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PubMed2002

The molecular mechanism of translocation through the nuclear pore complex is highly conserved.

In this report we investigated the activity of vertebrate nuclear transport factors in a primitive organism, Amoeba proteus, to better understand evolutionary changes in the transport mechanisms of organisms expected to have different requirements for nucleocytoplasmic exchange. It was initially determined that FxFG-containing nucleoporins and Ran, both of which are essential for nuclear import in vertebrates, as well as yeast, are also present and functional in amoebae. This suggests that there are fundamental similarities in the transport process; however, there are also significant differences. Transport substrates containing either the hnRNP A1 M9 shuttling signal (a GST/GFP/M9 fusion protein) or the classical bipartite NLS (colloidal gold coated with BSA-bipartite NLS conjugates), both of which are effectively transported in vertebrate cells, are excluded from the nucleus when microinjected into amoebae. However, when these substrates are injected along with transportin or importin alpha/beta, respectively, the vertebrate receptors for these signals, they readily accumulate in the nucleoplasm. These results indicate that although the molecular recognition of substrates is not well conserved between vertebrates and amoebae, vertebrate transport receptors are functional in A. proteus, showing that the translocation machinery is highly conserved. Since selected nuclear import pathways can be investigated in the absence of competing endogenous transport, A. proteus might provide a useful in vivo system for investigating specific molecular interactions involved in trafficking.

3T3 Cells↗
PubMed1982

Isolation and characterization of a proteinaceous subnuclear fraction composed of nuclear matrix, peripheral lamina, and nuclear pore complexes from embryos of Drosophila melanogaster.

Morphologically intact nuclei have been prepared from embryos of Drosophila melanogaster by a simple and rapid procedure. These nuclei have been further treated with high concentrations of DNase I and RNase A followed by sequential extraction with 2% Triton X-100 and 1 M NaCl to produce a structurally and biochemically distinct preparation designated Drosophila subnuclear fraction I (DSNF-I). As seen by phase-contrast microscopy, DSNF-I is composed of material which closely resembles unfractionated nuclei; residual internal nuclear structures including nucleolar remnants are clearly visible. By transmission electron microscopy, nuclear lamina, pore complexes, and a nuclear matrix are similarly identified. Biochemically, DSNF-I is composed almost entirely of protein (greater than 93%). SDS PAGE analysis reveals several major polypeptides; species at 174,000, 74,000, and 42,000 predominate. A polypeptide coincident with the Coomassie Blue-stainable 174-kdalton band has been shown by a novel technique of lectin affinity labeling to be a glycoprotein; a glycoprotein of similar or identical molecular weight has been found to be a component of nuclear envelope fractions isolated from the livers of rats, guinea pigs, opossums, and chickens. Antisera against several of the polypeptides in DSNF-I have been obtained from rabbits, and all of them show only little or no cross-reactivity with Drosophila cytoplasmic fractions. Initial results of immunocytochemical studies, while failing to positively localize either the 174- or 16-kdalton polypeptides, demonstrate a nuclear localization of the 74-kdalton antigen in all of several interphase cell types obtained from both Drosophila embryos and third-instar larvae.

Animals↗
PubMed1995

High content of a nuclear pore complex protein in cytoplasmic annulate lamellae of Xenopus oocytes.

The Xenopus laevis oocyte and egg represent an established model system to study nucleocytoplasmic transport and the assembly of the nuclear envelope (NE) and its pore complexes (PC). PCs, however, are not restricted to the NE but are also known to occur in cytoplasmic annulate lamellae (AL) in a variety of cells, including the Xenopus oocyte. However, the proportion of PCs found in such AL relative to those located in the NE, is unknown. In this study we have analyzed and quantified cytoplasmic AL in the full-grown (stage VI) Xenopus oocyte by immunolocalization at the light and electron microscopic level. Moreover, we have developed a method to enrich AL from enucleated oocytes, and have quantified a PC marker protein, nucleoporin p62, in both cytoplasmic AL and the NE. For this purpose we have used a specific monoclonal antibody (A225) which recognizes an epitope localized between amino acids 251 and 268 of Xenopus p62. We show that the number of PCs and p62 molecules present in AL far exceeds that of the NE. The possible implications of these findings to nucleocytoplasmic transport and nuclear PC (NPC) assembly are discussed.

Amino Acid Sequence↗
PubMed2003

Unconventional tethering of Ulp1 to the transport channel of the nuclear pore complex by karyopherins.

The ubiquitin-like protein SUMO-1 (small ubiquitin-related modifier 1) is covalently attached to substrate proteins by ligases and cleaved by isopeptidases. Yeast has two SUMO-1-deconjugating enzymes, Ulp1 and Ulp2, which are located at nuclear pores and in the nucleoplasm, respectively. Here we show that the catalytic C-domain of Ulp1 must be excluded from the nucleoplasm for cell viability. This is achieved by the noncatalytic N-domain, which tethers Ulp1 to the nuclear pores. The bulk of cellular Ulp1 is not associated with nucleoporins but instead associates with three karyopherins (Pse1, Kap95 and Kap60), in a complex that is not dissociated by RanGTP in vitro. The Ulp1 N-domain has two distinct binding sites for Pse1 and Kap95/Kap60, both of which are required for anchoring to the nuclear pore complex. We propose that Ulp1 is tethered to the nuclear pores by a Ran-insensitive interaction with karyopherins associated with nucleoporins. This location could allow Ulp1 to remove SUMO-1 from sumoylated cargo proteins during their passage through the nuclear pore channel.

