Association and colocalization of Eps15 with adaptor protein-2 and clathrin

Recruitment of activated G protein-coupled receptors to pre-existing clathrin-coated pits in living cells

The process of clathrin-mediated endocytosis tightly regulates signaling of the superfamily of seven-transmembrane G protein-coupled receptors (GPCRs). A fundamental question in the cell biology of membrane receptor endocytosis is whether activated receptors can initiate the formation of clathrin-coated pits (CPs) or whether they are simply mobilized to pre-existing CPs. Here, using various approaches, including a dynamic assay to monitor the distribution of CPs and GPCR-beta-arrestin complexes in live HeLa cells, we demonstrate for the first time that activated GPCRs do not initiate the de novo formation of CPs but instead are targeted to pre-existing CPs.

Checkpoints for vesicular traffic?

During interphase the transport of material between different intracellular organelles requires accurate regulation of fusiogenic domains. Recent studies on hepatic endosomes indicated that compartmentalized Cdk2 – cyclin E complexes act by braking fusion events. These Cdk2 complexes integrate tyrosine phosphorylation and dephosphory lation inputs, resulting in the control of the number of rounds of fusion at discrete domains. This leads to changes in the intracellular location of internalized receptors and ultimately their biological response.Key words: vesicular traffic, Cdk2, receptors tyrosine kinases.

c-Cbl ubiquitinates the EGF receptor at the plasma membrane and remains receptor associated throughout the endocytic route

Cbl family members have an evolutionarily conserved role in attenuating receptor tyrosine kinase function. Their negative regulatory capacity depends on a Ring finger domain that interacts with ubiquitin conjugating enzymes. Cbl molecules constitute a novel type of E3 or ubiquitin ligase family that is recruited to phosphotyrosine motifs. Ubiquitination of the receptor system is coupled to its downregulation, but it is unclear at which point in the endocytic pathway Cbl molecules come into play. Using low temperature and a dynamin mutant, we find that c-Cbl associates with and ubiquitinates the activated epidermal growth factor (EGF) receptor at the plasma membrane in the absence of endocytosis. With the aid of confocal microscopy and immunogold electron microscopy, we could demonstrate that c-Cbl associates with the EGF receptor at the plasma membrane prior to receptor recruitment into clathrin-coated pits and remains associated throughout the clathrin-mediated endocytic pathway. c-Cbl and the EGF receptor also colocalize in internal vesicles of multivesicular endosomes. Our data are consistent with a role for c-Cbl in clathrin-mediated endocytosis of tyrosine kinase receptors, as well as their intracellular sorting.

Shc mediates ligand-induced internalization of epidermal growth factor receptors

In order to clarify the physiological relevance of the interaction between Shc and adaptins, components of plasma membrane-coated pit adaptor complex AP2, we investigated the role of Shc in ligand-induced endocytosis of epidermal growth factor (EGF) receptors. In vitro peptide binding assay showed that alpha-adaptin bound to the wild-type peptide corresponding to amino acids 346-355 of Shc, RDLFDMKPFE, but not to the mutant peptide in which both phenylalanines at 349 and 354 were substituted for alanines (FA). Using adenovirus vectors carrying a herpes simplex virus epitope-tagged 52-kDa wild-type Shc and Shc FA, we examined the interaction between Shc, AP2, and EGF receptors in intact cells. Alpha-adaptin bound to wild-type Shc in an EGF-dependent manner, whereas EGF-dependent association of alpha-adaptin with Shc FA was markedly reduced. In addition, EGF increased the amount of alpha-adaptin coprecipitated with EGF receptors in cells expressing wild-type Shc but not Shc FA. These results suggest that EGF stimulates Shc-AP2 complex formation and association of Shc-AP2 complexes with EGF receptors. Internalization assay showed that (125)I-EGF internalization was reduced in cells overexpressing Shc FA. Immunofluorescence study showed that punctate staining along the plasma membrane border as well as punctate pattern characteristic of cytoplasmic vesicles near the plasma membrane was enhanced in cells expressing wild-type Shc. These results suggest, therefore, the implication of Shc in ligand-induced endocytosis of EGF receptors in intact cells.

Molecular requirements for the internalisation step of endocytosis: insights from yeast

Molecular genetic studies of endocytosis using the unicellular eukaryote Saccharomyces cerevisiae (budding yeast) have led to the identification of many cellular components, both proteins and lipids, required for this process. While initially, many of these requirements (e.g. for actin, various actin-associated proteins, the ubiquitin conjugation system, and for ergosterol and sphingolipids) appeared to differ from known requirements for endocytosis in higher eukaryotes (e.g. clathrin, AP-2, dynamin), it now seems that endocytosis in higher and lower eukaryotes share many requirements. Often, what were initially identified as actin cytoskeleton-associated proteins in S. cerevisiae, are now revealing themselves as clathrin-coated pit- and vesicle-associated proteins in higher eukaryotes. So rather than delineating two endocytic pathways, one actin-based and one clathrin-based, the combined studies on higher and lower eukaryotes are revealing interesting interplay in both systems between the actin cytoskeleton, clathrin coats, and lipids in the formation of endocytic vesicles at the plasma membrane. Recent results from the yeast system show that the Arp2/3p complex, Wiskott-Aldrich syndrome protein (WASP), and WASP-interacting protein (WIP), proteins involved in the nucleation step of actin filament assembly, play a major role in the formation of endocytic vesicles. This discovery suggests models whereby endocytic vesicles may be actively pushed from the plasma membrane and into the cell by newly forming and rapidly extending actin filaments.

Modification of AMPA receptor clustering regulates cerebellar synaptic plasticity

Cerebellar long-term depression (LTD) induced at parallel fiber-Purkinje neuron synapses is proposed to underlie certain types of motor learning. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, which mediate chemical transmission in these synapses, are clustered on the postsynaptic membrane. By increasing local density of the receptors, clustering is believed to increase synaptic efficacy. This article focuses on molecular mechanisms regulating the synaptic AMPA receptor clustering in Purkinje cells, which could underlie the expression of cerebellar LTD. Synaptic AMPA receptor clusters in dendritic spines of Purkinje cells are disrupted upon protein kinase C (PKC)-mediated phosphorylation of serine 880 in the C-terminal domain of GluR2. Phosphorylation of this residue causes significant reduction in the affinity of GluR2 C-terminal tail for glutamate receptor interacting protein (GRIP), a molecule known to be crucial for AMPA receptor clustering. Consequently, AMPA receptors on the synaptic membrane are destabilized and internalized by endocytosis. Based on these findings, a model for the expression of cerebellar LTD is proposed, in which a decrease in the number of postsynaptic AMPA receptors, initiated by phosphorylation of GluR2 serine 880, is the major mechanism underlying cerebellar LTD.

