Production of recombinant human G protein-coupled estrogen receptor (GPER) and establishment of a ligand binding assay using graphene quantum dots (GQDs)

G protein-coupled estrogen receptor (GPER) is a member of the GPCR family and a key mediator of the rapid, nongenomic actions of estrogens as membrane estrogen receptors. In this study, we established a procedure for the expression and purification of recombinant human membrane estrogen receptor (hGPER) protein via the expression system using the methylotrophic yeast Pichia pastoris. By optimizing codon usage, we successfully expressed hGPER at a level that can be purified by column chromatography. The recombinant protein was purified via three chromatography steps. Purified hGPER showed specific estrogen-binding activity (Kd = 9.9 nM and Bmax = 1.76 nM) in a radiolabeled steroid-binding assay. We subsequently established a homogeneous assay for hGPER ligands by conjugating semiconductor nanoparticles known as graphene quantum dots (GQDs) to hGPER. GQDs coupled with hGPER (GQD-hGPER) caused a decrease in fluorescence at 520 nm from E2-BSA-FITC, which was activated by 370 nm light upon the addition of free estradiol to the reaction mixture. Fluorescence was decreased by the administration of hGPER ligands but not by steroids that do not interact with hGPER. Thus, we successfully established a ligand-binding assay for hGPER that is suitable for screening potential compounds. hGPER is a promising candidate for drug discovery for nongenomic estrogen-stimulating effects. The homogeneous assay established in this study will be usable for that purpose.