Two adjacent docking sites in the yeast Hog1 mitogen-activated protein (MAP) kinase differentially interact with the Pbs2 MAP kinase kinase and the Ptp2 protein tyrosine phosphatase

Acetic acid-induced stress granules function as scaffolding complexes for Hog1 activation by Pbs2

Stress-activated protein kinases (SAPKs) respond to a wide variety of stressors. In most cases, the pathways through which specific stress signals are transmitted to the SAPK are not known. We show that the yeast SAPK Hog1 is activated by acetic acid through an intracellular mechanism that does not involve stimulation of the high osmolarity glycerol (HOG) signaling pathway beyond its basal level. Rather, acetic acid treatment drives the formation of stress granules, which function as a scaffold to bring Hog1 together with Pbs2, its immediately upstream activating kinase, in a stable assembly that leverages the basal activity of Pbs2 to phosphorylate Hog1. Deletion analysis of stress granule components revealed that the assembly is critical for both the acetic acid-induced activation of Hog1 and its association with Pbs2. Activated Hog1 remains associated with stress granules, which may have implications for its targeting.

Efficient genes identification via quantitative trait loci analysis by crossbreeding of sake and laboratory yeast

Saccharomyces cerevisiae, a unicellular eukaryotic microorganism, includes various strains used in alcoholic beverage production, like sake, shochu/awamori, and wine yeasts. Despite being the same "Saccharomyces cerevisiae", each strain has unique genes and mutations that make them suitable for specific production processes. We focused on sake yeast, Saccharomyces cerevisiae, suitable for sake making. To identify genes and mutations contributing to sake yeast's characteristics more efficiently, we improved the quantitative trait loci (QTL) analysis system. This genetic statistical method used spore-separating haploid strains (F1 segregant haploids) from crossing sake yeast and laboratory yeast haploid strains. We increased the number of F1 segregant haploids for QTL analysis from 100 to 400 and set DNA markers uniformly across the genome (approximately 12 Mbp) at 5,267 locations using single nucleotide polymorphisms (SNPs) spaced about 3 kb apart. Additionally, a small-scale sake making test using 400 F1 segregant haploids and QTL analysis of ethanol concentration in sake sample identified the PBS2 gene and its causative mutation (amino acid substitution at position 545). The PBS2 gene was also implicated in producing organic acids (fumaric, succinic, and malic acids) and inorganic acids (phosphoric acid) for sake. These findings validated the improved QTL analysis system as effective genes screening method. KEY POINTS: • A new QTL analysis system was constructed using sake and laboratory yeast. • PBS2 gene involved in the ethanol-producing capacity of Saccharomyces cerevisiae was identified. • PBS2 gene was also involved in the organic acid concentration in sake.

Engineering transcriptional regulatory networks for improving second-generation fuel ethanol production in Saccharomyces cerevisiae

Presently, Saccharomyces cerevisiae demonstrates proficient co-fermentation of glucose and xylose, marking a significant advancement in second-generation fuel ethanol production. However, the presence of high concentrations of inhibitors in industrial lignocellulose hydrolysates and post-glucose effect caused by glucose consumption hinders severely impedes yeast robustness and xylose utilization for ethanol fermentation. Even worse, the antagonism between xylose utilization ability and strain robustness was observed, which proposes a difficult challenge in the production of second-generation fuel ethanol by S. cerevisiae. This review introduces the effect of engineering transcriptional regulatory networks on enhancing xylose utilization, improving strain robustness, alleviating antagonism between xylose utilization and strain robustness, and reducing post-glucose effect. Additionally, we provide an outlook on the developmental trends in this field, offering insights into future directions for increasing the production of second-generation fuel ethanol in S. cerevisiae.

Positive feedback induces switch between distributive and processive phosphorylation of Hog1

Cellular decision making often builds on ultrasensitive MAPK pathways. The phosphorylation mechanism of MAP kinase has so far been described as either distributive or processive, with distributive mechanisms generating ultrasensitivity in theoretical analyses. However, the in vivo mechanism of MAP kinase phosphorylation and its activation dynamics remain unclear. Here, we characterize the regulation of the MAP kinase Hog1 in Saccharomyces cerevisiae via topologically different ODE models, parameterized on multimodal activation data. Interestingly, our best fitting model switches between distributive and processive phosphorylation behavior regulated via a positive feedback loop composed of an affinity and a catalytic component targeting the MAP kinase-kinase Pbs2. Indeed, we show that Hog1 directly phosphorylates Pbs2 on serine 248 (S248), that cells expressing a non-phosphorylatable (S248A) or phosphomimetic (S248E) mutant show behavior that is consistent with simulations of disrupted or constitutively active affinity feedback and that Pbs2-S248E shows significantly increased affinity to Hog1 in vitro. Simulations further suggest that this mixed Hog1 activation mechanism is required for full sensitivity to stimuli and to ensure robustness to different perturbations.

Host- and virus-induced gene silencing of HOG1-MAPK cascade genes in Rhizophagus irregularis inhibit arbuscule development and reduce resistance of plants to drought stress

Arbuscular mycorrhizal (AM) fungi can form beneficial associations with the most terrestrial vascular plant species. AM fungi not only facilitate plant nutrient acquisition but also enhance plant tolerance to various environmental stresses such as drought stress. However, the molecular mechanisms by which AM fungal mitogen-activated protein kinase (MAPK) cascades mediate the host adaptation to drought stimulus remains to be investigated. Recently, many studies have shown that virus-induced gene silencing (VIGS) and host-induced gene silencing (HIGS) strategies are used for functional studies of AM fungi. Here, we identify the three HOG1 (High Osmolarity Glycerol 1)-MAPK cascade genes RiSte11, RiPbs2 and RiHog1 from Rhizophagus irregularis. The expression levels of the three HOG1-MAPK genes are significantly increased in mycorrhizal roots of the plant Astragalus sinicus under severe drought stress. RiHog1 protein was predominantly localized in the nucleus of yeast in response to 1 M sorbitol treatment, and RiPbs2 interacts with RiSte11 or RiHog1 directly by pull-down assay. Importantly, VIGS or HIGS of RiSte11, RiPbs2 or RiHog1 hampers arbuscule development and decreases relative water content in plants during AM symbiosis. Moreover, silencing of HOG1-MAPK cascade genes led to the decreased expression of drought-resistant genes (RiAQPs, RiTPSs, RiNTH1 and Ri14-3-3) in the AM fungal symbiont in response to drought stress. Taken together, this study demonstrates that VIGS or HIGS of AM fungal HOG1-MAPK cascade inhibits arbuscule development and expression of AM fungal drought-resistant genes under drought stress.

Two activating phosphorylation sites of Pbs2 MAP2K in the yeast HOG pathway are differentially dephosphorylated by four PP2C phosphatases Ptc1-Ptc4

To cope with an increased external osmolarity, the budding yeast Saccharomyces cerevisiae activates the Hog1 mitogen-activated kinase (MAPK) through the High-Osmolarity Glycerol (HOG) pathway, which governs adaptive responses to osmostress. In the HOG pathway, two apparently redundant upstream branches, termed SLN1 and SHO1, activate cognate MAP3Ks Ssk2/22 and Ste11, respectively. These MAP3Ks, when activated, phosphorylate and thus activate the Pbs2 MAP2K, which in turn phosphorylates and activates Hog1. Previous studies have shown that protein tyrosine phosphatases (PTP) and the serine/threonine protein phosphatases type 2C (PP2C) negatively regulate the HOG pathway to prevent its excessive and inappropriate activation, which is detrimental to cell growth. The tyrosine phosphatases Ptp2 and Ptp3 dephosphorylate Hog1 at Tyr-176, whereas the PP2Cs Ptc1 and Ptc2 dephosphorylate Hog1 at Thr-174. In contrast, the identities of phosphatases that dephosphorylate Pbs2 remained less clear. Here, we examined the phosphorylation status of Pbs2 at the activating phosphorylation sites Ser-514 and Thr-518 (S514 and T518) in various mutants, both in the unstimulated and osmostressed conditions. Thus, we found that Ptc1-Ptc4 collectively regulate Pbs2 negatively, but each Ptc acts differently to the two phosphorylation sites in Pbs2. T518 is predominantly dephosphorylated by Ptc1, whereas the effect of Ptc2-Ptc4 could be seen only when Ptc1 is absent. Conversely, S514 can be dephosphorylated by any of Ptc1-4 to an appreciable extent. We also show that Pbs2 dephosphorylation by Ptc1 requires the adaptor protein Nbp2 that recruits Ptc1 to Pbs2, thus highlighting the complex processes involved in regulating adaptive responses to osmostress.

