Showing papers on "Heat shock protein published in 2010"

`The HSP70 chaperone machinery: J proteins as drivers of functional specificity

Abstract: Heat shock 70 kDa proteins (HSP70s) are ubiquitous molecular chaperones that function in a myriad of biological processes, modulating polypeptide folding, degradation and translocation across membranes, and protein-protein interactions. This multitude of roles is not easily reconciled with the universality of the activity of HSP70s in ATP-dependent client protein-binding and release cycles. Much of the functional diversity of the HSP70s is driven by a diverse class of cofactors: J proteins. Often, multiple J proteins function with a single HSP70. Some target HSP70 activity to clients at precise locations in cells and others bind client proteins directly, thereby delivering specific clients to HSP70 and directly determining their fate.

Heat shock factors: integrators of cell stress, development and lifespan

Abstract: Heat shock factors (HSFs) are essential for all organisms to survive exposures to acute stress. They are best known as inducible transcriptional regulators of genes encoding molecular chaperones and other stress proteins. Four members of the HSF family are also important for normal development and lifespan-enhancing pathways, and the repertoire of HSF targets has thus expanded well beyond the heat shock genes. These unexpected observations have uncovered complex layers of post-translational regulation of HSFs that integrate the metabolic state of the cell with stress biology, and in doing so control fundamental aspects of the health of the proteome and ageing.

Heat shock proteins (chaperones) in fish and shellfish and their potential role in relation to fish health: a review.

Abstract: Heat shock proteins (HSPs), also known as stress proteins and extrinsic chaperones, are a suite of highly conserved proteins of varying molecular weight (c. 16-100 kDa) produced in all cellular organisms when they are exposed to stress. They develop following up-regulation of specific genes, whose transcription is mediated by the interaction of heat shock factors with heat shock elements in gene promoter regions. HSPs function as helper molecules or chaperones for all protein and lipid metabolic activities of the cell, and it is now recognized that the up-regulation in response to stress is universal to all cells and not restricted to heat stress. Thus, other stressors such as anoxia, ischaemia, toxins, protein degradation, hypoxia, acidosis and microbial damage will also lead to their up-regulation. They play a fundamental role in the regulation of normal protein synthesis within the cell. HSP families, such as HSP90 and HSP70, are critical to the folding and assembly of other cellular proteins and are also involved in regulation of kinetic partitioning between folding, translocation and aggregation within the cell. HSPs also have a wider role in relation to the function of the immune system, apoptosis and various facets of the inflammatory process. In aquatic animals, they have been shown to play an important role in health, in relation to the host response to environmental pollutants, to food toxins and in particular in the development of inflammation and the specific and non-specific immune responses to bacterial and viral infections in both finfish and shrimp. With the recent development of non-traumatic methods for enhancing HSP levels in fish and shrimp populations via heat, via provision of exogenous HSPs or by oral or water administration of HSP stimulants, they have also, in addition to the health effects, been demonstrated to be valuable in contributing to reducing trauma and physical stress in relation to husbandry events such as transportation and vaccination.

Heat shock protein 70 (hsp70) as an emerging drug target.

