The yeast RNA1 gene product necessary for RNA processing is located in the cytosol and apparently excluded from the nucleus

Cryo-EM of the Nucleosome Core Particle Bound to Ran-RCC1 Reveals a Dynamic Complex

Ras-related nuclear protein (Ran) is a member of the Ras superfamily of small guanosine triphosphatases (GTPases) and a regulator of multiple cellular processes. In healthy cells, the GTP-bound form of Ran is concentrated at chromatin, creating a Ran•GTP gradient that provides the driving force for nucleocytoplasmic transport, mitotic spindle assembly, and nuclear envelope formation. The Ran•GTP gradient is maintained by the regulator of chromatin condensation 1 (RCC1), a guanine nucleotide exchange factor that accelerates GDP/GTP exchange in Ran. RCC1 interacts with nucleosomes, which are the fundamental repeating units of eukaryotic chromatin. Here, we present a cryo-EM analysis of a trimeric complex composed of the nucleosome core particle (NCP), RCC1, and Ran. While the contacts between RCC1 and Ran in the complex are preserved compared with a previously determined structure of RCC1-Ran, our study reveals that RCC1 and Ran interact dynamically with the NCP and undergo rocking motions on the nucleosome surface. Furthermore, the switch 1 region of Ran, which plays an important role in mediating conformational changes associated with the substitution of GDP and GTP nucleotides in Ras family members, appears to undergo disorder-order transitions and forms transient contacts with the C-terminal helix of histone H2B. Nucleotide exchange assays performed in the presence and absence of NCPs are not consistent with an active role for nucleosomes in nucleotide exchange, at least in vitro. Instead, the nucleosome stabilizes RCC1 and serves as a hub that concentrates RCC1 and Ran to promote efficient Ran•GDP to Ran•GTP conversion.

A deletion/duplication in the Ligon lintless-2 locus induces siRNAs that inhibit cotton fiber cell elongation

Most cultivated cotton (Gossypium hirsutum L.) varieties have two types of seed fibers: short fuzz fiber strongly adhered to the seed coat, and long lint fiber used in the textile industry. The Ligon lintless-2 (Li2) cotton mutant has a normal vegetative phenotype but produces very short lint fiber on the seeds. The Li2 mutation is controlled by a single dominant gene. We discovered a large structural rearrangement at the end of chromosome D13 in the Li2 mutant based on whole-genome sequencing and genetic mapping of segregating populations. The rearrangement contains a 177-kb deletion and a 221-kb duplication positioned as a tandem inverted repeat. The gene Gh_D13G2437 is located at the junction of the inverted repeat in the duplicated region. During transcription such structure spontaneously forms self-complementary hairpin RNA of Gh_D13G2437 followed by production of small interfering RNA (siRNA). Gh_D13G2437 encodes a Ran-Binding Protein 1 (RanBP1) that preferentially expresses during cotton fiber elongation. The abundance of siRNA produced from Gh_D13G2437 reciprocally corresponds with the abundance of highly homologous (68%-98% amino acid sequence identity) RanBP1 family transcripts during fiber elongation, resulting in a shorter fiber phenotype in the Li2. Overexpression of Gh_D13G2437 in the Li2 mutant recovered the long lint fiber phenotype. Taken together, our findings revealed that siRNA-induced silencing of a family of RanBP1s inhibit elongation of cotton fiber cells in the Li2 mutant.

Molecular and cellular dynamics of early embryonic cell divisions in Volvox carteri

Cell division is fundamental to all organisms and the green alga used here exhibits both key animal and plant functions. Specifically, we analyzed the molecular and cellular dynamics of early embryonic divisions of the multicellular green alga Volvox carteri (Chlamydomonadales). Relevant proteins related to mitosis and cytokinesis were identified in silico, the corresponding genes were cloned, fused to yfp, and stably expressed in Volvox, and the tagged proteins were studied by live-cell imaging. We reveal rearrangements of the microtubule cytoskeleton during centrosome separation, spindle formation, establishment of the phycoplast, and generation of previously unknown structures. The centrosomes participate in initiation of spindle formation and determination of spindle orientation. Although the nuclear envelope does not break down during early mitosis, intermixing of cytoplasm and nucleoplasm results in loss of nuclear identity. Finally, we present a model for mitosis in Volvox. Our study reveals enormous dynamics, clarifies spatio-temporal relationships of subcellular structures, and provides insight into the evolution of cell division.

Association of RanGAP to nuclear pore complex component, RanBP2/Nup358, is required for pupal development in Drosophila

Ran's GTPase-activating protein (RanGAP) is tethered to the nuclear envelope (NE) in multicellular organisms. We investigated the consequences of RanGAP localization in human tissue culture cells and Drosophila. In tissue culture cells, disruption of RanGAP1 NE localization surprisingly has neither obvious impacts on viability nor nucleocytoplasmic transport of a model substrate. In Drosophila, we identified a region within nucleoporin dmRanBP2 required for direct tethering of dmRanGAP to the NE. A dmRanBP2 mutant lacking this region shows no apparent growth defects during larval stages but arrests at the early pupal stage. A direct fusion of dmRanGAP to the dmRanBP2 mutant rescues this arrest, indicating that dmRanGAP recruitment to dmRanBP2 per se is necessary for the pupal ecdysis sequence. Our results indicate that while the NE localization of RanGAP is widely conserved in multicellular organisms, the targeting mechanisms are not. Further, we find a requirement for this localization during pupal development.

Exportins can inhibit major mitotic assembly events in vitro: membrane fusion, nuclear pore formation, and spindle assembly

Xenopus

egg extracts are a powerful in vitro tool for studying complex biological processes, including nuclear reconstitution, nuclear membrane and pore assembly, and spindle assembly. Extracts have been further used to demonstrate a moonlighting regulatory role for nuclear import receptors or importins on these cell cycle assembly events. Here we show that exportins can also play a role in these events. Addition of Crm1, Exportin-t, or Exportin-5 decreased nuclear pore assembly in vitro. RanQ69L-GTP, a constitutively active form of RanGTP, ameliorated inhibition. Both Crm1 and Exportin-t inhibited fusion of nuclear membranes, again counteracted by RanQ69L-GTP. In mitotic extracts, Crm1 and Exportin-t negatively impacted spindle assembly. Pulldowns from the extracts using Crm1- or Exportin-t-beads revealed nucleoporins known to be essential for both nuclear pore and spindle assembly, with RanQ69L-GTP decreasing a subset of these target interactions. This study suggests a model where exportins, like importins, can regulate major mitotic assembly events.

