A FRET-based competitive binding assay using coumestrol and the ligand-binding domain of human estrogen receptor alpha tagged with mTurquoise2 efficiently expressed in E. coli with ethanol

Abstract

The estrogen receptor (ER) is a nuclear receptor and one of the most extensively researched targets in the study of endocrine-disrupting chemicals (EDCs). Many biosensors and bioassays for estrogenic EDCs use the ligand-binding domain of human ERα (LBD-hERα) as a biological recognition element. However, the LBD-hERα is poorly stable and difficult to produce as a functional LBD-hERα in the E. coli expression system. In this study, we efficiently expressed the functional LBD-hERα tagged with the cyan fluorescent protein, mTurquoise2 (LBD-hERα-mTq2) by the addition of ethanol (3 %) to E. coli suspension during protein expression (> 40 times more compared to without ethanol). We found that ethanol not only promoted the proper folding of LBD-hERα-mTq2, but also prevented the proteolysis of poorly folded recombinant proteins. We established a FRET-based binding assay between a fluorescent estrogen, coumestrol, and the LBD-hERα-mTq2, in which the formation of the complex exhibits a significant degree of FRET. A subsequent competitive binding assay with diethylstilbestrol demonstrates that our system successfully functions as a simple and reliable bioassay to detect estrogenic EDCs.

Keywords: Estrogen receptor; Ethanol-assisted protein folding; FRET; Fluorescent protein; Ligand-binding domain; coumestrol; mTurquoise2.