G-Protein Coupled Receptor Signaling

## Introduction

G-protein coupled receptors (GPCRs) are the largest superfamily of cell-surface receptors, with over 800 members in humans. Approximately 34% of all FDA-approved drugs target GPCRs, making them the most therapeutically exploited receptor family. They mediate responses to hormones, neurotransmitters, light, odors, and tastants.

## Receptor Structure

GPCRs share a conserved architecture: seven transmembrane alpha-helices (TM1–TM7) connected by three intracellular and three extracellular loops. The extracellular domain and upper transmembrane regions form the ligand-binding pocket (orthosteric site). The intracellular domain, particularly intracellular loop 3 and the C-terminal tail, couples to heterotrimeric G-proteins composed of G-alpha, G-beta, and G-gamma subunits.

## The G-Protein Cycle

Ligand binding induces a conformational change—primarily an outward swing of TM6—that promotes GDP-to-GTP exchange on the G-alpha subunit (acting as a guanine nucleotide exchange factor). The activated G-alpha-GTP and G-beta-gamma dimer dissociate and independently modulate downstream effectors:

- **Gs**: Stimulates adenylyl cyclase, increasing cAMP → PKA activation
- **Gi/o**: Inhibits adenylyl cyclase, decreasing cAMP; G-beta-gamma opens GIRK channels
- **Gq/11**: Activates phospholipase C-beta, generating IP3 (Ca2+ release) and DAG (PKC activation)
- **G12/13**: Activates RhoGEFs → Rho GTPases for cytoskeletal remodeling

GTP hydrolysis by intrinsic GTPase activity returns G-alpha to the GDP-bound inactive state, terminating signaling. RGS (regulators of G-protein signaling) proteins accelerate this hydrolysis.

## Desensitization and Internalization

Sustained agonist exposure triggers phosphorylation by G-protein receptor kinases (GRKs), recruiting beta-arrestins. Beta-arrestins block G-protein coupling (homologous desensitization) and promote receptor internalization via clathrin-coated pits into endosomes for recycling or lysosomal degradation.

## Therapeutic Relevance

- **Beta-blockers** (propranolol): Antagonize Gs-coupled beta-adrenergic receptors
- **Opioids** (fentanyl): Activate Gi-coupled mu-opioid receptors
- **Antihistamines** (cetirizine): Block Gq-coupled histamine H1 receptors
- **Triptans** (sumatriptan): Activate Gi-coupled 5-HT1B/1D receptors

## Key Takeaways

- GPCRs have seven-transmembrane domains and couple to heterotrimeric G-proteins
- Different G-alpha subtypes (Gs, Gi, Gq, G12/13) activate distinct signaling pathways
- GRKs and beta-arrestins mediate receptor desensitization and internalization
- One-third of all approved drugs target GPCRs