Characterization of the carbohydrate moieties of the functional unit RvH1-a of Rapana venosa haemocyanin using HPLC/electrospray ionization MS and glycosidase digestion

Structural insights into a functional unit from an immunogenic mollusk hemocyanin

Mollusk hemocyanins, among the largest known proteins, are used as immunostimulants in biomedical and clinical applications. The hemocyanin of the Chilean gastropod Concholepas concholepas (CCH) exhibits unique properties, which makes it safe and effective for human immunotherapy, as observed in animal models of bladder cancer and melanoma, and dendritical cell vaccine trials. Despite its potential, the structure and amino acid sequence of CCH remain unknown. This study reports two sequence fragments of CCH, representing three complete functional units (FUs). We also determined the high-resolution (1.5 Å) X-ray crystal structure of an "FU-g type" from the CCHB subunit. This structure enables in-depth analysis of chemical interactions at the copper-binding center and unveils an unusual, truncated N-glycosylation pattern. These features are linked to eliciting more robust immunological responses in animals, offering insights into CCH's enhanced immunostimulatory properties and opening new avenues for its potential applications in biomedical research and therapies.

Assessment of the In Vitro and In Vivo Antitumor Activity of Hemocyanins from Helix aspersa, Helix lucorum, and Rapana venosa in a Graffi Myeloid Tumor Model

Hemocyanins are oxygen-transporting glycoproteins in the hemolymph of some invertebrate species that attracted scientific interest as potential anticancer agents. The present study aims to assess the in vitro and in vivo anticancer activity of hemocyanins isolated from Helix aspersa, Helix lucorum, and Rapana venosa in the Graffi myeloid tumor model. The in vitro antitumor activity of the hemocyanins was determined by a MTT test and cytomorphological analysis by fluorescent and transmission electron microscopy. The in vivo effects of the hemocyanins were examined in hamsters transplanted with Graffi tumor. The serum antibody titers against the tested hemocyanins and tumor antigen were determined by ELISA. Histopathological assessment of the morphological features related to antitumor effect, immune system response, and toxicity in some internal organs was performed. The results of in vitro studies indicated that the tested hemocyanins induced significant antiproliferative and apoptogenic effects. The in vivo investigations demonstrated a protective antitumor effect, expressed in reduced transplantability, suppression of tumor growth and metastasis, reduced mortality, prolonged survival time, and absence of toxic side effects. The present study indicated that the antitumor activity of the studied hemocyanins was due to both immune stimulation and direct effects on the tumor cells, and they displayed their potential as therapeutic agents against hematological malignances.

Antitumor Activity of Bioactive Compounds from Rapana venosa against Human Breast Cell Lines

This study is the first report describing the promising antitumor activity of biologically active compounds isolated from the hemolymph of marine snail Rapana venosa-a fraction with Mw between 50 and 100 kDa and two structural subunits (RvH1 and RvH2), tested on a panel of human breast cell lines-six lines of different molecular subtypes of breast cancer MDA-MB-231, MDA-MB-468, BT-474, BT-549, SK-BR-3, and MCF-7 and the non-cancerous MCF-10A. The fraction with Mw 50-100 kDa (HRv 50-100) showed good antitumor activity manifested by a significant decrease in cell viability, altered morphology, autophagy, and p53 activation in treated cancer cells. An apparent synergistic effect was observed for the combination of HRv 50-100 with cis-platin for all tested cell lines. The combination of HRv 50-100 with cisplatin and/or tamoxifen is three times more effective compared to treatment with classical chemotherapeutics alone. The main proteins in the active fraction, with Mw at ~50 kDa, ~65 kDa, ~100 kDa, were identified by MALDI-MS, MS/MS analyses, and bioinformatics. Homology was established with known proteins with antitumor potential detected in different mollusc species: peroxidase-like protein, glycoproteins Aplysianin A, L-amino acid oxidase (LAAO), and the functional unit with Mw 50 kDa of RvH. Our study reveals new perspectives for application of HRv 50-100 as an antitumor agent used alone or as a booster in combination with different chemotherapies.

