Nuclear pore complex: structure, function, and regulation

Targeting Proteins in Nucleus through Dual-Regulatory Pathways Acting in Cytoplasm

Nuclear proteins have been regarded as attractive targets for exploiting therapeutic agents. However, those agents cannot efficiently pass through nuclear pores and it is also difficult to overcome the crowded nuclear environment to react with proteins. Herein, we propose a novel strategy acting in the cytoplasm to regulate nuclear proteins based on their signaling pathways, instead of directly entering into nuclei. A multifunctional complex PKK-TTP/hs carries human telomerase reverse transcriptase (hTERT) small interfering RNA (defined as hs) for gene silencing in the cytoplasm, which reduced the import of nuclear protein. At the same time, it could generate reactive oxygen species (ROS) under light irradiation, which raised the export of nuclear proteins by promoting proteins translocation. Through this dual-regulatory pathway, we successfully reduced nuclear protein (hTERT proteins) in vivo (42.3%). This work bypasses the challenge of directly entering into the nucleus and provides an effective strategy for regulating nuclear proteins.

Changes in nuclear pore numbers control nuclear import and stress response of mouse hearts

Nuclear pores are essential for nuclear-cytoplasmic transport. Whether and how cells change nuclear pores to alter nuclear transport and cellular function is unknown. Here, we show that rat heart muscle cells (cardiomyocytes) undergo a 63% decrease in nuclear pore numbers during maturation, and this changes their responses to extracellular signals. The maturation-associated decline in nuclear pore numbers is associated with lower nuclear import of signaling proteins such as mitogen-activated protein kinase (MAPK). Experimental reduction of nuclear pore numbers decreased nuclear import of signaling proteins, resulting in decreased expression of immediate-early genes. In a mouse model of high blood pressure, reduction of nuclear pore numbers improved adverse heart remodeling and reduced progression to lethal heart failure. The decrease in nuclear pore numbers in cardiomyocyte maturation and resulting functional changes demonstrate how terminally differentiated cells permanently alter their handling of information flux across the nuclear envelope and, with that, their behavior.

Endocytic protein intersectin1-S shuttles into nucleus to suppress the DNA replication in breast cancer

Breast cancer is the most common type of cancer worldwide. However, the well-known molecular biomarkers are not enough to meet the needs of precision medicine. In search for novel targets in this regard, we reported ITSN1 (intersectin1) as one of the candidates through mRNA microarray analysis. In the present study, we reported that endocytic protein ITSN1-S exists not only in the cytoplasm but also in nuclei of breast cancer cells. ITSN1-S' functional nuclear localization signal is within its residues 306-312. Its nuclear export signal (NES) resides within its SH3 domains. We also found, the interaction between the CC domain of nuclear ITSN1-S and the NT domain of nuclear DNA helicase II (NDH II) directly suppressed the DNA replication and nascent DNA synthesis by inhibiting the R-loops resolution in breast cancer cells. Furthermore, the interaction between the EH domains of cytoplasmic ITSN1-S and PI3KC2α inhibit cell migration and invasion by inactivating the PI3KC2α-AKT pathway. Our results were confirmed in both ITSN1 gene knockout cells and in vivo assays. Finally, our clinical data showed a potential application of the combined consideration of the cytoplasmic and nuclear ITSN1-S as an independent prognosis factor. In conclusion, our study revealed ITSN1-S' novel positioning in the nuclei of breast cancer cells, its function in suppressing DNA replication, and its potential application in improved breast cancer prognosis.

Two nuclear localization signals regulate intracellular localization of the duck enteritis virus UL13 protein

Duck enteritis virus (DEV) multifunctional tegument protein UL13 is predicted to be a conserved herpesvirus protein kinase; however, little is known about its subcellular localization signal. In this study, through transfection of 2 predicted nuclear signals of DEV UL13 fused to enhanced green fluorescent protein, 2 bipartite nuclear localization signals (NLS) were identified. We found that ivermectin blocked the NLS-mediated nuclear import of DEV UL13, showing that the nuclear localization signal of DEV UL13 is a classical importin α- and β-dependent process. We constructed a DEV UL13 mutant strain in which the NLS of DEV UL13 was deleted to explore whether deletion of the NLS affects viral replication. Amino acids 4 to 7 and 90 to 96 were predicted to be NLSs, further proving that nuclear import occurs via a classical importin α- and β-dependent process. We also found that the NLS of pUL13 had no effect on DEV replication in cell culture. Our study enhances the understanding of DEV pUL13. Taken together, these results provide significant information regarding the biological function of pUL13 during DEV infection.

Non-AUG start codons responsible for ABO weak blood group alleles on initiation mutant backgrounds

Histo-blood group ABO gene polymorphism is crucial in transfusion medicine. We studied the activity and subcellular distribution of ABO gene-encoded A glycosyltransferases with N-terminal truncation. We hypothesized that truncated enzymes starting at internal methionines drove the synthesis of oligosaccharide A antigen in those already described alleles that lack a proper translation initiation codon. Not only we tested the functionality of the mutant transferases by expressing them and assessing their capacity to drive the appearance of A antigen on the cell surface, but we also analyzed their subcellullar localization, which has not been described before. The results highlight the importance of the transmembrane domain because proteins deprived of it are not able to localize properly and deliver substantial amounts of antigen on the cell surface. Truncated proteins with their first amino acid well within the luminal domain are not properly localized and lose their enzymatic activity. Most importantly, we demonstrated that other codons than AUG might be used to start the protein synthesis rather than internal methionines in translation-initiation mutants, explaining the molecular mechanism by which transferases lacking a classical start codon are able to synthesize A/B antigens.

Heterogeneity of T-Tubules in Pig Hearts

Background

T-tubules are invaginations of the sarcolemma that play a key role in excitation-contraction coupling in mammalian cardiac myocytes. Although t-tubules were generally considered to be effectively absent in atrial myocytes, recent studies on atrial cells from larger mammals suggest that t-tubules may be more numerous than previously supposed. However, the degree of heterogeneity between cardiomyocytes in the extent of the t-tubule network remains unclear. The aim of the present study was to investigate the t-tubule network of pig atrial myocytes in comparison with ventricular tissue.

Methods

Cardiac tissue was obtained from young female Landrace White pigs (45–75 kg, 5–6 months old). Cardiomyocytes were isolated by arterial perfusion with a collagenase-containing solution. Ca²⁺ transients were examined in field-stimulated isolated cells loaded with fluo-4-AM. Membranes of isolated cells were visualized using di-8-ANEPPS. T-tubules were visualized in fixed-frozen tissue sections stained with Alexa-Fluor 488-conjugated WGA. Binary images were obtained by application of a threshold and t-tubule density (TTD) calculated. A distance mapping approach was used to calculate half-distance to nearest t-tubule (HD_TT).

Results & Conclusion

The spatio-temporal properties of the Ca²⁺ transient appeared to be consistent with the absence of functional t-tubules in isolated atrial myocytes. However, t-tubules could be identified in a sub-population of atrial cells in frozen sections. While all ventricular myocytes had TTD >3% (mean TTD = 6.94±0.395%, n = 24), this was true of just 5/22 atrial cells. Mean atrial TTD (2.35±0.457%, n = 22) was lower than ventricular TTD (P<0.0001). TTD correlated with cell-width (r = 0.7756, n = 46, P<0.0001). HD_TT was significantly greater in the atrial cells with TTD ≤3% (2.29±0.16 μm, n = 17) than in either ventricular cells (1.33±0.05 μm, n = 24, P<0.0001) or in atrial cells with TTD >3% (1.65±0.06 μm, n = 5, P<0.05). These data demonstrate considerable heterogeneity between pig cardiomyocytes in the extent of t-tubule network, which correlated with cell size.

