Hierarchical functional specificity of cytosolic heat shock protein 70 (Hsp70) nucleotide exchange factors in yeast

Hsp70: A Multifunctional Chaperone in Maintaining Proteostasis and Its Implications in Human Disease

Hsp70, a 70 kDa molecular chaperone, plays a crucial role in maintaining protein homeostasis. It interacts with the DnaJ family of co-chaperones to modulate the functions of client proteins involved in various cellular processes, including transmembrane transport, extracellular vesicle trafficking, complex formation, and proteasomal degradation. Its presence in multiple cellular organelles enables it to mediate stress responses, apoptosis, and inflammation, highlighting its significance in disease progression. Initially recognized for its essential roles in protein folding, disaggregation, and degradation, later studies have demonstrated its involvement in several human diseases. Notably, Hsp70 is upregulated in multiple cancers, where it promotes tumor proliferation and serves as a tumor immunogen. Additionally, epichaperome networks stabilize protein-protein interactions in large and long-lived assemblies, contributing to both cancer progression and neurodegeneration. However, extracellular Hsp70 (eHsp70) in the tumor microenvironment can activate immune cells, such as natural killer (NK) cells, suggesting its potential in immunotherapeutic interventions, including CAR T-cell therapy. Given its multifaceted roles in cellular physiology and pathology, Hsp70 holds immense potential as both a biomarker and a therapeutic target across multiple human diseases. This review highlights the structural and functional importance of Hsp70, explores its role in disease pathogenesis, and discusses its potential in diagnostic and therapeutic applications.

Comparative analysis of BAG1 and BAG2: Insights into their structures, functions and implications in disease pathogenesis

As BAG family members, Bcl-2 associated athanogene family protein 1 (BAG1) and 2 (BAG2) are implicated in multiple cellular processes, including apoptosis, autophagy, protein folding and homeostasis. Although structurally similar, they considerably differ in many ways. Unlike BAG2, BAG1 has four isoforms (BAG1L, BAG1M, BAG1S and BAG1 p29) displaying different expression features and functional patterns. BAG1 and BAG2 play different cellular functions by interacting with different molecules to participate in the regulation of various diseases, including cancer/tumor and neurodegenerative diseases. Commonly, BAG1 acts as a protective factor to predict a good prognosis of patients with some types of cancer or a risk factor in some other cancers, while BAG2 is regarded as a risk factor to promote cancer/tumor progression. In neurodegenerative diseases, BAG2 commonly acts as a neuroprotective factor. In this review, we summarized the differences in molacular structure and biological function between BAG1 and BAG2, as well as the influences of them on pathogenesis of diseases, and explore the prospects for their clinical therapy application by specifying the activators and inhibitors of BAG1 and BAG2, which might provide a better understanding of the underlying pathogenesis and developing the targeted therapy strategies for diseases.

Early steps of protein disaggregation by Hsp70 chaperone and class B J-domain proteins are shaped by Hsp110

Hsp70 is a key cellular system counteracting protein misfolding and aggregation, associated with stress, ageing, and disease. Hsp70 solubilises aggregates and aids protein refolding through substrate binding and release cycles regulated by co-chaperones: J-domain proteins (JDPs) and nucleotide exchange factors (NEFs). Here, we elucidate the collaborative impact of Hsp110 NEFs and different JDP classes throughout Hsp70-dependent aggregate processing. We show that Hsp110 plays a major role at initial stages of disaggregation, determining its final efficacy. The NEF catalyses the recruitment of thick Hsp70 assemblies onto aggregate surface, which modifies aggregates into smaller species more readily processed by chaperones. Hsp70 stimulation by Hsp110 is much stronger with class B than class A JDPs and requires the auxiliary interaction between class B JDP and the Hsp70 EEVD motif. Furthermore, we demonstrate for the first time that Hsp110 disrupts the JDP-Hsp70 interaction. Such destabilisation of chaperone complexes at the aggregate surface might improve disaggregation, but also lead to the inhibition above the sub-stoichiometric Hsp110 optimum. Thus, balanced interplay between the co-chaperones and Hsp70 is critical to unlock its disaggregating potential.

The analysis of inducible family members in the water flea Daphnia magna led to the identification of an uncharacterized lineage of heat shock protein 70

To explore the function and evolutionary relationships of inducible heat shock protein 70 (Hsp70) in Daphnia magna, cDNAs of four Hsp70 family members (DmaHsp70, DmaHsp70-2, DmaHsp70-12, DmaHsp70-14) were cloned. While all DmaHsp70s possess three function domains, it is noteworthy that only DmaHsp70 ends with a "EEVD" motif. Phylogenetic analysis indicates that the Hsp70-12 lineage is distanced from the rest, and therefore it is an uncharacterized lineage of Hsp70. The differences in isoelectric point and 3-dimensional (3D) conformation of the N-terminal nucleotide binding domain (NBD) of DmaHsp70s further support the theory. DmaHsp70s exhibit varied motif distribution patterns and the logo sequences of motifs have diverse signature characteristics, indicating that different mechanisms are involved in the regulation of ATP binding and hydrolysis for the DmaHsp70s. Protein-protein network together with the predicted subcellular locations of DmaHsp70s suggest that they likely fulfill distinct roles in cells. The transcription of four DmaHsp70s were changed during the recovery stage after thermal stress or oxidative stress. But the expression pattern of them were dissimilar. Collectively, these results collectively elucidated the identification of a previously uncharacterizedHsp70 lineage in animal and extended our understanding of the Hsp70 family.

Effects of HSP70 chaperones Ssa1 and Ssa2 on Ste5 scaffold and the mating mitogen-activated protein kinase (MAPK) pathway in Saccharomyces cerevisiae

Ste5 is a prototype of scaffold proteins that regulate activation of mitogen-activated protein kinase (MAPK) cascades in all eukaryotes. Ste5 associates with many proteins including Gβγ (Ste4), Ste11 MAPKKK, Ste7 MAPKK, Fus3 and Kss1 MAPKs, Bem1, Cdc24. Here we show that Ste5 also associates with heat shock protein 70 chaperone (Hsp70) Ssa1 and that Ssa1 and its ortholog Ssa2 are together important for Ste5 function and efficient mating responses. The majority of purified overexpressed Ste5 associates with Ssa1. Loss of Ssa1 and Ssa2 has deleterious effects on Ste5 abundance, integrity, and localization particularly when Ste5 is expressed at native levels. The status of Ssa1 and Ssa2 influences Ste5 electrophoresis mobility and formation of high molecular weight species thought to be phosphorylated, ubiquitinylated and aggregated and lower molecular weight fragments. A Ste5 VWA domain mutant with greater propensity to form punctate foci has reduced predicted propensity to bind Ssa1 near the mutation sites and forms more punctate foci when Ssa1 Is overexpressed, supporting a dynamic protein quality control relationship between Ste5 and Ssa1. Loss of Ssa1 and Ssa2 reduces activation of Fus3 and Kss1 MAPKs and FUS1 gene expression and impairs mating shmoo morphogenesis. Surprisingly, ssa1, ssa2, ssa3 and ssa4 single, double and triple mutants can still mate, suggesting compensatory mechanisms exist for folding. Additional analysis suggests Ssa1 is the major Hsp70 chaperone for the mating and invasive growth pathways and reveals several Hsp70-Hsp90 chaperone-network proteins required for mating morphogenesis.

