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

Peptide-based approaches to treat lupus and other autoimmune diseases

After a long period where the potential of therapeutic peptides was let into oblivion and even dismissed, there is a revival of interest in peptides as potential drug candidates. Novel strategies for limiting metabolism and improve their bioavailability, and alternative routes of administration have emerged. This resulted in a large number of peptide-based drugs that are now being marketed in different indications. Regarding autoimmunity, successful data have been reported in numerous mouse models of autoimmune inflammation, yet relatively few clinical trials based on synthetic peptides are currently underway. This review reports on peptides that show much promises in appropriate mouse models of autoimmunity and describes in more detail clinical trials based on peptides for treating autoimmune patients. A particular emphasis is given to the 21-mer peptide P140/Lupuzor that has completed successfully phase I, phase IIa and phase IIb clinical trials for systemic lupus erythematosus.

Molecular chaperones and stress-inducible protein-sorting factors coordinate the spatiotemporal distribution of protein aggregates

The deposition of misfolded proteins in cytoplasmic protein bodies requires the concerted action of stress-inducible protein-sorting factors and molecular chaperones. Protein sequestration during acute stress is a cellular strategy that adjusts the flux of misfolded proteins to the capacities of the protein quality control system.

Acute stress causes a rapid redistribution of protein quality control components and aggregation-prone proteins to diverse subcellular compartments. How these remarkable changes come about is not well understood. Using a phenotypic reporter for a synthetic yeast prion, we identified two protein-sorting factors of the Hook family, termed Btn2 and Cur1, as key regulators of spatial protein quality control in Saccharomyces cerevisiae. Btn2 and Cur1 are undetectable under normal growth conditions but accumulate in stressed cells due to increased gene expression and reduced proteasomal turnover. Newly synthesized Btn2 can associate with the small heat shock protein Hsp42 to promote the sorting of misfolded proteins to a peripheral protein deposition site. Alternatively, Btn2 can bind to the chaperone Sis1 to facilitate the targeting of misfolded proteins to a juxtanuclear compartment. Protein redistribution by Btn2 is accompanied by a gradual depletion of Sis1 from the cytosol, which is mediated by the sorting factor Cur1. On the basis of these findings, we propose a dynamic model that explains the subcellular distribution of misfolded proteins as a function of the cytosolic concentrations of molecular chaperones and protein-sorting factors. Our model suggests that protein aggregation is not a haphazard process but rather an orchestrated cellular response that adjusts the flux of misfolded proteins to the capacities of the protein quality control system.

Identification of CSPα clients reveals a role in dynamin 1 regulation

Cysteine string protein α (CSPα), a presynaptic cochaperone for Hsc70, is required for synapse maintenance. Deletion of CSPα leads to neuronal dysfunction, synapse loss, and neurodegeneration. We utilized unbiased, systematic proteomics to identify putative CSPα protein clients. We found 22 such proteins whose levels are selectively decreased in CSPα knockout synapses. Of these putative CSPα protein clients, two directly bind to the CSPα chaperone complex and are bona fide clients. They are the t-SNARE SNAP-25 and the GTPase dynamin 1, which are necessary for synaptic vesicle fusion and fission, respectively. Using hippocampal cultures, we show that CSPα regulates the stability of client proteins and synaptic vesicle number. Our analysis of CSPα-dynamin 1 interactions reveals unexpectedly that CSPα regulates the polymerization of dynamin 1. CSPα, therefore, participates in synaptic vesicle endocytosis and may facilitate exo- and endocytic coupling. These findings advance the understanding of how synapses are functionally and structurally maintained.

HSPBs: small proteins with big implications in human disease

Although initially somewhat ignored, recent studies have now clearly established that the diverse members of the human family of small HSPs (HSPB1-HSPB10) play crucial roles in a wide range of cell types to maintain the integrity and function of tissues, in particular that of nervous and muscular tissue. The 10 human HSPBs clearly have overlapping and non-overlapping functional characteristics. Their ability to self- and hetero-oligomerise provides the cells with a large array of potentially different, specific functions. Single HSPB members can have a multitude of functions (moonlighting) and act on different "clients", thus affecting a wide range of different processes or structures that can ultimately affect the rate of aging of tissues and entire organisms. This is underscored by the findings that some inherited diseases involve mutations in several HSPB members that cause premature (mostly muscle and neuronal) tissue degeneration. Inversely, cancer cell resistance to different anticancer therapies is associated with elevated expression of several HSPBs. Still, many unanswered questions exist about the precise functioning of HSPBs, their collaboration with other HSPB members as well as their functions within the entire cellular chaperone network. Also, better insight in the regulation of expression of the various members and how their function is modulated post-translationally is needed. Such may be crucially important to develop means to intervene with their function for therapeutic purposes, which would require functional down-regulation in cancer but up-regulation in, for instance, cardiac or degenerative neuro/neuromuscular diseases. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

A chaperone trap contributes to the onset of cystic fibrosis

Protein folding is the primary role of proteostasis network (PN) where chaperone interactions with client proteins determine the success or failure of the folding reaction in the cell. We now address how the Phe508 deletion in the NBD1 domain of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein responsible for cystic fibrosis (CF) impacts the binding of CFTR with cellular chaperones. We applied single ion reaction monitoring mass spectrometry (SRM-MS) to quantitatively characterize the stoichiometry of the heat shock proteins (Hsps) in CFTR folding intermediates in vivo and mapped the sites of interaction of the NBD1 domain of CFTR with Hsp90 in vitro. Unlike folding of WT-CFTR, we now demonstrate the presence of ΔF508-CFTR in a stalled folding intermediate in stoichiometric association with the core Hsps 40, 70 and 90, referred to as a ‘chaperone trap’. Culturing cells at 30 C resulted in correction of ΔF508-CFTR trafficking and function, restoring the sub-stoichiometric association of core Hsps observed for WT-CFTR. These results support the interpretation that ΔF508-CFTR is restricted to a chaperone-bound folding intermediate, a state that may contribute to its loss of trafficking and increased targeting for degradation. We propose that stalled folding intermediates could define a critical proteostasis pathway branch-point(s) responsible for the loss of function in misfolding diseases as observed in CF.

