The classic signalling route for G alpha (q) is activation of phospholipase C beta thereby triggering phosphoinositide hydrolysis, calcium mobilization and protein kinase C activation. This provides a path to calcium-regulated kinases and phosphatases, GEFs, MAP kinase cassettes and other proteins that mediate cellular responses ranging from granule secretion, integrin activation, and aggregation in platelets. Gq participates in many other signalling events including direct interaction with RhoGEFs that stimulate RhoA activity and inhibition of PI3K. Both in vitro and in vivo, the G-protein Gq seems to be the predominant mediator of the activation of platelets. Moreover, G alpha (q) can stimulate the activation of Burton tyrosine kinase (Ma Y C et al. 1998). Regulator of G-protein Signalling (RGS) proteins can regulate the activity of G alpha (z) (Soundararajan M et al. 2008).
View original pathway at Reactome.
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Hydrolysis of phosphatidyl inositol-bisphosphate (PIP2) by phospholipase C (PLC) produces diacylglycerol (DAG) and inositol triphosphate (IP3). Both are potent second messengers. IP3 diffuses into the cytosol, but as DAG is a hydrophobic lipid it remains within the plasma membrane. IP3 stimulates the release of calcium ions from the smooth endoplasmic reticulum, while DAG activates the conventional and unconventional protein kinase C (PKC) isoforms, facilitating the translocation of PKC from the cytosol to the plasma membrane. The effects of DAG are mimicked by tumor-promoting phorbol esters. DAG is also a precursor for the biosynthesis of prostaglandins, the endocannabinoid 2-arachidonoylglycerol and an activator of a subfamily of TRP-C (Transient Receptor Potential Canonical) cation channels 3, 6, and 7.
This is the inactive form of the receptor, before protease activation. Proteinase (protease) activated receptors are activated by the cleavage of an N-terminal extracellular segment by serine proteases, particularly thrombin which activates PAR1, 3 and 4. The cleaved fragment is an activating ligand for the receptor; synthetic peptide mimics of the N-terminal fragment can activate uncleaved receptors.
This is the inactive form of the receptor, before protease activation. Proteinase (protease) activated receptors are activated by the cleavage of an N-terminal extracellular segment by serine proteases, particularly thrombin which activates PAR1, 3 and 4. The cleaved fragment is an activating ligand for the receptor; synthetic peptide mimics of the N-terminal fragment can activate uncleaved receptors.
This is the inactive form of the receptor, before protease activation. Proteinase (protease) activated receptors are activated by the cleavage of an N-terminal extracellular segment by serine proteases, particularly thrombin which activates PAR1, 3 and 4. The cleaved fragment is an activating ligand for the receptor; synthetic peptide mimics of the N-terminal fragment can activate uncleaved receptors.
This is the inactive form of the receptor, before protease activation. Proteinase (protease) activated receptors are activated by the cleavage of an N-terminal extracellular segment by serine proteases, particularly thrombin which activates PAR1, 3 and 4. The cleaved fragment is an activating ligand for the receptor; synthetic peptide mimics of the N-terminal fragment can activate uncleaved receptors.
The classical role of the G-protein beta/gamma dimer was believed to be the inactivation of the alpha subunit, Gbeta/gamma was viewed as a negative regulator of Galpha signalling. It is now known that Gbeta/gamma subunits can directly modulate many effectors, including some also regulated by G alpha.
