The Rho family GTPases and their regulators play critical roles in various actin remodeling functions including cytokinesis. We have previously reported that the Rho exchange factor ECT2 is a critical regulator of cytokinesis. The N-terminal half of ECT2 (ECT2-N) contains the domains related to the human repair protein XRCC1 and yeast cyclin B6 (CLB6) as well as two BRCA1 C-terminal (BRCT) domains, which are widespread to a number of repair and checkpoint control proteins. ECT2-N, which lacks the catalytic domain, can function as a dominant negative form and strongly inhibits cytokinesis. In this period, we found that ECT2-N inhibits a very late stage of cytokinesis through the BRCT domains. Cytoplasmic localization of ECT2-N strongly stimulated the activity of ECT2-N to inhibit cytokinesis. Both the BRCT domains in ECT2-N were essential for the cytokinesis inhibition by ECT2-N. Cells expressing ECT2-N exhibited normal mitosis and centrosome duplication. Several events involving actin remodeling during cytokinesis are also normal. These include cell rounding, cleavage furrow formation and ingression, daughter cell repositioning and cell spreading. However, severing of the intercellular bridge did not occur in ECT2-N-expressing cell, and subsequent collapse of the contractile ring resulted in the fusion of two emerging daughter cells. We conclude that the BRCT domains of ECT2 play an important role in the regulation of the terminal phase of cytokinesis.
