Title: The Elasticity of the Myosin Motor and Myofilaments in the Muscle Sarcomere
Abstract: During muscle contraction, the myosin motors emerging from the myosin filament in each half-sarcomere form cross-bridges with the opposing actin filament, pulling it towards the center of the sarcomere through a structural working stroke. Motors are mechanically coupled via their filament attachments, and the co-operative action of this coupled system is the basic functional unit of muscle, so determining the elastic properties of the half-sarcomere components is crucial for understanding the mechanism of this collective motor. To that end we used mechanical and X-ray diffraction measurements on single fibers and whole muscles of the frog. 4kHz length oscillations were imposed on single fibers (4°C, 2.15µm sarcomere length) to determine how the half-sarcomere compliance (Chs) is modulated by force. The results indicate the presence of an elastic element in parallel with the array of cross-bridges with a compliance of 200 nm/MPa, ca20 times larger than that attributed to the cross-bridges. X-ray diffraction from whole frog muscles allows precise measurements of the spacing of the high-order myosin- and actin-based reflections, which report changes in the strain of the two filaments. We found that, at forces >0.4 the isometric tetanic force, the filament compliance is constant and contributes 13.1±1.1 nm/MPa to Chs, from which a cross-bridge compliance of 0.35±0.13 nm/pN is calculated. This value is not significantly different from that of the myosin head alone, 0.38±0.06 nm/pN, estimated from changes in the intensity of the 14.5-nm X-ray reflection from the axial repeat of myosin heads during rapid length oscillations in rigor fibers. We conclude that cross-bridge compliance is low and fully accounted for by the compliance of the myosin head, without a significant contribution from the rod connecting the head to the filament backbone. Supported by MIUR-PRIN and FIRB (ITALY).