Optogenetics for sensors: On-demand fluorescent labeling of histone epigenetics

Post-translational modifications of histones to a large extent determine the functional state of chromatin loci. Dynamic visualization of histone modifications with genetically encoded fluorescent sensors makes it possible to monitor changes in the epigenetic state of a single living cell. At the same time, the sensors can potentially compete with endogenous factors recognizing these modifications. Thus, prolonged binding of the sensors to chromatin can affect normal epigenetic regulation. Here, we report an optogenetic sensor for live-cell visualization of histone H3 methylated at lysine-9 (H3K9me3) named MPP8-LAMS (MPP8-based light-activated modification sensor). MPP8-LAMS consists of several fusion protein parts (from N- to C-terminus): i) nuclear export signal (NES), ii) far-red fluorescent protein Katushka, iii) H3K9me3-binding reader domain of the human M phase phosphoprotein 8 (MPP8), iv) the light-responsive AsLOV2 domain, which exposes a nuclear localization signal (NLS) upon blue light stimulation. In the dark, due to the NES, MPP8-LAMS is localized in the cytosol. Under blue light illumination, MPP8-LAMS underwent an efficient translocation from cytosol to nucleus, enabling visualization of H3K9me3-enriched loci. Such an on-demand visualization minimizes potential impact on cell physiology as most of the time the sensor is separated from its target. In general, the present work extends the application of optogenetics to the area of advanced use of genetically encoded sensors.

Nuclear envelope budding and its cellular functions

The nuclear pore complex (NPC) has long been assumed to be the sole route across the nuclear envelope, and under normal homeostatic conditions it is indeed the main mechanism of nucleo-cytoplasmic transport. However, it has also been known that e.g. herpesviruses cross the nuclear envelope utilizing a pathway entitled nuclear egress or envelopment/de-envelopment. Despite this, a thread of observations suggests that mechanisms similar to viral egress may be transiently used also in healthy cells. It has since been proposed that mechanisms like nuclear envelope budding (NEB) can facilitate the transport of RNA granules, aggregated proteins, inner nuclear membrane proteins, and mis-assembled NPCs. Herein, we will summarize the known roles of NEB as a physiological and intrinsic cellular feature and highlight the many unanswered questions surrounding these intriguing nuclear events.

Physical forces modulate interphase nuclear size

The eukaryotic nucleus exhibits remarkable plasticity in size, adjusting dynamically to changes in cellular conditions such as during development and differentiation, and across species. Traditionally, the supply of structural constituents to the nuclear envelope has been proposed as the principal determinant of nuclear size. However, recent experimental and theoretical analyses have provided an alternative perspective, which emphasizes the crucial role of physical forces such as osmotic pressure and chromatin repulsion forces in regulating nuclear size. These forces can be modulated by the molecular profiles that traverse the nuclear envelope and assemble in the macromolecular complex. This leads to a new paradigm wherein multiple nuclear macromolecules that are not limited to only the structural constituents of the nuclear envelope, are involved in the control of nuclear size and related functions.

The E3 ubiquitin ligase CHIP drives monoubiquitylation-mediated nuclear import of the tumor suppressor PTEN

Monoubiquitylation is a principal mechanism driving nuclear translocation of the protein PTEN (phosphatase and tensin homolog deleted on chromosome ten). In this study, we describe a novel mechanism wherein the protein CHIP (C-terminus of Hsc70-interacting protein) mediates PTEN monoubiquitylation, leading to its nuclear import. Western blot analysis revealed a rise in both nuclear and total cellular PTEN levels under monoubiquitylation-promoting conditions, an effect that was abrogated by silencing CHIP expression. We established time-point kinetics of CHIP-mediated nuclear translocation of PTEN using immunocytochemistry and identified a role of karyopherin α1 (KPNA1) in facilitating nuclear transport of monoubiquitylated PTEN. We further established a direct interaction between CHIP and PTEN inside the nucleus, with CHIP participating in either polyubiquitylation or monoubiquitylation of nuclear PTEN. Finally, we showed that oxidative stress enhanced CHIP-mediated nuclear import of PTEN, which resulted in increased apoptosis, and decreased cell viability and proliferation, whereas CHIP knockdown counteracted these effects. To the best of our knowledge, this is the first report elucidating non-canonical roles for CHIP on PTEN, which we establish here as a nuclear interacting partner of CHIP.

Carbonic anhydrase IX subcellular localization in normoxic and hypoxic SH-SY5Y neuroblastoma cells is assisted by its C-terminal protein interaction domain

The human carbonic anhydrase IX (CA IX) is a hypoxia-induced transmembrane protein belonging to the α-CA enzyme family. It has a crucial role in pH regulation in hypoxic cells and acts by buffering intracellular acidosis induced by hypoxia. Indeed, it is frequently expressed in cancer cells, where it contributes to tumor progression. CA IX is also able to localize in the nucleus, where it contributes to 47S rRNA precursor genes transcription; however, the mechanisms assisting its nuclear translocation still remain unclear. The aim of our study was to deepen the understanding of the mechanisms involved in CA IX subcellular distribution. To this purpose, we implemented a site-directed mutagenesis approach targeting the C-terminal domain of CA IX and evaluated the subcellular distribution of the wild-type and mutant proteins in the SH-SY5Y cell line. The mutant proteins showed impaired binding ability and altered subcellular distribution in both normoxic and hypoxic conditions. Our data suggest that CA IX nuclear translocation depends on its transit through the secretory and the endocytic pathways.

Nuclear import of Mas-related G protein-coupled receptor member D induces pathological cardiac remodeling

Alamandine (Ala), a ligand of Mas-related G protein-coupled receptor, member D (MrgD), alleviates angiotensin II (AngII)-induced cardiac hypertrophy. However, the specific physiological and pathological role of MrgD is not yet elucidated. Here, we found that MrgD expression increased under various pathological conditions. Then, MrgD knockdown prevented AngII-induced cardiac hypertrophy and fibrosis via inactivating Gα_i-mediacted downstream signaling pathways, including the phosphorylation of p38 (p-P38), while MrgD overexpression induced pathological cardiac remodeling. Next, Ala, like silencing MrgD, exerted its cardioprotective effects by inhibiting Ang II-induced nuclear import of MrgD. MrgD interacted with p-P38 and promoted its entry into the nucleus under Ang II stimulation. Our results indicated that Ala was a blocking ligand of MrgD that inhibited downstream signaling pathway, which unveiled the promising cardioprotective effect of silencing MrgD expression on alleviating cardiac remodeling. Video Abstract.

Identification and functional characterization of a bipartite nuclear localization signal in ANKRD11

ANKRD11 gene encodes for the large nuclear protein essential for multiple system development including the nervous system. However, the molecular basis for the proper nuclear localization of ANKRD11 has not yet been elucidated. In this study, we have identified a functional bipartite nuclear localization signal (bNLS) between residues 53 and 87 of ANKRD11. Using biochemical approaches, we discovered two major binding sites in this bipartite NLS for Importin α1. Through site-directed mutagenesis and functional analysis, we further found that this bipartite NLS is sufficient for nuclear import of overexpressing GFP in HeLa cells and necessary for nuclear localization of ANKRD11. Importantly, our study provides a possible pathogenic mechanism for certain clinical variants located within the bipartite nuclear localization signal of ANKRD11.

