BAM folds OMPs using a budding mechanism.

Abstract

In Gram-negative bacteria, the biogenesis of beta-barrel outer membrane proteins (OMPs) is mediated by the beta-barrel assembly machinery (BAM). The mechanism employed by BAM is complex and so far is not fully resolved. Here, we report the structures of BAM in nanodiscs, where we observe an outward-open state. Mutations in the barrel domain of BamA reveal that plasticity in BAM is essential, particularly along the lateral seam of the barrel domain, which is further supported by molecular dynamics simulations that show conformational dynamics in BAM are modulated by the accessory proteins. We also report a series of cryo-EM structures detailing the near complete folding of EspP by BAM, which reveals EspP threading from the underside of BAM and sequentially incorporating into the barrel domain of BamA. Together, our studies provide a complete view of OMP biogenesis by BAM, overwhelmingly supporting that OMPs are folded using the budding mechanism.