O-glycosylation mimics N-glycosylation in the 16-kDa fragment of bovine pro-opiomelanocortin. The major O-glycan attached to Thr-45 carries SO4-4GalNAc beta 1-4GlcNAc beta 1-, which is the archetypal non-reducing epitope in the N-glycans of pituitary glycohormones

Trefoil factor family domains represent highly efficient conformational determinants for N-linked N,N'-di-N-acetyllactosediamine (LacdiNAc) synthesis

The disaccharide N,N'-di-N-acetyllactose diamine (LacdiNAc, GalNAcβ1-4GlcNAcβ) is found in a limited number of extracellular matrix glycoproteins and neuropeptide hormones indicating a protein-specific transfer of GalNAc by the glycosyltransferases β4GalNAc-T3/T4. Whereas previous studies have revealed evidence for peptide determinants as controlling elements in LacdiNAc biosynthesis, we report here on an entirely independent conformational control of GalNAc transfer by single TFF (Trefoil factor) domains as high stringency determinants. Human TFF2 was recombinantly expressed in HEK-293 cells as a wild type full-length probe (TFF2-Fl, containing TFF domains P1 and P2), as single P1 or P2 domain probes, as a series of Cys/Gly mutant forms with aberrant domain structures, and as a double point-mutated probe (T68Q/F59Q) lacking aromatic residues within a hydrophobic patch. The N-glycosylation probes were analyzed by mass spectrometry for their glycoprofiles. In agreement with natural gastric TFF2, the recombinant full-length and single domain probes expressed nearly exclusively fucosylated LacdiNAc on bi-antennary complex-type chains indicating that a single TFF domain was sufficient to induce transfer of this modification. Contrasting to this, the Cys/Gly mutants showed strongly reduced LacdiNAc levels and instead preponderant LacNAc expression. The probe with point mutations of two highly conserved aromatic residues in loop 3 (T68Q/F59Q) revealed that these are essential determinant components, as the probe lacked LacdiNAc expression. The structural features of the LacdiNAc-inducing determinant on human TFF2 are discussed on the basis of crystal structures of porcine TFF2, and a series of extracellular matrix-related LacdiNAc-positive glycoproteins detected as novel candidate proteins in the secretome of HEK-293 cells.

Expression of LacdiNAc groups on N-glycans among human tumors is complex

Aberrant glycosylation of proteins and lipids is one of the characteristic features of malignantly transformed cells. The GalNAcβ1 → 4GlcNAc (LacdiNAc or LDN) group at the nonreducing termini of both N- and O-glycans is not generally found in mammalian cells. We previously showed that the expression level of the LacdiNAc group in N-glycans decreases dramatically during the progression of human breast cancer. In contrast, the enhanced expression of the LacdiNAc group has been shown to be associated with the progression of human prostate, ovarian, and pancreatic cancers. Therefore, the expression of the disaccharide group appears to be dependent on types of tumors. The mechanism of formation of the LacdiNAc group in human tumors and cancer cells has been studied, and two β4-N-acetylgalacto-saminyltransferases (β4GalNAcTs), β4GalNAcT3 and β4GalNAcT4, have been shown to be involved in the biosynthesis of this disaccharide group in a tissue-dependent manner. Transfection of the β4GalNAcT3 gene brought about significant changes in the malignant phenotypes of human neuroblastoma, indicating that this disaccharide group is important for suppressing the tumor growth.

Peptide-specific transfer of N-acetylgalactosamine to O-linked glycans by the glycosyltransferases β1,4-N-acetylgalactosaminyl transferase 3 (β4GalNAc-T3) and β4GalNAc-T4

N- and O-linked oligosaccharides on pro-opiomelanocortin both bear the unique terminal sequence SO(4)-4-GalNAcβ1,4GlcNAcβ. We previously demonstrated that protein-specific transfer of GalNAc to N-linked oligosaccharides on glycoprotein substrates is dependent on the presence of both an oligosaccharide acceptor and a peptide recognition motif consisting of a cluster of basic amino acids. We characterized how two β1,4-N-acetylgalactosaminyltransferases, β4GalNAc-T3 and β4GalNAc-T4, require the presence of both the peptide recognition motif and the N-linked oligosaccharide acceptors to transfer GalNAc in β1,4-linkage to GlcNAc in vivo and in vitro. We now show that β4GalNAc-T3 and β4GalNAc-T4 are able to utilize the same peptide motif to selectively add GalNAc to β1,6-linked GlcNAc in core 2 O-linked oligosaccharide structures to form Galβ1,3(GalNAcβ1,4GlcNAcβ1,6)GalNAcαSer/Thr. The β1,4-linked GalNAc can be further modified with 4-linked sulfate by either GalNAc-4-sulfotransferase 1 (GalNAc-4-ST1) (CHST8) or GalNAc-4-ST2 (CHST9) or with α2,6-linked N-acetylneuraminic acid by α2,6-sialyltransferase 1 (ST6Gal1), thus generating a family of unique GalNAcβ1,4GlcNAcβ (LacdiNAc)-containing structures on specific glycoproteins.

