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 acid (RA) or adenovirus E1A during the differentiation of F9 cells. We show here that RA and E1A induce phosphorylation of the E1A-associated 300 kDa protein (p300) during the differentiation of F9 cells. The region of E1A that is required for interaction with cellular protein p300 overlaps with the region of E1A required for E1A to induce expression of the c-jun gene. Treatment of F9 cells with RA or infection of the cells by adenovirus led to a decrease in the electrophoretic mobility of p300. Phosphatase treatment of p300 from RA-treated or adenovirus-infected 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 sufficient for RA- and E1A-mediated up-regulation of the c-jun gene. The mobility of DRF complexes was changed by E1A or RA and the complexes were supershifted by addition of a polyclonal p300 antiserum. Moreover, overexpression of p300 resulted in an increase in the level of DRF1 complex. p300 fused to the DNA binding domain of the E2 protein of papilloma virus stimulated E2-dependent reporter activity in response to RA or E1A in F9 cells. Our results suggest that p300 is part of the DRF complexes, that it is differentially phosphorylated in undifferentiated versus differentiated cells and that it is likely involved in regulating transcription of the c-jun gene during F9 cell differentiation.