Towards understanding the extensive diversity of protein N-glycan structures in eukaryotes

N‐glycosylation is an important post‐translational modification of proteins that has been highly conserved during evolution and is found in Eukaryota, Bacteria and Archaea. In eukaryotes, N‐glycan processing is sequential, involving multiple specific steps within the secretory pathway as proteins travel through the endoplasmic reticulum and the Golgi apparatus. In this review, we first summarize the different steps of the N‐glycan processing and further describe recent findings regarding the diversity of N‐glycan structures in eukaryotic clades. This comparison allows us to explore the different regulation mechanisms of N‐glycan processing among eukaryotic clades. Recent findings regarding the regulation of protein N‐glycosylation are highlighted, especially the regulation of the biosynthesis of complex‐type N‐glycans through manganese and calcium homeostasis and the specific role of transmembrane protein 165 (TMEM165) for which homologous sequences have been identified in several eukaryotic clades. Further research will be required to characterize the function of TMEM165 homologous sequences in different eukaryotic clades.

Mollusc N-glycosylation: Structures, Functions and Perspectives

Molluscs display a sophisticated N-glycan pattern on their proteins, which is, in terms of involved structural features, even more diverse than that of vertebrates. This review summarises the current knowledge of mollusc N-glycan structures, with a focus on the functional aspects of the corresponding glycoproteins. Furthermore, the potential of mollusc-derived biomolecules for medical applications is addressed, emphasising the importance of mollusc research.

De Novo Structural Determination of the Oligosaccharide Structure of Hemocyanins from Molluscs

A number of studies have shown that glycosylation of proteins plays diverse functions in the lives of organisms, has crucial biological and physiological roles in pathogen–host interactions, and is involved in a large number of biological events in the immune system, and in virus and bacteria recognition. The large amount of scientific interest in glycoproteins of molluscan hemocyanins is due not only to their complex quaternary structures, but also to the great diversity of their oligosaccharide structures with a high carbohydrate content (2–9%). This great variety is due to their specific monosaccharide composition and different side chain composition. The determination of glycans and glycopeptides was performed with the most commonly used methods for the analysis of biomolecules, including peptides and proteins, including Matrix Assisted Laser Desorption/Ionisation–Time of Flight (MALDI-TOF-TOF), Liquid Chromatography - Electrospray Ionization-Mass Spectrometry (LC/ESI-MS), Liquid Chromatography (LC-Q-trap-MS/MS) or Nano- Electrospray Ionization-Mass Spectrometry (nano-ESI-MS) and others. The molluscan hemocyanins have complex carbohydrate structures with predominant N-linked glycans. Of interest are identified structures with methylated hexoses and xyloses arranged at different positions in the carbohydrate moieties of molluscan hemocyanins. Novel acidic glycan structures with specific glycosylation positions, e.g., hemocyanins that enable a deeper insight into the glycosylation process, were observed in Rapana venosa, Helix lucorum, and Haliotis tuberculata. Recent studies demonstrate that glycosylation plays a crucial physiological role in the immunostimulatory and therapeutic effect of glycoproteins. The remarkable diversity of hemocyanin glycan content is an important feature of their immune function and provides a new concept in the antibody–antigen interaction through clustered carbohydrate epitopes.

Identification and characterization of glycosylation sites on Litopenaeus vannamei hemocyanin

The respiratory glycoprotein hemocyanin has been implicated in immune-related functions. Using lectin blotting, we show that the binding of shrimp (Litopenaeus vannamei) hemocyanin to concanavalin A decreases markedly with O-glycosidase treatment but not with PNGase F. Twelve O-glycosylation sites, three on the large hemocyanin subunit and nine on the small hemocyanin subunit (HMCs), were identified by LC-MS/MS. Importantly, when the glycosylation sites at Thr-537, Ser-539, and Thr-542 on the C terminus of HMCs were replaced with alanine, the resultant mutant hemocyanin had reduced carbohydrate content, coupled with a fourfold reduction in bacterial agglutination and 0.2-fold reduction in antibacterial activities toward Vibrio parahaemolyticus and Staphylococcus aureus. These results suggest that the glycosylation sites on shrimp hemocyanin are closely related to its immunological functions.

N-glycan structures of β-HlH subunit of Helix lucorum hemocyanin

The carbohydrate structures of molluscan hemocyanins have recently received particular interest due to their specific monosaccharide composition, as well as their immunostimulatory properties and application in clinical studies. For the first time, we investigated N-glycans of the structural subunit β-HlH of hemocyanin isolated from Helix lucorum. In total, 32 different glycans were enzymatically liberated and characterized by tandem mass spectrometry using a Q-Trap mass spectrometer. Our study revealed a highly heterogeneous mixture of glycans with composition Hex_3-7HexNAc_2-5MeHex_0-4Pent_0-1Fuc_0-1. The oligosaccharide chains are mostly modified at the inner core by β1-2-linked xylose to β-mannose, by α1-6-fucosylation of the innermost GlcNAc residue (the Asn-bound GlcNAc), and by methylation. The glycans of β-HlH mainly contain a terminal MeHex residue; in some cases even two, three or four of these residues occur. Several carbohydrate chains in β-HlH are core-fucosylated without Xyl and also possess a high degree of methylation. This study shows the presence of mono- and bi-antennary N-glycans as well as hybrid type structures with or without core-fucosylation.

