The position of tropomyosin in muscle thin filaments

Abstract

The contraction of vertebrate skeletal muscle is controlled by the action of Ca²⁺ on muscle thin filaments. At low Ca²⁺ concentrations (<10⁻⁶M) the regulatory proteins of the thin filament, tropomyosin and troponin, relax muscle by preventing the interaction of myosin and actin. At higher Ca²⁺ levels this inhibition is removed, when Ca²⁺ binds to troponin. Tropomyosin, a long coiled-coil α-helical molecule, is located in each of the two long-pitch helical grooves of actin, but troponin, a globular molecule, is attached at intervals of 38 nm (refs 1, 2). By combining evidence from X-ray diffraction studies of muscle and from electron microscopy, several authors have proposed that tropomyosin moves to block or allow attachment of myosin heads to actin^3–5. We investigate here an alternative way of combining the data, which if valid may have important consequences for our understanding of the regulation of muscle contraction⁶.