Wu Trafficking of GPCRs

1. Auflage 2015
ISBN: 978-0-12-802955-8
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, Band Volume 132, 328 Seiten

Reihe: Progress in Molecular Biology and Translational Science

G protein-coupled receptors (GPCRs) constituteÿtheÿlargest superfamily of cell surface receptors that regulate a variety of cell functions. Over the past few decades great progress has been made in defining the roles of intracellular trafficking in controlling the functionality of the receptors as well as in the development of various human diseases. This volume of Progress in Molecular Biology and Translational Scienceÿreviews the recent understanding of GPCR trafficking regulators and molecular mechanisms. - Written by future leaders in the pain field - Covers a wide range of targets - Contains provocative ideas about GPCR trafficking

Dr. Guangyu Wu, the editor of this book, is a professor in the Department of Pharmacology and Toxicology at the Medical College of Georgia, Georgia Regents University. The long term goal of Dr. Wu's research effort is to define the molecular mechanisms underlying the intracellular trafficking and signal propagation of GPCRs. Dr. Wu's research at the earlier stages, as a graduate student and a post-doctoral fellow, was mainly on the GPCR pharmacology and signal transduction pathways. Since starting his own laboratory in the Department of Pharmacology at Louisiana State University Health Sciences Center New Orleans, Dr. Wu has focused his research on the regulation of GPCR export from the endoplasmic reticulum through the Golgi apparatus to the cell surface through studying the function of well-defined transport machinery at different intracellular organelles and defining specific motifs embedded within the receptors that direct receptor export from the ER/Golgi or cell surface transport.

Autoren/Hrsg.

