Partial purification and characterization of the glucosidases involved in the processing of asparagine-linked oligosaccharides

Expression plasticity and evolutionary changes extensively shape the sugar-mimic alkaloid adaptation of nondigestive glucosidase in lepidopteran mulberry-specialist insects

During the co-evolutionary arms race between plants and herbivores, insects evolved systematic adaptive plasticity to minimize the chemical defence effects of their host plants. Previous studies mainly focused on the expressional plasticity of enzymes in detoxification and digestion. However, the expressional response and adaptive evolution of other fundamental regulators against host phytochemicals are largely unknown. Glucosidase II (GII), which is composed of a catalytic GIIα subunit and a regulatory GIIβ subunit, is an evolutionarily conserved enzyme that regulates glycoprotein folding. In this study, we found that GIIα expression of the mulberry-specialist insect was significantly induced by mulberry leaf extract, 1-deoxynojirimycin (1-DNJ), whereas GIIβ transcripts were not significantly changed. Moreover, positive selection was detected in GIIα when the mulberry-specialist insects diverged from the lepidopteran order, whereas GIIβ was mainly subjected to purifying selection, thus indicating an asymmetrically selective pressure of GII subunits. In addition, positively selected sites were enriched in the GIIα of mulberry-specialist insects and located around the 1-DNJ-binding sites and in the C-terminal region, which could result in conformational changes that affect catalytic activity and substrate-binding efficiency. These results show that expression plasticity and evolutionary changes extensively shape sugar-mimic alkaloids adaptation of nondigestive glucosidase in lepidopteran mulberry-specialist insects. Our study provides novel insights into a deep understanding of the sequestration and adaptation of phytophagous specialists to host defensive compounds.

Insight into glucosidase II from the red marine microalga Porphyridium sp. (Rhodophyta)

N-glycosylation of proteins is one of the most important post-translational modifications that occur in various organisms, and is of utmost importance for protein function, stability, secretion, and loca-lization. Although the N-linked glycosylation pathway of proteins has been extensively characterized in mammals and plants, not much information is available regarding the N-glycosylation pathway in algae. We studied the α 1,3-glucosidase glucosidase II (GANAB) glycoenzyme in a red marine microalga Porphyridium sp. (Rhodophyta) using bioinformatic and biochemical approaches. The GANAB-gene was found to be highly conserved evolutionarily (compo-sed of all the common features of α and β subunits) and to exhibit similar motifs consistent with that of homolog eukaryotes GANAB genes. Phylogenetic analysis revealed its wide distribution across an evolutionarily vast range of organisms; while the α subunit is highly conserved and its phylogenic tree is similar to the taxon evolutionary tree, the β subunit is less conserved and its pattern somewhat differs from the taxon tree. In addition, the activity of the red microalgal GANAB enzyme was studied, including functional and biochemical characterization using a bioassay, indicating that the enzyme is similar to other eukaryotes ortholog GANAB enzymes. A correlation between polysaccharide production and GANAB activity, indicating its involvement in polysaccharide biosynthesis, is also demonstrated. This study represents a valuable contribution toward understanding the N-glycosylation and polysaccharide biosynthesis pathways in red microalgae.

Deciphering the roles of glycan processing in glycoprotein quality control through organic synthesis

Protein quality control (QC) in the endoplasmic reticulum (ER) comprises many aspects, including folding and transport of nascent proteins and degradation of misfolded proteins. Recent studies have revealed that high-mannose-type glycans play pivotal roles in the QC process. To gain knowledge of the molecular basis of this process with well-defined homogeneous compounds, we achieved a convergent synthesis of high-mannose-type glycans and their functionalized derivatives. A major part of our study focused on analyses of UDP-Glc: glycoprotein glucosyltransferase (UGGT) and ER glucosidase II, which play crucial roles in glycoprotein QC, to clarify their specificities. In addition, we established an in vitro assay system mimicking the in vivo condition, which is highly crowded due to the presence of various macromolecules.

Engineering Yarrowia lipolytica to produce glycoproteins homogeneously modified with the universal Man3GlcNAc2 N-glycan core

Yarrowia lipolytica is a dimorphic yeast that efficiently secretes various heterologous proteins and is classified as “generally recognized as safe.” Therefore, it is an attractive protein production host. However, yeasts modify glycoproteins with non-human high mannose-type N-glycans. These structures reduce the protein half-life in vivo and can be immunogenic in man. Here, we describe how we genetically engineered N-glycan biosynthesis in Yarrowia lipolytica so that it produces Man₃GlcNAc₂ structures on its glycoproteins. We obtained unprecedented levels of homogeneity of this glycanstructure. This is the ideal starting point for building human-like sugars. Disruption of the ALG3 gene resulted in modification of proteins mainly with Man₅GlcNAc₂ and GlcMan₅GlcNAc₂ glycans, and to a lesser extent with Glc₂Man₅GlcNAc₂ glycans. To avoid underoccupancy of glycosylation sites, we concomitantly overexpressed ALG6. We also explored several approaches to remove the terminal glucose residues, which hamper further humanization of N-glycosylation; overexpression of the heterodimeric Apergillus niger glucosidase II proved to be the most effective approach. Finally, we overexpressed an α-1,2-mannosidase to obtain Man₃GlcNAc₂ structures, which are substrates for the synthesis of complex-type glycans. The final Yarrowia lipolytica strain produces proteins glycosylated with the trimannosyl core N-glycan (Man₃GlcNAc₂), which is the common core of all complex-type N-glycans. All these glycans can be constructed on the obtained trimannosyl N-glycan using either in vivo or in vitro modification with the appropriate glycosyltransferases. The results demonstrate the high potential of Yarrowia lipolytica to be developed as an efficient expression system for the production of glycoproteins with humanized glycans.

