G protein-coupled receptors are the largest family of cell surface receptors, transmitting signals from a vast array of stimuli including hormones, neurotransmitters, light, and odorants. They are characterized by their seven-transmembrane domain structure and their coupling to heterotrimeric G proteins.
GPCR Structure
All GPCRs share a common architecture of seven alpha-helical transmembrane domains connected by three extracellular loops and three intracellular loops. The N-terminus is extracellular and often glycosylated, while the C-terminus is intracellular and contains phosphorylation sites important for regulation. The ligand-binding pocket varies between receptor classes: small ligands such as catecholamines bind within the transmembrane core, while larger peptide hormones bind to the extracellular loops and N-terminus.
GPCRs are classified into several families based on sequence homology. Class A rhodopsin-like receptors are the largest group, including adrenergic, dopamine, serotonin, and opioid receptors. Class B secretin-like receptors bind peptide hormones. Class C metabotropic glutamate receptors have a large extracellular Venus flytrap domain where glutamate binds.
G Protein Activation Cycle
Heterotrimeric G proteins consist of alpha, beta, and gamma subunits. In the inactive state, the alpha subunit binds GDP and the three subunits form a stable complex. Ligand-activated GPCR acts as a guanine nucleotide exchange factor, promoting the release of GDP and binding of GTP. The GTP-bound alpha subunit dissociates from the beta-gamma complex, and both components are free to regulate downstream effectors.
The signal is terminated when the alpha subunit hydrolyzes GTP to GDP through its intrinsic GTPase activity, allowing reassociation with the beta-gamma complex. Regulators of G protein signaling accelerate GTP hydrolysis, providing an additional layer of regulation. The GTPase activity of the alpha subunit is the off switch of GPCR signaling.
G Protein Subtypes
The alpha subunits are classified into four families based on sequence similarity and effector regulation. G-alpha-s stimulates adenylyl cyclase, increasing cAMP levels. It mediates the effects of many hormones including epinephrine through beta-adrenergic receptors, glucagon, and ACTH. G-alpha-i inhibits adenylyl cyclase, reducing cAMP. It mediates the effects of alpha-2 adrenergic receptors, muscarinic M2 receptors, and opioid receptors. G-alpha-q activates phospholipase C-beta, which cleaves phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate and diacylglycerol. G-alpha-12/13 regulates Rho GTPases involved in cytoskeletal reorganization and cell migration.
The cAMP Pathway
The canonical effector of G-alpha-s is adenylyl cyclase, which converts ATP to the second messenger cyclic AMP. cAMP activates protein kinase A, a tetramer of two regulatory and two catalytic subunits. cAMP binding to the regulatory subunits releases active catalytic subunits that phosphorylate serine and threonine residues on target proteins. PKA phosphorylates metabolic enzymes such as glycogen phosphorylase kinase, transcription factors such as CREB, and ion channels. The cAMP signal is terminated by phosphodiesterases that hydrolyze cAMP to AMP.
The Phospholipase C Pathway
G-alpha-q activates phospholipase C-beta, which hydrolyzes phosphatidylinositol 4,5-bisphosphate to produce inositol trisphosphate and diacylglycerol. IP3 diffuses to the endoplasmic reticulum and binds to IP3 receptors on the ER membrane, causing release of calcium ions into the cytoplasm. The resulting calcium increase activates calcium-binding proteins such as calmodulin, which in turn activates CaM kinases and other effectors. Diacylglycerol remains in the membrane and, together with calcium, activates protein kinase C. PKC phosphorylates serine and threonine residues on diverse target proteins involved in cell growth, differentiation, and secretion.
GPCR Desensitization
Prolonged agonist exposure leads to receptor desensitization. G protein-coupled receptor kinases phosphorylate the intracellular C-terminus of activated GPCRs. Beta-arrestin then binds the phosphorylated receptor, preventing further G protein coupling and targeting the receptor for internalization through clathrin-coated pits. Internalized receptors can be dephosphorylated and recycled to the membrane or degraded. Beta-arrestin also scaffolds signaling complexes, activating alternative pathways such as MAP kinase signaling independently of G proteins. This biased signaling is being exploited for drug development.
