Structures of the sugar chains of a major glycoprotein present in the egg jelly coat of a starfish, Asterias amurensis

Profiling N-glycans of the egg jelly coat of the sea urchin Paracentrotus lividus by MALDI-TOF mass spectrometry and capillary liquid chromatography electrospray ionization-ion trap tandem mass spectrometry systems

Sea urchin eggs are surrounded by a carbohydrate-rich layer, termed the jelly coat, that consists of polysaccharides and glycoproteins. In the present study, we describe two mass spectrometric strategies to characterize the N-glycosylation of the Paracentrotus lividus egg jelly coat, which has an alecithal-type extracellular matrix like mammalian eggs. Egg jelly was isolated, lyophilized, and dialyzed, followed by peptide N-glycosidase F (PNGase-F) treatment to release N-glycans from their protein chain. These N-glycans were then derivatized by permethylation reaction, and analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and capillary liquid chromatography electrospray ionization-ion trap tandem mass spectroscopy (CapLC ESI-Ion trap-MS/MS). N-glycans in the egg jelly coat glycoproteins were indicated by sodiated molecules at m/z 1579.8, 1783.9, 1988.0, 2192.0, and 2397.1 for permethylated oligosaccharides on MALDI-TOF MS. Fragmentation and structural characterization of these oligosaccharides were performed by ESI-Ion trap MS/MS. Then, MALDI-TOF-MS and ESI-Ion trap-MS/MS spectra were interpreted using the GlycoWorkbench software suite, a tool for building, displaying, and profiling glycan masses, to identify the original oligosaccharide structures. The oligosaccharides of the isolated egg jelly coat were mainly of the high mannose type.

Glucose Trimming ofN-Glycan in Endoplasmic Reticulum Is Indispensable for the Growth ofRaphanus sativusSeedling (kaiware radish)

Recently I found that glycosidase inhibitors such as castanospermine, deoxynojirimycin, swainsonine, 2-acetamindo 2,3-dideoxynojirimycin, and deoxymannojirimycin change the N-glycan structure of root glycoproteins, and that the glucosidase inhibitors castanospermine and deoxynojirimycin suppress the growth of Raphanus sativus seedlings (Mega, T., J. Biochem., 2004). The present study undertook to see whether the growth suppression is due to the inhibition of glucose trimming in endoplasmic reticulum (ER). The study, using three glucosidase inhibitors, castanospermine, N-methyl deoxynojirimycin, and deoxynojirimycin, upon the growth of R. sativus foliage leaf, made clear that glucose trimming is indispensable for plant growth, because the inhibition of glucose trimming correlated with leaf growth. On the other hand, processing inhibition in the Golgi apparatus by other glycosidase inhibitors had little effect on plant growth, although N-glycan processing was disrupted depending on inhibitor specificity. These results suggest that N-glycan processing after glucosidase processing is dispensable for plant growth and cell differentiation.

Glycobiology of reproductive processes in marine animals: the state of the art

Glycobiology is the study of complex carbohydrates in biological systems and represents a developing field of science that has made huge advances in the last half century. In fact, it combines all branches of biomedical research, revealing the vast and diverse forms of carbohydrate structures that exist in nature. Advances in structure determination have enabled scientists to study the function of complex carbohydrates in more depth and to determine the role that they play in a wide range of biological processes. Glycobiology research in marine systems has primarily focused on reproduction, in particular for what concern the chemical communication between the gametes. The current status of marine glycobiology is primarily descriptive, devoted to characterizing marine glycoconjugates with potential biomedical and biotechnological applications. In this review, we describe the current status of the glycobiology in the reproductive processes from gametogenesis to fertilization and embryo development of marine animals.

N-glycan moieties of the crustacean egg yolk protein and their glycosylation sites

