Domain swapping reveals the modular nature of Fos, Jun, and CREB proteins

The products of the Jun and Fos proto-oncogenes form a heterodimer that binds to and activates transcription from 12-O-tetradecanoylphorbol-13-acetate-responsive promoter elements (TGACTCA) and AP-1-binding sites (TGACATCA). These two proteins belong to a family of related transcription factors which contain similar domains required for protein dimerization and DNA binding but display different protein and DNA binding specificities. The basic region, required for DNA binding, is followed by a leucine zipper structure, a domain that mediates protein-protein interactions. To assess the role of these two domains in three related proteins, Fos, Jun, and CREB, we carried out extensive domain-swapping analysis. We found that (i) dimers formed by two Jun leucine zipper-containing proteins were unable to bind DNA as efficiently as a Fos-Jun combination, regardless of the source of the basic region; (ii) the Fos leucine zipper was unable to form either homo- or heterodimers with a chimeric protein containing a Fos leucine zipper; (iii) the Fos basic region was capable of binding to an AP-1 site; (iv) replacement of the Jun amino terminus with that of CREB had little effect on dimerization, whereas replacement with the amino terminus of Fos disrupted both protein-protein and protein-DNA interactions; (v) changes in relative affinities of the Fos and Jun basic regions for the AP-1 element were dependent on the secondary contributions of amino-terminal residues; and (vi) the Fos-Jun chimeric constructs cooperated in transcriptional transactivation of the Jun promoter in NIH 3T3 cells.