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.