Biogenesis of bacterial outer membrane proteins

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To obtain insight into OMP biogenesis, we recently developed a method to trap a modified form of an E. coli O157:H7 autotransporter called EspP stably bound to the Bam complex at a late stage of assembly. Using disulfide bond crosslinking, we found that when assembly stalls the C-terminal beta strand of the EspP beta barrel forms a rigid interface with the first beta strand of a laterally "open" form of the beta barrel of BamA, the central subunit of the Bam complex. In contrast, the N-terminal beta strand of the EspP derivative forms weaker, conformationally heterogeneous interactions with the lipid facing surface of the last two beta strands of BamA that likely represent intermediate assembly states. The results indicate that BamA forms a hybrid barrel with client proteins during their assembly. Based on our results, we proposed that BamA catalyzes the membrane insertion of partially folded beta barrels by a novel "swing" mechanism. We have also been using EspP as a model protein to study autotransporter biogenesis. In one major line of investigation we have been examining the mechanism by which the EspP passenger domain is translocated across the OM. It was originally proposed that the passenger domain is secreted through a channel formed by the covalently linked beta barrel domain (whence the name autotransporter), but results that we obtained from both biochemical and structural studies are inconsistent with this hypothesis. We found that the insertion of a small linker into the EspP passenger domain effectively creates a translocation intermediate by transiently stalling the translocation reaction (which is normally extremely rapid) near the site of the insertion. By using a site-specific photocrosslinking approach we found that residues adjacent to the stall point interact with BamA. These results support a model in which the Bam complex plays a major role in facilitating both the integration of the beta barrel domain into the OM and the translocation of the passenger domain across the OM. In a recent study, we found that the mutation of an unusual lipid-exposed lysine residue in the EspP beta barrel domain impairs a previously unidentified late folding step that follows both the membrane insertion of the beta barrel domain and the secretion of the passenger domain but that precedes proteolytic maturation. Our results demonstrate that beta 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 beta barrel domain is fully assembled, our results also provide evidence against the hypothesis that autotransporters are autonomous protein secretion systems.