Cooperation of molecular chaperones with the ubiquitin/proteasome system

The DnaJ protein OsDjA6 negatively regulates rice innate immunity to the blast fungus Magnaporthe oryzae

Rice blast, caused by Magnaporthe oryzae (synonym: Pyricularia oryzae), severely reduces rice production and grain quality. The molecular mechanism of rice resistance to M. oryzae is not fully understood. In this study, we identified a chaperone DnaJ protein, OsDjA6, which is involved in basal resistance to M. oryzae in rice. The OsDjA6 protein is distributed in the entire rice cell. The expression of OsDjA6 is significantly induced in rice after infection with a compatible isolate. Silencing of OsDjA6 in transgenic rice enhances resistance to M. oryzae and also results in an increased burst of reactive oxygen species after flg22 and chitin treatments. In addition, the expression levels of WRKY45, NPR1 and PR5 are increased in OsDjA6 RNAi plants, indicating that OsDjA6 may mediate resistance by affecting the salicylic acid pathway. Finally, we found that OsDjA6 interacts directly with the E3 ligase OsZFP1 in vitro and in vivo. These results suggest that the DnaJ protein OsDjA6 negatively regulates rice innate immunity, probably via the ubiquitination proteasome degradation pathway.

Cyclin-dependent kinase 5-mediated phosphorylation of CHIP promotes the tAIF-dependent death pathway in rotenone-treated cortical neurons

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase. Its dysregulation has been implicated in various neurodegenerative diseases. We previously reported that phosphorylation of the C-terminus of the Hsc70-interacting protein (CHIP) by Cdk5 promotes truncated apoptosis-inducing factor (tAIF)-mediated neuronal death induced by oxidative stress. Here, we determined whether this Cdk5-dependent cell death signaling pathway is present in experimental models of Parkinson's disease. First, we showed that rotenone activates Cdk5 in primary cultures of cortical neurons and causes tAIF-dependent neuronal cell death. This event was attenuated by negative regulation of endogenous Cdk5 activity by the pharmacological Cdk5 inhibitor, roscovitine, or by lentiviral knockdown of Cdk5. Cdk5 phosphorylates CHIP at Ser20 in rotenone-treated neurons. Consequently, overexpression of CHIPS20A, but not CHIPWT, attenuates tAIF-induced cell death in rotenone-treated cortical neurons. Taken together, these results indicate that phosphorylation of CHIP at Ser20 by Cdk5 activation inhibits CHIP-mediated tAIF degradation, thereby contributing to tAIF-induced neuronal cell death following rotenone treatment.

Chaperones and the Proteasome System: Regulating the Construction and Demolition of Striated Muscle

Protein folding factors (chaperones) are required for many diverse cellular functions. In striated muscle, chaperones are required for contractile protein function, as well as the larger scale assembly of the basic unit of muscle, the sarcomere. The sarcomere is complex and composed of hundreds of proteins and the number of proteins and processes recognized to be regulated by chaperones has increased dramatically over the past decade. Research in the past ten years has begun to discover and characterize the chaperones involved in the assembly of the sarcomere at a rapid rate. Because of the dynamic nature of muscle, wear and tear damage is inevitable. Several systems, including chaperones and the ubiquitin proteasome system (UPS), have evolved to regulate protein turnover. Much of our knowledge of muscle development focuses on the formation of the sarcomere but recent work has begun to elucidate the requirement and role of chaperones and the UPS in sarcomere maintenance and disease. This review will cover the roles of chaperones in sarcomere assembly, the importance of chaperone homeostasis and the cooperation of chaperones and the UPS in sarcomere integrity and disease.

Ecological disturbance influences adaptive divergence despite high gene flow in golden perch (Macquaria ambigua): Implications for management and resilience to climate change

Populations that are adaptively divergent but maintain high gene flow may have greater resilience to environmental change as gene flow allows the spread of alleles that have already been tested elsewhere. In addition, populations naturally subjected to ecological disturbance may already hold resilience to future environmental change. Confirming this necessitates ecological genomic studies of high dispersal, generalist species. Here we perform one such study on golden perch (Macquaria ambigua) in the Murray-Darling Basin (MDB), Australia, using a genome-wide SNP data set. The MDB spans across arid to wet and temperate to subtropical environments, with low to high ecological disturbance in the form of low to high hydrological variability. We found high gene flow across the basin and three populations with low neutral differentiation. Genotype-environment association analyses detected adaptive divergence predominantly linked to an arid region with highly variable riverine flow, and candidate loci included functions related to fat storage, stress and molecular or tissue repair. The high connectivity of golden perch in the MDB will likely allow locally adaptive traits in its most arid and hydrologically variable environment to spread and be selected in localities that are predicted to become arid and hydrologically variable in future climates. High connectivity in golden perch is likely due to their generalist life history and efforts of fisheries management. Our study adds to growing evidence of adaptation in the face of gene flow and highlights the importance of considering ecological disturbance and adaptive divergence in biodiversity management.

Pathways of cellular proteostasis in aging and disease

Ensuring cellular protein homeostasis, or proteostasis, requires precise control of protein synthesis, folding, conformational maintenance, and degradation. A complex and adaptive proteostasis network coordinates these processes with molecular chaperones of different classes and their regulators functioning as major players. This network serves to ensure that cells have the proteins they need while minimizing misfolding or aggregation events that are hallmarks of age-associated proteinopathies, including neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. It is now clear that the capacity of cells to maintain proteostasis undergoes a decline during aging, rendering the organism susceptible to these pathologies. Here we discuss the major proteostasis pathways in light of recent research suggesting that their age-dependent failure can both contribute to and result from disease. We consider different strategies to modulate proteostasis capacity, which may help develop urgently needed therapies for neurodegeneration and other age-dependent pathologies.

Mechanistic basis for a molecular triage reaction

Newly synthesized proteins are triaged between biosynthesis and degradation to maintain cellular homeostasis, but the decision-making mechanisms are unclear. We reconstituted the core reactions for membrane targeting and ubiquitination of nascent tail-anchored membrane proteins to understand how their fate is determined. The central six-component triage system is divided into an uncommitted client-SGTA complex, a self-sufficient targeting module, and an embedded but self-sufficient quality control module. Client-SGTA engagement of the targeting module induces rapid, private, and committed client transfer to TRC40 for successful biosynthesis. Commitment to ubiquitination is dictated primarily by comparatively slower client dissociation from SGTA and nonprivate capture by the BAG6 subunit of the quality control module. Our results provide a paradigm for how priority and time are encoded within a multichaperone triage system.

