Actomyosin organization in mitotic Dictyostelium amoebae

Immunofluorescence localization of actin and myosin during mitosis indicates the significant roles of these cytoskeletons for cytokinesis. High frequency mitosis was induced by synchronous culture using temperature shift (T. Kitanishi-Yumura and Y. Fukui, 1987), and high resolution fluorescence microscopy was performed by the agar-overlay method (S. Yumura and Y. Fukui, 1984). It was shown that actin and myosin are dissociated from the cortex in prophase and reassembled in anaphase to form unique cortical structures: the contractile ring and/or polar lamellipodial network. Conventional myosin (myosin-II) is only localized in the contractile ring, whereas a low-molecular weight isozyme (myosin-I) is localized in the polar leading edge (Y. Fukui, T. Lynch, H. Brzeska, and E. Korn, 1989). Actin is localized in both structures and also forms a unique array in the furrow oriented perpendicular to the plane of constriction (Y. Fukui and S. Inoué, manuscript in preparation). The study suggests that conventional myosin provides the major force of constriction, whereas myosin-I participates in projecting lamellipodia, and actin is involved in several different functions in different regions of the cytoplasm.