Identification of Mutations Responsible for Improved Xylose Utilization in an Adapted Xylose Isomerase Expressing Saccharomyces cerevisiae Strain

Economic conversion of biomass to biofuels and chemicals requires efficient and complete utilization of xylose. Saccharomyces cerevisiae strains engineered for xylose utilization are still considerably limited in their overall ability to metabolize xylose. In this study, we identified causative mutations resulting in improved xylose fermentation of an adapted S. cerevisiae strain expressing codon-optimized xylose isomerase and xylulokinase genes from the rumen bacterium Prevotella ruminicola. Genome sequencing identified single-nucleotide polymorphisms in seven open reading frames. Tetrad analysis showed that mutations in both PBS2 and PHO13 genes were required for increased xylose utilization. Single deletion of either PBS2 or PHO13 did not improve xylose utilization in strains expressing the xylose isomerase pathway. Saccharomyces can also be engineered for xylose metabolism using the xylose reductase/xylitol dehydrogenase genes from Scheffersomyces stipitis. In strains expressing the xylose reductase pathway, single deletion of PHO13 did show a significant increase xylose utilization, and further improvement in growth and fermentation was seen when PBS2 was also deleted. These findings will extend the understanding of metabolic limitations for xylose utilization in S. cerevisiae as well as understanding of how they differ among strains engineered with two different xylose utilization pathways.

The role of Acpbs2 in the asexual sporulation, stress response and carbon metabolism of Aspergillus cristatus

Aspergillus cristatus is the dominant fungus during the fermentation of Fuzhuan brick tea, hypotonic conditions only induced its sexual development to produce ascospores, while hypertonic conditions only induced its asexual development to produce conidia, indicating that osmotic stress can regulate spore production in A. cristatus. However, the underlying regulatory mechanism is unclear. In this study, the roles of Acpbs2, which is homologous to pbs2 from Saccharomyces cerevisiae, in sporulation, stress responses, the color of colonies, and carbon metabolism were explored in A. cristatus. Deletion mutants of Acpbs2 were obtained by homologous recombination. The time required to produce conidia was delayed, and the number of conidia produced was significantly reduced in hypertonic media in ΔAcpbs2 by phenotypic observations, indicating that Acpbs2 plays a positive role in asexual development. Stress sensitivity tests showed that the order of the sensitivity of ΔAcpbs2 to different osmotic regulators was 3 M NaCl > 3 M sucrose > 3 M sorbitol. Moreover, the deletion mutants were sensitive to high oxidative stress. The growth of the Acpbs2 deletion mutant was inhibited under alkaline-pH stress, indicating that Acpbs2 is involved in high pH stress tolerance. Additionally, compared with the wild type, the colony color of the Acpbs2 deletion mutant became lighter. All the above developmental defects were reversed by the reintroduction of the Acpbs2 gene in ΔAcpbs2. Transcriptome data showed that Acpbs2 regulated the expression of several genes related to conidial development, osmotic stress, oxidative stress, and carbon metabolism. More importantly, the interaction between Acpbs2 and its downstream gene Achog1 was verified by yeast two-hybrid assays. We speculated that this interaction might regulate the osmotic stress response, the oxidative stress response, and asexual sporulation in A. cristatus, which will be one of the focuses of our future research.

The yeast two-component SLN1 branch of the HOG pathway and the scaffolding activity of Pbs2 modulate the response to endoplasmic reticulum stress induced by tunicamycin

In addition to the UPR pathway, yeast cells require components of the HOG pathway to respond to ER stress. In this work, we found that unphosphorylated Sln1 and Ssk1 are required to mount an appropriate response to Tn. We also found that the MAPKKKs Ssk2 participates in the Tn response, but its osmo-redundant protein Ssk22 does not. We also found that the Pbs2 docking sites for Ssk2 (RDS-I and KD) are partially dispensable when mutated separately; however, the prevention of Ssk2 binding to Pbs2, by the simultaneous mutation of RDS-I and KD, caused strong sensitivity to Tn. In agreement with the lack of Hog1 phosphorylation during Tn treatment, a moderate resistance to Tn is obtained when a Pbs2 version lacking its kinase activity is expressed; however, the presence of mutual Pbs2-Hog1 docking sites is essential for the Tn response. Finally, we detected that Tn induced a transcriptional activation of some components of the SLN1 branch. These results indicate that the Tn response requires a complex formed by the MAPK module and components of the SLN1 branch but not their canonical osmoregulatory activities.

The specific inhibition of glycerol synthesis and the phosphorylation of a putative MAPK give insight into the mechanism of osmotic sensing in a dinoflagellate symbiont

Signaling pathways are fundamental for the establishment and maintenance of diverse symbioses. The symbiosis of cnidarians and dinoflagellate algae is the foundation for the ecological success of coral reefs, involving the transfer of photosynthetic products from symbiont to host. However, signal transduction pathways for this symbiosis remain uncharacterized. Cultured and natural cnidarian symbionts can produce glycerol, one of the main translocated photosynthates. Here, we investigate whether a signal transduction pathway may be involved in inducing glycerol synthesis in cultured symbionts under an osmotic stress model. We evaluated the effect of specific inhibitors of the main transduction pathways, p38, JNK, and ERK 1/2 in Brevolium minutum, the symbiont of the Aiptasia model system. We found that glycerol production and the specific activity of the enzyme Gpdh were selectively inhibited by a p38 MAPK inhibitor. Additionally, the phosphorylation of a putative p38-like protein was rapidly detected. Finally, we studied the presence of each of the components of the p38 MAPK pathway in silico, in genomes and transcriptomes reported up to date for different symbiont types. We propose a model for the arrangement of this pathway in the family of dinoflagellate symbionts known as Symbiodiniaceae.

Molecular cloning and sequence analysis of a mitogen-activated protein kinase gene in the Antarctic yeast Rhodotorula mucilaginosa AN5

BACKGROUND

The mitogen-activated protein kinase (MAPK) cascades play important roles in various signaling transduction networks of biotic and abiotic stress responses. However, MAPK signaling pathways in cold-active yeast Rhodotorula mucilaginosa have not been reported comprehensively.

METHODS AND RESULTS

In the present study, MAPK gene (RmMAPK) was first cloned and characterized from Antarctic sea ice yeast R. mucilaginosa AN5. The full length of the RmMAPK gene is 1086 bp and encodes a 361 amino acids protein with a predicted molecular mass of 40.9 kDa and a pI of 5.25. The RmMAPK contains 11 MAPK conserved subdomains and the phosphorylation motif TGY located in the activation loop of the kinase. Quantitative real-time PCR and western blot assay revealed that the expression and phosphorylation level of RmMAPK up-regulated rapidly and significantly when yeast cells were subjected to low temperature (4 °C), high salinity (120‰ NaCl) and heavy metal (2 mmol/L CuCl₂).

CONCLUSIONS

All data suggested that the MAPK cascades might act as a key function in response to extreme stresses, such as low temperature, high salinity and heavy metal.

A self-balancing circuit centered on MoOsm1 kinase governs adaptive responses to host-derived ROS in Magnaporthe oryzae

The production of reactive oxygen species (ROS) is a ubiquitous defense response in plants. Adapted pathogens evolved mechanisms to counteract the deleterious effects of host-derived ROS and promote infection. How plant pathogens regulate this elaborate response against ROS burst remains unclear. Using the rice blast fungus Magnaporthe oryzae, we uncovered a self-balancing circuit controlling response to ROS in planta and virulence. During infection, ROS induces phosphorylation of the high osmolarity glycerol pathway kinase MoOsm1 and its nuclear translocation. There, MoOsm1 phosphorylates transcription factor MoAtf1 and dissociates MoAtf1-MoTup1 complex. This releases MoTup1-mediated transcriptional repression on oxidoreduction-pathway genes and activates the transcription of MoPtp1/2 protein phosphatases. In turn, MoPtp1/2 dephosphorylate MoOsm1, restoring the circuit to its initial state. Balanced interactions among proteins centered on MoOsm1 provide a means to counter host-derived ROS. Our findings thereby reveal new insights into how M. oryzae utilizes a phosphor-regulatory circuitry to face plant immunity during infection.

