Distinct Phosphorylation Sites on the β2-Adrenergic Receptor Establish a Barcode That Encodes Differential Functions of β-Arrestin

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 beta-arrestins. These multifunctional adaptor proteins desensitize GPCRs, by reducing receptor coupling to G proteins and facilitating receptor internalization, and mediate GPCR signaling through beta-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 beta-arrestin engagement may aid in the discovery of therapeutic agents targeting individual beta-arrestin functions. The beta(2)-adrenergic receptor (beta(2)AR) 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 beta(2)AR at specific sites upon stimulation with an agonist that promotes signaling by both G protein-mediated and beta-arrestin-mediated pathways or with a biased ligand that promotes signaling only through beta-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 beta-arrestins and propose that the distinct phosphorylation patterns established by different GRKs establish a "barcode" that imparts distinct conformations to the recruited beta-arrestin, thus regulating its functional activities.