G beta-gamma complex

The individual subunits of the G protein complex were first identified in 1980 when the regulatory component of adenylate cyclase was successfully purified, yielding three polypeptides of different molecular weights.{{cite journal | vauthors = Northup JK, Sternweis PC, Smigel MD, Schleifer LS, Ross EM, Gilman AG | title = Purification of the regulatory component of adenylate cyclase | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 77 | issue = 11 | pages = 6516–20 | date = November 1980 | pmid = 6935665 | pmc = 350316 | doi = 10.1073/pnas.77.11.6516 | bibcode = 1980PNAS...77.6516N | jstor = 9587 | doi-access = free }} Initially, it was thought that G_α, the largest subunit, was the major effector regulatory subunit, and that G_βγ was largely responsible for inactivating the G_α subunit and enhancing membrane binding. However, downstream signalling effects of G_βγ were later discovered when the purified G_βγ complex was found to activate a cardiac muscarinic K+ channel.{{cite journal | vauthors = Logothetis DE, Kurachi Y, Galper J, Neer EJ, Clapham DE | title = The beta gamma subunits of GTP-binding proteins activate the muscarinic K+ channel in heart | journal = Nature | volume = 325 | issue = 6102 | pages = 321–6 | year = 1987 | pmid = 2433589 | doi = 10.1038/325321a0 | bibcode = 1987Natur.325..321L | s2cid = 4338529 }} Shortly after, the G_βγ complex associated with a mating factor receptor-coupled G protein in yeast was found to initiate a pheromone response.{{cite journal | vauthors = Whiteway M, Hougan L, Dignard D, Thomas DY, Bell L, Saari GC, Grant FJ, O'Hara P, MacKay VL | title = The STE4 and STE18 genes of yeast encode potential beta and gamma subunits of the mating factor receptor-coupled G protein | journal = Cell | volume = 56 | issue = 3 | pages = 467–77 | date = February 1989 | pmid = 2536595 | doi = 10.1016/0092-8674(89)90249-3 | s2cid = 53298578 }} Although these hypotheses were initially controversial, G_βγ has since been shown to directly regulate as many different protein targets as the G_α subunit.

Recently, possible roles of the G_βγ complex in retinal rod photoreceptors have been investigated, with some evidence for the maintenance of G_α inactivation. However, these conclusions were drawn from in vitro experiments under unphysiological conditions, and the physiological role of the G_βγ complex in vision is still unclear. Nevertheless, recent in vivo findings demonstrate the necessity of the transducin G_βγ complex in the functioning of rod photoreceptors under low light conditions.{{cite journal | vauthors = Kolesnikov AV, Rikimaru L, Hennig AK, Lukasiewicz PD, Fliesler SJ, Govardovskii VI, Kefalov VJ, Kisselev OG | title = G-protein betagamma-complex is crucial for efficient signal amplification in vision | journal = The Journal of Neuroscience | volume = 31 | issue = 22 | pages = 8067–77 | date = June 2011 | pmid = 21632928 | pmc = 3118088 | doi = 10.1523/JNEUROSCI.0174-11.2011 }}

The G_βγ subunit is a dimer composed of two polypeptides, however it acts functionally as a monomer, as the individual subunits do not separate, and have not been found to function independently.{{cite journal | vauthors = Hurowitz EH, Melnyk JM, Chen YJ, Kouros-Mehr H, Simon MI, Shizuya H | title = Genomic characterization of the human heterotrimeric G protein alpha, beta, and gamma subunit genes | journal = DNA Research | volume = 7 | issue = 2 | pages = 111–20 | date = April 2000 | pmid = 10819326 | doi = 10.1093/dnares/7.2.111 | doi-access = free }}

