The thermolability of nuclear protein import in tsBN2 cells is suppressed by microinjected Ran-GTP or Ran-GDP, but not by RanQ69L or RanT24N

Identification of a putative nuclear localization signal in the tumor suppressor maspin sheds light on its nuclear import regulation

The tumor suppressor activity of maspin (mammary serine protease inhibitor) has been associated with its nuclear localization. In this study we explore the regulation of maspin nuclear translocation. An in vitro nuclear import assay suggested that maspin can passively enter the nucleus. However, in silico analysis identified a putative maspin nuclear localization signal (NLS), which was able to mediate the nuclear translocation of a chimeric protein containing this NLS fused to five green fluorescent protein molecules in tandem (5GFP). Dominant‐negative Ran‐GTPase mutants RanQ69L or RanT24N suppressed this process. Unexpectedly, the full‐length maspin fused to 5GFP failed to enter the nucleus. As maspin's putative NLS is partially hidden in its three‐dimensional structure, we suggest that maspin nuclear transport could be conformationally regulated. Our results suggest that maspin nuclear translocation involves both passive and active mechanisms.

Akt-mediated phosphorylation increases the binding affinity of hTERT for importin α to promote nuclear translocation

Telomeres are essential for chromosome integrity and protection, and their maintenance requires the ribonucleoprotein enzyme telomerase. Previously, we have shown that human telomerase reverse transcriptase (hTERT) contains a bipartite nuclear localization signal (NLS; residues 222–240) that is responsible for nuclear import, and that Akt-mediated phosphorylation of residue S227 is important for efficient nuclear import of hTERT. Here, we show that hTERT binds to importin-α proteins through the bipartite NLS and that this heterodimer then forms a complex with importin-β proteins to interact with the nuclear pore complex. Depletion of individual importin-α proteins results in a failure of hTERT nuclear import, and the resulting cytoplasmic hTERT is degraded by ubiquitin-dependent proteolysis. Crystallographic analysis reveals that the bipartite NLS interacts with both the major and minor sites of importin-α proteins. We also show that Akt-mediated phosphorylation of S227 increases the binding affinity for importin-α proteins and promotes nuclear import of hTERT, thereby resulting in increased telomerase activity. These data provide details of a binding mechanism that enables hTERT to interact with the nuclear import receptors and of the control of the dynamic nuclear transport of hTERT through phosphorylation.

Nuclear import of hTERT requires a bipartite nuclear localization signal mediated by Akt phosphorylation

Sustained cell proliferation requires telomerase to maintain functional telomeres that are essential for chromosome integrity and protection. Although nuclear import of hTERT is required for telomerase activity to elongate telomeres in vivo, the molecular mechanism regulating nuclear localization of hTERT is unclear. Here we identify a bipartite nuclear localization signal (NLS) (amino acid residues 222–240) that is responsible for nuclear import of hTERT. Immunofluorescence imaging of hTERT revealed that mutations in any of the bipartite NLS sequences result in decreased nuclear fluorescence intensity compared to wild-type hTERT. We also show that Akt-mediated phosphorylation at serine 227 is necessary for directing nuclear translocation of hTERT. Interestingly, serine 227 is located between two clusters of basic amino acids in the bipartite NLS. Inactivation of Akt activity by a dominant-negative mutant or wortmannin treatment attenuated nuclear localization of hTERT. We further show that both bipartite NLS and serine 227 in hTERT are required for cellular immortalization of normal human foreskin fibroblast cells. Taken together, our findings reveal a novel regulatory mechanism of nuclear import of hTERT through a bipartite NLS mediated by Akt phosphorylation, which represents an alternative pathway for modulating telomerase activity in cancer.

System analysis shows distinct mechanisms and common principles of nuclear envelope protein dynamics

The ER–inner nuclear membrane trafficking of 15 integral membrane proteins followed by FRAP shows distinct ATP- and Ran-dependent translocation mechanisms.

