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Nuclear localization of Schizosaccharomyces pombe Mcm2/Cdc19p requires MCM complex assembly.

The minichromosome maintenance (MCM) proteins MCM2-MCM7 are conserved eukaryotic replication factors that assemble in a heterohexameric complex. In fission yeast, these proteins are nuclear throughout the cell cycle. In studying the mechanism that regulates assembly of the MCM complex, we analyzed the cis and trans elements required for nuclear localization of a single subunit, Mcm2p. Mutation of any single mcm gene leads to redistribution of wild-type MCM subunits to the cytoplasm, and this redistribution depends on an active nuclear export system. We identified the nuclear localization signal sequences of Mcm2p and showed that these are required for nuclear targeting of other MCM subunits. In turn, Mcm2p must associate with other MCM proteins for its proper localization; nuclear localization of MCM proteins thus requires assembly of MCM proteins in a complex. We suggest that coupling complex assembly to nuclear targeting and retention ensures that only intact heterohexameric MCM complexes remain nuclear.

Cell Cycle Proteins↗

Human sex reversal due to impaired nuclear localization of SRY. A clinical correlation.

SRY, an architectural transcription factor encoded by the sex-determining region of the Y chromosome, initiates testicular differentiation in mammalian embryogenesis. The protein contains a high-mobility group (HMG) box, a DNA-bending motif conserved among a broad class of nuclear proteins. Mutations causing human sex reversal (46, XY pure gonadal dysgenesis) are clustered in this domain. Basic N- and C-terminal regions of the HMG box are each proposed to provide nuclear localization signals. The significance of the C-terminal basic cluster (SRY residues 130-134) is uncertain, however, as its activity in cell culture varies with assay conditions. To test its importance, we have investigated a C-terminal sex-reversal mutation (R133W, position 78 of the HMG box). This de novo mutation impairs nuclear localization but not specific DNA binding or sharp DNA bending. Correlation between these properties and the phenotype of the patient suggests that nuclear localization of SRY is required for testicular differentiation and directed in part by the C-terminal basic cluster. To our knowledge, these results provide the first example of impaired organogenesis due to a nuclear localization signal mutation.

Amino Acid Sequence↗

Oncogenic forms of NOTCH1 lacking either the primary binding site for RBP-Jkappa or nuclear localization sequences retain the ability to associate with RBP-Jkappa and activate transcription.

Truncated forms of the NOTCH1 transmembrane receptor engineered to resemble mutant forms of NOTCH1 found in certain cases of human T cell leukemia/lymphoma (T-ALL) efficiently induce T-ALL when expressed in the bone marrow of mice. Unlike full-sized NOTCH1, two such truncated forms of the protein either lacking a major portion of the extracellular domain (DeltaE) or consisting only of the intracellular domain (ICN) were found to activate transcription in cultured cells, presumably through RBP-Jkappa response elements within DNA. Both truncated forms also bound to the transcription factor RBP-Jkappa in extracts prepared from human and murine T-ALL cell lines. Transcriptional activation required the presence of a weak RBP-Jkappa-binding site within the NOTCH1 ankyrin repeat region of the intracellular domain. Unexpectedly, a second, stronger RBP-Jkappa-binding site, which lies within the intracellular domain close to the transmembrane region and significantly augments association with RBP-Jkappa, was not needed for oncogenesis or for transcriptional activation. While ICN appeared primarily in the nucleus, DeltaE localized to cytoplasmic and nuclear membranes, suggesting that intranuclear localization is not essential for oncogenesis or transcriptional activation. In support of this interpretation, mutation of putative nuclear localization sequences decreased nuclear localization and increased transcriptional activation by membrane-bound DeltaE. Transcriptional activation by this mutant form of membrane-bound DeltaE was approximately equivalent to that produced by intranuclear ICN. These data are most consistent with NOTCH1 oncogenesis and transcriptional activation being independent of association with RBP-Jkappa at promoter sites.

Amino Acid Sequence↗

Regulation of heparan sulfate proteoglycan nuclear localization by fibronectin.