Gastrin is a hormone whose main function is to stimulate secretion of hydrochloric acid by the gastric mucosa, which results in gastrin formation inhibition. This hormone also acts as a mitogenic factor for gastrointestinal epithelial cells. Gastrin has two biologically active peptide forms, G34 and G17.Gastrin gene expression is upregulated in both a number of pre-malignant conditions and in established cancer through a variety of mechanisms. Depending on the tissue where it is expressed and the level of expression, differential processing of the polypeptide product leads to the production of different biologically active peptides. In turn, acting through the classical gastrin cholecystokinin B receptor CCK-BR, its isoforms and alternative receptors, these peptides trigger signalling pathways which influence the expression of downstream genes that affect cell survival, angiogenesis and invasion (Wank 1995, de Weerth et al. 1999, Grabowska & Watson 2007)
Phospholipase C (PLC) isozymes are a group of related proteins that cleave the polar head group from inositol phospholipids, typically in response to signals from cell surface receptors. They hydrolyze the highly phosphorylated lipid phosphatidylinositol 4,5-bisphosphate (PIP2) generating two products: inositol 1,4,5-trisphosphate (IP3), a universal calcium-mobilizing second messenger, and diacylglycerol (DAG), an activator of protein kinase C. PLC-beta isoforms are regulated by heterotrimeric GTP-binding proteins. PLC-beta 1 and 3 are widely expressed, with the highest concentrations found in (differing) specific regions of the brain. PLC-beta 2 is expressed at highest levels in cells of hematopoeitic origin; it is involved in leukocyte signaling and host defense. PLC-beta 4 is highly concentrated in cerebellar Purkinje and granule cells, the median geniculate body, whose axons terminate in the auditory cortex, and the lateral geniculate nucleus, where most retinal axons terminate in a visuotopic representation of each half of the visual field.
G alpha q protein (or Gq/11) consists of four family members (G-alpha 11, -alpha 14, -alpha 15 and -alpha q). It activates phospholipase C (PLC) (Dowal L et al, 2006). PLC hydrolyzes phosphatidylinositol (PIP2) to diacyl glycerol (DAG) and inositol triphosphate (IP3). DAG acts as a second messenger that activates protein kinase C (PKC) and IP3 can bind to IP3 receptors, particular calcium channels in the endoplasmic reticulum (ER). Calcium flow causes the cytosolic concentration of calcium to increase, causing a cascade of intracellular changes and activity.
The active form of G protein alpha subunit q (Gq-alpha) was found to activate phospholipase C beta-1 (PLC-beta1), in investigations using bovine membranes. Subsequently, all 4 human isoforms have been shown to be activated by Gq, though activation of PLCbeta-4 is limited. In recombinant assays, several activated rat G alpha q family members were found to stimulate human PLC-beta isoforms with the same rank order of decreasing potency. PLC-beta1 stimulation was slightly more than for PLC-beta3; PLC-beta3 stimulation was 10-fold greater than for beta-2. PLC-beta2 is expressed specifically in hematopoietic cells. PLC-beta acts directly on Gq to accelerate hydrolysis of bound GTP, thus PLC-betas are GTPase activating proteins (GAPs). The crystal structure of the C-terminal region from Turkey PLC-beta, revealed a novel fold composed almost entirely of three long helices forming a coiled-coil that dimerizes along its long axis in an antiparallel orientation. The extent of the dimer interface and gel exclusion chromatography data suggest that PLC-betas are functionally dimeric.
The Trio family of RhoA guanine nucleotide exchange factors (RhoGEFs) are directly activated by G alpha (q), possibly within a Gq:Trio:RhoA signalling complex, thereby linking Gq to RhoA-mediated processes such as cell migration, proliferation, and contraction. Like most other RhoGEFs, they have a tandem motif consisting of a Dbl homology (DH) and a pleckstrin homology (PH) domain. Trio and Duet have a number of other domains including an immunoglobin domains that may be involved in interacting with Rho, but the considerably smaller GEFT (p63RhoGEF) does not have any identifiable additional domains yet appears to be sufficient to mediate the activation of RhoA by G alpha (q). The structure represented by GEFT is proposed to represent the core of an ancient signal transduction pathway.
Phospholipase C activation is the classical signalling route for G alpha (q) but an additional mechanism is an inhibitory interaction between G alpha (q) and phosphatidylinositol 3-kinase alpha (PI3K alpha). There are several PI3K subtypes but only the p85 alpha/p110 alpha subtype (PI3K alpha) is a G alpha (q) effector (PMID: 18515384). Activated G alpha (q) inhibits PI3K alpha directly, in a GTP-dependent manner. G alpha(q) binding of PI3K competes with Ras, a PI3K activator (PMID: 16268778).
