Ran at a glance

B-type Plexins promote the GTPase activity of Ran to affect androgen receptor nuclear translocation in prostate cancer

Resistance to anti-androgen therapy for metastatic prostate cancer is a major clinical problem. Sema3C promotes resistance to androgen withdrawal via its receptor, PlexinB1. Activation of PlexinB1 promotes the ligand-independent nuclear translocation of the androgen receptor (AR), which may contribute to resistance to androgen deprivation therapy. However, the mechanism by which PlexinB1 promotes nuclear translocation is unclear. We show here that PlexinB1 and B2 regulate nuclear import by acting as GTPase activating proteins (GAPs) for the small RasGTPase Ran, a key regulator of nuclear trafficking. Purified PlexinB1/B2 protein catalyses the hydrolysis of RanGTP, and mutations in the GAP domain of PlexinB1 inhibit this activity. Activation of PlexinB1/B2 with Sema4D decreases the levels of RanGTP, while PlexinB1 or B2 depletion increases the levels of activated Ran in the cell. Ran directly associates with B-type plexins in a GTP-dependent manner. Sema4D is internalised by endocytosis, and PlexinB1 and Ran display overlapping patterns of expression. Furthermore, Sema4D/PlexinB1-induced AR nuclear translocation is dependent on the GAP domain of PlexinB1 and is blocked by the expression of non-functional Ran mutants. Depletion of PlexinB1 decreases the nuclear/cytoplasmic ratio of Ran, indicative of a higher RanGTP/GDP ratio. Plexins may promote the growth of androgen-independent prostate cancer through their activity as RanGAPs.

Small GTPase Ran: Depicting the nucleotide-specific conformational landscape of the functionally important C-terminus

The small GTPase Ran is the main regulator of the nucleo-cytoplasmic import and export through the nuclear pore complex. It functions as a molecular switch cycling between the GDP-bound inactive and GTP-bound active state. It consists of a globular (G) domain and a C-terminal region, which is bound to the G-domain in the inactive, GDP-bound states. Crystal structures of the GTP-bound active form complexed with Ran binding proteins (RanBP) show that the C-terminus undergoes a large conformational change, embracing Ran binding domains (RanBD). Whereas in the crystal structures of macromolecular complexes not containing RanBDs the structure of the C-terminal segment remains unresolved, indicating its large conformational flexibility. This movement could not have been followed either by experimental or simulation methods. Here, starting from the crystal structure of Ran in both GDP- and GTP-bound forms we show how rigid the C-terminal region in the inactive structure is during molecular dynamics (MD) simulations. Furthermore, we show how MD simulations of the active form are incapable of mapping the open conformations of the C-terminus. By using the MDeNM (Molecular Dynamics with excited Normal Modes) method, we were able to widely map the conformational surface of the C-terminus of Ran in the active GTP-bound form, which allows us to envisage how it can embrace RanBDs.

The Parapoynx stagnalis Nucleopolyhedrovirus (PastNPV), a Divergent Member of the Alphabaculovirus Group I Clade, Encodes a Homolog of Ran GTPase

We report the analysis of the genome of a novel Alphabaculovirus, Parapoynx stagnalis nucleopolyhedrovirus isolate 473 (PastNPV-473), from cadavers of the rice case bearer, Parapoynx stagnalis Zeller (Lepidoptera: Crambidae), collected in rice fields in Kerala, India. High-throughput sequencing of DNA from PastNPV occlusion bodies and assembly of the data yielded a circular genome-length contig of 114,833 bp with 126 annotated opening reading frames (ORFs) and six homologous regions (hrs). Phylogenetic inference based on baculovirus core gene amino acid sequence alignments indicated that PastNPV is a member of the group I clade of viruses in genus Alphabaculovirus, but different phylogenetic methods yielded different results with respect to the placement of PastNPV and four similarly divergent alphabaculoviruses in the group I clade. Branch lengths and Kimura-2-parameter pairwise nucleotide distances indicated that PastNPV-473 cannot be classified in any of the currently listed species in genus Alphabaculovirus. A unique feature of the PastNPV genome was the presence of an ORF encoding a homolog of Ran GTPase, a regulator of nucleocytoplasmic trafficking. PastNPV appears to have acquired a homolog of Ran relatively recently from a lepidopteran host via horizontal gene transfer.

RanGTPase links nucleo-cytoplasmic transport to the recruitment of cargoes into small extracellular vesicles

Small extracellular vesicle (sEV)-mediated intercellular communication regulates multiple aspects of growth and development in multicellular organisms. However, the mechanism underlying cargo recruitment into sEVs is currently unclear. We show that the key nucleo-cytoplasmic transport (NCT) protein-RanGTPase, in its GTP-bound form (RanGTP), is enriched in sEVs secreted by mammalian cells. This recruitment of RanGTP into sEVs depends on the export receptor CRM1 (also called XPO1). The recruitment of GAPDH, a candidate cargo protein, into sEVs is regulated by the RanGTP-CRM1axis in a nuclear export signal (NES)-dependent manner. Perturbation of NCT through overexpression or depletion of nuclear transport components affected the recruitment of Ran, CRM1 and GAPDH into sEVs. Our studies, thus, suggest a link between NCT, particularly the Ran-CRM1 axis, and recruitment of NES-containing cargoes into the sEVs. Collectively, these findings implicate RanGTPase as a link between NCT and sEV mediated intercellular communication.

Arabidopsis RAN GTPases are critical for mitosis during male and female gametogenesis

The development of male and female gametophytes is a prerequisite for successful propagation of angiosperms. The small GTPases RAN play fundamental roles in numerous cellular processes. Although RAN GTPases have been characterized in plants, their roles in cellular processes are far from understood. We report here that RAN GTPases in Arabidopsis are critical for gametophytic development. RAN1 loss-of-function showed no defects in gametophytic development likely due to redundancy. However, the expression of a dominant negative or constitutively active RAN1 resulted in gametophytic lethality. Genetic interference of RAN GTPases caused the arrest of pollen mitosis I and of mitosis of functional megaspores, implying a key role of properly regulated RAN activity in mitosis during gametophytic development.

Small-Molecule RAS Inhibitors as Anticancer Agents: Discovery, Development, and Mechanistic Studies

Mutations of RAS oncogenes are responsible for about 30% of all human cancer types, including pancreatic, lung, and colorectal cancers. While KRAS1 is a pseudogene, mutation of KRAS2 (commonly known as KRAS oncogene) is directly or indirectly associated with human cancers. Among the RAS family, KRAS is the most abundant oncogene related to uncontrolled cellular proliferation to generate solid tumors in many types of cancer such as pancreatic carcinoma (over 80%), colon carcinoma (40-50%), lung carcinoma (30-50%), and other types of cancer. Once described as 'undruggable', RAS proteins have become 'druggable', at least to a certain extent, due to the continuous efforts made during the past four decades. In this account, we discuss the chemistry and biology (wherever available) of the small-molecule inhibitors (synthetic, semi-synthetic, and natural) of KRAS proteins that were published in the past decades. Commercial drugs, as well as investigational molecules from preliminary stages to clinical trials, are categorized and discussed in this study. In summary, this study presents an in-depth discussion of RAS proteins, classifies the RAS superfamily, and describes the molecular mechanism of small-molecule RAS inhibitors.

