betabarrels

Betabarrels, more commonly referred to as beta-barrels or β-barrel proteins, describe a class of protein folds characterized by a cylindrical wall formed from beta-strands that assemble into a closed barrel spanning a lipid bilayer. Although the spelling betabarrels appears occasionally in texts, the conventional term in the literature is beta-barrel.

Beta-barrel proteins are most notably found in the outer membranes of Gram-negative bacteria, as well as in the outer membranes of mitochondria and chloroplasts in eukaryotes. They are typically composed of 8 to 24 antiparallel beta-strands that create a hollow pore or channel. The exterior surface is largely hydrophobic, matching the lipid bilayer, while the interior lining of the barrel pore is more hydrophilic, enabling selective passage of small molecules. Extracellular loops connecting the strands contribute to pore size and specificity and can participate in ligand binding or gating.

The assembly of beta-barrel proteins is facilitated by dedicated machinery, such as the beta-barrel assembly machinery (BAM) complex in bacteria, which assists folding and insertion into the outer membrane. Periplasmic chaperones also help prevent aggregation before insertion. Evolutionarily, beta-barrels represent a versatile scaffold capable of diverse functions, including passive diffusion through porins, enzymatic activity, and receptor roles.

Notable examples include porins such as Escherichia coli OmpF and OmpC, which form large, nonspecific channels, and mitochondrial porins such as VDAC, which regulate metabolite exchange between the mitochondrion and cytosol. Beta-barrels play a central role in membrane transport and microbial physiology.