Editing G protein-coupled receptor (section)

==Signaling==

[[File:GPCR mechanism.png|right|thumb|300px|G-protein-coupled receptor mechanism]]
If a receptor in an active state encounters a [[G protein]], it may activate it. Some evidence suggests that receptors and G proteins are actually pre-coupled.<ref name=" pmid=21873996 "/> For example, binding of G proteins to receptors affects the receptor's affinity for ligands. Activated G proteins are bound to [[guanosine triphosphate|GTP]].

Further signal transduction depends on the type of G protein. The enzyme [[adenylate cyclase]] is an example of a cellular protein that can be regulated by a G protein, in this case the G protein [[Gs alpha subunit|G<sub>s</sub>]]. Adenylate cyclase activity is activated when it binds to a subunit of the activated G protein. Activation of adenylate cyclase ends when the G protein returns to the [[guanosine diphosphate|GDP]]-bound state.

Adenylate cyclases (of which 9 membrane-bound and one cytosolic forms are known in humans) may also be activated or inhibited in other ways (e.g., Ca2+/[[calmodulin]] binding), which can modify the activity of these enzymes in an additive or synergistic fashion along with the G proteins.

The signaling pathways activated through a GPCR are limited by the [[protein primary structure|primary sequence]] and [[tertiary structure]] of the GPCR itself but ultimately determined by the particular [[protein conformation|conformation]] stabilized by a particular [[ligand (biochemistry)|ligand]], as well as the availability of [[transducer]] molecules. Currently, GPCRs are considered to utilize two primary types of transducers: [[G-proteins]] and [[arrestin|β-arrestins]]. Because β-arr's have high [[affinity (pharmacology)|affinity]] only to the [[phosphorylated]] form of most GPCRs (see above or below), the majority of signaling is ultimately dependent upon G-protein activation. However, the possibility for interaction does allow for G-protein-independent signaling to occur.

===G-protein-dependent signaling===

There are three main G-protein-mediated signaling pathways, mediated by four [[class (biology)|sub-classes]] of G-proteins distinguished from each other by [[sequence homology]] ([[Gαs|G<sub>αs</sub>]], [[Gαi|G<sub>αi/o</sub>]], [[Gαq|G<sub>αq/11</sub>]], and [[G12/G13 alpha subunits|G<sub>α12/13</sub>]]). Each sub-class of G-protein consists of multiple proteins, each the product of multiple [[genes]] or [[splice variant|splice variations]] that may imbue them with differences ranging from subtle to distinct with regard to signaling properties, but in general they appear reasonably grouped into four classes. Because the signal transducing properties of the various possible [[G beta-gamma complex|βγ combinations]] do not appear to radically differ from one another, these classes are defined according to the isoform of their α-subunit.<ref name="Wettschureck_2005"/>{{rp|1163}}

While most GPCRs are capable of activating more than one Gα-subtype, they also show a preference for one subtype over another. When the subtype activated depends on the ligand that is bound to the GPCR, this is called [[functional selectivity]] (also known as agonist-directed trafficking, or conformation-specific agonism). However, the binding of any single particular [[agonist]] may also initiate activation of multiple different G-proteins, as it may be capable of stabilizing more than one conformation of the GPCR's [[guanine nucleotide exchange factor|GEF]] domain, even over the course of a single interaction. In addition, a conformation that preferably activates one [[isoform]] of Gα may activate another if the preferred is less available. Furthermore, [[feedback]] pathways may result in [[post-translational modification|receptor modifications]] (e.g., phosphorylation) that alter the G-protein preference. Regardless of these various nuances, the GPCR's preferred coupling partner is usually defined according to the G-protein most obviously activated by the [[endogenous]] ligand under most [[physiological]] or [[experimental]] conditions.

