Traffic-independent function of the Sar1p/COPII machinery in proteasomal sorting of the cystic fibrosis transmembrane conductance regulator

ER exit in physiology and disease

The biosynthetic secretory pathway is comprised of multiple steps, modifications and interactions that form a highly precise pathway of protein trafficking and secretion, that is essential for eukaryotic life. The general outline of this pathway is understood, however the specific mechanisms are still unclear. In the last 15 years there have been vast advancements in technology that enable us to advance our understanding of this complex and subtle pathway. Therefore, based on the strong foundation of work performed over the last 40 years, we can now build another level of understanding, using the new technologies available. The biosynthetic secretory pathway is a high precision process, that involves a number of tightly regulated steps: Protein folding and quality control, cargo selection for Endoplasmic Reticulum (ER) exit, Golgi trafficking, sorting and secretion. When deregulated it causes severe diseases that here we categorise into three main groups of aberrant secretion: decreased, excess and altered secretion. Each of these categories disrupts organ homeostasis differently, effecting extracellular matrix composition, changing signalling events, or damaging the secretory cells due to aberrant intracellular accumulation of secretory proteins. Diseases of aberrant secretion are very common, but despite this, there are few effective therapies. Here we describe ER exit sites (ERES) as key hubs for regulation of the secretory pathway, protein quality control and an integratory hub for signalling within the cell. This review also describes the challenges that will be faced in developing effective therapies, due to the specificity required of potential drug candidates and the crucial need to respect the fine equilibrium of the pathway. The development of novel tools is moving forward, and we can also use these tools to build our understanding of the acute regulation of ERES and protein trafficking. Here we review ERES regulation in context as a therapeutic strategy.

COP I and II dependent trafficking controls ER-associated degradation in mammalian cells

Misfolded proteins and components of the endoplasmic reticulum (ER) quality control and ER associated degradation (ERAD) machineries concentrate in mammalian cells in the pericentriolar ER-derived quality control compartment (ERQC), suggesting it as a staging ground for ERAD. By tracking the chaperone calreticulin and an ERAD substrate, we have now determined that the trafficking to the ERQC is reversible and recycling back to the ER is slower than the movement in the ER periphery. The dynamics suggest vesicular trafficking rather than diffusion. Indeed, using dominant negative mutants of ARF1 and Sar1 or the drugs Brefeldin A and H89, we observed that COPI inhibition causes accumulation in the ERQC and increases ERAD, whereas COPII inhibition has the opposite effect. Our results suggest that targeting of misfolded proteins to ERAD involves COPII-dependent transport to the ERQC and that they can be retrieved to the peripheral ER in a COPI-dependent manner.

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.

Unraveling the Function of Lemur Tyrosine Kinase 2 Network

Lemur Tyrosine Kinase 2 (LMTK2) is a recently cloned transmembrane protein, actually a serine/threonine kinase named after the Madagascar primate lemur due to the long intracellular C-terminal tail. LMTK2 is relatively little known, compared to other kinases but its role has been increasingly recognized. Published data show that LMTK2 regulates key cellular events, including endocytic trafficking, nerve growth factor signaling, apoptosis, and Cl^- transport. Abnormalities in the expression and function of LMTK2 are associated with human disease, such as neurodegeneration, cancer and infertility. We summarized the current state of knowledge on LMTK2 structure, regulation, interactome, intracellular localization, and tissue expression and point out future research directions to better understand the role of LMTK2.

Sec13 is a positive regulator of VISA-mediated antiviral signaling

Viral infection triggers the innate antiviral immune response that rapidly produces type I interferons in most cell types to combat viruses invading. Upon viral infection, the cytoplasmic RNA sensors RIG-I/MDA5 recognize viral RNA, and then RIG-I/MDA5 is transported to mitochondria interacting with VISA through the CARD domain. From there, VISA recruits downstream antiviral signaling pathways molecules, such as TRAFs and TBK1. Eventually, IRF3 is phosphorylated and type I IFNs are induced to fight as the first line of defense against viruses. However, it remains unclear how VISA acts as a scaffold to assemble the signalosome in RIG-I-mediated antiviral signaling. Here, we demonstrated Sec13 as a novel component that was involved in VISA-mediated antiviral signaling pathway. The co-immunoprecipitation assays showed that Sec13 specifically interacts with VISA. Overexpression of Sec13 increases VISA's aggregation and ubiquitination and significantly enhances the phosphorylation and dimerization of IRF3, facilitating the IFN-β production. Conversely, the knockdown of Sec13 attenuates Sendai virus-induced and VISA-mediated IRF3 activation and the production of IFNβ, thus weakens antiviral immune activity.

N-Alpha-Acetyltransferases and Regulation of CFTR Expression

The majority of cystic fibrosis (CF)-causing mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) lead to the misfolding, mistrafficking, and degradation of the mutant protein. Inhibition of degradation does not effectively increase the amount of trafficking competent CFTR, but typically leads to increased ER retention of misfolded forms. Thus, the initial off pathway steps occur early in the processing of the protein. To identify proteins that interact with these early forms of CFTR, in vitro crosslink experiments identified cotranslational partners of the nascent chain of the severe misfolded mutant, G85E CFTR. The mutant preferentially interacts with a subunit of an N-alpha-acetyltransferase A. Based on recent reports that acetylation of the N-termini of some N-end rule substrates control their ubiquitination and subsequent degradation, a potential role for this modification in regulation of CFTR expression was assessed. Knockdown experiments identified two complexes, which affect G85E CFTR proteins levels, NatA and NatB. Effects of the knockdowns on mRNA levels, translation rates, and degradation rates established that the two complexes regulate G85E CFTR through two separate mechanisms. NatA acts indirectly by regulating transcription levels and NatB acts through a previously identified, but incompletely understood posttranslational mechanism. This regulation did not effect trafficking of G85E CFTR, which remains retained in the ER, nor did it alter the degradation rate of CFTR. A mutation predicted to inhibit N-terminal acetylation of CFTR, Q2P, was without effect, suggesting neither system acts directly on CFTR. These results contradict the prediction that N-terminal acetylation of CFTR determines its fitness as a proteasome substrate, but rather NatB plays a role in the conformational maturation of CFTR in the ER through actions on an unidentified protein.