The Cellular Location of Rad23, a Polyubiquitin Chain-Binding Protein, Plays a Key Role in Its Interaction with Substrates of the Proteasome

Well-studied structural motifs in Rad23 have been shown to bind polyubiquitin chains and the proteasome. These domains are predicted to enable Rad23 to transport polyubiquitylated (polyUb) substrates to the proteasome (Chen and Madura, 2002 [1]). The validation of this model, however, has been hindered by the lack of specific physiological substrates of Rad23. We report here that Rad23 can bind Ho-endonuclease (Ho-endo), a nuclear protein that initiates mating-type switching in Saccharomyces cerevisiae. We observed that the degradation of Ho-endo required export from the nucleus, in agreement with a previous report (Kaplun et al., 2003 [2]), and suggests that Rad23 can traffic proteins out of the nucleus. In agreement, the subcellular distribution of Rad23 is noticeably altered in genetic mutants that disrupt nucleocytoplasmic trafficking. Significantly, the location of Rad23 affected its binding to polyUb substrates. Mutations in nuclear export stabilized substrates, and caused accumulation in the nucleus. Importantly, Rad23 also accumulated in the nucleus in an export mutant, and bound to higher levels of polyUb proteins. In contrast, Rad23 is localized in the cytosol in rna1-1, a nucleocytoplasmic transport mutant, and it forms reduced binding to polyUb substrates. These and other studies indicate that substrates that are conjugated to polyubiquitin chains in the nucleus may rely on an export-dependent mechanism to be degraded by the proteasome. The evolutionary conservation of Rad23 and similar substrate-trafficking proteins predicts an important role for export in the turnover of nuclear proteins.

Exportin Crm1 is repurposed as a docking protein to generate microtubule organizing centers at the nuclear pore

Non-centrosomal microtubule organizing centers (MTOCs) are important for microtubule organization in many cell types. In fission yeast Schizosaccharomyces pombe, the protein Mto1, together with partner protein Mto2 (Mto1/2 complex), recruits the γ-tubulin complex to multiple non-centrosomal MTOCs, including the nuclear envelope (NE). Here, we develop a comparative-interactome mass spectrometry approach to determine how Mto1 localizes to the NE. Surprisingly, we find that Mto1, a constitutively cytoplasmic protein, docks at nuclear pore complexes (NPCs), via interaction with exportin Crm1 and cytoplasmic FG-nucleoporin Nup146. Although Mto1 is not a nuclear export cargo, it binds Crm1 via a nuclear export signal-like sequence, and docking requires both Ran in the GTP-bound state and Nup146 FG repeats. In addition to determining the mechanism of MTOC formation at the NE, our results reveal a novel role for Crm1 and the nuclear export machinery in the stable docking of a cytoplasmic protein complex at NPCs.

Release of condensin from mitotic chromosomes requires the Ran-GTP gradient in the reorganized nucleus

After mitosis, nuclear reorganization occurs together with decondensation of mitotic chromosomes and reformation of the nuclear envelope, thereby restoring the Ran-GTP gradient between the nucleus and cytoplasm. The Ran-GTP gradient is dependent on Pim1/RCC1. Interestingly, a defect in Pim1/RCC1 in Schizosaccharomyces pombe causes postmitotic condensation of chromatin, namely hypercondensation, suggesting a relationship between the Ran-GTP gradient and chromosome decondensation. However, how Ran-GTP interacts with chromosome decondensation is unresolved. To examine this interaction, we used Schizosaccharomyces japonicus, which is known to undergo partial breakdown of the nuclear membrane during mitosis. We found that Pim1/RCC1 was localized on nuclear pores, but this localization failed in a temperature-sensitive mutant of Pim1/RCC1. The mutant cells exhibited hypercondensed chromatin after mitosis due to prolonged association of condensin on the chromosomes. Conceivably, a condensin-dephosphorylation defect might cause hypercondensed chromatin, since chromosomal localization of condensin is dependent on phosphorylation by cyclin-dependent kinase (CDK). Indeed, CDK-phospho-mimic mutation of condensin alone caused untimely condensin localization, resulting in hypercondensed chromatin. Together, these results suggest that dephosphorylation of CDK sites of condensin might require the Ran-GTP gradient produced by nuclear pore-localized Pim1/RCC1.

Summary: A mutant of Pim1/RCC1 caused hypercondensed chromatin after mitosis due to prolonged association of condensin on chromosomes, suggesting that dephosphorylation of CDK sites of condensin might require Ran-GTP after mitosis.

Samp1, a RanGTP binding transmembrane protein in the inner nuclear membrane

Samp1 is a transmembrane protein of the inner nuclear membrane (INM), which interacts with the nuclear lamina and the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex in interphase and during mitosis, it localizes to the mitotic spindle. Samp1 was recently found to coprecipitate a protein complex containing Ran, a GTPase with fundamental regulatory functions both in interphase and in mitosis. To investigate the interaction between Samp1 and Ran in further detail, we have designed and expressed recombinant fusion proteins of the Chaetomium thermophilum homolog of Samp1 (Ct.Samp1) and human Ran. Pulldown experiments show that Samp1 binds directly to Ran and that Samp1 binds better to RanGTP compared to RanGDP. Samp1 also preferred RanGTP over RanGDP in living tsBN2 cells. We also show that the Ran binding domain is located between amino acids 75–135 in the nucleoplasmically exposed N-terminal tail of Samp1. This domain is unique for Samp1, without homology in any other proteins in fungi or metazoa. Samp1 is the first known transmembrane protein that binds to Ran and could provide a unique local binding site for RanGTP in the INM. Samp1 overexpression resulted in increased Ran concentrations in the nuclear periphery supporting this idea.

Nucleocytoplasmic Transport of RNAs and RNA-Protein Complexes

RNAs and ribonucleoprotein complexes (RNPs) play key roles in mediating and regulating gene expression. In eukaryotes, most RNAs are transcribed, processed and assembled with proteins in the nucleus and then either function in the cytoplasm or also undergo a cytoplasmic phase in their biogenesis. This compartmentalization ensures that sequential steps in gene expression and RNP production are performed in the correct order and it allows important quality control mechanisms that prevent the involvement of aberrant RNAs/RNPs in these cellular pathways. The selective exchange of RNAs/RNPs between the nucleus and cytoplasm is enabled by nuclear pore complexes, which function as gateways between these compartments. RNA/RNP transport is facilitated by a range of nuclear transport receptors and adaptors, which are specifically recruited to their cargos and mediate interactions with nucleoporins to allow directional translocation through nuclear pore complexes. While some transport factors are only responsible for the export/import of a certain class of RNA/RNP, others are multifunctional and, in the case of large RNPs, several export factors appear to work together to bring about export. Recent structural studies have revealed aspects of the mechanisms employed by transport receptors to enable specific cargo recognition, and genome-wide approaches have provided the first insights into the diverse composition of pre-mRNPs during export. Furthermore, the regulation of RNA/RNP export is emerging as an important means to modulate gene expression under stress conditions and in disease.

