Protein import into nuclei: association and dissociation reactions involving transport substrate, transport factors, and nucleoporins

HIV-1 matrix protein: a mysterious regulator of the viral life cycle

Significant progress has been achieved in the last few years concerning the human immunodeficiency virus (HIV-1) life cycle, mostly in the fields of cellular receptors for the virus, virus assembly and budding of virus particles from the cell surface. Meanwhile, some aspects, such as postentry events, virus maturation and the regulatory role of individual viral proteins remain poorly defined. This review summarizes some recent findings concerning the role of Gag Pr55 and its proteolytic processing in the HIV-1 life cycle with particular emphasis on the functions of matrix protein p17 (MA), the protein which plays a key role in regulation of the early and late steps of viral morphogenesis. Based on our recent observations, the possibility is discussed that two subsets of MA exist, one cleaved from the Gag precursor in the host cell (cMA), and the other cleaved in the virions (vMA). It is suggested that two MA fractions possess diverse functions and are involved in different stages of virus morphogenesis as key regulators of the viral life cycle.

New approach to prevent myocardial hypertrophy: the import blocking peptide

Calcineurin, a serine/threonine phosphatase, plays a crucial role in the development of myocardial hypertrophy. Calcineurin is a cytosolic phosphatase that dephosphorylates the nuclear factor of activated T cells (NFAT), a transcription factor. Until now, it has been postulated that dephosphorylated NFAT is shuttled into the nucleus. Recent evidence demonstrates that not only NFAT, but also calcineurin, is localized in the nucleus. Once calcineurin and NFAT enter the nucleus of cardiomyocytes, transcription of genes that are characteristic for myocardial hypertrophy (e.g., brain natriuretic peptide and atrial natriuretic peptide) occurs. Although the exact nuclear function of calcineurin remains unclear, its co-existence with NFAT is important for the full transcriptional activity of the calcineurin/NFAT signaling cascade. The principal effect of nuclear calcineurin is likely the prolonged nuclear retention period of NFAT. Potential effects of nuclear calcineurin include an antagonistic function to glycogen synthase kinase 3beta, which phosphorylates NFAT for its export out of the nucleus, or direct antagonization of the export of NFAT, catalyzed by the chromosome region maintenance 1, which would leave NFAT nuclear. The nuclear localization sequence (NLS) region at the amino acid sequence from position 172 to 183 of calcineurin Abeta is essential for shuttling calcineurin into the nucleus by importinbeta(1). A synthetic import blocking peptide (IBP) that mimics the nuclear localization sequence of calcineurin was generated. The NLS analog on IBP saturates the calcineurin binding site of importinbeta(1). This prevents the binding of calcineurin to importin and inhibits the nuclear shuttling of calcineurin. Inhibition of the calcineurin/importinbeta(1) interaction by competing synthetic peptides represents a new approach to the inhibition of the development of myocardial hypertrophy.

Identification of genes related to nasopharyngeal carcinoma with the help of pathway-based networks

cDNA microarray is a powerful tool to analyze simultaneously the expression levels of tens of thousands of genes. Compared with normal nasopharynx (NP) tissues, 2210 genes were highly differentially expressed in nasopharyngeal carcinoma (NPC) tissues detected by cDNA microarray. Since signal pathway is widely used to describe the complex relationship between genes, a pathway-based network was constructed to visualize the connection between the genes obtained from microarray data in this report. We analyzed the targeted genes that may have more important influence on this gene network with statistical methods and found that some genes might have significant influence on this network, especially Ras-related nuclear protein (RAN), carboxyl ester lipase (CEL), v-rel reticuloendotheliosis viral oncogene homolog A (RELA) genes. To verify the results from pathway-based selection, reverse transcription-polymerase chain reaction (RT-PCR) and real-time RT-PCR were performed to detect the expression levels of RAN, CEL and RELA genes and it was found that the RAN and CEL genes were significantly up-regulated in more than 80% of NPC tissues. To further elucidate the function of the RAN gene, RAN expression was specifically suppressed in a 5-8F NPC cell line by RNA interference (RNAi). As expected, the depletion of RAN could effectively block the proliferation of tumor cells. Therefore, our study may open up a new way to analyze the vast microarray data.

Studying nuclear protein import in yeast

The yeast Saccharomyces cerevisiae is a common model organism for biological discovery. It has become popularized primarily because it is biochemically and genetically amenable for many fundamental studies on eukaryotic cells. These features, as well as the development of a number of procedures and reagents for isolating protein complexes, and for following macromolecules in vivo, have also fueled studies on nucleo-cytoplasmic transport in yeast. One limitation of using yeast to study transport has been the absence of a reconstituted in vitro system that yields quantitative data. However, advances in microscopy and data analysis have recently enabled quantitative nuclear import studies, which, when coupled with the significant advantages of yeast, promise to yield new fundamental insights into the mechanisms of nucleo-cytoplasmic transport.

Association of nuclear pore FG-repeat domains to NTF2 import and export complexes

Transport into and out of the nucleus is regulated by the nuclear pore complex. Vital to this regulation are nuclear pore proteins with FG sequence repeats, which have been shown to be crucial for cell viability and which interact with nuclear transport receptors. Here we use molecular dynamics simulations to investigate the binding of FG-repeat peptides to the surface of NTF2, the Ran importer. The simulations, covering over 254 ns, agree with previous X-ray, mutational, NMR, and computational data in identifying four binding spots. They also serve to provide an all-atom view of binding at each spot, whereas FG-repeat binding has been only directly observed at a single spot. Furthermore, the simulations identify two novel binding spots in addition to the four others. All six binding spots broadly form a stripe across the surface of NTF2. The resulting regularity and proximity of binding spots on the surface may be necessary for identification of the transport receptor by the FG-repeats in the nuclear pore complex and for the successful transit of NTF2 through the pore.

Significant proportions of nuclear transport proteins with reduced intracellular mobilities resolved by fluorescence correlation spectroscopy

Nuclear transport requires freely diffusing nuclear transport proteins to facilitate movement of cargo molecules through the nuclear pore. We analyzed dynamic properties of importin alpha, importin beta, Ran and NTF2 in nucleus, cytoplasm and at the nuclear pore of neuroblastoma cells using fluorescence correlation spectroscopy. Mobile components were quantified by global fitting of autocorrelation data from multiple cells. Immobile components were quantified by analysis of photobleaching kinetics. Wild-type Ran was compared to various mutant Ran proteins to identify components representing GTP or GDP forms of Ran. Untreated cells were compared to cells treated with nocodazole or latrunculin to identify components associated with cytoskeletal elements. The results indicate that freely diffusing importin alpha, importin beta, Ran and NTF2 are in dynamic equilibrium with larger pools associated with immobile binding partners such as microtubules in the cytoplasm. These findings suggest that formation of freely diffusing nuclear transport intermediates is in competition with binding to immobile partners. Variation in concentrations of freely diffusing nuclear transport intermediates among cells indicates that the nuclear transport system is sufficiently robust to function over a wide range of conditions.

