EDEM accelerates ERAD by preventing aberrant dimer formation of misfolded α1-antitrypsin

Turnover of EDEM1, an ERAD-enhancing factor, is mediated by multiple degradation routes

Quality-based protein production and degradation in the endoplasmic reticulum (ER) are essential for eukaryotic cell survival. During protein maturation in the ER, misfolded or unassembled proteins are destined for disposal through a process known as ER-associated degradation (ERAD). EDEM1 is an ERAD-accelerating factor whose gene expression is upregulated by the accumulation of aberrant proteins in the ER, known as ER stress. Although the role of EDEM1 in ERAD has been studied in detail, the turnover of EDEM1 by intracellular degradation machinery, including the proteasome and autophagy, is not well understood. To clarify EDEM1 regulation in the protein level, degradation mechanism of EDEM1 was examined. Our results indicate that both ERAD and autophagy degrade EDEM1 alike misfolded degradation substrates, although each degradation machinery targets EDEM1 in different folded states of proteins. We also found that ERAD factors, including the SEL1L/Hrd1 complex, YOD1, XTP3B, ERdj3, VIMP, BAG6, and JB12, but not OS9, are involved in EDEM1 degradation in a mannose-trimming-dependent and -independent manner. Our results suggest that the ERAD accelerating factor, EDEM1, is turned over by the ERAD itself, similar to ERAD clients.

Zn2+-dependent functional switching of ERp18, an ER-resident thioredoxin-like protein

ERp18 is an endoplasmic reticulum (ER)-resident thioredoxin (Trx) family protein, similar to cytosolic Trx1. The Trx-like domain occupies a major portion of the whole ERp18 structure, which is postulated to be an ER paralog of cytosolic Trx1. Here, we elucidate that zinc ion (Zn2+) binds ERp18 through its catalytic motif, triggering oligomerization of ERp18 from a monomer to a trimer. While the monomeric ERp18 has disulfide oxidoreductase activity, the trimeric ERp18 acquires scavenger activity for hydrogen peroxide (H2O2) in the ER. Depletion of ERp18 thus causes the accumulation of H2O2, which is produced during the oxidative folding of nascent polypeptides in the ER. ERp18 knockdown in C. elegans without Prx4 and GPx7/8, both of which are also known to have H2O2 scavenging activity in the ER, shortened the lifespan, suggesting that ERp18 may form a primitive and essential H2O2 scavenging system for the maintenance of redox homeostasis in the ER.

Investigating the Role of 17-Beta Estradiol in the Regulation of the Unfolded Protein Response (UPR) in Pancreatic Beta Cells

Diabetes mellitus is clinically defined by chronic hyperglycemia. Sex differences in the presentation and outcome of diabetes exist with premenopausal women having a reduced risk of developing diabetes, relative to men, or women after menopause. Accumulating evidence shows a protective role of estrogens, specifically 17-beta estradiol, in the maintenance of pancreatic beta cell health; however, the mechanisms underlying this protection are still unknown. To elucidate these potential mechanisms, we used a pancreatic beta cell line (BTC6) and a mouse model of hyperglycemia-induced atherosclerosis, the ApoE-/-:Ins2+/Akita mouse, exhibiting sexual dimorphism in glucose regulation. In this study we hypothesize that 17-beta estradiol protects pancreatic beta cells by modulating the unfolded protein response (UPR) in response to endoplasmic reticulum (ER) stress. We observed that ovariectomized female and male ApoE-/-:Ins2+/Akita mice show significantly increased expression of apoptotic UPR markers. Sham operated female and ovariectomized female ApoE-/-:Ins2+/Akita mice supplemented with exogenous 17-beta estradiol increased the expression of adaptive UPR markers compared to non-supplemented ovariectomized female ApoE-/-:Ins2+/Akita mice. These findings were consistent to what was observed in cultured BTC6 cells, suggesting that 17-beta estradiol may protect pancreatic beta cells by repressing the apoptotic UPR and enhancing the adaptive UPR activation in response to pancreatic ER stress.

A Predominant Role of AtEDEM1 in Catalyzing a Rate-Limiting Demannosylation Step of an Arabidopsis Endoplasmic Reticulum-Associated Degradation Process

Endoplasmic reticulum-associated degradation (ERAD) is a key cellular process for degrading misfolded proteins. It was well known that an asparagine (N)-linked glycan containing a free α1,6-mannose residue is a critical ERAD signal created by Homologous to α-mannosidase 1 (Htm1) in yeast and ER-Degradation Enhancing α-Mannosidase-like proteins (EDEMs) in mammals. An earlier study suggested that two Arabidopsis homologs of Htm1/EDEMs function redundantly in generating such a conserved N-glycan signal. Here we report that the Arabidopsis irb1 (reversal of bri1) mutants accumulate brassinosteroid-insensitive 1-5 (bri1-5), an ER-retained mutant variant of the brassinosteroid receptor BRI1 and are defective in one of the Arabidopsis Htm1/EDEM homologs, AtEDEM1. We show that the wild-type AtEDEM1, but not its catalytically inactive mutant, rescues irb1-1. Importantly, an insertional mutation of the Arabidopsis Asparagine-Linked Glycosylation 3 (ALG3), which causes N-linked glycosylation with truncated glycans carrying a different free α1,6-mannose residue, completely nullifies the inhibitory effect of irb1-1 on bri1-5 ERAD. Interestingly, an insertional mutation in AtEDEM2, the other Htm1/EDEM homolog, has no detectable effect on bri1-5 ERAD; however, it enhances the inhibitory effect of irb1-1 on bri1-5 degradation. Moreover, AtEDEM2 transgenes rescued the irb1-1 mutation with lower efficacy than AtEDEM1. Simultaneous elimination of AtEDEM1 and AtEDEM2 completely blocks generation of α1,6-mannose-exposed N-glycans on bri1-5, while overexpression of either AtEDEM1 or AtEDEM2 stimulates bri1-5 ERAD and enhances the bri1-5 dwarfism. We concluded that, despite its functional redundancy with AtEDEM2, AtEDEM1 plays a predominant role in promoting bri1-5 degradation.

