Differential regulation of CFTRDeltaF508 degradation by ubiquitin ligases gp78 and Hrd1

gp78-regulated KAP1 phosphorylation induces radioresistance in breast cancer by facilitating PPP1CC/PPP2CA ubiquitination

Adjuvant radiation therapy is a common treatment for breast cancer, yet its effectiveness is often limited by radioresistance in patients. Identifying novel targets to combat this radioresistance is imperative. Recent investigations show that gp78 is upregulated in drug-resistant breast cancer cells. Our study reveals that gp78 markedly increased the phosphorylation of KAP1 and promoted DNA damage repair caused by ionizing radiation. Mechanistically, gp78 degrades phosphatases (PPP1CC/PPP2CA) in a ubiquitination-dependent manner. PPP1CC and PPP2CA are crucial regulators of KAP1 phosphorylation in response to DNA damage. Therefore, gp78 leads to a notable elevation in the phosphorylation of KAP1 by degrading phosphatases, thereby promoting the DNA damage repair process and increasing the radioresistance of tumor cells. The identification of gp78 as a pivotal regulator in radioresistance suggests a promising avenue for intervention. Combining blockade strategies targeting gp78 holds a signification potential for reversing radioresistance and improving the efficacy of breast cancer radiotherapy.

Targeting ubiquitination machinery in cystic fibrosis: Where do we stand?

Cystic Fibrosis (CF) is a genetic disease caused by mutations in CFTR gene expressing the anion selective channel CFTR located at the plasma membrane of different epithelial cells. The most commonly investigated variant causing CF is F508del. This mutation leads to structural defects in the CFTR protein, which are recognized by the endoplasmic reticulum (ER) quality control system. As a result, the protein is retained in the ER and degraded via the ubiquitin-proteasome pathway. Although blocking ubiquitination to stabilize the CFTR protein has long been considered a potential pharmacological approach in CF, progress in this area has been relatively slow. Currently, no compounds targeting this pathway have entered clinical trials for CF. On the other hand, the emergence of Orkambi initially, and notably the subsequent introduction of Trikafta/Kaftrio, have demonstrated the effectiveness of molecular chaperone-based therapies for patients carrying the F508del variant and even showed efficacy against other variants. These treatments directly target the CFTR variant protein without interfering with cell signaling pathways. This review discusses the limits and potential future of targeting protein ubiquitination in CF.

Small-molecule correctors divert CFTR-F508del from ERAD by stabilizing sequential folding states

Over 80% of people with cystic fibrosis (CF) carry the F508del mutation in the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride ion channel at the apical plasma membrane (PM) of epithelial cells. F508del impairs CFTR folding causing it to be destroyed by endoplasmic reticulum associated degradation (ERAD). Small-molecule correctors, which act as pharmacological chaperones to divert CFTR-F508del from ERAD, are the primary strategy for treating CF, yet corrector development continues with only a rudimentary understanding of how ERAD targets CFTR-F508del. We conducted genome-wide CRISPR/Cas9 knockout screens to systematically identify the molecular machinery that underlies CFTR-F508del ERAD. Although the ER-resident ubiquitin ligase, RNF5 was the top E3 hit, knocking out RNF5 only modestly reduced CFTR-F508del degradation. Sublibrary screens in an RNF5 knockout background identified RNF185 as a redundant ligase and demonstrated that CFTR-F508del ERAD is robust. Gene-drug interaction experiments illustrated that correctors tezacaftor (VX-661) and elexacaftor (VX-445) stabilize sequential, RNF5-resistant folding states. We propose that binding of correctors to nascent CFTR-F508del alters its folding landscape by stabilizing folding states that are not substrates for RNF5-mediated ubiquitylation.

Small molecule correctors divert CFTR-F508del from ERAD by stabilizing sequential folding states

Over 80% of people with cystic fibrosis (CF) carry the F508del mutation in the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride ion channel at the apical plasma membrane (PM) of epithelial cells. F508del impairs CFTR folding causing it to be destroyed by endoplasmic reticulum associated degradation (ERAD). Small molecule correctors, which act as pharmacological chaperones to divert CFTR-F508del from ERAD, are the primary strategy for treating CF, yet corrector development continues with only a rudimentary understanding of how ERAD targets CFTR-F508del. We conducted genome-wide CRISPR/Cas9 knockout screens to systematically identify the molecular machinery that underlies CFTR-F508del ERAD. Although the ER-resident ubiquitin ligase, RNF5 was the top E3 hit, knocking out RNF5 only modestly reduced CFTR-F508del degradation. Sublibrary screens in an RNF5 knockout background identified RNF185 as a redundant ligase, demonstrating that CFTR-F508del ERAD is highly buffered. Gene-drug interaction experiments demonstrated that correctors tezacaftor (VX-661) and elexacaftor (VX-445) stabilize sequential, RNF5-resistant folding states. We propose that binding of correctors to nascent CFTR-F508del alters its folding landscape by stabilizing folding states that are not substrates for RNF5-mediated ubiquitylation.

Differential Expression and Function of SVIP in Breast Cancer Cell Lines and In Silico Analysis of Its Expression and Prognostic Potential in Human Breast Cancer

The heterogeneity of cancer strongly suggests the need to explore additional pathways to target. As cancer cells have increased proteotoxic stress, targeting proteotoxic stress-related pathways such as endoplasmic reticulum stress is attracting attention as a new anticancer treatment. One of the downstream responses to endoplasmic reticulum stress is endoplasmic reticulum-associated degradation (ERAD), a major degradation pathway that facilitates proteasome-dependent degradation of unfolded or misfolded proteins. Recently, SVIP (small VCP/97-interacting protein), an endogenous ERAD inhibitor, has been implicated in cancer progression, especially in glioma, prostate, and head and neck cancers. Here, the data of several RNA-sequencing (RNA-seq) and gene array studies were combined to evaluate the SVIP gene expression analysis on a variety of cancers, with a particular focus on breast cancer. The mRNA level of SVIP was found to be significantly higher in primary breast tumors and correlated well with its promoter methylation status and genetic alterations. Strikingly, the SVIP protein level was found to be low despite increased mRNA levels in breast tumors compared to normal tissues. On the other hand, the immunoblotting analysis showed that the expression of SVIP protein was significantly higher in breast cancer cell lines compared to non-tumorigenic epithelial cell lines, while most of the key proteins of gp78-mediated ERAD did not exhibit such an expression pattern, except for Hrd1. Silencing of SVIP enhanced the proliferation of p53 wt MCF-7 and ZR-75-1 cells but not p53 mutant T47D and SK-BR-3 cells; however, it increased the migration ability of both types of cell lines. Importantly, our data suggest that SVIP may increase p53 protein levels in MCF7 cells by inhibiting Hrd1-mediated p53 degradation. Overall, our data reveal the differential expression and function of SVIP on breast cancer cell lines together with in silico data analysis.