O-linked N,N'-diacetyllactosamine (LacdiNAc)-modified glycans in extracellular matrix glycoproteins are specifically phosphorylated at subterminal N-acetylglucosamine

The terminal modification of glycans by β4 addition of N-acetylgalactosamine to N-acetylglucosamine with formation of the N,N-diacetyllactosediamine (LacdiNAc) moiety has been well documented for a number of N-linked glycoproteins and peptides, like neurohormones. Much less is known about O-glycoproteins in this regard because only human zona pellucida glycoprotein 3 (ZP3) and bovine proopiomelanocortin were reported to be LacdiNAc-modified. In searching for mammalian proteins modified with O-linked LacdiNAc we identified six positive species among nine endogenous and recombinant O-glycoproteins, which were extracellular matrix, or matrix-related proteins. These are ZP3 and the five novel LacdiNAc-positive species ECM1, AMACO, nidogen-1, α-dystroglycan, and neurofascin. The mass spectrometric analyses revealed a core 2-based tetrasaccharide as the common structural basis of O-linked LacdiNAc that could be further modified, similar to the type 2 LacNAc termini, with fucose, sialic acid, or sulfate. Here, we provide structural evidence for a novel type of mucin-type O-glycans that is strictly specific for LacdiNAc termini: sugar phosphorylation with formation of GalNAcβ1-4(phospho-)GlcNAc. The structural details of the phosphatase-labile compound were elucidated by MS(2) analysis of tetralysine complexes and by MS(n) measurements of the permethylated glycan alditols. Phospho-LacdiNAc was detected in human HEK-293 as well as in mouse myoblast cells and in bovine brain tissue.

A systematic study of site-specific GalNAc-type O-glycosylation modulating proprotein convertase processing

Site-specific GalNAc-type O-glycosylation is emerging as an important co-regulator of proprotein convertase (PC) processing of proteins. PC processing is crucial in regulating many fundamental biological pathways and O-glycans in or immediately adjacent to processing sites may affect recognition and function of PCs. Thus, we previously demonstrated that deficiency in site-specific O-glycosylation in a PC site of the fibroblast growth factor, FGF23, resulted in marked reduction in secretion of active unprocessed FGF23, which cause familial tumoral calcinosis and hyperostosis hyperphosphatemia. GalNAc-type O-glycosylation is found on serine and threonine amino acids and up to 20 distinct polypeptide GalNAc transferases catalyze the first addition of GalNAc to proteins making this step the most complex and differentially regulated steps in protein glycosylation. There is no reliable prediction model for O-glycosylation especially of isolated sites, but serine and to a lesser extent threonine residues are frequently found adjacent to PC processing sites. In the present study we used in vitro enzyme assays and ex vivo cell models to systematically address the boundaries of the region within site-specific O-glycosylation affect PC processing. The results demonstrate that O-glycans within at least ±3 residues of the RXXR furin cleavage site may affect PC processing suggesting that site-specific O-glycosylation is a major co-regulator of PC processing.

B4GALNT3 expression predicts a favorable prognosis and suppresses cell migration and invasion via β₁ integrin signaling in neuroblastoma

