9.A.14 The G-protein-coupled receptor (GPCR) Family
G protein-coupled receptors (GPCRs) constitute a large family involved in various types of signal transduction pathways triggered by hormones, odorants, peptides, proteins, and other types of ligands. The family is so diverse that many members lack apparent sequence similarity, although they all span the cell membrane seven times with an extracellular N- and a cytosolic C-terminus. Structure clustering of the predicted models for the 907 human GPCRs (5% of the total proteins encoded by the genome) suggests that GPCRs with similar structures tend to belong to a similar functional class, even when their sequences are diverse (Zhang et al. 2006). Wistrand et al. (2006) analyzed a divergent set of GPCRs and found distinct loop length patterns and differences in amino acid composition between cytosolic loops, extracellular loops, and membrane regions. Multiple high resolution GPCR structures have confirmed some features predicted by the original rhodopsin-based models, and they reveal ligand-binding modes and critical aspects of the receptor activation process (Audet and Bouvier 2012). At least some members of this family (e.g., 9.A.14.7.3) and at least some of the ionotropic ligand binding receptors (e.g., LIC; TC# 1.A.10.1.10) share an ANF receptor family ligand binding region/domain (M. Saier, unpublished observation). Interestingly, class A GPCRs appear to harness the energy of the transmembrane sodium potential to increase their sensitivity and selectivity (Shalaeva et al. 2019). This suggests, but does not prove a role in Na+ transport. GPCRs and their associated proteins play important roles in the development of cellular senescence (Santos-Otte et al. 2019).
Menon et al. (2011) demonstrated that opsin and rhodopsin are ATP-independent phospholipid flippases in photoreceptor discs. Reconstitution of opsin into large unilamellar vesicles promotes rapid flipping of phospholipid probes across the vesicle membrane. Subsequent work demonstrated that several other G-protein receptors (β1-adrenergic receptors (TC# 9.A.14.3.11), β2-adrenergric receptors (TC# 9.A.14.3.5) and adenosine A2A receptors (TC# 9.A.14.3.8) scramble lipids (Menon et al. 2011; Goren et al. 2014; Ernst and Menon 2015). It should be noted that all of the G-protein receptors integrate into the endoplasmic reticulum (ER) before entering secretory vesicles for export to the plasma membrane, and thus, they may serve as phospholipid flippases in the ER as well as the plasma membrane (Goren et al. 2014). Cholesterol interaction motifs in G protein-coupled receptors have been reviewed (Sarkar and Chattopadhyay 2020). Phospholipid scrambling by G protein-coupled receptors has been reviewed (Khelashvili and Menon 2021).
Close to the retinal ligand in rhodopsin, several water molecules help to organise a functionally important hydrogen bond network that undergoes significant changes during photo-activation (Lesca et al. 2018). Such water-mediated networks are critical for ligand binding to other GPCRs, and they are becoming increasingly important in drug discovery. GPCRs also contain a partially conserved water mediated hydrogen bond network that stabilises the ground state of the receptor, and rearrangement of this network leads to stabilization of the active state (Lesca et al. 2018).
Crystal structures are available for rhodopsin, adrenergic receptors, and adenosine receptors in both inactive and activated forms, as well as for chemokine, dopamine, and histamine receptors in inactive conformations. Katritch et al. (2012) reviewed common structural features, outlined the scope of structural diversity of GPCRs at different levels of homology, and briefly discussed the impact of the structures on drug discovery. A distinct modularity is observed between the extracellular (ligand-binding) and intracellular (signaling) regions. GPCRs comprise a consitutent family of the TOG superfamily which includes microbial rhodopsins (TC# 3.E.1) (Yee et al. 2013), and the conclusion of homology for members of these two families has been confirmed (Shalaeva et al. 2015). GPCRs and many other channel and transport proteins bind cholesterol to their intramembrane protein surfaces (Lee 2018). The dynamic aspects related to function have been considered (Wang et al. 2018).
