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PubMed2006

Subcellular localisation of human inositol 1,4,5-trisphosphate 3-kinase C: species-specific use of alternative export sites for nucleo-cytoplasmic shuttling indicates divergent roles of the catalytic and N-terminal domains.

The three isoforms of human Ins(1,4,5)P3 3-kinase (IP3K) show remarkable differences in their intracellular targeting. Whereas predominant targeting to the cytoskeleton and endoplasmic reticulum has been shown for IP3K-A and IP3K-B, rat IP3K-C shuttles actively between the nucleus and cytoplasm. In the present study we examined the expression and intracellular localisation of endogenous IP3K-C in different mammalian cell lines using an isoform-specific antibody. In addition, human IP3K-C, showing remarkable differences to its rat homologue in the N-terminal targeting domain, was tagged with EGFP and used to examine active transport mechanisms into and out of the nucleus. We found both a nuclear import activity residing in its N-terminal domain and a nuclear export activity sensitive to treatment with leptomycin B. Different from the rat isoform, an exportin 1-dependent nuclear export site of the human enzyme resides outside the N-terminal targeting domain in the catalytic enzyme domain. A phylogenetic survey of vertebrate IP3K sequences indicates that in each of the three isoforms a nuclear export signal has evolved in the catalytic domain either de novo (IP3K-A) or as a substitute for an earlier evolved corresponding N-terminal signal (IP3K-B and IP3K-C). In higher vertebrates, and in particular in primates, re-export of nuclear IP3K activity may be guaranteed by the mechanism discovered.

Active Transport, Cell Nucleus↗
PubMed2001

The cytoplasmic shuttling and subsequent degradation of p27Kip1 mediated by Jab1/CSN5 and the COP9 signalosome complex.

The fifth component of the COP9 signalosome complex, Jab1/CSN5, directly binds to and induces specific down-regulation of the cyclin-dependent kinase inhibitor p27 (p27(Kip1)). Nuclear-cytoplasmic translocation plays an important role because leptomycin B (LMB), a chemical inhibitor of CRM1-dependent nuclear export, prevents p27 degradation mediated by Jab1/CSN5. Here we show that Jab1/CSN5 functions as an adaptor between p27 and CRM1 to induce nuclear export and subsequent degradation. Jab1/CSN5, but not p27, contains a typical leucine-rich nuclear export signal (NES) sequence conserved among different species, through which CRM1 bound to Jab1/CSN5 in an LMB-sensitive manner. Alteration of conserved leucine residues to alanine within Jab1/CSN5-NES abolished the interaction with CRM1 in vitro and impaired LMB-sensitive nuclear export and the ability to induce p27 breakdown in cultured cells. A Jab1/CSN5 truncation mutant lacking NES reversed p27 down-regulation induced by the full-length Jab1/CSN5, indicating that this mutant functions as a dominant negative (DN-Jab1). Introduction of DN-Jab1 into proliferating fibroblasts increased the level of p27 protein, thereby inducing growth arrest of the cells. Random mutagenesis analysis revealed that specific aspartic acid, leucine, and asparagine residues contained in the Jab1/CSN5-binding domain of p27 were required for interaction with Jab1/CSN5 and for down-regulation of p27. Glycerol gradient and cell fractionation experiments showed that at least two different forms of Jab1/CSN5-containing complexes existed within the cell. One is the conventional 450-kDa COP9 signalosome (CSN) complex located in the nucleus, and the other is much smaller (around 100-kDa), containing only a subset of CSN components (CSN4-8 but not CSN1-3), and mainly located in the cytoplasm. Treatment of cells with LMB greatly reduced the level of the smaller complex, suggesting that it originated from the CSN complex by nuclear export. Besides Jab1/CSN5, CSN3, -6, -7, and -8 were capable of inducing p27 down-regulation, when ectopically expressed. These results indicate that cytoplasmic shuttling regulated by Jab1/CSN5 and other CSN components may be a new pathway to control the intracellular abundance of the key cell cycle regulator.

3T3 Cells↗
PubMed2005

Drosophila caliban, a nuclear export mediator, can function as a tumor suppressor in human lung cancer cells.