The nuclear envelope contains >100 transmembrane proteins that continuously exchange with the endoplasmic reticulum and move within the nuclear membranes. To better understand the organization and dynamics of this system, we compared the trafficking of 15 integral nuclear envelope proteins using FRAP. A surprising 30-fold range of mobilities was observed. The dynamic behavior of several of these proteins was also analyzed after depletion of ATP and/or Ran, two functions implicated in endoplasmic reticulum–inner nuclear membrane translocation. This revealed that ATP- and Ran-dependent translocation mechanisms are distinct and not used by all inner nuclear membrane proteins. The Ran-dependent mechanism requires the phenylalanine-glycine (FG)-nucleoporin Nup35, which is consistent with use of the nuclear pore complex peripheral channels. Intriguingly, the addition of FGs to membrane proteins reduces FRAP recovery times, and this also depends on Nup35. Modeling of three proteins that were unaffected by either ATP or Ran depletion indicates that the wide range in mobilities could be explained by differences in binding affinities in the inner nuclear membrane.

DNA-tumor virus entry--from plasma membrane to the nucleus

DNA-tumor viruses comprise enveloped and non-enveloped agents that cause malignancies in a large variety of cell types and tissues by interfering with cell cycle control and immortalization. Those DNA-tumor viruses that replicate in the nucleus use cellular mechanisms to transport their genome and newly synthesized viral proteins into the nucleus. This requires cytoplasmic transport and nuclear import of their genome. Agents that employ this strategy include adenoviruses, hepadnaviruses, herpesviruses, and likely also papillomaviruses, and polyomaviruses, but not poxviruses which replicate in the cytoplasm. Here, we discuss how DNA-tumor viruses enter cells, take advantage of cytoplasmic transport, and import their DNA genome through the nuclear pore complex into the nucleus. Remarkably, nuclear import of incoming genomes does not necessarily follow the same pathways used by the structural proteins of the viruses during the replication and assembly phases of the viral life cycle. Understanding the mechanisms of DNA nuclear import can identify new pathways of cell regulation and anti-viral therapies.

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.

Regulation of nuclear lamin polymerization by importin alpha

Nuclear lamins are integral components of the nuclear envelope and are important for the regulation of many aspects of nuclear function, including gene transcription and DNA replication. During interphase, the lamins form an intranuclear intermediate filament network that must be disassembled and reassembled when cells divide. Little is known about factors regulating this assembly/disassembly cycle. Using in vitro nuclear assembly and lamin assembly assays, we have identified a role for the nuclear transport factor importin alpha in the regulation of lamin assembly. Exogenous importin alpha inhibited nuclear lamin assembly in Xenopus interphase egg nuclear assembly assays. Fractionation of the egg extract used for nuclear assembly identified a high molecular weight complex containing the major egg lamin, XLB3, importin alpha, and importin beta. This complex could be dissociated by RanGTP or a competing nuclear localization sequence, indicating that lamin assembly is Ran- and importin alpha-dependent in the egg extract. We show that the addition of importin alpha to purified lamin B3 prevents the assembly of lamins in solution. Lamin assembly assays show that importin alpha prevents the self-association of lamins required to assemble lamin filaments into the typical paracrystals formed in vitro. These results suggest a role for importin alpha in regulating lamin assembly and possibly modulating the interactions of lamins with lamin-binding proteins.

A small-scale survey identifies selective and quantitative nucleo-cytoplasmic shuttling of a subset of CREM transcription factors

Elucidating dynamic aspects of intracellular localization of proteins is essential to decipher their functional interaction networks. Although transcription factors lacking a detectable cytoplasmic fraction have been generally considered compartmentalized in the nucleus, some were found to shuttle into the cytoplasm, suggesting functional interactions therein. To further investigate how common, specific and quantitative is this traffic, we have employed the heterokaryon assay for a small-scale survey of nuclear factors not previously tested for their nucleo-cytoplasmic motion. We show that a subset of cAMP response element (CRE) binding proteins of the CREM type shuttles within a biologically meaningful time frame, revealing a continuous flow into the cytoplasm that persists during signaling. Their dynamic behavior, not involving the classical Exportin-1 pathway, could be ascribed to C-terminal sequences, containing, in addition to the bZIP domain and the NLS, a nuclear export activity and an inhibitory activity at an adjacent site. Other proteins examined in this study either did not shuttle significantly or, like CREB and distinct CREM isoforms, shuttled with markedly delayed kinetics, denoting considerable selectivity of this traffic. These findings raise the possibility that events associated with bi-directional transport and periodic transit through the cytoplasm may modulate activities of select nuclear transcription factors.

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.

