What is GPCR?
G-protein coupled receptors (GPCRs) are cell surface receptors that play a critical role in cell signaling. GPCRs are the largest and most diverse protein family in the mammalian genome. They contain seven membrane helices, with an intracellular C-terminus connected by three intracellular and three extracellular loops and an extracellular N-terminus; thus giving rise to their other names 7-TM receptors, heptahelical receptors, and seven transmembrane receptors. They are comprised of about 800-1000 members, making up about 3 – 5 % of the human genome.
The GPCRs family is subdivided into 7 main classes including Class A rhodopsin-like, Class B secretin-like, Class C metabotropic glutamate/pheromone, Class F frizzled (FZD), Taste receptors (TAS1R, TAS2R), Vomeronasal receptors (VN1R, VN2R) and 7TM orphan receptors, in which class A rhodopsin like GPCRs is the largest. They make up about 48% percent of all GPCRs. The division is based on the type of stimuli that activates the GPCR and sequence similarity. Although most GPCRs have the same seven transmembrane structures, they all have differences in the N-terminus and manner in which their corresponding stimulus binds, thus giving rise to many different functions and sequences. The largest class, class A consists of light receptors and adrenaline receptors with a highly conserved Asp-Arg-Tyr motif at the cytoplasmic side of the third transmembrane domain. Class B consists of hormone and neuropeptide receptors. Class C class receptors are composed with GPCRs with an exceptionally large N-terminus.
As their name implies, GPCRs initiate signaling via G-proteins. The G-proteins coupled to GPCRs are heterotrimeric and consist of alpha (α), beta (β), and gamma (γ) subunits. The GPCR, in essence, is a guanine-nucleotide exchange factor (GEF) for the Gα subunit. GPCR signaling is initiated by the binding of an agonist to the GPCR, causing a conformational change of the GPCR, which results in the activation of GEF activity toward the Gα subunit. This GEF activity causes an increase in the dissociation of GDP from Gα, allowing the rapid exchange of GDP for GTP, which is present in high intracellular concentrations. The activated, GTP-bound Gα subunit then promotes dissociation of the heterotrimeric complex. The GTP-bound Gα subunits and Gβγ dimers then go on to activate a number of second messenger generating pathways including the activation of phospholipase C and the activation/inhibition of adenylate cyclase in addition to a variety of other pathways. The signaling is terminated by the intrinsic GTPase activity of the Gα subunit, which cleaves GTP to form GDP, inactivating Gα and resulting in the re-association of Gα with Gβγ in a heterotrimeric, inactive complex.
G protein–coupled receptors are found only in eukaryotes, such as yeast, choanoflagellates and animals. They mediate effects of light, neurotransmitters, lipids, proteins, amino acids, hormones, nucleotides, and chemokines. GPCRs family is predicted to be present throughout the majority of sequenced eukaryotic genomes. Classically GPCRs activate a chemosensory transduction pathway through a change in the associated heterotrimeric G-protein activity. Animal sensory cells from nematodes to vertebrates express hundreds of GPCR genes that play critical roles in both olfaction and gustation through heterotrimeric G-protein activation. Fungi and Amoebozoans, also utilize GPCRs in chemosensation. In yeast, this receptor class has been shown to play important roles in their nutrient and pheromone sensing pathways.
The human genome contains an estimated 900 GPCRs. They regulate a wide range of physiological processes including metabolism, neurotransmission, growth, and visual perception. While the primary amino acid sequence of GPCRs varies widely, all GPCRs share certain structural features. Solving the crystal structure for rhodopsin confirmed that GPCRs are composed of seven-transmembrane α-helices separated by three intracellular and three extracellular loops; the helices are oriented with an extracellular N-terminus and an intracellular C-terminus. GPCRs undergo cycles of activation and inactivation. In the inactive state, the heterotrimeric G-protein is composed of tightly associated GDP (guanosine-5’- diphosphate)-bound Gα and Gβγ subunits. Upon ligand binding, GPCRs undergo a conformational change, catalyzing Gα to exchange GDP for GTP (guanosine-5’- triphosphate). Then, GTP-bound Gα and Gβγ dissociate from the GPCR and activate downstream signaling effectors. Hydrolysis of Gα-GTP to Gα-GDP causes the reassociation of the heterotrimeric G-protein with the GPCR.
GPCR and the drug target
GPCRs constitute an important class of pharmaceutical targets because of their capacity to sense various signaling molecules and mediate a diverse range of cellular responses. It is remarkable that about 40% of known drugs work through the G proteincoupled receptors. Most of these pharmaceuticals, however, only act on a small percentage (<10%) of GPCRs that have been extensively studied. The vast number of GPCRs that are yet to be characterized are likely targets for new drugs that remain to be discovered. Progress in GPCR-targeted drug discovery is primarily hindered by the lack of comprehensive understanding of the structure-function relationship of this family of transmembrane proteins.
Endogenous and synthetic GPCR ligands are grouped into different classes on the basis of their effects on signaling: agonists (full or partial) increase receptor signaling, inverse agonists (full or partial) reduce the basal activity of the receptor, and neutral antagonists have no effect on receptor activity but can block the binding site and prevent other ligands from binding. Some receptors do not exhibit detectable basal activity, in which cases, receptor inverse agonists may not exist at all. Specific therapeutic benefits can be achieved by introducing ligands that either activate or block receptor signaling. For example, opioid receptor agonists such as morphine for treating pain, b2-adrenoceptor agonists such as salmeterol for treating asthma, and angiotensin receptor antagonists such as losartan for treating hypertension. These compounds are developed, in most cases, by screening of libraries composed of hundreds of thousands of available chemical entities. A more rational structure-based drug discovery approach has benefited significantly from the recent breakthroughs in structural understanding of the seven-transmembrane receptors.