Contractile Proteins of Muscle

Contractile protein of muscle is:

Correct Answer: Both Actin and Myosin

Both actin (thin filaments) and myosin (thick filaments) are the contractile proteins of muscle. Muscle contraction occurs through the sliding filament theory, where myosin heads pull actin filaments toward the center of the sarcomere.

Muscle Proteins and Their Roles

• Actin: Thin filament; forms the I-band and part of A-band; contractile protein
• Myosin: Thick filament; forms A-band; contractile protein with ATPase activity (myosin head)
• Troponin C: Regulatory protein; binds Ca²⁺ (calcium-sensitive component)
• Troponin T: Binds to tropomyosin
• Troponin I: Inhibitory subunit
• Tropomyosin: Regulatory protein; lies in the groove of actin; blocks myosin-binding sites at rest

Sliding Filament Theory

Ca²⁺ binds Troponin C → Tropomyosin shifts → Myosin-binding sites on actin exposed → Myosin head (with ATPase) binds actin → Power stroke → Sarcomere shortens → Muscle contracts.