Conserved stem II of the box C/D motif is essential for nucleolar localization and is required, along with the 15.5K protein, for the hierarchical assembly of the box C/D snoRNP

The 5' stem-loop of the U4 snRNA and the box C/D motif of the box C/D snoRNAs can both be folded into a similar stem-internal loop-stem structure that binds the 15.5K protein. The homologous proteins NOP56 and NOP58 and 61K (hPrp31) associate with the box C/D snoRNPs and the U4/U6 snRNP, respectively. This raises the intriguing question of how the two homologous RNP complexes specifically assemble onto similar RNAs. Here we investigate the requirements for the specific binding of the individual snoRNP proteins to the U14 box C/D snoRNPs in vitro. This revealed that the binding of 15.5K to the box C/D motif is essential for the association of the remaining snoRNP-associated proteins, namely, NOP56, NOP58, fibrillarin, and the nucleoplasmic proteins TIP48 and TIP49. Stem II of the box C/D motif, in contrast to the U4 5' stem-loop, is highly conserved, and we show that this sequence is responsible for the binding of NOP56, NOP58, fibrillarin, TIP48, and TIP49, but not of 15.5K, to the snoRNA. Indeed, the sequence of stem II was essential for nucleolar localization of U14 snoRNA microinjected into HeLa cells. Thus, the conserved sequence of stem II determines the specific assembly of the box C/D snoRNP.

Regulated nuclear targeting of cauliflower mosaic virus

The mature cauliflower mosaic virus (CaMV) capsid protein (CP), if expressed in the absence of other viral proteins, is transported into the plant cell nucleus by the action of a nuclear localization signal (NLS) close to the N terminus. In contrast, virus particles do not enter the nucleus, but dock at the nuclear membrane, a process inhibited by anti-NLS antibodies or by GTP gamma S, and apparently mediated by interaction of CP with host importin alpha. The very acidic N-terminal extension of the viral CP precursor inhibits nuclear targeting of the protein and hence the precursor is localized in the cytoplasm. We hypothesize that this provides a control mechanism which ensures that the CP precursor is used for virus assembly in the cytoplasm and that only mature virus particles reach the nuclear pore.

The importin-beta binding domain of snurportin1 is responsible for the Ran- and energy-independent nuclear import of spliceosomal U snRNPs in vitro

The nuclear localization signal (NLS) of spliceosomal U snRNPs is composed of the U snRNA's 2,2,7-trimethyl-guanosine (m3G)-cap and the Sm core domain. The m3G-cap is specifically bound by snurportin1, which contains an NH2-terminal importin-beta binding (IBB) domain and a COOH-terminal m3G-cap--binding region that bears no structural similarity to known import adaptors like importin-alpha (impalpha). Here, we show that recombinant snurportin1 and importin-beta (impbeta) are not only necessary, but also sufficient for U1 snRNP transport to the nuclei of digitonin-permeabilized HeLa cells. In contrast to impalpha-dependent import, single rounds of U1 snRNP import, mediated by the nuclear import receptor complex snurportin1-impbeta, did not require Ran and energy. The same Ran- and energy-independent import was even observed for U5 snRNP, which has a molecular weight of more than one million. Interestingly, in the presence of impbeta and a snurportin1 mutant containing an impalpha IBB domain (IBBimpalpha), nuclear U1 snRNP import was Ran dependent. Furthermore, beta-galactosidase (betaGal) containing a snurportin1 IBB domain, but not IBBimpalpha-betaGal, was imported into the nucleus in a Ran-independent manner. Our results suggest that the nature of the IBB domain modulates the strength and/or site of interaction of impbeta with nucleoporins of the nuclear pore complex, and thus whether or not Ran is required to dissociate these interactions.

Ran alters nuclear pore complex conformation

Transport across the nuclear membranes occurs through the nuclear pore complex (NPC), and is mediated by soluble transport factors including Ran, a small GTPase that is generally GDP-bound during import and GTP-bound for export. The dynamic nature of the NPC structure suggests a possible active role for it in driving translocation. Here we show that RanGTP but not RanGDP causes alterations of NPC structure when injected into the cytoplasm of Xenopus oocytes, including compaction of the NPC and extension of the cytoplasmic filaments. RanGTP caused accumulation of nucleoplasmin-gold along the length of extended cytoplasmic filaments, whereas RanGDP caused accumulation around the cytoplasmic rim of the NPC. This suggests a possible role for Ran in altering the conformation of the cytoplasmic filaments during transport.

