Dual role of the beta2-adrenergic receptor C terminus for the binding of beta-arrestin and receptor internalization

Homologous desensitization of beta2-adrenergic and other G-protein-coupled receptors is a two-step process. After phosphorylation of agonist-occupied receptors by G-protein-coupled receptor kinases, they bind beta-arrestins, which triggers desensitization and internalization of the receptors. Because it is not known which regions of the receptor are recognized by beta-arrestins, we have investigated beta-arrestin interaction and internalization of a set of mutants of the human beta2-adrenergic receptor. Mutation of the four serine/threonine residues between residues 355 and 364 led to the loss of agonist-induced receptor-beta-arrestin2 interaction as revealed by fluorescence resonance energy transfer (FRET), translocation of beta-arrestin2 to the plasma membrane, and receptor internalization. Mutation of all seven serine/threonine residues distal to residue 381 did not affect agonist-induced receptor internalization and beta-arrestin2 translocation. A beta2-adrenergic receptor truncated distal to residue 381 interacted normally with beta-arrestin2, whereas its ability to internalize in an agonist-dependent manner was compromised. A similar impairment of internalization was observed when only the last eight residues of the C terminus were deleted. Our experiments show that the C terminus distal to residue 381 does not affect the initial interaction between receptor and beta-arrestin, but its last eight amino acids facilitate receptor internalization in concert with beta-arrestin2.     

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.     

The SRC homology 2 domain of Rin1 mediates its binding to the epidermal growth factor receptor and regulates receptor endocytosis

Activated epidermal growth factor receptors (EGFRs) recruit intracellular proteins that mediate receptor signaling and endocytic trafficking. Rin1, a multifunctional protein, has been shown to regulate EGFR internalization (1). Here we show that EGF stimulation induces a specific, rapid, and transient membrane recruitment of Rin1 and that recruitment is dependent on the Src homology 2 (SH2) domain of Rin1. Immunoprecipitation of EGFR is accompanied by co-immunoprecipitation of Rin1 in a time- and ligand-dependent manner. Association of Rin1 and specifically the SH2 domain of Rin1 with the EGFR was dependent on tyrosine phosphorylation of the intracellular domain of the EGFR. The recruitment of Rin1, observed by light microscopy, indicated that although initially cytosolic, Rin1 was recruited to both plasma membrane and endosomes following EGF addition. Moreover, the expression of the SH2 domain of Rin1 substantially impaired the internalization of EGF without affecting internalization of transferrin. Finally, we found that Rin1 co-immunoprecipitated with a number of tyrosine kinase receptors but not with cargo endocytic receptors. These results indicate that Rin1 provides a link via its SH2 domain between activated tyrosine kinase receptors and the endocytic pathway through the recruitment and activation of Rab5a.     

Receptor/beta-arrestin complex formation and the differential trafficking and resensitization of beta2-adrenergic and angiotensin II type 1A receptors

Beta-arrestins target G protein-coupled receptors (GPCRs) for endocytosis via clathrin-coated vesicles. Beta-arrestins also become detectable on endocytic vesicles in response to angiotensin II type 1A receptor (AT1AR), but not beta2-adrenergic receptor (beta2AR), activation. The carboxyl-terminal tails of these receptors contribute directly to this phenotype, since a beta2AR bearing the AT1AR tail acquired the capacity to stimulate beta-arrestin redistribution to endosomes, whereas this property was lost for an AT1AR bearing the beta2AR tail. Using beta2AR/AT1AR chimeras, we tested whether the beta2AR and AT1AR carboxyl-terminal tails, in part via their association with beta-arrestins, might regulate differences in the intracellular trafficking and resensitization patterns of these receptors. In the present study, we find that beta-arrestin formed a stable complex with the AT1AR tail in endocytic vesicles and that the internalization of this complex was dynamin dependent. Internalization of the beta2AR chimera bearing the AT1AR tail was observed in the absence of agonist and was inhibited by a dominant-negative beta-arrestin1 mutant. Agonist-independent AT1AR internalization was also observed after beta-arrestin2 overexpression. After internalization, the beta2AR, but not the AT1AR, was dephosphorylated and recycled back to the cell surface. However, the AT1AR tail prevented beta2AR dephosphorylation and recycling. In contrast, although the beta2AR-tail promoted AT1AR recycling, the chimeric receptor remained both phosphorylated and desensitized, suggesting that receptor dephosphorylation is not a property common to all receptors. In summary, we show that the carboxyl-terminal tails of GPCRs not only contribute to regulating the patterns of receptor desensitization, but also modulate receptor intracellular trafficking and resensitization patterns.     

