The WD40 Protein BamB Mediates Coupling of BAM Complexes into Assembly Precincts in the Bacterial Outer Membrane

The β-barrel assembly machinery (BAM) complex is essential for localization of surface proteins on bacterial cells, but the mechanism by which it functions is unclear. We developed a direct stochastic optical reconstruction microscopy (dSTORM) methodology to view the BAM complex in situ. Single-cell analysis showed that discrete membrane precincts housing several BAM complexes are distributed across the E. coli surface, with a nearest neighbor distance of ∼200 nm. The auxiliary lipoprotein subunit BamB was crucial for this spatial distribution, and in situ crosslinking shows that BamB makes intimate contacts with BamA and BamB in neighboring BAM complexes within the precinct. The BAM complex precincts swell when outer membrane protein synthesis is maximal, visual proof that the precincts are active in protein assembly. This nanoscale interrogation of the BAM complex in situ suggests a model whereby bacterial outer membranes contain highly organized assembly precincts to drive integral protein assembly. Bacteria grow and divide by assembling new material into their surface membranes. Gunasinghe et al. used super-resolution microscopy and in situ crosslinking in live bacterial cells in order to visualize intimate contacts between BAM complexes suggesting a model whereby bacteria use highly organized precincts to drive membrane protein assembly.