Regulation and Mechanism of Bub1-Mediated Spindle Checkpoint Signaling
The spindle checkpoint is a surveillance mechanism that ensures the fidelity of chromosome segregation during mitosis and meiosis. Bub1 is a highly conserved protein serine/threonine kinase that plays multiple roles in the spindle checkpoint. The regulation and mechanism of Bub1 in spindle checkpoint were investigated. Bub1 is degraded during mitotic exit and the degradation of it is mediated by APC/C in complex with its activator Cdh1 (APC/CCdh1). Overexpression of Cdh1 reduces the protein levels of ectopically expressed Bub1 whereas depletion of Cdh1 by RNA interference (RNAi) increases the level of the endogenous Bub1 protein. Two KEN-box motifs on Bub1 are required for its degradation in vivo and ubiquitination in vitro. A Bub1 mutant protein with both KEN-boxes mutated is stable in cells. Kinetochore is the origin of spindle checkpoint signal and contains the catalytic machinery for generating the signal. We identify an ATP-dependent APC/CCdc20 inhibitory activity on metaphase chromosomes with unattached kinetochores. The Cdc20-S153A that cannot be phosphorylated by Bub1 is not inhibited by metaphase chromosomes, suggesting Bub1 is likely responsible for the inhibitory activity. Bub1 on unattached kinetochores is hyperphosphorylated and activated. Furthermore, the kinase-dead mutant of Bub1 cannot restore spindle checkpoint in Bub1-RNAi cells, demonstrating that the kinase activity of Bub1 is required for the spindle checkpoint. Plk1 is required for the generation of the tension-sensing 3F3/2 kinetochore epitope and facilitates kinetochore localization of Mad2 and other spindle checkpoint proteins. We investigate the mechanism by which Plk1 is recruited to kinetochores. We show that Plk1 binds to Bub1 in mitotic cells. The Plk1-Bub1 interaction requires the polo-box domain (PBD) of Plk1 and is enhanced by Cdk1-mediated phosphorylation of Bub1 at T609. The PBD-dependent binding of Plk1 to Bub1 facilitates phosphorylation of Bub1 by Plk1 in vitro. Depletion of Bub1 in HeLa cells by RNAi diminishes the kinetochore localization of Plk1. Ectopic expression of the wild-type Bub1, but not the Bub1-T609A mutant, in Bub1-RNAi cells restores the kinetochore localization of Plk1. Our results suggest that phosphorylation of Bub1 at T609 by Cdk1 creates a docking site for the PBD of Plk1 and facilitates the kinetochore recruitment of Plk1.