We previously showed that the Drosophila DNA binding homeodomain of Prospero included a 28 amino-acid sequence (HDA) that functions as a nuclear export signal. We describe here the identification of a protein we named Caliban, which can directly interact with the HDA. Caliban is homologous to human Sdccag1, which has been implicated in colon and lung cancer. Here we show that Caliban and Sdccag1 are mediators of nuclear export in fly and human cells, as interference RNA abrogates export of EYFP-HDA in normal fly and human lung cells. Caliban functions as a bipartite mediator nuclear export as the carboxy terminus binds HDA and the amino terminus itself functions as an NES, which directly binds the NES receptor Exportin. Finally, while non-cancerous lung cells have functional Sdccag1, five human lung carcinoma cell lines do not, even though Exportin still functions in these cells. Expression of fly Caliban in these human lung cancer cells restores EYFP-HDA nuclear export, reduces a cell's ability to form colonies on soft agar and reduces cell invasiveness. We suggest that Sdccag1 inactivation contributes to the transformed state of human lung cancer cells and that Caliban should be considered a candidate for use in lung cancer gene therapy.

Active Transport, Cell Nucleus↗
PubMed1997

Interaction of MAP kinase with MAP kinase kinase: its possible role in the control of nucleocytoplasmic transport of MAP kinase.

The mitogen-activated protein kinase (MAPK) cascade consisting of MAPK and its direct activator, MAPK kinase (MAPKK), is essential for signaling of various extracellular stimuli to the nucleus. Upon stimulation, MAPK is translocated to the nucleus, whereas MAPKK stays in the cytoplasm. It has been shown recently that the cytoplasmic localization of MAPKK is determined by its nuclear export signal (NES) in the near N-terminal region (residues 33-44). However, the mechanism determining the subcellular distribution of MAPK has been poorly understood. Here, we show that introduction of v-Ras, active STE11 or constitutively active MAPKK can induce nuclear translocation of MAPK in mammalian cultured cells. Furthermore, we show evidence suggesting that MAPK is localized to the cytoplasm through its specific association with MAPKK and that nuclear accumulation of MAPK is accompanied by dissociation of a complex between MAPK and MAPKK following activation of the MAPK pathway. We have identified the MAPK-binding site of MAPKK as its N-terminal residues 1-32. Moreover, a peptide encompassing the MAPK-binding site and the NES sequence of MAPKK has been shown to be sufficient to retain MAPK to the cytoplasm. These findings reveal the molecular basis regulating subcellular distribution of MAPK, and identify a novel function of MAPKK as a cytoplasmic anchoring protein for MAPK.

Animals↗
PubMed2003

Oxidative stress induces nucleo-cytoplasmic translocation of pancreatic transcription factor PDX-1 through activation of c-Jun NH(2)-terminal kinase.

Oxidative stress is induced in pancreatic beta-cells under diabetic conditions and causes beta-cell dysfunction. Antioxidant treatment of diabetic animals leads to recovery of insulin biosynthesis and increases the expression of its controlling transcription factor, pancreatic duodenal homeobox-1 (PDX-1), in pancreatic beta-cells. Here, we show that PDX-1 is translocated from the nuclei to the cytoplasm of pancreatic beta-cells in response to oxidative stress. When oxidative stress was charged upon beta-cell-derived HIT-T15 cells, both endogenous PDX-1 and exogenously introduced green fluorescent protein-tagged PDX-1 moved from the nuclei to the cytoplasm. The addition of a dominant negative form of c-Jun NH(2)-terminal kinase (JNK) inhibited oxidative stress-induced PDX-1 translocation, suggesting an essential role of JNK in mediating this phenomenon. Whereas the nuclear localization signal (NLS) in PDX-1 was not affected by oxidative stress, leptomycin B, a specific inhibitor of the classical leucine-rich nuclear export signal (NES), inhibited nucleo-cytoplasmic translocation of PDX-1 induced by oxidative stress. Moreover, we identified an NES at position 82-94 of the mouse PDX-1 protein. Thus, our present results revealed a novel mechanism that negatively regulates PDX-1 function. The identification of the NES, which overrides the function of the NLS in an oxidative stress-responsive, JNK-dependent manner, supports the complicated regulation of PDX-1 function in vivo and may further the understanding of beta-cell pathophysiology in diabetes.

