Efflux Pumps Represent Possible Evolutionary Convergence onto the Beta Barrel Fold

There are around 100 types of integral outer membrane proteins in each Gram negative bacteria. All of these proteins have the same fold—an up-down β-barrel. It has been suggested that all membrane β-barrels other than lysins are homologous. Here we suggest that β-barrels of efflux pumps have converged on this fold as well. By grouping structurally-solved outer membrane β-barrels (OMBBs) by sequence we find evidence that the membrane environment may have led to convergent evolution of the barrel fold. Specifically, the lack of sequence linkage to other barrels coupled with distinctive structural differences, such as differences in strand tilt and barrel radius, suggest that efflux pumps have evolutionarily converged on the barrel. Finally, we find a possible ancestor for the OMBB efflux pumps as they are related to periplasmic components of the same pumps.