Transforming acidic coiled-coil containing protein 3 suppresses influenza A virus replication by impeding viral endosomal trafficking and nuclear import

Transforming acidic coiled-coil containing protein 3 (TACC3) is a motor spindle protein that plays an essential role in stabilization of the mitotic spindle. In this study, we show that the overexpression of TACC3 reduces the viral titers of multiple influenza A viruses (IAVs). In contrast, the downregulation of TACC3 increases IAVs propagation. Next, we map the target steps of TACC3 requirement to the early stages of viral replication. By confocal microscopy and nuclear plasma separation experiment, we reveal that overexpression of TACC3 results in a substantial decrease of IAV NP accumulation in the nuclei of infected cells. We further show that viral attachment and internalization are not affected by TACC3 overexpression and detect that the early and late endosomal trafficking of IAV in TACC3 overexpression cells is slower than negative control cells. These results suggest that TACC3 exerts an impaired effect on the endosomal trafficking and nuclear import of vRNP, thereby negatively regulating IAV replication. Moreover, the infection of different IAV subtypes decreases the expression level of TACC3 in turn. Consequently, we speculate that IAV ensures the generation of offspring virions by antagonizing the expression of inhibitory factor TACC3. Collectively, our results establish TACC3 as an important inhibitory factor for replication of the IAV, suggesting that TACC3 could be a potential target for the development of future antiviral compounds.

Structures of importin-α bound to the wild-type and an internal deletion mutant of the bipartite nuclear localization signal of HIF-1α

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor that plays an important role as a master regulator of oxygen homeostasis. The activity of HIF-1 is regulated in part by dynamic intracellular trafficking of its α subunit (HIF-1α) that can shuttle between the nucleus and cytoplasm. It has been shown that nuclear localization of HIF-1α requires a variant of classic nuclear localization signal (NLS) and that an internal deletion of the amino acid residues (residues 724-751) in the NLS almost abolish the nuclear localization. Here we report the X-ray crystal structure of the nuclear import adaptor importin-α1 bound to the wild-type HIF-1α NLS at 1.8 Å resolution and of importin-α1 bound to the Δ724-751 mutant of the HIF-1α NLS at 1.9 Å resolution. In the wild-type structure, two basic clusters in the HIF-1α NLS made extensive interactions with importin-α1 on two sites (the major site and the minor site). In the mutant structure, the NLS residues still interacted extensively with the major site on importin-α1, but the interactions with the minor site were not observed. The structural data, together with computational analyses of binding free energies, indicate that the loss of the minor-site interactions inhibit nuclear accumulation of HIF-1α.

Nuclear import of IRF11 via the importin α/β pathway is essential for its antiviral activity

Interferon regulatory factor 11 (IRF11), an intriguing IRF member found only in fish species, has recently been shown to have antiviral properties that are dependent on its nuclear entry and DNA binding affinity. However, the mechanisms by which IRF11 enters the nucleus are unknown. In the present study, we found orthologs of IRF11 in lamprey and lancelet species by combining positional, phylogenetic and structural comparison data, showing that this gene has an ancient origin. The IRF11 gene (AjIRF11) from the Japanese eel, Anguilla japonica, was subsequently characterized, and it was found that AjIRF11 has antiviral activities against spring viremia of carp virus (SVCV), which are accomplished by regulating the production of type I IFN and IFN-stimulated genes. In addition to its known DNA binding residues in the α3 helix, two residues in Loop 1, His40 and Trp46, are also involved in DNA binding and activation of the IFN promoter. Using immunofluorescence microscopy and site-directed mutagenesis analysis, we confirmed that full nuclear localization of AjIRF11 requires the bipartite nuclear localization sequence (NLS) spanning residues 75 to 101, as well as the monopartite NLS situated between residues 119 and 122. Coimmunoprecipitation assays confirmed that AjIRF11 interacts with importin α via its NLSs and can also bind to importin β directly, implying that IRF11 can be imported to the nucleus by one or more transport receptors.

Crystallographic data of an importin-α3 dimer in which the two protomers are bridged by a bipartite nuclear localization signal

53BP1 (TP53-binding protein 1), a key player in DNA double-strand break repair, has a classical bipartite nuclear localization signal (NLS) of sequence 1666-GKRKLITSEEERSPAKRGRKS-1686 that binds to importin-α, a nuclear import adaptor protein. Nucleoporin Nup153 is involved in nuclear import of 53BP1, and the binding of Nup153 to importin-α has been proposed to promote efficient import of classical NLS-containing proteins. Here, the ARM-repeat domain of human importin-α3 bound to 53BP1 NLS was crystallized in the presence of a synthetic peptide corresponding to the extreme C-terminus of Nup153 (sequence: 1459-GTSFSGRKIKTAVRRRK-1475). The crystal belonged to space group I2, with unit-cell parameters a = 95.70, b = 79.60, c = 117.44 Å, β = 95.57°. The crystal diffracted X-rays to 1.9 Å resolution, and the structure was solved by molecular replacement. The asymmetric unit contained two molecules of importin-α3 and two molecules of 53BP1 NLS. Although no convincing density was observed for the Nup153 peptide, the electron density corresponding to 53BP1 NLS was unambiguous and continuous along the entire length of the bipartite NLS. The structure revealed a novel dimer of importin-α3, in which two protomers of importin-α3 are bridged by the bipartite NLS of 53BP1. In this structure, the upstream basic cluster of the NLS is bound to the minor NLS-binding site of one protomer of importin-α3, whereas the downstream basic cluster of the same chain of NLS is bound to the major NLS-binding site of another protomer of importin-α3. This quaternary structure is distinctly different from the previously determined crystal structure of mouse importin-α1 bound to the 53BP1 NLS. The atomic coordinates and structure factors have been deposited in the Protein Data Bank (accession code 8HKW).

Characterization of the nuclear import of human CHD4-NuRD complex

Chromatin remodeling enzymes form large multiprotein complexes that play central roles in regulating access to the genome. Here, we characterize the nuclear import of the human CHD4 protein. We show that CHD4 enters the nucleus by means of several importin alpha proteins (1, 5, 6, and 7), and rather independently of importin beta 1. Importin alpha 1 directly interacts with a monopartite "KRKR"-motif in the N-terminus of CHD4 (aa 304-307). However, alanine mutagenesis of this motif only leads to approx. 50% reduction in nuclear localization of CHD4, implying additional import mechanisms. Interestingly, we can show that CHD4 is already associated with NuRD core subunits such as MTA2, HDAC1, or RbAp46 in the cytoplasm, suggesting an assembly of the NuRD core complex before nuclear import. We propose that, in addition to the importin-alpha-dependent nuclear localization signal, CHD4 is dragged into the nucleus by a "piggyback mechanism" using the import signals of the associated NuRD subunits.

Alphaherpesvirus upregulates NDRG1 expression to facilitate the nuclear import of viral UL31 and UL34 proteins

Proteins UL31 and UL34 encoded by alphaherpesvirus are critical for viral primary envelopment and nuclear egress. We report here that pseudorabies virus (PRV), a useful model for research on herpesvirus pathogenesis, uses N-myc downstream regulated 1 (NDRG1) to assist the nuclear import of UL31 and UL34. PRV promoted NDRG1 expression through DNA damage-induced P53 activation, which was beneficial to viral proliferation. PRV induced the nuclear translocation of NDRG1, and its deficiency resulted in the cytosolic retention of UL31 and UL34. Therefore, NDRG1 assisted the nuclear import of UL31 and UL34. Furthermore, in the absence of the nuclear localization signal (NLS), UL31 could still translocate to the nucleus, and NDRG1 lacked an NLS, thus suggesting the existence of other mediators for the nuclear import of UL31 and UL34. We demonstrated that heat shock cognate protein 70 (HSC70) was the key factor in this process. UL31 and UL34 interacted with the N-terminal domain of NDRG1 and the C-terminal domain of NDRG1 bound to HSC70. Replenishment of HSC70_ΔNLS in HSC70-knockdown cells, or interference in importin α expression, abolished the nuclear translocation of UL31, UL34, and NDRG1. These results indicated that NDRG1 employs HSC70 to facilitate viral proliferation in the nuclear import of PRV UL31 and UL34. This article is protected by copyright. All rights reserved.

