Trafficking patterns of beta-arrestin and G protein-coupled receptors determined by the kinetics of beta-arrestin deubiquitination

Agonist-dependent internalization of G protein-coupled receptors via clathrin-coated pits is dependent on the adaptor protein beta-arrestin, which interacts with elements of the endocytic machinery such as AP2 and clathrin. For the beta(2)-adrenergic receptor (beta(2)AR) this requires ubiquitination of beta-arrestin by E3 ubiquitin ligase, Mdm2. Based on trafficking patterns and affinity of beta-arrestin, G protein-coupled receptors are categorized into two classes. For class A receptors (e.g. beta(2)AR), which recycle rapidly, beta-arrestin directs the receptors to clathrin-coated pits but does not internalize with them. For class B receptors (e.g. V2 vasopressin receptors), which recycle slowly, beta-arrestin internalizes with the receptor into endosomes. In COS-7 and human embryonic kidney (HEK)-293 cells, stimulation of the beta(2)AR or V2 vasopressin receptor leads, respectively, to transient or stable beta-arrestin ubiquitination. The time course of ubiquitination and deubiquitination of beta-arrestin correlates with its association with and dissociation from each type of receptor. Chimeric receptors, constructed by switching the cytoplasmic tails of the two classes of receptors (beta(2)AR and V2 vasopressin receptors), demonstrate reversal of the patterns of both beta-arrestin trafficking and beta-arrestin ubiquitination. To explore the functional consequences of beta-arrestin ubiquitination we constructed a yellow fluorescent protein-tagged beta-arrestin2-ubiquitin chimera that cannot be deubiquitinated by cellular deubiquitinases. This "permanently ubiquitinated" beta-arrestin did not dissociate from the beta(2)AR but rather internalized with it into endosomes, thus transforming this class A receptor into a class B receptor with respect to its trafficking pattern. Overexpression of this beta-arrestin ubiquitin chimera in HEK-293 cells also results in enhancement of beta(2)AR internalization and degradation. In the presence of N-ethylmaleimide (an inhibitor of deubiquitinating enzymes), coimmunoprecipitation of the receptor and beta-arrestin was increased dramatically, suggesting that deubiquitination of beta-arrestin triggers its dissociation from the receptor. Thus the ubiquitination status of beta-arrestin determines the stability of the receptor-beta-arrestin complex as well as the trafficking pattern of beta-arrestin.     

The interaction of beta-arrestin with the AP-2 adaptor is required for the clustering of beta 2-adrenergic receptor into clathrin-coated pits

Beta-arrestins are cytosolic proteins that regulate the signaling and the internalization of G protein-coupled receptors (GPCRs). Although termination of receptor coupling requires beta-arrestin binding to agonist-activated receptors, GPCR endocytosis involves the coordinate interactions between receptor-beta-arrestin complexes and other endocytic proteins such as adaptor protein 2 (AP-2) and clathrin. Clathrin interacts with a conserved motif in the beta-arrestin C-terminal tail; however, the specific molecular determinants in beta-arrestin that bind AP-2 have not been identified. Moreover, the respective contributions of the interactions of beta-arrestin with AP-2 and clathrin toward the targeting of GPCRs to clathrin-coated vesicles have not been established. Here, we identify specific arginine residues (Arg(394) and Arg(396)) in the beta-arrestin 2 C terminus that mediate beta-arrestin binding to AP-2 and show, in vitro, that these domains in beta-arrestin 1 and 2 interact equally well with AP-2 independently of clathrin binding. We demonstrate in HEK 293 cells by fluorescence microscopy that beta(2)-adrenergic receptor-beta-arrestin complexes lacking the beta-arrestin-clathrin binding motif are still targeted to clathrin-coated pits. In marked contrast, receptor-beta-arrestin complexes lacking the beta-arrestin/AP-2 interactions are not effectively compartmentalized in punctated areas of the plasma membrane. These results reveal that the binding of a receptor-beta-arrestin complex to AP-2, not to clathrin, is necessary for the initial targeting of beta(2)-adrenergic receptor to clathrin-coated pits.     

Mutations in the cytoplasmic domain of EGF receptor affect EGF binding and receptor internalization.

