NXT1 is necessary for the terminal step of Crm1-mediated nuclear export

Karyopherins in the Remodeling of Extracellular Matrix: Implications in Tendon Injury

Rotator Cuff Tendinopathies (RCT) are debilitating conditions characterized by alterations in the extracellular matrix (ECM) of the shoulder tendon, resulting in pain, discomfort, and functional limitations. Specific mediators, including HIF-1α, TGF-β, MMP-9 and others have been implicated in the morphological changes observed in the tendon ECM. These mediators rely on karyopherins, a family of nuclear proteins involved in nucleo-cytoplasmic transport; however, the role of karyopherins in RCT remains understudied despite their potential role in nuclear transport mechanisms. Also, the understanding regarding the precise contributions of karyopherins in RCT holds great promise for deciphering the underlying pathophysiological mechanisms of the disease and potentially fostering the development of targeted therapeutic strategies. This article critically discusses the implications, possibilities, and perspectives of karyopherins in the pathophysiology of RCT.

Specialization of the Drosophila nuclear export family protein Nxf3 for piRNA precursor export

The PIWI-interacting RNA (piRNA) pathway is a conserved small RNA-based immune system that protects animal germ cell genomes from the harmful effects of transposon mobilization. In Drosophila ovaries, most piRNAs originate from dual-strand clusters, which generate piRNAs from both genomic strands. Dual-strand clusters use noncanonical transcription mechanisms. Although transcribed by RNA polymerase II, cluster transcripts lack splicing signatures and poly(A) tails. mRNA processing is important for general mRNA export mediated by nuclear export factor 1 (Nxf1). Although UAP56, a component of the transcription and export complex, has been implicated in piRNA precursor export, it remains unknown how dual-strand cluster transcripts are specifically targeted for piRNA biogenesis by export from the nucleus to cytoplasmic processing centers. Here we report that dual-strand cluster transcript export requires CG13741/Bootlegger and the Drosophila nuclear export factor family protein Nxf3. Bootlegger is specifically recruited to piRNA clusters and in turn brings Nxf3. We found that Nxf3 specifically binds to piRNA precursors and is essential for their export to piRNA biogenesis sites, a process that is critical for germline transposon silencing. Our data shed light on how dual-strand clusters compensate for a lack of canonical features of mature mRNAs to be specifically exported via Nxf3, ensuring proper piRNA production.

Role of Host Genes in Influenza Virus Replication

At every step of their replication cycle influenza viruses depend heavily on their host cells. The multifaceted interactions that occur between the virus and its host cell determine the outcome of the infection, including efficiency of progeny virus production, tropism, and pathogenicity. In order to understand viral disease and develop therapies for influenza it is therefore pertinent to study the intricate interplay between influenza viruses and their required host factors. Here, we review the current knowledge on host cell factors required by influenza virus at the different stages of the viral replication cycle. We also discuss the roles of host factors in zoonotic transmission of influenza viruses and their potential for developing novel antivirals.

NXT1, a Novel Influenza A NP Binding Protein, Promotes the Nuclear Export of NP via a CRM1-Dependent Pathway

Influenza remains a serious worldwide public health problem. After infection, viral genomic RNA is replicated in the nucleus and packed into viral ribonucleoprotein, which will then be exported to the cytoplasm via a cellular chromosome region maintenance 1 (CRM1)-dependent pathway for further assembly and budding. However, the nuclear export mechanism of influenza virus remains controversial. Here, we identify cellular nuclear transport factor 2 (NTF2)-like export protein 1 (NXT1) as a novel binding partner of nucleoprotein (NP) that stimulates NP-mediated nuclear export via the CRM1-dependent pathway. NXT1-knockdown cells exhibit decreased viral replication kinetics and nuclear accumulated viral RNA and NP. By contrast, NXT1 overexpression promotes nuclear export of NP in a CRM1-dependent manner. Pull-down assays suggest the formation of an NXT1, NP, and CRM1 complex, and demonstrate that NXT1 binds to the C-terminal region of NP. These findings reveal a distinct mechanism for nuclear export of the influenza virus and identify the NXT1/NP interaction as a potential target for antiviral drug development.

RNA Export through the NPC in Eukaryotes

In eukaryotic cells, RNAs are transcribed in the nucleus and exported to the cytoplasm through the nuclear pore complex. The RNA molecules that are exported from the nucleus into the cytoplasm include messenger RNAs (mRNAs), ribosomal RNAs (rRNAs), transfer RNAs (tRNAs), small nuclear RNAs (snRNAs), micro RNAs (miRNAs), and viral mRNAs. Each RNA is transported by a specific nuclear export receptor. It is believed that most of the mRNAs are exported by Nxf1 (Mex67 in yeast), whereas rRNAs, snRNAs, and a certain subset of mRNAs are exported in a Crm1/Xpo1-dependent manner. tRNAs and miRNAs are exported by Xpot and Xpo5. However, multiple export receptors are involved in the export of some RNAs, such as 60S ribosomal subunit. In addition to these export receptors, some adapter proteins are required to export RNAs. The RNA export system of eukaryotic cells is also used by several types of RNA virus that depend on the machineries of the host cell in the nucleus for replication of their genome, therefore this review describes the RNA export system of two representative viruses. We also discuss the NPC anchoring-dependent mRNA export factors that directly recruit specific genes to the NPC.