Nuclear localization signal and protein context both mediate importin alpha specificity of nuclear import substrates

The "classical" nuclear protein import pathway depends on importin alpha and importin beta. Importin alpha binds nuclear localization signal (NLS)-bearing proteins and functions as an adapter to access the importin beta-dependent import pathway. In humans, only one importin beta is known to interact with importin alpha, while six alpha importins have been described. Various experimental approaches provided evidence that several substrates are transported specifically by particular alpha importins. Whether the NLS is sufficient to mediate importin alpha specificity is unclear. To address this question, we exchanged the NLSs of two well-characterized import substrates, the seven-bladed propeller protein RCC1, preferentially transported into the nucleus by importin alpha3, and the less specifically imported substrate nucleoplasmin. In vitro binding studies and nuclear import assays revealed that both NLS and protein context contribute to the specificity of importin alpha binding and transport.

Nuclear import time and transport efficiency depend on importin beta concentration

Although many components and reaction steps necessary for bidirectional transport across the nuclear envelope (NE) have been characterized, the mechanism and control of cargo migration through nuclear pore complexes (NPCs) remain poorly understood. Single-molecule fluorescence microscopy was used to track the movement of cargos before, during, and after their interactions with NPCs. At low importin β concentrations, about half of the signal-dependent cargos that interacted with an NPC were translocated across the NE, indicating a nuclear import efficiency of ∼50%. At high importin β concentrations, the import efficiency increased to ∼80% and the transit speed increased approximately sevenfold. The transit speed and import efficiency of a signal-independent cargo was also increased by high importin β concentrations. These results demonstrate that maximum nucleocytoplasmic transport velocities can be modulated by at least ∼10-fold by the importin β concentration and therefore suggest a potential mechanism for regulating the speed of cargo traffic across the NE.

Nuclear Localization Signal Receptor Affinity Correlates with in Vivo Localization in Saccharomyces cerevisiae

Nuclear localization signals (NLSs) target proteins into the nucleus through mediating interactions with nuclear import receptors. Here, we perform a quantitative analysis of the correlation between NLS receptor affinity and the steady-state distribution of NLS-bearing cargo proteins between the cytoplasm and the nucleus of live yeast, which reflects the relative import rates of various NLS sequences. We find that there is a complicated, but monotonic quantitative relationship between the affinity of an NLS for the import receptor, importin alpha, and the steady-state accumulation of the cargo in the nucleus. This analysis takes into consideration the impact of protein size. In addition, the hypothetical upper limit to an NLS affinity for the receptors is explored through genetic approaches. Overall, our results indicate that there is a correlation between the binding affinity of an NLS cargo for the NLS receptor, importin alpha, and the import rate for this cargo. This correlation, however, is not maintained for cargoes that bind to the NLS receptor with very weak or very strong affinity.

Flexible phenylalanine-glycine nucleoporins as entropic barriers to nucleocytoplasmic transport

Natively unfolded phenylalanine-glycine (FG)-repeat domains are alleged to form the physical constituents of the selective barrier-gate in nuclear pore complexes during nucleocytoplasmic transport. Presently, the biophysical mechanism behind the selective gate remains speculative because of a lack of information regarding the nanomechanical properties of the FG domains. In this work, we have applied the atomic force microscope to measure the mechanical response of individual and clusters of FG molecules. Single-molecule force spectroscopy reveals that FG molecules are unfolded and highly flexible. To provide insight into the selective gating mechanism, an experimental platform has been constructed to study the collective behavior of surface-tethered FG molecules at the nanoscale. Measurements indicate that the collective behavior of such FG molecules gives rise to an exponentially decaying long-range steric repulsive force. This finding indicates that the molecules are thermally mobile in an extended polymer brush-like conformation. This assertion is confirmed by observing that the brush-like conformation undergoes a reversible collapse transition in less polar solvent conditions. These findings reveal how FG-repeat domains may simultaneously function as an entropic barrier and a selective trap in the near-field interaction zone of nuclear pore complexes; i.e., selective gate.

Multiple importins function as nuclear transport receptors for the Rev protein of human immunodeficiency virus type 1

The Rev protein of human immunodeficiency virus type 1 is an RNA-binding protein that is required for nuclear export of unspliced and partially spliced viral mRNAs. Nuclear import of human immunodeficiency virus type 1 Rev has been suggested to depend on the classic nuclear transport receptor importin beta, but not on the adapter protein importin alpha. We now show that, similar to importin alpha, Rev is able to dissociate RanGTP from recycling importin beta, a reaction that leads to the formation of a novel import complex. Besides importin beta, the transport receptors transportin, importin 5, and importin 7 specifically interact with Rev and promote its nuclear import in digitonin-permeabilized cells. A single arginine-rich nuclear localization sequence of Rev is required for interaction with all importins tested so far. In contrast to the importin beta-binding domain of importin alpha, Rev interacts with an N-terminal fragment of importin beta. Transportin contains two independent binding sites for Rev. Hence, the mode of interaction of importin beta and transportin with Rev is clearly distinct from that with their classic import cargoes. Taken together, the viral protein takes advantage of multiple cellular transport pathways for its nuclear accumulation.

Kap120 functions as a nuclear import receptor for ribosome assembly factor Rpf1 in yeast

The nucleocytoplasmic exchange of macromolecules is mediated by receptors specialized in passage through the nuclear pore complex. The majority of these receptors belong to the importin beta protein family, which has 14 members in Saccharomyces cerevisiae. Nine importins carry various cargos from the cytoplasm into the nucleus, whereas four exportins mediate nuclear export. Kap120 is the only receptor whose transport cargo has not been found previously. Here, we characterize Kap120 as an importin for the ribosome maturation factor Rpf1, which was identified in a two-hybrid screen. Kap120 binds directly to Rpf1 in vitro and is released by Ran-GTP. At least three parallel import pathways exist for Rpf1, since nuclear import is defective in strains with the importins Kap120, Kap114, and Nmd5 deleted. Both kap120 and rpf1 mutants accumulate large ribosomal subunits in the nucleus. The nuclear accumulation of 60S ribosomal subunits in kap120 mutants is abolished upon RPF1 overexpression, indicating that Kap120 does not function in the actual ribosomal export step but rather in import of ribosome maturation factors.

