Self-regulation of 70-kilodalton heat shock proteins in Saccharomyces cerevisiae

What's Genetic Variation Got to Do with It? Starvation-Induced Self-Fertilization Enhances Survival in Paramecium

The pervasiveness of sex despite its well-known costs is a long-standing puzzle in evolutionary biology. Current explanations for the success of sex in nature largely rely on the adaptive significance of the new or rare genotypes that sex may generate. Less explored is the possibility that sex-underlying molecular mechanisms can enhance fitness and convey benefits to the individuals that bear the immediate costs of sex. Here, we show that the molecular environment associated with self-fertilization can increase stress resistance in the ciliate Paramecium tetraurelia. This advantage is independent of new genetic variation, coupled with a reduced nutritional input, and offers fresh insights into the mechanistic origin of sex. In addition to providing evidence that the molecular underpinnings of sexual reproduction and the stress response are linked in P. tetraurelia, these findings supply an integrative explanation for the persistence of self-fertilization in this ciliate.

Hsf1 on a leash - controlling the heat shock response by chaperone titration

Heat shock factor 1 (Hsf1) is an ancient transcription factor that monitors protein homeostasis (proteostasis) and counteracts disturbances by triggering a transcriptional programme known as the heat shock response (HSR). The HSR is transiently activated and upregulates the expression of core proteostasis genes, including chaperones. Dysregulation of Hsf1 and its target genes are associated with disease; cancer cells rely on a constitutively active Hsf1 to promote rapid growth and malignancy, whereas Hsf1 hypoactivation in neurodegenerative disorders results in formation of toxic aggregates. These central but opposing roles highlight the importance of understanding the underlying molecular mechanisms that control Hsf1 activity. According to current understanding, Hsf1 is maintained latent by chaperone interactions but proteostasis perturbations titrate chaperone availability as a result of chaperone sequestration by misfolded proteins. Liberated and activated Hsf1 triggers a negative feedback loop by inducing the expression of key chaperones. Until recently, Hsp90 has been highlighted as the central negative regulator of Hsf1 activity. In this review, we focus on recent advances regarding how the Hsp70 chaperone controls Hsf1 activity and in addition summarise several additional layers of activity control.

Growth-Regulated Hsp70 Phosphorylation Regulates Stress Responses and Prion Maintenance

Maintenance of protein homeostasis in eukaryotes under normal growth and stress conditions requires the functions of Hsp70 chaperones and associated cochaperones. Here, we investigate an evolutionarily conserved serine phosphorylation that occurs at the site of communication between the nucleotide-binding and substrate-binding domains of Hsp70. Ser151 phosphorylation in yeast Hsp70 (Ssa1) is promoted by cyclin-dependent kinase (Cdk1) during normal growth. Phosphomimetic substitutions at this site (S151D) dramatically downregulate heat shock responses, a result conserved with HSC70 S153 in human cells. Phosphomimetic forms of Ssa1 also fail to relocalize in response to starvation conditions, do not associate in vivo with Hsp40 cochaperones Ydj1 and Sis1, and do not catalyze refolding of denatured proteins in vitro in cooperation with Ydj1 and Hsp104. Despite these negative effects on HSC70/HSP70 function, the S151D phosphomimetic allele promotes survival of heavy metal exposure and suppresses the Sup35-dependent [PSI+ ] prion phenotype, consistent with proposed roles for Ssa1 and Hsp104 in generating self-nucleating seeds of misfolded proteins. Taken together, these results suggest that Cdk1 can downregulate Hsp70 function through phosphorylation of this site, with potential costs to overall chaperone efficiency but also advantages with respect to reduction of metal-induced and prion-dependent protein aggregate production.

Dual interaction of the Hsp70 J-protein cochaperone Zuotin with the 40S and 60S ribosomal subunits

Ribosome-associated J protein-Hsp70 chaperones promote nascent polypeptide folding and normal translational fidelity. Though known to span the ribosome subunits, understanding of J protein Zuo1 function is limited. New structural and crosslinking data allow more precise positioning of Saccharomyces cerevisiae Zuo1 near the 60S polypeptide exit site, pointing to interactions with ribosomal protein eL31 and 25S rRNA helix 24. The junction between the 60S-interacting and subunit-spanning helices is a hinge, positioning Zuo1 on the 40S, yet accommodating subunit rotation. Interaction between C-terminus of Zuo1 and 40S occurs via 18S rRNA expansion segment 12 (ES12) of helix 44, which originates at the decoding site. Deletions in either ES12 or C-terminus of Zuo1 alter stop codon readthrough and −1 frameshifting. Our study offers insight into how this cotranslational chaperone system may monitor decoding site activity and nascent polypeptide transit, thereby coordinating protein translation and folding.

In Silico Molecular Modeling and Docking Studies on Novel Mutants (E229V, H225P and D230C) of the Nucleotide-Binding Domain of Homo sapiens Hsp70

In this study, we explored the possibility of determining the synergistic interactions between nucleotide-binding domain (NBD) of Homo sapiens heat-shock 70 kDa protein (Hsp70) and E1A 32 kDa of adenovirus serotype 5 motif (PNLVP) in the efficiency of killing of tumor cells in cancer treatment. At present, the protein interaction between NBD and PNLVP motif is still unknown, but believed to enhance the rate of virus replication in tumor cells. Three mutant models (E229V, H225P and D230C) were built and simulated, and their interactions with PNLVP motif were studied. The PNLVP motif showed the binding energy and intermolecular energy values with the novel E229V mutant at -7.32 and -11.2 kcal/mol. The E229V mutant had the highest number of hydrogen bonds (7). Based on the root mean square deviation, root mean square fluctuation, hydrogen bonds, salt bridge, secondary structure, surface-accessible solvent area, potential energy and distance matrices analyses, it was proved that the E229V had the strongest and most stable interaction with the PNLVP motif among all the four protein-ligand complex structures. The knowledge of this protein-ligand complex model would help in designing Hsp70 structure-based drug for cancer therapy.

Expression Profiling and Cellular Localization of Stress Responsive Proteins in Squamous Cell Carcinoma of Human Esophagus

BACKGROUND

The ambiguity in relating expression dynamics of stress response proteins with human esophageal squamous cell carcinoma (ESCC) has sidelined the potential of stress proteins as therapeutic targets. This study was an attempt to unequivocally relate the stress protein dynamics with stage and propensity of ESCC.

METHODS

Surgically resected tumor and adjacent histologically normal tissue from 46 patients with esophageal squamous cell carcinoma were investigated in the present study. Expression of HSPs was analyzed by Western blotting and immunohistochemistry.