RAN and YBX1 are required for cell proliferation and IL-4 expression and linked to poor prognosis in oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) is one of the most common malignancies in the world, with a high mortality rate. RAN is a member of the Ras GTPase family and is overexpressed in a range of cancers, however, the relationship between RAN and OSCC is rarely reported. In this study, we found that RAN is overexpressed in OSCC tissues. RAN inhibition retarded OSCC cell proliferation and led to apoptosis and cell cycle arrest. Knockdown of RAN inhibited tumor growth in vivo. Strikingly, we found that RAN and oncogene Y-box binding protein-1 (YBX1) are positively associated with the immune infiltrates of CD4⁺ Th2 cells in multiple types of cancer, and can promote IL-4 expression. IL-4 treatment can partially rescue RAN knockdown-induced cell apoptosis in OSCC cells. Moreover, overexpression of RAN could rescue cell growth inhibition caused by knockdown of YBX1. Furthermore, patients with low expression of both RAN and YBX1 had better overall survival than others. Collectively, these findings indicate that RAN is a target of YBX1. RAN and YBX1 are required for cell proliferation and IL-4 expression. RAN and YBX1 are co-expressed and can serve as potential co-biomarkers for poor prognosis in OSCC.

GEF-independent Ran activation shifts a fraction of the protein to the cytoplasm and promotes cell proliferation

Ran (Ras-related nuclear protein) plays several important roles in nucleo-cytoplasmic transport, mitotic spindle formation, nuclear envelope/nuclear pore complex assembly, and other functions in the cytoplasm, as well as in cellular transformation when switched on. Unlike other members of the GTPase superfamily, Ran binds more tightly to GDP than to GTP due to the presence of an auto-inhibitory C-terminal tail. Multiple missense mutations in the C-terminus of Ran occur in cancers, but their biological significance remains unclear. Here, the quantitative GDP/GTP binding preference of four engineered mutations with unstable C-termini was analyzed using a devised mant-GDP dissociation assay. The results showed that the impact of different C-terminal mutations depends on multiple factors. Although these mutants were more GTP-loaded in human cells, they were shown to be more cytoplasmic, and to support nuclear transport with minimally or partially reduced efficiency. Further, several Ran cancer mutants were compromised in autoinhibition, slightly more GTP-bound, more cytoplasmic, and enhanced the proliferation of A549 and HeLa cells in vitro. Thus, our work reveals a new route of Ran activation independent of guanine nucleotide exchange factor (GEF), which may account for the hyper-proliferation induced by Ran cancer mutations.

Emerging molecular functions and novel roles for the DEAD-box protein Dbp5/DDX19 in gene expression

The DEAD-box protein (DBP) Dbp5, a member of the superfamily II (SFII) helicases, has multiple reported roles in gene expression. First identified as an essential regulator of mRNA export in Saccharomyces cerevisiae, the enzyme now has reported functions in non-coding RNA export, translation, transcription, and DNA metabolism. Localization of the protein to various cellular compartments (nucleoplasm, nuclear envelope, and cytoplasm) highlights the ability of Dbp5 to modulate different stages of the RNA lifecycle. While Dbp5 has been well studied for > 20 years, several critical questions remain regarding the mechanistic principles that govern Dbp5 localization, substrate selection, and functions in gene expression. This review aims to take a holistic view of the proposed functions of Dbp5 and evaluate models that accommodate current published data.

Evolution and function of bacterial RCC1 repeat effectors

Intracellular bacterial pathogens harbour genes, the closest homologues of which are found in eukaryotes. Regulator of chromosome condensation 1 (RCC1) repeat proteins are phylogenetically widespread and implicated in protein-protein interactions, such as the activation of the small GTPase Ran by its cognate guanine nucleotide exchange factor, RCC1. Legionella pneumophila and Coxiella burnetii, the causative agents of Legionnaires' disease and Q fever, respectively, harbour RCC1 repeat coding genes. Legionella pneumophila secretes the RCC1 repeat 'effector' proteins LegG1, PpgA and PieG into eukaryotic host cells, where they promote the activation of the pleiotropic small GTPase Ran, microtubule stabilisation, pathogen vacuole motility and intracellular bacterial growth as well as host cell migration. The RCC1 repeat effectors localise to the pathogen vacuole or the host plasma membrane and target distinct components of the Ran GTPase cycle, including Ran modulators and the small GTPase itself. Coxiella burnetii translocates the RCC1 repeat effector NopA into host cells, where the protein localises to nucleoli. NopA binds to Ran GTPase and promotes the nuclear accumulation of Ran(GTP), thus pertubing the import of the transcription factor NF-κB and innate immune signalling. Hence, divergent evolution of bacterial RCC1 repeat effectors defines the range of Ran GTPase cycle targets and likely allows fine-tuning of Ran GTPase activation by the pathogens at different cellular sites.

EIF4G1 and RAN as Possible Drivers for Malignant Pleural Mesothelioma

For malignant pleural mesothelioma (MPM) novel therapeutic strategies are urgently needed. In a previous study, we identified 51 putative cancer genes over-expressed in MPM tissues and cell lines. Here, we deepened the study on nine of them (ASS1, EIF4G1, GALNT7, GLUT1, IGF2BP3 (IMP3), ITGA4, RAN, SOD1, and THBS2) to ascertain whether they are truly mesothelial cancer driver genes (CDGs) or genes overexpressed in an adaptive response to the tumoral progression (“passenger genes”). Through a fast siRNA-based screening, we evaluated the consequences of gene depletion on migration, proliferation, colony formation capabilities, and caspase activities of four MPM (Mero-14, Mero-25, IST-Mes2, and NCI-H28) and one SV40-immortalized mesothelial cell line (MeT-5A) as a non-malignant model. The depletion of EIF4G1 and RAN significantly reduced cell proliferation and colony formation and increased caspase activity. In particular, the findings for RAN resemble those observed for other types of cancer. Thus, we evaluated the in vitro effects of importazole (IPZ), a small molecule inhibitor of the interaction between RAN and importin-β. We showed that IPZ could have effects similar to those observed following RAN gene silencing. We also found that primary cell lines from one out of three MPM patients were sensitive to IPZ. As EIF4G1 and RAN deserve further investigation with additional in vitro and in vivo studies, they emerged as promising CDGs, suggesting that their upregulation could play a role in mesothelial tumorigenesis and aggressiveness. Furthermore, present data propose the molecular pathways dependent on RAN as a putative pharmacological target for MPM patients in the view of a future personalized medicine.

