Characterization of a UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase that displays glycopeptide N-acetylgalactosaminyltransferase activity

Polypeptide N-acetylgalactosamine transferase 3: a post-translational writer on human health

Polypeptide N-acetylgalactosamine transferase 3 (ppGalNAc-T3) is an enzyme involved in the initiation of O-GalNAc glycan biosynthesis. Acting as a writer of frequent post-translational modification (PTM) on human proteins, ppGalNAc-T3 has key functions in the homeostasis of human cells and tissues. We review the relevant roles of this molecule in the biosynthesis of O-GalNAc glycans, as well as in biological functions related to human physiological and pathological conditions. With main emphasis in ppGalNAc-T3, we draw attention to the different ways involved in the modulation of ppGalNAc-Ts enzymatic activity. In addition, we take notice on recent reports of ppGalNAc-T3 having different subcellular localizations, highlight critical intrinsic and extrinsic functions in cellular physiology that are exerted by ppGalNAc-T3-synthesized PTMs, and provide an update on several human pathologies associated with dysfunctional ppGalNAc-T3. Finally, we propose biotechnological tools as new therapeutic options for the treatment of pathologies related to altered ppGalNAc-T3. KEY MESSAGES: ppGalNAc-T3 is a key enzyme in the human O-GalNAc glycans biosynthesis. enzyme activity is regulated by PTMs, lectin domain and protein-protein interactions. ppGalNAc-T3 is located in human Golgi apparatus and cell nucleus. ppGalNAc-T3 has a central role in cell physiology as well as in several pathologies. Biotechnological tools for pathological management are proposed.

Polypeptide N-acetylgalactosaminyltransferase-Associated Phenotypes in Mammals

Mucin-type O-glycosylation involves the attachment of glycans to an initial O-linked N-acetylgalactosamine (GalNAc) on serine and threonine residues on proteins. This process in mammals is initiated and regulated by a large family of 20 UDP-GalNAc: polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) (EC 2.4.1.41). The enzymes are encoded by a large gene family (GALNTs). Two of these genes, GALNT2 and GALNT3, are known as monogenic autosomal recessive inherited disease genes with well characterized phenotypes, whereas a broad spectrum of phenotypes is associated with the remaining 18 genes. Until recently, the overlapping functionality of the 20 members of the enzyme family has hindered characterizing the specific biological roles of individual enzymes. However, recent evidence suggests that these enzymes do not have full functional redundancy and may serve specific purposes that are found in the different phenotypes described. Here, we summarize the current knowledge of GALNT and associated phenotypes.

ppGalNAc-T4-catalyzed O-Glycosylation of TGF-β type Ⅱ receptor regulates breast cancer cells metastasis potential

GalNAc-type O-glycosylation, initially catalyzed by polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts), is one of the most abundant and complex posttranslational modifications of proteins. Emerging evidence has proven that aberrant ppGalNAc-Ts are involved in malignant tumor transformation. However, the exact molecular functions of ppGalNAc-Ts are still unclear. Here, the role of one isoform, ppGalNAc-T4, in breast cancer cell lines was investigated. The expression of ppGalNAc-T4 was found to be negatively associated with migration of breast cancer cells. Loss-of-function studies revealed that ppGalNAc-T4 attenuated the migration and invasion of breast cancer cells by inhibiting the epithelial-mesenchymal transition (EMT) process. Correspondingly, transforming growth factor beta (TGF-β) signaling, which is the upstream pathway of EMT, was impaired by ppGalNAc-T4 expression. ppGalNAc-T4 knockout decreased O-GalNAc modification of TGF-β type Ⅰ and Ⅱ receptor (TβR Ⅰ and Ⅱ) and led to the elevation of TGF-β receptor dimerization and activity. Importantly, a peptide from TβR Ⅱ was identified as a naked peptide substrate of ppGalNAc-T4 with a higher affinity than ppGalNAc-T2. Further, Ser31, corresponding to the extracellular domain of TβR Ⅱ, was identified as the O-GalNAcylation site upon in vitro glycosylation by ppGalNAc-T4. The O-GalNAc-deficient S31 A mutation enhanced TGF-β signaling activity and EMT in breast cancer cells. Together, these results identified a novel mechanism of ppGalNAc-T4-catalyzed TGF-β receptors O-GalNAcylation that suppresses breast cancer cell migration and invasion via the EMT process. Targeting ppGalNAc-T4 may be a potential therapeutic strategy for breast cancer treatment.

Mucin-Type O-GalNAc Glycosylation in Health and Disease

Mucin-type GalNAc O-glycosylation is one of the most abundant and unique post-translational modifications. The combination of proteome-wide mapping of GalNAc O-glycosylation sites and genetic studies with knockout animals and genome-wide analyses in humans have been instrumental in our understanding of GalNAc O-glycosylation. Combined, such studies have revealed well-defined functions of O-glycans at single sites in proteins, including the regulation of pro-protein processing and proteolytic cleavage, as well as modulation of receptor functions and ligand binding. In addition to isolated O-glycans, multiple clustered O-glycans have an important function in mammalian biology by providing structural support and stability of mucins essential for protecting our inner epithelial surfaces, especially in the airways and gastrointestinal tract. Here the many O-glycans also provide binding sites for both endogenous and pathogen-derived carbohydrate-binding proteins regulating critical developmental programs and helping maintain epithelial homeostasis with commensal organisms. Finally, O-glycan changes have been identified in several diseases, most notably in cancer and inflammation, where the disease-specific changes can be used for glycan-targeted therapies. This chapter will review the biosynthesis, the biology, and the translational perspectives of GalNAc O-glycans.

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.

The Cosmc-mediated effects of neutrophil elastase on T antigen expression in BEAS-2B cells

Mucin 5AC (MUC5AC) is a highly O-glycosylated mucin secreted by human bronchial epithelial cells during pulmonary inflammatory diseases. T antigen, a component of the MUC5AC glycans, is the product of the O-glycosylation transferase T-synthase and its chaperone Cosmc. Since the expression of Cosmc is mediated by signaling pathways and inflammatory factors affecting mucin O-glycosylation, we analyzed the impact of neutrophil elastase (NE)-mediated Cosmc and T antigen expression in BEAS-2B cells derived from human bronchial epithelial cells. The expression of Cosmc and T antigen in human lung tissue was analyzed by immunohistochemistry. Cellular immunohistochemistry and western blot analysis demonstrated elevated expression of T antigen in BEAS-2B cells after NE stimulation. Altered Cosmc expression in BEAS-2B cells after NE stimulation was analyzed by confocal microscopy, western blot analysis and quantitative RT-PCR. To assess the biological implications of Cosmc function for T-synthase activity and T antigen synthesis after NE stimulation, BEAS-2B cells were transfected with shRNA to silence the expression of Cosmc. The changes in signaling pathways were analyzed by western blotting. The expression of Cosmc and T antigen increased in lung tissue exposed to chronic inflammation. The expression of Cosmc and T antigen increased in NE-stimulated BEAS-2B cells. NE induced increases in T antigen expression and T-synthase transferase activity in BEAS-2B cells expressing Cosmc, highlighting the importance of Cosmc in the relationship between NE and T antigen. Cosmc and phosphatidylinositol-3-kinase (PI3K) played important roles in the signaling pathway that stimulated hyperexpression of T antigen.