The N-terminal domain of the mammalian nucleoporin p62 interacts with other nucleoporins of the FXFG family during interphase

Nuclear pore complexes (NPCs) provide the only sites for macromolecular transport between nucleus and cytoplasm. The nucleoporin p62, a component of higher eukaryotic NPCs, is located at the central gated channel and involved in nuclear trafficking of various cargos. p62 is organized into an N-terminal segment that contains FXFG repeats and binds the soluble transport factor NTF2, whereas the C-terminal portion associates with other nucleoporins and importin-beta1. We have now identified new components that interact specifically with the p62 N-terminal domain. Using the p62 N-terminal segment as bait, we affinity-purified nucleoporins Nup358, Nup214 and Nup153 from crude cell extracts. In ligand binding assays, the N-terminal p62 segment associated with Nup358 and p62, suggesting their direct binding to the p62 N-terminal portion. Furthermore, p62 was isolated in complex with Nup358, Nup214 and Nup153 from growing HeLa cells, indicating that the interactions Nup358/p62, Nup214/p62 and p62/Nup153 also occur in vivo. The formation of Nup358/p62 and p62/Nup153 complexes was restricted to interphase cells, whereas Nup214/p62 binding was detected in interphase as well as during mitosis. Our results support a model of complex interactions between FXFG containing nucleoporins, and we propose that some of these interactions may contribute to the movement of cargo across the NPC.

NuSAP, a mitotic RanGTP target that stabilizes and cross-links microtubules

Nucleolar and spindle-associated protein (NuSAP) was recently identified as a microtubule- and chromatin-binding protein in vertebrates that is nuclear during interphase. Small interfering RNA-mediated depletion of NuSAP resulted in aberrant spindle formation, missegregation of chromosomes, and ultimately blocked cell proliferation. We show here that NuSAP is enriched on chromatin-proximal microtubules at meiotic spindles in Xenopus oocytes. When added at higher than physiological levels to Xenopus egg extract, NuSAP induces extensive bundling of spindle microtubules and causes bundled microtubules within spindle-like structures to become longer. In vitro reconstitution experiments reveal two direct effects of NuSAP on microtubules: first, it can efficiently stabilize microtubules against depolymerization, and second, it can cross-link large numbers of microtubules into aster-like structures, thick fibers, and networks. With defined components we show that the activity of NuSAP is differentially regulated by Importin (Imp) alpha, Impbeta, and Imp7. While Impalpha and Imp7 appear to block the microtubule-stabilizing activity of NuSAP, Impbeta specifically suppresses aspects of the cross-linking activity of NuSAP. We propose that to achieve full NuSAP functionality at the spindle, all three importins must be dissociated by RanGTP. Once activated, NuSAP may aid to maintain spindle integrity by stabilizing and cross-linking microtubules around chromatin.

The mobile nucleoporin Nup2p and chromatin-bound Prp20p function in endogenous NPC-mediated transcriptional control

Nuclear pore complexes (NPCs) govern macromolecular transport between the nucleus and cytoplasm and serve as key positional markers within the nucleus. Several protein components of yeast NPCs have been implicated in the epigenetic control of gene expression. Among these, Nup2p is unique as it transiently associates with NPCs and, when artificially tethered to DNA, can prevent the spread of transcriptional activation or repression between flanking genes, a function termed boundary activity. To understand this function of Nup2p, we investigated the interactions of Nup2p with other proteins and with DNA using immunopurifications coupled with mass spectrometry and microarray analyses. These data combined with functional assays of boundary activity and epigenetic variegation suggest that Nup2p and the Ran guanylyl-nucleotide exchange factor, Prp20p, interact at specific chromatin regions and enable the NPC to play an active role in chromatin organization by facilitating the transition of chromatin between activity states.

Nuclear import and export of plant virus proteins and genomes

SUMMARY Nuclear import and export are crucial processes for any eukaryotic cell, as they govern substrate exchange between the nucleus and the cytoplasm. Proteins involved in the nuclear transport network are generally conserved among eukaryotes, from yeast and fungi to animals and plants. Various pathogens, including some plant viruses, need to enter the host nucleus to gain access to its replication machinery or to integrate their DNA into the host genome; the newly replicated viral genomes then need to exit the nucleus to spread between host cells. To gain the ability to enter and exit the nucleus, these pathogens encode proteins that recognize cellular nuclear transport receptors and utilize the host's nuclear import and export pathways. Here, we review and discuss our current knowledge about the molecular mechanisms by which plant viruses find their way into and out of the host cell nucleus.

From the trap to the basket: getting to the bottom of the nuclear pore complex

Nuclear pore complexes (NPCs) are large supramolecular assemblies that perforate the double-membraned nuclear envelope and serve as the sole gateways of molecular exchange between the cytoplasm and the nucleus in interphase cells. Combining novel specimen preparation regimes with innovative use of high-resolution scanning electron microscopy, Hans Ris produced in the late eighties stereo images of the NPC with unparalleled clarity and structural detail, thereby setting new standards in the field. Since that time, efforts undertaken to resolve the molecular structure and architecture, and the numerous interactions that occur between NPC proteins (nucleoporins), soluble transport receptors, and the small GTPase Ran, have led to a deeper understanding of the functional role of NPCs in nucleocytoplasmic transport. In spite of these breakthroughs, getting to the bottom of the actual cargo translocation mechanism through the NPC remains elusive and controversial. Here, we review recent insights into NPC function by correlating structural findings with biochemical data. By introducing new experimental and computational results, we reexamine how NPCs can discriminate between receptor-mediated and passive cargo to promote vectorial translocation in a highly regulated manner. Moreover, we comment on the importance and potential benefits of identifying and experimenting with individual key components implicated in the translocation mechanism. We conclude by dwelling on questions that we feel are pertinent to a more rational understanding of the physical aspects governing NPC mechanics. Last but not least, we substantiate these uncertainties by boldly suggesting a new direction in NPC research as a means to verify such novel concepts, for example, a de novo designed 'minimalist' NPC.

