G-protein-coupled receptors (GPCRs) represent one of the largest and most diverse groups of proteins in the genome. Activated receptors catalyse nucleotide exchange on a relatively small group of heterotrimeric G proteins to initiate intracellular signalling.
Biophysical studies of rhodopsin-family GPCRs have shown that receptor activation results in an outward movement of transmembrane helix VI, which opens a pocket for G protein binding. Increasing evidence suggests that the structure, conformation and specificity of the G protein binding site can be regulated by the identity of the bound ligand.
Several different models for receptor–G-protein association have been proposed. These proteins may be precoupled in a large signalling complex. Nucleotide exchange may follow a series of transition complexes. Receptors may function as dimers to activate G proteins.
Biophysical studies indicate that receptor-mediated GDP release requires a conformational change in the α5 helix of the Gα subunit and is associated with structural changes at the Gβ binding site. However, additional studies are required to fully describe the mechanism of receptor-mediated GDP release and the structure of the receptor–G-protein complex.
Binding of GTP induces a structural rearrangement of the receptor–G-protein complex that leads to dissociation of some, but not all, complexes. This observation is consistent with the existence of precoupled receptor–G-protein complexes that may not completely disassemble on G protein activation.
Fundamentally, the unanswered questions about G protein activation reflect a poor understanding of the structure of the complex. A goal of future studies will be to refine current models of the receptor–G-protein complex in terms of the available structural information until a crystal structure of the complex is solved.
Heterotrimeric G proteins have a crucial role as molecular switches in signal transduction pathways mediated by G-protein-coupled receptors. Extracellular stimuli activate these receptors, which then catalyse GTP–GDP exchange on the G protein α-subunit. The complex series of interactions and conformational changes that connect agonist binding to G protein activation raise various interesting questions about the structure, biomechanics, kinetics and specificity of signal transduction across the plasma membrane.
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- G-protein-coupled receptor
(GPCR). A member of the most diverse class of cell-surface receptors that mediate the actions of various hormones, neurotransmitters and sensory stimuli by activating heterotrimeric G proteins.
A guanine-nucleotide-binding protein and GTP hydrolase, the structural conformations and molecular interactions of which are governed by the identity of the bound nucleotide.
A GPCR that is activated by the photoisomerization of covalently bound 11-cis-retinal to all-trans-retinal.
- DRY motif
A highly conserved Asp-Arg-Tyr motif near the cytoplasmic face of GPCRs that has a crucial role in both the mechanisms of receptor and G protein activation.
- Palmitoylation site
A Cys residue to which palmitic acid is covalently attached, which anchors helix VIII to the membrane in GPCRs and creates a fourth intracellular loop.
- Inverse agonist
A ligand that decreases the intrinsic activity of a receptor (that is, has the opposite effect of an agonist).
- RGS protein
The regulator of G protein signalling (RGS) proteins bind Gα and accelerate GTP hydrolysis by stabilizing the transition-state conformation.
A non-hydrolysable analogue of GTP that is used to study the conformation and molecular interactions of activated Gα.
An all-β-sheet protein fold that is characterized by 4–8 blade-shaped β-sheets arranged toroidally around a central axis.
A motif of ∼40 amino acids ending in Trp-Asp, which is found in proteins as several repeated units that fold into a β-propeller and mediate protein–protein interactions.
A structural motif in proteins where two or more α-helices wrap around each other like the strands of a rope.
The transfer of either a farnesyl or a geranylgeranyl lipid moiety to the C-terminal Cys of a target protein.
- Site-directed spin labelling
A technique using site-directed Cys mutagenesis to provide a reactive thiol for the covalent attachment of a paramagnetic probe.
A heterotrimeric G protein of the visual transduction system that is composed of Gαtβ1γ1 and activates cGMP phosphodiesterase by binding its inhibitory subunit.
- Metarhodopsin II
(MII). An active signalling conformation of rhodopsin (formed after photo-isomerization of retinal) that catalyses nucleotide exchange on transducin.
- Pertussis toxin
The exotoxin produced by the bacterium Bordetella pertussis that catalyses ADP ribosylation of the C-terminal Cys of Gαi/o/t, preventing interaction with GPCRs.
- ADP ribosylation
An enzymatic covalent modification of a protein. ADP ribose is transferred from NADH.
- Ala-scanning mutagenesis
An experimental approach that uses systematic site-directed Ala mutagenesis in a protein to identify residues that are vital for function.
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Oldham, W., Hamm, H. Heterotrimeric G protein activation by G-protein-coupled receptors. Nat Rev Mol Cell Biol 9, 60–71 (2008). https://doi.org/10.1038/nrm2299
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