Transcriptional activity and nuclear localization of Cabut, the Drosophila ortholog of vertebrate TGF-β-inducible early-response gene (TIEG) proteins

Background

Cabut (Cbt) is a C₂H₂-class zinc finger transcription factor involved in embryonic dorsal closure, epithelial regeneration and other developmental processes in Drosophila melanogaster. Cbt orthologs have been identified in other Drosophila species and insects as well as in vertebrates. Indeed, Cbt is the Drosophila ortholog of the group of vertebrate proteins encoded by the TGF-ß-inducible early-response genes (TIEGs), which belong to Sp1-like/Krüppel-like family of transcription factors. Several functional domains involved in transcriptional control and subcellular localization have been identified in the vertebrate TIEGs. However, little is known of whether these domains and functions are also conserved in the Cbt protein.

Methodology/Principal Findings

To determine the transcriptional regulatory activity of the Drosophila Cbt protein, we performed Gal4-based luciferase assays in S2 cells and showed that Cbt is a transcriptional repressor and able to regulate its own expression. Truncated forms of Cbt were then generated to identify its functional domains. This analysis revealed a sequence similar to the mSin3A-interacting repressor domain found in vertebrate TIEGs, although located in a different part of the Cbt protein. Using β-Galactosidase and eGFP fusion proteins, we also showed that Cbt contains the bipartite nuclear localization signal (NLS) previously identified in TIEG proteins, although it is non-functional in insect cells. Instead, a monopartite NLS, located at the amino terminus of the protein and conserved across insects, is functional in Drosophila S2 and Spodoptera exigua Sec301 cells. Last but not least, genetic interaction and immunohistochemical assays suggested that Cbt nuclear import is mediated by Importin-α2.

Conclusions/Significance

Our results constitute the first characterization of the molecular mechanisms of Cbt-mediated transcriptional control as well as of Cbt nuclear import, and demonstrate the existence of similarities and differences in both aspects of Cbt function between the insect and the vertebrate TIEG proteins.

Mitogen activated protein kinase at the nuclear pore complex

Mitogen activated protein (MAP) kinases control eukaryotic proliferation, and import of kinases into the nucleus through the nuclear pore complex (NPC) can influence gene expression to affect cellular growth, cell viability and homeostatic function. The NPC is a critical regulatory checkpoint for nucleocytoplasmic traffic that regulates gene expression and cell growth, and MAP kinases may be physically associated with the NPC to modulate transport. In the present study, highly enriched NPC fractions were isolated and investigated for associated kinases and/or activity. Endogenous kinase activity was identified within the NPC fraction, which phosphorylated a 30 kD nuclear pore protein. Phosphomodification of this nucleoporin, here termed Nup30, was inhibited by apigenin and PD-98059, two MAP kinase antagonists as well as with SB-202190, a pharmacological blocker of p38. Furthermore, high throughput profiling of enriched NPCs revealed constitutive presence of all members of the MAP kinase family, extracellular regulated kinases (ERK), p38 and Jun N-terminal kinase. The NPC thus contains a spectrum of associated MAP kinases that suggests an intimate role for ERK and p38 in regulation of nuclear pore function.

Nuclear pore complex: biochemistry and biophysics of nucleocytoplasmic transport in health and disease

Nuclear pore complexes (NPCs) are the gateways connecting the nucleoplasm and cytoplasm. This structures are composed of over 30 different proteins and 60-125 MDa of mass depending on type of species. NPCs are bilateral pathways that selectively control the passage of macromolecules into and out of the nucleus. Molecules smaller than 40 kDa diffuse through the NPC passively while larger molecules require facilitated transport provided by their attachment to karyopherins. Kinetic studies have shown that approximately 1000 translocations occur per second per NPC. Maintaining its high selectivity while allowing for rapid translocation makes the NPC an efficient chemical nanomachine. In this review, we approach the NPC function via a structural viewpoint. Putting together different pieces of this puzzle, this chapter confers an overall insight into what molecular processes are engaged in import/export of active cargos across the NPC and how different transporters regulate nucleocytoplasmic transport. In the end, the correlation of several diseases and disorders with the NPC structural defects and dysfunctions is discussed.

Nucleoporin 153 arrests the nuclear import of hepatitis B virus capsids in the nuclear basket

Virtually all DNA viruses including hepatitis B viruses (HBV) replicate their genome inside the nucleus. In non-dividing cells, the genome has to pass through the nuclear pore complexes (NPCs) by the aid of nuclear transport receptors as e.g. importin β (karyopherin). Most viruses release their genome in the cytoplasm or at the cytosolic face of the NPC, as the diameter of their capsids exceeds the size of the NPC. The DNA genome of HBV is derived from reverse transcription of an RNA pregenome. Genome maturation occurs in cytosolic capsids and progeny capsids can deliver the genome into the nucleus causing nuclear genome amplification. The karyophilic capsids are small enough to pass the NPC, but nuclear entry of capsids with an immature genome is halted in the nuclear basket on the nuclear side of the NPC, and the genome remains encapsidated. In contrast, capsids with a mature genome enter the basket and consequently liberate the genome. Investigating the difference between immature and mature capsids, we found that mature capsids had to disintegrate in order to leave the nuclear basket. The arrest of a karyophilic cargo at the nuclear pore is a rare phenomenon, which has been described for only very few cellular proteins participating in nuclear entry. We analyzed the interactions causing HBV capsid retention. By pull-down assays and partial siRNA depletion, we showed that HBV capsids directly interact with nucleoporin 153 (Nup153), an essential protein of the nuclear basket which participates in nuclear transport via importin β. The binding sites of importin β and capsids were shown to overlap but capsid binding was 150-fold stronger. In cellulo experiments using digitonin-permeabilized cells confirmed the interference between capsid binding and nuclear import by importin β. Collectively, our findings describe a unique nuclear import strategy not only for viruses but for all karyophilic cargos.

Viral capsids facilitate protection of the enclosed viral genome and participate in the intracellular transport of the genome. At the site of replication capsids have to release the genome. The particular factors triggering genome liberation are not well understood. Like other karyophilic cargos, hepatitis B virus (HBV) capsids are transported through the nuclear pore using nuclear transport receptors of the importin ß superfamily. Unlike physiological cargos, HBV capsids become arrested within the nuclear basket, which is a filamentous structure on the nuclear side of the nuclear pore. Asking which interaction causes this unique strategy, we found that the capsids bind to a protein of the basket periphery, nucleoporin 153 (Nup153). The findings were confirmed in situ using digitonin-permeabilized cells that support physiological genome delivery into the nucleus. We observed that HBV capsids bound to Nup153 irrespective of the maturation of the encapsidated genome. But while capsids with an immature genome remained in arrested state, capsids with a mature genome disassembled and released their DNA.

