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

Abstract

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.

Figure 1

Concentration of client OmpA folded by BamA is greater than the concentration of folded BamA. The solid line marks the concentration at which there is a stoichiometric ratio of one OMP folded per BamA. The excess concentration of BamA-catalyzed folded OmpA indicates that BamA must catalyze the folding of multiple OmpA clients. The error bars indicate the standard deviation of five independent experiments; asterisks show a significance of p < 0.05 as a result of a Student’s t test.