STRUCTURAL DIFFERENCE BETWEEN RESTING AND RIGOR MUSCLE - EVIDENCE FROM INTENSITY CHANGES IN LOW-ANGLE EQUATORIAL X-RAY DIAGRAM

A large difference is observed in the relative intensities of the 10 and 11 equatorial X-ray reflections when low-angle diagrams from living striated muscles are compared with those from muscles in rigor at the same sarcomere length. The change indicates that a substantial amount of material (approximately 30% of the original mass of the myosin) moves from the vicinity of the thick filaments (situated at the lattice points of the hexagonal array) to the vicinity of the actin-containing filaments (situated at the trigonal points of the lattice). This finding has been confirmed by conventional electron microscopy and also by optical analysis of the patterns seen in the micrographs. This shift of material is interpreted in terms of the properties of the cross-bridges on the myosin filaments, which are believed to represent the heavy meromyosin part of the myosin molecules. If, in live relaxed muscle, the cross-bridges were not in contact with the actin filaments, and if, in rigor, they extended farther out from the thick filaments to attach themselves to sites on the actin filaments, then the changes in the patterns could be explained. It is suggested that the linear part of heavy meromyosin is flexibly attached to the backbone of the thick filaments and to the globular part of heavy meromyosin (which carries the actin-binding and ATPase sites), so that, by tilting farther outwards, this link can alter the radius at which the active end of the cross-bridge is situated. This model carries with it the strong implication that contraction is brought about by the structural changes which alter the effective orientation of the attachment of heavy meromyosin to actin. © 1968.