Transmembrane protein-free membranes fuse into xenopus nuclear envelope and promote assembly of functional pores

Post-mitotic reassembly of nuclear envelope (NE) and the endoplasmic reticulum (ER) has been reconstituted in a cell-free system based on interphase Xenopus egg extract. To evaluate the relative contributions of cytosolic and transmembrane proteins in NE and ER assembly, we replaced a part of native membrane vesicles with ones either functionally impaired by trypsin or N-ethylmaleimide treatments or with protein-free liposomes. Although neither impaired membrane vesicles nor liposomes formed ER and nuclear membrane, they both supported assembly reactions by fusing with native membrane vesicles. At membrane concentrations insufficient to generate full-sized functional nuclei, addition of liposomes and their fusion with membrane vesicles resulted in an extensive expansion of NE, further chromatin decondensation, restoration of the functionality, and spatial distribution of the nuclear pore complexes (NPCs), and, absent newly delivered transmembrane proteins, an increase in NPC numbers. This rescue of the nuclear assembly by liposomes was inhibited by wheat germ agglutinin and thus required active nuclear transport, similarly to the assembly of full-sized functional NE with membrane vesicles. Mechanism of fusion between liposomes and between liposomes and membrane vesicles was investigated using lipid mixing assay. This fusion required interphase cytosol and, like fusion between native membrane vesicles, was inhibited by guanosine 5'-3-O-(thio)triphosphate, soluble N-ethylmaleimide-sensitive factor attachment protein, and N-ethylmaleimide. Our findings suggest that interphase cytosol contains proteins that mediate the fusion stage of ER and NE reassembly, emphasize an unexpected tolerance of nucleus assembly to changes in concentrations of transmembrane proteins, and reveal the existence of a feedback mechanism that couples NE expansion with NPC assembly.

Localization of P-glycoprotein at the nuclear envelope of rat brain cells

P-glycoprotein is a plasma membrane drug efflux protein implicated in extrusion of cytotoxic compounds out of a cell. There is now evidence that suggests expression of this transporter at several subcellular sites, including the nucleus, mitochondria, and Golgi apparatus. This study investigated the localization and expression of P-glycoprotein at the nuclear membrane of rat brain microvessel endothelial (RBE4) and microglial (MLS-9) cell lines. Immunocytochemistry at the light and electron microscope levels using P-glycoprotein monoclonals antibodies demonstrated the localization of the protein at the nuclear envelope of RBE4 and MLS-9 cells. Western blot analysis revealed a single band of 170-kDa in purified nuclear membranes prepared from isolated nuclei of RBE4 and MLS-9 cells. These findings indicate that P-glycoprotein is expressed at the nuclear envelope of rat brain cells and suggest a role in multidrug resistance at this subcellular site.

In Vivo Study of HIV-1 Tat Arginine-rich Motif Unveils Its Transport Properties

Tat-derived peptides have attracted much interest as molecular carriers for intracellular delivery as they incorporate specific attributes required for efficient cargo delivery to sub-cellular domains. Little is known, however, about intracellular trafficking and interactions of Tat peptide-tagged cargoes, although some in vitro studies have suggested the relevance of active processes in Tat peptide-driven nuclear translocation. These issues are addressed by comparing Tat peptide-induced transport properties with well-established passive diffusion and active import benchmarks in living cells. Specifically, we examine several constructs of increasing molecular weight (MW) both below and above the threshold for passive diffusion through the nuclear pore. The resulting sub-cellular localization is analyzed by confocal imaging, and construct intracellular dynamics is investigated by fluorescence recovery after photobleaching (FRAP) real-time imaging. Our experiments yield the characteristic transport parameters of Tat peptide intra-cytoplasm dynamics and nucleus/cytoplasm shuttling. These results allow us to elucidate the mechanism of Tat peptide-driven nuclear permeation, demonstrating that it crosses the nuclear envelope (NE) by passive diffusion. Finally, we discuss the limitations of this route in terms of acceptable cargo size.

Functional domains of the TGF-β-inducible transcription factor Tieg3 and detection of two putative nuclear localization signals within the zinc finger DNA-binding domain

The recently identified TGF-beta-inducible early gene 3 (Tieg3) belongs to the gene family of Sp1/Klf-like transcription factors and is upregulated immediately after TGF-beta treatment. To explore the molecular mechanisms of Tieg3-mediated transcriptional control, GAL4-based luciferase assays were performed in order to determine regulatory domains within the Tieg3 protein. Using EGFP-fusion proteins, we monitored the intracellular localization and mapped putative nuclear localization signals (NLS). We provide evidence that the amino-terminus of Tieg3 is essential to repress the transcription and that the loss of the mSin3A interacting domain (SID) disrupts the repressive effects of Tieg3 in the oligodendroglial cell line OLI-neu. Herein we also demonstrate that the zinc finger containing DNA-binding domain (DBD) alone is able to activate the transcription of a reporter gene. Sequence analysis of the zinc finger region revealed no similarities to known activation domains. Analysis of the subcellular localization disclosed Tieg3 as a nuclear protein. Further, we identified the DBD as being essential for the nuclear localization of Tieg3. We detected two closely located putative bipartite NLS within the second and third zinc finger, which are conserved among the members of the Tieg family of proteins. Together these results may help to increase the understanding of Tieg3-mediated transcriptional control and to characterize this TGF-beta-induced Sp1/Klf-like transcription factor.

Determinants of the nucleocytoplasmic shuttling of muscle glycogen synthase

Muscle glycogen synthase (MGS) presents a nuclear speckled pattern in primary cultured human muscle and in 3T3-L1 cells deprived of glucose and with depleted glycogen reserves. Nuclear accumulation of the enzyme correlates inversely with cellular glycogen content. Although the glucose-induced export of MGS from the nucleus to the cytoplasm is blocked by leptomycin B, and therefore mediated by CRM1, no nuclear export signal was identified in the sequence of the protein. Deletion analysis shows that the region comprising amino acids 555-633 of human MGS, which encompasses an Arg-rich cluster involved in the allosteric activation of the enzyme by Glc6P, is crucial for its nuclear concentration and aggregation. Mutation of these Arg residues, which desensitizes the enzyme towards Glc6P, interferes with its nuclear accumulation. In contrast, the known phosphorylation sites of MGS that regulate its activity are not involved in the control of its subcellular distribution. Nuclear human MGS co-localizes with the promyelocytic leukaemia oncoprotein and p80-coilin, a marker of Cajal bodies. The subnuclear distribution of MGS is altered by incubation with transcription inhibitors. These observations suggest that, in addition to its metabolic function, MGS may participate in nuclear processes.

Subcellular localization of liver FBPase is modulated by metabolic conditions

In primary cultured hepatocytes, fructose-1,6-bisphosphatase (FBPase) localization is modulated by glucose, dihydroxyacetone (DHA) and insulin. In the absence of these substrates, FBPase was present in the cytoplasm, but the addition of glucose or DHA induced its translocation to the nucleus. As expected, we observed the opposite effect of glucose on glucokinase localization. The addition of insulin in the absence of glucose largely increased the amount of nuclear FBPase. Moreover, at high concentrations of glucose or DHA, FBPase shifted from the cytosol to the cell periphery and co-localized with GS. Interestingly, the synthesis of Glu-6-P and glycogen induced by DHA was not inhibited by insulin. These results indicate that FBPase is involved in glycogen synthesis from gluconeogenic precursors. Overall, these findings show that translocation may be a new integrative mechanism for gluconeogenesis and glyconeogenesis.

