N-linked glycosylation is the most important form of post-translational modification for proteins synthesized and folded in the Endoplasmic Reticulum (Stanley et al. 2009). An early study in 1999 revealed that about 50% of the proteins in the Swiss-Prot database at the time were N-glycosylated (Apweiler et al. 1999). It is now established that the majority of the proteins in the secretory pathway require glycosylation in order to achieve proper folding.
The addition of an N-glycan to a protein can have several roles (Shental-Bechor & Levy 2009). First, glycans enhance the solubility and stability of the proteins in the ER, the golgi and on the outside of the cell membrane, where the composition of the medium is strongly hydrophilic and where proteins, that are mostly hydrophobic, have difficulty folding properly. Second, N-glycans are used as signal molecules during the folding and transport process of the protein: they have the role of labels to determine when a protein must interact with a chaperon, be transported to the golgi, or targeted for degradation in case of major folding defects. Third, and most importantly, N-glycans on completely folded proteins are involved in a wide range of processes: they help determine the specificity of membrane receptors in innate immunity or in cell-to-cell interactions, they can change the properties of hormones and secreted proteins, or of the proteins in the vesicular system inside the cell.
All N-linked glycans are derived from a common 14-sugar oligosaccharide synthesized in the ER, which is attached co-translationally to a protein while this is being translated inside the reticulum. The process of the synthesis of this glycan, known as Synthesis of the N-glycan precursor or LLO, constitutes one of the most conserved pathways in eukaryotes, and has been also observed in some eubacteria. The attachment usually happens on an asparagine residue within the consensus sequence asparagine-X-threonine by an complex called oligosaccharyl transferase (OST).
After being attached to an unfolded protein, the glycan is used as a label molecule in the folding process (also known as Calnexin/Calreticulin cycle) (Lederkremer 2009). The majority of the glycoproteins in the ER require at least one glycosylated residue in order to achieve proper folding, even if it has been shown that a smaller portion of the proteins in the ER can be folded without this modification.
Once the glycoprotein has achieved proper folding, it is transported via the cis-Golgi through all the Golgi compartments, where the glycan is further modified according to the properties of the glycoprotein. This process involves relatively few enzymes but due to its combinatorial nature, can lead to several millions of different possible modifications. The exact topography of this network of reactions has not been established yet, representing one of the major challenges after the sequencing of the human genome (Hossler et al. 2006).
Since N-glycosylation is involved in an great number of different processes, from cell-cell interaction to folding control, mutations in one of the genes involved in glycan assembly and/or modification can lead to severe development problems (often affecting the central nervous system). All the diseases in genes involved in glycosylation are collectively known as Congenital Disorders of Glycosylation (CDG) (Sparks et al. 2003), and classified as CDG type I for the genes in the LLO synthesis pathway, and CDG type II for the others.

N-Linked glycoprotein is a highly interesting class of proteins for clinical and biological research. The large-scale characterization of N-linked glycoproteins accomplished by mass spectrometry-based glycoproteomics has provided valuable insights into the interdependence of glycoprotein structure and protein function. However, these studies focus mainly on the analysis of specific sample type, and lack integration of each glycoproteomic data from different tissues, body fluids or cell types. In this study, we established N-GlycositeAtlas, a comprehensive database resource of human N-linked glycoproteins and their glycosylation sites identified by mass spectrometry. N-GlycositeAtlas contains more than 30,000 glycosite-containing peptides (representing >14,000 N-glycosylation sites) from more than 7,000 N-glycoproteins from different biological sources including human-derived tissues, body fluids and cell lines from over 100 studies. The entire human N-glycoproteome database as well as 22 sub-databases associated with individual tissues or body fluids can be downloaded from the N-GlycositeAtlas website at nglycositeatlas.biomarkercenter.org.

Additional file 1: Table S1. The mass spectrometry-based human glycoprotein and glycosite database.

