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Conflicts of interest
Introduction G protein-coupled receptors (GPCRs), form the largest human membrane protein family, with 800 members overall. Many druggable targets for treatment of common diseases involve GPCRs that mediate therapeutic effects of 34% of all marketed drugs (Hauser, Attwood, Rask-Andersen, Schiöth, & Gloriam, 2017). The GPCR superfamily is classified into several classes, with rhodopsin-like class A being the largest and most studied so far. Class B includes the secretin-like and adhesion GPCRs, whereas class C contains metabotropic glutamate (mGlu), gamma-aminobutyric 2 deoxyglucose (GABAB), the calcium-sensing and taste receptors. In addition, smoothened and frizzled receptors are categorized as class F (Di Pizio et al., 2016; Moller, Moreno-Delgado, Pin, & Kniazeff, 2017). GPCRs are key players in cell communication and in the transduction of sensory signals of external origin, such as light, odorants and taste molecules, and endogenous stimuli such as hormones, neurotransmitters, (neuro)peptides, chemokines, purine ligands, and calcium ions among others molecules. Recent studies shed light on the conformational changes that follow GPCR activation and the structural state of the receptor necessary for its interactions with the three classes of proteins that preferentially bind active GPCRs: G proteins, GPCR kinases and β-arrestins (Gonzalez, Cordomí, Matsoukas, Zachmann, & Pardo, 2014; Gurevich & Gurevich, 2017; Vilardaga, Agnati, Fuxe, & Ciruela, 2010; Wang, Yu, Liu, Liu, & Song, 2017). GPCRs, also called seven-transmembrane (TM) receptors or heptahelical receptors, are cell surface proteins that share a common topology comprised by an extracellular (EC) N-terminus and seven TM α-helices connected by three intracellular (ICL) and three extracellular loops (ECL). They also have a disulphide bridge between ECL2 and TM3, and a cytoplasmic C-terminus containing an amphiphilic α-helix (H8) parallel to the cell membrane (Baltoumas, Theodoropoulou, & Hamodrakas, 2016; Gonzalez et al., 2014). The 7TM bundle can be divided into two module regions: EC and IC modules. The N-terminus ranges in length from relatively short, and often unstructured, as in many Class A members, to long and large globular domains with defined secondary structure, such as in class C GPCRs. The IC module includes the ICLs, a short helix 8 and an IC C-terminus, and is more conserved among GPCRs, compared to the EC region (Di Pizio et al., 2016; Katritch, Cherezov, & Stevens, 2012). The IC region is responsible for interaction with G proteins, β-arrestins, and other downstream effectors; in addition, it is vital for signal transduction and feedback modulation of receptor function (Moreira, 2014). Analysis of the known crystal structures of GPCRs shows that ligand binding affinity mostly occurs in a main cavity located between the extracellular segments of TMs 3, 5, 6, and 7 or in a minor binding cavity located between the EC segments of TMs 1, 2, 3, and 7. Despite these common pockets, different ligands penetrate to different depths within the TM bundle, and this structural core binds ligands in the EC region and transduces the information to the IC module. It is the most structurally conserved component of the GPCR structure, presenting characteristic hydrophobic patterns and several functionally important signature motifs (Di Pizio et al., 2016; Katritch et al., 2012). Structural studies with class A receptors, have revealed the existence of a conformational change upon receptor activation which includes a breaking of the ionic lock formed between TM6 and the D(E)RY motif in TM3, followed by movements of the TM5 and TM6 cytoplasmic segments and the ICL3, with some minor rearrangements in the TM3 helix (Baltoumas et al., 2016).
GPCRs form homodimers An emerging paradigm in GPCR research is the notion that these receptors do no act exclusively as monomers. Dimer formation has been shown to be mandatory for canonical receptor function in class C GPCRs (Kniazeff, Prézeau, Rondard, Pin, & Goudet, 2011; Moller et al., 2017). Xue et al. (2015) demonstrated that the mGluR2 dimer interface switches from TM4 to TM5 in the inactive state to TM6–TM6 interactions in the active conformation, revealing a key step in class C GPCR activation.