Role of Hsp70 in Post-Translational Protein Targeting: Tail-Anchored Membrane Proteins and Beyond

The Hsp70 family of molecular chaperones acts as a central 'hub' in the cell that interacts with numerous newly synthesized proteins to assist in their biogenesis. Apart from its central and well-established role in facilitating protein folding, Hsp70s also act as key decision points in the cellular chaperone network that direct client proteins to distinct biogenesis and quality control pathways. In this paper, we review accumulating data that illustrate a new branch in the Hsp70 network: the post-translational targeting of nascent membrane and organellar proteins to diverse cellular organelles. Work in multiple pathways suggests that Hsp70, via its ability to interact with components of protein targeting and translocation machineries, can initiate elaborate substrate relays in a sophisticated cascade of chaperones, cochaperones, and receptor proteins, and thus provide a mechanism to safeguard and deliver nascent membrane proteins to the correct cellular membrane. We discuss the mechanistic principles gleaned from better-studied Hsp70-dependent targeting pathways and outline the observations and outstanding questions in less well-studied systems.

Nucleotide Exchange Factors for Hsp70 Molecular Chaperones: GrpE, Hsp110/Grp170, HspBP1/Sil1, and BAG Domain Proteins

Molecular chaperones of the Hsp70 family are key components of the cellular protein-folding machinery. Substrate folding is accomplished by iterative cycles of ATP binding, hydrolysis, and release. The ATPase activity of Hsp70 is regulated by two main classes of cochaperones: J-domain proteins stimulate ATPase hydrolysis by Hsp70, while nucleotide exchange factors (NEFs) facilitate the conversion from the ADP-bound to the ATP-bound state, thus closing the chaperone folding cycle. NEF function can additionally be antagonized by ADP dissociation inhibitors. Beginning with the discovery of the prototypical bacterial NEF, GrpE, a large diversity of nucleotide exchange factors for Hsp70 have been identified, connecting it to a multitude of cellular processes in the eukaryotic cell. Here we review recent advances toward structure and function of nucleotide exchange factors from the Hsp110/Grp170, HspBP1/Sil1, and BAG domain protein families and discuss how these cochaperones connect protein folding with cellular quality control and degradation pathways.

Artificial optimization of bamboo Ppmar2 transposase and host factors effects on Ppmar2 transposition in yeast

Mariner-like elements (MLEs) are promising tools for gene cloning, gene expression, and gene tagging. We have characterized two MLE transposons from moso bamboo, Ppmar1 and Ppmar2. Ppmar2, is smaller in size and has higher natural activities, thus making it a more potential genomic tool compared to Ppmar1. Using a two-component system consisting of a transposase expression cassette and a non-autonomous transposon cotransformed in yeast, we investigated the transposition activity of Ppmar2 and created hyperactive transposases. Five out of 19 amino acid mutations in Ppmar2 outperformed the wild-type in terms of catalytic activities, especially with the S347R mutant having 6.7-fold higher transposition activity. Moreover, 36 yeast mutants with single-gene deletion were chosen to screen the effects of the host factors on Ppmar2NA transposition. Compared to the control strain (his3Δ), the mobility of Ppmar2 was greatly increased in 9 mutants and dramatically decreased in 7 mutants. The transposition ability in the efm1Δ mutant was 15-fold higher than in the control, while it was lowered to 1/66 in the rtt10Δ mutant. Transcriptomic analysis exhibited that EFM1 defection led to the significantly impaired DDR2, HSP70 expression and dramatically boosted JEN1 expression, whereas RTT10 defection resulted in significantly suppressed expression of UTP20, RPA190 and RRP5. Protein methylation, chromatin and RNA transcription may affect the Ppmar2NA transposition efficiency in yeast. Overall, the findings provided evidence for transposition regulation and offered an alternative genomic tool for moso bamboo and other plants.

Oxidation of two cysteines within yeast Hsp70 impairs proteostasis while directly triggering an Hsf1-dependent cytoprotective response

Neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and Huntington’s diseases affect millions of Americans every year. One factor linked to the formation of aggregates associated with these diseases is damage sustained to proteins by oxidative stress. Management of protein misfolding by the ubiquitous Hsp70 chaperone family can be modulated by modification of two key cysteines in the ATPase domain by oxidizing or thiol-modifying compounds. To investigate the biological consequences of cysteine modification on the Hsp70 Ssa1 in budding yeast, we generated cysteine null (cysteine to serine) and oxidomimetic (cysteine to aspartic acid) mutant variants of both C264 and C303 and demonstrate reduced ATP binding, hydrolysis, and protein folding properties in both the oxidomimetic and hydrogen peroxide–treated Ssa1. In contrast, cysteine nullification rendered Ssa1 insensitive to oxidative inhibition. Additionally, we determined the oxidomimetic ssa1-2CD (C264D, C303D) allele was unable to function as the sole Ssa1 isoform in yeast cells and also exhibited dominant negative effects on cell growth and viability. Ssa1 binds to and represses Hsf1, the major transcription factor controlling the heat shock response, and we found the oxidomimetic Ssa1 failed to stably interact with Hsf1, resulting in constitutive activation of the heat shock response. Consistent with our in vitro findings, ssa1-2CD cells were compromised for de novo folding, post-stress protein refolding, and in regulated degradation of a model terminally misfolded protein. Together, these findings pinpoint Hsp70 as a key link between oxidative stress and proteostasis, information critical to understanding cytoprotective systems that prevent and manage cellular insults underlying complex disease states.

Disease-associated mutations within the yeast DNAJB6 homolog Sis1 slow conformer-specific substrate processing and can be corrected by the modulation of nucleotide exchange factors

Molecular chaperones, or heat shock proteins (HSPs), protect against the toxic misfolding and aggregation of proteins. As such, mutations or deficiencies within the chaperone network can lead to disease. Dominant mutations within DNAJB6 (Hsp40)-an Hsp70 co-chaperone-lead to a protein aggregation-linked myopathy termed Limb-Girdle Muscular Dystrophy Type D1 (LGMDD1). Here, we used the yeast prion model client in conjunction with in vitro chaperone activity assays to gain mechanistic insights into the molecular basis of LGMDD1. Here, we show how mutations analogous to those found in LGMDD1 affect Sis1 (a functional homolog of human DNAJB6) function by altering the structure of client protein aggregates, interfering with the Hsp70 ATPase cycle, dimerization and substrate processing; poisoning the function of wild-type protein. These results uncover the mechanisms through which LGMDD1-associated mutations alter chaperone activity, and provide insights relevant to potential therapeutic interventions.

Multivalent protein-protein interactions are pivotal regulators of eukaryotic Hsp70 complexes

Heat shock protein 70 (Hsp70) is a molecular chaperone and central regulator of protein homeostasis (proteostasis). Paramount to this role is Hsp70's binding to client proteins and co-chaperones to produce distinct complexes, such that understanding the protein-protein interactions (PPIs) of Hsp70 is foundational to describing its function and dysfunction in disease. Mounting evidence suggests that these PPIs include both "canonical" interactions, which are universally conserved, and "non-canonical" (or "secondary") contacts that seem to have emerged in eukaryotes. These two categories of interactions involve discrete binding surfaces, such that some clients and co-chaperones engage Hsp70 with at least two points of contact. While the contributions of canonical interactions to chaperone function are becoming increasingly clear, it can be challenging to deconvolute the roles of secondary interactions. Here, we review what is known about non-canonical contacts and highlight examples where their contributions have been parsed, giving rise to a model in which Hsp70's secondary contacts are not simply sites of additional avidity but are necessary and sufficient to impart unique functions. From this perspective, we propose that further exploration of non-canonical contacts will generate important insights into the evolution of Hsp70 systems and inspire new approaches for developing small molecules that tune Hsp70-mediated proteostasis.

