The unfolded protein response transducer Ire1p contains a nuclear localization sequence recognized by multiple beta importins

The Ire1p transmembrane receptor kinase/endonuclease transduces the unfolded protein response (UPR) from the endoplasmic reticulum (ER) to the nucleus in Saccharomyces cerevisiae. In this study, we analyzed the capacity of a highly basic sequence in the linker region of Ire1p to function as a nuclear localization sequence (NLS) both in vivo and in vitro. This 18-residue sequence is capable of targeting green fluorescent protein to the nucleus of yeast cells in a process requiring proteins involved in the Ran GTPase cycle that facilitates nuclear import. Mutagenic analysis and importin binding studies demonstrate that the Ire1p linker region contains overlapping potential NLSs: at least one classical NLS (within sequences 642KKKRKR647 and/or 653KKGR656) that is recognized by yeast importin alpha (Kap60p) and a novel betaNLS (646KRGSRGGKKGRK657) that is recognized by several yeast importin beta homologues. Kinetic binding data suggest that binding to importin beta proteins would predominate in vivo. The UPR, and in particular ER stress-induced HAC1 mRNA splicing, is inhibited by point mutations in the Ire1p NLS that inhibit nuclear localization and also requires functional RanGAP and Ran GEF proteins. The NLS-dependent nuclear localization of Ire1p would thus seem to be central to its role in UPR signaling.

Apoptin is modified by SUMO conjugation and targeted to promyelocytic leukemia protein nuclear bodies

Apoptin, a protein of the chicken anemia virus (CAV), represents a novel potential anticancer therapeutic, because it induces apoptotic death specifically in tumor but not normal cells. The cellular localization appears to be crucial for apoptin's selective toxicity. In normal cells apoptin remains in the cytoplasm, whereas in transformed cells it migrates into the nucleus and kills the cell. However, the manner by which apoptin is able to distinguish between tumor and normal cells is unknown. Here, we report for the first time that apoptin interacts directly with the promyelocytic leukemia protein (PML) in tumor cells and accumulates in PML nuclear bodies (NBs), which are involved in apoptosis induction and viral replication. We also demonstrate that apoptin is sumoylated and that a sumoylation-deficient apoptin mutant is no longer recruited to PML-NBs, but localizes in the nuclear matrix. This mutant fails to bind PML, but can still induce apoptosis as efficiently as wild-type apoptin. Moreover, apoptin kills also PML-/- cells and promyelocytic leukemia cells with defective PML expression. Our results therefore suggest that apoptin kills tumor cells independently of PML and sumoylation, however, the interaction of apoptin with PML and small ubiquitin-like modifier (SUMO) proteins might be relevant for CAV replication.

Quantitative analysis of localization and nuclear aggregate formation induced by GFP-lamin A mutant proteins in living HeLa cells

Although A-type lamins are ubiquitously expressed, their role in the tissue-specificity of human laminopathies remains enigmatic. In this study, we generate a series of transfection constructs encoding missense lamin A mutant proteins fused to green fluorescent protein and investigate their subnuclear localization using quantitative live cell imaging. The mutant constructs used included the laminopathy-inducing lamin A rod domain mutants N195K, E358K, M371K, R386K, the tail domain mutants G465D, R482L, and R527P, and the Hutchinson-Gilford progeria syndrome-causing deletion mutant, progerin (LaA delta50). All mutant derivatives induced nuclear aggregates, except for progerin, which caused a more lobulated phenotype of the nucleus. Quantitative analysis revealed that the frequency of nuclear aggregate formation was significantly higher (two to four times) for the mutants compared to the wild type, although the level of lamin fusion proteins within nuclear aggregates was not. The distribution of endogenous A-type lamins was altered by overexpression of the lamin A mutants, coexpression experiments revealing that aberrant localization of the N195K and R386K mutants had no effect on the subnuclear distribution of histones H2A or H2B, or on nuclear accumulation of H2A overexpressed as a DsRed2 fusion protein. The GFP-lamin fusion protein-expressing constructs will have important applications in the future, enabling live cell imaging of nuclear processes involving lamins and how this may relate to the pathogenesis of laminopathies.