Protein Homeostasis Networks and the Use of Yeast to Guide Interventions in Alzheimer's Disease

Alzheimer's Disease (AD) is a progressive multifactorial age-related neurodegenerative disorder that causes the majority of deaths due to dementia in the elderly. Although various risk factors have been found to be associated with AD progression, the cause of the disease is still unresolved. The loss of proteostasis is one of the major causes of AD: it is evident by aggregation of misfolded proteins, lipid homeostasis disruption, accumulation of autophagic vesicles, and oxidative damage during the disease progression. Different models have been developed to study AD, one of which is a yeast model. Yeasts are simple unicellular eukaryotic cells that have provided great insights into human cell biology. Various yeast models, including unmodified and genetically modified yeasts, have been established for studying AD and have provided significant amount of information on AD pathology and potential interventions. The conservation of various human biological processes, including signal transduction, energy metabolism, protein homeostasis, stress responses, oxidative phosphorylation, vesicle trafficking, apoptosis, endocytosis, and ageing, renders yeast a fascinating, powerful model for AD. In addition, the easy manipulation of the yeast genome and availability of methods to evaluate yeast cells rapidly in high throughput technological platforms strengthen the rationale of using yeast as a model. This review focuses on the description of the proteostasis network in yeast and its comparison with the human proteostasis network. It further elaborates on the AD-associated proteostasis failure and applications of the yeast proteostasis network to understand AD pathology and its potential to guide interventions against AD.

Diverse Cell Stimulation Kinetics Identify Predictive Signal Transduction Models

Computationally understanding the molecular mechanisms that give rise to cell signaling responses upon different environmental, chemical, and genetic perturbations is a long-standing challenge that requires models that fit and predict quantitative responses for new biological conditions. Overcoming this challenge depends not only on good models and detailed experimental data but also on the rigorous integration of both. We propose a quantitative framework to perturb and model generic signaling networks using multiple and diverse changing environments (hereafter "kinetic stimulations") resulting in distinct pathway activation dynamics. We demonstrate that utilizing multiple diverse kinetic stimulations better constrains model parameters and enables predictions of signaling dynamics that would be impossible using traditional dose-response or individual kinetic stimulations. To demonstrate our approach, we use experimentally identified models to predict signaling dynamics in normal, mutated, and drug-treated conditions upon multitudes of kinetic stimulations and quantify which proteins and reaction rates are most sensitive to which extracellular stimulations.

The High Osmolarity Glycerol (HOG) Pathway Functions in Osmosensing, Trap Morphogenesis and Conidiation of the Nematode-Trapping Fungus Arthrobotrys oligospora

Hog1, a mitogen-activated protein kinase (MAPK), has been identified in diverse fungal species, and it regulates various cellular processes, such as osmoadaptation, nutrient-sensing, and pathogenesis. However, the roles that Hog1 plays in nematode-trapping fungi were previously unclear. Here, we characterized orthologs of Saccharomyces cerevisiae Hog1 and membrane mucin Msb2 in the nematode-trapping fungus Arthrobotrys oligospora. We generated gene deletion mutants of HOG1 and MSB2 in A. oligospora, and characterized their roles in osmosensing, growth, and trap morphogenesis. We found that both hog1 and msb2 mutants were highly sensitive to high osmolarity. Predation analyses further revealed that hog1 and msb2 deletion caused a reduction in trap formation and predation efficiency. Furthermore, HOG1 is required for conidiation in A. oligospora, demonstrating its critical role in this developmental pathway. In summary, this study demonstrated that the conserved Hog1 and Msb2 govern physiology, growth and development in the nematode-trapping fungus A. oligospora.

The High Osmolarity Glycerol Mitogen-Activated Protein Kinase regulates glucose catabolite repression in filamentous fungi

The utilization of different carbon sources in filamentous fungi underlies a complex regulatory network governed by signaling events of different protein kinase pathways, including the high osmolarity glycerol (HOG) and protein kinase A (PKA) pathways. This work unraveled cross-talk events between these pathways in governing the utilization of preferred (glucose) and non-preferred (xylan, xylose) carbon sources in the reference fungus Aspergillus nidulans. An initial screening of a library of 103 non-essential protein kinase (NPK) deletion strains identified several mitogen-activated protein kinases (MAPKs) to be important for carbon catabolite repression (CCR). We selected the MAPKs Ste7, MpkB, and PbsA for further characterization and show that they are pivotal for HOG pathway activation, PKA activity, CCR via regulation of CreA cellular localization and protein accumulation, as well as for hydrolytic enzyme secretion. Protein-protein interaction studies show that Ste7, MpkB, and PbsA are part of the same protein complex that regulates CreA cellular localization in the presence of xylan and that this complex dissociates upon the addition of glucose, thus allowing CCR to proceed. Glycogen synthase kinase (GSK) A was also identified as part of this protein complex and shown to potentially phosphorylate two serine residues of the HOG MAPKK PbsA. This work shows that carbon source utilization is subject to cross-talk regulation by protein kinases of different signaling pathways. Furthermore, this study provides a model where the correct integration of PKA, HOG, and GSK signaling events are required for the utilization of different carbon sources.

Filamentous fungi secrete an array of biotechnologically valuable enzymes, with enzyme production being inhibited in the presence of preferred carbon sources, such as glucose, in a process known as carbon catabolite repression (CCR). This work unravels upstream signalling events that regulate CCR in Aspergillus nidulans. Different mitogen-activated protein kinases (MAPKs) were identified and shown to be crucial for CCR and protein kinase A (PKA) activity, which is essential for carbon source utilisation in filamentous fungi. Furthermore, the MAPKs formed a protein complex with additional protein kinases, such as glycogen synthase kinase (GSK), which is important for glucose metabolism; resulting in the inhibition of CCR in the presence of non-preferred carbon sources. GSK was shown to potentially phosphorylate the MAPK PbsA of the high osmolarity glycerol (HOG) pathway. This study thus unravels the cross-talk between protein kinases from different signalling pathways that regulate carbon source utilisation in filamentous fungi.

Contribution of the mitogen-activated protein kinase Hog1 to the halotolerance of the marine yeast Debaryomyces hansenii

Halotolerant species are adapted to dealing continually with hyperosmotic environments, having evolved strategies that are uncommon in other organisms. The HOG pathway is the master system that regulates the cellular adaptation under these conditions; nevertheless, apart from the importance of Debaryomyces hansenii as an organism representative of the halotolerant class, its HOG1 pathway has been poorly studied, due to the difficulty of applying conventional recombinant DNA technology. Here we describe for the first time the phenotypic characterisation of a null HOG1 mutant of D. hansenii. Dhhog1Δ strain was found moderately resistant to 1 M NaCl and sensitive to higher concentrations. Under hyperosmotic shock, DhHog1 fully upregulated transcription of DhSTL1 and partially upregulated that of DhGPD1. High osmotic stress lead to long-term inner glycerol accumulation that was partially dependent on DhHog1. These observations indicated that the HOG pathway is required for survival under high external osmolarity but dispensable under low and mid-osmotic conditions. It was also found that DhHog1 can regulate response to alkali stress during hyperosmotic conditions and that it plays a role in oxidative and endoplasmic reticulum stress. Taken together, these results provide new insight into the contribution of this MAPK in halotolerance of this yeast.

Negative feedback-loop mechanisms regulating HOG- and pheromone-MAPK signaling in yeast

The pheromone response and the high osmolarity glycerol (HOG) pathways are considered the prototypical MAPK signaling systems. They are the best-understood pathways in eukaryotic cells, yet they continue to provide insights in how cells relate with the environment. These systems are subjected to tight regulatory circuits to prevent hyperactivation in length and intensity. Failure to do this may be a matter of life or death specially for unicellular organisms such as Saccharomyces cerevisiae. The signaling pathways are fine-tuned by positive and negative feedback loops exerted by pivotal control elements that allow precise responses to specific stimuli, despite the fact that some elements of the systems are common to different signaling pathways. Here we describe the experimentally proven negative feedback loops that modulate the pheromone response and the HOG pathways. As described in this review, MAP kinases are central mechanistic components of these feedback loops. They have the capacity to modulate basal signaling activity, a fast extranuclear response, and a longer-lasting transcriptional process.

