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1.
Secreted proteins are synthesized at the endoplasmic reticulum (ER), and a quality control mechanism in the ER is essential to maintain secretory pathway homeostasis. Newly synthesized soluble and integral membrane secreted proteins fold into their native conformations with the aid of ER molecular chaperones before they are transported to post-ER compartments. However, terminally mis-folded proteins may be retained in the ER and degraded by a process called ER-associated degradation (ERAD). Recent studies using yeast have shown that molecular chaperones both in the ER and in the cytosol play key roles during the ERAD of mis-folded proteins. One important role for chaperones during ERAD is to prevent substrate protein aggregation. Substrate selection is another important role for molecular chaperones during ERAD. 相似文献
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Endoplasmic-reticulum-associated protein degradation inside and outside of the endoplasmic reticulum
Summary Newly synthesized polypeptides that enter the endomembrane system encounter a folding environment in the lumen of the endoplasmic reticulum (ER) constituted by enzymes, lectinlike proteins, and molecular chaperones. The folding process is under scrutiny of this abundant catalytic machinery, and failure of the new arrivals to assume a stable and functional conformation is met with targeting to proteolytic destruction, a process which has been termed ER-associated degradation (ERAD). In recent years it became clear that, in most cases, proteolysis appears to take place in the cytosol after retro-translocation of the substrate proteins from the ER, and to depend on the ubiquitin-proteasome pathway. On the other hand, proteolytic activities within the ER that have been widely neglected so far may also contribute to the turnover of proteins delivered to ERAD. Thus, ERAD is being deciphered as a complex process that requires communication-dependent regulated proteolytic activities within both the ER lumen and the cytosol. Here we discuss some recent findings on ERAD and their implications on possible mechanisms involved.Abbreviations lAT
alpha-1-antitrypsin
- apoB
apolipoprotein B
- BiP
immunoglobulin-heavy-chain-binding protein
- CFTR
cystic fibrosis transmembrane conductance regulator
- CPY
carboxypeptidase Y
- ER
endoplasmic reticulum
- ERAD ER
associated degradation
- HMG-CoA
3-hydroxy-3-methylglutaryl coenzyme A
- MHC
major histocompatibility complex
- PDI
protein disulflde isomerase
- TCR
T cell antigen receptor 相似文献
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ER stress and diseases 总被引:1,自引:0,他引:1
Yoshida H 《The FEBS journal》2007,274(3):630-658
Proteins synthesized in the endoplasmic reticulum (ER) are properly folded with the assistance of ER chaperones. Malfolded proteins are disposed of by ER-associated protein degradation (ERAD). When the amount of unfolded protein exceeds the folding capacity of the ER, human cells activate a defense mechanism called the ER stress response, which induces expression of ER chaperones and ERAD components and transiently attenuates protein synthesis to decrease the burden on the ER. It has been revealed that three independent response pathways separately regulate induction of the expression of chaperones, ERAD components, and translational attenuation. A malfunction of the ER stress response caused by aging, genetic mutations, or environmental factors can result in various diseases such as diabetes, inflammation, and neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, and bipolar disorder, which are collectively known as 'conformational diseases'. In this review, I will summarize recent progress in this field. Molecules that regulate the ER stress response would be potential candidates for drug targets in various conformational diseases. 相似文献
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Endoplasmic reticulum (ER)-associated degradation (ERAD) is the process by which aberrant proteins in the ER lumen are exported back to the cytosol and degraded by the proteasome. Although ER molecular chaperones are required for ERAD, their specific role(s) in this process have been ill defined. To understand how one group of interacting lumenal chaperones facilitates ERAD, the fates of pro-alpha-factor and a mutant form of carboxypeptidase Y were examined both in vivo and in vitro. We found that these ERAD substrates are stabilized and aggregate in the ER at elevated temperatures when BiP, the lumenal Hsp70 molecular chaperone, is mutated, or when the genes encoding the J domain-containing proteins Jem1p and Scj1p are deleted. In contrast, deletion of JEM1 and SCJ1 had little effect on the ERAD of a membrane protein. These results suggest that one role of the BiP, Jem1p, and Scj1p chaperones is to maintain lumenal ERAD substrates in a retrotranslocation-competent state. 相似文献
