Induction-independent recruitment of CREB-binding protein to the c-fos serum response element through interactions between the bromodomain and Elk-1

Proliferative signals lead to the rapid and transient induction of the c-fos proto-oncogene by targeting the ternary complex assembled on the serum response element (SRE). Transactivation by both components of this complex, serum response factor (SRF) and the ternary complex factor Elk-1, can be potentiated by the coactivator CREB-binding protein (CBP). We report a novel interaction between the bromodomain of CBP, amino acids 1100-1286, and Elk-1. DNA binding and glutathione S-transferase pull-down assays demonstrate that binding requires Elk-1(1-212) but not the C-terminal transactivation domain. Competition and antibody controls show that the bromocomplex involves both SRF and Elk-1 on the c-fos SRE and uniquely Elk-1 on the E74 Ets binding site. Interestingly, methylation interference and DNA footprinting analyses show almost indistinguishable patterns between ternary and bromocomplexes, suggesting that CBP-(1100-1286) interacts via Elk-1 and does not require specific DNA contacts. Functionally, the bromocomplex blocks activation, because cotransfection of CBP-(1100-1286) reduces RasV12-driven activation of SRE and E74 luciferase reporters. Repression is relieved moderately or strongly by linking the bromodomain to the N- or C-terminal transactivation domains of CBP, respectively. These results are consistent with a model in which CBP is constitutively bound to the SRE in a higher order complex that would facilitate the rapid transcriptional activation of c-fos by signaling-driven phosphorylation.