Ran GTPase: A Key Player in Tumor Progression and Metastasis

Ran (Ras-related nuclear protein) GTPase is a member of the Ras superfamily. Like all the GTPases, Ran cycles between an active (GTP-bound) and inactive (GDP-bound) state. However, Ran lacks the CAAX motif at its C-terminus, a feature of other small GTPases that ensures a plasma membrane localization, and largely traffics between the nucleus and the cytoplasm. Ran regulates nucleo-cytoplasmic transport of molecules through the nuclear pore complex and controls cell cycle progression through the regulation of microtubule polymerization and mitotic spindle formation. The disruption of Ran expression has been linked to cancer at different levels - from cancer initiation to metastasis. In the present review, we discuss the contribution of Ran in the acquisition of three hallmarks of cancer, namely, proliferative signaling, resistance to apoptosis, and invasion/metastasis, and highlight its prognostic value in cancer patients. In addition, we discuss the use of this GTPase as a therapeutic target in cancer.

Divergent Evolution of Legionella RCC1 Repeat Effectors Defines the Range of Ran GTPase Cycle Targets

Legionella pneumophila is a ubiquitous environmental bacterium which, upon inhalation, causes a life-threatening pneumonia termed Legionnaires’ disease. The opportunistic pathogen grows in amoebae and macrophages by employing a “type IV” secretion system, which secretes more than 300 different “effector” proteins into the host cell, where they subvert pivotal processes. The function of many of these effector proteins is unknown, and their evolution has not been studied. L. pneumophila RCC1 repeat effectors target the small GTPase Ran, a molecular switch implicated in different cellular processes such as nucleocytoplasmic transport and microtubule cytoskeleton dynamics. We provide evidence that one or more RCC1 repeat genes are distributed in two main clusters of L. pneumophila strains and have divergently evolved to target different components of the Ran GTPase activation cycle at different subcellular sites. Thus, L. pneumophila employs a sophisticated strategy to subvert host cell Ran GTPase during infection.

Legionella pneumophila governs its interactions with host cells by secreting >300 different “effector” proteins. Some of these effectors contain eukaryotic domains such as the RCC1 (regulator of chromosome condensation 1) repeats promoting the activation of the small GTPase Ran. In this report, we reveal a conserved pattern of L. pneumophila RCC1 repeat genes, which are distributed in two main clusters of strains. Accordingly, strain Philadelphia-1 contains two RCC1 genes implicated in bacterial virulence, legG1 (Legionella eukaryotic gene 1), and ppgA, while strain Paris contains only one, pieG. The RCC1 repeat effectors localize to different cellular compartments and bind distinct components of the Ran GTPase cycle, including Ran modulators and the small GTPase itself, and yet they all promote the activation of Ran. The pieG gene spans the corresponding open reading frames of legG1 and a separate adjacent upstream gene, lpg1975. legG1 and lpg1975 are fused upon addition of a single nucleotide to encode a protein that adopts the binding specificity of PieG. Thus, a point mutation in pieG splits the gene, altering the effector target. These results indicate that divergent evolution of RCC1 repeat effectors defines the Ran GTPase cycle targets and that modulation of different components of the cycle might fine-tune Ran activation during Legionella infection.

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.

Beyond Rab GTPases Legionella activates the small GTPase Ran to promote microtubule polymerization, pathogen vacuole motility, and infection

Legionella spp. are amoebae-resistant environmental bacteria that replicate in free-living protozoa in a distinct compartment, the Legionella-containing vacuole (LCV). Upon transmission of Legionella pneumophila to the lung, the pathogens employ an evolutionarily conserved mechanism to grow in LCVs within alveolar macrophages, thus triggering a severe pneumonia termed Legionnaires' disease. LCV formation is a complex and robust process, which requires the bacterial Icm/Dot type IV secretion system and involves the amazing number of 300 different translocated effector proteins. LCVs interact with the host cell's endosomal and secretory vesicle trafficking pathway. Accordingly, in a proteomics approach as many as 12 small Rab GTPases implicated in endosomal and secretory vesicle trafficking were identified and validated as LCV components. Moreover, the small GTPase Ran and its effector protein RanBP1 have been found to decorate the pathogen vacuole. Ran regulates nucleo-cytoplasmic transport, spindle assembly, and cytokinesis, as well as the organization of non-centrosomal microtubules. In L. pneumophila-infected amoebae or macrophages, Ran and RanBP1 localize to LCVs, and the small GTPase is activated by the Icm/Dot substrate LegG1. Ran activation by LegG1 leads to microtubule stabilization and promotes intracellular pathogen vacuole motility and bacterial growth, as well as chemotaxis and migration of Legionella-infected cells.

Formation of a pathogen vacuole according to Legionella pneumophila: how to kill one bird with many stones

Legionella species are ubiquitous, waterborne bacteria that thrive in numerous ecological niches. Yet, in contrast to many other environmental bacteria, Legionella spp. are also able to grow intracellularly in predatory protozoa. This feature mainly accounts for the pathogenicity of Legionella pneumophila, which causes the majority of clinical cases of a severe pneumonia termed Legionnaires' disease. The pathomechanism underlying L. pneumophila infection is based on macrophage resistance, which in turn is largely defined by the opportunistic pathogen's resistance towards amoebae. L. pneumophila replicates in macrophages or amoebae in a unique membrane-bound compartment, the Legionella-containing vacuole (LCV). LCV formation requires the bacterial intracellular multiplication/defective for organelle trafficking (Icm/Dot) type IV secretion system and involves a plethora of translocated effector proteins, which subvert pivotal processes in the host cell. Of the ca. 300 different experimentally validated Icm/Dot substrates, about 50 have been studied and attributed a cellular function to date. The versatility and ingenuity of these effectors' mode of actions is striking. In this review, we summarize insight into the cellular functions and biochemical activities of well-characterized L. pneumophila effector proteins and the host pathways they target. Recent studies not only substantially increased our knowledge about pathogen-host interactions, but also shed light on novel biological mechanisms.

