Species-specificity of the BamA component of the bacterial outer membrane protein-assembly machinery.

The BamAprotein is the key component of the Bam complex, the assembly machinery for outer membrane proteins (OMP) in gram-negative bacteria. We previously demonstrated that BamArecognizes its OMP substrates in a species-specific manner in vitro. In this work, we further studied species specificity in vivo by testing the functioning of BamAhomologs of the proteobacteria Neisseria meningitidis, Neisseria gonorrhoeae, Bordetella pertussis, Burkholderia mallei, and Escherichia coli in E. coli and in N. meningitidis. We found that no BamAfunctioned in another species than the authentic one, except for N. gonorrhoeae BamA, which fully complemented a N. meningitidis bamAmutant. E. coli BamAwas not assembled into the N. meningitidisouter membrane. In contrast, the N. meningitidis BamAprotein was assembled into the outer membrane of E. coli to a significant extent and also associated with BamD, an essential accessory lipoprotein of the Bam complex.Various chimeras comprising swapped N-terminal periplasmic and C-terminal membrane-embedded domains of N. meningitidisand E. coli BamAproteins were also not functional in either host, although some of them were inserted in the OM suggesting that the two domains of BamAneed to be compatible in order to function. Furthermore, conformational analysis of chimeric proteins provided evidence for a 16-stranded β-barrel conformation of the membrane-embedded domain of BamA.