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Abstract
G-protein-coupled receptors (GPCRs) and G-proteins have a range of roles in many physiological and pathological processes and are among the most studied signaling proteins. A plethora of extracellular stimuli can activate the GPCR and can elicit distinct intracellular responses through the activation of specific transduction pathways. For many years, biologists thought that GPCR signaling occurred entirely on the plasma membrane. However, in recent decades, many lines of evidence have proved that the GPCRs and G-proteins may reside on endomembranes and can start or propagate signaling pathways through the organelles that form the secretory route. How these alternative intracellular signaling pathways of the GPCR and G-proteins influence the physiological and pathological function of the endomembranes is still under investigation. Here, we review the general role and classification of GPCRs and G-proteins with a focus on their signaling pathways in the membrane transport apparatus.
Highlights
The family of G-protein-coupled receptors (GPCRs) is the largest class of membrane proteins in mammals
We focus on elucidating the mechanisms of Gprotein signaling from GPCRs in various intracellular membrane compartments of the secretory pathway, on how these events vary from the ones in the plasma membrane, and on the potential physiological and pathological consequences of these intracellular regulatory events
GPCR receptor activation is triggered by a ligand binding to different domains, according to the GPCR class and to the biochemical proprieties of the ligand; this occurs to stimulate G protein signaling through the conformational change in the seven-transmembrane region of the receptor that enables the insertion of the α5 helix of the Gα subunit [28]
Summary
The family of G-protein-coupled receptors (GPCRs) is the largest class of membrane proteins in mammals. GPCRs are seven-transmembrane-domain protein receptors that initiate signal transduction upon binding to specific ligands. Their canonical transduction process requires a conformational switch of the receptor followed by the rearrangement of their principal effectors, i.e., the heterotrimeric G-proteins Gα, Gβ and Gγ, to start the signaling cascade. Over 800 GPCRs have been classified in the human genome [1], most of which can transmit a signal through specific ligand interactions and produce different biological responses. Half of the human GPCRs are olfactory receptors, while 350 mediate the effect of known ligands or chemical/physical stimuli and nearly. Scientists have assumed the canonical signaling route of the GPCR to be initiated only from the plasma membrane.
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