Exploring Regulation of Protein O-Glycosylation in Isogenic Human HEK293 Cells by Differential O-Glycoproteomics

Most proteins trafficking the secretory pathway of metazoan cells will acquire GalNAc-type O-glycosylation. GalNAc-type O-glycosylation is differentially regulated in cells by the expression of a repertoire of up to twenty genes encoding polypeptide GalNAc-transferase isoforms (GalNAc-Ts) that initiate O-glycosylation. These GalNAc-Ts orchestrate the positions and patterns of O-glycans on proteins in coordinated, but poorly understood ways - guided partly by the kinetic properties and substrate specificities of their catalytic domains, as well as by modulatory effects of their unique GalNAc-binding lectin domains. Here, we provide the hereto most comprehensive characterization of nonredundant contributions of individual GalNAc-T isoforms to the O-glycoproteome of the human HEK293 cell using quantitative differential O-glycoproteomics on a panel of isogenic HEK293 cells with knockout of GalNAc-T genes (GALNT1, T2, T3, T7, T10, or T11). We confirm that a major part of the O-glycoproteome is covered by redundancy, whereas distinct O-glycosite subsets are covered by nonredundant GalNAc-T isoform-specific functions. We demonstrate that the GalNAc-T7 and T10 isoforms function in follow-up of high-density O-glycosylated regions, and that GalNAc-T11 has highly restricted functions and essentially only serves the low-density lipoprotein-related receptors in linker regions (C6XXXTC1) between the ligand-binding repeats.

Members of the GalNAc-T family of enzymes utilize distinct Golgi localization mechanisms

Mucin-type O-glycosylation is an evolutionarily conserved and essential post-translational protein modification that is initiated in the Golgi apparatus by a family of enzymes known as the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts). GalNAc-Ts are type II membrane proteins which contain short N-terminal tails located in the cytoplasm, a transmembrane domain that crosses the Golgi membrane, to which is connected a stem region that tethers the C-terminal catalytic and lectin domains that reside in the Golgi lumen. Although mucin-type O-glycans have been shown to play critical roles in numerous biological processes, little is known about how the GalNAc-Ts are targeted to their site of action within the Golgi complex. Here, we investigate the essential protein domains required for Golgi localization of four representative members of the GalNAc-T family of enzymes. We find that GalNAc-T1 and -T2 require their cytoplasmic tail and transmembrane domains for proper Golgi localization, while GalNAc-T10 requires its transmembrane and luminal stem domains. GalNAc-T7 can use either its cytoplasmic tail or its luminal stem, in combination with its transmembrane domain, to localize to the Golgi. We determined that a single glutamic acid in the GalNAc-T10 cytoplasmic tail inhibits its ability to localize to the Golgi via a cytoplasmic tail-dependent mechanism. We therefore demonstrate that despite their similarity, different members of this enzyme family are directed to the Golgi by more than one set of targeting signals.

De novo expression of human polypeptide N-acetylgalactosaminyltransferase 6 (GalNAc-T6) in colon adenocarcinoma inhibits the differentiation of colonic epithelium

Aberrant expression of O-glycans is a hallmark of epithelial cancers. Mucin-type O-glycosylation is initiated by a large family of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) that target different proteins and are differentially expressed in cells and organs. Here, we investigated the expression patterns of all of the GalNAc-Ts in colon cancer by analyzing transcriptomic data. We found that GalNAc-T6 was highly up-regulated in colon adenocarcinomas but absent in normal-appearing adjacent colon tissue. These results were verified by immunohistochemistry, suggesting that GalNAc-T6 plays a role in colon carcinogenesis. To investigate the function of GalNAc-T6 in colon cancer, we used precise gene targeting to produce isogenic colon cancer cell lines with a knockout/rescue system for GALNT6 GalNAc-T6 expression was associated with a cancer-like, dysplastic growth pattern, whereas GALNT6 knockout cells showed a more normal differentiation pattern, reduced proliferation, normalized cell-cell adhesion, and formation of crypts in tissue cultures. O-Glycoproteomic analysis of the engineered cell lines identified a small set of GalNAc-T6-specific targets, suggesting that this isoform has unique cellular functions. In support of this notion, the genetically and functionally closely related GalNAc-T3 homolog did not show compensatory functionality for effects observed for GalNAc-T6. Taken together, these data strongly suggest that aberrant GalNAc-T6 expression and site-specific glycosylation is involved in oncogenic transformation.

Biological and biochemical properties of two Xenopus laevis N-acetylgalactosaminyltransferases with contrasting roles in embryogenesis

The biosynthesis of mucin-type O-linked glycans in animals is initiated by members of the large family of polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts), which play important roles in embryogenesis, organogenesis, adult tissue homeostasis and carcinogenesis. Until now, the mammalian forms of these enzymes have been the best characterized. However, two N-acetylgalactosaminyltransferases (xGalNAc-T6 and xGalNAc-T16) from the African clawed frog (Xenopus laevis), which are most homologous to those encoded by the human GALNT6 and GALNT16 (GALNTL1) genes, were shown to have contrasting roles in TGF-β/BMP signaling in embryogenesis. In this study we have examined these two enzymes further and show differences in their in vivo function during X. laevis embyrogenesis as evidenced by in situ hybridization and overexpression experiments. In terms of enzymatic activity, both enzymes were found to be active towards the EA2 peptide, but display differential activity towards a peptide based on the sequence of ActR-IIB, a receptor relevant to TGF-β/BMP signaling. In summary, these data demonstrate that these two enzymes from different branches of the N-acetylgalactosaminyltransferase do not only display differential substrate specificities, but also specific and distinct expression pattern and biological activities in vivo.

Expression and characterization of the first snail-derived UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase

UDP-GalNAc:polypeptide GalNAc transferase (ppGalNAcT; EC 2.4.1.41) catalyzes the first step in mucin-type O-glycosylation. To date, several members of this large enzyme family have been analyzed in detail. In this study we present cloning, expression and characterization of the first representative of this type of glycosyltransferase from mollusk origin, namely from Biomphalaria glabrata. The full length sequence of the respective gene was obtained by screening of a cDNA library using homology-based PCR. The entire gene codes for a protein consisting of 600 amino acids comprising the features of a typical type II membrane protein containing a cytoplasmic tail at the N-terminus, a transmembrane and a catalytic domain as well as a ricin-like motif at the C-terminus. Sequence comparison with ppGalNAcTs from various species revealed high similarities in terms of structural architecture. The enzyme is O-glycosylated but does not have any putative N-glycosylation sites. All four tested acceptor peptides were functional substrates, with Muc2 being the best one. Further biochemical parameters tested, confirmed a close relationship to the family of yet known ppGalNAcTs.

