Plasmid-encoded toxin (Pet) from enteroaggregative Escherichia coli is a member of the autotransporter subfamily termed SPATE (serine protease autotransporters of Enterobacteriaceae). Autotransporters, which are the most common Gram-negative secreted virulence factors, contain three functional domains: an amino terminal leader sequence, a mature protein or passenger domain, and a carboxy-terminal β domain. The leader sequence targets the protein to the periplasmic space and the β domain then forms a β-barrel pore in the outer membrane of the bacterium which allows the passenger domain to enter the external milieu. In some cases the passenger domain is cleaved from the β-barrel at the extracellular surface to release a soluble toxin. This is thought to be a self-contained process that does not require chaperones or ATP for folding and export of the passenger domain. Pet produces cytotoxic effects through cleavage of its target, the actin-binding protein α- fodrin. Pet is secreted into the extracellular environment, but its target lies within the cytosol. To reach its target, Pet moves from the cell surface to the ER where it triggers ER-associated degradation (ERAD) to enter the cytosol. ERAD is a normal cellular process in which improperly folded proteins are exported from the ER to the cytosol for degradation. Other toxins that utilize this pathway are AB toxins such as cholera toxin (CT) and ricin. The A subunits of these toxins are thermally unstable, and this facilitates their ERAD-dependent translocation into the cytosol. Pet, however, is not an AB toxin. We predict that thermal unfolding is not the mechanism Pet employs to exploit ERAD. It was necessary to purify the toxin first in order to study the structural properties and ER export of Pet. Surprisingly, purified Pet eluted as two close peaks by size exclusion chromatography. Both peaks were Pet as demonstrated through immunoblotting. The folding efficiency of autotransporters has not been extensively elucidated, and based on our purification results, we hypothesized that there is inefficiency in the folding of autotransporters, specifically Pet. A toxicity assay showed that Pet peak one did not display cytopathic activity while Pet peak two did. CD and fluorescence spectroscopy measurements also detected structural differences between the two variants of Pet and demonstrated that Pet peak one was an unfolded variant of Pet peak two. Native gel electrophoresis and biophysical measurements indicated that Pet peak one did not exist as a dimer or aggregate. Our results indicate there are two forms of Pet, and thus the folding process of autotransporters appears to be inherently inefficient. Active Pet (peak two) was used for further biophysical measurements and biochemical assays. Circular dichroism and fluorescence spectroscopy showed that the secondary and tertiary structures of Pet are maintained at physiological temperature, 37°C. Thermal unfolding of Pet occurred at temperatures above 50°C. Fluorescence quenching of Pet was also performed and demonstrated that, at 37°C, there are solvent-exposed aromatic amino acids. The slight structural alterations to Pet at physiological temperature as well as the exposed hydrophobic residues could trigger ERAD. In addition, a modeled structure of Pet revealed a hydrophobic loop which is surface-exposed and a likely target for toxin-ERAD interactions. The data suggests that translocation of Pet mediated by ERAD can occur by a mechanism different from certain AB toxins. An open, hydrophobic conformation likely triggers ERAD, but may also contribute to poor folding.
| Identifer | oai:union.ndltd.org:ucf.edu/oai:stars.library.ucf.edu:etd-4712 |
| Date | 01 January 2008 |
| Creators | Scaglione, Patricia |
| Publisher | STARS |
| Source Sets | University of Central Florida |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Electronic Theses and Dissertations |
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