Cell line specific modulation of extracellular aβ42 by Hsp40

Heat shock proteins (Hsps) are a set of molecular chaperones involved in cellular repair. They provide protective mechanisms that allow cells to survive potentially lethal insults, In response to a conditioning stress their expression is increased. Here we examined the connection between Hsps and Aβ(42), the amyloid peptide involved in the pathological sequence of Alzheimer's disease (AD). Extracellular Aβ(42) associates with neuronal cells and is a major constituent of senile plaques, one of the hallmarks of AD. Although Hsps are generally thought to prevent accumulation of misfolded proteins, there is a lack of mechanistic evidence that heat shock chaperones directly modulate Aβ(42) toxicity. In this study we show that neither extracellular Aβ(42) nor Aβ(42/)PrP(C) trigger the heat shock response in neurons. To address the influence of the neuroprotective heat shock response on cellular Aβ(42), Western analysis of Aβ(42) was performed following external Aβ(42) application. Five hours after a conditioning heat shock, Aβ(42) association with CAD cells was increased compared to control neurons. However, at forty-eight hours following heat shock Aβ(42) levels were reduced compared to that found for control cells. Moreover, transient transfection of the stress induced Hsp40, decreased CAD levels of Aβ(42). In contrast to CAD cells, hippocampal neurons transfected with Hsp40 retained Aβ(42) indicating that Hsp40 modulation of Aβ(42) proteostasis is cell specific. Mutation of the conserved HPD motif within Hsp40 significantly reduced the Hsp40-mediated Aβ(42) increase in hippocampal cultures indicating the importance of this motif in regulating cellular Aβ(42). Our data reveal a biochemical link between Hsp40 expression and Aβ(42) proteostasis that is cell specific. Therefore, increasing Hsp40 therapeutically with the intention of interfering with the pathogenic cascade leading to neurodegeneration in AD should be pursued with caution.

Mitochondrial chaperone DnaJA3 induces Drp1-dependent mitochondrial fragmentation

Mitochondrial morphology is dynamic and controlled by coordinated fusion and fission pathways. The role of mitochondrial chaperones in mitochondrial morphological changes and pathology is currently unclear. Here we report that altered levels of DnaJA3 (Tid1/mtHsp40) a mitochondrial member of the DnaJ protein family, and heat shock protein (Hsp) co-chaperone of matrix 70 kDa Hsp70 (mtHsp70/mortalin/HSPA9), induces mitochondrial fragmentation. Suppression of DnaJA3 induced mitochondrial fragmentation in HeLa cells. Elevated levels of DnaJA3 in normal Hs68 fibroblast cells and HeLa, SKN-SH, U87 and U251 cancer cell lines induces mitochondrial fragmentation. Mitochondrial fragmentation induction was not observed in HeLa cells when other DnaJA family members, or mitochondrial DnaJ protein HSC20, were ectopically expressed, indicating that the effects on mitochondrial morphology were specific to DnaJA3. We show that the DnaJ domain (amino acids 88-168) of DnaJA3 is sufficient for the induction of mitochondrial fragmentation. Furthermore, an H121Q point mutation of the DnaJ domain, which abrogates interaction and activation of mtHsp70 ATPase, eliminates fragmentation induced by DnaJA3. This suggests that DnaJA3 interaction with mtHsp70 may be critical in mitochondrial morphological changes. DnaJA3-induced mitochondrial fragmentation was dependent on fission factor dynamin-related protein 1 (Drp1). Ectopic expression of the mitofusins (Mfn1 and Mfn2), however, does not rescue DnaJA3-induced mitochondrial fragmentation. Lastly, elevated levels of DnaJA3 inducing mitochondrial fragmentation were associated with reduction in cell viability. Taken together, elevated DnaJA3 induces Drp1-depedendent mitochondrial fragmentation and decreased cell viability.

Chaperone networks in protein disaggregation and prion propagation

The oligomeric AAA+ chaperones Escherichia coli ClpB and Saccharomyces cerevisiae Hsp104 cooperate with cognate Hsp70/Hsp40 chaperone machineries in the reactivation of aggregated proteins in E. coli and S. cerevisiae. In addition, Hsp104 and Hsp70/Hsp40 are crucial for the maintenance of prion aggregates in yeast cells. While the bichaperone system efficiently solubilizes stress-generated amorphous aggregates, structurally highly ordered prion fibrils are only partially processed, resulting in the generation of fragmented prion seeds that can be transmitted to daughter cells for stable inheritance. Here, we describe and discuss the most recent mechanistic findings on yeast Hsp104 and Hsp70/Hsp40 cooperation in the remodeling of both types of aggregates, emphasizing similarities in the mechanism but also differences in the sensitivities towards chaperone activities.

Molecular chaperone involvement in chloroplast protein import

Chloroplasts are organelles of endosymbiotic origin that perform essential functions in plants. They contain about 3000 different proteins, the vast majority of which are nucleus-encoded, synthesized in precursor form in the cytosol, and transported into the chloroplasts post-translationally. These preproteins are generally imported via envelope complexes termed TOC and TIC (Translocon at the Outer/Inner envelope membrane of Chloroplasts). They must navigate different cellular and organellar compartments (e.g., the cytosol, the outer and inner envelope membranes, the intermembrane space, and the stroma) before arriving at their final destination. It is generally considered that preproteins are imported in a largely unfolded state, and the whole process is energy-dependent. Several chaperones and cochaperones have been found to mediate different stages of chloroplast import, in similar fashion to chaperone involvement in mitochondrial import. Cytosolic factors such as Hsp90, Hsp70 and 14-3-3 may assist preproteins to reach the TOC complex at the chloroplast surface, preventing their aggregation or degradation. Chaperone involvement in the intermembrane space has also been proposed, but remains uncertain. Preprotein translocation is completed at the trans side of the inner membrane by ATP-driven motor complexes. A stromal Hsp100-type chaperone, Hsp93, cooperates with Tic110 and Tic40 in one such motor complex, while stromal Hsp70 is proposed to act in a second, parallel complex. Upon arrival in the stroma, chaperones (e.g., Hsp70, Cpn60, cpSRP43) also contribute to the folding, assembly or onward intraorganellar guidance of the proteins. In this review, we focus on chaperone involvement during preprotein translocation at the chloroplast envelope. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Synergistic binding of DnaJ and DnaK chaperones to heat shock transcription factor σ32 ensures its characteristic high metabolic instability: implications for heat shock protein 70 (Hsp70)-Hsp40 mode of function

Escherichia coli heat shock transcription factor σ(32) is rapidly degraded by ATP-dependent proteases, such as FtsH and ClpYQ. Although the DnaK chaperone system (DnaK, DnaJ, and GrpE) promotes σ(32) degradation in vivo, the precise mechanism that is involved remains unknown. Our previous results indicated that σ(32) mutants containing amino acid substitution in the N-terminal half of Region 2.1 are markedly stabilized in vivo. Here, we report the further characterization of these mutants by examining purified σ(32) mutants in vitro. Surprisingly, I54A σ(32), a very stable mutant, is more susceptible to ClpYQ and FtsH proteases than wild-type σ(32), indicating that the stability of σ(32) does not always reflect its susceptibility to proteases. Co-precipitation and gel filtration analyses show that purified σ(32) mutants exhibit a reduced affinity for DnaJ, leading to a marked decrease in forming a complex with DnaK in the presence of DnaJ and ATP. Other mutants with modestly increased stability (A50S σ(32) and K51E σ(32)) show an intermediate efficiency of complex formation with DnaK, suggesting that defects in binding to DnaK and DnaJ are well correlated with metabolic stability; effective interaction with DnaK promotes σ(32) degradation in vivo. We argue that the stable and effective interaction of heat shock protein 70 (Hsp70) with a substrate polypeptide may generally require the simultaneous binding of heat shock protein 40 (Hsp40) to distinct sites on the substrate.

