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Two independent regions of HIV-1 Nef are required for connection with the endocytic pathway through binding to the mu 1 chain of AP1 complex.

The Nef protein from the human immunodeficiency virus (HIV) induces down-regulation of the CD4 and major histocompatibility complex class I molecules from the cell surface by interfering with the endocytic machinery. This work focuses on the interaction of HIV-1 Nef with the mu 1 chain of adaptor protein type 1 (AP1) complex and its contribution to the Nef-induced alterations of membrane trafficking. Two independent regions surrounding a disordered loop located in the C-terminal part of Nef are involved in mu 1 binding. Each region can separately interact with mu 1, and simultaneous point mutations within both regions are needed to abolish binding. We used CD8 chimeras in which the cytoplasmic tail was replaced by Nef mutants to show that these mu 1-binding sites contain determinants required to induce CD4 down-regulation and to target the chimera to the endocytic pathway by promoting AP1 complex recruitment. Ultrastructural analysis revealed that the CD8-Nef chimera provokes morphological alterations of the endosomal compartments and co-localizes with AP1 complexes. These data indicate that the recruitment by Nef of AP1 via binding to mu 1 participates in the connection of Nef with the endocytic pathway.

Adaptor Protein Complex 1↗

Leucine-specific, functional interactions between human immunodeficiency virus type 1 Nef and adaptor protein complexes.

The human immunodeficiency virus type 1 virulence protein Nef interacts with the endosomal sorting machinery via a leucine-based motif. Similar sequences within the cytoplasmic domains of cellular transmembrane proteins bind to the adaptor protein (AP) complexes of coated vesicles to modulate protein traffic, but the molecular basis of the interactions between these motifs and the heterotetrameric complexes is controversial. To identify the target of the Nef leucine motif, the native sequence was replaced with either leucine- or tyrosine-based AP-binding sequences from cellular proteins, and the interactions with AP subunits were correlated with function. Tyrosine motifs predictably modulated the interactions between Nef and the mu subunits of AP-1, AP-2, and AP-3; heterologous leucine motifs caused little change in these interactions. Conversely, leucine motifs mediated a ternary interaction between Nef and hemicomplexes containing the sigma1 plus gamma subunits of AP-1 or the sigma3 plus delta subunits of AP-3, whereas tyrosine motifs did not. Similarly, only leucine motifs supported the Nef-mediated association of AP-1 and AP-3 with endosomal membranes in cells treated with brefeldin A. Functionally, Nef proteins containing leucine motifs down-regulated CD4 from the cell surface and enhanced viral replication, whereas those containing tyrosine motifs were inactive. Apparently, the interaction of Nef with the mu subunits of AP complexes is insufficient for function. A leucine-specific mode of interaction that likely involves AP hemicomplexes is further required for Nef activity. The mu and hemicomplex interactions may cooperate to yield high avidity binding of AP complexes to Nef. This binding likely underlies the unusual ability of Nef to induce the stabilization of these complexes on endosomal membranes, an activity that correlates with enhancement of viral replication.

Adaptor Protein Complex mu Subunits↗

EpsinR: an AP1/clathrin interacting protein involved in vesicle trafficking.

EpsinR is a clathrin-coated vesicle (CCV) enriched 70-kD protein that binds to phosphatidylinositol-4-phosphate, clathrin, and the gamma appendage domain of the adaptor protein complex 1 (AP1). In cells, its distribution overlaps with the perinuclear pool of clathrin and AP1 adaptors. Overexpression disrupts the CCV-dependent trafficking of cathepsin D from the trans-Golgi network to lysosomes and the incorporation of mannose-6-phosphate receptors into CCVs. These biochemical and cell biological data point to a role for epsinR in AP1/clathrin budding events in the cell, just as epsin1 is involved in the budding of AP2 CCVs. Furthermore, we show that two gamma appendage domains can simultaneously bind to epsinR with affinities of 0.7 and 45 microM, respectively. Thus, potentially, two AP1 complexes can bind to one epsinR. This high affinity binding allowed us to identify a consensus binding motif of the form DFxDF, which we also find in gamma-synergin and use to predict that an uncharacterized EF-hand-containing protein will be a new gamma binding partner.

