The β-barrel assembly machine (BAM) mediates folding and insertion of integral
β-barrel outer membrane proteins (OMPs) in Gram-negative bacteria. Of the five
BAM subunits, only BamA and BamD are essential for cell viability. Here we
present the crystal structure of a fusion between BamA POTRA4-5 and BamD from
Rhodothermus marinus. The POTRA5 domain binds BamD between its tetratricopeptide
repeats 3 and 4. The interface structural elements are conserved in the
Escherichia coli proteins, which allowed structure validation by mutagenesis and
disulfide crosslinking in E. coli. Furthermore, the interface is consistent
with previously reported mutations that impair BamA-BamD binding. The structure
serves as a linchpin to generate a BAM model where POTRA domains and BamD form
an elongated periplasmic ring adjacent to the membrane with a central cavity
approximately 30 × 60 Å wide. We propose that nascent OMPs bind this
periplasmic ring prior to insertion and folding by BAM.