Hsp70 - a master regulator in protein degradation

Proteostasis, the controlled balance of protein synthesis, folding, assembly, trafficking and degradation, is a paramount necessity for cell homeostasis. Impaired proteostasis is a hallmark of ageing and of many human diseases. Molecular chaperones are essential for proteostasis in eukaryotic cells, and their function has traditionally been linked to protein folding, assembly and disaggregation. More recent findings suggest that chaperones also contribute to key steps in protein degradation. In particular, Hsp70 has an essential role in substrate degradation through the ubiquitin-proteasome system, as well as through different autophagy pathways. Accumulated knowledge suggests that the fate of an Hsp70 substrate is dictated by the combination of partners (cochaperones and other chaperones) that interact with Hsp70 in a given cell context.

Predicting the impact of Lynch syndrome-causing missense mutations from structural calculations

Accurate methods to assess the pathogenicity of mutations are needed to fully leverage the possibilities of genome sequencing in diagnosis. Current data-driven and bioinformatics approaches are, however, limited by the large number of new variations found in each newly sequenced genome, and often do not provide direct mechanistic insight. Here we demonstrate, for the first time, that saturation mutagenesis, biophysical modeling and co-variation analysis, performed in silico, can predict the abundance, metabolic stability, and function of proteins inside living cells. As a model system, we selected the human mismatch repair protein, MSH2, where missense variants are known to cause the hereditary cancer predisposition disease, known as Lynch syndrome. We show that the majority of disease-causing MSH2 mutations give rise to folding defects and proteasome-dependent degradation rather than inherent loss of function, and accordingly our in silico modeling data accurately identifies disease-causing mutations and outperforms the traditionally used genetic disease predictors. Thus, in conclusion, in silico biophysical modeling should be considered for making genotype-phenotype predictions and for diagnosis of Lynch syndrome, and perhaps other hereditary diseases.

The protein quality control system targets misfolded proteins for degradation. So far it has not been possible from sequence or structural data to predict the biological stability of a misfolded protein, or the effect of mutations on intracellular protein levels. Here we demonstrate that in silico saturation mutagenesis and biophysical calculations of the structural stability of the human mismatch repair protein MSH2 correlate with cellular protein levels, turnover and function. Of 24 different MSH2 variants, some of which are linked to Lynch syndrome, a destabilization of as little as 3 kcal/mol is sufficient to cause rapid degradation via the ubiquitin-proteasome pathway. Thus, biophysical modeling can, to a large extent, predict the metabolic stability of proteins. We also show that the same biophysical calculations can be used to distinguish with high accuracy neutral sequence variation from pathogenic variants, and that the calculations outperform several traditionally used disease predictors. We therefore suggest the method to be of potential value for patient stratification in Lynch syndrome, and perhaps other hereditary diseases.

Polyubiquitinylation Profile in Down Syndrome Brain Before and After the Development of Alzheimer Neuropathology

AIMS

Among the putative mechanisms proposed to be common factors in Down syndrome (DS) and Alzheimer's disease (AD) neuropathology, deficits in protein quality control (PQC) have emerged as a unifying mechanism of neurodegeneration. Considering that disturbance of protein degradation systems is present in DS and that oxidized/misfolded proteins require polyubiquitinylation for degradation via the ubiquitin proteasome system, this study investigated if dysregulation of protein polyubiquitinylation is associated with AD neurodegeneration in DS.

RESULTS

Postmortem brains from DS cases before and after development of AD neuropathology and age-matched controls were analyzed. By selectively isolating polyubiquitinated proteins, we were able to identify specific proteins with an altered pattern of polyubiquitinylation as a function of age. Interestingly, we found that oxidation is coupled with polyubiquitinylation for most proteins mainly involved in PQC and energy metabolism.

INNOVATION

This is the first study showing alteration of the polyubiquitinylation profile as a function of aging in DS brain compared with healthy controls. Understanding the onset of the altered ubiquitome profile in DS brain may contribute to identification of key molecular regulators of age-associated cognitive decline.

CONCLUSIONS

Disturbance of the polyubiquitinylation machinery may be a key feature of aging and neurodegeneration. In DS, age-associated deficits of the proteolytic system may further exacerbate the accumulation of oxidized/misfolded/polyubiquitinated proteins, which is not efficiently degraded and may become harmful to neurons and contribute to AD neuropathology. Antioxid. Redox Signal. 26, 280-298.

UBL/BAG-domain co-chaperones cause cellular stress upon overexpression through constitutive activation of Hsf1

As a result of exposure to stress conditions, mutations, or defects during synthesis, cellular proteins are prone to misfold. To cope with such partially denatured proteins, cells mount a regulated transcriptional response involving the Hsf1 transcription factor, which drives the synthesis of molecular chaperones and other stress-relieving proteins. Here, we show that the fission yeast Schizosaccharomyces pombe orthologues of human BAG-1, Bag101, and Bag102, are Hsp70 co-chaperones that associate with 26S proteasomes. Only a subgroup of Hsp70-type chaperones, including Ssa1, Ssa2, and Sks2, binds Bag101 and Bag102 and key residues in the Hsp70 ATPase domains, required for interaction with Bag101 and Bag102, were identified. In humans, BAG-1 overexpression is typically observed in cancers. Overexpression of bag101 and bag102 in fission yeast leads to a strong growth defect caused by triggering Hsp70 to release and activate the Hsf1 transcription factor. Accordingly, the bag101-linked growth defect is alleviated in strains containing a reduced amount of Hsf1 but aggravated in hsp70 deletion strains. In conclusion, we propose that the fission yeast UBL/BAG proteins release Hsf1 from Hsp70, leading to constitutive Hsf1 activation and growth defects.

CHIP promotes thyroid cancer proliferation via activation of the MAPK and AKT pathways

The carboxyl terminus of Hsp70-interacting protein (CHIP) is a U box-type ubiquitin ligase that plays crucial roles in various biological processes, including tumor progression. To date, the functional mechanism of CHIP in thyroid cancer remains unknown. Here, we obtained evidence of upregulation of CHIP in thyroid cancer tissues and cell lines. CHIP overexpression markedly enhanced thyroid cancer cell viability and colony formation in vitro and accelerated tumor growth in vivo. Conversely, CHIP knockdown impaired cell proliferation and tumor growth. Notably, CHIP promoted cell growth through activation of MAPK and AKT pathways, subsequently decreasing p27 and increasing cyclin D1 and p-FOXO3a expression. Our findings collectively indicate that CHIP functions as an oncogene in thyroid cancer, and is therefore a potential therapeutic target for this disease.

