Isolation of Chinese hamster ovary cell lines producing Man3GlcNAc2 asparagine-linked glycans

N-glycosylation of mannose receptor (CD206) regulates glycan binding by C-type lectin domains

The macrophage mannose receptor (MR, CD206) is a transmembrane endocytic lectin receptor, expressed in selected immune and endothelial cells, and is involved in immunity and maintaining homeostasis. Eight of the ten extracellular domains of the MR are C-type lectin domains (CTLDs) which mediate the binding of mannose, fucose, and GlcNAc in a calcium-dependent manner. Previous studies indicated that self-glycosylation of MR regulates its glycan binding. To further explore this structure-function relationship, we studied herein a recombinant version of mouse MR CTLD4-7 fused to human Fc-portion of IgG (MR-Fc). The construct was expressed in different glycosylation-mutant cell lines to study the influence of differential glycosylation on receptor glycan-binding properties. We conducted site-specific N- and O-glycosylation analysis and glycosylation site characterization using mass spectrometry by which several novel O-glycosylation sites were identified in mouse MR and confirmed in human full-length MR. This information guided experiments evaluating the receptor functionality by glycan microarray analysis in combination with glycan-modifying enzymes. Treatment of active MR-Fc with combinations of exoglycosidases, including neuraminidase and galactosidases, resulted in the loss of trans-binding (binding of MR CTLDs to non-MR glycans), due to unmasking of terminal, nonreducing GlcNAc in N-glycans of the MR CTLDs. Regalactosylation of N-glycans rescues mannose binding by MR-Fc. Our results indicate that glycans within the MR CTLDs act as a regulatory switch by masking and unmasking self-ligands, including terminal, nonreducing GlcNAc in N-glycans, which could control MR activity in a tissue- and cell-specific manner or which potentially affect bacterial pathogenesis in an immunomodulatory fashion.

Biochemical characterization, membrane association and identification of amino acids essential for the function of Alg11 from Saccharomyces cerevisiae, an alpha1,2-mannosyltransferase catalysing two sequential glycosylation steps in the formation of the lipid-linked core oligosaccharide

The biosynthesis of asparagine-linked glycans occurs in an evolutionarily conserved manner with the assembly of the unique lipid-linked oligosaccharide precursor Glc3Man9GlcNAc2-PP-Dol at the ER (endoplasmic reticulum). In the present study we characterize Alg11 from yeast as a mannosyltransferase catalysing the sequential transfer of two alpha1,2-linked mannose residues from GDP-mannose to Man3GlcNAc2-PP-Dol and subsequently to Man4GlcNAc2-PP-Dol forming the Man5GlcNAc2-PP-Dol intermediate at the cytosolic side of the ER before flipping to the luminal side. Alg11 is predicted to contain three hydrophobic transmembrane-spanning helices. Using Alg11 topology reporter fusion constructs, we show that only the N-terminal domain fulfils this criterion. Surprisingly, this domain can be deleted without disturbing glycosyltransferase function and membrane association, indicating also that the other two hydrophobic domains contribute to ER localization, but in a non-transmembrane manner. By site-directed mutagenesis we investigated amino acids important for transferase activity. We demonstrate that the first glutamate residue in the EX7E motif, conserved in a variety of glycosyltransferases, is more critical than the second, and loss of Alg11 function occurs only when both glutamate residues are exchanged, or when the mutation of the first glutamate residue is combined with replacement of another amino acid in the motif. This indicates that perturbations in EX7E are not restricted to the second glutamate residue. Moreover, Gly85 and Gly87, within a glycine-rich domain as part of a potential flexible loop, were found to be required for Alg11 function. Similarly, a conserved lysine residue, Lys319, was identified as being important for activity, which could be involved in the binding of the phosphate of the glycosyl donor.

