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Epsin binds to clathrin by associating directly with the clathrin-terminal domain. Evidence for cooperative binding through two discrete sites.

Epsin is a recently identified protein that appears to play an important role in clathrin-mediated endocytosis. The central region of epsin 1, the so-called DPW domain, binds to the heterotetrameric AP-2 adaptor complex by associating directly with the globular appendage of the alpha subunit. We have found that this central portion of epsin 1 also associates with clathrin. The interaction with clathrin is direct and not mediated by epsin-bound AP-2. Alanine scanning mutagenesis shows that clathrin binding depends on the sequence (257)LMDLADV located within the epsin 1 DPW domain. This sequence, related to the known clathrin-binding sequences in the adaptor beta subunits, amphiphysin, and beta-arrestin, facilitates the association of epsin 1 with the terminal domain of the clathrin heavy chain. Unexpectedly, inhibiting the binding of AP-2 to the GST-epsin DPW fusion protein by progressively deleting DPW triplets but leaving the LMDLADV sequence intact, diminishes the association of clathrin in parallel with AP-2. Because the beta subunit of the AP-2 complex also contains a clathrin-binding site, optimal association with soluble clathrin appears to depend on the presence of at least two distinct clathrin-binding sites, and we show that a second clathrin-binding sequence (480)LVDLD, located within the carboxyl-terminal segment of epsin 1, also interacts with clathrin directly. The LMDLADV and LVDLD sequences act cooperatively in clathrin recruitment assays, suggesting that they bind to different sites on the clathrin-terminal domain. The evolutionary conservation of similar clathrin-binding sequences in several metazoan epsin-like molecules suggests that the ability to establish multiple protein-protein contacts within a developing clathrin-coated bud is an important aspect of epsin function.

Adaptor Protein Complex alpha Subunits↗

Secreted Reelin molecules form homodimers.

During mammalian brain development, neurons are generated along the ventricle, migrate radially, and become aligned in defined patterns. These precise patterns of neuronal alignment are regulated by an extracellular matrix protein Reelin, and binding of Reelin to its receptors induces tyrosine phosphorylation of the intracellular adaptor protein disabled 1 (Dab1). We recently reported that Reelin molecules assemble to form a homomeric protein complex. Although the number of molecules in the full-length complex is unknown, recombinant N-terminal fragments, which contain the epitope for the function-blocking CR-50 antibody, assembled to form a complex of more than 40 monomers. When the N-terminus was deleted from Reelin, the truncated protein did not form a stable complex. To further characterize the Reelin assembly, we performed biochemical analysis of the full-length Reelin assembly in this study. Here, we report that a full-length Reelin forms a disulfide-linked homodimer. A chemical crosslinking experiment on secreted Reelin confirmed that only dimers are formed by the full-length protein. However, interestingly, chemical crosslinking of the N-terminus-truncated Reelin resulted in the formation of larger complexes, in addition to dimers, suggesting that the tertiary structure required for the proper and stable assembly/dimerization was altered by the truncation. The truncated protein did not induce efficient tyrosine phosphorylation of Dab1, although it bound well to the receptors. These findings demonstrate the functional importance of the N-terminal region of Reelin for proper dimerization and signaling. Proper but not simple extracellular crosslinking of the receptors by these dimers may be important for Reelin signaling to occur.

Animals↗

Suppression of insulin receptor substrate 1 (IRS-1) promotes mammary tumor metastasis.

The insulin receptor substrate (IRS) proteins are cytoplasmic adaptors that organize signaling complexes downstream of activated cell surface receptors. Here, we show that IRS-1 and IRS-2, despite significant homology, play critical yet distinct functions in breast cancer, and we identify specific signaling pathways that are influenced by IRS-1 using the polyoma virus middle-T (PyV-MT) transgenic mouse model of mammary carcinoma and Irs-1 null (Irs1(-/-)) mice. The absence of Irs-1 expression enhanced metastatic spread significantly without a significant effect on primary tumor growth. Orthotopic transplant studies revealed that the increased metastatic potential of Irs1-deficient tumor cells is cell autonomous. Mammary tumors that developed in PyV-MT::Irs1(-/-) mice exhibited elevated Irs-2 function and enhanced phosphatidylinositol 3-kinase/Akt/mTor activity, suggesting that one mechanism by which Irs-1 impedes metastasis is to suppress Irs-2-dependent signaling. In support of this mechanism, reduction of Irs-2 expression in Irs1(-/-) tumor cells restored mTor signaling to wild-type levels. PyV-MT::Irs1(-/-) tumors also exhibited a significant increase in vascular endothelial growth factor expression and microvessel density, which could facilitate their dissemination. The significance of our findings for human breast cancer is heightened by our observation that Irs-1 is inactivated in wild-type, metastatic mammary tumors by serine phosphorylation. Collectively, our findings reveal that inactivation of IRS-1 enhances breast cancer metastasis and support the novel hypothesis that IRS-1 has metastasis suppressor functions for breast cancer.