Vitellogenin (Vg) is the precursor of the egg yolk glycoprotein of crustaceans. In the prawn Macrobrachium rosenbergii, Vg is synthesized in the hepatopancreas, secreted to the hemolymph, and taken up by means of receptor-mediated endocytosis into the oocytes. The importance of glycosylation of Vg lies in its putative role in the folding, processing and transport of this protein to the egg yolk and in the fact that the N-glycan moieties could provide a source of carbohydrate during embryogenesis. The present study describes, for the first time, the structure of the glycan moieties and their sites of attachment to the Vg of M. rosenbergii. Bioinformatics analysis revealed seven putative N-glycosylation sites in M. rosenbergii Vg; two of these glycosylation sites are conserved throughout the Vgs of decapod crustaceans from the Pleocyemata suborder (N 159 and N 660). The glycosylation of six putative sites of M. rosenbergii Vg (N 151, N 159, N ,168 N ,614 N 660 and N 2300) was confirmed; three of the confirmed glycosylation sites are localized around the N-terminally conserved N-glycosylation site N 159. From a theoretical three-dimensional structure, these three N-glycosylated sites N 151, N 159, and N 168 were localized on the surface of the Vg consensus sequence. In addition, an uncommon high mannose N-linked oligosaccharide structure with a glucose cap (Glc1Man9GlcNAc2) was characterized in the secreted Vg. These findings thus make a significant contribution to the structural elucidating of the crustacean Vg glycan moieties, which may shed light on their role in protein folding and transport and in recognition between Vg and its target organ, the oocyte.

The presence of monoglucosylated N196-glycan is important for the structural stability of storage protein, arylphorin

Although N-glycosylation has been known to increase the stability of glycoproteins, it is difficult to assess the structural importance of glycans in the stabilization of glycoproteins. APA (Antheraea pernyi arylphorin) is an insect hexamerin that has two N-glycosylations at Asn196 and Asn344 respectively. The glycosylation of Asn344 is critical for the folding process; however, glycosylation of Asn196 is not. Interestingly, the N196-glycan (glycosylation of Asn196) remains in an immature form (Glc1Man9GlcNAc2). The mutation of Asn196 to glutamine does not change the ecdysone-binding activity relative to that of the wild-type. In the present study, we determined the crystal structure of APA, and all sugar moieties of the N196-glycan were clearly observed in the electron-density map. Although the sugar moieties of the glycan generally have high structural flexibility, most sugar moieties of the N196-glycan were well organized in the deep cleft of the subunit interface and mediated many inter- and intrasubunit hydrogen bonds. Analytical ultracentrifugation and GdmCl (guanidinium chloride) unfolding experiments revealed that the presence of the N196-glycan was important for stabilizing the hexameric state and overall stability of APA respectively. Our results could provide a structural basis for studying not only other glycoproteins that carry an immature N-glycan, but also the structural role of N-glycans that are located in the deep cleft of a protein.

Structural diversity of cytosolic free oligosaccharides in the human hepatoma cell line, HepG2

It is thought that free oligosaccharides in the cytosol are an outcome of quality control of glycoproteins by endoplasmic reticulum-associated degradation (ERAD). Although considerable amounts of free oligosaccharides accumulate in the cytosol, where they presumably have some function, detailed analyses of their structures have not yet been carried out. We isolated 21 oligosaccharides from the cytosolic fraction of HepG2 cells and analyzed their structures by the two-dimensional high-performance liquid chromatography (HPLC) sugar-mapping method. Sixteen novel oligosaccharides were identified in the cytosol in this study. All had a single N-acetylglucosamine at their reducing-end cores and could be expressed as (Man)n (GlcNAc)1. No free oligosaccharide with N,N'-diacetylchitobiose was detected in the cytosolic fraction of HepG2 cells. This suggested that endo-beta-N-acetylglucosaminidase was a key enzyme in the production of cytosolic free oligosaccharides. The 21 oligosaccharides were classified into three series--series 1: oligosaccharides processed from Manalpha1-2Manalpha1-6 (Manalpha1-2Manalpha1-3)Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3) Manbeta1-4GlcNAc (M9A') and Manalpha1-2Manalpha1-6(Manalpha1-3) Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3)Manbeta1-4GlcNAc (M8A') by digestion with cytosolic alpha-mannosidase; series 2: oligosaccharides processed with Golgi alpha-mannosidases in addition to endoplasmic reticulum (ER) and cytosolic alpha-mannosidases; and series 3: glucosylated oligosaccharides produced from Glc1Man9GlcNAc1 by hydrolysis with cytosolic alpha-mannosidase. The presence of the series "2" oligosaccharides suggests that some of the misfolded glycoproteins had been processed in pre-cis-Golgi vesicles and/or the Golgi apparatus. When the cells were treated with swainsonine to inhibit cytosolic alpha-mannosidase, the amounts of M9A' and M8A' increased remarkably, suggesting that these oligosaccharides were translocated into the cytosol. Four oligosaccharides of series "2" also increased. In contrast, there were obvious reductions in Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3)Manbeta1-4GlcNAc (M5B'), the end product from M9A' by digestion with cytosolic alpha-mannosidase, and Manalpha1-6(Manalpha1- 2Manalpha1-3)Manbeta1-4GlcNAc, derived from series "2" oligosaccharides by digestion with cytosolic alpha-mannosidase. Our data suggest that (1) some of the cytosolic oligosaccharides had been processed with Golgi alpha-mannosidases, (2) the major oligosaccharides translocated from the ER were M9A' and M8A', and (3) M5B' and Glc1M5B' were maintained at relatively high concentrations in the cytosol.

