Fluorescence complementation products with intensity levels mimicking the fu ll length intensities are obtained by introduction of improved folding capabilities with a mutation in position preceding the chromophore. This is particularly seen with the yellow variant of Green Fluorescent Protein (GFP) . An additive increase is obtained by splitting the GFP between amino acids 17 2 and 173. Screening for drugs capable of preventing interaction between proteins is performed by selecting the cells with the highest dynamic range through Fluorescence Activated Cell Sorting (FACS), as illustrated with the ability of FK506 to break the rapamycin induced interaction between FRB and FKBP.
TWO GREEN FLUORESCENT PROTEIN FRAGMENTS AND THEIR USE IN A METHOD FOR DETECTING PROTEIN - PROTEIN INTERACTIONS