Nuclear transport and subcellular localization of the dystrophin Dp71 and Dp40 isoforms in the PC12 cell line

The shortest dystrophins, Dp71 and Dp40, are transcribed from the DMD gene through an internal promoter located in intron 62. These proteins are the main product of the DMD gene in the nervous system and have been involved in various functions related to cellular differentiation and proliferation as well as other cellular processes. Dp71 mRNA undergoes alternative splicing that results in different Dp71 protein isoforms. The subcellular localization of some of these isoforms in the PC12 cell line has been previously reported, and a differential subcellular distribution was observed, which suggests a particular role for each isoform. With the aim of obtaining information on their function, this study identified factors involved in the nuclear transport of Dp71 and Dp40 isoforms in the PC12 cell line. Cell cultures were treated with specific nuclear import/export inhibitors to determine the Dp71 isoform transport routes. The results showed that all isoforms of Dp71 and Dp40 included in the analysis have the ability to enter the cell nucleus through α/β importin, and the main route of nuclear export for Dp71 isoforms is through the exportin CRM1, which is not the case for Dp40.

Importin KPNA2 confers HIV-1 pre-integration complex nuclear import by interacting with the capsid protein

For human immunodeficiency virus 1 (HIV-1) to infect non-dividing cells, pre-integration complex (PIC) must be transported into the nucleus within the replication cycle. We previously reported that the karyopherin β1 (KPNB1)-nucleoporin Pom121 pathway, related to the downstream process of PIC nuclear import, mediates efficient HIV-1 PIC nuclear import. Further, our earlier RNA transcriptome sequencing revealed that karyopherin α2 (KPNA2) was among the differentially expressed importin family members during monocyte to macrophage differentiation. Although PIC transport into the nucleus in HIV-1 has been widely studied, much remains to be understood about it. In this study, we confirmed our previous RNA sequencing results and found that HIV-1 replication was significantly lower in 293T cells with siRNA-mediated KPNA2 knockdown and higher in KPNA2-upregulated cells. Quantitative PCR indicated that viral replication was impaired during cDNA nuclear import. The N-terminal of the capsid protein p24 interacted with KPNA2, and KPNB1 participated in KPNA2-mediated PIC nuclear import. Disruption of the capsid-KPNA2 binding by overexpression of full-length p24 or p24 N-terminal impaired the PIC nuclear import. These results indicate that KPNA2 is an important upstream adaptor of the KPNB1-Pom121 axis, thereby mediating HIV-1 PIC nuclear transportation. KPNA2 is thus a potential target for HIV-1 antiviral treatment.

Hormone-Inducible Transport Reporter Assay to Study Nuclear Import Defects in Neurodegenerative Diseases

In the recent years, defective nuclear import has emerged as an important pathomechanism of neurodegenerative diseases, particularly in amyotrophic lateral sclerosis (ALS). Here, specific nuclear RNA binding proteins (RBPs) mislocalize and aggregate in the cytoplasm of neurons and glial cells in degenerating brain regions. Bona fide transport assays that measure nuclear import in a quantitative manner allow one to distinguish whether disease-linked RBP mutations that cause cytosolic RBP mislocalization directly result in reduced nuclear import or cause increased cytoplasmic localization of the RBP through other mechanisms. Here we describe the quantitative analysis of nuclear import rates of RBPs using a hormone-inducible system by live cell imaging.

Visualizing Nuclear Pore Complexes in Xenopus Egg Extracts

The nuclear pore complex (NPC) is the conduit in the nuclear envelope through which proteins and RNA are transported between the cytoplasm and nucleus. Xenopus egg extracts that support de novo assembly of nuclei have provided a robust system to study NPC structure and function because the biochemical composition of the extract can be easily manipulated. Here we describe how to assemble nuclei in Xenopus egg extract, how to visualize and analyze NPCs in both live and fixed samples, and different approaches to altering nucleocytoplasmic transport in extract.

Nucleocytoplasmic Shuttling of the Mechanosensitive Transcription Factors MRTF and YAP /TAZ

Myocardin-related transcription factor (MRTF) and the paralogous Hippo pathway effectors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) are transcriptional co-activators that play pivotal roles in myofibroblast generation and activation, and thus the pathogenesis of organ fibrosis. They are regulated by a variety of chemical and mechanical fibrogenic stimuli, primarily at the level of their nucleocytoplasmic shuttling. In this chapter we describe the tools and protocols that allow for exact, quantitative, and automated determination and analysis of the nucleocytoplasmic distribution of endogenous or heterologously expressed MRTF and YAP/TAZ, measured in large cell populations. Dynamic monitoring of nucleocytoplasmic ratios of transcription factors is a novel and important approach, suitable to address both the structural requirements and the regulatory mechanisms underlying transcription factor traffic and the consequent reprogramming of gene expression during fibrogenesis.

Novel small molecule inhibitor of Kpnβ1 induces cell cycle arrest and apoptosis in cancer cells

Karyopherin beta 1 (Kpnβ1) is a major nuclear import receptor that mediates the import of cellular cargoes into the nucleus. Recently it has been shown that Kpnβ1 is highly expressed in several cancers, and its inhibition by siRNA induces apoptotic cancer cell death, while having little effect on non-cancer cells. This study investigated the effect of a novel small molecule, Inhibitor of Nuclear Import-60 (INI-60), on cancer cell biology, as well as nuclear import activities associated with Kpnβ1, and cancer progression in vivo using cervical and oesophageal cancer cell lines. INI-60 treatment resulted in the inhibition of cancer cell proliferation, colony formation, migration and invasion, and induced a G1/S cell cycle arrest, followed by cancer cell death via apoptosis. Non-cancer cells were minimally affected by INI-60 at concentrations that inhibited cancer cells. INI-60 treatment altered the localisation of Kpnβ1 and its cargoes, NFκB/p65, NFAT and AP-1, and the overexpression of Kpnβ1 reduced INI-60 cytotoxicity. INI-60 also inhibited KYSE 30 oesophageal cancer cell line growth in vivo. Taken together, these results show that INI-60 inhibits the nuclear import of Kpnβ1 cargoes and interferes with cancer cell biology. INI-60 presents as a potential therapeutic approach for cancers of different tissue origins and warrants further investigation as a novel anti-cancer agent.

