Nucleocytoplasmic transport: factors and mechanisms

Role of glyceraldehyde-3-phosphate dehydrogenase binding to hepatitis B virus posttranscriptional regulatory element in regulating expression of HBV surface antigen

The hepatitis B virus (HBV) posttranscriptional regulatory element (HPRE) has been demonstrated to facilitate the cytoplasmic localization of unspliced transcripts. One cellular protein, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is known to combine with this element. However, its function on HPRE remains unclear. Here, we show that recombinant GAPDH protein binds to HPRE RNA in vitro in streptavidin pull-down assays. Functional analysis demonstrated that GAPDH inhibited HPRE function in a pDM138-HPRE chloramphenicol acetyltransferase reporter assay system. Overexpression of GAPDH depressed the expression of HBs antigen, as detected both in cells transiently expressing HBs-HPRE and in HepG2.2.15 cells. These data indicate that GAPDH may be involved in the posttranscriptional regulation of HBV, which is critical for the life cycle of HBV.

Roles of molecular chaperones in the nervous system

Heat shock proteins (HSPs) are induced not only by heat shock but also by various other environmental stresses. HSPs such as Hsp90, Hsp70, Hsp60, Hsp40 and Hsp28 are also expressed constitutively at normal growth temperatures and have basic and indispensable functions in the life cycle of proteins as molecular chaperones, as well as playing a role in protecting cells from deleterious stresses. Recently, Hsc70 and Hsp40 were found to be localized to the synapse in the mammalian central nervous system, indicating a synaptic role for these HSPs. Molecular chaperones are able to inhibit the aggregation of partially denatured proteins and refold them. In addition, molecular chaperones, especially Hsp70, protect the brain and heart from severe ischemia. In these respects, there are expectations for the use of molecular chaperones for protection against and therapeutic treatment of inherited diseases caused by protein misfolding. In this study, we review Hsp70 and Hsp40, and refer to the roles of these molecules in the synapse and cytoprotective functions of HSPs in stress tolerance and neurodegenerative diseases.

Presence of molecular chaperones, heat shock cognate (Hsc) 70 and heat shock proteins (Hsp) 40, in the postsynaptic structures of rat brain

The synaptic localization of molecular chaperones, heat shock cognate protein 70 (Hsc70) and Hsp40, was investigated immunohistochemically in the normal rat brain. Postsynaptic density (PSD) fractions contained a constitutive form of HSP70, heat shock cognate protein 70 (Hsc70 or p73) but not inducible form of HSP70 (p72). The immunoreactivities of Hsc70 (p73) were distributed throughout the rat brain, in neuronal somata, dendrites and axons. Their immunoreactivity in neurons was localized in the cytoplasmic matrix, dendrites, and spines at the electron microscopic level. Presynaptic terminals, but less frequently than postsynaptic staining, were also reactive. Postsynaptic areas immediately beneath the synaptic contact or PSDs were immunoreactive for Hsc70. The Hsp40 was highly concentrated in PSD fractions. The staining of Hsp40 immunoreactivity was punctate and distributed widely in the brain. Hsp40 immunoreactivity was localized in dendritic spines, especially in the subsynaptic web, with weak staining of PSDs at the electron microscopic level. Double immunofluorescent staining and confocal microscopy revealed that Hsc70 and Hsp40 were co-localized on somata and neuronal processes of cultured cerebral neurons, on which synaptophysin immunoreactive spots were scattered. These results suggest that Hsp40 and Hsc70 are co-localized at postsynaptic sites and postsynaptic chaperone activity may be mediated by these two heat shock proteins.