The O‑glycosylation mutant osteopontin alters lung cancer cell growth and migration in vitro and in vivo

Osteopontin (OPN) is an acidic, glycosylated and phosphorylated protein that plays an essential role in determining the aggressiveness and oncogenic potential of several types of cancer, including lung cancer. The OPN function is highly dependent on post-translational modification (PTM) and regulation of the processes that involve OPN can be mediated through glycosylation. However, the connection between OPN function and its O-glycosylation in lung cancer cells has yet to be investigated. In the present study, this issue was addressed by studying the effects of wild-type (WT) OPN and a triple mutant (TM) of OPN, which was mutated at three O-glycosylation sites in lung cancer cells. It was shown that OPN WT rather than OPN TM induced the OPN‑mediated signaling pathway. The OPN WT expression enhanced cap-dependent protein translation, NF-κB activity and glucose uptake, whereas a reduction was observed in cells treated with OPN TM. The results clearly demonstrated that unlike OPN WT, OPN TM did not increase lung cancer cell growth and migration both in vitro and in a xenograft mouse model. Thus, results of the present study suggested that targeting OPN by introducing OPN TM may be a good strategy for treating lung cancer.

Expression of core 3 synthase in human pancreatic cancer cells suppresses tumor growth and metastasis

Core 3-derived glycans, a major type of O-glycan expressed by normal epithelial cells of the gastrointestinal tract, are downregulated during malignancy because of loss of expression of functional β3-N-acetylglucosaminyltransferase-6 (core 3 synthase). We investigated the expression of core 3 synthase in normal pancreas and pancreatic cancer and evaluated the biological effects of re-expressing core 3 synthase in pancreatic cancer cells that had lost expression. We determined that pancreatic tumors and tumor cell lines have lost expression of core 3 synthase. Therefore, we re-expressed core 3 synthase in human pancreatic cancer cells (Capan-2 and FG) to investigate the contribution of core 3 glycans to malignant progression. Pancreatic cancer cells expressing core 3 synthase showed reduced in vitro cell proliferation, migration and invasion compared to vector control cells. Expression of core 3 O-glycans induced altered expression of β1 integrin, decreased activation of focal adhesion kinase, led to the downregulation of expression of several genes including REG1α and FGFR3 and altered lamellipodia formation. The addition of a GlcNAc residue by core 3 synthase leads to the extension of the tumor-associated Tn structure on MUC1. Orthotopic injection of FG cells expressing core 3 synthase into the pancreas of nude mice produced significantly smaller tumors and decreased metastasis to the surrounding tissues compared to vector control FG cells. These findings indicate that expression of core 3-derived O-glycans in pancreatic cancer cells suppresses tumor growth and metastasis through modulation of glycosylation of mucins and other cell surface and extracellular matrix proteins.

Aerosol delivery of lentivirus-mediated O-glycosylation mutant osteopontin suppresses lung tumorigenesis in K-ras (LA1) mice

BACKGROUND

Osteopontin (OPN) is a secreted glycophosphoprotein that has been implicated in the regulation of cancer development. The function of OPN is primarily regulated through post-translational modification such as glycosylation. As yet, however, the relationship between OPN glycosylation and lung cancer development has not been investigated. In this study, we addressed this issue by studying the effect of a triple mutant (TM) of OPN, which is mutated at three O-glycosylation sites, on lung cancer development in K-ras (LA1) mice, a murine model for human non-small cell lung cancer.

METHODS

Aerosolized lentivirus-based OPN TM was delivered into the lungs of K-ras (LA1) mice using a nose-only-inhalation chamber 3 times/wk for 4 wks. Subsequently, the effects of repeated delivery of OPN TM on lung tumorigenesis and its concomitant OPN-mediated signaling pathways were investigated.

RESULTS

Aerosol-delivered OPN TM inhibited lung tumorigenesis. In addition, the OPN-mediated Akt signaling pathway was inhibited. OPN TM also decreased NF-κB activity and the phosphorylation of 4E-BP1, while facilitating apoptosis in the lungs of K-ras (LA1) mice.

CONCLUSIONS

Our results show that aerosol delivery of OPN TM successfully suppresses lung cancer development in the K-ras (LA1) mouse model and, therefore, warrant its further investigation as a possible therapeutic strategy for non-small cell lung cancer.

Mucin-type O-glycosylation during development

Mucin-type O-glycosylation is an evolutionarily conserved protein modification present on membrane-bound and secreted proteins. Aberrations in O-glycosylation are responsible for certain human diseases and are associated with disease risk factors. Recent studies have demonstrated essential roles for mucin-type O-glycosylation in protein secretion, stability, processing, and function. Here, we summarize our current understanding of the diverse roles of mucin-type O-glycosylation during eukaryotic development. Appreciating how this conserved modification operates in developmental processes will provide insight into its roles in human disease and disease susceptibilities.

Control of mucin-type O-glycosylation: a classification of the polypeptide GalNAc-transferase gene family

Glycosylation of proteins is an essential process in all eukaryotes and a great diversity in types of protein glycosylation exists in animals, plants and microorganisms. Mucin-type O-glycosylation, consisting of glycans attached via O-linked N-acetylgalactosamine (GalNAc) to serine and threonine residues, is one of the most abundant forms of protein glycosylation in animals. Although most protein glycosylation is controlled by one or two genes encoding the enzymes responsible for the initiation of glycosylation, i.e. the step where the first glycan is attached to the relevant amino acid residue in the protein, mucin-type O-glycosylation is controlled by a large family of up to 20 homologous genes encoding UDP-GalNAc:polypeptide GalNAc-transferases (GalNAc-Ts) (EC 2.4.1.41). Therefore, mucin-type O-glycosylation has the greatest potential for differential regulation in cells and tissues. The GalNAc-T family is the largest glycosyltransferase enzyme family covering a single known glycosidic linkage and it is highly conserved throughout animal evolution, although absent in bacteria, yeast and plants. Emerging studies have shown that the large number of genes (GALNTs) in the GalNAc-T family do not provide full functional redundancy and single GalNAc-T genes have been shown to be important in both animals and human. Here, we present an overview of the GalNAc-T gene family in animals and propose a classification of the genes into subfamilies, which appear to be conserved in evolution structurally as well as functionally.

Glycosylation of α-dystroglycan: O-mannosylation influences the subsequent addition of GalNAc by UDP-GalNAc polypeptide N-acetylgalactosaminyltransferases

O-Linked glycosylation is a functionally and structurally diverse type of protein modification present in many tissues and across many species. α-Dystroglycan (α-DG), a protein linked to the extracellular matrix, whose glycosylation status is associated with human muscular dystrophies, displays two predominant types of O-glycosylation, O-linked mannose (O-Man) and O-linked N-acetylgalactosamine (O-GalNAc), in its highly conserved mucin-like domain. The O-Man is installed by an enzyme complex present in the endoplasmic reticulum. O-GalNAc modifications are initiated subsequently in the Golgi apparatus by the UDP-GalNAc polypeptide N-acetylgalactosaminyltransferase (ppGalNAc-T) enzymes. How the presence and position of O-Man influences the action of the ppGalNAc-Ts on α-DG and the distribution of the two forms of glycosylation in this domain is not known. Here, we investigated the interplay between O-Man and the addition of O-GalNAc by examining the activity of the ppGalNAc-Ts on peptides and O-Man-containing glycopeptides mimicking those found in native α-DG. These synthetic glycopeptides emulate intermediate structures, not otherwise readily available from natural sources. Through enzymatic and mass spectrometric methods, we demonstrate that the presence and specific location of O-Man can impact either the regional exclusion or the site of O-GalNAc addition on α-DG, elucidating the factors contributing to the glycosylation patterns observed in vivo. These results provide evidence that one form of glycosylation can influence another form of glycosylation in α-DG and suggest that in the absence of proper O-mannosylation, as is associated with certain forms of muscular dystrophy, aberrant O-GalNAc modifications may occur and could play a role in disease presentation.

UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases: completion of the family tree

The formation of mucin-type O-glycans is initiated by an evolutionarily conserved family of enzymes, the UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts). The human genome encodes 20 transferases; 17 of which have been characterized functionally. The complexity of the GalNAc-T family reflects the differential patterns of expression among the individual enzyme isoforms and the unique substrate specificities which are required to form the dense arrays of glycans that are essential for mucin function. We report the expression patterns and enzymatic activity of the remaining three members of the family and the further characterization of a recently reported isoform, GalNAc-T17. One isoform, GalNAcT-16 that is most homologous to GalNAc-T14, is widely expressed (abundantly in the heart) and has robust polypeptide transferase activity. The second isoform GalNAc-T18, most similar to GalNAc-T8, -T9 and -T19, completes a discrete subfamily of GalNAc-Ts. It is widely expressed and has low, albeit detectable, activity. The final isoform, GalNAc-T20, is most homologous to GalNAc-T11 but lacks a lectin domain and has no detectable transferase activity with the panel of substrates tested. We have also identified and characterized enzymatically active splice variants of GalNAc-T13 that differ in the sequence of their lectin domain. The variants differ in their affinities for glycopeptide substrates. Our findings provide a comprehensive view of the complexities of mucin-type O-glycan formation and provide insight into the underlying mechanisms employed to heavily decorate mucins and mucin-like domains with carbohydrate.

Multiple members of the UDP-GalNAc: polypeptide N-acetylgalactosaminyltransferase family are essential for viability in Drosophila

Mucin-type O-glycosylation represents a major form of post-translational modification that is conserved across most eukaryotic species. This type of glycosylation is initiated by a family of enzymes (GalNAc-Ts in mammals and PGANTs in Drosophila) whose members are expressed in distinct spatial and temporal patterns during development. Previous work from our group demonstrated that one member of this family is essential for viability and another member modulates extracellular matrix composition and integrin-mediated cell adhesion during development. To investigate whether other members of this family are essential, we employed RNA interference (RNAi) to each gene in vivo. Using this approach, we identified 4 additional pgant genes that are required for viability. Ubiquitous RNAi to pgant4, pgant5, pgant7, or the putative glycosyltransferase CG30463 resulted in lethality. Tissue-specific RNAi was also used to define the specific organ systems and tissues in which each essential family member is required. Interestingly, each essential pgant had a unique complement of tissues in which it was required. Additionally, certain tissues (mesoderm, digestive system, and tracheal system) required more than one pgant, suggesting unique functions for specific enzymes in these tissues. Expanding upon our RNAi results, we found that conventional mutations in pgant5 resulted in lethality and specific defects in specialized cells of the digestive tract, resulting in loss of proper digestive system acidification. In summary, our results highlight essential roles for O-glycosylation and specific members of the pgant family in many aspects of development and organogenesis.

Location, location, location: new insights into O-GalNAc protein glycosylation

O-GalNAc glycosylation of proteins confers essential structural, protective and signaling roles in eumetazoans. Addition of O-glycans onto proteins is an extremely complex process that regulates both sites of attachment and the types of oligosaccharides added. Twenty distinct polypeptide GalNAc-transferases (GalNAc-Ts) initiate O-glycosylation and fine-tuning their expression provides a mechanism for regulating this action. Recently, a new mode of regulation has emerged where activation of Src kinase selectively redistributes Golgi-localized GalNAc-Ts to the ER. This relocalization results in a strong increase in the density of O-glycan decoration. In this review, we discuss how different mechanisms can regulate the number and the types of O-glycans decorating proteins. In addition, we speculate how Src-dependent relocation of GalNAc-Ts could play an important role in cancerous cellular transformation.

Alterations in glycosylation as biomarkers for cancer detection

Glycoconjugates constitute a major class of biomolecules which include glycoproteins, glycosphingolipids and proteoglycans. Glycans are involved in several physiological and pathological conditions, such as host-pathogen interactions, cell differentiation, migration, tumour invasion and metastisation, cell trafficking and signalling. Cancer is associated with glycosylation alterations in glycoproteins and glycolipids. This review describes various aspects of protein glycosylation with the focus on alterations associated with human cancer. The application of these glycosylation modifications as biomarkers for cancer detection in tumour tissues and serological assays is summarised.

The role of mucin-type O-glycans in eukaryotic development

Newly emerging genetic studies have revealed that a subset of the family of glycosyltransferases responsible for the formation of mucin-type O glycans is essential for normal development. As additional genetic, biochemical and physical tools are developed to interrogate the complex structure and surface location of this under-studied class of carbohydrate, no doubt additional roles will be elucidated.

Isoform-specific O-glycosylation of osteopontin and bone sialoprotein by polypeptide N-acetylgalactosaminyltransferase-1

Mucin-type O-glycan biosynthesis is regulated by the family of UDP-GalNAc polypeptide:N-acetylgalactosaminlytransfersases (ppGalNAcTs) that catalyzes the first step in the pathway by transferring GalNAc to Ser or Thr residues in a protein from the sugar donor UDP-GalNAc. Because not all Ser/Thr residues are glycosylated, rules must exist that signal which hydroyxamino acids acquire sugar. To date, no universal consensus signal has emerged. Therefore, strategies to deduce the subset of proteins that will be glycosylated by distinct ppGalNAcTs must be developed. Mucin-type O-glycoproteins are present abundantly in bone, where we found multiple ppGalNAcT isoforms, including ppGalNAcT-1, to be highly expressed. Thus, we compared glycoproteins expressed in wild-type and Galnt1-null mice to identify bone-associated proteins that were glycosylated in a ppGalNAcT-1-dependent manner. A reduction in the apparent molecular masses of two SIBLINGs (small integrin binding ligand N-linked glycoproteins), osteopontin (OPN) and bone sialoprotein (BSP) in the Galnt1-null mice relative to those of the wild-type was observed. Several synthetic peptides derived from OPN and BSP sequences were designed to include either known or predicted (in silico) glycosylation sites. In vitro glycosylation assays of these peptides with recombinant ppGalNAcT-1, ppGalNAcT-2, or ppGalNAcT-3 demonstrated that both SIBLINGs contained Thr/Ser residues that were preferentially glycosylated by ppGalNAcT-1. In addition, lysates prepared from wild-type, but not those from Galnt1-null derived osteoblasts, could glycosylate these peptides efficiently, suggesting that OPN and BSP contain sites that are specific for ppGalNAcT-1. Our study presents a novel and systematic approach for identification of isoform-specific substrates of the ppGalNAcT family and suggests ppGalNAcT-1 to be indispensable for O-glycosylation at specific sites of the bone glycoproteins OPN and BSP.

