Different Orientation of Two Heads of a Myosin Crossbridge in Full-Filament Overlapped and Overstretched Muscles Obtained by X-Ray Fiber Diffraction

A novel method using the cylindrically averaged difference Patterson function was applied to correct a sampling effect due to the hexagonal filament array on the thick filament-based layer-line intensities from frog skeletal muscles at the full-filament overlap length. Using the corrected intensity data and the mixed structural model of a thick filament with two different axial periodicities of the myosin crossbridges, we performed an optimum search of azimuthal orientation of two heads of a myosin crossbridge and compared the optimum orientation to that from muscles stretched beyond filament overlap reported previously. The result showed that the myosin crossbriges in the regular repeating region had a similar configuration in both muscles. Two heads of a myosin crossbridge formed a windmill-shape when seen from the top of the filament and one head of a myosin crossbridge seemed to be almost in contact with another head in a pair at an adjacent crown level along the filament axis. One head was toward the converter domain of the other head, similar to regulated myosin heads in Tarantula muscles in which the intramolecular head-head interaction occurs. In the perturbed region, however, myosin crossbridges had different configurations in these muscles. In top view, two heads of a myosin crossbridge showed a U-shape structure in the overstreched muscles while a cross-shape structure in muscles with the full-filament overlap. One myosin head seemed to be in contact with the other head at the same axial crown level. The models suggest that the disposition of two-headed myosin crossbridges is stabilized by the head-head interaction at same or different axial crown levels. Probably this would be related to the inhibition mechanism of actomyosin interaction in the relaxed muscles.