Substrate ubiquitination retains misfolded membrane proteins in the endoplasmic reticulum for degradation

To maintain secretory pathway fidelity, misfolded proteins are commonly retained in the endoplasmic reticulum (ER) and selected for ER-associated degradation (ERAD). Soluble misfolded proteins use ER chaperones for retention, but the machinery that restricts aberrant membrane proteins to the ER is unclear. In fact, some misfolded membrane proteins escape the ER and traffic to the lysosome/vacuole. To this end, we describe a model substrate, SZ*, that contains an ER export signal but is also targeted for ERAD. We observe decreased ER retention when chaperone-dependent SZ* ubiquitination is compromised. In addition, appending a linear tetra-ubiquitin motif onto SZ* overrides ER export. By screening known ubiquitin-binding proteins, we then positively correlate SZ* retention with Ubx2 binding. Deletion of Ubx2 also inhibits the retention of another misfolded membrane protein. Our results indicate that polyubiquitination is sufficient to retain misfolded membrane proteins in the ER prior to ERAD.

Sun et al. characterize how misfolded membrane proteins are delivered for either ERAD or post-ER degradation in the secretory pathway. By using a model substrate that can access both pathways, they show that substrate retention requires chaperone-dependent substrate ubiquitination and interaction with a conserved ER membrane protein, Ubx2.

EDEM1 Drives Misfolded Protein Degradation via ERAD and Exploits ER-Phagy as Back-Up Mechanism When ERAD Is Impaired

Endoplasmic reticulum (ER)-associated degradation (ERAD) is the main mechanism of targeting ER proteins for degradation to maintain homeostasis, and perturbations of ERAD lead to pathological conditions. ER-degradation enhancing α-mannosidase-like (EDEM1) was proposed to extract terminally misfolded proteins from the calnexin folding cycle and target them for degradation by ERAD. Here, using mass-spectrometry and biochemical methods, we show that EDEM1 is found in auto-regulatory complexes with ERAD components. Moreover, the N-terminal disordered region of EDEM1 mediates protein–protein interaction with misfolded proteins, whilst the absence of this domain significantly impairs their degradation. We also determined that overexpression of EDEM1 can induce degradation, even when proteasomal activity is severely impaired, by promoting the formation of aggregates, which can be further degraded by autophagy. Therefore, we propose that EDEM1 maintains ER homeostasis and mediates ERAD client degradation via autophagy when either dislocation or proteasomal degradation are impaired.

The Crucial Role of Demannosylating Asparagine-Linked Glycans in ERADicating Misfolded Glycoproteins in the Endoplasmic Reticulum

Most membrane and secreted proteins are glycosylated on certain asparagine (N) residues in the endoplasmic reticulum (ER), which is crucial for their correct folding and function. Protein folding is a fundamentally inefficient and error-prone process that can be easily interfered by genetic mutations, stochastic cellular events, and environmental stresses. Because misfolded proteins not only lead to functional deficiency but also produce gain-of-function cellular toxicity, eukaryotic organisms have evolved highly conserved ER-mediated protein quality control (ERQC) mechanisms to monitor protein folding, retain and repair incompletely folded or misfolded proteins, or remove terminally misfolded proteins via a unique ER-associated degradation (ERAD) mechanism. A crucial event that terminates futile refolding attempts of a misfolded glycoprotein and diverts it into the ERAD pathway is executed by removal of certain terminal α1,2-mannose (Man) residues of their N-glycans. Earlier studies were centered around an ER-type α1,2-mannosidase that specifically cleaves the terminal α1,2Man residue from the B-branch of the three-branched N-linked Man₉GlcNAc₂ (GlcNAc for N-acetylglucosamine) glycan, but recent investigations revealed that the signal that marks a terminally misfolded glycoprotein for ERAD is an N-glycan with an exposed α1,6Man residue generated by members of a unique folding-sensitive α1,2-mannosidase family known as ER-degradation enhancing α-mannosidase-like proteins (EDEMs). This review provides a historical recount of major discoveries that led to our current understanding on the role of demannosylating N-glycans in sentencing irreparable misfolded glycoproteins into ERAD. It also discusses conserved and distinct features of the demannosylation processes of the ERAD systems of yeast, mammals, and plants.

How to Avoid a No-Deal ER Exit

Efficiency and fidelity of protein secretion are achieved thanks to the presence of different steps, located sequentially in time and space along the secretory compartment, controlling protein folding and maturation. After entering into the endoplasmic reticulum (ER), secretory proteins attain their native structure thanks to specific chaperones and enzymes. Only correctly folded molecules are allowed by quality control (QC) mechanisms to leave the ER and proceed to downstream compartments. Proteins that cannot fold properly are instead retained in the ER to be finally destined to proteasomal degradation. Exiting from the ER requires, in most cases, the use of coated vesicles, departing at the ER exit sites, which will fuse with the Golgi compartment, thus releasing their cargoes. Protein accumulation in the ER can be caused by a too stringent QC or by ineffective transport: these situations could be deleterious for the organism, due to the loss of the secreted protein, and to the cell itself, because of abnormal increase of protein concentration in the ER. In both cases, diseases can arise. In this review, we will describe the pathophysiology of protein folding and transport between the ER and the Golgi compartment.

Compensatory increases of select proteostasis networks after Hsp70 inhibition in cancer cells

Cancer cells thrive when challenged with proteotoxic stress by inducing components of the protein folding, proteasome, autophagy and unfolded protein response (UPR) pathways. Consequently, specific molecular chaperones have been validated as targets for anti-cancer therapies. For example, inhibition of Hsp70 family proteins (hereafter Hsp70) in rhabdomyosarcoma triggers UPR induction and apoptosis. To define how these cancer cells respond to compromised proteostasis, we compared rhabdomyosarcoma cells that were sensitive (RMS13) or resistant (RMS13-R) to the Hsp70 inhibitor MAL3-101. We discovered that endoplasmic reticulum-associated degradation (ERAD) and autophagy were activated in RMS13-R cells, suggesting that resistant cells overcome Hsp70 ablation by increasing misfolded protein degradation. Indeed, RMS13-R cells degraded ERAD substrates more rapidly than RMS cells and induced the autophagy pathway. Surprisingly, inhibition of the proteasome or ERAD had no effect on RMS13-R cell survival, but silencing of select autophagy components or treatment with autophagy inhibitors restored MAL3-101 sensitivity and led to apoptosis. These data indicate a route through which cancer cells overcome a chaperone-based therapy, define how cells can adapt to Hsp70 inhibition, and demonstrate the value of combined chaperone and autophagy-based therapies.This article has an associated First Person interview with the first author of the paper.

