Bacterial interaction with plants and animals, symbiotic or pathogenic, involves the transfer to host cells and tissues of a range of bacterial proteins whose biochemical activities are key to establishing both commensalism and infection. Export of protein molecules across the bacterial membranes takes place via a variety of mechanisms, from simple one-component systems to complex multicomponent pathways, with five types of nonhomologous protein secretion system characterized in pathogenic bacteria so far. In the October 3
Wai et al. dissected the secretion pathway of cytolysin A (ClyA), a pore-forming toxin with hemolytic and cytolytic properties produced by the gram-negative bacterium
Periplasmic ClyA is kept in monomeric form by a disulphide bond, whereas the relevant cysteine residues are reduced in the protein from the vesicles, which oligomerize to form the large complexes suggested to be the active form of pore assemblies. Since the redox status of periplasmic proteins depends on the activity of specific membrane-bound and periplasmic disulphide bond isomerases/oxidases, that ClyA can bypass this thiol-redox pathway suggests that bacteria can sort proteins when forming vesicles, excluding certain periplasmic proteins such as those catalyzing the formation of disulphide bonds.
"The results define a vesicle-mediated transport mechanism in bacteria that is responsible for the activation and delivery of pathogenic effector proteins," conclude the authors.
"The discovery that bacterial vesicles are not simply a phenomenon of in vitro growth should result in studies that will increase our knowledge of the interactions of bacteria with eukaryotic organisms," highlight Samuel Miller and colleagues at the University of Washington in an accompanying preview article.