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PubMed1999

Nuclear import and export: transport factors, mechanisms and regulation.

Molecules enter and exit the nucleus of eukaryotic cells through aqueous channels formed in the nuclear envelope by nuclear pore complexes (NPC). Proteins entering or leaving the nucleus use nuclear localization signals (NLSs), or nuclear export signals (NESs), respectively. Different types of NLSs and NESs are specifically recognized directly, or indirectly via adapters, by transport receptors. In recent years, many transport receptors, adapters, and the specific cargoes they carry into or out of the nucleus have been identified, revealing an increasing level of complexity. All the transport receptors identified are members of the same family and have in common the ability to shuttle between the nucleus and cytoplasm, and to interact with RanGTP and with nucleoporins at the NPC. The GTPase Ran regulates the interaction between receptors and cargoes, or adapters, and is crucial in providing directionality to nuclear import and export. One of the challenges for the future will be establishing the mechanisms of translocation through the NPC of receptors together with their cargoes. Nuclear import and export of many cargoes is regulated. The molecular mechanisms of regulation of nucleocytoplasmic transport during cell growth, development, viral infections, and different diseases constitute important areas of investigation.

Biological Transport↗
PubMed1999

Immunocytochemical detection of structural and regulatory proteins in rat adrenal nuclear matrix.

The nuclear matrix is a specific cell structure consisting of a residual nucleoskeleton that extends from the nucleoli to the nuclear envelope. The nuclear matrix of steroidogenic cells was isolated previously from a purified nuclear fraction. We present here an in situ extraction method, modified Lutz's method, for rat glandular adrenal cell nuclear matrix. This residual organelle was characterized and studied using immunocytochemical methods. The adrenal glands were removed, the cells prepared in suspension and deposited by cytospin onto Poly-L-lysine glass slides. The nuclear matrix was extracted with Nonidet P-40, DNase I and high and low ionic strength buffers. Structural proteins, nuclear lamins, coilin and fibrillarin were detected immunocytochemically. The adrenal fasciculata cells were easily identified by this method because of their large nuclei and abundant lipid droplets in the cytoplasm. After immunocytochemical detection by antibodies against lamins A and C, a marked brown layer at the periphery of the nucleus was observed. The intensity of the staining was lower using the antibody against nuclear lamin B. Immunocytochemical detection of the protein coilin revealed punctuated stained areas, 2-6 per nucleus, that probably correspond to the coiled bodies. The protein fibrillarin was detected at the nucleolus and coiled bodies. Our technique is simple, reveals well preserved adrenal nuclear matrices, and may be a useful method for immunocytochemical analysis and in situ hybridization.

Adrenal Glands↗
PubMed2002

[Nuclear pores: from yeast to higher eukaryotes].

In eukaryotes, bidirectional transport of macro-molecules between the cytoplasm and the nucleus occurs through elaborate supramolecular structures embedded in the nuclear envelope, the nuclear pore complexes (NPCs), whose overall architecture has been evolutionary conserved from yeast to vertebrates. In recent years, fast progress in characterizing the NPCs components (the nucleoporins or Nups) has been made in the yeast S. cerevisiae, and to a lesser extent in vertebrates. In addition, despite the low homology between most yeast and vertebrate nucleoporins, their organization and their topological mapping within the NPC substructures have been broadly conserved during evolution.

Animals↗
PubMed1992

[Cell fine structures observed by scanning electron microscopy].

The scanning electron microscope (SEM) provides vivid seemingly three dimensional images which are easier to understand for us than transmission electron microscopic images. For this point of view scanning electron microscopy is advantageous in morphological researches of cell fine structures. Nevertheless, there were few studies in this field, because SEM had much lower resolution than transmission electron microscope (TEM) and because there was no adequate method to reveal intracellular structures. In recent years, however, the resolution of SEM has been markedly improved and the specimen preparation techniques have also advanced. In this paper, some of our preparation technique for revealing cell surface structures or intracellular structures, in particular, osmium-DMSO-osmium method, and the results observed by these methods were described. 1) Nucleus. The nucleus was wrapped with a nuclear envelope that consisted of two membranes enclosing a narrow space. On the surface of the envelope many nuclear pores were observed. 2) Endoplasmic reticulum (ER). Rough ER consisted of flattened cisternae, arranged in parallel. The surface were studded with many ribosomes which were often arranged spirally to form polysomes. Smooth ER consisted of tubules. 3) Golgi complex. a) The Golgi stacks were all linked by anastomosing. b) Connection between Golgi stacks and rough ER was often observed. c) Cisternae in a Golgi stack were connected each other. 4) Mitochondria. The mitochondrion was bounded by 2 sheets of unit membrane and the inner membrane projected into the interior of the organelles to make mitochondrial cristae.

