HtpG is essential for the thermal stress management in cyanobacteria

Two new clades recovered at high temperatures provide novel phylogenetic and genomic insights into Candidatus Accumulibacter

Candidatus Accumulibacter, a key genus of polyphosphate-accumulating organisms, plays key roles in lab- and full-scale enhanced biological phosphorus removal (EBPR) systems. A total of 10 high-quality Ca. Accumulibacter genomes were recovered from EBPR systems operated at high temperatures, providing significantly updated phylogenetic and genomic insights into the Ca. Accumulibacter lineage. Among these genomes, clade IIF members SCELSE-3, SCELSE-4, and SCELSE-6 represent the to-date known genomes encoding a complete denitrification pathway, suggesting that Ca. Accumulibacter alone could achieve complete denitrification. Clade IIC members SSA1, SCUT-1, SCELCE-2, and SCELSE-8 lack the entire set of denitrifying genes, representing to-date known non-denitrifying Ca. Accumulibacter. A pan-genomic analysis with other Ca. Accumulibacter members suggested that all Ca. Accumulibacter likely has the potential to use dicarboxylic amino acids. Ca. Accumulibacter aalborgensis AALB and Ca. Accumulibacter affinis BAT3C720 seemed to be the only two members capable of using glucose for EBPR. A heat shock protein Hsp20 encoding gene was found exclusively in genomes recovered at high temperatures, which was absent in clades IA, IC, IG, IIA, IIB, IID, IIG, and II-I members. High transcription of this gene in clade IIC members SCUT-2 and SCUT-3 suggested its role in surviving high temperatures for Ca. Accumulibacter. Ambiguous clade identity was observed for newly recovered genomes (SCELSE-9 and SCELSE-10). Five machine learning models were developed using orthogroups as input features. Prediction results suggested that they belong to a new clade (IIK). The phylogeny of Ca. Accumulibacter was re-evaluated based on the laterally derived polyphosphokinase 2 gene, showing improved resolution in differentiating different clades.

HtpG Is a Metal-Dependent Chaperone Which Assists the DnaK/DnaJ/GrpE Chaperone System of Mycobacterium tuberculosis via Direct Association with DnaJ2

Protein folding is a crucial process in maintaining protein homeostasis, also known as proteostasis, in the cell. The requirement for the assistance of molecular chaperones in the appropriate folding of several proteins has already called into question the previously held view of spontaneous protein folding. These chaperones are highly ubiquitous cellular proteins, which not only help in mediating the proper folding of other nascent polypeptides but are also involved in refolding of the misfolded or the aggregated proteins. Hsp90 family proteins such as high-temperature protein G (HtpG) are abundant and ubiquitously expressed in both eukaryotic and prokaryotic cells. Although HtpG is known as an ATP-dependent chaperone protein in most organisms, function of this protein remains obscured in mycobacterial pathogens. Here, we aim to investigate significance of HtpG as a chaperone in the physiology of Mycobacterium tuberculosis. We report that M. tuberculosis HtpG (mHtpG) is a metal-dependent ATPase which exhibits chaperonin activity towards denatured proteins in coordination with the DnaK/DnaJ/GrpE chaperone system via direct association with DnaJ2. Increased expression of DnaJ1, DnaJ2, ClpX, and ClpC1 in a ΔhtpG mutant strain further suggests cooperativity of mHtpG with various chaperones and proteostasis machinery in M. tuberculosis. IMPORTANCE M. tuberculosis is exposed to variety of extracellular stressful conditions and has evolved mechanisms to endure and adapt to the adverse conditions for survival. mHtpG, despite being dispensable for M. tuberculosis growth under in vitro conditions, exhibits a strong and direct association with DnaJ2 cochaperone and assists the mycobacterial DnaK/DnaJ/GrpE (KJE) chaperone system. These findings suggest the potential role of mHtpG in stress management of the pathogen. Mycobacterial chaperones are responsible for folding of nascent protein as well as reactivation of protein aggregates. M. tuberculosis shows differential adaptive response subject to the availability of mHtpG. While its presence facilitates improved protein refolding via stimulation of the KJE chaperone activity, in the absence of mHtpG, M. tuberculosis enhances expression of DnaJ1/J2 cochaperones as well as Clp protease machinery for maintenance of proteostasis. Overall, this study provides a framework for future investigation to better decipher the mycobacterial proteostasis network in the light of stress adaptability and/or survival.

Uncoupling the Hsp90 and DnaK chaperone activities revealed the in vivo relevance of their collaboration in bacteria

Chaperone proteins are essential in all living cells to ensure protein homeostasis. Hsp90 is a major adenosine triphosphate (ATP)-dependent chaperone highly conserved from bacteria to eukaryotes. Recent studies have shown that bacterial Hsp90 is essential in some bacteria in stress conditions and that it participates in the virulence of pathogenic bacteria. In vitro, bacterial Hsp90 directly interacts and collaborates with the Hsp70 chaperone DnaK to reactivate model substrate proteins; however, it is still unknown whether this collaboration is relevant in vivo with physiological substrates. Here, we used site-directed mutagenesis on Hsp90 to impair DnaK binding, thereby uncoupling the chaperone activities. We tested the mutants in vivo in two bacterial models in which Hsp90 has known physiological functions. We found that the Hsp90 point mutants were defective to support (1) growth under heat stress and activation of an essential Hsp90 client in the aquatic bacterium Shewanella oneidensis and (2) biosynthesis of the colibactin toxin involved in the virulence of pathogenic Escherichia coli. Our study therefore demonstrates the essentiality of the direct collaboration between Hsp90 and DnaK in vivo in bacteria to support client folding. It also suggests that this collaboration already functional in bacteria has served as an evolutionary basis for a more complex Hsp70-Hsp90 collaboration found in eukaryotes.

BAG9 Confers Thermotolerance by Regulating Cellular Redox Homeostasis and the Stability of Heat Shock Proteins in Solanum lycopersicum

The Bcl-2-associated athanogene (BAG) family, a group of co-chaperones that share conservative domains in flora and fauna, is involved in plant growth, development, and stress tolerance. However, the function of tomato BAG genes on thermotolerance remains largely unknown. Herein, we found that the expression of BAG9 was induced during heat stress in tomato plants. Knockout of the BAG9 gene by CRISPR/Cas9 reduced, while its overexpression increased thermotolerance in tomato plants as reflected by the phenotype, photosynthesis rate, and membrane peroxidation. Heat-induced reactive oxygen species and oxidative/oxidized proteins were further increased in bag9 mutants and were normalized in BAG9 overexpressing plants. Furthermore, the activities of antioxidant enzymes, ascorbic acid (AsA)/dehydroascorbic acid (DHA), and reduced glutathione (GSH)/oxidized glutathione (GSSG) were reduced in bag9 mutants and were increased in BAG9 overexpressing plants under heat stress. Additionally, BAG9 interacted with Hsp20 proteins in vitro and in vivo. Accumulation of Hsp proteins induced by heat showed a reduction in bag9 mutants; meanwhile, it was increased in BAG9 overexpressing plants. Thus, BAG9 played a crucial role in response to heat stress by regulating cellular redox homeostasis and the stability of heat shock proteins.

