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PubMed1995

Targeting signals and subunit interactions in coated vesicle adaptor complexes.

There are two clathrin-coated vesicle adaptor complexes in the cell, one associated with the plasma membrane and one associated with the TGN. The subunit composition of the plasma membrane adaptor complex is alpha-adaptin, beta-adaptin, AP50, and AP17; while that of the TGN adaptor complex is gamma-adaptin, beta'-adaptin, AP47, and AP19. To search for adaptor targeting signals, we have constructed chimeras between alpha-adaptin and gamma-adaptin within their NH2-terminal domains. We have identified stretches of sequence in the two proteins between amino acids approximately 130 and 330-350 that are essential for targeting. Immunoprecipitation reveals that this region determines whether a construct coassemblies with AP50 and AP17, or with AP47 and AP19. These observations suggest that these other subunits may play an important role in targeting. In contrast, beta- and beta'-adaptins are clearly not involved in this event. Chimeras between the alpha- and gamma-adaptin COOH-terminal domains reveal the presence of a second targeting signal. We have further investigated the interactions between the adaptor subunits using the yeast two-hybrid system. Interactions can be detected between the beta/beta'-adaptins and the alpha/gamma-adaptins, between the beta/beta'-adaptins and the AP50/AP47 subunits, between alpha-adaptin and AP17, and between gamma-adaptin and AP19. These results indicate that the adaptor subunits act in concert to target the complex to the appropriate membrane.

Adaptor Protein Complex 1↗
PubMed1990

Conservation and diversity in families of coated vesicle adaptins.

The complete sequence of the beta adaptin subunit of the plasma membrane adaptor complex from coated vesicles has been elucidated. Complementary cDNA clones from human fibroblasts, rat lymphocytes, and bovine lymphocytes have been isolated, sequenced, and compared with each other and with beta adaptin sequences from rat brain (Kirchhausen, T., Nathanson, K.L., Matsui, W., Vaisberg, A., Chow, E.P., Burne, C., Keen, J.H., and Davis, A.E. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 2612-2616). Surprisingly, the 937-amino acid beta adaptin polypeptide is totally conserved between species. This remarkable homology contrasts with the absence of significant sequence similarity between the alpha (Robinson, M.S. (1989) J. Cell Biol. 108, 833-842) and beta adaptins of the plasma membrane adaptor complex. Diversity within each adaptin family is created by the expression of different genes and by tissue-specific differential splicing. The structures of the beta and alpha adaptins can both be divided into two globular domains interconnected by a variable and potentially flexible stalk domain.

Adaptor Protein Complex alpha Subunits↗
PubMed1992

The mechanism of receptor-mediated endocytosis: more questions than answers.

Receptor-mediated endocytosis occurs via clathrin-coated pits and is therefore coupled to the dynamic cycle of assembly and disassembly of the coat constituents. These coat proteins comprise part, but certainly not all, of the machinery involved in the recognition of membrane receptors and their selective packaging into transport vesicles for internalization. Despite considerable knowledge about the biochemistry of coated vesicles and purified coat proteins, little is known about the mechanisms of coated pit assembly, receptor-sorting and coated vesicle formation. Cell-free assays which faithfully reconstitute these events provide powerful new tools with which to elucidate the overall mechanism of receptor-mediated endocytosis.

Adaptor Protein Complex alpha Subunits↗
PubMed1992

Recruitment of coat proteins onto Golgi membranes in intact and permeabilized cells: effects of brefeldin A and G protein activators.

Brefeldin A (BFA) causes a rapid redistribution of coat proteins (e.g., gamma-adaptin) associated with the clathrin-coated vesicles that bud from the trans-Golgi network (TGN), while the clathrin-coated vesicles that bud from the plasma membrane are unaffected. gamma-Adaptin redistributes with the same kinetics as beta-COP, a coat protein associated with the non-clathrin-coated vesicles that bud from the Golgi complex. Upon removal of BFA, however, gamma-adaptin recovers its perinuclear distribution more rapidly. Redistribution of both proteins can be prevented by pretreating cells with AlF4-. Recruitment of adaptors from the cytosol onto the TGN membrane has been reconstituted in a permeabilized cell system and is increased by addition of GTP gamma S and blocked by addition of BFA. These results suggest a role for G proteins in the control of the clathrin-coated vesicle cycle at the TGN and further extend the similarities between clathrin-coated vesicles and non-clathrin-coated vesicles.

