Simplified schematic of actin backbones, shown as gray chains of actin molecules (balls), covered with smooth tropomyosin filaments. Troponin is shown in red (subunits not distinguished).

Calcium is required by two proteins, troponin and tropomyosin, that regulate muscle contraction by blocking the binding of myosin to filamentous actin. In a resting sarcomere, tropomyosin blocks ...