Heat shock proteins and the calcineurin-crz1 signaling regulate stress responses in fungi

The heat shock proteins (Hsps) act as a molecular chaperone to stabilize client proteins involved in various cell functions in fungi. Hsps are classified into different families such as HSP90, HSP70, HSP60, HSP40, and small HSPs (sHsps). Hsp90, a well-studied member of the Hsp family proteins, plays a role in growth, cell survival, and pathogenicity in fungi. Hsp70 and sHsps are involved in the development, tolerance to stress conditions, and drug resistance in fungi. Hsp60 is a mitochondrial chaperone, and Hsp40 regulates fungal ATPase machinery. In this review, we describe the cell functions, regulation, and the molecular link of the Hsps with the calcineurin-crz1 calcium signaling pathway for their role in cell survival, growth, virulence, and drug resistance in fungi and related organisms.

Involvement of the Heat Shock Protein HtpG of Salmonella Typhimurium in Infection and Proliferation in Hosts

Salmonella Typhimurium is a common pathogen infecting the gastrointestinal tract of humans and animals, causing host gastroenteritis and typhoid fever. Heat shock protein (HtpG) as a molecular chaperone is involved in the various cellular processes of bacteria, especially under environmental stress. However, the potential association of HtpG with S. Typhimurium infection remains unknown. In this study, we clarified that HtpG could also play a role as an effector in S. Typhimurium infection. RNA-seq indicated that the flagellar assembly pathway, infection pathway, and chemotaxis pathway genes of S. Typhimurium were downregulated after the mutation of HtpG, which resulted in compromises of S. Typhimurium motility, biofilm formation, adhesion, invasion, and inflammation-inducing ability. In addition, HtpG recombinant protein was capable of promoting the proliferation of S. Typhimurium in host cells and the resultant inflammation. Collectively, our results illustrated an important role of HtpG in S. Typhimurium infection.

The Bacterial Hsp90 Chaperone: Cellular Functions and Mechanism of Action

Heat shock protein 90 (Hsp90) is a molecular chaperone that folds and remodels proteins, thereby regulating the activity of numerous substrate proteins. Hsp90 is widely conserved across species and is essential in all eukaryotes and in some bacteria under stress conditions. To facilitate protein remodeling, bacterial Hsp90 collaborates with the Hsp70 molecular chaperone and its cochaperones. In contrast, the mechanism of protein remodeling performed by eukaryotic Hsp90 is more complex, involving more than 20 Hsp90 cochaperones in addition to Hsp70 and its cochaperones. In this review, we focus on recent progress toward understanding the basic mechanisms of bacterial Hsp90-mediated protein remodeling and the collaboration between Hsp90 and Hsp70. We describe the universally conserved structure and conformational dynamics of these chaperones and their interactions with one another and with client proteins. The physiological roles of Hsp90 in Escherichia coli and other bacteria are also discussed. We anticipate that the information gained from exploring the mechanism of the bacterial chaperone system will provide a framework for understanding the more complex eukaryotic Hsp90 system. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

pH-regulated chaperone function of cyanobacterial Hsp90 and Hsp70: Implications for light/dark regulation

We have shown that cyanobacterial chaperonins have pH-dependent anti-aggregation activity. The pH in cyanobacterial cytosol increases by one pH unit following a shift from darkness to light. In the present study, we examined whether other major chaperones such as Hsp90 (HtpG) and Hsp70 (DnaK2) from the cyanobacterium Synechococcus elongatus PCC7942 also display pH-dependent activity. Suppressing aggregation of various heat-denatured proteins, especially lactate dehydrogenase, at an equimolar ratio of cyanobacterial Hsp90 to protein substrate was found to be pH-dependent. Hsp90 showed the highest activity at pH 8.5 over the examined pH range of 7.0 to 8.5. pH affected the anti-aggregation activity of DnaK2 in a similar manner to that of Hsp90 in the presence of half equimolar DnaK2 to the protein substrate. The ATPase activity of cyanobacterial Hsp90 was pH-dependent, with a four-fold increase in activity when the pH was raised from 7.0 to 8.5. The ATPase activity of DnaK2 was also regulated by pH in a similar manner. Finally, an increase in pH from 7.0 to 8.5 enhanced activities of both Hsp90 and Hsp70 in protein-folding assistance by two- to three-fold. These results suggest that changes in pH may regulate chaperone function during a light-dark cycle in cyanobacterial cells.

Bacterial Hsp90 Facilitates the Degradation of Aggregation-Prone Hsp70-Hsp40 Substrates

In eukaryotes, the 90-kDa heat shock proteins (Hsp90s) are profusely studied chaperones that, together with 70-kDa heat shock proteins (Hsp70s), control protein homeostasis. In bacteria, however, the function of Hsp90 (HtpG) and its collaboration with Hsp70 (DnaK) remains poorly characterized. To uncover physiological processes that depend on HtpG and DnaK, we performed comparative quantitative proteomic analyses of insoluble and total protein fractions from unstressed wild-type (WT) Escherichia coli and from knockout mutants ΔdnaKdnaJ (ΔKJ), ΔhtpG (ΔG), and ΔdnaKdnaJΔhtpG (ΔKJG). Whereas the ΔG mutant showed no detectable proteomic differences with wild-type, ΔKJ expressed more chaperones, proteases and ribosomes and expressed dramatically less metabolic and respiratory enzymes. Unexpectedly, we found that the triple mutant ΔKJG showed higher levels of metabolic and respiratory enzymes than ΔKJ, suggesting that bacterial Hsp90 mediates the degradation of aggregation-prone Hsp70-Hsp40 substrates. Further in vivo experiments suggest that such Hsp90-mediated degradation possibly occurs through the HslUV protease.

A cyclic lipopeptide surfactin is a species-selective Hsp90 inhibitor that suppresses cyanobacterial growth

Heat shock protein 90 (Hsp90) is essential for eukaryotic cells, whereas bacterial homologs play a role under stresses and in pathogenesis. Identifying species-specific Hsp90 inhibitors is challenging because Hsp90 is evolutionarily conserved. We found that a cyclic lipopeptide surfactin inhibits the ATPase activity of Hsp90 from the cyanobacterium Synechococcus elongatus (S. elongatus) PCC 7942 but does not inhibit Escherichia coli (E. coli), yeast and human Hsp90s. Molecular docking simulations indicated that surfactin could bind to the N-terminal dimerization interface of the cyanobacterial Hsp90 in the ATP- and ADP-bound states, which provided molecular insights into the species-selective inhibition. The data suggest that surfactin inhibits a rate-limiting conformational change of S. elongatus Hsp90 in the ATP hydrolysis. Surfactin also inhibited the interaction of the cyanobacterial Hsp90 with a model substrate, and suppressed S. elongatus growth under heat stress, but not that of E. coli. Surfactin did not show significant cellular toxicity toward mammalian cells. These results indicate that surfactin inhibits the cellular function of Hsp90 specifically in the cyanobacterium. The present study shows that a cyclic peptide has a great specificity to interact with a specific homolog of a highly conserved protein family.