Mutations in the DNA-binding domain of the transcription factor Bright act as dominant negative proteins and interfere with immunoglobulin transactivation

Bright, for B cell regulator of immunoglobulin heavy chain transcription, binds A+T-rich sequences in the intronic enhancer regions of the murine heavy chain locus and 5'-flanking sequences of some variable heavy chain promoters. Most resting B cells do not express Bright; however, it is induced after stimulation with antigen or polyclonal mitogens. Bright activation results in up-regulation of mu transcription; however, it is not clear whether Bright function is critical for normal B cell development. To begin to address Bright function during B cell development, seven mutated forms of Bright were produced. Five of the seven mutants revealed little or no DNA binding activity. Furthermore, because Bright binds DNA as a dimer, two of the mutants formed complexes with wild type Bright and acted in a dominant negative fashion. Dominant negative Bright prevented the up-regulation of mu transcription in transfected Chinese hamster ovary cells transfected with wild type Bright. These data identify regions within Bright that are required for the DNA binding activity of Bright and for its function as a transcription factor.

Pores for thought: nuclear pore complex proteins

Nuclear pore complexes (NPCs) are enormous macromolecular structures that mediate the active exchange of proteins and RNPs between the nucleus and cytoplasm. Recent work has resulted in a windfall of identified NPC polypeptides, many with unique sequences. Several of the proteins have been shown to be part of extended cytoplasmic and nucleoplasmic NPC filaments. Biochemical, structural and genetic studies on NPC proteins are just beginning to allow an understanding of how they associate into a functional organelle.

Sequential morphological changes of erythrocyte apoptosis in Xenopus larvae exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

We previously demonstrated that 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment of Xenopus laevis during the early stages of life induces apoptosis in larval erythrocytes (Sakamoto et al., 1997). In the present study, an examination of these cells at the ultrastructural level was undertaken to elucidate the sequential morphological changes that occur during apoptosis. Xenopus embryos were exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin for 5 days shortly after fertilization. The circulating erythrocytes in larvae 12 days after fertilization were examined. Ultramicroscopic studies revealed four roughly defined stages of apoptosis. During the first stage, many small roundish vacuoles begin to appear in the cytoplasm. No noticeable changes can be found in the nucleus. In the second stage, the perinuclear cisterna become dilated, and huge cisternae can be seen in some erythrocytes. The roundish cytoplasmic vacuoles also become larger. Condensation of nuclear chromatin is not yet evident and the erythrocytes still maintain their elliptical shape. During the third stage, chromatin condensation and margination along the nuclear membrane becomes apparent. The nuclear pores gather in the diffuse chromatin region where the perinuclear cisterna is not dilated. The cytoplasm of some erythrocytes also becomes condensed and electron-dense. The normal arrangement of microtubules is disorderly and the erythrocytes deform into a roundish shape. Also, macrophages usually contact some part of the cell. In the final stage, those erythrocytes which show typical nuclear condensation, where neither nuclear or cytoplasmic fragmentation have occurred, are almost or completely phagocytosed by macrophages.

Characterization of the bipartite nuclear localization signal of protein LANA2 from Kaposi's sarcoma-associated herpesvirus

LANA2 is a nuclear latent protein detected exclusively in Kaposi's sarcoma-associated herpesvirus-infected B cells. The protein inhibits p53-dependent transactivation and apoptosis, suggesting an important role in the transforming activity of the virus. To explore the molecular mechanisms of its nuclear localization, fusion proteins of green fluorescent protein (EGFP) and deletion constructs of LANA2 were expressed in HeLa cells. Only the fragment comprising amino acid residues 355-440 of LANA2 localized in the cell nucleus. This fragment contains two closely located basic domains and forms a putative bipartite nuclear localization signal (NLS). The putative LANA2 NLS was able to target EGFP to the nucleus consistently. Site-directed mutation analyses demonstrated that LANA2 contains a functional bipartite NLS between amino acid positions 367 and 384. In addition, analysis of cells transfected with a cytoplasmic LANA2 mutant revealed that an appropriate subcellular localization may be crucial to regulate p53 activity.

Glycan-dependent signaling: O-linked N-acetylglucosamine

The addition of O-linked N-acetylglucosamine (O-GlcNAc) to target proteins may serve as a signaling modification analogous to protein phosphorylation. Like phosphorylation, O-GlcNAc is a dynamic modification occurring in the nucleus and cytoplasm. Various analytical methods have been developed to detect O-GlcNAc and distinguish it from glycosylation in the endomembrane system. Many target molecules have been identified; these targets are typically components of supramolecular complexes such as transcription factors, nuclear pore proteins, or cytoskeletal components. The enzymes responsible for O-GlcNAc addition and removal are highly conserved molecules having molecular features consistent with a signaling role. The O-GlcNAc transferase and O-GlcNAcase are likely to act in consort with kinases and phosphatases generating various isoforms of physiological substrates. These isoforms may differ in such properties as protein-protein interactions, protein stability, and enzymatic activity. Since O-GlcNAc plays a critical role in the regulation of signaling pathways of higher plants, the glycan modification is likely to perform similar signaling functions in mammalian cells. Glucose and amino acid metabolism generates hexosamine precursors that may be key regulators of a nutrient sensing pathway involving O-GlcNAc signaling. Altered O-linked GlcNAc metabolism may also occur in human diseases including neurodegenerative disorders, diabetes mellitus and cancer.

A functional nuclear localization sequence in the C-terminal domain of SHP-1

The Src homology 2 domain-containing protein tyrosine phosphatases SHP-1 and SHP-2 play an important role in many intracellular signaling pathways. Both SHP-1 and SHP-2 have been shown to interact with a diverse range of cytosolic and membrane-bound signaling proteins. Generally, SHP-1 and SHP-2 perform opposing roles in signaling processes; SHP-1 acts as a negative regulator of transduction in hemopoietic cells, whereas SHP-2 acts as a positive regulator. Intriguingly, SHP-1 has been proposed to play a positive regulating role in nonhemopoietic cells, although the mechanisms for this are not understood. Here we show that green fluorescent protein-tagged SHP-1 is unexpectedly localized within the nucleus of transfected HEK293 cells. In contrast, the highly related SHP-2 protein is more abundant within the cytoplasm of transfected cells. In accordance with this, endogenous SHP-1 is localized within the nucleus of several other nonhemopoietic cell types, whereas SHP-2 is distributed throughout the cytoplasm. In contrast, SHP-1 is confined to the cytoplasm of hemopoietic cells, with very little nuclear SHP-1 evident. Using chimeric SHP proteins and mutagenesis studies, the nuclear localization signal of SHP-1 was identified within the C-terminal domain of SHP-1 and found to consist of a short cluster of basic amino acids (KRK). Although the KRK motif resembles half of a bipartite nuclear localization signal, it appears to function independently and is absolutely required for nuclear import. Our findings show that SHP-1 and SHP-2 are distinctly localized within nonhemopoietic cells, with the localization of SHP-1 differing dramatically between nonhemopoietic and hemopoietic cell lineages. This implies that SHP-1 nuclear import is a tightly regulated process and indicates that SHP-1 may possess novel nuclear targets.

