N-glycan processing in ER quality control

Endoplasmic reticulum protein quality control and its relationship to environmental stress responses in plants

The endoplasmic reticulum (ER) has a sophisticated quality control (QC) system to eliminate improperly folded proteins from the secretory pathway. Given that protein folding is such a fastidious process and subject to adverse environmental conditions, the ER QC system appears to have been usurped to serve as an environmental sensor and responder in plants. Under stressful conditions, the ER protein folding machinery reaches a limit as the demands for protein folding exceed the capacity of the system. Under these conditions, misfolded or unfolded proteins accumulate in the ER, triggering an unfolded protein response (UPR). UPR mitigates ER stress by upregulating the expression of genes encoding components of the protein folding machinery or the ER-associated degradation system. In Arabidopsis thaliana, ER stress is sensed and stress signals are transduced by membrane-bound transcription factors, which are activated and mobilized under environmental stress conditions. Under acute or chronic stress conditions, UPR can also lead to apoptosis or programmed cell death. Despite recent progress in our understanding of plant protein QC, discovering how different environmental conditions are perceived is one of the major challenges in understanding this system. Since the ER QC system is one among many stress response systems in plants, another major challenge is determining the extent to which the ER QC system contributes to various stress responses in plants.

Redox control of endoplasmic reticulum function

The lumen of the endoplasmic reticulum constitutes a separate intracellular compartment with a special proteome and metabolome. The redox conditions of the organelle are also characteristically different from those of the other subcellular compartments. The luminal environment has been considered more oxidizing than the cytosol due to the presence of oxidative protein folding. However, recent observations suggest that redox systems in reduced and oxidized states are present simultaneously. The concerted action of membrane transporters and oxidoreductase enzymes maintains the oxidized state of the thiol-disulfide and the reduced state of the pyridine nucleotide redox systems, which are prerequisites for the normal redox reactions localized in the organelle. The powerful thiol-oxidizing machinery of oxidative protein folding continuously challenges the local antioxidant defense. Alterations of the luminal redox conditions, either in oxidizing or reducing direction, affect protein processing, are sensed by the accumulation of misfolded/unfolded proteins, and may induce endoplasmic reticulum stress and unfolded protein response. The activated signaling pathways attempt to restore the balance between protein loading and processing and induce programmed cell death if these attempts fail. Recent findings strongly support the involvement of redox-based endoplasmic reticulum stress in a plethora of human diseases, either as causative agents or as complications.

Requirement of a homolog of glucosidase II beta-subunit for EFR-mediated defense signaling in Arabidopsis thaliana

EFR is a plasma-membrane resident receptor responsible for recognition of microbial elongation factor Tu (EF-Tu) and thus triggering plant innate immunity to fend off phytopathogens. Functional EFR must be subject to the endoplasmic reticulum quality control (ERQC) machinery for the correct folding and proper assembly in order to reach its final destination. Genetic studies have demonstrated that ERD2b, a counterpart of the yeast or mammalian HDEL receptor ERD2 for retaining proteins in the endoplasmic reticulum (ER) lumen, is required for EFR function in plants (Li et al., 2009). In this study, we characterized the Arabidopsis glucosidase II beta-subunit via the HDEL motif against the non-redundant protein database. Data mining also revealed that the glucosidase II beta-subunit gene has a highly similar expression pattern to ERD2b and the other known ERQC components involved in EFR biogenesis. Importantly, the T-DNA insertion lines of the glucosidase II beta-subunit gene showed that EFR-controlled responses were substantially reduced or completely blocked in these mutants. The responses include seedling growth inhibition, induction of marker genes, MAP kinase activation, and callose deposition, triggered by peptide elf18, a full mimic of EF-Tu. Taken together, our data indicate a requirement of the glucosidase II beta-subunit for EFR function.

Elucidation of XA21-mediated innate immunity

In the early 1970s, the Xa21 gene from the wild rice species Oryza longistaminata drew attention of rice breeders because of its broad-spectrum resistance to diverse strains of a serious bacterial disease of rice in Asia and Africa, called 'bacterial blight disease', caused by the Gram-negative bacterium, Xanthomonas oryzae pv. oryzae (Xoo). In 1995, we isolated the gene controlling this resistance and in 2009 demonstrated that XA21 recognizes a highly conserved peptide, called 'Ax21' (activator of XA21-mediated immunity). Tyrosine sulfation of Ax21 is required for recognition by rice XA21. A decade of genetic, molecular and biochemical studies have uncovered key components of the XA21-mediated signalling cascade. Ax21 recognition by XA21 at the cell surface induces phosphorylation-mediated events, which are predicted to alter subcellular localization and/or DNA-binding activity of a WRKY family of transcription factors. Because XA21 is representative of the large number of predicted pattern recognition receptors (PRRs) in rice (n = 328), Arabidopsis (n = 35) and other plant species, further characterization of XA21-mediated signalling pathways will contribute to elucidation of these important defence responses.

Improved secretion of molecular chaperone-assisted human IgG in silkworm, and no alterations in their N-linked glycan structures

Human 29IJ6 IgG was expressed in silkworm using a Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid system. The mean amounts of 296IJ6 IgG produced in larval hemolymph and whole pupae were 30.1 microg/larva and 78.0 microg/pupa, respectively. The use of molecular chaperones including calreticulin (CRT), calnexin (CNX), and immunoglobulin heavy chain binding protein (BiP, GRP78) improved the production of 296IJ6 IgG secretion in the larvae fivefold. The total yield of recombinant 29IJ6 IgG was 239 microg/mL when coexpressed with CRT. However, the overexpression of molecular chaperones had negative effects on secretion. The N-linked glycans of secreted 296IJ6 IgG in silkworm hemolymph were dominated by paucimannose structures. Small amounts of GlcNAc residues linked to the Manalpha1,3 branch were detected. When molecular chaperones were coexpressed, the compositions of N-linked glycans in the IgG1 produced were unchanged compared with those produced without them. This suggests that N-glycosylation is controlled by a regulatory function in the Golgi apparatus even though the post-translational modification of 296IJ6 IgG was assisted by the coexpression of molecular chaperones. Therefore, if the glycosylation pathways that coexpress N-acetylglucosaminyltransferase, galactosyltransferase, and sialyltransferase could be improved, silkworm larvae might prove a useful system for producing human antibodies.