Bacterial Hsp90 Facilitates the Degradation of Aggregation-Prone Hsp70-Hsp40 Substrates

In eukaryotes, the 90-kDa heat shock proteins (Hsp90s) are profusely studied chaperones that, together with 70-kDa heat shock proteins (Hsp70s), control protein homeostasis. In bacteria, however, the function of Hsp90 (HtpG) and its collaboration with Hsp70 (DnaK) remains poorly characterized. To uncover physiological processes that depend on HtpG and DnaK, we performed comparative quantitative proteomic analyses of insoluble and total protein fractions from unstressed wild-type (WT) Escherichia coli and from knockout mutants ΔdnaKdnaJ (ΔKJ), ΔhtpG (ΔG), and ΔdnaKdnaJΔhtpG (ΔKJG). Whereas the ΔG mutant showed no detectable proteomic differences with wild-type, ΔKJ expressed more chaperones, proteases and ribosomes and expressed dramatically less metabolic and respiratory enzymes. Unexpectedly, we found that the triple mutant ΔKJG showed higher levels of metabolic and respiratory enzymes than ΔKJ, suggesting that bacterial Hsp90 mediates the degradation of aggregation-prone Hsp70-Hsp40 substrates. Further in vivo experiments suggest that such Hsp90-mediated degradation possibly occurs through the HslUV protease.

Regulation of the groESL1 transcription by the HrcA repressor and a novel transcription factor Orf7.5 in the cyanobacterium Synechococcus elongatus PCC7942

The CIRCE/HrcA system is highly conserved in cyanobacterial genomes. We have shown that heat-shock induction of the groESL1 operon in the cyanobacterium Synechocystis sp. PCC6803 is negatively regulated by the CIRCE/HrcA system. In Synechococcus elongatus PCC7942, a novel heat shock protein, Orf7.5, is involved in positive regulation of the groESL1 transcription. However, Orf7.5 is not conserved in some cyanobacteria, including Synechocystis sp. PCC6803. The purpose of this study is to evaluate the functional conservation of the CIRCE/HrcA system in S. elongatus PCC7942 and to understand the interplay between the CIRCE/HrcA system and the Orf7.5 regulatory system. We constructed single and double mutants of S. elongatus orf7.5, hrcA and orf7.5/hrcA and heat induction of the groESL1 transcription in these mutants was analyzed. Unexpectedly, derepression of the groESL1 transcription in an hrcA mutant was not observed. In all these mutants, the transcription was greatly suppressed under both normal and heat stress conditions, indicating that both HrcA and Orf7.5 are involved in regulation of the groESL1 transcription in a positive way. Consistent with the decrease in the groESL1 mRNA level, all the single and double mutants showed a great loss of acquired thermotolerance. Heat induction of the orf7.5 promoter activity was totally diminished in the orf7.5 mutant, indicating that Orf7.5 activates its own transcription. Yeast two hybrid analysis showed that the principle sigma factor RpoD1 interacts with Orf7.5. These results indicate that Orf7.5 enhances the transcription of groESL1 and orf7.5 by interacting with RpoD1.

Hsp90 and Hsp70 chaperones: Collaborators in protein remodeling

Heat shock proteins 90 (Hsp90) and 70 (Hsp70) are two families of highly conserved ATP-dependent molecular chaperones that fold and remodel proteins. Both are important components of the cellular machinery involved in protein homeostasis and participate in nearly every cellular process. Although Hsp90 and Hsp70 each carry out some chaperone activities independently, they collaborate in other cellular remodeling reactions. In eukaryotes, both Hsp90 and Hsp70 function with numerous Hsp90 and Hsp70 co-chaperones. In contrast, bacterial Hsp90 and Hsp70 are less complex; Hsp90 acts independently of co-chaperones, and Hsp70 uses two co-chaperones. In this review, we focus on recent progress toward understanding the basic mechanisms of Hsp90-mediated protein remodeling and the collaboration between Hsp90 and Hsp70, with an emphasis on bacterial chaperones. We describe the structure and conformational dynamics of these chaperones and their interactions with each other and with client proteins. The physiological roles of Hsp90 in Escherichia coli and other bacteria are also discussed. We anticipate that the information gained from exploring the mechanism of the bacterial chaperone system will provide the groundwork for understanding the more complex eukaryotic Hsp90 system and its modulation by Hsp90 co-chaperones.

Stimulation of the ATPase activity of Hsp90 by zerumbone modification of its cysteine residues destabilizes its clients and causes cytotoxicity

Hsp90 is an ATP-dependent molecular chaperone that assists folding and conformational maturation/maintenance of many proteins. It is a potential cancer drug target because it chaperones oncoproteins. A prokaryotic homolog of Hsp90 (HtpG) is essential for thermo-tolerance in some bacteria and virulence of zoonotic pathogens. To identify a new class of small molecules which target prokaryotic and eukaryotic Hsp90s, we studied the effects of a naturally occurring cyclic sesquiterpene, zerumbone, which inhibits proliferation of a wide variety of tumor cells, on the activity of Hsp90. Zerumbone enhanced the ATPase activity of cyanobacterial Hsp90 (Hsp90SE), yeast Hsp90, and human Hsp90α. It also enhanced the catalytic efficiency of Hsp90SE by greatly increasing kcat. Mass analysis showed that zerumbone binds to cysteine side chains of Hsp90SE covalently. Mutational studies identified 3 cysteine residues (one per each domain of Hsp90SE) that are involved in the enhancement, suggesting the presence of allosteric sites in the middle and C-terminal domains of Hsp90SE. Treatment of cyanobacterial cells with zerumbone caused them to become very temperature-sensitive, a phenotype reminiscent of cyanobacterial Hsp90 mutants, and also decreased the cellular level of linker polypeptides that are clients for Hsp90SE. Zerumbone showed cellular toxicity on cancer-derived mammalian cells by inducing apoptosis. In addition, zerumbone inhibited the binding of Hsp90/Cdc37 to client kinases. Altogether, we conclude that modification of cysteine residues of Hsp90 by zerumbone enhances its ATPase activity and inhibits physiological Hsp90 function. The activation of Hsp90 may provide new strategies to inhibit its chaperone function in cells.

The Noncompetitive Effect of Gambogic Acid Displaces Fluorescence-Labeled ATP but Requires ATP for Binding to Hsp90/HtpG

The heat shock protein 90 (Hsp90) family plays a critical role in maintaining the homeostasis of the intracellular environment for human and prokaryotic cells. Hsp90 orthologues were identified as important target proteins for cancer and plant disease therapies. It was shown that gambogic acid (GBA) has the potential to inhibit human Hsp90. However, it is unknown whether it is also able to act on the bacterial high-temperature protein (HtpG) analogue. In this work, we screened GBA and nine other novel potential Hsp90 inhibitors using a miniaturized high-throughput protein microarray-based assay and found that GBA shows an inhibitory effect on different Hsp90s after dissimilarity analysis of the protein sequence alignment. The dissociation constant of GBA and HtpG Xanthomonas (XcHtpG) computed from microscale thermophoresis is 682.2 ± 408 μM in the presence of ATP, which is indispensable for the binding of GBA to XcHtpG. Our results demonstrate that GBA is a promising Hsp90/HtpG inhibitor. The work further demonstrates that our assay concept has great potential for finding new potent Hsp/HtpG inhibitors.

Hsp90 Is Essential under Heat Stress in the Bacterium Shewanella oneidensis

The Hsp90 chaperone is essential in eukaryotes and activates a large array of client proteins. In contrast, its role is still elusive in bacteria, and only a few Hsp90 bacterial clients are known. Here, we found that Hsp90 is essential in the model bacterium Shewanella oneidensis under heat stress. A genetic screen for Hsp90 client proteins identified TilS, an essential protein involved in tRNA maturation. Overexpression of TilS rescued the growth defect of the hsp90 deletion strain under heat stress. In vivo, the activity and the amount of TilS were significantly reduced in the absence of Hsp90 at high temperature. Furthermore, we showed that Hsp90 interacts with TilS, and Hsp90 prevents TilS aggregation in vitro at high temperature. Together, our results indicate that TilS is a client of Hsp90 in S. oneidensis. Therefore, our study links the essentiality of bacterial Hsp90 at high temperature with the identification of a client.

