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PubMed2000

C. elegans nuclear envelope proteins emerin, MAN1, lamin, and nucleoporins reveal unique timing of nuclear envelope breakdown during mitosis.

Emerin, MAN1, and LAP2 are integral membrane proteins of the vertebrate nuclear envelope. They share a 43-residue N-terminal motif termed the LEM domain. We found three putative LEM domain genes in Caenorhabditis elegans, designated emr-1, lem-2, and lem-3. We analyzed emr-l, which encodes Ce-emerin, and lem-2, which encodes Ce-MAN1. Ce-emerin and Ce-MAN1 migrate on SDS-PAGE as 17- and 52-kDa proteins, respectively. Based on their biochemical extraction properties and immunolocalization, both Ce-emerin and Ce-MAN1 are integral membrane proteins localized at the nuclear envelope. We used antibodies against Ce-MAN1, Ce-emerin, nucleoporins, and Ce-lamin to determine the timing of nuclear envelope breakdown during mitosis in C. elegans. The C. elegans nuclear envelope disassembles very late compared with vertebrates and Drosophila. The nuclear membranes remained intact everywhere except near spindle poles during metaphase and early anaphase, fully disassembling only during mid-late anaphase. Disassembly of pore complexes, and to a lesser extent the lamina, depended on embryo age: pore complexes were absent during metaphase in >30-cell embryos but existed until anaphase in 2- to 24-cell embryos. Intranuclear mRNA splicing factors disassembled after prophase. The timing of nuclear disassembly in C. elegans is novel and may reflect its evolutionary position between unicellular and more complex eukaryotes.

Amino Acid Sequence↗
PubMedDate not supplied

The calcium store in the nuclear envelope.

The nuclear envelope has a relatively small volume, but is connected up to the vastly larger endoplasmic reticulum. The Ca2+ concentration in the lumen of the interconnected nuclear envelope and endoplasmic reticulum network is in the resting state maintained at a level of more than 100 microM. There are specific Ca2+ release channels present in the inner nuclear membrane that can be activated by inositol trisphosphate or cADP ribose. The system, therefore, allows selective release of Ca2+ into the nucleoplasm which could be important for the control of specific types of gene expression.

Animals↗
PubMed1987

Permeability measurements with closed vesicles from rat liver nuclear envelopes.

Closed nuclear envelope ghosts in the physiological orientation were prepared from rat liver and nuclei as previously described. Here we report transport measurements of various proteins and ribonucleic acids across the envelope of these vesicles. Histones were accumulated rapidly in the ghosts, in contrast to other, nonnuclear, proteins. Triton X-100 removal of the external nuclear membrane from loaded vesicles, as well as comparative studies with open vesicles, excluded the effects of external adsorption. The exchange rate of histones across the nuclear envelope is strongly depressed in the presence of GTP and GDP. The vesicles contain the translocation mechanism for poly(A)-containing RNA. The translocation of poly(A), messenger RNA, and ribosomal RNA was investigated after entrapment of these nucleic acids during the preparation of vesicles. Our data show that the complete export of only poly(A)-containing RNA from the vesicles is enhanced in the presence of 2 mM ATP. This RNA, as well as poly(A), is transported unidirectionally.

Animals↗
PubMed1994

Integral membrane proteins and dynamic organization of the nuclear envelope.

The nuclear envelope is a complex structure consisting of nuclear membranes, nuclear pore complexes and lamina. Several integral membrane proteins specific to the nuclear pore membrane and the inner nuclear membrane are known. Pore membrane proteins are probably important for organization and assembly of the nuclear pore complex, while proteins of the inner nuclear membrane are likely to play major roles in the structure and dynamics of the nuclear lamina and chromatin. Biochemical studies are now identifying potential binding partners for some of these integral membrane proteins, and analysis of nuclear envelope assembly at the end of mitosis is providing important insights into their functions.

Journal Article↗
PubMed1977

Ribonucleic acid stimulation of mammalian liver nuclear-envelope nucleoside triphosphatase. A possible enzymic marker for the nuclear envelope.

