Specificity of O-glycosylation by bovine colostrum UDP-GalNAc: polypeptide alpha-N-acetylgalactosaminyltransferase using synthetic glycopeptide substrates

Quantitative assessment of successive carbohydrate additions to the clustered O-glycosylation sites of IgA1 by glycosyltransferases

Mucin-type O-glycosylation occurs on many proteins that transit the Golgi apparatus. These glycans impact structure and function of many proteins and have important roles in cellular biosynthetic processes, signaling and differentiation. Although recent technological advances have enhanced our ability to profile glycosylation of glycoproteins, limitations in the understanding of the biosynthesis of these glycan structures remain. Some of these limitations stem from the difficulty to track the biosynthetic process of mucin-type O-glycosylation, especially when glycans occur in dense clusters in repeat regions of proteins, such as the mucins or immunoglobulin A1 (IgA1). Here, we describe a series of nano-liquid chromatography (LC)-mass spectrometry (MS) analyses that demonstrate the range of glycosyltransferase enzymatic activities involved in the biosynthesis of clustered O-glycans on IgA1. By utilizing nano-LC-MS relative quantitation of in vitro reaction products, our results provide unique insights into the biosynthesis of clustered IgA1 O-glycans. We have developed a workflow to determine glycoform-specific apparent rates of a human UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltrasnfersase (GalNAc-T EC 2.4.1.41) and demonstrated how pre-existing glycans affect subsequent activity of glycosyltransferases, such as core 1 galactosyltransferase and α2,3- and α2,6-specific sialyltransferases, in successive additions in the biosynthesis of clustered O-glycans. In the context of IgA1, these results have potential to provide insight into the molecular mechanisms implicated in the pathogenesis of IgA nephropathy, an autoimmune renal disease involving aberrant IgA1 O-glycosylation. In a broader sense, these methods and workflows are applicable to the studies of the concerted and competing functions of other glycosyltransferases that initiate and extend mucin-type core 1 clustered O-glycosylation.

Site-specific analysis of von Willebrand factor O-glycosylation

BACKGROUND

O-glycosylation of von Willebrand factor (VWF) affects many of its functions; however, there is currently no information on the occupancy of the 10 putative O-glycosylation sites.

OBJECTIVES

The aim of this study was the site-specific analysis of VWF O-glycosylation.

METHODS

Tryptic VWF-O-glycopeptides were isolated by lectin affinity chromatography and/or by reverse-phase high-performance liquid chromatography. Subsequently, the purified glycopeptides were analyzed by glycosidase digestion and mass spectrometry.

RESULTS

We found that all 10 predicted O-glycosylation sites in VWF are occupied. The majority of the glycan structures on all glycosylation sites is represented by disialyl core 1 O-glycan. The presence of core 2 O-glycan was also confirmed; interestingly, this structure was not evenly distributed among all 10 glycosylation sites. Analysis of the glycopeptides flanking the A1 domain revealed that generally more core-2-type O-glycan was present on the C-terminal Cluster 2 glycopeptide (encompassing T(1468) , T(1477) , S(1486) and T(1487) ) compared with the N-terminal Cluster 1 glycopeptide (encompassing T(1248) , T(1255) , T(1256) and S(1263) ). Disialosyl motifs were present on both glycopeptides flanking the A1 domain and on the glycosylation site T(2298) in the C1 domain. In addition, we identify sulfation of core 2 O-glycans and the presence of the rare Tn antigen.

CONCLUSIONS

This is the first study to describe the qualitative and semi-quantitative distribution of O-glycan structures on all 10 O-glycosylation sites, which will provide a valuable starting point for further studies exploring the functional and structural implications of O-glycosylation in VWF.

A high-throughput O-glycopeptide discovery platform for seromic profiling

Biomarker microarrays are becoming valuable tools for serological screening of disease-associated autoantibodies. Post-translational modifications (PTMs) such as glycosylation extend the range of protein function, and a variety of glycosylated proteins are known to be altered in disease progression. Here, we have developed a synthetic screening microarray platform for facile display of O-glycosylated peptides (O-PTMs). By introduction of a capping step during chemical solid-phase glycopeptide synthesis, selective enrichment of N-terminal glycopeptide end products was achieved on an amine-reactive hydrogel-coated microarray glass surface, allowing high-throughput display of large numbers of glycopeptides. Utilizing a repertoire of recombinant glycosyltransferases enabled further diversification of the array libraries in situ and display of a new level of potential biomarker candidates for serological screening. As proof-of-concept, we have demonstrated that MUC1 glycopeptides could be assembled and used to detect autoantibodies in vaccine-induced disease-free breast cancer patients and in patients with confirmed disease at time of diagnosis.

Site directed processing: role of amino acid sequences and glycosylation of acceptor glycopeptides in the assembly of extended mucin type O-glycan core 2

BACKGROUND

The assembly of Ser/Thr-linked O-glycans of mucins with core 2 structures is initiated by polypeptide GalNAc-transferase (ppGalNAc-T), followed by the action of core 1 beta3-Gal-transferase (C1GalT) and core 2 beta6-GlcNAc-transferase (C2GnT). Beta4-Gal-transferase (beta4GalT) extends core 2 and forms the backbone structure for biologically important epitopes. O-glycan structures are often abnormal in chronic diseases. The goal of this work is to determine if the activity and specificity of these enzymes are directed by the sequences and glycosylation of substrates.

METHODS

We studied the specificities of four enzymes that synthesize extended O-glycan core 2 using as acceptor substrates synthetic mucin derived peptides and glycopeptides, substituted with GalNAc or O-glycan core structures 1, 2, 3, 4 and 6.

RESULTS

Specific Thr residues were found to be preferred sites for the addition of GalNAc, and Pro in the +3 position was found to especially enhance primary glycosylation. An inverse relationship was found between the size of adjacent glycans and the rate of GalNAc addition. All four enzymes could distinguish between substrates having different amino acid sequences and O-glycosylated sites. A short glycopeptide Galbeta1-3GalNAcalpha-TAGV was identified as an efficient C2GnT substrate.