Reticulons Regulate the ER Inheritance Block during ER Stress

Segregation of functional organelles during the cell cycle is crucial to generate healthy daughter cells. In Saccharomyces cerevisiae, ER stress causes an ER inheritance block to ensure cells inherit a functional ER. Here, we report that formation of tubular ER in the mother cell, the first step in ER inheritance, depends on functional symmetry between the cortical ER (cER) and perinuclear ER (pnER). ER stress induces functional asymmetry, blocking tubular ER formation and ER inheritance. Using fluorescence recovery after photobleaching, we show that the ER chaperone Kar2/BiP fused to GFP and an ER membrane reporter, Hmg1-GFP, behave differently in the cER and pnER. The functional asymmetry and tubular ER formation depend on Reticulons/Yop1, which maintain ER structure. LUNAPARK1 deletion in rtn1Δrtn2Δyop1Δ cells restores the pnER/cER functional asymmetry, tubular ER generation, and ER inheritance blocks. Thus, Reticulon/Yop1-dependent changes in ER structure are linked to ER inheritance during the yeast cell cycle.

Signal dependent ER export of lemur tyrosine kinase 2

BACKGROUND

The membrane anchored kinase, LMTK2, is a serine/threonine kinase predominantly localized to endosomal compartments. LMTK2 has been shown to be involved in the trafficking of the CFTR ion channel, the androgen receptor, as well as modulating neurodegeneration. As a membrane anchored protein, LMTK2 must be exported from the ER, yet the mechanisms whereby LMTK2 is sequestered within the ER for efficient export are unknown.

METHODS

Sequence analysis of the carboxyl tail of LMTK2 revealed a putative di-acidic ER export motif. Site-directed mutagenesis was utilized to ablate this potential motif. Subcellular fractionation, immunofluorescence microscopy, and transferrin recycling assays were used to determine the consequence of mutating LMTK2's export motif.

RESULTS

Mutation of the di-acidic export motif led to ER retention of LMTK2, and an increase in protein half-life and a concomitant loss of LMTK2 from its appropriate terminal destination. Loss of LMTK2 from endosomal compartments by preventing its release from the ER is linked to a reduction in transferrin recycling.

CONCLUSIONS

We have identified a di-acidic ER export motif within the carboxyl tail of the membrane anchored kinase LMTK2. This sequence is used by LMTK2 for its efficient export from the ER.

ERAD of proteins containing aberrant transmembrane domains requires ubiquitylation of cytoplasmic lysine residues

Clearance of misfolded proteins from the endoplasmic reticulum (ER) is mediated by the ubiquitin-proteasome system in a process known as ER-associated degradation (ERAD). The mechanisms through which proteins containing aberrant transmembrane domains are degraded by ERAD are poorly understood. To address this question, we generated model ERAD substrates based on CD8 with either a non-native transmembrane domain but a folded ER luminal domain (CD8^TMD*), or the native transmembrane domain but a misfolded luminal domain (CD8^LUM*). Although both chimeras were degraded by ERAD, we found that the location of the folding defect determined the initial site of ubiquitylation. Ubiquitylation of cytoplasmic lysine residues was required for the extraction of CD8^TMD* from the ER membrane during ERAD, whereas CD8^LUM* continued to be degraded in the absence of cytoplasmic lysine residues. Cytoplasmic lysine residues were also required for degradation of an additional ERAD substrate containing an unassembled transmembrane domain and when a non-native transmembrane domain was introduced into CD8^LUM*. Our results suggest that proteins with defective transmembrane domains are removed from the ER through a specific ERAD mechanism that depends upon ubiquitylation of cytoplasmic lysine residues.

Summary: Proteins containing defective transmembrane domains are removed from the endoplasmic reticulum through a specific mechanism that depends upon the ubiquitylation of cytoplasmic lysine residues.

The ER Stress Surveillance (ERSU) pathway regulates daughter cell ER protein aggregate inheritance

Stress induced by cytoplasmic protein aggregates can have deleterious consequences for the cell, contributing to neurodegeneration and other diseases. Protein aggregates are also formed within the endoplasmic reticulum (ER), although the fate of ER protein aggregates, specifically during cell division, is not well understood. By simultaneous visualization of both the ER itself and ER protein aggregates, we found that ER protein aggregates that induce ER stress are retained in the mother cell by activation of the ER Stress Surveillance (ERSU) pathway, which prevents inheritance of stressed ER. In contrast, under conditions of normal ER inheritance, ER protein aggregates can enter the daughter cell. Thus, whereas cytoplasmic protein aggregates are retained in the mother cell to protect the functional capacity of daughter cells, the fate of ER protein aggregates is determined by whether or not they activate the ERSU pathway to impede transmission of the cortical ER during the cell cycle.

A Role for Macro-ER-Phagy in ER Quality Control

The endoplasmic-reticulum quality-control (ERQC) system shuttles misfolded proteins for degradation by the proteasome through the well-defined ER-associated degradation (ERAD) pathway. In contrast, very little is known about the role of autophagy in ERQC. Macro-autophagy, a collection of pathways that deliver proteins through autophagosomes (APs) for degradation in the lysosome (vacuole in yeast), is mediated by autophagy-specific proteins, Atgs, and regulated by Ypt/Rab GTPases. Until recently, the term ER-phagy was used to describe degradation of ER membrane and proteins in the lysosome under stress: either ER stress induced by drugs or whole-cell stress induced by starvation. These two types of stresses induce micro-ER-phagy, which does not use autophagic organelles and machinery, and non-selective autophagy. Here, we characterize the macro-ER-phagy pathway and uncover its role in ERQC. This pathway delivers 20-50% of certain ER-resident membrane proteins to the vacuole and is further induced to >90% by overexpression of a single integral-membrane protein. Even though such overexpression in cells defective in macro-ER-phagy induces the unfolded-protein response (UPR), UPR is not needed for macro-ER-phagy. We show that macro-ER-phagy is dependent on Atgs and Ypt GTPases and its cargo passes through APs. Moreover, for the first time the role of Atg9, the only integral-membrane core Atg, is uncoupled from that of other core Atgs. Finally, three sequential steps of this pathway are delineated: Atg9-dependent exit from the ER en route to autophagy, Ypt1- and core Atgs-mediated pre-autophagsomal-structure organization, and Ypt51-mediated delivery of APs to the vacuole.

