How to get across the target host membrane: bacterial type III toxin translocons

Type III secretion/translocation systems are essential factors of bacteria-host interactions in most gram-negative animal and plant pathogens. They are composed of more than 20 proteins and accomplish the passage of toxins from the bacterial cytoplasm toward the target cell. These sophisticated nanomachines span the two bacterial membranes and protrude out from the bacterial surface in a needle-like structure. Two pore-forming proteins are inserted into the plasma membrane of the host-cell, forming a sub-structure called bacterial type III toxin translocon (B4T). The translocators possessing -helical transmembrane domains are kept soluble by their common chaperons within the bacterial cytoplasm. Their activities toward biological membranes are described here as well as their own toxic activity within the host cytoplasm. Finally, the roles and importance of a third, helper, protein necessary for the function of the translocators in vivo are discussed