Nuclear import by karyopherin-βs: recognition and inhibition

Cell biological mechanisms of activity-dependent synapse to nucleus translocation of CRTC1 in neurons

Previous studies have revealed a critical role for CREB-regulated transcriptional coactivator (CRTC1) in regulating neuronal gene expression during learning and memory. CRTC1 localizes to synapses but undergoes activity-dependent nuclear translocation to regulate the transcription of CREB target genes. Here we investigate the long-distance retrograde transport of CRTC1 in hippocampal neurons. We show that local elevations in calcium, triggered by activation of glutamate receptors and L-type voltage-gated calcium channels, initiate active, dynein-mediated retrograde transport of CRTC1 along microtubules. We identify a nuclear localization signal within CRTC1, and characterize three conserved serine residues whose dephosphorylation is required for nuclear import. Domain analysis reveals that the amino-terminal third of CRTC1 contains all of the signals required for regulated nucleocytoplasmic trafficking. We fuse this region to Dendra2 to generate a reporter construct and perform live-cell imaging coupled with local uncaging of glutamate and photoconversion to characterize the dynamics of stimulus-induced retrograde transport and nuclear accumulation.

Sorting nexin 9 differentiates ligand-activated Smad3 from Smad2 for nuclear import and transforming growth factor β signaling

Sorting nexin 9 (SNX9) is shown to differentiate Smad3 from Smad2 nuclear delivery by mediating the association of phosphorylated Smad3 with importin 8 and the nuclear membrane. While the absence of SNX9 had negligible effects on transforming growth factor β receptor activity or Smad2 signaling, Smad3-dependent targets and phenotypes were inhibited.

Transforming growth factor β (TGFβ) is a pleiotropic protein secreted from essentially all cell types and primary tissues. While TGFβ’s actions reflect the activity of a number of signaling networks, the primary mediator of TGFβ responses are the Smad proteins. Following receptor activation, these cytoplasmic proteins form hetero-oligomeric complexes that translocate to the nucleus and affect gene transcription. Here, through biological, biochemical, and immunofluorescence approaches, sorting nexin 9 (SNX9) is identified as being required for Smad3-dependent responses. SNX9 interacts with phosphorylated (p) Smad3 independent of Smad2 or Smad4 and promotes more rapid nuclear delivery than that observed independent of ligand. Although SNX9 does not bind nucleoporins Nup153 or Nup214 or some β importins (Imp7 or Impβ), it mediates the association of pSmad3 with Imp8 and the nuclear membrane. This facilitates nuclear translocation of pSmad3 but not SNX9.

Mechanisms Regulating Protein Localization

Cellular functions are dictated by protein content and activity. There are numerous strategies to regulate proteins varying from modulating gene expression to post-translational modifications. One commonly used mode of regulation in eukaryotes is targeted localization. By specifically redirecting the localization of a pool of existing protein, cells can achieve rapid changes in local protein function. Eukaryotic cells have evolved elegant targeting pathways to direct proteins to the appropriate cellular location or locations. Here, we provide a general overview of these localization pathways, with a focus on nuclear and mitochondrial transport, and present a survey of the evolutionarily conserved regulatory strategies identified thus far. We end with a description of several specific examples of proteins that exploit localization as an important mode of regulation.

The Interaction of FABP with Kapα

Gene-activating lipophilic compounds are carried into the nucleus when loaded on fatty-acid-binding proteins (FABP). Some of these proteins are recognized by the α-Karyopherin (Kapα) through its nuclear localization signal (NLS) consisting of three positive residues that are not in a continuous sequence. The Importin system can distinguish between FABP loaded with activating and non-activating compounds. In the present study, we introduced molecular dynamics as a tool for clarifying the mechanism by which FABP4, loaded with activating ligand (linoleate) is recognized by Kapα. In the first phase, we simulated the complex between Kapα^ΔIBB (termed “Armadillo”) that was crystallized with two NLS hepta-peptides. The trajectory revealed that the crystal-structure orientation of the peptides is rapidly lost and new interactions dominate. Though, the NLS sequence of FABP4 is cryptic, since the functional residues are not in direct sequence, implicating more than one possible conformation. Therefore, four possible docked conformations were generated, in which the NLS of FABP4 is interacting with either the major or the minor sites of Kapα, and the N → C vectors are parallel or anti-parallel. Out of these four basic starting positions, only the FABP4-minor site complex exhibited a large number of contact points. In this complex, the FABP interacts with the minor and the major sites, suppressing the self-inhibitory interaction of the Kapα, rendering it free to react with Kapβ. Finally, we propose that the transportable conformation generated an extended hydrophobic domain which expanded out of the boundary of the FABP4, allowing the loaded linoleate to partially migrate out of the FABP into a joint complex in which the Kapα contributes part of a combined binding pocket.

Multiple exportins influence thyroid hormone receptor localization

The thyroid hormone receptor (TR) undergoes nucleocytoplasmic shuttling and regulates target genes involved in metabolism and development. Previously, we showed that TR follows a CRM1/calreticulin-mediated nuclear export pathway. However, two lines of evidence suggest TR also follows another pathway: export is only partially blocked by leptomycin B (LMB), a CRM1-specific inhibitor; and we identified nuclear export signals in TR that are LMB-resistant. To determine whether other exportins are involved in TR shuttling, we used RNA interference and fluorescence recovery after photobleaching shuttling assays in transfected cells. Knockdown of exportins 4, 5, and 7 altered TR shuttling dynamics, and when exportins 5 and 7 were overexpressed, TR distribution shifted toward the cytosol. To further assess the effects of exportin overexpression, we examined transactivation of a TR-responsive reporter gene. Our data indicate that multiple exportins influence TR localization, highlighting a fine balance of nuclear import, retention, and export that modulates TR function.

Nucleocytoplasmic transport in the midzone membrane domain controls yeast mitotic spindle disassembly

During anaphase B, Imp1-mediated transport of the AAA-ATPase Cdc48 protein at the membrane domain surrounding the mitotic spindle midzone promotes spindle midzone dissolution in fission yeast.

