Protein Phosphatase 2A (B55α) Prevents Premature Activation of Forkhead Transcription Factor FoxM1 by Antagonizing Cyclin A/Cyclin-dependent Kinase-mediated Phosphorylation

The forkhead transcription factor FoxM1 controls expression of a large number of genes that are specifically expressed during the G(2) phase of the cell cycle. Throughout most of the cell cycle, FoxM1 activity is restrained by an autoinhibitory mechanism, involving a repressor domain present in the N-terminal part of the protein. Activation of FoxM1 in G(2) is achieved by Cyclin A/Cyclin-dependent kinase (Cdk)-mediated phosphorylation, which alleviates autoinhibition by the N-terminal repressor domain. Here, we show that FoxM1 interacts with B55 alpha, a regulatory subunit of protein phosphatase 2A (PP2A). B55 alpha binds the catalytic subunit of PP2A, and this promotes dephosphorylation and inactivation of FoxM1. Indeed, we find that overexpression of B55 alpha results in decreased FoxM1 activity. Inversely, depletion of B55 alpha results in premature activation of FoxM1. The activation of FoxM1 that is observed upon depletion of B55 alpha is fully dependent on Cyclin A/Cdk-mediated phosphorylation of FoxM1. Taken together, these data demonstrate that B55 alpha acts to antagonize Cyclin A/Cdk-dependent activation of FoxM1, to ensure that FoxM1 activity is restricted to the G(2) phase of the cell cycle.