Functional and physical interaction between yeast Hsp90 and Hsp70

HOP stabilizes the HSFA1a and plays a main role in the onset of thermomorphogenesis

Living organisms have the capacity to respond to environmental stimuli, including warm conditions. Upon sensing mild temperature, plants launch a transcriptional response that promotes morphological changes, globally known as thermomorphogenesis. This response is orchestrated by different hormonal networks and by the activity of different transcription factors, including the heat shock factor A1 (HSFA1) family. Members of this family interact with heat shock protein 70 (HSP70) and heat shock protein 90 (HSP90); however, the effect of this binding on the regulation of HSFA1 activity or of the role of cochaperones, such as the HSP70-HSP90 organizing protein (HOP) on HSFA1 regulation, remains unknown. Here, we show that AtHOPs are involved in the folding and stabilization of the HSFA1a and are required for the onset of the transcriptional response associated to thermomorphogenesis. Our results demonstrate that the three members of the AtHOP family bind in vivo to the HSFA1a and that the expression of multiple HSFA1a-responsive-responsive genes is altered in the hop1 hop2 hop3 mutant under warm temperature. Interestingly, HSFA1a is accumulated at lower levels in the hop1 hop2 hop3 mutant, while control levels are recovered in the presence of the proteasome inhibitor MG132 or the synthetic chaperone tauroursodeoxycholic acid (TUDCA). This uncovers the HSFA1a as a client of HOP complexes in plants and reveals the participation of HOPs in HSFA1a stability.

The role of the co-chaperone HOP in plant homeostasis during development and stress

Proteins need to acquire their native structure in order to become fully functional. In specific cases, the active conformation is obtained spontaneously; nevertheless, many proteins need the assistance of chaperones and co-chaperones to be properly folded. These proteins help to maintain protein homeostasis under control conditions and under different stresses. HOP (HSP70-HSP90 organizing protein) is a highly conserved family of co-chaperones that assist HSP70 and HSP90 in the folding of specific proteins. In the last few years, findings in mammals and yeast have revealed novel functions of HOP and re-defined the role of HOP in protein folding. Here, we provide an overview of the most important aspects of HOP regulation and function in other eukaryotes and analyse whether these aspects are conserved in plants. In addition, we highlight the HOP clients described in plants and the role of HOP in plant development and stress response.

Hsp90, a team player in protein quality control and the stress response in bacteria

SUMMARYHeat shock protein 90 (Hsp90) participates in proteostasis by facilitating protein folding, activation, disaggregation, prevention of aggregation, degradation, and protection against degradation of various cellular proteins. It is highly conserved from bacteria to humans. In bacteria, protein remodeling by Hsp90 involves collaboration with the Hsp70 molecular chaperone and Hsp70 cochaperones. In eukaryotes, protein folding by Hsp90 is more complex and involves collaboration with many Hsp90 cochaperones as well as Hsp70 and Hsp70 cochaperones. This review focuses primarily on bacterial Hsp90 and highlights similarities and differences between bacterial and eukaryotic Hsp90. Seminal research findings that elucidate the structure and the mechanisms of protein folding, disaggregation, and reactivation promoted by Hsp90 are discussed. Understanding the mechanisms of bacterial Hsp90 will provide fundamental insight into the more complex eukaryotic chaperone systems.

Quantitative proteomic analysis reveals unique Hsp90 cycle-dependent client interactions

Hsp90 is an abundant and essential molecular chaperone that mediates the folding and activation of client proteins in a nucleotide-dependent cycle. Hsp90 inhibition directly or indirectly impacts the function of 10-15% of all proteins due to degradation of client proteins or indirect downstream effects. Due to its role in chaperoning oncogenic proteins, Hsp90 is an important drug target. However, compounds that occupy the ATP-binding pocket and broadly inhibit function have not achieved widespread use due to negative effects. More selective inhibitors are needed; however, it is unclear how to achieve selective inhibition. We conducted a quantitative proteomic analysis of soluble proteins in yeast strains expressing wild-type Hsp90 or mutants that disrupt different steps in the client folding pathway. Out of 2,482 proteins in our sample set (approximately 38% of yeast proteins), we observed statistically significant changes in abundance of 350 (14%) of those proteins (log2 fold change ≥ 1.5). Of these, 257/350 (∼73%) with the strongest differences in abundance were previously connected to Hsp90 function. Principal component analysis of the entire dataset revealed that the effects of the mutants could be separated into 3 primary clusters. As evidence that Hsp90 mutants affect different pools of clients, simultaneous co-expression of 2 mutants in different clusters restored wild-type growth. Our data suggest that the ability of Hsp90 to sample a wide range of conformations allows the chaperone to mediate folding of a broad array of clients and that disruption of conformational flexibility results in client defects dependent on those states.

Insights into Hsp90 mechanism and in vivo functions learned from studies in the yeast, Saccharomyces cerevisiae

The molecular chaperone Hsp90 (Heat shock protein, 90 kDa) is an abundant and essential cytosolic protein required for the stability and/or folding of hundreds of client proteins. Hsp90, along with helper cochaperone proteins, assists client protein folding in an ATP-dependent pathway. The laboratory of Susan Lindquist, in collaboration with other researchers, was the first to establish the yeast Saccharomyces cerevisiae as a model organism to study the functional interaction between Hsp90 and clients. Important insights from studies in her lab were that Hsp90 is essential, and that Hsp90 functions and cochaperone interactions are highly conserved between yeast and mammalian cells. Here, we describe key mechanistic insights into the Hsp90 folding cycle that were obtained using the yeast system. We highlight the early contributions of the laboratory of Susan Lindquist and extend our analysis into the broader use of the yeast system to analyze the understanding of the conformational cycle of Hsp90 and the impact of altered Hsp90 function on the proteome.

Molecular mechanisms of chaperone-directed protein folding: Insights from atomistic simulations

Molecular chaperones, a family of proteins of which Hsp90 and Hsp70 are integral members, form an essential machinery to maintain healthy proteomes by controlling the folding and activation of a plethora of substrate client proteins. This is achieved through cycles in which Hsp90 and Hsp70, regulated by task-specific co-chaperones, process ATP and become part of a complex network that undergoes extensive compositional and conformational variations. Despite impressive advances in structural knowledge, the mechanisms that regulate the dynamics of functional assemblies, their response to nucleotides, and their relevance for client remodeling are still elusive. Here, we focus on the glucocorticoid receptor (GR):Hsp90:Hsp70:co-chaperone Hop client-loading and the GR:Hsp90:co-chaperone p23 client-maturation complexes, key assemblies in the folding cycle of glucocorticoid receptor (GR), a client strictly dependent upon Hsp90/Hsp70 for activity. Using a combination of molecular dynamics simulation approaches, we unveil with unprecedented detail the mechanisms that underpin function in these chaperone machineries. Specifically, we dissect the processes by which the nucleotide-encoded message is relayed to the client and how the distinct partners of the assemblies cooperate to (pre)organize partially folded GR during Loading and Maturation. We show how different ligand states determine distinct dynamic profiles for the functional interfaces defining the interactions in the complexes and modulate their overall flexibility to facilitate progress along the chaperone cycle. Finally, we also show that the GR regions engaged by the chaperone machinery display peculiar energetic signatures in the folded state, which enhance the probability of partial unfolding fluctuations. From these results, we propose a model where a dynamic cross-talk emerges between the chaperone dynamics states and remodeling of client-interacting regions. This factor, coupled to the highly dynamic nature of the assemblies and the conformational heterogeneity of their interactions, provides the basis for regulating the functions of distinct assemblies during the chaperoning cycle.

