Thin filaments are composed of which combination of proteins?

• A. Actin, Troponin, Tropomyosin, Nebulin
• B. Myosin, Actin, Titin, Dystrophin
• C. Actin, Myosin, Tropomyosin, Nebulin
• D. Actin, Tropomyosin, Myosin, Titin

Answer: (A)

Thin filaments are built around actin, with regulatory proteins that control when myosin can bind. Tropomyosin sits along the actin filament, and the troponin complex attaches to tropomyosin to regulate access to the binding sites in response to calcium. Nebulin is a large structural protein that runs along the length of the thin filament, helping to set and stabilize its length. Put together, actin, tropomyosin, troponin, and nebulin describe the components typically associated with thin filaments.

Proteins such as myosin and titin are associated with thick filaments or broader sarcomere structure, and dystrophin links the cytoskeleton to the cell membrane—so they aren’t part of the thin filament composition.