The G-Protein βγ Complex

doi:10.1016/S0898-6568(98)00006-0

Cellular Signalling

Volume 10, Issue 7, July 1998, Pages 447-455

https://doi.org/10.1016/S0898-6568(98)00006-0 Get rights and content

Abstract

The vast majority of signalling pathways in mammalian cells are mediated by heterotrimeric (αβγ) G proteins. Reviewed here is regulation of signal transduction by the βγ complex at different protein interfaces: subunit–subunit, receptor–G protein and G protein–effector. The role of diverse β and γ subunit types in achieving specificity in signalling and potentially unidentified functions for these subunits also are discussed.

Introduction

Much of the initial focus after the discovery and purification of heterotrimeric G-proteins was directed at the G-protein α subunit because it was a GTPase that acted on both adenylate cyclase and cGMP phosphodiesterase 1, 2. Even after the identification of the G-protein β and γ subunits 3, 4, 5, their role in signalling was somewhat underappreciated. However, two pieces of suggestive evidence hinted at a broader role for these subunits: (1) the β and γ subunits seemed heterogeneous in different mammalian tissues, and (2) the tightly bound βγ complex was an obligatory requirement for receptor activation of a G protein 6, 7, 8. As the structures of the β and γ subunits began to be elucidated through molecular-cloning methods 9, 10, the unexpected finding that a βγ complex could regulate the function of a heart K⁺-ion-conducting channel pointed to a major function for these subunits as modulators of effector function [11]. In the decade following these initial results, the βγ complex began to be treated as an equal to the α subunit and as a separate arm in the G-protein-signalling pathway. However, there are hints that several functions of this subunit complex and the molecular mechanisms underlying those functions remain unelucidated. This review focusses on certain aspects of the biology of G-protein β and γ subunits. Recent reviews on these subunits and G-protein signalling contain related literature 12, 13, 14, 15.

Section snippets

Diversity

Conclusive evidence for diversity of β and γ subunit types in mammalian cells came from molecular cloning of their cDNAs. The initial isolation of the cDNAs encoding the β subunits indicated that two different genes encoded the 36,000 M_r and 35,000 M_r proteins seen in purified G proteins [6], named β1 and β2 16, 17. The expression profiles of the β1 and β2 RNAs in mammalian tissues matched the presence of the 36,000 M_r and 35,000 M_r proteins. Although the cDNA for Gtγ was the first

Genes and their expression

Six β subunits (including a splice variant [33]) and 11 γ subunits have been identified so far. Their amino acid sequences are aligned together in Figure 1A,B. This comparison of the β and γ subunit sequences indicates evidence for subfamilies in both cases. In regard to the β subunits, β1–β4 are very similar (Fig. 1A); β5 and a longer form of β5 show much less homology to the other β subtypes. This divergence in primary structure is reflected at the level of expression—β5 is expressed only in

Assembly and structure

The identification of several β and γ subunits raised questions regarding the nature of association among these subunits. Because many were expressed in the same cell, associations could be promiscuous or selective among the variety of β and γ subtypes. Several approaches—expression in cell lines, in vitro translation and recombinant protein purification—have now consistently demonstrated selectivity in association between β and γ subtypes 44, 45, 46. The use of the yeast two-hybrid system has

The prenyl group

As mentioned before, the γ subunits are post-translationally modified by a lipid group—a farnesyl or geranylgeranyl moiety. The isoprenoid moiety is attached to a Cys four amino acids from the C terminus [34]. This prenylation is followed by proteolytic cleavage of the last three residues of the γ subunit and carboxy methylation. The function of these lipid modifications, which are present on other proteins including the Ras family, has been thought to be to target the proteins to membranes.

Modulation of receptor function

Early experiments with G proteins indicated that the βγ complex is a requirement for interaction of the G protein with receptors. The α subunit alone did not effectively interact with the receptor in different assays 8, 77, 78. Although the identification of domains on the α subunit that interacted with the receptor indicated that the α subunit contacted the receptor directly [79], it was unclear why the βγ complex was required for G-protein coupling to the receptor. The ability of the βγ

Modulation of effector function

After the initial demonstration that the βγ complex activates the muscarinic K⁺ channel, a number of other effectors have been shown to be regulated by the βγ complex. Among them are adenylate cyclases [88], phospholipase C β (PLC β) 75, 89, voltage-gated Ca²⁺ channels (N and P/Q) 90, 91 and a brain Na⁺ channel [92]. In all these cases, there is evidence for direct interaction between the βγ complex and the effector 46, 93, 94, 95, 96, 97. Other proteins that directly interact with the βγ