Transportin is a major nuclear import receptor for c-Fos: a novel mode of cargo interaction

c-Fos, a component of the transcription factor AP-1, is rapidly imported into the nucleus after translation. We established an in vitro system using digitonin-permeabilized cells to analyze nuclear import of c-Fos in detail. Two import receptors of the importin beta superfamily, importin beta itself and transportin, promote import of c-Fos in vitro. Under conditions where importin beta-dependent transport was blocked, c-Fos still accumulated in the nucleus in the presence of cytosol. Inhibition of the transportin-dependent pathway, in contrast, abolished import of c-Fos. Furthermore, c-Fos mutants that interact with transportin but not with importin beta were efficiently imported in the presence of cytosol. Hence, transportin appears to be the predominant import receptor for c-Fos. A detailed biochemical characterization revealed that the interaction of transportin with c-Fos is distinct from the interaction with its established import cargoes, the M9 sequence of heterogeneous nuclear ribonucleoprotein A1 or the nuclear localization sequence of some basic proteins. Likewise, the binding sites on importin beta for its classic import cargo and for c-Fos can be separated. In summary, c-Fos employs a novel mode of receptor-cargo interaction. Hence, transportin may be as versatile as importin beta in recognizing different nuclear import cargoes.

Introduction to nucleocytoplasmic transport: molecules and mechanisms

Nucleocytoplasmic transport, the exchange of matter between nucleus and cytoplasm, plays a fundamental role in human and other eukaryotic cells, affecting almost every aspect of health and disease. The only gate for the transport of small and large molecules as well as supramolecular complexes between nucleus and cytoplasm is the nuclear pore complex (NPC). The NPC is not a normal membrane transport protein (transporter). Composed of 500 to 1000 peptide chains, the NPC features a mysterious functional duality. For most molecules, it constitutes a molecular sieve with a blurred cutoff at approx 10 nm, but for molecules binding to phenylalanine-glycine (FG) motifs, the NPC appears to be a channel of approx 50 nm diameter, permitting bidirectional translocation at high speed. To achieve this, the NPC cooperates with soluble factors, the nuclear transport receptors, which shuttle between nuclear contents and cytoplasm. Here, we provide a short introduction to nucleocytoplasmic transport by describing first the structure and composition of the nuclear pore complex. Then, mechanisms of nucleocytoplasmic transport are discussed. Finally, the still essentially unresolved mechanisms by which nuclear transport receptors and transport complexes are translocated through the nuclear pore complex are considered, and a novel translocation model is suggested.

Binding Dynamics of Isolated Nucleoporin Repeat Regions to Importin-β

The nuclear pore complex, through the interaction of its proteins with transport receptors, controls the transport of large molecules into and out of the cell's nucleus. There is ample evidence for proteins with FG sequence repeats playing an essential role in this control. Previous studies have elucidated binding spots for FG sequence repeats on the surface of the transport receptor importin-beta by X-ray crystallography and mutational studies. Molecular dynamics simulations have been performed to characterize the interaction of FG sequence repeats with the transport receptor. Observed binding spots have been verified and novel sites discovered, suggesting that importin-beta features many more binding spots than suspected so far. The observed binding spots are in accord with several models of nucleocytoplasmic transport, and the large number of binding spots on importin-beta may be necessary for the pore complex to distinguish between importin-beta and inert proteins, and to allow for its passage through the pore.

Oxadiazols: a new class of rationally designed anti-human immunodeficiency virus compounds targeting the nuclear localization signal of the viral matrix protein

Despite recent progress in anti-human immunodeficiency virus (HIV) therapy, drug toxicity and emergence of drug-resistant isolates during long-term treatment of HIV-infected patients necessitate the search for new targets that can be used to develop novel antiviral agents. One such target is the process of nuclear translocation of the HIV preintegration complex. Previously we described a class of arylene bis(methylketone) compounds that inhibit HIV-1 nuclear import by targeting the nuclear localization signal (NLS) in the matrix protein (MA). Here we report a different class of MA NLS-targeting compounds that was selected using computer-assisted drug design. The leading compound from this group, ITI-367, showed potent anti-HIV activity in cultures of T lymphocytes and macrophages and also inhibited HIV-1 replication in ex vivo cultured lymphoid tissue. The virus carrying inactivating mutations in MA NLS was resistant to ITI-367. Analysis by real-time PCR demonstrated that the compound specifically inhibited nuclear import of viral DNA, measured by two-long terminal repeat circle formation. Evidence of the existence of this mechanism was provided by immunofluorescent microscopy, using fluorescently labeled HIV-1, which demonstrated retention of the viral DNA in the cytoplasm of drug-treated macrophages. Compounds inhibiting HIV-1 nuclear import may be attractive candidates for further development.

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.

Identification of karyopherin beta as an immunogenic antigen of the malaria parasite using immune mice and human sera

A differential immunoscreening of the lambdagt11 Plasmodium falciparum genomic expression library was carried out using anti-P. yoelii sera (convalescent-phase mouse sera) and immune sera collected from healthy adults, to identify novel cross-reactive and possibly protective antigens of the parasite. One clone, with an insert size of 1132 bp that reacted strongly with both the sera was selected. The insert was found to be a part of the P. falciparum karyopherin beta (PfKbeta) homologue. RT-PCR and Northern blot analysis confirmed the expression of PfKbeta in the blood stages of the parasite. The approximately 110 kDa protein was localized in the cytoplasm at the ring and trophozoite, and in the parasitophorous vacuole at the schizont stage. Two large fragments of PfKbeta representing the N- and C-terminal halves were expressed in E. coli. The recombinant proteins were highly immunogenic in mice, and also found to be the target for immune response in natural infections of Plasmodium spp. Anti-sera against the protein showed a low level of anti-parasitic activity. Immunization with recombinant PfKbeta fragments was only partially protective against a heterologous challenge infection in mice. Our results show that the parasite releases a highly immunogenic, cytoplasmic protein into the host which may not contribute to the development of protective immunity.

Translocation through the nuclear pore complex: selectivity and speed by reduction-of-dimensionality

Translocation through the nuclear pore complex (NPC), a large transporter spanning the nuclear envelope, is a passive, diffusion-driven process, paradoxically enhanced by binding. To account for this mystery, several models have been suggested. However, recent experiments with modified NPCs make reconsideration necessary. Here, we suggest that nuclear transport receptors (NTRs) such as the karyopherins, in accordance with their peculiar boat-like structure, act as nanoscopic ferries transporting cargos through the NPC by sliding on a surface of phenylalanine glycine (FG) motifs. The dense array of FG motifs that covers the cytoplasmic filaments of the NPC is thought to continue on the wall of the large channel permeating the central framework of the NPC and on parts of the nuclear filaments to yield a coherent FG surface. Nuclear transport receptors are assumed to bind to the FG surface at filaments or at the channel entrance and then to rapidly search the FG surface by a two-dimensional random walk for the channel exit where they are released. The passage of neutral molecules is restricted to a narrow tube in the center of the central channel by a loose network of peptide chains. The model features virtual gating, is compatible with but not dependent on FG affinity gradients and tolerates deletions and transpositions of FG motifs. Implications of the model are discussed and tests are suggested.