Binding of epidermal growth factor (EGF) to its receptor results in a cascade of events that culminate in cell division. The receptor is present on the cell surface in two forms of high and low affinity binding for EGF. EGF binding activates the receptor's intracellular tyrosine kinase activity and subsequently causes the receptor to be rapidly internalized into the cell via clathrin-coated pits. We have cloned the EGF receptor cDNA into a retroviral expression vector and made mutations in vitro to investigate the function of different receptor domains. Deletion of cytoplasmic sequences abolishes high but not low affinity sites as well as impairing the ability of the protein to internalize into cells. Thus, cytoplasmic sequences must be involved in the regulation of high affinity sites and are required for EGF-induced receptor internalization. A four amino acid insertion mutation at residue 708 abolishes the protein-tyrosine kinase activity of the immunoprecipitated receptor. However, this receptor mutant exhibits both the high and low affinity states, internalizes efficiently and is able to cause cells to undergo DNA synthesis in response to EGF. Another four amino acid insertion mutation (residue 888) abolishes protein-tyrosine kinase activity, high affinity binding, internalization and mitogenic responsiveness. Finally, a chimaeric receptor composed of the extracellular EGF binding domain and the cytoplasmic v-abl kinase region transforms Rat-I cells. This chimaeric receptor possesses intrinsic protein tyrosine kinase activity which cannot be regulated by EGF. Moreover, EGF fails to induce the internalization of the chimaeric receptor.     

Internalization of the human CRF receptor 1 is independent of classical phosphorylation sites and of beta-arrestin 1 recruitment.

The corticotropin releasing factor receptor 1 (CRFR1) belongs to the superfamily of G-protein coupled receptors. Though CRF is involved in the aetiology of several stress-related disorders, including depression and anxiety, details of CRFR1 regulation such as internalization remain uncharacterized. In the present study, agonist-induced internalization of CRFR1 in HEK293 cells was visualized by confocal microscopy and quantified using the radioligand 125I-labelled sauvagine. Recruitment of beta-arrestin 1 in response to receptor activation was demonstrated by confocal microscopy. The extent of 125I-labelled sauvagine stimulated internalization was significantly impaired by sucrose, indicating the involvement of clathrin-coated pits. No effect on the extent of internalization was observed in the presence of the second messenger dependent kinase inhibitors H-89 and staurosporine, indicating that cAMP-dependent protein kinase and protein kinase C are not prerequisites for CRFR1 internalization. Surprisingly, deletion of all putative phosphorylation sites in the C-terminal tail, as well as a cluster of putative phosphorylation sites in the third intracellular loop, did not affect receptor internalization. However, these mutations almost abolished the recruitment of beta-arrestin 1 following receptor activation. In conclusion, we demonstrate that CRFR1 internalization is independent of phosphorylation sites in the C-terminal tail and third intracellular loop, and the degree of beta-arrestin 1 recruitment.     

Identification of NSF as a beta-arrestin1-binding protein. Implications for beta2-adrenergic receptor regulation

Previous studies have demonstrated that beta-arrestin1 serves to target G protein-coupled receptors for internalization via clathrin-coated pits and that its endocytic function is regulated by dephosphorylation at the plasma membrane. Using the yeast two-hybrid system, we have identified a novel beta-arrestin1-binding protein, NSF (N-ethylmaleimide-sensitive fusion protein), an ATPase essential for many intracellular transport reactions. We demonstrate that purified recombinant beta-arrestin1 and NSF interact in vitro and that these proteins can be coimmunoprecipitated from cells. beta-Arrestin1-NSF complex formation exhibits a conformational dependence with beta-arrestin1 preferentially interacting with the ATP bound form of NSF. In contrast to the beta-arrestin1-clathrin interaction, however, the phosphorylation state of beta-arrestin1 does not affect NSF binding. Functionally, overexpression of NSF in HEK 293 cells significantly enhances agonist-mediated beta2-adrenergic receptor (beta2-AR) internalization. Furthermore, when coexpressed with a beta-arrestin1 mutant (betaarr1S412D) that mimics a constitutively phosphorylated form of beta-arrestin1 and that acts as a dominant negative with regards to beta2-AR internalization, NSF rescues the betaarr1S412D-mediated inhibition of beta2-AR internalization. The demonstration of beta-arrestin1-NSF complex formation and the functional consequences of NSF overexpression suggest a hitherto unappreciated role for NSF in facilitating clathrin coat-mediated G protein-coupled receptor internalization.     

