Abstract

This work describes the potential requirement for a nuclear localization signal (NLS) in the regulation of Ect2, a RhoA GEF that activates RhoA for cytokinesis. The NLS has not previously been studied for its requirement in cells. Earlier studies led to a model where Ect2 is in an autoinhibited conformation mediated by Cdk1 phosphorylation, and that removal of this phosphate during anaphase is required for the open, active conformation. Further, Cyk4, a central spindle protein, binds to BRCT domains in the N-terminus of Ect2, which, together with lipid-binding is required for Ect2 activity. Interestingly the NLS is adjacent to the Cdk1 site, and in vitro studies showed that importin-binding is competed by phosphorylation. One model is that importin-binding facilitates the open conformation of Ect2 and work in concert with Cyk4-binding. Another, non-mutually exclusive model is that the NLS is required to mediate the localization of Ect2 to the re-forming nuclei in the daughter cells at the end of cytokinesis for abscission. To test these models, we generated point mutations in the NLS and performed rescue assays. We found that the NLS is required for the function of Ect2 in cytokinesis by regulating abscission. Further, point mutations in the Cdk1 phosphorylation site did cause obvious cytokinesis defects, suggesting this model needs to be revised.