The cochaperone HspBP1 inhibits the CHIP ubiquitin ligase and stimulates the maturation of the cystic fibrosis transmembrane conductance regulator

Protein stability and aggregation in Parkinson's disease

Parkinson's disease (PD), the second most common age-related neurodegenerative disease, results in abnormalities in motor functioning. Many fundamental questions regarding its aetiology remain unanswered. Pathologically, it is not until 70-80% of the dopaminergic neurons from the substantia nigra pars compacta are lost before clinical symptoms are observed. Thus research into PD is complicated by this apparent paradox in that what appears to be the beginning of the disease at the clinical level is really the end point neurochemically. Consequently, we can only second guess when the disease started and what initiated it. The causation is probably complex, with contributions from both genetic and environmental factors. Intracellular proteinaceous inclusions, Lewy bodies and Lewy neurites, found in surviving dopaminergic neurons, are the key pathological characteristic of PD. Their presence points to an inability within these terminally differentiated cells to deal with aggregating proteins. Recent advances in our knowledge of the underlying disease process have come about from studies on models based on genes associated with rare hereditary forms of PD, and mitochondrial toxins that mimic the behavioural effects of PD. The reason that dopaminergic neurons are particularly sensitive may be due to the additional cellular stress caused by the breakdown of the inherently chemically unstable neurotransmitter, dopamine. In the present review, I discuss the proposal that in sporadic disease, interlinked problems of protein processing and inappropriate mitochondrial activity seed the foundation for age-related increased levels of protein damage, and a reduced ability to deal with the damage, leading to inclusion formation and, ultimately, cell toxicity.

Cystic fibrosis transmembrane regulator missing the first four transmembrane segments increases wild type and DeltaF508 processing

We previously generated an adenoassociated viral gene therapy vector, rAAV-Delta264 cystic fibrosis transmembrane conductance regulator (CFTR), missing the first four transmembrane domains of CFTR. When infected into monkey lungs, Delta264 CFTR increased the levels of endogenous wild type CFTR protein. To understand this process, we transfected Delta264 CFTR plasmid cDNA into COS7 cells, and we noted that protein expression from the truncation mutant is barely detectable when compared with wild type or DeltaF508 CFTR. Delta264 CFTR protein expression increases dramatically when cells are treated with proteasome inhibitors. Cycloheximide experiments show that Delta264 CFTR is degraded faster than DeltaF508 CFTR. VCP and HDAC6, two proteins involved in retrograde translocation from endoplasmic reticulum to cytosol for proteasomal and aggresomal degradation, coimmunoprecipitate with Delta264 CFTR. In cotransfection studies in COS7 cells and in transfection of Delta264 CFTR into cells stably expressing wild type and DeltaF508 CFTR, Delta264 CFTR increases wild type CFTR protein and increases levels of maturation of immature band B to mature band C of DeltaF508 CFTR. Thus the adenoassociated viral vector, rAAV-Delta264 CFTR, is a highly promising cystic fibrosis gene therapy vector because it increases the amount of mature band C protein both from wild type and DeltaF508 CFTR and associates with key elements in quality control mechanism of CFTR.

Role of calnexin in the ER quality control and productive folding of CFTR; differential effect of calnexin knockout on wild-type and DeltaF508 CFTR

Cystic fibrosis (CF) is caused by the mutation in CF transmembrane conductance regulator (CFTR), a cAMP-dependent Cl(-) channel at the plasma membrane of epithelium. The most common mutant, DeltaF508 CFTR, has competent Cl(-) channel function, but fails to express at the plasma membrane since it is retained in the endoplasmic reticulum (ER) by the ER quality control system. Here, we show that calnexin (CNX) is not necessary for the ER retention of DeltaF508 CFTR. Our data show that CNX knockout (KO) does not affect the biosynthetic processing, cellular localization or the Cl(-) channel function of DeltaF508 CFTR. Importantly, cAMP-induced Cl(-) current in colonic epithelium from CNX KO/DeltaF508 CFTR mice was comparable with that of DeltaF508 CFTR mice, indicating that CNX KO failed to rescue the ER retention of DeltaF508 CFTR in vivo. Moreover, we show that CNX assures the efficient expression of WT CFTR, but not DeltaF508 CFTR, by inhibiting the proteasomal degradation, indicating that CNX might stimulate the productive folding of WT CFTR, but not DeltaF508 CFTR, which has folding defects.

