Two-step positioning of a cleavage furrow by cortexillin and myosin II

Background: Myosin II, a conventional myosin, is dispensable for mitotic di vision in Dictyostelium if the cells are attached to a substrate, but is re quired when the cells are growing in suspension. Only a small fraction of m yosin Il-null cells fail to divide when attached to a substrate. Cortexilli ns are actin-bundling proteins that translocate to the midzone of mitotic c ells and are important for the formation of a cleavage furrow, even in atta ched cells. Here, we investigated how myosin II and cortexillin I cooperate to determine the position of a cleavage furrow. Results: Using a green fluorescent protein (GFP)-cortexillin I fusion prote in as a marker for priming of a cleavage furrow, we found that positioning of a cleavage furrow occurred in two steps. In the first step, which was in dependent of myosin II and substrate, cortexillin I delineated a zone aroun d the equatorial region of the cell. Myosin II then focused the cleavage fu rrow to the middle of this cortexillin I zone. If asymmetric cleavage in th e absence of myosin II partitioned a cell into a binucleate and an anucleat e portion, cell-surface ruffles were induced along the cleavage furrow, whi ch led to movement of the anucleate portion along the connecting strand tow ards the binucleate one. Conclusions: In myosin Ii-null cells, cleavage furrow positioning occurs in two steps: priming of the furrow region and actual cleavage, which may pro ceed in the middle or at one border of the cortexillin ring. A control mech anism acting at late cytokinesis prevents cell division into an anucleate a nd a binucleate portion, causing a displaced furrow to regress if it become s aberrantly located on top of polar microtubule asters.