Concepts and principles of O-linked glycosylation

Glycan antagonists and inhibitors: a fount for drug discovery

Glycans, the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids, represent a relatively unexploited area for drug development compared with other macromolecules. This review describes the major classes of glycans synthesized by animal cells, their mode of assembly, and available inhibitors for blocking their biosynthesis and function. Many of these agents have proven useful for studying the biological activities of glycans in isolated cells, during embryological development, and in physiology. Some are being used to develop drugs for treating metabolic disorders, cancer, and infection, suggesting that glycans are excellent targets for future drug development.

An essential role of sialylated O-linked sugar chains in the recognition of mouse CD99 by paired Ig-like type 2 receptor (PILR)

The paired Ig-like type 2 receptor (PILR), which comprises both inhibitory and activating isoforms, is well conserved among most mammalians. The inhibitory PILRalpha possesses an ITIM in its cytoplasmic domain, whereas the activating PILRbeta does not have an ITIM but transduces activating signals by associating with the ITAM-bearing DAP12 adapter molecule. Both mouse PILRalpha and PILRbeta recognize mouse CD99, which is broadly expressed on various cells, including lymphocytes, and is involved in the regulation of immune responses. We herein report that sialylated O-linked sugar chains on CD99 are essential for the recognition by PILR. Mutations of one of two O-glycosylation sites on CD99 significantly reduced recognition of CD99 by the activating PILRbeta, whereas recognition by the inhibitory PILRalpha was not affected. In contrast, mutations of both O-glycosylation sites on CD99 completely abrogated the recognition by both PILRalpha and PILRbeta. PILR did not recognize CD99 treated with neuraminidase, and CD99 expressed on cells transfected with core 2 beta-1,6-N-acetylglucosaminyltransferase was not recognized by PILR. NK cells expressing endogenous activating PILRbeta receptors mediated cytotoxicity against cells expressing wild-type CD99 but not cells expressing mutant CD99 that lacked O-glycosylation sites. These findings indicate that sialylated O-linked sugar structures on CD99 play an important role in the recognition of PILR.

Analysis of O-glycosylation

Secreted as well as membrane-associated eukaryotic proteins are most commonly glycosylated. Saccharides are attached to proteins mainly through N-and O-glycosydic bonds or as part of the glycosylphosphatidyinositol-membrane anchor. In contrast to N-glycosylation, which involves the co-translational transfer in the endoplasmic reticulum (ER) of the glycan portion of Glc3Man9GlcNAc2-PP-dolichol to suitable Asn residues on nascent polypeptides, O-glycosylation begins with the addition of a single monosaccharide. Contrary to N-glycosylation, which involves an asparagine residue in the sequon Asn-Xaa-Thr/Ser (Xaa can be any amino acid except Pro, and it is rarely Cys), no particular sequence motif has been described for O-glycosylation. This may reflect the fact that: (1) the specificity of the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase is presently unknown; and (2) seems to be modulated by sequence context, secondary structure, and surface accessibility (1). An internet server, accessible at http://www.cbs.dtu.dk/netOglyc/cbsnetOglyc.html , produces neural network predictions of mucin-type GalNAc O-glycosylation sites in mammalian proteins based on 299 known and verified mucin-type O-glycosylation sites. The sequence context of glycosylated threonines was found to differ from that of serine, and the sites were found to cluster. Nonclustered sites had a sequence context different from that of clustered sites, and charged residues were disfavored at position -1 and +3.

GPX5, the selenium-independent glutathione peroxidase-encoding single copy gene is differentially expressed in mouse epididymis

Using various molecular approaches, including reverse transcription–polymerase chain reaction (RT–PCR), rapid amplification of cDNA ends–PCR, sequencing, northern and western blotting, we found that the mouse GPX5 gene gives rise to at least three different transcripts that are not expressed at the same levels in the mouse epididymis. In addition to the major GPX5 transcript, we show that minor GPX5 transcripts exist, arising either from precocious termination of transcription or an alternative splicing event within intron 4 of the 5 exon-encoding GPX5 single copy gene. Furthermore, we demonstrate that variants of the GPX5 protein that are correlated with the shorter GPX5 transcripts can be detected in caput epididymidis protein extracts and that the various GPX5 isoforms are subject to differential post-transcriptional maturation processes in the mouse epididymis that essentially involve the addition of O-glycosyl extensions. Using a sensitive poly-A+ mRNA tissue blot, as well as RT–PCR and northern assays, we further show that in addition to being expressed in the epididymis, the GPX5 gene is also expressed, albeit at lower levels, in other tissues of the male genital tract, including the testis and prostate. Finally, we present evidence suggesting that the GPX5 gene is expressed in a temporally regulated manner during mouse embryonic development.

Deficiencies in subunits of the Conserved Oligomeric Golgi (COG) complex define a novel group of Congenital Disorders of Glycosylation

Processing of the glycan structures on glycoproteins by different glycosylation enzymes depends on, among other, the non-uniform distribution of these enzymes within the Golgi stacks. This compartmentalization is achieved by a balance between anterograde and retrograde vesicular trafficking. If the balance is disturbed, the glycosylation machinery is mislocalized, which can cause Congenital Disorders of Glycosylation type II (CDG-II), as illustrated by the identification of congenital defects in the Conserved Oligomeric Golgi (COG) complex in humans. We collected findings from different COG deficient cell types, such as CHO, yeast and human fibroblasts to hypothesize about structure and function of the COG complex, and compared the phenotypes and genotypes of the patients known with a COG deficiency. Among 35 CDG-II patients we found 5 patients with a COG defect. COG defects are a novel group of CDG-II with deficient N- as well as O-glycosylation.

Initiation of protein O glycosylation by the polypeptide GalNAcT-1 in vascular biology and humoral immunity

Core-type protein O glycosylation is initiated by polypeptide N-acetylgalactosamine (GalNAc) transferase (ppGalNAcT) activity and produces the covalent linkage of serine and threonine residues of proteins. More than a dozen ppGalNAcTs operate within multicellular organisms, and they differ with respect to expression patterns and substrate selectivity. These distinctive features imply that each ppGalNAcT may differentially modulate regulatory processes in animal development, physiology, and perhaps disease. We found that ppGalNAcT-1 plays key roles in cell and glycoprotein selective functions that modulate the hematopoietic system. Loss of ppGalNAcT-1 activity in the mouse results in a bleeding disorder which tracks with reduced plasma levels of blood coagulation factors V, VII, VIII, IX, X, and XII. ppGalNAcT-1 further supports leukocyte trafficking and residency in normal homeostatic physiology as well as during inflammatory responses, in part by providing a scaffold for the synthesis of selectin ligands expressed by neutrophils and endothelial cells of peripheral lymph nodes. Animals lacking ppGalNAcT-1 are also markedly impaired in immunoglobulin G production, coincident with increased germinal center B-cell apoptosis and reduced levels of plasma B cells. These findings reveal that the initiation of protein O glycosylation by ppGalNAcT-1 provides a distinctive repertoire of advantageous functions that support vascular responses and humoral immunity.

