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PubMed1999

cORF and RcRE, the Rev/Rex and RRE/RxRE homologues of the human endogenous retrovirus family HTDV/HERV-K.

cORF, a protein encoded by the human endogenous retrovirus family HTDV/HERV-K, contains amino acid motifs which resemble the nuclear import and export signals of the viral regulatory proteins Rev (human immunodeficiency virus) and Rex (human T-cell leukemia virus [HTLV]). In this study, we demonstrated that cORF indeed has a Rev-like function and mapped the cORF-responsive RNA element to a sequence in the 3' long terminal repeat, a localization similar to RxRE, the responsive element in HTLV type 1. Accordingly, we have given the element the designation RcRE. cORF and RcRE stabilize unspliced and incompletely spliced viral transcripts and enhance their nuclear export via the CRM1 export pathway. So far, HTDV/HERV-K is the only endogenous retrovirus family with a complex regulation at the posttranscriptional level. It may be regarded as an intermediate in the evolution from simple to complex retroviruses.

Amino Acid Sequence↗
PubMed2004

Many fingers on the mitotic trigger: post-translational regulation of the Cdc25C phosphatase.

The mitotic inducer Cdc25 phosphatase controls the activation of Cdc2/cyclin B protein kinase and entry into mitosis in eukaryotic cells. Cdc25C is highly regulated by multiple post-translational modifications within its N-terminal regulatory domain and site-specific protein interactions. Phosphorylation of one inhibitory site targeted by multiple kinases determines the timing of Cdc25C activation and arrests cells in G2 in response to checkpoint, stress, developmental and extracellular signals. In mitosis, phosphorylation of several Ser/Thr residues and Pin1-catalysed peptidyl-proline isomerisation produces activation. Phosphorylation of one activating site is antagonistic to the proximal inhibitory site and maintains Cdc25C activity during mitosis. Phosphorylation and interacting proteins also modulate the nuclear import and export signals on Cdc25C, inducing dramatic changes in its localisation within the cell. Thus, the regulation of Cdc25C activity and localization integrates multiple signals that govern the decision to enter mitosis.

Active Transport, Cell Nucleus↗
PubMed2000

Cooperative signals governing ARF-mdm2 interaction and nucleolar localization of the complex.

The ARF tumor suppressor protein stabilizes p53 by antagonizing its negative regulator, Mdm2 (Hdm2 in humans). Both mouse p19(ARF) and human p14(ARF) bind to the central region of Mdm2 (residues 210 to 304), a segment that does not overlap with its N-terminal p53-binding domain, nuclear import or export signals, or C-terminal RING domain required for Mdm2 E3 ubiquitin ligase activity. The N-terminal 37 amino acids of mouse p19(ARF) are necessary and sufficient for binding to Mdm2, localization of Mdm2 to nucleoli, and p53-dependent cell cycle arrest. Although a nucleolar localization signal (NrLS) maps within a different segment (residues 82 to 101) of the human p14(ARF) protein, binding to Mdm2 and nucleolar import of ARF-Mdm2 complexes are both required for cell cycle arrest induced by either the mouse or human ARF proteins. Because many codons of mouse ARF mRNA are not recognized by the most abundant bacterial tRNAs, we synthesized ARF minigenes containing preferred bacterial codons. Using bacterially produced ARF polypeptides and chemically synthesized peptides conjugated to Sepharose, residues 1 to 14 and 26 to 37 of mouse p19(ARF) were found to interact independently and cooperatively with Mdm2, while residues 15 to 25 were dispensable for binding. Paradoxically, residues 26 to 37 of mouse p19(ARF) are also essential for ARF nucleolar localization in the absence of Mdm2. However, the mobilization of the p19(ARF)-Mdm2 complex into nucleoli also requires a cryptic NrLS within the Mdm2 C-terminal RING domain. The Mdm2 NrLS is unmasked upon ARF binding, and its deletion prevents import of the ARF-Mdm2 complex into nucleoli. Collectively, the results suggest that ARF binding to Mdm2 induces a conformational change that facilitates nucleolar import of the ARF-Mdm2 complex and p53-dependent cell cycle arrest. Hence, the ARF-Mdm2 interaction can be viewed as bidirectional, with each protein being capable of regulating the subnuclear localization of the other.