Adaptor Proteins, Vesicular Transport↗

Modulation of cellular protein trafficking by human immunodeficiency virus type 1 Nef: role of the acidic residue in the ExxxLL motif.

The nef gene contributes to the replication of primate lentiviruses by altering the trafficking of cellular proteins involved in adaptive immunity (class I and II major histocompatibility complex [MHC]) and viral transmission (CD4 and DC-SIGN). A conserved acidic leucine-based sequence (E(160)xxxLL) within human immunodeficiency virus type 1 (HIV-1) Nef binds to the cellular adaptor protein (AP) complexes, which mediate protein sorting into endosomal vesicles. The leucine residues in this motif are required for the down-regulation of CD4 and for the up-regulation of DC-SIGN and the invariant chain of MHC class II, but the role of the acidic residue is unclear. Here, substitution of E160 with uncharged residues impaired the ability of Nef to up-regulate the expression of the invariant chain and DC-SIGN at the cell surface, whereas substitution with a basic residue was required for a similar effect on the down-regulation of CD4. All substitutions of E160 relieved the Nef-mediated block to transferrin uptake. E160 was required for the efficient interaction of Nef with AP-1 and AP-3 and for the stabilization of these complexes on endosomal membranes in living cells. Systematic mutation of the ExxxLL sequence together with correlation of binding and functional data leads to the hypotheses that AP-1 and AP-3 are major cofactors for the effect of Nef on the trafficking of transferrin, are less important but contribute to the modulation of the invariant chain and DC-SIGN, and are least critical for the modulation of CD4. The data suggest that the E160 residue plays a differential role in the modulation of leucine-dependent Nef-targets and support a model in which distinct AP complexes are used by Nef to modulate different cellular proteins.

Adaptor Protein Complex 1↗

Synaptotagmin (Syt) IX is an essential determinant for protein sorting to secretory granules in mast cells.

The secretory granules (SGs) of secretory cells of the hematopoietic lineage, such as the mast cells, are lysosome-related organelles whose membrane proteins travel through the plasma membrane and the endocytic system. Therefore, a mechanism must exist to prevent proteins destined to recycling or to the trans-Golgi network (TGN) from reaching the SGs. We now show that synaptotagmin (Syt) IX, a Syt homologue that is required for recycling from the endocytic recycling compartment (ERC) in rat basophilic leukemia (RBL-2H3) cultured mast cells, is involved in segregating recycling proteins from the SGs. By using as a marker the recycling protein TGN38, which cycles between the TGN, plasma membrane, and the ERC, we show that knock-down of Syt IX results in mistargeting of HA-tagged TGN38 to the SGs. We further demonstrate that Syt IX binds directly the small GTPase ARF1 and associates with the clathrin adaptor complex AP-1. These results therefore implicate Syt IX as an essential factor for the correct sorting of SGs proteins. Moreover, they place Syt IX as part of the machinery that is involved in the formation of transport carriers that mediate SGs protein sorting.

ADP-Ribosylation Factor 1↗

HIV-1 Nef stabilizes AP-1 on membranes without inducing ARF1-independent de novo attachment.

HIV-1 Nef affects the trafficking of numerous cellular proteins to optimize viral replication and evade host defenses. The adaptor protein (AP) complexes, which form part of the cytoplasmic coat of endosomal vesicles, are key cellular co-factors for Nef. Nef binds these complexes and alters their physiologic cycle of attachment and release from membranes. Specifically, while AP-1 normally becomes cytosolic when attachment events are blocked by inhibition of the GTPase cycle of ADP-ribosylation factor-1 (ARF1), the complex remains membrane-associated in Nef-expressing cells. To investigate the mechanism of this effect, we used a permeabilized cell system to detect the de novo attachment of exogenous AP-1 to endosomal membranes. Nef did not mediate de novo attachment independently of ARF1, despite its ability to maintain the association of AP-1 with endosomal membranes when the activity of ARF1 was blocked. We conclude that Nef stabilizes AP complexes on endosomal membranes after ARF1-dependent attachment. This stabilization may facilitate coat formation and stimulate the trafficking of multiple cellular proteins.