β1,4-N-acetylgalactosaminyltransferase III (B4GALNT3) promotes the formation of GalNAcβ1,4GlcNAc (LacdiNAc or LDN). Drosophila β1,4-N-acetylgalactosaminyltransferase A (B4GALNTA) contributes to the synthesis of LDN, which helps regulate neuronal development. In this study, we investigated the expression and role of B4GALNT3 in human neuroblastoma (NB). We used IHC analysis to examine 87 NB tumors, and we identified correlations between B4GALNT3 expression and clinicopathologic factors, including patient survival. Effects of recombinant B4GALNT3 on cell behavior and signaling were studied in SK-N-SH and SH-SY5Y NB cells. Increased expression of B4GALNT3 in NB tumors correlated with a favorable histologic profile (P < 0.001, χ² test) and early clinical staging (P = 0.041, χ² test) and was a favorable prognostic factor for survival as evaluated by univariate and multivariate analyses. Reexpression of B4GALNT3 in SK-N-SH and SH-SY5Y cells suppressed cell proliferation, colony formation, migration, and invasion. Moreover, B4GALNT3 increased the LacdiNAc modification of β₁ integrin, leading to decreased phosphorylation of focal adhesion kinase (FAK), Src, paxillin, Akt, and ERK1/2. B4GALNT3-mediated suppression of cell migration and invasion were substantially reversed by concomitant expression of constitutively active Akt or MEK. We conclude that B4GALNT3 predicts a favorable prognosis for NB and suppresses the malignant phenotype via decreasing β₁ integrin signaling.

N-Linked Oligosaccharides on the Low Density Lipoprotein Receptor Homolog SorLA/LR11 Are Modified with Terminal GalNAc-4-SO4 in Kidney and Brain

Sorting protein-related receptor (SorLA/LR11) is a highly conserved mosaic receptor that is expressed by cells in a number of different tissues including principal cells of the collecting ducts in the kidney and neurons in the central and peripheral nervous systems. SorLA/LR11 has features that indicate it serves as a sorting receptor shuttling between the plasma membrane, endosomes, and the Golgi. We have found that a fraction of SorLA/LR11 that is synthesized in the kidney and the brain bears N-linked oligosaccharides that are modified with terminal beta1,4-linked GalNAc-4-SO(4). Oligosaccharides located in the vacuolar sorting (Vps) 10p domain (Vps10p domain) are modified with beta1,4-linked GalNAc when the Vps10p domain is expressed in cells along with either of two recently cloned protein-specific beta1,4GalNAc-transferases, GalNAcTIII and GalNAcTIV. Either of two sequences with basic amino acids located within the Vps10p domain is able to mediate recognition by these beta1,4GalNAc-transferases. The highly specific modification of oligosaccharides in the Vps10p domain of SorLA/LR11 with terminal GalNAc-4-SO(4) suggests that this unusual modification may modulate the interaction of SorLA/LR11 with proteins and influence their trafficking.

Glycosylation-related Gene Expression in Prion Diseases

Several lines of evidence indicate that some glycoconjugates are efficient effectors of the cellular prion protein (PrP(C)) conversion into its pathogenic (PrP(Sc)) isoform. To assess how glycoconjugate glycan moieties participate in the biogenesis of PrP(Sc), an exhaustive comparative analysis of the expression of about 200 glycosylation-related genes was performed on prion-infected or not, hypothalamus-derived GT1 cells by hybridization of DNA microarrays, semiquantitative RT-PCR, and biochemical assays. A significant up- (30-fold) and down- (17-fold) regulation of the expression of the ChGn1 and Chst8 genes, respectively, was observed in prion-infected cells. ChGn1 and Chst8 are involved in the initiation of the synthesis of chondroitin sulfate and in the 4-O-sulfation of non-reducing N-acetylgalactosamine residues, respectively. A possible role for a hyposulfated chondroitin in PrP(Sc) accumulation was evidenced at the protein level and by determination of chondroitin and heparan sulfate amounts. Treatment of Sc-GT1 cells with a heparan mimetic (HM2602) induced an important reduction of the amount of PrP(Sc), associated with a total reversion of the transcription pattern of the N-acetylgalactosamine-4-O-sulfotransferase 8. It suggests a link between the genetic control of 4-O-sulfation and PrP(Sc) accumulation.

Murine and human zona pellucida 3 derived from mouse eggs express identical O-glycans

Murine sperm initiate fertilization by binding to the outer covering of the egg known as the murine zona pellucida (mZP). This binding is thought to require the interaction of O-glycans linked to a specific mZP glycoprotein (mZP3) with egg-binding proteins coating the sperm plasma membrane. The precise molecular basis of this interaction remains to be resolved. In this study, we analyzed the O-glycosylation of the individual mZP glycoproteins by using ultrasensitive MS methods. We found that the majority of the O-glycans that are linked to mZP3 are core type 2 sequences terminated with sialic acid, lacNAc (Galbeta1-4GlcNAc), lacdiNAc (Gal-NAcbeta1-4GlcNAc), Galalpha1-3Gal, and NeuAcalpha2-3[GalNAcbeta1-4]Galbeta1-4 (Sda antigen). Many of these terminal sequences have been implicated previously in murine sperm-egg binding. Core type 1 O-glycans are also present and are generally unmodified, although some are terminated with sialic acid, beta-linked N-acetylhexosamine, or NeuAcalpha2-3[GalNAcbeta1-4]Galbeta1-4. Eggs expressing human ZP (huZP) glycoprotein huZP3, derived from transgenic mice, bind murine but not human sperm, implying that huZP3 acquires the same O-glycans as native mZP3. Sequencing of huZP3-associated O-glycans confirms that this implication is correct. The data obtained in this investigation may prove to be very useful for studies to determine the precise molecular basis of initial murine sperm-egg binding.

