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(A) Each monomer consists of a 16-stranded antiparallel β barrel that forms a transmembrane water-filled channel. (B) The monomers tightly associate to form trimers, which have three separate channels for the diffusion of small solutes through the bacterial outer membrane. A long loop of polypeptide chain (shown in red), which connects two β strands, protrudes into the lumen of each channel, narrowing it to a cross-section of 0.6 x 1 nm. (Adapted from M.S. Weiss et al., FEBS Lett.280: 379-382, 1991.)

(1) The E. coli OmpA protein (8 β strands), which serves as a receptor for a bacterial virus. (2) The E. coli OMPLA protein (12 β strands), is a lipase that hydrolyses lipid molecules. The amino acids that catalyze the enzymatic reaction (shown in red) protrude from the outside surface of the barrel. (3) A porin from the bacterium Rhodobacter capsulatus, which forms water-filled pores across the outer membrane (16 β strands). The diameter of the channel is restricted by loops (shown in blue) that protrude into the channel. (4) The E. coli FepA protein (22 β strands), which transports iron ions. The inside of the barrel is completely filled by a globular protein domain (shown in blue) that contains an iron-binding site. This domain is thought to change its conformation to transport the bound iron, but the molecular details of the changes are not known.