BamA Alone Accelerates Outer Membrane Protein Folding In Vitro through a Catalytic Mechanism

β-Barrel assembly machinery protein A (BamA) plays a critical role in the biogenesis of outer membrane proteins (OMPs); however, a mechanistic understanding of its function is lacking. Here, we report an in vitro assay that investigates whether the mechanism of BamA-catalyzed OMP folding is stoichiometric or catalytic. We found that BamA accelerates the folding of OMPs in vitro via a catalytic mechanism, similar to the activity of the full multiprotein β-barrel assembly machinery (BAM) complex in vivo. As BamA alone can repeatedly facilitate the folding of OMPs, we suggest the additional BAM components accelerate this basal activity to biologically relevant time scales.