N-glycan processing in ER quality control

Unglycosylation at Asn-633 made extracellular domain of E-cadherin folded incorrectly and arrested in endoplasmic reticulum, then sequentially degraded by ERAD

The human E-cadherin is a single transmembrane domain protein involved in Ca(2+)-dependent cell-cell adhesion. In a previous study, we demonstrated that all of four potential N-glycosylation sites in E-cadherin are occupied by N-glycans in human breast carcinoma cells in vivo and the elimination of N-glycan at Asn-633 dramatically affected E-cadherin expression and made it degraded. In this study we investigated the molecular mechanism of E-cadherin, which lacks N-glycosylation at Asn-633 (M4), degradation and the role of the N-glycan at Asn-633 in E-cadherin folding. We treated cells stably expressed M4 E-cadherin with MG123, DMM, respectively. Either MG132 or DMM could efficiently block degradation of M4 E-cadherin. M4 E-cadherin was recognized as the substrate of ERAD and was retro-translocated from ER lumen to cytoplasm by p97. It was observed that the ration of M4 E-cadherin binding to calnexin was significantly increased compared with that of other variants, suggesting that it was a misfolded protein, though cytoplasmic domain of M4 E-cadherin could associate with beta-catenin. Furthermore, we found that N-glycans of M4 E-cadherin were modified in immature high mannose type, suggesting that it could not depart to Golgi apparatus. In conclusion, this study revealed that N-glycosylation at Asn-633 is essential for E-cadherin expression, folding and trafficking.

Endoplasmic reticulum chaperones are involved in the morphogenesis of rotavirus infectious particles

The final assembly of rotavirus particles takes place in the endoplasmic reticulum (ER). In this work, we evaluated by RNA interference the relevance to rotavirus assembly and infectivity of grp78, protein disulfide isomerase (PDI), grp94, calnexin, calreticulin, and ERp57, members of the two ER folding systems described herein. Silencing the expression of grp94 and Erp57 had no effect on rotavirus infectivity, while knocking down the expression of any of the other four chaperons caused a reduction in the yield of infectious virus of about 50%. In grp78-silenced cells, the maturation of the oligosaccharide chains of NSP4 was retarded. In cells with reduced levels of calnexin, the oxidative folding of VP7 was impaired and the trimming of NSP4 was accelerated, and in calreticulin-silenced cells, the formation of disulfide bonds of VP7 was also accelerated. The knockdown of PDI impaired the formation and/or rearrangement of the VP7 disulfide bonds. All these conditions also affected the correct assembly of virus particles, since compared with virions from control cells, they showed an altered susceptibility to EGTA and heat treatments, a decreased specific infectivity, and a diminished reactivity to VP7 with monoclonal antibody M60, which recognizes only this protein when its disulfide bonds have been correctly formed. In the case of grp78-silenced cells, the virus produced bound less efficiently to MA104 cells than virus obtained from control cells. All these results suggest that these chaperones are involved in the quality control of rotavirus morphogenesis. The complexity of the steps of rotavirus assembly that occur in the ER provide a useful model for studying the organization and operation of the complex network of chaperones involved in maintaining the quality control of this organelle.

Distinct classes of trafficking rBAT mutants cause the type I cystinuria phenotype

Most mutations in the rBAT subunit of the heterodimeric cystine transporter rBAT-b(0,+)AT cause type I cystinuria. Trafficking of the transporter requires the intracellular assembly of the two subunits. Without its partner, rBAT, but not b(0,+)AT, is rapidly degraded. We analyzed the initial biogenesis of wild-type rBAT and type I cystinuria rBAT mutants. rBAT was degraded, at least in part, via the ERAD pathway. Assembly with b(0,+)AT within the endoplasmic reticulum (ER) blocked rBAT degradation and could be independent of the calnexin chaperone system. This system was, however, necessary for post-assembly maturation of the heterodimer. Without b(0,+)AT, wild-type and rBAT mutants were degraded with similar kinetics. In its presence, rBAT mutants showed strongly reduced (L89P) or no transport activity, failed to acquire complex N-glycosylation and to oligomerize, suggesting assembly and/or folding defects. Most of the transmembrane domain mutant L89P did not heterodimerize with b(0,+)AT and was degraded. However, the few [L89P]rBAT-b(0,+)AT heterodimers were stable, consistent with assembly, but not folding, defects. Mutants of the rBAT extracellular domain (T216M, R365W, M467K and M467T) efficiently assembled with b(0,+)AT but were subsequently degraded. Together with earlier results, the data suggest a two-step biogenesis model, with the early assembly of the subunits followed by folding of the rBAT extracellular domain. Defects on either of these steps lead to the type I cystinuria phenotype.