Abstract: Heat shock protein 70 (Hsp70) is a molecular chaperone that is expressed in response to stress. In this role, Hsp70 binds to its protein substrates and stabilize them against denaturation or aggregation until conditions improve.1 In addition to its functions during a stress response, Hsp70 has multiple responsibilities during normal growth; it assists in the folding of newly synthesized proteins,2, 3 the subcellular transport of proteins and vesicles,4 the formation and dissociation of complexes,5 and the degradation of unwanted proteins.6, 7 Thus, this chaperone broadly shapes protein homeostasis by controlling protein quality control and turnover during both normal and stress conditions.8 Consistent with these diverse activities, genetic and biochemical studies have implicated it in a range of diseases, including cancer, neurodegeneration, allograft rejection and infection. This review provides a brief review of Hsp70 structure and function and then explores some of the emerging opportunities (and challenges) for drug discovery. Hsp70 is Highly Conserved Members of the Hsp70 family are ubiquitously expressed and highly conserved; for example, the major Hsp70 from Escherichia coli, termed DnaK, is approximately 50% identical to human Hsp70s.9 Eukaryotes often express multiple Hsp70 family members with major isoforms found in all the cellular compartments: Hsp72 (HSPA1A) and heat shock cognate 70 (Hsc70/HSPA8) in the cytosol and nucleus, BiP (Grp78/HSPA5) in the endoplasmic reticulum and mtHsp70 (Grp75/mortalin/HSPA9) in mitochondria. Some of the functions of the cytosolic isoforms, Hsc70 and Hsp72, are thought to be redundant, but the transcription of Hsp72 is highly responsive to stress and Hsc70 is constitutively expressed. In the ER and mitochondria, the Hsp70 family members are thought to fulfill specific functions and have unique substrates, with BiP playing key roles in the folding and quality control of ER proteins and mtHsp70 being involved in the import and export of proteins from the mitochondria. For the purposes of this review, we will often use Hsp70 as a generic term to encompass the shared properties of the family members.

The small heat shock protein B8 (HspB8) promotes autophagic removal of misfolded proteins involved in amyotrophic lateral sclerosis (ALS)

Abstract: Several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), are characterized by the presence of misfolded proteins, thought to trigger neurotoxicity. Some familial forms of ALS (fALS), clinically indistinguishable from sporadic ALS (sALS), are linked to superoxide dismutase 1 (SOD1) gene mutations. It has been shown that the mutant SOD1 misfolds, forms insoluble aggregates and impairs the proteasome. Using transgenic G93A-SOD1 mice, we found that spinal cord motor neurons, accumulating mutant SOD1 also over-express the small heat shock protein HspB8. Using motor neuronal fALS models, we demonstrated that HspB8 decreases aggregation and increases mutant SOD1 solubility and clearance, without affecting wild-type SOD1 turnover. Notably, HspB8 acts on mutant SOD1 even when the proteasome activity is specifically blocked. The pharmacological blockage of autophagy resulted in a dramatic increase of mutant SOD1 aggregates. Immunoprecipitation studies, performed during autophagic flux blockage, demonstrated that mutant SOD1 interacts with the HspB8/Bag3/Hsc70/CHIP multiheteromeric complex, known to selectively activate autophagic removal of misfolded proteins. Thus, HspB8 increases mutant SOD1 clearance via autophagy. Autophagy activation was also observed in lumbar spinal cord of transgenic G93A-SOD1 mice since several autophago-lysosomal structures were present in affected surviving motor neurons. Finally, we extended our observation to a different ALS model and demonstrated that HspB8 exerts similar effects on a truncated version of TDP-43, another protein involved both in fALS and in sALS. Overall, these results indicate that the pharmacological modulation of HspB8 expression in motor neurons may have important implications to unravel the molecular mechanisms involved both in fALS and in sALS.

The proteomic response of the mussel congeners Mytilus galloprovincialis and M. trossulus to acute heat stress: implications for thermal tolerance limits and metabolic costs of thermal stress.

