Different requirements for formation of Jun: Jun and Jun: Fos complexes.

The cFos proto-oncoprotein associates with cJun to form a heterodimer with increased DNA binding and transcriptional activities. It has been suggested that dimerization of these proteins is mediated by the interdigitation of an orderly repeat of leucine residues forming a leucine zipper. In agreement with this model, we find that binding to the AP-1 site requires dimerization of these proteins. Although cFos, itself, does not seem to dimerize and bind to the AP-1 site, Jun: Fos heterodimers have higher stability than Jun homodimers, which accounts for their increased DNA binding activity. Mutational analysis indicates that at least three of the repeated leucines of cJun are important for homodimer formation. However, these residues can be mutated without affecting formation of Jun: Fos heterodimers. In addition, several other residues present between the leucines are also important for both homo- and heterodimerization. These findings provide support for the recent proposal that these proteins dimerize via formation of a coiled coil and suggest that residues other than leucines provide specificity for this interaction. Assuming that dimerization is required for proper alignment of the DNA recognition sites, we generated a cJun mutant containing a small insertion between the dimerization and the DNA recognition domains. This mutant fails to bind DNA, but it acts as a trans-dominant inhibitor of cJun and cFos because it still dimerizes with the wild-type proteins.