Secretion of virulence factors in Gram-negative bacteria involves transportation of the protein across two membranes to reach the cell exterior. There have been four secretion systems described in animal enteropathogens, such as Salmonella and Yersinia, with further sequence similarities in plant pathogens like Ralstonia and Erwinia [<cite idref="PUB00003585"/>].<p>The type III secretion system is of great interest, as it is used to transport virulence factors from the pathogen directly into the host cell and is only triggered when the bacterium comes into close contact with the host. The protein subunits of the system are very similar to those of bacterial flagellar biosynthesis. However, while the latter forms a ring structure to allow secretion of flagellin and is an integral part of the flagellum itself [<cite idref="PUB00003585"/>], type III subunits in the outer membrane translocate secreted proteins through a channel-like structure.</p> <p>Exotoxins secreted by the type III system do not possess a secretion signal, and are considered unique for this reason [<cite idref="PUB00003585"/>]. Yersinia secrete a Rho GTPase-activating protein, YopE [<cite idref="PUB00006631"/>, <cite idref="PUB00006632"/>], that disrupts the host cell actin cytoskeleton. YopE is regulated by another bacterial gene, SycE [<cite idref="PUB00006679"/>], that enables the exotoxin to remain soluble in the bacterial cytoplasm. A similar protein, exoenzyme S from <taxon tax_id="287">Pseudomonas aeruginosa</taxon>, has both ADP-ribosylation and GTPase activity [<cite idref="PUB00006632"/>, <cite idref="PUB00006679"/>].</p>