GAP Activity, but Not Subcellular Targeting, Is Required for Arabidopsis RanGAP Cellular and Developmental Functions

The Ran GTPase activating protein (RanGAP) is important to Ran signaling involved in nucleocytoplasmic transport, spindle organization, and postmitotic nuclear assembly. Unlike vertebrate and yeast RanGAP, plant RanGAP has an N-terminal WPP domain, required for nuclear envelope association and several mitotic locations of Arabidopsis thaliana RanGAP1. A double null mutant of the two Arabidopsis RanGAP homologs is gametophyte lethal. Here, we created a series of mutants with various reductions in RanGAP levels by combining a RanGAP1 null allele with different RanGAP2 alleles. As RanGAP level decreases, the severity of developmental phenotypes increases, but nuclear import is unaffected. To dissect whether the GAP activity and/or the subcellular localization of RanGAP are responsible for the observed phenotypes, this series of rangap mutants were transformed with RanGAP1 variants carrying point mutations abolishing the GAP activity and/or the WPP-dependent subcellular localization. The data show that plant development is differentially affected by RanGAP mutant allele combinations of increasing severity and requires the GAP activity of RanGAP, while the subcellular positioning of RanGAP is dispensable. In addition, our results indicate that nucleocytoplasmic trafficking can tolerate both partial depletion of RanGAP and delocalization of RanGAP from the nuclear envelope.

In vivo biochemical analyses reveal distinct roles of β-importins and eEF1A in tRNA subcellular traffic

Huang et al. developed in vivo β-importin complex co-IP assays to study the interactions of β-importins with tRNAs. Los1 (exportin-t) interacts with both unspliced and spliced tRNAs. In contrast, Msn5 (exportin-5) primarily interacts with spliced aminoacylated tRNAs. They demonstrate that Tef1/2 assembles with Msn5–tRNA complexes in a RanGTP-dependent manner.

Bidirectional tRNA movement between the nucleus and the cytoplasm serves multiple biological functions. To gain a biochemical understanding of the mechanisms for tRNA subcellular dynamics, we developed in vivo β-importin complex coimmunoprecipitation (co-IP) assays using budding yeast. Our studies provide the first in vivo biochemical evidence that two β-importin family members, Los1 (exportin-t) and Msn5 (exportin-5), serve overlapping but distinct roles in tRNA nuclear export. Los1 assembles complexes with RanGTP and tRNA. Both intron-containing pre-tRNAs and spliced tRNAs, regardless of whether they are aminoacylated, assemble into Los1–RanGTP complexes, documenting that Los1 participates in both primary nuclear export and re-export of tRNAs to the cytoplasm. In contrast, β-importin Msn5 preferentially assembles with RanGTP and spliced, aminoacylated tRNAs, documenting its role in tRNA nuclear re-export. Tef1/2 (the yeast form of translation elongation factor 1α [eEF1A]) aids the specificity of Msn5 for aminoacylated tRNAs to form a quaternary complex consisting of Msn5, RanGTP, aminoacylated tRNA, and Tef1/2. Assembly and/or stability of this quaternary complex requires Tef1/2, thereby facilitating efficient re-export of aminoacylated tRNAs to the cytoplasm.

A non-canonical mechanism for Crm1-export cargo complex assembly

The transport receptor Crm1 mediates the export of diverse cargos containing leucine-rich nuclear export signals (NESs) through complex formation with RanGTP. To ensure efficient cargo release in the cytoplasm, NESs have evolved to display low affinity for Crm1. However, mechanisms that overcome low affinity to assemble Crm1-export complexes in the nucleus remain poorly understood. In this study, we reveal a new type of RanGTP-binding protein, Slx9, which facilitates Crm1 recruitment to the 40S pre-ribosome-associated NES-containing adaptor Rio2. In vitro, Slx9 binds Rio2 and RanGTP, forming a complex. This complex directly loads Crm1, unveiling a non-canonical stepwise mechanism to assemble a Crm1-export complex. A mutation in Slx9 that impairs Crm1-export complex assembly inhibits 40S pre-ribosome export. Thus, Slx9 functions as a scaffold to optimally present RanGTP and the NES to Crm1, therefore, triggering 40S pre-ribosome export. This mechanism could represent one solution to the paradox of weak binding events underlying rapid Crm1-mediated export.

Components and regulation of nuclear transport processes

The spatial separation of DNA replication and gene transcription in the nucleus and protein translation in the cytoplasm is a uniform principle of eukaryotic cells. This compartmentalization imposes a requirement for a transport network of macromolecules to shuttle these components in and out of the nucleus. This nucleo‐cytoplasmic transport of macromolecules is critical for both cell physiology and pathology. Consequently, investigating its regulation and disease‐associated alterations can reveal novel therapeutic approaches to fight human diseases, such as cancer or viral infection. The characterization of the nuclear pore complex, the identification of transport signals and transport receptors, as well as the characterization of the Ran system (providing the energy source for efficient cargo transport) has greatly facilitated our understanding of the components, mechanisms and regulation of the nucleo‐cytoplasmic transport of proteins in our cells. Here we review this knowledge with a specific emphasis on the selection of disease‐relevant molecular targets for potential therapeutic intervention.

Atomic basis of CRM1-cargo recognition, release and inhibition

CRM1 or XPO1 is the major nuclear export receptor in the cell, which controls the nuclear-cytoplasmic localization of many proteins and RNAs. CRM1 is also a promising cancer drug target as the transport receptor is overexpressed in many cancers where some of its cargos are misregulated and mislocalized to the cytoplasm. Atomic level understanding of CRM1 function has greatly facilitated recent drug discovery and development of CRM1 inhibitors to target a variety of malignancies. Numerous atomic resolution CRM1 structures are now available, explaining how the exporter recognizes nuclear export signals in its cargos, how RanGTP and cargo bind with positive cooperativity, how RanBP1 causes release of export cargos in the cytoplasm and how diverse inhibitors such as Leptomycin B and the new KPT-SINE compounds block nuclear export. This review summarizes structure-function studies that explain CRM1-cargo recognition, release and inhibition.