Isolation and structure-bioactivity characterization of glycosylated N-pro-opiomelanocortin isoforms

The N-terminal fragment of mouse pro-opiomelanocortin (N-POMC) was isolated from AtT-20 cell-conditioned medium on the basis of immunoreactivity to an anti-POMC1-50 monoclonal antibody by a concentration step, a cation exchange step, reversed phase high-performance liquid chromatography (HPLC) and size exclusion HPLC. Two groups of N-POMC isoforms with a molecular weight (MW) of approximately 11 kDa and 13 kDa, respectively, were identified by mass spectrometry and N-terminal amino acid sequencing. C-terminal sequencing indicated that 11 kDa isoforms correspond to POMC1-74 and 13 kDa isoforms to POMC1-95. Isoforms from both groups enhanced the prolactin mRNA content (measured by means of TaqMan real-time reverse transcription-polymerase chain reaction) in cultured rat pituitary cell aggregates in a dose-dependent manner, but not all of them showed this activity. POMC1-74 compounds were significantly more potent than POMC1-95 isoforms. The observed effects were abolished by coincubation with the monoclonal anti-POMC1-50 antibody, showing the specificity of this biological action. Incorporation of bromodeoxyuridine into DNA of immunostained lactotrophs was enhanced by only a minor part of the isoforms. Some of these had no effect on prolactin mRNA expression. The N-POMC isoforms appeared to be N- and at least in part O-glycosylated. After enzymatic N-deglycosylation of selected N-POMC isoforms, the stimulatory effect on the prolactin mRNA level was depressed (in case of the POMC1-95 isoforms) or totally abolished (in case of the POMC1-74 isoforms). The present findings show that N-POMC is a mixture of differentially glycosylated isoforms, that the isoforms of POMC1-74 are the biologically more effective forms and that different isoforms induce different biological responses in the same cell population. The data also show the essential role of N-glycosylation in the biological response.

Glycosylated and phosphorylated proteins--expression in yeast and oocytes of Xenopus: prospects and challenges--relevance to expression of thermostable proteins

Phosphorylation and glycosylation are important posttranslational events in the biosynthesis of proteins. The different degrees of phosphorylation and glycosylation of proteins have been an intriguing phenomenon. Advances in genetic engineering have made it possible to control the degree of glycosylation and phosphorylation of proteins. Structural biology of phosphorylated and glycosylated proteins has been advancing at a much slower pace due to difficulties in using high-resolution NMR studies in solution phase. Major difficulties have arisen from the inherent mobilities of phosphorylated and glycosylated side chains. This paper reviews molecular and structural biology of phosphorylated and glycosylated proteins expressed in eukaryotic expression systems which are especially suited for large-scale production of these proteins. In our laboratory, we have observed that eukaryotic expression systems are particularly suited for the expression of thermostable light-activated proteins, e.g., bacteriorhodopsins and plastocyanins.

Molecular cloning and expression of two distinct human N-acetylgalactosamine 4-O-sulfotransferases that transfer sulfate to GalNAc beta 1-->4GlcNAc beta 1-->R in both N- and O-glycans