Quantitative proteomics analysis by iTRAQ revealed underlying changes in thermotolerance of Arthrospira platensis

Growth temperature is a critical factor that affects cultivation of Arthrospira platensis which is a type of cyanobacterium widely known as Spirulina that has significant commercial value. To investigate the molecular mechanism underlying the thermotolerance of Spirulina, differential protein expression profiling was carried out using iTRAQ-based quantitative proteomic analysis. This study only analyzed changes in thylakoids. Among the 2085 proteins quantified, 43 differentially expressed proteins were selected based on the fold change cutoff scores of ≥2 or ≤0.5 for up-regulation or down-regulation, respectively. An analysis of these 43 proteins found that 23% of them are photosynthetic system proteins which include photosynthetic enzymes and pigment proteins. The dynamic change of these proteins indicates that photosynthetic system functions were profoundly affected under heat stress and the light-dependent reactions were probably the most sensitive to temperature changes. Meanwhile, to cope with the low energy production due to impaired photosynthesis there was a considerable down-shift in protein synthesis which is a very energy demanding process. The impaired photosynthesis led to low energy generation that was compensated by a down-shift in translation (the most energy-demanding process) and an up-shift of glycolysis. The reduction of many ribosome proteins may lead to a loss in translation efficiency; therefore, Spirulina may adopted a different mechanism to increase translational elongation under heat stress to compensate for this loss, such as elevate L7/L12 proteins. Changes were also found in the classical heat shock proteins, the ROS scavenging system, DNA-binding proteins, and some membrane proteins. In conclusion, this research demonstrate that heat stress induces profound changes in cellular physiology and shed light on the mechanism of the heat stress response and thermotolerance of Arthrospira platensis.

BIOLOGICAL SIGNIFICANCE

Arthrospira platensis, widely known as Spirulina, is a type of cyanobacteria which is cultivated at large scale for it great commercial value. It has become a consensus that continually increasing temperature due to global warming is bringing serious threat to agriculture, including the Spirulina cultivation. High temperature not only limits biomass accumulation by Spirulina, but also changes the composition of nutrition. Therefore there is a greater need than ever before to understand how Spirulina tolerates and cope with high temperature. In this study, we for the first time applied the iTRAQ-based quantitative proteomic technology to investigate the thermotolerance of Spirulina. Our results showed that many biological processes were altered by heat stress. Most significantly, we found that heat stress harmed the photosynthesis ability and caused low energy production, and to deal with this situation, energy demanding processes like protein synthesis were down-shifted and the alternative energy metabolism process glycolysis was up-shifted. Our results also show other important proteins, like the classical heat shock proteins and some antioxidant proteins, are also increased. Thus our study sheds light to our understanding of the mechanism underlying the thermotolerance of Spirulina.

Goniothalamin enhances the ATPase activity of the molecular chaperone Hsp90 but inhibits its chaperone activity

Hsp90 is an ATP-dependent molecular chaperone that is involved in important cellular pathways such as signal transduction pathways. It is a potential cancer drug target because it plays a critical role for stabilization and activation of oncoproteins. Thus, small molecule compounds that control the Hsp90 function are useful to elucidate potential lead compounds against cancer. We studied effect of a naturally occurring styryl-lactone goniothalamin on the activity of Hsp90. Although many drugs targeting Hsp90 inhibit the ATPase activity of Hsp90, goniothalamin enhanced rather than inhibited the ATPase activity of a cyanobacterial Hsp90 (HtpG) and a yeast Hsp90. It increased both K(m) and k(cat) of the Hsp90s. Domain competition assays and tryptophan fluorescence measurements with various truncated derivatives of HtpG indicated that goniothalamin binds to the N-terminal domain of HtpG. Goniothalamin did not influence on the interaction of HtpG with a non-native protein or the anti-aggregation activity of HtpG significantly. However, it inhibited the activity of HtpG that assists refolding of a non-native protein in cooperation with the Hsp70 chaperone system. This is the first report to show that a small molecule that binds to the N-terminal domain of Hsp90 activates its ATPase activity, while inhibiting the chaperone function of Hsp90.

Divergence with gene flow in a population of thermophilic bacteria: a potential role for spatially varying selection

A fundamental goal of evolutionary biology is to understand how ecological diversity arises and is maintained in natural populations. We have investigated the contributions of gene flow and divergent selection to the distribution of genetic variation in an ecologically differentiated population of a thermophilic cyanobacterium (Mastigocladus laminosus) found along the temperature gradient of a nitrogen-limited stream in Yellowstone National Park. For most loci sampled, gene flow appears to be sufficient to prevent substantial genetic divergence. However, one locus (rfbC) exhibited a comparatively low migration rate as well as other signatures expected for a gene experiencing spatially varying selection, including an excess of common variants, an elevated level of polymorphism and extreme genetic differentiation along the gradient. rfbC is part of an expression island involved in the production of the polysaccharide component of the protective envelope of the heterocyst, the specialized nitrogen-fixing cell of these bacteria. SNP genotyping in the vicinity of rfbC revealed a ~5-kbp region including a gene content polymorphism that is tightly associated with environmental temperature and therefore likely contains the target of selection. Two genes have been deleted both in the predominant haplotype found in the downstream region of White Creek and in strains from other Yellowstone populations of M. laminosus, which may result in the production of heterocysts with different envelope properties. This study implicates spatially varying selection in the maintenance of variation related to thermal performance at White Creek despite on-going or recent gene flow.

Translational control of small heat shock genes in mesophilic and thermophilic cyanobacteria by RNA thermometers

Cyanobacteria constitute a heterogeneous phylum of oxygen-producing, photosynthetic prokaryotes. They are susceptible to various stress conditions like heat, salt, or light stress, all inducing the cyanobacterial heat shock response (HSR). Cyanobacterial small heat shock proteins (sHsps) are known to preserve thylakoid membrane integrity under stress conditions, thereby protecting the photosynthesis machinery. In Synechocystis sp PCC 6803, synthesis of the sHsp Hsp17 is regulated by an RNA thermometer (RNAT) in the 5′-untranslated region (5′-UTR) of the hsp17 mRNA. RNATs are direct temperature sensors that control expression of many bacterial heat shock and virulence genes. They hinder translation at low temperatures by base pairing, thus blocking ribosome access to the mRNA.

 

To explore the temperature range in which RNATs act, we studied various RNAT candidates upstream of sHsp genes from mesophilic and thermophilic cyanobacteria. The mesophilic cyanobacteria Anabaena variabilis and Nostoc sp chromosomally encode two sHsps each. Reporter gene studies suggested RNAT-mediated post-transcriptional regulation of shsp expression in both organisms. Detailed structural analysis of the two A. variabilis candidates revealed two novel RNAT types. The first, avashort, regulates translation primarily by masking of the AUG translational start codon. The second, featuring an extended initial hairpin, thus named avalong, presumably makes use of complex tertiary interaction. The 5′-UTR of the small heat shock gene hspA in the thermophile Thermosynechococcus elongatus is predicted to adopt an extended secondary structure. Structure probing revealed that the ribosome binding site was blocked at temperatures below 55 °C. The results of this study demonstrate that cyanobacteria commonly use RNATs to control expression of their small heat shock genes.

