The yeast heat shock transcription factor changes conformation in response to superoxide and temperature

The Snf5-Hsf1 transcription module synergistically regulates stress responses and pathogenicity by maintaining ROS homeostasis in Sclerotinia sclerotiorum

The SWI/SNF complex is guided to the promoters of designated genes by its co-operator to activate transcription in a timely and appropriate manner to govern development, pathogenesis, and stress responses in fungi. Nevertheless, knowledge of the complexes and their co-operator in phytopathogenic fungi is still fragmented. We demonstrate that the heat shock transcription factor SsHsf1 guides the SWI/SNF complex to promoters of heat shock protein (hsp) genes and antioxidant enzyme genes using biochemistry and pharmacology. This is accomplished through direct interaction with the complex subunit SsSnf5 under heat shock and oxidative stress. This results in the activation of their transcription and mediates histone displacement to maintain reactive oxygen species (ROS) homeostasis. Genetic results demonstrate that the transcription module formed by SsSnf5 and SsHsf1 is responsible for regulating morphogenesis, stress tolerance, and pathogenicity in Sclerotinia sclerotiorum, especially by directly activating the transcription of hsp genes and antioxidant enzyme genes counteracting plant-derived ROS. Furthermore, we show that stress-induced phosphorylation of SsSnf5 is necessary for the formation of the transcription module. This study establishes that the SWI/SNF complex and its co-operator cooperatively regulate the transcription of hsp genes and antioxidant enzyme genes to respond to host and environmental stress in the devastating phytopathogenic fungi.

The novel EhHSTF7 transcription factor displays an oligomer state and recognizes a heat shock element in the Entamoeba histolytica parasite

The heat shock response is a conserved mechanism that allows cells to respond and survive stress damage and is transcriptionally regulated by the heat shock factors and heat shock elements. The P-glycoprotein confer the multidrug resistance phenotype; Entamoeba histolytica has the largest multidrug resistance gene family described so far; one of these genes, the EhPgp5 gene, has an emetine-inducible expression. A functional heat shock element was localized in the EhPgp5 gene promoter, indicating transcriptional regulation by heat shock factors. In this work, we determined the oligomer state of EhHSTF7 and the recognition of the heat shock element of the EhPgp5 gene. The EhHSTF7 recombinant protein was obtained as monomer and oligomer. In silico molecular docking predicts protein-DNA binding between EhHSTF7 and 5'-GAA-3' complementary bases. The rEhHSTF7 protein specifically binds to the heat shock element of the EhPgp5 gene in gel shift assays. The competition assays with heat shock element mutants indicate that 5'-GAA-3' complementary bases are necessary for the rEhHSTF7 binding. Finally, the siRNA-mediated knockdown of Ehhstf7 expression causes downregulation of EhPgp5 expression, suggesting that EhHSTF7 is likely to play a key role in the E. histolytica multidrug resistance. This is the first report of a transcription factor that recognizes a heat shock element from a gene involved in drug resistance in parasites. However, further analysis needs to demonstrate the biological relevance of the EhHSTF7 and the rest of the heat shock factors of E. histolytica, to understand the underlying regulation of transcriptional control in response to stress.

Does Inter-Organellar Proteostasis Impact Yeast Quality and Performance During Beer Fermentation?

During beer production, yeast generate ethanol that is exported to the extracellular environment where it accumulates. Depending on the initial carbohydrate concentration in the wort, the amount of yeast biomass inoculated, the fermentation temperature, and the yeast attenuation capacity, a high concentration of ethanol can be achieved in beer. The increase in ethanol concentration as a consequence of the fermentation of high gravity (HG) or very high gravity (VHG) worts promotes deleterious pleiotropic effects on the yeast cells. Moderate concentrations of ethanol (5% v/v) change the enzymatic kinetics of proteins and affect biological processes, such as the cell cycle and metabolism, impacting the reuse of yeast for subsequent fermentation. However, high concentrations of ethanol (> 5% v/v) dramatically alter protein structure, leading to unfolded proteins as well as amorphous protein aggregates. It is noteworthy that the effects of elevated ethanol concentrations generated during beer fermentation resemble those of heat shock stress, with similar responses observed in both situations, such as the activation of proteostasis and protein quality control mechanisms in different cell compartments, including endoplasmic reticulum (ER), mitochondria, and cytosol. Despite the extensive published molecular and biochemical data regarding the roles of proteostasis in different organelles of yeast cells, little is known about how this mechanism impacts beer fermentation and how different proteostasis mechanisms found in ER, mitochondria, and cytosol communicate with each other during ethanol/fermentative stress. Supporting this integrative view, transcriptome data analysis was applied using publicly available information for a lager yeast strain grown under beer production conditions. The transcriptome data indicated upregulation of genes that encode chaperones, co-chaperones, unfolded protein response elements in ER and mitochondria, ubiquitin ligases, proteasome components, N-glycosylation quality control pathway proteins, and components of processing bodies (p-bodies) and stress granules (SGs) during lager beer fermentation. Thus, the main purpose of this hypothesis and theory manuscript is to provide a concise picture of how inter-organellar proteostasis mechanisms are connected with one another and with biological processes that may modulate the viability and/or vitality of yeast populations during HG/VHG beer fermentation and serial repitching.

Regulation of the heat shock transcription factor Hsf1 in fungi: implications for temperature-dependent virulence traits

The impact of fungal pathogens on human health is devastating. For fungi and other pathogens, a key determinant of virulence is the capacity to thrive at host temperatures, with elevated temperature in the form of fever as a ubiquitous host response to defend against infection. A prominent feature of cells experiencing heat stress is the increased expression of heat shock proteins (Hsps) that play pivotal roles in the refolding of misfolded proteins in order to restore cellular homeostasis. Transcriptional activation of this heat shock response is orchestrated by the essential heat shock transcription factor, Hsf1. Although the influence of Hsf1 on cellular stress responses has been studied for decades, many aspects of its regulation and function remain largely enigmatic. In this review, we highlight our current understanding of how Hsf1 is regulated and activated in the model yeast Saccharomyces cerevisiae, and highlight exciting recent discoveries related to its diverse functions under both basal and stress conditions. Given that thermal adaption is a fundamental requirement for growth and virulence in fungal pathogens, we also compare and contrast Hsf1 activation and function in other fungal species with an emphasis on its role as a critical regulator of virulence traits.

