Role of N-linked polymannose oligosaccharides in targeting glycoproteins for endoplasmic reticulum-associated degradation

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.

Nucleocytoplasmic plant lectins

During the last decade it was unambiguously shown that plants synthesize minute amounts of carbohydrate-binding proteins upon exposure to stress situations like drought, high salt, hormone treatment, pathogen attack or insect herbivory. In contrast to the 'classical' plant lectins, which are typically found in storage vacuoles or in the extracellular compartment this new class of lectins is located in the cytoplasm and the nucleus. Based on these observations the concept was developed that lectin-mediated protein-carbohydrate interactions in the cytoplasm and the nucleus play an important role in the stress physiology of the plant cell. Hitherto, six families of nucleocytoplasmic lectins have been identified. This review gives an overview of our current knowledge on the occurrence of nucleocytoplasmic plant lectins. The carbohydrate-binding properties of these lectins and potential ligands in the nucleocytoplasmic compartment are discussed in view of the physiological role of the lectins in the plant cell.

Arginine 383 is a crucial residue in ABCG2 biogenesis

ABCG2 is an ATP-binding cassette half-transporter initially identified in multidrug-resistant cancer cell lines and recently suggested to play an important role in pharmacokinetics. Here we report studies of a conserved arginine predicted to localize near the cytoplasmic side of TM1. First, we determined the effect of losing charge and bulk at this position via substitutions with glycine and alanine. The R383G mutant when transfected into HEK cells was not detectable on immunoblot or by functional assay, while the R383A mutant exhibited detectable but significantly decreased levels compared to wild-type, partial retention in the ER and altered glycosylation. Efflux of the ABCG2-substrates mitoxantrone and pheophorbide a was observed. Our experiments suggested rapid degradation of the R383A mutant by the proteasome via a kifunensine-insensitive pathway. Interestingly, overnight treatment of the R383A mutant with mitoxantrone assisted in protein maturation as evidenced by a shift to the N-glycosylated form. The R383A mutant when expressed in insect cells, though detected on the surface, had no measurable ATPase activity. In addition, substitution with the positively charged lysine resulted in significantly decreased protein expression levels in HEK cells, while retaining function. In conclusion, arginine 383 is a crucial residue for ABCG2 biogenesis, where even the most conservative mutations have a large impact.

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.

The effect of individual N-glycans on enzyme activity

In a series of investigations, N-glycosylation has proven to be a key determinant of enzyme secretion, activity, binding affinity and substrate specificity, enabling a protein to fine-tune its activity. In the majority of cases elimination of all putative N-glycosylation sites of an enzyme results in significantly reduced protein secretion levels, while removal of individual N-glycosylation sites often leads to the expression of active enzymes showing markedly reduced catalytic activity, with the decreased activity often commensurate with the number of glycosylation sites available, and the fully deglycosylated enzymes showing only minimal activity relative to their glycosylated counterparts. On the other hand, several cases have also recently emerged where deglycosylation of an enzyme results in significantly increased catalytic activity, binding affinity and altered substrate specificity, highlighting the very unique and diverse roles that individual N-glycans play in regulating enzyme function.

Plant glycosidases acting on protein-linked oligosaccharides

Glycosidases have been used as invaluable tools in glycobiology research for decades, and their role in glycoprotein maturation has been amply studied. The molecular biological coverage of this large group of enzymes has only recently reached an appreciable level. In this review, we present an overview of plant glycosidases, whose DNA/protein sequence has been identified and for which recombinant enzymes have been characterized. The physiological role in the maturation of glycoproteins is discussed as well as the biotechnological prospects arising from knowing the enzymes responsible for the removal of terminal N-acetylglucosamine residues. The current knowledge on plant fucosidases and of the first bits of information on glycosidases acting on arabinogalactan proteins is presented.

Kex2 protease converts the endoplasmic reticulum alpha1,2-mannosidase of Candida albicans into a soluble cytosolic form

Cytosolic α-mannosidases are glycosyl hydrolases that participate in the catabolism of cytosolic free N-oligosaccharides. Two soluble α-mannosidases (E-I and E-II) belonging to glycosyl hydrolases family 47 have been described in Candida albicans. We demonstrate that addition of pepstatin A during the preparation of cell homogenates enriched α-mannosidase E-I at the expense of E-II, indicating that the latter is generated by proteolysis during cell disruption. E-I corresponded to a polypeptide of 52 kDa that was associated with mannosidase activity and was recognized by an anti-α1,2-mannosidase antibody. The N-mannan core trimming properties of the purified enzyme E-I were consistent with its classification as a family 47 α1,2-mannosidase. Differential density-gradient centrifugation of homogenates revealed that α1,2-mannosidase E-I was localized to the cytosolic fraction and Golgi-derived vesicles, and that a 65 kDa membrane-bound α1,2-mannosidase was present in endoplasmic reticulum and Golgi-derived vesicles. Distribution of α-mannosidase activity in a kex2Δ null mutant or in wild-type protoplasts treated with monensin demonstrated that the membrane-bound α1,2-mannosidase is processed by Kex2 protease into E-I, recognizing an atypical cleavage site of the precursor. Analysis of cytosolic free N-oligosaccharides revealed that cytosolic α1,2-mannosidase E-I trims free Man₈GlcNAc₂ isomer B into Man₇GlcNAc₂ isomer B. This is believed to be the first report demonstrating the presence of soluble α1,2-mannosidase from the glycosyl hydrolases family 47 in a cytosolic compartment of the cell.

