The structural basis of the different affinities of two types of acidic N-glycosidic glycopeptides for concanavalin A--sepharose

Characterization of Mannoprotein Structural Diversity in Wine Yeast Species

The structure of yeast cell wall (CW) mannoproteins (MPs) influences their impact on wine properties. Yeast species produce a diverse range of MPs, but the link between properties and specific structural features has been ill-characterized. This study compared the protein and polysaccharide moieties of MP-rich preparations from four strains of four different enologically relevant yeast species, named Saccharomyces boulardii (SB62), Saccharomyces cerevisiae (SC01), Metschnikowia fructicola (MF77), and Torulaspora delbrueckii (TD70), and a commercial MP preparation. Monosaccharide determination revealed that SB62 MPs contained the highest mannose/glucose ratio followed by SC01, while polysaccharide size distribution analyses showed maximum molecular weights ranging from 1349 kDa for MF77 to 483 kDa for TD70. Protein identification analysis led to the identification of unique CW proteins in SB62, SC01, and TD70, as well as some proteins shared between different strains. This study reveals MP composition diversity within wine yeasts and paves the way toward their industrial exploitation.

Lectin binding and gel secretion within Lorenzinian electroreceptors of Polyodon

We imaged the carbohydrate-selective spatial binding of 8 lectins in the ampullary organs (AOs) of electroreceptors on the rostrum of freshwater paddlefish (Polyodon spathula), by fluorescence imaging and morphometry of frozen sections. A focus was candidate sites of secretion of the glycoprotein gel filling the lumen of AOs. The rostrum of Polyodon is an electrosensory appendage anterior of the head, covered with >50,000 AOs, each homologous with the ampulla of Lorenzini electroreceptors of marine rays and sharks. A large electrosensory neuroepithelium (EN) lines the basal pole of each AO’s lumen in Polyodon; support cells occupy most (97%) of an EN’s apical area, along with electrosensitive receptor cells. (1) Lectins WGA or SBA labeled the AO gel. High concentrations of the N-acetyl-aminocarbohydrate ligands of these lectins were reported in canal gel of ampullae of Lorenzini, supporting homology of Polyodon AOs. In cross sections of EN, WGA or SBA labeled cytoplasmic vesicles and organelles in support cells, especially apically, apparently secretory. Abundant phalloidin+ microvilli on the apical faces of support cells yielded the brightest label by lectins WGA or SBA. In parallel views of the apical EN surface, WGA labeled only support cells. We concluded that EN support cells massively secrete gel from their apical microvilli (and surface?), containing amino carbohydrate ligands of WGA or SBA, into the AO lumen. (2) Lectins RCA120 or ConA also labeled EN support cells, each differently. RCA120-fluorescein brightly labeled extensive Golgi tubules in the apical halves of EN cells. ConA did not label microvilli, but brightly labeled small vesicles throughout support cells, apparently non-secretory. (3) We demonstrated “sockets” surrounding the basolateral exteriors of EN receptor cells, as candidate glycocalyces. (4) We explored whether additional secretions may arise from non-EN epithelial cells of the interior ampulla wall. (5) Model: Gel is secreted mainly by support cells in the large EN covering each AO’s basal pole. Secreted gel is pushed toward the pore, and out. We modeled gel velocity as increasing ~11x, going distally in AOs (toward the narrowed neck and pore), due to geometrical taper of the ampulla wall. Gel renewal and accelerated expulsion may defend against invasion of the AO lumen by microbes or small parasites. (6) We surveyed lectin labeling of accessory structures, including papilla cells in AO necks, striated ectoderm epidermis, and sheaths on afferent axons or on terminal glia.

Sialylated and sulfated N-Glycans in MDCK and engineered MDCK cells for influenza virus studies

The Madin-Darby canine kidney (MDCK) cell line is an in vitro model for influenza A virus (IAV) infection and propagation. MDCK-SIAT1 (SIAT1) and humanized MDCK (hCK) cell lines are engineered MDCK cells that express N-glycans with elevated levels of sialic acid (Sia) in α2,6-linkage (α2,6-Sia) that are recognized by many human IAVs. To characterize the N-glycan structures in these cells and the potential changes compared to the parental MDCK cell line resulting from engineering, we analyzed the N-glycans from these cells at different passages, using both mass spectrometry and specific lectin and antibody binding. We observed significant differences between the three cell lines in overall complex N-glycans and terminal galactose modifications. MDCK cells express core fucosylated, bisected complex-type N-glycans at all passage stages, in addition to expressing α2,6-Sia on short N-glycans and α2,3-Sia on larger N-glycans. By contrast, SIAT1 cells predominantly express α2,6-Sia glycans and greatly reduced level of α2,3-Sia glycans. Additionally, they express bisected, sialylated N-glycans that are scant in MDCK cells. The hCK cells exclusively express α2,6-Sia glycans. Unexpectedly, hCK glycoproteins bound robustly to the plant lectin MAL-1, indicating α2,3-Sia glycans, but such binding was not Sia-dependent and closely mirrored that of an antibody that recognizes glycans with terminal 3-O-sulfate galactose (3-O-SGal). The 3-O-SGal epitope is highly expressed in N-glycans on multiple hCK glycoproteins. These results indicate vastly different N-glycomes between MDCK cells and the engineered clones that could relate to IAV infectivity.