From cradle to twilight: the carboxyl terminus directs the fate of the A(2A)-adenosine receptor

The extended carboxyl terminus of the A(2A)-adenosine receptor is known to engage several proteins other than those canonically involved in signalling by GPCRs (i.e., G proteins, G protein-coupled receptor kinases/GRKs, arrestins). The list includes the deubiquinating enzyme USP4, α-actinin, the guanine nucleotide exchange factor for ARF6 ARNO, translin-X-associated protein, calmodulin, the neuronal calcium binding protein NECAB2 and the synapse associated protein SAP102. However, if the fate of the A(2A)-receptor is taken into account - from its birthplace in the endoplasmic reticulum to its presumed site of disposal in the lysosome, it is evident that many more proteins must interact with the A(2A)-adenosine receptor. There are several arguments that support the conjecture that these interactions will preferentially occur with the carboxyl terminus of the A(2A)-adeonsine receptor: (i) the extended carboxyl terminus (of 122 residues=) offers the required space to accommodate companions; (ii) analogies can be drawn with other receptors, which engage several of these binding partners with their C-termini. This approach allows for defining the nature of the unknown territory. As an example, we posit a chaperone/coat protein complex-II (COPII) exchange model that must occur on the carboxyl terminus of the receptor. This model accounts for the observation that a minimum size of the C-terminus is required for correct folding of the receptor. It also precludes premature recruitment of the COPII-coat to a partially folded receptor.

Targeting a host process as an antiviral approach against dengue virus

The re-emergence of dengue virus as a significant human pathogen has lead to an increasing need for effective antivirals. Development of therapeutic agents with the ability to attenuate both the duration and severity of disease in patients after infection is particularly desirable in dengue endemic resource-poor settings. The reliance of dengue virus on endogenous processes during the late stages of infection prompts the development of molecules to interfere with and exploit these dependencies as potential antiviral therapies. Here we focus on the importance of N-linked glycan processing in infectious virion morphogenesis.

N-glycan-dependent quality control of the Na,K-ATPase beta(2) subunit

Bulky hydrophilic N-glycans stabilize the proper tertiary structure of glycoproteins. In addition, N-glycans comprise the binding sites for the endoplasmic reticulum (ER)-resident lectins that assist correct folding of newly synthesized glycoproteins. To reveal the role of N-glycans in maturation of the Na,K-ATPase beta(2) subunit in the ER, the effects of preventing or modifying the beta(2) subunit N-glycosylation on trafficking of the subunit and its binding to the ER lectin chaperone, calnexin, were studied in MDCK cells. Preventing N-glycosylation abolishes binding of the beta(2) subunit to calnexin and results in the ER retention of the subunit. Furthermore, the fully N-glycosylated beta(2) subunit is retained in the ER when glycan-calnexin interactions are prevented by castanospermine, showing that N-glycan-mediated calnexin binding is required for correct subunit folding. Calnexin binding persists for several hours after translation is stopped with cycloheximide, suggesting that the beta(2) subunit undergoes repeated post-translational calnexin-assisted folding attempts. Homology modeling of the beta(2) subunit using the crystal structure of the alpha(1)-beta(1) Na,K-ATPase shows the presence of a relatively hydrophobic amino acid cluster proximal to N-glycosylation sites 2 and 7. Combined, but not separate, removal of sites 2 and 7 dramatically impairs calnexin binding and prevents the export of the beta(2) subunit from the ER. Similarly, hydrophilic substitution of two hydrophobic amino acids in this cluster disrupts both beta(2)-calnexin binding and trafficking of the subunit to the Golgi. Therefore, the hydrophobic residues in the proximity of N-glycans 2 and 7 are required for post-translational calnexin binding to these N-glycans in incompletely folded conformers, which, in turn, is necessary for maturation of the Na,K-ATPase beta(2) subunit.

Apolipoprotein B100 biogenesis: a complex array of intracellular mechanisms regulating folding, stability, and lipoprotein assemblyThis paper is one of a selection of papers published in this special issue entitled “Canadian Society of Biochemistry, Molecular & Cellular Biology 52nd Annual Meeting — Protein Folding: Principles and Diseases” and has undergone the Journal's usual peer review process

Apolipoprotein B100 (apoB) is a large amphipathic lipid-binding protein that is synthesized by hepatocytes and used to assemble and stabilize very low density lipoproteins (VLDL). It may have been derived through evolution from other lipid-associating proteins such as microsomal triglyceride transfer protein or vitellogenin. The correct folding of apoB requires assistance from chaperone proteins in co-translational lipidation, disulfide bond formation, and glycosylation. Any impairment in these processes results in co-translational targeting of the misfolded apoB molecule for proteasomal degradation. In fact, most of the regulation of apoB production is mediated by intracellular degradation. ApoB that misfolds post-translationally, perhaps as a result of oxidative stress, may be eliminated through autophagy. This review focuses on the proposed pentapartite domain structure of apoB, the role that each domain plays in the binding of lipid species and regulation of apoB synthesis, and the process of VLDL assembly. The factors involved in the recognition, ubiquitination, and proteasomal delivery of defective apoB molecules are also discussed.

