Tyrosine phosphorylation of the beta2 subunit of clathrin adaptor complex AP-2 reveals the role of a di-leucine motif in the epidermal growth factor receptor trafficking

Regulated secretion of mutant p53 negatively affects T lymphocytes in the tumor microenvironment

Several studies have demonstrated the role of the oncogenic mutant p53 in promoting tumor progression; however, there is limited information on the effects of secreted oncogenic mutant p53 on the tumor microenvironment and tumor immune escape. In this study, we found that secretion of mutant p53, determined by exosome content, is dependent on its N-terminal dileucine motif via its binding to β-adaptin, and inhibited by the CHK2-mediated-Ser 20 phosphorylation. Moreover, we observed that the mutant p53 caused downregulation and dysfunction of CD4+ T lymphocytes in vivo and downregulated the levels and activities of rate-limiting glycolytic enzymes in vitro. Furthermore, inhibition of mutant p53 secretion by knocking down AP1B1 or mutation of dileucine motif could reverse the quantity and function of CD4+ T lymphocytes and restrain the tumor growth. Our study demonstrates that the tumor-derived exosome-mediated secretion of oncogenic mutant p53 inhibits glycolysis to alter the immune microenvironment via functional suppression of CD4+ T cells, which may be the underlying mechanism for tumor immune escape. Therefore, targeting TDE-mediated p53 secretion may serve as a potential therapeutic target for cancer treatment.

The clathrin adaptor complex-1 and Rab12 regulate post-golgi trafficking of WT epidermal growth factor receptor (EGFR)

The epidermal growth factor receptor (EGFR) plays important roles in cancer progression and is one of the major drug targets for targeted cancer therapy. Although fundamentally important, how newly synthesized EGFR is delivered to the cell surface to perform its cellular functions remains to be further investigated. In this study, we found using the approaches of gene knockout, siRNA knockdown, streptavidin pull-down, and co-immunoprecipitation assays that the clathrin adaptor complex-1 (AP-1) and Rab12 interact with EGFR and regulate the export of EGFR out of the trans-Golgi network (TGN). In addition, the tyrosine residue at the 998 position on human EGFR is critical to bind to AP-1, and this residue is important for TGN export of EGFR. We demonstrate that AP-1 and Rab12 are important for epidermal growth factor-induced phosphorylation of EGFR, cell elongation, and proliferation, suggesting that AP-1-mediated and Rab12-mediated post-Golgi trafficking is important for EGFR signaling. Moreover, TGN export of the constitutively activated mutant form of EGFR (EGFR^L858R) is independent of AP-1 and Rab12. Our results reveal insights into the molecular mechanisms that mediate the TGN-to-cell surface delivery of EGFR and indicate that TGN export of WT EGFR and EGFR^L858R depends on different cellular factors.

Mechanism of p38 MAPK-induced EGFR endocytosis and its crosstalk with ligand-induced pathways

Ligand binding triggers clathrin-mediated and, at high ligand concentrations, clathrin-independent endocytosis of EGFR. Clathrin-mediated endocytosis (CME) of EGFR is also induced by stimuli activating p38 MAPK. Mechanisms of both ligand- and p38-induced endocytosis are not fully understood, and how these pathways intermingle when concurrently activated remains unknown. Here we dissect the mechanisms of p38-induced endocytosis using a pH-sensitive model of endogenous EGFR, which is extracellularly tagged with a fluorogen-activating protein, and propose a unifying model of the crosstalk between multiple EGFR endocytosis pathways. We found that a new locus of p38-dependent phosphorylation in EGFR is essential for the receptor dileucine motif interaction with the σ2 subunit of clathrin adaptor AP2 and concomitant receptor internalization. p38-dependent endocytosis of EGFR induced by cytokines was additive to CME induced by picomolar EGF concentrations but constrained to internalizing ligand-free EGFRs due to Grb2 recruitment by ligand-activated EGFRs. Nanomolar EGF concentrations rerouted EGFR from CME to clathrin-independent endocytosis, primarily by diminishing p38-dependent endocytosis.

Endocytosis of BRASSINOSTEROID INSENSITIVE1 Is Partly Driven by a Canonical Tyr-Based Motif

Clathrin-mediated endocytosis (CME) and its core endocytic machinery are evolutionarily conserved across all eukaryotes. In mammals, the heterotetrameric adaptor protein complex-2 (AP-2) sorts plasma membrane (PM) cargoes into vesicles via the recognition of motifs based on Tyr or di-Leu in their cytoplasmic tails. However, in plants, very little is known about how PM proteins are sorted for CME and whether similar motifs are required. In Arabidopsis (Arabidopsis thaliana), the brassinosteroid (BR) receptor BR INSENSITIVE1 (BRI1) undergoes endocytosis, which depends on clathrin and AP-2. Here, we demonstrate that BRI1 binds directly to the medium AP-2 subunit (AP2M). The cytoplasmic domain of BRI1 contains five putative canonical surface-exposed Tyr-based endocytic motifs. The Tyr-to-Phe substitution in Y₈₉₈KAI reduced BRI1 internalization without affecting its kinase activity. Consistently, plants carrying the BRI1^Y898F mutation were hypersensitive to BRs. Our study demonstrates that AP-2-dependent internalization of PM proteins via the recognition of functional Tyr motifs also operates in plants.

An internally eGFP-tagged α-adaptin is a fully functional and improved fiduciary marker for clathrin-coated pit dynamics

Clathrin mediated endocytosis (CME) has been extensively studied in living cells by quantitative total internal reflection fluorescence microscopy (TIRFM). Fluorescent protein fusions to subunits of the major coat proteins, clathrin light chains or the heterotetrameric adaptor protein (AP2) complexes, have been used as fiduciary markers of clathrin coated pits (CCPs). However, the functionality of these fusion proteins has not been rigorously compared. Here, we generated stable cells lines overexpressing mRuby‐CLCa and/or μ2‐eGFP, σ2‐eGFP, two markers currently in use, or a novel marker generated by inserting eGFP into the unstructured hinge region of the α subunit (α‐eGFP). Using biochemical and TIRFM‐based assays, we compared the functionality of the AP2 markers. All of the eGFP‐tagged subunits were efficiently incorporated into AP2 and displayed greater accuracy in image‐based CCP analyses than mRuby‐CLCa. However, overexpression of either μ2‐eGFP or σ2‐eGFP impaired transferrin receptor uptake. In addition, μ2‐eGFP reduced the rates of CCP initiation and σ2‐eGFP perturbed AP2 incorporation into CCPs and CCP maturation. In contrast, CME and CCP dynamics were unperturbed in cells overexpressing α‐eGFP. Moreover, α‐eGFP was a more sensitive and accurate marker of CCP dynamics than mRuby‐CLCa. Thus, our work establishes α‐eGFP as a robust, fully functional marker for CME.

