Association of respiratory syncytial virus M protein with viral nucleocapsids is mediated by the M2-1 protein

Cytoplasmic inclusions in respiratory syncytial virus-infected cells comprising viral nucleocapsid proteins (L, N, P, and M2-1) and the viral genome are sites of viral transcription. Although not believed to be necessary for transcription, the matrix (M) protein is also present in these inclusions, and we have previously shown that M inhibits viral transcription. In this study, we have investigated the mechanisms for the association of the M protein with cytoplasmic inclusions. Our data demonstrate for the first time that the M protein associates with cytoplasmic inclusions via an interaction with the M2-1 protein. The M protein colocalizes with M2-1 in the cytoplasm of cells expressing only the M and M2-1 proteins and directly interacts with M2-1 in a cell-free binding assay. Using a cotransfection system, we confirmed that the N and P proteins are sufficient to form cytoplasmic inclusions and that M2-1 localizes to these inclusions; additionally, we show that M associates with cytoplasmic inclusions only in the presence of the M2-1 protein. Using truncated mutants, we show that the N-terminal 110 amino acids of M mediate the interaction with M2-1 and the subsequent association with nucleocapsids. The interaction of M2-1 with M and, in particular, the N-terminal region of M may represent a target for novel antivirals that block the association of M with nucleocapsids, thereby inhibiting virus assembly.

Impaired nuclear import and viral incorporation of Vpr derived from a HIV long-term non-progressor

We previously reported an epidemiologically linked HIV-1 infected patient cohort in which a long-term non-progressor (LTNP) infected two recipients who then exhibited normal disease progression. Expression of patient-derived vpr sequences from each of the three cohort members in mammalian cells tagged with GFP revealed a significant reduction in Vpr nuclear import and virion incorporation uniquely from the LTNP, whereas Vpr from the two progressing recipients displayed normal localisation and virion incorporation, implying a link between efficient Vpr nuclear import and HIV disease progression. Importantly, an F72L point mutation in the LTNP was identified for the first time as being uniquely responsible for decreased Vpr nuclear import.

Interferon gamma is recognised by importin alpha/beta: enhanced nuclear localising and transactivation activities of an interferon gamma mimetic

Interferon (IFN) gamma's ability to localise in the nucleus and function in gene activation has been known for some time, although the role of the conventional nuclear transporting importin molecules is unclear. Here, we demonstrate for the first time the direct recognition of IFNgamma and an IFNgamma mimetic peptide by IMPalpha and the IMPalpha/beta heterodimer, where the IFNgamma mimetic shows higher affinity. Significantly, this correlates well both with in vivo ability to target green fluorescent protein to the nucleus in transfected cells as determined by quantitative confocal laser scanning microscopy, as well as GAS promoter activity of a luciferase reporter. This has important implications for IFNgamma's anti-viral action, and the potential use of the IFNgamma mimetic in antiviral therapies.

Efficient gene delivery using reconstituted chromatin enhanced for nuclear targeting

Nonviral gene delivery is hampered by difficulties associated with transporting negatively charged DNA through the cell membrane and, more importantly, the nuclear envelope of target cells. Here we show for the first time that chromatin reconstituted with histone H2B proteins optimized for nuclear targeting can be used as an efficient means to deliver DNA to the nucleus of intact living mammalian cells, resulting in high levels of transgene expression that were approximately 6-fold more than those achieved by commercial liposomal preparations. The high efficiency is due in part to DNA condensation and protection against degradation in the reconstituted chromatin, as well as its ability to interact with high affinity with the importin proteins of the cellular nuclear import machinery. "Chromofection," gene delivery by protein transduction using chromatin enhanced for nuclear targeting represents an efficient means to deliver DNA to a wide variety of cell types, with the potential to treat complex genetic disorders.