Changes in structural features of free N-glycan and endoglycosidase activity during tomato fruit ripening

In developing plants, free N-glycans occur ubiquitously at micromolar concentrations. Such oligosaccharides have been proposed to be signaling molecules in plant development. As a part of a study to elucidate the physiological roles of de-N-glycosylation machinery involved in fruit ripening, we analyzed changes in the amounts and structural features of free N-glycans in tomato fruits at four ripening stages. The amount of high-mannose type free N-glycans increased significantly in accordance with fruit ripening, and the relative amounts of high-molecular size N-glycans, such as Man(8-9)GlcNAc(1), became predominant. These observations suggest that the de-N-glycosylation machinery, including endo-beta-N-acetylglucosaminidase (ENGase) activity, is stimulated in the later stages of fruit ripening. But contrary to expectation, we found that total ENGase activities in the tomato fruits did not vary significantly with the ripening process, suggesting that ENGase activity must be maintained at a certain level, and that the expression of alpha-mannosidase involved in the clearance of free N-glycans decreases during tomato fruit ripening.

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.

Diversity of degradation signals in the ubiquitin-proteasome system

The ubiquitin-proteasome system degrades an enormous variety of proteins that contain specific degradation signals, or 'degrons'. Besides the degradation of regulatory proteins, almost every protein suffers from sporadic biosynthetic errors or misfolding. Such aberrant proteins can be recognized and rapidly degraded by cells. Structural and functional data on a handful of degrons allow several generalizations regarding their mechanism of action. We focus on different strategies of degron recognition by the ubiquitin system, and contrast regulatory degrons that are subject to signalling-dependent modification with those that are controlled by protein folding or assembly, as frequently occurs during protein quality control.

Sugar-assisted glycopeptide ligation with complex oligosaccharides: scope and limitations

We have previously shown sugar-assisted ligation (SAL) to be a useful method for the convergent construction of glycopeptides. However to date SAL has only been carried out on systems where the thiol auxiliary is attached to a monosaccharide. For SAL to be truly applicable to the construction of fully elaborated glycopeptides and glycoproteins, it must be possible to carry out the reaction when the thiol auxiliary is attached to more elaborate sugars, as these are frequently what are observed in nature. Here we examine the effects of glycosylation at C-3, C-4, and C-6 of the C-2 auxiliary-containing glycan. Model glycopeptides where synthesized chemoenzymatically and reacted with peptide thioesters used in our previous work. These studies reveal that SAL is sensitive to extended glycosylation on the auxiliary-containing sugar. While it is possible to carry out SAL with extended glycosylation at C-4 and C-6, the presence of glycosylation at C-3 prevents the ligation from occurring. Additionally, with glycosylation at C-4 the ligation efficiency is affected by the identity of the N-terminal AA, while the nature of the C-terminal residue of the peptide thioester does not appear to affect ligation efficiency. These studies provide useful guidelines in deciding when it is appropriate to use SAL in the synthesis of complex glycopeptides and glycoproteins and how to choose ligation junctions for optimal yield.

Intracellular devastation in heart failure

End-stage heart failure is characterized by a number of abnormalities at the cellular level, which include changes in excitation-contraction coupling, alterations in contractile proteins and activation/deactivation of signaling pathways. Even though many of these changes are adaptive to the high workload and stress in heart failure, a significant number of these alterations are deeply deleterious to the cardiac cell. In this article, we will review the changes in calcium cycling that occur in myopathic hearts and how they can be effectively targeted. We will also focus on protein misfolding in the setting of cardiac dysfunction.

Accumulation of free complex-type N-glycans in MKN7 and MKN45 stomach cancer cells

During the N-glycosylation reaction, it has been shown that ‘free’ N-glycans are generated either from lipid-linked oligosaccharides or from misfolded glycoproteins. In both cases, occurrence of high mannose-type free glycans is well-documented, and the molecular mechanism for their catabolism in the cytosol has been studied. On the other hand, little, if anything, is known with regard to the accumulation of more processed, complex-type free oligosaccharides in the cytosol of mammalian cells. During the course of comprehensive analysis of N-glycans in cancer cell membrane fractions [Naka et al. (2006) J. Proteome Res. 5, 88–97], we found that a significant amount of unusual, complex-type free N-glycans were accumulated in the stomach cancer-derived cell lines, MKN7 and MKN45. The most abundant and characteristic glycan found in these cells was determined to be NeuAcα2-6Galβ1-4GlcNAcβ1-2Manα1-3Manβ1-4GlcNAc. Biochemical analyses indicated that those glycans found were cytosolic glycans derived from lysosomes due to low integrity of the lysosomal membrane. Since the accumulation of these free N-glycans was specific to only two cell lines among the various cancer cell lines examined, these cytosolic N-glycans may serve as a specific biomarker for diagnosis of specific tumours. A cytosolic sialidase, Neu2, was shown to be involved in the degradation of these sialoglycans, indicating that the cytosol of mammalian cells might be equipped for metabolism of complex-type glycans.

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.

Modulation of activity by Arg407: structure of a fungal alpha-1,2-mannosidase in complex with a substrate analogue

Class I alpha-mannosidases (glycoside hydrolase family GH47) play key roles in the maturation of N-glycans and the ER-associated degradation of unfolded glycoproteins. The 1.95 A resolution structure of a fungal alpha-1,2-mannosidase in complex with the substrate analogue methyl-alpha-D-lyxopyranosyl-(1',2)-alpha-D-mannopyranoside (LM) shows the intact disaccharide spanning the -1/+1 subsites, with the D-lyxoside ring in the -1 subsite in the 1C4 chair conformation, and provides insight into the mechanism of catalysis. The absence of the C5' hydroxymethyl group on the D-lyxoside moiety results in the side chain of Arg407 adopting two alternative conformations: the minor one interacting with Asp375 and the major one interacting with both the D-lyxoside and the catalytic base Glu409, thus disrupting its function. Chemical modification of Asp375 has previously been shown to inactivate the enzyme. Taken together, the data suggest that Arg407, which belongs to the conserved sequence motif RPExxE, may act to modulate the activity of the enzyme. The proposed mechanism for modulating the activity is potentially a general mechanism for this superfamily.