Subunits of the heterotrimeric transcription factor NF-Y are imported into the nucleus by distinct pathways involving importin beta and importin 13

The transcriptional activator NF-Y is a heterotrimeric complex composed of NF-YA, NF-YB, and NF-YC, which specifically binds the CCAAT consensus present in about 30% of eukaryotic promoters. All three subunits contain evolutionarily conserved core regions, which comprise a histone fold motif (HFM) in the case of NF-YB and NF-YC. Our results of in vitro binding studies and nuclear import assays reveal two different transport mechanisms for NF-Y subunits. While NF-YA is imported by an importin beta-mediated pathway, the NF-YB/NF-YC heterodimer is translocated into the nucleus in an importin 13-dependent manner. We define a nonclassical nuclear localization signal (ncNLS) in NF-YA, and mutational analysis indicates that positively charged amino acid residues in the ncNLS are required for nuclear targeting of NF-YA. Importin beta binding is restricted to the monomeric, uncomplexed NF-YA subunit. In contrast, the nuclear import of NF-YB and NF-YC requires dimer formation. Only the NF-YB/NF-YC dimer, but not the monomeric components, are recognized by importin 13 and are imported into the nucleus. Importin 13 competes with NF-YA for binding to the NF-YB/NF-YC dimer. Our data suggest that a distinct binding platform derived from the HFM of both subunits, NF-YB/NF-YC, mediates those interactions.

Nanoparticle-mediated wild-type p53 gene delivery results in sustained antiproliferative activity in breast cancer cells

Gene expression with nonviral vectors is usually transient and lasts for only a few days. Therefore, repeated injection of the expression vector is required to maintain a therapeutic protein concentration in the target tissue. Biodegradable nanoparticles (approximately 200 nm diameter) formulated using a biocompatible polymer, poly(D,L-lactide-co-glycolide) (PLGA), have the potential for sustained gene delivery. Our hypothesis is that nanoparticle-mediated gene delivery would result in sustained gene expression, and hence better efficacy with a therapeutic gene. In this study, we have determined the antiproliferative activity of wild-type (wt) p53 gene-loaded nanoparticles in a breast cancer cell line. Nanoparticles containing plasmid DNA were formulated using a multiple-emulsion-solvent evaporation technique. To understand the mechanism of sustained gene expression with nanoparticles, we monitored the intracellular trafficking of both the nanoparticles and the nanoparticle-entrapped DNA, and also determined p53 mRNA levels over a period of time. Cells transfected with wt-p53 DNA-loaded nanoparticles demonstrated a sustained and significantly greater antiproliferative effect than those with naked wt-p53 DNA or wt-p53 DNA complexed with a commercially available transfecting agent (Lipofectamine). Cells transfected with wt-p53 DNA-loaded nanoparticles demonstrated sustained p53 mRNA levels compared to cells which were transfected with naked wt-p53 DNA or the wt-p53 DNA-Lipofectamine complex, thus explaining the sustained antiproliferative activity of nanoparticles. Studies with fluorescently labeled DNA using confocal microscopy and quantitative analyses using a microplate reader demonstrated sustained intracellular localization of DNA with nanoparticles, suggesting the slow release of DNA from nanoparticles localized inside the cells. Cells which were transfected with naked DNA demonstrated transient intracellular DNA retention. In conclusion, nanoparticle-mediated wt-p53 gene delivery results in sustained antiproliferative activity, which could be therapeutically beneficial in cancer treatment.

Crystallization and preliminary X-ray diffraction analysis of importin-alpha complexed with NLS peptidomimetics

Importin-alpha is the nuclear import receptor that recognizes cargo proteins with nuclear localization sequences (NLSs). The study of NLS peptidomimetics can provide a better understanding of the requirements for the molecular recognition of cargo proteins by importin-alpha, and potentially engender a large number of applications in medicine. Importin-alpha was crystallized with a set of six NLS peptidomimetics, and X-ray diffraction data were collected in the range 2.1-2.5 A resolution. Preliminary electron density calculations show that the ligands are present in the crystals.

Coat proteins of Rice tungro bacilliform virus and Mungbean yellow mosaic virus contain multiple nuclear-localization signals and interact with importin alpha

Transport of the viral genome into the nucleus is an obligatory step in the replication cycle of plant pararetro- and geminiviruses. In both these virus types, the multifunctional coat protein (CP) is thought to be involved in this process. Here, a green fluorescent protein tagging approach was used to demonstrate nuclear import of the CPs of Rice tungro bacilliform virus (RTBV) and Mungbean yellow mosaic virus--Vigna (MYMV) in Nicotiana plumbaginifolia protoplasts. In both cases, at least two nuclear localization signals (NLSs) were identified and characterized. The NLSs of RTBV CP are located within both N- and C-terminal regions (residues 479KRPK/497KRK and 744KRK/758RRK), and those of MYMV CP within the N-terminal part (residues 3KR and 41KRRR). The MYMV and RTBV CP NLSs resemble classic mono- and bipartite NLSs, respectively. However, the N-terminal MYMV CP NLS and both RTBV CP NLSs show peculiarities in the number and position of basic residues. In vitro pull-down assays revealed interaction of RTBV and MYMV CPs with the nuclear import factor importin alpha, suggesting that both CPs are imported into the nucleus via an importin alpha-dependent pathway. The possibility that this pathway could serve for docking of virions to the nucleus is discussed.

Identification of nuclear import and export signals within Fli-1: roles of the nuclear import signals in Fli-1-dependent activation of megakaryocyte-specific promoters

The Ets factor Friend leukemia integration 1 (Fli-1) is an important regulator of megakaryocytic (Mk) differentiation. Here, we demonstrate two novel nuclear localization signals (NLSs) within Fli-1: one (NLS1) is located at the N terminus, and another (NLS2) is within the Ets domain. Nuclear accumulation of Fli-1 reflected the combined functional effects of the two discrete NLSs. Each NLS can independently direct nuclear transport of a carrier protein, with mutations within the NLSs affecting nuclear accumulation. NLS1 has a bipartite motif, whereas the NLS2 region contains a nonclassical NLS. Both NLSs bind importin alpha (IMPalpha) and IMPbeta, with NLS1 and NLS2 being predominantly recognized by IMPalpha and IMPbeta, respectively. Fli-1 also contains one nuclear export signal. Leptomycin B abolished its cytoplasmic accumulation, showing CRM1 dependency. We demonstrate that Ets domain binding to specific target DNA effectively blocks IMP binding, indicating that the targeted DNA binding plays a role in localizing Fli-1 to its destination and releasing IMPs for recycling back to the cytoplasm. Finally, by analyzing full-length Fli-1 carrying NLS1, NLS2, and combined NLS1-NLS2 mutations, we conclude that two functional NLSs exist in Fli-1 and that each NLS is sufficient to target Fli-1 to the nucleus for activation of Mk-specific genes.