RESULTS

HSP expression was observed in all 46 cases both in adjacent normal and tumor tissues. The expression and the localization of individual HSP showed no significant correlation with depth of invasion, tumor grade, and pathological stage of the tumor. HSP 27 was the most abundant protein followed by HSP 90 and HSP 70. The HSP 27 localized exclusively in the cytoplasm of adjacent normal and tumor cells. HSP 70 showed dispersed expression with predominating nuclear localization in both normal and tumor tissue cells and HSP 90 was localized in cytoplasm of adjacent normal and in nucleus of tumor cells in majority of the cases.

CONCLUSION

Our data advocate lack of relationship between stress protein expression and the progression of ESCC. The data renew the prospect of anti-HSP drugs as therapeutic resources in light of the possibility that their use would continue to sensitize cancer cells towards drug induced apoptosis for tumor regression.

Expression of hsrω-RNAi transgene prior to heat shock specifically compromises accumulation of heat shock-induced Hsp70 in Drosophila melanogaster

A delayed organismic lethality was reported in Drosophila following heat shock when developmentally active and stress-inducible noncoding hsrω-n transcripts were down-regulated during heat shock through hs-GAL4-driven expression of the hsrω-RNAi transgene, despite the characteristic elevation of all heat shock proteins (Hsp), including Hsp70. Here, we show that hsrω-RNAi transgene expression prior to heat shock singularly prevents accumulation of Hsp70 in all larval tissues without affecting transcriptional induction of hsp70 genes and stability of their transcripts. Absence of the stress-induced Hsp70 accumulation was not due to higher levels of Hsc70 in hsrω-RNAi transgene-expressing tissues. Inhibition of proteasomal activity during heat shock restored high levels of the induced Hsp70, suggesting very rapid degradation of the Hsp70 even during the stress when hsrω-RNAi transgene was expressed ahead of heat shock. Unexpectedly, while complete absence of hsrω transcripts in hsrω (66) homozygotes (hsrω-null) did not prevent high accumulation of heat shock-induced Hsp70, hsrω-RNAi transgene expression in hsrω-null background blocked Hsp70 accumulation. Nonspecific RNAi transgene expression did not affect Hsp70 induction. These observations reveal that, under certain conditions, the stress-induced Hsp70 can be selectively and rapidly targeted for proteasomal degradation even during heat shock. In the present case, the selective degradation of Hsp70 does not appear to be due to down-regulation of the hsrω-n transcripts per se; rather, this may be an indirect effect of the expression of hsrω-RNAi transgene whose RNA products may titrate away some RNA-binding proteins which may also be essential for stability of the induced Hsp70.

A Novel Protein Interaction between Nucleotide Binding Domain of Hsp70 and p53 Motif

Currently, protein interaction of Homo sapiens nucleotide binding domain (NBD) of heat shock 70 kDa protein (PDB: 1HJO) with p53 motif remains to be elucidated. The NBD-p53 motif complex enhances the p53 stabilization, thereby increasing the tumor suppression activity in cancer treatment. Therefore, we identified the interaction between NBD and p53 using STRING version 9.1 program. Then, we modeled the three-dimensional structure of p53 motif through homology modeling and determined the binding affinity and stability of NBD-p53 motif complex structure via molecular docking and dynamics (MD) simulation. Human DNA binding domain of p53 motif (SCMGGMNR) retrieved from UniProt (UniProtKB: P04637) was docked with the NBD protein, using the Autodock version 4.2 program. The binding energy and intermolecular energy for the NBD-p53 motif complex were -0.44 Kcal/mol and -9.90 Kcal/mol, respectively. Moreover, RMSD, RMSF, hydrogen bonds, salt bridge, and secondary structure analyses revealed that the NBD protein had a strong bond with p53 motif and the protein-ligand complex was stable. Thus, the current data would be highly encouraging for designing Hsp70 structure based drug in cancer therapy.

Heat shock response improves heterologous protein secretion in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a widely used platform for the production of heterologous proteins of medical or industrial interest. However, heterologous protein productivity is often low due to limitations of the host strain. Heat shock response (HSR) is an inducible, global, cellular stress response, which facilitates the cell recovery from many forms of stress, e.g., heat stress. In S. cerevisiae, HSR is regulated mainly by the transcription factor heat shock factor (Hsf1p) and many of its targets are genes coding for molecular chaperones that promote protein folding and prevent the accumulation of mis-folded or aggregated proteins. In this work, we over-expressed a mutant HSF1 gene HSF1-R206S which can constitutively activate HSR, so the heat shock response was induced at different levels, and we studied the impact of HSR on heterologous protein secretion. We found that moderate and high level over-expression of HSF1-R206S increased heterologous α-amylase yield 25 and 70 % when glucose was fully consumed, and 37 and 62 % at the end of the ethanol phase, respectively. Moderate and high level over-expression also improved endogenous invertase yield 118 and 94 %, respectively. However, human insulin precursor was only improved slightly and this only by high level over-expression of HSF1-R206S, supporting our previous findings that the production of this protein in S. cerevisiae is not limited by secretion. Our results provide an effective strategy to improve protein secretion and demonstrated an approach that can induce ER and cytosolic chaperones simultaneously.

Effect of the steroid K-canrenoate on hsp70 expression and tissue damage in transgenic Drosophila melanogaster (hsp70-lacZ) Bg9

In the present study the effect of 0.1, 0.2, 0.4, 0.8, and 1.0 µL/mL of the steroid K-canrenoate was evaluated in the third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ) Bg(9) for 6, 24, and 48 hours of duration. The treatment of 0.1, 0.2, and 0.4 µL/mL of K-canrenoate did not induce the activity of hsp70 significantly compared to the control. The treatments of 0.8 and 1.0 µL/mL of K-canrenoate not only caused tissue damage but also induced a significant increase in the expression of hsp70 for the different durations of exposure. The results of the present study suggest that the K-canrenoate at 0.8 and 1.0 µL/mL is cytotoxic and caused tissue damage in the third instar larvae of transgenic D. melanogaster (hsp70-lacZ) Bg(9).