Identification of a novel conserved sorting motif required for retromer-mediated endosome-to-TGN retrieval

The cation-independent mannose 6-phosphate receptor (CIMPR) cycles between the trans-Golgi network (TGN) and endosomes to mediate sorting of lysosomal hydrolases. The endosome-to-TGN retrieval of the CIMPR requires the retromer complex. Genetic, biochemical and structural data support the hypothesis that the retromer can directly bind to the tail of the CIMPR, to sort the CIMPR into vesicles and tubules for retrieval to the TGN. Presently, however, no known retromer sorting motif in the tail of the CIMPR has been identified. Using CD8-reporter proteins carrying the cytoplasmic tail of the CIMPR we have systematically dissected the CIMPR tail to identify a novel, conserved aromatic-containing sorting motif that is critical for the endosome-to-TGN retrieval of the CIMPR and for the interaction with retromer and the clathrin adaptor AP-1.

Histochemical demonstration of two subtypes of O-linked oligosaccharides in the rat gastric glands

The gastric glands synthesize glycoproteins whose oligosaccharides are linked to the peptide core mainly by the O-glycosidic bond, specifically removed by beta-elimination procedure. Our aim was to research the possibility of the existence of two subtypes of O-linked oligosaccharides with a different behavior to the removal procedure. The lectins from peanut (PNA) and Maackia amurensis (MAA-I) were histochemically used as markers of the O-linked oligosaccharides. Sections were also pretreated with beta-elimination and/or peptide N-Glycosidase F (PNGase-F) for the specific removal of O- and N-linked oligosaccharides, respectively. The lectin GNA, which mainly labels to N-linked oligosaccharides, was used to test the correct working of PNGase-F. To test the possibility that the beta-elimination treatment could remove the terminal sialic acid residues, the lectin LFA was used. The surface epithelium was negative to PNA, while it became strongly positive when beta-elimination was performed for 1 day. This staining was resistant to PNGase-F, suggesting that PNA was labeling to O-linked oligosaccharides. However, after beta-elimination for 5 days this staining is not observed. A similar pattern appeared with MAA-I. We propose the existence of two subtypes of O-linked oligosaccharides: labile and resistant. The labile O-linked oligosaccharides are removed with beta-elimination for 1 day, unmasking the PNA-positive oligosaccharides. These oligosaccharides are resistant O-linked oligosaccharides because staining is abolished with longer treatment of beta-elimination. The results with MAA-I also support this suggestion. In summary, the labile O-linked oligosaccharides are removed with short treatment, while the resistant O-linked oligosaccharides need a stronger procedure (for 5 days).

Premelanosome amyloid-like fibrils are composed of only golgi-processed forms of Pmel17 that have been proteolytically processed in endosomes

Melanin pigments are synthesized within specialized organelles called melanosomes and polymerize on intraluminal fibrils that form within melanosome precursors. The fibrils consist of proteolytic fragments derived from Pmel17, a pigment cell-specific integral membrane protein. The intracellular pathways by which Pmel17 accesses melanosome precursors and the identity of the Pmel17 derivatives within fibrillar melanosomes have been a matter of debate. We show here that antibodies that detect Pmel17 within fibrillar melanosomes recognize only the luminal products of proprotein convertase cleavage and not the remaining products linked to the transmembrane domain. Moreover, antibodies to the N and C termini detect only Pmel17 isoforms present in early biosynthetic compartments, which constitute a large fraction of detectable steady state Pmel17 in cell lysates because of slow early biosynthetic transport and rapid consumption by fibril formation. Using an antibody to a luminal epitope that is destroyed upon modification by O-linked oligosaccharides, we show that all post-endoplasmic reticulum Pmel17 isoforms are modified by Golgi-associated oligosaccharide transferases, and that only processed forms contribute to melanosome biogenesis. These data indicate that Pmel17 follows a single biosynthetic route from the endoplasmic reticulum through the Golgi complex and endosomes to melanosomes, and that only fragments encompassing previously described functional luminal determinants are present within the fibrils. These data have important implications for the site and mechanism of fibril formation.

Identification of six novel T-1 conotoxins from Conus pulicarius by molecular cloning

Cone snails are a group of ancient marine gastropods with highly sophisticated defense and prey strategies using conotoxins in their venom. Conotoxins are a diverse array of small peptides, mostly with multiple disulfide bridges. Using a 3' RACE approach, we identified six novel peptides from the venom ducts of a worm-hunting cone snail Conus pulicarius. These peptides are named Pu5.1-Pu5.6 as their primary structures show the typical pattern of T-1 conotoxin family, a large and diverse group of peptides widely distributed in venom ducts of all major feeding types of Conus. Except for the conserved signal peptide sequences in the precursors and unique arrangement of Cys residues (CC-CC) in mature domains, the six novel T-1 conotoxins show remarkable sequence diversity in their pro and mature regions and are, thus, likely to be functionally diversified. Here, we present a simple and fast strategy of gaining novel disulfide-rich conotoxins via molecular cloning and our detailed sequence analysis will pave the way for the future functional characterization of toxin-receptor interaction.

Structure elucidation of glycoproteins by direct nanoESI MS and MS/MS analysis of proteolytic glycopeptides

Bovine ribonuclease B (RNAse B) and asialofetuin (FETUA) were subjected to in-capillary tryptic digest (Pohlentz et al. Proteomics. 2005, 5, 1758-1763) and the obtained glycopeptides were analyzed, respectively, by nanoelectrospray ionization mass spectrometry and collision-induced dissociation (CID) during the ongoing digest. For RNAse, B glycans of the high-mannose type (Man(4) to Man(9)) attached to either a tetra- or a hexapeptide containing the sole N-glycosylation site of the protein were detected. Glycopeptides derived from all three N-glycosylation sites of FETUA were observed, and the corresponding CID spectra proved the respective glycans to be oligosaccharides of the triantennary complex type. Moreover, an O-glycopeptide carrying Gal-GalNAc at T(280) could be unambiguously identified. An in-solution tryptic/chymotryptic digest of human transferrin (TRFE) was analyzed directly for glycopeptides subsequent to the addition of methanol and formic acid. Disialylated diantennary glycans were observed in glycopeptides of both N-glycosylation sites of TRFE. These results demonstrate the feasibility of direct structure determination of glycopeptides in proteolytic mixtures without any further refurbishment.

Post-translational modifications of Epstein Barr virus BARF1 oncogene-encoded polypeptide

Epstein-Barr virus is associated with several human lymphomas and carcinomas, and its BARF1 oncogene encodes a protein that is thought to play an important role in carcinogenesis. A BARF1 recombinant adenovirus expression system, which led us to discover the macromolecular size of the cleaved and secreted form of the BARF1 protein in the native state and its mitogenic capacity on various cell lines in culture, was used further to investigate the structure and maturation of the BARF1 protein. We recently reported biophysical studies that showed dimer-based oligomerization of the BARF1 polypeptide. Here, new data are presented that confirm post-translational modifications predicted from the BARF1 sequence: phosphorylation on serine and threonine, and N- and O-glycosylation. The N- and O-glycans were partially characterized and it was demonstrated that both modifications are required for active secretion of the BARF1 protein via the classical pathway.

