Showing papers on "Importin published in 2002"
TL;DR: Just as heat shock proteins function as chaperones for exposed hydrophobic patches, importins act as chapers for exposed basic domains, and it is suggested that this represents a major and general cellular function of importins.
Abstract: Many nuclear transport pathways are mediated by importin β-related transport receptors. Here, we identify human importin (Imp) 4b as well as mouse Imp4a, Imp9a and Imp9b as novel family members. Imp4a mediates import of the ribosomal protein (rp) S3a, while Imp9a and Imp9b import rpS7, rpL18a and apparently numerous other substrates. Ribosomal proteins, histones and many other nuclear import substrates are very basic proteins that aggregate easily with cytoplasmic polyanions such as RNA. Imp9 effectively prevents such precipitation of, for example, rpS7 and rpL18a by covering their basic domains. The same applies to Imp4, Imp5, Imp7 and Impβ and their respective basic import substrates. The Impβ–Imp7 heterodimer appears specialized for the most basic proteins, such as rpL4, rpL6 and histone H1, and is necessary and sufficient to keep them soluble in a cytoplasmic environment prior to rRNA or DNA binding, respectively. Thus, just as heat shock proteins function as chaperones for exposed hydrophobic patches, importins act as chaperones for exposed basic domains, and we suggest that this represents a major and general cellular function of importins.
284 citations
TL;DR: A direct interaction of importin‐α5 with tyrosine‐phosphorylated STAT1 dimers is demonstrated, and evidence that a nuclear localization signal (NLS) exists in an inactive state within a STAT1 monomer is provided, and it is demonstrated that STAT1 binding to specific target DNA effectively blocks importin-α5 binding.
Abstract: Signal transducers and activators of transcription (STATs) reside in a latent state in the cytoplasm of the cell, but accumulate in the nucleus in response to cytokines or growth factors. Localization in the nucleus occurs following STAT tyrosine phosphorylation and dimerization. In this report we demonstrate a direct interaction of importin-α5 with tyrosine-phosphorylated STAT1 dimers, and provide evidence that a nuclear localization signal (NLS) exists in an inactive state within a STAT1 monomer. A mutation in STAT1 leucine 407 (L407A) is characterized, which generates a protein that is accurately tyrosine phosphorylated in response to interferon, dimerizes and binds DNA, but does not localize to the nucleus. The import defect of STAT1(L407A) appears to be a consequence of the inability of this protein to be recognized by its import shuttling receptor. In addition, we demonstrate that STAT1 binding to specific target DNA effectively blocks importin-α5 binding. This result may play a role in localizing STAT1 to its destination in the nucleus, and in releasing importin-α5 from STAT1 for recycling back to the cytoplasm.
264 citations
TL;DR: It is found that the vast majority of translation initiation factors, all three elongation factors and the termination factor eRF1 are strictly excluded from nuclei, suggesting that nuclear translation is actively suppressed by the nuclear export machinery.
Abstract: Importin β-type transport receptors mediate the vast majority of transport pathways between cell nucleus and cytoplasm. We identify here the translation elongation factor 1A (eEF1A) as the predominant nuclear export substrate of RanBP21/exportin 5 (Exp5). This cargo–exportin interaction is rather un usual in that eEF1A binds the exportin not directly, but instead via aminoacylated tRNAs. Exp5 thus represents the second directly RNA-binding exportin and mediates tRNA export in parallel with exportin-t. It was suggested recently that 10–15% of the cellular translation would occur in the nucleus. Our data rule out such a scenario and instead suggest that nuclear translation is actively suppressed by the nuclear export machinery. We found that the vast majority of translation initiation factors (eIF2, eIF2B, eIF3, eIF4A1, eIF5 and eIF5B), all three elongation factors (eEF1A, eEF1B and eEF2) and the termination factor eRF1 are strictly excluded from nuclei. Besides Exp5 and importin 13, CRM1 and as yet unidentified exportins also contribute to the depletion of translation factors from nuclei.
236 citations
TL;DR: The structure and biochemical experiments show that RanGAP does not act through an arginine finger, that the basic machinery for fast GTP hydrolysis is provided exclusively by Ran and that correct positioning of the catalytic glutamine is essential for catalysis.