Recently, cDNAs encoding human chondroitin 4-O-sulfotransferase-1 and -2 (C4ST-1 and C4ST-2) were cloned based on their similarity to HNK-1 sulfotransferase (HNK-1ST) (Hiraoka, N., Nakagawa, H., Ong, E., Akama, T.O., Fukuda, M.N., and Fukuda, M. [2000] Molecular cloning and expression of two distinct human chondroitin 4-O-sulfotransferases that belong to the HNK-1 sulfotransferase gene family. J. Biol. Chem., 275, 20188--20196). In the present study, we identified two additional novel sulfotransferases by searching the expression sequence tag and genomic DNA database for enzymes similar to C4ST-1 and C4ST-2. These newly cloned enzymes, termed GalNAc4ST-1 and GalNAc4ST-2, belong to the HNK-1ST gene family having 40--42% identity with C4ST-1. GalNAc4ST-1 and -2 do not add sulfate to HNK-1 precursor glycans, chondroitin, or desulfated dermatan sulfate. Instead, both enzymes can transfer sulfate to the 4-position of GalNAc in the context of GalNAc beta 1-->4GlcNAc beta 1-->R attached to both N-linked and core 2 branched O-linked oligosaccharides. GalNAc4ST-1 and -2 transcripts are highly expressed in the pituitary gland and trachea, respectively, and GalNAc4ST-1 and -2 transcripts are reciprocally expressed in other tissues as well. Moreover, both enzymes can transfer sulfate to lutropin, a pituitary glycoprotein hormone. These combined results indicate that GalNAc4ST-1 and -2 play critical roles in forming sulfo-->4GalNAc beta 1-->4GlcNAc beta 1-->R in both N-glycans and O-glycans in a tissue-specific manner.

Molecular cloning and expression of the pituitary glycoprotein hormone N-acetylgalactosamine-4-O-sulfotransferase

N-Linked oligosaccharides terminating with the sequence SO(4)-4-GalNAcbeta1,4GlcNAcbeta1,2Manalpha are present on the pituitary hormones lutropin (LH), thyrotropin, and pro-opiomelanocortin. The sulfated structures on LH are essential for expression of its biologic function in vivo. We have cloned the N-acetylgalactosamine-4-sulfotransferase (GalNAc-4-ST1, GenBank(TM) accession number ), which mediates sulfate addition to the N-linked oligosaccharides on LH and other pituitary glycoproteins with terminal (beta1,4-linked GalNAc based on its homology to HNK-1 sulfotransferase (HNK-1 ST). GalNAc-4-ST1 displays 23% identity to HNK-1 ST and 28% to chondroitin 4-sulfotransferase 1 (C4ST-1) and 26% to chondroitin 4-sulfotransferase 2 (C4ST-2). The cDNA predicts a type II transmembrane protein of 424 amino acids with four potential N-linked glycosylation sites and a single membrane-spanning domain. GalNAc-4-ST1 has putative 5'-phosphosulfonate and 3'-phosphate binding sites. Three more carboxyl-terminal regions of unknown function also show a high degree of identity with HNK-1 ST, C4ST-1, and C4ST-2. The membrane-bound form of GalNAc-4-ST1 transfers sulfate to GalNAcbeta1, 4GlcNAcbeta-R but not to chondroitin, whereas truncated forms of GalNAc-4-ST1 that are released into the medium transfer sulfate to both GalNAcbeta1,4GlcNAcbeta-R and chondroitin. The first 118 amino acids of GalNAc-4-ST1 appear to contribute to both its activity and specificity for terminal beta1,4-linked GalNAc. GalNAc-4-ST1 also efficiently transfers sulfate to N-linked oligosaccharides on native LH and other glycoproteins terminating with beta1,4-linked GalNAc. A single transcript of 2.4 kilobases is most highly expressed in the pituitary and other regions of the central nervous system. The GalNAc-4-ST1 gene is located on human chromosome 19q13.1.

Schistosome glysoconjugates

Schistosomes are trematodes known as blood flukes that cause schistosomiasis in people and animals. The male and female worms reside mainly in intestinal veins where they lay eggs that result in a wide-ranging pathology in infected individuals. A growing body of evidence indicates that carbohydrates on glycoproteins, glycolipids and glycosaminoglycans synthesized by the parasite are targets of humoral immunity and may play a role in modulating host immune responses. Carbohydrate antigens may provide protective immunity against infection. In addition, recent evidence indicates that glycoconjugates and carbohydrate-binding proteins from the parasites and their hosts participate in egg adhesion and granuloma formation involved in disease pathology. This review will highlight our current knowledge of the glycoconjugates synthesized by the parasites and their immunological and biological properties. There is increasing anticipation in the field that information about the glycobiology of these parasites may lead to carbohydrate-based vaccines and diagnostics for the disease and perhaps new therapies for treating infected individuals.

