Which protein binds calcium and, together with tropomyosin, regulates the interaction of actin and myosin?

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

Answer: (A)

Calcium regulation of the thin filament is being tested. Calcium binds to the troponin complex, specifically to the troponin C subunit. When calcium attaches there, the troponin-tropomyosin system changes shape so that tropomyosin moves away from the myosin-binding sites on actin. That exposes those sites and allows myosin heads to attach to actin and perform the power stroke, initiating contraction. This is why the troponin complex is the calcium-binding regulator that works with tropomyosin to control actin–myosin interaction. Tropomyosin alone sits along actin and blocks binding sites until troponin moves it; actin and myosin themselves are the filaments that generate force but do not regulate binding in response to calcium.