Nucleo-cytoplasmic shuttling of the hdm2 oncoprotein regulates the levels of the p53 protein via a pathway used by the human immunodeficiency virus rev protein

The hdm2 gene is overexpressed in a variety of human tumors. Its gene product localizes predominantly to the nucleus, where it acts as an inhibitor of the p53 tumor suppressor gene product. It is shown here that the hdm2 oncoprotein constantly shuttles between the nucleus and the cytoplasm. Shuttling of hdm2 does not depend on its interaction with p53. Nuclear export of hdm2 is mediated by a signal sequence similar to the nuclear export signal of the rev protein from human immunodeficiency virus and other lentiviruses. Mutation of this signal sequence abolishes detectable nucleo-cytoplasmic shuttling. When fused to a carrier protein, the hdm2 signal sequence can mediate nuclear export after intranuclear microinjection into HeLa cells. The export of hdm2 can be blocked by a competitive inhibitor of rev export, arguing that the export pathways for hdm2 and rev are either overlapping or identical. Inhibition of its export modifies the ability of hdm2 to block p53-mediated transcriptional activation, and hdm2's export function is required to accelerate the degradation of p53. Thus the rev nuclear export pathway may be used to regulate an oncogene product's activity and modulate cellular growth.

Nuclear import and export of influenza virus nucleoprotein

Influenza virus nucleoprotein (NP) shuttles between the nucleus and the cytoplasm. A nuclear localization signal (NLS) has been identified in NP at amino acids 327 to 345 (J. Davey et al., Cell 40:667-675, 1985). However, some NP mutants that lack this region still localize to the nucleus, suggesting an additional NLS in NP. We therefore investigated the nucleocytoplasmic transport of NP from influenza virus A/WSN/33 (H1N1). NP deletion constructs lacking the 38 N-terminal amino acids, as well as those lacking the 38 N-terminal amino acids and the previously identified NLS, localized to both the cytoplasm and the nucleus. Nuclear localization of a protein containing amino acids 1 to 38 of NP fused to LacZ proved that these 38 amino acids function as an NLS. Within this region, we identified two basic amino acids, Lys7 and Arg8, that are crucial for NP nuclear import. After being imported into the nucleus, the wild-type NP and the NP-LacZ fusion construct containing amino acids 1 to 38 of NP were both transported back to the cytoplasm, where they accumulated. These data indicate that NP has intrinsic structural features that allow nuclear import, nuclear export, and cytoplasmic accumulation in the absence of any other viral proteins. Further, the information required for nuclear import and export is located in the 38 N-terminal amino acids of NP, although other NP nuclear export signals may exist. Treatment of cells with a protein kinase C inhibitor increased the amounts of nuclear NP, whereas treatment of cells with a phosphorylation stimulator increased the amounts of cytoplasmic NP. These findings suggest a role of phosphorylation in nucleocytoplasmic transport of NP.

Export of hepatitis B virus RNA on a Rev-like pathway: inhibition by the regenerating liver inhibitory factor IkappaB alpha

Nuclear export of hepatitis B virus (HBV) RNA is mediated by a specific RNA element but, in contrast to lentivirus genomic RNA, does not depend on viral proteins. We show that nonetheless, the export of HBV RNA can be blocked by competitive inhibitors of Rev-mediated lentivirus RNA export, suggesting that the export pathways of both viral species share components and might be driven by the same nuclear export machinery. HBV RNA export is also inhibited by overexpression of IkappaB alpha, as reported previously for the export of human immunodeficiency virus RNA. Since IkappaB alpha is strongly overexpressed during liver regeneration, its inhibition of HBV RNA export might contribute to elimination or silent persistence of HBV.

Nup93, a vertebrate homologue of yeast Nic96p, forms a complex with a novel 205-kDa protein and is required for correct nuclear pore assembly

