Glycobiology

Sulfated N-glycans from human immunoglobulin A (IgA) were recently discovered via glycomic approaches. However, their site-specific description is still pending. Certain N-glycan structures at specific N-glycosylation sites in IgA are crucial for microbial neutralization and effector functions. For instance, sialylated N-glycans on the C-terminal tailpiece mediate anti-viral activity by interfering with sialic-acid-binding viruses. Sulfated N-glycan epitopes can be ligands for viral proteins and thus play a role in the immune response. In this study, we performed a site-specific screening for sulfated N-glycans in two commercially available human serum IgA samples employing an in-depth N-glycoproteomic approach, previously developed by us. We found evidence of complex-type and hybrid-type N-glycans containing sulfated N-acetylhexosamine (sulfated HexNAc) attached to the N-glycosylation sites in the tailpiece and the CH2 domain of both IgA subclasses. A detailed comparison of the N-glycosylation profiles of human serum IgA samples from two suppliers showed such N-glycans with sulfated HexNAc consistently in higher abundance in the tailpiece region. Surprisingly, also complex-type N-glycan compositions bearing O-acetylated sialic acid were identified in the tailpiece. These findings have not been described before for a site-specific glycopeptide analysis. Overall, our work provides a methodology for performing a dedicated site-specific search for sulfated and O-acetylated N-glycans that can be easily transferred, e.g. to human IgA derived from mucosal tissues, milk, or saliva. Our future aim is to include sulfated N-glycans into longitudinal studies of IgA N-glycosylation and to investigate their role as a biomarker and a treatment option. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=199 SRC="FIGDIR/small/597690v1_ufig1.gif" ALT="Figure 1"> View larger version (45K): org.highwire.dtl.DTLVardef@3d0214org.highwire.dtl.DTLVardef@1537f95org.highwire.dtl.DTLVardef@dd11eeorg.highwire.dtl.DTLVardef@1c43744_HPS_FORMAT_FIGEXP M_FIG C_FIG

Flagella are essential for motility and pathogenicity in many bacteria. The main component of the flagellar filament, flagellin, often undergoes post-translational modifications, with glycosylation being a common occurrence. In Pseudomonas aeruginosa PAO1, the b-type flagellin is O-glycosylated with a structure that includes a rhamnose, a phospho-group and a previous unknown moiety. This structure resembles the well-characterized glycan (Type A) in Clostridioides difficile strain 630, which features an N-acetylglucosamine linked to an N-methylthreonine via a phosphodiester bond. This study aimed to characterize the b-type glycan structure in Pseudomonas aeruginosa PAO1 using a set of mass spectrometry experiments. For this purpose, we used wildtype P. aeruginosa PAO1 and several gene mutants from the b-type glycan biosynthetic cluster. Moreover, we compared the mass spectrometry characteristics of the b-type glycan with those of in vitro modified Type A-peptides from C. difficile strain 630{Delta}erm. Our results demonstrate that the thus far unknown moiety of the b-type glycan in P. aeruginosa consists of an N,N-dimethylthreonine. These data allowed us to refine our model of the flagellin glycan biosynthetic pathway in both P. aeruginosa PAO1 and C. difficile strain 630.

Xyloglucan (XyG) is a major hemicellulosic polymer in primary cell walls of dicotyledonous plants but represents only a minor constituent of cell walls from graminaceous monocotyledons (Poaceae). Our current information on XyG biosynthesis in vitro comes exclusively from studies on dicotyledonous plants. While XyG has been reported in grass cell walls, there are no studies of XyG biosynthesis in vitro in grasses. In this report, we investigated XyG structure and biosynthesis in etiolated wheat seedlings and showed that their walls contain small amounts (4-14%) of XyG. Furthermore, structural analysis using electrospray ionization mass spectrometry (ESI-MS) and high pH anion exchange chromatography (HPAEC) revealed that wheat XyG may be of XXGGG-type. Interestingly, detergent extracts from root microsomes were able to fucosylate tamarind XyG in vitro in a similar way as fucosyltransferase activity from Arabidopsis thaliana (AtFUT1) and pea (PsFUT1). Endoglucanase digestion of the [14C]fucosylated-tamarind XyG formed by the wheat fucosyltransferase activity released radiolabeled oligosaccharides that co-eluted with authentic fucoslyated XyG oligosaccharides (XXFG and XLFG). Although wheat fucosyltransferase activity was low, it appeared to be specific to XyG and required divalent ions (Mg2+ or Mn2+) for full activity. Together, these results suggest that the XyG fucosylation mechanism is conserved between monocots and dicots.