Cyanobacterial heat-shock response: role and regulation of molecular chaperones

Cyanobacteria constitute a morphologically diverse group of oxygenic photoautotrophic microbes which range from unicellular to multicellular, and non-nitrogen-fixing to nitrogen-fixing types. Sustained long-term exposure to changing environmental conditions, during their three billion years of evolution, has presumably led to their adaptation to diverse ecological niches. The ability to maintain protein conformational homeostasis (folding–misfolding–refolding or aggregation–degradation) by molecular chaperones holds the key to the stress adaptability of cyanobacteria. Although cyanobacteria possess several genes encoding DnaK and DnaJ family proteins, these are not the most abundant heat-shock proteins (Hsps), as is the case in other bacteria. Instead, the Hsp60 family of proteins, comprising two phylogenetically conserved proteins, and small Hsps are more abundant during heat stress. The contribution of the Hsp100 (ClpB) family of proteins and of small Hsps in the unicellular cyanobacteria (Synechocystis and Synechococcus) as well as that of Hsp60 proteins in the filamentous cyanobacteria (Anabaena) to thermotolerance has been elucidated. The regulation of chaperone genes by several cis-elements and trans-acting factors has also been well documented. Recent studies have demonstrated novel transcriptional and translational (mRNA secondary structure) regulatory mechanisms in unicellular cyanobacteria. This article provides an insight into the heat-shock response: its organization, and ecophysiological regulation and role of molecular chaperones, in unicellular and filamentous nitrogen-fixing cyanobacterial strains.

Physical interaction between bacterial heat shock protein (Hsp) 90 and Hsp70 chaperones mediates their cooperative action to refold denatured proteins

In eukaryotes, heat shock protein 90 (Hsp90) is an essential ATP-dependent molecular chaperone that associates with numerous client proteins. HtpG, a prokaryotic homolog of Hsp90, is essential for thermotolerance in cyanobacteria, and in vitro it suppresses the aggregation of denatured proteins efficiently. Understanding how the non-native client proteins bound to HtpG refold is of central importance to comprehend the essential role of HtpG under stress. Here, we demonstrate by yeast two-hybrid method, immunoprecipitation assays, and surface plasmon resonance techniques that HtpG physically interacts with DnaJ2 and DnaK2. DnaJ2, which belongs to the type II J-protein family, bound DnaK2 or HtpG with submicromolar affinity, and HtpG bound DnaK2 with micromolar affinity. Not only DnaJ2 but also HtpG enhanced the ATP hydrolysis by DnaK2. Although assisted by the DnaK2 chaperone system, HtpG enhanced native refolding of urea-denatured lactate dehydrogenase and heat-denatured glucose-6-phosphate dehydrogenase. HtpG did not substitute for DnaJ2 or GrpE in the DnaK2-assisted refolding of the denatured substrates. The heat-denatured malate dehydrogenase that did not refold by the assistance of the DnaK2 chaperone system alone was trapped by HtpG first and then transferred to DnaK2 where it refolded. Dissociation of substrates from HtpG was either ATP-dependent or -independent depending on the substrate, indicating the presence of two mechanisms of cooperative action between the HtpG and the DnaK2 chaperone system.

High-temperature protein G is an essential virulence factor of Leptospira interrogans

Leptospira interrogans is a global zoonotic pathogen and is the causative agent of leptospirosis, an endemic disease of humans and animals worldwide. There is limited understanding of leptospiral pathogenesis; therefore, further elucidation of the mechanisms involved would aid in vaccine development and the prevention of infection. HtpG (high-temperature protein G) is the bacterial homolog to the highly conserved molecular chaperone Hsp90 and is important in the stress responses of many bacteria. The specific role of HtpG, especially in bacterial pathogenesis, remains largely unknown. Through the use of an L. interrogans htpG transposon insertion mutant, this study demonstrates that L. interrogans HtpG is essential for virulence in the hamster model of acute leptospirosis. Complementation of the htpG mutant completely restored virulence. Surprisingly, the htpG mutant did not appear to show sensitivity to heat or oxidative stress, phenotypes common in htpG mutants in other bacterial species. Furthermore, the mutant did not show increased sensitivity to serum complement, reduced survival within macrophages, or altered protein or lipopolysaccharide expression. The underlying cause for attenuation thus remains unknown, but HtpG is a novel leptospiral virulence factor and one of only a very small number identified to date.

Biochemical characterization and cooperation with co-chaperones of heat shock protein 90 from Schizosaccharomyces pombe

The characterization of Hsp90 from the fission yeast Schizosaccharomyces pombe was performed. Hsp90 of S. pombe existed as a dimer and exhibited ATP-dependent conformational changes. It captured unfolded proteins in the ATP-free open conformation and protected them from thermal aggregation. Hsp90 of S. pombe was also able to refold thermally denatured firefly luciferase. The co-chaperones Sti1 and Aha1 bound Hsp90 and modulated its activity. Because the affinity of Sti1 was higher than that of Aha1, the effect of Sti1 appeared to dominate when both co-chaperones existed simultaneously.

A novel transcriptional regulator, Sll1130, negatively regulates heat-responsive genes in Synechocystis sp. PCC6803

A conserved hypothetical protein, Sll1130, is a novel transcription factor that regulates the expression of major heat-responsive genes in Synechocystis sp. PCC6803. Synechocystis exhibited an increased thermotolerance due to disruption of sll1130. Δsll1130 cells recovered much faster than wild-type cells after they were subjected to heat shock (50°C) for 30 min followed by recovery at 34°C for 48 h. In Δsll1130 cultures, 70% of the cells were viable compared with the wild-type culture in which only 30% of the cells were viable. DNA microarray analysis revealed that in Δsll1130, expression of the heat-responsive genes such as htpG, hspA, isiA, isiB and several hypothetical genes were up-regulated. Sll1130 binds to a conserved inverted-repeat (GGCGATCGCC) located in the upstream region of the above genes. In addition, both the transcript and protein levels of sll1130 were immediately down-regulated upon shift of wild-type cells from 34 to 42°C. Collectively the results of the present study suggest that Sll1130 is a heat-responsive transcriptional regulator that represses the expression of certain heat-inducible genes at optimum growth temperatures. Upon heat shock, a quick drop in the Sll1130 levels leads to de-repression of the heat-shock genes and subsequent thermal acclimation. On the basis of the findings of the present study, we present a model which describes the heat-shock response involving Sll1130.

Heat shock response in photosynthetic organisms: membrane and lipid connections

The ability of photosynthetic organisms to adapt to increases in environmental temperatures is becoming more important with climate change. Heat stress is known to induce heat-shock proteins (HSPs) many of which act as chaperones. Traditionally, it has been thought that protein denaturation acts as a trigger for HSP induction. However, increasing evidence has shown that many stress events cause HSP induction without commensurate protein denaturation. This has led to the membrane sensor hypothesis where the membrane's physical and structural properties play an initiating role in the heat shock response. In this review, we discuss heat-induced modulation of the membrane's physical state and changes to these properties which can be brought about by interaction with HSPs. Heat stress also leads to changes in lipid-based signaling cascades and alterations in calcium transport and availability. Such observations emphasize the importance of membranes and their lipids in the heat shock response and provide a new perspective for guiding further studies into the mechanisms that mediate cellular and organismal responses to heat stress.