ADP-Ribosylation Factor 1↗

Activation-induced expression of carcinoembryonic antigen-cell adhesion molecule 1 regulates mouse T lymphocyte function.

Carcinoembryonic Ag cell adhesion molecule 1 (CEACAM1) consists of highly related homologs in humans and rodents that are characterized by significant alternate splicing generating isoforms capable of negative intracellular signaling by virtue of two immunoreceptor tyrosine-based inhibition motifs in its cytoplasmic (cyt) tail. Although human T cells have been recently observed to express CEACAM1, the expression and function of CEACAM1 in mouse T cells have not been defined. Although resting mouse spleen T cells exhibited no evidence of CEACAM1 on the cell surface, CEACAM1 was rapidly up-regulated on CD4+ and CD8+ T cells after activation with either Con A or anti-CD3 without a requirement for either de novo transcription or translation due to the fact that CEACAM1 was present intracellularly before activation. Using a GST-CEACAM1-cytoplasmic tail fusion protein, it was shown that the cytoplasmic tail of CEACAM1 bound the src homology domain-containing phosphatase 1 and adaptor protein 1 complex in its phosphorylated and nonphosphorylated states, respectively. CEACAM1 ligation with an anti-CEACAM1 mAb resulted in inhibition of an allogeneic MLR and anti-CD3 plus anti-CD28 Ab-induced proliferation of spleen T cells in vitro and inhibition of a delayed-type hypersensitivity response to oxazolone in vivo. Inhibition of the delayed-type hypersensitivity response required that the anti-CEACAM1-specific mAb be present at the time of T cell sensitization. These studies support a role for CEACAM1 as a novel class of immunoreceptor tyrosine-based inhibition motif-bearing regulatory molecules on T cells that are active during early phases of the immune response in mice.

Adaptor Proteins, Vesicular Transport↗

Clathrin assembly lymphoid myeloid leukemia (CALM) protein: localization in endocytic-coated pits, interactions with clathrin, and the impact of overexpression on clathrin-mediated traffic.

The clathrin assembly lymphoid myeloid leukemia (CALM) gene encodes a putative homologue of the clathrin assembly synaptic protein AP180. Hence the biochemical properties, the subcellular localization, and the role in endocytosis of a CALM protein were studied. In vitro binding and coimmunoprecipitation demonstrated that the clathrin heavy chain is the major binding partner of CALM. The bulk of cellular CALM was associated with the membrane fractions of the cell and localized to clathrin-coated areas of the plasma membrane. In the membrane fraction, CALM was present at near stoichiometric amounts relative to clathrin. To perform structure-function analysis of CALM, we engineered chimeric fusion proteins of CALM and its fragments with the green fluorescent protein (GFP). GFP-CALM was targeted to the plasma membrane-coated pits and also found colocalized with clathrin in the Golgi area. High levels of expression of GFP-CALM or its fragments with clathrin-binding activity inhibited the endocytosis of transferrin and epidermal growth factor receptors and altered the steady-state distribution of the mannose-6-phosphate receptor in the cell. In addition, GFP-CALM overexpression caused the loss of clathrin accumulation in the trans-Golgi network area, whereas the localization of the clathrin adaptor protein complex 1 in the trans-Golgi network remained unaffected. The ability of the GFP-tagged fragments of CALM to affect clathrin-mediated processes correlated with the targeting of the fragments to clathrin-coated areas and their clathrin-binding capacities. Clathrin-CALM interaction seems to be regulated by multiple contact interfaces. The C-terminal part of CALM binds clathrin heavy chain, although the full-length protein exhibited maximal ability for interaction. Altogether, the data suggest that CALM is an important component of coated pit internalization machinery, possibly involved in the regulation of clathrin recruitment to the membrane and/or the formation of the coated pit.

Adaptor Proteins, Vesicular Transport↗

Early embryonic death of mice deficient in gamma-adaptin.

