pathogen Fusarium graminearum

Fusarium graminearum is a necrotrophic plant pathogen of cereals that produces mycotoxins such as deoxynivalenol (DON) and zearalenone (ZEA) in grains. The stress-activated mitogen-activated protein kinase FgOS-2 is a central regulator in F. graminearum and controls, among others, virulence and DON and ZEA production. Here, we characterized the ATF/CREB-activating transcription factor FgAtf1, a regulator that functions downstream of FgOS-2. We created deletion and overexpression mutants of Fgatf1, the latter being also in an FgOS-2 deletion mutant. FgAtf1 localizes to the nucleus and appears to interact with FgOS-2 under osmotic stress conditions. Deletion mutants in Fgatf1 (ΔFgatf1) are more sensitive to osmotic stress and less sensitive to oxidative stress compared with the wild type. Furthermore, sexual reproduction is delayed. ΔFgatf1 strains produced higher amounts of DON under in vitro induction conditions than that of the wild type. However, during wheat infection, DON production by ΔFgatf1 is strongly reduced. The ΔFgatf1 strains displayed strongly reduced virulence to wheat and maize. Interestingly, constitutive expression of Fgatf1 in the wild type led to hypervirulence on wheat, maize, and Brachypodium distachyon. Moreover, constitutive expression of Fgatf1 in the ΔFgOS-2 mutant background almost complements ΔFgOS-2-phenotypes. These data suggest that FgAtf1 may be the most important transcription factor regulated by FgOS-2.

Phosphorelay-dependent and -independent regulation of MAPKKK by the Mcs4 response regulator in fission yeast

In a “two-component system,” extracellular stimuli are transmitted by the transfer of a phosphoryl group from a sensor histidine kinase to a response regulator (RR), a mechanism referred to as phosphorelay. In the fission yeast Schizosaccharomyces pombe, peroxide stress signals are transmitted by phosphorelay to the Mcs4 RR, which activates the Spc1 MAP kinase (MAPK) cascade. We previously demonstrated that a glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) physically interacts with Mcs4 and promotes phosphorelay signaling to Mcs4. Independently of the phosphorelay mechanism, Mcs4 also plays a critical role in osmostress signaling, as a part of the stable ternary complex with the Wis4 and Win1 MAPK kinase kinases (MAPKKKs). Interestingly, GAPDH dissociates from Mcs4 upon osmostress, while oxidative stress promotes their association. The Mcs4 RR may serve as a switching hub that mediates activation of the Wis4-Win1 MAPKKK heteromer in response to different forms of stress.

Chronological lifespan extension by Ecl1 family proteins depends on Prr1 response regulator in fission yeast

ecl1+, ecl2+ and ecl3+ genes encode highly homologous small proteins, and their over-expressions confer both H2O2 stress resistance and chronological lifespan extension on Schizosaccharomyces pombe. However, the mechanisms of how these Ecl1 family proteins function have not been elucidated. In this study, we conducted microarray analysis and identified that the expression of genes involved in sexual development and stress responses was affected by the over-expression of Ecl1 family proteins. In agreement with the mRNA expression profile, the cells over-expressing Ecl1 family proteins showed high mating efficiency and resistant phenotype to H2O2. We showed that the H2O2-resistant phenotype depends on catalase Ctt1, and over-expression of ctt1+ does not affect chronological lifespan. Furthermore, we showed that six genes, ste11+, spk1+, hsr1+, rsv2+, hsp9+ and lsd90+, whose expressions are increased in cells over-expressing Ecl1 family proteins are involved in chronological lifespan in fission yeast. Among these genes, the induction of ste11+ and hsr1+ was dependent on a transcription factor Prr1, and we showed that the extensions of chronological lifespan by Ecl1 family proteins are remarkably diminished in prr1 deletion mutant. From these results, we propose that Ecl1-family proteins conduct H2O2 stress resistance and chronological lifespan extension in ctt1+- and prr1+-dependent manner, respectively.

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.