Biochemical characterization and membrane topology of Alg2 from Saccharomyces cerevisiae as a bifunctional alpha1,3- and 1,6-mannosyltransferase involved in lipid-linked oligosaccharide biosynthesis

N-Linked glycosylation involves the ordered, stepwise synthesis of the unique lipid-linked oligosaccharide precursor Glc(3)Man(9) GlcNAc(2)-PP-Dol on the endoplasmic reticulum (ER), catalyzed by a series of glycosyltransferases. Here we characterize Alg2 as a bifunctional enzyme that is required for both the transfer of the alpha1,3- and the alpha1,6-mannose-linked residue from GDP-mannose to Man(1)GlcNAc(2)-PP-Dol forming the Man(3)GlcNAc(2)-PP-Dol intermediate on the cytosolic side of the ER. Alg2 has a calculated mass of 58 kDa and is predicted to contain four transmembrane-spanning helices, two at the N terminus and two at the C terminus. Contradictory to topology predictions, we prove that only the two N-terminal domains fulfill this criterion, whereas the C-terminal hydrophobic sequences contribute to ER localization in a nontransmembrane manner. Surprisingly, none of the four domains is essential for transferase activity because truncated Alg2 variants can exert their function as long as Alg2 is associated with the ER by either its N- or C-terminal hydrophobic regions. By site-directed mutagenesis we demonstrate that an EX(7)E motif, conserved in a variety of glycosyltransferases, is not important for Alg2 function in vivo and in vitro. Instead, we identify a conserved lysine residue, Lys(230), as being essential for activity, which could be involved in the binding of the phosphate of the glycosyl donor.

Specificity of DC-SIGN for mannose- and fucose-containing glycans

The dendritic cell specific C-type lectin dendritic cell specific ICAM-3 grabbing non-integrin (DC-SIGN) binds to "self" glycan ligands found on human cells and to "foreign" glycans of bacterial or parasitic pathogens. Here, we investigated the binding properties of DC-SIGN to a large array of potential ligands in a glycan array format. Our data indicate that DC-SIGN binds with K(d)<2muM to a neoglycoconjugate in which Galbeta1-4(Fucalpha1-3)GlcNAc (Le(x)) trisaccharides are expressed multivalently. A lower selective binding was observed to oligomannose-type N-glycans, diantennary N-glycans expressing Le(x) and GalNAcbeta1-4(Fucalpha1-3)GlcNAc (LacdiNAc-fucose), whereas no binding was observed to N-glycans expressing core-fucose linked either alpha1-6 or alpha1-3 to the Asn-linked GlcNAc of N-glycans. These results demonstrate that DC-SIGN is selective in its recognition of specific types of fucosylated glycans and subsets of oligomannose- and complex-type N-glycans.

Biosynthesis of lipid-linked oligosaccharides in Saccharomyces cerevisiae: Alg13p and Alg14p form a complex required for the formation of GlcNAc(2)-PP-dolichol

N-Glycosylation in the endoplasmic reticulum is an essential protein modification and highly conserved in evolution from yeast to man. Here we identify and characterize two essential yeast proteins having homology to bacterial glycosyltransferases, designated Alg13p and Alg14p, as being required for the formation of GlcNAc(2)-PP-dolichol (Dol), the second step in the biosynthesis of the unique lipid-linked core oligosaccharide. Down-regulation of each gene led to a defect in protein N-glycosylation and an accumulation of GlcNAc(1)-PP-Dol in vivo as revealed by metabolic labeling with [(3)H]glucosamine. Microsomal membranes from cells repressed for ALG13 or ALG14, as well as detergent-solubilized extracts thereof, were unable to catalyze the transfer of N-acetylglucosamine from UDP-GlcNAc to [(14)C]GlcNAc(1)-PP-Dol, but did not impair the formation of GlcNAc(1)-PP-Dol or GlcNAc-GPI. Immunoprecipitating Alg13p from solubilized extracts resulted in the formation of GlcNAc(2)-PP-Dol but required Alg14p for activity, because an Alg13p immunoprecipitate obtained from cells in which ALG14 was down-regulated lacked this activity. In Western blot analysis it was demonstrated that Alg13p, for which no well defined transmembrane segment has been predicted, localizes both to the membrane and cytosol; the latter form, however, is enzymatically inactive. In contrast, Alg14p is exclusively membrane-bound. Repression of the ALG14 gene causes a depletion of Alg13p from the membrane. By affinity chromatography on IgG-Sepharose using Alg14-ZZ as bait, we demonstrate that Alg13-myc co-fractionates with Alg14-ZZ. The data suggest that Alg13p associates with Alg14p to a complex forming the active transferase catalyzing the biosynthesis of GlcNAc(2)-PP-Dol.

