Abstract
The transcription factor E2F1 participates in cell cycle control through transcriptional activation of genes that promote S-phase entry. E2F1 is also linked to the expression of proapoptotic genes, and the loss of E2F1 activity facilitates tumor progression by reducing cellular apoptosis. Phosphorylation controlled by protein kinases and phosphatases is the major posttranslational modification and regulates the cellular levels and transactivator function of E2F1. Here, we characterize the regulatory roles of serine-375 (S375), one of the major phosphorylation sites of E2F1. Cyclin-dependent kinases such as CDK8 phosphorylate at S375 of E2F1, which is dephosphorylated by protein phosphatase 2A (PP2A) containing the B55 regulatory subunit. The PP2A adapter protein IER5 binds to both PP2A/B55 and E2F1 and assists dephosphorylation at S375 by PP2A. S375-dephosphorylated E2F1 exhibits higher DNA-binding affinity than the phosphorylated form. Although the promoter regions of proapoptotic genes are less occupied by E2F1 in cells, an increase in S375-dephosphorylated E2F1 induces preferential binding of E2F1 to the proapoptotic gene promoters and their expression. Our data identify PP2A/B55-IER5 as a critical regulator of E2F1 and suggest that the phosphorylation state of E2F1 is an important determinant for the expression of proapoptotic genes.