Transport of proteins and RNAs in and out of the nucleus

Intracellular localization of RORα is isoform and cell line-dependent

The retinoid-related orphan receptor alpha (RORalpha) belongs to the nuclear receptor superfamily and comprises four isoforms generated by different promotor usage and alternative splicing. To better understand its function, the subcellular distribution of RORalpha was investigated. We could show that subcellular distribution of RORalpha is cell line and isoform-dependent. Isoform specific differences were mediated by the A/B domains which with the exception of RORalpha1 contain a signal that mediates cytoplasmic localization. The lack of this signal in RORalpha1 results in a complete nuclear localization and prevents cell membrane association observed for RORalpha2, 3, and 4. The region responsible for membrane association was identified as the C-terminal alpha-helix 12. Furthermore, the hinge region/ligand binding domain mediates nuclear localization. Our results show that isoform specific activity of RORalpha is not only regulated by different expression and DNA binding affinities but also by different subcellular distribution. Different access to the nucleus reveals an important mechanism regulating the activity of this constitutively active nuclear receptor.

A novel nucleocytoplasmic shuttling sequence of DAZAP1, a testis-abundant RNA-binding protein

Deleted in Azoospermia Associated Protein 1 (DAZAP1) is a ubiquitous RNA-binding protein highly expressed in the human and the mouse testes. It shows a dynamic subcellular localization during spermatogenesis, present predominantly in the nuclei of late-stage spermatocytes and round spermatids and translocated to the cytoplasm during spermatid elongation. To test the hypothesis that DAZAP1 shuttles between the nucleus and the cytoplasm, we studied the nuclear transport of DAZAP1 in somatic cells using immunostaining, heterokaryon formation, and mutagenesis. DAZAP1 is detected exclusively in the nucleus and has the ability to shuttle between the nucleus and the cytoplasm using a highly conserved 25 amino acid segment, designated ZNS, at its C terminus. ZNS shares no sequence homology with other known nuclear localization or export signals. Attachment of ZNS to a red fluorescent protein DsRed2 confers the nucleocytoplasmic shuttling ability to that protein. The nuclear localization of DAZAP1 depends on active transcription. In the presence of an RNA polymerase II inhibitor, DAZAP1 is retained in the cytoplasm. DAZAP1 colocalizes with hnRNP A1 and hnRNP C1 in the nucleus and is a component of the heterogeneous nuclear ribonucleoprotein particles. Our results suggest that DAZAP1 plays a key role in mRNA transport during spermatogenesis.

RhoA signaling in phorbol ester-induced apoptosis

Exposure of cells to phorbol ester activates protein kinase C (PKC) to induce apoptosis or differentiation, depending on the cellular context. In erythroblastic cell lines, TF-1 and D2, upregulation of the RhoA signaling promotes phorbol ester-induced apoptosis through activating Rho-associated kinase (ROCK)/phosphorylation of myosin light chain (MLC), thus generating membrane contraction force. As a result, cell adhesion is inhibited and death receptor-mediated death pathway is activated in these cells with a concurrent changes in nucleocytoplasmic signaling for protein trafficking. A microtubule-regulated GEF-H1, which is a specific RhoA activator, was identified to contribute to RhoA activation in these cells. Thus, a cytoskeleton-regulated RhoA signaling cooperates with PKC activation constitutes a cellular context to determine the cell fate in response to phorbol ester stimulation.

A combined effect of the KPNA3 and KPNB3 genes on susceptibility to schizophrenia

Recent studies suggest that both the KPNB3 gene and the KPNA3 gene in the long arm of chromosome 13 (13q) are associated with schizophrenia. Because these two genes belong to the same family of karyopherins, their combined effect on illness was investigated among 238 Chinese family trios consisting of fathers, mothers and affected offspring with schizophrenia. We detected three single nucleotide polymorphisms (SNPs), including rs626716 at the KPNB3 locus, and rs3782929 and rs3736830 at the KPNA3 locus. The transmission disequilibrium test (TDT) showed allelic association for rs626716 (X2=10.77, P=0.001) and for rs3782929 (X2=4.89, P=0.027) but not for rs3736830 (X2=0.29, P=0.59). Although the conditional test did not show association either for the rs626716-rs3782929 combinations (X2=1.329, d.f.=2, P=0.514) or for the rs626716-rs3736830 combinations (X2=0.606, d.f.=2, P=0.739), the 1-d.f. test showed association for the rs626716(C)-rs3782929(G) combination (X2=10.79, P=0.001) and for the rs626716(C)-rs3736830(G) combination (X2=8.64, P=0.003). The present work suggests that the combination of the KPNA3 gene and the KPNB3 gene may increase a genetic risk for schizophrenia.

The clinical virology of hepatitis B

Hepatitis B virus (HBV) is a major human health problem as approximately 8% of the world’s population are chronic carriers and there are over a million HBV-related deaths annually. Treatment of HBV is extremely difficult, as the unique viral replication strategy results in both a continual source of stable DNA molecules that are the template for viral replication and gene expression, and a pool of viral quasispecies from which different isolates may emerge as selection pressures alter. Although the use of antiviral therapies has improved outcomes significantly for many chronically infected individuals, the emergence of drug-resistant and immune/vaccine-escape viruses ensures there is a continuing need for the development of new and imaginative approaches to control and eventually eradicate HBV.

Characterization of functional domains of equine infectious anemia virus Rev suggests a bipartite RNA-binding domain

Equine infectious anemia virus (EIAV) Rev is an essential regulatory protein that facilitates expression of viral mRNAs encoding structural proteins and genomic RNA and regulates alternative splicing of the bicistronic tat/rev mRNA. EIAV Rev is characterized by a high rate of genetic variation in vivo, and changes in Rev genotype and phenotype have been shown to coincide with changes in clinical disease. To better understand how genetic variation alters Rev phenotype, we undertook deletion and mutational analyses to map functional domains and to identify specific motifs that are essential for EIAV Rev activity. All functional domains are contained within the second exon of EIAV Rev. The overall organization of domains within Rev exon 2 includes a nuclear export signal, a large central region required for RNA binding, a nonessential region, and a C-terminal region required for both nuclear localization and RNA binding. Subcellular localization of green fluorescent protein-Rev mutants indicated that basic residues within the KRRRK motif in the C-terminal region of Rev are necessary for targeting of Rev to the nucleus. Two separate regions of Rev were necessary for RNA binding: a central region encompassing residues 57 to 130 and a C-terminal region spanning residues 144 to 165. Within these regions were two distinct, short arginine-rich motifs essential for RNA binding, including an RRDRW motif in the central region and the KRRRK motif near the C terminus. These findings suggest that EIAV Rev utilizes a bipartite RNA-binding domain.

The nuclear import of RNA helicase A is mediated by importin-α3

RNA helicase A (RHA), an ATPase/helicase, regulates the gene expression at various steps including transcriptional activation and RNA processing. RHA is known to shuttle between the nucleus and cytoplasm. We identified the nuclear localization signal (NLS) of RHA and analyzed the nuclear import mechanisms. The NLS of RHA (RHA-NLS) consisting of 19 amino acid residues is highly conserved through species and does not have the consensus classical NLS. In vitro nuclear import assays revealed that the nuclear import of RHA was Ran-dependent and mediated with the classical importin-alpha/beta-dependent pathway. The binding assay indicated that the basic residues in RHA-NLS were used for interaction with importin-alpha. Furthermore, the nuclear import of RHA-NLS was supported by importin-alpha1 and preferentially importin-alpha3. Our results indicate that the nuclear import of RHA is mediated by the importin-alpha3/importin-beta-dependent pathway and suggest that the specificity for importin may regulate the functions of cargo proteins.