Abstract: The Mediterranean blue mussel, Mytilus galloprovincialis , an invasive species in California, has displaced the more heat-sensitive native congener, Mytilus trossulus , from its former southern range, possibly due to climate change. By comparing the response of their proteomes to acute heat stress we sought to identify responses common to both species as well as differences that account for greater heat tolerance in the invasive. Mussels were acclimated to 13°C for four weeks and exposed to acute heat stress (24°C, 28°C and 32°C) for 1 h and returned to 13°C to recover for 24 h. Using two-dimensional gel electrophoresis and tandem mass spectrometry we identified 47 and 61 distinct proteins that changed abundance in M. galloprovincialis and M. trossulus , respectively. The onset temperatures of greater abundance of some members of the heat shock protein (Hsp) 70 and small Hsp families were lower in M. trossulus . The abundance of proteasome subunits was lower in M. galloprovincialis but greater in M. trossulus in response to heat. Levels of several NADH-metabolizing proteins, possibly linked to the generation of reactive oxygen species (ROS), were lower at 32°C in the cold-adapted M. trossulus whereas proteins generating NADPH, important in ROS defense, were higher in both species. The abundance of oxidative stress proteins was lower at 32°C in M. trossulus only, indicating that its ability to combat heat-induced oxidative stress is limited to lower temperatures. Levels of NAD-dependent deacetylase (sirtuin 5), which are correlated with lifespan, were lower in M. trossulus in response to heat stress. In summary, the expression patterns of proteins involved in molecular chaperoning, proteolysis, energy metabolism, oxidative damage, cytoskeleton and deacetylation revealed a common loci of heat stress in both mussels but also showed a lower sensitivity to high-temperature damage in the warm-adapted M. galloprovincialis , which is consistent with its expanding range in warmer waters. * AAT : aspartate aminotransferase cdc42 : cell division control protein homolog 42 cMDH : cytosolic malate dehydrogenase CoA : coenzyme A EGFR : epidermal growth factor EST : expressed sequence tag ETC : electron transport chain Hsp : heat shock protein IDH : isocitrate dehydrogenase IPG : immobilized pH gradient MAPK : mitogen-activated protein kinase mMDH : mitochondrial malate dehydrogenase MOWSE : molecular weight search MS : mass spectrometry MVP : major vault protein NAD(H) : nicotinamide adenine dinucleotide (reduced form) NADP(H) : nicotinamide adenine dinucleotide phosphate (reduced form) PCA : principle component analysis PDH : pyruvate dehydrogenase PEPCK : phosphoenolpyruvate carboxykinase pI : isoelectric point PMF : peptide mass fingerprint PTM : post-translational modification ROS : reactive oxygen species sHsps : small heat shock proteins SOD : superoxide dismutase TFA : trifluoroacetic acid T on : onset temperature 2-D GE : two-dimensional gel electrophoresis

A novel, small molecule inhibitor of Hsc70/Hsp70 potentiates Hsp90 inhibitor induced apoptosis in HCT116 colon carcinoma cells

Abstract: The anti-apoptotic function of the 70 kDa family of heat shock proteins and their role in cancer is well documented. Dual targeting of Hsc70 and Hsp70 with siRNA induces proteasome-dependent degradation of Hsp90 client proteins and extensive tumor specific apoptosis as well as the potentiation of tumor cell apoptosis following pharmacological Hsp90 inhibition. We have previously described the discovery and synthesis of novel adenosine-derived inhibitors of the 70 kDa family of heat shock proteins; the first inhibitors described to target the ATPase binding domain. The in vitro activity of VER-155008 was evaluated in HCT116, HT29, BT474 and MDA-MB-468 carcinoma cell lines. Cell proliferation, cell apoptosis and caspase 3/7 activity was determined for VER-155008 in the absence or presence of small molecule Hsp90 inhibitors. VER-155008 inhibited the proliferation of human breast and colon cancer cell lines with GI50s in the range 5.3–14.4 μM, and induced Hsp90 client protein degradation in both HCT116 and BT474 cells. As a single agent, VER-155008 induced caspase-3/7 dependent apoptosis in BT474 cells and non-caspase dependent cell death in HCT116 cells. VER-155008 potentiated the apoptotic potential of a small molecule Hsp90 inhibitor in HCT116 but not HT29 or MDA-MB-468 cells. In vivo, VER-155008 demonstrated rapid metabolism and clearance, along with tumor levels below the predicted pharmacologically active level. These data suggest that small molecule inhibitors of Hsc70/Hsp70 phenotypically mimic the cellular mode of action of a small molecule Hsp90 inhibitor and can potentiate the apoptotic potential of a small molecule Hsp90 inhibitor in certain cell lines. The factors determining whether or not cells apoptose in response to Hsp90 inhibition or the combination of Hsp90 plus Hsc70/Hsp70 inhibition remain to be determined.

