[Direct proof of the existence of Ca2+-induced structural changes in miosin-containing thick filaments of vertebrate skeletal muscles].

Frog sartorius muscles were skinned with the help of detergent Triton X-100 in relaxing solution and then rigorized in the solution free of ATP and calcium (Ca2+-free rigor). Some of the X-ray diffraction patterns obtained from such muscles differed drastically from the patterns of usual "calcium" rigor state of the muscle. Analysis of the X-ray diffraction patterns of Ca2+-free rigor state of the muscle allows the following conclusions to be made. (1) It is proved that the so-called forbidden meridional reflections localised in groups at the successive orders of the repeat period of about 430 A are due to diffraction on the lattice formed by minor proteins of the thick filaments. This must be taken into account while interpreting the intensity changes of the meridional 143 and 215 A reflections in the contracting muscle. (2) The shafts of the myosin-containing filaments may exist in two different structural states; the transition between these states takes place on the change of calcium concentration. (3) The deflection of myosin cross-bridges from the shafts of the filaments is possible at either of two conformations of the myosin shafts. (4) The formation of actomyosin rigor-complexes does not necessarily leads to a structural change in the myosin filament. (5) Existence of Ca2+-induced structural changes in the thick filaments indicates possible myosin-linked regulations in the vertebrate skeletal muscle alongside with the actin-linked ones.