The cell surface and extracellular matrix polysaccharide, heparan sulfate (HS) conveys chemical information to control or influence crucial biological processes. Attempts to describe its structure-function relationships with HS binding proteins in a classical lock and key type manner, however, have been unsuccessful. HS chains are synthesized in a non-template driven process in the ER and Golgi apparatus, involving a large number of enzymes capable of fine-tuning structures. Changes in the localization of HS-modifying enzymes throughout the Golgi, rather than protein expression levels, were found to correlate with changes in the structure of HS. Following brefeldin A treatment, the HS-modifying enzymes localized preferentially in COPII vesicles and at the trans-Golgi. Further, shortly after treatment with heparin, the HS-modifying enzyme moved from cis to trans-Golgi, which coincided with increased HS trisulfated disaccharide content. Finally, it was shown that COPI subunits and Sec24 gene expression changed. Collectively, these findings highlight that the ER-Golgi dynamics of HS-modifying enzymes via vesicular trafficking processes are critical prerequisite for the complete delineation of HS biosynthesis.

Here we present the GLYCAM Bacterial Carbohydrate Builder (https://glycam.org/cb), an enhanced version of the GLYCAM-Web Carbohydrate Builder1 structure modeller that integrates support for modelling bacterial glycans, enabling the straightforward generation of three-dimensional structural models. The tool integrates bacterial monosaccharide parametrisations into a curated, user-friendly web-based resource. It provides an intuitive interface for the generation of carbohydrate sequences and generates 3D structural models in PDB file format, as well as the input files required for performing molecular dynamics simulations with the AMBER software package. The current implementation includes a library of 18 bacterial monosaccharides, which can be used in combination with the already parametrised eukaryotic sugars to construct complex bacterial glycans. Common derivatives, including acetylation, methylation, and sulfation are also supported. By validating and integrating bacterial sugar parameters into the GLYCAM-Web Carbohydrate Builder, this work reduces the technical barriers associated with bacterial glycan modelling and facilitates computational studies of complex bacterial glycoconjugates.

The glycan distribution on cells is governed by the stochastic activity of different families of enzymes that are together called glycoEnzymes. These include ~400 gene products or 2% of the proteome, that have recently been curated in an ontology called GlycoEnzOnto. With the goal of making this ontology more accessible to the larger biomedical and biotechnology community, we organized a web resource called GlycoEnzDB, presenting this enzyme classification both in terms of enzyme function and the pathways that they participate in. This information is linked to i) Figures from the "Essentials of Glycobiology" textbook, ii) General gene, enzyme and pathway data appearing in external databases, iii) Manual and generative-artificial intelligence (AI) based text describing the function and pathways regulated by these entities, iv) Single-cell expression data across cell lines, normal human cell-types and tissue, and v) CRISPR-knockout/activation/inactivation and Transcription factor activity predictions. Whereas these data are curated for human glycoEnzymes, the knowledge framework may be extended to other species also. The user-friendly web interface is accessible at www.virtualglycome.org/glycoenzdb.

Tetraspanin proteins play an important role in many cellular processes as they are key organizers of different receptors on the plasma membrane. Most tetraspanins are highly glycosylated at their large extracellular loop, but the function of this post-translational modification remains largely unstudied. In this study we investigated the effects of glycosylation of CD37 and CD53, two tetraspanins important for cellular and humoral immunity. Broad and cell-specific repertoires of N-glycosylated CD37 and CD53 were observed in human B cells. We generated different glycosylation mutants of CD37 and CD53 and analyzed their localization, nanoscale organization and partner protein interaction capacity. Abrogation of glycosylation in CD37 revealed the importance of this modification for CD37 surface expression, whereas neither surface expression nor nanoscale organization of CD53 was affected by its glycosylation. CD37 interaction with its known partner proteins, CD20 and IL-6R, was not affected by glycosylation, other than via its changed subcellular localization. Surprisingly, glycosylation was found to inhibit the interaction between CD53 and its partner proteins CD45 and CD20. Together, our data show that tetraspanin glycosylation affects their function in immune cells, which adds another layer of regulation to tetraspanin-mediated membrane organization.