Dynamic changes in localization of Chromobox (Cbx) family members during the maternal to embryonic transition

The Chromobox domain (Cbx) gene family, consisting of Polycomb and Heterochromatin Protein 1 genes, is involved in transcriptional repression, cell cycle regulation and chromatin remodeling. We report the first study of gene expression and protein localization of the Cbx genes in in vitro produced bovine embryos. All but one gene (Cbx6) were expressed. This was confirmed by immunolocalization for HP1alpha, beta, gamma, and Pc2, 3. HP1beta was found in the nuclei of embryos from the two-cell stage onwards, whereas HP1gamma showed diffuse cytoplasmic/nuclear localization at the two- and eight-cell stages, and predominantly nuclear localization at the four-cell stage and the 16-cell stage onwards. Leptomycin B (LMB), a specific inhibitor of the nuclear export protein CRM-1 (chromosomal regional maintenance-1), was found to increase nuclear localization of HP1gamma at the eight-cell stage, and to prevent progression past this stage of embryogenesis. This indicates that HP1gamma possesses a CRM-1-dependent nuclear export pathway which may represent part of the basis of HP1gamma's ability to shuttle between the nucleus and the cytoplasm in dynamic fashion. HP1alpha was expressed in embryonic nuclei at all stages, but was found to relocalise from euchromatin to heterochromatin during the maternal to embryonic transition (MET). In contrast, Pc2 and Pc3 were evenly distributed between cytoplasm and nucleus until the eight- and sixteen-cell stages or the morula stage, respectively, before relocating preferentially to the cytoplasm. Collectively, the results suggest that dynamic changes of the nuclear-cytoplasmic and subnuclear distribution of members of the Cbx family may be central to the MET.

HIV-1 infection induces changes in expression of cellular splicing factors that regulate alternative viral splicing and virus production in macrophages

BACKGROUND

Macrophages are important targets and long-lived reservoirs of HIV-1, which are not cleared of infection by currently available treatments. In the primary monocyte-derived macrophage model of infection, replication is initially productive followed by a decline in virion output over ensuing weeks, coincident with a decrease in the levels of the essential viral transactivator protein Tat. We investigated two possible mechanisms in macrophages for regulation of viral replication, which appears to be primarily regulated at the level of tat mRNA: 1) differential mRNA stability, used by cells and some viruses for the rapid regulation of gene expression and 2) control of HIV-1 alternative splicing, which is essential for optimal viral replication.

RESULTS

Following termination of transcription at increasing times after infection in macrophages, we found that tat mRNA did indeed decay more rapidly than rev or nef mRNA, but with similar kinetics throughout infection. In addition, tat mRNA decayed at least as rapidly in peripheral blood lymphocytes. Expression of cellular splicing factors in uninfected and infected macrophage cultures from the same donor showed an inverse pattern over time between enhancing factors (members of the SR family of RNA binding proteins) and inhibitory factors (members of the hnRNP family). While levels of the SR protein SC35 were greatly up-regulated in the first week or two after infection, hnRNPs of the A/B and H groups were down-regulated. Around the peak of virus production in each culture, SC35 expression declined to levels in uninfected cells or lower, while the hnRNPs increased to control levels or above. We also found evidence for increased cytoplasmic expression of SC35 following long-term infection.

CONCLUSION

While no evidence of differential regulation of tat mRNA decay was found in macrophages following HIV-1 infection, changes in the balance of cellular splicing factors which regulate alternative viral pre-mRNA splicing were observed. These changes correlated with changes in Tat expression and virus production and could play an important role in viral persistence in macrophages. This mechanism could provide a novel target for control of infection in this critical cell type, which would be necessary for eventual eradication of the virus from infected individuals.

The N-terminal basic domain of the HIV-1 matrix protein does not contain a conventional nuclear localization sequence but is required for DNA binding and protein self-association

The HIV p17 or matrix (MA) protein has long been implicated in the process of nuclear import of the HIV genome and thus the ability of the virus to infect nondividing cells such as macrophages. While it has been demonstrated that MA is not absolutely required for this process, debate continues to surround the subcellular targeting properties of MA and its potential contribution to nuclear import of the HIV cDNA. Through the use of in vitro techniques we have determined that, despite the ability of MA to interact with importins, the full-length protein fails to enter the nucleus of cells. While MA does contain a region of basic amino acids within its N-terminus which can confer nuclear accumulation of a fusion protein, we show that this is due to nuclear retention mediated by DNA binding and does not represent facilitated import. Importantly, we show that the 26KK residues of MA, previously thought to be part of a nuclear localization sequence, are absolutely required for a number of MA's functions including its ability to bind DNA and RNA and its propensity to form high-order multimers/protein aggregates. The results presented here indicate that the N-terminal basic domain of MA does not appear likely to play a role in HIV cDNA nuclear import; rather this region appears to be a crucial structural and functional motif whose integrity is required for a number of other roles performed by MA during viral infection.