Animals↗

Endothelial Ca2+ waves preferentially originate at specific loci in caveolin-rich cell edges.

Stimulation of endothelial cells (ECs) with ATP evoked an increase in intracellular Ca2+ concentration ([Ca2+]i). In a single bovine aortic EC, the [Ca2+]i rise started at a specific peripheral locus and propagated throughout the entire cell as a Ca2+ wave. The initiation locus was constant upon repeated stimulation with ATP or other agonists (bradykinin and thrombin). The Ca2+ wave was unaffected by the removal of extracellular Ca2+, demonstrating its dependence on intracellular Ca2+ release. Microinjection of heparin into the cell inhibited the ATP-induced Ca2+ responses, indicating that the Ca2+ wave is at least partly mediated by the inositol 1,4, 5-trisphosphate receptor. Immunofluorescence staining revealed that caveolin, a marker protein for caveolae, is distributed heterogeneously in the cell and that Ca2+ waves preferentially originate at caveolin-rich cell edges. In contrast to caveolin, internalized transferrin and subunits of the clathrin-associated adaptor complexes such as adaptor protein-1 and -2 were diffusely distributed. Disruption of microtubules by Colcemid led to redistribution of caveolin away from the edges into the perinuclear center of the cell, and the ATP-induced [Ca2+]i increase was initiated on the rim of the centralized caveolin. Thus, caveolae may be involved in the initiation of ATP-induced Ca2+ waves in ECs.

Adaptor Protein Complex alpha Subunits↗

PKC beta regulates BCR-mediated IKK activation by facilitating the interaction between TAK1 and CARMA1.

The B cell antigen receptor (BCR)-mediated activation of IkappaB kinase (IKK) and nuclear factor-kappaB require protein kinase C (PKC)beta; however, the mechanism by which PKCbeta regulates IKK is unclear. Here, we demonstrate that another protein kinase, TGFbeta-activated kinase (TAK)1, is essential for IKK activation in response to BCR stimulation. TAK1 interacts with the phosphorylated CARMA1 (also known as caspase recruitment domain [CARD]11, Bimp3) and this interaction is mediated by PKCbeta. IKK is also recruited to the CARMA1-Bcl10-mucosal-associated lymphoid tissue 1 adaptor complex in a PKCbeta-dependent manner. Hence, our data suggest that phosphorylation of CARMA1, mediated by PKCbeta, brings two key protein kinases, TAK1 and IKK, into close proximity, thereby allowing TAK1 to phosphorylate IKK.

Amino Acid Sequence↗

Association of insulin-like growth factor 1 receptor with EHD1 and SNAP29.

Ligand-induced receptor-mediated endocytosis plays a central role in regulating signaling conveyed by tyrosine kinase receptors. This process depends on the recruitment of the adaptor protein 2 (AP-2) complex, clathrin, dynamin, and other accessory proteins to the ligand-bound receptor. We show here that besides AP-2 and clathrin, two other proteins participate in the endocytic process of the insulin-like growth factor receptor (IGF-1R); they are EHD1, an Eps15 homology (EH) domain-containing protein 1, and SNAP29, a synaptosomal-associated protein. EHD1 and SNAP29 form complexes with alpha-adaptin of AP-2 and co-localize in endocytic vesicles, indicating a role for them in endocytosis. EHD1 and SNAP29 interact directly with each other and are present in complexes with IGF-1R. After IGF-1 induction, EHD1 and IGF-1R co-localize intracellularly. Overexpression of EHD1 in Chinese hamster ovary cells represses IGF-1-mediated signaling, as measured by mitogen-activated protein kinase phosphorylation and Akt phosphorylation, indicating that EHD1 plays a role as a down-regulator in IGF-1 signaling pathway.