3T3 Cells↗
PubMed1998

Nuclear import and export of viruses and virus genomes.

Many viruses replicate in the nucleus of their animal and plant host cells. Nuclear import, export, and nucleo-cytoplasmic shuttling play a central role in their replication cycle. Although the trafficking of individual virus proteins into and out of the nucleus has been well studied for some virus systems, the nuclear transport of larger entities such as viral genomes and capsids has only recently become a subject of molecular analysis. In this review, the general concepts emerging are discussed and a survey is provided of current information on both plant and animal viruses. Summarizing the main findings in this emerging field, it is evident that most viruses that enter or exit the nucleus take advantage of the cell's nuclear import and export machinery. With a few exceptions, viruses seem to cross the nuclear envelope through the nuclear pore complexes, making use of cellular nuclear import and export signals, receptors, and transport factors. In many cases, they capitalize on subtle control systems such as phosphorylation that regulate traffic of cellular components into and out of the nucleus. The large size of viral capsids and their composition (they contain large RNA and DNA molecules for which there are few precedents in normal nuclear transport) make the processes unique and complicated. Prior capsid disassembly (or deformation) is required before entry of viral genomes and accessory proteins can occur through nuclear pores. Capsids of different virus families display diverse uncoating programs which culminate in genome transfer through the nuclear pores.

Amino Acid Sequence↗
PubMed2006

Topoisomerase II binds importin alpha isoforms and exportin/CRM1 but does not shuttle between the nucleus and cytoplasm in proliferating cells.

Resistance to anticancer drugs that target DNA topoisomerase II (topo II) isoforms alpha and/or beta is associated with decreased nuclear and increased cytoplasmic topo IIalpha. Earlier studies have confirmed that functional nuclear localization and export signal sequences (NLS and NES) are present in both isoforms. In this study, we show that topo II alpha and beta bind and are imported into the nucleus by importin alpha1, alpha3, and alpha5 in conjunction with importin beta. Topo IIalpha also binds exportin/CRM1 in vitro. However, wild-type topo IIalpha has only been observed in the cytoplasm of cells that are entering plateau phase growth. This suggests that topo IIalpha may shuttle between the nucleus and the cytoplasm with the equilibrium towards the nucleus in proliferating cells but towards the cytoplasm in plateau phase cells. The CRM1 inhibitor Leptomycin B increases the nuclear localization of GFP-tagged topo IIalpha with a mutant NLS, suggesting that its export is being inhibited. However, homokaryon shuttling experiments indicate that fluorescence-tagged wild-type topo II alpha and beta proteins do not shuttle in proliferating Cos-1 or HeLa cells. We conclude that topo II alpha and beta nuclear export is inhibited in proliferating cells so that these proteins do not shuttle.

Animals↗
PubMed2001

Dynamics of leptomycin B-sensitive nucleocytoplasmic flux of parathyroid hormone-related protein.

Parathyroid hormone-related protein is responsible for hypercalcemia induced by various tumors. The similarity of its N-terminus to that of parathyroid hormone enables parathyroid hormone-related protein to share parathyroid hormone's signaling properties, but the rest of the molecule possesses distinct functions including a role in the nucleus/nucleolus in reducing apoptosis and enhancing cell proliferation. We have previously shown that parathyroid hormone-related protein nuclear import is mediated by importin beta1. Here we use fluorescence recovery after photobleaching for the first time to show that, in living cells, parathyroid hormone-related protein is exported from the nucleus in a leptomycin B-sensitive manner, implicating CRM1 as the parathyroid hormone-related protein nuclear export receptor. Leptomycin B treatment significantly reduced the rate of nuclear export 4 -10-fold, thereby increasing parathyroid hormone-related protein concentration in the nucleus/nucleolus about 2-fold. Intriguingly, this also led to a 2-fold reduced nuclear import rate. Inhibiting the nuclear export of a protein able to shuttle between nucleus and cytoplasm through distinct receptors thus can also affect nuclear import, indicating that the subcellular localization of a protein containing distinct nuclear import and export signals is the product of an integrated system. Although there have been several recent studies examining the dynamics of intranuclear transport using fluorescence recovery after photobleaching, this represents, to our knowledge, the first use of the technique to examine the kinetics of nucleocytoplasmic flux in living cells.

