Distribution of mass in relaxed frog skeletal muscle and its redistribution upon activation

Five orders of equatorial reflection were recorded from both relaxed and fully activated intact frog sartorius muscle using synchrotron x-ray radiation. Electron density maps of the myofilament lattice in axial projection were calculated from the integrated intensities by Fourier synthesis, using all possible phase combinations. These maps were evaluated systematically in terms of their compatibility with electron microscopically and biochemically derived properties of the lattice structure and with the minimum wavelength principle. For the relaxed state, one phase combination emerged as most consistent with these constraints: it shows a thick filament with a compact core surrounded by an annular shell of density. The distribution of mass suggests that the S-2 moiety of the myosin molecule is an integral part of the thick-filament backbone and the S-1 moiety makes up the shell and is tilted or slewed around the backbone. For the active state, there are two feasible maps, which differ according to whether or not the activation process is associated with phase inversion in two of the reflections. Both maps represent patterns of redistribution of mass upon activation in which the thick-filament backbone is practically unaffected and there is movement of density from the annular shell towards the thin filaments. In addition to this outward radial flux of density from the thick-filament periphery, the pattern of net mass transfer involves a pronounced azimuthal component in both cases. The total net mass transfer is equivalent to approximately 20% (no phase change) or approximately 40% (with phase change) of the S-1 mass. From the observed systematic increase in peak widths of the higher orders, the size of the crystalline domain in the myofilament lattice in the relaxed sartorius is estimated to be greater than 650 nm and the variations in myofilament lattice spacing among different myofibrils to be about +/- 3%. Furthermore, in the activated state, the equilibrium positions of the myofilaments are no longer well ordered, but are distributed statistically about the lattice points with a standard deviation of approximately 3 nm.