Plant leucine aminopeptidases moonlight as molecular chaperones to alleviate stress-induced damage

Leucine aminopeptidases (LAPs) are present in animals, plants, and microbes. In plants, there are two classes of LAPs. The neutral LAPs (LAP-N and its orthologs) are constitutively expressed and detected in all plants, whereas the stress-induced acidic LAPs (LAP-A) are expressed only in a subset of the Solanaceae. LAPs have a role in insect defense and act as a regulator of the late branch of wound signaling in Solanum lycopersicum (tomato). Although the mechanism of LAP-A action is unknown, it has been presumed that LAP peptidase activity is essential for regulating wound signaling. Here we show that plant LAPs are bifunctional. Using three assays to monitor protein protection from heat-induced damage, it was shown that the tomato LAP-A and LAP-N and the Arabidopsis thaliana LAP1 and LAP2 are molecular chaperones. Assays using LAP-A catalytic site mutants demonstrated that LAP-A chaperone activity was independent of its peptidase activity. Furthermore, disruption of the LAP-A hexameric structure increased chaperone activity. Together, these data identify a new class of molecular chaperones and a new function for the plant LAPs as well as suggesting new mechanisms for LAP action in the defense of solanaceous plants against stress.

The lid domain of Caenorhabditis elegans Hsc70 influences ATP turnover, cofactor binding and protein folding activity

Hsc70 is a conserved ATP-dependent molecular chaperone, which utilizes the energy of ATP hydrolysis to alter the folding state of its client proteins. In contrast to the Hsc70 systems of bacteria, yeast and humans, the Hsc70 system of C. elegans (CeHsc70) has not been studied to date.

We find that CeHsc70 is characterized by a high ATP turnover rate and limited by post-hydrolysis nucleotide exchange. This rate-limiting step is defined by the helical lid domain at the C-terminus. A certain truncation in this domain (CeHsc70-Δ545) reduces the turnover rate and renders the hydrolysis step rate-limiting. The helical lid domain also affects cofactor affinities as the lidless mutant CeHsc70-Δ512 binds more strongly to DNJ-13, forming large protein complexes in the presence of ATP. Despite preserving the ability to hydrolyze ATP and interact with its cofactors DNJ-13 and BAG-1, the truncation of the helical lid domain leads to the loss of all protein folding activity, highlighting the requirement of this domain for the functionality of the nematode's Hsc70 protein.

The small heat shock proteins family: the long forgotten chaperones

Small heat shock proteins are a rather heterogeneous family of ATP-independent chaperones, some of which have been proven to block protein aggregation and help the cells to survive stressful conditions. Although much less studied than high molecular weight HSPs like HSP70/HSPA or HSP90/HSPC, their implication in physio-pathological processes and human diseases is now well evidenced, as it will be discussed in the different reviews of this special issue. In this mini-review we will just present a general introduction about the small heat shock proteins family. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Chaperones divide yeast proteins into classes of expression level and evolutionary rate

It has long been known that many proteins require folding via molecular chaperones for their function. Although it has become apparent that folding imposes constraints on protein sequence evolution, the effects exerted by different chaperone classes are so far unknown. We have analyzed data of protein interaction with the chaperones in Saccharomycescerevisiae using network methods. The results reveal a distinct community structure within the network that was hitherto undetectable with standard statistical tools. Sixty-four yeast chaperones comprise ten distinct modules that are defined by interaction specificity for their 2,691 interacting proteins. The classes of interacting proteins that are in turn defined by their dedicated chaperone modules are distinguished by various physiochemical protein properties and are characterized by significantly different protein expression levels, codon usage, and amino acid substitution rates. Correlations between substitution rate, codon bias, and gene expression level that have long been known for yeast are apparent at the level of the chaperone-defined modules. This indicates that correlated expression, conservation, and codon bias levels for yeast genes are attributable to previously unrecognized effects of protein folding. Proteome-wide categories of chaperone–substrate specificity uncover novel hubs of functional constraint in protein evolution that are conserved across 20 fungal genomes.

Primary hyperoxalurias: disorders of glyoxylate detoxification

Glyoxylate detoxification is an important function of human peroxisomes. Glyoxylate is a highly reactive molecule, generated in the intermediary metabolism of glycine, hydroxyproline and glycolate mainly. Glyoxylate accumulation in the cytosol is readily transformed by lactate dehydrogenase into oxalate, a dicarboxylic acid that cannot be metabolized by mammals and forms tissue-damaging calcium oxalate crystals. Alanine-glyoxylate aminotransferase, a peroxisomal enzyme in humans, converts glyoxylate into glycine, playing a central role in glyoxylate detoxification. Cytosolic and mitochondrial glyoxylate reductase also contributes to limit oxalate production from glyoxylate. Mitochondrial hydroxyoxoglutarate aldolase is an important enzyme of hydroxyproline metabolism. Genetic defect of any of these enzymes of glyoxylate metabolism results in primary hyperoxalurias, severe human diseases in which toxic levels of oxalate are produced by the liver, resulting in progressive renal damage. Significant advances in the pathophysiology of primary hyperoxalurias have led to better diagnosis and treatment of these patients, but current treatment relies mainly on organ transplantation. It is reasonable to expect that recent advances in the understanding of the molecular mechanisms of disease will result into better targeted therapeutic options in the future.

Suppression of protein aggregation by chaperone modification of high molecular weight complexes

Protein misfolding and aggregation are associated with many neurodegenerative diseases, including Huntington’s disease. The cellular machinery for maintaining proteostasis includes molecular chaperones that facilitate protein folding and reduce proteotoxicity. Increasing the protein folding capacity of cells through manipulation of DNAJ chaperones has been shown to suppress aggregation and ameliorate polyglutamine toxicity in cells and flies. However, to date these promising findings have not been translated to mammalian models of disease. To address this issue, we developed transgenic mice that over-express the neuronal chaperone HSJ1a (DNAJB2a) and crossed them with the R6/2 mouse model of Huntington’s disease. Over-expression of HSJ1a significantly reduced mutant huntingtin aggregation and enhanced solubility. Surprisingly, this was mediated through specific association with K63 ubiquitylated, detergent insoluble, higher order mutant huntingtin assemblies that decreased their ability to nucleate further aggregation. This was dependent on HSJ1a client binding ability, ubiquitin interaction and functional co-operation with HSP70. Importantly, these changes in mutant huntingtin solubility and aggregation led to improved neurological performance in R6/2 mice. These data reveal that prevention of further aggregation of detergent insoluble mutant huntingtin is an additional level of quality control for late stage chaperone-mediated neuroprotection. Furthermore, our findings represent an important proof of principle that DNAJ manipulation is a valid therapeutic approach for intervention in Huntington’s disease.