Human TREX2 components PCID2 and centrin 2, but not ENY2, have distinct functions in protein export and co-localize to the centrosome

TREX-2 is a five protein complex, conserved from yeast to humans, involved in linking mRNA transcription and export. The centrin 2 subunit of TREX-2 is also a component of the centrosome and is additionally involved in a distinctly different process of nuclear protein export. While centrin 2 is a known multifunctional protein, the roles of other human TREX-2 complex proteins other than mRNA export are not known. In this study, we found that human TREX-2 member PCID2 but not ENY2 is involved in some of the same cellular processes as those of centrin 2 apart from the classical TREX-2 function. PCID2 is present at the centrosome in a subset of HeLa cells and this localization is centrin 2 dependent. Furthermore, the presence of PCID2 at the centrosome is prevalent throughout the cell cycle as determined by co-staining with cyclins E, A and B. PCID2 but not ENY2 is also involved in protein export. Surprisingly, siRNA knockdown of PCID2 delayed the rate of nuclear protein export, a mechanism distinct from the effects of centrin 2, which when knocked down inhibits export. Finally we showed that co-depletion of centrin 2 and PCID2 leads to blocking rather than delaying nuclear protein export, indicating the dominance of the centrin 2 phenotype. Together these results represent the first discovery of specific novel functions for PCID2 other than mRNA export and suggest that components of the TREX-2 complex serve alternative shared roles in the regulation of nuclear transport and cell cycle progression.

Nuclear export of proteins and drug resistance in cancer

The intracellular location of a protein is crucial to its normal functioning in a cell. Cancer cells utilize the normal processes of nuclear-cytoplasmic transport through the nuclear pore complex of a cell to effectively evade anti-neoplastic mechanisms. CRM1-mediated export is increased in various cancers. Proteins that are exported in cancer include tumor-suppressive proteins such as retinoblastoma, APC, p53, BRAC1, FOXO proteins, INI1/hSNF5, galectin-3, Bok, nucleophosmin, RASSF2, Merlin, p21(CIP), p27(KIP1), N-WASP/FAK, estradiol receptor and Tob, drug targets topoisomerase I and IIα and BCR-ABL, and the molecular chaperone protein Hsp90. Here, we review in detail the current processes and known structures involved in the export of a protein through the nuclear pore complex. We also discuss the export receptor molecule CRM1 and its binding to the leucine-rich nuclear export signal of the cargo protein and the formation of a nuclear export trimer with RanGTP. The therapeutic potential of various CRM1 inhibitors will be addressed, including leptomycin B, ratjadone, KOS-2464, and specific small molecule inhibitors of CRM1, N-azolylacrylate analogs, FOXO export inhibitors, valtrate, acetoxychavicol acetate, CBS9106, and SINE inhibitors. We will also discuss examples of how drug resistance may be reversed by targeting the exported proteins topoisomerase IIα, BCR-ABL, and galectin-3. As effective and less toxic CRM1 export inhibitors become available, they may be used as both single agents and in combination with current chemotherapeutic drugs. We believe that the future development of low-toxicity, small-molecule CRM1 inhibitors may provide a new approach to treating cancer.

On Message Ribonucleic Acids Targeting to Mitochondria

Mitochondria are subcellular organelles that provide energy for a variety of basic cellular processes in eukaryotic cells. Mitochondria maintain their own genomes and many of their endosymbiont genes are encoded by nuclear genomes. The crosstalk between the mitochondrial and nuclear genomes ensures mitochondrial biogenesis, dynamics and maintenance. Mitochondrial proteins are partly encoded by nucleus and synthesized in the cytosol and partly in the mitochondria coded by mitochondrial genome. The efficiency of transport systems that transport nuclear encoded gene products such as proteins and mRNAs to the mitochondrial vicinity to allow for their translation and/or import are recently receiving wide attention. There is currently no concrete evidence that nuclear encoded mRNA is transported into the mitochondria, however, they can be transported onto the mitochondrial surface and translated at the surface of mitochondria utilizing cytosolic machinery. In this review we present an overview of the recent advances in the mRNA transport, with emphasis on the transport of nuclear-encoded mitochondrial protein mRNA into the mitochondria.

The ins and outs of nuclear trafficking: unusual aspects in apicomplexan parasites

Apicomplexa is a phylum within the kingdom Protista that contains some of the most significant threats to public health. One of the members of this phylum, Toxoplasma gondii, is amenable to molecular genetic analyses allowing for the identification of factors critical for colonization and disease. A pathway found to be important for T. gondii pathogenesis is the Ran network of nuclear trafficking. Bioinformatics analysis of apicomplexan genomes shows that while Ran is well conserved, the key regulators of Ran--Regulator of Chromosome Condensation 1 and Ran GTPase activating protein--are either highly divergent or absent. Likewise, several import and export receptor molecules that are crucial for nuclear transport are either not present or have experienced genetic drift such that they are no longer recognizable by bioinformatics tools. In this minireview we describe the basics of nuclear trafficking and compare components within apicomplexans to defined systems in humans and yeast. A detailed analysis of the nuclear trafficking network in these eukaryotes is required to understand how this potentially unique cellular biological pathway contributes to host-parasite interactions.

A novel nucleocytoplasmic traffic GTPase identified as a functional target of the bipartite geminivirus nuclear shuttle protein

SUMMARY

In contrast to the accumulated data on nuclear transport mechanisms of macromolecules, little is known concerning the regulated release of nuclear-exported complexes and their subsequent trans-cytoplasmic movement. The bipartite begomovirus nuclear shuttle protein (NSP) facilitates the nuclear export of viral DNA and cooperates with the movement protein (MP) to transport viral DNA across the plant cell wall. Here, we identified a cellular NSP-interacting GTPase (NIG) with biochemical properties consistent with a nucleocytoplasmic transport role. We show that NIG is a cytosolic GTP-binding protein that accumulates around the nuclear envelope and possesses intrinsic GTPase activity. NIG interacts with NSP in vitro and in vivo (under transient expression), and redirects the viral protein from the nucleus to the cytoplasm. We propose that NIG acts as a positive contributor to geminivirus infection by modulating NSP nucleocytoplasmic shuttling and hence facilitating MP-NSP interaction in the cortical cytoplasm. In support of this, overexpression of NIG in Arabidopsis enhances susceptibility to geminivirus infection. In addition to highlighting the relevance of NIG as a cellular co-factor for NSP function, our findings also have implications for general nucleocytoplasmic trafficking of cellular macromolecules.