Animals↗
PubMed2006

Cell-type-specific regulation of degradation of hypoxia-inducible factor 1 alpha: role of subcellular compartmentalization.

The hypoxia-inducible factor-1 alpha (HIF-1 alpha) is a transcription factor that mediates adaptive cellular responses to decreased oxygen availability (hypoxia). At normoxia, HIF-1 alpha is targeted by the von Hippel-Lindau tumor suppressor protein (pVHL) for degradation by the ubiquitin-proteasome pathway. In the present study we have observed distinct cell-type-specific differences in the ability of various tested pVHL-interacting subfragments to stabilize HIF-1 alpha and unmask its function at normoxia. These properties correlated with differences in subcellular compartmentalization and degradation of HIF-1 alpha. We observed that the absence or presence of nuclear localization or export signals differently affected the ability of a minimal HIF-1 alpha peptide spanning residues 559 to 573 of mouse HIF-1 alpha to stabilize endogenous HIFalpha and induce HIF-driven reporter gene activity in two different cell types (primary mouse endothelial and HepG2 hepatoma cells). Degradation of HIF-1 alpha occurred mainly in the cytoplasm of HepG2 cells, whereas it occurs with equal efficiency in nuclear and cytoplasmic compartments of primary endothelial cells. Consistent with these observations, green fluorescent protein-HIF-1 alpha is differently distributed during hypoxia and reoxygenation in hepatoma and endothelial cells. Consequently, we propose that differential compartmentalization of degradation of HIF-1 alpha and the subcellular distribution of HIF-1 alpha may account for cell-type-specific differences in stabilizing HIF-1 alpha protein levels under hypoxic conditions.

Amino Acid Sequence↗
PubMed2003

Dimerization of MLH1 and PMS2 limits nuclear localization of MutLalpha.

DNA mismatch repair maintains genomic stability by detecting and correcting mispaired DNA sequences and by signaling cell death when DNA repair fails. The mechanism by which mismatch repair coordinates DNA damage and repair with cell survival or death is not understood, but it suggests the need for regulation. Since the functions of mismatch repair are initiated in the nucleus, we asked whether nuclear transport of MLH1 and PMS2 is limiting for the nuclear localization of MutLalpha (the MLH1-PMS2 dimer). We found that MLH1 and PMS2 have functional nuclear localization signals (NLS) and nuclear export sequences, yet nuclear import depended on their C-terminal dimerization to form MutLalpha. Our studies are consistent with the idea that dimerization of MLH1 and PMS2 regulates nuclear import by unmasking the NLS. Limited nuclear localization of MutLalpha may thus represent a novel mechanism by which cells fine-tune mismatch repair functions. This mechanism may have implications in the pathogenesis of hereditary non-polyposis colon cancer.

Active Transport, Cell Nucleus↗
PubMed1997

Fragile X mental retardation protein: nucleocytoplasmic shuttling and association with somatodendritic ribosomes.

Fragile X syndrome, a leading cause of inherited mental retardation, is attributable to the unstable expansion of a CGG-repeat within the FMR1 gene that results in the absence of the encoded protein. The fragile X mental retardation protein (FMRP) is a ribosome-associated RNA-binding protein of uncertain function that contains nuclear localization and export signals. We show here detailed cellular localization studies using both biochemical and immunocytochemical approaches. FMRP was highly expressed in neurons but not glia throughout the rat brain, as detected by light microscopy. Although certain structures, such as hippocampus, revealed a strong signal, the regional variation in staining intensity appeared to be related to neuron size and density. In human cell lines and mouse brain, FMRP co-fractionated primarily with polysomes and rough endoplasmic reticulum. Ultrastructural studies in rat brain revealed high levels of FMRP immunoreactivity in neuronal perikarya, where it is concentrated in regions rich in ribosomes, particularly near or between rough endoplasmic reticulum cisternae. Immunogold studies also provided evidence of nucleocytoplasmic shuttling of FMRP, which was localized in neuronal nucleoplasm and within nuclear pores. Moreover, labeling was observed in large- and small-caliber dendrites, in dendritic branch points, at the origins of spine necks, and in spine heads, all known locations of neuronal polysomes. Dendritic localization, which was confirmed by co-fractionation of FMRP with synaptosomal ribosomes, suggests a possible role of FMRP in the translation of proteins involved in dendritic structure or function and relevant for the mental retardation occurring in fragile X syndrome.