Abstract: GTPase-activating proteins (GAPs) increase the rate of GTP hydrolysis on guanine nucleotide-binding proteins by many orders of magnitude. Studies with Ras and Rho have elucidated the mechanism of GAP action by showing that their catalytic machinery is both stabilized by GAP binding and complemented by the insertion of a so-called 'arginine finger' into the phosphate-binding pocket. This has been proposed as a universal mechanism for GAP-mediated GTP hydrolysis. Ran is a nuclear Ras-related protein that regulates both transport between the nucleus and cytoplasm during interphase, and formation of the mitotic spindle and/or nuclear envelope in dividing cells. Ran-GTP is hydrolysed by the combined action of Ran-binding proteins (RanBPs) and RanGAP. Here we present the three-dimensional structure of a Ran-RanBP1-RanGAP ternary complex in the ground state and in a transition-state mimic. The structure and biochemical experiments show that RanGAP does not act through an arginine finger, that the basic machinery for fast GTP hydrolysis is provided exclusively by Ran and that correct positioning of the catalytic glutamine is essential for catalysis.
203 citations
TL;DR: Using a permeabilized cell import assay, it is demonstrated that importin beta (1-485) can import PTHrP-coupled cargo in a Ran-dependent manner and it is proposed that this region contains a prototypical nuclear import receptor domain, which could have evolved into the modern importin Beta superfamily.
202 citations
TL;DR: It is concluded that RanBP2 is the major nucleoporin component of the cytoplasmic filaments of the NPC, and that these filaments do not have an essential role in importin α/β– or transportin-dependent import.
Abstract: The nuclear pore complex (NPC) mediates bidirectional macromolecular traffic between the nucleus and cytoplasm in eukaryotic cells Eight filaments project from the NPC into the cytoplasm and are proposed to function in nuclear import We investigated the localization and function of two nucleoporins on the cytoplasmic face of the NPC, CAN/Nup214 and RanBP2/Nup358 Consistent with previous data, RanBP2 was localized at the cytoplasmic filaments In contrast, CAN was localized near the cytoplasmic coaxial ring Unexpectedly, extensive blocking of RanBP2 with gold-conjugated antibodies failed to inhibit nuclear import Therefore, RanBP2-deficient NPCs were generated by in vitro nuclear assembly in RanBP2-depleted Xenopus egg extracts NPCs were formed that lacked cytoplasmic filaments, but that retained CAN These nuclei efficiently imported nuclear localization sequence (NLS) or M9 substrates NPCs lacking CAN retained RanBP2 and cytoplasmic filaments, and showed a minor NLS import defect NPCs deficient in both CAN and RanBP2 displayed no cytoplasmic filaments and had a strikingly immature cytoplasmic appearance However, they showed only a slight reduction in NLS-mediated import, no change in M9-mediated import, and were normal in growth and DNA replication We conclude that RanBP2 is the major nucleoporin component of the cytoplasmic filaments of the NPC, and that these filaments do not have an essential role in importin α/β– or transportin-dependent import
194 citations
TL;DR: It is demonstrated that nucleosome assembly protein 1 (Nap1p), a protein previously implicated in the deposition of histones H2A and H2B, is also involved in the transport of these two histones, and a model in which Nap1p links the nuclear transport of H2 a and H 2B to chromatin assembly is proposed.
Abstract: Import of core histones into the nucleus is a prerequisite for their deposition onto DNA and the assembly of chromatin. Here we demonstrate that nucleosome assembly protein 1 (Nap1p), a protein previously implicated in the deposition of histones H2A and H2B, is also involved in the transport of these two histones. We demonstrate that Nap1p can bind directly to Kap114p, the primary karyopherin/importin responsible for the nuclear import of H2A and H2B. Nap1p also serves as a bridge between Kap114p and the histone nuclear localization sequence (NLS). Nap1p acts cooperatively to increase the affinity of Kap114p for these NLSs. Nuclear accumulation of histone NLS–green fluorescent protein (GFP) reporters was decreased in Δnap1 cells. Furthermore, we demonstrate that Nap1p promotes the association of the H2A and H2B NLSs specifically with the karyopherin Kap114p. Localization studies demonstrate that Nap1p is a nucleocytoplasmic shuttling protein, and genetic experiments suggest that its shuttling is important for maintaining chromatin structure in vivo. We propose a model in which Nap1p links the nuclear transport of H2A and H2B to chromatin assembly.
