BamE Modulates the Escherichia coli Beta-Barrel Assembly Machine Component BamA

Biogenesis of the outer membrane (OM) is an essential process in Gram-negative bacteria. One of the key steps of OM biogenesis is the assembly of integral outer membrane beta-barrel proteins (OMPs) by a protein machine called the Barn complex. In Escherichia coli, the Barn complex is composed of the essential proteins BamA and BamD and three nonessential lipoproteins, BamB, BamC, and BamE. Both BamC and BamE are important for stabilizing the interaction between BamA and BamD. We used comprehensive genetic analysis to clarify the interplay between BamA and the BamCDE subcomplex. Combining a Delta bamE allele with mutations in genes that encode other OMP assembly factors leads to severe synthetic phenotypes, suggesting a critical function for BamE. These synthetic phenotypes are not nearly as severe in a Delta bamC background, suggesting that the functions of BamC and BamE are not completely overlapping. This unique function of BamE is related to the conformational state of BamA. In wild-type cells, BamA is sensitive to externally added proteinase K. Strikingly, when Delta bamE mutant cells are treated with proteinase K, BamA is degraded beyond detection. Taken together, our findings suggest that BamE modulates the conformation of BamA, likely through its interactions with BamD.