A yeast protein similar to bacterial two-component regulators

Genetic and biochemical analysis of the SLN1 pathway in Saccharomyces cerevisiae

The histidine kinase-based signal transduction pathway was first uncovered in bacteria and is a prominent form of regulation in prokaryotes. However, this type of signal transduction is not unique to prokaryotes; over the last decade two-component signal transduction pathways have been identified and characterized in diverse eukaryotes, from unicellular yeasts to multicellular land plants. A number of small but important differences have been noted in the architecture and function of eukaryotic pathways. Because of the powerful genetic approaches and facile molecular analysis associated with the yeast system, the SLN1 osmotic response pathway in Saccharomyces cerevisiae is particularly useful as a eukaryotic pathway model. This chapter provides an overview of genetic and biochemical methods that have been important in elucidating the stimulus-response events that underlie this pathway and in understanding the details of a eukaryotic His-Asp phosphorelay.

Interactions among HAMP domain repeats act as an osmosensing molecular switch in group III hybrid histidine kinases from fungi

The members of group III hybrid histidine kinases (HHK) are ubiquitous in fungi. Group III HHK have been implicated to function as osmosensors in the high osmolarity glycerol (HOG) pathway that is essential for fungal survival under high osmolarity stress. Recent literature suggests that group III HHK are also involved in conidia formation, virulence in several filamentous fungi, and are an excellent molecular target for antifungal agents. Thus, group III HHK constitute a very important group of sensor kinases. Structurally, group III HHK are distinct from Sln1p, the osmosensing HHK that regulates the HOG pathway in Saccharomyces cerevisiae. Group III HHK lack any transmembrane domain and typically contain HAMP domain repeats at the N terminus. Until now, it is not clear how group III HHK function as an osmosensor to regulate the HOG pathway. To investigate this, we undertook molecular characterization of DhNIK1, an ortholog from osmotolerant yeast Debaryomyces hansenii. We show here that DhNIK1 could complement sln1 mutation in S. cerevisiae thereby confirming its role as a bona fide osmosensor. We further investigated the role of HAMP domains by deleting them systematically. Our results clearly indicate that the HAMP4 domain is crucial for osmosensing by DhNik1p. Most importantly, we also show that the alternative interaction among the HAMP domains regulates the activity of DhNik1p like an "on-off switch" and thus provides, for the first time, an insight into the molecular mechanism of osmosensing by this group of HHKs.

Effects of osmolytes on the SLN1-YPD1-SSK1 phosphorelay system from Saccharomyces cerevisiae

The multistep His-Asp phosphorelay system in Saccharomyces cerevisiae allows cells to adapt to osmotic, oxidative, and other environmental stresses. The pathway consists of a hybrid histidine kinase SLN1, a histidine-containing phosphotransfer (HPt) protein YPD1, and two response regulator proteins, SSK1 and SKN7. Under nonosmotic stress conditions, the SLN1 sensor kinase is active, and phosphoryl groups are shuttled through YPD1 to SSK1, therefore maintaining the response regulator protein in a constitutively phosphorylated state. The cellular response to hyperosmotic stress involves rapid efflux of water and changes in intracellular ion and osmolyte concentration. In this study, we examined the individual and combined effects of NaCl and glycerol on phosphotransfer rates within the SLN1-YPD1-SSK1 phosphorelay. The results show that the combined effects of glycerol and NaCl on the phosphotransfer reaction rates are different from the individual effects of glycerol and NaCl. The combinatory effect is likely more representative of the in vivo changes that occur during hyperosmotic stress. In addition, the effect of osmolyte concentration on the half-life of the phosphorylated SSK1 receiver domain in the presence/absence of YPD1 was evaluated. Our findings demonstrate that increasing osmolyte concentrations negatively affect the YPD1 x SSK1-P interaction, thereby facilitating dephosphorylation of SSK1 and activating the HOG1 MAP kinase cascade. In contrast, at the highest osmolyte concentrations, reflective of the osmoadaptation phase of the signaling pathway, the kinetics of the phosphorelay favor production of SSK1-P and inhibition of the HOG1 pathway.

Dic2 and Dic3 loci confer osmotic adaptation and fungicidal sensitivity independent of the HOG pathway in Cochliobolus heterostrophus

Previously, we identified three gene loci, Dic1, Dic2, and Dic3, that confer high-osmolarity adaptation and dicarboximide/phenylpyrrole fungicide sensitivity in Cochliobolus heterostrophus. Dic1 encoded a group III histidine kinase, but the other genes were not characterized. In the present study, we revealed that both Dic2 and Dic3 are involved in the Skn7 pathway. Dic2 encoded an Skn7-type response regulator, ChSkn7. Strain N4502 contained D359N in the response regulator domain of ChSkn7. Strain E4503 contained a deletion of 50 amino acids in the DNA-binding domain. Strain N4507 was a null mutant of the ChSkn7 gene. All of the dic2 mutant strains showed similar levels of sensitivity to high osmolarity and similar levels of resistance to fungicides. These results strongly suggested that both the DNA-binding domain and response regulator domain are essential for Skn7 function in osmotic adaptation and fungicide sensitivity. A western blot analysis revealed that Dic3 is not involved in the regulation of Hog1-type MAPKs. The Chssk1/dic3 double mutant strains clearly showed greater resistance to fungicides than the single mutant strains. An additive effect was also observed in the high-osmolarity experiments. On the other hand, the dic3/dic2 double mutant strains did not show higher levels of resistance to fungicides and greater sensitivity to KCl than the single mutant strains. These results strongly suggested that the dic3 locus confer high-osmolarity adaptation and fungicide sensitivity independently from Ssk1-Hog1 pathway, but not the Skn7 pathway. Moreover, the dic3 strain and all dic2 strains showed similar levels of sensitivity to high-osmolarity stress and similar levels of resistance to fungicides, suggesting Dic3 to have an essential role in the Skn7 pathway. Our results provide new insight into the functions of the Skn7 pathway in filamentous fungi.

Modulation of the Rcs-mediated signal transfer by conformational flexibility

The Rcs (regulator of capsule synthesis) signalling complex comprises the membrane-associated hybrid sensor kinases RcsC and RcsD, the transcriptional regulator RcsB and the two co-inducers RcsA and RcsF. Acting as a global regulatory network, the Rcs phosphorelay controls multiple cellular pathways including capsule synthesis, cell division, motility, biofilm formation and virulence mechanisms. Signal-dependent communication of the individual Rcs domains showing histidine kinase, phosphoreceiver, phosphoryl transfer and DNA-binding activities is characteristic and essential for the modulation of signal transfer. We have analysed the structures of core elements of the Rcs network including the RcsC-PR (phosphoreceiver domain of RcsC) and the RcsD-HPt (histidine phosphotransfer domain of RcsD), and we have started to characterize the dynamics and recognition mechanisms of the proteins. RcsC-PR represents a typical CheY-like α/β/α sandwich fold and it shows a large conformational flexibility near the active-site residue Asp875. NMR analysis revealed that RcsC-PR is able to adopt preferred conformations upon Mg2+ co-ordination, BeF3− activation, phosphate binding and RcsD-HPt recognition. In contrast, the α-helical structure of RcsD-HPt is conformationally stable and contains a recognition area in close vicinity to the active-site His842 residue. Our studies indicate the importance of protein dynamics and conformational exchange for the differential response to the variety of signals perceived by complex regulatory networks.

