Fig. 5

Possible, currently hypothetical, mechanisms for OMP assembly by the BAM complex. a BamA-assisted. OMPs may fold via a pathway similar to that observed in vitro, with BamA acting as a membrane ‘disruptase’ to assist folding. b BamA-budding. OMP assembly involves the formation of a hybrid barrel by sequential insertion of β-strands templated by the β1/β16 strands of BamA. When the final substrate β-strand has been inserted, the nascent OMP buds off from the BamA barrel to complete folding. c Barrel-elongation. Interaction of the nascent OMP with the periplasmic BAM region promotes a ‘lateral open’ BAM state, exposing the β1 strand of BamA. BamA β1 then templates β-sheet formation in the nascent OMP, possibly via β-hairpin units. Folding is completed by concerted OMP insertion and tertiary structure formation, releasing the BamA barrel and allowing BAM to return to the ground state. In all models BamA is involved in destabilising the membrane to aid insertion and folding (not shown). The lipoproteins BamB–E and the chaperone SurA have been omitted for clarity. Note that there is currently little direct experimental evidence to favour one model over another (see main text for more details)