In Gram-negative bacteria, assembly of the β-barrel outer membrane protein (OMPs) relies on the β-barrel assembly machinery (BAM) complex. This highly sophisticated nanomachine integrates and assembles OMPs into the outer membrane and is composed of five subunits: the integral membrane protein BamA, and four lipoproteins called BamB, BamC, BamD, and BamE. Recently, crystal structures were reported for the entire BamABCDE complex, as well as the complex lacking BamB, (BamACDE). While these structural snapshots provide crucial clues as to how the BAM complex functions, the exact function of each subunit remains unclear.
In order to better understand these individual roles we developed a new procedure (termed the Escherichia coli microsome membrane (EMM) assembly assay) which can monitor the assembly reaction of OMPs into the outer membrane. We used OmpC as a substrate protein, which forms a trimer upon assembly into the outer membrane. The EMM assembly assay revealed that BamB contributes to efficient trimer formation of OmpC, and BamC is the substrate protein release factor of the BAM complex.