3T3 Cells↗

GADD34-PP1c recruited by Smad7 dephosphorylates TGFbeta type I receptor.

The cascade of phosphorylation is a pivotal event in transforming growth factor beta (TGFbeta) signaling. Reversible phosphorylation regulates fundamental aspects of cell activity. TGFbeta-induced Smad7 binds to type I receptor (TGFbeta type I receptor; TbetaRI) functioning as a receptor kinase antagonist. We found Smad7 interacts with growth arrest and DNA damage protein, GADD34, a regulatory subunit of the protein phosphatase 1 (PP1) holoenzyme, which subsequently recruits catalytic subunit of PP1 (PP1c) to dephosphorylate TbetaRI. Blocking Smad7 expression by RNA interference inhibits association of GADD34-PP1c complex with TbetaRI, indicating Smad7 acts as an adaptor protein in the formation of the PP1 holoenzyme that targets TbetaRI for dephosphorylation. SARA (Smad anchor for receptor activation) enhances the recruitment PP1c to the Smad7-GADD34 complex by controlling the specific subcellular localization of PP1c. Importantly, GADD34-PP1c recruited by Smad7 inhibits TGFbeta-induced cell cycle arrest and mediates TGFbeta resistance in responding to UV light irradiation. The dephosphorylation of TbetaRI mediated by Smad7 is an effective mechanism for governing negative feedback in TGFbeta signaling.

Activin Receptors, Type I↗

Crystal structure of the p14/MP1 scaffolding complex: how a twin couple attaches mitogen-activated protein kinase signaling to late endosomes.

Signaling pathways in eukaryotic cells are often controlled by the formation of specific signaling complexes, which are coordinated by scaffold and adaptor proteins. Elucidating their molecular architecture is essential to understand the spatial and temporal regulation of cellular signaling. p14 and MP1 form a tight (K(d) = 12.8 nM) endosomal adaptor/scaffold complex, which regulates mitogen-activated protein kinase (MAPK) signaling. Here, we present the 1.9-A crystal structure of a biologically functional p14/MP1 complex. The overall topology of the individual MP1 and p14 proteins is almost identical, having a central five-stranded beta-sheet sandwiched between a two-helix and a one-helix layer. Formation of the p14/MP1 heterodimer proceeds by beta-sheet augmentation and yields a unique, almost symmetrical, complex with several potential protein-binding sites on its surface. Mutational analysis allowed identification of the p14 endosomal adaptor motif, which seems to orient the complex relative to the endosomal membrane. Two highly conserved and hydrophobic protein-binding sites are located on the opposite "cytoplasmic" face of the p14/MP1 heterodimer and might therefore function as docking sites for the target proteins extracellular regulated kinase (ERK) 1 and MAPK/ERK kinase 1. Furthermore, detailed sequence analyses revealed that MP1/p14, together with profilins, define a protein superfamily of small subcellular adaptor proteins, named ProflAP. Taken together, the presented work provides insight into the spatial regulation of MAPK signaling, illustrating how p14 and MP1 collaborate as an endosomal adaptor/scaffold complex.

Animals↗

ICP27 recruits Aly/REF but not TAP/NXF1 to herpes simplex virus type 1 transcription sites although TAP/NXF1 is required for ICP27 export.

Herpes simplex virus type 1 (HSV-1) protein ICP27 interacts with the cellular export adaptor protein Aly/REF, which is part of the exon junction complex implicated in cellular mRNA export. We previously reported that Aly/REF was no longer associated with splicing factor SC35 sites during infection but instead colocalized with ICP27 in distinct structures. Here we show that these structures colocalize with ICP4 and are sites of HSV-1 transcription. ICP27 mutants with lesions in the region required for the interaction with Aly/REF failed to recruit Aly/REF to viral transcription sites; however, ICP27 export to the cytoplasm was unimpaired, indicating that the interaction of ICP27 with Aly/REF is not required for ICP27 shuttling. ICP27 has also been shown to interact with the cellular mRNA export receptor TAP/NXF1. We report that ICP27 interacts directly with TAP/NXF1 and does not require Aly/REF to bridge the interaction. The C terminus of ICP27 is required; however, the N-terminal leucine-rich region also contributes to the interaction of ICP27 with TAP/NXF1. In contrast to the results found for Aly/REF, mutants that failed to interact with TAP/NXF1 were not exported to the cytoplasm, and TAP/NXF1 was not recruited to sites of HSV-1 transcription. Therefore, the interaction of ICP27 with TAP/NXF1 occurs after ICP27 leaves viral transcription sites. We conclude that ICP27 and the viral RNAs to which it binds are exported via the TAP/NXF1 export receptor.

