Structure-function analysis of RhoA GAP RGA-3/4 during cytokinesis in the C. elegans embryo

Cytokinesis partitions the contents of a cell into two daughter cells. Cytokinesis is accomplished by constriction of an equatorial cortical contractile ring enriched in actin and myosin filaments. In animal cells, the position of the contractile ring is determined by the microtubule-based mitotic spindle. Yet, how upstream signaling events direct contractile ring precisely at the cell equator remains poorly understood. Contractile ring assembly is controlled by the small GTPase RhoA. Active RhoA promotes contractile ring assembly by stimulating the formation of actin myosin filaments. RhoA is activated by the guanine nucleotide exchange factor (GEF) ECT-2 and inactivated by the GTPase activating protein (GAP) MP-GAP/RGA-3/4. The balance of the GEF and the GAP activities serves as important mechanism that regulates the RhoA zone dimension and defines the position of the division plane. Most of the efforts to understand contractile ring assembly have focused on the molecular mechanism that leads to RhoA activation. Little is known about how the GAP that inactivates RhoA, MP-GAP/RGA-3/4 is regulated. Here, using the Caenorhabditis elegans embryo as a model system, we identify a region in RGA-3 that is essential for its function. Deletion of this C-terminal region blocks RGA-3 recruitment to the cell cortex and leads to hypercontractility during cytokinesis. Altogether these results identify a region on RGA-3 that is crucial for RhoGAP activity and provide some insight into how RGA-3 is controlled during cytokinesis