Substitutions in the BamA β-Barrel Domain Overcome the Conditional Lethal Phenotype of a ΔbamB ΔbamE Strain of Escherichia coli

BamA interacts with the BamBCDE lipoproteins, and together they constitute the essential beta-barrel assembly machine (BAM) of Escherichia coli. The simultaneous absence of BamB and BamE confers a conditional lethal phenotype and a severe beta-barrel outer membrane protein (OMP) biogenesis defect. Without BamB and BamE, wild-type BamA levels are significantly reduced, and the folding of the BamA beta-barrel, as assessed by the heat-modifiability assay, is drastically compromised. Single-amino-acid substitutions in the beta-barrel domain of BamA improve both bacterial growth and OMP biogenesis in a bamB bamE mutant and restore BamA levels close to the BamB(+) BamE(+) level. The substitutions alter BamA beta-barrel folding, and folding in the mutants becomes independent of BamB and BamE. Remarkably, BamA beta-barrel alterations also improve OMP biogenesis in cells lacking the major periplasmic chaperone, SurA, which, together with BamB, is thought to facilitate the transfer of partially folded OMPs to the soluble POTRA (polypeptide-transport-associated) domain of BamA. Unlike the bamB bamE mutant background, the absence of BamB or SurA does not affect BamA beta-barrel folding. Thus, substitutions in the outer membrane-embedded BamA beta-barrel domain overcome OMP biogenesis defects that occur at the POTRA domain of BamA in the periplasm. Based on the structure of FhaC, the altered BamA residues are predicted to lie on a highly conserved loop that folds inside the beta-barrel and in regions pointing outside the beta-barrel, suggesting that they influence BamA function by both direct and indirect mechanisms.