O-glycan regulation of apoptosis and proliferation in colorectal cancer cell lines

Cell growth pathways are mediated through protein–glycan interactions including O-glycosylation. Investigation of these growth pathways can be carried out using appropriate inhibitors to identify stage-specific events. We have adopted this approach to study a group of benzyl-O-N-acetyl-D-galactosamine analogues in human colorectal cancer cell lines. Exposure to O-glycan inhibitors resulted in the induction of apoptosis, a block in proliferation, accumulation of intracellular aryl-glycans and changes in related genes as detected by gene array. Colorectal cancer cell lines susceptible to the inhibitors showed growth arrest with all compounds. However, a differential action of each inhibitor was detected in the pattern of genes affected and in the structure of aryl-glycans formed.

O-linked glycosylation ensures the normal conformation of the autotransporter adhesin involved in diffuse adherence

The Escherichia coli adhesin involved in diffuse adherence (AIDA-I) is one of the few glycosylated proteins found in Escherichia coli. Glycosylation is mediated by a specific heptosyltransferase encoded by the aah gene, but little is known about the role of this modification and the mechanism involved. In this study, we identified several peptides of AIDA-I modified by the addition of heptoses by use of mass spectrometry and N-terminal sequencing of proteolytic fragments of AIDA-I. One threonine and 15 serine residues were identified as bearing heptoses, thus demonstrating for the first time that AIDA-I is O-glycosylated. We observed that unglycosylated AIDA-I is expressed in smaller amounts than its glycosylated counterpart and shows extensive signs of degradation upon heat extraction. We also observed that unglycosylated AIDA-I is more sensitive to proteases and induces important extracytoplasmic stress. Lastly, as was previously shown, we noted that glycosylation is required for AIDA-I to mediate adhesion to cultured epithelial cells, but purified mature AIDA-I fused to GST was found to bind in vitro to cells whether or not it was glycosylated. Taken together, our results suggest that glycosylation is required to ensure a normal conformation of AIDA-I and may be only indirectly necessary for its cell-binding function.

Effect of pH on the association behavior in aqueous solutions of pig gastric mucin

In this study, dynamic light scattering (DLS), turbidity, and rheo-small angle light scattering (rheo-SALS) methods have been utilized to examine the impact of pH (1 < or = pH < or = 7) on aqueous solutions of noncommercial purified pig gastric mucin. The asymmetric flow field-flow fractionation (AFFFF) measurements established that the mucin sample has a high molecular weight and is polydisperse. DLS measurements on dilute solutions of mucin disclosed large interchain aggregates at pH 2, where the polymer has a low charge density or is uncharged. At lower or higher values of pH, mucin is charged and the tendency of forming interpolymer complexes is affected. In the semidilute concentration regime, pronounced junction zones ('lumps' of polymer) are evolved and a heterogeneous connected network is formed at pH 2, whereas the association structures are disintegrated (smaller 'lumps') at lower or higher pH values due to electrostatic repulsive interactions, and a more homogeneous network is evolved. The DLS and viscosity results at pH 1 indicate the development of a fragmented network, composed of contracted chains that are decorated by some positive charges. The effect of shear flow on the structure of semidilute solutions of mucin was investigated with the aid of rheo-SALS methods. The scattered intensity revealed a strong upturn at low values of the wave vector (q) for mucin solutions at pH 2 and pH 4, which suggests the evolution of large association domains. At these pH values, a flow-induced anisotropy in the 2D SALS patterns in the form of elliptical shapes was observed at high shear rates.

Intracellular processing, glycosylation, and cell surface expression of human metapneumovirus attachment glycoprotein

The biosynthesis and posttranslational processing of human metapneumovirus attachment G glycoprotein were investigated. After pulse-labeling, the G protein accumulated as three species with molecular weights of 45,000, 50,000, and 53,000 (45K, 50K, and 53K, respectively). N-Glycosidase digestion indicated that these forms represent the unglycosylated precursor and N-glycosylated intermediate products, respectively. After an appropriate chase, these three naive forms were further processed to a mature 97K form. The presence of O-linked sugars in mature G protein was confirmed by O-glycanase digestion and lectin-binding assay using Arachis hypogaea (peanut agglutinin), an O-glycan-specific lectin. In addition, in the O-glycosylation-deficient cell line (CHO ldlD cell), the G protein could not be processed to the mature form unless the exogenous Gal and GalNAc were supplemented, which provided added evidence supporting the O-linked glycosylation of G protein. The maturation of G was completely blocked by monensin but was partially sensitive to brefeldin A (BFA), suggesting the O-linked glycosylation of G initiated in the trans-Golgi compartment and terminated in the trans-Golgi network. Enzymatic deglycosylation analysis confirmed that the BFA-G was a partial mature form containing N-linked oligosaccharides and various amounts of O-linked carbohydrate side chains. The expression of G protein at the cell surface could be detected by indirect immunofluorescence staining assay. Furthermore, cell surface immunoprecipitation displayed an efficient intracellular transport of G protein.

Glycomic mapping of O- and N-linked glycans from major rat sublingual mucin

Carbohydrate moieties of salivary mucins play various roles in life processes, especially as a microbial trapping agent. While structural details of the salivary O-glycans from several mammalian sources are well studied, very little information is currently available for the corresponding N-glycans. The existence of N-glycans alongside O-glycans on mucin isolated from rat sublingual gland has previously been implicated by total glycosyl compositional analysis but the respective structural data are both lacking. The advent of facile glycomic mapping and sequencing methods by mass spectrometry (MS) has enabled a structural reinvestigation into many previously unsolved issues. For the first time, high energy collision induced dissociation (CID) MALDI-MS/MS as implemented on a TOF/TOF instrument was applied to permethyl derivatives of mucin type O-glycans and N-glycans, from which the linkage specific fragmentation pattern could be established. The predominant O-glycans carried on the rat sublingual mucin were defined as sialylated core 3 and 4 types whereas the N-glycans were determined to be non-bisected hybrid types similarly carrying a sialylated type II chain. The masking effect of terminal sialylation on the tight binding of rat sublingual mucin to Galbeta1-->4GlcNAc specific lectins and three oligomannose specific lectins were clearly demonstrated in this study.

Between-species analysis of short-repeat modules in cell wall and sex-related hydroxyproline-rich glycoproteins of Chlamydomonas

Protein diversification is commonly driven by single amino acid changes at random positions followed by selection, but, in some cases, the structure of the gene itself favors the occurrence of particular kinds of mutations. Genes encoding hydroxyproline-rich glycoproteins (HRGPs) in green organisms, key protein constituents of the cell wall, carry short-repeat modules that are posited to specify proline hydroxylation and/or glycosylation events. We show here, in a comparison of two closely related Chlamydomonas species-Chlamydomonas reinhardtii (CC-621) and Chlamydomonas incerta (CC-1870/3871)-that these modules are prone to misalignment and hence to both insertion/deletion and endoduplication events, and that the dynamics of the rearrangements are constrained by purifying selection on the repeat patterns themselves, considered either as helical or as longitudinal face modules. We suggest that such dynamics may contribute to evolutionary diversification in cell wall architecture and physiology. Two of the HRGP genes analyzed (SAG1 and SAD1) encode the mating-type plus and minus sexual agglutinins, displayed only by gametes, and we document that these have undergone far more extensive divergence than two HRGP genes (GP1 and VSP3) that encode cell wall components-an example of the rapid evolution that characterizes sex-related proteins in numerous lineages. Strikingly, the central regions of the agglutinins of both mating types have diverged completely, by selective endoduplication of repeated motifs, since the two species last shared a common ancestor, suggesting that these events may have participated in the speciation process.