Hemocytes and plasma of the eastern oyster (Crassostrea virginica) display a diverse repertoire of sulfated and blood group A-modified N-glycans

The eastern oyster (Crassostrea virginica) has become a useful model system for glycan-dependent host-parasite interactions due to the hijacking of the oyster galectin CvGal1 for host entry by the protozoan parasite Perkinsus marinus, the causative agent of Dermo disease. In this study, we examined the N-glycans of both the hemocytes, which via CvGal1 are the target of the parasite, and the plasma of the oyster. In combination with HPLC fractionation, exoglycosidase digestion, and fragmentation of the glycans, mass spectrometry revealed that the major N-glycans of plasma are simple hybrid structures, sometimes methylated and core α1,6-fucosylated, with terminal β1,3-linked galactose; a remarkable high degree of sulfation of such glycans was observed. Hemocytes express a larger range of glycans, including core-difucosylated paucimannosidic forms, whereas bi- and triantennary glycans were found in both sources, including structures carrying sulfated and methylated variants of the histo-blood group A epitope. The primary features of the oyster whole hemocyte N-glycome were also found in dominin, the major plasma glycoprotein, which had also been identified as a CvGal1 glycoprotein ligand associated with hemocytes. The occurrence of terminal blood group moieties on oyster dominin and on hemocyte surfaces can account in part for their affinity for the endogenous CvGal1.

Methylation--an uncommon modification of glycans

A methyl (Me) group on a sugar residue is a rarely reported event. Until now, this type of modification has been found in the animal kingdom only in worms and molluscs, whereas it is more frequently present in some species of bacteria, fungi, algae and plants, but not in mammals. The monosaccharides involved as well as the positions of the Me groups on the sugar vary with species. Methylation appears to play a role in some recognition events, but details are still unknown. This review summarises the current knowledge on methylation of sugars in all types of organism.

Complicated N-linked glycans in simple organisms

Although countless genomes have now been sequenced, the glycomes of the vast majority of eukaryotes still present a series of unmapped frontiers. However, strides are being made in a few groups of invertebrate and unicellular organisms as regards their N-glycans and N-glycosylation pathways. Thereby, the traditional classification of glycan structures inevitably approaches its boundaries. Indeed, the glycomes of these organisms are rich in surprises, including a multitude of modifications of the core regions of N-glycans and unusual antennae. From the actually rather limited glycomic information we have, it is nevertheless obvious that the biotechnological, developmental and immunological relevance of these modifications, especially in insect cell lines, model organisms and parasites means that deciphering unusual glycomes is of more than just academic interest.

Method for analysis of different oligosacchiride structures

In this study, an improved, rapid, high yield synthesis of N,N'-4,4'-bis(benzyl-2-boronic acid)-bipyridinium dibromide (o-BBV) is described. The obtained o-BVV is applied in a two-component saccharide sensing system (complex) where it serves as a fluorescence quencher and a saccharide receptor. This system was applied to different natural oligosaccharides isolated from molluscan Rapana venosa (RvH1-a) and arthropodan Carcinus aestuarii (CaeH) hemocyanins (Hcs) and cyclodextrins (CDs). The carbohydrate contents of both Hcs were calculated in our previous work to be 1,6 % and 7 % for CaeH and RvH1-a, respectively. We propose that the difference in fluorescence increase of the native CaeH and RvH1-a when titrating them with the complex is due to the fact that the carbohydrate content of CaeH is lower and the carbohydrate chains are buried in between the structural subunits of the native molecule, while the glycans of the functional unit RvH1-a are exposed on the surface of the molecule leading to a 4-fold fluorescence's intensity change.

