ATP-Induced Axial Movement of Myosin Heads in Living Thick Filaments Recorded with a Gas Environmental Chamber attached to the Electron Microscope

Using a gas environmental (hydration) chamber, in which biological specimens can be kept in wet state, we succeeded in recording images of ‘living’ muscle thick filaments with gold position markers attached to the myosin heads. The position of individual myosin heads did not change appreciably with time in the absence of ATP, indicating stability of the myosin head mean position. On application of ATP, the position of individual myosin heads was found to move by ∼20 nm along the filament axis, while no appreciable movement of the filaments was detected. The ATP-induced myosin head movement was not observed in filaments in which ATPase activity of the myosin heads was eliminated. Application of ADP produced no appreciable myosin head movement. These results show that the ATP-induced myosin head movement takes place in the absence of the thin filaments. Since ATP reacts rapidly with the myosin head (M) to form the complex (M ADP Pi) having average lifetime of >10 s, the observed myosin head movement may be mostly associated with reaction, M + ATP → M ADP Pi. This work will open a new research field to study dynamic structural changes of individual biomolecules which are kept in ‘living’ state in an electron microscope.