[Show abstract][Hide abstract] ABSTRACT: Sequencing of the human genome has led to most genes being available in BAC or PAC vectors. However, limited functional information has been assigned to most of these genes. Techniques for the manipulation and transfer of complete functional units on large DNA fragments into human cells are crucial for the analysis of complete genes in their natural genomic context. One limitation of the functional studies using these vectors is the low transfection frequency.
We have constructed a shuttle vector, pPAC7, which contains both the EBNA-1 gene and oriP from the Epstein-Barr virus allowing stable maintenance of PAC clones in the nucleus of human cells. The pPAC7 vector also contains the EGFP reporter gene, which allows direct monitoring of the presence of PAC constructs in transfected cells, and the Bsr-cassette that allows highly efficient and rapid selection in mammalian cells by use of blasticidin. Positive selection for recombinant PAC clones is obtained in pPAC7 because the cloning sites are located within the SacBII gene. We show regulated expression of the CDH3 gene carried as a 132 kb genomic insert cloned into pPAC7, demonstrating that the pPAC7 vector can be used for functional studies of genes in their natural genomic context. Furthermore, the results from the transfection of a range of pPAC7 based constructs into two human cell lines suggest that the transfection efficiencies are not only dependent on construct size.
The shuttle vector pPAC7 can be used to transfer large genomic constructs into human cells. The genes transferred could potentially contain all long-range regulatory elements, including their endogenous regulatory promoters. Introduction of complete genes in PACs into human cells would potentially allow complementation assays to identify or verify the function of genes affecting cellular phenotypes.
Full-text · Article · Oct 2009 · BMC Biotechnology
[Show abstract][Hide abstract] ABSTRACT: 1The amino acid composition of purified Bacillus cereus phospholipase C is reported. The enzyme contains one methionine residue and two fragments are obtained after cyanogen bromide cleavage. The sequence of the amino-terminal fragment (25 residues) is reported.2Antisera were raised against the enzyme and purified by affinity chromatography. The antisera were monospecific and gave one precipitation line with purified as well as with crude phospholipase C, showing that no antigenic contaminants were present in the purified preparations used as antigen. The antibodies were purified to the extent that about two molecules neutralized one enzyme molecule.3The enzyme is quite resistant to denaturation by urea, sodium dodecyl sulphate or heat (in the presence of 1 mM Zn2+).4Phospholipase C hydrolyses phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine. Under the conditions used phosphatidylglycerol, cardiolipin, phosphatidylinositol, sphingomyelin, lysophosphatidylcholine and lysophosphatidyl ethanolamine were not substrates. Replacement of Zn2+ by Co2+ or Ni2+ or variation of pH (7.2–8.3), did not change the range of substrates. Phosphatidylcholine was the best substrate among the isolated phospholipids and dicaproylphosphatidylcholine was clearly a better substrate than dipalmitoylphosphatidylcholine.
[Show abstract][Hide abstract] ABSTRACT: RNA interference (RNAi) is a powerful genetic tool for loss-of-function studies in mammalian cells and is also considered a potentially powerful therapeutic modality for the treatment of a variety of human diseases. During the past 3 years a number of systems for conditional RNAi have been developed that allow controlled expression of short hairpin RNA (shRNA) triggers of RNAi. The simplest strategy relies on tet-operable polymerase III-promoted shRNAs and co-expression of the tetracycline regulatory protein, TetR. In this study we have combined these features into a single lentiviral vector that upon delivery to target cells allows robust induction of shRNAs, even with low levels of doxycycline; importantly, we show minimal leakiness in the absence of inducer. We have exploited the regulatory properties of our system by targeting an essential cellular gene, the nuclear RNaseIII endonuclease Drosha. Drosha is the core catalytic component of the "microprocessor complex" and cleaves the primary microRNA (miRNA) transcripts into their pre-miRNA hairpin intermediates. We anticipate that our vector will facilitate functional studies of miRNA biogenesis.
Full-text · Article · Jun 2007 · Molecular Therapy
[Show abstract][Hide abstract] ABSTRACT: The coagulation trigger tissue factor has been implicated in tumor growth, angiogenesis, and metastasis. In this study, we explore the effects of ex vivo and in vivo delivery of short interfering RNA (siRNA) targeting tissue factor on B16 melanoma colonization of the lung in a murine model for metastasis. The purposes of this work are to establish a noncytotoxic in vivo model for investigation of tissue factor function and provide preclinical assessment of the therapeutic potential of tissue factor siRNA for prevention of metastasis.