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Jason C. Young 《Disease models & mechanisms》2014,7(3):319-329
Protein-folding diseases are an ongoing medical challenge. Many diseases within this group are genetically determined, and have no known cure. Among the examples in which the underlying cellular and molecular mechanisms are well understood are diseases driven by misfolding of transmembrane proteins that normally function as cell-surface ion channels. Wild-type forms are synthesized and integrated into the endoplasmic reticulum (ER) membrane system and, upon correct folding, are trafficked by the secretory pathway to the cell surface. Misfolded mutant forms traffic poorly, if at all, and are instead degraded by the ER-associated proteasomal degradation (ERAD) system. Molecular chaperones can assist the folding of the cytosolic domains of these transmembrane proteins; however, these chaperones are also involved in selecting misfolded forms for ERAD. Given this dual role of chaperones, diseases caused by the misfolding and aberrant trafficking of ion channels (referred to here as ion-channel-misfolding diseases) can be regarded as a consequence of insufficiency of the pro-folding chaperone activity and/or overefficiency of the chaperone ERAD role. An attractive idea is that manipulation of the chaperones might allow increased folding and trafficking of the mutant proteins, and thereby partial restoration of function. This Review outlines the roles of the cytosolic HSP70 chaperone system in the best-studied paradigms of ion-channel-misfolding disease – the CFTR chloride channel in cystic fibrosis and the hERG potassium channel in cardiac long QT syndrome type 2. In addition, other ion channels implicated in ion-channel-misfolding diseases are discussed.KEY WORDS: Chaperone, Cystic fibrosis, Long QT syndrome, Degradation, Intracellular trafficking, Protein folding 相似文献
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Most proteins in the secretory pathway are translated, folded, and subjected to quality control at the endoplasmic reticulum (ER). These processes must be flexible enough to process diverse protein conformations, yet specific enough to recognize when a protein should be degraded. Molecular chaperones are responsible for this decision making process. ER associated chaperones assist in polypeptide translocation, protein folding, and ER associated degradation (ERAD). Nevertheless, we are only beginning to understand how chaperones function, how they are recruited to specific substrates and assist in folding/degradation, and how unique chaperone classes make quality control "decisions". 相似文献
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Brodsky JL 《Methods (San Diego, Calif.)》2005,35(4):354-359
The endoplasmic reticulum (ER) represents the first compartment into which nascent secreted proteins traffic, and not coincidentally the ER lumen houses a high concentration of factors that facilitate protein folding, such as molecular chaperones. To off-set the potentially lethal consequences of mis-folded secreted protein accumulation, aberrant proteins may be selected for degradation via a process known as ER associated degradation (ERAD). After their selection ERAD substrates are retro-translocated back to the cytoplasm and then degraded by the 26S proteasome. Key features of the selection, retro-translocation, and degradation steps that constitute the ERAD pathway were elucidated through the development of an in vitro ERAD assay. In this assay the fates of two yeast proteins can be distinguished after their translocation, or import into ER-derived microsomes. Whereas a wild type, glycosylated protein ("Gp(alpha)F") is stable, a non-glycosylated version of the same protein ("p(alpha)F") is rapidly degraded when microsomes containing radiolabeled forms of these substrates are incubated in cytosol and ATP. The purpose of this chapter is first to discuss the experimental findings from the use of the in vitro assay, and then to describe the assay in detail. Finally, future potential uses of the in vitro system are illustrated. 相似文献