Adaptor Protein Complex alpha Subunits↗
PubMed1992

100-kD proteins of Golgi- and trans-Golgi network-associated coated vesicles have related but distinct membrane binding properties.

The 100-110-kD proteins (alpha-, beta-, beta'-, and gamma-adaptins) of clathrin-coated vesicles and the 110-kD protein (beta-COP) of the nonclathrin-coated vesicles that mediate constitutive transport through the Golgi have homologous protein sequences. To determine whether homologous processes are involved in assembly of the two types of coated vesicles, the membrane binding properties of their coat proteins were compared. After treatment of MDBK cells with the fungal metabolite Brefeldin A (BFA), beta-COP was redistributed to the cytoplasm within 15 s, gamma-adaptin and clathrin in the trans-Golgi network (TGN) dispersed within 30 s, but the alpha-adaptin and clathrin present on coated pits and vesicles derived from the plasma membrane remained membrane associated even after a 15-min exposure to BFA. In PtK1 cells and MDCK cells, BFA did not affect beta-COP binding or Golgi morphology but still induced redistribution of gamma-adaptin and clathrin from TGN membranes to the cytoplasm. Thus BFA affects the binding of coat proteins to membranes in the Golgi region (Golgi apparatus and TGN) but not plasma membranes. However, the Golgi binding interactions of beta-COP and gamma-adaptin are distinct and differentially sensitive to BFA. BFA treatment did not release gamma-adaptin or clathrin from purified clathrin-coated vesicles, suggesting that their distribution to the cytoplasm after BFA treatment of cells was due to interference with their rebinding to TGN membranes after a normal cycle of disassembly. This was confirmed using an in vitro assay in which gamma-adaptin binding to TGN membranes was blocked by BFA and enhanced by GTP gamma S, similar to the binding of beta-COP to Golgi membranes. These results suggest the involvement of GTP-dependent proteins in the association of the 100-kD coat proteins with membranes in the Golgi region of the cell.

Adaptor Protein Complex alpha Subunits↗
PubMed1995

Different domains of the AP-1 adaptor complex are required for Golgi membrane binding and clathrin recruitment.

The assembly of clathrin-coated buds on the Golgi requires the recruitment of the heterotetrameric AP-1 adaptor complex, which is dependent on both guanine nucleotides and the small GTP-binding protein ADP-ribosylation factor (ARF). Here, we have investigated the structural domains of the AP-1 complex necessary for ARF-mediated translocation of the adaptor complex onto Golgi membranes and the subsequent recruitment of clathrin onto the membrane. Controlled proteolysis of purified AP-1, derived from bovine adrenal coated vesicles, was used to generate AP-1 core fragments composed of the amino-terminal trunk regions of the beta 1 and gamma subunits and associated mu 1 and sigma 1 subunits, and lacking either the beta 1 subunit carboxyl-terminal appendage or both beta 1 and gamma subunit appendages. On addition of these truncated fragments to AP-1-depleted adrenal cytosol, both types of core fragments were efficiently recruited onto Golgi membranes in the presence of GTP gamma S. Recruitment of both core fragments was inhibited by the fungal metabolite brefeldin A, indicative of an ARF-dependent process. Limited tryptic digestion of recruited, intact cytosolic AP-1 resulted in the quantitative release of the globular carboxyl-terminal appendage domains of the beta 1 and gamma subunits. The adaptor core complex remained associated with the Golgi membranes. Recruitment of cytosolic clathrin onto the Golgi membranes was strictly dependent on the presence of intact AP-1. Tryptic removal of the beta 1 subunit appendage prevented subsequent clathrin recruitment. We conclude that the structural determinants required for the ARF-mediated binding of cytosolic AP-1 onto Golgi membranes are contained within the adaptor core, and that the carboxyl-terminal appendage domains of the beta 1 and gamma subunits do not play any role in this process. Subsequent recruitment of cytosolic clathrin, however, requires an intact beta 1 subunit.

