AN ATOMIC MODEL OF THE UNREGULATED THIN FILAMENT OBTAINED BY X-RAY FIBER DIFFRACTION ON ORIENTED ACTIN-TROPOMYOSIN GELS

We present a model of the actin-tropomyosin complex in which the radia l and azimuthal position of tropomyosin was adjusted to fit the X-ray fiber diffraction patterns from oriented actin-tropomysin gels at a re solution of 1/8 Angstrom(-1). We used the recently published atomic F- actin model for the calculations. The atomic model of tropomyosin was obtained by model-building a coiled coiled-coil structure from the tro pomyosin sequence. The resulting atomic model is strongly preferred an d shows strong electrostatic interactions between charged side-chains of tropomyosin residues and actin residues in subdomain 3 and subdomai n 4. Furthermore, calculations of enthalpies based upon electrostatic interactions indicate that there is a favored rotational position of t he tropomyosin core at the calculated azimuthal and radial position gi ven by the X-ray refinement. Rotations of the tropomyosin strand out o f this position turn strongly attractive electrostatic interactions in to repulsive forces. The resulting binding radius of 39 Angstrom and t he determined azimuthal position of tropomyosin are in good agreement with electron microscopy reconstructions and neutron diffraction exper iments. Furthermore, the calculated position of tropomyosin would stil l partly block the rigor interaction of myosin cross-bridges with acti n, whereas it very likely allows undisturbed binding of the cross-brid ges in a weak binding state.