Electron Microscopic Studies of Myosin Molecules from Chicken Gizzard Muscle II: The Effect of Thiophosphorylation of the 20K-Dalton Light Chain on the ATP-Induced Change in the Conformation of Myosin Monomers1

The conformation of thiophosphorylated myosin molecules of chicken gizzard muscle was studied by electron microscopy with the rotary shadowing technique and by the light scattering method. In the absence of ATP, the radius of gyration (RG) of gizzard thiophosphorylated myosin was 478 A, and was essentially equal to that (474 A) of unthiophosphorylated myosin. In the presence of ATP, it was 355 A, and was much larger than that (146 A) of unthiophosphorylated myosin. In the presence of ATP, 84 percent of the unthiophosphorylated myosin molecules had intramolecular loops at their tails, but only 23 percent of the thiophosphorylated myosin molecules had them. There were two flexible regions in the unthiophosphorylated myosin tail. The considerable flexibility at both regions remained even when the light chain was thiophosphorylated. The two globular heads of the unthiophosphorylated myosin molecules had a tendency to bend back towards the tail in the presence of ATP, but this tendency was reduced by the light chain thiophosphorylation. The myosin molecules with "looped" tails were mostly, if not all, in one of two mirror-image forms, and the mirror-image asymmetry was independent of the thiophosphorylation.