Dysfunction of the cholesterol biosynthetic pathway in Huntington's disease

The expansion of a polyglutamine tract in the ubiquitously expressed huntingtin protein causes Huntington's disease (HD), a dominantly inherited neurodegenerative disease. We show that the activity of the cholesterol biosynthetic pathway is altered in HD. In particular, the transcription of key genes of the cholesterol biosynthetic pathway is severely affected in vivo in brain tissue from HD mice and in human postmortem striatal and cortical tissue; this molecular dysfunction is biologically relevant because cholesterol biosynthesis is reduced in cultured human HD cells, and total cholesterol mass is significantly decreased in the CNS of HD mice and in brain-derived ST14A cells in which the expression of mutant huntingtin has been turned on. The transcription of the genes of the cholesterol biosynthetic pathway is regulated via the activity of sterol regulatory element-binding proteins (SREBPs), and we found an approximately 50% reduction in the amount of the active nuclear form of SREBP in HD cells and mouse brain tissue. As a consequence, mutant huntingtin reduces the transactivation of an SRE-luciferase construct even under conditions of SREBP overexpression or in the presence of an exogenous N-terminal active form of SREBP. Finally, the addition of exogenous cholesterol to striatal neurons expressing mutant huntingtin prevents their death in a dose-dependent manner. We conclude that the cholesterol biosynthetic pathway is impaired in HD cells, mice, and human subjects, and that the search for HD therapies should also consider cholesterol levels as both a potential target and disease biomarker.

A hot pepper gene encoding WRKY transcription factor is induced during hypersensitive response to Tobacco mosaic virus and Xanthomonas campestris

Plant WRKY transcription factors were previously implicated in the alteration of gene expression in response to various pathogens. The WRKY proteins constitute a large family of plant transcription factors, whose precise functions have yet to be elucidated. Using a domain-specific differential display procedure, we isolated a WRKY gene, which is rapidly induced during an incompatible interaction between hot pepper and Tobacco mosaic virus (TMV) or Xanthomonas campestris pv . vesicatoria (Xcv). The full-length cDNA of CaWRKY-a (Capsicum annuum WRKY-a) encodes a putative polypeptide of 546 amino acids, containing two WRKY domains with a zinc finger motif. The expression of CaWRKY-a could be rapidly induced by not only chemical elicitor such as salicylic acid (SA) or ethephon but also wounding treatments. The nuclear localization of CaWRKY-a was determined in transient expression system using tobacco BY-2 cells by polyethylene glycol (PEG)-mediated transformation experiment. With oligonucleotide molecules containing the putative W-box sequences as a probe, we confirmed that CaWRKY-a protein had W-box-binding activity. These results suggest that CaWRKY-a might be involved as a transcription factor in plant defense-related signal transduction pathways.

Cloning and characterization of rat importin 9: implication for its neuronal function

We describe the structure of the rat importin 9 gene, together with its transcripts and the encoded protein with its putative functional domains. The importin 9 gene contains 24 exons in a genomic region spanning >52,000 bp. It is transcribed into two mRNAs, generated by means of alternative polyadenylation site usage arranged in tandem. Both transcripts possess the same noncanonical polyadenylation signal (AGUAAA) in rat, this hexamer being conserved in all vertebrates examined. Additionally, intron 8 is bordered by AT-AC dinucleotides. Importin 9 is expressed throughout adult rat tissues, but the 114-kDa Importin 9 protein was detected only in the brain. The localization of the Importin 9 protein was examined by immunohistochemistry in both adult rat tissues and primary hippocampal cell cultures. The strongest labeling was detected in vivo in areas populated by neurons in high density and also in the dendritic processes emanating from these cells. This protein was clearly concentrated in the nuclei of these cells, although their cytoplasms too were heavily labeled. Strong cytoplasmic and very strong nuclear staining was found in a vast majority of the cells with neuronal morphology in vitro. Cultured cells with glial morphology generally exhibited a weaker cytoplasmic labeling. In these cells, the signal decorated the nuclear envelope without nuclear staining and gradually dwindled toward the cell periphery. These results hint at the cell- or tissue-type specific functions of this type of importin protein.

Amino acid and peptide bioconjugates of copper(II) and zinc(II) complexes with a modified N,N-bis(2-picolyl)amine ligand

Four chelating nitrogen ligands 2-5 derived from N,N-bis(2-picolyl)amine (bpa, 1) were synthesized, namely, (PyCH(2))(2)N-CH(2)-p-C(6)H(4)-CO(2)R (R = Me, 2, and R = H, 3) and (PyCH(2))(2)N-(CH(2))(n)-CO(2)H (n = 2, 4, and n = 5, 5). Amino acid conjugates 6 and 7 were formed by condensation of 3 with H-Phe-OMe and H-betaAla-OMe, respectively. Cu(II) and Zn(II) complexes of 1-7 were prepared and fully characterized. The X-ray structures of 1(Zn), 2(Zn), 4(Cu), and 7(Cu) were determined. The Zn complexes 1(Zn) and 2(Zn) as well as 7(Cu) show a distorted trigonal bipyramidal coordination environment in the solid state. An octahedral complex is observed for 4(Cu) which forms chains along the crystallographic b axis by intermolecular coordination of the carboxylic acid to the metal ion of a neighboring complex. Ligand 3 was used to prepare the peptide bioconjugate 8 (3-Ahx-Pro-Lys-Lys-Lys-Arg-Lys-Phe-NH(2)) with a nuclear localization signal (nls) heptapeptide by solid phase synthesis. Cu(II) and Zn(II) complexes of 8 were synthesized in situ and studied by FAB-MS, ESI-MS, UV/vis, and EPR (for 8(Cu)), and FAB-MS, ESI-MS, and NMR (for 8(Zn)). All spectroscopic results clearly support metal coordination to the bpa ligand in the bioconjugates 8(M), even in the presence of other potential ligands from amino acid side chains of the peptide. We suggest metal-peptide conjugates like 8(M) as artificial metallochaperones because they have the potential to deliver metal ions to specific compartments in the cell as determined by the peptide moieties.

Dissection of a novel nuclear localization signal in open reading frame 29 of varicella-zoster virus

Open reading frame 29 (ORF29) of varicella-zoster virus (VZV) encodes a 120-kDa single-stranded DNA binding protein (ORF29p) that is not packaged in the virion and is expressed during latency. During lytic infection, ORF29p is localized primarily to infected cell nuclei. In contrast, ORF29p is found exclusively in the cytoplasm in neurons of the dorsal root ganglia obtained at autopsy from seropositive latently infected patients. ORF29p accumulates in the nuclei of neurons in dorsal root ganglia obtained at autopsy from patients with active zoster. The localization of this protein is, therefore, tightly correlated with the proposed VZV lytic/latent switch. In this report, we have investigated the nuclear import mechanism of ORF29p. We identified a novel nuclear targeting domain bounded by amino acids 9 to 154 of ORF29p that functions independent of other VZV-encoded factors. In vitro import assays in digitonin-permeabilized HeLa cells reveal that ORF29p is transported into the nucleus by a Ran-, karyopherin alpha- and beta-dependent mechanism. These data are further supported by the demonstration that a glutathione S-transferase-karyopherin alpha fusion interacts with ORF29p, but not with a protein containing a point mutation in its nuclear localization signal (NLS). Therefore, the region of ORF29p responsible for its nuclear targeting is also involved in the association with karyopherin alpha. As a result of this interaction, this noncanonical NLS appears to hijack the classical cellular nuclear import machinery. Elucidation of the mechanisms governing ORF29p nuclear targeting could shed light on the VZV reactivation process.

