Which proteins regulate the interaction between actin and myosin by controlling access to binding sites on actin?

• A. Tropomyosin and the troponin complex (troponin C, I, T)
• B. Titin
• C. Nebulin
• D. Dystrophin

Answer: (A)

Access to actin’s myosin-binding sites on the thin filament is controlled by tropomyosin and the troponin complex. Tropomyosin runs along actin filaments and, when the muscle is relaxed, blocks the binding sites that myosin would grab onto. The troponin complex—made up of troponin C, I, and T—binds calcium. When calcium levels rise, calcium binds troponin C, causing a shape change that moves tropomyosin away from the binding sites, exposing them so myosin heads can attach and generate a contraction. When calcium falls again, the troponin-tropomyosin arrangement returns to block the sites, and relaxation occurs. Other proteins listed have different roles in the muscle cell—titin provides elasticity and alignment within the sarcomere, nebulin helps determine thin filament length, and dystrophin links the cytoskeleton to the extracellular matrix.