N-terminal tyrosine modulation of the endocytic adaptor function of the beta-arrestins

The highly homologous beta-arrestin1 and -2 adaptor proteins play important roles in the function of G protein-coupled receptors. Either beta-arrestin variant can function as a molecular chaperone for clathrin-mediated receptor internalization. This role depends primarily upon two distinct, contiguous C-terminal beta-arrestin motifs recognizing clathrin and the beta-adaptin subunit of AP2. However, a molecular basis is lacking to explain the different endocytic efficacies of the two beta-arrestin isoforms and the observation that beta-arrestin N-terminal substitution mutants can act as dominant negative inhibitors of receptor endocytosis. Despite the near identity of the beta-arrestins throughout their N termini, sequence variability is present at a small number of residues and includes tyrosine to phenylalanine substitutions. Here we show that corresponding N-terminal (Y/F)VTL sequences in beta-arrestin1 and -2 differentially regulate mu-adaptin binding. Our results indicate that the beta-arrestin1 Tyr-54 lessens the interaction with mu-adaptin and moreover is a Src phosphorylation site. A gain of endocytic function is obtained with the beta-arrestin1 Y54F substitution, which improves both the beta-arrestin1 interaction with mu-adaptin and the ability to enhance beta2-adrenergic receptor internalization. These data indicate that beta-arrestin2 utilizes mu-adaptin as an endocytic partner, and that the inability of beta-arrestin1 to sustain a similar degree of interaction with mu-adaptin may result from coordination of Tyr-54 by neighboring residues or its modification by Src kinase. Additionally, these naturally occurring variations in beta-arrestins may also differentially regulate the composition of the signaling complexes organized on the receptor.     

Feedback regulation of beta-arrestin1 function by extracellular signal-regulated kinases.

The functions of beta-arrestin1 to facilitate clathrin-mediated endocytosis of the beta2-adrenergic receptor and to promote agonist-induced activation of extracellular signal-regulated kinases (ERK) are regulated by its phosphorylation/dephosphorylation at Ser-412. Cytoplasmic beta-arrestin1 is almost stoichiometrically phosphorylated at Ser-412. Dephosphorylation of beta-arrestin1 at the plasma membrane is required for targeting a signaling complex that includes the agonist-occupied receptors to the clathrin-coated pits. Here we demonstrate that beta-arrestin1 phosphorylation and function are modulated by an ERK-dependent negative feedback mechanism. ERK1 and ERK2 phosphorylate beta-arrestin1 at Ser-412 in vitro. Inhibition of ERK activity by a dominant-negative MEK1 mutant significantly attenuates beta-arrestin1 phosphorylation, thereby increasing the concentration of dephosphorylated beta-arrestin1. Under such conditions, beta-arrestin1-mediated beta2-adrenergic receptor internalization is enhanced as is its ability to bind clathrin. In contrast, if ERK-mediated phosphorylation is increased by transfection of a constitutively active MEK1 mutant, receptor internalization is inhibited. Our results suggest that dephosphorylated beta-arrestin1 mediates endocytosis-dependent ERK activation. Following activation, ERKs phosphorylate beta-arrestin1, thereby exerting an inhibitory feedback control of its function.     

Growth factor-induced binding of dynamin to signal transduction proteins involves sorting to distinct and separate proline-rich dynamin sequences.