Heparan sulfate proteoglycans (HSPG) regulate multiple cellular processes and mediate the cellular uptake of numerous molecules. While heparan sulphate glycosaminoglycan chains are known to modulate receptor binding of several heparin-binding proteins, here we show that distinct extracellular matrices direct HSPG to the nucleus. We analyzed HSPG localization in primary corneal fibroblasts, cultured on fibronectin or collagen type I matrices, using confocal laser scanning microscopy and cell fractionation. Image analysis revealed that the nuclear localization of HSPG core proteins was greater when cells were cultured on fibronectin versus collagen. Matrices containing the heparin-binding domain of fibronectin, but not the integrin-activating domain, demonstrated increased nuclear staining of core proteins. Furthermore, activation of protein kinase C with phorbol 12-myristate 13-acetate inhibited nuclear targeting of HSPG in cells on fibronectin, whereas inhibition of protein kinase C with Ro-31-8220 greatly enhanced nuclear localization of HSPG in cells on both collagen and fibronectin. We propose a matrix-dependent mechanism for nuclear localization of cell surface HSPG involving protein kinase C-mediated signaling. Nuclear localization of HSPG might play important roles in regulating nuclear function.

Animals↗

G protein-coupled receptor kinase 5 contains a DNA-binding nuclear localization sequence.

G protein-coupled receptor kinases (GRKs) mediate desensitization of agonist-occupied G protein-coupled receptors (GPCRs). Here we report that GRK5 contains a DNA-binding nuclear localization sequence (NLS) and that its nuclear localization is regulated by GPCR activation, results that suggest potential nuclear functions for GRK5. As assessed by fluorescence confocal microscopy, transfected and endogenous GRK5 is present in the nuclei of HEp2 cells. Mutation of basic residues in the catalytic domain of GRK5 (between amino acids 388 and 395) results in the nuclear exclusion of the mutant enzyme (GRK5(Delta)(NLS)), demonstrating that GRK5 contains a functional NLS. The nuclear localization of GRK5 is subject to dynamic regulation. Calcium ionophore treatment or activation of Gq-coupled muscarinic-M3 receptors promotes the nuclear export of the kinase in a Ca(2+)/calmodulin (Ca(2+)/CaM)-dependent fashion. Ca(2+)/CaM binding to the N-terminal CaM binding site of GRK5 mediates this effect. Furthermore, GRK5, but not GRK5(Delta)(NLS) or GRK2, binds specifically and directly to DNA in vitro. Consistent with their presence in the nuclei of transfected cells, all the GRK4, but not GRK2, subfamily members contain putative NLSs. These results suggest that the GRK4 subfamily of GRKs may play a signaling role in the nucleus and that GRK4 and GRK2 subfamily members perform divergent cellular functions.

Active Transport, Cell Nucleus↗

Cell cycle-dependent and DNA damage-inducible nuclear localization of FEN-1 nuclease is consistent with its dual functions in DNA replication and repair.

Flap endonuclease-1 (FEN-1), a 43-kDa protein, is a structure-specific and multifunctional nuclease. It plays important roles in RNA primer removal of Okazaki fragments during DNA replication, DNA base excision repair, and maintenance of genome stability. Three functional motifs of the enzyme were proposed to be responsible for its nuclease activities, interaction with proliferating cell nuclear antigen, and nuclear localization. In this study, we demonstrate in HeLa cells that a signal located at the C terminus (the nuclear localization signal (NLS) motif) facilitates nuclear localization of the enzyme during S phase of the cell cycle and in response to DNA damage. Truncation of the NLS motif prevents migration of the protein from the cytoplasm to the nucleus, while having no effect on the nuclease activities and its proliferating cell nuclear antigen interaction capability. Site-directed mutagenesis further revealed that a mutation of the KRK cluster to three alanine residues completely blocked the localization of FEN-1 into the nucleus, whereas mutagenesis of the KKK cluster led to a partial defect of nuclear localization in HeLa cells without observable phenotype in yeast. Therefore, the KRKXXXXXXXXKKK motif may be a bipartite NLS driving the protein into nuclei. Yeast RAD27Delta cells transformed with human mutant M(krk) survived poorly upon methyl methanesulfonate treatment or when they were incubated at an elevated temperature.

Cell Cycle↗

Phosphorylation of dystrophin Dp71d by Ca2+/calmodulin-dependent protein kinase II modulates the Dp71d nuclear localization in PC12 cells.