185 citations
TL;DR: It is demonstrated that specific NLSs in STATs mediate direct interactions of STAT dimers with importin α, which activates the nuclear import process, and this interaction was very stable and was dependent on lysines 410 and 413 of STAT1.
180 citations
TL;DR: It is demonstrated that both IMA-2 and IMB-1, the homologues of vertebrate importin alpha and beta, are essential for both spindle assembly and nuclear formation in early embryos.
Abstract: The small GTPase Ran has been found to play pivotal roles in several aspects of cell function. We have investigated the role of the Ran GTPase cycle in spindle formation and nuclear envelope assembly in dividing Caenorhabditis elegans embryos in real time. We found that Ran and its cofactors RanBP2, RanGAP, and RCC1 are all essential for reformation of the nuclear envelope after cell division. Reducing the expression of any of these components of the Ran GTPase cycle by RNAi leads to strong extranuclear clustering of integral nuclear envelope proteins and nucleoporins. Ran, RanBP2, and RanGAP are also required for building a mitotic spindle, whereas astral microtubules are normal in the absence of these proteins. RCC1(RNAi) embryos have similar abnormalities in the initial phase of spindle formation but eventually recover to form a bipolar spindle. Irregular chromatin structures and chromatin bridges due to spindle failure were frequently observed in embryos where the Ran cycle was perturbed. In addition, connection between the centrosomes and the male pronucleus, and thus centrosome positioning, depends upon the Ran cycle components. Finally, we have demonstrated that both IMA-2 and IMB-1, the homologues of vertebrate importin α and β, are essential for both spindle assembly and nuclear formation in early embryos.
174 citations
TL;DR: The 1.9 Å resolution crystal structure of yeast NTF2‐N77Y bound to a FxFG‐nucleoporin core is described, which provides a basis for understanding this interaction and its role in nuclear trafficking.
Abstract: Interactions with nucleoporins containing FxFG‐repeat cores are crucial for the nuclear import of RanGDP mediated by nuclear transport factor 2 (NTF2). We describe here the 1.9 A resolution crystal structure of yeast NTF2‐N77Y bound to a FxFG‐nucleoporin core, which provides a basis for understanding this interaction and its role in nuclear trafficking. The two identical FxFG binding sites on the dimeric molecule are formed by residues from each chain of NTF2. Engineered mutants at the interaction interface reduce the binding of NTF2 to nuclear pores and cause reduced growth rates and Ran mislocalization when substituted for the wild‐type protein in yeast. Comparison with the crystal structure of FG‐nucleoporin cores bound to importin‐β and TAP/p15 identified a number of common features of their binding sites. The structure of the binding interfaces on these transport factors provides a rationale for the specificity of their interactions with nucleoporins that, combined with their weak binding constants, facilitates rapid translocation through NPCs during nuclear trafficking.
165 citations
TL;DR: H3 and H4 are the first histones to be assembled onto DNA, and results show that their import is mediated by at least two import pathways, and it is demonstrated that cytosolic Kap123p is associated with acetylated H4.
TL;DR: The N-terminal part of the linker region of Dengue2 NS5 is critical for nuclear localization and is recognized with high affinity by the conventional NLS-binding importin α/β heterodimeric nuclear import receptor.
TL;DR: It is concluded that, following Sm protein assembly, the SMN complex persists until the final stages of cytoplasmic snRNP maturation and may provide somatic cell RNPs with an alternative NLS.
Abstract: The survival of motor neuron (SMN) protein is mutated in patients with spinal muscular atrophy (SMA). SMN is part of a multiprotein complex required for biogenesis of the Sm class of small nuclear ribonucleoproteins (snRNPs). Following assembly of the Sm core domain, snRNPs are transported to the nucleus via importin beta. Sm snRNPs contain a nuclear localization signal (NLS) consisting of a 2,2,7-trimethylguanosine (TMG) cap and the Sm core. Snurportin1 (SPN) is the adaptor protein that recognizes both the TMG cap and importin beta. Here, we report that a mutant SPN construct lacking the importin beta binding domain (IBB), but containing an intact TMG cap-binding domain, localizes primarily to the nucleus, whereas full-length SPN localizes to the cytoplasm. The nuclear localization of the mutant SPN was not a result of passive diffusion through the nuclear pores. Importantly, we found that SPN interacts with SMN, Gemin3, Sm snRNPs and importin beta. In the presence of ribonucleases, the interactions with SMN and Sm proteins were abolished, indicating that snRNAs mediate this interplay. Cell fractionation studies showed that SPN binds preferentially to cytoplasmic SMN complexes. Notably, we found that SMN directly interacts with importin beta in a GST-pulldown assay, suggesting that the SMN complex might represent the Sm core NLS receptor predicted by previous studies. Therefore, we conclude that, following Sm protein assembly, the SMN complex persists until the final stages of cytoplasmic snRNP maturation and may provide somatic cell RNPs with an alternative NLS.
