GPCRs: The Gatekeepers of Cellular Signalling
G protein-coupled receptors (GPCRs) are a type of membrane receptors in eukaryotes. They are the largest and most diverse membrane receptors. Membrane receptors are proteins that are embedded in within the cell membrane and bind to signalling molecules. GPCRs are found in all multicellular organisms, and are responsible for several physiological processes, including vision, smell, taste, hormone signalling, and cell growth.
GPCRs are activated by a variety of signalling molecules, including hormones, neurotransmitters, and light. When a signalling molecule binds to a GPCR, it causes the receptor to change shape. This change in shape activates a G protein, which is a molecular switch that relays the signal inside the cell.
G proteins are heterotrimeric proteins, meaning they are composed of three subunits: alpha, beta, and gamma. The alpha subunit is responsible for activating downstream signalling pathways. The beta and gamma subunits are regulatory subunits that modulate the activity of the alpha subunit.
The downstream signalling pathways activated by GPCRs are diverse and can vary depending on the type of G protein involved. Some common downstream signalling pathways include:
- The cAMP pathway: This pathway is activated by Gαs proteins, which stimulate the production of the second messenger cyclic AMP (cAMP). cAMP activates protein kinase A (PKA), which phosphorylates a variety of target proteins, leading to changes in gene expression, cell metabolism, and cell growth.
- The IP3/DAG pathway: This pathway is activated by Gαq/11 proteins, which stimulate the production of the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 causes the release of calcium ions from intracellular stores, while DAG activates protein kinase C (PKC), which also phosphorylates a variety of target proteins.
- The phospholipase C pathway: This pathway is activated by Gαi/o proteins, which inhibit the activity of phospholipase C. Phospholipase C is an enzyme that breaks down the membrane lipid phosphatidylinositol bisphosphate (PIP2) to form IP3 and DAG.
GPCRs are essential for a wide range of physiological processes, and they are a major target for drug development. Many of the drugs that we use today, including beta-blockers, anti-anxiety medications, and antidepressants, work by targeting GPCRs.
Conclusion
GPCRs are complex and versatile receptors that play a critical role in cellular signaling. By understanding how GPCRs work, we can develop new drugs that target these receptors to treat a variety of diseases.
Sources
- G Protein-Coupled Receptors: https://en.wikipedia.org/wiki/G_protein-coupled_receptor
- The Biology of G Protein-Coupled Receptors: https://www.ncbi.nlm.nih.gov/books/NBK22301/
- G Protein-Coupled Receptors as Drug Targets: https://www.nature.com/articles/nrd4236
