Mammalian Polo-like Kinase 1 (Plk1) Promotes Proper Chromosome Segregation by Phosphorylating and Delocalizing the PBIP1·CENP-Q Complex from Kinetochores

Abstract Mammalian polo-like kinase 1 (Plk1) is critically required for proper M-phase progression. Plk1 is self-recruited to prekinetochores/kinetochores by phosphorylating and binding to the T78 motif of a kinetochore scaffold protein, PBIP1 (also called CENP-U/50), which forms a stable complex with another kinetochore component, CENP-Q. However, the mechanism underlying how Plk1 localization to this site is regulated remains largely unknown. Here, we demonstrated that the PBIP1-CENP-Q complex becomes hyperphosphorylated and rapidly delocalized from kinetochores as cells enter mitosis. Plk1 phosphorylated the CENP-Q subunit of the PBIP1-CENP-Q complex at multiple sites, and mutation of nine Plk1-dependent phosphorylation sites to Ala (9A) enhanced CENP-Q association with chromatin and prolonged CENP-Q localization to kinetochores. Conversely, mutation of the nine sites to phospho-mimicking Asp/Glu (9D/E) residues dissociated CENP-Q from chromatin and kept the CENP-Q (9D/E) mutant from localizing to kinetochores. Strikingly, both the 9A and 9D/E mutants induced a defect in proper chromosome segregation, suggesting that both timely localization and delocalization of the PBIP1-CENP-Q complex to and from kinetochores are critical for normal M-phase progression. Notably, although Plk1 did not alter the level of PBIP1 and CENP-Q ubiquitination, Plk1-dependent phosphorylation and delocalization of these proteins from kinetochores appeared to indirectly lead to their degradation in mitotic cytosol. Thus, we propose that Plk1 regulates the timing of the delocalization and ultimate destruction of the PBIP1-CENP-Q complex, and that these processes are important not only for promoting Plk1-dependent mitotic progression, but also for resetting the timing of Plk1 recruitment to prekinetochores in the next cell cycle.