Role Of Β-arrestins Research Articles

Sensitivity to Δ9-tetrahydrocannabinol is selectively enhanced in beta-arrestin2−/− mice

Little is known about the roles of beta-arrestins in the regulation of brain CB1 cannabinoid receptors. This study investigated the role of beta-arrestin2 in cannabinoid behavioral effects using beta-arrestin2 -/- mice and their wild-type counterparts. A variety of cannabinoid ligands from different chemical classes that exhibit a variety of efficacies for activation of CB1 receptors were investigated, including Delta-tetrahydrocannabinol, CP55940, methanandamide, JWH-073, and O-1812. Delta-tetrahydrocannabinol produced both greater antinociception and greater decreases in body temperature in beta-arrestin2 -/- compared with beta-arrestin2 +/+ mice. No significant differences were, however, present in either assay for the other CB1 agonists. Antagonist radioligand binding indicated no difference in the density of cannabinoid CB1 receptors in the cerebellum, cortex, or hippocampus of beta-arrestin2 +/+ and -/- mice. These data demonstrate that beta-arrestin2 may regulate cannabinoid CB1 receptor sensitivity in an agonist-specific manner.

Differential regulation of class IA phosphoinositide 3-kinase catalytic subunits p110α and β by protease-activated receptor 2 and β-arrestins

PAR-2 (protease-activated receptor 2) is a GPCR (G-protein-coupled receptor) that can elicit both G-protein-dependent and -independent signals. We have shown previously that PAR-2 simultaneously promotes Galphaq/Ca2+-dependent activation and beta-arrestin-1-dependent inhibition of class IA PI3K (phosphoinositide 3-kinase), and we sought to characterize further the role of beta-arrestins in the regulation of PI3K activity. Whereas the ability of beta-arrestin-1 to inhibit p110alpha (PI3K catalytic subunit alpha) has been demonstrated, the role of beta-arrestin-2 in PI3K regulation and possible differences in the regulation of the two catalytic subunits (p110alpha and p110beta) associated with p85alpha (PI3K regulatory subunit) have not been examined. In the present study we have demonstrated that: (i) PAR-2 increases p110alpha- and p110beta-associated lipid kinase activities, and both p110alpha and p110beta are inhibited by over-expression of either beta-arrestin-1 or -2; (ii) both beta-arrestin-1 and -2 directly inhibit the p110alpha catalytic subunit in vitro, whereas only beta-arrestin-2 directly inhibited p110beta; (iii) examination of upstream pathways revealed that PAR-2-induced PI3K activity required the small GTPase Cdc (cell-division cycle)42, but not tyrosine phosphorylation of p85; and (iv) beta-arrestins inhibit PAR-2-induced Cdc42 activation. Taken together, these results indicated that beta-arrestins could inhibit PAR-2-stimulated PI3K activity, both directly and through interference with upstream pathways, and that the two beta-arrestins differ in their ability to inhibit the p110alpha and p110beta catalytic subunits. These results are particularly important in light of the growing interest in PAR-2 as a pharmacological target, as commonly used biochemical assays that monitor G-protein coupling would not screen for beta-arrestin-dependent signalling events.

β-arrestin and morphine tolerance

吗啡耐受的形成可归因于受体的脱敏、内化和受体下游信号的调控.许多研究表明β-arrestin在阿片受体的脱敏、内化和吗啡耐受形成过程中起关键作用.敲除小鼠控制阿片受体内化的蛋白β-arrestin2基因后,吗啡的镇痛作用明显增强而耐受减弱.最近研究还发现δ阿片受体(DOR)的激动可以导致β-arrestin1向核内转移,同时促进依赖β-arrestin1的p27和c-fos的转录,表明β-arrestin具有将受体信号传入细胞核内的信使作用.本综述简要概括了近年来关于β-arrestin与吗啡耐受的研究进展。

Highlights From The Literature

Highlights From The Literature

Diversifying Role for β-Arrestin

Intense investigation is focused on understanding mechanisms that control signaling through G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors (GPCRs), because they are known to be productive targets for therapeutic drugs. Nelson et al . show that β-arrestins, which limit signaling by GPCRs coupled to G proteins of the G s class, serve a very similar function in control of signaling by M1 muscarinic receptors. The muscarinic receptors use a different class of G protein linked to distinct signaling machinery. C. D. Nelson, S. J. Perry, D. S. Regier, S. M. Prescott, M. K. Topham, R. J. Lefkowitz, Targeting of diacylglycerol degradation to M1 muscarinic receptors by β-arrestins. Science 315 , 663-666 (2007). [Abstract] [Full Text] E. F. Grady, β-Arrestin, a two-fisted terminator. Science 315 , 605-606 (2007). [Summary] [Full Text]

