Restoration of 3-D structure of insect flight muscle from a rotationally averaged 2-D X-ray diffraction pattern

The contractile machinery of muscle, especially that of skeletal muscle, has a very regular array of contractile protein filaments, and gives rise to a very complex and informative diffraction pattern when irradiated with X-rays. However, the analysis of the diffraction patterns is often difficult, because (1) only rotationally averaged diffraction patterns can be obtained, resulting in substantial loss of information, and (2) the contractile machinery contains two different sets of protein filaments (actin and myosin) with different helical symmetries, and the reflections originating from them are often overlapped. These problems may be solved if the real-space 3-D structure of the contractile machinery is directly calculated from the diffraction pattern. Here we demonstrate that, by using the conventional phase-retrieval algorithm (hybrid input-output), the real-space 3-D structure of the contractile machinery can be well restored from a single rotationally averaged 2-D diffraction pattern. In this calculation, we used a model structure of insect flight muscle, known to have a very regular structure. Possibilities of extending this technique to the actual muscle diffraction patterns is discussed.