Current understanding of HOG-MAPK pathway in Aspergillus fumigatus

Aspergillus fumigatus is an important opportunistic fungal pathogen that causes lethal systemic invasive aspergillosis. It must be able to adapt to stress in the microenvironment during host invasion and systemic spread. The high-osmolarity glycerol (HOG) mitogen-activated protein kinase (MAPK) signaling pathway is a key element that controls adaptation to environmental stress. It plays a critical role in the virulence of several fungal pathogens. In this review, we summarize the current knowledge about the functions of different components of the HOG-MAPK pathway in A. fumigatus through mutant analysis or inferences from the genome annotation, focusing on their roles in adaptation to stress, regulation of infection-related morphogenesis, and effect on virulence. We also briefly compare the functions of the HOG pathway in A. fumigatus with those in the model fungi Saccharomyces cerevisiae and Aspergillus nidulans as well as several other human and plant pathogens including Candida albicans, Cryptococcus neoformans, and Magnaporthe oryzae. The genes described in this review mainly include tcsB, fos1, skn7, sho1, pbs2, and sakA whose deletion mutants have already been established in A. fumigatus. Among them, fos1 has been considered a virulence factor in A. fumigatus, indicating that components of the HOG pathway may be suitable as targets for developing new fungicides. However, quite a few of the genes of this pathway, such as sskA (ssk1), sskB, steC, and downstream regulator genes, are not well characterized. System biology approaches may contribute to a more comprehensive understanding of HOG pathway functions with dynamic details.

ROS homeostasis during development: an evolutionary conserved strategy

The balance between cellular proliferation and differentiation is a key aspect of development in multicellular organisms. Recent studies on Arabidopsis roots revealed distinct roles for different reactive oxygen species (ROS) in these processes. Modulation of the balance between ROS in proliferating cells and elongating cells is controlled at least in part at the transcriptional level. The effect of ROS on proliferation and differentiation is not specific for plants but appears to be conserved between prokaryotic and eukaryotic life forms. The ways in which ROS is received and how it affects cellular functioning is discussed from an evolutionary point of view. The different redox-sensing mechanisms that evolved ultimately result in the activation of gene regulatory networks that control cellular fate and decision-making. This review highlights the potential common origin of ROS sensing, indicating that organisms evolved similar strategies for utilizing ROS during development, and discusses ROS as an ancient universal developmental regulator.

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.

Predicting the fission yeast protein interaction network

A systems-level understanding of biological processes and information flow requires the mapping of cellular component interactions, among which protein-protein interactions are particularly important. Fission yeast (Schizosaccharomyces pombe) is a valuable model organism for which no systematic protein-interaction data are available. We exploited gene and protein properties, global genome regulation datasets, and conservation of interactions between budding and fission yeast to predict fission yeast protein interactions in silico. We have extensively tested our method in three ways: first, by predicting with 70-80% accuracy a selected high-confidence test set; second, by recapitulating interactions between members of the well-characterized SAGA co-activator complex; and third, by verifying predicted interactions of the Cbf11 transcription factor using mass spectrometry of TAP-purified protein complexes. Given the importance of the pathway in cell physiology and human disease, we explore the predicted sub-networks centered on the Tor1/2 kinases. Moreover, we predict the histidine kinases Mak1/2/3 to be vital hubs in the fission yeast stress response network, and we suggest interactors of argonaute 1, the principal component of the siRNA-mediated gene silencing pathway, lost in budding yeast but preserved in S. pombe. Of the new high-quality interactions that were discovered after we started this work, 73% were found in our predictions. Even though any predicted interactome is imperfect, the protein network presented here can provide a valuable basis to explore biological processes and to guide wet-lab experiments in fission yeast and beyond. Our predicted protein interactions are freely available through PInt, an online resource on our website (www.bahlerlab.info/PInt).

Identification of a novel response regulator, Crr1, that is required for hydrogen peroxide resistance in Candida albicans

Candida albicans colonises numerous niches within humans and thus its success as a pathogen is dependent on its ability to adapt to diverse growth environments within the host. Two component signal transduction is a common mechanism by which bacteria respond to environmental stimuli and, although less common, two component-related pathways have also been characterised in fungi. Here we report the identification and characterisation of a novel two component response regulator protein in C. albicans which we have named CRR1 (Candida Response Regulator 1). Crr1 contains a receiver domain characteristic of response regulator proteins, including the conserved aspartate that receives phosphate from an upstream histidine kinase. Significantly, orthologues of CRR1 are present only in fungi belonging to the Candida CTG clade. Deletion of the C. albicans CRR1 gene, or mutation of the predicted phospho-aspartate, causes increased sensitivity of cells to the oxidising agent hydrogen peroxide. Crr1 is present in both the cytoplasm and nucleus, and this localisation is unaffected by oxidative stress or mutation of the predicted phospho-aspartate. Furthermore, unlike the Ssk1 response regulator, Crr1 is not required for the hydrogen peroxide-induced activation of the Hog1 stress-activated protein kinase pathway, or for the virulence of C. albicans in a mouse model of systemic disease. Taken together, our data suggest that Crr1, a novel response regulator restricted to the Candida CTG clade, regulates the response of C. albicans cells to hydrogen peroxide in a Hog1-independent manner that requires the function of the conserved phospho-aspartate.