Inter-cellular transport of ran GTPase

Ran, a member of the Ras-GTPase superfamily, has a well-established role in regulating the transport of macromolecules across the nuclear envelope (NE). Ran has also been implicated in mitosis, cell cycle progression, and NE formation. Over-expression of Ran is associated with various cancers, although the molecular mechanism underlying this phenomenon is unclear. Serendipitously, we found that Ran possesses the ability to move from cell-to-cell when transiently expressed in mammalian cells. Moreover, we show that the inter-cellular transport of Ran is GTP-dependent. Importantly, Ran displays a similar distribution pattern in the recipient cells as that in the donor cell and co-localizes with the Ran binding protein Nup358 (also called RanBP2). Interestingly, leptomycin B, an inhibitor of CRM1-mediated export, or siRNA mediated depletion of CRM1, significantly impaired the inter-cellular transport of Ran, suggesting a function for CRM1 in this process. These novel findings indicate a possible role for Ran beyond nucleo-cytoplasmic transport, with potential implications in inter-cellular communication and cancers.

Arabidopsis RAN1 mediates seed development through its parental ratio by affecting the onset of endosperm cellularization

Although previous studies have demonstrated that endosperm development is influenced by its parental genome constitution, the genetic basis and molecular mechanisms that control parent-of-origin effects require further elucidation. Here we show that the Ras-related nuclear protein 1 (RAN1) regulates endosperm development in Arabidopsis thaliana. Reciprocal crosses between wild-type (WT) and transgenic lines misexpressing RAN1 (msRAN1) gave rise to small F1 seeds when RAN1 down-regulated/up-regulated individuals were used as a male/female parent; in contrast, F1 seeds were aborted when RAN1 down-regulated/up-regulated plants were used as a female/male parent, suggesting that seed development is affected by the parental genome ratio of RAN1. Whereas RAN1 expression in wild-type plants is reduced before the onset of endosperm cellularization, F1 seeds from reciprocal crosses between WT and msRAN1 showed abnormal endosperm cellularization and ectopic expression of RAN1. The expression of MINISEED3 (MINI3)-a gene that also controls endosperm cellularization-was also affected in these reciprocal crosses, and the misregulation of MINI3 activity rescued F1 seeds when msRAN1 plants were used in reciprocal crosses. Taken together, our results suggest that the parental ratio of RAN1 regulates the onset of endosperm cellularization through its genetic interaction with MINI3.

TPX2: of spindle assembly, DNA damage response, and cancer

For more than 15 years, TPX2 has been studied as a factor critical for mitosis and spindle assembly. These functions of TPX2 are attributed to its Ran-regulated microtubule-associated protein properties and to its control of the Aurora A kinase. Overexpressed in cancers, TPX2 is being established as marker for the diagnosis and prognosis of malignancies. During interphase, TPX2 resides preferentially in the nucleus where its function had remained elusive until recently. The latest finding that TPX2 plays a role in amplification of the DNA damage response, combined with the characterization of TPX2 knockout mice, open new perspectives to understand the biology of this protein. This review provides an historic overview of the discovery of TPX2 and summarizes its cytoskeletal and signaling roles with relevance to cancer therapies. Finally, the review aims to reconcile discrepancies between the experimental and pathological effects of TPX2 overexpression and advances new roles for compartmentalized TPX2.

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.

Beta-like importins mediate the nuclear translocation of mitogen-activated protein kinases

The rapid nuclear translocation of signaling proteins upon stimulation is important for the regulation of de novo gene expression. We have studied the stimulated nuclear shuttling of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinases (MAPKs) and found that they translocate into the nucleus in a Ran-dependent, but NLS- or NTS-independent, manner, unrelated to their catalytic activity. We show that this translocation involves three β-like importins, importins 3, 7, and 9 (Imp3/7/9). Knockdown of these importins inhibits the nuclear translocation of the MAPKs and, thereby, activation of their transcription factor targets. We further demonstrate that the translocation requires the stimulated formation of heterotrimers composed of Imp3/Imp7/MAPK or Imp3/Imp9/MAPK. JNK1/2 and p38α/β bind to either Imp7 or Imp9 upon stimulated posttranslational modification of the two Imps, while Imp3 joins the complex after its stimulation-induced phosphorylation. Once formed, these heterotrimers move to the nuclear envelope, where importin 3 remains, while importins 7 and 9 escort the MAPKs into the nucleus. These results suggest that β-like importins are central mediators of stimulated nuclear translocation of signaling proteins and therefore add a central level of regulation to stimulated transcription.

Overexpressed TPX2 causes ectopic formation of microtubular arrays in the nuclei of acentrosomal plant cells

TPX2 performs multiple roles in microtubule organization. Previously, it was shown that plant AtTPX2 binds AtAurora1 kinase and colocalizes with microtubules in a cell cycle-specific manner. To elucidate the function of TPX2 further, this work analysed Arabidopsis cells overexpressing AtTPX2-GFP. Distinct arrays of bundled microtubules, decorated with AtTPX2-GFP, were formed in the vicinity of the nuclear envelope and in the nuclei of overexpressing cells. The microtubular arrays showed reduced sensitivity to anti-microtubular drugs. TPX2-mediated formation of nuclear/perinuclear microtubular arrays was not specific for the transition to mitosis and occurred independently of Aurora kinase. The fibres were not observed in cells with detectable programmed cell death and, in this respect, they differed from TPX2-dependent microtubular assemblies functioning in mammalian apoptosis. Colocalization and co-purification data confirmed the interaction of importin with AtTPX2-GFP. In cells with nuclear foci of overexpressed AtTPX2-GFP, strong nuclear signals for Ran and importin diminished when microtubular arrays were assembled. This observation suggests that TPX2-mediated microtubule formation might be triggered by a Ran cycle. Collectively, the data suggest that in the acentrosomal plant cell, in conjunction with importin, overexpressed AtTPX2 reinforces microtubule formation in the vicinity of chromatin and the nuclear envelope.

Activation of Ran GTPase by a Legionella effector promotes microtubule polymerization, pathogen vacuole motility and infection

The causative agent of Legionnaires' disease, Legionella pneumophila, uses the Icm/Dot type IV secretion system (T4SS) to form in phagocytes a distinct "Legionella-containing vacuole" (LCV), which intercepts endosomal and secretory vesicle trafficking. Proteomics revealed the presence of the small GTPase Ran and its effector RanBP1 on purified LCVs. Here we validate that Ran and RanBP1 localize to LCVs and promote intracellular growth of L. pneumophila. Moreover, the L. pneumophila protein LegG1, which contains putative RCC1 Ran guanine nucleotide exchange factor (GEF) domains, accumulates on LCVs in an Icm/Dot-dependent manner. L. pneumophila wild-type bacteria, but not strains lacking LegG1 or a functional Icm/Dot T4SS, activate Ran on LCVs, while purified LegG1 produces active Ran(GTP) in cell lysates. L. pneumophila lacking legG1 is compromised for intracellular growth in macrophages and amoebae, yet is as cytotoxic as the wild-type strain. A downstream effect of LegG1 is to stabilize microtubules, as revealed by conventional and stimulated emission depletion (STED) fluorescence microscopy, subcellular fractionation and Western blot, or by microbial microinjection through the T3SS of a Yersinia strain lacking endogenous effectors. Real-time fluorescence imaging indicates that LCVs harboring wild-type L. pneumophila rapidly move along microtubules, while LCVs harboring ΔlegG1 mutant bacteria are stalled. Together, our results demonstrate that Ran activation and RanBP1 promote LCV formation, and the Icm/Dot substrate LegG1 functions as a bacterial Ran activator, which localizes to LCVs and promotes microtubule stabilization, LCV motility as well as intracellular replication of L. pneumophila.

