Which protein forms the thin filaments in muscle fibers?

Which protein forms the thin filaments in muscle fibers?

• A. Myosin
• B. Titin
• C. Actin
• D. Nebulin

Answer: (C)

Thin filaments are built primarily from actin. Actin monomers assemble into filamentous actin (F-actin), forming the backbone of the thin filament along the length of the sarcomere. Regulatory proteins sit on this actin scaffold—the tropomyosin-troponin complex—so that calcium can regulate whether myosin heads on the thick filament can bind. When calcium is present, the troponin-troopmyosin complex moves to expose myosin-binding sites on actin, allowing contraction to proceed. In contrast, the thick filaments are made mainly of myosin, and other proteins like titin provide elasticity and structural anchoring while nebulin helps set the length and alignment of the thin filaments. So actin is the protein that forms the thin filaments.

Thin filaments are built primarily from actin. Actin monomers assemble into filamentous actin (F-actin), forming the backbone of the thin filament along the length of the sarcomere. Regulatory proteins sit on this actin scaffold—the tropomyosin-troponin complex—so that calcium can regulate whether myosin heads on the thick filament can bind. When calcium is present, the troponin-troopmyosin complex moves to expose myosin-binding sites on actin, allowing contraction to proceed. In contrast, the thick filaments are made mainly of myosin, and other proteins like titin provide elasticity and structural anchoring while nebulin helps set the length and alignment of the thin filaments. So actin is the protein that forms the thin filaments.