Inhibition of the receptor-binding function of clathrin adaptor protein AP-2 by dominant-negative mutant mu2 subunit and its effects on endocytosis.
(25/2017)
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. (+info)
Characterization of a single-copy Arabidopsis gene encoding a protein showing limited similarity to the N-terminus of the mammalian clathrin-assembly protein AP180.
(26/2017)
The Arabidopsis 194 gene encoding a protein containing sequence similarity to an N-terminal region of the clathrin-assembly protein AP180 has been identified in a 4.9-kb region of genomic DNA upstream of the gene encoding the high mobility group protein HMG-I/Y. The gene consists of 12 exons and 11 introns, identified by comparison with partial cDNAs and using the NetPlantGene programme, and encodes a protein of 584 amino acid residues. The C-terminal region of the protein contains 8 tandem repeats of a 17-amino-acid-residue sequence. Southern blot analysis of genomic DNA from Columbia and Landsberg ecotypes of Arabidopsis indicates the presence of a single copy of the 194 gene. The 194 gene is expressed in all organs of Arabidopsis including roots, stems, leaves, flowers, and developing siliques, as determined by northern blot analysis. (+info)
Two elements target SIV Nef to the AP-2 clathrin adaptor complex, but only one is required for the induction of CD4 endocytosis.
(27/2017)
The simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) Nef proteins induce the endocytosis of CD4 and class I MHC molecules. Here we show that SIV Nef interacts with the AP-2 adaptor complex via two elements located in the N-terminal region of the Nef molecule, but only the N-distal element is required to induce CD4 endocytosis. This N-distal AP-2 targeting element contains no canonical endocytic signals and probably contacts the AP-2 complex via a novel interaction surface. The data support a model where SIV Nef induces CD4 endocytosis by promoting the normal interactions between the di-leucine sorting signal in the CD4 cytoplasmic domain and AP-2, but does not substitute for the CD4-AP-2 adaptor interaction. Neither element is important for the induction of class I MHC endocytosis, thus indicating that different mechanisms underlie the induction of class I MHC and CD4 endocytosis by Nef. In contrast to SIV Nef, HIV-1 Nef interacts with AP-2 via a surface containing a di-leucine endocytosis signal in the C-terminal disordered loop of Nef. The fact that genetic selection maintains similar molecular interactions via different surfaces in SIV and HIV-1 Nef proteins indicates that these interactions have critical roles for the viral life cycle in vivo. (+info)
Role of phosphoinositide 3-kinase in activation of ras and mitogen-activated protein kinase by epidermal growth factor.
(28/2017)
The paradigm for activation of Ras and extracellular signal-regulated kinase (ERK)/mitogen-activated protein (MAP) kinase by extracellular stimuli via tyrosine kinases, Shc, Grb2, and Sos does not encompass an obvious role for phosphoinositide (PI) 3-kinase, and yet inhibitors of this lipid kinase family have been shown to block the ERK/MAP kinase signalling pathway under certain circumstances. Here we show that in COS cells activation of both endogenous ERK2 and Ras by low, but not high, concentrations of epidermal growth factor (EGF) is suppressed by PI 3-kinase inhibitors; since Ras activation is less susceptible than ERK2 activation, PI 3-kinase-sensitive events may occur both upstream of Ras and between Ras and ERK2. However, strong elevation of PI 3-kinase lipid product levels by expression of membrane-targeted p110alpha is by itself never sufficient to activate Ras or ERK2. PI 3-kinase inhibition does not affect EGF-induced receptor autophosphorylation or adapter protein phosphorylation or complex formation. The concentrations of EGF for which PI 3-kinase inhibitors block Ras activation induce formation of Shc-Grb2 complexes but not detectable EGF receptor phosphorylation and do not activate PI 3-kinase. The activation of Ras by low, but mitogenic, concentrations of EGF is therefore dependent on basal, rather than stimulated, PI 3-kinase activity; the inhibitory effects of LY294002 and wortmannin are due to their ability to reduce the activity of PI 3-kinase to below the level in a quiescent cell and reflect a permissive rather than an upstream regulatory role for PI 3-kinase in Ras activation in this system. (+info)
Endocytosis: an assembly protein for clathrin cages.