Characterization of a novel activated Ran GTPase mutant and its ability to induce cellular transformation

Ran (Ras-related nuclear) protein, a member of the Ras superfamily of GTPases, is best known for its roles in nucleocytoplasmic transport, mitotic spindle fiber assembly, and nuclear envelope formation. Recently, we have shown that the overexpression of Ran in fibroblasts induces cellular transformation and tumor formation in mice (Ly, T. K., Wang, J., Pereira, R., Rojas, K. S., Peng, X., Feng, Q., Cerione, R. A., and Wilson, K. F. (2010) J. Biol. Chem. 285, 5815-5826). Here, we describe a novel activated Ran mutant, Ran(K152A), which is capable of an increased rate of GDP-GTP exchange and an accelerated GTP binding/GTP hydrolytic cycle compared with wild-type Ran. We show that its expression in NIH-3T3 fibroblasts induces anchorage-independent growth and stimulates cell invasion, as well as activates signaling pathways that lead to extracellular regulated kinase (ERK) activity. Furthermore, Ran(K152A) expression in the human mammary SKBR3 adenocarcinoma cell line gives rise to an enhanced transformed phenotype and causes a robust stimulation of both ERK and the N-terminal c-Jun kinase (JNK). Microarray analysis reveals that the expression of the gene encoding SMOC-2 (secreted modular calcium-binding protein-2), which has been shown to synergize with different growth factors, is increased by at least 50-fold in cells stably expressing Ran(K152A) compared with cells expressing control vector. Knocking down SMOC-2 expression greatly reduces the ability of Ran(K152A) to stimulate anchorage-independent growth in NIH-3T3 cells and in SKBR3 cells and also inhibits cell invasion in fibroblasts. Collectively, our findings highlight a novel connection between the hyper-activation of the small GTPase Ran and the matricellular protein SMOC-2 that has important consequences for oncogenic transformation.

Physical breakdown of the nuclear envelope is not necessary for breaking its barrier function

During mitosis in higher eukaryotic cells, nuclear envelope breakdown (NEBD) occurs and leads to the disassembly of the nuclear membrane and nuclear pore complexes (NPC). This brings about a mixing of nuclear and cytoplasmic macromolecules (open mitosis). On the other hand, in many fungi, mitosis occurs without NEBD (closed mitosis). In a recent study, we reported a novel phenomenon in a closed mitosis organism, Schizosaccharomyces pombe: mixing of nuclear and cytoplasmic proteins occurred in meiosis without breakdown of the nuclear membrane or disassembly of nuclear pore complexes. We designated this event virtual nuclear envelope breakdown (V-NEBD). The key event in V-NEBD is nuclear translocation of Rna1, a RanGAP1 homolog in S. pombe. This leads to collapse of the Ran-GTP gradient across the nuclear envelope (NE) and occurs coincidently with V-NEBD. Thus, the barrier function of the NE can be abated without its physical breakdown through modulation of the Ran-GTP gradient.

Thousands of rab GTPases for the cell biologist

Rab proteins are small GTPases that act as essential regulators of vesicular trafficking. 44 subfamilies are known in humans, performing specific sets of functions at distinct subcellular localisations and tissues. Rab function is conserved even amongst distant orthologs. Hence, the annotation of Rabs yields functional predictions about the cell biology of trafficking. So far, annotating Rabs has been a laborious manual task not feasible for current and future genomic output of deep sequencing technologies. We developed, validated and benchmarked the Rabifier, an automated bioinformatic pipeline for the identification and classification of Rabs, which achieves up to 90% classification accuracy. We cataloged roughly 8.000 Rabs from 247 genomes covering the entire eukaryotic tree. The full Rab database and a web tool implementing the pipeline are publicly available at www.RabDB.org. For the first time, we describe and analyse the evolution of Rabs in a dataset covering the whole eukaryotic phylogeny. We found a highly dynamic family undergoing frequent taxon-specific expansions and losses. We dated the origin of human subfamilies using phylogenetic profiling, which enlarged the Rab repertoire of the Last Eukaryotic Common Ancestor with Rab14, 32 and RabL4. Furthermore, a detailed analysis of the Choanoflagellate Monosiga brevicollis Rab family pinpointed the changes that accompanied the emergence of Metazoan multicellularity, mainly an important expansion and specialisation of the secretory pathway. Lastly, we experimentally establish tissue specificity in expression of mouse Rabs and show that neo-functionalisation best explains the emergence of new human Rab subfamilies. With the Rabifier and RabDB, we provide tools that easily allows non-bioinformaticians to integrate thousands of Rabs in their analyses. RabDB is designed to enable the cell biology community to keep pace with the increasing number of fully-sequenced genomes and change the scale at which we perform comparative analysis in cell biology.

Intracellular compartmentalisation via membrane-delimited organelles is a fundamental feature of the eukaryotic cell. Understanding its origins and specialisation into functionally distinct compartments is a major challenge in evolutionary cell biology. We focus on the Rab enzymes, critical organisers of the trafficking pathways that link the endomembrane system. Rabs form a large family of evolutionarily related proteins, regulating distinct steps in vesicle transport. They mark pathways and organelles due to their specific subcellular and tissue localisation. We propose a solution to the problem of identifying and annotating Rabs in hundreds of sequenced genomes. We developed an accurate bioinformatics pipeline that is able to take into account pre-existing and often inconsistent, manual annotations. We made it available to the community in form of a web tool, as well as a database containing thousands of Rabs assigned to sub-families, which yields clear functional predictions. Thousands of Rabs allow for a new level of analysis. We illustrate this by characterising for the first time the global evolutionary dynamics of the Rab family. We dated the emergence of subfamilies and suggest that the Rab family expands by duplicates acquiring new functions.