During each cell cycle, the mitotic spindle is efficiently assembled to achieve chromosome segregation and then rapidly disassembled as cells enter cytokinesis. Although much has been learned about assembly, how spindles disassemble at the end of mitosis remains unclear. Here we demonstrate that nucleocytoplasmic transport at the membrane domain surrounding the mitotic spindle midzone, here named the midzone membrane domain (MMD), is essential for spindle disassembly in Schizosaccharomyces pombe cells. We show that, during anaphase B, Imp1-mediated transport of the AAA-ATPase Cdc48 protein at the MMD allows this disassembly factor to localize at the spindle midzone, thereby promoting spindle midzone dissolution. Our findings illustrate how a separate membrane compartment supports spindle disassembly in the closed mitosis of fission yeast.

Reprint of "Nuclear transport factors: global regulation of mitosis"

The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear import receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator – the γ-TuRC complex – and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores toward the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.

Conservation of inner nuclear membrane targeting sequences in mammalian Pom121 and yeast Heh2 membrane proteins

This study examines whether active transport to the inner nuclear membrane, as shown for yeast membrane proteins Heh1 and Heh2, is conserved in metazoans. In support of this, the nuclear localization signal of metazoan Pom121 shares biochemical, structural, and functional properties with those of Heh1 and Heh2, and a Heh2-derived reporter protein targets to the inner membrane in Hek293T cells.

Endoplasmic reticulum–synthesized membrane proteins traffic through the nuclear pore complex (NPC) en route to the inner nuclear membrane (INM). Although many membrane proteins pass the NPC by simple diffusion, two yeast proteins, ScSrc1/ScHeh1 and ScHeh2, are actively imported. In these proteins, a nuclear localization signal (NLS) and an intrinsically disordered linker encode the sorting signal for recruiting the transport factors for FG-Nup and RanGTP-dependent transport through the NPC. Here we address whether a similar import mechanism applies in metazoans. We show that the (putative) NLSs of metazoan HsSun2, MmLem2, HsLBR, and HsLap2β are not sufficient to drive nuclear accumulation of a membrane protein in yeast, but the NLS from RnPom121 is. This NLS of Pom121 adapts a similar fold as the NLS of Heh2 when transport factor bound and rescues the subcellular localization and synthetic sickness of Heh2ΔNLS mutants. Consistent with the conservation of these NLSs, the NLS and linker of Heh2 support INM localization in HEK293T cells. The conserved features of the NLSs of ScHeh1, ScHeh2, and RnPom121 and the effective sorting of Heh2-derived reporters in human cells suggest that active import is conserved but confined to a small subset of INM proteins.

Transcriptional control of autophagy-lysosome function drives pancreatic cancer metabolism

Activation of cellular stress response pathways to maintain metabolic homeostasis is emerging as a critical growth and survival mechanism in many cancers. The pathogenesis of pancreatic ductal adenocarcinoma (PDA) requires high levels of autophagy, a conserved self-degradative process. However, the regulatory circuits that activate autophagy and reprogram PDA cell metabolism are unknown. Here we show that autophagy induction in PDA occurs as part of a broader transcriptional program that coordinates activation of lysosome biogenesis and function, and nutrient scavenging, mediated by the MiT/TFE family of transcription factors. In human PDA cells, the MiT/TFE proteins--MITF, TFE3 and TFEB--are decoupled from regulatory mechanisms that control their cytoplasmic retention. Increased nuclear import in turn drives the expression of a coherent network of genes that induce high levels of lysosomal catabolic function essential for PDA growth. Unbiased global metabolite profiling reveals that MiT/TFE-dependent autophagy-lysosome activation is specifically required to maintain intracellular amino acid pools. These results identify the MiT/TFE proteins as master regulators of metabolic reprogramming in pancreatic cancer and demonstrate that transcriptional activation of clearance pathways converging on the lysosome is a novel hallmark of aggressive malignancy.

Proteolytic cleavage, trafficking, and functions of nuclear receptor tyrosine kinases

Intracellular localization has been reported for over three-quarters of receptor tyrosine kinase (RTK) families in response to environmental stimuli. Internalized RTK may bind to non-canonical substrates and affect various cellular processes. Many of the intracellular RTKs exist as fragmented forms that are generated by γ-secretase cleavage of the full-length receptor, shedding, alternative splicing, or alternative translation initiation. Soluble RTK fragments are stabilized and intracellularly transported into subcellular compartments, such as the nucleus, by binding to chaperone or transcription factors, while membrane-bound RTKs (full-length or truncated) are transported from the plasma membrane to the ER through the well-established Rab- or clathrin adaptor protein-coated vesicle retrograde trafficking pathways. Subsequent nuclear transport of membrane-bound RTK may occur via two pathways, INFS or INTERNET, with the former characterized by release of receptors from the ER into the cytosol and the latter characterized by release of membrane-bound receptor from the ER into the nucleoplasm through the inner nuclear membrane. Although most non-canonical intracellular RTK signaling is related to transcriptional regulation, there may be other functions that have yet to be discovered. In this review, we summarize the proteolytic processing, intracellular trafficking and nuclear functions of RTKs, and discuss how they promote cancer progression, and their clinical implications.

Hepatitis C virus and host cell nuclear transport machinery: a clandestine affair

There is growing evidence that factors encoded by cytoplasmic replicating viruses functionally interact with components of the nucleocytoplasmic transport apparatus. They do so either to access the cell nucleus, thus affecting genes expression, or to interfere with nuclear transport functionality, hindering host immune response. Recent studies revealed that the hepatitis C virus (HCV) makes no exception, interacting with the host cell nuclear transport machinery at two different levels. On the one hand, small amounts of both core and NS5A localize within the host cell nucleus during productive infection, modulating gene expression and signaling functions to promote persistent infection. On the other hand, HCV infection causes a profound redistribution of certain nucleoproteins to the close proximity of endoplasmic reticulum membrane-derived viral replication factories, where viral RNA amplification occurs. These nucleoporins are believed to form nuclear pore complex-like structures, as suggested by their ability to recruit nuclear localization sequence-bearing proteins. Thus, both processes are linked to virus-induced persistence and pathogenesis, representing possible targets for the development of novel anti-HCV therapeutics.