Active Transport, Cell Nucleus↗
PubMed2003

The apolipoprotein B mRNA editing complex performs a multifunctional cycle and suppresses nonsense-mediated decay.

The C to U editing of apolipoprotein B (apoB) mRNA is mediated by a minimal complex composed of an RNA-binding cytidine deaminase (APOBEC1) and a complementing specificity factor (ACF). This editing generates a premature termination codon and a truncated open reading frame. We demonstrate that the APOBEC1-ACF holoenzyme mediates a multifunctional cycle. The atypical APOBEC1 nuclear localization signal is involved in RNA binding and is used to import ACF into the nucleus as cargo. APOBEC1 alone induces nonsense-mediated decay (NMD). The APOBEC1-ACF complex edits and remains associated with the edited RNA to protect it from NMD. The APOBEC1 nuclear export signal is involved in the export of ACF and the edited apoB mRNA together, to the site of translation.

APOBEC-1 Deaminase↗
PubMed1998

Distinct nuclear import and export pathways mediated by members of the karyopherin beta family.

Transport of proteins into and out of the nucleus occurs through nuclear pore complexes (NPCs) and is mediated by the interaction of transport factors with nucleoporins at the NPC. Nuclear import of proteins containing classical nuclear localization signals (NLSs) is mediated by a heterodimeric protein complex, composed of karyopherin alpha and beta1, that docks via beta1 the NLS-protein to the NPC. The GTPase Ran; the RanGDP binding protein, p10; and the RanGTP binding protein, RanBP1 are involved in translocation of the docked NLS-protein into the nucleus. Recently, new distinct nuclear import and export pathways that are mediated by members of the karyopherin beta family have been discovered. Karyopherin beta2 mediates import of mRNA binding proteins, whereas karyopherin beta3 and beta4 mediate import of a set of ribosomal proteins. Two other beta karyopherin family members, CRM1 and CAS, mediate export of proteins containing leucine-rich nuclear export signals (NES) and reexport of karyopherin alpha, respectively. This growing family contains new members that constitute potential transport factors for cargoes yet to be identified in the future. The common features of the members of karyopherin beta family are the ability to bind RanGTP and the ability to interact directly with nucleoporins at the NPC. The challenge for the future will be to identify the distinct or, perhaps, overlapping cargo(es) for each member of the karyopherin beta superfamily and to characterize the molecular mechanisms of translocation of karyopherins together with their cargoes through the NPC.

Animals↗
PubMed2003

Dynamic association of RNA-editing enzymes with the nucleolus.

ADAR1 and ADAR2 are editing enzymes that deaminate adenosine to inosine in long double stranded RNA duplexes and specific pre-mRNA transcripts. Here, we show that full-length and N-terminally truncated forms of ADAR1 are simultaneously expressed in HeLa and COS7 cells owing to the usage of alternative starting methionines. Because the N-terminus of ADAR1 contains a nuclear export signal, the full-length protein localizes predominantly in the cytoplasm, whereas the N-terminally truncated forms are exclusively nuclear and accumulate in the nucleolus. ADAR2, which lacks a region homologous to the N-terminal domain of ADAR1, localizes exclusively to the nucleus and similarly accumulates in the nucleolus. Within the nucleolus, ADAR1 and ADAR2 co-localize in a novel compartment. Photobleaching experiments demonstrate that, in live cells, ADAR1 and ADAR2 are in constant flux in and out of the nucleolus. When cells express the editing-competent glutamate receptor GluR-B RNA, endogenous ADAR1 and ADAR2 de-localize from the nucleolus and accumulate at sites where the substrate transcripts accumulate. This suggests that ADAR1 and ADAR2 are constantly moving through the nucleolus and might be recruited onto specific editing substrates present elsewhere in the cell.

Adenosine Deaminase↗
PubMed2003

Nucleocytoplasmic shuttling of antigen in mammalian cells conferred by a soluble versus insoluble single-chain antibody fragment equipped with import/export signals.

