| G protein-coupled receptor kinase | |||||||||
|---|---|---|---|---|---|---|---|---|---|
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| Identifiers | |||||||||
| EC no. | 2.7.11.16 | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDBPDBePDBsum | ||||||||
| Gene Ontology | AmiGO /QuickGO | ||||||||
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G protein-coupled receptor kinases (GPCRKs,GRKs) are afamily ofprotein kinases within theAGC (protein kinase A,protein kinase G,protein kinase C) group of kinases. Like all AGC kinases, GRKs use ATP to add phosphate toSerine andThreonine residues in specific locations of target proteins. In particular, GRKsphosphorylateintracellular domains ofG protein-coupled receptors (GPCRs). GRKs function in tandem witharrestin proteins to regulate the sensitivity of GPCRs for stimulating downstreamheterotrimeric G protein andG protein-independent signaling pathways.[2][3]
| Name | Notes | Gene | OMIM |
|---|---|---|---|
| G protein-coupled receptor kinase 1 | Rhodopsin kinase | GRK1 | 180381 |
| G protein-coupled receptor kinase 2 | β-Adrenergic receptor kinase 1 (βARK1) | ADRBK1 | 109635 |
| G protein-coupled receptor kinase 3 | β-Adrenergic receptor kinase 2 (βARK2) | ADRBK2 | 109636 |
| G protein-coupled receptor kinase 4 | Polymorphism associated withhypertension[4] | GRK4 | 137026 |
| G protein-coupled receptor kinase 5 | Polymorphism associated with cardioprotection[5] | GRK5 | 600870 |
| G protein-coupled receptor kinase 6 | Knockout mice are supersensitive todopaminergic drugs[6] | GRK6 | 600869 |
| G protein-coupled receptor kinase 7 | Cone opsin kinase | GRK7 | 606987 |
GRKs reside normally in an inactive state, but their kinase activity is stimulated by binding to a ligand-activated GPCR (rather than by regulatory phosphorylation as is common in other AGC kinases). Because there are only seven GRKs (only 4 of which are widely expressed throughout the body) but over 800 human GPCRs, GRKs appear to have limited phosphorylation site selectivity and are regulated primarily by the GPCR active state.[3]
G protein-coupled receptor kinases phosphorylate activated G protein-coupled receptors, which promotes the binding of anarrestin protein to the receptor. Phosphorylated serine and threonine residues in GPCRs act asbinding sites for and activators of arrestin proteins. Arrestin binding to phosphorylated, active receptors prevents receptor stimulation ofheterotrimeric G protein transducer proteins, blocking their cellular signaling and resulting in receptordesensitization. Arrestin binding also directs receptors to specific cellularinternalization pathways, removing the receptors from the cell surface and also preventing additional activation. Arrestin binding to phosphorylated, active receptor also enables receptor signaling through arrestin partner proteins. Thus the GRK/arrestin system serves as a complex signaling switch for G protein-coupled receptors.[3]
GRKs can be regulated by signaling events in cells, both in direct feedback mechanisms where receptor signals alter GRK activity over time, and due to signals emanating from distinct pathways from a particular GPCR/GRK system of interest. For example, GRK1 is regulated by the calcium sensor protein recoverin: calcium-bound recoverin binds directly to GRK1 to inhibit its ability to phosphorylate and desensitize rhodopsin, the visual GPCR in the retina, in light-activated retinal rod cells since light activation raises intracellular calcium in these cells, whereas in dark-adapted eyes, calcium levels are low in rod cells and GRK1 is not inhibited by recoverin.[7] The non-visual GRKs are inhibited instead by the calcium-binding proteincalmodulin.[2] GRK2 and GRK3 share a carboxyl terminal pleckstrin homology (PH) domain that binds to G protein beta/gamma subunits, and GPCR activation of heterotrimeric G proteins releases this free beta/gamma complex that binds to GRK2/3 to recruit these kinases to the cell membrane precisely at the location of the activated receptor, augmenting GRK activity to regulate the activated receptor.[2][3] GRK2 activity can be modulated by its phosphorylation by protein kinase A or protein kinase C, and by post-translational modification of cysteines by S-nitrosylation.[8][9]
X-ray crystal structures have been obtained for several GRKs (GRK1, GRK2, GRK4, GRK5 and GRK6), alone or bound to ligands.[10] Overall, GRKs share sequence homology and domain organization in which the central protein kinase catalytic domain is preceded by a domain with homology to the active domain ofRegulator of G protein Signaling proteins, RGS proteins (the RGS-homology – RH – domain) and is followed by a variable carboxyl terminal tail regulatory region.[3] In the folded proteins, the kinase domain forms a typical bi-lobe kinase structure with a central ATP-binding active site.[3] The RH domain is composed of alpha-helical region formed from the amino terminal sequence plus a short stretch of sequence following the kinase domain that provides 2 additional helices, and makes extensive contacts with one side of the kinase domain.[10] Modeling and mutagenesis suggests that the RH domain senses GPCR activation to open the kinase active site.[11]
GRK1 is involved withrhodopsin phosphorylation and deactivation in vision, together witharrestin-1, also known as S-antigen. Defects in GRK1 result inOguchi stationary night blindness. GRK7 similarly regulates coneopsin phosphorylation and deactivation incolor vision, together withcone arrestin, also known as arrestin-4 or X-arrestin.[3]
GRK2 was first identified as an enzyme that phosphorylated thebeta-2 adrenergic receptor, and was originally called the beta adrenergic receptor kinase (βARK, or ββARK1). GRK2 is overexpressed in heart failure, and GRK2 inhibition could be used to treatheart failure in the future.[12]
Polymorphisms in the GRK4 gene have been linked to both genetic and acquiredhypertension, acting in part through kidneydopamine receptors.[4] GRK4 is the most highly expressed GRK at the mRNA level, in maturingspermatids, but mice lacking GRK4 remain fertile so its role in these cells remains unknown.[13]
In humans, a GRK5 sequence polymorphism at residue 41 (leucine rather than glutamine) that is most common in individuals with African ancestry leads to elevated GRK5-mediated desensitization of airway beta2-adrenergic receptors, a drug target inasthma.[14] In zebrafish and in humans, loss of GRK5 function has been associated with heart defects due toheterotaxy, a series of developmental defects arising from improper left-right laterality duringorganogenesis.[15]
In the mouse, GRK6 regulation of D2dopamine receptors in thestriatum region of the brain alters sensitivity topsychostimulant drugs that act through dopamine, and GRK6 has been implicated in Parkinson's disease and in thedyskinesia side effects of anti-parkinson therapy with the drugL-DOPA.[16][17]
GRKs also phosphorylate non-GPCR substrates. GRK2 and GRK5 can phosphorylate some tyrosine kinase receptors, including the receptor for platelet-derived growth factor (PDGF) and insulin-like growth factor (IGF).[18][19]
GRKs also regulate cellular responses independent of theirkinase activity. In particular,G protein-coupled receptor kinase 2 is known to interact with a diverse repertoire of non-GPCR partner proteins, but other GRKs also have non-GPCR partners.[20] The RGS-homology (RH) domain of GRK2 and GRK3 binds to heterotrimeric G protein subunits of the Gq family, but despite these RH domains being unable to act as GTPase-activating proteins like traditional RGS proteins to turn off G protein signaling, this binding reduces Gq signaling by sequestering active G proteins away from their effector proteins such as phospholipase C-beta.[21]