Human cytomegalovirus DNA polymerase catalytic subunit pUL54 possesses independently acting nuclear localization and ppUL44 binding motifs

The catalytic subunit of human cytomegalovirus (HCMV) DNA polymerase pUL54 is a 1242-amino-acid protein, whose function, stimulated by the processivity factor, phosphoprotein UL44 (ppUL44), is essential for viral replication. The C-terminal residues (amino acids 1220-1242) of pUL54 have been reported to be sufficient for ppUL44 binding in vitro. Although believed to be important for functioning in the nuclei of infected cells, no data are available on either the interaction of pUL54 with ppUL44 in living mammalian cells or the mechanism of pUL54 nuclear transport and its relationship with that of ppUL44. The present study examines for the first time the nuclear import pathway of pUL54 and its interaction with ppUL44 using dual color, quantitative confocal laser scanning microscopy on live transfected cells and quantitative gel mobility shift assays. We showed that of two nuclear localization signals (NLSs) located at amino acids 1153-1159 (NLSA) and 1222-1227 (NLSB), NLSA is sufficient to confer nuclear localization on green fluorescent protein (GFP) by mediating interaction with importin alpha/beta. We also showed that pUL54 residues 1213-1242 are sufficient to confer ppUL44 binding abilities on GFP and that pUL54 and ppUL44 can be transported to the nucleus as a complex. Our work thus identified distinct sites within the HCMV DNA polymerase, which represent potential therapeutic targets and establishes the molecular basis of UL54 nuclear import.

Human cytomegalovirus (HCMV) DNA polymerase processivity factor ppUL44 dimerizes in the cytosol before translocation to the nucleus

Replication of the human cytomegalovirus genome takes place in the nuclei of infected cells and is mediated by a viral-encoded DNA polymerase complex formed by the catalytic subunit pUL54 and the processivity factor ppUL44. Although it has recently been shown that the dimerization ability of recombinant pUL44 appears to be crucial for effective DNA binding in vitro, whether ppUL44 can dimerize or not in a cellular context is unknown. Here, we show, by using co-immunoprecipitation and dual-color live imaging approaches on cells expressing fluorescent and differently tagged ppUL44 fusion proteins, that ppUL44 dimerizes in the cytoplasm via its N-terminal domain, before translocating to the nucleus. Furthermore, we show that nuclear translocation of differently tagged ppUL44 heterodimers can occur even when one subunit carries a nonfunctional nuclear localization signal. Importantly, the latter cotransfection assay represents a system to test small-molecule inhibitors for their ability to impair ppUL44 dimerization.

Protein transduction: cell penetrating peptides and their therapeutic applications

Cell penetrating proteins or peptides (CPPs) have the ability to cross the plasma membranes of mammalian cells in an apparently energy- and receptor-independent fashion. Although there is much debate over the mechanism by which this "protein transduction" occurs, the ability of CPPs to translocate rapidly into cells is being exploited to deliver a broad range of therapeutics including proteins, DNA, antibodies, oligonucleotides, imaging agents and liposomes in a variety of situations and biological systems. The current review looks at the delivery of many such molecules by various CPPs, and their potential therapeutic application in a wide range of areas. CPP ability to deliver different cargoes in a relatively efficient and non-invasive manner has implications as far reaching as drug delivery, gene transfer, DNA vaccination and beyond. Although many questions remain to be answered and limitations on the use of CPPs exist, it is clear that this emerging technology has much to offer in a clinical setting.

Tumor-specific nuclear targeting: promises for anti-cancer therapy?

Recent developments in anti-cancer gene therapy suggest that the idea of a magic bullet for cancer may not be a pipe dream. Viral-based anti-cancer vectors for gene therapy have been used preferentially in this regard, but recent results from clinical trials have raised serious concerns as to their safety. For this reason, the development of non-viral vectors able to deliver drugs or suicide genes specifically to cancer cells is of paramount importance. In this context, great interest has been raised by recent reports that several proteins, including viral protein 3 (VP3 or Apoptin) from Chicken Anemia Virus, are capable of selectively killing tumor cells. Intriguingly, VP3's anti-cancer activity is strongly linked to its ability to localize more efficiently in the nucleus of cancer and transformed cells than that of normal, non-transformed cells with a tumor cell-specific nuclear targeting signal (tNTS) located at the C-terminus of the protein. Clearly, the VP3 tNTS is an exciting prospect to enhance non-viral-mediated cancer cell killing. This review will discuss recent advances in the understanding of the mechanism responsible for VP3 tumor-specific nuclear localization, including its specific phosphorylation, and the implications for the enhancement of anti-cancer therapy. It also proposes alternative strategies to develop tNTSs for anti-cancer therapies.

