Review
Expanding roles for β-arrestins as scaffolds and adapters in GPCR signaling and trafficking

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Abstract

β-arrestins play previously unsuspected and important roles as adapters and scaffolds that localize signaling proteins to ligand-activated G-protein-coupled receptors. As with the paradigmatic role of the β-arrestins in uncoupling receptors from G proteins (desensitization), these novel functions involve the interaction of β-arrestin with phosphorylated heptahelical receptors. β-arrestins interact with at least two main classes of signaling proteins. First, interaction with molecules such as clathrin, AP-2 and NSF directs the clathrin-mediated internalization of G-protein-coupled receptors. Second, interaction with molecules such as Src, Raf, Erk, ASK1 and JNK3 appears to regulate several pathways that result in the activation of MAP kinases. These recent discoveries indicate that the β-arrestins play widespread roles as scaffolds and/or adapter molecules that organize a variety of complex signaling pathways emanating from heptahelical receptors. It is likely that additional roles for the β-arrestins remain to be discovered.

Introduction

G-protein-coupled receptors (GPCRs) constitute a family of seven-transmembrane-domain proteins that transmit extracellular signals to the interior of the cell by activating a variety of signaling pathways 1., 2., 3.. GPCRs lack intrinsic kinase activity, but the stimulation of receptors with agonist leads to an increase in the GTPase activity of the bound heterotrimeric G proteins. G-protein activation in turn modulates downstream effectors, such as the second messenger generating adenylyl cyclases and phospholipases, as well as ion channels. Phosphorylation of the receptors by G-protein-coupled receptor kinases (GRKs) followed by recruitment of β-arrestins to agonist-occupied receptors often terminates or attenuates such signaling by blocking G proteins from further interaction with the receptors (desensitization) 4., 5., 6., 7.. However, evidence has been accumulating to indicate that the β-arrestins also play novel roles as adapters, which recruit additional signaling molecules to ligand-activated receptors (Table 1). Thus, the β-arrestins appear to be involved in both termination of some aspects of GPCR signaling as well as in the initiation of others.

The arrestin gene family in mammals consists of at least four members: two visual arrestins (rod and cone), β-arrestin1 and β-arrestin2. Expression of the visual arrestins is largely restricted to the retina, whereas the β-arrestins are expressed in a wide variety of tissues. The arrestins are evolutionarily conserved and are present in all mammals, as well as in Drosophila melanogaster and Caernorhabditis elegans. Visual arrestin was originally discovered as a protein that was required to inhibit rhodopsin signaling 8., 9.. On the basis of its ability to also inhibit β2-adrenergic receptor (β2-AR) activation of G proteins in a reconstituted system [10], related molecules were sought and found in other tissues, leading to the discovery of β-arrestins 1 and 2 11., 12.. These molecules are responsible for the desensitization of numerous GPCRs. β-arrestin1 [13] and β-arrestin2 [14radical dotradical dot] knockout mice have been generated, and β-arrestin2 knockout mice show a marked potentiation of signaling through the μ-opioid GPCR [14radical dotradical dot]. When treated with the μ-opioid agonist morphine, these mice exhibit a marked prolongation and potentiation of the analgesic effects induced by this narcotic. However, they don't become tolerant to morphine, even after repeated administration, though physical dependence does develop [15]. Thus, β-arrestin-mediated desensitization of the receptor is closely related to and/or responsible for the clinical phenomenon of opiate tolerance.

Section snippets

β-arrestin is an adapter required for clathrin-mediated internalization of heptahelical receptors

Many GPCRs are internalized into endocytic vesicles after agonist stimulation. Ligand-stimulated receptor internalization appears to function in both dephosphorylation (resensitization) and signaling of activated receptors. Several endocytic mechanisms, including clathrin-mediated and caveolae-mediated endocytosis, appear to play roles for GPCRs. Two discoveries led to the hypothesis that the β-arrestins function as endocytic adapter proteins in addition to their previously described role in

Novel roles for β-arrestins as scaffolds linking G-protein-coupled receptors to mitogen-activated protein kinase cascades

β-arrestins also appear to function as molecular adapters in the modulation of several additional GPCR signaling cascades. The c-Jun N-terminal kinases (JNKs) and the extracellular-regulated kinases (ERKs) belong to a large family of kinases collectively known as the mitogen-activated protein kinases (MAPKs). Fig. 1 describes the key members of MAPK cascades—including the upstream kinases, MAPK kinases (MAP2Ks) and MAP2K kinases (MAP3Ks)—and indicates which members of each subset are involved

Regulation of JNK signaling

Activation of the JNK pathway is a critical intracellular mediator of the stress response. Originally named because activation of the pathway ultimately leads to phophoryl-ation of the transcription factor c-Jun, the JNKs are now known to phosphorylate many targets, including some that are non-nuclear [26radical dot]. The JNK family consists of at least three proteins genes (JNK1, JNK2 and JNK3), which can be further divided into p54 and p46 isoforms because of alternative splicing. The JNK pathway

Regulation of ERK signaling

Activation of the ERKs is involved in the mitogenic response to the activation of numerous receptors, including the GPCRs and RTKs. Modulation of this MAPK pathway typically involves the formation of multiprotein complexes at activated receptors that mediate the response to receptor stimulation. The ERK signaling module is made up of a multicomponent cascade, similar to the JNK pathway, that includes multiple kinases, each of which phosphorylates the next downstream kinase in the pathway (Fig. 1

Conclusions

Although the traditional role of the β-arrestins in preventing further signaling via G proteins continues to be actively studied and validated, novel signaling and scaffolding roles for the β-arrestins are becoming increasingly apparent. We have described a number of recently appreciated protein–protein interactions that appear to play important roles in signaling by β-arrestins and GPCRs. Fig. 2 diagrams some of the proteins that are present in GPCR–β-arrestin complexes. It is not known how

Acknowledgements

We thank S Laporte for help with computer modeling of the β-arrestin2 crystal structure and D Addison and M Holben for excellent secretarial assistance. RJ Lefkowitz is an Investigator of the Howard Hughes Medical Institute.

References and recommended reading

Papers of particular interest, published within the annual period of review,have been highlighted as:

  • radical dot of special interest

  • radical dotradical dot of outstanding interest

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      Citation Excerpt :

      Ligand-bound GPCRs are specifically phosphorylated by GPCR kinase (GRK), a subfamily of protein kinase A/G/C like kinases, which promotes the recruitment to the phosphorylated receptor of the cytosolic proteins β-arrestin1 (β-arr1) and β-arr2 (Lagerstrom and Schioth, 2008; Luttrell and Lefkowitz, 2002). In this old view, the binding of β-arrs to GPCRs sterically outcompetes G protein coupling and hampers further G-protein signaling (Black et al., 2016; Miller and Lefkowitz, 2001). β-arrs also promote removal of activated GPCRs on the plasma membrane to drive their internalization (Luttrell and Lefkowitz, 2002; Miller and Lefkowitz, 2001).

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