HtpG is involved in the pathogenesis of Edwardsiella tarda

Hsp90 is a molecular chaperone that is involved in diverse cellular processes including protein folding/repairing and signal transduction. Edwardsiella tarda is a serious fish pathogen that affects fish aquaculture worldwide. The aim of this study was to investigate the potential importance of HtpG, the prokaryotic homologue of Hsp90, in the pathogenesis of E. tarda. E. tarda HtpG is 627-residue in length and contains domain structures that are conserved among Hsp90 family members. Quantitative real time RT-PCR analysis indicated that expression of htpG is induced by heat shock and oxidative stress. Recombinant HtpG (rHtpG) purified from Escherichia coli exhibits apparent ATPase activity, which is optimal at 40°C. Mutation of htpG (i) affects bacterial growth at elevated temperature and renders the cells more sensitive to stress induced by reactive oxygen species, (ii) causes dramatic reduction in blood dissemination and general bacterial virulence, (iii) weakens the ability of E. tarda to block head kidney macrophage activation and to resist against the bactericidal effect of macrophages, and (iv) upregulates the expression of pro-inflammatory cytokines in macrophages. Taken together, these results indicate that HtpG is a biologically active protein that is required for E. tarda to cope with various stress conditions especially that encountered in vivo the host system during infection.

Heat shock protein 90 from Escherichia coli collaborates with the DnaK chaperone system in client protein remodeling

Molecular chaperones are proteins that assist the folding, unfolding, and remodeling of other proteins. In eukaryotes, heat shock protein 90 (Hsp90) proteins are essential ATP-dependent molecular chaperones that remodel and activate hundreds of client proteins with the assistance of cochaperones. In Escherichia coli, the activity of the Hsp90 homolog, HtpG, has remained elusive. To explore the mechanism of action of E. coli Hsp90, we used in vitro protein reactivation assays. We found that E. coli Hsp90 promotes reactivation of heat-inactivated luciferase in a reaction that requires the prokaryotic Hsp70 chaperone system, known as the DnaK system. An Hsp90 ATPase inhibitor, geldanamycin, inhibits luciferase reactivation demonstrating the importance of the ATP-dependent chaperone activity of E. coli Hsp90 during client protein remodeling. Reactivation also depends upon the ATP-dependent chaperone activity of the DnaK system. Our results suggest that the DnaK system acts first on the client protein, and then E. coli Hsp90 and the DnaK system collaborate synergistically to complete remodeling of the client protein. Results indicate that E. coli Hsp90 and DnaK interact in vivo and in vitro, providing additional evidence to suggest that E. coli Hsp90 and the DnaK system function together.

Cyclic lipopeptide antibiotics bind to the N-terminal domain of the prokaryotic Hsp90 to inhibit the chaperone activity

Chemical arrays were employed to screen ligands for HtpG, the prokaryotic homologue of Hsp (heat-shock protein) 90. We found that colistins and the closely related polymyxin B interact physically with HtpG. They bind to the N-terminal domain of HtpG specifically without affecting its ATPase activity. The interaction caused inhibition of chaperone function of HtpG that suppresses thermal aggregation of substrate proteins. Further studies were performed with one of these cyclic lipopeptide antibiotics, colistin sulfate salt. It inhibited the chaperone function of the N-terminal domain of HtpG. However, it inhibited neither the chaperone function of the middle domain of HtpG nor that of other molecular chaperones such as DnaK, the prokaryotic homologue of Hsp70, and small Hsp. The addition of colistin sulfate salt increased surface hydrophobicity of the N-terminal domain of HtpG and induced oligomerization of HtpG and its N-terminal domain. These structural changes are discussed in relation to the inhibition of the chaperone function.

Identification of in vivo HSP90-interacting proteins reveals modularity of HSP90 complexes is dependent on the environment in psychrophilic bacteria

Heat shock protein 90 (HSP90) is a conserved molecular chaperone that functions as part of complexes in which different client proteins target it to diverse sets of substrates. In this paper, HSP90 complexes were investigated in γ-proteobacteria from mild (Shewanella oneidensis) and cold environments (Shewanella frigidimarina and Psychrobacter frigidicola), to determine changes in HSP90 interactions with client proteins in response to the adaptation to cold environments. HSP90 participation in cold adaptation was determined using the specific inhibitor 17-allylamino-geldanamycin. Then, HSP90 was immunoprecipitated from bacterial cultures, and the proteins in HSP90 complexes were analyzed by two-dimensional gel electrophoresis and identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. According to HSP90-associated protein analysis, only 15 common proteins were found in both species from the same genus, S. oneidensis and S. frigidimarina, whereas a significant higher number of common proteins were found in both psychrophilic species S. frigidimarina and P. frigidicola 21 (p < 0.001). Only two HSP90-interacting proteins, the chaperone proteins DnaK and GroEL, were common to the three species. Interestingly, some proteins related to energy metabolism (isocitrate lyase, succinyl-CoA synthetase, alcohol dehydrogenase, NAD(+) synthase, and malate dehydrogenase) and some translation factors only interacted with HSP90 in psychrophilic bacteria. We can conclude that HSP90 and HSP90-associated proteins might take part in the mechanism of adaptation to cold environments, and interestingly, organisms living in similar environments conserve similar potential HSP90 interactors in opposition to phylogenetically closely related organisms of the same genus but from different environments.

HtpG, the prokaryotic homologue of Hsp90, stabilizes a phycobilisome protein in the cyanobacterium Synechococcus elongatus PCC 7942

HtpG, a homologue of HSP90, is essential for thermotolerance in cyanobacteria. It is not known how it plays this important role. We obtained evidence that HtpG interacts with linker polypeptides of phycobilisome in the cyanobacterium Synechococcus elongatus PCC 7942. In an htpG mutant, the 30 kDa rod linker polypeptide was reduced. In vitro studies with purified HtpG and phycobilisome showed that HtpG interacts with the linker polypeptide as well as other linker polypeptides to suppress their thermal aggregation with a stoichiometry of one linker polypeptide/HtpG dimer. We constructed various domain-truncated derivatives of HtpG to identify putative chaperone sites at which HtpG binds linker polypeptides. The middle domain and the N-terminal domain, although less efficiently, prevented the aggregation of denatured polypeptides, while the C-terminal domain did not. Truncation of the C-terminal domain that is involved in the dimerization of HtpG led to decrease in the anti-aggregation activity, while fusion of the N-terminal domain to the middle domain lowered the activity. In vitro studies with HtpG and the isolated 30 kDa rod linker polypeptide provided basically similar results to those with HtpG and phycobilisome. ADP inhibited the anti-aggregation activity, indicating that a compact ADP conformational state provides weaker aggregation protection compared with the others.

Enhanced thermotolerance of E. coli by expressed OsHsp90 from rice (Oryza sativa L.)