Assembly of RNP granules in stressed and aging oocytes requires nucleoporins and is coordinated with nuclear membrane blebbing

Protective cellular responses to stress and aging in the germline are essential for perpetuation of a species; however, relatively few studies have focused on how germ cells respond to stress and aging. We have previously shown that large ribonucleoprotein (RNP) complexes assemble in oocytes of Caenorhabditis during extended meiotic arrest or after environmental stress. Here we explore the regulation of these dynamic RNPs and demonstrate their assembly is coordinated with dramatic, nuclear membrane blebbing in oocytes. Our ultrastructural analyses reveal distinct changes in the endoplasmic reticulum, and the first evidence for the assembly of stacked annulate lamellae in Caenorhabditis. We further show several nucleoporins are required for the complete assembly of RNP granules, and a disruption in RNP granule assembly coupled with a low frequency of nuclear blebbing in arrested oocytes negatively impacts embryonic viability. Our observations support a model where nuclear membrane blebbing is required to increase the trafficking of nucleoporins to the cell cortex in stressed or meiotically arrested cells and to facilitate the recruitment of RNA and protein components of RNPs into large complexes. These new insights may have general implications for better understanding how germ cells preserve their integrity when fertilization is delayed and how cells respond to stress.

Nuclear translocation of RanGAP1 coincides with virtual nuclear envelope breakdown in fission yeast meiosis

In higher eukaryotes, mitosis proceeds with nuclear envelope breakdown (NEBD) and disassembly of the nuclear pore complex (NPC); this is designated "open" mitosis. On the other hand, in many fungi, mitosis and chromosome segregation takes place without NEBD; this is designated "closed" mitosis. In a recent study on Schizosaccharomyces pombe, a closed mitosis organism, we reported a novel phenomenon that is equivalent to NEBD: a mixing of nuclear proteins and cytoplasmic proteins occurred transiently for a few minutes in meiosis without physical breakdown of the nuclear envelope. We designated this event virtual nuclear envelope breakdown (V-NEBD). In S. pombe, nuclear translocation of Rna1, a RanGAP1 homolog in S. pombe, occurs during meiosis, and this translocation of Rna1 leads to collapse of the Ran-GTP gradient across the nuclear envelope and occurs coincidently with V-NEBD. Here, we describe possible roles of RanGAP1 in V-NEBD in S. pombe and provide insights into the roles V-NEBD may play in meiosis.

Identification and expression localization of a Ran homologue in mollusc abalone, Haliotis diversicolor supertexta

Ran protein is a central molecular in several key nuclear functions, including nucleocytoplasmic transport, cell-cycle progression and nuclear envelope assembly. In this study, we have isolated and characterized a Ran homologue from a gastropod abalone which we named ab-Ran. The full-length cDNA consists of 1239 bp with an ORF encoding a 220 amino acid protein. The deduced amino acid sequence of ab-Ran shows highly similar to that of other Ran members (84-88%). Moreover, the ab-Ran contains five conserved regions and four carboxy-terminal residues CAAX-box. RT-PCR analysis showed that the ab-Ran was ubiquitously expressed in abalone tissues. The intracellular localization examined by immunofluorescence and immunohistochemistry staining displayed that ab-Ran was largely concentrated in the nuclei and partially in the cytoplasm. To the best of our knowledge, this is the first identification and characterization of a Ran homologue in mollusk.

Nucleo-cytoplasmic transport of proteins and RNA in plants

Transport of macromolecules between the nucleus and the cytoplasm is an essential necessity in eukaryotic cells, since the nuclear envelope separates transcription from translation. In the past few years, an increasing number of components of the plant nuclear transport machinery have been characterised. This progress, although far from being completed, confirmed that the general characteristics of nuclear transport are conserved between plants and other organisms. However, plant-specific components were also identified. Interestingly, several mutants in genes encoding components of the plant nuclear transport machinery were investigated, revealing differential sensitivity of plant-specific pathways to impaired nuclear transport. These findings attracted attention towards plant-specific cargoes that are transported over the nuclear envelope, unravelling connections between nuclear transport and components of signalling and developmental pathways. The current state of research in plants is summarised in comparison to yeast and vertebrate systems, and special emphasis is given to plant nuclear transport mutants.

Nuclear compartmentalization is abolished during fission yeast meiosis

In eukaryotic cells, the nuclear envelope partitions the nucleus from the cytoplasm. The fission yeast Schizosaccharomyces pombe undergoes closed mitosis in which the nuclear envelope persists rather than being broken down, as in higher eukaryotic cells. It is therefore assumed that nucleocytoplasmic transport continues during the cell cycle. Here we show that nuclear transport is, in fact, abolished specifically during anaphase of the second meiotic nuclear division. During that time, both nucleoplasmic and cytoplasmic proteins disperse throughout the cell, reminiscent of the open mitosis of higher eukaryotes, but the architecture of the S. pombe nuclear envelope itself persists. This functional alteration of the nucleocytoplasmic barrier is likely induced by spore wall formation, because ectopic induction of sporulation signaling leads to premature dispersion of nucleoplasmic proteins. A photobleaching assay demonstrated that nuclear envelope permeability increases abruptly at the onset of anaphase of the second meiotic division. The permeability was not altered when sporulation was inhibited by blocking the trafficking of forespore-membrane vesicles from the endoplasmic reticulum to the Golgi. The evidence indicates that yeast gametogenesis produces vesicle transport-mediated forespore membranes by inducing nuclear envelope permeabilization.

Virtual breakdown of the nuclear envelope in fission yeast meiosis

Asymmetric localization of Ran regulators (RanGAP1 and RanGEF/RCC1) produces a gradient of RanGTP across the nuclear envelope. In higher eukaryotes, the nuclear envelope breaks down as the cell enters mitosis (designated "open" mitosis). This nuclear envelope breakdown (NEBD) leads to collapse of the RanGTP gradient and the diffusion of nuclear and cytoplasmic macromolecules in the cell, resulting in irreversible progression of the cell cycle. On the other hand, in many fungi, chromosome segregation takes place without NEBD (designated "closed" mitosis). Here we report that in the fission yeast Schizosaccharomyces pombe, despite the nuclear envelope and the nuclear pore complex remaining intact throughout both the meiotic and mitotic cell cycles, nuclear proteins diffuse into the cytoplasm transiently for a few minutes at the onset of anaphase of meiosis II. We also found that nuclear protein diffusion into the cytoplasm occurred coincidently with nuclear localization of Rna1, an S. pombe RanGAP1 homolog that is usually localized in the cytoplasm. These results suggest that nuclear localization of RanGAP1 and depression of RanGTP activity in the nucleus may be mechanistically tied to meiosis-specific diffusion of nuclear proteins into the cytoplasm. This nucleocytoplasmic shuffling of RanGAP1 and nuclear proteins represents virtual breakdown of the nuclear envelope.

