EGF-mCherry Fusion Protein Expressed in <i>E. coli</i> Shows Product Heterogeneity but a High Biological Activity

The epidermal growth factor receptor (EGFR) is a transmembrane protein involved in cell signaling processes, and dysregulation of its activity often drives tumor growth. EGFR is a clinically validated tumor marker and target for antibodies and tyrosine kinase inhibitors. We demonstrate that a fusion protein of the natural ligand epidermal growth factor (EGF) with the fluorescent reporter mCherry can be expressed in the cytosol of E. coli in high yields and with a high biological activity. Biophysical characterization by mass spectrometry analysis confirmed three disulfide bonds that are crucial for protein structure. Biolayer interferometry data of the protein–protein interaction of EGF-mCherry with the soluble EGFR are comparable to that of unmodified EGF. Cell culture experiments demonstrated that this fusion replicates all important features of the natural ligand. Finally, fluorescent assays based on EGF-mCherry provided a simple and convenient method to compare EGFR levels on cells and to determine competition of EGFR-binding molecules. These assays will help to rank competitive properties of EGFR inhibitors