Figure 1.

Models of thin filament from striated muscles. A. Helical arrangement of the globular protomers (monomers) in an actin filament.¹⁴⁾ The protomers are drawn as spheres. The centers of the protomers are helically arranged as shown in the lower panel, in which a short-pitched left-handed helix is drawn in a dotted line. The crossover points of the long-pitched right-handed double helix are indicated by arrows. There are 13 protomers (monomers) per crossover. B–C. The models of actin-tropomyosin-troponin proposed by the Ebashi group.^21–25) In the initial model (B),^21,23) troponin was drawn as a sphere for simplicity. In the model refined by Ohtsuki (C), the spatial relationship between the end of tropomyosin molecule and the troponin-binding site of tropomyosin was incorporated, and the relative positions of troponin molecules on the opposite sides of actin double helix were assumed to follow the helical symmetry of actin and stagger by 27.3 Å.^24,25) The 27.3 Å staggering of troponin was shown experimentally,⁸⁴⁾ and its detail is described in the section 4.