The cta3+ gene that encodes a cation-transporting P-type ATPase is induced by salt stress under control of the Wis1-Sty1 MAPKK-MAPK cascade in fission yeast

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.

Exomer Is Part of a Hub Where Polarized Secretion and Ionic Stress Connect

Plasma membrane and membranous organelles contribute to the physiology of the Eukaryotic cell by participating in vesicle trafficking and the maintenance of ion homeostasis. Exomer is a protein complex that facilitates vesicle transport from the trans-Golgi network to the plasma membrane, and its absence leads to the retention of a set of selected cargoes in this organelle. However, this retention does not explain all phenotypes observed in exomer mutants. The Schizosaccharomyces pombe exomer is composed of Cfr1 and Bch1, and cfr1Δ and bch1Δ were sensitive to high concentrations of potassium salts but not sorbitol, which showed sensitivity to ionic but not osmotic stress. Additionally, the activity of the plasma membrane ATPase was higher in exomer mutants than in the wild-type, pointing to membrane hyperpolarization, which caused an increase in intracellular K⁺ content and mild sensitivity to Na⁺, Ca²⁺, and the aminoglycoside antibiotic hygromycin B. Moreover, in response to K⁺ shock, the intracellular Ca²⁺ level of cfr1Δ cells increased significantly more than in the wild-type, likely due to the larger Ca²⁺ spikes in the mutant. Microscopy analyses showed a defective endosomal morphology in the mutants. This was accompanied by an increase in the intracellular pools of the K⁺ exporting P-type ATPase Cta3 and the plasma membrane Transient Receptor Potential (TRP)-like Ca²⁺ channel Pkd2, which were partially diverted from the trans-Golgi network to the prevacuolar endosome. Despite this, most Cta3 and Pkd2 were delivered to the plasma membrane at the cell growing sites, showing that their transport from the trans-Golgi network to the cell surface occurred in the absence of exomer. Nevertheless, shortly after gene expression in the presence of KCl, the polarized distribution of Cta3 and Pkd2 in the plasma membrane was disturbed in the mutants. Finally, the use of fluorescent probes suggested that the distribution and dynamics of association of some lipids to the plasma membrane in the presence of KCl were altered in the mutants. Thus, exomer participation in the response to K⁺ stress was multifaceted. These results supported the notion that exomer plays a general role in protein sorting at the trans-Golgi network and in polarized secretion, which is not always related to a function as a selective cargo adaptor.

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

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

Quantitative proteomics reveals dynamic responses of Synechocystis sp. PCC 6803 to next-generation biofuel butanol

Butanol is a promising biofuel, and recent metabolic engineering efforts have demonstrated the use of photosynthetic cyanobacterial hosts for its production. However, cyanobacteria have very low tolerance to butanol, limiting the economic viability of butanol production from these renewable producing systems. The existing knowledge of molecular mechanism involved in butanol tolerance in cyanobacteria is very limited. To build a foundation necessary to engineer robust butanol-producing cyanobacterial hosts, in this study, the responses of Synechocystis PCC 6803 to butanol were investigated using a quantitative proteomics approach with iTRAQ - LC-MS/MS technologies. The resulting high-quality dataset consisted of 25,347 peptides corresponding to 1452 unique proteins, a coverage of approximately 40% of the predicted proteins in Synechocystis. Comparative quantification of protein abundances led to the identification of 303 differentially regulated proteins by butanol. Annotation and GO term enrichment analysis showed that multiple biological processes were regulated, suggesting that Synechocystis probably employed multiple and synergistic resistance mechanisms in dealing with butanol stress. Notably, the analysis revealed the induction of heat-shock protein and transporters, along with modification of cell membrane and envelope were the major protection mechanisms against butanol. A conceptual cellular model of Synechocystis PCC 6803 responses to butanol stress was constructed to illustrate the putative molecular mechanisms employed to defend against butanol stress.