Yeast and vertebrate nuclear pores display significant morphological similarity by electron microscopy, but sequence similarity between the respective proteins has been more difficult to observe. Herein we have identified a vertebrate nucleoporin, Nup93, in both human and Xenopus that has proved to be an evolutionarily related homologue of the yeast nucleoporin Nic96p. Polyclonal antiserum to human Nup93 detects corresponding proteins in human, rat, and Xenopus cells. Immunofluorescence and immunoelectron microscopy localize vertebrate Nup93 at the nuclear basket and at or near the nuclear entry to the gated channel of the pore. Immunoprecipitation from both mammalian and Xenopus cell extracts indicates that a small fraction of Nup93 physically interacts with the nucleoporin p62, just as yeast Nic96p interacts with the yeast p62 homologue. However, a large fraction of vertebrate Nup93 is extracted from pores and is also present in Xenopus egg extracts in complex with a newly discovered 205-kDa protein. Mass spectrometric sequencing of the human 205-kDa protein reveals that this protein is encoded by an open reading frame, KIAAO225, present in the human database. The putative human nucleoporin of 205 kDa has related sequence homologues in Caenorhabditis elegans and Saccharomyces cerevisiae. The analyze the role of the Nup93 complex in the pore, nuclei were assembled that lack the Nup93 complex after immunodepletion of a Xenopus nuclear reconstitution extract. The Nup93-complex-depleted nuclei are clearly defective for correct nuclear pore assembly. From these experiments, we conclude that the vertebrate and yeast pore have significant homology in their functionally important cores and that, with the identification of Nup93 and the 205-kDa protein, we have extended the knowledge of the nearest-neighbor interactions of this core in both yeast and vertebrates.

Nuclear export of the E1B 55-kDa and E4 34-kDa adenoviral oncoproteins mediated by a rev-like signal sequence

The E1B 55-kDa and E4 34-kDa oncoproteins of adenovirus type 5 (abbreviated here as E1B-55kD and E4-34kD) promote the export of viral mRNA and inhibit the export of most cellular mRNA species. We show that the intracellular complex containing E1B-55kD and E4-34kD continuously shuttles between the nucleus and the cytoplasm, and may thus serve as a nucleocytoplasmic transporter for viral mRNA. We present evidence that within this complex, it is the E4-34kD protein that directs both nuclear import and nuclear export. E4-34kD contains a functional nuclear export signal similar to corresponding sequences found in the retroviral proteins rev and rex. This sequence element is required for nuclear export of the complex, and it can function autonomously when fused to a carrier protein and microinjected in HeLa cell nuclei. When E4-34kD is expressed alone, a portion of the protein that contains a predicted arginine-rich amphipathic alpha-helical structure mediates nuclear retention of the protein. This retention, however, can be abolished by the association with E1B-55kD or by a specific point mutation within the arginine-rich motif. The export of E4-34kD can be blocked by an HTLV-rex derived competitive inhibitor and overexpressed E4-34kD inhibits rev-mediated transport, suggesting that the export pathways accessed by the adenoviral and retroviral proteins share components. The interplay between two polypeptides as well as the involvement of a dominant nuclear retention domain are novel features that might contribute to the efficiency and regulation of the adenovirus export system.

A nuclear export signal in Kap95p is required for both recycling the import factor and interaction with the nucleoporin GLFG repeat regions of Nup116p and Nup100p

During nuclear import, cytosolic transport factors move through the nuclear pore complex (NPC) to the nuclear compartment. Kap95p is required during import for docking the nuclear localization signal-receptor and ligand to the NPC. Recycling of this factor back to the cytoplasm is necessary for continued rounds of import; however, the mechanism for Kap95p recycling is unknown. We have determined that recycling of Kap95p requires a nuclear export signal (NES). A region containing the NES in Kap95p was sufficient to mediate active nuclear export in a microinjection assay. Moreover, the NES was necessary for function. Mutation of the NES in Kap95p resulted in a temperaturesensitive import mutant, and immunofluorescence microscopy experiments showed that the mutated Kap95p was not recycled but instead localized in the nucleus and at the nuclear envelope. Srp1p, the yeast nuclear localization signal-receptor, also accumulated in the nuclei of the arrested kap95 mutant cells. Wild-type and NES-mutated Kap95p both bound Gsp1p (the yeast Ran/TC4 homologue), Srp1p, and the FXFG repeat region of the nucleoporin Nup1p. In contrast, the NES mutation abolished Kap95p interaction with the GLFG repeat regions from the nucleoporins Nup116p and Nup100p. In vivo interaction was demonstrated by isolation of Kap95p from yeast nuclear lysates in either protein A-tagged Nup116p or protein A-tagged Nup100p complexes. The protein A-tagged Nup116p complex also specifically contained Gle2p. These results support a model in which a step in the recycling of Kap95p is mediated by interaction of an NES with GLFG regions. Analysis of genetic interactions suggests Nup116p has a primary role in Kap95p recycling, with Nup100p compensating in the absence of Nup116p. This finding highlights an important role for a subfamily of GLFG nucleoporins in nuclear export processes.

