Mechanisms of receptor-mediated nuclear import and nuclear export

Activation of the DNA-dependent protein kinase stimulates nuclear export of the androgen receptor in vitro

The androgen receptor undergoes nuclear import in response to ligand, but the mechanism by which it undergoes nuclear export is poorly understood. We developed a permeabilized cell assay to characterize nuclear export of the androgen receptor in LNCaP prostate cancer cells. We found that nuclear export of endogenous androgen receptor can be stimulated by short double-stranded DNA oligonucleotides. This androgen receptor export pathway is dependent on ATP hydrolysis and is enhanced by phosphatase inhibition with okadaic acid. Fluorescence recovery after photobleaching in permeabilized cells, under the conditions that stimulate androgen receptor export, suggested that double-stranded DNA-dependent export does not simply reflect the relief of a nuclear retention mechanism. A radiolabeled androgen was used to show that the androgen receptor remains ligand-bound during translocation through the nuclear pore complex. A specific inhibitor to the DNA-dependent protein kinase, NU7026, inhibits androgen receptor export and phosphorylation. In living cells, NU7026 treatment increases androgen-dependent transcription from endogenous genes that are regulated by androgen receptor. We suggest that DNA-dependent protein kinase phosphorylation of the androgen receptor, or an interacting component, helps target the androgen receptor for export from the nucleus.

The mengovirus leader protein blocks interferon-alpha/beta gene transcription and inhibits activation of interferon regulatory factor 3

Viral infection of mammalian cells triggers the synthesis and secretion of type I interferons (i.e. IFN-alpha/beta), which induce the transcription of genes that cause cells to adopt an antiviral state. Many viruses have adapted mechanisms to evade IFN-alpha/beta-mediated responses. The leader protein of mengovirus, a picornavirus, has been implicated as an IFN-alpha/beta antagonist. Here, we show that the leader inhibits the transcription of IFN-alpha/beta and that both the presence of a zinc finger motif in its N-terminus and phosphorylation of threonine-47 are required for this function. Transcription of IFN-alpha/beta genes relies on the activity of a number of transcription factors, including interferon regulatory factor 3 (IRF-3). We show that the leader interferes with the transactivation activity of IRF-3 by interfering with its dimerization. Accordingly, mutant viruses with a disturbed leader function were impaired in their ability to suppress IFN-alpha/beta transcription in vivo. By consequence, the leader mutant viruses had an impaired ability to replicate and spread in normal mice but not in IFNAR-KO mice, which are incapable of mounting an IFN-alpha/beta-dependent antiviral response. These results suggest that the leader, by suppressing IRF3-mediated IFN-alpha/beta production, plays an important role in replication and dissemination of mengovirus in its host.

Determinants that control the distinct subcellular localization of p38alpha-PRAK and p38beta-PRAK complexes

p38alpha and p38beta MAPKs (mitogen-activated protein kinases) share about 80% of their protein sequence identity, but have quite different biological functions. One such difference is in regulating the subcellular localization of their downstream kinases, such as PRAK (p38-regulated/activated protein kinase or MK5). The p38alpha-PRAK complex is found in the nucleus, whereas the p38beta-PRAK complex is exclusively localized to the cytosol. By generating a series of chimeric and point mutants of p38alpha and p38beta, we found two amino acid residues (Asp(145) and Leu(156) in p38alpha, Gly(145) and Val(156) in p38beta) that determine the distinct subcellular locations of p38alpha-PRAK and p38beta-PRAK. The subcellular localization of MK2 (MAPK-activated protein kinase 2), another downstream kinase of p38, was regulated in the same manner as that of PRAK. We found that nuclear import, but not export, determines the subcellular localization of p38alpha-PRAK and p38beta-PRAK. The published structure of the p38alpha-MK2 complex suggests Leu(156) of p38alpha is involved in the interaction with the nuclear localization signal in PRAK. The difference at this residue between p38alpha and p38beta may affect the nuclear localization signal in PRAK differently, and thereby influence the import of the complexes. Asp(145) in p38alpha (or Gly(145) in p38beta) is located on a different surface patch, and further random mutagenesis revealed that mutation of Asp(145), Thr(123), and Gln(325), the residues that can directly interact with importin alpha as predicted by modeling, but not mutation of the other 7 amino acid residues that cannot reach importin alpha, re-locate p38alpha-PRAK to the cytosol, suggesting that interaction with import machinery is involved in determining the subcellular localization of the p38alpha-PRAK and p38beta-PRAK complexes. Last, we show that nuclear localization of PRAK is required for its role in inhibiting the proliferation of NIH3T3 cells. In conclusion, multiple determinants control the distinct subcellular localization of p38alpha-PRAK and p38beta-PRAK complexes, and the location of PRAK plays a role in its function.

Architecture of a coat for the nuclear pore membrane

The symmetric core of the nuclear pore complex can be considered schematically as a series of concentric cylinders. A peripheral cylinder coating the pore membrane contains the previously characterized, elongated heptamer that harbors Sec13-Nup145C in its middle section. Strikingly, Sec13-Nup145C crystallizes as a hetero-octamer in two space groups. Oligomerization of Sec13-Nup145C was confirmed biochemically. Importantly, the numerous interacting surfaces in the hetero-octamer are evolutionarily highly conserved, further underlining the physiological relevance of the oligomerization. The hetero-octamer forms a slightly curved, yet rigid rod of sufficient length to span the entire height of the proposed membrane-adjacent cylinder. In concordance with the dimensions and symmetry of the nuclear pore complex core, we suggest that the cylinder is constructed of four antiparallel rings, each ring being composed of eight heptamers arranged in a head-to-tail fashion. Our model proposes that the hetero-octamer would vertically traverse and connect the four stacked rings.

Multiple nuclear localization signals function in the nuclear import of the transcription factor Nrf2

Nuclear factor erythroid 2-related factor 2 (Nrf2) mediates the transcriptional response of cells to oxidative stress and is translocated into the nucleus following, or concomitant with, its activation by electrophiles or reactive oxygen species. The mechanism of its translocation into the nucleus is not entirely elucidated. Here we have identified two novel nuclear localization signal (NLS) motifs in murine Nrf2, one located near the N-terminal region (amino acid residues 42-53) and the other (residues 587-593) located near the C-terminal region. Imaging of green fluorescent protein (GFP)-tagged Nrf2 revealed that mutation(s) in any of these sequences resulted in decreased nuclear fluorescence intensity compared with the wild-type Nrf2 when Nrf2 activation was induced with the electrophile tert-butylhydroquinone. The mutations also impaired Nrf2-induced transactivation of antioxidant response element-driven reporter gene expression to the same extent as the Nrf2 construct bearing mutation in a previously identified bipartite NLS that maps at residues 494-511. When linked to GFP or to GFP-PEPCK-C each of the novel NLS motifs was sufficient to drive nuclear translocation of the fusion proteins. Co-immunoprecipitation assays demonstrated that importins alpha5 and beta1 associate with Nrf2, an interaction that was blocked by the nuclear import inhibitor SN50. SN50 also blocked tert-butylhydroquinone-induced nuclear fluorescence of GFP-Nrf2 in cells transfected with wild-type GFP-Nrf2. Overall these results reveal that multiple NLS motifs in Nrf2 function in its nuclear translocation in response to pro-oxidant stimuli and that the importin alpha-beta heterodimer nuclear import receptor system plays a critical role in the import process.