DnaJ (Hsp40 protein) binding to folded substrate impacts KplE1 prophage excision efficiency

Temperate phages mediate gene transfer and can modify the properties of their host organisms through the acquisition of novel genes, a process called lysogeny. The KplE1 prophage is one of the 10 prophage regions in Escherichia coli K12 MG1655. KplE1 is defective for lysis but fully competent for site-specific recombination. The TorI recombination directionality factor is strictly required for prophage excision from the host genome. We have previously shown that DnaJ promotes KplE1 excision by increasing the affinity of TorI for its site-specific recombination DNA target. Here, we provide evidence of a direct association between TorI and DnaJ using in vitro cross-linking assays and limited proteolysis experiments that show that this interaction allows both proteins to be transiently protected from trypsin digestion. Interestingly, NMR titration experiments showed that binding of DnaJ involves specific regions of the TorI structure. These regions, mainly composed of α-helices, are located on a surface opposite the DNA-binding site. Taken together, we propose that DnaJ, without the aid of DnaK/GrpE, is capable of increasing the efficiency of KplE1 excision by causing a conformational stabilization that allows TorI to adopt a more favorable conformation for binding to its specific DNA target.

Mutations affecting the cytoplasmic functions of the co-chaperone DNAJB6 cause limb-girdle muscular dystrophy

Limb-girdle muscular dystrophy type 1D (LGMD1D) was linked to chromosome 7q36 over a decade ago, but its genetic cause has remained elusive. Here we studied nine LGMD-affected families from Finland, the United States and Italy and identified four dominant missense mutations leading to p.Phe93Leu or p.Phe89Ile changes in the ubiquitously expressed co-chaperone DNAJB6. Functional testing in vivo showed that the mutations have a dominant toxic effect mediated specifically by the cytoplasmic isoform of DNAJB6. In vitro studies demonstrated that the mutations increase the half-life of DNAJB6, extending this effect to the wild-type protein, and reduce its protective anti-aggregation effect. Further, we show that DNAJB6 interacts with members of the CASA complex, including the myofibrillar myopathy-causing protein BAG3. Our data identify the genetic cause of LGMD1D, suggest that its pathogenesis is mediated by defective chaperone function and highlight how mutations in a ubiquitously expressed gene can exert effects in a tissue-, isoform- and cellular compartment-specific manner.

Protein homeostasis, aging and Alzheimer's disease

Alzheimer’s disease (AD) is one key medical challenge of the aging society and despite a great amount of effort and a huge collection of acquired data on molecular mechanisms that are associated with the onset and progression of this devastating disorder, no causal therapy is in sight. The two main hypotheses of AD, the amyloid cascade hypothesis and the Tau hypothesis, are still in the focus of AD research. With aging as the accepted main risk factor of the most important non familial and late onset sporadic forms of AD, it is now mandatory to discuss more intensively aspects of cellular aging and aging biochemistry and its impact on neurodegeneration. Since aging is accompanied by changes in cellular protein homeostasis and an increasing demand for protein degradation, aspects of protein folding, misfolding, refolding and, importantly, protein degradation need to be linked to AD pathogenesis. This is the purpose of this short review.

Structure and mechanistic insights into novel iron-mediated moonlighting functions of human J-protein cochaperone, Dph4

J-proteins are obligate cochaperones of Hsp70s and stimulate their ATPase activity via the J-domain. Although the functions of J-proteins have been well understood in the context of Hsp70s, their additional co-evolved "physiological functions" are still elusive. We report here the solution structure and mechanism of novel iron-mediated functional roles of human Dph4, a type III J-protein playing a vital role in diphthamide biosynthesis and normal development. The NMR structure of Dph4 reveals two domains: a conserved J-domain and a CSL-domain connected via a flexible linker-helix. The linker-helix modulates the conformational flexibility between the two domains, regulating thereby the protein function. Dph4 exhibits a unique ability to bind iron in tetrahedral coordination geometry through cysteines of its CSL-domain. The oxidized Fe-Dph4 shows characteristic UV-visible and electron paramagnetic resonance spectral properties similar to rubredoxins. Iron-bound Dph4 (Fe-Dph4) also undergoes oligomerization, thus potentially functioning as a transient "iron storage protein," thereby regulating the intracellular iron homeostasis. Remarkably, Fe-Dph4 exhibits vital redox and electron carrier activity, which is critical for important metabolic reactions, including diphthamide biosynthesis. Further, we observed that Fe-Dph4 is conformationally better poised to perform Hsp70-dependent functions, thus underlining the significance of iron binding in Dph4. Yeast Jjj3, a functional ortholog of human Dph4 also shows a similar iron-binding property, indicating the conserved nature of iron sequestration across species. Taken together, our findings provide invaluable evidence in favor of additional co-evolved specialized functions of J-proteins, previously not well appreciated.

Genetic analysis of complex interactions among components of the mitochondrial import motor and translocon in Saccharomyces cerevisiae

A highly conserved, Hsp70-based, import motor, which is associated with the translocase on the matrix side of the inner mitochondrial membrane, is critical for protein translocation into the matrix. Hsp70 is tethered to the translocon via interaction with Tim44. Pam18, the J-protein co-chaperone, and Pam16, a structurally related protein with which Pam18 forms a heterodimer, are also critical components of the motor. Their N termini are important for the heterodimer's translocon association, with Pam18's and Pam16's N termini interacting in the intermembrane space and the matrix, respectively. Here, using the model organism Saccharomyces cerevisiae, we report the identification of an N-terminal segment of Tim44, important for association of Pam16 with the translocon. We also report that higher amounts of Pam17, a nonessential motor component, are found associated with the translocon in both PAM16 and TIM44 mutants that affect their interaction with one another. These TIM44 and PAM16 mutations are also synthetically lethal with a deletion of PAM17. In contrast, a deletion of PAM17 has little, or no genetic interaction with a PAM18 mutation that affects translocon association of the Pam16:Pam18 heterodimer, suggesting a second role for the Pam16:Tim44 interaction. A similar pattern of genetic interactions and enhanced Pam17 translocon association was observed in the absence of the C terminus of Tim17, a core component of the translocon. We suggest the Pam16:Tim44 interaction may play two roles: (1) tethering the Pam16:Pam18 heterodimer to the translocon and (2) positioning the import motor for efficient engagement with the translocating polypeptide along with Tim17 and Pam17.

