Optical anisotropy decay studies of the dynamic structure of myosin filaments

We applied flash-induced absorption and phosphorescence anisotropy decay methods to the study of rotational motions of myosin heads in solution), myofibrils and muscle fibers); myosin heads were selectively labeled with a triplet probe EMI (5-eosinylmaleimide). EMI-labeled subfragment 1 (S1) showed a single exponential decay of anisotropy over two decades; the analysis indicated that if S1 is modeled as a prolate ellipsoid of revolution, the major axis was 16-17 nm and the minor axis 4.7-4.5 nm. The decay curve of myosin filaments could be simulated by double-exponentials-plus-constant approximation. The data could be analyzed by a double-cone model), in which we assumed that a head part (S1), wobbles in the first cone and a rod portion next to the head also wobbles in the second cone. The semiangle of each cone was estimated to be 35 and 48 degrees, respectively. We found that myosin heads in myofibrils under relaxing conditions extensively rotated as in myosin filaments in solution. When the spacing between thick and thin filaments was artificially reduced by the increase of osmotic pressure with the addition of polyvinylpyrrolidone (PVP), restriction of the angular range of the rotational motion was observed. Under rigor conditions no motion was observed in a 10 microsecond time scale, indicating that the heads were immobilized by binding to thin filaments. Preliminary results on the rotational motions of myosin heads in muscle fibers are also reported.