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In eukaryotic cells, the endoplasmic reticulum (ER) is a membrane-enclosed interconnected organelle responsible for the synthesis, folding, modification, and quality control of numerous secretory and membrane proteins. The processes of protein folding and maturation are highly assisted and scrutinized but are also sensitive to changes in ER homeostasis, such as Ca(2+) depletion, oxidative stress, hypoxia, energy deprivation, metabolic stimulation, altered glycosylation, activation of inflammation, as well as increases in protein synthesis or the expression of misfolded proteins or unassembled protein subunits. Only properly folded proteins can traffic to the Golgi apparatus, whereas those that misfold are directed to ER-associated degradation (ERAD) or to autophagy. The accumulation of unfolded/misfolded proteins in the ER activates signaling events to orchestrate adaptive cellular responses. This unfolded protein response (UPR) increases the ER protein-folding capacity, reduces global protein synthesis, and enhances ERAD of misfolded proteins. 相似文献
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Pekka Määttänen Kalle Gehring John J.M. Bergeron David Y. Thomas 《Seminars in cell & developmental biology》2010,21(5):500-511
The mechanism, in molecular terms of protein quality control, specifically of how the cell recognizes and discriminates misfolded proteins, remains a challenge. In the secretory pathway the folding status of glycoproteins passing through the endoplasmic reticulum is marked by the composition of the N-glycan. The different glycoforms are recognized by specialized lectins. The folding sensor UGGT acts as an unusual molecular chaperone and covalently modifies the Man9 N-glycan of a misfolded protein by adding a glucose moiety and converts it to Glc1Man9 that rebinds the lectin calnexin. However, further links between the folding status of a glycoprotein and the composition of the N-glycan are unclear. There is little unequivocal evidence for other proteins in the ER recognizing the N-glycan and also acting as molecular chaperones. Nevertheless, based upon a few examples, we suggest that this function is carried out by individual proteins in several different complexes. Thus, calnexin binds the protein disulfide isomerase ERp57, that acts upon Glc1Man9 glycoproteins. In another example the protein disulfide isomerase ERdj5 binds specifically to EDEM (which is probably a mannosidase) and a lectin OS9, and reduces the disulfide bonds of bound glycoproteins destined for ERAD. Thus the glycan recognition is performed by a lectin and the chaperone function performed by a specific partner protein that can recognize misfolded proteins. We predict that this will be a common arrangement of proteins in the ER and that members of protein foldase families such as PDI and PPI will bind specifically to lectins in the ER. Molecular chaperones BiP and GRp94 will assist in the folding of proteins bound in these complexes as well as in the folding of non-glycoproteins. 相似文献
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Nakatsukasa K Okada S Umebayashi K Fukuda R Nishikawa S Endo T 《The Journal of biological chemistry》2004,279(48):49762-49772
The protein quality control system in the endoplasmic reticulum (ER) ensures that only properly folded proteins are deployed throughout the cells. When nonnative proteins accumulate in the ER, the unfolded protein response is triggered to limit further accumulation of nonnative proteins and the ER is cleared of accumulated nonnative proteins by the ER-associated degradation (ERAD). In the yeast ER, aberrant nonnative proteins are mainly directed for the ERAD, but a distinct fraction of them instead receive O-mannosylation. In order to test whether O-mannosylation might also be a mechanism to process aberrant proteins in the ER, here we analyzed the effect of O-mannosylation on two kinds of model aberrant proteins, a series of N-glycosylation site mutants of prepro-alpha-factor and a pro-region-deleted derivative of Rhizopus niveus aspartic proteinase-I (Deltapro) both in vitro and in vivo. O-Mannosylation increases solubilities of the aberrant proteins and renders them less dependent on the ER chaperone, BiP, for being soluble. The release from ER chaperones allows the aberrant proteins to exit out of the ER for the normal secretory pathway transport. When the gene for Pmt2p, responsible for the O-mannosylation of these aberrant proteins, and that for the ERAD were simultaneously deleted, the cell exhibited enhanced unfolded protein response. O-Mannosylation may therefore function as a fail-safe mechanism for the ERAD by solubilizing the aberrant proteins that overflowed from the ERAD pathway and reducing the load for ER chaperones. 相似文献
