Formation of a β-barrel membrane protein is catalyzed by the interior surface of the assembly machine protein BamA

The beta-barrel assembly machine (Barn) complex in Gram-negative bacteria and its counterparts in mitochondria and chloroplasts fold and insert outer membrane beta-barrel proteins. BamA, an essential component of the complex, is itself a beta-barrel and is proposed to play a central role in assembling other barrel substrates. Here, we map the path of substrate insertion by the Barn complex using site-specific crosslinking to understand the molecular mechanisms that control beta-barrel folding and release. We find that the C-terminal strand of the substrate is stably held by BamA and that the N-terminal strands of the substrate are assembled inside the BamA beta-barrel. Importantly, we identify contacts between the assembling beta-sheet and the BamA interior surface that determine the rate of substrate folding. Our results support a model in which the interior wall of BamA acts as a chaperone to catalyze beta-barrel assembly.