G proteins play an important role in signalling pathways, activating intracellular signalling pathways.
G proteins are bound to a G-protein coupled receptor (GPCR) - a transmembrane protein allowing the transduction of extracellular signalling ligands to intracellular signalling pathways. On the ligand binding to the receptor protein, a conformational change is induced on an intracellular connecting loop, to which the G protein is bound. This allows it to dissociate and activate its signalling pathway.
On dissociation, the G protein migrates along the membrane, separating into $$G_\alpha$$ and $$G_{\beta\gamma}$$ subunits. The subunits are able to activate different signalling pathways, with $$G_\alpha$$ activating adenylyl cyclase leading to changes in gene transcription via CREB, and $$G_{\beta\gamma}$$ activating phospholipase C. It is unlikely all of the targets of the G-protein subunits have been identified, and it is probable that there are other targets.
Deactivation of a G protein coupled receptor involves phosphorylation by a kinase (ERK), which induces the signal to turn off as the G protein is unable to bind to the GPCR. This phosphorylation signals the G protein coupled receptor is ready for recycling. Arrestin binds to the phosphorylated domain, and the receptor is internalised by clathrin-mediated endocytosis. With the receptor embedded in an endosome, the cell ensures the ligand dissociates, sending it for degradation in a lysosome. The phosphates and arrestin are then removed, and the receptor reinserted into the membrane, where it is ready to be activated by a new ligand (agonist).
G proteins are deactivated by GTPase-activating proteins (GAPs) and GTP exchange factors (GEFs), allowing them to convert GTP to GDP, ceasing their signalling ability. GAPs and GEFs allow the acceleration of the intrinsic GTPase activity that the G proteins have, allowing signalling to be turned off more rapidly, preventing unwanted signalling from occurring (such as in the eye with rhodopsin).