Unique N-glycan moieties of the 66-kDa cell wall glycoprotein from the red microalga Porphyridium sp

We report here the structural determination of the N-linked glycans in the 66-kDa glycoprotein, part of the unique sulfated complex cell wall polysaccharide of the red microalga Porphyridium sp. Structures were elucidated by a combination of normal phase/reverse phase HPLC, positive ion MALDI-TOF MS, negative ion electrospray ionization, and MS/MS. The sugar moieties of the glycoprotein consisted of at least four fractions of N-linked glycans, each composed of the same four monosaccharides, GlcNAc, Man, 6-O-MeMan, and Xyl, with compositions Man(8-9)Xyl(1-2)Me(3)GlcNAc(2). The present study is the first report of N-glycans with the terminal Xyl attached to the 6-mannose branch of the 6-antenna and to the 3-oxygen of the penultimate (core) GlcNAc. Another novel finding was that all four glycans contain three O-methylmannose residues in positions that have never been reported before. Although it is known that some lower organisms are able to methylate terminal monosaccharides in glycans, the present study on Porphyridium sp. is the first describing an organism that is able to methylate non-terminal mannose residues. This study will thus contribute to understanding of N-glycosylation in algae and might shed light on the evolutionary development from prokaryotes to multicellular organisms. It also may contribute to our understanding of the red algae polysaccharide formation. The additional importance of this research lies in its potential for biotechnological applications, especially in evaluating the use of microalgae as cell factories for the production of therapeutic proteins.

Glycan structures of the structural subunit (HtH1) of Haliotis tuberculata hemocyanin

The oligosaccharide structures of the structural subunit HtH1 of Haliotis tuberculata hemocyanin (HtH) were studied by mass spectral sequence analysis of the glycans. The proposed structures are based on MALDI-TOF-MS data before and after treatment with the specific exoglycosidases β1-3,4,6-galactosidase and α1-6(>2,3,4) fucosidase followed by sequence analysis via electrospray ionization MS/MS-spectra. In total, 15 glycans were identified as a highly heterogeneous group of structures. As in most molluscan hemocyanins, the glycans of HtH1 contain a terminal MeHex, but more interestingly, a novel structural motif was observed: MeHex[Fuc(α1-3)-]GlcNAc, including thus MeHex and (α1-3)-Fuc residues being linked to an internal GlcNAc residue. While the functional unit (FU) c (HtH1-c) is completely lacking any potential glycosylation site, FU-h possesses a second exposed sugar attachment site between beta-strands 8 and 9 within the beta sandwich domain compared to the other FUs. The glycosylation pattern/sites show a high degree of conservation. In FU-h two prominent potential glycosylation sites can be detected. The finding that HtH1 is not able to form multidecameric structures in vivo could be explained by the presence of the exposed glycan on the surface of FU-h.

Glycan structures and antiviral effect of the structural subunit RvH2 of Rapana hemocyanin

Molluscan hemocyanins are very large biological macromolecules and they act as oxygen-transporting glycoproteins. Most of them are glycoproteins with molecular mass around 9000 kDa. The oligosaccharide structures of the structural subunit RvH2 of Rapana venosa hemocyanin (RvH) were studied by sequence analysis of glycans using MALDI-TOF-MS and tandem mass spectrometry on a Q-Trap mass spectrometer after enzymatical liberation of the N-glycans from the polypeptides. Our study revealed a highly heterogeneous mixture of glycans of the compositions Hex(0-9) HexNAc(2-4) Hex(0-3) Pent(0-3) Fuc(0-3). A novel type of N-glycan, with an internal fucose residue connecting one GalNAc(β1-2) and one hexuronic acid, was detected, as also occurs in subunit RvH1. A glycan with the same structure but with two deoxyhexose residues was observed as a doubly charged ion. Antiviral effects of the native molecules of RvH and also of Helix lucorum hemocyanin (HlH), of their structural subunits, and of the glycosylated functional unit RvH2-e and the non-glycosylated unit RvH2-c on HSV virus type 1 were investigated. Only glycosylated FU RvH2-e exhibits this antiviral activity. The carbohydrate chains of the FU are likely to interact with specific regions of glycoproteins of HSV, through van der Waals interactions in general or with certain amino acid residues in particular. Several clusters of these residues can be identified on the surface of RvH2-e.