Expression of a malarial Hsp70 improves defects in chaperone-dependent activities in ssa1 mutant yeast

Plasmodium falciparum causes the most virulent form of malaria and encodes a large number of molecular chaperones. Because the parasite encounters radically different environments during its lifecycle, many members of this chaperone ensemble may be essential for P. falciparum survival. Therefore, Plasmodium chaperones represent novel therapeutic targets, but to establish the mechanism of action of any developed therapeutics, it is critical to ascertain the functions of these chaperones. To this end, we report the development of a yeast expression system for PfHsp70-1, a P. falciparum cytoplasmic chaperone. We found that PfHsp70-1 repairs mutant growth phenotypes in yeast strains lacking the two primary cytosolic Hsp70s, SSA1 and SSA2, and in strains harboring a temperature sensitive SSA1 allele. PfHsp70-1 also supported chaperone-dependent processes such as protein translocation and ER associated degradation, and ameliorated the toxic effects of oxidative stress. By introducing engineered forms of PfHsp70-1 into the mutant strains, we discovered that rescue requires PfHsp70-1 ATPase activity. Together, we conclude that yeast can be co-opted to rapidly uncover specific cellular activities mediated by malarial chaperones.

ATPase domain of Hsp70 exhibits intrinsic ATP-ADP exchange activity

The chaperone activity of Hsp70 in protein folding and its conformational switching are regulated through the hydrolysis of ATP and the ATP-ADP exchange cycle. It was reported that, in the presence of physiological concentrations of ATP (approximately 5 mM) and ADP (approximately 0.5 mM), Hsp70 catalyzes ATP-ADP exchange through transfer of gamma-phosphate between ATP and ADP, via an autophosphorylated intermediate, whereas it only catalyzes the hydrolysis of ATP in the absence of ADP. To clarify the functional domain of the ATP-ADP exchange activity of Hsp70, we isolated the 44-kD ATPase domain of Hsp70 after limited proteolysis with alpha-chymotrypsin (EC 3.4.21.1). The possibility of ATP-ADP exchange activity of a contaminating nucleoside diphosphate kinase (EC 2.7.4.6) was monitored throughout the experiments. The purified 44-kD ATPase domain exhibited intrinsic ATP-ADP exchange by catalyzing the transfer of gamma-phosphate between ATP and ADP with acid-stable autophosphorylation at Thr204.

Sphingoid base is required for translation initiation during heat stress in Saccharomyces cerevisiae

Sphingolipids are required for many cellular functions including response to heat shock. We analyzed the yeast lcb1-100 mutant, which is conditionally impaired in the first step of sphingolipid biosynthesis and shows a strong decrease in heat shock protein synthesis and viability. Transcription and nuclear export of heat shock protein mRNAs is not affected. However, lcb1-100 cells exhibited a strong decrease in protein synthesis caused by a defect in translation initiation under heat stress conditions. The essential lipid is sphingoid base, not ceramide or sphingoid base phosphates. Deletion of the eIF4E-binding protein Eap1p in lcb-100 cells restored translation of heat shock proteins and increased viability. The translation defect during heat stress in lcb1-100 was due at least partially to a reduced function of the sphingoid base-activated PKH1/2 protein kinases. In addition, depletion of the translation initiation factor eIF4G was observed in lcb1-100 cells and ubiquitin overexpression allowed partial recovery of translation after heat stress. Taken together, we have shown a requirement for sphingoid bases during the recovery from heat shock and suggest that this reflects a direct lipid-dependent signal to the cap-dependent translation initiation apparatus.

Inhibition of Hsp90 function delays and impairs recovery from heat shock

The induction of the heat shock response as well as its termination is autoregulated by heat shock protein activities. In this study we have investigated whether Hsp90 functional protein levels influence the characteristics and duration of the heat shock response. Treatment of cells with several benzoquinone ansamycin inhibitors of Hsp90 (geldanamycin, herbimycin A) activated a heat shock response in the absence of heat shock, as reported previously. Pretreatment of cells with the Hsp90 inhibitors significantly delayed the rate of restoration of normal protein synthesis following a brief heat shock. Concurrently, the rate of Hsp synthesis and accumulation was substantially increased and prolonged. The cessation of heat shock protein synthesis did not occur until the levels of Hsp70 were substantially elevated relative to its standard threshold for autoregulation. The elevated levels of HSPS 22-28 (the small HSPS) and Hsp70 are not able to promote thermotolerance when Hsp90 activity is repressed by ansamycins; rather a suppression of thermotolerance is observed. These results suggest that a multicomponent protein chaperone complex involving both Hsp90 and Hsp70 signals the cessation of heat shock protein synthesis, the restoration of normal translation, and likely the establishment of thermotolerance. Impaired function of either component is sufficient to alter the heat shock response.

A new method for manipulating transgenes: engineering heat tolerance in a complex, multicellular organism

BACKGROUND

Heat-shock proteins (hsps) are thought to protect cells against stresses, especially due to elevated temperatures. But while genetic manipulation of hsp gene expression can protect microorganisms and cultured metazoan cells against lethal stress, this has so far not been demonstrated in multicellular organisms. Testing whether expression of an hsp transgene contributes to increased stress tolerance is complicated by a general problem of transgene analysis: if the transgene cannot be targeted to a precise site in the genome, newly observed phenotypes may be due to either the action of the transgene or mutations caused by the transgene insertion.

RESULTS

To study the relationship between heat tolerance and hsp expression in Drosophila melanogaster, we have developed a novel method for transgene analysis, based upon the site-specific FLP recombinase. The method employs site-specific sister chromatid exchange to create an allelic series of transgene insertions that share the same integration site, but differ in transgene copy number. Phenotypic differences between members of this series can be confidently attributed to the transgenes. Using such an allelic series and a novel thermotolerance assay for Drosophila embryos, we investigated the role of the 70 kD heat-shock protein, Hsp 70, in thermotolerance. At early embryonic stages, Hsp70 accumulation was rate-limiting for thermotolerance, and elevated Hsp70 expression increased survival at extreme temperatures.

CONCLUSION

Our results provide an improved method for analyzing transgenes and demonstrate that, in Drosophila, Hsp70 is a critical thermotolerance factor. They show, moreover, that manipulating the expression of a single hsp can be sufficient to improve the stress tolerance of a complex multicellular organism.

Transcriptional Regulation of the Metazoan Stress Protein Response

This review provides an updated account of the regulation of the metazoan stress protein response. Where indicated, observations made with yeasts are also included. However, a discussion of the plant stress protein response is intentionally omitted (for a review, see 1). The stress protein response, as discussed hereafter, is understood to relate to the response by virtually all cells to heat and other stressors that results in the induced expression of so-called heat shock or stress genes. The protein products of these genes localize largely to the cytoplasm, nucleus, or organelles. An analogous response controls the expression of related genes, whose products reside in the endoplasmic reticulum. The response, termed ER stress response or unfolded protein response, is mediated by a separate regulation system that is not discussed in this review. Note, however, that recent work suggests the existence of commonalities between the regulatory systems controlling the stress protein and ER stress responses (2).

