G-Protein Coupled Receptor Signaling

G-protein coupled receptors (GPCRs) are a large family of cell-surface receptors with a characteristic seven-transmembrane architecture that transduce a vast range of signals, including hormones, neurotransmitters, odorants, and light. On activation by their ligand they engage intracellular heterotrimeric G proteins, which in turn regulate effector enzymes and ion channels that generate second messengers.

Definition

G-protein coupled receptor signalling is the process by which a seven-transmembrane receptor, upon binding its ligand, acts as a guanine-nucleotide exchange factor for a heterotrimeric G protein, triggering exchange of GDP for GTP on the G-alpha subunit and thereby activating downstream effectors.

Scope

The topic covers the structure of seven-transmembrane receptors, the heterotrimeric G-protein cycle, the main G-protein classes and their effectors, and the mechanisms of receptor desensitization through kinases and arrestins. It is treated as a biochemical and molecular subject within signal transduction mechanisms.

Core questions

• How does ligand binding at the cell surface activate an intracellular G protein?
• How do different G-protein classes produce different cellular responses?
• How is GPCR signalling switched off and the receptor desensitized?

Key concepts

• Seven-transmembrane (heptahelical) architecture
• Heterotrimeric G protein (alpha, beta, gamma)
• GTP/GDP cycle
• Effectors (adenylyl cyclase, phospholipase C)
• G-protein subclasses (Gs, Gi, Gq)
• GPCR kinases (GRKs)
• Arrestins and desensitization

Mechanisms

Ligand binding stabilises an active conformation of the seven-transmembrane receptor, which then catalyses the exchange of GDP for GTP on the alpha subunit of an associated heterotrimeric G protein. The GTP-bound alpha subunit dissociates from the beta-gamma dimer, and both moieties regulate downstream effectors: for example, Gs stimulates and Gi inhibits adenylyl cyclase to change cyclic AMP levels, while Gq activates phospholipase C to generate inositol trisphosphate and diacylglycerol. The intrinsic GTPase activity of the alpha subunit hydrolyses GTP to GDP, terminating the signal and allowing reassembly of the inactive heterotrimer. Sustained stimulation leads to receptor phosphorylation by G-protein-coupled receptor kinases and the recruitment of arrestins, which uncouple the receptor from G proteins and promote its internalisation, producing desensitization.

Clinical relevance

GPCRs are the targets of a large share of marketed medicines because they mediate so many physiological signals. This entry describes their signalling mechanisms at a reference level and is not a basis for individual diagnostic or treatment decisions.

Evidence & guidelines

Understanding of GPCR signalling rests on biochemical, structural, and pharmacological research and authoritative reviews and textbooks rather than clinical practice guidelines.

History

The biochemical dissection of hormone-stimulated adenylyl cyclase by Rodbell and the identification of the transducing G proteins by Gilman established the central role of heterotrimeric G proteins, work recognised by a Nobel Prize. Lefkowitz and Kobilka's purification and cloning of adrenergic receptors revealed the conserved seven-transmembrane architecture and later the structural basis of receptor activation, and studies of GPCR kinases and arrestins clarified how the receptors are desensitized.

Key figures

• Robert Lefkowitz
• Brian Kobilka
• Martin Rodbell
• Alfred G. Gilman
• Heidi Hamm

Seminal works

• pierce-2002
• oldham-2008
• lefkowitz-2005

Frequently asked questions

Why are GPCRs called 'seven-transmembrane' receptors?

Their single polypeptide chain crosses the plasma membrane seven times, forming seven membrane-spanning helices; this shared architecture defines the family and creates the binding pocket and intracellular surfaces needed to engage G proteins.

What does the G protein actually do?

It acts as a molecular switch: when the activated receptor loads it with GTP, it splits into subunits that turn effector enzymes and channels on or off, and it switches itself off by hydrolysing the GTP back to GDP.