Structure of hemocyanin from garden snail Helix lucorum

Hemocyanins are giant extracellular oxygen carriers in the hemolymph of many molluscs and arthropods with different quaternary structure. They are represented in the hemolymph of molluscs with one, two or three isoforms, as decameric, didecameric, multidecameric and tubules aggregates. We describe here the structure of the hemocyanin Helix lucorum (HlH), species in the series of molluscan hemocyanins. In contrast with other molluscan hemocyanins, three different hemocyanin isopolypeptides were isolated from the hemolymph of the garden snail H. lucorum, named as beta-HlH, alpha(D)-HlH and alpha(N)-HlH. Their molecular masses were determined by size exclusion chromatography to be 1068 kDa (beta-HlH) and 1079 kDa (alpha(D)-HlH, and alpha(N)-HlH). Native HlH exhibits a predominant didecameric structure as revealed by electron microscopy and additionally few tridecamers are shown in the electron micrographs of HlH resulting from the association of a further decamer with one didecamer. The three isoforms are represented mainly as homogeneous didecamers, but they have different behaviour after dissociation and reassociation in the pH-stabilizing buffer, containing 20 mM CaCl(2). All isoforms were reassociated into didecamers and tubules with different length, but in contrast to alpha(D)-HlH isoform, longer tubules were observed in beta-HlH. Moreover the structure of beta-HlH was analysed after limited proteolysis with trypsin followed by FPLC and HPLC separation of the cleavage products. Eight different functional units were identified by their N-terminal sequences and molecular masses. The protein characteristics, including UV absorption at 340 nm, fluorescence and CD spectra of the native molecule and its units confirmed the structure of multimer protein complexes.

Determination of 3-O- and 4-O-methylated monosaccharide constituents in snail glycans

The N- and O-glycans of Arianta arbustorum, Achatina fulica, Arion lusitanicus and Planorbarius corneus were analysed for their monosaccharide pattern by reversed-phase HPLC after labelling with 2-aminobenzoic acid or 3-methyl-1-phenyl-2-pyrazolin-5-one and by gas chromatography-mass spectrometry. Glucosamine, galactosamine, mannose, galactose, glucose, fucose and xylose were identified. Furthermore, three different methylated sugars were detected: 3-O-methyl-mannose and 3-O-methyl-galactose were confirmed to be a common snail feature; 4-O-methyl-galactose was detected for the first time in snails.

PROTEIN N-GLYCOSYLATION OF GASTROPODS

Glycosylation plays an important role in several types of recognition processes associated with fertilisation and development, allergies, pathological events and cell death. Whereas the amino acid sequence of a protein is fixed by the DNA, the glycosylation abilities depend on enzymes and substrates currently present in the cell.During the last decades our knowledge on glycosylation - the structure of glycans as well as the corresponding biochemical pathways including the responsible enzymes - especially on glycans of mammalian origin increased enormously. The glycosylation capabilities of other species were under investigation only if their glycans were for any reason connected to human life (e.g. some recognition processes of pathogens or allergy on food or plant glycans) or if they were potent candidates for cell culture systems for the expression of therapeutic agents (some insect, yeast and plant cells). However, in the meantime there is an increasing interest also in invertebrate glycosylation.Snails in particular show a broad spectrum of glycosylation abilities within their N-glycosylation pattern. In one case this has been shown to be involved in an intermediate host - parasite recognition process. For other snail species, it was found that they share many structural elements of N-glycans with mammals, plants, insects or nematodes. Sometimes several of these elements are present within one single structure.Here we present an overview of the current knowledge of N-glycosylation of snails, the glycan structures and the corresponding enzymes involved in the biosynthetic glycosylation pathway.

Identification of glycosylated sites in Rapana hemocyanin by mass spectrometry and gene sequence, and their antiviral effect

Molluscan hemocyanins (Hcs) have recently received particular interest due to their significant immunostimulatory properties. This is mainly related to their high carbohydrate content and specific monosaccharide composition. We have now analyzed the oligosaccharides and the carbohydrate linkage sites of the Rapana venosa hemocyanin (RvH) using different approaches. We analyzed a number of glycopeptides by LC/ESI-MS/MS and identified the sugar chains and peptide sequences of 12 glycopeptides. Additionally, the potential carbohydrate linkage sites of 2 functional units, RvH-b and RvH-c, were determined by gene sequence analysis. Only RvH-c shows a potential N-glycosylation site. During this study, we discovered a highly conserved linker-intron, separating the coding exons of RVH-b and RvH-c. Following reports on antiviral properties from arthropod hemocyanin, we conducted a preliminary study of the antiviral activity of RvH and the functional units RvH-b and RvH-c. We show that the glycosylated FU RvH-c has antiviral properties against the respiratory syncytial virus (RSV), whereas native RvH and the nonglycosylated FU RvH-b have not. This is the first report of the fact that also molluscan hemocyanin functional units possess antiviral activity.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