The role of heat shock proteins (HSPs) in type 2 diabetes mellitus pathophysiology

Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by sustained hyperglycemia caused by impaired insulin signaling and secretion. Metabolic stress, caused by an inappropriate diet, is one of the major hallmarks provoking inflammation, endoplasmic reticulum (ER) stress, and mitochondrial dysfunction. Heat shock proteins (HSPs) are a group of highly conserved proteins that have a crucial role in chaperoning damaged and misfolded proteins to avoid disruption of cellular homeostasis under stress conditions. To do this, HSPs interact with diverse intra-and extracellular pathways among which are the insulin signaling, insulin secretion, and apoptosis pathways. Therefore, HSP dysfunction, e.g. HSP70, may lead to disruption of the pathways responsible for insulin secretion and uptake. Consistently, the altered expression of other HSPs and genetic polymorphisms in HSP-producing genes in diabetic subjects has made HSPs hot research in T2DM. This paper provides a comprehensive overview of the role of different HSPs in T2DM pathogenesis, affected cellular pathways, and the potential therapeutic strategies targeting HSPs in T2DM.

Domain Architecture Based Methods for Comparative Functional Genomics Toward Therapeutic Drug Target Discovery

Genes duplicate, mutate, recombine, fuse or fission to produce new genes, or when genes are formed from de novo, novel functions arise during evolution. Researchers have tried to quantify the causes of these molecular diversification processes to know how these genes increase molecular complexity over a period of time, for instance protein domain organization. In contrast to global sequence similarity, protein domain architectures can capture key structural and functional characteristics, making them better proxies for describing functional equivalence. In Prokaryotes and eukaryotes it has proven that, domain designs are retained over significant evolutionary distances. Protein domain architectures are now being utilized to categorize and distinguish evolutionarily related proteins and find homologs among species that are evolutionarily distant from one another. Additionally, structural information stored in domain structures has accelerated homology identification and sequence search methods. Tools for functional protein annotation have been developed to discover, protein domain content, domain order, domain recurrence, and domain position as all these contribute to the prediction of protein functional accuracy. In this review, an attempt is made to summarise facts and speculations regarding the use of protein domain architecture and modularity to identify possible therapeutic targets among cellular activities based on the understanding their linked biological processes.

Saccharomyces cerevisiae as a tool for deciphering Hsp90 molecular chaperone function

Yeast is a valuable model organism for their ease of genetic manipulation, rapid growth rate, and relative similarity to higher eukaryotes. Historically, Saccharomyces cerevisiae has played a major role in discovering the function of complex proteins and pathways that are important for human health and disease. Heat shock protein 90 (Hsp90) is a molecular chaperone responsible for the stabilization and activation of hundreds of integral members of the cellular signaling network. Much important structural and functional work, including many seminal discoveries in Hsp90 biology are the direct result of work carried out in S. cerevisiae. Here, we have provided a brief overview of the S. cerevisiae model system and described how this eukaryotic model organism has been successfully applied to the study of Hsp90 chaperone function.

Aha1 regulates Hsp90's conformation and function in a stoichiometry-dependent way

The heat shock protein 90 (Hsp90) is a molecular chaperone, which plays a key role in eukaryotic protein homeostasis. Co-chaperones assist Hsp90 in client maturation and in regulating essential cellular processes such as cell survival, signal transduction, gene regulation, hormone signaling, and neurodegeneration. Aha1 (activator of Hsp90 ATPase) is a unique co-chaperone known to stimulate the ATP hydrolysis of Hsp90, but the mechanism of their interaction is still unclear. In this report, we show that one or two Aha1 molecules can bind to one Hsp90 dimer and that the binding stoichiometry affects Hsp90's conformation, kinetics, ATPase activity, and stability. In particular, a coordination of two Aha1 molecules can be seen in stimulating the ATPase activity of Hsp90 and the unfolding of the middle domain, whereas the conformational equilibrium and kinetics are hardly affected by the stoichiometry of bound Aha1. Altogether, we show a regulation mechanism through the stoichiometry of Aha1 going far beyond a regulation of Hsp90's conformation.

J-domain Proteins form Binary Complexes with Hsp90 and Ternary Complexes with Hsp90 and Hsp70

Hsp90 and Hsp70 are highly conserved molecular chaperones that help maintain proteostasis by participating in protein folding, unfolding, remodeling and activation of proteins. Both chaperones are also important for cellular recovery following environmental stresses. Hsp90 and Hsp70 function collaboratively for the remodeling and activation of some client proteins. Previous studies using E. coli and S. cerevisiae showed that residues in the Hsp90 middle domain directly interact with a region in the Hsp70 nucleotide binding domain, in the same region known to bind J-domain proteins. Importantly, J-domain proteins facilitate and stabilize the interaction between Hsp90 and Hsp70 both in E. coli and S. cerevisiae. To further explore the role of J-domain proteins in protein reactivation, we tested the hypothesis that J-domain proteins participate in the collaboration between Hsp90 and Hsp70 by simultaneously interacting with Hsp90 and Hsp70. Using E. coli Hsp90, Hsp70 (DnaK), and a J-domain protein (CbpA), we detected a ternary complex containing all three proteins. The interaction involved the J-domain of CbpA, the DnaK binding region of E. coli Hsp90, and the J-domain protein binding region of DnaK where Hsp90 also binds. Additionally, results show that E. coli Hsp90 interacts with E. coli J-domain proteins, DnaJ and CbpA, and that yeast Hsp90, Hsp82, interacts with a yeast J-domain protein, Ydj1. Together these results suggest that the complexes may be transient intermediates in the pathway of collaborative protein remodeling by Hsp90 and Hsp70.

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.

Hsp90 mutants with distinct defects provide novel insights into cochaperone regulation of the folding cycle

Molecular chaperones play a key role in maintaining proteostasis and cellular health. The abundant, essential, cytosolic Hsp90 (Heat shock protein, 90 kDa) facilitates the folding and activation of hundreds of newly synthesized or misfolded client proteins in an ATP-dependent folding pathway. In a simplified model, Hsp70 first helps load client onto Hsp90, ATP binding results in conformational changes in Hsp90 that result in the closed complex, and then less defined events result in nucleotide hydrolysis, client release and return to the open state. Cochaperones bind and assist Hsp90 during this process. We previously identified a series of yeast Hsp90 mutants that appear to disrupt either the 'loading', 'closing' or 'reopening' events, and showed that the mutants had differing effects on activity of some clients. Here we used those mutants to dissect Hsp90 and cochaperone interactions. Overexpression or deletion of HCH1 had dramatically opposing effects on the growth of cells expressing different mutants, with a phenotypic shift coinciding with formation of the closed conformation. Hch1 appears to destabilize Hsp90-nucleotide interaction, hindering formation of the closed conformation, whereas Cpr6 counters the effects of Hch1 by stabilizing the closed conformation. Hch1 and the homologous Aha1 share some functions, but the role of Hch1 in inhibiting progression through the early stages of the folding cycle is unique. Sensitivity to the Hsp90 inhibitor NVP-AUY922 also correlates with the conformational cycle, with mutants defective in the loading phase being most sensitive and those defective in the reopening phase being most resistant to the drug. Overall, our results indicate that the timing of transition into and out of the closed conformation is tightly regulated by cochaperones. Further analysis will help elucidate additional steps required for progression through the Hsp90 folding cycle and may lead to new strategies for modulating Hsp90 function.

