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Nuclear localization of glyceraldehyde-3-phosphate dehydrogenase is not involved in the initiation of apoptosis induced by 1-Methyl-4-phenyl-pyridium iodide (MPP+).

Nuclear localization of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is implicated in the process of apoptosis. To study the function of GAPDH, we expressed GAPDH C-terminally fused with or without nuclear localization signal (NLS) in SH-SY5Y and NB41A3 cells using a retrovirus expression system. GAPDH carrying NLS (GAPDH-NLS) was expressed mainly in the nucleus. However, expression of GAPDH-NLS did not cause any difference in cell survival rate as compared to that of the vector alone or GAPDH without NLS. Treatment with 1-Methyl-4-phenyl-pyridium iodide (MPP+) caused no difference in the cell survival rate or in the pattern or extent of apoptosis among the three transductants. In the cells expressing GAPDH without NLS, MPP+ did not cause visible translocation of GAPDH into nucleus before the onset of apoptosis. Since GAPDH is known to comprise a CRM1-mediated nuclear export signal, we blocked the nuclear export of GAPDH by treatment with leptomycin B, an inhibitor of CRM1-mediated nuclear export. The treatment did not cause any difference in apoptosis among the three transductants. An additional treatment with MPP+ induced no apoptotic difference in these cells. Thus, we have concluded that a simple nuclear localization of GAPDH does not induce apoptosis, and that MPP+-induced apoptosis is not caused by nuclear translocation of GAPDH.

1-Methyl-4-phenylpyridinium↗

Mammalian suppressor-of-fused modulates nuclear-cytoplasmic shuttling of Gli-1.

Sonic hedgehog, Patched and Gli are components of a mammalian signalling pathway that has been conserved during evolution and which has a central role in the control of pattern formation and cellular proliferation during development. Here we identify the human Suppressor-of-Fused (SUFUH) complementary DNA and show that the gene product interacts physically with the transcriptional effector GLI-1, can sequester GLI-1 in the cytoplasm, but can also interact with GLI-1 on DNA. Functionally, SUFUH inhibits transcriptional activation by GLI-1, as well as osteogenic differentiation in response to signalling from Sonic hedgehog. Localization of GLI-1 is influenced by the presence of a nuclear-export signal, and GLI-1 becomes constitutively nuclear when this signal is mutated or nuclear export is inhibited. These results show that SUFUH is a conserved negative regulator of GLI-1 signalling that may affect nuclear-cytoplasmic shuttling of GLI-1 or the activity of GLI-1 in the nucleus and thereby modulate cellular responses.

Adult↗

Novel mechanism of the co-regulation of nuclear transport of SmgGDS and Rac1.

The armadillo protein SmgGDS promotes guanine nucleotide exchange by small GTPases containing a C-terminal polybasic region (PBR), such as Rac1 and RhoA. Because the PBR resembles a nuclear localization signal (NLS) sequence, we investigated the nuclear transport of SmgGDS with Rac1 or RhoA. We show that the Rac1 PBR has significant NLS activity when it is fused to green fluorescent protein (GFP) or in the context of full-length Rac1. In contrast, the RhoA PBR has very poor NLS activity when it is fused to GFP or in the context of full-length RhoA. The nuclear accumulation of both Rac1 and SmgGDS is enhanced by Rac1 activation and diminished by mutation of the Rac1 PBR. Conversely, SmgGDS nuclear accumulation is diminished by interactions with RhoA. An SmgGDS nuclear export signal sequence that we identified promotes SmgGDS nuclear export. These results suggest that SmgGDS. Rac1 complexes accumulate in the nucleus because the Rac1 PBR has NLS activity and because Rac1 supplies the appropriate GTP-dependent signal. In contrast, SmgGDS.RhoA complexes accumulate in the cytoplasm because the RhoA PBR does not have NLS activity. This model may be applicable to other armadillo proteins in addition to SmgGDS, because we demonstrate that activated Rac1 and RhoA also provide stimulatory and inhibitory signals, respectively, for the nuclear accumulation of p120 catenin. These results indicate that small GTPases with a PBR can regulate the nuclear transport of armadillo proteins.