Angiotensin Type 1A Receptors on Glial Cells in Rostral Ventrolateral Medulla and Hypertension

During the 1970s and early 1980s, all the components of the renin–angiotensin system (RAS) were identified within the brain.1 Subsequent physiological studies have shown that angiotensin II (Ang II) can act at various sites within the brain stem and hypothalamus to regulate cardiovascular function and fluid and electrolyte balance. Further, there is increasing evidence that brain Ang II can contribute to increased sympathetic activity in hypertension and heart failure.2 In recent years, a number of investigators have studied the physiological effects of alteration of the expression of genes encoding various components of the RAS. As reviewed recently by Davisson,2 early studies by Ganten and colleagues showed that transgenic rats containing the mouse renin ( Ren -2) gene were hypertensive. Although there is an increased expression of Ang II in the hypothalamus and medulla oblongata of such animals, the extent to which the hypertension was caused by increased levels of central or peripheral Ang II was not clear. Later, with the development of new methods that enabled brain-selective expression of particular genes, transgenic mice in which the angiotensin type 1A (AT1A) receptors were overexpressed in the brain but not in peripheral tissues were produced.3 These animals exhibit exaggerated pressor responses to intracerebroventricular administration of Ang II, but have a normal resting arterial blood pressure.3 Thus, this indicates that an increased density of AT1A receptors alone is not sufficient to produce hypertension. Other genetic models, however, have demonstrated that overactivity of the brain RAS can produce hypertension. In transgenic mice containing both the human angiotensinogen …

Role of β-arrestin 1 in the metastatic progression of colorectal cancer

G protein-coupled receptor ligand-dependent transactivation of growth factor receptors has been implicated in human cancer cell proliferation, migration, and cell survival. For example, prostaglandin E(2) (PGE(2))-induced transactivation of the EGF receptor (EGFR) in colorectal carcinoma cells is mediated by means of a c-Src-dependent mechanism and regulates cell proliferation and migration. Recent evidence indicates that beta-arrestin 1 may act as an important mediator in G protein-coupled receptor-induced activation of c-Src. Whether beta-arrestin 1 serves a functional role in these events is, however, unknown. We investigated the effects of PGE(2) on colorectal cancer cells expressing WT and mutant beta-arrestin 1. Here we report that PGE(2) induces the association of a prostaglandin E receptor 4/beta-arrestin 1/c-Src signaling complex resulting in the transactivation of the EGFR and downstream Akt (PKB) signaling. The interaction of beta-arrestin 1 and c-Src is critical for the regulation of colorectal carcinoma cell migration in vitro as well as metastatic spread of disease to the liver in vivo. These results show that the prostaglandin E/beta-arrestin 1/c-Src signaling complex is a crucial step in PGE(2)-mediated transactivation of the EGFR and may play a pivotal role in tumor metastasis. Furthermore, our data implicate a functional role for beta-arrestin 1 as a mediator of cellular migration and metastasis.

Dissociation of β-arrestin from internalized bradykinin B2 receptor is necessary for receptor recycling and resensitization

Beta-arrestins are multifunctional adaptors that bind agonist-activated G protein-coupled receptors (GPCRs), mediate their desensitization and internalization, and control the rate at which receptors recycle back at the plasma membrane ready for subsequent stimulation. The activation of the bradykinin (BK) type 2 receptor (B2R) results in the rapid desensitization and internalization of the receptor. Little is known, however, about the role of β-arrestin in regulating the intracellular trafficking and the resensitization of the B2R. Using confocal microscopy, we show that BK stimulation of COS-7 cells expressing B2R induces the colocalization of the agonist-activated receptor with β-arrestin into endosomes. Fluorescent imaging and ligand binding experiments also reveal that upon agonist removal, β-arrestin rapidly dissociates from B2R into endosomes, and that receptors return back to the plasma membrane, fully competent for reactivating B2R signaling as measured by NO production upon a second BK challenge. However, when the receptor is mutated in its C-terminal domain to increase its avidity for β-arrestin, B2R remains associated with β-arrestin into endosomes, and receptors fail to recycle to the plasma membrane postagonist wash. Similarly, the recycling of receptors is prevented when a β-arrestin mutant exhibiting increased avidity for agonist-bound GPCRs is expressed with B2R. Stabilizing receptor/β-arrestin complexes into endosomes results in the dampening of the BK-mediated NO production. These results provide evidence for the involvement of β-arrestin in the intracellular trafficking of B2R, and highlight the importance of receptor recycling in reestablishing B2R signaling.

