Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

RGS Protein Family

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_527

Synonyms

Historical Background

Signal transduction by G protein–coupled receptors (GPCRs) was considered for many years (Gilman 1987) to be a three-component system: the cell-surface receptor to receive external input from hormones and neurotransmitters, the heterotrimeric G protein to transduce this input to the intracellular compartment by its structural changes upon the exchange of guanosine triphosphate (GTP) for guanosine diphosphate (GDP), and effector proteins (such as  adenylyl cyclase, phospholipase C, and ion channels) to propagate the signal forward as changes in cell membrane potential and/or intracellular second messenger levels. However, for many physiological responses mediated by GPCRs, including the visual response controlled by the photoreceptor, rhodopsin (Arshavsky and Pugh 1998), intracellular signaling was known to be far shorter in duration than the time observed for the isolated components to revert to ground state in vitro (i.e., the time required for the heterotrimeric G-protein α subunit to hydrolyze GTP and return to its GDP-bound, inactive state). A critical fourth component to this system was discovered to be a large family of “regulators of G-protein signaling” proteins, also known as RGS proteins (Willard et al. 2008), that dramatically accelerate GTP hydrolysis by Gα subunits and thereby hasten signal termination (Fig. 1).
RGS Protein Family, Fig. 1

Role of RGS proteins in GPCR signaling as negative regulators. Ligand-activated GPCRs act as guanine nucleotide exchange factors for the inactive, GDP- and Gβγ-bound Gα subunit. The resultant release of GDP, and subsequent binding of the more abundant GTP, leads to a conformational change within Gα, eliminating the high-affinity binding site for Gβγ. The GTP-bound Gα and released Gβγ subunits are then able to bind effector proteins to propagate intracellular signaling. The intrinsic GTP hydrolysis activity of Gα subunits is greatly accelerated by the binding of RGS proteins, leading to the release of inorganic phosphate (Pi) and reassembly of the inactive Gα·GDP/Gβγ heterotrimer

One of the earliest reports of cloning a human RGS protein-encoding gene was that of G0S8 (now known as RGS2) in 1990. Trapped as one of a number of putative “G0/G1-switch genes” from mitogen-treated, primary human T-lymphocytes (Siderovski et al. 1990), G0S8/RGS2 was subsequently observed to encode a protein with sequence similarity to kinases specific for activated GPCRs (e.g., β-adrenergic receptor kinase,  rhodopsin kinase) and to the yeast “supersensitivity-to-phermone” protein SST2 (Siderovski et al. 1994). Functional complementation of yeast deficient in SST2 by overexpression of the human G0S8/RGS2 gene (Siderovski et al. 1996) provided one of the first clues that this emergent gene family encoded negative regulators of signal transduction acting downstream of GPCR activation; other groups working in disparate systems came to the same conclusion contemporaneously (Druey et al. 1996; Koelle and Horvitz 1996). These reports were quickly followed by a definitive demonstration of the biochemical activity underlying the negative regulatory function of RGS proteins: namely, acceleration of the intrinsic GTPase activity of Gα subunits using purified proteins and radiolabeled GTP (Berman et al. 1996a).