A role for RanBP1 in the release of CRM1 from the nuclear pore complex in a terminal step of nuclear export

We recently developed an assay in which nuclear export of the shuttling transcription factor NFAT (nuclear factor of activated T cells) can be reconstituted in permeabilized cells with the GTPase Ran and the nuclear export receptor CRM1. We have now used this assay to identify another export factor. After preincubation of permeabilized cells with a Ran mutant that cannot hydrolyze GTP (RanQ69L), cytosol supports NFAT export, but CRM1 and Ran alone do not. The RanQ69L preincubation leads to accumulation of CRM1 at the cytoplasmic periphery of the nuclear pore complex (NPC) in association with the p62 complex and Can/Nup214. RanGTP-dependent association of CRM1 with these nucleoporins was reconstituted in vitro. By biochemical fractionation and reconstitution, we showed that RanBP1 restores nuclear export after the RanQ69L preincubation. It also stimulates nuclear export in cells that have not been preincubated with RanQ69L. RanBP1 as well as Ran-binding domains of the cytoplasmic nucleoporin RanBP2 promote the release of CRM1 from the NPC. Taken together, our results indicate that RanGTP is important for the targeting of export complexes to the cytoplasmic side of the NPC and that RanBP1 and probably RanBP2 are involved in the dissociation of nuclear export complexes from the NPC in a terminal step of transport.

beta-catenin can be transported into the nucleus in a Ran-unassisted manner

The nuclear accumulation of beta-catenin plays an important role in the Wingless/Wnt signaling pathway. This study describes an examination of the nuclear import of beta-catenin in living mammalian cells and in vitro semi-intact cells. When injected into the cell cytoplasm, beta-catenin rapidly migrated into the nucleus in a temperature-dependent and wheat germ agglutinin-sensitive manner. In the cell-free import assay, beta-catenin rapidly migrates into the nucleus without the exogenous addition of cytosol, Ran, or ATP/GTP. Cytoplasmic injection of mutant Ran defective in its GTP hydrolysis did not prevent beta-catenin import. Studies using tsBN2, a temperature-sensitive mutant cell line that possesses a point mutation in the RCC1 gene, showed that the import of beta-catenin is insensitive to nuclear Ran-GTP depletion. These results show that beta-catenin possesses the ability to constitutively translocate through the nuclear pores in a manner similar to importin beta in a Ran-unassisted manner. We further showed that beta-catenin also rapidly exits the nucleus in homokaryons, suggesting that the regulation of nuclear levels of beta-catenin involves both nuclear import and export of this molecule.

The structure of the Q69L mutant of GDP-Ran shows a major conformational change in the switch II loop that accounts for its failure to bind nuclear transport factor 2 (NTF2)

We report the 2.3 A resolution X-ray crystal structure of the GDP-bound form of the RanQ69L mutant that is used extensively in studies of nucleocytoplasmic transport and cell-cycle progression. When the structure of GDP-RanQ69L from monoclinic crystals with P21 symmetry was compared with the structure of wild-type Ran obtained from monoclinic crystals, the Q69L mutant showed a large conformational change in residues 68-74, which are in the switch II region of the molecule which changes conformation in response to nucleotide state and which forms the major interaction interface with nuclear transport factor 2 (NTF2, sometimes called p10). This conformational change alters the positions of key residues such as Lys71, Phe72 and Arg76 that are crucial for the interaction of GDP-Ran with NTF2 and indeed, solution binding studies were unable to detect any interaction between NTF2 and GDP-RanQ69L under conditions where GDP-Ran bound effectively. This interaction between NTF2 and GDP-Ran is required for efficient nuclear protein import and may function between the docking and translocation steps of the pathway.

