Functionality of Melatonin Receptors: Internalization.

The main step of classical desensitization of a receptor, by mean of its disappearance from the plasma membrane, is its internalization. This is a key factor in the regulation of agonist-mediated signaling pathways, as it most of the time stops the activation of the receptor. Internalization is thus important to evaluate, as a complementary information for a natural ligand or an alternative synthetic agonist. Enzyme fragment complementation is an elegant but delicate way to measure this phenomenon, by fusing two complementary parts of an enzyme to two partners, and to measure the activity of the reconstituted enzyme upon complexation of the partners. In the present chapter, using two parts of β-galactosidase, one fused to the C-terminus of the MT1 receptor, the other to an endosomal protein, one can measure the formation of the complex; thus, the transfer of the receptor to the endosome from which MT1 will be recirculated.