Metformin-stimulated Mannose Transport in Dermal Fibroblasts

The biguanide drug metformin stimulates AMP-activated protein kinase, a master regulator of cellular energy metabolism, and has antihyperglycemic activity due to attenuation of gluconeogenesis in hepatocytes and 2-fold stimulation of glucose transport by skeletal muscle. Here we identify a metformin-stimulated d-mannose transport (MSMT) activity in dermal fibroblasts. MSMT increased mannose uptake 1.8-fold and had greater affinity for mannose than basal mannose transport activity. It was attributed to robust stimulation of a transporter expressed weakly in untreated cells. MSMT was not explained by greater glucose transporter activity because metformin unexpectedly decreased transport of 2-deoxy-d-glucose and 3-O-methyl-d-glucose by fibroblasts. Effective inhibitors of MSMT retained specificity for the 3-, 4-, and 6-OH groups of the mannose ring but not the 2-OH group. Thus, MSMT could be strongly inhibited by glucose and 2-deoxy-d-glucose even though the latter was not a good transport substrate. MSMT was significant because in the presence of 2.5 microm mannose, metformin corrected experimentally induced deficiencies in the synthesis of glucose(3)mannose(9)GlcNAc(2)-P-P-dolichol and N-linked glycosylation. MSMT was also identified in congenital disorder of glycosylation types Ia and Ib fibroblasts, and metformin acted synergistically with 100 microm mannose to correct lipid-linked oligosaccharide synthesis and N-glycosylation in the Ia cells. In conclusion, metformin activates a novel fibroblast mannose-selective transport system. This suggests that AMP-activated protein kinase may be a regulator of mannose metabolism and implies a therapy for congenital disorders of glycosylation-Ia.

Activation of glycogen phosphorylase with 5-aminoimidazole-4-carboxamide riboside (AICAR). Assessment of glycogen as a precursor of mannosyl residues in glycoconjugates

The experimental evaluation of the contribution of glycogen phosphorylase (GP) to biochemical pathways is limited to methods that raise cAMP, activating the cAMP-dependent protein kinase/phosphorylase kinase/GP cascade. Such methods convert the unphosphorylated form, "GPb," which catalyzes glycogenolysis only in the presence of appropriate allosteric activators such as AMP, to the phosphorylated, constitutively activated form, "GPa." However, activation of GP in this way is indirect, requires a functional cAMP kinase cascade, and is complicated by other actions of cAMP. Here, we demonstrate a strategy for the experimental manipulation of GP in intact dermal fibroblasts, involving activation by the membrane-permeable adenosine analog 5-aminoimidazole-4-carboxamide riboside (AICAR) and inhibition by caffeine and Pfizer compound CP-91149, which bind to GP at distinct sites. Potential complications because of activation of AMP-activated protein kinase by AICAR were assessed with metformin, which activates this kinase but does not activate GP. Using this strategy, we show that glycogen can be a significant and regulatable precursor of mannosyl units in lipid-linked oligosaccharides and glycoproteins.

Coupling of the dolichol-P-P-oligosaccharide pathway to translation by perturbation-sensitive regulation of the initiating enzyme, GlcNAc-1-P transferase

