Distinct conformational changes in β-arrestin report biased agonism at seven-transmembrane receptors

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Abstract
β-arrestins critically regulate G protein-coupled receptors (GPCRs), also known as seven-transmembrane receptors (7TMRs), both by inhibiting classical G protein signaling and by initiating distinct β-arrestin-mediated signaling. The recent discovery of β-arrestin-biased ligands and receptor mutants has allowed characterization of these independent “G protein-mediated” and “β-arrestin-mediated” signaling mechanisms of 7TMRs. However, the molecular mechanisms underlying the dual functions of β-arrestins remain unclear. Here, using an intramolecular BRET (bioluminescence resonance energy transfer)-based biosensor of β-arrestin 2 and a combination of biased ligands and/or biased mutants of three different 7TMRs, we provide evidence that β-arrestin can adopt multiple “active” conformations. Surprisingly, phosphorylation-deficient mutants of the receptors are also capable of directing similar conformational changes in β-arrestin as is the wild-type receptor. This indicates that distinct receptor conformations induced and/or stabilized by different ligands can promote distinct and functionally specific conformations in β-arrestin even in the absence of receptor phosphorylation. Our data thus highlight another interesting aspect of 7TMR signaling—i.e., functionally specific receptor conformations can be translated to downstream effectors such as β-arrestins, thereby governing their functional specificity.