Structure of the high mannose oligosaccharides of a human IgM myeloma protein. I. The major oligosaccharides of the two high mannose glycopeptides

Multi-isotype Glycoproteomic Characterization of Serum Antibody Heavy Chains Reveals Isotype- and Subclass-Specific N-Glycosylation Profiles

Antibodies are critical glycoproteins that bridge the innate and adaptive immune systems to provide protection against infection. The isotype/subclass of the antibody, the co-translational N-glycosylation on the CH2 domain, and the remodeling of the N-linked glycans during passage through the ER and Golgi are the known variables within the Fc domain that program antibody effector function. Through investigations of monoclonal therapeutics, it has been observed that addition or removal of specific monosaccharide residues from antibody N-glycans can influence the potency of antibodies, highlighting the importance of thoroughly characterizing antibody N-glycosylation. Although IgGs usually have a single N-glycosylation site and are well studied, other antibody isotypes, e.g. IgA and IgM, that are the first responders in certain diseases, have two to five sites/monomer of antibody, and little is known about their N-glycosylation. Here we employ a nLC-MS/MS method using stepped-energy higher energy collisional dissociation to characterize the N-glycan repertoire and site occupancy of circulating serum antibodies. We simultaneously determined the site-specific N-linked glycan repertoire for IgG1, IgG4, IgA1, IgA2, and IgM in individual healthy donors. Compared with IgG1, IgG4 displayed a higher relative abundance of G1S1F and a lower relative abundance of G1FB. IgA1 and IgA2 displayed mostly biantennary N-glycans. IgA2 variants with the either serine (S93) or proline (P93) were detected. In digests of the sera from a subset of donors, we detected an unmodified peptide containing a proline residue at position 93; this substitution would strongly disfavor N-glycosylation at N92. IgM sites N46, N209, and N272 displayed mostly complex glycans, whereas sites N279 and N439 displayed higher relative abundances of high-mannose glycoforms. This multi-isotype approach is a crucial step toward developing a platform to define disease-specific N-glycan signatures for different isotypes to help tune antibodies to induce protection. Data are available via ProteomeXchange with identifier PXD010911.

Quantitation of human milk proteins and their glycoforms using multiple reaction monitoring (MRM)

Human milk plays a substantial role in the child growth, development and determines their nutritional and health status. Despite the importance of the proteins and glycoproteins in human milk, very little quantitative information especially on their site-specific glycosylation is known. As more functions of milk proteins and other components continue to emerge, their fine-detailed quantitative information is becoming a key factor in milk research efforts. The present work utilizes a sensitive label-free MRM method to quantify seven milk proteins (α-lactalbumin, lactoferrin, secretory immunoglobulin A, immunoglobulin G, immunoglobulin M, α1-antitrypsin, and lysozyme) using their unique peptides while at the same time, quantifying their site-specific N-glycosylation relative to the protein abundance. The method is highly reproducible, has low limit of quantitation, and accounts for differences in glycosylation due to variations in protein amounts. The method described here expands our knowledge about human milk proteins and provides vital details that could be used in monitoring the health of the infant and even the mother. Graphical Abstract The glycopeptides EICs generated from QQQ.

A Microarray-Matrix-assisted Laser Desorption/Ionization-Mass Spectrometry Approach for Site-specific Protein N-glycosylation Analysis, as Demonstrated for Human Serum Immunoglobulin M (IgM)

We demonstrate a new approach for the site-specific identification and characterization of protein N-glycosylation. It is based on a nano-liquid chromatography microarray-matrix assisted laser desorption/ionization-MS platform, which employs droplet microfluidics for on-plate nanoliter reactions. A chromatographic separation of a proteolytic digest is deposited at a high frequency on the microarray. In this way, a short separation run is archived into thousands of nanoliter reaction cavities, and chromatographic peaks are spread over multiple array spots. After fractionation, each other spot is treated with PNGaseF to generate two correlated traces within one run, one with treated spots where glycans are enzymatically released from the peptides, and one containing the intact glycopeptides. Mining for distinct glycosites is performed by searching for the predicted deglycosylated peptides in the treated trace. An identified peptide then leads directly to the position of the "intact" glycopeptide clusters, which are located in the adjacent spots. Furthermore, the deglycosylated peptide can be sequenced efficiently in a simple collision-induced dissociation-MS experiment. We applied the microarray approach to a detailed site-specific glycosylation analysis of human serum IgM. By scanning the treated spots with low-resolution matrix assisted laser desorption/ionization-time-of-flight-MS, we observed all five deglycosylated peptides, including the one originating from the secretory chain. A detailed glycopeptide characterization was then accomplished on the adjacent, untreated spots with high mass resolution and high mass accuracy using a matrix assisted laser desorption ionization-Fourier transform-MS. We present the first detailed and comprehensive mass spectrometric analysis on the glycopeptide level for human polyclonal IgM with high mass accuracy. Besides complex type glycans on Asn 395, 332, 171, and on the J chain, we observed oligomannosidic glycans on Asn 563, Asn 402 and minor amounts of oligomannosidic glycans on the glycosite Asn 171. Furthermore, hybrid type glycans were found on Asn 402, Asn 171 and in traces Asn 332.