RGS Protein Activities

The signature enzymatic activity of RGS proteins is the acceleration of GTP hydrolysis by activated, GTP-bound Gα subunits. This acceleration can be made on certain GTPase-deficient Gα mutants as well (e.g., point mutation to the arginine, such as Arg-178 of Gαi1, that helps stabilize the γ-phosphate leaving group) but not on classical, GTPase-dead, Gln-to-Leu Gα mutants (e.g., Gαi1 Q204L) (Berman et al. 1996a). Following discovery of this signature biochemical activity of RGS proteins, a crystal structure of RGS4 bound to Gαi1 in a transition state-mimetic form was reported (Tesmer et al. 1997). The ability to observe the interaction between a RGS protein and its Gα target to high-resolution solidified early speculation that RGS proteins employ solely their highly conserved, ∼120 amino-acid “RGS domain” to stabilize Gα in its transition state along the path to GTP hydrolysis (Berman et al. 1996b); this “GTPase-accelerating protein” or “GAP” mechanism is distinctly different from that exhibited by the GAPs of Ras-superfamily GTPases. The GAP activity of certain RGS proteins, including RGS4, is thought to be modulated in a cellular context by the binding of phosphatidylinositol head-groups or calmodulin to a “B-site,” within the RGS domain but distinct from the Gα-binding interface (or “A-site”) (Fig. 2); engagement of the B-site with phosphatidylinositol-3,4,5-trisphosphate (PIP3) is considered to inhibit A-site GAP function in an allosteric fashion, whereas the binding of calmodulin (in a calcium-dependent manner) to the B-site removes the inhibitory influence of PIP3 (Tu and Wilkie 2004).
RGS Protein Family, Fig. 2

Predicted structural determinants of the allosteric control over RGS protein GAP activity. (a) For some RGS proteins, the binding of the RGS domain “B-site” with phosphatidylinositol-3,4,5-trisphosphate (PIP3) is thought to allosterically inhibit the GTPase-accelerating activity of the RGS domain Gα-binding “A-site”; in a calcium-dependent fashion, the binding of calmodulin to the B-site is thought to remove the inhibitory influence of PIP3 on A-site GAP function. (b) Visualization of the predicted functional sites within the RGS domain of RGS4 (Protein Data Bank id 1AGR) responsible for allosteric control of GAP activity. Highlighted (orange/dark grey) regions depict lysines thought to be required for PIP3 binding, while solid surface (cyan/light grey) areas depict the proposed A- and B-sites. The alpha-helical secondary structure that comprises the conserved RGS domain fold is displayed in red/black as ribbon tracing within the translucent surface rendering of the domain. Rotation about the vertical axis by 90o and 180o are shown in consecutive panels from left to right

Observations of RGS protein overexpression leading to accelerated on-rates of GPCR signal transduction without affecting response sensitivity or amplitude have been presented in the literature (e.g., Doupnik et al. 1997) as paradoxical findings that run counter to expectations that RGS protein GAP activity should only serve to accelerate the off-rate of GPCR signaling and, thereby, blunt signaling. It is possible that RGS proteins contribute more than just GAP activity to the functioning of the GPCR/G–protein/effector axis, especially since many RGS proteins contain multiple protein/protein-interaction domains in addition to the signature RGS domain (see below). More recently, by combining Gα mutations that accelerate intrinsic GTPase activity and that eliminate sensitivity to RGS domain GAP activity (while preserving all other Gα functions), it has been definitively demonstrated that accelerated GTP hydrolysis alone is sufficient to elicit observations of increased signaling onset and recovery times (Lambert et al. 2010). This finding, however, does not exclude the possibility that conventional (“GAP-active”) RGS proteins and/or other RGS domain-containing proteins exhibit additional functional effects on GPCR-initiated signal transduction in a cellular context.

