Transduction of Receptor Signalsby b-ArrestinsRobert J. Lefkowitz1,2*and Sudha K. Shenoy2The transmission of extracellular signals to the interior of the cell is a function of plasmamembrane receptors, of which the seven transmembrane receptor family is by far thelargest and most versatile. Classically, these receptors stimulate heterotrimeric G proteins,which control rates of generation of diffusible second messengers and entry of ions at theplasma membrane. Recent evidence, however, indicates another previously unappreciatedstrategy used by the receptors to regulate intracellular signaling pathways. They direct therecruitment, activation, and scaffolding of cytoplasmic signaling complexes via twomultifunctional adaptor and transducer molecules, b-arrestins 1 and 2. This mechanismregulates aspects of cell motility, chemotaxis, apoptosis, and likely other cellular functionsthrough a rapidly expanding list of signaling pathways.Transmission of extracellular signalsacrosstheplasmamembranebyreceptor-mediated signaling is one ofthe most fundamental cellular processes.However, only a small number of paradig-matic, generally operative mechanisms toaccomplish this goal have been delineated.Examples include gating of ion channels,stimulation of heterotrimeric G proteins, andactivation of receptor tyrosine kinases. How-ever, recent findings have revealed another.A regulatory process, originally discoveredas the means by which seven transmembranereceptor (7TMR) activation of G proteins isBdesensitized[ or turned off, quite surprisinglyhas been found to serve also as a parallelmeans of signal transduction. This newly ap-preciated signaling mechanism involves twofamilies of proteins, the G protein–coupledreceptor kinases (GRKs) and b-arrestins. Thelatter serve as multifunctional adaptor andscaffold proteins that recruit a broad spec-trum of signaling molecules and assemblies tothe receptors in a strictly activation-dependentfashion.Historical Perspective andClassical ParadigmsIn the mid-1980s, it was discovered that the pro-totypic adenylyl cyclase–coupled b2-adrenergicreceptor (b2AR) for catecholamines and thevisual sensing protein rhodopsin shared con-served structural and regulatory features,including sequence similarity, a seven trans-membrane (TM) architecture, and a mecha-nism for ‘‘desensitization.’’ This immediatelysuggested that all so-called G protein–coupledreceptors (GPCRs) might be members of thesame gene family and share these attributes—ahypothesis that was quickly confirmed [see (1)and references therein for a review of thishistorical material].Today, we know that 7TMRs representthe largest (2), most versatile, and most ubiq-uitous of the several families of membranereceptors. Moreover, they are the most com-mon target of therapeutic drugs (3). In re-sponse to a remarkable range of stimuli,including neurotransmitters, hormones, ions,and sensory stimuli, these receptors regulatethe metabolism, secretory properties, electri-cal activity, shape, and motility of virtuallyall mammalian cells.Studies of rhodopsin and the b2AR alsorevealed that G protein–mediated signaling isattenuated or desensitized by a highly conservedprocess (4) that involves phosphorylation ofthe activated receptors by specific protein ki-nases, such as rhodopsin kinase (now knownREVIEW1Howard Hughes Medical Institute,2Duke UniversityMedical Center, Durham, NC 27710, USA.*To whom correspondence should be addressed.E-mail: [email protected] 1. A list of b -arrestin–interacting proteins. ARF, ADP ribosylation factor; ARNO, ARFnucleotide exchange factor; IkBa, inhibitor of nuclear factor kB; PDE4D, phosphodiesterase 4D;PP2A, protein phosphatase 2A; Ral, members of the Ras superfamily of small guanosinetriphosphatases (GTPases); Ral-GDS, Ral guanosine diphosphate (GDP) dissociation stimulator;RhoA, a small GTPase; small G/GEFs, small GTPase and guanine nucleotide exchange factors.Binding protein b-Arrestin isoform Functional consequence Ref.Trafficking proteinsClathrin b-Arrestin 1, 2 Endocytosis (57)AP2 b-Arrestin 1, 2 Endocytosis (58)NSF b-Arrestin 1 Endocytosis; recycling (59)Small G/GEFsARF6 b-Arrestin 2d1 Endocytosis (60)ARNO b-Arrestin 2 Endocytosis (60)Ral-GDS b-Arrestin 1, 2 Ral-mediated cytoskeletal changes (61)RhoA b-Arrestin 1 Angiotensin II-dependent stressfiber formation(62)Signaling proteinsMAPK cascadecomponentsASK1 b-Arrestin 1, 2 JNK3 and p38 activation (24)c-Raf-1 b-Arrestin 1, 2 ERK activation (22, 23)JNK3 b-Arrestin 2d91 Stabilization of pJNK on endosomes (24)ERK2 b-Arrestin 1, 2 Stabilization of pERK on endosomes (22, 23, 26)Nonreceptortyrosine kinasesc-Src b-Arrestin 1, 2 Endocytosis, ERK activation (15)Yes b-Arrestin 1 Gaq activation and GLUT4transport(18)Hck b-Arrestin 1 Exocytosis of granules in neutrophils (17)Fgr b-Arrestin 1 Exocytosis of granules in neutrophils (17)OthersMdm2 b-Arrestin 1, 2 Ubiquitination, endocytosis (10)IkBab-Arrestin 1, 2 Stabilization of IkBa upon b2ARand TNFR stimulation(63, 64)PDE4D family b-Arrestin 1, 2 cAMP degradation (65)Dishevelled b-Arrestin 1 Increase in TCF/LEF transcription (66)Dishevelled b-Arrestin 2 Endocytosis of Frizzled4 (42)PP2A b-Arrestin 1 Ser412dephosphorylation (49)22 APRIL 2005 VOL 308 SCIENCE www.sciencemag.org512as GRK1) and the b-adrenergic receptor ki-nase or bARK (nowknown as GRK2), re-spectively. However,the receptor turn-offmechanism was foundto require more thanstimulus-dependentreceptor or rhodop-sin phosphorylation.In vitro, rhodopsinkinase–catalyzedphosphorylation ofactivated rhodopsinledtoonlypartialquenching of rhodop-sin signaling. A highlyabundant and immuno-genic retinal protein,now known as ‘‘ar-restin,’’ was found topotentiate the signal-dampening effects ofrhodopsin kinase phos-phorylation on rhodop-sin. Simultaneously,purification of bARKwas found to lead toprogressive loss of its ability to desensitizeb2ARs, in vitro. This suggested that someother factor necessary for the desensitiza-tion was being lost. Visual arrestin (alsocalled arrestin 1), albeit at high concen-trations, would restore this activity. Becauseexpression of arrestin is limited to the retina,structural and functional homologs werehypothesized to exist in other tissues. Clon-ing of visual arrestin led to the identifica-tion of similar genes encoding b-arrestin 1and b-arrestin 2 (also known as arrestin 2and 3).b-Arrestins 1 and 2 have marked specificityfor binding phosphorylated b2AR as opposedto