Effects of ATP Analogs on the Low-Angle X-Ray Diffraction Pattern of Insect Flight Muscle

Some years ago Reedy, Holmes, and Tregear (1965) noted that ATP had a dramatic effect upon the low-angle X-ray diffraction pattern of gly-cerinated fibers of the flight muscle from the giant water bug Lethocerus maximus. A strong meridional reflection appeared at a spacing of 145 A in the presence of ATP. When the ATP was washed out, this reflection disappeared and its place was taken by a 380 A layer line. With the help of electron microscopy these changes were shown to be due to an alteration in the orientation of the cross-bridges. In the absence of ATP (rigor state) the cross-bridges are at 45° to the filaments. In the presence of ATP the bridges appear to be free from the actin and are right-angled to the thick (myosin) filament (Fig. 1).