Liver - master and servant of serum proteome

Hepatocytes synthesise the majority of serum proteins. This production occurs in the endoplasmic reticulum (ER) and is adjusted by complex local and systemic regulatory mechanisms. Accordingly, serum levels of hepatocyte-made proteins constitute important biomarkers that reflect both systemic processes and the status of the liver. For example, C-reactive protein is an established marker of inflammatory reaction, whereas transferrin emerges as a liver stress marker and an attractive mortality predictor. The high protein flow through the ER poses a continuous challenge that is handled by a complex proteostatic network consisting of ER folding machinery, ER stress response, ER-associated degradation and autophagy. Various disorders disrupt this delicate balance and result in protein accumulation in the ER. These include chronic hepatitis B infection with overproduction of hepatitis B surface antigen or inherited alpha1-antitrypsin deficiency that give rise to ground glass hepatocytes and alpha1-antitrypsin aggregates, respectively. We review these ER storage disorders and their downstream consequences. The interaction between proteotoxic stress and other ER challenges such as lipotoxicity is also discussed. Collectively, this article aims to sharpen our view of liver hepatocytes as the central hubs of protein metabolism.

The degradation pathway of a model misfolded protein is determined by aggregation propensity

Protein homeostasis in the secretory pathway is maintained by a hierarchy of quality control checkpoints, including endoplasmic reticulum–associated degradation (ERAD), which leads to the destruction of misfolded proteins in the ER, as well as post-ER proteolysis. Although most aberrant proteins are degraded by ERAD, some misfolded proteins escape the ER and are degraded instead by lysosomal/vacuolar proteases. To date, it remains unclear how misfolded membrane proteins are selected for these different fates. Here we designed a novel model substrate, SZ*, to investigate how substrate selection is mediated in yeast. We discovered that SZ* is degraded by both the proteasome and vacuolar proteases, the latter of which occurs after ER exit and requires the multivesicular body pathway. By interrogating how various conditions affect the fate of SZ*, we also discovered that heat-shock and substrate overexpression increase ERAD targeting. These conditions also increase substrate aggregation. We next found that aggregation of the membrane-free misfolded domain in SZ* is concentration dependent, and fusion of this misfolded domain to a post-ER quality control substrate instead targets the substrate for ERAD. Our data indicate that a misfolded membrane protein with a higher aggregation propensity is preferentially retained in the ER and targeted for ERAD.

Iminosugar antivirals: the therapeutic sweet spot

Many viruses require the host endoplasmic reticulum protein-folding machinery in order to correctly fold one or more of their glycoproteins. Iminosugars with glucose stereochemistry target the glucosidases which are key for entry into the glycoprotein folding cycle. Viral glycoproteins are thus prevented from interacting with the protein-folding machinery leading to misfolding and an antiviral effect against a wide range of different viral families. As iminosugars target host enzymes, they should be refractory to mutations in the virus. Iminosugars therefore have great potential for development as broad-spectrum antiviral therapeutics. We outline the mechanism giving rise to the antiviral activity of iminosugars, the current progress in the development of iminosugar antivirals and future prospects for this field.

Redox diversity in ERAD-mediated protein retrotranslocation from the endoplasmic reticulum: a complex puzzle

Misfolded and incorrectly assembled proteins in the secretory pathway are eliminated by ubiquitylation and proteasomal degradation in a process known as ER-associated degradation (ERAD). Retrotranslocation of diverse substrates including misfolded proteins and viruses occurs through channels in the ER membrane, which are also utilized for host cell penetration by A/B class protein toxins such as cholera toxin, ricin or K28. According to the current view, disulfide-bonded proteins must either be reduced or rearranged to ensure translocation competence and entry into the cytosol from the ER. As the underlying mechanisms are still largely mysterious, we here focus on the redox status and disulfide isomerization of ERAD substrates and the role of oxidoreductases in the essential process of ER-to-cytosol retrotranslocation.

Aberrant disulphide bonding contributes to the ER retention of alpha1-antitrypsin deficiency variants

Mutations in alpha1-antitrypsin (AAT) can cause the protein to polymerise and be retained in the endoplasmic reticulum (ER) of hepatocytes. The ensuing systemic AAT deficiency leads to pulmonary emphysema, while intracellular polymers are toxic and cause chronic liver disease. The severity of this process varies considerably between individuals, suggesting the involvement of mechanistic co-factors and potential for therapeutically beneficial interventions. We show in Hepa1.6 cells that the mildly polymerogenic I (Arg39Cys) AAT mutant forms aberrant inter- and intra-molecular disulphide bonds involving the acquired Cys39 and the only cysteine residue in the wild-type (M) sequence (Cys232). Substitution of Cys39 to serine partially restores secretion, showing that disulphide bonding contributes to the intracellular retention of I AAT. Covalent homodimers mediated by inter-Cys232 bonding alone are also observed in cells expressing the common Z and other polymerising AAT variants where conformational behaviour is abnormal, but not in those expressing M AAT. Prevention of such disulphide linkage through the introduction of the Cys232Ser mutation or by treatment of cells with reducing agents increases Z AAT secretion. Our results reveal that disulphide interactions enhance intracellular accumulation of AAT mutants and implicate the oxidative ER state as a pathogenic co-factor. Redox modulation, e.g. by anti-oxidant strategies, may therefore be beneficial in AAT deficiency-associated liver disease.

The Role of Lectin-Carbohydrate Interactions in the Regulation of ER-Associated Protein Degradation

Proteins entering the secretory pathway are translocated across the endoplasmic reticulum (ER) membrane in an unfolded form. In the ER they are restricted to a quality control system that ensures correct folding or eventual degradation of improperly folded polypeptides. Mannose trimming of N-glycans on newly synthesized proteins plays an important role in the recognition and sorting of terminally misfolded glycoproteins for ER-associated protein degradation (ERAD). In this process misfolded proteins are retrotranslocated into the cytosol, polyubiquitinated, and eventually degraded by the proteasome. The mechanism by which misfolded glycoproteins are recognized and recruited to the degradation machinery has been extensively studied during last decade. In this review, we focus on ER degradation-enhancing α-mannosidase-like protein (EDEM) family proteins that seem to play a key role in the discrimination between proteins undergoing a folding process and terminally misfolded proteins directed for degradation. We describe interactions of EDEM proteins with other components of the ERAD machinery, as well as with various protein substrates. Carbohydrate-dependent interactions together with N-glycan-independent interactions seem to regulate the complex process of protein recognition and direction for proteosomal degradation.