Interaction with PI3-kinase contributes to the cytotoxic activity of apoptin

Apoptin, a small protein from the chicken anemia virus, has attracted attention because of its specificity in killing tumor cells. Localization of apoptin in the nucleus of tumor cells has been shown to be vital for proapoptotic activity, however, targeted expression of apoptin in the nucleus of normal cells does not harm the cells, indicating that nuclear localization of apoptin is insufficient for its cytotoxicity. Here, we demonstrate for the first time that apoptin interacts with the SH3 domain of p85, the regulatory subunit of phosphoinositide 3-kinase (PI3-K), through its proline-rich region. Apoptin derivatives devoid of this proline-rich region do not interact with p85, are unable to activate PI3-K, and show impaired apoptosis induction. Moreover, apoptin mutants containing the proline-rich domain are sufficient to elevate PI3-K activity and to induce apoptosis in cancer cells. Downregulation of p85 leads to nuclear exclusion of apoptin and impairs cell death induction, indicating that interaction with the p85 PI3-K subunit essentially contributes to the cytotoxic activity of apoptin.

Subcellular distribution of importins correlates with germ cell maturation

Importin proteins regulate access to the nucleus by recognizing and transporting distinct cargo proteins. Building on studies in Drosophila and Caenorhabditis elegans, we hypothesized that regulated expression and subcellular localization of specific importins may be linked to mammalian gonadal differentiation. We identified distinct developmental and cellular localization patterns for importins beta1, alpha3, alpha4 and RanBP5 (importin beta3) in fetal and postnatal murine testes using Western blotting and immunohistochemistry. Importin beta1 protein is detected in selected germ and somatic cells in fetal gonads, with a striking perinuclear staining evident from embryonic day (E) 14.5 within testicular gonocytes. RanBP5 exhibits age- and gender-specific subcellular localization within fetal gonads. At E12.5, RanBP5 protein is cytoplasmic in gonocytes but predominantly nuclear in oogonia, but by E14.5 RanBP5 appears nuclear in gonocytes and cytoplasmic in oogonia. In postnatal testes, importin alpha3 and alpha4 in spermatocytes, spermatids, and Sertoli cells display cytoplasmic and nuclear localization, respectively.

Nucleocytoplasmic distribution of rabies virus P-protein is regulated by phosphorylation adjacent to C-terminal nuclear import and export signals

Nucleocytoplasmic distribution of the rabies virus phosphoprotein is implicated in the evasion of cellular antiviral mechanisms by rabies virus and has been reported to depend on an N-terminal nuclear export sequence and a C-terminal nuclear localization sequence. This paper identifies a second nuclear export sequence that is located between key residues of the nuclear localization sequence in the phosphoprotein C-terminal domain. The C-terminal domain confers predominantly nuclear localization in unstimulated transfected cells, indicating that the nuclear localization sequence is the dominant signal at steady state. However, protein kinase-C activation or mutagenesis to mimic protein kinase-C phosphorylation at a site proximal to the C-terminal nuclear localization/export sequences shifts the targeting activity of the C-terminal domain toward nuclear exclusion, indicating that the nuclear export sequence becomes the dominant signal in activated cells. Mapping of these sequences within the three-dimensional structure of the C-terminal domain indicates that their activities may be coregulated by phosphorylation and/or conformational changes in the domain. The data are consistent with a model in which intimate positioning of the nuclear localization sequence, export sequence, and phosphorylation site within a single domain provides a switch mechanism to rapidly and efficiently balance the reciprocal import and export signals in response to cellular stimuli.

