Structure of an E. coli OmpF monomer. (A) Side view of a single β-barrel
of the OmpF trimers to highlight the orientation of the protein arrangement in the
outer membrane (“EC” is the extracellular side, and “Peri” the periplasmic side of the
outer membrane); the front part of the strands has are not shown to make a view on
the constricting L3 loop (orange) possible. (B) Cross-section of the OmpF monomer
observed from the side of the periplasm; clearly visible is the narrowing of the porin
channel by the internal pore residues in the constriction zone. Responsible residues
the acidic residues of the L3 loop (in red) and a cluster of basic amino acids of the
opposite barrel wall (in blue). (Cowan et al., 1995