How does the actin-myosin system use the energy liberated by adenosine triphosphate (ATP) hydrolysis to slide or generate force? To answer this question, conformational changes of the myosin head have been extensively studied, but not yet clarified. The myosin molecule has been visualized in the electron microscope by rotary-shadowing or negative-staining techniques. It was found that the typical shape of myosin heads is straight and pear-shaped (Elliott and Offer 1978). On the other hand, when the myosin heads are bound to actin or in crystalline form, they have bent configuration, which was shown by the three-dimensional image reconstruction from electron micrographs(Toyoshima and T. Wakabayashi 1985; Winkelmann et al. 1991). These findings led to the question as to whether the shapes of heads can change. Many physicochemiacl studies have indicated that changes of the shape of the myosin head occur during hydrolysis of ATP. In contrast, previous X-ray change in the head structure on biding of an ATP analogue (Stone and Mendelson 1986) or on binding of an ATP analogue (Stone and Mendelson 1986) or on binding to 'invisible' F-actin (Curmi et al. 1988), suggesting that no significant change occures in the head during activity. In this chapter we present complementary evidence for a structural change in the myosin head (subfragment 1, S1) during active hydrolysis of MgATP by electron microcopy and X-ray small-angle scattering. An electron microscopic study using the shadowing technique (Tokunaga et al. 1991) showed that there are two types of the myosin head structure, a straight one and a bent one. Examination under various conditions suggests that the heads are in an equilibrium of the straight and bent shapes and that nucleotides shift the equilibrium. Small-angle synchrotron X-ray scattering studies (K. Wakabayashi et al. 1992) have revealed distinct conformational changes of the myosin head (S1) in solution during hydrolysis of ATP.
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