β₂-Adrenergic receptors: Structure, regulation and signaling by partial and full agonists

Abstract

It is believed that G-protein-coupled receptors are the most promising target proteins for drug research. The β₂-adrenergic receptor has been analyzed extensively in terms of its structure, regulation and signaling as a prototype G-protein-coupled receptor. Recent development of a recombinant β₂-adrenergic receptor reveals that the receptor exists in different conformations in addition to the resting and active forms. Furthermore, different conformations that are important for coupling with G-proteins are induced in a ligand-specific manner. After the receptor binds an agonist, the receptor is phosphorylated by G-protein-coupled receptor kinase or second messenger-dependent kinase. β-Arrestin is a cytosolic protein that binds to receptors phosphorylated by G-protein-coupled receptor kinase, but not those phosphorylated by second messenger-dependent kinase, and plays important roles in agonist-induced desensitization. However, β-arrestin has recently been recognized as a scaffold protein that helps to achieve efficient transmission of internalization and signaling. Receptor agonists can be divided into two groups: full and partial agonists. Partial agonists have an advantage over full agonists because desensitization is less likely to be induced following stimulation with a partial agonist. However, the effects of partial agonists on β-arrestin-mediated signaling and internalization have not been fully examined so far. Information regarding the binding sites of partial agonists is still not sufficient to explain why they cannot fully activate the receptor. In the present study, recent progress regarding the structure, regulation and signaling of β₂ adrenergic receptors and about differences between full and partial agonists will be summarized.