The Yeast Hsp110 Sse1 Functionally Interacts with the Hsp70 Chaperones Ssa and Ssb

Sse1, Hsp110 chaperone of yeast, controls the cellular fate during endoplasmic reticulum stress

Sse1 is a cytosolic Hsp110 molecular chaperone of yeast, Saccharomyces cerevisiae. Its multifaceted roles in cellular protein homeostasis as a nucleotide exchange factor (NEF), as a protein-disaggregase and as a chaperone linked to protein synthesis (CLIPS) are well documented. In the current study, we show that SSE1 genetically interacts with IRE1 and HAC1, the endoplasmic reticulum-unfolded protein response (ER-UPR) sensors implicating its role in ER protein homeostasis. Interestingly, the absence of this chaperone imparts unusual resistance to tunicamycin-induced ER stress which depends on the intact Ire1-Hac1 mediated ER-UPR signaling. Furthermore, cells lacking SSE1 show inefficient ER-stress-responsive reorganization of translating ribosomes from polysomes to monosomes that drive uninterrupted protein translation during tunicamycin stress. In consequence, the sse1Δ strain shows prominently faster reversal from ER-UPR activated state indicating quicker restoration of homeostasis, in comparison to the wild-type (WT) cells. Importantly, Sse1 plays a critical role in controlling the ER-stress-mediated cell division arrest, which is escaped in sse1Δ strain during chronic tunicamycin stress. Accordingly, sse1Δ strain shows significantly higher cell viability in comparison to WT yeast imparting the stark fitness following short-term as well as long-term tunicamycin stress. These data, all together, suggest that cytosolic chaperone Sse1 is an important modulator of ER stress response in yeast and it controls stress-induced cell division arrest and cell death during overwhelming ER stress induced by tunicamycin.

A first-in-class inhibitor of Hsp110 molecular chaperones of pathogenic fungi

Proteins of the Hsp110 family are molecular chaperones that play important roles in protein homeostasis in eukaryotes. The pathogenic fungus Candida albicans, which causes infections in humans, has a single Hsp110, termed Msi3. Here, we provide proof-of-principle evidence supporting fungal Hsp110s as targets for the development of new antifungal drugs. We identify a pyrazolo[3,4-b] pyridine derivative, termed HLQ2H (or 2H), that inhibits the biochemical and chaperone activities of Msi3, as well as the growth and viability of C. albicans. Moreover, the fungicidal activity of 2H correlates with its inhibition of in vivo protein folding. We propose 2H and related compounds as promising leads for development of new antifungals and as pharmacological tools for the study of the molecular mechanisms and functions of Hsp110s.

Hsp70 and Hsp110 Chaperones Promote Early Steps of Proteasome Assembly

Whereas assembly of the 20S proteasome core particle (CP) in prokaryotes apparently occurs spontaneously, the efficiency of this process in eukaryotes relies on the dedicated assembly chaperones Ump1, Pba1-Pba2, and Pba3-Pba4. For mammals, it was reported that CP assembly initiates with formation of a complete α-ring that functions as a template for β subunit incorporation. By contrast, we were not able to detect a ring composed only of a complete set of α subunits in S. cerevisiae. Instead, we found that the CP subunits α1, α2, and α4 each form independent small complexes. Purification of such complexes containing α4 revealed the presence of chaperones of the Hsp70/Ssa and Hsp110/Sse families. Consistently, certain small complexes containing α1, α2, and α4 were not formed in strains lacking these chaperones. Deletion of the SSE1 gene in combination with deletions of PRE9 (α3), PBA3, or UMP1 genes resulted in severe synthetic growth defects, high levels of ubiquitin-conjugates, and an accumulation of distinct small complexes with α subunits. Our study shows that Hsp70 and Hsp110 chaperones cooperate to promote the folding of individual α subunits and/or their assembly with other CP subunits, Ump1, and Pba1-Pba4 in subsequent steps.

Interdomain interactions dictate the function of the Candida albicans Hsp110 protein Msi3

Heat shock proteins of 110 kDa (Hsp110s), a unique class of molecular chaperones, are essential for maintaining protein homeostasis. Hsp110s exhibit a strong chaperone activity preventing protein aggregation (the "holdase" activity) and also function as the major nucleotide-exchange factor (NEF) for Hsp70 chaperones. Hsp110s contain two functional domains: a nucleotide-binding domain (NBD) and substrate-binding domain (SBD). ATP binding is essential for Hsp110 function and results in close contacts between the NBD and SBD. However, the molecular mechanism of this ATP-induced allosteric coupling remains poorly defined. In this study, we carried out biochemical analysis on Msi3, the sole Hsp110 in Candida albicans, to dissect the unique allosteric coupling of Hsp110s using three mutations affecting the domain-domain interface. All the mutations abolished both the in vivo and in vitro functions of Msi3. While the ATP-bound state was disrupted in all mutants, only mutation of the NBD-SBDβ interfaces showed significant ATPase activity, suggesting that the full-length Hsp110s have an ATPase that is mainly suppressed by NBD-SBDβ contacts. Moreover, the high-affinity ATP-binding unexpectedly appears to require these NBD-SBD contacts. Remarkably, the "holdase" activity was largely intact for all mutants tested while NEF activity was mostly compromised, although both activities strictly depended on the ATP-bound state, indicating different requirements for these two activities. Stable peptide substrate binding to Msi3 led to dissociation of the NBD-SBD contacts and compromised interactions with Hsp70. Taken together, our data demonstrate that the exceptionally strong NBD-SBD contacts in Hsp110s dictate the unique allosteric coupling and biochemical activities.

Diversity in heat shock protein families: functional implications in virus infection with a comprehensive insight of their role in the HIV-1 life cycle

Heat shock proteins (HSPs) are a group of cellular proteins that are induced during stress conditions such as heat stress, cold shock, UV irradiation and even pathogenic insult. They are classified into families based on molecular size like HSP27, 40, 70 and 90 etc, and many of them act as cellular chaperones that regulate protein folding and determine the fate of mis-folded or unfolded proteins. Studies have also shown multiple other functions of these proteins such as in cell signalling, transcription and immune response. Deregulation of these proteins leads to devastating consequences, such as cancer, Alzheimer's disease and other life threatening diseases suggesting their potential importance in life processes. HSPs exist in multiple isoforms, and their biochemical and functional characterization still remains a subject of active investigation. In case of viral infections, several HSP isoforms have been documented to play important roles with few showing pro-viral activity whereas others seem to have an anti-viral role. Earlier studies have demonstrated that HSP40 plays a pro-viral role whereas HSP70 inhibits HIV-1 replication; however, clear isoform-specific functional roles remain to be established. A detailed functional characterization of all the HSP isoforms will uncover their role in cellular homeostasis and also may highlight some of them as potential targets for therapeutic strategies against various viral infections. In this review, we have tried to comprehend the details about cellular HSPs and their isoforms, their role in cellular physiology and their isoform-specific functions in case of virus infection with a specific focus on HIV-1 biology.

