Expression and purification of the ectodomain of erythropoietin receptor fused to mCitrine or mTFP1 fluorescent protein in Escherichia coli.

The erythropoietin receptor (EPOR) is a single-pass transmembrane protein that homo-dimerizes upon binding with its renal ligand erythropoietin (EPO) to trigger downstream signaling. Its extracellular ectodomain mediates ligand binding. Therefore, a fusion protein of the EPOR ectodomain can be useful for various in vitro assays, such as a binding assay with an EPO-like peptide, if overexpressed in Escherichia coli (E.coli). In this study, we hypothesized that fusion proteins of the EPOR ectodomain with mCitrine or mTFP1, expressed in bacteria, could enable in vitro Förster resonance energy transfer experiments. Two fusion proteins, EPOR-mCitrine and EPOR-mTFP1, were overexpressed in E. coli but obtained as inclusion bodies. Urea solubilization and stepwise dialysis yielded soluble fusion proteins. Circular dichroism spectroscopy revealed that EPOR-mCitrine had greater secondary structure content than EPOR-mTFP1. When combined with recombinant human EPO, the hydrodynamic radius of EPOR-mCitrine changed, as measured using dynamic light scattering, confirming binding. This interaction was further validated using isothermal titration calorimetry. We propose that bacterially produced EPOR-mCitrine is a useful in vitro tool for measuring EPO binding.