Cytoplasmic Ubiquitin-Specific Protease 19 (USP19) Modulates Aggregation of Polyglutamine-Expanded Ataxin-3 and Huntingtin through the HSP90 Chaperone

Ubiquitin-specific protease 19 (USP19) is one of the deubiquitinating enzymes (DUBs) involved in regulating the ubiquitination status of substrate proteins. There are two major isoforms of USP19 with distinct C-termini; the USP19_a isoform has a transmembrane domain for anchoring to the endoplasmic reticulum, while USP19_b contains an EEVD motif. Here, we report that the cytoplasmic isoform USP19_b up-regulates the protein levels of the polyglutamine (polyQ)-containing proteins, ataxin-3 (Atx3) and huntingtin (Htt), and thus promotes aggregation of their polyQ-expanded species in cell models. Our data demonstrate that USP19_b may orchestrate the stability, aggregation and degradation of the polyQ-expanded proteins through the heat shock protein 90 (HSP90) chaperone system. USP19_b directly interacts with HSP90 through its N-terminal CS (CHORD and SGT1)/P23 domains. In conjunction with HSP90, the cytoplasmic USP19 may play a key role in triage decision for the disease-related polyQ-expanded substrates, suggesting a function of USP19 in quality control of misfolded proteins by regulating their protein levels.

Human Heat shock protein 40 (Hsp40/DnaJB1) promotes influenza A virus replication by assisting nuclear import of viral ribonucleoproteins

A unique feature of influenza A virus (IAV) life cycle is replication of the viral genome in the host cell nucleus. The nuclear import of IAV genome is an indispensable step in establishing virus infection. IAV nucleoprotein (NP) is known to mediate the nuclear import of viral genome via its nuclear localization signals. Here, we demonstrate that cellular heat shock protein 40 (Hsp40/DnaJB1) facilitates the nuclear import of incoming IAV viral ribonucleoproteins (vRNPs) and is important for efficient IAV replication. Hsp40 was found to interact with NP component of IAV RNPs during early stages of infection. This interaction is mediated by the J domain of Hsp40 and N-terminal region of NP. Drug or RNAi mediated inhibition of Hsp40 resulted in reduced nuclear import of IAV RNPs, diminished viral polymerase function and attenuates overall viral replication. Hsp40 was also found to be required for efficient association between NP and importin alpha, which is crucial for IAV RNP nuclear translocation. These studies demonstrate an important role for cellular chaperone Hsp40/DnaJB1 in influenza A virus life cycle by assisting nuclear trafficking of viral ribonucleoproteins.

Dichlorvos exposure results in large scale disruption of energy metabolism in the liver of the zebrafish, Danio rerio

Background

Exposure to dichlorvos (DDVP), an organophosphorus pesticide, is known to result in neurotoxicity as well as other metabolic perturbations. However, the molecular causes of DDVP toxicity are poorly understood, especially in cells other than neurons and muscle cells. To obtain a better understanding of the process of non-neuronal DDVP toxicity, we exposed zebrafish to different concentrations of DDVP, and investigated the resulting changes in liver histology and gene transcription.

Results

Functional enrichment analysis of genes affected by DDVP exposure identified a number of processes involved in energy utilization and stress response in the liver. The abundance of transcripts for proteins involved in glucose metabolism was profoundly affected, suggesting that carbon flux might be diverted toward the pentose phosphate pathway to compensate for an elevated demand for energy and reducing equivalents for detoxification. Strikingly, many transcripts for molecules involved in β-oxidation and fatty acid synthesis were down-regulated. We found increases in message levels for molecules involved in reactive oxygen species responses as well as ubiquitination, proteasomal degradation, and autophagy.

To ensure that the effects of DDVP on energy metabolism were not simply a consequence of poor feeding because of neuromuscular impairment, we fasted fish for 29 or 50 h and analyzed liver gene expression in them. The patterns of gene expression for energy metabolism in fasted and DDVP-exposed fish were markedly different.

Conclusion

We observed coordinated changes in the expression of a large number of genes involved in energy metabolism and responses to oxidative stress. These results argue that an appreciable part of the effect of DDVP is on energy metabolism and is regulated at the message level. Although we observed some evidence of neuromuscular impairment in exposed fish that may have resulted in reduced feeding, the alterations in gene expression in exposed fish cannot readily be explained by nutrient deprivation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1941-2) contains supplementary material, which is available to authorized users.

Novel endogenous angiogenesis inhibitors and their therapeutic potential

Angiogenesis, the formation of new blood vessels from the pre-existing vasculature is essential for embryonic development and tissue homeostasis. It also plays critical roles in diseases such as cancer and retinopathy. A delicate balance between pro- and anti-angiogenic factors ensures normal physiological homeostasis. Endogenous angiogenesis inhibitors are proteins or protein fragments that are formed in the body and have the ability to limit angiogenesis. Many endogenous angiogenesis inhibitors have been discovered, and the list continues to grow. Endogenous protein/peptide inhibitors are relatively less toxic, better tolerated and have a lower risk of drug resistance, which makes them attractive as drug candidates. In this review, we highlight ten novel endogenous protein angiogenesis inhibitors discovered within the last five years, including ISM1, FKBPL, CHIP, ARHGAP18, MMRN2, SOCS3, TAp73, ZNF24, GPR56 and JWA. Although some of these proteins have been well characterized for other biological functions, we focus on their new and specific roles in angiogenesis inhibition and discuss their potential for therapeutic application.

Recent advances in the molecular diagnostics of gastric cancer

Gastric cancer (GC) is the third most common cause of cancer-related death in the world, representing a major global health issue. Although the incidence of GC is declining, the outcomes for GC patients remain dismal because of the lack of effective biomarkers to detect early GC and predict both recurrence and chemosensitivity. Current tumor markers for GC, including serum carcinoembryonic antigen and carbohydrate antigen 19-9, are not ideal due to their relatively low sensitivity and specificity. Recent improvements in molecular techniques are better able to identify aberrant expression of GC-related molecules, including oncogenes, tumor suppressor genes, microRNAs and long non-coding RNAs, and DNA methylation, as novel molecular markers, although the molecular pathogenesis of GC is complicated by tumor heterogeneity. Detection of genetic and epigenetic alterations from gastric tissue or blood samples has diagnostic value in the management of GC. There are high expectations for molecular markers that can be used as new screening tools for early detection of GC as well as for patient stratification towards personalized treatment of GC through prediction of prognosis and drug-sensitivity. In this review, the studies of potential molecular biomarkers for GC that have been reported in the publicly available literature between 2012 and 2015 are reviewed and summarized, and certain highlighted papers are examined.

Expression and significance of CHIP in canine mammary gland tumors

CHIP (Carboxy terminus of Hsc70 Interacting Protein) is an E3 ubiquitin ligase that can induce ubiquitination and degradation of several oncogenic proteins. The expression of CHIP is frequently lower in human breast cancer than in normal breast tissue. However, the expression and role of CHIP in the canine mammary gland tumor (CMGT) remain unclear. We investigated the potential correlation between CHIP expression and mammary gland tumor prognosis in female dogs. CHIP expression was measured in 54 dogs by immunohistochemistry and real-time RT-PCR. CHIP protein expression was significantly correlated with the histopathological diagnosis, outcome of disease and tumor classification. The transcriptional level of CHIP was significantly higher in normal tissues (P=0.001) and benign tumors (P=0.009) than it in malignant tumors. CHIP protein expression was significantly correlated with the transcriptional level of CHIP (P=0.0102). The log-rank test survival curves indicated that patients with low expression of CHIP had shorter overall periods of survival than those with higher CHIP protein expression (P=0.050). Our data suggest that CHIP may play an important role in the formation and development of CMGTs and serve as a valuable prognostic marker and potential target for genetic therapy.