Regulation of heat shock proteins 70 and their role in plant immunity

Heat shock proteins 70 (HSP70s) are steadily gaining more attention in the field of plant biotic interactions. Though their regulation and activity in plants are much less well characterized than are those of their counterparts in mammals, accumulating evidence indicates that the role of HSP70-mediated defense mechanisms in plant cells is indispensable. In this review, we summarize current knowledge of HSP70 post-translational control in plants. We comment on the phytohormonal regulation of HSP70 expression and protein abundance, and identify a prominent role for cytokinin in HSP70 control. We outline HSP70s' subcellular localizations, chaperone activity, and chaperone-mediated protein degradation. We focus on the role of HSP70s in plant pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity, and discuss the contribution of different HSP70 subfamilies to plant defense against pathogens.

Proteomics of Two Thermotolerant Isolates of Trichoderma under High-Temperature Stress

Several species of the soil borne fungus of the genus Trichoderma are known to be versatile, opportunistic plant symbionts and are the most successful biocontrol agents used in today's agriculture. To be successful in field conditions, the fungus must endure varying climatic conditions. Studies have indicated that a high atmospheric temperature coupled with low humidity is a major factor in the inconsistent performance of Trichoderma under field conditions. Understanding the molecular modulations associated with Trichoderma that persist and deliver under abiotic stress conditions will aid in exploiting the value of these organisms for such uses. In this study, a comparative proteomic analysis, using two-dimensional gel electrophoresis (2DE) and matrix-assisted laser desorption/time-of-flight (MALDI-TOF-TOF) mass spectrometry, was used to identify proteins associated with thermotolerance in two thermotolerant isolates of Trichoderma: T. longibrachiatum 673, TaDOR673 and T. asperellum 7316, TaDOR7316; with 32 differentially expressed proteins being identified. Sequence homology and conserved domains were used to identify these proteins and to assign a probable function to them. The thermotolerant isolate, TaDOR673, seemed to employ the stress signaling MAPK pathways and heat shock response pathways to combat the stress condition, whereas the moderately tolerant isolate, TaDOR7316, seemed to adapt to high-temperature conditions by reducing the accumulation of misfolded proteins through an unfolded protein response pathway and autophagy. In addition, there were unique, as well as common, proteins that were differentially expressed in the two isolates studied.

Ectopic BH3-only protein Bim acts as a co-chaperone to positively regulate Hsp70 in yeast

The chaperone heat shock protein 70 (Hsp70) is conserved from bacteria to humans and is crucial for avoiding protein misfolding under stress. Bim functions mainly as one of the Bcl-2 family pro-apoptotic members, was identified to be a co-chaperone of Hsp70. Herein, we reported that ectopic Bim could constitute the interactions with intrinsic Hsp70 and translate its positive co-chaperone activity in vitro to the yeast growth promotion and help Hsp70 to fold its client Ras-like protein. With the help of a specific Hsp70/Bim disruptor, we illustrated that Hsp70/Bim dimers rescue yeast from heat shock. In an organism lacks apoptotic Bcl-2 factors, the pro-apoptotic Bim in mammalian cells exhibits pro-survival functions.

Suppression of aggregate and amyloid formation by a novel intrinsically disordered region in metazoan Hsp110 chaperones

Molecular chaperones maintain proteostasis by ensuring the proper folding of polypeptides. Loss of proteostasis has been linked to numerous neurodegenerative disorders including Alzheimer's, Parkinson's, and Huntington's disease. Hsp110 is related to the canonical Hsp70 class of protein-folding molecular chaperones and interacts with Hsp70 as a nucleotide exchange factor (NEF). In addition to its NEF activity, Hsp110 possesses an Hsp70-like substrate-binding domain (SBD) whose biological roles remain undefined. Previous work in Drosophila melanogaster has implicated the sole Hsp110 gene (Hsc70cb) in proteinopathic neurodegeneration. We hypothesize that in addition to its role as an Hsp70 NEF, Drosophila Hsp110 may function as a protective protein "holdase," preventing the aggregation of unfolded polypeptides via the SBD-β subdomain. We demonstrate for the first time that Drosophila Hsp110 effectively prevents aggregation of the model substrate citrate synthase. We also report the discovery of a redundant and heretofore unknown potent holdase capacity in a 138-amino-acid region of Hsp110 carboxyl terminal to both SBD-β and SBD-α (henceforth called the C-terminal extension). This sequence is highly conserved in metazoan Hsp110 genes, completely absent from fungal representatives, and is computationally predicted to contain an intrinsically disordered region (IDR). We demonstrate that this IDR sequence within the human Hsp110s, Apg-1 and Hsp105α, inhibits the formation of amyloid Aβ-42 and α-synuclein fibrils in vitro but cannot mediate fibril disassembly. Together these findings establish capacity for metazoan Hsp110 chaperones to suppress both general protein aggregation and amyloidogenesis, raising the possibility of exploitation of this IDR for therapeutic benefit.

The RNA fold interactome of evolutionary conserved RNA structures in S. cerevisiae

RNA-binding proteins play key roles in regulation of gene expression via recognition of structural features in RNA molecules. Here we apply a quantitative RNA pull-down approach to 186 evolutionary conserved RNA structures and report 162 interacting proteins. Unlike global RNA interactome capture, we associate individual RNA structures within messenger RNA with their interacting proteins. Of our binders 69% are known RNA-binding proteins, whereas some are previously unrelated to RNA binding and do not harbor canonical RNA-binding domains. While current knowledge about RNA-binding proteins relates to their functions at 5′ or 3′-UTRs, we report a significant number of them binding to RNA folds in the coding regions of mRNAs. Using an in vivo reporter screen and pulsed SILAC, we characterize a subset of mRNA-RBP pairs and thus connect structural RNA features to functionality. Ultimately, we here present a generic, scalable approach to interrogate the increasing number of RNA structural motifs.

Previous study identified in vivo structured mRNA regions in Saccharomyces cerevisiae by dimethyl sulfate-sequencing. Here the authors use quantitative proteomics to identify protein interactors of 186 RNA folds in S. cerevisiae, providing functional links between RNA binding proteins and distinct mRNA fold.

Genetic Selection Based on a Ste6*C-HA-Ura3 Substrate Identifies New Cytosolic Quality Control Alleles in Saccharomyces cerevisiae

Protein quality control in the cytosol (CytoQC) is an important cellular pathway consisting of a network of components which monitor the folding of cytosolic proteins and ensure the efficient removal of aberrant ones. Our understanding of CytoQC mechanisms is limited in part by the ability of current approaches to identify new genes in the pathway. In this study, we developed a CytoQC reporter substrate, Ste6*C-HA-Ura3, for a new genetic selection of spontaneous CytoQC mutations in the yeast Saccharomyces cerevisiae In addition to UBR1, which encodes for a known CytoQC E3 ligase, we identified six new CytoQC candidates. In the preliminary characterization of two mutants, we found that Doa4 is involved in the degradation of misfolded substrates while Pup2 functions in the selectivity of CytoQC and ERAD substrates. Overall, the strategy demonstrates the potential to identify novel genes and advance our understanding of CytoQC.

Hitchhiking on vesicles: a way to harness age-related proteopathies?

Central to proteopathies and leading to most age-related neurodegenerative disorders is a failure in protein quality control (PQC). To harness the toxicity of misfolded and damaged disease proteins, such proteins are either refolded, degraded by temporal PQC, or sequestered by spatial PQC into specific, organelle-associated, compartments within the cell. Here, we discuss the impact of vesicle trafficking pathways in general, and syntaxin 5 in particular, as key players in spatial PQC directing misfolded proteins to the surface of vacuole and mitochondria, which facilitates their clearance and detoxification. Since boosting vesicle trafficking genetically can positively impact on spatial PQC and make cells less sensitive to misfolded disease proteins, we speculate that regulators of such trafficking might serve as therapeutic targets for age-related neurological disorders.

