The structure of F-actin and of actin filaments isolated from muscle

The filaments of the contractile apparatus have been isolated from a wide variety of smooth and striated muscles and examined in negatively-stained preparations in the electron microscope. In all cases there are thin filaments which are indistinguishable from the filaments in F-actin preparations. The filament consists of two helically-wound strands composed of subunits which appear to be alike and approximately spherical. The arrangement of the subunits corresponds to that of the scattering centres in one of the possible structures for actin deduced by Selby & Bear (1956) from the moderate-angle X-ray diffraction pattern of intact dried muscle. The number of globular subunits per turn of the helix (if integral) is 13 (cf. either 13 or 15 in the models proposed from diffraction data). The spacing of the subunits along each strand is 56·5 A (cf. 55 A in the models proposed from diffraction data). The cross-over points of the two twisted strands are spaced at intervals of 349 A along the filament (cf. either 351 A or 406 A in the models proposed from diffraction data). The overall diameter of the filament is about 80 A. It is shown in the case of rabbit skeletal muscle that this result is consistent with the quantity of actin in that muscle. There is good evidence that each of the globular subunits seen in the electron microscope represents one actin monomer. The structure of actin alone does not account for (i) the approximately 400 A axial periodicity observed in the I-substance of fibrils in the electron microscope, or (ii) the reflection at about 400 A observed in the axial diffraction pattern of intact muscle. It is suggested that both (i) and (ii) could be due to the combination of actin with other material, possibly tropomyosin B.

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