X-Ray Evidence for a Conformational Change in the Actin-containing Filaments of Vertebrate Striated Muscle

The control of vertebrate striated muscle from rest to activity is regulated by the combination of the regulatory proteins tropomyosin and troponin on the thin actin filament (see reviews by Ebashi and Endo, 1968; Ebashi et al., 1969). In the absence of calcium the regulatory proteins inhibit the actin-myosin interaction. Activation of muscle is caused by calcium binding to troponin which then removes the inhibition. Since there is only one troponin molecule for evmy seven actin molecules (Weber and Bremel, 1971) and troponin does not interact with actin, then the control of activation is pre-sumably mediated by the tropomyosin molecules which run along the grooves in the actin filaments (Hanson and Lowy, 1963; Moore et al., 1971). I t is, therefore, of importance to investigate any structural changes that occur in the thin filaments when actin-myosin interaction is allowed to take place, for such changes may give us information about he control mechanism. X-ray diffraction for the study of muscle has the great advantage that the structure can be studied at the molecular level without destruction of the specimen. The low-angle X-ray pattern (corresponding to spacings above about 20A) from vertebrate striated muscle consists of layer lines and meridional reflections that arise both from the myosin filaments and from the actin filaments (Huxley and Brown, 1967), so it is possible, by observing changes in the low-angle diffraction pattern, to follow changes occur-ring in the thin filament structure when the muscle contracts or passes into rigor. The great disadvan-tage of X-ray diffraction studies on muscle is that they do not give information directly about the muscle structure; the structure must first be as-sumed from other studies and the structure can be accepted if it gives a diffraction pattern like the observed pattern. The earlier studies of the muscle low-angle X-ray diffraction patterns (Elliott et al., 1967; Huxley and Brown, 1967) showed the gross structure of the thin filaments was remarkably insensitive to the state of the muscle. The axial period of the helical