Molecular basis for the recognition of snurportin 1 by importin beta

The nuclear import of uridine-rich ribonucleoproteins is mediated by the transport adaptor snurportin 1 (SNP1). Similar to importin alpha, SNP1 uses an N-terminal importin beta binding (sIBB) domain to recruit the receptor importin beta and gain access to the nucleus. In this study, we demonstrate that the sIBB domain has a bipartite nature, which contains two distinct binding determinants for importin beta. The first determinant spans residues 25-65 and includes the previously identified importin alpha IBB (alphaIBB) region of homology. The second binding determinant encompasses residues 1-24 and resembles region 1011-1035 of the nucleoporin 153 (Nup153). The two binding determinants synergize within the sIBB domain to confer a low nanomolar binding affinity for importin beta (K(d) approximately 2 nm) in an interaction that, in vitro, is displaced by RanGTP. We propose that in vivo the synergy of Nup153 and nuclear RanGTP promotes translocation of uridine-rich ribonucleoproteins into the nucleus.

Nuclear translocation of hypoxia-inducible factors (HIFs): involvement of the classical importin alpha/beta pathway

Hypoxia-inducible factors are the key elements in the essential process of oxygen homeostasis of vertebrate cells. Stabilisation and subsequent nuclear localisation of HIF-alpha subunits results in the activation of target genes such as vegf, epo and glut1. The passage of transcription factors e.g. HIF-1alpha into the nucleus through the nuclear pore complex is regulated by nuclear transport receptors. Therefore nucleocytoplasmic shuttling can regulate transcriptional activity by facilitating the cellular traffic of transcription factors between both compartments. Here, we report on the identification of specific interactions of hypoxia-inducible factors with nuclear transport receptors importin alpha/beta. HIF-1alpha, -1beta, and HIF-2alpha are binding to importin alpha1, alpha3, alpha5, and alpha7. The direct interaction of HIF-1alpha to alpha importins is dependent on a functional nuclear localisation signal within the C-terminal region of the protein. In contrast, the supposed N-terminal NLS is not effective. Our findings provide new insight into the mechanism of the regulation of nuclear transport of hypoxia-inducible factors.

Crossing the nuclear envelope: hierarchical regulation of nucleocytoplasmic transport

Transport of macromolecules between the nucleus and cytoplasm is a critical cellular process for eukaryotes, and the machinery that mediates nucleocytoplasmic exchange is subject to multiple levels of control. Regulation is achieved by modulating the expression or function of single cargoes, transport receptors, or the transport channel. Each of these mechanisms has increasingly broad impacts on transport patterns and capacity, and this hierarchy of control directly affects gene expression, signal transduction, development, and disease.

Arx1 is a nuclear export receptor for the 60S ribosomal subunit in yeast

We previously showed that nuclear export of the large (60S) ribosomal subunit relies on Nmd3 in a Crm1-dependent manner. Recently the general mRNA export factor, the Mtr2/Mex67 heterodimer, was shown to act as an export receptor in parallel with Crm1. These observations raise the possibility that nuclear export of the 60S subunit in Saccharomyces cerevisiae requires multiple export receptors. Here, we show that the previously characterized 60S subunit biogenesis factor, Arx1, also acts as an export receptor for the 60S subunit. We found that deletion of ARX1 was synthetic lethal with nmd3 and mtr2 mutants and was synthetic sick with several nucleoporin mutants. Deletion of ARX1 led to accumulation of pre-60S particles in the nucleus that were enriched for Nmd3, Crm1, Mex67, and Mtr2, suggesting that in the absence of Arx1, 60S export is impaired even though the subunit is loaded with export receptors. Finally, Arx1 interacted with several nucleoporins in yeast two-hybrid as well as in vitro assays. These results show that Arx1 can directly bridge the interaction between the pre-60S particle and the NPC and thus is a third export receptor for the 60S subunit in yeast.

Competition between targeting signals in hybrid proteins provides information on their relative in vivo affinities for subcellular compartments

After their translation and folding in the cytoplasm, proteins may be imported into an organelle, associate with a membrane, or rather become part of large, highly localised cytoplasmic structures such as the cytoskeleton. The localisation of a protein is governed by the strength of binding to its immediate target, such as an import receptor for an organelle or a major component of the cytoskeleton, e.g. actin. We have experimentally provided a set of actin-binding proteins with competing targeting information and expressed them at various concentrations to analyse the strength of the signal that governs their subcellular localisation. Our microscopic observations indicate that organellar sorting signals override the targeting preference of most cytoskeletal proteins. Among these signals, the nuclear localisation signal of SV40 is strongest, followed by the oligomerised PHB domain that targets vacuolin to the endosomal surface, and finally the tripeptide SKL mediating transport into the peroxisome. The actin-associated protein coronin, however, can only be misled by the nuclear localisation signal. Interestingly, the targeting behaviour of this model set of hybrid proteins in living Dictyostelium amoebae correlates surprisingly well with the affinities of their constituent signals derived from in vitro experiments conducted in various other organisms. Accordingly, this approach allows estimating the in vivo affinity of a protein to its target even if the latter is not known, as in the case of vacuolin.

Mutations affecting spindle pole body and mitotic exit network function are synthetically lethal with a deletion of the nucleoporin NUP1 in S. cerevisiae

Nuclear pore complexes (NPCs) are embedded in the nuclear envelope of eukaryotic cells and function to regulate passage of macromolecules in and out of the nucleus. Nup1 is one of 30 nucleoporins comprising the NPC of the yeast Saccharomyces cerevisiae and is located on the nucleoplasmic face of the NPC where it plays a role in mRNA export and protein transport. In order to further characterize the function of Nup1 we used a genetic approach to identify mutations that are synthetically lethal in combination with a deletion of NUP1 (nup1Delta). We have identified one such nup1 lethal mutant (nle6) as a temperature sensitive allele of nud1. NUD1 encodes a component of the yeast spindle pole body (SPB) and acts as scaffolding for the mitotic exit network (MEN). We observe that nle6/nud1 mutant cells have a normal distribution of NPCs within the nuclear envelope and exhibit normal rates of nuclear protein import at both the permissive and restrictive temperatures. nup1Delta also exhibits synthetic lethality with bub2Delta and bfa1Delta, both of which encode proteins that colocalize with Nud1 at spindle pole bodies and function in the mitotic exit network. However, we do not observe genetic interactions among nle6/nud1, bub2Delta, or bfa1Delta and mutations in the nucleoporin encoding genes NUP60 or NUP170, nor is nup1Delta synthetically lethal with the absence of components downstream in the mitotic exit network, including Lte1, Swi5, and Dbf2. Our results suggest a novel functional connection between Nup1 and proteins comprising both the spindle pole body and early mitotic exit network.

