Non-imaging-based β-arrestin recruitment assays. (A) BRET assay. The GPCR is tagged with a RLuc, and β-arrestin is tagged with GFP, or vice versa. Upon β-arrestin recruitment, the two tags come into close proximity and the light emitted from the RLuc reaction excites the GFP, which then emits a detectable signal at a higher wavelength. (B) Tango™ assay. β-arrestin is fused to a protease, while GPCR is extended at its C-terminus with a protease cleavage site followed by a transcription factor (TF). Upon β-arrestin recruitment, the TF fused to the receptor is cleaved and enters the nucleus to regulate the transcription of a reporter gene. (C) PathHunter™ assay. β-arrestin is fused to a deletion mutant of β-galactosidase that is catalytically inactive, and GPCR is tagged with a small fragment derived from the deleted sequence of the enzyme (ProLink™). Upon GPCR-β-arrestin interaction, the two parts of β-galactosidase are brought into close proximity, which results in cleavage of the substrate and generation of a chemiluminescent signal.