Intracellular expression of the plasmid-encoded toxin from enteroaggregative Escherichia coli.

The plasmid-encoded toxin (Pet) from enteroaggregative Escherichia coli is a serine protease autotransporter that acts as an enterotoxin and cytotoxin. When applied to epithelial cells in culture, purified toxin induces cell elongation and rounding, followed by exfoliation of cells from the substratum. These effects are accompanied by loss of actin stress fibers and electrophysiologic changes. Although it has been hypothesized that Pet has an intracellular site of action, evidence for this is indirect. In addition, Pet has recently been shown to cleave spectrin in vitro and in vivo. If Pet requires intracellular localization to execute its toxic effects, then intracellular expression of the protein could induce cytopathic effects similar to those observed when the toxin is applied to the cell surface. To test this hypothesis, we expressed the mature Pet toxin (comprising only the passenger domain of the Pet precursor) in the cytoplasm of HEp-2 cells by using mammalian expression vectors. Separately, we expressed the Pet passenger domain mutated at the catalytic serine (PetS260A), a construct that has been reported to lack toxic effects. Forty-eight hours after transient transfection of pcDNA3.1-pet in HEp-2 cells, we observed cell elongation and other morphological changes similar to those induced by applied toxin. Cells transfected with pcDNA3.1 vector alone appeared normal, while cells expressing the PetS260A mutant displayed similar (though less pronounced) changes compared with those in cells expressing pcDNA3.1-pet. Notably, intracellular expression of Pet was accompanied by condensation of the spectrin cytoskeleton. These studies corroborate an intracellular site of action for the Pet toxin, further implicate a role for spectrin in Pet intoxication, and provide a powerful tool for Pet structure and function analyses.

Structure-function analysis of the enteroaggregative Escherichia coli plasmid-encoded toxin autotransporter using scanning linker mutagenesis.

The plasmid-encoded toxin (Pet) from enteroaggregative Escherichia coli is a cytopathic serine protease, which is prototypical of a large family of bacterial autotransporter toxins. To further elucidate the structure-function relationships of this toxin, we employed transposon-based scanning linker mutagenesis. A subset of insertions throughout the Pet mature toxin (passenger) domain reduced secretion to the extracellular space. Many of these mutants were undetectable, but secretion of a subset of mutants with insertions in the N-terminal half of the toxin could be restored to wild type secretion levels if cultured in the presence of 0.1% Triton X-100. Secretion of two mutants with insertions at the extreme C terminus was partially restored when co-expressed with a minimal clone of EspP, a related autotransporter protein. Several well secreted mutants with insertions in the N-terminal third of the molecule reduced protease activity over 20-fold, suggesting that the protease domain is located within this N-terminal region of Pet. We have also identified two insertional mutants in the middle of the passenger domain that were proteolytic but no longer cytopathic; these mutants displayed decreased binding and internalization upon incubation with HEp-2 cells. Our data suggest the existence of separate functional domains mediating Pet proteolysis, secretion, and cell interaction.

The serine protease motif of EspC from enteropathogenic Escherichia coli produces epithelial damage by a mechanism different from that of Pet toxin from enteroaggregative E. coli.

EspC (Escherichia coli secreted protein C) of enteropathogenic E. coli (EPEC) shows the three classical domains of the autotransporter proteins and has a conserved serine protease motif belonging to the SPATE (serine protease autotransporters of Enterobacteriaceae) subfamily. EspC and its homolog Pet in enteroaggregative E. coli (EAEC) bear the same sequence within the serine protease motif, and both proteins produce enterotoxic effects, suggesting that like Pet, EspC could be internalized to reach and cleave the calmodulin-binding domain of fodrin, causing actin cytoskeleton disruption. Even though both proteins cause cytoskeleton damage by virtue of their serine protease motifs, the following evidence supports the hypothesis that the mechanisms are different. (i) To obtain similar cytotoxic and cytoskeletal effects, a threefold-higher EspC concentration and a twofold-higher exposure time are needed. (ii) EspC internalization into epithelial cells takes more time (6 h) than Pet internalization (30 min), and the distributions of the two proteins inside the cells are also different. (iii) Both proteins have affinity for fodrin and cleave it, but the cleavage sites are different; EspC produces two cleavages, while Pet produces just one. (iv) EspC does not cause fodrin redistribution within epithelial cells. (v) An EspC serine protease motif mutant, but not a Pet serine protease mutant, competes with EspC by blocking cytoskeletal damage. All these data suggest that the protein conformational structure is very important for the activity of the catalytic site, influencing its interaction with the target protein and its internalization. The differences between these proteins may explain the reduced ability of EspC to cause cytopathic effects. However, these differences may confer a specialized role on EspC in the pathogenesis of EPEC, which is different from that of Pet in EAEC pathogenesis.