STRADalpha regulates LKB1 localization by blocking access to importin-alpha, and by association with Crm1 and exportin-7

LKB1, a serine/threonine kinase, regulates cell polarity, metabolism, and cell growth. The activity and cellular distribution of LKB1 are determined by cofactors, STRADalpha and MO25. STRADalpha induces relocalization of LKB1 from the nucleus to the cytoplasm and stimulates its catalytic activity. MO25 stabilizes the STRADalpha/LKB1 interaction. We investigated the mechanism of nucleocytoplasmic transport of LKB1 in response to its cofactors. Although LKB1 is imported into the nucleus by importin-alpha/beta, STRADalpha and MO25 passively diffuse between the nucleus and the cytoplasm. STRADalpha induces nucleocytoplasmic shuttling of LKB1. STRADalpha facilitates nuclear export of LKB1 by serving as an adaptor between LKB1 and exportins CRM1 and exportin7. STRADalpha inhibits import of LKB1 by competing with importin-alpha for binding to LKB1. MO25 stabilizes the LKB1-STRADalpha complex but it does not facilitate its nucleocytoplasmic shuttling. Strikingly, the STRADbeta, isoform which differs from STRADalpha in the N- and C-terminal domains that are responsible for interaction with export receptors, does not efficiently relocalize LKB1 from the nucleus to the cytoplasm. These results identify a multifactored mechanism to control LKB1 localization, and they suggest that the STRADbeta-LKB1 complex might possess unique functions in the nucleus.

Signal sequences for targeting of gene therapy products to subcellular compartments: the role of CRM1 in nucleocytoplasmic shuttling of the protein switch

PURPOSE

The purpose of this study was to understand the mechanism of nuclear export of the protein switch, used for controlled intracellular delivery of gene products, by studying the involvement of classical export receptor CRM1.

METHOD

Transient transfections of protein switch constructs, isolated nuclear export and import signals were carried out. Effect of leptomycin B (inhibitor of export receptor) and geldanamycin (inhibitor of Hsp90) on localization of these constructs was studied using fluorescence microscopy. Putative nuclear export signals in the glucocorticoid and progesterone receptor ligand binding domains were identified and studied.

RESULTS

It was observed that treatment with leptomycin B caused nuclear accumulation of the protein switch constructs. However, geldanamycin did not have any pronounced effect on the localization. The isolated nuclear export signal from glucocorticoid receptor localized mostly in the cytoplasm, while its mutated version was present everywhere.

CONCLUSION

The localization controlled protein switch constructs are exported out of the nucleus by the classical CRM1 receptors. The ligand binding domain of these protein switch constructs plays an important role in maintaining these constructs in the cytoplasm in the absence of ligand, as well the re-export back to the cytoplasm from the nucleus after ligand washout.

Formation of a Tap/NXF1 homotypic complex is mediated through the amino-terminal domain of Tap and enhances interaction with nucleoporins

Nuclear export of mRNAs is mediated by the Tap/Nxt1 pathway. Tap moves its RNA cargo through the nuclear pore complex by direct interaction with nucleoporin phenylalanine-glycine repeats. This interaction is strengthened by the formation of a Tap/Nxt1 heterodimer. We now present evidence that Tap can form a multimeric complex with itself and with other members of the NXF family. We also show that the homotypic Tap complex can interact with both Nxt1 and nucleoporins in vitro. The region mediating this oligomerization is localized to the first 187 amino acids of Tap, which overlaps with its RNA-binding domain. Removal of this domain greatly reduces the ability of Tap to bind nucleoporins in vitro and in vivo. This is the first report showing that the Tap amino terminus modulates the interaction of Tap with nucleoporins. We speculate that this mechanism has a regulatory role for RNA export independent of RNA binding.

CRM1-mediated nuclear export: to the pore and beyond

CRM1 (chromosome region maintenance 1; also referred to as exportin1 or Xpo1) is a member of the importin beta superfamily of nuclear transport receptors, recognizing proteins bearing a leucine-rich nuclear export sequence. CRM1 is the major receptor for the export of proteins out of the nucleus and is also required for transport of many RNAs. Besides its established role in nuclear export, CRM1 is also implicated in various steps during mitosis, widening its functional spectrum within the cell.

Geldanamycin, an inhibitor of Hsp90, blocks cytoplasmic retention of progesterone receptors and glucocorticoid receptors via their respective ligand binding domains

Steroid hormone receptors (SHRs), such as glucocorticoid receptors (GR) and progesterone receptors (PR), are shuttling proteins that undergo continuous nuclear import and export. Various mechanisms have been proposed to explain the localization of SHRs. It has been suggested that the ligand-binding domain (LBD) of SHRs is important in determining the subcellular localization. We have studied the localization of GR-LBD and PR-LBD alone, as well as of full-length GR and PR in the presence of geldanamycin (GA), a benzoquinoid ansamycin that specifically inhibits heat shock protection (Hsp90), using transient transfections and fluorescent microscopy. Our studies have indicated that GR-LBD and PR-LBD are retained in the cytoplasm via interaction with Hsp90. It was observed that in the unliganded state, treatment with GA translocates these LBDs to the nucleus. Similar results were obtained for full-length PR and GR. Additionally, it was found that after ligand induction, GA accelerated reexport of SHRs after ligand washout, implicating Hsp90 in nuclear retention of SHRs in the washout state. We also propose that a recently found "export" signal present in the LBD of SHRs is involved in interactions with Hsp90 and hence cytoplasmic retention of these receptors. After ligand induction, Hsp90 also may play a role in nuclear retention of SHRs following hormone washout.

