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https://hdl.handle.net/2440/60005
Type: | Thesis |
Title: | Characterisation of a novel subtilase Cytotoxin from Shiga Toxigenic Escherichia Coli. |
Author: | Chong, Damien Christopher Chen Sau |
Issue Date: | 2009 |
School/Discipline: | School of Molecular and Biomedical Science |
Abstract: | Subtilase cytotoxin (SubAB) is the prototype of a novel class of AB₅ cytotoxins produced by Shiga-toxigenic Escherichia coli (STEC). The A subunit (SubA) is a serine protease that cleaves the ER chaperone BiP causing cell death by a previouslyundetermined mechanism. The B subunits of AB₅toxins typically recognise host cell glycan receptors and direct the subcellular transport of the A subunit. Although the function of SubA and its intracellular substrate have been elucidated, the B subunit (SubB) is relatively uncharacterised. The subcellular trafficking pathway of SubAB was initially examined. SubAB conjugated to Oregon Green 488 (SubAB-OG) was internalised by Vero cells by 5 min, and co-localised with its ER target BiP within 30 min. When Vero cells were incubated with SubAB-OG and either Alexa Fluor 594-conjugated Cholera toxin B subunit (CtxBAF594) or Texas Red-conjugated Shiga toxin B subunit (StxB-TR), individual cells exhibited differential toxin uptake. This was shown to be cell cycle-dependent, in which, SubAB-OG was preferentially internalised by cells migrating through G1 and early S phases. In contrast, CtxB-AF594 was taken up by cells in S through M phases and by a majority of cells in G1, while StxB-TR endocytosis occurred in cells traversing G1. Fluorescent SubAB co-localised with the clathrin marker transferrin, but not with Caveolin-1 (a marker for cholesterol-associated caveolae) and was subsequently trafficked via a retrograde pathway to the TGN, Golgi and ER. The clathrin inhibitor phenylarsine oxide prevented SubAB entry and BiP cleavage in SubAB-treated Vero, HeLa and N2A cells, while cholesterol depletion did not, demonstrating that, unlike either Stx or Ctx, SubAB internalisation is exclusively clathrin-dependent. Identification of the SubB receptor was initially approached using toxin overlay assays in which Vero cell glycolipid extracts were separated by thin-layer chromatography and overlaid with SubAB. SubAB exhibited a high affinity for particular acidic species in the ganglioside fraction. However, none co-migrated with commercial glycolipid standards. SubAB-OG also exhibited an affinity for the oligosaccharide structures of chimeric LPS from GM₂ and GM₃ bacterial receptor mimic constructs in an LPS toxin overlay assay. Glycan array analysis revealed that SubB possessed a unique affinity for carbohydrate receptors with a terminal Neu5Gcα(2→3)Galβ disaccharide. Monovalent receptor analogues with distal Neu5Gc or Neu5Gcα(2→3)Galβ and highly-sialylated α₁-AGP did not prevent endocytosis of SubAB-OG, BiP cleavage or cytotoxicity in Vero cells. This indicated that SubAB has a greater affinity for the host cell receptors than the receptor analogues and may engage multiple receptors displayed on a lipid bilayer. In addition to mediating toxin binding and subcellular trafficking, CtxB and StxB can also potentiate the immune response to co-administered antigen. Accordingly, the systemic immunomodulatory properties of SubB administered by the i.p. route were assessed in mice. Using SubAA₂₇₂ as a bystander antigen, SubB significantly increased mouse anti-SubAA₂₇₂ titres to levels that were comparable to those obtained using Alum adjuvant. However, when admixed with structurally-unrelated OVA, SubB did not significantly affect anti-OVA titres whereas Alum and CtxB did. This indicated that SubB may function as a systemic carrier protein (rather than an adjuvant) for particular antigens. |
Advisor: | Paton, Adrienne Webster Paton, James Cleland |
Dissertation Note: | Thesis (Ph.D.) - University of Adelaide, School of Molecular and Biomedical Science, 2009 |
Keywords: | AB5; Adjuvant; Carbohydrate; Cell cycle; Immunomodulation; Receptor; Retrograde; Shiga toxigenic Escherichia coli; Subtilase cytotoxin; Trafficking |
Provenance: | Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text. |
Appears in Collections: | Research Theses |
Files in This Item:
File | Description | Size | Format | |
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01front.pdf | 205.97 kB | Adobe PDF | View/Open | |
02chapters1-2.pdf | 4.02 MB | Adobe PDF | View/Open | |
03chapters3-ref.pdf | 4.05 MB | Adobe PDF | View/Open |
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