Square-wave voltammetry assays for glycoproteins on nanoporous gold

Electrochemical enzyme-linked lectinsorbent assays (ELLA) were developed using nanoporous gold (NPG) as a solid support for protein immobilization and as an electrode for the electrochemical determination of the product of the reaction between alkaline phosphatase (ALP) and p-aminophenyl phosphate (p-APP), which is p-aminophenol (p-AP). Glycoproteins or concanavalin A (Con A) and ALP conjugates were covalently immobilized onto lipoic acid self-assembled monolayers on NPG. The binding of Con A - ALP (or soybean agglutinin - ALP) conjugate to glycoproteins covalently immobilized on NPG and subsequent incubation with p-APP substrate was found to result in square-wave voltammograms whose peak difference current varied with the identity of the glycoprotein. NPG presenting covalently bound glycoproteins was used as the basis for a competitive electrochemical assay for glycoproteins in solution (transferrin and IgG). A kinetic ELLA based on steric hindrance of the enzyme-substrate reaction and hence reduced enzymatic reaction rate after glycoprotein binding is demonstrated using immobilized Con A-ALP conjugates. Using the immobilized Con A-ALP conjugate, the binding affinity of immunoglobulin G (IgG) was found to be 105 nM, and that for transferrin was found to be 650 nM. Minimal interference was observed in the presence of 5 mg mL-1 BSA as a model serum protein in both the kinetic and competitive ELLA. Inhibition studies were performed with methyl D-mannoside for the binding of TSF and IgG to Con A-ALP; IC50 values were found to be 90 μM and 286 μM, respectively. Surface coverages of proteins were estimated using solution depletion and the BCA protein concentration assay.

Lectin microarrays: concept, principle and applications

The lectin microarray is a novel platform for glycan analysis, having emerged only in recent years. Unlike other conventional methods, e.g., liquid chromatography and mass spectrometry, it enables rapid and high-sensitivity profiling of complex glycan features without the need for liberation of glycans. Target samples include an extensive range of glycoconjugates involved in cells, tissues, body fluids, as well as synthetic glycans and their mimics. Various procedures for rapid differential glycan profiling have been developed for glycan-related biomarkers. Such glycoproteomics targeting allows precise diagnosis of chronic diseases potentially related to cancer. Application of this method to evaluation of various types of stem cells resulted in the discovery of a new pluripotent cell-specific glycan marker. To explore this technology a more fundamental and extensive understanding of lectins is necessary in relation to the structural uniqueness of glycans. In this chapter, the essence of the lectin microarray is described with some focus on an evanescent-field-activated fluorescence detection principle as a system to achieve in situ (i.e., washing free) aqueous-phase observation under equilibrium conditions. The developed lectin microarray system allows even researchers with poor experience in glycan profiling to perform extensive high-throughput analysis targeting various forms of glycans and even cells.

Glycoproteomic analysis of embryonic stem cells: identification of potential glycobiomarkers using lectin affinity chromatography of glycopeptides

Numerous studies have recently focused on the identification of specific glycan biomarkers, given the important roles that protein linked glycans play, for example, during development and disease progression. The identification of protein glycobiomarkers, which are part of a very complex proteome, has involved the use of fractionation techniques such as lectin affinity chromatography. In this study, the glycoproteomic characterization of pluripotent murine embryonic stem cells (ES) and from ES cells that were differentiated into embroid bodies (EB) was performed using immobilized Concanavalin A (ConA). This procedure allowed the isolation of glycopeptides that express biantennary and hybrid N-linked structures (ConA2 fraction) as well as high mannose glycans (ConA3 fraction) that were abundant in both ES and EB stages. A total of 293 unique N-linked glycopeptide sequences (from 180 glycoproteins) were identified in the combined data sets from ES and EB cells. Of these glycopeptides, a total of 119 sequences were identified exclusively in only one of the lectin-bound fractions (24 in the ES-ConA2, 15 in the ES-ConA3, 16 in the EB-ConA2, and 64 in the EB-ConA3). Results from this study allowed the identification of individual N-glycosylation sites of proteins that express specific glycan types. The absence of some of these lectin-bound glycopeptides in a cell stage suggested that they were derived from proteins that were either expressed exclusively on a defined developmental stage or were expressed in both cell stages but carried the lectin-bound oligosaccharides in only one of them. Therefore, these lectin-bound glycopeptides can be considered as stage-specific glycobiomarkers.

Fluorescent glycosylamides produced by microscale derivatization of free glycans for natural glycan microarrays

A novel strategy for creating naturally derived glycan microarrays has been developed. Glycosylamines are prepared from free reducing glycans and stabilized by reaction with acryloyl chloride to generate a glycosylamide in which the reducing monosaccharide has a closed-ring structure. Ozonolysis of the protected glycan yields an active aldehyde, to which a bifunctional fluorescent linker is coupled by reductive amination. The fluorescent derivatives are easily coupled through a residual primary alkylamine to generate glycan microarrays. This strategy preserves structural features of glycans required for antibody recognition and allows development of natural arrays of fluorescent glycans in which the cyclic pyranose structure of the reducing-end sugar residue is retained.

Generation of a natural glycan microarray using 9-fluorenylmethyl chloroformate (FmocCl) as a cleavable fluorescent tag

Glycan microarray technology has become a successful tool for studying protein-carbohydrate interactions, but a limitation has been the laborious synthesis of glycan structures by enzymatic and chemical methods. Here we describe a new method to generate quantifiable glycan libraries from natural sources by combining widely used protease digestion of glycoproteins and Fmoc chemistry. Glycoproteins including chicken ovalbumin, bovine fetuin, and horseradish peroxidase (HRP) were digested by Pronase, protected by FmocCl, and efficiently separated by 2D-HPLC. We show that glycans from HRP glycopeptides separated by HPLC and fluorescence monitoring retained their natural reducing end structures, mostly core alpha1,3-fucose and core alpha1,2-xylose. After simple Fmoc deprotection, the glycans were printed on NHS-activated glass slides. The glycans were interrogated using plant lectins and antibodies in sera from mice infected with Schistosoma mansoni, which revealed the presence of both IgM and IgG antibody responses to HRP glycopeptides. This simple approach to glycopeptide purification and conjugation allows for the development of natural glycopeptide microarrays without the need to remove and derivatize glycans and potentially compromise their reducing end determinants.