Dual role of heat shock proteins as regulators of apoptosis and innate immunity.

Abstract: Stress or heat shock proteins (HSPs) 70 and 90 are powerful chaperones whose expression is induced in response to a wide variety of physiological and environmental insults. These proteins have different functions depending on their intracellular or extracellular location. Intracellular HSPs have a protective function. They allow the cells to survive potentially lethal conditions. The cytoprotective functions of HSPs can largely be explained by their anti-apoptotic properties. HSP70 and HSP90 can directly interact with different proteins of the tightly regulated programmed cell death machinery and thereby block the apoptotic process at distinct key points. In cancer cells, where the expression of HSP70 and/or HSP90 is frequently abnormally high, they participate in oncogenesis and in resistance to chemotherapy. Therefore, the inhibition of HSPs has become an interesting strategy in cancer therapy. In contrast to intracellular HSPs, extracellularly located or membrane-bound HSPs mediate immunological functions. They can elicit an immune response providing a link between innate and adaptive immune systems. In cancer, most immunotherapeutical approaches based on extracellular HSPs exploit their carrier function for immunogenic peptides. This review will focus on the roles of HSP70 and HSP90 in apoptosis and in innate immunity and how these functions are being exploited in cancer therapy.

Temporal expression of heat shock genes during cold stress and recovery from chill coma in adult Drosophila melanogaster.

Abstract: A common physiological response of organisms to environmental stresses is the increase in expression of heat shock proteins (Hsps). In insects, this process has been widely examined for heat stress, but the response to cold stress has been far less studied. In the present study, we focused on 11 Drosophila melanogaster Hsp genes during the stress exposure and recovery phases. The temporal gene expression of adults was analyzed during 9 h of cold stress at 0 degrees C and during 8 h of recovery at 25 degrees C. Increased expression of some, but not all, Hsp genes was elicited in response to cold stress. The transcriptional activity of Hsp genes was not modulated during the cold stress, and peaks of expression occurred during the recovery phase. On the basis of their response, we consider that Hsp60, Hsp67Ba and Hsc70-1 are not cold-inducible, whereas Hsp22, Hsp23, Hsp26, Hsp27, Hsp40, Hsp68, Hsp70Aa and Hsp83 are induced by cold. This study suggests the importance of the recovery phase for repairing chilling injuries, and highlights the need to further investigate the contributions of specific Hsp genes to thermal stress responses. Parallels are drawn between the stress response networks resulting from heat and cold stress.

Quaternary dynamics and plasticity underlie small heat shock protein chaperone function

Abstract: Small Heat Shock Proteins (sHSPs) are a diverse family of molecular chaperones that prevent protein aggregation by binding clients destabilized during cellular stress. Here we probe the architecture and dynamics of complexes formed between an oligomeric sHSP and client by employing unique mass spectrometry strategies. We observe over 300 different stoichiometries of interaction, demonstrating that an ensemble of structures underlies the protection these chaperones confer to unfolding clients. This astonishing heterogeneity not only makes the system quite distinct in behavior to ATP-dependent chaperones, but also renders it intractable by conventional structural biology approaches. We find that thermally regulated quaternary dynamics of the sHSP establish and maintain the plasticity of the system. This extends the paradigm that intrinsic dynamics are crucial to protein function to include equilibrium fluctuations in quaternary structure, and suggests they are integral to the sHSPs' role in the cellular protein homeostasis network.