HRD1 in human malignant neoplasms: Molecular mechanisms and novel therapeutic strategy for cancer

In tumor cells, the endoplasmic reticulum (ER) plays an essential role in maintaining cellular proteostasis by stimulating unfolded protein response (UPR) underlying stress conditions. ER-associated degradation (ERAD) is a critical pathway of the UPR to protect cells from ER stress-induced apoptosis and the elimination of unfolded or misfolded proteins by the ubiquitin-proteasome system (UPS). 3-Hydroxy-3-methylglutaryl reductase degradation (HRD1) as an E3 ubiquitin ligase plays an essential role in the ubiquitination and dislocation of misfolded protein in ERAD. In addition, HRD1 can target other normal folded proteins. In various types of cancer, the expression of HRD1 is dysregulated, and it targets different molecules to develop cancer hallmarks or suppress the progression of the disease. Recent investigations have defined the role of HRD1 in drug resistance in types of cancer. This review focuses on the molecular mechanisms of HRD1 and its roles in cancer pathogenesis and discusses the worthiness of targeting HRD1 as a novel therapeutic strategy in cancer.

Ubiquitination-Proteasome System (UPS) and Autophagy Two Main Protein Degradation Machineries in Response to Cell Stress

In response to environmental stimuli, cells make a series of adaptive changes to combat the injury, repair the damage, and increase the tolerance to the stress. However, once the damage is too serious to repair, the cells will undergo apoptosis to protect the overall cells through suicidal behavior. Upon external stimulation, some intracellular proteins turn into unfolded or misfolded protein, exposing their hydrophobic regions to form protein aggregation, which may ultimately produce serious damage to the cells. Ubiquitin plays an important role in the degradation of these unnatural proteins by tagging with ubiquitin chains in the ubiquitin-proteasome or autophagy system. If the two processes fail to eliminate the abnormal protein aggregates, the cells will move to apoptosis and death. Dysregulation of ubiquitin-proteasome system (UPS) and autophagy may result in the development of numerous diseases. This review focuses on the molecular mechanisms of UPS and autophagy in clearance of intracellular protein aggregates, and the relationship between dysregulation of ubiquitin network and diseases.

Novel regulation mechanism of adrenal cortisol and DHEA biosynthesis via the endogen ERAD inhibitor small VCP-interacting protein

Endoplasmic reticulum-associated degradation (ERAD) is a well-characterized mechanism of protein quality control by removal of misfolded or unfolded proteins. The tight regulation of ERAD is critical for protein homeostasis as well as lipid metabolism. Although the mechanism is complex, all ERAD branches converge on p97/VCP, a key protein in the retrotranslocation step. The multifunctionality of p97/VCP relies on its multiple binding partners, one of which is the endogenous ERAD inhibitor, SVIP (small VCP-interacting protein). As SVIP is a promising target for the regulation of ERAD, we aimed to assess its novel physiological roles. We revealed that SVIP is highly expressed in the rat adrenal gland, especially in the cortex region, at a consistently high level during postnatal development, unlike the gradual increase in expression seen in developing nerves. Steroidogenic stimulators caused a decrease in SVIP mRNA expression and increase in SVIP protein degradation in human adrenocortical H295R cells. Interestingly, silencing of SVIP diminished cortisol secretion along with downregulation of steroidogenic enzymes and proteins involved in cholesterol uptake and cholesterol biosynthesis. A certain degree of SVIP overexpression mainly increased the biosynthesis of cortisol as well as DHEA by enhancing the expression of key steroidogenic proteins, whereas exaggerated overexpression led to apoptosis, phosphorylation of eIF2α, and diminished adrenal steroid hormone biosynthesis. In conclusion, SVIP is a novel regulator of adrenal cortisol and DHEA biosynthesis, suggesting that alterations in SVIP expression levels may be involved in the deregulation of steroidogenic stimulator signaling and abnormal adrenal hormone secretion.

RETREG1/FAM134B mediated autophagosomal degradation of AMFR/GP78 and OPA1 -a dual organellar turnover mechanism

Turnover of cellular organelles, including endoplasmic reticulum (ER) and mitochondria, is orchestrated by an efficient cellular surveillance system. We have identified a mechanism for dual regulation of ER and mitochondria under stress. It is known that AMFR, an ER E3 ligase and ER-associated degradation (ERAD) regulator, degrades outer mitochondrial membrane (OMM) proteins, MFNs (mitofusins), via the proteasome and triggers mitophagy. We show that destabilized mitochondria are almost devoid of the OMM and generate "mitoplasts". This brings the inner mitochondrial membrane (IMM) in the proximity of the ER. When AMFR levels are high and the mitochondria are stressed, the reticulophagy regulatory protein RETREG1 participates in the formation of the mitophagophore by interacting with OPA1. Interestingly, OPA1 and other IMM proteins exhibit similar RETREG1-dependent autophagosomal degradation as AMFR, unlike most of the OMM proteins. The "mitoplasts" generated are degraded by reticulo-mito-phagy - simultaneously affecting dual organelle turnover.Abbreviations: AMFR/GP78: autocrine motility factor receptor; BAPTA: 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; BFP: blue fluorescent protein; CCCP: carbonyl cyanide m-chlorophenyl hydrazone; CNBr: cyanogen bromide; ER: endoplasmic reticulum; ERAD: endoplasmic-reticulum-associated protein degradation; FL: fluorescence, GFP: green fluorescent protein; HA: hemagglutinin; HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; IMM: inner mitochondrial membrane; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MFN: mitofusin, MGRN1: mahogunin ring finger 1; NA: numerical aperature; OMM: outer mitochondrial membrane; OPA1: OPA1 mitochondrial dynamin like GTPase; PRNP/PrP: prion protein; RER: rough endoplasmic reticulum; RETREG1/FAM134B: reticulophagy regulator 1; RFP: red fluorescent protein; RING: really interesting new gene; ROI: region of interest; RTN: reticulon; SEM: standard error of the mean; SER: smooth endoplasmic reticulum; SIM: structured illumination microscopy; SQSTM1/p62: sequestosome 1; STED: stimulated emission depletion; STOML2: stomatin like 2; TOMM20: translocase of outer mitochondrial membrane 20; UPR: unfolded protein response.