Sequestration of mutated alpha1-antitrypsin into inclusion bodies is a cell-protective mechanism to maintain endoplasmic reticulum function

A variant alpha1-antitrypsin with E342K mutation has a high tendency to form intracellular polymers, and it is associated with liver disease. In the hepatocytes of individuals carrying the mutation, alpha1-antitrypsin localizes both to the endoplasmic reticulum (ER) and to membrane-surrounded inclusion bodies (IBs). It is unclear whether the IBs contribute to cell toxicity or whether they are protective to the cell. We found that in hepatoma cells, mutated alpha1-antitrypsin exited the ER and accumulated in IBs that were negative for autophagosomal and lysosomal markers, and contained several ER components, but not calnexin. Mutated alpha1-antitrypsin induced IBs also in neuroendocrine cells, showing that formation of these organelles is not cell type specific. In the presence of IBs, ER function was largely maintained. Increased levels of calnexin, but not of protein disulfide isomerase, inhibited formation of IBs and lead to retention of mutated alpha1-antitrypsin in the ER. In hepatoma cells, shift of mutated alpha1-antitrypsin localization to the ER by calnexin overexpression lead to cell shrinkage, ER stress, and impairment of the secretory pathway at the ER level. We conclude that segregation of mutated alpha1-antitrypsin from the ER to the IBs is a protective cell response to maintain a functional secretory pathway.

Topology of molecular machines of the endoplasmic reticulum: a compilation of proteomics and cytological data

The endoplasmic reticulum (ER) is a key organelle of the secretion pathway involved in the synthesis of both proteins and lipids destined for multiple sites within and without the cell. The ER functions to both co- and post-translationally modify newly synthesized proteins and lipids and sort them for housekeeping within the ER and for transport to their sites of function away from the ER. In addition, the ER is involved in the metabolism and degradation of specific xenobiotics and endogenous biosynthetic products. A variety of proteomics studies have been reported on different subcompartments of the ER providing an ER protein dictionary with new data being made available on many protein complexes of relevance to the biology of the ER including the ribosome, the translocon, coatomer proteins, cytoskeletal proteins, folding proteins, the antigen-processing machinery, signaling proteins and proteins involved in membrane traffic. This review examines proteomics and cytological data in support of the presence of specific molecular machines at specific sites or subcompartments of the ER.

Uracil nucleotides: from metabolic intermediates to neuroprotection and neuroinflammation

Uracil nucleotides (i.e., UTP and UDP) have been known for years as fundamental intermediates in the de novo synthesis of the other pyrimidine nucleotides, which altogether represent key building blocks for nucleic acid synthesis. In addition, their sugar conjugates (i.e., UDP-glucose and UDP-galactose) enter in several biochemical routes, for example leading to glycogen biosynthesis, and protein and lipid glycosylation, which in turn contribute to the synthesis of essential components of the cellular plasma membrane. More recently, the existence of a "pyrimidinergic transmission" has arisen from the discovery that several purinergic G protein-coupled P2Y receptors can be activated also or exclusively by uracil nucleotides and sugar conjugates. The number of these receptors is continuously growing over years with the discovery that previously "orphan" G protein-coupled receptors are actually responding to this class of molecules. Therefore, new unforeseen effects mediated by uracil derivatives have emerged, in particular in the nervous system, and previously unexplored avenues for the pharmacological manipulation of this system are currently under investigation. In this commentary we shall try to put together our current knowledge on the biochemical and receptor-mediated effects of uracil nucleotide derivatives with a specific focus on the nervous system in order to depict a clearer view of the importance of the pyrimidinergic system in both physiological and pathological conditions.