Site-specific N-glycosylation of the S-locus receptor kinase and its role in the self-incompatibility response of the brassicaceae

The S-locus receptor kinase SRK is a highly polymorphic transmembrane kinase of the stigma epidermis. Through allele-specific interaction with its pollen coat-localized ligand, the S-locus cysteine-rich protein SCR, SRK is responsible for recognition and inhibition of self pollen in the self-incompatibility response of the Brassicaceae. The SRK extracellular ligand binding domain contains several potential N-glycosylation sites that exhibit varying degrees of conservation among SRK variants. However, the glycosylation status and functional importance of these sites are currently unclear. We investigated this issue in transgenic Arabidopsis thaliana stigmas that express the Arabidopsis lyrata SRKb variant and exhibit an incompatible response toward SCRb-expressing pollen. Analysis of single- and multiple-glycosylation site mutations of SRKb demonstrated that, although five of six potential N-glycosylation sites in SRKb are glycosylated in stigmas, N-glycosylation is not important for SCRb-dependent activation of SRKb. Rather, N-glycosylation functions primarily to ensure the proper and efficient subcellular trafficking of SRK to the plasma membrane. The study provides insight into the function of a receptor that regulates a critical phase of the plant life cycle and represents a valuable addition to the limited information available on the contribution of N-glycosylation to the subcellular trafficking and function of plant receptor kinases.

The amazing ubiquitin-proteasome system: structural components and implication in aging

Proteome quality control (PQC) is critical for the maintenance of cellular functionality and it is assured by the curating activity of the proteostasis network (PN). PN is constituted of several complex protein machines that under conditions of proteome instability aim to, firstly identify, and then, either rescue or degrade nonnative polypeptides. Central to the PN functionality is the ubiquitin-proteasome system (UPS) which is composed from the ubiquitin-conjugating enzymes and the proteasome; the latter is a sophisticated multi-subunit molecular machine that functions in a bimodal way as it degrades both short-lived ubiquitinated normal proteins and nonfunctional polypeptides. UPS is also involved in PQC of the nucleus, the endoplasmic reticulum and the mitochondria and it also interacts with the other main cellular degradation axis, namely the autophagy-lysosome system. UPS functionality is optimum in the young organism but it is gradually compromised during aging resulting in increasing proteotoxic stress; these effects correlate not only with aging but also with most age-related diseases. Herein, we present a synopsis of the UPS components and of their functional alterations during cellular senescence or in vivo aging. We propose that mild UPS activation in the young organism will, likely, promote antiaging effects and/or suppress age-related diseases.

Modulation of the unfolded protein response impedes tumor cell adaptation to proteotoxic stress: a PERK for hepatocellular carcinoma therapy

Background

Functional disturbances of the endoplasmic reticulum (ER) lead to activation of the unfolded protein response (UPR), which is involved in the consecutive steps of carcinogenesis. In human hepatocellular carcinoma (HCC), the UPR is shown to be activated; however, little is known about the UPR kinetics and effects of UPR modulation in HCC.

Methods

We sequentially monitored the UPR over time in an orthotopic mouse model for HCC and explored the effects of UPR modulation on cell viability and proliferation in vitro and in the mouse model.

Results

The expression of ER-resident chaperones peaked during tumor initiation and increased further during tumor progression, predominantly within the nodules. A peak in Ire1 signaling was observed during tumor initiation. The Perk pathway was activated during tumor progression, and the proapoptotic target Chop was upregulated from week 5 and continued to rise, especially in the tumors. The Atf6 pathway was modestly activated only after tumor initiation. Consistent with the UPR activation, electron microscopy demonstrated ER expansion and reorganization in HCC cells in vivo. Strikingly, under ER stress or hypoxia, the Perk inhibitor and not the Ire1 inhibitor reduced cell viability and proliferation via escalating proteotoxic stress in vitro. Notably, the Perk inhibitor significantly decreased tumor burden in the mouse model.

Conclusion

We provide the first evaluation of the UPR dynamics in a long-term cancer model and identified a small molecule inhibitor of Perk as a promising strategy for HCC therapy.

Electronic supplementary material

The online version of this article (doi:10.1007/s12072-014-9582-0) contains supplementary material, which is available to authorized users.

The role of EDEM2 compared with EDEM1 in ricin transport from the endoplasmic reticulum to the cytosol

EDEM1 [ER (endoplasmic reticulum)-degradation-enhancing α-mannosidase I-like protein 1] and EDEM2 are crucial regulators of ERAD (ER-associated degradation) that extracts non-native glycoproteins from the calnexin chaperone system. Ricin is a potent plant cytotoxin composed of an A-chain (RTA) connected by a disulfide bond to a cell-binding lectin B-chain (RTB). After endocytic uptake, the toxin is transported retrogradely to the ER, where the enzymatically active RTA is translocated to the cytosol in a similar manner as misfolded ER proteins. This transport is promoted by EDEM1. In the present study we report that EDEM2 is also involved in ricin retrotranslocation out of the ER. However, the role of EDEM1 and EDEM2 in ricin transport to the cytosol seems to differ. EDEM2 promotes ricin retrotranslocation irrespectively of ER translocon accessibility; moreover, co-immunoprecipitation and pull-down studies revealed that more ricin can interact with EDEM2 in comparison with EDEM1. On the other hand, interactions of both lectins with RTA are dependent on the structure of the RTA. Thus our data display a newly discovered role for EDEM2. Moreover, analysis of the involvement of EDEM1 and EDEM2 in ricin retrotranslocation to the cytosol may provide crucial information about general mechanisms of the recognition of ERAD substrates in the ER.

CD4 and BST-2/tetherin proteins retro-translocate from endoplasmic reticulum to cytosol as partially folded and multimeric molecules

CD4 and BST-2/Tetherin are cellular membrane proteins targeted to degradation by the HIV-1 protein Vpu. In both cases proteasomal degradation following recruitment into the ERAD pathway has been described. CD4 is a type I transmembrane glycoprotein, with four extracellular immunoglobulin-like domains containing three intrachain disulfide bridges. BST-2/Tetherin is an atypical type II transmembrane glycoprotein with an N-terminal transmembrane domain and a C-terminal glycophosphatidylinositol anchor, which dimerizes through three interchain bridges. We investigated spontaneous and Vpu-induced retro-translocation of CD4 and BST-2/Tetherin using our novel biotinylation technique in living cells to determine ER-to-cytosol retro-translocation of proteins. We found that CD4 retro-translocates with oxidized intrachain disulfide bridges, and only upon proteasomal inhibition does it accumulate in the cytosol as already reduced and deglycosylated molecules. Similarly, BST-2/Tetherin is first exposed to the cytosol as a dimeric oxidized complex and then becomes deglycosylated and reduced to monomers. These results raise questions on the required features of the putative retro-translocon, suggesting alternative retro-translocation mechanisms for membrane proteins in which complete cysteine reduction and unfolding are not always strictly required before ER to cytosol dislocation.