1. The specific activity of rat and pig liver nuclear-envelope nucleoside triphosphatase (EC 3.6.1.3) decreases when the system is depleted of RNA. The activity can be restored by adding high concentrations of yeast RNA to the assay medium. 2. Exogenous RNA also increases the activity of the enzyme in control envelopes (not RNA-depleted). The effect appears to be largely specific for poly(A) and poly(G); it is not stimulated by rRNA or tRNA preparations, ribonuclease-hydrolysed RNA, AMP, or double- or single-stranded DNA. 3. Inhibitors of the enzyme, in concentrations at which half-maximal inhibition of the enzyme is achieved, do not affect the percentage stimulation of the enzyme by yeast RNA. 4. The simulation is abolished by the inclusion of 150 mM-KCl or -NaCl in the assay medium, but not by increasing the assay pH to 8.5. 5. The results are discussed in the light of the possible role of the nucleoside triphosphatase in vivo in nucleo-cytoplasmic ribonucleoprotein translocation. 6. It is proposed that poly(G)-stimulated Mg2+-activated adenosine triphosphatase activity should be adopted as an enzymic marker for the nuclear envelope.

Adenosine Triphosphatases↗
PubMed2006

PLCgamma is enriched on poly-phosphoinositide-rich vesicles to control nuclear envelope assembly.

Nuclear envelope assembly is an essential event in each cell cycle but the proteins and lipids involved in its regulation remain mostly unknown. Assembly involves membrane fusions but neither specific SNAREs nor Rab GTPases have been identified in its control. We report that a precursor membrane population (MV1) required for NE assembly has a unique lipid composition consisting prominently of poly-phosphatidylinositides. The lipid composition was determined by adapting HPLC electrospray ionisation tandem mass spectrometry to phosphoinositide analysis, revealing the capacity of this technique to document dynamic lipid transitions of functional importance in natural membrane populations. MV1 is >100-fold enriched in endogenous PLCgamma and >25-fold enriched in the PLC substrate phosphatidylinositol bisphosphate (PtdInsP2) compared to the second membrane population, derived largely from endoplasmic reticulum (ER), that contributes most of the NE. During NE formation PLCgamma becomes transiently phosphorylated at the tyrosine 783 site indicative of its activation. In addition specific inhibition of PLCgamma blocks nuclear envelope formation. In vivo, PLCgamma is concentrated on vesicles of similar size to purified MV1. These associate with nuclei during the period of NE formation and are distinct from ER membranes. The unprecedented concentration of PLCgamma and its substrate PtdInsP2 in a subset of membranes that binds to only two regions of the nucleus, and activation of PLCgamma by GTP during initial stages of NE formation provide a mechanism for temporal control of NE assembly and offer an explanation for how such a process of membrane fusion can be spatially regulated.

Amino Acid Sequence↗
PubMed2002

The nuclear envelope, lamins and nuclear assembly.

The nuclear lamina is composed of both A- and B-type lamins and lamin-binding proteins. Many lamin-binding proteins are integral proteins of the inner nuclear membrane. Lamins and inner nuclear membrane proteins are important for a variety of cell functions, including nuclear assembly, replication, transcription, and nuclear integrity. Recent advances in the field in the past year include the identification of a family of spectrin-repeat-containing inner nuclear membrane proteins and other novel inner-membrane proteins, and the discovery of a nuclear membrane fusion complex. There is also growing evidence that A- and B-type lamins and their binding partners have distinct roles during nuclear assembly and interphase.

A Kinase Anchor Proteins↗
PubMed2000

GTP hydrolysis by Ran is required for nuclear envelope assembly.

Nuclear formation in Xenopus egg extracts requires cytosol and is inhibited by GTP gamma S, indicating a requirement for GTPase activity. Nuclear envelope (NE) vesicle fusion is extensively inhibited by GTP gamma S and two mutant forms of the Ran GTPase, Q69L and T24N. Depletion of either Ran or RCC1, the exchange factor for Ran, from the assembly reaction also inhibits this step of NE formation. Ran depletion can be complemented by the addition of Ran loaded with either GTP or GDP but not with GTP gamma S. RCC1 depletion is only complemented by RCC1 itself or by RanGTP. Thus, generation of RanGTP by RCC1 and GTP hydrolysis by Ran are both required for the extensive membrane fusion events that lead to NE formation.

Amino Acid Substitution↗
PubMed2003

Cell cycle dynamics of the nuclear envelope.