Cell Nucleus↗

A direct interaction between the adaptor protein Cbl-b and the kinase zap-70 induces a positive signal in T cells.

Engagement of the T-cell receptor (TCR)-CD3 complex induces a rapid increase in the activities of Src-family and Syk/Zap-70-family kinases [1] [2]. These activated kinases then induce the tyrosine phosphorylation of multiple intracellular proteins, eventually leading to T-cell activation. One of the prominent substrates for these kinases is the adaptor protein Cbl [3] and recent studies suggest that Cbl negatively regulates upstream kinases such as Syk and Zap-70 [4] [5]. Cbl-b, a homologue of Cbl, is widely expressed in many tissues and cells including hematopoietic cells [6] [7]. Cbl-b undergoes rapid tyrosine phosphorylation upon stimulation of the TCR and cytokine receptors [8] [9]. The role of Cbl-b is unclear, however. Here, we show that overexpression of Cbl-b in T cells induced the constitutive activation of the transcription factor nuclear factor of activated T cells (NFAT). A loss-of-function mutation in Cbl-b disrupted the interaction between Cbl-b and Zap-70 and nearly completely abrogated the Cbl-b-mediated activation of NFAT. Unlike the proposed role of Cbl as a negative regulator, our results suggest that the Cbl homologue Cbl-b has a positive role in T-cell signaling, most likely via a direct interaction with the upstream kinase Zap-70.

Adaptor Proteins, Signal Transducing↗

Membrane traffic: how do GGAs fit in with the adaptors?

The AP-1 adaptor complex has been cast as the major player in clathrin coat formation for vesicular transport from the trans-Golgi to the endocytic pathway. But new results on 'GGA' proteins have raised doubts about this paradigm and suggest both a new sorting mechanism and an unexpected complexity in the roles of clathrin.

ADP-Ribosylation Factor 1↗

Quantitative analysis of phosphotyrosine signaling networks triggered by CD3 and CD28 costimulation in Jurkat cells.

The mechanism by which stimulation of coreceptors such as CD28 contributes to full activation of TCR signaling pathways has been intensively studied, yet quantitative measurement of costimulation effects on functional TCR signaling networks has been lacking. In this study, phosphotyrosine networks triggered by CD3, CD28, or CD3 and CD28 costimulation were analyzed by site-specific quantitative phosphoproteomics, resulting in identification of 101 tyrosine and 3 threonine phosphorylation sites and quantification of 87 sites across four cell states. As expected, CD3 stimulation induced phosphorylation of CD3 chains and upstream components of TCR pathways such as Zap70, while CD28 stimulation induced phosphorylation of CD28, Vav-1, and other adaptor proteins including downstream of tyrosine kinase 1, Grb2-associated protein 2 (Grap2), and Wiskott-Aldrich syndrome protein. CD3 and CD28 costimulation induced a complex response including decreased threonine phosphorylation in the ERK1 and ERK2 activation loops and increased phosphorylation of selected tyrosine sites on ERK1/2, p38, phospholipase C-gamma, Src homology 2 domain-containing transforming protein 1, Grap2, and Vav-1, potentiating T cell activation. Hierarchical clustering and self-organizing maps were used to identify modules of coregulated phosphorylation sites within the network. Quantitative information in our study suggests quantitative and qualitative contribution by costimulation of CD28 on CD3-stimulated TCR signaling networks via enhanced phosphorylation of phospholipase C-gamma/Src homology 2 domain-containing transforming protein 1/Grap2/Vav-1 and their effects on downstream components including MAPKs.