Osmostress enhances activating phosphorylation of Hog1 MAP kinase by mono-phosphorylated Pbs2 MAP2K

The MAP kinase (MAPK) Hog1 is the central regulator of osmoadaptation in yeast. When cells are exposed to high osmolarity, the functionally redundant Sho1 and Sln1 osmosensors, respectively, activate the Ste11‐Pbs2‐Hog1 MAPK cascade and the Ssk2/Ssk22‐Pbs2‐Hog1 MAPK cascade. In a canonical MAPK cascade, a MAPK kinase kinase (MAP3K) activates a MAPK kinase (MAP2K) by phosphorylating two conserved Ser/Thr residues in the activation loop. Here, we report that the MAP3K Ste11 phosphorylates only one activating phosphorylation site (Thr‐518) in Pbs2, whereas the MAP3Ks Ssk2/Ssk22 can phosphorylate both Ser‐514 and Thr‐518 under optimal osmostress conditions. Mono‐phosphorylated Pbs2 cannot phosphorylate Hog1 unless the reaction between Pbs2 and Hog1 is enhanced by osmostress. The lack of the osmotic enhancement of the Pbs2‐Hog1 reaction suppresses Hog1 activation by basal MAP3K activities and prevents pheromone‐to‐Hog1 crosstalk in the absence of osmostress. We also report that the rapid‐and‐transient Hog1 activation kinetics at mildly high osmolarities and the slow and prolonged activation kinetics at severely high osmolarities are both caused by a common feedback mechanism.

A phosphorylated transcription factor regulates sterol biosynthesis in Fusarium graminearum

Sterol biosynthesis is controlled by transcription factor SREBP in many eukaryotes. Here, we show that SREBP orthologs are not involved in the regulation of sterol biosynthesis in Fusarium graminearum, a fungal pathogen of cereal crops worldwide. Instead, sterol production is controlled in this organism by a different transcription factor, FgSR, that forms a homodimer and binds to a 16-bp cis-element of its target gene promoters containing two conserved CGAA repeat sequences. FgSR is phosphorylated by the MAP kinase FgHog1, and the phosphorylated FgSR interacts with the chromatin remodeling complex SWI/SNF at the target genes, leading to enhanced transcription. Interestingly, FgSR orthologs exist only in Sordariomycetes and Leotiomycetes fungi. Additionally, FgSR controls virulence mainly via modulating deoxynivalenol biosynthesis and responses to phytoalexin.

Interaction between the transmembrane domains of Sho1 and Opy2 enhances the signaling efficiency of the Hog1 MAP kinase cascade in Saccharomyces cerevisiae

To cope with increased extracellular osmolarity, the budding yeast Saccharomyces cerevisiae activates the Hog1 mitogen-activated protein kinase (MAPK), which controls a variety of adaptive responses. Hog1 is activated through the high-osmolarity glycerol (HOG) pathway, which consists of a core MAPK cascade and two independent upstream branches (SHO1 and SLN1 branches) containing distinct osmosensing machineries. In the SHO1 branch, a homo-oligomer of Sho1, the four-transmembrane (TM) osmosensor, interacts with the transmembrane co-osmosensors, Hkr1 and Msb2, and the membrane anchor protein Opy2, through their TM domains, and activates the Ste20-Ste11-Pbs2-Hog1 kinase cascade. In this study, we isolated and analyzed hyperactive mutants of Sho1 and Opy2 that harbor mutations within their TM domains. Several hyperactive mutations enhanced the interaction between Sho1 and Opy2, indicating the importance of the TM-mediated interaction between Sho1 and Opy2 for facilitating effective signaling. The interaction between the TM domains of Sho1 and Opy2 will place their respective cytoplasmic binding partners Pbs2 and Ste11 in close proximity. Indeed, genetic analyses of the mutants showed that the Sho1-Opy2 interaction enhances the activation of Pbs2 by Ste11, but not Hog1 by Pbs2. Some of the hyperactive mutants had mutations at the extracellular ends of either Sho1 TM4 or Opy2 TM, and defined the Sho1-Opy2 binding site 1 (BS1). Chemical crosslinking and mutational analyses revealed that the cytoplasmic ends of Sho1 TM1 and Opy2 TM also interact with each other, defining the Sho1-Opy2 binding site 2 (BS2). A geometric consideration constrains that one Opy2 molecule must interact with two adjacent Sho1 molecules in Sho1 oligomer. These results raise a possibility that an alteration of the conformation of the Sho1-Opy2 complex might contributes to the osmotic activation of the Hog1 MAPK cascade.

Hog1 Regulates Stress Tolerance and Virulence in the Emerging Fungal Pathogen Candida auris

Candida auris has recently emerged as an important, multidrug-resistant fungal pathogen of humans. Comparative studies indicate that despite high levels of genetic divergence, C. auris is as virulent as the most pathogenic member of the genus, Candida albicans However, key virulence attributes of C. albicans, such as morphogenetic switching, are not utilized by C. auris, indicating that this emerging pathogen employs alternative strategies to infect and colonize the host. An important trait required for the pathogenicity of many fungal pathogens is the ability to adapt to host-imposed stresses encountered during infection. Here, we investigated the relative resistance of C. auris and other pathogenic Candida species to physiologically relevant stresses and explored the role of the evolutionarily conserved Hog1 stress-activated protein kinase (SAPK) in promoting stress resistance and virulence. In comparison to C. albicans, C. auris is relatively resistant to hydrogen peroxide, cationic stress, and cell-wall-damaging agents. However, in contrast to other Candida species examined, C. auris was unable to grow in an anaerobic environment and was acutely sensitive to organic oxidative-stress-inducing agents. An analysis of C. aurishog1Δ cells revealed multiple roles for this SAPK in stress resistance, cell morphology, aggregation, and virulence. These data demonstrate that C. auris has a unique stress resistance profile compared to those of other pathogenic Candida species and that the Hog1 SAPK has pleiotropic roles that promote the virulence of this emerging pathogen.IMPORTANCE The rapid global emergence and resistance of Candidaauris to current antifungal drugs highlight the importance of understanding the virulence traits exploited by this human fungal pathogen to cause disease. Here, we characterize the stress resistance profile of C. auris and the role of the Hog1 stress-activated protein kinase (SAPK) in stress resistance and virulence. Our findings that C. auris is acutely sensitive to certain stresses may facilitate control measures to prevent persistent colonization in hospital settings. Furthermore, our observation that the Hog1 SAPK promotes C. auris virulence akin to that reported for many other pathogenic fungi indicates that antifungals targeting Hog1 signaling would be broad acting and effective, even on emerging drug-resistant pathogens.

Intracellular mechanism by which arsenite activates the yeast stress MAPK Hog1

Stress-activated MAPKs (SAPKs) respond to a wide variety of stressors. In most cases, the pathways through which specific stress signals are transmitted to the SAPKs are not known. In this study, we delineate the intracellular signaling pathway by which the trivalent toxic metalloid arsenite [As(III)] activates the yeast SAPK Hog1. We demonstrate that, to activate Hog1, As(III) must enter the cell through the glycerol channel Fps1 and must be metabolized to methyl arsenite [MAs(III)] by the dimeric methyltransferase Mtq2:Trm112. We found that Mtq2:Trm1 displays SAM-dependent methyltransferase activity toward both As(III) and MAs(III). Additionally, we present genetic and biochemical evidence that MAs(III), but not As(III), is a potent inhibitor of the protein tyrosine phosphatases (Ptp2 and Ptp3) that normally maintain Hog1 in an inactive state. Inhibition of Ptp2 and Ptp3 by MAs(III) results in elevated Hog1 phosphorylation without activation of the protein kinases that act upstream of the SAPK and raises the possibility that other Hog1-activating stressors act intracellularly at different points along the canonical Hog1 activation pathway. Finally, we show that arsenate [As(V)], a pentavalent form of arsenic, also activates Hog1, but through a pathway that is distinct from that of As(III) and involves activation of the Hog1 MEK Pbs2.

