Myosin II-independent processes in mitotic cells of Dictyostelium discoideum: redistribution of the nuclei, re-arrangement of the actin system and formation of the cleavage furrow.

Mitosis was studied in multinucleated and mononucleated mutant cells of Dictyostelium discoideum that lack myosin II (Manstein et al. (1989) EMBO J. 8, 923-932). Multinucleated cells were produced by culture in suspension, mononucleated cells were enriched by growth on a solid surface (DeLozanne and Spudich (1987) Science 236, 1086-1091). The multinucleated cells disclosed interactions of mitotic complexes with the cell cortex that were not apparent in normal, mononucleated cells. During the anaphase stage, entire mitotic complexes consisting of spindle, microtubule asters, and separated sets of chromosomes were translocated to the periphery of the cells. These complexes were appended at a distance of about 3 microns from the cell surface, in a way that the spindle became orientated in parallel to the surface. Subsequently, lobes of the cell surface were formed around the asters of microtubules. These lobes were covered with tapered protrusions rich in coronin, an actin associated protein that typically accumulates in dynamic cell-surface projections (DeHostos et al. (1991) EMBO J. 10, 4097-4104). During their growth on a solid surface, mononucleated myosin II-null cells passed through the mitotic cleavage stages with a speed comparable to wild-type cells. Cytokinesis as linked to mitosis is distinguishable by several parameters from traction mediated cytofission, which results in the pinching off of pieces of a multinucleated cell in the interphase. The possibility is discussed that cells can cleave during mitosis without forming a contractile ring at the site of the cleavage furrow.