Electrical dimension of the nuclear envelope

Eukaryotic chromosomes are confined to the nucleus, which is separated from the rest of the cell by two concentric membranes known as the nuclear envelope (NE). The NE is punctuated by holes known as nuclear pore complexes (NPCs), which provide the main pathway for transport of cellular material across the nuclear-cytoplasmic boundary. The single NPC is a complicated octameric structure containing more than 100 proteins called nucleoporins. NPCs function as transport machineries for inorganic ions and macromolecules. The most prominent feature of an individual NPC is a large central channel, ~7 nm in width and 50 nm in length. NPCs exhibit high morphological and functional plasticity, adjusting shape to function. Macromolecules ranging from 1 to >100 kDa travel through the central channel into (and out of) the nucleoplasm. Inorganic ions have additional pathways for communication between cytosol and nucleus. NE can turn from a simple sieve that separates two compartments by a given pore size to a smart barrier that adjusts its permeabiltiy to the metabolic demands of the cell. Early microelectrode work characterizes the NE as a membrane barrier of highly variable permeability, indicating that NPCs are under regulatory control. Electrical voltage across the NE is explained as the result of electrical charge separation due to selective barrier permeability and unequal distribution of charged macromolecules across the NE. Patch-clamp work discovers NE ion channel activity associated with NPC function. From comparison of early microelectrode work with patch-clamp data and late results obtained by the nuclear hourglass technique, it is concluded that NPCs are well-controlled supramolecular structures that mediate transport of macromolecules and small ions by separate physical pathways, the large central channel and the small peripheral channels, respectively. Electrical properties of the two pathways are still unclear but could have great impact on the understanding of signal transfer across NE and gene expression.

Nuclear import of insulin-like growth factor-binding protein-3 and -5 is mediated by the importin beta subunit

Although insulin-like growth factor-binding protein (IGFBP)-3 and IGFBP-5 are known to modulate cell growth by reversibly sequestering extracellular insulin-like growth factors, several reports have suggested that IGFBP-3, and possibly also IGFBP-5, have important insulin-like growth factor-independent effects on cell growth. These effects may be related to the putative nuclear actions of IGFBP-3 and IGFBP-5, which we have recently shown are transported to the nuclei of T47D breast cancer cells. We now describe the mechanism for nuclear import of IGFBP-3 and IGFBP-5. In digitonin-permeabilized cells, where the nuclear envelope remained intact, nuclear translocation of wild-type IGFBP-3 appears to occur by a nuclear localization sequence (NLS)-dependent pathway mediated principally by the importin beta nuclear transport factor and requiring both ATP and GTP hydrolysis. Under identical conditions, an NLS mutant form of IGFBP-3, IGFBP-3[(228)KGRKR --> MDGEA], was unable to translocate to the nucleus. In cells where both the plasma membrane and nuclear envelope were permeabilized, wild-type IGFBP-3, but not the mutant form, accumulated in the nucleus, implying that the NLS was also involved in mediating binding to nuclear components. By fusing wild-type and mutant forms of NLS sequences (IGFBP-3 [215-232] and IGFBP-5 [201-218]) to the green fluorescent protein, we identified the critical residues of the NLS necessary and sufficient for nuclear accumulation. Using a Western ligand binding assay, wild-type IGFBP-3 and IGFBP-5, but not an NLS mutant form of IGFBP-3, were shown to be recognized by importin beta and the alpha/beta heterodimer but only poorly by importin alpha. Together these results suggest that the NLSs within the C-terminal domain of IGFBP-3 and IGFBP-5 are required for importin-beta-dependent nuclear uptake and probably also accumulation through mediating binding to nuclear components.

Patch-clamp detection of macromolecular translocation along nuclear pores

The present paper reviews the application of patch-clamp principles to the detection and measurement of macromolecular translocation along the nuclear pores. We demonstrate that the tight-seal 'gigaseal' between the pipette tip and the nuclear membrane is possible in the presence of fully operational nuclear pores. We show that the ability to form a gigaseal in nucleus-attached configurations does not mean that only the activity of channels from the outer membrane of the nuclear envelope can be detected. Instead, we show that, in the presence of fully operational nuclear pores, it is likely that the large-conductance ion channel activity recorded derives from the nuclear pores. We conclude the technical section with the suggestion that the best way to demonstrate that the nuclear pores are responsible for ion channel activity is by showing with fluorescence microscopy the nuclear translocation of ions and small molecules and the exclusion of the same from the cisterna enclosed by the two membranes of the envelope. Since transcription factors and mRNAs, two major groups of nuclear macromolecules, use nuclear pores to enter and exit the nucleus and play essential roles in the control of gene activity and expression, this review should be useful to cell and molecular biologists interested in understanding how patch-clamp can be used to quantitate the translocation of such macromolecules into and out of the nucleus.

Interaction of the transcription factor Sp1 with the nuclear pore protein p62 requires the C-terminal domain of p62

The transcription factor Sp1 plays an important role in the expression of many cellular genes. In studies of proteins that associate with Sp1, a 62-kDa glycoprotein was found in immunoprecipitates of Sp1. This protein was detected in these immunoprecipitates by the monoclonal antibody, RL2, which was originally raised against nuclear pore proteins but was subsequently found to recognize an epitope that contains O-linked N-acetylglucosamine (O-GlcNAc). The association of this protein with Sp1 could be blocked by SDS denaturation of the protein complex. Western blot analysis of the Sp1 immunoprecipitate using antibodies to p62 nucleoporin indicated that this nuclear pore protein associates with Sp1. Furthermore, immunoprecipitation of p62 nucleoporin resulted in the coprecipitation of Sp1. Recombinant p62, expressed as a GST-fusion protein using a vaccinia virus system, also interacted with both recombinant and native Sp1. This interaction between p62 and Sp1 required the C-terminus of p62 and the C-terminus was able to bind Sp1, albeit less efficiently than native p62. A mammalian two-hybrid interaction assay was devised in which p62 was fused to the Gal4 DNA-binding domain. This system also indicated that p62, through its C-terminus, interacts with Sp1 in the living cell. We propose that this interaction of a nuclear pore protein with Sp1 may reflect the nuclear organization required to bring transcribable DNA in contact with the transcription factors.

Intracellular distribution of an integral nuclear pore membrane protein fused to green fluorescent protein--localization of a targeting domain

The 121-kDa pore membrane protein (POM121) is a bitopic integral membrane protein specifically located in the pore membrane domain of the nuclear envelope with its short N-terminal tail exposed on the luminal side and its major C-terminal portion adjoining the nuclear pore complex. In order to locate a signal for targeting of POM121 to the nuclear pores, we overexpressed selected regions of POM121 alone or fused to the green fluorescent protein (GFP) in transiently transfected COS-1 cells or in a stably transfected neuroblastoma cell line. Microscopic analysis of the GFP fluorescence or immunostaining was used to determine the intracellular distribution of the overexpressed proteins. The endofluorescent GFP tag had no effect on the distribution of POM121, since the chimerical POM121-GFP fusion protein was correctly targeted to the nuclear pores of both COS-1 cells and neuroblastoma cells. Based on the differentiated intracellular sorting of the POM121 variants, we conclude that the first 128 amino acids of POM121 contains signals for targeting to the continuous endoplasmic reticulum/nuclear envelope membrane system but not specifically to the nuclear pores and that a specific nuclear pore targeting signal is located between amino acids 129 and 618 in the endoplasmically exposed portion of POM121.