Production of infectious hepatitis C virus by using RNA polymerase I-mediated transcription

In this study, we used an RNA polymerase I (Pol I) transcription system for development of a reverse genetics protocol to produce hepatitis C virus (HCV), which is an uncapped positive-strand RNA virus. Transfection with a plasmid harboring HCV JFH-1 full-length cDNA flanked by a Pol I promoter and Pol I terminator yielded an unspliced RNA with no additional sequences at either end, resulting in efficient RNA replication within the cytoplasm and subsequent production of infectious virions. Using this technology, we developed a simple replicon trans-packaging system, in which transient transfection of two plasmids enables examination of viral genome replication and virion assembly as two separate steps. In addition, we established a stable cell line that constitutively produces HCV with a low mutation frequency of the viral genome. The effects of inhibitors of N-linked glycosylation on HCV production were evaluated using this cell line, and the results suggest that certain step(s), such as virion assembly, intracellular trafficking, and secretion, are potentially up- and downregulated according to modifications of HCV envelope protein glycans. This Pol I-based HCV expression system will be beneficial for a high-throughput antiviral screening and vaccine discovery programs.

Congenital disorders of glycosylation in hepatology: the example of polycystic liver disease

Autosomal dominant polycystic liver disease (PCLD) is a rare progressive disorder characterized by an increased liver volume due to many (>20) fluid-filled cysts of biliary origin. Disease causing mutations in PRKCSH or SEC63 are found in approximately 25% of the PCLD patients. Both gene products function in the endoplasmic reticulum, however, the molecular mechanism behind cyst formation remains to be elucidated. As part of the translocon complex, SEC63 plays a role in protein import into the ER and is implicated in the export of unfolded proteins to the cytoplasm during ER-associated degradation (ERAD). PRKCSH codes for the beta-subunit of glucosidase II (hepatocystin), which cleaves two glucose residues of Glc(3)Man(9)GlcNAc(2) N-glycans on proteins. Hepatocystin is thereby directly involved in the protein folding process by regulating protein binding to calnexin/calreticulin in the ER. A separate group of genetic diseases affecting protein N-glycosylation in the ER is formed by the congenital disorders of glycosylation (CDG). In distinct subtypes of this autosomal recessive multisystem disease specific liver symptoms have been reported that overlap with PCLD. Recent research revealed novel insights in PCLD disease pathology such as the absence of hepatocystin from cyst epithelia indicating a two-hit model for PCLD cystogenesis. This opens the way to speculate about a recessive mechanism for PCLD pathophysiology and shared molecular pathways between CDG and PCLD. In this review we will discuss the clinical-genetic features of PCLD and CDG as well as their biochemical pathways with the aim to identify novel directions of research into cystogenesis.

Enhancing the secretion of recombinant proteins by engineering N-glycosylation sites

N-glycosylation is important for the folding and quality control of membrane and secretory proteins. We used mutagenesis to introduce N-glycosylation sequons in recombinant proteins to improve their secretion in HEK293 cells. Seven recombinant proteins, with or without endogenous N-glycosylation sequons, were tested by this method. Our results indicate that N-glycosylation sequons located at the N- or C-terminal are glycosylated at high rates and thus the N- and C-terminal may be convenient sites for effectively attaching oligosaccharide chains. More importantly, introduction of oligosaccharide chains at such positions has been found to improve the secretion levels for the majority of the recombinant proteins in our studies, regardless of endogenous N-glycosylation, presumably by improving their folding in the endoplasmic reticulum.

Overexpression of the endoplasmic reticulum chaperone BiP3 regulates XA21-mediated innate immunity in rice

Recognition of pathogen-associated molecular patterns by pattern recognition receptors (PRRs) activates the innate immune response. Although PRR-mediated signaling events are critical to the survival of plants and animals, secretion and localization of PRRs have not yet been clearly elucidated. Here we report the in vivo interaction of the endoplasmic reticulum (ER) chaperone BiP3 with the rice XA21 PRR, which confers resistance to the Gram negative bacterium, Xanthomonas oryzae pv. oryzae (Xoo). We show that XA21 is glycosylated and is primarily localized to the ER and also to the plasma membrane (PM). In BiP3-overexpressing rice plants, XA21-mediated immunity is compromised, XA21 stability is significantly decreased, and XA21 proteolytic cleavage is inhibited. BiP3 overexpression does not affect the general rice defense response, cell death or brassinolide-induced responses. These results indicate that BiP3 regulates XA21 protein stability and processing and that this regulation is critical for resistance to Xoo.

Molecular cloning of calnexin and calreticulin in the Pacific oyster Crassostrea gigas and its expression in response to air exposure

Calnexin (CNX) and calreticulin (CRT) are endoplasmic reticulum (ER) chaperones. CNX is a type I transmembrane protein and CRT is a soluble CNX homologue. In the ER, CNX and CRT are important for Ca(2+) homeostasis and protein maturation. Here, we describe the full-length cDNA of the first mollusk CNX (cgCNX) and a second mollusk CRT (cgCRT) from the oyster Crassostrea gigas. CgCNX, containing 3255bp, was composed of a 1764bp open reading frame (ORF) that encodes a 588-amino acid protein. CgCRT, containing 1727bp, was composed of a 1242bp ORF that encodes a 414-amino acid protein. CgCNX and cgCRT contains an N-terminal 21- and 16-amino acid sequence, respectively, which is characteristic of a signal sequence. At the C-terminus, cgCRT also contains the KDEL (-Lys-Asp-Glu-Leu) peptide motif suggesting that cgCRT localizes in the ER. Northern blot analysis showed that both cgCNX and cgCRT mRNAs are induced by air exposure. The expression patterns of cgCNX mRNA differed from those of cgCRT during air exposure. This suggests that these two molecular chaperones have different roles in the response to air exposure.