Intron-dependent enzymatic formation of modified nucleosides in eukaryotic tRNAs: a review

In eukaryotic cells, especially in yeast, several genes encoding tRNAs contain introns. These are removed from pre-tRNAs during the maturation process by a tRNA-specific splicing machinery that is located within the nucleus at the nuclear envelope. Before and after the intron removal, several nucleoside modifications are added in a stepwise manner, but most of them are introduced prior to intron removal. Some of these early nucleoside modifications are catalyzed by intron-dependent enzymes while most of the others are catalyzed in an intron-independent manner. In the present paper, we review all known cases where the nucleoside modifications were shown to depend strictly on the presence of an intron. These are pseudouridines at anticodon positions 34, 35 and 36 and 5-methylcytosine at position 34 of several eukaryotic tRNAs. One common property of the corresponding intron-dependent modifying enzymes is that their activities are essentially dependent on the local specific architecture of the pre-tRNA molecule that comprises the anticodon stem and loop prolonged by the intron domain. Thus introns clearly serve as internal (cis-type) RNAs that guide nucleoside modifications by providing transient target sites in tRNA for selected nuclear modifying enzymes. This situation may be similar to the recently discovered (trans-type) snoRNA-guided process of ribose methylations of ribosomal RNAs within the nucleolus of eukaryotic cells.

Phosphorylation of Srp1p, the yeast nuclear localization signal receptor, in vitro and in vivo

Srp1p, the protein encoded by SRP1 of the yeast Saccharomyces cerevisiae, is a yeast nuclear localization signal (NLS) receptor protein. We have previously reported isolation of a protein kinase from yeast extracts that phosphorylates Srp1p complexed with NLS peptides/proteins. From partial amino acid sequences of the four subunits of the purified kinase, we have now identified this protein kinase to be identical to yeast casein kinase II (CKII). It was previously thought that autophosphorylation of the 36 kDa subunit of the yeast enzyme was stimulated by the substrate, GST-Srp1p. However, with the use of a more refined system, no stimulation of autophosphorylation of the 36 kDa subunit of yeast CKII was observed. Biochemical and mutational analyses localized the in vitro phosphorylation site of Srp1p by CKII to serine 67. It was shown that, in the absence of NLS peptides/proteins, phosphorylation of the intact Srp1p protein is very weak, but deletion of the C-terminal end causes great stimulation of phosphorylation without NLS peptides/proteins. Thus, the CKII phosphorylation site is apparently masked in the intact protein structure by the presence of a C-terminal region, probably between amino acids 403 and 516. Binding of NLS peptides/proteins most likely causes a change in protein conformation, exposing the CKII phosphorylation site. Mutational alterations of serine 67, the CKII phosphorylation site, to valine (S67V) and aspartic acid (S67D) were not found to cause any significant deleterious effects on cell growth. Analysis of in vivo phosphorylation showed that at least 30% of the wild type Srp1p molecules are phosphorylated in growing cells, and that the phosphorylation is mostly at the serine 67 CKII site. The ability of Srp1p purified from E coli and treated with calf intestinal phosphatase to bind a SV40 T-antigen NLS peptide was compared with that of Srp1p which was almost fully phosphorylated by CKII. No significant difference was observed. It appears that NLS binding does not require any phosphorylation of Srp1p, either by CKII or by some other protein kinase.