Cse1p-binding dynamics reveal a binding pattern for FG-repeat nucleoporins on transport receptors

Nuclear pore proteins with phenylalanine-glycine repeats are vital to the functional transport of molecules across the nuclear pore complex. The current study investigates the binding of these FG-nucleoporins to the Cse1p:Kap60p:RanGTP nuclear export complex. Fourteen binding spots for FG-nucleoporin peptides are revealed on the surface of Cse1p, and 5 are revealed on the Kap60p surface. Taken together, and along with binding data for two other transport receptors, the data suggest that the ability to bind FG-nucleoporins by itself is not enough to ensure viable nuclear transport. Rather, it is proposed that the density of binding spots on the transport receptor surface is key in determining transport viability. The number of binding spots on the transport receptor surface should be large enough to ensure multiple, simultaneous FG-repeat binding, and their arrangement should be close enough to ensure multiple binding from the same FG-nucleoporin.

Nucleocytoplasmic trafficking of the molecular chaperone Hsp104 in unstressed and heat-shocked cells

Hsp104 is a molecular chaperone in yeast that restores solubility and activity to inactivated proteins after severe heat shock. We investigated the mechanisms that influence Hsp104 subcellular distribution in both unstressed and heat-shocked cells. In unstressed cells, Hsp104 and a green fluorescent protein-Hsp104 fusion protein were detected in both the nucleus and the cytoplasm. We demonstrate that a 17-amino-acid sequence of Hsp104 nuclear localization sequence 17 (NLS17) is sufficient to target a reporter molecule to the nucleus and is also necessary for normal Hsp104 subcellular distribution. The nuclear targeting function of NLS17 is genetically dependent on KAP95 and KAP121. In addition, wild-type Hsp104, but not an NLS17-mutated Hsp104 variant, accumulated in the nucleus of cells depleted for the general export factor Xpo1. Interestingly, severe, nonlethal heat shock enhances the nuclear levels of Hsp104 in an NLS17-independent manner. Under these conditions, we demonstrate that karyopherin-mediated nuclear transport is impaired, while the integrity of the nuclear-cytoplasmic barrier remains intact. Based on these observations, we propose that Hsp104 continues to access the nucleus during severe heat shock using a karyopherin-independent mechanism.

Nucleocytoplasmic Shuttling of the Zinc Finger Protein EZI Is Mediated by Importin-7-dependent Nuclear Import and CRM1-independent Export Mechanisms

Nucleocytoplasmic translocation constitutes a foundation for nuclear proteins to exert their proper functions and hence for various biological reactions to occur normally in eukaryotic cells. We reported previously that EZI/Zfp467, a 12 zinc finger motif-containing protein, localizes predominantly in the nucleus, yet the underlying mechanism still remains elusive. Here we constructed a series of mutant forms of EZI and examined their subcellular localization. The results delineated a non-canonical nuclear localization signal in the region covering the 9th to the 12th zinc fingers, which was necessary for nuclear accumulation of EZI as well as sufficient to confer nuclear localizing ability to a heterologous protein. We also found that the N-terminal domain of EZI is necessary for its nuclear export, the process of which was not sensitive to the CRM1 inhibitor leptomycin B. An interaction proteomics approach and the following co-immunoprecipitation experiments identified the nuclear import receptor importin-7 as a molecule that associated with EZI and, importantly, short interfering RNA-mediated knockdown of importin-7 expression completely abrogated nuclear accumulation of EZI. Taken together, these results identify EZI as a novel cargo protein for importin-7 and demonstrate a nucleocytoplasmic shuttling mechanism that is mediated by importin-7-dependent nuclear localization and CRM1-independent nuclear export.

Formation of a Tap/NXF1 homotypic complex is mediated through the amino-terminal domain of Tap and enhances interaction with nucleoporins

Nuclear export of mRNAs is mediated by the Tap/Nxt1 pathway. Tap moves its RNA cargo through the nuclear pore complex by direct interaction with nucleoporin phenylalanine-glycine repeats. This interaction is strengthened by the formation of a Tap/Nxt1 heterodimer. We now present evidence that Tap can form a multimeric complex with itself and with other members of the NXF family. We also show that the homotypic Tap complex can interact with both Nxt1 and nucleoporins in vitro. The region mediating this oligomerization is localized to the first 187 amino acids of Tap, which overlaps with its RNA-binding domain. Removal of this domain greatly reduces the ability of Tap to bind nucleoporins in vitro and in vivo. This is the first report showing that the Tap amino terminus modulates the interaction of Tap with nucleoporins. We speculate that this mechanism has a regulatory role for RNA export independent of RNA binding.

Heterodimerization with Jun family members regulates c-Fos nucleocytoplasmic traffic

c-Fos proto-oncoprotein forms AP-1 transcription complexes with heterodimerization partners such as c-Jun, JunB, and JunD. Thereby, it controls essential cell functions and exerts tumorigenic actions. The dynamics of c-Fos intracellular distribution is poorly understood. Hence, we have combined genetic, cell biology, and microscopic approaches to investigate this issue. In addition to a previously characterized basic nuclear localization signal (NLS) located within the central DNA-binding domain, we identified a second NLS within the c-Fos N-terminal region. This NLS is non-classic and its activity depends on transportin 1 in vivo. Under conditions of prominent nuclear localization, c-Fos can undergo nucleocytoplasmic shuttling through an active Crm-1 exportin-independent mechanism. Dimerization with the Jun proteins inhibits c-Fos nuclear exit. The strongest effect is observed with c-Jun probably in accordance with the relative stabilities of the different c-Fos:Jun dimers. Retrotransport inhibition is not caused by binding of dimers to DNA and, therefore, is not induced by indirect effects linked to activation of c-Fos target genes. Monomeric, but not dimeric, Jun proteins also shuttle actively. Thus, our work unveils a novel regulation operating on AP-1 by demonstrating that dimerization is crucial, not only for active transcription complex formation, but also for keeping them in the compartment where they exert their transcriptional function.