Hsp70/Hsp90 Organising Protein (Hop): Coordinating Much More than Chaperones

The Hsp70/Hsp90 organising protein (Hop, also known as stress-inducible protein 1/STI1/STIP1) has received considerable attention for diverse cellular functions in both healthy and diseased states. There is extensive evidence that intracellular Hop is a co-chaperone of the major chaperones Hsp70 and Hsp90, playing an important role in the productive folding of Hsp90 client proteins, although recent evidence suggests that eukaryotic Hop is regulatory within chaperone complexes rather than essential. Consequently, Hop is implicated in many key signalling pathways, including aberrant pathways leading to cancer. Hop is also secreted, and it is now well established that Hop interacts with the prion protein, PrP^C, to mediate multiple signalling events. The intracellular and extracellular forms of Hop most likely represent two different isoforms, although the molecular determinants of these divergent functions are yet to be identified. There is also a growing body of research that reports the involvement of Hop in cellular activities that appear independent of either chaperones or PrP^C. While the various cellular functions of Hop have been described, its biological function remains elusive. However, recent knockout studies in mammals suggest that Hop has an important role in embryonic development. This review provides a critical overview of the latest molecular, cellular and biological research on Hop, critically evaluating its function in healthy systems and how this function is adapted in diseased states.

Ydj1 interaction at nucleotide-binding-domain of yeast Ssa1 impacts Hsp90 collaboration and client maturation

Hsp90 constitutes one of the major chaperone machinery in the cell. The Hsp70 assists Hsp90 in its client maturation though the underlying basis of the Hsp70 role remains to be explored. In the present study, using S. cerevisiae strain expressing Ssa1 as sole Ssa Hsp70, we identified novel mutations in the nucleotide-binding domain of yeast Ssa1 Hsp70 (Ssa1-T175N and Ssa1-D158N) that adversely affect the maturation of Hsp90 clients v-Src and Ste11. The identified Ssa1 amino acids critical for Hsp90 function were also found to be conserved across species such as in E.coli DnaK and the constitutive Hsp70 isoform (HspA8) in humans. These mutations are distal to the C-terminus of Hsp70, that primarily mediates Hsp90 interaction through the bridge protein Sti1, and proximal to Ydj1 (Hsp40 co-chaperone of Hsp70 family) binding region. Intriguingly, we found that the bridge protein Sti1 is critical for cellular viability in cells expressing Ssa1-T175N (A1-T175N) or Ssa1-D158N (A1-D158N) as sole Ssa Hsp70. The growth defect was specific for sti1Δ, as deletion of none of the other Hsp90 co-chaperones showed lethality in A1-T175N or A1-D158N. Mass-spectrometry based whole proteome analysis of A1-T175N cells lacking Sti1 showed an altered abundance of various kinases and transcription factors suggesting compromised Hsp90 activity. Further proteomic analysis showed that pathways involved in signaling, signal transduction, and protein phosphorylation are markedly downregulated in the A1-T175N upon repressing Sti1 expression using doxycycline regulatable promoter. In contrast to Ssa1, the homologous mutations in Ssa4 (Ssa4-T175N/D158N), the stress inducible Hsp70 isoform, supported cell growth even in the absence of Sti1. Overall, our data suggest that Ydj1 competes with Hsp90 for binding to Hsp70, and thus regulates Hsp90 interaction with the nucleotide-binding domain of Hsp70. The study thus provides new insight into the Hsp70-mediated regulation of Hsp90 and broadens our understanding of the intricate complexities of the Hsp70-Hsp90 network.

Hsp70-Hsp90 constitutes major cellular chaperone machinery in cells. The Hsp70 plays critical role in Hsp90 chaperoning pathway. We have now identified novel mutations in the nucleotide-binding domain of yeast Ssa1 Hsp70 (Ssa1-T175N and Ssa1-D158N) that adversely affect Hsp90 client maturation. As compared to wt Ssa1, the identified Ssa1 mutants bind relatively better with Ydj1, and poorly support growth in the absence of Sti1, when present as the sole source of Ssa Hsp70 in S. cerevisiae. The cells expressing Ssa1-T175N as sole Ssa Hsp70 show downregulation of pathways involved in signaling, signal transduction, and protein phosphorylation upon repressing Sti1. The study shows that Ydj1 interaction at the nucleotide-binding domain of Ssa1 Hsp70 influences Hsp90 function.

In silico analysis of the HSP90 chaperone system from the African trypanosome, Trypanosoma brucei

African trypanosomiasis is a neglected tropical disease caused by Trypanosoma brucei (T. brucei) and spread by the tsetse fly in sub-Saharan Africa. The trypanosome relies on heat shock proteins for survival in the insect vector and mammalian host. Heat shock protein 90 (HSP90) plays a crucial role in the stress response at the cellular level. Inhibition of its interactions with chaperones and co-chaperones is being explored as a potential therapeutic target for numerous diseases. This study provides an in silico overview of HSP90 and its co-chaperones in both T. brucei brucei and T. brucei gambiense in relation to human and other trypanosomal species, including non-parasitic Bodo saltans and the insect infecting Crithidia fasciculata. A structural analysis of T. brucei HSP90 revealed differences in the orientation of the linker and C-terminal domain in comparison to human HSP90. Phylogenetic analysis displayed the T. brucei HSP90 proteins clustering into three distinct groups based on subcellular localizations, namely, cytosol, mitochondria, and endoplasmic reticulum. Syntenic analysis of cytosolic HSP90 genes revealed that T. b. brucei encoded for 10 tandem copies, while T. b. gambiense encoded for three tandem copies; Leishmania major (L. major) had the highest gene copy number with 17 tandem copies. The updated information on HSP90 from recently published proteomics on T. brucei was examined for different life cycle stages and subcellular localizations. The results show a difference between T. b. brucei and T. b. gambiense with T. b. brucei encoding a total of twelve putative HSP90 genes, while T. b. gambiense encodes five HSP90 genes. Eighteen putative co-chaperones were identified with one notable absence being cell division cycle 37 (Cdc37). These results provide an updated framework on approaching HSP90 and its interactions as drug targets in the African trypanosome.

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.

