Green fluorescent protein mutant as label in homogeneous assays for biomolecules

The green fluorescent protein (GFP) and its mutants have been extensively used to study various cellular processes and, more recently, as labels in binding assays. We have employed a mutant of GFP, an enhanced GFP (EGFP), in the development of homogeneous assays for biotin and cortisol. To demonstrate the feasibility of using EGFP as a label with different kinds of binders in the development of homogeneous assays, we employed the biotin-avidin and an antigen-antibody as the binding pairs. Biotin and cortisol were chemically conjugated to EGFP. A quenching of fluorescence intensity of EGFP was observed upon binding of avidin to the EGFP-biotin conjugate. The percentage fluorescence quenching observed decreased as the concentration of free biotin in the sample increased due to the fewer binding sites on avidin available for binding to the EGFP-biotin conjugate. A dose-response curve for biotin was generated by relating percentage fluorescence quenched with free biotin in the sample. To determine whether EGFP can undergo a similar type of homogeneous change when used as a label for antigen-antibody type of binding, cortisol was selected as a model analyte. In the presence of an anti-cortisol antibody the fluorescence signal of the EGFP-cortisol conjugate was quenched. A dose-response curve for cortisol was generated by relating the quenching in the fluorescence signal with varying amounts of free cortisol in the sample. This is the first time that GFP or one of its mutants has been employed as a label in homogeneous assays, thus enhancing the versatility of employing GFP or its mutants in a number of bioanalytical applications, such as clinical analysis and high-throughput screening systems.