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PubMed2000

Different patterns of DNA fragmentation and degradation of nuclear matrix proteins during apoptosis induced by radiation, hyperthermia or etoposide.

Several nuclear matrix proteins are substrates for proteolytic cleavage during apoptosis. Using Western blotting, the temporal patterns of cleavage of three nuclear matrix proteins (lamin B, NUMA and the nucleoporin TPR) were compared in HL60 cells induced to undergo apoptosis after irradiation, heat shock or treatment with etoposide. Flow cytometry was used to compare the kinetics of post-cleavage degradation of lamin B, NUMA and TPR after irradiation, and to correlate DNA fragmentation with protein degradation in cells induced to undergo apoptosis with different agents. During radiation-induced apoptosis, cleavage and subsequent degradation of lamin B, NUMA and TPR occurred with different kinetics. Low-molecular-weight DNA fragmentation occurred subsequent to the initiation of NUMA cleavage, coincided with lamin B cleavage, but occurred before more extensive degradation of lamin B and NUMA. A similar sequence was observed for cells treated with etoposide. However, during heat-induced apoptosis, cleavage of lamin B and NUMA occurred much sooner compared to other agents, with NUMA cleaved into multiple fragments within 15 min after heating. We conclude that the hierarchical sequence and kinetics of degradative events contributing to nuclear disassembly during apoptosis are highly dependent on the inducing agent. Furthermore, the nuclear pore complex, like the nuclear lamina and internal nuclear matrix, is a target for proteolytic cleavage.

Antigens, Nuclear↗
PubMed1999

Signal transduction networks and the biology of plant cells.

The development of plant transformation in the mid-1980s and of many new tools for cell biology, molecular genetics, and biochemistry has resulted in enormous progress in plant biology in the past decade. With the completion of the genome sequence of Arabidopsis thaliana just around the corner, we can expect even faster progress in the next decade. The interface between cell biology and signal transduction is emerging as a new and important field of research. In the past we thought of cell biology strictly in terms of organelles and their biogenesis and function, and researchers focused on questions such as, how do proteins enter chloroplasts? or, what is the structure of the macromolecules of the cell wall and how are these molecules secreted? Signal transduction dealt primarily with the perception of light (photomorphogenesis) or hormones and with the effect such signals have on enhancing the activity of specific genes. Now we see that the fields of cell biology and signal transduction are merging because signals pass between organelles and a single signal transduction pathway usually involves multiple organelles or cellular structures. Here are some examples to illustrate this new paradigm. How does abscisic acid (ABA) regulate stomatal closure? This pathway involves not only ABA receptors whose location is not yet known, but cation and anion channels in the plasma membrane, changes in the cytoskeleton, movement of water through water channels in the tonoplast and the plasma membrane, proteins with a farnesyl tail that can be located either in the cytosol or attached to a membrane, and probably unidentified ion channels in the tonoplast. In addition there are highly localized calcium oscillations in the cytoplasm resulting from the release of calcium stored in various compartments. The activities of all these cellular structures need to be coordinated during ABA-induced stomatal closure. For another example of the interplay between the proteins of signal transduction pathways and cytoplasmic structures, consider how plants mount defense responses against pathogens. Elicitors produced by pathogens bind to receptors on the plant plasma membrane or in the cytosol and eventually activate a large number of genes. This results in the coordination of activities at the plasma membrane (production of reactive oxygen species), in the cytoskeleton, localized calcium oscillations, and the modulation of protein kinases and protein phosphatases whose locations remain to be determined. The movement of transcription factors into the nucleus to activate the defense genes requires their release from cytosolic anchors and passage through the nuclear pore complexes of the nuclear envelope. This review does not cover all the recent progress in plant signal transduction and cell biology; it is confined to the topics that were discussed at a recent (November 1998) workshop held in Santiago at which lecturers from Chile, the USA and the UK presented recent results from their laboratories.

Plant Cells↗
PubMed1995

Sequence and characterization of cytoplasmic nuclear protein import factor p97.