AtfA bZIP-type transcription factor regulates oxidative and osmotic stress responses in Aspergillus nidulans

The aim of the study was to demonstrate that the bZIP-type transcription factor AtfA regulates different types of stress responses in Aspergillus nidulans similarly to Atf1, the orthologous 'all-purpose' transcription factor of Schizosaccharomyces pombe. Heterologous expression of atfA in a S. pombe Deltaatf1 mutant restored the osmotic stress tolerance of fission yeast in surface cultures to the same level as recorded in complementation studies with the atf1 gene, and a partial complementation of the osmotic and oxidative-stress-sensitive phenotypes was also achieved in submerged cultures. AtfA is therefore a true functional ortholog of fission yeast's Atf1. As demonstrated by RT-PCR experiments, elements of both oxidative (e.g. catalase B) and osmotic (e.g. glycerol-3-phosphate dehydrogenase B) stress defense systems were transcriptionally regulated by AtfA in a stress-type-specific manner. Deletion of atfA resulted in oxidative-stress-sensitive phenotypes while the high-osmolarity stress sensitivity of the fungus was not affected significantly. In A. nidulans, the glutathione/glutathione disulfide redox status of the cells as well as apoptotic cell death and autolysis seemed to be controlled by regulatory elements other than AtfA. In conclusion, the orchestrations of stress responses in the aspergilli and in fission yeast share several common features, but further studies are needed to answer the important question of whether a fission yeast-like core environmental stress response also operates in the euascomycete genus Aspergillus.

Growth at high pH and sodium and potassium tolerance in media above the cytoplasmic pH depend on ENA ATPases in Ustilago maydis

Potassium and Na(+) effluxes across the plasma membrane are crucial processes for the ionic homeostasis of cells. In fungal cells, these effluxes are mediated by cation/H(+) antiporters and ENA ATPases. We have cloned and studied the functions of the two ENA ATPases of Ustilago maydis, U. maydis Ena1 (UmEna1) and UmEna2. UmEna1 is a typical K(+) or Na(+) efflux ATPase whose function is indispensable for growth at pH 9.0 and for even modest Na(+) or K(+) tolerances above pH 8.0. UmEna1 locates to the plasma membrane and has the characteristics of the low-Na(+)/K(+)-discrimination ENA ATPases. However, it still protects U. maydis cells in high-Na(+) media because Na(+) showed a low cytoplasmic toxicity. The UmEna2 ATPase is phylogenetically distant from UmEna1 and is located mainly at the endoplasmic reticulum. The function of UmEna2 is not clear, but we found that it shares several similarities with Neurospora crassa ENA2, which suggests that endomembrane ENA ATPases may exist in many fungi. The expression of ena1 and ena2 transcripts in U. maydis was enhanced at high pH and at high K(+) and Na(+) concentrations. We discuss that there are two modes of Na(+) tolerance in fungi: the high-Na(+)-content mode, involving ENA ATPases with low Na(+)/K(+) discrimination, as described here for U. maydis, and the low-Na(+)-content mode, involving Na(+)-specific ENA ATPases, as in Neurospora crassa.

Comparison of gene expression signatures of diamide, H2O2 and menadione exposed Aspergillus nidulans cultures--linking genome-wide transcriptional changes to cellular physiology

BACKGROUND

In addition to their cytotoxic nature, reactive oxygen species (ROS) are also signal molecules in diverse cellular processes in eukaryotic organisms. Linking genome-wide transcriptional changes to cellular physiology in oxidative stress-exposed Aspergillus nidulans cultures provides the opportunity to estimate the sizes of peroxide (O2(2-)), superoxide (O2*-) and glutathione/glutathione disulphide (GSH/GSSG) redox imbalance responses.