Glycosylation is one of the most complex post-translational modifications (PTMs) of proteins in eukaryotic cells. Glycosylation plays an important role in biological processes ranging from protein folding and subcellular localization, to ligand recognition and cell-cell interactions. Experimental identification of glycosylation sites is expensive and laborious. Hence, there is significant interest in the development of computational methods for reliable prediction of glycosylation sites from amino acid sequences.We explore machine learning methods for training classifiers to predict the amino acid residues that are likely to be glycosylated using information derived from the target amino acid residue and its sequence neighbors. We compare the performance of Support Vector Machine classifiers and ensembles of Support Vector Machine classifiers trained on a dataset of experimentally determined N-linked, O-linked, and C-linked glycosylation sites extracted from O-GlycBase version 6.00, a database of 242 proteins from several different species. The results of our experiments show that the ensembles of Support Vector Machine classifiers outperform single Support Vector Machine classifiers on the problem of predicting glycosylation sites in terms of a range of standard measures for comparing the performance of classifiers. The resulting methods have been implemented in EnsembleGly, a web server for glycosylation site prediction.Ensembles of Support Vector Machine classifiers offer an accurate and reliable approach to automated identification of putative glycosylation sites in glycoprotein sequences.

Asparagine-linked glycosylation of proteins by the oligosaccharyltransferase (OST) occurs when acceptor sites or sequons (N-x?P-T/S) on nascent polypeptides enter the lumen of the rough endoplasmic reticulum. Metazoan organisms assemble two isoforms of the OST that have different catalytic subunits (STT3A or STT3B) and partially non-overlapping cellular roles. Potential glycosylation sites move past the STT3A complex, which is associated with the translocation channel, at the protein synthesis elongation rate. Here, we investigated whether close spacing between acceptor sites in a nascent protein promotes site skipping by the STT3A complex. Biosynthetic analysis of four human glycoproteins revealed that closely spaced sites are efficiently glycosylated by an STT3B-independent process unless the sequons contain non-optimal sequence features, including extreme close spacing between sequons (e.g. NxTNxT) or the presence of paired NxS sequons (e.g. NxSANxS). Many, but not all, glycosylation sites that are skipped by the STT3A complex can be glycosylated by the STT3B complex. Analysis of a murine glycoprotein database revealed that closely spaced sequons are surprisingly common, and are enriched for paired NxT sites when the gap between sequons is less than three residues.

N-linked glycans are required to maintain appropriate biological functions on proteins. Underglycosylation leads to many diseases in plants and animals; therefore, characterizing the extent of glycosylation on proteins is an important step in understanding, diagnosing, and treating diseases. To determine the glycosylation site occupancy, protein N-glycosidase F (PNGase F) is typically used to detach the glycan from the protein, during which the formerly glycosylated asparagine undergoes deamidation to become an aspartic acid. By comparing the abundance of the resulting peptide containing aspartic acid against the one containing non-glycosylated asparagine, the glycosylation site occupancy can be evaluated. However, this approach can give inaccurate results when spontaneous chemical deamidation of the non-glycosylated asparagine occurs. To overcome this limitation, we developed a new method to measure the glycosylation site occupancy that does not rely on converting glycosylated peptides to their deglycosylated forms. Specifically, the overall protein concentration and the non-glycosylated portion of the protein are quantified simultaneously by using heavy isotope-labeled internal standards coupled with LC-MS analysis, and the extent of site occupancy is accurately determined. The efficacy of the method was demonstrated by quantifying the occupancy of a glycosylation site on bovine fetuin. The developed method is the first work that measures the glycosylation site occupancy without using PNGase F, and it can be done in parallel with glycopeptide analysis because the glycan remains intact throughout the workflow.

HeLa cell line is frequently used in biomedical research, however little is known about N-glycan structures expressed on individual glycoproteins of this complex sample. We characterized site-specific N-glycosylation of HeLa N-glycoproteins using a complex workflow based on high and low energy tandem mass spectrometry experiments and rigorous data evaluation. The analyses revealed high amount of bovine serum contaminants compromising previous results focusing on released glycan analysis. We reliably identified 43 (human) glycoproteins, 69 N-glycosylation sites and 178 glycopeptides following an acetone precipitation based sample enrichment step. HeLa glycoproteins were found to be highly fucosylated and in several cases localization of the fucose (core or antenna) could also be determined based on low energy tandem mass spectra. High-mannose sugars were expressed in high amounts as expected in case of a cancer cell line. Our method enabled the detailed characterization of site-specific N-glycosylation of several glycoproteins expressed in HeLa. Furthermore, we were the first to experimentally prove the existence of 31 glycosylation sites, where previously presence of glycosylation was only predicted based on the existence of the consensus sequon.