Controlling protein compartmentalization to overcome disease

Over the past decade, considerable progress has been made to improve our understanding of the intracellular transport of proteins. Mechanisms of nuclear import and export involving classical receptors have been studied. Signal sequences required for directing a protein molecule to a specific cellular compartment have been defined. Knowledge of subcellular trafficking of proteins has also increased our understanding of diseases caused due to mislocalization of proteins. A specific protein on deviating from its native cellular compartment may result in disease due to loss of its normal functioning and aberrant activity in the "wrong" compartment. Mislocalization of proteins results in diseases that range from metabolic disorders to cancer. In this review we discuss some of the diseases caused due to mislocalization. We further focus on application of nucleocytoplasmic transport to drug delivery. Various rationales to treat diseases by exploiting intracellular transport machinery have been proposed. Although the pathways for intracellular movement of proteins have been defined, these have not been adequately utilized for management of diseases involving mislocalized proteins. This review stresses the need for designing drug delivery systems utilizing these mechanisms as this area is least exploited but offers great potential.

Mutations in tap uncouple RNA export activity from translocation through the nuclear pore complex

Interactions between transport receptors and phenylalanine-glycine (FG) repeats on nucleoporins drive the translocation of receptor-cargo complexes through nuclear pores. Tap, a transport receptor that mediates nuclear export of cellular mRNAs, contains a UBA-like and NTF2-like folds that can associate directly with FG repeats. In addition, two nuclear export sequences (NESs) within the NTF2-like region can also interact with nucleoporins. The Tap-RNA complex was shown to bind to three nucleoporins, Nup98, p62, and RanBP2, and these interactions were enhanced by Nxt1. Mutations in the Tap-UBA region abolished interactions with all three nucleoporins, whereas the effect of point mutations within the NTF2-like domain of Tap known to disrupt Nxt1 binding or nucleoporin binding were nucleoporin dependent. A mutation in any of these Tap domains was sufficient to reduce RNA export but was not sufficient to disrupt Tap interaction with the NPC in vivo or its nucleocytoplasmic shuttling. However, shuttling activity was reduced or abolished by combined mutations within the UBA and either the Nxt1-binding domain or NESs. These data suggest that Tap requires both the UBA- and NTF2-like domains to mediate the export of RNA cargo, but can move through the pores independently of these domains when free of RNA cargo.

Identification of a novel Rev-interacting cellular protein

BACKGROUND

Human cell types respond differently to infection by human immunodeficiency virus (HIV). Defining specific interactions between host cells and viral proteins is essential in understanding how viruses exploit cellular functions and the innate strategies underlying cellular control of HIV replication. The HIV Rev protein is a post-transcriptional inducer of HIV gene expression and an important target for interaction with cellular proteins. Identification of Rev-modulating cellular factors may eventually contribute to the design of novel antiviral therapies.

RESULTS

Yeast-two hybrid screening of a T-cell cDNA library with Rev as bait led to isolation of a novel human cDNA product (16.4.1). 16.4.1-containing fusion proteins showed predominant cytoplasmic localization, which was dependent on CRM1-mediated export from the nucleus. Nuclear export activity of 16.4.1 was mapped to a 60 amino acid region and a novel transport signal identified. Interaction of 16.4.1 with Rev in human cells was shown in a mammalian two-hybrid assay and by colocalization of Rev and 16.4.1 in nucleoli, indicating that Rev can recruit 16.4.1 to the nucleus/nucleoli. Rev-dependent reporter expression was inhibited by overexpressing 16.4.1 and stimulated by siRNAs targeted to 16.4.1 sequences, demonstrating that 16.4.1 expression influences the transactivation function of Rev.

CONCLUSION

These results suggest that 16.4.1 may act as a modulator of Rev activity. The experimental strategies outlined in this study are applicable to the identification and biological characterization of further novel Rev-interacting cellular factors.

NXT2 is required for embryonic heart development in zebrafish

Background

NXT2 is a member of NXT family proteins that are generally involved in exporting nuclear RNA in eukaryotic cells. It is not known if NXT2 has any function in specific biological processes.

Results

A zebrafish mutant exhibiting specific heart defects during embryogenesis was generated by animal cloning-mediated retroviral insertions. Molecular analysis indicated that the mutant phenotype was caused by a disruption of NXT2. Whole-mount RNA in situ hybridization showed that NXT2 transcripts were clearly detectable in embryonic heart as well as other tissues. Further analysis revealed that expression level of one form of alternative splicing NXT2 mRNA transcripts was significantly reduced, resulting in deficient myocardial cell differentiation and the malformation of cardiac valve at the atrioventricular boundary. The defects could be reproduced by morpholino anti-sense oligo knockdown of NXT2.

Conclusion

NXT2 has a critical role in maintaining morphogenetic integrity of embryonic heart in vertebrate species.

Efficient NES-dependent protein nuclear export requires ongoing synthesis and export of mRNAs

The mechanisms regulating nuclear export of proteins are not fully understood. To investigate whether the efficiency of protein nuclear export may depend on ongoing RNA synthesis and/or mRNA nuclear export, we used a microinjection approach with a fluorescent reporter protein containing a nuclear export signal (NES) and scored protein export in human fibroblasts under conditions when the synthesis or export of mRNAs was inhibited. We show that inhibition of transcription significantly attenuated generic NES-dependent nuclear export. Furthermore, digestion of endogenous nuclear RNAs by co-microinjection of RNAse A inhibited NES-dependent nuclear export. Finally, nuclear export of the NES reporter protein was significantly inhibited in cells in which nuclear export of mRNAs had been specifically blocked by microinjection of anti-TAP antibodies or by expression of a dominant negative form of NUP160. These results demonstrate a novel role for ongoing synthesis and export of mRNAs in NES-dependent protein nuclear export.

