G-protein coupled receptors (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 GPCR families are subdivided into 5 main classes, 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. Type C class receptors are composed with GPCRs with an exceptionally large N-terminus. The other two classes D and E are composed of yeast pheromone receptors and cAMP receptors.
Fig. 1 Schematic diagram of a GPCR (Moukhametzianov, Warne et al. 2011).
To date the most novel sequences for GPCRs have been identified by search methods such as BLAST analysis or polymerase chain reaction. Using this approach has been useful but it is still not enough to discover sequences of GPCRs that are more distantly related to the identified ones. This makes it difficult to determine their structure and sequences. Successfully crystallizing GPCR’s is very important because their three dimensional structures can help identify their novel ligands, and this can provide information for drug discoveries. So far there are only a few known structures of GPCRs, bovine rhodopsin was the first discovered, in 2000. Rhodopsin is a light activated photoreceptor in the eye. Its crystallization was a breakthrough because it led to discoveries of other systems that are not entirely related to structural studies. The discovery of Beta 2 receptor followed in 2007. Beta 1 receptor and Adenosine receptor structures were both determined in 2008. CXCR4 chemokine receptor and dopamine D3 receptor structures were subsequently determined in 2010. Finally, histamine H1 receptor was determined in 2011. GPCRs are unstable outside of the cell membrane and they are known to adopt many conformations, making it very difficult to successfully crystallize them. Their conformation makes them very flexible and fragile, making it very hard to obtain good crystals. GPCR’s were believed to be monomers. But studies have shown that they do tend to dimerize or become part of larger complexes. Dimerization is just a minimal structure for function, because studies have shown effects of ligands leading to quaternary structures.
GPCRs play a major role in intracellular communication. For example, they mediate effects of light, neurotransmitters, lipids, proteins, amino acids, hormones, nucleotides, and chemokines, and are capable of coupling to at least 18 different G-alpha proteins. They also play major roles in diseases such as cancer, HIV, metabolic diseases, and neurological diseases. Even though GPCRs play major roles in many diseases, mutations involving GPCR’s are rare. About 1/1000 people suffer from them. Most of the disorders they are involved in, involve antibodies directed against GPCRs. Due to their association to many diseases, they are desired targets for the development of novel drugs. They are also known to be very druggable, having many therapeutics directed towards them. Currently they make up about 2/3 of the drug discovery programs and about 25% of the top drugs in the market today. Regardless of making up such a large number in the drug industry not much is known about GPCRs, because of their instability and toxic effect on their host cells. This has given rise to the large interest and attention that the research community has given to them. Developing an easy and fast way to study GPCRs can give a broad range of information about their structure, function and interaction with their stimuli.
The activation of a GPCRs leads to a cascade of responses within the cell. Activation of GPCRs leads to the activation of G-proteins that are involved in another cascade of messages involving the alpha, beta and gamma subunits. These proteins function as the on and off switch of cell signaling, guanosine diphosphate (GDP) the inactive state and guanosine triphosphate (GTP) the active state--GPCRs function as the keyhole of cell interaction and their stimuli being the key. They play such a major role in cell signaling. Understanding how they function and what roles they are involved in when it comes to human diseases is what makes them such important targets for research.