Intracellular protein transport and sorting by vesicles in the secretory and endocytic pathways requires the formation of a protein coat on the membrane. The heterotetrameric adaptor protein complex 1 (AP-1) promotes the formation of clathrin-coated vesicles at the trans-Golgi network. AP-1 interacts with various sorting signals in the cytoplasmic tails of cargo molecules, thus indicating a function in protein sorting. We generated mutants of the gamma-adaptin subunit of AP-1 in mice to investigate its role in post-Golgi vesicle transport and sorting processes. gamma-Adaptin-deficient embryos develop until day 3.5 post coitus and die during the prenidation period, revealing that AP-1 is essential for viability. In heterozygous mice the amount of AP-1 complexes is reduced to half of controls. Free beta1- or micro1 chains were not detectable, indicating that they are unstable unless they are part of AP-1 complexes. Heterozygous mice weigh less then their wild-type littermates and show impaired T cell development.

Adaptor Protein Complex gamma Subunits↗

ARF1.GTP, tyrosine-based signals, and phosphatidylinositol 4,5-bisphosphate constitute a minimal machinery to recruit the AP-1 clathrin adaptor to membranes.

At the trans-Golgi network, clathrin coats containing AP-1 adaptor complexes are formed in an ARF1-dependent manner, generating vesicles transporting cargo proteins to endosomes. The mechanism of site-specific targeting of AP-1 and the role of cargo are poorly understood. We have developed an in vitro assay to study the recruitment of purified AP-1 adaptors to chemically defined liposomes presenting peptides corresponding to tyrosine-based sorting motifs. AP-1 recruitment was found to be dependent on myristoylated ARF1, GTP or nonhydrolyzable GTP-analogs, tyrosine signals, and small amounts of phosphoinositides, most prominently phosphatidylinositol 4,5-bisphosphate, in the absence of any additional cytosolic or membrane bound proteins. AP-1 from cytosol could be recruited to a tyrosine signal independently of the lipid composition, but the rate of recruitment was increased by phosphatidylinositol 4,5-bisphosphate. The results thus indicate that cargo proteins are involved in coat recruitment and that the local lipid composition contributes to specifying the site of vesicle formation.

ADP-Ribosylation Factor 1↗

Regulation of activity and localization of the WNK1 protein kinase by hyperosmotic stress.

Mutations within the WNK1 (with-no-K[Lys] kinase-1) gene cause Gordon's hypertension syndrome. Little is known about how WNK1 is regulated. We demonstrate that WNK1 is rapidly activated and phosphorylated at multiple residues after exposure of cells to hyperosmotic conditions and that activation is mediated by the phosphorylation of its T-loop Ser382 residue, possibly triggered by a transautophosphorylation reaction. Activation of WNK1 coincides with the phosphorylation and activation of two WNK1 substrates, namely, the protein kinases STE20/SPS1-related proline alanine-rich kinase (SPAK) and oxidative stress response kinase-1 (OSR1). Small interfering RNA depletion of WNK1 impairs SPAK/OSR1 activity and phosphorylation of residues targeted by WNK1. Hyperosmotic stress induces rapid redistribution of WNK1 from the cytosol to vesicular structures that may comprise trans-Golgi network (TGN)/recycling endosomes, as they display rapid movement, colocalize with clathrin, adaptor protein complex 1 (AP-1), and TGN46, but not the AP-2 plasma membrane-coated pit marker nor the endosomal markers EEA1, Hrs, and LAMP1. Mutational analysis suggests that the WNK1 C-terminal noncatalytic domain mediates vesicle localization. Our observations shed light on the mechanism by which WNK1 is regulated by hyperosmotic stress.

Amino Acid Sequence↗

Avl9p, a member of a novel protein superfamily, functions in the late secretory pathway.

The branching of exocytic transport routes in both yeast and mammalian cells has complicated studies of the late secretory pathway, and the mechanisms involved in exocytic cargo sorting and exit from the Golgi and endosomes are not well understood. Because cargo can be sorted away from a blocked route and secreted by an alternate route, mutants defective in only one route do not exhibit a strong secretory phenotype and are therefore difficult to isolate. In a genetic screen designed to isolate such mutants, we identified a novel conserved protein, Avl9p, the absence of which conferred lethality in a vps1Delta apl2Delta strain background (lacking a dynamin and an adaptor-protein complex 1 subunit). Depletion of Avl9p in this strain resulted in secretory defects as well as accumulation of Golgi-like membranes. The triple mutant also had a depolarized actin cytoskeleton and defects in polarized secretion. Overexpression of Avl9p in wild-type cells resulted in vesicle accumulation and a post-Golgi defect in secretion. Phylogenetic analysis indicated evolutionary relationships between Avl9p and regulators of membrane traffic and actin function.

