Structural mechanism of GPCR-arrestin interaction: recent breakthroughs.

G protein-coupled receptors (GPCRs) are a major membrane receptor family with important physiological and pathological functions. In the classical signaling pathway, ligand-activated GPCRs couple to G proteins, thereby inducing G protein-dependent signaling pathways and phosphorylation by G protein-coupled receptor kinases (GRKs). This leads to an interaction with arrestins, which results in GPCR desensitization. Recently, non-classical GPCR signaling pathways, mediated by GPCR-bound arrestins, have been identified. Consequently, arrestins play important roles in GPCR signaling not only with respect to desensitization but also in relation to G protein-independent signal transduction. These findings have led to efforts to develop functionally biased (i.e. signal transduction biased) GPCR-targeting drugs. One of these efforts is aimed at understanding the structural mechanism of functionally biased GPCR signaling, which includes understanding the G protein-selectivity or arrestin-selectivity of GPCRs. This goal has not yet been achieved; however, great progress has been made during the last 3 years toward understanding the structural mechanism of GPCR-mediated arrestin activation. This review will discuss the recent breakthroughs in the conformational understanding of GPCR-arrestin interaction.