Optical microwell assay of membrane transport kinetics

In optical single transporter recording, membranes are firmly attached to flat solid substrates containing small wells or test compartments (TC). Transport of fluorescent molecules through TC-spanning membrane patches is induced by solution change and recorded by confocal microscopy. Previously, track-etched membrane filters were used to create solid substrates containing populations of randomly distributed TCs. In this study the possibilities offered by orderly TC arrays as created by laser microdrilling were explored. A theoretical framework was developed taking the convolution of membrane transport, solution change, and diffusion into account. The optical properties of orderly TC arrays were studied and the kinetics of solution change measured. Export and import through the nuclear pore complex (NPC) was analyzed in isolated envelopes of Xenopus oocyte nuclei. In accordance with previous reports nuclear transport receptor NTF2, which binds directly to NPC proteins, was found to be translocated much faster than "inert" molecules of similar size. Unexpectedly, NXT1, a homolog of NTF2 reportedly unable to bind to NPC proteins directly, was translocated as fast as NTF2. Thus, microstructured TC arrays were shown to provide optical single transporter recording with a new basis.

hRIP, a cellular cofactor for Rev function, promotes release of HIV RNAs from the perinuclear region

Human immunodeficiency virus Rev facilitates the cytoplasmic accumulation of viral RNAs that contain a Rev binding site. A human Rev-interacting protein (hRIP) was originally identified based on its ability to interact with the Rev nuclear export signal (NES) in yeast two-hybrid assays. To date, however, the function of hRIP and a role for hRIP in Rev-directed RNA export have remained elusive. Here we ablate hRIP activity with a dominant-negative mutant or RNA interference and analyze Rev function by RNA in situ hybridization. We find, unexpectedly, that in the absence of functional hRIP, Rev-directed RNAs mislocalize and aberrantly accumulate at the nuclear periphery, where hRIP is localized. In contrast, in the absence of Rev or the Rev cofactor CRM1, Rev-directed RNAs remain nuclear. We further show that the RNA mislocalization pattern resulting from loss of hRIP activity is highly specific to Rev function: the intracellular distribution of cellular poly(A)(+) mRNA, nuclear proteins, and, most important, NES-containing proteins, are unaffected. Thus, hRIP is an essential cellular Rev cofactor, which acts at a previously unanticipated step in HIV-1 RNA export: movement of RNAs from the nuclear periphery to the cytoplasm.

Regulation of nuclear import and export by the GTPase Ran

This review focuses on the control of nuclear import and export pathways by the small GTPase Ran. Transport of signal-containing cargo substrates is mediated by receptors that bind to the cargo proteins and RNAs and deliver them to the appropriate cellular compartment. Ran is an evolutionarily conserved member of the Ras superfamily that regulates all receptor-mediated transport between the nucleus and the cytoplasm. We describe the identification and characterization of the RanGTPase and its binding partners: the guanine nucleotide exchange factor, RanGEF; the GTPase activating protein, RanGAP; the soluble import and export receptors; Ran-binding domain-(RBD) containing proteins; and NTF2 and related factors.

Nesprin-1alpha self-associates and binds directly to emerin and lamin A in vitro

Nesprin-1alpha is a spectrin repeat (SR)-containing, transmembrane protein of the inner nuclear membrane, and is highly expressed in muscle cells. A yeast two-hybrid screen for nesprin-1alpha-interacting proteins showed that nesprin-1alpha interacted with itself. Blot overlay experiments revealed that nesprin-1alpha's third SR binds the fifth SR. The carboxy-terminal half of nesprin-1alpha directly bound lamin A, a nuclear intermediate filament protein. Biochemical analysis demonstrated that nesprin-1alpha dimers bind directly to the nucleoplasmic domain of emerin, an inner nuclear membrane protein, with an affinity of 4 nM. Binding was optimal for full nucleoplasmic dimers of nesprin-1alpha, since nesprin fragments SR1-5 and SR5-7 bound emerin as monomers with affinities of 53 nM and 250 mM, respectively. We propose that membrane-anchored nesprin-1alpha antiparallel dimers interact with both emerin and lamin A to provide scaffolding at the inner nuclear membrane.

Nuclear export of mRNA by TAP/NXF1 requires two nucleoporin-binding sites but not p15

Metazoan NXF1/p15 heterodimers promote export of bulk mRNA through nuclear pore complexes (NPC). NXF1 interacts with the NPC via two distinct structural domains, the UBA-like domain and the NTF2-like scaffold, which results from the heterodimerization of the NTF2-like domain of NXF1 with p15. Both domains feature a single nucleoporin-binding site, and they act synergistically to promote NPC translocation. Whether the NTF2-like scaffold (and thereby p15) contributes only to NXF1/NPC association or is also required for other functions, e.g., to impart directionality to the export process by regulating NXF1/NPC or NXF1/cargo interactions, remains unresolved. Here we show that a minimum of two nucleoporin-binding sites is required for NXF1-mediated export of cellular mRNA. These binding sites can be provided by an NTF2-like scaffold followed by a UBA-like domain (as in the wild-type protein) or by two NTF2-like scaffolds or two UBA-like domains in tandem. In the latter case, the export activity of NXF1 is independent of p15. Thus, as for the UBA-like domain, the function of the NTF2-like scaffold is confined to nucleoporin binding. More importantly, two copies of either of these domains are sufficient to promote directional transport of mRNA cargoes across the NPC.