In the retinal binding pocket of rhodopsin, a Schiff base links the retinal ligand covalently to the Lys296 side chain. Light transforms the inverse agonist 11-cis-retinal into the agonist all-trans-retinal, leading to the active Meta II state. Crystal structures of Meta II and the active conformation of the opsin apoprotein revealed two openings of the 7-transmembrane (TM) bundle towards the hydrophobic core of the membrane, one between TMS1/TMS7 and one between TMS5/TMS6, respectively. Computational analysis revealed a putative ligand channel connecting the openings and traversing the binding pocket. Single amino acids lining the channel were replaced, and 11-cis-retinal uptake and all-trans-retinal release were measured (Piechnick et al., 2012). Most mutations slow or accelerate both uptake and release, often with opposite effects, and mutations closer to the Lys296 active site show larger effects. The mutations do not probe local channel permeability but affect global protein dynamics, with the focal point in the ligand pocket. Piechnick et al. (2012) proposed a model for the retinal/receptor interaction in which the active receptor conformation sets the open state of the channel for 11-cis-retinal and all-trans-retinal, with positioning of the ligand at the active site as the kinetic bottleneck. Although other G protein-coupled receptors lack the covalent link to the protein, the access of ligands to their binding pocket may follow similar schemes.
Rhodopsin contains a pocket within its seven TMSs which bears the inactivating 11-cis-retinal bound by a protonated Schiff-base to Lys296 in TMS7. Light-induced 11-cis-/all-trans-isomerization leads to the Schiff-base deprotonated active Meta II intermediate. With Meta II decay, the Schiff-base bond is hydrolyzed, all-trans-retinal is released from the pocket, and the apoprotein opsin reloads with new 11-cis-retinal. The crystal structure of opsin in its active Ops* conformation provides the basis for computational modeling of retinal release and uptake. The ligand-free 7 TMS bundle of opsin opens into the hydrophobic membrane layer through openings A (between TM1 and 7) and B (between TM5 and 6). A continuous channel through the protein connects these two openings and has in its central part the retinal binding pocket. The channel traverses the receptor over a distance of ca. 70 Å and is between 11.6 and 3.2 Å wide. Both openings are lined with aromatic residues, but the central part is polar. Four constrictions within the channel are so narrow that they must stretch to allow passage of the retinal beta-ionone-ring. Constrictions are at openings A and B, respectively, and at Trp265 and Lys296 within the retinal pocket. Unidirectional passage may involve uptake of 11-cis-retinal through A and release of photolyzed all-trans-retinal through B (Hildebrand et al. 2009).
Ion Channel-Coupled Receptors (ICCRs) are artificial proteins comprised of a G protein-coupled receptor and a fused ion channel, engineered to couple channel gating to ligand binding. These biological entities have potential use in drug screening and functional characterization, in addition to providing tools in the synthetic biology repertoire as synthetic K+-selective ligand-gated channels. The ICCR concept has been validated with fusion proteins between the K+ channel Kir6.2 and muscarinic M2 or dopaminergic D2 receptors. Caro et al. (2011) extended the concept to the longer β2-adrenergic receptor which, unlike M2 and D2 receptors, displayed barely detectable surface expression and did not couple to Kir6.2 when unmodified. However, a Kir6.2-binding protein, the N-terminal transmembrane domain of the sulfonylurea receptor, greatly increased plasma membrane expression of β2 constructs.
Odorant and taste receptors account for over half of the GPCR repertoire in man. These receptors are widely expressed throughout the body and function beyond the oronasal cavity - with roles including nutrient sensing, autophagy, muscle regeneration, regulation of gut motility, protective airway reflexes, bronchodilation, and respiratory disease. Foster et al. 2013 summarized the evidence for expression and function of odorant and taste receptors in tissues beyond the nose and mouth.
The murine cytomegalovirus (MCMV) M78 protein (TC# 9.B.14.18.1) is a member of the β-herpesvirus 'UL78 family' of seven transmembrane receptors (7TMRs). These receptors are required for efficient cell-cell spread of their respective viruses in tissue culture, and M78 knockout viruses are attenuated for replication in vivo. M78 forms dimers, a property common to several cellular 7TMRs. M78 traffics to the cell surface, but is rapidly and constitutively endocytosed. M78 co-loclaizes with markers for both the clathrin-dependent and lipid raft/caveolae-mediated internalization pathways. In MCMV-infected cells, the subcellular localization of M78 is modified during the course of infection, which may be related to the incorporation of M78 into the virion envelope during the course of virion maturation (Sharp et al. 2009). The 7 TMS beta-Herpesvirus M78 Protein (UL78) (TC# 9.A.14.18.1) Family is a subfamily of the GPCR family within the TOG superfamily.
