Which regulatory proteins control actin filaments during muscle contraction?

• A. Myosin
• B. Tropomyosin and Troponin
• C. Titin
• D. Dystrophin

Answer: (B)

In skeletal muscle, the access of myosin to actin is regulated by the tropomyosin-troponin complex. In relaxed muscle, tropomyosin sits in the grooves of the actin filament and blocks the myosin-binding sites, preventing cross-bridge formation. When calcium levels rise, calcium binds to troponin (primarily troponin C), causing a shift in the troponin-tropomyosin complex that moves tropomyosin away from those binding sites. With the sites exposed, myosin heads can attach to actin and generate force through the cross-bridge cycle using ATP. The other proteins mentioned play important roles in contraction and structure—myosin is the motor that pulls, titin provides elasticity and alignment within the sarcomere, and dystrophin helps anchor the cytoskeleton to the cell membrane—but they do not regulate the exposure of actin’s myosin-binding sites. Therefore, tropomyosin and troponin are the regulators that control actin filament readiness for contraction.