Roles of periplasmic chaperone proteins in the biogenesis of serine protease autotransporters of Enterobacteriaceae

Host-Toxin Interactions Involving EspC and Pet, Two Serine Protease Autotransporters of the Enterobacteriaceae

EspC and Pet are toxins secreted by the diarrheagenic enteropathogenic and enteroaggregative Escherichia coli pathotypes, respectively. Both toxins have a molecular mass around 110 kDa and belong to the same protein family called Serine Protease Autotransporters of the Enterobacteriaceae (SPATE). Furthermore, both toxins act within the cytosol of intoxicated epithelial cells to disrupt the architecture of the actin cytoskeleton. This cytopathic and enterotoxic effect results from toxin cleavage of the actin-binding protein fodrin, although the two toxins recognize different cleavage sites on fodrin. EspC and Pet also have dramatically different mechanisms of entering the target cell which appear dependent upon the E. coli pathotype. In this review, we compare/contrast EspC and Pet in regards to their mode of delivery into the target cell, their effects on fodrin and the actin cytoskeleton, and their possible effects on the physiology of the intestinal epithelial cell.

Interaction of FkpA, a peptidyl-prolyl cis/trans isomerase with EspP autotransporter protein

The serine protease autotransporters of Enterobacteriaceae (SPATEs) represent a large class of proteases with contributions to virulence. They are synthesized with a C-terminal domain that forms a β-barrel pore in the outer membrane implicated in translocation of the N-terminal 'passenger' domain across the outer membrane. The most recent model for autotransporter secretion comprises entry to the periplasm via the Sec apparatus, followed by the insertion of the C-terminus into the outer membrane as a β-barrel protein and accompanied by translocation of the passenger domain to the bacterial cell surface, all of this with the assistance of the Bam complex insertase/foldase and periplasmic chaperone proteins. We have recently observed direct involvement of periplasmic chaperones in the biogenesis of EspP, a prototypical autotransporter protein produced by Escherichia coli O157:H7. Using molecular and biophysical approaches we demonstrated for the first time, direct protein-protein interactions between the periplasmic SurA and DegP chaperones and either the EspP-β or EspP passenger domains. Such chaperone interactions took place on conserved aromatic residues on the SPATE family. In this report, we now demonstrate direct binding of the periplasmic chaperone FkpA to the EspP passanger domain in Surface Plasmon Resonance experiments with relatively high affinity. We also provide evidence of interaction between the SurA and Skp chaperones with the Bam. These findings in conjunction with newly published data support the role of chaperones in preventing misfolding of AT passenger domains before translocation throughout the Bam complex.

A host-specific factor is necessary for efficient folding of the autotransporter plasmid-encoded toxin

Autotransporters are the most common virulence factors secreted from Gram-negative pathogens. Until recently, autotransporter folding and outer membrane translocation were thought to be self-mediated events that did not require accessory factors. Here, we report that two variants of the autotransporter plasmid-encoded toxin are secreted by a lab strain of Escherichia coli. Biophysical analysis and cell-based toxicity assays demonstrated that only one of the two variants was in a folded, active conformation. The misfolded variant was not produced by a pathogenic strain of enteroaggregative E. coli and did not result from protein overproduction in the lab strain of E. coli. Our data suggest a host-specific factor is required for efficient folding of plasmid-encoded toxin.