Regulation of CFTR Biogenesis by the Proteostatic Network and Pharmacological Modulators

Cystic fibrosis (CF) is the most common lethal inherited disease among Caucasians in North America and a significant portion of Europe. The disease arises from one of many mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator, or CFTR. The most common disease-associated allele, F508del, along with several other mutations affect the folding, transport, and stability of CFTR as it transits from the endoplasmic reticulum (ER) to the plasma membrane, where it functions primarily as a chloride channel. Early data demonstrated that F508del CFTR is selected for ER associated degradation (ERAD), a pathway in which misfolded proteins are recognized by ER-associated molecular chaperones, ubiquitinated, and delivered to the proteasome for degradation. Later studies showed that F508del CFTR that is rescued from ERAD and folds can alternatively be selected for enhanced endocytosis and lysosomal degradation. A number of other disease-causing mutations in CFTR also undergo these events. Fortunately, pharmacological modulators of CFTR biogenesis can repair CFTR, permitting its folding, escape from ERAD, and function at the cell surface. In this article, we review the many cellular checkpoints that monitor CFTR biogenesis, discuss the emergence of effective treatments for CF, and highlight future areas of research on the proteostatic control of CFTR.

SVIP alleviates CCl4-induced liver fibrosis via activating autophagy and protecting hepatocytes

Prolonged parenchymal cell death leads to activation of fibrogenic cells and extracellular matrix accumulation and eventually liver fibrosis. Autophagy, a major catabolic process of intracellular degradation and recycling, participates in hepatic fibrosis. However, the precise role of autophagy in the pathogenesis of hepatic fibrosis is controversial. The present study aims to investigate the key role of small VCP/p97 interacting protein (SVIP) against CCl₄-induced hepatic fibrosis via activating autophagy. Autophagy could be activated by SVIP in HepG2 cells, but starvation cannot increase SVIP expression in vitro and in vivo. Moreover, SVIP expression, in agreement with autophagic activity and the volume of lipid droplets, first increases and then decreases during the progression of liver fibrosis with CCl₄ treatment in vivo and in vivo. Further, overexpression of SVIP can protect HepG2 cells from the toxicity of CCl₄, which could be enhanced by starvation. Finally, starvation keeps SVIP and autophagy at such high levels in the rat livers that markedly delays the progress of hepatic fibrosis. Probably, the protective effect of SVIP is associated with stabilizing nuclear factor (erythroid-derived 2)-related factor 2 (Nrf2) and transcription factor EB (TFEB). The current study provides insight into the biological role of SVIP and autophagy in regulating hepatic fibrosis, targeting SVIP might be a novel therapeutic strategy in the future.

Protein Quality Control in the Endoplasmic Reticulum and Cancer

The endoplasmic reticulum (ER) is an essential compartment of the biosynthesis, folding, assembly, and trafficking of secretory and transmembrane proteins, and consequently, eukaryotic cells possess specialized machineries to ensure that the ER enables the proteins to acquire adequate folding and maturation for maintaining protein homeostasis, a process which is termed proteostasis. However, a large variety of physiological and pathological perturbations lead to the accumulation of misfolded proteins in the ER, which is referred to as ER stress. To resolve ER stress and restore proteostasis, cells have evolutionary conserved protein quality-control machineries of the ER, consisting of the unfolded protein response (UPR) of the ER, ER-associated degradation (ERAD), and autophagy. Furthermore, protein quality-control machineries of the ER play pivotal roles in the control of differentiation, progression of cell cycle, inflammation, immunity, and aging. Therefore, severe and non-resolvable ER stress is closely associated with tumor development, aggressiveness, and response to therapies for cancer. In this review, we highlight current knowledge in the molecular understanding and physiological relevance of protein quality control of the ER and discuss new insights into how protein quality control of the ER is implicated in the pathogenesis of cancer, which could contribute to therapeutic intervention in cancer.

Dissection of the Role of VIMP in Endoplasmic Reticulum-Associated Degradation of CFTRΔF508

Endoplasmic reticulum (ER)-associated protein degradation (ERAD) is an important quality control mechanism that eliminates misfolded proteins from the ER. The Derlin-1/VCP/VIMP protein complex plays an essential role in ERAD. Although the roles of Derlin-1 and VCP are relatively clear, the functional activity of VIMP in ERAD remains to be understood. Here we investigate the role of VIMP in the degradation of CFTRΔF508, a cystic fibrosis transmembrane conductance regulator (CFTR) mutant known to be a substrate of ERAD. Overexpression of VIMP markedly enhances the degradation of CFTRΔF508, whereas knockdown of VIMP increases its half-life. We demonstrate that VIMP is associated with CFTRΔF508 and the RNF5 E3 ubiquitin ligase (also known as RMA1). Thus, VIMP not only forms a complex with Derlin-1 and VCP, but may also participate in recruiting substrates and E3 ubiquitin ligases. We further show that blocking CFTRΔF508 degradation by knockdown of VIMP substantially augments the effect of VX809, a drug that allows a fraction of CFTRΔF508 to fold properly and mobilize from ER to cell surface for normal functioning. This study provides insight into the role of VIMP in ERAD and presents a potential target for the treatment of cystic fibrosis patients carrying the CFTRΔF508 mutation.

Unfolded Protein Response of the Endoplasmic Reticulum in Tumor Progression and Immunogenicity

The endoplasmic reticulum (ER) is a pivotal regulator of folding, quality control, trafficking, and targeting of secreted and transmembrane proteins, and accordingly, eukaryotic cells have evolved specialized machinery to ensure that the ER enables these proteins to acquire adequate folding and maturation in the presence of intrinsic and extrinsic insults. This adaptive capacity of the ER to intrinsic and extrinsic perturbations is important for maintaining protein homeostasis, which is termed proteostasis. Failure in adaptation to these perturbations leads to accumulation of misfolded or unassembled proteins in the ER, which is termed ER stress, resulting in the activation of unfolded protein response (UPR) of the ER and the execution of ER-associated degradation (ERAD) to restore homeostasis. Furthermore, both of the two axes play key roles in the control of tumor progression, inflammation, immunity, and aging. Therefore, understanding UPR of the ER and subsequent ERAD will provide new insights into the pathogenesis of many human diseases and contribute to therapeutic intervention in these diseases.

