Association of epidermal growth factor receptors with coated pit adaptins via a tyrosine phosphorylation-regulated mechanism

Budding uninhibited by benzimidazoles-1 (BUB1) regulates EGFR signaling by reducing EGFR internalization

EGFR signaling initiates upon ligand binding which leads to activation and internalization of the receptor-ligand complex. Here, we evaluated if BUB1 impacted EGFR signaling by regulating EGFR receptor internalization and activation. BUB1 was ablated genomically (siRNA) or biochemically (2OH-BNPP1) in cells. EGF ligand was used to initiate EGFR signaling while disuccinimidyl suberate (DSS) was used for cross linking cellular proteins. EGFR signaling was measured by western immunoblotting and receptor internalization was evaluated by fluorescent microscopy (pEGFR (pY1068) colocalization with early endosome marker EEA1). siRNA mediated BUB1 depletion led to an overall increase in total EGFR levels and more phospho-EGFR (Y845, Y1092, and Y1173) dimers while the amount of total EGFR (non-phospho) dimers remained unchanged. BUB1 inhibitor (BUB1i) decreased EGF mediated EGFR signaling including pEGFR Y845, pAKT S473 and pERK1/2 in a time dependent manner. Additionally, BUB1i also reduced EGF mediated pEGFR (Y845) dimers (asymmetric dimers) without affecting total EGFR dimers (symmetric dimers) indicating that dimerization of inactive EGFR is not affected by BUB1. Furthermore, BUB1i blocked EGF mediated EGFR degradation (increase in EGFR half-life) without impacting half-lives of HER2 or c-MET. BUB1i also reduced co-localization of pEGFR with EEA1 positive endosomes suggesting that BUB1 might modulate EGFR endocytosis. Our data provide evidence that BUB1 protein and its kinase activity may regulate EGFR activation, endocytosis, degradation, and downstream signaling without affecting other members of the receptor tyrosine kinase family.

Interferon Receptor Trafficking and Signaling: Journey to the Cross Roads

Like most plasma membrane proteins, type I interferon (IFN) receptor (IFNAR) traffics from the outer surface to the inner compartments of the cell. Long considered as a passive means to simply control subunits availability at the plasma membrane, an array of new evidence establishes IFNAR endocytosis as an active contributor to the regulation of signal transduction triggered by IFN binding to IFNAR. During its complex journey initiated at the plasma membrane, the internalized IFNAR complex, i.e. IFNAR1 and IFNAR2 subunits, will experience post-translational modifications and recruit specific effectors. These finely tuned interactions will determine not only IFNAR subunits destiny (lysosomal degradation vs. plasma membrane recycling) but also the control of IFN-induced signal transduction. Finally, the IFNAR system perfectly illustrates the paradigm of the crosstalk between membrane trafficking and intracellular signaling. Investigating the complexity of IFN receptor intracellular routes is therefore necessary to reveal new insight into the role of IFNAR membrane dynamics in type I IFNs signaling selectivity and biological activity.

Annexin A1 promotes the nuclear localization of the epidermal growth factor receptor in castration-resistant prostate cancer

Epidermal growth factor receptor is a cancer driver whose nuclear localization has been associated with the progression of prostate cancer to the castration-resistant phenotype. Previous reports indicated a functional interaction between this receptor and the protein Annexin A1, which has also been associated with aggressive tumors. The molecular pathogenesis of castration-resistant prostate cancer remains largely unresolved, and herein we have demonstrated the correlation between the expression levels and localization of the epidermal growth factor receptor and Annexin A1 in prostate cancer samples and cell lines. Interestingly, a higher expression of both proteins was detected in castration-resistant prostate cancer cell lines and the strongest correlation was seen at the nuclear level. We verified that Annexin A1 interacts with the epidermal growth factor receptor, and by using prostate cancer cell lines knocked down for Annexin A1, we succeeded in demonstrating that Annexin A1 promotes the nuclear localization of epidermal growth factor receptor. Finally, we showed that Annexin A1 activates an autocrine signaling in castration-resistant prostate cells through the formyl peptide receptor 1. The inhibition of such signaling by Cyclosporin H inhibits the nuclear localization of epidermal growth factor receptor and its downstream signaling. The present work sheds light on the functional interaction between nuclear epidermal growth factor receptor and nuclear Annexin A1 in castration-resistant prostate cancer. Therefore, strategies to inhibit the nuclear localization of epidermal growth factor receptor through the suppression of the Annexin A1 autocrine loop could represent an important intervention strategy for castration-resistant prostate cancer.

Adaptor protein 1 B mu subunit does not contribute to the recycling of kAE1 protein in polarized renal epithelial cells

Mutations in the gene encoding the kidney anion exchanger 1 (kAE1) can lead to distal renal tubular acidosis (dRTA). dRTA mutations reported within the carboxyl (C)-terminal tail of kAE1 result in apical mis-targeting of the exchanger in polarized renal epithelial cells. As kAE1 physically interacts with the μ subunit of epithelial adaptor protein 1 B (AP-1B), we investigated the role of heterologously expressed μ1B subunit of the AP-1B complex for kAE1 retention to the basolateral membrane in polarized porcine LLC-PK1 renal epithelial cells that are devoid of endogenous AP-1B. We confirmed the interaction and close proximity between kAE1 and μ1B using immunoprecipitation and proximity ligation assay, respectively. Expressing the human μ1B subunit in these cells decreased significantly the amount of cell surface kAE1 at the steady state, but had no significant effect on kAE1 recycling and endocytosis. We show that (i) heterologous expression of μ1B displaces the physical interaction of endogenous GAPDH with kAE1 WT supporting that both AP-1B and GAPDH proteins bind to an overlapping site on kAE1 and (ii) phosphorylation of tyrosine 904 within the potential YDEV interaction motif does not alter the kAE1/AP-1B interaction. We conclude that μ1B subunit is not involved in recycling of kAE1.

Interdependent epidermal growth factor receptor signalling and trafficking

Epidermal growth factor (EGF) receptor (EGFR) signalling regulates diverse cellular functions, promoting cell proliferation, differentiation, migration, cell growth and survival. EGFR signalling is critical during embryogenesis, in particular in epithelial development, and disruption of the EGFR gene results in epithelial immaturity and perinatal death. EGFR signalling also functions during wound healing responses through accelerating wound re-epithelialisation, inducing cell migration, proliferation and angiogenesis. Upregulation of EGFR signalling is often observed in carcinomas and has been shown to promote uncontrolled cell proliferation and metastasis. Therefore aberrant EGFR signalling is a common target for anticancer therapies. Various reports indicate that EGFR signalling primarily occurs at the plasma membrane and EGFR degradation following endocytosis greatly attenuates signalling. Other studies argue that EGFR internalisation is essential for complete activation of downstream signalling cascades and that endosomes can serve as signalling platforms. The aim of this review is to discuss current understanding of intersection between EGFR signalling and trafficking.