Analysis of glycoprotein processing in the endoplasmic reticulum using synthetic oligosaccharides

Protein quality control (QC) in the endoplasmic reticulum (ER) comprises many steps, including folding and transport of nascent proteins as well as degradation of misfolded proteins. Recent studies have revealed that high-mannose-type glycans play a pivotal role in the QC process. To gain knowledge about the molecular basis of this process with well-defined homogeneous compounds, we achieved a convergent synthesis of high-mannose-type glycans and their functionalized derivatives. We focused on analyses of UDP-Glc: glycoprotein glucosyltransferase (UGGT) and ER Glucosidase II, which play crucial roles in glycoprotein QC; however, their specificities remain unclear. In addition, we established an in vitro assay system mimicking the in vivo condition which is highly crowded because of the presence of various biomacromolecules.

The secreted plant N-glycoproteome and associated secretory pathways

N-Glycosylation is a common form of eukaryotic protein post-translational modification, and one that is particularly prevalent in plant cell wall proteins. Large scale and detailed characterization of N-glycoproteins therefore has considerable potential in better understanding the composition and functions of the cell wall proteome, as well as those proteins that reside in other compartments of the secretory pathway. While there have been numerous studies of mammalian and yeast N-glycoproteins, less is known about the population complexity, biosynthesis, structural variation, and trafficking of their plant counterparts. However, technical developments in the analysis of glycoproteins and the structures the glycans that they bear, as well as valuable comparative analyses with non-plant systems, are providing new insights into features that are common among eukaryotes and those that are specific to plants, some of which may reflect the unique nature of the plant cell wall. In this review we present an overview of the current knowledge of plant N-glycoprotein synthesis and trafficking, with particular reference to those that are cell wall localized.

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.

Binding residues and catalytic domain of soluble Saccharomyces cerevisiae processing alpha-glucosidase I

Alpha-glucosidase I initiates the trimming of newly assembled N-linked glycoproteins in the lumen of the endoplasmic reticulum (ER). Site-specific chemical modification of the soluble alpha-glucosidase I from yeast using diethylpyrocarbonate (DEPC) and tetranitromethane (TNM) revealed that histidine and tyrosine are involved in the catalytic activity of the enzyme, as these residues could be protected from modification using the inhibitor deoxynojirimycin. Deoxynojirimycin could not prevent inactivation of enzyme treated with N-bromosuccinimide (NBS) used to modify tryptophan residues. Therefore, the binding mechanism of yeast enzyme contains different amino acid residues compared to its mammalian counterpart. Catalytically active polypeptides were isolated from endogenous proteolysis and controlled trypsin hydrolysis of the enzyme. A 37-kDa nonglycosylated polypeptide was isolated as the smallest active fragment from both digests, using affinity chromatography with inhibitor-based resins (N-methyl-N-59-carboxypentyl- and N-59-carboxypentyl-deoxynojirimycin). N-terminal sequencing confirmed that the catalytic domain of the enzyme is located at the C-terminus. The hydrolysis sites were between Arg(521) and Thr(522) for endogenous proteolysis and residues Lys(524) and Phe(525) for the trypsin-generated peptide. This 37-kDa polypeptide is 1.9 times more active than the 98-kDa protein when assayed with the synthetic trisaccharide, alpha-D-Glc1,2alpha-D-Glc1,3alpha-D-Glc-O(CH2)(8)COOCH(3), and is not glycosylated. Identification of this relatively small fragment with catalytic activity will allow mechanistic studies to focus on this critical region and raises interesting questions about the relationship between the catalytic region and the remaining polypeptide.

Two distinct domains of the beta-subunit of glucosidase II interact with the catalytic alpha-subunit

Recent purification and cDNA cloning of the endoplasmic reticulum processing enzyme glucosidase II have revealed that it is composed of two soluble proteins: a catalytic alpha-subunit and a beta-subunit of unknown function, both of which are highly conserved in mammals. Since the beta-subunit, which contains a C-terminal His-Asp-Glu-Leu (HDEL) motif, may function to link the catalytic subunit to the KDEL receptor as a retrieval mechanism, we sought to map the regions of the mouse beta-subunit protein responsible for mediating the association with the alpha-subunit. By screening a panel of recombinant beta-subunit glutathione S-transferase fusion proteins for the ability to precipitate glucosidase II activity, we have identified two non-overlapping interaction domains (ID1 and ID2) within the beta-subunit. ID1 encompasses 118 amino acids at the N-terminus of the mature polypeptide, spanning the cysteine-rich element in this region. ID2, located near the C-terminus, is contained within amino acids 273-400, a region occupied in part by a stretch of acidic residues. Variable usage of 7 alternatively spliced amino acids within ID2 was found not to influence the association of the two sub-units. We theorize that the catalytic subunit of glucosidase II binds synergistically to ID1 and ID2, explaining the high associative stability of the enzyme complex.

Regulation of prolactin receptor glycosylation and its role in receptor location

In unstimulated mammary epithelial cells from virgin mice, the prolactin receptor exists as two isoforms: a 78 and a 70 kDa species. Both proteins are reduced to a single 61 kDa molecule after N-glycanase F treatment, indicating that their size difference is solely a result of carbohydrate content. Membrane fractionation experiments reveal that the smaller species is exclusively intracellular, while the larger one is located on the cell surface. Nitric oxide (NO) stimulates the migration of prolactin receptors from an internal pool to the plasmalemma in only 30 min and this redistribution is associated with an increase in molecular weight. Redistribution is blocked by swainsonine, but not by castanospermine or 1-deoxymannojirimycin, suggesting that the glycosylation step involved with translocation is either alpha-mannosidase II or N-acetylglucosamine (NAG) transferase I. The former is unaffected by NO but the activity of the latter is doubled 30 min after exposure to NO. These data suggest that prolactin receptors are retained intracellularly because of their incomplete N-glycosylation and that NO triggers their redistribution by stimulating the completion of this process, in part by increasing NAG transferase I activity.

