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 (simi lar 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) tilti ng on actin, and two-thirds keeping rigor contact with actin while the ligh t chain domain (LCD) tilts axially from similar to 105 degrees to similar t o 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 3 5 degrees axial angle, giving an similar to 13 nm interaction distance and an similar to 4-6 nm working stroke.