Phosphorylation-dependent trafficking of plasma membrane proteins in animal and plant cells

In both unicellular and multicellular organisms, transmembrane (TM) proteins are sorted to and retained at specific membrane domains by endomembrane trafficking mechanisms that recognize sorting signals in the these proteins. The trafficking and distribution of plasma membrane (PM)-localized TM proteins (PM proteins), especially of those PM proteins that show an asymmetric distribution over the PM, has received much attention, as their proper PM localization is crucial for elementary signaling and transport processes, and defects in their localization often lead to severe disease symptoms or developmental defects. The subcellular localization of PM proteins is dynamically regulated by post-translational modifications, such as phosphorylation and ubiquitination. These modificaitons mostly occur on sorting signals that are located in the larger cytosolic domains of the cargo proteins. Here we review the effects of phosphorylation of PM proteins on their trafficking, and present the key examples from the animal field that have been subject to studies for already several decades, such as that of aquaporin 2 and the epidermal growth factor receptor. Our knowledge on cargo trafficking in plants is largely based on studies of the family of PIN FORMED (PIN) carriers that mediate the efflux of the plant hormone auxin. We will review what is known on the subcellular distribution and trafficking of PIN proteins, with a focus on how this is modulated by phosphorylation, and identify and discuss analogies and differences in trafficking with the well-studied animal examples.

LRP6 protein regulates low density lipoprotein (LDL) receptor-mediated LDL uptake

Genetic variations in LRP6 gene are associated with high serum LDL cholesterol levels. We have previously shown that LDL clearance in peripheral B-lymphocytes of the LRP6(R611C) mutation carriers is significantly impaired. In this study we have examined the role of wild type LRP6 (LRP6(WT)) and LRP6(R611C) in LDL receptor (LDLR)-mediated LDL uptake. LDL binding and uptake were increased when LRP6(WT) was overexpressed and modestly reduced when it was knocked down in LDLR-deficient CHO (ldlA7) cells. These findings implicated LRP6 in LDLR-independent cellular LDL binding and uptake. However, LRP6 knockdown in wild type CHO cells resulted in a much greater decline in LDL binding and uptake compared with CHO-ldlA7 cells, suggesting impaired function of the LDLR. LDLR internalization was severely diminished when LRP6 was knocked down and was restored after LRP6 was reintroduced. Further analysis revealed that LRP6(WT) forms a complex with LDLR, clathrin, and ARH and undergoes a clathrin-mediated internalization after stimulation with LDL. LDLR and LRP6 internalizations as well as LDL uptake were all impaired in CHO-k1 cells expressing LRP6(R611C). These studies identify LRP6 as a critical modulator of receptor-mediated LDL endocytosis and introduce a mechanism by which variation in LRP6 may contribute to high serum LDL levels.

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.

Endocytosis and intracellular trafficking of ErbBs

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

Endocytosis and intracellular trafficking of ErbBs

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

Vascular endothelial growth factor and semaphorin induce neuropilin-1 endocytosis via separate pathways

The neuropilin (Nrp)1 receptor is essential for both nervous and vascular system development. Nrp1 is unusually versatile, because it transmits both chemoattractive and repulsive signals in response to vascular endothelial growth factor (VEGF)-A and class 3 semaphorins, respectively. Both Nrp1 and VEGF receptor 2 undergo ligand-dependent endocytosis. We sought to establish the endocytic pathway of Nrp1 and to determine whether uptake is required for its signaling. Whereas Nrp1 underwent clathrin-dependent endocytosis in response to VEGFA(165) treatment, semaphorin 3C (sema3C) induced lipid raft-dependent endocytosis. The myosin VI PDZ (postsynaptic density 95, Disk large, Zona occludens-1) adaptor protein synectin was essential for Nrp1 trafficking. Sema3C failed to inhibit migration of synectin(-/-) endothelial cells, mirroring the lower migratory response of these cells to VEGFA(165). These results show that the endocytic pathway of Nrp1 is determined by its ligand and that the trafficking of Nrp1 is essential for its signaling.

YRRL motifs in the cytoplasmic domain of the thrombopoietin receptor regulate receptor internalization and degradation

Thrombopoietin (Tpo), acting through the c-Mpl receptor, promotes the survival and proliferation of hematopoietic stem and progenitor cells and drives megakaryocyte differentiation. The proproliferation and survival signals activated by Tpo must therefore be tightly regulated to prevent uncontrolled cell growth. In this work, we determined the mechanisms that control Tpo-stimulated c-Mpl internalization and defined the processes leading to its degradation. Stimulation of BaF-Mpl cells with Tpo leads to rapid, clathrin-dependent endocytosis of the receptor. Using small interfering RNA (siRNA), we found that inhibition of adaptor protein 2 (AP2), which mediates endocytosis of transmembrane proteins, strongly attenuates Tpo-stimulated c-Mpl internalization. AP2 interacts with YXXPhi motifs and we identified 2 such motifs in c-Mpl (Y(8)RRL and Y(78)RRL) and investigated Tpo-stimulated internalization of receptors bearing point mutations at these sites. After Tpo stimulation, internalization was greatly reduced in c-Mpl Y(78)F and c-Mpl Y(8+78)F, and these cell lines also exhibited increased proliferation and increased strength and duration of Jak2, STAT5, AKT, and ERK1/2 activation in response to Tpo. We also found that the Y(8)RRL motif regulates Tpo-stimulated lysosomal degradation of c-Mpl. Our data establishes that c-Mpl cytoplasmic YRRL motifs are responsible for both Tpo-mediated internalization via interactions with AP2 and lysosomal targeting after endocytosis.