Beta-arrestin binding to the beta2-adrenergic receptor requires both receptor phosphorylation and receptor activation

Homologous desensitization of beta2-adrenergic receptors has been shown to be mediated by phosphorylation of the agonist-stimulated receptor by G-protein-coupled receptor kinase 2 (GRK2) followed by binding of beta-arrestins to the phosphorylated receptor. Binding of beta-arrestin to the receptor is a prerequisite for subsequent receptor desensitization, internalization via clathrin-coated pits, and the initiation of alternative signaling pathways. In this study we have investigated the interactions between receptors and beta-arrestin2 in living cells using fluorescence resonance energy transfer. We show that (a) the initial kinetics of beta-arrestin2 binding to the receptor is limited by the kinetics of GRK2-mediated receptor phosphorylation; (b) repeated stimulation leads to the accumulation of GRK2-phosphorylated receptor, which can bind beta-arrestin2 very rapidly; and (c) the interaction of beta-arrestin2 with the receptor depends on the activation of the receptor by agonist because agonist withdrawal leads to swift dissociation of the receptor-beta-arrestin2 complex. This fast agonist-controlled association and dissociation of beta-arrestins from prephosphorylated receptors should permit rapid control of receptor sensitivity in repeatedly stimulated cells such as neurons.     

Cholesterol oxidation switches the internalization pathway of endothelin receptor type A from caveolae to clathrin-coated pits in Chinese hamster ovary cells

We investigated the mechanism of endothelin receptor type A (ETA) internalization in Chinese hamster ovary cells using two assays; flow cytometric quantification of cell surface myc-ETA and in situ localization of Cy5-labeled ET-1. In both assays, agonist-dependent internalization of myc-ETA was inhibited by nystatin and filipin, both of which disrupt internalization via caveolae, whereas it was barely affected by chlorpromazine and hypertonic sucrose, both of which disrupt internalization via clathrin-coated pits. In addition to myc-ETA, ET-1 caused intracellular translocation of caveolin-1 and this translocation was also blocked by nystatin but not by chlorpromazine. These results strongly argue that ETA is internalized via caveolae but not clathrin-coated pits. Treatment of the cells with cholesterol oxidase reduced cellular cholesterol and caused intracellular translocation of caveolin-1 but did not affect cell surface localization of myc-ETA. In cholesterol oxidase-treated cells, however, both chlorpromazine and hypertonic sucrose effectively blocked ET-1-induced myc-ETA internalization and nystatin was less effective than in untreated cells. Accordingly, expression of a dominant negative form of beta-arrestin blocked myc-ETA internalization in cholesterol oxidase-treated cells but not in untreated cells. These results suggest that, in Chinese hamster ovary cells, 1) agonist-occupied ETA can be internalized either via caveolae or clathrin-coated pits; 2) of the two, the former is the default pathway; and 3) the oxidative state of cell surface cholesterol is one of the factors involved in the pathway selection.     

Beta-arrestin scaffolding of phosphatidylinositol 4-phosphate 5-kinase Ialpha promotes agonist-stimulated sequestration of the beta2-adrenergic receptor

Members of the seven-transmembrane receptor (7TMR) superfamily are sequestered from the plasma membrane following stimulation both to limit cellular responses as well as to initiate novel G protein-independent signaling pathways. The best studied mechanism for 7TMR internalization is via clathrin-coated pits, where clathrin and adaptor protein complex 2 nucleate and polymerize upon encountering the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP(2)) to form the outer layer of the clathrin-coated vesicle. Activated receptors are recruited to clathrin-coated pits by beta-arrestins, scaffolding proteins that interact with agonist-occupied 7TMRs as well as adaptor protein complex 2 and clathrin. We report here that following stimulation of the beta2-adrenergic receptor (beta2-AR), a prototypical 7TMR, beta-arrestins bind phosphatidylinositol 4-phosphate 5-kinase (PIP5K) Ialpha, a PIP(2)-producing enzyme. Furthermore, beta-arrestin2 is required to form a complex with PIP5K Ialpha and agonist-occupied beta2-AR, and beta-arrestins synergize with the kinase to produce PIP(2) in response to isoproterenol stimulation. Interestingly, beta-arrestins themselves bind PIP(2), and a beta-arrestin mutant deficient in PIP(2) binding no longer internalizes 7TMRs, fails to interact with PIP5K Ialpha, and is not associated with PIP kinase activity assayed in vitro. However, a chimeric protein in which the core kinase domain of PIP5K Ialpha has been fused to the same beta-arrestin mutant rescues internalization of beta2-ARs. Collectively, these data support a model in which beta-arrestins direct the localization of PIP5K Ialpha and PIP(2) production to agonist-activated 7TMRs, thereby regulating receptor internalization.     