Animals↗
PubMed1992

Ultrastructural findings of congenital dyserythropoietic sickle cell beta thal-associated anemia.

The ultrastructural findings of erythroblasts and reticulocytes in one case of congenital dyserythropoiethic anemia (CDA) associated with a haemoglobinopathy, sickle cell beta thalassemia minor (Type V CDA), is described. The observations can be summarized as follows: 1) A lot of large breaks are present in the erythroblast nuclear envelope. 2) Nuclear membrane evaginations are filled with dense loose chromatin. 3) Electron-transparent areas (moth eaten chromatin) are evident in dense chromatin. 4) Electron-dense granular material, related to altered haemoglobin chain storage, is evident in the nucleus and in the cytoplasm. 5) Iron deposits are present in mitochondrial matrix. 6) Myelinic figures are present in reticulocyte cytoplasm. For the first time the ultrastructural findings in this type of associated CDA are described and related to the double origin of clinical symptoms.

Anemia, Dyserythropoietic, Congenital↗
PubMed1992

[Ultrastructure of boar sperm membranes prepared by freeze-fracturing].

The objective of the present paper was to describe the topographic orientation of intramembraneous particles (IMP) in the membranes of freshly ejaculated boar spermatozoa applying the method of freeze fracturing. Disc clusters of IMP's could be distinguished in the acrosome-covering plasma membrane (PF). The border of the head to a distance of about 0.3 micron seemed to contain no IMP's (Figs. 1, 2). In the postacrosomal region in an anterior direction from the posterior ring the IMP's were found to be arranged in palissade slant rows. Statistical measurements of 20 spermatozoa (Tab. I) indicated that the slant rows extended to the greatest distance in the lateral part of the head, up to 0.67 micron (+/- 1.45-0.39) from the posterior ring. In the middle of the head the rows extended to a distance of 0.18 micron (+/- 0.10-0.42). The transition spot between the zone of sparse IMP's and the zone of densely arranged IMP's was at a distance of 1.63 microns (+/- 1.87-0.69) in an anterior direction from the posterior ring (Figs. 3, 4). In the flagellum in the plasma membrane (PF) the first spot of an ample occurrence of IMP's is located in the first mitochondrion in the spiral. The IMP clusters follow the cicumferential orientation of mitochondria in the mitochondrial spiral. The IMP's were missing in the spaces between the spiral coils (Fig. 5). The membranes of the mitochondria contain a large amount of IMP's. This activity is transferred to the clinging part of the plasma membrane (Fig. 6). A larger accumulation of IMP's can be seen at the spot where the plasma membrane covers the annulus (Fig. 7). In the plasma membrane of the main segment of the flagellum the IMP's are distributed irregularly in PF, and also in EF (Fig. 8). The EF surfaces of the plasma membrane had in general an indistinct structural organisation of IMP's. The distribution of IMP's in acrosomal membranes was found to be irregular. The postacrosomal lamina in the freeze fracturing did not look like a morphologically identifiable structure. In the nuclear envelope delimiting the posterior nuclear space, nuclear pores could be identified (Fig. 9). No nuclear pores could be seen in the nuclear envelope clinging to the posterior part of the nucleus. At this spot smaller IMP's were observed which could be a part of the proteins of the filaments connecting the concave surface of the head with the convex articular surface of the flagellum. These filaments connect the head and the flagellum (Fig. 10).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1997

Death process of primitive erythrocytes and phagocytosis by liver macrophages of the mouse embryo.

