PathHunter® Total GPCR Internalization cell lines are engineered to co-express the ProLink™ (PK) tagged GPCR, and an Enzyme Acceptor (EA) tag localized to the endosomes. Activation of the GPCR-PK induces internalization of the receptor in EA-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.