Muscle Segment Homeobox Genes Direct Embryonic Diapause by Limiting Inflammation in the Uterus

Embryonic diapause is a reproductive strategy widespread in the animal kingdom. This phenomenon is defined by a temporary arrest in blastocyst growth and metabolic activity within a quiescent uterus without implantation until the environmental and maternal milieu become favorable for pregnancy to progress. We found that uterine Msx expression persists during diapause across species; their inactivation in the mouse uterus results in termination of diapause with the development of implantation-like responses ("pseudoimplantation") that ultimately succumbed to resorption. To understand the cause of this failure, we compared proteome profiles between floxed and Msx-deleted uteri. In deleted uteri, several functional networks, including transcription/translation, ubiquitin-proteasome, inflammation, and endoplasmic reticulum stress, were dysregulated. Computational modeling predicted intersection of these pathways on an enhanced inflammatory signature. Further studies showed that this signature was reflected in increased phosphorylated IκB levels and nuclear NFκB in deleted uteri. This was associated with enhanced proteasome activity and endoplasmic reticulum stress. Interestingly, treatment with anti-inflammatory glucocorticoid (dexamethasone) reduced the inflammatory signature with improvement of the diapause phenotype. These findings highlight an unexpected role of uterine Msx in limiting aberrant inflammatory responses to maintain embryonic diapause.

Heat shock protein responses to aging and proteotoxicity in the olfactory bulb

The olfactory bulb is one of the most vulnerable brain regions in age-related proteinopathies. Proteinopathic stress is mitigated by the heat shock protein (Hsp) family of chaperones. Here, we describe age-related decreases in Hsc70 in the olfactory bulb of the female rat and higher levels of Hsp70 and Hsp25 in middle and old age than at 2-4 months. To model proteotoxic and oxidative stress in the olfactory bulb, primary olfactory bulb cultures were treated with the proteasome inhibitors lactacystin and MG132 or the pro-oxidant paraquat. Toxin-induced increases were observed in Hsp70, Hsp25, and Hsp32. To determine the functional consequences of the increase in Hsp70, we attenuated Hsp70 activity with two mechanistically distinct inhibitors. The Hsp70 inhibitors greatly potentiated the toxicity of sublethal lactacystin or MG132 but not of paraquat. Although ubiquitinated protein levels were unchanged with aging in vivo or with sublethal MG132 in vitro, there was a large, synergistic increase in ubiquitinated proteins when proteasome and Hsp70 functions were simultaneously inhibited. Our study suggests that olfactory bulb cells rely heavily on Hsp70 chaperones to maintain homeostasis during mild proteotoxic, but not oxidative insults, and that Hsp70 prevents the accrual of ubiquitinated proteins in these cells. The olfactory bulb is affected in the early phases of many age-related neurodegenerative disorders. Here, we described the impact of aging on multiple heat shock proteins (Hsps), such as Hsp70, in the female rat olfactory bulb in vivo. Using multiple proteasome and Hsp70 inhibitors (see schematic), we found that proteotoxicity elicited a compensatory increase in Hsp70 in primary olfactory bulb cells in vitro. Hsp70 then reduced the proteotoxic buildup of ubiquitinated proteins and robustly protected against cell death according to three independent viability assays. Thus, olfactory bulb neurons can mount impressive natural adaptations to proteotoxic injury, perhaps explaining why neurodegenerative disorders are so delayed in onset and so slow to progress.

CHIP: a co-chaperone for degradation by the proteasome

Protein homeostasis relies on a balance between protein folding and protein degradation. Molecular chaperones like Hsp70 and Hsp90 fulfil well-defined roles in protein folding and conformational stability via ATP dependent reaction cycles. These folding cycles are controlled by associations with a cohort of non-client protein co-chaperones, such as Hop, p23 and Aha1. Pro-folding co-chaperones facilitate the transit of the client protein through the chaperone mediated folding process. However, chaperones are also involved in ubiquitin-mediated proteasomal degradation of client proteins. Similar to folding complexes, the ability of chaperones to mediate protein degradation is regulated by co-chaperones, such as the C terminal Hsp70 binding protein (CHIP). CHIP binds to Hsp70 and Hsp90 chaperones through its tetratricopeptide repeat (TPR) domain and functions as an E3 ubiquitin ligase using a modified RING finger domain (U-box). This unique combination of domains effectively allows CHIP to network chaperone complexes to the ubiquitin-proteasome system. This chapter reviews the current understanding of CHIP as a co-chaperone that switches Hsp70/Hsp90 chaperone complexes from protein folding to protein degradation.

Heat shock protein 70 regulates degradation of the mumps virus phosphoprotein via the ubiquitin-proteasome pathway

Mumps virus (MuV) infection induces formation of cytoplasmic inclusion bodies (IBs). Growing evidence indicates that IBs are the sites where RNA viruses synthesize their viral RNA. However, in the case of MuV infection, little is known about the viral and cellular compositions and biological functions of the IBs. In this study, pulldown purification and N-terminal amino acid sequencing revealed that stress-inducible heat shock protein 70 (Hsp72) was a binding partner of MuV phosphoprotein (P protein), which was an essential component of the IB formation. Immunofluorescence and immunoblotting analyses revealed that Hsp72 was colocalized with the P protein in the IBs, and its expression was increased during MuV infection. Knockdown of Hsp72 using small interfering RNAs (siRNAs) had little, if any, effect on viral propagation in cultured cells. Knockdown of Hsp72 caused accumulation of ubiquitinated P protein and delayed P protein degradation. These results show that Hsp72 is recruited to IBs and regulates the degradation of MuV P protein through the ubiquitin-proteasome pathway.

IMPORTANCE

Formation of cytoplasmic inclusion bodies (IBs) is a common characteristic feature in mononegavirus infections. IBs are considered to be the sites of viral RNA replication and transcription. However, there have been few studies focused on host factors recruited to the IBs and their biological functions. Here, we identified stress-inducible heat shock protein 70 (Hsp72) as the first cellular partner of mumps virus (MuV) phosphoprotein (P protein), which is an essential component of the IBs and is involved in viral RNA replication/transcription. We found that the Hsp72 mobilized to the IBs promoted degradation of the MuV P protein through the ubiquitin-proteasome pathway. Our data provide new insight into the role played by IBs in mononegavirus infection.