Characterization of the Bex gene family in humans, mice, and rats

To better understand the development of ventral mesencephalic dopamine neurons, we performed subtractive hybridization screens to find ventral mesencephalic genes expressed at rat embryonic day 10 when these neurons begin to differentiate. The most commonly identified genes in these screens were members of the Bex (Brain expressed X-linked) gene family, rat Bex1 (Rex3), and a novel gene, rat Bex4. After identifying these genes, we then sought to characterize the Bex gene family. Two additional novel Bex genes (human Bex5 and mouse Bex6) were discovered through genomic databases. Bex5 is present in humans and monkeys, but not rodents, while Bex6 exists in mice, but not humans. Bex4 and Bex5 are localized to the X chromosome, are expressed in brain, and are similar in sequence. Bex4 and Bex5 are 54% and 56% identical to human Bex3 (pHGR74, NADE). Mouse Bex6 is on chromosome 16 and is 67% identical to mouse Bex4. Human Bex gene expression was studied with tissue expression arrays probed with specific oligonucleotides. Human Bex1 and Bex2 have similar expression patterns in the central nervous system with high levels in pituitary, cerebellum, and temporal lobe, and Bex1 is widely expressed outside of the central nervous system with high expression in the liver. Human Bex4 is highly expressed in heart, skeletal muscle, and liver, while Bex3 and Bex5 are more widely expressed. The subcellular localization of the Bex proteins varies from nuclear (rat Bex1) to cytoplasmic (rat Bex3, human Bex5, and mouse Bex6) and to both nuclear and cytoplasmic (rat Bex2 and rat Bex4). Rat Bex3, rat Bex4, human Bex5, and mouse Bex6 are degraded by the proteasome, while rat Bex1 or Bex2 are not. Rat Bex3 protein can likely bind transition metals through a histidine-rich domain. Because this gene family was originally named Bex and because these genes are unified by sequence similarity and gene structure, we believe the Bex nomenclature should prevail over nomenclature based on function (NADE) that has not been extended to the other Bex genes. We conclude that the Bex gene family members are highly homologous but differ in their expression patterns, subcellular localization, and degradation by the proteasome.

Nuclear import in viral infections

The separation of transcription in the nucleus and translation in the cytoplasm requires nucleo-cytoplasmic exchange of proteins and RNAs. Viruses have evolved strategies to capitalize on the nucleo-cytoplasmic trafficking machinery of the cell. Here, we first discuss the principal mechanisms of receptor-mediated nuclear import of proteinaceous cargo through the nuclear pore complex, the gate keeper of the cell nucleus. We then focus on viral strategies leading to nuclear import of genomes and subgenomic particles. Nucleo-cytoplasmic transport is directly important for those viruses that are replicating in the nucleus, such as DNA tumor viruses and RNA viruses, including parvoviruses, the DNA retroviruses hepadnaviruses, RNA-retrotransposons and retroviruses, adenoviruses, herpesviruses, papovaviruses, and particular negative-sense RNA viruses, such as the orthomyxovirus influenza virus. The viral strategies of nuclear import turn out to be surprisingly diverse. Their investigation continues to give insight into how nucleic acids pass in and out of the nucleus.

Internal ribosome entry segment-mediated translation during apoptosis: the role of IRES-trans-acting factors

During apoptosis, there is a reduction in translation initiation caused by caspase cleavage of several of the factors required for the cap-dependent scanning mechanism. Under these circumstances, many proteins that are required for apoptosis are instead translated by the alternative method of internal ribosome entry. This mechanism requires the formation of a complex RNA structural element and in the presence of internal ribosome entry segment (IRES)-trans-acting factors (ITAFs), the ribosome is recruited to the RNA. The interactions of several ITAFs with IRESs have been investigated in detail, and several mechanisms of action have been noted, including acting as chaperones, stabilising and remodelling the RNA structure. Structural remodelling by PTB in particular will be discussed, and how this protein is able to facilitate recruitment of the ribosome to several IRESs by causing previously occluded sites to become more accessible.

Characterization of the nuclear localization and nuclear export signals of bovine herpesvirus 1 VP22

The bovine herpesvirus 1 (BHV-1) tegument protein VP22 is predominantly localized in the nucleus after viral infection. To analyze subcellular localization in the absence of other viral proteins, a plasmid expressing BHV-1 VP22 fused to enhanced yellow fluorescent protein (EYFP) was constructed. The transient expression of VP22 fused to EYFP in COS-7 cells confirmed the predominant nuclear localization of VP22. Analysis of the amino acid sequence of VP22 revealed that it does not have a classical nuclear localization signal (NLS). However, by constructing a series of deletion derivatives, we mapped the nuclear targeting domain of BHV-1 VP22 to amino acids (aa) 121 to 139. Furthermore, a 4-aa motif, 130PRPR133, was able to direct EYFP and an EYFP dimer (dEYFP) or trimer (tEYFP) predominantly into the nucleus, whereas a deletion or mutation of this arginine-rich motif abrogated the nuclear localization property of VP22. Thus, 130PRPR133 is a functional nonclassical NLS. Since we observed that the C-terminal 68 aa of VP22 mediated the cytoplasmic localization of EYFP, an analysis was performed on these C-terminal amino acid sequences, and a leucine-rich motif, 204LDRMLKSAAIRIL216, was detected. Replacement of the leucines in this putative nuclear export signal (NES) with neutral amino acids resulted in an exclusive nuclear localization of VP22. Furthermore, this motif was able to localize EYFP and dEYFP in the cytoplasm, and the nuclear export function of this NES could be blocked by leptomycin B. This demonstrates that this leucine-rich motif is a functional NES. These data represent the first identification of a functional NLS and NES in a herpesvirus VP22 homologue.

Nup153 affects entry of messenger and ribosomal ribonucleoproteins into the nuclear basket during export

A specific messenger ribonucleoprotein (RNP) particle, Balbiani ring (BR) granules in the dipteran Chironomus tentans, can be visualized during passage through the nuclear pore complex (NPC). We have now examined the transport through the nuclear basket preceding the actual translocation through the NPC. The basket consists of eight fibrils anchored to the NPC core by nucleoprotein Nup153. On nuclear injection of anti-Nup153, the transport of BR granules is blocked. Many granules are retained on top of the nuclear basket, whereas no granules are seen in transit through NPC. Interestingly, the effect of Nup153 seems distant from the antibody-binding site at the base of the basket. We conclude that the entry into the basket is a two-step process: an mRMP first binds to the tip of the basket fibrils and only then is it transferred into the basket by a Nup153-dependent process. It is indicated that ribosomal subunits follow a similar pathway.

