Figure 8. A C-terminal C1 domain-GAP module targets CYK-4 to the rachis surface/bridges to enable oocyte celluarization.

(A) Schematics compare the closure of compartment bridges during oocyte cellularization in the germline (left) to cytokinesis (shown here in a two-cell stage embryo, right). (Left) During bridge closure in the germline, the CYK-4 C1 domain and the Rho GTPase binding interface collaborate to recruit CYK-4 to the rachis surface/bridges. In the germline, bridges are not bisected by a central spindle-like microtubule bundles and the kinesin-6, ZEN-4, although present, is not essential for CYK-4 targeting or cellularization. (Right) During cytokinesis, cortical contractility is patterned by the anaphase spindle. Both ZEN-4 and CYK-4 are required to assemble the central spindle and contractile ring. Targeting to the central spindle and other ZEN-4/microtubule-dependent mechanisms may help deliver CYK-4 to the cell surface. We speculate that the C-terminal module-based mechanism that operates in the germline may also contribute to targeting CYK-4 to the plasma membrane during cytokinesis. (B) Our data suggest that CYK-4 associated with the rachis surface promotes RhoA activation. If active RhoA^RHO-1 generated by membrane-associated CYK-4 enables the recruitment of additional CYK-4 via interaction with its C1-GAP module, this would result in a positive feedback that promotes RhoA activation and CYK-4 recruitment.