The Hsp70 chaperone network

The 70-kDa heat shock proteins (Hsp70s) are ubiquitous molecular chaperones that act in a large variety of cellular protein folding and remodelling processes. They function virtually at all stages of the life of proteins from synthesis to degradation and are thus crucial for maintaining protein homeostasis, with direct implications for human health. A large set of co-chaperones comprising J-domain proteins and nucleotide exchange factors regulate the ATPase cycle of Hsp70s, which is allosterically coupled to substrate binding and release. Moreover, Hsp70s cooperate with other cellular chaperone systems including Hsp90, Hsp60 chaperonins, small heat shock proteins and Hsp100 AAA+ disaggregases, together constituting a dynamic and functionally versatile network for protein folding, unfolding, regulation, targeting, aggregation and disaggregation, as well as degradation. In this Review we describe recent advances that have increased our understanding of the molecular mechanisms and working principles of the Hsp70 network. This knowledge showcases how the Hsp70 chaperone system controls diverse cellular functions, and offers new opportunities for the development of chemical compounds that modulate disease-related Hsp70 activities.

Activity of the yeast cytoplasmic Hsp70 nucleotide-exchange factor Fes1 is regulated by reversible methionine oxidation

Cells employ a vast network of regulatory pathways to manage intracellular levels of reactive oxygen species (ROS). An effectual means used by cells to control these regulatory systems are sulfur-based redox switches, which consist of protein cysteine or methionine residues that become transiently oxidized when intracellular ROS levels increase. Here, we describe a methionine-based oxidation event involving the yeast cytoplasmic Hsp70 co-chaperone Fes1. We show that Fes1 undergoes reversible methionine oxidation during excessively-oxidizing cellular conditions, and we map the site of this oxidation to a cluster of three methionine residues in the Fes1 core domain. Making use of recombinant proteins and a variety of in vitro assays, we establish that oxidation inhibits Fes1 activity and, correspondingly, alters Hsp70 activity. Moreover, we demonstrate in vitro and in cells that Fes1 oxidation is reversible and is regulated by the cytoplasmic methionine sulfoxide reductase Mxr1 (MsrA) and a previously unidentified cytoplasmic pool of the reductase Mxr2 (MsrB). We speculate that inactivation of Fes1 activity during excessively-oxidizing conditions may help maintain protein-folding homeostasis in a suboptimal cellular folding environment. The characterization of Fes1 oxidation during cellular stress provides a new perspective as to how the activities of the cytoplasmic Hsp70 chaperones may be attuned by fluctuations in cellular ROS levels and provides further insight into how cells use methionine-based redox switches to sense and respond to oxidative stress.

Cellular sequestrases maintain basal Hsp70 capacity ensuring balanced proteostasis

Maintenance of cellular proteostasis is achieved by a multi-layered quality control network, which counteracts the accumulation of misfolded proteins by refolding and degradation pathways. The organized sequestration of misfolded proteins, actively promoted by cellular sequestrases, represents a third strategy of quality control. Here we determine the role of sequestration within the proteostasis network in Saccharomyces cerevisiae and the mechanism by which it occurs. The Hsp42 and Btn2 sequestrases are functionally intertwined with the refolding activity of the Hsp70 system. Sequestration of misfolded proteins by Hsp42 and Btn2 prevents proteostasis collapse and viability loss in cells with limited Hsp70 capacity, likely by shielding Hsp70 from misfolded protein overload. Btn2 has chaperone and sequestrase activity and shares features with small heat shock proteins. During stress recovery Btn2 recruits the Hsp70-Hsp104 disaggregase by directly interacting with the Hsp70 co-chaperone Sis1, thereby shunting sequestered proteins to the refolding pathway.

Fine Tuning: Effects of Post-Translational Modification on Hsp70 Chaperones

The discovery of heat shock proteins shaped our view of protein folding in the cell. Since their initial discovery, chaperone proteins were identified in all domains of life, demonstrating their vital and conserved functional roles in protein homeostasis. Chaperone proteins maintain proper protein folding in the cell by utilizing a variety of distinct, characteristic mechanisms to prevent aberrant intermolecular interactions, prevent protein aggregation, and lower entropic costs to allow for protein refolding. Continued study has found that chaperones may exhibit alternative functions, including maintaining protein folding during endoplasmic reticulum (ER) import and chaperone-mediated degradation, among others. Alternative chaperone functions are frequently controlled by post-translational modification, in which a given chaperone can switch between functions through covalent modification. This review will focus on the Hsp70 class chaperones and their Hsp40 co-chaperones, specifically highlighting the importance of post-translational control of chaperones. These modifications may serve as a target for therapeutic intervention in the treatment of diseases of protein misfolding and aggregation.

Hsp70-nucleotide exchange factor (NEF) Fes1 has non-NEF roles in degradation of gluconeogenic enzymes and cell wall integrity

Fes1 is a conserved armadillo repeat-containing Hsp70 nucleotide exchange factor important for growth at high temperature, proteasomal protein degradation and prion propagation. Depleting or mutating Fes1 induces a stress response and causes defects in these processes that are ascribed solely to disruption of Fes1 regulation of Hsp70. Here, we find Fes1 was essential for degradation of gluconeogenic enzymes by the vacuole import and degradation (Vid) pathway and for cell wall integrity (CWI), which is crucial for growth at high temperature. Unexpectedly, Fes1 mutants defective in physical or functional interaction with Hsp70 retained activities that support Vid and CWI. Fes1 and the Fes1 mutants bound to the Vid substrate Fbp1 in vitro and captured Slt2, a signaling kinase that regulates CWI, from cell lysates. Our data show that the armadillo domain of Fes1 binds proteins other than Hsp70, that Fes1 has important Hsp70-independent roles in the cell, and that major growth defects caused by depleting Fes1 are due to loss of these functions rather than to loss of Hsp70 regulation. We uncovered diverse functions of Fes1 beyond its defined role in regulating Hsp70, which points to possible multi-functionality among its conserved counterparts in other organisms or organelles.

Fes1, a yeast homolog of human nucleotide exchange factor HspBP1, binds and regulates Hsp70, a universally conserved protein that helps maintain health of proteins in cells. Fes1 is believed to function only by helping Hsp70 release ADP and substrates and cells lacking Fes1 are sick. We find Fes1 is essential for protein degradation by a vacuolar pathway (Vid) and for cell wall integrity (CWI), and it interacts with a Vid substrate and a regulator of CWI. Fes1 mutants that cannot regulate Hsp70 can still support Vid and CWI, interact with proteins involved in these processes and restore cell health. Thus, Fes1 binds proteins other than Hsp70 and has important functions beyond regulating Hsp70 that are needed for optimal cell fitness.

Roles of the nucleotide exchange factor and chaperone Hsp110 in cellular proteostasis and diseases of protein misfolding

Cellular protein homeostasis (proteostasis) is maintained by a broad network of proteins involved in synthesis, folding, triage, repair and degradation. Chief among these are molecular chaperones and their cofactors that act as powerful protein remodelers. The growing realization that many human pathologies are fundamentally diseases of protein misfolding (proteopathies) has generated interest in understanding how the proteostasis network impacts onset and progression of these diseases. In this minireview, we highlight recent progress in understanding the enigmatic Hsp110 class of heat shock protein that acts as both a potent nucleotide exchange factor to regulate activity of the foldase Hsp70, and as a passive chaperone capable of recognizing and binding cellular substrates on its own, and its integration into the proteostasis network.

