ATF-2 and C/EBPalpha Can Form a Heterodimeric DNA Binding Complex in Vitro. Functional Implications for Transcriptional Regulation

We screened an expression cDNA library with a radiolabeled C/EBPalpha fusion protein and isolated three independent cDNAs encoding ATF-2, a bZIP protein that binds cAMP response elements (CRE). This interaction requires the respective bZIP domains, which form a typical bZIP heterodimer with altered DNA binding selectivity. C/EBPalpha and ATF-2 homodimers bind CRE sites, but ATF-2:C/EBPalpha heterodimers do not. Heterodimers bind an asymmetric sequence composed of one consensus half-site for each monomer, and may thus have a unique regulatory function. As predicted, co-transfection of ATF-2 with C/EBPalpha results in decreased activation of transcription driven from consensus C/EBP-binding sites. In contrast, C/EBPalpha and ATF-2 function cooperatively to activate transcription driven by the asymmetric sequence. Both factors are expressed in liver, where immunoprecipitation experiments show that ATF-2 co-precipitates with C/EBPalpha. These results are consistent with the interpretation that C/EBPalpha and ATF-2 can associate in vivo. Moreover, the formation of ATF-2:C/EBPbeta heterodimers suggests that cross-family dimerization with ATF-2 may be a general property for C/EBP family proteins.