Spodoptera frugiperda mRNA export factor interacts with and mediates the nuclear import of Autographa californica multiple nucleopolyhedrovirus ORF34 (Ac34)

Autographa californica multiple nucleopolyhedrovirus orf34 (ac34) is one of the unique genes of alphabaculoviruses. For successful alphabaculovirus replication, viral proteins must be transported to the nucleus. Our previous study showed that the nuclear localization of Ac34 was required for optimal production of budded virions. To investigate the mechanism of Ac34 nuclear import, mass spectrometric analysis was performed to identify potential proteins that may be involved in the nuclear import of Ac34. The result indicated that Spodoptera frugiperda mRNA export factor (SfMEF) may interact with Ac34 during baculovirus infection. Co-immunoprecipitation assays confirmed that Ac34 could interact with SfMEF in the absence of other baculovirus proteins. The deletion of ac34 did not affect the subcellular localization of SfMEF; however, knocking down Sfmef prevented the nuclear import of Ac34 in virus-infected cells. The mutations of C116 or C119 in a potential CCCH zinc finger motif (C¹¹⁶-X₂-C¹¹⁹-X₈-C¹²⁸-X₂-H¹³¹) of Ac34 led to an exclusive cytoplasmic distribution of Ac34, in consistent with our previous finding of mutations of C128 or H131 in this motif. Co-immunoprecipitation analysis showed that the above mutations in the potential zinc finger motif disrupted the interaction between Ac34 and SfMEF, and the loss of the interaction resulted in decreased BV production. Our findings demonstrated that SfMEF interacts with and mediates the nuclear import of Ac34, which is a new nucleocytoplasmic transport pathway used by alphabaculovirus to achieve successful viral replication within the nucleus of the infected cells.

Importin-9 regulates chromosome segregation and packaging in Drosophila germ cells

Germ cells undergo distinct nuclear processes as they differentiate into gametes. Although these events must be coordinated to ensure proper maturation, the stage-specific transport of proteins in and out of germ cell nuclei remains incompletely understood. Our efforts to genetically characterize Drosophila genes that exhibit enriched expression in germ cells led to the finding that loss of the highly conserved Importin β/karyopherin family member Importin-9 (Ipo9, herein referring to Ranbp9) results in female and male sterility. Immunofluorescence and fluorescent in situ hybridization revealed that Ipo9KO mutants display chromosome condensation and segregation defects during meiosis. In addition, Ipo9KO mutant males form abnormally structured sperm and fail to properly exchange histones for protamines. Ipo9 physically interacts with proteasome proteins, and Ipo9 mutant males exhibit disruption of the nuclear localization of several proteasome components. Thus, Ipo9 coordinates the nuclear import of functionally related factors necessary for the completion of gametogenesis. This article has an associated First Person interview with the first author of the paper.

Inhibition of Kpnβ1 mediated nuclear import enhances cisplatin chemosensitivity in cervical cancer

Background

Inhibition of nuclear import via Karyopherin beta 1 (Kpnβ1) shows potential as an anti-cancer approach. This study investigated the use of nuclear import inhibitor, INI-43, in combination with cisplatin.

Methods

Cervical cancer cells were pre-treated with INI-43 before treatment with cisplatin, and MTT cell viability and apoptosis assays performed. Activity and localisation of p53 and NFκB was determined after co-treatment of cells.

Results

Pre-treatment of cervical cancer cells with INI-43 at sublethal concentrations enhanced cisplatin sensitivity, evident through decreased cell viability and enhanced apoptosis. Kpnβ1 knock-down cells similarly displayed increased sensitivity to cisplatin. Combination index determination using the Chou-Talalay method revealed that INI-43 and cisplatin engaged in synergistic interactions. p53 was found to be involved in the cell death response to combination treatment as its inhibition abolished the enhanced cell death observed. INI-43 pre-treatment resulted in moderately stabilized p53 and induced p53 reporter activity, which translated to increased p21 and decreased Mcl-1 upon cisplatin combination treatment. Furthermore, cisplatin treatment led to nuclear import of NFκB, which was diminished upon pre-treatment with INI-43. NFκB reporter activity and expression of NFκB transcriptional targets, cyclin D1, c-Myc and XIAP, showed decreased levels after combination treatment compared to single cisplatin treatment and this associated with enhanced DNA damage.

Conclusions

Taken together, this study shows that INI-43 pre-treatment significantly enhances cisplatin sensitivity in cervical cancer cells, mediated through stabilization of p53 and decreased nuclear import of NFκB. Hence this study suggests the possible synergistic use of nuclear import inhibition and cisplatin to treat cervical cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-07819-3.

Importin β1 mediates nuclear import of the factors associated with nonsense-mediated RNA decay

In eukaryotic cells, nonsense-mediated RNA decay (NMD) is an essential physiological mechanism coupled to translation, regulating the stability of abnormal RNA containing premature termination codon (PTC) and a significant fraction of normal transcriptomes. So far, the molecular regulation mechanism of NMD pathway is far from fully elucidated. Previously, we observed the interaction between importin β1 (Impβ1) and UPF1, a core factor of NMD. Here, we demonstrated that Impβ1 knockdown stabilized NMD reporters, and Impβ1 and UPF1 interacted and co-regulated an extensive number of target transcripts. Furthermore, Impβ1 affected the interaction between UPF1 and SMG5 or MAGOH, and the nuclear distributions of UPF1, SMG1, SMG5 and MAGOH. Besides, Ran knockdown extremely promoted the dissociation of UPF1 from SMG5 or MAGOH. Our findings provide molecular insight into the potential function of Impβ1in nonsense-mediated RNA decay.

HNF1A-MODY Mutations in Nuclear Localization Signal Impair HNF1A-Import Receptor KPNA6 Interactions

Mutations in hepatocyte nuclear factor (HNF)1A gene cause the most common form of Maturity-onset diabetes of the young (MODY), a monogenic subtype of diabetes mellitus. Functional characterization of mutant proteins reveals that mutations may disrupt DNA binding capacity, transactivation ability and nuclear localization of HNF1A depending on the position of the mutation. Previously identified Arg271Trp and Ser345Tyr mutations in HNF1A were found to be defective in nuclear localization. Arg271 residue resides in a region similar to classical nuclear localization signal (NLS) motif, while Ser345 does not. Importin α family members recognize NLS motifs on cargo proteins and subsequently translocate them into nucleus. Here, we first investigated the nuclear localization mechanism of wild type HNF1A protein. For this purpose, we analyzed the interaction of HNF1A with three mouse homolog importin α proteins (KPNA2, KPNA4 and KPNA6) by co-immunoprecipitation assay and molecular docking simulation. Hereby, KPNA6 was identified as the main import receptor, which is responsible for the transport of HNF1A into the nucleus. Immunolocalization studies in mouse pancreatic cells (Min6) also confirmed the co-localization of HNF1A and KPNA6 in the cytoplasm. Secondly, the interaction between KPNA6 and mutant HNF1A proteins (Arg271Trp and Ser345Tyr) was assessed. Co-immunoprecipitation studies revealed a reduced interaction compared to wild type HNF1A. Our study demonstrated for the first time that HNF1A transcription factor is recognized and transported by importin/karyopherin import family, and mutations in NLS motifs may disrupt the interaction leading to nuclear localization abnormalities and MODY phenotype.

Hypusination of Eif5a regulates cytoplasmic TDP-43 aggregation and accumulation in a stress-induced cellular model

TAR DNA-binding protein 43 (TDP-43) is a nuclear RNA/DNA binding protein involved in mRNA metabolism. Aberrant mislocalization to the cytoplasm and formation of phosphorylated/aggregated TDP-43 inclusions remains the hallmark pathology in a spectrum of neurodegenerative diseases, including frontotemporal disorders and Alzheimer's disease. Eukaryotic Translation Initiation Factor 5A undergoes a unique post-translation modification of lysine to hypusine (K50), which determines eIF5A binding partners. We used a sodium arsenite-induced cellular stress model to investigate the role of hypusinated eIF5A (eIF5AHypK50) in governing TDP-43 cytoplasmic mislocalization and accumulation in stress granule. Our proteomics and functional data provide evidence that eIF5A interacts with TDP-43 in a hypusine-dependent manner. Additionally, we showed that following stress TDP-43 interactions with eIF5AHypK50 were induced both in the cytoplasm and stress granules. Pharmacological reduction of hypusination or mutations of lysine residues within the hypusine loop decreased phosphorylated and insoluble TDP-43 levels. The proteomic and biochemical analysis also identified nuclear pore complex importins KPNA1/2, KPNB1, and RanGTP as interacting partners of eIF5AHypK50. These findings are the first to provide a novel pathway and potential therapeutic targets that require further investigation in models of TDP-43 proteinopathies.

