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 o f 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 su rface (DeLozanne and Spudich (1987) Science 236, 1086-1091). The multi nucleated cells disclosed interactions of mitotic complexes with the c ell cortex that were not apparent in normal, mononucleated cells. Duri ng the anaphase stage, entire mitotic complexes consisting of spindle, microtubule asters, and separated sets of chromosomes were translocat ed to the periphery of the cells. These complexes were appended at a d istance of about 3 mu m from the cell surface, in a way that the spind le became orientated in parallel to the surface. Subsequently, lobes o f the cell surface were formed around the asters of microtubules. Thes e lobes were covered with tapered protrusions rich in coronin, an acti n associated protein that typically accumulates in dynamic cell-surfac e projections (DeHostos et al, (1991) EMBO J. 10, 4097-4104). During t heir growth on a solid surface, mononucleated myosin II-null cells pas sed through the mitotic cleavage stages with a speed comparable to wil d-type cells. Cytokinesis as linked to mitosis is distinguishable by s everal 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 wi thout forming a contractile ring at the site of the cleavage furrow.