Threonine and Serine Linkages in Mucopolysaccharides and Glycoproteins

Cytosolic O-glycosylation is abundant in nerve terminals

Phosphorylation plays a key role in regulating growth cone migration and protein trafficking in nerve terminals. Here we show that nerve terminal proteins contain another abundant post-translational modification: beta-N-acetylglucosamine linked to hydroxyls of serines or threonines (O-GlcNAc(1)). O-GlcNAc modifications are essential for embryogenesis and mounting evidence suggests that O-GlcNAc is a regulatory modification that affects many phosphorylated proteins. We show that the activity and expression of O-GlcNAc transferase (OGT) and N-acetyl-beta-D-glucosaminidase (O-GlcNAcase), the two enzymes regulating O-GlcNAc modifications, are present in nerve terminal structures (synaptosomes) and are particularily abundant in the cytosol of synaptosomes. Numerous synaptosome proteins are highly modified with O-GlcNAc. Although most of these proteins are present in low abundance, we identified by proteomic analysis three neuron-specific O-GlcNAc modified proteins: collapsin response mediator protein-2 (CRMP-2), ubiquitin carboxyl hydrolase-L1 (UCH-L1) and beta-synuclein. CRMP-2, which is involved in growth cone collapse, is a major O-GlcNAc modified protein in synaptosomes. All three proteins are implicated in regulatory cascades that mediate intracellular signaling or neurodegenerative diseases. We propose that O-GlcNAc modifications in the nerve terminal help regulate the functions of these and other synaptosome proteins, and that O-GlcNAc may play a role in neurodegenerative disease.

Keratan sulfate glycosaminoglycans in murine eosinophil-specific granules

We examined the presence of sialyl glycoconjugates in specific granules from murine bone marrow eosinophils. Lectin cytochemistry using Maackia amurensis lectin II (MAL II) specific for sialyl alpha-2,3 galactose residues demonstrated positive labeling in both immature and mature specific granules. Pretreatment with Clostridium neuraminidase or keratanase II eliminated the positive labeling of MAL II in the specific granules. High iron diamine-thiocarbohydrazide-silver proteinate physical development (HID-TCH-SP-PD) staining, which is specific for sulfated glycoconjugates, also positively labeled immature specific granules lacking crystalloids but not mature granules with crystalloids. Pretreatment with a combination of chondroitinase ABC and keratanase, or a combination of chondroitinase ABC and keratanase II, eliminated the positive labeling obtained with HID-TCH-SP-PD. These results indicate that the sialyl residues detected by MAL II are expressed as terminal sugar residues of keratan sulfate proteoglycan, which appears to be of the corneal type in view of its sensitivity to keratanase and keratanase II. (J Histochem Cytochem 47:481-488, 1999)

Identification and structural analysis of the tetrasaccharide NeuAc alpha(2-->6)Gal beta(1-->4)GlcNAc beta(1-->3)Fuc alpha 1-->O-linked to serine 61 of human factor IX

O-Linked fucose has been found attached to Thr/Ser residues within the sequence Cys-X-X-Gly-Gly-Thr/Ser-Cys in the N-terminal EGF domains of several coagulation/fibrinolytic proteins. Carbohydrate composition and mass spectrometric analyses of tryptic and thermolytic peptides containing the corresponding site (Ser-61) in the first EGF domain of human factor IX indicated the presence of a tetrasaccharide containing one residue each of sialic acid, galactose, N-acetylglucosamine, and fucose. The Ser-61 tetrasaccharide was not susceptible to alpha-fucosidase digestion. Fragments generated during mass spectrometric analysis indicated that fucose was the attachment sugar residue. The involvement of fucose in the carbohydrate-peptide linkage was confirmed by two-dimensional 1H NMR spectroscopic analysis of the glycopeptide containing factor IX residues 57-65. The complete structure of the tetrasaccharide was obtained by methylation analysis and two-dimensional 1H TOCSY and ROESY experiments as NeuAc alpha(2-->6)Gal beta(1-->4)GlcNAc beta(1-->3)Fuc alpha 1-->O-Ser61.

Glycopeptidolipids from Mycobacterium fortuitum: a variant in the structure of C-mycoside

Strains from the Mycobacterium fortuitum complex contain surface species-specific lipids allowing their precise identification. In M. fortuitum biovar. peregrinum two major glycopeptidolipids, of the C-mycoside type, were characterized by a combination of chemical analyses, NMR, and FAB mass spectrometry. Important information was obtained by mass spectrometry both on their molecular weight and on the peptide and saccharide sequences without any derivatization. The basic structure of the two compounds was shown to be [formula: see text] The disaccharide part linked O-glycosidically to alaninol was either 3,4-di-O-methyl-alpha-L-rhamnopyranosyl (1----2) 3,4-di-O-methyl-alpha-L-rhamnopyranoside (mycoside I) or 3-O-methyl-alpha-L-rhamnopyranosyl (1----2) 3,4-di-O-methyl-alpha-L-rhamnopyranoside (mycoside II). This is an unusual structure of a C-mycoside since neither 6-deoxytalose nor its derivatives are present. Moreover, the oligosaccharide part is linked to the alaninol residue instead of the allo-threonine.