Two separate regions essential for nuclear import of the hnRNP D nucleocytoplasmic shuttling sequence

Heterogeneous nuclear ribonucleoprotein (hnRNP) D/AUF1 functions in mRNA genesis in the nucleus and modulates mRNA decay in the cytoplasm. Although it is primarily nuclear, it shuttles between the nucleus and cytoplasm. We studied the nuclear import and export of the last exon-encoding sequence common to all its isoforms by its expression as a green fluorescent protein-fusion protein in HeLa cells and by heterokaryon assay. The C-terminal 19-residue sequence (SGYGKVSRRGGHQNSYKPY) was identified as an hnRNP D nucleocytoplasmic shuttling sequence (DNS). In vitro nuclear transport using permeabilized cells indicated that nuclear import of DNS is mediated by transportin-1 (Trn-1). DNS accumulation in the nucleus was dependent on Trn-1, Ran, and energy in multiple rounds of nuclear transport. Use of DNS with deletions, alanine scanning mutagenesis and point mutations revealed that two separate regions (the N-terminal seven residues and the C-terminal two residues) are crucial for in vivo and in vitro transport as well as for interaction with Trn-1. The N- and C-terminal motifs are conserved in the shuttling sequences of hnRNP A1 and JKTBP.

The KPNA3 gene may be a susceptibility candidate for schizophrenia

The present study investigated the possible association of the KPNA3 locus in the 13q14 region with schizophrenia. We detected 7 single nucleotide polymorphisms (SNPs) on 13q14, one (rs6313) present at the HTR2A locus and the other 6 at the KPNA3 locus, among 124 British family trios consisting of mother, father and affected offspring with schizophrenia. The transmission disequilibrium test (TDT) showed allelic association for rs3736830 (chi(2)=8.66, P=0.003), rs2181185 (chi(2)=3.86, P=0.049) and rs626716 (chi(2)=5.82, P=0.016), but not for rs6313 (chi(2)=0.009, P=0.926). The global P-value was 0.029 for 1000 permutations with the TDT. The 2-SNP haplotype analysis showed a disease association for the rs2273816-rs3736830 haplotypes (chi(2)=7.63, d.f.=2, P=0.022), the rs3736830-rs2181185 haplotypes (chi(2)=10.30, d.f.=2, P=0.006) and the rs2181185-rs3782929 haplotypes (chi(2)=9.26, d.f.=2, P=0.01). The global P-value was 0.034 for 1000 permutations with the 2-SNP haplotype analysis. The 6-SNP haplotype system also showed a weak association with the illness (chi(2)=15.62, d.f.=8, P=0.048), although the 1-d.f. test did not show the association for nine individual haplotypes when a P-value was corrected by the Bonferroni corrections. The present study suggests that the KPNA3 may contribute genetically to schizophrenia in a small effect size.

Antagonistic regulation of Yan nuclear export by Mae and Crm1 may increase the stringency of the Ras response

Phosphorylation of Yan, a major target of Ras signaling, leads to Crm1-dependent Yan nuclear export, a response that is regulated by Yan polymerization. Yan SAM (sterile alpha motif) domain mutations preventing polymerization result in Ras-independent, but Crm1-dependent Yan nuclear export, suggesting that polymerization prevents Yan export. Mae, which depolymerizes Yan, competes with Crm1 for binding to Yan. Phosphorylation of Yan favors Crm1 in this competition and counteracts inhibition of nuclear export by Mae. These findings suggest that, prior to Ras activation, the Mae/Yan interaction blocks premature nuclear export of Yan monomers. After activation, transcriptional up-regulation of Mae apparently leads to complete depolymerization and export of Yan.

β-Catenin can act as a nuclear import receptor for its partner transcription factor, lymphocyte enhancer factor-1 (lef-1)

Nuclear accumulation of beta-catenin plays an important role in the Wnt signaling pathway. In the nucleus, beta-catenin acts as a transcriptional co-activator for TCF/LEF family of transcription factors. It has been shown that lef-1 contains a typical basic type nuclear localization signal (NLS) and is transported into the nucleus by the conventional import pathway. In this study, we found that a mutant lef-1 lacking the classical NLS accumulated in the nucleus of living cells, when beta-catenin was co-expressed. In addition, in a cell-free import assay, lef-1 migrated into the nucleus in the presence of beta-catenin alone without any other soluble factors. In contrast, another mutant lef-1 lacking the beta-catenin binding domain failed to migrate into the nucleus, even in the presence of beta-catenin. These findings indicate that beta-catenin alone can mediate the nuclear import of lef-1 through the direct binding. Collectively, we propose that there are two distinct pathways for the nuclear import of lef-1: importin alpha/beta-mediated and beta-catenin-mediated one, which provides a novel paradigm for Wnt signaling pathway.

MicroRNA biogenesis: coordinated cropping and dicing

The recent discovery of microRNAs (miRNAs) took many by surprise because of their unorthodox features and widespread functions. These tiny, approximately 22-nucleotide, RNAs control several pathways including developmental timing, haematopoiesis, organogenesis, apoptosis, cell proliferation and possibly even tumorigenesis. Among the most pressing questions regarding this unusual class of regulatory miRNA-encoding genes is how miRNAs are produced in cells and how the genes themselves are controlled by various regulatory networks.

NUP98 Fusion in Human Leukemia: Dysregulation of the Nuclear Pore and Homeodomain Proteins

NUP98 is fused to a variety of partner genes, including abdominal B-like HOX, in human myeloid and T-cell malignancies via chromosomal translocation involving 11p15. NUP98 encodes a 98-kd nucleoporin that is a component of the nuclear pore complex and functions in nucleocytoplasmic transport, with its N-terminal GLFG repeats used as a docking site for karyopherins. Disruption of NUP98 may affect the nuclear pore function, and the abnormal expression and altered function of fusion partners may also be critical for leukemia development. Recent studies using mouse models expressing NUP98-HOX have confirmed its leukemogenic potential, and cooperative genes for NUP98-HOXA9 in leukemogenesis have been identified in these studies.Thus, the NUP98 chimera is a unique molecule that provides valuable information regarding nuclear pore function and the role of the homeobox protein in leukemogenesis/carcinogenesis.

A novel role for Vpr of human immunodeficiency virus type 1 as a regulator of the splicing of cellular pre-mRNA

Vpr, one of the accessory gene products of human immunodeficiency virus type 1 (HIV-1), affects aspects of both viral and cellular proliferation, being involved in long terminal repeat (LTR) activation, arrest of the cell cycle at the G2 phase, and apoptosis. We have discovered a novel role for Vpr as a regulator of the splicing of pre-mRNA both in vivo and in vitro. We found, by RT-PCR and RNase protection analysis, that Vpr caused the accumulation of incompletely spliced forms of alpha-globin 2 and beta-globin pre-mRNAs in cells that had been transiently transfected with a Vpr expression vector. We postulated that this novel effect of Vpr might occur via a pathway that is distinct from arrest of the cell cycle at G2. By analyzing splicing reactions in vitro, we showed that Vpr inhibited the splicing of beta-globin pre-mRNA in vitro. The splicing of intron 1 of alpha-globin 2 pre-mRNA was modestly inhibited by Vpr but the splicing of intron 2 was unaffected. Interestingly, an experimental infection system which utilizes high-titered HIV-1/vesticular stomatitis virus G protein showed that Vpr expressed from an HIV-1 provirus was sufficient to accumulate endogenous alpha-globin 2 pre-mRNA. Thus, it is likely that Vpr contributes to selective inhibition of the splicing of cellular pre-mRNA.