The relative contribution of mannose salvage pathways to glycosylation in PMI-deficient mouse embryonic fibroblast cells

Mannose for mammalian glycan biosynthesis can be imported directly from the medium, derived from glucose or salvaged from endogenous or external glycans. All pathways must generate mannose 6-phosphate, the activated form of mannose. Imported or salvaged mannose is directly phosphorylated by hexokinase, whereas fructose 6-phosphate from glucose is converted to mannose 6-phosphate by phosphomannose isomerase (PMI). Normally, PMI provides the majority of mannose for glycan synthesis. To assess the contribution of PMI-independent pathways, we used PMI-null fibroblasts to study N-glycosylation of DNase I, a highly sensitive indicator protein. In PMI-null cells, imported mannose and salvaged mannose make a significant contribution to N-glycosylation. When these cells were grown in mannose-free medium along with the mannosidase inhibitor, swainsonine, to block the salvage pathways, N-glycosylation of DNase I was almost completely eliminated. Adding approximately 13 microm mannose to the medium completely restored normal glycosylation. Treatment with bafilomycin A(1), an inhibitor of lysosomal acidification, also markedly reduced N-glycosylation of DNase I, but in this case only 8 microm mannose was required to restore full glycosylation, indicating that a nonlysosomal source of mannose made a significant contribution. Glycosylation levels were greatly also reduced in glycoconjugate-free medium, when endosomal membrane trafficking was blocked by expression of a mutant SKD1. From these data, we conclude that PMI-null cells can salvage mannose from both endogenous and external glycoconjugates via lysosomal and nonlysosomal degradation pathways.

Two distinct pathways for cyclooxygenase-2 protein degradation

Cyclooxygenases (COX-1 and COX-2) are N-glycosylated, endoplasmic reticulum-resident, integral membrane proteins that catalyze the committed step in prostanoid synthesis. COX-1 is constitutively expressed in many types of cells, whereas COX-2 is usually expressed inducibly and transiently. The control of COX-2 protein expression occurs at several levels, and overexpression of COX-2 is associated with pathologies such as colon cancer. Here we have investigated COX-2 protein degradation and demonstrate that it can occur through two independent pathways. One pathway is initiated by post-translational N-glycosylation at Asn-594. The N-glycosyl group is then processed, and the protein is translocated to the cytoplasm, where it undergoes proteasomal degradation. We provide evidence from site-directed mutagenesis that a 27-amino acid instability motif (27-IM) regulates posttranslational N-glycosylation of Asn-594. This motif begins with Glu-586 8 residues upstream of the N-glycosylation site and ends with Lys-612 near the C terminus at Leu-618. Key elements of the 27-IM include a helix involving residues Glu-586 to Ser-596 with Asn-594 near the end of this helix and residues Leu-610 and Leu-611, which are located in an apparently unstructured downstream region of the 27-IM. The last 16 residues of the 27-IM, including Leu-610 and Leu-611, appear to promote N-glycosylation of Asn-594 perhaps by causing this residue to become exposed to appropriate glycosyl transferases. A second pathway for COX-2 protein degradation is initiated by substrate-dependent suicide inactivation. Suicide-inactivated protein is then degraded. The biochemical steps have not been resolved, but substrate-dependent degradation is not inhibited by proteasome inhibitors or inhibitors of lysosomal proteases. The pathway involving the 27-IM occurs at a constant rate, whereas degradation through the substrate-dependent process is coupled to the rate of substrate turnover.

Glucosylated free oligosaccharides are biomarkers of endoplasmic- reticulum α-glucosidase inhibition

The inhibition of ER (endoplasmic reticulum) α-glucosidases I and II by imino sugars, including NB-DNJ (N-butyl-deoxynojirimycin), causes the retention of glucose residues on N-linked oligosaccharides. Therefore, normal glycoprotein trafficking and processing through the glycosylation pathway is abrogated and glycoproteins are directed to undergo ERAD (ER-associated degradation), a consequence of which is the production of cytosolic FOS (free oligosaccharides). Following treatment with NB-DNJ, FOS were extracted from cells, murine tissues and human plasma and urine. Improved protocols for analysis were developed using ion-exchange chromatography followed by fluorescent labelling with 2-AA (2-aminobenzoic acid) and purification by lectin-affinity chromatography. Separation of 2-AA-labelled FOS by HPLC provided a rapid and sensitive method that enabled the detection of all FOS species resulting from the degradation of glycoproteins exported from the ER. The generation of oligosaccharides derived from glucosylated protein degradation was rapid, reversible, and time- and inhibitor concentration-dependent in cultured cells and in vivo. Long-term inhibition in cultured cells and in vivo indicated a slow rate of clearance of glucosylated FOS. In mouse and human urine, glucosylated FOS were detected as a result of transrenal excretion and provide unique and quantifiable biomarkers of ER-glucosidase inhibition.

Chemical synthesis of a glycoprotein having an intact human complex-type sialyloligosaccharide under the Boc and Fmoc synthetic strategies

The chemical synthesis of complex glycoproteins is an ongoing challenge in protein chemistry. We have examined the synthesis of a single glycoform of monocyte chemotactic protein-3 (MCP-3), a CC-chemokine that consists of 76 amino acids and one N-glycosylation site. A three-segment native chemical ligation strategy was employed using unprotected peptides and glycopeptide. Importantly, the synthesis required the development of methods for the generation of sialylglycopeptide-alphathioesters. For the sialylglycopeptide-alphathioester segment, we examined and successfully implemented approaches using Fmoc-SPPS and Boc-SPPS. To avoid use of hydrogen fluoride, the Boc approach utilized minimal side chain protection and direct thiolysis of the resin bound peptide. Using these strategies, we successfully synthesized a glycoprotein having an intact and homogeneous complex-type sialyloligosaccharide.