The group III two-component histidine kinase of filamentous fungi is involved in the fungicidal activity of the bacterial polyketide ambruticin

We have shown that the plant pathogen Alternaria brassicicola exhibited very high susceptibility to ambruticin VS4 and to a lesser extent to the phenylpyrrole fungicide fludioxonil. These compounds are both derived from natural bacterial metabolites with antifungal properties and are thought to exert their toxicity by interfering with osmoregulation in filamentous fungi. Disruption of the osmosensor group III histidine kinase gene AbNIK1 (for A. brassicola NIK1) resulted in high levels of resistance to ambruticin and fludioxonil, while a mutant isolate characterized by a single-amino-acid substitution in the HAMP domain of the kinase only exhibited moderate resistance. Moreover, the natural resistance of Saccharomyces cerevisiae to these antifungal molecules switched to sensitivity in strains expressing AbNIK1p. We also showed that exposure to fludioxonil and ambruticin resulted in abnormal phosphorylation of a Hog1-like mitogen-activated protein kinase (MAPK) in A. brassicicola. Parallel experiments carried out with wild-type and mutant isolates of Neurospora crassa revealed that, in this species, ambruticin susceptibility was dependent on the OS1-RRG1 branch of the phosphorelay pathway downstream of the OS2 MAPK cascade but independent of the yeast Skn7-like response regulator RRG2. These results show that the ability to synthesize a functional group III histidine kinase is a prerequisite for the expression of ambruticin and phenylpyrrole susceptibility in A. brassicicola and N. crassa and that, at least in the latter species, improper activation of the high-osmolarity glycerol-related pathway could explain their fungicidal properties.

Phosphorylated Ssk1 prevents unphosphorylated Ssk1 from activating the Ssk2 mitogen-activated protein kinase kinase kinase in the yeast high-osmolarity glycerol osmoregulatory pathway

In Saccharomyces cerevisiae, external high osmolarity activates the Hog1 mitogen-activated protein kinase (MAPK), which controls various aspects of osmoadaptation. Ssk1 is a homolog of bacterial two-component response regulators and activates the Ssk2 MAPK kinase kinase upstream of Hog1. It has been proposed that unphosphorylated Ssk1 (Ssk1-OH) is the active form and that Ssk1 phosphorylated (Ssk1 approximately P) at Asp554 by the Sln1-Ypd1-Ssk1 multistep phosphorelay mechanism is the inactive form. In this study, we show that constitutive activation of Ssk2 occurs when Ssk1 phosphorylation is blocked by either an Ssk1 mutation at the phosphorylation site or an Ssk1 mutation that inhibits its interaction with Ypd1, the donor of phosphate to Ssk1. Thus, Ssk1-OH is indeed necessary for Ssk2 activation. However, overexpression of wild-type Ssk1 or of an Ssk1 mutant that cannot bind Ssk2 prevents constitutively active Ssk1 mutants from activating Ssk2. Therefore, Ssk1 has a dual function as both an activator of Ssk2 and an inhibitor of Ssk1 itself. We also found that Ssk1 exists mostly as a dimer within cells. From mutant phenotypes, we deduce that only the Ssk1-OH/Ssk1-OH dimer can activate Ssk2 efficiently. Hence, because Ssk1 approximately P binds to and inhibits Ssk1-OH, moderate fluctuation of the level of Ssk1-OH does not lead to nonphysiological and detrimental activation of Hog1.

Detection and measurement of two-component systems that control dimorphism and virulence in fungi

Systemic dimorphic fungi include six phylogenetically related ascomycetes. These organisms grow in a mold form in the soil on most continents around the world. After the mold spores, which are the infectious particles, are inhaled into the lung of a susceptible mammalian host, they undergo a morphological change into a pathogenic yeast form. The ability to convert to the yeast form is essential for this class of fungal agents to be pathogenic and produce disease. Temperature change is one key stimulus that triggers the phase transition from mold (25 degrees ) to yeast (37 degrees ). Genes that are expressed only in the pathogenic yeast form of these fungi have been identified to help explain how and why this phase transition is required for virulence. However, the regulators of yeast-phase specific genes, especially of phase transition from mold to yeast, have remained poorly understood. We used Agrobacterium-mediated gene transfer for insertional mutagenesis to create mutants that are defective in the phase transition and to identify genes that regulate this critical event. We discovered that a hybrid histidine kinase senses environmental signals such as temperature and regulates phase transition, dimorphism, and virulence in members of this fungal family. This chapter describes our approach to the identification and analysis of this global regulator.

Yeast osmoregulation

Osmoregulation is the active control of the cellular water balance and encompasses homeostatic mechanisms crucial for life. The osmoregulatory system in the yeast Saccharomyces cerevisiae is particularly well understood. Key to yeast osmoregulation is the production and accumulation of the compatible solute glycerol, which is partly controlled by the high osmolarity glycerol (HOG) signaling system. Genetic analyses combined with studies on protein-protein interactions have revealed the wiring scheme of the HOG signaling network, a branched mitogen-activated protein (MAP) kinase (MAPK) pathway that eventually converges on the MAPK Hog1. Hog1 is activated following cell shrinking and controls posttranscriptional processes in the cytosol as well as gene expression in the nucleus. HOG pathway activity can easily and rapidly be controlled experimentally by extracellular stimuli, and signaling and adaptation can be separated by a system of forced adaptation. This makes yeast osmoregulation suitable for studies on system properties of signaling and cellular adaptation via mathematical modeling. Computational simulations and parallel quantitative time course experimentation on different levels of the regulatory system have provided a stepping stone toward a holistic understanding, revealing how the HOG pathway can combine rigorous feedback control with maintenance of signaling competence. The abundant tools make yeast a suitable model for an integrated analysis of cellular osmoregulation. Maintenance of the cellular water balance is fundamental for life. All cells, even those in multicellular organisms with an organism-wide osmoregulation, have the ability to actively control their water balance. Osmoregulation encompasses homeostatic processes that maintain an appropriate intracellular environment for biochemical processes as well as turgor of cells and organism. In the laboratory, the osmoregulatory system is studied most conveniently as a response to osmotic shock, causing rapid and dramatic changes in the extracellular water activity. Those rapid changes mediate either water efflux (hyperosmotic shock), and hence cell shrinkage, or influx (hypoosmotic shock), causing cell swelling. The yeast S. cerevisiae, as a free-living organism experiencing both slow and rapid changes in extracellular water activity, has proven a suitable and genetically tractable experimental system in studying the underlying signaling pathways and regulatory processes governing osmoregulation. Although far from complete, the present picture of yeast osmoregulation is both extensive and detailed (de Nadal et al., 2002; Hohmann, 2002; Klipp et al., 2005). Simulations using mathematical models combined with time course measurements of different molecular processes in signaling and adaptation have allowed elucidation of the first system properties on the yeast osmoregulatory network.

