Characterization of Schizosaccharomyces pombe ER alpha-mannosidase: a reevaluation of the role of the enzyme on ER-associated degradation

Engineering the endoplasmic reticulum secretory pathway in Trichoderma reesei for improved cellulase production

The filamentous fungus Trichoderma reesei is an extraordinarily efficient cell factory of industrial cellulase for production of biofuels and other bio-based products because of its excellent potential to secrete cellulolytic enzymes. Engineering the protein secretory pathway may be a powerful means for efficient protein production. However, it is uncertain whether this engineering approach could improve cellulase production in T. reesei. Herein, the endoplasmic reticulum (ER) secretory pathway was engineered for the production of cellulolytic enzymes by multiple strategies, including: (I) overexpression of the key components of protein folding (Pdi1, Ero1 and BiP); (II) overexpression of the glycosylation-related elements (Gpt1 and Gls2); (III) knockout of the ER mannosidase I (Mns1) encoding gene mns1. By utilizing these ER engineering strategies, the secretion of β-glucosidase was remarkably elevated in the engineered strains, ranging from 29.2 % to 112.5 %. Furthermore, it was found that engineering these components also regulated the ER stress resistance. More importantly, the total cellulase production was increased with varying degrees, which reached a maximum of 149.4 %, using the filter paper assay (FPA) as a characterization method. These results demonstrated that engineering the ER secretory pathway can enhance protein secretion, particularly for cellulase production, which shed light for the development of high-efficient cellulolytic enzymes for economically feasible bioethanol production from lignocellulosic biomass.

Substrate specificities of α1,2- and α1,3-galactosyltransferases and characterization of Gmh1p and Otg1p in Schizosaccharomyces pombe

In the fission yeast Schizosaccharomyces pombe, α1,2- and α1,3-linked D-galactose (Gal) residues are transferred to N- and O-linked oligosaccharides of glycoproteins by galactosyltransferases. Although the galactomannans are important for cell-cell communication in S. pombe (e.g., in non-sexual aggregation), the mechanisms underlying galactosylation in cells remain unclear. S. pombe has 10 galactosyltransferase-related genes: seven belonging to glycosyltransferase (GT) family 34 and three belonging GT family 8. Disruption of all 10 α-galactosyltransferases (strain Δ10GalT) has been shown to result in a complete lack of α-Gal residues. Here, we have investigated the function and substrate specificities of galactosyltransferases in S pombe by using strains expressing single α-galactosyltransferases in the Δ10GalT background. High-performance liquid chromatography (HPLC) analysis of pyridylaminated O-linked oligosaccharides showed that two GT family 34 α1,2-galactosyltransferases (Gma12p and Gmh6p) and two GT family 8 α1,3-galactosyltransferases (Otg2p and Otg3p) are involved in galactosylation of O-linked oligosaccharide. Moreover, 1H-NMR of N-glycans revealed that three GT family 34 α1,2-galactosyltransferases (Gmh1p, Gmh2p, and Gmh3p) are required for galactosylation of N-linked oligosaccharides. Furthermore, HPLC and lectin-blot analysis revealed that Otg1p showed α1,3-galactosyltransferase activity under conditions of co-expression with Gmh6p, indicating that α-1,2-linked galactose is required for the galactosylation activity of Otg1p in S. pombe. In conclusion, eight galactosyltransferases have been shown to have activity in S. pombe with different substrate specificities. These findings will be useful for genetically tailoring the galactosylation of both N- and O- glycans in fission yeast.

In Vitro Mannosidase Assay of EDEMs: ER Degradation-Enhancing α-Mannosidase-Like Proteins

Quality control of newly synthesized glycoproteins is tightly regulated by sugar processing of N-glycans and by recognition of specific glycan structures by lectins in the endoplasmic reticulum (ER). Mannose trimming and its recognition determine the targeting of misfolded glycoproteins for ER-associated degradation. ER degradation-enhancing α-mannosidase-like (EDEM) proteins in mammals and their homologue Htm1p/Mnl1p in Saccharomyces cerevisiae are involved in this process. To analyze the function of EDEM proteins, we expressed and purified recombinant EDEM3 from HEK293 cells and assessed its mannose-trimming activity in vitro.

