Motions of tropomyosin. Crystal as metaphor

Abstract

Movements of tropomyosin play an essential role in muscle regulation. This fibrous protein is a two-chain alpha-helical coiled coil that bonds head to tail to form cables wound in the two long grooves of the actin helix. The regulatory switch consists of tropomyosin and a "globular" Ca2+-sensitive protein complex called troponin. The structure of the tropomyosin filaments has now been determined by x-ray crystallography to approximately 15 A resolution. The complete sequence of alpha-tropomyosin is known; by using mercury markers on the cysteine residues the ends of the molecules in the filaments have been identified. Details of the coiled-coil structure have also been visualized by refinement of models against the diffraction data. The average pitch of the coiled coil is approximately 137 A, so that each tropomyosin molecule can make similar contacts with seven actin monomers. The electron density map also indicates that departures from the alpha-helical coiled coil occur in a few localized regions of the molecule, especially at the overlapping ends. Motions of tropomyosin in the crystal lattice are displaced by the character of the Bragg reflections and the strong diffuse scatter. These effects depend markedly on temperature. It appears that the molecular filaments fluctuate freely in a direction perpendicular to their axes. Moreover, the C-terminal half of the molecule "unfolds" to some degree at less than physiological temperatures. Crystallographic results on co-crystals of tropomyosin and a component of troponin (TnT) suggest that this subunit consists of structurally distinct domains, so that the troponin complex is not in fact simply "globular". The interactions of the extended alpha-helical region of TnT may "stiffen" tropomyosin and influence its motions. We picture the tropomyosin/troponin switch in muscle as a restless cable, perpetually making and breaking bonds as it vibrates on the thin filament. These movements of tropomyosin probably depend on two aspects of its design: the regular pattern of coiled-coil linkages with actin; and the aperiodic features that allow flexibility and motion.