A protein kinase CK2 site flanking the nuclear targeting signal enhances nuclear transport of human cytomegalovirus ppUL44

The processivity factor of the human cytomegalovirus (HCMV) DNA polymerase phosphoprotein ppUL44 plays an essential role in viral replication, showing nuclear localization in infected cells. The present study examines ppUL44's nuclear import pathway for the first time, ectopic expression of ppUL44 revealing a strong nuclear localization in transfected COS-7 and other cell types, implying that no other HCMV proteins are required for nuclear transportation and retention. We show that of the two potential nuclear localization signals (NLSs) located at amino acids 162-168 (NLS1) and 425-431 (NLS2), NLS2 is necessary and sufficient to confer nuclear localization. Moreover, using enzyme-linked immunosorbent assays and gel mobility shift assays, we show that NLS2 is recognized with high affinity by the importin (IMP) alpha/beta heterodimer. Using gel mobility shift and transient transfection assays, we find that flanking sequences containing a cluster of potential phosphorylation sites, including a consensus site for protein kinase CK2 (CK2) at Ser413 upstream of the NLS, increase NLS2-dependent IMP binding and nuclear localization, suggesting a role for these sites in enhancing UL44 nuclear transport. Results from site-directed mutagenic analysis and live-cell imaging of green fluorescent protein (GFP)-UL44 fusion protein-expressing cells treated with the CK2-specific inhibitor 4,5,6,7-tetrabromobenzotriazole are consistent with phosphorylation of Ser413 enhancing ppUL44 nuclear transport.

Importin alpha mRNAs have distinct expression profiles during spermatogenesis

Importin proteins control access to the cell nucleus by mediating the nuclear transport of specific cargoes. We hypothesized that developmental regulation of gene expression may be partially effected by changes in the nuclear transport machinery complement, manifested as regulated expression of importin alpha family genes. We first clarified the identity of the five known mouse importin alpha genes relative to those for human and then determined their expression throughout postnatal rodent testis using PCR and in situ hybridization. Distinct expression patterns were observed for each. At 10 dpp, all importin alpha mRNAs were detected in spermatogonia. In the adult mouse testis, importins alpha1 and alpha3 were detected in spermatogonia and early pachytene spermatocytes. Importin alpha4 mRNA was identified in pachytene spermatocytes, alpha6 mRNA in round spermatids, and alpha2 mRNA in both of these. The distinct importin alpha expression patterns are consistent with their having specific roles and transport cargoes during spermatogenesis.

CRM1-dependent nuclear export of dengue virus type 2 NS5

The dengue virus multidomain RNA polymerase NS5 has been observed in the nucleus in mammalian infected cell systems. We previously showed that NS5 nuclear localization is mediated by two nuclear targeting signals within the NS5 interdomain region that are recognized by distinct members of the importin superfamily of intracellular transporters. Intriguingly, we have recently found that NS5 also possesses the ability to be exported from the nucleus by the importin family member CRM1 (exportin 1) both in Vero cells transfected to express NS5, and in dengue virus type 2 infected Vero cells, based on use of the CRM1-specific inhibitor leptomycin B (LMB). LMB treatment of Vero cells resulted in increased nuclear accumulation in both systems, and interestingly in the latter, resulted in an alteration in the kinetics of virus production. Our results imply that subcellular trafficking of NS5 at particular times in the infectious cycle may be central to the kinetics of virus production; perturbing this trafficking may represent a viable approach to develop new antiviral therapeutics.

