Regulation of glycosylation. The influence of protein structure on N-linked oligosaccharide processing

Analysis of protein landscapes around N-glycosylation sites from the PDB repository for understanding the structural basis of N-glycoprotein processing and maturation

Asparagine-linked glycans (N-glycans) are attached onto nascent glycoproteins in the endoplasmic reticulum (ER) and subsequently processed by a set of processing enzymes in the ER and Golgi apparatus. Accumulating evidence has shown that not all N-glycans on glycoproteins are uniformly processed into mature forms (hybrid and complex types in mammals) through the ER and Golgi apparatus, and a certain set of glycans remains unprocessed as an "immature" form (high-mannose type in mammals). Much attention has been paid to environmental factors regulating N-glycoprotein maturation, such as the expression levels of glycosyltransferases/glycosidases. On the other hand, the influence of the 3D structure of the carrier glycoprotein on N-glycan maturation has been investigated mostly using individual model glycoproteins. To obtain more insights into N-glycoprotein maturation, we herein analyze glycoprotein structures extracted from the Protein Data Bank. We confirm that site-specific N-glycan processing is largely explained by the solvent accessibility of the glycosylated Asn residue and of the conjugated N-glycan. Potential bias of protein structural features toward immature or mature forms was explored within a range of concentric circles of fully folded glycoproteins. There does appear to be bias in amino acid composition and secondary structure. Most notably, γ-branched amino acid residues (Asn+Asp+Leu) occur more frequently on unstructured loop regions of immature glycoproteins. Structural features of the protein surface around the N-glycosylated site do seem to affect N-glycan processing and maturation.

N-Linked Glycan Branching and Fucosylation Are Increased Directly in Hcc Tissue As Determined through in Situ Glycan Imaging

Hepatocellular carcinoma (HCC) remains as the fifth most common cancer in the world and accounts for more than 700,000 deaths annually. Changes in serum glycosylation have long been associated with this cancer but the source of that material is unknown and direct glycan analysis of HCC tissues has been limited. Our laboratory previously developed a method of in situ tissue based N-linked glycan imaging that bypasses the need for microdissection and solubilization of tissue prior to analysis. We used this methodology in the analysis of 138 HCC tissue samples and compared the N-linked glycans in cancer tissue with either adjacent untransformed or tissue from patients with liver cirrhosis but no cancer. Ten glycans were found significantly elevated in HCC tissues as compared to cirrhotic or adjacent tissue. These glycans fell into two major classes, those with increased levels of fucosylation and those with increased levels of branching with or without any fucose modifications. In addition, increased levels of fucosylated glycoforms were associated with a reduction in survival time. This work supports the hypothesis that the increased levels of fucosylated N-linked glycans in HCC serum are produced directly from the cancer tissue.

Influence of protein/glycan interaction on site-specific glycan heterogeneity

To study how the interaction between N-linked glycans and the surrounding amino acids influences oligosaccharide processing, we used protein disulfide isomerase (PDI), a glycoprotein bearing 5 N-glycosylation sites, as a model system and expressed it transiently in a Chinese hamster ovary (CHO)-S cell line. PDI was produced as both secreted Sec-PDI and endoplasmic reticulum-retained glycoprotein (ER)-PDI, to study glycan processing by ER and Golgi resident enzymes. Quantitative site-specific glycosylation profiles were obtained, and flux analysis enabled modeling site-specific glycan processing. By altering the primary sequence of PDI, we changed the glycan/protein interaction and thus the site-specific glycoprofile because of the improved enzymatic fluxes at enzymatic bottlenecks. Our results highlight the importance of direct interactions between N-glycans and surface-exposed amino acids of glycoproteins on processing in the ER and the Golgi and the possibility of changing a site-specific N-glycan profile by modulating such interactions and thus the associated enzymatic fluxes. Altering the primary protein sequence can therefore be used to glycoengineer recombinant proteins.-Losfeld, M.-E., Scibona, E., Lin, C.-W., Villiger, T. K., Gauss, R., Morbidelli, M., Aebi, M. Influence of protein/glycan interaction on site-specific glycan heterogeneity.

Substrate recognition and catalysis by GH47 α-mannosidases involved in Asn-linked glycan maturation in the mammalian secretory pathway

Maturation of Asn-linked oligosaccharides in the eukaryotic secretory pathway requires the trimming of nascent glycan chains to remove all glucose and several mannose residues before extension into complex-type structures on the cell surface and secreted glycoproteins. Multiple glycoside hydrolase family 47 (GH47) α-mannosidases, including endoplasmic reticulum (ER) α-mannosidase I (ERManI) and Golgi α-mannosidase IA (GMIA), are responsible for cleavage of terminal α1,2-linked mannose residues to produce uniquely trimmed oligomannose isomers that are necessary for ER glycoprotein quality control and glycan maturation. ERManI and GMIA have similar catalytic domain structures, but each enzyme cleaves distinct residues from tribranched oligomannose glycan substrates. The structural basis for branch-specific cleavage by ERManI and GMIA was explored by replacing an essential enzyme-bound Ca²⁺ ion with a lanthanum (La³⁺) ion. This ion swap led to enzyme inactivation while retaining high-affinity substrate interactions. Cocrystallization of La³⁺-bound enzymes with Man₉GlcNAc₂ substrate analogs revealed enzyme-substrate complexes with distinct modes of glycan branch insertion into the respective enzyme active-site clefts. Both enzymes had glycan interactions that extended across the entire glycan structure, but each enzyme engaged a different glycan branch and used different sets of glycan interactions. Additional mutagenesis and time-course studies of glycan cleavage probed the structural basis of enzyme specificity. The results provide insights into the enzyme catalytic mechanisms and reveal structural snapshots of the sequential glycan cleavage events. The data also indicate that full steric access to glycan substrates determines the efficiency of mannose-trimming reactions that control the conversion to complex-type structures in mammalian cells.

