Ulla-Maja Bailey

Analysis of Congenital Disorder of Glycosylation-Id in a Yeast Model System Shows Diverse Site-Specific Under-glycosylation of Glycoproteins

Asparagine-linked glycosylation is a common post-translational modification of proteins in eukaryotes. Mutations in the human ALG3 gene cause changed levels and altered glycan structures on mature glycoproteins and are the cause of a severe congenital disorder of glycosylation (CDG-Id). Diverse glycoproteins are also under-glycosylated in Saccharomyces cerevisae alg3 mutants. Here we analyzed site-specific glycosylation occupancy in this yeast model system using peptide-N-glycosidase F to label glycosylation sites with an asparagine-aspartate conversion that creates a new endoproteinase AspN cleavage site, followed by proteolytic digestion, and detection of peptides and glycopeptides by LC-ESI-MS/MS. We used this analytical method to identify and measure site-specific glycosylation occupancy in alg3 mutant and wild type yeast strains. We found decreased site-specific N-glycosylation occupancy in the alg3 knockout strain preferentially at Asn-Xaa-Ser sequences located in secondary structural elements, features previously associated with poor glycosylation efficiency. Furthermore, we identified 26 previously experimentally unverified glycosylation sites. Our results provide insights into the underlying mechanisms of disease in CDG-Id, and our methodology will be useful in site-specific glycosylation analysis in many model systems and clinical applications.

Automated measurement of site-specific N-glycosylation occupancy with SWATH-MS

Asparagine‐linked glycosylation is a common post‐translational modification of proteins catalyzed by oligosaccharyltransferase that is important in regulating many aspects of protein function. Analysis of protein glycosylation, including glycoproteomic measurement of the site‐specific extent of glycosylation, remains challenging. Here, we developed methods combining enzymatic deglycosylation and protease digestion with SWATH‐MS to enable automated measurement of site‐specific occupancy at many glycosylation sites. Deglycosylation with peptide‐endoglycosidase H, leaving a remnant N‐acetylglucosamine on asparagines previously carrying high‐mannose glycans, followed by trypsin digestion allowed robust automated measurement of occupancy at many sites. Combining deglycosylation with the more general peptide‐N‐glycosidase F enzyme with AspN protease digest allowed robust automated differentiation of nonglycosylated and deglycosylated forms of a given glycosylation site. Ratiometric analysis of deglycosylated peptides and the total intensities of all peptides from the corresponding proteins allowed relative quantification of site‐specific glycosylation occupancy between yeast strains with various isoforms of oligosaccharyltransferase. This approach also allowed robust measurement of glycosylation sites in human salivary glycoproteins. This method for automated relative quantification of site‐specific glycosylation occupancy will be a useful tool for research with model systems and clinical samples.

Identification of salivary N-glycoproteins and measurement of glycosylation site occupancy by boronate glycoprotein enrichment and liquid chromatography/electrospray ionization tandem mass spectrometry.

RATIONALE Diseases including cancer and congenital disorders of glycosylation have been associated with changes in the site-specific extent of protein glycosylation. Saliva can be non-invasively sampled and is rich in glycoproteins, giving it the potential to be a useful biofluid for the discovery and detection of disease biomarkers associated with changes in glycosylation. METHODS Saliva was collected from healthy individuals and glycoproteins were enriched using phenylboronic acid based glycoprotein enrichment resin. Proteins were deglycosylated with peptide-N-glycosidase F and digested with AspN or trypsin. Desalted peptides and deglycosylated peptides were separated by reversed-phase liquid chromatography and detected with on-line electrospray ionization quadrupole-time-of-flight mass spectrometry using a 5600 TripleTof instrument. Site-specific glycosylation occupancy was semi-quantitatively determined from the abundance of deglycosylated and nonglycosylated versions of each given peptide. RESULTS Glycoprotein enrichment identified 67 independent glycosylation sites from 24 unique proteins, a 3.9-fold increase in the number of glycosylation sites identified. Enrichment of glycoproteins rather than glycopeptides allowed detection of both deglycosylated and nonglycosylated versions of each peptide, and thereby robust measurement of site-specific occupancy at 21 asparagines. Healthy individuals showed limited biological variability in occupancy, with partially modified sites having characteristics consistent with inefficient glycosylation by oligosaccharyltransferase. Inclusion of negative controls without enzymatic deglycosylation controlled for spontaneous chemical deamidation, and identified asparagines previously incorrectly annotated as glycosylated. CONCLUSIONS We developed a sample preparation and mass spectrometry detection strategy for rapid and efficient measurement of site-specific glycosylation occupancy on diverse salivary glycoproteins suitable for biomarker discovery and detection of changes in glycosylation occupancy in human disease.