Structural basis for the presence of a monoglucosylated oligosaccharide in mature glycoproteins

Arylphorin is an insect hexameric storage protein. The structures of the oligosaccharides attached to this protein have recently been determined. However, their precise functions remain to be established. Proteolysis and MALDI MS studies disclose that the amino acid residues Asn196 and Asn344 are N-glycosylated with Glc(1)Man(9)GlcNAc(2) and Man(5-6)GlcNAc(2) oligosaccharides, respectively. Interestingly, significant variations in the amounts of glycans involving Glc(1)Man(9)GlcNAc(2) are evident in arylphorins purified from larvae reared at different seasons. The data suggest that the metabolism of larvae and local protein structure contribute to glycan development. Three-dimensional model of the protein speculated that N-glycosidic linkage to Asn196 in the Glc(1)Man(9)GlcNAc(2) structure was buried inside the twofold axis of the hexamer, whereas oligosaccharide linkages to Asn344 were completely exposed to solvent. This finding is in agreement with previous biochemical data showing that limited Glc(1)Man(9)GlcNAc(2) was released by protein-N-glycosidase F under non-denaturing conditions, in contrast to Man(5-6)GlcNAc(2) oligosaccharides.

Structural characterization of the N-glycans of gp273, the ligand for sperm-egg interaction in the mollusc bivalve Unio elongatulus

Gp273, a glycoprotein of the egg extracellular coats of the mollusc bivalve Unio elongatulus, is the ligand molecule for sperm-egg interaction during fertilization. In this study we have analyzed the N-glycans from gp273. N-glycans were enzymatically released by PNGase F digestion and their structures were elucidated by normal phase HPLC profiling of the 2-aminobenzamide-labeled N-glycans, MALDI-TOF mass spectrometry and 1H NMR spectroscopy. The combined data revealed that the N-glycans of gp273 consist of Glc1Man9GlcNAc2 and Man9GlcNAc2. In Unio, the presence of noncomplex-type N-glycans parallels the inefficacy of these glycans in the ligand function. Their role in the protection of the polypeptide chain from proteolytic attack is suggested by the electrophoretic patterns obtained after enzymatic digestion of the native and the N-deglycosylated protein. These results are discussed in the light of the evolution of the recognition and adhesion properties of oligosaccharide chains in the fertilization process.

Calnexin discriminates between protein conformational states and functions as a molecular chaperone in vitro

Although calnexin is thought to function as a molecular chaperone for glycoproteins, a prevalent view is that it cannot distinguish between protein conformational states, binding solely through its lectin site to monoglucosylated oligosaccharides. Using purified components in vitro, calnexin effectively prevented the aggregation not only of glycoproteins bearing monoglucosylated oligosaccharides but also proteins lacking N-glycans, an effect enhanced by ATP. It also suppressed the thermal denaturation of nonglycosylated proteins and enhanced their refolding in conjunction with other cellular components. Calnexin formed stable complexes with unfolded conformers of these proteins but not with the native molecules. Therefore, in addition to being a lectin, calnexin functions as a bona fide molecular chaperone capable of interacting with polypeptide segments of folding glycoproteins.

Novel structures of N-linked high-mannose type oligosaccharides containing alpha-D-galactofuranosyl linkages in Aspergillus niger alpha-D-glucosidase