The G_β subunit is a member of the β-propeller family of proteins, which typically possess four to eight antiparallel β-sheets arranged in the shape of a propeller.{{cite journal | vauthors = Sondek J, Bohm A, Lambright DG, Hamm HE, Sigler PB | title = Crystal structure of a G-protein beta gamma dimer at 2.1A resolution | journal = Nature | volume = 379 | issue = 6563 | pages = 369–74 | date = January 1996 | pmid = 8552196 | doi = 10.1038/379369a0 | bibcode = 1996Natur.379..369S | s2cid = 4321948 }} G_β contains a seven-bladed β-propeller, each blade arranged around a central axis and composed of four antiparallel β-sheets. The amino acid sequence contains seven WD repeat motifs of about 40 amino acids, each highly conserved and possessing the Trp-Asp dipeptide that gives the repeat its name.

The G_γ subunit is considerably smaller than G_β, and is unstable on its own, requiring interaction with G_β to fold, explaining the close association of the dimer. In the G_βγ dimer, the G_γ subunit wraps around the outside of G_β, interacting through hydrophobic associations, and exhibits no tertiary interactions with itself. The N terminus helical domains of the two subunits form a coiled coil with one another that typically extends away from the core of the dimer.

To date, five β-subunit and eleven γ-subunit genes have been identified in mammals. The G_β genes have very similar sequences, while significantly greater variation is seen in the G_γ genes, indicating that the functional specificity of the G_βγ dimer may be dependent on the type of G_γ subunit involved.

Of additional structural interest is the discovery of a so-called “hotspot” present on the surface of the G_βγ dimer; a specific site of the protein that binds to diverse range of peptides and is thought to be a contributing factor in the ability of G_βγ to interact with a wide variety of effectors.{{cite journal | vauthors = Scott JK, Huang SF, Gangadhar BP, Samoriski GM, Clapp P, Gross RA, Taussig R, Smrcka AV | title = Evidence that a protein-protein interaction 'hot spot' on heterotrimeric G protein betagamma subunits is used for recognition of a subclass of effectors | journal = The EMBO Journal | volume = 20 | issue = 4 | pages = 767–76 | date = February 2001 | pmid = 11179221 | pmc = 145424 | doi = 10.1093/emboj/20.4.767 }}{{cite journal | vauthors = Gulati S, Jin H, Masuho I, Orban T, Cai Y, Pardon E, Martemyanov KA, Kiser PD, Stewart PL, Ford CP, Steyaert J, Palczewski K | title = Targeting G protein-coupled receptor signaling at the G protein level with a selective nanobody inhibitor | journal = Nature Communications | volume = 9 | issue = 1 | pages = 1996 | year = 2018 | pmid = 29777099 | pmc = 5959942 | doi = 10.1038/s41467-018-04432-0 | bibcode = 2018NatCo...9.1996G }}

Synthesis of the subunits occurs in the cytosol. Folding of the β-subunit is thought to be aided by the chaperone CCT (chaperonin containing tailless-complex polypeptide 1), which also prevents aggregation of folded subunits.{{cite journal | vauthors = Wells CA, Dingus J, Hildebrandt JD | title = Role of the chaperonin CCT/TRiC complex in G protein betagamma-dimer assembly | journal = The Journal of Biological Chemistry | volume = 281 | issue = 29 | pages = 20221–32 | date = July 2006 | pmid = 16702223 | doi = 10.1074/jbc.M602409200 | doi-access = free }} A second chaperone, PhLP (phosducin-like protein), binds to the CCT/G_β complex, and is phosphorylated, allowing CCT to dissociate and G_γ to bind. Finally, PhLP is released, exposing the binding site for G_α, allowing for formation of the final trimer at the endoplasmic reticulum, where it is targeted to the plasma membrane.{{cite journal | vauthors = Lukov GL, Baker CM, Ludtke PJ, Hu T, Carter MD, Hackett RA, Thulin CD, Willardson BM | title = Mechanism of assembly of G protein betagamma subunits by protein kinase CK2-phosphorylated phosducin-like protein and the cytosolic chaperonin complex | journal = The Journal of Biological Chemistry | volume = 281 | issue = 31 | pages = 22261–74 | date = August 2006 | pmid = 16717095 | doi = 10.1074/jbc.M601590200 | doi-access = free }} G_γ subunits are known to be prenylated (covalently modified by the addition of lipid moieties) prior to addition to G_β, which itself has not been found to be modified. This prenylation is thought to be involved in directing the interaction of the subunit both with membrane lipids and other proteins.{{cite journal | vauthors = Wedegaertner PB, Wilson PT, Bourne HR | title = Lipid modifications of trimeric G proteins | journal = The Journal of Biological Chemistry | volume = 270 | issue = 2 | pages = 503–6 | date = January 1995 | pmid = 7822269 | doi = 10.1074/jbc.270.2.503 | doi-access = free }}