Improved method for drawing of a glycan map, and the first page of glycan atlas, which is a compilation of glycan maps for a whole organism

Glycan Atlas is a set of glycan maps over the whole body of an organism. The glycan map that includes data of glycan structure and quantity displays micro-heterogeneity of the glycans in a tissue, an organ, or cells. The two-dimensional glycan mapping is widely used for structure analysis of N-linked oligosaccharides on glycoproteins. In this study we developed a comprehensive method for the mapping of both N- and O-glycans with and without sialic acid. The mapping data of 150 standard pyridylaminated glycans were collected. The empirical additivity rule which was proposed in former reports was able to adapt for this extended glycan map. The adapted rule is that the elution time of pyridylamino glycans on high performance liquid chromatography (HPLC) is expected to be the simple sum of the partial elution times assigned to each monosaccharide residue. The comprehensive mapping method developed in this study is a powerful tool for describing the micro-heterogeneity of the glycans. Furthermore, we prepared 42 pyridylamino (PA-) glycans from human serum and were able to draw the map of human serum N- and O-glycans as an initial step of Glycan Atlas editing.

Activation of the lectin pathway of complement in pig-to-human xenotransplantation models

BACKGROUND

Natural IgM containing anti-Gal antibodies initiates classic pathway complement activation in xenotransplantation. However, in ischemia-reperfusion injury, IgM also induces lectin pathway activation. The present study was therefore focused on lectin pathway as well as interaction of IgM and mannose-binding lectin (MBL) in pig-to-human xenotransplantation models.

METHODS

Activation of the different complement pathways was assessed by cell enzyme-linked immunosorbent assay using human serum on wild-type (WT) and α-galactosyl transferase knockout (GalTKO)/hCD46-transgenic porcine aortic endothelial cells (PAEC). Colocalization of MBL/MASP2 with IgM, C3b/c, C4b/c, and C6 was investigated by immunofluorescence in vitro on PAEC and ex vivo in pig leg xenoperfusion with human blood. Influence of IgM on MBL binding to PAEC was tested using IgM depleted/repleted and anti-Gal immunoabsorbed serum.

RESULTS

Activation of all the three complement pathways was observed in vitro as indicated by IgM, C1q, MBL, and factor Bb deposition on WT PAEC. MBL deposition colocalized with MASP2 (Manders' coefficient [3D] r=0.93), C3b/c (r=0.84), C4b/c (r=0.86), and C6 (r=0.80). IgM colocalized with MBL (r=0.87) and MASP2 (r=0.83). Human IgM led to dose-dependently increased deposition of MBL, C3b/c, and C6 on WT PAEC. Colocalization of MBL with IgM (Pearson's coefficient [2D] rp=0.88), C3b/c (rp=0.82), C4b/c (rp=0.63), and C6 (rp=0.81) was also seen in ex vivo xenoperfusion. Significantly reduced MBL deposition and complement activation was observed on GalTKO/hCD46-PAEC.

CONCLUSION

Colocalization of MBL/MASP2 with IgM and complement suggests that the lectin pathway is activated by human anti-Gal IgM and may play a pathophysiologic role in pig-to-human xenotransplantation.

Different phylogenetic profile and reduced mannose-sensitive adherence capacity characterize commensal Escherichia coli in IgA deficient individuals

In IgA deficiency, secretory IgA (S-IgA) is absent from intestinal secretions. S-IgA carbohydrate chains act as receptors for the mannose specific (MS) adhesin fim of Escherichia coli. In IgA deficient (IgAd) individuals, commensal E. coli express less MS adherence to epithelial cells, due both to reduced carriage of the fimH adhesin gene, reduced capacity to switch it on, and reduced adherence of adhesin-expressing bacteria. Here, we show that commensal E. coli microbiota of IgA deficient individuals belong to phylogenetic group A and display low MS adherence. In healthy individuals, group B2 with strong MS adherence dominate.

Recognition roles of the carbohydrate glycotopes of human and bovine lactoferrins in lectin-N-glycan interactions

BACKGROUND

Lactoferrin is an iron-binding protein belonging to the transferrin family. In addition to iron homeostasis, lactoferrin is also thought to have anti-microbial, anti-inflammatory, and anticancer activities. Previous studies showed that all lactoferrins are glycosylated in the human body, but the recognition roles of their carbohydrate glycotopes have not been well addressed.

METHODS

The roles of human and bovine lactoferrins involved in lectin-N-glycan recognition processes were analyzed by enzyme-linked lectinosorbent assay with a panel of applied and microbial lectins.

RESULTS AND CONCLUSIONS

Both native and asialo human/bovine lactoferrins reacted strongly with four Man-specific lectins - Concanavalia ensiformis agglutinin, Morniga M, Pisum sativum agglutinin, and Lens culinaris lectin. They also reacted well with PA-IIL, a LFuc>Man-specific lectin isolated from Pseudomonas aeruginosa. Both human and bovine lactoferrins also recognized a sialic acid specific lectin-Sambucus nigra agglutinin, but not their asialo products. Both native and asialo bovine lactoferrins, but not the human ones, exhibited strong binding with a GalNAc>Gal-specific lectin-Wisteria floribunda agglutinin. Human native lactoferrins and its asialo products bound well with four Gal>GalNAc-specific type-2 ribosome inactivating protein family lectins-ricin, abrin-a, Ricinus communis agglutinin 1, and Abrus precatorius agglutinin (APA), while the bovine ones reacted only with APA.