Dynamin, a 100 kDa GTPase, is critical for endocytosis, synaptic transmission and neurogenesis. Endocytosis accompanies receptor processing and plays an essential role in attenuating receptor tyrosine kinase signal transduction. Dynamin has been demonstrated to be involved in the endocytic processing at the cell surface and may play a general role in coupling receptor activation to endocytosis. Src homology (SH) domain dependent protein-protein interactions are important to tyrosine kinase receptor signal transduction. The C-terminus of dynamin contains two clusters of SH3 domain binding proline motifs; these motifs may interact with known SH3 domain proteins during tyrosine kinase receptor activation. We demonstrate here that SH3 domain-containing signal transduction proteins, such as phospholipase C gamma-1 (PLC gamma-1), do indeed bind to dynamin in a growth factor inducible manner. The induction of PLC gamma-1 binding to dynamin occurs within minutes of the addition of platelet derived growth factor (PDGF) to cells. Binding of these signal transduction proteins to dynamin involves specific sorting to individual proline motif clusters and appears to be responsible for co-immunoprecipitation of tyrosine phosphorylated PDGF receptors with dynamin following PDGF stimulation of mammalian cells. The binding of dynamin to SH3 domain-containing proteins may therefore be important for formation of the protein complex required for the endocytic processing of activated tyrosine kinase receptors.     

Actin-binding protein 1 regulates B cell receptor-mediated antigen processing and presentation in response to B cell receptor activation

The BCR serves as both signal transducer and Ag transporter. Binding of Ags to the BCR induces signaling cascades and Ag processing and presentation, two essential cellular events for B cell activation. BCR-initiated signaling increases BCR-mediated Ag-processing efficiency by increasing the rate and specificity of Ag transport. Previous studies showed a critical role for the actin cytoskeleton in these two processes. In this study, we found that actin-binding protein 1 (Abp1/HIP-55/SH3P7) functioned as an actin-binding adaptor protein, coupling BCR signaling and Ag-processing pathways with the actin cytoskeleton. Gene knockout of Abp1 and overexpression of the Src homology 3 domain of Abp1 inhibited BCR-mediated Ag internalization, consequently reducing the rate of Ag transport to processing compartments and the efficiency of BCR-mediated Ag processing and presentation. BCR activation induced tyrosine phosphorylation of Abp1 and translocation of both Abp1 and dynamin 2 from the cytoplasm to plasma membrane, where they colocalized with the BCR and cortical F-actin. Mutations of the two tyrosine phosphorylation sites of Abp1 and depolymerization of the actin cytoskeleton interfered with BCR-induced Abp1 recruitment to the plasma membrane. The inhibitory effect of a dynamin proline-rich domain deletion mutant on the recruitment of Abp1 to the plasma membrane, coimmunoprecipitation of dynamin with Abp1, and coprecipitation of Abp1 with GST fusion of the dyanmin proline-rich domain demonstrate the interaction of Abp1 with dynamin 2. These results demonstrate that the BCR regulates the function of Abp1 by inducing Abp1 phosphorylation and actin cytoskeleton rearrangement, and that Abp1 facilitates BCR-mediated Ag processing by simultaneously interacting with dynamin and the actin cytoskeleton.     

G-protein-coupled receptor kinase specificity for beta-arrestin recruitment to the beta2-adrenergic receptor revealed by fluorescence resonance energy transfer

The small family of G-protein-coupled receptor kinases (GRKs) regulate cell signaling by phosphorylating heptahelical receptors, thereby promoting receptor interaction with beta-arrestins. This switches a receptor from G-protein activation to G-protein desensitization, receptor internalization, and beta-arrestin-dependent signal activation. However, the specificity of GRKs for recruiting beta-arrestins to specific receptors has not been elucidated. Here we use the beta(2)-adrenergic receptor (beta(2)AR), the archetypal nonvisual heptahelical receptor, as a model to test functional GRK specificity. We monitor endogenous GRK activity with a fluorescence resonance energy transfer assay in live cells by measuring kinetics of the interaction between the beta(2)AR and beta-arrestins. We show that beta(2)AR phosphorylation is required for high affinity beta-arrestin binding, and we use small interfering RNA silencing to show that HEK-293 and U2-OS cells use different subsets of their expressed GRKs to promote beta-arrestin recruitment, with significant GRK redundancy evident in both cell types. Surprisingly, the GRK specificity for beta-arrestin recruitment does not correlate with that for bulk receptor phosphorylation, indicating that beta-arrestin recruitment is specific for a subset of receptor phosphorylations on specific sites. Moreover, multiple members of the GRK family are able to phosphorylate the beta(2)AR and induce beta-arrestin recruitment, with their relative contributions largely determined by their relative expression levels. Because GRK isoforms vary in their regulation, this partially redundant system ensures beta-arrestin recruitment while providing the opportunity for tissue-specific regulation of the rate of beta-arrestin recruitment.     