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.

IPO3-mediated Nonclassical Nuclear Import of NF-κB Essential Modulator (NEMO) Drives DNA Damage-dependent NF-κB Activation

Activation of IκB kinase (IKK) and NF-κB by genotoxic stresses modulates apoptotic responses and production of inflammatory mediators, thereby contributing to therapy resistance and premature aging. We previously reported that genotoxic agents induce nuclear localization of NF-κB essential modulator (NEMO) via an undefined mechanism to arbitrate subsequent DNA damage-dependent IKK/NF-κB signaling. Here we show that a nonclassical nuclear import pathway via IPO3 (importin 3, transportin 2) mediates stress-induced NEMO nuclear translocation. We found putative nuclear localization signals in NEMO whose mutations disrupted stress-inducible nuclear translocation of NEMO and IKK/NF-κB activation in stably reconstituted NEMO-deficient cells. RNAi screening of both importin α and β family members, as well as co-immunoprecipitation analyses, revealed that a nonclassical importin β family member, IPO3, was the only importin that was able to associate with NEMO and whose reduced expression prevented genotoxic stress-induced NEMO nuclear translocation, IKK/NF-κB activation, and inflammatory cytokine transcription. Recombinant IPO3 interacted with recombinant NEMO but not the nuclear localization signal mutant version and induced nuclear import of NEMO in digitonin-permeabilized cells. We also provide evidence that NEMO is disengaged from IKK complex following genotoxic stress induction. Thus, the IPO3 nuclear import pathway is an early and crucial determinant of the IKK/NF-κB signaling arm of the mammalian DNA damage response.

Structural Characterization of the Chaetomium thermophilum TREX-2 Complex and its Interaction with the mRNA Nuclear Export Factor Mex67:Mtr2

The TREX-2 complex integrates mRNA nuclear export into the gene expression pathway and is based on a Sac3 scaffold to which Thp1, Sem1, Sus1, and Cdc31 bind. TREX-2 also binds the mRNA nuclear export factor, Mex67:Mtr2, through the Sac3 N-terminal region (Sac3N). Here, we characterize Chaetomium thermophilum TREX-2, show that the in vitro reconstituted complex has an annular structure, and define the structural basis for interactions between Sac3, Sus1, Cdc31, and Mex67:Mtr2. Crystal structures show that the binding of C. thermophilum Sac3N to the Mex67 NTF2-like domain (Mex67^NTF2L) is mediated primarily through phenylalanine residues present in a series of repeating sequence motifs that resemble those seen in many nucleoporins, and Mlp1 also binds Mex67:Mtr2 using a similar motif. Deletion of Sac3N generated growth and mRNA export defects in Saccharomyces cerevisiae, and we propose TREX-2 and Mlp1 function to facilitate export by concentrating mature messenger ribonucleoparticles at the nuclear pore entrance.

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Chaetomium thermophilum TREX-2 has an annular structure resembling the letter Q

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Structure of interfaces between TREX-2 components Sac3, Sus1, and Cdc31 defined

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Sac3N phenylalanines dominate C. thermophilum TREX-2 binding to Mex67 NTF2L domain

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TREX-2 facilitates mRNA export by concentrating mature mRNPs at nuclear pores

Crystal structure of the karyopherin Kap121p bound to the extreme C-terminus of the protein phosphatase Cdc14p

In Saccharomyces cerevisiae, the protein phosphatase Cdc14p is an antagonist of mitotic cyclin-dependent kinases and is a key regulator of late mitotic events such as chromosome segregation, spindle disassembly and cytokinesis. The activity of Cdc14p is controlled by cell-cycle dependent changes in its association with its competitive inhibitor Net1p (also known as Cfi1p) in the nucleolus. For most of the cell cycle up to metaphase, Cdc14p is sequestered in the nucleolus in an inactive state. During anaphase, Cdc14p is released from Net1p, spreads into the nucleus and cytoplasm, and dephosphorylates key mitotic targets. Although regulated nucleocytoplasmic shuttling of Cdc14p has been suggested to be important for exit from mitosis, the mechanism underlying Cdc14p nuclear trafficking remains poorly understood. Here we show that the C-terminal region (residues 517-551) of Cdc14p can function as a nuclear localization signal (NLS) in vivo and also binds to Kap121p (also known as Pse1p), an essential nuclear import carrier in yeast, in a Gsp1p-GTP-dependent manner in vitro. Moreover we report a crystal structure, at 2.4 Å resolution, of Kap121p bound to the C-terminal region of Cdc14p. The structure and structure-based mutational analyses suggest that either the last five residues at the extreme C-terminus of Cdc14p (residues 547-551; Gly-Ser-Ile-Lys-Lys) or adjacent residues with similar sequence (residues 540-544; Gly-Gly-Ile-Arg-Lys) can bind to the NLS-binding site of Kap121p, with two residues (Ile in the middle and Lys at the end of the five residues) of Cdc14p making key contributions to the binding specificity. Based on comparison with other structures of Kap121p-ligand complexes, we propose "IK-NLS" as an appropriate term to refer to the Kap121p-specific NLS.

Identification and characterization of nuclear and nucleolar localization signals in 58-kDa microspherule protein (MSP58)

BACKGROUND

MSP58 is a nucleolar protein associated with rRNA transcription and cell proliferation. Its mechanism of translocation into the nucleus or the nucleolus, however, is not entirely known. In order to address this lack, the present study aims to determine a crucial part of this mechanism: the nuclear localization signal (NLS) and the nucleolar localization signal (NoLS) associated with the MSP58 protein.

