Figure 5. Tropomyosin in the B and M state.

(A) Top views along the filament axis at the three positions indicated by vertical markers in (B). Upon myosin binding, the position of tropomyosin is rotated azimuthally by ca. 30° as indicated by the arrows. The density of F-actin with tropomyosin was calculated at 8 Å resolution based on the tropomyosin model in its B-state⁴³. In addition to the rotation, the radial position of tropomyosin is reduced from 43 Å to 40 Å. Scale bar, 5 nm. (B) Side view orthogonal to the filament axis. The vectors defined by the positions of the Cα atoms of two pseudorepeats are depicted as black bars. The angle between the tropomyosin filament and the actin filament remains unchanged upon myosin binding at ~20°. Scale bar, 2.5 nm (C) Overlay of the two states depicted in (B) with actin and myosin faded. Displacement of tropomyosin can be described as a shift along the surface of the actin filament with additional lateral movement. MyoE, tropomyosin and actin are salmon, light green and blue, respectively. Scale bar, 2.5 nm.