Which regulatory proteins control access to myosin-binding sites on actin?

• A. Tropomyosin and troponin
• B. CapZ and Tropomodulin
• C. Dystrophin and GPC
• D. Myosin and actin

Answer: (A)

Access to the myosin-binding sites on actin is controlled by the troponin–tropomyosin complex. In resting muscle, tropomyosin sits in the grooves of the actin filament and covers the myosin-binding sites, preventing cross-bridge formation. When calcium levels rise, calcium binds to troponin C, causing a shift in the troponin-tropomyosin complex that moves tropomyosin away from those sites. That unblocks the sites, allowing myosin heads to bind to actin and generate contraction. The other proteins mentioned don’t regulate site exposure: CapZ and Tropomodulin cap filament ends; dystrophin and glycoprotein complex link the cytoskeleton to the membrane; and myosin and actin are the binding partners, not regulators of site accessibility.