Reconstitution of Nup157 and Nup145N into the Nup84 Complex*[boxs]

About 30 different nucleoporins (Nups) constitute the nuclear pore complex. We have affinity-purified 28 of these nuclear pore proteins and identified new nucleoporin interactions by this analysis. We found that Nup157 and Nup170, two members of the large structural Nups, and the Gly-Leu-Phe-Gly nucleoporin Nup145N specifically co-purified with members of the Nup84 complex. In addition, Nup145N co-enriched during Nup157 purification. By in vitro reconstitution, we demonstrate that Nup157 and Nup145N form a nucleoporin subcomplex. Moreover, we show that Nup157 and Nup145N bind to the heptameric Nup84 complex. This assembly thus represents approximately one-third of all nucleoporins. To characterize Nup157 structurally, we purified and analyzed it by electron microscopy. Nup157 is a hollow sphere that resembles a clamp or a gripping hand. Thus, we could reconstitute an interaction between a large structural Nup, an FG repeat Nup, and a major structural module of the nuclear pore complex.

Structural basis for the high-affinity binding of nucleoporin Nup1p to the Saccharomyces cerevisiae importin-beta homologue, Kap95p

Macromolecules are transported across the nuclear envelope most frequently by karyopherin/importin-beta superfamily members that are constructed from HEAT repeats. Transport of Kap95p (yeast importin-beta), the principal carrier for protein import, through nuclear pore complexes is facilitated by interactions with nucleoporins containing FG repeats. However, Nup1p interacts more strongly with Kap95p than other FG-nucleoporins. To establish the basis of this increased affinity, we determined the structure of Kap95p complexed with Nup1p residues 963-1076 that contain the high-affinity Kap95p binding site. Nup1p binds Kap95p at three sites between the outer A-helices of HEAT repeats 5, 6, 7 and 8. At each site, phenylalanine residues from Nup1p are buried in hydrophobic depressions between adjacent HEAT repeats. Although the Nup1p and generic FG-nucleoporin binding sites on Kap95p overlap, Nup1p binding differs markedly and has contributions from additional hydrophobic residues, together with interactions generated by the intimate contact of the linker between Nup1 residues 977-987 with Kap95p. The length and composition of this linker is crucial and suggests how differences in affinity for Kap95p both between and within FG-nucleoporins arise.

Nuclear import of the stem-loop binding protein and localization during the cell cycle

A key factor involved in the processing of histone pre-mRNAs in the nucleus and translation of mature histone mRNAs in the cytoplasm is the stem-loop binding protein (SLBP). In this work, we have investigated SLBP nuclear transport and subcellular localization during the cell cycle. SLBP is predominantly nuclear under steady-state conditions and localizes to the cytoplasm during S phase when histone mRNAs accumulate. Consistently, SLBP mutants that are defective in histone mRNA binding remain nuclear. As assayed in heterokaryons, export of SLBP from the nucleus is dependent on histone mRNA binding, demonstrating that SLBP on its own does not possess any nuclear export signals. We find that SLBP interacts with the import receptors Impalpha/Impbeta and Transportin-SR2. Moreover, complexes formed between SLBP and the two import receptors are disrupted by RanGTP. We have further shown that SLBP is imported by both receptors in vitro. Three sequences in SLBP required for Impalpha/Impbeta binding were identified. Simultaneous mutation of all three sequences was necessary to abolish SLBP nuclear localization in vivo. In contrast, we were unable to identify an in vivo role for Transportin-SR2 in SLBP nuclear localization. Thus, only the Impalpha/Impbeta pathway contributes to SLBP nuclear import in HeLa cells.

Importin alpha: a multipurpose nuclear-transport receptor

The importin alpha/beta heterodimer targets hundreds of proteins to the nuclear-pore complex (NPC) and facilitates their translocation across the nuclear envelope. Importin alpha binds to classical nuclear localization signal (cNLS)-containing proteins and links them to importin beta, the karyopherin that ferries the ternary complex through the NPC. A second karyopherin, the exportin CAS, recycles importin alpha back to the cytoplasm. In this article, we discuss control mechanisms that importin alpha exerts over the assembly and disassembly of the ternary complex and we describe how new groups of importin alpha genes arose during the evolution of metazoan animals to function in development and differentiation. We also describe activities of importin alpha that seem to be distinct from its housekeeping functions in nuclear transport.

Zinc finger domain of Snail functions as a nuclear localization signal for importin β-mediated nuclear import pathway

Snail, a DNA-binding zinc finger protein, functions as a transcriptional repressor for genes including E-cadherin during development and the acquisition of tumor cell invasiveness. Human Snail is a 264-amino acid nuclear protein with an amino-terminal basic amino acid-rich domain (SNAG domain) and a carboxyl-terminal DNA-binding domain (zinc finger domain). A series of fusion proteins composed of green fluorescent protein (GFP) and portions of the Snail protein were generated, and their subcellular localization was examined. Fusion of the four zinc fingers to GFP led to the targeting of GFP to the nucleus, demonstrating that the zinc finger domain is sufficient for nuclear localization. Using an in vitro transport system, the nuclear import of Snail was reconstituted by importin (karyopherin) beta in the presence of Ran and NTF2. We further demonstrated that Snail binds directly to importin beta in a zinc finger domain-dependent manner. These results indicate that zinc finger domain of Snail functions as a nuclear localization signal and Snail can be transported into the nucleus in an importin beta-mediated manner.