The G_βγ complex is an essential element in the GPCR signaling cascade. It has two main states for which it performs different functions. When G_βγ is interacting with G_α it functions as a negative regulator. In the heterotrimer form, the G_βγ dimer increases the affinity of G_α for GDP, which causes the G protein to be in an inactive state.{{cite journal | vauthors = Brandt DR, Ross EM | title = GTPase activity of the stimulatory GTP-binding regulatory protein of adenylate cyclase, Gs. Accumulation and turnover of enzyme-nucleotide intermediates | journal = The Journal of Biological Chemistry | volume = 260 | issue = 1 | pages = 266–72 | date = January 1985 | doi = 10.1016/S0021-9258(18)89726-5 | pmid = 2981206 | url = http://www.jbc.org/cgi/pmidlookup?view=long&pmid=2981206 | doi-access = free }} For the G_α subunit to become active, the nucleotide exchange must be induced by the GPCR. Studies have shown that it is the G_βγ dimer that demonstrates specificity for the appropriate receptor and that the G_γ subunit actually enhances the interaction of the G_α subunit with the GPCR.{{cite journal | vauthors = Im MJ, Holzhöfer A, Böttinger H, Pfeuffer T, Helmreich EJ | title = Interactions of pure beta gamma-subunits of G-proteins with purified beta 1-adrenoceptor | journal = FEBS Letters | volume = 227 | issue = 2 | pages = 225–9 | date = January 1988 | pmid = 2828119 | doi = 10.1016/0014-5793(88)80903-7 | s2cid = 84523709 | doi-access = free }}{{cite journal | vauthors = Kisselev O, Gautam N | title = Specific interaction with rhodopsin is dependent on the gamma subunit type in a G protein | journal = The Journal of Biological Chemistry | volume = 268 | issue = 33 | pages = 24519–22 | date = November 1993 | doi = 10.1016/S0021-9258(19)74493-7 | pmid = 8227005 | url = http://www.jbc.org/cgi/pmidlookup?view=long&pmid=8227005 | doi-access = free }} The GPCR is activated by an extracellular ligand and subsequently activates the G protein heterotrimer by causing a conformational change in the G_α subunit. This causes the replacement of GDP with GTP as well as the physical dissociation of the G_α and the G_βγ complex.{{cite journal | vauthors = Digby GJ, Lober RM, Sethi PR, Lambert NA | title = Some G protein heterotrimers physically dissociate in living cells | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 47 | pages = 17789–94 | date = November 2006 | pmid = 17095603 | pmc = 1693825 | doi = 10.1073/pnas.0607116103 | bibcode = 2006PNAS..10317789D | doi-access = free }}

Once separated, both G_α and G_βγ are free to participate in their own distinct signaling pathways. G_βγ does not go through any conformational changes when it dissociates from G_α and it acts as a signaling molecule as a dimer.{{cite journal | vauthors = Lin Y, Smrcka AV | title = Understanding molecular recognition by G protein βγ subunits on the path to pharmacological targeting | journal = Molecular Pharmacology | volume = 80 | issue = 4 | pages = 551–7 | date = October 2011 | pmid = 21737569 | pmc = 3187535 | doi = 10.1124/mol.111.073072 }} The G_βγ dimer has been found to interact with many different effector molecules by protein-protein interactions. Different combinations of the G_β and G_γ subtypes can influence different effectors and work exclusively or synergistically with the G_α subunit.

