Scheme 1.

Activation and trafficking process of a G protein-coupled receptor (GPCR). (A) When a GPCR is activated, GTP takes the place of GDP at the GPCR-coupled G protein α subunit, allowing its release from the βγ subunit. Then, the complex of GPCR/agonist/α subunit/GTP activates a specific biological response, with the participation of specific partners. (B) As a regulatory (desensitization) process, β-arrestin is recruited, allowing GPCR translocation through endosomal pathways where it can be recycled at the membrane or translocated in the nucleus to activate specific genes. GPCR can also be internalized in lysosomes, allowing degradation.