Transduced p16INK4a peptides inhibit hypophosphorylation of the retinoblastoma protein and cell cycle progression prior to activation of Cdk2 complexes in late G1

Progression of cells through the G1 phase of the cell cycle requires cyclin D:Cdk4/6 and cyclin E:Cdk2 complexes; however, the duration and ordering of these complexes remain unclear. To address this, we synthesized a peptidyl mimetic of the CDK4/6 inhibitor, p16INK4a that contained an NH2-terminal TAT protein transduction domain. Transduction of TAT-p16 wild-type peptides into cells resulted in the loss of active, hypophosphorylated pRb and elicited an early G1 cell cycle arrest, provided cyclin E:Cdk2 complexes were inactive. We conclude that cyclin D:Cdk4/6 activity is required for early G1 phase cell cycle progression up to, but not beyond, activation of cyclin E:Cdk2 complexes at the restriction point and is thus nonredundant with cyclin E:Cdk2 in late G1.