RESULTS

We have identified and characterized two NLSs in MSP58. The first is located between residues 32 and 56 (NLS1) and constitutes three clusters of basic amino acids (KRASSQALGTIPKRRSSSRFIKRKK); the second is situated between residues 113 and 123 (NLS2) and harbors a monopartite signal (PGLTKRVKKSK). Both NLS1 and NLS2 are highly conserved among different vertebrate species. Notably, one bipartite motif within the NLS1 (residues 44-56) appears to be absolutely necessary for MSP58 nucleolar localization. By yeast two-hybrid, pull-down, and coimmunoprecipitation analysis, we show that MSP58 binds to importin α1 and α6, suggesting that nuclear targeting of MSP58 utilizes a receptor-mediated and energy-dependent import mechanism. Functionally, our data show that both nuclear and nucleolar localization of MSP58 are crucial for transcriptional regulation on p21 and ribosomal RNA genes, and context-dependent effects on cell proliferation.

CONCLUSIONS

Results suggest that MSP58 subnuclear localization is regulated by two nuclear import signals, and that proper subcellular localization of MSP58 is critical for its role in transcriptional regulation. Our study reveals a molecular mechanism that controls nuclear and nucleolar localization of MSP58, a finding that might help future researchers understand the MSP58 biological signaling pathway.

Nuclear transport factors: global regulation of mitosis

The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear import receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator - the γ-TuRC complex - and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores toward the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.

Nuclear entry of DNA viruses

DNA viruses undertake their replication within the cell nucleus, and therefore they must first deliver their genome into the nucleus of their host cells. Thus, trafficking across the nuclear envelope is at the basis of DNA virus infections. Nuclear transport of molecules with diameters up to 39 nm is a tightly regulated process that occurs through the nuclear pore complex (NPC). Due to the enormous diversity of virus size and structure, each virus has developed its own strategy for entering the nucleus of their host cells, with no two strategies alike. For example, baculoviruses target their DNA-containing capsid to the NPC and subsequently enter the nucleus intact, while the hepatitis B virus capsid crosses the NPC but disassembles at the nuclear side of the NPC. For other viruses such as herpes simplex virus and adenovirus, although both dock at the NPC, they have each developed a distinct mechanism for the subsequent delivery of their genome into the nucleus. Remarkably, other DNA viruses, such as parvoviruses and human papillomaviruses, access the nucleus through an NPC-independent mechanism. This review discusses our current understanding of the mechanisms used by DNA viruses to deliver their genome into the nucleus, and further presents the experimental evidence for such mechanisms.

Hyper-dependence of breast cancer cell types on the nuclear transporter Importin β1

We previously reported that overexpression of members of the Importin (Imp) superfamily of nuclear transporters results in increased nuclear trafficking through conventional transport pathways in tumour cells. Here we show for the first time that the extent of overexpression of Impβ1 correlates with disease state in the MCF10 human breast tumour progression system. Excitingly, we find that targeting Impβ1 activity through siRNA is >30 times more efficient in decreasing the viability of malignant ductal carcinoma cells compared to isogenic non-transformed counterparts, and is highly potent and tumour selective at subnanomolar concentrations. Tumour cell selectivity of the siRNA effects was unique to Impβ1 and not other Imps, with flow cytometric analysis showing >60% increased cell death compared to controls concomitant with reduced nuclear import efficiency as indicated by confocal microscopic analysis. This hypersensitivity of malignant cell types to Impβ1 knockdown raises the exciting possibility of anti-cancer therapies targeted at Impβ1.

Importin β2 Mediates the Spatio-temporal Regulation of Anillin through a Noncanonical Nuclear Localization Signal

The compartmentalization of cell cycle regulators is a common mechanism to ensure the precise temporal control of key cell cycle events. For instance, many mitotic spindle assembly factors are known to be sequestered in the nucleus prior to mitotic onset. Similarly, the essential cytokinetic factor anillin, which functions at the cell membrane to promote the physical separation of daughter cells at the end of mitosis, is sequestered in the nucleus during interphase. To address the mechanism and role of anillin targeting to the nucleus in interphase, we identified the nuclear targeting motif. Here, we show that anillin is targeted to the nucleus by importin β2 in a Ran-dependent manner through an atypical basic patch PY nuclear localization signal motif. We show that although importin β2 binding does not regulate anillin's function in mitosis, it is required to prevent the cytosolic accumulation of anillin, which disrupts cellular architecture during interphase. The nuclear sequestration of anillin during interphase serves to restrict anillin's function at the cell membrane to mitosis and allows anillin to be rapidly available when the nuclear envelope breaks down to remodel the cellular architecture necessary for successful cell division.

Nuclear-cytoplasmatic shuttling of proteins in control of cellular oxygen sensing

In order to pass through the nuclear pore complex, proteins larger than ∼40 kDa require specific nuclear transport receptors. Defects in nuclear-cytoplasmatic transport affect fundamental processes such as development, inflammation and oxygen sensing. The transcriptional response to O2 deficiency is controlled by hypoxia-inducible factors (HIFs). These are heterodimeric transcription factors of each ∼100-120 kDa proteins, consisting of one out of three different O2-labile α subunits (primarily HIF-1α) and a more constitutive 1β subunit. In the presence of O2, the α subunits are hydroxylated by specific prolyl-4-hydroxylase domain proteins (PHD1, PHD2, and PHD3) and an asparaginyl hydroxylase (factor inhibiting HIF-1, FIH-1). The prolyl hydroxylation causes recognition by von Hippel-Lindau tumor suppressor protein (pVHL), ubiquitination, and proteasomal degradation. The activity of the oxygen sensing machinery depends on dynamic intracellular trafficking. Nuclear import of HIF-1α and HIF-1β is mainly mediated by importins α and β (α/β). HIF-1α can shuttle between nucleus and cytoplasm, while HIF-1β is permanently inside the nucleus. pVHL is localized to both compartments. Nuclear import of PHD1 relies on a nuclear localization signal (NLS) and uses the classical import pathway involving importin α/β receptors. PHD2 shows an atypical NLS, and its nuclear import does not occur via the classical pathway. PHD2-mediated hydroxylation of HIF-1α occurs predominantly in the cell nucleus. Nuclear export of PHD2 involves a nuclear export signal (NES) in the N-terminus and depends on the export receptor chromosome region maintenance 1 (CRM1). Nuclear import of PHD3 is mediated by importin α/β receptors and depends on a non-classical NLS. Specific modification of the nuclear translocation of the three PHD isoforms could provide a promising strategy for the development of new therapeutic substances to tackle major diseases.

