Molecular Mechanisms of Bacterial Virulence: Type III Secretion and Pathogenicity Islands
Figure 1. Schematic diagram of type I, type II, and type III secretion systems. All systems use the energy of ATP hydrolysis to drive secretion. Type I and type III secrete proteins across both the inner membrane and the cell envelope (outer membrane) in one step; secreted proteins do not make an intermediate stop in the periplasm, as they do in type II secretion. Type I and type III systems are also similar in that they do not remove any part of the secreted protein. In contrast, the N-terminus of proteins secreted by the general secretory pathway is removed upon transfer to the periplasm; the N-terminal signal sequence can be seen in the periplasm, and the extracellular protein is clearly different from the intracellular protein by virtue of its absence. Type I systems are composed of far fewer components than type III systems; this is indicated by the number of distinct proteins (indicated by shape and size) in the figure. Type II and type III systems share a similar cell envelope component, as indicated by sequence homology; this is reflected in the shape of a cell envelope component in the figure.