Emerging paradigms of β-arrestin-dependent seven transmembrane receptor signaling

Abstract

β-Arrestins, originally discovered to desensitize activated seven transmembrane receptors (7TMRs; also known as G-protein-coupled receptors, GPCRs), are now well established mediators of receptor endocytosis, ubiquitylation and G protein-independent signaling. Recent global analyses of β-arrestin interactions and β-arrestin-dependent phosphorylation events have uncovered several previously unanticipated roles of β-arrestins in a range of cellular signaling events. These findings strongly suggest that the functional roles of β-arrestins are much broader than currently understood. Biophysical studies aimed at understanding multiple active conformations of the 7TMRs and the β-arrestins have begun to unravel the mechanistic basis for the diverse functional capabilities of β-arrestins in cellular signaling.

Figure 1

Multifaceted roles of β-arrestins and GRKs in 7TMR signaling and regulation. (a) The classical paradigm of G protein-dependent signaling via 7TMRs where binding of an agonist leads to conformational changes in the receptor. Activated receptor in turn couples to and activates heterotrimeric G proteins. Upon activation, heterotrimeric G proteins dissociate and activated G_α and G_βγ lead to generation of downstream signaling. (b) GRK-mediated phosphorylation and β-arrestin-mediated desensitization of 7TMRs. Agonist occupied activated receptors are phosphorylated by GRKs at serine/threonines primarily in the C-terminus but also in the intracellular loops. Phosphorylated receptors recruit multifunctional adaptor proteins β-arrestins which sterically hinder further G protein coupling to the receptor and in turn lead to receptor desensitization. (c) Novel roles of β-arrestins as endocytosis adaptors, E3 ubiquitin ligase adaptors and the new paradigm of β-arrestin dependent signaling downstream of 7TMRs. Receptor bound β-arrestins also recruit several components of clathrin dependent internalization machinery to the activated receptors and subsequently promote receptor endocytosis via clathrin coated pits. Moreover, β-arrestins also act as adaptors for a number of different E3 ubiquitin ligases to facilitate receptor ubiquitination. Surprisingly, β-arrestins are also capable of scaffolding a number of signaling molecules such as c-Src, Akt and ERK in order to initiate G protein-independent signaling downstream of activated 7TMRs. Please note that the different binding partners of β-arrestins shown in the figure probably do not bind β-arrestins simultaneously.

Figure 1

Multifaceted roles of β-arrestins and GRKs in 7TMR signaling and regulation. (a) The classical paradigm of G protein-dependent signaling via 7TMRs where binding of an agonist leads to conformational changes in the receptor. Activated receptor in turn couples to and activates heterotrimeric G proteins. Upon activation, heterotrimeric G proteins dissociate and activated G_α and G_βγ lead to generation of downstream signaling. (b) GRK-mediated phosphorylation and β-arrestin-mediated desensitization of 7TMRs. Agonist occupied activated receptors are phosphorylated by GRKs at serine/threonines primarily in the C-terminus but also in the intracellular loops. Phosphorylated receptors recruit multifunctional adaptor proteins β-arrestins which sterically hinder further G protein coupling to the receptor and in turn lead to receptor desensitization. (c) Novel roles of β-arrestins as endocytosis adaptors, E3 ubiquitin ligase adaptors and the new paradigm of β-arrestin dependent signaling downstream of 7TMRs. Receptor bound β-arrestins also recruit several components of clathrin dependent internalization machinery to the activated receptors and subsequently promote receptor endocytosis via clathrin coated pits. Moreover, β-arrestins also act as adaptors for a number of different E3 ubiquitin ligases to facilitate receptor ubiquitination. Surprisingly, β-arrestins are also capable of scaffolding a number of signaling molecules such as c-Src, Akt and ERK in order to initiate G protein-independent signaling downstream of activated 7TMRs. Please note that the different binding partners of β-arrestins shown in the figure probably do not bind β-arrestins simultaneously.