Gp78 E3 Ubiquitin Ligase: Essential Functions and Contributions in Proteostasis

As per the requirement of metabolism and fitness, normal cellular functions are controlled by several proteins, and their interactive molecular and signaling events at multiple levels. Protein quality control (PQC) mechanisms ensure the correct folding and proper utilization of these proteins to avoid their misfolding and aggregation. To maintain the optimum environment of complex proteome PQC system employs various E3 ubiquitin ligases for the selective degradation of aberrant proteins. Glycoprotein 78 (Gp78) is an E3 ubiquitin ligase that prevents multifactorial deleterious accumulation of different misfolded proteins via endoplasmic reticulum-associated degradation (ERAD). However, the precise role of Gp78 under stress conditions to avoid bulk misfolded aggregation is unclear, which can act as a crucial resource to establish the dynamic nature of the proteome. Present article systematically explains the detailed molecular characterization of Gp78 and also addresses its various cellular physiological functions, which could be crucial to achieving protein homeostasis. Here, we comprehensively represent the current findings of Gp78, which shows its PQC roles in different physiological functions and diseases; and thereby propose novel opportunities to better understand the unsolved questions for therapeutic interventions linked with different protein misfolding disorders.

The evolving role of ubiquitin modification in endoplasmic reticulum-associated degradation

The endoplasmic reticulum (ER) serves as a warehouse for factors that augment and control the biogenesis of nascent proteins entering the secretory pathway. In turn, this compartment also harbors the machinery that responds to the presence of misfolded proteins by targeting them for proteolysis via a process known as ER-associated degradation (ERAD). During ERAD, substrates are selected, modified with ubiquitin, removed from the ER, and then degraded by the cytoplasmic 26S proteasome. While integral membrane proteins can directly access the ubiquitination machinery that resides in the cytoplasm or on the cytoplasmic face of the ER membrane, soluble ERAD substrates within the lumen must be retrotranslocated from this compartment. In either case, nearly all ERAD substrates are tagged with a polyubiquitin chain, a modification that represents a commitment step to degrade aberrant proteins. However, increasing evidence indicates that the polyubiquitin chain on ERAD substrates can be further modified, serves to recruit ERAD-requiring factors, and may regulate the ERAD machinery. Amino acid side chains other than lysine on ERAD substrates can also be modified with ubiquitin, and post-translational modifications that affect substrate ubiquitination have been observed. Here, we summarize these data and provide an overview of questions driving this field of research.

Ubiquitination and the Regulation of Membrane Proteins

Newly synthesized transmembrane proteins undergo a series of steps to ensure that only the required amount of correctly folded protein is localized to the membrane. The regulation of protein quality and its abundance at the membrane are often controlled by ubiquitination, a multistep enzymatic process that results in the attachment of ubiquitin, or chains of ubiquitin to the target protein. Protein ubiquitination acts as a signal for sorting, trafficking, and the removal of membrane proteins via endocytosis, a process through which multiple ubiquitin ligases are known to specifically regulate the functions of a number of ion channels, transporters, and signaling receptors. Endocytic removal of these proteins through ubiquitin-dependent endocytosis provides a way to rapidly downregulate the physiological outcomes, and defects in such controls are directly linked to human pathologies. Recent evidence suggests that ubiquitination is also involved in the shedding of membranes and associated proteins as extracellular vesicles, thereby not only controlling the cell surface levels of some membrane proteins, but also their potential transport to neighboring cells. In this review, we summarize the mechanisms and functions of ubiquitination of membrane proteins and provide specific examples of ubiquitin-dependent regulation of membrane proteins.

Androgen Mediated Regulation of Endoplasmic Reticulum-Associated Degradation and its Effects on Prostate Cancer

The endoplasmic reticulum (ER) comprises thirty percent of the newly translated proteins in eukaryotic cells. The quality control mechanism within the ER distinguishes between properly and improperly folded proteins and ensures that unwanted proteins are retained in the ER and subsequently degraded through ER-associated degradation (ERAD). Besides cleaning of misfolded proteins ERAD is also important for physiological processes by regulating the abundance of normal proteins of the ER. Thus it is important to unreveal the regulation patterns of ERAD. Here, we describe that ERAD pathway is regulated by androgen, where its inhibitor SVIP was downregulated, all other ERAD genes were upregulated. Consistently, androgen treatment increased the degradation rate of ERAD substrates. Using several independent techniques, we showed that this regulation is through androgen receptor transactivation. ERAD genes found to be upregulated in prostate cancer tissues and silencing expression of Hrd1, SVIP, and gp78 reduced the in vitro migration and malignant transformation of LNCaP cells. Our data suggests that expression levels of ERAD components are regulated by androgens, that promotes ERAD proteolytic activity, which is positively related with prostate tumorigenesis.

SYVN1, NEDD8, and FBXO2 Proteins Regulate ΔF508 Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Ubiquitin-mediated Proteasomal Degradation

We previously reported that delivery of a microRNA-138 mimic or siRNA against SIN3A to cultured cystic fibrosis (ΔF508/ΔF508) airway epithelia partially restored ΔF508-cystic fibrosis transmembrane conductance regulator (CFTR)-mediated cAMP-stimulated Cl^- conductance. We hypothesized that dissecting this microRNA-138/SIN3A-regulated gene network would identify individual proteins contributing to the rescue of ΔF508-CFTR function. Among the genes in the network, we rigorously validated candidates using functional CFTR maturation and electrolyte transport assays in polarized airway epithelia. We found that depletion of the ubiquitin ligase SYVN1, the ubiquitin/proteasome system regulator NEDD8, or the F-box protein FBXO2 partially restored ΔF508-CFTR-mediated Cl^- transport in primary cultures of human cystic fibrosis airway epithelia. Moreover, knockdown of SYVN1, NEDD8, or FBXO2 in combination with corrector compound 18 further potentiated rescue of ΔF508-CFTR-mediated Cl^- conductance. This study provides new knowledge of the CFTR biosynthetic pathway. It suggests that SYVN1 and FBXO2 represent two distinct multiprotein complexes that may degrade ΔF508-CFTR in airway epithelia and identifies a new role for NEDD8 in regulating ΔF508-CFTR ubiquitination.

Profiling of Host Cell Response to Successive Canine Parvovirus Infection Based on Kinetic Proteomic Change Identification

Canine parvovirus (CPV) reproduces by co-opting the resources of host cells, inevitably causing cytotoxic effects to the host cells. Feline kidney F81 cells are sensitive to CPV infection and show disparate growing statuses at different time points post-infection. This study analysed the response of F81 cells to CPV infection at successive infection time points by iTRAQ-based quantitative proteomics. Differentially expressed proteins (DEPs) during 60 h of infection and at selected time points post-infection were identified by an analysis of variance test and a two-tailed unpaired t test, respectively. DEPs with similar quantitative changes were clustered by hierarchical clustering and analysed by gene ontology enrichment, revealing that 12 h and 60 h post-infection were the optimal times to analyse the autonomous parvovirus replication and apoptosis processes, respectively. Using the Metacore^TM database, 29 DEPs were enriched in a network involved in p53 regulation. Besides, a significantly enriched pathway suggests that the CPV-induced cytopathic effect was probably due to the deficiency of functional CFTR caused by CPV infection. This study uncovered the systemic changes in key cellular factors involved in CPV infection and help to understand the molecular mechanisms of the anti-cancer activity of CPV and the cytopathic effects induced by CPV infection.