Endocytosis in cellular uptake of drug delivery vectors: Molecular aspects in drug development

Drug delivery vectors are widely applied to increase drug efficacy while reducing the side effects and potential toxicity of a drug. They allow for patient-tailored therapy, dose titration, and therapeutic drug monitoring. A major part of drug delivery systems makes use of large nanocarriers: liposomes or virus-like particles (VLPs). These systems allow for a relatively large amount of cargo with good stability of vectors, and they offer multiple options for targeting vectors in vivo. Here we discuss endocytic pathways that are available for drug delivery by large nanocarriers. We focus on molecular aspects of the process, including an overview of potential molecular targets for studies of drug delivery vectors and for future solutions allowing targeted drug delivery.

Regulation of EGFR Endocytosis by CBL During Mitosis

The overactivation of epidermal growth factor (EGF) receptor (EGFR) is implicated in various cancers. Endocytosis plays an important role in EGFR-mediated cell signaling. We previously found that EGFR endocytosis during mitosis is mediated differently from interphase. While the regulation of EGFR endocytosis in interphase is well understood, little is known regarding the regulation of EGFR endocytosis during mitosis. Here, we found that contrary to interphase cells, mitotic EGFR endocytosis is more reliant on the activation of the E3 ligase CBL. By transfecting HeLa, MCF-7, and 293T cells with CBL siRNA or dominant-negative 70z-CBL, we found that at high EGF doses, CBL is required for EGFR endocytosis in mitotic cells, but not in interphase cells. In addition, the endocytosis of mutant EGFR Y1045F-YFP (mutation at the direct CBL binding site) is strongly delayed. The endocytosis of truncated EGFR Δ1044-YFP that does not bind to CBL is completely inhibited in mitosis. Moreover, EGF induces stronger ubiquitination of mitotic EGFR than interphase EGFR, and mitotic EGFR is trafficked to lysosomes for degradation. Furthermore, we showed that, different from interphase, low doses of EGF still stimulate EGFR endocytosis by non-clathrin mediated endocytosis (NCE) in mitosis. Contrary to interphase, CBL and the CBL-binding regions of EGFR are required for mitotic EGFR endocytosis at low doses. This is due to the mitotic ubiquitination of the EGFR even at low EGF doses. We conclude that mitotic EGFR endocytosis exclusively proceeds through CBL-mediated NCE.

Epidermal growth factor receptor: Structure-function informing the design of anticancer therapeutics

Research on the epidermal growth factor (EGF) family and the family of receptors (EGFR) has progressed rapidly in recent times. New crystal structures of the ectodomains with different ligands, the activation of the kinase domain through oligomerisation and the use of fluorescence techniques have revealed profound conformational changes on ligand binding. The control of cell signaling from the EGFR-family is complex, with heterodimerisation, ligand affinity and signaling cross-talk influencing cellular outcomes. Analysis of tissue homeostasis indicates that the control of pro-ligand processing is likely to be as important as receptor activation events. Several members of the EGFR-family are overexpressed and/or mutated in cancer cells. The perturbation of EGFR-family signaling drives the malignant phenotype of many cancers and both inhibitors and antagonists of signaling from these receptors have already produced therapeutic benefits for patients. The design of affibodies, antibodies, small molecule inhibitors and even immunotherapeutic drugs targeting the EGFR-family has yielded promising new approaches to improving outcomes for cancer patients. In this review, we describe recent discoveries which have increased our understanding of the structure and dynamics of signaling from the EGFR-family, the roles of ligand processing and receptor cross-talk. We discuss the relevance of these studies to the development of strategies for designing more effective targeted treatments for cancer patients.

Annexins-Coordinators of Cholesterol Homeostasis in Endocytic Pathways

The spatiotemporal regulation of calcium (Ca2+) storage in late endosomes (LE) and lysosomes (Lys) is increasingly recognized to influence a variety of membrane trafficking events, including endocytosis, exocytosis, and autophagy. Alterations in Ca2+ homeostasis within the LE/Lys compartment are implicated in human diseases, ranging from lysosomal storage diseases (LSDs) to neurodegeneration and cancer, and they correlate with changes in the membrane binding behaviour of Ca2+-binding proteins. This also includes Annexins (AnxA), which is a family of Ca2+-binding proteins participating in membrane traffic and tethering, microdomain organization, cytoskeleton interactions, Ca2+ signalling, and LE/Lys positioning. Although our knowledge regarding the way Annexins contribute to LE/Lys functions is still incomplete, recruitment of Annexins to LE/Lys is greatly influenced by the availability of Annexin bindings sites, including acidic phospholipids, such as phosphatidylserine (PS) and phosphatidic acid (PA), cholesterol, and phosphatidylinositol (4,5)-bisphosphate (PIP2). Moreover, the cytosolic portion of LE/Lys membrane proteins may also, directly or indirectly, determine the recruitment of Annexins to LE. Strikingly, within LE/Lys, AnxA1, A2, A6, and A8 differentially contribute to cholesterol transport along the endocytic route, in particular, cholesterol transfer between LE and other compartments, positioning Annexins at the centre of major pathways mediating cellular cholesterol homeostasis. Underlying mechanisms include the formation of membrane contact sites (MCS) and intraluminal vesicles (ILV), as well as the modulation of LE-cholesterol transporter activity. In this review, we will summarize the current understanding how Annexins contribute to influence LE/Lys membrane transport and associated functions.

The EGFR odyssey - from activation to destruction in space and time

When cell surface receptors engage their cognate ligands in the extracellular space, they become competent to transmit potent signals to the inside of the cell, thereby instigating growth, differentiation, motility and many other processes. In order to control these signals, activated receptors are endocytosed and thoroughly curated by the endosomal network of intracellular vesicles and proteolytic organelles. In this Review, we follow the epidermal growth factor (EGF) receptor (EGFR) from ligand engagement, through its voyage on endosomes and, ultimately, to its destruction in the lysosome. We focus on the spatial and temporal considerations underlying the molecular decisions that govern this complex journey and discuss how additional cellular organelles - particularly the ER - play active roles in the regulation of receptor lifespan. In summarizing the functions of relevant molecules on the endosomes and the ER, we cover the order of molecular events in receptor activation, trafficking and downregulation, and provide an overview of how signaling is controlled at the interface between these organelles.

Spatial and Temporal Regulation of Receptor Tyrosine Kinase Activation and Intracellular Signal Transduction

Epidermal growth factor (EGF) and insulin receptor tyrosine kinases (RTKs) exemplify how receptor location is coupled to signal transduction. Extracellular binding of ligands to these RTKs triggers their concentration into vesicles that bud off from the cell surface to generate intracellular signaling endosomes. On the exposed cytosolic surface of these endosomes, RTK autophosphorylation selects the downstream signaling proteins and lipids to effect growth factor and polypeptide hormone action. This selection is followed by the recruitment of protein tyrosine phosphatases that inactivate the RTKs and deliver them by membrane fusion and fission to late endosomes. Coincidentally, proteinases inside the endosome cleave the EGF and insulin ligands. Subsequent inward budding of the endosomal membrane generates multivesicular endosomes. Fusion with lysosomes then results in RTK degradation and downregulation. Through the spatial positioning of RTKs in target cells for EGF and insulin action, the temporal extent of signaling, attenuation, and downregulation is regulated.

