Phosphorylation of Xenopus p31 potentiates mitotic checkpoint exit

p31 plays an important role in spindle assembly checkpoint (SAC) silencing. However, how p31’s activity is regulated remains unclear. Here we show that the timing of M-phase exit in Xenopus egg extracts (XEEs) depends upon SAC activity, even under conditions that are permissive for spindle assembly. p31 antagonizes the SAC, promoting XEE progression into anaphase after spindles are fully formed. We further show that mitotic p31 phosphorylation by Inhibitor of nuclear factor k-B kinase-b (IKK-b) enhances this role in SAC silencing. Together, our findings implicate IKK-b in the control of anaphase timing in XEE through p31 activation and SAC downregulation.