Lunapark Is a Component of a Ubiquitin Ligase Complex Localized to the Endoplasmic Reticulum Three-way Junctions

The endoplasmic reticulum (ER) network comprises sheets and tubules that are connected by dynamic three-way junctions. Lunapark (Lnp) localizes to and stabilizes ER three-way junctions by antagonizing the small GTPase Atlastin, but how Lnp shapes the ER network is unclear. Here, we used an affinity purification approach and mass spectrometry to identify Lnp as an interacting partner of the ER protein quality control ubiquitin ligase gp78. Accordingly, Lnp purified from mammalian cells has a ubiquitin ligase activity in vitro Intriguingly, biochemical analyses show that this activity can be attributed not only to associated ubiquitin ligase, but also to an intrinsic ubiquitin ligase activity borne by Lnp itself. This activity is contained in the N-terminal 45 amino acids of Lnp although this segment does not share homology to any known ubiquitin ligase motifs. Despite its interaction with gp78, Lnp does not seem to have a broad function in degradation of misfolded ER proteins. On the other hand, the N-terminal ubiquitin ligase-bearing motif is required for the ER three-way junction localization of Lnp. Our study identifies a new type of ubiquitin ligase and reveals a potential link between ubiquitin and ER morphology regulation.

Five Questions (with their Answers) on ER-Associated Degradation

Production of a functional proteome is a major burden for our cells. Native proteins operate inside and outside the cells to eventually warrant life and adaptation to metabolic and environmental changes, there is no doubt that production and inappropriate handling of misfolded proteins may cause severe disease states. This review focuses on protein destruction, which is, paradoxically, a crucial event for cell and organism survival. It regulates the physiological turnover of proteins and the clearance of faulty biosynthetic products. It mainly relies on the intervention of two catabolic machineries, the ubiquitin proteasome system and the (auto)lysosomal system. Here, we have selected five questions dealing with how, why and when proteins produced in the mammalian endoplasmic reticulum are eventually selected for destruction.

Ubiquitin mediated regulation of the E3 ligase GP78 by Mahogunin in trans affects mitochondrial homeostasis

Cellular quality control provides an efficient surveillance system to regulate mitochondrial turn-over. This study elucidates a novel interaction of the cytosolic E3 ligase, MGRN1 with the ER ubiquitin E3 ligase, GP78. Loss of Mgrn1 function has been implicated in late-onset spongiform neurodegeneration, congenital heart defects amongst several developmental defects. MGRN1 ubiquitinates GP78 in trans via non-canonical K11 linkages. This helps maintain constitutively low levels of GP78 in healthy cells, in turn downregulating mitophagy. GP78, however, does not regulate MGRN1. When mitochondria are stressed, cytosolic Ca2+ increases.This leads to reduced interaction between MGRN1 and GP78 and its compromised ubiquitination. Chelating Ca2+ restores association between the two ligases and the trans ubiquitination. Catalytic inactivation of MGRN1 results in elevated levels of GP78 and consequential increase in the initiation of mitophagy. This is significant because functional depletion of MGRN1 by membrane-associated disease causing prion protein, CtmPrP affects polyubiquitination and degradation of GP78, also leading to an increase in mitophagy events. This suggests that MGRN1 participates in mitochondrial quality control and could contribute to neurodegeneration in a sub-set of CtmPrP mediated prion diseases.

Selective multifaceted E3 ubiquitin ligases barricade extreme defense: Potential therapeutic targets for neurodegeneration and ageing

Efficient and regular performance of Ubiquitin Proteasome System and Autophagy continuously eliminate deleterious accumulation of nonnative protiens. In cellular quality control system, E3 ubiquitin ligases are significant employees for defense mechanism against abnormal toxic proteins. Few findings indicate that lack of functions of E3 ubiquitin ligases can be a causative factor of neurodevelopmental disorders, neurodegeneration, cancer and ageing. However, the detailed molecular pathomechanism implying E3 ubiquitin ligases in cellular functions in multifactorial disease conditions are not well understood. This article systematically represents the unique characteristics, molecular nature, and recent developments in the knowledge of neurobiological functions of few crucial E3 ubiquitin ligases. Here, we review recent literature on the roles of E6-AP, HRD1 and ITCH E3 ubiquitin ligases in the neuro-pathobiological mechanisms, with precise focus on the processes of neurodegeneration, and thereby propose new lines of potential targets for therapeutic interventions.

The E3 ubiquitin ligase gp78 protects against ER stress in zebrafish liver

Enhanced endoplasmic reticulum (ER)-associated protein degradation (ERAD) activity by the unfolded protein response (UPR) represents one of the mechanisms for restoring ER homeostasis. In vitro evidence indicates that the mammalian gp78 protein is an E3 ubiquitin ligase that facilitates ERAD by polyubiquitinating and targeting proteins for proteasomal degradation under both physiologic and stress conditions. However, the in vivo function of gp78 in maintaining ER protein homeostasis remains untested. Here we show that like its mammalian counterpart, the zebrafish gp78 is also an E3 ubiquitin ligase as revealed by in vitro ubiquitination assays. Expression analysis uncovered that gp78 is highly expressed in several organs, including liver and brain, of both larval and adult fish. Treatment of larvae or adult fish with tunicamycin induces ER stress and upregulates the expression of several key components of the gp78 ERAD complex in the liver. Moreover, liver-specific overexpression of the dominant-negative form of gp78 (gp78-R2M) renders liver more sensitive to tunicamycin-induced ER stress and enhances the expression of sterol response element binding protein (Srebp)-target genes, which was largely suppressed in fish overexpressing wild-type gp78. Together, these data indicate that gp78 plays a critical role in protecting against ER stress in liver.