Identification of the CD45-associated 116-kDa and 80-kDa proteins as the alpha- and beta-subunits of alpha-glucosidase II

CD45 is an abundant, highly glycosylated transmembrane protein-tyrosine phosphatase expressed on hematopoietic cells. Herein we demonstrate that two proteins of 116 kDa and 80 kDa copurify with CD45 from mouse T cells. Microsequence analysis of the 116-kDa protein revealed high similarity to an incomplete human open reading frame that has been suggested to correspond to the catalytic alpha-subunit of glucosidase II. We determined the nucleotide sequence of the mouse cDNA and observed that it encodes a protein product nearly identical to its human homologue and shares an active site consensus sequence with Family 31 glucosidases. Amino acid sequencing of the 80-kDa protein, followed by molecular cloning, revealed high homology to human and bovine cDNAs postulated to encode the beta-subunit of glucosidase II. Antisera developed to the mouse beta-subunit allowed us to demonstrate that the interaction between CD45 and glucosidase II can be reconstituted in vitro in an endoglycosidase H-sensitive manner. The strong interaction between glucosidase II and CD45 may provide a paradigm for investigating novel aspects of the biology of these proteins.

Synthesis of alpha-D-Glcp-(1-->2)-alpha-D-Glcp-(1-->3) -alpha-D-Glcp-O-(CH2)8 COOCH3 for use in the assay of alpha-glucosidase I activity

The chemical synthesis of alpha-D-Glcp-(1-->2)-alpha-D-Glcp p-(1-->3) -alpha-D-Glcp-O-(CH2)8 COOCH3 (9), a substrate specific for alpha-glucosidase I, is reported. This enzyme removes the terminal alpha-D-Glcp unit to produce alpha-D-Glcp-(1-->3)-alpha-D-Glcp-O-(CH2)8 COOCH3 (10). This is the first synthetic substrate described for glucosidase I that allows kinetic evaluation of substrates and inhibitors of this enzyme. Tetramethylrhodamine was coupled to 9 through an ethylenediamine linker to produce a brilliant red derivative. Addition of this fluorescent dye did not affect enzyme binding to the substrate, as determined by a comparison of the Km value (1.3 mM). The fluorescent label allows visual detection of 2-3 pmol of product by TLC.

Glucose trimming and reglucosylation determine glycoprotein association with calnexin in the endoplasmic reticulum

To determine the role of N-linked oligosaccharides in the folding of glycoproteins, we analyzed the processing of in vitro translated influenza hemagglutinin (HA) in dog pancreas microsomes. We found that binding to calnexin, a membrane-bound molecular chaperone, was specific to molecules that possessed monoglucosylated core glycans. In the microsomes, these were generated either by glucose removal from the original triglucosylated core oligosaccharide by glucosidases I and II or by reglucosylation of already unglucosylated high mannose glycans. Release of fully folded HA from calnexin required the removal of the remaining glucose by glucosidase II. The results provided an explanation for trimming and reglucosylation activities in the endoplasmic reticulum and established a direct correlation between glycosylation and folding.

How N-linked oligosaccharides affect glycoprotein folding in the endoplasmic reticulum

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Characterization of the maltase activity of glucosidase II from rat liver. Kinetic model

Glucosidase II is a key enzyme in the processing of N-glycoproteins since it removes the two glucose residues from the protein-linked oligosaccharide Glc2Man9GlcNAc2-R. We have studied the kinetics of the purified enzyme, using maltose as substrate. Analysis of data fitting to single and double-hyperbolic equations and the Eadie-Hofstee profile indicate that the enzyme has two binding (active) sites for the hydrolysis of maltose. The Km and Vmax values for the high-affinity site were 0.43 mM and 691 mU/mg, respectively, whereas the values for the low-affinity site were 57.7 mM and 2888 mU/mg, respectively. The Vmax/Km ratios were 1607 and 50.1 ml/min per g for the high- and low-affinity sites, respectively. A new kinetic model for this enzyme is proposed from the equilibria corresponding to the partial competitive inhibition produced by maltose on p-nitrophenyl-glucosidase activity. The amino acid composition of the enzyme has been established.

Effect of bromoconduritol on glucosidase II from rat liver. A new kinetic model for the binding and hydrolysis of the substrate

Bromoconduritol inhibits the p-nitrophenyl-glucosidase and maltase activities of glucosidase II purified from rat liver, an enzyme that removes the two alpha-1,3-linked glucose residues of the protein-bound oligosaccharide Glc2Man9GlcNAc2 in the processing of N-glycoproteins. The inactivation process exhibits pseudo-first-order kinetics. Previously, we have demonstrated the occurrence of two binding (active) sites in the glucosidase II for the substrates p-nitrophenyl alpha-D-glucopyranoside (pNphGlc) and maltose (high- and low-affinity sites). The inhibition kinetic studies with bromoconduritol indicate that the high- and low-affinity sites for pNphGlc correspond to high- and low-affinity sites for maltose, respectively. Bromoconduritol has no effect on the binding of the substrates (pNphGlc and maltose) to the high-affinity site, although it does modify the low-affinity site and hinders the binding of the two indicated substrates to this site. These results, together with previous reports, have prompted us to propose a new kinetic model of binding and hydrolysis of the physiological substrate of the enzyme, in which the outermost glucose residue would bind and be released at the high-affinity site, whereas the innermost glucose residue would do so at the low-affinity site.