βγ Complex in disease

There is now convincing evidence that mutations in G-protein-coupled receptors and α subunits lie at the basis of human diseases. No known mutation in the G-protein β or γ subunits is associated with a disease as yet. Such mutations seem highly likely, considering the diversity of these two families of proteins and the conservation of their primary structure across mammalian species. The first indication for an indirect association between βγ and a disease process arises in the case of early

Final note: specificity

The G-protein βγ complex, which was originally thought to be the inactive membrane-anchoring partner in the G pro- tein, has turned out to be a versatile molecule with multifarious roles. Clear answers as to why there is so much diversity in these molecules and why their structures are conserved evolutionarily are not yet there. The indications are that some of these diverse structures are required for specific contact with receptors and effectors. How does the same βγ complex mediate so many

Acknowledgements

Work in the authors’ laboratory was supported by the National Institutes of Health. N. G. is an Established Investigator of the American Heart Association.

References (135)

A.G. Gilman
Cell
(1984)
D.R. Manning et al.
J. Biol. Chem.
(1983)
J.D. Hildebrandt et al.
J. Biol. Chem.
(1984)
P.C. Sternweis et al.
J. Biol. Chem.
(1984)
J.D. Hildebrandt et al.
J. Biol. Chem.
(1985)
B.K. Fung
J. Biol. Chem.
(1983)
E.J. Helmreich et al.
Biochim. Biophys. Acta
(1996)
T.T.d. Amatruda et al.
J. Biol. Chem.
(1988)
J.D. Robishaw et al.
J. Biol. Chem.
(1989)
J.J. Cali et al.
J. Biol. Chem.
(1992)

K. Ray et al.

J. Biol. Chem.

(1995)

S. Kalyanaraman et al.

Biochem. Biophys. Res. Commun.

(1995)

R. Morishita et al.

J. Biol. Chem.

(1995)

O.C. Ong et al.

J. Biol. Chem.

(1995)

N.J. Ryba et al.

J. Biol. Chem.

(1995)

E. von Weizsacker et al.

Biochem. Biophys. Res. Commun.

(1992)

A.J. Watson et al.

J. Biol. Chem.

(1994)

M.D. Wilcox et al.

J. Biol. Chem.

(1995)

A.J. Watson et al.

J. Biol. Chem.

(1996)

S.L. Moores et al.

J. Biol. Chem.

(1991)

L. Tao et al.

Exp. Eye Res.

(1993)

O.C. Ong et al.

Genomics

(1997)

P.E. Danielson et al.

Genomics

(1994)

C.J. Schmidt et al.

J. Biol. Chem.

(1992)

K. Yan et al.

J. Biol. Chem.

(1996)

A. Garritsen et al.

J. Biol. Chem.

(1994)

D.J. Spring et al.

J. Biol. Chem.

(1994)

C.J. Schmidt et al.

J. Biol. Chem.

(1991)

A.N. Pronin et al.

FEBS Lett.

(1993)

M.A. Wall et al.

Cell

(1995)

K. Yan et al.

J. Biol. Chem.

(1997)

W.F. Simonds et al.

J. Biol. Chem.

(1991)

A.M. Schultz et al.

Biochem. Biophys. Res. Commun.

(1987)

O. Kisselev et al.

J. Biol. Chem.

(1995)

H. Yasuda et al.

J. Biol. Chem.

(1996)

M.A. Lindorfer et al.

J. Biol. Chem.

(1996)

Y. Fukada et al.

J. Biol. Chem.

(1994)

V.A. Florio et al.

J. Biol. Chem.

(1989)

M.J. Im et al.

FEBS Lett.

(1988)

W.J. Phillips et al.

J. Biol. Chem.

(1992)

A.B. Fawzi et al.

J. Biol. Chem.

(1991)

O. Kisselev et al.

J. Biol. Chem.

(1993)

O.G. Kisselev et al.

J. Biol. Chem.

(1994)

A. Wise et al.

J. Biol. Chem.

(1997)

J.M. Taylor et al.

J. Biol. Chem.

(1996)

J.Y. Ma et al.

Neuron

(1997)

J.L. Blank et al.

J. Biol. Chem.

(1992)

T. Asano et al.

FEBS Lett.

(1986)

T. Katada et al.

J. Biol. Chem.

(1987)

L. Stryer

Annu. Rev. Neurosci.