Chemical rescue of deltaF508-CFTR mimics genetic repair in cystic fibrosis bronchial epithelial cells

In a previous study of sodium 4-phenylbutyrate (4-PBA)-responsive proteins in cystic fibrosis (CF) IB3-1 bronchial epithelial cells, we identified 85 differentially expressed high abundance proteins from whole cellular lysate (Singh, O. V., Vij, N., Mogayzel, P. J., Jr., Jozwik, C., Pollard, H. B., and Zeitlin, P. L. (2006) Pharmacoproteomics of 4-phenylbutyrate-treated IB3-1 cystic fibrosis bronchial epithelial cells. J. Proteome Res. 5, 562-571). In the present work we hypothesize that a subset of heat shock proteins that interact with cystic fibrosis transmembrane conductance regulator (CFTR) in common during chemical rescue and genetic repair will identify therapeutic networks for targeted intervention. Immunocomplexes were generated from total cellular lysates, and three subcellular fractions (endoplasmic reticulum (ER), cytosol, and plasma membrane) with anti-CFTR polyclonal antibody from CF (IB3-1), chemically rescued CF (4-PBA-treated IB3-1), and genetically repaired CF (IB3-1/S9 daughter cells repaired by gene transfer with adeno-associated virus-(wild type) CFTR). CFTR-interacting proteins were analyzed on two-dimensional gels and identified by mass spectrometry. A set of 16 proteins known to act in ER-associated degradation were regulated in common and functionally connected to the protein processing, protein folding, and inflammatory response. Some of these proteins were modulated exclusively in ER, cytosol, or plasma membrane. A subset of 4-PBA-modulated ER-associated degradation chaperones (GRP94, HSP84, GRP78, GRP75, and GRP58) was observed to associate with the immature B form of CFTR in ER. HSP70 and HSC70 interacted with the C band (mature form) of CFTR at the cell surface. We conclude that chemically rescued CFTR associates with a specific set of HSP70 family proteins that mark therapeutic interactions and can be useful to correct both ion transport and inflammatory phenotypes in CF subjects.

The activities and function of molecular chaperones in the endoplasmic reticulum

Most proteins in the secretory pathway are translated, folded, and subjected to quality control at the endoplasmic reticulum (ER). These processes must be flexible enough to process diverse protein conformations, yet specific enough to recognize when a protein should be degraded. Molecular chaperones are responsible for this decision making process. ER associated chaperones assist in polypeptide translocation, protein folding, and ER associated degradation (ERAD). Nevertheless, we are only beginning to understand how chaperones function, how they are recruited to specific substrates and assist in folding/degradation, and how unique chaperone classes make quality control "decisions".

The role of the UPS in cystic fibrosis

CF is an inherited autosomal recessive disease whose lethality arises from malfunction of CFTR, a single chloride (Cl^-) ion channel protein. CF patients harbor mutations in the CFTR gene that lead to misfolding of the resulting CFTR protein, rendering it inactive and mislocalized. Hundreds of CF-related mutations have been identified, many of which abrogate CFTR folding in the endoplasmic reticulum (ER). More than 70% of patients harbor the ΔF508 CFTR mutation that causes misfolding of the CFTR proteins. Consequently, mutant CFTR is unable to reach the apical plasma membrane of epithelial cells that line the lungs and gut, and is instead targeted for degradation by the UPS. Proteins located in both the cytoplasm and ER membrane are believed to identify misfolded CFTR for UPS-mediated degradation. The aberrantly folded CFTR protein then undergoes polyubiquitylation, carried out by an E1-E2-E3 ubiquitin ligase system, leading to degradation by the 26S proteasome. This ubiquitin-dependent loss of misfolded CFTR protein can be inhibited by the application of ‘corrector’ drugs that aid CFTR folding, shielding it from the UPS machinery. Corrector molecules elevate cellular CFTR protein levels by protecting the protein from degradation and aiding folding, promoting its maturation and localization to the apical plasma membrane. Combinatory application of corrector drugs with activator molecules that enhance CFTR Cl^- ion channel activity offers significant potential for treatment of CF patients.

Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; ).