N-Acetylglucosamine exists in various forms and linkage patterns within organisms, playing a crucial role in numerous vital biological processes. However, research focusing on {beta}-1,2-linked N-acetylglucosamine within complex biantennary N-glycans on the cell surface remain largely unexplored. Currently, there is a lack of efficient tools and methods capable of directly modifying terminal N-acetylglucosamine on living cells. In this study, we identified a novel {beta}-N-acetylhexosaminidase from Elizabethkingia meningoseptica, which demonstrated favorable enzymatic stability within a pH range of 5-8 and at temperatures from 4 {degrees}C to 37 {degrees}C. This enzyme specifically cleaves non-reducing terminal {beta}-1,2-N-acetylglucosamine, enabling direct removal of this modification from oligosaccharides and native glycoproteins. More importantly, we demonstrate for the first time that this enzyme successfully removes {beta}-1,2-linked N-acetylglucosamine on living cell surfaces. Given its microbial origin and potential utility in living cell glycan editing, we have named it emNagII or cell-surface glycan-editing N-acetylglucosaminidase (csgeNagII). Through sequence analysis and alanine scanning mutagenesis, we identified a predicted active pocket containing the catalytic residue pair Asp317-Glu318, with Asp317 being essential for enzymatic activity. In summary, our findings provide an effective and reliable method for the targeted removal of {beta}-1,2-N-acetylglucosamine on living cell surfaces, establishing a foundation for further functional studies and practical applications.

Human extracellular 6-O-endosulfatases Sulf-1 and Sulf-2 are the only enzymes that post-synthetically alter the 6-O sulfation of heparan sulfate proteoglycans (HSPG), which regulates interactions of HSPG with many proteins. Oncogenicity of Sulf-2 in different cancers has been documented and we have shown that Sulf-2 is associated with poor survival outcomes in head and neck squamous cell carcinoma (HNSCC). In spite of its importance, limited information is available on direct protein-protein interactions of the Sulf-2 protein in the tumor microenvironment. In this study, we used monoclonal antibody (mAb) affinity purification and mass spectrometry to identify galectin-3-binding protein (LG3BP) as a highly specific binding partner of Sulf-2 in the secretome of HNSCC cell lines. We validated their direct interaction in vitro using recombinant proteins and have shown that the chondroitin sulfate (CS) covalently bound to the Sulf-2 influences the binding to LG3BP. We confirmed importance of the CS chain for the interaction by generating a mutant Sulf-2 protein that lacks the CS. Importantly, we have shown that the LG3BP inhibits Sulf-2 activity in vitro in a concentration dependent manner. As a consequence, the addition of LG3BP to a spheroid cell culture inhibited invasion of the HNSCC cells into Matrigel. Thus, Sulf-2 interaction with LG3BP has functional relevance, and may regulate physiological activity of the Sulf-2 enzyme as well as its activity in the tumor microenvironment.

The receptor binding domain (RBD) of the SARS-CoV-2 spike protein is a conserved domain and a target for neutralizing antibodies. We defined the carbohydrate content of recombinant RBD produced in different mammalian cells. We found a higher degree of complex type N-linked glycans, with less sialylation and more fucosylation, when the RBD was produced in Human embryonic kidney cells compared to the same protein produced in Chinese hamster ovary cells. The carbohydrates on the RBD proteins were enzymatically modulated and the effect on antibody reactivity was evaluated with serum samples from SARS-CoV-2 positive patients. Removal of all carbohydrates diminished antibody reactivity while removal of only sialic acids or terminal fucoses improved the reactivity. The RBD produced in Lec3.2.8.1-cells, which generate carbohydrate structures devoid of sialic acids and with reduced fucose content, exhibited enhanced antibody reactivity verifying the importance of these specific monosaccharides. The results can be of importance for the design of future vaccine candidates, indicating that it might be possible to enhance the immunogenicity of recombinant viral proteins.

The Escherichia coli surfaceome consists mainly of the large surface organelles expressed by the organism to navigate and interact with the surrounding environment. The current study focuses on type I fimbriae and flagella. These large polymeric surface organelles are composed of hundreds to thousands of subunits, with their large size often preventing them from being studied in their native form. Recent studies are accumulating which demonstrate the glycosylation of surface proteins or virulence factors in pathogens, including E. coli. Using biochemical and glycobiological techniques, including biotin-hydrazide labelling of glycans and chemical and glycosidase treatments, we demonstrate i) the presence of a well-defined and chemically resistant FimA oligomer in several strains of pathogenic and non-pathogenic E. coli, ii) the major subunit of type I fimbriae, FimA, in pathogenic and laboratory strains is recognized by concanavalin A, iii) standard methods to remove N-glycans (PNGase F) or a broad-specificity mannosidase fail to remove the glycan structure, despite the treatments resulting in altered migration in SDS-PAGE, iv) PNGase F treatment results in a novel 32 kDa band recognized by anti-FliC antiserum. While the exact identity of the glycan(s) and their site of attachment currently elude detection by conventional glycomics/glycoproteomics, the current findings highlight a potential additional layer of complexity of the surface (glyco)proteome of the commensal or adhesive and invasive E. coli strains studied.