Regulation of protein turnover by heat shock proteins

Protein turnover reflects the balance between synthesis and degradation of proteins, and it is a crucial process for the maintenance of the cellular protein pool. The folding of proteins, refolding of misfolded proteins, and also degradation of misfolded and damaged proteins are involved in the protein quality control (PQC) system. Correct protein folding and degradation are controlled by many different factors, one of the most important of which is the heat shock protein family. Heat shock proteins (HSPs) are in the class of molecular chaperones, which may prevent the inappropriate interaction of proteins and induce correct folding. On the other hand, these proteins play significant roles in the degradation pathways, including endoplasmic reticulum-associated degradation (ERAD), the ubiquitin-proteasome system, and autophagy. This review focuses on the emerging role of HSPs in the regulation of protein turnover; the effects of HSPs on the degradation machineries ERAD, autophagy, and proteasome; as well as the role of posttranslational modifications in the PQC system.

ALS-related misfolded protein management in motor neurons and muscle cells

Amyotrophic Lateral Sclerosis (ALS) is the most common form of adult-onset motor neuron disease. It is now considered a multi-factorial and multi-systemic disorder in which alterations of the crosstalk between neuronal and non-neuronal cell types might influence the course of the disease. In this review, we will provide evidence that dysfunctions of affected muscle cells are not only a marginal consequence of denervation associated to motor neurons loss, but a direct consequence of cell muscle toxicity of mutant SOD1. In muscle, the misfolded state of mutant SOD1 protein, unlike in motor neurons, does not appear to have direct effects on protein aggregation and mitochondrial functionality. Muscle cells are, in fact, more capable than motor neurons to handle misfolded proteins, suggesting that mutant SOD1 toxicity in muscle is not mediated by classical mechanisms of intracellular misfolded proteins accumulation. Several recent works indicate that a higher activation of molecular chaperones and degradative systems is present in muscle cells, which for this reason are possibly able to better manage misfolded mutant SOD1. However, several alterations in gene expression and regenerative potential of skeletal muscles have also been reported as a consequence of the expression of mutant SOD1 in muscle. Whether these changes in muscle cells are causative of ALS or a consequence of motor neuron alterations is not yet clear, but their elucidation is very important, since the understanding of the mechanisms involved in mutant SOD1 toxicity in muscle may facilitate the design of treatments directed toward this specific tissue to treat ALS or at least to delay disease progression.

Contribution of chaperones to STAT pathway signaling

Aberrant STAT signaling is associated with the development and progression of many cancers and immune related diseases. Recent findings demonstrate that proteostasis modulators under clinical investigation for cancer therapy have a significant impact on STAT signaling, which may be critical for mediating their anti-cancer effects. Chaperones are critical for protein folding, stability and function and, thus, play an essential role in the maintenance of proteostasis. In this review we discuss the role of chaperones in STAT and tyrosine kinase (TK) protein folding, modulation of STAT and TK activity, and degradation of TKs. We highlight the important role of chaperones in STAT signaling, and how this knowledge has provided a framework for the development of new therapeutic avenues of targeting STAT signaling related pathologies.

Heat shock proteins in neurodegenerative disorders and aging

Many members of the heat shock protein family act in unison to refold or degrade misfolded proteins. Some heat shock proteins also directly interfere with apoptosis. These homeostatic functions are especially important in proteinopathic neurodegenerative diseases, in which specific proteins misfold, aggregate, and kill cells through proteotoxic stress. Heat shock protein levels may be increased or decreased in these disorders, with the direction of the response depending on the individual heat shock protein, the disease, cell type, and brain region. Aging is also associated with an accrual of proteotoxic stress and modulates expression of several heat shock proteins. We speculate that the increase in some heat shock proteins in neurodegenerative conditions may be partly responsible for the slow progression of these disorders, whereas the increase in some heat shock proteins with aging may help delay senescence. The protective nature of many heat shock proteins in experimental models of neurodegeneration supports these hypotheses. Furthermore, some heat shock proteins appear to be expressed at higher levels in longer-lived species. However, increases in heat shock proteins may be insufficient to override overwhelming proteotoxic stress or reverse the course of these conditions, because the expression of several other heat shock proteins and endogenous defense systems is lowered. In this review we describe a number of stress-induced changes in heat shock proteins as a function of age and neurodegenerative pathology, with an emphasis on the heat shock protein 70 (Hsp70) family and the two most common proteinopathic disorders of the brain, Alzheimer's and Parkinson's disease.

Overexpression of monoubiquitin improves photosynthesis in transgenic tobacco plants following high temperature stress

The ubiquitin/26S proteasome system (Ub/26S) is implicated in abiotic stress responses in plants. In this paper, transgenic tobacco plants overexpressing Ta-Ub2 from wheat were used to study the functions of Ub in the improvement of photosynthesis under high temperature (45°C) stress. We observed higher levels of Ub conjugates in transgenic plants under high temperature stress conditions compared to wild type (WT) as a result of the constitutive overexpression of Ta-Ub2, suggesting increased protein degradation by the 26S proteasome system under high temperature stress. Overexpressing Ub increased the photosynthetic rate (Pn) of transgenic tobacco plants, consistent with the improved ATPase activity in the thylakoid membrane and enhanced efficiency of PSII photochemistry. The higher D1 protein levels following high temperature stress in transgenic plants than WT were also observed. These findings imply that Ub may be involved in tolerance of photosynthesis to high temperature stress in plants. Compared with WT, the transgenic plants showed lower protein carbonylation and malondialdehyde (MDA) levels, less reactive oxygen species (ROS) accumulation, but higher antioxidant enzyme activity under high temperature stress. These findings suggest that the improved antioxidant capacity of transgenic plants may be one of the most important mechanisms underlying Ub-regulated high temperature tolerance.

Chaperoning proteins for destruction: diverse roles of Hsp70 chaperones and their co-chaperones in targeting misfolded proteins to the proteasome

Molecular chaperones were originally discovered as heat shock-induced proteins that facilitate proper folding of proteins with non-native conformations. While the function of chaperones in protein folding has been well documented over the last four decades, more recent studies have shown that chaperones are also necessary for the clearance of terminally misfolded proteins by the Ub-proteasome system. In this capacity, chaperones protect misfolded degradation substrates from spontaneous aggregation, facilitate their recognition by the Ub ligation machinery and finally shuttle the ubiquitylated substrates to the proteasome. The physiological importance of these functions is manifested by inefficient proteasomal degradation and the accumulation of protein aggregates during ageing or in certain neurodegenerative diseases, when chaperone levels decline. In this review, we focus on the diverse roles of stress-induced chaperones in targeting misfolded proteins to the proteasome and the consequences of their compromised activity. We further discuss the implications of these findings to the identification of new therapeutic targets for the treatment of amyloid diseases.

