See also G Protein-Coupled Receptors:

Introduction:

G proteins are membrane-linked macromolecules whose activities are controlled by binding of the guanosine nucleotides GTP and GDP, which act as molecular switches. Bound GTP turns on the signaling capacities of the G protein and hydrolysis of GTP or exchange for GDP turns off the signal by returning the G protein to an inactive form. There are 2 major categories of G proteins, large G proteins and small G proteins. Small G proteins such as Ras, consist of a single polypeptide chain and is a key participant in the activation of important proliferation inducing signal transduction cascade triggered by binding of ligands to their receptor tyrosine kinases. Large G proteins are composed of alpha, beta, and gamma subunits and are critically involved in many processes like vision, olfaction, glucose metabolism and luekocyte chemotaxis. 

All G proteins are active when bound to GTP and inactive when boudn to GDP. When a ligand binds to a GPCR and activates its associated G protein, the G protein exchanges GDP for GTP and dissociates into two parts consisting of the Galpha subunit bound to GTP, and the Gbeta and Ggama subunits. The signal can then be transmitted by either the Galpha or the Gbeta/gamma components, stimulating effector prtoeins. The hydrolysis of bound GTP to GDP by Galpha causes reassociation of the therotrimer, restoring the “off” state of the system. 

Often, the effector prteins activated by G proteins produce a seocnd messenger. Two of the most common effectors are adenylyl cyclase and phospholipase C, which produce cAMP and IP3 plus DAG, respectively. 

G Protein Families

Gs family

All receptors that act via cyclic AMP are coupled to a stimulatory G protein (GS)which activates adenylyl cyclase and thereby increases cyclic AMP concentration. Cyclic AMP in turn activates cyclic AMP dependent protein kinase (PKA) which catalyzes the transfer of the terminal P from ATP to specific serines or threonines of selected targeted proteins, thereby regulating their activity.

Gi family

Another G protein, called inhibitory G protein (Gi) by directly regulating ion channels rather than by decreasing cyclic AMP content.  activation inhibits adenylate cyclase activity.

Gq family

Another G protein called Gq activate the plasma membrane bound enzyme phospholipase C-B. The phosphlipase cleaves an inositol phospholipid called phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)into two products: 1. inositol 1,4,5-trisphosphate (IP3) and 2. diacylglycerol (DAG). IP3 is a small water soluble molecule that cleaves the PM and opens IP3-gated Ca2+ release channels in the ER membrane. DAG remains embedded in the membrane and activates a serine/threonine protein kinase called protein kinase C (PKC) so named because it is Ca2+ dependent.

Regulation of GPCR Signaling

Desensitization of G-Protein Linked Receptors

–RGS proteins (Regulators of G protein Signaling) which enhance the intrinsic GPAase activity of the alpha subunit.

–GRKs (G-protein linked receptor kinases) GRKs phosphorylate agonist occupied G protein coupled receptors which then binds to a member of the arrestin family of proteins. The bound arrestin prevents the receptor from interacting with G proteins and it can also serve as an adaptor protein to couple the receptor to clathrin coated pits, inducing receptor mediated endocytosis. GRKs are an example of homologous desensitization since only receptors occupied by agonists serve as a substrate for the kinase.

GRKs include a rhodopsin specific kinase involved in rod photoreceptor desensitization. In vision transduction responses, which are the fastest G protein mediated responses known in vertebrates, receptor activation caused by light leads to a fall rather than a rise in the level of a cyclic GMP. Gated Na+ channels are kept open in the dark by cyclic GMP that is bound. Light causes a hyperpolarization (which inhibits synaptic signaling) rather than a depolarization of the PM (which could stimulate signaling). Hyperpolarization (an increase in the membrane potential) results because the activation by light of rhodopsin G linked receptors leads to a fall in cyclic GMP concentration and the closure of the Na channels. Specifically, the chromophore 11-cis retinal or rhodopsin isomerizes to trans retinal when it absorbs a single photon. This conformation change then alters the G-protein transducin (Gt) causing its ? unit to dissociate and activate cyclic GMP phosphodiesterase which then hydrolyzes cyclic GMP. A rhodopsin specific kinase (RK) phosphorylates the cytosolic tail of activated rhodopsin and arrestin then binds to the phosphorylated rhodopsin so that the cells can revert to a resting dark state.

PKA or PKC which phosphorylates the receptor. This is an example of heterologous desensitization since PKA or PKC phosphorylates and desensitizes different types of receptor substrates.

See J. Wess, “Molecular basis of receptor/G-protein-coupling selectivity. Pharmacol. Ther. 80 (1998), pp. 231-264.; . L. De Vries and M. Gist Farquhar, RGS proteins: more than just GAPs for heterotrimeric G proteins. Trends Cells Biol. 9 (1999), pp. 138-144; L. De Vries, B. Zheng, T. Fischer, E. Elenko and M.G. Farquhar, “The regulator of G protein signaling family” Annue. Rev. Pharmacol. Toxicol. 40 (2000), pp. 235-271.