GENERAL SIGNIFICANCE

This study provides essential knowledge regarding the different roles of bioactive sites of lactoferrins in lectin-N-glycan recognition processes.

Specific antibody activity, glycan heterogeneity and polyreactivity contribute to the protective activity of S-IgA at mucosal surfaces

An explanation of the principles and mechanisms involved in peaceful co-existence between animals and the huge, diverse, and ever-changing microbiota that resides on their mucosal surfaces represents a challenging puzzle that is fundamental in everyday survival. In addition to mechanical barriers and a variety of innate defense factors, mucosal immunoglobulins (Igs) provide protection by two complementary mechanisms: specific antibody activity and innate, Ig glycan-mediated binding, both of which serve to contain the mucosal microbiota in its physiological niche. Thus, the interaction of bacterial ligands with IgA glycans constitutes a discrete mechanism that is independent of antibody specificity and operates primarily in the intestinal tract. This mucosal site is by far the most heavily colonized with an enormously diverse bacterial population, as well as the most abundant production site for antibodies, predominantly of the IgA isotype, in the entire immune system. In embodying both adaptive and innate immune mechanisms within a single molecule, S-IgA maintains comprehensive protection of mucosal surfaces with economy of structure and function.

The impact of glycosylation on the biological function and structure of human immunoglobulins

Immunoglobulins are the major secretory products of the adaptive immune system. Each is characterized by a distinctive set of glycoforms that reflects the wide variation in the number, type, and location of their oligosaccharides. In a given physiological state, glycoform populations are reproducible; therefore, disease-associated alterations provide diagnostic biomarkers (e.g., for rheumatoid arthritis) and contribute to disease pathogenesis. The oligosaccharides provide important recognition epitopes that engage with lectins, endowing the immunoglobulins with an expanded functional repertoire. The sugars play specific structural roles, maintaining and modulating effector functions that are physiologically relevant and can be manipulated to optimize the properties of therapeutic antibodies. New molecular models of all the immunoglobulins are included to provide a basis for informed and critical discussion. The models were constructed by combining glycan sequencing data with oligosaccharide linkage and dynamics information from the Glycobiology Institute experimental database and protein structural data from "The Protein Data Bank."

Mannose-binding lectin binds IgM to activate the lectin complement pathway in vitro and in vivo

Recent evidence has implicated a role for the MBL-dependent lectin pathway in gastrointestinal and myocardial ischemia/reperfusion (I/R)-induced injury. However, previous studies have implicated IgM and the classical pathway as initiators of complement activation following I/R. Thus, we investigated the potential interaction between MBL and IgM leading to complement activation. Using surface plasmon resonance, we demonstrate that MBL does bind human IgM. Subsequently, functional complement activation was demonstrated in vitro following sensitization of human RBCs with mouse anti-human CD59 IgM and more lysis was observed with MBL sufficient sera compared to MBL deficient (KO) sera. Similarly, treatment of human endothelial cells with mouse anti-human CD59 IgM, MBL and MASP-2 activated and deposited C4. These data suggest that the presence of both IgM and MBL can activate the lectin pathway in vitro. Serum ALT levels increased significantly in sIgM/MBL-A/C KO mice reconstituted with WT plasma compared to sIgM/MBL-A/C KO mice reconstituted with MBL-A/C KO plasma following gastrointestinal (G) I/R. Similarly, intestinal C3 deposition was greater in sIgM/MBL-A/C KO mice reconstituted with WT plasma compared to sIgM/MBL-A/C KO mice treated with MBL-A/C KO plasma. These data indicate for the first time that both IgM and MBL-A/C are required for GI/R-induced complement activation and subsequent injury.

Degalactosylated and/or Denatured IgA, but Not Native IgA in Any Form, Bind to Mannose-Binding Lectin

Mannose-binding lectin (MBL) is reported to bind to agalactosyl IgG, but not to normally galactosylated (native) IgG. It was recently reported that serum polymeric IgA in its native form reacts with MBL, whereas a more recent report has claimed that native IgD and IgE, and possibly IgM, do not. This led us to investigate whether IgA is truly reactive with MBL. To accomplish this, we collected purified human Igs, of various classes, subclasses, and allotypes, and tested their ability to bind to MBL using an ELISA method. Among these preparations, only one (monoclonal IgA2m(2):Kur) exhibited significant MBL binding. In particular, polymeric or monomeric forms of our normal serum IgA preparation lacked any ability to bind to MBL whatsoever. However, all the Ig preparations which had not bound to MBL became able to do so when they were degalactosylated with a galactosidase treatment, and the binding was further enhanced by acidic denaturation of the Igs. Among the degalactosylated and/or acid-denatured IgA, the IgA2 subclass exhibited a higher level of MBL binding than did IgA1. Our results suggest that MBL does not bind to native Igs (viewed in principle as "self" components), and that only Igs with abnormal glycosylation (degalactosylated forms) and/or denaturation would be MBL reactive.

