Citron kinase controls abscission through RhoA and Anillin.

The small GTPase RhoA plays a crucial role in the different stages of cytokinesis, including contractile ring formation, cleavage furrow ingression and midbody abscission. Citron kinase (CIT-K) is a conserved protein required for cytokinesis from insects to mammals, currently considered a cytokinesis-specific effector of active RhoA. In agreement with previous suggestions, we show here that, as in the case of Drosophila cells, even in mammalian cells CIT-K is specifically required for abscission. However, in contrast with the current view, we provide evidence that, during late cytokinesis, CIT-K is an upstream regulator rather than a downstream effector of RhoA. In addition, we show that CIT-K is capable to physically and functionally interact with the actin-binding protein Anillin. Active RhoA and the Anillin are displaced from the midbody in CIT-K depleted cells, while only Anillin, but not CIT-K, is affected if RhoA is inactivated in late cytokinesis. The overexpression of CIT-K and of Anillin leads to abscission delay. However, the delay produced by CIT-K overexpression can be reverted by RhoA inactivation, while the delay produced by Anillin overexpression is RhoA-independent. Altogether, these results indicate that CIT-K is a crucial abscission regulator that may promote midbody stability through active RhoA and Anillin.