Subcellular distribution in rat liver of a novel broad-specificity (alpha 1----2, alpha 1----3 and alpha 1----6) mannosidase active on oligomannose glycans

Recently, the purification to homogeneity was reported of a novel broad-specificity alpha-mannosidase from rat liver microsomal membranes [P. Bonay and R. C. Hughes (1991) Eur. J. Biochem. 197, 229-238]. The enzyme catalyzed the ordered removal of alpha 1----2-, alpha 1----3- and alpha 1----6-linked mannose residues from MannGlcNAc oligosaccharide substrates where n = 4-9. We now show by subcellular fractionation and immunocytochemistry that the novel mannosidase is present in the endoplasmic reticulum, Golgi apparatus and endosomes. Enzyme activity is enriched in a heavy Golgi membrane fraction and to lesser extent in an intermediate density Golgi membrane fraction containing GlcNAc transferase I activity and in a 'late' endosomal fraction. Low levels of enzyme activity were detectable in endoplasmic reticulum membranes and in 'early' endosomes but not in receptor-enriched and ligand-free endosomes. Assays of enzymic activity using Golgi membrane fractions in the absence and presence of Triton X-100 showed that the active site of the enzyme faces the lumen of the membrane vesicles. Antibodies directed against the purified mannosidase showed no immunological cross-reaction to known endoplasmic reticulum and Golgi mannosidases. Conversely, the purified mannosidase was not recognized by antibodies directed against endoplasmic reticulum mannosidase nor Golgi mannosidase IA.

Glucosidase II from rat liver microsomes. Kinetic model for binding and hydrolysis

Glucosidase II is an enzyme involved in glycoprotein biosynthesis, releasing both alpha-1,3-linked glucose residues from the protein-linked oligosaccharide Glc2Man9GlcNAc2-R in the processing of N-glycans. We studied the kinetic properties of the enzyme, purified to homogeneity and, for the first time, we have been able to demonstrate the occurrence of two active sites in this enzyme and to establish the mechanisms of binding and hydrolysis of the physiological substrate at its active site(s). The analyses of data fitting to single and double hyperbolic equations and the Eadie-Hofstee profile, together with the inhibition kinetics, demonstrate that the enzyme has two different active sites. The Km and Vmax. values for the high-affinity site (site 1) were 0.78 mM and 437 munits/mg respectively, whereas the values for the low-affinity site (site 2) were 481 mM and 13797 munits/mg respectively, for the p-nitrophenyl alpha-D-glucopyranoside substrate. The Vmax./Km ratios, which indicate the efficacy of an active site for a substrate, were 560 and 28.7 ml/min per g for active sites 1 and 2, respectively. The inhibition type, with respect to site 1, for glucose, maltose, D-glucone-delta-lactone, CaCl2 and MgCl2 was pure-competitive, partial-competitive, parabolic, non-competitive and non-competitive respectively. Ki values for glucose, maltose, CaCl2 and MgCl2 were 6.75, 2.05, 10.60 and 14.20 mM respectively. Thus glucose would bind to active site 1, maltose to site 2 (and near to site 1) and D-glucone-delta-lactone to either site 1 or 2. The following hydrolysis mechanism for the physiological substrate (Glc2Man9GlcNAc2-protein) of glucosidase II may be concluded from all the foregoing kinetic evidence: the external glucose would be the first released residue, at active site 2, thereafter producing Glc1Man9GlcNAc2-protein; the remaining glucose would be released at active site 1, delivering the Man9GlcNAc2-protein product, which would leave the enzyme.

Glucosidase II from control and ethanol-treated rats. Purification and properties

Liver glucosidase II from control and ethanol-treated rats was purified and its physical and catalytic properties studied. No significant variation was found in the purification and properties of the enzyme from either source (ethanol-treated and control rats), except for activity after storage. Glucosidase II was purified to electrophoretic homogeneity from liver microsomes by solubilization, protamine sulphate precipitation, and anion exchange (DEAE-Sephacel) and affinity (Con A-Sepharose-4B) chromatographies. The native enzyme molecule is a tetramer (Mr 425 +/- 10 kDa) with identical subunits (Mr 106 +/- 3 kDa). Km values, determined at pH 6.8 for the p-nitrophenyl-glucosidase activity of glucosidase II from control and ethanol-treated rats, were 1.20 +/- 0.12 and 1.14 +/- 0.13 mM, respectively. The Arrhenius plot was linear, and the value for the apparent activation energy, calculated from this plot, was 56.64 kJ/mol. The p-nitrophenyl-glucosidase activity of glucosidase II from control and ethanol-treated rats was inhibited to the same extent by NH4+, by the divalent cations Ca2+ and Mg2+, and by methanol, ethanol, 2-propanol, n-propanol, isobutanol and n-butanol.