The molecular characterisation of chicken pituitary N-terminal pro-opiomelanocortin (POMC)

Monoclonal antibodies (Mabs) specifically recognizing the chicken pituitary corticotropes were used to isolate a population of closely related peptides from crude chicken pituitary extracts. A homogeneous N-terminal sequence homologous to the extreme N-terminus of mammalian and amphibian pro-opiomelanocortin (POMC) was revealed. Further physicochemical analysis proved the existence of a series of C-terminally truncated peptides including 3 major molecular species corresponding to Ser1-Gly64, Ser1-Arg73 and Ser1-Gly105 respectively. The two latter molecules were shown to be N-glycosylated at position Asn67, with mass spectrometric data indicating a carbohydrate structure of the oligomannose 5 type, in addition to two more complex structures. No evidence was found in favour of O-glycosylation on Ser47. Degenerated PCR primers were deduced from the above protein sequence and from the known chicken adrenocorticotropic hormone (ACTH) sequence. The nucleotide sequence obtained by reversed transcription PCR (RT-PCR) completely confirmed the new amino acid sequence data including pro-gamma-MSH, the joining peptide and ACTH.

Inhibition of L- and P-selectin by a rationally synthesized novel core 2-like branched structure containing GalNAc-Lewisx and Neu5Acalpha2-3Galbeta1-3GalNAc sequences

The selectins interact in important normal and pathological situations with certain sialylated, fucosylated glycoconjugate ligands containing sialyl Lewisx(Neu5Acalpha2-3Galbeta1-4(Fucalpha1-3)GlcN Ac). Much effort has gone into the synthesis of sialylated and sulfated Lewisxanalogs as competitive ligands for the selectins. Since the natural selectin ligands GlyCAM-1 and PSGL-1 carry sialyl Lewisxas part of a branched Core 2 O-linked structure, we recently synthesized Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-6(SE-3Galbeta1++ +-3)GalNAc1alphaOMe and found it to be a moderately superior ligand for L and P-selectin (Koenig et al. , Glycobiology 7, 79-93, 1997). Other studies have shown that sulfate esters can replace sialic acid in some selectin ligands (Yeun et al. , Biochemistry, 31, 9126-9131, 1992; Imai et al. , Nature, 361, 555, 1993). Based upon these observations, we hypothesized that Neu5Acalpha2-3Galbeta1-3GalNAc might have the capability of interacting with L- and P-selectin. To examine this hypothesis, we synthesized Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-6(Neu5Acalpha2++ +-3Galbeta1-3)-GalNAc alpha1-OB, which was found to be 2- to 3-fold better than sialyl Lexfor P and L selectin, respectively. We also report the synthesis of an unusual structure GalNAcbeta1-4(Fucalpha1- 3)GlcNAcbeta1-OMe (GalNAc-Lewisx-O-methyl glycoside), which also proved to be a better inhibitor of L- and P-selectin than sialyl Lewisx-OMe. Combining this with our knowledge of Core 2 branched structures, we have synthesized a molecule that is 5- to 6-fold better at inhibiting L- and P-selectin than sialyl Lewisx-OMe, By contrast to unbranched structures, substitution of a sulfate ester group for a sialic acid residue in such a molecule resulted in a considerable loss of inhibition ability. Thus, the combination of a sialic acid residue on the primary (beta1-3) arm, and a modified Lexunit on the branched (beta1-6) arm on an O-linked Core 2 structure generated a monovalent synthetic oliogosaccharide inhibitor superior to SLexfor both L- and P-selectin.

Distribution of glycoproteins with beta-N-acetylgalactosaminylated N-linked sugar chains among bovine tissues

Only a small number of glycoproteins has been reported to contain N-linked sugar chains with GalNAcbeta1-->4GlcNAc structure. Our previous studies showed that most glycoproteins from bovine milk fat globule membranes contain beta-N-acetylgalactosaminylated N-linked sugar chains [Sato et al., J. Biochem. 114 (1993) 890-900]. In order to study how widely this glycosylation occurs, lectin blot analysis of membrane glycoproteins from 12 bovine tissues was performed using Wistaria floribunda agglutinin (WFA), which interacts with oligosaccharides terminating with N-acetylgalactosamine. The WFA-positive bands were detected in samples from most tissues except for intestine although the number and reactivity of bands to lectin varied among the tissues. Upon pretreatment of blotted filters with Bacillus beta-N-acetylgalactosaminidase or N-glycanase, no lectin binding was observed. WFA-agarose column chromatography of oligosaccharides released by hydrazinolysis from membrane glycoproteins of bovine tissues except for intestine revealed that a few to 18% of the released oligosaccharides bind and are eluted from the column with 100 mM N-acetylgalactosamine. These results indicate that many glycoproteins from a variety of bovine tissues contain N-linked sugar chains with GalNAcbeta1-->4GlcNAc structure, suggesting a wider occurrence of this glycosylation in bovine tissues.

