Small-molecule modulation of β-arrestins.

β-arrestins are multifunctional regulators of G protein-coupled receptor (GPCR) signaling, orchestrating diverse downstream signaling events and physiological responses across the vast GPCR superfamily. While GPCR pharmacology has advanced to target orthosteric and allosteric sites, as well as G proteins and GRKs, comparable chemical tools to study β-arrestins remain lacking. Here, we report the discovery of small-molecule inhibitors that selectively target β-arrestins and delineate their mechanism of action through integrated pharmacological, biochemical, biophysical, and structural analyses. These inhibitors disrupt β-arrestin-engagement with agonist-activated GPCRs, impairing desensitization, internalization, and β-arrestin-dependent functions while sparing G protein-receptor coupling. Cryo-EM, MD simulations, and structure-guided mutagenesis reveal that one modulator, Cmpd-5, engages a cryptic pocket formed by the middle, C-, and lariat loops of β-arrestin1-a critical receptor-binding interface-stabilizing a distinct conformation incompatible with GPCR engagement. Together, these findings provide a mechanistic framework for β-arrestin modulation, introducing transducer-targeted strategies to fine-tune GPCR signaling and guide the development of pathway-specific therapeutics.