Comparative glycoproteomics of N-linked complex-type glycoforms containing sialic acid in human serum

This study describes a simple and efficient approach for comparative analysis of sialylated glycoforms of proteins containing differentially branched complex-type glycans. The analytical protocol is based on glycopeptide selection from tryptic digests with serial lectin affinity chromatography (SLAC), quantification with global internal standard technology, fractionation of deglycosylated peptides with reversed-phase chromatography, and peptide sequencing with tandem mass spectrometry. Fractionation of complex tri- and tetraantennary N-linked glycoforms from biantennary N-linked glycoforms bearing terminal sialic acid residues was achieved using a set of serial lectin columns with immobilized Sambucus nigra agglutinin and concanavalin A. These two fractions from the affinity selection were differentially labeled, mixed, and then deglycosylated with the enzyme PNGase F. The deglycosylated sample was further fractionated by reversed-phase chromatography and analyzed by electrospray ionization mass spectrometry. The SLAC strategy was applied to tryptic digests of human serum, and it was found that most sialylated glycopeptides identified carry more biantennary glycans than tri- and tetraantennary glycans, and the relative amount of biantennary glycan versus tri- and tetraantennary glycans was different at separate glycosylation sites within the same glycoprotein.

One-step biotinylation procedure for carbohydrates to study carbohydrate–protein interactions

Protein-carbohydrate interactions play crucial roles in numerous biological processes. To study these interactions, we developed a simple and fast procedure for the biotinylation of carbohydrates based on reductive amination. The method allows complete and stable biotinylation of small quantities of oligosaccharides and includes a rapid and simple procedure to remove excess labeling reagent. After biotinylation, the structural and biological integrity of the glycans was intact as determined by HPLC, mass spectrometry, and a plant lectin assay. By using the human C-type lectin DC-SIGN (dendritic cell-specific ICAM-3-grabbing nonintegrin), we demonstrate that the biotinylated glycans can be used in a glycan array to determine binding specificities of lectins. Moreover, we show that fluorescent beads coated with selected biotinylated glycans bind to DC-SIGN-expressing dendritic cells in vitro. Finally, by using biotinylated high-mannose N-glycans, we could visualize DC-SIGN-expressing cells in lymph node tissue. The availability of easy biotinylation methods for oligosaccharides such as those described here greatly facilitates the functional analysis of lectins. In addition, the biotinylated glycans will be great tools for investigating functional lectin receptors in situ.

Purification and diagnostic utility of a recombinant hepatitis E virus capsid protein expressed in insect larvae

We report here the expression and purification of a truncated form of the hepatitis E virus ORF2 protein (ORF2delta111/deltaTM), from the fat bodies of Spodoptera litura larvae infected with a recombinant baculovirus. The purified protein migrated as a doublet of approximately 56 kDa on SDS-PAGE and was found to be glycosylated by staining with concanavalin A-linked horseradish peroxidase. The protein was used in a sensitive and specific enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies to HEV. The results showed complete concordance with those obtained using a commercial kit for the detection of anti-HEV antibodies. Antigen expression in the insect larvae system presents a rapid and low-cost method that obviates the need for expensive tissue culture scale-ups or special equipment.

Intracellular accumulation of antithrombin Morioka (C95R), a novel mutation causing type I antithrombin deficiency

Antithrombin (AT) is a major plasma protease inhibitor with three intramolecular disulfide bonds, and its deficiency is associated with increased venous thrombosis. Recently, we found a novel missense mutation named AT Morioka (C95R), which causes the loss of one of the three disulfide bonds. In this study, we prepared Chinese hamster ovary cells stably overexpressing wild type or mutant AT and examined the intracellular fate of the ATs. In pulse-chase experiments, newly synthesized wild type AT was secreted into the medium with a half-life of approximately 1.5 h. In contrast, most of the mutant type AT was not secreted during the chase period of 9 h and, surprisingly, was not degraded in the cells. The kinetics of the secretion suggests that the mutant was secreted about 50 times more slowly into the medium. Most of the mutant AT in the cells had high mannose type oligosaccharides, suggesting that it was retained in the endoplasmic reticulum (ER). In addition, half of the mutant AT existed in a dimeric form with an intermolecular disulfide bond. On immunoelectron microscopy, the mutant AT was found to have accumulated in variously sized structures surrounded by a single membrane in the cytoplasm. Immunogold particles exhibiting calnexin immunoreactivity were detected on the membranes. Ribosomes were attached to some of the small structures that had accumulated the mutant AT. Further, we prepared Chinese hamster ovary cells stably overexpressing another mutant AT in which two cysteine residues at 21 and 95, responsible for disulfide bond formation, were substituted for arginines. In pulse-chase experiments, the mutant AT (C21C,C95R) was secreted faster than that of AT Morioka (C95R) into the medium. These results suggest that AT Morioka remained for a long time in ER without being degraded and accumulated in newly formed membrane structures derived from the ER. The dimerization of AT Morioka (C95R) through Cys-21 seems to be critical for its intracellular accumulation.

Expression and purification of glycosylated bovine beta-casein (L70S/P71S) in Pichia pastoris

Post-translational glycosylation of bovine beta-casein (L70S/P71S) that results in Asn(68)-linked glycan on the protein was obtained in up to 30% of total beta-casein expressed in the methylotrophic yeast Pichia pastoris. Among the growth/induction media used, buffered minimal glycerol (BMG)/buffered minimal methanol (BMM) media were best for the production of glycosylated bovine beta-casein, indicating pH-dependent glycosylation. Glycosylated bovine beta-casein (L70S/P71S) expressed in P. pastoris was purified to homogeneity by the combination of ammonium sulfate fractionation, Concanavalin A--Sepharose affinity column, and Mono Q anion-exchange FPLC. The purified glycosylated bovine beta-casein was specific only to Concanavalin A, and the oligosaccharide structure of glycosylated beta-casein was of high-mannose type. Unlike the hyperglycosylation that occurred in yeast, the majority of bovine beta-casein was not hyperglycosylated in P. pastoris, and its molecular weight was estimated to be 33.6 kDa. Glycosylated bovine beta-casein was normally phosphorylated to the same degree as native bovine beta-casein.