Glycosylation is the most common postranslational modification. Defects in this pathway lead to autosomal recessive disorders, called congenital disorders of glycosylation (CDG). Up to date about 50 CDGs have been identified and more are expected to be discovered. This category of metabolic disorders can be divided into four basic groups depending on where the glycosylation process occurs on the molecule. The depicts pathway depicts the group of N-glycosylation. Generally, N-glycosylation processes spread over three cellular compartments - cytosol, endoplasmic reticulum and Golgi apparatus. The associated mortality rate in combination with the limited treatment options for CDG, points out the relevance for further investigations of this pathway. Disorders resulting from an enzyme defect are highlighted in pink. This pathway was inspired by Chapter 30 of the book of Blau (ISBN 3642403360 (978-3642403361)).

Mucins are a family of high molecular weight, heavily glycosylated proteins (glycoconjugates) produced by epithelial tissues in most metazoa. Mucins' key characteristic is their ability to form gels; therefore they are a key component in most gel-like secretions, serving functions from lubrication to cell signalling to forming chemical barriers. To date, there are approximately 20 genes that express mucins. Mature mucins are composed of two distinct regions:
(1) The amino- and carboxy-terminal regions are very lightly glycosylated, but rich in cysteines. The cysteine residues participate in establishing disulfide linkages within and among mucin monomers.
(2) A large central region rich in serine, threonine and proline residues called the variable number of tandem repeat (VNTR) region which can become heavily O-glycosylated with hundreds of O-GalNAc glycans.
N-acetyl-galactosamine (GalNAc) is the first glycan to be attached, forming the simplest mucin O-glycan. After this, several different pathways are possible generating "core" structures. Four core structures are commonly formed, several others are possible but infrequent. O-linked glycans are often capped by the addition of a sialic acid residue, terminating the addition of any more O-glycans (Brockhausen et al, 2009; Tarp and Clausen, 2008).

The global pandemic of severe acute pneumonia syndrome (COVID-19) caused by SARS-CoV-2 urgently calls for prevention and intervention strategies. The densely glycosylated spike (S) protein highly exposed on the viral surface is a determinant for virus binding and invasion into host cells as well as elicitation of a protective host immune response. Herein, we characterized the site-specific N-glycosylation of SARS-CoV-2 S protein using stepped collision energy (SCE) mass spectrometry (MS). Following digestion with two complementary proteases to cover all potential N-glycosylation sequons and integrated N-glycoproteomics analysis, we revealed the N-glycosylation profile of SARS-CoV-2 S proteins at the levels of intact N-glycopeptides and glycosites, along with the glycan composition and site-specific number of glycans. All 22 potential canonical N-glycosites were identified in S protein protomer. Of those, 18 N-glycosites were conserved between SARS-CoV and SARS-CoV-2 S proteins. Nearly all glycosites were preserved among the 753 SARS-CoV-2 genome sequences available in the public influenza database Global Initiative on Sharing All Influenza Data. By comparison, insect cell-expressed SARS-CoV-2 S protein contained 38 N-glycans, which were primarily assigned to the high-mannose type N-glycans, whereas the human cell-produced protein possessed up to 140 N-glycans largely belonging to the complex type. In particular, two N-glycosites located in the structurally exposed receptor-binding domain of S protein exhibited a relatively conserved N-glycan composition in human cells. This N-glycosylation profiling and determination of differences between distinct expression systems could shed light on the infection mechanism and promote development of vaccines and targeted drugs.

1. Enveloped viruses hijack not only host translation processes, but also its glycosylation machinery and to a variable extent cover viral surface proteins with tolerogenic host-like structures. SARS-CoV-2 surface protein S presents as a trimer on the viral surface and is covered by a dense shield of N-linked glycans, and a few O-glycosites have been reported. The biosynthetic nature of O-glycosylation is highly variable and is affected by initiating enzyme repertoires in different tissues and cell types, and therefore warrants a more thorough investigation. Here, we used our well-established O-glycoproteomic workflows to map the precise positions of O-linked glycosylation sites on three different entities of protein S – insect cell or human cell-produced ectodomains, or insect cell-derived receptor binding domain (RBD). In total 25 O-glycosites were identified, with similar patterns in the two ectodomains of different cell origin, and a distinct pattern of the monomeric RBD. Strikingly, 16 out of 25 O-glycosites were located within three amino acids from known N-glycosites. However, O-glycosylation was primarily found on peptides that were unoccupied by N-glycans, and otherwise had low overall occupancy. This suggests possible complimentary functions of O-glycans in immune shielding and negligible effects of O-glycosylation on subunit vaccine design for SARS-CoV-2

This dataset is an outcome of a series of experiments conducted to investigate the significance of glycosylation on the function of the human calcium-sensing receptor (CaSR). The CaSR is a family C G protein-coupled receptor that plays an important role in regulating whole body extracellular calcium (Ca2+o) homeostasis. The exact location of the binding sites for Ca2+o (and other cations) is still unknown, but experimental and in silico analysis has provided evidence that they may be in the extracellular domain and the transmembrane domain. Post translational modifications leads to the covalent attachment of a number of negatively charged sugars, which may provide a negatively charged surface for polyvalent cations to interact with.