C57BL/6 mice were evaluated for pulmonary metastases following tail vein injection of B16 cells transfected with either active or inactive siRNA. Mice receiving cells transfected with active siRNA had significantly lower numbers of pulmonary tumors compared with mice injected with control cells (transfected with inactive siRNA). The average time point at which the mice started to exhibit tumor-associated stress was also increased significantly from 22 days for the control group to 27 days for the experimental group (P = 0.01). In a therapeutically more relevant model, where the siRNA was delivered i.p. and the cells (untransfected) by tail vein injection, an inhibitory effect on metastasis was observed when the siRNA treatment was initiated either before or at the time of cell injection.
The results suggest that tissue factor has a crucial function in promoting lung tumor metastasis of blood-borne tumor cells in the early stages of the tumor take process and further suggest that treatment with tissue factor siRNA may become a viable clinical strategy for prevention of tumor metastasis.
Full-text · Article · Aug 2006 · Clinical Cancer Research
[Show abstract][Hide abstract] ABSTRACT: Tissue factor (TF) is a transmembrane glycoprotein that binds its zymogen cofactor, Factor VIIa (FVIIa) on the cell surface. Together (TF/FVIIa) they activate Factor X (FX) and Factor IX (FIX) and start the extrinsic pathway of blood coagulation. As such, the TF/FVIIa complex plays an important role in normal physiology as well as in thrombotic diseases such as unstable angina (UA), disseminated intravascular coagulation (DIC), and deep vein thrombosis (DVT). In addition to its function as an initiator of coagulation, TF/FVIIa plays an important role in inflammation. Expression of TF on the cell surface and its appearance as a soluble molecule are characteristic features of acute and chronic inflammation in conditions such as sepsis and atherosclerosis. Here we demonstrate that BCX-3607, a small molecule potent inhibitor of TF/FVIIa, reduces thrombus weight in an animal model of DVT. BCX-3607 also decreases the level of interleukin-6 (IL-6) in a LPS-stimulated mouse model of endotoxemia. Additionally, in vitro studies indicate that BCX-3607 blocks the generation of TF/FVIIa-induced IL-8 mRNA in human keratinocytes and reduces the TF/FVIIa-mediated generation of IL-6 and IL-8 in human umbilical vein endothelial cells (HUVEC). Therefore, BCX-3607 might block the TF/FVIIa-mediated coagulation and inflammation associated with pathological conditions.
No preview · Article · Feb 2006 · Thrombosis Research
[Show abstract][Hide abstract] ABSTRACT: The present study investigated the hepatic regulation of fatty acid metabolism in hTNFalpha transgenic mice. Reduced hepatic mRNA levels and activities of carnitine palmitoyltransferase-II (CPT-II) and mitochondrial HMG-CoA synthase were observed, accompanied by decreased fatty acid oxidation, fatty acyl-CoA oxidase and fatty acid synthase (FAS) activities and down-regulated gene expression of mitochondrial acetyl-CoA carboxylase 2 (ACC2). The mRNA levels of peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARdelta were reduced. The hepatic fatty acid composition was altered, with increased amounts of saturated and polyunsaturated fatty acids. The relative amounts of Delta(9) desaturated fatty acids were decreased, as was Delta(9)desaturase mRNA. The CPT-I mRNA level remained unchanged. The PPARalpha targeted genes CPT-II and HMG-CoA synthase are potential regulators of mitochondrial fatty acid oxidation and ketogenesis in hTNFalpha transgenic mice, and the increased propionyl-CoA level found is a possible inhibitor of these processes. Reduced mitochondrial and peroxisomal fatty acid oxidation may explain the increased hepatic triglyceride level induced by TNFalpha. This is not due to de novo fatty acid synthesis as both FAS activity and gene expression of ACC2 were reduced.
