INTERPRETATION OF THE X-RAY-DIFFRACTION PATTERN FROM RELAXED SKELETAL-MUSCLE AND MODELING OF THE THICK FILAMENT STRUCTURE

The first part of this paper is devoted to the model-building studies of our high resolution meridional X-ray diffraction patterns (in the region from 11500 to 1150 angstrom-1) obtained from relaxed frog muscle. A one-dimensional model of thick filament was proposed which basically consists of two symmetrical arrays of 50 crossbridge crown projections. In the proximate and central zones of the filament the crossbridge crowns are regularly shifted with a 429 angstrom period and appear as triplets with a 130 angstrom distance between crowns, while the crowns in the distal parts of filament are regularly ordered with a 143 angstrom repeat. The centre-to-centre distance between regions with crossbridge perturbations is 7050 angstrom. The length of each crown projection is about 125 angstrom. The model includes also (1) C-protein component represented in each half of the filament by seven stripes of about 350 angstrom long and located 429 angstrom apart, (2) a uniform density of filament backbone of about 1.5-mu-m length, and (3) 13 high density stripes in a central zone located with 223 angstrom period. The final model explains very well the positions and intensities of the main meridional reflections. A three-dimensional model of crossbridge configuration is described in the second part of the work. The model was constructed by using the intensity profiles of the first six myosin layer lines of the X-ray pattern from stretched muscle and taking into account the crossbridge perturbations and the axial size of crossbridge crown obtained from the one-dimensional studies. It was found that both myosin heads are tilted in opposite directions along the filament and wrap around the filament backbone. This crossbridge configuration corresponds well to the findings of Haselgrove. To take into account interference effects between thick filaments we proposed a model of the filament hexagonal lattice with disorder of the second kind. The cross-sectional width of crystalline domains in stretched muscle was estimated to be about 0.11-mu-m. The models presented may be helpful in the interpretation of the X-ray diffraction patterns from contracting muscle.