Quantitative analysis of protein-protein interactions by native page/fluorimaging

We have developed a new quantitative native PAGE mobility shift assay, which allows for the measurement of binding affinities for interacting protein pairs, one of which is fluorescently labelled. We have used it to examine recognition of the Simian virus 40 (SV40) large tumour T-antigen (T-ag) nuclear localisation sequence (NLS) by members of the importin (Imp) superfamily of nuclear transport proteins. We demonstrate that the T-ag NLS binds to the Imp alpha/beta heterodimer in NLS-dependent manner, determining that it binds with eight-fold higher affinity (340 nM), when compared to Imp alpha alone, consistent with autoinhibition of Imp alpha when not complexed with Imp beta. The mobility shift assay is able to detect nM binding affinities, making it a sensitive and useful tool to analyse protein-protein interactions in solution.

Drivers of Germ Cell Maturation

Spermatogenesis requires progression of germ line stem cells through a precisely ordered differentiation pathway to form spermatozoa. Diverse and dynamic signals from the transforming growth factor-beta (TGF-beta) superfamily influence many stages of germ cell development. For example, interactions between several TGF-beta superfamily ligands (bone morphogenetic proteins, activin, and glial-derived neurotrophic growth factor [GDNF]) appear to govern the onset of spermatogenesis, and we are exploring how germ cells interpret these competing signals. We examined the in vivo impact of activin on testis development using two mouse models, the inhba-/- mouse (which lacks the gene encoding the activin A subunit and dies at birth) and BK mice, with inhbb (encoding the activin betaB subunit) replacing inhba (which survive to adulthood and show delayed fertility onset in males). Distinct effects on Sertoli cell and germ cell populations during fetal and early postnatal development were measured. We recognize that specific proteins, including downstream targets of TGF-beta signals, such as Smads, must move into the nucleus to implement the gene transcription changes required for development. We hypothesized that changes at the level of cellular nuclear transport machinery may be required to mediate this. Examination of proteins involved in classical nuclear import, the importins, revealed that each importin has a developmentally regulated expression pattern in male germ cells. Because each importin binds a selected range of cargo proteins and mediates their nucleocytoplasmic passage, our findings suggest that each importin ferries cargo required for discrete stages of spermatogenesis.

Nuclear import of the respiratory syncytial virus matrix protein is mediated by importin beta1 independent of importin alpha

The matrix (M) protein of respiratory syncytial virus (RSV) plays an important role in virus assembly through specific interactions with RSV nucleocapsids and envelope glycoproteins in the cytoplasm as well as with the host cell membrane. We have previously shown that M localizes to the nucleus of infected cells at an early stage in the RSV infection cycle, where it may be instrumental in inhibiting host cell processes. The present study uses transient expression of M as well as a truncated green fluorescent protein (GFP) fusion derivative to show for the first time that M is able to localize in the nucleus in the absence of other RSV gene products, through the action of amino acids 110-183, encompassing the nucleic acid binding regions of the protein, that are sufficient to target GFP to the nucleus. Using native PAGE, ELISA-based binding assays, a novel Alphascreen assay, and an in vitro nuclear transport assay, we show that M is recognized directly by the importin beta1 nuclear import receptor, which mediates its nuclear import in concert with the guanine nucleotide-binding protein Ran. Retention of M in the nucleus through binding to nuclear components, probably mediated by the putative zinc finger domain of M, also contributes to M nuclear accumulation. This is the first report of the importin binding and nuclear import properties of a gene product from a negative sense RNA virus, with implications for the function of RSV M and possibly other viral M proteins in the nucleus of infected cells.

Laminopathy-inducing lamin A mutants can induce redistribution of lamin binding proteins into nuclear aggregates

Lamins, members of the family of intermediate filaments, form a supportive nucleoskeletal structure underlying the nuclear envelope and can also form intranuclear structures. Mutations within the A-type lamin gene cause a variety of degenerative diseases which are collectively referred to as laminopathies. At the molecular level, laminopathies have been shown to be linked to a discontinuous localization pattern of A-type lamins, with some laminopathies containing nuclear lamin A aggregates. Since nuclear aggregate formation could lead to the mislocalization of proteins interacting with A-type lamins, we set out to examine the effects of FLAG-lamin A N195K and R386K protein aggregate formation on the subnuclear distribution of the retinoblastoma protein (pRb) and the sterol responsive element binding protein 1a (SREBP1a) after coexpression as GFP-fusion proteins in HeLa cells. We observed strong recruitment of both proteins into nuclear aggregates. Nuclear aggregate recruitment of the NPC component nucleoporin NUP153 was also observed and found to be dependent on the N-terminus. That these effects were specific was implied by the fact that a number of other coexpressed karyophilic GFP-fusion proteins, such as the nucleoporin NUP98 and kanadaptin, did not coaggregate with FLAG-lamin A N195K or R386K. Immunofluorescence analysis further indicated that the precursor form of lamin A, pre-lamin A, could be found in intranuclear aggregates. Our results imply that redistribution into lamin A-/pre-lamin A-containing aggregates of proteins such as pRb and SREBP1a could represent a key aspect underlying the molecular pathogenesis of certain laminopathies.