Characterization of the nuclear import of human MutLalpha

DNA mismatch repair (MMR) is essential for the maintenance of replication fidelity. Its major task is to recognize mismatches as well as insertion/deletion loops of newly synthesized DNA strands. Although different players of human MMR have been identified, the regulation of essential steps of MMR is poorly understood. Because MMR is initiated in the nucleus, nuclear import might be a mechanism to regulate MMR. Nuclear targeting is accomplished by conserved signal sequences called nuclear localization signals (NLS), which represent clusters of positively charged amino acids (aa). hMLH1 contains two clusters of positively charged amino acids, which are candidate NLS sequences (aa 469-472 and 496-499), while hPMS2 contains one (aa 574-580). To study the effect of these clusters on nuclear import, NLS mutants of hMLH1 and hPMS2 were generated and expressed in 293T cells. The subcellular localization of the mutant constructs was monitored by confocal laser microscopy. We demonstrated that missense mutations of two signal sequences, one in hMLH1 and one in hPMS2, lead to impaired nuclear import, which was especially prominent for mutants of the hMLH1 residues K471 and R472; and hPMS2 residues K577 and R578.

Comprehensive analysis of a multidimensional liquid chromatography mass spectrometry dataset acquired on a quadrupole selecting, quadrupole collision cell, time-of-flight mass spectrometer: II. New developments in Protein Prospector allow for reliable and comprehensive automatic analysis of large datasets

A thorough analysis of the protein interaction partners of the yeast GTPase Gsp1p was carried out by a multidimensional chromatography strategy of strong cation exchange fractionation of peptides followed by reverse phase LC-ESI-MSMS using a QSTAR instrument. This dataset was then analyzed using the latest developmental version of Protein Prospector. The Prospector search results were also compared with results from the search engine "Mascot" using a new results comparison program within Prospector named "SearchCompare." The results from this study demonstrate that the high quality data produced on a quadrupole selecting, quadrupole collision cell, time-of-flight (QqTOF) geometry instrument allows for confident assignment of the vast majority of interpretable spectra by current search engines.

Coat proteins of Rice tungro bacilliform virus and Mungbean yellow mosaic virus contain multiple nuclear-localization signals and interact with importin alpha

Transport of the viral genome into the nucleus is an obligatory step in the replication cycle of plant pararetro- and geminiviruses. In both these virus types, the multifunctional coat protein (CP) is thought to be involved in this process. Here, a green fluorescent protein tagging approach was used to demonstrate nuclear import of the CPs of Rice tungro bacilliform virus (RTBV) and Mungbean yellow mosaic virus--Vigna (MYMV) in Nicotiana plumbaginifolia protoplasts. In both cases, at least two nuclear localization signals (NLSs) were identified and characterized. The NLSs of RTBV CP are located within both N- and C-terminal regions (residues 479KRPK/497KRK and 744KRK/758RRK), and those of MYMV CP within the N-terminal part (residues 3KR and 41KRRR). The MYMV and RTBV CP NLSs resemble classic mono- and bipartite NLSs, respectively. However, the N-terminal MYMV CP NLS and both RTBV CP NLSs show peculiarities in the number and position of basic residues. In vitro pull-down assays revealed interaction of RTBV and MYMV CPs with the nuclear import factor importin alpha, suggesting that both CPs are imported into the nucleus via an importin alpha-dependent pathway. The possibility that this pathway could serve for docking of virions to the nucleus is discussed.

Human mRNA cap methyltransferase: alternative nuclear localization signal motifs ensure nuclear localization required for viability

A characteristic feature of gene expression in eukaryotes is the addition of a 5'-terminal 7-methylguanine cap (m7GpppN) to nascent pre-mRNAs in the nucleus catalyzed by capping enzyme and cap methyltransferase. Small interfering RNA (siRNA) knockdown of cap methyltransferase in HeLa cells resulted in apoptosis as measured by terminal deoxynucleotidyltransferase-mediated dUTP-tetramethylrhodamine nick end labeling assay, demonstrating the importance of mRNA 5'-end methylation for mammalian cell viability. Nuclear localization of cap methyltransferase is mediated by interaction with importin-alpha, which facilitates its transport and selective binding to transcripts containing 5'-terminal GpppN. The methyltransferase 96-144 region has been shown to be necessary for importin binding, and N-terminal fusion of this sequence to nonnuclear proteins proved sufficient for nuclear localization. The targeting sequence was narrowed to amino acids 120 to 129, including a required 126KRK. Although full-length methyltransferase (positions 1 to 476) contains the predicted nuclear localization signals 57RKRK, 80KKRK, 103KKRKR, and 194KKKR, mutagenesis studies confirmed functional motifs only at positions 80, 103, and the previously unrecognized 126KRK. All three motifs can act as alternative nu clear targeting signals. Expression of N-truncated cap methyltransferase (120 to 476) restored viability of methyltransferase siRNA knocked-down cells. However, an enzymatically active 144-476 truncation mutant missing the three nuclear localization signals was mostly cytoplasmic and ineffective in preventing siRNA-induced loss of viability.

Enhanced nuclear import and transfection efficiency of plasmid DNA using streptavidin-fused importin-β

In order to enhance the nuclear import of exogenous genes, novel plasmid DNA/importin-beta conjugates, which consist of a biotinylated plasmid DNA and a recombinant streptavidin-fused importin-beta, were prepared. The spacer length between plasmid DNA and biotin and the number of introduced biotin were adjusted. The microinjection of plasmid DNA/importin-beta conjugates into the cytoplasm of NIH3T3 cells resulted in the nuclear localization of conjugates and the higher expression efficiency, compared to intact plasmid DNA alone. These results indicate that plasmid DNA/importin-beta conjugates would be an important tool to enhance the nuclear localization of exogenous DNA in non-viral gene delivery system.

Role of Nup98 in nuclear entry of human immunodeficiency virus type 1 cDNA

Human immunodeficiency virus type 1 (HIV-1), like other lentiviruses, can infect non-dividing cells. The lentiviruses are most likely to have evolved a nuclear import strategy to import HIV-1 cDNA and viral protein complex through the nuclear pore complex (NPC) formed by nucleoporin proteins (Nup). In this study, we found that synthesis of integrated and 2LTR but not full-length form of HIV-1 cDNA was clearly impaired in culture via transduction of vesicular stomatitis virus matrix protein (VSV M), an inhibitor protein, through binding to the phenylalanine-glycine (FG) repeat region of Nup98. The impairment of synthesis of integrated and 2LTR DNA with VSV M was restored by ectopic overexpression of Nup98. A series of experiments using Nup98-depleted NPC by the small interfering RNA (siRNA) technique showed specific impairment of NPC structure and some functions, including nuclear import of HIV-1 cDNA. Our results suggest that Nup98 on the NPC specifically participates in the nuclear entry of HIV-1 cDNA following HIV-1 entry.

