Type III secretion system

The type III secretion system (T3SS) is a membrane-embedded nanomachine found in several Gram-negative bacteria. Upon contact between bacteria and host cells, the syringe-like T3SS (Figure 1) transfers proteins termed effectors from the bacterial cytosol to the cytoplasm or the plasma membrane of a single target cell. This is a major difference from secretion systems that merely release molecules into the extracellular milieu, where they act on potentially distant target cells expressing the relevant surface receptors. The syringe architecture is conserved at the structural and functional level and supports injection into a great variety of hosts and tissues. However, the pool of effectors is species specific and determines the outcome of the interaction, via modulation of target-cell function. The type III secretion system (T3SS) creates a channel through bacterial and host membranes, allowing for the injection of bacterial effector proteins directly into the host cell. Puhar and Sansonetti discuss the structure and evolution of the T3SS, the functions of the effectors, and the immune response in plants and animals to effector injection.