NGF signals through TrkA to increase clathrin at the plasma membrane and enhance clathrin-mediated membrane trafficking

Neurotrophin (NT) signals may be moved from axon terminals to neuron cell bodies via signaling endosomes-organelles in which NTs continue to be bound to their activated receptors. Suggesting that clathrin-coated membranes serve as one source of signaling endosomes, in earlier studies we showed that nerve growth factor (NGF) treatment increased clathrin at the plasma membrane and resulted in colocalization of clathrin with TrkA, the receptor tyrosine kinase for NGF. Strikingly, however, we also noted that most clathrin puncta at the surface of NGF-treated cells did not colocalize with TrkA, raising the possibility that NGF induces a general increase in clathrin-coated membrane formation. To explore this possibility further, we examined the distribution of clathrin in NGF- and BDNF-treated cells. NGF signaling in PC12 cells robustly redistributed the adaptor protein AP2 and the clathrin heavy chain (CHC) to surface membranes. Using confocal and epifluorescence microscopy, as well as biochemical assays, we showed the redistribution of clathrin to be attributable to the activation of TrkA. Significantly, NGF signaled through TrkA to induce an increase in clathrin-mediated membrane trafficking, as revealed in the increased endocytosis of transferrin. In that BDNF treatment increased AP2 and clathrin at the surface membranes of hippocampal neurons, these findings may represent a physiologically significant response to NTs. We conclude that NT signaling increases clathrin-coated membrane formation and clathrin-mediated membrane trafficking and speculate that this effect contributes to their trophic actions via the increased internalization of receptors and other proteins that are present in clathrin-coated membranes.

Disabled-2 colocalizes with the LDLR in clathrin-coated pits and interacts with AP-2

Disabled-2 (Dab2) is a widely expressed relative of Disabled-1, a neuron-specific signal-transduction protein that binds to and receives signals from members of the low-density lipoprotein receptor (LDLR) family. Members of the LDLR family internalize through clathrin-coated pits and vesicles to endosomes, from where they return to the cell surface through the secretory pathway. In this study, we show that the Dab2 phosphotyrosine-binding domain binds peptides containing the sequence FXN-PXY. This core sequence is found in the intracellular domains of LDLR family members and is important for receptor internalization. Dab2 transiently colocalizes with the LDLR in clathrin-coated pits, but is absent from endosomes and lysosomes. Dab2 is alternatively spliced and its localization depends on a region of the protein that contains two DPF motifs that are present in the p96 Dab2 protein and absent in the p67 splice variant. This region is sufficient to confer Dab2 binding to the alpha-adaptin subunit of the clathrin adaptor protein, AP-2. Overexpression of p96 but not of p67 Dab2 disrupts the localization of AP-2. These findings suggest that in addition to previously reported signal-transduction functions, Dab2 could also act as an adaptor protein that may regulate protein trafficking.

Hydrogen peroxide reversibly inhibits epidermal growth factor (EGF) receptor internalization and coincident ubiquitination of the EGF receptor and Eps15

Recently, we demonstrated that hydrogen peroxide (H2O2) inhibits the internalization of the epidermal growth factor (EGF) receptor and the EGF-induced mono-ubiquitination of EGF receptor pathway substrate clone #15 (Eps15) in fibroblasts. In addition, it was suggested that EGF receptor internalization might be inhibited by H2O2 by inhibition of ubiquitination of proteins involved in endocytosis. Here, we show that H2O2 also inhibits the poly-ubiquitination of the EGF receptor in fibroblasts. Furthermore, recovery of the cells resulted in re-establishment of ubiquitination of both the EGF receptor and Eps15 and coincided with restoration of internalization of those receptors that had bound EGF in the presence of H2O2. In addition, EGF receptor internalization was inhibited by the sulphydryl reagent N-ethylmaleimide (NEM), indicating that intact SH groups might be required for receptor-mediated endocytosis. Furthermore, H2O2 rapidly induced an increase in the cellular ratio of GSSG:GSH (oxidized glutathione:reduced glutathione) and removal of H2O2 resulted in a fast restoration of the ratio of GSSG:GSH. Therefore, these results suggest a relation between the inhibition of internalization ubiquitination and an increase in GSSG:GSH ratio, which strengthens the hypothesis that H2O2 inhibits EGF receptor internalization by an inhibition of ubiquitination of proteins involved in EGF receptor-mediated endocytosis.

Epidermal growth factor and membrane trafficking. EGF receptor activation of endocytosis requires Rab5a

Activated epidermal growth factor receptors recruit various intracellular proteins leading to signal generation and endocytic trafficking. Although activated receptors are rapidly internalized into the endocytic compartment and subsequently degraded in lysosomes, the linkage between signaling and endocytosis is not well understood. Here we show that EGF stimulation of NR6 cells induces a specific, rapid and transient activation of Rab5a. EGF also enhanced translocation of the Rab5 effector, early endosomal autoantigen 1 (EEA1), from cytosol to membrane. The activation of endocytosis, fluid phase and receptor mediated, by EGF was enhanced by Rab5a expression, but not by Rab5b, Rab5c, or Rab5a truncated at the NH(2) and/or COOH terminus. Dominant negative Rab5a (Rab5:N34) blocked EGF-stimulated receptor-mediated and fluid-phase endocytosis. EGF activation of Rab5a function was dependent on tyrosine residues in the COOH-terminal domain of the EGF receptor (EGFR). Removal of the entire COOH terminus by truncation (c'973 and c'991) abrogated ligand-induced Rab5a activation of endocytosis. A "kinase-dead" EGFR failed to stimulate Rab5a function. However, another EGF receptor mutant (c'1000), with the kinase domain intact and a single autophosphorylation site effectively signaled Rab5 activation. These results indicate that EGFR and Rab5a are linked via a cascade that results in the activation of Rab5a and that appears essential for internalization. The results point to an interdependent relationship between receptor activation, signal generation and endocytosis.

NGF signals through TrkA to increase clathrin at the plasma membrane and enhance clathrin-mediated membrane trafficking

Neurotrophin (NT) signals may be moved from axon terminals to neuron cell bodies via signaling endosomes-organelles in which NTs continue to be bound to their activated receptors. Suggesting that clathrin-coated membranes serve as one source of signaling endosomes, in earlier studies we showed that nerve growth factor (NGF) treatment increased clathrin at the plasma membrane and resulted in colocalization of clathrin with TrkA, the receptor tyrosine kinase for NGF. Strikingly, however, we also noted that most clathrin puncta at the surface of NGF-treated cells did not colocalize with TrkA, raising the possibility that NGF induces a general increase in clathrin-coated membrane formation. To explore this possibility further, we examined the distribution of clathrin in NGF- and BDNF-treated cells. NGF signaling in PC12 cells robustly redistributed the adaptor protein AP2 and the clathrin heavy chain (CHC) to surface membranes. Using confocal and epifluorescence microscopy, as well as biochemical assays, we showed the redistribution of clathrin to be attributable to the activation of TrkA. Significantly, NGF signaled through TrkA to induce an increase in clathrin-mediated membrane trafficking, as revealed in the increased endocytosis of transferrin. In that BDNF treatment increased AP2 and clathrin at the surface membranes of hippocampal neurons, these findings may represent a physiologically significant response to NTs. We conclude that NT signaling increases clathrin-coated membrane formation and clathrin-mediated membrane trafficking and speculate that this effect contributes to their trophic actions via the increased internalization of receptors and other proteins that are present in clathrin-coated membranes.