Ran-binding protein 3 links Crm1 to the Ran guanine nucleotide exchange factor

Ran-binding protein 3 (RanBP3) is an approximately 55-kDa protein that functions as a cofactor for Crm1-mediated nuclear export. RanBP3 stimulates export by enhancing the affinity of Crm1 for Ran.GTP and cargo. However, important additional functions for this cofactor may exist. We now report that RanBP3 associates with the Ran-specific guanine nucleotide exchange factor, regulator of chromosome condensation 1 (RCC1). This interaction was stimulated by the addition of Ran; moreover, Ran.GDP, Ran.GTP, and Ran without nucleotide could all stimulate complex formation between RanBP3 and RCC1 even though binding of Ran.GDP to RanBP3 alone was undetectable. RanBP3 could also promote binding of Crm1 to RCC1 in the presence of Ran. Binding of RanBP3 to RCC1 increased the catalytic activity of RCC1 toward Ran, and importantly, the ability of RanBP3 to stimulate RCC1 was not affected by the presence of Crm1. These data indicate that RanBP3 acts as a scaffold protein to promote the efficient assembly of export complexes. By tethering Crm1 to catalytically enhanced RCC1, RanBP3 may lower the entropic barrier for the loading of Ran.GTP onto Crm1. We propose that this provides an additional mechanism by which RanBP3 facilitates export.

Complex formation between Tap and p15 affects binding to FG-repeat nucleoporins and nucleocytoplasmic shuttling

Mammalian Tap-p15 and yeast Mex67p-Mtr2p are conserved and essential mRNA export factor complexes that transport mRNPs through the nuclear pore. Here, we report that the small subunit p15 affects the binding of the large subunit Tap to repeat nucleoporins. BIAcore measurements revealed that recombinant Tap binds with high affinity (K(d) in the nm range) to repeat nucleoporins and dissociates from them very slowly. In contrast, when recombinant Tap was bound to p15, the derived heterodimeric complex exhibited a significant lower affinity to FG-repeat nucleoporins (K(d) in the microm range). Furthermore, when recombinant Tap lacking the N-terminal nuclear localization sequences (TapDeltaNLS) was microinjected in mammalian cells, it did not shuttle; however, TapDeltaNLS with bound p15 efficiently shuttles between nucleus and cytoplasm. We conclude that heterodimerization of Tap and p15 is required for shuttling of the functional Tap-p15 mRNA exporter complex.

Formation of Tap/NXT1 heterodimers activates Tap-dependent nuclear mRNA export by enhancing recruitment to nuclear pore complexes

The Tap protein has been shown to activate the nuclear export of mRNA species bearing retroviral constitutive transport elements and is also believed to play an essential role in the sequence nonspecific export of cellular mRNAs. However, it has remained unclear how Tap activity is regulated in vivo. Here, we report that the small NXT1/p15-1 protein functions as a critical cofactor for Tap-mediated mRNA export in both human and invertebrate cells. In the absence of NXT1 binding, the Tap protein is unable to effectively interact with components of the nuclear pore complex and both Tap nucleocytoplasmic shuttling and the nuclear export of mRNA molecules tethered to Tap are therefore severely attenuated. Formation of a Tap/NXT1 heterodimer enhances nucleoporin binding both in vitro and in vivo and induces the formation of a Tap/NXT1/nucleoporin ternary complex that is likely to be a key intermediate in the process of nuclear mRNA export. The critical importance of NXT1 for the nuclear export of poly(A)(+) RNA is emphasized by the finding that specific inhibition of the expression of the Drosophila homolog of human NXT1, by using RNA interference, results in the nuclear accumulation of poly(A)(+) RNA in cultured insect cells. These data suggest that NXT1 may act as a molecular switch that regulates the ability of Tap to mediate nuclear mRNA export by controlling the interaction of Tap with components of the nuclear pore.

NXF1/p15 heterodimers are essential for mRNA nuclear export in Drosophila

The conserved family of NXF proteins has been implicated in the export of messenger RNAs from the nucleus. In metazoans, NXFs heterodimerize with p15. The yeast genome encodes a single NXF protein (Mex67p), but there are multiple nxf genes in metazoans. Whether metazoan NXFs are functionally redundant, or their multiplication reflects an adaptation to a greater substrate complexity or to tissue-specific requirements has not been established. The Drosophila genome encodes one p15 homolog and four putative NXF proteins (NXF1 to NXF4). Here we show that depletion of the endogenous pools of NXF1 or p15 from Drosophila cells inhibits growth and results in a rapid and robust accumulation of polyadenylated RNAs within the nucleus. Fluorescence in situ hybridizations show that export of both heat-shock and non-heat-shock mRNAs, as well as intron-containing and intronless mRNAs is inhibited. Depleting endogenous NXF2 or NXF3 has no apparent phenotype. Moreover, NXF4 is not expressed at detectable levels in cultured Drosophila cells. We conclude that Dm NXF1/p15 heterodimers only (but not NXF2-NXF4) mediate the export of the majority of mRNAs in Drosophila cells and that the other members of the NXF family play more specialized or different roles.

Transport into and out of the nucleus

A defining characteristic of eukaryotic cells is the possession of a nuclear envelope. Transport of macromolecules between the nuclear and cytoplasmic compartments occurs through nuclear pore complexes that span the double membrane of this envelope. The molecular basis for transport has been revealed only within the last few years. The transport mechanism lacks motors and pumps and instead operates by a process of facilitated diffusion of soluble carrier proteins, in which vectoriality is provided by compartment-specific assembly and disassembly of cargo-carrier complexes. The carriers recognize localization signals on the cargo and can bind to pore proteins. They also bind a small GTPase, Ran, whose GTP-bound form is predominantly nuclear. Ran-GTP dissociates import carriers from their cargo and promotes the assembly of export carriers with cargo. The ongoing discovery of numerous carriers, Ran-independent transport mechanisms, and cofactors highlights the complexity of the nuclear transport process. Multiple regulatory mechanisms are also being identified that control cargo-carrier interactions. Circadian rhythms, cell cycle, transcription, RNA processing, and signal transduction are all regulated at the level of nucleocytoplasmic transport. This review focuses on recent discoveries in the field, with an emphasis on the carriers and cofactors involved in transport and on possible mechanisms for movement through the nuclear pores.