Two-component response regulators Ssk1p and Skn7p additively regulate high-osmolarity adaptation and fungicide sensitivity in Cochliobolus heterostrophus

Filamentous ascomycetous fungi possess many histidine kinases and two conserved response regulators, Ssk1p and Skn7p, in their two-component signaling systems. We previously reported that the fungus unique group III histidine kinase regulates high-osmolarity adaptation and iprodione/fludioxonil fungicide sensitivity by controlling the phosphorylation of Hog1-type mitogen-activated protein kinase (MAPK) in filamentous ascomycetes. Here, we have characterized the response regulator genes ChSsk1 and ChSkn7 in the southern corn leaf blight fungus Cochliobolus heterostrophus. Both ChSsk1- and ChSkn7-disrupted mutants showed little sensitivity to high-osmolarity stress and moderate resistance to the iprodione/fludioxonil fungicides. The phosphorylation of Hog1-type MAPK BmHog1p induced by high-osmolarity stress and fungicide treatments was only regulated by ChSsk1p, indicating that ChSkn7p has roles in high-osmolarity adaptation and fungicide sensitivity that are independent from the activation of BmHog1p. The Chssk1 Chskn7 double mutants clearly showed higher sensitivity to osmolar stress and higher resistance to fungicides than the single mutants. The dose responses of the double mutants fit well with those of the group III histidine kinase-deficient strain. These results suggest that in filamentous ascomycetes, the Ssk1- and Skn7-type response regulators control high-osmolarity adaptation and fungicide sensitivity additively with differential mechanisms under the regulation of the group III histidine kinase. This study provides evidence that filamentous fungi have a unique two-component signaling system that is different from that of yeast and is responsible for high-osmolarity adaptation and fungicide sensitivity.

Activation of the HOG pathway upon cold stress in Saccharomyces cerevisiae

When Saccharomyces cerevisiae cells are exposed to hyper-osmotic stress, the high-osmolarity glycerol response (HOG) pathway is activated to induce osmotic responses. The HOG pathway consists of two upstream osmosensing branches, the SLN1 and SHO1 branches, and a downstream MAP kinase cascade. Although the mechanisms by which these upstream branches transmit signals to the MAP kinase cascade are well understood, the mechanisms by which they sense and respond to osmotic changes are elusive. Here we show that the HOG pathway is activated in an SLN1 branch-dependent manner when cells are exposed to cold stress (0 degrees C treatment). Dimethyl sulfoxide (DMSO) treatment, which rigidifies the cell membrane, also activates the HOG pathway in both SLN1 branch- and SHO1 branch-dependent manners. Moreover, cold stress, as well as hyper-osmotic stress, exhibits a synergistic effect with DMSO treatment on HOG pathway activation. On the other hand, ethanol treatment, which fluidizes the cell membrane, partially represses the cold stress-induced HOG pathway activation. Our results suggest that both osmosensing branches respond to the rigidification of the cell membrane to activate the HOG pathway.

Hypersaline stress induces the turnover of phosphatidylcholine and results in the synthesis of the renal osmoprotectant glycerophosphocholine in Saccharomyces cerevisiae

The role of phosphatidylcholine turnover during hypersaline stress is investigated in Saccharomyces cerevisiae. In the wild-type strain, 2180-1A hypersaline stress induced the rapid turnover of phosphatidylcholine, a major membrane lipid. Yeast cells were grown in the presence of [14C]-choline to label phosphatidylcholine. Upon shifting the cells to medium with 0.8 M NaCl, phosphatidylcholine levels were diminished by c. 30% within 20 min to yield glycerophosphocholine, a methylamine osmoprotectant that has been previously identified in renal cells. High-performance liquid chromatography studies showed that osmotically mediated glycerophosphocholine production was enhanced if 10 mM choline was added as a supplement to synthetic dextrose medium with 1.6 M NaCl, but glycine betaine was not detected. Enhanced glycerophosphocholine production also correlated with improved growth in media containing 1.6 M NaCl and choline. Enhanced growth is specific to methylamines: salt-stressed cells supplemented with 10 mM choline or glycine betaine showed enhanced growth relative to unsupplemented control cultures, but other additives had no effect on growth or adversely affected it. Nutritional effects are ruled out because yeast cannot use choline or glycine betaine as carbon or nitrogen sources in normal or high-salt medium. Finally, enhanced growth in hypersaline media with choline or glycine betaine is dependent on the choline permease Hnm1. These results in yeast highlight a similarity with mammalian renal cells, namely that phosphatidylcholine turnover contributes to osmotic adaptation via synthesis of the osmoprotectant glycerophosphocholine.

Global control of dimorphism and virulence in fungi

Microbial pathogens that normally inhabit our environment can adapt to thrive inside mammalian hosts. There are six dimorphic fungi that cause disease worldwide, which switch from nonpathogenic molds in soil to pathogenic yeast after spores are inhaled and exposed to elevated temperature. Mechanisms that regulate this switch remain obscure. We show that a hybrid histidine kinase senses host signals and triggers the transition from mold to yeast. The kinase also regulates cell-wall integrity, sporulation, and expression of virulence genes in vivo. This global regulator shapes how dimorphic fungal pathogens adapt to the mammalian host, which has broad implications for treating and preventing systemic fungal disease.

Methylglyoxal as a Signal Initiator for Activation of the Stress-activated Protein Kinase Cascade in the Fission Yeast Schizosaccharomyces pombe

Methylglyoxal (MG) is a typical 2-oxoaldehyde derived from glycolysis. We have recently found that MG activates transcription factors such as Yap1 and Msn2, and triggers a Hog1 mitogen-activated protein kinase cascade in Saccharomyces cerevisiae. Regarding the activation of Hog1 by MG, we found that Sln1, an osmosensor possessing histidine kinase activity, functions as a sensor of MG (Maeta, K., Izawa, S., and Inoue, Y. (2005) J. Biol. Chem. 280, 253-260). To gain further insight into the role of MG as a signal initiator, here we analyze the response of Schizosaccharomyces pombe to extracellular MG. Spc1, a stress-activated protein kinase (SAPK), was phosphorylated following the treatment with MG. No phosphorylation was observed in a wis1Delta mutant. The His-to-Asp phosphorelay system consisting of three histidine kinases (Phk1, Phk2, and Phk3), a phosphorelay protein (Spy1), and a response regulator (Mcs4) exists upstream of the Spc1-SAPK pathway. The phosphorylation of Spc1 following MG treatment was observed in phk1Deltaphk2Deltaphk3Delta and spy1Delta cells, but not in mcs4Delta cells. These results suggest that S. pombe has an alternative module(s) that directs the MG signal to the SAPK pathway via Mcs4. Additionally, we found that the transcription factor Pap1 is concentrated in the nucleus in response to MG, independent of the Spc1-SAPK pathway.

Group III histidine kinase is a positive regulator of Hog1-type mitogen-activated protein kinase in filamentous fungi

We previously reported that the group III histidine kinase Dic1p in the maize pathogen Cochliobolus heterostrophus is involved in resistance to dicarboximide and phenylpyrrole fungicides and in osmotic adaptation. In addition, exposure to the phenylpyrrole fungicide fludioxonil led to improper activation of Hog1-type mitogen-activated protein kinases (MAPKs) in some phytopathogenic fungi, including C. heterostrophus. Here we report, for the first time, the relationship between the group III histidine kinase and Hog1-related MAPK: group III histidine kinase is a positive regulator of Hog1-related MAPK in filamentous fungi. The phosphorylation pattern of C. heterostrophus BmHog1p (Hog1-type MAPK) was analyzed in wild-type and dic1-deficient strains by Western blotting. In the wild-type strain, phosphorylated BmHog1p was detected after exposure to both iprodione and fludioxonil at a concentration of 1 microg/ml. In the dic1-deficient strains, phosphorylated BmHog1p was not detected after exposure to 10 microg/ml of the fungicides. In response to osmotic stress (0.4 M KCl), a trace of phosphorylated BmHog1p was found in the dic1-deficient strains, whereas the band representing active BmHog1p was clearly detected in the wild-type strain. Similar results were obtained for Neurospora crassa Os-2p MAPK phosphorylation in the mutant of the group III histidine kinase gene os-1. These results indicate that group III histidine kinase positively regulates the activation of Hog1-type MAPKs in filamentous fungi. Notably, the Hog1-type MAPKs were activated at high fungicide (100 microg/ml) and osmotic stress (0.8 M KCl) levels in the histidine kinase mutants of both fungi, suggesting that another signaling pathway activates Hog1-type MAPKs in these conditions.