Hedgehog, Smoothened, and β-Arrestin

Hedgehog (Hh) proteins carry signals that are essential for pattern formation during vertebrate embryogenesis. Extracellular Hh molecules bind to a receptor on the cell surface and activate Smoothened, a membrane-spanning protein, which transmits signals to the cell interior. β-Arrestin proteins are inhibitors of heterotrimeric GTP-binding protein (G protein)-coupled receptor (GPCR) signaling and also promote internalization and signaling by GPCRs. Chen et al. find that in mammalian cells, activated Smoothened molecules preferentially associate with β-arrestin 2. Reducing levels of β-arrestin 2 inhibited internalization of Smoothened. Furthermore, Wilbanks et al. find that loss of β-arrestin 2 in zebrafish causes developmental abnormalities similar to those of mutants in the Hh signaling pathway. Overexpression of β-arrestin 2 could partially rescue some defects in embryos with deficient Hh signaling, and loss of β-arrestin 2 decreased expression of Hh-responsive genes. Together, the findings provide insight into the roles of β-arrestin 2 during development and the mechanisms by which Hh signaling influences developmental processes from embryogenesis to cancer. W. Chen, X.-R. Ren, C. D. Nelson, L. S. Barak, J. K. Chen, P. A. Beachy, F. de Sauvage, R. J. Lefkowitz, Activity-dependent internalization of Smoothened mediated by β-arrestin 2 and GRK2. Science 306 , 2257-2260 (2004). [Abstract] [Full Text] A. M. Wilbanks, G. B. Fralish, M. L. Kirby, L. S. Barak, Y.-X. Li, M. G. Caron, β-Arrestin 2 regulates zebrafish development through the Hedgehog signaling pathway. Science 306 , 2264-2267 (2004). [Abstract] [Full Text]

Constitutive Protease-activated Receptor-2-mediated Migration of MDA MB-231 Breast Cancer Cells Requires Both β-Arrestin-1 and -2

Protease-activated receptor-2 (PAR-2) is activated by trypsin-like serine proteases and can promote cell migration through an ERK1/2-dependent pathway, involving formation of a scaffolding complex at the leading edge of the cell. Previous studies also showed that expression of a dominant negative fragment of beta-arrestin-1 reduces PAR-2-stimulated internalization, ERK1/2 activation, and cell migration; however, this reagent may block association of many proteins, including beta-arrestin-2 with clathrin-coated pits. Here we investigate the role of PAR-2 in the constitutive migration of a metastatic breast cancer cell line, MDA MB-231, and use small interfering RNA to determine the contribution of each beta-arrestin to this process. We demonstrate that a trypsin-like protease secreted from MDA MB-231 cells can promote cell migration through autocrine activation of PAR-2 and this correlates with constitutive localization of PAR-2, beta-arrestin-2, and activated ERK1/2 to pseudopodia. Addition of MEK-1 inhibitors, trypsin inhibitors, a scrambled PAR-2 peptide, and silencing of beta-arrestins with small interfering RNA also reduce base-line migration of MDA MB-231 cells. In contrast, a less metastatic PAR-2 expressing breast cancer cell line does not exhibit constitutive migration, pseudopodia formation, or trypsin secretion; in these cells PAR-2 is more uniformly distributed around the cell periphery. These data demonstrate a requirement for both beta-arrestins in PAR-2-mediated motility and suggest that autocrine activation of PAR-2 by secreted proteases may contribute to the migration of metastatic tumor cells through beta-arrestin-dependent ERK1/2 activation.