Intramolecular masking of nuclear localization signals: analysis of importin binding using a novel AlphaScreen-based method

Active nuclear import of proteins requires the recognition of a nuclear localization sequence (NLS) by members of the importin (IMP) family of proteins. We have developed a modified AlphaScreen-based assay able to estimate the solution binding affinities of such interactions using biotinylated IMPs and His6-tagged NLS-containing proteins. We describe this assay in detail as well as its application in documenting the phenomenon of intramolecular masking of NLSs using recombinant green fluorescent protein (GFP) fusion proteins containing sequences from the SV40 large tumor T antigen (T-ag). We also use it to examine, for the first time, IMP binding to the cancer cell-specific proapoptotic factor viral protein 3 (VP3) from the chicken anemia virus (CAV). High-affinity binding of the IMPalpha/beta heterodimer to the T-ag NLS was observed when the GFP tag was fused to its N terminus but not to its C terminus. Effects of flanking residues were also observed in GFP-T-ag fusion derivatives containing the Thr128 NLS-inactivating mutation, whereby the absence of flanking sequences N terminal to the T-ag NLS appeared to decrease the specificity of the mutation in terms of oblating IMPalpha/beta binding. IMPbeta, but not IMPalpha or the IMPalpha/beta heterodimer, was found to bind to CAV VP3 with high affinity. Interestingly, GFP-VP3(74-121) bound to IMPbeta with threefold higher affinity than the full-length protein, GFP-VP3(1-121), implying that the NLS is masked to a significant extent in the context of full-length protein. This may represent a regulatory mechanism to control nuclear import in a tumor cell-specific fashion.

Expression of nuclear transport importins beta 1 and beta 3 is regulated during rodent spermatogenesis

Spermatogenic differentiation requires progressive gene expression changes, and proteins required for this must be transported into the nucleus. Many of these contain a nuclear localization signal and are likely to be transported by importin protein family members, each of which recognizes and transports distinct cargo proteins. We hypothesized that importins, as modulators of protein nuclear access, would display distinct expression profiles during spermatogenesis, indicating their potential to regulate key steps in cellular differentiation. This was tested throughout testicular development in rodents. Real-time PCR analysis of postnatal mouse testes revealed changing expression levels of Knpb1 (encoding importin beta 1) and Ranbp5 (encoding beta 3) mRNAs, with Knpb1 highest at 26 days postpartum and Ranbp5 highest in Day 26 and adult testis. Their distinctive cellular expression patterns visualized using in situ hybridization and immunohistochemistry were identical in mouse and rat testes where examined. Within the seminiferous epithelium, Knpb1 mRNA and importin beta1 protein were detected within mitotic Sertoli and germ cells during fetal and early postnatal development, becoming restricted to spermatogonia and spermatocytes in adulthood. Importin beta 3 protein in fetal germ cells displayed a striking difference in intracellular localization between male and female gonads. In adult testes, Ranbp5 mRNA was detected in round spermatids and importin beta 3 protein in elongating spermatids. This is the first comprehensive in situ demonstration of developmentally regulated synthesis of nuclear transport components. The contrasting expression patterns of importins beta 1 and 3 identify them as candidates for regulating nuclear access of factors required for developmental switches.