Human serum IgM glycosylation: identification of glycoforms that can bind to mannan-binding lectin

The glycoprotein IgM is the major antibody produced in the primary immune response to antigens, circulating in the serum both as a pentamer and a hexamer. Pentameric IgM has a single J chain, which is absent in the hexamer. The mu (heavy) chain of IgM has five N-linked glycosylation sites. Asn-171, Asn-332, and Asn-395 are occupied by complex glycans, whereas Asn-402 and Asn-563 are occupied by oligomannose glycans. The glycosylation of human polyclonal IgM from serum has been analyzed. IgM was found to contain 23.4% oligomannose glycans GlcNAc2Man5-9, consistent with 100% occupancy of Asn-402 and 17% occupancy of the variably occupied site at Asn-563. Mannan-binding lectin (MBL) is a member of the collectin family of proteins, which bind to oligomannose and GlcNAc-terminating structures. A commercial affinity chromatography resin containing immobilized MBL has been reported to be useful for partial purification of mouse and also human IgM. Human IgM glycoforms that bind to immobilized MBL were isolated; these accounted for only 20% of total serum IgM. Compared with total serum IgM, the MBL-binding glycoforms contained 97% more GlcNAc-terminating structures and 8% more oligomannose structures. A glycosylated model of pentameric IgM was constructed, and from this model, it became evident that IgM has two distinct faces, only one of which can bind to antigen, as the J chain projects from the non-antigen-binding face. Antigen-bound IgM does not bind to MBL, as the target glycans appear to become inaccessible once IgM has bound antigen. Antigen-bound IgM pentamers therefore do not activate complement via the lectin pathway, but MBL might have a role in the clearance of aggregated IgM.

Control of bisecting GlcNAc addition to N-linked sugar chains

In the present study, experimental control of the formation of bisecting GlcNAc was investigated, and the competition between beta-1,4-GalT (UDP-galactose:N-acetylglucosamine beta-1, 4-galactosyltransferase) and GnT-III (UDP-N-acetylglucosamine:beta-d-mannoside beta-1, 4-N-acetylglucosaminyltransferase) was examined. We isolated a beta-1,4-GalT-I single knockout human B cell clone producing monoclonal IgM and several transfectant clones that overexpressed beta-1,4-GalT-I or GnT-III. In the beta-1,4-GalT-I-single knockout cells, the extent of bisecting GlcNAc addition to the sugar chains of IgM was increased, where beta-1,4-GalT activity was reduced to about half that in the parental cells, and GnT-III activity was unaltered. In the beta-1,4-GalT-I transfectants, the extent of bisecting GlcNAc addition was reduced although GnT-III activity was not altered significantly. In the GnT-III transfectants, the extent of bisecting GlcNAc addition increased along with the increase in levels of GnT-III activity. The extent of bisecting GlcNAc addition to the sugar chains of IgM was significantly correlated with the level of intracellular beta-1,4-GalT activity relative to that of GnT-III. These results were interpreted as indicating that beta-1, 4-GalT competes with GnT-III for substrate in the cells.

Substrate specificities of recombinant murine Golgi alpha1, 2-mannosidases IA and IB and comparison with endoplasmic reticulum and Golgi processing alpha1,2-mannosidases

The catalytic domains of murine Golgi alpha1,2-mannosidases IA and IB that are involved in N-glycan processing were expressed as secreted proteins in P.pastoris . Recombinant mannosidases IA and IB both required divalent cations for activity, were inhibited by deoxymannojirimycin and kifunensine, and exhibited similar catalytic constants using Manalpha1,2Manalpha-O-CH3as substrate. Mannosidase IA was purified as a 50 kDa catalytically active soluble fragment and shown to be an inverting glycosidase. Recombinant mannosidases IA and IB were used to cleave Man9GlcNAc and the isomers produced were identified by high performance liquid chromatography and proton-nuclear magnetic resonance spectroscopy. Man9GlcNAc was rapidly cleaved by both enzymes to Man6GlcNAc, followed by a much slower conversion to Man5GlcNAc. The same isomers of Man7GlcNAc and Man6GlcNAc were produced by both enzymes but different isomers of Man8GlcNAc were formed. When Man8GlcNAc (Man8B isomer) was used as substrate, rapid conversion to Man5GlcNAc was observed, and the same oligosaccharide isomer intermediates were formed by both enzymes. These results combined with proton-nuclear magnetic resonance spectroscopy data demonstrate that it is the terminal alpha1, 2-mannose residue missing in the Man8B isomer that is cleaved from Man9GlcNAc at a much slower rate. When rat liver endoplasmic reticulum membrane extracts were incubated with Man9GlcNAc2, Man8GlcNAc2was the major product and Man8B was the major isomer. In contrast, rat liver Golgi membranes rapidly cleaved Man9GlcNAc2to Man6GlcNAc2and more slowly to Man5GlcNAc2. In this case all three isomers of Man8GlcNAc2were formed as intermediates, but a distinctive isomer, Man8A, was predominant. Antiserum to recombinant mannosidase IA immunoprecipitated an enzyme from Golgi extracts with the same specificity as recombinant mannosidase IA. These immunodepleted membranes were enriched in a Man9GlcNAc2to Man8GlcNAc2-cleaving activity forming predominantly the Man8B isomer. These results suggest that mannosidases IA and IB in Golgi membranes prefer the Man8B isomer generated by a complementary mannosidase that removes a single mannose from Man9GlcNAc2.