An Arrestin Role at the IGF-1 Receptor

Povsic et al. investigated the role of β-arrestins in the activation of phosphatidylinositol 3-kinase (PI3K) by insulin-like growth factor 1 (IGF-1) and discovered that, unexpectedly, this involved a mechanism independent of the tyrosine kinase activity of the IGF-1 receptor. β-Arrestins are recruited to heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) upon agonist binding and are involved in GPCR desensitization, as well as in activating various signaling pathways, and they are best known for their association with GPCRs. However, β-arrestin1 is also recruited to two related receptor tyrosine kinases--the insulin receptor and the IGF-1 receptor. IGF-1 signaling, which has been implicated in cell differentiation and proliferation, involves the activation of PI3K. Using two mouse embryo fibroblast lines that differed only in β-arrestin1 expression, Povsic et al. showed that IGF-1-mediated activation of PI3Kα; the subsequent recruitment to the plasma membrane, phosphorylation, and activation of Akt; and the inhibition of apoptosis by IGF-1 after serum withdrawal all depended on β-arrestin1. Pharmacologic analysis indicated that activation of PI3K and Akt was independent of IGF-1 receptor tyrosine kinase activity or the phosphorylation of the adaptor protein insulin receptor substrate. PI3K activation was also independent of Src and of IGF-1 activation of G i (the family of G protein α subunits that can inhibit adenylyl cyclases). Thus, β-arrestins appear to mediate PI3K activation in response to IGF-1 through a novel pathway that is independent of the tyrosine kinase activity of the IGF-1 receptor. T. J. Povsic, T. A. Kohout, R. J. Lefkowitz, β-Arrestin1 mediates insulin-like growth factor 1 (IGF-1) activation of phosphatidylinositol 3-kinase (PI3K) and anti-apoptosis. J. Biol. Chem . 278 , 51334-51339 (2003). [Abstract] [Full Text]

Regulation of G protein-coupled receptor endocytosis and trafficking by Rab GTPases

G protein-coupled receptors (GPCRs) are integral membrane proteins that, in response to activation by extracellular stimuli, regulate intracellular second messenger levels via their coupling to heterotrimeric G proteins. GPCR activation also initiates a series of molecular events that leads to G protein-coupled receptor kinase-mediated receptor phosphorylation and the binding of β-arrestin proteins to the intracellular face of the receptor. β-Arrestin binding not only contributes to the G protein-uncoupling of GPCRs, but also mediates the targeting of many GPCRs for endocytosis in clathrin-coated pits. Several GPCRs internalize as a stable complex with β-arrestin and the stability of this complex appears to regulate, at least in part, whether the receptors are dephosphorylated in early endosomes and recycled back to the cell surface as fully functional receptors, retained in early endosomes or targeted for degradation in lysosomes. More recently, it has become appreciated that the movement of GPCRs through functionally distinct intracellular membrane compartments is regulated by a variety of Rab GTPases and that the activity of these Rab GTPases may influence GPCR function. Moreover, it appears that GPCRs are not simply passive cargo molecules, but that GPCR activation may directly influence Rab GTPase activity and as such, GPCRs may directly control their own targeting between intracellular compartments. This review provides a synopsis of the current knowledge regarding the role of β-arrestins and Rab GTPases in regulating the intracellular trafficking and function of GPCRs.

A Broader Role for β-Arrestins

β-Arrestins are multifunctional proteins that desensitize heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors and help link such receptors to cellular signaling pathways, as well as promote endocytosis of the receptors. In two reports, Chen et al. (see the Perspective by Spiegel) show that β-arrestins are even more versatile than previously thought. β-arrestin 2 is required to promote internalization of Frizzled proteins (the cell surface receptors for Wnt glycoproteins). In this role, β-arrestins do not bind directly to receptors, as they do with G protein-coupled receptors. Rather, β-arrestin 2 binds to a protein called disheveled (Dvl) that is recruited to activated Frizzled receptor proteins in the cell membrane. Once Dvl brings β-arrestin to Frizzled receptor protein complexes, the receptors are recruited into clathrin-coated pits for internalization. β-arrestins also promote internalization of a completely different type of receptor, the transforming growth factor-β receptor type III. In this case, β-arrestin binds to a receptor subunit that is phosphorylated by another receptor subunit. W. Chen, D. ten Berge, J. Brown, S. Ahn, L. A. Hu, W. E. Miller, M. G. Caron, L. S. Barak, R. Nusse, R. J. Lefkowitz, Dishevelled 2 recruits β-arrestin 2 to mediate Wnt5A-stimulated endocytosis of Frizzled 4. Science 301 , 1391-1394 (2003). [Abstract] [Full Text] W. Chen, K. C. Kirkbride, T. How, C. D. Nelson, J. Mo, J. P. Frederick, X.-F. Wang, R. J. Lefkowitz, G. C. Blobe, β-Arrestin 2 mediates endocytosis of type III TGF-β receptor and down-regulation of its signaling. Science 301 , 1394-1397 (2003). [Abstract] [Full Text] A. Spiegel, ß-Arrestin--Not just for G protein-coupled receptors. Science 301 , 1338-1339 (2003). [Summary] [Full Text]

