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J. Cell Biol.,
Volume 141, Number 2, April 20, 1998 483-492
* Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710; and The small GTPase racE is essential for cytokinesis in Dictyostelium. We found that this requirement is restricted to cells grown in suspension. When
attached to a substrate, racE null cells form an actomyosin contractile ring and complete cytokinesis normally. Nonetheless, racE null cells fail completely in cytokinesis when in suspension. To understand this conditional
requirement for racE, we developed a method to observe cytokinesis in suspension. Using this approach,
we found that racE null cells attempt cytokinesis in suspension by forming a contractile ring and cleavage furrow. However, the cells form multiple blebs and fail in
cytokinesis by regression of the cleavage furrow. We
believe this phenotype is caused by the extremely low
level of cortical tension 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 their content of F-actin. Instead, mitotic racE null cells contain
abnormal F-actin aggregates. These results suggest that
racE is essential for the organization of the cortical cytoskeleton to maintain proper cortical 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.
Department of
Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708
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