CYK-4: A Rho family GTPase activating protein (GAP) required for central spindle formation and cytokinesis

During cytokinesis of animal cells, the mitotic spindle plays at least two roles. Initially, the spindle positions the contractile ring. Subsequently, the central spindle, which is composed of microtubule bundles that form du ring anaphase, promotes a late step in cytokinesis. How the central spindle assembles and functions in cytokinesis is poorly understood. The cyk-4 gen e has been identified by genetic analysis in Caenorhabditis elegans. Embryo s from cyk-4(t1689ts) mutant hermaphrodites initiate, but fail to complete, cytokinesis. These embryos also fail to assemble the central spindle. We s how that the cyk-4 gene encodes a GTPase activating protein (GAP) for Rho f amily GTPases. CYK-4 activates GTP hydrolysis by RhoA, Rac1, and Cdc42 in v itro. RNA-mediated interference of RhoA, Rad, and Cdc42 indicates that only RhoA is essential for cytokinesis and, thus, RhoA is the likely target of CYK-4 GAP activity for cytokinesis. CYK-4 and a CYK-4:GFP fusion protein lo calize to the central spindle and persist at cell division remnants. CYK-4 localization is dependent on the kinesin-like protein ZEN-4/CeMKLP1 and vic e versa. These data suggest that CYK-4 and ZEN-4/CeMKLP1 cooperate in centr al spindle assembly. Central spindle localization of CYK-4 could accelerate GTP hydrolysis by RhoA, thereby allowing contractile ring disassembly and completion of cytokinesis.