Here we have generated 3D structures of glycoforms of the spike (S) glycoprotein from SARS-CoV-2, based on reported 3D structures and glycomics data for the protein produced in HEK293 cells. We also analyze structures for glycoforms representing those present in the nascent glycoproteins (prior to enzymatic modifications in the Golgi), as well as those that are commonly observed on antigens present in other viruses. These models were subjected to molecular dynamics (MD) simulation to determine the extent to which glycan microheterogeneity impacts the antigenicity of the S glycoprotein. Lastly, we have identified peptides in the S glycoprotein that are likely to be presented in human leukocyte antigen (HLA) complexes, and discuss the role of S protein glycosylation in potentially modulating the adaptive immune response to the SARS-CoV-2 virus or to a related vaccine. The 3D structures show that the protein surface is extensively shielded from antibody recognition by glycans, with the exception of the ACE2 receptor binding domain, and also that the degree of shielding is largely insensitive to the specific glycoform. Despite the relatively modest contribution of the glycans to the total molecular weight (17% for the HEK293 glycoform) the level of surface shielding is disproportionately high at 42%.

Mutations in LMAN1 and MCFD2 cause the combined deficiency of FV and FVIII (F5F8D). LMAN1 and MCFD2 form a protein complex that transport FV and FVIII from the endoplasmic reticulum to the Golgi. Although both proteins are required for the cargo receptor function, little is known about specific roles of LMAN1 and MCFD2 in transporting FV/FVIII. We used different LMAN1 and MCFD2 deficient cell lines to investigate the LMAN1/MCFD2-dependent FV/FVIII secretion pathway. LMAN1 deficiency led to more profound decreases in FV/FVIII secretion in HEK293T and HepG2 cells than in HCT116 cells, suggesting regulation of cargo transport by the LMAN1/MCFD2 pathway varies in different cell types. Using these cell lines, we developed functional assays to accurately assess pathogenicity of recently reported potential LMAN1 and MCFD2 missense mutations. LMAN1 with mutations abolishing carbohydrate binding can still partially rescue FV/FVIII secretion, suggesting that N-glycan binding is not absolutely required for FV/FVIII transport. Surprisingly, overexpression of either WT or mutant MCFD2 is sufficient to rescue FV/FVIII secretion defects in LMAN1 deficient cells. These results suggest that cargo binding and transport are carried out by MCFD2 and that LMAN1 primarily serves as a shuttling carrier of MCFD2. Finally, overexpression of both LMAN1 and MCFD2 does not further increase FV/FVIII secretion, suggesting that the amount of the LMAN1-MCFD2 receptor complex is not a rate-limiting factor in ER-Golgi transport of FV/FVIII. This study provides new insight into the molecular mechanism of F5F8D and intracellular trafficking of FV and FVIII. Key PointsO_LIEfficient ER-to-Golgi transport of FV and FVIII requires the LMAN1-MCFD2 cargo receptor complex. C_LIO_LIMCFD2 functions as a primary interacting partner of FV/FVIII cargo and LMAN1 primarily serves as a shuttling carrier of MCFD2. C_LI