A journey to the world of glycobiology

Finding of the deletion phenomenon of certain oligosaccharides in human milk and its correlation to the blood types of the donors opened a way to elucidate the biochemical basis of blood types in man. This success led to the idea of establishing reliable techniques to elucidate the structures and functions of the N-linked sugar chains of glycoproteins. N-Linked sugar chains were first released quantitatively as oligosaccharides by enzymatic and chemical means, and labelled by reduction with NaB3H4. After fractionation, structures of the radioactive oligosaccharides were determined by a series of methods developed for the studies of milk oligosaccharides. By using such techniques, structural rules hidden in the N-linked sugar chains, and organ- and species-specific N-glycosylation of glycoproteins, which afforded a firm basis to the development of glycobiology, were elucidated. Finding of galactose deficiency in the N-linked sugar chains of serum IgG from patients with rheumatoid arthritis, and malignant alteration of N-glycosylation in various tumors opened a new research world called glycopathology. However, recent studies revealed that several structural exceptions occur in the sugar chains of particular glycoproteins. Finding of the occurrence of the Galbeta1-4Fucalpha1- group linked at the C-6 position of the proximal N-acetylglucosamine residue of the hybrid type sugar chains of octopus rhodopsin is one of such examples. This finding indicated that the fucosyl residue of the fucosylated trimannosyl core should no more be considered as a stop signal as has long been believed. Furthermore, recent studies on dystroglycan revealed that the sugar chains, which do not fall into the current classification of N and O-linked sugar chains, are essential for the expression of the functional role of this glycoprotein. It was found that expression of many glycoproteins is altered by aging. Among the alterations of the glycoprotein patterns found in the brain nervous system, the most prominent evidence was found in P0. This protein is produced in non-glycosylated form in the spinal cord of young mammals. However, it starts to be N-glycosylated in the spinal cord of aged animals. These evidences indicate that various unusual sugar chains occur as minor components in mammals, and play important roles in particular tissues.

Asn-linked sugar chain structures of recombinant human thrombopoietin produced in Chinese hamster ovary cells

Human thrombopoietin (TPO) that regulates the numbers of megakaryocytes and platelets is a heavily N- and O-glycosylated glycoprotein hormone with partial homology to human erythropoietin (EPO). We prepared recombinant human TPO produced in Chinese hamster ovary (CHO) cells and analyzed the sugar chain structures quantitatively using 2-aminobenzamide labeling, sequential glycosidase digestion and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF/MS). We found bi-, tri- and tetraantennary complex-type sugar chains with one or two N-acetyllactosamine repeats, which are common to recombinant human EPO produced in CHO cells. On the other hand, there were triantennary sugar chains with one or two N-acetyllactosamine repeats that were specific to the recombinant human TPO, and their distributions of branch structures were also different. These results suggested that proximal protein structure should determine the branch structure of Asn-linked sugar chains in addition to the glycosyltransferases subset.

Influence of glycosylation on the clearance of recombinant human sex hormone-binding globulin from rabbit blood

Human sex hormone-binding globulin (hSHBG) is a plasma glycoprotein that binds sex steroids with high affinity. Variations in hSHBG glycosylation contribute to its electrophoretic microheterogeneity, but the functional significance of different SHBG glycoforms is unknown. Carbohydrates may influence the biological activities and half-lives of glycoproteins and we have examined how oligosaccharides at specific sites influence the plasma clearance of hSHBG in vivo. To accomplish this, fully-glycosylated hSHBG, or hSHBG mutants lacking specific oligosaccharides chains, were expressed in Chinese hamster ovary (CHO) cells and purified by immunoaffinity chromatography. The purified recombinant proteins were then biotinylated to study their plasma half-lives after intravenous injection into rabbits. When compared to hSHBG isolated from serum, recombinant hSHBG migrates with a slightly larger average molecular size during denaturing polyacrylamide gel electrophoresis. This is due to a greater proportion (33-39% vs. 3%) of more highly branched N-linked oligosaccharides on the recombinant proteins. When injected into rabbits, the disappearance of recombinant hSHBG showed two exponential components, as previously shown for natural hSHBG in the same animal model. The mean +/- S.E.M. plasma half-lives of recombinant hSHBG (t 1/2alpha 0.11+/-0.03 h and t 1/2beta 18.94+/-1.65 h) are shorter than previously measured for natural hSHBG (t 1/2alpha 3.43+/-0.72 h and t 1/2beta 38.18+/-7.22 h) and this is likely due to differences in the composition of their N-linked oligosaccharides. An O-linked chain at Thr7 does not influence the plasma clearance of hSHBG in the presence or absence of N-linked carbohydrates at Asn351 and Asn367. However, a 1.5-1.6 fold (p<0.03) increase in plasma half-life of variants lacking both N-glycosylation sites was observed and this is probably due to the fact these variants are not recognized by the asialoglycoprotein receptor-mediated clearance system. Removal of either N-glycosylation consensus site also increased (p<0.0001) the plasma half-life of hSHBG by 2.3 2.4 fold. Thus, the metabolic clearance of hSHBG appears to be determined by the number of N-linked oligosaccharides rather than their location.

