PHOSPHORYLATION OF THE ADENOVIRUS E1A-ASSOCIATED 300 KDA PROTEIN IN RESPONSE TO RETINOIC ACID AND E1A DURING THE DIFFERENTIATION OF F9 CELLS

Transcription of the c-jun gene is up-regulated by either retinoic aci d (RA) or adenovirus E1A during the differentiation of F9 cells, We sh ow here that RA and E1A induce phosphorylation of the E1A-associated 3 00 kDa protein (p300) during the differentiation of F9 cells. The regi on of EIA that is required for interaction with cellular protein p300 overlaps with the region of E1A required for EIA to induce expression of the c-jun gene. Treatment of F9 cells with RA or infection of the c ells by adenovirus led to a decrease in the electrophoretic mobility o f p300. Phosphatase treatment of p300 from RA-treated or adenovirus-in fected F9 cells reversed the changes in migration of p300, indicating that RA- and E1A-mediated changes in the mobility of p300 were due to phosphorylation. We also found factors, designated DRF1 and DRF2, that bound specifically to a sequence element that is necessary and suffic ient for RA- and E1A-mediated up-regulation of the c-jun gene. The mob ility of DRF complexes was changed by E1A or RA and the complexes were supershifted by addition of a polyclonal p300 antiserum. Moreover, ov erexpression of p300 resulted in an increase in the level of DRF1 comp lex. p300 fused to the DNA binding domain of the E2 protein of papillo ma virus stimulated E2-dependent reporter activity in response to RA o r E1A in F9 cells. Our results suggest that p300 is part of the DRF co mplexes, that it is differentially phosphorylated in undifferentiated versus differentiated cells and that it is likely involved in regulati ng transcription of the c-jun gene during F9 cell differentiation.