Separate responses of karyopherins to glucose and amino acid availability regulate nucleocytoplasmic transport

The subcellular distribution of yeast β-importins inverts upon acute glucose deprivation, likely due to collapse of the RanGTP nuclear–cytoplasmic gradient. This redistribution of β-importins likely results in rapid widespread alterations of the traffic of macromolecules between the nucleus and cytoplasm in response to glucose limitation.

The importin-β family members (karyopherins) mediate the majority of nucleocytoplasmic transport. Msn5 and Los1, members of the importin-β family, function in tRNA nuclear export. tRNAs move bidirectionally between the nucleus and the cytoplasm. Nuclear tRNA accumulation occurs upon amino acid (aa) or glucose deprivation. To understand the mechanisms regulating tRNA subcellular trafficking, we investigated whether Msn5 and Los1 are regulated in response to nutrient availability. We provide evidence that tRNA subcellular trafficking is regulated by distinct aa-sensitive and glucose-sensitive mechanisms. Subcellular distributions of Msn5 and Los1 are altered upon glucose deprivation but not aa deprivation. Redistribution of tRNA exportins from the nucleus to the cytoplasm likely provides one mechanism for tRNA nuclear distribution upon glucose deprivation. We extended our studies to other members of the importin-β family and found that all tested karyopherins invert their subcellular distributions upon glucose deprivation but not aa deprivation. Glucose availability regulates the subcellular distributions of karyopherins likely due to alteration of the RanGTP gradient since glucose deprivation causes redistribution of Ran. Thus nuclear–cytoplasmic distribution of macromolecules is likely generally altered upon glucose deprivation due to collapse of the RanGTP gradient and redistribution of karyopherins between the nucleus and the cytoplasm.

Nuclear localization signals of varicella zoster virus ORF4

The varicella zoster virus (VZV) ORF4 protein, one of immediate-early genes protein, is associated with the tegument in purified virions. ORF4 protein functions at both transcriptional and post-transcriptional levels, present during different phase of whole VZV life cycle. ORF4 protein acts as a nucleocytoplasm shuttle protein, the precise nuclear location signals (NLS) and molecular mechanisms of nucleocytoplasm transport are not elucidated. At this study, we constructed a series of mutants, used fluorescence microscopy and Co-IP analysis to identify an unconventional bipartite NLS ((130)RKHRDRSLSNRRRRP(144)) in VZV ORF4. This study also demonstrates that nuclear import of VZV ORF4 occurs via a Ran-dependent pathway with importin-α5 and importin-β1. Additionally, NLS function of ORF4 is independent from VZV ORF62 protein. ORF62 protein cannot influence the intracellular distribution of ORF4 protein without NLS. So interaction between ORF4 and ORF62 protein is speculated to occur in nucleus. Thus, NLS is indispensable for the post-transcriptional function of ORF4.

Ran GTPase in nuclear envelope formation and cancer metastasis

Ran is a small ras-related GTPase that controls the nucleocytoplasmic exchange of macromolecules across the nuclear envelope. It binds to chromatin early during nuclear formation and has important roles during the eukaryotic cell cycle, where it regulates mitotic spindle assembly, nuclear envelope formation and cell cycle checkpoint control. Like other GTPases, Ran relies on the cycling between GTP-bound and GDP-bound conformations to interact with effector proteins and regulate these processes. In nucleocytoplasmic transport, Ran shuttles across the nuclear envelope through nuclear pores. It is concentrated in the nucleus by an active import mechanism where it generates a high concentration of RanGTP by nucleotide exchange. It controls the assembly and disassembly of a range of complexes that are formed between Ran-binding proteins and cellular cargo to maintain rapid nuclear transport. Ran also has been identified as an essential protein in nuclear envelope formation in eukaryotes. This mechanism is dependent on importin-β, which regulates the assembly of further complexes important in this process, such as Nup107-Nup160. A strong body of evidence is emerging implicating Ran as a key protein in the metastatic progression of cancer. Ran is overexpressed in a range of tumors, such as breast and renal, and these perturbed levels are associated with local invasion, metastasis and reduced patient survival. Furthermore, tumors with oncogenic KRAS or PIK3CA mutations are addicted to Ran expression, which yields exciting future therapeutic opportunities.

The molecular architecture of the plant nuclear pore complex

The nucleus contains the cell's genetic material, which directs cellular activity via gene regulation. The physical barrier of the nuclear envelope needs to be permeable to a variety of macromolecules and signals. The most prominent gateways for the transport of macromolecules are the nuclear pore complexes (NPCs). The NPC is the largest multiprotein complex in the cell, and is composed of multiple copies of ~30 different proteins called nucleoporins. Although much progress has been made in dissecting the NPC structure in vertebrates and yeast, the molecular architecture and physiological function of nucleoporins in plants remain poorly understood. In this review, we summarize the current knowledge regarding the plant NPC proteome and address structural and functional aspects of plant nucleoporins, which support the fundamental cellular machinery.

Cex1p facilitates Rna1p-mediated dissociation of the Los1p-tRNA-Gsp1p-GTP export complex

Nuclear tRNA export plays an essential role in key cellular processes such as regulation of protein synthesis, cell cycle progression, response to nutrient availability and DNA damage and development. Like other nuclear export processes, assembly of the nuclear tRNA export complex in the nucleus is dependent on Ran-GTP/Gsp1p-GTP, and dissociation of the export receptor-tRNA-Ran-GTP/Gsp1p-GTP complex in the cytoplasm requires RanBP1/Yrb1p and RanGAP/Rna1p to activate the GTPase activity of Ran-GTP/Gsp1p-GTP. The Saccharomyces cerevisiae Cex1p and Human Scyl1 have also been proposed to participate in unloading of the tRNA export receptors at the cytoplasmic face of the nuclear pore complex (NPC). Here, we provide evidence suggesting that Cex1p is required for activation of the GTPase activity of Gsp1p and dissociation of the receptor-tRNA-Gsp1p export complex in S. cerevisiae. The data suggest that Cex1p recruits Rna1p from the cytoplasm to the NPC and facilitates Rna1p activation of the GTPase activity of Gsp1p by enabling Rna1p to gain access to Gsp1p-GTP bound to the export receptor tRNA complex. It is possible that this tRNA unloading mechanism is conserved in evolutionarily diverse organisms and that other Gsp1p-GTP-dependent export processes use a pathway-specific component to recruit Rna1p to the NPC.

