On display: autotransporter secretion and application.

The classical monomeric autotransporters are ubiquitously used by Gram-negative bacteria to export virulence and colonization factors to their cell surface or into their surroundings. They are expressed as monomeric proteins that pass the inner and outer membrane in two consecutive steps facilitated by the Sec translocon and the Bam complex, respectively. In this mini-review we discuss how autotransporters translocate their secreted functional domains across the outer membrane. We highlight the interactions with the Bam complex and discuss how specific features of the recently solved structure of Bam lead to a mechanistic model for autotransporter secretion. Furthermore, the autotransporter secretion pathway is the system of choice for surface display of heterologous proteins for biotechnical and biomedical purposes. We summarize recent advances in the application of autotransporters with a focus on outer membrane vesicle vaccine development and discuss its limitations in secreting more complex heterologous proteins. Finally, we present an exciting new technology to circumvent secretion limitations by ligating heterologous proteins of interest to autotransporters that are displayed on the cell surface.