The Type III Secretion System

The type-III secretion system (T3SS) is an export machine used by pathogenic Gram-negative bacteria to deliver proteins straight into the eukaryotic cytosol with the aim to subvert the host cell defense. After the discovery of T3S in 1990, significant progress has been made in the understanding of its structure, assembly and function. The basic structure consisting of the membrane-embedded basal body, the needle and the tip structure has been analyzed in more details. In the last years, the structure of several structural components has been solved and important insights into the assembly process have been gained. Furthermore, comparing the T3SS of pathogenic bacteria with the flagellum the relationship between these two structures becomes obvious. Besides the structural similarities, the assembly of these two nanomachines shows some commonalities like for example the length control of external structures like the T3 needle and the flagellar hook. In addition to the export machine the complete T3SS also includes the pore forming translocator proteins, effector proteins and a set of specific chaperones. Here, we review different aspects of the T3SS focusing on the structure and assembly of this fascinating nanomachine.