Fraying of A-filaments into three subfilaments.

Since Huxley established their bipolar structure, direct electron microsocpy of A-filaments isolated from vertebrate skeletal muscle has yielded little further interpretable information about the mode of packing of the myosin molecules within the filament. Using A-filaments prepared from rat psoas muscle we have now found it possible to induce clear fraying of these filaments into subfilaments, by exposure of the preparation to very low ionic strength before contrasting with uranyl acetate. The number of such filaments observed is generally (and never in excess of) three. Considerations of symmetry suggest that the formation of these frayed filaments is compatible only with a 'three stranded' model for the native A-filament, a finding in agreement with the balance of evidence recently published using other techniques.