Cystic fibrosis transmembrane conductance regulator (CFTR) functionality is dependent on coatomer protein I (COPI)

BACKGROUND INFORMATION

Cystic fibrosis results from mutations in the ABC transporter CFTR (cystic fibrosis transmembrane conductance regulator), which functions as a cAMP-regulated anion channel. The most prevalent mutation in CFTR, the Phe(508) deletion, results in the generation of a trafficking and functionally deficient channel. The cellular machineries involved in modulating CFTR trafficking and function have not been fully characterized. In the present study, we identified a role for the COPI (coatomer protein I) cellular trafficking machinery in the development of the CFTR polypeptide into a functional chloride channel. To examine the role of COPI in CFTR biosynthesis, we employed the cell line ldlF, which harbours a temperature-sensitive mutation in epsilon-COP, a COPI subunit, to inhibit COPI function and then determined whether the CFTR polypeptide produced from the transfected gene developed into a cAMP-regulated chloride channel.

RESULTS

When COPI was inactivated in the ldlF cells by an elevated temperature pulse (39 degrees C), the CFTR polypeptide was detected on the cell surface by immunofluorescence microscopy and cell-surface biotinylation. Therefore, CFTR proceeded upstream in the secretory pathway in the absence of COPI function, a result demonstrated previously by others. In contrast, electrophysiological measurements indicated an absence of cAMP-stimulated chloride efflux, suggesting that channel function was impaired. In comparison, expression of CFTR at the same elevated temperature (39 degrees C) in an epsilon-COP-rescued cell line [ldlF(ldlF)], which has an introduced wild-type epsilon-COP gene in addition to the mutant epsilon-COP gene, showed restoration of cAMP-stimulated channel activity, confirming the requirement of COPI for channel function.

CONCLUSIONS

These results therefore suggest that generation of the folded-functional conformation of CFTR requires COPI.

CHIP chaperones wild type p53 tumor suppressor protein

Wild type p53 exists in a constant state of equilibrium between wild type and mutant conformation and undergoes conformational changes at elevated temperature. We have demonstrated that the co-chaperone CHIP (carboxyl terminus of Hsp70-interacting protein), which suppressed aggregation of several misfolded substrates and induced the proteasomal degradation of both wild type and mutant p53, physically interacts with the amino terminus of WT53 and prevented it from irreversible thermal inactivation. CHIP preferentially binds to the p53 mutant phenotype and restored the DNA binding activity of heat-denatured p53 in an ATP-independent manner. In cells under elevated temperatures that contained a higher level of p53 mutant phenotype, CHIP restored the native-like conformation of p53 in the presence of geldanamycin, whereas CHIP-small interfering RNA considerably increased the mutant form. Further, under elevated temperatures, the levels of CHIP and p53 were higher in nucleus, and chromatin immunoprecipitation shows the presence of p53 and CHIP together upon the DNA binding site in the p21 and p53 promoters. We propose that CHIP might be a direct chaperone of wild type p53 that helps p53 in maintaining wild type conformation under physiological condition as well as help resurrect p53 mutant phenotype into a folded native state under stress condition.

Chaperone functions of the E3 ubiquitin ligase CHIP

The carboxyl terminus of the Hsc70-interacting protein (CHIP) is an Hsp70 co-chaperone as well as an E3 ubiquitin ligase that protects cells from proteotoxic stress. The abilities of CHIP to interact with Hsp70 and function as a ubiquitin ligase place CHIP at a pivotal position in the protein quality control system, where its entrance into Hsp70-substrate complexes partitions nonnative proteins toward degradation. However, the manner by which Hsp70 substrates are selected for ubiquitination by CHIP is not well understood. We discovered that CHIP possesses an intrinsic chaperone activity that enables it to selectively recognize and bind nonnative proteins. Interestingly, the chaperone function of CHIP is temperature-sensitive and is dramatically enhanced by heat stress. The ability of CHIP to recognize nonnative protein structure may aid in selection of slow folding or misfolded polypeptides for ubiquitination.

Pharmacoproteomics of 4-phenylbutyrate-treated IB3-1 cystic fibrosis bronchial epithelial cells

4-Phenylbutyrate (4-PBA) is an oral butyrate derivative that has recently been approved for treatment of urea cycle disorders and is under investigation in clinical trials of cancer, hemoglobinopathies, and cystic fibrosis (CF). We hypothesized that proteome profiling of IB3-1 cystic fibrosis bronchial epithelial cells treated with 4-PBA would identify butyrate-responsive cellular chaperones, protein processing enzymes, and cell trafficking molecules associated with the amelioration of the chloride transport defect in these cells. Protein profiles were analyzed by two-dimensional gel electrophoresis and mass spectrometry. Over a pI range of 4-7 and molecular weight from 20 to 150 kDa a total of 85 differentially expressed proteins were detected. Most of the identified proteins were chaperones, catalytic enzymes, and proteins comprising structural elements, cellular defense, protein biosynthesis, trafficking activity, and ion transport. Subsets of these proteins were confirmed by immunoblot analysis. These data represent a first-draft of the pharmacoproteomics map of 4-PBA treated cystic fibrosis bronchial epithelial cells.

