What is actin?

• A. Thick filaments
• B. Thin filaments
• C. Nerve tissue
• D. A protein

Answer: (B)

Actin is the main protein that makes up the thin filaments in muscle fibers. In each sarcomere, thin filaments are built from actin polymers arranged in a helical structure, and they work together with regulatory proteins (tropomyosin and troponin) to control contraction. During muscle shortening, thin filaments slide past thick filaments made of myosin, and cross-bridges form as myosin heads bind to actin sites. While actin is indeed a protein found in many cells, its specific role in muscle is as the component of thin filaments, not the thick filaments or other tissues. Hence actin is best described as thin filaments.