Gerald N, Dai J, Ting-Beall H P, De Lozanne A
Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
J Cell Biol. 1998 Apr 20;141(2):483-92. doi: 10.1083/jcb.141.2.483.
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.
小GTP酶racE对盘基网柄菌的胞质分裂至关重要。我们发现这种需求仅限于悬浮培养的细胞。当附着在底物上时，racE基因敲除细胞形成肌动球蛋白收缩环并正常完成胞质分裂。然而，racE基因敲除细胞在悬浮状态下胞质分裂完全失败。为了理解对racE的这种条件性需求，我们开发了一种在悬浮状态下观察胞质分裂的方法。使用这种方法，我们发现racE基因敲除细胞在悬浮状态下试图通过形成收缩环和分裂沟来进行胞质分裂。然而，细胞形成多个泡状突起，并由于分裂沟的退缩而在胞质分裂中失败。我们认为这种表型是由于与野生型细胞相比，racE基因敲除细胞中皮质张力极低所致。racE基因敲除细胞皮质张力降低并非由其F-肌动蛋白含量减少引起。相反，有丝分裂的racE基因敲除细胞含有异常的F-肌动蛋白聚集体。这些结果表明，racE对于皮质细胞骨架的组织以维持适当的皮质完整性至关重要。racE的这一功能独立于对底物的附着，但可以被由附着到底物诱导的其他信号通路所绕过。
