Looks can be deceiving: recent insights into the mechanism of protein secretion by the autotransporter pathway

Autotransporters are a large superfamily of cell surface proteins produced by Gram-negative bacteria that consist of an N-terminal extracellular domain (passenger domain') and a C-terminal -barrel domain that resides in the outer membrane (OM). Although it was originally proposed that the passenger domain is translocated across the OM through a channel formed exclusively by the covalently linked -barrel domain, this idea has been strongly challenged by a variety of observations. Recent experimental results have suggested a new model in which both the translocation of the passenger domain and the membrane integration of the -barrel domain are facilitated by the Bam complex, a highly conserved heteroligomer that plays a general role in OM protein assembly. Other factors, including periplasmic chaperones and inner membrane proteins, have also recently been implicated in the biogenesis of at least some members of the autotransporter superfamily. New results have raised intriguing questions about the energetics of the secretion reaction and the relationship between the assembly of autotransporters and the assembly of other classes of OM proteins. Concomitantly, new mechanistic and structural insights have expanded the utility of the autotransporter pathway for the surface display of heterologous peptides and proteins of interest.