The biarsenical dye Lumio exhibits a reduced ability to specifically detect tetracysteine-containing proteins within live cells

Investigating the localisation of proteins within live cells via fluorescence microscopy typically involves the fusion of the protein of interest to a large fluorescent protein such as green fluorescent protein (GFP). Alternate fluorescent labelling technologies such as the fluorescent biarsenical dye molecules (e.g. FlAsH, ReAsH) are preferable to the use of large fusion proteins in many respects and allow greater flexibility in terms of the location of the labelling site. We assessed the ability of the FlAsH-derived biarsenical dye molecule Lumio to label a range of tetracysteine containing proteins within live cells and report that although in some circumstances Lumio is capable of positively detecting such proteins, the sensitivity and specificity of labelling is significantly reduced, making the Lumio-labelling system unsuitable for the detection of a wide range of protein within live cells.

Nucleocytoplasmic transport of DNA: enhancing non-viral gene transfer

Gene therapy, the correction of dysfunctional or deleted genes by supplying the lacking component, has long been awaited as a means to permanently treat or reverse many genetic disorders. To achieve this, therapeutic DNA must be delivered to the nucleus of cells using a safe and efficient delivery vector. Although viral-based vectors have been utilized extensively due to their innate ability to deliver DNA to intact cells, safety considerations, such as pathogenicity, oncogenicity and the stimulation of an immunological response in the host, remain problematical. There has, however, been much progress in the development of safe non-viral gene-delivery vectors, although they remain less efficient than the viral counterparts. The major limitations of non-viral gene transfer reside in the fact that it must be tailored to overcome the intracellular barriers to DNA delivery that viruses already master, including the cellular and nuclear membranes. In particular, nuclear transport of the therapeutic DNA is known to be the rate-limiting step in the gene-delivery process. Despite this, much progress had been made in recent years in developing novel means to overcome these barriers and efficiently deliver DNA to the nuclei of intact cells. This review focuses on the nucleocytoplasmic delivery of DNA and mechanisms to enhance to non-viral-mediated gene transfer.

Targeted delivery to the nucleus

Macromolecules and supramolecular complexes are frequently required to enter and exit the nucleus during normal cell function, but access is restricted and exchange to and from the nucleus is tightly controlled. We describe the mechanisms which regulate nuclear import of endogenous molecules and indicate how viruses exploit these mechanisms during their life cycle. Opportunities exist to make use of natural pathways for delivery of therapeutic entities, in particular to develop safe and effective methods for gene therapy, although past attempts to design non-viral nuclear delivery systems have met with limited success. To increase the likelihood of success scientists will need an appreciation of the mechanisms by which viruses deliver their genomes to the nucleus, and will need a commitment to control the architecture of non-viral delivery systems at the molecular level. Effective delivery systems will require several attributes to facilitate endosomal escape, microtubular transport and uptake through the nuclear pore complex. The published literature provides a strong foundation for design of nuclear targeting systems. The challenge faced by delivery scientists is to assemble a system which is as effective as, for example, the adenovirus but which lacks its immunogenicity. This article reviews the relevant literature and indicates key areas for future research.

Nuclear targeting of the growth hormone receptor results in dysregulation of cell proliferation and tumorigenesis

Growth hormone receptor (GHR) has been demonstrated to be nuclear localized both in vivo and in vitro, but the significance of this observation has remained elusive. Here we show that nuclear GHR is strongly correlated with proliferative status in vivo by using a liver regeneration model. In vitro, nuclear translocation of the GH receptor is GH-dependent and appears to be mediated by the Importin system. Constitutive nuclear targeting of GHR in murine pro-B cells is associated with constitutive activation of STAT5, a transforming agent in lymphoma and other cell types. This activation is abrogated by inhibition of JAK2 and appears to be driven by autocrine murine GH action coupled with enhanced nuclear uptake of phospho-STAT5. Nuclear targeting induces dysregulated cell cycle progression in the pro-B cell line, associated with constitutive up-regulation of the proliferation inducers Survivin and Mybbp, the metastasis related Dysadherin, and other tumor markers. GHR nuclear-targeted cells generate aggressive metastatic tumors when injected into nude mice, which display nuclear localized GHR strikingly similar to that seen in human lymphomas. We conclude that aberrant nuclear localization of GHR is a marker of high proliferative status and is sufficient to induce tumorigenesis and tumor progression.