Disruption of AMPA receptor GluR2 clusters following long-term depression induction in cerebellar Purkinje neurons

Cerebellar long-term depression (LTD) is thought to play an important role in certain types of motor learning. However, the molecular mechanisms underlying this event have not been clarified. Here, using cultured Purkinje cells, we show that stimulations inducing cerebellar LTD cause phosphorylation of Ser880 in the intracellular C-terminal domain of the AMPA receptor subunit GluR2. This phosphorylation is accompanied by both a reduction in the affinity of GluR2 to glutamate receptor interacting protein (GRIP), a molecule known to be critical for AMPA receptor clustering, and a significant disruption of postsynaptic GluR2 clusters. Moreover, GluR2 protein released from GRIP is shown to be internalized. These results suggest that the dissociation of postsynaptic GluR2 clusters and subsequent internalization of the receptor protein, initiated by the phosphorylation of Ser880, are the mechanisms underlying the induction of cerebellar LTD.

Characterization of Cdk2-cyclin E complexes in plasma membrane and endosomes of liver parenchyma. Insulin-dependent regulation

Rat liver parenchyma Golgi/endosomes fractions harbor a tyrosine-phosphorylated 34-kDa protein. Screening of Golgi, endosomes (ENs), plasmalemma (PM), and cytosolic (Cyt) fractions revealed the presence of the mitotic kinase Cdk2 in ENs, PM, and Cyt. The fluid phase endocytic marker horseradish peroxidase gained access to the endosomal Cdk2, confirming its localization. Cdk2 was shown to be associated to cyclin E and was active in ENs and PM fractions. The administration of a single dose of insulin (1.5 microgram/100 g, body weight) induced a time-dependent activation of the insulin receptor kinase in these structures. Insulin receptor-kinase activation was followed by the inhibition of immunoprecipitated Cdk2-cyclin E kinase activity in PM and the progressive disappearance of cyclin E. In marked contrast, no such effect was observed in ENs. The injection of a phosphotyrosyl phosphatase inhibitor (bpV(phen)) increased the levels of cyclin E in ENs and PM. A massive recruitment of p27(kip1) was observed in the Cdk2-cyclin E complexes isolated from PM and Cyt but not from ENs. In vitro, Cdk2-cyclin E complexes have the capacity to inhibit the formation of hybrid structures containing horseradish peroxidase and radioiodinated epidermal growth factor. Therefore, in the PM and ENs of adult rat liver, an active and regulated pool of the mitotic kinase Cdk2-cyclin E and some yet to be defined effectors are present. Cdk2 may contribute to the modulation of transport events and/or maintenance of the topology of endocytic elements.

The function of the Drosophila fat facets deubiquitinating enzyme in limiting photoreceptor cell number is intimately associated with endocytosis

Fat facets is a deubiquitinating enzyme required in a cell communication pathway that limits to eight the number of photoreceptor cells in each facet of the Drososphila compound eye. Genetic data support a model whereby Faf removes ubiquitin, a polypeptide tag for protein degradation, from a specific ubiquitinated protein thus preventing its degradation. Here, mutations in the liquid facets gene were identified as dominant enhancers of the fat facets mutant eye phenotype. The liquid facets locus encodes epsin, a vertebrate protein associated with the clathrin endocytosis complex. The results of genetic experiments reveal that fat facets and liquid facets facilitate endocytosis and function in common cells to generate an inhibitory signal that prevents ectopic photoreceptor determination. Moreover, it is demonstrated that the fat facets mutant phenotype is extraordinarily sensitive to the level of liquid facets expression. We propose that Liquid facets is a candidate for the critical substrate of Fat facets in the eye.

Regulation of AMPA receptor-mediated synaptic transmission by clathrin-dependent receptor internalization

Redistribution of postsynaptic AMPA- (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-) subtype glutamate receptors may regulate synaptic strength at glutamatergic synapses, but the mediation of the redistribution is poorly understood. We show that AMPA receptors underwent clathrin-dependent endocytosis, which was accelerated by insulin in a GluR2 subunit-dependent manner. Insulin-stimulated endocytosis rapidly decreased AMPA receptor numbers in the plasma membrane, resulting in long-term depression (LTD) of AMPA receptor-mediated synaptic transmission in hippocampal CA1 neurons. Moreover, insulin-induced LTD and low-frequency stimulation-(LFS-) induced homosynaptic CA1 LTD were found to be mutually occlusive and were both blocked by inhibiting postsynaptic clathrin-mediated endocytosis. Thus, controlling postsynaptic receptor numbers through endocytosis may be an important mechanism underlying synaptic plasticity in the mammalian CNS.

Mapping of Eps15 domains involved in its targeting to clathrin-coated pits

Clathrin-coated pit (CCP) formation occurs as a result of the targeting and assembly of cytosolic coat proteins, mainly the plasma membrane clathrin-associated protein complex (AP-2) and clathrin, to the intracellular face of the plasma membrane. In the present study, the mechanisms by which Eps15, an AP-2-binding protein, is targeted to CCPs was analyzed by following the intracellular localization of Eps15 mutants fused to the green fluorescent protein. Our previous results indicated that the N-terminal Eps15 homology (EH) domains are required for CCP targeting. We now show that EH domains are, however, not sufficient for targeting to CCPs. Similarly, neither the central coiled-coil nor the C-terminal AP-2 binding domains were able to address green fluorescent protein to CCPs. Thus, targeting of Eps15 to CCPs likely results from the collaboration between EH domains and another domain of the protein. An Eps15 mutant lacking the coiled-coil domain localized to CCPs showing that Eps15 dimerization is not strictly required. In contrast, Eps15 mutants lacking all AP-2 binding sites showed a dramatic decrease in plasma membrane staining, showing that AP-2 binding sites, together with EH domains, play an important role in targeting Eps15 into CCPs. Finally, the effect of the Eps15 mutants on clathrin-dependent endocytosis was tested by both immunofluorescence and flow cytometry. The results obtained showed that inhibition of transferrin uptake was observed only with mutants able to interfere with CCP assembly.

The internalization and endosomal trafficking of the EGF receptor in response to EGF is delayed in the waved-2 mouse liver

The EGF receptor in waved-2 mice contains a point mutation that renders it kinase-deficient. We investigated how the waved-2 mutation affects the internalization and endosomal trafficking of the receptor in vivo in response to EGF. When the waved-2 mice were injected with EGF, there was approximately 50% less tyrosine phosphorylation detected in whole-liver homogenate compared to wild-type background mice. Although EGF increased the EGF receptor levels in the early and late liver endosomal fractions of waved-2 mice, its trafficking was delayed compared to wild-type mice. Ubiquitination of the EGF receptor may affect its endosomal sorting. We found that a similar amount of EGF receptor was immunoprecipitated from the endosomal fractions of EGF-treated waved-2 and wild-type with anti-ubiquitin antibody. These results demonstrate that the waved-2 EGF receptor can become ubiquitinated and can be trafficked to the late endosomes, although it appears that its kinase deficiency delays this process.

