The cell cycle-dependent regulation of Forkhead box M1 transcriptional activity by phosphorylation

The Forkhead box M1 (FoxM1) transcription factor is critical for expression of the genes essential for G1/S transition and mitotic progression. However, the mechanism of temporal activation of different subset genes by FoxM1 remains unknown. In this thesis, I show that both phosphorylation status and transcriptional activity of FoxM1 increase as cells progress from S to G2/M phases. Dephosphorylation of FoxM1 coincides with exit from mitosis. I demonstrated that Polo-like kinase 1 (PLK1) interacts with FoxM1 at M phase to phosphorylate the C-terminal transcriptional activation domain of FoxM1 and stimulate its activity. Furthermore, using mass spectrometry analysis to study the in vivo phosphorylation sites within the hyperphosphorylated FoxM1, I identified a conserved phosphorylation site (Ser-251) within the DNA binding domain of FoxM1 that is critical for the initial activation of FoxM1 by Cdk1. Disruption of Ser-251 inhibits phosphorylation of FoxM1 and dramatically decreases its transcriptional activity. Cells expressing the S251A mutant exhibit reduced expression of the G2/M phase genes and impaired mitotic progression. Together, my results demonstrate that the transcriptional activity of FoxM1 is controlled in a cell cycle-dependent fashion by temporally regulated phosphorylation and dephosphorylation events, and that the phosphorylation at Ser-251 is critical for the activation of FoxM1.