IgM polymerization inhibits the Golgi-mediated processing of the mu-chain carboxy-terminal glycans

Secreted glycoproteins generally contain oligosaccharides of the complex type. However, several molecules have been described in which individual glycans are processed differently from one another. Folding, assembly and oligomerization could affect the maturation of certain glycans by hindering them to the Golgi processing machinery. We have tested this possibility by analysing a panel of engineered murine mu chains secreted as mu2L2 monomers or as polymers, and having or not the carboxy-terminal glycan (Asn563). In secreted IgM polymers, Asn563 bears high-mannose oligosaccharides, typical of endoplasmic reticulum resident proteins, while complex sugars are found at the other four sites (Brenckle and Kornfeld, 1980 Arch. Biochem. Biophys. 243, 605-618). Polymeric and monomeric IgM contain mu chains whose glycans are processed differently. We show here that this is mainly due to the differential processing at the Asn563 glycan, which undergoes Golgi-mediated processing when IgM are secreted in the monomeric form. These results indicate that the oligomerization-dependent accessibility to the sugar modifying enzymes can be one of the key features that dictate the extent of oligosaccharide processing in multimeric glycoproteins. The presence of high mannose glycans at Asn563 implies that IgM polymerization takes place before encountering mannosidase II, likely in a pre-Golgi compartment.

Abnormal expression and glycosylation of the large and small isoforms of myelin-associated glycoprotein in dysmyelinating quaking mutants

The relative expression of large (L) and small (S) isoforms of the myelin-associated glycoprotein (MAG) and their glycosylation were compared in developing spinal cord of quaking and control mice. Using antisera specific for L- and S-MAG, respectively, it was shown that S-MAG is the principal isoform in quaking mice at all ages between 13 and 72 days, although L-MAG was just detectable by western blotting at the early ages. Both L- and S-MAG have higher apparent molecular weights in quaking mice than in controls. Experiments involving lectin binding and glycosidase treatment demonstrated that the higher molecular weight of MAG in the quaking mutant was due to a higher content of N-acetylneuraminic acid residues linked alpha 2-3 to galactose as well as to more branching of oligosaccharide moieties indicated by a higher content of subterminal galactose residues. The total sialic acid measured by HPAE-chromatography in purified quaking MAG was 40% higher than in control MAG. By contrast, quaking MAG contained less of the adhesion-related, HNK-1 carbohydrate epitope. Another difference was that a lower molecular weight form of MAG with predominantly high mannose oligosaccharides was prominent in young quaking mice, but not in controls. The abnormalities of MAG expression related to splicing of its mRNA and glycosylation may contribute to the myelin pathology in quaking mutants.

Studies on the aetiology of coeliac disease: no evidence for lectin-like components in wheat gluten

In an approach to examine the lectin-hypothesis in the pathogenesis of coeliac disease, the presence of lectin-like components in three wheat gluten preparations known to induce coeliac disease, gliadin, Frazer fraction III and an acetic acid/ethanol extract of gluten, was investigated. Lectin-like components in these wheat gluten preparations were traced in binding studies employing a variety of model glycoproteins glycosylated with the different types of N-linked oligosaccharides, i.e., those of the high mannose-, complex- and hybrid-type. Binding affinity of wheat proteins to these glycoproteins was analyzed by affinity dotting and blotting techniques and was compared to that of the well characterized lectins Galanthus nivalis agglutinin, Concanavalin A and wheat germ agglutinin. Though the three wheat gluten preparations exhibited binding reactivity for distinct model glycoproteins, no correlation was found between the type of N-glycosylation of the model glycoproteins and their binding capability to the different wheat gluten preparations. Moreover, binding of the three gluten preparations to the model glycoproteins could not be inhibited by competitive saccharides (methyl-alpha-D-mannopyranoside, N-acetyl-D-glucosamine, mannan). Enzymatic deglycosylation of the ligand glycoproteins with endo-beta-N-acetylglucosaminidase H (Endo H, EC 3.2.1.96) or peptide N-glycosidase F (PNGase F, EC 3.5.1.52) abolished their binding reactivity for the plant lectins, but did not affect binding of the wheat gluten preparations. These results give no evidence for the presence of lectin-like components in wheat gluten preparations and do question the 'lectin hypothesis' of coeliac disease.

The conformational effects of N-glycosylation on the tailpiece from serum IgM

1H-NMR spectroscopy has been used to study the conformation and dynamics of the isolated tailpiece from human serum immunoglobulin M, a 22-residue peptide containing a single asparagine glycosylation site. The peptide is isolated as a set of glycoforms, varying only in the sequence of the oligosaccharide attached at the glycosylation site. The oligosaccharides present have the general formula (Man)n(GlcNAc)2, with 45% having n = 6, 45% having n = 8 and 10% having n = 7 and/or 9. They have been identified and their NMR parameters compared to those found for the isolated oligosaccharides in free solution. The conformation and dynamics of the peptide component have also been studied, using NOE data and hydrogen-exchange experiments, and the results compared to those obtained from the aglycosyl peptide of the same sequence. The presence of the peptide is found to have no measurable effect on the conformation of the oligosaccharides. However, the presence of oligosaccharide causes a decrease in the conformational mobility of the backbone and sidechains of the peptide in the region of the glycosylation site. This is proposed to result from interactions between the oligosaccharide core and the amino acid side chains. Further, the conformation of the N-glycosidic linkage has been shown to be both rigid and planar. Thus, the conformational space available to an N-linked oligosaccharide in a glycoprotein relative to the protein may depend to a large extent upon the flexibility of the asparagine side chain. Various roles for the different glycoforms of the tail peptide are discussed.

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.