The ectopic expression of antibody fragments within mammalian cells is a challenging approach for interfering with or even blocking the biological function of the intracellular target. For this purpose, single-chain Fv (scFv) fragments are generally preferred. Here, by transfecting several mammalian cell lines, we compared the intracellular behavior of two scFvs (13R4 and 1F4) that strongly differ in their requirement of disulphide bonding for the formation of active molecules in bacteria. The scFv 13R4, which is correctly folded in the bacterial cytoplasm, was solubly expressed in all cell lines tested and was distributed in their cytoplasm and nucleus, as well. In addition, by appending to the 13R4 molecules the SV40 T-antigen nuclear localisation signal (NLS) tag, cytoplasmic-coexpressed antigen was efficiently retargeted to the nucleus. Compared to the scFv 13R4, the scFv 1F4, which needs to be secreted in bacteria for activity, accumulated, even with the NLS tag, as insoluble aggregates within the cytoplasm of the transfected cells, thereby severely disturbing fundamental functions of cell physiology. Furthermore, by replacing the NLS tag with a leucine-rich nuclear export signal (NES), the scFv 13R4 was exclusively located in the cytoplasm, whereas the similarly modified scFv 1F4 still promoted cell death. Coexpression of NES-tagged 13R4 fragments with nuclear antigen promoted its efficient retargeting to the cytoplasm. This dominant effect of the NES tag was also observed after exchange of the nuclear signals between the scFv 13R4 and its antigen. Taken together, the results indicate that scFvs that are active in the cytoplasm of bacteria may behave similarly in mammalian cells and that the requirement of their conserved disulphide bridges for activity is a limiting factor for mediating the nuclear import/export of target in a mammalian cell context. The described shuttling effect of antigen conferred by a soluble scFv may represent the basis of a reliable in vivo assay of effective protein- protein interactions.

Active Transport, Cell Nucleus↗
PubMed2005

Identification of domains of ataxia-telangiectasia mutated required for nuclear localization and chromatin association.

Ataxia-telangiectasia mutated (ATM) is essential for rapid induction of cellular responses to DNA double strand breaks (DSBs). In this study, we mapped a nuclear localization signal (NLS), 385KRKK388, within the amino terminus of ATM and demonstrate its recognition by the conventional nuclear import receptor, the importin alpha1/beta1 heterodimer. Although mutation of this NLS resulted in green fluorescent protein (GFP) x ATM(NLSm) localizing predominantly within the cytoplasm, small amounts of nuclear GFP x ATM(NLSm) were still sufficient to elicit a DNA damage response. Insertion of an heterologous nuclear export signal between GFP and ATM(NLSm) resulted in complete cytoplasmic localization of ATM, concomitantly reducing the level of substrate phosphorylation and increasing radiosensitivity, which indicates a functional requirement for ATM nuclear localization. Interestingly, the carboxyl-terminal half of ATM, containing the kinase domain, which localizes to the cytoplasm, could not autophosphorylate itself or phosphorylate substrates, nor could it correct radiosensitivity in response to DSBs even when targeted to the nucleus by insertion of an exogenous NLS, demonstrating that the ATM amino terminus is required for optimal ATM function. Moreover, we have shown that the recruitment/retention of ATM at DSBs requires its kinase activity because a kinase-dead mutant of GFP x ATM failed to form damage-induced foci. Using deletion mutation analysis we mapped a domain in ATM (amino acids 5-224) required for its association with chromatin, which may target ATM to sites of DNA damage. Combined, these data indicate that the amino terminus of ATM is crucial not only for nuclear localization but also for chromatin association, thereby facilitating the kinase activity of ATM in vivo.

Animals↗
PubMed2004

Adapter protein SH2-B beta undergoes nucleocytoplasmic shuttling: implications for nerve growth factor induction of neuronal differentiation.

