The mammalian proto-oncogene products Fos, Jun and Myc, and the yeast activator GCN4 belong to the "leucine zipper" class of transcription regulation proteins. These proteins bind to DNA as dimers, and dimerization is mediated through their leucine zipper sequences. Dimerization is highly specific; only certain leucine zipper sequences bind to one another. Point mutations in the leucine zipper can disrupt dimerization. A synthetic, 33-amino-acid peptide corresponding to the GCN4 leucine zipper (GCN4-pl) forms a stable, helical dimer. The high- resolution x-ray crystal structure of this peptide determined in the sponsor's laboratory reveals that the leucine zipper is a parallel, helical coiled coil. This peptide provides a simple model for investigating the specific molecular recognition of leucine zipper regions and other coiled coils. The long-term goal of the proposed research is to understand the energetic and structural basis for the stability and specificity of GCN4 dimerization. Amino acid substitutions will be made initially at Asn16 in GCN-4-pl. This residue forms a unique buried hydrogen bond in the dimer interface, and it is conserved in the leucine zippers of Jun and CREB. The thermal stability of the mutant dimers will be measured by circular dichroism (CD), selected mutants will be crystallized, and their x-ray crystal structures will be determined. The crystal structures will be examined in order to understand the relative stabilities of the mutants. The studies of this simple model system will provide unique insights into the physical basis for the specificity of dimerization of leucine zipper proteins.