The nuclear transporter importin 13 is critical for cell survival during embryonic stem cell differentiation

Nuclear transporter Importin (Imp, Ipo) 13 is known to transport various mammalian cargoes into/out of the nucleus, but its role in directing cell-fate is unclear. Here we examine the role of Imp13 in the maintenance of pluripotency and differentiation of embryonic stem cells (ESCs) for the first time, using an embryonic body (EB)-based model. When induced to differentiate, Ipo13^-/- ESCs displayed slow proliferation, reduced EB size, and lower expression of the proliferation marker KI67, concomitant with an increase in the number of TUNEL⁺ nuclei compared to wildtype ESCs. At days 5 and 10 of differentiation, Ipo13^-/- EBs also showed enhanced loss of the pluripotency transcript OCT3/4, and barely detectable clusters of OCT3/4 positive cells. Day 5 Ipo13^-/- EBs further exhibited reduced levels of the mesodermal markers Brachyury and Mixl1, correlating with reduced numbers of haemoglobinised cells generated. Our findings suggest that Imp13 is critical to ESC survival as well as early post-gastrulation differentiation.

Ran participates in deltamethrin stress through regulating the nuclear import of Nrf2

The small GTPase Ran has a variety of biological functions, one of the most prominent of which is to regulate nucleocytoplasmic transport. In our previous study, it was suggested that Ran is involved in the deltamethrin (DM) stress. In addition, Keap1-Nrf2-ARE pathway was also confirmed to be associated with DM stress. We report here that under DM stress, interfering Ran or nuclear transport factor Ntf2 by RNAi could suppress the nuclear import of nuclear transcription factor Nrf2 which then down-regulates the expressions of detoxification enzyme genes (Cyp4d20, Cyp4ae1, GstD5, Sod3, etc.), ultimately resulting in a significant apoptosis of Drosophila Kc cells. In contrast, after overexpressing Ran in Kc cells, Nrf2 has a higher concentration in the nucleus, and the expressions of detoxification enzyme genes are up-regulated, while the DM-induced apoptosis is significantly lower than that of the control group. Additionally, we preliminary found silencing Ntf2 or Ran could prevent the nuclear import of transcription factor Dif under DM stress, subsequently decreased expressions of antimicrobial peptide genes (Drsl1). In summary, our data mainly indicates that Ran may participate in DM stress through regulating the nuclear import of Nrf2, which could help to study the mechanism of deltamethrin resistance.

MxB impedes the NUP358-mediated HIV-1 pre-integration complex nuclear import and viral replication cooperatively with CPSF6

BACKGROUND

The human myxovirus resistance 2 (Mx2/MxB) protein was originally found to regulate cytoplasmic-nuclear transport but was recently reported to restrict HIV-1 replication by binding to HIV-1 capsid (CA), preventing uncoating, the nuclear import of pre-integration complex (PIC) and viral DNA integration. This work explores the mechanisms of MxB-mediated HIV-1 inhibition.

RESULTS

We demonstrated that MxB represses NUP358-mediated PIC nuclear import and HIV-1 replication. Moreover, MxB's effects on PIC nuclear import and HIV-1 replication depend critically on cofactor cleavage and polyadenylation specificity factor subunit 6 (CPSF6). MxB binds nucleoporin NUP358, blocks NUP358-CA interaction, thereby impeding the nuclear import of HIV-1 PIC with CPSF6 binding to PIC. More intriguingly, CPSF6's role in nuclear import depends on MxB, being a facilitator of HIV-1 nuclear import on its own, but becoming an inhibitor when MxB is present.

CONCLUSIONS

Our work establishes that MxB impedes the NUP358-mediated HIV-1 nuclear import and viral replication cooperatively with CPSF6.

Nuclear transporter karyopherin subunit alpha 3 levels modulate Porcine circovirus type 2 replication in PK-15 cells

Entering the nucleus is important for Porcine circovirus type 2 (PCV2) replication. Karyopherins (KPNs) mediate the nuclear import of many cytoplasmic proteins. Our previous study showed that KPNA3 is involved in interferon production during PCV2 infection induced by Poly I:C and ISD (Interferon stimulatory DNA). However, it remains unclear whether PCV2 replication is associated with KPNA3. In the present study, knockdown of KPNA3 promoted the replication of PCV2, whereas overexpression of KPNA3 inhibited PCV2 replication in PK-15 cells. Furthermore, KPNA3 knockdown inhibited IRF3 and reduced the expression of antiviral genes including IFN-β, ISG54, Mx1 and ISG56, while the opposite results were obtained after KPNA3 overexpression. KPNA3 knockdown also promoted p65 nuclear translocation and increased the mRNA expression of IL-10 and IL-1β. These results suggested that KPNA3 facilitates IRF3 entry into the nucleus and the production of an antiviral response, resulting in PCV2 replication inhibition and blockage of NF-κB signal activation.

Karyopherins and condensates

Several aggregation-prone RNA-binding proteins, including FUS, EWS, TAF15, hnRNP A1, hnRNP A2, and TDP-43, are mutated in neurodegenerative diseases. The nuclear-cytoplasmic distribution of these proteins is controlled by proteins in the karyopherin family of nuclear transport factors (Kaps). Recent studies have shown that Kaps not only transport these proteins but also inhibit their self-association/aggregation, acting as molecular chaperones. This chaperone activity is impaired for disease-causing mutants of the RNA-binding proteins. Here, we review physical data on the mechanisms of self-association of several disease-associated RNA-binding proteins, through liquid-liquid phase separation and amyloid fiber formation. In each case, we relate these data to biophysical, biochemical, and cell biological data on the inhibition of self-association by Kaps. Our analyses suggest that Kaps may be effective chaperones because they contain large surfaces with diverse physical properties that enable them to engage multiple different regions of their cargo proteins, blocking self-association.

Switching of cardiac troponin I between nuclear and cytoplasmic localization during muscle differentiation

The nuclear accumulation of proteins may depend on the presence of short targeting sequences, which are known as nuclear localization signals (NLSs). Here, we found that NLSs are predicted in some cytosolic proteins and examined the hypothesis that these NLSs may be functional under certain conditions. As a model, human cardiac troponin I (hcTnI) was used. After expression in cultured non-muscle or undifferentiated muscle cells, hcTnI accumulated inside nuclei. Several NLSs were predicted and confirmed by site-directed mutagenesis in hcTnI. Nuclear import occurred via the classical karyopherin-α/β nuclear import pathway. However, hcTnI expressed in cultured myoblasts redistributed from the nucleus to the cytoplasm, where it was integrated into forming myofibrils after the induction of muscle differentiation. It appears that the dynamic retention of proteins inside cytoplasmic structures can lead to switching between nuclear and cytoplasmic localization.

