Elsevier

Biochemical Pharmacology

Volume 61, Issue 11, 1 June 2001, Pages 1329-1337
Biochemical Pharmacology

Commentary
Gγ-like (ggl) domains: new frontiers in g-protein signaling and β-propeller scaffolding2

https://doi.org/10.1016/S0006-2952(01)00633-5Get rights and content

Abstract

The standard model of signal transduction from G-protein-coupled receptors (GPCRs) involves guanine nucleotide cycling by a heterotrimeric G-protein assembly composed of Gα, Gβ, and Gγ subunits. The WD-repeat β-propeller protein Gβ and the alpha-helical, isoprenylated polypeptide Gγ are considered obligate dimerization partners; moreover, conventional Gβγ heterodimers are considered essential to the functional coupling of Gα subunits to receptors. However, our recent discovery of a Gβ5 binding site (the Gγ-like or “GGL” domain) within several regulators of G-protein signaling (RGS) proteins revealed the potential for functional GPCR/Gα coupling in the absence of a conventional Gγ subunit. In addition, we posit that the interaction between Gβ5 isoforms and the GGL domains of RGS proteins represents a general mode of binding between β-propeller proteins and their partners, extending beyond the realm of G-protein-linked signal transduction.

Introduction

Signal transduction controls a wide variety of cellular activities, ranging from release of hormones and neurotransmitters, modulation of transmembrane ion flux, and activation or repression of gene transcription, to integrated responses of cellular survival, proliferation, and differentiation. One major class of signal transduction pathways is that controlled by heterotrimeric “G-proteins” [1], [2], [3]. Loss-of-function and gain-of-function mutations to GPCRs and downstream regulators cause a variety of human diseases, including vision pathologies such as retinitis pigmentosa [4], [5] and endocrine disorders such as pseudohypoparathyroidism and McCune-Albright syndrome [6], [7], [8], [9]. Whooping cough and fatal diarrhea, characteristic of infection by Bordetella pertussis and Vibrio cholerae, respectively, are caused by direct effects on G-protein activity catalyzed by pathogen-produced exotoxins [10], [11]. Perturbation of G-protein signaling is also central to the actions of many drugs, from anti-asthmatics and anti-hypertensives to anti-depressants and anti-psychotics [12], [13]. Thus, a better understanding of the molecular machinery underlying G-protein-coupled signal transduction is key to its continued exploitation for drug discovery and the amelioration of human disease.

In the “standard” model of heterotrimeric G-protein signal transduction, serpentine cell-surface GPCRs are coupled to a membrane-associated, heterotrimeric G protein composed of α, β, and γ subunits (Fig. 1A). Upon binding extracellular ligand, the GPCR becomes a GEF through conformational changes in its intracellular loops, thus promoting replacement of bound GDP for GTP on the Gα subunit [17]. The binding of GTP changes the conformation of three “switch” regions within Gα [18], [19], allowing its dissociation from Gβγ. Both GTP-bound Gα and free Gβγ subunits initiate signals by interactions with downstream effector proteins, until the intrinsic GTPase activity of Gα returns the protein to the GDP-bound state (Fig. 1). Reassociation of Gβγ with GDP-bound Gα obscures critical effector contact sites [20], [21], thereby terminating all effector activations. In this manner, therefore, the duration of heterotrimeric G-protein-coupled signaling is controlled by the lifetime of the G-protein α subunit in the GTP-bound state.

The recent discovery of the “regulators of G-protein signaling” or RGS proteins [22], [23], [24], [25] has added several new levels of complexity to this standard model of GPCR signaling [16]. At the simplest level, RGS proteins act via their hallmark, alpha-helical RGS-box [14], [26], [27] to accelerate the intrinsic GTPase activity of Gα subunits [28], [29], [30] and thus attenuate signals derived from GTP-bound Gα and free Gβγ subunits (Fig. 1A). While small RGS proteins such as GAIP, RGS1, GOS8/RGS2, and RGS4 encompass little more than an RGS-box [31], [32], [33], other RGS proteins are composed of multiple domains which bestow additional functionality. As one example, F-subfamily RGS proteins [34], typified by p115-RhoGEF [35] and PDZ-RhoGEF [36], bear DH and PH domains C-terminal to a Gα12/13-specific RGS-box; these proteins not only accelerate Gα12/13 GTPase activity, but also act concomitantly as Gα-effectors, since RGS-box occupancy by Gα-GTP stimulates the guanine-nucleotide exchange activity of the DH/PH tandem directed toward the monomeric G-protein Rho [36], [37]. The D-subfamily members, RGS12 and RGS14, are also presumed to play a role in coordinating cross-talk between heterotrimeric and Ras-superfamily G-proteins [38], given the recent identification of putative Ras-binding (RBD) and novel Gα-binding (GoLoco) domains within both RGS proteins [39], [40], [41], as well as PDZ [30] and PTB domains [16], [42] unique to RGS12. However, the most radical affront to the standard model of GPCR signaling has come from the identification of the Gγ-like or “GGL” domain within the C-subfamily RGS proteins [43]—a discovery that has presaged not only the existence of novel G-protein subunit assemblies but also a potentially universal mode of interaction with β-propeller proteins.

