Abstract: A method of monitoring the amount of primary or secondary ligand in a cell, using a green fluorescent protein complex and fluorescence resonance energy transfer between two GFP molecules to monitor the primary or secondary ligand, is disclosed. Further disclosed is a method of screening a molecule, such as a peptide, for primary ligand-binding activity, which also uses a green fluorescent protein complex and fluorescence resonance energy transfer between two GFP molecules, to screen the molecule or peptide. A green fluorescent protein complex having a first green fluorescent protein, a primary ligand-binding peptide, and a second green fluorescent protein, is also disclosed.