Seven oligosaccharides were isolated from alpha-D-glucosidase (EC 3.2.1.20) from Aspergillus niger, and the structures of these oligosaccharides were studied by 1H NMR spectroscopy. After treatment of the alpha-D-glucosidase with N-glycosidase F, seven major oligosaccharide peaks were detected by Dionex anion-exchange HPLC. The structures corresponding to the three peaks OS-1, OS-2, and OS-4 were determined to be Man8GlcNAc2, Man9GlcNAc2, and GlcMan9GlcNAc2, respectively, from 1H NMR spectra of the isolated fractions. Each of the four oligosaccharides OS-5, OS-6, OS-7-1, and OS-7-2 contained an alpha-D-galactofuranosyl residue (Galf) linked to Man(A) via an alpha-(1-->2)-linkage. OS-7 was found to consist of two oligosaccharides. The structures of these four oligosaccharides were determined to be GalfMan5GlcNAc2, GalfMan6GlcNAc2, GalfMan7GlcNAc2, and GalfMan8GlcNAc2 by 1H NMR spectroscopy and compositional analysis. The Galf structure of GalfMan5GlcNAc2 was found to be identical to that of an oligosaccharide previously isolated from the alpha-D-galactosidase of the same strain. The structure of OS-3 remains undetermined.

Protein glycosylation. Structural and functional aspects

During the last decade, there have been enormous advances in our knowledge of glycoproteins and the stage has been set for the biotechnological production of many of them for therapeutic use. These advances are reviewed, with special emphasis on the structure and function of the glycoproteins (excluding the proteoglycans). Current methods for structural analysis of glycoproteins are surveyed, as are novel carbohydrate-peptide linking groups, and mono- and oligo-saccharide constituents found in these macromolecules. The possible roles of the carbohydrate units in modulating the physicochemical and biological properties of the parent proteins are discussed, and evidence is presented on their roles as recognition determinants between molecules and cells, or cell and cells. Finally, examples are given of changes that occur in the carbohydrates of soluble and cell-surface glycoproteins during differentiation, growth and malignancy, which further highlight the important role of these substances in health and disease.

Structures of asparagine linked oligosaccharides of immunoglobulins (IgY) isolated from egg-yolk of Japanese quail

Structures of the Asn linked oligosaccharides of quail egg-yolk immunoglobulin (IgY) were determined in this study. Asn linked oligosaccharides were cleaved from IgY by hydrazinolysis and labelled with p-aminobenzoic acid ethyl ester (ABEE) after N-acetylation. The ABEE labelled oligosaccharides were then fractionated by a combination of Concanavalin A-agarose column chromatography and anion exchange, normal phase and reversed phase HPLC before their structures were determined by sequential exoglycosidase digestion, methylation analysis, HPLC, and 500 MHz 1H-NMR spectroscopy. Quail IgY contained only neutral oligosaccharides of the following categories: the glucosylated oligomannose type (0.6% Glc alpha 1-3Glc alpha 1-3Man9GlcNAc2; 35.6%, Glc alpha 1-3Man7-9GlcNAc2). oligomannose type (15.0%, with the structure Man5-9GlcNAc2) and biantennary complex type with core structures of -Man alpha 1-3(-Man alpha 1-6)Man beta 1-4GlcNAc beta 1-4GlcNAc (9.9%), -Man alpha 1-3 (GlcNAc beta 1-4)(-Man alpha 1-6)Man beta 1-4GlcNAc beta 1-4GlcNAc (25.1%) and -Man alpha 1-3(GlcNAc beta 1-4)(-Man alpha 1-6)Man beta 1-4GlcNAc beta 1-4(Fuc alpha 1-6)GlcNAc (11.4%). Although never found in mammalian proteins, glucosylated oligosaccharides (Glc1Man7-9GlcNAc2) have been located previously in hen IgY.

Novel N-linked oligo-mannose type oligosaccharides containing an alpha-D-galactofuranosyl linkage found in alpha-D-galactosidase from Aspergillus niger

Structures of oligosaccharides from Aspergillus niger alpha-D-galactosidase [EC 3.2.1.22] were studied. Purified alpha-D-galactosidase was treated with N-glycosidase F, and six kinds of oligosaccharides were isolated by gel chromatography and anion-exchange chromatography. The structures of the oligosaccharides were determined by 1H-NMR and compositional analysis to be Man5GlcNAc2, Man6GlcNAc2, Man9GlcNAc2, GlcMan9GlcNAc2, GalMan4GlcNAc2 and GalMan5GlcNAc2. From mild acid hydrolysis, methylation analysis and ROESY spectral analysis, it was ascertained that the galactosyl residue in two oligosaccharides was in the furanose form and was bound to mannose at the nonreducing end with an alpha 1-2 linkage (GalfMan4GlcNAc2 and GalfMan5GlcNAc2).