We have shown that the splicing isoform of Dp71 (Dp71d) localizes to the nucleus of PC12 cells, an established cell line derived from a rat pheochromocytoma; however, the mechanisms governing its nuclear localization are unknown. As protein phosphorylation modulates the nuclear import of proteins, and as Dp71d presents several potential sites for phosphorylation, we analyzed whether Dp71d is phosphorylated in PC12 cells and the role of phosphorylation on its nuclear localization. We demonstrated that Dp71d is phosphorylated under basal conditions at serine and threonine residues by endogenous protein kinases. Dp71d phosphorylation was activated by 2-O-tetradecanoyl phorbol 13-acetate (TPA), but this effect was blocked by EGTA. Supporting the role of intracellular calcium on Dp71d phosphorylation, we observed that the stimulation of calcium influx by cell depolarization increased Dp71d phosphorylation, and that the calcium-calmodulin inhibitor N-(6-aminohexyl)-1-naphthalenesulfonamide (W-7) blocked such induction. The blocking action of bisindolylmaleimide I (Bis I), a specific inhibitor for Ca2+/diacylglicerol-dependent protein kinase (PKC), on Dp71d phosphorylation suggested the participation of PKC in this event. In addition, transfection experiments with Ca2+/calmodulin-dependent protein kinase II (CaMKII) expression vectors as well as the use of KN-62, a CaMKII-specific inhibitor, demonstrated that CaMKII is also involved in Dp71d phosphorylation. Stimulation of Dp71d phosphorylation by cell depolarization and/or the overexpression of CaMKII favored the Dp71d nuclear accumulation. Overall, our results indicate that CAMKII-mediated Dp71d phosphorylation modulates its nuclear localization.

Active Transport, Cell Nucleus↗

Complex nuclear localization signals in the matrix protein of vesicular stomatitis virus.

The matrix (M) protein of vesicular stomatitis virus (VSV) functions from within the nucleus to inhibit bi-directional nucleocytoplasmic transport. Here, we show that M protein can be imported into the nucleus by an active transport mechanism, even though it is small enough (approximately 27 kDa) to diffuse through nuclear pore complexes. We map two distinct nuclear localization signal (NLS)-containing regions of M protein, each of which is capable of directing the nuclear localization of a heterologous protein. One of these regions, comprising amino acids 47-229, is also sufficient to inhibit nucleocytoplasmic transport. Two amino acids that are conserved among the matrix proteins of vesiculoviruses are important for nuclear localization, but are not essential for the inhibitory activity of M protein. Thus, different regions of M protein function for nuclear localization and for inhibitory activity.

Active Transport, Cell Nucleus↗

FBI-1 enhances transcription of the nuclear factor-kappaB (NF-kappaB)-responsive E-selectin gene by nuclear localization of the p65 subunit of NF-kappaB.

The POZ domain is a highly conserved protein-protein interaction motif found in many regulatory proteins. Nuclear factor-kappaB (NF-kappaB) plays a key role in the expression of a variety of genes in response to infection, inflammation, and stressful conditions. We found that the POZ domain of FBI-1 (factor that binds to the inducer of short transcripts of human immunodeficiency virus-1) interacted with the Rel homology domain of the p65 subunit of NF-kappaB in both in vivo and in vitro protein-protein interaction assays. FBI-1 enhanced NF-kappaB-mediated transcription of E-selectin genes in HeLa cells upon phorbol 12-myristate 13-acetate stimulation and overcame gene repression by IkappaB alpha or IkappaB beta. In contrast, the POZ domain of FBI-1, which is a dominant-negative form of FBI-1, repressed NF-kappaB-mediated transcription, and the repression was cooperative with IkappaB alpha or IkappaB beta. In contrast, the POZ domain tagged with a nuclear localization sequence polypeptide of FBI-1 enhanced NF-kappaB-responsive gene transcription, suggesting that the molecular interaction between the POZ domain and the Rel homology domain of p65 and the nuclear localization by the nuclear localization sequence are important in the transcription enhancement mediated by FBI-1. Confocal microscopy showed that FBI-1 increased NF-kappaB movement into the nucleus and increased the stability of NF-kappaB in the nucleus, which enhanced NF-kappaB-mediated transcription of the E-selectin gene. FBI-1 also interacted with IkappaB alpha and IkappaB beta.