Inhibition of post-translational N-glycosylation by HRD1 that controls the fate of ABCG5/8 transporter

N-glycosylation of proteins in endoplasmic reticulum is critical for protein quality control. We showed here a post-translational N-glycosylation affected by the HRD1 E3 ubiquitin ligase. Both WT- and E3-defective C329S-HRD1 decreased the level of high mannose form of ABCG8, a protein that heterodimerizes with ABCG5 to control sterol balance. Meanwhile, HRD1 increased the non-glycosylated ABCG8 regardless of its E3 activity, thereby suppressing full maturation of ABCG5/8 transporter. Pulse chase and mutational analysis indicated that HRD1 inhibits STT3B-dependent post-translational N-glycosylation of ABCG8. Whereas, HRD1 had only slight effect on the N-glycosylation status of ABCG5; rather it accelerated ABCG5 degradation in an E3 activity-dependent manner. Finally, RMA1, another E3 ubiquitin ligase, accelerated the degradation of both ABCG5 and ABCG8 via E3 activity-dependent manner. HRD1 and RMA1 may therefore be negative regulators of disease-associated transporter ABCG5/ABCG8. The findings also highlight the unexpected E3 activity-independent role of HRD1 in the regulation of N-glycosylation.

Autoadaptive ER-associated degradation defines a preemptive unfolded protein response pathway

Folding-defective proteins must be cleared efficiently from the endoplasmic reticulum (ER) to prevent perturbation of the folding environment and to maintain cellular proteostasis. Misfolded proteins engage dislocation machineries (dislocons) built around E3 ubiquitin ligases that promote their transport across the ER membrane, their polyubiquitylation, and their proteasomal degradation. Here, we report on the intrinsic instability of the HRD1 dislocon and the constitutive, rapid turnover of the scaffold protein HERP. We show that HRD1 dislocon integrity relies on the presence of HRD1 clients that interrupt, in a dose-dependent manner, the UBC6e/RNF5/p97/proteasome-controlled relay that controls HERP turnover. We propose that ER-associated degradation (ERAD) deploys autoadaptive regulatory pathways, collectively defined as ERAD tuning, to rapidly adapt degradation activity to misfolded protein load and to preempt the unfolded protein response (UPR) activation.

The mammalian endoplasmic reticulum-associated degradation system

The endoplasmic reticulum (ER) is the site of synthesis for nearly one-third of the eukaryotic proteome and is accordingly endowed with specialized machinery to ensure that proteins deployed to the distal secretory pathway are correctly folded and assembled into native oligomeric complexes. Proteins failing to meet this conformational standard are degraded by ER-associated degradation (ERAD), a complex process through which folding-defective proteins are selected and ultimately degraded by the ubiquitin-proteasome system. ERAD proceeds through four tightly coupled steps involving substrate selection, dislocation across the ER membrane, covalent conjugation with polyubiquitin, and proteasomal degradation. The ERAD machinery shows a modular organization with central ER membrane-embedded ubiquitin ligases linking components responsible for recognition in the ER lumen to the ubiquitin-proteasome system in the cytoplasm. The core ERAD machinery is highly conserved among eukaryotes and much of our basic understanding of ERAD organization has been derived from genetic and biochemical studies of yeast. In this article we discuss how the core ERAD machinery is organized in mammalian cells.

Ubiquitination and degradation of CFTR by the E3 ubiquitin ligase MARCH2 through its association with adaptor proteins CAL and STX6

Golgi-localized cystic fibrosis transmembrane conductance regulator (CFTR)-associated ligand (CAL) and syntaxin 6 (STX6) regulate the abundance of mature, post-ER CFTR by forming a CAL/STX6/CFTR complex (CAL complex) that promotes CFTR degradation in lysosomes. However, the molecular mechanism underlying this degradation is unknown. Here we investigated the interaction of a Golgi-localized, membrane-associated RING-CH E3 ubiquitin ligase, MARCH2, with the CAL complex and the consequent binding, ubiquitination, and degradation of mature CFTR. We found that MARCH2 not only co-immunoprecipitated and co-localized with CAL and STX6, but its binding to CAL was also enhanced by STX6, suggesting a synergistic interaction. In vivo ubiquitination assays demonstrated the ubiquitination of CFTR by MARCH2, and overexpression of MARCH2, like that of CAL and STX6, led to a dose-dependent degradation of mature CFTR that was blocked by bafilomycin A1 treatment. A catalytically dead MARCH2 RING mutant was unable to promote CFTR degradation. In addition, MARCH2 had no effect on a CFTR mutant lacking the PDZ motif, suggesting that binding to the PDZ domain of CAL is required for MARCH2-mediated degradation of CFTR. Indeed, silencing of endogenous CAL ablated the effect of MARCH2 on CFTR. Consistent with its Golgi localization, MARCH2 had no effect on ER-localized ΔF508-CFTR. Finally, siRNA-mediated silencing of endogenous MARCH2 in the CF epithelial cell line CFBE-CFTR increased the abundance of mature CFTR. Taken together, these data suggest that the recruitment of the E3 ubiquitin ligase MARCH2 to the CAL complex and subsequent ubiquitination of CFTR are responsible for the CAL-mediated lysosomal degradation of mature CFTR.

A unique IBMPFD-related P97/VCP mutation with differential binding pattern and subcellular localization

p97/VCP is a hexameric AAA type ATPase that functions in a variety of cellular processes such as endoplasmic reticulum associated degradation (ERAD), organelle biogenesis, autophagy and cell-cycle regulation. Inclusion body myopathy associated with Paget disease of the bone and frontotemporal dementia (IBMPFD) is an autosomal dominant disorder which has been attributed to mutations in p97/VCP. Several missense mutations affecting twelve different amino acids have been identified in IBMPFD patients and some of them were suggested to be involved in the observed pathology. Here, we analyzed the effect of all twelve p97/VCP variants on ERAD substrates and their cofactor binding abilities. While all mutants cause ERAD substrate accumulation, P137L mutant p97/VCP differs from other IBMPFD mutants by having a unique solubility profile and subcellular localization. Intriguingly, although almost all mutants exhibit enhanced p47 and Ufd1-Npl4 binding, the P137L mutation completely abolishes p97/VCP interactions with Ufd1, Npl4 and p47, while retaining its gp78 binding. While recombinant R155C mutant protein consistently interacts with both Ufd1 and VIM of gp78, P137L mutant protein lost binding ability to Ufd1 but not to VIM in vitro. The differential impairments in p97/VCP interactions with its functional partners and function should help our understanding of the molecular pathogenesis of IBMPFD.

Role of the SEL1L:LC3-I complex as an ERAD tuning receptor in the mammalian ER

Several regulators of endoplasmic reticulum (ER)-associated degradation (ERAD) have a shorter half-life compared to conventional ER chaperones. At steady state, they are selectively removed from the ER by poorly defined events collectively referred to as ERAD tuning. Here we identify the complex comprising the type-I transmembrane protein SEL1L and the cytosolic protein LC3-I as an ERAD tuning receptor regulating the COPII-independent, vesicle-mediated removal of the lumenal ERAD regulators EDEM1 and OS-9 from the ER. Expression of folding-defective polypeptides enhances the lumenal content of EDEM1 and OS-9 by inhibiting their SEL1L:LC3-I-mediated segregation. This raises ERAD activity in the absence of UPR-induction. The mouse hepatitis virus (MHV) subverts ERAD tuning for replication. Consistently, SEL1L or LC3 silencing impair the MHV life cycle. Collectively, our data provide new molecular information about the ERAD tuning mechanisms that regulate ERAD in mammalian cells at the post translational level and how these mechanisms are hijacked by a pathogen.