G_βγ signaling is diverse, inhibiting or activating many downstream events depending on its interaction with different effectors. Researchers have discovered that G_βγ regulates ion channels, such as G protein-gated inward rectifier channels, as well as calcium channels.{{cite journal | vauthors = Ikeda SR | title = Voltage-dependent modulation of N-type calcium channels by G-protein beta gamma subunits | journal = Nature | volume = 380 | issue = 6571 | pages = 255–8 | date = March 1996 | pmid = 8637575 | doi = 10.1038/380255a0 | bibcode = 1996Natur.380..255I | s2cid = 4325047 }} In human PBMC, G_βγ complex has been shown to activate phosphorylation of ERK1/2.{{Cite journal|last1=Saroz|first1=Yurii|last2=Kho|first2=Dan T.|last3=Glass|first3=Michelle|last4=Graham|first4=Euan Scott|last5=Grimsey|first5=Natasha Lillia|date=2019-10-19|title=Cannabinoid Receptor 2 (CB 2 ) Signals via G-alpha-s and Induces IL-6 and IL-10 Cytokine Secretion in Human Primary Leukocytes|journal=ACS Pharmacology & Translational Science|language=en|pages=414–428|doi=10.1021/acsptsci.9b00049|issn=2575-9108|doi-access=free|volume=2|issue=6|pmid=32259074|pmc=7088898}} Another example of G_βγ signaling is its effect of activating or inhibiting adenylyl cyclase leading to the intracellular increase or decrease of the secondary messenger cyclic AMP.{{cite journal | vauthors = Tang WJ, Gilman AG | title = Type-specific regulation of adenylyl cyclase by G protein beta gamma subunits | journal = Science | volume = 254 | issue = 5037 | pages = 1500–3 | date = December 1991 | pmid = 1962211 | doi = 10.1126/science.1962211 | bibcode = 1991Sci...254.1500T }} For more examples of G_βγ signaling see table. However, the full extent of G_βγ signaling has not yet been discovered.

The G_βγ subunit plays a variety of roles in cell signalling processes and as such researchers are now examining its potential as a therapeutic drug target for the treatment of many medical conditions. However, it is recognized that there are a number of considerations to keep in mind when designing a drug which targets the G_βγ subunit:

1. The G_βγ subunit is essential for the formation of heterotrimeric G protein through its association with the G_α subunit allowing the G proteins coupling to the GPCR. Therefore, any agent inhibiting the G_βγ subunits signalling effects must not interfere with the heterotrimeric G protein formation or G_α subunit signalling.
2. G_βγ expression is universal throughout almost all the cells of the body so any agent acting to inhibit this subunit could elicit numerous side effects.
3. Small molecule inhibitors that target the coupling of G_βγ to specific effectors and do not interfere with normal G protein cycling/ heterotrimeric formation, have the potential to work as therapeutic agents in treating some specific diseases.

Research has been conducted on how altering the actions of G_βγ subunits could be beneficial for the treatment of certain medical conditions. G_βγ signalling has been examined for its role in a variety of conditions including heart failure, inflammation and leukemia.{{cite journal | vauthors = Runne C, Chen S | title = PLEKHG2 promotes heterotrimeric G protein βγ-stimulated lymphocyte migration via Rac and Cdc42 activation and actin polymerization | journal = Molecular and Cellular Biology | volume = 33 | issue = 21 | pages = 4294–307 | date = November 2013 | pmid = 24001768 | pmc = 3811901 | doi = 10.1128/MCB.00879-13 }}