Hsp90 cochaperone Aha1 downregulation rescues misfolding of CFTR in cystic fibrosis

The pathways that distinguish transport of folded and misfolded cargo through the exocytic (secretory) pathway of eukaryotic cells remain unknown. Using proteomics to assess global cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein interactions (the CFTR interactome), we show that Hsp90 cochaperones modulate Hsp90-dependent stability of CFTR protein folding in the endoplasmic reticulum (ER). Cell-surface rescue of the most common disease variant that is restricted to the ER, DeltaF508, can be initiated by partial siRNA silencing of the Hsp90 cochaperone ATPase regulator Aha1. We propose that failure of DeltaF508 to achieve an energetically favorable fold in response to the steady-state dynamics of the chaperone folding environment (the "chaperome") is responsible for the pathophysiology of CF. The activity of cargo-associated chaperome components may be a common mechanism regulating folding for ER exit, providing a general framework for correction of misfolding disease.

Small heat-shock proteins select deltaF508-CFTR for endoplasmic reticulum-associated degradation

Secreted proteins that fail to achieve their native conformations, such as cystic fibrosis transmembrane conductance regulator (CFTR) and particularly the DeltaF508-CFTR variant can be selected for endoplasmic reticulum (ER)-associated degradation (ERAD) by molecular chaperones. Because the message corresponding to HSP26, which encodes a small heat-shock protein (sHsp) in yeast was up-regulated in response to CFTR expression, we examined the impact of sHsps on ERAD. First, we observed that CFTR was completely stabilized in cells lacking two partially redundant sHsps, Hsp26p and Hsp42p. Interestingly, the ERAD of a soluble and a related integral membrane protein were unaffected in yeast deleted for the genes encoding these sHsps, and CFTR polyubiquitination was also unaltered, suggesting that Hsp26p/Hsp42p are not essential for polyubiquitination. Next, we discovered that DeltaF508-CFTR degradation was enhanced when a mammalian sHsp, alphaA-crystallin, was overexpressed in human embryonic kidney 293 cells, but wild-type CFTR biogenesis was unchanged. Because alphaA-crystallin interacted preferentially with DeltaF508-CFTR and because purified alphaA-crystallin suppressed the aggregation of the first nucleotide-binding domain of CFTR, we suggest that sHsps maintain the solubility of DeltaF508-CFTR during the ERAD of this polypeptide.

Calreticulin Negatively Regulates the Cell Surface Expression of Cystic Fibrosis Transmembrane Conductance Regulator

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-dependent Cl- channel at the plasma membrane, and its malfunction results in cystic fibrosis, the most common lethal genetic disease in Caucasians. Quality control of CFTR is strictly regulated by several molecular chaperones. Here we show that calreticulin (CRT), which is a lectin-like chaperone in the endoplasmic reticulum (ER), negatively regulates the cell surface CFTR. RNA interference-based CRT knockdown induced the increase of CFTR expression. Consistently, this effect was observed in vivo. CRT heterozygous (CRT+/-) mice had a higher endogenous expression of CFTR than the wild-type mice. Moreover, CRT overexpression induced cell surface expression of CRT, and it significantly decreased the cell surface expression and function of CFTR. CRT overexpression destabilized the cell surface CFTR by enhancing endocytosis, leading to proteasomal degradation. Deletion of the carboxyl domain of CRT, which results in its ER export, increased the negative effect and enhanced the interaction with CFTR. Thus, CRT in the post-ER compartments may act as a negative regulator of the cell surface CFTR.