Subunit exchange of alphaA-crystallin

alpha-Crystallin, the major protein in the mammalian lens, is a molecular chaperone that can bind denaturing proteins and prevent their aggregation. Like other structurally related small heat shock proteins, each alpha-crystallin molecule is composed of an average of 40 subunits that can undergo extensive reorganization. In this study we used fluorescence resonance energy transfer to monitor the rapid exchange of recombinant alpha-crystallin subunits. We labeled alphaA-crystallin with stilbene iodoacetamide (4-acetamido-4'-((iodoacetyl)amino)stilbene-2,2'-disulfonic acid), which serves as an energy donor and with lucifer yellow iodoacetamide, which serves as an energy acceptor. Upon mixing the two populations of labeled alphaA-crystallin, we observed a reversible, time-dependent decrease in stilbene iodoacetamide emission intensity and a concomitant increase in lucifer yellow iodoacetamide fluorescence. This result is indicative of an exchange reaction that brings the fluorescent alphaA-crystallin subunits close to each other. We further showed that the exchange reaction is strongly dependent on temperature, with a rate constant of 0.075 min-1 at 37 degrees C and an activation energy of 60 kcal/mol. The subunit exchange is independent of pH and calcium concentration but decreases at low and high ionic strength, suggesting the involvement of both ionic and hydrophobic interactions. It is also markedly reduced by the binding of large denatured proteins. The degree of inhibition is directly proportional to the molecular mass and the amount of bound polypeptide, suggesting an interaction of several alphaA-crystallin subunits with multiple binding sites of the denaturing protein. Our findings reveal a dynamic organization of alphaA-crystallin subunits, which may be a key factor in preventing protein aggregation during denaturation.

Glycogenin, the primer of glycogen synthesis, binds to actin

We have studied the intracellular localization of glycogenin by fusing green fluorescent protein (GFP) to the N-terminus of rabbit muscle glycogenin and expressing the chimeric protein in C2C12, COS-1 and rat hepatic cells. The fusion protein showed a nuclear and cytosolic distribution and partially co-localized with actin in the cytosol. Disruption of the actin cytoskeleton with cytochalasin D led to a change in the pattern of green fluorescence, which coincided with that observed for the remaining non-depolymerized actin. The distribution of the single point mutant K324A was completely uniform and was not affected by this drug. These findings indicate that rabbit muscle glycogenin binds to actin through the heptapeptide 321DNIKKKL327, a common motif found in other actin-binding proteins, which is located at the C-terminal end of this protein, and suggest that the actin cytoskeleton plays an important role in glycogen metabolism.

Muscle glycogen synthase translocates from the cell nucleus to the cystosol in response to glucose

We have studied the intracellular localization of muscular glycogen synthase by fusing the green fluorescent protein (GFP) of the jelly-fish Aequorea victoria to the N-terminus of human muscle glycogen synthase (HMGS), and expressing the chimeric protein in C2C12, COS-1 cells, and primary cultured rat hepatocytes. In contrast to what we have recently found for the hepatic glycogen synthase (Fernandez-Novell et al. (1997) Biochem. J. 321, 227-231), the GFP/HMGS fusion protein is localized to the nucleus of the cell in the absence of glucose, and in the presence of the sugar it is essentially found in the cytosol. Insulin is not required for the translocation of the enzyme.

Kinetic characterization of the human retinoblastoma protein bipartite nuclear localization sequence (NLS) in vivo and in vitro. A comparison with the SV40 large T-antigen NLS

The retinoblastoma (RB) tumor suppressor is a nuclear phosphoprotein important for cell growth control and able to bind specifically to viral oncoproteins such as the SV40 large tumor antigen (T-ag). Human RB possesses a bipartite nuclear localization sequence (NLS) consisting of two clusters of basic amino acids within amino acids 860-877, also present in mouse and Xenopus homologs, which resembles that of nucleoplasmin. The T-ag NLS represents a different type of NLS, consisting of only one stretch of basic amino acids. To compare the nuclear import kinetics conferred by the bipartite NLS of RB to those conferred by the T-ag NLS, we used beta-galactosidase fusion proteins containing the NLSs of either RB or T-ag. The RB NLS was able to target beta-galactosidase to the nucleus both in vivo (in microinjected cells of the HTC rat hepatoma line) and in vitro (in mechanically perforated HTC cells). Mutational substitution of the proximal basic residues of the NLS abolished nuclear targeting activity, confirming its bipartite character. Nuclear accumulation of the RB fusion protein was half-maximal within about 8 min in vivo, maximal levels being between 3-4-fold those in the cytoplasm, which was less than 50% of the maximal levels attained by the T-ag fusion protein, while the initial rate of nuclear import of the RB protein was also less than half that of T-ag. Nuclear import conferred by both NLSs in vitro was dependent on cytosol and ATP and inhibited by the nonhydrolyzable GTP analog GTPgammaS. Using an ELISA-based binding assay, we determined that the RB bipartite NLS had severely reduced affinity, compared with the T-ag NLS, for the high affinity heterodimeric NLS-binding protein complex importin 58/97, this difference presumably representing the basis of the reduced maximal nuclear accumulation and import rate in vivo. The results support the hypothesis that the affinity of NLS recognition by NLS-binding proteins is critical in determining the kinetics of nuclear protein import.

Cationic lipid-based gene delivery systems: pharmaceutical perspectives

Gene delivery systems are designed to control the location of administered therapeutic genes within a patient's body. Successful in vivo gene transfer may require (i) the condensation of plasmid and its protection from nuclease degradation, (ii) cellular interaction and internalization of condensed plasmid, (iii) escape of plasmid from endosomes (if endocytosis is involved), and (iv) plasmid entry into cell nuclei. Expression plasmids encoding a therapeutic protein can be, for instance, complexed with cationic liposomes or micelles in order to achieve effective in vivo gene transfer. A thorough knowledge of pharmaceutics and drug delivery, bio-engineering, as well as cell and molecular biology is required to design optimal systems for gene therapy. This mini-review provides a critical discussion on cationic lipid-based gene delivery systems and their possible uses as pharmaceuticals.

Molecular cloning and expression of a chloride ion channel of cell nuclei

Ion channels are known to be present on the plasma membrane of virtually all cells and have been found on the membranes of various intracellular organelles. However, until recently they were believed not to occur at the nuclear membrane. In this study we describe the molecular cloning and characterization of a nuclear ion channel protein, designated nuclear chloride channel-27 (NCC27), from the human myelomonocytic cell line, U937. NCC27 is a novel chloride ion channel protein that was found to localize principally to the cell nucleus. Its only known homologue is a bovine chloride ion channel protein (p64) believed to localize to internal organelles. NCC27 therefore represents the first human member of a new class of organellar chloride ion channel proteins.

Development of targeted delivery systems for nucleic acid drugs

Our increased understanding of disease pathogenesis is the basis for developing novel nucleic acid drugs. The main challenge encountered in this development is how to maintain therapeutically meaningful concentrations of the drugs in the vicinity of their targets for the desired periods. The intrinsic difficulty arises from the fact that nucleic acid drugs are not readily transported across membranes. Hence, their delivery and transport characteristics at the whole body, organ and cellular levels need to be thoroughly examined. Liposomes and receptor-mediated polycation systems are promising carriers for their delivery in vivo. There are many barriers to be overcome for successful antisense and gene therapies. Along with other factors, disposition, stability against nucleases, binding to cell surface receptor and internalization, and intracellular trafficking affect the in vivo delivery and efficacy of nucleic acid drugs. This review article discusses the delivery and transport of these compounds.

