Which regulator binds calcium and interacts with actin to control contraction?

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

Answer: (A)

Contraction is controlled by a calcium-sensitive regulatory complex on actin. When calcium levels rise, calcium binds to troponin C, a subunit of the troponin complex attached to actin. This binding causes a change in the troponin complex that slides tropomyosin away from the myosin-binding sites on actin. With those sites exposed, myosin heads can form cross-bridges and the power stroke proceeds, producing contraction. In resting muscle, tropomyosin blocks those sites and prevents interaction. The other proteins, actin and myosin, are the contractile filaments themselves, not regulators. So the regulator that binds calcium and interacts with actin to control contraction is troponin.