CONCLUSIONS

The activities of four enzymes assembling the extended core 2 structure are affected by the amino acid sequence and presence of carbohydrates on nearby residues in acceptor glycopeptides. In particular, the sequences and O-glycosylation patterns direct the addition of the first and second sugar residues by ppGalNAc-T and C1GalT which act in a site directed fashion.

GENERAL SIGNIFICANCE

Knowledge of site directed processing enhances our understanding of the control of O-glycosylation in normal cells and in disease.

MUC1 and the MUCs: A Family of Human Mucins with Impact in Cancer Biology

Mucins represent a family of glycoproteins characterized by repeat domains and a dense O-glycosylation. During the last two decades, the gene and peptide structures of various mucins as well as their glycosylation states were partly elucidated. Characteristic tumor-associated alterations of the expression patterns and glycosylation profiles were observed in biochemical, immunochemical, and histological studies and are discussed in the light of efforts to use the most prominent member in this family, MUC1, as a tumor target in anti-tumor strategies. Within this context the present review, focusing on MUC1, describes recent work on the regulation of mucin biosynthesis by cytokines and hormones, the role of mucins in cell adhesion, and their interaction with the immune system. Important aspects of clinical diagnostics based on mucin antigens are discussed, including the application of tumor serum assays and the significance of numerous studies revealing correlations between the expression of peptide cores or mucin-associated carbohydrates and clinicopathological parameters like tumor progression and prognosis.

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.

UDP-Gal: GlcNAc-R beta1,4-galactosyltransferase--a target enzyme for drug design. Acceptor specificity and inhibition of the enzyme

Galactosyltransferases are important enzymes for the extension of the glycan chains of glycoproteins and glycolipids, and play critical roles in cell surface functions and in the immune system. In this work, the acceptor specificity and several inhibitors of bovine beta1,4-Gal-transferase T1 (beta4GalT, EC 2.4.1.90) were studied. Series of analogs of N-acetylglucosamine (GlcNAc) and GlcNAc-carrying glycopeptides were synthesized as acceptor substrates. Modifications were made at the 3-, 4- and 6-positions of the sugar ring of the acceptor, in the nature of the glycosidic linkage, in the aglycone moiety and in the 2-acetamido group. The acceptor specificity studies showed that the 4-hydroxyl group of the sugar ring was essential for beta4GalT activity, but that the 3-hydroxyl could be replaced by an electronegative group. Compounds having the anomeric beta-configuration were more active than those having the alpha-configuration, and O-, S- and C-glycosyl compounds were all active as substrates. The aglycone was a major determinant for the rate of Gal-transfer. Derivatives containing a 2-naphthyl aglycone were inactive as substrates although quinolinyl groups supported activity. Several compounds having a bicyclic structure as the aglycone were found to bind to the enzyme and inhibited the transfer of Gal to control substrates. The best small hydrophobic GlcNAc-analog inhibitor was found to be 1-thio-N-butyrylGlcNbeta-(2-naphthyl) with a K(i) of 0.01 mM. These studies help to delineate beta4GalT-substrate interactions and will aid in the development of biologically applicable inhibitors of the enzyme.

Differential O-glycosylation in cortical and medullary thymocytes

Differentiation of T lymphocytes is characterized by variable expression of CD8/CD4 co-receptor molecules and changes in the glycosylation pattern. In this work, O-glycosylation was analyzed in microsomes from murine thymocytes purified with the PNA and Amaranthus leucocarpus (ALL) lectins, specific for the T antigen (Gal beta1,3GalNAc1,0 Ser/Thr) in cortical and medullary thymocytes, respectively. Three peptides were used as acceptors for UDP-N-acetylgalactosamine: polypeptide N-acetylgalactosaminyl-transferase (GalNAc transferase); the peptide motif TTSAPTTS was the best glycosylated one. Cortical ALL-PNA+ thymocytes showed two-fold higher GalNAc transferase activity than ALL+PNA- thymocytes; however, capillary electrophoresis showed a higher proportion of di- versus mono-glycosylated peptides for ALL+PNA- than for ALL-PNA+. We compared the GalNAc transferase activity of thymocytes from dexamethasone-treated mice versus control mice. GalNAc transferase activity was six-fold higher in thymocytes from control mice than from dexamethasone-treated mice; the rate of di-glycosylated peptides for dexamethosone-resistant ALL+ was two-fold higher than for ALL- thymocytes. Our results confirm an upregulated biosynthesis of O-glycosidically linked glycans on T cell surface glycoproteins, and suggest that the modification of GalNAc transferase activity plays a relevant role during the maturation process of thymic cells.

The effect of TNF-alpha on glycosylation pathways in bovine synoviocytes

Synoviocytes are fibroblastic cells that line joint cavities. These cells synthesize numerous cell-surface and extracellular-matrix glycoproteins that are required for maintenance of the joint. Joint inflammation, such as occurs in arthritis, has been shown to have major effects on synoviocyte proliferation and on the biosynthesis of glycoproteins. The structures of the carbohydrate moieties of glycoproteins, however, and the enzymes involved in their synthesis have not yet been described for synoviocytes. Therefore, to characterize the cell-surface glycoconjugates, synoviocytes were isolated from bovine ankles, and the cells were grown in primary cultures. Lectin-binding assays were used to identify exposed N- and O-glycan carbohydrate determinants on synoviocytes, and specific enzyme assays were used to identify some of the glycosyltransferases involved in the synthesis of the glycan chains. A number of the enzymes that synthesize N- and O-linked oligosaccharides were found to be active in cell-free extracts of synoviocytes, including those that synthesize core-1-based O-glycans and the more complex bi-antennary N-glycans. To understand the molecular events underlying the inflammatory response in the synovium of arthritis patients, we examined the effect of the inflammatory cytokine tumour necrosis factor alpha (TNF-alpha) on synoviocytes and on glycosylation profiles. TNF-alpha treatment, which induces apoptosis in synoviocytes, was accompanied by changes in lectin-binding patterns, indicating alterations in the expression of cell-surface oligosaccharides. Concurrently, changes in specific enzyme activities were observed in treated cells. Two enzymes potentially important to the inflammatory process, core 2 beta6-GlcNAc-transferase and beta4-Gal-transferase, increased after TNF-alpha treatment. This is the first study of glycoprotein biosynthesis in synoviocytes, and it shows that synoviocytes have a characteristic glycosylation phenotype that is altered in the presence of inflammatory cytokines.

