C/ATF, A MEMBER OF THE ACTIVATING TRANSCRIPTION FACTOR FAMILY OF DNA-BINDING PROTEINS, DIMERIZES WITH CAAT ENHANCER-BINDING PROTEINS AND DIRECTS THEIR BINDING TO CAMP RESPONSE ELEMENTS

Members of the C/EBP family of basic-leucine zipper (bZip) transcription factors form heterodimers and bind to the CAAT box and other sequence-related enhancer motifs. Using a P-32-labeled protein probe consisting of the bZip domain of C/EBPbeta, we isolated a clone encoding C/EBP-related ATF (C/ATF), a bZip protein that heterodimerizes with C/EBP-like proteins but belongs to the CREB/ATF family. C/ATF homodimers do not bind to typical C/EBP DNA sites. Instead they bind to palindromic cAMP response elements such as that of the somatostatin gene. In addition, C/ATF-C/EBPbeta heterodimers bind to a subclass of asymmetric cAMP response elements exemplified by those in the phosphoenolpyruvate carboxykinase and proenkephalin genes. Transient transfection studies indicate that interactions between C/ATF and C/EBPbeta are the basis for a functional cross talk between these two families of transcription factors that may be important for the integration of hormonal and developmental stimuli that determine the expression of subsets of genes in specific cellular phenotypes.