Protein quality control in the nucleus

In their natural environment, cells are regularly exposed to various stress conditions that may lead to protein misfolding, but also in the absence of stress, misfolded proteins occur as the result of mutations or failures during protein synthesis. Since such partially denatured proteins are prone to aggregate, cells have evolved several elaborate quality control systems to deal with these potentially toxic proteins. First, various molecular chaperones will seize the misfolded protein and either attempt to refold the protein or target it for degradation via the ubiquitin-proteasome system. The degradation of misfolded proteins is clearly compartmentalized, so unique degradation pathways exist for misfolded proteins depending on whether their subcellular localization is ER/secretory, mitochondrial, cytosolic or nuclear. Recent studies, mainly in yeast, have shown that the nucleus appears to be particularly active in protein quality control. Thus, specific ubiquitin-protein ligases located in the nucleus, target not only misfolded nuclear proteins, but also various misfolded cytosolic proteins which are transported to the nucleus prior to their degradation. In comparison, much less is known about these mechanisms in mammalian cells. Here we highlight recent advances in our understanding of nuclear protein quality control, in particular regarding substrate recognition and proteasomal degradation.

In vitro effect of cadmium and copper on separated blood leukocytes of Dicentrarchus labrax

The immunotoxic effects of heavy metals on blood leukocytes of sea bass (Dicentrarchus labrax) were examined. The cells, separated by a discontinuous Percoll-gradients, were exposed in vitro to various sublethal concentrations of cadmium and copper (10(-7)M, 10(-5)M, and 10(-3)M) and their immunotoxic effect was then evaluated by measuring neutral red uptake, MTT assay, DNA fragmentation and Hsp70 gene expression. First of all, we demonstrated that the cells treated in vitro could incorporate Cd and Cu. A relationship between heavy metal exposure and dose-time-dependent alterations in responses of leukocytes from blood was found for both metals, but copper was more immunotoxic than cadmium in all assays performed. A significant reduction in the cells׳ ability to uptake neutral red and viability by MTT assay was recorded, indicating that both cadmium and copper could change the membrane permeability, inducing cellular apoptosis when the concentration of metals reached 10(-3)M. The apoptotic effect may also explain the high level of cytotoxicity found when the leukocytes were exposed to higher concentration of metals. These results demonstrated that toxic effect of copper and cadmium affect on the mechanisms of cell-mediated immunity reducing the immune defences of the organism.

A chaperone-assisted degradation pathway targets kinetochore proteins to ensure genome stability

Cells are regularly exposed to stress conditions that may lead to protein misfolding. To cope with this challenge, molecular chaperones selectively target structurally perturbed proteins for degradation via the ubiquitin-proteasome pathway. In mammals the co-chaperone BAG-1 plays an important role in this system. BAG-1 has two orthologues, Bag101 and Bag102, in the fission yeast Schizosaccharomyces pombe. We show that both Bag101 and Bag102 interact with 26S proteasomes and Hsp70. By epistasis mapping we identify a mutant in the conserved kinetochore component Spc7 (Spc105/Blinkin) as a target for a quality control system that also involves, Hsp70, Bag102, the 26S proteasome, Ubc4 and the ubiquitin-ligases Ubr11 and San1. Accordingly, chromosome missegregation of spc7 mutant strains is alleviated by mutation of components in this pathway. In addition, we isolated a dominant negative version of the deubiquitylating enzyme, Ubp3, as a suppressor of the spc7-23 phenotype, suggesting that the proteasome-associated Ubp3 is required for this degradation system. Finally, our data suggest that the identified pathway is also involved in quality control of other kinetochore components and therefore likely to be a common degradation mechanism to ensure nuclear protein homeostasis and genome integrity.

The accumulation of misfolded proteins represents a considerable threat to the health of individual cells and has been linked to severe diseases, including cancer and neurodegenerative disorders. To cope with this threat, especially under stress conditions, cells have evolved efficient quality control mechanisms. In general, these rely on molecular chaperones to either seize and refold misfolded proteins, or target them for degradation via the ubiquitin-proteasome system. At present, our understanding of what determines whether a chaperone commits to a folding or a degradation mode is limited. However, studies suggest that association with certain regulatory co-chaperones contributes to this process. Here, we show that certain BAG-1-type co-chaperones function in quality control by targeting misfolded kinetochore components for proteolysis. The presented genetic and biochemical data show that specific ubiquitin conjugating enzymes and ubiquitin-protein ligases maintain nuclear protein homeostasis and are required for upholding genome integrity.

Alterations in the Atlantic cod (Gadus morhua) hepatic thiol-proteome after methylmercury exposure

Proteomic studies in general have demonstrated that the most effective and thorough analysis of biological samples requires subfractionation and/or enrichment prior to downstream processing. In the present study, Atlantic cod (Gadus morhua) liver samples were fractionated using activated thiol sepharose to isolate hepatic proteins containing free/reactive cysteines. This subset of proteins is of special interest when studying the physiological effects attributed to methylmercury (MeHg) exposure. Methylmercury is a persistent environmental contaminant that has a potent affinity toward thiol groups, and can directly bind proteins via available cysteine residues. Further, alterations in the cod thiol-proteome following MeHg exposure (2 mg/kg body weight) were explored with two-dimensional gel electrophoresis combined with downstream mass spectrometry analyses for protein identifications. Thirty-five protein spots were found to respond to MeHg exposure, and 13 of these were identified when searching cod-specific databases with acquired mass spectrometry data. Among the identified thiol-containing proteins, some are known to respond to MeHg treatment, including constituents of the cytoskeleton, and proteins involved in oxidative stress responses, protein synthesis, protein folding, and energy metabolism. Methylmercury also appeared to affect cod heme metabolism/turnover, producing significantly altered levels of hemoglobin and hemopexin in liver following metal exposure. The latter finding suggests that MeHg may also affect the hematological system in Atlantic cod.

Inclusion body formation, macroautophagy, and the role of HDAC6 in neurodegeneration

The failure to clear misfolded or aggregated proteins from the cytoplasm of nerve cells and glia is a common pathogenic event in a variety of neurodegenerative disorders. This might be causally related to defects in the major proteolytic systems, i.e., the ubiquitin-proteasomal system and the autophagic pathway. Large protein aggregates and defective organelles are excluded from the proteasome. They can be degraded only by macroautophagy, which is a highly selective process. It requires p62 to act as a bridge connecting ubiquitinated protein aggregates and autophagosomes, and the tubulin deacetylase histone deacetylase 6 (HDAC6). HDAC6 has recently been identified as a constituent in Lewy bodies of Parkinson disease and glial cytoplasmic inclusions of multiple system atrophy. It is considered a sensor of proteasomal inhibition and a cellular stress surveillance factor, and plays a central role in autophagy by controlling the fusion process of autophagosomes with lysosomes. Upon proteasomal inhibition, HDAC6 is relocated and recruited to polyubiquitin-positive aggresomes. Tubulin acetylation is a major consequence of HDAC6 inhibition, and HDAC6 inhibition restores microtubule (MT)-dependent transport mechanisms in neurons. This suggests the involvement of HDAC6 in neurodegenerative diseases. Furthermore, the protein tau seems to be a substrate for HDAC6. Tau acetylation impairs MT assembly and promotes tau fibrillization in vitro. It has been suggested that acetylation and phosphorylation of tau at multiples sites may act synergistically in the pathogenesis of tau fibrillization. In this review, we will survey the process of aggresome formation, macroautophagy and the role of autophagosomal proteins and HDAC6 in inclusion body formation.