Folding and Quality Control of Glycoproteins

Folding of proteins is essential so that they can exert their functions. For proteins that transit the secretory pathway, folding occurs in the endoplasmic reticulum (ER) and various chaperone systems assist in acquiring their correct folding/subunit formation. N-glycosylation is one of the most conserved posttranslational modification for proteins, and in eukaryotes it occurs in the ER. Consequently, eukaryotic cells have developed various systems that utilize N-glycans to dictate and assist protein folding, or if they consistently fail to fold properly, to destroy proteins for quality control and the maintenance of homeostasis of proteins in the ER.

Misfolding of Lysosomal α-Galactosidase a in a Fly Model and Its Alleviation by the Pharmacological Chaperone Migalastat

Fabry disease, an X-linked recessive lysosomal disease, results from mutations in the GLA gene encoding lysosomal α-galactosidase A (α-Gal A). Due to these mutations, there is accumulation of globotriaosylceramide (GL-3) in plasma and in a wide range of cells throughout the body. Like other lysosomal enzymes, α-Gal A is synthesized on endoplasmic reticulum (ER) bound polyribosomes, and upon entry into the ER it undergoes glycosylation and folding. It was previously suggested that α-Gal A variants are recognized as misfolded in the ER and undergo ER-associated degradation (ERAD). In the present study, we used Drosophila melanogaster to model misfolding of α-Gal A mutants. We did so by creating transgenic flies expressing mutant α-Gal A variants and assessing development of ER stress, activation of the ER stress response and their relief with a known α-Gal A chaperone, migalastat. Our results showed that the A156V and the A285D α-Gal A mutants underwent ER retention, which led to activation of unfolded protein response (UPR) and ERAD. UPR could be alleviated by migalastat. When expressed in the fly’s dopaminergic cells, misfolding of α-Gal A and UPR activation led to death of these cells and to a shorter life span, which could be improved, in a mutation-dependent manner, by migalastat.

Histone H3 lysine 36 methyltransferase mobilizes NER factors to regulate tolerance against alkylation damage in fission yeast

The Set2 methyltransferase and its target, histone H3 lysine 36 (H3K36), affect chromatin architecture during the transcription and repair of DNA double-stranded breaks. Set2 also confers resistance against the alkylating agent, methyl methanesulfonate (MMS), through an unknown mechanism. Here, we show that Schizosaccharomyces pombe (S. pombe) exhibit MMS hypersensitivity when expressing a set2 mutant lacking the catalytic histone methyltransferase domain or a H3K36R mutant (reminiscent of a set2-null mutant). Set2 acts synergistically with base excision repair factors but epistatically with nucleotide excision repair (NER) factors, and determines the timely nuclear accumulation of the NER initiator, Rhp23, in response to MMS. Set2 facilitates Rhp23 recruitment to chromatin at the brc1 locus, presumably to repair alkylating damage and regulate the expression of brc1+ in response to MMS. Set2 also show epistasis with DNA damage checkpoint proteins; regulates the activation of Chk1, a DNA damage response effector kinase; and acts in a similar functional group as proteins involved in homologous recombination. Consistently, Set2 and H3K36 ensure the dynamicity of Rhp54 in DNA repair foci formation after MMS treatment. Overall, our results indicate a novel role for Set2/H3K36me in coordinating the recruitment of DNA repair machineries to timely manage alkylating damage.

Cell wall polysaccharides released during the alcoholic fermentation by Schizosaccharomyces pombe and S. japonicus: quantification and characterization

The present work demonstrates that yeasts belonging to the Schizosaccharomyces genus release a high quantity of polysaccharides of cell wall origin starting from the onset of the alcoholic fermentation. By the end of the alcoholic fermentation, all of the Schizosaccharomyces yeast strains released a quantity of polysaccharides approximately 3-7 times higher than that released by a commercial Saccharomyces cerevisiae yeast strain under the same fermentative conditions of synthetic juice. A higher content of polysaccharide was found in media fermented by Schizosaccharomyces japonicus with respect to that of Schizosaccharomyces pombe. Some of the strains evaluated were also able to produce high levels of pyruvic acid, which has been shown to be an important compound for color stability of wine. The presence of strains with different malic acid consumption patterns along with high polysaccharide release would enable production of naturally modified wines with enhanced mouth feel and reduced acidity. The chemical analysis of the released polysaccharides demonstrated divergence between the two yeast species S. pombe and S. japonicus. A different mannose/galactose ratio and a different percentage of proteins was observed on the polysaccharides released by S. pombe as compared to S. japonicus. Analysis of the proteins released in the media revealed the presence of a glycoprotein with a molecular size around 32-33 kDa only for the species S. japonicus. Mass spectrometry analysis of carbohydrate moieties showed similar proportions among the N-glycan chains released in the media by both yeast species but differences between the two species were also observed. These observations suggest a possible role of rapid MALDI-TOF screening of N-glycans compositional fingerprint as a taxonomic tool for this genus. Polysaccharides release in the media, in particular galactomannoproteins in significant amounts, could make these yeasts particularly interesting also for the industrial production of exogenous polysaccharide preparations.