In the endoplasmic reticulum (ER), O-glycosylation by O-fucose, O-glucose, and O-GlcNAc occurs in the epidermal growth factor-like (EGF) domains of secreted or transmembrane glycoproteins. Previous studies focusing on Notch receptors have revealed the pivotal role of these O-glycans in the cell surface expression of Notch or secretion of truncated Notch fragments. Although it has been demonstrated that O-fucose, O-glucose, and O-GlcNAc stabilize individual EGF domains, their role in the secretory pathway after the completion of the folding process remains unexplored. In this study, we used delta-like 1 homolog (DLK1) containing six consecutive EGF domains as a model glycoprotein to investigate the role of EGF domain-specific O-glycans in the secretory pathway. Semi-quantitative site-specific glycoproteomics of recombinantly expressed DLK1 revealed multiple O-fucose and O-glucose modifications in addition to an unusual EOGT-dependent O-hexose modification. Consistent with the results of the secretion assay, inactivation of the glycosyltransferases modifying O-fucose and O-glucose, but not the newly identified O-hexose, perturbed the transport of DLK1 from the ER during retention using the selective hooks (RUSH) system. Importantly, the absence of O-fucose did not result in an apparent loss of O-glucose modification within the same EGF domain, and vice versa. Given that protein O-fucosyltransferase 1 and protein O-glucosyltransferase 1 activities depend on the folded state of the EGF domains, O-glycans affected DLK1 transport independently of the folding process required for O-glycosylation in the ER. These findings highlight the distinct roles of O-glycans in facilitating the transport of DLK1 from the ER to the cell surface.

The intracellular transport system is an evolutionally conserved, essential, and highly regulated network of organelles and transport vesicles that traffic protein and lipid cargoes within the cell. The events of vesicle formation, budding and fusion are orchestrated by the trafficking machinery - an elaborate set of proteins including small GTPases, vesicular coats, tethers, and SNAREs. The Golgi - the central organelle in this transport network, receives, modifies and sorts secretory and endocytic cargo. Glycosylation is one of the major modifications that occur within the Golgi, which houses enzymes and other components of glycosylation machinery. According to the current Golgi maturation model, Golgi resident proteins are constantly recycled from the late (trans) Golgi compartments to the early compartment (cis) by the evolutionary conserved vesicular trafficking machinery. The key modulator of vesicular trafficking and glycosylation at the Golgi is the Conserved Oligomeric Golgi (COG) complex - its interaction vesicular trafficking machinery particularly Golgi SNAREs (STX5, GS28 (GOSR1), GS15 (BET1L) and YKT6) that drive fusion of incoming vesicles. Since the COG complex functions upstream of SNARE-mediated vesicle fusion, we hypothesize that depletion of Golgi v-SNAREs would mirror defects observed in COG deficient cells. To test this, we created single and double knockouts (KO) of GS28 and GS15 in HEK293T cells and analyzed resulting mutants using a comprehensive set of biochemical, mass-spectrometry (MS) and microscopy approaches. Deletion of GS28 significantly affected GS15, but not the other two partners, STX5 and YKT6. Surprisingly, our analysis revealed that COG dysfunction is more deleterious for Golgi function than disrupting the canonical Golgi SNARE complex. Quantitative MS analysis of STX5-interacting SNAREs revealed unexpected flexibility of Golgi SNARE pairing in mammalian cells. We uncovered two novel non-canonical Golgi SNARE complexes - STX5/VTI1B/GS15/YKT6 and STX5/SNAP29/VAMP7 which were upregulated in GS28 KO cells. Analysis of cells co-depleted for GS28/SNAP29 or GS28/VTI1B SNAREs revealed escalated defects in Golgi glycosylation, indicating that upregulation of these complexes functionally substitutes deleted GS28. Our data points to the remarkable plasticity in the intra-Golgi membrane fusion machinery which is controlled by the COG complex.

Synthesis of glycosaminoglycans such as heparan sulfate (HS) and chondroitin sulfate (CS) occurs in the lumen of the Golgi but the relationship between Golgi structural integrity and glycosaminoglycan synthesis is not clear. In this study, we disrupted the Golgi structure by knocking out GRASP55 and GRASP65 and determined its effect on the synthesis, sulfation, and secretion of HS and CS. We found that GRASP depletion increased HS synthesis while decreasing CS synthesis in cells, altered HS and CS sulfation, and reduced both HS and CS secretion. Using proteomics, RNA-seq and biochemical approaches, we identified EXTL3, a key enzyme in the HS synthesis pathway, whose level is upregulated in GRASP knockout cells; while GalNacT1, an essential CS synthesis enzyme, is robustly reduced. In addition, we found that GRASP depletion decreased HS sulfation via the reduction of PAPSS2, a bifunctional enzyme in HS sulfation. Our study provides the first evidence that Golgi structural defect may significantly alter the synthesis and secretion of glycosaminoglycans.

