Cells (e.g. HepG2 hepatocellular cancer cells) are transfected with a plasmid that encodes the desired single intracellular protein (e.g. glucose transporter 1). Before the transfection, the plasmid is modified by inserting a special sequence that determines the integration of an unnatural reactive amino acid (BCN-lysine) into the transfected protein. After protein synthesis, the live cells are treated with a cell permeable fluorogen carrying a reactive tetrazine group that forms a covalent adduct with the reactive lysine. In this way, the molecular interactions of the labeled protein can be directly followed by fluorescence microscopy (e.g. translocation towards the cell membrane) and the effects of inhibitors or enhancers of the glucose transporters can be analyzed in live cells. The major significance of the method is, that it may be useful in testing potential antitumor compounds that inhibit glucose uptake/metabolism.