The role of p97/Cdc48p in endoplasmic reticulum-associated degradation: from the immune system to yeast

Quality control mechanisms in the endoplasmic reticulum prevent deployment of aberrant or unwanted proteins to distal destinations and target them to degradation by a process known as endoplasmic reticulum-associated degradation, or ERAD. Attempts to characterize ERAD by identifying a specific component have revealed that the most general characteristic of ERAD is that the protein substrates are initially translocated to the ER and eventually eliminated in the cytosol by the ubiquitin-proteasome pathway. Hence, dislocation from the ER back to the cytosol is a hallmark in ERAD and p97/Cdc48p, a cytosolic AAA-ATPase that is essential for ERAD, appears to provide the driving force for this process. Moreover, unlike many ERAD components that participate in degradation of either lumenal or membrane substrates, p97/Cdc48p has a more general role in that it is required for ERAD of both types of substrates. Although p97/Cdc48p is not dedicated exclusively to ERAD, its ability to physically associate with ERAD substrates, with VIMP and with the E3 gp78 suggest that the p97/Cdc48Ufdl/Npl4 complex acts as a coordinator that maintains coupling between the different steps in ERAD.

Therapy through chaperones: sense or antisense? Cystic fibrosis as a model disease

Massive production and accumulation of a single abnormal protein may constitute a major toxic burden for the cell and even compromise the organism's long-term viability. Consequently, adaptation and survival have forced evolution to create 'quality control' mechanisms that detect, monitor, and often degrade such abnormally folded gene products, in which molecular chaperones are key players. Notwithstanding this, there are numerous examples of misfolded proteins which, in spite of being recognized as aberrant and efficiently discarded by cellular quality control, still retain some of the functional properties of their wild-type counterparts, so that their maintenance in the cell would be beneficial for the organism. Herein are described the cellular roles of molecular chaperones and some new insights on the mechanisms by which they influence the development of human diseases caused by mutations that lead to protein misfolding. A special emphasis is given to cystic fibrosis, a classical genetic disorder resulting from the retention and degradation of a mutant, albeit functional, protein by the endoplasmic reticulum quality control. This particular system has been a good example to describe the mechanisms that are likely to be shared by a number of protein substrates, to define the common characteristics of the mutants, as well as to identify the mechanistic intervenients in their retention and degradation. Finally, new approaches aimed at correcting protein folding defects are discussed, including the potential of molecular chaperones (e.g., through RNA interference) as novel therapeutic targets, and the usage of chemical or pharmacological chaperones as new therapeutic agents.

Development of a sensitive assay for the measurement of antibodies against heat shock protein binding protein 1 (HspBP1): increased levels of anti-HspBP1 IgG are prevalent in HIV infected subjects

The 70 kDa heat shock protein (Hsp70) is generally considered to be an intracellular protein, however, there is evidence that Hsp70 can be found in the extracellular environment. Hsp70 and antibodies against Hsp70 have been reported in human serum. Recent evidence has shown that Hsp70 antibodies are elevated in HIV infected individuals. This study reports on the antibody levels against a co-chaperone, HspBP1, that regulates Hsp70 activity. We have developed a solid-phase enzyme linked assay for the determination of anti-HspBP1 IgG antibodies. We report here that HspBP1 antibodies are present in human serum and the levels are elevated approximately twofold in HIV infected patients. There was no correlation between HspBP1 antibody levels and clinical parameters nor was there a relation between anti-Hsp70 levels and anti-HspBP1 levels. The presence of HspBP1 antibodies in human serum suggests that the protein may also be present in the serum. The increased level of HspBP1 antibodies in HIV infected individuals suggests a relationship directly to the virus or indirectly to secondary consequences of HIV infection.

BAG-2 acts as an inhibitor of the chaperone-associated ubiquitin ligase CHIP

Cellular protein quality control involves a close interplay between molecular chaperones and the ubiquitin/proteasome system. We recently identified a degradation pathway, on which the chaperone Hsc70 delivers chaperone clients, such as misfolded forms of the cystic fibrosis transmembrane conductance regulator (CFTR), to the proteasome. The cochaperone CHIP is of central importance on this pathway, because it acts as a chaperone-associated ubiquitin ligase. CHIP mediates the attachment of a ubiquitin chain to a chaperone-presented client protein and thereby stimulates its proteasomal degradation. To gain further insight into the function of CHIP we isolated CHIP-containing protein complexes from human HeLa cells and analyzed their composition by peptide mass fingerprinting. We identified the Hsc70 cochaperone BAG-2 as a main component of CHIP complexes. BAG-2 inhibits the ubiquitin ligase activity of CHIP by abrogating the CHIP/E2 cooperation and stimulates the chaperone-assisted maturation of CFTR. The activity of BAG-2 resembles that of the previously characterized Hsc70 cochaperone and CHIP inhibitor HspBP1. The presented data therefore establish multiple mechanisms to control the destructive activity of the CHIP ubiquitin ligase in human cells.