Interaction of J-protein co-chaperone Jac1 with Fe-S scaffold Isu is indispensable in vivo and conserved in evolution

The ubiquitous mitochondrial J-protein Jac1, called HscB in Escherichia coli, and its partner Hsp70 play a critical role in the transfer of Fe-S clusters from the scaffold protein Isu to recipient proteins. Biochemical results from eukaryotic and prokaryotic systems indicate that formation of the Jac1-Isu complex is important for both targeting of the Isu for Hsp70 binding and stimulation of Hsp70's ATPase activity. However, in apparent contradiction, we previously reported that an 8-fold decrease in Jac1's affinity for Isu1 is well tolerated in vivo, raising the question as to whether the Jac1:Isu interaction actually plays an important biological role. Here, we report the determination of the structure of Jac1 from Saccharomyces cerevisiae. Taking advantage of this information and recently published data from the homologous bacterial system, we determined that a total of eight surface-exposed residues play a role in Isu binding, as assessed by a set of biochemical assays. A variant having alanines substituted for these eight residues was unable to support growth of a jac1-Δ strain. However, replacement of three residues caused partial loss of function, resulting in a significant decrease in the Jac1:Isu1 interaction, a slow growth phenotype, and a reduction in the activity of Fe-S cluster-containing enzymes. Thus, we conclude that the Jac1:Isu1 interaction plays an indispensable role in the essential process of mitochondrial Fe-S cluster biogenesis.

DNAJB6 chaperones PP2A mediated dephosphorylation of GSK3β to downregulate β-catenin transcription target, osteopontin

Elevated levels of the oncoprotein, osteopontin (OPN), are associated with poor outcome of several types of cancers including melanoma. We have previously reported an important involvement of DNAJB6, a member of heat-shock protein 40 (HSP40) family, in negatively impacting tumor growth. The current study was prompted by our observations reported here which revealed a reciprocal relationship between DNAJB6 and OPN in melanoma specimens. The 'J domain' is the most conserved domain of HSP40 family of proteins. Hence, we assessed the functional role of the J domain in activities of DNAJB6. We report that the J domain of DNAJB6 is involved in mediating OPN suppression. Deletion of the J domain renders DNAJB6 incapable of impeding malignancy and suppressing OPN. Our mechanistic investigations reveal that DNAJB6 binds HSPA8 (heat-shock cognate protein, HSC70) and causes dephosphorylation of glycogen synthase kinase 3β (GSK3β) at Ser 9 by recruiting protein phosphatase, PP2A. This dephosphorylation activates GSK3β, leading to degradation of β-catenin and subsequent loss of TCF/LEF (T cell factor1/lymphoid enhancer factor1) activity. Deletion of the J domain abrogates assembly of this multiprotein complex and renders GSK3β inactive, thus, stabilizing β-catenin, a transcription co-activator for OPN expression. Our in-vitro and in-vivo functional analyses show that silencing OPN expression in the background of deletion of the J domain renders the resultant tumor cells less malignant despite the presence of stabilized β-catenin. Thus, we have uncovered a new mechanism for regulation of GSK3β activity leading to inhibition of Wnt/β-catenin signaling.

DNAJB6 chaperones PP2A mediated dephosphorylation of GSK3β to downregulate β-catenin transcription target, osteopontin

Elevated levels of the oncoprotein, osteopontin (OPN), are associated with poor outcome of several types of cancers including melanoma. We have previously reported an important involvement of DNAJB6, a member of heat-shock protein 40 (HSP40) family, in negatively impacting tumor growth. The current study was prompted by our observations reported here which revealed a reciprocal relationship between DNAJB6 and OPN in melanoma specimens. The 'J domain' is the most conserved domain of HSP40 family of proteins. Hence, we assessed the functional role of the J domain in activities of DNAJB6. We report that the J domain of DNAJB6 is involved in mediating OPN suppression. Deletion of the J domain renders DNAJB6 incapable of impeding malignancy and suppressing OPN. Our mechanistic investigations reveal that DNAJB6 binds HSPA8 (heat-shock cognate protein, HSC70) and causes dephosphorylation of glycogen synthase kinase 3β (GSK3β) at Ser 9 by recruiting protein phosphatase, PP2A. This dephosphorylation activates GSK3β, leading to degradation of β-catenin and subsequent loss of TCF/LEF (T cell factor1/lymphoid enhancer factor1) activity. Deletion of the J domain abrogates assembly of this multiprotein complex and renders GSK3β inactive, thus, stabilizing β-catenin, a transcription co-activator for OPN expression. Our in-vitro and in-vivo functional analyses show that silencing OPN expression in the background of deletion of the J domain renders the resultant tumor cells less malignant despite the presence of stabilized β-catenin. Thus, we have uncovered a new mechanism for regulation of GSK3β activity leading to inhibition of Wnt/β-catenin signaling.

XPORT-dependent transport of TRP and rhodopsin

TRP channels have emerged as key biological sensors in vision, taste, olfaction, hearing, and touch. Despite their importance, virtually nothing is known about the folding and transport of TRP channels during biosynthesis. Here, we identify XPORT (exit protein of rhodopsin and TRP) as a critical chaperone for TRP and its G protein-coupled receptor (GPCR), rhodopsin (Rh1). XPORT is a resident ER and secretory pathway protein that interacts with TRP and Rh1, as well as with Hsp27 and Hsp90. XPORT promotes the targeting of TRP to the membrane in Drosophila S2 cells, a finding that provides a critical first step toward solving a longstanding problem in the successful heterologous expression of TRP. Mutations in xport result in defective transport of TRP and Rh1, leading to retinal degeneration. Our results identify XPORT as a molecular chaperone and provide a mechanistic link between TRP channels and their GPCRs during biosynthesis and transport.

The C-terminal helices of heat shock protein 70 are essential for J-domain binding and ATPase activation

The J-domain co-chaperones work together with the heat shock protein 70 (HSP70) chaperone to regulate many cellular events, but the mechanism underlying the J-domain-mediated HSP70 function remains elusive. We studied the interaction between human-inducible HSP70 and Homo sapiens J-domain protein (HSJ1a), a J domain and UIM motif-containing co-chaperone. The J domain of HSJ1a shares a conserved structure with other J domains from both eukaryotic and prokaryotic species, and it mediates the interaction with and the ATPase cycle of HSP70. Our in vitro study corroborates that the N terminus of HSP70 including the ATPase domain and the substrate-binding β-subdomain is not sufficient to bind with the J domain of HSJ1a. The C-terminal helical α-subdomain of HSP70, which was considered to function as a lid of the substrate-binding domain, is crucial for binding with the J domain of HSJ1a and stimulating the ATPase activity of HSP70. These fluctuating helices are likely to contribute to a proper conformation of HSP70 for J-domain binding other than directly bind with the J domain. Our findings provide an alternative mechanism of allosteric activation for functional regulation of HSP70 by its J-domain co-chaperones.