The HIV-1 Capsid: More than Just a Delivery Package

Productive HIV infection requires integration of viral genes into the host genome. But how viral DNA gets to the nucleus in the first place remains one of the most controversial yet deceptively simple questions in HIV post-entry biology. This is illustrated in cartoons of viral entry, which often depict the entry process as an 'explosion' of the HIV capsid in the cytosol and independent movement of viral DNA through nuclear pores and into the nucleus. HIV enters the cell cytosol with two encapsidated RNA strands and must undergo reverse transcription (RT) to synthesise DNA. Even here there is no consensus for where, when or how RT happens. HIV must get into the nucleus, which in a non-dividing cell requires transport through the nuclear pore. Finally, the virus must 'uncoat': shed its protein capsid to allow its DNA to be spliced with that of the host. Where the virus uncoats and whether this is a single or multi-step process are similarly hotly debated. Understanding these processes is further complicated by three broad factors. First, that there are inter-relationships between these processes that may ensure HIV undergoes the right step at the right place at the right time. Second, the host has cofactors which the virus is dependent upon and must recruit but also immune factors that can sense and inhibit virus and so must be avoided. Third, HIV post-entry biology is cell-type dependent-meaning that factors which are essential in one cell type can be redundant in another.

A Lysine Residue Essential for Geminivirus Replication Also Controls Nuclear Localization of the Tomato Yellow Leaf Curl Virus Rep Protein

Geminiviruses are single-stranded DNA (ssDNA) viruses that infect a wide range of plants. To promote viral replication, geminiviruses manipulate the host cell cycle. The viral protein Rep is essential to reprogram the cell cycle and then initiate viral DNA replication by interacting with a plethora of nuclear host factors. Even though many protein domains of Rep have been characterized, little is known about its nuclear targeting. Here, we show that one conserved lysine in the N-terminal part of Rep is pivotal for nuclear localization of the Rep protein from Tomato yellow leaf curl virus (TYLCV), with two other lysines also contributing to its nuclear import. Previous work had identified that these residues are essential for Rep from Tomato golden mosaic virus (TGMV) to interact with the E2 SUMO-conjugating enzyme (SCE1). We here show that mutating these lysines leads to nuclear exclusion of TYLCV Rep without compromising its interaction with SCE1. Moreover, the ability of TYLCV Rep to promote viral DNA replication also depends on this highly conserved lysine independently of its role in nuclear import of Rep. Our data thus reveal that this lysine potentially has a broad role in geminivirus replication, but its role in nuclear import and SCE1 binding differs depending on the Rep protein examined.IMPORTANCE Nuclear activity of the replication initiator protein (Rep) of geminiviruses is essential for viral replication. We now define that one highly conserved lysine is important for nuclear import of Rep from three different begomoviruses. To our knowledge, this is the first time that nuclear localization has been mapped for any geminiviral Rep protein. Our data add another key function to this lysine residue, besides its roles in viral DNA replication and interaction with host factors, such as the SUMO E2-conjugating enzyme.

Structural and biochemical characterization of the recognition of the 53BP1 nuclear localization signal by importin-α

53BP1 (TP53-binding protein 1) plays a key role in DNA double-strand break repair by promoting non-homologous end joining (NHEJ) especially during G1 phase of the cell cycle. Nuclear import of 53BP1 is required for proper localization of 53BP1 and maintenance of genome integrity. 53BP1 has a classical bipartite nuclear localization signal (NLS) of sequence 1666-GKRKLITSEEERSPAKRGRKS-1686. Ser1678 within the 53BP1 NLS can be phosphorylated by CDK1/cyclin B, and a phosphomimetic substitution of Ser1678 with aspartate has been shown to negatively regulate nuclear import of 53BP1. Here, the X-ray crystal structures of the nuclear import adaptor importin-α1 bound to the wild-type 53BP1 NLS and the S1678D mutant of 53BP1 NLS are reported at resolutions of 1.9 and 1.7 Å, respectively. In the wild-type structure, not only the two basic clusters of the 53BP1 NLS but also the linker region between the basic clusters made extensive interactions with importin-α1. In the mutant structure, the linker region between the basic clusters in the 53BP1 NLS made fewer interactions with importin-α1 than those observed in the wild-type structure. However, biochemical binding assays using purified proteins showed that the 53BP1 mutation S1678D reduces the binding affinity to importin-α1 only to a modest extent. Implications of these findings for regulatory mechanism of 53BP1 nuclear import are discussed.

Nucleocytoplasmic shuttling of the human parainfluenza virus type 2 phosphoprotein

Human parainfluenza virus type 2 phosphoprotein (P) is an essential component of viral polymerase. The P gene encodes both P and accessory V proteins by a specific gene editing mechanism. Therefore, the N-terminal 164 amino acids of P protein are common to V protein. Interestingly, while P protein is located in the cytoplasm, V protein is found mainly in the nucleus. Using deletion mutants, we show the presence of a nuclear localization signal (NLS) in the P/V common domain, and a nuclear export signal (NES) in the C-terminal P specific region. The NLS region makes a complex with importin α5 or 7. In the presence of leptomycin B, P protein is retained in the nucleus, indicating that it contains a CRM1-dependent NES. We identified the NLS (⁶⁵PVKPRRKK⁷²) and the NES (²²⁵IIELLKGLDL²³⁴) using β-galactosidase fusion proteins. Moreover, nucleocytoplasmic shuttling of P protein appears to be important for efficient viral polymerase activity.

Nuclear import of transcriptional corepressor BCOR occurs through interaction with karyopherin α expressed in human periodontal ligament

Mutations in the gene encoding BCL-6 corepressor (BCOR) are responsible for oculofaciocardiodental (OFCD) syndrome, which is a rare X-linked dominant disorder characterized by radiculomegaly of permanent teeth as the most typical symptom. To function as a transcriptional corepressor, BCOR needs to enter the nucleus; however, the molecular pathway for its nuclear translocation during dental root formation remains unclear. The purpose of this study was to determine the mechanism underlying BCOR transport into the nucleus. Our results showed that human periodontal ligament (PDL) cells expressed karyopherin α (KPNA)2, KPNA4, and KPNA6 belonging to a family of nuclear import proteins, which interacted with BCOR in the immunoprecipitation assay. Site-directed mutagenesis targeting the two nuclear localization signals (NLSs) within BCOR reduced its nuclear translocation; however, co-expression of KPNA2, KPNA4, or KPNA6 with BCOR carrying a previously described mutation which eliminated one of the two NLSs significantly increased nuclear accumulation of the mutant BCOR, indicating participation of KPNA in BCOR nuclear translocation. Comparative expression profiling of PDL cells isolated from normal and OFCD patients revealed significant downregulation of SMAD4, GLI1, and nuclear factor 1-C (NFIC) mRNA expression, suggesting that BCOR mutations cause hyperactive root formation in OFCD syndrome by inhibiting SMAD4-Hedgehog-NFIC signaling implicated in dental root development. Our study contributes to understanding of the mechanisms providing nuclear import of BCOR during root formation.

NCAM140 is translocated into the nucleus by an importin-β1-dependent mechanism

The neural cell adhesion molecule (NCAM) is important for neural development and for plasticity in adult brain. Previous studies demonstrated a calmodulin-dependent import of a transmembrane fragment of NCAM into the nucleus that regulates gene expression. In a protein macroarray we identified importin-β1 as a potential interaction partner of NCAM's cytoplasmic tail. The interaction was verified and an importin-β1-dependent import of NCAM into the nucleus could be demonstrated using quantitative immunofluorescence analysis. Generation of NCAM deletion mutants revealed that the last amino acids of the cytoplasmic region of NCAM are dispensable whereas other parts of NCAM's cytoplasmic tail take part in its nuclear translocation. With this study we propose an alternative nuclear route for NCAM via the classical importin-mediated import.

