INTERACTION AND FUNCTIONAL COLLABORATION OF P300 AND C EBP-BETA/

Transcriptional coactivators such as p300 and CREB-binding protein (CB P) function as important elements in the transcription factor network, linking individual transactivators via protein-protein interactions t o the basal transcriptional machinery. We have investigated whether p3 00 plays a role in transactivation mediated by C/EBP beta, a conserved member of the C/EBP family. We show that C/EBP beta-dependent transac tivation is strongly inhibited by adenovirus E1A but not by E1A mutant s defective in p300 binding. Ectopic expression of p300 reverses the E 1A-dependent inhibition and increases the transactivation potential of C/EBP beta. Furthermore, we show that C/EBP beta and p300 interact wi th each other and demonstrate that the sequences responsible for inter action map to the E1A binding region of p300 and the amino terminus of C/EBP beta. Finally, we show that the minimal C/EBP beta binding site of p300 acts as a dominant-negative inhibitor of C/EBP beta. These ob servations identify p300 as a bona fide coactivator for C/EBP beta. C/ EBP beta is highly expressed in the myelomonocytic lineage of the hema topoietic system and cooperates with Myb to activate mim-1, a gene spe cifically expressed during myelomonocytic differentiation. Recent evid ence has shown that Myb recruits CBP (and presumably p300) as a coacti vator and, in contrast to C/EBP beta, interacts with the CREB binding site of p300-CBP. We show that p300 not only stimulates the activity o f Myb and C/EBP beta individually but also increases the synergy betwe en them. Thus, our results reveal a novel function of p300: in additio n to linking specific transcription factors to the basal transcription al machinery, p300 also mediates the cooperation between transactivato rs interacting with different domains of p300.