Nuclear import of Ran is mediated by the transport factor NTF2

A concentration gradient of the GTP-bound form of the GTPase Ran across nuclear pores is essential for the transport of many proteins and nucleic acids between the nuclear and cytoplasmic compartments of eukaryotic cells [1] [2] [3] [4]. The mechanisms responsible for the dynamics and maintenance of this Ran gradient have been unclear. We now show that Ran shuttles between the nucleosol and cytosol, and that cytosolic Ran accumulates rapidly in the nucleus in a saturable manner that is dependent on temperature and on the guanine-nucleotide exchange factor RCC1. Nuclear import in digitonin-permeabilized cells in the absence of added factors was minimal. The addition of energy and nuclear transport factor 2 (NTF2) [5] was sufficient for the accumulation of Ran in the nucleus. An NTF2 mutant that cannot bind Ran [6] was unable to facilitate Ran import. A GTP-bound form of a Ran mutant that cannot bind NTF2 was not a substrate for import. A dominant-negative importin-beta mutant inhibited nuclear import of Ran, whereas addition of transportin, which accumulates in the nucleus, enhanced NTF2-dependent Ran import. We conclude that NTF2 functions as a transport receptor for Ran, permitting rapid entry into the nucleus where GTP-GDP exchange mediated by RCC1 [7] converts Ran into its GTP-bound state. The Ran-GTP can associate with nuclear Ran-binding proteins, thereby creating a Ran gradient across nuclear pores.

Nucleocytoplasmic shuttling factors including Ran and CRM1 mediate nuclear export of NFAT In vitro

We have developed a permeabilized cell assay to study the nuclear export of the shuttling transcription factor NFAT, which contains a leucine-rich export signal. The assay uses HeLa cells that are stably transfected with NFAT fused to the green fluorescent protein (GFP). Nuclear export of GFP-NFAT in digitonin-permeabilized cells occurs in a temperature- and ATP-dependent manner and can be quantified by flow cytometry. In vitro NFAT export requires the GTPase Ran, which is released from cells during the digitonin permeabilization. At least one additional rate-limiting export factor is depleted from permeabilized cells by a preincubation at 30 degrees C in the absence of cytosol. This activity can be provided by cytosolic or nucleoplasmic extracts in a subsequent export step. Using this assay, we have purified a second major export activity from cytosol. We found that it corresponds to CRM1, a protein recently reported to be a receptor for certain leucine-rich export sequences. CRM1 appears to be imported into the nucleus by a Ran-dependent mechanism that is distinct from conventional signaling pathways. Considered together, our studies directly demonstrate by fractionation and reconstitution that nuclear export of NFAT is mediated by multiple nucleocytoplasmic shuttling factors, including Ran and CRM1.

Ran-unassisted nuclear migration of a 97-kD component of nuclear pore-targeting complex

A 97-kD component of nuclear pore-targeting complex (the beta-subunit of nuclear pore-targeting complex [PTAC]/importin/karyopherin) mediates the import of nuclear localization signal (NLS)-containing proteins by anchoring the NLS receptor protein (the alpha-subunit of PTAC/importin/karyopherin) to the nuclear pore complex (NPC). The import requires a small GTPase Ran, which interacts directly with the beta-subunit. The present study describes an examination of the behavior of the beta-subunit in living cells and in digitonin-permeabilized cells. In living cells, cytoplasmically injected beta-subunit rapidly migrates into the nucleus. The use of deletion mutants reveals that nuclear migration of the beta-subunit requires neither Ran- nor alpha-subunit-binding but only the NPC-binding domain of this molecule, which is also involved in NLS-mediated import. Furthermore, unlike NLS-mediated import, a dominant-negative Ran, defective in GTP-hydrolysis, did not inhibit nuclear migration of the beta-subunit. In the digitonin-permeabilized cell-free import assay, the beta-subunit transits rapidly through the NPC into the nucleus in a saturating manner in the absence of exogenous addition of soluble factors. These results show that the beta-subunit undergoes translocation at the NPC in a Ran-unassisted manner when it does not carry alpha-subunit/NLS substrate. Therefore, a requirement for Ran arises only when the beta-subunit undergoes a translocation reaction together with the alpha-subunit/NLS substrate. The results provide an insight to the yet unsolved question regarding the mechanism by which proteins are directionally transported through the NPC, and the role of Ran in this process.