Purification of RanGDP, RanGTP, and RanGMPPNP by ion exchange chromatography

Ran is a small GTPase that cycles between a guanosine diphosphate (GDP)-bound form (RanGDP) and a guanosine triphosphate (GTP)-bound form (RanGTP) and plays important roles in nuclear transport and mitosis. For studies of Ran function and its interactions with partner proteins, pure RanGDP and RanGTP complexes are critical. Ran complexed with the nonhydrolyzable GTP analog, GMPPNP (RanGMPPNP), is used instead of RanGTP when inhibition of hydrolysis is required. In this study, we demonstrate that the binding of Ran to a UNO Q ion exchange column is remarkably sensitive to small shifts in MgCl(2) concentration, and we use this property to purify recombinant RanGTP, RanGMPPNP, and RanGDP complexes. At 10 mM MgCl(2), Ran was found predominantly in the flow-through and, thus, was separated from the vast majority of bacterial proteins. After reducing the concentration of MgCl(2) to 5 mM, further purification of RanGTP, RanGMPPNP, and RanGDP was achieved by loading onto ion exchange columns and elution with an NaCl gradient. Purity of the resulting preparations was confirmed by releasing the bound nucleotide and checking it against a known nucleotide by high-performance liquid chromatography (HPLC). To further confirm the purity and function of the Ran preparations, appropriate protein-binding, enzymatic, and nuclear import assays were carried out. These methods should facilitate studies of cellular processes involving Ran by providing pure functional Ran-nucleotide complexes.

Metastable network model of protein transport through nuclear pores

To reconcile the observed selectivity and the high rate of translocation of cargo-importin complexes through nuclear pores, we propose that the core of the nuclear pore complex is blocked by a metastable network of phenylalanine and glycine nucleoporins. Although the network arrests the unfacilitated passage of objects larger than its mesh size, cargo-importin complexes act as catalysts that reduce the free energy barrier between the cross-linked and the dissociated states of the Nups, and open the network. Using Brownian dynamics simulations we calculate the distribution of passage times through the network for inert particles and cargo-importin complexes of different sizes and discuss the implications of our results for experiments on translocation of proteins through the nuclear pore complex.

Inhibition of nuclear import by the proapoptotic protein CC3

We report here that the normal cellular protein CC3/TIP30, when in excess, inhibits nuclear import in vitro and in vivo. CC3 binds directly to the karyopherins of the importin beta family in a RanGTP-insensitive manner and associates with nucleoporins in vivo. CC3 inhibits the nuclear import of proteins possessing either the classical nuclear localization signal or the M9 signal recognized by transportin. CC3 also inhibits nuclear translocation of transportin itself. Cells modified to express higher levels of CC3 have a slower rate of nuclear import and, as described earlier, show an increased sensitivity to death signals. A mutant CC3 protein lacking proapoptotic activity has a lower affinity for transportin, is displaced from it by RanGTP, and fails to inhibit nuclear import in vitro and in vivo. Together, our results support a correlation between the ability of CC3 to form a RanGTP-resistant complex with importins, inhibit nuclear import, and induce apoptosis. Significantly, a dominant-negative form of importin beta1 shown previously to inhibit multiple transport pathways induces rapid cell death, strongly indicating that inhibition of nuclear transport serves as a potent apoptotic signal.

AMP-activated protein kinase-regulated phosphorylation and acetylation of importin alpha1: involvement in the nuclear import of RNA-binding protein HuR

Nuclear import of HuR, a shuttling RNA-binding protein, is associated with reduced stability of its target mRNAs. Increased function of the AMP-activated protein kinase (AMPK), an enzyme involved in responding to metabolic stress, was recently shown to reduce the cytoplasmic levels of HuR. Here, we provide evidence that importin alpha1, an adaptor protein involved in nuclear import, contributes to the nuclear import of HuR through two AMPK-modulated mechanisms. First, AMPK triggered the acetylation of importin alpha1 on Lys(22), a process dependent on the acetylase activity of p300. Second, AMPK phosphorylated importin alpha1 on Ser(105). Accordingly, expression of importin alpha1 proteins bearing K22R or S105A mutations failed to mediate the nuclear import of HuR in intact cells. Our results point to importin alpha1 as a critical downstream target of AMPK and key mediator of AMPK-triggered HuR nuclear import.

ZAP is a CRM1-dependent nucleocytoplasmic shuttling protein

The zinc finger antiviral protein (ZAP) is a recently isolated host antiviral factor. It specifically inhibits the replication of Moloney murine leukemia virus (MMLV) and Sindbis virus (SIN) by preventing the accumulation of viral RNA in the cytoplasm. In this report, we demonstrate that ZAP is predominantly localized in the cytoplasm at steady state but shuttles between the nucleus and the cytoplasm in a CRM1-dependent manner. Two nuclear localization sequences (NLS) and one nuclear export sequence (NES) were identified. One NLS was mapped to amino acids 68-RARVCRRK-75 and the other mapped to a region including amino acids K405 and K406. The NES was mapped to amino acids 284-LEDVSVDV-291. These findings help to understand why ZAP specifically prevents the accumulation of viral RNA in the cytoplasm. These findings also suggest possible functions of ZAP in the nucleus.

Exportin 7 defines a novel general nuclear export pathway

Most transport pathways between cell nucleus and cytoplasm are mediated by nuclear transport receptors of the importin beta family. These receptors are in continuous circulation between the two compartments and transfer cargo molecules from one side of the nuclear envelope to the other. RanBP16 is a family member from higher eukaryotes of so far unknown function. We now show that it exports p50RhoGAP from the nucleus and thereby confines this activity to the cytoplasm. It also accounts for nuclear exclusion of 14-3-3sigma, which in turn is known to anchor, for example, cyclin-dependent kinases in the cytoplasm. Our data further suggest that RanBP16 exports several additional cargoes. It thus appears to be a nuclear export mediator with broad substrate specificity and we will therefore refer to it as exportin 7 (Exp7). Finally, we demonstrate that Exp7-dependent nuclear export signals differ fundamentally from the leucine-rich, CRM1-dependent ones: First, they are not just short linear sequences, but instead include folded motifs. Second, basic residues are critical for Exp7 recruitment.

The karyopherin Msn5/Kap142 requires Nup82 for nuclear export and performs a function distinct from translocation in RPA protein import

Protein transport between the nucleus and cytoplasm requires interactions between nuclear pore complex proteins (nucleoporins) and soluble nuclear transport factors (karyopherins, importins, and exportins). Exactly how these interactions contribute to the nucleocytoplasmic transport of substrates remains unclear. Using a synthetic lethal screen with the nucleoporin NUP1, we have identified a conditional allele of NUP82, encoding an essential nuclear pore complex protein in Saccharomyces cerevisiae. This nup82-3 allele also exhibits synthetic genetic interactions with mutants of the karyopherin MSN5. nup82-3 mutants accumulate the Msn5 export substrate Pho4 within the nucleus at non-permissive temperatures. The nuclear import of the RPA complex subunit Rfa2 is impaired in nup82-3 and in mutants of the karyopherin KAP95, but is not affected by the loss of MSN5. Interestingly, deletion of MSN5 results in retention of Rfa2-GFP within the nucleus under conditions in which it normally diffuses out. These data provide evidence that Nup82 is important for Msn5-mediated nuclear protein export and Kap95-mediated protein import. In addition, Msn5 may play a role independent of import in the localization of Rfa2.