Nuclear location sequence-mediated binding of karyophilic proteins to the nuclear pore complexes is one of the earliest steps in nuclear protein import. We previously identified two cytosolic proteins that reconstitute this step in a permeabilized cell assay: the 54/56-kD NLS receptor and p97. A monoclonal antibody to p97 localizes the protein to the cytoplasm and the nuclear envelope. p97 is extracted from nuclear envelopes under the same conditions as the O-glycosylated nucleoporins indicating a tight association with the pore complex. The antibody inhibits import in a permeabilized cell assay but does not affect binding of karyophiles to the nuclear pore complex. Immunodepletion of p97 renders the cytosol inactive for import and identifies at least three other cytosolic proteins that interact with p97. cDNA cloning of p97 shows that it is a unique protein containing 23 cysteine residues. Recombinant p97 binds zinc and a bound metal ion is required for the nuclear envelope binding activity of the protein.

Amino Acid Sequence↗
PubMed1987

Electron microscopic studies of giant nucleus-like structure formed by lambda DNA introduced into the cytoplasm of Xenopus laevis fertilized eggs and embryos.

When bacteriophage lambda DNA was injected into the cytoplasm of the fertilized egg of Xenopus laevis, giant nucleus-like structures were assembled around the injected DNA. These nucleus-like structures survived during cleavage and were partitioned into blastomeres at the blastula stage. The nucleus-like structures formed in the uncleaved fertilized eggs and the blastula cells were both surrounded by a bilayer nuclear membrane with nuclear pore complexes. The ultrastructural features of the lambda DNA-induced nucleus-like structure were considerably different from those of the normal blastula nucleus: although the nuclear pore complexes appeared to be normal, the 'nucleoplasm' was much too homogeneous as compared with that of the normal nucleus.

Animals↗
PubMed2004

Nup88 mRNA overexpression is associated with high aggressiveness of breast cancer.

The nuclear pore complex protein Nup88 is overexpressed in tumor cells. Immunohistochemical studies have shown that this overexpression is linked to higher aggressiveness of colorectal carcinoma and to enhanced metastatic potential of melanoma cells. However, the antibodies so far developed against Nup88 have the drawback of recognizing a number of other, up to now unspecified antigens besides Nup88. For this reason, we devised the present study on Nup88 expression at the mRNA level. RNA was extracted from fresh tumor tissue corresponding to 122 breast cancer patients. Nup88 mRNA expression was measured by means of differential RT-PCR, standardizing against a constitutive internal control gene (beta-actin). The results were dichotomized into "high" and "low" expression levels, using the median value as cut-off. High Nup88 mRNA expression levels correlated significantly with ductal and tubular histology (p = 0.012), histologic and nuclear grade 3 of tumors (p < 0.001), absence of hormone receptor expression (p < 0.001), expression of the c-erb-B2 oncogene (p < 0.001), expression of mutant p53 protein (p < 0.001), high proliferation (defined by Ki67 labeling index >20%, p < 0.001), DNA aneuploidy (p < 0.001) as well as the most important ominous clinical prognostic factor, axillary node invasion (p < 0.001). We also found an inverse correlation (p < 0.001) with expression of the H-MAM (mammaglobin) gene, a marker of low biologic and clinical aggressiveness of breast cancer. All of these factors, without exception, define a highly aggressive tumor phenotype. These findings appear to be specific to Nup88 and not to nuclear pore proteins in general. Indeed, analysis of Nup107 (which is a limiting component of the nuclear pore complex) under the same conditions in the same tumors did not yield comparable results.

Adenocarcinoma↗
PubMed1997

Mediators of nuclear protein import target karyophilic proteins to pore complexes of cytoplasmic annulate lamellae.

Nuclear pore complexes are constitutive structures of the nuclear envelope in eukaryotic cells and represent the sites where transport of molecules between nucleus and cytoplasm takes place. However, pore complexes of similar structure, but with largely unknown functional properties, are long known to occur also in certain cytoplasmic cisternae that have been termed annulate lamellae (AL). To analyze the capability of the AL pore complex to interact with the soluble mediators of nuclear protein import and their karyophilic protein substrates, we have performed a microinjection study in stage VI oocytes of Xenopus laevis. In these cells AL are especially abundant and can easily be identified by light and electron microscopy. Following injection into the cytoplasm, fluorochrome-labeled mediators of two different nuclear import pathways, importin beta and transportin, not only associate with the nuclear envelope but also with AL. Likewise, nuclear localization signals (NLS) of the basic and M9 type, but not nuclear export signals, confer targeting and transient binding of fluorochrome-labeled proteins to cytoplasmic AL. Mutation or deletion of the NLS signals prevents these interactions. Furthermore, binding to AL is abolished by dominant negative inhibitors of nuclear protein import. Microinjections of gold-coupled NLS-bearing proteins reveal specific gold decoration at distinct sites within the AL pore complex. These include such at the peripheral pore complex-attached fibrils and at the central "transporter" and closely resemble those of "transport intermediates" found in electron microscopic studies of the nuclear pore complex (NPC). These data demonstrate that AL can represent distinct sites within the cytoplasm of transient accumulation of nuclear proteins and that the AL pore complex shares functional binding properties with the NPC.