The N-terminal region of eukaryotic translation initiation factor 5A signals to nuclear localization of the protein

The eukaryotic translation initiation factor 5A (eIF5A) is a ubiquitous protein of eukaryotic and archaeal organisms which undergoes hypusination, a unique post-translational modification. We have generated a polyclonal antibody against murine eIF5A, which in immunocytochemical assays in B16-F10 cells revealed that the endogenous protein is preferentially localized to the nuclear region. We therefore analyzed possible structural features present in eIF5A proteins that could be responsible for that characteristic. Multiple sequence alignment analysis of eIF5A proteins from different eukaryotic and archaeal organisms showed that the former sequences have an extended N-terminal segment. We have then performed in silico prediction analyses and constructed different truncated forms of murine eIF5A to verify any possible role that the N-terminal extension might have in determining the subcellular localization of the eIF5A in eukaryotic organisms. Our results indicate that the N-terminal extension of the eukaryotic eIF5A contributes in signaling this protein to nuclear localization, despite of bearing no structural similarity with classical nuclear localization signals.

Rumble in the nuclear jungle: compartmentalization, trafficking, and nuclear action of plant immune receptors

Plants and animals have evolved structurally related innate immune sensors inside cells to detect the presence of microbial molecules. An evolutionary ancient folding machinery becomes engaged for the synthesis of autorepressed receptor forms in both kingdoms. The receptors act as regulatory signal transduction switches and are activated upon direct or indirect perception of non-self structures. Recent findings indicate that nucleo-cytoplasmic partitioning and nuclear activity is critical for the function of several plant immune sensors, thereby linking receptor function to transcriptional reprogramming of host cells for pathogen defense. This implies short signalling pathways and reveals parallels with regulatory control mechanisms of animal steroid receptors.

Ginger ingredients reduce viability of gastric cancer cells via distinct mechanisms

Ginger has been used throughout the world as spice, food and traditional herb. We found that 6-gingerol, a phenolic alkanone isolated from ginger, enhanced the TRAIL-induced viability reduction of gastric cancer cells while 6-gingerol alone affected viability only slightly. 6-Gingerol facilitated TRAIL-induced apoptosis by increasing TRAIL-induced caspase-3/7 activation. 6-Gingerol was shown to down-regulate the expression of cIAP1, which suppresses caspase-3/7 activity, by inhibiting TRAIL-induced NF-kappaB activation. As 6-shogaol has a chemical structure similar to 6-gingerol, we also assessed the effect of 6-shogaol on the viability of gastric cancer cells. Unlike 6-gingerol, 6-shogaol alone reduced the viability of gastric cancer cells. 6-Shogaol was shown to damage microtubules and induce mitotic arrest. These findings indicate for the first time that in gastric cancer cells, 6-gingerol enhances TRAIL-induced viability reduction by inhibiting TRAIL-induced NF-kappaB activation while 6-shogaol alone reduces viability by damaging microtubules.

Analysis of a predicted nuclear localization signal: implications for the intracellular localization and function of the Saccharomyces cerevisiae RNA-binding protein Scp160

Gene expression is controlled by RNA-binding proteins that modulate the synthesis, processing, transport and stability of various classes of RNA. Some RNA-binding proteins shuttle between the nucleus and cytoplasm and are thought to bind to RNA transcripts in the nucleus and remain bound during translocation to the cytoplasm. One RNA-binding protein that has been hypothesized to function in this manner is the Saccharomyces cerevisiae Scp160 protein. Although the steady-state localization of Scp160 is cytoplasmic, previous studies have identified putative nuclear localization (NLS) and nuclear export (NES) signals. The goal of this study was to test the hypothesis that Scp160 is a nucleocytoplasmic shuttling protein. We exploited a variety of yeast export mutants to capture any potential nuclear accumulation of Scp160 and found no evidence that Scp160 enters the nucleus. These localization studies were complemented by a mutational analysis of the predicted NLS. Results indicate that key basic residues within the predicted NLS of Scp160 can be altered without severely affecting Scp160 function. This finding has important implications for understanding the function of Scp160, which is likely limited to the cytoplasm. Additionally, our results provide strong evidence that the presence of a predicted nuclear localization signal within the sequence of a protein should not lead to the assumption that the protein enters the nucleus in the absence of additional experimental evidence.

A RanGAP protein physically interacts with the NB-LRR protein Rx, and is required for Rx-mediated viral resistance

Race-specific disease resistance in plants is mediated by the products of host disease resistance (R) genes. Plant genomes possess hundreds of R gene homologs encoding nucleotide-binding and leucine-rich repeat (NB-LRR) proteins. NB-LRR proteins induce a disease resistance response following recognition of pathogen-encoded avirulence (Avr) proteins. However, little is known about the general mechanisms by which NB-LRR proteins recognize Avr proteins or how they subsequently induce defense responses. The Rx NB-LRR protein of potato confers resistance to potato virus X (PVX). Using a co-purification strategy, we have identified a Ran GTPase-activating protein (RanGAP2) as an Rx-interacting protein. We show by co-immunoprecipitation that this interaction is mediated in planta through the putative signaling domain at the Rx amino terminus. Overexpression of RanGAP2 results in activation of certain Rx derivatives. Likewise, knocking down RanGAP2 expression in Nicotiana benthamiana by virus-induced gene silencing compromises Rx-mediated resistance to PVX. Thus, we have demonstrated a novel role for a RanGAP in the function of a plant disease resistance response.

Retention and mobility of the mammalian lamin B receptor in the plant nuclear envelope

BACKGROUND INFORMATION

In a previous study, we showed that GFP (green fluorescent protein) fused to the N-terminal 238 amino acids of the mammalian LBR (lamin B receptor) localized to the NE (nuclear envelope) when expressed in the plant Nicotiana tabacum. The protein was located in the NE during interphase and migrated with nuclear membranes during cell division. Targeting and retention of inner NE proteins requires several mechanisms: signals that direct movement through the nuclear pore complex, presence of a transmembrane domain or domains and retention by interaction with nuclear or nuclear-membrane constituents.

RESULTS

Binding mutants of LBR-GFP were produced to investigate the mechanisms for the retention of LBR in the NE. FRAP (fluorescence recovery after photobleaching) analysis of mutant and wild-type constructs was employed to examine the retention of LBR-GFP in the plant NE. wtLBR-GFP (wild-type LBR-GFP) was shown to have significantly lower mobility in the NE than the lamin-binding domain deletion mutant, which showed increased mobility in the NE and was also localized to the endoplasmic reticulum and punctate structures in some cells. Modification of the chromatin-binding domain resulted in the localization of the protein in nuclear inclusions, in which it was immobile.

CONCLUSIONS

As expression of truncated LBR-GFP in plant cells results in altered targeting and retention compared with wtLBR-GFP, we conclude that plant cells can recognize the INE (inner NE)-targeting motif of LBR. The altered mobility of the truncated protein suggests that not only do plant cells recognize this signal, but also have nuclear proteins that interact weakly with LBR.

