The crystal structure of porin from Rhodobacter capsulatus strain 37b4 has been solved at 3.0 Å (1 Å = 0.1 nm) resolution by multiple isomorphous replacement and solvent-flattening. The three pores of the trimer are well denned in the electron density map. Each pore consists of a 16-stranded β-barrel which traverses the membrane as a tube. Near its center the tube is narrowed by chain segments protruding from the inner wall of the barrel that form an eye-let with an irregular cross-section of about 6 Å by 10 Å. The eye-let has an axial length of about 10 Å; it defines the exclusion limit for diffusing particles.