Most F508del-CFTR is targeted to degradation at an early folding checkpoint and independently of calnexin

Biosynthesis and folding of multidomain transmembrane proteins is a complex process. Structural fidelity is monitored by endoplasmic reticulum (ER) quality control involving the molecular chaperone calnexin. Retained misfolded proteins undergo ER-associated degradation (ERAD) through the ubiquitin-proteasome pathway. Our data show that the major degradation pathway of the cystic fibrosis transmembrane conductance regulator (CFTR) with F508del (the most frequent mutation found in patients with the genetic disease cystic fibrosis) from the ER is independent of calnexin. Moreover, our results demonstrate that inhibition of mannose-processing enzymes, unlike most substrate glycoproteins, does not stabilize F508del-CFTR, although wild-type (wt) CFTR is drastically stabilized under the same conditions. Together, our data support a novel model by which wt and F508del-CFTR undergo ERAD from two distinct checkpoints, the mutant being disposed of independently of N-glycosidic residues and calnexin, probably by the Hsc70/Hsp70 machinery, and wt CFTR undergoing glycan-mediated ERAD.

HSJ1 Is a Neuronal Shuttling Factor for the Sorting of Chaperone Clients to the Proteasome

Protein degradation in eukaryotic cells usually involves the attachment of a ubiquitin chain to a substrate protein and its subsequent sorting to the proteasome. Molecular mechanisms underlying the sorting process only recently began to emerge and rely on a cooperation of chaperone machineries and ubiquitin-chain recognition factors [1-3]. Here, we identify isoforms of the cochaperone HSJ1 as neuronal shuttling factors for ubiquitylated proteins. HSJ1 combines a J-domain that stimulates substrate loading onto the Hsc70 chaperone with ubiquitin interaction motifs (UIMs) involved in binding ubiquitylated chaperone clients. HSJ1 prevents client aggregation, shields clients against chain trimming by ubiquitin hydrolases, and stimulates their sorting to the proteasome. In this way, HSJ1 isoforms participate in ER-associated degradation (ERAD) and protect neurons against cytotoxic protein aggregation.

Processing of CFTR: traversing the cellular maze--how much CFTR needs to go through to avoid cystic fibrosis?

Biosynthesis of the cystic fibrosis transmembrane conductance regulator (CFTR), like other proteins aimed at the cell surface, involves transport through a series of membranous compartments, the first of which is the endoplasmic reticulum (ER), where CFTR encounters the appropriate environment for folding, oligomerization, maturation, and export from the ER. After exiting the ER, CFTR has to traffic through complex pathways until it reaches the cell surface. Although not yet fully understood, the fine details of these pathways are starting to emerge, partially through identification of an increasing number of CFTR-interacting proteins (CIPs) and the clarification of their roles in CFTR trafficking and function. These aspects of CFTR biogenesis/degradation and by membrane traffic and CIPs are discussed in this review. Following this description of complex pathways and multiple checkpoints to which CFTR is subjected in the cell, the basic question remains of how much CFTR has to overcome these barriers and be functionally expressed at the plasma membrane to avoid CF. This question is also discussed here.

The chaperone-associated ubiquitin ligase CHIP is able to target p53 for proteasomal degradation

The cellular level of the tumor suppressor p53 is tightly regulated through induced degradation via the ubiquitin/proteasome system. The ubiquitin ligase Mdm2 plays a pivotal role in stimulating p53 turnover. However, recently additional ubiquitin ligases have been identified that participate in the degradation of the tumor suppressor. Apparently, multiple degradation pathways are employed to ensure proper destruction of p53. Here we show that the chaperone-associated ubiquitin ligase CHIP is able to induce the proteasomal degradation of p53. CHIP-induced degradation was observed for mutant p53, which was previously shown to associate with the chaperones Hsc70 and Hsp90, and for the wild-type form of the tumor suppressor. Our data reveal that mutant and wild-type p53 transiently associate with molecular chaperones and can be diverted onto a degradation pathway through this association.