GSK3β controls epithelial-mesenchymal transition and tumor metastasis by CHIP-mediated degradation of Slug

Glycogen synthase kinase 3 beta (GSK3β) is highly inactivated in epithelial cancers and is known to inhibit tumor migration and invasion. The zinc-finger-containing transcriptional repressor, Slug, represses E-cadherin transcription and enhances epithelial-mesenchymal transition (EMT). In this study, we find that the GSK3β-pSer9 level is associated with the expression of Slug in non-small cell lung cancer. GSK3β-mediated phosphorylation of Slug facilitates Slug protein turnover. Proteomic analysis reveals that the carboxyl terminus of Hsc70-interacting protein (CHIP) interacts with wild-type Slug (wtSlug). Knockdown of CHIP stabilizes the wtSlug protein and reduces Slug ubiquitylation and degradation. In contrast, nonphosphorylatable Slug-4SA is not degraded by CHIP. The accumulation of nondegradable Slug may further lead to the repression of E-cadherin expression and promote cancer cell migration, invasion and metastasis. Our findings provide evidence of a de novo GSK3β-CHIP-Slug pathway that may be involved in the progression of metastasis in lung cancer.

The DNAJB6 and DNAJB8 protein chaperones prevent intracellular aggregation of polyglutamine peptides

Fragments of proteins containing an expanded polyglutamine (polyQ) tract are thought to initiate aggregation and toxicity in at least nine neurodegenerative diseases, including Huntington's disease. Because proteasomes appear unable to digest the polyQ tract, which can initiate intracellular protein aggregation, preventing polyQ peptide aggregation by chaperones should greatly improve polyQ clearance and prevent aggregate formation. Here we expressed polyQ peptides in cells and show that their intracellular aggregation is prevented by DNAJB6 and DNAJB8, members of the DNAJ (Hsp40) chaperone family. In contrast, HSPA/Hsp70 and DNAJB1, also members of the DNAJ chaperone family, did not prevent peptide-initiated aggregation. Intriguingly, DNAJB6 and DNAJB8 also affected the soluble levels of polyQ peptides, indicating that DNAJB6 and DNAJB8 inhibit polyQ peptide aggregation directly. Together with recent data showing that purified DNAJB6 can suppress fibrillation of polyQ peptides far more efficiently than polyQ expanded protein fragments in vitro, we conclude that the mechanism of DNAJB6 and DNAJB8 is suppression of polyQ protein aggregation by directly binding the polyQ tract.

Design principles of protein biosynthesis-coupled quality control

The protein biosynthetic machinery, composed of ribosomes, chaperones, and localization factors, is increasingly found to interact directly with factors dedicated to protein degradation. The coupling of these two opposing processes facilitates quality control of nascent polypeptides at each stage of their maturation. Sequential checkpoints maximize the overall fidelity of protein maturation, minimize the exposure of defective products to the bulk cellular environment, and protect organisms from protein misfolding diseases.

The Not4 RING E3 Ligase: A Relevant Player in Cotranslational Quality Control

The Not4 RING E3 ligase is a subunit of the evolutionarily conserved Ccr4-Not complex. Originally identified in yeast by mutations that increase transcription, it was subsequently defined as an ubiquitin ligase. Substrates for this ligase were characterized in yeast and in metazoans. Interestingly, some substrates for this ligase are targeted for polyubiquitination and degradation, while others instead are stable monoubiquitinated proteins. The former are mostly involved in transcription, while the latter are a ribosomal protein and a ribosome-associated chaperone. Consistently, Not4 and all other subunits of the Ccr4-Not complex are present in translating ribosomes. An important function for Not4 in cotranslational quality control has emerged. In the absence of Not4, the total level of polysomes is reduced. In addition, translationally arrested polypeptides, aggregated proteins, and polyubiquitinated proteins accumulate. Its role in quality control is likely to be related on one hand to its importance for the functional assembly of the proteasome and on the other hand to its association with the RNA degradation machines. Not4 is in an ideal position to signal to degradation mRNAs whose translation has been aborted, and this defines Not4 as a key player in the quality control of newly synthesized proteins.

Hsp90: still a viable target in prostate cancer

Heat shock protein 90 (Hsp90) is a molecular chaperone that regulates the maturation, activation and stability of critical signaling proteins that drive the development and progression of prostate cancer, including the androgen receptor. Despite robust preclinical data demonstrating anti-tumor activity of first-generation Hsp90 inhibitors in prostate cancer, poor clinical responses initially cast doubt over the clinical utility of this class of agent. Recent advances in compound design and development, use of novel preclinical models and further biological insights into Hsp90 structure and function have now stimulated a resurgence in enthusiasm for these drugs as a therapeutic option. This review highlights how the development of new-generation Hsp90 inhibitors with improved physical and pharmacological properties is unfolding, and discusses the potential contexts for their use either as single agents or in combination, for men with metastatic prostate cancer.

Molecular mechanism of mutant p53 stabilization: the role of HSP70 and MDM2

Numerous p53 missense mutations possess gain-of-function activities. Studies in mouse models have demonstrated that the stabilization of p53 R172H (R175H in human) mutant protein, by currently unknown factors, is a prerequisite for its oncogenic gain-of-function phenotype such as tumour progression and metastasis. Here we show that MDM2-dependent ubiquitination and degradation of p53 R175H mutant protein in mouse embryonic fibroblasts is partially inhibited by increasing concentration of heat shock protein 70 (HSP70/HSPA1-A). These phenomena correlate well with the appearance of HSP70-dependent folding intermediates in the form of dynamic cytoplasmic spots containing aggregate-prone p53 R175H and several molecular chaperones. We propose that a transient but recurrent interaction with HSP70 may lead to an increase in mutant p53 protein half-life. In the presence of MDM2 these pseudoaggregates can form stable amyloid-like structures, which occasionally merge into an aggresome. Interestingly, formation of folding intermediates is not observed in the presence of HSC70/HSPA8, the dominant-negative K71S variant of HSP70 or HSP70 inhibitor. In cancer cells, where endogenous HSP70 levels are already elevated, mutant p53 protein forms nuclear aggregates without the addition of exogenous HSP70. Aggregates containing p53 are also visible under conditions where p53 is partially unfolded: 37°C for temperature-sensitive variant p53 V143A and 42°C for wild-type p53. Refolding kinetics of p53 indicate that HSP70 causes transient exposure of p53 aggregate-prone domain(s). We propose that formation of HSP70- and MDM2-dependent protein coaggregates in tumours with high levels of these two proteins could be one of the mechanisms by which mutant p53 is stabilized. Moreover, sequestration of p73 tumour suppressor protein by these nuclear aggregates may lead to gain-of-function phenotypes.