Endocytosis of Ubiquitylation-Deficient EGFR Mutants via Clathrin-Coated Pits is Mediated by Ubiquitylation

Signaling by epidermal growth factor receptor (EGFR) is controlled by endocytosis. However, mechanisms of EGFR endocytosis remain poorly understood. Here, we found that the EGFR mutant lacking known ubiquitylation, acetylation and clathrin adaptor AP-2-binding sites (21KRΔAP2) was internalized at relatively high rates via the clathrin-dependent pathway in human duodenal adenocarcinoma HuTu-80 cells. RNA interference analysis revealed that this residual internalization is strongly inhibited by depletion of Grb2 and the E2 ubiquitin-conjugating enzyme UbcH5b/c, and partially affected by depletion of the E3 ubiquitin ligase Cbl and ubiquitin-binding adaptors, indicating that an ubiquitylation process is involved. Several new ubiquitin conjugation sites were identified by mass spectrometry in the 21KRΔAP2 mutant, suggesting that cryptic ubiquitylation may mediate endocytosis of this mutant. Total internal reflection fluorescence microscopy imaging of HuTu-80 cells transfected with labeled ubiquitin adaptor epsin1 demonstrated that the ubiquitylation-deficient EGFR mutant was endocytosed through a limited population of epsin-enriched clathrin-coated pits (CCPs), although with a prolonged CCP lifetime. Native EGFR was recruited with the same efficiency into CCPs containing either AP-2 or epsin1 that were tagged with fluorescent proteins by genome editing of MDA-MD-231 cells. We propose that two redundant mechanisms, ubiquitylation and interaction with AP-2, contribute to EGFR endocytosis via CCPs in a stochastic fashion.

Proteolytic cleavage, trafficking, and functions of nuclear receptor tyrosine kinases

Intracellular localization has been reported for over three-quarters of receptor tyrosine kinase (RTK) families in response to environmental stimuli. Internalized RTK may bind to non-canonical substrates and affect various cellular processes. Many of the intracellular RTKs exist as fragmented forms that are generated by γ-secretase cleavage of the full-length receptor, shedding, alternative splicing, or alternative translation initiation. Soluble RTK fragments are stabilized and intracellularly transported into subcellular compartments, such as the nucleus, by binding to chaperone or transcription factors, while membrane-bound RTKs (full-length or truncated) are transported from the plasma membrane to the ER through the well-established Rab- or clathrin adaptor protein-coated vesicle retrograde trafficking pathways. Subsequent nuclear transport of membrane-bound RTK may occur via two pathways, INFS or INTERNET, with the former characterized by release of receptors from the ER into the cytosol and the latter characterized by release of membrane-bound receptor from the ER into the nucleoplasm through the inner nuclear membrane. Although most non-canonical intracellular RTK signaling is related to transcriptional regulation, there may be other functions that have yet to be discovered. In this review, we summarize the proteolytic processing, intracellular trafficking and nuclear functions of RTKs, and discuss how they promote cancer progression, and their clinical implications.

A phosphotyrosine switch for cargo sequestration at clathrin-coated buds

The AP-2 clathrin adaptor complex oversees endocytic cargo selection in two parallel but independent manners. First, by physically engaging peptide-based endocytic sorting signals, a subset of clathrin-dependent transmembrane cargo is directly collected into assembling buds. Synchronously, by interacting with an assortment of clathrin-associated sorting proteins (CLASPs) that independently select different integral membrane cargo for inclusion within the incipient bud, AP-2 handles additional cargo capture indirectly. The distal platform subdomain of the AP-2 β2 subunit appendage is a privileged CLASP-binding surface that recognizes a cognate, short α-helical interaction motif. This signal, found in the CLASPs β-arrestin and the autosomal recessive hypercholesterolemia (ARH) protein, docks into an elongated groove on the β2 appendage platform. Tyr-888 is a critical constituent of this spatially confined β2 appendage contact interface and is phosphorylated in numerous high-throughput proteomic studies. We find that a phosphomimetic Y888E substitution does not interfere with incorporation of expressed β2-YFP subunit into AP-2 or alter AP-2 deposition at surface clathrin-coated structures. The Y888E mutation does not affect interactions involving the sandwich subdomain of the β2 appendage, indicating that the mutated appendage is folded and operational. However, the Y888E, but not Y888F, switch selectively uncouples interactions with ARH and β-arrestin. Phyogenetic conservation of Tyr-888 suggests that this residue can reversibly control occupancy of the β2 platform-binding site and, hence, cargo sorting.

Proteomic analysis of the epidermal growth factor receptor (EGFR) interactome and post-translational modifications associated with receptor endocytosis in response to EGF and stress

Aberrant expression, activation, and stabilization of epidermal growth factor receptor (EGFR) are causally associated with several human cancers. Post-translational modifications and protein-protein interactions directly modulate the signaling and trafficking of the EGFR. Activated EGFR is internalized by endocytosis and then either recycled back to the cell surface or degraded in the lysosome. EGFR internalization and recycling also occur in response to stresses that activate p38 MAP kinase. Mass spectrometry was applied to comprehensively analyze the phosphorylation, ubiquitination, and protein-protein interactions of wild type and endocytosis-defective EGFR variants before and after internalization in response to EGF ligand and stress. Prior to internalization, EGF-stimulated EGFR accumulated ubiquitin at 7 K residues and phosphorylation at 7 Y sites and at S(1104). Following internalization, these modifications diminished and there was an accumulation of S/T phosphorylations. EGFR internalization and many but not all of the EGF-induced S/T phosphorylations were also stimulated by anisomycin-induced cell stress, which was not associated with receptor ubiquitination or elevated Y phosphorylation. EGFR protein interactions were dramatically modulated by ligand, internalization, and stress. In response to EGF, different E3 ubiquitin ligases became maximally associated with EGFR before (CBL, HUWE1, and UBR4) or after (ITCH) internalization, whereas CBLB was distinctively most highly EGFR associated following anisomycin treatment. Adaptin subunits of AP-1 and AP-2 clathrin adaptor complexes also became EGFR associated in response to EGF and anisomycin stress. Mutations preventing EGFR phosphorylation at Y(998) or in the S(1039) region abolished or greatly reduced EGFR interactions with AP-2 and AP-1, and impaired receptor trafficking. These results provide new insight into spatial, temporal, and mechanistic aspects of EGFR regulation.

