DYNAMIC ELECTRON-MICROSCOPY OF ATP-INDUCED MYOSIN HEAD MOVEMENT IN LIVING MUSCLE THICK FILAMENTS

Although muscle contraction is known to result from movement of the my osin heads on the thick filaments while attached to the thin filaments , the myosin head movement coupled with ATP hydrolysis still remains t o be investigated, Using a gas environmental (hydration) chamber, in w hich biological specimens can be kept in wet state, we succeeded in re cording images of living muscle thick filaments with gold position mar kers attached to the myosin heads. The position of individual myosin h eads did not change appreciably with time in the absence of ATP, indic ating stability of the myosin head mean position. On application of AT P, the position of individual myosin heads was found to move by approx imate to 20 nm along the filament axis, whereas no appreciable movemen t of the filaments was detected, The ATP-induced myosin head movement was not observed in filaments in which ATPase activity of the myosin h eads was eliminated, Application of ADP produced no appreciable myosin head movement, These results show that the ATP-induced myosin head mo vement takes place in the absence of the thin filaments, Because ATP r eacts rapidly with the myosin head (M) to form the complex (M.ADP.P-i) with an average lifetime of >10 s, the observed myosin head movement may be mostly associated with reaction, M + ATP --> M.ADP.P-i. This wo rk will open a new research field to study dynamic structural changes of individual biomolecules, which are kept in a living state in an ele ctron microscope.