TL;DR: This paper showed that 20 S proteasomes are imported as precursor complexes into the nucleus by using the srp1-49 mutant which is deficient in nuclear import of cargos with classical nuclear localization sequences (cNLS).
TL;DR: It is shown that Npap60 (also called Nup50), a protein previously believed to be a structural component of the NPC, is a Ran binding protein and a cofactor for importin-α:β-mediated import.
TL;DR: The technique of fluorescence recovery after photobleaching is used to demonstrate the ability of P THrP to shuttle between cytoplasm and nucleus and to visualize directly the transport of PTHrP into the nucleus in living cells.
Abstract: PTH-related protein (PTHrP) was first discovered as a circulating factor secreted by certain cancers and is responsible for the syndrome of humoral hypercalcemia of malignancy induced by various tumors. The similarity of its N terminus to that of PTH enables PTHrP to share the signaling properties of PTH, but the rest of the molecule possesses distinct functions, including a role in the nucleus/nucleolus in reducing apoptosis and enhancing cell proliferation. PTHrP nuclear import is mediated by importin β1. In this study we use the technique of fluorescence recovery after photobleaching to demonstrate the ability of PTHrP to shuttle between cytoplasm and nucleus and to visualize directly the transport of PTHrP into the nucleus in living cells. Endogenous and transfected PTHrP was demonstrated to colocalize with microtubule structures in situ using various high-resolution microscopic approaches, as well as in in vitro binding studies, where importin β1, but not importinα , enhanced the microtubular associa...
TL;DR: It is found that importin α migrated into the nucleus without the addition of importin β, Ran or any other soluble factors in an in vitro transport assay, suggesting that the nuclear import machinery for importinα at individual nuclear pore complexes may be regulated by reaction(s) that require ATP hydrolysis.
Abstract: A classical nuclear localization signal (NLS)‐containing protein is transported into the nucleus via the formation of a NLS‐substrate/importin α/β complex. In this study, we found that importin α migrated into the nucleus without the addition of importin β, Ran or any other soluble factors in an in vitro transport assay. A mutant importin α lacking the importin β‐binding domain efficiently entered the nucleus. Competition experiments showed that this import pathway for importin α is distinct from that of importin β. These results indicate that importin α alone can enter the nucleus via a novel pathway in an importin β‐ and Ran‐independent manner. Furthermore, this process is evolutionarily conserved as similar results were obtained in Saccharomyces cerevisiae . Moreover, the import rate of importin α differed among individual nuclei of permeabilized cells, as demonstrated by time‐lapse experiments. This heterogeneous nuclear accumulation of importin α was affected by the addition of ATP, but not ATPγS. These results suggest that the nuclear import machinery for importin α at individual nuclear pore complexes may be regulated by reaction(s) that require ATP hydrolysis.
TL;DR: It is suggested that RanGTP functions in the importin α/β and transportin import pathways to promote the transport of large cargo by enhancing the ability of import complexes to traverse diffusionally restricted areas of the NPC.
Abstract: Previous work has shown that the transport of some small protein cargoes through the nuclear pore complex (NPC) can occur in vitro in the absence of nucleoside triphosphate hydrolysis. We now demonstrate that in the importin α/β and transportin import pathways, efficient in vitro transport of large proteins, in contrast to smaller proteins, requires hydrolyzable GTP and the small GTPase Ran. Morphological and biochemical analysis indicates that the presence of Ran and GTP allows large cargo to efficiently cross central regions of the NPC. We further demonstrate that this function of RanGTP at least partly involves its direct binding to importin β and transportin. We suggest that RanGTP functions in these pathways to promote the transport of large cargo by enhancing the ability of import complexes to traverse diffusionally restricted areas of the NPC.