Animals↗
PubMed2002

The novel human HUEL (C4orf1) protein shares homology with the DNA-binding domain of the XPA DNA repair protein and displays nuclear translocation in a cell cycle-dependent manner.

We have previously isolated and characterized a novel human gene HUEL (C4orf1) that is ubiquitously expressed in a wide range of human fetal, adult tissues and cancer cell lines. HUEL maps to region 4p12-p13 within the short arm of chromosome 4 whose deletion is frequently associated with bladder and other carcinomas. Here we present the genomic organization, sizes and boundaries of exons and introns of HUEL. The GC-rich upstream genomic region and 5' untranslated region (UTR) together constitute a CpG island, a hallmark of housekeeping genes. The 3250 bp HUEL cDNA incorporates a 1704 bp ORF that translates into a hydrophilic protein of 568-amino acids (aa), detected as a band of approximately 70 kDa by Western blotting. We have isolated the murine homolog of HUEL which exhibits 89% nucleotide and 94% amino acid identity to its human counterpart. The HUEL protein shares significant homology with the minimal DNA-binding domain (DNA-BD) of the DNA repair protein encoded by the xeroderma pigmentosum group A (XPA) gene. Other notable features within HUEL include the putative nuclear receptor interaction motif, nuclear localization and export signals, zinc finger, leucine zipper and acidic domains. Mimosine-mediated cell cycle synchronization of PLC/PRF/5 liver cancer cells clearly portrayed translocation of HUEL into the nucleus specifically during the S phase of the cell cycle. Yeast two-hybrid experiments revealed interactions of HUEL with two partner proteins (designated HIPC and HIPB) bearing similarity to a mitotically phosphorylated protein and to reverse transcriptase. Co-immunoprecipitation assays validated the interaction between HUEL and HIPC proteins in mammalian cells. HUEL is likely to be an evolutionarily conserved, housekeeping gene that plays a role intimately linked with cellular replication, DNA synthesis and/or transcriptional regulation.

Active Transport, Cell Nucleus↗
PubMed2001

Expression of homologues for p53 and p73 in the softshell clam (Mya arenaria), a naturally-occurring model for human cancer.

Homologues for human p53 (Hsp53) and p73 (Hsp73) genes were cloned and expression patterns for their corresponding proteins analysed in tissues from normal and leukemic softshell clams (Mya arenaria). These are the first structural and functional data for p53 and p73 cDNAs and gene products in a naturally occurring, non-mammalian disease model. Core sequence of the predicted clam p53 (Map53) and p73 (Map73) proteins is virtually identical and includes the following highly conserved regions: the transcriptional activation domain (TAD), MDM2 binding site, ATM phosphorylation site, proline rich domain, DNA binding domains (DBDs) II-V, nuclear import and export signals and the tetramerization domain. The core sequence is a structural mosaic of the corresponding human proteins, with the TAD and DBDs resembling Hsp53 and Hsp73, respectively. This suggests that Map53 and Map73 proteins may function similarly to human proteins. Clam proteins have either a short (Map53) or long (Map73) C-terminal extension. These features suggest that Map53 and Map73 may be alternate splice variants of a p63/p73-like ancestral gene. Map73 is significantly upregulated in hemocytes and adductor muscle from leukemic clams. In leukemic hemocytes, both proteins are absent from the nucleus and sequestered in the cytoplasm. This observation suggests that a non-mutational p53/p73-dependent mechanism may be involved in the clam disease. Further studies of these gene products in clams may reveal p53/p73-related molecular mechanisms that are held in common with Burkitt's lymphoma or other human cancers.

Amino Acid Sequence↗
PubMed2000

Fetal Alz-50 clone 1 (FAC1) protein interacts with the Myc-associated zinc finger protein (ZF87/MAZ) and alters its transcriptional activity.