Adaptor Proteins, Signal Transducing↗

Regulation of APP-dependent transcription complexes by Mint/X11s: differential functions of Mint isoforms.

Mints/X11s are neuron-specific (Mints 1 and 2) and ubiquitous (Mint 3) adaptor proteins composed of isoform-specific N-terminal sequences and common C-terminal phosphotyrosine-binding (PTB) and PDZ domains. We now show that all three Mints bind to the cytoplasmic tail of amyloid-beta precursor protein (APP) and presenilins and strongly increase the levels of cellular APP in transfected cells. Immunocytochemistry revealed that in neurons, Mints 1 and 2 were colocalized with APP in the trans-Golgi network, with lower levels throughout the cell body and neurites. Using an APP-dependent transactivation assay that uses a fusion protein of APP coupled to the potent transcription factor Gal4/VP16, we examined the effects of Mints on the proteolytic processing and putative transcriptional function of APP. Although all Mints were biochemically similar, only Mints 1 and 2 but not Mint 3 strongly inhibited transactivation by APP-Gal4/VP16. Inhibition was enhanced by a mutation of the first PDZ domain and by deletion of the PDZ domains or the N-terminal sequences but abolished by inactivation of the PTB- and PDZ domains. Mint 1 also inhibited transactivation by the "precleaved" cytoplasmic tail of APP fused to Gal4/VP16, whereas Fe65 (which binds to APP as strongly as Mints) enhanced transactivation. Our data suggest that Mints 1 and 2 but not Mint 3 have a specific effect on APP function that cannot be explained simply by their interaction with presenilins and occurs at least partly after cleavage of APP. In view of their biochemical similarity, the functional differences among Mints are unexpected, suggesting that Mints 1 and 2 have a brain-specific function related to APP that is not executed by the ubiquitous Mint 3.

Adaptor Proteins, Signal Transducing↗

Interaction of endocytic signals from the HIV-1 envelope glycoprotein complex with members of the adaptor medium chain family.

The envelope glycoprotein (Env) complex of HIV-1 undergoes rapid internalization from the plasma membrane of human cells by virtue of a tyrosine-based endocytic signal (RQGYSPL, residues 704-710) in the cytosolic tail of the protein (J. F. Rowell et al., J. Immunol. 155, 473-488, 1995). Here we demonstrate that this tyrosine-based signal interacts with the mu 2 (medium) chain of the AP-2 clathrin-associated adaptor, a protein complex involved in endocytosis of cell surface receptors. The same signal is also capable of interacting with two other members of the adaptor medium chain family, mu 1 and mu 3A, which are components of the AP-1 and AP-3 adaptor complexes, respectively. Interactions with mu 1 and mu 3A might be responsible for the targeting of the internalized envelope glycoprotein to lysosomes or to the basolateral plasma membrane of polarized epithelial cells. A second potential tyrosine-based signal (LFSYHRL, residues 760-766) also interacts with mu 1, mu 2, and mu 3A, although it is less important for internalization in vivo probably due to its position within the cytosolic tail. Overexpression of chimeric proteins having the HIV-1 Env cytosolic tail increases expression of the transferrin receptor on the cell surface, probably due to saturation of the cellular pool of mu 2 by the overexpressed proteins. These observations suggest that HIV-1 Env utilizes the protein sorting machinery of the host cells for internalization and sorting at various steps of the endocytic and biosynthetic pathways.

Adaptor Proteins, Vesicular Transport↗

Centaurin-alpha 1 associates in vitro and in vivo with nucleolin.

Centaurin-alpha(1) was originally described as a binding partner for phosphoinositides. In spite of the presence of a putative ADP-ribosylation factor (ARF) GTPase-activating protein (GAP) domain, no ARF-GAP activity has been attributed to centaurin-alpha(1) so far. Thus the function of this protein remains to be determined. In order to better understand its intracellular role, we aimed to identify centaurin-alpha(1) partners. Using affinity chromatography followed by mass spectrometry analysis, we identified several potential centaurin-alpha(1) protein partners. Nucleolin, a nucleolar protein involved in ribosome biosynthesis, was the main centaurin-alpha(1) interacting protein. The interaction between centaurin-alpha(1) and nucleolin was confirmed by Western blot analysis and GST pull down assays. Moreover, we have shown that ectopically expressed centaurin-alpha(1) associates in vivo with endogenous nucleolin in human embryonic kidney 293 cells. In addition, the association between nucleolin and centaurin-alpha(1) was disrupted by RNAse treatment, indicating that RNA integrity was necessary for their binding. This suggested that centaurin-alpha(1) was part of a ribonucleoprotein complex.