The nuclear envelope (NE) consists of an inner and an outer membrane, nuclear pore complexes, and the underlying nuclear lamina, a filamentous scaffold structure formed by lamins. The inner membrane is linked to the lamina and chromatin by its integral membrane proteins, such as lamin B receptor (LBR), emerin, and various isoforms of lamina-associated polypeptides (LAP) 1 and 2, which bind lamins and/or chromatin. During mitosis, the NE is disassembled upon phosphorylation of its core components, and the NE is torn apart by a dynein-driven microtubule-dependent mechanism. Nuclear reassembly after sister chromatid separation requires a timely coordinated and dephosphorylation-dependent association of lamin-binding proteins and lamins with chromosomal proteins and targeting of membranes to specific sites on chromosomes. Various chromatin-binding domains in lamina proteins, such as the LEM domain, present in all LAP2 isoforms and in emerin, as well as unique regions in lamina proteins and in specific LAP2 isoforms have been implicated in defined steps of NE reformation. Furthermore, novel mechanisms of membrane fusion involving Ran GTPase are just beginning to emerge.

Animals↗
PubMed1998

Ion Permeability of the Nuclear Envelope.

The nuclear envelope mediates nucleocytoplasmic communication. Nuclear pores transport proteins and RNA into and out of the nucleus. The pore is believed to allow free ion diffusion. Using an electrophysiological approach, we show the possible semipermeable properties of the envelope. To accomplish these functions we hypothesize a mechanism in which the pore complex acts as a molecular diaphragm.

Journal Article↗
PubMed1981

Temperature-induced changes in nuclear pore complex frequencies, nuclear envelope surface areas, and nuclear volumes in light-synchronized Euglena.

An autotrophic culture of Euglena, synchronized using a day:night (D:N), 14:10-h cycle, was subjected to a 21.5 leads to 31.5 degrees C temperature shift and then to a reversed shift in temperature after three D:N cycles at 31.5 degrees C. Nuclear pore complex (NPC) number per square micrometre and nuclear surface area and volume determinations were made on G1 cells at various intervals. Cells sampled immediately prior to the 21.5 leads to 31.5 degrees C shift had a mean value of 37.68 NPC . micron-2 nuclear envelope surface area, 30.40 NPCs/micron2 after three D:N cycles at 31.5 degrees C and 39.98 NPCs/micron2 after three D:N cycles at the resumed culture temperature of 21.5 degrees. Thus temperature changes affect NPC numbers per square micrometre and these changes are reversible. Mean nuclear surface area was 125.76 micron2 immediately prior to the 21.5 leads to 31.5 degrees C shift, and decreased over two D:N cycles at 31.5 degrees C to 101.30 micron2 by the end of the third D:N cycle. Nuclear envelope surface area, one and two D:N cycles after the 31.5 leads to 21.5 degrees C shift, was approximately equal that prior to the 21.5 leads to 31.5 degrees C shift. After the third D:N cycle, however, nuclear surface area had increased to 173.05 micron2. The changes in nuclear surface area resulted in large differences in the estimates of the total number of NPCs per nucleus. Euglena immediately prior to the 21.5 leads to 31.5 degrees C temperature shift had 4739 NPCs/nucleus; immediately prior to the 31.5 leads to 21.5 degrees C shift had 3079 NPCs/nucleus; and had 6919 NPCs/nucleus at 21.5 degrees C and three D:N cycles after the 31.5 leads to 21.5 degrees C shift. Estimates of the number of NPCs per cubic micrometre of nuclear volume were almost identical between these samples.

Animals↗
PubMed1980

Nuclear envelope of Chinese hamster ovary cells. Re-formation of the nuclear envelope following mitosis.

We have developed a technique for isolating nuclei and nuclear envelope(s) (NE) from Chinese hamster ovary (CHO) cells which does not depend on the use of detergents to solubilize contaminating cytoplasm. In our procedure NE are prepared from purified nuclei by nuclease digestion and subsequent high salt-sucrose gradient centrifugation. The nuclei and NE fractions are free of significant contamination by other subcellular organelles as judged by electron microscopy and enzyme analysis. Examination of the peptide and glycopeptide composition of the NE fraction by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals a very complex coomassie blue staining profile with prominent bands in the 55 000-75 000 molecular weight range. Using this NE isolation technique, we have examined the breakdown and re-formation of the NE during a limited stage (late G2, M, and early G1) of the replicative cycle in synchronized populations of CHO cells. Our data demonstrate that a minimum of 60% of the early G1 NE protein and a minimum of 50% of the early G1 NE phospholipid were present in the cell during the preceding G2 phase of the cell cycle and were reutilized in the re-formation of the NE occurring during late M and early G1. Our evidence suggests that the vast majority of the newly synthesized peptides and glycopeptides of the NE which appear in the daughter NE are synthesized during the early G1 phase of the replicative cycle. Examination of the NE peptides by one-dimensional gel electrophoresis suggests that no reproducible changes in NE peptide composition can be correlated with specific phases of the cell cycle.