Drosophila TIM binds importin α1, and acts as an adapter to transport PER to the nucleus

Regulated nuclear entry of clock proteins is a conserved feature of eukaryotic circadian clocks and serves to separate the phase of mRNA activation from mRNA repression in the molecular feedback loop. In Drosophila, nuclear entry of the clock proteins, PERIOD (PER) and TIMELESS (TIM), is tightly controlled, and impairments of this process produce profound behavioral phenotypes. We report here that nuclear entry of PER-TIM in clock cells, and consequently behavioral rhythms, require a specific member of a classic nuclear import pathway, Importin α1 (IMPα1). In addition to IMPα1, rhythmic behavior and nuclear expression of PER-TIM require a specific nuclear pore protein, Nup153, and Ran-GTPase. IMPα1 can also drive rapid and efficient nuclear expression of TIM and PER in cultured cells, although the effect on PER is mediated by TIM. Mapping of interaction domains between IMPα1 and TIM/PER suggests that TIM is the primary cargo for the importin machinery. This is supported by attenuated interaction of IMPα1 with TIM carrying a mutation previously shown to prevent nuclear entry of TIM and PER. TIM is detected at the nuclear envelope, and computational modeling suggests that it contains HEAT-ARM repeats typically found in karyopherins, consistent with its role as a co-transporter for PER. These findings suggest that although PER is the major timekeeper of the clock, TIM is the primary target of nuclear import mechanisms. Thus, the circadian clock uses specific components of the importin pathway with a novel twist in that TIM serves a karyopherin-like role for PER.

In Drosophila, circadian rhythms are driven by a negative feedback loop that includes the key regulators, period (per) and timeless (tim). To generate this feedback loop, PER and TIM proteins first accumulate in the cytoplasm and then translocate to the nucleus where PER represses transcription. Thus, the nuclear import of PER-TIM proteins is a critical step to separate the phases of activation and repression of mRNA synthesis. In this study, we discovered that a member of the nuclear import machinery, importin α1 is an essential component of this feedback loop. Flies lacking importin α1 (IMPα1) display arrhythmic behavior and cytoplasmic expression of both PER and TIM at all times. In cultured S2 cells, IMPα1 expression directly facilitates nuclear import of TIM, but the effect on PER appears to be indirect. TIM expression is detected at the nuclear envelope and it interacts with other components of the nuclear transport machinery, which we show are also required for nuclear expression of TIM-PER and for behavioral rhythms. Our results thus suggest that TIM functions to link PER to the nuclear import machinery through IMPα1. Altogether, this study provides the mechanistic basis of a crucial step in the circadian clock mechanism.

Identification of proteins associated with Aha1 in HeLa cells by quantitative proteomics

The identification of the activator of heat shock protein 90 (Hsp90) ATPase's (Aha1) protein-protein interaction (PPI) network will provide critical insights into the relationship of Aha1 with multi-molecular complexes and shed light onto Aha1's interconnections with Hsp90-regulated biological functions. Flag-tagged Aha1 was over-expressed in HeLa cells and isolated by anti-Flag affinity pull downs, followed by trypsin digestion and identification co-adsorbing proteins by liquid chromatography-tandem mass spectroscopy (LC-MS/MS). A probability-based identification of Aha1 PPIs was generated from the LC-MS/MS analysis by using a relative quantification strategy, spectral counting (SC). By comparing the SC-based protein levels between Aha1 pull-down samples and negative controls, 164 Aha1-interacting proteins were identified that were quantitatively enriched in the pull-down samples over the controls. The identified Aha1-interacting proteins are involved in a wide number of intracellular bioprocesses, including DNA maintenance, chromatin structure, RNA processing, translation, nucleocytoplasmic and vesicle transport, among others. The interactions of 33 of the identified proteins with Aha1 were further confirmed by Western blotting, demonstrating the reliability of our affinity-purification-coupled quantitative SC-MS strategy. Our proteomic data suggests that Aha1 may participate in diverse biological pathways to facilitate Hsp90 chaperone functions in response to stress.

Identification and characterization of nuclear and nucleolar localization signals in the adeno-associated virus serotype 2 assembly-activating protein

Assembly-activating protein (AAP) of adeno-associated virus serotype 2 (AAV2) is a nucleolar-localizing protein that plays a critical role in transporting the viral capsid VP3 protein to the nucleolus for assembly. Here, we identify and characterize AAV2 AAP (AAP2) nuclear (NLS) and nucleolar (NoLS) localization signals near the carboxy-terminal region of AAP2 (amino acid positions 144 to 184) (AAP2(144-184)). This region contains five basic-amino-acid-rich (BR) clusters, KSKRSRR (AAP2BR1), RRR (AAP2BR2), RFR (AAP2BR3), RSTSSR (AAP2BR4), and RRIK (AAP2BR5), from the amino terminus to the carboxy terminus. We created 30 AAP2BR mutants by arginine/lysine-to-alanine mutagenesis or deletion of AAP2BRs and 8 and 1 green fluorescent protein (GFP)-AAP2BR and β-galactosidase-AAP2BR fusion proteins, respectively, and analyzed their intracellular localization in HeLa cells by immunofluorescence microscopy. The results showed that AAP2(144-184) has redundant multipartite NLSs and that any combinations of 4 AAP2BRs, but not 3 or less, can constitute a functional NLS-NoLS; AAP2BR1 and AAP2BR2 play the most influential role for nuclear localization, but either one of the two AAP2BRs is dispensable if all 4 of the other AAP2BRs are present, resulting in 3 different, overlapping NLS motifs; and the NoLS is shared redundantly among the five AAP2BRs and functions in a context-dependent manner. AAP2BR mutations not only resulted in aberrant intracellular localization, but also attenuated AAP2 protein expression to various degrees, and both of these abnormalities have a significant negative impact on capsid production. Thus, this study reveals the organization of the intermingling NLSs and NoLSs in AAP2 and provides insights into their functional roles in capsid assembly.