Human mannose-binding protein inhibits infection of HeLa cells by Chlamydia trachomatis

The role that collectin (mannose-binding protein) may play in the host's defense against chlamydial infection was investigated. Recombinant human mannose-binding protein was used in the inhibition of cell culture infection by Chlamydia trachomatis (C/TW-3/OT, E/UW-5/Cx, and L2/434/Bu), Chlamydia pneumoniae (AR-39), and Chlamydia psittaci (6BC). Mannose-binding protein (MBP) inhibited infection of all chlamydial strains by at least 50% at 0.098 microg/ml for TW-3 and UW-5, and at 6.25 microg/ml for 434, AR-39, and 6BC. The ability of MBP to inhibit infection with strain L2 was not affected by supplementation with complement or addition of an L2-specific neutralizing monoclonal antibody. Enzyme-linked immunosorbent assay and dot blot analyses showed MBP bound to the surface of the organism to exert inhibition, which appeared to block the attachment of radiolabeled organisms to HeLa cells. Immunoblotting and affinity chromatography indicated that MBP binds to the 40-kDa glycoprotein (the major outer membrane protein) on the outer surface of the chlamydial elementary body. Hapten inhibition assays with monosaccharides and defined oligosaccharides showed that the inhibitory effects of MBP were abrogated by mannose or high-mannose type oligomannose-oligosaccharide. The latter carbohydrate is the ligand of the 40-kDa glycoprotein of C. trachomatis L2, which is known to mediate attachment, suggesting that the MBP binds to high mannose moieties on the surface of chlamydial organisms. These results suggest that MBP plays a role in first-line host defense against chlamydial infection in humans.

Intracellular transport, cell-surface exposure and release of recombinant Tamm-Horsfall glycoprotein

Human Tamm-Horsfall glycoprotein (T-H), first described as the major urinary glycoprotein, is a glycosylphosphatidyl-inositol (GPI)-anchored membrane protein which mainly resides at the luminal face of cells of the thick ascending limb of Henle's loop (TAL) and early distal convoluted tubules of nephron. Since no human renal cell-line producing T-H is available, T-H cDNA was transfected in HeLa cells and a cell line was selected in which 95% of the cells stably expressed T-H, in order to elucidate the biosynthesis, mechanisms regulating the transport of T-H along the exocytic pathway, exposure at the cell surface and release in soluble form. Treatment of cells with an exogenous reducing agent results in a drastic delay in the conversion from precursor to mature T-H. Since the accumulating T-H-precursor carries glycans not yet processed by Golgi-mannosidases, we propose that the formation of a correct set of intrachain disulphide bonds is required for T-H exit out the endoplasmic reticulum. Even the treatment of cells with an inhibitor of GPI-anchor biosynthesis results in an intracellular accumulation of T-H precursor, loss of T-H localization into Golgi apparatus and reduced surface exposure. These results indicate that the GPI-anchor addition is necessary for T-H delivery to the cell-surface. The release rate of new synthesized T-H shows an initial lag time very likely depending on the time required for T-H surface exposure. A portion of released T-H appears to contain ethanolamine, a component of GPI anchor, indicating that, at least in HeLa cells, a GPI-specific phospholipase contributes to the T-H release. Exposure of cells to monensin and brefeldin A results in a loss of accumulation of T-H in the Golgi perinuclear region and a reduced delivery to the cell surface. Under monensin treatment an intermediate T-H form non-exposed at the cell surface is released in the medium, indicating that a soluble T-H may be produced inside the cell under conditions that alter the Golgi apparatus. If such an event occurs in polarized kidney cells, a T-H release from the basolateral face may be postulated, inasmuch as the GPI-anchor is an apical sorting signal. Since T-H is a powerful autoantigen, the accumulation of soluble T-H in the interstitium of TAL may cause the formation of immunocomplexes.

Secretion of alpha1,3-galactosyltransferase by cultured cells and presence of enzyme in animal sera

Glycosyltransferases are normally synthesized as membrane-anchored proteins. However, we recently found that the murine enzyme UDP-Gal:Gal beta1 -->4GLcNAc (Gal to Gal) alpha1,3 galactosyltransferase (alpha1,3GT) is secreted in a soluble form into media by mouse teratocarcinoma F9 cells (Cho SK, Yeh J-C, Cho M, Cummings RD (1996) J Biol Chem 271: 3238-46). To study the biosynthesis of this enzyme and whether secretion of the soluble enzyme is a general phenomenon, a solid-phase assay was developed for the alpha1,3GT activity. A recombinant and soluble form of the murine alpha1,3GT was produced in H293 cells (H293-alpha1,3GT) to aid in optimizing the assay. Desialylated orosomucoid was used as an immobilized acceptor in coated microtiter plates. The formation of product was detected by a biotinylated human-derived anti-alpha-Gal IgG and streptavidin conjugated to either alkaline phosphatase or the recombinant bioluminescent protein aequorin. Enzyme activity was dependent on the concentrations of asialoorosomucoid, UDP-Gal, alpha1,3GT and the time of incubation. The assay was also useful in monitoring alpha1,3GT activity during enzyme enrichment procedures. Using this assay, we found that alpha1,3GT activity was present in both cell extracts and culture media of several mammalian cell lines. Enzyme activity was also present in the sera from several mammals, but activity was absent in the sera from either humans or baboons. Our results demonstrate the development of a novel assay for the alpha1,3GT and provide evidence that secretion of the enzyme is a common biological phenomenon.

Expression of human H-type alpha1,2-fucosyltransferase encoding for blood group H(O) antigen in Chinese hamster ovary cells. Evidence for preferential fucosylation and truncation of polylactosamine sequences