The role of beta-arrestins in the termination and transduction of G-protein-coupled receptor signals.

beta-Arrestins are versatile adapter proteins that form complexes with most G-protein-coupled receptors (GPCRs) following agonist binding and phosphorylation of receptors by G-protein-coupled receptor kinases (GRKs). They play a central role in the interrelated processes of homologous desensitization and GPCR sequestration, which lead to the termination of G protein activation. beta-arrestin binding to GPCRs both uncouples receptors from heterotrimeric G proteins and targets them to clathrin-coated pits for endocytosis. Recent data suggest that beta-arrestins also function as GPCR signal transducers. They can form complexes with several signaling proteins, including Src family tyrosine kinases and components of the ERK1/2 and JNK3 MAP kinase cascades. By recruiting these kinases to agonist-occupied GPCRs, beta-arrestins confer distinct signaling activities upon the receptor. beta-arrestin-Src complexes have been proposed to modulate GPCR endocytosis, to trigger ERK1/2 activation and to mediate neutrophil degranulation. By acting as scaffolds for the ERK1/2 and JNK3 cascades, beta-arrestins both facilitate GPCR-stimulated MAP kinase activation and target active MAP kinases to specific locations within the cell. Thus, their binding to GPCRs might initiate a second wave of signaling and represent a novel mechanism of GPCR signal transduction.

Classical and new roles of beta-arrestins in the regulation of G-protein-coupled receptors.

In the classical model of G-protein-coupled receptor (GPCR) regulation, arrestins terminate receptor signalling. After receptor activation, arrestins desensitize phosphorylated GPCRs, blocking further activation and initiating receptor internalization. This function of arrestins is exemplified by studies on the role of arrestins in the development of tolerance to, but not dependence on, morphine. Arrestins also link GPCRs to several signalling pathways, including activation of the non-receptor tyrosine kinase SRC and mitogen-activated protein kinase. In these cascades, arrestins function as adaptors and scaffolds, bringing sequentially acting kinases into proximity with each other and the receptor. The signalling roles of arrestins have been expanded even further with the discovery that the formation of stable receptor-arrestin complexes initiates photoreceptor apoptosis in Drosophila, leading to retinal degeneration. Here we review our current understanding of arrestin function, discussing both its classical and newly discovered roles.

Beta-Arrestin 1 and 2 differentially regulate heptahelical receptor signaling and trafficking.

The two widely coexpressed isoforms of beta-arrestin (termed beta arrestin 1 and 2) are highly similar in amino acid sequence. The beta-arrestins bind phosphorylated heptahelical receptors to desensitize and target them to clathrin-coated pits for endocytosis. To better define differences in the roles of beta-arrestin 1 and 2, we prepared mouse embryonic fibroblasts from knockout mice that lack one of the beta-arrestins (beta arr1-KO and beta arr2-KO) or both (beta arr1/2-KO), as well as their wild-type (WT) littermate controls. These cells were analyzed for their ability to support desensitization and sequestration of the beta(2)-adrenergic receptor (beta(2)-AR) and the angiotensin II type 1A receptor (AT(1A)-R). Both beta arr1-KO and beta arr2-KO cells showed similar impairment in agonist-stimulated beta(2)-AR and AT(1A)-R desensitization, when compared with their WT control cells, and the beta arr1/2-KO cells were even further impaired. Sequestration of the beta(2)-AR in the beta arr2-KO cells was compromised significantly (87% reduction), whereas in the beta arr1-KO cells it was not. Agonist-stimulated internalization of the AT(1A)-R was only slightly reduced in the beta arr1-KO but was unaffected in the beta arr2-KO cells. In the beta arr1/2-KO cells, the sequestration of both receptors was dramatically reduced. Comparison of the ability of the two beta-arrestins to sequester the beta(2)-AR revealed beta-arrestin 2 to be 100-fold more potent than beta-arrestin 1. Down-regulation of the beta(2)-AR was also prevented in the beta arr1/2-KO cells, whereas no change was observed in the single knockout cells. These findings suggest that sequestration of various heptahelical receptors is regulated differently by the two beta-arrestins, whereas both isoforms are capable of supporting receptor desensitization and down-regulation.

