Downsizing proto-oncogene cFos to short helix-constrained peptides that bind Jun

The oncogenic transcription factor Activator Protein-1 (AP-1) is a DNA binding protein that assembles through dimerization of Fos and Jun protein subunits, their leucine-rich helical sequences entwining into a coiled coil structure. This study reports on downsizing the protooncogene cFos protein (380 residues) to shorter peptides (37-25 residues), modified with helix-inducing constraints to enhance binding to Jun. A crystal structure is reported for a 37-residue Fos-derived peptide (FosW) bound to Jun. This guided iterative downsizing of FosW to shorter peptide sequences, constrained into stable water-soluble alpha helices by connecting amino acid sidechains to form cyclic pentapeptide components. Structural integrity in the presence and absence of Jun was assessed by circular dichroism spectroscopy, while thermodynamics of binding to cFos was measured by isothermal titration calorimetry. A 25-residue constrained peptide, one-third shorter yet 25 % more helical than the structurally characterised 37-residue Fos-derived peptide, retained 80 % of the binding free energy due to pre-organisation in a Jun-binding helix conformation, with entropy gain (TΔS = + 3.2 kcal/mol) compensating for enthalpy loss. Attaching a cell penetrating peptide (TAT48-57) and a nuclear localisation signal (SV40) promoted cell uptake, localisation to the nucleus, and inhibition of the proliferation of
two breast cancer cell lines.