Clathrin required for phosphorylation and internalization of β₂-adrenergic receptor by G protein-coupled receptor kinase 2 (GRK2)

Clathrin is a major component of clathrin-coated pits and serves as a binding scaffold for endocytic machinery through the binding of a specific sequence known as the clathrin-binding motif. This motif is also found in cellular signaling proteins other than endocytic components, including G protein-coupled receptor kinase 2 (GRK2), which phosphorylates G protein-coupled receptors and promotes uncoupling of receptor-G protein interaction. However, the functions of clathrin in the regulation of GRK2 are unknown. Here we demonstrated that overexpression of GRK2 mutated at the clathrin-binding motif with alanine (GRK2-5A) results in inhibition of phosphorylation and internalization of the β₂-adrenergic receptor (β₂AR). However, the interaction of β₂AR with GRK2-5A is the same as that of wild type GRK2 as determined by bioluminescence resonance energy transfer. Furthermore, GRK2-5A phosphorylates rhodopsin essentially to the same extent as wild type GRK2 in vitro. Depletion of the clathrin heavy chain using small interference RNA inhibits agonist-induced phosphorylation and internalization of β₂AR. Thus, clathrin works as a regulator of GRK2 in cells. These results indicate that clathrin is a novel player in cellular functions in addition to being a component of endocytosis.