Troponin-tropomyosin complex

Interaction of the troponin-tropomyosin complex with the myosin binding site in the actin strands. Top panel: Interaction of the myosin with actin is physically blocked by the presence of the protein tropomyosin, which lies in the groove of the thin filament formed by the two F-actin strands, occupying the myosin binding site. Bottom panel: Calcium (Ca2+) binds to troponin C, leading to a conformational change which moves the tropomyosin molecule and makes the myosin binding site accessible to the myosin head. The other troponins also contribute to this process. Troponin T serves to attach the troponin complex to actin and tropomyosin, and troponin I is an inhibitory protein which inhibits the ATPase of actomyosin.