Inhibition of HSP90 Preserves Blood-Brain Barrier Integrity after Cortical Spreading Depression

Cortical spreading depression (CSD) is a pathophysiological mechanism underlying headache disorders, including migraine. Blood-brain barrier (BBB) permeability is increased during CSD. Recent papers have suggested that heat shock proteins (HSP) contribute to the integrity of the blood-brain barrier. In this study, the possible role of HSP90 in CSD-associated blood-brain barrier leak at the endothelial cell was investigated using an in vitro model, for the blood-endothelial barrier (BEB), and an in vivo model with an intact BBB. We measured barrier integrity using trans endothelial electric resistance (TEER) across a monolayer of rodent brain endothelial cells (bEnd.3), a sucrose uptake assay, and in situ brain perfusion using female Sprague Dawley rats. CSD was induced by application of 60 mM KCl for 5 min in in vitro experiments or cortical injection of KCl (1 M, 0.5 µL) through a dural cannula in vivo. HSP90 was selectively blocked by 17-AAG. Our data showed that preincubation with 17-AAG (1 µM) prevented the reduction of TEER values caused by the KCl pulse on the monolayer of bEnd.3 cells. The elevated uptake of 14C-sucrose across the same endothelial monolayer induced by the KCl pulse was significantly reduced after preincubation with HSP90 inhibitor. Pre-exposure to 17-AAG significantly mitigated the transient BBB leak after CSD induced by cortical KCl injection as determined by in situ brain perfusion in female rats. Our results demonstrated that inhibition of HSP90 with the selective agent 17-AAG reduced CSD-associated BEB/BBB paracellular leak. Overall, this novel observation supports HSP90 inhibition mitigates KCl-induced BBB permeability and suggests the development of new therapeutic approaches targeting HSP90 in headache disorders.

Grp94 works upstream of BiP in protein remodeling under heat stress

Hsp90 and Hsp70 are highly conserved molecular chaperones that promote the proper folding and activation of substrate proteins that are often referred to as clients. The two chaperones functionally collaborate to fold specific clients in an ATP-dependent manner. In eukaryotic cytosol, initial client folding is done by Hsp70 and its co-chaperones, followed by a direct transfer of client refolding intermediates to Hsp90 for final client processing. However, the mechanistic details of collaboration of organelle specific Hsp70 and Hsp90 are lacking. This work investigates the collaboration of the endoplasmic reticulum (ER) Hsp70 and Hsp90, BiP and Grp94 respectively, in protein remodeling using in vitro refolding assays. We show that under milder denaturation conditions, BiP collaborates with its co-chaperones to refold misfolded proteins in an ATP-dependent manner. Grp94 does not play a major role in this refolding reaction. However, under stronger denaturation conditions that favor aggregation, Grp94 works in an ATP-independent manner to bind and hold misfolded clients in a folding competent state for subsequent remodeling by the BiP system. We also show that the collaboration of Grp94 and BiP is not simply a reversal of the eukaryotic refolding mechanism since a direct interaction of Grp94 and BiP is not required for client transfer. Instead, ATP binding but not hydrolysis by Grp94 facilitates the release of the bound client, which is then picked up by the BiP system for subsequent refolding in a Grp94-independent manner.

Novel function of the C-Terminal region of the Hsp110 family member Osp94 in unfolded protein refolding

Osp94, a member of the Hsp110/Sse1 family of heat shock proteins, has a longer C-terminus than Hsc70/Hsp70, composed of the loop region with partial SBDβ (L), and SBDα and the C-terminal extension (H), but the functions of these domains are poorly understood. Osp94 suppressed heat-induced aggregation of luciferase (Luc). Osp94-bound heat-inactivated Luc was reactivated in the presence of rabbit reticulocyte lysate (RRL) and/or a combination of Hsc70 and Hsp40. Targeted deletion mutagenesis revealed that the SBDβ and H domains of Osp94 are critical for protein disaggregation and RRL-mediated refolding. Reactivation of Hsp90-bound heat-inactivated Luc was abolished in the absence of RRL but compensated by PA28α, a proteasome activator. Interestingly, the LH domain also reactivated heat-inactivated Luc, independent of PA28α. Biotin-tag cross-linking experiments indicated that the LH domain and PA28α interact with Luc bound by Hsp90 during refolding. A chimera protein in which the H domain was exchanged for PA28α also mediated disaggregation and reactivation of heat-inactivated Luc. These results indicate that Osp94 acts as a holdase and that the C-terminal region plays a PA28α-like role in the refolding of unfolded proteins.

Hsp70 in Redox Homeostasis

Cellular redox homeostasis is precisely balanced by generation and elimination of reactive oxygen species (ROS). ROS are not only capable of causing oxidation of proteins, lipids and DNA to damage cells but can also act as signaling molecules to modulate transcription factors and epigenetic pathways that determine cell survival and death. Hsp70 proteins are central hubs for proteostasis and are important factors to ameliorate damage from different kinds of stress including oxidative stress. Hsp70 members often participate in different cellular signaling pathways via their clients and cochaperones. ROS can directly cause oxidative cysteine modifications of Hsp70 members to alter their structure and chaperone activity, resulting in changes in the interactions between Hsp70 and their clients or cochaperones, which can then transfer redox signals to Hsp70-related signaling pathways. On the other hand, ROS also activate some redox-related signaling pathways to indirectly modulate Hsp70 activity and expression. Post-translational modifications including phosphorylation together with elevated Hsp70 expression can expand the capacity of Hsp70 to deal with ROS-damaged proteins and support antioxidant enzymes. Knowledge about the response and role of Hsp70 in redox homeostasis will facilitate our understanding of the cellular knock-on effects of inhibitors targeting Hsp70 and the mechanisms of redox-related diseases and aging.

The switch from client holding to folding in the Hsp70/Hsp90 chaperone machineries is regulated by a direct interplay between co-chaperones

Folding of stringent clients requires transfer from Hsp70 to Hsp90. The co-chaperone Hop physically connects the chaperone machineries. Here, we define its role from the remodeling of Hsp70/40-client complexes to the mechanism of client transfer and the conformational switching from stalled to active client-processing states of Hsp90. We show that Hsp70 together with Hsp40 completely unfold a stringent client, the glucocorticoid receptor ligand-binding domain (GR-LBD) in large assemblies. Hop remodels these for efficient transfer onto Hsp90. As p23 enters, Hsp70 leaves the complex via switching between binding sites in Hop. Current concepts assume that to proceed to client folding, Hop dissociates and the co-chaperone p23 stabilizes the Hsp90 closed state. In contrast, we show that p23 functionally interacts with Hop, relieves the stalling Hsp90-Hop interaction, and closes Hsp90. This reaction allows folding of the client and is thus the key regulatory step for the progression of the chaperone cycle.

The endoplasmic reticulum chaperone BiP is a closure-accelerating cochaperone of Grp94

Hsp70 and Hsp90 chaperones provide protein quality control to the cytoplasm, endoplasmic reticulum (ER), and mitochondria. Hsp90 activity is often enhanced by cochaperones that drive conformational changes needed for ATP-dependent closure and capture of client proteins. Hsp90 activity is also enhanced when working with Hsp70, but, in this case, the underlying mechanistic explanation is poorly understood. Here we examine the ER-specific Hsp70/Hsp90 paralogs (BiP/Grp94) and discover that BiP itself acts as a cochaperone that accelerates Grp94 closure. The BiP nucleotide binding domain, which interacts with the Grp94 middle domain, is responsible for Grp94 closure acceleration. A client protein initiates a coordinated progression of steps for the BiP/Grp94 system, in which client binding to BiP causes a conformational change that enables BiP to bind to Grp94 and accelerate its ATP-dependent closure. Single-molecule fluorescence resonance energy transfer measurements show that BiP accelerates Grp94 closure by stabilizing a high-energy conformational intermediate that otherwise acts as an energetic barrier to closure. These findings provide an explanation for enhanced activity of BiP and Grp94 when working as a pair, and demonstrate the importance of a high-energy conformational state in controlling the timing of the Grp94 conformational cycle. Given the high conservation of the Hsp70/Hsp90 system, other Hsp70s may also serve dual roles as both chaperones and closure-accelerating cochaperones to their Hsp90 counterparts.

