The thin filament is the actin-based protein polymer that interacts with myosin to generate muscle contraction. The thin filament contains different actin-associated proteins in striated muscle and in smooth muscle. In striated (skeletal or cardiac) muscle it is interdigitated between the myosin-based thick filaments in a highly ordered lattice. It is approximately 100 A in diameter (see Microfilament) and can have a length of ~110 |im. The striated muscle thin filament contains the regulatory proteins tropomyosin and troponin in addition to actin. X-ray fiber diffraction has been used to generate an atomic model of the actin-tropomyosin component of the thin filament (1). Skeletal striated muscle thin filaments also contain the giant molecule nebulin, which is believed to function as a regulator of thin filament length (2). Most thin filaments have a defined stoichiometry of one tropomyosin coiled-coil dimer and one troponin complex for every seven actin subunits. The troponin complex contains three subunits: troponin C (for calcium-binding), troponin I (for inhibitory), and troponin T (for tropomyosin-binding). The release of Ca by the sarcoplasmic reticulum of striated muscle leads to the binding of Ca by troponin T, and then to an azimuthal rotation of the tropomyosin-troponin complex about the actin filament. This unblocks the myosin-binding site on the surface of the actin filament, as proposed in the "steric-blocking" model for thin filament regulation (3). Recent evidence has suggested that the initial binding of myosin to F-actin is required for an additional shift of the tropomyosin-troponin complex that leads to full activation of the thin filament (4).
Within smooth muscle, the thin filament contains the actin-binding proteins caldesmon and calponin, in addition to tropomyosin. The regulatory role of these accessory proteins in smooth muscle contraction (where the main regulation resides on the myosin filaments) is much less understood than it is for striated muscle thin filaments, and structural studies have suggested that the roles of calponin (5) and caldesmon (6) in smooth muscle cannot be the same as that played by troponin in striated muscle.
