We are developing an allosteric sensor for detection of protein-protein interactions by the insertion of a protein of interest into a surface loop in the green fluorescent protein (GFP). We envision that the insertion of an unstructured protein will result in GFP retaining its fluorescent properties; however, when the same inserted protein regains structure in the presence of its binding partner, the GFP fluorescence will be perturbed. To evaluate this approach, a leucine zipper peptide, Fos is inserted into the a specific surface loop of the GFP β-barrel. Upon the introduction of the companion leucine zipper Jun, heterodimerization between the two leucine zippers lead a concomitant change in the GFP barrel structure, leading to decreased fluorescence. Results show that the titration with the Jun leucine zipper leads to a proportional decrease in GFP fluorescence and titration produced an IC₅₀~ 87.5 nM, a value similar to the literature values of the Fos/Jun binding constant.