The Structural Role of Tropomyosin in Regulating Thin Filament Activation of Actin-Myosin Interaction.

Abstract

Myosin motor-heads projecting from muscle thick filaments interact cyclically with actin-based thin filament tracks, thereby driving inter-filament sliding that powers muscle contraction. Here, controlled recruitment of myosin heads from thick filaments leads pre-powerstroke myosin to bind weakly to actin. Myosin then isomerizes into strongly bound post-powerstroke conformations on actin, thus producing crossbridge motion in active muscles. In striated muscles, this process is regulated by a steric mechanism involving coiled-coil tropomyosin controlling access to myosin-binding sites on actin. Biochemical and structural studies suggest the regulatory mechanism involves tropomyosin occupying three average configurations on the actin thin filament, dependent on Ca²⁺, troponin, and myosin binding. Once Ca²⁺-activation of muscle occurs, tropomyosin pivots away from a troponin subunit-I (TnI) imposed B-state (myosin-blocking) position to a C-state position on actin, allowing initial weak myosin-binding to actin. The thin filament reconfiguration only partially relieves tropomyosin-troponin imposed steric inhibition of the myosin binding. However, the initial weak myosin-binding causes further tropomyosin translocation to an M-state position as myosin transitions from its pre-powerstroke to its post-powerstroke conformation, thereby fully activating the thin filament and resulting in contraction. This review summarizes the evolving structural evidence that has accumulated over many years, and which has shaped our current understanding of the troponin-tropomyosin steric regulatory mechanism that governs muscle contractility.