THE ADENOVIRUS E1A PROTEIN OVERRIDES THE REQUIREMENT FOR CELLULAR RASIN INITIATING DNA-SYNTHESIS

The adenovirus E1A protein can induce cellular DNA synthesis in growth -arrested cells by interacting with the cellular protein p300 or pRb. In addition, serum- and growth factor-dependent cells require ras acti vity to initiate DNA Synthesis and recently we have shown that Balb/c 3T3 Cells can be blocked in either early or late G(1) following microi njection of an anti-ras antibody. In this study, the E1A 243 amino aci d protein is shown through microinjection not only to shorten the G(0) to S phase interval but, what is more important, to override the inhi bitory effects exerted by the anti-ras antibody in either early or lat e G(1). Specifically, whether E1A is co-injected with anti-ras into qu iescent cells or injected 18 h following a separate injection of anti- ras after serum stimulation, it efficiently induces cellular DNA synth esis in cells that would otherwise be blocked in G(0)/G(1). Moreover, injection of a mutant form of E1A that can no longer associate with p3 00 is just as efficient as wild-type E1A in Stimulating DNA synthesis in cells whose ras activity has been neutialized by anti-ras. The resu lts presented here show that E1A is capable of overriding the requirem ent of cellular ras activity in promoting the entry of cells into S ph ase. Moreover, the results suggest the possibility that pRb and/or pRb -related proteins may function in a ras-dependent pathway that enables E1A to achieve this activity.