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PubMed2004

Plant nuclear envelope proteins.

Compared to research in the animal field, the plant NE has been clearly under-investigated. The available data so far indicate similarities as well as striking differences that raise interesting questions about the function and evolution of the NE in different kingdoms. Despite a seemingly similar structure and organization of the NE, many of the proteins that are integral components of the animal NE appear to lack homologues in plant cells. The sequencing of the Arabidopsis genome has not led to the identification of homologues of animal NE components, but has indicated that the plant NE must have a distinct protein composition different from that found in metazoan cells. Besides providing a selective barrier between the nucleoplasm and the cytoplasm, the plant NE functions as a scaffold for chromatin but the scaffolding components are not identical to those found in animal cells. The NE comprises an MTOC in higher plant cells, a striking difference to the organization of microtubule nucleation in other eukaryotic cells. Nuclear pores are present in the plant NE, but identifiable orthologues of most animal and yeast nucleoporins are presently lacking. The transport pathway through the nuclear pores via the action of karyopherins and the Ran cycle is conserved in plant cells. Interestingly, RanGAP is sequestered to the NE in plant cells and animal cells, yet the targeting domains and mechanisms of attachment are different between the two kingdoms. At present, only a few proteins localized at the plant NE have been identified molecularly. Future research will have to expand the list of known protein components involved in building a functional plant NE.

Active Transport, Cell Nucleus↗
PubMed2004

Import and export at the nuclear envelope.

Eukaryotic cells transport a myriad of molecules between the nucleus and cytoplasm and have evolved a number of related biochemical pathways to achieve this, many of which have been elucidated in recent years. One central and common component to all the pathways is the NPC. NPC components appear to play vital roles in transport and the NPC is structurally dynamic, but whether its role is as a facilitator, a controller or both is yet to be decided and awaits further analysis on the role of individual components in specific pathways.

Active Transport, Cell Nucleus↗
PubMed2005

Nuclear envelope breakdown may deliver an inhibitor of protein phosphatase 1 which triggers cyclin B translation in starfish oocytes.

In vertebrates, enhanced translation of mRNAs in oocytes and early embryos entering M-phase is thought to occur through polyadenylation, involving binding, hyperphosphorylation and proteolytic degradation of Aurora-activated CPEB. In starfish, an unknown component of the oocyte nucleus is required for cyclin B synthesis following the release of G2/prophase block by hormonal stimulation. We have found that CPEB cannot be hyperphosphorylated following hormonal stimulation in starfish oocytes from which the nucleus has been removed. Activation of Aurora kinase, known to interact with protein phosphatase 1 and its specific inhibitor Inh-2, is also prevented. The microinjection of Inh-2 restores Aurora activation, CPEB hyperphosphorylation and cyclin B translation in enucleated oocytes. Nevertheless, we provide evidence that CPEB is in fact hyperphosphorylated by cdc2, without apparent involvement of Aurora or MAP kinase, and that cyclin B synthesis can be stimulated without previous degradation of phosphorylated CPEB. Thus, the regulation of cyclin B synthesis necessary for progression through meiosis can be explained by an equilibrium between CPEB phosphorylation and dephosphorylation, and both aspects of this control may rely on the sole activation of Cdc2 and subsequent nuclear breakdown.

Animals↗
PubMed1989

Nuclear envelope assembly around sperm chromatin in cell-free preparations from Drosophila embryos.

Chicken sperm chromatin initiated an assembly of interphase-like nuclei in a cell-free cytoplasmic preparation from 1-6 h old Drosophila melanogaster embryos. The formation of these interphase-like nuclei from the condensed sperm chromatin happened in a series of distinct steps. Anti-Drosophila lamin monoclonal antibody stained the assembled nuclei in a pattern indistinguishable from normal Drosophila nuclei. This assembly process required an ATP regenerating system and could be blocked by the addition of novobiocin into the cell-free extract.

Adenosine Triphosphate↗
PubMed1984

Functional histone antibody fragments traverse the nuclear envelope.

Factors important in the translocation process of proteins across the nuclear membrane were studied by microinjecting either fluoresceinated nonimmune IgG and F(ab)2 or the corresponding molecules, prepared from antisera to histones, into the nucleus and cytoplasm of human fibroblasts. Intact IgG from both preparations remained at the site of injection regardless of whether it was injected into the nucleus or the cytoplasm. In contrast, nonimmune F(ab)2 distributed uniformly throughout the cell. The F(ab)2 derived from affinity-pure antihistone moves into the nucleus after cytoplasmic injection and remains in the nucleus after nuclear microinjection. The migration of the antihistone F(ab)2 into the nucleus results in inhibition of uridine incorporation in the nuclei of the microinjected cells. We conclude that non-nuclear proteins, devoid of specific signal sequences, traverse the nuclear membrane and accumulate in the nucleus provided their radius of gyration is less than 55A and the nucleus contains binding sites for these molecules. These findings support the model of "quasibifunctional binding sites" as a driving force for nuclear accumulation of proteins. The results also indicate that active F(ab)2 fragments, microinjected into somatic cells, can bind to their antigenic sites suggesting that microinjection of active antibody fragments can be used to study the location and function of nuclear components in living cells.

Biological Transport↗
PubMed2004

Spatial and temporal control of nuclear envelope assembly by Ran GTPase.