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.

Candida albicans SRR1, a putative two-component response regulator gene, is required for stress adaptation, morphogenesis, and virulence

We report here the identification and characterization of a previously uncharacterized, two-component response regulator gene (orf19.5843) from Candida albicans. Because of its apparent functions in stress adaptation, we have named this gene SRR1 (stress response regulator 1). Disruption of SRR1 causes defects in hyphal development, reduced resistance to stress, and severe virulence attenuation in the mouse model of disseminated candidiasis.

The MAP kinase MpkA controls cell wall integrity, oxidative stress response, gliotoxin production and iron adaptation in Aspergillus fumigatus

The saprophytic fungus Aspergillus fumigatus is the most important air-borne fungal pathogen. The cell wall of A. fumigatus has been studied intensively as a potential target for development of effective antifungal agents. A major role in maintaining cell wall integrity is played by the mitogen-activated protein kinase (MAPK) MpkA. To gain a comprehensive insight into this central signal transduction pathway, we performed a transcriptome analysis of the ΔmpkA mutant under standard and cell wall stress conditions. Besides genes involved in cell wall remodelling, protection against ROS and secondary metabolism such as gliotoxin, pyomelanin and pseurotin A, also genes involved in siderophore biosynthesis were regulated by MpkA. Consistently, northern and western blot analyses indicated that iron starvation triggers phosphorylation and thus activation of MpkA. Furthermore, localization studies indicated that MpkA accumulates in the nucleus under iron depletion. Hence, we report the first connection between a MAPK pathway and siderophore biosynthesis. The measurement of amino acid pools and of the pools of polyamines indicated that arginine was continuously converted into ornithine to fuel the siderophore pool in the ΔmpkA mutant strain. Based on our data, we propose that MpkA fine-tunes the balance between stress response and energy consuming cellular processes.

Two-component mediated peroxide sensing and signal transduction in fission yeast

Two-component related proteins play a major role in regulating the oxidative stress response in the fission yeast, Schizosaccharomyces pombe. For example, the peroxide-sensing Mak2 and Mak3 histidine kinases regulate H(2)O(2)-induced activation of the Sty1 stress-activated protein kinase pathway, and the Skn7-related response regulator transcription factor, Prr1, is essential for activation of the core oxidative stress response genes. Here, we investigate the mechanism by which the S. pombe two-component system senses H(2)O(2), and the potential role of two-component signaling in the regulation of Prr1. Significantly, we demonstrate that PAS and GAF domains present in the Mak2 histidine kinase are essential for redox-sensing and activation of Sty1. In addition, we find that Prr1 is required for the transcriptional response to a wide range of H(2)O(2) concentrations and, furthermore, that two-component regulation of Prr1 is specifically required for the response of cells to high levels of H(2)O(2). Significantly, this provides the first demonstration that the conserved two-component phosphorylation site on Skn7-related proteins influences resistance to oxidative stress and oxidative stress-induced gene expression. Collectively, these data provide new insights into the two-component mediated sensing and signaling mechanisms underlying the response of S. pombe to oxidative stress.

Aspergillus nidulans transcription factor AtfA interacts with the MAPK SakA to regulate general stress responses, development and spore functions

Fungi utilize a phosphorelay system coupled to a MAP kinase module for sensing and processing environmental signals. In Aspergillus nidulans, response regulator SskA transmits osmotic and oxidative stress signals to the stress MAPK (SAPK) SakA. Using a genetic approach together with GFP tagging and molecular bifluorescence we show that SakA and ATF/CREB transcription factor AtfA define a general stress-signalling pathway that plays differential roles in oxidative stress responses during growth and development. AtfA is permanently localized in the nucleus, while SakA accumulates in the nucleus in response to oxidative or osmotic stress signals or during normal spore development, where it physically interacts with AtfA. AtfA is required for expression of several genes, the conidial accumulation of SakA and the viability of conidia. Furthermore, SakA is active (phosphorylated) in asexual spores, remaining phosphorylated in dormant conidia and becoming dephosphorylated during germination. SakA phosphorylation in spores depends on certain (SskA) but not other (SrrA and NikA) components of the phosphorelay system. Constitutive phosphorylation of SakA induced by the fungicide fludioxonil prevents both, germ tube formation and nuclear division. Similarly, Neurospora crassa SakA orthologue OS-2 is phosphorylated in intact conidia and gets dephosphorylated during germination. We propose that SakA-AtfA interaction regulates gene expression during stress and conidiophore development and that SAPK phosphorylation is a conserved mechanism to regulate transitions between non-growing (spore) and growing (mycelia) states.

