Ras regulates the association of serum response factor and CCAAT/enhancer-binding protein beta

The serum response element (SRE) is a promoter element essential for transc riptional activation of immediate early genes, such as c-fos and early grow th response-1, by mitogenic signals, Several transcription factors bind the SRE, including the serum response factor (SRF), the ternary complex factor , and the CCAAT/enhancer-binding protein beta (C/EBP beta). The C/EBP beta mRNA encodes three translation products of 38, 35, and 20 kDa, p35-C/EBP be ta activates transcription of the SRE in an SRF-dependent fashion, whereas p20-C/EBP beta, which initiates at an internal in-frame methionine, lacks a transactivation domain and inhibits transcription. We show that SRF and C/ EBP beta interact in vivo through the DNA binding domain of SRF and the C t erminus of C/EEP beta common to p35/38 and p20. Therefore, like the ternary complex factor, C/EBP beta may be recruited to the SRE not only by binding to the DNA, which is not a high affinity site, but also by protein-protein interactions with SRF, Strikingly, in both the mammalian two-hybrid assay and in vivo coimmunoprecipitations, the association of SRF and p35-C/EBP be ta but not p20-C/EBP beta is dramatically stimulated by activated Ras, Furt hermore, mutation of the threonine within a mitogen-activated protein kinas e consensus motif in the C terminus of C/EBP beta eliminates the response t o Ras, These results suggest a new mechanism by which mitogenic signals may influence transcription activity of the SRE by selectively promoting prote in-protein interactions between SRF and the transactivator p35-C/EBP beta.