Active Transport, Cell Nucleus↗

Identification and characterization of a functional nuclear localization signal in the HIV-1 integrase interactor LEDGF/p75.

Human lens epithelium-derived growth factor (LEDGF)/p75 protein forms a specific nuclear complex with human immunodeficiency virus type 1 (HIV-1) integrase and is essential for nuclear localization and chromosomal association of the viral protein. We now studied nuclear import of LEDGF/p75 in live and semipermeabilized cells. We show that nuclear import of LEDGF/p75 is GTP-, Ran-, importin-alpha/beta-, and energy-dependent and that the protein competes with the canonical SV40 large T antigen nuclear localization signal (NLS) for nuclear import receptors. We identified the NLS of LEDGF/p75 through deletion analysis and site-directed mutagenesis. The LEDGF/p75 NLS, 148GRKRKAEKQ156, belongs to the canonical SV40-like family. Fusion of this short peptide to the amino terminus of Escherichia coli beta-galactosidase rendered the fusion protein nuclear, confirming that the LEDGF/p75 NLS is transferable. Moreover, a single amino acid change in the NLS was sufficient to exclude the mutant LEDGF/p75 protein from the nucleus and abolish nuclear import of HIV-1 integrase.

Alternative Splicing↗

A novel nuclear localization signal in the auxiliary domain of apobec-1 complementation factor regulates nucleocytoplasmic import and shuttling.

C to U editing of the nuclear apolipoprotein B (apoB) transcript is mediated by a core enzyme containing a catalytic deaminase, apobec-1, and an RNA binding subunit, apobec-1 complementation factor (ACF). ACF expression is predominantly nuclear, including mutant proteins with deletions of a putative nuclear localization signal. We have now identified a novel 41-residue motif (ANS) in the auxiliary domain of ACF that functions as an authentic nuclear localization signal. ANS-green fluorescence protein and ANS-beta-galactosidase chimeras were both expressed exclusively in the nucleus, whereas wild-type chimeras or an ACF deletion mutant lacking the ANS were cytoplasmic. Nuclear accumulation of ACF is transcription-dependent, temperature-sensitive, and reversible, features reminiscent of a shuttling protein. ACF relocates to the cytoplasm after actinomycin D treatment, an effect blocked by the CRM1 inhibitor leptomycin B. Heterokaryon assays confirmed directly that ACF shuttles in vivo. ACF binds to the protein carrier, transportin 2 in vivo, and colocalizes to the nucleus as determined by confocal microscopy. Co-immunoprecipitation experiments revealed that transportin 2 binds directly to the ANS motif. These data suggest that directed nuclear localization and compartmentalization of the core complex of the apoB RNA editing enzyme is regulated through a dominant targeting sequence (ANS) contained within ACF.

Active Transport, Cell Nucleus↗

Overlapping but separable determinants of DNA binding and nuclear localization map to the C-terminal end of the Caenorhabditis elegans DAF-12 DNA binding domain.

Proteins are commonly viewed as modular assemblies of functional domains. We analyzed a loss-of-function mutation in the Caenorhabditis elegans intracellular receptor DAF-12, a conservative substitution of an arginine to a lysine at position 197 (R197K). Arg(197) resides in region similar to a nuclear localization signal, just downstream of the receptor minimal zinc finger DNA binding domain (DBD) core. We found that the R197K, but not mutations of neighboring arginine or lysine residues, dramatically reduced DAF-12 transcriptional regulatory activity in a yeast reporter assay. This reduction in regulatory activity correlated with greatly decreased DNA binding affinity in vitro, suggesting a role for the DAF-12 DBD C-terminal region (dbdC), and specifically for Arg(197), in DNA binding. Remarkably, three basic residues immediately contiguous with Arg(197) played little role in DNA binding and rather affected nuclear localization; in contrast, Arg(197) itself was dispensable for nuclear localization. Thus, DAF-12 dbdC harbors overlapping but separable determinants of DNA binding and nuclear localization in a single small region.

Amino Acid Sequence↗

Evidence of nuclear localization of neuronal nitric oxide synthase in cultured astrocytes of rats.