Heart failure can be characterized by a loss of β adrenergic receptor (βAR) signalling in heart cells.{{cite journal | vauthors = Brodde OE, Michel MC | title = Adrenergic and muscarinic receptors in the human heart | journal = Pharmacological Reviews | volume = 51 | issue = 4 | pages = 651–90 | date = December 1999 | pmid = 10581327 | url = http://pharmrev.aspetjournals.org/cgi/pmidlookup?view=long&pmid=10581327 }} When the βAR is stimulated by catecholamines such as adrenaline and noradrenaline, there is normally an increase in the contractility of the heart. However, in heart failure there are sustained and elevated levels of catecholamines which result in chronic desensitization of the βAR receptor. This leads to a decrease in the strength of heart contractions. Some research suggests that this chronic desensitization is due to the over activation of a kinase, G protein-coupled receptor kinase 2 (GRK2), which phosphorylates and deactivates certain G protein coupled receptors .{{cite journal | vauthors = Hata JA, Koch WJ | title = Phosphorylation of G protein-coupled receptors: GPCR kinases in heart disease | journal = Molecular Interventions | volume = 3 | issue = 5 | pages = 264–72 | date = August 2003 | pmid = 14993440 | doi = 10.1124/mi.3.5.264 }} When the G protein coupled receptor is activated, the G_βγ subunit recruits GRK2 which then phosphorylates and desensitizes GPCRs like the βAR.{{cite journal | vauthors = Pitcher JA, Inglese J, Higgins JB, Arriza JL, Casey PJ, Kim C, Benovic JL, Kwatra MM, Caron MG, Lefkowitz RJ | title = Role of beta gamma subunits of G proteins in targeting the beta-adrenergic receptor kinase to membrane-bound receptors | journal = Science | volume = 257 | issue = 5074 | pages = 1264–7 | date = August 1992 | pmid = 1325672 | doi = 10.1126/science.1325672 | bibcode = 1992Sci...257.1264P }} Preventing the interaction of the βγ subunit with GRK2 has therefore been studied as a potential target for increasing heart contractile function. The developed molecule GRK2ct is a protein inhibitor which inhibits the signalling properties of G_βγ subunit but does not interfere with alpha subunit signalling.{{cite journal | vauthors = Koch WJ, Hawes BE, Inglese J, Luttrell LM, Lefkowitz RJ | title = Cellular expression of the carboxyl terminus of a G protein-coupled receptor kinase attenuates G beta gamma-mediated signaling | journal = The Journal of Biological Chemistry | volume = 269 | issue = 8 | pages = 6193–7 | date = February 1994 | doi = 10.1016/S0021-9258(17)37587-7 | pmid = 8119963 | url = http://www.jbc.org/cgi/pmidlookup?view=long&pmid=8119963 | doi-access = free }} The over expression of GRK2ct has been shown to significantly rescue cardiac function in murine models of heart failure by blocking G_βγ subunit signalling.{{cite journal | vauthors = Rockman HA, Chien KR, Choi DJ, Iaccarino G, Hunter JJ, Ross J, Lefkowitz RJ, Koch WJ | title = Expression of a beta-adrenergic receptor kinase 1 inhibitor prevents the development of myocardial failure in gene-targeted mice | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 95 | issue = 12 | pages = 7000–5 | date = June 1998 | pmid = 9618528 | pmc = 22717 | doi = 10.1073/pnas.95.12.7000 | bibcode = 1998PNAS...95.7000R | doi-access = free }} In another study, biopsies were taken from patients with heart failure and virally induced overexpression of GRK2ct in the heart myocytes. Other tests showed an improvement in cardiac cell contractile function by inhibiting G_βγ.{{cite journal | vauthors = Williams ML, Hata JA, Schroder J, Rampersaud E, Petrofski J, Jakoi A, Milano CA, Koch WJ | title = Targeted beta-adrenergic receptor kinase (betaARK1) inhibition by gene transfer in failing human hearts | journal = Circulation | volume = 109 | issue = 13 | pages = 1590–3 | date = April 2004 | pmid = 15051637 | doi = 10.1161/01.CIR.0000125521.40985.28 | doi-access = free }}