NTF2 monomer-dimer equilibrium

Nuclear transport factor 2 (NTF2) mediates nuclear import of RanGDP, a central component of many nuclear trafficking pathways. NTF2 is a homodimer and each chain has independent binding sites for RanGDP and nuclear pore proteins (nucleoporins) that contain FxFG sequence repeats. We show here that the monomer-dimer dissociation constant for NTF2 obtained by sedimentation equilibrium ultracentrifugation is in the micromolar range, indicating that a substantial proportion of cellular NTF2 may be monomeric. To investigate the functional significance of NTF2 dimerization, we engineered a series of point mutations at the dimerization interface and one of these (M118E) remained monomeric below concentrations of 150 microM. CD spectra and X-ray crystallography showed that M118E-NTF2 preserved the wild-type NTF2 fold, although its thermal stability was 20 deg. C lower than that of the wild-type. M118E-NTF2 bound both RanGDP and FxFG nucleoporins less strongly, suggesting that dissociation of the NTF2 dimer could facilitate RanGDP release and thus nucleotide exchange after it had been transported into the nucleus. Moreover, colloidal gold coated with M118E-NTF2 showed reduced binding to Xenopus oocyte nuclear pores. Overall, our results indicate that dimer formation is important for NTF2 function and give insight into the formation of heterodimers by mRNA export factors such as TAP1 and NXT1 that contain NTF2-homology domains.

RNA export mediated by tap involves NXT1-dependent interactions with the nuclear pore complex

Nuclear export of ribonucleoprotein complexes requires cis-acting signals and recognition by receptors that mediate translocation through the nuclear pore complex. Translocation is likely to involve a series of physical interactions between the ribonucleoprotein complex and nucleoporins within the nuclear pore complex. Here, we have characterized the function of NXT1 in the context of the Tap-dependent RNA export pathway. Tap has been implicated in the nuclear export of RNA transcripts derived from Mason-Pfizer monkey virus that contain the constitutive transport element. We demonstrate that NXT1 stimulates binding of a Tap-RNA complex to nucleoporins in vitro, and we provide mutational analysis that shows these interactions are necessary for nuclear export of an intron-containing viral mRNA in vivo. Tap contains separate domains for binding to nucleoporins and NXT1, both of which are critical for its export function. RNA export is mediated by a heterodimer of Tap and NXT1, and the function of NXT1 on this pathway is to regulate the affinity of the Tap-RNA complex for nucleoporins within the nuclear pore complex. We propose that NXT1-dependent binding of the Tap-RNA complex to the nucleoporin p62, which we have reconstituted in vitro using recombinant proteins, represents a single step of the translocation reaction.

DNA binding domains in diverse nuclear receptors function as nuclear export signals

BACKGROUND

The nuclear receptor superfamily of transcription factors directs gene expression through DNA sequence-specific interactions with target genes. Nuclear import of these receptors involves recognition of a nuclear localization signal (NLS) by importins, which mediate translocation into the nucleus. Nuclear receptors lack a leucine-rich nuclear export signal (NES), and export is insensitive to leptomycin B, indicating that nuclear export is not mediated by Crm1.

RESULTS

We set out to define the NES in the glucocorticoid receptor (GR) and to characterize the export pathway. We found that the 69 amino acid DNA binding domain (DBD) of GR, which is unrelated to any known NES, is necessary and sufficient for export. Mutational analysis revealed that a 15 amino acid sequence between the two zinc binding loops in the GR-DBD confers nuclear export to a GFP reporter protein, and alanine-scanning mutagenesis was used to identify the residues within this sequence that are critical for export. The DBD is highly related (41%-88% identity) in steroid, nonsteroid, and orphan nuclear receptors, and we found that the DBDs from ten different nuclear receptors all function as export signals. DBD-dependent nuclear export is saturable, and prolonged nuclear localization of the GR increases its transcriptional activity.

CONCLUSIONS

Multiple members of the nuclear receptor superfamily use a common pathway to exit the nucleus. We propose that NLS-mediated import and DBD-mediated export define a shuttling cycle that integrates the compartmentalization and activity of nuclear receptors.

In vitro analysis of nuclear transport mediated by the C-terminal shuttle domain of Tap

The Tap protein of higher eukaryotes is implicated in the nuclear export of type D retroviral mRNA and some cellular mRNAs. Here we have developed an in vitro assay to study nuclear export mediated by the C-terminal shuttle domain of Tap involving the rapamycin-induced attachment of this transport domain to a nuclear green fluorescent protein-containing reporter. We found that export by the Tap transport domain does not involve cytosolic transport factors including the GTPase Ran. The transport domain directly binds to several nucleoporins positioned in different regions of the nuclear pore complex. These results argue that a direct interaction of the Tap transport domain with nucleoporins is responsible for its nucleocytoplasmic translocation. We found that the karyopherin beta-related export receptor CRM1 competes with the Tap transport domain for binding to Nup214 but not for binding to Nup62 or Nup153, suggesting that the Tap and CRM1 nuclear export pathways converge at the cytoplasmic periphery of the nuclear pore complex. Because the rates of in vitro nuclear import and export by the Tap transport domain are very similar, the directionality of mRNA export mediated by Tap probably is determined by mechanisms other than simple binding of the Tap transport domain to nucleoporins.

