Two types of demembranated cell models have been established using Characeae internodal cells; the tonoplast-free cell model and the plasmalemma-permeabilized cell model. In these models, both ATP and Mg
2+ were essential for the maintenance of cytoplasmic streaming. Ca
2+ inhibited the streaming in both models. However, the Ca
2+-sensitivity of the streaming in the plasmalemma-permeabilized model was 1000 times higher than the tonoplast-free model, suggesting that some Ca
2+-sensitizing component was dispersed or disorganized by the disintegration of the tonoplast. Using the actin bundles present in the tonoplast-free cells, the streaming was reconstituted with the endoplasm from other Characeae cells or the rabbit skeletal muscle, scallop or
Physarum myosin. Ca
2+ accelearated the streaming reconstituted with scallop myosin but inhibited that reconstituted with
Physarum myosin. These opposite effects of Ca
2+ on reconstituted streamings agree with the effects of Ca
2+ on the respective actomyosin ATPases.
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