Hazardous effects of effluent from the chrome plating industry: 70 kDa heat shock protein expression as a marker of cellular damage in transgenic Drosophila melanogaster (hsp70-lacZ)

Hazardous effects of an effluent from the chrome plating industry were examined by exposing transgenic Drosophila melanogaster (hsp70-lacZ) to various concentrations (0.05, 0.1, 1.0, 10.0, and 100.0 micro L/mL) of the effluent through diet. The emergence pattern of adult flies was affected, along with impaired reproductive performance at the higher dietary concentrations of the effluent. Interestingly, the effect of the effluent was more pronounced in male than in female flies. The effect of the effluent on development of adult flies was concurrent with the expression pattern of the heat shock protein 70 gene (hsp70), both in larval tissues and in the reproductive organs of adult flies. We observed a dose- and time-dependent expression of hsp70 in third instar larvae exposed for different time intervals. Absence of hsp70 expression in larvae exposed to 0.1 micro L/mL of the effluent indicated that this is the highest nontoxic concentration for Drosophila. The stress gene assay in the reproductive organs of adult flies revealed hsp70 expression in the testis of male flies only. However, trypan blue dye exclusion tests in these tissues indicate tissue damage in the male accessory gland of adult flies, which was further confirmed by ultrastructural observations. In the present study we demonstrate the utility of transgenic Drosophila as an alternative animal model for evaluating hazardous effects of the effluent from the chrome plating industry and further reveal the cytoprotective role of hsp70 and its expression as an early marker in environmental risk assessment.

Saccharomyces cerevisiae Hsp70 mutations affect [PSI+] prion propagation and cell growth differently and implicate Hsp40 and tetratricopeptide repeat cochaperones in impairment of [PSI+]

We previously described an Hsp70 mutant (Ssa1-21p), altered in a conserved residue (L483W), that dominantly impairs yeast [PSI(+)] prion propagation without affecting growth. We generated new SSA1 mutations that impaired [PSI(+)] propagation and second-site mutations in SSA1-21 that restored normal propagation. Effects of mutations on growth did not correlate with [PSI(+)] phenotype, revealing differences in Hsp70 function required for growth and [PSI(+)] propagation and suggesting that Hsp70 interacts differently with [PSI(+)] prion aggregates than with other cellular substrates. Complementary suppression of altered activity between forward and suppressing mutations suggests that mutations that impair [PSI(+)] affect a similar Hsp70 function and that suppressing mutations similarly overcome this effect. All new mutations that impaired [PSI(+)] propagation were located in the ATPase domain. Locations and homology of several suppressing substitutions suggest that they weaken Hsp70's substrate-trapping conformation, implying that impairment of [PSI(+)] by forward mutations is due to altered ability of the ATPase domain to regulate substrate binding. Other suppressing mutations are in residues important for interactions with Hsp40 or TPR-containing cochaperones, suggesting that such interactions are necessary for the impairment of [PSI(+)] propagation caused by mutant Ssa1p.

Antagonistic interactions between yeast [PSI(+)] and [URE3] prions and curing of [URE3] by Hsp70 protein chaperone Ssa1p but not by Ssa2p

The yeast [PSI(+)], [URE3], and [PIN(+)] genetic elements are prion forms of Sup35p, Ure2p, and Rnq1p, respectively. Overexpression of Sup35p, Ure2p, or Rnq1p leads to increased de novo appearance of [PSI(+)], [URE3], and [PIN(+)], respectively. This inducible appearance of [PSI(+)] was shown to be dependent on the presence of [PIN(+)] or [URE3] or overexpression of other yeast proteins that have stretches of polar residues similar to the prion-determining domains of the known prion proteins. In a similar manner, [PSI(+)] and [URE3] facilitate the appearance of [PIN(+)]. In contrast to these positive interactions, here we find that in the presence of [PIN(+)], [PSI(+)] and [URE3] repressed each other's propagation and de novo appearance. Elevated expression of Hsp104 and Hsp70 (Ssa2p) had little effect on these interactions, ruling out competition between the two prions for limiting amounts of these protein chaperones. In contrast, we find that constitutive overexpression of SSA1 but not SSA2 cured cells of [URE3], uncovering a specific interaction between Ssa1p and [URE3] and a functional distinction between these nearly identical Hsp70 isoforms. We also find that Hsp104 abundance, which critically affects [PSI(+)] propagation, is elevated when [URE3] is present. Our results are consistent with the notion that proteins that have a propensity to form prions may interact with heterologous prions but, as we now show, in a negative manner. Our data also suggest that differences in how [PSI(+)] and [URE3] interact with Hsp104 and Hsp70 may contribute to their antagonistic interactions.

The yeast hsp70 homologue Ssa is required for translation and interacts with Sis1 and Pab1 on translating ribosomes

The 70-kDa heat shock proteins are molecular chaperones that participate in a variety of cellular functions. This chaperone function is stimulated by interaction with hsp40 proteins. The Saccharomyces cerevisiae gene encoding the essential hsp40 homologue, SIS1, appears to function in translation initiation. Mutations in ribosomal protein L39 (rpl39) complement loss-of-function mutations in SIS1 as well as PAB1 (poly(A)-binding protein), suggesting a functional interaction between these proteins. However, while a direct interaction between Sis1 and Pab1 is not detectable, both of these proteins physically interact with the essential Ssa (and not Ssb) family of hsp70 proteins. This interaction is mediated by the variable C-terminal domain of Ssa. Subcellular fractionations demonstrate that the binding of Ssa to ribosomes is dependent upon its C terminus and that its interaction with Sis1 and Pab1 occurs preferentially on translating ribosomes. Consistent with a function in translation, depletion of Ssa protein produces a general translational defect that appears similar to loss of Sis1 and Pab1 function. This translational effect of Ssa appears mediated, at least in part, by its affect on the interaction of Pab1 with the translation initiation factor, eIF4G, which is dramatically reduced in the absence of functional Ssa protein.