RCC1 uses a conformationally diverse loop region to interact with the nucleosome: a model for the RCC1-nucleosome complex

The binding of RCC1 (regulator of chromosome condensation 1) to chromatin is critical for cellular processes such as mitosis, nucleocytoplasmic transport, and nuclear envelope formation because RCC1 recruits the small GTPase Ran (Ras-related nuclear protein) to chromatin and sets up a Ran-GTP gradient around the chromosomes. However, the molecular mechanism by which RCC1 binds to nucleosomes, the repeating unit of chromatin, is not known. We have used biochemical approaches to test structural models for how the RCC1 beta-propeller protein could bind to the nucleosome. In contrast to the prevailing model, RCC1 does not appear to use the beta-propeller face opposite to its Ran-binding face to interact with nucleosomes. Instead, we find that RCC1 uses a conformationally flexible loop region we have termed the switchback loop in addition to its N-terminal tail to bind to the nucleosome. The juxtaposition of the RCC1 switchback loop to its Ran binding surface suggests a novel mechanism for how nucleosome-bound RCC1 recruits Ran to chromatin. Furthermore, this model accounts for previously unexplained observations for how Ran can interact with the nucleosome both dependent and independent of RCC1 and how binding of the nucleosome can enhance RCC1's Ran nucleotide exchange activity.

Screening of potential molecular targets for colorectal cancer therapy

Colorectal cancer is a leading cause of cancer death worldwide. To identify molecular targets for colorectal cancer therapy, we tested small interfering RNAs (siRNAs) against 97 genes whose expression was elevated in human colorectal cancer tissues for the ability to promote apoptosis of human colorectal cancer cells (HT-29 cells). The results indicate that the downregulation of PSMA7 (proteasome subunit, alpha-type, 7) and RAN (ras-related nuclear protein) most efficiently induced apoptosis of HT-29 cells. PSMA7 and RAN were highly expressed in colorectal cancer cell lines compared with normal colon tissues. Furthermore, PSMA7 and RAN were overexpressed in not only colon tumor tissues but also the other tumor tissues. Moreover, in vivo delivery of PSMA7 siRNA and RAN siRNA markedly induced apoptosis in HT-29 xenograft tumors in mice. Thus, silencing of PSMA7 and RAN induces cancer cells to undergo apoptosis, and PSMA7 and RAN might be promising new molecular targets for drug and RNA interference-based therapeutics against colorectal cancer.

Karyopherin binding interactions and nuclear import mechanism of nuclear pore complex protein Tpr

BACKGROUND

Tpr is a large protein with an extended coiled-coil domain that is localized within the nuclear basket of the nuclear pore complex. Previous studies 1 involving antibody microinjection into mammalian cells suggested a role for Tpr in nuclear export of proteins via the CRM1 export receptor. In addition, Tpr was found to co-immunoprecipitate with importins alpha and beta from Xenopus laevis egg extracts 2, although the function of this is unresolved. Yeast Mlp1p and Mlp2p, which are homologous to vertebrate Tpr, have been implicated in mRNA surveillance to retain unspliced mRNAs in the nucleus34. To augment an understanding of the role of Tpr in nucleocytoplasmic trafficking, we explored the interactions of recombinant Tpr with the karyopherins CRM1, importin beta and importin alpha by solid phase binding assays. We also investigated the conditions required for nuclear import of Tpr using an in vitro assay.

RESULTS

We found that Tpr binds strongly and specifically to importin alpha, importin beta, and a CRM1 containing trimeric export complex, and that the binding sites for importins alpha and beta are distinct. We also determined that the nuclear import of Tpr is dependent on cytosolic factors and energy and is efficiently mediated by the importin alpha/beta import pathway.

CONCLUSION

Based on the binding and nuclear import assays, we propose that Tpr is imported into the nucleus by the importin alpha/beta heterodimer. In addition, we suggest that Tpr can serve as a nucleoporin binding site for importin beta during import of importin beta cargo complexes and/or importin beta recycling. Our finding that Tpr bound preferentially to CRM1 in an export complex strengthens the notion that Tpr is involved in protein export.

Nup358 interacts with APC and plays a role in cell polarization

Asymmetric localization of adenomatous polyposis coli (APC) to the ends of a subset of microtubules located in the leading edges is essential for the establishment of front-rear polarity during cell migration. APC is known to associate with microtubules in three ways: through interaction with the plus-end tracking protein EB1, direct binding through a C-terminal basic region, and through interaction with the plus-end motor kinesin-2. Here we report that the middle region of APC has a previously unidentified microtubule plus-end-targeting function, suggesting an additional microtubule-binding mode for APC. Through the same region, APC interacts with Nup358 (also called RanBP2), a microtubule-binding nucleoporin. Ectopic expression of the middle region of APC is sufficient to recruit endogenous Nup358 to the plus ends of microtubules. Furthermore, our results indicate that Nup358 cooperates with kinesin-2 to regulate the localization of APC to the cell cortex through a nuclear-transport-independent mechanism. Using RNA interference and a scratch-induced wound-healing assay we demonstrate that Nup358 functions in polarized cell migration. These results reveal a more active role for structural nucleoporins in regulating fundamental cellular processes than previously anticipated.

Identification of phosphorylation-dependent binding partners of aquaporin-2 using protein mass spectrometry

Vasopressin-mediated control of water permeability in the renal collecting duct occurs in part through regulation of the distribution of aquaporin-2 (AQP2) between the apical plasma membrane and intracellular membrane compartments. Phosphorylation of Ser-256 at AQP2's cytoplasmic COOH-terminus is well-accepted as a critical step for translocation. The aim of this study was to identify binding partners to phosphorylated versus nonphosphorylated forms of the AQP2 COOH-terminus via a targeted comparative proteomic approach. Cytosol from inner medullary collecting ducts isolated from rat kidneys was incubated with "bait" peptides, representing the COOH-terminal AQP2 tail in its nonphosphorylated and phosphorylated forms, to capture differentially bound proteins prior to LC-MS/MS analysis. Mass spectrometric results were confirmed by immunoblotting. Immunoprecipitation was performed using an AQP2 COOH-terminal antibody combined with immunblotting against the proposed binding partners to demonstrate interactions with native AQP2. Our studies confirmed previously identified interactions between AQP2 and hsc70, hsp70-1 and -2, as well as annexin II. These proteins were found to bind less to the Ser-256-phosphorylated AQP2 than to the nonphosphorylated form. In contrast, another heat shock protein, hsp70-5 (BiP/grp78), bound to phosphorylated AQP2 more avidly than to nonphosphorylated AQP2. Immunogold EM studies demonstrated that BiP is present not only in the ER but also in the cytoplasm and apical plasma membrane of rat collecting duct cells. Furthermore, confocal immunofluorescence studies showed partial colocalization of BiP with AQP2 in non-ER compartments. These results suggest that phosphorylation of AQP2 at Ser-256 may regulate AQP2 trafficking in part by mediating differential binding of hsp70 family proteins to the COOH-terminal tail.