The second open reading frame of the avian reovirus S1 gene encodes a transcription-dependent and CRM1-independent nucleocytoplasmic shuttling protein

It was previously shown that the second open reading frame of the avian reovirus S1 gene encodes a 146-amino-acid nonstructural protein, designated p17, which has no known function and no sequence similarity to other known proteins. The results presented in this report demonstrate that p17 accumulates in the nucleoplasm of infected and transfected cells. An examination of the deduced amino acid sequence of p17 revealed the presence of a putative monopartite nuclear localization signal (NLS) between residues 119 and 128. Mutagenesis analysis revealed both that this sequence is indeed a functional NLS and that two of its basic residues are critical for the normal nuclear distribution of p17. An interspecies heterokaryon assay further showed that p17 shuttles continuously between the nucleus and the cytoplasm and that this activity is restricted to its NLS-containing C-terminal tail. Finally, an analysis of the intracellular distribution of p17 in the presence of inhibitors of both RNA polymerase II and CRM1 further revealed that the nucleocytoplasmic distribution of p17 is coupled to transcriptional activity and that the viral protein exits the nucleus via a CRM1-independent pathway.

A CRM1-dependent nuclear export pathway is involved in the regulation of IRF-5 subcellular localization

Interferon regulatory factors (IRFs) are involved in gene regulation in many biological processes including the antiviral, growth regulatory, and immune modulatory functions of the interferon system. Several studies have demonstrated that IRF-3, IRF-5, and IRF-7 specifically contribute to the innate antiviral response to virus infection. It has been reported that virus-specific phosphorylation leads to IRF-5 nuclear localization and up-regulation of interferon, cytokine, and chemokine gene expression. Two nuclear localization signals have been identified in IRF-5, both of which are sufficient for nuclear translocation and retention in virus-infected cells. In the present study, we demonstrate that a CRM1-dependent nuclear export pathway is involved in the regulation of IRF-5 subcellular localization. IRF-5 possesses a functional nuclear export signal (NES) that controls dynamic shuttling between the cytoplasm and the nucleus. The NES element is dominant in unstimulated cells and results in the predominant cytoplasmic localization of IRF-5. Mutation of two leucine residues in the NES motif to alanine, or three adjacent Ser/Thr residues to the phosphomimetic Asp, results in constitutively nuclear IRF-5 and suggests that phosphorylation of adjacent Ser/Thr residues may contribute to IRF-5 nuclear accumulation in virus-induced cells. IKK-related kinases TBK1 and IKKepsilon have been shown to phosphorylate and activate IRF-3 and IRF-7, leading to the production of type 1 interferons and the development of a cellular antiviral state. We examined the phosphorylation and activation of IRF-5 by TBK1 and IKKepsilon kinases. Although IRF-5 is phosphorylated by IKKepsilon and TBK1 in co-transfected cells, the phosphorylation of IRF-5 did not lead to IRF-5 nuclear localization or activation.

PARP-10, a novel Myc-interacting protein with poly(ADP-ribose) polymerase activity, inhibits transformation

The proto-oncoprotein c-Myc functions as a transcriptional regulator that controls different aspects of cell behavior, including proliferation, differentiation, and apoptosis. In addition, Myc proteins have the potential to transform cells and are deregulated in the majority of human cancers. Several Myc-interacting factors have been described that mediate part of Myc's functions in the control of cell behavior. Here, we describe the isolation of a novel 150 kDa protein, designated PARP-10, that interacts with Myc. PARP-10 possesses domains with homology to RNA recognition motifs and to poly(ADP-ribose) polymerases (PARP). Molecular modeling and biochemical analysis define a PARP domain that is capable of ADP-ribosylating PARP-10 itself and core histones, but neither Myc nor Max. PARP-10 is localized to the nuclear and cytoplasmic compartments that is controlled at least in part by a Leu-rich nuclear export sequence (NES). Functionally, PARP-10 inhibits c-Myc- and E1A-mediated cotransformation of rat embryo fibroblasts, a function that is independent of PARP activity but that depends on a functional NES. Together, our findings define a novel PARP enzyme involved in the control of cell proliferation.

Belle is a Drosophila DEAD-box protein required for viability and in the germ line

DEAD-box proteins are ATP-dependent RNA helicases that function in various stages of RNA processing and in RNP remodeling. Here, we report identification and characterization of the Drosophila protein Belle (Bel), which belongs to a highly conserved subfamily of DEAD-box proteins including yeast Ded1p, Xenopus An3, mouse PL10, human DDX3/DBX, and human DBY. Mutations in DBY are a frequent cause of male infertility in humans. Bel can substitute in vivo for Ded1p, an essential yeast translation factor, suggesting a requirement for Bel in translation initiation. Consistent with an essential cellular function, strong loss of function mutations in bel are recessive lethal with a larval growth defect phenotype. Hypomorphic bel mutants are male-sterile. Bel is also closely related to the Drosophila DEAD-box protein Vasa (Vas), a germ line-specific translational regulator. We find that Bel and Vas colocalize in nuage and at the oocyte posterior during oogenesis, and that bel function is required for female fertility. However, unlike Vas, Bel is not specifically enriched in embryonic pole cells. We conclude that the DEAD-box protein Bel has evolutionarily conserved roles in fertility and development.

Casein kinase II phosphorylation regulates alphaNAC subcellular localization and transcriptional coactivating activity

The subcellular localization of the alphaNAC coactivator is regulated, but the signaling pathways controlling its nucleocytoplasmic shuttling and coactivation function are not completely characterized. We report here that casein kinase II (CK2) phosphorylated alphaNAC on several phosphoacceptor sites, especially in an amino-terminal cluster. Deletion or mutation of the clustered CK2 sites induced nuclear accumulation of alphaNAC in cells. alphaNAC also localized to the nucleus when endogenous CK2 activity was inhibited by quercetin or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). These observations suggested that phosphorylation by CK2 might play a signaling role in the nuclear export of alphaNAC. Interestingly, inhibition of the chromosome region maintenance 1 (CRM1) exportin by leptomycin B (LMB) led to accumulation of alphaNAC in the nucleus. We conclude that CK2 phosphorylation of the N-terminal cluster corresponds to the signal for alphaNAC's nuclear export via a CRM1-dependent pathway. Finally, the nuclear accumulation of the protein resulting from the lack of CK2 phosphorylation mediated a slight but significant increase of the alphaNAC coactivating function on AP-1 transcriptional activity. Thus, alphaNAC's exit from the nucleus and capacity to potentiate transcription appear dependent on its phosphorylation status.

New transport peptides broaden the horizon of applications for peptidic pharmaceuticals

Protein transduction domains (PTDs) have proven to be an invaluable tool to transduce a wide variety of cargo's including peptides across the plasma membrane and into intact tissue. The PTDs are able to deliver biologically active molecules both in vitro and in vivo. This study describes many new polybasic PTDs of which some are just as potent as the PTDs derived from extracellular RNAses or other published PTDs. Large differences in potency became apparent when the PTDs are coupled to particular cargoes. Therefore, the unique characteristic of a PTD may only become apparent when it is selected for a particular application. Rules for optimization of PTDs for particular applications are now emerging and open the way for a new generation of drug delivery agents. Because fixation artifacts and irreversible membrane binding may cause misinterpretation of the amount of internalization of polybasic peptides, we have developed an enzyme transduction assay based on the intracellular loading of a cell permeable substrate. In this assay, a fluorescent signal is generated by internalized enzyme in intact cells and not by membrane-bound or extracellular enzyme.

