Two pairs of oppositely charged amino acids from Jun and Fos confer heterodimerization to GCN4 leucine zipper.

The preferential assembly of Jun and Fos into heterodimers has been shown to be mainly driven by 16 amino acids (8 from each protein) situated in positions e and g of the leucine zipper coiled-coil structures of the two proteins (O'Shea, E. K., Rutkowski, R., and Kim, P. S. (1992) Cell 68, 699-708). Using a similar approach, we show that among these residues two pairs of oppositely charged amino acids account in fact for most of the additional free energy of heterodimerization in this system. These residues are 2 glutamic acid side chains in positions g1 and e2 of the Fos leucine zipper and 2 lysine residues in the equivalent positions of the Jun zipper. These amino acids were placed in the context of a GCN4 leucine zipper using peptide synthesis. These peptides contain unique cysteine residues enabling the formation of covalent dimers. The gain in heterodimer free energy has been determined both by cysteine-linked dimer formation under redox conditions and by thermal melting experiments of covalent dimers using circular dichroism experiments. The two pairs of oppositely charged residues (Glu,Glu and Lys,Lys) in positions g1 and e2 contribute at least -1.9 kcal/mol of additional free energy, accounting for a 50-fold excess of the heterodimer with respect to one of the homodimers. Thermal denaturation studies as a function of pH and ionic strength suggest that electrostatic effects should indeed be a major driving force for heterodimerization. On the contrary, peptides harboring the 12 amino acids from Jun and Fos in the other e and g positions (i.e. in e1, g2, e3, g3, e4, and g4) show only a moderate tendency to form heterodimers.