Tropomyosin Crystal Dynamics

There are few protein crystals seriously studied which give poorer X-ray diffraction patterns than tropomyosin (Caspar et al., 1969). This protein is the archetype of the α-helical coiled-coil and is the only fibrous protein known to form macroscopic crystals. The fact that more than 95% of the lattice volume is water can account for the disorder and mosaic spread of the crystal. The very open form of the lattice results from cross-connected molecular filaments. The same bonding sites appear to be used in a variety of polymorphic nets formed by tropomyosin. The packing of tropomyosin in the fibrous aggregates obtained with divalent cations may be generated from net forms by aligning the molecular filaments. All these in vitro structures are built from polar filaments made up of 400 Å-long tropomyosin molecules bonded head-to-tail. In muscle, tropomyosin appears to form the same polar filaments which are coiled in the grooves of...