====Gα signaling====

# The effector of both the G<sub>αs</sub> and G<sub>αi/o</sub> pathways is the [[cyclic amp|cyclic-adenosine monophosphate]] (cAMP)-generating enzyme [[adenylyl cyclase|adenylate cyclase]], or AC. While there are ten different AC gene products in mammals, each with subtle differences in [[tissue (biology)|tissue]] distribution or function, all [[catalyze]] the conversion of [[cytosolic]] [[adenosine triphosphate]] (ATP) to cAMP, and all are directly stimulated by G-proteins of the G<sub>αs</sub> class. In contrast, however, interaction with Gα subunits of the G<sub>αi/o</sub> type inhibits AC from generating cAMP. Thus, a GPCR coupled to G<sub>αs</sub> counteracts the actions of a GPCR coupled to G<sub>αi/o</sub>, and vice versa. The level of cytosolic cAMP may then determine the activity of various [[cyclic nucleotide-gated ion channel|ion channels]] as well as members of the [[Serine/threonine-specific protein kinase|ser/thr-specific]] [[protein kinase A|protein kinase&nbsp;A]] (PKA) family. Thus cAMP is considered a [[second messenger system|second messenger]] and PKA a secondary [[effector (biology)|effector]].
# The effector of the G<sub>αq/11</sub> pathway is [[phospholipase C|phospholipase C-β]] (PLCβ), which catalyzes the cleavage of membrane-bound [[phosphatidylinositol 4,5-bisphosphate]] (PIP2) into the second messengers [[inositol trisphosphate|inositol (1,4,5) trisphosphate]] (IP3) and [[diglyceride|diacylglycerol]] (DAG). IP3 acts on [[inositol trisphosphate receptor|IP3 receptors]] found in the membrane of the [[endoplasmic reticulum]] (ER) to elicit [[Ca2+|Ca<sup>2+</sup>]] release from the ER, while DAG diffuses along the [[plasma membrane]] where it may activate any membrane localized forms of a second ser/thr kinase called [[protein kinase C]] (PKC). Since many isoforms of PKC are also activated by increases in intracellular Ca<sup>2+</sup>, both these pathways can also converge on each other to signal through the same secondary effector. Elevated intracellular Ca<sup>2+</sup> also binds and [[allosterically]] activates proteins called [[calmodulin]]s, which in turn tosolic [[small GTPase]], [[Rho family of GTPases|Rho]]. Once bound to GTP, Rho can then go on to activate various proteins responsible for [[cytoskeleton]] regulation such as [[Rho-associated protein kinase|Rho-kinase]] (ROCK). Most GPCRs that couple to G<sub>α12/13</sub> also couple to other sub-classes, often G<sub>αq/11</sub>.

====Gβγ signaling====

The above descriptions ignore the effects of [[G beta-gamma complex|Gβγ]]–signalling, which can also be important, in particular in the case of activated G<sub>αi/o</sub>-coupled GPCRs. The primary effectors of Gβγ are various ion channels, such as [[G protein-coupled inwardly-rectifying potassium channel|G-protein-regulated inwardly rectifying K<sup>+</sup> channels]] (GIRKs), [[P-type calcium channel|P]]/[[Q-type calcium channel|Q]]- and [[N-type calcium channel|N-]]type [[voltage-dependent calcium channel|voltage-gated Ca<sup>2+</sup> channels]], as well as some isoforms of AC and PLC, along with some [[PI3K|phosphoinositide-3-kinase]] (PI3K) isoforms.

