PathHunter® Total GPCR Internalization cell lines are engineered to co-express the Enzyme Acceptor (EA) tagged GPCR, and a ProLink™ (PK) tag localized to the endosomes. Activation of the GPCR-EA induces internalization of the receptor in PK-tagged endosomes, forcing complementation of the two β-galactosidase enzyme fragments (EA and PK). The resulting functional enzyme hydrolyzes substrate to generate a chemiluminescent signal.