Yeast Hog1 proteins are sequestered in stress granules during high-temperature stress

The yeast high-osmolarity glycerol (HOG) pathway plays a central role in stress responses. It is activated by various stresses, including hyperosmotic stress, oxidative stress, high-temperature stress and exposure to arsenite. Hog1, the crucial MAP kinase of the pathway, localizes to the nucleus in response to high osmotic concentrations, i.e. high osmolarity; but, otherwise, little is known about its intracellular dynamics and regulation. By using the methylotrophic yeast Candida boidinii, we found that CbHog1-Venus formed intracellular dot structures after high-temperature stress in a reversible manner. Microscopic observation revealed that CbHog1-mCherry colocalized with CbPab1-Venus, a marker protein of stress granules. Hog1 homologs in Pichia pastoris and Schizosaccharomyces pombe also exhibited similar dot formation under high-temperature stress, whereas Saccharomyces cerevisiae Hog1 (ScHog1)-GFP did not. Analysis of CbHog1-Venus in C. boidinii revealed that a β-sheet structure in the N-terminal region was necessary and sufficient for its localization to stress granules. Physiological studies revealed that sequestration of activated Hog1 proteins in stress granules was responsible for downregulation of Hog1 activity under high-temperature stress.This article has an associated First Person interview with the first author of the paper.

The phosphorelay signal transduction system in Candida glabrata: an in silico analysis

Signaling systems allow microorganisms to sense and respond to different stimuli through the modification of gene expression. The phosphorelay signal transduction system in eukaryotes involves three proteins: a sensor protein, an intermediate protein and a response regulator, and requires the transfer of a phosphate group between two histidine-aspartic residues. The SLN1-YPD1-SSK1 system enables yeast to adapt to hyperosmotic stress through the activation of the HOG1-MAPK pathway. The genetic sequences available from Saccharomyces cerevisiae were used to identify orthologous sequences in Candida glabrata, and putative genes were identified and characterized by in silico assays. An interactome analysis was carried out with the complete genome of C. glabrata and the putative proteins of the phosphorelay signal transduction system. Next, we modeled the complex formed between the sensor protein CgSln1p and the intermediate CgYpd1p. Finally, phosphate transfer was examined by a molecular dynamic assay. Our in silico analysis showed that the putative proteins of the C. glabrata phosphorelay signal transduction system present the functional domains of histidine kinase, a downstream response regulator protein, and an intermediate histidine phosphotransfer protein. All the sequences are phylogenetically more related to S. cerevisiae than to C. albicans. The interactome suggests that the C. glabrata phosphorelay signal transduction system interacts with different proteins that regulate cell wall biosynthesis and responds to oxidative and osmotic stress the same way as similar systems in S. cerevisiae and C. albicans. Molecular dynamics simulations showed complex formation between the response regulator domain of histidine kinase CgSln1 and intermediate protein CgYpd1 in the presence of a phosphate group and interactions between the aspartic residue and the histidine residue. Overall, our research showed that C. glabrata harbors a functional SLN1-YPD1-SSK1 phosphorelay system.

Reconstruction of the High-Osmolarity Glycerol (HOG) Signaling Pathway from the Halophilic Fungus Wallemia ichthyophaga in Saccharomyces cerevisiae

The basidiomycetous fungus Wallemia ichthyophaga grows between 1.7 and 5.1 M NaCl and is the most halophilic eukaryote described to date. Like other fungi, W. ichthyophaga detects changes in environmental salinity mainly by the evolutionarily conserved high-osmolarity glycerol (HOG) signaling pathway. In Saccharomyces cerevisiae, the HOG pathway has been extensively studied in connection to osmotic regulation, with a valuable knock-out strain collection established. In the present study, we reconstructed the architecture of the HOG pathway of W. ichthyophaga in suitable S. cerevisiae knock-out strains, through heterologous expression of the W. ichthyophaga HOG pathway proteins. Compared to S. cerevisiae, where the Pbs2 (ScPbs2) kinase of the HOG pathway is activated via the SHO1 and SLN1 branches, the interactions between the W. ichthyophaga Pbs2 (WiPbs2) kinase and the W. ichthyophaga SHO1 branch orthologs are not conserved: as well as evidence of poor interactions between the WiSho1 Src-homology 3 (SH3) domain and the WiPbs2 proline-rich motif, the absence of a considerable part of the osmosensing apparatus in the genome of W. ichthyophaga suggests that the SHO1 branch components are not involved in HOG signaling in this halophilic fungus. In contrast, the conserved activation of WiPbs2 by the S. cerevisiae ScSsk2/ScSsk22 kinase and the sensitivity of W. ichthyophaga cells to fludioxonil, emphasize the significance of two-component (SLN1-like) signaling via Group III histidine kinase. Combined with protein modeling data, our study reveals conserved and non-conserved protein interactions in the HOG signaling pathway of W. ichthyophaga and therefore significantly improves the knowledge of hyperosmotic signal processing in this halophilic fungus.

Scaffold Protein Ahk1, Which Associates with Hkr1, Sho1, Ste11, and Pbs2, Inhibits Cross Talk Signaling from the Hkr1 Osmosensor to the Kss1 Mitogen-Activated Protein Kinase

In the budding yeast Saccharomyces cerevisiae, osmostress activates the Hog1 mitogen-activated protein kinase (MAPK), which regulates diverse osmoadaptive responses. Hkr1 is a large, highly glycosylated, single-path transmembrane protein that is a putative osmosensor in one of the Hog1 upstream pathways termed the HKR1 subbranch. The extracellular region of Hkr1 contains both a positive and a negative regulatory domain. However, the function of the cytoplasmic domain of Hkr1 (Hkr1-cyto) is unknown. Here, using a mass spectrometric method, we identified a protein, termed Ahk1 (Associated with Hkr1), that binds to Hkr1-cyto. Deletion of the AHK1 gene (in the absence of other Hog1 upstream branches) only partially inhibited osmostress-induced Hog1 activation. In contrast, Hog1 could not be activated by constitutively active mutants of the Hog1 pathway signaling molecules Opy2 or Ste50 in ahk1Δ cells, whereas robust Hog1 activation occurred in AHK1(+) cells. In addition to Hkr1-cyto binding, Ahk1 also bound to other signaling molecules in the HKR1 subbranch, including Sho1, Ste11, and Pbs2. Although osmotic stimulation of Hkr1 does not activate the Kss1 MAPK, deletion of AHK1 allowed Hkr1 to activate Kss1 by cross talk. Thus, Ahk1 is a scaffold protein in the HKR1 subbranch and prevents incorrect signal flow from Hkr1 to Kss1.

Binding of the Extracellular Eight-Cysteine Motif of Opy2 to the Putative Osmosensor Msb2 Is Essential for Activation of the Yeast High-Osmolarity Glycerol Pathway

To adapt to environmental high osmolarity, the budding yeast Saccharomyces cerevisiae activates the Hog1 mitogen-activated protein kinase, which regulates diverse osmoadaptive responses. Hog1 is activated through the high-osmolarity glycerol (HOG) pathway, which consists of independent upstream signaling routes termed the SLN1 branch and the SHO1 branch. Here, we report that the extracellular cysteine-rich (CR) domain of the transmembrane-anchor protein Opy2 binds to the Hkr1-Msb2 homology (HMH) domain of the putative osmosensor Msb2 and that formation of the Opy2-Msb2 complex is essential for osmotic activation of Hog1 through the MSB2 subbranch of the SHO1 branch. By analyzing the phenotypes of mutants with Opy2 cysteine-to-alanine mutations, we deduced that the CR domain forms four intramolecular disulfide bonds. To probe for the potential induction of conformational changes in the Opy2-Msb2 complex by osmostress, we constructed mutants with a site-specific Cys-to-Ala mutation of the Opy2 CR domain and mutants with a Cys substitution of the Msb2 HMH domain. Each of these mutants had a reduced cysteine. These mutants were then combinatorially cross-linked using chemical cross-linkers of different lengths. Cross-linking between Opy2 Cys48 and Msb2 Cys1023 was sensitive to osmotic changes, suggesting that osmostress induced a conformational change. We therefore propose that the Opy2-Msb2 complex might serve as an osmosensor.