Glycopeptide-preferring polypeptide GalNAc transferase 10 (ppGalNAc T10), involved in mucin-type O-glycosylation, has a unique GalNAc-O-Ser/Thr-binding site in its catalytic domain not found in ppGalNAc T1 or T2

Mucin-type O-gly co sy la tion is initiated by a large family of UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferases (ppGalNAc Ts) that transfer GalNAc from UDP-GalNAc to the Ser and Thr residues of polypeptide acceptors. Some members of the family prefer previously gly co sylated peptides (ppGalNAc T7 and T10), whereas others are inhibited by neighboring gly co sy la tion (ppGalNAc T1 and T2). Characterizing their peptide and glycopeptide substrate specificity is critical for understanding the biological role and significance of each isoform. Utilizing a series of random peptide and glycopeptide substrates, we have obtained the peptide and glycopeptide specificities of ppGalNAc T10 for comparison with ppGalNAc T1 and T2. For the glycopeptide substrates, ppGalNAc T10 exhibited a single large preference for Ser/Thr-O-GalNAc at the +1 (C-terminal) position relative to the Ser or Thr acceptor site. ppGalNAc T1 and T2 revealed no significant enhancements suggesting Ser/Thr-O-GalNAc was inhibitory at most positions for these isoforms. Against random peptide substrates, ppGalNAc T10 revealed no significant hydrophobic or hydrophilic residue enhancements, in contrast to what has been reported previously for ppGalNAc T1 and T2. Our results reveal that these transferases have unique peptide and glycopeptide preferences demonstrating their substrate diversity and their likely roles ranging from initiating transferases to filling-in transferases.

A mucin-type O-glycosyltransferase modulates cell adhesion during Drosophila development

Cell-cell and cell-matrix adhesion are crucial during many stages of eukaryotic development. Here, we provide the first example that mucin-type O-linked glycosylation is involved in a developmentally regulated cell adhesion event in Drosophila melanogaster. Mutations in one member of the evolutionarily conserved family of enzymes that initiates O-linked glycosylation alter epithelial cell adhesion in the Drosophila wing blade. A transposon insertion mutation in pgant3 or RNA interference to pgant3 resulted in blistered wings, a phenotype characteristic of genes involved in integrin-mediated cell interactions. Expression of wild type pgant3 in the mutant background rescued the wing blistering phenotype, whereas expression of another family member (pgant35A) did not, revealing a unique requirement for pgant3. pgant3 mutants displayed reduced O-glycosylation along the basal surface of larval wing imaginal discs, which was restored with wild type pgant3 expression, suggesting that reduced glycosylation of basal proteins is responsible for disruption of adhesion in the adult wing blade. Glycosylation reactions demonstrated that PGANT3 glycosylates certain extracellular matrix (ECM) proteins. Immunoprecipitation experiments revealed that PGANT3 glycosylates tiggrin, an ECM protein known to bind integrin. We propose that this glycosyltransferase is uniquely responsible for glycosylating tiggrin in the wing disc, thus modulating proper cell adhesion through integrin-ECM interactions. This study provides the first evidence for the role of O-glycosylation in a developmentally regulated, integrin-mediated, cell adhesion event and reveals a novel player in wing blade formation during Drosophila development.

Conservation of peptide acceptor preferences between Drosophila and mammalian polypeptide-GalNAc transferase ortholog pairs

UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltrans- ferases (ppGalNAc Ts) comprise a large family of glycosyltransferases that initiate mucin-type protein O-glycosylation, transferring alpha-GalNAc to Thr and Ser residues of polypeptide acceptors. Families of ppGalNAc Ts are found across diverse eukaryotes with orthologs identifiable from mammals to single-cell organisms. The peptide substrate specificity and specific protein targets of the individual ppGalNAc T family members remain poorly understood. Previously, we reported a series of oriented random peptide substrate libraries for quantitatively determining the peptide substrate specificities of the mammalian ppGalNAc T1 and T2 (Gerken TA, Raman J, Fritz TA, Jamison O. 2006. Identification of common and unique peptide substrate preferences for the UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferases T1 & T2 (ppGalNAc T1 & T2) derived from oriented random peptide substrates. J Biol Chem. 281:32403-32416). With these substrates, previously unknown features of the transferases were revealed. Utilizing these and a new lengthened set of random peptides, studies have now been performed on PGANT5 and PGANT2, the Drosophila orthologs of T1 and T2. The results from these studies suggest that the major peptide substrate determinants for these transferases are contained within 2 to 3 residues flanking the site of glycosylation. It is further found that the mammalian and fly T1 orthologs display very similar peptide substrate preferences, while the T2 orthologs are nearly indistinguishable, suggesting similar peptide preferences amongst orthologous pairs have been maintained across evolution. This conclusion is further supported by sequence homology comparisons of each of the transferase orthologs, showing that the peptide substrate and UDP binding site residues are more highly conserved between species relative to their remaining catalytic and lectin domain residues.

Expression of UDP-N-acetyl-D-galactosamine: polypeptide N-acetylgalactosaminyltransferase-6 in gastric mucosa, intestinal metaplasia, and gastric carcinoma

Aberrant mucin O-glycosylation is often observed in cancer and is characterized by the expression of immature simple mucin-type carbohydrate antigens. UDP-N-acetyl-d-galactosamine:polypeptide N-acetylgalactosaminyltransferase-6 (ppGalNAc-T6) is one of the enzymes responsible for the initial step in O-glycosylation. This study evaluated the expression of ppGalNAc-T6 in human gastric mucosa, intestinal metaplasia, and gastric carcinomas. Our results showed that ppGalNAc-T6 is expressed in normal gastric mucosa and in intestinal metaplasia. A heterogeneous expression and staining pattern for this enzyme was observed in gastric carcinomas. ppGalNAc-T6 was expressed in 79% of the cases, and its expression level was associated with the presence of venous invasion. Our results provide evidence that ppGalNAc-T6 is an IHC marker associated with venous invasion in gastric carcinoma and may contribute to the understanding of the molecular mechanisms that underlie aberrant glycosylation in gastric carcinogenesis and in gastric carcinoma.

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.

Glycosyltransferases: structures, functions, and mechanisms

Glycosyltransferases catalyze glycosidic bond formation using sugar donors containing a nucleoside phosphate or a lipid phosphate leaving group. Only two structural folds, GT-A and GT-B, have been identified for the nucleotide sugar-dependent enzymes, but other folds are now appearing for the soluble domains of lipid phosphosugar-dependent glycosyl transferases. Structural and kinetic studies have provided new insights. Inverting glycosyltransferases utilize a direct displacement S(N)2-like mechanism involving an enzymatic base catalyst. Leaving group departure in GT-A fold enzymes is typically facilitated via a coordinated divalent cation, whereas GT-B fold enzymes instead use positively charged side chains and/or hydroxyls and helix dipoles. The mechanism of retaining glycosyltransferases is less clear. The expected two-step double-displacement mechanism is rendered less likely by the lack of conserved architecture in the region where a catalytic nucleophile would be expected. A mechanism involving a short-lived oxocarbenium ion intermediate now seems the most likely, with the leaving phosphate serving as the base.

