MASTL promotes cyclin B1 destruction by enforcing Cdc20-independent binding of cyclin B1 to the APC/C.

When cells enter mitosis, the anaphase-promoting complex/cyclosome (APC/C) is activated by phosphorylation and binding of Cdc20. The RXXL destruction box (D-box) of cyclin B1 only binds Cdc20 after release of the spindle checkpoint in metaphase, initiating cyclin B1 ubiquitination upon chromosome bi-orientation. However, we found that cyclin B1, through Cdk1 and Cks, is targeted to the phosphorylated APC/C(Cdc20) at the start of prometaphase, when the spindle checkpoint is still active. Here, we show that MASTL is essential for cyclin B1 recruitment to the mitotic APC/C and that this occurs entirely independently of Cdc20. Importantly, MASTL-directed binding of cyclin B1 to spindle checkpoint-inhibited APC/C(Cdc20) critically supports efficient cyclin B1 destruction after checkpoint release. A high incidence of anaphase bridges observed in response to MASTL RNAi may result from cyclin B1 remaining after securin destruction, which is insufficient to keep MASTL-depleted cells in mitosis but delays the activation of separase.