Regulation of c-SRC activity and function by the adapter protein CAS

SRC family kinases play essential roles in a variety of cellular functions, including proliferation, survival, differentiation, and apoptosis. The activities of these kinases are regulated by intramolecular interactions and by heterologous binding partners that modulate the transition between active and inactive structural conformations. p130(CAS) (CAS) binds directly to both the SH2 and SH3 domains of c-SRC and therefore has the potential to structurally alter and activate this kinase. In this report, we demonstrate that overexpression of full-length CAS in COS-1 cells induces c-SRC-dependent tyrosine phosphorylation of multiple endogenous cellular proteins. A carboxy-terminal fragment of CAS (CAS-CT), which contains the c-SRC binding site, was sufficient to induce c-SRC-dependent protein tyrosine kinase activity, as measured by tyrosine phosphorylation of cortactin, paxillin, and, to a lesser extent, focal adhesion kinase. A single amino acid substitution located in the binding site for the SRC SH3 domain of CAS-CT disrupted CAS-CT's interaction with c-SRC and inhibited its ability to induce tyrosine phosphorylation of cortactin and paxillin. Murine C3H10T1/2 fibroblasts that expressed elevated levels of tyrosine phosphorylated CAS and c-SRC-CAS complexes exhibited an enhanced ability to form colonies in soft agar and to proliferate in the absence of serum or growth factors. CAS-CT fully substituted for CAS in mediating growth in soft agar but was less effective in promoting serum-independent growth. These data suggest that CAS plays an important role in regulating specific signaling pathways governing cell growth and/or survival, in part through its ability to interact with and modulate the activity of c-SRC.     

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.     

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.     

Phosphorylation of beta-arrestin2 regulates its function in internalization of beta(2)-adrenergic receptors

Beta-arrestins mediate agonist-dependent desensitization and internalization of G protein-coupled receptors. Previously, we have shown that phosphorylation of beta-arrestin1 by ERKs at Ser-412 regulates its association with clathrin and its function in promoting clathrin-mediated internalization of the receptor. In this paper we report that beta-arrestin2 is also phosphorylated, predominantly at residues Thr-383 and Ser-361. Isoproterenol stimulation of the beta(2)-adrenergic receptor promotes dephosphorylation of beta-arrestin2. Mutation of beta-arrestin2 phosphorylation sites to aspartic acid decreases the association of beta-arrestin2 with clathrin, thereby reducing its ability to promote internalization of the beta(2)-adrenergic receptor. Its ability to bind and desensitize the beta(2)-adrenergic receptor is, however, unaltered. These results suggest that, analogous to beta-arrestin1, phosphorylation/dephosphorylation of beta-arrestin2 regulates clathrin-mediated internalization of the beta(2)-adrenergic receptor. In contrast to beta-arrestin1, which is phosphorylated by ERK1 and ERK2, phosphorylation of beta-arrestin2 at Thr-383 is shown to be mediated by casein kinase II. Recently, it has been reported that phosphorylation of visual arrestin at Ser-366 prevents its binding to clathrin. Thus it appears that the function of all arrestin family members in mediating internalization of G protein-coupled receptors is regulated by distinct phosphorylation/dephosphorylation mechanisms.     

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.     

Agonist-dependent recruitment of phosphoinositide 3-kinase to the membrane by beta-adrenergic receptor kinase 1. A role in receptor sequestration

