Myosin II-independent cytokinesis in Dictyostelium: its mechanism and implications

Similar to higher animal cells, ameba tells of the cellular slime mold Dirt yostelium discoideum form contractile rings containing filaments of myosin II during mitosis, and it is generally believed that contraction of these r ings bisects the cells both on substrates and in suspension. In suspension, mutant cells lacking the single myosin II heavy chain gene cannot carry ou t cytokinesis, become large and multinucleate, and eventually lyze, support ing the idea that myosin II plays critical roles in cytokinesis. These muta nt cells are however viable on substrates, Detailed analyses of these mutan t cells on substrates revealed that, in addition to "classic" cytokinesis w hich depends on myosin II ("cytokinesis A"), Dictyostelium has two distinct , novel methods of cytokinesis, 1) attachment-assisted mitotic cleavage emp loyed by myosin II null cells on substrates ("cytokinesis B"), and 2) cytof ission, a cell cycle-independent division of adherent cells ("cytokinesis C "), Cytokinesis A, B, and C lose their function and demand fewer protein fa ctors in this order. Cytokinesis B is of particular importance for future s tudies. Similar to cytokinesis A, cytokinesis B involves formation of a cle avage furrow in the equatorial region, and it may be a primitive but basic mechanism of efficiently bisecting a cell in a cell cycle-coupled manner. A nalysis of large, multinucleate myosin II null cells suggested that interac tions between astral microtubules and cortices positively induce polar prot rusive activities in telophase. A model is proposed to explain how such pol ar activities drive cytokinesis B, and how cytokinesis B is coordinated wit h cytokinesis A in wild type cells.