Disruption of SPT23 results in increased heat sensitivity due to plasma membrane damage in Pichia pastoris

The ability to adapt to environmental changes is a necessary strategy for cell survival. Spt23 is responsible for regulation of Δ-9 desaturase expression in Pichia pastoris. Disruption of SPT23 leads to a remarkable decrease in cellular unsaturated fatty acids. In this study, we found that deletion of SPT23 resulted in growth defects under high temperature culture conditions and heat treatment induced the expression of SPT23. By measuring expression changes of heat shock proteins, protein levels and cellular localization of Hsf1, it was revealed that the sensitivity of spt23Δ to high temperature was independent of the heat shock response. Addition of the osmotic stabilizer sorbitol can restore the growth defects of spt23Δ under heat conditions. In addition, loss of SPT23 led to increased plasma membrane permeability, decreased plasma membrane integrity, depolarization, ergosterol synthesis defects and cell wall component disorder, which suggested that the sensitivity to heat treatment in spt23Δ was due to plasma membrane damage. Taken together, our results give new insights into the relationship between Spt23 and high temperature environmental stress.

Exploring the boundaries of direct detection and characterization of labile isomers - a case study of copper(ii)-dipeptide systems

The investigation of the linkage isomers of biologically essential and kinetically labile metal complexes in aqueous solutions poses a challenge, as these microspecies cannot be separately studied. Therefore, derivatives are commonly used to initially determine the stability or spectral characteristics of at least one of the isomers. Here we directly detect the isomers, describe the metal ion coordination sphere, speciation and thermodynamic parameters by a synergistic application of temperature dependent EPR and CD spectroscopic measurements in copper(ii)-dipeptide systems including His-Gly and His-Ala ligands. The ΔH = (-23 ± 4) kJ mol^-1 value of the standard enthalpy change corresponding to the peptide-type to histamine-type isomerisation equilibrium of the [CuL]⁺ complex was corroborated by several techniques. The preferential coordination of the side-chains was observed at lower temperatures, whereas, metal-binding of the backbone atoms became favourable upon increasing temperature. This study exemplifies the necessity of using temperature dependent multiple methodologies for a reliable description of similar systems for upstream applications.

The Fundamental Role of Reactive Oxygen Species in Plant Stress Response

Chemical, physical, and biotic factors continuously vary in the natural environment. Such parameters are considered as stressors if the magnitude of their change exceeds the current acclimation norm of the plant. Activation of genetic programs allows for conditional expansion of the acclimation norm and depends on specific sensing mechanisms, intracellular communication, and regulation. The redox and reactive oxygen species (ROS) network plays a fundamental role in directing the acclimation response. These highly reactive compounds like H₂O₂ are generated and scavenged under normal conditions and participate in realizing a basal acclimation level. Spatial and temporal changes in ROS levels and redox state provide valuable information for regulating epigenetic processes, transcription factors (TF), translation, protein turnover, metabolic pathways, and cross-feed, e.g., into hormone-, NO-, or Ca²⁺-dependent signaling pathways. At elevated ROS levels uncontrolled oxidation reactions compromise cell functions, impair fitness and yield, and in extreme cases may cause plant death.

Sulfide exposure results in enhanced sqr transcription through upregulating the expression and activation of HSF1 in echiuran worm Urechis unicinctus

Sulfide is a natural, widely distributed, poisonous substance. Sulfide: quinone oxidoreductase (SQR) is responsible for the initial oxidation of sulfide in mitochondria. To study transcriptional regulation of sqr after sulfide exposure, a 2.6-kb sqr upstream sequence from echiuran worm Urechis unicinctus was cloned by genome walking. Bioinformatics analysis showed 3 heat shock elements (HSEs) in proximal promoter region of the sqr upstream sequence. Moreover, an Hsf1 cDNA in U. unicinctus (UuHsf1) was isolated with a full-length sequence of 2334 bp and its polyclonal antibody was prepared using U. unicinctus HSF1 (UuHSF1) expressed prokaryotically with whole sequence of its open reading frame (ORF). In vivo ChIP and in vitro EMSA assays revealed UuHSF1 could interact with the sqr proximal promoter region. Transient transfection and mutation assays indicated that UuHSF1 bound specifically to HSE (-155bp to -143bp) and enhanced the transcription of sqr. Furthermore, sulfide treatment experiments demonstrated that sulfide could increase the expression of HSF1 protein, and induce trimerization of the protein which binds to HSEs and then activate sqr transcription. Quantitative real-time PCR analysis revealed sqr mRNA level increased significantly after U. unicinctus was exposed to sulfide for 6h, which corresponded to content changes of both trimeric HSF1 and HSF1-HSE complex. We concluded that UuHSF1 is a transcription factor of sqr and sulfide could induce sqr transcription by upregulating the expression and activation of HSF1 in U. unicinctus exposed to sulfide.

Cellular redox regulation, signaling, and stress response in plants

Cellular and organellar redox states, which are characterized by the balance between oxidant and antioxidant pool sizes, play signaling roles in the regulation of gene expression and protein function in a wide variety of plant physiological processes including stress acclimation. Reactive oxygen species (ROS) and ascorbic acid (AsA) are the most abundant oxidants and antioxidants, respectively, in plant cells; therefore, the metabolism of these redox compounds must be strictly and spatiotemporally controlled. In this review, we provided an overview of our previous studies as well as recent advances in (1) the molecular mechanisms and regulation of AsA biosynthesis, (2) the molecular and genetic properties of ascorbate peroxidases, and (3) stress acclimation via ROS-derived oxidative/redox signaling pathways, and discussed future perspectives in this field.