Two-component signal transduction in human fungal pathogens

Signal transduction pathways provide mechanisms for adaptation to stress conditions. One of the most studied of these pathways is the HOG1 MAP kinase pathway that in Saccharomyces cerevisiae is used to adapt cells to osmostress. The HOG1 MAPK has also been studied in Candida albicans, and more recently observations on the Hog1p functions have been described in two other human pathogens, Aspergillus fumigatus and Cryptococcus neoformans. The important, but not surprising, concept is that this pathway is used for different yet similar functions in each of these fungi, given their need to adapt to different environmental signals. Current studies of C. albicans focus upon the identification of two-component signal proteins that, in both C. albicans and S. cerevisiae, regulate the HOG1 MAPK. In C. albicans, these proteins regulate cell wall biosynthesis (and, therefore, adherence to host cells), osmotic and oxidant adaptation, white-opaque switching, morphogenesis, and virulence of the organism.

The Sho1 adaptor protein links oxidative stress to morphogenesis and cell wall biosynthesis in the fungal pathogen Candida albicans

The Sho1 adaptor protein is an important element of one of the two upstream branches of the high-osmolarity glycerol (HOG) mitogen-activated protein (MAP) kinase pathway in Saccharomyces cerevisiae, a signal transduction cascade involved in adaptation to stress. In the present work, we describe its role in the pathogenic yeast Candida albicans by the construction of mutants altered in this gene. We report here that sho1 mutants are sensitive to oxidative stress but that Sho1 has a minor role in the transmission of the phosphorylation signal to the Hog1 MAP kinase in response to oxidative stress, which mainly occurs through a putative Sln1-Ssk1 branch of the HOG pathway. Genetic analysis revealed that double ssk1 sho1 mutants were still able to grow on high-osmolarity media and activate Hog1 in response to this stress, indicating the existence of alternative inputs of the pathway. We also demonstrate that the Cek1 MAP kinase is constitutively active in hog1 and ssk1 mutants, a phenotypic trait that correlates with their resistance to the cell wall inhibitor Congo red, and that Sho1 is essential for the activation of the Cek1 MAP kinase under different conditions that require active cell growth and/or cell wall remodeling, such as the resumption of growth upon exit from the stationary phase. sho1 mutants are also sensitive to certain cell wall interfering compounds (Congo red, calcofluor white), presenting an altered cell wall structure (as shown by the ability to aggregate), and are defective in morphogenesis on different media, such as SLAD and Spider, that stimulate hyphal growth. These results reveal a role for the Sho1 protein in linking oxidative stress, cell wall biogenesis, and morphogenesis in this important human fungal pathogen.

The transcriptional response of Saccharomyces cerevisiae to Pichia membranifaciens killer toxin

The transcriptional response of Saccharomyces cerevisiae to Pichia membranifaciens killer toxin (PMKT) was investigated. We explored the global gene expression responses of the yeast S. cerevisiae to PMKT using DNA microarrays, real time quantitative PCR, and Northern blot. We identified 146 genes whose expression was significantly altered in response to PMKT in a non-random functional distribution. The majority of induced genes, most of them related to the high osmolarity glycerol (HOG) pathway, were core environmental stress response genes, showing that the coordinated transcriptional response to PMKT is related to changes in ionic homeostasis. Hog1p was observed to be phosphorylated in response to PMKT implicating the HOG signaling pathway. Individually deleted mutants of both up- (99) and down-regulated genes (47) were studied for altered sensitivity; it was observed that the deletion of up-regulated genes generated hypersensitivity (82%) to PMKT. Deletion of down-regulated genes generated wild-type (36%), resistant (47%), and hypersensitive (17%) phenotypes. This is the first study that shows the existence of a transcriptional response to the poisoning effects of a killer toxin.

Mechanism of activation for transcription factor PhoB suggested by different modes of dimerization in the inactive and active states

Response regulators (RRs), which undergo phosphorylation/dephosphorylation at aspartate residues, are highly prevalent in bacterial signal transduction. RRs typically contain an N-terminal receiver domain that regulates the activities of a C-terminal DNA binding domain in a phosphorylation-dependent manner. We present crystallography and solution NMR data for the receiver domain of Escherichia coli PhoB which show distinct 2-fold symmetric dimers in the inactive and active states. These structures, together with the previously determined structure of the C-terminal domain of PhoB bound to DNA, define the conformation of the active transcription factor and provide a model for the mechanism of activation in the OmpR/PhoB subfamily, the largest group of RRs. In the active state, the receiver domains dimerize with 2-fold rotational symmetry using their alpha4-beta5-alpha5 faces, while the effector domains bind to DNA direct repeats with tandem symmetry, implying a loss of intramolecular interactions.

Cell wall integrity signaling in Saccharomyces cerevisiae

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca(2+) signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.

Protein-protein interaction affinity plays a crucial role in controlling the Sho1p-mediated signal transduction pathway in yeast

Protein-protein interactions are required for most cellular functions, yet little is known about the relationship between protein-protein interaction affinity and biological activity. To investigate this issue, we engineered a series of mutants that incrementally reduced the affinity of the yeast Sho1p SH3 domain for its in vivo target, the MAP kinase kinase Pbs2p. We demonstrate a strong linear correlation between the binding energy of these mutants and quantitative in vivo outputs from the HOG high-osmolarity response pathway controlled by Sho1p. In addition, we find that reduction in binding affinity for the correct target within this pathway causes a proportional increase in misactivation of the related mating pheromone response pathway and that strong binding affinity alone does not guarantee efficient biological activity. Our experiments also indicate that a second binding surface on the Sho1p SH3 domain is required for its proper in vivo function.

An Os-1 family histidine kinase from a filamentous fungus confers fungicide-sensitivity to yeast

Three groups of fungicides (phenylpyrroles, dicarboximides, aromatic hydrocarbons) are effective against filamentous fungi. The target of these fungicides is the osmotic stress signal transduction pathway, which is dependent on the Os-1 family of two-component histidine kinases. These fungicides usually have no fungicidal effect on the yeast Saccharomyces cerevisiae. In this report, we found that expression of Hik1, an Os-1 orthologue from rice blast fungus, can confer fungicide-sensitivity to yeast. This requires both the histidine kinase and the response regulator domains of Hik1. Analysis of yeast mutants indicated that this sensitivity is Hog1- and Ssk1-dependent. In addition, our studies revealed an interaction between Hik1 and Ypd1. These observations suggest that Hik1 is a direct target of the fungicides or is a mediator of fungicide action and that the fungicidal effect is transmitted to the Hog1 pathway via Ypd1.