TL;DR: Advances in the area of enhancing DNA targeting to the nucleus are discussed, including the use of modular nuclear targeting signals recognised by the cellular nuclear import machinery and DNA condensing agents to facilitate passage through the nuclear pore.
Abstract: Summary Gene therapy involves the introduction of DNA-encoding therapeutic gene products into appropriate cells of an affected individual. The limitations of the approach relate largely to the poor efficiency of the delivery of the therapeutic DNA to the nucleus. This review examines recent work in the area of non-viral gene transfer, building on developments in the field of nuclear protein import and their application in the field of non-viral gene transfer. In particular, advances in the area of enhancing DNA targeting to the nucleus are discussed, including the use of modular nuclear targeting signals recognised by the cellular nuclear import machinery and DNA condensing agents to facilitate passage through the nuclear pore. Optimising nuclear DNA delivery through these and other strategies should assist greatly in rendering gene therapy a viable and realistic possibility for treating disease.
TL;DR: This is the first demonstration that acetylation may alter nuclear partitioning by direct interference with nuclear import receptor recognition and indicates that E1A may exert its pleiotropic effects on cellular transformation in part by affecting cytoplasmic processes.
TL;DR: The results suggest that certain canonical NLSs within a nucleoprotein complex, such as the Vp1 NLS, can be masked from functioning by binding to the nucleic acid component and that the availability of an NLS that is not masked and can become exposed for importin binding is a general feature of the nuclear entry of the nucleop protein complexes, including those of other animal viruses.
Abstract: For nuclear entry of large nucleoprotein complexes, it is thought that one key nuclear localization signal (NLS) of a protein component becomes exposed to mediate importin recognition. We show that the nuclear entry of simian virus 40 involves a dynamic interplay between two distinct interiorly situated capsid NLSs, the Vp1 NLS and the Vp3 NLS, and the selective exposure and importin recognition of the Vp3 NLS. The Vp3 NLS-null mutants assembled normally into virion-like particles (VLP) in mutant DNA-transfected cells. When used to infect a new host, the null VLP entered the cell normally but was impaired in viral DNA nuclear entry due to a lack of recognition by the importin alpha 2/beta heterodimer, leading to reduced viability. Both Vp3 and Vp1 NLSs directed importin interaction in vitro, but the Vp1 NLS, which overlaps the Vp1 DNA binding domain, did not bind importins in the presence of DNA. The results suggest that certain canonical NLSs within a nucleoprotein complex, such as the Vp1 NLS, can be masked from functioning by binding to the nucleic acid component and that the availability of an NLS that is not masked and can become exposed for importin binding, such as the Vp3 NLS, is a general feature of the nuclear entry of the nucleoprotein complexes, including those of other animal viruses.
TL;DR: It is demonstrated that Pab1p associates nonspecifically with immobilized baits via RNA and binds to the FG repeat regions of Nups directly via the Nup85p subunit, and possible roles of these novel interactions in the mechanisms of nucleocytoplasmic transport are discussed.
TL;DR: Rescued sterility of α 2 null males and female infertility was rescued by a mutant importin α 2 transgene lacking a site that is normally phosphorylated in ovaries, suggesting that male and female gametogenesis have distinct requirements for importinα2.
Abstract: Importin α’s mediate the nuclear transport of many classical nuclear localization signal (cNLS)-containing proteins. Multicellular animals contain multiple importin α genes, most of which fall into three conventional phylogenetic clades, here designated α1, α2, and α3. Using degenerate PCR we cloned Drosophila melanogaster importin α 1 , α 2 , and α 3 genes, demonstrating that the complete conventional importin α gene family arose prior to the split between invertebrates and vertebrates. We have begun to analyze the genetic interactions among conventional importin α genes by studying their capacity to rescue the male and female sterility of importin α 2 null flies. The sterility of α 2 null males was rescued to similar extents by importin α 1 , α 2 , and α 3 transgenes, suggesting that all three conventional importin α’s are capable of performing the important role of importin α2 during spermatogenesis. In contrast, sterility of α 2 null females was rescued only by importin α 2 transgenes, suggesting that it plays a paralog-specific role in oogenesis. Female infertility was also rescued by a mutant importin α 2 transgene lacking a site that is normally phosphorylated in ovaries. These rescue experiments suggest that male and female gametogenesis have distinct requirements for importin α2.
TL;DR: It is shown that importin-beta, an effector of Ran involved in nucleocytoplasmic transport and mitotic spindle assembly, is required for NE assembly induced by Ran and may coordinate these processes during cell division.
