THE SV40 LARGE T-ANTIGEN AND ADENOVIRUS E1A ONCOPROTEINS INTERACT WITH DISTINCT ISOFORMS OF THE TRANSCRIPTIONAL COACTIVATOR, P300

p300 is a nuclear phosphoprotein likely to be involved in the control of cell growth. Here we show that SV40 large T antigen (Tag) forms a s pecific complex with p300. In various Tag-expressing cell lines, the a ffinity of Tag for p300 was restricted to a newly identified unphospho rylated but ubiquitinated form of the protein. Further, Tag did not as sociate with p300 in an SV40 Tag-producing cell line (REV2) in which t he original transformed phenotype (SV52) is reverted. Biochemical stud ies demonstrate that both the phosphorylation and the ubiquitination p rofile of p300 are altered in REV2 with respect to the wild-type fully transformed SV52 parental cells, wherein Tag-p300 complexes are readi ly detected. In contrast to Tag, the adenovirus early expression produ ct E1a interacts with both phosphorylated and unphosphorylated forms o f p300. In addition, when REV2 cells were infected with adenovirus, E1 a-p300 complexes were detected, suggesting that the p300 expressed in REV2 has lost the affinity for Tag, but not for E1a. We then compared the ability of Tag and E1a to affect the transcription levels of the c AMP-responsive promoter (CRE), which is modulated in vivo by p300, in REV2 cells. We found that Tag repressed the CRE promoter in all of the cell lines in which Tag-p300 complexes were detected, but not in REV2 cells. In contrast, E1a efficiently inhibited CRE-directed transcript ion in this cell line. The data thus indicate that the different speci ficities exhibited by Tag and E1a towards the various forms of p300 ar e reflected in vivo as a difference in the ability of these viral onco proteins to modulate the expression of CRE-containing genes.