The Conventional RGS Protein Subfamilies

The “conventional” members of the RGS protein family (Table 1) exhibit Gα-directed GAP activity and have been numbered from RGS1 to RGS21 (excluding RGS15, which turned out to be RGS3). The majority of these proteins target Gα subunits of the Gαi and Gαq subfamilies (Soundararajan et al. 2008), albeit with notable exceptions as described below. These proteins have been divided into subfamilies based on overall protein architecture and RGS domain sequence similarity (Willard et al. 2008). The R4-subfamily is the largest by membership, consisting of RGS1, -2, -3, -4, -5, -8, -13, -16, -18, and -21, yet the smallest by individual protein size; most members merely consist of the ∼120 amino-acid RGS domain with short N- and C-terminal polypeptide extensions (e.g., RGS21 is only 152 amino acids in length). An exception to this small size is the R4-subfamily member RGS3, given that alternative isoforms of this protein are expressed that include N-terminal PDZ and C2 domains. Within the R4-subfamily, RGS2 is unique in acting as a potent GAP solely on Gαq subfamily members (and not Gαi subunits) in vitro (Kimple et al. 2009), although inhibition of Gi-coupled GPCR signaling can be observed in a cellular context upon RGS2 overexpression (Ingi et al. 1998). RZ-subfamily members (RGS17, -19, and -20) are also small polypeptides but are distinct from the R4-subfamily in containing cysteine-rich N-termini thought to be reversibly palmitoylated for differential subcellular trafficking. As the name suggests, RZ-subfamily members have particular selectivity for Gαz subunits, although this is not exclusive, and binding of (and GAP activity on) Gαi subunits is also manifested (e.g., Soundararajan et al. 2008).
RGS Protein Family, Table 1

The conventional RGS proteins

Family

Name

GenBank locus

UniProt id

Entrez gene id

Distinguishing characteristic(s)

R4

RGS1

NM_002922

Q08116

5996

Implicated in multiple sclerosis

R4

RGS2

NM_002923

P41220

5997

Selective for Gαq; modulator of anxiety and vasoconstrictor signaling

R4

RGS3

NM_144489

P49796

5998

Isoforms can contain PDZ and C2 domains

R4

RGS4

NM_005613

P49798

5999

Associated with susceptibility to schizophrenia

R4

RGS5

NM_003617

O15539

8490

Expressed in pericytes; associated with neovascularization

R7

RGS6

NM_004296

P49758

9628

Potential modulator of parasympathetic activation in heart

R7

RGS7

NM_002924

P49802

6000

Implicated in CNS opioid and muscarinic acetylcholine signaling

R4

RGS8

NM_033345

P57771

85397

Directly binds GPCR loops (or indirectly via spinophilin); may control stable cell-surface GPCR expression

R7

RGS9

NM_003835

O75916

8787

Key deactivator of retinal phototransduction cascade

R12

RGS10

NM_002925

O43665

6001

Phosphorylation and palmitoylation control nuclear localization and Gα substrate selectivity