Transcription factors mediate their regulatory effects through interaction with DNA and numerous nuclear proteins. The fetal Alz-50 clone 1 (FAC1) protein, a novel DNA-binding protein with the capacity to repress transcription, is likely to function through a similar mechanism (1). Using the two-hybrid yeast screen, we have shown that FAC1 interacts with the myc-associated zinc finger protein (ZF87/MAZ). This association was confirmed in vitro with recombinant protein. The ZF87/MAZ interaction domain was mapped to the region containing a putative nuclear localization signal (NLS) and nuclear export sequence (NES) of FAC1, using deletion mutants of the FAC1 protein. FAC1, on the other hand, recognizes a conformational interface that includes the proline/alanine-rich domain of ZF87/MAZ and the first zinc finger. Cotransfection of NIH3T3 cells with ZF87/MAZ and a luciferase reporter containing the SV40 promoter and enhancer results in an increase in transcriptional activation, suggesting ZF87/MAZ is able to recognize its consensus binding site present in the SV40 promoter. Cotransfection with FAC1 reduces the level of ZF87/MAZ-induced activation of the SV40 promoter in a dose dependent manner. A mutant FAC1, lacking the ZF87/MAZ interaction domain, does not alter ZF87/MAZ activation of the SV40 promoter. These data demonstrate that interaction between FAC1 and ZF87/MAZ alters the transactivation capacity of ZF87/MAZ. By immunoblot analysis, FAC1 and ZF87/MAZ exhibit similar tissue distribution and co-localize to pathologic structures in Alzheimer's disease brain. Coexpression of FAC1 and ZF87/MAZ suggest that interaction of these two proteins will have biological implications for gene regulation in neurodegeneration.

3T3 Cells↗
PubMed2006

SEL1L expression in non-small cell lung cancer.

SEL1L gene product plays a role in cell transformation and tumor progression in human breast, pancreas, esophageal, and prostate cancer. SEL1L expression was evaluated in a series of 76 surgically resected non-small cell lung carcinomas to investigate its clinical significance. SEL1L is scarcely detectable in normal lung, whereas in the initial stages of cell transformation, it becomes consistently expressed with evident staining in bronchial squamous metaplasia and in associated dysplastic changes. SEL1L immunoreactivity can be detected both in the cytoplasm and less commonly in the nuclei; the subcellular location correlates with tumor histotype, with cytoplasmic immunoreactivity being most prevalent in squamous cell carcinomas (P = .0005) and nuclear immunoreactivity being associated with adenocarcinomas (P = .02). Nuclear import and export signals are present in the SEL1L coding sequence, justifying the different subcellular location of the protein. SEL1L immunoreactivity was inversely correlated with tumor grade (P = .05); when considering only the adenocarcinomas, a stronger association was found (P = .006). SEL1L messenger RNA and protein evaluation in lung cancer cell lines confirmed the expression of the gene and the dual subcellular location of the protein in lung tumors. The data here reported suggest that, in non-small cell lung carcinoma, SEL1L may be an indicator of cell transformation, thus having important biologic and clinical implications.

Adult↗
PubMed2003

Trafficking of viral genomic RNA into and out of the nucleus: influenza, Thogoto and Borna disease viruses.

Most RNA viruses that lack a DNA phase replicate in the cytoplasm. However, several negative-stranded RNA viruses such as influenza, Thogoto, and Borna disease viruses replicate their RNAs in the nucleus, taking advantage of the host cell's nuclear machinery. A challenge faced by these viruses is the trafficking of viral components into and out of the nucleus through the nuclear membrane. The genomic RNAs of these viruses associate with proteins to form large complexes called viral ribonucleoproteins (vRNPs), which exceed the size limit for passive diffusion through the nuclear pore complex (NPC). To insure efficient transport across the nuclear membrane, these viruses use nuclear import and export signals exposed on the vRNPs. These signals recruit the cellular import and export complexes, which are responsible for the translocation of the vRNPs through the NPC. The ability to control the direction of vRNP trafficking throughout the viral life cycle is critical. Various mechanisms, ranging from simple post-translational modification to complex, sequential masking-and-exposure of localization signals, are used to insure the proper movement of the vRNPs.

Active Transport, Cell Nucleus↗
PubMed1997

RNA movement between the nucleus and the cytoplasm.

The past year has seen significant advances in our understanding of the mechanism of RNA movement between the nucleus and the cytoplasm. The emerging view is that proteins bind to and escort RNAs to their proper subcellular location. The discovery of peptide signals that target nuclear export and the identification of novel protein mediators of RNA export are examples of significant recent discoveries.

