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The Journal of Cell Biology, Vol 39, 620-629, Copyright © 1968 by Rockefeller University Press

ARTICLE

ELECTRON MICROSCOPIC INVESTIGATIONS OF ACTOMYOSIN AS A FUNCTION OF IONIC STRENGTH



N. Ikemoto 1, S. Kitagawa 1, A. Nakamura 1, and J. Gergely 1

1 From the Department of Muscle Research, Institute of Biological and Medical Sciences, Retina Foundation, and the Kennedy Memorial Laboratories, Massachusetts General Hospital, and the Department of Biological Chemistry, Harvard Medical School, Boston, Massachusetts 02114.

Dr. Ikemoto's present address is the Department of Biology, Faculty of Science, Okayama University, Okayama, Japan. Dr. Kitagawa's present address is the Research Institute of Catalysis, Hokkaido University, Sapporo, Japan

Natural actomyosin at µ = 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 µ = 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 µ = 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 µ = 0.1; (b) natural actomyosin digested with trypsin at µ = 0.6, which was followed by removal of LMM aggregates at low ionic strength, is essentially the same as acto-HMM at µ = 0.1; and (c) digestion with trypsin of natural actomyosin at µ = 0.2 for varying periods of time leads to a separation of arrowhead-bearing filaments from LMM aggregates.

Submitted on June 11, 1968
Revised on July 29, 1968


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