p21 Disrupts the interaction between cdk2 and the E2F-p130 complex

In nonproliferating or growth-arrested cells, the transcription factor E2F remains bound to the retinoblastoma-related protein p130. Accumulation of this E2F-p130 complex correlates with an arrest of the cell cycle progression. Progression through G1 phase is associated with a cyclin-dependent binding of the cyclin-dependent kinase cdk2 to the E2F-p130 complex. By fractionating mouse L-cell extracts, we have obtained a partially purified preparation of the E2F-p130 complex that also contains cdk2. Incubation of this complex with recombinant p21 results in a disruption of the interaction between cdk2 and the E2F-p130 complex in extracts of a cell line that expresses a temperature-sensitive mutant of p53. Incubation at the permissive temperature (32 degrees C) results in an induction of p21 synthesis. An increase in the level of p21 in these cells correlates with a loss of cdk2 from the cdk2-containing E2F-p130 complex. We also show that the expression of a reporter gene containing E2F sites in the promoter region is reduced by the coexpression of p21. Since p21 is believed to be a mediator of p53, we speculated that the p21-mediated disruption of the cdk2-containing E2F-p130 complex plays a role in the growth suppression function of p53.

HPV16 E7 oncoprotein induces expression of a 110 kDa heat shock protein

Heat shock protein genes are induced by various kinds of stress. Besides stress, the heat shock family gene hsp70 has been shown to be induced by growth-stimulating agents such as the DNA virus oncoproteins and serum. Here, we report cloning of a novel cDNA that encodes a 100 kDa heat shock protein-related polypeptide as a human papillomavirus oncoprotein E7-inducible gene. E7 induces expression of this heat shock protein at the level of RNA synthesis. Moreover, the induction of this heat shock protein-mRNA was dependent on the conserved region 2 of the E7 protein, which is essential for binding to the proteins of the retinoblastoma family.