Seven-transmembrane receptors (7TMRs; also known as G protein-coupled receptors) are the largest class of receptors in the human genome and are common targets for therapeutics. Originally identified as mediators of 7TMR desensitization, β-arrestins (arrestin 2 and arrestin 3) are now recognized as true adaptor proteins that transduce signals to multiple effector pathways. Signalling that is mediated by β-arrestins has distinct biochemical and functional consequences from those mediated by G proteins, and several biased ligands and receptors have been identified that preferentially signal through either G protein-or β-arrestin-mediated pathways. These ligands are not only useful tools for investigating the biochemistry of 7TMR signalling, they also have the potential to be developed into new classes of therapeutics.Seven-transmembrane receptors (7TMRs), also called G protein-coupled receptors (GPCRs), are the most common class of receptors, with more than 800 members identified in the human genome 1 . They are also the most commonly targeted receptor class for medicinal therapeutics 2 . Drugs that activate 7TMRs are thought to modulate the proportion of receptors that are in an active signalling conformation relative to those in an inactive, non-signalling conformation. Based on the classical model for 7TMR activity, agonist binding to the 7TMR causes the receptor to adopt a conformation that results in the activation of associated heterotrimeric G proteins. This activation involves the exchange of bound GDP for GTP by the Gα subunit of the G protein, leading to dissociation of the heterotrimeric protein complex into Gα and Gβγ subunits. This dissociation then promotes the production of and consequent Correspondence to R.J.L. lefko001@receptor-biol.duke.edu. Competing interests statementThe authors declare competing financial interests: see web version for details. DATABASES IUPHAR Databse of Receptors and Ion NIH Public Access Author ManuscriptNat Rev Drug Discov. Author manuscript; available in PMC 2010 November 1. Published in final edited form as:Nat Rev Drug Discov. 2010 May ; 9(5): 373-386. doi:10.1038/nrd3024. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript signalling by second messenger systems, such as those involving cyclic AMP, diacylglycerol and calcium 3 . Signalling by the activated conformation of the 7TMR is terminated by phosphorylation of the cytoplasmic loops and tail of the 7TMR, which is catalysed predominantly by GPCR kinases (GRKs). This results in the binding of arrestins (most commonly β-arrestin 1 and β-arrestin 2) and consequent desensitization followed by internalization into clathrin-coated pits 4 . Thus, in the classical model, heterotrimeric G proteins mediate signal transduction via the receptor, and β-arrestins mediate receptor desensitization and internalization (FIG. 1a).This classical model, however, is both over-simplified and incomplete. Over the past decade there has been a new appreciation regarding the capacity of β-arrestins to act not o...

Phosphorylation of G protein–coupled receptors (GPCRs, which are also known as seven-transmembrane spanning receptors) by GPCR kinases (GRKs) plays essential roles in the regulation of receptor function by promoting interactions of the receptors with β-arrestins. These multifunctional adaptor proteins desensitize GPCRs, by reducing receptor coupling to G proteins and facilitating receptor internalization, and mediate GPCR signaling through β-arrestin–specific pathways. Detailed mapping of the phosphorylation sites on GPCRs targeted by individual GRKs and an understanding of how these sites regulate the specific functional consequences of β-arrestin engagement may aid in the discovery of therapeutic agents targeting individual β-arrestin functions. The β2-adrenergic receptor (β2AR) has many serine and threonine residues in the carboxyl-terminal tail and the intracellular loops, which are potential sites of phosphorylation. We monitored the phosphorylation of the β2AR at specific sites upon stimulation with an agonist that promotes signaling by both G protein–mediated and β-arrestin–mediated pathways or with a biased ligand that promotes signaling only through β-arrestin–mediated events in the presence of the full complement of GRKs or when either GRK2 or GRK6 was depleted. We correlated the specific and distinct patterns of receptor phosphorylation by individual GRKs with the functions of β-arrestins and propose that the distinct phosphorylation patterns established by different GRKs establish a “barcode” that imparts distinct conformations to the recruited β-arrestin, thus regulating its functional activities.

Ubiquitously expressed seven-transmembrane receptors (7TMRs) classically signal through heterotrimeric G proteins and are commonly referred to as G protein-coupled receptors. It is now recognized that 7TMRs also signal through β-arrestins, which act as versatile adapters controlling receptor signaling, desensitization, and trafficking. Most endogenous receptors appear to signal in a balanced fashion using both β-arrestin and G protein-mediated pathways. Some 7TMRs are thought to be nonsignaling “decoys” because of their inability to activate typical G protein signaling pathways; it has been proposed that these receptors act to scavenge ligands or function as coreceptors. Here we demonstrate that ligand binding to the decoy receptor CXCR7 does not result in activation of signaling pathways typical of G proteins but does activate MAP kinases through β-arrestins in transiently transfected cells. Furthermore, we observe that vascular smooth muscle cells that endogenously express CXCR7 migrate to its ligand interferon-inducible T-cell alpha chemoattractant (ITAC), an effect that is significantly attenuated by treatment with either a CXCR7 antagonist or β-arrestin depletion by siRNA. This example of an endogenous “β-arrestin-biased” 7TMR that signals through β-arrestin in the absence of G protein activation demonstrates that some 7TMRs encoded in the genome have evolved to signal through β-arrestin exclusively and suggests that other receptors that are currently thought to be orphans or decoys may also signal through such nonclassical pathways.