===G-protein-independent signaling===

Although they are classically thought of working only together, GPCRs may signal through G-protein-independent mechanisms, and heterotrimeric G-proteins may play functional roles independent of GPCRs. GPCRs may signal independently through many proteins already mentioned for their roles in G-protein-dependent signaling such as [[arrestin|β-arrs]], [[G protein-coupled receptor kinase|GRKs]], and [[Src (gene)|Srcs]]. Such signaling has been shown to be physiologically relevant, for example, [[arrestin|β-arrestin]] signaling mediated by the chemokine receptor [[CXCR3]] was necessary for full efficacy chemotaxis of activated T cells.<ref>{{cite journal | vauthors = Smith JS, Nicholson LT, Suwanpradid J, Glenn RA, Knape NM, Alagesan P, Gundry JN, Wehrman TS, Atwater AR, Gunn MD, MacLeod AS, Rajagopal S | title = Biased agonists of the chemokine receptor CXCR3 differentially control chemotaxis and inflammation | journal = Science Signaling | volume = 11 | issue = 555 | pages = eaaq1075 | date = November 2018 | pmid = 30401786 | pmc = 6329291 | doi = 10.1126/scisignal.aaq1075 }}</ref> In addition, further scaffolding proteins involved in [[subcellular localization]] of GPCRs (e.g., [[PDZ (biology)|PDZ-domain]]-containing proteins) may also act as signal transducers. Most often the effector is a member of the [[MAPK]] family.

====Examples====

In the late 1990s, evidence began accumulating to suggest that some GPCRs are able to signal without G proteins. The [[MAPK1|ERK2]] mitogen-activated protein kinase, a key signal transduction mediator downstream of receptor activation in many pathways, has been shown to be activated in response to cAMP-mediated receptor activation in the [[slime mold]] [[Dictyostelium discoideum|''D. discoideum'']] despite the absence of the associated G protein α- and β-subunits.<ref>{{cite journal | vauthors = Kim JY, Haastert PV, Devreotes PN | title = Social senses: G-protein-coupled receptor signaling pathways in Dictyostelium discoideum | journal = Chemistry & Biology | volume = 3 | issue = 4 | pages = 239–43 | date = April 1996 | pmid = 8807851 | doi = 10.1016/S1074-5521(96)90103-9 | doi-access = free }}</ref>

In mammalian cells, the much-studied β<sub>2</sub>-adrenoceptor has been demonstrated to activate the ERK2 pathway after arrestin-mediated uncoupling of G-protein-mediated signaling. Therefore, it seems likely that some mechanisms previously believed related purely to receptor desensitisation are actually examples of receptors switching their signaling pathway, rather than simply being switched off.

In kidney cells, the [[bradykinin receptor B2]] has been shown to interact directly with a protein tyrosine phosphatase. The presence of a tyrosine-phosphorylated [[immunoreceptor tyrosine-based inhibitory motif|ITIM]] (immunoreceptor tyrosine-based inhibitory motif) sequence in the B2 receptor is necessary to mediate this interaction and subsequently the antiproliferative effect of bradykinin.<ref>{{cite journal | vauthors = Duchene J, Schanstra JP, Pecher C, Pizard A, Susini C, Esteve JP, Bascands JL, Girolami JP | title = A novel protein-protein interaction between a G protein-coupled receptor and the phosphatase SHP-2 is involved in bradykinin-induced inhibition of cell proliferation | journal = The Journal of Biological Chemistry | volume = 277 | issue = 43 | pages = 40375–83 | date = October 2002 | pmid = 12177051 | doi = 10.1074/jbc.M202744200 | doi-access = free }}</ref>

====GPCR-independent signaling by heterotrimeric G-proteins====

Although it is a relatively immature area of research, it appears that heterotrimeric G-proteins may also take part in non-GPCR signaling. There is evidence for roles as signal transducers in nearly all other types of receptor-mediated signaling, including [[integrins]], [[receptor tyrosine kinases]] (RTKs), [[cytokine receptors]] ([[JAK-STAT signaling pathway|JAK/STATs]]), as well as modulation of various other "accessory" proteins such as [[guanine nucleotide exchange factor|GEFs]], [[guanosine nucleotide dissociation inhibitors|guanine-nucleotide dissociation inhibitors]] (GDIs) and [[protein phosphatases]]. There may even be specific proteins of these classes whose primary function is as part of GPCR-independent pathways, termed activators of G-protein signalling (AGS). Both the ubiquity of these interactions and the importance of Gα vs. Gβγ subunits to these processes are still unclear.