Key to Arrestin-3 Activation and Signaling

Key to Arrestin-3 Activation and Signaling

Arrestins comprise a class of proteins originally discovered for their ability to bind to activated and phosphorylated G protein-coupled receptors (GPCRs). Arrestins prevent the binding of G proteins to the GPCR, thereby blocking G protein-mediated signaling. Recent work, however, has shown that some arrestins can also initiate receptor-independent signaling, leading Vanderbilt Basic Sciences investigators Tina Iverson, Seva Gurevich, and Eugenia Gurevich to partner with their collaborators Candice Klug (Medical College of Wisconsin) and Chad Brautigam (University of Texas – Soutwestern) in an investigation of the structural basis of this type of arrestin signaling. They obtained a high resolution crystal structure of a complex of arrestin-3 with inositol hexakisphosphate (IP₆) a small molecule activator of receptor-independent signaling. The structure revealed that IP₆ binding caused arrestin-3 to trimerize and that the resulting complex also contained six IP₆ molecules, all located at the interface between two arrestin-3 monomers. The IP₆ bound to arrestin-3 in the same location as the phosphate groups of phosphorylated GPCRs, disrupting a structure known as the polar core and inducing a rotation between the two primary domains of arrestin-3. These conformational changes were typical of arrestin-3 activation. In addition, the arrestin-3-IP₆ complex was stabilized by hydrophobic interactions between a short stretch of α-helix contributed by each arrestin-3- monomer. These α-helical segments were derived from a region of arrestin-3 known as the finger loop, which is notable for binding, in α-helical conformation, to a hydrophobic pocket found in activated GPCRs. This finding suggests that the finger loop plays a critical role in arrestin-dependent signaling. Further work using mutants that disrupt the IP₆ binding sites and finger loop demonstrated the importance of these structures for IP₆ binding, trimer formation, and arrestin-3-mediated signaling via the c-Jun N-terminal Kinase (cJUN) pathway. Together, the results provide key insights into arrestin activation via both phosphate group interactions and the finger loop, explaining how GPCR phosphorylation and activation are recognized by arrestins as well as how these same mechanisms can lead to receptor-independent signaling. The work is published in the journal Nature Communications [Q. Chen et al. Nat. Commun., 2017, 8:1426 DOI: 10.1038/s41467-017-01218-8].