There are 11 potential N-linked glycosylation sites in the CaSR’s extracellular domain. A single conservative asparagine to glutatmine mutation was undertaken to disrupt individual N-linked glycosylation consensus sequences. Alanine scanning mutations were also used to assess the significance of potential O-linked glycosylation sites (small clusters of Ser/The residues near the membrane anchor points of the exo-loops in the transmembrane domain). These mutant receptors were then transiently expressed in HEK293 cells. The impact of each mutation on cell expression and function was assessed by the following techniques:

• Western blotting: Hand cast SDS-PAGE gel and Membrane transfer was undertaken using the BioRad mini-protean system, Precast gradient gels (4 – 15%) were TGX precast gels from Biorad. Western blotting films were scanned in .JPG and .PNG format. Quantification of bands was undertaken using GNU Imaging Manipulation Program (GIMP) imaging software.

• Cell surface expression ELISAs: Antibody based enzyme-linked immunosorbent assays (ELISAs) were used. The chromogenic substrate for horseradish peroxidase (HRP) was 3,3’,5,5’-Tetramethylbenzidine (TMB). Absorbance at 450 nM was read using an EnVision Multilabel Reader and exported using .XLS files. Data were represented and analysed using GraphPad Prism 5.

• Microfluorimetry with fura-2 AM (a calcium-sensitive fluorophore): Fluorescent images were taken on a Zeiss Axiovert 200M microscope with a Carl Zeiss monochrome HSm digital camera using a 63x Zeiss long-distance objective. Excitation filters, emission filters and dichroic mirrors were all obtained from Zeiss. The fluorescent light source was provided by a Lambda Sutter DG-4 wavelength switcher. The ratiometric (340/380 nm) region-of-interest data were exported using Stallion software (.SLD files) and subsequently processed using GraphPad Prism 5.0.

The creation of the mutant cells and expression/function experiments were carried out by Dr Sarah Brennan and Karolina Windloch. Data analysis was undertaken by Dr Sarah Brennan. For further information, refer to the associated thesis.

The majority of cell surface and secreted proteins are modified by glycans and this glycosylation process plays an important role in the development of multicellular organisms. These glycan modifications enable communication between cells and the extracellular matrix via interactions with specific glycan-binding lectins and the regulation of receptor-mediated signalling. Glycosylation plays an important role in myogenesis and the development of muscle tissue but our molecular understanding of the precise glycans, catalytic enzymes and lectins involved remain only partially understood. Here, we quantified dynamic remodeling of the membrane-associated proteome during in vitro myogenesis. We observed wide-spread changes in the abundance of several receptors and important enzymes regulating glycosylation. Quantification of released N-linked glycans via glycomic analysis confirmed remodeling of the glycome with a switch in sialic acid linkages, and changes in di-galactosylation and paucimannosylation. Quantitative glycoproteomic analysis with stable isotope labelling, enrichment of glycopeptides and analysis via multiple fragmentation approaches identified precise glycoproteins containing these regulated glycans including integrins and growth factor receptors. Myogenesis was also associated with the regulation of several lectins including the up-regulation of Galectin-1 (LGALS1). CRISPR/Cas9-mediated deletion of Lgals1 inhibited differentiation and myotube formation suggesting an early defect in the myogenic program. We also observed similar changes in N-glycosylation and the up-regulation of LGALS1 during postnatal skeletal muscle development in mice. Finally, treatment of new-born mice with recombinant adeno-associated viruses to express LGALS1 in the musculature resulted in enhanced muscle mass. Our data will be a valuable resource to further understand the role of glycosylation and lectins on myogenesis and may aid in the development of intervention strategies aimed at promoting healthy muscle.