AP2 clathrin adaptor complex, but not AP1, controls the access of the major histocompatibility complex (MHC) class II to endosomes

Newly synthesized MHC II alpha- and beta-chains associated with the invariant chain chaperone (Ii) enter the endocytic pathway for Ii degradation and loading with peptides before transport to the cell surface. It is unclear how alphabetaIi complexes are sorted from the Golgi apparatus and directed to endosomes. However, indirect evidence tends to support direct transport involving the AP1 clathrin adaptor complex. Surprisingly, we show here that knocking down the production of AP1 by RNA interference did not affect the trafficking of alphabetaIi complexes. In contrast, AP2 depletion led to a large increase in surface levels of alphabetaIi complexes, inhibited their rapid internalization, and strongly delayed the appearance of mature MHC II in intracellular compartments. Thus, in the cell systems studied here, rapid internalization of alphabetaIi complexes via an AP2-dependent pathway represents a key step for MHC II delivery to endosomes and lysosomes.

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.

Compartmentalized signal transduction by receptor tyrosine kinases

Signal transduction through receptor tyrosine kinases is believed to occur mainly at the plasma membrane. Ligands bind to their cognate receptors and trigger autophosphorylation events, which are detected by intracellular signalling molecules. However, ligands, such as epidermal growth factor and insulin, induce the rapid internalization of their receptors into endosomes. Although this event is traditionally thought to attenuate the ligand-induced response, in this article the authors discuss an alternative scenario in which selective and regulated signal transduction from receptor tyrosine kinases occurs within the endosome.

Stimulation-Dependent Recycling of Integrin β1 Regulated by ARF6 and Rab11

In comparison to the internalization pathways of endocytosis, the recycling pathways are less understood. Even less defined is the process of regulated recycling, as few examples exist and their underlying mechanisms remain to be clarified. In this study, we examine the endocytic recycling of integrin beta1, a process that has been suggested to play an important role during cell motility by mediating the redistribution of integrins to the migrating front. External stimulation regulates the endocytic itinerary of beta1, mainly at an internal compartment that is likely to be a subset of the recycling endosomes. This stimulation-dependent recycling is regulated by ARF6 and Rab11, and also requires the actin cytoskeleton in an ARF6-dependent manner. Consistent with these observations being relevant for cell motility, mutant forms of ARF6 that affect either actin rearrangement or recycling inhibit the motility of a breast cancer cell line.

Sorting motifs in receptor trafficking

A variety of receptors have been analyzed in sufficient detail to identify sorting motifs. Initial studies focused on the identification of sequences in the cytoplasmic tails of the LDL and transferrin receptors that mediated their internalization. These motifs have since been found in the cytoplasmic domains of a wide variety of receptors and provide for numerous sorting functions. This review will outline the early studies on LDL and transferrin receptors and will then focus on two classes of signaling receptors, receptor tyrosine kinases (EGF and the insulin receptors) and heterotrimeric G-protein coupled receptors (beta2-adrenergic receptors). The identification of sorting motifs and proteins that bind these motifs will be discussed. Importantly, the studies identify a variety of potential targets for modulating the sorting and hence activity of these medically important receptors.

Effect of clathrin heavy chain- and alpha-adaptin-specific small inhibitory RNAs on endocytic accessory proteins and receptor trafficking in HeLa cells

To assess the contribution of individual endocytic proteins to the assembly of clathrin coated pits, we depleted the clathrin heavy chain and the alpha-adaptin subunit of AP-2 in HeLa-cells using RNA interference. 48 h after transfection with clathrin heavy chain-specific short interfering RNA both, the heavy and light chains were depleted by more than 80%. Residual clathrin was mainly membrane-associated, and an increase in shallow pits was noted. The membrane-association of adaptors, clathrin assembly lymphoid myeloid leukemia protein (CALM), epsin, dynamin, and Eps15 was only moderately affected by the knockdown and all proteins still displayed a punctate staining distribution. Clathrin depletion inhibited the uptake of transferrin but not that of the epidermal growth factor. However, efficient sorting of the epidermal growth factor into hepatocyte growth factor-regulated tyrosine kinase substrate-positive endosomes was impaired. Depletion of alpha-adaptin abolished almost completely the plasma membrane association of clathrin. Binding of Eps15 to membranes was strongly and that of CALM moderately reduced. Whereas the uptake of transferrin was efficiently blocked in alpha-adaptin knockdown cells, the internalization and sorting of the epidermal growth factor was not significantly impaired. Since neither clathrin nor AP-2 is essential for the internalization of EGF, we conclude that it is taken up by an alternative mechanism.

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

Tyrosine phosphorylation of the beta2 subunit of clathrin adaptor complex AP-2 was detected in three types of cells treated with epidermal growth factor (EGF). The tyrosine phosphorylation was observed during recruitment of EGF receptors into coated pits at 4 degrees C and reached maximum at 37 degrees C at post-recruitment stages of endocytosis. An inhibitor of EGF receptor kinase completely abolished this phosphorylation in all cell types, whereas the inhibitor of Src family kinases partially inhibited beta2 phosphorylation in A-431 cells but not in HeLa cells. By using beta2 subunit tagged with yellow fluorescent protein that is effectively assembled into AP-2 complex, the major phosphorylation site of beta2 was mapped to Tyr-6. Analysis of cells expressing dominant-interfering mutant mu2 subunit of AP-2 suggested that beta2 phosphorylation is partially mediated by the receptor interaction with the mu2 subunit. Mutation of leucine residues 1010 and 1011 motif in the EGF receptor resulted in the severe inhibition of beta2 tyrosine phosphorylation. From these data, we propose that interactions of the EGF receptor with AP-2 mediated by the receptor 974YRAL and di-leucine motifs may contribute to beta2 tyrosine phosphorylation. Surprisingly, mutation of the Leu-1010/Leu-1011 motif resulted in impaired degradation of EGF receptors, suggesting the role of this motif in lysosomal targeting of the receptor.

β1integrins are distributed in adhesion structures with fibronectin and caveolin and in coated pits

Integrins are found in adhesion structures, which link the extracelullar matrix to cytoskeletal proteins. Here, we attempt to further define the distribution of β1integrins in the context of their association with matrix proteins and other cell surface molecules relevant to the endocytic process. We find that β1integrins colocalize with fibronectin in fibrillar adhesion structures. A fraction of caveolin is also organized along these adhesion structures. The extracellular matrix protein laminin is not concentrated in these structures. The α4β1integrin exhibits a distinct distribution from other β1integrins after cells have adhered for 1 h to extracellular matrix proteins but is localized in adhesion structures after 24 h of adhesion. There are differences between the fibronectin receptors: α5β1integrins colocalize with adaptor protein-2 in coated pits, while α4β1integrins do not. This parallels our earlier observation that of the two laminin receptors, α1β1and α6β1, only αaβ1integrins colocalize with adaptor protein-2 in coated pits. Calcium chelation or inhibition of mitogen-activated protein kinase kinase, protein kinase C, or src did not affect localization of α1β1and α5β1integrins in coated pits. Likewise, the integrity of coated-pit structures or adhesion structures is not required for integrin and adaptor protein-2 colocalization. This suggests a robust and possibly constitutive interaction between these integrins and coated pits.Key words: adhesion, endocytosis, extracellular matrix, microscopy, confocal, signalling.