The human H(O) blood group is specified by the structure Fucalpha1-2Galbeta1-R, but the factors regulating expression of this determinant on cell surface glycoconjugates are not well understood. To learn more about the regulation of H blood group expression, cDNA encoding the human H-type GDPFuc:beta-D-galactoside alpha1, 2-fucosyltransferase (alpha1,2FT) was stably transfected into Chinese hamster ovary (CHO) cells. The new cell line, designated CHO(alpha1,2)FT, expressed surface neoglycans containing the H antigen. The structures of the fucosylated neoglycans in CHO(alpha1, 2)FT cells and the distribution of these glycans on glycoproteins were characterized. Seventeen percent of the [3H]Gal-labeled glycopeptides from CHO(alpha1,2)FT cells bound to the immobilized H blood group-specific lectin Ulex europaeus agglutinin-I (UEA-I), whereas none from parental CHO cells bound to the lectin. The glycopeptides from CHO(alpha1,2)FT cells binding to UEA-I contained polylactosamine [3Galbeta1-4GlcNAcbeta1-]n with the terminal sequence Fucalpha1-2Galbeta1- 4GlcNAc-R. Fucosylation of the polylactosamine sequences on complex-type N-glycans in CHO(alpha1, 2)FT cells caused a decrease in both sialylation and length of polylactosamine. Unexpectedly, only small amounts of terminal fucosylation was found in diantennary complex-type N-glycans. The O-glycans and glycolipids were not fucosylated by the H-type alpha1, 2FT. Two major high molecular weight glycoproteins, one of which was shown to be the lysosome-associated membrane glycoprotein LAMP-1, preferentially contained the H-type structure and were bound by immobilized UEA-I. These results demonstrate that in CHO cells the expressed H-type alpha1,2FT does not indiscriminately fucosylate terminal galactosyl residues in complex-type N-glycans, but it favors glycans containing polylactosamine and dramatically alters their length and sialylation.

An N-linked high-mannose type oligosaccharide, expressed at the major outer membrane protein of Chlamydia trachomatis, mediates attachment and infectivity of the microorganism to HeLa cells

The structure of the carbohydrate of the 40-kD major outer membrane component of Chlamydia trachomatis and its role in defining infectivity of the organism were investigated. The oligosaccharides were released from the glycoprotein by N-glycanase digestion, coupled to a 2-aminopyridyl residue, and subjected to two-dimensional sugar mapping technique. The major fractions consisted of "high-mannose type" oligosaccharides containing 8-9 mannose residues. Bi- and tri-antennary "complex type" oligosaccharides having terminal galactose were detected as minor components. These oligosaccharides were N-linked and contained no sialic acid. This structural profile is consistent with our previous characterization based on lectin-binding and glycosidase digestion. Functional specificity of identified chlamydial oligosaccharides was analyzed using glycopeptides fractionated from ovalbumin and structurally defined oligosaccharides from other sources. The glycopeptide fraction having high-mannose type oligosaccharide, as compared to those having complex or hybrid-type, showed a stronger inhibitory effect on attachment and infectivity of chlamydial organisms to HeLa cells. Among high-mannose type oligosaccharides, the strongest inhibition was observed with mannose 8 as compared with mannose 6, 7, or 9. These results indicate that a specific high-mannose type oligosaccharide linked to the major outer membrane protein of C. trachomatis mediates attachment and infectivity of the organism to HeLa cells.

Characterization of fucosyllactose determinant-bearing glycoproteins probed by a Biomphalaria alexandrina lectin in Schistosoma mansoni cercariae

Utilizing a Biomphalaria alexandrina-derived lectin (BaSII) of proven specificity to a Schistosoma mansoni-associated fucosyllactose [(Fuc alpha 1-2) Gal beta 1-4 Glc] determinant, two determinant-bearing glycoproteins of 40 and 37 kDa were found to be synthesized by the cercarial stage of the parasite. The two glycoproteins were isolated by BaSII affinity column chromatography from extracts of cercariae metabolically radiolabelled with 35S-methionine. Treatments with endoglycosidases, alkaline borohydride, as well as concanavalin A column chromatography and analysis by two-dimensional gels indicated that the two glycoproteins are synthesized as a single 33 kDa polypeptide backbone that is differentially glycosylated with one and/or two determinant-bearing N-linked complex-type glycan units of either the biantennary, or, to a lesser extent, the tri- or tetra-antennary types. The two glycoproteins lack other conventional high mannose-type or O-linked glycans, and the distinct structures of the complex-type oligosaccharides accounted solely for the expression of three isomorphs for each determinant-bearing glycoprotein. Based on the structural relatedness of the fucosyllactose determinant to the antigenic mammalian blood group H trisaccharide, our observations may have implications in mechanisms of host-parasite interactions as well as immunoprophylaxis.

Structure of glycopeptides isolated from bovine milk component PP3

The heat-stable acid-soluble phosphoglycoprotein component PP3 was isolated from the bovine milk proteose peptone fraction by concanavalin A affinity chromatography. Glycopeptides were released by pronase digestion of the milk component PP3 and were subsequently separated by high-pH anion-exchange chromatography on CarboPac PA-1. The primary structures of the glycan and peptide moieties of eight N-glycopeptides have been established by combining methylation analysis, mass spectrometry, 400-MHz 1H-NMR spectroscopy, and peptide sequence analysis. All the analyzed fractions contained biantennary N-acetyllactosamine-type carbohydrate chains, some of them with a GalNAc(beta 1-4)GlcNAc or a NeuAc(alpha 2-6)GalNAc(beta 1-4)GlcNAc group. This particular sequence did or did not replace the Gal(beta 1-4)GlcNAc group usually found in most N-linked glycans. Moreover, the sialylated Gal and GalNAc residues were only found on the Man(alpha 1-3) antenna.

Sex hormone-binding globulin/androgen-binding protein: steroid-binding and dimerization domains

Plasma sex hormone-binding globulin (SHBG) and testicular androgen-binding protein (ABP) are homodimeric glycoproteins that share the same primary structure, and differ only with respect to the types of oligosaccharides associated with them. The biological significance of these differences is not understood, but enzymatically deglycosylated SHBG and a non-glycosylated SHBG mutant both bind steroids normally. Various affinity-labelling experiments, and studies of recombinant SHBG mutants have indicated that a region encompassing and including Met-139 in human SHBG represents an important component of its steroid-binding site. Analyses of chimeric proteins comprising various portions of human SHBG and rat ABP have also indicated that residues important for the much higher affinity of human SHBG for steroid ligands are probably located within the N-terminal portion of these molecules. Recent studies of SHBG mutants have confirmed this, and a deletion mutant containing only the first 205 N-terminal residues of human SHBG has been produced which dimerizes and binds steroids appropriately. The introduction of amino-acid substitutions between Lys-134 and Phe-148 of SHBG has also indicated that residues including and immediately N-terminal of Met-139 may influence steroid-binding specificity, while those immediately C-terminal of Met-139 represent at least a part of the dimerization domain. These studies have also demonstrated that dimerization is induced by the presence of steroid ligand in the binding site, and that divalent cations play an important role in this process. Together, these data have led us to conclude that SHBG is a modular protein, which comprises an N-terminal steroid-binding and dimerization domain, and a C-terminal domain containing a highly-conserved consensus sequence for glycosylation that may be required for other biological activities, such as cell-surface recognition.