Comparative study of the N-glycoprotein synthesis through dolichol intermediates in mitochondria, Golgi apparatus-rich fraction and endoplasmic reticulum-rich fraction

1. Glycosylation of endogenous dolichol acceptors was higher in mitochondria than in C 30,000 g (Golgi apparatus-rich fraction) and C 100,000 g (endoplasmic reticulum-rich fraction). 2. In mitochondria, N-glycoprotein biosynthesized were composed of high mannose type and non-fucosylated biantennary complex type while in C 30,000 g and C 100,000 g preparations, they contained biantennary complex type as tri and tetraantennary complex type oligosaccharides in both fucosylated and non-fucosylated forms.

Structural studies of the asparagine-linked sugar chains of two immunoglobulin M's purified from a patient with Waldenström's macroglobulinemia

The structures of the sugar chains present in two human monoclonal IgM molecules purified from the serum of a patient with Waldenström's macroglobulinemia have been determined. The asparagine-linked sugar chains were liberated as oligosaccharides by hydrazinolysis and labeled by reduction with NaB3H4 after N-acetylation. Their structures were studied by serial lectin column chromatography and sequential exoglycosidase digestion in combination with methylation analysis. These two IgM's were shown to contain almost the same sugar chains. The sugar chains were a mixture of a series of high-mannose-type and biantennary complex-type oligosaccharides. The complex-type oligosaccharides contain Man alpha 1----6(+/- GlcNAc beta 1----4)(Man alpha 1----3)Man beta 1----4GlcNAc beta 1----4(Fuc alpha 1----6)GlcNAc as their core and GlcNAc beta 1----, Gal beta 1----4GlcNAc beta 1---- and Neu5Ac alpha 2----6Gal beta 1----4GlcNAc beta 1---- groups in their outer chain moieties.

One-step purification of murine IgM and human alpha 2-macroglobulin by affinity chromatography on immobilized snowdrop bulb lectin

A new mannose-specific plant lectin (GNA) isolated from the snowdrop bulb was immobilized on Sepharose 4B and employed for the purification of certain glycoproteins with high-mannose type glycan chains. Murine IgM bound tightly to this column and was eluted with 0.1 M methyl alpha-D-mannoside whereas bovine and murine IgG were not bound. When a murine hybridoma serum containing IgM monoclonal antibody was applied to this column, highly purified IgM antibody was obtained after elution with methyl alpha-D-mannoside. On the contrary, human IgM was not bound by this column despite reports that it contains high-mannose type glycan chains. alpha 2-Macroglobulin was the sole glycoprotein present in human serum which was bound by the immobilized snowdrop lectin column. It appears that only glycoproteins containing multiple Man(alpha 1,3)Man units are bound to the immobilized lectin.

Jacalin, the human IgA1 and IgD precipitating lectin, also binds IgA2 of both allotypes

The lectin jacalin from jackfruit seeds shows a human IgA-subclass specificity by gel precipitation and Western blotting. However, its reactivity with IgA2 is a matter of controversy. We further studied the immunoglobulin isotype specificity of jacalin by affinity chromatography with myeloma sera and by inhibition of jacalin binding to solid-phase IgA1 by purified monoclonal immunoglobulins. The lectin proved to bind IgA2 of both allotypes with a lower apparent affinity than for IgA1 and IgD.

Characterization of novel glycoprotein components of synaptic membranes and postsynaptic densities, gp65 and gp55, with a monoclonal antibody

A monoclonal antibody, mab SMgp65, which recognises two major glycoprotein components of isolated forebrain synaptic subfractions has been raised. The mab has been used to study the cellular and subcellular localisation of these novel glycoproteins and for the partial characterisation of both molecular species. Western blots show that the mab reacts with two diffuse glycoprotein bands (gp) of apparent Mr 65,000, gp65, and 55,000, gp55. Both glycoproteins are membrane-bound, only detectable in CNS tissue and exist solely in a concanavalin A (con A) binding form. Digestion with endoglycosidase H lowers the Mr of both glycoproteins by some 5-7 kDa. Gp65 and gp55 are enriched in synaptic membrane (SM), light membrane (LM) and microsomal fractions. However, whilst gp65 is enriched in isolated postsynaptic densities (psds) gp55 is conspicuously absent from this fraction. Regional distribution studies show a marked variation in the level of gp65. Gp65 is concentrated in several forebrain regions notably cerebral cortex, hippocampus and striatum, is present only in low levels in cerebellum and is barely detectable in pons and medulla. In contrast gp55 is present in all regions studied, but is most concentrated in cerebellum. Immunocytochemical studies show intense staining of regions rich in gp65, but no staining of regions deficient in this glycoprotein. This suggests that the mab recognises gp65, but not gp55 in fixed tissue sections. Exposure of tissue sections to Triton X-100 increases the intensity of gp65-like immunoreactivity, but does not alter its pattern of subcellular distribution. Higher resolution studies show the immunoreactivity to be localised to subsets of neurites, many being axonal. The reaction deposits also extend into the synaptic region of the immunoreactive neurones. Cultured cerebellar granule cells, but not astrocytes express gp55. The results are discussed in terms of the molecular properties and localisation of these two novel glycoproteins.