Active Transport, Cell Nucleus↗

ARM domain-dependent nuclear import of adenomatous polyposis coli protein is stimulated by the B56 alpha subunit of protein phosphatase 2A.

Inactivating mutations in the adenomatous polyposis coli (APC) gene correlate with progression of colon cancer and familial adenomatous polyposis. The APC tumor suppressor contributes to chromosome segregation and turnover of the oncogenic transcriptional activator beta-catenin, and these activities are impaired by truncating cancer mutations. APC was recently identified as a shuttling protein whose subcellular distribution is regulated by two nuclear localization signals (NLSs) and multiple nuclear export signals (NESs). Here, we show that mutant disease-linked truncated forms of APC, most of which lack the two central NLSs and certain NES sequences, retain nuclear-cytoplasmic shuttling activity. Nuclear export of truncated APC is mediated by a dominant N-terminal NES. Nuclear import of NLS-deficient APC mutants is facilitated by the N-terminal ARM domain. Furthermore, co-expression of the ARM-binding protein, B56 alpha, increased the nuclear localization of mutant and wild-type APC. The minimal B56 alpha-responsive sequence mapped to APC amino acids 302-625. B56 alpha is a regulatory subunit of protein phosphatase 2A; however, its ability to shift APC to the nucleus was independent of phosphatase activity. We conclude that APC nuclear import is regulated by the ARM domain through its interaction with B56 alpha and postulate that APC/B56 alpha complexes target the dephosphorylation of specific proteins within the nucleus.

Adenomatous Polyposis Coli Protein↗

Transforming growth factor beta-independent shuttling of Smad4 between the cytoplasm and nucleus.

Smad4 plays a pivotal role in all transforming growth factor beta (TGF-beta) signaling pathways. Here we describe six widely expressed alternatively spliced variants of human Smad4 with deletions of different exons in the linker, the region of Smad4 that separates the two well-conserved MH1 and MH2 domains. All these Smad4 variants form complexes with activated Smad2 and Smad3 and are incorporated into DNA-binding complexes with the transcription factor Fast-1, regardless of the amount of linker they contain. However, sequences encoded by exons 5 to 7 in the linker are essential for transcriptional activation. Most importantly, our observation that different Smad4 isoforms have different subcellular localizations has led us to the identification of a functional CRM1-dependent nuclear export signal in the Smad4 linker and a constitutively active nuclear localization signal in the N-terminal MH1 domain. In the absence of TGF-beta signaling, we conclude that Smad4 is rapidly and continuously shuttling between the nucleus and the cytoplasm, the distribution of Smad4 between the nucleus and the cytoplasm being dictated by the relative strengths of the nuclear import and export signals. We demonstrate that inhibition of CRM1-mediated nuclear export by treatment of cells with leptomycin B results in endogenous Smad4 accumulating very rapidly in the nucleus. Endogenous Smad2 and Smad3 are completely unaffected by leptomycin B treatment, indicating that the nucleocytoplasmic shuttling is specific for Smad4. We propose that, upon TGF-beta signaling, complex formation between Smad4 and activated Smad2 or -3 leads to nuclear accumulation of Smad4 through inhibition of its nuclear export. We demonstrate that after prolonged TGF-beta signaling Smad2 becomes dephosphorylated and Smad2 and Smad4 accumulate back in the cytoplasm.

Active Transport, Cell Nucleus↗

Nucleocytoplasmic shuttling of receptor-interacting protein 3 (RIP3): identification of novel nuclear export and import signals in RIP3.

Receptor-interacting protein 3 (RIP3), a member of the RIP Ser/Thr kinase family, has been characterized as a pro-apoptotic protein involved in the tumor necrosis factor receptor-1 signaling pathway. In this study, we have mapped a minimal region of RIP3 sufficient for apoptosis induction to a fragment of 31 amino acids in length. This minimal region also functions as an unconventional nuclear localization signal sufficient to confer the import of full-length RIP3 to the nucleus to trigger apoptosis, suggesting that RIP3 is able to play an apoptosis-inducing role in the nucleus. In addition, we have characterized two novel leucine-rich nuclear export signals (NESs) that are responsible for the nuclear export of RIP3 to the cytoplasm via a chromosome region maintenance 1 (CRM1)-dependent pathway and an extra leucine-rich NES in the N terminus of RIP3 that contributes to the cytoplasmic distribution in a CRM1-independent manner. Thus, we provide the first evidence that RIP3 acts a nucleocytoplasmic shuttling protein, which presents a possible link between death receptor signaling and nuclear apoptosis.

