Which feature distinguishes β-barrel transmembrane proteins from α-helical transmembrane proteins?
Two fundamental integral protein folds crossing lipid bilayers differ in secondary structure forming membrane-spanning segments. Alpha-helical type uses hydrophobic alpha-helices of nineteen to twenty seven residues where backbone carbonyl amide hydrogen bonds internal satisfying polarity, side chains aliphatic contacting lipid tails, found predominantly in plasma membrane and inner membranes of Gram-negative bacteria, mitochondria inner membrane, endoplasmic reticulum; multispan examples Band 3 fourteen passes, GPCR seven passes, glycophorin one pass. Beta-barrel type folds as antiparallel amphipathic beta-strands seven to twenty two residues where alternating residues face lipid hydrophobic and lumen hydrophilic, strands connected by short turns and long loops, hydrogen bond network closes cylinder first to last strand. Barrels exclusively discovered in outer membranes of Gram-negative bacteria as porins OmpF OmpC PhoE, mitochondria VDAC, chloroplast outer envelope Toc75, inserted by BAM and SAM machinery. Functionally alpha-helical receptors transporters, beta-barrel nonspecific channels for small polar solutes. Thus formation of membrane-spanning beta-strands distinguishes beta-barrel architecture from amphipathic alpha-helices.
Ref: Schulz, Beta-Barrel Membrane Proteins – Structure and Assembly, Biochim Biophys Acta 2002.