Structural studies have revealed that inactive rhodopsin-like class A GPCRs (Franco et al. 2016; Cong et al. 2017) harbor a conserved binding site for Na+ ions in the center of their transmembrane domain, accessible from the extracellular space. Vickery et al. 2017 showed that the opening of a conserved hydrated channel in activated state receptors allows the Na+ to egress from its binding site into the cytosol. Coupled with protonation changes, this ion movement occurs without significant energy barriers, and can be driven by physiological transmembrane ion and voltage gradients. They proposed that Na+ ion exchange with the cytosol is a key step in GPCR activation, and that this transition locks receptors in long-lived active-state conformations (Vickery et al. 2017). Thus, while some GPCRs are lipid flippases, class A GPCRs are ion channels, and some GPCRs may function as water channels. Opsin may also be capable of transporting retinal across the membrane.
Rhodopsins are photoreceptive proteins and key tools in optogenetics (Kandori 2020). Although rhodopsin was originally named as a red-colored pigment for vision, the modern meaning of rhodopsin encompasses photoactive proteins containing a retinal chromophore in animals and microbes. Animal and microbial rhodopsins respectively possess 11-cis and all-trans retinal, respectively. As cofactors bound with their animal and microbial rhodopsin (seven transmembrane alpha-helices) environments, 11-cis and all-trans retinal undergo photoisomerization into all-trans and 13-cis retinal forms as part of their functional cycle. While animal rhodopsins are G protein coupled receptors, the function of microbial rhodopsins is highly divergent. Many of the microbial rhodopsins are able to transport ions in a passive or an active manner. These light-gated channels or light-driven pumps represent the main tools for respectively effecting neural excitation and silencing in the emerging field of optogenetics (Kandori 2020).
Transmembrane receptors, of which GPCRs are the largest group, act as cellular antennae that interpret information from the extracellular environment. Extracellular vesicles (EVs) are nanocarriers that can transport functionally competent transmembrane receptors, ligands, and a cargo of signal proteins. Roles for EVs in GPCR signal transduction have been reviewed (Bebelman et al. 2020). Their relevance to current GPCR and EV paradigms were discussed. GPCRs can be labeled and identified using tetrafunctional probes, consisting of (1) a ligand of interest, (2) 2-aryl-5-carboxytetrazole (ACT) as a photoreactive group, (3) a hydrazine-labile cleavable linker, and (4) biotin for enrichment (Miyajima et al. 2020).
Class A (rhodopsin-like) G protein-coupled receptors (GPCRs) are constitutive phospholipid scramblases as evinced after their reconstitution into liposomes. Yet phospholipid scrambling is not detectable in the resting plasma membrane of mammalian cells that is replete with GPCRs. Morra et al. 2022 considered whether cholesterol limits the ability of GPCRs to scramble lipids. A previous Markov State Model (MSM) analysis of molecular dynamics simulations of membrane-embedded opsin indicated that phospholipid headgroups traverse a dynamically revealed hydrophilic groove between TMSs 6 and 7 while their tails remain in the bilayer. Comparative MSM analyses of 150-mus simulations of opsin in cholesterol-free and cholesterol-rich membranes revealed that cholesterol inhibits phospholipid scrambling by occupying the TMS 6/7 interface and stabilizing the closed groove conformation while itself undergoing flip-flop. This mechanism may explain the inability of GPCRs to scramble lipids at the plasma membrane (Morra et al. 2022).
Generalized transport reactions catalyzed by opsin and/or certain other GPCRs are:
lipid (inner leaflet) ⇌ lipid (outer leaflet)
Na+ (out) ⇌ Na+ (in)
retinal (out) ⇌ retinal (in)
water (out) ⇌ water (in)