Analysis ofO-glycosylation site occupancy in bovine ?-casein glycoforms separated by two-dimensional gel electrophoresis

The ability of two-dimensional gel electrophoresis (2-DE) to separate glycoproteins was exploited to separate distinct glycoforms of kappa-casein that differed only in the number of O-glycans that were attached. To determine where the glycans were attached, the individual glycoforms were digested in-gel with pepsin and the released glycopeptides were identified from characteristic sugar ions in the tandem mass spectrometry (MS) spectra. The O-glycosylation sites were identified by tandem MS after replacement of the glycans with ammonia / aminoethanethiol. The results showed that glycans were not randomly distributed among the five potential glycosylation sites in kappa-casein. Rather, glycosylation of the monoglycoform could only be detected at a single site, T152. Similarly the diglycoform appeared to be modified exclusively at T152 and T163, while the triglycoform was modified at T152, T163 and T154. While low levels of glycosylation at other sites cannot be excluded the hierarchy of site occupation between glycoforms was clearly evident and argues for an ordered addition of glycans to the protein. Since all five potential O-glycosylation sites can be glycosylated in vivo, it would appear that certain sites remain latent until other sites are occupied. The determination of glycosylation site occupancy in individual glycoforms separated by 2-DE revealed a distinct pattern of in vivo glycosylation that has not been recognized previously.

Role of Peptide Sequence and Neighboring Residue Glycosylation on the Substrate Specificity of the Uridine 5'-Diphosphate−α-N-acetylgalactosamine:PolypeptideN-acetylgalactosaminyl Transferases T1 and T2: Kinetic Modeling of the Porcine and Canine Submaxillary Gland Mucin Tandem Repeats†

A large family of uridine 5'-diphosphate (UDP)-alpha-N-acetylgalactosamine (GalNAc):polypeptide N-acetylgalactosaminyl transferases (ppGalNAc Ts) initiates mucin-type O-glycan biosynthesis at serine and threonine. The peptide substrate specificities of individual family members are not well characterized or understood, leaving an inability to rationally predict or comprehend sites of O-glycosylation. Recently, a kinetic modeling approach demonstrated neighboring residue glycosylation as a major factor modulating the O-glycosylation of the porcine submaxillary gland mucin 81 residue tandem repeat by ppGalNAc T1 and T2 [Gerken et al. (2002) J. Biol. Chem. 277, 49850-49862]. To confirm the general applicability of this model and its parameters, the ppGalNAc T1 and T2 glycosylation kinetics of the 80+ residue tandem repeat from the canine submaxillary gland mucin was obtained and characterized. To reproduce the glycosylation patterns of both mucins (comprising 50+ serine/threonine residues), specific effects of neighboring peptide sequence, in addition to the previously described effects of neighboring residue glycosylation, were required of the model. Differences in specificity of the two transferases were defined by their sensitivities to neighboring proline and nonglycosylated hydroxyamino acid residues, from which a ppGalNAc T2 motif was identified. Importantly, the model can approximate the previously reported ppGalNAc T2 glycosylation kinetics of the IgA1 hinge domain peptide [Iwasaki, et al. (2003) J. Biol. Chem. 278, 5613-5621], further validating both the approach and the ppGalNAc T2 positional weighting parameters. The characterization of ppGalNAc transferase specificity by this approach may prove useful for the search for isoform-specific substrates, the creation of isoform-specific inhibitors, and the prediction of mucin-type O-glycosylation sites.

A genetic approach to Mammalian glycan function

The four essential building blocks of cells are proteins, nucleic acids, lipids, and glycans. Also referred to as carbohydrates, glycans are composed of saccharides that are typically linked to lipids and proteins in the secretory pathway. Glycans are highly abundant and diverse biopolymers, yet their functions have remained relatively obscure. This is changing with the advent of genetic reagents and techniques that in the past decade have uncovered many essential roles of specific glycan linkages in living organisms. Glycans appear to modulate biological processes in the development and function of multiple physiologic systems, in part by regulating protein-protein and cell-cell interactions. Moreover, dysregulation of glycan synthesis represents the etiology for a growing number of human genetic diseases. The study of glycans, known as glycobiology, has entered an era of renaissance that coincides with the acquisition of complete genome sequences for multiple organisms and an increased focus upon how posttranslational modifications to protein contribute to the complexity of events mediating normal and disease physiology. Glycan production and modification comprise an estimated 1% of genes in the mammalian genome. Many of these genes encode enzymes termed glycosyltransferases and glycosidases that reside in the Golgi apparatus where they play the major role in constructing the glycan repertoire that is found at the cell surface and among extracellular compartments. We present a review of the recently established functions of glycan structures in the context of mammalian genetic studies focused upon the mouse and human species. Nothing tends so much to the advancement of knowledge as the application of a new instrument. The native intellectual powers of men in different times are not so much the causes of the different success of their labours, as the peculiar nature of the means and artificial resources in their possession. T. Hager: Force of Nature (1)