Chaperones that cure yeast artificial [PSI+] and their prion-specific effects

The [PSI(+)] nonsense-suppressor determinant of Saccharomyces cerevisiae results from the ability of Sup35 (eRF3) translation termination factor to undergo prion-like aggregation [1]. Although this process is autocatalytic, in vivo it depends on the chaperone Hsp104, whose lack or overexpression can cure [PSI(+)] [2]. Overproduction of the chaperone protein Ssb1 increased the [PSI(+)] curing by excess Hsp104, although it had no effect on its own, and excess chaperone protein Ssa1 protected [PSI(+)] against Hsp104 [3,4]. We used an artificial [PSI(+)(PS)] based on the Sup35 prion-forming domain from yeast Pichia methanolica [5] to find other prion-curing factors. Both [PSI(+)(PS)] and [PSI(+)] have prion 'strains', differing in their suppressor efficiency and mitotic stability. We show that [PSI(+)(PS)] and a 'weak' strain of [PSI(+)] can be cured by overexpression of chaperones Ssa1, Ssb1 and Ydj1. The ability of different chaperones to cure [PSI(+)(PS)] showed significant prion strain specificity, which could be related to variation in Sup35 prion structure. Our results imply that homologs of these chaperones may be active against mammalian prion and amyloid diseases.

A role for cytosolic hsp70 in yeast [PSI(+)] prion propagation and [PSI(+)] as a cellular stress

[PSI(+)] is a prion (infectious protein) of Sup35p, a subunit of the Saccharomyces cerevisiae translation termination factor. We isolated a dominant allele, SSA1-21, of a gene encoding an Hsp70 chaperone that impairs [PSI(+)] mitotic stability and weakens allosuppression caused by [PSI(+)]. While [PSI(+)] stability is normal in strains lacking SSA1, SSA2, or both, SSA1-21 strains with a deletion of SSA2 cannot propagate [PSI(+)]. SSA1-21 [PSI(+)] strains are hypersensitive to curing of [PSI(+)] by guanidine-hydrochloride and partially cured of [PSI(+)] by rapid induction of the heat-shock response but not by growth at 37 degrees. The number of inheritable [PSI(+)] particles is significantly reduced in SSA1-21 cells. SSA1-21 effects on [PSI(+)] appear to be independent of Hsp104, another stress-inducible protein chaperone known to be involved in [PSI(+)] propagation. We propose that cytosolic Hsp70 is important for the formation of Sup35p polymers characteristic of [PSI(+)] from preexisting material and that Ssa1-21p both lacks and interferes with this activity. We further demonstrate that the negative effect of heat stress on [PSI(+)] phenotype directly correlates with solubility of Sup35p and find that in wild-type strains the presence of [PSI(+)] causes a stress that elevates basal expression of Hsp104 and SSA1.

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.

The heat shock response in yeast: differential regulations and contributions of the Msn2p/Msn4p and Hsf1p regulons

The heat shock transcription factor Hsf1p and the stress-responsive transcription factors Msn2p and Msn4p are activated by heat shock in the yeast Saccharomyces cerevisiae. Their respective contributions to heat shock protein induction have been analysed by comparison of mutants and wild-type strains using [35S]-methionine labelling and two-dimensional gel electrophoresis. Among 52 proteins induced by a shift from 25 degrees C to 38 degrees C, half of them were found to be dependent upon Msn2p and/or Msn4p (including mostly antioxidants and enzymes involved in carbon metabolism), while the other half (including mostly chaperones and associated proteins) were dependent upon Hsf1p. The two sets of proteins overlapped only slightly. Three proteins were induced independently of these transcription factors, suggesting the involvement of other transcription factor(s). The Ras/cAMP/PKA signalling pathway cAMP had a negative effect on the induction of the Msn2p/Msn4p regulon, but did not affect the Hsf1p regulon. Thus, the two types of transcription factor are regulated differently and control two sets of functionally distinct proteins, suggesting two different physiological roles in the heat shock cellular response.

A nuclear export signal prevents Saccharomyces cerevisiae Hsp70 Ssb1p from stimulating nuclear localization signal-directed nuclear transport

Hsp70 has been implicated in nuclear localization signal (NLS)-directed nuclear transport. Saccharomyces cerevisiae contains distinct SSA and SSB gene families of cytosolic Hsp70s. The nucleocytoplasmic localization of Ssa1p and Ssb1p was investigated using green fluorescent protein (GFP) fusions. Whereas GFP-Ssa1p localized both to the nucleus and cytoplasm, GFP-Ssb1p appeared only in the cytosol. The C-terminal domain of Ssb1p contains a leucine-rich nuclear export signal (NES) that is necessary and sufficient to direct nuclear export. The accumulation of GFP-Ssb1p in the nuclei of xpo1-1 cells suggests that Ssb1p shuttles across the nuclear envelope. Elevated levels of SSA1 but not SSB1 suppressed the NLS-GFP nuclear localization defects of nup188-Delta cells. Studies with Ssa1p/Ssb1p chimeras revealed that the Ssb1p NES is sufficient and necessary to inhibit the function of Ssa- or Ssb-type Hsp70s in nuclear transport. Thus, NES-less Ssb1p stimulates nuclear transport in nup188-Delta cells and NES-containing Ssa1p does not. We conclude that the differential function of Ssa1p and Ssb1p in nuclear transport is due to the NES-directed export of the Ssb1p and not to functional differences in their ATPase or peptide binding domains.

Analysis of the heat-shock response displayed by two Chaetomium species originating from different thermal environments

Three features of the heat shock response, reorganization of protein expression, intracellular accumulation of trehalose, and alteration in unsaturation degree of fatty acids were investigated in the thermophilic fungus Chaetomium thermophile and compared to the response displayed by a closely related mesophilic species, C. brasiliense. Thermophilic heat shock response paralleled the mesophilic response in many respects like (i) the temperature difference observed between normothermia and the upper limit of translational activity, (ii) the transient nature of the heat shock response at the level of protein expression including both the induction of heat shock proteins (HSPs) as well as the repression of housekeeping proteins, (iii) the presence of representatives of high-molecular-weight HSPs families, (iv) intracellular accumulation of trehalose, and finally (v) modifications in fatty acid composition. On the other hand, a great variability between the two organisms was observed for the proteins expressed during stress, in particular a protein of the HSP60 family that was only observed in C. thermophile. This peptide was also present constitutively at normal temperature and may thus fulfil thermophilic functions. It is shown that accumulation of trehalose does not play a part in thermophily but is only a stress response. C. thermophile contains less polyunsaturated fatty acids at normal temperature than C. brasiliense, a fact that can be directly related to thermophily. When subjected to heat stress, both organisms tended to accumulate shorter and less unsaturated fatty acids.