Kinetic properties of nuclear transport conferred by the retinoblastoma (Rb) NLS

The retinoblastoma (RB) tumor suppressor is a nuclear phosphoprotein central to control of cellular proliferation. We have previously shown that human RB possesses an evolutionarily conserved bipartite nuclear localization sequence (NLS) (KRSAEGSNPPKPLKKLR877) resembling that of nucleoplasmin. Here we analyze the kinetic properties of the RB NLS in detail with respect to recognition by cellular nuclear import factors, the importins (IMPs), and nuclear transport properties, comparing results to those for the NLSs from SV40 large tumor antigen (T-ag) and the Xenopus laevis phosphoprotein N1N2. Binding affinities of different IMPalpha subunits for the Rb NLS, in the absence or presence of IMPbeta subunits were determined, and NLS-dependent nuclear import reconstituted in vitro for the first time using purified IMPalpha/beta subunits together with recombinant human RanGDP and nuclear transport factor 2 (NTF2). RB NLS-mediated transport had a strict requirement for all components, with high NTF2 concentrations inhibiting transport. As in the case of transport mediated by the T-ag- and N1N2-NLSs, nuclear import of an RB-NLS containing beta-Gal fusion protein was reduced or abolished when anti-IMPalpha or beta antibody was added to cytosolic extract, respectively, confirming that RB NLS-mediated nuclear import occurs through action of IMPalpha/beta. We conclude that although mediated by IMPalpha/beta, and similar in most respects to transport mediated by the similarly bipartite N1N2 NLS, nuclear import conferred by the RB NLS has distinct properties, in part due to the affinity of its interaction with IMPalpha.

Interaction of the nuclear localizing cytolytic granule serine protease granzyme B with importin alpha or beta: modulation by the serpin inhibitor PI-9

Conditional on perforin-dependent delivery to the nucleus of target cells, the cytolytic granule serine protease granzyme B (GrB) plays a central role in eliciting the nuclear events of apoptosis, as shown by the fact that reducing GrB nuclear entry prevents nuclear apoptosis. Apart from a requirement for cytosolic factors and lack of dependence on the guanine-nucleotide-binding protein Ran, little is known regarding the nuclear import pathway of GrB. In this study we use quantitative yeast two-hybrid and direct binding assays to show that GrB can be recognized independently by either of the nuclear import receptor family members importin (IMP) alpha and beta1, but that these proteins either alone or in combination cannot replace exogenous cytosol to reconstitute GrB nuclear import in vitro. Whereas antibodies to IMP(alpha) inhibit transport, indicating that IMP(alpha) is required for GrB nuclear import, those to IMP(beta) enhance transport, implying that IMP(beta) inhibits GrB nuclear import; consistent with this, the addition of recombinant IMP(beta) but not IMP(alpha) reduces maximal nuclear accumulation in the presence of cytosol. Intriguingly, complexation of GrB with its specific serpin inhibitor PI-9 was found to prevent recognition by IMP(beta) but not by IMP(alpha), and eliminate the apparent requirement for IMP(alpha) for nuclear import. We conclude that GrB nuclear import exhibits complex regulation by IMPs; that heterodimerization with PI-9 can modulate the interaction has implications for protection against apoptosis.

Apoptin nuclear accumulation is modulated by a CRM1-recognized nuclear export signal that is active in normal but not in tumor cells

Tumor cell-specific activity of chicken anemia virus viral protein 3 (VP3 or apoptin) is believed to be dependent on its ability to localize in the nucleus of transformed but not of primary or nontransformed cells. The present study characterizes the signals responsible for the novel nucleocytoplasmic trafficking properties of VP3 using two isogenic tumor/nontumor cell pairs. In addition to the tumor cell-specific nuclear targeting signal, comprising two stretches of basic amino acids in the VP3 COOH terminus which are highly efficient in tumor but not in normal cells, we define the CRM1-recognized nuclear export sequence (NES) within the VP3 tumor cell-specific nuclear targeting signal for the first time. Intriguingly, the NES (amino acids 97-105) is functional in normal but not in tumor cells through the action of the threonine 108 phosphorylation site adjacent to the NES which inhibits its action. In addition, we characterize a leucine-rich sequence (amino acids 33-46) that assists VP3 nuclear accumulation by functioning as a nuclear retention sequence, conferring association with promyelocytic leukemia nuclear bodies. This unique combination of signals is the basis of the tumor cell-specific nuclear targeting abilities of VP3.