Structural characterization of intact, branched oligosaccharides by high performance liquid chromatography and liquid secondary ion mass spectrometry

We report results of a mass-spectrometric-based strategy for determining the detailed structural features of N-linked oligosaccharides from glycoproteins. The method was used to characterize a series of intact, high mannose oligosaccharides isolated from human immunoglobulin M (IgM). The IgM was purified from a patient with Waldenstrom's macroglobulinemia. The strategy included releasing the oligosaccharides by digestion of the purified glycoprotein with endoglycosidase H, separating the released oligosaccharides by high resolution gel filtration, and derivatizing the resulting reducing termini with the uv-absorbing moiety, ethyl p-aminobenzoate. This particular derivative facilitates HPLC detection and provides centers for protonation and deprotonation enhancing liquid secondary ion mass spectra. Positive and negative ion spectra contained molecular species of similar abundance. However, fragment ion peaks yielding sequence information were significantly more prominent in the negative ion mass spectra. Furthermore, it was obvious that the fragmentation patterns differed substantially for linear and branched oligomers. For linear oligosaccharides, a smooth envelope of fragment ions was observed; from low to high mass there was an ordered decrease in ion abundance from both the reducing and nonreducing termini. This pattern of fragment ions was not observed for branched oligosaccharides since in these cases fragments at certain masses could not arise by single bond cleavages. Therefore, these fragments were either significantly reduced in abundance or absent as compared with identical fragments formed from linear molecules. Importantly, 200 pmol of an oligosaccharide could be derivatized, separated, and detected by mass spectrometry, allowing identification of previously unreported minor components of the IgM oligosaccharides. Therefore, this experimental strategy is particularly useful for the purification and detailed structural characterization of low abundance oligosaccharides isolated from heterogeneous biological samples.

Lectin affinity high-performance liquid chromatography: interactions of N-glycanase-released oligosaccharides with leukoagglutinating phytohemagglutinin, concanavalin A, Datura stramonium agglutinin, and Vicia villosa agglutinin

We have developed a lectin affinity high-performance liquid chromatography technique for analysis of oligosaccharides using columns of silica-bound lectins. Purified leukoagglutinating phytohemagglutinin (L-PHA), concanavalin A (Con A), Datura stramonium agglutinin (DSA), and Vicia villosa agglutinin (VVA) were covalently coupled to periodate-oxidized diol-silica by reductive amination. Homogeneous oligosaccharides of known structure, purified following release from Asn with N-glycanase and reduction with NaBH4, were tested for their ability to interact with the silica-bound lectins. The characteristic elution position obtained for each oligosaccharide was reproducible and correlated with specific structural features. The oligosaccharide specificities displayed by silica-bound L-PHA, Con A, and DSA were virtually identical to those established utilizing lectin-agarose conjugates. Analysis of oligosaccharides by lectin affinity HPLC allowed further definition of the specificity of VVA for N-glycanase-released, reduced oligosaccharides. Lectin affinity HPLC is rapid and convenient, providing an important structure-specific dimension to oligosaccharide analysis. This technique is particularly useful when utilized in conjunction with anion-exchange and ion-suppression amine adsorption HPLC methods, which fractionate on the basis of charge and size, respectively. In addition to their utility for oligosaccharide characterization, these affinity columns demonstrate the high degree of oligosaccharide specificity displayed by plant and animal lectins.

Major carbohydrate structures at five glycosylation sites on murine IgM determined by high resolution 1H-NMR spectroscopy

Mouse myeloma immunoglobulin IgM heavy chains were cleaved with cyanogen bromide into nine peptide fragments, four of which contain asparagine-linked glycosylation. Three glycopeptides contain a single site, including Asn 171, 402, and 563 in the intact heavy chain. Another glycopeptide contains two sites at Asn 332 and 364. The carbohydrate containing fragments were treated with Pronase and fractionated by elution through Bio-Gel P-6. The major glycopeptides from each site were analyzed by 500 MHz 1H-NMR and the carbohydrate compositions determined by gas-liquid chromatography. The oligosaccharide located at Asn 171 is a biantennary complex and is highly sialylated. The amount of sialic acid varies, and some oligosaccharides contain alpha 1,3-galactose linked to the terminal beta 1,4-galactose. The oligosaccharides at Asn 332, Asn 364, an Asn 402 are all triantennary and are nearly completely sialylated on two branches and partially sialylated on the triantennary branch linked beta 1,4 to the core mannose. The latter is sialylated about 40% of the time for all three glycosylation sites. The major oligosaccharide located at Asn 563 is of the high mannose type. The 1H-NMR determination of structures at Asn 563 suggests that the high mannose oligosaccharide contains only three mannose residues.

Characterization of MOPC 315 IgA oligosaccharide processing intermediates

The structures of alpha 1,2-mannose containing partially processed asparagine-linked oligosaccharides on the alpha-chain of MOPC 315 IgA were characterized using specific glycosidases and acetolysis. Man6GlcNAc2, a substrate for a Golgi alpha 1,2-mannosidase, was found to be a single isomeric structure. Likewise, Man7-9GlcNAc2 were single isomers indicating an ordered sequence of removal of alpha 1,2-linked mannose residues on this murine immunoglobulin heavy chain.