Characterization of N-glycosylation profiles from mammalian and insect cell derived chikungunya VLP

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus that causes severe arthralgia. The envelope of CHIKV is composed of 240 copies of two glycoproteins: E1 and E2. In this work, we have characterized the N-glycosylation patterns of CHIKV virus-like particles (VLPs), containing both E1 and E2 proteins, derived from mammalian and insect cells using hydrophilic interaction liquid chromatography (HILIC) with fluorescence (FL) and mass spectrometry (MS) detection. While HEK293 derived CHIKV VLPs contain oligomannose, hybrid and complex glycans, VLPs derived from SfBasic predominantly contain oligomannose glycans. This strong host dependence of N-glycosylation pattern resembles other alphaviruses such as SINV. The VLPs from HEK293 and SfBasic, with significantly different N-glycosylation profiles, are valuable reagents enabling future in-depth correlation studies between immunogenicity and glycosylation. In addition, the characterization tools presented here allow one to monitor glycosylation during vaccine process development and ensure process consistency.

Analysis of site-specific N-glycan remodeling in the endoplasmic reticulum and the Golgi

The hallmark of N-linked protein glycosylation is the generation of diverse glycan structures in the secretory pathway. Dynamic, non-template-driven processes of N-glycan remodeling in the endoplasmic reticulum and the Golgi provide the cellular setting for structural diversity. We applied newly developed mass spectrometry-based analytics to quantify site-specific N-glycan remodeling of the model protein Pdi1p expressed in insect cells. Molecular dynamics simulation, mutational analysis, kinetic studies of in vitro processing events and glycan flux analysis supported the defining role of the protein in N-glycan processing.

Enhanced Aromatic Sequons Increase Oligosaccharyltransferase Glycosylation Efficiency and Glycan Homogeneity

N-Glycosylation plays an important role in protein folding and function. Previous studies demonstrate that a phenylalanine residue introduced at the n-2 position relative to an Asn-Xxx-Thr/Ser N-glycosylation sequon increases the glycan occupancy of the sequon in insect cells. Here, we show that any aromatic residue at n-2 increases glycan occupancy in human cells and that this effect is dependent upon oligosaccharyltransferase substrate preferences rather than differences in other cellular processing events such as degradation or trafficking. Moreover, aromatic residues at n-2 alter glycan processing in the Golgi, producing proteins with less complex N-glycan structures. These results demonstrate that manipulating the sequence space surrounding N-glycosylation sequons is useful both for controlling glycosylation efficiency, thus enhancing glycan occupancy, and for influencing the N-glycan structures produced.

Structural plasticity of the Semliki Forest virus glycome upon interspecies transmission

Cross-species viral transmission subjects parent and progeny alphaviruses to differential post-translational processing of viral envelope glycoproteins. Alphavirus biogenesis has been extensively studied, and the Semliki Forest virus E1 and E2 glycoproteins have been shown to exhibit differing degrees of processing of N-linked glycans. However the composition of these glycans, including that arising from different host cells, has not been determined. Here we determined the chemical composition of the glycans from the prototypic alphavirus, Semliki Forest virus, propagated in both arthropod and rodent cell lines, by using ion-mobility mass spectrometry and collision-induced dissociation analysis. We observe that both the membrane-proximal E1 fusion glycoprotein and the protruding E2 attachment glycoprotein display heterogeneous glycosylation that contains N-linked glycans exhibiting both limited and extensive processing. However, E1 contained predominantly highly processed glycans dependent on the host cell, with rodent and mosquito-derived E1 exhibiting complex-type and paucimannose-type glycosylation, respectively. In contrast, the protruding E2 attachment glycoprotein primarily contained conserved under-processed oligomannose-type structures when produced in both rodent and mosquito cell lines. It is likely that glycan processing of E2 is structurally restricted by steric-hindrance imposed by local viral protein structure. This contrasts E1, which presents glycans characteristic of the host cell and is accessible to enzymes. We integrated our findings with previous cryo-electron microscopy and crystallographic analyses to produce a detailed model of the glycosylated mature virion surface. Taken together, these data reveal the degree to which virally encoded protein structure and cellular processing enzymes shape the virion glycome during interspecies transmission of Semliki Forest virus.