Polypeptide binding specificities of Saccharomyces cerevisiae oligosaccharyltransferase accessory proteins Ost3p and Ost6p

Asparagine‐linked glycosylation is a common and vital co‐ and post‐translocational modification of diverse secretory and membrane proteins in eukaryotes that is catalyzed by the multiprotein complex oligosaccharyltransferase (OTase). Two isoforms of OTase are present in Saccharomyces cerevisiae, defined by the presence of either of the homologous proteins Ost3p or Ost6p, which possess different protein substrate specificities at the level of individual glycosylation sites. Here we present in vitro characterization of the polypeptide binding activity of these two subunits of the yeast enzyme, and show that the peptide‐binding grooves in these proteins can transiently bind stretches of polypeptide with amino acid characteristics complementary to the characteristics of the grooves. We show that Ost6p, which has a peptide‐binding groove with a strongly hydrophobic base lined by neutral and basic residues, binds peptides enriched in hydrophobic and acidic amino acids. Further, by introducing basic residues in place of the wild type neutral residues lining the peptide‐binding groove of Ost3p, we engineer binding of a hydrophobic and acidic peptide. Our data supports a model of Ost3/6p function in which they transiently bind stretches of nascent polypeptide substrate to inhibit protein folding, thereby increasing glycosylation efficiency at nearby asparagine residues.

Analysis of the extreme diversity of salivary alpha-amylase isoforms generated by physiological proteolysis using liquid chromatography–tandem mass spectrometry

Saliva is a crucial biofluid for oral health and is also of increasing importance as a non-invasive source of disease biomarkers. Salivary alpha-amylase is an abundant protein in saliva, and changes in amylase expression have been previously associated with a variety of diseases and conditions. Salivary alpha-amylase is subject to a high diversity of post-translational modifications, including physiological proteolysis in the oral cavity. Here we developed methodology for rapid sample preparation and non-targeted LC-ESI-MS/MS analysis of saliva from healthy subjects and observed an extreme diversity of alpha-amylase proteolytic isoforms. Our results emphasize the importance of consideration of post-translational events such as proteolysis in proteomic studies, biomarker discovery and validation, particularly in saliva.

Sequence-based protein stabilization in the absence of glycosylation.

Asparagine-linked N-glycosylation is a common modification of proteins that promotes productive protein folding and increases protein stability. Although N-glycosylation is important for glycoprotein folding, the precise sites of glycosylation are often not conserved between protein homologues. Here we show that, in Saccharomyces cerevisiae, proteins upregulated during sporulation under nutrient deprivation have few N-glycosylation sequons and in their place tend to contain clusters of like-charged amino-acid residues. Incorporation of such sequences complements loss of in vivo protein function in the absence of glycosylation. Targeted point mutation to create such sequence stretches at glycosylation sequons in model glycoproteins increases in vitro protein stability and activity. A dependence on glycosylation for protein stability or activity can therefore be rescued with a small number of local point mutations, providing evolutionary flexibility in the precise location of N-glycans, allowing protein expression under nutrient-limiting conditions, and improving recombinant protein production.

Mixed disulfide formation in vitro between a glycoprotein substrate and yeast oligosaccharyltransferase subunits Ost3p and Ost6p

Oligosaccharyltransferase (OTase) glycosylates selected asparagine residues in secreted and membrane proteins in eukaryotes, and asparagine (N)-glycosylation affects the folding, stability and function of diverse glycoproteins. The range of acceptor protein substrates that are efficiently glycosylated depends on the action of several accessory subunits of OTase, including in yeast the homologous proteins Ost3p and Ost6p. A model of Ost3p and Ost6p function has been proposed in which their thioredoxin-like active site cysteines form transient mixed disulfide bonds with cysteines in substrate proteins to enhance the glycosylation of nearby asparagine residues. We tested aspects of this model with a series of in vitro assays. We developed a whole protein mixed disulfide interaction assay that showed that Ost6p could form mixed disulfide bonds with selected cysteines in pre-reduced yeast Gas1p, a model glycoprotein substrate of Ost3p and Ost6p. A complementary peptide affinity chromatography assay for mixed disulfide bond formation showed that Ost3p could also form mixed disulfide bonds with cysteines in selected reduced tryptic peptides from Gas1p. Together, these assays showed that the thioredoxin-like active sites of Ost3p and Ost6p could form transient mixed disulfide bonds with cysteines in a model substrate glycoprotein, consistent with the function of Ost3p and Ost6p in modulating N-glycosylation substrate selection by OTase in vivo.

