The peptide moiety of blood-group-specific glycoproteins. Some amino-acid sequences in the regions carrying the carbohydrate chains

Homology between ABH-carrier α2-seminoglycoprotein and Mac-2 binding protein

alpha2-seminoglycoprotein (alpha2-SGP), purified from human seminal plasma, is a carrier of glycoprotein for the ABO blood grouping. The alpha2-SGP exists in the secretions of the seminal vesicle and various glands. However, the function of alpha2-SGP is, as yet, unknown. In this study, we determined that two internal amino acid sequences of 8 and 12 residues of alpha2-SGP were Ala-Val-Asp-Thr-Trp-Ser-Trp-Gly and Thr-Leu-Gln-Ala-Leu-Glu-Phe-His-Thr-Val-Pro-Phe. These sequences were completely coincident with the domain 3 of human Mac-2 binding protein (M2BP), which was identified as a tumor-associated antigen. In addition, we also confirmed an alpha2-SGP binding activity to galectin-3 that was one of a ligand for M2BP, and the immunological cross-reactivity between alpha2-SGP and M2BP. These findings strongly suggested that alpha2-SGP was identical with M2BP.

Mucin-type glycoproteins

Considerable advances have been made in recent years in our understanding of the biochemistry of mucin-type glycoproteins. This class of compounds is characterized mainly by a high level of O-linked oligosaccharides. Initially, the glycoproteins were solely known as the major constituents of mucus. Recent studies have shown that mucins from the gastrointestinal tract, lungs, salivary glands, sweat glands, breast, and tumor cells are structurally related to high-molecular-weight glycoproteins, which are produced by epithelial cells as membrane proteins. During mucin synthesis, an orchestrated sequence of events results in giant molecules of Mr 4 to 6 x 10(6), which are stored in mucous granules until secretion. Once secreted, mucin forms a barrier, not only to protect the delicate epithelial cells against the extracellular environment, but also to select substances for binding and uptake by these epithelia. This review is designed to critically examine relations between structure and function of the different compounds categorized as mucin glycoproteins.

O glycosylation of glycoprotein G of human respiratory syncytial virus is specified within the divergent ectodomain

cDNAs encoding the G glycoprotein of respiratory syncytial virus and the hemagglutinin-neuraminidase (HN) glycoprotein of parainfluenza virus type 3 were modified by site-specific mutagenesis and restriction fragment replacement to encode chimeric proteins consisting of the cytoplasmic and transmembrane domains of one protein fused to the ectodomain of the other. In the case of the HN ectodomain attached to the G transmembrane and cytoplasmic domains, cell surface expression of the chimera was reduced. Otherwise, the presence of the heterologous transmembrane and cytoplasmic domains had little effect on the processing of the HN or G ectodomain, as assayed by the acquisition of N-linked and O-linked carbohydrates, transport to the cell surface and, in the case of HN, folding, oligomerization, and hemadsorption activity. These results showed that the synthesis and processing of each ectodomain did not require the homologous transmembrane and cytoplasmic domains. In particular, O glycosylation of the G protein was specified fully by its ectodomain, even though this domain is highly divergent among the respiratory syncytial virus antigenic subgroups. In addition, whereas the cytoplasmic and transmembrane domains of the G protein were relatively highly conserved, they were nonetheless fully replaceable without significantly affecting processing.

Mucus glycoprotein gels. Role of glycoprotein polymeric structure and carbohydrate side-chains in gel-formation

The structure of mucus glycoprotein gels from the pig gastrointestinal tract was investigated by mechanical spectroscopy. Gastric, duodenal, and colonic mucus had the same mechanical profile, characteristic of a viscoelastic gel. The gel structure collapsed on destruction of the polymeric structure of the component glycoprotein by reduction with 0.2M mercaptoethanol or after proteolysis with papain. The progressive weakening of mechanical properties and the decrease in polymeric glycoprotein content were measured as functions of time of reduction. A linear correlation was obtained between the gel quality [defined by tan delta, the ratio of the loss modulus (G'') to the storage modulus (G')] and the proportion of polymeric to subunit glycoprotein in the mucus. Purified mucus glycoprotein, at the same concentration as that in native mucus, resulted in a gel with mechanical properties no different from those of the respective native secretion, demonstrating that the glycoprotein alone could reproduce the gel-forming properties of mucus. After proteolytic digestion, all native secretions and reconstituted mucus showed an absence of Newtonian behaviour in the frequency dependence of dynamic viscosity at low frequencies. This provided evidence that the noncovalent interactions, characteristic of the native gel matrix, were still present after proteolytic digestion when the nonglycosylated protein core accessible to proteinases had been removed. These results were interpreted to show (a) a common mechanism for gel-formation in gastric, duodenal, and colonic mucus; (b) that the polymeric structure of mucus glycoproteins confers the three-dimensional structure necessary for formation of the gel network; and (c) that noncovalent interactions which arise between the glycoprotein molecules by relatively stable interdigitation of the carbohydrate side-chains are involved in formation of the gel network.