With immunocytochemistry, we have observed the nuclear localization of neuronal nitric oxide synthase (nNOS) in cultured cerebral cortical astrocytes of rats. During the early six days in the subcultures of these cells, nNOS-immunoreactivity was mainly distributed in the cytoplasm. However, nNOS-immunoreactivity was mainly distributed in the nucleus at day 7, and this nuclear localization lasted about ten hours. Meanwhile, inducible nitric oxide synthase expression was significantly inhibited in these cells. Thereafter, nNOS-immunoreactivity was mainly distributed in the cytoplasm again. By confocal microscopy and western blot analysis, the phenomenon of nNOS nuclear localization was further confirmed; and the activity of nNOS in nuclear protein extracts from astrocytes of day 7-subculture could be detected using electron spin resonance (ESR) technique. These results may represent a new pathway of nitric oxide/nNOS participating in inducible nitric oxide synthase gene transcription regulation.

Animals↗

The long amino-terminal tail domain of annexin XI is necessary for its nuclear localization.

Annexin XI is a newly identified annexin which localizes mainly in the nucleus of rat embryonic fibroblasts. There are no typical nuclear localization signals (NLS) in the molecule. To define the region responsible for its nuclear localization, a series of mutants and chimeric cDNA were constructed. These were transiently expressed in COS-7 cells, and the subcellular distributions of the mutants and chimeric proteins were determined by indirect immunofluorescence microscopy. Wild-type annexin XI was located predominantly within the nucleus. Deletion of the N-terminal tail domain (residues 3-196) changed the distribution of the protein from the nucleus to the cytoplasm whereas deletion of the C-terminal core domain (residues 208-504) still kept the protein sorting to the nucleus. Three other mutants lacking 60-80 amino acids in the N-terminal region (residues 3-61, 61-115, and 115-197, respectively) no longer efficiently imported into the nucleus. Furthermore, Escherichia coli beta-galactosidase polypeptide was efficiently localized to the nucleus only when fused with the whole N-terminal region of annexin XI (residues 1-207), not with part of the N-terminal region. In primary cultured rat hepatocytes, annexin XI was distributed in the cytoplasm but not in the nucleus. These results suggest that the whole N-terminal tail domain of annexin XI is necessary and sufficient for its nuclear localization, and may function as NLS in a cell-type specific manner.

Animals↗

Improved nuclear localization of DNA-binding polyamides.

Regulation of endogenous genes by DNA-binding polyamides requires effective nuclear localization. Previous work employing confocal microscopy to study uptake of fluorophore-labeled polyamides has demonstrated the difficulty of predicting a priori the nuclear uptake of a given polyamide. The data suggest that dye identity influences uptake sufficiently such that a dye-conjugate cannot be used as a proxy for unlabeled analogs. Polyamides capable of nuclear localization unaided by fluorescent dyes are desirable due to size and other limitations of fluorophores. Recently, a polyamide-fluorescein conjugate targeted to the hypoxia response element (HRE) was found to inhibit vascular endothelial growth factor (VEGF) expression in cultured HeLa cells. The current study uses inhibition of VEGF expression as a biological read-out for effective nuclear localization of HRE-targeted polyamides. We synthesized a focused library of non-fluorescent, HRE-targeted polyamides in which the C-terminus 'tail' has been systematically varied. Members of this library bind the HRE with affinities comparable or superior to that of the fluorescein-labeled analog. Although most library members demonstrate modest or no biological activity, two non-fluorescent polyamides are reported with activity rivaling that of the previously reported fluorescein-labeled polyamide. We also show evidence that promoter occupancy by HIF-1, the transcription factor that binds the HRE, is inhibited by HRE-targeted polyamides.

Binding Sites↗

A variant of nuclear localization signal of bipartite-type is required for the nuclear translocation of hypoxia inducible factors (1alpha, 2alpha and 3alpha).