Analysis of asparagine-linked oligosaccharides by sequential lectin affinity chromatography

Lectins are proteins that specifically bind to a particular carbohydrate structure. Affinity chromatography with immobilized lectins is a quite effective technique not only for the fractionation of glycoproteins or oligosaccharides but also their structural assessment. In this article, we focus on the separation of glycopeptides and oligosaccharides derived from glycoproteins by affinity chromatography on immobilized lectin columns.

alpha 1-Acid glycoprotein binds human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein via N-linked glycans

In the present study, we demonstrate a specific low-affinity interaction between recombinant precursor gp160 (rgp160) or surface unit gp120 (rgp120) of human immunodeficiency virus type 1 (HIV-1) and alpha 1-acid glycoprotein (AGP), a human glycoprotein displaying complex type N-glycans. Binding of rgp160/rgp120 to agarose-coupled AGP was dose-dependent, saturable, calcium-, pH- and temperature-dependent. Binding was inhibited by soluble AGP, asialo-AGP, fetuin, beta-D-GlcNAc47-BSA, alpha-D-Man20-BSA, mannan, complex-type asialo-agalacto-tetraanternary precursor oligosaccharide from human AGP and oligomannose 9 from porcine thyroglobulin; fully deglycosylated AGP was not inhibitory. The three AGP glycoforms separated on immobilized ConA bound rgp160 to the same extent as did unfractionated AGP. These findings extend our previous results on the carbohydrate-binding properties of HIV-1 envelope (Env) glycoprotein in that they demonstrate the involvement of AGP glycan moieties in the binding to rgp160/rgp120. Preincubation of rgp160 with AGP or mannan significantly reduced its binding to monocyte-derived macrophages (MDM), suggesting that AGP may play a role in preventing binding of soluble or virus-bound Env glycoprotein to CD4+ monocytic cells.

Inhibitors of N-linked oligosaccharide processing glucosidases interfere with oligodendrocyte differentiation in culture

Previous studies have demonstrated that inhibitors of glycoprotein processing glucosidases interfere with the development of oligodendrocyte properties in primary cultures of embryonic rat brain cells (Bhat, J Neurosci Res 20:158-164, 1988). The present study examines the effect of castanospermine, an inhibitor of the processing glucosidases, on the development and differentiation of isolated oligodendrocyte progenitor cells. Treatment of oligodendrocyte progenitors with castanospermine did not affect the developmental progression of the precursors to become committed oligodendrocytes as revealed by comparable increases in the percentages of cells positive for galactocerebroside (a surface marker for terminally differentiated oligodendrocytes) in control and drug-treated cultures. On the other hand, there was an impairment of the expression of differentiated properties of oligodendrocytes [i.e., sulfolipid synthesis, myelin basic protein (MBP)] and 2'3'-cyclic nucleotide 3'-phosphohydrolase in the drug-treated cultures. Immunocytochemical analysis with anti-MBP antibodies revealed a reduced number of MBP-positive cells in inhibitor-treated cultures. Furthermore, a majority of MBP-positive cells in such cultures displayed immunoreactive MBP in their cell body and not the processes, unlike in control cultures where both cell body and the processes of oligodendrocytes stained intensely for MBP. The strong inhibitory effect of castanospermine on the expression of oligodendrocyte-specific activities was contrasted with a relatively smaller effect of swainsonine, a mannosidase inhibitor on oligodendrocyte differentiation. Both castanospermine and swainsonine, however, effectively blocked the formation of complex-type oligosaccharides, suggesting thereby a lack of correlation between the inhibition of the formation of complex-type oligosaccharides and oligodendrocyte differentiation. It is suggested, therefore, that early trimming reactions involving the removal of glucose residues from the high mannose oligosaccharides in the endoplasmic reticulum may be essential for the cell surface localization and function of glycoproteins critically involved in surface interactions of oligodendrocytes with each other and/or with the substratum.

New state markers for alcoholism. Comparison of carbohydrate deficient transferrin (CDT) and alcohol mediated (triantennary) transferrin (AMT)

Carbohydrate deficient transferrin (CDTect-RIA, Pharmacia) was compared with an Immunoluminometric assay for isotransferrin separated by a short column Con-A sepharose which we have called alcohol mediated triantennary transferrin (AMT). 101 in-patients with alcohol dependency syndrome (alcohol consumption of more than 60 g/day) were grouped according to the time of abstinence A1 (0-7 days), A2 (8-14 days), A3 (> or = 15 days). Serum samples were obtained at admission (U0) and under abstinent conditions after 10-20 days (U1). All groups were controlled for AMT, CDT, GGT, MCV. Control groups were 30 in-patients with non alcoholic liver disease (NALD) and 31 healthy volunteers (alcohol consumption of less than 20g/day). Results showed for CDT and AMT highly significant differences between short abstinence period (group A1) and more than two weeks abstinence (group A3) alcoholics and between group A1 and healthy controls. In group A1 CDT was significantly elevated (P < or = .001) compared to NALD group whereas AMT showed no differences. CDT (cut off 22 mg/l) showed a high diagnostic specificity (A1/controls 97%, A1/NALD 83%, A1/A3 78%) but only a diagnostic sensitivity of 61%. AMT (cut off 260 mg/l) revealed a diagnostic test sensitivity of 74%. The diagnostic test specificity of AMT was inferior to CDT (A1/controls 74%, A1/NALD 50%, A1/A3 70%). Initial CDT and AMT values in alcoholics were highly correlated (P < or = .001) with time of abstinence. CDT and AMT decline was correlated with time of abstinence. CDT was proved for high significant (P < or = .001) decline over a longer period of abstinence (11-30 days) while AMT decline was significant (P = .008) only in early abstinence (0-10 days). Presence of a withdrawal syndrome was highly correlated (P < or = .01) with CDT values above 22 mg/l and AMT values above 260 mg/l. Furthermore in selected follow up cases it was shown that AMT seemed to be a more sensitive indicator for short alcoholic relapses than CDT.