Insertion of a classical nuclear import signal into the matrix domain of the Rous sarcoma virus Gag protein interferes with virus replication

The Rous sarcoma virus Gag protein undergoes transient nuclear trafficking during virus assembly. Nuclear import is mediated by a nuclear targeting sequence within the MA domain. To gain insight into the role of nuclear transport in replication, we investigated whether addition of a "classical " nuclear localization signal (NLS) in Gag would affect virus assembly or infectivity. A bipartite NLS derived from nucleoplasmin was inserted into a region of the MA domain of Gag that is dispensable for budding and infectivity. Gag proteins bearing the nucleoplasmin NLS insertion displayed an assembly defect. Mutant virus particles (RC.V8.NLS) were not infectious, although they were indistinguishable from wild-type virions in Gag, Gag-Pol, Env, and genomic RNA incorporation and Gag protein processing. Unexpectedly, postinfection viral DNA synthesis was also normal, as similar amounts of two-long-terminal-repeat junction molecules were detected for RC.V8.NLS and wild type, suggesting that the replication block occurred after nuclear entry of proviral DNA. Phenotypically revertant viruses arose after continued passage in culture, and sequence analysis revealed that the nucleoplasmin NLS coding sequence was deleted from the gag gene. To determine whether the nuclear targeting activity of the nucleoplasmin sequence was responsible for the infectivity defect, two critical basic amino acids in the NLS were altered. This virus (RC.V8.KR/AA) had restored infectivity, and the MA.KR/AA protein showed reduced nuclear localization, comparable to the wild-type MA protein. These data demonstrate that addition of a second NLS, which might direct MA and/or Gag into the nucleus by an alternate import pathway, is not compatible with productive virus infection.

The unique C-terminal tail of the mitogen-activated protein kinase ERK5 regulates its activation and nuclear shuttling

ERK5 is unique among mitogen-activated protein kinases (MAPKs) in that it contains a large C-terminal tail. We addressed the question of how this tail could affect the signaling capacity of ERK5. Gradual deletion of the C-terminal domains resulted in a drastic increase of ERK5 kinase activity, which was dependent on the up-stream MAPK cascade, thus indicating a possible auto-inhibitory function of the tail. It is interesting that ERK5 was able to autophosphorylate its own tail. Moreover, ERK5, which was found to be expressed in virtually all kinds of cell lines, localized to nuclear as well as cytoplasmic compartments. The localization of ERK5 was determined by its C-terminal domains, which were also required for appropriate nucleocytoplasmic shuttling. Taken together, these results indicate that ERK5 signaling is directed by the presence of its unique C-terminal tail, which might be the key to understanding the key role of ERK5 in MAPK signaling.

Requirement of DDX3 DEAD box RNA helicase for HIV-1 Rev-RRE export function

A single transcript in its unspliced and spliced forms directs the synthesis of all HIV-1 proteins. Although nuclear export of intron-containing cellular transcripts is restricted in mammalian cells, HIV-1 has evolved the viral Rev protein to overcome this restriction for viral transcripts. Previously, CRM1 was identified as a cellular cofactor for Rev-dependent export of intron-containing HIV-1 RNA. Here, we present evidence that Rev/CRM1 activity utilizes the ATP-dependent DEAD box RNA helicase, DDX3. We show that DDX3 is a nucleo-cytoplasmic shuttling protein, which binds CRM1 and localizes to nuclear membrane pores. Knockdown of DDX3 using either antisense vector or dominant-negative mutants suppressed Rev-RRE-function in the export of incompletely spliced HIV-1 RNAs. Plausibly, DDX3 is the human RNA helicase which functions in the CRM1 RNA export pathway analogously to the postulated role for Dbp5p in yeast mRNA export.

Anti-HSV activity of lactoferrin and lactoferricin is dependent on the presence of heparan sulphate at the cell surface

Lactoferrin (LF) is a multifunctional glycoprotein, which plays an important role in immune regulation and defense mechanisms against bacteria, fungi, and viruses. Lactoferricin (Lfcin) is a potent antimicrobial peptide generated from the N-terminal part of LF by pepsin cleavage. In this study, we investigated the mechanisms of the anti-herpes simplex virus (anti-HSV) activity of LF and Lfcin. The results demonstrated that LF and Lfcin inhibited the entry of HSV into Vero cells. LF had no effect against HSV after the virus had entered the cells, while Lfcin exerted antiviral activity also after the initial binding of the virus to the host cell. The distribution of LF and Lfcin in the cells was investigated by immunogold-labeling and transmission electron microscope (TEM). LF was found mainly at the cell surface in cells expressing heparan sulphate. Lfcin was randomly distributed intracellularly. LF must be present at the cell surface to exert antiviral activity, while Lfcin exert its antiviral activity also when found mainly intracellularly. Both LF and Lfcin were dependent on the presence of heparan sulphate at the cell surface to exert their antiviral activity.

Nuclear efflux of heterogeneous nuclear ribonucleoprotein C1/C2 in apoptotic cells: a novel nuclear export dependent on Rho-associated kinase activation

Using a proteomic approach, we searched for protein changes dependent on Rho-associated kinase (ROCK) during phorbol-12-myristate-13-acetate (PMA)-induced apoptosis. We found that heterogeneous nuclear ribonucleoprotein C1 and C2 (hnRNP C1/C2), two nuclear restricted pre-mRNA binding proteins, are translocated to the cytosolic compartment in a ROCK-dependent manner in PMA-induced pro-apoptotic cells, where nuclear envelopes remain intact. The subcellular localization change of hnRNP C1/C2 appears to be dependent on ROCK-mediated cytoskeletal change and independent of caspase execution and new protein synthesis. Such a ROCK-dependent translocation is also seen in TNFalpha-induced apoptotic NIH3T3 cells. By overexpressing the dominant active form of ROCK, we showed that a ROCK-mediated signal is sufficient to induce translocation of hnRNP C1/C2. Deletion experiments indicated that the C-terminal 40-amino-acid region of hnRNP C1/C2 is required for ROCK-responsive translocation. By using nuclear yellow fluorescent protein (YFP) fusion, we determined that the C-terminal 40-amino-acid region of hnRNP C1/C2 is a novel nuclear export signal responsive to ROCK-activation. We conclude that a novel nuclear export is activated by the ROCK signaling pathway to exclude hnRNP C1/C2 from nucleus, by which the compartmentalization of specific hnRNP components is disturbed in apoptotic cells.

Ligand dependent and independent internalization and nuclear translocation of fibroblast growth factor (FGF) receptor 1

Basic fibroblast growth factor (FGF-2) is one of the prototype members of a rapidly expanding family of polypeptides. FGF-2 acts on cells via a dual-receptor system consisting of high-affinity tyrosine kinase receptors (FGFR) and low-affinity receptors comprised of heparan sulfate proteoglycans. Following ligand binding and subsequent internalization, both FGF-2 and FGFR1 are translocated to the nucleus where they have activities distinct from those expressed at the cell surface. Despite the growing number of growth factors and receptors shown to translocate to the nucleus, little is known about the mechanisms of internalization and translocation and how these processes are regulated. In the studies reported in this paper, we examined the roles of clathrin-dependent and -independent endocytosis in the uptake of FGFR1 and one of its ligands, FGF-2. While the uptake of FGF-2 occurred at least partly by a caveolar-dependent mechanism, that of FGFR1 was independent of both caveolae and coated pits. Surprisingly, neither the uptake of FGF-2 nor FGFR1 required the activity of the receptor tyrosine kinase. In addition, we identified a cell cycle-dependent pathway of FGFR1 nuclear translocation that appears to be independent of ligand binding.

