Electron microscopic investigations of actomyosin as a function of ionic strength.

Natural actomyosin at micro = 0.6 appears in various forms, including the regular arrowhead structures originally reported by Huxley (1), when it has been stained negatively with 1% uranyl acetate. In addition to the arrowheads, thin whiskers, 700-1200 A in length and 20 A in width, attached to the arm of the arrowheads have been demonstrated. The dimensions of the whiskers and arms of the arrowheads are practically the same as those of the light meromyosin (LMM) and the heavy meromyosin (HMM) moieties of the single myosin molecule, respectively. Changes in the electron microscopically distinguishable elements during aggregation of natural actomyosin on reduction of the ionic strength have been observed. At micro = 0.4, partial aggregation of the LMM whiskers begins to result in some parallel alignment of the arrowhead-bearing filaments (acto-HMM). In the range of micro = 0.3-0.1, the LMM whiskers merge into smooth filaments which are arranged alternatingly with arrowhead-bearing filaments. Thus, lateral aggregation of composite actomyosin filaments (acto-HMM + LMM whiskers) results with the LMM moieties as links. This view is supported by the following facts: (a) acto-HMM is devoid of whiskers and does not show lateral aggregation at micro = 0.1; (b) natural actomyosin digested with trypsin at micro = 0.6, which was followed by removal of LMM aggregates at low ionic strength, is essentially the same as acto-HMM at micro = 0.1; and (c) digestion with trypsin of natural actomyosin at micro = 0.2 for varying periods of time leads to a separation of arrowhead-bearing filaments from LMM aggregates.