Evolution of the β-barrel assembly machinery.

Proteins from the Omp85 family have roles in membrane biogenesis, and the archetypal protein of this family is the bacterial outer membrane protein BamA. Through evolution, BamA has acquired membrane protein partner subunits, but distinct partner subunits are evident in the various bacterial lineages. As a result, experimental work on several species of bacteria has revealed varietal forms of the β-barrel assembly machinery (BAM complex). This scenario extends even into mitochondria and plastids, organelles of eukaryotic cells that evolved from intracellular bacterial ancestors. In addition to the BAM complex, other molecular machines, namely the two-partner secretion system (TPS) and the translocation and assembly module (the TAM), probably evolved from gene duplication events involving BamA. We discuss what is known about the diverse composition of the BAM complex in various bacterial lineages, and how this diversity impacts on our understanding of the mechanism underlying the assembly of bacterial outer membranes.