RhoA and the Rho guanine nucleotide exchange factor ECT-2 are involved in both polarization and cytokinesis. During cytokinesis, interactions of ECT-2 with the Rho GTPase-activating protein CYK-4 promote RhoA activation. A novel protein, NOP-1, acts in parallel with CYK-4 to promote RhoA activation during polarization and cytokinesis.The GTPase RhoA is a central regulator of cellular contractility in a wide variety of biological processes. During these events, RhoA is activated by guanine

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... tide exchange factors (GEFs). These molecules are highly regulated to ensure that RhoA activation occurs at the proper time and place. During cytokinesis, RhoA is activated by the RhoGEF ECT-2. In human cells, ECT-2 activity requires its association with CYK-4, which is a component of the centralspindlin complex. In contrast, in early Caenorhabditis elegans embryos, not all ECT-2–dependent functions require CYK-4. In this study, we identify a novel protein, NOP-1, that functions in parallel with CYK-4 to promote RhoA activation. We use mutations in nop-1 and cyk-4 to dissect cytokinesis and cell polarization. NOP-1 makes a significant, albeit largely redundant, contribution to cytokinesis. In contrast, NOP-1 is required for the preponderance of RhoA activation during the establishment phase of polarization.