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

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 (similar to 30% of all heads) depart markedly in axial and azimuthal angles from Rayment's 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 similar to 105 degrees to similar to 70 degrees. The results suggest the MD tilts and slews on actin from weak to strong binding, followed by swinging of the LCD through an similar to 35 degrees axial angle, giving an similar to 13 nm interaction distance and an similar to 4-6 nm working stroke.