The evolutionarily conserved N-terminal region of Cbl is sufficient to enhance down-regulation of the epidermal growth factor receptor

The mammalian proto-oncoprotein Cbl and its homologues in Caenorhabditis elegans and Drosophila are evolutionarily conserved negative regulators of the epidermal growth factor receptor (EGF-R). Overexpression of wild-type Cbl enhances down-regulation of activated EGF-R from the cell surface. We report that the Cbl tyrosine kinase-binding (TKB) domain is essential for this activity. Whereas wild-type Cbl enhanced ligand-dependent EGF-R ubiquitination, down-regulation from the cell surface, accumulation in intracellular vesicles, and degradation, a Cbl TKB domain-inactivated mutant (G306E) did not. Furthermore, the transforming truncation mutant Cbl-N (residues 1-357), comprising only the Cbl TKB domain, functioned as a dominant negative protein. It colocalized with EGF-R in intracellular vesicular structures, yet it suppressed down-regulation of EGF-R from the surface of cells expressing endogenous wild-type Cbl. Therefore, Cbl-mediated down-regulation of EGF-R requires the integrity of both the N-terminal TKB domain and additional C-terminal sequences. A Cbl truncation mutant comprising amino acids 1-440 functioned like wild-type Cbl in down-regulation assays. This mutant includes the evolutionarily conserved TKB and RING finger domains but lacks the less conserved C-terminal sequences. We conclude that the evolutionarily conserved N terminus of Cbl is sufficient to effect enhancement of EGF-R ubiquitination and down-regulation from the cell surface.

Hydrogen peroxide inhibits epidermal growth factor receptor internalization in human fibroblasts

Several cellular signal transduction cascades are affected by oxidative stress. In this study, the effect of hydrogen peroxide (H2O2) on the endocytosis of the epidermal growth factor (EGF) receptor was investigated. Exposure of HER14 cells to H2O2 resulted in a concentration-dependent inhibition of EGF receptor internalization. Binding studies demonstrated that this H2O2-induced inhibition in internalization was not due to altered binding of EGF to its receptor. Addition of H2O2 at different time points during internalization showed that EGF receptor internalization was rapidly reduced, suggesting that one of the first steps in the internalization process is inhibited. In addition, H2O2 inhibited the internalization of a different receptor, the chicken hepatic lectin receptor, in a concentration-dependent manner as well. Treatment of cells with another inducer of oxidative stress, cumene hydroperoxide, also resulted in a decreased internalization. Finally, we showed that H2O2 inhibited EGF-induced mono-ubiquitination of the EGF receptor pathway substrate clone 15, a process that normally occurs during EGF receptor endocytosis. These results clearly show that oxidative stress interferes with EGF signaling.

Coat proteins regulating membrane traffic

This review focuses on the roles of coat proteins in regulating the membrane traffic of eukaryotic cells. Coat proteins are recruited to the donor organelle membrane from a cytosolic pool by specific small GTP-binding proteins and are required for the budding of coated vesicles. This review first describes the four types of coat complexes that have been characterized so far: clathrin and its adaptors, the adaptor-related AP-3 complex, COPI, and COPII. It then discusses the ascribed functions of coat proteins in vesicular transport, including the physical deformation of the membrane into a bud, the selection of cargo, and the targeting of the budded vesicle. It also mentions how the coat proteins may function in an alternative model for transport, namely via tubular connections, and how traffic is regulated. Finally, this review outlines the evidence that related coat proteins may regulate other steps of membrane traffic.

Role for Drs2p, a P-type ATPase and potential aminophospholipid translocase, in yeast late Golgi function

ADP-ribosylation factor appears to regulate the budding of both COPI and clathrin-coated transport vesicles from Golgi membranes. An arf1Delta synthetic lethal screen identified SWA3/DRS2, which encodes an integral membrane P-type ATPase and potential aminophospholipid translocase (or flippase). The drs2 null allele is also synthetically lethal with clathrin heavy chain (chc1) temperature-sensitive alleles, but not with mutations in COPI subunits or other SEC genes tested. Consistent with these genetic analyses, we found that the drs2Delta mutant exhibits late Golgi defects that may result from a loss of clathrin function at this compartment. These include a defect in the Kex2-dependent processing of pro-alpha-factor and the accumulation of abnormal Golgi cisternae. Moreover, we observed a marked reduction in clathrin-coated vesicles that can be isolated from the drs2Delta cells. Subcellular fractionation and immunofluorescence analysis indicate that Drs2p localizes to late Golgi membranes containing Kex2p. These observations indicate a novel role for a P-type ATPase in late Golgi function and suggest a possible link between membrane asymmetry and clathrin function at the Golgi complex.

The EH network

The EH domain is an evolutionary conserved protein-protein interaction domain present in a growing number of proteins from yeast to mammals. Even though the domain was discovered just 5 years ago, a great deal has been learned regarding its three-dimensional structure and binding specificities. Moreover, a number of cellular ligands of the domain have been identified and demonstrated to define a complex network of protein-protein interactions in the eukaryotic cell. Interestingly, many of the EH-containing and EH-binding proteins display characteristics of endocytic "accessory" proteins, suggesting that the principal function of the EH network is to regulate various steps in endocytosis. In addition, recent evidence suggests that the EH network might work as an "integrator" of signals controlling cellular pathways as diverse as endocytosis, nucleocytosolic export, and ultimately cell proliferation.

The N terminus of amphiphysin II mediates dimerization and plasma membrane targeting

Amphiphysin I and II are nerve terminal-enriched proteins containing SH3 domains that interact with dynamin and synaptojanin. The amphiphysins may function in synaptic vesicle endocytosis by targeting synaptojanin and dynamin to emerging endocytic buds through SH3 domain-independent interactions with clathrin and AP2. We have recently identified and cloned several amphiphysin II splice variants that differentially incorporate clathrin-binding domains. To determine whether these domains function in membrane targeting, we used immunofluorescence to examine the potential localization of amphiphysin II variants to clathrin-coated pits on plasma membranes purified from transfected COS-7 cells. Full-length amphiphysin II targets to the plasma membrane where it partially co-localizes with clathrin. However, splice variants and deletion constructs lacking clathrin-binding domains still target to the plasma membrane, and removal of clathrin from the membrane does not affect amphiphysin II distribution. Surprisingly, plasma membrane targeting was dependent on the presence of a 31-amino acid alternatively spliced sequence at the N terminus of amphiphysin II, a result confirmed using subcellular fractionation. In binding assays, the 31-amino acid sequence was also found to facilitate amphiphysin dimerization mediated through the N terminus. Taken together, these data support a role for the N terminus of amphiphysin II in membrane targeting during endocytosis.

