Figure 2.

Schematic of type 5 secretion systems. Extracellular passengers are in blue, outer membrane β-barrels in purple and linkers connecting the β-barrel to the passenger in red. The passengers of T5aSSs are typically β-helical structures. T5cSSs or TAAs are formed by three identical chains. The passengers of TAAs like YadA consist of a coiled coil stalk and an N-terminal β-structured head domain [102], though more complicated architectures exist. T5eSSs, or inverse autotransporters, such as intimin and invasin have passengers consisting of tandem immunoglobulin-like domains often capped by a lectin-like domain. The transmembrane of all three classes consist of a 12-standed β-barrel (with three chains each contributing four β-strands in the case of TAAs) with the linker(s) contained within the hydrophilic lumen of the barrel [102]. The periplasmic LysM domain of inverse autotransporters is shown in orange. The positions of the N- and C-termini are indicated. The figure is adapted from [103].