Glycoprotein misfolding in the endoplasmic reticulum: identification of released oligosaccharides reveals a second ER-associated degradation pathway for Golgi-retrieved proteins

Endoplasmic reticulum-associated degradation (ERAD) is a key cellular process whereby misfolded proteins are removed from the endoplasmic reticulum (ER) for subsequent degradation by the ubiquitin/proteasome system. In the present work, analysis of the released, free oligosaccharides (FOS) derived from all glycoproteins undergoing ERAD, has allowed a global estimation of the mechanisms of this pathway rather than following model proteins through degradative routes. Examining the FOS produced in endomannosidase-compromised cells following α-glucosidase inhibition has revealed a mechanism for clearing Golgi-retrieved glycoproteins that have failed to enter the ER quality control cycle. The Glc3Man7GlcNAc2 FOS species has been shown to be produced in the ER lumen by a mechanism involving a peptide: N-glycanase-like activity, and its production was sensitive to disruption of Golgi-ER trafficking. The detection of this oligosaccharide was unaffected by the overexpression of EDEM1 or cytosolic mannosidase, both of which increased the production of previously characterised cytosolically localised FOS. The lumenal FOS identified are therefore distinct in their production and regulation compared to FOS produced by the conventional route of misfolded glycoproteins directly removed from the ER. The production of such lumenal FOS is indicative of a novel degradative route for cellular glycoproteins that may exist under certain conditions.

SERPINA2 is a novel gene with a divergent function from SERPINA1

Serine protease inhibitors (SERPINs) are a superfamily of highly conserved proteins that play a key role in controlling the activity of proteases in diverse biological processes. The SERPIN cluster located at the 14q32.1 region includes the gene coding for SERPINA1, and a highly homologous sequence, SERPINA2, which was originally thought to be a pseudogene. We have previously shown that SERPINA2 is expressed in different tissues, namely leukocytes and testes, suggesting that it is a functional SERPIN. To investigate the function of SERPINA2, we used HeLa cells stably transduced with the different variants of SERPINA2 and SERPINA1 (M1, S and Z) and leukocytes as the in vivo model. We identified SERPINA2 as a 52 kDa intracellular glycoprotein, which is localized at the endoplasmic reticulum (ER), independently of the variant analyzed. SERPINA2 is not significantly regulated by proteasome, proposing that ER localization is not due to misfolding. Specific features of SERPINA2 include the absence of insoluble aggregates and the insignificant response to cell stress, suggesting that it is a non-polymerogenic protein with divergent activity of SERPINA1. Using phylogenetic analysis, we propose an origin of SERPINA2 in the crown of primates, and we unveiled the overall conservation of SERPINA2 and A1. Nonetheless, few SERPINA2 residues seem to have evolved faster, contributing to the emergence of a new advantageous function, possibly as a chymotrypsin-like SERPIN. Herein, we present evidences that SERPINA2 is an active gene, coding for an ER-resident protein, which may act as substrate or adjuvant of ER-chaperones.

Stress-independent activation of XBP1s and/or ATF6 reveals three functionally diverse ER proteostasis environments

The unfolded protein response (UPR) maintains endoplasmic reticulum (ER) proteostasis through the activation of transcription factors such as XBP1s and ATF6. The functional consequences of these transcription factors for ER proteostasis remain poorly defined. Here, we describe methodology that enables orthogonal, small-molecule-mediated activation of the UPR-associated transcription factors XBP1s and/or ATF6 in the same cell independent of stress. We employ transcriptomics and quantitative proteomics to evaluate ER proteostasis network remodeling owing to the XBP1s and/or ATF6 transcriptional programs. Furthermore, we demonstrate that the three ER proteostasis environments accessible by activating XBP1s and/or ATF6 differentially influence the folding, trafficking, and degradation of destabilized ER client proteins without globally affecting the endogenous proteome. Our data reveal how the ER proteostasis network is remodeled by the XBP1s and/or ATF6 transcriptional programs at the molecular level and demonstrate the potential for selective restoration of aberrant ER proteostasis of pathologic, destabilized proteins through arm-selective UPR activation.

A shared endoplasmic reticulum-associated degradation pathway involving the EDEM1 protein for glycosylated and nonglycosylated proteins

Studies of misfolded protein targeting to endoplasmic reticulum-associated degradation (ERAD) have largely focused on glycoproteins, which include the bulk of the secretory proteins. Mechanisms of targeting of nonglycosylated proteins are less clear. Here, we studied three nonglycosylated proteins and analyzed their use of known glycoprotein quality control and ERAD components. Similar to an established glycosylated ERAD substrate, the uncleaved precursor of asialoglycoprotein receptor H2a, its nonglycosylated mutant, makes use of calnexin, EDEM1, and HRD1, but only glycosylated H2a is a substrate for the cytosolic SCF(Fbs2) E3 ubiquitin ligase with lectin activity. Two nonglycosylated BiP substrates, NS-1κ light chain and truncated Igγ heavy chain, interact with the ERAD complex lectins OS-9 and XTP3-B and require EDEM1 for degradation. EDEM1 associates through a region outside of its mannosidase-like domain with the nonglycosylated proteins. Similar to glycosylated substrates, proteasomal inhibition induced accumulation of the nonglycosylated proteins and ERAD machinery in the endoplasmic reticulum-derived quality control compartment. Our results suggest a shared ERAD pathway for glycosylated and nonglycosylated proteins composed of luminal lectin machinery components also capable of protein-protein interactions.

Histone deacetylase inhibitor (HDACi) suberoylanilide hydroxamic acid (SAHA)-mediated correction of α1-antitrypsin deficiency

α1-Antitrypsin (α1AT) deficiency (α1ATD) is a consequence of defective folding, trafficking, and secretion of α1AT in response to a defect in its interaction with the endoplasmic reticulum proteostasis machineries. The most common and severe form of α1ATD is caused by the Z-variant and is characterized by the accumulation of α1AT polymers in the endoplasmic reticulum of the liver leading to a severe reduction (>85%) of α1AT in the serum and its anti-protease activity in the lung. In this organ α1AT is critical for ensuring tissue integrity by inhibiting neutrophil elastase, a protease that degrades elastin. Given the limited therapeutic options in α1ATD, a more detailed understanding of the folding and trafficking biology governing α1AT biogenesis and its response to small molecule regulators is required. Herein we report the correction of Z-α1AT secretion in response to treatment with the histone deacetylase (HDAC) inhibitor suberoylanilide hydroxamic acid (SAHA), acting in part through HDAC7 silencing and involving a calnexin-sensitive mechanism. SAHA-mediated correction restores Z-α1AT secretion and serpin activity to a level 50% that observed for wild-type α1AT. These data suggest that HDAC activity can influence Z-α1AT protein traffic and that SAHA may represent a potential therapeutic approach for α1ATD and other protein misfolding diseases.

