Dynamic X-ray diffraction of skeletal muscle contraction: structural change of actin filaments.

The X-ray diffraction pattern recorded during contraction shows that the force generation of a muscle proceeds upon interaction of the actin and myosin heads in the incommensurate structural framework of the thin and thick filaments. In this molecular framework the binding of myosin heads to actin filaments is thought to occur on a random basis. Such an actomyosin structure would not produce constructive interference between scattered X-rays from the bound heads and the thin filaments. The characteristic intensity changes of the thin filament layer lines that occur during contraction suggest strongly that the actin structure is varied by interaction with the myosin heads, in a manner that is quite different from that in the rigor state. Variability of the time courses of the intensity changes of the various layer lines indicates that structural change within the thin filament does not take place uniformly and that some different structural processes are involved during contraction At the plateau of isometric contraction, the thin filament structure as a whole assumes a more four-stranded nature due to the changes in the actin structure and tropomyosin position. Our present results imply that the changes of actin structure induced by interaction with myosin heads would be responsible for the regulation as well as force generation in muscle contraction.