The conundrum of UDP-Glc entrance into the yeast ER lumen

UDP-Glc entrance into the endoplasmic reticulum (ER) of eukaryotic cells is a key step in the quality control of glycoprotein folding, a mechanism requiring transfer of a Glc residue from the nucleotide sugar (NS) to glycoprotein folding intermediates by the UDP-Glc:glycoprotein glucosyltransferase (UGGT). According to a bioinformatics search there are only eight genes in the Schizosaccharomyces pombe genome belonging to the three Pfam families to which all known nucleotide-sugar transporters (NSTs) of the secretory pathway belong. The protein products of two of them (hut1⁺ and yea4⁺) localize to the ER, those of genes gms1⁺, vrg4⁺, pet1⁺, pet2⁺ and pet3⁺ to the Golgi, whereas that of gms2⁺ has an unknown location. Here we demonstrate that (1) Δhut1 and Δgpt1 (UGGT null) mutants share several phenotypic features; (2) Δhut1 mutants show a 50% reduction in UDP-Glc transport into ER-derived membranes; (3) in vivo UDP-Glc ER entrance occurred in Δhut1Δyea4Δgms2 mutants and in cells in which Δhut1 disruption was combined with that of each of four of the genes encoding Golgi-located proteins. Therefore, disruption of all genes whose products localize to the ER or have an unknown location did not obliterate UDP-Glc ER entrance. We conclude that the hut1⁺ gene product is involved in UDP-Glc entrance into the ER, but that at least another as yet unknown NST displaying an unconventional sequence operates in the yeast secretory pathway. This conclusion agrees with our previous results showing that UDP-Glc entrance into the yeast ER does not follow the classical NST antiport mechanism.

Glycosylation-directed quality control of protein folding

Membrane-bound and soluble proteins of the secretory pathway are commonly glycosylated in the endoplasmic reticulum. These adducts have many biological functions, including, notably, their contribution to the maturation of glycoproteins. N-linked glycans are of oligomeric structure, forming configurations that provide blueprints to precisely instruct the folding of protein substrates and the quality control systems that scrutinize it. O-linked mannoses are simpler in structure and were recently found to have distinct functions in protein quality control that do not require the complex structure of N-linked glycans. Together, recent studies reveal the breadth and sophistication of the roles of these glycan-directed modifications in protein biogenesis.

A sweet code for glycoprotein folding

Glycoprotein synthesis is initiated in the endoplasmic reticulum (ER) lumen upon transfer of a glycan (Glc3Man9GlcNAc2) from a lipid derivative to Asn residues (N-glycosylation). N-Glycan-dependent quality control of glycoprotein folding in the ER prevents exit to Golgi of folding intermediates, irreparably misfolded glycoproteins and incompletely assembled multimeric complexes. It also enhances folding efficiency by preventing aggregation and facilitating formation of proper disulfide bonds. The control mechanism essentially involves four components, resident lectin-chaperones (calnexin and calreticulin) that recognize monoglucosylated polymannose protein-linked glycans, lectin-associated oxidoreductase acting on monoglucosylated glycoproteins (ERp57), a glucosyltransferase that creates monoglucosylated epitopes in protein-linked glycans (UGGT) and a glucosidase (GII) that removes the glucose units added by UGGT. This last enzyme is the only mechanism component sensing glycoprotein conformations as it creates monoglucosylated glycans exclusively in not properly folded glycoproteins or in not completely assembled multimeric glycoprotein complexes. Glycoproteins that fail to properly fold are eventually driven to proteasomal degradation in the cytosol following the ER-associated degradation pathway, in which the extent of N-glycan demannosylation by ER mannosidases play a relevant role in the identification of irreparably misfolded glycoproteins.