Purification and biochemical characterization of Msi3, an essential Hsp110 molecular chaperone in Candida albicans

Hsp110s are unique and essential molecular chaperones in the eukaryotic cytosol. They play important roles in maintaining cellular protein homeostasis. Candida albicans is the most prevalent yeast opportunistic pathogen that causes fungal infections in humans. As the only Hsp110 in Candida albicans, Msi3 is essential for the growth and infection of Candida albicans. In this study, we have expressed and purified Msi3 in nucleotide-free state and carried out biochemical analyses. Sse1 is the major Hsp110 in budding yeast S. cerevisiae and the best characterized Hsp110. Msi3 can substitute Sse1 in complementing the temperature-sensitive phenotype of S. cerevisiae carrying a deletion of SSE1 gene although Msi3 shares only 63.4% sequence identity with Sse1. Consistent with this functional similarity, the purified Msi3 protein shares many similar biochemical activities with Sse1 including binding ATP with high affinity, changing conformation upon ATP binding, stimulating the nucleotide-exchange for Hsp70, preventing protein aggregation, and assisting Hsp70 in refolding denatured luciferase. These biochemical characterizations suggested that Msi3 can be used as a model for studying the molecular mechanisms of Hsp110s.

Direct involvement of Hsp70 ATP hydrolysis in Ubr1-dependent quality control

Chaperones can mediate both protein folding and degradation. This process is referred to as protein triage, which demands study to reveal mechanisms of quality control for both basic scientific and translational purposes. In yeast, many misfolded proteins undergo chaperone-dependent ubiquitination by the action of the E3 ligases Ubr1 and San1, allowing detailed study of protein triage. In cells, both HSP70 and HSP90 mediated substrate ubiquitination, and the canonical ATP cycle was required for HSP70’s role: we have found that ATP hydrolysis by HSP70, the nucleotide exchange activity of Sse1, and the action of J-proteins are all needed for Ubr1-mediated quality control. To discern whether chaperones were directly involved in Ubr1-mediated ubiquitination, we developed a bead-based assay with covalently immobilized but releasable misfolded protein to obviate possible chaperone effects on substrate physical state or transport. In this in vitro assay, only HSP70 was required, along with its ATPase cycle and relevant cochaperones, for Ubr1-mediated ubiquitination. The requirement for the HSP70 ATP cycle in ubiquitination suggests a possible model of triage in which efficiently folded proteins are spared, while slow-folding or nonfolding proteins are iteratively tagged with ubiquitin for subsequent degradation.

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.

Single-Gene Deletions Contributing to Loss of Heterozygosity in Saccharomyces cerevisiae: Genome-Wide Screens and Reproducibility

Loss of heterozygosity (LOH) is a phenomenon commonly observed in cancers; the loss of chromosomal regions can be both causal and indicative of underlying genome instability. Yeast has long been used as a model organism to study genetic mechanisms difficult to study in mammalian cells. Studying gene deletions leading to increased LOH in yeast aids our understanding of the processes involved, and guides exploration into the etiology of LOH in cancers. Yet, before in-depth mechanistic studies can occur, candidate genes of interest must be identified. Utilizing the heterozygous Saccharomyces cerevisiae deletion collection (≈ 6500 strains), 217 genes whose disruption leads to increased LOH events at the endogenously heterozygous mating type locus were identified. Our investigation to refine this list of genes to candidates with the most definite impact on LOH includes: secondary testing for LOH impact at an additional locus, gene ontology analysis to determine common gene characteristics, and positional gene enrichment studies to identify chromosomal regions important in LOH events. Further, we conducted extensive comparisons of our data to screens with similar, but distinct methodologies, to further distinguish genes that are more likely to be true contributors to instability due to their reproducibility, and not just identified due to the stochastic nature of LOH. Finally, we selected nine candidate genes and quantitatively measured their impact on LOH as a benchmark for the impact of genes identified in our study. Our data add to the existing body of work and strengthen the evidence of single-gene knockdowns contributing to genome instability.

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.

Genome-wide identification of HSP70/110 genes in sea cucumber Apostichopus japonicus and comparative analysis of their involvement in aestivation

HSP70/110s are a subgroup of heat shock proteins and play crucial roles in protein homeostasis. HSP70/110s can enhance cell survival in response to a multitude of stressful stimuli, of which the most studied one is heat stress. To perform a systematic study of HSP70/110s in sea cucumber Apostichopus japonicus, 15 HSP70/110 genes, including 13 HSP70s and two HSP110s, were identified and characterized from the transcriptome and genome of sea cucumber. Moderate expansion and conserved structure were found by the phylogenetic and syntenic analysis. Differential expression patterns of HSP70/110s were observed in adult individuals during aestivation, with the comparison of juvenile individuals without aestivation in chronic heat stress. Tissue-specific expression profiles were found both in adult and juvenile individuals, which might indicate that the functional tissues (intestine and respiratory tree) could be restored to normal physiological activity prior to protecting and sporting tissues (body wall and muscle). Differential expression profiles were also observed between the adult and juvenile individuals, which was mainly due to the hypometabolism in aestivation. Taken together, tissue-specific pattern and individual-specific pattern were observed in the HSP70/110 expression profiles in sea cucumber during aestivation. These findings could provide early insight into the involvement of HSP70/110s in the aestivation of marine invertebrate.

Endogenous epitope tagging of heat shock protein 70 isoform Hsc70 using CRISPR/Cas9

Heat shock protein 70 (Hsp70) is an evolutionarily well-conserved molecular chaperone involved in several cellular processes such as folding of proteins, modulating protein-protein interactions, and transport of proteins across the membrane. Binding partners of Hsp70 (known as "clients") are identified on an individual basis as researchers discover their particular protein of interest binds to Hsp70. A full complement of Hsp70 interactors under multiple stress conditions remains to be determined. A promising approach to characterizing the Hsp70 "interactome" is the use of protein epitope tagging and then affinity purification followed by mass spectrometry (AP-MS/MS). AP-MS analysis is a widely used method to decipher protein-protein interaction networks and identifying protein functions. Conventionally, the proteins are overexpressed ectopically which interferes with protein complex stoichiometry, skewing AP-MS/MS data. In an attempt to solve this issue, we used CRISPR/Cas9-mediated gene editing to integrate a tandem-affinity (TAP) epitope tag into the genomic locus of HSC70. This system offers several benefits over existing expression systems including native expression, no requirement for selection, and homogeneity between cells. This cell line, freely available to chaperone researchers, will aid in small and large-scale protein interaction studies as well as the study of biochemical activities and structure-function relationships of the Hsc70 protein.