Recent insights into the biological roles of mucin-type O-glycosylation

In this special issue of the Glycoconjugate Journal focusing on glycosciences and development, we summarize recent advances in our understanding of the role of mucin-type O-glycans in development and disease. The presence of this widespread protein modification has been known for decades, yet identification of its biological functions has been hampered by the redundancy and complexity of the enzyme family controlling the initiation of O-glycosylation, as well as the diversity of extensions of the core sugar. Recent studies in organisms as diverse as mammals and Drosophila have yielded insights into the function of this highly abundant and evolutionarily-conserved protein modification. Gaining an understanding of mucin-type O-glycans in these diverse systems will elucidate crucial conserved processes underlying many aspects of development and homeostasis.

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.

Function of conserved aromatic residues in the Gal/GalNAc-glycosyltransferase motif of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase 1

UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (GalNAc transferases), which initiate mucin-type O-glycan biosynthesis, have broad acceptor substrate specificities, and it is still unclear how they recognize peptides with different sequences. To increase our understanding of the catalytic mechanism of GalNAc-T1, one of the most ubiquitous isozymes, we studied the effect of substituting six conserved aromatic residues in the highly conserved Gal/GalNAc-glycosyltransferase motif with leucine on the catalytic properties of the enzyme. Our results indicate that substitutions of Trp302 and Phe325 have little impact on enzyme function and that substitutions of Phe303 and Tyr309 could be made with only limited impact on the interaction(s) with donor and/or acceptor substrates. By contrast, Trp328 and Trp316 are essential residues for enzyme functions, as substitution with leucine, at either site, led to complete inactivation of the enzymes. The roles of these tryptophan residues were further analyzed by evaluating the impact of substitutions with additional amino acids. All evaluated substitutions at Trp328 resulted in enzymes that were completely inactive, suggesting that the invariant Trp328 is essential for enzymatic activity. Trp316 mutant enzymes with nonaromatic replacements were again completely inactive, whereas two mutant enzymes containing a different aromatic amino acid, at position 316, showed low catalytic activity. Somewhat surprisingly, a kinetic analysis revealed that these two amino acid substitutions had a moderate impact on the enzyme's affinity for the donor substrate. By contrast, the drastically reduced affinity of the Trp316 mutant enzymes for the acceptor substrates suggests that Trp316 is important for this interaction.

The lectin domains of polypeptide GalNAc-transferases exhibit carbohydrate-binding specificity for GalNAc: lectin binding to GalNAc-glycopeptide substrates is required for high density GalNAc-O-glycosylation

Initiation of mucin-type O-glycosylation is controlled by a large family of UDP GalNAc:polypeptide N-acetylgalactosaminyltransferases (GalNAc-transferases). Most GalNAc-transferases contain a ricin-like lectin domain in the C-terminal end, which may confer GalNAc-glycopeptide substrate specificity to the enzyme. We have previously shown that the lectin domain of GalNAc-T4 modulates its substrate specificity to enable unique GalNAc-glycopeptide specificities and that this effect is selectively inhibitable by GalNAc; however, direct evidence of carbohydrate binding of GalNAc-transferase lectins has not been previously presented. Here, we report the direct carbohydrate binding of two GalNAc-transferase lectin domains, GalNAc-T4 and GalNAc-T2, representing isoforms reported to have distinct glycopeptide activity (GalNAc-T4) and isoforms without apparent distinct GalNAc-glycopeptide specificity (GalNAc-T2). Both lectins exhibited specificity for binding of free GalNAc. Kinetic and time-course analysis of GalNAc-T2 demonstrated that the lectin domain did not affect transfer to initial glycosylation sites, but selectively modulated velocity of transfer to subsequent sites and affected the number of acceptor sites utilized. The results suggest that GalNAc-transferase lectins serve to modulate the kinetic properties of the enzymes in the late stages of the initiation process of O-glycosylation to accomplish dense or complete O-glycan occupancy.

Identification of common and unique peptide substrate preferences for the UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferases T1 and T2 derived from oriented random peptide substrates

A large family of UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferases (ppGalNAc Ts) catalyzes the first step of mucin-type protein O-glycosylation by transferring GalNAc to serine and threonine residues of acceptor polypeptides. The acceptor peptide substrate specificity and specific protein targets of the individual ppGalNAc T family members remain poorly characterized and poorly understood, despite the fact that mutations in two individual isoforms are deleterious to man and the fly. In this work a series of oriented random peptide substrate libraries, based on the GAGAXXXTXXXAGAGK sequence motif (where X = randomized positions), have been used to obtain the first comprehensive determination of the peptide substrate specificities of the mammalian ppGalNAc T1 and T2 isoforms. ppGalNAc T-glycosylated random peptides were isolated by lectin affinity chromatography, and transferase amino acid preferences were determined by Edman amino acid sequencing. The results reveal common and unique position-sensitive features for both transferases, consistent with previous reports of the preferences of ppGalNAc T1 and T2. The random peptide substrates also reveal additional specific features that have never been described before that are consistent with the x-ray crystal structures of the two transferases and furthermore are reflected in a data base analysis of in vivo O-glycosylation sites. By using the transferase-specific preferences, optimum and selective acceptor peptide substrates have been generated for each transferase. This approach represents a relatively complete, facile, and reproducible method for obtaining ppGalNAc T peptide substrate specificity. Such information will be invaluable for identifying isoform-specific peptide acceptors, creating isoform-specific substrates, and predicting O-glycosylation sites.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update covering the period 1999-2000

This review describes the use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates and continues coverage of the field from the previous review published in 1999 (D. J. Harvey, Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates, 1999, Mass Spectrom Rev, 18:349-451) for the period 1999-2000. As MALDI mass spectrometry is acquiring the status of a mature technique in this field, there has been a greater emphasis on applications rather than to method development as opposed to the previous review. The present review covers applications to plant-derived carbohydrates, N- and O-linked glycans from glycoproteins, glycated proteins, mucins, glycosaminoglycans, bacterial glycolipids, glycosphingolipids, glycoglycerolipids and related compounds, and glycosides. Applications of MALDI mass spectrometry to the study of enzymes acting on carbohydrates (glycosyltransferases and glycosidases) and to the synthesis of carbohydrates, are also covered.

The chemistry and biology of mucin-type O-linked glycosylation

Mucin-type O-linked glycosylation is a fundamental post-translational modification that is involved in a variety of important biological processes. However, the lack of chemical tools to study mucin-type O-linked glycosylation has hindered our molecular understanding of O-linked glycans in many biological contexts. The review discusses the significance of mucin-type O-linked glycosylation initiated by the polypeptide N-acetylgalactosaminyltransferases in biology and development of chemical tools to study these enzymes and their substrates.