Using evidence derived primarily from studies using Xenopus egg extracts, a model for the role of Ran in multiple stages during NE assembly can be proposed (Figure 2). Ran is concentrated on chromatin prior to NE assembly and recruits RCC1 that generates Ran-GTP locally. Recruitment of RCC1 to chromatin may be a specialized mechanism to initiate NE assembly following fertilization of the egg, whereas in somatic cells, RCC1 may be present on chromatin throughout mitosis. Ran-GTP recruits vesicles to the surface of chromatin, and promotes vesicle fusion to form the double membrane of the NE. Ran-GTP may recruit membrane vesicles to chromatin through binding to integral membrane proteins through importin-beta. A transient complex would be formed between Ran-GTP, importin-beta and the target protein, which would be released locally to promote assembly of a precursor complex. GTP hydrolysis by Ran would release importin-beta, but may also play a role in vesicle fusion. Ran-GTP also promotes NPC assembly by releasing nucleoporins such as Nup107 from inhibitory complexes with importin-beta. In vertebrate cells undergoing mitosis, the majority of Ran molecules are excluded from the chromosomes and dispersed into the cytoplasm. Relocalization of Ran to chromatin at the end of mitosis may co-ordinate the initiation of NE assembly with disassembly of the mitotic spindle. The function of Ran in this transition is likely to be coupled to changes in the activity of cyclin-dependent protein kinases and other activities that control the progression of the cell cycle. Thus, changes in the localization of Ran and its regulators provide temporal and spatial control of NE assembly at the end of mitosis.

Animals↗
PubMed2006

Loss of emerin at the nuclear envelope disrupts the Rb1/E2F and MyoD pathways during muscle regeneration.

Emery-Dreifuss muscular dystrophy (EDMD1) is caused by mutations in either the X-linked gene emerin (EMD) or the autosomal lamin A/C (LMNA) gene. Here, we describe the derivation of mice lacking emerin in an attempt to derive a mouse model for EDMD1. Although mice lacking emerin show no overt pathology, muscle regeneration in these mice revealed defects. A bioinformatic array analysis of regenerating Emd null muscle revealed abnormalities in cell-cycle parameters and delayed myogenic differentiation, which were associated with perturbations to transcriptional pathways regulated by the retinoblastoma (Rb1) and MyoD genes. Temporal activation of MyoD transcriptional targets was significantly delayed, whereas targets of the Rb1/E2F transcriptional repressor complex remained inappropriately active. The inappropriate modulation of Rb1/MyoD transcriptional targets was associated with up-regulation of Rb1, MyoD and their co-activators/repressors transcripts, suggesting a compensatory effort to overcome a molecular block to differentiation at the myoblast/myotube transition during regeneration. This compensation appeared to be effective for MyoD transcriptional targets, although was less effective for Rb1 targets. Analysis of Rb1 phosphorylation states showed prolonged hyper-phosphorylation at key developmental stages in Emd null myogenic cells, both in vivo and in vitro. We also analyzed the same pathways in Lmna null muscle, which shows extensive dystrophy. Surprisingly, Lmna null muscle did not show the same perturbations to Rb- and MyoD-dependent pathways. We did observe increased transcriptional expression of Lap2alpha and delayed expression of Rb1, which may regulate alternative transcriptional pathways in the Lmna null myoblasts. We suggest that the dominant LMNA mutations seen in many clinically disparate laminopathies may similarly alter Rb function, with regard to either the timing of exit from the cell cycle or terminal differentiation programs or both.

Animals↗
PubMed1981

Effects of cholesterol on the properties of the membranes of isolated sheep liver nuclei and nuclear envelopes.

The exchangeability of cholesterol between sheep liver nuclear membranes and liposomes, and the effect of cholesterol on the fluidity of the membrane lipid were studied. In intact nuclei, the cholesterol/phospholipid ratio increased from 0.102 to 0.145 mol/mol on incubation with cholesterol-rich liposomes, with a time for half-maximal uptake of 4.2 h. In isolated envelopes under the same conditions, the ratio increased from 0.110 to 0.266 mol/mol with a time for half-maximal uptake of about 1.9 h. Moreover, the approximate order parameter of the spin label 5-(N-oxyl-4',4'-dimethyloxazolidino)-stearic acid was 0.677 in intact nuclei and 0.723 in isolated envelopes prior to exchange; after exchange, these values increased to 0.717 and 0.756, respectively. These differences between the preparations could not be attributed to differences in the capacity for cholesterol uptake between the two nuclear membranes, or to a slow rate of exchange between them; the presence of an intact nuclear matrix appeared both to disorder the lipid partially and to inhibit cholesterol uptake. The differences indicate that conclusions based on physical studies of the membrane lipid in isolated envelopes are not necessarily applicable to the intact nucleus.

Animals↗
PubMed2005

[The significance of anti-nuclear envelope (gp210) antibody in primary biliary cirrhosis].

Primary biliary cirrhosis (PBC) is considered to be an autoimmune disease selectively targeted for interlobular bile ducts. While anti-mitochondrial antibodies are specifically detected in more than 90% of PBC patients, anti-nuclear envelope-gp210 antibodies are also specifically detected in 20-30% of PBC patients. In this review, we present 1, T cells specific for mitochondrial major epitope, PDC-E2 163-176, cross-react with peptides derived from nuclear envelope-gp210 protein, 2, PBC patients who have sustained high antibody titers to gp210 are at high risk for the progression to end-stage hepatic failure. These evidences may be very important for the epitope spreading of autoantigens from PDC-E2 to nuclear antigens and for the identification of target antigens on biliary epithelial cells which are recognized by cytotoxic T cells in PBC.

Autoantibodies↗