Glycodynamics of Mucin Biosynthesis in Gastrointestinal Tumor Cells

Glycoproteins found in the secretions and on the surfaces of cancer cells include mucins and mucin-like glycoproteins. These molecules have been shown to carry antigens that are characteristically expressed on cancer cells, including Tn and T antigens and Lewis epitopes. The structures of O-glycans are often abnormal in gastrointestinal tumors, or else are present in abnormal amounts, and these structures greatly contribute to the phenotype and biology of cancer cells. It has been shown that glycans of cancer cells have functional importance in cell adhesion, invasion and metastasis. The possible mechanisms leading to these cancer-specific changes in carbohydrate structures (termed glycodynamics) involve altered mRNA expression and catalytic activities of glycosyltransferases and sulfotransferases found in tissues and cells of gastrointestinal tumors. In a number of cases it has been possible to correlate enzyme changes with oligosaccharide structures. Different mechanisms have been suggested leading to the synthesis of cancer-specific Lewis, T and Tn antigens, but the regulation of cancer mucin antigens generally appears to be very complex and is poorly understood. The expression levels of specific mucin antigens and enzymes in gastro-intestinal tumors have diagnostic as well as prognostic value. These antigens also have potential for cancer immunotherapy. However, we first need to unravel the complexity of the control of glycosylation in cancer cells. Most importantly, studies of the functional implications of the glycodynamics in cancer cells, as related to cell adhesion and impact on the immune system will provide promising directions for future research.

Mucin core O-glycosylation is modulated by neighboring residue glycosylation status. Kinetic modeling of the site-specific glycosylation of the apo-porcine submaxillary mucin tandem repeat by UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases T1 and T2

The influence of peptide sequence and environment on the initiation and elongation of mucin O-glycosylation is not well understood. The in vivo glycosylation pattern of the porcine submaxillary gland mucin (PSM) tandem repeat containing 31 O-glycosylation sites (Gerken, T. A., Gilmore, M., and Zhang, J. (2002) J. Biol. Chem. 277, 7736-7751) reveals a weak inverse correlation with hydroxyamino acid density (and by inference the density of glycosylation) with the extent of GalNAc glycosylation and core-1 substitution. We now report the time course of the in vitro glycosylation of the apoPSM tandem repeat by recombinant UDP-GalNAc:polypeptide alpha-GalNAc transferases (ppGalNAc transferase) T1 and T2 that confirm these findings. A wide range of glycosylation rates are found, with several residues showing apparent plateaus in glycosylation. An adjustable kinetic model that reduces the first-order rate constants proportional to neighboring glycosylation status, plus or minus three residues of the site of glycosylation, was found to reasonably reproduce the experimental rate data for both transferases, including apparent plateaus in glycosylation. The unique, transferase-specific, positional weighting constants reveal information on the peptide/glycopeptide recognition site for each transferase. Both transferases displayed high sensitivities to neighboring Ser/Thr glycosylation, whereas ppGalNAc T2 displayed additional high sensitivities to the presence of nonglycosylated Ser/Thr residues. This is the first demonstration of the ability to model mucin O-glycosylation kinetics, confirming that under the appropriate conditions neighboring glycosylation status can be a significant factor modulating the first step of mucin O-glycan biosynthesis.

Characterization of a novel human UDP-GalNAc transferase, pp-GalNAc-T10

A novel member of the human UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase (pp-GalNAc-T) gene family was cloned as a homolog of human pp-GalNAc-T7, and designated pp-GalNAc-T10. pp-GalNAc-T10 transcript was found in the small intestine, stomach, pancreas, ovary, thyroid gland and spleen. In a polypeptide GalNAc-transfer assay, recombinant pp-GalNAc-T10 transferred GalNAc onto a panel of mucin-derived peptide substrates. Furthermore, pp-GalNAc-T10 demonstrated strong transferase activity with glycopeptide substrates.

Unmasking a new recognition signal for O-linked glycosylation in the chorionic gonadotropin beta subunit

hCGbeta subunit is distinguished among the other members of the family of the glycoprotein hormones by the presence of four serine O-linked oligosaccharide units in the last 25 amino acids. This carboxy terminal peptide (CTP) influences the intracellular behavior of the subunit and is important for maintaining the biological half-life of hCG. To examine how the O-linked oligosaccharides affect the metabolic behavior of hCG, we generated a CGbeta mutant devoid of the native O-linked acceptor sites. An alternative site not used in the native subunit was glycosylated and the structure of this oligosaccharide differed from the wild-type O-linked carbohydrates. This glycosylation occurred at serine 130 in the CTP and in contrast to the wild type O-linked oligosaccharides, sialic acid is a major component of the alternatively linked carbohydrate. The data show that deleting the native acceptor sites exposes a new site for O-glycosylation and promotes a differential intracellular processing of the beta subunit. These results support the hypothesis that the CTP participates in the folding of the newly synthesized subunit, which is manifested by the post-translational changes reported here.

Molecular cloning and characterization of a novel member of the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase family, pp-GalNAc-T12

We cloned in silico a novel human UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (pp-GalNAc-T), pp-GalNAc-T12. The deduced amino acid sequence of pp-GalNAc-T12 contains all conserved motifs in pp-GalNAc-T family proteins. Quantitative real time polymerase chain reaction analysis revealed that the pp-GalNAc-T12 transcript was expressed mainly in digestive organs such as stomach, small intestine and colon. The recombinant pp-GalNAc-T12 transferred GalNAc to the mucin-derived peptides such as the Muc1a, Muc5AC, EA2 peptides and the GalNAc-Muc5AC glycopeptide. Since mucins are glycoproteins mainly produced in the digestive organs, our results suggest that pp-GalNAc-T12 plays an important role in the initial step of mucin-type oligosaccharide biosynthesis in digestive organs.