A microtubule-facilitated nuclear import pathway for cancer regulatory proteins

Nuclear protein import is dependent on specific targeting signals within cargo proteins recognized by importins (IMPs) that mediate translocation through the nuclear pore. Recent evidence, however, implicates a role for the microtubule (MT) network in facilitating nuclear import of the cancer regulatory proteins parathyroid hormone-related protein (PTHrP) and p53 tumor suppressor. Here we assess the extent to which MT and actin integrity may be generally required for nuclear protein import for the first time. We examine 10 nuclear-localizing proteins with diverse IMP-dependent nuclear import pathways, our results indicating that the cytoskeleton does not have a general mechanistic role in nuclear localization sequence-dependent nuclear protein import. Of the proteins examined, only the p110(Rb) tumor suppressor protein Rb, together with p53 and PTHrP, was found to require MT integrity for optimal nuclear import. Fluorescence recovery after photobleaching experiments indicated that the MT-dependent nuclear transport pathway increases both the rate and extent of Rb nuclear import but does not affect Rb nuclear export. Dynamitin overexpression experiments implicate the MT motor dynein in the import process. The results indicate that, additional to IMP/diffusion-dependent processes, certain cancer regulatory proteins utilize an MT-enhanced pathway for accelerated nuclear import that is presumably required for their nuclear functions.

An importin alpha/beta-recognized bipartite nuclear localization signal mediates targeting of the human herpes simplex virus type 1 DNA polymerase catalytic subunit pUL30 to the nucleus

Although the 1235 amino acids human herpes simplex virus type 1 (HSV-1) DNA polymerase catalytic subunit, pUL30, is essential for HSV-1 replication in the nucleus of host cells, little information is available regarding its nuclear import mechanism. The present study addresses this issue directly, characterizing pUL30's nuclear import pathway for the first time using quantitative confocal laser scanning microscopy (CLSM) on living cells, and fluorescent binding assays. In addition to a previously described nuclear localization signal (NLS) located within the pUL30 binding site for the polymerase accessory protein (PAP) pUL42, that appears to be dispensable for nuclear targeting, pUL30 possesses three putative basic NLSs. Intriguingly, the core of pUL30-NLS2 (residues 1114-1120) is highly homologous to that of the recently described NLS, similarly located upstream of the PAP binding site, of the human cytomegalovirus (HCMV) DNA polymerase catalytic subunit, pUL54. Here we show for the first time that pUL30-NLS2 itself is only partially functional in terms of nuclear import due to residue P1118 present in position 3 of the NLS core. Intriguingly, pUL30-NLS2 together with pUL30-NLS3 (residues 1133-1136) represents a fully functional bipartite NLS (pUL30-NLSbip), required for nuclear targeting of pUL30, and able to confer nuclear localization on heterologous proteins by conferring high-affinity interaction with the importin (IMP) alpha/beta heterodimer. Since nuclear targeting of HSV-1 proteins forming the replication fork is crucial for viral replication, the pUL30-NLSbip emerges for the first time as a viable therapeutic target.

Dynein light chain association sequences can facilitate nuclear protein import

Nuclear localization sequence (NLS)-dependent nuclear protein import is not conventionally held to require interaction with microtubules (MTs) or components of the MT motor, dynein. Here we report for the first time the role of sequences conferring association with dynein light chains (DLCs) in NLS-dependent nuclear accumulation of the rabies virus P-protein. We find that P-protein nuclear accumulation is significantly enhanced by its dynein light chain association sequence (DLC-AS), dependent on MT integrity and association with DLCs, and that P-protein-DLC complexes can associate with MT cytoskeletal structures. We also find that P-protein DLC-AS, as well as analogous sequences from other proteins, acts as an independent module that can confer enhancement of nuclear accumulation to proteins carrying the P-protein NLS, as well as several heterologous NLSs. Photobleaching experiments in live cells demonstrate that the MT-dependent enhancement of NLS-mediated nuclear accumulation by the P-protein DLC-AS involves an increased rate of nuclear import. This is the first report of DLC-AS enhancement of NLS function, identifying a novel mechanism regulating nuclear transport with relevance to viral and cellular protein biology. Importantly, this data indicates that DLC-ASs represent versatile modules to enhance nuclear delivery with potential therapeutic application.

