Structural architecture of an outer membrane channel as determined by electron crystallography

Abstract

PORINS are a family of membrane channels commonly found in the outer membranes of Gram-negative bacteria where they serve as diffusional pathways for waste products, nutrients and anti-biotics, and can also be receptors for bacteriophages^1,2. Porin channels have been shown in vitro to be voltage-gated^3–6. They can exhibit slight selectivities for certain solutes; for example PhoE porin has some selectivity for anionic and phosphate-containing compounds^1,7. Unlike many known membrane proteins which often contain long stretches of hydrophobic segments that are believed to traverse the membrane in a helical conformation, porins are found to have charged residues distributed almost uniformly along their primary sequences and have most of their secondary structure in a β-sheet conformation^8–10. We have made crystalline patches of PhoE porin embedded in a lipid bilayer and have used these to determine the structure of PhoE porin by electron crystallography to a resolution of 6Å. The basic structure consists of a trimer of elliptically shaped, cylindrical walls of β sheet. Each cylinder has an inner lining, formed by parts of the polypeptide, that defines the channel size. The structure provides a clue as to how deletions of segments of polypeptide, which are found in certain mutants, can result in an actual increase in the channel size.