Which regulatory protein binds to actin and calcium to regulate contraction?

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

Answer: (B)

Calcium regulation of contraction on the thin filament involves the troponin-tropomyosin complex. When calcium levels rise, calcium binds to the troponin C subunit of the troponin complex, causing a shape change that moves tropomyosin away from the myosin-binding sites on actin. This exposure allows myosin heads to bind actin and perform power strokes, leading to contraction. The troponin complex is the regulatory protein here because it directly senses calcium and communicates with the actin-tropomyosin arrangement to control access to the binding sites.

Tropomyosin, while it sits along actin and blocks binding sites when calcium is low, does not respond to calcium itself. Actin is the filament that serves as the track for myosin, not the regulator of contraction. Myosin is the motor that pulls on actin to generate force, not the regulator that responds to calcium.