RESULTS

Genome-wide transcriptional changes triggered by diamide, H2O2 and menadione in A. nidulans vegetative tissues were recorded using DNA microarrays containing 3533 unique PCR-amplified probes. Evaluation of LOESS-normalized data indicated that 2499 gene probes were affected by at least one stress-inducing agent. The stress induced by diamide and H2O2 were pulse-like, with recovery after 1 h exposure time while no recovery was observed with menadione. The distribution of stress-responsive gene probes among major physiological functional categories was approximately the same for each agent. The gene group sizes solely responsive to changes in intracellular O2(2-), O2*- concentrations or to GSH/GSSG redox imbalance were estimated at 7.7, 32.6 and 13.0 %, respectively. Gene groups responsive to diamide, H2O2 and menadione treatments and gene groups influenced by GSH/GSSG, O2(2-) and O2*- were only partly overlapping with distinct enrichment profiles within functional categories. Changes in the GSH/GSSG redox state influenced expression of genes coding for PBS2 like MAPK kinase homologue, PSK2 kinase homologue, AtfA transcription factor, and many elements of ubiquitin tagging, cell division cycle regulators, translation machinery proteins, defense and stress proteins, transport proteins as well as many enzymes of the primary and secondary metabolisms. Meanwhile, a separate set of genes encoding transport proteins, CpcA and JlbA amino acid starvation-responsive transcription factors, and some elements of sexual development and sporulation was ROS responsive.

CONCLUSION

The existence of separate O2(2-), O2*- and GSH/GSSG responsive gene groups in a eukaryotic genome has been demonstrated. Oxidant-triggered, genome-wide transcriptional changes should be analyzed considering changes in oxidative stress-responsive physiological conditions and not correlating them directly to the chemistry and concentrations of the oxidative stress-inducing agent.

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

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

K+ fluxes in Schizosaccharomyces pombe

All living cells accumulate high concentrations of K+ in order to keep themselves alive. To this end they have developed a great diversity of transporters. The internal level of K+ is the result of the net balance between the activities of the K+ influx and the K+ efflux transporters. Potassium fluxes have been extensively studied and characterized in Saccharomyces cerevisiae. However, this is not the case in the fission yeast and, in addition, the information available indicates that both yeasts present substantial and interesting differences. In this paper we have reviewed and summarized the information on K+ fluxes in Schizosaccharomyces pombe. We have included some unpublished results recently obtained in our laboratory and, in particular, we have highlighted the significant differences found between the well-known yeast S. cerevisiae and the fission yeast Sch. pombe.

An inventory of the P-type ATPases in the fission yeast Schizosaccharomyces pombe

The analysis of the Schizosaccharomyces pombe genome revealed the presence of 14 putative P-type ATPases. The clustering of ATPases resembles that of Saccharomyces cerevisiae, indicating that the main classes of pumps were already present before the split of the Archiascomycetes from the other Ascomycota. The overall amino acid identity between fission and budding yeast P-type ATPases is generally low (30-50%). This is similar to the fungus-plant and fungus-animal comparisons, suggesting that fungal ATPases underwent an extensive process of diversification. Unlike Sac. cerevisiae, fission yeast lacks Na(+)-ATPases, has a single heavy-metal ATPase and three ATPases of unknown specificity. The observed divergence within these fungi might reflect physiological differences, including adaptation to environmental stresses.

Role of fission yeast Tup1-like repressors and Prr1 transcription factor in response to salt stress

In Schizosaccharomyces pombe, the Sty1 mitogen-activated protein kinase and the Atf1 transcription factor control transcriptional induction in response to elevated salt concentrations. Herein, we demonstrate that two repressors, Tup11 and Tup12, and the Prr1 transcription factor also function in the response to salt shock. We find that deletion of both tup genes together results in hypersensitivity to elevated cation concentrations (K(+) and Ca(2+)) and we identify cta3(+), which encodes an intracellular cation transporter, as a novel stress gene whose expression is positively controlled by the Sty1 pathway and negatively regulated by Tup repressors. The expression of cta3(+) is maintained at low levels by the Tup repressors, and relief from repression requires the Sty1, Atf1, and Prr1. Prr1 is also required for KCl-mediated induction of several other Sty1-dependent genes such as gpx1(+) and ctt1(+). Surprisingly, the KCl-mediated induction of cta3(+) expression occurs independently of Sty1 in a tup11Delta tup12Delta mutant and so the Tup repressors link induction to the Sty1 pathway. We also report that in contrast to a number of other Sty1- and Atf1-dependent genes, the expression of cta3(+) is induced only by high salt concentrations. However, in the absence of the Tup repressors this specificity is lost and a range of stresses induces cta3(+) expression.