Molecular interactions between the importin alpha/beta heterodimer and proteins involved in vertebrate nuclear protein import

We have used in vitro binding assays to examine specific interactions between a number of cytoplasmic and nuclear pore proteins involved in nuclear protein import in vertebrates. We demonstrate that nuclear transport factor 2 (NTF2), nucleoporin p62 and the Ras-like GTPase Ran bind to the importin heterodimer via its beta subunit. The binding behaviour of p62 truncation mutants indicated that importin-beta interacts primarily with the alpha-helical coiled-coil rod domain of nucleoporin p62 and not with the N-terminal domain that contains a number of degenerate repeats based on the xFxFG sequence motif. The binding of Ran to importin-beta was sensitive to its nucleotide state, with RanGTP binding strongly, whereas RanGDP binding could not be detected using our assay conditions. RanGTP, but not RanGDP, was able to displace p62 bound to the importin alpha/beta complex, suggesting that the binding sites for p62 and RanGTP on importin-beta overlap. Moreover, RanGTP, but not RanGDP, weakened the interaction between importin-alpha and importin-beta in a concentration-dependent manner. NTF2 bound to the importin heterodimer but did not displace p62, suggesting that the NTF2 and p62 binding sites on importin-beta do not overlap. The set of interactions we observed was not altered by the binding of NLS-containing substrates such as transcription factor IIIA to the importin heterodimer. Our results are consistent with models for nuclear protein import in which Ran nucleotide exchange modulates the binding of the importin-substrate complexes during translocation through nuclear pore complexes.

Differential expression and sequence-specific interaction of karyopherin alpha with nuclear localization sequences

The process of nuclear protein transport requires the interaction of several different proteins, either directly or indirectly with nuclear localization or targeting sequences (NLS). Recently, a number of karyopherins alpha, or NLS-binding proteins, have been identified. We have found that the karyopherins hSRP1 and hSRP1alpha are differentially expressed in various leukocyte cell lines and could be induced in normal human peripheral blood lymphocytes. We show that the two karyopherins bind with varied specificities in a sequence specific manner to different NLSs and that the sequence specificity is modulated by other cytosolic proteins. There was a correlation between binding of karyopherins alpha to different NLSs and their ability to be imported into the nucleus. Taken together, these data provide evidence for multiple levels of control of the nuclear import process.

Identification of protein p270/Tpr as a constitutive component of the nuclear pore complex-attached intranuclear filaments

Using a monoclonal antibody, mAb 203-37, we have identified a polypeptide of M(r) approximately 270 kD (p270) as a general constituent of the intranuclear filaments attached to the nucleoplasmic annulus of the nuclear pore complex (NPC) in diverse kinds of vertebrate cells. Using cDNA cloning and immunobiochemistry, we show that human protein p270 has a predicted molecular mass of 267 kD and is essentially identical to the coiled-coil dominated protein Tpr reported by others to be located on the outer, i.e., cytoplasmic surface of NPCs (Byrd, D.A., D.J. Sweet, N. Pante, K.N. Konstantinov, T. Guan, A.C.S. Saphire, P.J. Mitchell, C.S. Cooper, U. Aebi, and L. Gerace. 1994. J. Cell Biol. 127: 1515-1526). To clarify this controversial localization, we have performed immunoelectron microscopy in diverse kinds of mammalian and amphibian cells with a series of antibodies raised against different epitopes of human and Xenopus laevis p270/Tpr. In these experiments, the protein has been consistently and exclusively detected in the NPC-attached intranuclear filaments, and p270/Tpr-containing filament bundles have been traced into the nuclear interior for up to 350 nm. No reaction has been noted at the cytoplasmic side of NPCs with any of the p270/Tpr antibodies, whereas control antibodies such as those against protein RanBP2/Nup358 specifically decorate the cytoplasmic annulus of NPCs. Pore complexes of cytoplasmic annulate lamellae in various mammalian and amphibian cells are also devoid of immunodetectable protein p270/Tpr. We conclude that this coiled-coil protein is a general and ubiquitous component of the intranuclear NPC-attached filaments and discuss its possible functions.

