Dynamic aspects of the contractile system in Physarum Plasmodium: II. Contractility of triton cell models in accordance with the contraction and relaxation phases of the plasmodia

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Abstract
The contractility of Physarum plasmodium was investigated using cell models that were prepared by treating thin‐spread plasmodia with ice‐cold 0.2% Triton X‐100. Cell models obtained from the anterior regions of the thin‐spread plasmodia in the contraction phase retained many birefringent cytoplasmic fibrils. The fibrils vigorously contracted on addition of ATP, inducing simultaneous contraction of the whole cell models. In contrast, cell models prepared from the anterior regions in the relaxation phase scarcely contained the birefringent fibrils and exhibited only weak contractility on addition of ATP. The posterior regions of the thinspread plasmodia, which were composed of ramified plasmodial strands, always retained many fibrils when treated with the Triton solution and showed intensive contraction on addition of ATP.SDS‐polyacrylamide gel electrophoresis showed that the model was enriched for actin and myosin. About 40% of the actin was extracted from the plasmodium by the Triton treatment, while scarcely any myosin was extracted.Fragmin, a F‐actin‐fragmenting factor, caused the birefringent fibrils to diminish in the presence of Ca2+, but more than 30 minutes was required for their complete disappearance. The birefringent fibrils weakened by 30‐minute fragmin treatment disappeared immediately on addition of ATP or AMP‐PNP.