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.

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.

Profiling Ssb-Nascent Chain Interactions Reveals Principles of Hsp70-Assisted Folding

The yeast Hsp70 chaperone Ssb interacts with ribosomes and nascent polypeptides to assist protein folding. To reveal its working principle, we determined the nascent chain-binding pattern of Ssb at near-residue resolution by in vivo selective ribosome profiling. Ssb associates broadly with cytosolic, nuclear, and hitherto unknown substrate classes of mitochondrial and endoplasmic reticulum (ER) nascent proteins, supporting its general chaperone function. Ssb engages most substrates by multiple binding-release cycles to a degenerate sequence enriched in positively charged and aromatic amino acids. Timely association with this motif upon emergence at the ribosomal tunnel exit requires ribosome-associated complex (RAC) but not nascent polypeptide-associated complex (NAC). Ribosome footprint densities along orfs reveal faster translation at times of Ssb binding, mainly imposed by biases in mRNA secondary structure, codon usage, and Ssb action. Ssb thus employs substrate-tailored dynamic nascent chain associations to coordinate co-translational protein folding, facilitate accelerated translation, and support membrane targeting of organellar proteins.

A toxic imbalance of Hsp70s in Saccharomyces cerevisiae is caused by competition for cofactors

Molecular chaperones are responsible for managing protein folding from translation through degradation. These crucial machines ensure that protein homeostasis is optimally maintained for cell health. However, 'too much of a good thing' can be deadly, and the excess of chaperones can be toxic under certain cellular conditions. For example, overexpression of Ssa1, a yeast Hsp70, is toxic to cells in folding-challenged states such as [PSI+]. We discovered that overexpression of the nucleotide exchange factor Sse1 can partially alleviate this toxicity. We further argue that the basis of the toxicity is related to the availability of Hsp70 cofactors, such as Hsp40 J-proteins and nucleotide exchange factors. Ultimately, our work informs future studies about functional chaperone balance and cautions against therapeutic chaperone modifications without a thorough examination of cofactor relationships.

ER stress causes widespread protein aggregation and prion formation

ER stress results in widespread aggregation of proteins that are not localized to the ER or are part of the secretory system. Hamdan et al. demonstrate that amorphous and amyloidogenic protein aggregation is an indirect consequence of perturbing ER homeostasis.

Disturbances in endoplasmic reticulum (ER) homeostasis create a condition termed ER stress. This activates the unfolded protein response (UPR), which alters the expression of many genes involved in ER quality control. We show here that ER stress causes the aggregation of proteins, most of which are not ER or secretory pathway proteins. Proteomic analysis of the aggregated proteins revealed enrichment for intrinsically aggregation-prone proteins rather than proteins which are affected in a stress-specific manner. Aggregation does not arise because of overwhelming proteasome-mediated degradation but because of a general disruption of cellular protein homeostasis. We further show that overexpression of certain chaperones abrogates protein aggregation and protects a UPR mutant against ER stress conditions. The onset of ER stress is known to correlate with various disease processes, and our data indicate that widespread amorphous and amyloid protein aggregation is an unanticipated outcome of such stress.

How and why do toxic conformers of aberrant proteins accumulate during ageing?

Ageing can be defined as a gradual decline in cellular and physical functions accompanied by an increased sensitivity to the environment and risk of death. The increased risk of mortality is causally connected to a gradual, intracellular accumulation of so-called ageing factors, of which damaged and aggregated proteins are believed to be one. Such aggregated proteins also contribute to several age-related neurodegenerative disorders e.g. Alzheimer’s, Parkinson’s, and Huntington’s diseases, highlighting the importance of protein quality control (PQC) in ageing and its associated diseases. PQC consists of two interrelated systems: the temporal control system aimed at refolding, repairing, and/or removing aberrant proteins and their aggregates and the spatial control system aimed at harnessing the potential toxicity of aberrant proteins by sequestering them at specific cellular locations. The accumulation of toxic conformers of aberrant proteins during ageing is often declared to be a consequence of an incapacitated temporal PQC system—i.e. a gradual decline in the activity of chaperones and proteases. Here, we review the current knowledge on PQC in relation to ageing and highlight that the breakdown of both temporal and spatial PQC may contribute to ageing and thus comprise potential targets for therapeutic interventions of the ageing process.

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.

Proteomic response to linoleic acid hydroperoxide in Saccharomyces cerevisiae

Yeast AP-1 transcription factor (Yap1p) and the enigmatic oxidoreductases Oye2p and Oye3p are involved in counteracting lipid oxidants and their unsaturated breakdown products. In order to uncover the response to linoleic acid hydroperoxide (LoaOOH) and the roles of Oye2p, Oye3p and Yap1p, we carried out proteomic analysis of the homozygous deletion mutants oye3Δ, oye2Δ and yap1Δ alongside the diploid parent strain BY4743. The findings demonstrate that deletion of YAP1 narrowed the response to LoaOOH, as the number of proteins differentially expressed in yap1Δ was 70% of that observed in BY4743. The role of Yap1p in regulating the major yeast peroxiredoxin Tsa1p was demonstrated by the decreased expression of Tsa1p in yap1Δ. The levels of Ahp1p and Hsp31p, previously shown to be regulated by Yap1p, were increased in LoaOOH-treated yap1Δ, indicating their expression is also regulated by another transcription factor(s). Relative to BY4743, protein expression differed in oye3Δ and oye2Δ under LoaOOH, underscored by superoxide dismutase (Sod1p), multiple heat shock proteins (Hsp60p, Ssa1p, and Sse1p), the flavodoxin-like protein Pst2p and the actin stabiliser tropomyosin (Tpm1p). Proteins associated with glycolysis were increased in all strains following treatment with LoaOOH. Together, the dataset reveals, for the first time, the yeast proteomic response to LoaOOH, highlighting the significance of carbohydrate metabolism, as well as distinction between the roles of Oye3p, Oye2p and Yap1p.