The endoplasmic reticulum cation P-type ATPase Cta4p is required for control of cell shape and microtubule dynamics

Here we describe the phenotypic characterization of the cta4+ gene, encoding a novel member of the P4 family of P-type ATPases of fission yeast. The cta4Delta mutant is temperature sensitive and cold sensitive lethal and displays several morphological defects in cell polarity and cytokinesis. Microtubules are generally destabilized in cells lacking Cta4p. The microtubule length is decreased, and the number of microtubules per cell is increased. This is concomitant with an increase in the number of microtubule catastrophe events in the midzone of the cell. These defects are likely due to a general imbalance in cation homeostasis. Immunofluorescence microscopy and membrane fractionation experiments revealed that green fluorescent protein-tagged Cta4 localizes to the ER. Fluorescence resonance energy transfer experiments in living cells using the yellow cameleon indicator for Ca2+ indicated that Cta4p regulates the cellular Ca2+ concentration. Thus, our results reveal a link between cation homeostasis and the control of cell shape, microtubule dynamics, and cytokinesis, and appoint Ca2+ as a key ion in controlling these processes.

Potassium- or sodium-efflux ATPase, a key enzyme in the evolution of fungi

Potassium is the most abundant cation in cells. Therefore, plant-associated fungi and intracellular parasites are permanently or circumstantially exposed to high K(+) and must avoid excessive K(+) accumulation activating K(+) efflux systems. Because high K(+) and high pH are compatible in natural environments, free-living organisms cannot keep a permanent transmembrane DeltapH and cannot rely only on K(+)/H(+) antiporters, as do mitochondria. This study shows that the Schizosaccharomyces pombe CTA3 is a K(+)-efflux ATPase, and that other fungi are furnished with Na(+)-efflux ATPases, which also pump Na(+). All these fungal ATPases, including those pumping only Na(+), form a phylogenetic group, IID or ENA, among P-type ATPases. By searching in databases and partial cloning of ENA genes in species of Zygomycetes and Basidiomycetes, the authors conclude that probably all fungi have these genes. This study indicates that fungal K(+)- or Na(+)-ATPases evolved from an ancestral K(+)-ATPase, through processes of gene duplication. In yeast hemiascomycetes these duplications have occurred recently and produced bifunctional ATPases, whereas in Neurospora, and probably in other euascomycetes, they occurred earlier in evolution and produced specialized ATPases. In Schizosaccharomyces, adaptation to Na(+) did not involve the duplication of the K(+)-ATPase and thus it retains an enzyme which is probably close to the original one. The parasites Leishmania and Trypanosoma have ATPases phylogenetically related to fungal K(+)-ATPases, which are probably functional homologues of the fungal enzymes.

Vacuolar H(+)-ATPase, but not mitochondrial F(1)F(0)-ATPase, is required for NaCl tolerance in Saccharomyces cerevisiae

Salt tolerance in Saccharomyces cerevisiae is a complex trait, involving regulation of membrane polarization, Na(+) efflux and sequestration of Na(+) in the vacuole. Since transmembrane transport energized by H(+)-adenosine triphosphatases (ATPases) is common to all of these tolerance mechanisms, the objective of this study was to characterize the responses of the plasma membrane H(+)-ATPase, vacuolar H(+)-ATPase and mitochondrial F(1)F(0)-ATPase to NaCl stress. We hypothesized that since the vacuolar ATPase is responsible for generating the proton motive force required for import of cations (such as Na(+)) into the vacuole, strains lacking this activity should be hypersensitive to NaCl. We found that strains lacking vacuolar ATPase activity were in fact hypersensitive to NaCl, while strains lacking ATP synthase were not. This effect was specific to the ionic component of NaCl stress, since the mutant strains were indistinguishable from wild-type and complemented strains in the presence of sorbitol.