Assembly and function of type III secretory systems

Type III secretion systems allow Yersinia spp., Salmonella spp.. Shigella s pp., Bordetella spp., and Pseudomonas aeruginosa and enteropathogenic Esche richia coli adhering at the surface of a eukaryotic cell to inject bacteria l proteins across the two bacterial membranes and the eukaryotic cell membr ane to destroy or subvert the target cell. These systems consist of a secre tion apparatus, made of similar to 25 proteins, and an array of proteins re leased by this apparatus. Some of these released proteins are "effectors," which are delivered into the cytosol of the target cell, whereas the others are "translocators," which help the effecters to cross the membrane of the eukaryotic cell. Most of the effecters act on the cytoskeleton or on intra cellular-signaling cascades. A protein injected by the enteropathogenic E. coli serves as a membrane receptor for the docking of the bacterium itself at the surface of the cell. Type III secretion systems also occur in plant pathogens where they are involved both in causing disease in susceptible ho sts and in eliciting the so-called hypersensitive response in resistant or nonhost plants. They consist of 15-20 Hrp proteins building a secretion app aratus and two groups of effecters: harpins and avirulence proteins. Harpin s are presumably secreted in the extracellular compartment, whereas avirule nce proteins are thought to be targeted into plant cells. Although a cohere nt picture is clearly emerging, basic questions remain to be answered. In p articular, little is known about how the type III apparatus fits together t o deliver proteins in animal cells. It is even more mysterious for plant ce lls where a thick wall has to be crossed. In spite of these haunting questi ons, type TIT secretion appears as a fascinating trans-kingdom communicatio n device.