Major finding: Phosphorylation of GPCRs on distinct sites dictates β-arrestin conformation and function.
Concept: GPCR kinase-induced phosphorylation patterns confer specificity and complexity to GPCR signaling.
Impact: GPCR drug design strategies must consider specific barcodes and corresponding β-arrestin states.
β-Arrestins are adaptor proteins that are key regulators of G protein-coupled receptors (GPCR), which constitute the largest signaling family in the genome. Following GPCR activation, phosphorylation of GPCR intracellular loops or cytoplasmic tails by GPCR kinases (GRK) recruits β-arrestins, which uncouple the G protein from the receptor and induce receptor desensitization and internalization. Once bound to a GPCR, β-arrestins can serve as scaffolds for other proteins, and have been linked to oncogenic signaling pathways and metastatic cell behavior. Because GRKs differentially regulate GPCR function, Nobles and colleagues hypothesized that individual GRKs phosphorylate distinct GPCR sites, constituting a “barcode” which would impact the conformation and downstream function of β-arrestins. Indeed, the authors show that GRK2 and GRK6 differentially regulate β-arrestin2-dependent internalization and ERK signaling downstream of the β2-adrenergic G-coupled receptor (β2AR). To demonstrate that these differences were due to distinct conformations of β-arrestin, the authors utilized a β-arrestin2 bioluminescence resonance energy transfer (BRET)-based biosensor, which displays quantitatively distinct readouts dependent on the proximity of the β-arrestin2 N- and C-termini. The BRET assay indicated that knockdown of GRK2 and GRK6 produces 2 distinct β-arrestin2 conformations. Proteomic analysis of β2AR phosphorylation sites following GRK2 or GRK6 depletion revealed that these GRKs phosphorylate distinct regions and sites, suggesting a general mechanism for specific induction of β-arrestin conformations and downstream signaling events by individual GRKs. Because of the large degree of heterogeneity among GPCRs and the existence of other GRKs and β-arrestins, future studies will be necessary to characterize the effects of GRK-mediated phosphorylation on all aspects of β-arrestin function. However, because GPCRs are attractive drug targets, barcodes for desirable cellular conditions and β-arrestin states can be exploited in future drug screens.
Note: Research Watch is written by Cancer Discovery Science Writers. Readers are encouraged to consult the original articles for full details.
