Virulence of the gastric pathogen
Helicobacter pylori (Hp) is directly linked to the pathogen''s ability to glycosylate proteins; for example,
Hp flagellin proteins are heavily glycosylated with the unusual nine-carbon sugar pseudaminic acid, and this modification is absolutely essential for
Hp to synthesize functional flagella and colonize the host''s stomach. Although
Hp''s glycans are linked to pathogenesis,
Hp''s glycome remains poorly understood; only the two flagellin glycoproteins have been firmly characterized in
Hp. Evidence from our laboratory suggests that
Hp synthesizes a large number of as-yet unidentified glycoproteins. Here we set out to discover
Hp''s glycoproteins by coupling glycan metabolic labeling with mass spectrometry analysis. An assessment of the subcellular distribution of azide-labeled proteins by Western blot analysis indicated that glycoproteins are present throughout
Hp and may therefore serve diverse functions. To identify these species,
Hp''s azide-labeled glycoproteins were tagged via Staudinger ligation, enriched by tandem affinity chromatography, and analyzed by multidimensional protein identification technology. Direct comparison of enriched azide-labeled glycoproteins with a mock-enriched control by both SDS-PAGE and mass spectrometry-based analyses confirmed the selective enrichment of azide-labeled glycoproteins. We identified 125 candidate glycoproteins with diverse biological functions, including those linked with pathogenesis. Mass spectrometry analyses of enriched azide-labeled glycoproteins before and after cleavage of O-linked glycans revealed the presence of Staudinger ligation-glycan adducts in samples only after beta-elimination, confirming the synthesis of O-linked glycoproteins in
Hp. Finally, the secreted colonization factors urease alpha and urease beta were biochemically validated as glycosylated proteins via Western blot analysis as well as by mass spectrometry analysis of cleaved glycan products. These data set the stage for the development of glycosylation-based therapeutic strategies, such as new vaccines based on natively glycosylated
Hp proteins, to eradicate
Hp infection. Broadly, this report validates metabolic labeling as an effective and efficient approach for the identification of bacterial glycoproteins.
Helicobacter pylori (
Hp)
1 infection poses a significant health risk to humans worldwide. The Gram-negative, pathogenic bacterium
Hp colonizes the gastric tract of more than 50% of humans (
1). Approximately 15% of infected individuals develop duodenal ulcers and 1% of infected individuals develop gastric cancer (
2). Current treatment to clear infection requires “triple therapy” (
3), a combination of multiple antibiotics that is often associated with negative side effects (
4). Because of poor patient compliance and the evolution of antibiotic resistance, existing antibiotics are no longer effective at eradicating
Hp infection (
4). New treatment methods are needed to eliminate
Hp from the human gastric tract.Recent work has focused on gaining insights into the pathogenesis of
Hp to aid the development of new treatments. The most recent findings in this area have conclusively revealed that glycosylation of proteins in
Hp is required for pathogenesis.
Hp use complex flagella, comprised of flagellin proteins, to navigate the host''s gastric mucosa (
5,
6). The flagellin proteins are heavily glycosylated with the unusual nine-carbon sugar pseudaminic acid, found exclusively in mucosal-associated pathogens (
Hp (
7),
Campylobacter jejuni (
8) and
Pseudomonas aeruginosa (
9)). This modification is absolutely essential for the formation of functional flagella on
Hp (
7,
10). Deletion of any one of the enzymes in the pseudaminic acid biosynthetic pathway results in
Hp that lack flagella, are nonmotile, and are unable to colonize the host''s stomach (
7). Although pseudaminic acid is critical for
Hp virulence, it is absent from humans (
11,
12). Therefore, insights into
Hp''s pathogenesis have revealed that
Hp''s glycan pseudaminic acid is a bona fide target of therapeutic intervention. This is one of a number of examples linking protein glycosylation to virulence in medically significant bacterial pathogens (
13,
14).Despite these findings,
Hp''s glycome remains poorly understood overall. Only the two flagellin glycoproteins have been firmly characterized in
Hp (
7) to date. Nine other candidate glycoproteins have been identified in
Hp, but their glycosylation status has not been biochemically confirmed (
15). The relative paucity of information regarding
Hp''s glycoproteins is due in part to the previously held belief that protein glycosylation could not occur in bacteria (
13,
16,
17). However, even after Szymanski (
18,
19), Koomey (
20), Guerry (
21), Logan (
7), Comstock and others (
13,
16,
17) disproved this belief by firmly establishing the synthesis of glycoproteins in bacteria, the study of bacterial glycoproteins has presented unique challenges for analytical study (
14,
22). For example, the unusual structures of bacterial glycans, which often contain amino- and deoxy-carbohydrates exclusively found in bacteria (
12,
23–
25), hampers their identification using existing tools. Though methods such as the use of glycan-binding reagents (
20,
24,
26,
27) and periodic acid/hydrazide glycan labeling (
15) have successfully detected glycoproteins in a range of bacteria, they present limitations. Glycan binding-based methods are often limited because of the unavailability of lectins or antibodies with binding specificity for glycosylated proteins in the bacteria of interest (
14,
22). Periodic acid/hydrazide-based labeling is plagued by a lack of specificity for glycosylated proteins (
15). Thus, an efficient and robust approach to discover
Hp''s glycoproteins is needed.In previous work, we established that the chemical technique known as metabolic oligosaccharide engineering (MOE), which was developed by Bertozzi (
28,
29), Reutter (
30), and others for the study of mammalian glycoproteins, is a powerful approach to label and detect
Hp''s glycoproteins (
31). Briefly,
Hp metabolically processes the unnatural, azide-containing sugar peracetylated
N-azidoacetylglucosamine (Ac
4GlcNAz) (
32), an analog of the common metabolic precursor
N-acetylglucosamine (GlcNAc), into cellular glycoproteins (). Elaboration of azide-labeled glycoproteins via Staudinger ligation (
33) with a phosphine probe conjugated to a FLAG peptide (Phos-FLAG) (
34) followed by visualization with an anti-FLAG antibody () revealed a glycoprotein fingerprint containing a large number of as-yet unidentified
Hp glycoproteins that merit further investigation (
31).
Open in a separate windowMetabolic oligosaccharide engineering facilitates labeling and detection of
Hp''s glycoproteins. Supplementation of
Hp with Ac
4GlcNAz leads to metabolic labeling of
Hp''s N-linked and O-linked glycoproteins with azides. Azide-modified glycoproteins covalently labeled with Phos-FLAG can be detected via Western blot analysis with anti-FLAG antibody to yield
Hp''s glycoprotein fingerprint, which contains a large number of as-yet unidentified glycoproteins.Here we describe a glycoproteomic identification strategy for the selective detection, isolation, and discovery of
Hp''s glycoproteins. In particular, we demonstrate that glycan metabolic labeling coupled with mass spectrometry analysis is an efficient and robust chemical approach to identify novel glycoproteins in
Hp. This work characterizes glycosylated virulence factors in
Hp, thus opening the door to new vaccination and antibiotic therapies to eradicate
Hp infection. Broadly, this work validates metabolic oligosaccharide engineering as a complementary method to discover bacterial glycoproteins.
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