Structure of Hsp90-Hsp70-Hop-GR reveals the Hsp90 client-loading mechanism

Maintaining a healthy proteome is fundamental for the survival of all organisms¹. Integral to this are Hsp90 and Hsp70, molecular chaperones that together facilitate the folding, remodelling and maturation of the many 'client proteins' of Hsp90². The glucocorticoid receptor (GR) is a model client protein that is strictly dependent on Hsp90 and Hsp70 for activity^3-7. Chaperoning GR involves a cycle of inactivation by Hsp70; formation of an inactive GR-Hsp90-Hsp70-Hop 'loading' complex; conversion to an active GR-Hsp90-p23 'maturation' complex; and subsequent GR release⁸. However, to our knowledge, a molecular understanding of this intricate chaperone cycle is lacking for any client protein. Here we report the cryo-electron microscopy structure of the GR-loading complex, in which Hsp70 loads GR onto Hsp90, uncovering the molecular basis of direct coordination by Hsp90 and Hsp70. The structure reveals two Hsp70 proteins, one of which delivers GR and the other scaffolds the Hop cochaperone. Hop interacts with all components of the complex, including GR, and poises Hsp90 for subsequent ATP hydrolysis. GR is partially unfolded and recognized through an extended binding pocket composed of Hsp90, Hsp70 and Hop, revealing the mechanism of GR loading and inactivation. Together with the GR-maturation complex structure⁹, we present a complete molecular mechanism of chaperone-dependent client remodelling, and establish general principles of client recognition, inhibition, transfer and activation.

Transcriptome analysis provides the first insight into the molecular basis of temperature plasticity in Banggai cardinalfish, Pterapogon kauderni

Banggai cardinalfish, Pterapogon kauderni, is a tropical fish listed as an endangered species by IUCN. Its distribution and survival condition are extremely limited, and the changes of living environment caused by global warming may seriously threaten its geographical distribution. In order to understand the survival temperature range and the potential mechanism of temperature plasticity of P. kauderni, transcriptome analysis was performed under five temperature conditions (18 °C, 22 °C, 26 °C, 30 °C and 34 °C). A total of 432,444,497 clean reads were obtained from the mix tissues of whole head, viscera (except intestine), and muscle. All clean data were spliced into 194,832 unigenes. Compared with 26 °C, 57, 107, 187 and 174 differentially expressed genes (DEGs) were obtained at 18 °C, 22 °C, 30 °C and 34 °C, respectively. Gene Ontology (GO) analysis showed the most highly enriched in the DEGs were cellular processes, binding, metabolic processes and biological regulation. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated circadian rhythm, protein processing in endoplasmic reticulum, influenza A and prion disease were significantly enriched. 47 genes that may be related to temperature stress were identified, such as Per1, MLP, IGFBP1, HSP70, HSP90α, HSPA4, DNAJB1, CALR. This is the first RNA-Seq study of P. kauderni. This information should be valuable for further targeted studies on temperature tolerance, thereby assisting the protection and development of P. kauderni.

Comprehensive Pan-Cancer Analysis of Heat Shock Protein 110, 90, 70, and 60 Families

Background: Here we carried out a panoramic analysis of the expression and prognosis of HSP110, HSP90, HSP70, and HSP60 families in 33 types of cancer, with the aim of deepening the systematic understanding of heat shock proteins (HSPs) in cancer.

Materials and Methods: Next-generation sequencing data of multiple tumors were downloaded from TCGA, CCLE and Oncomine databases. RStudio 3.6.1 was used to analyze HSP110, HSP90, HSP70 and HSP60 families based on their expression in 33 types of cancer. The validations in vivo (stomach adenocarcinoma and colon adenocarcinoma tissues) were performed by qRT-PCR.

Results: HSPs were differentially expressed in different cancers. The results revealed mainly positive correlations among the expressions of HSPs in different cancers. Expressions of HSP family members were generally associated with poor prognosis in respiratory, digestive, urinary and reproductive system tumors and associated with good prognosis in cholangiocarcinoma, pheochromocytoma and paraganglioma. TCGA mutation analysis showed that HSP gene mutation rate in cancers was 0–23%. CCLE mutation analysis indicated that HSP gene mutation rate in 828 cell lines from 15 tumors was 0–17%. CNV analysis revealed that HSPs have different degrees of gene amplifications and deletions in cancers. Gene mutations of 15 HSPs influenced their protein expressions in different cancers. Copy number amplifications and deletions of 22 HSPs also impacted protein expression levels in pan-cancer. HSP gene mutation was generally a poor prognosis factor in cancers, except for uterine corpus endometrial carcinoma. CNVs in 14 HSPs showed varying influences on survival status in different cancers. HSPs may be involved in the activation and inhibition of multiple cancer-related pathways. HSP expressions were closely correlated with 22 immune cell infiltrations in different cancers. The qRT-PCR validation results in vivo showed that HSPA2 was down-regulated in stomach adenocarcinoma and colon adenocarcinoma; HSPA7 and HSPA1A also were down-regulated in colon adenocarcinoma. HSPA2-HSPA7 (r = 0.031, p = 0.009) and HSPA1A-HSPA7 (r = 0.516, p < 0.001) were positive correlation in colon adenocarcinoma.

Conclusion: These analysis and validation results show that HSP families play an important role in the occurrence and development of various tumors and are potential tumor diagnostic and prognostic biomarkers as well as anti-cancer therapeutic targets.

Regulation of the endocytosis and prion-chaperoning machineries by yeast E3 ubiquitin ligase Rsp5 as revealed by orthogonal ubiquitin transfer

Attachment of the ubiquitin (UB) peptide to proteins via the E1-E2-E3 enzymatic machinery regulates diverse biological pathways, yet identification of the substrates of E3 UB ligases remains a challenge. We overcame this challenge by constructing an "orthogonal UB transfer" (OUT) cascade with yeast E3 Rsp5 to enable the exclusive delivery of an engineered UB (xUB) to Rsp5 and its substrate proteins. The OUT screen uncovered new Rsp5 substrates in yeast, such as Pal1 and Pal2, which are partners of endocytic protein Ede1, and chaperones Hsp70-Ssb, Hsp82, and Hsp104 that counteract protein misfolding and control self-perpetuating amyloid aggregates (prions), resembling those involved in human amyloid diseases. We showed that prion formation and effect of Hsp104 on prion propagation are modulated by Rsp5. Overall, our work demonstrates the capacity of OUT to deconvolute the complex E3-substrate relationships in crucial biological processes such as endocytosis and protein assembly disorders through protein ubiquitination.

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.