In vitro reconstitution of a heterotrimeric nucleoporin complex consisting of recombinant Nsp1p, Nup49p, and Nup57p

The yeast nucleoporins Nsp1p, Nup49p, and Nup57p form a complex at the nuclear pores which is involved in nucleocytoplasmic transport. To investigate the molecular basis underlying complex formation, recombinant full-length Nup49p and Nup57p and the carboxyl-terminal domain of Nsp1p, which lacks the FXFG repeat domain, were expressed in Escherichia coli. When the three purified proteins were mixed together, they spontaneously associated to form a 150-kDa complex of 1:1:1 stoichiometry. In this trimeric complex, Nup57p fulfills the role of an organizing center, to which Nup49p and Nsp1p individually bind. For this interaction to occur, only two heptad repeat regions of the Nsp1p carboxyl-terminal domain are required, each region being about 50 amino acids in length. Finally, the reconstituted complex has the capability to bind to full-length Nic96p but not to mutant forms which also do not interact in vivo. When added to permeabilized yeast cells, the complex associates with the nuclear envelope and the nuclear pores. We conclude that Nsp1p, Nup49p, and Nup57p can reconstitute a complex in vitro which is competent for further assembly with other components of nuclear pores.

GLE2, a Saccharomyces cerevisiae homologue of the Schizosaccharomyces pombe export factor RAE1, is required for nuclear pore complex structure and function

To identify and characterize novel factors required for nuclear transport, a genetic screen was conducted in the yeast Saccharomyces cerevisiae. Mutations that were lethal in combination with a null allele of the gene encoding the nucleoporin Nup100p were isolated using a colony-sectoring assay. Three complementation groups of gle (for GLFG lethal) mutants were identified. In this report, the characterization of GLE2 is detailed. GLE2 encodes a 40.5-kDa polypeptide with striking similarity to that of Schizosaccharomyces pombe RAE1. In indirect immunofluorescence and nuclear pore complex fractionation experiments, Gle2p was associated with nuclear pore complexes. Mutated alleles of GLE2 displayed blockage of polyadenylated RNA export; however, nuclear protein import was not apparently diminished. Immunofluorescence and thin-section electron microscopic analysis revealed that the nuclear pore complex and nuclear envelope structure was grossly perturbed in gle2 mutants. Because the clusters of herniated pore complexes appeared subsequent to the export block, the structural perturbations were likely indirect consequences of the export phenotype. Interestingly, a two-hybrid interaction was detected between Gle2p and Srp1p, the nuclear localization signal receptor, as well as Rip1p, a nuclear export signal-interacting protein. We propose that Gle2p has a novel role in mediating nuclear transport.

Nic96p is required for nuclear pore formation and functionally interacts with a novel nucleoporin, Nup188p

The amino-terminal domain of Nic96p physically interacts with the Nsp1p complex which is involved in nucleocytoplasmic transport. Here we show that thermosensitive mutations mapping in the central domain of Nic96p inhibit nuclear pore formation at the nonpermissive temperature. Furthermore, the carboxyterminal domain of Nic96p functionally interacts with a novel nucleoporin Nup188p in an allele-specific fashion, and when ProtA-Nup188p was affinity purified, a fraction of Nic96p was found in physical interaction. Although NUP188 is not essential for viability, a null mutant exhibits striking abnormalities in nuclear envelope and nuclear pore morphology. We propose that Nic96p is a multivalent protein of the nuclear pore complex linked to several nuclear pore proteins via its different domains.

The thermolability of nuclear protein import in tsBN2 cells is suppressed by microinjected Ran-GTP or Ran-GDP, but not by RanQ69L or RanT24N

The nuclear protein regulator of chromosome condensation 1 (RCC1) stimulates guanine nucleotide exchange on a protein, Ran, that is required for nuclear protein import. In the present report, we confirm that RCC1 is also required for nuclear protein import in tsBN2 hamster cells in vivo. The thermolability of nuclear protein import in tsBN2 cells was suppressed by microinjection of purified Ran-GTP into the cytoplasm, but Ran-GDP also relieved the import deficiency, suggesting either that both forms of Ran are active in import in vivo or that tsBN2 cells at restrictive temperature retain a mechanism to convert Ran-GDP to Ran-GTP. To distinguish between these possibilities, nuclear protein import in tsBN2 cells was tested in the presence of Ran mutants, one deficient in GTP hydrolysis (RanQ69L), and one with weak binding to GDP and little or no binding to GTP (RanT24N). Microinjection of the mutant RanQ69L inhibited import in vivo in either the GTP- or GDP-bound form at both the permissive and nonpermissive temperatures. RanT24N-GDP inhibited import in vivo at the permissive temperature and failed to stimulate nuclear protein import at the nonpermissive temperature. The implications of these results for the roles of RCC1 and Ran in nuclear protein import in vivo are discussed.

