The most abundant protein of the outer membrane, it forms a pore through it. Forms extremely stable complexes with apparent masses of 150, 50, 45 and 38 kDa. Found in a ring-shaped complex of 7 nm diameter with a 2 nm channel through the middle. Complete denaturation requires temperatures over 110 degrees Celsius. Ignicoccus hospitalis is unique in having an energized outer membrane while ATP synthesis occurs within the periplasmic space.