Cytoprotective Co-chaperone BcBAG1 Is a Component for Fungal Development, Virulence, and Unfolded Protein Response (UPR) of Botrytis cinerea

The Bcl-2 associated athanogene (BAG) family is an evolutionarily conserved group of co-chaperones that confers stress protection against a variety of cellular insults extending from yeasts, plants to humans. Little is known, however, regarding the biological role of BAG proteins in phytopathogenic fungi. Here, we identified the unique BAG gene (BcBAG1) from the necrotrophic fungal pathogen, Botrytis cinerea. BcBAG1 is the homolog of Arabidopsis thaliana AtBAG4, and ectopic expression of BcBAG1 in atbag4 knock-out mutants restores salt tolerance. BcBAG1 deletion mutants (ΔBcbag1) exhibited decreased conidiation, enhanced melanin accumulation and lost the ability to develop sclerotia. Also, BcBAG1 disruption blocked fungal conidial germination and successful penetration, leading to a reduced virulence in host plants. BcBAG1 contains BAG (BD) domain at C-terminus and ubiquitin-like (UBL) domain at N-terminus. Complementation assays indicated that BD can largely restored pathogenicity of ΔBcbag1. Abiotic stress assays showed ΔBcbag1 was more sensitive than the wild-type strain to NaCl, calcofluor white, SDS, tunicamycin, dithiothreitol (DTT), heat and cold stress, suggesting BcBAG1 plays a cytoprotective role during salt stress, cell wall stress, and ER stress. BcBAG1 negatively regulated the expression of BcBIP1, BcIRE1 and the splicing of BcHAC1 mRNA, which are core regulators of unfolded protein response (UPR) during ER stress. Moreover, BcBAG1 interacted with HSP70-type chaperones, BcBIP1 and BcSKS2. In summary, this work demonstrates that BcBAG1 is pleiotropic and not only essential for fungal development, hyphal melanization, and virulence, but also required for response to multiple abiotic stresses and UPR pathway of B. cinerea.

Thiol stress-dependent aggregation of the glycolytic enzyme triose phosphate isomerase in yeast and human cells

The eukaryotic cytosolic proteome is vulnerable to changes in proteostatic and redox balance caused by temperature, pH, oxidants, and xenobiotics. Cysteine-containing proteins are especially at risk, as the thiol side chain is subject to oxidation, adduction, and chelation by thiol-reactive compounds. The thiol-chelating heavy metal cadmium is a highly toxic environmental pollutant demonstrated to induce the heat shock response and recruit protein chaperones to sites of presumed protein aggregation in the budding yeast Saccharomyces cerevisiae. However, endogenous targets of cadmium toxicity responsible for these outcomes are largely unknown. Using fluorescent protein fusion to cytosolic proteins with known redox-active cysteines, we identified the yeast glycolytic enzyme triose phosphate isomerase as being aggregation-prone in response to cadmium and to glucose depletion in chronologically aging cultures. Cadmium-induced aggregation was limited to newly synthesized Tpi1 that was recruited to foci containing the disaggregase Hsp104 and the peroxiredoxin chaperone Tsa1. Misfolding of nascent Tpi1 in response to both cadmium and glucose-depletion stress required both cysteines, implying that thiol status in this protein directly influences folding. We also demonstrate that cadmium proteotoxicity is conserved between yeast and human cells, as HEK293 and HCT116 cell lines exhibit recruitment of the protein chaperone Hsp70 to visible foci. Moreover, human TPI, mutations in which cause a glycolytic deficiency syndrome, also forms aggregates in response to cadmium treatment, suggesting that this conserved enzyme is folding-labile and may be a useful endogenous model for investigating thiol-specific proteotoxicity.

The yeast stress inducible Ssa Hsp70 reduces α-synuclein toxicity by promoting its degradation through autophagy

The mechanism underlying the role of Hsp70s in toxicity associated with intracellular accumulation of toxic protein inclusions is under intense investigation. In current study, we examined the roles of all different isoforms of yeast cytosolic Ssa Hsp70 on α-synuclein mediated cellular toxicity. The study showed that yeast cells expressing stress-inducible Ssa3 or Ssa4 as sole Ssa Hsp70 isoforms, reduced α-synuclein toxicity better than those expressing a constitutive counterpart. The protective effect of stress-inducible Ssa Hsp70s was not α-syn specific, but more general to other inclusion forming proteins such as polyQ. We show that the protective effect is not by induction of a general stress response in Ssa3 cells rather by promoting α-synuclein degradation through autophagy. The present study revealed that effect of Hsp70s was isoform dependent, and that autophagy protects Ssa3 cells from the deleterious effects of toxic protein inclusions.

The absence of specific yeast heat-shock proteins leads to abnormal aggregation and compromised autophagic clearance of mutant Huntingtin proteins

The functionality of a protein depends on its correct folding, but newly synthesized proteins are susceptible to aberrant folding and aggregation. Heat shock proteins (HSPs) function as molecular chaperones that aid in protein folding and the degradation of misfolded proteins. Trinucleotide (CAG) repeat expansion in the Huntingtin gene (HTT) results in the expression of misfolded Huntingtin protein (Htt), which contributes to the development of Huntington’s disease. We previously found that the degradation of mutated Htt with polyQ expansion (Htt103QP) depends on both ubiquitin proteasome system and autophagy. However, the role of heat shock proteins in the clearance of mutated Htt remains poorly understood. Here, we report that cytosolic Hsp70 (Ssa family), its nucleotide exchange factors (Sse1 and Fes1), and a Hsp40 co-chaperone (Ydj1) are required for inclusion body formation of Htt103QP proteins and their clearance via autophagy. Extended induction of Htt103QP-GFP leads to the formation of a single inclusion body in wild-type yeast cells, but mutant cells lacking these HSPs exhibit increased number of Htt103QP aggregates. Most notably, we detected more aggregated forms of Htt103QP in sse1Δ mutant cells using an agarose gel assay. Increased protein aggregates are also observed in these HSP mutants even in the absence Htt103QP overexpression. Importantly, these HSPs are required for autophagy-mediated Htt103QP clearance, but are less critical for proteasome-dependent degradation. These findings suggest a chaperone network that facilitates inclusion body formation of misfolded proteins and the subsequent autophagic clearance.

Hsp70-Hsp110 chaperones deliver ubiquitin dependent and independent substrates to the 26S proteasome for proteolysis

In protein quality control, proteotoxic misfolded proteins are recognized by molecular chaperones, ubiquitylated by dedicated quality-control ligases and delivered to 26S proteasome for degradation. The chaperone Hsp70 and its nucleotide exchange factor Hsp110 functions in the degradation of misfolded proteins by the ubiquitin-proteasome system via poorly understood mechanisms. Here we report that yeast Hsp110 (Sse1 and Sse2) functions in the degradation of Hsp70-associated ubiquitin conjugates at the post-ubiquitylation step and is required for the proteasomal degradation of ubiquitin-independent substrates. Hsp110 associates with the 19S regulatory particle of the 26S proteasome and interacts with Hsp70 to facilitate the delivery of Hsp70 substrates for proteasomal degradation. Using a highly defined ubiquitin-independent proteasome substrate we show that the mere introduction of a single Hsp70-binding site renders its degradation dependent on Hsp110. The findings define a dedicated and chaperone-dependent pathway for the efficient shuttling of cellular proteins to the proteasome with profound implications for understanding protein quality control and cellular stress management.