EGF receptor trafficking: consequences for signaling and cancer

•

EGF receptor endocytic traffic can regulate signaling and cell survival.

•

Signaling from activated EGFR occurs at the endosome as well as the cell surface.

•

Endocytosis can have positive and negative effects on signaling and tumorigenesis.

•

EGFR traffic promoted by antineoplastic therapy is important in tumor resistance.

The ligand-stimulated epidermal growth factor receptor (EGFR) has been extensively studied in the analysis of molecular mechanisms regulating endocytic traffic and the role of that traffic in signal transduction. Although such studies have largely focused on mitogenic signaling and dysregulated traffic in tumorigenesis, there is growing interest in the potential role of EGFR traffic in cell survival and the consequent response to cancer therapy. Here we review recent advances in our understanding of molecular mechanisms regulating ligand-stimulated EGFR activation, internalization, and post-endocytic sorting. The role of EGFR overexpression/mutation and new modulators of EGFR traffic in cancer and the response to cancer therapeutics are also discussed. Finally, we speculate on the relationship between EGFR traffic and cell survival.

Cargo recognition in clathrin-mediated endocytosis

The endosomal system is expansive and complex, characterized by swift morphological transitions, dynamic remodeling of membrane constituents, and intracellular positioning changes. To properly navigate this ever-altering membrane labyrinth, transmembrane protein cargoes typically require specific sorting signals that are decoded by components of protein coats. The best-characterized sorting process within the endosomal system is the rapid internalization of select transmembrane proteins within clathrin-coated vesicles. Endocytic signals consist of linear motifs, conformational determinants, or covalent modifications in the cytosolic domains of transmembrane cargo. These signals are interpreted by a diverse set of clathrin-associated sorting proteins (CLASPs) that translocate from the cytosol to the inner face of the plasma membrane. Signal recognition by CLASPs is highly cooperative, involving additional interactions with phospholipids, Arf GTPases, other CLASPs, and clathrin, and is regulated by large conformational changes and covalent modifications. Related sorting events occur at other endosomal sorting stations.

Endocytosis of EGFR requires its kinase activity and N-terminal transmembrane dimerization motif

EGFR signaling is attenuated by endocytosis and degradation of receptor-ligand complexes in lysosomes. Endocytosis of EGFR is known to be regulated by multiple post-translational modifications. The observation that prevention of these modifications does not block endocytosis completely, suggests the involvement of other mechanism(s). Recently, receptor clustering has been suggested to induce internalization of multiple types of membrane receptors. However, the mechanism of clustering-induced internalization remains unknown. We have used biparatopic antibody fragments from llama (VHHs) to induce EGFR clustering without stimulating tyrosine kinase activity. Using this approach, we have found an essential role for the N-terminal GG4-like dimerization motif in the transmembrane domain (TMD) for clustering-induced internalization. Moreover, conventional EGF-induced receptor internalization depends exclusively on this TMD dimerization and kinase activity. Mutations in this dimerization motif eventually lead to reduced EGFR degradation and sustained signaling. We propose a novel role for the TMD dimerization motif in the negative-feedback control of EGFR. The widely conserved nature of GG4-like dimerization motifs in transmembrane proteins suggests a general role for these motifs in clustering-induced internalization.

Advances in analysis of low signal-to-noise images link dynamin and AP2 to the functions of an endocytic checkpoint

Numerous endocytic accessory proteins (EAPs) mediate assembly and maturation of clathrin-coated pits (CCPs) into cargo-containing vesicles. Analysis of EAP function through bulk measurement of cargo uptake has been hampered due to potential redundancy among EAPs and, as we show here, the plasticity and resilience of clathrin-mediated endocytosis (CME). Instead, EAP function is best studied by uncovering the correlation between variations in EAP association to individual CCPs and the resulting variations in maturation. However, most EAPs bind to CCPs in low numbers, making the measurement of EAP association via fused fluorescent reporters highly susceptible to detection errors. Here, we present a framework for unbiased measurement of EAP recruitment to CCPs and their direct effects on CCP dynamics. We identify dynamin and the EAP-binding α-adaptin appendage domain of the AP2 adaptor as switches in a regulated, multistep maturation process and provide direct evidence for a molecular checkpoint in CME.

Endocytosis of receptor tyrosine kinases

Endocytosis is the major regulator of signaling from receptor tyrosine kinases (RTKs). The canonical model of RTK endocytosis involves rapid internalization of an RTK activated by ligand binding at the cell surface and subsequent sorting of internalized ligand-RTK complexes to lysosomes for degradation. Activation of the intrinsic tyrosine kinase activity of RTKs results in autophosphorylation, which is mechanistically coupled to the recruitment of adaptor proteins and conjugation of ubiquitin to RTKs. Ubiquitination serves to mediate interactions of RTKs with sorting machineries both at the cell surface and on endosomes. The pathways and kinetics of RTK endocytic trafficking, molecular mechanisms underlying sorting processes, and examples of deviations from the standard trafficking itinerary in the RTK family are discussed in this work.

Flotillins regulate membrane mobility of the dopamine transporter but are not required for its protein kinase C dependent endocytosis

Flotillins were proposed to mediate clathrin-independent endocytosis, and recently, flotillin-1 was implicated in the protein kinase C (PKC)-triggered endocytosis of the dopamine transporter (DAT). Since endocytosis of DAT was previously shown to be clathrin-mediated, we re-examined the role of clathrin coat proteins and flotillin in DAT endocytosis using DAT tagged with the hemagglutinin epitope (HA) in the extracellular loop and a quantitative HA antibody uptake assay. Depletion of flotillin-1, flotillin-2 or both flotillins together by small interfering RNAs (siRNAs) did not inhibit PKC-dependent internalization and degradation of HA-DAT. In contrast, siRNAs to clathrin heavy chain and μ2 subunit of clathrin adaptor complex AP-2 as well as a dynamin inhibitor Dyngo-4A significantly decreased PKC-dependent endocytosis of HA-DAT. Similarly, endocytosis and degradation of DAT that is not epitope-tagged were highly sensitive to the clathrin siRNAs and dynamin inhibition but were not affected by flotillin knockdown. Very little co-localization of DAT with flotillins was observed in cells ectopically expressing DAT and in cultured mouse dopaminergic neurons. Depletion of flotillins increased diffusion rates of HA-DAT in the plasma membrane, suggesting that flotillin-organized microdomains may regulate the lateral mobility of DAT. We propose that clathrin-mediated endocytosis is the major pathway of PKC-dependent internalization of DAT, and that flotillins may modulate functional association of DAT with plasma membrane rafts rather than mediate DAT endocytosis.