The role of E3 in pH protection during alphavirus assembly and exit

Alphaviruses are small enveloped viruses whose surface is covered by spikes composed of trimers of E2/E1 glycoprotein heterodimers. During virus entry, the E2/E1 dimer dissociates within the acidic endosomal environment, freeing the E1 protein to mediate fusion of the viral and endosome membranes. E2 is synthesized as a precursor, p62, which is cleaved by furin in the late secretory pathway to produce mature E2 and a small peripheral glycoprotein, E3. The immature p62/E1 dimer is acid resistant, but since p62 is cleaved before exit from the acidic secretory pathway, low pH-dependent binding of E3 to the spike complex is believed to prevent premature fusion. Based on analysis of the structure of the Chikungunya virus E3/E2/E1 complex, we hypothesized that interactions of E3 residues Y47 and Y48 with E2 are important in this binding. We then directly tested the in vivo role of E3 in pH protection by alanine substitutions of E3 Y47 and Y48 (Y47/48A) in Semliki Forest virus. The mutant was nonviable and was blocked in E1 transport to the plasma membrane and virus production. Although the Y47/48A mutant initially formed the p62/E1 heterodimer, the dimer dissociated during transport through the secretory pathway. Neutralization of the pH in the secretory pathway successfully rescued dimer association, E1 transport, and infectious particle production. Further mutagenesis identified the critical contact as the cation-π interaction of E3 Y47 with E2. Thus, E3 mediates pH protection of E1 during virus biogenesis via interactions strongly dependent on Y47 at the E3-E2 interface.

A key interaction between the alphavirus envelope proteins responsible for initial dimer dissociation during fusion

Alphaviruses such as Semliki Forest virus (SFV) are enveloped viruses whose surface is covered by an organized lattice composed of trimers of E2-E1 heterodimers. The E1 envelope protein, a class II fusion protein, contains the hydrophobic fusion loop and refolds to drive virus fusion with the endosome membrane. The E2 protein is synthesized as a precursor p62, whose processing by furin primes the heterodimer for dissociation during virus entry. Dissociation of the E2-E1 heterodimer is an essential step during low-pH-triggered fusion, while the dissociation of the immature p62-E1 dimer is relatively pH resistant. Previous structural studies described an "acid-sensitive region" in E2 that becomes disordered at low pH. Within this region, the conserved E2 H170 is in position to form a hydrogen bond with the underlying E1 S57. Here we experimentally tested the role of this interaction in regulating dimer dissociation in mature and immature virus. Alanine substitutions of E1 S57 and E2 H170 destabilized the heterodimer and produced a higher pH threshold for exposure of the E1 fusion loop and for fusion of the immature virus. E1 S57K or S57D mutations were lethal and caused transport and assembly defects that were partially abrogated by neutralization of the exocytic pathway. The lethal phenotype of E1 S57K was rescued by second-site mutations at E2 H170/M171. Together, our results define a key role for the E1 S57-E2 H170 interaction in dimer stability and the pH dependence of fusion and provide evidence for stepwise dissociation of the E2-E1 dimer at low pH.

N-linked glycans on dengue viruses grown in mammalian and insect cells

This study compared the ability of mosquito and mammalian cell-derived dengue virus (DENV) to infect human dendritic cell-specific ICAM3-grabbing non-integrin (DC-SIGN)-expressing cells and characterized the structure of envelope (E) protein N-linked glycans on DENV derived from the two cell types. DENVs derived from both cell types were equally effective at infecting DC-SIGN-expressing human monocytes and dendritic cells. The N-linked glycans on mosquito cell-derived virus were a mix of high-mannose and paucimannose glycans. In virus derived from mammalian cells, the N-linked glycans were a mix of high-mannose and complex glycans. These results indicate that N-linked glycans are incompletely processed during DENV egress from cells, resulting in high-mannose glycans on viruses derived from both cell types. Studies with full-length and truncated E protein demonstrated that incomplete processing was most likely a result of the poor accessibility of glycans on the membrane-anchored protein.

Role of N-linked glycosylation for sindbis virus infection and replication in vertebrate and invertebrate systems

Each Sindbis virus (SINV) surface glycoprotein has two sites for N-linked glycosylation (E1 positions 139 and 245 [E1-139 and E1-245] and E2 positions 196 and 318 [E2-196 and E2-318]). Studies of SINV strain TE12 mutants with each site eliminated identified the locations of carbohydrates by cryo-electron microscopy (S. V. Pletnev et al., Cell 105:127-136, 2001). In the current study, the effects of altered glycosylation on virion infectivity, growth in cells of vertebrates and invertebrates, heparin binding, virulence in mice, and replication in mosquitoes were assessed. Particle-to-PFU ratios for E1-139 and E2-196 mutant strains were similar to that for TE12, but this ratio for the E1-245 mutant was 100-fold lower than that for TE12. Elimination of either E2 glycosylation site increased virus binding to heparin and increased replication in BHK cells. Elimination of either E1 glycosylation site had no effect on heparin binding but resulted in an approximately 10-fold decrease in virus yield from BHK cells compared to the TE12 amount. No differences in pE2 processing were detected. E2-196 and E2-318 mutants were more virulent in mice after intracerebral inoculation, while E1-139 and E1-245 mutants were less virulent. The E1-245 mutant showed impaired replication in C7/10 mosquito cells and in Culex quinquefasciatus after intrathoracic inoculation. We conclude that the increased replication and virulence of E2-196 and E2-318 mutants are primarily due to increased efficiency of binding to heparan sulfate on mammalian cells. Lack of glycosylation at E1-139 or E1-245 impairs replication in vertebrate cells, while E1-245 also severely affects replication in invertebrate cells.