The sugar-binding ability of ERGIC-53 is enhanced by its interaction with MCFD2

Combined deficiency of factors V and VIII (F5F8D) is a bleeding disorder caused by mutations in LMAN1 or MCFD2. LMAN1 encodes ERGIC-53, a cargo receptor with an L-type lectin domain, and MCFD2 is a EF-hand-containing protein. We prepared a biotinylated, soluble form of ERGIC-53, which we labeled with R-phycoerythrin conjugated streptavidin. By flow cytometry, sERGIC-53-SA bound to HeLaS3 cells in the presence of calcium but only after preincubation with MCFD2. Treating the cells with endo H or incubating them with high mannose-type oligosaccharides, especially M(8B), abrogated sERGIC-53-SA binding. Surface plasmon resonance experiments demonstrated that MCFD2 specifically bound to sERGIC-53 and 2 MCFD2 mutants found in F5F8D patients had a K(a) that was 3 or 4 orders of magnitude lower for sERGIC-53 than for wild-type MCFD2. The K(a) of sERGIC-53 and MCFD2 was measured at several pH values and calcium concentrations, and we found that at a calcium concentration less than 0.2 mM, this interaction became significantly weaker. These results demonstrate that the binding of ERGIC-53 to sugar is enhanced by its interaction with MCFD2, and defects in this interaction in F5F8D patients may be the cause for reduced secretion of factors V and VIII.

Molecular basis of sugar recognition by the human L-type lectins ERGIC-53, VIPL, and VIP36

ERGIC-53, VIPL, and VIP36 are related type 1 membrane proteins of the mammalian early secretory pathway. They are classified as L-type lectins because of their luminal carbohydrate recognition domain, which exhibits homology to leguminous lectins. These L-type lectins have different intracellular distributions and dynamics in the endoplasmic reticulum-Golgi system of the secretory pathway and interact with N-glycans of glycoproteins in a Ca(2+)-dependent manner, suggesting a role in glycoprotein sorting and trafficking. To understand the function of these lectins, knowledge of their carbohydrate specificity is crucial but only available for VIP36 (Kamiya, Y., Yamaguchi, Y., Takahashi, N., Arata, Y., Kasai, K. I., Ihara, Y., Matsuo, I., Ito, Y., Yamamoto, K., and Kato, K. (2005) J. Biol. Chem. 280, 37178-37182). Here we provide a comprehensive and quantitative analysis of sugar recognition of the carbohydrate recognition domains of ERGIC-53 and VIPL in comparison with VIP36 using a pyridylaminated sugar library in conjunction with frontal affinity chromatography. Frontal affinity chromatography revealed selective interaction of VIPL and VIP36 with the deglucosylated trimannose in the D1 branch of high-mannose-type oligosaccharides but with different pH dependence. ERGIC-53 bound high-mannose-type oligosaccharides with low affinity and broad specificity, not discriminating between monoglucosylated and deglucosylated high-mannosetype oligosaccharides. Based on the sugar-binding properties in conjunction with known features of these proteins, we propose a model for the action of the three lectins in glycoprotein guidance and trafficking. Moreover, structure-based mutagenesis revealed that the sugar-binding properties of these L-type lectins can be switched by single amino acid substitutions.

Free N-linked oligosaccharide chains: formation and degradation

There is growing evidence that N-linked glycans play pivotal roles in protein folding and intra- and/or intercellular trafficking of N-glycosylated proteins. It has been shown that during the N-glycosylation of proteins, significant amounts of free oligosaccharides (free OSs) are generated in the lumen of the endoplasmic reticulum (ER) by a mechanism which remains to be clarified. Free OSs are also formed in the cytosol by enzymatic deglycosylation of misfolded glycoproteins, which are subjected to destruction by a cellular system called "ER-associated degradation (ERAD)." While the precise functions of free OSs remain obscure, biochemical studies have revealed that a novel cellular process enables them to be catabolized in a specialized manner, that involves pumping free OSs in the lumen of the ER into the cytosol where further processing occurs. This process is followed by entry into the lysosomes. In this review we summarize current knowledge about the formation, processing and degradation of free OSs in eukaryotes and also discuss the potential biological significance of this pathway. Other evidence for the occurrence of free OSs in various cellular processes is also presented.

Bodipy-VAD-Fmk, a useful tool to study yeast peptide N-glycanase activity

In this paper the development of a fluorescent activity based probe, Bodipy-VAD-Fmk, for visualization of yeast peptide N-glycanase is described. The activity based probe is used to assess the efficacy of known and new chitobiose-based electrophilic traps to bind yeast peptide N-glycanase.

Stimulation of ERAD of misfolded null Hong Kong alpha1-antitrypsin by Golgi alpha1,2-mannosidases

Terminally misfolded or unassembled proteins are degraded by the cytoplasmic ubiquitin-proteasome pathway in a process known as ERAD (endoplasmic reticulum-associated protein degradation). Overexpression of ER alpha1,2-mannosidase I and EDEMs target misfolded glycoproteins for ERAD, most likely due to trimming of N-glycans. Here we demonstrate that overexpression of Golgi alpha1,2-mannosidase IA, IB, and IC also accelerates ERAD of terminally misfolded human alpha1-antitrypsin variant null (Hong Kong) (NHK), and mannose trimming from the N-glycans on NHK in 293 cells. Although transfected NHK is primarily localized in the ER, some NHK also co-localizes with Golgi markers, suggesting that mannose trimming by Golgi alpha1,2-mannosidases can also contribute to NHK degradation.

