Tomographic 3D Reconstruction of Quick-Frozen, Ca2+-Activated Contracting 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 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 ∼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.