Gram-negative bacteria are surrounded by a double membrane that protects the inner membrane very efficiently against harmful compounds. In order to secret proteins across both membranes of the cell envelope, these bacteria evolved a number of different secretion systems, known as type I to type VI secretion systems. In our section, we are studying the mechanism of type V secretion, the most-widely used of these secretion pathways. Within type V secretion systems there are two different versions, the so-called autotransporters and the two-partner secretion (TPS) pathway. Both systems depend on Sec-mediated transport across the inner membrane. Subsequently, a β-barrel is formed in the outer membrane that facilitates transport across this second membrane. For autotransporters, this β-barrel is part of the secreted protein, whereas for the TPS system the β-barrel is a separate protein containing a periplasmic extension that probably recognizes the secreted protein.
Interestingly, these secreted proteins are generally very large, but are, despite their size, efficiently secreted to the cell surface where they are retained (surface display) or truly released in the medium or host. This characteristic makes these transporters also interesting from a biotechnological point of view. Currently, we exploit this system for the delivery of heterologous antigens at the surface of live attenuated bacteria or outer membrane vesicles. The antigen-decorated bacteria or vesicles can be used as mucosal vaccines. Recent data in mouse infection models demonstrate the immunogenicity and protective efficacy of the prototype vaccines tested.
In addition, we are studying the role of the β-barrel in membrane transport and the size limitations for this transport process. In addition, we have substituted various domains of the secreted portion for heterologous domains and have shown that in general these chimeric constructs are still efficiently secreted to the cell surface. Finally, we are studying substrate specificity for the TPS system of the important pathogen Neisseria meningitidis, which contains several of these secretion systems.