Chapter 3 Regulators of G Protein Signaling Proteins as Central Components of G Protein‐Coupled Receptor Signaling Complexes

doi:10.1016/S1877-1173(09)86003-1

Progress in Molecular Biology and Translational Science

Volume 86, 2009, Pages 49-74

https://doi.org/10.1016/S1877-1173(09)86003-1 Get rights and content

The regulators of G protein signaling (RGS) proteins bind directly to G protein alpha (Gα) subunits to regulate the signaling functions of Gα and their linked G protein‐coupled receptors (GPCRs). Recent studies indicate that RGS proteins also interact with GPCRs, not just G proteins, to form preferred functional pairs. Interactions between GPCRs and RGS proteins may be direct or indirect (via a linker protein) and are dictated by the receptors, rather than the linked G proteins. Emerging models suggest that GPCRs serve as platforms for assembling an overlapping and distinct constellation of signaling proteins that perform receptor‐specific signaling tasks. Compelling evidence now indicates that RGS proteins are central components of these GPCR signaling complexes. This review will outline recent discoveries of GPCR/RGS pairs as well as new data in support of the idea that GPCRs serve as platforms for the formation of multiprotein signaling complexes.

Introduction

Neurotransmitters and hormones exert their activity by relaying messages across the plasma membrane and inside cells via specific G protein‐coupled receptors (GPCRs). In turn, GPCRs activate heterotrimeric G proteins and linked intracellular signaling pathways.¹ Early models of G protein signaling proposed that GPCRs preferentially bound and activated one specific G protein. However, our understanding of these pathways has evolved in recent years to include a new appreciation for an unexpected complexity of GPCR/G protein signal transduction. Emerging evidence suggests that following agonist stimulation, some receptors can activate multiple G proteins and regulatory proteins to trigger various signaling pathways.² In some cases, signaling occurs in the absence of agonist due to constitutive receptor activity.3, 4 Extensive cross talk between G protein‐linked and other signaling pathways also is well documented, further complicating GPCR signal transduction.5, 6, 7, 8 To preserve specificity and fidelity, these complex receptor‐initiated signals must be tightly regulated at multiple levels. A large number of regulatory proteins have been identified in recent years that modulate GPCR and G protein signaling. Prominent among these are the regulators of G protein signaling (RGS) proteins, a diverse family of multifunctional proteins that regulate GPCR signal transduction at the level of the receptor, the G protein and the effector. This review will focus on our current understanding of RGS protein interactions with receptors and their regulation of receptor signaling.

Section snippets

Overview of RGS Proteins

A primary function of RGS proteins is to regulate the lifetime of G protein signaling events. Agonist activation of a GPCR triggers the exchange of GDP for GTP on a bound Gα, thereby stimulating the protein to initiate a downstream signaling cascade. The duration of the signaling event is determined by the lifetime of GTP bound to the Gα subunit that, in turn, is dictated by the intrinsic GTPase activity of the Gα. In some cases, Gα GTPase activity may be accelerated when a Gα interacts with

RGS Protein Interactions with GPCRs

Compelling evidence from many independent studies now indicates that RGS proteins selectively interact with GPCRs to form functional pairs (Table I). These studies have demonstrated that RGS protein interactions with receptors may be G protein‐dependent, G protein‐independent, or both—though which of these mechanisms applies in individual cases remains to be clearly established. Considerable information is now available regarding how RGS proteins interact with G proteins.¹³ Early studies using

GPCRs Serve as Platforms for Molecular Signaling

As outlined above, RGS proteins and GPCRs form preferred functional pairs, either through direct or indirect interactions. This information changes the way that we think about GPCR signaling and impacts working models of how these receptors and their linked G proteins and downstream signaling pathways are regulated. Recently solved crystal structures of GPCRs indicate that these seven‐transmembrane‐spanning proteins have the surface area and capacity to form multiprotein complexes. In this way,

Summary and Perspectives

This review has summarized the many reports of RGS protein involvement in signaling by GPCRs and other cell surface receptors. From these reports, investigators are gaining an appreciation for the essential role that RGS proteins play in receptor signaling beyond their recognized role as simple inhibitors of G protein signaling. New models that portray GPCRs as multifunctional platforms that mediate diverse and overlapping signaling pathways are supported by a large and growing body of

Acknowledgments

K.L.M. was supported by an American Heart Association predoctoral fellowship (AHA0715465B). J.R.H. was supported by grants from the National Institutes of Health (R01NS037112 and R01NS049195).