Active Transport, Cell Nucleus↗

Nuclear localization of Tob is important for regulation of its antiproliferative activity.

TOB: is a member of an antiproliferative gene family that includes btg1, pc3/tis21/btg2, pc3b, ana/btg3, and tob2. Exogenous overexpression of the family proteins suppresses cell proliferation. These proteins participate in transcriptional regulation of several genes. Here, we show that Tob is a nuclear protein that is imported into the nucleus through a nuclear localization signal (NLS)-mediated mechanism. Mutation in the NLS sequence of Tob affects its nuclear localization and impairs antiproliferative activity. Additionally, Tob contains a nuclear export signal (NES). In oncogenic ErbB2-transformed cells, nuclear export of Tob is facilitated by NES-mediated mechanism, resulting in decrease of its antiproliferative activity. These results indicate that regulation of nuclear localization of Tob is important for its antiproliferative activity.

Animals↗

Modulation of Alp4 function in Schizosaccharomyces pombe induces novel phenotypes that imply distinct functions for nuclear and cytoplasmic gamma-tubulin complexes.

The gamma-tubulin complex acts as a nucleation unit for microtubule assembly. It remains unknown, however, how spatial and temporal regulation of the complex activity affects microtubule-mediated cellular processes. Alp4 is one of the essential components of the S. pombe gamma-tubulin complex. We show here that overproduction of a carboxy-terminal form of Alp4 (Alp4C) and its derivatives tagged to a nuclear localization signal or to a nuclear export signal affect localization of gamma-tubulin complexes and induces novel phenotypes that reflect distinct functions of nuclear and cytoplasmic gamma-tubulin complexes. Nuclear Alp4C induces a Wee1-dependent G2 delay, reduces the levels of the gamma-tubulin complex at the spindle pole body, and results in defects in mitotic progression including spindle assembly, cytoplasmic microtubule disassembly, and chromosome segregation. In contrast, cytoplasmic Alp4C induces oscillatory nuclear movement and affects levels of cell polarity markers, Bud6 and Tip1, at the cell ends. These results demonstrate that regulation of nuclear gamma-tubulin complex activity is essential for cell cycle progression through the G2/M boundary and M phase, whereas regulation of cytoplasmic gamma-tubulin complex activity is important for nuclear positioning and cell polarity control during interphase.

Cell Cycle Proteins↗

Nuclear import and export signals are essential for proper cellular trafficking and function of ZIC3.

Missense, frameshift and nonsense mutations in the zinc finger transcription factor ZIC3 cause heterotaxy as well as isolated congenital heart disease. Previously, we developed transactivation and subcellular localization assays to test the function of ZIC3 point mutations. Aberrant cytoplasmic localization suggested that the pathogenesis of ZIC3 mutations results, at least in part, from failure of appropriate cellular trafficking. To further investigate this hypothesis, the nucleocytoplasmic shuttling properties of ZIC3 have been examined. Subcellular localization assays designed to span the entire open-reading frame of wild-type and mutant ZIC3 proteins identified the presence of nucleocytoplasmic transport signals. ZIC3 domain mapping indicates that a relatively large region containing the zinc finger binding sites and a known GLI interacting domain is required for transport to the nucleus. Site-directed mutagenesis of critical residues within two putative nuclear localization signals (NLSs) leads to loss of nuclear localization. No further decrease was observed when both NLS sites were mutated, suggesting that mutation of either NLS site is sufficient for loss of importin-mediated nuclear localization. Additionally, we identify a cryptic CRM-1-dependent nuclear export signal (NES) within ZIC3, and identify a mutation within this region in a patient with heterotaxy. These results provide the first evidence that control of cellular trafficking of ZIC3 is critical for function and suggest a possible mechanism for transcriptional control during left-right patterning. Identification of mutations in mapped NLS or NES domains in heterotaxy patients demonstrates the functional importance of these domains in cardiac morphogenesis and allows for integration of structural analysis with developmental function.