Animals↗
PubMed1989

Ultrastructural analysis of cytoplasmic intermediate filaments and the nuclear lamina in the mouse plasmacytoma cell line MPC-11 after the induction of vimentin synthesis.

We examined cytoplasmic intermediate filaments (IFs) and the nuclear lamina in cells of the mouse plasmacytoma cell line MPC-11 (lacking both IF proteins and lamins A and C) after induction of vimentin synthesis with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) by means of whole-mount immunogold electron microscopy (IEM). The technique of IEM was modified to allow analysis of the cytoskeleton and nuclear lamina of cells grown in suspension culture employing antibodies against vimentin and lamin B. IEM showed that newly synthesized vimentin assembled into IFs which formed anastomosing networks throughout the cytoplasm, radiating primarily from the nucleus. The filaments decorated by gold-conjugated antibodies appeared to make contact with the lipid-depleted nuclear envelope residue either by directly terminating on it or through an indirect link via short fibers of varying diameter. Some filaments terminated on the subunits of the nuclear pore complexes but they did not pass through the pores. In the absence of lamins A and C, lamin B formed a nuclear lamina consisting of a globular-filamentous network anchoring the nuclear pore complexes.

Animals↗
PubMed2004

RanGAP1*SUMO1 is phosphorylated at the onset of mitosis and remains associated with RanBP2 upon NPC disassembly.

The RanGTPase activating protein RanGAP1 has essential functions in both nucleocytoplasmic transport and mitosis. In interphase, a significant fraction of vertebrate SUMO1-modified RanGAP1 forms a stable complex with the nucleoporin RanBP2/Nup358 at nuclear pore complexes. RanBP2 not only acts in the RanGTPase cycle but also is a SUMO1 E3 ligase. Here, we show that RanGAP1 is phosphorylated on residues T409, S428, and S442. Phosphorylation occurs before nuclear envelope breakdown and is maintained throughout mitosis. Nocodazole arrest leads to quantitative phosphorylation. The M-phase kinase cyclin B/Cdk1 phosphorylates RanGAP1 efficiently in vitro, and T409 phosphorylation correlates with nuclear accumulation of cyclin B1 in vivo. We find that phosphorylated RanGAP1 remains associated with RanBP2/Nup358 and the SUMO E2-conjugating enzyme Ubc9 in mitosis, hence mitotic phosphorylation may have functional consequences for the RanGTPase cycle and/or for RanBP2-dependent sumoylation.

Amino Acid Sequence↗
PubMed2006

FG-rich repeats of nuclear pore proteins form a three-dimensional meshwork with hydrogel-like properties.

Nuclear pore complexes permit rapid passage of cargoes bound to nuclear transport receptors, but otherwise suppress nucleocytoplasmic fluxes of inert macromolecules >/=30 kilodaltons. To explain this selectivity, a sieve structure of the permeability barrier has been proposed that is created through reversible cross-linking between Phe and Gly (FG)-rich nucleoporin repeats. According to this model, nuclear transport receptors overcome the size limit of the sieve and catalyze their own nuclear pore-passage by a competitive disruption of adjacent inter-repeat contacts, which transiently opens adjoining meshes. Here, we found that phenylalanine-mediated inter-repeat interactions indeed cross-link FG-repeat domains into elastic and reversible hydrogels. Furthermore, we obtained evidence that such hydrogel formation is required for viability in yeast.

Active Transport, Cell Nucleus↗
PubMed1998

Ultrastructural localization of interferon-inducible double-stranded RNA-activated enzymes in human cells.