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

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

Transforming growth factor-beta receptors interact with AP2 by direct binding to beta2 subunit

Transforming growth factor-beta (TGF-beta) superfamily members regulate a wide range of biological processes by binding to two transmembrane serine/threonine kinase receptors, type I and type II. We have previously shown that the internalization of these receptors is inhibited by K(+) depletion, cytosol acidification, or hypertonic medium, suggesting the involvement of clathrin-coated pits. However, the involvement of the clathrin-associated adaptor complex AP2 and the identity of the AP2 subunit that binds the receptors were not known. Herein, we have studied these issues by combining studies on intact cells with in vitro assays. Using fluorescence photobleaching recovery to measure the lateral mobility of the receptors on live cells (untreated or treated to alter their coated pit structure), we demonstrated that their mobility is restricted by interactions with coated pits. These interactions were transient and mediated through the receptors' cytoplasmic tails. To measure direct binding of the receptors to specific AP2 subunits, we used yeast two-hybrid screens and in vitro biochemical assays. In contrast to most other plasma membrane receptors that bind to AP2 via the mu2 subunit, AP2/TGF-beta receptor binding was mediated by a direct interaction between the beta2-adaptin N-terminal trunk domain and the cytoplasmic tails of the receptors; no binding was observed to the mu2, alpha, or sigma2 subunits of AP2 or to mu1 of AP1. The data uniquely demonstrate both in vivo and in vitro the ability of beta2-adaptin to directly couple TGF-beta receptors to AP2 and to clathrin-coated pits, providing the first in vivo evidence for interactions of a transmembrane receptor with beta2-adaptin.

The mu2 subunit of the clathrin adaptor AP-2 binds to FDNPVY and YppØ sorting signals at distinct sites

The endocytic sorting signal on the low-density lipoprotein receptor for clathrin-mediated internalization is the sequence FDNPVY in the receptor's cytosolic tail. We have used a combination of surface plasmon resonance and crosslinking with a photoactivated peptide probe to demonstrate the interaction between FDNPVY-containing peptides and the mu2 chain of purified AP-2 clathrin adaptors (the complexes responsible for plasma membrane sorting). We show that recognition of the FDNPVY signal is mediated by a binding site in the mu2-subunit that is distinct from the site for the more general YppØ sorting signal, another tyrosine-based sequence also recognized by mu2-adaptin. These results suggest the possibility that low-density lipoprotein receptor uptake may be modulated specifically and independently of other proteins in the clathrin pathway.

Adaptor protein-2 exhibits alpha 1 beta 1 or alpha 6 beta 1 integrin-dependent redistribution in rhabdomyosarcoma cells

Downregulation of several signaling pathways, such as those stimulated by growth factor receptors, occurs by internalization of signaling receptors through clathrin-coated pits. The first step in internalization or endocytosis is interaction with AP-2, which results in coated pit formation by assembly of clathrin to AP-2. Changes in endocytosis are reflected in the distribution of AP-2 molecules at the cell surface. Integrins are receptors which mediate attachment to the extracellular matrix and also stimulate numerous intracellular signaling pathways; however, it is not known how signaling through integrins is terminated or downregulated. Endocytosis through clathrin-coated pits offers an attractive mechanism for this. This work explores the relationship between AP-2 and beta(1) integrins. RD cells grown for 24 h on collagen or laminin exhibit a redistribution of AP-2 to the cell periphery relative to those grown on fibronectin or polylysine. The total AP-2 protein levels in the cells are unaffected. Blocking alpha(1)beta(1) integrin ligand binding on collagen prevents this redistribution fully. On laminin where alpha(1)beta(1) and alpha(6)beta(1) integrins are engaged, both receptors must be simultaneously blocked to prevent AP-2 redistribution, confirming that the redistribution depends on the specific engagement of the receptors. Immunofluorescence reveals that the majority of alpha(1)beta(1) integrins colocalize with alpha(6)beta(1) integrins in linear structures identified as focal adhesions. A separate fraction of alpha(1)beta(1) integrins colocalize with AP-2 in coated pits. Interestingly, alpha(6)beta(1) integrins are not located in coated pits, demonstrating that integrin colocalization with AP-2 is not necessary to induce redistribution of AP-2.

Novel mechanism for regulation of epidermal growth factor receptor endocytosis revealed by protein kinase A inhibition

Current models put forward that the epidermal growth factor receptor (EGFR) is efficiently internalized via clathrin-coated pits only in response to ligand-induced activation of its intrinsic tyrosine kinase and is subsequently directed into a lysosomal-proteasomal degradation pathway by mechanisms that include receptor tyrosine phosphorylation and ubiquitylation. Herein, we report a novel mechanism of EGFR internalization that does not require ligand binding, receptor kinase activity, or ubiquitylation and does not direct the receptor into a degradative pathway. Inhibition of basal protein kinase A (PKA) activity by H89 and the cell-permeable substrate peptide Myr-PKI induced internalization of 40-60% unoccupied, inactive EGFR, and its accumulation into early endosomes without affecting endocytosis of transferrin and mu-opioid receptors. This effect was abrogated by interfering with clathrin function. Thus, the predominant distribution of inactive EGFR at the plasma membrane is not simply by default but involves a PKA-dependent restrictive condition resulting in receptor avoidance of endocytosis until it is stimulated by ligand. Furthermore, PKA inhibition may contribute to ligand-induced EGFR endocytosis because epidermal growth factor inhibited 26% of PKA basal activity. On the other hand, H89 did not alter ligand-induced internalization of EGFR but doubled its half-time of down-regulation by retarding its segregation into degradative compartments, seemingly due to a delay in the receptor tyrosine phosphorylation and ubiquitylation. Our results reveal that PKA basal activity controls EGFR function at two levels: 1) residence time of inactive EGFR at the cell surface by a process of "endocytic evasion," modulating the accessibility of receptors to stimuli; and 2) sorting events leading to the down-regulation pathway of ligand-activated EGFR, determining the length of its intracellular signaling. They add a new dimension to the fine-tuning of EGFR function in response to cellular demands and cross talk with other signaling receptors.

