Structures and functions of autotransporter proteins in microbial pathogens

Since their discovery more than 20 years ago the autotransporter protein superfamily has been growing continuously and currently represents the largest protein family in (pathogenic) Gram-negative bacteria. Autotransporter proteins (AT) adhere to a common structural principle and are composed of a C-terminal β-barrel-shaped 'translocator' domain and an N-terminal 'passenger' domain. The translocator is anchored in the outer membrane and is indispensable for the N-terminal passenger part to traverse the outer membrane. Most if not all AT harbor a chaperone segment that increases protein stability and may be located in the passenger or translocator domain. The passenger mediates the specific virulence function(s) of the particular AT. Accordingly, passenger domains of AT can be quite variable. Interestingly, AT have been identified as the first glycosylated proteins in Gram-negative bacteria. Despite the considerable efforts invested in the characterization of autotransporter biogenesis, various aspects such as the participation of accessory proteins, the fate of the translocator, or the translocation of glycosylated proteins still remain only poorly understood. In addition, recent evidence indicates that the prefix 'auto' might be slightly exaggerated. Here, we will selectively discuss novel insights at various stages of AT biogenesis.