Hsp90 Co-chaperones Form Plastic Genetic Networks Adapted to Client Maturation

Heat shock protein 90 (Hsp90) is a molecular chaperone regulating the activity of diverse client proteins together with a plethora of different co-chaperones. Whether these functionally cooperate has remained enigmatic. We analyze all double mutants of 11 Saccharomyces cerevisiae Hsp90 co-chaperones in vivo concerning effects on cell physiology and the activation of specific client proteins. We find that client activation is supported by a genetic network with weak epistasis between most co-chaperones and a few modules with strong genetic interactions. These include an epistatic module regulating protein translation and dedicated epistatic networks for specific clients. For kinases, the bridging of Hsp70 and Hsp90 by Sti1/Hop is essential for activation, whereas for steroid hormone receptors, an epistatic module regulating their dwell time on Hsp90 is crucial, highlighting the specific needs of different clients. Thus, the Hsp90 system is characterized by plastic co-chaperone networks fine-tuning the conformational processing in a client-specific manner.

Violacein-Induced Chaperone System Collapse Underlies Multistage Antiplasmodial Activity

Antimalarial drugs with novel modes of action and wide therapeutic potential are needed to pave the way for malaria eradication. Violacein is a natural compound known for its biological activity against cancer cells and several pathogens, including the malaria parasite, Plasmodium falciparum (Pf). Herein, using chemical genomic profiling (CGP), we found that violacein affects protein homeostasis. Mechanistically, violacein binds Pf chaperones, PfHsp90 and PfHsp70-1, compromising the latter's ATPase and chaperone activities. Additionally, violacein-treated parasites exhibited increased protein unfolding and proteasomal degradation. The uncoupling of the parasite stress response reflects the multistage growth inhibitory effect promoted by violacein. Despite evidence of proteotoxic stress, violacein did not inhibit global protein synthesis via UPR activation-a process that is highly dependent on chaperones, in agreement with the notion of a violacein-induced proteostasis collapse. Our data highlight the importance of a functioning chaperone-proteasome system for parasite development and differentiation. Thus, a violacein-like small molecule might provide a good scaffold for development of a novel probe for examining the molecular chaperone network and/or antiplasmodial drug design.

Disrupting progression of the yeast Hsp90 folding pathway at different transition points results in client-specific maturation defects

The protein molecular chaperone Hsp90 (Heat shock protein, 90 kilodalton) plays multiple roles in the biogenesis and regulation of client proteins impacting myriad aspects of cellular physiology. Amino acid alterations located throughout Saccharomyces cerevisiae Hsp90 have been shown to result in reduced client activity and temperature-sensitive growth defects. Although some Hsp90 mutants have been shown to affect activity of particular clients more than others, the mechanistic basis of client-specific effects is unknown. We found that Hsp90 mutants that disrupt the early step of Hsp70 and Sti1 interaction, or show reduced ability to adopt the ATP-bound closed conformation characterized by Sba1 and Cpr6 interaction, similarly disrupt activity of three diverse clients, Utp21, Ssl2, and v-src. In contrast, mutants that appear to alter other steps in the folding pathway had more limited effects on client activity. Protein expression profiling provided additional evidence that mutants that alter similar steps in the folding cycle cause similar in vivo consequences. Our characterization of these mutants provides new insight into how Hsp90 and cochaperones identify and interact with diverse clients, information essential for designing pharmaceutical approaches to selectively inhibit Hsp90 function.

Chaperone Networks in Fungal Pathogens of Humans

The heat shock proteins (HSPs) function as chaperones to facilitate proper folding and modification of proteins and are of particular importance when organisms are subjected to unfavourable conditions. The human fungal pathogens are subjected to such conditions within the context of infection as they are exposed to human body temperature as well as the host immune response. Herein, the roles of the major classes of HSPs are briefly reviewed and their known contributions in human fungal pathogens are described with a focus on Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus. The Hsp90s and Hsp70s in human fungal pathogens broadly contribute to thermotolerance, morphological changes required for virulence, and tolerance to antifungal drugs. There are also examples of J domain co-chaperones and small HSPs influencing the elaboration of virulence factors in human fungal pathogens. However, there are diverse members in these groups of chaperones and there is still much to be uncovered about their contributions to pathogenesis. These HSPs do not act in isolation, but rather they form a network with one another. Interactions between chaperones define their specific roles and enhance their protein folding capabilities. Recent efforts to characterize these HSP networks in human fungal pathogens have revealed that there are unique interactions relevant to these pathogens, particularly under stress conditions. The chaperone networks in the fungal pathogens are also emerging as key coordinators of pathogenesis and antifungal drug tolerance, suggesting that their disruption is a promising strategy for the development of antifungal therapy.

Structural Communication between the E. coli Chaperones DnaK and Hsp90

The 70 kDa and 90 kDa heat shock proteins Hsp70 and Hsp90 are two abundant and highly conserved ATP-dependent molecular chaperones that participate in the maintenance of cellular homeostasis. In Escherichia coli, Hsp90 (Hsp90Ec) and Hsp70 (DnaK) directly interact and collaborate in protein remodeling. Previous work has produced a model of the direct interaction of both chaperones. The locations of the residues involved have been confirmed and the model has been validated. In this study, we investigate the allosteric communication between Hsp90Ec and DnaK and how the chaperones couple their conformational cycles. Using elastic network models (ENM), normal mode analysis (NMA), and a structural perturbation method (SPM) of asymmetric and symmetric DnaK-Hsp90Ec, we extract biologically relevant vibrations and identify residues involved in allosteric signaling. When one DnaK is bound, the dominant normal modes favor biological motions that orient a substrate protein bound to DnaK within the substrate/client binding site of Hsp90Ec and release the substrate from the DnaK substrate binding domain. The presence of one DnaK molecule stabilizes the entire Hsp90Ec protomer to which it is bound. Conversely, the symmetric model of DnaK binding results in steric clashes of DnaK molecules and suggests that the Hsp90Ec and DnaK chaperone cycles operate independently. Together, this data supports an asymmetric binding of DnaK to Hsp90Ec.

The Hsp70-Hsp90 go-between Hop/Stip1/Sti1 is a proteostatic switch and may be a drug target in cancer and neurodegeneration

The Hsp70 and Hsp90 molecular chaperone systems are critical regulators of protein homeostasis (proteostasis) in eukaryotes under normal and stressed conditions. The Hsp70 and Hsp90 systems physically and functionally interact to ensure cellular proteostasis. Co-chaperones interact with Hsp70 and Hsp90 to regulate and to promote their molecular chaperone functions. Mammalian Hop, also called Stip1, and its budding yeast ortholog Sti1 are eukaryote-specific co-chaperones, which have been thought to be essential for substrate ("client") transfer from Hsp70 to Hsp90. Substrate transfer is facilitated by the ability of Hop to interact simultaneously with Hsp70 and Hsp90 as part of a ternary complex. Intriguingly, in prokaryotes, which lack a Hop ortholog, the Hsp70 and Hsp90 orthologs interact directly. Recent evidence shows that eukaryotic Hsp70 and Hsp90 can also form a prokaryote-like binary chaperone complex in the absence of Hop, and that this binary complex displays enhanced protein folding and anti-aggregation activities. The canonical Hsp70-Hop-Hsp90 ternary chaperone complex is essential for optimal maturation and stability of a small subset of clients, including the glucocorticoid receptor, the tyrosine kinase v-Src, and the 26S/30S proteasome. Whereas many cancers have increased levels of Hop, the levels of Hop decrease in the aging human brain. Since Hop is not essential in all eukaryotic cells and organisms, tuning Hop levels or activity might be beneficial for the treatment of cancer and neurodegeneration.