Sites of asparagine-linked oligosaccharides in porcine 32 kDa enamelin

The 32 kDa enamelin protein isolated from developing porcine enamel was previously shown to contain eight different asparagine-linked oligosaccharides. However, only three consensus attachment sites were evident in this protein. In this study, glycopeptides containing all three potential glycosylation sites (72-Asn, 79-Asn and 91-Asn) were purified from 32 kDa enamelin. The oligosaccharides were isolated from each glycopeptide following digestion with N-oligosaccharide glycopeptidase, labeled with 2-aminopyridine at the reducing ends, and then characterized by reverse phase HPLC. All three potential sites were found to be glycosylated heterogeneously (i.e., five biantennary complexes at 72-Asn, two biantennary complexes at 79-Asn, three triantennary complexes at 91-Asn), accounting for all eight oligosaccharides characterized previously. These results indicate that 32 kDa enamelin has a complex pattern of asparagine-linked glycosylation localized within a small region (20 residues) of the protein. The functional significance of this glycosylation remains to be established.

Recombinant glycodelin carrying the same type of glycan structures as contraceptive glycodelin-A can be produced in human kidney 293 cells but not in chinese hamster ovary cells

We have produced human recombinant glycodelin in human kidney 293 cells and in Chinese hamster ovary (CHO) cells. Structural analyses by lectin immunoassays and fast atom bombardment mass spectrometry showed that recombinant human glycodelin produced in CHO cells contains only typical CHO-type glycans and is devoid of any of the N, N'-diacetyllactosediamine (lacdiNAc)-based chains previously identified in glycodelin-A (GdA). By contrast, human kidney 293 cells produced recombinant glycodelin with the same type of carbohydrate structures as GdA. The presence of a beta1-->4-N-acetylgalactosaminyltransferase functioning in the synthesis of lacdiNAc-based glycans in human kidney 293 cells is concluded to be the cause of the occurrence of lacdiNAc-based glycans on glycodelin produced in these cells. Furthermore, human kidney 293 cells were found to be particularly suited for the production of recombinant glycodelin when they were cultured in high glucose media. Lowering the glucose concentration and the addition of glucosamine resulted in higher relative amounts of oligomannosidic-type glycans and complex glycans with truncated antennae. Human glycodelin is an attractive candidate for the development of a contraceptive agent, and this study gives valuable information for selecting the proper expression system and cell culture conditions for the production of a correctly glycosylated recombinant form.

Differential effects of changes in the length of a signal/anchor domain on membrane insertion, subunit assembly, and intracellular transport of a type II integral membrane protein

The length requirement for a functional uncleaved signal/anchor (S/A) domain of the paramyxovirus hemagglutinin-neuraminidase (HN) type II glycoprotein was analyzed. HN mutants with progressive NH2-terminal S/A deletions or insertions were expressed in HeLa cells, and the membrane targeting, folding, tetramer assembly, and intracellular transport of the proteins were examined. Changing the length of the S/A by two residues resulted in HN mutants that displayed aberrant endoplasmic reticulum (ER) membrane targeting or translocation. This phenotype did not simply reflect upper or lower limitations on the size of a functional S/A, because normal signaling was restored by further alterations involving three or four residues. Likewise, ER-to-Golgi transport of mutants containing deletions of one or two S/A residues was delayed (approximately 30% of WT) or blocked, but transport was restored for a mutant with a total of three deleted residues. HN mutants with S/A insertions of three or four Leu residues differed from wild-type HN by having heterogeneous Golgi-specific carbohydrate modifications. Differences in ER-to-Golgi transport of the mutants did not strictly correlate with defects in either native folding of the ectodomain or the assembly of two dimers into a tetramer. Together, these data suggest that efficient entry into and exit from the ER are sensitive to changes in the HN S/A that may reflect alterations to a structural requirement along one side of an alpha-helix.

N-glycosylation of recombinant human interferon-gamma produced in different animal expression systems

Recombinant human interferon-gamma (IFN-gamma) was expressed in Chinese hamster ovary cells, baculovirus-infected Sf9 insect cells and the mammary gland of transgenic mice. The N-linked carbohydrate populations associated with both Asn25 and Asn97 glycosylation sites were characterized by matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) in combination with exoglycosidase array sequencing. A site-specific analysis of dual (2N) and single (1N) site-occupancy variants of IFN-gamma derived from Chinese hamster ovary cells showed that N-glycans were predominantly of the complex bi- and triantennary type. Although Asn25-linked glycans were substituted with a core fucose residue, Asn97 N-glycans were predominantly non-fucosylated, and truncated complex and high-mannose oligosaccharide chains were also evident. Transgenic mouse derived IFN-gamma exhibited considerable site-specific variation in N-glycan structures. Asn25-linked carbohydrates were of the complex, core fucosylated type, Asn97-linked carbohydrates were mainly of the oligomannose type, with smaller proportions of hybrid and complex N-glycans. Carbohydrates associated with both glycosylation sites of IFN-gamma from Sf9 insect cells were mainly tri-mannosyl core structures, with fucosylation confined to the Asn25 site. These data demonstrate the profound influence of host cell type and protein structure on the N-glycosylation of recombinant proteins.