Atomic force microscopy visualizes ATP-dependent dissociation of multimeric TATA-binding protein before translocation into the cell nucleus

The TATA-binding protein (TBP) is a universal transcription factor which plays an essential role in eukaryotic gene expression. As a karyophilic molecule, this cytosolic protein reaches its DNA-binding site through the transport channel of the nuclear pore complex. As occurs with other major cellular proteins, TBP forms multimers in solution, which is a limiting factor for nuclear translocation. While studying the nuclear translocation of TBP, we detected ATP-dependent multimerization of TBP with atomic force microscopy. In physiological solutions containing ATP, 14-molecule multimers dissociated into four-molecule multimers with a half-maximum dissociation constant of 10 microM. Electrophysiological experiments using isolated cell nuclei of cultured kidney cells revealed that TBP translocates into the cell nucleus only in the presence of ATP. When ATP was replaced with its slowly hydrolysing analogue, ATP[gamma-S] [i.e. adenosine 5'-o-(3-thiotriphosphate)], the aggregates remained intact and nuclear translocation was not possible. Taken together, our investigations suggest that TBP exhibits ATPase activity similar to that observed in relation to molecular chaperons. This activity secures physiological translocation of the transcription factor into the nucleus.

Aldosterone activates the nuclear pore transporter in cultured kidney cells imaged with atomic force microscopy

Nuclear pore complexes (NPC), located in the nuclear envelope, functionally connect the cell nucleus with the cytoplasm and serve as a crucial pathway for macromolecule exchange. A Madin-Darby canine kidney (MDCK) clone that resembles principal cells of the collecting duct was shown recently to respond to sustained aldosterone exposure with a significant increase in the NPC number per nucleus. The present study elucidates the molecular nature of the NPC pathway and its regulation by aldosterone applying atomic force microscopy. We imaged individual NPC in situ and searched for a putative so-called transporter in the NPC centre. In aldosterone-depleted cells we found numerous macromolecules docked to individual NPC waiting for translocation into the nucleoplasm (standby mode=inactive pore). In contrast, in aldosterone-treated cells NPC were frequently found free of macromolecules, indicating that the translocation process kept pace with docking under hormone-stimulated conditions (transport mode=active pore). In the NPC centre we detected a ring-like structure with a central invagination. We assume that the ring is the putative transporter and that the invagination is the channel entrance used for translocation of macromolecules. Transporters were found in open and closed configurations. In conclusion, the results provide evidence for the existence of a nuclear transporter as part of the translocation machinery of an individual NPC. Aldosterone increases the activity of the nuclear transporter and thus facilitates steroid-mediated gene expression.

Tyrosine kinase-dependent release of an adenovirus preterminal protein complex from the nuclear matrix

Adenovirus (Ad) replicative complexes form at discrete sites on the nuclear matrix (NM) through the interaction of Ad preterminal protein (pTP). The NM is a highly salt-resistant fibrillar network which is known to anchor transcription, mRNA splicing, and DNA replication complexes. Incubation of rATP with NM to which pTP was bound caused the release of pTP as a pTP-NM complex with a size of 220 to 230 kDa; incubation with 5' adenylylimidodiphosphate (rAMP-PNP) showed no significant release, indicating that rATP hydrolysis was required. With NM extracts, it was shown that a pTP-NM complex which was capable of binding Ad origin DNA could be reconstituted in vitro. A number of high-molecular-weight NM proteins ranging in size from 120 to 200 kDa were identified on Far Western blots for their ability to bind pTP. rATP-dependent release of pTP from the NM was inhibited in a dose-dependent fashion by the addition of tyrosine kinase inhibitors, such as quercetin, methyl-2,5-dihydroxycinnamate, or genistein. NM-mediated phosphorylation of a poly(Glu, Tyr) substrate was also significantly abrogated by the addition of these compounds. rATP-dependent release of Ad DNA termini bound to the NM via pTP was also blocked by the addition of these inhibitors. These results indicate that a tyrosine kinase mechanism controls the release of pTP from its binding sites on the NM. These data support the concept that phosphorylation may play a key role in the modulation of pTP binding sites on the NM.

Regulation of cytosolic calcium levels in vascular smooth muscle

Ca2+ plays an important role in the contraction of skeletal, cardiac, and smooth muscle, as well as in a number of important processes, such as secretion and neuronal activity. In this review, I focus on the various mechanisms by which cytosolic Ca2+ concentration is regulated in vascular smooth muscle, in the resting state and during activation. Particular attention is paid to the calcium pumps of the plasmalemma and the sarcoplasmic reticulum, to the inositol 1,4,5-trisphosphate- and ryanodine-sensitive calcium channels of the sarcoplasmic reticulum, and to voltage-dependent and voltage-independent calcium channels of the plasmalemma.

The regulation of protein transport to the nucleus by phosphorylation

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Cyclin-dependent kinase site-regulated signal-dependent nuclear localization of the SW15 yeast transcription factor in mammalian cells

Control over the nuclear transport of transcription factors (TFs) represents a level of gene regulation integral to cellular processes such as differentiation, transformation and signal transduction. The Saccharomyces cerevisiae TF SWI5 is excluded from the nucleus in a cell cycle-dependent fashion, mediated by phosphorylation by the cyclin-dependent kinase (cdk) CDC28. Nuclear entry occurs in G1. beta-galactosidase fusion proteins carrying SWI5 amino acids 633-682, including the nuclear localization sequence (NLS: Lys-Lys-Tyr-Glu-Asn-Val-Val-Ile-Lys-Arg-Ser-Pro-Arg-Lys-Arg-Gly-Arg-Pro- Arg-Lys655) were analyzed for subcellular localization in appropriate temperature-sensitive yeast strains blocked in G1 or G2/M using indirect immunofluorescence, and for nuclear import kinetics in living rat hepatoma or Vero African green monkey kidney cells microinjected with fluorescently labeled bacterially expressed protein and quantitative confocal laser microscopy. Cell cycle-dependent nuclear localization in yeast was both NLS and cdk site-dependent, whereby mutation of the cdk site serines (Ser646 and Ser664) to alanine resulted in constitutive nuclear localization. In mammalian cells, the SWI5 fusion proteins were similarly transported to the nucleus in an NLS-dependent fashion, while the mutation to Ala of the cdk site serines increased the maximal level of nuclear accumulation from about 1- to over 8-fold. We suggest that phosphorylation at the cdk sites inhibits nuclear transport of SWI5, consistent with our previous observations for the inhibition of SV40 large tumor antigen nuclear transport by phosphorylation by the cdk cdc2. The results indicate for the first time that a yeast NLS and, fascinatingly, its regulatory mechanisms are functional in higher eukaryotes, implying the universal nature of regulatory signals for protein transport to the nucleus.