Histidine kinase two-component response regulator proteins regulate reproductive development, virulence, and stress responses of the fungal cereal pathogens Cochliobolus heterostrophus and Gibberella zeae

Histidine kinase (HK) phosphorelay signaling is a major mechanism by which fungi sense their environment. The maize pathogen Cochliobolus heterostrophus has 21 HK genes, 4 candidate response regulator (RR) genes (SSK1, SKN7, RIM15, REC1), and 1 gene (HPT1) encoding a histidine phosphotransfer domain protein. Because most HKs are expected to signal through RRs, these were chosen for deletion. Except for pigment and slight growth alterations for rim15 mutants, no measurable altered phenotypes were detected in rim15 or rec1 mutants. Ssk1p is required for virulence and affects fertility and proper timing of sexual development of heterothallic C. heterostrophus. Pseudothecia from crosses involving ssk1 mutants ooze masses of single ascospores, and tetrads cannot be found. Wild-type pseudothecia do not ooze. Ssk1p represses asexual spore proliferation during the sexual phase, and lack of it dampens asexual spore proliferation during vegetative growth, compared to that of the wild type. ssk1 mutants are heavily pigmented. Mutants lacking Skn7p do not display any of the above phenotypes; however, both ssk1 and skn7 mutants are hypersensitive to oxidative and osmotic stresses and ssk1 skn7 mutants are more exaggerated in their spore-type balance phenotype and more sensitive to stress than single mutants. ssk1 mutant phenotypes largely overlap hog1 mutant phenotypes, and in both types of mutant, the Hog1 target gene, MST1, is not induced. ssk1 and hog1 mutants were examined in the homothallic cereal pathogen Gibberella zeae, and pathogenic and reproductive phases of development regulated by Ssk1 and Hog1 were found to mirror, but also vary from, those of C. heterostrophus.

Proteomic Evaluation of Cellular Responses of Saccharomyces cerevisiae to Formic Acid Stress

Formic acid is a representative carboxylic acid that inhibits bacterial cell growth, and thus it is generally considered to constitute an obstacle to the reuse of renewable biomass. In this study, Saccharomyces cerevisiae was used to elucidate changes in protein levels in response to formic acid. Fifty-seven differentially expressed proteins in response to formic acid toxicity in S. cerevisiae were identified by 1D-PAGE and nano-liquid chromatography-tandem mass spectrometry (nano-LC-MS/MS) analyses. Among the 28 proteins increased in expression, four were involved in the MAP kinase signal transduction pathway and one in the oxidative stress-induced pathway. A dramatic increase was observed in the number of ion transporters related to maintenance of acid-base balance. Regarding the 29 proteins decreased in expression, they were found to participate in transcription during cell division. Heat shock protein 70, glutathione reductase, and cytochrome c oxidase were measured by LC-MS/MS analysis. Taken together, the inhibitory action of formic acid on S. cerevisiae cells might disrupt the acid-base balance across the cell membrane and generate oxidative stress, leading to repressed cell division and death. S. cerevisiae also induced expression of ion transporters, which may be required to maintain the acid-base balance when yeast cells are exposed to high concentrations of formic acid in growth medium.

MAP kinase kinase kinase (MAPKKK)-dependent and -independent activation of Sty1 stress MAPK in fission yeast

In fission yeast, the Sty1/Spc1/Phh1 mitogen-activated protein kinase (MAPK) pathway is known to be involved in multiple-stress responses. It is currently thought that the Sty1 MAPK cascade is mediated by histidine kinases and phosphorelay proteins in response to oxidative stress signals. However, studies of the exact transduction mechanism of multiple-stress responses are lacking. Thus, in response to various stimuli, we monitored the Sty1 MAPK pathway through the downstream transcription factor Atf1 in living cells using a highly sensitive luciferase reporter gene. Surprisingly, in cadmium and low glucose (LG) medium, Atf1 activation was observed even in the absence of all of the four fission yeast MAPK kinase kinases (MAPKKKs); whereas in osmotic stress, Atf1 activation was abolished. Thus, the osmotic stress likely mediates the MAPK activation via MAPKKKs, whereas a cadmium or LG condition activates the MAPK in a MAPKKK-independent manner. On the other hand, knockout of tyrosine phosphatase gene pyp1(+) abolished the Atf1 response to cadmium and LG, but not to osmotic stress, suggesting that Pyp1 is a sensor for cadmium and LG.

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.