Nucleocytoplasmic recycling of the nuclear localization signal receptor alpha subunit in vivo is dependent on a nuclear export signal, energy, and RCC1

Nuclear protein import requires a nuclear localization signal (NLS) receptor and at least three other cytoplasmic factors. The alpha subunit of the NLS receptor, Rag cohort 1 (Rch1), enters the nucleus, probably in a complex with the beta subunit of the receptor, as well as other import factors and the import substrate. To learn more about which factors and/or events end the import reaction and how the import factors return to the cytoplasm, we have studied nucleocytoplasmic shuttling of Rch1 in vivo. Recombinant Rch1 microinjected into Vero or tsBN2 cells was found primarily in the cytoplasm. Rch1 injected into the nucleus was rapidly exported in a temperature-dependent manner. In contrast, a mutant of Rch1 lacking the first 243 residues accumulated in the nuclei of Vero cells after cytoplasmic injection. After nuclear injection, the truncated Rch1 was retained in the nucleus, but either Rch1 residues 207-217 or a heterologous nuclear export signal, but not a mutant form of residues 207-217, restored nuclear export. Loss of the nuclear transport factor RCC1 (regulator of chromosome condensation) at the nonpermissive temperature in the thermosensitive mutant cell line tsBN2 caused nuclear accumulation of wild-type Rch1 injected into the cytoplasm. However, free Rch1 injected into nuclei of tsBN2 cells at the nonpermissive temperature was exported. These results suggested that RCC1 acts at an earlier step in Rch1 recycling, possibly the disassembly of an import complex that contains Rch1 and the import substrate. Consistent with this possibility, incubation of purified RanGTP and RCC1 with NLS receptor and import substrate prevented assembly of receptor/substrate complexes or stimulated their disassembly.

A dual-specificity phosphatase Cdc25B is an unstable protein and triggers p34(cdc2)/cyclin B activation in hamster BHK21 cells arrested with hydroxyurea

By incubating at 30 degrees C in the presence of an energy source, p34(cdc2)/cyclin B was activated in the extract prepared from a temperature-sensitive mutant, tsBN2, which prematurely enters mitosis at 40 degrees C, the nonpermissive temperature (Nishimoto, T. , E. Eilen, and C. Basilico. 1978. Cell. 15:475-483), and wild-type cells of the hamster BHK21 cell line arrested in S phase, without protein synthesis. Such an in vitro activation of p34(cdc2)/cyclin B, however, did not occur in the extract prepared from cells pretreated with protein synthesis inhibitor cycloheximide, although this extract still retained the ability to inhibit p34(cdc2)/cyclin B activation. When tsBN2 cells arrested in S phase were incubated at 40 degrees C in the presence of cycloheximide, Cdc25B, but not Cdc25A and C, among a family of dual-specificity phosphatases, Cdc25, was lost coincidentally with the lack of the activation of p34(cdc2)/cyclin B. Consistently, the immunodepletion of Cdc25B from the extract inhibited the activation of p34(cdc2)/cyclin B. Cdc25B was found to be unstable (half-life < 30 min). Cdc25B, but not Cdc25C, immunoprecipitated from the extract directly activated the p34(cdc2)/cyclin B of cycloheximide-treated cells as well as that of nontreated cells, although Cdc25C immunoprecipitated from the extract of mitotic cells activated the p34(cdc2)/cyclin B within the extract of cycloheximide-treated cells. Our data suggest that Cdc25B made an initial activation of p34(cdc2)/cyclin B, which initiates mitosis through the activation of Cdc25C.

Interferon-gamma-dependent nuclear import of Stat1 is mediated by the GTPase activity of Ran/TC4

In response to interferon-gamma (IFN-gamma), Stat1 enters the nucleus, where it activates transcription. In order to better understand the mechanism of the extracellular signal-induced protein import into the nucleus, we have established an in vivo assay system that uses recombinant Stat1 protein as a model transport substrate. Using this system, we found that Stat1 is actively transported through the nuclear pores in an IFN-gamma-dependent manner and tyrosine (Tyr701) phosphorylation of Stat1 is actually required for its nuclear import. When the antibody against Ran, which was identified as an essential factor for active nuclear protein transport, was injected, the IFN-gamma-dependent nuclear transport of Stat1 was completely inhibited. Furthermore, nuclear import of Stat1 was suppressed by microinjection of two mutant Ran proteins, one defective in GTP hydrolysis (G19V) and the other with little or no binding to GTP (T24N), both of which are known to act as dominant negative inhibitors of nuclear import. These results indicate that the conditional nuclear import of Stat1 requires GTP hydrolysis by Ran.