Primitive erythrocytes from the yolk sac are nucleated cells which have a short life span of several days in the embryonic circulation of the mouse. The death process of primitive erythrocytes was ultrastructurally investigated in embryonic mice. At 12 days of gestation, primitive erythrocytes accounted for 96.1% of the circulating erythrocytes. The percentages in 13-, 14-, 15- and 18-day-embryos were 43.8%, 15.4%, 7.7% and 0.0%, respectively. Between 13 and 15 days, anucleate primitive erythrocytes made up from 1.5% to 5.9% of circulating erythrocytes. During the gestational period, the nuclei of primitive erythrocytes markedly decreased in volume, and the chromatin underwent condensation and marginated to crescents along the nuclear envelope. Following nuclear fragmentation and dissolution of condensed chromatin, nuclear residues were finally formed in aging primitive erythrocytes. Formation of annulate lamellae was also observed, associated with the nuclear shrinkage. The nuclei had an apoptotic-like appearance, but TUNEL-positive reactions could not be identified in any nuclei of the primitive erythrocytes. Dying erythrocytes with nuclear residues were phagocytosed in toto by hepatic macrophages and disposed of from embryonic circulation without enucleation.

Animals↗
PubMed1993

The analysis of 40 kDa nuclear protein, p40, in interphase cells and mitotic cells.

We previously reported that the monoclonal antibody M108 recognized a 40 kDa protein both in the nucleus and the cytoplasm. This nuclear 40 kDa antigen was located in the nuclear envelope in interphase cells and in the perichromosomal region during mitosis. Now, we have analyzed this nuclear 40 kDa protein (p40) further, through morphological and biochemical approaches. At the beginning of mitosis, the perinuclear p40 detached from the nuclear envelope and moved to surround the condensing chromatin, while in the late stage of mitosis, the perichromosomal p40 moved back to the reassembled nuclear envelope. Most of the perichromosomal p40 on the metaphase chromosome was solubilized only by DNase I treatment, not by either high salt or detergent treatment. On the other hand, the perinuclear p40 was not solubilized by DNase1 alone, or high salt detergent alone. Sequential treatments with DNase I and high salt detergent were required to extract p40 in interphase nuclei. These results suggest that p40 was associated both with the nuclear envelope and chromatin DNA in interphase nuclei, while it bound only to chromatin DNA in mitosis.

Animals↗
PubMed1987

Monoclonal antibodies identify a group of nuclear pore complex glycoproteins.

Using monoclonal antibodies we identified a group of eight polypeptides of rat liver nuclear envelopes that have common epitopes. Most or all of these proteins are structurally distinct, as shown by tryptic peptide mapping and analysis with polyclonal antibodies. While these polypeptides are relatively tightly bound to nuclear membranes, only one is an integral membrane protein. The eight antigens cofractionate with the nuclear pore complex under various conditions of ionic strength and detergent. It can be seen by immunofluorescence microscopy that the monoclonal antibodies reacting with these antigens stain the nuclear surface of interphase cells in a finely punctate pattern. When the nuclear envelope is disassembled and subsequently reformed during mitosis, the proteins are reversibly dispersed throughout the cytoplasm in the form of minute foci. By EM immunogold localization on isolated nuclear envelopes, the monoclonal antibodies label exclusively the nuclear pore complex, at both its nucleoplasmic and cytoplasmic margins. Considered together, our biochemical and localization data indicate that the eight nuclear envelope polypeptides are pore complex components. As shown in the accompanying paper (Holt, G. D., C. M. Snow, A. Senior, R. S. Haltiwanger, L. Gerace, and G. W. Hart, J. Cell Biol., 104:1157-1164) these eight polypeptides contain a novel form of glycosylation, O-linked N-acetylglucosamine. The relative abundance and disposition of these O-linked glycoproteins in the pore complex are consistent with their having a role in nucleocytoplasmic transport.

Animals↗
PubMed1984

Are annulate lamellae in the Drosophila embryo the result of overproduction of nuclear pore components?

Annulate lamellae are cytoplasmic organelles composed of stacked sheets of membrane containing pores that are structurally indistinguishable from nuclear pores. The functions of annulate lamellae are not well understood. Although they may be found in virtually any eucaryotic cell, they occur most commonly in transformed and embryonic tissues. In Drosophila, annulate lamellae are found in the syncytial blastoderm embryo as it is cleaved to form the cellular blastoderm. The cytological events of the cellularization process are well documented, and may be used as temporal landmarks when studying changes in annulate lamellae. By using morphometric techniques to analyze electron micrographs of embryos, we are able to calculate the number of pores per nucleus in nuclear envelopes and annulate lamellae during progressive stages of cellularization. We find that annulate lamellae pores remain at a low level while nuclear envelopes are expanding and acquiring pores in early interphase. Once nuclear envelopes are saturated with pores, however, the number of annulate lamellae pores increases more than 10-fold in 9 min. Over the next 30 min it gradually declines to the initial low level. On the basis of these results, we propose (a) that pore synthesis and assembly continues after nuclear envelopes have been saturated with pores; (b) that these supernumerary pores accumulate transiently in cytoplasmic annulate lamellae; and (c) that because these pores are not needed by the embryo they are subsequently degraded.