Metabolic and chaperone gene loss marks the origin of animals: evidence for Hsp104 and Hsp78 chaperones sharing mitochondrial enzymes as clients

The evolution of animals involved acquisition of an emergent gene repertoire for gastrulation. Whether loss of genes also co-evolved with this developmental reprogramming has not yet been addressed. Here, we identify twenty-four genetic functions that are retained in fungi and choanoflagellates but undetectable in animals. These lost genes encode: (i) sixteen distinct biosynthetic functions; (ii) the two ancestral eukaryotic ClpB disaggregases, Hsp78 and Hsp104, which function in the mitochondria and cytosol, respectively; and (iii) six other assorted functions. We present computational and experimental data that are consistent with a joint function for the differentially localized ClpB disaggregases, and with the possibility of a shared client/chaperone relationship between the mitochondrial Fe/S homoaconitase encoded by the lost LYS4 gene and the two ClpBs. Our analyses lead to the hypothesis that the evolution of gastrulation-based multicellularity in animals led to efficient extraction of nutrients from dietary sources, loss of natural selection for maintenance of energetically expensive biosynthetic pathways, and subsequent loss of their attendant ClpB chaperones.

Molecular mechanisms of herbicide-inducible gene expression of tobacco CYP71AH11 metabolizing the herbicide chlorotoluron

Tobacco cytochrome P450 (CYP) 71AH11 metabolized the herbicide chlorotoluron, and its mRNA level was increased in tobacco culture cells by the treatment of 2,4-D. In order to clarify molecular mechanisms of induced gene expression of CYP71AH11 by herbicide treatment, a 1574-bp 5'-flanking region of CYP71AH11 was cloned, ligated to the reporter β-glucuronidase (GUS) gene, and then transformed into tobacco plants. The GUS activity in the transgenic tobacco plants was highly induced by bromoxynil treatment, followed by 2,4-D. Chlorotoluron was slightly increased the GUS activity. The bromoxynil-increased GUS activity was partially repressed by the antioxidants, suggesting that reactive oxygen species may be involved in activation of the 5'-flanking region of CYP71AH11 by bromoxynil treatment. Deletion and mutation assays showed that the region CD (-1281 to -770bp from the start codon of CYP71AH11) was important, but not sufficient, for response to bromoxynil. Electrophoretic mobility shift assays and southwestern blotting revealed that the sequences AAAAAG, and GAACAAAC and GAAAATTC in the CD region were important for interaction to the nuclear proteins of <30 and ≈75 kDa, respectively. Particularly, interaction between AAAAAG and <30 kDa proteins was increased by bromoxynil treatment. These results gave a cue for understanding the bromoxynil-induced gene expression of CYP71AH11, which may contribute to herbicide tolerance and selectivity in crop plants.

Arabidopsis heat shock factor HsfA1a directly senses heat stress, pH changes, and hydrogen peroxide via the engagement of redox state

Arabidopsis heat shock factor HsfA1a is present in a latent, monomeric state under normal conditions; its activation involves heat stress-induced trimerization, binding to heat shock element in target promoters, and the acquisition of transcriptional competence. HsfA1a is an important regulator for heat stress-induced gene expression and thermotolerance. However, it is not clear whether HsfA1a is directly activated by stress and the mechanisms of the stress signaling are poorly understood. We analyzed HsfA1a activation by trimerization and DNA-binding assays in vitro and in vivo in response to heat stress, low/high pH, and hydrogen peroxide treatments. Our results show that purified recombinant HsfA1a was activated by these stress treatments in vitro. The same treatments also induced the binding to HSP18.2 and HSP70 promoters as examined by chromatin immunoprecipitation, and the HsfA1a DNA binding paralleled the mRNA expression of its target genes induced by different stresses. Stress-induced DNA-binding could be reversed, both in vitro and in vivo, by subsequent incubation with reducing agents (DTT, NADPH). These data suggest that HsfA1a can directly sense stress and become activated, and this process is dependent on the redox state. An N-terminal deletion of the amino acid residues from 48 to 74 negatively affected pH- and hydrogen peroxide-, but not heat-stress sensing.

Biology of the heat shock response and protein chaperones: budding yeast (Saccharomyces cerevisiae) as a model system

The eukaryotic heat shock response is an ancient and highly conserved transcriptional program that results in the immediate synthesis of a battery of cytoprotective genes in the presence of thermal and other environmental stresses. Many of these genes encode molecular chaperones, powerful protein remodelers with the capacity to shield, fold, or unfold substrates in a context-dependent manner. The budding yeast Saccharomyces cerevisiae continues to be an invaluable model for driving the discovery of regulatory features of this fundamental stress response. In addition, budding yeast has been an outstanding model system to elucidate the cell biology of protein chaperones and their organization into functional networks. In this review, we evaluate our understanding of the multifaceted response to heat shock. In addition, the chaperone complement of the cytosol is compared to those of mitochondria and the endoplasmic reticulum, organelles with their own unique protein homeostasis milieus. Finally, we examine recent advances in the understanding of the roles of protein chaperones and the heat shock response in pathogenic fungi, which is being accelerated by the wealth of information gained for budding yeast.

