A ROLE FOR DICTYOSTELIUM RACE IN CORTICAL TENSION AND CLEAVAGE FURROWPROGRESSION

The small GTPase racE is essential for cytokinesis in Dictyostelium. W e found that this requirement is restricted to cells grown in suspensi on. When attached to a substrate, racE null cells form an actomyosin c ontractile ring and complete cytokinesis normally. Nonetheless, racE n ull cells fail completely in cytokinesis when in suspension. To unders tand this conditional requirement for racE, we developed a method to o bserve cytokinesis in suspension. Using this approach, we found that r acE null cells attempt cytokinesis in suspension by forming a contract ile ring and cleavage furrow. However, the cells form multiple blebs a nd fail in cytokinesis by regression of the cleavage furrow. We believ e this phenotype is caused by the extremely low level of cortical tens ion found in racE null cells compared to wild-type cells. The reduced cortical tension of racE null cells is not caused by a decrease in the ir content of F-actin. Instead, mitotic racE null cells contain abnorm al F-actin aggregates. These results suggest that racE is essential fo r the organization of the cortical cytoskeleton to maintain proper cor tical integrity. This function of racE is independent of attachment to a substrate, but can be bypassed by other signaling pathways induced by adhesion to a substrate.