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G protein-coupled receptors (GPCRs) are transmembrane proteins that span the cell membrane seven times. They are found embedded in the plasma membrane of cells throughout the body and function as molecular switches that respond to external signals. GPCRs represent one of the largest families of cell surface receptors involved in cellular communication.

When a ligand binds to a GPCR, it causes a conformational change in the receptor's structure. This change activates an associated G protein inside the cell, triggering a cascade of intracellular signals. The activated G protein then interacts with downstream effector molecules to produce cellular responses.

G proteins are intracellular signaling molecules that couple activated GPCRs to downstream cellular responses. When activated by a GPCR, G proteins exchange GDP for GTP and dissociate into active subunits. These subunits then interact with effector proteins to amplify and propagate the signal throughout the cell.

GPCRs can trigger diverse cellular responses including changes in gene expression, enzyme activity, and ion channel function. These receptors regulate processes such as vision, smell, taste, and hormone responses. GPCR signaling also influences molecular factors affecting cell division and cellular differentiation through intracellular signaling cascades.

Unlike receptor tyrosine kinases that have intrinsic enzymatic activity, GPCRs lack catalytic domains and instead couple to G proteins to initiate signaling. GPCRs are characterized by their seven transmembrane domains and their ability to activate heterotrimeric G proteins. This indirect signaling mechanism allows GPCRs to produce diverse cellular outcomes through multiple downstream pathways.

GPCRs are important therapeutic targets because they regulate critical physiological processes and are involved in numerous diseases. Approximately 30-40% of FDA-approved drugs target GPCRs, making them among the most successful drug targets. Their involvement in signal transduction pathways that control cellular functions makes them valuable for treating conditions ranging from hypertension to cancer.

After GPCR activation, intracellular signaling cascades are initiated through G protein-mediated pathways. These cascades can lead to changes in second messenger levels, activation of protein kinases, and alterations in gene expression through regulation of expression at multiple steps. Signal termination occurs through GTPase activity and receptor desensitization mechanisms.