Biochemistry and lectin binding properties of mammalian salivary mucous glycoproteins

The molecules responsible for the highly viscous properties of mucus are secretory glycoproteins referred to as mucins. Salivary mucins are characterized by a high sugar to protein ratio and are of a broad range of molecular weight from 7 x 10(4) to millions. With a few exceptions, they contain up to 30% of hexosamine (galactosamine and glucosamine), 8-33% of sialic acid, trace to 15% of galactose or fucose and little or no mannose. The size of carbohydrate side chains of these glycoproteins ranges from one to about fifteen units of sugar. These carbohydrate side chains are usually O-glycosidically linked through N-acetylgalactosamine to a peptidyl serine or threonine. In some instances, ester sulfate groups, mainly on N-acetylglucosamine, are also a structural feature. In many of these glycoproteins, the saccharide sequence is the same as that which determines the specificity of blood groups. Carbohydrate sequence analysis shows that salivary mucins exhibit considerable polydispersity, great diversity and remarkable structural flexibility not only among animal species but also within the same mucin molecule. Based on their lectin-binding ability, they can be used for purification of lectins, and lectins coupled to resin may be useful for the isolation of mucin-type glycoproteins. The epithelial mucous secretions modulate oral microbial flora; many secretory components serve as lectin-receptors for the attachment of microbes. The judicious use of lectins with widely differing binding characteristics has already been valuable in the in situ localization of salivary glycoproteins, in elucidating structural details, recording sugar density within a given tissue section, and defining host-parasite interactions. It is hoped that their use, together with monoclonal antibody (158) and tissue culture techniques (159, 160) will further clarify the roles of individual secretory mucous glycoproteins in health and disease.

An epitope residing in carbohydrate chains of a sea squirt antigen termed Gi-rep

O-Glycosidically linked oligosaccharides (Gp-1 beta-b) were liberated from a large size glycopeptide (GP) fraction (Gp-1) in a Pronase digest of a sea squirt antigen termed Gi-rep by the treatment with 0.1 N NaOH per 1 mol/L of NaBH4. Gp-1 beta-b as well as Gp-1 and the intact antigen was capable of inducing a remarkable erythema in the skin of patients with asthma with sea squirt allergy at its concentration of 10 micrograms/ml. N-Glycoside GP fractions (Gp-1 beta-a and Gp-2 beta) having the allergenic activity were also prepared as alkali-resistant GPs from Gp-1 and the other relatively small size GP fraction (Gp-2) of the Pronase digest, respectively, after the alkali treatment. Chemically, the three allergenically active preparations were rich in galactosamine, glucosamine, and fucose in common, although the N-linked carbohydrates were much larger in size than O-linked carbohydrates. Accordingly, it has been expected that the epitope of the sea squirt antigen corresponds to certain oligosaccharide units residing in both of the O- and N-linked carbohydrate chains of the antigen. This suggestion was consistent with the observation that the allergenic activity of Gi-rep was significantly unstable to the periodate oxidation but substantially stable not only to acid, alkali, and heat treatments but also to the enzymatic proteolysis with Pronase E.

O-linked oligosaccharides of mouse egg ZP3 account for its sperm receptor activity

Previously, we reported that ZP3, one of three different glycoproteins present in the mouse egg's zona pellucida, serves as a sperm receptor. Furthermore, small glycopeptides derived from egg ZP3 retain full sperm receptor activity, suggesting a role for carbohydrate, rather than polypeptide chain in receptor function. Here, we report that removal of O-linked oligosaccharides from ZP3 destroys its sperm receptor activity, whereas removal of N-linked oligosaccharides has no effect. A specific size class of O-linked oligosaccharides, recovered following mild alkaline hydrolysis and reduction of ZP3, is shown to possess sperm receptor activity and to bind to sperm. The results presented strongly suggest that mouse sperm bind to eggs via O-linked oligosaccharides present on ZP3.