Roles of Hsp70s in Stress Responses of Microorganisms, Plants, and Animals

Hsp70s (heat shock protein 70s) are a class of molecular chaperones that are highly conserved and ubiquitous in organisms ranging from microorganisms to plants and humans. Most research on Hsp70s has focused on the mechanisms of their functions as molecular chaperones, but recently, studies on stress responses are coming to the forefront. Hsp70s play key roles in cellular development and protecting living organisms from environmental stresses such as heat, drought, salinity, acidity, and cold. Moreover, functions of human Hsp70s are related to diseases including neurological disorders, cancer, and virus infection. In this review, we provide an overview of the specific roles of Hsp70s in response to stress, particularly abiotic stress, in all living organisms.

N-Myristoylation of the Rpt2 subunit of the yeast 26S proteasome is implicated in the subcellular compartment-specific protein quality control system

Ubiquitination is the posttranslational modification of a protein by covalent attachment of ubiquitin. Controlled proteolysis via the ubiquitin-proteasome system (\UPS) alleviates cellular stress by clearing misfolded proteins. In budding yeast, UPS within the nucleus degrades the nuclear proteins as well as proteins imported from the cytoplasm. While the predominantly nuclear localization of the yeast proteasome is maintained by the importin-mediated transport, N-myristoylation of the proteasome subunit Rpt2 was indicated to cause dynamic nucleo-cytoplasmic localization of proteasomes. Here, we quantitatively analyzed the ubiquitinated peptides using anti-K-ε-GG antibody in yeast cell lines with or without a mutation in the N-myristoylation site of Rpt2 and detected upregulated ubiquitination of proteins with nucleo-cytoplasmic localizations in the mutant strains. Moreover, both the protein and ubiquitinated peptide levels of two Hsp70 family chaperones involved in the nuclear import of misfolded proteins, Ssa and Sse1, were elevated in the mutant strains, whereas levels of an Hsp70 family chaperone involved in the nuclear export, Ssb, were reduced. Taken together, our results indicate that N-myristoylation of Rpt2 is involved in controlled proteolysis via regulation of the nucleo-cytoplasmic localization of the yeast proteasome.

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.

Hsp70 clears misfolded kinases that partitioned into distinct quality-control compartments

Hsp70 facilitates maturation of newly synthesized kinases and assists degradation of kinases under normal and stressed conditions. Hsp70 degrades misfolded kinases that partition into different quality-control compartments by promoting their ubiquitination, thus protecting cells from proteotoxic stress.

Hsp70 aids in protein folding and directs misfolded proteins to the cellular degradation machinery. We describe discrete roles of Hsp70,SSA1 as an important quality-control machinery that switches functions to ameliorate the cellular environment. SSA1 facilitates folding/maturation of newly synthesized protein kinases by aiding their phosphorylation process and also stimulates ubiquitylation and degradation of kinases in regular protein turnover or during stress when kinases are denatured or improperly folded. Significantly, while kinases accumulate as insoluble inclusions upon SSA1 inhibition, they form soluble inclusions upon Hsp90 inhibition or stress foci during heat stress. This suggests formation of inclusion-specific quality-control compartments under various stress conditions. Up-regulation of SSA1 results in complete removal of these inclusions by the proteasome. Elevation of the cellular SSA1 level accelerates kinase turnover and protects cells from proteotoxic stress. Upon overexpression, SSA1 targets heat-denatured kinases toward degradation, which could enable them to recover their functional state under physiological conditions. Thus active participation of SSA1 in the degradation of misfolded proteins establishes an essential role of Hsp70 in deciding client fate during stress.

BiP and its nucleotide exchange factors Grp170 and Sil1: mechanisms of action and biological functions

BiP (immunoglobulin heavy-chain binding protein) is the endoplasmic reticulum (ER) orthologue of the Hsp70 family of molecular chaperones and is intricately involved in most functions of this organelle through its interactions with a variety of substrates and regulatory proteins. Like all Hsp70 family members, the ability of BiP to bind and release unfolded proteins is tightly regulated by a cycle of ATP binding, hydrolysis, and nucleotide exchange. As a characteristic of the Hsp70 family, multiple DnaJ-like co-factors can target substrates to BiP and stimulate its ATPase activity to stabilize the binding of BiP to substrates. However, only in the past decade have nucleotide exchange factors for BiP been identified, which has shed light not only on the mechanism of BiP-assisted folding in the ER but also on Hsp70 family members that reside throughout the cell. We will review the current understanding of the ATPase cycle of BiP in the unique environment of the ER and how it is regulated by the nucleotide exchange factors, Grp170 (glucose-regulated protein of 170kDa) and Sil1, both of which perform unanticipated roles in various biological functions and disease states.

Quantitative proteomics of the yeast Hsp70/Hsp90 interactomes during DNA damage reveal chaperone-dependent regulation of ribonucleotide reductase

The highly conserved molecular chaperones Hsp90 and Hsp70 are indispensible for folding and maturation of a significant fraction of the proteome, including many proteins involved in signal transduction and stress response. To examine the dynamics of chaperone-client interactions after DNA damage, we applied quantitative affinity-purification mass spectrometry (AP-MS) proteomics to characterize interactomes of the yeast Hsp70 isoform Ssa1 and Hsp90 isoform Hsp82 before and after exposure to methyl methanesulfonate. Of 256 proteins identified and quantified via (16)O(/18)O labeling and LC-MS/MS, 142 are novel Hsp70/90 interactors. Nearly all interactions remained unchanged or decreased after DNA damage, but 5 proteins increased interactions with Ssa1 and/or Hsp82, including the ribonucleotide reductase (RNR) subunit Rnr4. Inhibiting Hsp70 or 90 chaperone activity destabilized Rnr4 in yeast and its vertebrate homolog hRMM2 in breast cancer cells. In turn, pre-treatment of cancer cells with chaperone inhibitors sensitized cells to the RNR inhibitor gemcitabine, suggesting a novel chemotherapy strategy. All MS data have been deposited in the ProteomeXchange with identifier PXD001284.

BIOLOGICAL SIGNIFICANCE

This study provides the dynamic interactome of the yeast Hsp70 and Hsp90 under DNA damage which suggest key roles for the chaperones in a variety of signaling cascades. Importantly, the cancer drug target ribonucleotide reductase was shown to be a client of Hsp70 and Hsp90 in both yeast and breast cancer cells. As such, this study highlights the potential of a novel cancer therapeutic strategy that exploits the synergy of chaperone and ribonucleotide reductase inhibitors.