Protein glycosylation is a highly important, yet a poorly understood protein post-translational modification. Thousands of possible glycan structures and compositions create potential for tremendous site heterogeneity and analytical challenge. A lack of suitable analytical methods for large-scale analyses of intact glycopeptides has ultimately limited our abilities to both address the degree of heterogeneity across the glycoproteome and to understand how it contributes biologically to complex systems. Here we show that N-glycoproteome site-specific microheterogeneity can be captured at a global level via glycopeptide profiling with activated ion electron transfer dissociation (AI-ETD), enabling characterization of nearly 2,100 N-glycosites (> 7,500 unique N-glycopeptides) from mouse brain tissue. Moreover, we have used this unprecedented scale of glycoproteomic data to develop several new visualizations that will prove useful for analyzing intact glycopeptides in future studies. Our data reveal that N-glycosylation profiles can differ between subcellular regions and structural domains and that N-glycosite heterogeneity manifests in several different forms, including dramatic differences in glycosites on the same protein.

Understanding the glycosylation of envelope spike (S) protein of SARS-CoV-2 is important in defining the antigenic surface of this key viral target. However, the underlying protein architecture may significantly influence glycan occupancy and processing. There is, therefore, potential for different recombinant fragments of S protein to display divergent glycosylation. Here, we show that the receptor binding domain (RBD), when expressed as a monomer, exhibits O-linked glycosylation which is not recapitulated in the native-like soluble trimeric protein. We unambiguously assign O-linked glycosylation by homogenizing N-linked glycosylation using the enzymatic inhibitor, kifunensine, and then analyzing the resulting structures by electron transfer higher-energy collision dissociation (EThcD) in an Orbitrap Eclipse Tribrid instrument. In the native-like trimer, we observe a single unambiguous O-linked glycan at T323 which displays very low occupancy. In contrast, several sites of O-linked glycosylation can be identified when RBD is expressed as a monomer, with T323 being almost completely occupied. We ascribe this effect to the relaxation of steric restraints arising from quaternary protein architecture. Our analytical approach has also highlighted that fragmentation ions arising from trace levels of truncated N-linked glycans can be misassigned as proximal putative O-linked structures, particularly where a paucity of diagnostic fragments were obtained. Overall, we and show that in matched expression systems quaternary protein architecture limits O-linked glycosylation of the spike protein

The characterization of glycosylation is required for many protein therapeutics. The emergence of antibody and antibody-like molecules with multiple glycan attachment sites has rendered glycan analysis increasingly more complicated. Reliance on site-specific glycopeptide analysis is therefore necessary to fully analyze multi-glycosylated biotherapeutics. Established glycopeptide methodologies have generally utilized a priori knowledge of the glycosylation states of the investigated protein(s), database searching of results generated from data-dependent liquid chromatography-tandem mass spectrometry workflows, and extracted ion quantitation of the individual identified species. However, the inherent complexity of glycosylation makes predicting all glycoforms on all glycosylation sites extremely challenging, if not impossible. That is, only the "knowns" are assessed. Here, we describe an agnostic methodology to qualitatively and quantitatively assess both "known" and "unknown" site-specific glycosylation for biotherapeutics that contain multiple glycosylation sites. The workflow uses data-independent, all ion fragmentation to generate glycan oxonium ions, which are then extracted across the entirety of the chromatographic timeline to produce a glycan-specific "fingerprint" of the glycoprotein sample. We utilized both HexNAc and sialic acid oxonium ion profiles to quickly assess the presence of Fab glycosylation in a therapeutic monoclonal antibody, as well as for high-throughput comparisons of multi-glycosylated protein drugs derived from different clones to a reference product. An automated method was created to rapidly assess oxonium profiles between samples, and to provide a quantitative assessment of similarity.

Reanalysis of submissions PXD005411, PXD005413, PXD005412, PXD005553, PXD005555, PXD005565 and PXD019937 using Glyco-Decipher. Identification results of peptide-spectrum matches supporting Glyco-Decipher manuscript (Glyco-Decipher: glycan database-independent peptide matching enables discovery of new glycans and in-depth characterization of site-specific N-glycosylation). Recently, several elegant bioinformatics tools have been developed to identify glycopeptides from tandem mass spectra for site-specific glycoproteomics studies. These glycan database-dependent tools have substantially improved glycoproteomics analysis but fail to identify glycopeptides with unexpected glycans. We present a platform called Glyco-Decipher to interpret the glycoproteomics data of N-linked glycopeptides. It adopts a glycan database-independent peptide matching scheme that allows the unbiased profiling of glycans and the discovery of new glycans linked with modifications. Reanalysis of several large-scale datasets showed that Glyco-Decipher outperformed the open search method in glycan blind searching and the popular glycan database-dependent software tools in glycopeptide identification. Our glycan database-independent search also revealed that modified glycans are responsible for a large fraction of unassigned glycopeptide spectra in shotgun glycoproteomics.