A common docking site for response regulators on the yeast phosphorelay protein YPD1

In Saccharomyces cerevisiae, a multi-component phosphorelay signal transduction pathway mediates cellular responses to environmental stress. A histidine-containing phosphotransfer protein, YPD1, represents a bifurcation point between the SLN1-YPD1-SSK1 pathway responsible for osmotic stress responses and the SLN1-YPD1-SKN7 pathway involved in cell wall biosynthesis and cell cycle control. The phosphorelay protein YPD1 must physically interact with and transfer phosphoryl groups between three homologous response regulator domains, designated SLN1-R1, SSK1-R2, and SKN7-R3. In this comparative study, the molecular basis of interaction was examined between YPD1 and each of the three response regulator domains utilizing alanine scanning mutagenesis combined with a yeast two-hybrid assay. Results from the yeast two-hybrid assay indicate that all three response regulator domains bind to a common area, largely hydrophobic in nature, on the surface of YPD1. We postulate that other YPD1 surface residues surrounding this common docking site are involved in making specific interactions with one or more of the response regulator domains.

A two-component histidine kinase of the rice blast fungus is involved in osmotic stress response and fungicide action

We isolated and characterized a histidine kinase gene (HIK1) from the rice blast fungus, Pyricularia oryzae (Magnaporthe grisea). The deduced amino acid sequence of HIK1 showed highest similarity (85.7%) to a hybrid-type histidine kinase, Os-1/Nik-1 of Neurospora crassa. Disruption of HIK1 caused no defect in cell growth on normal media and in pathogenicity to rice plants. The Deltahik1 strain acquired resistance to three groups of fungicides (phenylpyrroles, dicarboximides, and aromatic hydrocarbons) similar to os-1 mutants of N. crassa. The Deltahik1 strain showed increased sensitivity to high concentrations of sugars although its salt sensitivity was not elevated, suggesting that the rice blast fungus can distinguish osmostresses caused by high sugar concentrations and high salt concentrations. In contrast, os-1 mutants of N. crassa are sensitive to high concentrations of both salts and sugars. These findings suggest that P. oryzae and N. crassa partially differ in their os (osmosensitive) signal transduction pathway.

Membrane fluidity and its roles in the perception of environmental signals

Poikilothermic organisms are exposed to frequent changes in environmental conditions and their survival depends on their ability to acclimate to such changes. Changes in ambient temperature and osmolarity cause fluctuations in the fluidity of cell membranes. Such fluctuations are considered to be critical to the initiation of the regulatory reactions that ultimately lead to acclimation. The mechanisms responsible for the perception of changes in membrane fluidity have not been fully characterized. However, the analysis of genome-wide gene expression using DNA microarrays has provided a powerful new approach to studies of the contribution of membrane fluidity to gene expression and to the identification of environmental sensors. In this review, we focus on the mechanisms that regulate membrane fluidity, on putative sensors that perceive changes in membrane fluidity, and on the subsequent expression of genes that ensures acclimation to a new set of environmental conditions.

Solution Structure of the Escherichia coli YojN Histidine-phosphotransferase Domain and its Interaction with Cognate Phosphoryl Receiver Domains

The Rcs signaling system in Escherichia coli controls a variety of physiological functions, including capsule synthesis, cell division and motility. The activity of the central regulator RcsB is modulated by phosphorylation through the sensor kinases YojN and RcsC, with the YojN histidine phosphotransferase (HPt) domain representing the catalytic unit that coordinates the potentially reversible phosphotransfer reaction between the receiver domains of the RcsB and RcsC proteins. Heteronuclear high-resolution NMR spectroscopy was employed to determine the solution structure of the YojN-HPt domain and to map the interaction with its two cognate receiver domains. The solution structure of YojN-HPt exhibits a well-ordered and rigid protein core consisting of the five helices alphaI to alphaV. The helices alphaII to alphaV form a four-helix bundle signature motif common to proteins of similar function, and helix alphaI forms a cap on top of the bundle. The helix alphaII is separated by a proline induced kink into two parts with different orientations and dynamic behavior that is potentially important for complex formation with other proteins. The N-terminal part of YojN-HPt spanning the first 26 amino acid residues seems to contain neither a regular secondary structure nor a stable tertiary structure and is disordered in solution. The identified YojN-HPt recognition sites for the regulator RcsB and for the isolated receiver domain of the RcsC kinase largely overlap in defined regions of the helices alphaII and alphaIII, but show significant differences. Using the residues with the largest chemical shift changes obtained from titration experiments, we observed a dissociation constant of approximately 200microM for YojN-HPt/RcsC-PR and of 40microM for YojN-HPt/RcsB complexes. Our data indicate the presence of a recognition area in close vicinity to the active-site histidine residue of HPt domains as a determinant of specificity in signal-transduction pathways.

Autophosphorylation activity of the Arabidopsis ethylene receptor multigene family

Receptors for the gaseous phytohormone ethylene show sequence similarity to bacterial two-component histidine kinases. These receptors are encoded by a multigene family that can be divided into subfamilies 1 and 2. It has been previously shown that a subfamily 1 Arabidopsis thaliana ethylene receptor, ETR1, autophosphorylates in vitro on a conserved histidine residue (1). However, sequence comparisons between the five ethylene receptor family members suggest that subfamily 2 members do not have all the motifs necessary for histidine kinase activity. Further, a tobacco subfamily 2 receptor, NTHK1, autophosphorylates on serines and threonines in vitro (2). Here we show that all five Arabidopsis ethylene receptor proteins autophosphorylate in vitro. We analyzed the nature of the phosphorylated amino acids by acid/base stability and bi-dimensional thin layer electrophoresis and demonstrated that unlike ETR1 all other ethylene receptors autophosphorylate predominantly on serine residues. ERS1, the only other subfamily 1 receptor, is able to phosphorylate on both histidine and serine residues in the presence of Mn2+. However, histidine autophosphorylation is lost when ERS1 is assayed in the presence of both Mg2+ and Mn2+, suggesting that this activity may not occur in vivo. Furthermore, mutation of the histidine residue conserved in two-component systems does not abolish serine autophosphorylation, eliminating the possibility of a histidine to serine phosphotransfer. Our biochemical observations complement the recently published genetic data that histidine kinase activity is not necessary for ethylene receptor function in plants and suggest that ethylene signal transduction does not occur through a phosphorelay mechanism.

Saccharomyces cerevisiae histidine phosphotransferase Ypd1p shuttles between the nucleus and cytoplasm for SLN1-dependent phosphorylation of Ssk1p and Skn7p

Sln1p is a plasma membrane-localized two-component histidine kinase that functions as an osmotic stress sensor in Saccharomyces cerevisiae. Changes in osmotic pressure modulate Sln1p kinase activity, which, together with Ypd1p, a phosphorelay intermediate, changes the phosphorylation status of two response regulators, Ssk1p and Skn7p. Ssk1p controls the activity of the HOG1 mitogen-activated protein kinase pathway. Skn7p is a nuclearly localized transcription factor that regulates genes involved in cell wall integrity and other processes. Subcellular compartmentalization may therefore play an important role in eukaryotic two-component pathway regulation. We have studied the subcellular localization of SLN1 pathway components and find that Ypd1p is a dynamic protein with a role in shuttling the osmotic stress signal from Sln1p to Ssk1p in the cytosol and to Skn7p in the nucleus. The need to translocate the signal into different intracellular compartments contributes a spatial dimension to eukaryotic two-component pathways compared to the prototypical two-component pathways of prokaryotes.