Animals↗
PubMed2001

Chromatin docking and exchange activity enhancement of RCC1 by histones H2A and H2B.

The Ran guanosine triphosphatase (GTPase) controls nucleocytoplasmic transport, mitotic spindle formation, and nuclear envelope assembly. These functions rely on the association of the Ran-specific exchange factor, RCC1 (regulator of chromosome condensation 1), with chromatin. We find that RCC1 binds directly to mononucleosomes and to histones H2A and H2B. RCC1 utilizes these histones to bind Xenopus sperm chromatin, and the binding of RCC1 to nucleosomes or histones stimulates the catalytic activity of RCC1. We propose that the docking of RCC1 to H2A/H2B establishes the polarity of the Ran-GTP gradient that drives nuclear envelope assembly, nuclear transport, and other nuclear events.

Active Transport, Cell Nucleus↗
PubMed1961

Structure in nucleated erythrocytes.

The structure of the nucleated erythrocyte of frog and chicken has been investigated by electron microscopy and correlated with the distribution of haemoglobin and DNA-containing material determined by haem absorption and Feulgen staining in the light microscope. The nuclei of both species are found to contain haemoglobin which is continuous with the haemoglobin in the cytoplasm through holes or pores in the nuclear envelope. In addition the nucleus of the frog erythrocyte sometimes contains a single invagination which is lined by the nuclear envelope. The structure of the nuclear envelope and the pores and the organisation of the nucleus are similar to those already described for other somatic cells. Erythrocytes differ from the cells previously studied in that a continuity, via the nuclear pores, of chemical substance in the interior of the nucleus and in the cytoplasm can be directly demonstrated. This is due to the fact that the cytoplasm of erythrocytes is simple, consisting predominantly of haemoglobin, and that haemoglobin is easily recognised by its specific absorption. The static pictures obtained by electron microscopy have been supplemented by observations in phase-contrast of the changes in refraction of the cell contents due to the diffusion of the haemoglobin from the nucleus into the cytoplasm during haemolysis.

Cell Nucleus↗
PubMed1993

Preventing re-replication of DNA in a single cell cycle: evidence for a replication licensing factor.

Xenopus egg extracts treated with the protein kinase inhibitor 6-dimethylaminopurine (6-DMAP) are unable to support the initiation of DNA replication. Nuclei assembled in 6-DMAP extracts behave as though they are in G2, and will not undergo another round of DNA replication until passage through mitosis. 6-DMAP extracts are functionally devoid of a replication factor that modifies chromatin in early G1 before nuclear envelope assembly, but which is itself incapable of crossing the nuclear envelope. This chromatin modification is capable of supporting only a single round of semiconservative replication. The behavior of this replication factor is sufficient to explain why eukaryotic DNA is replicated once and only once in each cell cycle, and conforms to the previous model of a Replication Licensing Factor. Cell cycle analysis shows that this putative Licensing Factor is inactive during metaphase, but becomes rapidly activated on exit from metaphase when it can modify chromatin before nuclear envelope assembly is complete.

Adenine↗
PubMed1999

Function and assembly of nuclear pore complex proteins.

