Figure 3.

Models for YopK functions. (A) Controlling fidelity. During the middle stage, YopN is associated with the basal body to prevent premature release of late substrates. This blockade is released upon cell contact by translocating YopN into host cells. YopK now associates with the basal body to prevent aberrant injection of middle substrates. (B) An OFF switch. During the late stage, YopK is injected into the host cell and acts to down-regulate injection of the other late substrates. Two models are shown to depict how this may happen. In the Signal transduction model, YopK would interact with the pore complex and cause a conformational change in the pore, which then triggers structural changes along the length of the injectisome to provide a signal to the basal body. Further transport of late substrates is then inhibited. In the Plug model, such structural changes are not necessarily induced. Rather, YopK binding to the pore causes a physical blockade to the channel such that substrates cannot pass through the pore.