Analysis of ER-associated glycoprotein degradation using synthetic glycopeptide probes

Quality control of proteins is an essential process for maintaining normal cell activity. It ensures that only correctly folded proteins are produced and terminally misfolded proteins are eliminated by degradation. ER-associated degradation (ERAD) of misfolded proteins is an important aspect of protein quality control system. Recent studies have revealed that glycoprotein glycans play significant roles in this process. It includes polyubiquitination, deglycosylation, and proteasomal degradation. In the present study, a systematic analysis of these steps was carried out using chemically synthesized glycopeptides. We revealed that N-linked glycopeptides are degraded by 20S proteasome, but with drastically reduced rate compared to non-glycosylated peptide. This result strongly suggests that deglycosylating activity of peptide:N-glycanase (PNGase) is important for the facile degradation of glycoproteins. Our study showed, for the first time, that PNGase cleaves truncated glycans as short as chitobiose from peptide. However, this cleavage required the presence of hydrophobic region nearby N-glycosylation site. Furthermore, analysis of interactions with F-box protein Fbs1 was conducted with fluorescent correlation spectroscopy (FCS). It was shown that the presence of Fbs1 perturb the activity of PNGase toward high-mannose-type glycopeptides.

Gaucher disease

Although Gaucher disease is a rare disorder, recent developments in novel means for therapeutic intervention have invigorated both academic research and pharmaceutical industry discovery programmes. The common mutations found in the lysosomal enzyme deficient in Gaucher disease, beta-glucocerebrosidase, earmark these proteins for destruction by the endoplasmic reticulum-localised protein folding machinery, resulting in enzyme insufficiency, lysosomal glycolipid storage and subsequent pathology. However, many of these mutants can be rescued from global misfolding to preserve glycolipid substrate binding and eventual catalysis in the lysosome, by the addition of subinhibitory concentrations of pharmacologically active small molecules. This novel, chaperon-mediated approach has benefited from insights into the molecular understanding of beta-glucocerebrosidase structure, drug design and development in cellular models for disease.

Fluorescently labeled inhibitor for profiling cytoplasmic peptide:N-glycanase

Cytoplasmic peptide:N-glycanase (PNGase) is an enzyme that removes N-glycans from misfolded glycoproteins. The function of cytoplasmic PNGase plays a significant role in the degradation of misfolded glycoproteins, which is critical for cell viability. Recently, we reported that haloacetoamidyl derivatives of high-mannose-type oligosaccharides selectively modify the catalytic cysteine of cytoplasmic PNGase and serve as its specific inhibitor. Interestingly, a drastically simplified chloroacetamidyl chitobiose derivative [(GlcNAc)(2)-ClAc] was also reactive to PNGase. In our work, it was conjugated to a hydrophobic fluorophore in order to render (GlcNAc)(2)-ClAc cells permeable. We demonstrated that this compound [BODIPY-(GlcNAc)(2)-ClAc] specifically binds to cytoplasmic PNGase from budding yeast (Png1). To date, only Z-VAD-fmk is known as an inhibitor of PNGase. BODIPY-(GlcNAc)(2)-ClAc and Z-VAD-fmk share the same binding site on Png1, while BODIPY-(GlcNAc)(2)-ClAc has markedly stronger inhibitory activity. The functional analysis of PNGase using Z-VAD-fmk should be carefully interpreted because of its intrinsic property as a caspase inhibitor. In sharp contrast, chloroacetamidyl chitobiose was not reactive to caspase. In addition, BODIPY-(GlcNAc)(2)-ClAc did not bind either chitobiose-binding lectins or PNGase from other sources. Moreover, fluorescent microscopy clearly showed that BODIPY-(GlcNAc)(2)-ClAc was efficiently introduced into cells. These results suggest that this compound could be an in vivo inhibitor of cytoplasmic PNGase.

Characterization and subcellular localization of human neutral class IIα-mannosidase cytosolic enzymes/free oligosaccharides/glycosidehydrolase family 38/M2C1/N-glycosylation

A glycosyl hydrolase family 38 enzyme, neutral alpha-mannosidase, has been proposed to be involved in hydrolysis of cytosolic free oligosaccharides originating either from ER-misfolded glycoproteins or the N-glycosylation process. Although this enzyme has been isolated from the cytosol, it has also been linked to the ER by subcellular fractionations. We have studied the subcellular localization of neutral alpha-mannosidase by immunofluorescence microscopy and characterized the human recombinant enzyme with natural substrates to elucidate the biological function of this enzyme. Immunofluorescence microscopy showed neutral alpha-mannosidase to be absent from the ER, lysosomes, and autophagosomes, and being granularly distributed in the cytosol. In experiments with fluorescent recovery after photo bleaching, neutral alpha-mannosidase had slower than expected two-phased diffusion in the cytosol. This result together with the granular appearance in immunostaining suggests that portion of the neutral alpha-mannosidase pool is somehow complexed. The purified recombinant enzyme is a tetramer and has a neutral pH optimum for activity. It hydrolyzed Man(9)GlcNAc to Man(5)GlcNAc in the presence of Fe(2+), Co(2+), and Mn(2+), and uniquely to neutral alpha-mannosidases from other organisms, the human enzyme was more activated by Fe(2+) than Co(2+). Without activating cations the main reaction product was Man(8)GlcNAc, and Cu(2+) completely inhibited neutral alpha-mannosidase. Our findings from enzyme-substrate characterizations and subcellular localization studies support the suggested role for neutral alpha-mannosidase in hydrolysis of soluble cytosolic oligomannosides.

