A protein of muscle that together with tropomyosin forms a regulatory protein complex and permits muscular contraction when combined with calcium ions?

• A. Troponin
• B. Tropomyosin
• C. Actin
• D. Myosin

Answer: (A)

Muscle contraction is controlled by the troponin-tropomyosin complex on actin filaments. When calcium ions rise, they bind to the troponin C subunit, causing the troponin-tropomyosin complex to shift and expose the myosin-binding sites on actin. This allows myosin heads to attach and perform the cross-bridge cycle, using ATP to generate force and drive contraction. Troponin is the regulatory protein that, with tropomyosin, responds to calcium to enable contraction. The other proteins don’t provide this calcium-dependent regulation on their own: tropomyosin blocks binding sites when calcium isn’t present, actin is simply the filament, and myosin is the motor protein that pulls, not the regulator.