Cloning and characterization of the histidine kinase gene Dic1 from Cochliobolus heterostrophus that confers dicarboximide resistance and osmotic adaptation

A gene for a putative two-component histidine kinase, which is homologous to os-1 from Neurospora crassa, was cloned and sequenced from the plant-pathogenic fungus Cochliobolus heterostrophus. The predicted protein possessed the conserved histidine kinase domain, the response regulator domain, and six tandem repeats of 92-amino-acids at the N-terminal end that are found in histidine kinases from other filamentous fungi. Introduction of the histidine kinase gene complemented the deficiency of the C. heterostrophus dic1 mutant, suggesting that the Dic1 gene product is a histidine kinase. Dic1 mutants are resistant to dicarboximide and phenylpyrrole fungicides, and they are sensitive to osmotic stress. We previously classified dic1 alleles into three types, based on their phenotypes. To explain the phenotypic differences among the dic1 mutant alleles, we cloned and sequenced the mutant dic1 genes and compared their sequences with that of the wild-type strain. Null mutants for Dic1, and mutants with a deletion or point mutation in the N-terminal repeat region, were highly sensitive to osmotic stress and highly resistant to both fungicides. A single amino acid change within the kinase domain or the regulator domain altered the sensitivity to osmotic stress and conferred moderate resistance to the fungicides. These results suggest that this predicted protein, especially its repeat region, has an important function in osmotic adaptation and fungicide resistance.

Phosphorelay-regulated degradation of the yeast Ssk1p response regulator by the ubiquitin-proteasome system

In Saccharomyces cerevisiae, a phosphorelay signal transduction pathway composed of Sln1p, Ypd1p, and Ssk1p, which are homologous to bacterial two-component signal transducers, is involved in the osmosensing mechanism. In response to high osmolarity, the phosphorelay system is inactivated and Ssk1p remains unphosphorylated. Unphosphorylated Ssk1p binds to and activates the Ssk2p mitogen-activated protein (MAP) kinase kinase kinase, which in turn activates the downstream components of the high-osmolarity glycerol response (HOG) MAP kinase cascade. Here, we report a novel inactivation mechanism for Ssk1p involving degradation by the ubiquitin-proteasome system. Degradation is regulated by the phosphotransfer from Ypd1p to Ssk1p, insofar as unphosphorylated Ssk1p is degraded more rapidly than phosphorylated Ssk1p. Ubc7p/Qri8p, an endoplasmic reticulum-associated ubiquitin-conjugating enzyme, is involved in the phosphorelay-regulated degradation of Ssk1p. In ubc7Delta cells in which the degradation is hampered, the dephosphorylation and/or inactivation process of the Hog1p MAP kinase is delayed compared with wild-type cells after the hyperosmotic treatment. Our results indicate that unphosphorylated Ssk1p is selectively degraded by the Ubc7p-dependent ubiquitin-proteasome system and that this mechanism downregulates the HOG pathway after the completion of the osmotic adaptation.

Sit4 phosphatase is functionally linked to the ubiquitin-proteasome system

Using a synthetic lethality screen we found that the Sit4 phosphatase is functionally linked to the ubiquitin-proteasome system. Yeast cells harboring sit4 mutations and an impaired proteasome (due to pre1-1 pre4-1 mutations) exhibited defective growth on minimal medium. Nearly identical synthetic effects were found when sit4 mutations were combined with defects of the Rad6/Ubc2- and Cdc34/Ubc3-dependent ubiquitination pathways. Under synthetic lethal conditions, sit4 pre or sit4 ubc mutants formed strongly enlarged unbudded cells with a DNA content of 1N, indicating a defect in the maintenance of cell integrity during starvation-induced G(1) arrest. Sit4-related synthetic effects could be cured by high osmotic pressure or by the addition of certain amino acids to the growth medium. These results suggest a concerted function of the Sit4 phosphatase and the ubiquitin-proteasome system in osmoregulation and in the sensing of nutrients. Further analysis showed that Sit4 is not a target of proteasome-dependent protein degradation. We could also show that Sit4 does not contribute to regulation of proteasome activity. These data suggest that both Sit4 phosphatase and the proteasome act on a common target protein.

The transient receptor potential channel on the yeast vacuole is mechanosensitive

Ca2+ is released from the vacuole into the yeast cytoplasm on an osmotic upshock, but how this upshock is perceived was unknown. We found the vacuolar channel, Yvc1p, to be mechanosensitive, showing that the Ca2+ conduit is also the sensing molecule. Although fragile, the yeast vacuole allows limited direct mechanical examination. Pressures at tens of millimeters of Hg (1 mmHg = 133 Pa) activate the 400-pS Yvc1p conductance in whole-vacuole recording mode as well as in the excised cytoplasmic-side-out mode. Raising the bath osmolarity activates this channel and causes vacuolar shrinkage and deformation. It appears that, on upshock, a transient osmotic force activates Yvc1p to release Ca2+ from the vacuole. Mechanical activation of Yvc1p occurs regardless of Ca2+ concentration and is apparently independent of its known Ca2+ activation, which we now propose to be an amplification mechanism (Ca2+-induced Ca2+ release). Yvc1p is a member of the transient receptor potential-family channels, several of which have been associated with mechanosensation in animals. The possible use of Yvc1p as a molecular model to study mechanosensation in general is discussed.

MEK kinase 2 and the adaptor protein Lad regulate extracellular signal-regulated kinase 5 activation by epidermal growth factor via Src

Lad is an SH2 domain-containing adaptor protein that binds MEK kinase 2 (MEKK2), a mitogen-activated protein kinase (MAPK) kinase kinase for the extracellular signal-regulated kinase 5 (ERK5) and JNK pathways. Lad and MEKK2 are in a complex in resting cells. Antisense knockdown of Lad expression and targeted gene disruption of MEKK2 expression results in loss of epidermal growth factor (EGF) and stress stimuli-induced activation of ERK5. Activation of MEKK2 and the ERK5 pathway by EGF and stress stimuli is dependent on Src kinase activity. The Lad-binding motif is encoded within amino acids 228 to 282 in the N terminus of MEKK2, and expression of this motif blocks Lad-MEKK2 interaction, resulting in inhibition of Src-dependent activation of MEKK2 and ERK5. JNK activation by EGF is similarly inhibited by loss of Lad or MEKK2 expression and by blocking the interaction of MEKK2 and Lad. Our studies demonstrate that Src kinase activity is required for ERK5 activation in response to EGF, MEKK2 expression is required for ERK5 activation by Src, Lad and MEKK2 association is required for Src activation of ERK5, and EGF and Src stimulation of ERK5-regulated MEF2-dependent promoter activity requires a functional Lad-MEKK2 signaling complex.

Ssk1p response regulator binding surface on histidine-containing phosphotransfer protein Ypd1p

Ypd1p, a histidine-containing phosphotransfer protein, plays an important role in a branched His-Asp phosphorelay signal transduction pathway that regulates cellular responses to hyperosmotic stress in Saccharomyces cerevisiae. Ypd1p is required for phosphoryl group transfer from the membrane-bound Sln1p sensor histidine kinase to two downstream response regulator proteins, Ssk1p and Skn7p. To investigate the molecular basis for interaction of Ypd1p with these response regulator domains, we used an approach that coupled alanine-scanning mutagenesis of surface-exposed residues in Ypd1p with a yeast two-hybrid interaction screen. Mutated residues that adversely affected the interaction of Ypd1p with the C-terminal response regulator domain of Ssk1p were identified and found to cluster on or near the alphaA helix in Ypd1p. Our results, supported by analysis of a modeled complex, identify a binding site on Ypd1p for response regulators that is composed of a cluster of conserved hydrophobic residues surrounded by less conserved polar residues. We propose that molecular interactions involving Ypd1p are mediated primarily through hydrophobic contacts, whereas binding specificity and strength of interaction may be influenced by select polar side chain interactions.