Extensive proteomic remodeling is induced by eukaryotic translation elongation factor 1Bγ deletion in Aspergillus fumigatus

The opportunistic pathogen Aspergillus fumigatus is ubiquitous in the environment and predominantly infects immunocompromised patients. The functions of many genes remain unknown despite sequencing of the fungal genome. A putative translation elongation factor 1Bγ (eEF1Bγ, termed elfA; 750 bp) is expressed, and exhibits glutathione S-transferase activity, in A. fumigatus. Here, we demonstrate the role of ElfA in the oxidative stress response, as well as a possible involvement in translation and actin cytoskeleton organization, respectively. Comparative proteomics, in addition to phenotypic analysis, under basal and oxidative stress conditions, demonstrated a role for A. fumigatus elfA in the oxidative stress response. An elfA-deficient strain (A. fumigatus ΔelfA) was significantly more sensitive to the oxidants H2O2, diamide, and 4,4'-dipyridyl disulfide (DPS) than the wild-type. This was further supported with the identification of differentially expressed proteins of the oxidative stress response, including; mitochondrial peroxiredoxin Prx1, molecular chaperone Hsp70 and mitochondrial glycerol-3-phosphate dehydrogenase. Phenotypic analysis also revealed that A. fumigatus ΔelfA was significantly more tolerant to voriconazole than the wild-type. The differential expression of two aminoacyl-tRNA synthetases suggests a role for A. fumigatus elfA in translation, while the identification of actin-bundling protein Sac6 and vacuolar dynamin-like GTPase VpsA link A. fumigatus elfA to the actin cytoskeleton. Overall, this work highlights the diverse roles of A. fumigatus elfA, with respect to translation, oxidative stress and actin cytoskeleton organization. In addition to this, the strategy of combining targeted gene deletion with comparative proteomics for elucidating the role of proteins of unknown function is further revealed.

The C-terminal amino acid of the MHC-I heavy chain is critical for binding to Derlin-1 in human cytomegalovirus US11-induced MHC-I degradation

Derlin-1 plays a critical role in endoplasmic reticulum-associated protein degradation (ERAD) of a particular subset of proteins. Although it is generally accepted that Derlin-1 mediates the export of ERAD substrates from the ER to the cytosol, little is known about how Derlin-1 interacts with these substrates. Human cytomegalovirus (HCMV) US11 exploits Derlin-1-dependent ERAD to degrade major histocompatibility complex class I (MHC-I) molecules and evade immune surveillance. US11 requires the cytosolic tail of the MHC-I heavy chain to divert MHC-I molecules into the ERAD pathway for degradation; however, the underlying mechanisms remain unknown. Here, we show that the cytosolic tail of the MHC-I heavy chain, although not required for interaction with US11, is required for tight binding to Derlin-1 and thus for US11-induced dislocation of the MHC-I heavy chain to the cytosol for proteasomal degradation. Surprisingly, deletion of a single C-terminal amino acid from the cytosolic tail disrupted the interaction between MHC-I molecules and Derlin-1, rendering mutant MHC-I molecules resistant to US11-induced degradation. Consistently, deleting the C-terminal cytosolic region of Derlin-1 prevented it from binding to MHC-I molecules. Taken together, these results suggest that the cytosolic region of Derlin-1 is involved in ERAD substrate binding and that this interaction is critical for the Derlin-1-mediated dislocation of the MHC-I heavy chain to the cytosol during US11-induced MHC-I degradation.

COPII machinery cooperates with ER-localized Hsp40 to sequester misfolded membrane proteins into ER-associated compartments

Proteins that fail to fold in the endoplasmic reticulum (ER) are subjected to ER-associated degradation (ERAD). Certain transmembrane ERAD substrates are segregated into specialized ER subdomains, termed ER-associated compartments (ERACs), before targeting to ubiquitin-proteasome degradation. The traffic-independent function of several proteins involved in COPII-mediated ER-to-Golgi transport have been implicated in the segregation of exogenously expressed human cystic fibrosis transmembrane conductance regulator (CFTR) into ERACs in Saccharomyces cerevisiae. Here we focus on the properties of COPII components in the sequestration of enhanced green fluorescent protein (EGFP)-CFTR into ERACs. It has been demonstrated that the temperature-sensitive growth defects in many COPII mutants can be suppressed by overexpressing other genes involved in COPII vesicle formation. However, we show that these suppression abilities are not always correlated with the ability to rescue the ERAC formation defect, suggesting that COPII-mediated EGFP-CFTR entry into ERACs is independent of its ER-to-Golgi trafficking function. In addition to COPII machinery, we find that ER-associated Hsp40s are also involved in the sequestration process by directly interacting with EGFP-CFTR. COPII components and ER-associated Hsp40, Hlj1p, act in the same pathway to sequester EGFP-CFTR into ERACs. Our findings point to an as-yet-undefined role of COPII proteins in the formation of ERACs.

Eeyarestatin 1 interferes with both retrograde and anterograde intracellular trafficking pathways

BACKGROUND

The small molecule Eeyarestatin I (ESI) inhibits the endoplasmic reticulum (ER)-cytosol dislocation and subsequent degradation of ERAD (ER associated protein degradation) substrates. Toxins such as ricin and Shiga/Shiga-like toxins (SLTx) are endocytosed and trafficked to the ER. Their catalytic subunits are thought to utilise ERAD-like mechanisms to dislocate from the ER into the cytosol, where a proportion uncouples from the ERAD process, recovers a catalytic conformation and destroys their cellular targets. We therefore investigated ESI as a potential inhibitor of toxin dislocation.