Structural mechanisms of chaperone mediated protein disaggregation

The ClpB/Hsp104 and Hsp70 classes of molecular chaperones use ATP hydrolysis to dissociate protein aggregates and complexes, and to move proteins through membranes. ClpB/Hsp104 are members of the AAA+ family of proteins which form ring-shaped hexamers. Loops lining the pore in the ring engage substrate proteins as extended polypeptides. Interdomain rotations and conformational changes in these loops coupled to ATP hydrolysis unfold and pull proteins through the pore. This provides a mechanism that progressively disrupts local secondary and tertiary structure in substrates, allowing these chaperones to dissociate stable aggregates such as β-sheet rich prions or coiled coil SNARE complexes. While the ClpB/Hsp104 mechanism appears to embody a true power-stroke in which an ATP powered conformational change in one protein is directly coupled to movement or structural change in another, the mechanism of force generation by Hsp70s is distinct and less well understood. Both active power-stroke and purely passive mechanisms in which Hsp70 captures spontaneous fluctuations in a substrate have been proposed, while a third proposed mechanism-entropic pulling-may be able to generate forces larger than seen in ATP-driven molecular motors without the conformational coupling required for a power-stroke. The disaggregase activity of these chaperones is required for thermotolerance, but unrestrained protein complex/aggregate dissociation is potentially detrimental. Disaggregating chaperones are strongly auto-repressed, and are regulated by co-chaperones which recruit them to protein substrates and activate the disaggregases via mechanisms involving either sequential transfer of substrate from one chaperone to another and/or simultaneous interaction of substrate with multiple chaperones. By effectively subjecting substrates to multiple levels of selection by multiple chaperones, this may insure that these potent disaggregases are only activated in the appropriate context.

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

Heat shock protein 70 (Hsp70) molecular chaperones play critical roles in protein homeostasis. In the budding yeast Saccharomyces cerevisiae, cytosolic Hsp70 interacts with up to three types of nucleotide exchange factors (NEFs) homologous to human counterparts: Sse1/Sse2 (Heat shock protein 110 (Hsp110)), Fes1 (HspBP1), and Snl1 (Bag-1). All three NEFs stimulate ADP release; however, it is unclear why multiple distinct families have been maintained throughout eukaryotic evolution. In this study we investigate NEF roles in Hsp70 cell biology using an isogenic combinatorial collection of NEF deletion mutants. Utilizing well characterized model substrates, we find that Sse1 participates in most Hsp70-mediated processes and is of particular importance in protein biogenesis and degradation, whereas Fes1 contributes to a minimal extent. Surprisingly, disaggregation and resolubilization of thermally denatured firefly luciferase occurred independently of NEF activity. Simultaneous deletion of SSE1 and FES1 resulted in constitutive activation of heat shock protein expression mediated by the transcription factor Hsf1, suggesting that these two factors are important for modulating stress response. Fes1 was found to interact in vivo preferentially with the Ssa family of cytosolic Hsp70 and not the co-translational Ssb homolog, consistent with the lack of cold sensitivity and protein biogenesis phenotypes for fes1Δ cells. No significant consequence could be attributed to deletion of the minor Hsp110 SSE2 or the Bag homolog SNL1. Together, these lines of investigation provide a comparative analysis of NEF function in yeast that implies Hsp110 is the principal NEF for cytosolic Hsp70, making it an ideal candidate for therapeutic intervention in human protein folding disorders.

Global transcript and phenotypic analysis of yeast cells expressing Ssa1, Ssa2, Ssa3 or Ssa4 as sole source of cytosolic Hsp70-Ssa chaperone activity

Background

Cytosolic Hsp70 is a ubiquitous molecular chaperone that is involved in responding to a variety of cellular stresses. A major function of Hsp70 is to prevent the aggregation of denatured proteins by binding to exposed hydrophobic regions and preventing the accumulation of amorphous aggregates. To gain further insight into the functional redundancy and specialisation of the highly homologous yeast Hsp70-Ssa family we expressed each of the individual Ssa proteins as the sole source of Hsp70 in the cell and assessed phenotypic differences in prion propagation and stress resistance. Additionally we also analysed the global gene expression patterns in yeast strains expressing individual Ssa proteins, using microarray and RT-qPCR analysis.

Results

We confirm and extend previous studies demonstrating that cells expressing different Hsp70-Ssa isoforms vary in their ability to propagate the yeast [PSI⁺] prion, with Ssa3 being the most proficient. Of the four Ssa family members the heat inducible isoforms are more proficient in acquiring thermotolerance and we show a greater requirement than was previously thought, for cellular processes in addition to the traditional Hsp104 protein disaggregase machinery, in acquiring such thermotolerance. Cells expressing different Hsp70-Ssa isoforms also display differences in phenotypic response to exposure to cell wall damaging and oxidative stress agents, again with the heat inducible isoforms providing better protection than constitutive isoforms. We assessed global transcriptome profiles for cells expressing individual Hsp70-Ssa isoforms as the sole source of cytosolic Hsp70, and identified a significant difference in cellular gene expression between these strains. Differences in gene expression profiles provide a rationale for some phenotypic differences we observed in this study. We also demonstrate a high degree of correlation between microarray data and RT-qPCR analysis for a selection of genes.

Conclusions

The Hsp70-Ssa family provide both redundant and variant-specific functions within the yeast cell. Yeast cells expressing individual members of the Hsp70-Ssa family as the sole source of Ssa protein display differences in global gene expression profiles. These changes in global gene expression may contribute significantly to the phenotypic differences observed between the Hsp70-Ssa family members.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-194) contains supplementary material, which is available to authorized users.

The Hsp70/90 cochaperone, Sti1, suppresses proteotoxicity by regulating spatial quality control of amyloid-like proteins

Escape of aberrant proteins from protein quality control leads to accumulation of toxic protein species. Sti1 interacts with Hsp70 to mediate spatial PQC of amyloid-like proteins by regulating their distribution in different intracellular protein-handling depots. Sti1 suppresses proteotoxicity by targeting amyloid-like proteins to perinuclear foci.

Conformational diseases are associated with the conversion of normal proteins into aggregation-prone toxic conformers with structures similar to that of β-amyloid. Spatial distribution of amyloid-like proteins into intracellular quality control centers can be beneficial, but cellular mechanisms for protective aggregation remain unclear. We used a high-copy suppressor screen in yeast to identify roles for the Hsp70 system in spatial organization of toxic polyglutamine-expanded Huntingtin (Huntingtin with 103Q glutamine stretch [Htt103Q]) into benign assemblies. Under toxic conditions, Htt103Q accumulates in unassembled states and speckled cytosolic foci. Subtle modulation of Sti1 activity reciprocally affects Htt toxicity and the packaging of Htt103Q into foci. Loss of Sti1 exacerbates Htt toxicity and hinders foci formation, whereas elevation of Sti1 suppresses Htt toxicity while organizing small Htt103Q foci into larger assemblies. Sti1 also suppresses cytotoxicity of the glutamine-rich yeast prion [RNQ+] while reorganizing speckled Rnq1–monomeric red fluorescent protein into distinct foci. Sti1-inducible foci are perinuclear and contain proteins that are bound by the amyloid indicator dye thioflavin-T. Sti1 is an Hsp70 cochaperone that regulates the spatial organization of amyloid-like proteins in the cytosol and thereby buffers proteotoxicity caused by amyloid-like proteins.