In mammalian cells, inhibition of translation interferes with synthesis of the lipid-linked oligosaccharide (LLO) Glc3Man9GlcNAc2-P-P-dolichol as measured with radioactive sugar precursors. Conflicting hypotheses have been proposed, and the fundamental basis for this regulation has remained elusive. Here, fluorophore-assisted carbohydrate electrophoresis (FACE) was used to measure LLO concentrations directly in cells treated with translation blockers. Further, LLO biosynthetic enzymes were assayed in vitro with endogenous acceptor substrates using either cells gently permeabilized with streptolysin-O (SLO) or microsomes from homogenized cells. In Chinese hamster ovary (CHO)-K1 cells treated with translation blockers, FACE did not detect changes in concentrations of Glc3Man9GlcNAc2-P-P-dolichol or early LLO intermediates. These results do not support earlier proposals for feedback repression of LLO initiation by accumulated Glc3Man9GlcNAc2-P-P-dolichol, or inhibition of a GDP-mannose dependent transferase. With microsomes from cells treated with translation blockers, there was no interference with LLO initiation by GlcNAc-1-P transferase (GPT), mannose-P-dolichol synthase, glucose-P-dolichol synthase, or LLO synthesis in vitro, as reported previously. Surprisingly, inhibition of all of these was detected with the SLO in vitro system. Additional experiments with the SLO system showed that the three transferases shared a limited pool of dolichol-P that was trapped as Glc3Man9GlcNAc2-P-P-dolichol by translation arrest. Overexpression of GPT was unable to reverse the effects of translation arrest on LLO initiation, and experiments with FACE and the SLO system showed that overexpressed GPT was not functional in vivo, although it was highly active in microsomal assays. Thus, the combined use of the SLO in vitro system and FACE showed that LLO biosynthesis depends upon a limited primary pool of dolichol-P. Physical perturbation associated with microsome preparation appears to make available a secondary pool of dolichol-P, masking inhibition by translation arrest, as well as activating a nonfunctional fraction of GPT. The implications of these results for the organization of the LLO pathway are discussed.

Analyses of dolichol pyrophosphate-linked oligosaccharides in cell cultures and tissues by fluorophore-assisted carbohydrate electrophoresis

Lipid-linked oligosaccharides (LLOs) are the precursors of asparagine (N)-linked glycans, which are essential information carriers in many biological systems, and defects in LLO synthesis cause Type I congenital disorders of glycosylation. Due to the low abundance of LLOs and the limitations of the chemical and physical methods previously used to detect them, simple and sensitive nonradioactive methods for LLO analysis are lacking. Thus, almost all studies of LLO synthesis have relied on metabolic labeling of the oligosaccharides with radioactive sugar precursors. We report that LLOs in cell cultures and tissues can be easily detected and quantified with a sensitivity of 1-2 pmol by fluorophore-assisted carbohydrate electrophoresis (FACE). These analyses required efficient removal of contaminants, most likely trace quantities of glycogen breakdown products, that interfered with FACE. Studies with CHO-K1 cells showed that LLOs detected by FACE and by metabolic labeling had similar turnover rates. Glc(3)Man(9)GlcNAc(2)-P-P-dolichol was the most prominent LLO detected by FACE in normal cultured cells and mouse tissues. However, the relative amounts of Glc(0-2)Man(5-9)GlcNAc(2)-P-P-dolichol intermediates in tissues, such as liver and kidney, were unexpectedly greater than for cultured cells. IV injection of D-mannose, raising the circulatory concentration by three- to fourfold, did not affect LLO composition. Thus, the relative accumulation of LLO intermediates in mouse liver and kidney is not likely due to inadequate D-mannose in the circulation. In summary, FACE is a facile, accurate, and sensitive method for LLO analysis, permitting investigations not feasible by metabolic labeling.

Requirement of the Lec35 gene for all known classes of monosaccharide-P-dolichol-dependent glycosyltransferase reactions in mammals

The Lec35 gene product (Lec35p) is required for utilization of the mannose donor mannose-P-dolichol (MPD) in synthesis of both lipid-linked oligosaccharides (LLOs) and glycosylphosphatidylinositols, which are important for functions such as protein folding and membrane anchoring, respectively. The hamster Lec35 gene is shown to encode the previously identified cDNA SL15, which corrects the Lec35 mutant phenotype and predicts a novel endoplasmic reticulum membrane protein. The mutant hamster alleles Lec35.1 and Lec35.2 are characterized, and the human Lec35 gene (mannose-P-dolichol utilization defect 1) was mapped to 17p12-13. To determine whether Lec35p was required only for MPD-dependent mannosylation of LLO and glycosylphosphatidylinositol intermediates, two additional lipid-mediated reactions were investigated: MPD-dependent C-mannosylation of tryptophanyl residues, and glucose-P-dolichol (GPD)-dependent glucosylation of LLO. Both were found to require Lec35p. In addition, the SL15-encoded protein was selective for MPD compared with GPD, suggesting that an additional GPD-selective Lec35 gene product remains to be identified. The predicted amino acid sequence of Lec35p does not suggest an obvious function or mechanism. By testing the water-soluble MPD analog mannose-beta-1-P-citronellol in an in vitro system in which the MPD utilization defect was preserved by permeabilization with streptolysin-O, it was determined that Lec35p is not directly required for the enzymatic transfer of mannose from the donor to the acceptor substrate. These results show that Lec35p has an essential role for all known classes of monosaccharide-P-dolichol-dependent reactions in mammals. The in vitro data suggest that Lec35p controls an aspect of MPD orientation in the endoplasmic reticulum membrane that is crucial for its activity as a donor substrate.

