Figure 1.

Organization and biogenesis of autotransporters. A. The typical organization of an autotransporter (AT) comprising an N-terminal signal sequence (SS), a passenger domain and a translocation unit (TU). The passenger domain includes an N-terminal part that bears the activity of the AT and a C-terminal domain, called the autochaperone domain, which is important for efficient translocation across the outer membrane. The TU also has two distinct domains; the N-terminal region is structured as an α-helix, whereas the C-terminal region is structured as a β-barrel. B. The biogenesis of an AT has four main steps: translocation across the inner membrane, periplasmic transport, insertion into and translocation across the outer membrane and, lastly, processing of the passenger domain.