ROS signaling as common element in low oxygen and heat stresses

The activation of the oxidative metabolism in plants under low oxygen conditions has prompted controversial views. The presence of a ROS component in the transcriptome in response to low oxygen has been observed and an overlap with heat stress has been proved. It has been also demonstrated that ROS are produced during both anoxia and heat, but the site of their production remain contentious. Membrane NADPH oxidase and mitochondrial electron transport flow have been indicated as possible ROS generation systems. Both anoxia and heat have been shown to induce the transcription of Heat Shock Factors (HSFs) and Heat Shock Proteins (HSPs), among which HSFA2 and some of its targets. HSFA2 over-expressing plant has been shown to be more tolerant to anoxia, while the knockout hsfa2 lose the capability of wild type plants to cross-acclimate to anoxia through mild heat pre-treatment. The production of ROS seems to be an integral part of the anoxia and heat response, where HSFs likely play a central role in activating the HSP pathway. This mechanism is suggested to result in enhanced plant tolerance to both anoxia and heat.

Transcriptional regulation in Saccharomyces cerevisiae: transcription factor regulation and function, mechanisms of initiation, and roles of activators and coactivators

Here we review recent advances in understanding the regulation of mRNA synthesis in Saccharomyces cerevisiae. Many fundamental gene regulatory mechanisms have been conserved in all eukaryotes, and budding yeast has been at the forefront in the discovery and dissection of these conserved mechanisms. Topics covered include upstream activation sequence and promoter structure, transcription factor classification, and examples of regulated transcription factor activity. We also examine advances in understanding the RNA polymerase II transcription machinery, conserved coactivator complexes, transcription activation domains, and the cooperation of these factors in gene regulatory mechanisms.

The response to heat shock and oxidative stress in Saccharomyces cerevisiae

A common need for microbial cells is the ability to respond to potentially toxic environmental insults. Here we review the progress in understanding the response of the yeast Saccharomyces cerevisiae to two important environmental stresses: heat shock and oxidative stress. Both of these stresses are fundamental challenges that microbes of all types will experience. The study of these environmental stress responses in S. cerevisiae has illuminated many of the features now viewed as central to our understanding of eukaryotic cell biology. Transcriptional activation plays an important role in driving the multifaceted reaction to elevated temperature and levels of reactive oxygen species. Advances provided by the development of whole genome analyses have led to an appreciation of the global reorganization of gene expression and its integration between different stress regimens. While the precise nature of the signal eliciting the heat shock response remains elusive, recent progress in the understanding of induction of the oxidative stress response is summarized here. Although these stress conditions represent ancient challenges to S. cerevisiae and other microbes, much remains to be learned about the mechanisms dedicated to dealing with these environmental parameters.

Association of constitutive hyperphosphorylation of Hsf1p with a defective ethanol stress response in Saccharomyces cerevisiae sake yeast strains

Modern sake yeast strains, which produce high concentrations of ethanol, are unexpectedly sensitive to environmental stress during sake brewing. To reveal the underlying mechanism, we investigated a well-characterized yeast stress response mediated by a heat shock element (HSE) and heat shock transcription factor Hsf1p in Saccharomyces cerevisiae sake yeast. The HSE-lacZ activity of sake yeast during sake fermentation and under acute ethanol stress was severely impaired compared to that of laboratory yeast. Moreover, the Hsf1p of modern sake yeast was highly and constitutively hyperphosphorylated, irrespective of the extracellular stress. Since HSF1 allele replacement did not significantly affect the HSE-mediated ethanol stress response or Hsf1p phosphorylation patterns in either sake or laboratory yeast, the regulatory machinery of Hsf1p is presumed to function differently between these types of yeast. To identify phosphatases whose loss affected the control of Hsf1p, we screened a series of phosphatase gene deletion mutants in a laboratory strain background. Among the 29 mutants, a Δppt1 mutant exhibited constitutive hyperphosphorylation of Hsf1p, similarly to the modern sake yeast strains, which lack the entire PPT1 gene locus. We confirmed that the expression of laboratory yeast-derived functional PPT1 recovered the HSE-mediated stress response of sake yeast. In addition, deletion of PPT1 in laboratory yeast resulted in enhanced fermentation ability. Taken together, these data demonstrate that hyperphosphorylation of Hsf1p caused by loss of the PPT1 gene at least partly accounts for the defective stress response and high ethanol productivity of modern sake yeast strains.

Induction and transcriptional regulation of laccases in fungi

Fungal laccases are phenol oxidases widely studied for their use in several industrial applications, including pulp bleaching in paper industry, dye decolourisation, detoxification of environmental pollutants and revalorization of wastes and wastewaters. The main difficulty in using these enzymes at industrial scale ensues from their production costs. Elucidation of the components and the mechanisms involved in regulation of laccase gene expression is crucial for increasing the productivity of native laccases in fungi. Laccase gene transcription is regulated by metal ions, various aromatic compounds related to lignin or lignin derivatives, nitrogen and carbon sources. In this manuscript, most of the published results on fungal laccase induction, as well as analyses of both the sequences and putative functions of laccase gene promoters are reviewed. Analyses of promoter sequences allow defining a correlation between the observed regulatory effects on laccase gene transcription and the presence of specific responsive elements, and postulating, in some cases, a mechanism for their functioning. Only few reports have investigated the molecular mechanisms underlying laccase regulation by different stimuli. The reported analyses suggest the existence of a complex picture of laccase regulation phenomena acting through a variety of cis acting elements. However, the general mechanisms for laccase transcriptional regulation are far from being unravelled yet.

Differential effects of hydrogen peroxide and ascorbic acid on the aerobic thermosensitivity of yeast cells grown under aerobic and anoxic conditions

We have previously demonstrated that in aerobically-grown cells of the yeast Saccharomyces cerevisiae, hydrogen peroxide (H(2)O(2)) increases and ascorbic acid decreases cellular thermosensitivity, as determined by the inducibility of a heat shock (HS)-reporter gene. In this work, we reveal that the aerobic thermosensitivity of anaerobically-grown yeast cells also increases in the presence of H(2)O(2), albeit differentially between cells with two different lipid profiles. In comparison to aerobically-grown fermenting cells treated with the same H(2)O(2) concentration, both these types of anaerobically-grown cells were found to be considerably less sensitive to aerobic heat shock and considerably more thermotolerant. Paradoxically, and in contrast to ascorbate-pretreated aerobically-grown yeast cells, when anaerobically-grown cells were heat-shocked aerobically in the presence of the same ascorbic acid concentration, they exhibited increased thermosensitivity and decreased intrinsic thermotolerance with respect to their untreated counterparts. These findings are discussed with respect to what is currently known about the redox and physiological status of yeast cells grown aerobically and cells reoxygenated following anoxic growth.