N-glycans are not required for the efficient degradation of the mutant Saccharomyces cerevisiae CPY* in Schizosaccharomyces pombe

In eukaryotic cells, aberrant proteins generated in the endoplasmic reticulum (ER) are degraded by the ER-associated degradation (ERAD) pathway. Here, we report on the ERAD pathway of the fission yeast Schizosaccharomyces pombe. We constructed and expressed Saccharomyces cerevisiae wild-type CPY (ScCPY) and CPY-G255R mutant (ScCPY*) in S. pombe. While ScCPY was glycosylated and efficiently transported to the vacuoles in S. pombe, ScCPY* was retained in the ER and was not processed to the matured form in these cells. Cycloheximide chase experiments revealed that ScCPY* was rapidly degraded in S. pombe, and its degradation depended on Hrd1p and Ubc7p homologs. We also found that Mnl1p and Yos9p, proteins that are essential for ERAD in S. cerevisiae, were not required for ScCPY* degradation in S. pombe. Moreover, the null-glycosylation mutant of ScCPY, CPY*0000, was rapidly degraded by the ERAD pathway. These results suggested that N-linked oligosaccharides are not important for the recognition of luminal proteins for ERAD in S. pombe cells.

Demonstration that endoplasmic reticulum-associated degradation of glycoproteins can occur downstream of processing by endomannosidase

During quality control in the ER (endoplasmic reticulum), nascent glycoproteins are deglucosylated by ER glucosidases I and II. In the post-ER compartments, glycoprotein endo-α-mannosidase provides an alternative route for deglucosylation. Previous evidence suggests that endomannosidase non-selectively deglucosylates glycoproteins that escape quality control in the ER, facilitating secretion of aberrantly folded as well as normal glycoproteins. In the present study, we employed FOS (free oligosaccharides) released from degrading glycoproteins as biomarkers of ERAD (ER-associated degradation), allowing us to gain a global rather than single protein-centred view of ERAD. Glucosidase inhibition was used to discriminate between glucosidase- and endomannosidase-mediated ERAD pathways. Endomannosidase expression was manipulated in CHO (Chinese-hamster ovary)-K1 cells, naturally lacking a functional version of the enzyme, and HEK (human embryonic kidney)-293T cells. Endomannosidase was shown to decrease the levels of total FOS, suggesting decreased rates of ERAD. However, following pharmacological inhibition of ER glucosidases I and II, endomannosidase expression resulted in a partial switch between glucosylated FOS, released from ER-confined glycoproteins, to deglucosylated FOS, released from endomannosidase-processed glycoproteins transported from the Golgi/ERGIC (ER/Golgi intermediate compartment) to the ER. Using this approach, we have identified a previously unknown pathway of glycoprotein flow, undetectable by the commonly employed methods, in which secretory cargo is targeted back to the ER after being processed by endomannosidase.

Glucosidase II and N-glycan mannose content regulate the half-lives of monoglucosylated species in vivo

A decrease in N-glycan mannose content significantly diminishes in vivo glucosidase II–mediated deglucosylation rates but does not affect in vivo UDP-glucose:glycoprotein glucosyltransferase–mediated glucosylation, thus increasing the possibility of displaying monoglucosylated structures able to interact with calnexin/calreticulin for longer time periods.

Glucosidase II (GII) sequentially removes the two innermost glucose residues from the glycan (Glc₃Man₉GlcNAc₂) transferred to proteins. GII also participates in cycles involving the lectin/chaperones calnexin (CNX) and calreticulin (CRT) as it removes the single glucose unit added to folding intermediates and misfolded glycoproteins by the UDP-Glc:glycoprotein glucosyltransferase (UGGT). GII is a heterodimer in which the α subunit (GIIα) bears the active site, and the β subunit (GIIβ) modulates GIIα activity through its C-terminal mannose 6-phosphate receptor homologous (MRH) domain. Here we report that, as already described in cell-free assays, in live Schizosaccharomyces pombe cells a decrease in the number of mannoses in the glycan results in decreased GII activity. Contrary to previously reported cell-free experiments, however, no such effect was observed in vivo for UGGT. We propose that endoplasmic reticulum α-mannosidase–mediated N-glycan demannosylation of misfolded/slow-folding glycoproteins may favor their interaction with the lectin/chaperone CNX present in S. pombe by prolonging the half-lives of the monoglucosylated glycans (S. pombe lacks CRT). Moreover, we show that even N-glycans bearing five mannoses may interact in vivo with the GIIβ MRH domain and that the N-terminal GIIβ G2B domain is involved in the GIIα–GIIβ interaction. Finally, we report that protists that transfer glycans with low mannose content to proteins have nevertheless conserved the possibility of displaying relatively long-lived monoglucosylated glycans by expressing GIIβ MRH domains with a higher specificity for glycans with high mannose content.