Nutritionally essential fatty acids and biologically indispensable cyclooxygenases

The study of cyclooxygenases (COXs), targets of aspirin and related drugs, is rooted in the discovery of essential fatty acids (EFAs). There are two COXs that convert EFAs, primarily arachidonic acid, to prostaglandins. Each COX is involved with distinct biologies. COX-1 expression is constitutive while COX-2 is inducible. The two COXs might have evolved partly to permit prostaglandin formation at different tissue sites. However, COX-2 is sometimes induced in cells already expressing COX-1, and in these instances, COX-2 functions while COX-1 is latent. This can occur because of unique biochemical properties of COX-2 that enable cells to form prostaglandins when arachidonic acid comprises a small fraction of available fatty acids and the concentrations of peroxides that are necessary for COX to function are low.

Ero1 and redox homeostasis in the endoplasmic reticulum

Living cells must be able to respond to physiological and environmental fluctuations that threaten cell function and viability. A cellular event prone to disruption by a wide variety of internal and external perturbations is protein folding. To ensure protein folding can proceed under a range of conditions, the cell has evolved transcriptional, translational, and posttranslational signaling pathways to maintain folding homeostasis during cell stress. This review will focus on oxidative protein folding in the endoplasmic reticulum (ER) and will discuss the features of the main facilitator of biosynthetic disulfide bond formation, Ero1. Ero1 plays an essential role in setting the redox potential in the ER and regulation of Ero1 activity is central to maintain redox homeostasis and proper ER folding activity.

Endoplasmic reticulum (ER) mannosidase I is compartmentalized and required for N-glycan trimming to Man5-6GlcNAc2 in glycoprotein ER-associated degradation

We had previously shown that endoplasmic reticulum (ER)-associated degradation (ERAD) of glycoproteins in mammalian cells involves trimming of three to four mannose residues from the N-linked oligosaccharide Man(9)GlcNAc(2). A possible candidate for this activity, ER mannosidase I (ERManI), accelerates the degradation of ERAD substrates when overexpressed. Although in vitro, at low concentrations, ERManI removes only one specific mannose residue, at very high concentrations it can excise up to four alpha1,2-linked mannose residues. Using small interfering RNA knockdown of ERManI, we show that this enzyme is required for trimming to Man(5-6)GlcNAc(2) and for ERAD in cells in vivo, leading to the accumulation of Man(9)GlcNAc(2) and Glc(1)Man(9)GlcNAc(2) on a model substrate. Thus, trimming by ERManI to the smaller oligosaccharides would remove the glycoprotein from reglucosylation and calnexin binding cycles. ERManI is strikingly concentrated together with the ERAD substrate in the pericentriolar ER-derived quality control compartment (ERQC) that we had described previously. ERManI knockdown prevents substrate accumulation in the ERQC. We suggest that the ERQC provides a high local concentration of ERManI, and passage through this compartment would allow timing of ERAD, possibly through a cycling mechanism. When newly made glycoproteins cannot fold properly, transport through the ERQC leads to trimming of a critical number of mannose residues, triggering a signal for degradation.

[Effect of green tea flavonols on the function of the endoplasmic reticulum]

The various (e.g. anti-tumor and anti-diabetic) health effects of green tea attributed to its flavonols, primarily to epigallocatechin-gallate, got into the focus of interest. The endoplasmic reticulum, which plays key role in the metabolism of carcinogens, in the synthesis of secreted or cell surface proteins as well as in the glucose production, might be a potential target for anti-tumor and anti-diabetic agents. Therefore, it is an important question how the flavonols affect its functions. Experiments carried out in microsomes and hepatoma cells revealed that flavonols inhibit glucuronide transport in the endoplasmic reticulum, which may reduce the reactivation of carcinogens; they inhibit glucosidase II, which may cause endoplasmic reticulum stress and apoptosis in hepatoma cells; and they hinder glucose efflux, which may decrease hepatic glucose production and blood glucose level. These observations are useful for further investigation of the relevant transport processes and transporters and also contribute to the better understanding of the mechanisms of flavanol effects.