Nonlysosomal, pre-Golgi degradation of unassembled asialoglycoprotein receptor subunits: a TLCK- and TPCK-sensitive cleavage within the ER

The human asialoglycoprotein receptor subunit H2a is cotranslationally inserted into the ER membrane. When expressed together with subunit H1 in mouse fibroblasts part forms a hetero-oligomer that is transported to the cell surface, but when expressed alone it is all rapidly degraded. Degradation is insensitive to lysosomotropic agents and the undegraded precursor is last detected in the ER region of the cell. Small amounts of an intermediate 35-kD degradation product can be detected (Amara, J. F., G. Lederkremer, and H. F. Lodish. 1989. J. Cell Biol. 109:3315). We show here that the oligosaccharides on both precursor H2a and the 35-kD fragment are Man6-9GlcNAc2, structures typically found in pre-Golgi compartments. Subcellular fractionation shows that the intermediate degradation product does not cofractionate with the lysosomal enzyme beta-galactosidase, but is found in a part of the ER that contains ribosomes. Thus the intermediate degradation product is localized in the ER, indicating that the initial degradation event does take place in the ER. All degradation of H2a, including the initial endoproteolytic cleavage generating the 35-kD intermediate, is blocked by the protease inhibitors N-tosyl-L-lysine chloromethyl ketone and N-tosyl-L-phenylalanine chloromethyl ketone. These drugs do not inhibit ER-to-Golgi transport of H1. Depleting the cells of ATP or inhibiting protein synthesis allows the initial endoproteolytic cleavage to occur, but blocks further degradation of the 35-kD intermediate; thus we can convert all cellular H2 into the 35-kD intermediate. Approximately 50% of H2b, a splicing variant differing from H2a by a five amino acid deletion, can be transported to the cell surface, and the rest appears to be degraded by the same pathway as H2a, both when expressed alone in fibroblasts and together with H1 in HepG2 cells. Addition of N-tosyl-L-lysine chloromethyl ketone or N-tosyl-L-phenylalanine chloromethyl ketone blocks degradation of the approximately 50% that is not transported, but does not affect the fraction of H2b that moves to the Golgi region. Thus, a protein destined for degradation will not be transported to the Golgi region if degradation is inhibited.

Bcl-2 is an inner mitochondrial membrane protein that blocks programmed cell death

The t(14; 18) chromosomal translocation of human follicular B-cell lymphoma juxtaposes the bcl-2 gene with the immunoglobulin heavy chain locus. The bcl-2 immunoglobulin fusion gene is markedly deregulated resulting in inappropriately elevated levels of bcl-2 RNA and protein. Transgenic mice bearing a bcl-2 immunoglobulin minigene demonstrate a polyclonal expansion of resting yet responsive IgM-IgD B cells which display prolonged cell survival but no increase in cell cycling. Moreover, deregulated bcl-2 extends the survival of certain haematopoietic cell lines following growth-factor deprivation. By using immunolocalization studies we now demonstrate that Bcl-2 is an integral inner mitochondrial membrane protein of relative molecular mass 25,000 (25k). Overexpression of Bcl-2 blocks the apoptotic death of a pro-B-lymphocyte cell line. Thus, Bcl-2 is unique among proto-oncogenes, being localized to mitochondria and interfering with programmed cell death independent of promoting cell division.

Purification to homogeneity and properties of mannosidase II from mung bean seedlings

Mannosidase II was purified from mung bean seedlings to apparent homogeneity by using a combination of techniques including DEAE-cellulose and hydroxyapatite chromatography, gel filtration, lectin affinity chromatography, and preparative gel electrophoresis. The release of radioactive mannose from GlcNAc[3H]Man5GlcNAc was linear with time and protein concentration with the purified protein, did not show any metal ion requirement, and had a pH optimum of 6.0. The purified enzyme showed a single band on SDS gels that migrated with the Mr 125K standard. The enzyme was very active on GlcNAcMan5GlcNAc but had no activity toward Man5GlcNAc, Man9GlcNAc, Glc3Man9GlcNAc, or other high-mannose oligosaccharides. It did show slight activity toward Man3GlcNAc. The first product of the reaction of enzyme with GlcNAcMan5GlcNAc, i.e., GlcNAcMan4GlcNAc, was isolated by gel filtration and subjected to digestion with endoglucosaminidase H to determine which mannose residue had been removed. This GlcNAcMan4GlcNAc was about 60% susceptible to Endo H indicating that the mannosidase II preferred to remove the alpha 1,6-linked mannose first, but 40% of the time removed the alpha 1,3-linked mannose first. The final product of the reaction, GlcNAcMan3GlcNAc, was characterized by gel filtration and various enzymatic digestions. Mannosidase II was very strongly inhibited by swainsonine and less strongly by 1,4-dideoxy-1,4-imino-D-mannitol. It was not inhibited by deoxymannojirimycin.

Asparagine-linked glycoprotein biosynthesis in rat brain: identification of glucosidase I, glucosidase II, and and endomannosidase (glucosyl mannosidase)

Previous studies from this laboratory provided evidence, largely based upon the presence of a novel alpha-D-mannosidase, suggesting that the biosynthesis of N-linked glycoproteins may be different in brain as compared to other tissues (Tulsiani, D. R. P., and Touster, O. (1985) J. Biol. Chem. 260, 13,081-13,087). In the present report we describe studies on the enzymes involved in early processing reactions. These studies indicate that the brain, like other tissues, contains glucosidases I and II. The two glucosidases were separated as distinct activities with some overlapping by chromatography on a DE-52 column. The differential inhibition studies and substrate specificity studies support our conclusion that, as in other tissues, rat brain glucosidase I cleaves alpha 1,2-linked terminal glucosyl residues, whereas glucosidase II prefers alpha 1,3-linked glucosyl residues. In addition to these two processing glucosidases, we have characterized an endo enzyme (glucosyl mannosidase) in rat brain. The endomannosidase cleaves a disaccharide (glucosyl alpha 1,3-mannose) from monoglucosylated oligosaccharides (GlcMan7-9GlcNAc). Little or no activity was observed when di- or triglucosylated oligosaccharide was used as a substrate. The pH optimum of the glucosyl mannosidase is 6.2-6.8. The enzyme appears to be an intrinsic microsomal membrane component, since washing of the microsomal membranes with salt solution did not release the enzyme in soluble form. A mixture of Triton X-100 and sodium deoxycholate is required for complete solubilization of the enzyme. The solubilized enzyme is eluted from a Bio-Gel A-1.5m column as a single peak with an apparent molecular weight of 380,000.