R7

RGS11

NM_183337

O94810

8786

Modulator of retinal ON-bipolar cell light response

R12

RGS12

NM_198229

O14924

6002

Contains PDZ, PTB, RBD, and GoLoco domains; scaffold for Ras/Raf/MAPK cascade

R4

RGS13

NM_002927

O14921

6003

Modulator of GPCR signaling in mast cells / allergic responses

R12

RGS14

NM_006480

O43566

10636

Contains RBD and GoLoco domains; scaffold for Ras/Raf/MAPK cascade

R4

RGS16

NM_002928

O15492

6004

Feeding and fasting controls expression in periportal hepatocytes

RZ

RGS17

NM_012419

Q9UGC6

26575

Implicated in lung tumorigenesis

R4

RGS18

NM_130782

Q9NS28

64407

Expressed in leukocytes, megakaryocytes, and platelets

RZ

RGS19

NM_005873

P49795

10287

Implicated in Wnt/β-catenin signaling

RZ

RGS20

NM_170587

O76081

8601

Modulator of mu-opioid receptor signaling

R4

RGS21

NM_001039152

Q2M5E4

431704

Expressed in lingual taste buds

R7-subfamily members (RGS6, -7, -9, and -11) are known to play key roles in the regulation of various neuronal processes such as nociception, motor control, reward behavior, and vision. These four proteins share an expression pattern biased to neuronal tissues, as well as a unique multi-domain protein architecture composed of DEP (Dishevelled/EGL-10/Pleckstrin) and GGL (G-gamma-like) domains present N-terminal to a central RGS domain. The DEP domain mediates interaction with unique membrane anchor proteins R7BP and R9AP, whereas the GGL domain (as its name implies) binds a neuronal-specific Gβ subunit, Gβ5, to form an obligate dimeric configuration akin to conventional Gβ/Gγ subunits (Snow et al. 1998). While the R12-subfamily member RGS10 consists of little more than an RGS domain, the other two members of this subfamily (RGS12 and RGS14) share elaborate multi-domain architectures. C-terminal to their RGS domains, both RGS12 and RGS14, possess a tandem repeat of Ras-binding domains (RBDs) and a single GoLoco motif; the first of the two RBDs binds selectively to activated H-Ras (Willard et al. 2007), whereas the GoLoco motif is known to bind Gαi subunits in their GDP-bound inactive state (Kimple et al. 2002). Unlike RGS14, RGS12 also possesses N-terminal PDZ and PTB domains which play important roles in the functional organization of an H-Ras-Raf-MAPK signaling cascade required for nerve growth factor (NGF)-mediated axonogenesis by dorsal root ganglion neurons (Willard et al. 2007).

Other RGS Domain-Containing Proteins

There are an equivalent number of “nonconventional” RGS proteins (Table 2) that, while possessing the highly conserved nine alpha-helical structure of the RGS domain (Tesmer et al. 1997; Soundararajan et al. 2008), either have already been identified in other functional contexts or have yet to be identified as bona fide Gα-directed GAPs. With respect to the latter situation, AKAP-10 (also known as D-AKAP2) and RGS22 possess more than one RGS domain, but to date neither have been convincingly shown to bind to (nor accelerate the GTPase activity of) Gα subunits; this is also true of three sorting nexins (SNX13, -14, and -25) and the RA-subfamily members (Axin, Axin2) that each possess a single, central RGS domain of poorly characterized or controversial Gα-modulatory function.
RGS Protein Family, Table 2

Other proteins containing RGS domain(s)

Family

Name

GenBank locus

UniProt id

Entrez gene id

Distinguishing characteristic(s)

 

AKAP10

NM_007202

O43572

11216

Contains 2 RGS domains which interact with Rab4 and Rab11 GTPases

 

RGS22

NM_015668

Q9BYZ4

26166

Contains 3 RGS domains; specifically expressed in testes

SNX

SNX13

NM_015132

Q9Y5W8

23161

Also known as RGS-PX1; controversial report of Gαs-directed GAP activity

SNX

SNX14

NM_153816

Q9Y5W7

57231

Also known as RGS-PX2

SNX

SNX25

NM_031953

Q9H3E2

83891

Sorting nexin-25; speculated to bind phosphatidylinositols with its PX domain

RA

Axin

NM_003502

O15169

8312

Involved in Wnt signaling; component of β-catenin destruction complex

RA

Axin2

NM_004655

Q9Y2T1

8313

Also known as conductin; regulator of centrosome cohesion

GRK

GRK1

NM_002929

Q15835

6011

Also known as rhodopsin kinase

GRK

GRK2

NM_001619

P25098

156

Also known as β-adrenergic receptor kinase-1 (βARK1); RGS domain binds activated Gαq/11

GRK

GRK3

NM_005160

P35626

157

Also known as β-adrenergic receptor kinase-2 (βARK2)

GRK

GRK4

NM_182982

P32298

2868

Linked to genetic and acquired hypertension

GRK

GRK5

NM_005308

P34947

2869

Modulator of NFκB signaling via IκBα interaction

GRK

GRK6

NM_002082

P43250

2870

Involved in phosphorylation and desensitization of CXCR4

GRK

GRK7

NM_139209

Q8WTQ7

131890

Involved in cone phototransduction

GEF

ARHGEF1

NM_004706

Q92888

9138

Also known as p115 RhoGEF; Gα12/13-dependent exchange factor for RhoA GTPase

GEF

ARHGEF11

NM_014784

O15085

9826

Also known as PDZ-RhoGEF

GEF

ARHGEF12

NM_015313

Q9NZN5

23365

Also known as LARG or “leukemia-associated RhoGEF”

