Structural insight into the formation of lipoprotein-β-barrel complexes by the β-barrel assembly machinery

ABSTRACT The β-barrel assembly machinery (BAM) inserts outer membrane β-barrel proteins (OMPs) in the outer membrane of Gram-negative bacteria. In Enterobacteriacea, BAM also mediates export of the stress sensor lipoprotein RcsF to the cell surface by assembling RcsF-OMP complexes. Although BAM has been the focus of intense research due to its essential activity in generating and maintaining the outer membrane, how it functions remains poorly understood. Here, we report the crystal structure of the key BAM component BamA in complex with RcsF. BamA adopts an inward-open conformation, with the lateral gate to the membrane closed. The globular domain of RcsF is lodged deep inside the lumen of the BamA barrel, binding regions proposed to undergo an outward and lateral opening during OMP insertion. Our structural and biochemical data indicate a push-and-pull mechanism for RcsF export upon conformational cycling of BamA and provide a mechanistic explanation for how RcsF uses its interaction with BamA to detect envelope stress. Our data also suggest that the flux of incoming OMP substrates is involved in the control of BAM activity. Overall, the structural insights gleaned here elucidate a fundamental biological process and suggest a new avenue for antibiotic development.