G-protein-coupled receptor dimerization: modulation of receptor function.

G-protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors in the human genome that respond to a plethora of signals, including neurotransmitters, peptide hormones, and odorants, to name a few. They couple to second messenger signaling cascade mechanisms via heterotrimeric G-proteins. Recently, many studies have revealed that GPCRs exist as dimers, which may be present as homo- or heterodimers/oligomers. These recent findings have been met with skepticism, since they are contradictory to the dogma that GPCRs function as monomers. Although the existence of GPCR dimers/oligomers was predicted from early pharmacological and biochemical studies, further studies to critically evaluate this phenomenon were impeded by the lack of appropriate reagents. The availability of cDNAs for GPCRs, of highly selective ligands and of antibodies for these receptors has made it possible to visualize and investigate the functional effects of GPCR oligomers. Pharmacological studies, along with biochemical techniques, such as cross-linking and immunoprecipitation with differentially epitope-tagged receptors, have been employed to demonstrate the oligomerization of a number of GPCRs. Moreover, recent biophysical techniques, such as bioluminescence and fluorescence resonance energy transfer, now make it possible to examine GPCR dimerization/oligomerization in living cells. In this review, we provide a brief overview of some of the techniques employed to describe GPCR dimers, and we discuss their respective limitations. We also examine the implications of dimerization/oligomerization on GPCR function. In addition, we discuss domains of the receptors that are thought to facilitate dimerization/oligomerization. Finally, we consider recent evidence for the subcellular localization of the dimer/oligomer assembly.