Ivermectin inhibits DNA polymerase UL42 of pseudorabies virus entrance into the nucleus and proliferation of the virus in vitro and vivo

Pseudorabies virus (PRV) is an important viral pathogen of pigs that causes huge losses in pig herds worldwide. Ivermectin is a specific inhibitor of importin-α/β-dependent nuclear transport and shows antiviral potential against several RNA viruses by blocking the nuclear localization of viral proteins. Since the replication of DNA viruses is in the nucleus, ivermectin may be functional against DNA virus infections if the DNA polymerase or other important viral proteins enter the nucleus via the importin-α/β-mediated pathway. Here, we determined whether ivermectin suppresses PRV replication in hamster kidney BHK-21 cells and investigated the effect of ivermectin on the subcellular localization of the PRV UL42 protein, the accessory subunit of PRV DNA polymerase. Also, an in vivo anti-PRV assay was conducted in mice. Our data demonstrate that ivermectin treatment inhibits PRV infection in cells in a dose-dependent manner. Treatment of PRV-infected cells with ivermectin significantly suppressed viral DNA synthesis and progeny virus production. Ivermectin disrupted the nuclear localization of UL42 by targeting the nuclear localization signal of the protein in transfected cells. Ivermectin treatment increased the survival rates of mice infected with PRV and relieved infection as indicated by lower clinical scores and fewer gross lesions in the brain. Together, our results suggest that ivermectin may be a therapeutic or preventative agent against PRV infection.

Investigating nucleo-cytoplasmic shuttling of the human DEAD-box helicase DDX3

The human DEAD-box helicase DDX3 is a multi-functional protein involved in the regulation of gene expression and additional non-conventional roles as signalling adaptor molecule that are independent of its enzymatic RNA remodeling activity. It is a nucleo-cytoplasmic shuttling protein and it has previously been suggested that dysregulation of its subcellular localization could contribute to tumourigenesis. Indeed, both tumour suppressor and oncogenic functions have been attributed to DDX3. In this study, we investigated the regulation of DDX3's nucleocytoplasmic shuttling. We confirmed that an N-terminal conserved Nuclear Export Signal (NES) is required for export of human DDX3 from the nucleus, and identified three regions within DDX3 that can independently facilitate its nuclear import. We also aimed to identify conditions that alter DDX3's subcellular localisation. Viral infection, cytokine treatment and DNA damage only induced minor changes in DDX3's subcellular distribution as determined by High Content Analysis. However, DDX3's nuclear localization increased in early mitotic cells (during prophase) concomitant with an increase in DDX3 expression levels. Our results are likely to have implications for the proposed use of (nuclear) DDX3 as a prognostic biomarker in cancer.

The transmembrane nucleoporin Pom121 ensures efficient HIV-1 pre-integration complex nuclear import

HIV-1 hijacks host classical cargo nuclear transportation, or nonclassical pathways by directly interacting with importin-β family proteins or nucleoporins for efficient pre-integration complex (PIC) nuclear import. Recently, an N-terminal truncated form of nucleoporin Pom121c (601-987 aa) was reported to inhibit HIV-1 replication. In contrast, we found that HIV-1 replication was significantly decreased in 293T and TZM-b1 cells with siRNA-mediated Pom121 knockdown. Quantitative PCR indicated that viral replication was impaired at the step of cDNA nuclear import. Furthermore, we found that karyopherin-β1 (KPNB1), which belongs to the importin-β family, interacts with Pom121 and is involved in Pom121-mediated PIC nuclear import. Rescue experiment indicated that the FG-repeats and the following α-helix in Pom121 are required for its role in HIV-1 PIC nuclear import. Taken together, our results showed that full-length Pom121 enables efficient PIC nuclear import, and suggested that this process may rely on KPNB1 dependent classical cargo nuclear transportation way.

Influence of acute promyelocytic leukemia therapeutic drugs on nuclear pore complex density and integrity

During cell division, a large number of nuclear proteins are released into the cytoplasm due to nuclear envelope breakdown. Timely nuclear import of these proteins following exit from mitosis is critical for establishment of the G1 nuclear environment. Dysregulation of post-mitotic nuclear import may affect the fate of newly divided stem or progenitor cells and may lead to cancer. Acute promyelocytic leukemia (APL) is a malignant disorder that involves a defect in blood cell differentiation at the promyelocytic stage. Recent studies suggest that pharmacological concentrations of the APL therapeutic drugs, all-trans retinoic acid (ATRA) and arsenic trioxide (ATO), affect post-mitotic nuclear import of the APL-associated oncoprotein PML/RARA. In the present study, we have investigated the possibility that ATRA and ATO affect post-mitotic nuclear import through interference with components of the nuclear import machinery. We observe reduced density and impaired integrity of nuclear pore complexes after ATRA and/or ATO exposure. Using a post-mitotic nuclear import assay, we demonstrate distinct import kinetics among different nuclear import pathways while nuclear import rates were similar in the presence or absence of APL therapeutic drugs.

Identification of a novel trafficking pathway exporting a replication protein, Orc2 to nucleus via classical secretory pathway in Plasmodium falciparum

Malaria parasites use an extensive secretory pathway to traffic a number of proteins within itself and beyond. In higher eukaryotes, Endoplasmic Reticulum (ER) membrane bound transcription factors such as SREBP are reported to get processed en route and migrate to nucleus under the influence of specific cues. However, a protein constitutively trafficked to the nucleus via classical secretory pathway has not been reported. Herein, we report the presence of a novel trafficking pathway in an apicomplexan, Plasmodium falciparum where a homologue of an Origin Recognition Complex 2 (Orc2) goes to the nucleus following its association with the ER. Our work highlights the unconventional role of ER in protein trafficking and reports for the first time an ORC homologue getting trafficked through such a pathway to the nucleus where it may be involved in DNA replication and other ancillary functions. Such trafficking pathways may have a profound impact on the cell biology of a malaria parasite and have significant implications in strategizing new antimalarials.

Let me in: Control of HIV nuclear entry at the nuclear envelope

The nuclear envelope is a physical barrier that isolates the cellular DNA from the rest of the cell, thereby limiting pathogen invasion. The Human Immunodeficiency Virus (HIV) has a remarkable ability to enter the nucleus of non-dividing target cells such as lymphocytes, macrophages and dendritic cells. While this step is critical for replication of the virus, it remains one of the less understood aspects of HIV infection. Here, we review the viral and host factors that favor or inhibit HIV entry into the nucleus, including the viral capsid, integrase, the central viral DNA flap, and the host proteins CPSF6, TNPO3, Nucleoporins, SUN1, SUN2, Cyclophilin A and MX2. We review recent perspectives on the mechanism of action of these factors, and formulate fundamental questions that remain. Overall, these findings deepen our understanding of HIV nuclear import and strengthen the favorable position of nuclear HIV entry for antiviral targeting.

Identification of a novel mutation (Ala66Thr) of SRY gene causes XY pure gonadal dysgenesis by affecting DNA binding activity and nuclear import

Sex-determining region of the Y chromosome (SRY) gene plays a crucial role in male sexual differentiation and development. Several mutations in the SRY gene have been reported in the high mobility group (HMG) box domain and can cause gonadal dysgenesis symptoms. In this study, we report that a novel missense mutation in the SRY gene, a G to A transition within the HMG box, causes the Ala66Thr amino acid substitution in a female patient presenting 46,XY karyotype with pure gonadal dysgenesis. The G to A base transition was not found in the SRY sequence after the screening of 100 normal males. Furthermore, Ala66Thr mutation drastically reduced the binding capacity of SRY to DNA sequences, whereas wild-type SRY protein showed the normal binding capacity to DNA sequences in vitro. We also found that the mutant SRY protein was partly localized in cytoplasm, whereas wild-type SRY protein was strictly localized in cell nucleus. In addition, we analyzed the three-dimensional structure of SRY protein by homology modeling methods. In conclusion, we identified a novel SRY mutation in a 46,XY female patient with pure gonadal dysgenesis, demonstrating the importance of the Ala66Thr mutation in DNA binding activity and nuclear transport.