The Heat-Inducible Transcription Factor HsfA2 Enhances Anoxia Tolerance in Arabidopsis

Abstract: Anoxia induces several heat shock proteins, and a mild heat pretreatment can acclimatize Arabidopsis (Arabidopsis thaliana) seedlings to subsequent anoxic treatment. In this study, we analyzed the response of Arabidopsis seedlings to anoxia, heat, and combined heat + anoxia stress. A significant overlap between the anoxic and the heat responses was observed by whole-genome microarray analysis. Among the transcription factors induced by both heat and anoxia, the heat shock factor A2 (HsfA2), known to be involved in Arabidopsis acclimation to heat and to other abiotic stresses, was strongly induced by anoxia. Heat-dependent acclimation to anoxia is lost in an HsfA2 knockout mutant (hsfa2) as well as in a double mutant for the constitutively expressed HsfA1a/HsfA1b (hsfA1a/1b), indicating that these three heat shock factors cooperate to confer anoxia tolerance. Arabidopsis seedlings that overexpress HsfA2 showed an increased expression of several known targets of this transcription factor and were markedly more tolerant to anoxia as well as to submergence. Anoxia failed to induce HsfA2 target proteins in wild-type seedlings, while overexpression of HsfA2 resulted in the production of HsfA2 targets under anoxia, correlating well with the low anoxia tolerance experiments. These results indicate that there is a considerable overlap between the molecular mechanisms of heat and anoxia tolerance and that HsfA2 is a player in these mechanisms.

Independent evolution of the core domain and its flanking sequences in small heat shock proteins

Abstract: Small heat shock proteins (sHsps) are molecular chaperones involved in maintaining protein homeostasis; they have also been implicated in protein folding diseases and in cancer. In this protein family, a conserved core domain, the so-called α-crystallin or Hsp20 domain, is flanked by highly variable, nonconserved sequences that are essential for chaperone function. Analysis of 8714 sHsps revealed a broad variation of primary sequences within the superfamily as well as phyla-dependent differences. Significant variations were found in the number of sHsps per genome, their amino acid composition, and the length distribution of the different sequence parts. Reconstruction of the evolutionary tree for the sHsp superfamily shows that the flanking regions fall into several subgroups, indicating that they were remodeled several times in parallel but independent of the evolution of the α-crystallin domain. The evolutionary history of sHsps is thus set apart from that of other protein families in that two exon boundary-independent strategies are combined: the evolution of the conserved α-crystallin domain and the independent evolution of the N- and C-terminal sequences. This scenario allows for increased variability in specific small parts of the protein and thus promotes functional and structural differentiation of sHsps, which is not reflected in the general evolutionary tree of species.

Heat shock protein 90 in neurodegenerative diseases

Abstract: Hsp90 is a molecular chaperone with important roles in regulating pathogenic transformation. In addition to its well-characterized functions in malignancy, recent evidence from several laboratories suggests a role for Hsp90 in maintaining the functional stability of neuronal proteins of aberrant capacity, whether mutated or over-activated, allowing and sustaining the accumulation of toxic aggregates. In addition, Hsp90 regulates the activity of the transcription factor heat shock factor-1 (HSF-1), the master regulator of the heat shock response, mechanism that cells use for protection when exposed to conditions of stress. These biological functions therefore propose Hsp90 inhibition as a dual therapeutic modality in neurodegenerative diseases. First, by suppressing aberrant neuronal activity, Hsp90 inhibitors may ameliorate protein aggregation and its associated toxicity. Second, by activation of HSF-1 and the subsequent induction of heat shock proteins, such as Hsp70, Hsp90 inhibitors may redirect neuronal aggregate formation, and protect against protein toxicity. This mini-review will summarize our current knowledge on Hsp90 in neurodegeneration and will focus on the potential beneficial application of Hsp90 inhibitors in neurodegenerative diseases.

The heat shock factor family and adaptation to proteotoxic stress.