c-Src regulates clathrin adapter protein 2 interaction with beta-arrestin and the angiotensin II type 1 receptor during clathrin- mediated internalization

Beta-arrestins are multifunctional adapters involved in the internalization and signaling of G protein-coupled receptors (GPCRs). They target receptors to clathrin-coated pits (CCPs) through binding with clathrin and clathrin adapter 2 (AP-2) complex. They also act as transducers of signaling by recruiting c-Src kinase to certain GPCRs. Here we sought to determine whether c-Src regulates the recruitment of AP-2 to beta-arrestin and the angiotensin II (Ang II) type 1 receptor (AT1R) during internalization. We show that the agonist stimulation of native AT1R in vascular smooth muscle cells (VSMCs) induces the formation of an endogenous complex containing c-Src, beta-arrestins and AP-2. In vitro studies using coimmunoprecipitation experiments and a yeast three-hybrid assay reveal that c-Src stabilizes the agonist-independent association between beta-arrestin2 and the beta-subunit of AP-2 independently of the kinase activity of c-Src. However, although c-Src expression promoted the rapid dissociation of AP-2 from both beta-arrestin and AT1R after receptor stimulation, a kinase-inactive mutant of c-Src failed to induce the dissociation of AP-2 from the agonist-occupied receptor. Thus, the consequence of c-Src in regulating the dissociation of AP-2 from the receptor was also examined on the internalization of AT1R by depleting c-Src in human embryonic kidney (HEK) 293 cells using a small interfering RNA strategy. Experiments in c-Src depleted cells reveal that AT1R remained mostly colocalized with AP-2 at the plasma membrane after Ang II stimulation, consistent with the observed delay in receptor internalization. Moreover, coimmunoprecipitation experiments in c-Src depleted HEK 293 cells and VSMCs showed an increased association of AP-2 to the agonist-occupied AT1R and beta-arrestin, respectively. Together, our results support a role for c-Src in regulating the dissociation of AP-2 from agonist-occupied AT1R and beta-arrestin during the clathrin-mediated internalization of receptors and suggest a novel function for c-Src kinase in the internalization of AT1R.     

Protease-activated receptor-1 down-regulation: a mutant HeLa cell line suggests novel requirements for PAR1 phosphorylation and recruitment to clathrin-coated pits

Protease-activated receptor-1 (PAR1), a G protein-coupled receptor (GPCR) for thrombin, is irreversibly activated by a proteolytic mechanism, then internalized and degraded in lysosomes. The latter is critical for temporal fidelity of thrombin signaling. Toward understanding PAR1 down-regulation, we first investigated the pathway of PAR1 internalization. Activated PAR1 was rapidly recruited to clathrin-coated pits, where it colocalized with transferrin receptor (TfnR). Dominant-negative dynamin and clathrin hub mutants both blocked PAR1 internalization. Blockade of PAR1 internalization with dynamin K44A also inhibited activation-dependent PAR1 degradation. Thus activated PAR1 internalizes via clathrin-coated pits together with receptors that recycle and is then sorted away from such receptors and delivered to lysosomes. In the course of these studies we identified a mutant HeLa cell line, designated JT1, that was defective in PAR1 internalization. PAR1 signaled robustly in JT1 cells but was not phosphorylated or recruited to clathrin-coated pits after activation. Internalization of TfnR was intact in JT1 cells and internalization of beta(2)-adrenergic receptor, a GPCR that internalizes and recycles, was present but perhaps reduced. Taken together, these studies suggest that PAR1 is internalized in a dynamin- and clathrin-dependent manner like TfnR and beta(2)-adrenergic receptor but requires a distinct gene product for recruitment into this pathway.     

beta-arrestins regulate interleukin-8-induced CXCR1 internalization.