Actins↗

The clathrin adaptor complex 1 directly binds to a sorting signal in Ste13p to reduce the rate of its trafficking to the late endosome of yeast.

Yeast trans-Golgi network (TGN) membrane proteins maintain steady-state localization by constantly cycling to and from endosomes. In this study, we examined the trafficking itinerary and molecular requirements for delivery of a model TGN protein A(F-->A)-alkaline phosphatase (ALP) to the prevacuolar/endosomal compartment (PVC). A(F-->A)-ALP was found to reach the PVC via early endosomes (EEs) with a half-time of approximately 60 min. Delivery of A(F-->A)-ALP to the PVC was not dependent on either the GGA or adaptor protein 1 (AP-1) type of clathrin adaptors, which are thought to function in TGN to PVC and TGN to EE transport, respectively. Surprisingly, in cells lacking the function of both GGA and AP-1 adaptors, A(F-->A)-ALP transport to the PVC was dramatically accelerated. A 12-residue cytosolic domain motif of A(F-->A)-ALP was found to mediate direct binding to AP-1 and was sufficient to slow TGN-->EE-->PVC trafficking. These results suggest a model in which this novel sorting signal targets A(F-->A)-ALP into clathrin/AP-1 vesicles at the EE for retrieval back to the TGN.

Adaptor Protein Complex 1↗

In vitro assays of Arf1 interaction with GGA proteins.

ADP-ribosylation factor 1 (Arf1) is a GTP-binding protein that regulates membrane traffic. This function of Arf1 is, at least in part, mediated by Arf1 x GTP binding to coat proteins such as coatomer, clathrin adaptor protein (AP) complexes 1 and 3, and gamma-adaptin homology-Golgi associated Arf-binding (GGA) proteins. Binding to Arf1 x GTP recruits these coat proteins to membranes, leading to the formation of transport vesicles. Whereas coatomer and the AP complexes are hetero-oligomers, GGAs are single polypeptide chains. Therefore, working with recombinant GGAs is straightforward compared to the other Arf1 effectors. Consequently, the GGAs have been used as a model for studying Arf1 interactions with effectors and as reagents to determine Arf1 x GTP levels in cells. In this chapter, we describe in vitro assays for analysis of GGA interaction with Arf1 x GTP and for determining intracellular Arf1 x GTP levels.

ADP-Ribosylation Factor 1↗

Mapping the GRIF-1 binding domain of the kinesin, KIF5C, substantiates a role for GRIF-1 as an adaptor protein in the anterograde trafficking of cargoes.

Gamma-aminobutyric acid, type A (GABAA) receptor interacting factor-1 (GRIF-1) and N-acetylglucosamine transferase interacting protein (OIP) 106 are both members of a newly identified coiled-coil family of proteins. They are kinesin-associated proteins proposed to function as adaptors in the anterograde trafficking of organelles to synapses. Here we have studied in more detail the interaction between the prototypic kinesin heavy chain, KIF5C, kinesin light chain, and GRIF-1. The GRIF-1 binding site of KIF5C was mapped using truncation constructs in yeast two-hybrid interaction assays, co-immunoprecipitations, and co-localization studies following expression in mammalian cells. Using these approaches, it was shown that GRIF-1 and the KIF5C binding domain of GRIF-1, GRIF-1-(124-283), associated with the KIF5C non-motor domain. Refined studies using yeast two-hybrid interactions and co-immunoprecipitations showed that GRIF-1 and GRIF-1-(124-283) associated with the cargo binding region within the KIF5C non-motor domain. Substantiation that the GRIF-1-KIF5C interaction was direct was shown by fluorescence resonance energy transfer analyses using fluorescently tagged GRIF-1 and KIF5C constructs. A significant fluorescence resonance energy transfer value was found between the C-terminal EYFP-tagged KIF5C and ECFP-GRIF-1, the C-terminal EYFP-tagged KIF5C non-motor domain and ECFP-GRIF-1, but not between the N-terminal EYFP-tagged KIF5C nor the EYFP-KIF5C motor domain and ECFP-GRIF-1, thus confirming direct association between the two proteins at the KIF5C C-terminal and GRIF-1 N-terminal regions. Co-immunoprecipitation and confocal imaging strategies further showed that GRIF-1 can bind to the tetrameric kinesin light-chain/kinesin heavy-chain complex. These findings support a role for GRIF-1 as a kinesin adaptor molecule requisite for the anterograde delivery of defined cargoes such as mitochondria and/or vesicles incorporating beta2 subunit-containing GABAA receptors, in the brain.