Evidence for host-driven selection of the HIV type 1 vpr gene in vivo during HIV disease progression in a transfusion-acquired cohort

An epidemiologically linked HIV-1-infected cohort, in which a nonprogressor donor infected two recipients who progressed to AIDS, was examined. Sequence analysis, over time, of HIV-1 vpr gene quasispecies from uncultured peripheral blood cells revealed an insertion of arginine at position 90 altering a highly conserved C-terminal motif, believed to play a role in Vpr nuclear targeting. Full genome analysis from each patient showed no gene defects in other gene regions, implying that the mutational selection was unique to the vpr gene. A detailed analysis of the vpr quasispecies showed very little amino acid diversity in the nonprogressing donor, whereas, following viral transmission, the amino acid diversity increased dramatically over time in tandem with disease progression in the two recipients. Although the R insertion at position 90 was present in all three individuals, the variable degree of additional amino acid changes over time may have influenced HIV disease in the nonprogressor donor and the two progressing recipients. These data provide the first evidence in favor of vpr gene evolution over time, which was host-driven. The status of the nonprogressing donor was consistent with a highly protective B-57 HLA type, which was absent in the two progressing recipients, implying a role for host HLA type and other immunologic selective pressures in vpr gene selection in vivo.

Identification of Domains of Ataxia-telangiectasia Mutated Required for Nuclear Localization and Chromatin Association

Ataxia-telangiectasia mutated (ATM) is essential for rapid induction of cellular responses to DNA double strand breaks (DSBs). In this study, we mapped a nuclear localization signal (NLS), 385KRKK388, within the amino terminus of ATM and demonstrate its recognition by the conventional nuclear import receptor, the importin alpha1/beta1 heterodimer. Although mutation of this NLS resulted in green fluorescent protein (GFP) x ATM(NLSm) localizing predominantly within the cytoplasm, small amounts of nuclear GFP x ATM(NLSm) were still sufficient to elicit a DNA damage response. Insertion of an heterologous nuclear export signal between GFP and ATM(NLSm) resulted in complete cytoplasmic localization of ATM, concomitantly reducing the level of substrate phosphorylation and increasing radiosensitivity, which indicates a functional requirement for ATM nuclear localization. Interestingly, the carboxyl-terminal half of ATM, containing the kinase domain, which localizes to the cytoplasm, could not autophosphorylate itself or phosphorylate substrates, nor could it correct radiosensitivity in response to DSBs even when targeted to the nucleus by insertion of an exogenous NLS, demonstrating that the ATM amino terminus is required for optimal ATM function. Moreover, we have shown that the recruitment/retention of ATM at DSBs requires its kinase activity because a kinase-dead mutant of GFP x ATM failed to form damage-induced foci. Using deletion mutation analysis we mapped a domain in ATM (amino acids 5-224) required for its association with chromatin, which may target ATM to sites of DNA damage. Combined, these data indicate that the amino terminus of ATM is crucial not only for nuclear localization but also for chromatin association, thereby facilitating the kinase activity of ATM in vivo.

Towards safe, non-viral therapeutic gene expression in humans

The potential dangers of using viruses to deliver and integrate DNA into host cells in gene therapy have been poignantly highlighted in recent clinical trials. Safer, non-viral gene delivery approaches have been largely ignored in the past because of their inefficient delivery and the resulting transient transgene expression. However, recent advances indicate that efficient, long-term gene expression can be achieved by non-viral means. In particular, integration of DNA can be targeted to specific genomic sites without deleterious consequences and it is possible to maintain transgenes as small episomal plasmids or artificial chromosomes. The application of these approaches to human gene therapy is gradually becoming a reality.

Regulation of Nuclear Transport: Central Role in Development and Transformation?

Transport of macromolecules into and out of the nucleus is generally effected by targeting signals that are recognized by specific members of the importin/exportin transport receptor family. The latter mediate passage through the nuclear envelope-embedded nuclear pore complexes (NPCs) by conferring interaction with NPC constituents, as well as with other components of the nuclear transport machinery, including the guanine nucleotide-binding protein Ran. Importantly, nuclear transport is regulated at multiple levels via a diverse range of mechanisms, such as the modulation of the accessibility and affinity of target signal recognition by importins/exportins, with phosphorylation/dephosphorylation as a major mechanism. Alteration of the level of the expression of components of the nuclear transport machinery also appears to be a key determinant of transport efficiency, having central importance in development, differentiation and transformation.