No preview · Article · Jul 2005 · Biochimica et Biophysica Acta
[Show abstract][Hide abstract] ABSTRACT: Tissue factor (TF) is the membrane receptor of the serine protease coagulation factor VIIa (FVIIa). Formation of the TF/FVIIa complex initiates the coagulation cascade. We used short hairpin RNA (shRNA)-mediated RNA interference to knock down TF expression in the human metastatic melanoma cell line LOX-L. After transfection with the shRNA construct, 3 stable clones with significantly downregulated TF expression were established. They exhibited decreased proliferation in vitro as determined by (14)C thymidine incorporation and soft agar assay. The in vivo metastatic potential was assessed in an experimental pulmonary metastasis model in which cells from different clones were injected into the tail vein of nude mice. The incidence of pulmonary tumors was significantly lower in mice receiving shRNA-expressing cells (33% +/- 15%) than in control mice injected with wild-type cells or cells stably transfected with empty expression vector (90% +/- 10%). The mice injected with TF-downregulated cells had markedly longer survival time (69 +/- 17 days) compared to the control mice (35.6 +/- 5 days; p = 0.03). Thus, reduction of TF levels in LOX-L cells significantly delayed and reduced lung tumor formation. As a first step in elucidating the molecular basis for this effect, we compared the global gene expression profile in TF-downregulated cells and control cells by using cDNA microarray analysis. Forty-four known human genes were found to be significantly upregulated (> 2-fold; p < 0.05) and 228 genes significantly downregulated (>or= 3-fold; p < 0.05) in TF-downregulated cells compared to control cells. The differentially expressed genes encode proteins functioning in transcription, translation, cell communication and cell growth/death. The results provide a basis for investigating molecular mechanisms underlying the effects of TF on the metastatic capacity of LOX-L melanoma cells.
Full-text · Article · Dec 2004 · International Journal of Cancer
[Show abstract][Hide abstract] ABSTRACT: We recently cloned a putative DEAD-box helicase MDDX28 and found that it was localized to the nuclei and mitochondria of COS-1 cells. The mitochondrial localization of MDDX28 is largely diffuse. We have, however, used immunofluorescence and immunogold cytochemistry to show that the MDDX28 protein is localized in a distinct mitochondrial subcompartment in 5-10% of COS-1 cells. This proportion increases to approximately 35% after treatment with ethidium bromide, suggesting upregulation following transcription inhibition. To our knowledge, this is the first example of protein relocation in the mitochondria caused by transcription inhibition. The mitochondrial subcompartmentation of MDDX28 was negatively affected by mutations in a RNA-binding domain and three basic domains previously shown to be important in transcription-dependent intranuclear localization. Furthermore, immunogold cytochemistry and fractionation of rat liver indicated that the protein is a part of an RNA-protein (RNP) complex interacting peripherally with the mitochondrial inner membrane. Our results reveal new principles for regulation of protein localization in the mitochondria and suggest parallels between the function of the MDDX28 protein in the nucleus and mitochondria.
No preview · Article · Nov 2004 · Experimental Cell Research
[Show abstract][Hide abstract] ABSTRACT: C-terminal Src kinase (Csk) controls the Src family kinase Lck, which is essential for T cell antigen receptor (TCR)-mediated signaling. For the first time, we here report the effects of acute elimination of Csk in Jurkat T cells and primary T cells using short interfering (si) RNA. In both cell types, 70-85% knockdown of Csk was achieved within 48 h. No alterations in surface expression of CD3, CD4 or CD8, or in Lck protein level were observed. Phosphorylation of Y505 in Lck was markedly reduced and a concomitant 4-5-fold increase in Lck Y394 phosphorylation was observed both in normal and Jurkat T cells. Kinase assays revealed 2-3-fold higher Lck activity. In Jurkat cells, basal levels of zeta chain phosphorylation were elevated, and spontaneous NFAT-AP-1 activation occurred, indicating aberrant Lck kinase activity. After TCR triggering, Csk knockdown cells revealed faster and stronger, but not sustained, phosphorylation of Lck Y394 and zeta chains compared to control. TCR-induced activation of NFAT-AP-1 and TCR/CD28-stimulated IL-2 secretion occurred at weaker stimuli and with augmented responses in Csk knockdown Jurkat and primary T cells, respectively. Altogether, these data suggest that acute elimination of Csk in T cells without evolution of compensatory mechanisms results in aberrant Lck activity and augmented TCR-stimulated responses.