Biogenesis of transport intermediates in the endocytic pathway

Evidence is accumulating that membrane traffic between organelles can be achieved by different types of intermediates. Small (< 100 nm) and short-lived vesicles mediate transport from the plasma membrane or the trans-Golgi network to endosomes, and formation of these vesicles depends on specific adapter complexes. In contrast, transport from early to late endosomes is achieved by relatively large (approximately 0.5 microm), long-lived and multivesicular intermediates, and their biogenesis depends on endosomal COP-I proteins. Here, we review recent work on the formation of these different transport intermediates, and we discuss, in particular, coat proteins, sorting signals contained in cargo molecules and the emerging role of lipid in vesicle biogenesis.

Splice variants of intersectin are components of the endocytic machinery in neurons and nonneuronal cells

We recently identified and cloned intersectin, a protein containing two Eps15 homology (EH) domains and five Src homology 3 (SH3) domains. Using a newly developed intersectin antibody, we demonstrate that endogenous COS-7 cell intersectin localizes to clathrin-coated pits, and transfection studies suggest that the EH domains may direct this localization. Through alternative splicing in a stop codon, a long form of intersectin is generated with a C-terminal extension containing Dbl homology (DH), pleckstrin homology (PH), and C2 domains. Western blots reveal that the long form of intersectin is expressed specifically in neurons, whereas the short isoform is expressed at lower levels in glia and other nonneuronal cells. Immunofluorescence analysis of cultured hippocampal neurons reveals that intersectin is found at the plasma membrane where it is co-localized with clathrin. Ibp2, a protein identified based on its interactions with the EH domains of intersectin, binds to clathrin through the N terminus of the heavy chain, suggesting a mechanism for the localization of intersectin at clathrin-coated pits. Ibp2 also binds to the clathrin adaptor AP2, and antibodies against intersectin co-immunoprecipitate clathrin, AP2, and dynamin from brain extracts. These data suggest that the long and short forms of intersectin are components of the endocytic machinery in neurons and nonneuronal cells.

The ubiquitin-proteasome system and endocytosis

Internalization of membrane proteins has been studied for more than three decades without solving all the underlying mechanisms. Our knowledge of clathrin-mediated endocytosis is certainly sufficient to understand the basic principles. However, more detailed insight is required to recognize why different proteins enter clathrin-coated pits with different rates and affinities. In addition to clathrin coat components, at least two adaptor systems and even more accessory proteins have been described to preselect membrane proteins before they can enter cells. Recent experimental data have identified the ubiquitin-proteasome system as a regulatory system for endocytosis. This system is well-known for its basic regulatory function in protein degradation, and controls a magnitude of key events. The ubiquitin-proteasome system is now identified as a regulator of the endocytosis of selected membrane proteins. In this review, we will discuss the complexity and implications of this mechanism for receptor-mediated endocytosis.

Inhibition of clathrin-coated pit assembly by an Eps15 mutant

Recent data have shown that Eps15, a newly identified component of clathrin-coated pits constitutively associated with the AP-2 complex, is required for receptor-mediated endocytosis. However, its precise function remains unknown. Interestingly, Eps15 contains three EH (Eps15-Homology) domains also found in proteins required for the internalization step of endocytosis in yeast. Results presented here show that EH domains are required for correct coated pit targeting of Eps15. Furthermore, when cells expressed an Eps15 mutant lacking EH domains, the plasma membrane punctate distribution of both AP-2 and clathrin was lost, implying the absence of coated pits. This was further confirmed by the fact that dynamin, a GTPase found in coated pits, was homogeneously redistributed on the plasma membrane and that endocytosis of transferrin, a specific marker of clathrin-dependent endocytosis, was strongly inhibited. Altogether, these results strongly suggest a role for Eps15 in coated pit assembly and more precisely a role for Eps15 in the docking of AP-2 onto the plasma membrane. This hypothesis is supported by the fact that a GFP fusion protein encoding the ear domain of (alpha)-adaptin, the AP-2 binding site for Eps15, was efficiently targeted to plasma membrane coated pits.

The downregulation of CD4 and MHC-I by primate lentiviruses: a paradigm for the modulation of cell surface receptors

The human and simian immunodeficiency viruses (HIV and SIV) downregulate the cell surface expression of CD4, their primary receptor, and of class I histocompatibility complex (MHC-I), a critical mediator of immune recognition. While the first of these effects seems important to preserve viral infectivity, the second likely promotes immune evasion. Three HIV-1 proteins, Nef, Env and Vpu, contribute to downregulate CD4, Env forms a complex with CD4 in the endoplasmic reticulum, thereby retaining the receptor in this compartment. Nef and Vpu, on the other hand, act as connectors between CD4 and specific intracellular trafficking pathways, targeting the receptor for degradation in the lysosome and the proteasome, respectively. Some of the downstream partners of the viral proteins in these events have been identified, and include the adaptor complex of clathrin-coated pits, the beta subunit of COP-I coatomer, and the ubiquitin pathway-related h-beta TrCP protein. HIV-induced MHC-I downregulation, mostly the effect of Nef, also reflects a redistribution of this receptor, with its accumulation in the Golgi. The modalities of this process, however, are as yet imperfectly understood. New evidence indicates that the mechanisms employed by primate lentiviruses to downmodulate CD4 and MHC-I are also exploited by a number of cellular regulatory processes.

The interaction of epsin and Eps15 with the clathrin adaptor AP-2 is inhibited by mitotic phosphorylation and enhanced by stimulation-dependent dephosphorylation in nerve terminals

Clathrin-mediated endocytosis was shown to be arrested in mitosis due to a block in the invagination of clathrin-coated pits. A Xenopus mitotic phosphoprotein, MP90, is very similar to an abundant mammalian nerve terminal protein, epsin, which binds the Eps15 homology (EH) domain of Eps15 and the alpha-adaptin subunit of the clathrin adaptor AP-2. We show here that both rat epsin and Eps15 are mitotic phosphoproteins and that their mitotic phosphorylation inhibits binding to the appendage domain of alpha-adaptin. Both epsin and Eps15, like other cytosolic components of the synaptic vesicle endocytic machinery, undergo constitutive phosphorylation and depolarization-dependent dephosphorylation in nerve terminals. Furthermore, their binding to AP-2 in brain extracts is enhanced by dephosphorylation. Epsin together with Eps15 was proposed to assist the clathrin coat in its dynamic rearrangements during the invagination/fission reactions. Their mitotic phosphorylation may be one of the mechanisms by which the invagination of clathrin-coated pits is blocked in mitosis and their stimulation-dependent dephosphorylation at synapses may contribute to the compensatory burst of endocytosis after a secretory stimulus.

Ras caught in another affair: the exchange factors for Ral

The Ral guanine nucleotide exchange factors are direct targets of Ras, providing a mechanism for Ral activation by extracellular signals. In addition, Ral can be activated by a Ras-independent pathway. Ral guanine nucleotide exchange factors contribute to cellular transformation induced by oncogenic Ras through an Erk-independent mechanism which may involve activation of transcription.