The functional role of neutrophils during acute inflammatory responses is regulated by two high affinity interleukin-8 receptors (CXCR1 and CXCR2) that are rapidly desensitized and internalized upon binding their cognate chemokine ligands. The efficient re-expression of CXCR1 on the surface of neutrophils following agonist-induced internalization suggests that CXCR1 surface receptor turnover may involve regulatory pathways and intracellular factors similar to those regulating beta2-adrenergic receptor internalization and re-expression. To examine the internalization pathway utilized by ligand-activated CXCR1, a CXCR1-GFP construct was transiently expressed in two different cell lines, HEK 293 and RBL-2H3 cells. While interleukin-8 stimulation promoted CXCR1 sequestration in RBL-2H3 cells, receptor internalization in HEK 293 cells required co-expression of G protein-coupled receptor kinase 2 and beta-arrestin proteins. The importance of beta-arrestins in CXCR1 internalization was confirmed by the ability of a dominant negative beta-arrestin 1-V53D mutant to block internalization of CXCR1 in RBL-2H3 cells. A role for dynamin was also demonstrated by the lack of CXCR1 internalization in dynamin I-K44A dominant negative mutant-transfected RBL-2H3 cells. Agonist-promoted co-localization of transferrin and CXCR1-GFP in endosomes of RBL-2H3 cells confirmed that receptor internalization occurs via clathrin-coated vesicles. Our data provides a direct link between agonist-induced internalization of CXCR1 and a requirement for G protein-coupled receptor kinase 2, beta-arrestins, and dynamin during this process.     

Endocytosis of activated receptors and clathrin-coated pit formation: deciphering the chicken or egg relationship

The fundamental mechanisms by which receptors once targeted for endocytosis are found in coated pits is an important yet unresolved question. Specifically, are activated receptors simply trapped on encountering preexisting coated pits, subsequently being rapidly internalized? Or do the receptors themselves, by active recruitment, gather soluble coat and cytosolic components and initiate the rapid assembly of new coated pits that then mediate their internalization? To explore this question, we studied the relationship between activation of IgE-bound high affinity Fc receptors (FCepsilonRI) and coated pit formation. Because these receptors are rapidly internalized via clathrin-coated pits only when cross-linked by the binding of multivalent antigens, we were able to separate activation from internalization by using an immobilized antigen. The FCepsilonRIs, initially uniformly distributed over the cell surface. relocalized and aggregated on the antigen-exposed membrane. The process was specific for the antigen, and temperature- and time-dependent. This stimulation initiated a cascade of cellular responses typical of FCepsilonRI signaling including membrane ruffling, cytoskeletal rearrangements, and, in the presence of Ca2+, exocytosis. Despite these responses, no change in coated pit disposition or in the distribution of clathrin and assembly protein AP2 was detected, as monitored by immunoblotting and by quantitative (vertical sectioning) confocal microscopy analysis of immunofluorescently stained cells. Specifically, there was no decrease in the density of clathrin-coated pits in regions of the cell membrane not in contact with the antigen, and there was no apparent increase in clathrin-coated pits in proximity to stimulated FCepsilonRI receptors as would have been expected if the receptors were inducing formation of new pits by active recruitment. These results indicate that de novo formation of clathrin-coated pits is not a prerequisite for rapid internalization or a direct response to stimulation of FCepsilonRI receptors. Therefore, increases in coated pits reported to occur in response to activation of some signaling receptors must be consequences of the signal transduction processes, rather than strictly serving the purpose of the internalization of the receptors.     