Effect of beta-O-glucosylation on L-Ser and L-Thr diamides: a bias toward alpha-helical conformations

Beta-D-O-glucosylation produces a remarkable effect on the peptide backbone of the model peptides derived from serine and threonine. Consequently, this type of glycosylation is responsible for the experimentally observed shift from extended conformations (model peptides) towards the folded conformations (model glycopeptides). The conclusion has been solidly assessed by a combined NMR/MD protocol. Interestingly, the MD (molecular dynamics) results for the glycopeptides point towards the existence of water-bridging molecules between the sugar and peptide moieties, which could explain the stabilization of the folded conformers in aqueous solution.

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."

O-linked glycosylation at threonine 27 protects the copper transporter hCTR1 from proteolytic cleavage in mammalian cells

The major human copper uptake protein, hCTR1, has 190 amino acids and a predicted mass of 21 kDa. hCTR1 antibodies recognize multiple bands in SDS-PAGE centered at 35 kDa. Part of this increased mass is due to N-linked glycosylation at Asn-15. We show that in mammalian cells the N15Q mutant protein trafficked to the plasma membrane and mediated copper uptake at 75% of the rate of wild-type hCTR1. We demonstrate that the extracellular amino terminus of hCTR1 also contains O-linked polysaccharides. Glycosidase treatment that removed O-linked sugars reduced the apparent mass of hCTR1 or N15Q mutant protein by 1-2 kDa. Expression of amino-terminal truncations and alanine substitution mutants of hCTR1 in HEK293 and MDCK cells localized the site of O-linked glycosylation to Thr-27. Expression of alanine substitutions at Thr-27 resulted in proteolytic cleavage of hCTR1 on the carboxyl side of the T27A mutations. This cleavage produced a 17-kDa polypeptide missing approximately the first 30 amino acids of hCTR1. Expression of wild-type hCTR1 in mutant Chinese hamster ovary cells that were unable to initiate O-glycosylation also resulted in hCTR1 cleavage to produce the 17-kDa polypeptide. The 17-kDa hCTR1 polypeptide was located in the plasma membrane and mediated copper uptake at about 50% that of the rate of wild-type hCTR1. Thus, O-linked glycosylation at Thr-27 is necessary to prevent proteolytic cleavage that removes half of the extracellular amino terminus of hCTR1 and significantly impairs transport activity of the remaining polypeptide.

Membrane association is a determinant for substrate recognition by PMT4 protein O-mannosyltransferases

Protein O-mannosylation represents an evolutionarily conserved, essential posttranslational modification with immense impact on a variety of cellular processes. In humans, O-mannosylation defects result in Walker-Warburg syndrome, a severe recessive congenital muscular dystrophy associated with defects in neuronal migration that produce complex brain and eye abnormalities. In mouse and yeasts, loss of O-mannosylation causes lethality. Protein O-mannosyltransferases (PMTs) initiate the assembly of O-mannosyl glycans. The evolutionarily conserved PMT family is classified into PMT1, PMT2, and PMT4 subfamilies, which mannosylate distinct target proteins. In contrast to other types of glycosylation, signal sequences for O-mannosylation have not been identified to date. In the present study, we identified signals that determine PMT4-dependent O-mannosylation. Using specific model proteins, we demonstrate that in yeast Pmt4p mediates O-mannosylation of Ser/Thr-rich membrane-attached proteins. The nature of the membrane-anchoring sequence is nonrelevant, as long as it is flanked by a Ser/Thr-rich domain facing the endoplasmic reticulum lumen. Our work shows that, in contrast to several other types of glycosylation, PMT4 O-mannosylation signals are not just linear protein's primary structure sequences but rather are highly complex. Based on these findings, we performed in silico analyses of the Saccharomyces cerevisiae proteome and identified previously undescribed Pmt4p substrates. This tool for proteome-wide identification of O-mannosylated proteins is of general interest because several of these proteins are major players of a wide variety of cellular processes.

Glycosylation of therapeutic proteins in different production systems

Glycosylation plays an important role in a number of therapeutic proteins, including monoclonal antibodies. The enzymatic activity of a therapeutic protein is mainly determined by the protein structure, whereas the pharmacokinetics, pharmacodistribution, solubility, stability, enhancement of effector function and receptor binding are all influenced by the carbohydrate moiety. Hyperglycosylated proteins show increased serum half-life, are less sensitive to proteolysis and more heat-stable compared with the non-glycosylated forms. Molecular engineering of the TNK-tissue plasminogen activator molecule results in a more complex type of glycosylation and increases the half-life of the protein, which allows a single bolus injection at a lower dose for the treatment of acute myocardial infarction. Antibody-dependent cell cytotoxicity (ADCC) is determined partially by the specific N-glycosylation of the Fc domain of the monoclonal antibody. Specific glycoforms of monoclonal antibodies, which interact solely with the FcgammaRIIIa receptor of natural killer cells, result in superior ADCC compared with heterogeneous glycoforms that interact with different Fc receptors. This demonstrates that glycoengineering for directed glycosylation of therapeutic proteins can improve the therapeutic effect. While the amino acid sequence of the therapeutic protein is determined by the nucleotide sequence of the inserted gene, glycosylation depends on the glycosylating enzymes in the endoplasmatic reticulum and the Golgi apparatus of the eukaryotic host cell. In addition, the glycosylation of the therapeutic protein is affected by the culture medium used, the efficiency of protein expression and the physiological status of the host cell.

CONCLUSION

For a given protein, changes in the type of host cell, composition of the culture media and fermentation conditions during process development will most likely result in changes in the site occupation and heterogeneity of glycosylation. This, of course, can influence the therapeutic profile. Therefore, the early selection of the host cell and selection of upstream parameters are key in the process development of a product.

Dermokine: an extensively differentially spliced gene expressed in epithelial cells

Studies performed to discover genes overexpressed in inflammatory diseases identified dermokine as being upregulated in such disease conditions. Dermokine is a gene that was first observed as expressed in the differentiated layers of skin. Its two major isoforms, alpha and beta, are transcribed from different promoters of the same locus, with the alpha isoform representing the C terminus of the beta isoform. Recently, additional transcript variants have been identified. Extensive in silico analysis and reverse transcriptase (RT)-PCR cloning has confirmed the existence of these variants in human cells and tissues, identified a new human isoform as well as the gamma isoform in mouse. Recombinant expression and analysis of the C-terminal truncated isoform indicate that the molecule is O-linked glycosylated and forms multimers in solution. In situ hybridization and immunohistochemistry has shown that the gene is differentially expressed in various cells and tissues, other than the skin. These results show that the dermokine gene is expressed in epithelial tissues other than the skin and this expression is transcriptionally and posttranscriptionally complex.