Immunological potential of Helix vulgaris and Rapana venosa hemocyanins

A new hemocyanin was isolated from the hemolymph of garden snails Helix vulgaris, composed of two isoforms, HvH1 and HvH2 separated on an ion exchange column DEAE-Sepharose 6CL. Structural and immunological properties of Helix vulgaris hemocyanin were studied in comparison with molluscan Hcs Rapana venosa and Megathura crenulata. The possibility of using HvH and RvH as carriers of small molecules (haptens) in immunizing protocols was studied in comparison with KLH, which is a widely used, highly immunogenic carrier protein. By using HvH as a carrier of the well-known hapten TNBS (2,4,6-trinitrobenzene sulfonic acid), an increasing with time production of hapten-specific TFN-gamma was detected in splenocyte cultures of mice, which lasted longer than in case of KLH and RvH carriers. Also, use of HvH or RvH as a carrier of the hapten ProT alpha[101-109] (i.e., the synthetic C-terminal fragment of the poorly immunogenic protein prothymosin alpha) showed that antisera of higher titres than that of the control conjugate (ProT alpha[101-109]-KLH) were obtained immediately after the second bleeding. HvH and RvH may prove to be useful for the development of new antiviral, antibacterial and antitumor vaccines, since they seem to launch strong and specific immune response against the conjugated antigens.

Oligosaccharide structure of a functional unit RvH1-b of Rapana venosa hemocyanin using HPLC/electrospray ionization mass spectrometry

In the present study the structures of two glycopeptides (G1 and G1'), isolated from FU RvH(1)-b and two glycopeptides (G2 and G3), isolated from the structural subunit RvH(1) of Rapana venosa hemocyanin, were determined. To structurally characterize the site-specific carbohydrate heterogeneity and binding site of the N-linked glycopeptide(s), a combination of capillary reversed-phase chromatography and ion trap mass spectrometry was used. The amino acid sequences of glycopeptides G1 and G1' determined by Edman degradation and MS/MS sequencing demonstrated that the oligosaccharides are linked to N-glycosylation sites. Two peptides (a glycosylated (G1) and non-glycosylated one) were identified in this fraction and no linkage sites were observed in the latter one. Based on the sequencing of the glycosylated fractions G1, G1', G2 and G3, the carbohydrate structure Man(alpha1-6)Man(alpha1-3)Man(beta1-4)GlcNAc(beta1-4)[Fuc(alpha1-6)]GlcNAc-R could be identified for glycopeptides G1 and G3, and only the typical core structure Man(alpha1-6)Man(alpha1-3)Man(beta1-4)GlcNAc(beta1-4)GlcNAc-R was found for G1' and G2. The Fuc residue found in glycopeptides G1 and G3 is attached to N-acetyl-glucosamine of the carbohydrate core, as often found in other glycoproteins.

Involvement of glycan chains in the antigenicity of Rapana thomasiana hemocyanin

Functional unit (FU) RtH2-e from Rapana thomasiana hemocyanin (Hc) was degraded into small fragments with chymotrypsin. The glycopeptides were separated from the non-glycosylated peptides by chromatography on Concanavalin-A-Sepharose and characterized by mass spectrometry. The glycan part of the glycopeptides (all with common peptide stretch of 14 amino acids) consists of the classical trimannosyl-N,N-diacetylchitobiose core for N-glycosylation, predominantly extended with a unique tetrasaccharide that is branched on fucose. In inhibition ELISA experiments, the glycopeptides interfered in the complex formation between FU RtH2-e and rabbit antibodies against Rapana Hc (about 30% of inhibition). The inhibition also was retained after treatment of the glycopeptides with pronase in order to completely destroy the peptide part. The inhibitory effect of the non-glycosylated peptides, on the other hand, was very low. This study thus demonstrates that the glycans attached to FU RtH2-e contribute to the antigenicity of Rapana Hc.

