Removal of phosphorylation sites from the β₂-adrenergic receptor delays onset of agonist-promoted desensitization

Abstract

Eukaryotic cells have evolved a variety of mechanisms for dampening their responsiveness to hormonal stimulation in the face of sustained activation. The mechanisms for such processes, collectively referred to as desensitization, often involve alterations in the properties and number of cell-surface hormone receptors^1–3. It has been speculated that phosphorylation–dephosphorylation reactions, which are known to regulate the catalytic activities of enzymes, also regulate the function of receptors⁴. Highly specific receptor kinases, such as rhodopsin kinase⁵ and β-adrenergic receptor kinase⁶, which show stimulus-dependent phosphorylation of receptors have been described. Direct evidence for a causal relationship between receptor phosphorylation and desensitization has been lacking however. Here we report that prevention of agonist-stimulated β₂-adrenergic receptor (β₂AR) phosphorylation by truncation of its serine and threonine-rich phosphate acceptor segment delays the onset of desensitization. We also show that selective replacement of these serine and theronine residues by alanine and glycine delays desensitization even further. These data provide the first direct evidence that one molecular mechanism of desensitization of G-protein-coupled receptors involves their agonist-induced phosphorylation.