Beta-1,4-galactosylation of N-glycans is a complex process

Most beta-1,4-galactosyltransferase (beta-1,4-GalT)-knockout mice die after birth. Although several defects were found transiently in these animals, the primary cause of death is obscure. Not only beta-1,4-linked galactose residues on N-glycans, but also beta-1, 4-GalT activities were found in some of the tissues. Recently, five human genes which encode beta-1,4-GalTs have been cloned, and the possible presence of such novel beta-1,4-GalTs in mice is considered to bring about survival of the mutant animal beyond birth. In order to understand the semi-lethal nature of this animal, it is inevitable to clarify how individual novel beta-1,4-GalTs are involved in the biosynthesis of glycoconjugates based on their acceptor-substrate specificities.

Regulation of the dolichol pathway in human fibroblasts by the endoplasmic reticulum unfolded protein response

Accumulation of unfolded proteins within the endoplasmic reticulum (ER) of eukaryotic cells triggers the unfolded protein response (UPR), which activates transcription of several genes encoding ER chaperones and folding enzymes. This study reports that conversion of dolichol-linked Man(2-5)GlcNAc(2) intermediates into mature Glc(3)Man(9)GlcNAc(2) oligosaccharides in primary human adult dermal fibroblasts is also stimulated by the UPR. This stimulation was not evident in several immortal cell lines and did not require a cytoplasmic stress response. Inhibition of dolichol-linked Glc(3)Man(9)GlcNAc(2) synthesis by glucose deprivation could be counteracted by the UPR, improving the transfer of Glc(3)Man(9)GlcNAc(2) to asparagine residues on nascent polypeptides. Glycosidic processing of asparagine-linked Glc(3)Man(9)GlcNAc(2) in the ER leads to the production of monoglucosylated oligosaccharides that promote interaction with the lectin chaperones calreticulin and calnexin. Thus, control of the dolichol-linked Glc(3)Man(9)GlcNAc(2) supply gives the UPR the potential to maintain efficient protein folding in the ER without new synthesis of chaperones or folding enzymes.

Chinese hamster ovary cells with reduced hexokinase activity maintain normal GDP-mannose levels

Parental Chinese hamster ovary (CHO) cells were mutagenized and subjected first to a mannose suicide selection technique and second to a screen of individual colonies grown on polyester discs for reduced mannose incorporation into protein. The incorporation of radioactivity for the selection and the screen was conducted at 41.5 degrees C instead of the normal growth temperature of 34 degrees C in order to allow for the isolation of temperature-sensitive lesions. This selection/screening procedure resulted in the isolation of M15-4 cells, which had three- to five-fold lower incorporation of [2-3H]mannose into mannose 6-phosphate, mannose 1-phosphate, GDP-mannose, oligosaccharide-lipid, and glycoprotein at 41.5 degrees C. We detected no difference in the qualitative pattern of mannose-labeled lipid-linked oligosaccharides compared to parental cells. M15-4 cells synthesized dolichol. The defect of M15-4 cells was determined to be in hexokinase activity; crude cytosolic extracts were eight- to nine-fold lower in hexokinase activity in M15-4 cells compared to parental cells. As a result of this defect, incorporation of labeled mannose from the medium was significantly decreased. However, the level of GDP-mannose in M15-4 cells was 70% of normal. The phenotype of M15-4 was a lower specific activity of labeled GDP-mannose, not a substantial reduction in the level of GDP-mannose. Consistent with these results, no alterations in the glycosylation of a model glycoprotein, G protein of vesicular stomatitis virus, were observed. These cells grew slower than parental cells, especially in low-glucose medium.