A gene encoding the rice (Oryza sativa L.) 90-kDa heat shock protein (OsHsp90) was introduced into Escherichia coli using the pGEX-6p-3 expression vector with a glutathione-S-transferase (GST) tag to analyze the possible function of this protein under heat stress for the first time. We compared the survivability of E. coli (BL21) cells transformed with a recombinant plasmid containing GST-OsHsp90 fusion protein with control E. coli cells transformed with the plasmid containing GST and the wild type BL21 under heat shock after isopropyl beta-D: -thiogalactopyranoside induction. Cells expressing GST-OsHsp90 demonstrated thermotolerance at 42, 50, and 70 degrees C, treatments that were more harmful to cells expressing GST and the wild type. Further studies were carried out to analyze the heat-induced characteristics of OsHsp90 at 42, 50, and 70 degrees C in vitro. When cell lysates from E. coli transformants were heated at these heat stresses, expressed GST-OsHsp90 prevented the denaturation of bacterial proteins treated with 42 degrees C heat shocks, and partially prevented that of proteins treated at 50 and 70 degrees C; meanwhile, cells expressing GST-OsHsp90 withstood the duration at 50 degrees C. These results indicate that OsHsp90 functioned as a chaperone, binding to a subset of substrates, and maintained E. coli growth well at high temperatures.

Protein expression profile of Gluconacetobacter diazotrophicus PAL5, a sugarcane endophytic plant growth-promoting bacterium

This is the first broad proteomic description of Gluconacetobacter diazotrophicus, an endophytic bacterium, responsible for the major fraction of the atmospheric nitrogen fixed in sugarcane in tropical regions. Proteomic coverage of G. diazotrophicus PAL5 was obtained by two independent approaches: 2-DE followed by MALDI-TOF or TOF-TOF MS and 1-DE followed by chromatography in a C18 column online coupled to an ESI-Q-TOF or ESI-IT mass spectrometer. The 583 identified proteins were sorted into functional categories and used to describe potential metabolic pathways for nucleotides, amino acids, carbohydrates, lipids, cofactors and energy production, according to the Enzyme Commission of Enzyme Nomenclature (EC) and Kyoto Encyclopedia of genes and genomes (KEGG) databases. The identification of such proteins and their possible insertion in conserved biochemical routes will allow comparisons between G. diazotrophicus and other bacterial species. Furthermore, the 88 proteins classified as conserved unknown or unknown constitute a potential target for functional genomic studies, aiming at the understanding of protein function and regulation of gene expression. The knowledge of metabolic fundamentals and coordination of these actions are crucial for the rational, safe and sustainable interference on crops. The entire dataset, including peptide sequence information, is available as Supporting Information and is the major contribution of this work.

Interaction of the molecular chaperone HtpG with uroporphyrinogen decarboxylase in the cyanobacterium Synechococcus elongatus PCC 7942

Uroporphyrinogen decarboxylase (HemE) is important due to its location at the first branch-point in tetrapyrrole biosynthesis. We detected a complex formation between full-length polypeptides of HtpG and HemE by biochemical studies in vivo and in vitro. The interaction suppressed the enzyme activity, suggesting a regulatory role of HtpG in tetrapyrrole biosynthesis.

A Quantitative Proteomic Analysis of Light Adaptation in a Globally Significant Marine Cyanobacterium Prochlorococcus marinus MED4

In this study, we conducted biological and technical replicate proteomic experiments using isobaric tags for relative and absolute quantification (iTRAQ), to elucidate the light adaptation strategies of Prochlorococcus marinus MED4. The MED4 strain is adapted to an oceanic environment characterized by low nutrient levels, and ever-changing light intensities. Approximately 11% of the proteome was identified, with an average coefficient of variation of iTRAQ quantification values of 0.15. Fifteen proteins were deemed to be statistically and significantly differentially expressed in changing light intensities, particularly the down-regulation of photosystem-related proteins, and the up-regulation of the stress-related chaperone GroEL in high light compared to low light.

Studies on the role of HtpG in the tetrapyrrole biosynthesis pathway of the cyanobacterium Synechococcus elongatus PCC 7942

In cyanobacterium Synechococcus elongatus PCC 7942, we observed that htpG-overexpression caused remarkable growth inhibition. In addition, subcellular fractionation experiments showed that HtpG was localized in the membrane fraction. To understand its function in cyanobacteria, we carried out yeast two-hybrid screening to identify specific proteins interacting with HtpG, and found out, HemE, uroporphyrinogen decarboxylase. When compared to the wild-type strain, the htpG-null mutant and -overexpressing strains exhibited higher and lower cytosolic HemE activity, based on the coproporphyrin production, respectively. These results strongly suggest that HtpG is involved in the regulation of tetrapyrrole biosynthesis through interacting with HemE protein.

The heat shock response in the cyanobacterium Synechocystis sp. Strain PCC 6803 and regulation of gene expression by HrcA and SigB

We report on the genome-wide response, based on DNA microarrays, of the cyanobacterium Synechocystis sp. PCC 6803 wild type and DeltasigB to a 15 min heat shock. Approximately 9% of the genes in wild type and DeltasigB were significantly regulated (P < 0.001) following this treatment, with chaperones induced the most. The absence of sigB had no dramatic effect on specific genes induced by heat shock, but did affect the level of transcription of the chaperones. In addition, sigE was induced in DeltasigB. Comparison of global gene expression of the wild type and the hrcA mutant at 30 degrees C enabled us to examine the HrcA regulon, which included groESL and groEL2. Several genes belonging to specific functional groups (e.g., pilus biogenesis/assembly and phototaxis, biosynthesis of aromatic amino acids, murien sacculus and peptidoglycan, surface polysaccharides, and the Sec pathway) were differentially regulated following heat shock. We used results from knock-out mutants in sigB, sigD and sigE to construct a model of the network of group 2 sigma factor regulation upon each other. In this network, SigB represented the major node and SigE a secondary node. Overall, we determined that transcription of the heat-shock genes are regulated to various degrees by SigB, SigE and HrcA.

Comparative genomics and evolution of the HSP90 family of genes across all kingdoms of organisms

BACKGROUND

HSP90 proteins are essential molecular chaperones involved in signal transduction, cell cycle control, stress management, and folding, degradation, and transport of proteins. HSP90 proteins have been found in a variety of organisms suggesting that they are ancient and conserved. In this study we investigate the nuclear genomes of 32 species across all kingdoms of organisms, and all sequences available in GenBank, and address the diversity, evolution, gene structure, conservation and nomenclature of the HSP90 family of genes across all organisms.

RESULTS

Twelve new genes and a new type HSP90C2 were identified. The chromosomal location, exon splicing, and prediction of whether they are functional copies were documented, as well as the amino acid length and molecular mass of their polypeptides. The conserved regions across all protein sequences, and signature sequences in each subfamily were determined, and a standardized nomenclature system for this gene family is presented. The proeukaryote HSP90 homologue, HTPG, exists in most Bacteria species but not in Archaea, and it evolved into three lineages (Groups A, B and C) via two gene duplication events. None of the organellar-localized HSP90s were derived from endosymbionts of early eukaryotes. Mitochondrial TRAP and endoplasmic reticulum HSP90B separately originated from the ancestors of HTPG Group A in Firmicutes-like organisms very early in the formation of the eukaryotic cell. TRAP is monophyletic and present in all Animalia and some Protista species, while HSP90B is paraphyletic and present in all eukaryotes with the exception of some Fungi species, which appear to have lost it. Both HSP90C (chloroplast HSP90C1 and location-undetermined SP90C2) and cytosolic HSP90A are monophyletic, and originated from HSP90B by independent gene duplications. HSP90C exists only in Plantae, and was duplicated into HSP90C1 and HSP90C2 isoforms in higher plants. HSP90A occurs across all eukaryotes, and duplicated into HSP90AA and HSP90AB in vertebrates. Diplomonadida was identified as the most basal organism in the eukaryote lineage.