EGF receptor downregulation depends on a trafficking motif in the distal tyrosine kinase domain

On binding to its receptor, epidermal growth factor (EGF) initiates a cascade of events leading to cell proliferation or differentiation. In addition, the EGF receptor itself is downregulated to attenuate mitogenic signaling. Downregulation occurs through trafficking of receptors to lysosomes, culminating in proteolytic destruction of both the receptor and ligand; however, endocytic sorting mechanisms that underlie lysosomal targeting remain obscure. The goal of this study was to explore one aspect of the molecular basis for ligand-induced lysosomal targeting and degradation of EGF receptors. In this study, we identify a tyrosine-leucine motif ((954)YLVI) that is essential for transit of ligand-receptor complexes to lysosomes. When this motif is mutated, HEK 293 cells expressing the mutant receptors demonstrate impaired lysosomal targeting and downregulation compared with wild-type receptors. (954)YLVI is highly conserved among EGF receptors from various mammalian and invertebrate species and is critical for receptor downregulation. We propose that (954)YLVI works in concert with at least two additional regions within the EGF receptor cytoplasmic domain that are essential for efficiently targeting ligand-receptor complexes to the lysosome.

Mu 2 binding directs the cystic fibrosis transmembrane conductance regulator to the clathrin-mediated endocytic pathway

The cystic fibrosis transmembrane conductance regulator (CFTR) contains a conserved tyrosine-based internalization motif, (1424)YDSI, which interacts with the endocytic clathrin adaptor complex, AP-2, and is required for its efficient endocytosis. Although direct interactions between several endocytic sequences and the medium chain and endocytic clathrin adaptor complexes have been shown by protein-protein interaction assays, whether all these interactions occur in vivo or are physiologically important has not always been addressed. Here we show, using both in vitro and in vivo assays, a physiologically relevant interaction between CFTR and the mu subunit of AP-2. Cross-linking experiments were performed using photoreactive peptides containing the YDSI motif and purified adaptor complexes. CFTR peptides cross-linked a 50-kDa subunit of purified AP-2 complexes, the apparent molecular mass of mu 2. Furthermore, isolated mu 2 bound to the sorting motif, YDSI, both in cross-linking experiments and glutathione S-transferase pull-down experiments, confirming that mu 2 mediates the interaction between CFTR and AP-2 complexes. Inducible overexpression of dominant-negative mu 2 in HeLa cells results in AP-2 complexes that fail to interact with CFTR. Moreover, internalization of CFTR in mutant cells is greatly reduced compared with wild type HeLa cells. These results indicate that the AP-2 endocytic complex selectively interacts with the conserved tyrosine-based internalization signal in the carboxyl terminus of CFTR, YDSI. Furthermore, this interaction is mediated by the mu 2 subunit of AP-2 and mutations in mu 2 that block its interaction with YDSI inhibit the incorporation of CFTR into the clathrin-mediated endocytic pathway.

Activation of preformed EGF receptor dimers by ligand-induced rotation of the transmembrane domain

The epidermal growth factor receptor plays crucial roles throughout the development of multicellular organisms, and inappropriate activation of the receptor is associated with neoplastic transformation of many cell types. The receptor is thought to be activated by ligand-induced homodimerisation. Here, however, we show by chemical cross-linking and sucrose density-gradient centrifugation that in the absence of bound ligand the receptor has an ability to form a dimer and exists as a preformed dimer on the cell surface. We also analysed the receptor dimerisation by inserting cysteine residues at strategic positions about the putative alpha-helix axis of the extracellular juxtamembrane region. The mutant receptors spontaneously formed disulphide bridges and transformed NIH3T3 cells in the absence of ligand, depending upon the positions of the cysteine residue inserted. Kinetic analyses of the disulphide bonding indicate that EGF binding induces flexible rotation or twist of the juxtamembrane region of the receptor in the plane parallel with the lipid bilayer. The binding of an ATP competitor to the intracellular domain also induced similar flexible rotation of the juxtamembrane region. All the disulphide-bonded dimers had flexible ligand-binding domains with the same biphasic affinities for EGF as the wild-type. These results demonstrate that ligand binding to the flexible extracellular domains of the receptor dimer induce rotation or twist of the juxtamembrane regions, hence the transmembrane domains, and dissociate the dimeric, inactive form of the intracellular domains. The flexible rotation of the intracellular domains may be necessary for the intrinsic catalytic kinase to become accessible to the multiple tyrosine residues present in the regulatory domain and various substrates, and may be a common property of many cell-surface receptors, such as the insulin receptor.

Self-assembly and binding of a sorting nexin to sorting endosomes

The fate of endocytosed membrane proteins and luminal contents is determined by a materials processing system in sorting endosomes. Endosomal retention is a mechanism that traps specific proteins within this compartment, and thereby prevents their recycling. We report that a sorting nexin SNX1, a candidate endosomal retention protein, self-assembles in vitro and in vivo, and has this property in common with its yeast homologue Vps5p. A comparison of SNX1 expressed in bacterial and in mammalian systems and analyzed by size-exclusion chromatography indicates that in cytosol SNX1 tetramers are part of a larger complex with additional proteins. An endosomal retention function would require that SNX1 bind to endosomal membranes, yet the complexes that we analyzed were largely soluble and little SNX1 was found in pellet fractions. Using green fluorescent protein fusions, endocytic compartment markers and fluorescence recovery after photobleaching, we found that there is an equilibrium between free cytoplasmic and early/sorting endosome-bound pools of green fluorescent protein-SNX1. Fluorescence resonance energy transfer indicated that spectral variants of green fluorescent protein-SNX1 were oligomeric in vivo. In cell extracts, these green fluorescent protein-SNX1 oligomers corresponded to tetrameric and larger complexes of green fluorescent protein-SNX1. Using video microscopy, we observed small vesicle docking and tubule budding from large green fluorescent protein-SNX1 coated endosomes, which are features consistent with their role as sorting endosomes. http://www.biologists.com/JCS/movies/jcs2058.html

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.