Active Transport, Cell Nucleus↗
PubMed1999

The nuclear pore complex protein Nup88 is overexpressed in tumor cells.

It has been previously shown (J. Schneider et al., Br. J. Cancer, 77: 1015-1020, 1998) that an antibody directed against Candida albicans proteins (C6) recognizes specifically a protein in human ovarian tumors. We have now performed an immunoscreening of a human cDNA library using this antibody and identified the antigen as Nup88, a protein localized preferentially at the nuclear membrane and probably implicated in nucleocytoplasmic transport. Our results show that Nup88 is strongly expressed in a series of human tumor cell lines compared with nontransformed cell lines at the RNA and the protein levels. Furthermore, we observed that in 76% of 21 ovarian tumors analyzed, the protein is also overexpressed in malignant tissue when compared with healthy adjacent tissue. Nup88 may, therefore, be considered as a putative marker for tumor growth and is probably related to increased cell cycling.

Antibodies, Monoclonal↗
PubMed2005

Versatility at the nuclear pore complex: lessons learned from the nucleoporin Nup153.

The vertebrate pore protein Nup153 plays pivotal roles in nuclear pore function. In addition to being important to pore architecture, Nup153 is a key participant in both import and export. The scope of Nup153 function also extends beyond the canonical view of the pore as a trafficking gateway. During the transition into mitosis, Nup153 directs proteins involved in membrane remodeling to the nuclear envelope. As cells exit mitosis, Nup153 is recruited to the chromosomal surface, where nuclear pores are formed anew in a complicated process still under much experimental scrutiny. In addition, Nup153 is targeted for protease cleavage during apoptosis and in response to certain viral infections, providing molecular insight into pore reconfiguration during cell response. Overall, the versatile nature of Nup153 underscores an emerging view of the nuclear pore at the nexus of many key cellular processes.

Animals↗
PubMed2002

The mouse interferon-inducible gene Ifi204 product interacts with the Tpr protein, a component of the nuclear pore complex.

We have used yeast two-hybrid screening to isolate cDNA-encoding proteins interacting with the protein encoded by the interferon (IFN)-inducible gene Ifi204. Four independent overlapping clones were isolated from an NIH3T3 cDNA library. The largest clone encoded a protein (1203 amino acids in length) sharing 94% identity with the C-terminal portion of the human translocated promoter region (Tpr) protein. Northern blot analysis revealed a 7.5-kilobase mRNA present in both mouse and human cell lines. In addition, in vivo interaction was demonstrated by coimmunoprecipitation experiments. Anti-Tpr polyclonal monospecific antibodies (Ab) used for immunofluorescence staining labeled the nuclear envelope (NE) in a punctate pattern characteristic of nucleoporins and also yielded staining throughout the nuclear interior. The intranuclear Tpr occurred in apparently discrete foci. When superimposed on optical sections obtained with anti-p204 Abs, these colocalized, with the sole exception of the nucleolar compartment stained by the anti-p204 Abs only. Although the specific function of Tpr is not defined, it appears to mediate p204 translocation from the cytoplasmic to the nuclear compartment following IFN treatment.

3T3 Cells↗
PubMed1995

The nuclear pore complex protein p62 is one of several sialic acid-containing proteins of the nuclear envelope.

While investigating the glycosylation of nuclear envelope proteins of neuroblastoma cells, we found several proteins that bound the sialic acid-specific Sambucus nigra agglutinin. The strongest signals were obtained for proteins with apparent molecular masses of 66 and 180 kDa. The specificity of the lectin binding was checked by acylneuraminyl hydrolase treatment of nuclear envelope proteins, which prohibited S. nigra agglutinin binding. Digestion of nuclear envelope proteins with the N-glycosidase F revealed that sialic acid was N-glycosidically linked to the 180-kDa protein and very probably O-glycosidically linked to the 66-kDa protein. Upon extraction, the latter behaved like the nucleoporin p62 in that it was partly extracted by high ionic strength buffers, could not be solubilized by nonionic detergent, and was completely removed from the nuclear envelope with urea. Two-dimensional gel electrophoretic comparison showed that the S. nigra agglutinin-binding protein and p62 have an identical isoelectric point of about 5.0 and an identical apparent molecular mass of 66 kDa. This, together with the binding of the anti-nucleoporin antibody, demonstrated the identity of the 66-kDa sialoprotein and p62. S. nigra agglutinin inhibits nuclear protein transport in neuroblastoma cells, strongly suggesting a functional significance of sialylation of p62.

Agglutinins↗
PubMed1976

Microtubules associated with nuclear pore complexes and coated pits in the CNS.

Using a new albumin prefixation technique, microtubules have been observed in close association with the nuclear pores of neurons and glia. Thus, microtubules may be involved in such phenomena as anchoring, migration or rotation of the nucleus or in chemical messenger transport between nucleus and cytoplasm. Microtubules are also seen running close to the coated pits of dendrites. The implications are discussed.

Animals↗