Isolation of Bovine Immunoglobulins Resistant to Peptic Digestion: New Perspectives in the Prevention of Failure in Passive Immunization of Neonatal Calves

In calves, neonatal mortality and disease susceptibility are greatly influenced by failure in passive immunization, normally provided by colostrum ingestion just after birth. Formulations projected to replace natural colostrum have not been successful, and one of the possible reasons for such failure is that orally administered Ig are probably digested in the gastrointestinal tract, so they are not absorbed as intact functional molecules. With the aim of finding an adequate colostrum substitute, we used columns of immobilized jacalin, a lectin known by its ability to bind O-linked oligosaccharides, to obtain a colostral Ig population putatively protected against enzymatic cleavage by the presence of sugar chains. Immunoglobulin G1 is a major constituent of colostrum Ig bound to jacalin (JB-Ig). This preparation contains 10% of the total colostral Ig and is typically 3 to 6 times more resistant to pepsin digestion than the Ig contained in the fraction that is not bound to jacalin, which presumably does not contain O-glycans. Mass spectrometry analysis demonstrated that the tryptic peptides obtained from JB-Ig and unbound Ig were similar, indicating that their distinct susceptibility to enzyme hydrolysis was associated with differences in their sugar chains. Therefore, the present research suggests that the bovine colostrum JB-Ig has potential application in the immunotherapy of neonatal calves that have not been supplied with colostrum.

Sodium butyrate alters erythropoietin glycosylation via multiple mechanisms

Recombinant human erythropoietin (rHuEPO) produced in a human kidney fibrosarcoma cell line, HT1080, was used as a model to study the effects of sodium butyrate (SB) on protein glycosylation. Treatment with 2 mM SB resulted in complex changes with respect to sugar nucleotide pools including an increase in UDP-Gal and a decrease in UDP-GlcNac. In addition, polylactosamine structures present on rHuEPO increased after SB treatment. To determine if these phenotypic changes correlated with changes in mRNA abundance, we profiled mRNA levels over a 24-h period in the presence or absence of SB using oligonucleotide microarrays. By filtering our data through a functional glycomics gene list associated with the processes of glycan degradation, glycan synthesis, and sugar nucleotide synthesis and transport we identified 26 genes with significantly altered mRNA levels. We were able to correlate the changes in message in six of these genes with measurable phenotypic changes within our system including: neu1, b3gnt6, siat4b, b3gnt1, slc17a5, and galt. Interestingly, for the two genes: cmas and gale, our measurable phenotypic changes did not correlate with changes in mRNA expression. These data demonstrate both the utility and pit falls of coupling biochemical analysis with high throughput oligonucleotide microarrays to predict how changes in cell culture environments will impact glycoprotein oligosaccharide content.

Expression of the O-linked N-acetylglucosamine containing epitope H in normal myometrium and uterine smooth muscle cell tumors

Epitope H contains an O-linked N-acetylglucosamine (O-GlcNAc) residue in a specific conformation and/or environment recognized by monoclonal antibody H (mAbH). We have previously shown that epitope H is present in more than one polypeptide and in various types of normal and pathological cells. In the present study, we focused on uterine smooth muscle cell tumors and their adjacent normal myometrium to gain further insight into the expression patterns of epitope H in human tissues. The indirect immunoperoxidase method was applied using the mAbH and the monoclonal anti-cytokeratin 8 antibody (AbCK8) in 50 cases of typical uterine leiomyomas and in five cases of uterine leiomyosarcomas, with four cases belonging to Group II A and one to Group III according to Bell et al. [6]. Western immunoblotting was applied using mAbH and AbCK8 in five cases of uterine leiomyomas and their adjacent myometrium. The main results were as follows: (1) epitope H showed intense immunohistochemical expression in 46% (23/50) and moderate expression in 54% (27/50) of uterine leiomyomas, (2) epitope H showed intense immunohistochemical expression in 40% (2/5) and moderate expression in 60% (3/5) of uterine leiomyosarcomas, (3) epitope H showed no difference in the immunohistochemical expression between leiomyomas and their adjacent myometrium and between leiomyosarcomas and their adjacent myometrium, (4) immunohistochemical expression of cytokeratin 8 was not detected in the normal and neoplastic smooth muscle cells, (5) Western immunoblotting showed that in the smooth muscle cells of the myometrium and leiomyomas, epitope H is localized in four polypeptides with molecular weights of 100, 61, 59, and 54 kDa, and (6) Western immunoblotting did not detect cytokeratin 8 in the normal and neoplastic smooth muscle cells. The present results indicate fluctuations of the epitope expression levels in uterine smooth muscle cell tumors and their adjacent myometrium. These fluctuations may be of interest for gaining insight into the pathogenesis of uterine smooth muscle cell tumors, since O-GlcNAc glycosylation is involved in cell cycle and apoptosis pathways and may modify proteins involved in oncogenesis (tumor suppressor proteins and oncoproteins) and proteins with important biological functions such as cytoskeletal proteins, transcription factors, and heat-shock proteins. Furthermore, the present results indicate that cytokeratin 8, without being present in the cells of the myometrium, leiomyomas and leiomyosarcomas, shares its epitope H, which contains its unique sugar O-N-acetylglucosamine residue, with four other unrelated polypeptides produced by the normal and neoplastic smooth muscle cells. This should be considered when using anti-cytokeratin 8 antibodies in immunohistochemistry against smooth muscle cell tumors to avoid false positive immunohistochemical results.

The location and characterisation of the O-linked glycans of the human insulin receptor

O-linked glycosylation is a post-translational and post-folding event involving exposed S/T residues at beta-turns or in regions with extended conformation. O-linked sites are difficult to predict from sequence analyses compared to N-linked sites. Here we compare the results of chemical analyses of isolated glycopeptides with the prediction using the neural network prediction method NetOGlyc3.1, a procedure that has been reported to correctly predict 76% of O-glycosylated residues in proteins. Using the heavily glycosylated human insulin receptor as the test protein six sites of mucin-type O-glycosylation were found at residues T744, T749, S757, S758, T759, and T763 compared to the three sites (T759 and T763- correctly, T756- incorrectly) predicted by the neural network method. These six sites occur in a 20 residue segment that begins nine residues downstream from the start of the insulin receptor beta-chain. This region which also includes N-linked glycosylation sites at N742 and N755, is predicted to lack secondary structure and is followed by residues 765-770, the known linear epitope for the monoclonal antibody 18-44.

High-sensitivity profiling of glycoproteins from human blood serum through multiple-lectin affinity chromatography and liquid chromatography/tandem mass spectrometry

We report here the use of high-performance lectin affinity enrichment of glycoproteins at microscale levels using a series of silica-bound lectins. The potential of this approach is being demonstrated for the glycoprotein enrichment from microliter volumes of human blood serum. Individual injections of sample to the affinity microcolumns packed with four lectin materials with different glycan specificities (Con A, SNA-I, UEA-I, PHA-L), followed by off-line reversed-phase pre-fractionation and nano-LC/MS/MS, permitted identification of 108 proteins in the lectin-bound fractions spanning a concentration dynamic range of 7-10 orders of magnitude. In contrast, multi-lectin microcolumn affinity chromatography, an alternative enrichment approach allowed identification of only 67 proteins. An attractive feature of high-performance lectin affinity chromatography at microscale levels is the substantial reduction of sample losses that are commonly experienced with extensive sample preparation needed for larger sample volumes.

