Porins form aqueous channels for the diffusion of small hydrophillic molecules across the outer membrane. Individual 16-strand anti-parallel beta-barrels form a central pore, and trimerizes thru mainly hydrophobic interactions at the interface. Trimers are stabilized by hytrophillic clamping of Loop L2. Loop 3 bends into the pore, creating an elliptical constriction of about 7 x 11A, large enough to allow passage of a glucose molecule without steric hindrance. Removal of the C-terminal residue (usuallly F) destabilizes the trimer and removal of the 16th beta-sheet abolishes trimerization. Unlike typical membrane proteins, porins lack long hydrophobic stretches. Short turns are found at the smooth, periplasmic end, longer irregular loops are found at the rough, extracellular end. C-terminal residue forms salt bridge with N-terminus.
Jiyao W et al.(2023). "The conserved domain database in 2023.",