Essential role for complex N-glycans in forming an organized layer of bronchial epithelium

Mice lacking the complex subset of N-glycans due to inactivation of the Mgat1 gene die at mid-gestation, making it difficult to identify specific biological functions for this class of cell surface carbohydrates. To circumvent this embryonic lethality and to uncover tissue-specific functions for complex N-glycans, WW6 embryonic stem cells with inactivated Mgat1 alleles were tracked in chimeric embryos. The Mgat1 gene encodes N-acetylglucosaminyltransferase I (Glc-NAc-TI; EC 2.4.1.101), the transferase that initiates the synthesis of complex N-glycans. WW6 cells carry an inert beta-globin transgene that allows their identification in chimeras by DNA-DNA in situ hybridization. Independent Mgat1-/- and Mgat1+/- mutant WW6 isolates contributed like parent WW6 cells to the tissues of embryonic day (E) 10.5 to E16.5 chimeras. However, a cell type-specific difference was observed in lung. Homozygous null Mgat1-/- WW6 cells did not contribute to the epithelial layer in more than 99% bronchi. This deficiency was corrected by transfection of a Mgat1 transgene. Interestingly, heterozygous Mgat1+/- WW6 cells were also deficient in populating the layer of bronchial epithelium. Furthermore, examination of lung bud in E9.5 Mgat1-/- mutant embryos showed complete absence of an organized epithelial cell layer in the bronchus. Thus, complex N-glycans are required to form a morphologically recognizable bronchial epithelium, revealing an in vivo, cell type-specific function for this class of N-glycans.

Expression cloning of a novel suppressor of the Lec15 and Lec35 glycosylation mutations of Chinese hamster ovary cells

Lec15 and Lec35 are recessive Chinese hamster ovary (CHO) cell glycosylation mutations characterized by inefficient synthesis and utilization, respectively, of mannose-P-dolichol (MPD). Consequently, Lec15 and Lec35 cells accumulate Man5GlcNAc2-P-P-dolichol and glucosaminyl-acylphosphatidylinositol. This report describes the cloning of a suppressor (termed SL15) of the Lec15 and Lec35 mutations from a CHO cDNA library by functional expression in Lec15 cells, employing phytohemagglutinin/swainsonine selection. The SL15 protein has a predicted molecular weight of 26,693 with two potential membrane spanning regions and a likely C-terminal endoplasmic reticulum retention signal (Lys-Lys-Glu-Gln). Lec15 cells transfected with SL15 have normal levels of MPD synthase activity in vitro and convert Man5GlcNAc2-P-P-dolichol to Glc0-3Man9GlcNAc2-P-P-dolichol in vivo. Surprisingly, SL15 also corrects the defective mannosylation in Lec35 cells. The SL15 protein bears no apparent similarity to Saccharomyces cerevisiae MPD synthase (the DPM1 protein), but is highly similar to the hypothetical F38E1.9 protein encoded on Caenorhabditis elegans chromosome 5. These results indicate a novel function for the SL15 protein and suggest that MPD synthesis is more complex than previously suspected.

Glycosylation of a novel member of the immunoglobulin gene superfamily expressed in rat carcinoma cell lines