Osmosensing and scaffolding functions of the oligomeric four-transmembrane domain osmosensor Sho1

The yeast high osmolarity glycerol (HOG) pathway activates the Hog1 MAP kinase, which coordinates adaptation to high osmolarity conditions. Here we demonstrate that the four-transmembrane (TM) domain protein Sho1 is an osmosensor in the HKR1 sub-branch of the HOG pathway. Crosslinking studies indicate that Sho1 forms planar oligomers of the dimers-of-trimers architecture by dimerizing at the TM1/TM4 interface and trimerizing at the TM2/TM3 interface. High external osmolarity induces structural changes in the Sho1 TM domains and Sho1 binding to the cytoplasmic adaptor protein Ste50, which leads to Hog1 activation. Besides its osmosensing function, the Sho1 oligomer serves as a scaffold. By binding to the TM proteins Opy2 and Hkr1 at the TM1/TM4 and TM2/TM3 interface, respectively, Sho1 forms a multi-component signalling complex that is essential for Hog1 activation. Our results illuminate how the four TM domains of Sho1 dictate the oligomer structure as well as its osmosensing and scaffolding functions.

The unique characteristics of HOG pathway MAPKs in the extremely halotolerant Hortaea werneckii

HwHog1A/B, Hortaea werneckii homologues of the MAP kinase Hog1 from Saccharomyces cerevisiae, are vital for the extreme halotolerance of H. werneckii. In mesophilic S. cerevisiae, Hog1 is phosphorylated already at low osmolyte concentrations, and regulates expression of a similar set of genes independent of osmolyte type. To understand how HwHog1 kinases activity is regulated in H. werneckii, we studied HwHog1A/B activation in vivo, by following phosphorylation of HwHog1A/B in H. werneckii exposed to various osmolytes, and in vitro, by measuring kinase activities of recombinant HwHog1A, HwHog1B and Hog1ΔC. To this end, highly pure and soluble recombinant Hog1 homologues were isolated from insect cells. Our results demonstrate that HwHog1A/B are, in general, transiently phosphorylated in cells shocked with ≥3 M osmolyte, yet constitutive phosphorylation is observed at extreme NaCl and KCl concentrations. Importantly, phosphorylation profiles differ depending on the osmolyte type. Additionally, phosphorylated recombinant HwHog1A/B show lower specific kinase activities compared to Hog1ΔC. In summary, HOG pathway MAPKs in the extremely halotolerant H. werneckii show unique characteristics compared to S. cerevisiae homologues. The reported findings contribute to defining the key determinants of H. werneckii osmotolerance, which is important for its potential transfer to economically relevant microorganisms and crops.

Metabolic respiration induces AMPK- and Ire1p-dependent activation of the p38-Type HOG MAPK pathway

Evolutionarily conserved mitogen activated protein kinase (MAPK) pathways regulate the response to stress as well as cell differentiation. In Saccharomyces cerevisiae, growth in non-preferred carbon sources (like galactose) induces differentiation to the filamentous cell type through an extracellular-signal regulated kinase (ERK)-type MAPK pathway. The filamentous growth MAPK pathway shares components with a p38-type High Osmolarity Glycerol response (HOG) pathway, which regulates the response to changes in osmolarity. To determine the extent of functional overlap between the MAPK pathways, comparative RNA sequencing was performed, which uncovered an unexpected role for the HOG pathway in regulating the response to growth in galactose. The HOG pathway was induced during growth in galactose, which required the nutrient regulatory AMP-dependent protein kinase (AMPK) Snf1p, an intact respiratory chain, and a functional tricarboxylic acid (TCA) cycle. The unfolded protein response (UPR) kinase Ire1p was also required for HOG pathway activation in this context. Thus, the filamentous growth and HOG pathways are both active during growth in galactose. The two pathways redundantly promoted growth in galactose, but paradoxically, they also inhibited each other's activities. Such cross-modulation was critical to optimize the differentiation response. The human fungal pathogen Candida albicans showed a similar regulatory circuit. Thus, an evolutionarily conserved regulatory axis links metabolic respiration and AMPK to Ire1p, which regulates a differentiation response involving the modulated activity of ERK and p38 MAPK pathways.

In fungal species, differentiation to the filamentous/hyphal cell type is critical for entry into host cells and virulence. Comparative RNA sequencing was used to explore the pathways that regulate differentiation to the filamentous cell type in yeast. This approach uncovered a role for the stress-response MAPK pathway, HOG, during the increased metabolic respiration that induces filamentous growth. In this context, the AMPK Snf1p and ER stress kinase Ire1p regulated the HOG pathway. Cross-modulation between the HOG and filamentous growth (ERK-type) MAPK pathways optimized the differentiation response. The regulatory circuit described here may extend to behaviors in metazoans.

Yeast osmosensors Hkr1 and Msb2 activate the Hog1 MAPK cascade by different mechanisms

To cope with environmental high osmolarity, the budding yeast Saccharomyces cerevisiae activates the mitogen-activated protein kinase (MAPK) Hog1, which controls an array of osmoadaptive responses. Two independent, but functionally redundant, osmosensing systems involving the transmembrane sensor histidine kinase Sln1 or the tetraspanning membrane protein Sho1 stimulate the Hog1 MAPK cascade. Furthermore, the Sho1 signaling branch itself also involves the two functionally redundant osmosensors Hkr1 and Msb2. However, any single osmosensor (Sln1, Hkr1, or Msb2) is sufficient for osmoadaptation. We found that the signaling mechanism by which Hkr1 or Msb2 stimulated the Hog1 cascade was specific to each osmosensor. Specifically, activation of Hog1 by Msb2 required the scaffold protein Bem1 and the actin cytoskeleton. Bem1 bound to the cytoplasmic domain of Msb2 and thus recruited the kinases Ste20 and Cla4 to the membrane, where either of them can activate the kinase Ste11. The cytoplasmic domain of Hkr1 also contributed to the activation of Ste11 by Ste20, but through a mechanism that involved neither Bem1 nor the actin cytoskeleton. Furthermore, we found a PXXP motif in Ste20 that specifically bound to the Sho1 SH3 (Src homology 3) domain. This interaction between Ste20 and Sho1 contributed to the activation of Hog1 by Hkr1, but not by Msb2. These differences between Hkr1 and Msb2 may enable differential regulation of these two proteins and provide a mechanism through Msb2 to connect regulation of the cytoskeleton with the response to osmotic stress.

The HOG signal transduction pathway in the halophilic fungus Wallemia ichthyophaga: identification and characterisation of MAP kinases WiHog1A and WiHog1B

The high-osmolarity glycerol (HOG) pathway is one of the several MAP kinase cascades in fungi. It is the main signal transduction system that is responsible for cellular stress responses, and has primarily been studied in the context of osmotic stress. In the present study, we provide the first insights into the HOG pathway of the obligatory halophilic basidiomycetous fungus Wallemia ichthyophaga, with the characterisation of its two Hog1-like kinases: WiHog1A and WiHog1B. These share high similarity to Hog1 kinase from Saccharomyces cerevisiae (ScHog1) at the level of amino-acid sequence. While WiHog1A could not optimally complement the function of ScHog1, WiHog1B was a fully functional Hog1-like kinase and could improve the halotolerance of the yeast, compared to the wild-type or the ScHog1-expressing hog1Δ strain. In W. ichthyophaga cells, Hog1 was constitutively phosphorylated under optimal osmotic conditions and dephosphorylated when the cells were challenged with hypo-osmolar or hyperosmolar stress. This pattern of phosphorylation kinetics is opposite to that of yeast. Transcriptional analysis of these two kinases in W. ichthyophaga shows that WiHOG1B is more responsive to changes in NaCl concentrations than WiHOG1A. Our identification and characterisation of these Hog1-like kinases from W. ichthyophaga confirm the existence of the HOG signalling pathway and its role in osmosensing in this halophilic fungus.