Use of a new polyclonal antibody to study the distribution and glycosylation of the sodium-coupled bicarbonate transporter NCBE in rodent brain

NCBE (SLC4A10) is a member of the SLC4 family of bicarbonate transporters, several of which play important roles in intracellular-pH regulation and transepithelial HCO(3)(-) transport. Here we characterize a new antibody that was generated in rabbit against a fusion protein consisting of maltose-binding protein and the first 135 amino acids (aa) of the N-terminus of human NCBE. Western blotting--both of purified peptides representing the initial approximately 120 aa of the transporters and of full-length transporters expressed in Xenopus oocytes--demonstrated that the antibody is specific for NCBE versus the two most closely related proteins, NDCBE (SLC4A8) and NBCn1 (SLC4A7). Western blotting of tissue in four regions of adult mouse brain indicates that NCBE is expressed most abundantly in cerebral cortex (CX), cerebellum (CB) and hippocampus (HC), and less so in subcortex (SCX). NCBE protein was present in CX, CB, and HC microdissected to avoid choroid plexus. Immunocytochemistry shows that NCBE is present at the basolateral membrane of embryonic day 18 (E18) fetal and adult choroid plexus. NCBE protein is present by Western blot and immunocytochemistry in cultured and freshly dissociated HC neurons but not astrocytes. By Western blot, nearly all NCBE in mouse and rat brain is highly N-glycosylated (approximately 150 kDa). PNGase F reduces the molecular weight (MW) of natural NCBE in mouse brain or human NCBE expressed in oocytes to approximately the predicted MW of the unglycosylated protein. In oocytes, mutating any one of the three consensus N-glycosylation sites reduces glycosylation of the other two, and the triple mutant exhibits negligible functional expression.

Fbs1 protects the malfolded glycoproteins from the attack of peptide:N-glycanase

Fbs1 is a cytosolic lectin putatively operating as a chaperone as well as a substrate-recognition subunit of the SCF(Fbs1) ubiquitin ligase complex. To provide structural and functional basis of preferential binding of Fbs1 to unfolded glycoproteins, we herein characterize the interaction of Fbs1 with a heptapeptide carrying Man3GlcNAc2 by nuclear magnetic resonance (NMR) spectroscopy and other biochemical methods. Inspection of the NMR data obtained by use of the isotopically labeled glycopeptide indicated that Fbs1 interacts with sugar-peptide junctions, which are shielded in native glycoprotein, in many cases, but become accessible to Fbs1 in unfolded glycoproteins. Furthermore, Fbs1 was shown to inhibit deglycosylation of denatured ribonuclease B by a cytosolic peptide:N-glycanase (PNGase). On the basis of these data, we suggest that Fbs1 captures malfolded glycoproteins, protecting them from the attack of PNGase, during the chaperoning or ubiquitinating operation in the cytosol.

Engineering eukaryotic protein factories

The biopharmaceuticals market is currently outperforming the pharmaceuticals market and is now valued at US$ 48 billion with an average annual growth of 19%. Behind this success is a 100-fold increase in productivities of eukaryotic expression systems. However, the productivity per cell has remained unchanged for more than 10 years. The engineering of the ER-resident protein folding machinery is discussed together with an overview of signal transduction pathways activated by heterologous protein overexpression to increase cell specific productivities.

Purification and initiation of structural characterization of human peripheral myelin protein 22, an integral membrane protein linked to peripheral neuropathies

