Structure and structural change of the myosin head.

The ATPase site of myosin was located by three-dimensional electron microscopy using the avidin-biotin system. The site is about 5 nm from the tip of the myosin head, about 4 nm apart from the actin-binding site of myosin. Other functional sites on the myosin head were located by electron microscopy with the avidin-biotin system, monoclonal antibodies and site-directed antibodies. These findings enable us to estimate the domain structure of the head. The shape of the myosin heads was examined by electron microscopy using rotary-shadowing and uni-directional shadowing technique, and two types were seen: a straight one and a bent one. Bending occurs at 12 +/- 2 nm from the head-rod junction. This location corresponds with the images by three-dimensional electron microscopy and electron micrography of the crystal. The bending region locates at the boundary of domains. Bent heads increase in the presence of ADP-Vi. The location and angle of bending were almost the same under all the conditions examined. These findings suggest that the heads of straight and bent shapes are in equilibrium, and that ADP-Vi shifts the equilibrium to the bent shape. The bending of the myosin head may play an important role in the molecular mechanism of muscle contraction.