Suppression of polyglutamine protein toxicity by co-expression of a heat-shock protein 40 and a heat-shock protein 110

A network of heat-shock proteins mediates cellular protein homeostasis, and has a fundamental role in preventing aggregation-associated neurodegenerative diseases. In a Drosophila model of polyglutamine (polyQ) disease, the HSP40 family protein, DNAJ-1, is a superior suppressor of toxicity caused by the aggregation of polyQ containing proteins. Here, we demonstrate that one specific HSP110 protein, 70 kDa heat-shock cognate protein cb (HSC70cb), interacts physically and genetically with DNAJ-1 in vivo, and that HSC70cb is necessary for DNAJ-1 to suppress polyglutamine-induced cell death in Drosophila. Expression of HSC70cb together with DNAJ-1 significantly enhanced the suppressive effects of DNAJ-1 on polyQ-induced neurodegeneration, whereas expression of HSC70cb alone did not suppress neurodegeneration in Drosophila models of either general polyQ disease or Huntington's disease. Furthermore, expression of a human HSP40, DNAJB1, together with a human HSP110, APG-1, protected cells from polyQ-induced neural degeneration in flies, whereas expression of either component alone had little effect. Our data provide a functional link between HSP40 and HSP110 in suppressing the cytotoxicity of aggregation-prone proteins, and suggest that HSP40 and HSP110 function together in protein homeostasis control.

Mutational analysis of Sse1 (Hsp110) suggests an integral role for this chaperone in yeast prion propagation in vivo

The yeast Hsp110 chaperone Sse1 is a conserved protein that is a noncanonical member of the Hsp70 protein superfamily. Sse1 influences the cellular response to heat stress and has also been implicated in playing a role in the propagation of prions in yeast. Sse1 can seemingly exert its effects in vivo through direct or indirect actions by influencing the nucleotide exchange activity of canonical cytosolic Hsp70s. Using a genetic screen based on the inability to propagate the yeast [PSI(+)] prion, we have identified 13 new Sse1 mutants that are predicted to alter chaperone function through a variety of different mechanisms. Not only are these new Sse1 mutants altered in the ability to propagate and cure yeast prions but also to varying degrees in the ability to grow at elevated temperatures. The expression levels of chaperone proteins known to influence yeast prion propagation are unaltered in the Sse1 mutants, suggesting that the observed phenotypic effects are caused by direct functional alterations in these mutants. Mapping the location of the mutants onto the Sse1 crystal structure suggests that more than one functional alteration in Sse1 may result in changes in prion propagation and ability to function at elevated temperatures. All Sse1 mutants isolated provide essential functions in the cell under normal growth conditions, further demonstrating that essential chaperone functions in vivo can to some degree at least be detached from those related to propagation of prions. Our results suggest that Sse1 can influence prion propagation through a variety of different mechanisms.

High molecular weight stress proteins: Identification, cloning and utilisation in cancer immunotherapy

Although the large stress/heat shock proteins (HSPs), i.e. Hsp110 and Grp170, were identified over 30 years ago, these abundant and highly conserved molecules have received much less attention compared to other conventional HSPs. Large stress proteins act as molecular chaperones with exceptional protein-holding capability and prevent the aggregation of proteins induced by thermal stress. The chaperoning properties of Hsp110 and Grp170 are integral to the ability of these molecules to modulate immune functions and are essential for developing large chaperone complex vaccines for cancer immunotherapy. The potent anti-tumour activity of the Hsp110/Grp170-tumour protein antigen complexes demonstrated in preclinical studies has led to a phase I clinical trial through the National Cancer Institute's rapid access to intervention development (RAID) programme that is presently underway. Here we review aspects of the structure and function of these large stress proteins, their roles as molecular chaperones in the biology of cell stress, and prospects for their use in immune regulation and cancer immunotherapy. Lastly, we will discuss the recently revealed immunosuppressive activity of scavenger receptor A that binds to Hsp110 and Grp170, as well as the feasibility of targeting this receptor to promote T-cell activation and anti-tumour immunity induced by large HSP vaccines and other immunotherapies.

The ribosome as a hub for protein quality control

Cells face a constant challenge as they produce new proteins. The newly synthesized polypeptides must be folded properly to avoid aggregation. If proteins do misfold, they must be cleared to maintain a functional and healthy proteome. Recent work is revealing the complex mechanisms that work cotranslationally to ensure protein quality control during biogenesis at the ribosome. Indeed, the ribosome is emerging as a central hub in coordinating these processes, particularly in sensing the nature of the nascent protein chain, recruiting protein folding and translocation components, and integrating mRNA and nascent chain quality control. The tiered and complementary nature of these decision-making processes confers robustness and fidelity to protein homeostasis during protein synthesis.

Hsp110 is required for spindle length control

Systematic affinity purification combined with mass spectrometry analysis of N- and C-tagged cytoplasmic Hsp70/Hsp110 chaperones was used to identify new roles of Hsp70/Hsp110 in the cell. This allowed the mapping of a chaperone-protein network consisting of 1,227 unique interactions between the 9 chaperones and 473 proteins and highlighted roles for Hsp70/Hsp110 in 14 broad biological processes. Using this information, we uncovered an essential role for Hsp110 in spindle assembly and, more specifically, in modulating the activity of the widely conserved kinesin-5 motor Cin8. The role of Hsp110 Sse1 as a nucleotide exchange factor for the Hsp70 chaperones Ssa1/Ssa2 was found to be required for maintaining the proper distribution of kinesin-5 motors within the spindle, which was subsequently required for bipolar spindle assembly in S phase. These data suggest a model whereby the Hsp70-Hsp110 chaperone complex antagonizes Cin8 plus-end motility and prevents premature spindle elongation in S phase.

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

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

The yeast Hsp70 Ssa1 is a sensor for activation of the heat shock response by thiol-reactive compounds

Diverse thiol-reactive compounds are found to activate the Hsf1-regulated heat shock response in Saccharomyces cerevisiae. The highly conserved cytosolic Hsp70 protein chaperone is shown to act as a sensor for these molecules through a pair of reactive cysteine residues in the nucleotide-binding domain.