Nuclear localization of dengue virus nonstructural protein 5 through its importin alpha/beta-recognized nuclear localization sequences is integral to viral infection

Dengue virus nonstructural protein 5 (NS5) is a large multifunctional protein with a central role in viral replication. We previously identified two nuclear localization sequences (NLSs) within the central region of dengue virus type-2 (DENV-2) NS5 ('aNLS' and 'bNLS') that are recognized by the importin alpha/beta and importin beta1 nuclear transporters, respectively. Here, we demonstrate the importance of the kinetics of NS5 nuclear localization to virus production for the first time and show that the aNLS is responsible. Site-specific mutations in the bipartite-type aNLS or bNLS region were introduced into a reporter plasmid encoding green fluorescent protein fused to the N-terminus of DENV-2 NS5, as well as into DENV-2 genomic length complementary DNA. Mutation of basic residues in the highly conserved region of the bNLS did not affect nuclear import of NS5. In contrast, mutations in either basic cluster of the aNLS decreased NS5 nuclear accumulation and reduced virus production, with the greatest reduction observed for mutation of the second cluster (K(387)K(388)K(389)); mutagenesis of both clusters abolished NS5 nuclear import and DENV-2 virus production completely. The latter appeared to relate to the impaired ability of virus lacking nuclear-localizing NS5, as compared with wild-type virus expressing nuclear-localizing NS5, to reduce interleukin-8 production as part of the antiviral response. The results overall indicate that NS5 nuclear localization through the aNLS is integral to viral infection, with significant implications for other flaviviruses of medical importance, such as yellow fever and West Nile viruses.

Apoptosis induces neuronal apolipoprotein-E synthesis and localization in apoptotic bodies

Neuronal apoptosis is crucial for central nervous system development and also contributes to neurodegenerative disease. Apolipoprotein-E (apoE) regulates brain lipid transport and specific neuronal functions and previous research, investigating non-neuronal cell types, identified an association between apoptosis and increased apoE expression. In the present study we used the human SK-N-SH neuronal cell line to investigate potential changes in apoE expression during apoptosis which occurs as a consequence of extended culture (up to 5 days) without replenishing trophic factors. Standard and real-time PCR analysis indicated a significant 6-fold increase in apoE mRNA after 3 days which was correlated with caspase-3 activation, TUNEL positivity and the formation of apoptotic bodies. ApoE protein levels were low in the absence of apoptosis but increased by 8-fold when apoptosis was induced. Analysis of cellular debris that accumulated in the culture supernatants indicated that apoE levels became progressively concentrated in apoptotic bodies. These data indicate that apoE is up-regulated during neuronal apoptosis and raise the possibility that apoE may play a role in the clearance of apoptotic bodies through apoE-receptor interactions.

Regulating the functions of the HIV-1 matrix protein

The HIV-1 structural protein matrix (MA) is involved in a number of essential steps during infection and appears to possess multiple, seemingly conflicting targeting signals. Although MA has long been known to be crucial for virion assembly, details regarding this function, and the domains responsible for mediating it, are still emerging. MA has also been implicated in nuclear import of HIV cDNA and is purported to contain a nuclear targeting signal. Little is known about how these opposing plasma membrane and nuclear targeting signals are regulated and which signals predominate at various stages of infection. Additionally, MA has recently been implicated in a number of novel roles during infection including viral entry/uncoating, cytoskeletal-mediated transport, and targeting viral assembly to lipid rafts. Here we discuss our current understanding of MA's functions during infection and explore the recent advancements made in elucidating the mechanism of these processes. It appears that MA possesses a cache of targeting signals that are likely to be regulated throughout the infectious cycle by a combination of structural and biochemical modifications including phosphorylation, myristoylation, and multimerization. The ability of HIV to modify the properties of MA at specific stages of infection is central to the multifunctional behavior of MA and the efficiency of HIV infection. The recently reported success of drugs specifically designed to block MA function (Haffar O, Dubrovsky L, and Lowe R et al. J Virol 2005;79:13028-13036) confirms the importance of this protein for HIV infection and highlights a potentially new avenue in multivalent drug therapy.