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An increasing body of data links endoplasmic reticulum (ER) function to autophagy. Not surprisingly, then, some aberrant proteins in the ER can be destroyed either via ER associated degradation (ERAD), which is proteasome-mediated, or via autophagy. One such substrate is the "Z" variant of the alpha-1 protease inhibitor (A1Pi), variably known as A1Pi-Z or AT-Z ("anti-trypsin, Z variant"). The wild type protein is primarily synthesized in the liver and is secreted. In contrast, AT-Z, like other ERAD substrates, is retro-translocated from the ER and delivered to the proteasome. However, AT-Z can form high molecular weight polymers that are degraded via autophagy, and cells that accumulate AT-Z polymers ultimately succumb, which leads to liver disease. Therefore, identifying genes that have an impact AT-Z turnover represents an active area of research. To this end, a yeast expression system for AT-Z has proven valuable. For example, a recent study using this system indicates that the activity of a proteasome assembly chaperone (PAC) is critical for maximal AT-Z turnover, which suggests a new role for PACs. Because PACs are conserved, it will be critical to analyze whether these dedicated chaperones are implicated in other diseases associated with ERAD and autophagy. 相似文献
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The endoplasmic reticulum (ER) quality control pathway destroys misfolded and unassembled proteins in the ER. Most substrates of this ER-associated degradation (ERAD) pathway are constitutively targeted for destruction through recognition of poorly understood structural hallmarks of misfolding. However, the normal yeast ER membrane protein 3-hydroxy-3-methylglutaryl-CoA reductase (Hmg2p) undergoes ERAD that is physiologically regulated by sterol pathway signals. We have proposed that Hmg2p ERAD occurs by a regulated transition to an ERAD quality control substrate. Consistent with this, we had previously shown that Hmg2p is strongly stabilized by chemical chaperones such as glycerol, which stabilize misfolded proteins. To understand the features of Hmg2p that permit regulated ERAD, we have thoroughly characterized the effects of chemical chaperones on Hmg2p. These agents caused a reversible, immediate, direct change in Hmg2p degradation consistent with an effect on Hmg2p structure. We devised an in vitro limited proteolysis assay of Hmg2p in its native membranes. In vitro, chemical chaperones caused a dramatic, rapid change in Hmg2p structure to a less accessible form. As in the living cell, the in vitro action of chemical chaperones was highly specific for Hmg2p and completely reversible. To evaluate the physiological relevance of this model behavior, we used the limited proteolysis assay to examine the effects of changing in vivo degradation signals on Hmg2p structure. We found that changes similar to those observed with chemical chaperones were brought about by alteration of natural degradation signal. Thus, Hmg2p can undergo significant, reversible structural changes that are relevant to the physiological control of Hmg2p ERAD. These findings support the idea that Hmg2p regulation is brought about by regulated alteration of folding state. Considering the ubiquitous nature of quality control pathways in biology, it may be that this strategy of regulation is widespread. 相似文献
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Machiko Sakoh-Nakatogawa Shuh-ichi Nishikawa Toshiya Endo 《The Journal of biological chemistry》2009,284(18):11815-11825
The endoplasmic reticulum (ER) has a strict protein quality control system.
Misfolded proteins generated in the ER are degraded by the ER-associated
degradation (ERAD). Yeast Mnl1p consists of an N-terminal mannosidase homology
domain and a less conserved C-terminal domain and facilitates the ERAD of
glycoproteins. We found that Mnl1p is an ER luminal protein with a cleavable
signal sequence and stably interacts with a protein-disulfide isomerase (PDI).