The heat shock transcription factor HSF1 governs the response to heat shock, oxidative stresses, and xenobiotics through unknown mechanisms. We demonstrate that diverse thiol-reactive molecules potently activate budding yeast Hsf1. Hsf1 activation by thiol-reactive compounds is not consistent with the stresses of misfolding of cytoplasmic proteins or cytotoxicity. Instead, we demonstrate that the Hsp70 chaperone Ssa1, which represses Hsf1 in the absence of stress, is hypersensitive to modification by a thiol-reactive probe. Strikingly, mutation of two conserved cysteine residues to serine in Ssa1 rendered cells insensitive to Hsf1 activation and subsequently induced thermotolerance by thiol-reactive compounds, but not by heat shock. Conversely, substitution with the sulfinic acid mimic aspartic acid resulted in constitutive Hsf1 activation. Cysteine 303, located within the nucleotide-binding domain, was found to be modified in vivo by a model organic electrophile, demonstrating that Ssa1 is a direct target for thiol-reactive molecules through adduct formation. These findings demonstrate that Hsp70 is a proximal sensor for Hsf1-mediated cytoprotection and can discriminate between two distinct environmental stressors.

A lysine-rich region within fungal BAG domain-containing proteins mediates a novel association with ribosomes

Heat shock protein 70 (Hsp70) is a highly conserved molecular chaperone that assists in the folding of nascent chains and the repair of unfolded proteins through iterative cycles of ATP binding, hydrolysis, and nucleotide exchange tightly coupled to polypeptide binding and release. Cochaperones, including nucleotide exchange factors (NEFs), modulate the rate of ADP/ATP exchange and serve to recruit Hsp70 to distinct processes or locations. Among three nonrelated cytosolic NEFs in Saccharomyces cerevisiae, the Bag-1 homolog SNL1 is unique in being tethered to the endoplasmic reticulum (ER) membrane. We demonstrate here a novel physical association between Snl1 and the intact ribosome. This interaction is both independent of and concurrent with binding to Hsp70 and is not dependent on membrane localization. The ribosome binding site is identified as a short lysine-rich motif within the amino terminus of the Snl1 BAG domain distinct from the Hsp70 interaction region. Additionally, we demonstrate a ribosome association with the Candida albicans Snl1 homolog and localize this putative NEF to a perinuclear/ER membrane, suggesting functional conservation in fungal BAG domain-containing proteins. We therefore propose that the Snl1 family of NEFs serves a previously unknown role in fungal protein biogenesis based on the coincident recruitment of ribosomes and Hsp70 to the ER membrane.

Human heat shock protein 105/110 kDa (Hsp105/110) regulates biogenesis and quality control of misfolded cystic fibrosis transmembrane conductance regulator at multiple levels

Heat shock protein 105/110-kDa (Hsp105/110), a member of the Hsp70 super family of molecular chaperones, serves as a nucleotide exchange factor for Hsc70, independently prevents the aggregation of misfolded proteins, and functionally relates to Hsp90. We investigated the roles of human Hsp105α, the constitutively expressed isoform, in the biogenesis and quality control of the cystic fibrosis transmembrane conductance regulator (CFTR). In the endoplasmic reticulum (ER), Hsp105 facilitates CFTR quality control at an early stage in its biosynthesis but promotes CFTR post-translational folding. Deletion of Phe-508 (ΔF508), the most prevalent mutation causing cystic fibrosis, interferes with de novo folding of CFTR, impairing its export from the ER and accelerating its clearance in the ER and post-Golgi compartments. We show that Hsp105 preferentially associates with and stabilizes ΔF508 CFTR at both levels. Introduction of the Hsp105 substrate binding domain potently increases the steady state level of ΔF508 CFTR by reducing its early-stage degradation. This in turn dramatically enhances ΔF508 CFTR cell surface functional expression in cystic fibrosis airway epithelial cells. Although other Hsc70 nucleotide exchange factors such as HspBP1 and BAG-2 inhibit CFTR post-translational degradation in the ER through cochaperone CHIP, Hsp105 has a primary role promoting CFTR quality control at an earlier stage. The Hsp105-mediated multilevel regulation of ΔF508 CFTR folding and quality control provides new opportunities to understand how chaperone machinery regulates the homeostasis and functional expression of misfolded proteins in the cell. Future studies in this direction will inform therapeutics development for cystic fibrosis and other protein misfolding diseases.

The architecture of functional modules in the Hsp90 co-chaperone Sti1/Hop

Sti1/Hop is a modular protein required for the transfer of client proteins from the Hsp70 to the Hsp90 chaperone system in eukaryotes. It binds Hsp70 and Hsp90 simultaneously via TPR (tetratricopeptide repeat) domains. Sti1/Hop contains three TPR domains (TPR1, TPR2A and TPR2B) and two domains of unknown structure (DP1 and DP2). We show that TPR2A is the high affinity Hsp90-binding site and TPR1 and TPR2B bind Hsp70 with moderate affinity. The DP domains exhibit highly homologous α-helical folds as determined by NMR. These, and especially DP2, are important for client activation in vivo. The core module of Sti1 for Hsp90 inhibition is the TPR2A-TPR2B segment. In the crystal structure, the two TPR domains are connected via a rigid linker orienting their peptide-binding sites in opposite directions and allowing the simultaneous binding of TPR2A to the Hsp90 C-terminal domain and of TPR2B to Hsp70. Both domains also interact with the Hsp90 middle domain. The accessory TPR1-DP1 module may serve as an Hsp70-client delivery system for the TPR2A-TPR2B-DP2 segment, which is required for client activation in vivo.

The response to heat shock and oxidative stress in Saccharomyces cerevisiae

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

Unique peptide substrate binding properties of 110-kDa heat-shock protein (Hsp110) determine its distinct chaperone activity

The molecular chaperone 70-kDa heat-shock proteins (Hsp70s) play essential roles in maintaining protein homeostasis. Hsp110, an Hsp70 homolog, is highly efficient in preventing protein aggregation but lacks the hallmark folding activity seen in Hsp70s. To understand the mechanistic differences between these two chaperones, we first characterized the distinct peptide substrate binding properties of Hsp110s. In contrast to Hsp70s, Hsp110s prefer aromatic residues in their substrates, and the substrate binding and release exhibit remarkably fast kinetics. Sequence and structure comparison revealed significant differences in the two peptide-binding loops: the length and properties are switched. When we swapped these two loops in an Hsp70, the peptide binding properties of this mutant Hsp70 were converted to Hsp110-like, and more impressively, it functionally behaved like an Hsp110. Thus, the peptide substrate binding properties implemented in the peptide-binding loops may determine the chaperone activity differences between Hsp70s and Hsp110s.

