Visualization of actin polymerization and depolymerization cycles during polyamine-induced cytokinesis in living Amoeba proteus

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Abstract
Microinjection of spermine induces cytokinesis of Amoeba proteus. Within 30–60 s after spermine injection cells form one, or less commonly, two cleavage furrows and within the following 4–10 min the constrictions are completed. The resulting nucleated cell parts show normal streaming and locomotion, whereas the non-nucleated cell parts remain stationary and later degenerate. The intracellular distribution of fully polymerization-competent fluorescently labelled muscle actin was followed by image intensification. Double injection experiments initially using labelled actin and 30 min later spermine revealed a ring-like structure of enhanced fluorescence corresponding to the constricting cleavage furrow. Immediately after cleavage was completed, the ring disappeared. Electron microscopy of cells fixed during spermine-induced cytokinesis showed numerous well aligned actin and myosin filaments in the developing cleavage furrow. These filaments are a specialized manifestation of the cell cortex. The results demonstrate that cycles of actin and myosin polymerization and depolymerization and the parallel alignment of preexisting filaments (crosslinking) represent a basic mechanism in the generation of the motive force during cytokinesis.