Clathrin, adaptors and eps15 in endosomes containing activated epidermal growth factor receptors

Activation of the epidermal growth factor receptor (EGFR) by EGF results in binding of clathrin adaptor protein complex AP-2 to the receptor cytoplasmic tail. The transient interaction with AP-2 is thought to be responsible for the selective recruitment of the EGFR into coated pits during endocytosis. In this study we found that EGF-induced EGFR/AP-2 association, measured by co-immunoprecipitation, persists after receptor internalization. Double-label immunofluorescence of EGF-treated A-431 and COS-1 cells revealed the presence of AP-2, clathrin and eps15, another component of the plasma membrane coated pits, in the large perinuclear endosomes loaded with EGFRs. By optical sectioning and image deconvolution, the immunoreactivities were seen to be distributed within vesicular and tubular elements of these endosomes. In addition, these compartments contained the transferrin receptors and a EEA.1 protein, markers of early endosomes. Furthermore, Golgi clathrin adaptor complex AP-1 was found in EGFR-containing endosomes and EGFR immunoprecipitates in A-431 cells. The direct interaction of the EGFR with micro1 as well as micro2 subunits of AP-1 and AP-2, correspondingly, was shown using the yeast two-hybrid assay. Brefeldin A, a drug that releases AP-1 from the trans-Golgi membranes, had no effect on AP-1 association with endosomes and its co-precipitation with EGFR. Taken together, the data suggest that endosomal EGFR-AP complexes make up a significant portion of the total amount of these complexes detectable by co-immunoprecipitation. It can be proposed that APs are capable of binding to the endosomal membrane via a mechanism that requires AP interaction with the intracellular tails of multimeric receptors like activated EGFR, which in turn allows recruitment of clathrin and eps15. The hypothesis that the competition between adaptor complexes for binding to the receptor tails in endosomes may regulate of the sorting of receptors is discussed.

Eps15 is recruited to the plasma membrane upon epidermal growth factor receptor activation and localizes to components of the endocytic pathway during receptor internalization

Eps15 is a substrate for the tyrosine kinase of the epidermal growth factor receptor (EGFR) and is characterized by the presence of a novel protein:protein interaction domain, the EH domain. Eps15 also stably binds the clathrin adaptor protein complex AP-2. Previous work demonstrated an essential role for eps15 in receptor-mediated endocytosis. In this study we show that, upon activation of the EGFR kinase, eps15 undergoes dramatic relocalization consisting of 1) initial relocalization to the plasma membrane and 2) subsequent colocalization with the EGFR in various intracellular compartments of the endocytic pathway, with the notable exclusion of coated vesicles. Relocalization of eps15 is independent of its binding to the EGFR or of binding of the receptor to AP-2. Furthermore, eps15 appears to undergo tyrosine phosphorylation both at the plasma membrane and in a nocodazole-sensitive compartment, suggesting sustained phosphorylation in endocytic compartments. Our results are consistent with a model in which eps15 undergoes cycles of association:dissociation with membranes and suggest multiple roles for this protein in the endocytic pathway.

Endocytosis: EH domains lend a hand

A number of proteins that have been implicated in endocytosis feature a conserved protein-interaction module known as an EH domain. The three-dimensional structure of an EH domain has recently been solved, and is likely to presage significant advances in understanding molecular mechanisms of endocytosis.

Identification of a novel ubiquitin conjugation motif, required for ligand-induced internalization of the growth hormone receptor

In addition to its role in selective protein degradation, the conjugation of ubiquitin to proteins has also been implicated in the internalization of plasma membrane proteins, including the alpha-factor receptor Ste2p, uracil permease Fur4p, epithelial sodium channel ENaC and the growth hormone receptor (GHR). Binding of GH to its receptor induces receptor dimerization, resulting in the activation of signal transduction pathways and an increase of GHR ubiquitination. Previously, we have shown that the ubiquitin conjugation system mediates GH-induced GHR internalization. Here, we present evidence that a specific domain of the GHR regulates receptor endocytosis via the ubiquitin conjugation system. This ubiquitin-dependent endocytosis (UbE) motif consists of the amino acid sequence DSWVEFIELD and is homologous to sequences in other proteins, several of which are known to be ubiquitinated. In addition, we show that GH internalization by a truncated GHR is independent of the presence of lysine residues in the cytosolic domain of this receptor, while internalization still depends on an intact ubiquitin conjugation system. Thus, GHR internalization requires the recruitment of the ubiquitin conjugation system to the GHR UbE motif rather than the conjugation of ubiquitin to the GHR itself.

Intersectin, a novel adaptor protein with two Eps15 homology and five Src homology 3 domains

We screened a Xenopus laevis oocyte cDNA expression library with a Src homology 3 (SH3) class II peptide ligand and identified a 1270-amino acid-long protein containing two Eps15 homology (EH) domains, a central coiled-coil region, and five SH3 domains. We named this protein Intersectin, because it potentially brings together EH and SH3 domain-binding proteins into a macromolecular complex. The ligand preference of the EH domains were deduced to be asparajine-proline-phenylalanine (NPF) or cyclized NPF (CX1-2NPFXXC), depending on the type of phage-displayed combinatorial peptide library used. Screens of a mouse embryo cDNA library with the EH domains of Intersectin yielded clones for the Rev-associated binding/Rev-interacting protein (RAB/Rip) and two novel proteins, which we named Intersectin-binding proteins (Ibps) 1 and 2. All three proteins contain internal and C-terminal NPF peptide sequences, and Ibp1 and Ibp2 also contain putative clathrin-binding sites. Deletion of the C-terminal sequence, NPFL-COOH, from RAB/Rip eliminated EH domain binding, whereas fusion of the same peptide sequence to glutathione S-transferase generated strong binding to the EH domains of Intersectin. Several experiments support the conclusion that the free carboxylate group contributes to binding of the NPFL motif at the C terminus of RAB/Rip to the EH domains of Intersectin. Finally, affinity selection experiments with the SH3 domains of Intersectin identified two endocytic proteins, dynamin and synaptojanin, as potential interacting proteins. We propose that Intersectin is a component of the endocytic machinery.

Recognition specificity of individual EH domains of mammals and yeast

The Eps homology (EH) domain is a recently described protein binding module that is found, in multiple or single copies, in several proteins in species as diverse as human and yeast. In this work, we have investigated the molecular details of recognition specificity mediated by this domain family by characterizing the peptide-binding preference of 11 different EH domains from mammal and yeast proteins. Ten of the eleven EH domains could bind at least some peptides containing an Asn-Pro-Phe (NPF) motif. By contrast, the first EH domain of End3p preferentially binds peptides containing an His-Thr/Ser-Phe (HT/SF) motif. Domains that have a low affinity for the majority of NPF peptides reveal some affinity for a third class of peptides that contains two consecutive amino acids with aromatic side chains (FW or WW). This is the case for the third EH domain of Eps15 and for the two N-terminal domains of YBL47c. The consensus sequences derived from the peptides selected from phage-displayed peptide libraries allows for grouping of EH domains into families that are characterized by different NPF-context preference. Finally, comparison of the primary sequence of EH domains with similar or divergent specificity identifies a residue at position +3 following a conserved tryptophan, whose chemical characteristics modulate binding preference.