METHODOLOGY AND PRINCIPAL FINDINGS

Using cytotoxicity measurements, we found no role for ES(I) as an inhibitor of toxin dislocation from the ER, but instead found that for SLTx, ESI treatment of cells was protective by reducing the rate of toxin delivery to the ER. Microscopy of the trafficking of labelled SLTx and its B chain (lacking the toxic A chain) showed a delay in its accumulation at a peri-nuclear location, confirmed to be the Golgi by examination of SLTx B chain metabolically labelled in the trans-Golgi cisternae. The drug also reduced the rate of endosomal trafficking of diphtheria toxin, which enters the cytosol from acidified endosomes, and delayed the Golgi-specific glycan modifications and eventual plasma membrane appearance of tsO45 VSV-G protein, a classical marker for anterograde trafficking.

CONCLUSIONS AND SIGNIFICANCE

ESI acts on one or more components that function during vesicular transport, whilst at least one retrograde trafficking pathway, that of ricin, remains unperturbed.

Saccharomyces cerivisiae as a model system for kidney disease: what can yeast tell us about renal function?

Ion channels, solute transporters, aquaporins, and factors required for signal transduction are vital for kidney function. Because mutations in these proteins or in associated regulatory factors can lead to disease, an investigation into their biogenesis, activities, and interplay with other proteins is essential. To this end, the yeast, Saccharomyces cerevisiae, represents a powerful experimental system. Proteins expressed in yeast include the following: 1) ion channels, including the epithelial sodium channel, members of the inward rectifying potassium channel family, and cystic fibrosis transmembrane conductance regulator; 2) plasma membrane transporters, such as the Na(+)-K(+)-ATPase, the Na(+)-phosphate cotransporter, and the Na(+)-H(+) ATPase; 3) aquaporins 1-4; and 4) proteins such as serum/glucocorticoid-induced kinase 1, phosphoinositide-dependent kinase 1, Rh glycoprotein kidney, and trehalase. The variety of proteins expressed and studied emphasizes the versatility of yeast, and, because of the many available tools in this organism, results can be obtained rapidly and economically. In most cases, data gathered using yeast have been substantiated in higher cell types. These attributes validate yeast as a model system to explore renal physiology and suggest that research initiated using this system may lead to novel therapeutics.

Cargo loading at the ER

Trafficking of newly synthesized cargo through the early secretory pathway defines and maintains the intracellular organization of eukaryotic cells as well as the organization of tissues and organs. The importance of this pathway is underlined by the increasing number of mutations in key components of the ER export machinery that are causative of a diversity of human diseases. Here we discuss the molecular mechanisms that dictate cargo selection during vesicle budding. While, in vitro reconstitution assays, unicellular organisms such as budding yeast, and mammalian cell culture still have much to offer in terms of gaining a full understanding of the molecular basis for secretory cargo export, such assays have to date been limited to analysis of smaller, freely diffusible cargoes. The export of large macromolecular complexes from the ER such as collagens (up to 300 nm) or lipoproteins (~500 nm) presents a clear problem in terms of maintaining both selectivity and efficiency of export. It has also become clear that in order to translate our knowledge of the molecular basis for ER export to a full understanding of the implications for normal development and disease progression, the use of metazoan models is essential. Combined, these approaches are now starting to shed light not only on the mechanisms of macromolecular cargo export from the ER but also reveal the implications of failure of this process to human development and disease.

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.

γ-protocadherins are enriched and transported in specialized vesicles associated with the secretory pathway in neurons

Gamma protocadherins (Pcdh-γs) resemble classical cadherins and have the potential to engage in cell-cell interactions with homophilic properties. Emerging evidence suggests non-conventional roles for some protocadherins in neural development. We sought to determine whether Pcdh-γ trafficking in neurons is consistent with an intracellular role for these molecules. Here we show that, in contrast to the largely surface localization of classical cadherins, endogenous Pcdh-γs are primarily intracellular in rat neurons in vivo and are equally distributed within organelles of subsynaptic dendritic and axonal compartments. A strikingly higher proportion of Pcdh-γ-containing organelles in synaptic compartments was observed at postnatal day 16. To determine the origin of Pcdh-γ-trafficking organelles, we isolated organelles with Pcdh-γ antibody-coupled magnetic beads from brain organelle suspensions. Vesicles with high levels of COPII and endoplasmic reticulum-Golgi intermediate compartment (ERGIC) components were isolated with the Pcdh-γ antibody but not with the classical cadherin antibody. In cultured hippocampal neurons, Pcdh-γ immunolabeling partially overlapped with calnexin- and COPII-positive puncta in dendrites. Mobile Pcdh-γ-GFP profiles dynamically codistributed with a DsRed construct coupled to ER retention signals by live imaging. Pcdh-γ expression correlated with accumulations of tubulovesicular and ER-like organelles in dendrites. Our results are consistent with the possibility that Pcdh-γs could have a unique function within the secretory pathway in addition to their documented surface roles.

The endoplasmic reticulum-associated degradation of the epithelial sodium channel requires a unique complement of molecular chaperones

The epithelial sodium channel (ENaC) is composed of a single copy of an alpha-, beta-, and gamma-subunit and plays an essential role in water and salt balance. Because ENaC assembles inefficiently after its insertion into the ER, a substantial percentage of each subunit is targeted for ER-associated degradation (ERAD). To define how the ENaC subunits are selected for degradation, we developed novel yeast expression systems for each ENaC subunit. Data from this analysis suggested that ENaC subunits display folding defects in more than one compartment and that subunit turnover might require a unique group of factors. Consistent with this hypothesis, yeast lacking the lumenal Hsp40s, Jem1 and Scj1, exhibited defects in ENaC degradation, whereas BiP function was dispensable. We also discovered that Jem1 and Scj1 assist in ENaC ubiquitination, and overexpression of ERdj3 and ERdj4, two lumenal mammalian Hsp40s, increased the proteasome-mediated degradation of ENaC in vertebrate cells. Our data indicate that Hsp40s can act independently of Hsp70 to select substrates for ERAD.