Nuclear export of the nonenveloped parvovirus virion is directed by an unordered protein signal exposed on the capsid surface

It is uncertain whether nonenveloped karyophilic virus particles may actively traffic from the nucleus outward. The unordered amino-terminal domain of the VP2 major structural protein (2Nt) of the icosahedral parvovirus minute virus of mice (MVM) is internal in empty capsids, but it is exposed outside of the shell through the fivefold axis of symmetry in virions with an encapsidated single-stranded DNA genome, as well as in empty capsids subjected to a heat-induced structural transition. In productive infections of transformed and normal fibroblasts, mature MVM virions were found to efficiently exit from the nucleus prior to cell lysis, in contrast to the extended nuclear accumulation of empty capsids. Newly formed mutant viruses lacking the three phosphorylated serine residues of 2Nt were hampered in their exit from the human transformed NB324K nucleus, in correspondence with the capacity of 2Nt to drive microinjected phosphorylated heated capsids out of the nucleus. However, in normal mouse A9 fibroblasts, in which the MVM capsid was phosphorylated at similar sites but with a much lower rate, the nuclear exit of virions and microinjected capsids harboring exposed 2Nt required the infection process and was highly sensitive to inhibition of the exportin CRM1 in the absence of a demonstrable interaction. Thus, the MVM virion exits the nucleus by accessing nonconventional export pathways relying on cell physiology that can be intensified by infection but in which the exposure of 2Nt remains essential for transport. The flexible 2Nt nuclear transport signal may illustrate a common structural solution used by nonenveloped spherical viruses to propagate in undamaged host tissues.

Sm proteins, the constituents of the spliceosome, are components of nuage and mitochondrial cement in Xenopus oocytes

A conserved feature of germ cells in many animal species is the presence of perinuclear electron-dense material called the "nuage" that is believed to be a precursor of germinal (or polar or P) granules. In Xenopus oogenesis the nuage is first observed near the nuclear envelope and subsequently in close contact with mitochondria, at which stage it is called the mitochondrial cement. In this study, we found that, in Xenopus pre-stage I and stage I oocytes, nuage and mitochondrial cement contain the spliceosomal Sm proteins, Xcat2 mRNA, and DEAD-box RNA helicase XVLG1. Other components of Cajal bodies or splicing machinery such as coilin, SMN protein, and snRNAs are absent from the nuage and mitochondrial cement. We suggest that Xenopus Sm proteins have adapted to a role independent of pre-mRNA splicing and that instead of binding to their traditional spliceosomal partner such as snRNA, they bind mRNAs that are the components of germinal granules (i.e., Xcat2 mRNA) and facilitate the transport of these mRNAs from the nucleus to the nuage that is a precursor of germinal granules. In addition, the presence of Vasa-like DEAD-box helicase in Xenopus nuage suggests involvement of nuage in the microRNA and/or RNAi pathway, similar to the role of nuage in Drosophila.

A small-scale survey identifies selective and quantitative nucleo-cytoplasmic shuttling of a subset of CREM transcription factors

Elucidating dynamic aspects of intracellular localization of proteins is essential to decipher their functional interaction networks. Although transcription factors lacking a detectable cytoplasmic fraction have been generally considered compartmentalized in the nucleus, some were found to shuttle into the cytoplasm, suggesting functional interactions therein. To further investigate how common, specific and quantitative is this traffic, we have employed the heterokaryon assay for a small-scale survey of nuclear factors not previously tested for their nucleo-cytoplasmic motion. We show that a subset of cAMP response element (CRE) binding proteins of the CREM type shuttles within a biologically meaningful time frame, revealing a continuous flow into the cytoplasm that persists during signaling. Their dynamic behavior, not involving the classical Exportin-1 pathway, could be ascribed to C-terminal sequences, containing, in addition to the bZIP domain and the NLS, a nuclear export activity and an inhibitory activity at an adjacent site. Other proteins examined in this study either did not shuttle significantly or, like CREB and distinct CREM isoforms, shuttled with markedly delayed kinetics, denoting considerable selectivity of this traffic. These findings raise the possibility that events associated with bi-directional transport and periodic transit through the cytoplasm may modulate activities of select nuclear transcription factors.

Role of the tetradecapeptide repeat domain of human histone deacetylase 6 in cytoplasmic retention

Histone deacetylase 6 (HDAC6) contains tandem catalytic domains and a ubiquitin-binding zinc finger and displays deacetylase activity toward acetylated microtubules. Here we show that unlike its orthologs from Caenorhabditis elegans, Drosophila, and mouse, human HDAC6 possesses a tetradecapeptide repeat domain located between the second deacetylase domain and the C-terminal ubiquitin-binding motif. Related to this structural difference, the cytoplasmic localization of human, but not murine, HDAC6 is resistant to treatment with leptomycin B (LMB). Although it is dispensable for the deacetylase and ubiquitin binding activities of human HDAC6, the tetradecapeptide repeat domain displays acetyl-microtubule targeting ability. Moreover, it forms a unique structure and is required for the LMB-resistant cytoplasmic localization of human HDAC6. Besides the tetradecapeptide repeat domain, human HDAC6 possesses two LMB-sensitive nuclear export signals and a nuclear localization signal. These results thus indicate that the cytoplasmic localization for murine and human HDAC6 proteins is differentially regulated and suggest that the tetradecapeptide repeat domain serves as an important sequence element to stably retain human HDAC6 in the cytoplasm.

Two African swine fever virus proteins derived from a common precursor exhibit different nucleocytoplasmic transport activities

African swine fever virus (ASFV), a large icosahedral deoxyvirus, is the causative agent of an economically relevant hemorrhagic disease that affects domestic pigs. The major purpose of the present study was to investigate the nuclear transport activities of the ASFV p37 and p14 proteins, which result from the proteolytic processing of a common precursor. Experiments were performed by using yeast-based nucleocytoplasmic transport assays and by analysis of the subcellular localization of different green fluorescent and Myc fusion proteins in mammalian cells. The results obtained both in yeast and mammalian cells clearly demonstrated that ASFV p14 protein is imported into the nucleus but not exported to the cytoplasm. The ability of p37 protein to be exported from the nucleus to the cytoplasm of both yeast and mammalian cells was also demonstrated, and the results clearly indicate that p37 nuclear export is dependent on the interaction of the protein with the CRM-1 receptor. In addition, p37 was shown to exhibit nuclear import activity in mammalian cells. The p37 protein nuclear import and export abilities described here constitute the first report of a nucleocytoplasmic shuttling protein encoded by the ASFV genome. Overall, the overlapping results obtained for green fluorescent protein fusions and Myc-tagged proteins undoubtedly demonstrate that ASFV p37 and p14 proteins exhibit nucleocytoplasmic transport activities. These findings are significant for understanding the role these proteins play in the replication cycle of ASFV.