Full-text · Article · Aug 2004 · European Journal of Immunology
[Show abstract][Hide abstract] ABSTRACT: Binding of the coagulation protease factor VIIa to its receptor Tissue Factor (TF) induces intracellular signals in several cell types including HaCaT keratinocytes. TF belongs to the cytokine receptor family, but is most likely not alone in transferring the complete TF/FVIIa signal over the plasma membrane. The protease activated receptor PAR2 is involved in factor VIIa and factor Xa signal transduction. Our results indicate that the epidermal growth factor receptor (EGFR) and the proline rich tyrosine kinase 2 (PYK2) participate in TF/FVIIa signalling as formation of the TF/FVIIa complex increased the phosphorylation of these proteins. Both FVIIa protease activity and available TF were necessary for generation of the signal. Increased tyrosine phosphorylation of the EGFR was observed following TF/FVIIa complex formation on the cell surface. The EGFR kinase inhibitor tyrphostin AG1478 abrogated the TF/FVIIa-complex induced MAP kinase activation and mRNA increase of egr-1, heparin-binding EGF, and interleukin-8 following FVIIa addition. Using specific antibodies, increased phosphorylation of PYK2 tyrosine residues 402 and 580 was observed. The first site is the major autophosphorylation site and the docking site for Src family kinases. The second site is important for the kinase activity. The Src family kinase Yes and the tyrosine phosphatase SHP-2 were detected in immunoprecipitates using either anti-PYK2 or anti-EGFR antibodies. Their coprecipitation with EGFR increased in the presence of FVIIa. Moreover, the coprecipitation of EGFR and PYK2 increased with FVIIa stimulation. Together, these data suggest that EGFR, PYK2, Yes, and SHP-2 are involved in transduction of the TF/FVIIa signal possibly via transactivation of the EGF receptor.
No preview · Article · Aug 2004 · Thrombosis and Haemostasis
[Show abstract][Hide abstract] ABSTRACT: Randomly designed siRNA targeting different positions within the same mRNA display widely differing activities. We have performed a statistical analysis of 46 siRNA, identifying various features of the 19bp duplex that correlate significantly with functionality at the 70% knockdown level and verified these results against an independent data set of 34 siRNA recently reported by others. Features that consistently correlated positively with functionality across the two data sets included an asymmetry in the stability of the duplex ends (measured as the A/U differential of the three terminal basepairs at either end of the duplex) and the motifs S1, A6, and W19. The presence of the motifs U1 or G19 was associated with lack of functionality. A selection algorithm based on these findings strongly differentiated between the two functional groups of siRNA in both data sets and proved highly effective when used to design siRNA targeting new endogenous human genes.
Preview · Article · May 2004 · Biochemical and Biophysical Research Communications
[Show abstract][Hide abstract] ABSTRACT: A novel mutation in the factor VII gene resulting in procoagulant activity of 7.5% and antigen levels of 23% is presented. Single-stranded conformational polymorphism and DNA sequencing analysis revealed heterozygous shifts, and mutations were detected in exons 5, 7 and 8. The mutant L204P in exon 7 was novel, while the common polymorphisms, H115H and R353Q, were located in exons 5 and 8, respectively. The molecular effect of the L204P mutation was characterized using recombinant mammalian expression in Chinese hamster ovary cells. Low levels (4 ng/ml) of secreted mutant protein were found in transiently transfected cells compared to wild-type factor VII (83 ng/ml). Metabolic labeling demonstrated that the rate of mutant protein synthesis was similar to that of wild-type FVII, and the mutant protein accumulated intracellularly with no signs of increased degradation during a four-hour chase. No interaction between secreted P204 protein and immobilized soluble tissue factor was detected using surface plasmon reso-nance. The activation rate of recombinant mutant FVII protein was strongly reduced compared to wild-type FVII. A 9-fold reduction in the rate of FX activation was detected whereas Km was nearly the same for wild-type and the mutant. This slow rate was caused by a correspondingly lowered rate of P204 activation. A synthetic peptide sequence comprising amino acids 177-206 blocked binding of FVIIa to the TF-chip, and the subsequent factor X activation with an IC(50) value of 0.5 micro M in a chromogenic factor Xa assay. Additionally, evaluation of the peptide by surface plasmon resonance analysis resulted in inhibition of complex formation with an apparent K(I) of 7 micro M.
No preview · Article · Feb 2004 · Thrombosis and Haemostasis
[Show abstract][Hide abstract] ABSTRACT: Tumour necrosis factor (TNF) is involved in the pathogenesis of several diseases. In mice, human TNF signals only through p55, one of two murine TNF receptors. We here report a study of growth, viability and morphological alterations in transgenic mice expressing a low constitutive and tissue-restricted level of human TNF in vivo. The transgene was expressed solely in T cells. The transgenic mice showed a marked failure to thrive and a rapid cellular depletion in spleen and thymus. Slight fibrosis was seen in most tissues investigated, in addition to immature adipose tissue and irregular lymphocytic areas. Serum levels of hTNF were only slightly increased in the transgenic mice, enough, however, to cause an inflammatory reaction. All the symptoms were abrogated by an inhibitory hTNF antibody, demonstrating the essential role of hTNF in this phenotype. Transgenic mice constitute a multidimensional system allowing observation of disease processes over time in all tissues. The effects of hTNF were seen first and foremost in the lymphoid organs of the transgenic mice, verifying their cells as major targets at low levels of hTNF expression in the T-cell compartments. Chronic, low levels of TNF expression cause profound disturbances in lymphoid tissue development resulting in cachexia and premature death.