Developmental regulation of a pregnancy-specific oligosaccharide structure, NeuAcalpha2,6GalNAcbeta1,4GlcNAc, on select members of the rat placental prolactin family

Successful pregnancy is dependent upon an array of signaling proteins secreted by the trophoblast cells of the placenta. Among these is a group of proteins related to pituitary prolactin, known as the prolactin/growth hormone family. These proteins are expressed at specific times during gestation and synthesized in distinct trophoblast cell types in the rat placenta. We report here that select members of this family, prolactin-like protein (PLP-A), PLP-B, PLP-C, decidual/trophoblast PRP, and placental lactogen I variant, only which are expressed in the spongiotrophoblast, late in rat placental development bear Asn-linked oligosaccharides terminating with NeuAcalpha2,6GalNAcbeta1,4GlcNAcbeta-R. This reflects the concurrent expression of these prolactin/growth hormone family members with the peptide-specific beta1,4GalNAc-transferase and an alpha2,6-sialyltransferase, which can add sialic acid to terminal beta1,4-linked GalNAc. We have determined that at least one of the prolactin-like proteins, PLP-A, is recognized by the protein-specific GalNAc-transferase. The presence of NeuAcalpha2, 6GalNAcbeta1,4GlcNAcbeta-R on only a limited number of glycoproteins synthesized by the spongiotrophoblasts between mid gestation and birth reflects the need for both the GalNAc-transferase and the peptide recognition determinant for efficient addition of GalNAc. Thus, expression of the GalNAc-transferase and specific members of the prolactin/growth hormone family is developmentally regulated in the rat placenta, suggesting a physiological role for the terminal NeuAcalpha2,6GalNAcbeta1,4GlcNAcbeta-R sequence on Asn-linked oligosaccharides of these proteins.

Cleavage of the transferrin receptor is influenced by the composition of the O-linked carbohydrate at position 104

A soluble form of the human transferrin receptor (TfR) resulting from proteolytic cleavage at Arg 100 has been measured in human blood. In tissue culture cells elimination of the O-linked carbohydrate at Thr 104, four amino acids from the cleavage site, results in enhanced cleavage of the TfR (Rutledge et al., 1994, Blood, 83:580-586). In the present set of studies, the influence of amino acid substitution and the composition of the oligosaccharide at amino acid 104 on the cleavage of the TfR was examined. Site-directed mutagenesis was used to generate six different amino acids at position 104 which varied in size and charge. Measurement of the soluble TfR in the conditioned medium of the transfected cells of each mutant TfR showed that the large and charged side chains inhibited TfR cleavage the most. Otherwise the properties of the mutant TfRs were indistinguishable from the wild-type TfR in that the affinity of transferrin for these receptors, the extent of disulfide bond formation of the TfRs, and the proportion of TfRs at the cell surface were similar to that of the wild-type TfR. Removal of the sialic acid component of the carbohydrate from wild-type TfR by treatment of live cells with neuraminidase enhances TfR cleavage. Expression of wild-type TfR in CHO IdlD cells (a glycosylation defective cell line) also shows enhanced cleavage under conditions that produce truncated or no O-linked carbohydrates. Treatment of IdlD cells with neuraminidase reveals that the sialic acid of the O-linked carbohydrate protects against TfR cleavage, whereas the core sugars Gal-NAc and Gal do not protect as much. These results show that the terminal charged sialic acid residues are important for protection from proteolytic cleavage and suggest that cleavage could be regulated in the cell by removal of all or part of the carbohydrate.

Getting the glycosylation right: implications for the biotechnology industry

Glycosylation is the most extensive of all the posttranslational modifications, and has important functions in the secretion, antigenicity and clearance of glycoproteins. In recent years major advances have been made in the cloning of glycosyltransferase enzymes, in understanding the varied biological functions of carbohydrates, and in the accurate analysis of glycoprotein heterogeneity. In this review we discuss the impact of these advances on the choice of a recombinant host cell line, in optimizing cell culture processes, and in choosing the appropriate level of glycosylation analysis for each stage of product development.