The protein kinase PKR and the 2',5'-oligoadenylate (2-5A) synthetase are two interferon-induced and double-stranded RNA-activated enzymes which are implicated in the mechanism of action of interferon. Their distribution was undertaken here at the ultrastructural level by the immunogold procedure, following the use of specific monoclonal antibodies directed against PKR and 69- and 100-kDa forms of the 2-5A synthetase. These enzymes were detected as a pool of nonaggregated proteins scattered throughout the cell and as aggregates often associated with electron-dense doughnut-like structures showing a similar aspect whatever their subcellular localization: the cytoplasm, the nuclear envelope, and the nucleus. In general, the 2-5A synthetases were present in much more lower amounts than the PKR, probably due to the difficulty of detecting traces of proteins by electron microscopy. To circumvent this, we used a human lymphoblastoid cell line overexpressing the 69-kDa form of the 2-5A synthetase. In such cells, the synthetase was then clearly observed in both the cytoplasm and the nucleus; isolated or small clusters of gold particles were numerous in the cell mainly over the RNP fibrils of the interchromatin space, nucleolus, and ribosomes. Interestingly, gold particles were also found to be associated with the membranes of nuclear envelope and rough endoplasmic reticulum probably due to the myristilated motif of this form of 2-5A synthetase. Finally, intensely labeled electron-opaque dots sometimes associated with the nuclear pore complexes were present in the nucleus and in the cytoplasm of cells which might suggest their transport from the nucleus to the cytoplasm or reciprocally through the nuclear pore complexes. These observations indicate the wider distribution of the dsRNA-activated enzymes in the cell, thus pointing out their potential implication in as yet undetermined physiological function(s) necessary for various cellular metabolic reactions.

2',5'-Oligoadenylate Synthetase↗
PubMed2006

Flexible phenylalanine-glycine nucleoporins as entropic barriers to nucleocytoplasmic transport.

Natively unfolded phenylalanine-glycine (FG)-repeat domains are alleged to form the physical constituents of the selective barrier-gate in nuclear pore complexes during nucleocytoplasmic transport. Presently, the biophysical mechanism behind the selective gate remains speculative because of a lack of information regarding the nanomechanical properties of the FG domains. In this work, we have applied the atomic force microscope to measure the mechanical response of individual and clusters of FG molecules. Single-molecule force spectroscopy reveals that FG molecules are unfolded and highly flexible. To provide insight into the selective gating mechanism, an experimental platform has been constructed to study the collective behavior of surface-tethered FG molecules at the nanoscale. Measurements indicate that the collective behavior of such FG molecules gives rise to an exponentially decaying long-range steric repulsive force. This finding indicates that the molecules are thermally mobile in an extended polymer brush-like conformation. This assertion is confirmed by observing that the brush-like conformation undergoes a reversible collapse transition in less polar solvent conditions. These findings reveal how FG-repeat domains may simultaneously function as an entropic barrier and a selective trap in the near-field interaction zone of nuclear pore complexes; i.e., selective gate.

Active Transport, Cell Nucleus↗
PubMed1997

A nuclear localization signal of human aryl hydrocarbon receptor nuclear translocator/hypoxia-inducible factor 1beta is a novel bipartite type recognized by the two components of nuclear pore-targeting complex.

Aryl hydrocarbon receptor nuclear translocator (ARNT) is a component of the transcription factors, aryl hydrocarbon receptor (AhR) and hypoxia-inducible factor 1, which transactivate their target genes, such as CYP1A1 and erythropoietin, in response to xenobiotic aromatic hydrocarbons and to low O2 concentration, respectively. Since ARNT was isolated as a factor required for the nuclear translocation of AhR from the cytoplasm in response to xenobiotics, the subcellular localization of ARNT has been of great interest. In this investigation, we analyzed the subcellular distribution of ARNT using transient expression of a fusion gene with beta-galactosidase and microinjection of recombinant proteins containing various fragments of ARNT in the linker region of glutathione S-transferase/green fluorescent protein. We found a clear nuclear localization of ARNT in the absence of exogenous ligands to AhR, and identified the nuclear localization signal (NLS) of amino acid residues 39-61. The characterized NLS consists of 23 amino acids, and can be classified as a novel variant of the bipartite type on the basis of having two separate regions responsible for efficient nuclear translocation activity, but considerable deviation of the sequence from the consensus of the classical bipartite type NLSs. Like the well characterized NLS of the SV40 T-antigen, this variant bipartite type of ARNT NLS was also mediated by the two components of nuclear pore targeting complex, PTAC58 and PTAC97, to target to the nuclear rim in an in vitro nuclear transport assay.