Structural basis for RanGTP independent entry of spliceosomal U snRNPs into the nucleus

The nuclear import of assembled spliceosomal subunits, the uridine-rich small nuclear ribonucleoprotein particles (U snRNPs), is mediated by a nuclear import receptor adaptor couple of importin beta (Imp beta) and snurportin1 (SPN1). In contrast to any other characterized active nuclear import, the Imp beta/SPN1/U snRNP complex does not require RanGTP for the terminal release from the nuclear basket of the nuclear pore complex (NPC). The crystal structure of Imp beta (127-876) in complex with the Imp beta-binding (IBB) domain of SPN1 (1-65) at 2.8-A resolution reveals that Imp beta adopts an open conformation, which is unique for a functional Imp beta/cargo complex, and rather surprisingly, it resembles the conformation of the Imp beta/RanGTP complex. As binding of RanGTP to Imp beta usually triggers the release of import complexes from the NPC, we propose that by already mimicking a conformation similar to Imp beta/RanGTP the independent dissociation of Imp beta/SPN1 from the nuclear basket is energetically aided.

Subcellular distribution of human RDM1 protein isoforms and their nucleolar accumulation in response to heat shock and proteotoxic stress

The RDM1 gene encodes a RNA recognition motif (RRM)-containing protein involved in the cellular response to the anti-cancer drug cisplatin in vertebrates. We previously reported a cDNA encoding the full-length human RDM1 protein. Here, we describe the identification of 11 human cDNAs encoding RDM1 protein isoforms. This repertoire is generated by alternative pre-mRNA splicing and differential usage of two translational start sites, resulting in proteins with long or short N-terminus and a great diversity in the exonic composition of their C-terminus. By using tagged proteins and fluorescent microscopy, we examined the subcellular distribution of full-length RDM1 (renamed RDM1alpha), and other RDM1 isoforms. We show that RDM1alpha undergoes subcellular redistribution and nucleolar accumulation in response to proteotoxic stress and mild heat shock. In unstressed cells, the long N-terminal isoforms displayed distinct subcellular distribution patterns, ranging from a predominantly cytoplasmic to almost exclusive nuclear localization, suggesting functional differences among the RDM1 proteins. However, all isoforms underwent stress-induced nucleolar accumulation. We identified nuclear and nucleolar localization determinants as well as domains conferring cytoplasmic retention to the RDM1 proteins. Finally, RDM1 null chicken DT40 cells displayed an increased sensitivity to heat shock, compared to wild-type (wt) cells, suggesting a function for RDM1 in the heat-shock response.

Characterization of the nuclear localization signal of the hepatitis delta virus antigen

The delta antigen (HDAg) is the only protein encoded by the hepatitis delta virus (HDV) RNA genome. The HDAg contains an RNA binding domain, a dimerization domain, and a nuclear localization signal (NLS). The nuclear import of HDV RNPs is thought to be one of the first tasks of the HDAg during the HDV replication cycle. Using c-myc-PK fusions with several regions of the HDAg in transfection assays in Huh7 cells, we found that the HDAg NLS consists of a single stretch of 10 amino acids, EGAPPAKRAR, located in positions 66-75. Deletion and mutation analysis of this region showed that both the acidic glutamic acid residue at position 66 and the basic arginine residue at position 75 are essential for promoting nuclear import.

Crystal structure of nucleoporin Nic96 reveals a novel, intricate helical domain architecture

The nuclear pore complex (NPC) is an elaborate protein machine that mediates macromolecular transport across the nuclear envelope in all eukaryotes. The NPC is formed by nucleoporins that assemble in multiple copies around an 8-fold symmetry axis. Homology modeling suggests that most architectural nucleoporins are composed of simple beta-propeller and alpha-helical repeat domains. Here we present the crystal structure of Nic96, the Nup93 homolog in Saccharomyces cerevisiae, one of the major components of the NPC. This is the first structure of an alpha-helical nucleoporin domain. The protein folds into an elongated, mostly alpha-helical structure. Characteristically, non-canonical architectural features define the Nic96 structure. Sequence conservation among Nup93 homologs across all eukaryotes strongly suggests that the distinct topology is evolutionarily well maintained. We propose that the unique Nic96/Nup93 fold has a conserved function in all eukaryotes.

A saturated FG-repeat hydrogel can reproduce the permeability properties of nuclear pore complexes

The permeability barrier of nuclear pore complexes (NPCs) controls the exchange between nucleus and cytoplasm. It suppresses the flux of inert macromolecules > or = 30 kDa but allows rapid passage of even very large cargoes, provided these are bound to appropriate nuclear transport receptors. We show here that a saturated hydrogel formed by a single nucleoporin FG-repeat domain is sufficient to reproduce the permeability properties of NPCs. Importin beta and related nuclear transport receptors entered such hydrogel >1000x faster than a similarly sized inert macromolecule. The FG-hydrogel even reproduced import signal-dependent and importin-mediated cargo influx, allowing importin beta to accelerate the gel entry of a large cognate cargo more than 20,000-fold. Intragel diffusion of the importin beta-cargo complex occurred rapidly enough to traverse an NPC within approximately 12 ms. We extend the "selective phase model" to explain these effects.

Nuclear mRNA export requires specific FG nucleoporins for translocation through the nuclear pore complex

Trafficking of nucleic acids and large proteins through nuclear pore complexes (NPCs) requires interactions with NPC proteins that harbor FG (phenylalanine-glycine) repeat domains. Specialized transport receptors that recognize cargo and bind FG domains facilitate these interactions. Whether different transport receptors utilize preferential FG domains in intact NPCs is not fully resolved. In this study, we use a large-scale deletion strategy in Saccharomyces cerevisiae to generate a new set of more minimal pore (mmp) mutants that lack specific FG domains. A comparison of messenger RNA (mRNA) export versus protein import reveals unique subsets of mmp mutants with functional defects in specific transport receptors. Thus, multiple functionally independent NPC translocation routes exist for different transport receptors. Our global analysis of the FG domain requirements in mRNA export also finds a requirement for two NPC substructures—one on the nuclear NPC face and one in the NPC central core. These results pinpoint distinct steps in the mRNA export mechanism that regulate NPC translocation efficiency.

Constitutive nuclear import of latent and activated STAT5a by its coiled coil domain

Signal transducer and activator of transcription 5a (STAT5a) is a critical transcription factor for a number of physiological processes including hematopoiesis and mammary gland development. Cytokines such as growth hormone, prolactin, erythropoietin, and interleukin-2 stimulate the activation of STAT5a by tyrosine phosphorylation. Tyrosine phosphorylation confers a conformational change and the ability to bind specific target DNA. To execute its function as a signaling molecule and transcription factor, accurate cellular localization of STAT5a is essential. This study explores the nuclear trafficking of STAT5a both before phosphorylation and after tyrosine phosphorylation. With the use of live cell imaging we demonstrate the continuous shuttling of STAT5a in and out of the nucleus. Evaluation of a series of mutations and deletions identifies a region within the coiled coil domain of STAT5a that is critical for nuclear import of both unphosphorylated and tyrosine-phosphorylated forms. The mechanism that regulates transport of STAT5a through nuclear pore complexes into the nucleus is therefore independent of tyrosine phosphorylation. However, after tyrosine phosphorylation, STAT5a accumulates in the nucleus because of its retention by DNA binding. These findings should provide a foundation for further studies that involve targeting the activity of STAT5a.