Determination of the site-specific oligosaccharide distribution of the O-glycans attached to the porcine submaxillary mucin tandem repeat. Further evidence for the modulation of O-glycans side chain structures by peptide sequence

Little is known of the degree that polypeptide sequence and the local environment modulate the structures of O-linked glycans. Toward this understanding, the site-specific mono- (GalNAc-O-), di- (beta-Gal-1,3-alpha-GalNAc-O-), and trisaccharide (alpha-Fuc-1,2-beta-Gal-1,3-alpha-GalNAc-O-) distributions have been determined for 29 of the 31 O-glycosylated Ser/Thr residues in the tandem repeat domains of blood group A-negative porcine submaxillary gland mucin. The glycosylation patterns obtained from three individual animals are in agreement with earlier incomplete determinations on a pooled mucin (Gerken, T. A., Owens, C. L., and Pasumarthy, M. (1997) J. Biol. Chem. 272, 9709-9719; Gerken, T. A., Owens, C. L., and Pasumarthy, M. (1998) J. Biol. Chem. 273, 26580-26588), confirming that the addition of the peptide-linked GalNAc and its substitution by beta-1,3-Gal are sensitive to local peptide sequence in a highly reproducible manner in vivo. The present data further support earlier suggestions of an inverse correlation of the density of hydroxyamino acid residues (and by inference the density of peptide GalNAc) with the extent of substitution of the peptide-linked GalNAc by beta-1,3-Gal. This effect is highly correlated for Ser-linked glycans but not for Thr-linked glycans. A similar correlation is observed with respect to the in vivo peptide GalNAc glycosylation pattern. In contrast, the addition of alpha-1,2-Fuc to beta-Gal shows no apparent correlation with hydroxyamino acid density, although a marked elevation in the fucosylation of Ser-linked glycans compared with Thr-linked glycans is observed. The above effects may represent both steric and conformational factors acting to alter the relative accessibility and activity of the glycosyltransferases toward substrate. These results demonstrate that the porcine submaxillary gland core 1 beta 3-galactosyltransferase and alpha2-fucosyltransferase exhibit unique peptide/glycopeptide sensitivities that may provide mechanisms for the modulation of O-linked side chain structures.

From imino sugars to cancer glycoproteins

This review summarizes fifty years of carbohydrate chemistry, from studies of reaction mechanisms and development of synthetic methods to the design of specific compounds for investigations of cancer glycoproteins. Through these years of intensive research, many carbohydrate derivatives were shown to be biologically relevant, for example the fragments of bacterial polysaccharides and imino sugars that were potent inhibitors of glycosidases. Oligosaccharides and O- glycopeptides with GalNAc-Ser/Thr linkages were essential for studies of cancer immunology and the specificities and inhibition of cancer glycosyltransferases. Thus carbohydrate chemistry has provided the basis for our knowledge of the biochemistry and immunology of cancer cell glycoproteins.

Pathways of mucin O-glycosylation in normal and malignant rat colonic epithelial cells reveal a mechanism for cancer-associated Sialyl-Tn antigen expression

The Sialyl-Tn antigen (Sialyl alpha-Ser/Thr) is expressed as a cancer-associated antigen on the surface of cancer cells. Its presence is associated with a poor prognosis in patients with colorectal and other cancers. We previously reported that Sialyl-Tn expression in LSC human colon cancer cells could be explained by a specific lack of the activity of core 1 beta3-Gal-transferase (Brockhausen et al., Glycoconjugate J. 15, 595-603, 1998) and an inability to synthesize the common O-glycan core structures. To support this mechanism, or find other mechanisms to explain Sialyl-Tn antigen expression, we investigated the O-glycosylation pathways in clonal rat colon cancer cell lines that were selected for positive or negative expression of Sialyl-Tn antigen, and compared these pathways to those in normal rat colonic mucosa. Normal rat colonic mucosa had very active glycosyltransferases synthesizing O-glycan core structures 1 to 4. Several sialyl-, sulfo- and fucosyltransferases were also active. An M type core 2 beta6-GlcNAc-transferase was found to be present in rat colon mucosa and all of the rat colon cancer cells. O-glycosylation pathways in rat colon cancer cells were significantly different from normal rat colonic mucosa; for example, rat colon cancer cells lost the ability to synthesize O-glycan core 3. All rat colon cancer cell lines, regardless of the Sialyl-Tn phenotype, expressed glycosyltransferases assembling complex O-glycans of core 1 and core 2 structures (unlike human LSC colon cancer cells which lack core 1 beta3-Gal-transferase activity). It was the activity of CMP-sialic acid:GalNAc-mucin alpha6-sialyltransferase that coincided with Sialyl-Tn expression. Sialyl-Tn negative cells had a several fold higher activity of core 2 beta6-GlcNAc-transferase which synthesizes complex O-glycans that may mask adjacent Sialyl-Tn epitopes. The results suggest a new mechanism controlling Sialyl-Tn expression in cancer cells.

Order and maximum incorporation of N-acetyl-D-galactosamine into threonine residues of MUC2 core peptide with microsome fraction of human-colon-carcinoma LS174T cells

Mucin 2 (MUC2) is the major intestinal mucin. O-glycans are attached to MUC2 in a potentially diverse arrangement, which is crucial for their interaction with endogeneous and exogeneous lectins. In the present report, five oligopeptides [PTTTPITTTT(K), ITTTTTVTPT(K), TVTPTPTPTG(K), PTPTGTQTPT(K) and TQTPTTTPIT(K)] corresponding to portions of the MUC2 tandem repeat domain were synthesized, and incubated with UDP-N-acetyl-D-galactosamine (UDP-GalNAc) and detergent-soluble microsomes, prepared from the human colon carcinoma cell line LS174T. The products were fractionated by reverse-phase HPLC and characterized by matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry. Oligopeptides with GalNAc residues derived from PTTTPITTTT(K), containing consecutive threonine residues, were found to be glycosylated with 1-7 GalNAc residues per single peptide. The sequences of all glycopeptides were determined. The results indicated that the predominant sites of the first through to the sixth GalNAc incorporation were Thr(3), Thr(6), Thr(5), Thr(2), Thr(4) and Thr(1), respectively. An exception was the presence of a glycopeptide with three GalNAc residues at Thr(1), Thr(4) and Thr(5). Oligopeptides containing alternating threonine residues [TVTPTPTPTG(K) and PTPTGTQTPT(K)] were not fully glycosylated under the same conditions or even after prolonged incubations. Thus, the preferential order and maximum number of GalNAc incorporation into threonine residues of MUC2 core peptides depends on the peptide sequence, when the microsome fraction of LS174T cells is used as a source of N-acetyl-D-galactosaminyltransferases.

