Port of entry--the type III secretion translocon

Many Gram-negative plant and animal pathogenic bacteria use a specialized type III secretion system (TTSS) as a molecular syringe to inject effector proteins directly into the host cell. Protein translocation across the eukaryotic host cell membrane is presumably mediated by a bacterial translocon. The structure of this predicted transmembrane complex and the mechanism of transport are far from being understood. In bacterial pathogens of animals, several putative type III secretion translocon proteins (TTPs) have been identified. Interestingly, TTP sequences are not conserved among different bacterial species, however, there are structural similarities such as transmembrane segments and coiled-coil regions. Accumulating evidence suggests that TTPs are components of oligomeric protein channels that are inserted into the host cell membrane by the TTSS.