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

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.

Protein-Based Adjuvants for Vaccines as Immunomodulators of the Innate and Adaptive Immune Response: Current Knowledge, Challenges, and Future Opportunities

New-generation vaccines, formulated with subunits or nucleic acids, are less immunogenic than classical vaccines formulated with live-attenuated or inactivated pathogens. This difference has led to an intensified search for additional potent vaccine adjuvants that meet safety and efficacy criteria and confer long-term protection. This review provides an overview of protein-based adjuvants (PBAs) obtained from different organisms, including bacteria, mollusks, plants, and humans. Notably, despite structural differences, all PBAs show significant immunostimulatory properties, eliciting B-cell- and T-cell-mediated immune responses to administered antigens, providing advantages over many currently adopted adjuvant approaches. Furthermore, PBAs are natural biocompatible and biodegradable substances that induce minimal reactogenicity and toxicity and interact with innate immune receptors, enhancing their endocytosis and modulating subsequent adaptive immune responses. We propose that PBAs can contribute to the development of vaccines against complex pathogens, including intracellular pathogens such as Mycobacterium tuberculosis, those with complex life cycles such as Plasmodium falciparum, those that induce host immune dysfunction such as HIV, those that target immunocompromised individuals such as fungi, those with a latent disease phase such as Herpes, those that are antigenically variable such as SARS-CoV-2 and those that undergo continuous evolution, to reduce the likelihood of outbreaks.

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.

Comparative Proteomic Analysis of Slime from the Striped Pyjama Squid, Sepioloidea lineolata, and the Southern Bottletail Squid, Sepiadarium austrinum (Cephalopoda: Sepiadariidae)

Sepioloidea lineolata, the striped pyjama squid (family Sepiadariidae), is a small species of benthic bobtail squid distributed along the Southern Indo-Pacific coast of Australia. Like other sepiadariid squids, it is known to secrete large volumes of viscous slime when stressed. In order to identify key proteins involved in the function of sepiadariid slimes, we compared the slime proteome of Sepioloidea lineolata with that of a closely related species, Sepiadarium austrinum. Of the 550 protein groups identified in Sepioloidea lineolata slime, 321 had orthologs in Sepiadarium austrinum, and the abundance of these (iBAQ) was highly correlated between species. Both slimes were dominated by a small number of abundant proteins, and several of these were short secreted proteins with no homologues outside the class Cephalopoda. No mucins were identified within either species' slime, suggesting that it is structurally distinct from mucin polymer-based gels found in many vertebrate and echinoderm secretions. The extent of N-glycosylation in the slime of Sepioloidea lineolata was also studied via glycan cleavage with Peptide: N-glycosidase F (PNGase-F). Although very few (four) proteins showed strong evidence of N-glycosylation, we found that treatment with PNGase-F led to a slight increase in peptide identification rates compared with controls.

Multistep Fractionation and Mass Spectrometry Reveal Zwitterionic and Anionic Modifications of the N- and O-glycans of a Marine Snail

Various studies in the past have revealed that molluscs can produce a wide range of rather complex N-glycan structures, which vary from those occurring in other invertebrate animals; particularly methylated glycans have been found in gastropods, and there are some reports of anionic glycans in bivalves. Due to the high variability in terms of previously described structures and methodologies, it is a major challenge to establish glycomic workflows that yield the maximum amount of detailed structural information from relatively low quantities of sample. In this study, we apply differential release with peptide:N-glycosidases F and A followed by solid-phase extraction on graphitized carbon and reversed-phase materials to examine the glycome of Volvarina rubella (C. B. Adams, 1845), a margin snail of the clade Neogastropoda. The resulting four pools of N-glycans were fractionated on a fused core RP-HPLC column and subject to MALDI-TOF MS and MS/MS in conjunction with chemical and enzymatic treatments. In addition, selected N-glycan fractions, as well as O-glycans released by β-elimination, were analyzed by porous graphitized carbon-LC-MS and MS(n). This comprehensive approach enabled us to determine a number of novel modifications of protein-linked glycans, including N-methyl-2-aminoethylphosphonate on mannose and N-acetylhexosamine residues, core β1,3-linked mannose, zwitterionic moieties on core Galβ1,4Fuc motifs, additional mannose residues on oligomannosidic glycans, and bisubstituted antennal fucose; furthermore, typical invertebrate N-glycans with sulfate and core fucose residues are present in this gastropod.