Relation of N-glycosylation of apolipoprotein B-100 to cellular metabolism of low density lipoprotein

We studied the functional role of N-linked sugar chains of apolipoprotein (apo) B-100 of low density lipoprotein (LDL) in cholesterol metabolism. The N-linked sugar chains of apo B-100 of LDL obtained from four homozygous Watanabe heritable hyperlipidemic (WHHL) rabbits were liberated by hydrazinolysis, followed by NaB3H4 reduction and were fractionated by paper electrophoresis and column chromatography. They consisted of one neutral (N) and two acidic (A1, A2) fractions. The ratio of apo B-100 acidic fractions (A1+A2) varied among 4 WHHL rabbits. Serial measurements of serum cholesterol levels showed that they decreased with aging in each of 4 WHHL rabbits. We investigated the relation of the ratio of acidic sugar chains of apo B-100 to the serum cholesterol levels. Reciprocals of the serum cholesterol levels were significantly correlated with the ratio of acidic sugar chains of apo B-100 (r = 0.901, P < 0.001). To elucidate the role of N-linked sugar chains of apo B-100, we investigated cellular uptake of LDL in normal rabbit skin fibroblasts. The amounts of association, degradation and cholesteryl esterification of LDL with a lower ratio of acidic sugar chains at 37 degrees C were greater than those of LDL with a higher ratio of acidic sugar chains. These results suggest that N-glycosylation of apo B-100 may be related with serum cholesterol levels and N-linked sugar chains of apo B-100 may play an important role in cellular metabolism of LDL.

Glycosylation of two recombinant human uterine tissue plasminogen activator variants carrying an additional N-glycosylation site in the epidermal-growth-factor-like domain

Recombinant human uterine tissue plasminogen activator (tPA) glycosylation mutants carrying an additional N-glycosylation site in the epidermal-growth-factor-like domain due to the replacement of either Tyr67 by Asn (YN-tPA) or Gly60 by Ser (GS-tPA) were expressed in mouse epithelial cells (C127) in the presence of [6-3H]glucosamine. Glycopeptides comprising individual glycosylation sites were isolated and oligosaccharides attached were liberated by treatment with endo-beta-N-acetylglucosaminidase H or peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F. Oligosaccharide alditols obtained after reduction were either directly characterized by high-pH anion-exchange chromatography (high-mannose and hybrid-type glycans) or preparatively subfractionated after enzymic desialylation and separation from sulphated asialooligosaccharides (complex-type sugar chains). Individual (sub)fractions of glucans were studied by methylation analysis, liquid secondary-ion mass spectrometry and, in part, by exoglycosidase digestion, whereas corresponding deglycosylated peptides were identified by amino acid analysis and N-terminal amino acid sequencing. The results revealed that Asn117 of YN-tPA carried exclusively high-mannose-type glycans with five to nine mannose residues similar to wild-type tPA expressed in this cell line [Pfeiffer, G., Schmidt, M., Strube, K.-H. & Geyer, R. (1989) Eur. J. Biochem. 186, 273-286]. In contrast, Asn117 of GS-tPA carried only small amounts (about 25%) of high-mannose and hybrid-type species and predominantly complex-type sugar chains (about 75%) which were partially incomplete and mostly devoid of fucose. Newly introduced N-glycosylation sites at Asn67 (YN-tPA) or Asn58 (GS-tPA) as well as those at Asn184 and Asn448 were solely substituted by complex-type glycans. Each carbohydrate attachment site displayed a peculiar oligosaccharide pattern with regard to branching and substitution by Gal alpha 3-residues, sulphate groups, intersecting GlcNAc and lactosamine repeats. Our study clearly demonstrates that creation of a new glycosylation site at Asn58 influenced the oligosaccharide processing and, hence, the glycosylation pattern at Asn117, whereas introduction of a new site at Asn67 did not. The relative amounts of complex-type glycans at Asn117 of GS-tPA correlated with the degree of carbohydrate substitution of Asn58. Therefore, it can be concluded that the presence of a sugar chain at the position and not the Gly to Ser mutation itself is responsible for the observed alteration of GS-tPA glycosylation.

Control of IgG/Fc glycosylation: a comparison of oligosaccharides from chimeric human/mouse and mouse subclass immunoglobulin Gs