Recycling of EGFR and ErbB2 is associated with impaired Hrs tyrosine phosphorylation and decreased deubiquitination by AMSH

ErbB receptors play an important role in normal cellular growth, differentiation and development, but overexpression or poor downregulation can result in enhanced signaling and cancerous growth. ErbB signaling is terminated by clathrin-dependent receptor-mediated endocytosis, followed by incorporation in multi-vesicular bodies and subsequent degradation in lysosomes. In contrast to EGFR, ErbB2 displays poor ligand-induced downregulation and enhanced recycling, but the molecular mechanisms underlying this difference are poorly understood. Given our previous observation that both EGFR and an EGFR-ErbB2 chimera undergo Cbl-mediated K63-polyubiquitination, we investigated in the present study whether activation of the EGFR and the EGFR-ErbB2 chimera is associated with tyrosine phosphorylation of the ESCRT-0 complex subunit Hrs and AMSH-mediated deubiquitination. EGF stimulation of the EGFR resulted in efficient Hrs tyrosine phosphorylation and deubiquitination by the K63-polyubiquitin chain-specific deubiquitinating enzyme AMSH. In contrast, EGF activation of EGFR-ErbB2 showed significantly decreased Hrs tyrosine phosphorylation and deubiquitination by AMSH. To test whether this phenotype is the result of endosomal recycling, we induced recycling of the EGFR by stimulation with TGFα. Indeed, even though TGFα-stimulation of EGFR is associated with efficient ligand-stimulated K63-polyubiquitination, we observed that Hrs tyrosine phosphorylation as well as AMSH-mediated deubiquitination is significantly reduced under these conditions. Using various EGFR-ErbB2 chimeras, we demonstrate that enhanced recycling, decreased Hrs tyrosine phosphorylation and decreased AMSH mediated deubiquitination of EGFR-ErbB2 chimeras is primarily due to the presence of ErbB2 sequences or the absence of EGFR sequences C-terminal to the Cbl binding site. We conclude that endosomal recycling of the EGFR and ErbB2 receptors is associated with significantly impaired tyrosine phosphorylation of the ESCRT-0 subunit Hrs as well as decreased deubiquitination by AMSH, which is consistent with the finding that recycling receptors are not efficiently incorporated in the MVB pathway.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

ERBB2 is a target for USP8-mediated deubiquitination

Overexpression and poor downregulation of ErbB receptor tyrosine kinases are associated with enhanced signaling and tumorigenesis. Attenuation of EGF-receptor (EGFR) signaling is mediated by endocytosis and ubiquitination by the E3-ligase Cbl. En route to lysosomes, but before incorporation of the EGFR into internal vesicles of MVBs, the EGFR undergoes Usp8-mediated deubiquitination. ErbB2 displays enhanced recycling back to the cell surface, and therefore we hypothesized that Usp8 is not part of the ErbB2 trafficking pathway. Here, we demonstrate, in the context of a chimeric EGFR-ErbB2 receptor, that (i) EGF induces pY1091 Cbl binding site-dependent K63-polyubiquitination of EGFR-ErbB2, (ii) Cbl is tyrosine phosphorylated upon stimulation of EGFR-ErbB2 wt and Y1091F mutant receptor, (iii) EGF-induced activation of EGFR-ErbB2 induces Usp8 tyrosine phosphorylation, and (iv) ubiquitination of the EGFR-ErbB2 wt and Y1091F mutant is enhanced upon coexpression of catalytically inactive Usp8-C748A in the presence and absence of EGF. We further show that Usp8 tyrosine phosphorylation upon stimulation of EGFR-ErbB2 is (a) independent of Y1091, (b) dependent on Src- and EGFR-ErbB2-kinase activity, (c) enhanced upon coexpression of Usp8-C748A, and (d) partly dependent on the Microtubule Interacting and Transport (MIT) domain of Usp8. Our findings demonstrate that Usp8 is part of the ErbB2 endosomal trafficking pathway.

Multiple mechanisms collectively regulate clathrin-mediated endocytosis of the epidermal growth factor receptor

Four independent mechanisms for uptake of activated EGFR are identified by a combination of receptor mutagenesis and RNA interference approaches.

Endocytosis of the epidermal growth factor receptor (EGFR) is important for the regulation of EGFR signaling. However, EGFR endocytosis mechanisms are poorly understood, which precludes development of approaches to specifically inhibit EGFR endocytosis and analyze its impact on signaling. Using a combination of receptor mutagenesis and RNA interference, we demonstrate that clathrin-dependent internalization of activated EGFR is regulated by four mechanisms, which function in a redundant and cooperative fashion. These mechanisms involve ubiquitination of the receptor kinase domain, the clathrin adaptor complex AP-2, the Grb2 adaptor protein, and three C-terminal lysine residues (K1155, K1158, and K1164), which are acetylated, a novel posttranslational modification for the EGFR. Based on these findings, the first internalization-defective EGFR mutant with functional kinase and normal tyrosine phosphorylation was generated. Analysis of the signaling kinetics of this mutant revealed that EGFR internalization is required for the sustained activation of protein kinase B/AKT but not for the activation of mitogen-activated protein kinase.

Cellular signalling: Peptide hormones and growth factors

Peptide hormones and growth factors initiate signalling by binding to and activating their cell surface receptors. The activated receptors interact with and modulate the activity of cell surface enzymes and adaptor proteins which entrain a series of reactions leading to metabolic and proliferative signals. Rapid internalization of ligand-receptor complexes into the endosomal system both prolongs and augments events initiated at the cell surface. In addition endocytosis brings activated receptors into contact with a wider range of substrates giving rise to unique signalling events critical for modulating proliferation and apoptosis. Within the endosomal system, receptor function is regulated by lowering vacuolar pH, augmenting ligand proteolysis and promoting receptor kinase dephosphorylation. Ubiquitination-deubiquitination plays a key role in regulating receptor traffic through the endosomal system resulting in either recycling to the cell surface or degradation in multivesicular-lysosomal elements. From a clinical perspective there are several studies showing that manipulating endosomal processes may constitute a new therapeutic strategy.

Internalization and intracellular sorting of the EGF receptor: a model for understanding the mechanisms of receptor trafficking

The epidermal growth factor receptor (EGFR; also known as ErbB1) is one of four related receptor tyrosine kinases. These receptors (EGFR, ErbB2, ErbB3 and ErbB4) are frequently overexpressed in cancer and such overexpression is associated with poor clinical outcome. Understanding the mechanisms involved in growth-factor-receptor downregulation is medically important, as several drugs that interfere with the function and trafficking of ErbB proteins are currently being developed or are already in clinical trials. EGFR has become a model protein for understanding the biology and endocytosis of related growth-factor receptors, and the mechanisms involved in its endocytosis and degradation have been scrutinized for several decades. Nevertheless, the details and principles of these processes are still poorly understood and often controversial. In particular, the literature describing how the ubiquitylation and recruitment of EGFR to clathrin-coated pits are connected is inconsistent and confusing. In this Opinion article, we discuss the impact of signaling motifs, kinase activity and ubiquitylation on clathrin-dependent endocytosis and lysosomal sorting of EGFR. In addition, we discuss potential explanations for contradicting reports, and propose models for the recruitment of ligand-activated EGFR to clathrin-coated pits as well as for lysosomal sorting of ligand-activated EGFR.