Calnexin phosphorylation: linking cytoplasmic signalling to endoplasmic reticulum lumenal functions

Calnexin is an abundant integral membrane phosphoprotein of the endoplasmic reticulum (ER) of eukaryotic cells. The role of the luminal domain as an N-glycoprotein specific lectin has been well-established. Cytosolic C-terminal domain phosphorylation of calnexin has recently been elucidated in glycoprotein folding and quality control. Signalling of the presence of unfolded proteins from the lumen of the ER is mediated by the three ER membrane sensor proteins Ire1, ATF6 and PERK. The observation that the C-terminus of calnexin is differentially phosphorylated when glycoproteins are misfolded initiated our search for functional roles of calnexin phosphorylation. Recent studies have defined a role for phosphorylation at a proline-directed kinase site (Ser563) in ER protein quality control, while phosphorylation at a casein kinase 2 site (Ser534, Ser544) may be linked to transport functions. There are also four other abundant integral membrane phosphoproteins in the ER, and these may be components of other signalling pathways that link and coordinate other ER functions with the rest of the cell.

Ubiquitylation in the ERAD Pathway

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

ERAD substrates: which way out?

Global folding of polypeptides entering the endoplasmic reticulum (ER) starts as soon as they emerge from the narrow Sec61 translocon. Attainment of the native structure can take from several minutes to hours, depending on the gene product. Until then, non-native folding intermediates must be protected from molecular chaperones that recognize misfolded determinants and could prematurely interrupt folding programs by re-directing them to disposal pathways. On the other hand, futile folding attempts must actively be stopped to prevent intraluminal accumulation of defective cargo. This review describes recent advances in understanding how terminally misfolded polypeptides are extracted from the folding environment and directed to specific dislocons within the ER membrane for transfer to the cytoplasm for proteasome-mediated degradation.

ER stress response, peroxisome proliferation, mitochondrial unfolded protein response and Golgi stress response

The endoplasmic reticulum (ER) response has been thought a cytoprotective mechanism to cope with accumulation of unfolded proteins in the ER. Recent progress has made a quantum leap revealing that ER stress response can be regarded as an autoregulatory system adjusting the ER capacity to cellular demand. This Copernican change raised a novel fundamental question in cell biology: how do cells regulate the capacity of each organelle in accordance with cellular needs? Although this fundamental question has not been fully addressed yet, research about each organelle has been advancing. The proliferation of the peroxisome is regulated by the PPAR alpha pathway, whereas the abundance of mitochondria appears to be regulated by mitochondrial retrograde signaling and the mitochondrial unfolded protein response. The functional capacity of the Golgi apparatus may be regulated by the mechanism of the Golgi stress response. These observations strongly suggest the existence of an elaborate network of organelle autoregulation in eukaryotic cells.

Protein disulfide isomerase: a critical evaluation of its function in disulfide bond formation

Disulfide bond formation is probably involved in the biogenesis of approximately one third of human proteins. A central player in this essential process is protein disulfide isomerase or PDI. PDI was the first protein-folding catalyst reported. However, despite more than four decades of study, we still do not understand much about its physiological mechanisms of action. This review examines the published literature with a critical eye. This review aims to (a) provide background on the chemistry of disulfide bond formation and rearrangement, including the concept of reduction potential, before examining the structure of PDI; (b) detail the thiol-disulfide exchange reactions that are catalyzed by PDI in vitro, including a critical examination of the assays used to determine them; (c) examine oxidation and reduction of PDI in vivo, including not only the role of ERo1 but also an extensive assessment of the role of glutathione, as well as other systems, such as peroxide, dehydroascorbate, and a discussion of vitamin K-based systems; (d) consider the in vivo reactions of PDI and the determination and implications of the redox state of PDI in vivo; and (e) discuss other human and yeast PDI-family members.

N-glycan structures: recognition and processing in the ER

The processing of N-linked glycans determines secretory protein homeostasis in the eukaryotic cell. Folding and degradation of glycoproteins in the endoplasmic reticulum (ER) are regulated by molecular chaperones and enzymes recruited by specific oligosaccharide structures. Recent findings have identified several components of this protein quality control system that specifically modify N-linked glycans, thereby generating oligosaccharide structures recognized by carbohydrate-binding proteins, lectins. In turn, lectins direct newly synthesized polypeptides to the folding, secretion or degradation pathways. The "glyco-code of the ER" displays the folding status of a multitude of cargo proteins. Deciphering this code will be instrumental in understanding protein homeostasis regulation in eukaryotic cells and for intervention because such processes can have crucial importance for clinical and industrial applications.

Calnexin phosphorylation attenuates the release of partially misfolded alpha1-antitrypsin to the secretory pathway

Calnexin is a type I integral membrane phosphoprotein resident of the endoplasmic reticulum. Its intraluminal domain has been deduced to function as a lectin chaperone coordinating the timing of folding of newly synthesized N-linked glycoproteins of the secretory pathway. Its C-terminal cytosolic oriented extension has an ERK1 phosphorylation site at Ser(563) affecting calnexin association with the translocon. Here we find an additional function for calnexin phosphorylation at Ser(563) in endoplasmic reticulum quality control. A low dose of the misfolding agent l-azetidine 2-carboxylic acid slows glycoprotein maturation and diminishes the extent and rate of secretion of newly synthesized secretory alpha1-antitrypsin. Under these conditions the phosphorylation of calnexin is enhanced at Ser(563). Inhibition of this phosphorylation by the MEK1 inhibitor PD98059 enhanced the extent and rate of alpha1-antitrypsin secretion comparable with that achieved by inhibiting alpha-mannosidase activity with kifunensine. This is the first report in which the phosphorylation of calnexin is linked to the efficiency of secretion of a cargo glycoprotein.