IMPORTANCE

Adeno-associated virus (AAV) has become a popular and successful vector for in vivo gene therapy; however, its biology has yet to be fully understood. In this regard, the recent discovery of the assembly-activating protein (AAP), a nonstructural, nucleolar-localizing AAV protein essential for viral capsid assembly, has provided us a new opportunity to better understand the fundamental processes required for virion formation. Here, we identify clusters of basic amino acids in the carboxy terminus of AAP from AAV serotype 2 (AAV2) that act as nuclear and nucleolar localization signals. We also demonstrate their importance in maintaining AAP expression levels and efficient production of viral capsids. Insights into the functions of AAP can elucidate the requirements and process for AAV capsid assembly, which may lead to improved vector production for use in gene therapy. This study also contributes to the growing body of work on nuclear and nucleolar localization signals.

Components and regulation of nuclear transport processes

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

Karyopherin-centric control of nuclear pores based on molecular occupancy and kinetic analysis of multivalent binding with FG nucleoporins

Intrinsically disordered Phe-Gly nucleoporins (FG Nups) within nuclear pore complexes exert multivalent interactions with transport receptors (Karyopherins (Kaps)) that orchestrate nucleocytoplasmic transport. Current FG-centric views reason that selective Kap translocation is promoted by alterations in the barrier-like FG Nup conformations. However, the strong binding of Kaps with the FG Nups due to avidity contradicts rapid Kap translocation in vivo. Here, using surface plasmon resonance, we innovate a means to correlate in situ mechanistic (molecular occupancy and conformational changes) with equilibrium (binding affinity) and kinetic (multivalent binding kinetics) aspects of Karyopherinβ1 (Kapβ1) binding to four different FG Nups. A general feature of the FxFG domains of Nup214, Nup62, and Nup153 is their capacity to extend and accommodate large numbers of Kapβ1 molecules at physiological Kapβ1 concentrations. A notable exception is the GLFG domain of Nup98, which forms a partially penetrable cohesive layer. Interestingly, we find that a slowly exchanging Kapβ1 phase forms an integral constituent within the FG Nups that coexists with a fast phase, which dominates transport kinetics due to limited binding with the pre-occupied FG Nups at physiological Kapβ1 concentrations. Altogether, our data reveal an emergent Kap-centric barrier mechanism that may underlie mechanistic and kinetic control in the nuclear pore complex.

A backdoor to the nucleus that runs in the family?

Yoshimura and colleagues show that HEAT proteins that are involved in diverse cellular functions may facilitate their own translocation through the nuclear pore complex, owing to their structural similarity to nuclear transport receptors of the karyopherin β family.

Nuclear import of adenovirus DNA involves direct interaction of hexon with an N-terminal domain of the nucleoporin Nup214

In this study, we characterized the molecular basis for binding of adenovirus (AdV) to the cytoplasmic face of the nuclear pore complex (NPC), a key step during delivery of the viral genome into the nucleus. We used RNA interference (RNAi) to deplete cells of either Nup214 or Nup358, the two major Phe-Gly (FG) repeat nucleoporins localized on the cytoplasmic side of the NPC, and evaluated the impact on hexon binding and AdV infection. The accumulation of purified hexon trimers or partially disassembled AdV at the nuclear envelope (NE) was observed in digitonin-permeabilized cells in the absence of cytosolic factors. Both in vitro hexon binding and in vivo nuclear import of the AdV genome were strongly reduced in Nup214-depleted cells but still occurred in Nup358-depleted cells, suggesting that Nup214 is a major binding site of AdV during infection. The expression of an NPC-targeted N-terminal domain of Nup214 in Nup214-depleted cells restored the binding of hexon at the NE and the nuclear import of protein VII (pVII), indicating that this region is sufficient to allow AdV binding. We further narrowed the binding site to a 137-amino-acid segment in the N-terminal domain of Nup214. Together, our results have identified a specific region within the N terminus of Nup214 that acts as a direct NPC binding site for AdV.

IMPORTANCE

AdVs, which have the largest genome of nonenveloped DNA viruses, are being extensively explored for use in gene therapy, especially in alternative treatments for cancers that are refractory to traditional therapies. In this study, we characterized the molecular basis for binding of AdV to the cytoplasmic face of the NPC, a key step for delivery of the viral genome into the nucleus. Our data indicate that a 137-amino-acid region of the nucleoporin Nup214 is a binding site for the major AdV capsid protein, hexon, and that this interaction is required for viral DNA import. These findings provide additional insight on how AdV exploits the nuclear transport machinery for infection. The results could promote the development of new strategies for gene transfer and enhance understanding of the nuclear import of other viral DNA genomes, such as those of papillomavirus or hepatitis B virus that induce specific cancers.

Characterization of the nuclear import mechanism of the CCAAT-regulatory subunit Php4

Php4 is a nucleo-cytoplasmic shuttling protein that accumulates in the nucleus during iron deficiency. When present in the nucleus, Php4 associates with the CCAAT-binding protein complex and represses genes encoding iron-using proteins. Here, we show that nuclear import of Php4 is independent of the other subunits of the CCAAT-binding complex. Php4 nuclear import relies on two functionally independent nuclear localization sequences (NLSs) that are located between amino acid residues 171 to 174 (KRIR) and 234 to 240 (KSVKRVR). Specific substitutions of basic amino acid residues to alanines within these sequences are sufficient to abrogate nuclear targeting of Php4. The two NLSs are biologically redundant and are sufficient to target a heterologous reporter protein to the nucleus. Under low-iron conditions, a functional GFP-Php4 protein is only partly targeted to the nucleus in imp1Δ and sal3Δ mutant cells. We further found that cells expressing a temperature-sensitive mutation in cut15 exhibit increased cytosolic accumulation of Php4 at the nonpermissive temperature. Further analysis by pull-down experiments revealed that Php4 is a cargo of the karyopherins Imp1, Cut15 and Sal3. Collectively, these results indicate that Php4 can be bound by distinct karyopherins, connecting it into more than one nuclear import pathway.

