Cytoplasm has been isolated from single amoeba (Chaos carolinensis) in physiological solutions similar to rigor, contraction, and relaxation solutions designed to control the contractile state of vertebrate striated muscle. Contractions of the isolated cytoplasm are elicited by free calcium ion concentrations above ca. 7.0 x 10-7 M. Amoeba cytoplasmic contractility has been cycled repeatedly through stabilized (rigor), contracted, and relaxed states by manipulating the exogenous free calcium and ATP concentrations. The transition from stabilized state to relaxed state was characterized by a loss of viscoelasticity which was monitored as changes in the capacity of the cytoplasm to exhibit strain birefringence when stretched. When the stabilized cytoplasm was stretched, birefringent fibrils were observed. Thin sections of those fibrils showed thick (150–250 Å) and thin (70 Å) filaments aligned parallel to the long axis of fibrils visible with the light microscope. Negatively stained cytoplasm treated with relaxation solution showed dissociated thick and thin filaments morphologically identical with myosin aggregates and purified actin, respectively, from vertebrate striated muscle. In the presence of threshold buffered free calcium, ATP, and magnesium ions, controlled localized contractions caused membrane-less pseudopodia to extend into the solution from the cytoplasmic mass. These experiments shed new light on the contractile basis of cytoplasmic streaming and pseudopod extension, the chemical control of contractility in the amoeba cytoplasm, the site of application of the motive force for amoeboid movement, and the nature of the rheological transformations associated with the circulation of cytoplasm in intact amoeba.

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