Adaptor Proteins, Signal Transducing↗

Cutting edge: association of phospholipase C-gamma 2 Src homology 2 domains with BLNK is critical for B cell antigen receptor signaling.

To explore the mechanism(s) by which phospholipase C (PLC)-gamma 2 participates in B cell Ag receptor (BCR) signaling, we have studied the function of PLC-gamma 2 mutants in B cells deficient in PLC-gamma 2. Mutation of the N-terminal Src homology 2 domain [SH2(N)] resulted in the complete loss of inositol 1,4, 5-trisphosphate generation upon BCR engagement. A possible explanation for the SH2(N) requirement was provided by findings that this mutation abrogates the association of PLC-gamma 2 with an adaptor protein BLNK. Moreover, expression of a membrane-associated form (CD16/PLC-gamma 2) with SH2(N) mutation required coligation of BCR and CD16 for inositol 1,4,5-trisphosphate generation. Together, our results suggest a central role for the SH2(N) domain in directing PLC-gamma 2 into the close proximity of BCR signaling complex by its association with BLNK, whereby PLC-gamma 2 becomes tyrosine phosphorylated and thereby activated.

Adaptor Proteins, Signal Transducing↗

Re-localization of activated EGF receptor and its signal transducers to multivesicular compartments downstream of early endosomes in response to EGF.

The rapid internalization of receptor tyrosine kinases after ligand binding has been assumed to be a negative modulation of signal transduction. However, accumulating data indicate that signal transduction from internalized cell surface receptors also occurs from endosomes. We show that a substantial fraction of tyrosine-phosphorylated epidermal growth factor receptor (EGFR) and Shc, Grb2 and Cbl after internalization relocates from early endosomes to compartments which are negative for the early endosomes, recycling vesicle markers EEA1 and transferrin in EGF-stimulated cells. These compartments contained the multivesicular body and late endosome marker CD63, and the late endosome and lysosome marker LAMP-1, and showed a multivesicular morphology. Subcellular fractionation revealed that activated EGFR, adaptor proteins and activated ERK 1 and 2 were located in EEA1-negative and LAMP-1-positive fractions. Co-immunoprecipitations showed EGFR in complex with both Shc, Grb2 and Cbl. Treatment with the weak base chloroquine or inhibitors of lysosomal enzymes after EGF stimulation induced an accumulation of tyrosine-phosphorylated EGFR and Shc in EEA1-negative and CD63-positive vesicles after a 120-min chase period. This was accompanied by a sustained activation of ERK 1 and 2. These results suggest that EGFR signaling is not spatially restricted to the plasma membrane, primary vesicles and early endosomes, but is continuing from late endocytic trafficking organelles maturing from early endosomes.

Adaptor Proteins, Signal Transducing↗

A direct interaction between JNK1 and CrkII is critical for Rac1-induced JNK activation.

CrkII, a cellular homolog of v-crk, belongs to a family of adaptor proteins that play a central role in signal transduction cascades. We demonstrate that CrkII interacts directly with c-Jun N-terminal kinase 1 (JNK1). A proline-rich sequence of JNK1 is critical for the interaction of the kinase with the N-terminal Src homology 3 (SH3) domain of CrkII. JNK1 is localized with CrkII in membrane ruffles of Crk-overexpressing cells in a Rac1-dependent manner. A JNK1 mutant (K340A) that fails to interact with CrkII is defective in Rac/epidermal growth factor-induced activation, but remains responsive to UVC irradiation. Furthermore, CrkII recruits JNK1 to a p130Cas multiprotein complex where it may be activated through a hematopoietic progenitor kinase 1- and mitogen-activated protein kinase kinase 4-dependent pathway. Together, the results presented here argue for a new mechanism of regulation of the JNK pathway through the CrkII-p130Cas adaptor complex.

Amino Acid Substitution↗