Expression of biologically active procorticotrophin-releasing hormone (proCRH) in stably transfected CHO-K1 cells: characterization of nuclear proCRH

Corticotrophin-releasing hormone (CRH) is a 41 amino acid neuropeptide which is cleaved at a pair of dibasic amino acids from a larger precursor molecule (pre-proCRH) by the action of endopeptidases. In cells possessing a regulated secretory pathway, sorting of proneuropeptides and prohormones occurs within the trans-Golgi network, where they are finally packaged into secretory vesicles to be released in response to an external stimulus. Such cells also possess a constitutive secretory pathway, and neuropeptides are also translocated into this subcellular compartment. We have recently established stably transfected CHO-K1 cells expressing the rat pre-proCRH cDNA, and shown that proCRH was localized within the secretory pathway and the nucleus of transfected cells. Both the cytoplasmic and nuclear species of IR-CRH displayed an apparent molecular weight approximately 19 kDa, consistent with the size of the uncleaved CRH precursor molecule. In this paper, we further characterized the bitopological, i.e. nuclear and cytoplasmic localization of proCRH within transfected CHO-K1 cells. Immunoreactive nuclear CRH was not extractable using detergents (Triton X-100 and CHAPS), 10 mM salt washes or RNase digestion but could be abolished by digestion with DNase I. These results therefore suggest that nuclear proCRH is in close association with DNA/chromatin. Treatment of transfected cells with inhibitors of protein and RNA synthesis for up to 24 h had no effect upon immunoreactive nuclear CRH, indicating that it is very stable with a long half life. Brefeldin A treatment had no effect upon the nuclear translocation of newly synthesized proCRH, suggesting that late stages of the secretory pathway (i.e. post rough endoplasmic reticulum compartments) of the transfected cells do not play a role in proCRH nuclear transport. We also demonstrate that proCRH synthesized within stably transfected CHO-K1 cells is capable of stimulating ACTH release from primary cultures of anterior pituitary cells, therefore showing for the first time that the intact precursor is also biologically active and could act as an ACTH secretagogue in-vivo.

Disruption of the nucleoporin gene NUP133 results in clustering of nuclear pore complexes

We have characterized a protein with an estimated molecular mass of 130 kDa that is contained in a highly enriched yeast nuclear pore complex (NPC) fraction. Partial amino acid sequence from this protein has led us to a previously identified open reading frame on chromosome XI of Saccharomyces cerevisiae encoding a protein of 133 kDa. Due to its coenrichment with NPCs during cell fractionation and the phenotype observed in the disrupted strain, we propose to term the gene encoding this protein NUP133. Cells carrying a disrupted copy of NUP133 were temperature sensitive for growth. In addition, abnormal clustering of NPCs was observed. This phenotype is similar to that previously observed in the disruption of another nucleoporin gene, NUP145. We speculate that the gene product of NUP133, Nup133p, may functionally overlap with the NUP145 gene product, Nup145p, and that these proteins may be involved in maintaining the position of the NPC within the nuclear envelope.

Nuclear pores in the apoptotic cell

During apoptosis, nuclear pores undergo strong modifications, which are described here in five different apoptotic models. Conventional electron microscopy, supported by freeze-fracture analysis, showed a constant migration of nuclear pores towards the diffuse chromatin areas. In contrast, dense chromatin areas appear pore-free and are frequently surrounded by strongly dilated cisternae. A possible functional significance of this pore behaviour during apoptosis is discussed.

Preparation of isolated nuclei from K 562 haemopoietic cell line for high resolution scanning electron microscopy

The aim of the work is to visualise nuclear pore complexes (NPCs) in mammalian cells by high resolution scanning electron microscopy. A detergent-free isolation protocol was employed to obtain clean nuclei from the haemopoietic cell line K 562. Nuclear isolation was performed by mechanical homogenisation under hypotonic conditions followed by purification of the nuclear fraction. The isolated nuclei were attached to silicon chips, fixed, critical point dried, and sputter coated with a thin film (3-4 nm) of tantalum. Analysis of the nuclear surface by scanning electron microscopy (SEM) revealed a strong sensitivity of the outer nuclear membrane (ONM) to disruption during the isolation procedure. A significant reduction of the characteristic pattern of damage to the ONM was achieved by means of an isopicnic centrifugation on an isoosmolar balanced Percoll gradient. Analysis of the population of isolated nuclei by flow cytometry showed no signs of cell cycle specific losses of nuclei during isolation. The SEM investigations of the morphology of the nuclear envelope (NE) and of substructural details of NPCs and polyribosomes were performed using an in-lens field emission scanning electron microscope.

Nuclear pore complex ion channels (review)

It is currently thought that nuclear pore complexes (NPCs) primarily govern nucleocytoplasmic interactions via selective recognition and active transport of macromolecules. However, in various nuclear preparations, patch-clamp and fluorescence, luminiscence and ion microscopy support classical microelectrode measurements indicating that monoatomic ion flow across the nuclear envelope (NE) is strictly regulated. Gating of large conductance nuclear envelope ion channels (NICs) somewhat resembles that of gap junctional channels. In other respects, NICs are distinct in that they require cytosolic factors, are blocked by wheat germ agglutinin and are blocked and/or modified by antibodies to epitopes of NPC glycoproteins. Therefore, NIC activity, recorded as electrical current/conductance is likely to be intrinsic to NPCs. This observation suggests a potential use for the patch-clamp technique in establishing the mechanisms underlying nuclear pore gating in response to cytosolic and nucleosolic factors such as transcription and growth factors, oncogene and proto-oncogene products and receptors for retinoids, steroids and thyroid hormone. NIC activity may also be useful in evaluating the mechanisms of nuclear import of foreign nucleic acid material such as that contained in virons and viroids. Finally, in consideration to the electrophysiological data accumulated so far, the study of nuclear pore ion channel activity may help our understanding of other important issues such as cell suicide, programmed cell death or apoptosis.

Calcium and calcium-binding proteins in the nucleus

Calcium has long been known to play a role as a key cytoplasmic second messenger, but until relatively recently its possible involvement in nuclear signal transduction and the regulation of nuclear events has not been extensively studied. Evidence revealing the presence of transmembrane nuclear Ca2+ gradients and a variety of intranuclear Ca2+ binding proteins has fueled renewed interest in this key ion and its involvement in cell-cycle timing and division, gene expression, and protein activation. This review will offer an overview of the current state of knowledge and theory regarding calcium orchestration of nuclear functions and events and discuss possible future directions in this field of study.

NUP145 encodes a novel yeast glycine-leucine-phenylalanine-glycine (GLFG) nucleoporin required for nuclear envelope structure

We have isolated and characterized the gene encoding a fourth yeast glycine-leucine-phenylalanine-glycine (GLFG) repeat nucleoporin with a calculated molecular mass of 145.3 kD, and therefore termed NUP145. The amino-terminal half of Nup145p is similar to two previously identified GLFG nucleoporins, Nup116p and Nup100p (Wente, S. R., M. P. Rout, and G. Blobel. 1992. J. Cell Biol. 119:705-723). A deletion/disruption in the amino-terminal half of NUP145 (nup145 delta N) had only a slight effect on cell growth at temperatures between 17 and 37 degrees C. However, immunofluorescence microscopy of nup145 delta N cells with antinucleoporin antibodies showed that the characteristic punctate nuclear staining normally seen in wild-type yeast cells was reduced, with the majority of the signal located in one or two intense spots at the nuclear periphery. Thin section electron microscopy analysis revealed the presence of what appeared to be successive herniations of the nuclear envelope forming grape-like structures at primarily one site on the nup145 delta N nuclei. These successive herniations contained numerous NPC-like structures, correlating to the limited bright patches of anti-nucleoporin immunofluorescence signal. In some cases the successive herniations were small. Occasionally, however, multi-lobulated nuclei were seen. We suggest that the ultrastructural phenotype of nup145 delta N cells is due to a defective interaction of nup145 delta N NPCs with the surrounding pore membrane domain of the nuclear envelope. We have also analyzed the synthetic lethal phenotypes among GLFG nucleoporin mutant alleles, and found that strains harboring nup116 and either nup100 or nup145 mutations were not viable. This, in combination with the morphological analysis, may reflect overlapping yet distinct roles for these three GLFG nucleoporins in NPC-nuclear envelope interactions.