Are bacterial ‘autotransporters’ really transporters?

Autotransporters are bacterial outer membrane proteins that consist of a large N-terminal extracellular domain (‘passenger domain’) and a C-terminal β-barrel domain (‘β domain’). The β domain was originally proposed to function as a channel that transports its own passenger domain across the outer membrane. Results of recent structural, biochemical and molecular genetic studies, however, have challenged this idea. Here I describe an alternative model in which translocation of the passenger domain is mediated by an exogenous factor (possibly a newly identified factor necessary for assembly of outer membrane proteins called ‘Omp85/YaeT’), whereas the β domain only targets the protein to the outer membrane and serves as a membrane anchor.