Amino Acid Sequence↗
PubMed1998

The c-myc coding region determinant-binding protein: a member of a family of KH domain RNA-binding proteins.

The half-life of c- myc mRNA is regulated when cells change their growth rates or differentiate. Two regions within c- myc mRNA determine its short half-life. One is in the 3'-untranslated region, the other is in the coding region. A cytoplasmic protein, the coding region determinant-binding protein (CRD-BP), binds in vitro to the c- myc coding region instability determinant. We have proposed that the CRD-BP, when bound to the mRNA, shields the mRNA from endonucleolytic attack and thereby prolongs the mRNA half-life. Here we report the cloning and further characterization of the mouse CRD-BP, a 577 amino acid protein containing four hnRNP K-homology domains, two RNP domains, an RGG RNA-binding domain and nuclear import and export signals. The CRD-BP is closely related to the chicken beta-actin zipcode-binding protein and is similar to three other proteins, one of which is overexpressed in some human cancers. Recombinant mouse CRD-BP binds specifically to c- myc CRD RNA in vitro and reacts with antibody against human CRD-BP. Most of the CRD-BP in the cell is cytoplasmic and co-sediments with ribosomal subunits.

Amino Acid Sequence↗
PubMed2003

Novel p27(kip1) C-terminal scatter domain mediates Rac-dependent cell migration independent of cell cycle arrest functions.

Hepatocyte growth factor (HGF) signaling via its receptor, the proto-oncogene Met, alters cell proliferation and motility and has been associated with tumor metastasis. HGF treatment of HepG2 human hepatocellular carcinoma cells induces cell migration concomitant with increased levels of the p27(kip1) cyclin-cdk inhibitor. HGF signaling resulted in nuclear export of endogenous p27 to the cytoplasm, via Ser-10 phosphorylation, where it colocalized with F-actin. Introduction of transducible p27 protein (TATp27) was sufficient for actin cytoskeletal rearrangement and migration of HepG2 cells. TATp27 mutational analysis identified a novel p27 C-terminal domain required for cell migration, distinct from the N-terminal cyclin-cyclin-dependent kinase (cdk) binding domain. Loss or disruption of the p27 C-terminal domain abolished both actin rearrangement and cell migration. The cell-scattering activity of p27 occurred independently of its cell cycle arrest functions and required cytoplasmic localization of p27 via Ser-10 phosphorylation. Furthermore, Rac GTPase was necessary for p27-dependent migration but alone was insufficient for HepG2 cell migration. These results predicted a migration defect in p27-deficient cells. Indeed, p27-deficient primary fibroblasts failed to migrate, and reconstitution with TATp27 rescued the motility defect. These observations define a novel role for p27 in cell motility that is independent of its function in cell cycle inhibition.

Actins↗
PubMed1999

Biology of the fragile X mental retardation protein, an RNA-binding protein.

The fragile X syndrome, an X-linked disease, is the most frequent cause of inherited mental retardation. The syndrome results from the absence of expression of the FMR1 gene (fragile mental retardation 1) owing to the expansion of a CGG trinucleotide repeat located in the 5' untranslated region of the gene and the subsequent methylation of its CpG island. The FMR1 gene product (FMRP) is a cytoplasmic protein that contains two KH domains and one RGG box, characteristics of RNA-binding proteins. FMRP is associated with mRNP complexes containing poly(A)+mRNA within actively translating polyribosomes and contains nuclear localization and export signals making it a putative transporter (chaperone) of mRNA from the nucleus to the cytoplasm. FMRP is the archetype of a novel family of cytoplasmic RNA-binding proteins that includes FXR1P and FXR2P. Both of these proteins are very similar in overall structure to FMRP and are also associated with cytoplasmic mRNPs. Members of the FMR family are widely expressed in mouse and human tissues, albeit at various levels, and seem to play a subtle choreography of expression. FMRP is most abundant in neurons and is absent in muscle. FXR1P is strongly expressed in muscle and low levels are detected in neurons. The complex expression patterns of the FMR1 gene family in different cells and tissues suggest that independent, however similar, functions for each of the three FMR-related proteins might be expected in the selection and metabolism of tissue-specific classes of mRNA. The molecular mechanisms altered in cells lacking FMRP still remain to be elucidated as well as the putative role(s) of FXR1P and FXR2P as compensatory molecules.

Animals↗