ER-associated complexes (ERACs) containing aggregated cystic fibrosis transmembrane conductance regulator (CFTR) are degraded by autophagy

The ubiquitin-proteasome pathway and autophagy are the two major mechanisms responsible for the clearance of cellular proteins. We have used the yeast Saccharomyces cerevisiae as a model system and the cystic fibrosis transmembrane conductance regulator (CFTR) as a model substrate to study the interactive function of these two pathways in the degradation of misfolded proteins. EGFP-tagged human CFTR was introduced into yeast and expressed under a copper-inducible promoter. The localization and degradation of EGFP-CFTR in live cells were monitored by time-lapse imaging following its de novo synthesis. EGFP-CFTR first appears within the perinuclear and sub-cortical ER and is mobile within the plane of the membrane as assessed by fluorescence recovery after photobleaching (FRAP). This pool of EGFP-CFTR is subsequently degraded through a proteasome-dependent pathway that is inhibited in the pre1-1 yeast strain defective in proteasomal degradation. Prolonged expression of EGFP-CFTR leads to the sequestration of EGFP-CFTR molecules into ER structures called ER-associated complexes (ERACs). The sequestration of EGFP-CFTR into ERACs appears to be driven by aggregation since EGFP-CFTR molecules present within ERACs are immobile as measured by FRAP. Individual ERACs are cleared from cells through the autophagic pathway that is blocked in the atg6Delta and atg1Delta yeast strains defective in autophagy. Our results suggest that the proteasomal and the autophagic pathways function together to clear misfolded proteins from the ER.

Bap31 is an itinerant protein that moves between the peripheral endoplasmic reticulum (ER) and a juxtanuclear compartment related to ER-associated Degradation

Certain endoplasmic reticulum (ER)-associated degradation (ERAD) substrates with transmembrane domains are segregated from other ER proteins and sorted into a juxtanuclear subcompartment, known as the ER quality control compartment. Bap31 is an ER protein with three transmembrane domains, and it is assumed to be a cargo receptor for ER export of some transmembrane proteins, especially those prone to ERAD. Here, we show that Bap31 is a component of the ER quality control compartment and that it moves between the peripheral ER and a juxtanuclear ER or ER-related compartment distinct from the conventional ER-Golgi intermediate compartment. The third and second transmembrane domains of Bap31 are principally responsible for the movement to and recycling from the juxtanuclear region, respectively. This cycling was blocked by depolymerization of microtubules and disruption of dynein-dynactin function. Overexpression of Sar1p and Arf1 mutants affected Bap31 cycling, suggesting that this cycling pathway is related to the conventional vesicular transport pathways.

Endoplasmic reticulum-associated degradation of mutant CFTR requires a guanine nucleotide-sensitive step

Proteasome degradation of endoplasmic reticulum (ER)-misfolded proteins requires retrograde transport from ER to the cytosol. To date, it is not clear whether this event constitutes the exclusive ER degradation process for non-native membrane proteins. Here we describe the role of GTP in the degradation of DeltaF508-CFTR and the alpha subunit of the T-cell receptor (TCRalpha), representative misfolded ER membrane proteins. Selective intracellular GTP depletion extended the DeltaF508-CFTR half-life sixfold, whereas ATP depletion accelerated its turnover and inhibited only 80% of the proteasome activity that was not affected by GTP depletion. AlF(4)(-), a well-known inhibitor of heterotrimeric G proteins, but not of AlF(3), delayed the mutant CFTR turnover in vivo, in semi-intact cells and in ER-enriched microsomes, without affecting ER to Golgi cargo transport. DeltaF508-CFTR degradation was also inhibited by alkaline stripping of ER-associated membrane proteins. We propose that at the ER, GTP may participate in the disposal of misfolded membrane proteins through activation of heterotrimeric G proteins.

PERK-dependent compartmentalization of ERAD and unfolded protein response machineries during ER stress

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) activates the ER membrane kinases PERK and IRE1 leading to the unfolded protein response (UPR). We show here that UPR activation triggers PERK and IRE1 segregation from BiP and their sorting with misfolded proteins to the ER-derived quality control compartment (ERQC), a pericentriolar compartment that we had identified previously. PERK phosphorylates translation factor eIF2alpha, which then accumulates on the cytosolic side of the ERQC. Dominant negative PERK or eIF2alpha(S51A) mutants prevent the compartmentalization, whereas eIF2alpha(S51D) mutant, which mimics constitutive phosphorylation, promotes it. This suggests a feedback loop where eIF2alpha phosphorylation causes pericentriolar concentration at the ERQC, which in turn amplifies the UPR. ER-associated degradation (ERAD) is an UPR-dependent process; we also find that ERAD components (Sec61beta, HRD1, p97/VCP, ubiquitin) are recruited to the ERQC, making it a likely site for retrotranslocation. In addition, we show that autophagy, suggested to play a role in elimination of aggregated proteins, is unrelated to protein accumulation in the ERQC.

EDEM1 reveals a quality control vesicular transport pathway out of the endoplasmic reticulum not involving the COPII exit sites

Immature and nonnative proteins are retained in the endoplasmic reticulum (ER) by the quality control machinery. Folding-incompetent glycoproteins are eventually targeted for ER-associated protein degradation (ERAD). EDEM1 (ER degradation-enhancing alpha-mannosidase-like protein 1), a putative mannose-binding protein, targets misfolded glycoproteins for ERAD. We report that endogenous EDEM1 exists mainly as a soluble glycoprotein. By high-resolution immunolabeling and serial section analysis, we find that endogenous EDEM1 is sequestered in buds that form along cisternae of the rough ER at regions outside of the transitional ER. They give rise to approximately 150-nm vesicles scattered throughout the cytoplasm that are lacking a recognizable COPII coat. About 87% of the immunogold labeling was over the vesicles and approximately 11% over the ER lumen. Some of the EDEM1 vesicles also contain Derlin-2 and the misfolded Hong Kong variant of alpha-1-antitrypsin, a substrate for EDEM1 and ERAD. Our results demonstrate the existence of a vesicle budding transport pathway out of the rough ER that does not involve the canonical transitional ER exit sites and therefore represents a previously unrecognized passageway to remove potentially harmful misfolded luminal glycoproteins from the ER.