[Show abstract][Hide abstract] ABSTRACT: Elevated levels of coagulation factor VII (FVII) have been associated with increased risk for myocardial infarction (MI). The R353Q polymorphism of the FVII gene has been shown to modify plasma levels of FVII, and has in some studies also been associated with reduced risk for MI.
To examine the R353Q polymorphism of the FVII gene and the relation to myocardial infarction (MI), cardiovascular disease (CVD), and diabetes, and furthermore, to elucidate the association between the polymorphism and plasma levels of FVII coagulant activity (FVIIc), FVII antigen (FVIIag), activated FVII (FVIIa), and serum choline-containing phospholipids (PC).
In 560 elderly men characterised as hypercholesterolemic in 1972, we examined the R353Q polymorphism by melting curve analysis after real-time PCR. In a subgroup of 205 individuals, FVIIc, FVIIag, FVIIa, and PC were analysed.
There were no significant associations between genotype and the disease states, although we observed a lower number of MI cases among subjects with the Q allele, compared to the RR individuals (14% vs. 19%). FVIIag and FVIIc levels were lower in RQ compared to RR subjects, whereas for FVIIa the opposite was observed (p<0.001 for all). PC correlated positively with FVIIag (r=0.24, p<0.001), but negatively with FVIIa (r=-0.25, p<0.001). No genotype specific interactions were found for the association between FVII and PC.
No significant associations between the R353Q polymorphism and MI, CVD, or diabetes were observed, although the polymorphism strongly influenced plasma levels of FVII. Serum PC correlated significantly with FVIIag and inversely with FVIIa, independently of genotype.
No preview · Article · Jan 2004 · Thrombosis Research
[Show abstract][Hide abstract] ABSTRACT: We have dissected the molecular determinants involved in targeting the protein serine kinase PSKH1 to the endoplasmic reticulum (ER), the Golgi apparatus, and the plasma membrane (PM). Given this intracellular localization pattern, a potential role of PSKH1 in the secretory pathway was explored. The amino-terminal of PSKH1 revealed a striking similarity to the often acylated Src homology domain 4 (SH4)-harboring nonreceptor tyrosine kinases. Biochemical studies demonstrated that PSKH1 is myristoylated on glycine 2 and palmitoylated on cysteine 3. Dual amino-terminal acylation targets PSKH1 to Golgi as shown by colocalization with beta-COP and GM130, while nonpalmitoylated (myristoylated only) PSKH1 targets intracellular membranes colocalizing with protein disulphide isomerase (PDI, a marker for ER). Immunoelectron microscopy revealed that the dually acylated amino-terminal domain (in fusion with EGFP) was targeted to Golgi membranes as well as to the plasma membrane (PM), suggesting that the amino-terminal domain provides PSKH1 with membrane specificity dependent on its fatty acylation status. Subcellular fractionation by sucrose gradient analysis confirmed the impact of dual fatty acylation on endomembrane targeting, while cytosol and membrane fractioning revealed that myristoylation but not palmitoylation was required for general membrane association. A minimal region required for proper Golgi targeting of PSKH1 was identified within the first 29 amino acids. Expression of a PSKH1 mutant where the COOH-terminal kinase domain was swapped with green fluorescent protein and cysteine 3 was exchanged with serine resulted in disassembly of the Golgi apparatus as visualized by redistribution of beta-COP and GM130 to a diffuse cytoplasmic pattern, while leaving the tubulin skeleton intact. Our results suggest a structural and regulatory role of PSKH1 in maintenance of the Golgi apparatus, a key organelle within the secretory pathway.