Tolerance and stress response to ethanol in the yeast Saccharomyces cerevisiae

Eukaryotic cells have developed diverse strategies to combat the harmful effects of a variety of stress conditions. In the model yeast Saccharomyces cerevisiae, the increased concentration of ethanol, as the primary fermentation product, will influence the membrane fluidity and be toxic to membrane proteins, leading to cell growth inhibition and even death. Though little is known about the complex signal network responsible for alcohol stress responses in yeast cells, several mechanisms have been reported to be associated with this process, including changes in gene expression, in membrane composition, and increases in chaperone proteins that help stabilize other denatured proteins. Here, we review the recent progresses in our understanding of ethanol resistance and stress responses in yeast.

Sensing and responding to excess light

Plants and algae often absorb too much light-more than they can actually use in photosynthesis. To prevent photo-oxidative damage and to acclimate to changes in their environment, photosynthetic organisms have evolved direct and indirect mechanisms for sensing and responding to excess light. Photoreceptors such as phototropin, neochrome, and cryptochrome can sense excess light directly and relay signals for chloroplast movement and gene expression responses. Indirect sensing of excess light through biochemical and metabolic signals can be transduced into local responses within chloroplasts, into changes in nuclear gene expression via retrograde signaling pathways, or even into systemic responses, all of which are associated with photoacclimation.

A heat-activated MAP kinase (HAMK) as a mediator of heat shock response in tobacco cells

A heat-activated MAP kinase (HAMK), immunologically related to the extracellular signal-regulated kinase (ERK) super-family of protein kinases, has been identified in BY2 cells of tobacco. The activation of HAMK at 37 degrees C was transient and detected within 2 min and reached a maximum level within 5 min. Ca(2+) chelators and channel blockers, and the known inhibitors of MEK, a MAP kinase kinase, prevented the heat activation of HAMK. This suggests that HAMK activation is part of a heat-triggered MAP kinase cascade that requires Ca(2+) influx. The heat shock protein HSP70 accumulated at 37 degrees C, but not when HAMK activation was prevented with the inhibitors of MEK or with Ca(2+) chelators or channel blockers. As previously shown for heat activation of HAMK, heat-induced accumulation of HSP70 requires membrane fluidization and reorganization of cytoskeleton. We concluded that heat-triggered HAMK cascade might play an essential role in the launching of heat shock response and hsp gene expression in tobacco cells.

Alzheimer's Abeta fused to green fluorescent protein induces growth stress and a heat shock response

The 42 amino acid Alzheimer's Abeta peptide is involved in the progression of Alzheimer's disease. Here we describe the effects of intracellular Abeta, produced through its attachment to either end of a green fluorescent protein, in yeast. Cells producing Abeta exhibited a lower growth yield and a heat shock response, showing that Abeta fusions promote stress in cells and supporting the notion that intracellular Abeta is a toxic molecule. These studies have relevance in understanding the role of Abeta in the death of neuronal cells, and indicate that yeast may be a new tractable model system for the screening for inhibitors of the stress caused by Abeta.

Role of heat shock transcription factor in Saccharomyces cerevisiae oxidative stress response

The heat shock transcription factor Hsf1 of the yeast Saccharomyces cerevisiae regulates the transcription of a set of genes that contain heat shock elements (HSEs) in their promoters and function in diverse cellular processes, including protein folding. Here, we show that Hsf1 activates the transcription of various target genes when cells are treated with oxidizing reagents, including the superoxide anion generators menadione and KO(2) and the thiol oxidants diamide and 1-chloro-2,4-dinitrobenzene (CDNB). Similar to heat shock, the oxidizing reagents are potent inducers of both efficient HSE binding and extensive phosphorylation of Hsf1. The inducible phosphorylation of Hsf1 is regulated by the intramolecular domain-domain interactions and affects HSE structure-specific transcription. Unlike the heat shock, diamide, or CDNB response, menadione or KO(2) activation of Hsf1 is inhibited by cyclic-AMP-dependent protein kinase (PKA) activity, which negatively regulates the activator functions of other transcriptional regulators implicated in the oxidative stress response. These results demonstrate that Hsf1 is a member of the oxidative stress-responsive activators and that PKA is a general negative regulator in the superoxide anion response.

In the yeast heat shock response, Hsf1-directed induction of Hsp90 facilitates the activation of the Slt2 (Mpk1) mitogen-activated protein kinase required for cell integrity

Yeast is rendered temperature sensitive with loss of the C-terminal (CT) domain of heat shock transcription factor (Hsf1). This domain loss was found to abrogate heat stimulation of Slt2 (Mpk1), the mitogen-activated protein kinase that directs the reinforced cell integrity gene expression needed for high-temperature growth. In Hsf1 CT domain-deficient cells, Slt2 still undergoes Mkk1/2-directed dual-Thr/Tyr phosphorylation in response to the heat stimulation of cell integrity pathway signaling, but the low Hsp90 expression level suppresses any corresponding increase in Slt2 kinase activity due to Slt2 being a "client" of the Hsp90 chaperone. A non-Hsf1-directed Hsp90 overexpression restored the heat induction of Slt2 activity in these cells, as well as both Slt2-dependent (Rlm1, Swi4) and Slt2-independent (MBF) transcriptional activities. Their high-temperature growth was also rescued, not just by this Hsp90 overexpression but by osmotic stabilization, by the expression of a Slt2-independent form of the Rlm1 transcriptional regulator of cell integrity genes, and by a multicopy SLT2 gene vector. In providing the elevated Hsp90 needed for an efficient activation of Slt2, heat activation of Hsf1 indirectly facilitates (Slt2-directed) heat activation of yet another transcription factor (Rlm1). This provides an explanation as to why, in earlier transcript analysis compared to chromatin immunoprecipitation studies, many more genes of yeast displayed an Hsf1-dependent transcriptional activation by heat than bound Hsf1 directly. The levels of Hsp90 expression affecting transcription factor regulation by Hsp90 client protein kinases also provides a mechanistic model for how heat shock factor can influence the expression of several non-hsp genes in higher organisms.

