Rb function is required for E1A-induced S-phase checkpoint activation

It is widely accepted that adenoviral E1A exerts its influence on recipient cells through binding to the retinoblastoma (Rb) family proteins, followed by a global release of E2F factors from pocket-protein control. Our study challenges this simple paradigm by demonstrating previously unappreciated complexity. We show that E1A-expressing primary and transformed cells are characterized by the persistence of Rb-E2F1 complexes. We provide evidence that E1A causes Rb stabilization by interfering with its proteasomal degradation. Functional experiments supported by biochemical data reveal not only a dramatic increase in Rb and E2F1 protein levels in E1A-expressing cells but also demonstrate their activation throughout the cell cycle. We further show that E1A activates an Rb- and E2F1-dependent S-phase checkpoint that attenuates the growth of cells that became hyperploid through errors in mitosis and supports the fidelity DNA replication even in the absence of E2F complexes with other Rb family proteins, thereby functionally substituting for the loss of p53. Our results support the essential role of Rb and E2F1 in the regulation of genomic stability and DNA damage checkpoints.