Pathways of O-glycan biosynthesis in cancer cells

Glycoproteins with O-glycosidically linked carbohydrate chains of complex structures and functions are found in secretions and on the cell surfaces of cancer cells. The structures of O-glycans are often unusual or abnormal in cancer, and greatly contribute to the phenotype and biology of cancer cells. Some of the mechanisms of changes in O-glycosylation pathways have been determined in cancer model systems. However, O-glycan biosynthesis is a complex process that is still poorly understood. The glycosyltransferases and sulfotransferases that synthesize O-glycans appear to exist as families of related enzymes of which individual members are expressed in a tissue- and growth-specific fashion. Studies of their regulation in cancer may reveal the connection between cancerous transformation and glycosylation which may help to understand and control the abnormal biology of tumor cells. Cancer diagnosis may be based on the appearance of certain glycosylated epitopes, and therapeutic avenues have been designed to attack cancer cells via their glycans.

A novel human UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase, GalNAc-T7, with specificity for partial GalNAc-glycosylated acceptor substrates

A novel member of the human UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase gene family, designated GalNAc-T7, was cloned and expressed. GalNAc-T7 exhibited different properties compared to other characterized members of this gene family, in showing apparent exclusive specificity for partially GalNAc-glycosylated acceptor substrates. GalNAc-T7 showed no activity with a large panel of non-glycosylated peptides, but was selectively activated by partial GalNAc glycosylation of peptide substrates derived from the tandem repeats of human MUC2 and rat submaxillary gland mucin. The function of GalNAc-T7 is suggested to be as a follow-up enzyme in the initiation step of O-glycosylation.

Dynamic epigenetic regulation of initial O-glycosylation by UDP-N-Acetylgalactosamine:Peptide N-acetylgalactosaminyltransferases. site-specific glycosylation of MUC1 repeat peptide influences the substrate qualities at adjacent or distant Ser/Thr positions

In search of possible epigenetic regulatory mechanisms ruling the initiation of O-glycosylation by polypeptide:N-acetylgalactosaminyltransferases, we studied the influences of mono- and disaccharide substituents of glycopeptide substrates on the site-specific in vitro addition of N-acetylgalactosamine (GalNAc) residues by recombinant GalNAc-Ts (rGalNAc-T1, -T2, and -T3). The substrates were 20-mers (HGV20) or 21-mers (AHG21) of the MUC1 tandem repeat peptide carrying GalNAcalpha or Galbeta1-3GalNAcalpha at different positions. The enzymatic products were analyzed by MALDI mass spectrometry and Edman degradation for the number and sites of incorporated GalNAc. Disaccharide placed on the first position of the diad Ser-16-Thr-17 prevents glycosylation of the second, whereas disaccharide on the second position of Ser-16-Thr-17 and Thr-5-Ser-6 does not prevent GalNAc addition to the first. Multiple disaccharide substituents suppress any further glycosylation at the remaining sites. Glycosylation of Ser-16 is negatively affected by glycosylation at position -6 (Thr-10) or -10 (Ser-6) and is inhibited by disaccharide at position -11 (Thr-5), suggesting the occurrence of glycosylation-induced effects on distant acceptor sites. Kinetic studies revealed the accelerated addition of GalNAc to Ser-16 adjacent to GalNAc-substituted Thr-17, demonstrating positive regulatory effects induced by glycosylation on the monosaccharide level. These antagonistic effects of mono- and disaccharides could underlie a postulated regulatory mechanism.

The acceptor specificity of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases

The in vitro and in vivo specificity of the family of peptide:N-acetylgalactosaminyltransferases (GalNAcT) is analyzed on the basis of the reactivity and/or inhibitory activity of peptides and protein segments. The transferases appear to be multi-substrate enzymes with extended active sites containing a least nine subsites that interact cooperatively with a linear segment of at least nine amino acid residues on the acceptor polypeptide. Functional acceptor sites are located on the surface of the protein and extended conformations (beta-strand conformation) are preferred. The acceptor specificity of GalNAc-T can be predicted from the primary structure of the acceptor peptide with an accuracy of 70 to 80%. The same GalNAc-T enzymes catalyze the glycosylation of both serine and threonine residues. The higher in vitro catalytic efficiency toward threonine versus serine is the result of enhanced binding as well as increased reaction velocity, both effects being the result of steric interactions between the active site of the enzyme and the methyl group of threonine. Results from substrate binding studies suggest that GalNAc-T catalyzed transfer proceeds via an ordered sequential mechanism.

Cloning of a human UDP-N-acetyl-alpha-D-Galactosamine:polypeptide N-acetylgalactosaminyltransferase that complements other GalNAc-transferases in complete O-glycosylation of the MUC1 tandem repeat

A fourth human UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase, designated GalNAc-T4, was cloned and expressed. The genomic organization of GalNAc-T4 is distinct from GalNAc-T1, -T2, and -T3, which contain multiple coding exons, in that the coding region is contained in a single exon. GalNAc-T4 was placed at human chromosome 12q21.3-q22 by in situ hybridization and linkage analysis. GalNAc-T4 expressed in Sf9 cells or in a stably transfected Chinese hamster ovary cell line exhibited a unique acceptor substrate specificity. GalNAc-T4 transferred GalNAc to two sites in the MUC1 tandem repeat sequence (Ser in GVTSA and Thr in PDTR) using a 24-mer glycopeptide with GalNAc residues attached at sites utilized by GalNAc-T1, -T2, and -T3 (TAPPAHGVTSAPDTRPAPGSTAPPA, GalNAc attachment sites underlined). Furthermore, GalNAc-T4 showed the best kinetic properties with an O-glycosylation site in the P-selectin glycoprotein ligand-1 molecule. Northern analysis of human organs revealed a wide expression pattern. Immunohistology with a monoclonal antibody showed the expected Golgi-like localization in salivary glands. A single base polymorphism, G1516A (Val to Ile), was identified (allele frequency 34%). The function of GalNAc-T4 complements other GalNAc-transferases in O-glycosylation of MUC1 showing that glycosylation of MUC1 is a highly ordered process and changes in the repertoire or topology of GalNAc-transferases will result in altered pattern of O-glycan attachments.