A structural model of the Sgt2 protein and its interactions with chaperones and the Get4/Get5 complex

The insertion of tail-anchored transmembrane (TA) proteins into the appropriate membrane is a post-translational event that requires stabilization of the transmembrane domain and targeting to the proper destination. Sgt2 is a heat-shock protein cognate (HSC) co-chaperone that preferentially binds endoplasmic reticulum-destined TA proteins and directs them to the GET pathway via Get4 and Get5. Here, we present the crystal structure from a fungal Sgt2 homolog of the tetratrico-repeat (TPR) domain and part of the linker that connects to the C-terminal domain. The linker extends into the two-carboxylate clamp of the TPR domain from a symmetry-related molecule mimicking the binding to HSCs. Based on this structure, we provide biochemical evidence that the Sgt2 TPR domain has the ability to directly bind multiple HSC family members. The structure allows us to propose features involved in this lower specificity relative to other TPR containing co-chaperones. We further show that a dimer of Sgt2 binds a single Get5 and use small angle x-ray scattering to characterize the domain arrangement of Sgt2 in solution. These results allow us to present a structural model of the Sgt2-Get4/Get5-HSC complex.

Multiple hsp70 isoforms in the eukaryotic cytosol: mere redundancy or functional specificity?

Hsp70 molecular chaperones play a variety of functions in every organism, cell type and organelle, and their activities have been implicated in a number of human pathologies, ranging from cancer to neurodegenerative diseases. The functions, regulations and structure of Hsp70s were intensively studied for about three decades, yet much still remains to be learned about these essential folding enzymes. Genome sequencing efforts revealed that most genomes contain multiple members of the Hsp70 family, some of which co-exist in the same cellular compartment. For example, the human cytosol and nucleus contain six highly homologous Hsp70 proteins while the yeast Saccharomyces cerevisiae contains four canonical Hsp70s and three fungal-specific ribosome-associated and specialized Hsp70s. The reasons and significance of the requirement for multiple Hsp70s is still a subject of debate. It has been postulated for a long time that these Hsp70 isoforms are functionally redundant and differ only by their spatio-temporal expression patterns. However, several studies in yeast and higher eukaryotic organisms challenged this widely accepted idea by demonstrating functional specificity among Hsp70 isoforms. Another element of complexity is brought about by specific cofactors, such as Hsp40s or nucleotide exchange factors that modulate the activity of Hsp70s and their binding to client proteins. Hence, a dynamic network of chaperone/co-chaperone interactions has evolved in each organism to efficiently take advantage of the multiple cellular roles Hsp70s can play. We summarize here our current knowledge of the functions and regulations of these molecular chaperones, and shed light on the known functional specificities among isoforms.

Inferring physical protein contacts from large-scale purification data of protein complexes

Recent large-scale data sets of protein complex purifications have provided unprecedented insights into the organization of cellular protein complexes. Several computational methods have been developed to detect co-complexed proteins in these data sets. Their common aim is the identification of biologically relevant protein complexes. However, much less is known about the network of direct physical protein contacts within the detected protein complexes. Therefore, our work investigates whether direct physical contacts can be computationally derived by combining raw data of large-scale protein complex purifications. We assess four established scoring schemes and introduce a new scoring approach that is specifically devised to infer direct physical protein contacts from protein complex purifications. The physical contacts identified by the five methods are comprehensively benchmarked against different reference sets that provide evidence for true physical contacts. Our results show that raw purification data can indeed be exploited to determine high-confidence physical protein contacts within protein complexes. In particular, our new method outperforms competing approaches at discovering physical contacts involving proteins that have been screened multiple times in purification experiments. It also excels in the analysis of recent protein purification screens of molecular chaperones and protein kinases. In contrast to previous findings, we observe that physical contacts inferred from purification experiments of protein complexes can be qualitatively comparable to binary protein interactions measured by experimental high-throughput assays such as yeast two-hybrid. This suggests that computationally derived physical contacts might complement binary protein interaction assays and guide large-scale interactome mapping projects by prioritizing putative physical contacts for further experimental screens.

Bioinformatic approach to identify chaperone pathway relationship from large-scale interaction networks

We describe a computational protocol to identify functional modules and pathway relationship of chaperones based on physical interaction data derived from high-throughput proteomic experiments. The protocol first identifies interacting proteins shared by the different chaperone systems to organize the chaperones into functional modules. The chaperone functional modules represent groups of chaperones that are involved in mediating the folding of the shared interacting proteins. Either the chaperones in a module can function along a single folding pathway of a given substrate protein or the substrate protein might have two or more different folding pathways that the chaperones act on independently. As described in our computational protocol, probabilities of these pathway relationships between two chaperones in a two-component chaperone module can be determined using whole-genome expression and cellular pathways as reference. This protocol is potentially useful for identifying functional modules and pathway relationships in other biological systems that involve multiple proteins with many identified interactions.

Hsp110 chaperones control client fate determination in the hsp70-Hsp90 chaperone system

The Hsp110 family of protein chaperones was known to promote maturation of Hsp90 client proteins. The yeast Hsp110 ortholog Sse1 is now shown to influence the decision to fold or degrade substrates of the Hsp70–Hsp90 chaperone system when maturation is compromised.

Heat shock protein 70 (Hsp70) plays a central role in protein homeostasis and quality control in conjunction with other chaperone machines, including Hsp90. The Hsp110 chaperone Sse1 promotes Hsp90 activity in yeast, and functions as a nucleotide exchange factor (NEF) for cytosolic Hsp70, but the precise roles Sse1 plays in client maturation through the Hsp70–Hsp90 chaperone system are not fully understood. We find that upon pharmacological inhibition of Hsp90, a model protein kinase, Ste11ΔN, is rapidly degraded, whereas heterologously expressed glucocorticoid receptor (GR) remains stable. Hsp70 binding and nucleotide exchange by Sse1 was required for GR maturation and signaling through endogenous Ste11, as well as to promote Ste11ΔN degradation. Overexpression of another functional NEF partially compensated for loss of Sse1, whereas the paralog Sse2 fully restored GR maturation and Ste11ΔN degradation. Sse1 was required for ubiquitinylation of Ste11ΔN upon Hsp90 inhibition, providing a mechanistic explanation for its role in substrate degradation. Sse1/2 copurified with Hsp70 and other proteins comprising the “early-stage” Hsp90 complex, and was absent from “late-stage” Hsp90 complexes characterized by the presence of Sba1/p23. These findings support a model in which Hsp110 chaperones contribute significantly to the decision made by Hsp70 to fold or degrade a client protein.