The adapter protein SH2-B has been shown to bind to activated nerve growth factor (NGF) receptor TrkA and has been implicated in NGF-induced neuronal differentiation and the survival of sympathetic neurons. However, the mechanism by which SH2-B enhances and maintains neurite outgrowth is unclear. We examined the ability of truncation mutants to regulate neuronal differentiation and observed that certain truncation mutants localized in the nucleus rather than in the cytoplasm or at the plasma membrane as reported for wild-type SH2-B beta. Addition of the nuclear export inhibitor leptomycin B caused both overexpressed wild-type and endogenous SH2-B beta to accumulate in the nucleus of both PC12 cells and COS-7 cells as did deletion of a putative nuclear export sequence (amino acids 224 to 233) or mutation of two critical lysines in that sequence. Deleting or mutating the nuclear export signal caused SH2-B beta to lose its ability to enhance NGF-induced differentiation of PC12 cells. Neither the NGF-induced phosphorylation of ERKs 1 and 2 nor their subcellular distribution was altered in PC12 cells stably expressing the nuclear export-defective SH2-B beta(L231A, L233A). These data provide strong evidence that SH2-B beta shuttles constitutively between the nucleus and cytoplasm. However, SH2-B beta needs continuous access to the cytoplasm and/or plasma membrane to participate in NGF-induced neurite outgrowth. These data also suggest that the stimulatory effect of SH2-B beta on NGF-induced neurite outgrowth of PC12 cells is either downstream of ERKs or via some other pathway yet to be identified.

Active Transport, Cell Nucleus↗
PubMed2001

Calreticulin Is a receptor for nuclear export.

In previous work, we used a permeabilized cell assay that reconstitutes nuclear export of protein kinase inhibitor (PKI) to show that cytosol contains an export activity that is distinct from Crm1 (Holaska, J.M., and B.M. Paschal. 1995. Proc. Natl. Acad. Sci. USA. 95: 14739-14744). Here, we describe the purification and characterization of the activity as calreticulin (CRT), a protein previously ascribed to functions in the lumen of the ER. We show that cells contain both ER and cytosolic pools of CRT. The mechanism of CRT-dependent export of PKI requires a functional nuclear export signal (NES) in PKI and involves formation of an export complex that contains RanGTP. Previous studies linking CRT to downregulation of steroid hormone receptor function led us to examine its potential role in nuclear export of the glucocorticoid receptor (GR). We found that CRT mediates nuclear export of GR in permeabilized cell, microinjection, and transfection assays. GR export is insensitive to the Crm1 inhibitor leptomycin B in vivo, and it does not rely on a leucine-rich NES. Rather, GR export is facilitated by its DNA-binding domain, which is shown to function as an NES when transplanted to a green fluorescent protein reporter. CRT defines a new export pathway that may regulate the transcriptional activity of steroid hormone receptors.

Active Transport, Cell Nucleus↗
PubMed2003

Molecular evidence for the nuclear localization of FADD.

The Fas-associated death domain (FADD) adaptor protein FADD/Mort-1 is recruited by several members of the tumor necrosis factor receptor (TNFR) superfamily during cell death activated via death receptors. Since most studies have focused on the interaction of FADD with plasma membrane proteins, FADD's subcellular location is thought to be confined to the cytoplasm. In this report, we show for the first time that FADD is present in both the cytoplasm and the nucleus of cells, and that its nuclear localization relies on strong nuclear localization and nuclear export signals (NLS and NES, respectively) that reside in the death-effector domain (DED) of the protein. Specifically, we found that a conserved basic KRK35 sequence of the human protein is necessary for FADD's nuclear localization, since disruption of this motif leads to the confinement of FADD in the cytoplasm. Furthermore, we show that the leucine-rich motif LTELKFLCL28 in the DED is necessary for FADD's nuclear export. Functionally, mutation of the NES of FADD and its seclusion in the nucleus reduces the cell death-inducing efficacy of FADD reconstituted in FADD-deficient T cells.

Active Transport, Cell Nucleus↗
PubMed2006

Characterization of functional domains of equine infectious anemia virus Rev suggests a bipartite RNA-binding domain.

Equine infectious anemia virus (EIAV) Rev is an essential regulatory protein that facilitates expression of viral mRNAs encoding structural proteins and genomic RNA and regulates alternative splicing of the bicistronic tat/rev mRNA. EIAV Rev is characterized by a high rate of genetic variation in vivo, and changes in Rev genotype and phenotype have been shown to coincide with changes in clinical disease. To better understand how genetic variation alters Rev phenotype, we undertook deletion and mutational analyses to map functional domains and to identify specific motifs that are essential for EIAV Rev activity. All functional domains are contained within the second exon of EIAV Rev. The overall organization of domains within Rev exon 2 includes a nuclear export signal, a large central region required for RNA binding, a nonessential region, and a C-terminal region required for both nuclear localization and RNA binding. Subcellular localization of green fluorescent protein-Rev mutants indicated that basic residues within the KRRRK motif in the C-terminal region of Rev are necessary for targeting of Rev to the nucleus. Two separate regions of Rev were necessary for RNA binding: a central region encompassing residues 57 to 130 and a C-terminal region spanning residues 144 to 165. Within these regions were two distinct, short arginine-rich motifs essential for RNA binding, including an RRDRW motif in the central region and the KRRRK motif near the C terminus. These findings suggest that EIAV Rev utilizes a bipartite RNA-binding domain.