Agonist-mediated endocytosis of rat somatostatin receptor subtype 3 involves beta-arrestin and clathrin coated vesicles

Agonist-induced endocytosis of somatostatin receptors determines subsequent cellular responsiveness to peptide agonist and influences somatostatin receptor scintigraphy, a technique to image various tumours. We examined the internalization of sst3HSV, an epitope-tagged type 3 somatostatin receptor, in transfected rat neuroendocrine insulinoma cells. Stimulation of these cells with somatostatin induced trafficking of coexpressed enhanced green fluorescence protein/beta-arrestin1 fusion protein and sst3HSV to colocalize in the same endocytic vesicles. Coexpression of a dominant negative mutant of the arrestin fusion protein with the receptor blocked the internalization of sst3HSV. Stimulation with somatostatin also induced the transient translocation of alpha-adaptin, a component of the adaptor protein complex 2, to the plasma membrane. alpha-adaptin and clathrin colocalized with the receptor. By electron microscopy, we observed internalized sst3 in clathrin coated pits, endosomes and at the limiting membrane of multivesicular bodies, a location typical for receptors being recycled. Concordantly, we observed sst3HSV colocalized with Rab11 in a perinuclear compartment which is likely to correspond to the pericentriolar recycling endosome. Thus, agonist-induced endocytosis of sst3 depends on its interaction with beta-arrestin, involves the adaptor protein complex 2 and proceeds via clathrin coated vesicles to the recycling compartment.

Dual interaction of synaptotagmin with mu2- and alpha-adaptin facilitates clathrin-coated pit nucleation

The synaptic vesicle protein synaptotagmin was proposed to act as a major docking site for the recruitment of clathrin coats implicated in endocytosis, including the recycling of synaptic vesicles. We show here that the C2B domain of synaptotagmin binds mu2- and alpha-adaptin, two of the four subunits of the endocytic adaptor complex AP-2. mu2 represents the major interacting subunit of AP-2 within this complex. Its binding to synaptotagmin is mediated by a site in subdomain B that is distinct from the binding site for tyrosine-based sorting motifs located in subdomain A. The presence of the C2B domain of synaptotagmin at the surface of liposomes enhances the recruitment of AP-2 and clathrin. Conversely, perturbation of the interaction between synaptotagmin and AP-2 by synprint, the cytoplasmic synaptotagmin-binding domain of N-type calcium channels, inhibits transferrin internalization in living cells. We conclude that a dual interaction of synaptotagmin with the clathrin adaptor AP-2 plays a key physiological role in the nucleation of endocytic clathrin-coated pits.

Tyrosine phosphorylation of Eps15 is required for ligand-regulated, but not constitutive, endocytosis

Membrane receptors are internalized either constitutively or upon ligand engagement. Whereas there is evidence for differential regulation of the two processes, little is known about the molecular machinery involved. Previous studies have shown that an unidentified kinase substrate is required for endocytosis of the epidermal growth factor receptor (EGFR), the prototypical ligand-inducible receptor, but not of the transferrin receptor (TfR), the prototypical constitutively internalized receptor. Eps15, an endocytic protein that is tyrosine phosphorylated by EGFR, is a candidate for such a function. Here, we show that tyrosine phosphorylation of Eps15 is necessary for internalization of the EGFR, but not of the TfR. We mapped Tyr 850 as the major in vivo tyrosine phosphorylation site of Eps15. A phosphorylation-negative mutant of Eps15 acted as a dominant negative on the internalization of the EGFR, but not of the TfR. A phosphopeptide, corresponding to the phosphorylated sequence of Eps15, inhibited EGFR endocytosis, suggesting that phosphotyrosine in Eps15 serves as a docking site for a phosphotyrosine binding protein. Thus, tyrosine phosphorylation of Eps15 represents the first molecular determinant, other than those contained in the receptors themselves, which is involved in the differential regulation of constitutive vs. regulated endocytosis.

Inhibition of AT1 receptor internalization by concanavalin A blocks angiotensin II-induced ERK activation in vascular smooth muscle cells. Involvement of epidermal growth factor receptor proteolysis but not AT1 receptor internalization

Recent studies of beta(2)-adrenergic receptor suggest that agonist-promoted receptor internalization may play an important role in extracellular signal-regulated kinase (ERK) activation by G protein-coupled receptors. In the present study, we explored the effects of angiotensin II (Ang II) type-1 receptor (AT(1)) internalization on Ang II-induced activation of ERK using the receptor internalization blocker concanavalin A (ConA) and the carboxyl terminus-truncated receptor mutants with impaired internalization. ConA inhibited AT(1) receptor internalization without affecting ligand binding to the receptor, Ang II-induced generation of second messengers, and activation of tyrosine kinases Src and Pyk2 in vascular smooth muscle cells (VSMC). ConA blocked ERK activation evoked by Ang II and the calcium ionophore A23187. Impairment of AT(1) receptor internalization by truncating the receptor carboxyl terminus did not affect Ang II-induced ERK activation. ConA induced proteolytic cleavage of the epidermal growth factor (EGF) receptor at carboxyl terminus and abolished Ang II-induced transactivation of the EGF receptor, which is critical for ERK activation by Ang II in VSMC. ConA also induced proteolysis of erbB-2 but not platelet-derived growth factor receptor. Thus, ConA blocks Ang II-induced ERK activation in VSMC through a distinct mechanism, the ConA-mediated proteolysis of the EGF receptor.

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 processing of ligands by the class A scavenger receptor is dependent on signal information located in the cytoplasmic domain

The mechanisms that regulate the transport of the macrophage class A scavenger receptor during ligand uptake were investigated. Kinetic analysis of the changes in receptor phosphorylation demonstrated that serine phosphorylation increased during the internalization of acetyl-low density lipoproteins (LDL) by macrophages. The increase was maximal at about 2.5 min after the initiation of ligand uptake. Oxidized LDL also stimulated serine phosphorylation, but the relative increase was smaller and the time to maximum was shorter. Receptor mutants expressed in Chinese hamster ovary and COS cells showed that elimination of the potential phosphorylation site at Ser(21) increased acetyl-LDL metabolism, whereas inactivation of the site at Ser(49) reduced acetyl-LDL uptake. The increase in uptake by the Ser(21) mutant was due to an increase in surface receptor expression. In contrast, elimination of the site at Ser(49) did not affect receptor expression but slowed receptor internalization. To identify potential internalization signal sequences, beta-turn structure in the cytosolic domain was targeted for mutagenesis. Disruption of one region near Asp(25) inhibited receptor activity. The studies support a model whereby receptor internalization requires the presence of an internalization signal motif but that the rate of receptor internalization is governed by the pattern of receptor phosphorylation induced by the ligand.