Structural requirements for O-glycosylation of the mouse hepatitis virus membrane protein

The mouse hepatitis virus (MHV) membrane (M) protein contains only O-linked oligosaccharides. We have used this protein as a model to study the structural requirements for O-glycosylation. We show that MHV M is modified by the addition of a single oligosaccharide side chain at the cluster of 4 hydroxylamino acids present at its extreme amino terminus and identified Thr at position 5 as the functional acceptor site. The hydroxylamino acid cluster, which is quite conserved among O-glycosylated coronavirus M proteins, is not in itself sufficient for O-glycosylation. Downstream amino acids are required to introduce a functional O-glycosylation site into a foreign protein. In a mutagenic analysis O-glycosylation was found to be sensitive to some particular changes but no unique sequence motif for O-glycosylation could be identified. Expression of mutant M proteins in cells revealed that substitution of any 1 residue was tolerated, conceivably due to the occurrence of multiple UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc transferases). Indeed, MHV M served as a substrate for GalNac-T1, -T2, and -T3, as was demonstrated using an in situ glycosylation assay based on the co-expression of endoplasmic reticulum-retained forms of the GalNAc transferases with endoplasmic reticulum-resident MHV M mutants. The GalNAc transferases were found to have largely overlapping, but distinct substrate specificities. The requirement for a threonine as acceptor rather than a serine residue and the requirement for a proline residue three positions downstream of the acceptor site were found to be distinctive features.

Mass spectrometric determination of the sites of O-glycan attachment with low picomolar sensitivity

A sensitive protocol for unambiguously and positively identifying O-glycosylation sites in glycopeptides is described, based on beta-elimination of the glycan chain(s) using NH4OH. On glycan elimination, NH3 is incorporated into the amino acid residue(s) to which the glycan(s) had been attached, to yield a modified amino acid residue having a distinct mass. Electrospray ionization collision-induced dissociation tandem mass spectrometry allows the released, modified peptide to be sequenced and the site(s) of the modified amino acid residue(s) to be identified. The protocol has been optimized using a series of structurally related O-glycopeptides, and standard conditions are recommended for handling unknowns. We demonstrate that site determination can be achieved using as little as 1 pmol of starting material.

Influence of the amino acid sequence on the MUC5AC motif peptide O-glycosylation by human gastric UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase(s)

The present work was carried out to study the role of the peptide moiety in the addition of O-linked N-acetylgalactosamineto human apomucin using human crude microsomal homogenates from gastric mucosa (as enzyme source) and a series of peptide acceptors representative of tandem repeat domains deduced from the MUC5AC mucin gene (expressed in the gastric mucosa). Being rich in threonine and serine placed in clusters, these peptides provided several potential sites for O-glycosylation. The glycosylated products were analysed by a combination of electrospray mass spectrometry and capillary electrophoresis in order to isolate the glycopeptides and to determine their sequence by Edman degradation. The O-glycosylation of our MUC5AC motif peptides gave information on the specificity and activity of the gastric microsomal UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferase(s). The proline residues and the induced-conformations are of great importance for the recognition of MUC5AC peptides but they are not the only factors for the choice of the O-glycosylation sites. Moreover, for the di-glycosylated peptides, the flanking regions of the proline residues strongly influence the site of the second O-glycosylation.

NetOglyc: prediction of mucin type O-glycosylation sites based on sequence context and surface accessibility

The specificities of the UDP-GalNAc:polypeptide Nacetylgalactosaminyltransferases which link the carbohydrate GalNAc to the side-chain of certain serine and threonine residues in mucin type glycoproteins, are presently unknown. The specificity seems to be modulated by sequence context, secondary structure and surface accessibility. The sequence context of glycosylated threonines was found to differ from that of serine, and the sites were found to cluster. Non-clustered sites had a sequence context different from that of clustered sites. Charged residues were disfavoured at position -1 and +3. A jury of artificial neural networks was trained to recognize the sequence context and surface accessibility of 299 known and verified mucin type O-glycosylation sites extracted from O-GLYCBASE. The cross-validated NetOglyc network system correctly found 83% of the glycosylated and 90% of the non-glycosylated serine and threonine residues in independent test sets, thus proving more accurate than matrix statistics and vector projection methods. Predictions of O-glycosylation sites in the envelope glycoprotein gp120 from the primate lentiviruses HIV-1, HIV-2 and SIV are presented. The most conserved O-glycosylation signals in these evolutionary-related glycoproteins were found in their first hypervariable loop, V1. However, the strain variation for HIV-1 gp120 was significant. A computer server, available through WWW or E-mail, has been developed for prediction of mucin type O-glycosylation sites in proteins based on the amino acid sequence. The server addresses are http://www.cbs.dtu.dk/services/NetOGlyc/ and netOglyc@cbs.dtu.dk.