Gene duplications, deletions, and point mutations in peripheral myelin protein 22 (PMP22) are linked to several inherited peripheral neuropathies. However, the structural and biochemical properties of this very hydrophobic putative tetraspan integral membrane protein have received little attention, in part because of difficulties in obtaining milligram quantities of wild type and disease-linked mutant forms of the protein. In this study a fusion protein was constructed consisting of a fragment of lambda repressor, a decahistidine tag, an intervening TEV protease cleavage site, a Strep tag, and the human PMP22 sequence. This fusion protein was expressed in Escherichia coli at a level of 10-20 mg/L of protein. Following TEV cleavage of the fusion partner, PMP22 was purified and its structural properties were examined in several different types of detergent micelles using cross-linking, near and far-UV circular dichroism, and nuclear magnetic resonance (NMR) spectroscopy. PMP22 is highly helical and, in certain detergents, shows evidence of stable tertiary structure. The protein exhibits a strong tendency to dimerize. The 1H-15N TROSY NMR spectrum is well dispersed and contains signals from all regions of the protein. It appears that detergent-solubilized PMP22 is amenable to detailed structural characterization via crystallography or NMR. This work sets the stage for more detailed studies of the structure, folding, and misfolding of wild type and disease-linked mutants in order to unravel the molecular defects underlying peripheral neuropathies.

Involvement of endoplasmic reticulum stress in a novel Classic Galactosemia model

Inherited deficiency of galactose-1-phosphate uridyltransferase (GALT) activity in humans leads to a potentially lethal disorder called Classic Galactosemia. It is well known that patients often accumulate high levels of galactose metabolites such as galactose-1-phosphate (gal-1-p) in their tissues. However, specific targets of gal-1-p and other accumulated metabolites remain uncertain. In this study, we developed a new model system to study this toxicity using primary fibroblasts derived from galactosemic patients. GALT activity was reconstituted in these primary cells through lentivirus-mediated gene transfer. Gene expression profiling showed that GALT-deficient cells, but not normal cells, responded to galactose challenge by activating a set of genes characteristic of endoplasmic reticulum (ER) stress. Western blot analysis showed that the master regulator of ER stress, BiP, was up-regulated at least threefold in these cells upon galactose challenge. We also found that treatment of these cells with galactose, but not glucose or hexose-free media reduced Ca2+ mobilization in response to activation of Gq-coupled receptors. To explore whether the muted Ca2+ mobilization is related to reduced inositol turnover, we discovered that gal-1-p competitively inhibited human inositol monophosphatase (hIMPase1). We hypothesize that galactose intoxication under GALT-deficiency resulted from accumulation of toxic galactose metabolite products, which led to the accumulation of unfolded proteins, altered calcium homeostasis, and subsequently ER stress.

Substrate-specific requirements for UGT1-dependent release from calnexin

Newly synthesized glycoproteins displaying monoglucosylated N-glycans bind to the endoplasmic reticulum (ER) chaperone calnexin, and their maturation is catalyzed by the calnexin-associated oxidoreductase ERp57. Folding substrates are eventually released from calnexin, and terminal glucoses are removed from N-glycans. The UDP-glucose:glycoprotein glucosyltransferase (UGT1, UGGT, GT) monitors the folding state of polypeptides released from calnexin and adds back a glucose residue on N-glycans of nonnative polypeptides, thereby prolonging retention in the calnexin chaperone system for additional folding attempts. Here we show that for certain newly synthesized glycoproteins UGT1 deletion has no effect on binding to calnexin. These proteins must normally complete their folding program in one binding event. Other proteins normally undergo multiple binding events, and UGT1 deletion results in their premature release from calnexin. For other proteins, UGT1 deletion substantially delays release from calnexin, unexpectedly showing that UGT1 activity might be required for a structural maturation needed for substrate dissociation from calnexin and export from the ER.

Protein energetics in maturation of the early secretory pathway

The early secretory pathway (ESP) consisting of the endoplasmic reticulum (ER), pre-Golgi intermediates and the Golgi stack links protein synthesis to folding and vesicle trafficking to generate the membrane architecture of the eukaryotic cell. The fundamental principles that contribute to organization of the ESP remain largely unknown. We raise the possibility that assembly of the ESP is largely built on a foundation that is influenced by the kinetic and thermodynamic properties of the protein fold. Folding energetics may provide an adjustable platform for adaptor-dependent interactions with the transport machinery, suggesting the possibility that protein cargo energetics plays a central role in directing both trafficking patterns and global compartmental organization of the ESP. In this view, cargo energetics likely coordinates the composition and maturation of ER and Golgi compartments with the physiological state of the cell in different tissue and environmental settings.