Tyrosine phosphorylation regulates rapid endocytosis in adrenal chromaffin cells

Secretion of neurotransmitter at the synapse and in secretory cells depends on the availability of vesicles for exocytosis. Rapid endocytosis is responsible for initiating local vesicle recycling and is essential during sustained neurotransmission. Although exocytosis is triggered by Ca(2+) influx and modulated by serine/threonine kinases, relatively little is known about the regulation of rapid endocytosis. Our data suggest that rapid endocytosis is controlled by tyrosine phosphorylation. Treatment of bovine adrenal chromaffin cells with tyrphostin 23, a protein tyrosine kinase inhibitor, dramatically slowed the time course of rapid endocytosis. In contrast, there was no effect on either the amount or rate of exocytosis. Application of orthovanadate, Zn(2+), or poly(Glu, Tyr) (4:1), each of which is a tyrosine phosphatase inhibitor, reversed the effect of tyrphostin 23 on rapid endocytosis. Thus rapid endocytosis, like exocytosis, is subject to regulation by intracellular signaling pathways.

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.

An internalization-competent influenza hemagglutinin mutant causes the redistribution of AP-2 to existing coated pits and is colocalized with AP-2 in clathrin free clusters

Image correlation spectroscopy and cross correlation spectroscopy were used to demonstrate that approximately 25% of the internalization-competent influenza virus hemagglutinin mutant, HA+8, is colocalized with clathrin and AP-2 at the plasma membrane of intact cells, while wild-type HA (which is excluded from coated pits) does not colocalize with either protein. Clathrin and AP-2 clusters were saturated when HA+8 was overexpressed, and this was accompanied by a redistribution of AP-2 into existing coated pits. However, de novo coated pit formation was not observed. In nontreated cells, the number of clusters of clathrin or AP-2 colocalized with HA+8 was always comparable. Hypertonic treatment which disperses the clathrin lattices resulted in more clusters containing AP-2 and HA+8 than clathrin and HA+8. Less colocalization of HA+8 with clathrin was also observed after cytosol acidification, which causes the formation of deeply invaginated pits, where the HA+8 may be inaccessible to extracellular labeling by antibodies, and blocks coated vesicle budding. However, cytosol acidification elevated the number of clusters containing both HA+8 and AP-2, suggesting an increase in their level of association outside of the deep invaginations. Our results imply that AP-2 and HA+8 can colocalize in clusters devoid of clathrin, at least in cells treated to alter the clathrin lattice structure. Although we cannot ascertain whether this also occurs in untreated cells, we propose that AP-2 binding to membrane proteins carrying internalization signals can occur prior to the binding of AP-2 to clathrin. While such complexes can in principle serve to recruit clathrin for the formation of new coated pits, the higher affinity of the internalization signals for clathrin-associated AP-2 [Rapoport, I., et al. (1997) EMBO J. 16, 2240-2250] makes it more likely that once the AP-2-membrane protein complexes form, they are quickly recruited into existing coated pits.

Free clathrin triskelions are required for the stability of clathrin-associated adaptor protein (AP-2) coated pit nucleation sites

In this study image correlation spectroscopy was used to demonstrate the presence of two populations of clathrin in situ, on intact cells. In the periphery of the cell ~35% of the clathrin triskelions are free within the cytosol while ~65% are in large aggregates, presumably coated pits. Although endocytosis is inhibited at low temperature, free clathrin triskelions are still present and small AP-2 aggregates (of ~20 proteins), or coated pit nucleation sites, are still observed. Following hypertonic treatment, or cytoplasmic acidification, free clathrin triskelions within the cytosol are depleted and all of the clathrin becomes associated with the membrane. Under these conditions coated pit associated AP-2 remains while the smaller AP-2 aggregates, or coated pit nucleation sites, dissociate. This indicates that the stabilization of AP-2 coated pit nucleation sites requires the presence of free clathrin triskelions within the cytosol. Furthermore, this indicates that free clathrin is required for the early stages of coated pit formation and presumably the continuation of the clathrin-mediated endocytic process. We also provide indirect evidence that AP-2 binding to the membrane in coated pit nucleation sites may be regulated in part by binding to internalization-competent membrane receptors.Key words: adaptor protein (AP-2), clathrin, distribution, nucleation sites, endocytosis.

Epidermal growth factor receptor internalization rate is regulated by negative charges near the SH2 binding site Tyr992

This study examines the effects of mutations at and in the vicinity of tyrosine 992 of the epidermal growth factor receptor (EGFr) on epidermal growth factor- (EGF-) stimulated internalization of the receptor. Two regions of the EGFr adjacent to this domain have been defined previously as internalization domains. The present work shows that the mutation of negatively charged amino acid residues near Tyr992 to their uncharged analogues increases the rate of EGF receptor internalization. In addition, the conversion of Tyr992, which is an EGFr ligand-induced autophosphorylation site, to phenylalanine also increases the rate of receptor internalization. However, the mutation of Tyr992 to a glutamate residue does not alter the receptor internalization rate. In addition, the truncation of the EGFr at glutamate 996 reduces the internalization rate by half. This result confirms previous reports that residues immediately C-terminal to Glu996 are necessary to allow the normal rate of ligand-induced receptor endocytosis. The data suggest that negative charge in the vicinity of Tyr992, and potentially the phosphorylation of the EGFr at Tyr992, reduces the rate of ligand-induced receptor endocytosis. This reduction in internalization rate increases the lifetime of the activated EGFr in the plasma membrane by about 70%, thus suggesting that phosphorylation of Tyr992 acts to increase the signaling capacity of the EGF receptor even as it directly acts as an SH2 binding site.

Recognition of sorting signals by clathrin adaptors

Sorting of membrane proteins is generally mediated by cytosolic coats, which create a scaffold to form coated buds and vesicles and to selectively concentrate cargo by interacting with cytosolic signals. The classical paradigm is the interaction between clathrin coats and associated adaptor proteins, which cluster receptors with characteristic tyrosine and dileucine motifs during endocytosis. Clathrin in association with different sets of adaptors is found in addition at the trans-Golgi network and endosomes. Sequences similar to internalization signals also direct lysosomal and basolateral sorting, which implicates related clathrinadaptor coats in the respective sorting pathways. This review concentrates on the recognition of sorting signals by clathrin-associated adaptor proteins, an area of significant recent progress due to new methodological and conceptual approaches.