Involvement of protein tyrosine phosphatases BcPtpA and BcPtpB in regulation of vegetative development, virulence and multi-stress tolerance in Botrytis cinerea

Tyrosine phosphorylation and dephosphorylation have emerged as fundamentally important mechanisms of signal transduction and regulation in eukaryotic cells, governing many processes, but little has been known about their functions in filamentous fungi. In this study, we deleted two putative protein tyrosine phosphatase (PTP) genes (BcPTPA and BcPTPB) in Botrytis cinerea, encoding the orthologs of Saccharomyces cerevisiae Ptp2 and Ptp3, respectively. Although BcPtpA and BcPtpB have opposite functions in conidiation, they are essential for sclerotial formation in B. cinerea. BcPTPA and BcPTPB deletion mutants ΔBcPtpA-10 and ΔBcPtpB-4 showed significantly increased sensitivity to osmotic and oxidative stresses, and to cell wall damaging agents. Inoculation tests showed that both mutants exhibited dramatically decreased virulence on tomato leaves, apples and grapes. In S. cerevisiae, it has been shown that Ptp2 and Ptp3 negatively regulate the high-osmolarity glycerol (HOG) pathway and the cell wall integrity (CWI) pathway. Although both BcPtpA and BcPtpB were able to inactive Hog1 and Mpk1 in S. cerevisiae, in contrast to S. cerevisiae, they positively regulate phosphorylation of BcSak1 (the homologue of Hog1) and BcBmp3 (the homologue of Mpk1) in B. cinerea under stress conditions. These results demonstrated that functions of PTPs in B. cinerea are different from those in S. cerevisiae, and BcPtpA and BcPtpB play important roles in regulation of vegetative development, virulence and in adaptation to oxidative, osmotic and cell-wall damage stresses in B. cinerea.

Structural mechanism for the specific assembly and activation of the extracellular signal regulated kinase 5 (ERK5) module

Mitogen-activated protein kinase (MAPK) activation depends on a linear binding motif found in all MAPK kinases (MKK). In addition, the PB1 (Phox and Bem1) domain of MKK5 is required for extracellular signal regulated kinase 5 (ERK5) activation. We present the crystal structure of ERK5 in complex with an MKK5 construct comprised of the PB1 domain and the linear binding motif. We show that ERK5 has distinct protein-protein interaction surfaces compared with ERK2, which is the closest ERK5 paralog. The two MAPKs have characteristically different physiological functions and their distinct protein-protein interaction surface topography enables them to bind different sets of activators and substrates. Structural and biochemical characterization revealed that the MKK5 PB1 domain cooperates with the MAPK binding linear motif to achieve substrate specific binding, and it also enables co-recruitment of the upstream activating enzyme and the downstream substrate into one signaling competent complex. Studies on present day MAPKs and MKKs hint on the way protein kinase networks may evolve. In particular, they suggest how paralogous enzymes with similar catalytic properties could acquire novel signaling roles by merely changing the way they make physical links to other proteins.

Response to hyperosmotic stress

An appropriate response and adaptation to hyperosmolarity, i.e., an external osmolarity that is higher than the physiological range, can be a matter of life or death for all cells. It is especially important for free-living organisms such as the yeast Saccharomyces cerevisiae. When exposed to hyperosmotic stress, the yeast initiates a complex adaptive program that includes temporary arrest of cell-cycle progression, adjustment of transcription and translation patterns, and the synthesis and retention of the compatible osmolyte glycerol. These adaptive responses are mostly governed by the high osmolarity glycerol (HOG) pathway, which is composed of membrane-associated osmosensors, an intracellular signaling pathway whose core is the Hog1 MAP kinase (MAPK) cascade, and cytoplasmic and nuclear effector functions. The entire pathway is conserved in diverse fungal species, while the Hog1 MAPK cascade is conserved even in higher eukaryotes including humans. This conservation is illustrated by the fact that the mammalian stress-responsive p38 MAPK can rescue the osmosensitivity of hog1Δ mutations in response to hyperosmotic challenge. As the HOG pathway is one of the best-understood eukaryotic signal transduction pathways, it is useful not only as a model for analysis of osmostress responses, but also as a model for mathematical analysis of signal transduction pathways. In this review, we have summarized the current understanding of both the upstream signaling mechanism and the downstream adaptive responses to hyperosmotic stress in yeast.

Osmostress induces autophosphorylation of Hog1 via a C-terminal regulatory region that is conserved in p38α

Many protein kinases require phosphorylation at their activation loop for induction of catalysis. Mitogen-activated protein kinases (MAPKs) are activated by a unique mode of phosphorylation, on neighboring Tyrosine and Threonine residues. Whereas many kinases obtain their activation via autophosphorylation, MAPKs are usually phosphorylated by specific, dedicated, MAPK kinases (MAP2Ks). Here we show however, that the yeast MAPK Hog1, known to be activated by the MAP2K Pbs2, is activated in pbs2Δ cells via an autophosphorylation activity that is induced by osmotic pressure. We mapped a novel domain at the Hog1 C-terminal region that inhibits this activity. Removal of this domain provides a Hog1 protein that is partially independent of MAP2K, namely, partially rescues osmostress sensitivity of pbs2Δ cells. We further mapped a short domain (7 amino acid residues long) that is critical for induction of autophosphorylation. Its removal abolishes autophosphorylation, but maintains Pbs2-mediated phosphorylation. This 7 amino acids stretch is conserved in the human p38α. Similar to the case of Hog1, it’s removal from p38α abolishes p38α’s autophosphorylation capability, but maintains, although reduces, its activation by MKK6. This study joins a few recent reports to suggest that, like many protein kinases, MAPKs are also regulated via induced autoactivation.

A framework for mapping, visualisation and automatic model creation of signal-transduction networks

An intuitive formalism for reconstructing cellular networks from empirical data is presented, and used to build a comprehensive yeast MAP kinase network. The accompanying rxncon software tool can convert networks to a range of standard graphical formats and mathematical models.

Network mapping at the granularity of empirical data that largely avoids combinatorial complexity

Automatic visualisation and model generation with the rxncon open source software tool

Visualisation in a range of formats, including all three SBGN formats, as well as contingency matrix or regulatory graph

Comprehensive and completely references map of the yeast MAP kinase network in the rxncon format

Intracellular signalling systems are highly complex. This complexity makes handling, analysis and visualisation of available knowledge a major challenge in current signalling research. Here, we present a novel framework for mapping signal-transduction networks that avoids the combinatorial explosion by breaking down the network in reaction and contingency information. It provides two new visualisation methods and automatic export to mathematical models. We use this framework to compile the presently most comprehensive map of the yeast MAP kinase network. Our method improves previous strategies by combining (I) more concise mapping adapted to empirical data, (II) individual referencing for each piece of information, (III) visualisation without simplifications or added uncertainty, (IV) automatic visualisation in multiple formats, (V) automatic export to mathematical models and (VI) compatibility with established formats. The framework is supported by an open source software tool that facilitates integration of the three levels of network analysis: definition, visualisation and mathematical modelling. The framework is species independent and we expect that it will have wider impact in signalling research on any system.

Involvement of a putative response regulator Brrg-1 in the regulation of sporulation, sensitivity to fungicides, and osmotic stress in Botrytis cinerea

The response regulator protein is a core element of two-component signaling pathway. In this study, we investigated functions of BRRG-1 of Botrytis cinerea, a gene that encodes a putative response regulator protein, which is homologous to Rrg-1 in Neurospora crassa. The BRRG-1 gene deletion mutant ΔBrrg1-62 was unable to produce conidia. The mutant showed increased sensitivity to osmotic stress mediated by NaCl and KCl, and to oxidative stress generated by H(2)O(2). Additionally, the mutant was more sensitive to the fungicides iprodione, fludioxonil, and triadimefon than the parental strain. Western-blot analysis showed that the Bos-2 protein, the putative downstream component of Brrg-1, was not phosphorylated in the ΔBrrg1-62. Real-time polymerase chain reaction assays showed that expression of BOS-2 also decreased significantly in the mutant. All of the defects were restored by genetic complementation of the ΔBrrg1-62 with the wild-type BRRG-1 gene. Plant inoculation tests showed that the mutant did not show changes in pathogenicity on rapeseed leaves. These results indicated that Brrg-1 is involved in the regulation of asexual development, sensitivity to iprodione, fludioxonil, and triadimefon fungicides, and adaptation to osmotic and oxidative stresses in B. cinerea.