Yersinia effector YopJ inhibits yeast MAPK signaling pathways by an evolutionarily conserved mechanism

Yersinia effector, YopJ, inhibits the innate immune response by blocking MAP kinase and NFkappaB signaling pathways in mammalian cells. Herein, YopJ is shown to disrupt the MAP kinase signaling pathways in Saccharomyces cerevisiae. Expression of YopJ in yeast blocks the ability of yeast to respond to alpha factor by disrupting activation of the pheromone signaling pathway upstream of the activation of the MAPK Fus3p. YopJ also blocks the high osmolarity growth (HOG) MAP kinase pathway in yeast upstream of the activation of the MAPK Hog1p. YopJ is proposed to block the MAP kinase pathways in yeast in a similar manner to the way it blocks mammalian signaling pathways, implicating that a novel, evolutionarily conserved mechanism of regulation is utilized for signal transduction by these pathways.

Perception and signal transduction of cytokinins

Cytokinins are plant hormones implicated in diverse and essential processes in plant growth and development, and key genes for the metabolism and actions of cytokinins have recently been identified. Cytokinins are perceived by three histidine kinases--CRE1/WOL/AHK4, AHK2, and AHK3--which initiate intracellular phosphotransfer. The final destination of the transferred phosphoryl groups is response regulators. The type-B Arabidopsis response regulators (ARRs) are DNA-binding transcriptional activators that are required for cytokinin responses. On the other hand, the type-A ARRs act as repressors of cytokinin-activated transcription. How phosphorelay regulate response regulators and how response regulators control downstream events are open questions and discussed in this review.

Role of Ptc2 type 2C Ser/Thr phosphatase in yeast high-osmolarity glycerol pathway inactivation

Three type 2C Ser/Thr phosphatases (PTCs) are negative regulators of the yeast Saccharomyces cerevisiae high-osmolarity glycerol mitogen-activated protein kinase (MAPK) pathway. Ptc2 and Ptc3 are 75% identical to each other and differ from Ptc1 in having a noncatalytic domain. Previously, we showed that Ptc1 inactivates the pathway by dephosphorylating the Hog1 MAPK; Ptc1 maintains low basal Hog1 activity and dephosphorylates Hog1 during adaptation. Here, we examined the function of Ptc2 and Ptc3. First, deletion of PTC2 and/or PTC3 together with PTP2, encoding the protein tyrosine phosphatase that inactivates Hog1, produced a strong growth defect at 37 degrees C that was dependent on HOG1, providing further evidence that PTC2 and PTC3 are negative regulators. Second, overexpression of PTC2 inhibited Hog1 activation but did not affect Hog1-Tyr phosphorylation, suggesting that Ptc2 inactivates the pathway by dephosphorylating the Hog1 activation loop phosphothreonine (pThr) residue. Indeed, in vitro studies confirmed that Ptc2 was specific for Hog1-pThr. Third, deletion of both PTC2 and PTC3 led to greater Hog1 activation upon osmotic stress than was observed in wild-type strains, although no obvious change in Hog1 inactivation during adaptation was seen. These results indicate that Ptc2 and Ptc3 differ from Ptc1 in that they limit maximal Hog1 activity. The function of the Ptc2 noncatalytic domain was also examined. Deletion of this domain decreased V(max) by 1.6-fold and increased K(m) by 2-fold. Thus Ptc2 requires an additional amino acid sequence beyond the catalytic domain defined for PTCs for full activity.

His-Asp phosphorelay signal transduction in higher plants: receptors and response regulators for cytokinin signaling in Arabidopsis thaliana

Bacteria have devised phosphotransfer signaling mechanisms for eliciting a variety of adaptive responses to their environment. These mechanisms are collectively referred to as two-component regulatory systems. Each system generally consists of a sensor protein histidine kinase, which is anchored in the cell membrane, and a cytoplasmic response regulator, whose activity is modulated by the sensor. Most response regulators are transcription factors. In this review, we briefly introduce the established concept on bacterial two-component regulatory systems, using the Agrobacterium VirA-VirG system as an example, and give the evidence for the existence of quite similar systems in higher plants, such as the signal transduction induced by the phytohormone cytokinin. The Arabidopsis CRE1 histidine kinase and its related proteins AHK2 and AHK3 perceive cytokinins in the environment and transduce a signal, presumably through the AHP bridge components that carry the histidine-containing phosphotransfer (HPt) domain, to the ARR1 response regulator that transcriptionally activates genes immediately responsive to cytokinins. In addition, this signal transfer process appears to participate in cross-talk with signaling systems that respond to daylight and another phytohormone, ethylene, through an intracellular pool of several ARR1-like molecular species and the AHP components.

An Arabidopsis histidine kinase is essential for megagametogenesis

Cytokinin-Independent 1 (CKI1) belongs to a group of putative plant histidine kinases whose members do not appear to act as ethylene receptors. The deduced protein structure, combined with the observation that Arabidopsis callus cultures overexpressing CKI1 exhibit a "cytokinin-independent" cell division and greening phenotype, led to the hypothesis that CKI1 is involved in cytokinin signaling, perhaps acting as a cytokinin receptor. To test the function of CKI1, we used a reverse-genetic approach to identify plants carrying T-DNA insertions in CKI1. Two independent alleles were identified, which produce the same developmental phenotype. Analyses of populations segregating for the cki1-5 or cki1-6 T-DNA insertion alleles failed to reveal any homozygous cki1 plants, indicating that the homozygous mutant condition was lethal. Based on segregation distortion, transmission studies, a microscopy-based examination of developing female gametophytes, and mRNA expression data, we suggest that CKI1 function is required for megagametophyte development. Our work with CKI1 mutants indicates that signal transduction by means of a HisAsp phosphorelay system may play an important and previously unsuspected role in female gametophyte development in Arabidopsis.

The histidine kinase Hik33 perceives osmotic stress and cold stress in Synechocystis sp PCC 6803

The stress imposed on living organisms by hyperosmotic conditions and low temperature appears to be perceived via changes in the physical state of membrane lipids. We compared genome-wide patterns of transcription between wild-type Synechocystis sp. PCC 6803 and cells with a mutation in the histidine kinase Hik33 using a DNA microarray. Our results indicated that Hik33 regulated the expression of both osmostress-inducible and cold-inducible genes. The respective genes that were regulated by Hik33 under hyperosmotic and low-temperature conditions were, for the most part, different from one another. However, Hik33 also regulated the expression of a set of genes whose expression was induced both by osmotic stress and by cold stress. These results indicate that Hik33 is involved in responses to osmotic stress and low-temperature stress but that the mechanisms of the responses differ.