Tickets to ride: selecting cargo for clathrin-regulated internalization

Clathrin-mediated endocytosis oversees the constitutive packaging of selected membrane cargoes into transport vesicles that fuse with early endosomes. The process is responsive to activation of signalling receptors and ion channels, promptly clearing post-translationally tagged forms of cargo off the plasma membrane. To accommodate the diverse array of transmembrane proteins that are variably gathered into forming vesicles, a dedicated sorting machinery cooperates to ensure that non-competitive uptake from the cell surface occurs within minutes. Recent structural and functional data reveal remarkable plasticity in how disparate sorting signals are recognized by cargo-selective clathrin adaptors, such as AP-2. Cargo loading also seems to govern whether coats ultimately bud or dismantle abortively at the cell surface.

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.

RNA interference screen identifies Usp18 as a regulator of epidermal growth factor receptor synthesis

Elevated expression of epidermal growth factor receptor (EGFR) contributes to the progression of many types of cancer. Therefore, we developed a high-throughput screen to identify proteins that regulate the levels of EGFR in squamous cell carcinoma. Knocking down various ubiquitination-related genes with small interfering RNAs led to the identification of several novel genes involved in this process. One of these genes, Usp18, is a member of the ubiquitin-specific protease family. We found that knockdown of Usp18 in several cell lines reduced expression levels of EGFR by 50-80%, whereas the levels of other receptor tyrosine kinases remained unchanged. Overexpression of Usp18 elevated EGFR levels in a manner requiring the catalytic cysteine of Usp18. Analysis of metabolically radiolabeled cells showed that the rate of EGFR protein synthesis was reduced up to fourfold in the absence of Usp18. Interestingly, this dramatic reduction occurred despite no change in the levels of EGFR mRNA. This suggests that depletion of Usp18 inhibited EGFR mRNA translation. In fact, this inhibition required the presence of native 5' and 3' untranslated region sequences on EGFR mRNA. Together, our data provide evidence for the novel mechanism of EGFR regulation at the translational step of receptor synthesis.

ErbB2 and ErbB4 Cbl binding sites can functionally replace the ErbB1 Cbl binding site

Poor downregulation of ErbB receptors is associated with enhanced downstream signaling and tumorigenesis. It has been suggested that poor downregulation of ErbB-2, -3 and -4 receptors when compared to ErbB1 is due to decreased recruitment of Cbl E3 ligase proteins. However, a highly conserved Cbl binding site is not only present in ErbB1/EGFR (FLQRpY(1045)SSDP), but also in ErbB2 (PLQRpY(1091)SEDP) and ErbB4 (STQRpY(1103)SADP). We therefore replaced the ErbB1 Cbl binding site by that of ErbB2 and ErbB4. Whereas retrovirally infected NIH3T3 cells containing the EGFR Y1045F mutation showed dramatically impaired Cbl recruitment, EGFR ubiquitination and delayed EGFR degradation, replacement of the EGFR Cbl binding site by that of ErbB2 or ErbB4 did not affect Cbl recruitment, receptor-ubiquitination, -degradation, -downregulation or ligand degradation. We conclude that poor downregulation of ErbB2 and ErbB4 receptors is not due to sequence variations in the Cbl binding site of these receptors.

PTPRR protein tyrosine phosphatase isoforms and locomotion of vesicles and mice

Protein tyrosine phosphatases (PTPs) are central players in many different cellular processes and their aberrant activity is associated with multiple human pathologies. In this review, we present current knowledge on the PTPRR subfamily of classical PTPs that is expressed in neuronal cells and comprises receptor-type (PTPBR7, PTP-SL) as well as cytosolic (PTPPBSγ-37, PTPPBSγ-42) isoforms. The two receptor-type isoforms PTPBR7 and PTP-SL both localize in late endosomes and the Golgi area. PTPBR7, however, is additionally localized at the cell surface and on early endosomes. During cerebellar maturation, PTPBR7 expression in developing Purkinje cells ceases and is replaced by PTP-SL expression in the mature Purkinje cells. All PTPRR isoforms contain a kinase interacting motif that makes them mitogen-activated protein kinase phosphatases. The distinct subcellular localization of the different PTPRR isoforms may reflect differential roles in growth-factor-induced MAPK-mediated retrograde signaling cascades. Studies in PTPRR-deficient mice established that PTPRR isoforms are physiological regulators of MAPK phosphorylation levels. Surprisingly, PTPRR-deficient mice display defects in motor coordination and balancing skills, while cerebellar morphological abnormalities, which are often encountered in ataxic mouse models, are absent. This is reminiscent of the phenotype observed in a handful of mouse mutants that have alterations in cerebellar calcium ion homeostasis. Elucidation of the molecular mechanisms by which PTPRR deficiency imposes impairment of cerebellar neurons and motor coordination may provide candidate molecules for hereditary cerebellar ataxias that still await identification of the corresponding disease genes.

A structural explanation for the binding of endocytic dileucine motifs by the AP2 complex

Most transmembrane proteins are selected as transport vesicle cargo through the recognition of short, linear amino acid motifs in their cytoplasmic portions by vesicle coat proteins. In the case of clathrin-coated vesicles (CCVs) the motifs are recognised by clathrin adaptors. The AP2 adaptor complex (subunits α,β2,μ2,σ2) recognises both major endocytic motifs: YxxΦ motifs 1 and [DE]xxxL[LI] acidic dileucine motifs. Here we describe the binding of AP2 to the endocytic dileucine motif from CD4 2 . The major recognition events are the two leucine residues binding in hydrophobic pockets on σ2. The hydrophilic residue four residues upstream from the first leucine sits on a positively charged patch made from residues on σ2 and α subunits. Mutations in key residues inhibit the binding of AP2 to ‘acidic dileucine’ motifs displayed in liposomes containing PtdIns4,5P2, but do not affect binding to YxxΦ motifs via μ2. In the ‘inactive’ AP2 core structure 3 , both motif binding sites are blocked by different parts of the β2 subunit. To allow a dileucine motif to bind, the β2 N-terminus is displaced and becomes disordered; however, in this structure the YxxΦ binding site on μ2 remains blocked.