Unraveling G protein-coupled receptor endocytosis pathways using real-time monitoring of agonist-promoted interaction between beta-arrestins and AP-2

The most widely studied pathway underlying agonist-promoted internalization of G protein-coupled receptors (GPCRs) involves beta-arrestin and clathrin-coated pits. However, both beta-arrestin- and clathrin-independent processes have also been reported. Classically, the endocytic routes are characterized using pharmacological inhibitors and various dominant negative mutants, resulting sometimes in conflicting results and interpretational difficulties. Here, taking advantage of the fact that beta-arrestin binding to the beta2 subunit of the clathrin adaptor AP-2 (beta2-adaptin) is needed for the beta-arrestin-mediated targeting of GPCRs to clathrin-coated pits, we developed a bioluminescence resonance energy transfer-based approach directly assessing the molecular steps involved in the endocytosis of GPCRs in living cells. For 10 of the 12 receptors tested, including some that were previously suggested to internalize via clathrin-independent pathways, agonist stimulation promoted beta-arrestin 1 and 2 interaction with beta2-adaptin, indicating a beta-arrestin- and clathrin-dependent endocytic process. Detailed analyses of beta-arrestin interactions with both the receptor and beta2-adaptin also allowed us to demonstrate that recruitment of beta-arrestins to the receptor and the ensuing conformational changes are the leading events preceding AP-2 engagement and subsequent clathrin-mediated endocytosis. Among the receptors tested, only the endothelin A and B receptors failed to promote interaction between beta-arrestins and beta2-adaptin. However, both receptors recruited beta-arrestins upon agonist stimulation, suggesting a beta-arrestin-dependent but clathrin-independent route of internalization for these two receptors. In addition to providing a new tool to dissect the molecular events involved in GPCR endocytosis, the bioluminescence resonance energy transfer-based beta-arrestin/beta2-adaptin interaction assay represents a novel biosensor to assess receptor activation.     

beta -Arrestins regulate protease-activated receptor-1 desensitization but not internalization or Down-regulation.

The widely expressed beta-arrestin isoforms 1 and 2 bind phosphorylated G protein-coupled receptors (GPCRs) and mediate desensitization and internalization. Phosphorylation of protease-activated receptor-1 (PAR1), a GPCR for thrombin, is important for desensitization and internalization, however, the role of beta-arrestins in signaling and trafficking of PAR1 remains unknown. To assess beta-arrestin function we examined signaling and trafficking of PAR1 in mouse embryonic fibroblasts (MEFs) derived from beta-arrestin (betaarr) knockouts. Desensitization of PAR1 signaling was markedly impaired in MEFs lacking both betaarr1 and betaarr2 isoforms compared with wild-type cells. Strikingly, in cells lacking only betaarr1 PAR1 desensitization was also significantly impaired compared with betaarr2-lacking or wild-type cells. In wild-type MEFs, activated PAR1 was internalized through a dynamin- and clathrin-dependent pathway and degraded. Surprisingly, in cells lacking both betaarr1 and betaarr2 activated PAR1 was similarly internalized through a dynamin- and clathrin-dependent pathway and degraded, whereas the beta(2)-adrenergic receptor (beta(2)-AR) failed to internalize. A PAR1 cytoplasmic tail mutant defective in agonist-induced phosphorylation failed to internalize in both wild-type and beta-arrestin knockout cells. Thus, PAR1 appears to utilize a distinct phosphorylation-dependent but beta-arrestin-independent pathway for internalization through clathrin-coated pits. Together, these findings strongly suggest that the individual beta-arrestin isoforms can differentially regulate GPCR desensitization and further reveal a novel mechanism by which GPCRs can internalize through a dynamin- and clathrin-dependent pathway that is independent of arrestins.     

Post-endocytic fates of delta-opioid receptor are regulated by GRK2-mediated receptor phosphorylation and distinct beta-arrestin isoforms

