The nuclear localization of Ect2 is required for cytokinesis

In cytokinesis, a contractile ring constricts the cell to form two daughters. After ingression, the ring matures into the midbody, and an intercellular bridge connects the daughter cells until it is cut during abscission. Ring assembly requires the activation of RhoA GTPase at the equatorial cortex by Ect2, a guanine nucleotide exchange factor (GEF). However, it is unclear if RhoA must be inactivated after ring closure to complete cytokinesis. Here, we show that the nuclear sequestration of Ect2 after ingression is required for cytokinesis. Mutating the nuclear localization signal (NLS) in Ect2 causes it to remain at the midbody where it generates persistent active RhoA, leading to cytokinesis failure. Re-localizing the mutant to the nucleus using SV40NLS restores cytokinesis. Further, over-expression of mutant Ect2 causes cytokinesis failure, which is rescued by reducing GEF activity. We propose that persistent RhoA causes instability in the intercellular bridge, preventing abscission. Nuclear sequestration may regulate the function of other contractile proteins with NLS sequences.