EAST, an epidermal growth factor receptor- and Eps15-associated protein with Src homology 3 and tyrosine-based activation motif domains

We describe the cloning and characterization of a new cytoplasmic protein designated epidermal growth factor receptor-associated protein with SH3- and TAM domains (EAST). It contains an Src homology 3 domain in its midregion and a tyrosine-based activation motif in its COOH terminus. Antibodies to EAST recognize a 68-kDa protein that is present in most chicken tissues. An epidermal growth factor (EGF)-dependent association between the EGF receptor (EGFR) and EAST was shown by reciprocal immunoprecipitation/immunoblotting studies with specific antibodies. Activated EGFR catalyzed the tyrosine phosphorylation of EAST, as judged by an in vitro kinase assay with both immunoprecipitated and purified EGFR. Immunoprecipitation/immunoblotting experiments also demonstrated an association between EAST and eps15, an EGFR substrate associated with clathrin-coated pits and vesicles, which is essential in the endocytotic pathway. The association between EAST and eps15 was not affected by EGF treatment. In immunofluorescence microscopy, EAST was shown to partially colocalize with clathrin. The sequence of the NH2-terminal portion of EAST shows a high degree of similarity with a group of proteins involved in endocytosis or vesicle trafficking. Thus, EAST is a novel signal transduction component probably involved in EGF signaling and in the endocytotic machinery.

Src phosphorylates EAST, a novel EGF receptor-associated protein

We have recently found and characterized EAST, a novel EGF receptor-associated protein with SH3 and TAM domains. In this study we show that EAST becomes phosphorylated by Src kinase. This, in conjunction with our earlier observations on the close association between EAST and the endocytic machinery, suggests that EAST could be involved in Src-dependent effects on EGF receptor endocytosis.

The neural cell adhesion molecule L1 interacts with the AP-2 adaptor and is endocytosed via the clathrin-mediated pathway

Cell-cell interactions mediated via cell adhesion molecules (CAMs) are dynamically regulated during nervous system development. One mechanism to control the amount of cell surface CAMs is to regulate their recycling from the plasma membrane. The L1 subfamily of CAMs has a highly conserved cytoplasmic domain that contains a tyrosine, followed by the alternatively spliced RSLE (Arg-Ser-Leu-Glu) sequence. The resulting sequence of YRSL conforms to a tyrosine-based sorting signal that mediates clathrin-dependent endocytosis of signal-bearing proteins. The present study shows that L1 associates in rat brain with AP-2, a clathrin adaptor that captures plasma membrane proteins with tyrosine-based signals for endocytosis by coated pits. In vitro assays demonstrate that this interaction occurs via the YRSL sequence of L1 and the mu 2 chain of AP-2. In L1-transfected 3T3 cells, L1 endocytosis is blocked by dominant-negative dynamin that specifically disrupts clathrin-mediated internalization. Furthermore, endocytosed L1 colocalizes with the transferrin receptor (TfR), a marker for clathrin-mediated internalization. Mutant forms of L1 that lack the YRSL do not colocalize with TfR, indicating that the YRSL mediates endocytosis of L1. In neurons, L1 is endocytosed preferentially at the rear of axonal growth cones, colocalizing with Eps15, another marker for the clathrin endocytic pathway. These results establish a mechanism by which L1 can be internalized from the cell surface and suggest that an active region of L1 endocytosis at the rear of growth cones is important in L1-dependent axon growth.

The neural cell adhesion molecule L1 interacts with the AP-2 adaptor and is endocytosed via the clathrin-mediated pathway

Cell-cell interactions mediated via cell adhesion molecules (CAMs) are dynamically regulated during nervous system development. One mechanism to control the amount of cell surface CAMs is to regulate their recycling from the plasma membrane. The L1 subfamily of CAMs has a highly conserved cytoplasmic domain that contains a tyrosine, followed by the alternatively spliced RSLE (Arg-Ser-Leu-Glu) sequence. The resulting sequence of YRSL conforms to a tyrosine-based sorting signal that mediates clathrin-dependent endocytosis of signal-bearing proteins. The present study shows that L1 associates in rat brain with AP-2, a clathrin adaptor that captures plasma membrane proteins with tyrosine-based signals for endocytosis by coated pits. In vitro assays demonstrate that this interaction occurs via the YRSL sequence of L1 and the mu 2 chain of AP-2. In L1-transfected 3T3 cells, L1 endocytosis is blocked by dominant-negative dynamin that specifically disrupts clathrin-mediated internalization. Furthermore, endocytosed L1 colocalizes with the transferrin receptor (TfR), a marker for clathrin-mediated internalization. Mutant forms of L1 that lack the YRSL do not colocalize with TfR, indicating that the YRSL mediates endocytosis of L1. In neurons, L1 is endocytosed preferentially at the rear of axonal growth cones, colocalizing with Eps15, another marker for the clathrin endocytic pathway. These results establish a mechanism by which L1 can be internalized from the cell surface and suggest that an active region of L1 endocytosis at the rear of growth cones is important in L1-dependent axon growth.

Distinct saturable pathways for the endocytosis of different tyrosine motifs

Endocytosis of surface proteins through clathrin-coated pits requires an internalization signal in the cytoplasmic domain. Two types of internalization signal have been described: one requiring a tyrosine as the critical residue (tyrosine-based motif), and the other consisting of either two consecutive leucines or an isoleucine and leucine (dileucine motif). Although it seems that these signals are necessary and sufficient for endocytic targeting, the mechanism of recognition is not well understood. To examine this question, tetracycline-repressible cell lines were used to overexpress one of several receptors bearing a tyrosine-based internalization signal. By measuring the rates of endocytosis for either the overexpressed receptor, or that of other endogenous receptors, we were able to show that the endocytosis of identical receptors could be saturated, but a complete lack of competition exists between the transferrin receptor (TfR), the low-density lipoprotein receptor, and the epidermal growth factor receptor. Overexpression of any one of these receptors resulted in its redistribution toward the cell surface, implying that entry into coated pits is limited. During high levels of TfR expression, however, a significant increase in the amount of surface Lamp1, but not low-density lipoprotein receptor, epidermal growth factor receptor, or Lamp2, is detected. This suggests that Lamp1 and TfR compete for the same endocytic sites. Together, these results support the idea that there are at least three distinct saturable components involved in clathrin-mediated endocytosis.

Tyrosine phosphorylation of selected secretory carrier membrane proteins, SCAMP1 and SCAMP3, and association with the EGF receptor

Secretory carrier membrane proteins (SCAMPs) are ubiquitously expressed proteins of post-Golgi vesicles. In the presence of the tyrosine phosphatase inhibitor vanadate, or after overexpression in Chinese hamster ovary (CHO) cells, SCAMP1 and SCAMP3 are phosphorylated selectively on tyrosine residue(s). Phosphorylation is reversible after vanadate washout in situ or when isolated SCAMP3 is incubated with the recombinant tyrosine phosphatase PTP1B. Vanadate also causes the partial accumulation of SCAMP3, but not SCAMP1, in "patches" at or near the cell surface. A search for SCAMP kinase activities has shown that SCAMPs 1 and 3, but not SCAMP2, are tyrosine phosphorylated in EGF-stimulated murine fibroblasts overexpressing the EGF receptor (EGFR). EGF catalyzes the progressive phosphorylation of the SCAMPs up to 1 h poststimulation and may enhance colocalization of the EGFR and SCAMP3 within the cell interior. EGF also induces SCAMP-EGFR association, as detected by coimmunoprecipitation, and phosphorylation of SCAMP3 is stimulated by the EGFR in vitro. These results suggest that phosphorylation of SCAMPs, either directly or indirectly, may be functionally linked to the internalization/down-regulation of the EGFR.