Once internalized, some G protein-coupled receptors (GPCRs) can recycle back to the cell surface, while some of them are delivered to lysosomes for degradation. Because recycling and degradation represent two opposing receptor fates, understanding the mechanisms that determine post-endocytic fate of GPCRs is of great importance. Our recent work has verified that agonist-induced internalization of delta-opioid receptor (DOR) employs both phosphorylation-dependent and -independent mechanisms in HEK293 cells. To investigate whether these two internalization mechanisms work differently in receptor regulation, we monitored receptor post-endocytic fates using flow cytometry, surface receptor biotinylation and radioligand binding assays. Results showed that the internalized wild type DOR could either recycle to the cell surface or be degraded. Mutant DOR M4/5/6, which lacks all three G protein-coupled receptor kinase 2 (GRK2) phosphorylation sites, could also internalize upon agonist challenge although in a reduced level as compared with the wild type counterpart. However, the internalized mutant DOR could not recycle back to the cell surface and all mutant DOR was degraded after internalization. Inhibition of GRK2 expression by GRK2 RNAi also strongly attenuated recycling of DOR. Furthermore, overexpression of GRK2, which significantly increased receptor phosphorylation and internalization, also targeted more internalized receptors to the recycling pathway. These data suggest that GRK2-catalyzed receptor phosphorylation is critically involved in DOR internalization and recycling, and the phosphorylation-independent internalization leads to receptor degradation. Data obtained from beta-arrestin1 and beta-arrestin2 RNAi experiments indicated that both beta-arrestin1 and beta-arrestin2 participate in phosphorylation-dependent internalization and the subsequent recycling of DOR. However, phosphorylation-independent internalization and degradation of DOR were strongly blocked by beta-arrestin2 RNAi, but not beta-arrestin1 RNAi. Taken together, these data demonstrate for the first time that GRK2 phosphorylation-dependent internalization mediated by both beta-arrestin1 and beta-arrestin2 leads DOR to recycle, whereas GRK2-independent internalization mediated by beta-arrestin2 alone leads to receptor degradation. Thus, the post-endocytic fate of internalized DOR can be regulated by GRK2-catalyzed receptor phosphorylation as well as distinct beta-arrestin isoforms.     

Peptide ligand binding properties of the corticotropin-releasing factor (CRF) type 2 receptor: pharmacology of endogenously expressed receptors, G-protein-coupling sensitivity and determinants of CRF2 receptor selectivity

The CRF2 receptor is involved in stress responses, cardiovascular function and gastric motility. Endogenous agonists (urocortin (UCN) 2, UCN 3) and synthetic antagonists (astressin2-B, antisauvagine-30) are selective for CRF2 over the CRF1 receptor. Peptide ligand binding properties of the CRF2 receptor require further investigation, including ligand affinity for endogenously expressed receptors, the effect of receptor-G-protein coupling on ligand affinity, and the molecular basis of ligand selectivity. Ligand affinity for rat CRF(2a) in olfactory bulb and CRF(2b) in A7r5 cells was similar to that for the cloned human CRF(2a) receptor (within three-fold), except for oCRF (9.4- and 5.4-fold higher affinity in olfactory bulb and A7r5 cells, respectively). Receptor-G-protein uncoupling reduced agonist affinity only 1.2- to 6.5-fold (compared with 92-1300-fold for the CRF1 receptor). Ligand selectivity mechanisms were investigated using chimeric CRF2/CRF1 receptors. The juxtamembrane receptor domain determined selectivity of antisauvagine-30, the N-terminal-extracellular domain contributed to selectivity of UCN 3, and both domains contributed to selectivity of UCN 2 and astressin2-B. Therefore ligands differ in the contribution of receptor domains to their selectivity, and CRF2-selective antagonists bind the juxtamembrane domain. These findings will be important for identifying the CRF2 receptor in tissues and for developing ligands targeting the receptor, both of which will be useful in identifying the emerging physiological functions of the CRF2 receptor.     

Properties of secretin receptor internalization differ from those of the beta(2)-adrenergic receptor.

The endocytic pathway of the secretin receptor, a class II GPCR, is unknown. Some class I G protein-coupled receptors (GPCRs), such as the beta(2)-adrenergic receptor (beta(2)-AR), internalize in clathrin-coated vesicles and this process is mediated by G protein-coupled receptor kinases (GRKs), beta-arrestin, and dynamin. However, other class I GPCRs, for example, the angiotensin II type 1A receptor (AT(1A)R), exhibit different internalization properties than the beta(2)-AR. The secretin receptor, a class II GPCR, is a GRK substrate, suggesting that like the beta(2)-AR, it may internalize via a beta-arrestin and dynamin directed process. In this paper we characterize the internalization of a wild-type and carboxyl-terminal (COOH-terminal) truncated secretin receptor using flow cytometry and fluorescence imaging, and compare the properties of secretin receptor internalization to that of the beta(2)-AR. In HEK 293 cells, sequestration of both the wild-type and COOH-terminal truncated secretin receptors was unaffected by GRK phosphorylation, whereas inhibition of cAMP-dependent protein kinase mediated phosphorylation markedly decreased sequestration. Addition of secretin to cells resulted in a rapid translocation of beta-arrestin to plasma membrane localized receptors; however, secretin receptor internalization was not reduced by expression of dominant negative beta-arrestin. Thus, like the AT(1A)R, secretin receptor internalization is not inhibited by reagents that interfere with clathrin-coated vesicle-mediated internalization and in accordance with these results, we show that secretin and AT(1A) receptors colocalize in endocytic vesicles. This study demonstrates that the ability of secretin receptor to undergo GRK phosphorylation and beta-arrestin binding is not sufficient to facilitate or mediate its internalization. These results suggest that other receptors may undergo endocytosis by mechanisms used by the secretin and AT(1A) receptors and that kinases other than GRKs may play a greater role in GPCR endocytosis than previously appreciated.     