Histochemical application of mild alkaline hydrolysis for selective elimination of O-glycosidically linked glycoproteins

A new technique to eliminate O-glycosidically linked glycoprotein (mucin-type glycoprotein) selectively has been developed. Composite paraffin sections were collodionized before and after alkaline treatment with 0.5 M NaOH in 70% ethanol; the effect of this procedure on mucosubstances was examined using the periodic acid-Schiff reaction. Exposure to alkaline hydrolysis for 72 to 144 hours at 4 C led to a complete loss of periodic acid-Schiff reactivity of epithelial mucins in rat sublingual gland, stomach and small intestine, but that of fuzzy coat, thyroid colloid, collagen fibers and tracheal cartilage was well preserved. These results agreed fairly well with biochemical findings. The present study also revealed that materials prepared by freeze-substitution provided the most satisfactory results.

Glycosaminoglycans produced by human synovial cell cultures

Human synovial cells in culture are known to synthesize hyaluronic acid, but the production of sulfated glycosaminoglycans (GAG) has received less attention. Using 14C-glucosamine as a precursor, GAG content was studied in the medium, trypsin-solubilized pericellular layer, and cell residue fraction of cultured synovial cells derived from the synovial membranes of nonrheumatoid and rheumatoid joints. Over 90% of the total non-dialyzable counts appeared in the culture medium, for the most part in hyaluronic acid. The remaining nondialyzable counts were cell-associated, almost equally divided between the pericellular layer and cell residues. In these fractions, only part of the counts were in GAG susceptible to testicular hyaluronidase digestion, and GAG were significantly lower in the cell residue of the rheumatoid synovial cells compared to the nonrheumatoid cells. Analysis of the chondroitinase ABC and AC digestion products of these GAG indicated the presence of chondroitin-4 and -6 sulfates, and dermatan sulfate, but not heparan sulfate. Similar findings with respect to the identity of the GAG in nonrheumatoid and rheumatoid synovial cell culture media were obtained with 35SO4 as a precursor.

Isolation and characterization of goat submaxillary-mucin

Mucin from the submaxillary glands of goats was isolated by precipitation from an aqueous extract with a cationic detergent, followed by dissolution in CaCl2 solution, and fractionation on a DEAE-cellulose column, Five homogeneous, mucin fractions were isolated by NaCl-gradient elution of the column. One of the fractions was obtained in major yield (46%); it was homogeneous by paper electrophoresis, and contained carbohydrates (46.4%) and protein (48.9%). The sugars were fucose, galactose, glucose, mannose, 2-acetamido-2-deoxygalactose, 2-acetamido-2-deoxyglucose, and N-acetylneuraminic acid. The principal amino acids were alanine, glutamic acid, glycine, proline, serine, threonine, and valine. The proportions of these residues were determined.

The effect of thiols on Saccharomyces fragilis

Treatment of cell suspensions of Saccharomyces fragilis with 0.01 M beta-mercaptoethanol or dithiothreitol released a variety of substances of high and low molecular weight. Twenty-two high-molecular-weight glycoproteins were separated by a combination of chromatography on DEAE cellulose and polyacrylamide gel electrophoresis in presence of sodium dodecylsulphate. The carbohydrate components consisted of at least 95% mannose and the protein components had threonine and serine as the major amino acids. Only very small amounts of phosphorus were associated with the high-molecular-weight components. The low-molecular-weight substances werr probably released from the internal cell pool and uracil and hypoxanthine were identified as components of this fraction. It is suggested that in addition to breaking disulphide bridges in the cell wall the thiols may also render the plasmalemma permeable to certain low-molecular-weight substances. Such effects are not lethal since the yeast can be trained to grow in presence of 0.01 M mercaptoethanol.

Depolymerization and de-N-acetylation of glycosaminoglycuronans by the action of alkali in the presence of sodium borohydride

Depolymerization of Ch-4-S, Ch-6-S, HA and HP was observed by gel filtration after treatment with 2-10 N NaOH or Ba(OH)2 in the presence of NaBH4 at 80-83 degrees for 1-5 hr. Depolymerized products were isolated after these treatments. 4-Deoxy-alpha-L-threo-4-enohexopyranosyluronic acid residue as the nonreducing end group was demonstrated by the H-1 and H-4 proton signals in the NMR spectra and by the absorption at 225-230 nm. L-Gulonic acid, which was produced from D-glucuronic acid by the action of NaBH4, was detected as the reducing end group in the depolymerized products of Ch-6-S, but no hexosaminitol was detected. De-N-acetylation to the extent of 34-50% occurred after treatment with 2.5 N NaOH in the presence of NaBH4, and almost complete de-N-acetylation accompanied by considerable destruction was observed with Ch-4-S and Ch-6-S after treatment with 5.5-10.0 N NaOH. A reaction mechanism is proposed for the depolymerization of glycosaminoglycuronans by the action of alkali.