The mitogen-activated protein kinase kinase BOS5 is involved in regulating vegetative differentiation and virulence in Botrytis cinerea

We present a characterization of bos5 from Botrytis cinerea, a gene that encodes a mitogen-activated protein kinase kinase (MAPKK), which is homologous to OS-5 of Neurospora crassa. The bos5 gene deletion mutant exhibited reduced vegetative growth and strongly impaired conidiation. The mutant also exhibited increased sensitivity to the dicarboximide fungicide iprodione and to osmotic stress mediated by NaCl or KCl. Western-blot analysis showed that the BcSAK1 protein, the putative downstream component of BOS5, was not phosphorylated in the mutant. Plant inoculation tests showed that the mutants were unable to infect cucumber leaves. All of these defects were restored by genetic complementation of the Deltabcos5-21 mutant with the wild-type bos5 gene. These results indicated that BOS5 is involved in the regulation of vegetative differentiation, virulence, adaptation to iprodione and ionic stress in B. cinerea.

Transcriptomic response of the mycoparasitic fungus Trichoderma atroviride to the presence of a fungal prey

Background

Combating the action of plant pathogenic microorganisms by mycoparasitic fungi has been announced as an attractive biological alternative to the use of chemical fungicides since two decades. The fungal genus Trichoderma includes a high number of taxa which are able to recognize, combat and finally besiege and kill their prey. Only fragments of the biochemical processes related to this ability have been uncovered so far, however.

Results

We analyzed genome-wide gene expression changes during the begin of physical contact between Trichoderma atroviride and two plant pathogens Botrytis cinerea and Rhizoctonia solani, and compared with gene expression patterns of mycelial and conidiating cultures, respectively. About 3000 ESTs, representing about 900 genes, were obtained from each of these three growth conditions. 66 genes, represented by 442 ESTs, were specifically and significantly overexpressed during onset of mycoparasitism, and the expression of a subset thereof was verified by expression analysis. The upregulated genes comprised 18 KOG groups, but were most abundant from the groups representing posttranslational processing, and amino acid metabolism, and included components of the stress response, reaction to nitrogen shortage, signal transduction and lipid catabolism. Metabolic network analysis confirmed the upregulation of the genes for amino acid biosynthesis and of those involved in the catabolism of lipids and aminosugars.

Conclusion

The analysis of the genes overexpressed during the onset of mycoparasitism in T. atroviride has revealed that the fungus reacts to this condition with several previously undetected physiological reactions. These data enable a new and more comprehensive interpretation of the physiology of mycoparasitism, and will aid in the selection of traits for improvement of biocontrol strains by recombinant techniques.

Mechanism of Mpk1 mitogen-activated protein kinase binding to the Swi4 transcription factor and its regulation by a novel caffeine-induced phosphorylation

The Mpk1 mitogen-activated protein kinase (MAPK) of the cell wall integrity signaling pathway uses a noncatalytic mechanism to activate the SBF (Swi4/Swi6) transcription factor. Active Mpk1 forms a complex with Swi4, the DNA-binding subunit of SBF, conferring the ability to bind DNA. Because SBF activation is independent of Mpk1 catalytic activity but requires Mpk1 to be in an active conformation, we sought to understand how Mpk1 interacts with Swi4. Mutational analysis revealed that binding and activation of Swi4 by Mpk1 requires an intact D-motif-binding site, a docking surface common to MAPKs that resides distal to the phosphorylation loop but does not require the substrate-binding site, revealing a novel mechanism for MAPK target regulation. Additionally, we found that Mpk1 binds near the autoinhibitory C terminus of Swi4, suggesting an activation mechanism in which Mpk1 substitutes for Swi6 in promoting Swi4 DNA binding. Finally, we show that caffeine is an atypical activator of cell wall integrity signaling, because it induces phosphorylation of the Mpk1 C-terminal extension at Ser423 and Ser428. These phosphorylations were dependent on the DNA damage checkpoint kinases, Mec1/Tel1 and Rad53. Phosphorylation of Ser423 specifically blocked SBF activation by preventing Mpk1 association with Swi4, revealing a novel mechanism for regulating MAPK target specificity.

Msb2 signaling mucin controls activation of Cek1 mitogen-activated protein kinase in Candida albicans

We have characterized the role that the Msb2 protein plays in the fungal pathogen Candida albicans by the use of mutants defective in the putative upstream components of the HOG pathway. Msb2, in cooperation with Sho1, controls the activation of the Cek1 mitogen-activated protein kinase under conditions that damage the cell wall, thus defining Msb2 as a signaling element of this pathway in the fungus. msb2 mutants display altered sensitivity to Congo red, caspofungin, zymolyase, or tunicamycin, indicating that this protein is involved in cell wall biogenesis. Msb2 (as well as Sho1 and Hst7) is involved in the transmission of the signal toward Cek1 mediated by the Cdc42 GTPase, as revealed by the use of activated alleles (Cdc42(G12V)) of this protein. msb2 mutants have a stronger defective invasion phenotype than sho1 mutants when tested on certain solid media that use mannitol or sucrose as a carbon source or under hypoxia. Interestingly, Msb2 contributes to growth under conditions of high osmolarity when both branches of the HOG pathway are altered, as triple ssk1 msb2 sho1 mutants (but not any single or double mutant) are osmosensitive. However, this phenomenon is independent of the presence of Hog1, as Hog1 phosphorylation, Hog1 translocation to the nucleus, and glycerol accumulation are not affected in this mutant following an osmotic shock. These results reveal essential functions in morphogenesis, invasion, cell wall biogenesis, and growth under conditions of high osmolarity for Msb2 in C. albicans and suggest the divergence and specialization of this signaling pathway in filamentous fungi.

Glycosylation defects activate filamentous growth Kss1 MAPK and inhibit osmoregulatory Hog1 MAPK

The yeast filamentous growth (FG) MAP kinase (MAPK) pathway is activated under poor nutritional conditions. We found that the FG-specific Kss1 MAPK is activated by a combination of an O-glycosylation defect caused by disruption of the gene encoding the protein O-mannosyltransferase Pmt4, and an N-glycosylation defect induced by tunicamycin. The O-glycosylated membrane proteins Msb2 and Opy2 are both essential for activating the FG MAPK pathway, but only defective glycosylation of Msb2 activates the FG MAPK pathway. Although the osmoregulatory HOG (high osmolarity glycerol) MAPK pathway and the FG MAPK pathway share almost the entire upstream signalling machinery, osmostress activates only the HOG-specific Hog1 MAPK. Conversely, we now show that glycosylation defects activate only Kss1, while activated Kss1 and the Ptp2 tyrosine phosphatase inhibit Hog1. In the absence of Kss1 or Ptp2, however, glycosylation defects activate Hog1. When Hog1 is activated by glycosylation defects in ptp2 mutant, Kss1 activation is suppressed by Hog1. Thus, the reciprocal inhibitory loop between Kss1 and Hog1 allows only one or the other of these MAPKs to be stably activated under various stress conditions.

When expressed in yeast, mammalian mitogen-activated protein kinases lose proper regulation and become spontaneously phosphorylated

MAPKs (mitogen-activated protein kinases) are key components in cell signalling pathways. Under optimal growth conditions, their activity is kept off, but in response to stimulation it is dramatically evoked. Because of the high degree of evolutionary conservation at the levels of sequence and mode of activation, MAPKs are believed to share similar regulatory mechanisms in all eukaryotes and to be functionally substitutable between them. To assess the reliability of this notion, we systematically analysed the activity, regulation and phenotypic effects of mammalian MAPKs in yeast. Unexpectedly, all mammalian MAPKs tested were spontaneously phosphorylated in yeast. JNKs (c-Jun N-terminal kinases) lost their phosphorylation in pbs2Delta cells, but p38s and ERKs (extracellular-signal-regulated kinases) maintained their spontaneous phosphorylation even in pbs2Deltaste7Deltamkk1Deltamkk2Delta cells. Kinase-dead variants of ERKs and p38s were phosphorylated in strains lacking a single MEK (MAPK/ERK kinase), but not in pbs2Deltaste7Deltamkk1Deltamkk2Delta cells. Thus, in yeast, p38 and ERKs are phosphorylated via a combined mechanism of autophosphorylation and MEK-mediated phosphorylation (any MEK). We further addressed the mechanism allowing mammalian MAPKs to exploit yeast MEKs in the absence of any activating signal. We suggest that mammalian MAPKs lost during evolution a C-terminal region that exists in some yeast MAPKs. Indeed, removal of this region from Hog1 and Mpk1 rendered them spontaneously and highly phosphorylated. It implies that MAPKs possess an efficient inherent autoposphorylation capability that is suppressed in yeast MAPKs via a C-terminal domain and in mammalian MAPKs via as yet unknown means.