Nucleotide exchange factors Fes1 and HspBP1 mimic substrate to release misfolded proteins from Hsp70

Protein quality control depends on the tight regulation of interactions between molecular chaperones and polypeptide substrates. Substrate release from the chaperone Hsp70 is triggered by nucleotide-exchange factors (NEFs) that control folding and degradation fates via poorly understood mechanisms. We found that the armadillo-type NEFs budding yeast Fes1 and its human homolog HspBP1 employ flexible N-terminal release domains (RDs) with substrate-mimicking properties to ensure the efficient release of persistent substrates from Hsp70. The RD contacts the substrate-binding domain of the chaperone, competes with peptide substrate for binding and is essential for proper function in yeast and mammalian cells. Thus, the armadillo domain engages Hsp70 to trigger nucleotide exchange, whereas the RD safeguards the release of substrates. Our findings provide fundamental mechanistic insight into the functional specialization of Hsp70 NEFs and have implications for the understanding of proteostasis-related disorders, including Marinesco-Sjögren syndrome.

Identification of novel factors enhancing recombinant protein production in multi-copy Komagataella phaffii based on transcriptomic analysis of overexpression effects

The methylotrophic yeast Komagataella phaffii (Pichia pastoris) has been developed into a highly successful system for heterologous protein expression in both academia and industry. However, overexpression of recombinant protein often leads to severe burden on the physiology of K. phaffii and triggers cellular stress. To elucidate the global effect of protein overexpression, we set out to analyze the differential transcriptome of recombinant strains with 12 copies and a single copy of phospholipase A₂ gene (PLA₂) from Streptomyces violaceoruber. Through GO, KEGG and heat map analysis of significantly differentially expressed genes, the results indicated that the 12-copy strain suffered heavy cellular stress. The genes involved in protein processing and stress response were significantly upregulated due to the burden of protein folding and secretion, while the genes in ribosome and DNA replication were significantly downregulated possibly contributing to the reduced cell growth rate under protein overexpression stress. Three most upregulated heat shock response genes (CPR6, FES1, and STI1) were co-overexpressed in K. phaffii and proved their positive effect on the secretion of reporter enzymes (PLA₂ and prolyl endopeptidase) by increasing the production up to 1.41-fold, providing novel helper factors for rational engineering of K. phaffii.

Two chaperones locked in an embrace: structure and function of the ribosome-associated complex RAC

Chaperones, which assist protein folding are essential components of every living cell. The yeast ribosome-associated complex (RAC) is a chaperone that is highly conserved in eukaryotic cells. The RAC consists of the J protein Zuo1 and the unconventional Hsp70 homolog Ssz1. The RAC heterodimer stimulates the ATPase activity of the ribosome-bound Hsp70 homolog Ssb, which interacts with nascent polypeptide chains to facilitate de novo protein folding. In addition, the RAC-Ssb system is required to maintain the fidelity of protein translation. Recent work reveals important details of the unique structures of RAC and Ssb and identifies how the chaperones interact with the ribosome. The new findings start to uncover how the exceptional chaperone triad cooperates in protein folding and maintenance of translational fidelity and its connection to extraribosomal functions.

Substrate binding by the yeast Hsp110 nucleotide exchange factor and molecular chaperone Sse1 is not obligate for its biological activities

The highly conserved heat shock protein 70 (Hsp70) is a ubiquitous molecular chaperone essential for maintaining cellular protein homeostasis. The related protein Hsp110 (Sse1/Sse2 in Saccharomyces cerevisiae) functions as a nucleotide exchange factor (NEF) to regulate the protein folding activity of Hsp70. Hsp110/Sse1 also can prevent protein aggregation in vitro via its substrate-binding domain (SBD), but the cellular roles of this "holdase" activity are poorly defined. We generated and characterized an Sse1 mutant that separates, for the first time, its nucleotide exchange and substrate-binding functions. Sse1_sbd retains nucleotide-binding and nucleotide exchange activities while exhibiting severe deficiencies in chaperone holdase activity for unfolded polypeptides. In contrast, we observed no effect of the SBD mutation in reconstituted disaggregation or refolding reactions in vitro. In vivo, Sse1_sbd successfully heterodimerized with the yeast cytosolic Hsp70s Ssa and Ssb and promoted normal growth, with the exception of sensitivity to prolonged heat but not other proteotoxic stress. Moreover, Sse1_sbd was fully competent to support Hsp90-dependent signaling through heterologously expressed glucocorticoid receptor and degradation of a permanently misfolded protein, two previously defined roles for Sse1. We conclude that despite conservation among eukaryotic homologues, chaperone holdase activity is not an obligate function in the Hsp110 family.

Coordinated Hsp110 and Hsp104 Activities Power Protein Disaggregation in Saccharomyces cerevisiae

Protein aggregation is intimately associated with cellular stress and is accelerated during aging, disease, and cellular dysfunction. Yeast cells rely on the ATP-consuming chaperone Hsp104 to disaggregate proteins together with Hsp70. Hsp110s are ancient and abundant chaperones that form complexes with Hsp70. Here we provide in vivo data showing that the Saccharomyces cerevisiae Hsp110s Sse1 and Sse2 are essential for Hsp104-dependent protein disaggregation. Following heat shock, complexes of Hsp110 and Hsp70 are recruited to protein aggregates and function together with Hsp104 in the disaggregation process. In the absence of Hsp110, targeting of Hsp70 and Hsp104 to the aggregates is impaired, and the residual Hsp104 that still reaches the aggregates fails to disaggregate. Thus, coordinated activities of both Hsp104 and Hsp110 are required to reactivate aggregated proteins. These findings have important implications for the understanding of how eukaryotic cells manage misfolded and amyloid proteins.

A Differentiation Transcription Factor Establishes Muscle-Specific Proteostasis in Caenorhabditis elegans

Safeguarding the proteome is central to the health of the cell. In multi-cellular organisms, the composition of the proteome, and by extension, protein-folding requirements, varies between cells. In agreement, chaperone network composition differs between tissues. Here, we ask how chaperone expression is regulated in a cell type-specific manner and whether cellular differentiation affects chaperone expression. Our bioinformatics analyses show that the myogenic transcription factor HLH-1 (MyoD) can bind to the promoters of chaperone genes expressed or required for the folding of muscle proteins. To test this experimentally, we employed HLH-1 myogenic potential to genetically modulate cellular differentiation of Caenorhabditis elegans embryonic cells by ectopically expressing HLH-1 in all cells of the embryo and monitoring chaperone expression. We found that HLH-1-dependent myogenic conversion specifically induced the expression of putative HLH-1-regulated chaperones in differentiating muscle cells. Moreover, disrupting the putative HLH-1-binding sites on ubiquitously expressed daf-21(Hsp90) and muscle-enriched hsp-12.2(sHsp) promoters abolished their myogenic-dependent expression. Disrupting HLH-1 function in muscle cells reduced the expression of putative HLH-1-regulated chaperones and compromised muscle proteostasis during and after embryogenesis. In turn, we found that modulating the expression of muscle chaperones disrupted the folding and assembly of muscle proteins and thus, myogenesis. Moreover, muscle-specific over-expression of the DNAJB6 homolog DNJ-24, a limb-girdle muscular dystrophy-associated chaperone, disrupted the muscle chaperone network and exposed synthetic motility defects. We propose that cellular differentiation could establish a proteostasis network dedicated to the folding and maintenance of the muscle proteome. Such cell-specific proteostasis networks can explain the selective vulnerability that many diseases of protein misfolding exhibit even when the misfolded protein is ubiquitously expressed.

