mXBP/CRE-BP2 and c-Jun form a complex which binds to the cyclic AMP, but not to the 12-O-tetradecanoylphorbol-13-acetate, response element.

Proto-oncogene products c-Fos and c-Jun form a complex which binds with high affinity to the 12-O-tetradecanoylphorbol-13-acetate (TPA) response DNA element and which stimulates transcription of phorbol ester- inducible genes. We have previously identified, by screening a lambda gt11 expression library, murine protein mXBP, which binds to a sequence which overlaps the 3' end of the murine class II major histocompatibility complex A alpha gene X box, a conserved transcription element found upstream of all class II genes. Here, we demonstrate that the target sequence for mXBP is a consensus cyclic AMP response element (CRE). mXBP is a member of the leucine zipper family of DNA-binding proteins and has significant homology to oncoproteins c-Fos and c-Jun. The inferred amino acid sequence of mXBP shows near identity to human CRE-BP1, except it does not contain an internal proline-rich domain. Immunoprecipitation and glutaraldehyde cross-linking studies show that mXBP/CRE-BP2 can form a complex with c-Jun. Complex formation is dependent on intact leucine zipper domains in both proteins. mXBP-c-Jun complexes can coexist with c-Fos-c-Jun complexes and can bind with high affinity to CRE, but not to TPA response DNA element, sequences. These results suggest that changes in the expression of mXBP/CRE-BP2, c-Fos, and c-Jun, which alter the ratio of mXBP-c-Jun to c-Fos-c-Jun complexes, would affect the relative expression of cyclic AMP and phorbol ester-responsive genes. This provides support for a combinatorial model of gene regulation, whereby protein-protein interactions which alter the DNA binding specificity of protein complexes can expand the flexibility of cellular transcriptional responses.