Mechanisms of COPII vesicle formation and protein sorting

The evolutionarily conserved coat protein complex II (COPII) generates transport vesicles that mediate protein transport from the endoplasmic reticulum (ER). COPII coat is responsible for direct capture of cargo proteins and for the physical deformation of the ER membrane that drives the COPII vesicle formation. In addition to coat proteins, recent data have indicated that the Ras-like small GTPase Sar1 plays multiple roles, such as COPII coat recruitment, cargo sorting, and completion of the final fission. In the present review, we summarize current knowledge of COPII-mediated vesicle formation from the ER, as well as highlighting non-canonical roles of COPII components.

Cellular response to unfolded proteins in the endoplasmic reticulum of plants

Secretory and transmembrane proteins are synthesized in the endoplasmic reticulum (ER) in eukaryotic cells. Nascent polypeptide chains, which are translated on the rough ER, are translocated to the ER lumen and folded into their native conformation. When protein folding is inhibited because of mutations or unbalanced ratios of subunits of hetero-oligomeric proteins, unfolded or misfolded proteins accumulate in the ER in an event called ER stress. As ER stress often disturbs normal cellular functions, signal-transduction pathways are activated in an attempt to maintain the homeostasis of the ER. These pathways are collectively referred to as the unfolded protein response (UPR). There have been great advances in our understanding of the molecular mechanisms underlying the UPR in yeast and mammals over the past two decades. In plants, a UPR analogous to those in yeast and mammals has been recognized and has recently attracted considerable attention. This review will summarize recent advances in the plant UPR and highlight the remaining questions that have yet to be addressed.

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

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

Destabilization of the VCP-Ufd1-Npl4 complex is associated with decreased levels of ERAD substrates

p97/VCP associated with Ufd1-Npl4 is considered a key player in ER-associated degradation (ERAD). RNA interference (RNAi) of one component of the Ufd1-Npl4 heterodimer destabilizes the VCP-Ufd1-Npl4 complex inducing proteasome-dependent degradation of the other component and releasing free VCP. In contrast to RNAi of VCP, RNAi of Ufd1 or Npl4 depleting approximately 90% of the VCP-Ufd1-Npl4 complexes does not induce unfolded protein response, indicating that the Ufd1-Npl4 dimer is not involved in the regulation of ER function by VCP. RNAi of Ufd1 or Npl4 is associated with a 2-fold increase in the levels of polyubiquitinated proteins, which form dispersed aggregates often associated with calnexin-positive structures. However, contrary to the effects of proteasome inhibition, RNAi of Ufd1 or Npl4 does not induce an accumulation of alpha-TCR and delta-CD3, two ERAD substrates overexpressed in HeLa cells. Instead, a 60-70% decrease in their levels is observed. The decrease in alpha-TCR levels is associated with a 50% decrease of its half-life. Upregulation of the putative channel forming protein, derlin-1, may contribute to the increased degradation of ERAD substrates. To explain our findings, we propose a model, where association of emerging ERAD substrates with VCP-Ufd1-Npl4 is not required for their degradation but has a regulatory role.

Structure of the mouse peptide N-glycanase-HR23 complex suggests co-evolution of the endoplasmic reticulum-associated degradation and DNA repair pathways

Peptide N-glycanase removes N-linked oligosaccharides from misfolded glycoproteins as part of the endoplasmic reticulum-associated degradation pathway. This process involves the formation of a tight complex of peptide N-glycanase with Rad23 in yeast and the orthologous HR23 proteins in mammals. In addition to its function in endoplasmic reticulum-associated degradation, HR23 is also involved in DNA repair, where it plays an important role in damage recognition in complex with the xeroderma pigmentosum group C protein. To characterize the dual role of HR23, we have determined the high resolution crystal structure of the mouse peptide N-glycanase catalytic core in complex with the xeroderma pigmentosum group C binding domain from HR23B. Peptide N-glycanase features a large cleft between its catalytic cysteine protease core and zinc binding domain. Opposite the zinc binding domain is the HR23B-interacting region, and surprisingly, the complex interface is fundamentally different from the orthologous yeast peptide N-glycanase-Rad23 complex. Different regions on both proteins are involved in complex formation, revealing an amazing degree of divergence in the interaction between two highly homologous proteins. Furthermore, the mouse peptide N-glycanase-HR23B complex mimics the interaction between xeroderma pigmentosum group C and HR23B, thereby providing a first structural model of how the two proteins interact within the nucleotide excision repair cascade in higher eukaryotes. The different interaction interfaces of the xeroderma pigmentosum group C binding domains in yeast and mammals suggest a co-evolution of the endoplasmic reticulum-associated degradation and DNA repair pathways.

Establishment and characterization of a novel polarized MDCK epithelial cellular model for CFTR studies

F508del is the most common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that is responsible for the genetic disease Cystic Fibrosis (CF). It results in a major failure of CFTR to traffic to the apical membrane of epithelial cells, where it should function as a chloride (Cl-) channel. Most studies on localization, processing and cellular trafficking of wild-type (wt) and F508del-CFTR have been performed in non-epithelial cells. Notwithstanding, polarized epithelial cells possess distinctly organized and regulated membrane trafficking pathways. We have used Madin-Darby canine kidney (MDCK) type II cells (proximal tubular cells which do not express endogenous CFTR) to generate novel epithelial, polarized cellular models stably expressing wt- or F508del-CFTR through transduction with recombinant lentiviral vectors. Characterization of these cell lines shows that wt-CFTR is correctly processed and apically localized, producing a cAMP-activated Cl- conductance. In contrast, F508del-CFTR is mostly detected in itsimmature form, localized intracellularly and producing only residual Cl- conductance. These novel cell lines constitute bona fide models and significantly improved resources to investigate the molecular mechanisms of polarized membrane traffic of wt- and F508del-CFTR in the same cellular background. They are also useful to identify/validate novel therapeutic compounds for CF.

Endoplasmic reticulum-associated degradation

Secretory and transmembrane proteins enter the secretory pathway through the protein-conducting Sec61 channel in the membrane of the endoplasmic reticulum. In the endoplasmic reticulum, proteins fold, are frequently covalently modified, and oligomerize before they are packaged into transport vesicles that shuttle them to the Golgi complex. Proteins that misfold in the endoplasmic reticulum are selectively transported back across the endoplasmic reticulum membrane to the cytosol for degradation by proteasomes. Depending on the topology of the defect in the protein, cytosolic or lumenal chaperones are involved in its targeting to degradation. The export channel for misfolded proteins is likely also formed by Sec61p. Export may be powered by AAA-ATPases of the proteasome 19S regulatory particle or Cdc48p/p97. Exported proteins are frequently ubiquitylated prior to degradation and are escorted to the proteasome by polyubiquitin-binding proteins.