Oligosaccharyltransferase subunits bind polypeptide substrate to locally enhance N-glycosylation.

Oligosaccharyltransferase is a multiprotein complex that catalyzes asparagine-linked glycosylation of diverse proteins. Using yeast genetics and glycoproteomics, we found that transient interactions between nascent polypeptide and Ost3p/Ost6p, homologous subunits of oligosaccharyltransferase, were able to modulate glycosylation efficiency in a site-specific manner in vivo. These interactions were driven by hydrophobic and electrostatic complementarity between amino acids in the peptide-binding groove of Ost3p/Ost6p and the sequestered stretch of substrate polypeptide. Based on this dependence, we used in vivo scanning mutagenesis and in vitro biochemistry to map the precise interactions that affect site-specific glycosylation efficiency. We conclude that transient binding of substrate polypeptide by Ost3p/Ost6p increases glycosylation efficiency at asparagines proximal and C-terminal to sequestered sequences. We detail a novel mode of interaction between translocating nascent polypeptide and oligosaccharyltransferase in which binding to Ost3p/Ost6p segregates a short flexible loop of glycosylation-competent polypeptide substrate that is delivered to the oligosaccharyltransferase active site for efficient modification.

Deglycosylation systematically improves N-glycoprotein identification in liquid chromatography-tandem mass spectrometry proteomics for analysis of cell wall stress responses in Saccharomyces cerevisiae lacking Alg3p.

Post-translational modification of proteins with glycosylation is of key importance in many biological systems in eukaryotes, influencing fundamental biological processes and regulating protein function. Changes in glycosylation are therefore of interest in understanding these processes and are also useful as clinical biomarkers of disease. The presence of glycosylation can also inhibit protease digestion and lower the quality and confidence of protein identification by mass spectrometry. While deglycosylation can improve the efficiency of subsequent protease digest and increase protein coverage, this step is often excluded from proteomic workflows. Here, we performed a systematic analysis that showed that deglycosylation with peptide-N-glycosidase F (PNGase F) prior to protease digestion with AspN or trypsin improved the quality of identification of the yeast cell wall proteome. The improvement in the confidence of identification of glycoproteins following PNGase F deglycosylation correlated with a higher density of glycosylation sites. Optimal identification across the proteome was achieved with PNGase F deglycosylation and complementary proteolysis with either AspN or trypsin. We used this combination of deglycosylation and complementary protease digest to identify changes in the yeast cell wall proteome caused by lack of the Alg3p protein, a key component of the biosynthetic pathway of protein N-glycosylation. The cell wall of yeast lacking Alg3p showed specifically increased levels of Cis3p, a protein important for cell wall integrity. Our results showed that deglycosylation prior to protease digestion improved the quality of proteomic analyses even if protein glycosylation is not of direct relevance to the study at hand.

Selection against glycosylation sites in potential target proteins of the general HMWC N-glycosyltransferase in Haemophilus influenzae

The HMWABC system of non-typeable Haemophilus influenzae (NTHi) encodes the HMWA adhesin glycoprotein, which is glycosylated by the HMWC glycosyltransferase. HMWC is a cytoplasmic N-glycosyltransferase, homologues of which are widespread in the Pasteurellaceae. We developed an assay for nonbiased detection of glycoproteins in NTHi based on metabolic engineering of the Leloir pathway and growth in media containing radiolabelled monosaccharides. The only glycoprotein identified in NTHi by this assay was HMWA. However, glycoproteomic analyses ex vivo in Escherichia coli showed that HMWC of NTHi was a general glycosyltransferase capable of glycosylating selected asparagines in proteins other than its HMWA substrate, including Asn78 in E. coli 30S ribosomal protein S5. The equivalent residue in S5 homologues in H. influenzae or other sequenced Pasteurellaceae genomes is not asparagine, and these organisms also showed significantly fewer than expected potential sites of glycosylation in general. Expression of active HMWC in E. coli resulted in growth inhibition compared with expression of inactive enzyme, consistent with glycosylation by HMWC detrimentally affecting the function of some E. coli proteins. Together, this supports the presence of a selective pressure in the Pasteurellaceae against glycosylation sites that would be modified by the general N-glycosyltransferase activity of HMWC.