Proteasome nuclear import mediated by Arc3 can influence efficient DNA damage repair and mitosis in Schizosaccharomyces pombe

Proteasomes must efficiently remove their substrates throughout the cells in a timely manner as many of these proteins can be toxic. This study shows that proteasomes can do so efficiently because they are highly mobile. Furthermore this study uncovers that proteasome mobility requires functional Arc3, a subunit of the Arp2/3 complex.

Proteasomes must remove regulatory molecules and abnormal proteins throughout the cell, but how proteasomes can do so efficiently remains unclear. We have isolated a subunit of the Arp2/3 complex, Arc3, which binds proteasomes. When overexpressed, Arc3 rescues phenotypes associated with proteasome deficiencies; when its expression is repressed, proteasome deficiencies intensify. Arp2/3 is best known for regulating membrane dynamics and vesicular transport; thus, we performed photobleaching experiments and showed that proteasomes are readily imported into the nucleus but exit the nucleus slowly. Proteasome nuclear import is reduced when Arc3 is inactivated, leading to hypersensitivity to DNA damage and inefficient cyclin-B degradation, two events occurring in the nucleus. These data suggest that proteasomes display Arc3-dependent mobility in the cell, and mobile proteasomes can efficiently access substrates throughout the cell, allowing them to effectively regulate cell-compartment–specific activities.

Purification and biochemical characterisation of endoplasmic reticulum alpha1,2-mannosidase from Sporothrix schenckiil

Alpha 1,2-mannosidases from glycosyl hydrolase family 47 participate in N-glycan biosynthesis. In filamentous fungi and mammalian cells, alpha1,2-mannosidases are present in the endoplasmic reticulum (ER) and Golgi complex and are required to generate complex N-glycans. However, lower eukaryotes such Saccharomyces cerevisiae contain only one alpha1,2-mannosidase in the lumen of the ER and synthesise high-mannose N-glycans. Little is known about the N-glycan structure and the enzyme machinery involved in the synthesis of these oligosaccharides in the dimorphic fungus Sporothrix schenckii. Here, a membrane-bound alpha-mannosidase from S. schenckii was solubilised using a high-temperature procedure and purified by conventional methods of protein isolation. Analytical zymograms revealed a polypeptide of 75 kDa to be responsible for enzyme activity and this purified protein was recognised by anti-alpha1,2-mannosidase antibodies. The enzyme hydrolysed Man(9)GlcNAc(2) into Man(8)GlcNAc(2) isomer B and was inhibited preferentially by 1-deoxymannojirimycin. This alpha1,2-mannosidase was localised in the ER, with the catalytic domain within the lumen of this compartment. These properties are consistent with an ER-localised alpha1,2-mannosidase of glycosyl hydrolase family 47. Our results also suggested that in contrast to other filamentous fungi, S. schenckii lacks Golgi alpha1,2-mannosidases and therefore, the processing of N-glycans by alpha1,2-mannosidases is similar to that present in lower eukaryotes.