Protein folding and quality control in the ER

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

Bypass of glycan-dependent glycoprotein delivery to ERAD by up-regulated EDEM1

Extensive trimming of mannose residues targets a misfolded glycoprotein for endoplasmic reticulum–associated degradation (ERAD). Surprisingly, overexpression of EDEM1 or its up-regulation by the unfolded protein response bypasses this requirement. Delivery to OS9 in the ER-derived quality control compartment and ERAD becomes mannose trimming–independent, accelerating glycoprotein disposal.

Trimming of mannose residues from the N-linked oligosaccharide precursor is a stringent requirement for glycoprotein endoplasmic reticulum (ER)-associated degradation (ERAD). In this paper, we show that, surprisingly, overexpression of ER degradation–enhancing α-mannosidase-like protein 1 (EDEM1) or its up-regulation by IRE1, as occurs in the unfolded protein response, overrides this requirement and renders unnecessary the expression of ER mannosidase I. An EDEM1 deletion mutant lacking most of the carbohydrate-recognition domain also accelerated ERAD, delivering the substrate to XTP3-B and OS9. EDEM1 overexpression also accelerated the degradation of a mutant nonglycosylated substrate. Upon proteasomal inhibition, EDEM1 concentrated together with the ERAD substrate in the pericentriolar ER-derived quality control compartment (ERQC), where ER mannosidase I and ERAD machinery components are localized, including, as we show here, OS9. We suggest that a nascent glycoprotein can normally dissociate from EDEM1 and be rescued from ERAD by reentering calnexin-refolding cycles, a condition terminated by mannose trimming. At high EDEM1 levels, glycoprotein release is prevented and glycan interactions are no longer required, canceling the otherwise mandatory ERAD timing by mannose trimming and accelerating the targeting to degradation.

Role of malectin in Glc(2)Man(9)GlcNAc(2)-dependent quality control of α1-antitrypsin

In cells, human malectin stably interacted with newly synthesized AT^NHK, but not AT, via G2M9 glycans. The interaction of AT^NHK with malectin resulted in enhanced ERAD of AT^NHK and prevented the secretion of the misfolded glycoprotein. These findings provide evidence of a role of malectin in glycoprotein quality control via recognition of G2M9.

Malectin was first discovered as a novel endoplasmic reticulum (ER)–resident lectin from Xenopus laevis that exhibits structural similarity to bacterial glycosylhydrolases. Like other intracellular lectins involved in glycoprotein quality control, malectin is highly conserved in animals. Here results from in vitro membrane-based binding assays and frontal affinity chromatography confirm that human malectin binds specifically to Glc₂Man₉GlcNAc₂ (G2M9) N-glycan, with a K_a of 1.97 × 10⁵ M⁻¹, whereas binding to Glc₁Man₉GlcNAc₂ (G1M9), Glc₃Man₉GlcNAc₂ (G3M9), and other N-glycans is barely detectable. Metabolic labeling and immunoprecipitation experiments demonstrate that before entering the calnexin cycle, the folding-defective human α1-antitrypsin variant null Hong Kong (AT^NHK) stably associates with malectin, whereas wild-type α1-antitrypsin (AT) or N-glycan–truncated variant of AT^NHK (AT^NHK-Q3) dose not. Moreover, malectin overexpression dramatically inhibits the secretion of AT^NHK through a mechanism that involves enhanced ER-associated protein degradation; by comparison, the secretion of AT and AT^NHK-Q3 is only slightly affected by malectin overexpression. ER-stress induced by tunicamycin results in significantly elevated mRNA transcription of malectin. These observations suggest a possible role of malectin in regulating newly synthesized glycoproteins via G2M9 recognition.

A single point mutation in ricin A-chain increases toxin degradation and inhibits EDEM1-dependent ER retrotranslocation

Ricin is a potent plant cytotoxin composed of an A-chain [RTA (ricin A-chain)] connected by a disulfide bond to a cell binding lectin B-chain [RTB (ricin B-chain)]. After endocytic uptake, the toxin is transported retrogradely to the ER (endoplasmic reticulum) from where enzymatically active RTA is translocated to the cytosol. This transport is promoted by the EDEM1 (ER degradation-enhancing α-mannosidase I-like protein 1), which is also responsible for directing aberrant proteins for ERAD (ER-associated protein degradation). RTA contains a 12-residue hydrophobic C-terminal region that becomes exposed after reduction of ricin in the ER. This region, especially Pro250, plays a crucial role in ricin cytotoxicity. In the present study, we introduced a point mutation [P250A (substitution of Pro250 with alanine)] in the hydrophobic region of RTA to study the intracellular transport of the modified toxin. The introduced mutation alters the secondary structure of RTA into a more helical structure. Mutation P250A increases endosomal–lysosomal degradation of the toxin, as well as reducing its transport from the ER to the cytosol. Transport of modified RTA to the cytosol, in contrast to wild-type RTA, appears to be EDEM1-independent. Importantly, the interaction between EDEM1 and RTAP250A is reduced. This is the first reported evidence that EDEM1 protein recognition might be determined by the structure of the ERAD substrate.

The endoplasmic reticulum-associated degradation and disulfide reductase ERdj5

The endoplasmic reticulum (ER) is an organelle where secretory or membrane proteins are correctly folded with the aid of various molecular chaperones and oxidoreductases. Only correctly folded and assembled proteins are enabled to reach their final destinations, which are called as ER quality control (ERQC) mechanisms. ER-associated degradation (ERAD) is one of the ERQC mechanisms for maintaining the ER homeostasis and facilitates the elimination of misfolded or malfolded proteins accumulated in the ER. ERAD is mainly consisting of three processes: recognition of misfolded proteins for degradation in the ER, retrotranslocation of (possibly) unfolded substrates from the ER to the cytosol through dislocation channel, and their degradation in the cytosol via ubiquitin-protesome system. After briefly mentioned on productive folding of nascent polypeptides in the ER, we here overview the above three processes in ERAD system by highlighting on novel ERAD factors such as EDEM and ERdj5 in mammals and yeasts.