Active Transport, Cell Nucleus↗

Analysis of nucleocytoplasmic trafficking of the HuR ligand APRIL and its influence on CD83 expression.

Dendritic cells (DC) are the most potent antigen-presenting cells of the immune system and are able to sensitize even naïve T cells. Mature DC are characterized by expression of CD83, a surface molecule that is proposed to be involved in efficient T cell activation. It has been recently shown that CD83 mRNA is transported from the nucleus to the cytoplasm in a HuR- and CRM1-dependent manner. Therefore we here investigated the impact of two known protein ligands of HuR, pp32 and APRIL, on CD83 expression. Both pp32 (ANP32A) and APRIL (ANP32B) are shuttle proteins, and it has been reported earlier that these HuR ligands can act as adaptors that link HuR and the CRM1-specific nuclear export pathway. By employing RNA interference (RNAi) technology we demonstrate that pp32 is dispensable for CD83 expression, whereas APRIL contributes to the nuclear export and subsequent translation of CD83 mRNA. Furthermore, we have determined the nuclear import signal (NLS) as well as the nuclear export signal (NES) of human APRIL. Moreover, we analyzed the status of phosphorylation of endogenous APRIL and identified threonine 244 to be an as yet unrecognized phosphate acceptor. Finally, we were able to show that phosphorylation of this specific amino acid residue regulates the nuclear export of APRIL. In sum, we report here the signal sequences in APRIL that mediate its intracellular trafficking and provide evidence that this protein ligand of HuR is an important player in the post-transcriptional regulation of CD83 expression by affecting the nucleocytoplasmic translocation of CD83 mRNA.

Active Transport, Cell Nucleus↗

The nuclear accumulation of the Fanconi anemia protein FANCE depends on FANCC.

The Fanconi anemia (FA) protein FANCE is an essential component of the nuclear FA core complex, which is required for monoubiquitination of the downstream target FANCD2, an important step in the FA pathway of DNA cross-link repair. FANCE is predominantly localized in the nucleus and acts as a molecular bridge between the FA core complex and FANCD2, through direct binding of both FANCC and FANCD2. At present, it is poorly understood how the nuclear accumulation of FANCE is regulated and therefore we investigated the nuclear localization of this FA protein. We found that FANCE has a strong tendency to localize in the nucleus, since the addition of a nuclear export signal does not interfere with the nuclear localization of FANCE. We also demonstrate that the nuclear accumulation of FANCE does not rely solely on its nuclear localization signal motifs, but also on FANCC. The other FA proteins are not involved in the nuclear accumulation of FANCE, indicating a tight relationship between FANCC and FANCE, as suggested from their direct interaction. Finally, we show that the region of FANCE interacting with FANCC appears to be different from the region involved in binding FANCD2. This strengthens the idea that FANCE recruits FANCD2 to the core complex, without interfering with the binding of FANCC.

Active Transport, Cell Nucleus↗

Identification and characterization of the nuclear import and export signals of the mammalian Ste20-like protein kinase 3.

Mst3, a human Ste20-like protein kinase, has been recently demonstrated to undergo a caspase-mediated cleavage during apoptosis. The proteolytic cleavage of the C-terminus of Mst3 caused nuclear translocation of its kinase domain. This work provides evidence that Mst3 may contain a bipartite-like nuclear localization sequence (NLS) at the C-terminus of its kinase domain (residues 278-292). The removal of NLS from the kinase domain of Mst3 led to the cytoplasmic accumulation of EGFP-Mst3(Delta277). The presence of nuclear exporting signals in the Mst3 was also demonstrated by leptomycin B-treatment and serial deletion of the C-terminal regulatory domain of Mst3. A nuclear export signal was also postulated to be in the regions of amino acids 335-386. In conclusion, Mst3 contains both NLS and NES signals, which may cooperate to control the subcellular distribution of Mst3.

Active Transport, Cell Nucleus↗

Protein 14-3-3sigma interacts with and favors cytoplasmic subcellular localization of the glucocorticoid receptor, acting as a negative regulator of the glucocorticoid signaling pathway.