Nuclear localization of MBNL1: splicing-mediated autoregulation and repression of repeat-derived aberrant proteins

In some neurological diseases caused by repeat expansions such as myotonic dystrophy, the RNA-binding protein muscleblind-like 1 (MBNL1) accumulates in intranuclear inclusions containing mutant repeat RNA. The interaction between MBNL1 and mutant RNA in the nucleus is a key event leading to loss of MBNL function, yet the details of this effect have been elusive. Here, we investigated the mechanism and significance of MBNL1 nuclear localization. We found that MBNL1 contains two classes of nuclear localization signal (NLS), a classical bipartite NLS and a novel conformational NLS. Alternative splicing of exon 7 acts as a switch between these NLS types and couples MBNL1 activity and intracellular localization. Depending on its nuclear localization, MBNL1 promoted nuclear accumulation of mutant RNA containing a CUG or CAG repeat, some of which produced proteins containing homopolymeric tracts such as polyglutamine. Furthermore, MBNL1 repressed the expression of these homopolymeric proteins including those presumably produced through repeat-associated non-ATG (RAN) translation. These results suggest that nuclear retention of expanded RNA reflects a novel role of MBNL proteins in repressing aberrant protein expression and may provide pathological and therapeutic implications for a wide range of repeat expansion diseases associated with nuclear RNA retention and/or RAN translation.

Restricted expression of karyopherin alpha mRNA in the sea urchin suggests a role in neurogenesis

Karyopherin alpha (KAP-α) proteins are critical for the transport of many molecules into the nucleus. In this study, we identified three members of the KAP-α family in the sea urchin Lytechinus variegatus and described the developmental expression of these proteins. Although many importins are assumed to have ubiquitous expression, we found that all three genes were differentially expressed. Both LvKPNA1/5/6 and LvKPNA3/4 accumulated at high levels during cleavage, exhibiting cyclic expression as cells divided. By the blastula and gastrula stages expression decreased, remaining highest in the vegetal plate and archenteron, and by the prism/pluteus stages expression was restricted to the oral surface and gut. Expression of a third KAP-α gene, LvKPNA2/7, was examined in embryos from the mesenchyme blastula to pluteus stages. LvKPNA2/7 mRNA is present in vegetal cells of the mesenchyme blastula and, during gastrulation, it is localized to the archenteron and appears in additional groups of ectodermal cells. Prism/pluteus stage embryos expressed LvKPNA2/7 in the gut and scattered distribution of transcripts in the ciliary band resembled expression patterns of neural cells. We hypothesize that LvKPNA2/7 maintains pluripotency in the neural precursors prior to activation of neural differentiation and believe that this study is an important first step in an effort to better understand the roles of importins during embryogenesis.

Prosurvival function of the cellular apoptosis susceptibility/importin-α1 transport cycle is repressed by p53 in liver cancer

Proteins of the karyopherin superfamily including importins and exportins represent an essential part of the nucleocytoplasmic transport machinery. However, the functional relevance and regulation of karyopherins in hepatocellular carcinoma (HCC) is poorly understood. Here we identified cellular apoptosis susceptibility (CAS, exportin-2) and its transport substrate importin-α1 (imp-α1) among significantly up-regulated transport factor genes in HCC. Disruption of the CAS/imp-α1 transport cycle by RNAi in HCC cell lines resulted in decreased tumor cell growth and increased apoptosis. The apoptotic phenotype upon CAS depletion could be recapitulated by direct knockdown of the X-linked inhibitor of apoptosis (XIAP) and partially reverted by XIAP overexpression. In addition, XIAP and CAS mRNA expression levels were correlated in HCC patient samples (r=0.463; P<0.01), supporting the in vivo relevance of our findings. Furthermore, quantitative mass spectrometry analyses of murine HCC samples (p53-/- versus p53+/+) indicated higher protein expression of CAS and imp-α1 in p53-/- tumors. Consistent with a role of p53 in regulating the CAS/imp-α1 transport cycle, we observed that both transport factors were repressed upon p53 induction in a p21-dependent manner.

CONCLUSION

The CAS/imp-α1 transport cycle is linked to XIAP and is required to maintain tumor cell survival in HCC. Moreover, CAS and imp-α1 are targets of p53-mediated repression, which represents a novel aspect of p53's ability to control tumor cell growth in hepatocarcinogenesis.

Atomic basis of CRM1-cargo recognition, release and inhibition

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

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

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

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

A code for RanGDP binding in ankyrin repeats defines a nuclear import pathway

Regulation of nuclear import is fundamental to eukaryotic biology. The majority of nuclear import pathways are mediated by importin-cargo interactions. Yet not all nuclear proteins interact with importins, necessitating the identification of a general importin-independent nuclear import pathway. Here, we identify a code that determines importin-independent nuclear import of ankyrin repeats (ARs), a structural motif found in over 250 human proteins with diverse functions. AR-containing proteins (ARPs) with a hydrophobic residue at the 13th position of two consecutive ARs bind RanGDP efficiently, and consequently enter the nucleus. This code, experimentally tested in 17 ARPs, predicts the nuclear-cytoplasmic localization of over 150 annotated human ARPs with high accuracy and is acquired by the most common familial melanoma-associated CDKN2A mutation, leading to nuclear accumulation of mutant p16ink4a. The RaDAR (RanGDP/AR) pathway represents a general importin-independent nuclear import pathway and is frequently used by AR-containing transcriptional regulators, especially those regulating NF-κB/p53.