Neutral N-glycan patterns of the gastropods Limax maximus, Cepaea hortensis, Planorbarius corneus, Arianta arbustorum and Achatina fulica

The N-glycosylation potentials of Limax maximus, Cepaea hortensis, Planorbarius corneus, Arianta arbustorum and Achatina fulica were analysed by investigation of the N-glycan structures of the skin and viscera glycoproteins by a combination of HPLC and mass-spectrometry methods. It is one of the first steps to enlarge the knowledge on the glycosylation abilities of gastropods, which may help to establish new cell culture systems, to uncover new means for pest control for some species, and to identify carbohydrate-epitopes which may be relevant for immune response. All snails analysed contained mainly oligomannosidic and small paucimannosidic structures, often terminated with 3-O-methylated mannoses. The truncated structures carried modifications by beta1-2-linked xylose to the beta-mannose residue, and/or an alpha-fucosylation, mainly alpha1,6-linked to the innermost N-acetylglucosaminyl residue of the core. Many of these structures were missing the terminal N-acetylglucosamine, which has been shown to be a prerequisite for processing to complex N-glycans in the Golgi. In some species (Planorbarius corneus and Achatina fulica) traces of large structures, terminated by 3-O-methylated galactoses and carrying xylose and/or fucose residues, were also detected. In Planorbarius viscera low amounts of terminal alpha1-2-fucosylation were determined. Combining these results, gastropods seem to be capable to produce all kinds of structures ranging from those typical in mammals through to structures similar to those found in plants, insects or nematodes. The detailed knowledge of this very complex glycosylation system of the gastropods will be a valuable tool to understand the principle rules of glycosylation in all organisms.

A challenging insight on the structural unit 1 of molluscan Rapana venosa hemocyanin

Hemocyanins of mollusks are high molecular mass glycoproteins with a complex quaternary structure which still remains to be defined in detail for most of its species as far as number, spatial distribution and interactions of their structural units is concerned. In the present study, we isolated the functional units of the structural subunit RvH1 of Rapana venosa hemocyanin, combining enzymatic and non-enzymatic methods. Our results suggest that Hc's carbohydrate moieties play a basic role in the organization of the structural units, resulting from post-translational polymerization of the 50 kDa functional units and involving sugar moieties that link between them.

New insights in Rapana venosa hemocyanin N-glycosylation resulting from on-line mass spectrometric analyses

The N-glycosylation of structural unit 1 of Rapana venosa hemocyanin was studied. Enzymatically liberated N-glycans were analyzed by matrix-assisted laser desorption ionization-time-of-flight-mass spectrometry (MALDI-TOF-MS) and capillary electrophoresis (CE)-MS following 8-aminopyrene-1,3,6-trisulfonate labeling and labeling with 3-aminopyrazole, a new dedicated sugar reagent. Structural information was obtained by exoglycosidase sequencing, on-line MS/MS, permethylation, and amidation. A mixture of high-mannose and complex glycans with so far unknown and unusual acidic terminal structures was revealed. As the hemocyanin protein sequence is currently unknown, de novo sequencing of the glycopeptides had to be carried out. The N-glycans were therefore enzymatically removed with simultaneous partial (50%) (18)O-labeling of glycosylated asparagine residues prior to proteolysis. Following nano-liquid chromatography-MALDI-TOF-MS, the originally glycosylated peptides could be revealed and their sequences determined by MS/MS. The site occupancies were subsequently elucidated by precursor ion scanning of the intact glycopeptides using a Q-Trap mass spectrometer.

Glycosylation of Rapana thomasiana hemocyanin. Comparison with other prosobranch (gastropod) hemocyanins

The carbohydrate content and composition of hemocyanins (Hcs) of three prosobranchs (gastropods), Rapana thomasiana, Megathura crenulata and Haliotis tuberculata, were compared. The analyses were performed by gas-liquid chromatography after methanolysis, re-N-acetylation and trimethylsilylation. The two structural subunits of R. thomasiana Hc, RtH1 and RtH2, both showed 2.6% (w/w) carbohydrate content with very similar monosaccharide composition, indicative for N-glycosylation. The two isoforms of M. crenulata Hc (KLH), KLH1 and KLH2, on the other hand, definitely differed in glycosylation: KLH2 (3.4% carbohydrate, w/w) comprised relatively less mannose and more N-acetylgalactosamine than KLH1 (3.0% carbohydrate, w/w), in agreement with the fact that O-glycosylation has been observed in a functional unit (FU) of KLH2. For the Hc of the abalone H. tuberculata, with 4.5% (w/w) carbohydrate, appreciable amounts of 3-O-methyl-d-mannose and 3-O-methyl-d-galactose were detected, showing that the occurrence of methylated sugars is not restricted to the Hcs of pulmonates. From the structural subunit RtH2 of Rapana Hc the FUs RtH2-b and RtH2-d were isolated. On the basis of amino acid sequence analysis and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) of the respective native and PNGase-F-treated glycopeptides, one N-glycosylation site was found for each FU. This site was located at Asn-405 for RtH2-b and at Asn-394 for RtH2-d; the carbohydrate moiety corresponded to GlcNAc2Man6 and GlcNAc2Man5, respectively. A comparison was made with the N-glycosylation sites of other FUs of Rapana Hc.