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

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

Fluorescent glycosylamides produced by microscale derivatization of free glycans for natural glycan microarrays

A novel strategy for creating naturally derived glycan microarrays has been developed. Glycosylamines are prepared from free reducing glycans and stabilized by reaction with acryloyl chloride to generate a glycosylamide in which the reducing monosaccharide has a closed-ring structure. Ozonolysis of the protected glycan yields an active aldehyde, to which a bifunctional fluorescent linker is coupled by reductive amination. The fluorescent derivatives are easily coupled through a residual primary alkylamine to generate glycan microarrays. This strategy preserves structural features of glycans required for antibody recognition and allows development of natural arrays of fluorescent glycans in which the cyclic pyranose structure of the reducing-end sugar residue is retained.

Glycosylation- and phosphorylation-dependent intracellular transport of lysosomal hydrolases

Lysosomes contain more than 50 soluble hydrolases that are targeted to lysosomes in a mannose 6-phosphate (Man6P)-dependent manner. The phosphorylation of man- nose residues on high mannose-type oligosaccharides of newly synthesized lysosomal enzymes is catalyzed by two multimeric enzymes, GlcNAc-1-phosphotransferase and GlcNAc-1-phosphodiester-alpha-N-acetylglucosaminidase, allowing the binding to two distinct Man6P receptors in the Golgi apparatus. Inherited defects in the GlcNAc-1-phosphotransferase complex result in missorting and cellular loss of lysosomal enzymes, and the subsequent lysosomal dysfunction causes the lysosomal storage disorders mucolipidosis types II and III. Biosynthetic studies and the availability of Man6P receptor-deficient mouse models have provided new insights into the structural requirements for preferential binding of subsets of lysosomal enzymes to Man6P receptors as well as the identification of alternative targeting pathways.

Lectin-like ERAD players in ER and cytosol

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

Novel ubiquitin-dependent quality control in the endoplasmic reticulum

Proteins of the endomembrane system undergo assisted folding in the endoplasmic reticulum (ER), then quality-control and, if misfolded, ER-associated degradation (ERAD). Recent findings on the biogenesis of a type-I membrane protein (an LRP6 mutant) lead us to hypothesize the existence of a novel mechanism promoting folding of membrane proteins from the cytosolic side of the ER. The proposed folding mechanism involves cycles of chaperone binding through mono-ubiquitylation and de-ubiquitylation, followed eventually by poly-ubiquitylation and ERAD. This suggests a novel dual role for ubiquitylation in the ER - dependent on the type of ubiquitin chains involved - in folding and in degradation, and highlights the potential importance of de-ubiquitylating enzymes.

Unconventional splicing of XBP1 mRNA occurs in the cytoplasm during the mammalian unfolded protein response

XBP1 is a key transcription factor that regulates the mammalian unfolded protein response. Its expression is regulated by unconventional mRNA splicing that is carried out by endonuclease IRE1 and a specific, as yet unknown, RNA ligase in response to the accumulation of unfolded proteins in the ER. Conventional mRNA splicing occurs only in the nucleus, but it has remained unclear whether unconventional splicing of XBP1 mRNA takes place in the nucleus, cytoplasm or both. Here, we show that the catalytic domain of IRE1 contains a nuclear exclusion signal to prevent IRE1 from mislocalizing to the nucleus. In addition, RNA ligase, which joins XBP1 exons cleaved by IRE1 was detected in the cytoplasm but not in the nucleus. Moreover, the cytoplasm contained large amounts of unspliced XBP1 mRNA compared with the nucleus. Most unspliced XBP1 mRNA was converted to spliced mRNA by unconventional splicing even if de novo transcription was blocked, suggesting that cytoplasmic XBP1 mRNA, not nuclear XBP1 mRNA, is a major substrate for unconventional splicing. From these observations, we concluded that unconventional splicing of XBP1 mRNA occurs predominantly in the cytoplasm.

Dexamethasone induces a heat-stress response that ameliorates the conformational consequences on antithrombin of L-asparaginase treatment

BACKGROUND

L-asparaginase (L-ASP) treatment of patients with acute lymphoblastic leukemia causes a severe antithrombin deficiency by intracellular retention of this serpin within the endoplasmic reticulum (ER) of hepatic cells, and a subsequent risk of thrombosis. Interestingly, co-administration of dexamethasone with L-ASP seems to reduce the risk of thrombosis.

OBJECTIVES

We have investigated the effect of two corticoids, dexamethasone and prednisone, on the conformational consequences of L-ASP treatment on antithrombin.

PATIENTS/METHODS

Levels, activity, conformation and immunohistological features of antithrombin were studied in patients, cell and mice models. Because of the importance of the steroid receptor-heat stress response (HSR) axis, and the role of unfolded protein response (UPR) in conformational diseases, we also evaluated Hsp27, Hsp70, Hsp90, HSF-1 and ER chaperons (Grp78 and Grp94).

RESULTS

In all models, L-ASP alone or in combination with prednisone caused the intracellular retention of antithrombin associated with a severe deficiency. In contrast, the combination of L-ASP with dexamethasone ameliorated both the deficiency and intracellular retention of the serpin, which is associated with increased expression of heat shock proteins and ER-chaperons.

CONCLUSIONS

These results suggest a protective effect of dexamethasone on the conformational consequences of L-ASP on antithrombin as a result of exacerbated HSR and UPR that help to explain the reduced risk of thrombosis reported in patients that follow this scheme of treatment.