Mannose trimming is required for delivery of a glycoprotein from EDEM1 to XTP3-B and to late endoplasmic reticulum-associated degradation steps

Although the trimming of α1,2-mannose residues from precursor N-linked oligosaccharides is an essential step in the delivery of misfolded glycoproteins to endoplasmic reticulum (ER)-associated degradation (ERAD), the exact role of this trimming is unclear. EDEM1 was initially suggested to bind N-glycans after mannose trimming, a role presently ascribed to the lectins OS9 and XTP3-B, because of their in vitro affinities for trimmed oligosaccharides. We have shown before that ER mannosidase I (ERManI) is required for the trimming and concentrates together with the ERAD substrate and ERAD machinery in the pericentriolar ER-derived quality control compartment (ERQC). Inhibition of mannose trimming prevents substrate accumulation in the ERQC. Here, we show that the mannosidase inhibitor kifunensine or ERManI knockdown do not affect binding of an ERAD substrate glycoprotein to EDEM1. In contrast, substrate association with XTP3-B and with the E3 ubiquitin ligases HRD1 and SCF(Fbs2) was inhibited. Consistently, whereas the ERAD substrate partially colocalized upon proteasomal inhibition with EDEM1, HRD1, and Fbs2 at the ERQC, colocalization was repressed by mannosidase inhibition in the case of the E3 ligases but not for EDEM1. Interestingly, association and colocalization of the substrate with Derlin-1 was independent of mannose trimming. The HRD1 adaptor protein SEL1L had been suggested to play a role in N-glycan-dependent substrate delivery to OS9 and XTP3-B. However, substrate association with XTP3-B was still dependent on mannose trimming upon SEL1L knockdown. Our results suggest that mannose trimming enables delivery of a substrate glycoprotein from EDEM1 to late ERAD steps through association with XTP3-B.

Role of intramembrane charged residues in the quality control of unassembled T-cell receptor alpha-chains at the endoplasmic reticulum

Endoplasmic reticulum (ER)-associated degradation (ERAD) of unassembled T-cell receptor alpha-chain (TCRalpha) is reliant on the presence of two basic residues in the transmembrane (TM) segment of TCRalpha. The precise role of these residues in ER quality control is unclear. Here, we show that a TCRalpha mutant lacking these intramembrane charged residues has a tendency to form homooligomers through an interchain disulfide bond that involves a specific pair of cysteine residues. Covalent oligomerization of TCRalpha appears to stabilize it at the ER membrane. The presence of a single lysine residue at specific positions within the TCRalpha TM domain abolishes its oligomerization and causes its rapid degradation. Conversely, when TCRalpha oligomerization is induced by a bivalent compound, the degradation of TCRalpha is inhibited. Together, these results suggest that the intramembrane charged residues in TCRalpha do not function as a signal for substrate recognition in ERAD. Instead, their primary role is to reduce TCRalpha oligomerization, maintaining it in a retrotranslocation-competent state. Our results also suggest that the ERAD machinery is inefficient when coping with oligomerized substrates, indicating a requirement for chaperone-mediated protein disassembly in the ER lumen prior to retrotranslocation.

Sorting things out through endoplasmic reticulum quality control

The endoplasmic reticulum (ER) is a highly organized and specialized organelle optimized for the production of proteins. It is comprised of a highly interconnected network of tubules that contain a large set of resident proteins dedicated to the maturation and processing of proteins that traverse the eukaryotic secretory pathway. As protein maturation is an imperfect process, frequently resulting in misfolding and/or the formation of aggregates, proteins are subjected to a series of evaluation processes within the ER. Proteins deemed native are sorted for anterograde trafficking, while immature or non-native proteins are initially retained in the ER in an attempt to rescue the aberrant products. Terminally misfolded substrates are eventually targeted for turnover through the ER-associated degradation or ERAD pathway to protect the cell from the release of a defective product. A clearer picture of the identity of the machinery involved in these quality control evaluation processes and their mechanisms of actions has emerged over the past decade.

Novel thioredoxin-related transmembrane protein TMX4 has reductase activity

In the endoplasmic reticulum (ER), a number of thioredoxin (Trx) superfamily proteins are present to enable correct disulfide bond formation of secretory and membrane proteins via Trx-like domains. Here, we identified a novel transmembrane Trx-like protein 4 (TMX4), in the ER of mammalian cells. TMX4, a type I transmembrane protein, was localized to the ER and possessed a Trx-like domain that faced the ER lumen. A maleimide alkylation assay showed that a catalytic CXXC motif in the TMX4 Trx-like domain underwent changes in its redox state depending on cellular redox conditions, and, in the normal state, most of the endogenous TMX4 existed in the oxidized form. Using a purified recombinant protein containing the Trx-like domain of TMX4 (TMX4-Trx), we confirmed that this domain had reductase activity in vitro. The redox potential of this domain (-171.5 mV; 30 degrees C at pH 7.0) indicated that TMX4 could work as a reductase in the environment of the ER. TMX4 had no effect on the acceleration of ER-associated degradation. Because TMX4 interacted with calnexin and ERp57 by co-immunoprecipitation assay, the role of TMX4 may be to enable protein folding in cooperation with these proteins consisting of folding complex in the ER.

A dual role for EDEM1 in the processing of rod opsin

Mutations in rod opsin, the archetypal G-protein-coupled receptor, cause retinitis pigmentosa. The majority of mutations, e.g. P23H, cause protein misfolding, resulting in ER retention, induction of the unfolded protein response and degradation by ERAD. If misfolded rod opsin escapes degradation, it aggregates and forms intracellular inclusions. Therefore, it is important to identify the chaperones that mediate the folding or degradation of rod opsin. ER degradation enhancing alpha-mannosidase-like 1 (EDEM1) can enhance the release of terminally misfolded glycoproteins from the calnexin chaperone system. Here, we identify EDEM1 as a novel chaperone of rod opsin. EDEM1 expression promoted the degradation of P23H rod opsin and decreased its aggregation. By contrast, shRNA-mediated knockdown of EDEM1 increased both the amount of P23H rod opsin and its aggregation into inclusions. EDEM1 was detected in rod photoreceptor inner segments and EndoH-sensitive rod opsin co-immunoprecipitated with EDEM1 from retina, suggesting that rod opsin is a physiological EDEM1 client. Unexpectedly, EDEM1 binding to rod opsin was independent of mannose trimming and EDEM1 promoted the cell-surface expression of mutant rod opsin. Collectively, the data suggest that EDEM1 is a chaperone for rod opsin and that expression of EDEM1 can be used to promote correct folding, as well as enhanced degradation, of mutant proteins in the ER to combat protein-misfolding disease.