CONCLUSION

The present study presents the first comparative genomic study and evolutionary analysis of the HSP90 family of genes across all kingdoms of organisms. HSP90 family members underwent multiple duplications and also subsequent losses during their evolution. This study established an overall framework of information for the family of genes, which may facilitate and stimulate the study of this gene family across all organisms.

Interaction of a small heat shock protein with light-harvesting cyanobacterial phycocyanins under stress conditions

Phycobiliproteins such as phycocyanins are the most abundant proteins found in cyanobacteria which are assembled to form the phycobilisome. Here, we showed that a small heat shock protein, HspA, interacts directly with phycocyanins from the cyanobacterium Synechococcus sp. strain PCC 7942 in vitro and suppresses inactivation of their light-harvesting functions due to heat denaturation in the presence of hydrogen peroxide. Under the denaturing conditions, phycobilisomes were de-assembled to lighter complexes and then aggregated. HspA associated with phycocyanins in the dissociated complexes, and suppressed the aggregation. The specific interaction between a small heat shock protein and phycocyanins was further supported by the fact that HspA and alpha-crystallin protected isolated phycocyanins from denaturation, while HtpG and lysozyme did not. The maximum protection was observed at a molar ratio of four HspA monomer per one phycocyanin (alpha beta) monomer.

The SigBσfactor mediates high-temperature responses in the cyanobacteriumSynechocystissp. PCC6803

The sigma factors of RNA polymerase play central roles when bacteria adapt to different environmental conditions. We studied heat-shock responses in the cyanobacterium Synechocystis sp. PCC6803 using the sigma factor inactivation strains deltasigB, deltasigD and deltasigBD. The SigB factor was found to be important for short-term heat-shock responses and acquired thermotolerance. The normal high-temperature induction of the hspA gene depended on the SigB factor. The SigD sigma factor had a role in high-temperature responses as well, and the double inactivation strain deltasigBD grew more slowly at 43 degrees C than the deltasigB and deltasigD strains.

Constitutive Expression of Small Heat Shock Protein in an htpG Disruptant of the Cyanobacterium Synechococcus sp. PCC 7942

In cyanobacteria, a disruptant of hspA encoding a small heat shock protein homologue, shows decreased cell growth rates at moderately high temperatures, and loss of both basal and acquired thermo-tolerances, which resemble the phenotype of an htpG disruptant. In vitro studies have shown that both small heat shock protein and Hsp90 can bind and keep non-native proteins in a refolding-competent state under denaturing conditions. The aim of the present study is to elucidate whether constitutive expression of HspA can functionally replace HtpG, a prokaryotic homolog of Hsp90, in the cyanobacterium Synechococcus sp. PCC 7942. HspA did not improve the viability of the htpG disruptant at a lethal temperature, although it did that of the wild type. It did not improve an iron-starved phenotype of the mutant under normal growth conditions, a novel phenotype found in the present study. These results suggest that cellular function of HtpG may differ significantly from that of HspA.

Evolutionary origins of Hsp90 chaperones and a deep paralogy in their bacterial ancestors

The 82-90 kD family of molecular chaperone proteins has homologs in eukaryotes (Hsp90) and many eubacteria (HtpG) but not in Archaebacteria. We used representatives of all four different eukaryotic paralogs (cytosolic, endoplasmic reticulum (ER), chloroplast, mitochondrial) together with numerous eubacterial HtpG proteins for phylogenetic analyses to investigate their evolutionary origins. Our trees confirm that none of the organellar Hsp90s derives from the endosymbionts of early eukaryotes. Contrary to previous suggestions of distant origins through lateral gene transfer (LGT) all eukaryote Hsp90s are related to Gram-positive eubacterial HtpG proteins. The nucleocytosolic, ER and chloroplast Hsp90 paralogs are clearly mutually related. The origin of mitochondrial Hsp90 is more obscure, as these sequences are deeply nested within eubacteria. Our trees also reveal a deep split within eubacteria into a group of mainly long-branching sequences (including the eukaryote mitochondrial Hsp90s) and another group comprising exclusively short-branching HtpG proteins, from which the cytosolic/ER versions probably arose. Both versions are present in several eubacterial phyla, suggesting gene duplication very early in eubacterial evolution and multiple independent losses thereafter. We identified one probable case of LGT within eubacteria. However, multiple losses can simply explain the evolutionary pattern of the eubacterial HtpG paralogs and predominate over LGT. We suggest that the actinobacterial ancestor of eukaryotes harbored genes for both eubacterial HtpG paralogs, as the actinobacterium Streptomyces coelicolor still does; one could have given rise to the mitochondrial Hsp90 and the other, following another duplication event in the ancestral eukaryote, to the cytosolic and ER Hsp90 homologs.

Effects of high light on transcripts of stress-associated genes for the cyanobacteria Synechocystis sp. PCC 6803 and Prochlorococcus MED4 and MIT9313

Cyanobacteria constitute an ancient, diverse and ecologically important bacterial group. The responses of these organisms to light and nutrient conditions are finely controlled, enabling the cells to survive a range of environmental conditions. In particular, it is important to understand how cyanobacteria acclimate to the absorption of excess excitation energy and how stress-associated transcripts accumulate following transfer of cells from low- to high-intensity light. In this study, quantitative RT-PCR was used to monitor changes in levels of transcripts encoding chaperones and stress-associated proteases in three cyanobacterial strains that inhabit different ecological niches: the freshwater strain Synechocystis sp. PCC 6803, the marine high-light-adapted strain Prochlorococcus MED4 and the marine low-light-adapted strain Prochlorococcus MIT9313. Levels of transcripts encoding stress-associated proteins were very sensitive to changes in light intensity in all of these organisms, although there were significant differences in the degree and kinetics of transcript accumulation. A specific set of genes that seemed to be associated with high-light adaptation (groEL/groES, dnaK2, dnaJ3, clpB1 and clpP1) could be targeted for more detailed studies in the future. Furthermore, the strongest responses were observed in Prochlorococcus MED4, a strain characteristic of the open ocean surface layer, where hsp genes could play a critical role in cell survival.

Targeted inactivation of the hrcA repressor gene in cyanobacteria

To determine if the CIRCE/HrcA system operates in cyanobacteria, we have inactivated the hrcA repressor gene in Synechocystis sp. PCC 6803 by gene targeting. In the hrcA mutant, the groESL1 operon and the groEL2 gene, both of which have the CIRCE operator in their upstream regions, were derepressed at 30 degrees C without affecting expression of other major heat-shock genes. However, expression of these groE genes in the mutant was not fully derepressed. Their transcription increased further upon heat shock, and was initiated from the same sites as those used under normal conditions. This suggests that their expression is regulated by at least two different mechanisms, a negative one controlled by HrcA and an unknown positive one. The heat-induced expression of clpB1 and htpG was greatly repressed by the absence of HrcA. The hrcA mutant which constitutively overexpressed GroEL displayed improved cellular thermotolerance and also reduced photobleaching of phycocyanin under heat stress conditions.