A nuclear export sequence located on a beta-strand in fibroblast growth factor-1

Receptor-bound and endocytosed fibroblast growth factor-1 (FGF-1) is able to cross the vesicle membrane and translocate to cytosol and nucleus. This suggests an intracellular role of FGF-1, which also signals by activating transmembrane FGF receptors. Phosphorylation of internalized FGF-1 by nuclear protein kinase C delta induces rapid export from the nuclei by a leptomycin B-sensitive pathway. In the present work, we have searched for and identified a Leu-rich nuclear export sequence (NES) at the C terminus of FGF-1 required for its nuclear export and able to confer nuclear export activity to a reporter protein in an in vivo system. Mutants where hydrophobic amino acids within the NES were exchanged for alanine exhibited reduced or abolished nuclear export. As demonstrated in co-immunoprecipitation experiments, a complex containing FGF-1, exportin-1, and its co-factor Ran-GTP, was formed in vitro. Formation of this complex in vivo was demonstrated by a peroxisomal targeting assay. Formation of the FGF-1-exportin-1-Ran-GTP complex in vitro as well as nuclear export of FGF-1 in vivo was dependent on phosphorylation of FGF-1, and it was abolished by leptomycin B. The FGF-1 NES was found to be situated along a beta-strand, which has not been reported before, since NESs usually are alpha-helical.

Subcellular Localization Modulates Activation Function 1 Domain Phosphorylation in the Androgen Receptor

Although the steady-state distribution of the androgen receptor (AR) is predominantly nuclear in androgen-treated cells, androgen-bound AR shuttles between the nucleus and the cytoplasm. In the present study we have addressed how nucleocytoplasmic shuttling contributes to the regulation of AR. Nuclear transport signal fusions were used to force AR localization to the nucleus or cytoplasm of prostate cancer cells, and the effect of localization on shuttling, transcription, androgen binding, and phosphorylation was determined. Fusing the simian virus 40 nuclear localization signal or c-Abl nuclear export signal to AR resulted in androgen-independent localization to the nucleus or cytoplasm, respectively. AR forced to the nucleus was transcriptionally active on prostate-specific antigen and mouse mammary tumor virus promoters driving reporter genes. AR forced to the cytoplasm was largely inactive on the prostate-specific antigen promoter, but, surprisingly, AR was active on the mouse mammary tumor virus promoter and on two endogenous genes examined. Thus, highly transient nuclear localization of AR is sufficient to activate transcription. Androgen dissociation rates and the dissociation constant (KD) of AR for androgen were similar whether AR was localized to the cytoplasm or the nucleus, suggesting the ligand-binding cycle of AR is not strictly linked to its compartmentalization. Using phosphosite antibodies, we found that compartmentalization influences the phosphorylation state of AR. We show there is a bias for androgen-dependent phosphorylation of Ser81, Ser256, and Ser308 in the nucleus and androgen-independent phosphorylation of Ser94 in the cytoplasm. We propose that one function of nucleocytoplasmic shuttling is to integrate the signaling environment in the cytoplasm with AR activity in the nucleus.

Interphase nucleo-cytoplasmic shuttling and localization of SIRT2 during mitosis

The human NAD+-dependent protein deacetylase SIRT2 resides predominantly in the cytoplasm where it functions as a tubulin deacetylase. Here we report that SIRT2 maintains a largely cytoplasmic localization during interphase by active nuclear export in a Crm1-dependent manner. We identified a functional, leptomycin B-sensitive, nuclear export signal sequence within SIRT2. During the cell cycle, SIRT2 becomes enriched in the nucleus and is associated with mitotic structures, beginning with the centrosome during prophase, the mitotic spindle during metaphase, and the midbody during cytokinesis. Cells overexpressing wild-type or a catalytically inactive SIRT2 exhibit an increase in multinucleated cells. The findings suggest a novel mechanism of regulating SIRT2 function by nucleo-cytoplasmic shuttling, as well as a role for SIRT2 in the nucleus during interphase and throughout mitosis.

Acetate inhibits NFAT activation in T cellsvia importin β1 interference

Acetate is a principal short chain fatty acid produced by bacterial fermentation in the colon and a major end product of alcohol metabolism. In the present study, we assessed the effects of acetate on T cell activation and found that acetate inhibited NFAT activation but not NF-kappaB activation. Moreover, acetate impaired the nuclear translocation of NFAT but not that of NF-kappaB. Unlike cyclosporin A (CsA), acetate did not affect the dephosphorylation of NFAT and calcineurin activity. Acetate impaired the binding of NFAT to importin beta1, which is involved in NFAT nuclear translocation. NFAT is a critical transcription factor in cytokine and early response gene expression in activated T cells. Agents targeting NFAT such as CsA are used to suppress harmful immune responses in inflammatory diseases. Therefore, we also evaluated the efficacy of acetate in murine models of inflammatory diseases, and found that acetate administration (as well as administration of dexamethasone) attenuated trinitrobenzenesulfonic acid-induced colitis and dinitrofluorobenzene-induced dermatitis. These findings indicate for the first time that acetate inhibits NFAT activation by interfering with the interaction between NFAT and importin beta1 in T cells and that acetate can potentially act as an anti-inflammatory agent.

The Importance of N-terminal Polycysteine and Polybasic Sequences for G14α and G16α Palmitoylation, Plasma Membrane Localization, and Signaling Function

Plasma membrane targeting of G protein alpha (Galpha) subunits is essential for competent receptor-to-G protein signaling. Many Galpha are tethered to the plasma membrane by covalent lipid modifications at their N terminus. Additionally, it is hypothesized that Gq family members (Gqalpha,G11alpha,G14alpha, and G16alpha) in particular utilize a polybasic sequence of amino acids in their N terminus to promote membrane attachment and protein palmitoylation. However, this hypothesis has not been tested, and nothing is known about other mechanisms that control subcellular localization and signaling properties of G14alpha and G16alpha. Here we report critical biochemical factors that mediate membrane attachment and signaling function of G14alpha and G16alpha. We find that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences in their N termini and that the polycysteine sequence along with the adjacent polybasic region are both important for G16alpha-mediated signaling at the plasma membrane. Surprisingly, the isolated N termini of G14alpha and G16alpha expressed as peptides fused to enhanced green fluorescent protein each exhibit differential requirements for palmitoylation and membrane targeting; individual cysteine residues, but not the polybasic regions, determine lipid modification and subcellular localization. However, full-length G16alpha, more so than G14alpha, displays a functional dependence on single cysteines for membrane localization and activity, and its full signaling potential depends on the integrity of the polybasic sequence. Together, these findings indicate that G14alpha and G16alpha are palmitoylated at distinct polycysteine sequences, and that the adjacent polybasic domain is not required for Galpha palmitoylation but is important for localization and functional activity of heterotrimeric G proteins.