MAb E4 recognizes a 66-kDa glycoprotein, pE4, which is a member of the immunoglobulin gene superfamily. This protein is expressed at the cell surface in rat colon and mammary carcinomas, but only in trace amounts in normal adult rat tissues. Since expression of aberrant carbohydrate structures is often associated with malignant transformation, glycosylation of pE4 was analyzed. Reactivity of lectins with pE4 suggested the absence of N-acetylneuraminic acid, terminal galactose and O-linked glycan, and the presence of N-linked glycans. Tunicamycin treatment reduced the binding of MAb E4 to cancer cells suggesting that the E4 epitope is at least partially glycosylated. Digestions with neuraminidases, O-glycosidase and peptide-N-glycosidase F confirmed these results. Pronase treatment abolished the binding of MAb E4, indicating that E4 epitope involves not only a carbohydrate determinant but also a peptide moiety. Mild periodate oxidation abolished the binding of MAb E4, indicating that non-reducing terminus carbohydrates are part of the E4 epitope. Neutral sugar analysis revealed the absence of galactose and the presence of fucose. Since fucose is sensitive to periodate oxidation, this sugar could be the carbohydrate part of the determinant recognized by MAb E4. Reactivity of lectins specific for fucose indicated the presence of alpha(1-6)-fucose on pE4.

The molecular chaperone calnexin binds Glc1Man9GlcNAc2 oligosaccharide as an initial step in recognizing unfolded glycoproteins

Calnexin is a molecular chaperone that resides in the membrane of the endoplasmic reticulum. Most proteins that calnexin binds are N-glycosylated, and treatment of cells with tunicamycin or inhibitors of initial glucose trimming steps interferes with calnexin binding. To test if calnexin is a lectin that binds early oligosaccharide processing intermediates, a recombinant soluble calnexin was created. Incubation of soluble calnexin with a mixture of Glc0-3Man9GlcNAc2 oligosaccharides resulted in specific binding of the Glc1Man9GlcNAc2 species. Furthermore, Glc1Man5-7GlcNAc2 oligosaccharides bound relatively poorly, suggesting that, in addition to a requirement for the single terminal glucose residue, at least one of the terminal mannose residues was important for binding. To assess the involvement of oligosaccharide-protein interactions in complexes of calnexin and newly synthesized glycoproteins, alpha 1-antitrypsin or the heavy chain of the class I histocompatibility molecule were purified as complexes with calnexin and digested with endoglycosidase H. All oligosaccharides on either glycoprotein were accessible to this probe and could be removed without disrupting the association with calnexin. Furthermore, the addition of 1 M alpha-methyl glucoside or alpha-methyl mannoside had no effect on complex stability. These findings suggest that once complexes between calnexin and glycoproteins are formed, oligosaccharide binding does not contribute significantly to the overall interaction. However, it is likely that the binding of Glc1Man9GlcNAc2 oligosaccharides is a crucial event during the initial recognition of newly synthesized glycoproteins by calnexin.

Analysis of glycoforms present in two mouse IgG2a monoclonal antibody preparations

Lectins have been used for the determination of the oligosaccharide structures expressed by two monoclonal IgG antibodies, MN12 and RIV6. Dot blot experiments revealed the presence of terminal Fuc alpha (1-->6)GlcNAc, Gal beta (1-->3)GalNAc, Gal beta (1-->4)GlcNAc, Man alpha (1-->6, 1-->3)Man, NeuAc alpha (2-->6)Gal and NeuAc alpha (2-->6)GalNAc on both monoclonal antibodies. MN12 was shown to contain a carbohydrate moiety within the Fc region only. RIV6 contained carbohydrate moieties within both the Fc and Fab regions. Additional O-glycosidic linked carbohydrate chains were detected within the Fc region of both monoclonal antibodies. High mannose structures were also detected on both Mabs.

Chinese hamster ovarian cell glycoproteins that mediate type 1 piliated gram-negative bacterial adherence

We used Chinese hamster ovary (CHO) cell lines to define the structures of glycoproteins responsible for Type 1 piliated bacterial adherence. CSH 50 Escherichia coli, a Type 1 piliated bacteria, adhered significantly better than an isogenic nonpiliated E. coli to all CHO lines tested. CSH 50 E. coli adhered least well to CHO cells expressing intact complex type oligosaccharides on cell surface glycoproteins. CSH 50 adherence increased when shorter oligosaccharides were present and was maximal when mannose groups were present in terminal, nonreducing positions. Five high mannose type glycoproteins, with molecular weights of 79, 75, 55, 50, and 37 kD, were identified as high affinity ligands for Type 1 piliated bacteria. Our results suggest that alterations in cell surface carbohydrates may increase adherence of Type 1 piliated gram-negative bacteria to cells.