(A) Electron micrographs of purified type III apparatuses. About two dozen proteins are necessary to make the complete structure, which is seen in the three enlarged micrographs below. The large lower ring is embedded in the inner membrane, and the smaller upper ring is embedded in the outer membrane. The long projection at the top is a hollow tube, through which secreted proteins can travel. (B) During infection, contact of the tube tip with the plasma membrane of a host cell triggers secretion. Here, the plague bacillus Yersinia pestis delivers toxins to a macrophage. (A, from K. Tamano et al., EMBO J. 19:3876–3887, 2000, by permission of Oxford University Press.)