Adaptor Protein Complex 1↗

The mechanism of trans-activation of the MDR1 gene by human T-cell leukemia virus.

Overexpression of P-glycoprotein (P-gp), the protein product of the multidrug resistance gene (MDR1), confers a drug resistant phenotype on cells. We have recently demonstrated that the MDR1 promoter is transcriptionally activated by the HTLV-I tax protein, providing an explanation for the development of drug resistance in HTLV-I infections. Here we report that HTLV-I mediated MDR1 activation is dependent on the presence of an NF-IL6-binding site located between base pairs -148 and -141 relative to the transcription start site. This finding opens up the possibility of moderating P-gp expression through interference with NF-IL6 binding to its trans recognition element and subsequent repression of MDR1 transcription.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

Specific regulation of the adaptor protein complex AP-3 by the Arf GAP AGAP1.

Arf1 regulates membrane trafficking at several membrane sites by interacting with at least seven different vesicle coat proteins. Here, we test the hypothesis that Arf1-dependent coats are independently regulated by specific interaction with Arf GAPs. We find that the Arf GAP AGAP1 directly associates with and colocalizes with AP-3, a coat protein complex involved in trafficking in the endosomal-lysosomal system. Binding is mediated by the PH domain of AGAP1 and the delta and sigma3 subunits of AP-3. Overexpression of AGAP1 changes the cellular distribution of AP-3, and reduced expression of AGAP1 renders AP-3 resistant to brefeldin A. AGAP1 overexpression does not affect the distribution of other coat proteins, and AP-3 distribution is not affected by overexpression of other Arf GAPs. Cells overexpressing AGAP1 also exhibit increased LAMP1 trafficking via the plasma membrane. Taken together, these results support the hypothesis that AGAP1 directly and specifically regulates AP-3-dependent trafficking.

3T3 Cells↗

Anti-inflammatory mechanisms of phenanthroindolizidine alkaloids.

The molecular mechanisms for the anti-inflammatory activity of phenanthroindolizidine alkaloids were examined in an in vitro system mimicking acute inflammation by studying the suppression of lipopolysaccharide (LPS)/interferon-gamma (IFNgamma)-induced nitric oxide production in RAW264.7 cells. Two of the phenanthroindolizidine alkaloids, NSTP0G01 (tylophorine) and NSTP0G07 (ficuseptine-A), exhibited potent suppression of nitric oxide production and did not show significant cytotoxicity to the LPS/IFNgamma-stimulated RAW264.7 cells, in contrast to their respective cytotoxic effects on cancer cells. Tylophorine was studied further to investigate the responsible mechanisms. It was found to inhibit the induced protein levels of tumor necrosis factor-alpha, inducible nitric-oxide synthase (iNOS), and cyclooxygenase (COX)-II. It also inhibited the activation of murine iNOS and COX-II promoter activity. However, of the two common responsive elements of iNOS and COX-II promoters, nuclear factor-kappaB (NF-kappaB) and adaptor protein (AP)1, only AP1 activation was inhibited by tylophorine in the LPS/IFNgamma-stimulated RAW264.7 cells. Further studies showed that the tylophorine enhanced the phosphorylation of Akt and thus decreased the expression and phosphorylation levels of c-Jun protein, thereby causing the subsequent inhibition of AP1 activity. Furthermore, the tylophorine was able to block mitogen-activated protein/extracellular signal-regulated kinase kinase 1 activity and its downstream signaling activation of NF-kappaB and AP1. Thus, NSTP0G01 exerts its anti-inflammatory effects by inhibiting expression of the proinflammatory factors and related signaling pathways.

Adaptor Protein Complex 1↗