Amino Acid Sequence↗
PubMed1990

Cytoplasmic transport of ribosomal subunits microinjected into the Xenopus laevis oocyte nucleus: a generalized, facilitated process.

To study the biochemistry of ribonucleoprotein export from the nucleus, we characterized an in vivo assay in which the cytoplasmic appearance of radiolabeled ribosomal subunits was monitored after their microinjection into Xenopus oocyte nuclei. Denaturing gel electrophoresis and sucrose density gradient sedimentation demonstrated that injected subunits were transported intact. Consistent with the usual subcellular distribution of ribosomes, transport was unidirectional, as subunits injected into the cytoplasm did not enter the nucleus. Transport displayed properties characteristic of a facilitated, energy-dependent process; the rate of export was saturable and transport was completely inhibited either by lowering the temperature or by depleting nuclei of ATP; the effect of lowered temperature was completely reversible. Transport of injected subunits was likely a process associated with the nuclear pore complex, since export was also inhibited by prior or simultaneous injection of wheat germ agglutinin, a lectin known to inhibit active nuclear transport by binding to N-acetyl glucosamine-containing glycoproteins present in the NPC (Hart, G. W., R. S. Haltiwanger, G. D. Holt, and W. G. Kelly. 1989. Annu. Rev. Biochem. 58:841-874). Although GlcNAc modified proteins exist on both the nuclear and cytoplasmic sides of the nuclear pore complex, ribosomal subunit export was inhibited only when wheat germ agglutinin was injected into the nucleus. Finally, we found that ribosomal subunits from yeast and Escherichia coli were efficiently exported from Xenopus oocyte nuclei, suggesting that export of some RNP complexes may be directed by a collective biochemical property rather than by specific macromolecular primary sequences or structures.

Adenosine Triphosphate↗
PubMed1999

Taxol affects nuclear lamina and pore complex organization and inhibits import of karyophilic proteins into the cell nucleus.

Treatment of human carcinoma cells with Taxol induces focal unraveling of the nuclear lamina and extensive clustering or ectopic localization of the nuclear pore complexes. These striking aberrations develop when the cells are transferred to drug-free medium and are allowed to complete mitosis. As could be confirmed by terminal deoxynucleotidyl transferase-mediated nick end labeling assays, 4,6-diamidino-2-phenylindole staining, 5-bromo-2-deoxyuridine incorporation, and examination of the nuclear lamins by Western blotting, the malformation of the nuclear envelope is not a consequence of apoptosis or G1 arrest. In fact, Taxol-treated cells possessing a defective nuclear envelope remain alive and replication-competent for at least 24 h, undergoing programmed death 72 h after removal of the drug. While still in the nonapoptotic state, these cells lose the ability to import karyophilic proteins into the nucleus. Diminished nucleocytoplasmic transport through the nuclear pore complex can be readily demonstrated by in vitro assays involving digitonin-permeabilized cells or in vivo monitoring of nuclear factor-kappaB translocation upon stimulation with tumor necrosis factor-alpha. These observations reveal novel cellular targets of antimicrotubule drugs and may pave the way for improved schemes of anticancer treatment.

Apoptosis↗
PubMed2004

A novel allele of Saccharomyces cerevisiae NDC1 reveals a potential role for the spindle pole body component Ndc1p in nuclear pore assembly.

Both the spindle pole body (SPB) and the nuclear pore complex (NPC) are essential organelles embedded in the nuclear envelope throughout the life cycle of the budding yeast Saccharomyces cerevisiae. However, the mechanism by which these two multisubunit structures are inserted into the nuclear envelope during their biogenesis is not well understood. We have previously shown that Ndc1p is the only known integral membrane protein that localizes to both the SPBs and the NPCs and is required for SPB duplication. For this study, we generated a novel temperature-sensitive (ts) allele of NDC1 to investigate the role of Ndc1p at the NPCs. Yeast cells carrying this allele (ndc1-39) failed to insert the SPB into the nuclear envelope at the restrictive temperature. Importantly, the double mutation of ndc1-39 and NPC assembly mutant nic96-1 resulted in cells with enhanced growth defects. While nuclear protein import and NPC distribution in the nuclear envelope were unaffected, ndc1-39 mutants failed to properly incorporate the nucleoporin Nup49p into NPCs. These results provide evidence that Ndc1p is required for NPC assembly in addition to its role in SPB duplication. We postulate that Ndc1p is crucial for the biogenesis of both the SPBs and the NPCs at the step of insertion into the nuclear envelope.