Properties of Secretin Receptor Internalization Differ from Those of the β2-Adrenergic Receptor

The endocytic pathway of the secretin receptor, a class II GPCR, is unknown. Some class I G protein-coupled receptors (GPCRs), such as the beta(2)-adrenergic receptor (beta(2)-AR), internalize in clathrin-coated vesicles and this process is mediated by G protein-coupled receptor kinases (GRKs), beta-arrestin, and dynamin. However, other class I GPCRs, for example, the angiotensin II type 1A receptor (AT(1A)R), exhibit different internalization properties than the beta(2)-AR. The secretin receptor, a class II GPCR, is a GRK substrate, suggesting that like the beta(2)-AR, it may internalize via a beta-arrestin and dynamin directed process. In this paper we characterize the internalization of a wild-type and carboxyl-terminal (COOH-terminal) truncated secretin receptor using flow cytometry and fluorescence imaging, and compare the properties of secretin receptor internalization to that of the beta(2)-AR. In HEK 293 cells, sequestration of both the wild-type and COOH-terminal truncated secretin receptors was unaffected by GRK phosphorylation, whereas inhibition of cAMP-dependent protein kinase mediated phosphorylation markedly decreased sequestration. Addition of secretin to cells resulted in a rapid translocation of beta-arrestin to plasma membrane localized receptors; however, secretin receptor internalization was not reduced by expression of dominant negative beta-arrestin. Thus, like the AT(1A)R, secretin receptor internalization is not inhibited by reagents that interfere with clathrin-coated vesicle-mediated internalization and in accordance with these results, we show that secretin and AT(1A) receptors colocalize in endocytic vesicles. This study demonstrates that the ability of secretin receptor to undergo GRK phosphorylation and beta-arrestin binding is not sufficient to facilitate or mediate its internalization. These results suggest that other receptors may undergo endocytosis by mechanisms used by the secretin and AT(1A) receptors and that kinases other than GRKs may play a greater role in GPCR endocytosis than previously appreciated.

Role of β-Arrestins in the IntracelIular Trafficking of G-Protein-Coupled Receptors

Publisher Summary The regulation G-protein-coupled receptor (GR) signaling requires a coordinated and delicate balance between the processes governing receptor activation, desensitization, and resensitization. In the case of the β 2 -adrenergic receptor ( β 2 AR), the most rapid means of receptor desensitization is phosphorylation, which is achieved within seconds of agonist activation by either second-messenger-dependent protein kinases (for example, cyclic adenosine monophophate [cAMP]-dependent protein kinase, PKA) or GR kinases (GRKs), which specifically phosphorylate the agonist-activated form of the receptor. GRK-mediated phosphorylation of agonist activated β 2 ARs promotes the binding of arrestin proteins (for example, β -arrestin), which, when bound, further uncouple the receptor. While the mechanisms underlying receptor activation and desensitization have been fairly well characterized, until recently, the molecular mechanism (s) by which GR responsiveness can be re-established have remained unknown, except that receptor sequestration and dephosphorylation are likely involved in this process. However, recent studies now indicate that the same molecular intermediates required for receptor desensitization, GRK-mediated phosphorylation and β -arrestin binding, likely play an important role in receptor resensitization by serving as the signals and molecular determinants initiating β 2 ARs sequestration. Research also suggests that β -arrestins play a dual role in the regulation of β 2 AR responsiveness: they not only bind and uncouple the agonist-activated GRK-phosphorylated β 2 AR from its G protein, but also participate in directing the receptor for agonist-promoted sequestration. β -arrestins appear to act as adaptor-like proteins in β 2 AR trafficking and either serve to recruit other cellular proteins that participate in the mobilization of receptors to endocytic organelles or execute this function themselves.

Role of beta-arrestin in mediating agonist-promoted G protein-coupled receptor internalization.

beta-Arrestins are proteins that bind phosphorylated heterotrimeric GTP-binding protein (G protein)-coupled receptors (GPCRs) and contribute to the desensitization of GPCRs by uncoupling the signal transduction process. Resensitization of GPCR responsiveness involves agonist-mediated receptor sequestration. Overexpression of beta-arrestins in human embryonic kidney cells rescued the sequestration of beta 2-adrenergic receptor (beta 2AR) mutants defective in their ability to sequester, an effect enhanced by simultaneous overexpression of beta-adrenergic receptor kinase 1. Wild-type beta 2AR sequestration was inhibited by the overexpression of two beta-arrestin mutants. These findings suggest that beta-arrestins play an integral role in GPCR internalization and thus serve a dual role in the regulation of GPCR function.