Dynamic association of BAM complex modules includes surface exposure of the lipoprotein BamC.

摘要

The BAM complex drives assembly of β-barrel proteins into the outer membrane of gram-negative bacteria. It is composed of five subunits: BamA, BamB, BamC, BamD and BamE. We find that the BAM complex isolated from the outer membrane of Escherichia coli consists of a core complex of BamA:B:C:D:E and in addition, a BamA:B module and a BamC:D module. In the absence of BamC, these modules are destabilized resulting in increased protease susceptibility of BamD and BamB. While the N-terminus of BamC carries a highly conserved region crucial for stable interaction with BamD, immunofluorescence, immunoprecipitation and protease-sensitivity assays show that the C-terminal domain of BamC, comprised of two helix-grip motifs, is exposed on the surface of Escherichia coli. This unexpected topology of a bacterial lipoprotein is reminiscent of the analogous protein subunits from the mitochondrial β-barrel insertion machinery, the SAM complex. The modular arrangement and topological features provide new insight into the architecture of the BAM complex, towards a better understanding of the mechanism driving β-barrel membrane protein assembly.