Protein export from the nucleus

The evolution of the nucleus imposed on eukaryotic cells the necessity to strictly control exchange of molecules between the nucleus and the remainder of the cell, not only to protect and correctly transmit genetic information, but also to coordinate nuclear and cytoplasmic functions. Studies over the past 10 years have provided major insights into the molecular mechanisms responsible for transport of molecules between the nucleus and the cytoplasm. In addition, regulation of the nucleocytoplasmic distribution of diverse cellular factors has emerged as one of the most efficient mechanism to adapt gene expression to the cell environment, for example by controlling the access of transcriptional regulators to their target genes. In this review, we focus on the molecular basis of protein nuclear export that relies on interactions between targeting sequences present on the cargoes, specific export receptors or exportins and nuclear pore proteins, with special emphasis on the role of the Ran GTPase and associated proteins in this process.

Targeting of Ran: variation on a common theme?

The Ran GTPase plays a key role in nucleocytoplasmic transport. In its GTP-bound form, it directly interacts with members of the importin β family of nuclear transport receptors and modulates their association with cargo. Work in cell-free higher-eukaryote systems has demonstrated additional roles for Ran in spindle and nuclear envelope formation during mitosis. However, until recently, no Ran-target proteins in these cellular processes were known. Several groups have now identified importin β as one important target of Ran during mitotic spindle formation. This finding suggests that Ran uses the same effectors to regulate different cellular processes.

Structural basis for the recognition of a nucleoporin FG repeat by the NTF2-like domain of the TAP/p15 mRNA nuclear export factor

TAP-p15 heterodimers have been implicated in the export of mRNAs through nuclear pore complexes (NPCs). We report a structural analysis of the interaction domains of TAP and p15 in a ternary complex with a Phe-Gly (FG) repeat of an NPC component. The TAP-p15 heterodimer is structurally similar to the homodimeric transport factor NTF2, but unlike NTF2, it is incompatible with either homodimerization or Ran binding. The NTF2-like heterodimer functions as a single structural unit in recognizing an FG repeat at a hydrophobic pocket present only on TAP and not on p15. This FG binding site interacts synergistically with a second site at the C terminus of TAP to mediate mRNA transport through the pore. In general, our findings suggest that FG repeats bind with a similar conformation to different classes of transport factors.

Ran-binding protein 3 is a cofactor for Crm1-mediated nuclear protein export

Crm1 is a member of the karyopherin family of nucleocytoplasmic transport receptors and mediates the export of proteins from the nucleus by forming a ternary complex with cargo and Ran:GTP. This complex translocates through the nuclear pores and dissociates in the cytosol. The yeast protein Yrb2p participates in this pathway and binds Crm1, but its mechanism of action has not been established. We show that the human orthologue of Yrb2p, Ran-binding protein 3 (RanBP3), acts as a cofactor for Crm1-mediated export in a permeabilized cell assay. RanBP3 binds directly to Crm1, and the complex possesses an enhanced affinity for both Ran:GTP and cargo. RanBP3 shuttles between the nucleus and the cytoplasm by a Crm1-dependent mechanism, and the Crm1--RanBP3-NES-Ran:GTP quarternary complex can associate with nucleoporins. We infer that this complex translocates through the nuclear pore to the cytoplasm where it is disassembled by RanBP1 and Ran GTPase--activating protein.

Nuclear export of mRNA

Export of mRNA through nuclear pore complexes (NPC) is preceded by multiple and well coordinated processing steps, resulting in the formation of an export competent ribonucleoprotein complex (mRNP). Numerous factors involved in the translocation of the mRNP through the NPC and its release into the cytoplasm have been isolated mainly through genetic approaches in yeast, and putative functional homologues have been identified in metazoan systems. Understanding the mechanism of mRNA export relies, in part, on the functional characterization of these factors and the establishment of a complete network of molecular interactions. Here we summarize recent progress in the characterization of yeast and mammalian components implicated in the export of an mRNA from the nucleus to the cytoplasm.

Overexpression of TAP/p15 heterodimers bypasses nuclear retention and stimulates nuclear mRNA export

Human TAP and its yeast orthologue Mex67p are members of the multigene family of NXF proteins. A conserved feature of NXFs is a leucine-rich repeat domain (LRR) followed by a region related to the nuclear transport factor 2 (the NTF2-like domain). The NTF2-like domain of metazoan NXFs heterodimerizes with a protein known as p15 or NXT. A C-terminal region related to ubiquitin-associated domains (the UBA-like domain) is present in most, but not all NXF proteins. Saccharomyces cerevisiae Mex67p and Caenorhabditis elegans NXF1 are essential for the export of messenger RNA from the nucleus. Human TAP mediates the export of simian type D retroviral RNAs bearing the constitutive transport element, but the precise role of TAP and p15 in mRNA nuclear export has not yet been established. Here we show that overexpression of TAP/p15 heterodimers bypasses nuclear retention and stimulates the export of mRNAs that are otherwise exported inefficiently. This stimulation of mRNA export is strongly reduced by removing the UBA-like domain of TAP and abolished by deleting the LRR domain or the NTF2-like domain. Similar results are obtained when TAP/p15 heterodimers are directly tethered to the RNA export cargo. Our data indicate that formation of TAP/p15 heterodimers is required for TAP-mediated export of mRNA and show that the LRR domain of TAP plays an essential role in this process.

Nuclear pores and nuclear assembly

Communication between the nucleus and cytoplasm occurs through large macromolecular structures, the nuclear pores. Quantitative scanning transmission electron microscopy has estimated the mass of a nuclear pore to be 60 million Daltons in yeast and 120 million Daltons in vertebrates. The past two years were noteworthy in that they saw: 1) the purification of both the yeast and vertebrate nuclear pores, 2) the initial description of routes through the pore for specific transport receptors, 3) glimpses of intranuclear organization imposed by the nuclear pores and envelope and 4) the revelation of new and pivotal roles for the small GTPase Ran not only in nuclear import but in spindle assembly and nuclear membrane fusion.