GRKs are serine/threonine kinases that phosphorylate GPCRs leading to receptor desensitization. GRK5 appears to be the predominant regulator of PAR1 desensitization in endothelial cells.
GRK2 can inhibit GPCR signaling via phosphorylation-independent sequestration of Gq/11/14 subunits utilising its RGS homology (RH) domain. GRK2 may be an effector of activated Gq, initiating signalling cascades other than the classical PLC beta signalling associated with Gq.
When a ligand activates a G protein-coupled receptor, it induces a conformational change in the receptor (a change in shape) that allows the receptor to function as a guanine nucleotide exchange factor (GEF), stimulating the exchange of GDP for GTP on the G alpha subunit. In the traditional view of heterotrimeric protein activation, this exchange triggers the dissociation of the now active G alpha subunit from the beta:gamma dimer, initiating downstream signalling events. The G alpha subunit has intrinsic GTPase activity and will eventually hydrolyze the attached GTP to GDP, allowing reassociation with G beta:gamma. Additional GTPase-activating proteins (GAPs) stimulate the GTPase activity of G alpha, leading to more rapid termination of the transduced signal. In some cases the downstream effector may have GAP activity, helping to deactivate the pathway. This is the case for phospholipase C beta, which possesses GAP activity within its C-terminal region (Kleuss et al. 1994).
The classical view of G-protein signalling is that the G-protein alpha subunit dissociates from the beta:gamma dimer. Activated G alpha (q) and the beta:gamma dimer then participate in separate signaling cascades. Although G protein dissociation has been contested (e.g. Bassi et al. 1996), recent in vivo experiments have demonstrated that dissociation does occur, though possibly not to completion (Lambert 2008).
The classical model of G-protein signaling suggests that the G-protein dissociates upon GPCR activation. The active G alpha (q) subunit then participates in signaling, until its intrinsic GTPase activity degrades the bound GTP to GDP. The inactive G alpha (q):GDP complex has much higher affinity for the G beta:gamma complex and consequently reassociates.
G-Protein Coupled Receptors (GPCR) sense extracellular signals and activate different Guanine nucleotide binding proteins. Upon activation, the Guanine nucleotide-binding protein G(q) subunit alpha class (GNAQ/GNA11/GNA14/GNA15) can bind directly to the THSH3 domain of the non-receptor Tyrosine-protein kinase BTK in vitro and in vivo. This binding results in a conformational change in BTK, which leads to its activation. Physiologically, BTK plays a key role in B lymphocyte development, differentiation and signalling.
G-Protein Coupled Receptors (GPCR) sense extracellular signals and activate different Guanine nucleotide binding proteins. Upon activation, the Guanine nucleotide-binding protein G(q) subunit alpha class (GNAQ/GNA11/GNA14/GNA15) can bind directly to the non-receptor Tyrosine-protein kinase BTK. This binding breaks intramolecular interactions in BTK thereby making the kinase domain available for substrates. Physiologically, BTK plays a key role in B lymphocyte development, differentiation and signalling.
G-Protein Coupled Receptors (GPCR) sense extracellular signals and activate different Guanine nucleotide binding proteins. Upon activation, the Guanine nucleotide-binding protein G(q) subunit alpha class (GNAQ/GNA11/GNA14/GNA15) can bind to the non-receptor Tyrosine-protein kinase BTK. This binding results in a conformational change in BTK. Subsequently, the structurally modified BTK is released from GNAQ and is now catalytically active. Active BTK can trigger the downstream MAPK p38 pathway. Physiologically, BTK plays a key role in B lymphocyte development, differentiation and signalling.