Mechanism of Nef-induced CD4 endocytosis: Nef connects CD4 with the mu chain of adaptor complexes

The Nef protein of primate lentiviruses down-regulates the cell surface expression of CD4 and probably MHC I by connecting these receptors with the endocytic machinery. Here, we reveal that Nef interacts with the mu chains of adaptor complexes, key components of clathrin-coated pits. For human immunodeficiency virus type 2 (HIV-2) and simian immunodeficiency virus (SIV) Nef, this interaction occurs via tyrosine-based motifs reminiscent of endocytosis signals. Mutating these motifs prevents the binding of SIV Nef to the mu chain of plasma membrane adaptor complexes, abrogates its ability to induce CD4 internalization, suppresses the accelerated endocytosis of a chimeric integral membrane protein harboring Nef as its cytoplasmic domain and confers a dominant-negative phenotype to the viral protein. Taken together, these data identify mu adaptins as downstream mediators of the down-modulation of CD4, and possibly MHC I, by Nef.

Pan1p, yeast eps15, functions as a multivalent adaptor that coordinates protein-protein interactions essential for endocytosis

A genetic screen for factors required for endocytosis in the budding yeast Saccharomyces cerevisiae previously identified PAN1. Pan1p is a homologue of the mammalian protein eps15, which has been implicated in endocytosis by virtue of its association with the plasma membrane clathrin adaptor complex AP-2. Pan1p contains two eps15 homology (EH) domains, a protein-protein interaction motif also present in other proteins that function in membrane trafficking. To address the role of Pan1p and EH domains in endocytosis, a yeast two-hybrid screen was performed using the EH domain-containing region of Pan1p. This screen identified yAP180A, one of two yeast homologues of a class of clathrin assembly proteins (AP180) that exhibit in vitro clathrin cage assembly activity. In vitro binding studies using GST fusion proteins and yeast extracts defined distinct binding sites on yAP180A for Pan1p and clathrin. yAP180 proteins and Pan1p, like actin, localize to peripheral patches along the plasma membrane. Mammalian synaptojanin, a phosphatidylinositol polyphosphate-5-phosphatase, also has been implicated in endocytosis recently, and three synaptojanin-like genes have been identified in yeast. We observed genetic interactions between the yeast SJL1 gene and PAN1, which suggest a role for phosphoinositide metabolites in Pan1p function. Together with other studies, these findings suggest that Pan1p coordinates regulatory interactions between proteins required for both endocytosis and actin-cytoskeleton organization; these proteins include the yAP180 proteins, clathrin, the ubiquitin-protein ligase Rsp5p, End3p, and synaptojanin. We suggest that Pan1p (and by extension eps15) serves as a multivalent adaptor around which dynamic interactions between structural and regulatory components of the endocytic pathway converge.

AP-2/Eps15 interaction is required for receptor-mediated endocytosis

We have previously shown that the protein Eps15 is constitutively associated with the plasma membrane adaptor complex, AP-2, suggesting its possible role in endocytosis. To explore the role of Eps15 and the function of AP-2/Eps15 association in endocytosis, the Eps15 binding domain for AP-2 was precisely delineated. The entire COOH-terminal domain of Eps15 or a mutant form lacking all the AP-2-binding sites was fused to the green fluorescent protein (GFP), and these constructs were transiently transfected in HeLa cells. Overexpression of the fusion protein containing the entire COOH-terminal domain of Eps15 strongly inhibited endocytosis of transferrin, whereas the fusion protein in which the AP-2-binding sites had been deleted had no effect. These results were confirmed in a cell-free assay that uses perforated A431 cells to follow the first steps of coated vesicle formation at the plasma membrane. Addition of Eps15-derived glutathione-S-transferase fusion proteins containing the AP-2-binding site in this assay inhibited not only constitutive endocytosis of transferrin but also ligand-induced endocytosis of epidermal growth factor. This inhibition could be ascribed to a competition between the fusion protein and endogenous Eps15 for AP-2 binding. Altogether, these results show that interaction of Eps15 with AP-2 is required for efficient receptor-mediated endocytosis and thus provide the first evidence that Eps15 is involved in the function of plasma membrane-coated pits.

Assembly of clathrin coats disrupts the association between Eps15 and AP-2 adaptors

Eps15 is a phosphorylation substrate of the epidermal growth factor receptor kinase. In vivo, it is largely found in complex with AP-2, the plasma membrane clathrin adaptor protein complex. Although AP-2 is uniformly distributed across the surface of clathrin-coated pits and vesicles, Eps15 is preferentially found in the rims of endocytic clathrin-coated pits (1). This observation suggests that Eps15 may disengage from AP-2 during coat formation. Here we use two new anti-Eps15 antibodies to show that, contrary to our own earlier suggestion, coated vesicles isolated from brain do not contain detectable amounts of Eps15. Furthermore, when AP-2 complexes that are saturated with Eps15 are used for in vitro assembly of clathrin-AP-2 coats, normal structures are formed that contain the expected amounts of clathrin and AP-2, but the amount of Eps15 present is dramatically lower than that of AP-2. We propose that during coated pit formation, addition of clathrin to the growing edge at the rim of the pit releases Eps15 from AP-2.

Ubiquitin and the control of protein fate in the secretory and endocytic pathways

The modification of proteins by chains of ubiquitin has long been known to mediate targeting of cytosolic and nuclear proteins for degradation by proteasomes. In this article, we discuss recent developments that reveal the involvement of ubiquitin in the degradation of proteins retained within the endoplasmic reticulum (ER) and in the internalization of plasma membrane proteins. Both luminal and transmembrane proteins retained in the ER are now known to be retrotranslocated into the cytosol in a process that involves ER chaperones and components of the protein import machinery. Once exposed to the cytosolic milieu, retro-translocated proteins are degraded by the proteasome, in most cases following polyubiquitination. There is growing evidence that both the ubiquitin-conjugating machinery and proteasomes may be associated with the cytosolic face of the ER membrane and that they could be functionally coupled to the process of retrotranslocation. The ubiquitination of plasma membrane proteins, on the other hand, mediates internalization of the proteins, which in most cases is followed by lysosomal/vacuolar degradation. There is, however, a well-documented case of a plasma membrane protein (the c-Met receptor) for which ubiquitination results in proteasomal degradation. These recent findings imply that ubiquitin plays more diverse roles in the regulation of the fate of cellular proteins than originally anticipated.