TRANSACTIVATION-COMPETENT BOVINE PAPILLOMAVIRUS E2 PROTEIN IS SPECIFICALLY REQUIRED FOR EFFICIENT REPRESSION OF HUMAN-PAPILLOMAVIRUS ONCOGENE EXPRESSION AND FOR ACUTE GROWTH-INHIBITION OF CERVICAL-CARCINOMA CELL-LINES

The papillomavirus E2 proteins can function as sequence-slpecific tran sactivators or transrepressors of transcription and as cofactors in vi ral DNA replication. We previously demonstrated that acute expression of the bovine papillomavirus type 1 (BPV1) E2 protein in HeLa and HT-3 cervical carcinoma cell lines greatly reduced cellular proliferation by imposing a specific G(1)/S phase growth arrest. In this report, we analyzed the effects of a panel of point mutations in the BPV1 E2 prot ein to identify the functional requirements for acute growth inhibitio n. Disruption of E2-specific transactivation by mutations within eithe r the transactivation domain or the DNA binding domain severely impair ed E2-mediated growth inhibition in HeLa and HT-3 cells, even though t hese mutants retain various other E2 activities. This result indicates that functional transactivation activity is required for acute E2-med iated growth inhibition. HeLa cells, which contain a wild-type p53 gen e, and HT-3 cells, which contain a transactivation-defective p53 gene, exhibited similar responses to the E2 mutants, suggesting that identi cal functions of the E2 protein were required for growth arrest regard less of p53 status. Replacement of the E2 transactivation domain with that of the herpes simplex virus VP16 generated a chimeric transactiva tor that efficiently stimulated expression of an E2-responsive reporte r plasmid yet was completely defective for growth inhibition, suggesti ng that an E2-specific transactivation function is required for growth arrest. Surprisingly, the transactivation-defective E2 mutants were a lso markedly defective in their ability to repress transcription of th e native human papillomavirus type 18 (HPV18) E6/E7 oncogenes in HeLa cells and of the HPV18 promoter present in a transfected reporter plas mid. These mutants were also defective in their ability to increase p5 3 levels. Therefore, efficient repression of the HPV18 promoter in HeL a cells is not merely a consequence of the binding of an E2 protein to appropriately situated binding sites in the promoter.