Ran-dependent nuclear export mediators: a structural perspective

Nuclear export is an essential eukaryotic activity. It proceeds through nuclear pore complexes (NPCs) and is mediated by soluble receptors that shuttle between nucleus and cytoplasm. RanGTPase-dependent export mediators (exportins) constitute the largest class of these carriers and are functionally highly versatile. All of these exportins load their substrates in response to RanGTP binding in the nucleus and traverse NPCs as ternary RanGTP-exportin-cargo complexes to the cytoplasm, where GTP hydrolysis leads to export complex disassembly. The different exportins vary greatly in their substrate range. Recent structural studies of both protein- and RNA-specific exporters have illuminated how exportins bind their cargoes, how Ran triggers cargo loading and how export complexes are disassembled in the cytoplasm. Here, we review the current state of knowledge and highlight emerging principles as well as prevailing questions.

RanGAP is required for post-meiotic mitosis in female gametophyte development in Arabidopsis thaliana

RanGAP is the GTPase-activating protein of the small GTPase Ran and is involved in nucleocytoplasmic transport in yeast and animals via the Ran cycle and in mitotic cell division. Arabidopsis thaliana has two copies of RanGAP, RanGAP1 and RanGAP2. To investigate the function of plant RanGAP, T-DNA insertional mutants were analysed. Arabidopsis plants with a null mutant of either RanGAP1 or RanGAP2 had no observable phenotype. Analysis of segregating progeny showed that double mutants in RanGAP1 and RanGAP2 are female gametophyte defective. Ovule clearing with differential interference contrast optics showed that mutant female gametophytes were arrested at interphase, predominantly after the first mitotic division following meiosis. In contrast, mutant pollen developed and functioned normally. These results show that the two RanGAPs are redundant and indispensable for female gametophyte development in Arabidopsis but dispensable for pollen development. Nuclear division arrest during a mitotic stage suggests a role for plant RanGAP in mitotic cell cycle progression during female gametophyte development.

Probing a structural model of the nuclear pore complex channel through molecular dynamics

The central pore of a nuclear pore complex (NPC) is filled with unstructured proteins that contain many FG-repeats separated by hydrophilic regions. An example of such protein is nsp1. By simulating an array of nsp1 segments, we identified, in an earlier study, a spontaneously formed brushlike structure that promises to explain selective transport in the NPC channel. Here we report four (350,000 atom, 200 ns) simulations probing this structure via its interaction with transport receptor NTF2 as well as with an inert protein. NTF2 dimers are observed to gradually enter the brush, but the inert protein is not. Both NTF2 and the inert protein are found to bind to FG-repeats, but binding periods lasted more briefly for the inert protein. A simulation also investigated the behavior of a brush made of mutant nsp1 that is known to be less effective in NPC-selective transport, finding that this brush does not attract NTF2.

Targeting proteins to the plant nuclear envelope

The nuclear envelope and the nuclear pore are important structures that both separate and selectively connect the nucleoplasm and the cytoplasm. The requirements for specific targeting of proteins to the plant nuclear envelope and nuclear pore are poorly understood. How are transmembrane-domain proteins sorted to the nuclear envelope and nuclear pore membranes? What protein–protein interactions are involved in associating other proteins to the nuclear pore? Are there plant-specific aspects to these processes? We are using the case of the nuclear pore-associated Ran-cycle component RanGAP (Ran GTPase-activating protein) to address these fundamental questions. Plant RanGAP is targeted to the nuclear pore by a plant-specific mechanism involving two families of nuclear pore-associated proteins [WIP (WPP-domain-interacting protein) and WIT (WPP-domain-interacting tail-anchored protein)] not found outside the land plant lineage. One protein family (WIP or WIT) is sufficient for RanGAP targeting in differentiated root cells, whereas both families are necessary in meristematic cells. A C-terminal predicted transmembrane domain is sufficient for targeting WIP proteins to the nuclear envelope. Nuclear-envelope targeting of WIT proteins requires a coiled-coil domain and is facilitated by HSC70 (heat-shock cognate 70 stress protein) chaperones and a class of plant-specific proteins resembling the RanGAP-targeting domain (WPP proteins). Taken together, this sheds the first light on the requirements and interdependences of nuclear envelope and nuclear pore targeting in land plants.

Mechanism and a peptide motif for targeting peripheral proteins to the yeast inner nuclear membrane

Trm1 is a tRNA specific m(2)(2)G methyltransferase shared by nuclei and mitochondria in Saccharomyces cerevisiae. In nuclei, Trm1 is peripherally associated with the inner nuclear membrane (INM). We investigated the mechanism delivering/tethering Trm1 to the INM. Analyses of mutations of the Ran pathway and nuclear pore components showed that Trm1 accesses the nucleoplasm via the classical nuclear import pathway. We identified a Trm1 cis-acting sequence sufficient to target passenger proteins to the INM. Detailed mutagenesis of this region uncovered specific amino acids necessary for authentic Trm1 to locate at the INM. The INM information is contained within a sequence of less than 20 amino acids, defining the first motif for addressing a peripheral protein to this important subnuclear location. The combined studies provide a multi-step process to direct Trm1 to the INM: (i) translation in the cytoplasm; (ii) Ran-dependent import into the nucleoplasm; and (iii) redistribution from the nucleoplasm to the INM via the INM motif. Furthermore, we demonstrate that the Trm1 mitochondrial targeting and nuclear localization signals are in competition with each other, as Trm1 becomes mitochondrial if prevented from entering the nucleus.

Going green: plants' alternative way to position the Ran gradient

Ran is a multi-functional small GTPase of the Ras super-family involved in nucleocytoplasmic transport, mitotic spindle assembly, cell cycle control and nuclear envelope (NE) formation. Its roles are accomplished by the asymmetric distribution of its GTP- and GDP-bound forms, enabled by the specific localization of Ran accessory proteins, the Ran GTPase-activating protein RanGAP and the nucleotide exchange factor RCC1. Mammalian RanGAP1 is targeted to the NE during interphase and to the spindle and kinetochores during mitosis via a SUMOylated C-terminal domain and interaction with the nucleoporin Nup358/RanBP2. Arabidopsis RanGAP1 (AtRanGAP1) lacks the SUMOylated C-terminal domain of vertebrate RanGAP, but contains a plant-specific N-terminal domain (WPP domain), which is necessary and sufficient for its targeting to the NE in interphase. AtRanGAP1 has a mitotic trafficking pattern uniquely different from that of vertebrate RanGAP, which includes targeting to the outward-growing rim of the cell plate. The WPP domain is necessary and sufficient for this targeting. Now, a novel family of plant-specific, nuclear pore-associated proteins has been identified in Arabidopsis, which is essential for anchoring RanGAP to the Arabidopsis nuclear envelope at the root meristem. This suggests that RanGAP anchoring to the nuclear pore has been solved in two fundamentally different ways in animals and plants. These findings support a separate evolution of RanGAP targeting mechanisms in different kingdoms, possibly related to different functional geometries of the Ran gradient in animal and higher plant cell division.