General Structural and Functional Features of Molecular Chaperones

Molecular chaperones are a group of structurally diverse and highly conserved ubiquitous proteins. They play crucial roles in facilitating the correct folding of proteins in vivo by preventing protein aggregation or facilitating the appropriate folding and assembly of proteins. Heat shock proteins form the major class of molecular chaperones that are responsible for protein folding events in the cell. This is achieved by ATP-dependent (folding machines) or ATP-independent mechanisms (holders). Heat shock proteins are induced by a variety of stresses, besides heat shock. The large and varied heat shock protein class is categorised into several subfamilies based on their sizes in kDa namely, small Hsps (HSPB), J domain proteins (Hsp40/DNAJ), Hsp60 (HSPD/E; Chaperonins), Hsp70 (HSPA), Hsp90 (HSPC), and Hsp100. Heat shock proteins are localised to different compartments in the cell to carry out tasks specific to their environment. Most heat shock proteins form large oligomeric structures, and their functions are usually regulated by a variety of cochaperones and cofactors. Heat shock proteins do not function in isolation but are rather part of the chaperone network in the cell. The general structural and functional features of the major heat shock protein families are discussed, including their roles in human disease. Their function is particularly important in disease due to increased stress in the cell. Vector-borne parasites affecting human health encounter stress during transmission between invertebrate vectors and mammalian hosts. Members of the main classes of heat shock proteins are all represented in Plasmodium falciparum, the causative agent of cerebral malaria, and they play specific functions in differentiation, cytoprotection, signal transduction, and virulence.

Identification and Characterization of Plant-Interacting Targets of Tomato Spotted Wilt Virus Silencing Suppressor

Tomato spotted wilt virus (TSWV; species Tomato spotted wilt orthotospovirus) is an economically important plant virus that infects multiple horticultural crops on a global scale. TSWV encodes a non-structural protein NSs that acts as a suppressor of host RNA silencing machinery during infection. Despite extensive structural and functional analyses having been carried out on TSWV NSs, its protein-interacting targets in host plants are still largely unknown. Here, we systemically investigated NSs-interacting proteins in Nicotiana benthamiana via affinity purification and mass spectrometry (AP-MS) analysis. Forty-three TSWV NSs-interacting candidates were identified in N. benthamiana. Gene Ontology (GO) and protein–protein interaction (PPI) network analyses were carried out on their closest homologs in tobacco (Nicotiana tabacum), tomatoes (Solanum lycopersicum) and Arabidopsis (Arabidopsis thaliana). The results showed that NSs preferentially interacts with plant defense-related proteins such as calmodulin (CaM), importin, carbonic anhydrase and two heat shock proteins (HSPs): HSP70 and HSP90. As two major nodes in the PPI network, CaM and importin subunit α were selected for the further verification of their interactions with NSs via yeast two-hybrid (Y2H) screening. Our work suggests that the downstream signaling, transportation and/or metabolic pathways of host-NSs-interacting proteins may play critical roles in NSs-facilitated TSWV infection.

The Hsp70-Hsp90 co-chaperone Hop/Stip1 shifts the proteostatic balance from folding towards degradation

Hop/Stip1/Sti1 is thought to be essential as a co-chaperone to facilitate substrate transfer between the Hsp70 and Hsp90 molecular chaperones. Despite this proposed key function for protein folding and maturation, it is not essential in a number of eukaryotes and bacteria lack an ortholog. We set out to identify and to characterize its eukaryote-specific function. Human cell lines and the budding yeast with deletions of the Hop/Sti1 gene display reduced proteasome activity due to inefficient capping of the core particle with regulatory particles. Unexpectedly, knock-out cells are more proficient at preventing protein aggregation and at promoting protein refolding. Without the restraint by Hop, a more efficient folding activity of the prokaryote-like Hsp70-Hsp90 complex, which can also be demonstrated in vitro, compensates for the proteasomal defect and ensures the proteostatic equilibrium. Thus, cells may act on the level and/or activity of Hop to shift the proteostatic balance between folding and degradation.

Hop, also known as Stip1 or Sti1, facilitates substrate transfer between the Hsp70 and Hsp90 molecular chaperones. Characterization of proteostasis-related pathways in STIP1 knock-out cell lines reveals that in eukaryotes Stip1 modulates the balance between protein folding and degradation.

Thymoquinone attenuates doxorubicin-cardiotoxicity in rats

Contrary to the fact that doxorubicin is a powerful chemotherapeutic agent for the treatment of neoplastic diseases, cardiotoxicity is too important to be ignored. Thymoquinone serves as a powerful free radical scavenger. In the study, the effects of thymoquinone against doxorubicin-cardiotoxicity will be evaluated. Forty rats were divided into five groups. Group I: control group (n = 8); group II: olive oil group (n = 8); group III: thymoquinone group (n = 8); given 10 mg/kg thymoquinone intraperitoneally per day throughout the experiment; group IV: doxorubicin group (n = 8); injected with a single dose of 15 mg/kg ip doxorubicin on the 7th day of the experiment; group V: doxorubicin + thymoquinone group (n = 8); administered with 10 mg/kg thymoquinone per day during the experiment and 15 mg/kg doxorubicin ip on the 7th day. The experiment was planned for 14 days. Immunohistochemically, heat shock protein (HSP) 70 and HSP90, glucose-regulated protein 78 (GRP78), caspase-3 were stained. We made terminal deoxynucleotidyl transferase dUTP nick end labeling for apoptotic evaluation. Total oxidant status (TOS) levels and total antioxidant status (TAS) were measured in the heart tissue. Atrial natriuretic peptide (ANP) and pro-B type natriuretic peptide (proBNP) were evaluated. In the study, HSP70, HSP90, GRP78, and caspase-3 levels increased in group IV. TOS and TAS levels were significant compared to group I. Doxorubicin significantly increased ANP and NT-proBNP levels. Thymoquinone revealed significant differences in these values. Thymoquinone can be an important cardioprotective agent against doxorubicin-cardiotoxicity.

Mutations in the Hsp90 N Domain Identify a Site that Controls Dimer Opening and Expand Human Hsp90α Function in Yeast

Hsp90 is a highly conserved molecular chaperone important for the activity of many client proteins. Hsp90 has an N-terminal ATPase domain (N), a middle domain (M) that interacts with clients and a C-terminal dimerization domain (C). "Closing" of dimers around clients is regulated by ATP binding, co-chaperones, and post-translational modifications. ATP hydrolysis coincides with release of mature client and resetting the reaction cycle. Humans have two Hsp90s: hHsp90α and hHsp90β. Although 85% identical, hHsp90β supports Hsp90 function in yeast much better than hHsp90α. Determining the basis of this difference would provide important insight into functional specificity of seemingly redundant Hsp90s, and the evolution of eukaryotic Hsp90 systems and clientele. Here, we found host co-chaperones Sba1, Cpr6 and Cpr7 inhibited hHsp90α function in yeast, and we identified mutations clustering in the N domain that considerably improved hHsp90α function in yeast. The strongest of these rescuer mutations accelerated nucleotide-dependent lid closing, N-M domain docking, and ATPase. It also disrupted binding to Sba1, which prolongs the closed state, and promoted N-M undocking and lid opening. Our data suggest the rescuer mutations improve function of hHsp90α in yeast by accelerating return to the open state. Our findings imply hHsp90α occupies the closed state too long to function effectively in yeast, and define an evolutionarily conserved region of the N domain involved in resetting the Hsp90 reaction cycle.

