A postmitotic function and distinct localization mechanism for centralspindlin at a stable intercellular bridge.

Centralspindlin, a complex composed of the subunits ZEN-4 and CYK-4, recruits and regulates proteins that modulate the actin cytoskeleton to promote cleavage furrow formation and progression during cytokinesis. The ZEN-4 subunit is a kinesin that is proposed to function primarily by bundling microtubules and promoting transport of the complex to the midzone. ZEN-4 and CYK-4 are mutually dependent for localization to the midzone during cytokinesis. Once at the midzone, the CYK-4 subunit functions to recruit actin regulators and the scaffold anillin as well as to regulate RhoA and Rac via its intrinsic GAP domain, ultimately promoting actomyosin contractile ring assembly. We have revealed a distinct mechanism for centralspindlin localization and function at a stable, postmitotic intercellular bridge in the Caenorhabditis elegans gonad. Loss of zen-4 or cyk-4 function disrupts germ cell progression postmitotically. In contrast to the localization and recruitment relationships during mitosis, centralspindlin is maintained at the intercellular bridge by anillin, and CYK-4 is localized independently of ZEN-4 but not vice versa. We present evidence that centralspindlin function at the rachis bridge involves ZEN-4 action on the microtubules as opposed to the regulation of the actin cytoskeleton mediated by CYK-4 during cytokinesis.