On the design of broad based screening assays to identify potential pharmacological chaperones of protein misfolding diseases

Correcting aberrant folds that develop during protein folding disease states is now an active research endeavor that is attracting increasing attention from both academic and industrial circles. One particular approach focuses on developing or identifying small molecule correctors or pharmacological chaperones that specifically stabilize the native fold. Unfortunately, the limited screening platforms available to rapidly identify or validate potential drug candidates are usually inadequate or slow because the folding disease proteins in question are often transiently folded and/or aggregation-prone, complicating and/or interfering with the assay outcomes. In this review, we outline and discuss the numerous platform options currently being employed to identify small molecule therapeutics for folding diseases. Finally, we describe a new stability screening approach that is broad based and is easily applicable toward a very large number of both common and rare protein folding diseases. The label free screening method described herein couples the promiscuity of the GroEL binding to transient aggregation-prone hydrophobic folds with surface plasmon resonance enabling one to rapidly identify potential small molecule pharmacological chaperones.

Cellular chaperones and folding enzymes are vital contributors to membrane bound replication and movement complexes during plant RNA virus infection

Cellular chaperones and folding enzymes play central roles in the formation of positive-strand and negative-strand RNA virus infection. This article examines the key cellular chaperones and discusses evidence that these factors are diverted from their cellular functions to play alternative roles in virus infection. For most chaperones discussed, their primary role in the cell is to ensure protein quality control. They are system components that drive substrate protein folding, complex assembly or disaggregation. Their activities often depend upon co-chaperones and ATP hydrolysis. During plant virus infection, Hsp70 and Hsp90 proteins play central roles in the formation of membrane-bound replication complexes for certain members of the tombusvirus, tobamovirus, potyvirus, dianthovirus, potexvirus, and carmovirus genus. There are several co-chaperones, including Yjd1, RME-8, and Hsp40 that associate with the bromovirus replication complex, pomovirus TGB2, and tospovirus Nsm movement proteins. There are also examples of plant viruses that rely on chaperone systems in the endoplasmic reticulum (ER) to support cell-to-cell movement. TMV relies on calreticulin to promote virus intercellular transport. Calreticulin also resides in the plasmodesmata and plays a role in calcium sequestration as well as glycoprotein folding. The pomovirus TGB2 interacts with RME-8 in the endosome. The potexvirus TGB3 protein stimulates expression of ER resident chaperones via the bZIP60 transcription factor. Up-regulating factors involved in protein folding may be essential to handling the load of viral proteins translated along the ER. In addition, TGB3 stimulates SKP1 which is a co-factor in proteasomal degradation of cellular proteins. Such chaperones and co-factors are potential targets for antiviral defense.

Enhanced HSP70 lysine methylation promotes proliferation of cancer cells through activation of Aurora kinase B

Although heat-shock protein 70 (HSP70), an evolutionarily highly conserved molecular chaperone, is known to be post-translationally modified in various ways such as phosphorylation, ubiquitination and glycosylation, physiological significance of lysine methylation has never been elucidated. Here we identify dimethylation of HSP70 at Lys-561 by SETD1A. Enhanced HSP70 methylation was detected in various types of human cancer by immunohistochemical analysis, although the methylation was barely detectable in corresponding non-neoplastic tissues. Interestingly, methylated HSP70 predominantly localizes to the nucleus of cancer cells, whereas most of the HSP70 protein locates to the cytoplasm. Nuclear HSP70 directly interacts with Aurora kinase B (AURKB) in a methylation-dependent manner and promotes AURKB activity in vitro and in vivo. We also find that methylated HSP70 has a growth-promoting effect in cancer cells. Our findings demonstrate a crucial role of HSP70 methylation in human carcinogenesis.

Cellular responses to misfolded proteins and protein aggregates

Maintenance of the proteome is a major homeostatic task of the cell and disregulation of protein homeostasis can be deadly. The accumulation of different forms of misfolded protein can perturb protein homeostasis and cause extensive cell and tissue damage. The cell has various quality control systems to help prevent the accumulation of misfolded proteins and the complexity of the different mechanisms that have evolved is bewildering. The first order of business for all quality control systems is recognition of misfolded proteins, which is followed by a triage decision. In many cases, modular molecular chaperones function in different assemblies with degradatory or folding co-factors to direct a misfolded protein toward continued life or death. Herein, an overview of quality control mechanisms that triage soluble cytosolic proteins, protein aggregates, and ER-associated proteins is presented.

Heat shock protein 70 inhibits hydrogen peroxide-induced nucleolar fragmentation via suppressing cleavage and down-regulation of nucleolin

It has been reported that nucleolar fragmentation is a part of the overall apoptotic morphology, however, it is currently obscure whether and how nucleolar fragmentation can be induced by hydrogen peroxide (H(2)O(2)) and heat shock protein 70 (Hsp70) can prevent nucleolar fragmentation. To dissect these two questions, C(2)C(12) myogenic cells and immortalized mouse embryonic fibroblasts (MEFs) with heat shock transcriptional factor 1 (HSF1) null mutation were treated with heat shock response (HS) (42.5 ± 0.5°C for 1 h and recovery at 37°C for 24 h) and then were insulted with 0.5 mmol/L H(2)O(2). Morphological changes of nucleoli were observed under contrast microscope or electronic microscope. It was found that (1) stimulation with H(2)O(2)-induced nucleolar fragmentation by mediating cleavage and down-regulation of nucleolar protein, nucleolin in C(2)C(12) myocytes and MEFs; (2) HS suppressed nucleolar fragmentation by inducing the expression of Hsp70 in an HSF1-dependent manner as indicated by assays of transfection with Hsp70 antisense oligonucleotides (AS-ONs) or recombinant plasmids of full-length Hsp70 cDNA; (3) protection of Hsp70 against nucleolar fragmentation was related to its accumulation in nucleolus mediated by nuclear localization sequence and its inhibition against cleavage and down-regulation of nucleolin. These results suggested that H(2)O(2)-induced nucleolar fragmentation and HS or Hsp70 inhibit H(2)O(2)-induced nucleolar fragmentation through the translocation of Hsp70 into nucleolar and its protection against impairment of nucleolin.