3T3 Cells↗
PubMed1994

The sequence of a 22.4 kb DNA fragment from the left arm of yeast chromosome II reveals homologues to bacterial proline synthetase and murine alpha-adaptin, as well as a new permease and a DNA-binding protein.

We report the sequencing of a 22,470 bp DNA fragment from the left arm of Saccharomyces cerevisiae chromosome II. Thirteen open reading frames longer than 300 bp provisionally called YBL0520, YBL0401 to YBL0408 and YBL0410 to YBL0413 have been detected. Five genes were previously sequenced: COR1, encoding a core protein of the mitochondrial coenzyme QH2 cytochrome c reductase complex (Tzagaloff and Crivellone, 1986), PRS3, a proteasome subunit gene (Lee et al., 1992), ERD2, coding for a protein involved in the secretory pathway (Semeza et al., 1990), URA7, which encodes a CTP synthetase (Ozier-Kalogeropoulos et al., 1991) and the gene for the ribosomal protein L16 (Pan et al., 1993). Among the others, YBL0406 shows striking homologies to FUR4 (Jund et al., 1988) and DAL4 (Yoo et al., 1992), the uracyl and allantoin permeases; YBL0520 is a DNA-related protein, possibly involved in gene regulation; YBL0412 shares homologies with the mouse alpha-adaptins A and C; and YBL0413 is homologous to a protein of Pseudomonas aeruginosa that is likely to be involved in proline biosynthesis. YBL0401, internal to YBL0520, is probably not expressed.

Adaptor Protein Complex alpha Subunits↗
PubMed1993

Exocytosis of vacuolar apical compartment (VAC) in Madin-Darby canine kidney epithelial cells: cAMP is involved as second messenger.

Vacuolar apical compartment (VAC) is a transient organelle originally observed in Madin-Darby canine kidney (MDCK) epithelial cells impaired from forming cell-cell contacts. VACs are large vacuoles which contain microvilli and apical plasma membrane markers (among others, a 184-kDa plasma membrane protein, AP2), but exclude basolateral membrane markers. Upon reestablishment of cell-cell contacts, VACs are rapidly (within 1 h) exocytosed toward intercellular spaces, after which the apical plasma membrane drifts toward its final destination (Vega-Salas, Salas, and Rodriguez-Boulan. 1988. J. Cell Biol. 107, 1717-1728). In this work, we studied the role of cAMP as a mediator for the exocytosis of VACs. We shifted confluent cells from low to normal calcium medium (thus reestablishing cell-cell contacts and causing VAC exocytosis), a shift which resulted in a significant rise of cellular levels of both total intracellular and protein-bound cAMP. The 8-Br analog of cAMP (8-Br-cAMP) (5-50 microM) caused externalization of the intracellular compartment of AP2 as measured by radioimmunoassay. A similar effect was observed with 3-isobutyl-1-methylxanthine. 8-Br-cAMP also caused the appearance of AP2-positive VAC images in nonpermeabilized cells, namely, VACs that become accessible to extracellular antibodies upon fusion with the plasma membrane. Lanthanum, which abolishes the peak of intracellular free calcium during a calcium switch, failed to block the exocytosis. On the other hand, 12-O-tetradecanoylphorbol-13-acetate induced only a modest exocytic response. Finally, 8-Br-cAMP induced VAC exocytosis in sparse MDCK cells grown in normal calcium medium. These data indicate that cAMP is a mediator between the extracellular signal provided by cell-cell contacts and VAC exocytosis.

