High-end microscopy studies of G protein-coupled receptors (GPCRs) require installing onto the receptors bright and photostable dyes. Labeling must occur in quantitative yields， to allow stoichiometric data analysis， and in a minimally invasive fashion， to avoid perturbing GPCR function. We demonstrate here that the genetic incorporation of trans-cyclooct-2-ene lysine (TCO*) allows achieving quantitative single-residue labeling of the extracellular loops of the β-adrenergic and the muscarinic M class A GPCRs， as well as of the corticotropin releasing factor class B GPCR. Labeling occurs within a few minutes by reaction with dye-tetrazine conjugates on the surface of live cells and preserves the functio... More
High-end microscopy studies of G protein-coupled receptors (GPCRs) require installing onto the receptors bright and photostable dyes. Labeling must occur in quantitative yields， to allow stoichiometric data analysis， and in a minimally invasive fashion， to avoid perturbing GPCR function. We demonstrate here that the genetic incorporation of trans-cyclooct-2-ene lysine (TCO*) allows achieving quantitative single-residue labeling of the extracellular loops of the β-adrenergic and the muscarinic M class A GPCRs， as well as of the corticotropin releasing factor class B GPCR. Labeling occurs within a few minutes by reaction with dye-tetrazine conjugates on the surface of live cells and preserves the functionality of the receptors. To precisely quantify the labeling yields， we devise a method based on fluorescence fluctuation microscopy that extracts the number of labeling sites at the single-cell level. Further， we show that single-residue labeling is better suited for studies of GPCR diffusion than fluorescent-protein tags， since the latter can affect the mobility of the receptor. Finally， by performing dual-color competitive labeling on a single TCO* site， we devise a method to estimate the oligomerization state of a GPCR without the need for a biological monomeric reference， which facilitates the application of fluorescence methods to oligomerization studies. As TCO* and the dye-tetrazines used in this study are commercially available and the described microscopy techniques can be performed on a commercial microscope， we expect our approach to be widely applicable to fluorescence microscopy studies of membrane proteins in general.