As previously mentioned, early reports of the discovery of the RGS protein family highlighted the presence of an N-terminal RGS domain within the known family of serine/threonine kinases (GRK1 to 7; Table 2) that are specific for activated GPCRs (Siderovski et al. 1994; Siderovski et al. 1996); subsequent examination of this N-terminal RGS domain within GRK2 revealed in vitro Gαq binding selectivity and a cellular function in inhibiting Gq-coupled GPCR signaling, albeit with little (if any) Gαq-directed GAP activity. N-terminal RGS domains were also identified in guanine nucleotide exchange factors for the small GTPase RhoA (i.e., the GEF-subfamily of RGS proteins; namely, p115-RhoGEF/ARHGEF1, PDZ-RhoGEF/ARHGEF11, and LARG/ARHGEF12). This identification helped to explain the ability of G12/13-coupled GPCRs to activate RhoA in a cellular context; therefore, the GEF-subfamily of RGS proteins are emblematic of RGS domain-containing proteins that serve as effectors (i.e., propagating the signal forward) even while they also serve as GAPs for their upstream activators (i.e., activated Gα12·GTP and Gα13·GTP subunits).

Summary

Originally discovered as negative regulators of GPCR signal transduction owing to their Gα-directed GAP activity, the RGS proteins are now appreciated to possess multifaceted functions in cellular signaling networks. These multiple functions can arise from elaborate, multiple protein-domain architectures, unique binding partners, and their individual abilities to coordinate and/or modulate other signal transduction components, such as small Ras-superfamily GTPases. Unique expression patterns and Gα-binding selectivities of the RGS proteins underlie their individual involvement in distinct physiological and pathophysiological phenomena. What remains to be determined is whether RGS proteins can be selectively inhibited by small molecules and, even more speculatively, whether their activity could be enhanced by small molecules that usurp normal allosteric control over the Gα-binding A-site.