A simple and rapid microplate assay for glycoprotein-processing glycosidases

A simple and convenient microplate assay for glycosidases involved in the glycoprotein-processing reactions is described. The assay is based on specific binding of high-mannose-type oligosaccharide substrates to concanavalin A-Sepharose, while monosaccharides liberated by enzymatic hydrolysis do not bind to concanavalin A-Sepharose. By the use of radiolabeled substrates [( 3H]glucose for glucosidases and [3H]mannose for mannosidases), the radioactivity in the liberated monosaccharides can be determined as a measure of the enzymatic activity. This principle was employed earlier for developing assays for glycosidases previously reported (B. Saunier et al. (1982) J. Biol. Chem. 257, 14155-14161; T. Szumilo and A. D. Elbein (1985) Anal. Biochem. 151, 32-40). These authors have reported the separation of substrate from the product by concanavalin A-Sepharose column chromatography. This procedure is handicapped by the fact that it cannot be used for a large number of samples and is time consuming. We have simplified this procedure and adapted it to the use of a microplate (96-well plate). This would help in processing a large number of samples in a short time. In this report we show that the assay is comparable to the column assay previously reported. It is linear with time and enzyme concentration and shows expected kinetics with castanospermine, a known inhibitor of alpha-glucosidase I.

Plant glucosidase II catalyzes a transglucosylation reaction in addition to the hydrolytic reaction

When the purified plant glucosidase II was incubated with [3H]Glc2Man9GlcNAc in the presence of glycerol and the products were analyzed by gel filtration, a large peak of radioactivity emerged just before the glucose standard. The formation of this peak was dependent on both the presence of Glc2Man9GlcNAc and the presence of glycerol, and the amount of this product increased with time of incubation and amount of glucosidase II in the incubation. When the incubation was performed with [3H]Glc2Man9GlcNAc plus [14C]glycerol, the product contained both 14C and 3H. Strong acid hydrolysis of the purified product gave rise to [14C]glycerol and [3H]glucose. Various other chemical treatments and chromatographic techniques showed that the product was glucosyl----glycerol. Since the glucose was released by alpha-glucosidase, the product must be glucosyl-alpha-glycerol. This study demonstrates that the processing glucosidase II catalyzes a trans-glycosylation reaction in the presence of acceptors like glycerol. Since this transglycosylation reaction may give rise to unexpected products, investigators should be aware of its possible occurrence.

An investigation of the possible influence of neutral alpha-glucosidases on the clinical heterogeneity of glycogenosis type II

The lysosomal storage disorder glycogenosis type II, caused by a deficiency of lysosomal alpha-glucosidase, is very heterogeneous in its clinical presentation. It has been suggested that this heterogeneity may be due to differential expression of neutral alpha-glucosidases. We have therefore analysed the activity of the major neutral alpha-glucosidases in cultured fibroblasts or muscle cells from 26 patients with glycogenosis type II. The results indicate that there is no correlation between the expression of neutral alpha-glucosidase isoenzymes and the clinical phenotype of this disease.

Endolyn-78, a membrane glycoprotein present in morphologically diverse components of the endosomal and lysosomal compartments: implications for lysosome biogenesis

A monoclonal antibody (2C5) raised against rat liver lysosomal membranes was used to identify a 78-kD glycoprotein that is present in the membranes of both endosomes and lysosomes and, therefore, is designated endolyn-78. In cultures of rat hepatoma (Fu5C8) and kidney cells (NRK), this glycoprotein could not be labeled with [35S]methionine or with [32P]inorganic phosphate but was easily labeled with [35S]cysteine and [3H]mannose. Pulse-chase experiments and determinations of endoglycosidase H (endo H) sensitivity showed that endolyn-78 is derived from a precursor of Mr 58-62 kD that is processed to the mature form with a t1/2 of 15-30 min. The protein has a 22-kD polypeptide backbone that is detected after a brief pulse in tunicamycin-treated cells. During a chase in the presence of the drug, this is converted into an O-glycosylated product of 46 kD that despite the absence of N-linked oligosaccharides is effectively transferred to lysosomes. This demonstrates that the delivery of endolyn-78 to this organelle is not mediated by the mannose-6-phosphate receptor (MPR). Immunocytochemical experiments showed that endolyn-78 is present in the limiting membranes and the interior membranous structures of morphologically identifiable secondary lysosomes that contain the lysosomal hydrolase beta-glucuronidase, lack the MPR, and could not be labeled with alpha-2-macroglobulin at 18.5 degrees C, a temperature which prevents appearance of endocytosed markers in lysosomes. Endolyn-78 was present at low levels in the plasma membrane and in peripheral tubular endosomes, but was prominent in morphologically diverse components of the endosomal compartment (vacuolar endosomes and various types of multivesicular bodies) which acquired alpha-2-macroglobulin at 18.5 degrees C, and frequently contained substantial levels of the MPR and variable levels of beta-glucuronidase. On the other hand, the MPR was very rarely found in endolyn-containing structures that were not labeled with alpha-2-macroglobulin at the low temperature. Thus, the process of lysosomal maturation appears to involve the progressive delivery of lysosomal enzymes to various types of endosomes that may have already received some of the lysosomal membrane proteins. Although endolyn-78 would be one of the proteins added early to endosomes, other lysosomal membrane proteins may be added only to multivesicular endosomes that represent very advanced stages of maturation.