Protein disulphide isomerase family members show distinct substrate specificity: P5 is targeted to BiP client proteins

At least 17 members of the protein disulphide isomerase (PDI) family of oxidoreductases are present in the endoplasmic reticulum (ER) of mammalian cells. They are thought to catalyse disulphide formation to aid folding or to regulate protein function; however, little is known about their individual functions. Here, we show that some proteins that enter the ER are clients for single oxidoreductases, whereas others are clients for several PDI-like enzymes. We previously identified potential substrates for ERp57, and here identify substrates for ERp18 and ERp46. In addition, we analysed the specificity of substrates towards PDI, ERp72, ERp57, ERp46, ERp18 and P5. Strikingly, ERp18 shows specificity towards a component of the complement cascade, pentraxin-related protein PTX3, whereas ERp46 has specificity towards peroxiredoxin-4, a thioredoxin peroxidase. By contrast, most PDI family members react with Ero1alpha. Moreover, P5 forms a non-covalent complex with immunoglobulin heavy chain binding protein (BiP) and shows specificity towards BiP client proteins. These findings highlight cooperation between BiP and P5, and demonstrate that individual PDI family members recognise specific substrate proteins.

Differences in endoplasmic-reticulum quality control determine the cellular response to disease-associated mutants of proteolipid protein

Missense mutations in human PLP1, the gene encoding myelin proteolipid protein (PLP), cause dysmyelinating Pelizaeus-Merzbacher disease of varying severity. Although disease pathology has been linked to retention of misfolded PLP in the endoplasmic reticulum (ER) and induction of the unfolded protein response (UPR), the molecular mechanisms that govern phenotypic heterogeneity remain poorly understood. To address this issue, we examined the cellular response to missense mutants of PLP that are associated with distinct disease phenotypes. We found that the mild-disease-associated mutants, W162L and G245A, were cleared from the ER comparatively quickly via proteasomal degradation and/or ER exit. By contrast, the more ;aggressive' A242V mutant, which causes severe disease, was significantly more stable, accumulated at the ER and resulted in a specific activation of the UPR. On the basis of these findings, we propose that the rate at which mutant PLP proteins are cleared from the ER modulates disease severity by determining the extent to which the UPR is activated.

Positive contribution of ERdj5/JPDI to endoplasmic reticulum protein quality control in the salivary gland

In eukaryotic cells, most membrane and secretory proteins are modified post-translationally in the ER (endoplasmic reticulum) for correct folding and assembly. Disulfide-bond formation is one of the important modifications affecting folding and is catalysed by the PDI (protein disulfide isomerase) family proteins. ERdj5 [also known as JPDI (J-domain-containing PDI-like protein)] is a member of the PDI family proteins and has been reported to act as a reductase in ERAD (ER-associated degradation). However, the role of ERdj5 at the whole-body level remains unclear. Therefore in the present study we generated ERdj5-knockout mice {the mouse gene of ERdj5 is known as Dnajc10 [DnaJ (Hsp40) homologue, subfamily C, member 10]} and analysed them. Although ERdj5-knockout mice were viable and healthy, the ER stress response was activated in the salivary gland of the knockout mice more than that of control mice. Furthermore, in ERdj5-knockout cells, the expression of exogenous ERdj5 mitigated the ER stress caused by overproduction of alpha-amylase, which is one of the most abundant proteins in saliva and has five intramolecular disulfide bonds. This effect was dependent on the thioredoxin-like motifs of ERdj5. Thus we suggest that ERdj5 contributes to ER protein quality control in the salivary gland.

Lectin-like ERAD players in ER and cytosol

Protein quality control in the endoplasmic reticulum (ER) is an elaborate process conserved from yeast to mammals, ensuring that only newly synthesized proteins with correct conformations in the ER are sorted further into the secretory pathway. It is well known that high-mannose type N-glycans are involved in protein-folding events. In the quality control process, proteins that fail to achieve proper folding or proper assembly are degraded in a process known as ER-associated degradation (ERAD). The ERAD pathway comprises multiple steps including substrate recognition and targeting to the retro-translocation machinery, retrotranslocation from the ER into the cytosol, and proteasomal degradation through ubiquitination. Recent studies have documented the important roles of sugar-recognition (lectin-type) molecules for trimmed high-mannose type N-glycans and glycosidases in the ERAD pathways in both ER and cytosol. In this review, we discuss a fundamental system that monitors glycoprotein folding in the ER and the unique roles of the sugar-recognizing ubiquitin ligase and peptide:N-glycanase (PNGase) in the cytosolic ERAD pathway.

Autophagic elimination of misfolded procollagen aggregates in the endoplasmic reticulum as a means of cell protection

Type I collagen is a major component of the extracellular matrix, and mutations in the collagen gene cause several matrix-associated diseases. These mutant procollagens are misfolded and often aggregated in the endoplasmic reticulum (ER). Although the misfolded procollagens are potentially toxic to the cell, little is known about how they are eliminated from the ER. Here, we show that procollagen that can initially trimerize but then aggregates in the ER are eliminated by an autophagy-lysosome pathway, but not by the ER-associated degradation (ERAD) pathway. Inhibition of autophagy by specific inhibitors or RNAi-mediated knockdown of an autophagy-related gene significantly stimulated accumulation of aggregated procollagen trimers in the ER, and activation of autophagy with rapamycin resulted in reduced amount of aggregates. In contrast, a mutant procollagen which has a compromised ability to form trimers was degraded by ERAD. Moreover, we found that autophagy plays an essential role in protecting cells against the toxicity of the ERAD-inefficient procollagen aggregates. The autophagic elimination of aggregated procollagen occurs independently of the ERAD system. These results indicate that autophagy is a final cell protection strategy deployed against ER-accumulated cytotoxic aggregates that are not able to be removed by ERAD.

The mammalian UPR boosts glycoprotein ERAD by suppressing the proteolytic downregulation of ER mannosidase I

The secretory pathway provides a physical route through which only correctly folded gene products are delivered to the eukaryotic cell surface. The efficiency of endoplasmic reticulum (ER)-associated degradation (ERAD), which orchestrates the clearance of structurally aberrant proteins under basal conditions, is boosted by the unfolded protein response (UPR) as one of several means to relieve ER stress. However, the underlying mechanism that links the two systems in higher eukaryotes has remained elusive. Herein, the results of transient expression, RNAi-mediated knockdown and functional studies demonstrate that the transcriptional elevation of EDEM1 boosts the efficiency of glycoprotein ERAD through the formation of a complex that suppresses the proteolytic downregulation of ER mannosidase I (ERManI). The results of site-directed mutagenesis indicate that this capacity does not require that EDEM1 possess inherent mannosidase activity. A model is proposed in which ERManI, by functioning as a downstream effector target of EDEM1, represents a checkpoint activation paradigm by which the mammalian UPR coordinates the boosting of ERAD.