Abstract: The heat shock response was originally characterized as the induction of a set of major heat shock proteins encoded by heat shock genes. Because heat shock proteins act as molecular chaperones that facilitate protein folding and suppress protein aggregation, this response plays a major role in maintaining protein homeostasis. The heat shock response is regulated mainly at the level of transcription by heat shock factors (HSFs) in eukaryotes. HSF1 is a master regulator of the heat shock genes in mammalian cells, as is HSF3 in avian cells. HSFs play a significant role in suppressing protein misfolding in cells and in ameliorating the progression of Caenorhabditis elegans, Drosophila and mouse models of protein-misfolding disorders, by inducing the expression of heat shock genes. Recently, numerous HSF target genes were identified, such as the classical heat shock genes and other heat-inducible genes, called nonclassical heat shock genes in this study. Importance of the expression of the nonclassical heat shock genes was evidenced by the fact that mouse HSF3 and chicken HSF1 play a substantial role in the protection of cells from heat shock without inducing classical heat shock genes. Furthermore, HSF2 and HSF4, as well as HSF1, shown to have roles in development, were also revealed to be necessary for the expression of certain nonclassical heat shock genes. Thus, the heat shock response regulated by the HSF family should consist of the induction of classical as well as of nonclassical heat shock genes, both of which might be required to maintain protein homeostasis.

Targeting HSP70: the second potentially druggable heat shock protein and molecular chaperone?

Abstract: The HSF1-mediated stress response pathway is steadily gaining momentum as a critical source of targets for cancer therapy. Key mediators of this pathway include molecular chaperones such as heat shock protein (HSP) 90. There has been considerable progress in targeting HSP90 and the preclinical efficacy and signs of early clinical activity of HSP90 inhibitors have provided proof-of-concept for targeting this group of proteins. The HSP70 family of molecular chaperones are also key mediators of the HSF-1-stress response pathway and have multiple additional roles in protein folding, trafficking and degradation, as well as regulating apoptosis. Genetic and biochemical studies have supported the discovery of HSP70 inhibitors which have the potential for use as single agents or in combination to enhance the effects of classical chemotherapeutic or molecularly targeted agents including HSP90 inhibitors. Here we provide a perspective on the progress made so far in designing agents which target the HSP70 family.

The Arabidopsis Chaperone J3 Regulates the Plasma Membrane H+-ATPase through Interaction with the PKS5 Kinase

Abstract: The plasma membrane H+-ATPase (PM H+-ATPase) plays an important role in the regulation of ion and metabolite transport and is involved in physiological processes that include cell growth, intracellular pH, and stomatal regulation. PM H+-ATPase activity is controlled by many factors, including hormones, calcium, light, and environmental stresses like increased soil salinity. We have previously shown that the Arabidopsis thaliana Salt Overly Sensitive2-Like Protein Kinase5 (PKS5) negatively regulates the PM H+-ATPase. Here, we report that a chaperone, J3 (DnaJ homolog 3; heat shock protein 40-like), activates PM H+-ATPase activity by physically interacting with and repressing PKS5 kinase activity. Plants lacking J3 are hypersensitive to salt at high external pH and exhibit decreased PM H+-ATPase activity. J3 functions upstream of PKS5 as double mutants generated using j3-1 and several pks5 mutant alleles with altered kinase activity have levels of PM H+-ATPase activity and responses to salt at alkaline pH similar to their corresponding pks5 mutant. Taken together, our results demonstrate that regulation of PM H+-ATPase activity by J3 takes place via inactivation of the PKS5 kinase.

Heat Shock Protein Cognate 70-4 and an E3 Ubiquitin Ligase, CHIP, Mediate Plastid-Destined Precursor Degradation through the Ubiquitin-26S Proteasome System in Arabidopsis

Abstract: Plastid-targeted proteins pass through the cytosol as unfolded precursors. If proteins accumulate in the cytosol, they can form nonspecific aggregates that cause severe cellular damage. Here, we demonstrate that high levels of plastid precursors are degraded through the ubiquitin-proteasome system (UPS) in Arabidopsis thaliana cells. The cytosolic heat shock protein cognate 70-4 (Hsc70-4) and E3 ligase carboxy terminus of Hsc70-interacting protein (CHIP) were highly induced in plastid protein import2 plants, which had a T-DNA insertion at Toc159 and showed an albino phenotype and a severe defect in protein import into chloroplasts. Hsc70-4 and CHIP together mediated plastid precursor degradation when import-defective chloroplast-targeted reporter proteins were transiently expressed in protoplasts. Hsc70-4 recognized specific sequence motifs in transit peptides and thereby led to precursor degradation through the UPS. CHIP, which interacted with Hsc70-4, functioned as an E3 ligase in the Hsc70-4–mediated protein degradation. The physiological role of Hsc70-4 was confirmed by analyzing Hsc70-4 RNA interfernce plants in an hsc70-1 mutant background. Plants with lower Hsc70 levels exhibited abnormal embryogenesis, resulting in defective seedlings that displayed high levels of reactive oxygen species and monoubiquitinated Lhcb4 precursors. We propose that Hsc70-4 and CHIP mediate plastid-destined precursor degradation to prevent cytosolic precursor accumulation and thereby play a critical role in embryogenesis.