When particular GPCRs are activated by their specific chemokines G_βγ directly activates PI3Kγ which is involved in the recruitment of neutrophils that contribute to inflammation.{{cite journal | vauthors = Li Z, Jiang H, Xie W, Zhang Z, Smrcka AV, Wu D | title = Roles of PLC-beta2 and -beta3 and PI3Kgamma in chemoattractant-mediated signal transduction | journal = Science | volume = 287 | issue = 5455 | pages = 1046–9 | date = February 2000 | pmid = 10669417 | doi = 10.1126/science.287.5455.1046 | bibcode = 2000Sci...287.1046L }}{{cite journal | vauthors = Hirsch E, Katanaev VL, Garlanda C, Azzolino O, Pirola L, Silengo L, Sozzani S, Mantovani A, Altruda F, Wymann MP | title = Central role for G protein-coupled phosphoinositide 3-kinase gamma in inflammation | journal = Science | volume = 287 | issue = 5455 | pages = 1049–53 | date = February 2000 | pmid = 10669418 | doi = 10.1126/science.287.5455.1049 | bibcode = 2000Sci...287.1049H }}{{cite journal | vauthors = Stephens LR, Eguinoa A, Erdjument-Bromage H, Lui M, Cooke F, Coadwell J, Smrcka AS, Thelen M, Cadwallader K, Tempst P, Hawkins PT | title = The G beta gamma sensitivity of a PI3K is dependent upon a tightly associated adaptor, p101 | journal = Cell | volume = 89 | issue = 1 | pages = 105–14 | date = April 1997 | pmid = 9094719 | doi = 10.1016/S0092-8674(00)80187-7 | s2cid = 16852661 | doi-access = free }}{{cite journal | vauthors = Stephens L, Smrcka A, Cooke FT, Jackson TR, Sternweis PC, Hawkins PT | title = A novel phosphoinositide 3 kinase activity in myeloid-derived cells is activated by G protein beta gamma subunits | journal = Cell | volume = 77 | issue = 1 | pages = 83–93 | date = April 1994 | pmid = 8156600 | doi = 10.1016/0092-8674(94)90237-2 | s2cid = 53255676 }} It has been discovered that the inhibition of PI3Kγ significantly reduces inflammation. PI3Kγ is the intended target molecule in the prevention of inflammation as it is the common signalling effector of many different chemokine and receptor types involved in promoting inflammation. Although PI3Kγ is the intended target there are other isoforms of PI3 which perform different functions from PI3Kγ. Since PI3Kγ is specifically regulated by G_βγ, while other isoforms of PI3 are largely regulated by other molecules, inhibiting Gβγ signalling would provide the desired specificity of a therapeutic agent designed to treat inflammation.

The G_βγ subunit has been shown to activate a Rho guanine nucleotide exchange factor (RhoGef) gene PLEKHG2 which is upregulated in a number of leukemia cell lines and mouse models of leukemia.{{cite journal | vauthors = Ueda H, Nagae R, Kozawa M, Morishita R, Kimura S, Nagase T, Ohara O, Yoshida S, Asano T | title = Heterotrimeric G protein betagamma subunits stimulate FLJ00018, a guanine nucleotide exchange factor for Rac1 and Cdc42 | journal = The Journal of Biological Chemistry | volume = 283 | issue = 4 | pages = 1946–53 | date = January 2008 | pmid = 18045877 | doi = 10.1074/jbc.M707037200 | doi-access = free }} Lymphocyte chemotaxis as a result of Rac and CDC42 activation as well as actin polymerization is believed to be regulated by the G_βγ activated RhoGef. Therefore, a drug inhibiting the G_βγ could play a role in the treatment of leukemia.

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{{Intracellular signaling peptides and proteins}}

{{Acid anhydride hydrolases}}

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Category:G proteins

Category:Protein complexes