Fluorescence polarization assay to quantify protein-protein interactions: an update.

A fluorescence polarization assay can be used to evaluate the strength of a protein-protein interaction. A green fluorescent protein variant is fused to one of the protein partners. The formation of a complex is then deduced from an increase in fluorescence polarization, and the equilibrium dissociation constant of the complex is determined in a homogeneous aqueous environment. The assay is demonstrated by using the interaction of the S-protein and S-peptide fragments of ribonuclease A as a case study.