Figure 1

Multifaceted roles of β-arrestins and GRKs in 7TMR signaling and regulation. (a) The classical paradigm of G protein-dependent signaling via 7TMRs where binding of an agonist leads to conformational changes in the receptor. Activated receptor in turn couples to and activates heterotrimeric G proteins. Upon activation, heterotrimeric G proteins dissociate and activated G_α and G_βγ lead to generation of downstream signaling. (b) GRK-mediated phosphorylation and β-arrestin-mediated desensitization of 7TMRs. Agonist occupied activated receptors are phosphorylated by GRKs at serine/threonines primarily in the C-terminus but also in the intracellular loops. Phosphorylated receptors recruit multifunctional adaptor proteins β-arrestins which sterically hinder further G protein coupling to the receptor and in turn lead to receptor desensitization. (c) Novel roles of β-arrestins as endocytosis adaptors, E3 ubiquitin ligase adaptors and the new paradigm of β-arrestin dependent signaling downstream of 7TMRs. Receptor bound β-arrestins also recruit several components of clathrin dependent internalization machinery to the activated receptors and subsequently promote receptor endocytosis via clathrin coated pits. Moreover, β-arrestins also act as adaptors for a number of different E3 ubiquitin ligases to facilitate receptor ubiquitination. Surprisingly, β-arrestins are also capable of scaffolding a number of signaling molecules such as c-Src, Akt and ERK in order to initiate G protein-independent signaling downstream of activated 7TMRs. Please note that the different binding partners of β-arrestins shown in the figure probably do not bind β-arrestins simultaneously.

Figure 2

The global scope of the β-arrestin interactome and β-arrestin dependent phosphorylation events. (a) Subcellular and (b) functional distribution of β-arrestin interacting proteins with and without stimulation of the AT1aR by angiotensin II [68]. Note that the majority of β-arrestin interaction partners are distributed in the cytoplasm but a significant fraction are nuclear proteins highlighting the potential nuclear roles of β-arrestins. The functional distribution of β-arrestin binding proteins highlights their major roles in cellular signaling, cellular organization and nucleic acid binding. (c) Subcellular and (d) Functional distribution of the proteins which are phosphorylated upon activation of the AT1aR by a β-arrestin-biased ligand, SII-angiotensin [68]. Note that both the cellular and functional distribution of proteins which are phosphorylated/dephosphorylated upon SII stimulation in a β-arrestin dependent manner mimics the pattern of proteins identified in the interactomics screen.

Figure 3

An example of a β-arrestin dependent MAP kinase signaling network downstream of AT1aR. The proteins highlighted in red interact with β-arrestins, the proteins outlined in green are phosphorylated upon activation of AT1aR by a β-arrestin biased ligand, SII-angiotensin. The protein highlighted in red and outlined in green were present in both, the β-arrestin interactome and the β-arrestin phosphoproteome. The proteins outlined in blue are present in the corresponding MAP kinase pathways but are not identified in the proteomics screen. Please note that this figure highlights a potential β-arrestin dependent MAP kinase signaling network based on interactomics and phosphoproteomics studies and all the MAP kinase modules depicted here have not necessarily been documented to be regulated by β-arrestins. The dotted arrows indicate the pathways which have not yet been confirmed by independent studies to be directly activated by β-arrestins, the solid arrows indicate the pathways which have been studied in detail and which are well established to be modulated by β-arrestins.

Figure 4

A schematic representation of a simple conceptual framework to explain the mechanistic basis of β-arrestin biased signaling. (a) Binding of an unbiased ligand to the receptor induces an active conformation of the receptor (R*) whereas binding of a β-arrestin-biased ligand promotes a different active conformation of the receptor (R**). The distinct receptor conformations are coupled to corresponding active conformations of β-arrestin (β-arr * and β-arr **) which govern different functional outcomes. (b) Binding of an unbiased agonist to a β-arrestin biased receptor also induces a distinct conformation in the receptor which is likely to be similar to that induced by a β-arrestin-biased ligand to the wild-type receptor and a corresponding conformation in β-arrestin. The exact structural details of these multiple active conformations of receptors and β-arrestins remain to be deciphered by crystallography and other biophysical methods.