Tropomyosin: Crystal structure, polymorphism and molecular interactions

Abstract

The α-protein tropomyosin forms a variety of ordered aggregates. True crystals with a very open lattice and at least two other net forms are produced near the isoelectric point together with fibrous aggregates. Three distinctive types of tactoids with axial periodicities about 400 Å are formed with divalent cations. The electron microscope observations on the polymorphic forms have been related to X-ray diffraction measurements on the crystal lattice and the tactoids produced with magnesium. The X-ray pattern of one projection of the crystal has been interpreted at low resolution from a model composed of rod-shaped molecules arranged in accord with the electron micrographs. The tropomyosin molecules in the crystal are associated head-to-tail in polar filaments with a 400 Å period. The filaments are periodically bent as a consequence of the cross connections at two sites alternatively separated by about 230 Å and 170 Å in the 400 Å axial period. The principal conclusions of this study are: there is a specific polar end-to-end bonding of tropomyosin molecules which defines the period of about 400 Å observed in the polymorphic nets and tactoids; there are two cross-connecting sites which are involved in net formation; in many of the forms, the polar filaments are arranged in oppositely directed pairs; the molecular coiled-coil is often supercoiled in the polar filaments. The periodicity in the I band of muscle can be identified with the 400 Å repeat characteristic of the end-to-end association of tropomyosin. The polymorphism observed in vitro may be related to the structural and regulatory functions of tropomyosin in muscle.