The Structure of Actin-rich Filaments of Muscles According to X-ray Diffraction* by Cecily

In the accompanying paper 1 we have described the structure of paramyosin in molluscan muscles. A second type of fibrous muscle component is possibly of greater significance, since it has been found in all muscles thus far examined, in smooth and striated varieties from vertebrate and invertebrate animals (Schmitt, Bear, Hall, and Jakus, 1947). When first discovered by means of x-ray diffraction this second component was designated type II (Bear, 1945). Subsequent x-ray examination of actin films by Astbury and Spark (1947; see also Astbury, 1947, 1949) indicated that it is an actin-rich fibrous system, if not composed of actin alone. In what follows for brevity we shall call this fibrous system actin, although very complete knowledge of the diffraction properties of isolated actin has not been available during most of this investigation, which deals only with results obtained from intact dried muscles. In the original diffraction work unambiguous index assignments could not be made for the observed reflections. I t was clear, however, that the axial identity period was in the range 350 to 420 A. Subsequent electron-optical investigations of striated muscle have revealed a similar axial periodicity (ca. 400 A, cf. Draper and Hodge, 1949), which has come to be considered, though without detailed justification, as related to the same system observed by the early diffraction studies. Recently Philpott and Szent-Gy/Jrgyi (1954)