In smooth muscle, which molecule forms a complex with calcium to activate MLCK?

• A. Troponin C
• B. Myosin light-chain phosphatase
• C. Calmodulin
• D. Actin-tropomyosin

Answer: (C)

Calcium signaling in smooth muscle uses a dedicated calcium-binding protein as the sensor. When calcium rises, it binds to calmodulin, forming a Ca2+-calmodulin complex that activates myosin light-chain kinase (MLCK). This activation leads MLCK to phosphorylate the regulatory light chains of myosin, enabling myosin to interact with actin and power contraction. Relaxation happens as calcium falls, the Ca2+-calmodulin complex dissociates, MLCK activity decreases, and myosin light-chain phosphatase removes the phosphate, returning the system to a relaxed state. Troponin C, found in skeletal and cardiac muscle, isn’t the smooth muscle sensor; myosin light-chain phosphatase isn’t the calcium sensor but the enzyme that promotes relaxation; and actin-tropomyosin are structural filaments, not calcium sensors.