Can the different heat shock response thresholds found in fermenting and respiring yeast cells be attributed to their differential redox states?

In this study we used a heat-shock (HS) reporter gene to demonstrate that respiring cells are intrinsically less sensitive (by 5 degrees C) than their fermenting counterparts to a sublethal heat shock. We also used an oxidant-sensitive fluorescent probe to demonstrate that this correlates with lower levels of sublethal reactive oxygen species (ROS) accumulation in heat-stressed respiring cells. Moreover, this relationship between HS induction of the reporter gene and ROS accumulation extends to respiring cells that have had their ROS levels modified by treatment with the anti-oxidant ascorbic acid and the pro-oxidant H(2)O(2). Thus, by demonstrating that the ROS/HSR correlation previously demonstrated in fermenting cells also holds for respiring cells (despite their greater HS insensitivity and higher level of intrinsic thermotolerance), we provide evidence that the intracellular redox state may influence both the sensitivity of the heat-shock response (HSR) and stress tolerance in the yeast Saccharomyces cerevisiae.

The natural osmolyte trehalose is a positive regulator of the heat-induced activity of yeast heat shock transcription factor

In Saccharomyces cerevisiae, the intracellular concentration of trehalose increases rapidly in response to many environmental stresses, including heat shock. These high trehalose levels have been correlated with tolerance to adverse conditions and led to the model that trehalose functions as a chemical cochaperone. Here, we show that the transcriptional activity of Hsf1 during the heat shock response depends on trehalose. Strains with low levels of trehalose have a diminished transcriptional response to heat shock, while strains with high levels of trehalose have an enhanced transcriptional response to heat shock. The enhanced transcriptional response does not require the other heat-responsive transcription factors Msn2/4 but is dependent upon heat and Hsf1. In addition, the phosphorylation levels of Hsf1 correlate with both transcriptional activity and the presence of trehalose. These in vivo results support a new role for trehalose, where trehalose directly modifies the dynamic range of Hsf1 activity and therefore influences heat shock protein mRNA levels in response to stress.

The Hsp70 member, Ssa1, acts as a DNA-binding transcriptional co-activator of laccase in Cryptococcus neoformans

Hsp70 proteins are a well-known class of chaperones that have also been described to have roles in cellular regulation. Here, we show that a Cryptococcus neoformans Hsp70 homologue Ssa1 acts as a DNA-binding transcriptional co-activator of the fungal virulence factor, laccase, via binding to a GC-rich element within the 5'-UAS in response to glucose starvation, iron, copper, calcium and temperature. In addition, Ssa1 forms a regulatory complex with heat shock transcription factor and TATA-binding protein during laccase induction. Furthermore, deletion of Ssa1 results in reduced laccase and attenuated virulence using a mouse model. These results indicate that Hsp70 functions as a stress-related transcriptional co-activator required for fungal virulence.

Could heat shock transcription factors function as hydrogen peroxide sensors in plants?

BACKGROUND

Heat shock transcription factors (Hsfs) are modular transcription factors encoded by a large gene family in plants. They bind to the consensus sequence 'nGAAnnTCCn' found in the promoters of many defence genes, and are thought to function as a highly redundant and flexible gene network that controls the response of plants to different environmental stress conditions, including biotic and abiotic stresses. Hsf proteins encoded by different genes exhibit a high degree of complexity in their interactions. They can potentially bind and activate their own promoters, as well as the promoters of other members of their gene family, and they can form homo- or heterotrimers resulting in altered nuclear localization, as well as enhanced or suppressed transcription.

SCOPE

In this review, we summarize recent studies on Hsf function in Arabidopsis and tomato and present evidence obtained from microarray expression studies in Arabidopsis that the Hsf gene network is highly flexible and specialized, with specific members and/or member combinations controlling the response of plants to particular stress conditions. In addition, we describe recent studies that support the hypothesis that certain Hsfs function as molecular sensors that directly sense reactive oxygen species (ROS) and control the expression of oxidative stress response genes during oxidative stress.

Role of trehalose and heat in the structure of the C-terminal activation domain of the heat shock transcription factor

The heat shock transcription factor (HSF) is the primary transcriptional regulator of the heat shock response in eukaryotes. Saccharomyces cerevisiae HSF1 has two functional transcriptional activation domains, located N- and C-terminal to the central core of the protein. These activation domains have a low level of transcriptional activity prior to stress, but they acquire a high level of transcriptional activity in response to stresses such as heat. Previous studies on the N-terminal activation domain have shown that it can be completely disordered. In contrast, we show that the C-terminal activation domain of S. cerevisiae HSF1 does contain a certain amount of secondary structure as measured by circular dichroism (CD) and protease resistance. The alpha-helical content of the domain can be increased by the addition of the disaccharide trehalose but not by sucrose. Trehalose, but not sucrose, causes a blue shift in the fluorescence emission spectra, which is suggestive of an increase in tertiary structure. Trehalose, which is known to be a chemical chaperone, also increases proteases' resistance and promotes heat-induced increases in alpha-helicity. The latter is particularly intriguing because of the physiological role of trehalose in yeast. Trehalose levels are increased dramatically after heat shock, and this is thought to protect protein structure prior to the increase of heat shock protein levels. Our results suggest that the dramatic changes in S. cerevisiae HSF1 transcriptional activity in response to stress might be linked to the combined effects of trehalose and elevated temperatures in modifying the overall structure of HSF1's C-terminal activation domain.