Selective retention of monoglucosylated high mannose oligosaccharides by a class of mutant vesicular stomatitis virus G proteins

Cells infected with a temperature-sensitive mutant of vesicular stomatitis virus, ts045, or transfected with the plasmid vector pdTM12 produce mutant forms of the G protein that remain within the ER. The mutant G proteins were isolated by immunoprecipitation from cells metabolically labeled with [2-3H]mannose to facilitate analysis of the protein-linked oligosaccharides. The 3H-labeled glycopeptides recovered from the immunoprecipitated G proteins contained high mannose-type oligosaccharides. Structural analysis, however, indicated that 60-78% of the 3H-mannose-labeled oligosaccharides contained a single glucose residue and no fewer than eight mannose residues. The 3H-labeled ts045 oligosaccharides were deglucosylated and processed to complex-type units after the infected cells were returned to the permissive temperature. When shifted to the permissive temperature in the presence of a proton ionophore, the G protein oligosaccharides were deglucosylated but remained as high mannose-type units. The glucosylated state was observed, therefore, when the G protein existed in an altered conformation. The ts045 G protein oligosaccharides were deglucosylated in vitro by glucosidase II at both the permissive and nonpermissive temperatures. G protein isolated from ts045-infected cells labeled with [6-3H]galactose in the presence of cycloheximide contained 3H-glucose-labeled monoglucosylated oligosaccharides, indicating that the high mannose oligosaccharides were glucosylated in a posttranslational process. These results suggest that aberrant G proteins are selectively modified by resident ER enzymes to retain monoglucosylated oligosaccharides.

Purification and characterization of glucosidase II involved in N-linked glycoprotein processing in bovine mammary gland

Glucosidase II is an endoplasmic-reticulum-localized enzyme that cleaves the two internally alpha-1,3-linked glucosyl residues of the oligosaccharide Glc alpha 1----2Glc alpha 1----3Glc alpha 1----3Man5-9GlcNAc2 during the biosynthesis of asparagine-linked glycoproteins. We have purified this enzyme to homogeneity from the lactating bovine mammary gland. The enzyme is a high-mannose-type asparagine-linked glycoprotein with a molecular mass of approx. 290 kDa. Upon SDS/polyacrylamide-gel electrophoresis under reducing conditions, the purified enzyme shows two subunits of 62 and 64 kDa, both of which are glycosylated. The pH optimum is between 6.6 and 7.0. Specific polyclonal antibodies raised against the bovine mammary enzyme also recognize a similar antigen in heart, liver and the mammary gland of bovine, guinea pig, rat and mouse. These antibodies were used to develop a sensitive enzyme-linked immunosorbent assay for glucosidase II.

Characterization of calf liver glucosidase I and its inhibition by basic sugar analogs

Glucosidase I, the enzyme catalyzing the first step of N-linked oligosaccharide processing, has been purified from calf liver crude membranes [H. Hettkamp, G. Legler, and E. Bause, (1984) Eur. J. Biochem. 142, 85-90]. Binding experiments with concanavalin A-Sepharose suggest that glucosidase I is a glycoprotein with high-mannose carbohydrate chain(s). The enzyme has a subunit molecular mass of approximately 83 kDa and specifically hydrolyzes the terminal alpha-1,2-linked glucose residue from the natural Glc3-Man9-GlcNAc2 oligosaccharide. Studies with a variety of substrates modified in the aglycon moiety suggest that the Glc2 branch rather than the more distant domains of the substrate molecule are important for binding and hydrolysis. Glucosidase I does not require metal ions for activity and is strongly inhibited by 1-deoxynojirimycin (dNM) and its N-alkyl derivatives. Ki values range from 0.07 microM for N-methyl-dNM to 1.0 microM for dNM, measured at the pH-optimum of enzyme activity. The pH dependence of inhibition indicates that the cationic form of the inhibitors is the active species. Comparison of the Ki for N-decanoyl-dNM (approximately 70 microM) with that of N-decyl-dNM (approximately 0.4 microM) suggests that electrostatic interactions at the catalytic site of the enzyme are important for inhibitor binding. 1-Deoxymannojirimycin, previously assumed to be a specific mannosidase inhibitor, as well as its N-methyl and N-5-carboxypentyl derivatives, inhibit glucosidase I with Ki values around 190, 17, and 100 microM, respectively. This apparent lack of specificity shows that in vivo experiments on N-glycoprotein processing as well as the interpretation of results with these mannosidase inhibitors may give misleading results when these compounds are used in the millimolar range.

1,4-Dideoxy-1,4-imino-D-mannitol inhibits glycoprotein processing and mannosidase

