Thin filaments are made of which protein?

• A. Myosin protein
• B. Actin protein
• C. Troponin
• D. Nebulin

Answer: (B)

Thin filaments in muscle fibers are built from actin, specifically filamentous actin (F-actin) that forms the long, thin backbone of the filament. This actin backbone provides the binding sites for myosin heads, enabling cross-bridge formation and force generation during contraction. Along the actin strand sit regulatory proteins like tropomyosin and troponin, which control access to those binding sites in response to calcium. Nebulin helps stabilize and set the length of the thin filament, but it isn’t the filament itself. Myosin is the motor protein that makes up the thick filament. So actin is the protein that constructs the thin filament.