Tropomyosin-Troponin Assembly

Tropomyosin crystals illustrate dynamic interactions of this molecule in muscle (Cohen et al., 1971). The very properties which make these crystals biologically interesting, however, limit what can be seen by X-ray crystallography. The 400 Å long tropomyosin molecules are firmly bonded end-to-end to form an open mesh of cross-connected supercoiled filaments (Caspar et al., 1969). These filaments occupy only about 5% of the volume of the crystal, and their flexibility results in disorder of the crystal lattice. Moreover, the bonds which cross-connect the filaments are weak, and slight rotations at these bonds produce large changes in lattice dimensions. So far we have only been able to visualize the molecules in one projection to a resolution of about 20 Å. We can see changes in intermolecular interactions without being able to distinguish details of molecular conformation.