Int6 and Moe1 interact with Cdc48 to regulate ERAD and proper chromosome segregation

Int6/eIF3e is implicated in tumorigenesis, but its molecular functions remain unclear. We have studied its fission yeast homolog Yin6, reporting that it regulates proteolysis by controlling the assembly/localization of proteasomes, and binds directly to another conserved protein, Moe1. In the present study, we isolated Cdc48 as a Moe1-binding protein from a yeast two-hybrid screen, and confirmed biochemically that they form a stable complex in fission yeast. Overexpressing Moe1 or Yin6 partially rescued phenotypes of cdc48 mutants; conversely, overexpressing Cdc48 partially rescued phenotypes of moe1 or yin6 mutants. Mutants defective in both Cdc48 and the Yin6-Moe1 complex showed growth defects that were far more severe than either alone. These double mutants were severely deficient in endoplasmic reticulum associated degradation (ERAD), as they were hypersensitive to accumulation of misfolded proteins. In addition, their chromosomes showed frequent defects in spindle attachment and segregation--these mitotic defects correlated with Ase1 and Bir1/survivin mislocalization. These results suggest that Cdc48, Yin6 and Moe1 act in the same protein complex to concertedly control ERAD and chromosome segregation. Many of these properties are evolutionarily conserved in humans, since human Cdc48 rescued the lethality of the yeast cdc48Delta mutant, and Int6 and Moe1/eIF3d bind Cdc48 in human cells.

C(1)-/C(2)-aromatic-imino-glyco-conjugates: experimental and computational studies of binding, inhibition and docking aspects towards glycosidases isolated from soybean and jack bean

Several C(1)-imino conjugates of D: -galactose, D: -lactose and D: -ribose, where the nitrogen center was substituted by the salicylidene or naphthylidene, were synthesized and characterized. Similar C(2)-imino conjugates of D: -glucose have also been synthesized. All the glyco-imino-conjugates, which are transition state analogues, exhibited 100% inhibition of the activity towards glycosidases extracted from soybean and jack bean meal. Among these, a galactosyl-napthyl-imine-conjugate (1c) showed 50% inhibition of the activity of pure alpha-mannosidase from jack bean at 22 +/- 2.5 microM, and a ribosyl-naphthyl-imine-conjugate (3c) showed at 31 +/- 5.5 microM and hence these conjugates are potent inhibitors of glycosidases. The kinetic studies suggested non-competitive inhibition by these conjugates. The studies are also suggestive of the involvement of aromatic, imine and carbohydrate moieties of the glyco-imino-conjugates in the effective inhibition. The binding of glyco-imino-conjugate has been established by extensive studies carried out using fluorescence emission and isothermal titration calorimetry. The conformational changes resulted in the enzyme upon interaction of these derivatives has been established by studying the fluorescence quench of the enzyme by KI as well as from the secondary structural changes noticed in CD spectra. All these studies revealed the difference in the binding strengths of the naphthylidene vs. salicylidene as well as galactosyl vs. lactosyl moieties present in these conjugates. The differential inhibition of these glyco-conjugates has been addressed by quantifying the specific interactions present between the glyco-conjugates and the enzyme by using rigid docking studies.

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

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

The evolution of N-glycan-dependent endoplasmic reticulum quality control factors for glycoprotein folding and degradation

Asn-linked glycans (N-glycans) play important roles in the quality control (QC) of glycoprotein folding in the endoplasmic reticulum (ER) lumen and in ER-associated degradation (ERAD) of proteins by cytosolic proteasomes. A UDP-Glc:glycoprotein glucosyltransferase glucosylates N-glycans of misfolded proteins, which are then bound and refolded by calreticulin and/or calnexin in association with a protein disulfide isomerase. Alternatively, an alpha-1,2-mannosidase (Mns1) and mannosidase-like proteins (ER degradation-enhancing alpha-mannosidase-like proteins 1, 2, and 3) are part of a process that results in the dislocation of misfolded glycoproteins into the cytosol, where proteins are degraded in the proteasome. Recently we found that numerous protists and fungi contain 0-11 sugars in their N-glycan precursors versus 14 sugars in those of animals, plants, fungi, and Dictyostelium. Our goal here was to determine what effect N-glycan precursor diversity has on N-glycan-dependent QC systems of glycoprotein folding and ERAD. N-glycan-dependent QC of folding (UDP-Glc:glycoprotein glucosyltransferase, calreticulin, and/or calnexin) was present and active in some but not all protists containing at least five mannose residues in their N-glycans and was absent in protists lacking Man. In contrast, N-glycan-dependent ERAD appeared to be absent from the majority of protists. However, Trypanosoma and Trichomonas genomes predicted ER degradation-enhancing alpha-mannosidase-like protein and Mns1 orthologs, respectively, each of which had alpha-mannosidase activity in vitro. Phylogenetic analyses suggested that the diversity of N-glycan-dependent QC of glycoprotein folding (and possibly that of ERAD) was best explained by secondary loss. We conclude that N-glycan precursor length has profound effects on N-glycan-dependent QC of glycoprotein folding and ERAD.