Cloning and expression of a brain-specific putative UDP-GalNAc: polypeptide N-acetylgalactosaminyltransferase gene

We isolated a rat cDNA clone and its human orthologue, which are most homologous to UDP-GalNAc: polypeptide N-acetylgalactosaminyltransferase 9, by homology-based PCR from brain. Nucleotide sequence analysis of these putative GalNAc-transferases (designated pt-GalNAc-T) showed that they contained structural features characteristic of the GalNAc-transferase family. It was also found that human pt-GalNAc-T was identical to the gene WBSCR17, which is reported to be in the critical region of patients with Williams-Beuren Syndrome, a neurodevelopmental disorder, and to be predominantly expressed in brain and heart. In order to investigate the expression of pt-GalNAc-T in brain in more detail, we first examined that of human pt-GalNAc-T by Northern blot analysis and found the expression of the 5.0-kb mRNA to be most abundant in cerebral cortex with somewhat less abundant in cellebellum. The expression of rat pt-GalNAc-T was investigated more extensively. The brain-specific expression of 2.0-kb and 5.0-kb transcripts was demonstrated by Northern blot analysis. In situ hybridization in the adult brain revealed high levels of expression in cerebellum, hippocampus, thalamus, and cerebral cortex. Moreover, observation at high magnification revealed the expression to be associated with neurons, but not with glial cells. Analysis of the rat embryos also demonstrated that rat pt-GalNAc-T was expressed in the nervous system, including in the diencephalons, cerebellar primordium, and dorsal root ganglion. However, recombinant human pt-GalNAc-T, which was expressed in insect cells, did not glycosylate several peptides derived from mammalian mucins, suggesting that it may have a strict substrate specificity. The brain-specific expression of pt-GalNAc-T suggested its involvement in brain development, through O-glycosylation of proteins in the neurons.

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.

The expression pattern of UDP-N-acetyl-alpha-D-galactosamine-polypeptide N-acetyl-galactosaminyl transferase-3 in squamous cell carcinoma of the esophagus

OBJECTIVE

UDP-N-acetyl-alpha-D-galactosamine-polypeptide N-acetylgalactosaminyl transferase-3 (GalNAc-T3) regulates the initial glycosylation of mucin-type O-linked proteins. Although a different expression of GalNAc-T3 has been reported in various cancers, the expression has not been characterized in squamous cell carcinoma (SCC) of the esophagus.

METHODS

We have also evaluated the expression of this enzyme in surgically resected esophageal mucosa. By immunohistochemical staining using a specific antibody, we evaluated the expression of GalNAc-T3 in 66 esophageal SCC and 28 dysplasia samples, and analyzed the relationship between the expression of GalNAc-T3 and clinicopathological features.

RESULTS

GalNAc-T3 was positively detected in the majority of the cases of SCC, but not in dysplasia as well as the normal counterparts in resected esophagus. GalNAc-T3 was determined to be positive in 37 cases (68.5%) of differentiated carcinomas, but only in 4 cases (33.3%) of undifferentiated carcinomas (p < 0.05). Hematogeneous metastasis was observed in 13 of 41 (31.7%) GalNAc-T3-positive tumors, which was significantly more frequent than in negative tumors (2/25, 8%; p < 0.05). The number of metastatic nodes was significantly higher in tumors with GalNAc-T3-positive than GalNAc-negative expression (4.2 +/- 3.2 vs. 2.8 +/- 1.3, p < 0.05). The survival rate tended to be lower for patients with GalNAc-T3-positive tumors, although the difference was not statistically significant (p = 0.18).

CONCLUSION

GalNAc-T3 may play a positive role in the process of carcinogenesis and progression in esophageal SCC. Functional inhibition of GalNAc-T3 may be effective for the prevention and treatment of esophageal SCC.

Cloning, expression and properties of porcine trachea UDP-galnac: polypeptide N-acetylgalactosaminyl transferase

A UDP-GalNAc:polypeptide N-acetyl-galactosaminyl transferase which catalyses the transfer of GalNAc from UDP-GalNAc to serine and threonine residues in mucin polypeptide chains was purified to homogeneity from swine trachea epithelium (Mendicino J, Sangadala S: Mol Cell Biochem 185: 135-145, 1998). Peptides obtained by proteolysis of the purified enzyme were isolated, sequenced and used to prepare degenerate oligonucleotide primers. Amplified segments of a gene encoding GalNAc transferase were synthesised using the primers and a swine trachea epithelial cDNA library. Selected cDNA fragments were then used to screen the cDNA library, and a clone containing an open reading frame encoding 559 amino acids was isolated. The predicted amino acid sequence contains type II transmembrane region, three potential N-glycosylation sites as well as all of the isolated peptide sequences. The nucleotide sequence and predicted primary protein structure of the transferase were very similar to those of type T-1 GalNAc transferases. The isolated clone was transiently expressed in COS 7 cells and the recombinant enzyme, which contained an N-terminal hexa-histidine tag, was purified to homogeneity and its enzymatic properties were examined. The Vmax of the recombinant enzyme, 2.08 micromol/(min mg), was nearly the same as the native enzyme, 2.12 micromol/(min mg), when assayed with partially deglycosylated mucins as glycosyl acceptors. Both enzymes showed much higher activities when assayed with peptides prepared by limited acid hydrolysis of incompletely deglycosylated Cowper's gland, swine, and human respiratory mucins and tryptic peptides isolated from deglycosylated mucin polypeptide chains. However, as noted earlier (Mendicino J, Sangadala S: Mol Cell Biochem 185: 135-145, 1998), these enzymes showed very little activity with completely deglycosylated mucin polypeptide chains. When completely deglycosylated polypeptide chains were partially glycosylated by incubation with microsome preparations they were again good glycosyl acceptors for the T1-GalNAc transferases isolated from swine trachea. These results show for the first time that multiple isoforms of GalNAc transferases acting in sequence may be required for the complete O-glycosylation of mucin polypeptide chains, and those acting on the nacent polypeptide chain synthesize intermediates which can serve as glycosyl acceptors for other isoforms of the enzyme.

The beginnings of mucin biosynthesis: the crystal structure of UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferase-T1

UDP-GalNAc:polypeptide alpha-N-acetylgalactosaminyltransferases (ppGaNTases) initiate the formation of mucin-type, O-linked glycans by catalyzing the transfer of alpha-N-acetylgalactosamine from UDP-GalNAc to Ser or Thr residues of core proteins to form the Tn antigen (GalNAc-alpha-1-O-Ser/Thr). ppGaNTases are unique among glycosyltransferases in containing a C-terminal lectin domain. We present the x-ray crystal structure of a ppGaNTase, murine ppGaNTase-T1, and show that it folds to form distinct catalytic and lectin domains. The association of the two domains forms a large cleft in the surface of the enzyme that contains a Mn2+ ion complexed by invariant D209 and H211 of the "DXH" motif and by invariant H344. Each of the three potential lectin domain carbohydrate-binding sites (alpha, beta, and gamma) is located on the active-site face of the enzyme, suggesting a mechanism by which the transferase may accommodate multiple conformations of glycosylated acceptor substrates. A model of a mucin 1 glycopeptide substrate bound to the enzyme shows that the spatial separation between the lectin alpha site and a modeled active site UDP-GalNAc is consistent with the in vitro pattern of glycosylation observed for this peptide catalyzed by ppGaNTase-T1. The structure also provides a template for the larger ppGaNTase family, and homology models of several ppGaNTase isoforms predict dramatically different surface chemistries consistent with isoform-selective acceptor substrate recognition.

