β-Adrenergic Receptor Kinase (GRK2) Colocalizes with β-Adrenergic Receptors during Agonist-induced Receptor Internalization

Rapid regulation of G protein-coupled receptors appears to involve agonist-promoted receptor phosphorylation by G protein-coupled receptor kinases (GRKs). This is followed by binding of uncoupling proteins termed arrestins and transient receptor internalization. In this report we show that the beta-adrenergic receptor kinase (betaARK-1 or GRK2) follows a similar pattern of internalization upon agonist activation of beta2-adrenergic receptors (beta2AR) and that betaARK expression levels modulate receptor sequestration. Stable cotransfected cells expressing an epitope-tagged beta2AR and betaARK-1 show an increased rate and extent of beta2AR internalization compared with cells expressing receptor alone. Moreover, subcellular gradient fractionation studies suggest that betaARK colocalizes with the internalized receptors. In fact, double immunofluorescence analysis using confocal microscopy shows extensive colocalization of beta2AR and betaARK in intracellular vesicles upon receptor stimulation. Our results confirm a functional relationship between receptor phosphorylation and sequestration and indicate that betaARK does not only translocates from the cytoplasm to the plasma membrane in response to receptor occupancy, but shares endocytic mechanisms with the beta2AR. These data suggest a direct role for betaARK in the sequestration process and/or the involvement of receptor internalization in the intracellular trafficking of the kinase.