TL;DR: A model in which Stat5B shuttles between the nucleus and cytoplasm by two different mechanisms, one being a factor-independent constitutive shuttling by monomeric form, and the other, a factor stimulation-dependent one regulated by tyrosine phosphorylation and subsequent dimerization is proposed.
Abstract: In response to cytokine stimuli, Stats are phosphorylated and translocated to the nucleus to activate target genes. Then, most are dephosphorylated and returned to the cytoplasm. Using Ba/F3 cells, we found that the nuclear export of Stat5B by cytokine depletion was inhibited by leptomycin B (LMB), a specific inhibitor of nuclear export receptor chromosome region maintenance 1. Interestingly, LMB treatment in the absence of cytokine led to the accumulation of Stat5B in the nucleus, suggesting that Stat5B shuttles between the nucleus and the cytoplasm as a monomer without cytokine stimulation. This notion is supported by the observation that LMB-induced accumulation of Stat5B in the nucleus was also observed with Stat5B having a mutated tyrosine 699, which is essential for dimer formation. Using a series of mutant Stat5Bs, we identified a part of the coiled coil domain to be a critical region for monomer nuclear import and a more N-terminal region to be critical for the cytokine stimulation dependent import of Stat5B. Taken together, we propose a model in which Stat5B shuttles between the nucleus and cytoplasm by two different mechanisms, one being a factor-independent constitutive shuttling by monomeric form, and the other, a factor stimulation-dependent one regulated by tyrosine phosphorylation and subsequent dimerization.
TL;DR: The identification and characterization of the RanGTPase and its binding partners are described: the guanine nucleotide exchange factor, RanGEF; the GTPase activating protein, RanGAP; the soluble import and export receptors; Ran-binding domain-(RBD) containing proteins; and NTF2 and related factors.
Abstract: This review focuses on the control of nuclear import and export pathways by the small GTPase Ran. Transport of signal-containing cargo substrates is mediated by receptors that bind to the cargo proteins and RNAs and deliver them to the appropriate cellular compartment. Ran is an evolutionarily conserved member of the Ras superfamily that regulates all receptor-mediated transport between the nucleus and the cytoplasm. We describe the identification and characterization of the RanGTPase and its binding partners: the guanine nucleotide exchange factor, RanGEF; the GTPase activating protein, RanGAP; the soluble import and export receptors; Ran-binding domain-(RBD) containing proteins; and NTF2 and related factors.
TL;DR: The results suggest that the nature of the IBB domain modulates the strength and/or site of interaction of impβ with nucleoporins of the nuclear pore complex, and thus whether or not Ran is required to dissociate these interactions.
Abstract: 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.
TL;DR: Evidence is provided that SUMO modification in yeast, as has been suspected for vertebrates, plays an important role in nucleocytoplasmic trafficking.
TL;DR: It is demonstrated that two nuclear localization signals target EKLF to the nucleus and suggest this transport relies primarily on a novel zinc finger/importin protein interaction.
TL;DR: This study is the first to assess all the human importin α isoforms in documenting differential nuclear transport factor regulation during cell proliferation and differentiation and found differential importin regulation.
Abstract: We recently cloned six human importin a proteins that transport specific substrates in complex with importin beta into the nucleus. We now compared their absolute expression levels in different human cell lines. We examined their expression regulation during human cell proliferation and differentiation by means of specific antibodies. Proliferation inhibition by starvation of HeLa and HaCaT cells led to a marked decrease in the expression of various nuclear transport factors. In contrast, re-addition of serum increased alpha-importin expression. We analyzed two models for cell differentiation and found differential importin regulation. Stimulation of rat pancreatic AR42J cell differentiation towards a neuroendocrine phenotype with activin A or towards an acinar phenotype with dexamethasone, caused strong upregulation of importin alpha3 and alpha4 expression. Phorbol ester-induced differentiation of human leukemia (HL60) cells towards a macrophage phenotype led to downregulation of importin alpha1 and alpha4 expression after 72 hours. Similarly, importins alpha1 and alpha4 displayed a strong downregulation when HL60 cells were directed towards a neutrophil phenotype by DMSO treatment. This study is the first to assess all the human importin alpha isoforms in documenting differential nuclear transport factor regulation during cell proliferation and differentiation.