A protein that forms the regulatory complex with troponin regulating the interaction of actin and myosin by blocking cross-bridge formation unless troponin is combined with calcium ions?

• A. Tropomyosin
• B. Troponin
• C. Actin
• D. Myosin

Answer: (A)

Tropomyosin works with the troponin complex to control whether the binding sites on actin are available for myosin to attach. Tropomyosin runs along the grooves of actin filaments and blocks the myosin-binding sites when calcium isn’t bound. The troponin complex sits atop tropomyosin and, when calcium ions bind to troponin C, it causes a shift that moves tropomyosin away from those sites. That exposure lets cross-bridges form and muscles contract. Actin is the filament that provides the binding sites, myosin is the motor that forms the cross-bridges, and troponin is the regulator that responds to calcium—together with tropomyosin. So the protein that forms the regulatory complex with troponin is tropomyosin.