Cyclin B3 activates the Anaphase-Promoting Complex/Cyclosome in meiosis and mitosis

In mitosis and meiosis, chromosome segregation is triggered by the Anaphase-Promoting Complex/Cyclosome (APC/C), a multi-subunit ubiquitin ligase that targets proteins for degradation, leading to the separation of chromatids. APC/C activation requires phosphorylation of its APC3 and APC1 subunits, which allows the APC/C to bind its Cdc20 co-activator. The identity of the kinase(s) responsible for APC/C activation in vivo is unclear. Cyclin B3 is required for meiotic anaphase in flies, worms and vertebrates, but whether it activates the APC/C is unclear. We found that Drosophila Cyclin B3 (CycB3) collaborates with PP2A-B55/Tws in embryonic development, indicating that CycB3 also promotes anaphase in mitosis. Moreover, CycB3 promotes APC/C activity and anaphase in cells in culture. We show that CycB3 physically associates with the APC/C, is required for phosphorylation of APC3, and promotes APC/C association with its co-activators. We propose that CycB3-Cdk1 directly phosphorylates the APC/C to activate it in both meiosis and mitosis.