Fungal heat-shock proteins in human disease

Heat-shock proteins (hsps) have been identified as molecular chaperones conserved between microbes and man and grouped by their molecular mass and high degree of amino acid homology. This article reviews the major hsps of Saccharomyces cerevisiae, their interactions with trehalose, the effect of fermentation and the role of the heat-shock factor. Information derived from this model, as well as from Neurospora crassa and Achlya ambisexualis, helps in understanding the importance of hsps in the pathogenic fungi, Candida albicans, Cryptococcus neoformans, Aspergillus spp., Histoplasma capsulatum, Paracoccidioides brasiliensis, Trichophyton rubrum, Phycomyces blakesleeanus, Fusarium oxysporum, Coccidioides immitis and Pneumocystis jiroveci. This has been matched with proteomic and genomic information examining hsp expression in response to noxious stimuli. Fungal hsp90 has been identified as a target for immunotherapy by a genetically recombinant antibody. The concept of combining this antibody fragment with an antifungal drug for treating life-threatening fungal infection and the potential interactions with human and microbial hsp90 and nitric oxide is discussed.

Mutated Yeast Heat Shock Transcription Factor Activates Transcription Independently of Hyperphosphorylation

The homotrimeric heat shock transcription factor (HSF) binds to the heat shock element of target genes and regulates transcription in response to various stresses. The Hsf1 protein of Saccharomyces cerevisiae is extensively phosphorylated upon heat shock; a modification that is under positive regulation by its C-terminal regulatory domain (CTM). Hyperphosphorylation has been implicated in gene-specific transcriptional activation. Here, we surveyed genes whose heat shock response is reduced by a CTM mutation. The CTM is indispensable for transcription via heat shock elements bound by a single Hsf1 trimer but is dispensable for transcription via heat shock elements bound by Hsf1 trimers in a cooperative manner. Intragenic mutations located within or near the wing region of the winged helix-turn-helix DNA-binding domain suppress the temperature-sensitive growth phenotype associated with the CTM mutation and enable Hsf1 to activate transcription independently of hyperphosphorylation. Deletion of the wing partially restores the transcriptional defects of the unphosphorylated Hsf1. These results demonstrate a functional link between hyperphosphorylation and the wing region and suggest that this modification is involved in a conformational change of a single Hsf1 trimer to an active form.

Do mitochondria regulate the heat-shock response in Saccharomyces cerevisiae?

A mild heat shock induces the synthesis of heat-shock proteins (hsps), which protect cells from damage during more extreme heat exposure. The nature of the signals that induce transcription of heat shock-regulated genes remains conjectural. In this work we studied the role of mitochondria in regulating hsps synthesis in Saccharomyces cerevisiae. The results obtained clearly indicate that a mild heat shock elicits a hyperpolarization of the inner mitochondrial membrane and such an event is one of several signals triggering the chain of reactions that activates the expression of the HSP104 gene and probably the expression of other heat shock-regulated genes in S. cerevisiae. The uncouplers or mitochondrial inhibitors which are capable of dissipating the potential on the inner mitochondrial membrane under particular experimental conditions prevent the synthesis of Hsp104 induced by mild heat shock and thus inhibit the development of induced thermotolerance. It is suggested that cAMP-dependent protein kinase A is participating in the mitochondrial regulation of nuclear genes.

Hypoxia abolishes transience of the heat-shock response in the methylotrophic yeast Hansenula polymorpha

The heat-shock response is conserved amongst practically all organisms. Almost invariably, the massive heat-shock protein (Hsp) synthesis that it induces is subsequently down-regulated, making this a transient, not a sustained, stress response. This study investigated whether the heat-shock response displays any unusual features in the methylotrophic yeast Hansenula polymorpha, since this organism exhibits the highest growth temperature (49-50 degrees C) identified to date for any yeast and grows at 47 degrees C without either thermal death or detriment to final biomass yield. Maximal levels of Hsp induction were observed with a temperature upshift of H. polymorpha from 30 degrees C to 47-49 degrees C. This heat shock induces a prolonged growth arrest, heat-shock protein synthesis being down-regulated long before growth resumes at such high temperatures. A 30 degrees C to 49 degrees C heat shock also induced thermotolerance, although H. polymorpha cells in balanced growth at 49 degrees C were intrinsically thermotolerant. Unexpectedly, the normal transience of the H. polymorpha heat-shock response was suppressed completely by imposing the additional stress of hypoxia at the time of the 30 degrees C to 49 degrees C temperature upshift. Hypoxia abolishing the transience of the heat-shock response appears to operate at the level of Hsp gene transcription, since the heat-induced Hsp70 mRNA was transiently induced in a heat-shocked normoxic culture but displayed sustained induction in a culture deprived of oxygen at the time of temperature upshift.

Phosphorylation of the yeast heat shock transcription factor is implicated in gene-specific activation dependent on the architecture of the heat shock element

Heat shock transcription factor (HSF) binds to the heat shock element (HSE) and regulates transcription, where the divergence of HSE architecture provides gene- and stress-specific responses. The phosphorylation state of HSF, regulated by stress, is involved in the activation and inactivation of the transcription activation function. A domain designated as CTM (C-terminal modulator) of the Saccharomyces cerevisiae HSF is required for the activation of genes containing atypical HSE but not typical HSE. Here, we demonstrate that CTM function is conserved among yeast HSFs and is necessary not only for HSE-specific activation but also for the hyperphosphorylation of HSF upon heat shock. Moreover, both transcription and phosphorylation defects due to CTM mutations were restored concomitantly by a set of intragenic suppressor mutations. Therefore, the hyperphosphorylation of HSF is correlated with the activation of genes with atypical HSE but is not involved in that of genes with typical HSE. The function of CTM was circumvented in an HSF derivative lacking CE2, a yeast-specific repression domain. Taken together, we suggest that CTM alleviates repression by CE2, which allows HSF to be heat-inducibly phosphorylated and presume that phosphorylation is a prerequisite for the activator function of HSF when it binds to an atypical HSE.