Hypoxia inducible factors (HIF1, 2 and 3), consisting of alpha and beta subunits, play an essential role in various responses to hypoxia. Nuclear entry of alpha subunits is a necessary step for the formation of DNA-binding complex with beta subunit, which is constitutively localized in the nucleus. We show here that the nuclear accumulation of HIF2alpha induced by hypoxia is mediated through a novel variant of bipartite-type nuclear localization signal (NLS) in the C-terminus of the protein, which has an unusual length of spacer sequence between two adjacent basic domains. We further show that when the ubiquitin-proteasome system was deficient or inhibited, HIF2alpha accumulated in the nucleus even under normoxia, also mediated through the bipartite NLS. These findings indicate that the protein stability is critical for the nuclear localization of HIF2alpha and hypoxia is not a necessary factor for the process. Importantly, the NLS of HIF2alpha is also conserved in the other HIF family members, HIF1alpha and HIF3alpha. Mutational analyses proved that the NLS mediating the nuclear localization of HIF1alpha is indeed bipartite-, but not monopartite-type as thought before. Our results suggest that the newly identified NLS is crucial for the functional regulation of HIF family.

Amino Acid Sequence↗

Functional activity of the fanconi anemia protein FAA requires FAC binding and nuclear localization.

Fanconi anemia (FA) is an autosomal recessive disease characterized by genomic instability, cancer susceptibility, and cellular hypersensitivity to DNA-cross-linking agents. Eight complementation groups of FA (FA-A through FA-H) have been identified. Two FA genes, corresponding to complementation groups FA-A and FA-C, have been cloned, but the functions of the encoded FAA and FAC proteins remain unknown. We have recently demonstrated that FAA and FAC interact to form a nuclear complex. In this study, we have analyzed a series of mutant forms of the FAA protein with respect to functional activity, FAC binding, and nuclear localization. Mutation or deletion of the amino-terminal nuclear localization signal (NLS) of FAA results in loss of functional activity, loss of FAC binding, and cytoplasmic retention of FAA. Replacement of the NLS sequence with a heterologous NLS sequence, derived from the simian virus 40 T antigen, results in nuclear localization but does not rescue functional activity or FAC binding. Nuclear localization of the FAA protein is therefore necessary but not sufficient for FAA function. Mutant forms of FAA which fail to bind to FAC also fail to promote the nuclear accumulation of FAC. In addition, wild-type FAC promotes the accumulation of wild-type FAA in the nucleus. Our results suggest that FAA and FAC perform a concerted function in the cell nucleus, required for the maintenance of chromosomal stability.

Amino Acid Sequence↗

Nuclear translocation of IFN-gamma is an intrinsic requirement for its biologic activity and can be driven by a heterologous nuclear localization sequence.

We have previously identified a nuclear localization sequence (NLS) in interferon-gamma (IFN-gamma). This NLS functions intracellularly by forming a complex with its transcription factor Stat1alpha and the nuclear importer of Stat1alpha, the importin-alpha analog NPI-1. The stability of this complex and the subsequent nuclear translocation of the complexed Stat1alpha are dependent on the integrity of this NLS, showing that Stat1alpha nuclear import is mediated by the IFN-gamma NLS. In this study, to directly evaluate the intrinsic requirement of nuclear IFN-gamma toward its biologic activities, we engineered a chimeric in which the IFN-gamma NLS has been substituted by a heterologous NLS, namely, the prototypical NLS of the SV40 large T antigen, which would drive nuclear translocation of IFN-gamma in a sequence-nonspecific manner. The chimeric, IFN-gamma-SV, was equally active in antiviral and antiproliferative assays as the wild-type IFN-gamma. Interestingly, IFN-gamma-SV was also translocated to the nucleus and was also recovered intracellularly as a complex with the Stat1alpha importer NPI-1, like wild-type IFN-gamma. Comparison with an NLS deletion mutant showed that deletion or changes within the NLS motif of IFN-gamma were inconsequential to the high-affinity extracellular binding to the IFN-gamma receptor complex, yet the presence of an NLS was critical to the expression of the biologic activities of IFN-gamma and its NPI-1 complexation ability. Our data conclusively demonstrate that nuclear translocation of IFN-gamma is an intrinsic requirement for the full expression of the biologic activities of IFN-gamma and strengthen the conclusion that nuclear chaperoning of Stat1alpha is the primary role of IFN-gamma nuclear translocation. This type of ligand imprinting by sequestering of activated Stat may contribute to the specificity of Stat nuclear transcription.

Active Transport, Cell Nucleus↗