Nuclear pore complexes (NPCs) are extremely elaborate structures that mediate the bidirectional movement of macromolecules between the nucleus and cytoplasm. The current view of NPC organization features a massive symmetrical framework that is embedded in the double membranes of the nuclear envelope. It embraces a central channel of as yet ill-defined structure but which may accommodate particles with diameters up to 26 nm provided that they bear specific import/export signals. Attached to both faces of the central framework are peripheral structures, short cytoplasmic filaments, and a nuclear basket assembly, which interact with molecules transiting the NPC. The mechanisms of assembly and the nature of NPC structural intermediates are still poorly understood. However, mutagenesis and expression studies have revealed discrete sequences within certain NPC proteins that are necessary and sufficient for their appropriate targeting. In addition, some details are emerging from observations on cells undergoing mitosis where the nuclear envelope is disassembled and its components, including NPC subunits, are dispersed throughout the mitotic cytoplasm. At the end of mitosis, all of these components are reutilized to form nuclear envelopes in the two daughter cells. To date, it has been possible to define a time course of postmitotic assembly for a group of NPC components (CAN/Nup214, Nup153, POM121, p62 and Tpr) relative to the integral inner nuclear membrane protein LAP2 and the NPC membrane glycoprotein gp210. Nup153, a dynamic component of the nuclear basket, associates with chromatin towards the end of anaphase coincident with, although independent of, the inner nuclear membrane protein, LAP2. Assembly of the remaining proteins follows that of the nuclear membranes and occurs in the sequence POM121, p62, CAN/Nup214 and gp210/Tpr. Since p62 remains as a complex with three other NPC proteins (p58, p54, p45) during mitosis, and CAN/Nup214 maintains a similar interaction with its partner, Nup84, the relative timing of assembly of these additional four proteins may also be inferred. These observations suggest that there is a sequential association of NPC proteins with chromosomes during nuclear envelope reformation and the recruitment of at least eight of these precedes that of gp210. These findings support a model in which it is POM121 rather than gp210 that defines initial membrane-associated NPC assembly intermediates and which may therefore represent an essential component of the central framework of the NPC.

Animals↗
PubMed1996

Changes in ultrastructure of cytoplasm and nucleus during spermiogenesis in Chara vulgaris.

In the present study much attention was paid to the first and middle stages of spermiogenesis in Chara vulgaris. The spermiogenesis was divided into 8 phases (A-H). In telophase leading to spermatid formation (A phase) there are no structural changes, indicating that cells pass into a new functional stage-spermiogenesis.B phase: After the formation of a posttelophase nucleus, the difference is striking in chromatin of newly formed spermatids and chromatin nuclei in cycling cells. The nuclei of spermatids form big clusters of dense chromatin and nucleoli remain as in telophase. However, in cycling cells condensed chromatin forms fine reticulum while nucleoli contain abundant granular component. C phase: The movement of the nucleus to the side wall, characteristic for early spermiogenesis, is not preceded by changes in the structure of the nucleus. D phase: In D phase and later free spaces between plasmalemma and the cell wall appear. In these spaces two flagella have a typical structure (9 + 2). They are surrounded by plasmalemma with rhombus-shaped scales on its surface. The structure of nucleus is characterised by a thick, dense layer of chromatin and gradual disappearance of nucleoli by extrusion to the cytoplasm. In electron-transparent ground cytoplasm, numerous ribosomes forming clusters, spirals and chains are present. Numerous rough ER cisternae with light content also appear. Bodies similar to secondary lysosomes and intensified activity of Golgi structures are observed. E phase: Large amounts of proteins seem to be synthesized and accumulated in rough ER cisternae which are filled with an electron dense substance. The same kind of substance can be seen within the perinuclear space. One may assume that proteins migrate through the nuclear envelope to the cell nucleus. Probably at that moment the main proteins taking part in the reorganisation of chromatin structure and in the exchange of somatic proteins into generative ones are formed. Simultaneously synthesis of other types of proteins continues. Active Golgi structures producing numerous light and coated vesicles complete the picture of intensive metabolism. Parallelly reduction of cytoplasm takes place. F phase: The nuclear chromatin becomes netlike after DNP staining, without a thick layer of condensed chromatin close to the nuclear envelope. Simultaneously, all types of structures with positive contrast after EDTA migrate polarly from nucleus to the cytoplasm. Further reduction of cytoplasm takes place and protein synthesis continues as suggested by the presence of numerous free polysomes. G phase: The structure of chromatin fibrils undergoes a complete transformation. They become much thicker, parallel to each other, changing the orientation along with the formation of nucleus coils. The nucleus is devoid of most nucleoplasm structures. It has significantly elongated shape and shows the presence of manchette microtubules on the outside of nuclear envelope and lamina attached to the inner membrane of nuclear envelope. Condensed mitochondria are located near flagella and nucleus while plastids with starch grains lined up on the opposite side of the spermatozoid are also surrounded by microtubules. H phase: Nuclear fibrils form lamellar interconnections and the nucleus has a form of dense reticular structure surrounded by the nuclear envelope without pores.

Animals↗
PubMed2001

The N-terminal "beta-barrel" domain of 5-lipoxygenase is essential for nuclear membrane translocation.

