Extreme Dynamics in the BamA β-Barrel Seam

BamA is an essential component of the beta-barrel assembly machine (BAM) that is responsible for insertion and folding of beta-barrel outer membrane proteins (OMPs) in Gramnegative bacteria. BamA is an OMP itself, and its beta-barrel transmembrane domain is thought to catalyze OMP insertion and folding, although the molecular mechanism remains poorly understood. Crystal structures of BamA and complementary molecular dynamics simulations have shown that its beta-barrel seam (the interface between the first and last barrel strands) is destabilized. This has led to mechanistic models in which the BamA barrel seam functions as a lateral gate that opens and successively accepts beta-hairpins from a nascent OMP such that a nascent barrel can bud from BamA. Consistent with this model, disulfide locking of the BamA barrel seam is lethal in Escherichia coli. Here we show that disulfide locking of the BamA barrel has no effect on its ability to catalyze folding of a model OMP into liposomes. However, disulfide trapping experiments indicate that the BamA barrel is highly dynamic in the liposome membranes, with the beta-strands at the barrel seam undergoing register sliding by more than 14 angstrom both up and down the membrane. Remarkably, these extreme dynamics were also observed in the BamA barrel in the context of the native E. coli outer membrane. These results are consistent with a model in which the BamA barrel dynamics induce defects in the outer membrane that facilitate insertion of nascent OMPs.