STRUCTURAL-CHANGES IN ACTIN-TROPOMYOSIN DURING MUSCLE REGULATION - COMPUTER MODELING OF LOW-ANGLE X-RAY-DIFFRACTION DATA

The crystal structure of G-actin monomer has been used together with tropomyosin in a filament model to explain the low-angle X-ray diffraction data from relaxed and activated actin filaments. The four-subdomain actin monomer can be approximated quite well by a four-sphere unit. Orienting this unit and tropomyosin into a filament by searching for the best fit between the computed Fourier transform and the observed vertebrate skeletal muscle low-angle actin layer-lines from muscles at non-overlap sarcomere lengths produced models for the structural changes within the thin filaments (actin plus tropomyosin) between the resting state and the active states, which occur as a result of calcium-activation and independent of myosin interaction with actin. The models are very sensitive to changes in the positions of the centres of mass of the subdomains, but not to the exact shape of the objects used to represent them (e.g. spheres, ellipsoids etc.), as long as the volume is fixed, at the resolution here considered. It is concluded that, even with a four-subdomain structure for the actin molecules, the observed low-angle X-ray diffraction patterns cannot be explained without a substantial azimuthal swing of the tropomyosin strands when resting filaments are calcium-activated. The direction of this swing upon calcium-activation is away from a position close to the proposed major binding site of the myosin head on actin; a result consistent with the original ''steric blocking model'' of thin filament-based regulation in which the tropomyosin position on actin is crucial for regulation of the myosin crossbridge cycle on actin. Tropomyosin sterically hindering myosin attachment in the ''off'' state remains a possibility However, even in the ''on'' state, the tropomyosin position is close enough to the myosin-binding site to have an effect, where it could regulate the transition of the head from a weak to a strong state. In addition to this tropomyosin movement there are small, but plausible, actin subdomain movements. A tropomyosin shift on its own will not explain the data. Allowance for possible movement of actin subdomain 2 along with the tropomyosin shift still does not explain the data. An additional small movement of subdomain 1, the main myosin-binding subdomain, is postulated. (C) 1995 Academic Press Limited