Human Hsp70 molecular chaperone binds two calcium ions within the ATPase domain

BACKGROUND

The 70 kDa heat shock proteins (Hsp70) are a family of molecular chaperones, which promote protein folding and participate in many cellular functions. The Hsp70 chaperones are composed of two major domains. The N-terminal ATPase domain binds to and hydrolyzes ATP, whereas the C-terminal domain is required for polypeptide binding. Cooperation of both domains is needed for protein folding. The crystal structure of bovine Hsc70 ATPase domain (bATPase) has been determined and, more recently, the crystal structure of the peptide-binding domain of a related chaperone, DnaK, in complex with peptide substrate has been obtained. The molecular chaperone activity and conformational switch are functionally linked with ATP hydrolysis. A high-resolution structure of the ATPase domain is required to provide an understanding of the mechanism of ATP hydrolysis and how it affects communication between C- and N-terminal domains.

RESULTS

The crystal structure of the human Hsp70 ATPase domain (hATPase) has been determined and refined at 1. 84 A, using synchrotron radiation at 120K. Two calcium sites were identified: the first calcium binds within the catalytic pocket, bridging ADP and inorganic phosphate, and the second calcium is tightly coordinated on the protein surface by Glu231, Asp232 and the carbonyl of His227. Overall, the structure of hATPase is similar to bATPase. Differences between them are found in the loops, the sites of amino acid substitution and the calcium-binding sites. Human Hsp70 chaperone is phosphorylated in vitro in the presence of divalent ions, calcium being the most effective.

CONCLUSIONS

The structural similarity of hATPase and bATPase and the sequence similarity within the Hsp70 chaperone family suggest a universal mechanism of ATP hydrolysis among all Hsp70 molecular chaperones. Two calcium ions have been found in the hATPase structure. One corresponds to the magnesium site in bATPase and appears to be important for ATP hydrolysis and in vitro phosphorylation. Local changes in protein structure as a result of calcium binding may facilitate phosphorylation. A small, but significant, movement of metal ions and sidechains could position catalytically important threonine residues for phosphorylation. The second calcium site represents a new calcium-binding motif that can play a role in the stabilization of protein structure. We discuss how the information about catalytic events in the active site could be transmitted to the peptide-binding domain.

Transcriptional regulation of the yeast DnaJ homologue SIS1

The Saccharomyces cerevisiae SIS1 gene encodes an essential heat shock protein with similarity to the Escherichia coli DnaJ protein. In sis 1-85 and sis1-86 mutants, the sis1 RNA is induced to high levels at room temperature and without heat shock. The presence of wild type SIS1 in the sis1-85 mutant represses the overexpression of SIS1-85 protein. Furthermore, overexpression of wild type SIS1 reduces the beta-galactosidase activity expressed from a SIS1:lacZ fusion. These results suggest that SIS1 negatively regulates its own expression. The autoregulation of SIS1 transcription is mediated through a 39-base pair cis-element containing the SIS1 heat shock element plus additional flanking sequences on one side. Although SIS1 transcription is constitutive, it is transiently induced upon heat shock. In addition, SIS1 transcription is regulated by SSA (a class of HSP70 proteins) function. The elevated transcription of SIS1 in ssa1 ssa2 mutants is mediated solely through the SIS1 heat shock element. Therefore, the SIS1 autoregulatory element is different from the SSA-responsive element, suggesting that the mechanism involved in autoregulation of SIS1 is distinct from regulation of SIS1 by SSA proteins.

Induction and Regulation of Heat-Shock Gene Expression by an Amino Acid Analog in Soybean Seedlings

The effect of the proline analog azetidine-2-carboxylic acid (Aze) on the induction and the regulation of heat-shock (HS) mRNA accumulation and heat-shock protein (HSP) synthesis in soybean (Glycine max) seedlings was studied. Treatment with Aze elicited an HS-like response at the normal growth temperature, 28[deg]C, with seven of nine HS cDNA clones tested. Two cDNA clones, Gm-Hsp22.5 and pFS2033, share 78% identity; however, transcripts hybridizing to GmHsp22.5 but not pFS2033 accumulated with Aze treatment at 28[deg]C. Substantial incorporation of radioactive amino acid into high molecular weight HSPs but not low molecular weight HSPs was observed in vivo during Aze treatment at 28[deg]C. Low molecular weight HSPs were detected using antibodies raised against an abundant member of low molecular weight class I HSPs, indicating that low molecular weight HSPs were synthesized at normal growth temperatures during Aze treatment despite a lack of substantial in vivo radioactive amino acid incorporation. In summary, Aze treatment induced accumulation of most but not all HS mRNAs and HSPs in soybean seedlings; the observations presented here suggest differential regulation among various HS genes at the transcriptional and posttranscriptional levels.

The transcriptional regulation of heat shock genes: a plethora of heat shock factors and regulatory conditions

The inducible regulation of heat shock gene transcription is mediated by a family of heat shock factors (HSF) that respond to diverse forms of physiological and environmental stress including elevated temperature, amino acid analogs, heavy metals, oxidative stress, anti-inflammatory drugs, arachidonic acid, and a number of pathophysiological disease states. The vertebrate genome encodes a family of HSFs which are expressed ubiquitously, yet the DNA binding properties of each factor are negatively regulated and activated in response to specific conditions. This chapter will discuss the regulation of the HSF multi-gene family and the role of these transcriptional activators in the inducible expression of genes encoding heat shock proteins and molecular chaperones.

Inhibition mechanism of HSP70 induction in murine FM3A cells maintained at low culture temperature

We have shown previously that induction of HSP70 synthesis in murine FM3A and its mutant ts85 cells by heat shock is somehow modulated by culture temperature. In this study, we further examined activation of heat shock transcription factor (HSF) and induction kinetics of HSP70 synthesis and HSP70 mRNA in FM3A and ts85 cells maintained at 37 degrees C (37 degrees C-cells) and 33 degrees C (33 degrees C-cells). Upon exposure to heat shock, HSF was activated to a high level in 37 degrees C-FM3A cells, whereas HSF was activated only to a low level in the 33 degrees C-cells. The induction of HSP70 mRNA and HSP70 synthesis occurred successively in the 37 degrees C-cells but not in the 33 degrees C-cells. On the other hand, in both 37 and 33 degrees C-ts85 cells, activation of HSF, induction of HSP70 mRNA, and HSP70 synthesis occurred successively. Characteristically, protein synthesis in both 33 degrees C-FM3A and ts85 cells was significantly lower than in the respective 37 degrees C-cells, but constitutive HSP73 levels were similar among both the 37 and 33 degrees C-cells. Furthermore, inhibition of protein synthesis of FM3A cells did not influence the activation of HSF, but accelerated inactivation of the activated HSF. We discuss the possible inhibition mechanisms of activation of HSF in 33 degrees C-FM3A cells, regarding the function of HSP70 in both protein synthesis and repression of HSF.

