Cytokinesis in animal cells is mediated by a cortical actomyosinbased contractile ring. The GTPase RhoA is a critical regulator of this process as it activates both non-muscle myosin and a nucleator of actin filaments [1]. The site at which active RhoA and its effectors accumulate is controlled by the microtubule-based spindle during anaphase [2]. ECT-2, the guanine nucleotide exchange factor (GEF) that activates RhoA during cytokinesis is regulated by phosphorylation and subcellular localization [3–5]. ECT2 localization depends on interactions with CYK-4/MgcRacGAP, a Rho GTPase activating protein (GAP) domain containing protein [5, 6]. Here, we show that, contrary to expectations, the Rho GTPase activating protein (GAP) domain of CYK-4 promotes activation of RhoA during cytokinesis. Furthermore, we show that the primary phenotype caused by mutations in the GAP domain of CYK-4 is not caused by ectopic activation of CED-10/Rac1 and ARX-2/Arp2. However, inhibition of CED-10/Rac1 and ARX-2/Arp2 facilitates ingression of weak cleavage furrows. These results demonstrate that a GAP domain can contribute to activation of a small GTPase. Furthermore, cleavage furrow ingression is sensitive to the balance of contractile forces and cortical tension.

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