Characterization of an ERAD gene as VPS30/ATG6 reveals two alternative and functionally distinct protein quality control pathways: one for soluble Z variant of human alpha-1 proteinase inhibitor (A1PiZ) and another for aggregates of A1PiZ

The Z variant of human alpha-1 proteinase inhibitor (A1PiZ) is a substrate for endoplasmic reticulum-associated protein degradation (ERAD). To identify genes required for the degradation of this protein, A1PiZ degradation-deficient (add) yeast mutants were isolated. The defect in one of these mutants, add3, was complemented by VPS30/ATG6, a gene that encodes a component of two phosphatidylinositol 3-kinase (PtdIns 3-kinase) complexes: complex I is required for autophagy, whereas complex II is required for the carboxypeptidase Y (CPY)-to-vacuole pathway. We found that upon overexpression of A1PiZ, both PtdIns 3-kinase complexes were required for delivery of the excess A1PiZ to the vacuole. When the CPY-to-vacuole pathway was compromised, A1PiZ was secreted; however, disruption of autophagy led to an increase in aggregated A1PiZ rather than secretion. These results suggest that excess soluble A1PiZ transits the secretion pathway to the trans-Golgi network and is selectively targeted to the vacuole via the CPY-to-vacuole sorting pathway, but excess A1PiZ that forms aggregates in the endoplasmic reticulum is targeted to the vacuole via autophagy. These findings illustrate the complex nature of protein quality control in the secretion pathway and reveal multiple sites that recognize and sort both soluble and aggregated forms of aberrant or misfolded proteins.

Golgi inheritance in mammalian cells is mediated through endoplasmic reticulum export activities

Golgi inheritance during mammalian cell division occurs through the disassembly, partitioning, and reassembly of Golgi membranes. The mechanisms responsible for these processes are poorly understood. To address these mechanisms, we have examined the identity and dynamics of Golgi proteins within mitotic membranes using live cell imaging and electron microscopy techniques. Mitotic Golgi fragments, seen in prometaphase and telophase, were found to localize adjacent to endoplasmic reticulum (ER) export domains, and resident Golgi transmembrane proteins cycled rapidly into and out of these fragments. Golgi proteins within mitotic Golgi haze-seen during metaphase-were found to redistribute with ER markers into fragments when the ER was fragmented by ionomycin treatment. The temperature-sensitive misfolding mutant ts045VSVG protein, when localized to the Golgi at the start of mitosis, became trapped in the ER at the end of mitosis in cells shifted to 40 degrees C. Finally, reporters for Arf1 and Sar1 activity revealed that Arf1 and Sar1 undergo sequential inactivation during mitotic Golgi breakdown and sequential reactivation upon Golgi reassembly at the end of mitosis. Together, these findings support a model of mitotic Golgi inheritance that involves inhibition and subsequent reactivation of cellular activities controlling the cycling of Golgi components into and out of the ER.

Search and destroy: ER quality control and ER-associated protein degradation

Proteins synthesized in the endoplasmic reticulum (ER) encounter quality control checkpoints that verify their fitness to proceed in the secretory pathway. Molecules undergoing folding and assembly are kept out of the exocytic pathway until maturation is complete. Misfolded side products that inevitably form are removed from the mixture of conformers and returned to the cytosol for degradation. How unfolded proteins are recognized and how irreversibly misfolded proteins are sorted to ER-associated degradation pathways was poorly understood. Recent developments from a combination of genetic and biochemical analyses has revealed new insights into these mechanisms. The emerging view shows distinct pathways working in collaboration to filter the diverse range of unfolded proteins from the transport flow and to divert misfolded molecules for destruction.

The end of "naive reductionism": rise of systems biology or renaissance of physiology?

Systems biology is an emerging discipline focused on tackling the enormous intellectual and technical challenges associated with translating genome sequence into a comprehensive understanding of how organisms are built and run. Physiology and systems biology share the goal of understanding the integrated function of complex, multicomponent biological systems ranging from interacting proteins that carry out specific tasks to whole organisms. Despite this common ground, physiology as an academic discipline runs the real risk of fading into the background and being superseded organizationally and administratively by systems biology. My goal in this article is to discuss briefly the cornerstones of modern systems biology, specifically functional genomics, nonmammalian model organisms and computational biology, and to emphasize the need to embrace them as essential components of 21st-century physiology departments and research and teaching programs.

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

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

Mechanisms of lipid malabsorption in Cystic Fibrosis: the impact of essential fatty acids deficiency

Transport mechanisms, whereby alimentary lipids are digested and packaged into small emulsion particles that enter intestinal cells to be translocated to the plasma in the form of chylomicrons, are impaired in cystic fibrosis. The purpose of this paper is to focus on defects that are related to intraluminal and intracellular events in this life-limiting genetic disorder. Specific evidence is presented to highlight the relationship between fat malabsorption and essential fatty acid deficiency commonly found in patients with cystic fibrosis that are often related to the genotype. Given the interdependency of pulmonary disease, pancreatic insufficiency and nutritional status, greater attention should be paid to the optimal correction of fat malabsorption and essential fatty acid deficiency in order to improve the quality of life and extend the life span of patients with cystic fibrosis.

Protein folding and quality control in the endoplasmic reticulum

The endoplasmic reticulum (ER) is a highly versatile protein factory that is equipped with chaperones and folding enzymes essential for protein folding. ER quality control guided by these chaperones is essential for life. Whereas correctly folded proteins are exported from the ER, misfolded proteins are retained and selectively degraded. At least two main chaperone classes, BiP and calnexin/calreticulin, are active in ER quality control. Folding factors usually are found in complexes. Recent work emphasises more than ever that chaperones act in concert with co-factors and with each other.