Heat Shock Proteins: Cell Protection through Protein Triage

Abstract: Heat shock proteins (HSPs) are chaperones that catalyze the proper folding of nascent proteins and the refolding of denatured proteins. The ubiquitin-proteasome system is an error-checking system that directs improperly folded proteins for destruction. A coordinated interaction between the HSPs (renaturation) and the proteasome (degradation) must exist to assure protein quality control mechanisms. Although it still remains unknown how the decision of folding vs. degradation is taken, many pieces of evidence demonstrate that HSPs interact directly or indirectly with the proteasome, assuring quite selectively the proteasomal degradation of certain proteins under stress conditions. In this review, we will describe the different data that demonstrate a role for HSP90, HSP70, HSP27, and alpha-B-crystallin in the partitioning of proteins to either one of these pathways, referred as protein triage.

Identification of the key structural motifs involved in HspB8/HspB6-Bag3 interaction.

Abstract: The molecular chaperone HspB8 [Hsp (heat-shock protein) B8] is member of the B-group of Hsps. These proteins bind to unfolded or misfolded proteins and protect them from aggregation. HspB8 has been reported to form a stable molecular complex with the chaperone cohort protein Bag3 (Bcl-2-associated athanogene 3). In the present study we identify the binding regions in HspB8 and Bag3 crucial for their interaction. We present evidence that HspB8 binds to Bag3 through the hydrophobic groove formed by its strands β4 and β8, a region previously known to be responsible for the formation and stability of higher-order oligomers of many sHsps (small Hsps). Moreover, we demonstrate that two conserved IPV (Ile-Pro-Val) motifs in Bag3 mediate its binding to HspB8 and that deletion of these motifs suppresses HspB8 chaperone activity towards mutant Htt43Q (huntingtin exon 1 fragment with 43 CAG repeats). In addition, we show that Bag3 can bind to the molecular chaperone HspB6. The interaction between HspB6 and Bag3 requires the same regions that are involved in the HspB8–Bag3 association and HspB6–Bag3 promotes clearance of aggregated Htt43Q. Our findings suggest that the co-chaperone Bag3 might prevent the accumulation of denatured proteins by regulating sHsp activity and by targeting their substrate proteins for degradation. Interestingly, a mutation in one of Bag3 IPV motifs has recently been associated with the development of severe dominant childhood muscular dystrophy, suggesting a possible important physiological role for HspB–Bag3 complexes in this disease.

Roles of GRP78 in physiology and cancer.

Abstract: As one member of 70 kDa heat shock proteins, glucose-regulated protein 78 (GRP78) participates in protein folding, transportation and degradation. This sort of capacity can be enhanced by stresses under which GRP78 is induced rapidly. Unlike its homologues, GRP78 presents multifaceted subcellular position: When ER retention, it serves as the switch of unfolded protein response; When mitochondrial binding, it directly interacts with apoptotic executors; When cell surface residing, it recognizes extracellular ligands, transducing proliferative signals, especially in certain tumors. The close correlation between GRP78 and neoplasm provides us further insight into the event of carcinogenesis and cancer cell chemoresistance, indicating its prognostic predicting significance and validating potential therapeutics for clinical usage, especially because its small molecular inhibitors are emerging quickly these years. What's more, GRP78-related signaling may be helpful for clearer understanding of its biological mechanisms.