Oligosaccharide profiles were obtained for chimeric mouse-human antibodies corresponding to each of the human IgG subclasses 1-4, and mouse IgG2b antibodies each expressed in the mouse J558L cell line. These antibodies have specificity for the NIP hapten and form a matched set of IgGs. An IgG4 chimeric antibody (B72.3) produced in the chinese hamster ovary (CHO-K1) cell line was also analysed for carbohydrate. Additionally aglycosylated mutants of this IgG4 (B72.3) and anti-NIP mouse IgG2b were analysed. The total lack of carbohydrate found in the aglycosylated site-directed mutants human chimeric IgG4 B72.3 (Asn 297-->Gln) and mouse IgG2b (Asn 297-->Ala) demonstrates that there are no N-glycosylation sites other than Asn 297. Therefore glycosylation profiles for all the IgGs analysed reflect carbohydrate attached to this site. Factors such as cell type (A), template direction by the IgG heavy chains (B) and culture conditions (C) are shown to influence IgG glycosylation profiles. (A) The anti-NIP IgG antibodies expressed by the J558L cell line may have one or two Gal (alpha 1-->3) Gal residues per oligosaccharide unit, indicative of the presence of (alpha 1-->3) galactosyl transferase in the J558L mouse cell line. (B) The galactosylation profiles obtained for the IgG heavy chains, in particular the preference for galactosylation of the Man (alpha 1-->6) arm rather than the Man (alpha 1-->3) arm, contrary to the beta-galactosyltransferase specificity, suggest that the polypeptide chain may act as a template to influence the extent of galactosylation and hence the proportions of each oligosaccharide incorporated. The IgG2 antibody does not display this galactosylation preference. (C) The extent of galactosylation appears to be influenced by the growth conditions, with the highest levels of galactosylation being found for IgG produced by cells grown in still cultures, rather than cells grown as ascites or in hollow fibre bioreactors. It is concluded that though the profile of glycosylation is controlled predominantly by the glycosylation activity of the cell in which the IgG is expressed, differences between the IgG heavy chain templates of the various subclasses and culture conditions can also influence glycosylation.

Structure of the N- and O-glycans of the A-chain of human plasma alpha 2HS-glycoprotein as deduced from the chemical compositions of the derivatives prepared by stepwise degradation with exoglycosidases

The structure of the glycans of the A-chain of human plasma alpha 2HS-glycoprotein was established from the chemical compositions of its derivatives prepared by sequential enzymatic degradation of the carbohydrate moiety, from the determination of the kind and amount of the monosaccharides liberated after each step of the enzymatic digestion, and from the distinct specificity of the highly purified exoglycosidases. The exoglycosidases were three sialidases (Vibrio cholerae, fowl plague virus, and Arthrobacter ureafaciens), two beta-galactosidases (Streptococcus pneumoniae and bovine testis), one alpha-N-acetylgalactosaminidase, one beta-N-acetylglucosaminidase, and one alpha-mannosidase. Utilizing sialidases with different cleavage specificities, the number of alpha 2-3- and alpha 2-6-linked sialic acid residues could be separately determined. As to the beta-galactosidases, the enzyme isolated from S. pneumoniae cleaves only beta 1-4-linked galactose residues, whereas the bovine testes enzyme acts on both the beta 1-4- and beta 1-3-linked galactose residues. Jack bean beta-N-acetylglucosaminidase cleaves beta 1-2, beta 1-4, and beta 1-6 GlcNAc with higher activity for the beta 1-2. Jack bean alpha-mannosidase cleaves alpha 1-2, alpha 1-6, and alpha 1-3 Man with greater activity for alpha 1-2 and alpha 1-6. Bovine liver alpha-N-acetylgalactosaminidase cleaves O-linked GalNAc. On the basis of these results, the A-chain of alpha 2 HS-glycoprotein was found to possess two biantennary N-glycans and two O-linked trisaccharides.

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.

Structure of the N-linked oligosaccharides of the human transferrin receptor

Human transferrin receptor was isolated from placenta and from the hepatocarcinoma cell line Hep G2. Asparagine-linked oligosaccharides were released by treatment of tryptic glycopeptides with endo-beta-N-acetylglucosaminidase H or peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F. Oligosaccharide alditols were fractionated by anion-exchange high-performance liquid chromatography and by high-pH anion-exchange chromatography. Glycans from placental transferrin receptor were further characterized, after desialylation, by methylation analysis and, in part, by liquid secondary-ion mass spectrometry. Sialylation of placental transferrin receptor was examined by lectin affinity blotting with Sambucus nigra agglutinin and Maackia amurensis agglutinin. In order to trace possible inter-individual differences in N-glycosylation of the receptor, two preparations of placental transferrin receptor purified from two donors were compared. The results demonstrate that human transferrin receptor from placenta predominantly carries diantennary and triantennary N-acetyllactosaminic glycans as well as hybrid-type species, the galactose residues of which being almost completely substituted with (alpha 2-3)-linked sialic acid residues. Distinct differences were noted in the glycosylation pattern of the receptor from different individuals. Transferrin receptor from donor A carried predominantly diantennary and triantennary complex-type glycans, in part fucosylated at the innermost N-acetylglucosamine residue, in addition to small amounts of bisected and of incomplete diantennary species. Placental transferrin receptor from donor B predominantly carried triantennary N-acetyllactosaminic glycans without fucose and hybrid-type oligosaccharides with four or five mannose residues. Distinct from placental transferrin receptor, the receptor from Hep G2 cells contained larger amounts of oligomannosidic glycans with six to nine mannose residues and tetrasialylated complex-type oligosaccharides apart from mono-, di- and trisialylated species.