The AP-2 adaptor beta2 appendage scaffolds alternate cargo endocytosis

The independently folded appendages of the large alpha and beta2 subunits of the endocytic adaptor protein (AP)-2 complex coordinate proper assembly and operation of endocytic components during clathrin-mediated endocytosis. The beta2 subunit appendage contains a common binding site for beta-arrestin or the autosomal recessive hypercholesterolemia (ARH) protein. To determine the importance of this interaction surface in living cells, we used small interfering RNA-based gene silencing. The effect of extinguishing beta2 subunit expression on the internalization of transferrin is considerably weaker than an AP-2 alpha subunit knockdown. We show the mild sorting defect is due to fortuitous substitution of the beta2 chain with the closely related endogenous beta1 subunit of the AP-1 adaptor complex. Simultaneous silencing of both beta1 and beta2 subunit transcripts recapitulates the strong alpha subunit RNA interference (RNAi) phenotype and results in loss of ARH from endocytic clathrin coats. An RNAi-insensitive beta2-yellow fluorescent protein (YFP) expressed in the beta1 + beta2-silenced background restores cellular AP-2 levels, robust transferrin internalization, and ARH colocalization with cell surface clathrin. The importance of the beta appendage platform subdomain over clathrin for precise deposition of ARH at clathrin assembly zones is revealed by a beta2-YFP with a disrupted ARH binding interface, which does not restore ARH colocalization with clathrin. We also show a beta-arrestin 1 mutant, which engages coated structures in the absence of any G protein-coupled receptor stimulation, colocalizes with beta2-YFP and clathrin even in the absence of an operational clathrin binding sequence. These findings argue against ARH and beta-arrestin binding to a site upon the beta2 appendage platform that is later obstructed by polymerized clathrin. We conclude that ARH and beta-arrestin depend on a privileged beta2 appendage site for proper cargo recruitment to clathrin bud sites.

ARAP1 regulates endocytosis of EGFR

Signaling through the EGF receptor is regulated by endocytosis. ARAP1 is a protein with Arf guanosine triphosphatase-activating protein (GAP) and Rho GAP domains. We investigated the role of ARAP1 in EGF receptor endocytic trafficking. Following EGF treatment of cells, ARAP1 rapidly and transiently associated with the edge of the cell and punctate structures containing Rab5, rabaptin 5 and EGFR but not early embryonic antigen 1 (EEA1). EGF associated with the ARAP1-positive punctate structures prior to EEA1-positive early endosomes. Recruitment of ARAP1 to the punctate structures required active Rab5 and an additional signal from EGFR. Decreasing ARAP1 levels with small interfering RNA accelerated association of EGF with EEA1 endosomes and degradation of EGFR. Phosphorylation of extracellular-signal-regulated kinase (ERK) and c-Jun-amino-terminal kinase (JNK) was diminished and more transient in cells with reduced levels of ARAP1 than in controls. Based on these findings, we propose that ARAP1 regulates the endocytic traffic of EGFR and, consequently, the rate of EGFR signal attenuation.

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.

Phospholipase C-epsilon augments epidermal growth factor-dependent cell growth by inhibiting epidermal growth factor receptor down-regulation

The down-regulation of the epidermal growth factor (EGF) receptor is critical for the termination of EGF-dependent signaling, and the dysregulation of this process can lead to oncogenesis. In the present study, we suggest a novel mechanism for the regulation of EGF receptor down-regulation by phospholipase C-epsilon. The overexpression of PLC-epsilon led to an increase in receptor recycling and decreased the down-regulation of the EGF receptor in COS-7 cells. Adaptor protein complex 2 (AP2) was identified as a novel binding protein that associates with the PLC-epsilon RA2 domain independently of Ras. The interaction of PLC-epsilon with AP2 was responsible for the suppression of EGF receptor down-regulation, since a perturbation in this interaction abolished this effect. Enhanced EGF receptor stability by PLC-epsilon led to the potentiation of EGF-dependent growth in COS-7 cells. Finally, the knockdown of PLC-epsilon in mouse embryo fibroblast cells elicited a severe defect in EGF-dependent growth. Our results indicated that PLC-epsilon could promote EGF-dependent cell growth by suppressing receptor down-regulation.

Identification of EGF receptor C-terminal sequences 1005–1017 and di-leucine motif 1010LL1011 as essential in EGF receptor endocytosis

Most studies regarding the role of epidermal growth factor (EGF) receptor (EGFR) C-terminal domain in EGFR internalization are done in the context of EGFR kinase activation. We recently showed that EGF-induced EGFR internalization is directly controlled by receptor dimerization, rather than kinase activation. Here we studied the role of EGFR C-terminus in EGF-induced EGFR internalization with or without EGFR kinase activation. We showed that graduate truncation of EGFR from C-terminus to 1044 did not affect EGF-induced EGFR endocytosis with or without kinase activation. However, truncation to 991 or further completely inhibited EGFR endocytosis. Graduate truncation within 991-1044 progressively lower EGF-induced EGFR endocytosis with most significant effects observed for residues 1005-1017. The endocytosis patterns of mutant EGFRs are independent of EGFR kinase activation. The residues 1005-1017 were also required for EGFR internalization triggered by non-ligand-induced receptor dimerization. This indicates that residues 1005-1017 function as an internalization motif, rather than a dimerization motif, to mediate EGFR internalization. Furthermore, we showed that the di-leucine motif 1010LL1011 within this region is essential in mediating EGF-induced rapid EGFR internalization independent of kinase activation. We conclude that EGFR C-terminal sequences 1005-1017 and the 1010LL1011 motif are essential for EGF-induced EGFR endoytosis independent of EGFR kinase activation and autophosphorylation.

A di-leucine sorting signal in ZIP1 (SLC39A1) mediates endocytosis of the protein

It has been demonstrated that the plasma membrane expression of ZIP1 is regulated by endocytic mechanisms. In the zinc-replete condition, the level of surface expressed ZIP1 is low due to the rapid internalization of ZIP1. The present study aimed to identify a sorting signal(s) in ZIP1 that mediated endocytosis of ZIP1. Four potential sorting signals (three di-leucine-and one tyrosine-based) were found by searching the eukaryotic linear motif resource for functional sites in proteins (http://elm.eu.org). Site-directed mutagenesis and immunofluorescence microscopic analyses demonstrated that the di-leucine sorting signal, ETRALL144-149, located in the variable loop region of ZIP1, was required for the ZIP1 internalization and lysosomal degradation. Substitutions of alanines for the di-leucine residues (LL148,149/AA) severely impaired the internalization of ZIP1 and subsequent protein degradation, leading to an accumulation of the mutant ZIP1 on the cell surface, as well as inside the cell. Using chimeric proteins composed of an alpha-chain of interleukin-2 receptor fused to the peptides derived from the variable loop region of ZIP1, we found that the di-leucine sorting signal of ZIP1 was required and sufficient for endocytosis of the chimeric proteins.

Clathrin-Coated Pits: Vive La Différence?