Molecular chaperones protect proteins from misfolding and aggregation. In multi-cellular organisms, the composition and expression levels of chaperones vary between tissues. However, little is known of how such differential expression is regulated. We hypothesized that the cellular differentiation that regulates the cell-type specific expression program may be involved in establishing a cell-type specific chaperone network. To test this possibility, we addressed the myogenic commitment transcription factor HLH-1 (CeMyoD) that converts embryonic cells to muscle cells in Caenorhabditis elegans. We demonstrated that HLH-1 regulates the expression of muscle chaperones during muscle differentiation. Moreover, we showed that HLH-1-dependent expression of chaperones is required for embryonic development and muscle function. We propose that cellular differentiation results in cell-specific differences in the chaperone network that may be detrimental in terms of the susceptibility of neurons and muscle cells to protein misfolding diseases.

Non-canonical Interactions between Heat Shock Cognate Protein 70 (Hsc70) and Bcl2-associated Anthanogene (BAG) Co-Chaperones Are Important for Client Release

Heat shock cognate protein 70 (Hsc70) regulates protein homeostasis through its reversible interactions with client proteins. Hsc70 has two major domains: a nucleotide-binding domain (NBD), that hydrolyzes ATP, and a substrate-binding domain (SBD), where clients are bound. Members of the BAG family of co-chaperones, including Bag1 and Bag3, are known to accelerate release of both ADP and client from Hsc70. The release of nucleotide is known to be mediated by interactions between the conserved BAG domain and the Hsc70 NBD. However, less is known about the regions required for client release, and it is often assumed that this activity also requires the BAG domain. It is important to better understand this step because it determines how long clients remain in the inactive, bound state. Here, we report the surprising observation that truncated versions of either human Bag1 or Bag3, comprised only the BAG domain, promoted rapid release of nucleotide, but not client, in vitro Rather, we found that a non-canonical interaction between Bag1/3 and the Hsc70 SBD is sufficient for accelerating this step. Moreover, client release did not seem to require the BAG domain or Hsc70 NBD. These results suggest that Bag1 and Bag3 control the stability of the Hsc70-client complex using at least two distinct protein-protein contacts, providing a previously under-appreciated layer of molecular regulation in the human Hsc70 system.

Dual role of ribosome-associated chaperones in prion formation and propagation

Chaperones of the diverse ubiquitous Hsp70 family are involved in the regulation of ordered self-perpetuating protein aggregates (amyloids and prions), implicated in both devastating diseases and protein-based inheritance. Yeast ribosome-associated chaperone complex (RAC), composed of the Hsp40 protein Zuo1 and non-canonical Hsp70 protein Ssz1, mediates association of the Hsp70 chaperone Ssb with translating ribosomes. Ssb participates in co-translational protein folding, regulation of premature translation termination, and ribosome biogenesis. The loss of Ssb or disruption of RAC results in the increased formation of [PSI ⁺], a prion form of the translation termination factor Sup35 (eRF3). This implicates co-translational protein misfolding in de novo prion formation. However, RAC disruption also destabilizes pre-existing [PSI ⁺] prions, as Ssb, released from ribosomes to the cytosol in the absence of RAC, antagonizes the function of the major cytosolic chaperone, Ssa, in prion propagation. The mechanism of the Ssa/Ssb antagonism is currently under investigation and may include a competition for substrates and/or co-chaperones. Notably, yeast cells with wild-type RAC also release Ssb to the cytosol in certain unfavorable growth conditions, and Ssb contributes to increased prion loss in these conditions. This indicates that the circulation of Ssb between the ribosome and cytosol may serve as a physiological regulator of the formation and propagation of self-perpetuating protein aggregates. Indeed, RAC and Ssb modulate toxicity of some aggregating proteins in yeast. Mammalian cells lack the Ssb ortholog but contain a RAC counterpart, apparently recruiting other Hsp70 protein(s). Thus, amyloid modulation by ribosome-associated chaperones could be applicable beyond yeast.

Cytosolic splice isoform of Hsp70 nucleotide exchange factor Fes1 is required for the degradation of misfolded proteins in yeast

Yeast Hsp70 nucleotide exchange factor Fes1 is expressed by rare alternative splicing as two isoforms. Fes1L is targeted to the nucleus, and Fes1S localizes to the cytosol and is required for the efficient proteasomal degradation of cytosolic misfolded proteins, as well as of species that are imported into the nucleus for degradation.

Cells maintain proteostasis by selectively recognizing and targeting misfolded proteins for degradation. In Saccharomyces cerevisiae, the Hsp70 nucleotide exchange factor Fes1 is essential for the degradation of chaperone-associated misfolded proteins by the ubiquitin-proteasome system. Here we show that the FES1 transcript undergoes unique 3′ alternative splicing that results in two equally active isoforms with alternative C-termini, Fes1L and Fes1S. Fes1L is actively targeted to the nucleus and represents the first identified nuclear Hsp70 nucleotide exchange factor. In contrast, Fes1S localizes to the cytosol and is essential to maintain proteostasis. In the absence of Fes1S, the heat-shock response is constitutively induced at normally nonstressful conditions. Moreover, cells display severe growth defects when elevated temperatures, amino acid analogues, or the ectopic expression of misfolded proteins, induce protein misfolding. Importantly, misfolded proteins are not targeted for degradation by the ubiquitin-proteasome system. These observations support the notion that cytosolic Fes1S maintains proteostasis by supporting the removal of toxic misfolded proteins by proteasomal degradation. This study provides key findings for the understanding of the organization of protein quality control mechanisms in the cytosol and nucleus.

Prion aggregate structure in yeast cells is determined by the Hsp104-Hsp110 disaggregase machinery

3D structural analysis of a yeast [PSI+] prion model by correlative fluorescence and electron tomography reveals that prion aggregate structure depends on the levels of Hsp70 chaperones, the protein remodeling ATPase Hsp104, and the Hsp70 nucleotide exchange factor/disaggregase Sse1 (yeast Hsp110).

Prions consist of misfolded proteins that have adopted an infectious amyloid conformation. In vivo, prion biogenesis is intimately associated with the protein quality control machinery. Using electron tomography, we probed the effects of the heat shock protein Hsp70 chaperone system on the structure of a model yeast [PSI+] prion in situ. Individual Hsp70 deletions shift the balance between fibril assembly and disassembly, resulting in a variable shell of nonfibrillar, but still immobile, aggregates at the surface of the [PSI+] prion deposits. Both Hsp104 (an Hsp100 disaggregase) and Sse1 (the major yeast form of Hsp110) were localized to this surface shell of [PSI+] deposits in the deletion mutants. Elevation of Hsp104 expression promoted the appearance of this novel, nonfibrillar form of the prion aggregate. Moreover, Sse1 was found to regulate prion fibril length. Our studies reveal a key role for Sse1 (Hsp110), in cooperation with Hsp104, in regulating the length and assembly state of [PSI+] prion fibrils in vivo.

A Non-enveloped Virus Hijacks Host Disaggregation Machinery to Translocate across the Endoplasmic Reticulum Membrane

Mammalian cytosolic Hsp110 family, in concert with the Hsc70:J-protein complex, functions as a disaggregation machinery to rectify protein misfolding problems. Here we uncover a novel role of this machinery in driving membrane translocation during viral entry. The non-enveloped virus SV40 penetrates the endoplasmic reticulum (ER) membrane to reach the cytosol, a critical infection step. Combining biochemical, cell-based, and imaging approaches, we find that the Hsp110 family member Hsp105 associates with the ER membrane J-protein B14. Here Hsp105 cooperates with Hsc70 and extracts the membrane-penetrating SV40 into the cytosol, potentially by disassembling the membrane-embedded virus. Hence the energy provided by the Hsc70-dependent Hsp105 disaggregation machinery can be harnessed to catalyze a membrane translocation event.