Alleles↗
PubMed1996

A GTPase distinct from Ran is involved in nuclear protein import.

Signal-dependent transport of proteins into the nucleus is a multi-step process mediated by nuclear pore complexes and cytosolic transport factors. One of the cytosolic factors, Ran, is the only GTPase that has a characterized role in the nuclear import pathway. We have used a mutant form of Ran with altered nucleotide binding specificity to investigate whether any other GTPases are involved in nuclear protein import. D125N Ran (XTP-Ran) binds specifically to xanthosine triphosphate (XTP) and has a greatly reduced affinity for GTP, so it is no longer sensitive to inhibition by nonhydrolyzable analogues of GTP such as guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S). using in vitro transport assays, we have found that nuclear import supported by XTP-Ran is nevertheless inhibited by the addition of non-hydrolyzable GTP analogues. This in conjunction with the properties of the inhibitory effect indicates that at least one additional GTPase is involved in the import process. Initial characterization suggests that the inhibited GTPase plays a direct role in protein import and could be a component of the nuclear pore complex.

Biological Transport, Active↗
PubMed1997

Molecular mechanisms of nuclear protein transport.

Transport of proteins into and out of the nucleus occurs through nuclear pore complexes (NPC). A heterodimeric protein complex, composed of karyopherin alpha and beta (or importin alpha and beta) functions to target proteins containing a nuclear localization sequence (NLS) to the NPCs. Two additional proteins, the GTPase Ran and p10, are required to translocate the docked NLS protein into the nucleus. The alpha subunit of karyopherin functions as the NLS receptor, whereas the beta subunit mediates docking to nucleoporins that contain peptide repeats. During import the karyopherin heterodimer dissociates: karyopherin alpha and import substrates enter and accumulate in the nucleoplasm, whereas karyopherin beta accumulates at the nuclear pore complex. Ran-GTP induces dissociation of karyopherin alpha from beta by forming a complex with karyopherin beta and promotes the release of both karyopherin subunits from a docking site. Protein transport across the NPC may occur via guided diffusion involving the karyopherin-mediated docking and undocking of import substrate to multiple binding sites that extend from the cytoplasmic to the nucleoplasmic ends of the NPC.

Animals↗
PubMed1997

Interphase nuclei of many mammalian cell types contain deep, dynamic, tubular membrane-bound invaginations of the nuclear envelope.

The nuclear envelope consists of a double-membraned extension of the rough endoplasmic reticulum. In this report we describe long, dynamic tubular channels, derived from the nuclear envelope, that extend deep into the nucleoplasm. These channels show cell-type specific morphologies ranging from single short stubs to multiple, complex, branched structures. Some channels transect the nucleus entirely, opening at two separate points on the nuclear surface, while others terminate at or close to nucleoli. These channels are distinct from other topological features of the nuclear envelope, such as lobes or folds. The channel wall consists of two membranes continuous with the nuclear envelope, studded with features indistinguishable from nuclear pore complexes, and decorated on the nucleoplasmic surface with lamins. The enclosed core is continuous with the cytoplasm, and the lumenal space between the membranes contains soluble ER-resident proteins (protein disulphide isomerase and glucose-6-phosphatase). Nuclear channels are also found in live cells labeled with the lipophilic dye DiOC6. Time-lapse imaging of DiOC6-labeled cells shows that the channels undergo changes in morphology and spatial distribution within the interphase nucleus on a timescale of minutes. The presence of a cytoplasmic core and nuclear pore complexes in the channel walls suggests a possible role for these structures in nucleo-cytoplasmic transport. The clear association of a subset of these structures with nucleoli would also be consistent with such a transport role.

3T3 Cells↗