Quantifying GPCR internalization: A focus on the kisspeptin receptor

GPCR internalization is a critical regulatory step in determining receptor activity. While internalization terminates G protein-coupled signaling, it might be required for G protein-independent signaling. A large number of clinical therapies are based on preventing or promoting GPCR internalization. Thus, for any given GPCR, it is important to characterize its internalization and understand the factors that regulate such internalization. Here we describe different experimental protocols to evaluate the internalization of any GPCR transiently expressed in HEK 293 cells. The protocols describe the use of immunofluorescence and imaging techniques as well as flow cytometry. The techniques described use the FLAG-tagged kisspeptin receptor (KISS1R) as an example but are equally applicable to any other GPCR.