Inhibition of the receptor-binding function of clathrin adaptor protein AP-2 by dominant-negative mutant mu2 subunit and its effects on endocytosis

Although interactions between the mu2 subunit of the clathrin adaptor protein complex AP-2 and tyrosine-based internalization motifs have been implicated in the selective recruitment of cargo molecules into coated pits, the functional significance of this interaction for endocytosis of many types of membrane proteins remains unclear. To analyze the function of mu2-receptor interactions, we constructed an epitope-tagged mu2 that incorporates into AP-2 and is targeted to coated pits. Mutational analysis revealed that Asp176 and Trp421 of mu2 are involved in the interaction with internalization motifs of TGN38 and epidermal growth factor (EGF) receptor. Inducible overexpression of mutant mu2, in which these two residues were changed to alanines, resulted in metabolic replacement of endogenous mu2 in AP-2 complexes and complete abrogation of AP-2 interaction with the tyrosine-based internalization motifs. As a consequence, endocytosis of the transferrin receptor was severely impaired. In contrast, internalization of the EGF receptor was not affected. These results demonstrate the potential usefulness of the dominant-interfering approach for functional analysis of the adaptor protein family, and indicate that clathrin-mediated endocytosis may proceed in both a mu2-dependent and -independent manner.

Sorting of furin at the trans-Golgi network. Interaction of the cytoplasmic tail sorting signals with AP-1 Golgi-specific assembly proteins

The eukaryotic subtilisin-like endoprotease furin is found predominantly in the trans-Golgi network (TGN) and cycles between this compartment, the cell surface, and the endosomes. There is experimental evidence for endocytosis from the plasma membrane and transport from endosomes to the TGN, but direct exit from the TGN to endosomes via clathrin-coated vesicles has only been discussed but not directly shown so far. Here we present data showing that expression of furin promotes the first step of clathrin-coat assembly at the TGN, the recruitment of the Golgi-specific assembly protein AP-1 on Golgi membranes. Further, we report that furin indeed is present in isolated clathrin-coated vesicles. Packaging into clathrin-coated vesicles requires signal components in the furin cytoplasmic domain which can be recognized by AP-1 assembly proteins. We found that besides depending on the phosphorylation state of a casein kinase II site, interaction of the furin tail with AP-1 and its mu1subunit is mediated by a tyrosine motif and to less extent by a leucine-isoleucine signal, whereas a monophenylalanine motif is only involved in binding to the intact AP-1 complex. This study implies that high affinity interaction of AP-1 or mu1 with the cytoplasmic tail of furin needs a complex interplay of signal components rather than one distinct signal.

EGF receptor signaling stimulates SRC kinase phosphorylation of clathrin, influencing clathrin redistribution and EGF uptake

Epidermal growth factor (EGF) binding to its receptor causes rapid phosphorylation of the clathrin heavy chain at tyrosine 1477, which lies in a domain controlling clathrin assembly. EGF-mediated clathrin phosphorylation is followed by clathrin redistribution to the cell periphery and is the product of downstream activation of SRC kinase by EGF receptor (EGFR) signaling. In cells lacking SRC kinase, or cells treated with a specific SRC family kinase inhibitor, EGF stimulation of clathrin phosphorylation and redistribution does not occur, and EGF endocytosis is delayed. These observations demonstrate a role for SRC kinase in modification and recruitment of clathrin during ligand-induced EGFR endocytosis and thereby define a novel effector mechanism for regulation of endocytosis by receptor signaling.

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.

Interactions of the cytoplasmic domains of human and simian retroviral transmembrane proteins with components of the clathrin adaptor complexes modulate intracellular and cell surface expression of envelope glycoproteins

The cytoplasmic domains of the transmembrane (TM) envelope proteins (TM-CDs) of most retroviruses have a Tyr-based motif, YXXO, in their membrane-proximal regions. This signal is involved in the trafficking and endocytosis of membrane receptors via clathrin-associated AP-1 and AP-2 adaptor complexes. We have used CD8-TM-CD chimeras to investigate the role of the Tyr-based motif of human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency virus (SIV), and human T-leukemia virus type 1 (HTLV-1) TM-CDs in the cell surface expression of the envelope glycoprotein. Flow cytometry and confocal microscopy studies showed that this motif is a major determinant of the cell surface expression of the CD8-HTLV chimera. The YXXO motif also plays a key role in subcellular distribution of the envelope of lentiviruses HIV-1 and SIV. However, these viruses, which encode TM proteins with a long cytoplasmic domain, have additional determinants distal to the YXXO motif that participate in regulating cell surface expression. We have also used the yeast two-hybrid system and in vitro binding assays to demonstrate that all three retroviral YXXO motifs interact with the micro1 and micro2 subunits of AP complexes and that the C-terminal regions of HIV-1 and SIV TM proteins interact with the beta2 adaptin subunit. The TM-CDs of HTLV-1, HIV-1, and SIV also interact with the whole AP complexes. These results clearly demonstrate that the cell surface expression of retroviral envelope glycoproteins is governed by interactions with adaptor complexes. The YXXO-based signal is the major determinant of this interaction for the HTLV-1 TM, which contains a short cytoplasmic domain, whereas the lentiviruses HIV-1 and SIV have additional determinants distal to this signal that are also involved.

A leucine-based determinant in the epidermal growth factor receptor juxtamembrane domain is required for the efficient transport of ligand-receptor complexes to lysosomes

Ligand binding causes the epidermal growth factor (EGF) receptor to undergo accelerated internalization with eventual degradation in lysosomes. The goal of this study was to investigate the molecular basis of endocytic sorting, focussing on post-internalization events. We have identified a sequence located between amino acid residues 675 and 697, encompassing a dileucine motif at residues 679 and 680, that enhances endosome-to-lysosome transport when conformational restraints in the EGF receptor carboxyl terminus are removed by truncation. The same dileucine motif is also necessary for efficient lysosomal transport of ligand-occupied full-length EGF receptors. A L679A,L680A substitution diminished the degradation of occupied full-length EGF receptors without affecting internalization but had a significant effect on recycling. Rapid recycling of mutant receptors resulted in reduced intracellular retention of occupied EGF receptors and delayed down-regulation of cell surface receptors. We propose that the L679A,L680A substitution acts primarily to impair transport of ligand-receptor complexes through an early endosomal compartment, diverting occupied receptors to a recycling compartment at the expense of incorporation into lysosome transport vesicles. We also found that mutant receptors with truncations at the distal half of tyrosine kinase domain (residues 809-957) were not efficiently delivered to the cell surface but were destroyed in an endoplasmic reticulum-associated degradative pathway.