Animals↗
PubMed1981

The acylation of 1-acyl-sn-glycero-3-phosphorylcholine by glial and neuronal nuclei and derived neuronal nuclear envelopes: a comparison of nuclear and microsomal membranes.

A neuronal nuclear fraction (N1), a glial nuclear fraction (N2) and a fraction containing microsomal membranes (P3) were isolated from homogenates of cerebral cortices of 15-day-old rabbits. A nuclear envelope fraction (E) was prepared from fraction N1. In comparison with the parent fraction N1, fraction E had a much lower yield of protein (0.077 mg/g cerebral cortex), a low specific DNA content, an eightfold higher specific phospholipid content (0.85 mumol phospholipid phosphate/mg protein) and a very similar phospholipid distribution profile. Using 100, 50, and 25 microM 1-acyl-sn-glycero-3-phosphorylcholine (labelled with [3H]palmitate) and 100 microM oleoyl CoA, the activity of acyl-CoA:1-acyl-sn-glycero-3-phosphorylcholine acyltransferase was studied in vitro. Fractions N1 and N2 had specific activities which were two to three times the specific activities shown for fraction P3. Fraction E was particularly enriched in this acylation activity and had specific activities which were 6 times those of fraction N1 and 11-19 times those of fraction P3. The existence of nuclear acyl-CoA:1-acyl-sn-glycero-3-phosphorylcholine acyltransferase activity was indicated as was a particularly high concentration of this enzyme within the nuclear envelope. In assays of lysolecithin-lysolecithin transacylase activities, fraction N2 (glial nuclei) showed the highest specific activities, being three to four times those of fractions N1 or P3. This transacylase activity (N2) was as high as 40% of the corresponding acyltransferase activity measured in this fraction using oleoyl CoA as acyl donor.

Animals↗
PubMed1992

The two steps of nuclear import, targeting to the nuclear envelope and translocation through the nuclear pore, require different cytosolic factors.

We have isolated two cytosolic fractions from Xenopus oocytes that contain all of the activity necessary to support both steps of nuclear import in digitonin-permeabilized mammalian cells: binding at the nuclear envelope and translocation through the nuclear pore. The first cytosolic fraction (fraction A) interacts with an import-competent, but not a mutant, nuclear localization sequence-bearing conjugate and stimulates its accumulation at the nuclear envelope in an ATP-independent fashion. The second cytosolic fraction (fraction B) gives no discernible effect when added alone; but when added either together with fraction A, or after fraction A, stimulates the passage of the conjugate from the outer nuclear envelope to the interior of the nucleus in an ATP-dependent fashion.

Amino Acid Sequence↗
PubMed1991

Focal nuclear envelope lesions and specific nuclear lamin A/C dephosphorylation during infection with human cytomegalovirus.

Morphological analysis of cytomegalovirus-infected human fibroblasts reveals characteristic alterations of the nuclear envelope during budding of the nucleocapsids from the nucleus. These changes include focal nuclear lamina thickening and formation of "blebs" at the nuclear membranes. Using a specific monoclonal antibody that recognizes a phosphorylated epitope in nuclear lamins A/C, we show here that the cytopathological alterations at the neighborhood of the nuclear lamina are paralleled by a process of systemic dephosphorylation of the nuclear lamins.

Alkaline Phosphatase↗
PubMed1986

Characterization of multiple epoxide hydrolase activities in mouse liver nuclear envelope.

A nuclear envelope-associated epoxide hydrolase in mouse liver that hydrates trans-stilbene oxide has been identified and characterized. This epoxide hydrolase is distinct from the enzyme in nuclear envelopes that hydrates benzo[a]pyrene 4,5-oxide and other arene oxides. This distinction was demonstrated by the criteria of pH optima, response to specific inhibitors in vitro, and precipitation by specific antibodies. The new epoxide hydrolase had a pH optimum of 6.8, was poorly inhibited by trichloropropene oxide, was potently inhibited by 4-phenylchalcone oxide, and did not bind to antiserum against benzo[a]pyrene 4,5-oxide hydrolase. This nuclear enzyme is similar in many of its properties to cytosolic and microsomal trans-stilbene oxide hydrolases and may be nuclear envelope-bound form of these other epoxide hydrolases. It differed from these other trans-stilbene oxide hydrolases in that its affinities for both trans-stilbene oxide (measured as apparent Km) and 4-phenylchalcone oxide (measured as I50) were 4- to 20-fold lower than those of either the cytosolic or microsomal forms.

Animals↗