Free oligosaccharides in the cytosol of Caenorhabditis elegans are generated through endoplasmic reticulum-golgi trafficking

Free oligosaccharides (FOSs) in the cytosol of eukaryotic cells are mainly generated during endoplasmic reticulum (ER)-associated degradation (ERAD) of misfolded glycoproteins. We analyzed FOS of the nematode Caenorhabditis elegans to elucidate its detailed degradation pathway. The major FOSs were high mannose-type ones bearing 3-9 Man residues. About 94% of the total FOSs had one GlcNAc at their reducing end (FOS-GN1), and the remaining 6% had two GlcNAc (FOS-GN2). A cytosolic endo-beta-N-acetylglucosaminidase mutant (tm1208) accumulated FOS-GN2, indicating involvement of the enzyme in conversion of FOS-GN2 into FOS-GN1. The most abundant FOS in the wild type was Man(5)GlcNAc(1), the M5A' isomer (Manalpha1-3(Manalpha1-6)Manalpha1-6(Manalpha1-3)Manbeta1-4GlcNAc), which is different from the corresponding M5B' (Manalpha1-2Manalpha1-2Manalpha1-3(Manalpha1-6)Manbeta1-4GlcNAc) in mammals. Analyses of FOS in worms treated with Golgi alpha-mannosidase I inhibitors revealed decreases in Man(5)GlcNAc(1) and increases in Man(7)GlcNAc(1). These results suggested that Golgi alpha-mannosidase I-like enzyme is involved in the production of Man(5-6)-GlcNAc(1), which is unlike in mammals, in which cytosolic alpha-mannosidase is involved. Thus, we assumed that major FOSs in C. elegans were generated through Golgi trafficking. Analysis of FOSs from a Golgi alpha-mannosidase II mutant (tm1078) supported this idea, because GlcNAc(1)Man(5)GlcNAc(1), which is formed by the Golgi-resident GlcNAc-transferase I, was found as a FOS in the mutant. We concluded that significant amounts of misfolded glycoproteins in C. elegans are trafficked to the Golgi and are directly or indirectly retro-translocated into the cytosol to be degraded.

Localization and in vitro binding studies suggest that the cytoplasmic/nuclear tobacco lectin can interact in situ with high-mannose and complex N-glycans

The possible in vivo interaction of the Nicotiana tabacum agglutinin (Nictaba) with endogenous glycoproteins was corroborated using a combination of confocal/electron microscopy of an EGFP-Nictaba fusion protein expressed in tobacco Bright Yellow-2 (BY-2) cells and biochemical analyses. In vitro binding studies demonstrated that the expressed EGFP-Nictaba possesses carbohydrate-binding activity. Microscopic analyses confirmed the previously reported cytoplasmic/nuclear location of Nictaba in jasmonate-treated tobacco leaves and provided evidence for the involvement of a nuclear localization signal-dependent transport mechanism. In addition, it became evident that the lectin is not uniformly distributed over the nucleus and the cytoplasm of BY-2 cells. Far Western blot analysis of extracts from whole BY-2 cells and purified nuclei revealed that Nictaba interacts in a glycan inhibitable way with numerous proteins including many nuclear proteins. Enzymatic deglycosylation with PNGase F indicated that the observed interaction depends on the presence of N-glycans. Glycan array screening, which showed that Nictaba exhibits a strong affinity for high-mannose and complex N-glycans, provided a reasonable explanation for this observation. The cytoplasmic/nuclear localization of a plant lectin that has a high affinity for high-mannose and complex N-glycans and specifically interacts with conspecific glycoproteins suggests that N-glycosylated proteins might be more important in the cytoplasm and nucleus than is currently believed.

PGY repeats and N-glycans govern the trafficking of paranodin and its selective association with contactin and neurofascin-155

Formation of nodes of Ranvier requires contact of axons with myelinating glial cells, generating specialized axo-glial subdomains. Caspr/paranodin is required for the formation of septate-like junctions at paranodes, whereas the related caspr2 is essential for the organization of juxtaparanodes. The molecular mechanisms underlying the segregation of these related glycoproteins within distinct complexes are poorly understood. Exit of paranodin from the endoplasmic reticulum (ER) is mediated by its interaction with F3/contactin. Using domain swapping with caspr2, we mapped a motif with Pro-Gly-Tyr repeats (PGY) in the ectodomain of paranodin responsible for its ER retention. Deletion of PGY allows cell surface delivery of paranodin bypassing the calnexin-calreticulin quality control. Conversely, insertion of PGY in caspr2 or NrCAM blocks these proteins in the ER. PGY is a novel type of processing signal that compels chaperoning of paranodin by contactin. Contactin associated with paranodin is expressed at the cell surface with high-mannose N-glycans. Using mutant CHO lines altered in the processing of N-linked carbohydrates, we show that the high-mannose glycoform of contactin strongly binds neurofascin-155, its glial partner at paranodes. Thus, the unconventional processing of paranodin and contactin may determine the selective association of axo-glial complexes at paranodes.

Protein Glycosylation, Conserved from Yeast to Man: A Model Organism Helps Elucidate Congenital Human Diseases

Proteins can be modified by a large variety of covalently linked saccharides. The present review concentrates on two types, protein N-glycosylation and protein O-mannosylation, which, with only a few exceptions, are evolutionary conserved from yeast to man. They are also distinguished by some special features: The corresponding glycosylation processes start in the endoplasmatic reticulum, are continued in the Golgi apparatus, and require dolichol-activated precursors for the initial biosynthetic steps. With respect to the molecular biology of both types of protein glycosylation, the pathways and the genetic background of the reactions have most successfully been studied with the genetically easy-to-handle baker's yeast, Saccharomyces cerevisae. Many of the severe developmental disturbances in children are related to protein glycosylation, for example, the CDG syndrome (congenital disorders of glycosylation) as well as congenital muscular dystrophies with neuronal-cell-migration defects have been elucidated with the help of yeast.