Comparative proteomic analysis of an Aspergillus fumigatus mutant deficient in glucosidase I (AfCwh41)

Alpha-glucosidase I regulates trimming of the terminal alpha-1,2-glucose residue in the N-glycan processing pathway, which plays an important role in quality control systems in mammalian cells. Previously, we identified the gene encoding alpha-glucosidase I in the opportunistic human fungal pathogen Aspergillus fumigatus, namely Afcwh41. Deletion of the Afcwh41 gene results in a severe reduction of conidia formation, a temperature-sensitive deficiency of cell wall integrity, and abnormalities of polar growth and septation. An upregulation of the genes encoding Rho-type GTPases was also observed, which suggests activation of the cell wall integrity pathway in the mutant. Using 2D gel analysis, we revealed that the proteins involved in protein assembly, ubiquitin-mediated degradation and actin organization are altered in the DeltaAfcwh41 mutant. Evidence was obtained for a defect in the polarized localization of the actin cytoskeleton in the mutant. Our results suggest that blocking of the glucose trimming in A. fumigatus might induce accumulation of misfolded proteins in the endoplasmic reticulum; these misfolded proteins are probably required for cell wall synthesis and thus activate the cell wall integrity pathway, which then causes the abnormal polarity associated with the DeltaAfcwh41 mutant.

Cellular responses to endoplasmic reticulum stress and apoptosis

The endoplasmic reticulum (ER) is the cell organelle where secretory and membrane proteins are synthesized and folded. Correctly folded proteins exit the ER and are transported to the Golgi and other destinations within the cell, but proteins that fail to fold properly-misfolded proteins-are retained in the ER and their accumulation may constitute a form of stress to the cell-ER stress. Several signaling pathways, collectively known as unfolded protein response (UPR), have evolved to detect the accumulation of misfolded proteins in the ER and activate a cellular response that attempts to maintain homeostasis and a normal flux of proteins in the ER. In certain severe situations of ER stress, however, the protective mechanisms activated by the UPR are not sufficient to restore normal ER function and cells die by apoptosis. Most research on the UPR used yeast or mammalian model systems and only recently Drosophila has emerged as a system to study the molecular and cellular mechanisms of the UPR. Here, we review recent advances in Drosophila UPR research, in the broad context of mammalian and yeast literature.

Sodium selenite induces apoptosis by ROS-mediated endoplasmic reticulum stress and mitochondrial dysfunction in human acute promyelocytic leukemia NB4 cells

INTRODUCTION

In this study, we delineated the apoptotic signaling pathways activated by sodium selenite in NB4 cells.

MATERIALS AND METHODS

NB4 cells were treated with 20 microM sodium selenite for different times. The activation of caspases and ER stress markers, ROS levels, mitochondrial membrane potential and cell apoptosis induced by sodium selenite were analyzed by immunoblotting analysis, DCF fluorescence and flow cytometric respectively. siRNA was used to detect the effect of GADD153 on selenite-induced cell apoptosis.

CONCLUSIONS

Sodium selenite-induced reactive oxygen species generation is an early event that triggers endoplasmic reticulum stress mitochondrial apoptotic pathways in NB4 cells.

Mechanisms of lipase maturation

Lipases are acyl hydrolases that represent a diverse group of enzymes present in organisms ranging from prokaryotes to humans. This article focuses on an evolutionarily related family of extracellular lipases that include lipoprotein lipase, hepatic lipase and endothelial lipase. As newly synthesized proteins, these lipases undergo a series of co- and post-translational maturation steps occurring in the endoplasmic reticulum, including glycosylation and glycan processing, and protein folding and subunit assembly. This article identifies and discusses mechanisms that direct early and late events in lipase folding and assembly. Lipase maturation employs the two general chaperone systems operating in the endoplasmic reticulum, as well as a recently identified lipase-specific chaperone termed lipase maturation factor 1. We propose that the two general chaperone systems act in a coordinated manner early in lipase maturation in order to help create partially folded monomers; lipase maturation factor 1 then facilitates final monomer folding and subunit assembly into fully functional homodimers. Once maturation is complete, the lipases exit the endoplasmic reticulum and are secreted to extracellular sites, where they carry out a number of functions related to lipoprotein and lipid metabolism.

Synthesis and biological evaluation of a chitobiose-based peptide N-glycanase inhibitor library

Peptide N-glycanase (PNGase), the enzyme responsible for the deglycosylation of N-linked glycoproteins, has an active site related to that of cysteine proteases. Chitiobiose was equipped with electrophilic traps often used in cysteine protease inhibitors, and the resulting compounds were evaluated as PNGase inhibitors. We found that the electrophilic trap of the inhibitor has a great influence on the potency of the compounds with the chloromethyl ketone inhibitor being the first potent C-glycoside-based PNGase inhibitor.

Hepatic lipase maturation: a partial proteome of interacting factors

Tandem affinity purification (TAP) has been used to isolate proteins that interact with human hepatic lipase (HL) during its maturation in Chinese hamster ovary cells. Using mass spectrometry and Western blotting, we identified 28 proteins in HL-TAP isolated complexes, 16 of which localized to the endoplasmic reticulum (ER), the site of HL folding and assembly. Of the 12 remaining proteins located outside the ER, five function in protein translation or ER-associated degradation (ERAD). Components of the two major ER chaperone systems were identified, the BiP/Grp94 and the calnexin (CNX)/calreticulin (CRT) systems. All factors involved in CNX/CRT chaperone cycling were identified, including UDP-glucose:glycoprotein glucosyltransferase 1 (UGGT), glucosidase II, and the 57 kDa oxidoreductase (ERp57). We also show that CNX, and not CRT, is the lectin chaperone of choice during HL maturation. Along with the 78 kDa glucose-regulated protein (Grp78; BiP) and the 94 kDa glucose-regulated protein (Grp94), an associated peptidyl-prolyl cis-trans isomerase and protein disulfide isomerase were also detected. Finally, several factors in ERAD were identified, and we provide evidence that terminally misfolded HL is degraded by the ubiquitin-mediated proteasomal pathway. We propose that newly synthesized HL emerging from the translocon first associates with CNX, ERp57, and glucosidase II, followed by repeated posttranslational cycles of CNX binding that is mediated by UGGT. BiP/Grp94 may stabilize misfolded HL during its transition between cycles of CNX binding and may help direct its eventual degradation.

Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase has unusual substrate specificity and protects the parasite from stress

In this paper, we describe the range of N-linked glycan structures produced by wild-type and glucosidase II null mutant bloodstream form Trypanosoma brucei parasites and the creation and characterization of a bloodstream form Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase null mutant. These analyses highlight peculiarities of the Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase, including an unusually wide substrate specificity, ranging from Man(5)GlcNAc(2) to Man(9)GlcNAc(2) glycans, and an unusually high efficiency in vivo, quantitatively glucosylating the Asn263 N-glycan of variant surface glycoprotein (VSG) 221 and 75% of all non-VSG N glycosylation sites. We also show that although Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase is not essential for parasite growth at 37 degrees C, it is essential for parasite growth and survival at 40 degrees C. The null mutant was also shown to be hypersensitive to the effects of the N glycosylation inhibitor tunicamycin. Further analysis of bloodstream form Trypanosoma brucei under normal conditions and stress conditions suggests that it does not have a classical unfolded protein response triggered by sensing unfolded proteins in the endoplasmic reticulum. Rather, judging by its uniform Grp78/BiP levels, it appears to have an unregulated and constitutively active endoplasmic reticulum protein folding system. We suggest that the latter may be particularly appropriate for this organism, which has an extremely high flux of glycoproteins through its secretory pathway.

Sorting of lysosomal proteins

Lysosomes are composed of soluble and transmembrane proteins that are targeted to lysosomes in a signal-dependent manner. The majority of soluble acid hydrolases are modified with mannose 6-phosphate (M6P) residues, allowing their recognition by M6P receptors in the Golgi complex and ensuing transport to the endosomal/lysosomal system. Other soluble enzymes and non-enzymatic proteins are transported to lysosomes in an M6P-independent manner mediated by alternative receptors such as the lysosomal integral membrane protein LIMP-2 or sortilin. Sorting of cargo receptors and lysosomal transmembrane proteins requires sorting signals present in their cytosolic domains. These signals include dileucine-based motifs, DXXLL or [DE]XXXL[LI], and tyrosine-based motifs, YXXØ, which interact with components of clathrin coats such as GGAs or adaptor protein complexes. In addition, phosphorylation and lipid modifications regulate signal recognition and trafficking of lysosomal membrane proteins. The complex interaction of both luminal and cytosolic signals with recognition proteins guarantees the specific and directed transport of proteins to lysosomes.

Disrupted cell adhesion but not proliferation mediates cyst formation in polycystic liver disease

The pathogenesis of polycystic liver disease is not well understood. The putative function of the associated proteins, hepatocystin and Sec63p, do not give insight in their role in cystogenesis and their tissue-wide expression does not fit with the liver-specific phenotype of the disease. We designed this study with the specific aim to dissect whether pathways involved in polycystic kidney diseases are also implicated in polycystic liver disease. Therefore, we immunohistochemically stained cyst tissue specimen with antibodies directed against markers for apoptosis, proliferation, growth receptors, signaling and adhesion. We analyzed genotyped polycystic liver disease cyst tissue (n=21) compared with normal liver tissue (n=13). None of the cysts showed proliferation of epithelial cells. In addition, anti-apoptosis marker Bcl-2 revealed slight increase in expression, with variable increase of apoptosis marker active caspase 3. Growth factor receptors, EGFR and c-erbB-2, were overexpressed and mislocalized. We found EGFR staining in the nuclei of cyst epithelial cells regardless of mutational state of the patient. Further, in hepatocystin-mutant polycystic liver disease patients, apical membranous staining of c-erbB-2 and adhesion markers, MUC1 and CEA, was lost and the proteins appeared to be retained in cytoplasm of cyst epithelia. Finally, we found loss of adhesion molecules E-cadherin and Ep-CAM in cyst epithelium of all patients. Nevertheless, we observed normal beta-catenin expression. Our results show that polycystic liver disease cystogenesis is different from renal cystogenesis. Polycystic liver disease involves overexpression of growth factor receptors and loss of adhesion. In contrast, proliferation or deregulated apoptosis do not seem to be implicated. Moreover differential findings for PRKCSH- and SEC63-associated polycystic liver disease suggest a divergent mechanism for cystogenesis in these two groups.

Role of N-glycosylation in trafficking of apical membrane proteins in epithelia

Polarized distribution of plasma membrane transporters and receptors in epithelia is essential for vectorial functions of epithelia. This polarity is maintained by sorting of membrane proteins into apical or basolateral transport containers in the trans-Golgi network and/or endosomes followed by their delivery to the appropriate plasma membrane domains. Sorting depends on the recognition of sorting signals in proteins by specific sorting machinery. In the present review, we summarize experimental evidence for and against the hypothesis that N-glycans attached to the membrane proteins can act as apical sorting signals. Furthermore, we discuss the roles of N-glycans in the apical sorting event per se and their contribution to folding and quality control of glycoproteins in the endoplasmic reticulum or retention of glycoproteins in the plasma membrane. Finally, we review existing hypotheses on the mechanism of apical sorting and discuss the potential roles of the lectins, VIP36 and galectin-3, as putative apical sorting receptors.

Glycobiology in the cytosol: the bitter side of a sweet world

Progress in glycobiology has undergone explosive growth over the past decade with more of the researchers now realizing the importance of glycan chains in various inter- and intracellular processes. However, there is still an area of glycobiology awaiting exploration. This is especially the case for the field of "glycobiology in the cytosol" which remains rather poorly understood. Yet evidence is accumulating to demonstrate that the glycoconjugates and their recognition molecules (i.e. lectins) are often present in this subcellular compartment.