Asparagine-linked oligosaccharides of Semliki Forest virus grown in mosquito cells

The structure of the N-linked oligosaccharides of Semliki Forest viral glycoproteins produced in infected mosquito cells (C6/36) was investigated by biosynthetic labeling, enzymic deglycosylation using endo-beta-N-acetylglucosaminidases H, D, F/glycopeptidase F, exoglycosidase and analysis of the sugars on Concanavalin A-Sepharose columns and by gel filtration chromatography. The results demonstrated that the glycoproteins decorating the virus shed from infected cells have N-linked glycans with a trimannosyl core similar to the core glycans produced by vertebrate and yeast cells. However, the E1 glycoprotein produced by infected C6/36 cells exhibited both a trimannosyl core and a modified trimannosyl core most probably with terminal N-acetylglucosamine. The carbohydrate side chains of Semliki Forest envelope proteins displayed two types of structural heterogeneities existing either at different N-glycosylation sites as in the case of E2, or at the same N-glycosylation site as in the case of E1. In the presence of 1-deoxymannojirimycin, no structural heterogeneities in the glycan chains were found. This strongly suggests that the glycosylation events that lead to the observed sugar heterogeneities occur in the Golgi membranes.

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

Chinese hamster ovary lines with two mutations, one causing accumulation of Man5GlcNAc2-P-P-dolichol and a second resulting in defective N-acetylglucosaminyltransferase I activity, synthesize asparagine-linked glycans with the structure Man3GlcNAc2. As a result, the asparagine-linked glycans produced by these lines are smaller and less heterogeneous than those produced by other currently available animal cell lines.

NMR investigations of the N-linked oligosaccharides at individual glycosylation sites of human lutropin

Human lutropin or luteinizing hormone (hLH) is a heterodimeric glycoprotein, composed of two subunits. hLH alpha (N-glycosylated at Asn52 and Asn78) and hLH beta (N-glycosylated at Asn30). The sugar chains were liberated by hydrazinolysis from intact hLH beta and from glycopeptides obtained after tryptic digestion of hLH alpha, subsequently reduced and fractionated as alditols by anion-exchange and ion-suppression amine-adsorption HPLC and identified mainly by one-dimensional (1D) and two-dimensional (2D) 1H-NMR spectroscopy. The results indicate predominantly diantennary. N-acetyllactosamine-type structures at all three glycosylation sites. The oligosaccharides attached to Asn52 (hLH alpha) and Asn30 (hLH beta) show a remarkably similar pattern, with mainly chain-terminating 4-sulphated 2-deoxy-2-N-acetylamino-D-galactose (GalNAc) and a sulphated/sialylated structure as the major single component. However, virtually all N-glycans on the beta subunit bear a fucose residue alpha 1-6-linked to the proximal GlcNAc, whereas those at Asn52 (and Asn78) of the alpha subunit are predominantly non-fucosylated. The oligosaccharides at Asn78 (hLH alpha) are sialylated rather than sulphated and contain the unique sequence NeuAc alpha 2-6 GalNAc beta 1-4GlcNAc beta 1-2 Man alpha 1-3 as part of the majority of mono- and disialylated compounds. The major single constituent at Asn78 has the following structure: [formula, see text]

Effect of the carbohydrate moiety on the secondary structure of beta 2-glycoprotein. I. Implications for the biosynthesis and folding of glycoproteins

By use of six highly purified exoglycosidases with well-defined specificity, the oligosaccharide units of human plasma beta 2-glycoprotein I (beta 2I) were modified by sequential enzymatic degradation. The released monosaccharides (NeuAc, Gal, GlcNAc, and Man) were quantified, and the carbohydrate compositions of the resulting glycoprotein (gp) derivatives were determined. The gp was found to be both partially sialylated and galactosylated. These findings which are in agreement with earlier reports suggest that the carbohydrate moiety of beta 2I possesses more bi- than tri-antennas, probably three of the former and two of the latter carbohydrate units. Circular dichroic (CD) spectra of native beta 2I and its derivatives were measured in aqueous buffer and 2-chloroethanol (2-CE). Analysis of these spectra for elements of secondary structure showed beta 2I and most of the derivatives to contain predominantly beta-sheet and beta-turn structures. The lack of alpha-helical structures in aqueous buffer was noted. Removal of a large portion of the carbohydrate moiety did not alter the CD spectra or secondary structure of beta 2I in either aqueous buffer or in 2-CE. However, after enzymatic removal of approximately 96% of the carbohydrate moiety, large significant changes in the spectra and secondary structures were observed. In aqueous buffer a shift in the wavelength minimum occurred, accompanied by an increase in the magnitude of the molar ellipticity and the amount of beta-turn, with a reduction in random coil. One-third of the amino acids which were originally in random coil conformation assumed beta-turns after removal of 96% of the carbohydrate moiety.(ABSTRACT TRUNCATED AT 250 WORDS)

Oligosaccharide processing in the expression of human plasminogen cDNA by lepidopteran insect (Spodoptera frugiperda) cells

