Lateral gating mechanism and plasticity of the BAM complex in micelles and E. coli

The {beta}-barrel assembly machinery (BAM) mediates folding and insertion of the majority of OMPs in Gram-negative bacteria. BAM is a penta-heterooligomeric complex consisting of the central {beta}-barrel BamA and four interacting lipoproteins BamB, C, D, and E. The conformational switching of BamA between inward-open (IO) and lateral-open (LO) conformations is required for substrate recognition and folding. However, the mechanism for the lateral gating or how the structural details observed in vitro correspond with the cellular environment remains elusive. Here we addressed these questions by characterizing the conformational heterogeneity of BamAB, BamACDE and BamABCDE complexes in detergent micelles and or E. coli using pulsed dipolar electron spin resonance spectroscopy (PDS). We show that the binding of BamB does not induce any visible changes in BamA and the BamAB complex exists in the IO conformation. The BamCDE complex induces an IO to LO transition through a coordinated movement along the BamA barrel. However, the extracellular loop (L6) is unaffected by the presence of lipoproteins and exhibits a large segmental dynamics extending to the exit pore. PDS experiments with BamABCDE complex in intact E. coli confirmed the dynamic behavior of both the lateral gate and the L6 in the native environment. Our results demonstrate that the BamCDE complex plays a key role for the function by regulating lateral gating in BamA.