The carbohydrate 3D structure-prediction tools (builders) at GLYCAM-Web (glycam.org) are widely used for generating experimentally-consistent 3D structures of oligosaccharides suitable for data interpretation, hypothesis generation, simple visualization, and subsequent molecular dynamics (MD) simulation. The graphical user interface (GUI) enables users to create carbohydrate sequences (e.g. DGalpb1-4DGlcpb1-OH) that are converted to 3D models of the carbohydrate structures in multiple formats, including PDB and OFF (AMBER software format). The resulting structures are energy minimized prior to download and online visualization. There are advanced options for selecting which shapes (rotamers) of the oligosaccharide to generate, and for creating explicitly solvated structures for subsequent MD simulation. The GLYCAM-Web builders integrate known conformational preferences of oligosaccharides, summarized here, and employ the GLYCAM forcefield for energy minimization with algorithms tailored for speed and scalability. Even for large oligosaccharides (100 residues, ~2100 atoms) a 3D structure is typically returned to the user in less than a minute.

Semaphorin 3A (Sema3A) is a secreted protein that signals to cells through binding to neuropilin and plexin receptors and provides neurons with guidance cues key for axon pathfinding, and also controls cell migration in several other biological systems. Sema3A interacts with glycosaminoglycans (GAGs), an interaction that could localize the protein within tissues and involves the C-terminal domain of the protein. This domain comprises several furin cleavage sites that are processed during secretion and in previous works have hampered recombinant production of full-length wild type Sema3A, and the biochemical analysis of Sema3A interaction with GAGs. In this work, we have developed a strategy to purify the full-length protein in high yield and identified two sequences in the C-terminal domain, KRDRKQRRQR and KKGRNRR, which confer to the protein sub nM affinity for chondroitin sulfate and heparan sulfate polysaccharides. Using chemically defined oligosaccharides and solid phase binding assays, we report that Sema3A recognizes a (GlcA-GalNAc4S6S)2 motif but not a (GlcA2S-GalNAc6S)2 motif and is thus highly specific for type E chondroitin sulfate. Functionally, we found that Sema3A rigidified CS-E films that mimic the GAG presentation within extracellular matrices (ECMs), suggesting that Sema3A may have a previously unidentified function to cross-link and thus stabilize GAG-rich ECMs. Finally, we demonstrated that the full-length Sema3A is more potent at inhibiting neurite outgrowth than the truncated or mutant forms that were previously purified and that the GAG binding sites are required to achieve full activity. The results suggest that Sema3A can rigidify and cross-link GAG matrices, implicating Sema3A could function as an extracellular matrix organizer in addition to binding to and signaling through its cognate cell surface receptors.

Missense variants in O-GlcNAc transferase (OGT) result in OGT congenital disorder of glycosylation (OGT-CDG), an intellectual disability syndrome associated with O-GlcNAc dyshomeostasis and a range of neurodevelopmental defects. Inhibition of O-GlcNAcase (OGA), the enzyme responsible for removing protein O-GlcNAcylation, has been explored as a target for modulating brain O-GlcNAc homeostasis in neurodegenerative diseases and may also be a target for OGT-CDG. Here, we describe an OGT-CDG mouse line that exhibits microcephaly, motor deficits, and brain O-GlcNAc dyshomeostasis, closely mirroring patient symptoms. We genetically explored OGA as a target for OGT-CDG by crossing these mice with a line carrying catalytically inactive OGA. Encouragingly, this partially restored O-GlcNAc homeostasis in brain and blood, although it did not result in significant phenotypic rescue. These findings suggest that OGA inhibition can modulate enzymatic imbalance in OGT-CDG mice, and that blood can be used to monitor the effects of interventions targeting O-GlcNAc dyshomeostasis.

Motile pathogens often rely upon flagellar motility as an essential virulence factor and in many species the structural flagellin protein is glycosylated. This post-translational modification has been shown to be necessary for proper folding of the flagellin structural proteins and proper function of the flagellar filament in a number of bacterial species. Aeromonas hydrophila is a ubiquitous aquatic pathogen with a constitutively expressed polar flagellum. Using a suite of mass spectrometry techniques, the flagellin FlaA and FlaB structural proteins of A. hydrophila strain ATCC 7966T were shown to be glycosylated with significant metaheterogeneity: heterologous glycans were observed with variable site occupancy. The penta- and hexa-saccharide glycan chains contained a previously unreported pseudaminic acid derivative with a mass of 422 Da as the linking sugar, followed in sequence by two hexoses, an N-acetylglucosamine derivative, a deoxy N-acetylglucosamine derivative, and sometimes an additional N-acetylglucosamine.