Regulation of survival gene hsp70

Rapid expression of the survival gene, inducible heat shock protein 70 (hsp70), is critical for mounting cytoprotection against severe cellular stress, like elevated temperature. Hsp70 protein chaperones the refolding of heat-denatured peptides to minimize proteolytic degradation as a part of an eukaryotically conserved phenomenon referred to as the heat shock response. The physiologic stress associated with exercise, which can include elevated temperature, mechanical damage, hypoxia, lowered pH, and reactive oxygen species generation, may promote protein unfolding, leading to hsp70 gene expression in skeletal myofibers. Although the pre-transcriptional activation of hsp70 gene expression has been thoroughly reviewed, discussion of downstream hsp70 gene regulation is less extensive. The purpose of this brief review was to examine all levels of hsp70 gene regulation in response to heat stress and exercise with a special focus on skeletal myofibers where data are available. In general, while heat stress represses bulk gene expression, hsp70 mRNA expression is enhanced. Post-transcriptionally, intronless hsp70 mRNA circumvents a host of decay pathways, as well as heat stress-repressed pre-mRNA splicing and nuclear export. Pre-translationally, hsp70 mRNA is excluded from stress granules and preferentially translated during heat stress-repressed global cap-dependent translation. Post-translationally, nascent Hsp70 protein is thermodynamically stable at elevated temperatures, allowing for the commencement of chaperoning activity early after synthesis to attenuate the heat shock response and protect against subsequent injury. This review demonstrates that hsp70 mRNA expression is closely coupled with functional protein translation.

Ecological and molecular consequences of prolonged drought and subsequent rehydration in Folsomia candida (Collembola)

Drought tolerance in water-permeable, soil-living Collembola (e.g. Folsomia candida) is achieved due to a unique water vapour absorption mechanism, where accumulation of sugars and polyols is essential. However, the molecular mechanisms underlying such adaptation as well as the maintenance of this survival strategy and the responses to rehydration after prolonged drought in these soil-living Collembola are unclear. In the present study, the functional relationships between ecological drought responses and expression of related target genes were investigated in F. candida exposed to mild and severe drought for up to 5 weeks by relating survival, moulting and reproduction rate with mRNA-level expression of 7 target genes during drought, dehydration and rehydration. Prolonged drought and subsequent rehydration induced significant changes in gene expression which could be related to the fitness traits studied. In F. candida the ecological and molecular responses to mild drought differed from those of severe drought. From the changes in gene expression, where significantly increased expression of Glucose-6-phosphate-isomerase (gpi) and Heat shock protein 70 (hsp70) was dominating, it is proposed that protection of cellular structure and function during prolonged mild drought (98.2% RH) is partly achieved from a continuous accumulation of compatible osmolytes in F. candida. To achieve protection during and after prolonged severe drought (96.1% RH), components related to cell division and development such as inositol monophosphatase and one of the small heat shock proteins (sHsps), Heat shock protein23 (hsp23), seem to play an important role in F. candida.

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.

Quantitative proteomics reveals that Hsp90 inhibition preferentially targets kinases and the DNA damage response

Despite the increasing importance of heat shock protein 90 (Hsp90) inhibitors as chemotherapeutic agents in diseases such as cancer, their global effects on the proteome remain largely unknown. Here we use high resolution, quantitative mass spectrometry to map protein expression changes associated with the application of the Hsp90 inhibitor, 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG). In depth data obtained from five replicate SILAC experiments enabled accurate quantification of about 6,000 proteins in HeLa cells. As expected, we observed activation of a heat shock response with induced expression of molecular chaperones, which refold misfolded proteins, and proteases, which degrade irreparably damaged polypeptides. Despite the broad range of known Hsp90 substrates, bioinformatics analysis revealed that particular protein classes were preferentially affected. These prominently included proteins involved in the DNA damage response, as well as protein kinases and especially tyrosine kinases. We followed up on this observation with a quantitative phosphoproteomic analysis of about 4,000 sites, which revealed that Hsp90 inhibition leads to much more down- than up-regulation of the phosphoproteome (34% down versus 6% up). This study defines the cellular response to Hsp90 inhibition at the proteome level and sheds light on the mechanisms by which it can be used to target cancer cells.

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.

Chemical and biological approaches for adapting proteostasis to ameliorate protein misfolding and aggregation diseases: progress and prognosis

Maintaining the proteome to preserve the health of an organism in the face of developmental changes, environmental insults, infectious diseases, and rigors of aging is a formidable task. The challenge is magnified by the inheritance of mutations that render individual proteins subject to misfolding and/or aggregation. Maintenance of the proteome requires the orchestration of protein synthesis, folding, degradation, and trafficking by highly conserved/deeply integrated cellular networks. In humans, no less than 2000 genes are involved. Stress sensors detect the misfolding and aggregation of proteins in specific organelles and respond by activating stress-responsive signaling pathways. These culminate in transcriptional and posttranscriptional programs that up-regulate the homeostatic mechanisms unique to that organelle. Proteostasis is also strongly influenced by the general properties of protein folding that are intrinsic to every proteome. These include the kinetics and thermodynamics of the folding, misfolding, and aggregation of individual proteins. We examine a growing body of evidence establishing that when cellular proteostasis goes awry, it can be reestablished by deliberate chemical and biological interventions. We start with approaches that employ chemicals or biological agents to enhance the general capacity of the proteostasis network. We then introduce chemical approaches to prevent the misfolding or aggregation of specific proteins through direct binding interactions. We finish with evidence that synergy is achieved with the combination of mechanistically distinct approaches to reestablish organismal proteostasis.

Optimal functional levels of activation-induced deaminase specifically require the Hsp40 DnaJa1

The enzyme activation-induced deaminase (AID) deaminates deoxycytidine at the immunoglobulin genes, thereby initiating antibody affinity maturation and isotype class switching during immune responses. In contrast, off-target DNA damage caused by AID is oncogenic. Central to balancing immunity and cancer is AID regulation, including the mechanisms determining AID protein levels. We describe a specific functional interaction between AID and the Hsp40 DnaJa1, which provides insight into the function of both proteins. Although both major cytoplasmic type I Hsp40s, DnaJa1 and DnaJa2, are induced upon B-cell activation and interact with AID in vitro, only DnaJa1 overexpression increases AID levels and biological activity in cell lines. Conversely, DnaJa1, but not DnaJa2, depletion reduces AID levels, stability and isotype switching. In vivo, DnaJa1-deficient mice display compromised response to immunization, AID protein and isotype switching levels being reduced by half. Moreover, DnaJa1 farnesylation is required to maintain, and farnesyltransferase inhibition reduces, AID protein levels in B cells. Thus, DnaJa1 is a limiting factor that plays a non-redundant role in the functional stabilization of AID.

Heat shock protein 70 kDa chaperone/DnaJ cochaperone complex employs an unusual dynamic interface

The heat shock protein 70 kDa (Hsp70)/DnaJ/nucleotide exchange factor system assists in intracellular protein (re)folding. Using solution NMR, we obtained a three-dimensional structure for a 75-kDa Hsp70-DnaJ complex in the ADP state, loaded with substrate peptide. We establish that the J domain (residues 1-70) binds with its positively charged helix II to a negatively charged loop in the Hsp70 nucleotide-binding domain. The complex shows an unusual "tethered" binding mode which is stoichiometric and saturable, but which has a dynamic interface. The complex represents part of a triple complex of Hsp70 and DnaJ both bound to substrate protein. Mutagenesis data indicate that the interface is also of relevance for the interaction of Hsp70 and DnaJ in the ATP state. The solution complex is completely different from a crystal structure of a disulfide-linked complex of homologous proteins [Jiang, et al. (2007) Mol Cell 28:422-433].