The Drosophila RCC1 homolog, Bj1, regulates nucleocytoplasmic transport and neural differentiation during Drosophila development

The Bj1 gene encodes the Drosophila homolog of RCC1, the guanine-nucleotide exchange factor for RanGTPase. Here, we provide the first phenotypic characterization of a RCC1 homolog in a developmental model system. We identified Bj1 (dRCC1) in a genetic screen to identify mutations that alter central nervous system development. We find that zygotic dRCC1 mutant embryos exhibit specific defects in the development and differentiation of lateral CNS neurons although cell division and the cell cycle appear grossly normal. dRCC1 mutant nerve cords contain abnormally large cells with compartmentalized nuclei and exhibit increased transcription in the lateral CNS. As RCC1 is an important component of the nucleocytoplasmic transport machinery, we find that dRCC1 function is required for nuclear import of nuclear localization signal sequence (NLS)-carrying cargo molecules. Finally, we show that dRCC1 is required for cell proliferation and/or survival during germline, eye and wing development and that dRCC1 appears to facilitate apoptosis.

Importin 7 and importin alpha/importin beta are nuclear import receptors for the glucocorticoid receptor

The vertebrate glucocorticoid receptor (GR) is cytoplasmic without hormone and localizes to the nucleus after hormone binding. GR has two nuclear localization signals (NLS): NL1 is similar in sequence to the SV40 NLS; NL2 is poorly defined, residing in the ligand-binding domain. We found that GR displayed similar hormone-regulated compartmentalization in Saccharomyces cerevisiae and required the Sxm1 nuclear import receptor for NL2-mediated import. Two metazoan homologues of Sxm1, importin 7 and importin 8, bound both NL1 and NL2, whereas importin alpha selectively bound NL1. In an in vitro nuclear import assay, both importin 7 and the importin alpha-importin beta heterodimer could import a GR NL1 fragment. Under these conditions, full-length GR localized to nuclei in the presence but not absence of an unidentified component in cell extracts. Interestingly, importin 7, importin 8, and importin alpha bound GR even in the absence of hormone; thus, hormonal control of localization is exerted at a step downstream of import receptor binding.

Minimal nuclear pore complexes define FG repeat domains essential for transport

Translocation through nuclear pore complexes (NPCs) requires interactions between receptor-cargo complexes and phenylalanine-glycine (FG) repeats in multiple FG domain-containing NPC proteins (FG-Nups). We have systematically deleted the FG domains of 11 Saccharomyces cerevisiae FG-Nups in various combinations. All five asymmetrically localized FG domains deleted together were non-essential. However, specific combinations of symmetrically localized FG domains were essential. Over half the total mass of FG domains could be deleted without loss of viability or the NPC's normal permeability barrier. Significantly, symmetric deletions caused mild reductions in Kap95-Kap60-mediated import rates, but virtually abolished Kap104 import. These results suggest the existence of multiple translocation pathways.

Nuclear localization domains in human thyroid transcription factor 2

Thyroid transcription factor-2 (TTF2) is a nuclear protein involved in morphogenesis and gene expression in thyroid gland, belonging to the family of the forkhead/winged-helix transcription factors. In the present study we have investigated the sequence determinants for transport and accumulation into the nucleus of the TTF2 protein. By transient expression of fusion proteins constructed by joining different parts of TTF2 to the reporter gene of the jellyfish green fluorescent protein (GFP) and, in a separate set of deleted constructs, the glutathione S-transferase (GST) coding sequence, we have demonstrated that a basic amino acid stretch present at both ends of the DNA-binding domain is a bona fide nuclear localization signal (NLS). We have analyzed the subcellular localization of deleted GFP-GST-TTF2 fusion proteins and have shown that residues inside the forkhead domain (FHD) contributed to the complete nuclear TTF2 protein accumulation. Furthermore, by means of GST binding assays we have shown that distinct TTF2 fragments, containing the NLS, were able to bind the nuclear import receptor importin alpha. Taken together, our results provide the first documentation about nuclear targeting of a forkhead protein containing two identical NLS signal flanking the DNA-binding domain.

A pre-ribosome-associated HEAT-repeat protein is required for export of both ribosomal subunits

Rrp12p (Ypl012w) is unusual among characterized ribosome synthesis factors in being associated with late precursors to both the 40S and 60S subunits. Rrp12p is predominantly nuclear with nucleolar enrichment at steady state, but shuttled between the nucleus and cytoplasm in a heterokaryon assay. Strains depleted of Rrp12p are impaired in the nuclear export of both ribosomal subunits. Sequence analysis combined with fold recognition and modeling showed that Rrp12p is a member of a family of pre-ribosome-associated HEAT-repeat proteins. Like other HEAT-repeat transport factors, Rrp12p binds in vitro to nucleoporin FG-repeats of both the GLFG and FXFG families and to the GTPase Gsp1p (yeast RAN). Rrp12p also showed robust in vitro binding to a pre-rRNA transcript, in addition to poly(A) and poly(U). We propose that Rrp12p binds to the RNA components of the pre-ribosomes and promotes export of both subunits via its interactions with the nucleoporins and Gsp1p.

Structural basis for Nup2p function in cargo release and karyopherin recycling in nuclear import

The yeast nucleoporin Nup2p is associated primarily with the nuclear basket of nuclear pore complexes and is required for efficient importin-alpha:beta-mediated nuclear protein import as well as efficient nuclear export of Kap60p/importin-alpha. Residues 1-51 of Nup2p bind tightly to Kap60p and are required for Nup2p function in vivo. We have determined the 2.6 A resolution crystal structure of a complex between this region of Nup2p and the armadillo repeat domain of Kap60p. Nup2p binds along the inner concave groove of Kap60p, but its interaction interface is different from that employed for nuclear localization signal (NLS) recognition although there is some overlap between them. Nup2p binds Kap60p more strongly than NLSs and accelerates release of NLSs from Kap60p. Nup2p itself is released from Kap60p by Cse1p:RanGTP only in the presence of the importin-beta binding (IBB) domain of Kap60p. These data indicate that Nup2p increases the overall rate of nuclear trafficking by coordinating nuclear import termination and importin recycling as a concerted process.