Selective transport control on molecular velcro made from intrinsically disordered proteins

The selectivity and speed of many biological transport processes transpire from a 'reduction of dimensionality' that confines diffusion to one or two dimensions instead of three. This behaviour remains highly sought after on polymeric surfaces as a means to expedite diffusional search processes in molecular engineered systems. Here, we have reconstituted the two-dimensional diffusion of colloidal particles on a molecular brush surface. The surface is composed of phenylalanine-glycine nucleoporins (FG Nups)--intrinsically disordered proteins that facilitate selective transport through nuclear pore complexes in eukaryotic cells. Local and ensemble-level experiments involving optical trapping using a photonic force microscope and particle tracking by video microscopy, respectively, reveal that 1-µm-sized colloidal particles bearing nuclear transport receptors called karyopherins can exhibit behaviour that varies from highly localized to unhindered two-dimensional diffusion. Particle diffusivity is controlled by varying the amount of free karyopherins in solution, which modulates the multivalency of Kap-binding sites within the molecular brush. We conclude that the FG Nups resemble stimuli-responsive molecular 'velcro', which can impart 'reduction of dimensionality' as a means of biomimetic transport control in artificial environments.

Structure of transportin SR2, a karyopherin involved in human disease, in complex with Ran

Transportin SR2 (TRN-SR2) is a β-type karyopherin responsible for the nuclear import of specific cargoes, including serine/arginine-rich splicing factors. The protein has been implicated in a variety of human diseases, including HIV infection, primary biliary cirrhosis and limb-girdle muscular dystrophy 1F. Towards understanding its molecular mechanism, a 2.9 Å resolution crystal structure of human TRN-SR2 complexed with the small GTPase Ran has been determined. TRN-SR2 is composed of 20 α-helical HEAT repeats forming a solenoid-like fold. The first nine repeats form a `cradle' for the binding of RanGTP, revealing similarities but also differences with respect to the related importin 13 complex.

Transportin-1 and Transportin-2: protein nuclear import and beyond

Nearly 20 years after its identification as a new β-karyopherin mediating the nuclear import of the RNA-binding protein hnRNP A1, Transportin-1 is still commonly overlooked in comparison with its best known cousin, Importin-β. Transportin-1 is nonetheless a considerable player in nucleo-cytoplasmic transport. Over the past few years, significant progress has been made in the characterization of the nuclear localization signals (NLSs) that Transportin-1 recognizes, thereby providing the molecular basis of its diversified repertoire of cargoes. The recent discovery that mutations in the Transportin-dependent NLS of FUS cause mislocalization of this protein and result in amyotrophic lateral sclerosis illustrates the importance of Transportin-dependent import for human health. Besides, new functions of Transportin-1 are emerging in processes other than nuclear import. Here, we summarize what is known about Transportin-1 and the related β-karyopherin Transportin-2.

Tight regulation of a timed nuclear import wave of EKLF by PKCθ and FOE during Pro-E to Baso-E transition

Erythropoiesis is a highly regulated process during which BFU-E are differentiated into RBCs through CFU-E, Pro-E, PolyCh-E, OrthoCh-E, and reticulocyte stages. Uniquely, most erythroid-specific genes are activated during the Pro-E to Baso-E transition. We show that a wave of nuclear import of the erythroid-specific transcription factor EKLF occurs during the Pro-E to Baso-E transition. We further demonstrate that this wave results from a series of finely tuned events, including timed activation of PKCθ, phosphorylation of EKLF at S68 by P-PKCθ(S676), and sumoylation of EKLF at K74. The latter EKLF modifications modulate its interactions with a cytoplasmic ankyrin-repeat-protein FOE and importinβ1, respectively. The role of FOE in the control of EKLF nuclear import is further supported by analysis of the subcellular distribution patterns of EKLF in FOE-knockout mice. This study reveals the regulatory mechanisms of the nuclear import of EKLF, which may also be utilized in the nuclear import of other factors.

A bimodular nuclear localization signal assembled via an extended double-stranded RNA-binding domain acts as an RNA-sensing signal for transportin 1

The human RNA-editing enzyme adenosine deaminase acting on RNA (ADAR1) carries a unique nuclear localization signal (NLS) that overlaps one of its double-stranded RNA-binding domains (dsRBDs). This dsRBD-NLS is recognized by the nuclear import receptor transportin 1 (Trn1; also called karyopherin-β2) in an RNA-sensitive manner. Most Trn1 cargos bear a well-characterized proline-tyrosine-NLS, which is missing from the dsRBD-NLS. Here, we report the structure of the dsRBD-NLS, which reveals an unusual dsRBD fold extended by an additional N-terminal α-helix that brings the N- and C-terminal flanking regions in close proximity. We demonstrate experimentally that the atypical ADAR1-NLS is bimodular and is formed by the combination of the two flexible fragments flanking the folded domain. The intervening dsRBD acts only as an RNA-sensing scaffold, allowing the two NLS modules to be properly positioned for interacting with Trn1. We also provide a structural model showing how Trn1 can recognize the dsRBD-NLS and how dsRNA binding can interfere with Trn1 binding.

Nuclear PRAS40 couples the Akt/mTORC1 signaling axis to the RPL11-HDM2-p53 nucleolar stress response pathway

The Ribosomal Protein (RP)-HDM2-p53 pathway has been shown to play key roles in oncogene-induced apoptosis and senescence, but the mechanism regulating this pathway remains elusive. The Proline-Rich Akt Substrate of 40 kDA (PRAS40) has recently been identified as a binding partner and inhibitor of the mechanistic Target of Rapamycin Complex 1 (mTORC1). Although other inhibitors of mTORC1 are known tumor suppressors, PRAS40 promotes cell survival and tumorigenesis. Here we demonstrate that Akt- and mTORC1-mediated phosphorylation of PRAS40 at T246 and S221, respectively, promotes nuclear-specific association of PRAS40 with Ribosomal Protein L11 (RPL11). Importantly, silencing of PRAS40 induces upregulation of p53 in a manner dependent upon RPL11. This effect is rescued by wild type PRAS40, but not by the RPL11 binding-null PRAS40 T246A mutant. We find that PRAS40 negatively regulates the RPL11-HDM2-p53 nucleolar stress response pathway and suppresses induction of p53-mediated cellular senescence. This work identifies nuclear PRAS40 as a dual-input signaling checkpoint that links cell growth and proliferation to inhibition of cellular senescence. These findings may help to explain the pro-tumorigenic effect of PRAS40 and identify the PRAS40-RPL11 complex as a promising target for p53-restorative anti-cancer drug discovery.