Rapid and massive green fluorescent protein production leads to formation of protein Y-bodies in plant cells

Although high level of recombinant protein production can be achieved via transient expression in plant cells, the mechanism by which tolerance to the presence of recombinant protein is acquired remains unclear. Here we show that green fluorescent protein (GFP) encoded by an intron-optimized tobacco mosaic viral vector formed large membraneless GFP bodies called Y-bodies that demonstrated mainly perinuclear localization. The Y-bodies were heterogeneous in size, approaching the size of the cell nucleus. Experiments with extracted GFP and live cell imaging showed that Y-bodies included actively fluorescent, non-aggregated, tightly packed GFP molecules. The plant cells probably formed Y-bodies to exclude the recombinant protein from normal physiological turnover.

Sodium arsenite and cadmium chloride induction of proteasomal inhibition and HSP accumulation in Xenopus laevis A6 kidney epithelial cells

Sodium arsenite (NA) and cadmium chloride (CdCl(2)) are relatively abundant environmental toxicants that have multiple toxic effects including carcinogenesis, dysfunction of gene regulation and DNA and protein damage. In the present study, treatment of Xenopus laevis A6 kidney epithelial cells with concentrations of NA (20-30 μM) or CdCl(2) (100-200 μM) that induced HSP30 and HSP70 accumulation also produced an increase in the relative levels of ubiquitinated protein. Actin protein levels were unchanged in these experiments. In time course experiments, the levels of ubiquitinated protein and HSPs increased over a 24h exposure to NA or CdCl(2). Furthermore, treatment of cells with NA or CdCl(2) reduced the relative levels of proteasome chymotrypsin (CT)-like activity compared to control. Interestingly, pretreatment of cells with the HSP accumulation inhibitor, KNK437, prior to NA or CdCl(2) exposure decreased the relative levels of ubiquitinated protein as well as HSP30 and HSP70. A similar finding was made with ubiquitinated protein induced by proteasomal inhibitors, MG132 and celastrol, known to induce HSP accumulation in A6 cells. However, the NA- or CdCl(2)-induced decrease in proteasome CT-like activity was not altered by KNK437 pretreatment. This study has shown for the first time in poikilothermic vertebrates that NA and CdCl(2) can inhibit proteasomal activity and that there is a possible association between HSP accumulation and the mechanism of protein ubiquitination.

Molecular and functional characterization of the only known hemiascomycete ortholog of the carboxyl terminus of Hsc70-interacting protein CHIP in the yeast Yarrowia lipolytica

The carboxyl terminus of Hsc70-interacting protein (CHIP) is an Hsp70 co-chaperone and a U-box ubiquitin ligase that plays a crucial role in protein quality control in higher eukaryotes. The yeast Yarrowia lipolytica is the only known hemiascomycete where a CHIP ortholog is found. Here, we characterize Y. lipolytica's CHIP ortholog (Yl.Chn1p) and document its interactions with components of the protein quality control machinery. We show that Yl.Chn1p is non-essential unless Y. lipolytica is severely stressed. We sought for genetic interactions among key components of the Y. lipolytica protein quality control arsenal, including members of the Ssa-family of Hsp70 molecular chaperones, the Yl.Bag1p Hsp70 nucleotide exchange factor, the Yl.Chn1p and Yl.Ufd2p U-box ubiquitin ligases, the Yl.Doa10p and Yl.Hrd1p RING-finger ubiquitin ligases, and the Yl.Hsp104p disaggregating molecular chaperone. Remarkably, no synthetic phenotypes were observed among null alleles of the corresponding genes in most cases, suggesting that overlapping pathways efficiently act to enable Y. lipolytica cells to survive under harsh conditions. Yl.Chn1p interacts with mammalian and Saccharomyces cerevisiae members of the Hsp70 family in vitro, and these interactions are differently regulated by Hsp70 co-chaperones. We demonstrate notably that Yl.Chn1p/Ssa1p interaction is Fes1p-dependent and the formation of an Yl.Chn1p/Ssa1p/Sse1p ternary complex. Finally, we show that, similar to Sse1p, Yl.Chn1p can act as a "holdase" to prevent the aggregation of a heat-denatured protein.

Selection for intrabody solubility in mammalian cells using GFP fusions

Single-chain antibody fragments (scFv) expressed in the cytoplasm of mammalian cells, also called intrabodies, have many applications in functional proteomics. These applications are, however, limited by the aggregation-prone behaviour of many intrabodies. We show here that two scFv with highly homologous sequences and comparable soluble expression levels in Escherichia coli cytoplasm have different behaviours in mammalian cells. When over-expressed, one of the scFv aggregates in the cytoplasm whereas the second one is soluble and active. When expressed at low levels, using a retroviral vector, as a fusion with the green fluorescent protein (GFP) the former does not form aggregates and is degraded, resulting in weakly fluorescent cells, whereas the latter is expressed as a soluble protein, resulting in strongly fluorescent cells. These data suggest that the GFP signal can be used to evaluate the soluble expression of intrabodies in mammalian cells. When applied to a subset of an E.coli-optimised intrabody library, we showed that the population of GFP+ cells contains indeed soluble mammalian intrabodies. Altogether, our data demonstrate that the requirements for soluble intrabody expression are different in E.coli and mammalian cells, and that intrabody libraries can be directly optimised in human cells using a simple GFP-based assay.

Apoptosis-inducing factor: structure, function, and redox regulation

Apoptosis-inducing factor (AIF) is a flavin adenine dinucleotide-containing, NADH-dependent oxidoreductase residing in the mitochondrial intermembrane space whose specific enzymatic activity remains unknown. Upon an apoptotic insult, AIF undergoes proteolysis and translocates to the nucleus, where it triggers chromatin condensation and large-scale DNA degradation in a caspase-independent manner. Besides playing a key role in execution of caspase-independent cell death, AIF has emerged as a protein critical for cell survival. Analysis of in vivo phenotypes associated with AIF deficiency and defects, and identification of its mitochondrial, cytoplasmic, and nuclear partners revealed the complexity and multilevel regulation of AIF-mediated signal transduction and suggested an important role of AIF in the maintenance of mitochondrial morphology and energy metabolism. The redox activity of AIF is essential for optimal oxidative phosphorylation. Additionally, the protein is proposed to regulate the respiratory chain indirectly, through assembly and/or stabilization of complexes I and III. This review discusses accumulated data with respect to the AIF structure and outlines evidence that supports the prevalent mechanistic view on the apoptogenic actions of the flavoprotein, as well as the emerging concept of AIF as a redox sensor capable of linking NAD(H)-dependent metabolic pathways to apoptosis.