Figure 10.

Actin filament structure and regulation: (a) An untwisted version of an F-actin filament, (b) coiled-coil strands of tropomyosin. (c) Troponin structures with and without Ca²⁺ bound. (d–f) The whole thin filament (untwisted) to illustrate the assembly of the three components in (a–c) and their changing configurations in different states: (d) Off state, no Ca²⁺. (e) Closed state, Ca²⁺ bound to troponin alters troponin which shifts tropomyosin strands to where some heads can attach. (f) With more attached heads the tropomyosin is pushed over further to the open state and yet more heads can then attach to actin. (d–f) adapted from Paul et al. [39], with permission.