Section snippets

Discovery of the ggl domain and its binding partner, gβ5

In a continuing effort to identify and characterize novel RGS family members, we cloned the human RGS11 cDNA and performed a detailed bioinformatic analysis of its encoded polypeptide sequence. Between N-terminal DEP [44] and C-terminal RGS-box domains, we observed a 64 amino-acid region with striking similarity to G-protein γ subunits [43]. This GGL domain was also noted to be present in the related RGS proteins RGS6, RGS7, RGS9, and EGL-10 [43] (Fig. 2), denoted the “C-subfamily” by Farquhar

The true partner for gβ5?

The discovery of C-subfamily RGS proteins as avid binding partners for Gβ5 brings into question the relevance of recent research exploring the signaling capacity of Gβ5 in complex with conventional Gγ subunits. In their papers describing the original identification of Gβ5 and Gβ5L, Simon and colleagues [60], [61] suggested that Gγ2 is the most likely dimerization partner for both Gβ5 isoforms. However, this suggestion was based not on the frank isolation of Gβ5/Gγ2 dimers, but solely on an

The true function of gβ5/rgs complexes?

If the true in vivo partners for Gβ5 isoforms are GGL domain-containing RGS proteins, what function(s) in GPCR signaling is performed by these novel heterodimers? The RGS-box contained within such heterodimers has demonstrable GAP activity in vitro toward Gαt (in the case of RGS9 [57]) and Gαo (in the case of RGS6, -7, and -11 [43], [56]); thus, a potential role for Gβ5/RGS complexes in accelerating GPCR signaling deactivation by enhancing GTP hydrolysis can be predicted and, indeed, has been

The ggl domain as a modular β-propeller binding unit

We believe that the GGL domain represents a modular interaction site found within many different proteins that bind β-propeller partners, and not just a Gβ5 binding site restricted to certain RGS proteins. For example, we have recently identified two novel open-reading frames, CG15844 and CG18511, within the Drosophila melanogaster genome [47] that each possess a Gγ-like polypeptide sequence, yet lack an identifiable RGS-box (Fig. 2). While we have been unable to detect any additional

Conclusion

The discovery of the GGL domain as a novel Gβ binding partner is leading to a bifurcated view of G-protein-coupled signal transduction: a Gβ5/RGS heterodimer must now be placed into the context of GPCR/Gα/effector signaling alongside the conventional Gβγ subunit paradigm. Our recent identification of GGL domains in proteins outside the RGS C-subfamily suggests that it may be necessary to extend the concept of Gγ-like domains well past the current, narrow realm of heterotrimeric G-protein

Acknowledgements

Our thanks to Ryan Watkins for assistance in modelling the RACK1/PDE4D5 interaction and T. Kendall Harden for critical review of this commentary and unwavering support. The authors are supported by NIH Grants GM57391 (J.S.), and GM62338 (D.P.S.). J.S. is a Scholar of the Pew Charitable Trusts, and D.P.S. is a Scholar of the EJLB Foundation. J.S. is a Scholar of the Pew Charitable Trusts and D.P.S. is a scholar of the EJLB Foundation and a recipient of a New Investigator Award in The Basic

References (85)

  • D.M. Berman et al.

    GAIP and RGS4 are GTPase-activating proteins for the Gi subfamily of G protein α subunits

    Cell

    (1996)
  • B.E. Snow et al.

    GTPase activating specificity of RGS12 and binding specificity of an alternatively spliced PDZ (PSD-95/Dlg/ZO-1) domain

    J Biol Chem

    (1998)
  • B. Zheng et al.

    Divergence of RGS proteinsevidence for the existence of six mammalian RGS subfamilies

    Trends Biochem Sci

    (1999)
  • S. Fukuhara et al.

    A novel PDZ domain containing guanine nucleotide exchange factor links heterotrimeric G proteins to Rho

    J Biol Chem

    (1999)
  • B.E. Snow et al.

    Molecular cloning and expression analysis of rat Rgs12 and Rgs14

    Biochem Biophys Res Commun

    (1997)
  • D.P. Siderovski et al.

    The GoLoco motifa Gαi/o binding motif and potential guanine-nucleotide exchange factor

    Trends Biochem Sci

    (1999)
  • C. Ponting et al.

    Pleckstrin’s repeat performancea novel domain in G-protein signaling

    Trends Biochem Sci

    (1996)
  • S.J. Yarwood et al.

    The RACK1 signaling scaffold protein selectively interacts with the cAMP-specific phosphodiesterase PDE4D5 isoform

    J Biol Chem

    (1999)
  • B.A. Posner et al.

