FIG. 3.

Schematic overview of the type V secretion systems. The secretion pathway of the autotransporter proteins (type Va) is depicted at the bottom left of the diagram, the two-partner system (type Vb) is depicted in the center of the diagram, and the type Vc or AT-2 family is depicted on the right. The four functional domains of the proteins are shown: the signal sequence, the passenger domain, the linker region, and the β-domain. The autotransporter polyproteins are synthesized and generally exported through the cytoplasmic membrane via the Sec machinery. Interestingly, effector proteins with an unusual extended signal sequence, which purportedly mediates Srp-dependent export, are found in all three categories of type V secretion. Once through the inner membrane, the signal sequence is cleaved and the β-domain inserts into the outer membrane in a biophysically favored β-barrel structure that forms a pore in the outer membrane. After formation of the β-barrel, the passenger domain inserts into the pore and is translocated to the bacterial cell surface, where it may or may not undergo further processing.