Spatial and temporal coordination of mitosis by Ran GTPase

The small nuclear GTPase Ran controls the directionality of macromolecular transport between the nucleus and the cytoplasm. Ran also has important roles during mitosis, when the nucleus is dramatically reorganized to allow chromosome segregation. Ran directs the assembly of the mitotic spindle, nuclear-envelope dynamics and the timing of cell-cycle transitions. The mechanisms that underlie these functions provide insights into the spatial and temporal coordination of the changes that occur in intracellular organization during the cell-division cycle.

Two distinct interacting classes of nuclear envelope-associated coiled-coil proteins are required for the tissue-specific nuclear envelope targeting of Arabidopsis RanGAP

Ran GTPase plays essential roles in multiple cellular processes, including nucleocytoplasmic transport, spindle formation, and postmitotic nuclear envelope (NE) reassembly. The cytoplasmic Ran GTPase activating protein RanGAP is critical to establish a functional RanGTP/RanGDP gradient across the NE and is associated with the outer surface of the NE in metazoan and higher plant cells. Arabidopsis thaliana RanGAP association with the root tip NE requires a family of likely plant-specific nucleoporins combining coiled-coil and transmembrane domains (CC-TMD) and WPP domain-interacting proteins (WIPs). We have now identified, by tandem affinity purification coupled with mass spectrometry, a second family of CC-TMD proteins, structurally similar, yet clearly distinct from the WIP family, that is required for RanGAP NE association in root tip cells. A combination of loss-of-function mutant analysis and protein interaction data indicates that at least one member of each NE-associated CC-TMD protein family is required for RanGAP targeting in root tip cells, while both families are dispensable in other plant tissues. This suggests an unanticipated complexity of RanGAP NE targeting in higher plant cells, contrasting both the single nucleoporin anchor in metazoans and the lack of targeting in fungi and proposes an early evolutionary divergence of the underlying plant and animal mechanisms.

Nucleocytoplasmic transport of proteins

In eukaryotic cells, the movement of macromolecules between the nucleus and cytoplasm occurs through the nuclear pore complex (NPC)--a large protein complex spanning the nuclear envelope. The nuclear transport of proteins is usually mediated by a family of transport receptors known as karyopherins. Karyopherins bind to their cargoes via recognition of nuclear localization signal (NLS) for nuclear import or nuclear export signal (NES) for export to form a transport complex. Its transport through NPC is facilitated by transient interactions between the karyopherins and NPC components. The interactions of karyopherins with their cargoes are regulated by GTPase Ran. In the current review, we describe the NPC structure, NLS, and NES, as well as the model of classic Ran-dependent transport, with special emphasis on existing alternative mechanisms; we also propose a classification of the basic mechanisms of protein transport regulation.

Cse1p-binding dynamics reveal a binding pattern for FG-repeat nucleoporins on transport receptors

Nuclear pore proteins with phenylalanine-glycine repeats are vital to the functional transport of molecules across the nuclear pore complex. The current study investigates the binding of these FG-nucleoporins to the Cse1p:Kap60p:RanGTP nuclear export complex. Fourteen binding spots for FG-nucleoporin peptides are revealed on the surface of Cse1p, and 5 are revealed on the Kap60p surface. Taken together, and along with binding data for two other transport receptors, the data suggest that the ability to bind FG-nucleoporins by itself is not enough to ensure viable nuclear transport. Rather, it is proposed that the density of binding spots on the transport receptor surface is key in determining transport viability. The number of binding spots on the transport receptor surface should be large enough to ensure multiple, simultaneous FG-repeat binding, and their arrangement should be close enough to ensure multiple binding from the same FG-nucleoporin.

Kinetochore capture and bi-orientation on the mitotic spindle

Kinetochores are large protein complexes that are formed on chromosome regions known as centromeres. For high-fidelity chromosome segregation, kinetochores must be correctly captured on the mitotic spindle before anaphase onset. During prometaphase, kinetochores are initially captured by a single microtubule that extends from a spindle pole and are then transported poleward along the microtubule. Subsequently, microtubules that extend from the other spindle pole also interact with kinetochores and, eventually, each sister kinetochore attaches to microtubules that extend from opposite poles - this is known as bi-orientation. Here we discuss the molecular mechanisms of these processes, by focusing on budding yeast and drawing comparisons with other organisms.

Perinuclear and nuclear envelope localizations of Arabidopsis Ran proteins

Using phragmoplastin-interacting protein 1 (PhrIP1) as bait, we isolated an Arabidopsis cDNA encoding Ran2, a small Ras-like GTP-binding protein. The interaction between PhrIP1 and Ran2 was confirmed by an in vitro protein-protein interaction assay with purified Ran2 and PhrIP1. The plant Ran2 shares high sequence homology, 78 and 86% at the amino acid level, with human Ran/TC4 and C. elegans Ran, respectively. Our results obtained from enzyme assays and Western blot analysis show that Ran2 has intrinsic GTPase activity and is present in the soluble fraction of Arabidopsis seedling extract. Fluorescent microscopy using anti-Ran2 antibody revealed that the Ran protein is localized in the perinuclear region with the highest concentration at the nuclear envelope. In contrast to its animal counterparts that are present in the nucleoplasm, the Ran protein is absent inside the nucleus. These results suggest that plant Ran proteins may be involved in mediation of nucleocytoplasmic transport and assembly of the nuclear envelope after karyokinesis in plant cells.

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.