Interference with O-glycosylation in RMA lymphoma cells leads to a reduced in vivo growth of the tumor

Carbohydrate processing in cancer cells can influence the growth, metastatic potential, vascularization and immune recognition of such cells. Interference with N-glycosylation has been shown both to reduce the membrane expression of MHC class I and to increase the in vitro sensitivity of tumor cells to NK cell killing. We investigated the effect of O-glycosylation inhibition on the in vivo growth, phenotype and NK sensitivity of RMA lymphoma cells using benzyl N-acetyl-alpha-D-galactosamide (BAG). BAG-treated cells were found to have a strongly reduced local growth potential in vivo. However, inhibition of O-glycosylation caused this effect without any significant downregulation of MHC-I and increase in sensitivity to NK killing as seen after inhibition of N-glycosylation using Castanospermine. BAG treatment of RMA cells resulted in the removal of larger O-linked glycans and a high expression of the T-antigen (GalGalNAc), a target for natural antibodies (NAs) induced by the gastrointestinal bacterial flora. Whether the loss of larger O-linked glycans, and associated functions, or of biological effects of NA contributed to the antitumor effect remains to be established. The results support the idea that inhibitors of O- as well as N-linked glycosylation may be useful for the treatment of cancer, given that they can be specifically targeted to the tumor tissue.

Structures of human N-Acetylglucosamine kinase in two complexes with N-Acetylglucosamine and with ADP/glucose: insights into substrate specificity and regulation

N-Acetylglucosamine (GlcNAc), a major component of complex carbohydrates, is synthesized de novo or salvaged from lysosomally degraded glycoconjugates and from nutritional sources. The salvage pathway requires that GlcNAc kinase converts GlcNAc to GlcNAc-6-phosphate, a component utilized in UDP-GlcNAc biosynthesis or energy metabolism. GlcNAc kinase belongs to the sugar kinase/Hsp70/actin superfamily that catalyze phosphoryl transfer from ATP to their respective substrates, and in most cases catalysis is associated with a large conformational change in which the N-terminal small and C-terminal large domains enclose the substrates. Here we report two crystal structures of homodimeric human GlcNAc kinase, one in complex with GlcNAc and the other in complex with ADP and glucose. The active site of GlcNAc kinase is located in a deep cleft between the two domains of the V-shaped monomer. The enzyme adopts a "closed" configuration in the GlcNAc-bound complex and GlcNAc interacts with residues of both domains. In addition, the N-acetyl methyl group contacts residues of the other monomer in the homodimer, a unique feature compared to other members of the sugar kinase/Hsp70/actin superfamily. This contrasts an "open" configuration in the ADP/glucose-bound structure, where glucose cannot form these interactions, explaining its low binding affinity for GlcNAc kinase. Our results support functional implications derived from apo crystal structures of GlcNAc kinases from Chromobacter violaceum and Porphyromonas gingivalis and show that Tyr205, which is phosphorylated in thrombin-activated platelets, lines the GlcNAc binding pocket. This suggests that phosphorylation of Tyr205 may modulate GlcNAc kinase activity and/or specificity.

The precursor to B-type natriuretic peptide is an O-linked glycoprotein

Human pro-B-type natriuretic peptide (proBNP), the precursor for B-type natriuretic peptide (BNP), was expressed in Chinese hamster ovary cells (CHO) and compared by Western blot analysis to BNP cross-reacting material immunoprecipitated from the plasma of heart failure patients. Both recombinant and native forms co-migrated as a diffuse band centered around 25 kDa and were reduced to a 12 kDa species by treatment with a mixture of O-link deglycosylation enzymes. The 108-amino acid CHO-expressed protein was examined by tryptic mapping and LC-MS and found to be an O-linked glycoprotein. Determination of the sites of O-glycosyl addition by blank cycle sequencing of tryptic and Glu-C (Staphylococcus aureus V8 protease) peptides showed that there are seven sites of glycosylation confined to a 36-amino acid residue stretch within the center of the propeptide region. This data is consistent with previous observations of higher molecular weight isoforms of BNP.

Molecular mass of macromolecules and subunits and the quaternary structure of hemoglobin from the microcrustacean Daphnia magna

The molecular masses of macromolecules and subunits of the extracellular hemoglobin from the fresh-water crustacean Daphnia magna were determined by analytical ultracentrifugation, multiangle laser light scattering and electrospray ionization mass spectrometry. The hemoglobins from hypoxia-incubated, hemoglobin-rich and normoxia-incubated, hemoglobin-poor Daphnia magna were analyzed separately. The sedimentation coefficient of the macromolecule was 17.4 +/- 0.1 S, and its molecular mass was 583 kDa (hemoglobin-rich animals) determined by AUC and 590.4 +/- 11.1 kDa (hemoglobin-rich animals) and 597.5 +/- 49 kDa (hemoglobin-poor animals), respectively, determined by multiangle laser light scattering. Measurements of the hemoglobin subunit mass of hemoglobin-rich animals by electrospray ionization mass spectrometry revealed a significant peak at 36.482 +/- 0.0015 kDa, i.e. 37.715 kDa including two heme groups. The hemoglobin subunits are modified by O-linked glycosylation in the pre-A segments of domains 1. No evidence for phosphorylation of hemoglobin subunits was found. The subunit migration behavior during SDS/PAGE was shown to be influenced by the buffer system used (Tris versus phosphate). The subunit mass heterogeneity found using Tris buffering can be explained by glycosylation of hemoglobin subunits. Based on molecular mass information, Daphnia magna hemoglobin is demonstrated to consist of 16 subunits. The quaternary structure of the Daphnia magna hemoglobin macromolecule was assessed by three-dimensional reconstructions via single-particle analysis based on negatively stained electron microscopic specimens. It turned out to be much more complex than hitherto proposed: it displays D4 symmetry with a diameter of approximately 12 nm and a height of about 8 nm.