Analyses of a series of Mnl1p mutants revealed that interactions between the
C-terminal domain of Mnl1p and PDI, which include an intermolecular disulfide
bond, are essential for subsequent introduction of a disulfide bond into the
mannosidase homology domain of Mnl1p by PDI. This disulfide bond is essential
for the ERAD activity of Mnl1p and in turn stabilizes the prolonged
association of PDI with Mnl1p. Close interdependence between Mnl1p and PDI
suggests that these two proteins form a functional unit in the ERAD
pathway.The endoplasmic reticulum
(ER)2 is the first
organelle in the secretory pathway of eukaryotic cells and provides an optimum
environment for maturation of newly synthesized secretory and membrane
proteins. Protein folding/assembly in the ER is aided by molecular chaperones
and folding enzymes. Molecular chaperones in the ER assist folding of newly
synthesized proteins and prevent them from premature misfolding and/or
aggregate formation (1,
2). Protein folding in the ER
is often associated with formation of disulfide bonds, which contribute to
stabilization of native, functional states of proteins. Disulfide bond
formation could be a rate-limiting step of protein folding both in
vitro and in vivo
(3,
4), and the ER has a set of
folding enzymes including protein-disulfide isomerase (PDI) and its homologs
that catalyze disulfide bond formation
(5,
6).In parallel, protein folding/assembly in the ER relies on the inherent
failsafe mechanism, i.e. the ER quality control system, to ensure
that only correctly folded and/or assembled proteins can exit the ER.
Misfolded or aberrant proteins are retained in the ER for refolding by
ER-resident chaperones, whereas terminally misfolded proteins are degraded by
the mechanism known as ER-associated degradation (ERAD). The ERAD consists of
recognition and processing of aberrant substrate proteins, retrotranslocation
across the ER membrane, and subsequent proteasome-dependent degradation in the
cytosol. More than 20 different components have been identified to be involved
in this process in yeast and mammals
(7).The majority of proteins synthesized in the ER are glycoproteins, in which
N-linked glycans are not only important for folding but also crucial
for their ERAD if they fail in folding. Specifically, trimming of one or more
mannose residues of Man9GlcNAc2 oligosaccharide and
recognition of the modified mannose moiety represent a key step for selection
of terminally misfolded proteins for disposal
(8). A mannosidase I-like
protein, Mnl1p/Htm1p (yeast), and EDEM (mammals, ER degradation enhancing
α-mannosidase-like protein) were identified as candidates for lectins
that recognize ERAD substrates with modified mannose moieties
(9–11).
Both Mnl1p and EDEM contain an N-terminal mannosidase homology domain (MHD),
which lacks cysteine residues conserved among α1,2-mannosidase family
members and is proposed to function in recognition of mannose-trimmed
carbohydrate chains (supplemental Fig. S1). However, whether Mnl1p or EDEM
indeed functions as an ERAD-substrate-binding lectin or has a mannosidase
activity is still in debate
(11–15),
and Yos9p was suggested to take the role of ERAD-substrate binding lectin
(14,
16–18).
Mnl1p, but not EDEM, has a large C-terminal extension, which does not show any
homology to known functional domains and is conserved only among fungal Mnl1p
homologs (supplemental Fig. S1).After recognition of the modified mannose signal for degradation, aberrant
proteins are maintained or converted to be retrotranslocation competent by ER
chaperones including BiP (19).
PDI was also indicated to be involved in these steps in the ERAD by, for
example, its possible chaperone-like functions
(20–23).
The yeast PDI, Pdi1p, contains four thioredoxin-like domains, two of which
have a CGHC motif as active sites, followed by a C-terminal extension
containing the ER retention signal. During its catalytic cycle, PDI
transiently forms a mixed disulfide intermediate with its substrate through an
intermolecular disulfide bond between the cysteine residues of the active site
of PDI and the substrate molecule.Here we report identification of PDI as an Mnl1p-interacting protein.
Stable interactions between the C-terminal domain of Mnl1p and PDI involve
intermolecular disulfide bonds. Stably interacting PDI is required for
formation of the functionally essential intramolecular disulfide bond in the
MHD of Mnl1p, which in turn stabilizes and prolongs the Mnl1p-PDI
interactions. Possible roles for those stable interactions between Mnl1p and
PDI in the ERAD will be discussed. 相似文献
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