5-Lipoxygenase is the key enzyme in the formation of leukotrienes, which are potent lipid mediators of asthma pathophysiology. This enzyme translocates to the nuclear envelope in a calcium-dependent manner for leukotriene biosynthesis. Eight green fluorescent protein (GFP)-lipoxygenase constructs, representing the major human and mouse enzymes within this family, were constructed and their cDNAs transfected into human embryonic kidney 293 cells. Of these eight lipoxygenases, only the 5-lipoxygenase was clearly nuclear localized and translocated to the nuclear envelope upon stimulation with the calcium ionophore. The N-terminal "beta -barrel" domain of 5-lipoxygenase, but not the catalytic domain, was necessary and sufficient for nuclear envelope translocation. The GFP-N-terminal 5-lipoxygenase domain translocated faster than GFP-5-lipoxygenase. beta-Barrel/catalytic domain chimeras with 12- and 15-lipoxygenase indicated that only the N-terminal domain of 5-lipoxygenase could carry out this translocation function. Mutations of iron atom binding ligands (His550 or deletion of C-terminal isoleucine) that disrupt nuclear localization do not alter translocation capacity indicating distinct determinants of nuclear localization and translocation. Moreover, data show that GFP-5-lipoxygenase beta-barrel containing constructs can translocate to the nuclear membrane whether cytoplasmic or nuclear localized. Thus, the predicted beta-barrel domain of 5-lipoxygenase may function like the C2 domain within protein kinase C and cytosolic phospholipase A(2) with unique determinants that direct its localization to the nuclear envelope.

Active Transport, Cell Nucleus↗
PubMed1995

The nuclear membrane-associated honeycomb structure of the unicellular organism Amoeba proteus: on the search for homologies with the nuclear lamina of metazoa.

In the protozoon Amoeba proteus, a complex and highly organized structure with the morphology of a honeycomb is associated with the nucleoplasmic surface of the nuclear membrane. We have tested whether this structure exhibits similarity to the nuclear lamina of metazoic organisms. First, we have shown that the honeycomb layer is composed of 3 to 5 nm thick protein fibrils resistant to treatment with detergent, high salt, and digestion with nucleases, thus possessing properties typical for karyoskeletal elements. However, in contrast to the meshwork of lamin filaments in somatic cells of metazoic organisms, the honeycomb layer is not tightly anchored to the nucleoplasmic side of pore complexes, or to the inner nuclear membrane. Second, in microinjection experiments we investigated whether fluorescently labeled lamins of Xenopus laevis (lamins A and LI) and Drosophila melanogaster (lamin Dmo) were able to associate in vivo with the Amoeba proteus honeycomb structure. In microinjected amoeba these three lamins were efficiently transported into the nucleus, but did not associate with the nuclear envelope. Our results suggest that the Amoeba proteus nuclear envelope, including the honeycomb layer, does not contain proteins exhibiting high homologies to lamins of metazoan species thus preventing the localized assembly of microinjected lamins along the nuclear periphery.

3T3 Cells↗
PubMed1993

Highly localized tracks of human immunodeficiency virus type 1 Nef in the nucleus of cells of a human CD4+ T-cell line.

A human T-cell line constitutively expressing the nef gene from the human immunodeficiency virus type 1 SF2 isolate was used to examine the distribution of the Nef protein in the nucleus. High-resolution immunogold labeling/electron microscopic studies with polyclonal anti-Nef antibodies on nef+ and nef- cells revealed that a small fraction of Nef is in the nucleus and it is localized in specific curvilinear tracks that extend between the nuclear envelope and the nucleoplasm. An examination of the sequence of the SF2 nef gene revealed a putative nuclear targeting sequence that was previously found in several other eukaryotic nucleoplasmic proteins. The nuclear localization of Nef suggests a potential nuclear function for this protein. The presence of Nef in distinct nuclear tracks suggests that Nef is transported along a specific pathway that extends from the nuclear envelope into the nucleoplasm. A previous study [Meier, U. T. & Blobel, G. (1992) Cell 70, 127-138] has shown that the nucleolar protein of rat liver cells (Nopp140) shuttles from the nucleolus to the nuclear envelope on distinct tracks. The present study has suggested that the transport of a nucleoplasmic protein may also occur on distinct nuclear pathways.

Amino Acid Sequence↗