A novel mitosis-specific dynamic actin structure in Dictyostelium cells.

Cell division of various animal cells depends on their attachment to a substratum. Dictyostelium cells deficient in type II myosin, analogous to myosin in muscle, can divide on a substratum without the contractile ring. To investigate the mechanism of this substratum-dependent cytokinesis, the dynamics of actin in the ventral cortex were observed by confocal and total internal reflection fluorescence microscopy. Specifically during mitosis, we found novel actin-containing structures (mitosis-specific dynamic actin structures, MiDASes) underneath the nuclei and centrosomes. When the nucleus divided, the MiDAS also split in two and followed the movement of the daughter nuclei. At that time, the distal ends of astral microtubules reached mainly the MiDAS regions of the ventral cortex. An inhibitor of microtubules induced disappearance of MiDASes, leading to aborted cytokinesis, suggesting that astral microtubules are required for the formation and maintenance of MiDASes. Fluorescence recovery after photobleaching experiments revealed that the MiDAS was highly dynamic and comprised small actin-containing dot-like structures. Interference reflection microscopy and assays blowing away the cell bodies by jet streaming showed that MiDASes were major attachment sites of dividing cells. Thus, the MiDASes are strong candidates for scaffolds for substratum-dependent cytokinesis, serving to transmit mechanical force to the substratum.