The dissociation of ATP from hsp70 of Saccharomyces cerevisiae is stimulated by both Ydj1p and peptide substrates

hsp70 proteins of both eukaryotes and prokaryotes possess both ATPase and peptide binding activities. These two activities are crucial for the chaperone activity of hsp70 proteins. The activity of DnaK, the primary hsp70 of Escherichia coli, is modulated by the GrpE and DnaJ proteins. In the yeast Saccharomyces cerevisiae, the predominant cytosolic hsp70, Ssa1p, interacts with a DnaJ homologue, Ydj1p. In order to better understand the function of the Ssa1p/Ydj1p chaperone, the effects of polypeptide substrates and Ydj1p on Ssa1p ATPase activity were assessed using a combination of steady-state kinetic analysis and single turnover substrate hydrolysis experiments. Polypeptide substrates and Ydj1p both serve to stimulate ATPase activity of Ssa1p. The two types of effector are biochemically distinct, each conferring a characteristic K+ dependence on Ssa1p ATPase activity. However, in single turnover ATP hydrolysis experiments, both polypeptide substrates and Ydj1p destabilized the ATP.Ssa1p complex through a combination of accelerated hydrolysis of bound ATP and accelerated release of ATP from Ssa1p. The acceleration of ATP release by Ydj1p is a previously unidentified function of a DnaJ homologue. In the case of Ydj1p-stimulated Ssa1p, steady-state ATPase activity is increased less than 2-fold at physiological K+ concentrations, despite a 15-fold increase in the hydrolysis of bound ATP. The primary effect of Ydj1p appears to be to disfavor an ATP form of Ssa1p. On the other hand, peptide stimulation of Ssa1p ATPase activity was enhanced at physiological K+ concentrations, supporting the idea that cycles of ATP hydrolysis play an important role in the interaction of hsp70 with polypeptide substrates. The enhanced ATP dissociation caused by both polypeptide substrates and Ydj1p may play a role in the regulation of Ssa1p chaperone activity by altering the relative abundance of ATP-and ADP-bound forms.

Heat-shock proteins Hsp104 and Hsp70 reactivate mRNA splicing after heat inactivation

BACKGROUND

The heat-shock protein Hsp104 plays a crucial role in the survival of cells exposed to high temperatures and other severe stresses, but its specific functions and the biological pathways on which it operates have been unclear. Indeed, very little is known about the specific cellular processes in which any of the heat-shock proteins acts to affect thermotolerance. One essential process that is particularly sensitive to heat in many organisms is the splicing of intervening sequences from mRNA precursors.

RESULTS

We have examined the role of Hsp104 in the repair of splicing after disruption by heat shock. When splicing in the budding yeast Saccharomyces cerevisiae was disrupted by a brief heat shock, it recovered much more rapidly in wild-type strains than in strains containing hsp104 mutations. Constitutive expression of Hsp104 promoted the recovery of heat-damaged splicing in the absence of other protein synthesis, but did not protect splicing from the initial disruption, suggesting that Hsp104 functions to repair splicing after heat damage rather than to prevent the initial damage. A modest reduction in the recovery of splicing after heat shock in an hsp70 mutant suggested that Hsp70 may also function in the repair of splicing. The roles of Hsp104 and Hsp70 were confirmed by the ability of the purified proteins to restore splicing in extracts that had been heat-inactivated in vitro. Together, these two proteins were able to restore splicing to a greater degree than could be accomplished by an optimal concentration of either protein alone.

CONCLUSIONS

Our findings provide the first demonstration of the roles of heat-shock proteins in a biological process that is known to be particularly sensitive to heat in vivo. The results support previous genetic arguments that the Hsp104 and Hsp70 proteins have different, but related, functions in protecting cells from the toxic effects of high temperatures. Because Hsp104 and Hsp70 are able to function in vitro, after the heat-damaged substrate or substrates have been generated, neither protein is required to bind to its target(s) during heating in order to effect repair.

Heat-shock proteins as molecular chaperones

Functional proteins within cells are normally present in their native, completely folded form. However, vital processes of protein biogenesis such as protein synthesis and translocation of proteins into intracellular compartments require the protein to exist temporarily in an unfolded or partially folded conformation. As a consequence, regions buried when a polypeptide is in its native conformation become exposed and interact with other proteins causing protein aggregation which is deleterious to the cell. To prevent aggregation as proteins become unfolded, heat-shock proteins protect these interactive surfaces by binding to them and facilitating the folding of unfolded or nascent polypeptides. In other instances the binding of heat-shock proteins to interactive surfaces of completely folded proteins is a crucial part of their regulation. As heat shock and other stress conditions cause cellular proteins to become partially unfolded, the ability of heat-shock proteins to protect cells against the adverse effects of stress becomes a logical extension of their normal function as molecular chaperones.

Altered regulation of heat shock gene expression in heat resistant mouse cells

PURPOSE

The differences in the heat shock gene regulation between the RIF-1 cell line and its heat resistant derivative TR4 are further characterized.

METHODS AND MATERIALS

In vitro gel retardation assays were used to assess the presence of activated heat shock transcription factor in the two cell lines. The levels of the heat-inducible HSP 70.1, the constitutive HSC 70, the germ line-specific HSP 70.2, and the HSP 28 mRNAs in both untreated and iso-heated RIF-1 and TR4 cells were determined using the polymerase chain reaction coupled with the reverse transcriptase reaction. Induction and decay of induced heat shock protein synthesis was measured by 35S-methionine labeling of proteins.

RESULTS

Unheated TR4 cells display characteristics of heat shocked RIF-1 cells. TR4 cells have a constitutively activated heat shock transcription factor and elevated levels of the HSP 70.1, HSC 70, and the HSP 28 mRNAs. Upon an equal heat dose of 45 degrees C, 15 min, the TR4 cells exhibited a more rapid onset in heat shock mRNA and protein induction than did the RIF-1 cells. During the recovery from heat shock, the activated heat shock transcription factor and the induced HSP70 mRNAs decayed more slowly in the TR4 cells, although the protein synthesis pattern of the TR4 cells returned to control levels more rapidly following heat shock than did protein synthesis of the RIF-1 cells.

CONCLUSION

Unheated TR4 cells are similar to heat shocked RIF-1 cells at the transcriptional level. Induced HSP70 expression is modulated by the severity of the heat treatment (or the degree of heat damage) perceived by the cells rather than by the absolute heat dose given. We propose that the unheated TR4 cells are locked into the "ON" state of the heat shock response.