Isolation and functional analysis of a gene, tcsB, encoding a transmembrane hybrid-type histidine kinase from Aspergillus nidulans

We cloned and characterized a novel Aspergillus nidulans histidine kinase gene, tcsB, encoding a membrane-type two-component signaling protein homologous to the yeast osmosensor synthetic lethal N-end rule protein 1 (SLN1), which transmits signals through the high-osmolarity glycerol response 1 (HOG1) mitogen-activated protein kinase (MAPK) cascade in yeast cells in response to environmental osmotic stimuli. From an A. nidulans cDNA library, we isolated a positive clone containing a 3,210-bp open reading frame that encoded a putative protein consisting of 1,070 amino acids. The predicted tcsB protein (TcsB) has two probable transmembrane regions in its N-terminal half and has a high degree of structural similarity to yeast Sln1p, a transmembrane hybrid-type histidine kinase. Overexpression of the tcsB cDNA suppressed the lethality of a temperature-sensitive osmosensing-defective sln1-ts yeast mutant. However, tcsB cDNAs in which the conserved phosphorylation site His(552) residue or the phosphorelay site Asp(989) residue had been replaced failed to complement the sln1-ts mutant. In addition, introduction of the tcsB cDNA into an sln1delta sho1delta yeast double mutant, which lacked two osmosensors, suppressed lethality in high-salinity media and activated the HOG1 MAPK. These results imply that TcsB functions as an osmosensor histidine kinase. We constructed an A. nidulans strain lacking the tcsB gene (tcsBdelta) and examined its phenotype. However, unexpectedly, the tcsBdelta strain did not exhibit a detectable phenotype for either hyphal development or morphology on standard or stress media. Our results suggest that A. nidulans has more complex and robust osmoregulatory systems than the yeast SLN1-HOG1 MAPK cascade.

Osmotic stress signaling and osmoadaptation in yeasts

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

Altered phosphotransfer in an activated mutant of the Saccharomyces cerevisiae two-component osmosensor Sln1p

The SLN1 two-component signaling pathway of Saccharomyces cerevisiae utilizes a multistep phosphorelay mechanism to control osmotic stress responses via the HOG1 mitogen-activated protein kinase pathway and the transcription factor Skn7p. Sln1p consists of a sensor kinase module that undergoes histidine autophosphorylation and a receiver module that autocatalytically transfers the phosphoryl group from histidine to aspartate. The Sln1p aspartyl phosphate is then transferred to Ypd1p, which in turn transfers the phosphoryl group to a conserved aspartate on one of two response regulators, Ssk1p and Skn7p. Activated alleles of SLN1 (sln1*) were previously identified that appear to increase the level of phosphorylation of downstream targets Ssk1p and Skn7p. In principle, the phenotype of sln1* alleles could arise from an increase in autophosphorylation or phosphotransfer activities or a decrease in an intrinsic or extrinsic dephosphorylation activity. Genetic analysis of the activated mutants has been unable to distinguish between these possibilities. In this report, we address this issue by analyzing phosphorelay and phosphohydrolysis reactions involving the Sln1p-associated receiver. The results are consistent with a model in which the activated phenotype of the sln1* allele, sln-22, arises from a shift in the phosphotransfer equilibrium from Sln1p to Ypd1p, rather than from impaired dephosphorylation of the system in response to osmotic stress.

Heat stress activates the yeast high-osmolarity glycerol mitogen-activated protein kinase pathway, and protein tyrosine phosphatases are essential under heat stress

The yeast high-osmolarity glycerol (HOG) mitogen-activated protein kinase (MAPK) pathway has been characterized as being activated solely by osmotic stress. In this work, we show that the Hog1 MAPK is also activated by heat stress and that Sho1, previously identified as a membrane-bound osmosensor, is required for heat stress activation of Hog1. The two-component signaling protein, Sln1, the second osmosensor in the HOG pathway, was not involved in heat stress activation of Hog1, suggesting that the Sho1 and Sln1 sensors discriminate between stresses. The possible function of Hog1 activation during heat stress was examined, and it was found that the hog1 delta strain does not recover as rapidly from heat stress as well as the wild type. It was also found that protein tyrosine phosphatases (PTPs) Ptp2 and Ptp3, which inactivate Hog1, have two functions during heat stress. First, they are essential for survival at elevated temperatures, preventing lethality due to Hog1 hyperactivation. Second, they block inappropriate cross talk between the HOG and the cell wall integrity MAPK pathways, suggesting that PTPs are important for maintaining specificity in MAPK signaling pathways.

Crystal structure of a cyanobacterial phytochrome response regulator

The two-component signal transduction pathway widespread in prokaryotes, fungi, molds, and some plants involves an elaborate phosphorelay cascade. Rcp1 is the phosphate receiver module in a two-component system controlling the light response of cyanobacteria Synechocystis sp. via cyanobacterial phytochrome Cph1, which recognizes Rcp1 and transfers its phosphoryl group to an aspartate residue in response to light. Here we describe the crystal structure of Rcp1 refined to a crystallographic R-factor of 18.8% at a resolution of 1.9 A. The structure reveals a tightly associated homodimer with monomers comprised of doubly wound five-stranded parallel beta-sheets forming a single-domain protein homologous with the N-terminal activator domain of other response regulators (e.g., chemotaxis protein CheY). The three-dimensional structure of Rcp1 appears consistent with the conserved activation mechanism of phosphate receiver proteins, although in this case, the C-terminal half of its regulatory domain, which undergoes structural changes upon phosphorylation, contributes to the dimerization interface. The involvement of the residues undergoing phosphorylation-induced conformational changes at the dimeric interface suggests that dimerization of Rcp1 may be regulated by phosphorylation, which could affect the interaction of Rcp1 with downstream target molecules.

Osmoregulation and fungicide resistance: the Neurospora crassa os-2 gene encodes a HOG1 mitogen-activated protein kinase homologue

Neurospora crassa osmosensitive (os) mutants are sensitive to high osmolarity and therefore are unable to grow on medium containing 4% NaCl. We found that os-2 and os-5 mutants were resistant to the phenylpyrrole fungicides fludioxonil and fenpiclonil. To understand the relationship between osmoregulation and fungicide resistance, we cloned the os-2 gene by using sib selection. os-2 encodes a putative mitogen-activated protein (MAP) kinase homologous to HOG1 and can complement the osmosensitive phenotype of a Saccharomyces cerevisiae hog1 mutant. We sequenced three os-2 alleles and found that all of them were null with either frameshift or nonsense point mutations. An os-2 gene replacement mutant also was generated and was sensitive to high osmolarity and resistant to phenylpyrrole fungicides. Conversely, os-2 mutants transformed with the wild-type os-2 gene could grow on media containing 4% NaCl and were sensitive to phenylpyrrole fungicides. Fludioxonil stimulated intracellular glycerol accumulation in wild-type strains but not in os-2 mutants. Fludioxonil also caused wild-type conidia and hyphal cells to swell and burst. These results suggest that the hyperosmotic stress response pathway of N. crassa is the target of phenylpyrrole fungicides and that fungicidal effects may result from a hyperactive os-2 MAP kinase pathway.

Transcription factors regulating the response to oxidative stress in yeast

A main avenue of defense against fungal infection uses oxidative killing of these and other microorganisms. Consequently, the ability of fungi to withstand an oxidative challenge has important implications for their ultimate pathogenicity in a host organism. Fungi also serve as an excellent model system for handling of reactive oxygen species in eukaryotic cells. For these reasons, a great deal of work has been invested in analyzing pathways involved in and the mechanisms regulating oxidative stress tolerance in fungi. The goal of this review is to discuss the current state of knowledge underlying the ability of fungal cells to mount a response to oxidative stress via activation of transcription factors. Studies in Saccharomyces cerevisiae have identified multiple transcriptional regulatory proteins that mediate tolerance to oxidative stress. Experiments focused on the fission yeast Schizosaccharomyces pombe have led to the discovery of protein kinase cascades highly related to mammalian stress-activated protein kinases. Recent studies on the pathogenic yeast Candida albicans have allowed analysis of the role of a critical oxidant-regulated transcription factor in this important human pathogen. Further understanding of oxidative stress resistance pathways in fungi is an important step toward understanding the molecular pathogenesis of these microorganisms.