(29/2017)
The protein AP180 is known to have clathrin-assembly activity in vitro. AP180 has now been found to be crucial for synaptic vesicle endocytosis and the maintenance of a uniform-size vesicle population in vivo. These results significantly advance our understanding of clathrin-mediated endocytosis in the synapse and elsewhere. (+info)
Splice variants of intersectin are components of the endocytic machinery in neurons and nonneuronal cells.
(30/2017)
We recently identified and cloned intersectin, a protein containing two Eps15 homology (EH) domains and five Src homology 3 (SH3) domains. Using a newly developed intersectin antibody, we demonstrate that endogenous COS-7 cell intersectin localizes to clathrin-coated pits, and transfection studies suggest that the EH domains may direct this localization. Through alternative splicing in a stop codon, a long form of intersectin is generated with a C-terminal extension containing Dbl homology (DH), pleckstrin homology (PH), and C2 domains. Western blots reveal that the long form of intersectin is expressed specifically in neurons, whereas the short isoform is expressed at lower levels in glia and other nonneuronal cells. Immunofluorescence analysis of cultured hippocampal neurons reveals that intersectin is found at the plasma membrane where it is co-localized with clathrin. Ibp2, a protein identified based on its interactions with the EH domains of intersectin, binds to clathrin through the N terminus of the heavy chain, suggesting a mechanism for the localization of intersectin at clathrin-coated pits. Ibp2 also binds to the clathrin adaptor AP2, and antibodies against intersectin co-immunoprecipitate clathrin, AP2, and dynamin from brain extracts. These data suggest that the long and short forms of intersectin are components of the endocytic machinery in neurons and nonneuronal cells. (+info)
Mu1B, a novel adaptor medium chain expressed in polarized epithelial cells.
(31/2017)
The apical and basolateral plasma membrane domains of polarized epithelial cells contain distinct sets of integral membrane proteins. Biosynthetic targeting of proteins to the basolateral plasma membrane is mediated by cytosolic tail determinants, many of which resemble signals involved in the rapid endocytosis or lysosomal targeting. Since these signals are recognized by adaptor proteins, we hypothesized that there could be epithelial-specific adaptors involved in polarized sorting. Here, we report the identification of a novel member of the adaptor medium chain family, named mu1B, which is closely related to the previously described mu1A (79% amino acid sequence identity). Northern blotting and in situ hybridization analyses reveal the specific expression of mu1B mRNA in a subset of polarized epithelial and exocrine cells. Yeast two-hybrid analyses show that mu1B is capable of interacting with generic tyrosine-based sorting signals. These observations suggest that mu1B may be involved in protein sorting events specific to polarized cells. (+info)
Disabled-2 inactivation is an early step in ovarian tumorigenicity.
(32/2017)
Disabled-2 (Dab2) functions in mitogenic signal transduction pathway, and is frequently activated by homozygous gene deletion in tumors, suggesting that Dab2 is a candidate tumor suppressor. Here, we surveyed the expression of Dab2, and report that Dab2 is expressed in a variety of tissues, and the level of expression is particularly high in ovary and breast. Dab2 expression was also detected in immortalized breast and ovarian epithelial cells. However, in more than a dozen established tumor cell lines derived from breast and ovarian epithelial tumors examined by Western blotting, Dab2 expression was undetectable in 90% of these cell lines. Histological staining of human ovarian tissues with specific anti-Dab2 antibodies indicated that Dab2 is highly expressed in the surface epithelial layer. In an immunohistological study of 26 ovarian carcinomas, 22 (85%) of the tumors were found to lose the expression of Dab2 in the tumor cells, which are epithelial origin. Loss of Dab2 expression is not correlated with tumor grade, suggesting that Dab2 is lost in an early stage of tumorigenicity. Indeed, loss of Dab2 correlates closely with morphological transformation of the surface epithelial cells. Additionally, loss of Dab2 protein occurs in hyperproliferative, but histological benign ovarian epithelium, suggesting that loss of Dab2 occurs in pre-malignant lesions. Thus, this study indicates that the loss of Dab2 expression is correlated with tumorigenicity of the cells disregarding the grade of the tumors, and loss of Dab2 expression is an early event in ovarian malignancies. (+info)