Identification of 9-O-acetyl-N-acetylneuraminic acid in normal canine pre-ocular tear film secreted mucins and its depletion in Keratoconjunctivitis sicca

O-Acetylated sialic acids have been reported in many sialoglycoproteins where they mediate a variety of immune and other biological events. We have previously demonstrated that the protective mucus barrier on the surface of the canine eye contains sialoglycoproteins. We have also investigated the occurrence of O-acetylated sialic acids in these ocular mucins. Mucus aspirated from the surface of normal dog eyes and those with keratoconjunctivitis sicca (KCS) was fractionated into three pools by density gradient centrifugation. Sialic acids comprised 0.6-0.9% of the dry weight of the mucins isolated. The sialic acid profile in these pools was examined using HPLC. O-Acetylated sialic acids, mainly Neu5,9Ac2, were detected in normal animals and made up 10-30% of the total sialic acids detected. A doubling of the sialic acid content was found in KCS mucins, but the level of 9-O-acetylated sialic acid was reduced below 4% of total. Histological analysis of conjunctival tissue from normal and KCS dogs showed the presence of sialic acids, detected with the alpha(2-6) sialic acid-specific lectin Sambucus nigra, in the goblet cells and corresponding to the staining pattern for MUC5AC, the major ocular-secreted mucin gene product. In KCS animals a disruption of the normal pattern of conjunctival goblet cells was seen with preservation of the pattern of lectin binding observed in normal animals. Thus the data demonstrate the presence of mono-O-Acetylated sialic acids in normal canine ocular mucins and a loss of this population of sialic acids in dry eye disease in spite of a significant increase in total sialic acids in KCS mucin.

Optimisation of the cellular metabolism of glycosylation for recombinant proteins produced by Mammalian cell systems

Many biopharmaceuticals are now produced as secreted glycoproteins from mammalian cell culture. The glycosylation profile of these proteins is essential to ensure structural stability and biological and clinical activity. However, the ability to control the glycosylation is limited by our understanding of the parameters that affect the heterogeneity of added glycan structures. It is clear that the glycosylation process is affected by a number of factors including the 3-dimensional structure of the protein, the enzyme repertoire of the host cell, the transit time in the Golgi and the availability of intracellular sugar-nucleotide donors. From a process development perspective there are many culture parameters that can be controlled to enable a consistent glycosylation profile to emerge from each batch culture. A further, but more difficult goal is to control the culture conditions to enable the enrichment of specific glycoforms identified with desirable biological activities. The purpose of this paper is to discuss the cellular metabolism associated with protein glycosylation and review the attempts to manipulate, control or engineer this metabolism to allow the expression of human glycosylation profiles in producer lines such as genetically engineered Chinese hamster ovary (CHO) cells.

Structure of the Epstein-Barr Virus Oncogene BARF1

The Epstein-Barr virus is a human gamma-herpesvirus that persistently infects more than 90% of the human population. It is associated with numerous epithelial cancers, principally undifferentiated nasopharyngeal carcinoma and gastric carcinoma. The BARF1 gene is expressed in a high proportion of these cancers. An oncogenic, mitogenic and immortalizing activity of the BARF1 protein has been shown. We solved the structure of the secreted BARF1 glycoprotein expressed in a human cell line by X-ray crystallography at a resolution of 2.3A. The BARF1 protein consists of two immunoglobulin (Ig)-like domains. The N-terminal domain belongs to the subfamily of variable domains whereas the C-terminal one is related to a constant Ig-domain. BARF1 shows an unusual hexamerisation involving two principal contacts, one between the C-terminal domains and one between the N-terminal domains. The C-terminal contact with an uncommonly large contact surface extends the beta-sandwich of the Ig-domain through the second molecule. The N-terminal contact involves Ig-domains with an unusual relative orientation but with a more classical contact surface with a size in the range of dimer interactions of Ig-domains. The structure of BARF1 is most closely related to CD80 or B7-1, a co-stimulatory molecule present on antigen presenting cells, from which BARF1 must have been derived during evolution. Still, domain orientation and oligomerization differ between BARF1 and CD80. It had been shown that BARF1 binds to hCSF-1, the human colony-stimulating factor 1, but this interaction has to be principally different from the one between CSF-1 and CSF-1 receptor.

The hemopexin and O-glycosylated domains tune gelatinase B/MMP-9 bioavailability via inhibition and binding to cargo receptors

Gelatinase B/matrix metalloproteinase-9 (MMP-9), a key regulator and effector of immunity, contains a C-terminal hemopexin domain preceded by a unique linker sequence of approximately 64 amino acid residues. This linker sequence is demonstrated to be an extensively O-glycosylated (OG) domain with a compact three-dimensional structure. The OG and hemopexin domains have no influence on the cleavage efficiency of MMP-9 substrates. In contrast, the hemopexin domain contains a binding site for the cargo receptor low density lipoprotein receptor-related protein-1 (LRP-1). Furthermore, megalin/LRP-2 is identified as a new functional receptor for the hemopexin domain of MMP-9, able to mediate the endocytosis and catabolism of the enzyme. The OG domain is required to correctly orient the hemopexin domain for inhibition by TIMP-1 and internalization by LRP-1 and megalin. Therefore, the OG and hemopexin domains down-regulate the bioavailability of active MMP-9 and the interactions with the cargo receptors are proposed to be the original function of hemopexin domains in MMPs.

Glycosylation of the severe acute respiratory syndrome coronavirus triple-spanning membrane proteins 3a and M

The severe acute respiratory syndrome coronavirus (SARS-CoV) open reading frame 3a protein has recently been shown to be a structural protein. The protein is encoded by one of the so-called group-specific genes and has no sequence homology with any of the known structural or group-specific proteins of coronaviruses. It does, however, have several similarities to the coronavirus M proteins; (i) they are triple membrane spanning with the same topology, (ii) they have similar intracellular localizations (predominantly Golgi), (iii) both are viral structural proteins, and (iv) they appear to interact with the E and S proteins, as well as with each other. The M protein plays a crucial role in coronavirus assembly and is glycosylated in all coronaviruses, either by N-linked or by O-linked oligosaccharides. The conserved glycosylation of the coronavirus M proteins and the resemblance of the 3a protein to them led us to investigate the glycosylation of these two SARS-CoV membrane proteins. The proteins were expressed separately using the vaccinia virus T7 expression system, followed by metabolic labeling. Pulse-chase analysis showed that both proteins were modified, although in different ways. While the M protein acquired cotranslationally oligosaccharides that could be removed by PNGaseF, the 3a protein acquired its modifications posttranslationally, and they were not sensitive to the N-glycosidase enzyme. The SARS-CoV 3a protein, however, was demonstrated to contain sialic acids, indicating the presence of oligosaccharides. O-glycosylation of the 3a protein was indeed confirmed using an in situ O-glycosylation assay of endoplasmic reticulum-retained mutants. In addition, we showed that substitution of serine and threonine residues in the ectodomain of the 3a protein abolished the addition of the O-linked sugars. Thus, the SARS-CoV 3a protein is an O-glycosylated glycoprotein, like the group 2 coronavirus M proteins but unlike the SARS-CoV M protein, which is N glycosylated.