STAT dynamics

The ability of transcription factors to gain entrance to the nucleus is critical to their role in gene expression. Signal transducers and activators of transcription (STATs) are latent DNA binding factors activated by specific tyrosine phosphorylation. There are seven mammalian STAT genes encoding proteins that display constitutive nuclear localization and/or conditional nuclear localization. This review will focus on STAT1 and STAT2 that are activated in response to interferon and exhibit conditional nuclear localization. The dynamic redistribution of STAT1 and STAT2 between the cytoplasm and the nucleus is coordinate with their gain of ability to bind DNA.

Nuclear import of c-Jun is mediated by multiple transport receptors

c-Jun and c-Fos are major components of the transcriptional complex AP-1. Here, we investigate the nuclear import pathway(s) of the transcription factor c-Jun. c-Jun bound specifically to the nuclear import receptors importin beta, transportin, importin 5, importin 7, importin 9, and importin 13. In digitonin-permeabilized cells, importin beta, transportin, importin 7, and importin 9 promoted efficient import of c-Jun into the nucleus. Importin alpha, by contrast, inhibited nuclear import of c-Jun in vitro. A single basic region preceding the leucine zipper of c-Jun functions as a nuclear localization signal (NLS) and was required for interaction with all tested import receptors. In vivo, nuclear import of a c-Jun reporter protein lacking the leucine zipper strictly depended on this NLS. In a leucine zipper-dependent manner, c-Jun with mutations in its NLS was still imported into the nucleus in a complex with endogenous leucine zipper proteins or, for example, with cotransfected c-Fos. Together, these results explain the highly efficient nuclear import of the transcription factor c-Jun.

Nuclear import and export of Venezuelan equine encephalitis virus nonstructural protein 2

Many RNA viruses, which replicate predominantly in the cytoplasm, have nuclear components that contribute to their life cycle or pathogenesis. We investigated the intracellular localization of the multifunctional nonstructural protein 2 (nsP2) in mammalian cells infected with Venezuelan equine encephalitis virus (VEE), an important, naturally emerging zoonotic alphavirus. VEE nsP2 localizes to both the cytoplasm and the nucleus of mammalian cells in the context of infection and also when expressed alone. Through the analysis of a series of enhanced green fluorescent protein fusions, a segment of nsP2 that completely localizes to the nucleus of mammalian cells was identified. Within this region, mutation of the putative nuclear localization signal (NLS) PGKMV diminished, but did not obliterate, the ability of the protein to localize to the nucleus, suggesting that this sequence contributes to the nuclear localization of VEE nsP2. Furthermore, VEE nsP2 specifically interacted with the nuclear import protein karyopherin-alpha1 but not with karyopherin-alpha2, -3, or -4, suggesting that karyopherin-alpha1 transports nsP2 to the nucleus during infection. Additionally, a novel nuclear export signal (NES) was identified, which included residues L526 and L528 of VEE nsP2. Leptomycin B treatment resulted in nuclear accumulation of nsP2, demonstrating that nuclear export of nsP2 is mediated via the CRM1 nuclear export pathway. Disruption of either the NLS or the NES in nsP2 compromised essential viral functions. Taken together, these results establish the bidirectional transport of nsP2 across the nuclear membrane, suggesting that a critical function of nsP2 during infection involves its shuttling between the cytoplasm and the nucleus.

Impact of protein acetylation in inflammatory lung diseases

Chronic inflammatory lung diseases are characterized by increased expression of multiple inflammatory genes following activation by proinflammatory transcription factors, such as nuclear factor kappaB (NF-kappaB) and AP-1. Gene expression is, at least in part, regulated by acetylation of core histones through the action of coactivators, such as CREB-binding protein (CBP), which have intrinsic histone acetyltransferase (HAT) activity. Conversely gene repression is mediated via a combination of histone deacetylases (HDAC) and other corepressors. In asthma, the level of HAT activity is elevated in bronchial biopsies, whereas HDAC activity levels are only partially reduced and inhaled corticosteroids are able to reduce the increased HAT activity back to those seen in normal subjects. In contrast, in chronic obstructive pulmonary disease (COPD), there is a greater reduction in HDAC activity and HDAC2 expression but no difference in HAT activity. HAT and HDAC are also reported to modify a large and expanding number of nonhistone proteins, including nuclear import proteins, chaperones, cytoskeletal proteins, and other transcriptional factors, such as NF-kappaB and signal transducer and activation of transcription (STAT). Acetylation regulates several aspects of protein function and stability leading to differing effects on inflammatory gene expression and cell recruitment involved in the pathogenesis of inflammatory diseases. This review will examine the impact of acetylation on the function of key proteins involved in airway inflammatory disease and the effects of current therapies on acetylation status of key proteins. Further appreciation of the role of these changes may lead to the development of novel therapeutic approaches to inflammatory lung diseases that are currently difficult to treat.

Reversibility in nucleocytoplasmic transport

Nucleocytoplasmic exchange of proteins and RNAs is mediated by receptors that usher their cargo through the nuclear pores. Peptide localization signals on each cargo determine the receptors with which it will interact. Those interactions are normally regulated by the small GTPase Ran. Hydrolysis of GTP provides the chemical energy required to create a bona fide thermodynamic pump that selectively and directionally accumulates its substrates across the nuclear envelope. A common perception is that cargo delivery is irreversible, e.g., a protein imported to the nucleus does not return to the cytoplasm except perhaps via a specific export receptor. Quantitative measurements using cell-free nuclei reconstituted in Xenopus egg extract show that nuclear accumulation follows first-order kinetics and reaches steady state at a level that follows a Michaelis-Menten function of the cytoplasmic cargo concentration. This saturation suggests that receptor-mediated translocation across the nuclear pore occurs bidirectionally. The reversibility of accumulation was demonstrated directly by exchange of the cytosolic medium and by fluorescence recovery after photobleaching. Based on our results, we offer a simple biophysical model that predicts the observed behavior. A far-reaching consequence is that the nuclear localization signal dictates the fate of a protein population rather than that of the individual molecules that bear it, which remain free to shuttle back and forth. This implies an open communication between the nucleus and cytoplasm and a ubiquitous mechanism for signaling in both directions.