Prediction, conservation analysis, and structural characterization of mammalian mucin-type O-glycosylation sites

O-GalNAc-glycosylation is one of the main types of glycosylation in mammalian cells. No consensus recognition sequence for the O-glycosyltransferases is known, making prediction methods necessary to bridge the gap between the large number of known protein sequences and the small number of proteins experimentally investigated with regard to glycosylation status. From O-GLYCBASE a total of 86 mammalian proteins experimentally investigated for in vivo O-GalNAc sites were extracted. Mammalian protein homolog comparisons showed that a glycosylated serine or threonine is less likely to be precisely conserved than a nonglycosylated one. The Protein Data Bank was analyzed for structural information, and 12 glycosylated structures were obtained. All positive sites were found in coil or turn regions. A method for predicting the location for mucin-type glycosylation sites was trained using a neural network approach. The best overall network used as input amino acid composition, averaged surface accessibility predictions together with substitution matrix profile encoding of the sequence. To improve prediction on isolated (single) sites, networks were trained on isolated sites only. The final method combines predictions from the best overall network and the best isolated site network; this prediction method correctly predicted 76% of the glycosylated residues and 93% of the nonglycosylated residues. NetOGlyc 3.1 can predict sites for completely new proteins without losing its performance. The fact that the sites could be predicted from averaged properties together with the fact that glycosylation sites are not precisely conserved indicates that mucin-type glycosylation in most cases is a bulk property and not a very site-specific one. NetOGlyc 3.1 is made available at www.cbs.dtu.dk/services/netoglyc.

Characterization of a UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase with an unusual lectin domain from the platyhelminth parasite Echinococcus granulosus

As part of a general project aimed at elucidating the initiation of mucin-type O-glycosylation in helminth parasites, we have characterized a novel ppGalNAc-T (UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase) from the cestode Echinococcus granulosus (Eg-ppGalNAc-T1). A full-length cDNA was isolated from a library of the tissue-dwelling larval stage of the parasite, and found to code for a 654-amino-acid protein containing all the structural features of ppGalNAc-Ts. Functional characterization of a recombinant protein lacking the transmembrane domain showed maximal activity at 28 degrees C, in the range 6.5-7.5 pH units and in the presence of Cu2+. In addition, it transferred GalNAc to a broad range of substrate peptides, derived from human mucins and O-glycosylated parasite proteins, including acceptors containing only serine or only threonine residues. Interestingly, the C-terminal region of Eg-ppGalNAc-T1 bears a highly unusual lectin domain, considerably longer than the one from other members of the family, and including only one of the three ricin B repeats generally present in ppGalNAc-Ts. Furthermore, a search for conserved domains within the protein C-terminus identified a fragment showing similarity to a recently defined domain, specialized in the binding of organic phosphates (CYTH). The role of the lectin domain in the determination of the substrate specificity of these enzymes suggests that Eg-ppGalNAc-T1 would be involved in the glycosylation of a special type of substrate. Analysis of the tissue distribution by in situ hybridization and immunohistochemistry revealed that this transferase is expressed in the hydatid cyst wall and the subtegumental region of larval worms. Therefore it could participate in the biosynthesis of O-glycosylated parasite proteins exposed at the interface between E. granulosus and its hosts.

Functional characterization of a novel Toxoplasma gondii glycosyltransferase: UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase-T3

We report the functional characterization of a new UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (ppGalNAc-T) (EC 2.4.1.41) from the human disease-causing parasite, Toxoplasma gondii. This glycosyltransferase is denoted as T. gondii ppGalNAc-T3. These enzymes are responsible for the initial step of mucin-type O-glycosylation: the transfer of GalNAc from the UDP-GalNAc nucleotide sugar donor onto a peptide acceptor. Following an in silico analysis of the publicly available T. gondii DNA database, we used molecular biology approaches to identify and isolate the cDNA encoding this enzyme. The resulting type II membrane protein contains N-terminal cytoplasmic, transmembrane, and C-terminal lumenal domains. Conceptual translation of the cDNA sequence also reveals a stem region and the presence of several important sequence motifs. When the recombinant construct was expressed in stably transfected Drosophila melanogaster S2 cells, the purified protein exhibited glycosyltransferase activity in vitro against glycopeptide, but not "naked" peptide, acceptors. In addition, using reverse transcriptase-PCR, T. gondii ppGalNAc-T3 mRNA was equivalently expressed during the tachyzoite and bradyzoite developmental stages. The identification of T. gondii ppGalNAc-T3 as a functional "follow-up" glycopeptide glycosyltransferase further confirms that this human parasite has its own enzymatic O-glycosylation machinery.

Characterization of a novel human UDP-GalNAc transferase, pp-GalNAc-T15

We have cloned, expressed and characterized a novel member of the human UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (pp-GalNAc-T) family, pp-GalNAc-T15. The pp-GalNAc-T15 transcript was ubiquitously expressed in human tissues. Recombinant pp-GalNAc-T15 transferred N-acetylgalactosamine (GalNAc) toward a panel of mucin-derived peptide substrates in vitro. Although pp-GalNAc-T15 showed significantly less catalytic activity than pp-GalNAc-T2, T15 transferred up to seven GalNAcs to the Muc5AC peptide, while T2 transferred up to five GalNAcs. These results clearly indicated that pp-GalNAc-T15 is a novel member of the human pp-GalNAc-T family with unique catalytic activity.

Small Molecule Inhibitors of Mucin-Type O-Linked Glycosylation from a Uridine-Based Library

The polypeptide N-acetyl-alpha-galactosaminyltransferases (ppGalNAcTs, also abbreviated ppGaNTases) initiate mucin-type O-linked glycosylation and therefore play pivotal roles in cell-cell communication and protection of tissues. In order to develop new tools for studying mucin-type O-linked glycosylation, we screened a 1338 member uridine-based library to identify small molecule inhibitors of ppGalNAcTs. Using a high-throughput enzyme-linked lectin assay (ELLA), two inhibitors of murine ppGalNAcT-1 (K(I) approximately 8 microM) were identified that also inhibit several other members of the family. The compounds did not inhibit other mammalian glycosyltransferases or nucleotide sugar utilizing enzymes, suggesting selectivity for the ppGalNAcTs. Treatment of cells with the compounds abrogated mucin-type O-linked glycosylation but not N-linked glycosylation and also induced apoptosis. These uridine analogs represent the first generation of chemical tools to study the functions of mucin-type O-linked glycosylation.

Characterization of a Novel Polypeptide N-Acetylgalactosaminyltransferase (dGalNAc-T3) from Drosophila

Polypeptide N-acetylgalactosaminyltransferases (GalNAc-transferases) catalyze the initial reaction of mucin-type O-glycosylation. Here, we report the first biochemical characterization of one of the Drosophila GalNAc-transferases, dGalNAc-T3. This enzyme retains conserved motifs essential for the catalytic activity, but is a novel isozyme in that it has several inserted sequences in its lectin-like domain. Northern hybridization analysis of this isozyme identified a 2.5-kb mRNA in Drosophila larva. Biochemical characterization was carried out using the recombinant soluble dGalNAc-T3 expressed in COS7 cells. dGalNAc-T3, which required Mn2+ for the activity, had a pH optimum ranging from pH 7.5 to 8.5, and glycosylated most effectively at 29-33 degrees C. Its Km for UDP-GalNAc was 10.7 microM, which is as low as that of mammalian isozymes. dGalNAc-T3 glycosylated the peptides containing a sequence of XTPXP or TTAAP most efficiently. The enzyme was irreversibly inhibited by p-chloromercuriphenylsulphonic acid, indicating the presence of essential Cys residues for the activity.