1-Methyl-3-isobutylxanthine↗
PubMed1995

Characterization of the human glucocorticoid receptor promoter.

To elucidate the functional elements that are involved in the regulation of the human glucocorticoid receptor (hGR) gene, transient expression, DNase I footprinting, and gel mobility shift analyses were conducted. We found that the hGR promoter region between -700 and +38 bp contained 11 footprinted sites. Deletion of the -374 to -183 bp region, which is highly conserved between human and mouse (93%), induced a 5-24-fold reduction in promoter activity in HeLa, NIH3T3, CV1, and HepG2 cells. Three footprints, FP5, FP6, and FP7, were shown to map to this region. In particular, the FP7 site was found to be within the -374 to -347 bp region. Deletion of this region triggered a significant decline in promoter activity in HeLa and NIH3T3 cells but not in HepG2 cells. AP2 was found to bind FP7. In HepG2 cells AP2 elicited transactivation of the hGR promoter activity. Transfection data revealed that the upstream GC box-rich fragment between -700 and -375 bp induced a 4-7-fold activation of the heterologous tk promoter in an orientation-independent manner. Our studies demonstrate that several transcription factors are involved in regulating GR expression and that AP2 could function as an important positive regulator of GR promoter activity.

3T3 Cells↗
PubMed1995

Ligand-induced endocytosis of epidermal growth factor receptors that are defective in binding adaptor proteins.

Ligand-activated epidermal growth factor receptors (EGFRs) associate with coated pit adaptor proteins (AP2) in vivo, implying a mechanism for receptor retention in coated pits during internalization. Using an in vitro binding assay, we localized the adaptor binding determinant to residues 970-991 of EGFRs and confirmed specificity by competition with a synthetic peptide corresponding to this sequence. A mutant EGFR lacking this AP2 binding determinant did not associate with AP2 in vivo but demonstrated internalization and down-regulation kinetics indistinguishable from its wild-type counterpart. Immunocytochemistry confirmed ligand-induced internalization of the mutant EGFR. These data suggest that endocytic determinants are distinct from AP2 binding determinants and that processes other than association with AP2 regulate endocytosis of EGFRs.

Adaptor Protein Complex 2↗
PubMed1995

The appendage domain of alpha-adaptin is a high affinity binding site for dynamin.

Dynamin is a GTPase that appears to be required for endocytosis. Even though this molecule is known to be in surface-coated pits, the identity of the resident coat proteins that account for this localization is not known. Here we show that dynamin is one of three synaptic terminal proteins that bind with specificity to the appendage domain of alpha-adaptin. Binding is sensitive to both salt and pH levels but is not affected by nucleotides. Using recombinant dynamin expressed in SF9 cells, we estimate that the binding affinity is approximately 200 nM. Binding does not require GTP, and the GTPase activity of dynamin is not stimulated by this interaction. These results suggest that the COOH terminus of alpha-adaptin may be a domain within AP2 that mediates the initial interactions between dynamin and surface-coated pits. This may be an essential step in the regulation of coated pit budding.

Adaptor Protein Complex 2↗
PubMed1989

Cloning of cDNAs encoding two related 100-kD coated vesicle proteins (alpha-adaptins).

Coat proteins of approximately 100-kD (adaptins) are components of the adaptor complexes which link clathrin to receptors in coated vesicles. The alpha-adaptins, which are found exclusively in endocytic coated vesicles, separate into two bands on SDS gels, designated A and C (Robinson, M. S., 1987. J. Cell Biol. 104:887-895). Two distinct cDNAs (sequences 1 and 2) encoding the two alpha-adaptins were cloned from a mouse brain cDNA library. Southern blotting indicates that there is one copy of each of the two alpha-adaptin genes, and that there are no additional closely related genes. Based on the size of the predicted protein products of the two genes (108 and 104 kD), the relative abundance of the two messages in brain and liver, and the reactivity of a sequence 1 fusion protein with different antibodies, it was possible to conclude that sequence 1 codes for A and sequence 2 for C. The two protein sequences are strikingly homologous to each other (84% identical amino acids), the major difference being an additional stretch of 41 amino acids, rich in prolines and acidic residues, inserted into the COOH-terminal half of A. In situ hybridization carried out on mouse brain sections indicates that the same cell type may express both transcripts, but that their relative expressions vary. Antipeptide antibodies are now being raised to find out whether the proteins are localized in functionally distinct populations of endocytic coated vesicles.