Carbohydrate specificity of an insecticidal lectin isolated from the leaves of Glechoma hederacea (ground ivy) towards mammalian glycoconjugates

Preliminary studies indicated that the potent insecticidal lectin, Gleheda, from the leaves of Glechoma hederacea (ground ivy) preferentially agglutinates human erythrocytes carrying the Tn (GalNAcalpha1-Ser/Thr) antigen. However, no details have been reported yet with respect to the fine specificity of the lectin. To corroborate the molecular basis of the insecticidal activity and physiological function of Gleheda, it is necessary to identify the recognition factors that are involved in the Gleheda-glycotope interaction. In the present study, the requirement of high-density multivalent carbohydrate structural units for Gleheda binding and a fine-affinity profile were evaluated using ELLSA (enzyme-linked lectinosorbent assay) with our extended glycan/ligand collections, a glycan array and molecular modelling. From the results, we concluded that a high-density of exposed multivalent Tn-containing glycoproteins (natural armadillo and asialo ovine salivary glycoproteins) were the most potent factors for Gleheda binding. They were, on a nanogram basis, 6.5x10(5), 1.5x10(4) and 3.1x10(3) times more active than univalent Gal (galactose), GalNAc (N-acetylgalactosamine) and Tn respectively. Among mono- and oligo-saccharides examined, simple clustered Tn (molecular mass <3000 Da) from ovine salivary glycoprotein was the best, being 37.5 and 1.7x10(3) times better than GalNAc and Gal respectively. GalNAc glycosides were significantly more active than Gal glycosides, indicating that the N-acetamido group at C-2 plays an important role in Gleheda binding. The results of glycan array support the conclusions drawn with respect to the specificity of Gleheda based on the ELLSA assays. These findings combined with the results of the molecular modelling and docking indicate the occurrence of a primary GalNAcalpha1-binding site in the Gleheda monomer. However, the extraordinary binding feature of Gleheda for glycoproteins demonstrates the importance of affinity enhancement by high-density multivalent glycotopes in the ligand-lectin interactions in biological processes.

Affinity monolithic capillary columns for glycomics/proteomics: 1. Polymethacrylate monoliths with immobilized lectins for glycoprotein separation by affinity capillary electrochromatography and affinity nano-liquid chromatography in either a single column or columns coupled in series

In this report, microcolumn separation schemes involving monolithic capillary columns with immobilized lectins, and relevant to nanoglycomics/nanoproteomics were introduced. Positive and neutral monoliths based on poly(glycidyl methacrylate-co-ethylene dimethacrylate) were designed for achieving lectin affinity chromatography (LAC) by nano-LC and CEC. The positive monoliths (i.e., monoliths with cationic sites) afforded relatively high permeability in nano-LC but lack predictable EOF magnitude and direction, while neutral monoliths provided a good compromise between reasonable permeability in nano-LC and predictable EOF in CEC. Lectin affinity nano-LC permitted the enrichment of classes of different glycoproteins having similar N-glycans recognized by the immobilized lectin, whereas lectin affinity CEC provided the simultaneous capturing and separation of different glycoproteins due to differences in charge-to-mass ratio. Also, this investigation demonstrated for the first time the coupling of lectin capillary columns in series (i.e., tandem columns) for enhanced separation of glycoproteins by LAC using the CEC modality. Furthermore, in the coupled columns format, glycoforms of a given glycoprotein were readily separated.

Mechanisms in protein O-glycan biosynthesis and clinical and molecular aspects of protein O-glycan biosynthesis defects: a review

BACKGROUND

Genetic diseases that affect the biosynthesis of protein O-glycans are a rapidly growing group of disorders. Because this group of disorders does not have a collective name, it is difficult to get an overview of O-glycosylation in relation to human health and disease. Many patients with an unsolved defect in N-glycosylation are found to have an abnormal O-glycosylation as well. It is becoming increasingly evident that the primary defect of these disorders is not necessarily localized in one of the glycan-specific transferases, but can likewise be found in the biosynthesis of nucleotide sugars, their transport to the endoplasmic reticulum (ER)/Golgi, and in Golgi trafficking. Already, disorders in O-glycan biosynthesis form a substantial group of genetic diseases. In view of the number of genes involved in O-glycosylation processes and the increasing scientific interest in congenital disorders of glycosylation, it is expected that the number of identified diseases in this group will grow rapidly over the coming years.

CONTENT

We first discuss the biosynthesis of protein O-glycans from their building blocks to their secretion from the Golgi. Subsequently, we review 24 different genetic disorders in O-glycosylation and 10 different genetic disorders that affect both N- and O-glycosylation. The key clinical, metabolic, chemical, diagnostic, and genetic features are described. Additionally, we describe methods that can be used in clinical laboratory screening for protein O-glycosylation biosynthesis defects and their pitfalls. Finally, we introduce existing methods that might be useful for unraveling O-glycosylation defects in the future.

N-linked oligosaccharides as outfitters for glycoprotein folding, form and function

Glycosylation, particularly N-linked glycosylation, profoundly affects protein folding, oligomerization and stability. The increased efficiency of folding of glycosylated proteins could be due to the chaperone-like activity of glycans, which is observed even when the glycan is not attached to the protein. Covalently linked glycans could also facilitate oligomerization by mediating inter-subunit interactions in the protein or stabilizing the oligomer in other ways. Glycosylation also affects the rate of fibril formation in prion proteins: N-glycans reduce the rate of fibril formation, and O-glycans affect the rate either way depending on factors such as position and orientation. It has yet to be determined whether there is any correlation among the sites of glycosylation and the ensuing effect in multiply glycosylated proteins. It is also not apparent whether there is a common pattern in the conservation of glycans in a related family of glycoproteins, but it is evident that glycosylation is a multifaceted post-translational modification. Indeed, glycosylation serves to "outfit" proteins for fold-function balance.

2-Deoxyglucose: An anticancer and antiviral therapeutic, but not any more a low glucose mimetic

2-Deoxyglucose (2-DG), a non-metabolizable glucose analogue, blocks glycolysis and inhibits protein glycosylation. It has been tested in multiple studies for possible application as an anticancer or antiviral therapeutic. The inhibitory effect of 2-DG on ATP generation made it a good candidate molecule as a calorie restriction mimetic as well. Furthermore, 2-DG has been utilized in numerous studies to simulate a condition of glucose starvation. Because 2-DG disrupts glucose metabolism, protein glycosylation, and ER quality control at the same time, a cellular or pathologic outcome could be easily misinterpreted without clear understanding of 2-DG's effect on each of these aspects. However, the effect of 2-DG on protein glycosylation has rarely been investigated. A recent study suggested that 2-DG causes hyperGlcNAcylation of proteins, while low glucose supply causes hypoGlcNAcylation. In certain aspects of cellular physiology, this difference could be disregarded, but in others, this may possibly cause totally different outcomes.

Glycoengineering: the effect of glycosylation on the properties of therapeutic proteins

Therapeutic proteins have revolutionized the treatment of many diseases but low activity or rapid clearance limits their utility. New approaches have been taken to design drugs with enhanced in vivo activity and half-life to reduce injection frequency, increase convenience, and improve patient compliance. One recently used approach is glycoengineering, changing protein-associated carbohydrate to alter pharmacokinetic properties of proteins. This technology has been applied to erythropoietin and resulted in the discovery of darbepoetin alfa (DA), a hyperglycosylated analogue of erythropoietin that contains two additional N-linked carbohydrates, a threefold increase in serum half-life and increased in vivo activity compared to recombinant human erythropoietin (rHuEPO). The increased serum half-life allows for less frequent dosing to maintain target hemoglobin levels in anemic patients. Carbohydrates on DA and other molecules can also increase molecular stability, solubility, increase in vivo biological activity, and reduce immunogenicity. These properties are discussed.