Field electron microscopic image of rotary-shadowed AMII. (Upper) WT in 10 mM imidazole (pH 7.0), 2.5 mM KCl, 1 mM MgCl2, and 1 mM ATP shows staggered bipolar octamers. (Lower) pWT in 10 mM imidazole (pH 7), 2.5 mM KCl, 2 mM MgCl2, and 1 mM ATP shows antiparallel bipolar octamers with clustered heads. (Scale bars, 100 nm.)