Diversity in the architecture of ATLs, a family of plant ubiquitin-ligases, leads to recognition and targeting of substrates in different cellular environments

Ubiquitin-ligases or E3s are components of the ubiquitin proteasome system (UPS) that coordinate the transfer of ubiquitin to the target protein. A major class of ubiquitin-ligases consists of RING-finger domain proteins that include the substrate recognition sequences in the same polypeptide; these are known as single-subunit RING finger E3s. We are studying a particular family of RING finger E3s, named ATL, that contain a transmembrane domain and the RING-H2 finger domain; none of the member of the family contains any other previously described domain. Although the study of a few members in A. thaliana and O. sativa has been reported, the role of this family in the life cycle of a plant is still vague. To provide tools to advance on the functional analysis of this family we have undertaken a phylogenetic analysis of ATLs in twenty-four plant genomes. ATLs were found in all the 24 plant species analyzed, in numbers ranging from 20–28 in two basal species to 162 in soybean. Analysis of ATLs arrayed in tandem indicates that sets of genes are expanding in a species-specific manner. To get insights into the domain architecture of ATLs we generated 75 pHMM LOGOs from 1815 ATLs, and unraveled potential protein-protein interaction regions by means of yeast two-hybrid assays. Several ATLs were found to interact with DSK2a/ubiquilin through a region at the amino-terminal end, suggesting that this is a widespread interaction that may assist in the mode of action of ATLs; the region was traced to a distinct sequence LOGO. Our analysis provides significant observations on the evolution and expansion of the ATL family in addition to information on the domain structure of this class of ubiquitin-ligases that may be involved in plant adaptation to environmental stress.

The predator becomes the prey: regulating the ubiquitin system by ubiquitylation and degradation

Ubiquitylation (also known as ubiquitination) regulates essentially all of the intracellular processes in eukaryotes through highly specific modification of numerous cellular proteins, which is often tightly regulated in a spatial and temporal manner. Although most often associated with proteasomal degradation, ubiquitylation frequently serves non-proteolytic functions. In light of its central roles in cellular regulation, it has not been surprising to find that many of the components of the ubiquitin system itself are regulated by ubiquitylation. This observation has broad implications for pathophysiology.

Ubiquitination of E3 ligases: self-regulation of the ubiquitin system via proteolytic and non-proteolytic mechanisms

Ubiquitin modification of many cellular proteins targets them for proteasomal degradation, but in addition can also serve non-proteolytic functions. Over the last years, a significant progress has been made in our understanding of how modification of the substrates of the ubiquitin system is regulated. However, little is known on how the ubiquitin system that is comprised of ∼1500 components is regulated. Here, we discuss how the biggest subfamily within the system, that of the E3 ubiquitin ligases that endow the system with its high specificity towards the numerous substrates, is regulated and in particular via self-regulation mediated by ubiquitin modification. Ligases can be targeted for degradation in a self-catalyzed manner, or through modification mediated by an external ligase(s). In addition, non-proteolytic functions of self-ubiquitination, for example activation of the ligase, of E3s are discussed.

Posttranslational negative regulation of glycosylated and non-glycosylated BCRP expression by Derlin-1

Human breast cancer resistance protein (BCRP)/MXR/ABCG2 is a well-recognized ABC half-transporter that is highly expressed at the apical membrane of many normal tissues and cancer cells. BCRP facilitates disposition of endogenous and exogenous harmful xenobiotics to protect cells/tissues from xenobiotic-induced toxicity. Despite the enormous impact of BCRP in the physiological and pathophysiological regulation of the transport of a wide variety of substrates, little is known about the factors that regulate posttranslational expression of BCRP. Here, we identified Derlin-1, a member of a family of proteins that bears homology to yeast Der1p, as a posttranslational regulator of BCRP expression. Overexpression of Derlin-1 suppressed ER to Golgi transport of wild-type (WT) BCRP that is known to be efficiently trafficked to the plasma membrane. On the other hand, protein expression of N596Q variant of BCRP, N-linked glycosylation-deficient mutant that preferentially undergoes ubiquitin-mediated ER-associated degradation (ERAD), was strongly suppressed by the overexpression of Derlin-1, whereas knockdown of Derlin-1 stabilized N596Q protein, suggesting a negative regulatory role of Derlin-1 for N596Q protein expression. Notably, knockdown of Derlin-1 also stabilized the expression of tunicamycin-induced deglycosylated WT BCRP protein, implying the importance of glycosylation state for the recognition of BCRP by Derlin-1. Thus, our data demonstrate that Derlin-1 is a negative regulator for both glycosylated and non-glycosylated BCRP expression and provide a novel posttranslational regulatory mechanism of BCRP by Derlin-1.

Applications of proteomic technologies for understanding the premature proteolysis of CFTR

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene, which encodes an ATP-dependent anion channel. Disease-causing mutations can affect channel biogenesis, trafficking or function, and result in reduced ion transport at the apical surface of many tissues. The most common CFTR mutation is a deletion of phenylalanine at position 508 (DeltaF508), which results in a misfolded protein that is prematurely targeted for degradation. This article focuses on how proteomic approaches have been utilized to explore the mechanisms of premature proteolysis in CF. Additionally, we emphasize the potential for proteomic-based technologies in expanding our understanding of CF pathophysiology and therapeutic approaches.

The Unfolded Protein Response, Degradation from Endoplasmic Reticulum and Cancer

The endoplasmic reticulum (ER) is an essential organelle involved in many cellular functions including protein folding and secretion, lipid biosynthesis and calcium homeostasis. Proteins destined for the cell surface or for secretion are made in the ER, where they are folded and assembled into multi-subunit complexes. The ER plays a vital role in cellular protein quality control by extracting and degrading proteins that are not correctly folded or assembled into native complexes. This process, known as ER-associated degradation (ERAD), ensures that only properly folded and assembled proteins are transported to their final destinations. Besides its role in protein folding and transport in the secretory pathway, the ER regulates the biosynthesis of cholesterol and other membrane lipids. ERAD is an important means to ensure that levels of the responsible enzymes are appropriately maintained. The ER is also a major organelle for oxygen and nutrient sensing as cells adapt to their microenvironment. Stresses that disrupt ER function leads to accumulation of unfolded proteins in the ER, a condition known as ER stress. Cells adapt to ER stress by activating an integrated signal transduction pathway called the unfolded protein response (UPR) (1). The UPR represents a survival response by the cells to restore ER homeostasis. If ER stress persists, cells activate mechanisms that result in cell death. Chronic ER stress is increasingly being recognized as a factor in many human diseases such as diabetes, neurodegenerative disorders and cancer. In this review we discuss the roles of the UPR and ERAD in cancer and suggest directions for future research.