The nucleoporin Nup214 sequesters CRM1 at the nuclear rim and modulates NFkappaB activation in Drosophila

CRM1-mediated protein export is an important determinant of the nuclear accumulation of many gene regulators. Here, we show that the NFkappaB transcription factor Dorsal is a substrate of CRM1 and requires the nucleoporin Nup214 for its nuclear translocation upon signaling. Nup214 bound to CRM1 directly and anchored it to the nuclear envelope. In nup214 mutants CRM1 accumulated in the nucleus and NES-protein export was enhanced. Nup214 formed complexes with Nup88 and CRM1 in vivo and Nup214 protected Nup88 from degradation at the nuclear rim. In turn, Nup88 was sufficient for targeting the complex to the nuclear pores. Overexpression experiments indicated that Nup214 alone attracts a fraction of CRM1 to the nuclear envelope but does not interfere with NES-GFP export. By contrast, overexpression of the Nup214-Nup88 complex trapped CRM1 and Dorsal to cytoplasmic foci and inhibited protein export and immune response activation. We hypothesize that variation in levels of the Nup214-Nup88 complex at the pore changes the amount of NPC-bound CRM1 and influences the relative strength and duration of NFkappaB signaling responses.

Cellular dynamics of small RNAs

This review highlights the unexpectedly complicated nuclear egress and nuclear import of small RNAs. Although nucleus/cytoplasm trafficking was thought to be restricted to snRNAs of many, but not all, eukaryotes, recent data indicate that such traffic may be more common than previously appreciated. First, in conflict with numerous previous reports, new information indicates that Saccharomyces cerevisiae snRNAs may cycle between the nucleus and the cytoplasm. Second, recent studies also provide evidence that other small RNAs that function exclusively in the nucleus-the budding yeast telomerase RNA and possibly small nucleolar RNAs-may exit to the cytoplasm, only to return to the nucleus. Third, nucleus/cytoplasm cycling of RNAs also occurs for RNAs that function solely in the cytoplasm, as it has been discovered that cytoplasmic tRNAs of budding yeast travel "retrograde" to the nucleus and, perhaps, back again to the cytoplasm to function in protein synthesis. Fourth, there is at least one example in ciliates of small double-stranded RNAs traveling multiple cycles between the cytoplasm and distinct nuclei to direct genome structure. This report discusses data that support or argue against nucleus/cytoplasm bidirectional movement for each category of small RNA and the possible roles that such movement may serve.

Extracellular signal-dependent nuclear import of STAT3 is mediated by various importin alphas

The signal transducer and activator of transcription 3 (STAT3) is a transcription factor that is involved in a variety of biological functions. STAT3 is activated by cytokines and growth factors via the phosphorylation of a tyrosine residue, dimerization, and subsequent nuclear translocation. However, the mechanism of its nuclear translocation is unclear. A study of the cytokine-stimulated import of STAT3 into the nucleus is reported herein. An oncostatin M (OSM)-dependent nuclear import assay system was first established in living cells. Using this system, we demonstrated that the microinjection of the importin alpha5/NPI-1 mutant, an anti-importin beta antibody, and the RanQ69L mutant inhibited the nuclear import of STAT3. Second, we showed that tyrosine-phosphorylated STAT3 associates, not only with importin alpha5/NPI-1 but also with other importin alphas, as a result of OSM stimulation, as evidenced by a solution binding assay. These findings suggest that the extracellular signal-dependent nuclear transport of STAT3 is mediated by various importin alphas, importin beta, and Ran.

Plant-specific mitotic targeting of RanGAP requires a functional WPP domain

The small GTPase Ran is involved in nucleocytoplasmic transport, spindle formation, nuclear envelope (NE) formation, and cell-cycle control. In vertebrates, these functions are controlled by a three-dimensional gradient of Ran-GTP to Ran-GDP, established by the spatial separation of Ran GTPase-activating protein (RanGAP) and the Ran guanine nucleotide exchange factor RCC1. While this spatial separation is established by the NE during interphase, it is orchestrated during mitosis by association of RCC1 with the chromosomes and RanGAP with the spindle and kinetochores. SUMOylation of vertebrate RanGAP1 is required for NE, spindle, and centromere association. Arabidopsis RanGAP1 (AtRanGAP1) lacks the SUMOylated C-terminal domain of vertebrate RanGAP, but contains a plant-specific N-terminal domain (WPP domain), which is necessary and sufficient for its targeting to the NE in interphase. Here we show that the human and plant RanGAP-targeting domains are kingdom specific. AtRanGAP1 has a mitotic trafficking pattern uniquely different from that of vertebrate RanGAP, which includes targeting to the outward-growing rim of the cell plate. The WPP domain is necessary and sufficient for this targeting. Point mutations in conserved residues of the WPP domain also abolish targeting to the nuclear rim and the cell plate, suggesting that the same mechanism is involved in both targeting events. These results indicate that plant and animal RanGAPs undergo different migration patterns during cell division, which require their kingdom-specific targeting domains.

Partial nuclear pore complex disassembly during closed mitosis in Aspergillus nidulans

BACKGROUND

Many organisms undergo closed mitosis and locate tubulin and mitotic kinases to nuclei only during mitosis. How this is regulated is unknown. Interestingly, the NIMA kinase of Aspergillus nidulans interacts with two nuclear pore complex (NPC) proteins and NIMA is required for mitotic localization of the Cdk1 kinase to nuclei. Therefore, we wished to define the mechanism by which the NPC is regulated during A. nidulans' closed mitosis.

RESULTS

The structural makeup of the NPC is dramatically changed during A. nidulans' mitosis. At least five NPC proteins disperse throughout the cell during mitosis while at least three structural components remain at the NPC. These modifications correlate with marked changes in the function of the NPC. Notably, during mitosis, An-RanGAP is not excluded from nuclei, and five other nuclear or cytoplasmic proteins investigated fail to locate as they do during interphase. Mitotic modification of the NPC requires NIMA and Cdk1 kinase activation. NIMA appears to be particularly important. Most strikingly, ectopic induction of NIMA promotes mitotic-like changes in NPC structure and function during S phase. Furthermore, NIMA locates to the NPC during entry into mitosis, and a dominant-negative version of NIMA that causes G2 delay dwells at the NPC.

CONCLUSIONS

We conclude that partial NPC disassembly under control of NIMA and Cdk1 in A. nidulans may represent a new mechanism for regulating closed mitoses. We hypothesize that proteins locate by their relative binding affinities within the cell during A. nidulans' closed mitosis, analogous to what occurs during open mitosis.

The plant nuclear envelope

This review summarizes our present knowledge about the composition and function of the plant nuclear envelope. Compared with animals or yeast, our molecular understanding of the nuclear envelope in higher plants is in its infancy. However, fundamental differences in the structure and function of the plant and animal nuclear envelope have already been found. Here, we compare and contrast these differences with respect to nuclear pore complexes, targeting of Ran signaling to the nuclear envelope, inner nuclear envelope proteins, and the role and fate of the nuclear envelope during mitosis. Further investigation of the emerging fundamental differences as well as the similarities between kingdoms might illuminate why there appears to be more than one blueprint for building a nucleus.