Structural characterization of the carbohydrates of the rat ovarian luteinizing hormone/chorionic gonadotropin receptor

The numbers and types of oligosaccharide present on the rat ovarian luteinizing hormone (LH)/chorionic gonadotropin (CG) receptor were determined by treating radiolabelled purified receptors with glycosidases and examining the changes in electrophoretic mobility and number of radiolabelled bands on SDS/PAGE. The purified receptor was also transferred to nitrocellulose after SDS/PAGE and probed with digoxigenin-labelled lectins. The following conclusions were drawn: (1) the rat ovarian LH/CG receptor contains at least two complex-type N-linked oligosaccharide chains, of which one is biantennary and the rest multiantennary. (2) The N-linked chains terminate in either unsubstituted galactose or sialic acid linked alpha 2-3 or alpha 2-6 to the penultimate galactose. (3) The N-linked oligosaccharides also contain internal poly(N-acetyl-lactosamine) sequences and fucose-linked alpha 1-6 to the proximal N-acetylglucosamine. (4) No O-linked carbohydrate moieties are present on the receptor molecule.

Different forms of 130 kD connective tissue protein are specific for boundaries in the nervous system and basement membrane of muscle cells in leech

The nervous system and muscle tissue of the leech express two different organ-specific forms of connective tissue protein. The nervous system-specific form appears in regional boundaries separating cell bodies, axonal tracts and areas of the neuropile during late embryogenesis. In contrast, the muscle-specific form appears earlier during development in the basement membrane of muscle cells. In extraction experiments both forms behave like extracellular matrix proteins and because of their molecular weight, are considered members of a group of cell type-specific 130 kD proteins (leech gp130s). However, the two forms differ in their posttranslational modification. As determined by Con A and lentil lectin affinity chromatography, only the nervous system-specific, but not the muscle-specific form, has fucosylated and high mannose N-linked carbohydrates. These differences in the developmental onset and glycosylation suggest that nervous system-specific and muscle-specific connective tissue proteins are regulated differently and participate in different molecular interactions.

Partial purification and characterization of mold antigens commonly found in foods

Rapid methods are needed for detection of molds in foods; therefore, an enzyme-linked immunosorbent assay was developed. The extracellular and mycelial antigens for Mucor, Aspergillus, Cladosporium, and Geotrichum species were partially purified and characterized. The molecular masses of the mycelial and extracellular antigens, as determined by size exclusion chromatography, ranged from 4.5 x 10(5) to 6.7 x 10(5) Da. There was only one main antigenic peak separated by Sepharose CL-4B and concanavalin A-Sepharose columns for Mucor, Cladosporium, and Geotrichum mycelial and extracellular antigens, but there were two for Aspergillus mycelial antigens and three for Aspergillus extracellular antigens. These antigens contained 10 to 50% protein which was part of the active site since protease digestion significantly decreased antigenic activity. Neutral sugars, ranging from 13 to 75%, made up the rest of the active site, and < 1% phosphate was detected in mycelial antigens. Geotrichum, Cladosporium, and Aspergillus antigens contained mainly glucose, galactose, and mannose. Mucor antigens contained these sugars plus fucose. The percentage of sugars differed between the mycelia and extracellular antigens. Enzymatic digestion and competitive inhibition tests using different sugar derivatives showed that galactosyl residues with beta linkages were immunodominant for Aspergillus, Geotrichum, and Cladosporium antigens and mannosyl residues with alpha linkages were immunodominant for Mucor antigens.

Heterogeneity of bovine lactotransferrin glycans. Characterization of alpha-D-Galp-(1-->3)-beta-D-Gal- and alpha-NeuAc-(2-->6)-beta-D-GalpNAc-(1-->4)- beta-D-GlcNAc-substituted N-linked glycans

Lactotransferrin isolated from a pool of mature bovine milk has been shown to contain N-glycosidically-linked glycans possessing N-acetylneuraminic acid, galactose, mannose, fucose, N-acetylglucosamine, and N-acetylgalactosamine. The glycopeptides obtained by Pronase digestion were fractionated by concanavalin A-Sepharose affinity chromatography into three fractions: slightly retained (A), retained (B), and strongly retained (C). The structure of the glycans of the three fractions has been determined by application of methanolysis, methylation analysis, fast atom bombardment-mass spectrometry, and 1H NMR spectroscopy. Diantennary structures without GalNAc were present as partially sialylated and partially (1-->6)-alpha-L-fucosylated structures in Fractions A and B. Sequences containing alpha-D-Galp-(1-->3)-beta-D-Gal on the alpha-D-Man-(1-->6) antenna, and beta-D-GalpNAc-(1-->4)-beta-D-GlcNAc and alpha-NeuAc-(2-->6)-beta-D-GalpNAc-(1-->4)-beta-D-GlcNAc on the alpha-D-Man-(1-->3) antenna were characterized in the oligosaccharide-alditols obtained by reductive cleavage of Fraction B. A series of Man4-9-GlcNAc structures were identified in Fraction C after endo-N-acetyl-beta-D-glucosaminidase digestion. These results show that the structures of bovine lactotransferrin glycans are more heterogeneous than those of previously characterized transferrin glycans.