1,4-Dideoxy-1,4-imino-D-mannitol (DIM) was synthesized chemically from benzyl-alpha-D-mannopyranoside [Fleet et al (1984) J. Chem. Soc. Chem. Commun., 1240-1241], and was tested in vitro as an inhibitor of various alpha-mannosidases and in cell culture as an inhibitor of glycoprotein processing. DIM proved to be an effective inhibitor of jack bean alpha-mannosidase, with 50% inhibition requiring 25 to 50 ng/ml inhibitor. It also inhibited lysosomal alpha-mannosidase, but in this case 50% inhibition required about 1 to 2 micrograms/ml. In both cases, the inhibition was of the competitive type when p-nitrophenyl-alpha-D-mannopyranoside was used as the substrate. The inhibition was better at higher pH values, suggesting that DIM was more effective when the nitrogen in the ring was in the unprotonated form. In addition, rat liver processing mannosidase I was also inhibited by DIM as measured by the release of [3H]mannose from [3H]mannose-labeled Man9GlcNAc. Glycoprotein processing was examined in influenza virus-infected MDCK cells. Infected cells were incubated in various concentrations of DIM and labeled with [2-3H]mannose. Viral and cell pellets were digested with Pronase and glycopeptides were isolated by gel filtration on columns of Bio-Gel P-4. The glycopeptides were then treated with endoglucosaminidase H (Endo H) and rechromatographed on the Bio-Gel column in order to distinguish complex from high-mannose structures. As the DIM concentration in the medium was raised, more and more of the [3H]mannose was incorporated into high-mannose oligosaccharides, and less and less radioactivity was in the complex chains. Most of the Endo H-released oligosaccharides induced by DIM were of the Man9GlcNAc structure, as determined by gel filtration, HPLC, and digestion by alpha-mannosidase. Thus, DIM also appears to inhibit mannosidase I in cell culture. However, about 15% of the Endo H-released oligosaccharides appear to be hybrid types of oligosaccharides, suggesting that DIM may also inhibit mannosidase II.

A simple and reliable assay for glycoprotein-processing glycosidases

A simple and reproducible assay to measure the activity of the glycoprotein-processing glycosidases, i.e., glucosidases and mannosidases, is described. This assay takes advantage of the fact that high-mannose and glucose-containing high-mannose oligosaccharides bind to columns of concanavalin A-Sepharose, but the liberated glucose and mannose residues emerge from these columns in the wash. Thus, using [3H]mannose-labeled Man9-N-acetylglucosamine (Man9GlcNAc) or [3H]glucose-labeled Glc3Man7-9-GlcNAc as substrates, the amount of radioactivity in the wash can be quickly and efficiently determined as a measure of enzyme activity. Although the use of this assay was reported previously [B. Saunier et al., 1982, J. Biol. Chem. 257, 14155-14161], the details of its use, its reproducibility, and the problems with interfering materials have not been thoroughly described. In this report, we show that the assay is linear with time and protein concentration, and shows the expected kinetics with various processing inhibitors. The assay works well with the microsomal enzyme preparation and with a solubilized enzyme fraction. In addition, methods are described for the preparation of various radioactive oligosaccharide substrates (i.e., Man9GlcNAc and Glc3Man7-9GlcNAc) using appropriate glycoprotein-processing inhibitors.

The effect of castanospermine on the oligosaccharide structures of glycoproteins from lymphoma cell lines

The effect of castanospermine on the processing of N-linked oligosaccharides was examined in the parent mouse lymphoma cell line and in a mutant cell line that lacks glucosidase II. When the parent cell line was grown in the presence of castanospermine at 100 micrograms/ml, glucose-containing high-mannose oligosaccharides were obtained that were not found in the absence of inhibitor. These oligosaccharides bound tightly to concanavalin A-Sepharose and were eluted in the same position as oligosaccharides from the mutant cells grown in the absence or presence of the alkaloid. The castanospermine-induced oligosaccharides were characterized by gel filtration on Bio-Gel P-4, by h.p.l.c. analysis, by enzymic digestions and by methylation analysis of [3H]mannose-labelled and [3H]galactose-labelled oligosaccharides. The major oligosaccharide released by endoglucosaminidase H in either parent or mutant cells grown in castanospermine was a Glc3Man7GlcNAc, with smaller amounts of Glc3Man8GlcNAc and Glc3Man9GlcNAc. On the other hand, in the absence of castanospermine the mutant produces mostly Glc2Man7GlcNAc. In addition to the above oligosaccharides, castanospermine stimulated the formation of an endoglucosaminidase H-resistant oligosaccharide in both cell lines. This oligosaccharide was characterized as a Glc2Man5GlcNAc2 (i.e., Glc(1,2)Glc(1,3)Man(1,2)Man(1,2)Man(1,3)[Man(1,6)]Man-GlcNAc-GlcNAc). Castanospermine was tested directly on glucosidase I and glucosidase II in lymphoma cell extracts by using [Glc-3H]Glc3Man9GlcNAc and [Glc-3H]Glc2Man9GlcNAc as substrates. Castanospermine was a potent inhibitor of both activities, but glucosidase I appeared to be more sensitive to inhibition.

Comparison between 1-deoxynojirimycin and N-methyl-1-deoxynojirimycin as inhibitors of oligosaccharide processing in intestinal epithelial cells

The alpha-glucosidase inhibitor N-methyl-1-deoxynojirimycin (MDJN) inhibits the synthesis of N-linked complex oligosaccharides in rat intestinal epithelial cells to the same extent as reported previously for 1-deoxynojirimycin (DJN) [Saunier, Kilker, Tkacz, Quaroni & Herscovics (1982) J. Biol. Chem. 257, 14155-14161]. Analysis of each of the endo-beta-N-acetylglucosaminidase H (endo H)-sensitive oligosaccharides separated by h.p.l.c. with yeast glucosidase I, which specifically removes the terminal glucose residue from oligosaccharides containing three glucose residues, and with jack-bean (Canavalia ensiformis) alpha-mannosidase, indicates that both inhibitors cause the accumulation of a mixture of glucosylated oligosaccharides containing one to three glucose residues and seven to nine, and even possibly six, mannose residues. About 70% of the endo H-sensitive oligosaccharides formed in the presence of MDJN contain three glucose residues, compared with only about 20% of the corresponding oligosaccharides of the DJN treated cells. It is concluded that both compounds inhibit the formation of N-linked complex oligosaccharides by interfering with the processing glucosidases. These compounds are valuable in the study of the role of oligosaccharides in glycoproteins.