Which of the following is one of the proteins found in the thin myofilament?
A. Myosin C. Actin B. Titin D. Dystrophin
The correct answer and explanation is:
Correct Answer: C. Actin
Explanation:
Muscle fibers are composed of repeating units called sarcomeres, which are the functional units of contraction. Each sarcomere contains two main types of myofilaments: thin and thick. These filaments are responsible for muscle contraction through their interaction.
The thin myofilament is primarily composed of three proteins:
- Actin – This is the main structural protein in thin filaments. It forms a double-helical chain that provides the binding sites for the myosin heads during muscle contraction. Actin interacts with myosin to generate the force needed for contraction.
- Tropomyosin – This is a regulatory protein that wraps around actin filaments and covers the myosin-binding sites on actin when the muscle is relaxed.
- Troponin – This complex binds to tropomyosin and actin, and it has a calcium-binding site. When calcium ions bind to troponin, it causes a conformational change that shifts tropomyosin away from the myosin-binding sites on actin, allowing contraction to occur.
In contrast, myosin is the main protein of the thick filaments, not thin. It has a long tail and a globular head, which binds to actin and ATP to generate muscle movement. Titin is a giant elastic protein that helps anchor the thick filaments to the Z-line and contributes to muscle elasticity and structural integrity. Dystrophin is a structural protein that connects the cytoskeleton of muscle cells to the surrounding extracellular matrix, but it is not part of the thin or thick filaments.
Therefore, actin is the correct answer as it is a primary component of the thin myofilament and plays a central role in muscle contraction by directly interacting with myosin during the sliding filament process.