Agonist-dependent desensitization of the beta-adrenergic receptor requires translocation and activation of the beta-adrenergic receptor kinase1 by liberated Gbetagamma subunits. Subsequent internalization of agonist-occupied receptors occurs as a result of the binding of beta-arrestin to the phosphorylated receptor followed by interaction with the AP2 adaptor and clathrin proteins. Receptor internalization is known to require D-3 phosphoinositides that are generated by the action of phosphoinositide 3-kinase. Phosphoinositide 3-kinases form a family of lipid kinases that couple signals via receptor tyrosine kinases and G-protein-coupled receptors. The molecular mechanism by which phosphoinositide 3-kinase acts to promote beta-adrenergic receptor internalization is not well understood. In the present investigation we demonstrate a novel finding that beta-adrenergic receptor kinase 1 and phosphoinositide 3-kinase form a cytosolic complex, which leads to beta-adrenergic receptor kinase 1-mediated translocation of phosphoinositide 3-kinase to the membrane in an agonist-dependent manner. Furthermore, agonist-induced translocation of phosphoinositide 3-kinase results in rapid interaction with the receptor, which is of functional importance, since inhibition of phosphoinositide 3-kinase activity attenuates beta-adrenergic receptor sequestration. Therefore, agonist-dependent recruitment of phosphoinositide 3-kinase to the membrane is an important step in the process of receptor sequestration and links phosphoinositide 3-kinase to G-protein-coupled receptor activation and sequestration.     

Ubiquitination of beta-arrestin links seven-transmembrane receptor endocytosis and ERK activation

Beta-arrestin2 and its ubiquitination play crucial roles in both internalization and signaling of seven-transmembrane receptors (7TMRs). To understand the connection between ubiquitination and the endocytic and signaling functions of beta-arrestin, we generated a beta-arrestin2 mutant that is defective in ubiquitination (beta-arrestin2(0K)), by mutating all of the ubiquitin acceptor lysines to arginines and compared its properties with the wild type and a stably ubiquitinated beta-arrestin2-ubiquitin (Ub) chimera. In vitro translated beta-arrestin2 and beta-arrestin2(0K) displayed equivalent binding to recombinant beta(2)-adrenergic receptor (beta(2)AR) reconstituted in vesicles, whereas beta-arrestin2-Ub bound approximately 4-fold more. In cellular coimmunoprecipitation assays, beta-arrestin2(0K) bound nonreceptor partners, such as AP-2 and c-Raf and scaffolded phosphorylated ERK robustly but displayed weak binding to clathrin. Moreover, beta-arrestin2(0K) was recruited only transiently to activated receptors at the membrane, did not enhance receptor internalization, and decreased the amount of phosphorylated ERK assimilated into isolated beta(2)AR complexes. Although the wild type beta-arrestin2 formed ERK signaling complexes with the beta(2)AR at the membrane, a stably ubiquitinated beta-arrestin2-Ub chimera not only stabilized the ERK signalosomes but also led to their endosomal targeting. Interestingly, in cellular fractionation assays, the ubiquitination state of beta-arrestin2 favors its distribution in membrane fractions, suggesting that ubiquitination increases the propensity of beta-arrestin for membrane association. Our findings suggest that although beta-arrestin ubiquitination is dispensable for beta-arrestin cytosol to membrane translocation and its "constitutive" interactions with some cytosolic proteins, it nevertheless is a prerequisite both for the formation of tight complexes with 7TMRs in vivo and for membrane compartment interactions that are crucial for downstream endocytic and signaling processes.     

Cell-type-specific pathways of neurotensin endocytosis

The neurotensin receptor subtype 1 (NTS1) is a G-protein-coupled receptor (GPCR) mediating a large number of central and peripheral effects of neurotensin. Upon stimulation, NTS1 is rapidly internalized and targeted to lysosomes. This process depends on the interaction of the phosphorylated receptor with β–arrestin. Little is known about other accessory endocytic proteins potentially involved. Here, we investigated the involvement of dynamin, amphiphysin, and intersectin in the internalization of NTS1 receptor-ligand complexes in transfected COS-7 and HEK 293 cells, by using the transferrin receptor as an internal control for the constitutive endocytic pathway. We found that NTS1 endocytosis was not only arrestin–dependent, but also dynamin–dependent in both COS-7 and HEK 293 cells, whereas internalization of the transferrin receptor was independent of arrestin but required dynamin. Overexpression of the SH3 domain of amphiphysin II had no effect on receptor internalization in either cell type. By contrast, overexpression of full-length intersectin or of its SH3 domain (but not of its EH domain) inhibited NTS1 internalization in COS-7 but not in HEK 293 cells. This difference between COS-7 and HEK 293 cells was not attributable to differences in endogenous intersectin levels between the two cell lines. Indeed, the same constructs inhibited transferrin endocytosis equally well in COS-7 and HEK 293 cells. However, immunogold electron microscopy revealed that internalized NTS1 receptors were associated with clathrin-coated pits in COS-7 cells but with smooth vesicles in HEK 293 cells, suggesting that NTS1 internalization proceeds via different endocytic pathways in these two cell types. This work was supported by grants to A.B. from CIHR and FRSQ.     