G Protein Coupled Receptors (GPCR) sense extracellular signals and activate different Guanine nucleotide binding proteins (G proteins). Upon activation, GPCRs can replace the GDP with GTP in the alpha subunit of G proteins. GTP binding modifies the conformation of G alpha proteins and activates them. The Regulator of G protein Signalling (RGS) are GTPase Accelerating Proteins (GAPs) that can directly inhibit the G alpha subunit activity. There are at least 25 different types of RGS proteins known. Several of these RGS proteins (1, 2, 3, 4, 5, 8, 13, 16, 17, 18, 19, 21) can bind and stabilize the transition state for GTP hydrolysis of Guanine nucleotide binding protein G(q) subunit alpha class (GNAQ/GNA11/GNA14/GNA15). Subsequently, this leads to GTP hydrolysis and inactivation of G alpha (q) and terminating downstream signalling (Neubig RR and Siderovski DP et al. 2002, Kach J et al., 2012). The primary function of G alpha (q) is activation of phospholipase C beta thereby triggering phosphoinositide hydrolysis, calcium mobilization and protein kinase C activation.
G Protein Coupled Receptors (GPCR) sense extracellular signals and activate different Guanine nucleotide binding proteins (G proteins). Upon activation, GPCRs can replace the GDP with GTP in the alpha subunit of G proteins. GTP binding modifies the conformation of G alpha proteins and activates them. The Regulator of G protein Signalling (RGS) are GTPase Accelerating Proteins (GAPs) that can directly inhibit the G alpha subunit activity. There are at least 25 different types of RGS proteins known. Several of these RGS proteins (1, 2, 3, 4, 5, 8, 13, 16, 17, 18, 19, 21) can bind and stabilize the transition state of Guanine nucleotide binding protein G(q) subunit alpha class (GNAQ/GNA11/GNA14/GNA15). Following this, the RGS domain of the proteins exert GAP activity on G alpha (q) and allosterically modulate residues within G-alpha subunit to accelerate the intrinsic GTPase activity that hydrolyses GTP to GDP. This inactivates G alpha (q) and terminates downstream signalling (Neubig & Siderovski 2002, Kach et al. 2012). The primary function of G alpha (q) is activation of phospholipase C beta thereby triggering phosphoinositide hydrolysis, calcium mobilization and protein kinase C activation.
G Protein Coupled Receptors (GPCR) sense extracellular signals and activate different Guanine nucleotide binding proteins (G proteins). Upon activation, GPCRs can replace the GDP with GTP in the alpha subunit of G proteins. GTP binding modifies the conformation of G alpha proteins and activates them. The Regulator of G protein Signalling (RGS) are GTPase Accelerating Proteins (GAPs) that can directly inhibit the G alpha subunit activity. There are at least 25 different types of RGS proteins known. Several of these RGS proteins (1, 2, 3, 4, 5, 8, 13, 16, 17, 18, 19, 21) can bind and stabilize the transition state of Guanine nucleotide binding protein G(q) subunit alpha class (GNAQ/GNA11/GNA14/GNA15). Subsequently, the RGS domain in the complex facilitates the hydrolyses of G alpha (q):GTP to G alpha (q):GDP. Following this, the complex dissociates releasing inactive G alpha (q) (Neubig & Siderovski 2002, Kach et al. 2012). The primary function of G alpha (q) is activation of phospholipase C beta thereby triggering phosphoinositide hydrolysis, calcium mobilization and protein kinase C activation.
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DataNodes
(q/11):Trio family
RhoGEFssignalling pathway
via PKC and MAPKG-protein Gq/11
(inactive)complexes that activate
Gq/11:Heterotrimeric G-protein Gq (active)complexes that activate
Gq/11:Heterotrimeric G-protein Gq (inactive)complexes that
activate Gq/11Annotated Interactions
(q/11):Trio family
RhoGEFsG-protein Gq/11
(inactive)G-protein Gq/11
(inactive)complexes that activate
Gq/11:Heterotrimeric G-protein Gq (active)complexes that activate
Gq/11:Heterotrimeric G-protein Gq (active)complexes that activate
Gq/11:Heterotrimeric G-protein Gq (inactive)complexes that activate
Gq/11:Heterotrimeric G-protein Gq (inactive)complexes that
activate Gq/11