Targeting the glycans of glycoproteins: a novel paradigm for antiviral therapy

Several chronic viral infections (such as HIV and hepatitis C virus) are highly prevalent and are a serious health risk. The adaptation of animal viruses to the human host, as recently exemplified by influenza viruses and the severe acute respiratory syndrome coronavirus, is also a continuous threat. There is a high demand, therefore, for new antiviral lead compounds and novel therapeutic concepts. In this Review, an original therapeutic concept for suppressing enveloped viruses is presented that is based on a specific interaction of carbohydrate-binding agents (CBAs) with the glycans present on viral-envelope glycoproteins. This approach may also be extended to other pathogens, including parasites, bacteria and fungi.

SPFH2 Mediates the Endoplasmic Reticulum-associated Degradation of Inositol 1,4,5-Trisphosphate Receptors and Other Substrates in Mammalian Cells

Inositol 1,4,5-trisphosphate (IP(3)) receptors are endoplasmic reticulum (ER) membrane calcium channels that, upon activation, become substrates for the ER-associated degradation (ERAD) pathway. Although it is clear that IP(3) receptors are polyubiquitinated upon activation and are transferred to the proteasome by a p97-based complex, currently nothing is known about the proteins that initially select activated IP(3) receptors for ERAD. Here, we sought to identify novel proteins that associate with and mediate the ERAD of endogenous activated IP(3) receptors. SPFH2, an uncharacterized SPFH domain-containing protein, rapidly associated with IP(3) receptors in a manner that preceded significant polyubiquitination and the association of p97 and related proteins. SPFH2 was found to be an ER membrane protein largely residing within the ER lumen and in resting and stimulated cells was linked to ERAD pathway components, apparently via endogenous substrates undergoing degradation. Suppression of SPFH2 expression by RNA interference markedly inhibited IP(3) receptor polyubiquitination and degradation and the processing of other ERAD substrates. Overall, these studies identify SPFH2 as a key ERAD pathway component and suggest that it may act as a substrate recognition factor.

N-glycan structure dictates extension of protein folding or onset of disposal

The endoplasmic reticulum (ER) is the site of folding for proteins that are resident in the ER or that are destined for the Golgi, endosomes, lysosomes, the plasma membrane, or secretion. Cotranslational addition of preassembled glucose(3)-mannose(9)-N-acetylglucosamine(2) core oligosaccharides (N-glycosylation) is a common event for polypeptides synthesized in this compartment. Protein-bound oligosaccharides are exposed to several ER glycanases that sequentially remove terminal glucose or mannose residues. Their activity must be tightly regulated because the N-glycan composition determines whether the associated protein is subjected to folding attempts in the ER lumen or whether it is retrotranslocated into the cytosol and degraded.

Glycoprotein folding and the role of EDEM1, EDEM2 and EDEM3 in degradation of folding-defective glycoproteins

Proteins synthesized in the endoplasmic reticulum (ER) lumen are exposed to several dedicated chaperones and folding factors that ensure efficient maturation. Nevertheless, protein folding remains error-prone and mutations in the polypeptide sequence may significantly reduce folding-efficiency. Folding-incompetent proteins carrying N-glycans are extracted from futile folding cycles in the calnexin chaperone system upon intervention of EDEM1, EDEM2 and EDEM3, three ER-stress-induced members of the glycosyl hydrolase 47 family. This review describes current knowledge about mechanisms regulating folding and disposal of glycoproteins.

FKBP22 is part of chaperone/folding catalyst complexes in the endoplasmic reticulum ofNeurospora crassa

FKBP22 is a dimeric protein in the lumen of the endoplasmic reticulum, which exhibits a chaperone as well as a PPIase activity. It binds via its FK506 binding protein (FKBP) domain directly to the Hsp70 chaperone BiP that stimulates the chaperone activity of FKBP22. Here we demonstrate additionally the association of FKBP22 with the molecular chaperones and folding catalysts Grp170, alpha-subunit of glucosidase II, PDI, ERp38, and CyP23. These proteins are associated with FKBP22 in at least two protein complexes. Furthermore, we report an essential role for FKBP22 in the development of microconidiophores in Neurospora crassa.