BamA ß16C strand and periplasmic turns are critical for outer membrane protein insertion and assembly.

ABSTRACT

Outer membrane (OM) ß-barrel proteins play important roles in importing nutrients, exporting wastes and conducting signals in Gram-negative bacteria, mitochondria and chloroplasts. The outer membrane proteins (OMPs) are inserted and assembled into the OM by OMP85 family proteins. In Escherichia coli, the ß-barrel assembly machinery (BAM) contains four lipoproteins such as BamB, BamC, BamD and BamE, and one OMP BamA, forming a 'top hat'-like structure. Structural and functional studies of the E. coli BAM machinery have revealed that the rotation of periplasmic ring may trigger the barrel ß1C-ß6C scissor-like movement that promote the unfolded OMP insertion without using ATP. Here, we report the BamA C-terminal barrel structure of Salmonella enterica Typhimurium str. LT2 and functional assays, which reveal that the BamA's C-terminal residue Trp, the ß16C strand of the barrel and the periplasmic turns are critical for the functionality of BamA. These findings indicate that the unique ß16C strand and the periplasmic turns of BamA are important for the outer membrane insertion and assembly. The periplasmic turns might mediate the rotation of the periplasmic ring to the scissor-like movement of BamA ß1C-ß6C, triggering the OMP insertion. These results are important for understanding the OMP insertion in Gram-negative bacteria, as well as in mitochondria and chloroplasts.