Specific method for the fragmentation of the polypeptide chain of glycoproteins. Distribution of carbohydrate chains on the peptide core of blood-group-specific glycoprotein

A method for specific fragmentation of the polypeptide backbone of glycoproteins at the glycosylated serine and threonine residues has been developed. The fragmentation includes beta-elimination of the carbohydrate chains, followed by bromination of the resulting enamine groups, and cleavage of the brominated amino acid residues by alkaline sodium borohydride. The method was employed for fragmentation of the peptide core of pig blood-group substance H. Essentially all the serine and threonine residues were shown to be O-glycosylated, and rather frequently either adjacent or separated by a single amino acid (mainly alanine). When they were separated by two or three amino acid residues, proline was preponderant.

Studies of the limited degradation of mucus glycoproteins. The effect of dilute hydrogen peroxide

1. The action of dilute H2O2 on a series of ovarian-cyst glycoproteins and glycopolypeptides was investigated. 2. Both native glycoproteins and the glycopolypeptides were carbohydrate-rich, of relatively low molecular weight and of simple structure. 3. At pH 5.6 and 37 degrees C, exposure to H2O2 for a limited time brought about a partial degradation, the molecular weight being decreased by 2-4-fold. 4. Carbohydrate analysis showed very little change in the oligosaccharide moiety, apart from a small decrease in sialic acid in some samples. 5. Amino acid analysis showed minor changes in serine, threonine and proline contents, but almost total loss of histidine. Concomitantly, there was a small gain in aspartic acid. 6. Myosin, examined at both pH 5.7 and 6.7, exhibited generally similar behaviour, there being losses of other amino acid residues as well as histidine: the viscosity was decreased to a low value, and a range of peptides of widely varying size was produced. 7. It is suggested that attack on the histidine residue, with partial conversion into aspartic acid, is accompanied by scission of the histidyl peptide bond.

Isolation and characterization of the major oligosaccharide of human platelet membrane glycoprotein GPIb

Treatment of intact human platelets with chymotrypsin released a glycopolypeptide that was shown to be derived from the major membrane glycoprotein, GPIb. The glycopolypeptide contained 59% carbohydrate on a molar basis and was rich in serine, threonine and proline. Almost all the carbohydrate could be released from the glycopolypeptide by treatment with alkali in the presence of NaBH4. The major component (comprising 80% of the released sugar) was purified and shown to be a hexasaccharide containing sialic acid, galactose, N-acetylglucosamine and N-acetylgalactosaminitol in the molar ratios 2:2:1:1. Two possible structures for this hexasaccharide are proposed on the basis of the known biosynthetic pathways of mucus-type glycoproteins. Our data is consistent with the occurrence of an O-glycosidically linked oligosaccharide on one amino acid in four of the glycopolypeptide. These results suggest that glycoprotein Ib can best be described as a membrane-bound mucus-type glycoproteins. Our data are consistent with the occurrence of an O- in the process by which platelets adhere to the exposed subendothelium of damaged blood-vessel walls. The possible role of the glycopolypeptide portion of GPIb in this process was investigated. Neither the major oligosaccharide nor the glycopolypeptide itself inhibited ristocetin-induced platelet agglutination at the concentrations tested. It is suggested that the carbohydrate moieties of GPIb molecules at the cell surface interact to form a barrier to macromolecules. Such a barrier could play a major role in modulating platelet function.