Wu, Guangyu

Leseproben

Chapter One Arrestins
Critical Players in Trafficking of Many GPCRs
Vsevolod V. Gurevich1; Eugenia V. Gurevich    Department of Pharmacology, Vanderbilt University, Nashville, Tennessee, USA
1 Corresponding author: email address: [e-mail] Abstract
Arrestins specifically bind active phosphorylated G protein-coupled receptors (GPCRs). Receptor binding induces the release of the arrestin C-tail, which in non-visual arrestins contains high-affinity binding sites for clathrin and its adaptor AP2. Thus, serving as a physical link between the receptor and key components of the internalization machinery of the coated pit is the best-characterized function of non-visual arrestins in GPCR trafficking. However, arrestins also regulate GPCR trafficking less directly by orchestrating their ubiquitination and deubiquitination. Several reports suggest that arrestins play additional roles in receptor trafficking. Non-visual arrestins appear to be required for the recycling of internalized GPCRs, and the mechanisms of their function in this case remain to be elucidated. Moreover, visual and non-visual arrestins were shown to directly bind N-ethylmaleimide-sensitive factor, an important ATPase involved in vesicle trafficking, but neither molecular details nor the biological role of these interactions is clear. Considering how many different proteins arrestins appear to bind, we can confidently expect the elucidation of additional trafficking-related functions of these versatile signaling adaptors. Keywords Arrestin GPCR Signaling Internalization Recycling Abbreviations AIP4 atrophin-1-interacting protein 4 AP2 adaptor protein 2 ß2AR ß2-adrenergic receptor GPCR G protein-coupled receptor GRK G protein-coupled receptor kinase Nedd4 neural precursor cell expressed developmentally down-regulated protein 4 1 Arrestins and GPCR Trafficking
Preferential binding of arrestins to active phosphorylated receptors was discovered about 30 years ago.1 The finding that arrestin binding suppresses receptor coupling to cognate G proteins was made soon after in the visual system.2 The mechanism turned out to be remarkably simple: direct competition between arrestin and G protein for overlapping sites.3,4 For some time, it appeared that the only function arrestins have is to bind active phosphorylated G protein-coupled receptors (GPCRs), precluding receptor interactions with G proteins by direct competition.3,4 The first described non-GPCR binding partners of arrestins were trafficking proteins: clathrin in 19965 and clathrin adaptor AP2 a few years later.6 These data demonstrated that arrestins play an essential role not only in GPCR desensitization7 but also in receptor endocytosis,8 via trafficking signals added by receptor-bound arrestins. The discovery that arrestins are ubiquitinated upon receptor binding and regulate ubiquitination of GPCRs9 revealed yet another mechanism, whereby arrestins regulate receptor trafficking indirectly. Here, we discuss several known mechanisms of arrestin effects on GPCR trafficking and highlight observations that suggest that there are many other mechanisms that still remain to be elucidated. 2 Non-visual Arrestins Mediate GPCR Internalization via Coated Pits
Arrestins promote GPCR internalization by virtue of recruitment of clathrin and AP2 via fairly well-mapped binding sites in the C-tail of non-visual arrestins5,6,10,11 (Fig. 1). Interestingly, the C-tail in the basal conformation of all arrestins is anchored to the N-domain,12–16 whereas receptor binding triggers its release.17–19 The expression of separated arrestin C-tail carrying these sites inhibits GPCR internalization, apparently by winning the competition with the arrestin–receptor complexes for clathrin and AP2.20 This finding provided the first clear evidence of functional significance of shielding of the arrestin C-tail in the basal conformation and its release upon receptor binding. In free arrestins, the C-tail is anchored to the body of the molecule, which makes it inaccessible, preventing its competition with the receptor-bound arrestins for the components of internalization machinery (reviewed in Ref. 21). Figure 1 Arrestins play many roles in GPCR trafficking. Arrestins (ARR) bind active phosphorylated GPCRs (shown as a seven-helix bundle). Receptor binding induces the release of the arrestin C-tail, which carries binding sites for clathrin (Clath) and adaptor protein-2 (AP2). The interactions of these sites with clathrin and AP2 promote receptor internalization via coated pits. Arrestins also recruit ubiquitin ligases Mdfm2, Nedd4, and AIP4 to the complex, which favors ubiquitination of both non-visual arrestins and at least some GPCRs. Arrestins also recruit certain deubiquitination enzymes (USP20 and USP33 are shown), facilitating receptor deubiquitination. The role of arrestin interactions with microtubules, centrosome, and N-ethylmaleimide-sensitive factor (NSF) in trafficking of GPCRs and/or other proteins remains to be elucidated. Another known mechanism of arrestin recruitment to the coated pit is its direct binding to phosphoinositides, which was reported to be necessary for GPCR internalization.22 Since resident coated pit protein AP2 is also recruited to this part of the membrane via phosphoinositide binding,23 one might think that as soon as the arrestin–receptor complex is formed, it has no choice but to move to the coated pit. However, this does not appear to be the case. In muscarinic M2 receptor, which was among the first shown to bind arrestins,24 two Ser/Thr clusters in the third cytoplasmic loop were identified as critical for arrestin binding and receptor desensitization.25 Yet the elimination of these clusters, and even dominant-negative dynamin K44A mutant that blocks the internalization of ß2AR in the same cells, did not prevent M2 endocytosis, suggesting that M2 receptor does not use coated pits and internalizes in an arrestin-independent manner.25 Interestingly, overexpression of non-visual arrestins can redirect some M2 to coated pits,25 suggesting that this receptor can use more than one route. Many other GPCRs were shown to have that choice. For example, chemokine receptor CCR5 uses both phosphorylation- and arrestin-dependent and -independent pathways.26 Cysteinyl leukotriene type 1 receptor internalizes normally in mouse embryonic fibroblasts lacking both non-visual arrestins, yet arrestin expression facilitates its internalization,27 apparently directing it to the arrestin-dependent pathway, which is usually not preferred, similar to M2 receptor.25 Metabotropic glutamate receptor mGluR1a constitutively internalizes via arrestin-independent mechanism, whereas its agonist-dependent internalization appears to be mediated by arrestin-2.28 Endogenous and overexpressed serotonin 5HT4 receptor internalizes via arrestin-dependent pathway, but the deletion of Ser/Thr cluster targeted by G protein-coupled receptor kinases (GRKs) redirects it to an alternative pathway and even facilitates its internalization.29 Thus, it appears that the ability of GPCRs to use more than one internalization pathway is a general rule, rather than an exception, likely representing one of the many backup mechanisms cells usually have. Many receptors have recognizable internalization motifs in their sequence, so arrestin binding simply adds new ones. The relative strength of these motifs, as well as the arrestin expression levels, likely determines the pathway(s) each receptor chooses in a particular cell. The dominant internalization pathway of a particular receptor is not necessarily the same in different cell types, or even at different functional states of the same cell (reviewed in Ref. 8). Variety, rather than uniformity, characterizes the world of GPCR signaling and trafficking.30 3 Visual Arrestins and Trafficking Proteins
In vertebrate rod photoreceptors, rhodopsin is localized on the discs, which are detached from the plasma membrane31 and therefore are topologically equivalent to vesicles with internalized non-visual GPCRs. Thus, vertebrate rhodopsin is not supposed to be internalized. Indeed, arrestin-1, which is the prevalent arrestin isoform in both rods and cones,32 does not have conventional clathrin- or AP2-binding elements in its C-tail.33 However, sequence comparison of arrestin-1 and non-visual subtypes shows that in the region homologous to AP2-binding motif in arrestin-2 and -3, only one positive charge is missing.34 Therefore, it is hardly surprising that arrestin-1 also binds AP2, albeit with ~ 30 times lower affinity.34 Constitutively active rhodopsin–K296E is a naturally occurring mutant that causes autosomal dominant retinitis pigmentosa in humans, apparently due to constitutive phosphorylation and formation of a stable complex with arrestin-1.35 The concentration of rhodopsin in the outer segment of rods reaches ~ 3 mM.31 Rods also...