<p>The type III secretion system of Gram-negative bacteria is used to transport virulence factors from the pathogen directly into the host cell [<cite idref="PUB00003585"/>] and is only triggered when the bacterium comes into close contact with the host. Effector proteins secreted by the type III system do not possess a secretion signal, and are considered unique because of this. Yersinia spp. secrete an effector protein called YopE through the type III needle [<cite idref="PUB00006632"/>]. This acts as a Rho GTPase-activating protein that disrupts the host cell actin cytoskeleton, and is regulated by a chaperone protein called SycE/YerA [<cite idref="PUB00007794"/>]. In the absence of the SycE chaperone, YopE is not transported through the needle and remains in the bacterial cytoplasm, so suggesting a crucial role for this moiety [<cite idref="PUB00006679"/>]. </p><p>Both the YopE regulator and SycE/YerA proteins share similarity with the exoenzyme S (ExoS) gene product of <taxon tax_id="287">Pseudomonas aeruginosa</taxon> [<cite idref="PUB00007797"/>]. ExoS has both ADP-ribosylating and GTPase activity, and is implicated as a virulence factor. As type III secretion in Pseudomonas is often associated with systemic and even fatal infections in susceptible patients [<cite idref="PUB00007798"/>], the proteins involved are of interest as vaccine and drug targets.</p> Type III secretion chaperone SycE, Gram-negative bacterial