The synthesis and in vivo assembly of functional antibodies in yeast

The yeast Saccharomyces cerevisiae can synthesize, process and secrete higher eukaryotic proteins. We have investigated the expression of immunoglobulin chains in yeast and demonstrate here the synthesis, processing and secretion of light and heavy chains, the glycosylation of heavy chain, the intracellular localization of these foreign proteins by immunofluorescence, and the detection of functional antibodies in cells co-expressing both chains. This may provide the basis of a microbial fermentation process for the production of monoclonal antibodies. The co-expression of light and heavy chains in Escherichia coli has been reported but functional antibodies were not assembled in vivo. Furthermore, only low-level assembly of these chains was found in vitro.

Primary structure of N-glycosidically linked asialoglycans of secretory immunoglobulins A from human milk

The asialoglycopeptides obtained from secretory immunoglobulins A from human milk have been separated by gel filtration and affinity chromatography on Concanavalin A-Sepharose and Lens culinaris agglutinin-Sepharose columns. Their structures have been determined by sugar analysis, methylation studies including mass spectrometry and 500-MHz 1H-NMR spectroscopy. The glycans are of the biantennary N-acetyllactosamine type differing in their degree of extension by fucosyl-N-acetyllactosamine residues. The overall structures of the glycopeptides are as follows: (Formula: see text) Most of the asialoglycopeptide structures possess an intersecting GlcNAc residue; they are suggested to be located on the alpha chain of the secretory immunoglobulins A of human milk. The non-intersected structures probably occur on the secretory piece. The methodology applied to the structural analysis adequately coped with the extremely high degree of heterogeneity shown by the structures.

A comparative study of the size-heterogeneous high mannose oligosaccharides of some insect vitellins

Comparative studies of the carbohydrate component from vitellins of the cockroaches Blattella germanica, Blaberus discoidalis, Periplaneta americana and Simploce capitata and the locust Locusta migratoria have been conducted. Chemical, enzymatic and chromatographic analyses show that each vitellin contains variably processed high mannose type oligosaccharides. While all have a common size range they occur as two distinct classes based on the proportion of individual saccharides present. Oligosaccharide size distribution is not a characteristic of an individual animal but of the species. Because oligosaccharide heterogeneity also occurs in B. germanica vitellogenin (the hemolymph precursor of vitellin), it does not result from structural changes during or after its uptake by the egg.

Biosynthesis of glycoproteins in human placenta: processing of oligosaccharides

The processing of the high-mannose asparagine-linked oligosaccharides synthesized by first-trimester human placenta has been investigated. Tissue was pulsed for 1 h with [2-3H]mannose and chased for zero, 45, 90, and 180 min in media containing unlabeled mannose. Glycopeptides, prepared by Pronase digestion of the delipidated membrane pellets at each time point, were treated with endo-beta-N-acetylglucosaminidase-H to release the high-mannose asparagine-linked oligosaccharides. The largest major processing intermediate isolated was Glc1Man9GlcNAc, which was converted into Man9GlcNAc, and then into Man8GlcNAc, Man7GlcNAc, Man6GlcNAc, and Man5GlcNAc. There was also a minor pathway in which mannosyl residues were removed prior to the glucose. By carrying out the detailed structural characterization of the individual processing intermediates, it was possible to demonstrate that processing of the Man9GlcNAc to Man5GlcNAc proceeded by the nonrandom removal of the alpha 1,2-linked mannosyl residues. Specifically, of 12 possible sequences of removal of the four alpha 1,2-linked mannosyl residues present in Man9GlcNAc, first-trimester human placenta utilized only two of these in the processing of asparagine-linked oligosaccharides. It is suggested that the limited number of processing pathways reflects a high degree of specificity of these reactions in human placenta.

Separation procedure and sugar composition of oligosaccharides in the surface glycoprotein of Friend murine leukemia virus

The sugar composition of the surface glycoprotein from Friend murine leukemia virus was determined by gas-liquid chromatography of the alditol acetates and by the thiobarbituric acid method, respectively. N-Acetylglucosamine, mannose, galactose, sialic acid and fucose were found in a molar ratio around 15.2:11.6:7.4:3.3:1.0. Ten oligosaccharide fractions were obtained from glycoprotein preparations by a suitable sequence of degradation (with pronase, endo-beta-N-acetylglucosaminidase H, neuraminidase, and by hydrazinolysis) and separation procedures (concanavalin A-affinity chromatography and gel filtration). The qualitative sugar composition of these fractions was analyzed by in vivo labelling with D-[6-(3)H]glucosamine, D-[2-(3)H]mannose, D-[6-(3)H]galactose, or L-[6-(3)H]fucose, and their molecular weights were estimated from the gel elution volumina. Four fractions of N-glycosidically linked oligosaccharides of the oligomannosidic ('high mannose') type oligomannosidic7-oligomannosidic10, about seven to ten sugar residues), two of the mixed (M11 and M12), and four of the N-acetyllactosaminic ('complex') type (N-acetyllactosaminic9, probably nine sugar residues; N-acetyllactosaminica-N-acetyllactosaminic c, size unknown) were thus identified.

The biosynthetic pathway of the asparagine-linked oligosaccharides of glycoproteins

This review deals with the structure and addition of the different types of oligosaccharides to asparagine residues in proteins. This process occurs in several steps, first an oligosaccharide which contains N-acetylglucosamine mannose and glucose is built up joined to dolichyl diphosphate. The oligosaccharide is then transferred to a polypeptide chain, loses its glucose, and is modified by removal of some monosaccharides and addition of others giving rise to a variety of saccharides.