Nuclear localization signals of the E-cadherin transcriptional repressor Snail

The E-cadherin transcriptional repressor, Snail, plays a critical role in driving the epithelial-mesenchymal transition programs that mark gastrulation as well as invasion of cancer cells. Recent data suggest that Snail is phosphorylated by GSK3-beta, resulting in beta-TRCP-mediated ubiquitination and proteasomal degradation. Accordingly, Wnt signaling inhibits Snail phosphorylation, and consequently increases Snail protein levels. In the present study, we examine the function of nuclear localization motifs embedded within the Snail sequence. A typical bipartite nuclear localization signal (NLS) motif is located at the N-terminal of Snail, where it overlaps with the SNAG domain (residues 8-16), while a basic cluster NLS motif is found proximal to zinc finger domains (residues 151-152). Mutational inactivation of these NLS signals resulted in decreased levels of nuclear and total Snail protein as well as attenuated Snail repressor activity on an E-cadherin promoter construct, suggesting that NLS motifs are essential for proper function. In the presence of GSK3 inhibitor LiCl, the cytoplasmic levels of the NLS mutants increased, suggesting that cytosolic Snail undergoes rapid phosphorylation and degradation. Given the highly conserved nature of the Snail NLS motifs (from Xenopus to human), these results indicate that nuclear localization signals regulate Snail expression and subcellular localization via GSK3-beta-dependent phosphorylation.

(LYS)(16)-based reducible polycations provide stable polyplexes with anionic fusogenic peptides and efficient gene delivery to post mitotic cells

Extracellular stability, endocytic escape, intracellular DNA release and nuclear translocation of DNA are all critical properties of non-viral vector/DNA particles. We have evaluated a (Lys)(16)-based linear, reducible polycation (RPC) in combination with an acid-dependent, anionic fusogenic peptide for gene delivery to dividing and post-mitotic cells. The RPC was formed from Cys(Lys)(16)Cys monomers. Molecular weight was 24,000 Da, corresponding to an average of 10.5 peptide monomers per RPC. Non-reducible polylysine (PLL) (27,000 Da) and monomeric (Lys)(16) peptide were evaluated for comparison. (Lys)(16)/DNA particles were disrupted at fusogenic peptide concentrations well below those used for gene delivery. By contrast, RPC/DNA an PLL/DNA particles were stable in the presence of high concentrations of the anionic peptide. Addition of 10% serum virtually abolished the transfection ability of (Lys)(16)/DNA/fusogenic peptide particles, but had little effect on RPC/DNA/fusogenic peptide particles. RPC/DNA/fusogenic peptide particles were highly effective for gene delivery to both cell lines and post-mitotic corneal endothelium. PLL/DNA/fusogenic peptide particles were moderately effective on cell lines, but gave no gene delivery with corneal endothelial cells. We conclude that (Lys)(16)-based RPC/DNA/fusogenic peptide particles provide a gene delivery system which is potentially stable in the extracellular environment and, on reductive depolymerisation, can release DNA plasmids for nuclear translocation.

Locating proteins in the cell using TargetP, SignalP and related tools

Determining the subcellular localization of a protein is an important first step toward understanding its function. Here, we describe the properties of three well-known N-terminal sequence motifs directing proteins to the secretory pathway, mitochondria and chloroplasts, and sketch a brief history of methods to predict subcellular localization based on these sorting signals and other sequence properties. We then outline how to use a number of internet-accessible tools to arrive at a reliable subcellular localization prediction for eukaryotic and prokaryotic proteins. In particular, we provide detailed step-by-step instructions for the coupled use of the amino-acid sequence-based predictors TargetP, SignalP, ChloroP and TMHMM, which are all hosted at the Center for Biological Sequence Analysis, Technical University of Denmark. In addition, we describe and provide web references to other useful subcellular localization predictors. Finally, we discuss predictive performance measures in general and the performance of TargetP and SignalP in particular.

Delta and Egfr expression are regulated by Importin-7/Moleskin in Drosophila wing development

Drosophila DIM-7 (encoded by the moleskin gene, msk) is the orthologue of vertebrate Importin-7. Both Importin-7 and Msk/DIM-7 function as nuclear import cofactors, and have been implicated in the control of multiple signal transduction pathways, including the direct nuclear import of the activated (phosphorylated) form of MAP kinase. We performed two genetic deficiency screens to identify deficiencies that similarly modified Msk overexpression phenotypes in both eyes and wings. We identified 11 total deficiencies, one of which removes the Delta locus. In this report, we show that Delta loss-of-function alleles dominantly suppress Msk gain-of-function phenotypes in the developing wing. We find that Msk overexpression increases both Delta protein expression and Delta transcription, though Msk expression alone is not sufficient to activate Delta protein function. We also find that Msk overexpression increases Egfr protein levels, and that msk gene function is required for proper Egfr expression in both developing wings and eyes. These results indicate a novel function for Msk in Egfr expression. We discuss the implications of these data with respect to the integration of Egfr and Delta/Notch signaling, specifically through the control of MAP kinase subcellular localization.

Nuclear import of influenza A viral ribonucleoprotein complexes is mediated by two nuclear localization sequences on viral nucleoprotein

Background

The influenza A virus replicates in the nucleus of its host cell. Thus, entry of the influenza genome into the cell nucleus is necessary for establishing infection. The genome of the influenza A virus consists of eight single-stranded, negative-sense RNA molecules, individually packed with several copies of the viral nucleoprotein (NP) into ribonucleoprotein particles (vRNPs). These vRNPs are large, rod-shaped complexes containing a core of NP, around which the RNA is helically wrapped. The vRNPs are the entities that enter the nucleus, and their nuclear import must be mediated by nuclear localization sequences (NLSs) exposed on the vRNPs. NP contains at least two putative NLSs, one at the N-terminus (NLS1) and one in the middle (NLS2) of the protein. These NP NLSs have been shown to mediate the nuclear import of recombinant NP molecules. However, it remains to be determined which NLS mediates the nuclear import of influenza vRNP complexes.

Results

To directly track the nuclear import of the influenza A genome, we developed an experimental assay based on digitonin-permeabilized cells and fluorescently-labeled vRNPs isolated from the influenza A virus. We used this assay to determine the contribution of the two proposed NLSs on NP to the nuclear import of influenza vRNP complexes. Peptides that mimic each of the two NLSs on NP were used to compete with vRNPs for their nuclear import receptors. In addition, antibodies against the two NP NLSs were used to block the NLSs on the vRNP complexes, and thereby inhibit vRNP nuclear import. Both peptide competition and antibody inhibition of either sequence resulted in decreased nuclear accumulation of vRNPs. The two sequences act independently of each other, as inhibition of only one of the two NLSs still resulted in significant, though diminished, nuclear import of vRNPs. Furthermore, when both sequences were blocked, vRNP nuclear import was almost completely inhibited. Antibody inhibition studies further showed that NLS1 on NP is the main contributor to the nuclear import of vRNPs.

Conclusion

Our results demonstrate that both NLS1 and NLS2 on NP can mediate the nuclear uptake of influenza A vRNPs.