Electron microscopic visualization of the cross-bridge movement coupled with ATP hydrolysis in muscle thick filaments in aqueous solution, reminiscences and future prospects.

Although it has been well established that muscle contraction results from cyclic attachment-detachment between the cross-bridges extending from the thick filaments and the sites on the thin filaments, the movement of the cross-bridges coupled with ATP hydrolysis still remains to be a matter for debate and speculation. The most straightforward way to elucidate this mystery is to record individual cross-bridge movement in response to ATP. Using a gas environmental chamber (EC, or hydration chamber), with which biological specimens retaining their physiological function can be observed under an electron microscope, my coworkers and I succeeded in recording the ATP-induced individual cross-bridge movement in two different kinds of synthetic thick filaments in 1997 and 2008. In the synthetic bipolar filaments consisting of rabbit skeletal muscle myosin, the amplitude of cross-bridge movement exhibits a peak at 5-7.5 nm, and the direction of cross-bridge movement is away from, but not towards, the filament bare region in the absence of thin filaments. After exhaustion of ATP, the cross-bridges return towards their initial position, indicating that the initial cross-bridge state may be analogous to that after completion of power stroke. These results constitute the first visualization of the cross-bridge recovery stroke, indicating that the EC is a powerful tool to open new horizons in the research fields of life sciences.