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

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 action 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.