Mass spectral evidence for N-glycans with branching on fucose in a molluscan hemocyanin

Glycopeptides, isolated from a trypsinolysate of functional unit (FU) RtH2-e of Rapana thomasiana hemocyanin subunit 2, were analysed by electrospray ionization mass spectrometry and MS/MS. From the molecular mass observed after deglycosylation, it was inferred that all glycopeptides shared the same peptide stretch 92-143 of FU RtH2-e with a glycosylation site at Asn-127. Besides the core structure Man(3)GlcNAc(2) for N-glycosylation, structures with a supplementary GlcNAc linked to either the Man(alpha1-3) or the Man(alpha1-6) arm and/or an additional tetrasaccharide unit connected to the other Man arm were observed, indicating the existence of microheterogeneity at the glycan level. The tetrasaccharide unit contains a central fucose moiety substituted with 3-O-methylgalactose and N-acetylgalactosamine, and linked to GlcNAc at the reducing end. This structure represents a novel N-glycan motif and is likely to be immunogenic. A second potential site for N-glycosylation in FU RtH2-e at Asn-17 was shown to be not glycosylated.

Structural and functional analysis of glycosylated Cu/Zn-superoxide dismutase from the fungal strain Humicola lutea 103

The fungal strain Humicola lutea 103 produces a naturally glycosylated Cu/Zn-superoxide dismutase (Cu/ZnSOD) (HLSOD). To improve its yield, the effect of increased concentration of Cu2+ (from 1 to 750 microg/ml) on growth and enzyme biosynthesis was studied. The primary structure of this fungal enzyme has been determined by Edman degradation of peptide fragments derived from proteolytic digest. A single chain of the protein, consisting of 152 amino acid residues, reveals a very high degree (74-85%) of structural homology in comparison to the amino acid sequences of other fungal Cu/ZnSODs. The difference of the molecular masses of H. lutea Cu/ZnSOD, measured by MALDI-MS (15,935 Da) and calculated by its amino acid sequence (15,716 Da), is attributed to the carbohydrate chain of one mole of N-acetylglucosamine, attached to the N-glycosylation site Asn23-Glu-Ser. HLSOD protected mice from mortality after experimental influenza A/Aichi/2/68 (H3N2) virus infection. Using the glycosylated HLSOD, the survival rate is increased by 66% (protective index=86.1%) and the survival time prolonged by 5.2 days, similar to the application of ribavarin, while non-glycosylated bovine SOD conferred lower protection.

Neutral N-glycans of the gastropod Arion lusitanicus

The neutral N-glycan structures of Arion lusitanicus (gastropod) skin, viscera and egg glycoproteins were examined after proteolytic digestion, release of the glycans from the peptides, fluorescent labelling with 2-aminopyridine and fractionation by charge, size and hydrophobicity to obtain pure glycan structures. The positions and linkages of the sugars in the glycan were analysed by two dimensional HPLC (size and hydrophobicity) and MALDI-TOF mass spectrometry before and after digestion with specific exoglycosidases. The most striking feature in the adult tissues was the high amount of oligomannosidic and small paucimannosidic glycans terminated with 3-O-methylated mannoses. The truncated structures often contained modifications of the inner core by beta1,2-linked xylose to the beta-mannose residue and/or an alpha-fucosylation (mainly alpha1,6-) of the innermost GlcNAc residue. Skin and viscera showed predominantly the same glycans, however, in different amounts. Traces of large structures carrying 3-O-methylated galactoses were also detected. The egg glycans contained mainly (approximately 75%) oligomannosidic structures and some paucimannosidic structures modified by xylose or alpha1,6-fucose, but in this case no methylation of any monosaccharide was detected. Thus, gastropods seem to be capable of producing many types of structures ranging from those typical in human to structures similar to those found in nematodes, and therefore will be a valuable model to understand the regulation of glycosylation. Furthermore, this opens the way for using this organism as a host for the production of recombinant proteins. The detailed knowledge on glycosylation also may help to identify targets for pest control.