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One structural feature in common among RGS proteins is an RGS domain (~ 120 amino acids), which is involved in binding to Gα-GTP and mediating GAP function. It has recently been shown that GPCRs can interact with RGS and Gα proteins, and serve as a scaffolding platform for them [15,16]. Some RGS proteins directly or indirectly interact with intracellular loop 3 or the C-terminus of GPCRs to inhibit Gα-mediated signaling [17,18].
Activation of seven-transmembrane-domain-possessing G protein-coupled receptors (GPCRs) by extracellular stimuli elicits intracellular responses. One class of GPCRs–protease-activated receptors (PARs)—is activated by endogenous proteases, such as thrombin and trypsin. Members of the regulator of G protein signaling (RGS) family stimulate GTP hydrolysis of G protein alpha (Gα) subunits, thereby inhibiting GPCR/Gα-mediated signaling. We previously reported that RGS2 and RGS4 inhibit PAR1/Gα-mediated signaling by interacting with PAR1 in a Gα-dependent manner. Here, employing the bioluminescence resonance energy transfer (BRET) technique, we identified RGS8 as a novel PAR1-interacting protein. Very little BRET activity was observed between PAR1-Venus (PAR1-Ven) and RGS8-Luciferase (RGS8-Luc) in the absence of Gα. However, in the presence of Gα_o, BRET activity was specifically and significantly increased. This interaction was confirmed by biochemical and immunofluorescence assays. Notably, RGS8 inhibited PAR1/Gα_i/o-mediated adenylyl cyclase and ERK activation, and prevented Gα_o-induced neurite outgrowth and activation of Necdin protein, a downstream target of Gα_o. Our findings suggest a novel function of RGS8 and reveal cellular mechanisms by which RGS8 mediates PAR1 inhibition.
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Citation Excerpt :
Compelling evidence now indicates that GPCRs are platforms where specific sets of proteins assemble to execute receptor-specific signaling events. RGS proteins are central components of GPCR signaling complexes that fine-tune G protein signaling and serve as multifunctional integrators of these pathways.2,5,12–14 The RGS protein family consists of almost 40 members that share a conserved RGS domain, which selectively binds to activated Gα-GTP subunits and, in nearly all cases, confers GAP activity.1–4,12
The regulators of G protein signaling (RGS) proteins are a diverse family of proteins that function as central components of G protein and other signaling pathways. In the brain, regulator of G protein signaling 14 (RGS14) is enriched in neurons in the hippocampus where the mRNA and protein are highly expressed. This brain region plays a major role in processing learning and forming new memories. RGS14 is an unusual RGS protein that acts as a multifunctional scaffolding protein to integrate signaling events and pathways essential for synaptic plasticity, including conventional and unconventional G protein signaling, mitogen-activated protein kinase, and, possibly, calcium signaling pathways. Within the hippocampus of primates and rodents, RGS14 is predominantly found in the enigmatic CA2 subfield. Principal neurons within the CA2 subfield differ from neighboring hippocampal regions in that they lack a capacity for long-term potentiation (LTP) of synaptic transmission, which is widely viewed as the cellular substrate of learning and memory formation. RGS14 was recently identified as a natural suppressor of LTP in hippocampal CA2 neurons as well as forms of learning and memory that depend on the hippocampus. Although CA2 has only recently been studied, compelling recent evidence implicates area CA2 as a critical component of hippocampus circuitry with functional roles in mediating certain types of learning and memory. This review will highlight the known functions of RGS14 in cell signaling and hippocampus physiology, and discuss potential roles for RGS14 in human cognition and disease.

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Chapter 3 Regulators of G Protein Signaling Proteins as Central Components of G Protein‐Coupled Receptor Signaling Complexes

Introduction

Section snippets

Overview of RGS Proteins

RGS Protein Interactions with GPCRs

GPCRs Serve as Platforms for Molecular Signaling

Summary and Perspectives

Acknowledgments

Trends Biochem Sci

Pharmacol Ther

Cell Signal

Cell Signal

Trends Pharmacol Sci

J Biol Chem

Semin Cell Dev Biol

J Biol Chem

Semin Cell Dev Biol

Semin Cell Dev Biol

J Biol Chem

Cell Signal

Methods Enzymol

J Biol Chem

J Biol Chem

Biochem Biophys Res Commun

J Biol Chem

J Biol Chem

Trends Pharmacol Sci

J Biol Chem

J Biol Chem

Cell

J Biol Chem

J Biol Chem

Cell Signal

Biochem Biophys Res Commun

Cell Signal

J Mol Biol

J Biol Chem

J Biol Chem

J Biol Chem

Eur J Pharmacol

Neuron

Biochem Biophys Res Commun

Cell Signal

Cell Signal

J Biol Chem

J Biol Chem

Eur J Pharmacol

Cell Signal

Neuron

J Biol Chem

J Biol Chem

J Biol Chem

J Biol Chem

Trends Biochem Sci

Adv Protein Chem

J Biol Chem

Constitutive activity and inverse agonists of G protein‐coupled receptors: a current perspective

Mol Pharmacol

Pharmacogenomic and structural analysis of constitutive G protein‐coupled receptor activity

Annu Rev Pharmacol Toxicol