Adaptor Protein Complex alpha Subunits↗
PubMed1992

Cytosol- and clathrin-dependent stimulation of endocytosis in vitro by purified adaptors.

Using stage-specific assays for receptor-mediated endocytosis of transferrin (Tfn) into perforated A431 cells we show that purified adaptors stimulate coated pit assembly and ligand sequestration into deeply invaginated coated pits. Late events in endocytosis involving membrane fission and coated vesicle budding which lead to the internalization of Tfn are unaffected. AP2, plasma membrane adaptors, are active at physiological concentrations, whereas AP1, Golgi adaptors, are inactive. Adaptor-dependent stimulation of Tfn sequestration requires cytosolic clathrin, but is unaffected by clathrin purified from coated vesicles suggesting that soluble and assembled clathrin pools are functionally distinct. In addition to adaptors and cytosolic clathrin other, as yet unidentified, cytosolic factors are also required for efficient coated pit invagination. These results provide new insight into the mechanisms and regulation of coated pit assembly and invagination.

Adaptor Protein Complex 1↗
PubMed1994

Programming gene expression in developing epidermis.

As the major proteins of adult keratinocytes, keratins provide biochemical markers for exploring mouse epidermal embryogenesis. Here, we used a modified method of whole-mount in situ hybridization to track skin-specific expression of endogenous keratin mRNAs throughout embryogenesis. To monitor transcriptional regulation, we coupled this with beta-galactosidase expression of a human epidermal keratin promoter-driven transgene. These studies have radically changed our perception of how the program of gene expression becomes established during epidermal development. Specifically, we have discovered that (1) basal keratin (K5 and K14) genes are first detected at E9.5 in a highly regional fashion, and surprisingly as early as the single layered ectodermal stage; (2) the early patterns do not correlate with morphogenesis per se, but rather with regional variations in the embryonic origin of underlying mesenchyme, supporting morphogenetic criteria that early inductive cues are mesenchymal; (3) epidermal keratin genes are expressed in periderm, supporting the notion that this layer arises from ectodermal stratification, even though it is simple epithelial-like in morphology and is subsequently sloughed during development; (4) later embryonic patterns of K5 and K14 gene expression parallel proliferative capacity and not stratification; and (5) K1 and K10 mRNAs are first detected as early as E13.5, and their patterns correlate with differentiation and not stratification. These patterns of epidermal gene expression led us to explore whether potential transcriptional regulators of these genes are expressed similarly. We show that AP2 (but not Sp1) cRNAs hybridize in a pattern similar to, but preceding that of basal keratin cRNAs. Finally, using gene expression in cultured cells, we demonstrate that AP2 has a strong inductive effect on basal keratin expression in a cellular environment that does not normally possess AP2 activity.

Adaptor Protein Complex 2↗
PubMed1995

Expression and localization of alpha-adaptin isoforms.

