Identification of a novel post-insertion step in the assembly of a bacterial outer membrane protein

Although the achinery (Bam) complex has been shown to facilitate the insertion of barrel proteins into the bacterial outer membrane (OM), the stage at which barrels fold is unknown. Here, we describe insights into barrel assembly that emerged from an analysis of a member of the autotransporter family of OM proteins (EspP) in Escherichia coli. EspP contains an extracellular passenger' domain that is translocated across the OM and then released from the covalently linked barrel domain in an intra-barrel cleavage reaction. We found that the mutation of an unusual lipid-exposed lysine residue impairs a previously unidentified late folding step that follows both the membrane insertion of the barrel domain and the secretion of the passenger domain but that precedes proteolytic maturation. Our results demonstrate that barrel assembly can be completed at a post-insertion stage and raise the possibility that interactions with membrane lipids can promote folding in vivo. Furthermore, by showing that the passenger domain is secreted before the barrel domain is fully assembled, our results also provide evidence against the long-standing hypothesis that autotransporters are autonomous protein secretion systems.