Serum levels of glycosyltransferases and related glycoproteins as indicators of cancer: biological and clinical implications

Many studies have suggested that malignant transformation is associated with fundamental changes in the cell surface; similar changes have been described for normal stem cells and cells of embryonic or fetal origin. There is now evidence that the tumor cell secretes or sheds glycoproteins and glycosyltransferases into the surrounding medium and into serum. There are claims that some of these serum glycoproteins and glycosyltransferases are associated with, or specifically related to, the extent of tumor growth and may serve as a cancer marker. A cancer-associated galactosyltransferase isoenzyme (GT-II) has been described and purified. Different isoelectric forms of fucosyltransferase have also been described as indicative of malignancy. The articles to be published in CRC Critical Reviews in Clinical Laboratory Sciences will analyze the evidence for the association of these membrane factors with tumor growth. In order to better understand the possible significance of altered glycoproteins and of increased or different forms of glycosyltransferases during tumor growth, recent data on glycoprotein synthesis will be discussed including the new concepts on the control of glycoprotein synthesis through lipid intermediates. The possible mechanisms whereby malignant transformation could alter glycoprotein synthesis will be discussed with particular emphasis on the significance of these alterations to the biology of the malignant cell. Changes in surface membrane glycoproteins have long been implicated in the ability of a cell to metastasize. Secretion and/or shedding of the cell surface may also be important in the process of metastasis and in altering the host immune response. Detection and the study of these "shed" materials in patients appear to be indicating a new approach to cancer biology detection and therapy.

Partial deglycosylation of blood-group-specific glycoproteins

The time course for the partial deglycosylation of blood-group-specific glycoproteins from human ovarian-cyst fluids with 0.25 M-H2SO4/acetic acid and 6 M-HCl in methanol was studied. Either reagent readily removed about 80% of the carbohydrate from the glycoproteins to leave non-diffusible glycopeptides that contain N-acetylgalactosamine as the predominant sugar. Some changes in amino acid distribution were observed during the deglycosylation, which were attributed to an accelerated break-up of the nonglycosylated regions of the parent glycoprotein. The N-acetylgalactosaminyl-peptides isolated were judged to be polydisperse by gel filtration, and ion-exchange chromatography divided the glycopeptide population into several fractions with differing amino acid compositions. A Lumbricus terrestris hexosaminidase preparation was successful in removing almost all the remaining sugar from the glycopeptides, but caused further rupture of the peptide. When a per O-acetylated glycoprotein was treated with the H2SO4/acetic acid reagent the glycopeptide contained, in addition to N-acetylgalactosamine, about 50% of the sialic acid present in the parent glycoprotein, indicating that most of this sugar is located near the peptide end of the carbohydrate chains.

Isolation and characterization of glycoproteins from canine tracheal mucus

Three homogeneous glycoproteins were isolated from reduced and S-carboxy-methylated canine tracheal pouch mucus by gel filtration and ion-exchange chromatography. Initial fractionation was carried out on Sephadex G-200; chromatography of the excluded Sephadex G-200 fraction on Bio-Gel A-15 m yielded two high molecular weight glycoprotein fractions. Following rechromatography on the same column, the main fraction behaved as an electrophoretically homogeneous high molecular weight (581 600) glycoprotein, with a high carbohydrate content (80%) and a single amino-terminal amino acid (arginine). Ion-exchange chromatography (DEAE-cellulose) of the included Sephadex G-200 fraction yielded two electrophoretically homogeneous glycoproteins of lower molecular weight (20 800 and 24 600, respectively). A single amino-terminal amino acid, glycine and alanine, respectively, was detected for each glycoprotein. Chemical analysis of these three glycoproteins revealed the presence of fucose, galactose, N-acetylgalactosamine, N-acetylglucosamine, N-acetylneuraminic acid and sulfate monoester. The high molecular weight glycoprotein had a higher hexose, sialic acid and sulfate content, per mg of protein, than the low molecular weight glycoproteins. The results of the alkaline borohydride treatment indicated that the majority of the carbohydrate chains of these glycoproteins are linked to the protein core through O-glycosidic bonds involving N-acetylgalactosamine and serine or threonine.

Cyanogen bromide fragments of bovine cervical mucus glycoprotein

Treatment of bovine cervical mucus glycoprotein with cyanogen bromide gives four fractions, with the molecular weights of the three major fractions being 230 000, 130 000, and 35 000. The results indicate that, as for other glycoproteins, there are different regions along the protein core, some of which have a high sugar content and others which contain considerably less carbohydrate; it seems likely that the regions of lower sugar content may be important to intermolecular linkages. The data suggest that the basic unit of the glycoprotein has a molecular weight of 550 000-600 000, with its protein core consisting of approx. 1200 amino acid residues.