Active Transport, Cell Nucleus↗
PubMed2000

Three leucine-rich sequences and the N-terminal region of double-stranded RNA-activated protein kinase (PKR) are responsible for its cytoplasmic localization.

The double-stranded RNA-activated-protein kinase PKR was originally identified as a ribosomal protein that regulates protein synthesis at the translational level. While PKR locates predominantly to the cytoplasm, nuclear or nucleolar species of PKR have been detected. Here, we demonstrate that PKR possesses three leucine-rich sequences resembling nuclear export signals (NESs). Enhanced green fluorescent protein (EGFP) fused to one of these sequences and transfected in COS-1 cells exhibited predominant cytoplasmic staining, which was abrogated by a leucine to alanine substitution. In addition, Leptomycin B (LMB), an inhibitor of NES-mediated nuclear export, inhibited the cytoplasmic localization of EGFP-NES, indicating the potential activity of these stretches as NESs. Although EGFP fused to a PKR with three NES mutations still located to the cytoplasm, an additional N-terminal deletion impaired the cytoplasmic predominance, suggesting that the N-terminal region is also required for localization. These results suggest that the cytoplasmic localization of PKR is regulated by NESs as well as the N-terminal sequence.

Active Transport, Cell Nucleus↗
PubMed2004

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

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

ATP-Binding Cassette Transporters↗
PubMed2003

The carboxy terminus of Prospero regulates its subcellular localization.

Subcellular localization of the transcription factor Prospero is dynamic. For example, the protein is cytoplasmic in neuroblasts, nuclear in sheath cells, and degraded in newly formed neurons. The carboxy terminus of Prospero, including the homeodomain and Prospero domain, plays roles in regulating these changes. The homeodomain has two distinct subdomains, which exclude proteins from the nucleus, while the intact homeo/Prospero domain masks this effect. One subdomain is an Exportin-dependent nuclear export signal requiring three conserved hydrophobic residues, which models onto helix 1. Another, including helices 2 and 3, requires proteasome activity to degrade nuclear protein. Finally, the Prospero domain is missing in pros(I13) embryos, thus unmasking nuclear exclusion, resulting in constitutively cytoplasmic protein. Multiple processes direct Prospero regulation of cell fate in embryonic nervous system development.

Active Transport, Cell Nucleus↗
PubMed1999

CRM1-mediated recycling of snurportin 1 to the cytoplasm.

Importin beta is a major mediator of import into the cell nucleus. Importin beta binds cargo molecules either directly or via two types of adapter molecules, importin alpha, for import of proteins with a classical nuclear localization signal (NLS), or snurportin 1, for import of m3G-capped U snRNPs. Both adapters have an NH2-terminal importin beta-binding domain for binding to, and import by, importin beta, and both need to be returned to the cytoplasm after having delivered their cargoes to the nucleus. We have shown previously that CAS mediates export of importin alpha. Here we show that snurportin 1 is exported by CRM1, the receptor for leucine-rich nuclear export signals (NESs). However, the interaction of CRM1 with snurportin 1 differs from that with previously characterized NESs. First, CRM1 binds snurportin 1 50-fold stronger than the Rev protein and 5,000-fold stronger than the minimum Rev activation domain. Second, snurportin 1 interacts with CRM1 not through a short peptide but rather via a large domain that allows regulation of affinity. Strikingly, snurportin 1 has a low affinity for CRM1 when bound to its m3G-capped import substrate, and a high affinity when substrate-free. This mechanism appears crucial for productive import cycles as it can ensure that CRM1 only exports snurportin 1 that has already released its import substrate in the nucleus.

Animals↗