Histone-mediated transduction as an efficient means for gene delivery

Gene delivery into the nucleus of eukaryotic cells is inefficient, largely because of the significant barriers within the target cell of the plasma membrane and nuclear envelope. Recently, a group of basic proteins, including the HIV-1 Tat protein and the four core histones, have been shown to enter cells through a novel energy- and receptor-independent manner. Here, we show that engineered histone H2B proteins are able to mediate the efficient delivery of either green fluorescent protein or DNA into HeLa cells through the process of "Histone-Mediated Transduction" (HMT), with further enhancement achieved by utilizing a dimer of histones H2B and H2A. Subsequent nuclear delivery was accelerated approximately two-fold by the addition of an optimized nuclear localization signal to histone H2B, thereby increasing the affinity of interaction with components of the cellular nuclear import machinery, resulting in increased expression of a reporter gene. Further, we demonstrate that the domains responsible for this histone transduction are located in the N-terminal tail and globular regions of histone H2B. HMT represents a new, efficient, and technically non-demanding means to deliver DNA to the nucleus of intact cells, including embryonic stem cells, which has important applications in gene therapy and cancer therapeutics.

Dengue virus RNA polymerase NS5: a potential therapeutic target?

Dengue fever (DF)/dengue haemorrhagic fever (DHF) is the most common arthropod-borne viral infection, where it is now estimated that 2.5-3 billion people world-wide are at risk of infection. Currently there is no available treatment, in the form of vaccine or drug, making eradication of the mosquito vector the only viable control measure, which has proved costly and of limited success. There are a number of different vaccines undergoing testing, but whilst a dengue vaccine is clearly desirable, there are several issues which make live-attenuated vaccines problematic. These include the phenomenon of antibody-dependent enhancement (ADE) and the possibility of recombination of attenuated vaccine strains with wild-type flavivirus members reverting vaccines to a virulent form. Until we gain a better understanding of these issues and their associated risks, the safety of any live dengue vaccine cannot be assured. It therefore may be safer and more feasible for therapeutic-based approaches to be developed as an alternative to live vaccines. As our understanding of dengue molecular biology expands, new potential targets for drugs are emerging. One of the most promising is the dengue non-structural protein 5 (NS5), the largest and most highly conserved of the dengue proteins. This review examines the unique properties of NS5, including its functions, interactions, subcellular localisation and regulation, and looks at ways in which some of these may be exploited in our quest for effective drugs.

Quantitative analysis of DNA-protein interactions using double-labeled native gel electrophoresis and fluorescence-based imaging

We have developed a sensitive, non-radioactive method to assess the interaction of transcription factors/DNA-binding proteins with DNA. We have modified the traditional radiolabeled DNA gel mobility shift assay to incorporate a DNA probe end-labeled with a Texas-red fluorophore and a DNA-binding protein tagged with the green fluorescent protein to monitor precisely DNA-protein complexation by native gel electrophoresis. We have applied this method to the DNA-binding proteins telomere release factor-1 and the sex-determining region-Y, demonstrating that the method is sensitive (able to detect 100 fmol of fluorescently labeled DNA), permits direct visualization of both the DNA probe and the DNA-binding protein, and enables quantitative analysis of DNA and protein complexation, and thereby an estimation of the stoichiometry of protein-DNA binding.

Central role of the respiratory syncytial virus matrix protein in infection

Respiratory syncytial virus is the major respiratory pathogen of infants and children worldwide, with no effective treatment or vaccine available. Steady progress has been made in understanding the respiratory syncytial virus life cycle and the consequences of infection, but many areas of respiratory syncytial virus biology remain poorly understood, including the role of subcellular localisation of respiratory syncytial virus gene products such as the matrix protein in the infected host cell. The matrix protein plays a central role in viral assembly and, intriguingly, has been observed to traffic into and out of the nucleus at specific times during the respiratory syncytial virus infectious cycle. Further, the matrix protein has been shown to be able to inhibit transcription, which may be a key to respiratory syncytial virus pathogenesis. This review will focus on the role of the matrix protein in respiratory syncytial virus infection and what is known of its nucleocytoplasmic trafficking, the understanding of which may lead to new therapeutic approaches to combat respiratory syncytial virus, and/or vaccine development.