P27(KIP1) BLOCKS CYCLIN E-DEPENDENT TRANSACTIVATION OF CYCLIN-A GENE-EXPRESSION

Cyclin E is necessary and rate limiting for the passage of mammalian c ells through the G(1) phase of the cell cycle. Control of cell cycle p rogression by cyclin E involves cdk2 kinase, which requires cyclin E f or catalytic activity. Expression of cyclin E/cdk2 leads to an activat ion of cyclin A gene expression, as monitored by reporter gene constru cts derived from the human cyclin A promoter. Promoter activation by c yclin E/cdk2 requires an E2F binding site in the cyclin A promoter. We show here that cyclin E/cdk2 kinase can directly bind to E2F/p107 com plexes formed on the cyclin A promoter-derived E2F binding site, and t his association is controlled by p27(KIP1), most likely through direct protein-protein interaction. These observations suggest that cyclin E /cdk2 associates with E2F/p107 complexes in late G(1) phase, once p27( KIP1) has decreased below a critical threshold level. Since a kinase-n egative mutant of cdk2 prevents promoter activation, it appears that t ranscriptional activation of the cyclin A gene requires an active cdk2 kinase tethered to its promoter region.