Biogenesis of beta-barrel proteins in evolutionary context

The vast majority of outer membrane (OM) proteins in Gram-negative bacteria belongs to the class of membrane-embedded beta-barrel proteins. Besides Gram-negative bacteria, the presence of beta-barrel proteins is restricted to the OM of the eukaryotic organelles mitochondria and chloroplasts that were derived from prokaryotic ancestors. The assembly of these proteins into the corresponding OM is in each case facilitated by a dedicated protein complex that contains a highly conserved central beta-barrel protein termed BamA/YaeT/Omp85 in Gram-negative bacteria and Tob55/Sam50 in mitochondria. However, little is known about the exact mechanism by which these complexes mediate the integration of beta-barrel precursors into the lipid bilayer. Interestingly, previous studies showed that during evolution, these complexes retained the ability to functionally assemble beta-barrel proteins from different origins. In this review we summarize the current knowledge on the biogenesis pathway of beta-barrel proteins in Gram-negative bacteria, mitochondria and chloroplasts and focus on the commonalities and divergences that evolved between the different beta-barrel assembly machineries. (C) 2014 Elsevier GmbH. All rights reserved.