The 19-amino acid cassette of cyclooxygenase-2 mediates entry of the protein into the endoplasmic reticulum-associated degradation system

Cyclooxygenase (COX) isoforms catalyze the committed step in prostaglandin biosynthesis. The primary structures of COX-1 and COX-2 are very similar except that COX-2 has a 19-amino acid (19-AA) segment of unknown function located just inside its C terminus. Here we provide evidence that the major role of the 19-AA cassette is to mediate entry of COX-2 into the ER-associated degradation system that transports ER proteins to the cytoplasm. COX-1 is constitutively expressed in many cells, whereas COX-2 is usually expressed inducibly and transiently. In murine NIH/3T3 fibroblasts, we find that COX-2 protein is degraded with a half-life (t(1/2)) of about 2 h, whereas COX-1 is reasonably stable (t(1/2) > 12 h); COX-2 degradation is retarded by 26 S proteasome inhibitors. Similarly, COX-1 expressed heterologously in HEK293 cells is quite stable (t(1/2) > 24 h), whereas COX-2 expressed heterologously is degraded with a t(1/2) of approximately 5 h, and its degradation is slowed by proteasome inhibitors. A deletion mutant of COX-2 was prepared lacking 18 residues of the 19-AA cassette. This mutant retains native COX-2 activity but, unlike native COX-2, is stable in HEK293 cells. Conversely, inserting the COX-2 19-AA cassette near the C terminus of COX-1 yields a mutant ins594-612 COX-1 that is unstable (t(1/2) approximately 3 h). Mutation of Asn-594, an N-glycosylation site at the beginning of the 19-AA cassette, stabilizes both COX-2 and ins594-612 COX-1; nonetheless, COX mutants that are glycosylated at Asn-594 but lack the remainder of the 19-amino acid cassette (i.e. del597-612 COX-2 and ins594-596 COX-1) are stable. Thus, although glycosylation of Asn-594 is necessary for COX-2 degradation, at least part of the remainder of the 19-AA insert is also required. Finally, kifunensine, a mannosidase inhibitor that can block entry of ER proteins into the ER-associated degradation system, retards COX-2 degradation.

Secretory expression of synthetic human Fas ligand extracellular domain gene in Pichia pastoris: influences of tag addition and N-glycosylation site deletion, and development of a purification method

Human Fas ligand is a medically important membrane glycoprotein that induces the apoptosis of harmful cells. A new secretory expression and purification method was devised for the production of a large amount of recombinant human Fas ligand extracellular domain (hFasLECD) by Pichia pastoris. The expression plasmid containing a synthetic hFasLECD gene designed using yeast optimal codons was constructed for the secretion of hFasLECD. The secreted product exhibited the specific binding activity toward soluble human Fas receptor extracellular domain-human IgG(1)-Fc domain fusion protein, and the receptor-ligand complex was immunoprecipitated by Protein A conjugated agarose-gel beads. The influences of the N- and C- terminal addition of FLAG/(His)(6) tag spaced by pentaglycine sequence and the sequentially accumulative deletions of N-glycosylation sites within hFasLECD were investigated. The secretion of functional hFasLECD was retained after the N-terminal tagging and the deletion of either single or double N-glycosylation sites. As judged from SDS-PAGE analysis of the culture supernatant, the N-terminal addition of FLAG-(Gly)(5) tag and the deletion of single N-glycosylation site via N184Q mutation increased the secretion level of the product. In contrast, the C-terminal tagged genes and all N-glycosylation sites deleted gene failed to direct the secretion of functional hFasLECD. The secreted products in the culture medium were purified using a cation-exchange chromatography and a gel-filtration chromatography. The purified hFasLECDs existed as trimers composed of a mixture of monomer species in different glycosylation states. Approximately five milligram of functional N-terminal FLAG-(Gly)(5) tagged hFasLECD N184Q mutant was obtained from one liter culture supernatant.

N-glycans are involved in the response of Caenorhabditis elegans to bacterial pathogens

Caenorhabditis elegans is becoming a popular tool for the study of glycan function particularly as it applies to development. More than 150 C. elegans genes have been identified as homologs of vertebrate genes involved in glycan metabolism. However, only a relatively small number of these genes have been expressed and studied in any detail. Oligomannose N-glycans (Man5-9GlcNAc2Asn), major components of the N-glycans of all eukaryotes including C. elegans, are essential, at least in part, for eukaryote survival, because they play an important role in protein quality control. In addition, vertebrates make hybrid (GlcNAcMan3-5GlcNAc2Asn) and complex (XGlcNAc2-6Man3GlcNAc2Asn) but little or no paucimannose (Man3-4GlcNAc2Asn)N-glycans, whereas plants, insects, and C. elegans make paucimannose but little or no hybrid nor complex N-glycans. UDP-GlcNAc:alpha3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (encoded by the gene Mgat1) controls the synthesis of hybrid, complex, and paucimannose N-glycans in all eukaryotes. C. elegans has three genes encoding beta1,2-N-acetylglucosaminyltransferase I (gly-12, gly-13, gly-14). To determine the functional requirement for this enzyme in worms, we generated seven worm strains with mutations in these three genes (gly-12, dpy-6 gly-13, gly-14, gly-12 gly-13, gly-14;gly-12, gly-14;dpy-6 gly-13 and gly-14;gly-12 gly-13). Whereas mice and Drosophila melanogaster with null mutations in Mgat1 suffer severe developmental abnormalities, all seven C. elegans strains with null mutations in the genes encoding beta1,2-N-acetylglucosaminyltransferase I develop normally and seem to have a wild-type phenotype. We now present evidence that beta1,2-N-acetylglucosaminyltransferase I-dependent N-glycans (consisting mainly of paucimannose N-glycans) play a role in the interaction of C. elegans with pathogenic bacteria, suggesting that these N-glycans are components of the worm's innate immune system.

