GPCRs are 7-transmembrane spanning proteins which are made in the endoplasmic reticulum (ER), matured as they travel through the Golgi apparatus, and finally appear at the cell surface in their functional form. Here they wait to be stimulated by their signalling molecule (an agonist). Since receptor theory was first conceived, we considered that GPCRs functioned alone, as single entities. With the advent of new technology, we could now observe protein-protein interactions in live cells using bioluminescence resonance energy transfer (BRET) methods. From this work it was discovered that GPCRs could exist in very close proximity to each other, and could therefore function as dimers (units of 2 single GPCRs). For many GPCRs, the dimers were formed in the ER as the starting point of their journey to the cell surface.
