E1A PROMOTES ASSOCIATION BETWEEN P300 AND PRB IN MULTIMERIC COMPLEXESREQUIRED FOR NORMAL BIOLOGICAL-ACTIVITY

The oncogenes of the small DNA tumor viruses encode transforming prote ins with multiple domains that influence the cell cycle and aspects of the transformed phenotype. Like other gene products of this type, the adenovirus E1A proteins influence the cell by binding to specific cel l growth control proteins. These include members of the retinoblastoma gene product (pRB) family, which are bound by the E1A region 2-specif ic site, and p300, which is bound at the E1A amino terminus. Binding a t these two sites is largely independent, and discrete transcription-r egulating functions remain intact in E1A products when only one or the other binding site is functional. In this report, immunoprecipitation with p300 antibodies reveals the presence of the pRB family proteins in p300 complexes when E1A is expressed in host cells, indicating that E1A can mediate physical contact between p300 and the pRB-related pro teins. The ability of E1A to induce proliferation efficiently in quies cent primary cells correlates closely with the ability to bind p300 an d individual members of the pRB family simultaneously in multimeric co mplexes, even though the E1A active sites can bind their target protei ns efficiently when separated on different molecules. Conservation of a spacer region between the two binding sites that is required for sim ultaneous binding and efficient induction of proliferation supports th e concept that the E1A protein structure has evolved to facilitate sim ultaneous binding. These results indicate that the E1A proteins are de signed not merely to sequester these cellular products, but also to br ing them into proximal association with each other in biologically sig nificant complexes.