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Receptor proteins that are associated, on their intracellular side, with proteins that are activated by binding to guanosine triphosphate.
Guanosine triophosphate is otherwise known as GTP and is a close "cousin" of ATP. The proteins that are activated by binding to GTP are known as G proteins and, of course, G protein-linked receptors initiate signal transduction pathways by facilitating the binding of a G protein subunit to GTP. G proteins, which are membrane proteins, are able to diffuse to target proteins that they then activate.
The presence of GTP in association with G proteins is unstable, and hydrolysis to GDP occurs, resulting in inactivation of G protein. This then is the means by which signal transduction is halted, only to be reestablished upon further signal reception by the G protein linked receptor.
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