References

  1. Arshavsky VY, Pugh Jr EN. Lifetime regulation of G protein-effector complex: emerging importance of RGS proteins. Neuron. 1998;20(1):11–4.PubMedCrossRefGoogle Scholar
  2. Berman DM, Wilkie TM, Gilman AG. GAIP and RGS4 are GTPase-activating proteins for the Gi subfamily of G protein alpha subunits. Cell. 1996a;86(3):445–52.PubMedCrossRefGoogle Scholar
  3. Berman DM, Kozasa T, Gilman AG. The GTPase-activating protein RGS4 stabilizes the transition state for nucleotide hydrolysis. J Biol Chem. 1996b;271(44):27209–12.PubMedCrossRefGoogle Scholar
  4. Doupnik CA, Davidson N, Lester HA, Kofuji P. RGS proteins reconstitute the rapid gating kinetics of gbetagamma-activated inwardly rectifying K+ channels. Proc Natl Acad Sci U S A. 1997;94(19):10461–6.PubMedPubMedCentralCrossRefGoogle Scholar
  5. Druey KM, Blumer KJ, Kang VH, Kehrl JH. Inhibition of G-protein-mediated MAP kinase activation by a new mammalian gene family. Nature. 1996;379(6567):742–6.PubMedCrossRefGoogle Scholar
  6. Gilman AG. G proteins: transducers of receptor-generated signals. Annu Rev Biochem. 1987;56:615–49.PubMedCrossRefGoogle Scholar
  7. Ingi T, Krumins AM, Chidiac P, Brothers GM, Chung S, Snow BE, et al. Dynamic regulation of RGS2 suggests a novel mechanism in G-protein signaling and neuronal plasticity. J Neurosci. 1998;18(18):7178–88.PubMedGoogle Scholar
  8. Kimple RJ, Kimple ME, Betts L, Sondek J, Siderovski DP. Structural determinants for GoLoco-induced inhibition of nucleotide release by Galpha subunits. Nature. 2002;416(6883):878–81.PubMedCrossRefGoogle Scholar
  9. Kimple AJ, Soundararajan M, Hutsell SQ, Roos AK, Urban DJ, Setola V, et al. Structural determinants of G-protein alpha subunit selectivity by regulator of G-protein signaling 2 (RGS2). J Biol Chem. 2009;284(29):19402–11.PubMedPubMedCentralCrossRefGoogle Scholar
  10. Koelle MR, Horvitz HR. EGL-10 regulates G protein signaling in the C. elegans nervous system and shares a conserved domain with many mammalian proteins. Cell. 1996;84(1):115–25.PubMedCrossRefGoogle Scholar
  11. Lambert NA, Johnston CA, Cappell SD, Kuravi S, Kimple AJ, Willard FS, et al. Regulators of G-protein signaling accelerate GPCR signaling kinetics and govern sensitivity solely by accelerating GTPase activity. Proc Natl Acad Sci U S A. 2010;107(15):7066–71.PubMedPubMedCentralCrossRefGoogle Scholar
  12. Siderovski DP, Blum S, Forsdyke RE, Forsdyke DR. A set of human putative lymphocyte G0/G1 switch genes includes genes homologous to rodent cytokine and zinc finger protein-encoding genes. DNA Cell Biol. 1990;9(8):579–87.PubMedCrossRefGoogle Scholar
  13. Siderovski DP, Heximer SP, Forsdyke DR. A human gene encoding a putative basic helix-loop-helix phosphoprotein whose mRNA increases rapidly in cycloheximide-treated blood mononuclear cells. DNA Cell Biol. 1994;13(2):125–47.PubMedCrossRefGoogle Scholar
  14. Siderovski DP, Hessel A, Chung S, Mak TW, Tyers M. A new family of regulators of G-protein-coupled receptors? Curr Biol. 1996;6(2):211–2.PubMedCrossRefGoogle Scholar
  15. Snow BE, Krumins AM, Brothers GM, Lee SF, Wall MA, Chung S, et al. A G protein gamma subunit-like domain shared between RGS11 and other RGS proteins specifies binding to Gbeta 5 subunits. Proc Natl Acad Sci U S A. 1998;95(22):13307–12.PubMedPubMedCentralCrossRefGoogle Scholar
  16. Soundararajan M, Willard FS, Kimple AJ, Turnbull AP, Ball LJ, Schoch GA, et al. Structural diversity in the RGS domain and its interaction with heterotrimeric G protein alpha-subunits. Proc Natl Acad Sci U S A. 2008;105(17):6457–62.PubMedPubMedCentralCrossRefGoogle Scholar
  17. Tesmer JJ, Berman DM, Gilman AG, Sprang SR. Structure of RGS4 bound to AlF4–activated G(i alpha1): stabilization of the transition state for GTP hydrolysis. Cell. 1997;89(2):251–61.PubMedCrossRefGoogle Scholar
  18. Tu Y, Wilkie TM. Allosteric regulation of GAP activity by phospholipids in regulators of G-protein signaling. Methods Enzymol. 2004;389:89–105.PubMedCrossRefGoogle Scholar
  19. Willard MD, Willard FS, Li X, Cappell SD, Snider WD, Siderovski DP. Selective role for RGS12 as a Ras/Raf/MEK scaffold in nerve growth factor-mediated differentiation. EMBO J. 2007;26(8):2029–40.PubMedPubMedCentralCrossRefGoogle Scholar
  20. Willard MD, Willard FS, Siderovski DP. The superfamily of ‘regulator of G-protein signaling’ (RGS) proteins. In: Bradshaw R, Dennis E, editors. Handbook of cell signaling. 2nd ed. San Diego: Elsevier; 2008.Google Scholar

Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Department of PharmacologyUniversity of North Carolina at Chapel HillChapel HillUSA