How non-enveloped viruses penetrate a host membrane to enter cells and cause disease remains an enigmatic step. To infect cells, the non-enveloped SV40 must transport across the ER membrane to reach the cytosol. In this study, we report that a cellular Hsp105-powered disaggregation machinery pulls SV40 into the cytosol, likely by uncoating the ER membrane-penetrating virus. Because this disaggregation machinery is thought to clarify cellular aggregated proteins, we propose that the force generated by this machinery can also be hijacked by a non-enveloped virus to propel its entry into the host.

GrpE, Hsp110/Grp170, HspBP1/Sil1 and BAG domain proteins: nucleotide exchange factors for Hsp70 molecular chaperones

Molecular chaperones of the Hsp70 family are key components of the cellular protein folding machinery. Substrate folding is accomplished by iterative cycles of ATP binding, hydrolysis and release. The ATPase activity of Hsp70 is regulated by two main classes of cochaperones: J-domain proteins stimulate ATPase hydrolysis by Hsp70, while nucleotide exchange factors (NEF) facilitate its conversion from the ADP-bound to the ATP-bound state, thus closing the chaperone folding cycle. Beginning with the discovery of the prototypical bacterial NEF GrpE, a large diversity of Hsp70 nucleotide exchange factors has been identified, connecting Hsp70 to a multitude of cellular processes in the eukaryotic cell. Here we review recent advances towards structure and function of nucleotide exchange factors from the Hsp110/Grp170, HspBP1/Sil1 and BAG domain protein families and discuss how these cochaperones connect protein folding with quality control and degradation pathways.

Blocking Hsp70 enhances the efficiency of amphotericin B treatment against resistant Aspergillus terreus strains

The polyene antifungal amphotericin B (AmB) is widely used to treat life-threatening fungal infections. Even though AmB resistance is exceptionally rare in fungi, most Aspergillus terreus isolates exhibit an intrinsic resistance against the drug in vivo and in vitro. Heat shock proteins perform a fundamental protective role against a multitude of stress responses, thereby maintaining protein homeostasis in the organism. In this study, we elucidated the role of heat shock protein 70 (Hsp70) family members and compared resistant and susceptible A. terreus clinical isolates. The upregulation of cytoplasmic Hsp70 members at the transcriptional as well as translational levels was significantly higher with AmB treatment than without AmB treatment, particularly in resistant A. terreus isolates, thereby indicating a role of Hsp70 proteins in the AmB response. We found that Hsp70 inhibitors considerably increased the susceptibility of resistant A. terreus isolates to AmB but exerted little impact on susceptible isolates. Also, in in vivo experiments, using the Galleria mellonella infection model, cotreatment of resistant A. terreus strains with AmB and the Hsp70 inhibitor pifithrin-μ resulted in significantly improved survival compared with that achieved with AmB alone. Our results point to an important mechanism of regulation of AmB resistance by Hsp70 family members in A. terreus and suggest novel drug targets for the treatment of infections caused by resistant fungal isolates.

The nucleotide exchange factors of Hsp70 molecular chaperones

Molecular chaperones of the Hsp70 family form an important hub in the cellular protein folding networks in bacteria and eukaryotes, connecting translation with the downstream machineries of protein folding and degradation. The Hsp70 folding cycle is driven by two types of cochaperones: J-domain proteins stimulate ATP hydrolysis by Hsp70, while nucleotide exchange factors (NEFs) promote replacement of Hsp70-bound ADP with ATP. Bacteria and organelles of bacterial origin have only one known NEF type for Hsp70, GrpE. In contrast, a large diversity of Hsp70 NEFs has been discovered in the eukaryotic cell. These NEFs belong to the Hsp110/Grp170, HspBP1/Sil1, and BAG domain protein families. In this short review we compare the structures and molecular mechanisms of nucleotide exchange factors for Hsp70 and discuss how these cochaperones contribute to protein folding and quality control in the cell.

Molecular chaperones: guardians of the proteome in normal and disease states

Proteins must adopt a defined three-dimensional structure in order to gain functional activity, or must they? An ever-increasing number of intrinsically disordered proteins and amyloid-forming polypeptides challenge this dogma. While molecular chaperones and proteases are traditionally associated with protein quality control inside the cell, it is now apparent that molecular chaperones not only promote protein folding in the "forward" direction by facilitating folding and preventing misfolding and aggregation, but also facilitate protein unfolding and even disaggregation resulting in the recovery of functional protein from aggregates. Here, we review our current understanding of ATP-dependent molecular chaperones that harness the energy of ATP binding and hydrolysis to fuel their chaperone functions. An emerging theme is that most of these chaperones do not work alone, but instead function together with other chaperone systems to maintain the proteome. Hence, molecular chaperones are the major component of the proteostasis network that guards and protects the proteome from damage. Furthermore, while a decline of this network is detrimental to cell and organismal health, a controlled perturbation of the proteostasis network may offer new therapeutic avenues against human diseases.

Sil1, a nucleotide exchange factor for BiP, is not required for antibody assembly or secretion

Sil1 is a nucleotide exchange factor for the ER chaperone BiP, and mutations in this gene lead to Marinesco–Sjögren syndrome, a multisystem disorder. Effects of loss of Sil1 on biosynthesis and secretion of antibodies, a well-characterized BiP client, are determined in a mouse model for this disease and patient-derived lymphoblastoid cell lines.

Sil1 is a nucleotide exchange factor for the endoplasmic reticulum chaperone BiP, and mutations in this gene lead to Marinesco–Sjögren syndrome (MSS), a debilitating autosomal recessive disease characterized by multisystem defects. A mouse model for MSS was previously produced by disrupting Sil1 using gene-trap methodology. The resulting Sil1^Gt mouse phenocopies several pathologies associated with MSS, although its ability to assemble and secrete antibodies, the best-characterized substrate of BiP, has not been investigated. In vivo antigen-specific immunizations and ex vivo LPS stimulation of splenic B cells revealed that the Sil1^Gt mouse was indistinguishable from wild-type age-matched controls in terms of both the kinetics and magnitude of antigen-specific antibody responses. There was no significant accumulation of BiP-associated Ig assembly intermediates or evidence that another molecular chaperone system was used for antibody production in the LPS-stimulated splenic B cells from Sil1^Gt mice. ER chaperones were expressed at the same level in Sil1^WT and Sil1^Gt mice, indicating that there was no evident compensation for the disruption of Sil1. Finally, these results were confirmed and extended in three human EBV-transformed lymphoblastoid cell lines from individuals with MSS, leading us to conclude that the BiP cofactor Sil1 is dispensable for antibody production.

Challenging muscle homeostasis uncovers novel chaperone interactions in Caenorhabditis elegans

Proteome stability is central to cellular function and the lifespan of an organism. This is apparent in muscle cells, where incorrect folding and assembly of the sarcomere contributes to disease and aging. Apart from the myosin-assembly factor UNC-45, the complete network of chaperones involved in assembly and maintenance of muscle tissue is currently unknown. To identify additional factors required for sarcomere quality control, we performed genetic screens based on suppressed or synthetic motility defects in Caenorhabditis elegans. In addition to ethyl methyl sulfonate-based mutagenesis, we employed RNAi-mediated knockdown of candidate chaperones in unc-45 temperature-sensitive mutants and screened for impaired movement at permissive conditions. This approach confirmed the cooperation between UNC-45 and Hsp90. Moreover, the screens identified three novel co-chaperones, CeHop (STI-1), CeAha1 (C01G10.8) and Cep23 (ZC395.10), required for muscle integrity. The specific identification of Hsp90 and Hsp90 co-chaperones highlights the physiological role of Hsp90 in myosin folding. Our work thus provides a clear example of how a combination of mild perturbations to the proteostasis network can uncover specific quality control modules.