Molecular evolution of the HSP70 multigene family

Eukaryotic genomes encode multiple 70-kDa heat-shock proteins (HSP70s). The Saccharomyces cerevisiae HSP70 family is comprised of eight members. Here we present the nucleotide sequence of the SSA3 and SSB2 genes, completing the nucleotide sequence data for the yeast HSP70 family. We have analyzed these yeast sequences as well as 29 HSP70s from 24 additional eukaryotic and prokaryotic species. Comparison of the sequences demonstrates the extreme conservation of HSP70s; proteins from the most distantly related species share at least 45% identity and more than one-sixth of the amino acids are identical in the aligned region (567 amino acids) among all proteins analyzed. Phylogenetic trees constructed by two independent methods indicate that ancient molecular and cellular events have given rise to at least four monophyletic groups of eukaryotic HSP70 proteins. Each group of evolutionarily similar HSP70s shares a common intracellular localization and is presumed to be comprised of functional homologues; these include heat-shock proteins of the cytoplasm, endoplasmic reticulum, mitochondria, and chloroplasts. HSP70s localized in mitochondria and plastids are most similar to the DnaK HSP70 homologues in purple bacteria and cyanobacteria, respectively, which is consistent with the proposed prokaryotic origin of these organelles. The analyses indicate that the major eukaryotic HSP70 groups arose prior to the divergence of the earliest eukaryotes, roughly 2 billion years ago. In some cases, as exemplified by the SSA genes encoding the cytoplasmic HSP70s of S. cerevisiae, more recent duplication events have given rise to subfamilies within the major groups. The S. cerevisiae SSB proteins comprise a unique subfamily not identified in other species to date. This subfamily appears to have resulted from an ancient gene duplication that occurred at approximately the same time as the origin of the major eukaryotic HSP70 groups.

Regulation of hsp70 induction in thermotolerant HeLa cells

Upon exposure to heat shock, non-thermotolerant (NT) HeLa cells transiently synthesize a large amount of 70-kDa heat-shock protein (hsp70), whereas thermotolerant (TT) cells synthesize a small amount of hsp70. When the hsp70 mRNA of HeLa cells was analyzed, it became apparent that hsp70 mRNA in TT cells did not increase following heat shock, whereas hsp70 mRNA in NT cells did increase dramatically. A further analysis of the activation of the heat-shock transcription factor (HSF) showed that significant activation of HSF was observed immediately after heat shock in both NT and TT cells. However, activated HSF was rapidly repressed in the TT cells, but not in the NT cells. Thus, the decreased induction of hsp70 synthesis observed in the TT HeLa cells may be due to the immediate repression of activated cellular HSF, which probably results in the reduced induction of hsp70 mRNA. The hsp70 content in the TT cells was usually higher than in the NT cells. However, after heat-shock treatment, the hsp70 content of the NT cells increased to nearly the level of the TT cells concomitant with the repression of hsp70 synthesis. The association of activated HSF with hsp70 was observed in both NT and TT cells, and the amount of HSF-hsp70 complex within the cell increased in proportion to the increase in hsp70 in the cells. These findings strongly suggest that the activity of HSF is negatively regulated by the intracellular content of hsp70 in these cells. Furthermore, in vitro experiments on the activation of HSF suggest that HSFs of NT and TT cells may have different properties, or an unknown factor may exist which regulates HSF activation in these cells.

Thermal response of yeast cells overexpressing hsp70 genes

We have performed experiments to examine whether the overexpression of different members of the hsp70 family (SSA1, SSA2 and SSA4) in yeast Saccharomyces cerevisiae protects cells from thermal stress. Yeast cells were transformed with plasmids containing the SSA1 or SSA4 gene which was placed under the control of a galactose-inducible GAL1 promoter. In galactose, transformed yeast cells successfully overexpressed these hsp70 proteins at 23 degrees C, their normal growth temperature. When cell survival after heat shock at 50 or 56 degrees C was examined, our results showed that overexpression of SSA1 or SSA4 protein did not protect yeast cells against thermal stress, nor affect the cells' ability to develop thermotolerance. In contrast, thermal sensitivity was modified significantly by growing cells in galactose. Cellular survival of these cells after a 50 degrees C, 30-min heat treatment was 10(4)-fold higher than that of the same strains grown in glucose. In order to study the effects of overexpression of hsp70 on the thermal response of yeast cells independent of the carbon source, we eliminated the glucose-galactose-associated differences in thermal sensitivity by cloning the aforementioned hsp70 genes under the control of a glyceraldehyde phosphatase GAP promoter. S. cerevisiae transformed with these plasmids overexpressed the appropriate hsp70 gene in glucose as well as in galactose at 23 degrees C. Again, the overexpression of hsp70 neither protected cells from thermal stress, nor had a significant effect on the development of thermotolerance.

Heat-shock-induced protein synthesis is responsible for the switch-off of hsp70 transcription in Tetrahymena

We had previously described that new RNA synthesis is required for expression of the heat shock protein HSP70. Here, we find that the HSP70 mRNA decreases its levels under stress conditions, heat shock (HS) or arsenite (As), and that its levels start to decline at the same time as maximal HSPs synthesis (including HSP70) occurs. This suggests that regulation of the hsp70 gene is mainly exerted at the transcriptional level. Accumulation of the HSP70 mRNA in cells stressed in presence of cycloheximide (CHX), indicates that (a) protein(s) non-existent before stress, possibly HSP70 itself (which is shown here to be relatively stable), is involved in negatively regulating hsp70 expression. Since degradation of the HSP70 mRNA is also shown to occur in cells heat-shocked under CHX, as seen from decay of its levels upon addition of actinomycin D (AMD), the protein(s) must repress hsp70 expression at the transcriptional level. Other conditions that affect normal protein synthesis, namely the translation inhibitor puromycin and the arginine-analog canavanine (shown here to be stress inducers in Tetrahymena pyriformis), also cause a delay in transcription-arrest of the HSP70 mRNA. Under severe stress conditions of HS (36 degrees C) or As (350 microM), the levels of HSP70 mRNA are higher than under mild stress conditions, however, no significant difference is seen in the pattern of HSP70 mRNA decay.

Heat shock protein response in phosphorus-deficient heat-stressed broiler chickens

1. During acute in vivo heat stress, a normal heat shock protein (HSP) response was not inducible in chickens deficient in inorganic phosphorus (P(i)-deficient). 2. Small quantities of HSP 70 and HSP 90 were induced, but little or no HSP 23 was induced in P(i)-deficient chickens compared to P(i)-adequate chickens. 3. Increased susceptibility of P(i)-deficient chickens to acute heat stress was attributed to their inability to produce an adequate HSP response.