beta-Barrel Membrane Protein Assembly by the Barn Complex

beta-barrel membrane proteins perform important functions in the outer membranes (OMs) of Gram-negative bacteria and of the mitochondria and chloroplasts of eukaryotes. The protein complexes that assemble these proteins in their respective membranes have been identified and shown to contain a component that has been conserved from bacteria to humans. beta-barrel proteins are handled differently from a-helical membrane proteins in the cell in order to efficiently transport them to their final locations in unfolded but folding-competent states. The mechanism by which the assembly complex then binds, folds, and inserts beta-barrels into the membrane is not well understood, but recent structural, biochemical, and genetic studies have begun to elucidate elements of how the complex provides a facilitated pathway for beta-barrel assembly. Ultimately, studies of the mechanism of beta-barrel assembly and comparison to the better-understood process of a-helical membrane protein assembly will reveal whether there are general principles that guide the folding and insertion of all membrane proteins.