Tropomyosin

Tropomyosin is an elongated, filament-forming protein that lies along the surface of actin filaments. In skeletal and cardiac muscle, it is an essential component of the thin filament, together with actin and the troponin complex. Tropomyosin molecules consist of two alpha-helical protein chains wrapped around each other in a coiled-coil structure and arranged end to end along the actin filament .^1,2

In striated muscle, tropomyosin helps regulate whether myosin can interact productively with actin. When a muscle is relaxed and intracellular calcium is low, interactions between troponin and tropomyosin position tropomyosin so that it inhibits productive binding of myosin heads to actin. This helps prevent force-generating cross-bridge cycling .^1,2

When calcium concentration rises during muscle activation, calcium binds to troponin C. The resulting structural changes within the troponin complex alter the position of tropomyosin on actin, allowing myosin heads greater access to actin. Myosin binding can then shift tropomyosin further toward an activated position, promoting additional actin-myosin interactions and contraction .^1,2

Tropomyosin should not be confused with troponin. Tropomyosin is the elongated protein that extends along the actin filament, whereas troponin is a three-protein regulatory complex associated with the thin filament. Troponin C senses calcium and the troponin complex communicates this signal to tropomyosin, while tropomyosin directly helps regulate myosin access to actin .^1,2