G Protein Coupled Receptors as Drug Targets: The Role of β-Arrestins

G protein coupled receptors (GPCRs) are extremely important drug targets and the β-arrestin intracellular scaffolding and adaptor proteins regulate major aspects of their pharmacology. β-arrestin binding to activated, GPCR kinase (GRK)-phosphorylated receptors has the capacity to terminate G protein coupling, internalize the receptors into clathrincoated vesicles and establish a secondary signaling complex independent of G protein signaling. These events appear to be differentially regulated by GRK phosphorylation, ubiquitination and potentially β-arrestin oligomerization, which are likely to be highly receptor and cell-type dependent. The role of β-arrestins in switching from G-protein dependent to independent signaling places them in a pivotal position to dictate the downstream effects of ligand binding. Consequently, we must appreciate the functioning of these molecules as we strive to discover and optimize new GPCR drug therapies for endocrine, metabolic and immune disorders.