A soluble form of the first extracellular domain of mouse type 2beta corticotropin-releasing factor receptor reveals differential ligand specificity

The heptahelical receptors for corticotropin-releasing factor (CRF), CRFR1 and CRFR2, display different specificities for CRF family ligands: CRF and urocortin I bind to CRFR1 with high affinity, whereas urocortin II and III bind to this receptor with very low affinities. In contrast, all the urocortins bind with high affinities, and CRF binds with lower affinity to CRFR2. The first extracellular domain (ECD1) of CRFR1 is important for ligand recognition. Here, we characterize a bacterially expressed soluble protein, ECD1-CRFR2beta, corresponding to the ECD1 of mouse CRFR2beta. The K(i) values for binding to ECD1-CRFR2beta are: astressin = 10.7 (5.4-21.1) nm, urocortin I = 6.4 (4.7-8.7) nm, urocortin II = 6.9 (5.8-8.3) nm, CRF = 97 (22-430) nm, urocortin III = sauvagine >200 nm. These affinities are similar to those for binding to a chimeric receptor in which the ECD1 of CRFR2beta replaces the ECD of the type 1B activin receptor (ALK4). The ECD1-CRFR2beta possesses a disulfide arrangement identical to that of the ECD1 of CRFR1, namely Cys(45)-Cys(70), Cys(60)-Cys(103), and Cys(84)-Cys(118). As determined by circular dichroism, ECD1-CRFR2beta undergoes conformational changes upon binding astressin. These data reinforce the importance of the ECD1 of CRF receptors for ligand recognition and raise the interesting possibility that different ligands having similar affinity for the full-length receptor may, nevertheless, have different affinities for microdomains of the receptor.     

Endocytosis and intracellular trafficking of ErbBs

This review article describes the pathways and mechanisms of endocytosis and post-endocytic sorting of the EGF receptor (EGFR/ErbB1) and other members of the ErbB family. Growth factor binding to EGFR accelerates its internalization through clathrin-coated pits which is followed by the efficient lysosomal targeting of internalized receptors and results in receptor down-regulation. The role of EGFR interaction with the Grb2 adaptor protein and Cbl ubiquitin ligase, and receptor ubiquitination in the clathrin-dependent internalization and sorting of EGFR in multivesicular endosomes is discussed. Activation and phosphorylation of ErbB2, ErbB3 and ErbB4 also results in their ubiquitination. However, these ErbBs are internalized and targeted to lysosomes less efficiently than EGFR. When overexpressed endocytosis-impaired ErbBs may inhibit the internalization and degradation of EGFR.     

Endocytosis and intracellular trafficking of ErbBs

This review article describes the pathways and mechanisms of endocytosis and post-endocytic sorting of the EGF receptor (EGFR/ErbB1) and other members of the ErbB family. Growth factor binding to EGFR accelerates its internalization through clathrin-coated pits which is followed by the efficient lysosomal targeting of internalized receptors and results in receptor down-regulation. The role of EGFR interaction with the Grb2 adaptor protein and Cbl ubiquitin ligase, and receptor ubiquitination in the clathrin-dependent internalization and sorting of EGFR in multivesicular endosomes is discussed. Activation and phosphorylation of ErbB2, ErbB3 and ErbB4 also results in their ubiquitination. However, these ErbBs are internalized and targeted to lysosomes less efficiently than EGFR. When overexpressed endocytosis-impaired ErbBs may inhibit the internalization and degradation of EGFR.