Messengers from the synapses to the nucleus (MSNs) that establish late phase of long-term potentiation (LTP) and long-term memory

The late stage of long-term potentiation (LTP) and long-term memory is believed to be largely governed by altered gene expression for its generation and maintenance, while the early stages of LTP and memory are controlled mainly by the phosphorylation-dephosphorylation of the synaptic proteins. For the altered gene expression, synaptic information must be transmitted from the synaptic sites to the nucleus. This article describes the presence of specific messenger molecules that transmit synaptic information to the nucleus; these molecules are referred to as MSNs (Messengers from Synapse to the Nucleus). In addition, recent studies have indicated that certain transcription factors localize at postsynaptic sites as well as the nucleus, and may function as MSNs.

Nuclear pores and macromolecular assemblies involved in nucleocytoplasmic transport

Transport in and out of the nucleus is mediated by nuclear pore complexes (NPCs). Information has recently been emerging both on the structure of many of the proteins involved in this transport and on the complexes formed between them. In addition to NPC-based complexes, such as those based on nucleoporins Nsp1p and p62, cytoplasmic protein macromolecular assemblies are also important for nucleocytoplasmic transport.

Molecular dissection of the nuclear pore complex

The nuclear pore complex (NPC) is an approximately 120 megadalton (MDa) supramolecular assembly embedded in the double-membraned nuclear envelope (NE) that mediates bidirectional molecular trafficking between the cytoplasm and the nucleus of interphase eukaryotic cells. The structure of the NPC has been studied extensively by electron microscopy (EM), and a consensus model of its basic framework has emerged. Over the past few years, there has been significant progress in dissecting the molecular constituents of the NPC and in identifying distinct NPC subcomplexes. The combination of well-characterized antibodies with different EM specimen preparation methods has allowed localization of several of these proteins within the three-dimensional (3-D) architecture of the NPC. Thus, the molecular dissection of the NPC is definitely on its way to being elucidated. Here, we review these findings and discuss the emerging structural concepts.

Vigilin contains a functional nuclear localisation sequence and is present in both the cytoplasm and the nucleus

Vigilin is a member of the KH protein family and contains 14 tandemly arranged potential RNA-binding domains. Between KH domains 2 and 3 we have identified a nuclear localization sequence by cloning this sequence into the NH2-terminal region of phage T7 RNA polymerase as a reporter protein and by showing its transfer into the nucleus. Furthermore we provide experimental evidence that Vigilin is present both in the nucleus and in the cytoplasm in similar concentrations. These observations support the notion that Vigilin may shuttle between nucleus and cytoplasm presumably in contact with RNA molecules.

A novel complex of nucleoporins, which includes Sec13p and a Sec13p homolog, is essential for normal nuclear pores

In a genetic screen for nucleoporin-interacting components, a novel nuclear pore protein Nup84p, which exhibits homology to mammalian Nup107p, was isolated. Nup84p forms a complex with five proteins, of which Nup120p, Nup85p, Sec13p, and a Sec13p homolog were identified. Upon isolation of Sec13p-ProtA, nucleoporins were still associated, but the major copurifying band was a 150 kDa protein, showing that Sec13p occurs in two complexes. Disruption of any of the genes encoding Nup84p, Nup85p, or Nup120p caused defects in nuclear membrane and nuclear pore complex organization, as well as in poly(A)+ RNA transport. Thus, the Nup84p complex in conjunction with Sec13-type proteins is required for correct nuclear pore biogenesis.