N-Butyldeoxynojirimycin is a broadly effective anti-HIV therapy significantly enhanced by targeted liposome delivery

OBJECTIVE

N-Butyldeoxynojirimycin (NB-DNJ), an inhibitor of HIV gp120 folding, was assessed as a broadly active therapy for the treatment of HIV/AIDS. Furthermore, to reduce the effective dose necessary for antiviral activity, NB-DNJ was encapsulated inside liposomes and targeted to HIV-infected cells.

METHODS

Thirty-one primary isolates of HIV (including drug-resistant isolates) were cultured in peripheral blood mononuclear cells to quantify the effect of NB-DNJ on viral infectivity. pH-sensitive liposomes capable of mediating the intracellular delivery of NB-DNJ inside peripheral blood mononuclear cells were used to increase drug efficacy.

RESULTS

NB-DNJ decreased viral infectivity with a single round of treatment by an average of 80% in HIV-1-infected and 95% in HIV-2-infected cultures. Two rounds of treatment reduced viral infectivity to below detectable levels for all isolates tested, with a calculated IC50 of 282 and 211 micromol/l for HIV-1 and HIV-2, respectively. When encapsulated inside liposomes, NB-DNJ inhibited HIV-1 with final concentrations in the nmol/l range (IC50 = 4 nmol/l), a 100 000-fold enhancement in IC50 relative to free NB-DNJ. Targeting liposomes to the gp120/gp41 complex with a CD4 molecule conjugated to the outer bilayer increased drug/liposome uptake five-fold in HIV-infected cells compared with uninfected cells. NB-DNJ CD4 liposomes demonstrated additional antiviral effects, reducing viral secretion by 81% and effectively neutralizing free viral particles to prevent further infections.

CONCLUSION

The use of targeted liposomes encapsulating NB-DNJ provides an attractive therapeutic option against all clades of HIV, including drug-resistant isolates, in an attempt to prevent disease progression to AIDS.

Regulation of protein O-glycosylation by the endoplasmic reticulum-localized molecular chaperone Cosmc

Regulatory pathways for protein glycosylation are poorly understood, but expression of branchpoint enzymes is critical. A key branchpoint enzyme is the T-synthase, which directs synthesis of the common core 1 O-glycan structure (T-antigen), the precursor structure for most mucin-type O-glycans in a wide variety of glycoproteins. Formation of active T-synthase, which resides in the Golgi apparatus, requires a unique molecular chaperone, Cosmc, encoded on Xq24. Cosmc is the only molecular chaperone known to be lost through somatic acquired mutations in cells. We show that Cosmc is an endoplasmic reticulum (ER)–localized adenosine triphosphate binding chaperone that binds directly to human T-synthase. Cosmc prevents the aggregation and ubiquitin-mediated degradation of the T-synthase. These results demonstrate that Cosmc is a molecular chaperone in the ER required for this branchpoint glycosyltransferase function and show that expression of the disease-related Tn antigen can result from deregulation or loss of Cosmc function.

N-glycan-mediated quality control in the endoplasmic reticulum is required for the expression of correctly folded delta-opioid receptors at the cell surface

A great majority of G protein-coupled receptors are modified by N-glycosylation, but the functional significance of this modification for receptor folding and intracellular transport has remained elusive. Here we studied these phenomena by mutating the two N-terminal N-glycosylation sites (Asn(18) and Asn(33)) of the human delta-opioid receptor, and expressing the mutants from the same chromosomal integration site in stably transfected inducible HEK293 cells. Both N-glycosylation sites were used, and their abolishment decreased the steady-state level of receptors at the cell surface. However, pulse-chase labeling, cell surface biotinylation, and immunofluorescence microscopy revealed that this was not because of intracellular accumulation. Instead, the non-N-glycosylated receptors were exported from the endoplasmic reticulum with enhanced kinetics. The results also revealed differences in the significance of the individual N-glycans, as the one attached to Asn(33) was found to be more important for endoplasmic reticulum retention of the receptor. The non-N-glycosylated receptors did not show gross functional impairment, but flow cytometry revealed that a fraction of them was incapable of ligand binding at the cell surface. In addition, the receptors that were devoid of N-glycans showed accelerated turnover and internalization and were targeted for lysosomal degradation. The results accentuate the importance of protein conformation-based screening before export from the endoplasmic reticulum, and demonstrate how the system is compromised when N-glycosylation is disrupted. We conclude that N-glycosylation of the delta-opioid receptor is needed to maintain the expression of fully functional and stable receptor molecules at the cell surface.

Assembly and trafficking of P2X purinergic receptors (Review)

P2X receptors are cation selective ion channels gated by the binding of extracellular ATP. Seven subtypes have been identified and they have widespread and overlapping distributions throughout the body. They form homo- and heterotrimeric complexes that differ in their functional properties and subcellular localization. They form part of larger signalling complexes, interacting with unrelated ion channels and other membrane and cytosolic proteins. Up- or down-regulation of their expression is associated with several disease states. This review aims to summarize recent work on the assembly and trafficking of this family of receptors.

N-Glycosylation of the Drosophila neural protein Chaoptin is essential for its stability, cell surface transport and adhesive activity

Glycosylation of proteins can modulate their function in a striking variety of systems, including immune responses, neuronal activities and development. The Drosophila protein, Chaoptin (Chp), is essential for the development and maintenance of photoreceptor cells. This protein is heavily glycosylated, but the possible role of this glycosylation is not well-understood. Here we show that mutations introduced into about 1/3 of 16 potential N-linked glycosylation sites within Chp impaired its cell adhesive activities when expressed in Drosophila S2 cells. Mutation of 2/3 of the glycosylation sites resulted in a marked decrease in Chp protein abundance. These results suggest that N-linked glycosylation of Chp is essential for its stability and activity.