THE CREB-BINDING PROTEIN (CBP) COOPERATES WITH THE SERUM RESPONSE FACTOR FOR TRANSACTIVATION OF THE C-FOS SERUM RESPONSE ELEMENT

The serum response element is one of the major promoter elements of th e immediate early response to extracellular signals. The serum respons e element includes two main binding sites for proteins: the Ets box, w hich binds p62(TCF), and the CArG box, which binds p67(SRF). These two proteins are direct targets for signal transduction pathways; p62(TCF ) is a nuclear end point of the Ras/mitogen-activated protein kinase p athway, and p67(SRF) is targeted by the Rho/Rac small G-proteins. The mechanism by which the signal is further transduced from the transcrip tion factors to the basal transcriptional machinery is poorly understo od. Recent data have suggested that the cAMP-responsive element-bindin g protein (CREB)-binding protein, a transcriptional adaptor involved i n the transactivation through a wide variety of enhancer elements, par ticipates in p62(TCF) activity., We here show that the CREB-binding pr otein also cooperates in the process of transactivation by p67(SRF). C otransfections of expression vectors for the CREB-binding protein incr eased the expression, in response to serum, of reporters under the con trol of the c-fos serum response element, Interestingly, the C-termina l moiety of the CREB-binding protein was not necessary to observe this effect. The cooperation did not require the Ets box in the serum resp onse element, and the CArG box was sufficient, indicating that the CRE B-binding protein is able to cooperate with p67(SRF) in, the absence o f an Ets protein, Co-immunoprecipitation experiments using cell extrac ts showed that p67(SRF) could be retained with antibodies directed aga inst the CREB-binding protein, suggesting that the two proteins form a multimolecular complex in live cells, The physical interaction betwee n p67(SRF) and the CREB-binding protein was further confirmed by two-h ybrid assays in mammalian cells, Our results indicate that the CREB-bi nding protein cooperates with p67(SRF) and, thus, suggest that the ser um response element is regulated by a multimolecular complex, which in cludes the CREB-binding protein, p67(SRF), and p62(TCF), With multiple interactions between the components of the complex.