O-linked protein glycosylation structure and function

There has been a recent resurgence of interest in the post-translational modification of serine and threonine hydroxyl groups by glycosylation, because the resulting O-linked oligosaccharide chains tend to be clustered over short stretches of peptide and hence they can present multivalent carbohydrate antigenic or functional determinants for antibody recognition, mammalian cell adhesion and microorganism binding. Co-operativity can greatly increase the affinity of interactions with antibodies or carbohydrate binding proteins. Thus, in addition to their known importance in bearing tumour associated antigens in the gastrointestinal and respiratory tracts, glycoproteins with O-linked chains have been implicated as ligands or co-receptors for selectins (mammalian carbohydrate binding proteins). Microorganisms may have adopted similar mechanisms for interactions with mammalian cells in infection, by having relatively low affinity ligands (adhesins) for carbohydrate binding, which may bind with higher affinity due to the multivalency of the host ligand and which are complemented by other virulence factors such as interactions with integrin-type molecules. In addition to specific adhesion signals from O-linked carbohydrate chains, multivalent O-glycosylation is involved in determining protein conformation and forming conjugate oligosaccharide-protein antigenic, and possible functional determinants.

Purification and characterization of the GalNAc-4-sulfotransferase responsible for sulfation of GalNAc beta 1,4GlcNAc-bearing oligosaccharides

The pituitary glycoprotein hormone lutropin is characterized by its pulsatile appearance in the bloodstream which is important for the expression of its biological activity in the ovary. We have previously shown that lutropin bears unique Asn-linked oligosaccharides terminating with GalNAc-4-SO4 which allow the hormone to be rapidly cleared from the bloodstream via a specific receptor in the liver, thus contributing to its pulsatile appearance in the circulation. Furthermore, we have found that carbonic anhydrase VI, synthesized by the submaxillary gland and secreted into the saliva, also bears Asn-linked oligosaccharides terminating with GalNAc-4-SO4, suggesting that this unique sulfated structure mediates other biological functions in addition to rapid clearance from the circulation. We report here the purification of a GalNAc-4-sulfotransferase which transfers sulfate to terminal beta 1,4-linked GalNAc on Asn-linked oligosaccharides. We show that the purified submaxillary gland enzyme has kinetic parameters identical to the pituitary enzyme, indicating that the same sulfotransferase is responsible for the sulfation of lutropin oligosaccharides in pituitary and carbonic anhydrase VI oligosaccharides in submaxillary gland. This GalNAc-4-sulfotransferase has an apparent molecular mass of 128 kDa and can be specifically photoaffinity radiolabeled with 3',5'-ADP, a competitive inhibitor of sulfotransferase activity. The acceptor specificity of this GalNAc-4-sulfotransferase indicates that it is able to transfer sulfate to terminal GalNAc beta 1,4GlcNAc on both N- and O-glycosidically linked oligosaccharides, suggesting that this enzyme is also responsible for the sulfation of O-linked glycans on proopiomelanocortin.

Differential expression of GalNAc-4-sulfotransferase and GalNAc-transferase results in distinct glycoforms of carbonic anhydrase VI in parotid and submaxillary glands

Differential expression of glycosyltransferases has the potential to generate functionally distinct glycoforms of otherwise identical proteins. We have previously demonstrated the presence of unique oligosaccharides terminating with GalNAc-4-SO4 on the pituitary glycoproteins lutropin (LH), thyroid stimulating hormone (TSH), and pro-opiomelanocortin (POMC). A glycoprotein hormone:GalNAc-transferase and a GalNAc-4-sulfotransferase are present in the pituitary and can account for the synthesis of these unique oligosaccharides on specific glycoproteins. Both transferases are coordinately expressed in a number of tissues in addition to pituitary, including submaxillary gland, lacrimal gland, and kidney, suggesting that additional glycoproteins bearing oligosaccharides terminating with GalNAc-4-SO4 are synthesized in these tissues. In this study we show that while the glycoprotein hormone:GalNAc-transferase and the GalNAc-4-sulfotransferase are coordinately expressed in bovine submaxillary gland, the GalNAc-transferase is expressed in the parotid gland in the absence of the GalNAc-4-sulfotransferase. The relative expression of these two transferases in submaxillary and parotid glands correlates with the presence of unique Asn-linked oligosaccharides on carbonic anhydrase VI (CA VI) synthesized in each of these tissues. The majority of Asn-linked oligosaccharides on CA VI synthesized in submaxillary gland terminate with GalNAc-4-SO4. In contrast, CA VI which is synthesized in bovine parotid gland bears oligosaccharides which terminate predominantly with beta 1,4-linked GalNAc which is not sulfated. The presence of different terminal residues on the Asn-linked oligosaccharides of submaxillary and parotid CA VI thus correlates with the complement of transferases in these glands and suggests differing biological roles for submaxillary and parotid CA VI.