(1986)

Cited by (158)

G protein gamma subunit, a hidden master regulator of GPCR signaling
2022, Journal of Biological Chemistry
Heterotrimeric G proteins (αβγ subunits) that are activated by G protein-coupled receptors (GPCRs) mediate the biological responses of eukaryotic cells to extracellular signals. The α subunits and the tightly bound βγ subunit complex of G proteins have been extensively studied and shown to control the activity of effector molecules. In contrast, the potential roles of the large family of γ subunits have been less studied. In this review, we focus on present knowledge about these proteins. Induced loss of individual γ subunit types in animal and plant models result in strikingly distinct phenotypes indicating that γ subtypes play important and specific roles. Consistent with these findings, downregulation or upregulation of particular γ subunit types result in various types of cancers. Clues about the mechanistic basis of γ subunit function have emerged from imaging the dynamic behavior of G protein subunits in living cells. This shows that in the basal state, G proteins are not constrained to the plasma membrane but shuttle between membranes and on receptor activation βγ complexes translocate reversibly to internal membranes. The translocation kinetics of βγ complexes varies widely and is determined by the membrane affinity of the associated γ subtype. On translocating, some βγ complexes act on effectors in internal membranes. The variation in translocation kinetics determines differential sensitivity and adaptation of cells to external signals. Membrane affinity of γ subunits is thus a parsimonious and elegant mechanism that controls information flow to internal cell membranes while modulating signaling responses.
Subtype-dependent regulation of Gβγ signalling
2021, Cellular Signalling
Citation Excerpt :
The human genome encodes 5 Gβ and 12 Gγ genes, resulting in significant potential structural and functional diversity in G protein heterotrimers. Among mammalian Gβ isoforms, Gβ1 to Gβ4 share more sequence homology than Gβ5 [6,7]. Gβ1-4 are 36 kDa proteins while Gβ5 is a 40 kDa protein, with only 50% sequence similarity to other Gβ subunits.
G protein-coupled receptors (GPCRs) transmit information to the cell interior by transducing external signals to heterotrimeric G protein subunits, Gα and Gβγ subunits, localized on the inner leaflet of the plasma membrane. Though the initial focus was mainly on Gα-mediated events, Gβγ subunits were later identified as major contributors to GPCR-G protein signalling. A broad functional array of Gβγ signalling has recently been attributed to Gβ and Gγ subtype diversity, comprising 5 Gβ and 12 Gγ subtypes, respectively. In addition to displaying selectivity towards each other to form the Gβγ dimer, numerous studies have identified preferences of distinct Gβγ combinations for specific GPCRs, Gα subtypes and effector molecules. Importantly, Gβ and Gγ subtype-dependent regulation of downstream effectors, representing a diverse range of signalling pathways and physiological functions have been found. Here, we review the literature on the repercussions of Gβ and Gγ subtype diversity on direct and indirect regulation of GPCR/G protein signalling events and their physiological outcomes. Our discussion additionally provides perspective in understanding the intricacies underlying molecular regulation of subtype-specific roles of Gβγ signalling and associated diseases.
Exome reports A de novo GNB2 variant associated with global developmental delay, intellectual disability, and dysmorphic features
2020, European Journal of Medical Genetics
Heterotrimeric G proteins are composed of α, β, and γ subunits and are involved in integrating signals between receptors and effector proteins. The 5 human Gβ proteins (encoded by GNB1, GNB2, GNB3, GNB4, and GNB5) are highly similar. Variants in GNB1 were identified as a genetic cause of developmental delay. De novo variant in GNB2 has recently been reported as a cause of sinus node dysfunction and atrioventricular block but not as a cause of developmental delay. Trio-based whole-exome sequencing was performed on an individual with global developmental delay, muscle hypotonia, multiple congenital joint contractures and dysmorphism such as brachycephalus, thick eyebrows, thin upper lip, micrognathia, prominent chin, and bilateral tapered fingers. We identified a de novo GNB2 variant c.229G>A, p.(Gly77Arg). Notably, pathogenic substitutions of the homologous Gly77 residue including an identical variant (p.Gly77Arg, p.Gly77Val, p.Gly77Ser, p.Gly77Ala) of GNB1, a paralog of GNB2, was reported in individuals with global developmental delay and hypotonia. Clinical features of our case overlap with those of GNB1 variants. Our study suggests that a GNB2 variant may be associated with syndromic global developmental delay.
The expanding roles and mechanisms of G protein–mediated presynaptic inhibition
2019, Journal of Biological Chemistry
Citation Excerpt :
This would allow researchers the ability to identify specific neuronal subtypes where Gβγ–SNARE could be a key regulatory mechanism. A growing body of work supports the notion of subunit specificity for a subset of effectors of the five known Gβ and 12 Gγ subunits, most of which can heterodimerize with each other in vitro, although in vivo evidence is more scarce (113, 114). Genetic studies show that despite considerable homology, Gβ and Gγ subunits perform specific roles in development.