ERAD ubiquitin ligases: multifunctional tools for protein quality control and waste disposal in the endoplasmic reticulum

In eukaryotic cells terminally misfolded proteins of the secretory pathway are retarded in the endoplasmic reticulum (ER) and subsequently degraded in a ubiquitin-proteasome-dependent manner. This highly conserved process termed ER-associated protein degradation (ERAD) ensures homeostasis in the secretory pathway by disposing faulty polypeptides and preventing their deleterious accumulation and eventual aggregation in the cell. The focus of this paper is the functional description of membrane-bound ubiquitin ligases, which are involved in all critical steps of ERAD. In the end we want to speculate on how the modular architecture of these entities ensures the specificity of substrate selection and possibly accomplishes the transport of misfolded polypeptides from the ER into the cytoplasm.

Regulation of the polycomb protein Ring1B by self-ubiquitination or by E6-AP may have implications to the pathogenesis of Angelman syndrome

The polycomb repressive complex (PRC) 1 protein Ring1B is an ubiquitin ligase that modifies nucleosomal histone H2A, a modification which plays a critical role in regulation of gene expression. We have shown that self-ubiquitination of Ring1B generates multiply branched, "noncanonical" polyubiquitin chains that do not target the ligase for degradation, but rather stimulate its activity toward histone H2A. This finding implies that Ring1B is targeted by a heterologous E3. In this study, we identified E6-AP (E6-associated protein) as a ligase that targets Ring1B for "canonical" ubiquitination and subsequent degradation. We further demonstrated that both the self-ubiquitination of Ring1B and its modification by E6-AP target the same lysines, suggesting that the fate of Ring1B is tightly regulated (e.g., activation vs. degradation) by the type of chains and the ligase that catalyzes their formation. As expected, inactivation of E6-AP affects downstream effectors: Ring1B and ubiquitinated H2A levels are increased accompanied by repressed expression of HoxB9, a PRC1 target gene. Consistent with these findings, E6-AP knockout mice display an elevated level of Ring1B and ubiquitinated histone H2A in various tissues, including cerebellar Purkinje neurons, which may have implications to the pathogenesis of Angelman syndrome, a neurodevelopmental disorder caused by deficiency of E6-AP in the brain.

Stringent requirement for HRD1, SEL1L, and OS-9/XTP3-B for disposal of ERAD-LS substrates

Soluble ERAD substrates require the Hrd1 E3 ligase for degradation compared with membrane-anchored peptides that use GP78.

Sophisticated quality control mechanisms prolong retention of protein-folding intermediates in the endoplasmic reticulum (ER) until maturation while sorting out terminally misfolded polypeptides for ER-associated degradation (ERAD). The presence of structural lesions in the luminal, transmembrane, or cytosolic domains determines the classification of misfolded polypeptides as ERAD-L, -M, or -C substrates and results in selection of distinct degradation pathways. In this study, we show that disposal of soluble (nontransmembrane) polypeptides with luminal lesions (ERAD-L_S substrates) is strictly dependent on the E3 ubiquitin ligase HRD1, the associated cargo receptor SEL1L, and two interchangeable ERAD lectins, OS-9 and XTP3-B. These ERAD factors become dispensable for degradation of the same polypeptides when membrane tethered (ERAD-L_M substrates). Our data reveal that, in contrast to budding yeast, tethering of mammalian ERAD-L substrates to the membrane changes selection of the degradation pathway.

Huntingtin interacts with the cue domain of gp78 and inhibits gp78 binding to ubiquitin and p97/VCP

Huntington's disease (HD) is caused by polyglutamine expansion in huntingtin (htt) protein, but the exact mechanism of HD pathogenesis remains uncertain. Recent evidence suggests that htt proteins with expanded polyglutamine tracts induce endoplasmic reticulum (ER) stress, probably by interfering with ER-associated degradation (ERAD). Here we report that mutant htt interacts and interferes with the function of gp78, an ER membrane-anchored ubiquitin ligase (E3) involved in ERAD. Mapping studies showed that the HEAT repeats 2&3 of htt interact with the cue domain of gp78. The interaction competitively reduces polyubiquitinated protein binding to gp78 and also sterically blocks gp78 interaction of p97/VCP, a molecular chaperone that is essential for ERAD. These effects of htt negatively regulate the function of gp78 in ERAD and are aggravated by polyglutamine expansion. Paradoxically, gp78 is still able to ubiquitinate and facilitate degradation of htt proteins with expanded polyglutamine. The impairment of ERAD by mutant htt proteins is associated with induction of ER stress. Our studies provide a novel molecular mechanism that supports the involvement of ER stress in HD pathogenesis.

Ubiquitylation in the ERAD Pathway

Ubiquitylation is a protein modification mechanism, which is found in a multitude of cellular processes like DNA repair and replication, cell signaling, intracellular trafficking and also, very prominently, in selective protein degradation. One specific protein degradation event in the cell concerns the elimination of misfolded proteins to prevent disastrous malfunctioning of cellular pathways. The most complex of these ubiquitylation dependent elimination pathways of misfolded proteins is associated with the endoplasmic reticulum (ER). Proteins, which enter the endoplasmic reticulum for secretion, are folded in this organelle and transported to their site of action. A rigid protein quality control check retains proteins in the endoplasmic reticulum, which fail to fold properly and sends them back to the cytosol for elimination by the proteasome. This requires crossing of the misfolded protein of the endoplasmic reticulum membrane and polyubiquitylation in the cytosol by the ubiquitin-activating, ubiquitin-conjugating and ubiquitin-ligating enzyme machinery.Ubiquitylation is required for different steps of the ER-associated degradation process (ERAD). It facilitates efficient extraction of the ubiquitylated misfolded proteins from and out of the ER membrane by the Cdc48-Ufd1-Npl4 complex and thereby triggers their retro translocation to the cytosol. In addition, the modification with ubiquitin chains guarantees guidance, recognition and binding of the misfolded proteins to the proteasome in the cytosol for efficient degradation.