Importin-beta is a GDP-to-GTP exchange factor of Ran: implications for the mechanism of nuclear import

Ran-GTP interacts strongly with importin-beta, and this interaction promotes the release of the importin-alpha-nuclear localization signal cargo from importin-beta. Ran-GDP also interacts with importin-beta, but this interaction is 4 orders of magnitude weaker than the Ran-GTP.importin-beta interaction. Here we use the yeast complement of nuclear import proteins to show that the interaction between Ran-GDP and importin-beta promotes the dissociation of GDP from Ran. The release of GDP from the Ran-GDP-importin-beta complex stabilizes the complex, which cannot be dissociated by importin-alpha. Although Ran has a higher affinity for GDP compared with GTP, Ran in complex with importin-beta has a higher affinity for GTP. This feature is responsible for the generation of Ran-GTP from Ran-GDP by importin-beta. Ran-binding protein-1 (RanBP1) activates this reaction by forming a trimeric complex with Ran-GDP and importin-beta. Importin-alpha inhibits the GDP exchange reaction by sequestering importin-beta, whereas RanBP1 restores the GDP nucleotide exchange by importin-beta by forming a tetrameric complex with importin-beta, Ran, and importin-alpha. The exchange is also inhibited by nuclear-transport factor-2 (NTF2). We suggest a mechanism for nuclear import, additional to the established RCC1 (Ran-guanine exchange factor)-dependent pathway that incorporates these results.

Dynamic release of nuclear RanGTP triggers TPX2-dependent microtubule assembly during the apoptotic execution phase

During apoptosis, the interphase microtubule network is dismantled then later replaced by a novel, non-centrosomal microtubule array. These microtubules assist in the peripheral redistribution of nuclear fragments in the apoptotic cell; however, the regulation of apoptotic microtubule assembly is not understood. Here, we demonstrate that microtubule assembly depends upon the release of nuclear RanGTP into the apoptotic cytoplasm because this process is blocked in apoptotic cells overexpressing dominant-negative GDP-locked Ran (T24N). Actin-myosin-II contractility provides the impetus for Ran release and, consequently, microtubule assembly is blocked in blebbistatin- and Y27632-treated apoptotic cells. Importantly, the spindle-assembly factor TPX2 (targeting protein for Xklp2), colocalises with apoptotic microtubules, and siRNA silencing of TPX2, but not of the microtubule motors Mklp1 and Kid, abrogates apoptotic microtubule assembly. These data provide a molecular explanation for the assembly of the apoptotic microtubule network, and suggest important similarities with the process of RanGTP- and TPX2-mediated mitotic spindle formation.

Inhibition of active nuclear transport is an intrinsic trigger of programmed cell death in trypanosomatids

The link between nucleocytoplasmic transport and apoptosis remains controversial. Nucleocytoplasmic exchange of molecules seems indeed essential for the initiation and execution of the apoptotic programme; but inhibition of nuclear transport factors may also represent a powerful apoptotic trigger. The GTPase Ran (together with its partners), first discovered to be essential in nucleocytoplasmic transport, has multiple key functions in cell biology, and particularly in spindle assembly, kinetochore function and nuclear envelope assembly. Among the Ran partners studied, NTF2 appears to be solely involved in nucleocytoplasmic transport. Here, we localised Ran and several of its partners, RanBP1, CAS and NTF2, at the nuclear membrane in the trypanosomatid Leishmania major. Remarkably, these proteins fused to GFP decorated a perinuclear 'collar' of about 15 dots, colocalising at nuclear pore complexes with the homologue of nucleoporin Sec13. In the other trypanosomatid Trypanosoma brucei, RNAi knockdown of the expression of the corresponding genes resulted in an apoptosis-like phenomenon. These phenotypes show that Ran and its partners have a key function in trypanosomatids like they have in mammals. Our data, notably those about TbNTF2 RNAi, support the idea that active nucleocytoplasmic transport is not essential for the initiation and execution of apoptosis, and, rather, the impairment of this transport constitutes an intrinsic signal for triggering PCD.

The spindle function of CDCA4

In an attempt to discover novel proteins functioning in both interphase nucleus and mitotic spindle as NuMA does, we carried out cDNA library screening with pooled autoimmune antibodies. Among positive clones we found a recently identified transcription regulatory protein (CDCA4) with the distinctive nuclear-mitotic apparatus distribution. CDCA4 localizes at metaphase spindle poles and the midzone in later stages. Additionally, an intensive CDCA4 accumulation parallel to spindle was observed in half of metaphase cells but not in later stages, implying a transient form of CDCA4 binding to midzone from anaphase. Mitotic arrest dissolved CDCA4 from centrosomes but during the spindle recovery, CDCA4 invariably colocalized with the microtubule nucleation foci as a component of microtubule organization center. RNA interference of CDCA4 resulted in significant increase of multinuclei and multipolar spindles, suggesting impaired function in chromosome segregation or cytokinesis. However, the spindle checkpoint and the centrosome cycle appeared not to be affected by such interference. Furthermore, CDCA4 depletion resulted in accelerated cell proliferation, perhaps due to the disruption of CDCA4 nuclear function as a transcription suppressor. Interphase CDCA4 is localized in nucleoli by immunofluorescence, although GFP-CDCA4 expressed in the nucleoplasm. An N-terminal KRKC domain appears to be the nuclear localization signal as identified by sequence alignment and the expression of truncated mutants. Taken together, our results suggested that as a novel nuclearmitotic apparatus protein, CDCA4 is involved in spindle organization from prometaphase. When anaphase begins, CDCA4 may play a different role as a midzone factor involved in chromosome segregation or cytokinesis.

Meiotic maturation: receptor trafficking is the key

Meiotic maturation and ovulation rates in Caenorhabditis elegans are regulated by a sperm-released gradient of major sperm protein (MSP). Recent work has provided insights into the modulation of the MSP signal by the trafficking of its receptor in oocytes.

Purification, crystallization and preliminary X-ray crystallographic analysis of Rab27a GTPase in complex with exophilin4/Slp2-a effector

By switching between GTP-active and GDP-inactive conformations, small Ras GTPases partly regulate membrane trafficking, cell growth and cytoskeleton dynamics. Among Rab GTPases, the Rab27 subfamily, which comprises Rab27a and Rab27b, controls the proper targeting of secretory vesicles to the plasma membrane. GppNHp-bound Rab27a in complex with the Rab27-binding domain of exophilin4/Slp2-a effector has been purified and crystallized for structural studies. The crystals belong to space group P2(1)2(1)2(1) and a complete data set was collected to a resolution of 1.8 A. Eventually, the structural characterization of the Rab27a-exophilin4/Slp2-a complex will clarify Rab27 recognition by its effectors prior to vesicle tethering and docking.