Which protein is the principal contractile component found in thin filaments?

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

Answer: (A)

Actin is the main contractile component of thin filaments. Thin filaments are built primarily from actin subunits that form long chains (F-actin) providing the binding sites for myosin heads during contraction. When the myosin heads attach to these sites and pull, the filaments slide past one another, producing force. The other proteins, tropomyosin and troponin, regulate whether those myosin-binding sites on actin are exposed in response to calcium, but they aren’t the force-generating parts. Myosin, found in thick filaments, is the motor protein that does the pulling. So, actin is the correct choice.