PHOSPHORYLATION OF E2F-1 BY CYCLIN A-CDK2

Transcription factor E2F-1 has a putative consensus sequence for phosp horylation by cyclin dependent kinase (Ser-Pro-X-Lys/Arg). Therefore, we studied the phosphorylation of E2F-1 in vivo and in vitro and its b iological functions. E2F-1 was prepared by immunoprecipitation with an ti-E2F-1 antibody from IMR32 lysates and was effectively phosphorylate d by human cyclin A-cdk2 which was expressed in insect cells using bac ulovirus system. GST-EZF-1 was phosphorylated by cyclin A-cdk2 more ef ficiently than by cyclin E-cdk2. Cyclin D1-cdk4 phosphorylated PRE but scarcely phosphorylated GST-E2F-1 or H1 histone. The 60 kd protein pr ecipitated with anti-E2F-1 antibody was phosphorylated in vivo. Phosph o-peptide mapping indicated that its cleavage profile was identical wi th that of E2F-1 phosphorylated by cyclin A-cdk2 in vitro. This 60 kd protein, which is likely to be E2F-1, was not phosphorylated during th e GO and early G1 phase, Phosphorylation of E2F-1 began from the S pha se while phosphorylation of pRB started nearly at G1/S. The in vivo ph osphorylation of E2F-1 was inhibited by butyrolactone I, a cyclin-depe ndent kinase inhibitor (Kitagawa et al., 1993, Oncogene, 8, 2425-2432) . The binding of E2F-1 to E2 promoter was found to be reduced by phosp horylation of E2F-1 by cyclin A-cdk2, suggesting that phosphorylation of E2F-1 may induce shut off of gene expression at the transcriptional level. These results suggest that E2F-1 is phosphorylated by cyclin A -cdkZ in the S phase in vivo as well as in vitro and that its phosphor ylation by cyclin A-cdkZ may modulate its activity.