    Regulators of G protein signaling 6 and 7. Purification of complexes with Gβ5 and assessment of their effects on G protein-mediated signaling pathways

    J Biol Chem

    (1999)
  • A.J. Watson et al.

    A fifth member of the mammalian G-protein β-subunit family. Expression in brain and activation of the β 2 isotype of phospholipase C

    J Biol Chem

    (1994)
  • A.J. Watson et al.

    A novel form of the G protein β subunit Gβ5 is specifically expressed in the vertebrate retina

    J Biol Chem

    (1996)
  • M.L. Bayewitch et al.

    Differential modulation of adenylyl cyclases I and II by various Gβ subunits

    J Biol Chem

    (1998)
  • S. Zhang et al.

    Selective activation of effector pathways by brain-specific G protein β5

    J Biol Chem

    (1996)
  • U. Maier et al.

    5γ2 is a highly selective activator of phospholipid-dependent enzymes

    J Biol Chem

    (2000)
  • M.B. Jones et al.

    Instability of the G-protein β5 subunit in detergent

    Anal Biochem

    (1999)
  • J.E. Fletcher et al.

    The G protein β5 subunit interacts selectively with the Gq α subunit

    J Biol Chem

    (1998)
  • M.A. Lindorfer et al.

    Differential activity of the G protein β52 subunit at receptors and effectors

    J Biol Chem

    (1998)
  • D.S. Witherow et al.

    Complexes of the G protein subunit Gβ5 with the regulators of G protein signaling RGS7 and RGS9characterization in native tissues and in transfected cells

    J Biol Chem

    (2000)
  • A. Kovoor et al.

    Co-expression of Gβ5 enhances the function of two Gγ subunit-like domain-containing regulators of G protein signaling proteins

    J Biol Chem

    (2000)
  • J. de Rooij et al.

    Mechanism of regulation of the Epac family of cAMP-dependent RapGEFs

    J Biol Chem

    (2000)
  • Y. Tu et al.

    Palmitoylation of a conserved cysteine in the regulator of G protein signaling (RGS) domain modulates the GTPase-activating activity of RGS4 and RGS10

    J Biol Chem

    (1999)
  • A. Gaskell et al.

    The three domains of a bacterial sialidasea β-propeller, an immunoglobulin module and a galactose-binding jelly-roll

    Structure

    (1995)
  • T.A. Springer

    An extracellular β-propeller module predicted in lipoprotein and scavenger receptors, tyrosine kinases, epidermal growth factor precursor, and extracellular matrix components

    J Mol Biol

    (1998)
  • A.G. Gilman

    G proteinstransducers of receptor-generated signals

    Annu Rev Biochem

    (1987)
  • P.J. Rosenfeld et al.

    A null mutation in the rhodopsin gene causes rod photoreceptor dysfunction and autosomal recessive retinitis pigmentosa

    Nat Genet

    (1992)
  • J.L. Patten et al.

    Mutation in the gene encoding the stimulatory G protein of adenylate cyclase in Albright’s hereditary osteodystrophy

    N Engl J Med

    (1990)
  • L.S. Weinstein et al.

    Mutations of the Gs α-subunit gene in Albright hereditary osteodystrophy detected by denaturing gradient gel electrophoresis

    Proc Natl Acad Sci USA

    (1990)
  • L.S. Weinstein et al.

    Activating mutations of the stimulatory G protein in the McCune-Albright syndrome

    N Engl J Med

    (1991)
  • W.F. Schwindinger et al.

    Identification of a mutation in the gene encoding the α subunit of the stimulatory G protein of adenylyl cyclase in McCune-Albright syndrome

    Proc Natl Acad Sci USA

    (1992)
  • J. Holmgren

    Actions of cholera toxin and the prevention and treatment of cholera

    Nature

    (1981)
  • E.L. Hewlett

    Pertussiscurrent concepts of pathogenesis and prevention

    Pediatr Infect Dis J

    (1997)
  • W. Roush

    Regulating G protein signaling

    Science

    (1996)
  • Cited by (0)

    2

    Abbreviations: DEP, dishevelled/EGL-10/pleckstrin-related domain; DH, dbl-homology domain; GAP, guanosine triphosphatase-activating protein; GEF, guanine nucleotide exchange factor; GGL, G-gamma-like; GIRK, G-protein-gated inwardly rectifying potassium channel; GPCR, G-protein-coupled receptor; G protein, guanine nucleotide binding protein; GTPase, guanosine triphosphatase; mAChR, muscarinic acetylcholine receptor; MAPK, mitogen-activated protein kinase; PDE, phosphodiesterase; PDZ, PSD-95/Discs-large/ZO-1 related domain; PH, pleckstrin-homology domain; PI3Kγ, gamma isoform of phosphoinositide 3-kinase; PLC-β, beta isoform of phospholipase C; PTB, phosphotyrosine-binding domain; RACK1, receptor for activated C kinase type-1; RBD, Ras-binding domain; RGS, regulators of G-protein signaling; and SAPK, stress-activated protein kinase.

    View full text