No preview · Article · Jan 2004 · Experimental Cell Research
[Show abstract][Hide abstract] ABSTRACT: The factor VII genes of five unrelated Finnish female patients, F1-F5, with moderate bleeding tendency, were screened for mutations using single strand conformational polymorphisms and DNA sequencing. Heterozygous shifts were detected in exons 5 and 8 for patient F1, and sequencing confirmed the presence of the silent dimorphism H115H, the polymorphism R353Q and the mutation A294V. The patient F1 was also heterozygous for a novel -59T/G transversion mutation in the Hepatocyte nuclear factor 4-binding site. The remaining four patients carried a -32A/C transversion mutation located in a footprint (-51 to -32) covering the major transcription initiation start site -51). There was also a consensus sequence match to an initiator response-like binding element covering -51. Two patients were homozygous and two heterozygous for this mutation. Plasma FVII:Ag and FVII:C levels were reduced in parallel. A strong reduction in binding affinity of a specific nuclear protein to the -32C-containing oligonucleotide was found by electrophoretic mobility shift assays on nuclear extracts from HepG2 cells. EDTA caused no reduced binding. A minimal promoter (-191 to +15) containing the wild-type sequence or the -32A/C or -59T/G mutations was cloned in front of the firefly luciferase reporter gene and transiently transfected into Hep3B cells. Reduced activities [23.0 +/- 3.1% (-32C), 55.4 +/- 6.3% (-59G), 100% (wild-type construct)] were found for the mutated promoters. Southwestern blotting and UV crosslinking analysis showed binding of three proteins (20, 20 and 50 kDa) to the putative initiator response element. The -32A/C mutant oligonucleotide bound two proteins.
No preview · Article · Sep 2003 · Thrombosis and Haemostasis
[Show abstract][Hide abstract] ABSTRACT: Dietary fat influences plasma levels of coagulation factor VII (FVII) and serum phospholipids (PL). It is, however, unknown if the fat-mediated changes in FVII are linked to PL. The present study aimed to investigate the effects of dietary fat on fasting and postprandial levels of activated FVII (FVIIa), FVII coagulant activity (FVIIc), FVII protein (FVIIag) and choline-containing PL (PC). In a randomized single-blinded crossover-designed study a high-fat diet (HSAFA), a low-fat diet (LSAFA), both rich in saturated fatty acids, and a high-fat diet rich in unsaturated fatty acids (HUFA) were consumed for 3 weeks. Twenty-five healthy females, in which postprandial responses were studied in a subset of twelve, were included. The HSAFA diet resulted in higher levels of fasting FVIIa and PC compared with the LSAFA and the HUFA diets (all comparisons P< or =0.01). The fasting PC levels after the LSAFA diet were also higher than after the HUFA diet (P<0.001). Postprandial levels of FVIIa and PC were highest on the HSAFA diet and different from LSAFA and HUFA (all comparisons P< or =0.05). Postprandial FVIIa was higher on the HUFA compared with the LSAFA diet (P<0.03), whereas the HUFA diet resulted in lower postprandial levels of PC than the LSAFA diet (P<0.001). Significant correlations between fasting levels of PC and FVIIc were found on all diets, whereas FVIIag was correlated to PC on the HSAFA and HUFA diet. The present results indicate that dietary fat, both quality and quantity, influences fasting and postprandial levels of FVIIa and PC. Although significant associations between fasting FVII and PC levels were found, our results do not support the assumption that postprandial FVII activation is linked to serum PC.
Full-text · Article · Aug 2003 · British Journal Of Nutrition
[Show abstract][Hide abstract] ABSTRACT: The human protein MDDX28 is a putative RNA helicase and a nucleocytoplasmic shuttling protein also localized to the mitochondria. Its localization is novel among RNA helicases. We have studied its intracellular targeting signals and show that the first 20 amino acids of MDDX28 are necessary and sufficient for both mitochondrial import and nuclear export of the protein. Mutation of the five leucines in the sequence to alanines abolished the mitochondrial targeting signal as well as greatly reducing the nuclear export signal, indicating that these signal sequences are highly overlapping. Two short stretches of basic amino acids separated by 44 residues were both necessary and sufficient for full nuclear localization. However, they were not absolutely essential, because the protein was present in 7% of the nuclei when both signals were mutated. This indicates that MDDX28 contains another unidentified weak nuclear localization signal(s). Three basic domains in the N-terminal half of the protein and its RNA binding ability were essential for nucleolar localization as well as transcription-inhibition-dependent localization to nuclear subcompartments. Two of these basic domains were the same as those constituting the nuclear localization signal, suggesting that they are responsible for bringing the protein into the nucleus to the sites of RNA binding. Our results indicate that MDDX28 nucleo-cytoplasmic shuttling is dependent on the availability of nascent RNA.