COPII-dependent export of cystic fibrosis transmembrane conductance regulator from the ER uses a di-acidic exit code

Cystic fibrosis (CF) is a childhood hereditary disease in which the most common mutant form of the CF transmembrane conductance regulator (CFTR) ΔF508 fails to exit the endoplasmic reticulum (ER). Export of wild-type CFTR from the ER requires the coat complex II (COPII) machinery, as it is sensitive to Sar1 mutants that disrupt normal coat assembly and disassembly. In contrast, COPII is not used to deliver CFTR to ER-associated degradation. We find that exit of wild-type CFTR from the ER is blocked by mutation of a consensus di-acidic ER exit motif present in the first nucleotide binding domain. Mutation of the code disrupts interaction with the COPII coat selection complex Sec23/Sec24. We propose that the di-acidic exit code plays a key role in linking CFTR to the COPII coat machinery and is the primary defect responsible for CF in ΔF508-expressing patients.

Macromolecular Interactions and Ion Transport in Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disease caused by autosomal recessive mutations of the CF transmembrane regulator, CFTR. CFTR functions in the plasma membrane of epithelial cells lining the lung, pancreas, liver, intestines, sweat duct, and the epididymis. The primary problem in CF is that mutations in CFTR affect its ability to be made, processed, and trafficked to the plasma membrane and/or its function as a Cl(-) channel and conductance regulator. Many proteins and processes normally interact with normal CFTR throughout its life cycle and mutant CFTR during the disease process. Understanding the function of these proteins and processes is expected to provide a clearer understanding of how normal CFTR is involved in salt movement and how mutant CFTR is handled by the cell and leads to the pathophysiology of CF. Recently, efforts to find therapies that correct defective CFTR have been intensifying. To facilitate our understanding of normal and mutant CFTR and the identification of new drug targets for developing novel therapies, a panel of experts was convened by the National Heart, Lung, and Blood Institute to explore the critical questions, challenges, and current opportunities to highlight new areas of research that would facilitate a integrated understanding of the processes and proteins that impact CFTR. The meeting highlighted the multiple pathways and interacting proteins involved in CFTR folding and biosynthesis, processing, and trafficking. A number of critical areas for future study were identified. Although these therapies are promising, a big question remains as to whether simply correcting defective CFTR will lead to significant improvement in patient health or whether the symptoms manifested in CF will require therapies in addition to those that target defective CFTR specifically.

Misfolded proteins are sorted by a sequential checkpoint mechanism of ER quality control

Misfolded proteins retained in the endoplasmic reticulum (ER) are degraded by the ER-associated degradation pathway. The mechanisms used to sort them from correctly folded proteins remain unclear. Analysis of substrates with defined folded and misfolded domains has revealed a system of sequential checkpoints that recognize topologically distinct domains of polypeptides. The first checkpoint examines the cytoplasmic domains of membrane proteins. If a lesion is detected, it is retained statically in the ER and rapidly degraded without regard to the state of its other domains. Proteins passing this test face a second checkpoint that monitors domains localized in the ER lumen. Proteins detected by this pathway are sorted from folded proteins and degraded by a quality control mechanism that requires ER-to-Golgi transport. Although the first checkpoint is obligatorily directed at membrane proteins, the second monitors both soluble and membrane proteins. Our data support a model whereby “properly folded” proteins are defined biologically as survivors that endure a series of distinct checkpoints.

Distinct roles for the Hsp40 and Hsp90 molecular chaperones during cystic fibrosis transmembrane conductance regulator degradation in yeast

Aberrant secreted proteins can be destroyed by ER-associated protein degradation (ERAD), and a prominent, medically relevant ERAD substrate is the cystic fibrosis transmembrane conductance regulator (CFTR). To better define the chaperone requirements during CFTR maturation, the protein was expressed in yeast. Because Hsp70 function impacts CFTR biogenesis in yeast and mammals, we first sought ER-associated Hsp40 cochaperones involved in CFTR maturation. Ydj1p and Hlj1p enhanced Hsp70 ATP hydrolysis but CFTR degradation was slowed only in yeast mutated for both YDJ1 and HLJ1, suggesting functional redundancy. In contrast, CFTR degradation was accelerated in an Hsp90 mutant strain, suggesting that Hsp90 preserves CFTR in a folded state, and consistent with this hypothesis, Hsp90 maintained the solubility of an aggregation-prone domain (NBD1) in CFTR. Soluble ERAD substrate degradation was unaffected in the Hsp90 or the Ydj1p/Hlj1p mutants, and surprisingly CFTR degradation was unaffected in yeast mutated for Hsp90 cochaperones. These results indicate that Hsp90, but not the Hsp90 complex, maintains CFTR structural integrity, whereas Ydj1p/Hlj1p catalyze CFTR degradation.

A striking quality control subcompartment in Saccharomyces cerevisiae: the endoplasmic reticulum-associated compartment

The folding of nascent secretory and membrane proteins is monitored by the endoplasmic reticulum (ER) quality control system. Misfolded proteins are retained in the ER and can be removed by ER-associated degradation. As a model for the ER quality control of multispanning membrane proteins in yeast, we have been studying mutant forms of Ste6p. Here, we identify mislocalized mutant forms of Ste6p that induce the formation of, and localize to, prominent structures that are absent in normal cells. We have named these structures ER-associated compartments (ERACs), based on their juxtaposition to and connection with the ER, as observed by fluorescence and electron microscopy. ERACs comprise a network of tubulo-vesicular structures that seem to represent proliferated ER membranes. Resident ER lumenal and membrane proteins are present in ERACs in addition to their normal ER localization, suggesting there is no barrier for their entry into ERACs. However, the forms of Ste6p in ERACs are excluded from the ER and do not enter the secretory pathway; instead, they are ultimately targeted for ER-associated degradation. The presence of ERACs does not adversely affect secretory protein traffic through the ER and does not lead to induction of the unfolded protein response. We propose that ERACs may be holding sites to which misfolded membrane proteins are specifically diverted so as not to interfere with normal cellular functions. We discuss the likelihood that related ER membrane proliferations that form in response to certain other mutant or unassembled membrane proteins may be substantially similar to ERACs.