Glycosylation is an abundant post-translational modification of both intracellular and extracellular proteins [1]. The majority of glycans are classified as N-linked chains, where the carbohydrate moiety is attached to asparagine residues, or O-linked chains, most commonly linked to a serine or threonine. N-linked glycosylation is initiated by the oligosaccharyltransferase complex with only two paralogs of the catalytic subunit, whereas O-glycan initiation is more complex. There are several types of O-linked glycosylation, but among the most diverse is the mucin or GalNAc type (hereafter referred to as O-glycosylation). O-glycosylation is initiated by 20 evolutionarily conserved polypeptide GalNAc-transferases (GalNAc-Ts), which catalyze the first step in the O-glycosylation of proteins by adding GalNAc residues to threonine, serine, and tyrosine amino acids (Fig 1A). Each of the GalNAc-Ts are differentially expressed in various tissues and have both distinct and overlapping peptide substrate specificities [2-12]. Thus, the repertoire of GalNAc-Ts expressed in a given cell determines the subset and O-glycosite pattern of glycosylated proteins [13]. Substantial efforts have been made to characterize and predict the substrate specificities of GalNAc-Ts in vitro, but understanding of the in vivo specificities of the individual GalNAc-Ts or their biological functions is limited [13-15]. This lack of insight prevents an understanding of how site-specific O-linked glycosylation affects diseases, such as metabolic disorders, cardiovascular disease, and various malignancies, that have been associated with GalNAc-Ts through genome-wide association studies and other linkage studies [16-26]. Therefore, it is imperative that we establish how O-glycosylation at specific sites in proteins affects protein function. A major task in achieving this goal is to identify the non-redundant biological functions of site-specific O-glycosylation. We and others recently developed new strategies for identifying specific sites on proteins that undergo O-glycosylation in different cell types and tissues [27-31]. Characterization of the O-glycoproteomic landscape in isolated human cells and multiple human cell lines suggests that more than 80 % of all proteins that traffic through the secretory pathway are O-glycoproteins [28, 30]. Probing the non-redundant contributions of individual GalNAc-Ts in cells with and without specific GalNAc-Ts [32-34] has revealed broad substrate specificities for some of the individual isoforms, whereas others seem to have very restricted substrate specificities [33-35]. Assessing all of the mapped O-glycosylation sites to identify associations between O-glycosites and protein annotations, we recently found that O-glycans are over-represented close to tandem repeat regions, protease cleavage sites, within propeptides, and on a select group of protein domains [28, 30, 36]. Although such general associations between the location of O-glycans and protein functions may direct future investigations, the strategy does not define the function of site-specific glycosylation. Further progress in discovering and defining novel functions of site-specific glycosylation events requires direct quantitative analysis of potential biological responses induced by the loss of distinct GalNAc-T isoforms, and such biological responses are not easily observed in single cell culture systems. Instead, more complex model systems can be used to examine and dissect the molecular mechanisms underlying the important biological functions of site-specific glycosylation. We previously used an organotypic tissue model equipped with genetically engineered cells to decipher the function of elongated O-glycans [29]. In the present study, we use the model combined with quantitative O-glycoproteomics and phosphoproteomics to perform open-ended discovery of the biological functions of site-specific glycosylation governed by GalNAc-Ts (Fig 1B). With this combinatorial strategy, we demonstrate that loss of individual GalNAc-T isoforms has distinct phenotypic consequences through their effect on distinct biological pathways, suggesting specific roles during epithelial formation.

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Protein glycosylation is a complex post-translational modification with crucial cellular functions in all domains of life. Currently, large-scale glycoproteomics approaches rely on glycan database dependent algorithms and are thus unsuitable for discovery-driven analyses of glycoproteomes. Therefore, we devised SugarPy, a glycan database independent Python module, and validated it on the glycoproteome of human breast milk. We further demonstrated its applicability by analyzing glycoproteomes with uncommon glycans stemming from the green algae Chalmydomonas reinhardtii and the archaeon Haloferax volcanii. Finally, SugarPy facilitated the novel characterization of glycoproteins from Cyanidioschyzon merolae.