Cellular distribution of constitutively active mutant parathyroid hormone (PTH)/PTH-related protein receptors and regulation of cyclic adenosine 3',5'-monophosphate signaling by beta-arrestin2

PTH promotes endocytosis of human PTH receptor 1 (PTH1Rc) by activating protein kinase C and recruiting beta-arrestin2. We examined the role of beta-arrestin2 in regulating the cellular distribution and cAMP signaling of two constitutively active PTH1Rc mutants, H223R and T410P. Overexpression of a beta-arrestin2-green fluorescent protein (GFP) conjugate in COS-7 cells inhibited constitutive cAMP accumulation by H223R and T410P in a dose-dependent manner, as well as the response to PTH of both mutant and wild-type PTH1Rcs. The cellular distribution of PTH1Rc-GFP conjugates, fluorescent ligands, and ssarrestin2-GFP was analyzed by fluorescence microscopy in HEK-293T cells. In cells expressing either receptor mutant, a ligand-independent mobilization of beta-arrestin2 to the cell membrane was observed. In the absence of ligand, H223R and wild-type PTH1Rcs were mainly localized on the cell membrane, whereas intracellular trafficking of T410P was also observed. While agonists promoted beta-arrestin2-mediated endocytosis of bot PTH1Rc mutants, antagonists were rapidly internalized only with T410P. The protein kinases inhibitor, staurosporine, significantly decreased internalization of ligand-PTH1Rc mutant complexes, although the recruitment of beta-arrestin2 to the cell membrane was unaffected. Moreover, in cells expressing a truncated wild-type PTH1Rc lacking the C-terminal cytoplasmic domain, agonists stimulated translocation of beta-arrestin2 to the cell membrane followed by ligand-receptor complex internalization without associated beta-arrestin2. In conclusion, cAMP signaling by constitutively active mutant and wild-type PTH1Rcs is inhibited by a receptor interaction with beta-arrestin2 on the cell membrane, possibly leading to uncoupling from G(s)alpha. This phenomenon is independent from protein kinases activity and the receptor C-terminal cytoplasmic domain. In addition, there are differences in the cellular localization and internalization features of constitutively active PTH1Rc mutants H223R and T410P.     

Internalization determinants of the parathyroid hormone receptor differentially regulate beta-arrestin/receptor association.

beta-Arrestins have been implicated in regulating internalization of the parathyroid hormone receptor (PTHR), but the structural features in the receptor required for this effect are unknown. In the present study performed in HEK-293 cells, we demonstrated that different topological domains of PTHR are implicated in agonist-dependent receptor internalization; truncation of the cytoplasmic tail (PTHR-TR), selective mutations of the cytoplasmic tail to remove the sites of parathyroid hormone (PTH)-stimulated phosphorylation (PTHR-PD), and mutations in the third transmembrane helix (N289A) or in the third cytoplasmic loop (K382A) resulted in a 30-60% reduction in (125)I-PTH-related protein internalization. To better define the role of these internalization determinants, we have tested the ability of these mutant PTHRs to associate with beta-arrestins by using three different methodological approaches: 1) ability of overexpression of beta-arrestins to restore the internalization of (125)I-PTH-related protein for the mutant PTHRs; 2) visualization of PTH-mediated trafficking of beta-arrestin1 and -2 fused to the green fluorescent protein with receptors by confocal microscopy; 3) quantification of beta-arrestin1-green fluorescent protein translocation by Western blot. Our data reveal that the receptor' cytoplasmic tail contains determinants of beta-arrestin interaction that are distinct from the phosphorylation sites and are sufficient for transient association of beta-arrestin2, but stable association requires receptor phosphorylation. Determinants in the receptor's core (Asn-289 and Lys-382) appear to regulate internalization of the receptor/beta-arrestin complex toward early endocytic endosomes during the initial step of endocytosis.