Tomographic 3D reconstruction of quick-frozen, Ca2+ activated insect flight muscle

Abstract

Motor actions of myosin were directly visualized by electron tomography of insect flight muscle quick-frozen during contraction. In 3D images, active crossbridges are usually single myosin heads, bound preferentially to actin target zones sited midway between troponins. Active attached bridges (˜30% of all heads) depart markedly in axial and azimuthal angles from Rayments rigor acto-S1 model, one-third requiring motor domain (MD) tilting on actin, and two-thirds keeping rigor contact with actin while the light chain domain (LCD) tilts axially from ˜105° to ˜70°. The results suggest the MD tilts and slews on actin from weak to strong binding, followed by swinging of the LCD through an ˜35° axial angle, giving an ˜13 nm interaction distance and an ˜4–6 nm working stroke