Throughout the past five decades, tremendous advancements have been made in our understanding of G protein signaling and presynaptic inhibition, many of which were published in the Journal of Biological Chemistry under the tenure of Herb Tabor as Editor-in-Chief. Here, we identify these critical advances, including the formulation of the ternary complex model of G protein–coupled receptor signaling and the discovery of Gβγ as a critical signaling component of the heterotrimeric G protein, along with the nature of presynaptic inhibition and its physiological role. We provide an overview for the discovery and physiological relevance of the two known Gβγ–mediated mechanisms for presynaptic inhibition: first, the action of Gβγ on voltage-gated calcium channels to inhibit calcium influx to the presynaptic active zone and, second, the direct binding of Gβγ to the SNARE complex to displace synaptotagmin downstream of calcium entry, which has been demonstrated to be important in neurons and secretory cells. These two mechanisms act in tandem with each other in a synergistic manner to provide more complete spatiotemporal control over neurotransmitter release.
GPCR regulation of secretion
2018, Pharmacology and Therapeutics
Citation Excerpt :
There are 5 different Gβ and 12 different Gγ subunits (Downes & Gautam, 1999; Hildebrandt, 1997; Khan et al., 2013; Simon, Strathmann, & Gautam, 1991). Gβ subunits are made up of an N-terminal α helix of approximately 20 amino acids and 7 blades of four anti-parallel β strands forming a propeller like WD repeat (Clapham & Neer, 1997b; Gautam, Downes, Yan, & Kisselev, 1998b; Smrcka, 2008a; Sondek, Bohm, Lambright, Hamm, & Sigler, 1996) (Fig. 6). Gβ1-4 share up to 90% amino acid sequence identity whereas Gβ5 is only 50% identical (Betty, Harnish, Rhodes, & Cockett, 1998; Smrcka, 2008a).
Modulation of neurotransmitter exocytosis by activated G_i/o coupled G-protein coupled receptors (GPCRs) is a universal regulatory mechanism used both to avoid overstimulation and to influence circuitry. One of the known modulation mechanisms is the interaction between Gβγ and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNAREs). There are 5 Gβ and 12 Gγ subunits, but specific Gβγs activated by a given GPCR and the specificity to effectors, such as SNARE, in vivo are not known. Although less studied, Gβγ binding to the exocytic fusion machinery (i.e. SNARE) provides a more direct regulatory mechanism for neurotransmitter release. Here, we review some recent insights in the architecture of the synaptic terminal, modulation of synaptic transmission, and implications of G protein modulation of synaptic transmission in diseases. Numerous presynaptic proteins are involved in the architecture of synaptic terminals, particularly the active zone, and their importance in the regulation of exocytosis is still not completely understood. Further understanding of the Gβγ-SNARE interaction and the architecture and mechanisms of exocytosis may lead to the discovery of novel therapeutic targets to help patients with various disorders such as hypertension, attention-deficit/hyperactivity disorder, post-traumatic stress disorder, and acute/chronic pain.
A Combined In Vitro/In Silico Approach to Identifying Off-Target Receptor Toxicity
2018, iScience
Many xenobiotics can bind to off-target receptors and cause toxicity via the dysregulation of downstream transcription factors. Identification of subsequent off-target toxicity in these chemicals has often required extensive chemical testing in animal models. An alternative, integrated in vitro/in silico approach for predicting toxic off-target functional responses is presented to refine in vitro receptor identification and reduce the burden on in vivo testing. As part of the methodology, mathematical modeling is used to mechanistically describe processes that regulate transcriptional activity following receptor-ligand binding informed by transcription factor signaling assays. Critical reactions in the signaling cascade are identified to highlight potential perturbation points in the biochemical network that can guide and optimize additional in vitro testing. A physiologically based pharmacokinetic model provides information on the timing and localization of different levels of receptor activation informing whole-body toxic potential resulting from off-target binding.

View all citing articles on Scopus

^†: Present address: Anti-Infectives Research Department, Smith Kline Beecham Pharmaceuticals, Collegeville, PA 19426, USA.

View full text

TOPICAL REVIEWThe G-Protein βγ Complex

Abstract

Introduction

Section snippets

Diversity

Genes and their expression

Assembly and structure

The prenyl group

Modulation of receptor function

Modulation of effector function

βγ Complex in disease

Final note: specificity

Acknowledgements

Cell

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Biochim. Biophys. Acta

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Biochem. Biophys. Res. Commun.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Biochem. Biophys. Res. Commun.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Exp. Eye Res.

Genomics

Genomics

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

FEBS Lett.

Cell

J. Biol. Chem.

J. Biol. Chem.

Biochem. Biophys. Res. Commun.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

FEBS Lett.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

J. Biol. Chem.

Neuron

J. Biol. Chem.

FEBS Lett.

J. Biol. Chem.

Annu. Rev. Neurosci.

TOPICAL REVIEW
The G-Protein βγ Complex