Saccharomyces cerevisiae alpha1,6-mannosyltransferase has a catalytic potential to transfer a second mannose molecule

In yeast, the N-linked oligosaccharide modification in the Golgi apparatus is initiated by alpha1,6-mannosyltransferase (encoded by the OCH1 gene) with the addition of mannose to the Man(8)GlcNAc(2) or Man(9)GlcNAc(2) endoplasmic reticulum intermediates. In order to characterize its enzymatic properties, the soluble form of the recombinant Och1p was expressed in the methylotrophic yeast Pichia pastoris as a secreted protein, after truncation of its transmembrane region and fusion with myc and histidine tags at the C-terminus, and purified using a metal chelating column. The enzymatic reaction was performed using various kinds of pyridylaminated (PA) sugar chains as acceptor, and the products were separated by high performance liquid chromatography. The recombinant Och1p efficiently transferred a mannose to Man(8)GlcNAc(2)-PA and Man(9)GlcNAc(2)-PA acceptors, while Man(5)GlcNAc(2)-PA, which completely lacks alpha1,2-linked mannose residues, was not used as an acceptor. At high enzyme concentrations, a novel product was detected by HPLC. Analysis of the product revealed that a second mannose was attached at the 6-O-position of alpha1,3-linked mannose branching from the alpha1,6-linked mannose that is attached to beta1,4-linked mannose of Man(10)GlcNAc(2)-PA produced by the original activity of Och1p. Our results indicate that Och1p has the potential to transfer two mannoses from GDP-mannose, and strictly recognizes the overall structure of high mannose type oligosaccharide.

N-linked glycan recognition and processing: the molecular basis of endoplasmic reticulum quality control

Nascent polypeptides emerging into the lumen of the endoplasmic reticulum (ER) are N-glycosylated on asparagines in Asn-Xxx-Ser/Thr motifs. Processing of the core oligosaccharide eventually determines the fate of the associated polypeptide by regulating entry into and retention by the calnexin chaperone system, or extraction from the ER folding environment for disposal. Recent advances have shown that at least two N-glycans are necessary for protein access to the calnexin chaperone system and that polypeptide cycling in the system is a rather rare event, which, for folding-defective polypeptides, is activated only upon persistent misfolding. Additionally, dismantling of the polypeptide-bound N-glycan interrupts futile folding attempts, and elicits preparation of the misfolded chain for dislocation into the cytosol and degradation.

EDEM3, a soluble EDEM homolog, enhances glycoprotein endoplasmic reticulum-associated degradation and mannose trimming

Quality control in the endoplasmic reticulum ensures that only properly folded proteins are retained in the cell through mechanisms that recognize and discard misfolded or unassembled proteins in a process called endoplasmic reticulum-associated degradation (ERAD). We previously cloned EDEM (ER degradation-enhancing alpha-mannosidase-like protein) and showed that it accelerates ERAD of misfolded glycoproteins. We now cloned mouse EDEM3, a soluble homolog of EDEM. EDEM3 consists of 931 amino acids and has all the signature motifs of Class I alpha-mannosidases (glycosyl hydrolase family 47) in its N-terminal domain and a protease-associated motif in its C-terminal region. EDEM3 accelerates glycoprotein ERAD in transfected HEK293 cells, as shown by increased degradation of misfolded alpha1-antitrypsin variant (null (Hong Kong)) and of TCRalpha. Overexpression of EDEM3 also greatly stimulates mannose trimming not only from misfolded alpha1-AT null (Hong Kong) but also from total glycoproteins, in contrast to EDEM, which has no apparent alpha1,2-mannosidase activity. Furthermore, overexpression of the E147Q EDEM3 mutant, which has the mutation in one of the conserved acidic residues essential for enzyme activity of alpha1,2-mannosidases, abolishes the stimulation of mannose trimming and greatly decreases the stimulation of ERAD by EDEM3. These results show that EDEM3 has alpha1,2-mannosidase activity in vivo, suggesting that the mechanism whereby EDEM3 accelerates glycoprotein ERAD is different from that of EDEM.