The type III secretion system of Pseudomonas aeruginosa: infection by injection

Abstract

The Pseudomonas aeruginosa type III secretion system consists of five functional parts: proteins that constitute the secretion machine itself, proteins that translocate the secreted proteins into host cells, proteins that regulate the secretion process, proteins that bind the secreted proteins to facilitate the secretion process, called chaperone proteins, and proteins that are actually injected into host cells, called effector proteins. Only four effector proteins are known to be secreted by the P. aeruginosa type III secretion system: ExoS, ExoT, ExoU and ExoY. ExoS is a bifunctional toxin with GTPase-activating and ADP ribosyl transferase activities. These activities lead to the disruption of the actin cytoskeleton and apoptosis-like cell death. ExoT is 76% identical to ExoS and also has both GTPase-activating and ADP ribosyl transferase activities; however, the ADP ribosyl transferase activity of ExoT is directed against different substrates from that of ExoS. These activities lead to the disruption of the actin cytoskeleton and apoptosis-like cell death. ExoU has phospholipase A₂ activity that results in the rapid lysis of many mammalian cell types. ExoY is an adenylyl cyclase. Its role in pathogenesis is unclear.