Polypeptide:N-acetylgalactosaminyltransferase activities towards the mucin MUC5AC peptide motif using microsomal preparations of normal and tumoral digestive mucosa

The selected-acceptor substrate peptide (TTSAPTTS), deduced from the human mucin gene MUC5AC (expressed essentially in the human gastric and tracheobronchial mucosa), was used to assay polypeptide:N-acetylgalactosaminyltransferases (GalNAc transferases) of different microsomal preparations, obtained from gastric and colonic mucosa in normal and tumoral situations. The O-glycosylated products, analyzed by capillary electrophoresis and electrospray mass spectrometry, showed a variable number of GalNAc O-linked to the different hydroxy amino acids of TTSAPTTS, depending on the tissue studied. Our observations were consistent with the existence of more than one form of GalNAc transferases which were expressed differentially in the gastrointestinal tract (stomach and/or colon). The levels of enzyme activities showed a tissue-specific pattern as they were high in normal colonic tissue and low in colon cancer. On the other hand, in the tumoral gastric tissue (displaying intestinal metaplasia) a high level of GalNAc transferase activities was obtained, similar to that found in the normal colon. Moreover, slight discrepancies (activities and number of O-linked GalNAc) were only detected between normal gastric and tumoral colonic preparations. Thus, the data indicated that the dedifferentiation of the gastric cancer tissue may induce GalNAc transferase activities similar to those in the normal colonic, tissue and that colonic and gastric tissues may contain families of glycosyltransferases involved specifically in reaction towards particular peptide or protein substrates. In addition, the analysis by capillary electrophoresis and electrospray mass spectrometry revealed, in tumoral gastric as well as in normal colonic tissues, a high dipeptidylaminotransferase activity inducing an elongation of TTSAPTTS by dithreonine. This activity was low in normal gastric and tumoral colonic tissues.

Synthetic methods of glycopeptide assembly, and biological analysis of glycopeptide products

The technology of glycopeptide synthesis has recently developed into a fully mature science capable of creating diverse glycopeptides of biological interest, even in combinatorial displays. This has allowed biochemists to investigate substrate specificity in the biosynthetic processing and immunology of various protein glycoforms. The construction of all the mucin core structures and a variety of cancer-related glycopeptides has facilitated detailed analysis of the interaction between MHC-bound glycopeptides and T cell receptors. Novel dendritic neoglycopeptide ligands have been shown to demonstrate high affinity for carbohydrate receptors and these interactions are highly dendrimer specific. Large complex N-linked oligosaccharides have been introduced into glycopeptides using synthetic or chemoenzymatic procedures, both methods affording pure glycopeptides corresponding to a single glycoform in preparative quantities. The improved availability of glycosyl transferases has led to increased use of chemoenzymatic synthesis. Chemical ligation has been introduced as a method of attaching glycans to peptide templates. Combinatorial synthesis and the analysis of resin-bound glycopeptide libraries have been successfully carried out by applying the ladder synthesis principle. Direct quantitative glycosylation of peptide templates on solid phase has paved the way for the synthesis of templated glycopeptide mixtures as libraries of libraries.

Localization of O-glycosylation sites on glycopeptide fragments from lactation-associated MUC1. All putative sites within the tandem repeat are glycosylation targets in vivo

Since there is no consensus sequence directing the initial GalNAc incorporation into mucin peptides, O-glycosylation sites are not reliably predictable. We have developed a mass spectrometric sequencing strategy that allows the identification of in vivo O-glycosylation sites on mucin-derived glycopeptides. Lactation-associated MUC1 was isolated from human milk and partially deglycosylated by trifluoromethanesulfonic acid to the level of core GalNAc residues. The product was fragmented by the Arg-C-specific endopeptidase clostripain to yield tandem repeat icosapeptides starting with the PAP motif. PAP20 glycopeptides were subjected to sequencing by post-source decay matrix-assisted laser desorption ionization mass spectrometry or by solid phase Edman degradation to localize the glycosylation sites. The masses of C- or N-terminal fragments registered for the mono- to pentasubstituted PAP20 indicated that GalNAc was linked to the peptide at Ser5,Thr6 (GSTA) and Thr14 (VTSA) but contrary to previous in vitro glycosylation studies also at Thr19 and Ser15 located within the PDTR or VTSA motifs, respectively. Quantitative data from solid phase Edman sequencing revealed no preferential glycosylation of the threonines. These discrepancies between in vivo and in vitro glycosylation patterns may be explained by assuming that O-glycosylation of adjacent peptide positions is a dynamically regulated process that depends on changes of the substrate qualities induced by glycosylation at vicinal sites.

Determination of the site-specific O-glycosylation pattern of the porcine submaxillary mucin tandem repeat glycopeptide. Model proposed for the polypeptide:galnac transferase peptide binding site

The heterogeneously glycosylated 81-residue tryptic tandem repeat glycopeptide from porcine submaxillary mucin (PSM) has been isolated and its glycosylation pattern determined by amino acid sequencing. Key to these studies is the ability to trim the structurally heterogeneous PSM oligosaccharide side chains to homogeneous GalNAc monosaccharide side chains by mild trifluoromethanesulfonic acid treatment. Trypsin treatment of trifluoromethanesulfonic acid-treated PSM releases the 81-residue tandem repeat as an ensemble of 81-residue glycopeptides with different glycosylation patterns. Automated amino acid sequencing using Edman degradative chemistry of the repeat was used to determine the extent of glycosylation of nearly every Ser and Thr residue. The Thr residues are all highly glycosylated within the range of 73-90%, giving an average Thr glycosylation of 83%. In contrast, the Ser residues display a wide range of glycosylations, ranging between 33 and 95%, giving an average Ser glycosylation of 74%. These data are consistent with the known elevated glycosylation of Thr peptides over Ser peptides for the porcine UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferase. It is also observed that the extent of glycosylation of the repeat correlates poorly with published predictive methods. An examination of the sequences surrounding the glycosylation sites reveals that nearly all of the highly glycosylated sites have a penultimate Gly residue, whereas those that are less highly glycosylated have medium to large side chain penultimate residues. As observed by others, glycosylation also appears to be modulated by the presence of Pro residues. On the basis of these findings we suggest that the acceptor peptide binds the transferase in a beta-like conformation and that penultimate residue side chain steric interactions may play a role in determining extent that a given Ser or Thr is glycosylated. A model for the GalNAc transferase peptide binding site is proposed.