Dbp5, Gle1-IP6 and Nup159: a working model for mRNP export

Gene expression is a stepwise process involving distinct cellular processes including transcription, mRNA (mRNA) processing, mRNA export, and translation. As mRNAs are being synthesized, proteins associate with the RNA to form messenger ribonucleoprotein particles (mRNPs). Previous studies have demonstrated that the RNA-binding protein composition of these mRNPs is dynamic, changing as the mRNP moves through the different steps of gene expression, and playing a critical role in these events. An important step during this maturation process occurs at the cytoplasmic face of the nuclear pore complex (NPC) where the export protein Gle1 bound to inositol hexakisphosphate (IP 6) spatially activates the ATP-hydrolysis and mRNP-remodeling activity of the DEAD-box protein Dbp5. Recent work from our laboratory and others has provided important insights into the function and regulation of Dbp5. These include a more detailed explanation of the mechanism of Dbp5 RNP remodeling, the role of Gle1-IP6 in stimulating Dbp5 ATPase activity, and the identification of a novel paradigm for regulation of Dbp5 by Nup159. Based on in vitro biochemical assays, X-ray crystallography, and corresponding in vivo phenotypes, we propose here an updated model of the Dbp5 cycle during mRNP export through the NPC. This takes into account all available data and provides a platform for future studies.

Protein folding and quality control in the ER

The endoplasmic reticulum (ER) uses an elaborate surveillance system called the ER quality control (ERQC) system. The ERQC facilitates folding and modification of secretory and membrane proteins and eliminates terminally misfolded polypeptides through ER-associated degradation (ERAD) or autophagic degradation. This mechanism of ER protein surveillance is closely linked to redox and calcium homeostasis in the ER, whose balance is presumed to be regulated by a specific cellular compartment. The potential to modulate proteostasis and metabolism with chemical compounds or targeted siRNAs may offer an ideal option for the treatment of disease.

A chloroplast-targeted DnaJ protein AtJ8 is negatively regulated by light and has rapid turnover in darkness

The DnaJ proteins (also called as J proteins, J domain proteins or HSP40 proteins) function as molecular co-chaperones for the HSP70 proteins. We assessed the expression of the small chloroplast-targeted DnaJ protein, the AtJ8 protein, by subjecting the wild type Arabidopsis plants to different illumination conditions. It is shown that the expression of the transcripts and proteins of the ATJ8 gene is primarily regulated at the level of transcription. When plants were incubated under high light for 3h, both the transcripts and proteins were completely abolished. Upon transfer of plants to darkness, the transcripts started rapidly accumulating, and subsequently, the AtJ8 protein became visible after 2h in darkness. Conversely, incubation of plants in darkness or under low light intensities induced expression of the ATJ8 transcripts and proteins. Feeding plants with sugars clearly decreased the transcript and protein levels, and incubation with cycloheximide revealed a rapid turnover for AtJ8 in darkness. Moreover, the AtJ8 protein was found to be nearly missing from the var1 mutant, which lacks the FTSH5 protease. It is concluded that AtJ8 is expressed mainly in darkness, is prone to a rapid turnover but is partially stabilized by the FTSH proteases.

DnaJ homolog Hdj2 facilitates Japanese encephalitis virus replication

Background

Japanese encephalitis virus (JEV) is a member of the mosquito-borne Flaviviridae family of viruses that causes human encephalitis. Upon infection of a new host, replication of viral RNA involves not only the viral RNA-dependent RNA polymerase (RdRp), but also host proteins. Host factors involved in JEV replication are not well characterized.

Results

We identified Hdj2, a heat-shock protein 40 (Hsp40)/DnaJ homolog, from a mouse brain cDNA library interacting with JEV nonstructural protein 5 (NS5) encoding viral RdRp using yeast two-hybrid system. Specific interaction of Hdj2 with NS5 was confirmed by coimmunoprecipitation and colocalization in JEV-infected cells. Overexpression of Hdj2 in JEV-infected cells led to an increase of RNA synthesis, and the virus titer was elevated approximately 4.5- to 10-fold. Knocking down of Hdj2 by siRNA reduced the virus production significantly.

Conclusions

We conclude that Hdj2 directly associates with JEV NS5 and facilitates viral replication. This study is the first to demonstrate Hdj2 involved in JEV replication, providing insight into a potential therapeutic target and cell-based vaccine development of JEV infection.

Ordered assembly of heat shock proteins, Hsp26, Hsp70, Hsp90, and Hsp104, on expanded polyglutamine fragments revealed by chemical probes

In Saccharomyces cerevisae, expanded polyglutamine (polyQ) fragments are assembled into discrete cytosolic aggregates in a process regulated by the molecular chaperones Hsp26, Hsp70, Hsp90, and Hsp104. To better understand how the different chaperones might cooperate during polyQ aggregation, we used sequential immunoprecipitations and mass spectrometry to identify proteins associated with either soluble (Q25) or aggregation-prone (Q103) fragments at both early and later times after induction of their expression. We found that Hsp26, Hsp70, Hsp90, and other chaperones interact with Q103, but not Q25, within the first 2 h. Further, Hsp70 and Hsp90 appear to be partially released from Q103 prior to the maturation of the aggregates and before the recruitment of Hsp104. To test the importance of this seemingly ordered process, we used a chemical probe to artificially enhance Hsp70 binding to Q103. This treatment retained both Hsp70 and Hsp90 on the polyQ fragment and, interestingly, limited subsequent exchange for Hsp26 and Hsp104, resulting in incomplete aggregation. Together, these results suggest that partial release of Hsp70 may be an essential step in the continued processing of expanded polyQ fragments in yeast.

Influence of prion variant and yeast strain variation on prion-molecular chaperone requirements

Prions of budding yeast serve as a tractable model of amyloid behavior. Here we address the issue of the effect of yeast strain variation on prion stability, focusing also on the effect of amyloid conformation and the involvement of the co-chaperone Sis1, an essential J-protein partner of Hsp70. We found, despite an initial report to the contrary, that yeast strain background has little effect on the requirement for particular Sis1 domains for stable propagation of the prion [RNQ+], if the level of Sis1 expression is controlled. On the other hand, some variation in prion behavior was observed between yeast strains, in particular, the stability of certain [PSI+] variants. Future examination of such yeast strain-specific phenomena may provide useful insights into the basis of prion/chaperone dynamics.