Structures of asparagine-linked oligosaccharides from hen egg-yolk antibody (IgY). Occurrence of unusual glucosylated oligo-mannose type oligosaccharides in a mature glycoprotein

Asparagine-linked oligosaccharides present on hen egg-yolk immunoglobulin, termed IgY, were liberated from the protein by hydrazinolysis. After N-acetylation, the oligosaccharides were labelled with a UV-absorbing compound, p-aminobenzoic acid ethyl ester (ABEE). The ABEE-derivatized oligosaccharides were fractionated by anion exchange, normal phase and reversed phase HPLC, and their structures were determined by a combination of sugar composition analysis, methylation analysis, negative ion FAB-MS, 500 MHz 1H-NMR and sequential exoglycosidase digestions. IgY contained monoglucosylated oligomannose type oligosaccharides with structures of Glc alpha 1-3Man7-9-GlcNAc-GlcNAc, oligomannose type oligosaccharides with the size range of Man5-9GlcNAc-GlcNAc, and biantennary complex type oligosaccharides with core region structure of Man alpha 1-6(+/- GlcNAc beta 1-4)(Man alpha 1-3)Man beta 1-4GlcNAc beta 1-4(+/- Fuc alpha 1-6)GlcNAc. The glucosylated oligosaccharides, Glc1Man8GlcNAc2 and Glc1Man7GlcNAc2, have not previously been reported in mature glycoproteins from any source.

Structural studies of the asparagine-linked sugar chains of two immunoglobulin M's purified from a patient with Waldenström's macroglobulinemia

The structures of the sugar chains present in two human monoclonal IgM molecules purified from the serum of a patient with Waldenström's macroglobulinemia have been determined. The asparagine-linked sugar chains were liberated as oligosaccharides by hydrazinolysis and labeled by reduction with NaB3H4 after N-acetylation. Their structures were studied by serial lectin column chromatography and sequential exoglycosidase digestion in combination with methylation analysis. These two IgM's were shown to contain almost the same sugar chains. The sugar chains were a mixture of a series of high-mannose-type and biantennary complex-type oligosaccharides. The complex-type oligosaccharides contain Man alpha 1----6(+/- GlcNAc beta 1----4)(Man alpha 1----3)Man beta 1----4GlcNAc beta 1----4(Fuc alpha 1----6)GlcNAc as their core and GlcNAc beta 1----, Gal beta 1----4GlcNAc beta 1---- and Neu5Ac alpha 2----6Gal beta 1----4GlcNAc beta 1---- groups in their outer chain moieties.

Physiological inducers of the acrosome reaction

This article reviews recent studies on physiological inducers of the acrosome reaction in starfish. Upon encountering the jelly coat of eggs, starfish sperm undergo the acrosome reaction in response to a cooperation of three jelly components: a sulfated glycoprotein named acrosome reaction-inducing substance (ARIS), a group of steroidal saponins named Co-ARIS, and an oligopeptide presumably having an activity to increase the intracellular pH of sperm. ARIS induces the acrosome reaction in high Ca2+ or high pH sea water. In normal sea water, both ARIS and Co-ARIS are required for the induction. In addition to ARIS and Co-ARIS, a third jelly component, the oligopeptide, is necessary to mimic the full capacity of the jelly coat to induce the acrosome reaction. ARIS and Co-ARIS cooperatively increase the intracellular Ca2+ by stimulating Ca2+ channels, while the oligopeptide increases the intracellular pH by stimulating Na+/H+ exchange systems. When sperm meet the eggs, both changes are simultaneously achieved in them and thus they undergo the acrosome reaction.

Characterization of N-linked gluco-oligomannose type of carbohydrate chains of glycoproteins from the ovary of the starfish Asterias rubens (L.)

Glycoproteins were isolated from the ovary of the starfish Asterias rubens (L.). After delipidation, sugar analysis revealed the presence of mannose, glucose and N-acetylglucosamine in a molar ratio of 9.0:1.3:2.3. Subsequently, hydrazinolysis, re-N-acetylation, reduction and high-voltage paper electrophoresis were carried out, resulting in a mixture of neutral oligosaccharide alditols which was fractionated on Bio-Gel P-4. The alditols, investigated by 500-MHz 1H-NMR spectroscopy, turned out to be of the oligomannose type or of the gluco-oligomannose type containing 9 mannose and 1-3 glucose residues. The most abundant compounds were established to be: (Formula: see text) and (Formula: see text).