Sensitivity to Hsp90-targeting drugs can arise with mutation to the Hsp90 chaperone, cochaperones and plasma membrane ATP binding cassette transporters of yeast

The Hsp90 molecular chaperone catalyses the final activation step of many of the most important regulatory proteins of eukaryotic cells. The antibiotics geldanamycin and radicicol act as highly selective inhibitors of in vivo Hsp90 function through their ability to bind within the ADP/ATP binding pocket of the chaperone. Drugs based on these compounds are now being developed as anticancer agents, their administration having the potential to inactivate simultaneously several of the targets critical for counteracting multistep carcinogenesis. This investigation used yeast to show that cells can be rendered hypersensitive to Hsp90 inhibitors by mutation to Hsp90 itself (within the Hsp82 isoform of yeast Hsp90, the point mutations T101I and A587T); with certain cochaperone defects and through the loss of specific plasma membrane ATP binding cassette transporters (Pdr5p, and to a lesser extent, Snq2p). The T101I hsp82 and A587T hsp82 mutations do not cause higher drug affinity for purified Hsp90 but may render the in vivo chaperone cycle more sensitive to drug inhibition. It is shown that these mutations render at least one Hsp90-dependent process (deactivation of heat-induced heat shock factor activity) more sensitive to drug inhibition in vivo.

In vivo and in vitro activation of temperature-responsive plant map kinases

Alfalfa cells possess two temperature-responsive Mitogen-Activated Protein Kinases (MAPKs), SAMK (Stress-Activated MAP Kinase) activated at 4 degrees C and HAMK (Heat shock-Activated MAP Kinase) activated at 37 degrees C. Both are inactive at 25 degrees C. We show here that SAMK is activated when cells are transferred from 37 degrees C to 25 degrees C, and HAMK is activated when cells are transferred from 4 degrees C to 25 degrees C. Moreover, we show that heat activation of HAMK also occurs in cell-free extracts. We conclude that (i) SAMK or HAMK activation does not require a particular temperature but a relative temperature shift, and (ii) that either HAMK itself or one or more of its upstream activators can sense temperature change directly.

HSF and Msn2/4p can exclusively or cooperatively activate the yeast HSP104 gene

In an effort to understand how an accurate level of stress-specific expression is obtained, we studied the promoter of the yeast HSP104 gene. Through 5' deletions, we defined a 334 bp fragment upstream of the first coding AUG as sufficient and essential for maximal basal activity and a 260 bp fragment as sufficient and essential for heat shock responsiveness. These sequences contain heat shock elements (HSEs) and stress response elements (STREs) that cooperate to achieve maximal inducible expression. However, in the absence of one set of factors (e.g. in msn2Deltamsn4Delta cells) proper induction is obtained exclusively through HSEs. We also show that HSP104 is constitutively derepressed in ras2Delta cells. This derepression is achieved exclusively through activation of STREs, with no role for HSEs. Strikingly, in ras2Deltamsn2Deltamsn4Delta cells the HSP104 promoter is also derepressed, but in this strain derepression is mediated through HSEs, showing the flexibility and adaptation of the promoter. Thus, appropriate transcription of HSP104 is usually obtained through cooperation between the Msn2/4/STRE and the HSF/ HSE systems, but each factor could activate the promoter alone, backing up the other. Transcription control of HSP104 is adaptive and robust, ensuring proper expression under extreme conditions and in various mutants.

Structural analysis of yeast HSF by site-specific crosslinking

We have introduced cysteine substitutions into the yeast HSF1 gene at a variety of locations. Most have no phenotypic effect, and therefore provide site-specific probes for thiol-specific reagents. Crosslinking of single mutants identifies locations where equivalent regions of individual monomers can approach each other in the HSF trimer. Crosslinking of double mutants indicates regions that can approach closely within a single subunit. Results for the DNA binding domain and trimerization domain are consistent with known structural information, and provide essential controls on the validity of the technique. In contrast to these two domains, the N-terminal and C-terminal domains, wherein lie the transcriptional activators, are highly flexible, and do not appear to be in stable contact with any other portions of the protein. None of these patterns are affected by the conformational change that is induced by superoxide or heat shock. We suggest a new model for the mechanism of HSF regulation that accomodates the structural information provided by these studies.

Complex regulation of the yeast heat shock transcription factor

The yeast heat shock transcription factor (HSF) is regulated by posttranslational modification. Heat and superoxide can induce the conformational change associated with the heat shock response. Interaction between HSF and the chaperone hsp70 is also thought to play a role in HSF regulation. Here, we show that the Ssb1/2p member of the hsp70 family can form a stable, ATP-sensitive complex with HSF-a surprising finding because Ssb1/2p is not induced by heat shock. Phosphorylation and the assembly of HSF into larger, ATP-sensitive complexes both occur when HSF activity decreases, whether during adaptation to a raised temperature or during growth at low glucose concentrations. These larger HSF complexes also form during recovery from heat shock. However, if HSF is assembled into ATP-sensitive complexes (during growth at a low glucose concentration), heat shock does not stimulate the dissociation of the complexes. Nor does induction of the conformational change induce their dissociation. Modulation of the in vivo concentrations of the SSA and SSB proteins by deletion or overexpression affects HSF activity in a manner that is consistent with these findings and suggests the model that the SSA and SSB proteins perform distinct roles in the regulation of HSF activity.