Regulation of Citron kinase by CDK1 and Aurora B regulates midbody formation and stability

Many cell division events are regulated by protein phosphorylation, which can result from cross-regulatory mechanisms among mitotic kinases and phosphatases that have yet to be fully elucidated. Here, we report the characterization of a novel mechanism by which CDK1 and Aurora B (AURKB) kinases regulate the distribution and interactions of Citron kinase (CIT-K, encoded by CIT). We show that CDK1 phosphorylates serine 440 and AURKB phosphorylates serine 699, both residues located adjacent to or within the CIT-K coiled coil domain. S440 and S699 temporal phosphorylation profiles reflect the activity of the kinases responsible for their phosphorylation. Functional analyses using phosphorylation site mutants indicate that S699 phosphorylation is important for CIT-K localization and successful cytokinesis, whereas perturbing S440 phosphorylation leads to abnormal midbody formation and accumulation of post-mitotic midbody remnants (MBRs). Furthermore, we found that phosphorylation at either residue reduces the ability of CIT-K to interact with its midbody partners AURKB, KIF14 and MKLP1 (also known as KIF23). Taken together, our findings indicate that phosphorylation of CIT-K by CDK1 and AURKB regulates midbody formation and MBR stability by controlling the association of CIT-K with its partners. They expand our understanding of the mechanisms that regulate abscission and can lead to further insights into the role of MBRs in post-mitotic events.