Because of the discovery of coated pits and vesicles more than 40 years ago and the identification of clathrin as a major component of the coat, it has been assumed that clathrin-coated pits (CCPs) are responsible for the uptake of most plasma membrane receptors undergoing internalization. The recent molecular characterization of clathrin-independent routes of endocytosis confirms that several alternative endocytic pathways operate at the plasma membrane of mammalian cells. This heterogeneous view of endocytosis has been expanded still further by recent studies, suggesting that different subpopulations of CCPs responsible for the internalization of specific sets of cargo may coexist. In the present review, we have discussed the experimental evidence in favor or against the existence of distinct parallel clathrin-dependent pathways at the plasma membrane.

UBPY-mediated Epidermal Growth Factor Receptor (EGFR) De-ubiquitination Promotes EGFR Degradation

Whereas poly-ubiquitination targets protein substrates for proteasomal degradation, mono-ubiquitination is known to regulate protein trafficking in the endosomal system and to target cargo proteins for lysosomal degradation. The role of the de-ubiquitinating enzymes AMSH and UBPY in endosomal trafficking of cargo proteins such as the epidermal growth factor receptor (EGFR) has only very recently been the subject of study and is already a matter of debate. Although one report (Mizuno, E., Iura, T., Mukai, A., Yoshimori, T., Kitamura, N., and Komada, M. (2005) Mol. Biol. Cell 16, 5163-5174) concludes that UBPY negatively regulates EGFR degradation by de-ubiquitinating the EGFR on endosomes, another report (Row, P. E., Prior, I. A., McCullough, J., Clague, M. J., and Urbe, S. (2006) J. Biol. Chem. 281, 12618-12624) concludes that UBPY-mediated EGFR de-ubiquitination is essential for EGFR degradation. Here, we demonstrate that Usp8/UBPY, the mammalian ortholog of budding yeast Ubp4/Doa4, constitutively co-precipitates in a bivalent manner with the EGFR. Moreover, UBPY is a substrate for Src-family tyrosine kinases that are activated after ligand-induced EGFR activation. Using overexpression of three different recombinant dominant negative UBPY mutants (UBPY C748A mutant, UBPY 1-505, and UBPY 640-1080) in NIH3T3 and HEK293 cells, we demonstrate that UBPY affects both constitutive and ligand-induced (i) EGFR ubiquitination, (ii) EGFR expression levels, and (iii) the appearance of intermediate EGFR degradation products as well as (iv) downstream mitogen-activated protein kinase signal transduction. Our findings provide further evidence in favor of the model that UBPY-mediated EGFR de-ubiquitination promotes EGFR degradation.

Epidermal growth factor-dependent phosphorylation of the GGA3 adaptor protein regulates its recruitment to membranes

The Golgi-localized, Gamma-ear-containing, Arf-binding (GGA) proteins are monomeric clathrin adaptors that mediate the sorting of transmembrane cargo at the trans-Golgi network and endosomes. Here we report that one of these proteins, GGA3, becomes transiently phosphorylated upon activation of the epidermal growth factor (EGF) receptor. This phosphorylation takes place on a previously unrecognized site in the "hinge" segment of the protein, S368, and is strictly dependent on the constitutive phosphorylation of another site, S372. The EGF-induced phosphorylation of S368 does not require internalization of the EGF receptor or association of GGA3 with membranes. This phosphorylation can be blocked by inhibitors of both the mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways that function downstream of the activated EGF receptor. Phosphorylation of GGA3 on S368 causes an increase in the hydrodynamic radius of the protein, indicating a transition to a more asymmetric shape. Mutation of S368 and S372 to a phosphomimic aspartate residue decreases the association of GGA3 with membranes. These observations indicate that EGF signaling elicits phosphorylation events that regulate the association of GGA3 with organellar membranes.

Binding of AP-2 adaptor complex to brain membrane is regulated by phosphorylation of proteins

Phosphorylation of proteins appears as a key process in early steps of clathrin coated vesicle formation. Here, we report that treatment of post-nuclear fraction with alkaline phosphatase induced redistribution of alpha subunits of AP-2 adaptor complex to cytosol and this effect was higher in the alpha2 subunit. A high serine phosphorylation status of alpha subunits correlated with the higher affinity of AP-2 to membranes. Using a simple binding assay, where membranes were incubated with either purified adaptors or cytosols, we observed an inhibitory effect of tyrphostin, a tyrosine kinase inhibitor, on the binding of AP-2 to membranes, but also an unexpected decrease induced by the phosphatase inhibitor cyclosporine. We also show an inhibitory effect of ATP mediated by cytosolic proteins, although it could not be related to the phosphorylation of AP-2, suggesting an action upstream a cascade of phosphorylations that participate in the regulation of the assembly of AP-2 to membranes.

Growth factor receptor binding protein 2-mediated recruitment of the RING domain of Cbl to the epidermal growth factor receptor is essential and sufficient to support receptor endocytosis

Knockdown of growth factor receptor binding protein 2 (Grb2) by RNA interference strongly inhibits clathrin-mediated endocytosis of the epidermal growth factor receptor (EGFR). To gain insights into the function of Grb2 in EGFR endocytosis, we have generated cell lines in which endogenous Grb2 was replaced by yellow fluorescent protein (YFP)-tagged Grb2 expressed at the physiological level. In these cells, Grb2-YFP fully reversed the inhibitory effect of Grb2 knockdown on EGFR endocytosis and, moreover, trafficked together with EGFR during endocytosis. Overexpression of Grb2-binding protein c-Cbl did not restore endocytosis in Grb2-depleted cells. However, EGFR endocytosis was rescued in Grb2-depleted cells by chimeric proteins consisting of the Src homology (SH) 2 domain of Grb2 fused to c-Cbl. The "knockdown and rescue" analysis revealed that the expression of Cbl-Grb2/SH2 fusions containing RING finger domain of Cbl restores normal ubiquitylation and internalization of the EGFR in the absence of Grb2, consistent with the important role of the RING domain in EGFR endocytosis. In contrast, the carboxy-terminal domain of Cbl, when attached to Grb2 SH2 domain, had 4 times smaller endocytosis-rescue effect compared with the RING-containing chimeras. Together, the data suggest that the interaction of Cbl carboxy terminus with CIN85 has a minor and a redundant role in EGFR internalization. We concluded that Grb2-mediated recruitment of the functional RING domain of Cbl to the EGFR is essential and sufficient to support receptor endocytosis.

Cargo recognition during clathrin-mediated endocytosis: a team effort

Transmembrane proteins destined to endosomes are selectively accumulated in clathrin-coated pits at the plasma membrane and rapidly internalized in clathrin-coated vesicles. The recognition of specific sequence motifs in transmembrane cargo by coated-pit proteins confers specificity on the endocytic process. Interaction of membrane cargo with the clathrin adaptor protein complex AP-2 is the major mechanism of cargo sorting into coated pits in mammalian cells. Recent studies have revealed a variety of alternative mechanisms of cargo recruitment involving additional adaptor proteins. These alternative mechanisms appear to be particularly important during clathrin-mediated endocytosis of signaling receptors.

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.