Regulation of Hsp70 function by HspBP1: structural analysis reveals an alternate mechanism for Hsp70 nucleotide exchange

HspBP1 belongs to a family of eukaryotic proteins recently identified as nucleotide exchange factors for Hsp70. We show that the S. cerevisiae ortholog of HspBP1, Fes1p, is required for efficient protein folding in the cytosol at 37 degrees C. The crystal structure of HspBP1, alone and complexed with part of the Hsp70 ATPase domain, reveals a mechanism for its function distinct from that of BAG-1 or GrpE, previously characterized nucleotide exchange factors of Hsp70. HspBP1 has a curved, all alpha-helical fold containing four armadillo-like repeats unlike the other nucleotide exchange factors. The concave face of HspBP1 embraces lobe II of the ATPase domain, and a steric conflict displaces lobe I, reducing the affinity for nucleotide. In contrast, BAG-1 and GrpE trigger a conserved conformational change in lobe II of the ATPase domain. Thus, nucleotide exchange on eukaryotic Hsp70 occurs through two distinct mechanisms.

Cooperation of molecular chaperones with the ubiquitin/proteasome system

Molecular chaperones and energy-dependent proteases have long been viewed as opposing forces that control protein biogenesis. Molecular chaperones are specialized in protein folding, whereas energy-dependent proteases such as the proteasome mediate efficient protein degradation. Recent data, however, suggest that molecular chaperones directly cooperate with the ubiquitin/proteasome system during protein quality control in eukaryotic cells. Modulating the intracellular balance of protein folding and protein degradation may open new strategies for the treatment of human diseases that involve chaperone pathways such as cancer and diverse amyloid diseases.

Distinct but overlapping functions of Hsp70, Hsp90, and an Hsp70 nucleotide exchange factor during protein biogenesis in yeast

Hsp70 and Hsp90 molecular chaperones play essential roles in protein expression and maturation, and while catalyzing protein folding they can "decide" to target mis-folded substrates for degradation. In this report, we show for the first time distinct but partially overlapping requirements for Hsp90, Hsp70, and an Hsp70 nucleotide exchange factor (NEF) at different steps during the biogenesis of a model substrate, firefly luciferase (FFLux), in yeast. By examining the inducible expression of FFLux in wild type cells and in specific yeast mutants, we find that the Fes1p NEF is required for efficient FFLux folding, whereas the Hsp70, Ssa1p, is required for both protein folding and stability, and to maintain maximal FFLux mRNA levels. In contrast, Hsp90 function was primarily necessary to express the FFLux-encoding gene from an inducible promoter. Together, these data indicate previously unknown roles for these proteins and point to the complexity with which chaperones and cochaperones function in the cell.

A foldable CFTR{Delta}F508 biogenic intermediate accumulates upon inhibition of the Hsc70-CHIP E3 ubiquitin ligase

CFTRΔF508 exhibits a correctable protein-folding defect that leads to its misfolding and premature degradation, which is the cause of cystic fibrosis (CF). Herein we report on the characterization of the CFTRΔF508 biogenic intermediate that is selected for proteasomal degradation and identification of cellular components that polyubiquitinate CFTRΔF508. Nonubiquitinated CFTRΔF508 accumulates in a kinetically trapped, but folding competent conformation, that is maintained in a soluble state by cytosolic Hsc70. Ubiquitination of Hsc70-bound CFTRΔF508 requires CHIP, a U box containing cytosolic cochaperone. CHIP is demonstrated to function as a scaffold that nucleates the formation of a multisubunit E3 ubiquitin ligase whose reconstituted activity toward CFTR is dependent upon Hdj2, Hsc70, and the E2 UbcH5a. Inactivation of the Hsc70–CHIP E3 leads CFTRΔF508 to accumulate in a nonaggregated state, which upon lowering of cell growth temperatures, can fold and reach the cell surface. Inhibition of CFTRΔF508 ubiquitination can increase its cell surface expression and may provide an approach to treat CF.

Endoplasmic reticulum-associated protein degradation—one model fits all?

The endoplasmic reticulum (ER) is the eukaryotic organelle where most secretory proteins are folded for subsequent delivery to their site of action. Proper folding of newly synthesized proteins is monitored by a stringent ER quality control system. This system recognizes misfolded or unassembled proteins and prevents them from reaching their final destination. Instead, they are extracted from the ER, polyubiquitinated and degraded by the cytosolic proteasome. With the identification of novel components and substrates, a more and more complex picture of this process emerges in which distinct pathways target different sets of substrates for destruction.