␤-Arrestins are cytosolic proteins that mediate homologous desensitization of G protein-coupled receptors (GPCRs) by binding to agonist-occupied receptors and by uncoupling them from heterotrimeric G proteins. The recent finding that ␤-arrestins bind to some mitogen-activated protein (MAP) kinases has suggested that they might also function as scaffolds for GPCR-stimulated MAP kinase activation. To define the role of ␤-arrestins in the regulation of ERK MAP kinases, we examined the effect of

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... rrestin overexpression on ERK1/2 activation and nuclear signaling in COS-7 cells expressing angiotensin II type 1a receptors (AT1aRs). Expression of either ␤-arrestin1 or ␤-arrestin2 reduced angiotensin-stimulated phosphatidylinositol hydrolysis but paradoxically increased angiotensin-stimulated ERK1/2 phosphorylation. The increase in ERK1/2 phosphorylation in ␤-arrestin-expressing cells correlated with activation of a ␤-arrestin-bound pool of ERK2. The ␤-arrestindependent increase in ERK1/2 phosphorylation was accompanied by a significant reduction in ERK1/2-mediated, Elk1-driven transcription of a luciferase reporter. Analysis of the cellular distribution of phospho-ERK1/2 by confocal immunofluorescence microscopy and cellular fractionation revealed that overexpression of ␤-arrestin resulted in a significant increase in the cytosolic pool of phospho-ERK1/2 and a corresponding decrease in the nuclear pool of phospho-ERK1/2 following angiotensin stimulation. ␤-Arrestin overexpression resulted in formation of a cytoplasmic pool of ␤-arrestinbound phospho-ERK, decreased nuclear translocation of phospho-ERK1/2, and inhibition of Elk1-driven luciferase transcription even when ERK1/2 was activated by overexpression of cRaf-1 in the absence of AT1aR stimulation. These data demonstrate that ␤-arrestins facilitate GPCR-mediated ERK activation but inhibit ERKdependent transcription by binding to phospho-ERK1/2, leading to its retention in the cytosol. The G protein-coupled receptor (GPCR) 1 superfamily is composed of a diverse array of membrane receptors that share a conserved seven-transmembrane domain architecture. In response to receptor occupancy, GPCRs promote the activation of heterotrimeric G proteins by catalyzing the exchange of GDP for GTP on the G␣ subunit and dissociation of the G␣ subunit from the G␤␥ subunit heterodimer. Once dissociated, free G␣-GTP and G␤␥ subunits regulate the activity of enzymatic effectors, such as adenylyl cyclases and phospholipase C isoforms. For the majority of GPCRs, productive G protein coupling in the continued presence of agonist is terminated by receptor phosphorylation followed by the binding of arrestins (1, 2) . Specialized G protein-coupled receptor kinases phosphorylate agonist-occupied GPCRs, increasing their affinity for arrestins. Upon binding the receptor, arrestins sterically block further coupling between GPCR and G protein. In addition, the two non-visual arrestins, ␤-arrestin1 and 2, target GPCRs for endocytosis by linking the GPCR to components of the cellular endocytic machinery, including clathrin and AP-2 (3, 4). Besides their well characterized roles in GPCR desensitization and sequestration, recent evidence suggests that ␤-arrestins may also contribute to GPCR signaling by functioning as adaptors or scaffolds for the recruitment of signaling molecules into complex with agonist-occupied receptors. By binding to both the nonreceptor tyrosine kinase, c-Src, and to agonist-occupied ␤ 2adrenergic receptors, ␤-arrestin1 can confer tyrosine kinase activity upon the receptor (5). Similarly, ␤-arrestins are involved in recruiting c-Src to the neurokinin-1 receptor in KNRK kidney epithelial cells (6) and recruiting the Src family kinases, Hck and c-Fgr, to the CXCR-1 receptor in neutrophils (7) . Recent reports also indicate that ␤-arrestins can interact directly with component kinases of the extracellular signal-regulated kinase 1/2 (ERK) and c-Jun N-terminal kinase 3 (JNK) mitogen-activated protein (MAP) kinase cascades (6, 8, 9) . ␤-Arrestins have been shown to form complexes with angiotensin II type 1a receptor (AT1aR), cRaf-1 and ERK (9), with protease-activated receptor type 2 (PAR-2), Raf-1 and ERK1/2 (8), and with neurokinin-1 receptor, c-Src and ERK1/2 (6). ␤-Arrestin2 can also serve as a scaffold for the component kinases of the JNK3 cascade, facilitating JNK3 activation by binding directly to JNK3 and the MAP kinase kinase kinase, Ask1 (9). Although it is increasingly clear that ␤-arrestins can bind to some MAP kinases and facilitate their activation by GPCRs,