Site-specific epitope tagging of G protein-coupled receptors by bioorthogonal modification of a genetically encoded unnatural amino acid.

We developed a general strategy for labeling expressed membrane proteins with a peptide epitope tag and detecting the tagged proteins in native cellular membranes. First, we genetically encoded the unnatural amino acid p-azido-L-phenylalanine (azF) at various specific sites in a G protein-coupled receptor (GPCR), C-C chemokine receptor 5 (CCR5). The reactive azido moiety facilitates Staudinger ligation to a triarylphosphine-conjugated FLAG peptide. We then developed a whole-cell-based enzyme-linked immunosorbent assay approach to detect the modified azF-CCR5 using anti-FLAG mAb. We optimized conditions to achieve labeling and detection of low-abundance GPCRs in live cells. We also performed an accessibility screen to identify azF positions on CCR5 amenable to labeling. Finally, we demonstrate a preparative strategy for obtaining pure bioorthogonally modified GPCRs suitable for single-molecule detection fluorescence experiments. This peptide epitope tagging strategy, which employs genetic encoding and bioorthogonal labeling of azF in live cells, should be useful for studying biogenesis of polytopic membrane proteins and GPCR signaling mechanisms.