There are two alpha-adaptin genes, alpha A and alpha C, which in brain encode proteins of of M(r) 108 x 10(3) and 104 x 10(3), respectively. Although both mRNAs can be detected on northern blots of brain and liver, the higher molecular mass polypeptide can only be detected on western blots of brain. Here we explain these observations by showing that alpha A is alternatively spliced and that the protein product in most tissues is different from the one expressed in brain in that it is missing 21 amino acids within the hinge region, giving it a similar mobility to that of alpha C. Monospecific antibodies were raised against the various alpha-adaptin isoforms and used to compare their distribution in cells and tissues. Both alpha A and alpha c are co-assembled into the same coated pits, and the larger isoform of alpha A is co-assembled with the smaller isoforms of alpha-adaptin, both in cells that naturally express it an in transfected cells. Examination of brain and spinal cord sections, labelled either for the larger isoform of alpha A or for alpha C, reveals that that the two are to some extent differentially distributed, consistent with previous in situ hybridisation studies. This finding, combined with the observation that there is considerable variability in the relative expression of the two isoforms in different tissues, indicates that the two genes are switched on in response to different stimuli. Moreover, the larger isoform of alpha A appears to be more efficiently concentrated in the nerve terminals than alpha C, which is found not only at the terminals but also diffusely distributed in the cell bodies and dendrites. This suggests that alpha C may play more of a role in the recycling of membrane components throughout the cell.

3T3 Cells↗
PubMed1994

Microtubule reorganization and lysosome redistribution by a viral v-src oncogene, in mouse Balb/3T3 cells expressing human EGF receptor.

The epidermal growth factor (EGF)-induced endocytosis of its receptor is an obligatory pathway for the cellular regulation of the EGF-specific receptor (EGF-R). BNER4 is a mouse Balb/3T3 cell line transfected with human EGF-R complementary DNA (cDNA). B4/src-13 and B4/src-24 are BNER4 cells transfected with a viral oncogene v-src. Indirect immunofluorescence study demonstrated that EGF-R was mostly localized at the perinuclear region in BNER4 cells at 60 min after EGF addition, whereas it was diffusely distributed throughout the cytoplasm in its v-src transfectants. Double indirect immunofluorescence study further confirmed that EGF-R was localized in lysosomes in BNER4 and B4/src-13 cells at 60 min after EGF addition. Intracellular distribution of the Golgi apparatus, clathrin-coated vesicles and early endosomes were similar in all cell lines. However, the lysosomes detected by anti-lysosomal membrane protein (LGP85) antibodies were diffusely distributed throughout the cytoplasm in the v-src transfectacts. By contrast, in the parental BNER4 cells, the lysosomes were mostly localized in the perinuclear region. The organization of microtubules, but not of actin, was markedly different between BNER4 cells and its v-src transfectants. Nocodazole, which depolymerizes microtubules, altered the distribution of the lysosomes and EGF-R in BNER4 cells. Both intracellular lysosome distribution and microtubule organization in nocodazole-treated BNER4 cells were found to be similar to those in its v-src transfectants without nocodazole treatment. These findings support the notion that changes in lysosome distribution may be correlated with microtubule reorganization by v-src in mouse Balb/3T3 cells.

3T3 Cells↗
PubMed1992

Lysosomal acid phosphatase is internalized via clathrin-coated pits.

The presence of lysosomal acid phosphatase (LAP) in coated pits at the plasma membrane was investigated by immunocytochemistry in thymidine kinase negative mouse L-cells (Ltk-) and baby hamster kidney (BHK) cells overexpressing human LAP (Ltk-LAP and BHK-LAP cells). Double immunogold labeling showed that at various stages of invaginating coated pits LAP colocalized with clathrin and plasma membrane adaptors (HA-2 adaptors). Quantitation of the immunogold label showed similar density of wild-type LAP in coated over non-coated areas of the plasma membrane, whereas an internalization-deficient, truncated mutant of LAP which lacks the cytoplasmic tail was less efficiently included into coated pits. Internalization of anti-LAP antibodies into endosomal vesicles was accompanied by rapid dissociation of the coat proteins as shown by an immunofluorescence assay. The role of clathrin-coated vesicles in internalization of LAP was further corroborated by microinjecting monoclonal antibodies against clathrin or HA-2 adaptors into BHK-LAP cells. Internalization of LAP as detected by an immunofluorescence assay was transiently blocked by microinjected antibodies against clathrin or HA-2 adaptors, whereas unrelated antibodies did not affect internalization. These data suggest that LAP is included into clathrin-coated pits of the plasma membrane for rapid internalization.

Acid Phosphatase↗