The glucocorticoid receptor (GR) alpha interacts with the highly conserved 14-3-3 family proteins. The latter bind phosphorylated serine/threonine residues of "partner" molecules and influence many signal transduction events by altering their subcellular localization and/or protecting them from proteolysis. To examine the physiologic role of 14-3-3 on the glucocorticoid-signaling pathway, we studied the nucleocytoplasmic shuttling and transactivation properties of GRalpha in a cell line replete with or devoid of 14-3-3sigma. We found that endogenous 14-3-3sigma helped localize green fluorescent protein-fused GRalpha in the cytoplasm in the absence of ligand and potentiated its nuclear export after ligand withdrawal. 14-3-3sigma also suppressed the transcriptional activity of GRalpha on a glucocorticoid-responsive promoter. Disruption of the classic nuclear export signal of 14-3-3sigma inactivated its ability to influence the nucleocytoplasmic trafficking and transactivation activity of GRalpha, whereas introduction of a mutation inactivating the binding activity of 14-3-3sigma to some of its partner proteins did not. 14-3-3sigma bound the ligand-binding domain of GRalpha through its COOH-terminal portion, in a partially ligand-dependent fashion, while it did not interact with "ligand-binding domain" of GRbeta at all. These results suggest that 14-3-3sigma functions as a negative regulator in the glucocorticoid signaling pathway, possibly by shifting the subcellular localization/circulation of this receptor toward the cytoplasm through its nuclear export signal. Since 14-3-3 proteins play significant roles in numerous cellular activities, such as cell cycle progression, growth, differentiation, and apoptosis, these actions might indirectly influence the transcriptional activity of GRalpha. Conversely, through its 14-3-3 protein interactions, GRalpha may influence these processes.

14-3-3 Proteins↗

A threshold nuclear level of the v-Rel oncoprotein is required for transformation of avian lymphocytes.

The net distribution of eukaryotic transcription factors between the cytoplasm and the nucleus provides an effective mechanism for controlling gene expression. We have utilized cis-acting signals for both nuclear import and nuclear export to experimentally manipulate the distribution of the v-Rel oncoprotein between the nucleus and the cytoplasm. The respective abilities of the v-Rel oncoprotein to localize to the nucleus in chicken embryo fibroblasts, to activate kappaB-dependent transcription in yeast, and to transform avian lymphoid cells were each markedly reduced by the fusion of a cis-acting nuclear export signal onto v-Rel. Our results demonstrate that a threshold nuclear function of v-Rel is required for manifestation of its oncogenic properties. In contrast, while increased expression of the avian IkappaB-alpha protein was able to prevent nuclear localization of v-Rel in chicken embryo fibroblasts, coexpression of IkappaB-alpha with v-Rel in the target cell for v-Rel mediated transformation did not reduce the ability of v-Rel to transform avian lymphoid cells or alter the distribution of v-Rel between the nucleus and the cytoplasm in v-Rel-transformed cells. Our results suggest that the ability of IkappaB-alpha to inhibit nuclear localization of v-Rel is affected by cell-type specific differences between fibroblasts and lymphoid cells.

Animals↗

Optical recording of signal-mediated protein transport through single nuclear pore complexes.

Optical single-transporter recording, a recently established fluorescence microscopic method, was used to study the selective transport of proteins through single nuclear pore complexes (NPCs) of Xenopus oocytes. Recombinant proteins containing either a nuclear localization signal (import protein) or a nuclear export signal (export protein) were generated as transport substrates. To approximate in vivo conditions as closely as possible, a Xenopus egg extract was applied to the cytosolic side and a Xenopus oocyte nuclear extract to the nuclear side of the NPCs. It was found that protein transport through functionally isolated, "patched" NPCs depended on signal sequences, extracts, and metabolic energy, as in vivo. All NPCs were competent for both import and export. The transport direction was strictly determined by the transport signal, and at none of the conditions explored was the import protein exported or the export protein imported, even when the application sides of the extracts were reversed. The mean transport rates of the single NPC were approximately 2 dimers/s for the import protein and approximately 4 dimers/s for the export protein ( approximately 15 microM substrate concentration, 22-24 degrees C), in good agreement with in vivo rates estimated for mammalian cells by microinjection experiments. The study shows that optical single-transporter recording permits the analysis of membrane transport processes not previously accessible to single-transporter recording and thus provides additional possibilities for the elucidation of nucleocytoplasmic transport mechanisms.