Anomalous N-glycan structures with an internal fucose branched to GlcA and GlcN residues isolated from a mollusk shell-forming fluid

This report describes the structural details of a unique N-linked valence epitope on the major protein within the extrapallial (EP) fluid of the mollusk, Mytilus edulis. Fluids from this area are considered to be responsible for shell expansion by a self-assembly process that provides an organic framework for the growth of CaCO3 crystals. Previous reports from our laboratories have described the purification and amino acid sequence of this EP protein, which was found to be a glycoprotein (EPG) of approximately 28 KDa with 14.3% carbohydrate on a single N-linked consensus site. Described herein is the de novo sequence of the major glycan and its glycomers. The sequence was determined by ion trap sequential mass spectrometry (ITMS(n)) resolving structure by tracking precursor-product relationships through successive rounds of collision induced disassociation (CID), thereby spatially resolving linkage and branching details within the confines of the ion trap. Three major glycomers were detected, each possessing a 6-linked fucosylated N-linked core. Two glycans possessed four and five identical antennae, while the third possessed four antennas, but with an additional methylfucose 2-linked to the glucuronic acid moiety, forming a pentasaccharide. The tetrasaccharide structure was: 4-O-methyl-GlcA(1-4)[GlcNAc(1-3)]Fuc(1-4)GlcNAc, while the pentasaccharide was shown to be as follows: mono-O-methyl-Fuc(1-2)-4-O-methyl-GlcA(1-4)[GlcNAc(1-3)]Fuc(1-4)GlcNAc. Samples were differentially deuteriomethylated (CD3/CH3) to localize indigenous methylation, further analyzed by high resolution mass spectrometry (HRMS) to confirm monomer compositions, and finally gas chromatography mass spectrometry (GC-MS) to assign structural and stereoisomers. The interfacial shell surface location of this major extrapallial glycoprotein, its calcium and heavy metal binding properties and unique structure suggests a probable role in shell formation and possibly metal ion detoxification. A closely related terminal tetrasaccharide structure has been reported in spermatozoan glycolipids of freshwater bivalves.

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.

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.

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.

The system of xylogalactans from the red seaweed Jania rubens (Corallinales, Rhodophyta)

The main acidic polysaccharides from the red seaweed Jania rubens share the general characteristics of corallinans (agar-like xylogalactans). After fractionation by ion-exchange chromatography, ten fractions were separated and characterized by sugar composition, other components, methylation, ethylation, desulfation-methylation, and NMR analyses. The main group of fractions carry the agaran disaccharidic repeating unit [-->3)-beta-D-Gal-(1-->4)-alpha-L-Gal-(1-->] substituted mainly on O-6 of the beta-D-Gal unit by beta-xylosyl side stubs, and less with sulfate or methoxyl groups, and also on O-2 of the alpha-l-Gal unit with methoxyl or sulfate, or less on O-3 of the same unit with methoxyl groups. These features are somehow common to the four members of the order already studied. However, a sugar uncommon to the order appears in moderate proportions for all the fractions: it is 3,6-anhydro-l-galactose (partly sulfated or methoxylated on O-2) replacing the L-Gal unit. Besides, several other structural features never found in the order (and uncommon in any polysaccharide) appear in some minor fractions: the presence of side stubs of 2,3-di- and 3-O-methyl-D-galactose, and also part of the 3-O-methyl-L-galactose acting as side stubs. These results show that, although the main features of the corallinean xylogalactans are common to all the species studied, each one has minor characteristics of its own.

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.

Conformational stabilization at the active site of molluskan (Rapana thomasiana) hemocyanin by a cysteine-histidine thioether bridge A study by mass spectrometry and molecular modeling

In some type-3 copper proteins (molluskan hemocyanin, catechol oxidase and fungal tyrosinase) one of the histidine residues, liganding the Cu(A) atom of the dinuclear copper active site, is covalently linked to a cysteine residue by a thioether bridge. The purpose of this study was to disclose the function of this bridge. Mass spectral analysis of a peptide, isolated from Rapana thomasiana (gastropodan mollusk) hemocyanin, indicated a stabilization of the peptide structure in the region of the bridge. Molecular modeling of three thioether containing type-3 copper proteins using the dead-end elimination method showed that the concerned histidine would be very flexible if not linked to the cysteine. Also, the side chain orientation of the histidine is rather exceptional, as evidenced by statistical data from the protein databank. It is suggested that the role of the bridge is to fix the histidine in an orientation that is optimal for coordination of the Cu(A) atom.

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.

Glycoproteomics based on tandem mass spectrometry of glycopeptides

Next to the identification of proteins and the determination of their expression levels, the analysis of post-translational modifications (PTM) is becoming an increasingly important aspect in proteomics. Here, we review mass spectrometric (MS) techniques for the study of protein glycosylation at the glycopeptide level. Enrichment and separation techniques for glycoproteins and glycopeptides from complex (glyco-)protein mixtures and digests are summarized. Various tandem MS (MS/MS) techniques for the analysis of glycopeptides are described and compared with respect to the information they provide on peptide sequence, glycan attachment site and glycan structure. Approaches using electrospray ionization and matrix-assisted laser desorption/ionization (MALDI) of glycopeptides are presented and the following fragmentation techniques in glycopeptide analysis are compared: collision-induced fragmentation on different types of instruments, metastable fragmentation after MALDI ionization, infrared multi-photon dissociation, electron-capture dissociation and electron-transfer dissociation. This review discusses the potential and limitations of tandem mass spectrometry of glycopeptides as a tool in structural glycoproteomics.