The low-angle x-ray diagram of vertebrate striated muscle and its behaviour during contraction and rigor.

Frog sartorius muscle gives a rich and informative low-angle X-ray diagram. Though the interpretation of this diagram was originally rather difficult, information now available from other structural studies, especially from electron microscopy, provides the framework in which its significance can be understood, and the data often provide powerful criteria for testing the validity of particular interpretations of the electron microscope observations. More important still, X-ray diffraction can, in principal, be used to study muscle directly under different physiological conditions, including active contraction, at the level of structure where changes may be taking place, without having to fix or alter the preparation in any way. Previous low-angle X-ray studies have been hampered by the difficulty of recording the pattern with sufficient speed and resolution. We have developed new types of low-angle camera (partly in collaboration with Dr K. C. Holmes) which give rather large gains in both these factors and we have used them to study the muscle patterns—including those given by contracting muscles-in some detail. Part of the X-ray diagram arises from the helical arrangement of actin monomers in the thin filaments of muscle; the pitch of this helix is approximately 2 x 360 A to 2 x 370 A. Other reflections come from the helical arrangement of myosin cross-bridges on the thick filaments which lie approximately on a 6/2 helix of pitch 429 A. Further low-angle reflections arise from additional components of the thick and thin filaments and these have repeat periodicities different from those of the associated helical structures. Other reflections still arise from the fixed lengths of the filaments. Muscles in rigor (when the cross-bridges are permanently attached to the thin filaments) give X-ray diagrams which differ very considerably from those from resting muscle. The changes take place largely in the myosin component and the results indicate that a co-operative re-organization of the helical arrangement of myosin cross-bridges may occur when they bind to the sites on the actin filaments in such a way as to maximize the number of points of near-registration. The actin filaments appear to behave as relatively invariant structures though small changes in pitch cannot be excluded. In an actively contracting muscle, the over-all repeating periodicities along both the myosin and the actin filaments remain virtually constant (apart from an approximately 1 % increase in the myosin subunit spacing). However, large changes in intensity take place in certain of the low-angle reflections, and show that movement of cross-bridges takes place during contraction. The behaviour of the pattern indicates that a limited change in tilt and/or longitudinal position of the cross-bridges is accompanied by more substantial changes in their azimuthal and possibly radial positions. The changes in position of individual cross-bridges are not synchronized with each other. No changes have been detected so far in the actin pattern during contraction.