The multiple functions of the co-chaperone stress inducible protein 1

Stress inducible protein 1 (STI1) is a co-chaperone acting with Hsp70 and Hsp90 for the correct client proteins' folding and therefore for the maintenance of cellular homeostasis. Besides being expressed in the cytosol, STI1 can also be found both in the cell membrane and the extracellular medium playing several relevant roles in the central nervous system (CNS) and tumor microenvironment. During CNS development, in association with cellular prion protein (PrP^c), STI1 regulates crucial events such as neuroprotection, neuritogenesis, astrocyte differentiation and survival. In cancer, STI1 is involved with tumor growth and invasion, is undoubtedly a pro-tumor factor, being considered as a biomarker and possibly therapeutic target for several malignancies. In this review, we discuss current knowledge and new findings on STI1 function as well as its role in tissue homeostasis, CNS and tumor progression.

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.

Biophysical analysis of Plasmodium falciparum Hsp70-Hsp90 organising protein (PfHop) reveals a monomer that is characterised by folded segments connected by flexible linkers

Plasmodium falciparum causes the most lethal form of malaria. The cooperation of heat shock protein (Hsp) 70 and 90 is thought to facilitate folding of select group of cellular proteins that are crucial for cyto-protection and development of the parasites. Hsp70 and Hsp90 are brought into a functional complex that allows substrate exchange by stress inducible protein 1 (STI1), also known as Hsp70-Hsp90 organising protein (Hop). P. falciparum Hop (PfHop) co-localises and occurs in complex with the parasite cytosolic chaperones, PfHsp70-1 and PfHsp90. Here, we characterised the structure of recombinant PfHop using synchrotron radiation circular dichroism (SRCD) and small-angle X-ray scattering. Structurally, PfHop is a monomeric, elongated but folded protein, in agreement with its predicted TPR domain structure. Using SRCD, we established that PfHop is unstable at temperatures higher than 40°C. This suggests that PfHop is less stable at elevated temperatures compared to its functional partner, PfHsp70-1, that is reportedly stable at temperatures as high as 80°C. These findings contribute towards our understanding of the role of the Hop-mediated functional partnership between Hsp70 and Hsp90.

Hop depletion reduces HSF1 levels and activity and coincides with reduced stress resilience

Heat-shock factor 1 (HSF1) regulates the transcriptional response to stress and controls expression of molecular chaperones required for cell survival. Here we report that HSF1 is regulated by the abundance of the Hsp70-Hsp90 organizing protein (Hop/STIP1). HSF1 levels were significantly reduced in Hop-depleted HEK293T cells. HSF1 transcriptional activity at the Hsp70 promoter, and binding of a biotinylated HSE oligonucleotide under both basal and heat-shock conditions were significantly reduced. Hop-depleted HEK293T cells were more sensitive to the HSF1 inhibitor KRIBB11 and showed reduced short-term proliferation, and reduced long-term survival under basal and heat-shock conditions. HSF1 nuclear localization was reduced in response to heat-shock and the nuclear staining pattern in Hop-depleted cells was punctate. Taken together, these data suggest that Hop regulates HSF1 function under both basal and stress conditions through a mechanism involving changes in levels, activity and subcellular localization, and coincides with reduced cellular fitness.

The Yeast Hsp70 Cochaperone Ydj1 Regulates Functional Distinction of Ssa Hsp70s in the Hsp90 Chaperoning Pathway

Heat-shock protein (Hsp) 90 assists in the folding of diverse sets of client proteins including kinases and growth hormone receptors. Hsp70 plays a major role in many Hsp90 functions by interacting and modulating conformation of its substrates before being transferred to Hsp90s for final maturation. Each eukaryote contains multiple members of the Hsp70 family. However, the role of different Hsp70 isoforms in Hsp90 chaperoning actions remains unknown. Using v-Src as an Hsp90 substrate, we examined the role of each of the four yeast cytosolic Ssa Hsp70s in regulating Hsp90 functions. We show that the strain expressing stress-inducible Ssa3 or Ssa4, and the not constitutively expressed Ssa1 or Ssa2, as the sole Ssa Hsp70 isoform reduces v-Src-mediated growth defects. The study shows that although different Hsp70 isoforms interact similarly with Hsp90s, v-Src maturation is less efficient in strains expressing Ssa4 as the sole Hsp70. We further show that the functional distinction between Ssa2 and Ssa4 is regulated by its C-terminal domain. Further studies reveal that Ydj1, which is known to assist substrate transfer to Hsp70s, interacts relatively weakly with Ssa4 compared with Ssa2, which could be the basis for poor maturation of the Hsp90 client in cells expressing stress-inducible Ssa4 as the sole Ssa Hsp70. The study thus reveals a novel role of Ydj1 in determining the functional distinction among Hsp70 isoforms with respect to the Hsp90 chaperoning action.

Comprehensive fitness maps of Hsp90 show widespread environmental dependence

Gene-environment interactions have long been theorized to influence molecular evolution. However, the environmental dependence of most mutations remains unknown. Using deep mutational scanning, we engineered yeast with all 44,604 single codon changes encoding 14,160 amino acid variants in Hsp90 and quantified growth effects under standard conditions and under five stress conditions. To our knowledge, these are the largest determined comprehensive fitness maps of point mutants. The growth of many variants differed between conditions, indicating that environment can have a large impact on Hsp90 evolution. Multiple variants provided growth advantages under individual conditions; however, these variants tended to exhibit growth defects in other environments. The diversity of Hsp90 sequences observed in extant eukaryotes preferentially contains variants that supported robust growth under all tested conditions. Rather than favoring substitutions in individual conditions, the long-term selective pressure on Hsp90 may have been that of fluctuating environments, leading to robustness under a variety of conditions.

Promotion of Hyperthermic-Induced rDNA Hypercondensation in Saccharomyces cerevisiae

Ribosome biogenesis is tightly regulated through stress-sensing pathways that impact genome stability, aging and senescence. In Saccharomyces cerevisiae, ribosomal RNAs are transcribed from rDNA located on the right arm of chromosome XII. Numerous studies reveal that rDNA decondenses into a puff-like structure during interphase, and condenses into a tight loop-like structure during mitosis. Intriguingly, a novel and additional mechanism of increased mitotic rDNA compaction (termed hypercondensation) was recently discovered that occurs in response to temperature stress (hyperthermic-induced) and is rapidly reversible. Here, we report that neither changes in condensin binding or release of DNA during mitosis, nor mutation of factors that regulate cohesin binding and release, appear to play a critical role in hyperthermic-induced rDNA hypercondensation. A candidate genetic approach revealed that deletion of either HSP82 or HSC82 (Hsp90 encoding heat shock paralogs) result in significantly reduced hyperthermic-induced rDNA hypercondensation. Intriguingly, Hsp inhibitors do not impact rDNA hypercondensation. In combination, these findings suggest that Hsp90 either stabilizes client proteins, which are sensitive to very transient thermic challenges, or directly promotes rDNA hypercondensation during preanaphase. Our findings further reveal that the high mobility group protein Hmo1 is a negative regulator of mitotic rDNA condensation, distinct from its role in promoting premature condensation of rDNA during interphase upon nutrient starvation.