Biochemical characterization of the Ran-RanBP1-RanGAP system: are RanBP proteins and the acidic tail of RanGAP required for the Ran-RanGAP GTPase reaction?

RanBP type proteins have been reported to increase the catalytic efficiency of the RanGAP-mediated GTPase reaction on Ran. Since the structure of the Ran-RanBP1-RanGAP complex showed RanBP1 to be located away from the active site, we reinvestigated the reaction using fluorescence spectroscopy under pre-steady-state conditions. We can show that RanBP1 indeed does not influence the rate-limiting step of the reaction, which is the cleavage of GTP and/or the release of product P(i). It does, however, influence the dynamics of the Ran-RanGAP interaction, its most dramatic effect being the 20-fold stimulation of the already very fast association reaction such that it is under diffusion control (4.5 x 10(8) M(-1) s(-1)). Having established a valuable kinetic system for the interaction analysis, we also found, in contrast to previous findings, that the highly conserved acidic C-terminal end of RanGAP is not required for the switch-off reaction. Rather, genetic experiments in Saccharomyces cerevisiae demonstrate a profound effect of the acidic tail on microtubule organization during mitosis. We propose that the acidic tail of RanGAP is required for a process during mitosis.

A panoramic view of yeast noncoding RNA processing

Predictive analysis using publicly available yeast functional genomics and proteomics data suggests that many more proteins may be involved in biogenesis of ribonucleoproteins than are currently known. Using a microarray that monitors abundance and processing of noncoding RNAs, we analyzed 468 yeast strains carrying mutations in protein-coding genes, most of which have not previously been associated with RNA or RNP synthesis. Many strains mutated in uncharacterized genes displayed aberrant noncoding RNA profiles. Ten factors involved in noncoding RNA biogenesis were verified by further experimentation, including a protein required for 20S pre-rRNA processing (Tsr2p), a protein associated with the nuclear exosome (Lrp1p), and a factor required for box C/D snoRNA accumulation (Bcd1p). These data present a global view of yeast noncoding RNA processing and confirm that many currently uncharacterized yeast proteins are involved in biogenesis of noncoding RNA.

The auto-inhibitory function of importin alpha is essential in vivo

Proteins that contain a classical nuclear localization signal (NLS) are recognized in the cytoplasm by a heterodimeric import receptor composed of importin/karyopherin alpha and beta. The importin alpha subunit recognizes classical NLS sequences, and the importin beta subunit directs the complex to the nuclear pore. Recent work shows that the N-terminal importin beta binding (IBB) domain of importin alpha regulates NLS-cargo binding in the absence of importin beta in vitro. To analyze the in vivo functions of the IBB domain, we created a series of mutants in the Saccharomyces cerevisiae importin alpha protein. These mutants dissect the two functions of the N-terminal IBB domain, importin beta binding and auto-inhibition. One of these importin alpha mutations, A3, decreases auto-inhibitory function without impacting binding to importin beta or the importin alpha export receptor, Cse1p. We used this mutant to show that the auto-inhibitory function is essential in vivo and to provide evidence that this auto-inhibitory-defective importin alpha remains bound to NLS-cargo within the nucleus. We propose a model where the auto-inhibitory activity of importin alpha is required for NLS-cargo release and the subsequent Cse1p-dependent recycling of importin alpha to the cytoplasm.

Nuclear RanGTP is not required for targeting small nucleolar RNAs to the nucleolus

The small GTPase Ran is the central regulator of macromolecular transport between the cytoplasm and the nucleus. Recent work has suggested that RanGTP also plays an important role in regulating some intra-nuclear processes. In this study, we have investigated whether RanGTP is required for the intra-nuclear transport of RNAs. Specifically, we directly analyzed the nucleolar localization of Box C/D and Box H/ACA small nucleolar RNAs (snoRNAs) in mammalian (tsBN2) cells, Saccharomyces cerevisiae and Xenopus oocytes under conditions that deplete nuclear RanGTP and prevent RNA export to the cytoplasm. Our data suggest that depletion of nuclear RanGTP does not significantly alter the nucleolar localization of U3 snoRNA in tsBN2 cells. Complementary studies in the budding yeast S. cerevisiae using conditional Ran mutants as well as mutants in Ran regulatory proteins also indicate that disruption of the Ran gradient or of Ran itself does not detectably affect the nucleolar localization of snoRNAs. Finally, microinjection into Xenopus oocytes was used to clearly demonstrate that a specific pool of snoRNAs could still be efficiently targeted to the nucleolus even when the RanGTP gradient was disrupted by microinjection of mutant Ran proteins. Taken together, our data from three phylogenetically distinct experimental systems suggest that nuclear RanGTP, which is essential for trafficking of RNAs between the nuclear and cytoplasmic compartments, is not required for nuclear retention or nucleolar localization of snoRNAs.

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.

Coupling of termination, 3' processing, and mRNA export

In a screen to identify genes required for mRNA export in Saccharomyces cerevisiae, we isolated an allele of poly(A) polymerase (PAP1) and novel alleles encoding several other 3' processing factors. Many newly isolated and some previously described mutants (rna14-48, rna14-49, rna14-64, rna15-58, and pcf11-1 strains) are defective in polymerase II (Pol II) termination but, interestingly, retain the ability to polyadenylate these improperly processed transcripts at the nonpermissive temperature. Deletion of the cis-acting sequences required to couple 3' processing and termination also produces transcripts that fail to exit the nucleus, suggesting that all of these processes (cleavage, termination, and export) are coupled. We also find that several but not all mRNA export mutants produce improperly 3' processed transcripts at the nonpermissive temperature. 3' maturation defects in mRNA export mutants include improper Pol II termination and/or the previously characterized hyperpolyadenylation of transcripts. Importantly, not all mRNA export mutants have defects in 3' processing. The similarity of the phenotypes of some mRNA export mutants and 3' processing mutants indicates that some factors from each process may mechanistically interact to couple mRNA processing and export. Consistent with this assumption, we present evidence that Xpo1p interacts in vivo with several 3' processing factors and that the addition of recombinant Xpo1p to in vitro processing reaction mixtures stimulates 3' maturation. Of the core 3' processing factors tested (Rna14p, Rna15p, Pcf11p, Hrp1p, Fip1p, and Cft1p), only Hrp1p shuttles. Overexpression of Rat8p/Dbp5p suppresses both 3' processing and mRNA export defects found in xpo1-1 cells.