Animals↗

Leptomycin B-sensitive nuclear export of MAPKAP kinase 2 is regulated by phosphorylation.

UNLABELLED: To study the intracellular localization of MAPKAP kinase 2 (MK2), which carries a putative bipartite nuclear localization signal (NLS), we constructed a green fluorescent protein-MAPKAP kinase 2 fusion protein (GFP-MK2). In transfected cells, this protein is located predominantly in the nucleus; unexpectedly, upon stress, it rapidly translocates to the cytoplasm. This translocation can be blocked by the p38 MAP kinase inhibitor SB203580, indicating its regulation by phosphorylation. Molecular mimicry of MK2 phosphorylation at T317 in GFP-MK2 led to a mutant which is located almost exclusively in the cytoplasm of the cell, whereas the mutant T317A shows no stress-induced redistribution. Since leptomycin B, which inhibits the interaction of exportin 1 with the Rev-type leucine-rich nuclear export signal (NES), blocks stress-dependent translocation of GFP-MK2, it is supposed that phosphorylation-induced export of the protein causes the translocation. We have identified the region responsible for nuclear export in MK2 which is partially overlapping with and C-terminal to the autoinhibitory motif. This region contains a cluster of hydrophobic amino acids in the characteristic spacing of a leucine-rich Rev-type NES which is necessary to direct GFP-MK2 to the cytoplasm. However, unlike the Rev-type NES, this region alone is not sufficient for nuclear export. The data obtained indicate that MK2 contains a constitutively active NLS and a stress-regulated signal for nuclear export. KEYWORDS: nuclear export/nuclear import/protein phosphorylation/signal transduction/stress response

3T3 Cells↗

Nuclear export of signal recognition particle RNA in mammalian cells.

In mammalian cells the signal recognition particle (SRP) consists of a approximately 300 nucleotide RNA and six proteins. Although the molecular structure and functional cycle of the SRP are both very well understood, far less is known about how the SRP is first assembled in the cell. Recent work has suggested that SRP assembly begins in the nucleoli. When NRK (rat fibroblast) cells were treated with leptomycin B (LMB), a specific inhibitor of the CRM1 nuclear export receptor, the level of SRP RNA increased in the nucleoli, as did the level of nucleolar 28S ribosomal RNA. Moreover, when a hamster cell line carrying a temperature-sensitive mutation in the guanine nucleotide exchange factor of the GTPase Ran (Ran-GEF) was shifted to the non-permissive temperature, the nucleolar level of SRP RNA increased. These results indicate that the steady-state concentration of SRP RNA in the nucleolus is sensitive to perturbations in nuclear import/export pathways.

Active Transport, Cell Nucleus↗

Nuclear export of the transcription factor NirA is a regulatory checkpoint for nitrate induction in Aspergillus nidulans.

NirA, the specific transcription factor of the nitrate assimilation pathway of Aspergillus nidulans, accumulates in the nucleus upon induction by nitrate. NirA interacts with the nuclear export factor KapK, which bridges an interaction with a protein of the nucleoporin-like family (NplA). Nitrate induction disrupts the NirA-KapK interaction in vivo, whereas KapK associates with NirA when this protein is exported from the nucleus. A KpaK leptomycin-sensitive mutation leads to inducer-independent NirA nuclear accumulation in the presence of the drug. However, this does not lead to constitutive expression of the genes controlled by NirA. A nirA(c)1 mutation leads to constitutive nuclear localization and activity, remodeling of chromatin, and in vivo binding to a NirA upstream activation sequence. The nirA(c)1 mutation maps in the nuclear export signal (NES) of the NirA protein. The NirA-KapK interaction is nearly abolished in NirA(c)1 and NirA proteins mutated in canonical leucine residues in the NirA NES. The latter do not result in constitutively active NirA protein, which implies that nuclear retention is necessary but not sufficient for NirA activity. The results are consistent with a model in which activation of NirA by nitrate disrupts the interaction of NirA with the NplA/KapK nuclear export complex, thus resulting in nuclear retention, leading to AreA-facilitated DNA binding of the NirA protein and subsequent chromatin remodeling and transcriptional activation.

Active Transport, Cell Nucleus↗