A comparison has been made between the Asn289-linked oligosaccharide structures of human plasma plasminogen and a recombinant human plasminogen, expressed in lepidopteran insect (Spodoptera frugiperda) cells, after infection of these cells with a recombinant baculovirus containing the entire human plasminogen cDNA. Using anion-exchange liquid chromatography mapping of the oligosaccharide units cleaved from the proteins by glycopeptidase F, compared with elution positions of standard oligosaccharide structures, coupled with monosaccharide compositional analysis, we find that the human plasma protein contained only bisialo-biantennary complex-type carbohydrate and asialo-biantennary complex carbohydrate, confirming earlier work published by this laboratory. The glycosylation pattern of the insect cell expressed recombinant human plasminogen showed considerable microheterogeneity, with identifiable high-mannose carbohydrate (Man9GlcNAc2) and truncated high-mannose oligosaccharide (Man5GlcNAc2, Man4GlcNAc2, and Man3GlcNAc2). Of major importance, approximately 40% of the oligosaccharide population consisted of complex carbohydrate (bisialo-biantennary), identical in structure with that of the human plasma protein. This is the first direct identification of complex carbohydrate in proteins produced in insect cells and demonstrates that trimming and processing of high-mannose carbohydrate into complex-type oligosaccharide can occur. Our data indicate that both normal and alternate pathways exist in these cells for incorporation and trimming of high-mannose oligosaccharides and that mannosidases, as well as galactosyl-, hexosaminidasyl-, and sialyltransferases are present, and/or can be induced, in these cells. From these observations, we conclude that amino acid sequences and/or protein conformational properties can control oligosaccharide processing events.

Heterogeneity of N-linked sugar chains of apolipoprotein B-100 in Watanabe heritable hyperlipidemic and fasting rabbits

We have elucidated the structures of N-linked sugar chains of human apolipoprotein (apo) B-100 (Arch Biochem Biophys 1989; 273:197-205). To investigate the role of the carbohydrate moieties of apolipoprotein B-100, we determined the structures of the N-linked sugar chains of apo B-100 purified from low density lipoprotein (LDL) of a Watanabe heritable hyperlipidemic (WHHL) rabbit and compared them with those of fasting Japanese White rabbits. The N-linked oligosaccharides of apo B-100 were liberated by hydrozinolysis, followed by NaB3H4 reduction, and were fractionated by paper electrophoresis and Bio-Gel P-4 column chromatography. These consisted of one neutral fraction (N) and two acidic fractions (A1 and A2) in both WHHL and fasting rabbits. N contained high mannose type oligosaccharides consisting of Man5GlcNAc2 to Man9GlcNAc2. A1 and A2 contained monosialylated and disialylated biantennary complex type oligosaccharides, respectively. The molar ratios of N, A1, and A2 were 5:2:2 in the WHHL rabbit and 4:2:5 in fasting rabbits. The content of sialic acid residues in the WHHL rabbit was calculated to be 0.64 by taking the value of that in fasting rabbits as 1.0. These results show the heterogeneity of N-linked sugar chains of apo B-100 in WHHL and fasting rabbits and suggest the possibility that the characteristics of LDL in WHHL rabbits may be altered by making the surface charge more positive than in fasting rabbits.

Alterations in the Domain Structure of Tissue-Type Plasminogen Activator Change the Nature of Asparagine-Linked Glycosylation

The formation of N-linked oligosaccharides of eukaryotic glycoproteins starts with the attachment of a common precursor at the recognition site Asn-X-Ser/Thr. Subsequent processing, by yet unknown controlling factors, leads to the formation of three different glycans: the high mannose type, the complex type and the hybrid type. In order to gain insight into the processing mechanisms, we studied the glycan pattern of a panel of related molecules constructed by insertion, duplication or deletion of the domains encoded by the cDNA of a fibrinolytic glycoprotein, tissue-type plasminogen activator (t-PA). These variant molecules are identical in regard to the glycosylation sites originally situated in particular domains, but differ with respect to the sequential alignment of the domains. The variant and native t-PA genes were transfected into mouse C127 cells and their carbohydrate structures analyzed by the susceptibility to specific endoglycosidases and by reaction with sugar-specific lectins. We found that with one exception, all mutant activators lack the high mannose glycan found at asn 117 of native t-PA. The exception was a molecule that retains the original domain arrangement up to and through the glycosylation site at asn 117. These results demonstrate for the first time that structural alterations in the primary sequence distal to the actual glycosylation site can result in altered processing of N-linked oligosacharides.

Characterisation of the asparagine-linked oligosaccharides from Trypanosoma brucei type-I variant surface glycoproteins

The complete primary structures of the Asn-linked oligosaccharides from the conserved glycosylation site of the type-I variant surface glycoproteins of Trypanosoma brucei MITat 1.4 and MITat 1.6 were determined using a combination of exoglycosidase digestions, permethylation analysis, acetolysis and 1H NMR. Both variants contained almost exclusively oligomannose-type oligosaccharides, identical in structure to those of mammalian glycoproteins. The oligosaccharides ranged in size from (Man)9(GlcNAc)2 to (Man)5(GlcNAc)2. The relative abundance of each component was similar in both variants. The major components were (Man)8(GlcNAc)2 and (Man)7(GlcNAc)2 with slightly less (Man)9(GlcNAc)2 and (Man)6(GlcNAc)2 and much less (Man)5(GlcNAc)2. Both variants also contained the same structural isomers. The close similarity of the oligomannose series indicates identical processing at the conserved site in both variants.