Structural diversity of cytosolic free oligosaccharides in the human hepatoma cell line, HepG2

It is thought that free oligosaccharides in the cytosol are an outcome of quality control of glycoproteins by endoplasmic reticulum-associated degradation (ERAD). Although considerable amounts of free oligosaccharides accumulate in the cytosol, where they presumably have some function, detailed analyses of their structures have not yet been carried out. We isolated 21 oligosaccharides from the cytosolic fraction of HepG2 cells and analyzed their structures by the two-dimensional high-performance liquid chromatography (HPLC) sugar-mapping method. Sixteen novel oligosaccharides were identified in the cytosol in this study. All had a single N-acetylglucosamine at their reducing-end cores and could be expressed as (Man)n (GlcNAc)1. No free oligosaccharide with N,N'-diacetylchitobiose was detected in the cytosolic fraction of HepG2 cells. This suggested that endo-beta-N-acetylglucosaminidase was a key enzyme in the production of cytosolic free oligosaccharides. The 21 oligosaccharides were classified into three series--series 1: oligosaccharides processed from Manalpha1-2Manalpha1-6 (Manalpha1-2Manalpha1-3)Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3) Manbeta1-4GlcNAc (M9A') and Manalpha1-2Manalpha1-6(Manalpha1-3) Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3)Manbeta1-4GlcNAc (M8A') by digestion with cytosolic alpha-mannosidase; series 2: oligosaccharides processed with Golgi alpha-mannosidases in addition to endoplasmic reticulum (ER) and cytosolic alpha-mannosidases; and series 3: glucosylated oligosaccharides produced from Glc1Man9GlcNAc1 by hydrolysis with cytosolic alpha-mannosidase. The presence of the series "2" oligosaccharides suggests that some of the misfolded glycoproteins had been processed in pre-cis-Golgi vesicles and/or the Golgi apparatus. When the cells were treated with swainsonine to inhibit cytosolic alpha-mannosidase, the amounts of M9A' and M8A' increased remarkably, suggesting that these oligosaccharides were translocated into the cytosol. Four oligosaccharides of series "2" also increased. In contrast, there were obvious reductions in Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3)Manbeta1-4GlcNAc (M5B'), the end product from M9A' by digestion with cytosolic alpha-mannosidase, and Manalpha1-6(Manalpha1- 2Manalpha1-3)Manbeta1-4GlcNAc, derived from series "2" oligosaccharides by digestion with cytosolic alpha-mannosidase. Our data suggest that (1) some of the cytosolic oligosaccharides had been processed with Golgi alpha-mannosidases, (2) the major oligosaccharides translocated from the ER were M9A' and M8A', and (3) M5B' and Glc1M5B' were maintained at relatively high concentrations in the cytosol.

Identification of a protein from rust fungi transferred from haustoria into infected plant cells

The formation of haustoria is one of the hallmarks of the interaction of obligate biotrophic fungi with their host plants. In addition to their role in nutrient uptake, it is hypothesized that haustoria are actively involved in establishing and maintaining the biotrophic relationship. We have identified a 24.3-kDa protein that exhibited a very unusual allocation. Rust transferred protein 1 from Uromyces fabae (Uf-RTP1p) was not only detected in the host parasite interface, the extrahaustorial matrix, but also inside infected plant cells by immunofluorescence and electron microscopy. Uf-RTP1p does not exhibit any similarity to sequences currently listed in the public databases. However, we identified a homolog of Uf-RTP1p in the related rust fungus Uromyces striatus (Us-RTP1p). The localization of Uf-RTP1p and Us-RTP1p inside infected plant cells was confirmed, using four independently raised polyclonal antibodies. Depending on the developmental stage of haustoria, Uf-RTP1p was found in increasing amounts in host cells, including the host nucleus. Putative nuclear localization signals (NLS) were found in the predicted RTP1p sequences. However, functional efficiency could only be verified for the Uf-RTP1p NLS by means of green fluorescent protein fusions in transformed tobacco protoplasts. Western blot analysis indicated that Uf-RTP1p and Us-RTP1p most likely enter the host cell as N-glycosylated proteins. However, the mechanism by which they cross the extrahaustorial membrane and accumulate in the host cytoplasm is unknown. The localization of RTP1p suggests that it might play an important role in the maintenance of the biotrophic interaction.

Sugar-binding Properties of VIP36, an Intracellular Animal Lectin Operating as a Cargo Receptor

The vesicular integral protein of 36 kDa (VIP36) is an intracellular animal lectin that acts as a putative cargo receptor, which recycles between the Golgi and the endoplasmic reticulum. Although it is known that VIP36 interacts with glycoproteins carrying high mannose-type oligosaccharides, detailed analyses of the sugar-binding specificity that discriminates isomeric oligosaccharide structures have not yet been performed. In the present study, we have analyzed, using the frontal affinity chromatography (FAC) method, the sugar-binding properties of a recombinant carbohydrate recognition domain of VIP36 (VIP36-CRD). For this purpose, a pyridylaminated sugar library, consisting of 21 kinds of oligosaccharides, including isomeric structures, was prepared and subjected to FAC analyses. The FAC data have shown that glucosylation and trimming of the D1 mannosyl branch interfere with the binding of VIP36-CRD. VIP36-CRD exhibits a bell-shaped pH dependence of sugar binding with an optimal pH value of approximately 6.5. By inspection of the specificity and optimal pH value of the sugar binding of VIP36 and its subcellular localization, together with the organellar pH, we suggest that VIP36 binds glycoproteins that retain the intact D1 mannosyl branch in the cis-Golgi network and recycles to the endoplasmic reticulum where, due to higher pH, it releases its cargos, thereby contributing to the quality control of glycoproteins.

Structural approaches to the study of oligosaccharides in glycoprotein quality control

High-mannose-type oligosaccharides have been shown to play important roles in protein quality control. Several intracellular proteins, such as lectins, chaperones and glycan-processing enzymes, are involved in this process. These include calnexin/calreticulin, UDP-glucose:glycoprotein glucosyltransferase (UGGT), cargo receptors (such as VIP36 and ERGIC-53), mannosidase-like proteins (e.g. EDEM and Htm1p) and ubiquitin ligase (Fbs). They are thought to recognize high-mannose-type glycans with subtly different structures, although the precise specificities are yet to be clarified. In order to gain a clear understanding of these protein-carbohydrate interactions, comprehensive synthesis of high-mannose-type glycans was conducted. In addition, two approaches to the synthesis of artificial glycoproteins with homogeneous oligosaccharides were investigated. Furthermore, a novel substrate of UGGT was discovered.