Gemin4 is an essential gene in mice, and its overexpression in human cells causes relocalization of the SMN complex to the nucleoplasm

Gemin4 is a member of the Survival Motor Neuron (SMN) protein complex, which is responsible for the assembly and maturation of Sm-class small nuclear ribonucleoproteins (snRNPs). In metazoa, Sm snRNPs are assembled in the cytoplasm and subsequently imported into the nucleus. We previously showed that the SMN complex is required for snRNP import in vitro, although it remains unclear which specific components direct this process. Here, we report that Gemin4 overexpression drives SMN and the other Gemin proteins from the cytoplasm into the nucleus. Moreover, it disrupts the subnuclear localization of the Cajal body marker protein, coilin, in a dose-dependent manner. We identified three putative nuclear localization signal (NLS) motifs within Gemin4, one of which is necessary and sufficient to direct nuclear import. Overexpression of Gemin4 constructs lacking this NLS sequestered Gemin3 and, to a lesser extent Gemin2, in the cytoplasm but had little effect on the nuclear accumulation of SMN. We also investigated the effects of Gemin4 depletion in the laboratory mouse, Mus musculus. Gemin4 null mice die early in embryonic development, demonstrating that Gemin4 is an essential mammalian protein. When crossed onto a severe SMA mutant background, heterozygous loss of Gemin4 failed to modify the early postnatal mortality phenotype of SMA type I (Smn^−/−;SMN2^+/+) mice. We conclude that Gemin4 plays an essential role in mammalian snRNP biogenesis, and may facilitate import of the SMN complex (or subunits thereof) into the nucleus.

Summary:Gemin4 loss-of-function is recessive lethal in mice, whereas in cell culture its overexpression results in a dominant, gain-of-function relocalization of SMN and other Gemin proteins to the nucleus.

Inhibitors of the integrase-transportin-SR2 interaction block HIV nuclear import

Background

Combination antiretroviral therapy efficiently suppresses HIV replication in infected patients, transforming HIV/AIDS into a chronic disease. Viral resistance does develop however, especially under suboptimal treatment conditions such as poor adherence. As a consequence, continued exploration of novel targets is paramount to identify novel antivirals that do not suffer from cross-resistance with existing drugs. One new promising class of targets are HIV protein–cofactor interactions. Transportin-SR2 (TRN-SR2) is a β-karyopherin that was recently identified as an HIV-1 cofactor. It has been implicated in nuclear import of the viral pre-integration complex and was confirmed as a direct binding partner of HIV-1 integrase (IN). Nevertheless, consensus on its mechanism of action is yet to be reached.

Results

Here we describe the development and use of an AlphaScreen-based high-throughput screening cascade for small molecule inhibitors of the HIV-1 IN–TRN-SR2 interaction. False positives and nonspecific protein–protein interaction inhibitors were eliminated through different counterscreens. We identified and confirmed 2 active compound series from an initial screen of 25,608 small molecules. These compounds significantly reduced nuclear import of fluorescently labeled HIV particles.

Conclusions

Alphascreen-based high-throughput screening can allow the identification of compounds representing a novel class of HIV inhibitors. These results corroborate the role of the IN–TRN-SR2 interaction in nuclear import. These compounds represent the first in class small molecule inhibitors of HIV-1 nuclear import.

A Homokaryon Assay for Nucleocytoplasmic Shuttling Activity of HBV Core Protein

Hepatitis B virus (HBV) core protein (HBc) can be present in both nucleus and cytoplasm. The arginine-rich domain (ARD) at the cytoplasmic tail of HBc contains both a nuclear localization signal (NLS) and nuclear export signal (NES). We established a homokaryon assay to detect the dynamic trafficking of HBc between nucleus and cytoplasm in hepatocytes. Using immunofluorescence assay (IFA) and PEG-induced cell-cell fusion, we demonstrated that a chimeric reporter protein of SV40 large T antigen, when fused in-frame with HBc ARD, can shuttle from a donor nuclei (green) to the recipient nuclei (red) in the context of binucleated or polynucleated hybrid cells. The shuttling activity driven by HBc ARD can be measured quantitatively by this IFA method.

Prolonged exposure to particulate chromate inhibits RAD51 nuclear import mediator proteins

Particulate hexavalent chromium (Cr(VI)) is a human lung carcinogen and a human health concern. The induction of structural chromosome instability is considered to be a driving mechanism of Cr(VI)-induced carcinogenesis. Homologous recombination repair protects against Cr(VI)-induced chromosome damage, due to its highly accurate repair of Cr(VI)-induced DNA double strand breaks. However, recent studies demonstrate Cr(VI) inhibits homologous recombination repair through the misregulation of RAD51. RAD51 is an essential protein in HR repair that facilitates the search for a homologous sequence. Recent studies show prolonged Cr(VI) exposure prevents proper RAD51 subcellular localization, causing it to accumulate in the cytoplasm. Since nuclear import of RAD51 is crucial to its function, this study investigated the effect of Cr(VI) on the RAD51 nuclear import mediators, RAD51C and BRCA2. We show acute (24h) Cr(VI) exposure induces the proper localization of RAD51C and BRCA2. In contrast, prolonged (120h) exposure increased the cytoplasmic localization of both proteins, although RAD51C localization was more severely impaired. These results correlate temporally with the previously reported Cr(VI)-induced RAD51 cytoplasmic accumulation. In addition, we found Cr(VI) does not inhibit interaction between RAD51 and its nuclear import mediators. Altogether, our results suggest prolonged Cr(VI) exposure inhibits the nuclear import of RAD51C, and to a lesser extent, BRCA2, which results in the cytoplasmic accumulation of RAD51. Cr(VI)-induced inhibition of nuclear import may play a key role in its carcinogenic mechanism since the nuclear import of many tumor suppressor proteins and DNA repair proteins is crucial to their function.

A set of enhanced green fluorescent protein concatemers for quantitative determination of nuclear localization signal strength

Regulated transport of proteins between nucleus and cytoplasm is an important process in the eukaryotic cell. In most cases, active nucleo-cytoplasmic protein transport is mediated by nuclear localization signal (NLS) and/or nuclear export signal (NES) motifs. In this study, we developed a set of vectors expressing enhanced GFP (EGFP) concatemers ranging from 2 to 12 subunits (2xEGFP to 12xEGFP) for analysis of NLS strength. As shown by in gel GFP fluorescence analysis and αGFP Western blotting, EGFP concatemers are expressed as fluorescent full-length proteins in eukaryotic cells. As expected, nuclear localization of concatemeric EGFPs decreases with increasing molecular weight. By oligonucleotide ligation this set of EGFP concatemers can be easily fused to NLS motifs. After determination of intracellular localization of EGFP concatemers alone and fused to different NLS motifs we calculated the size of a hypothetic EGFP concatemer showing a defined distribution of EGFP fluorescence between nucleus and cytoplasm (n/c ratio = 2). Clear differences of the size of the hypothetic EGFP concatemer depending on the fused NLS motif were observed. Therefore, we propose to use the size of this hypothetic concatemer as quantitative indicator for comparing strength of different NLS motifs.

Nuclear Import and Export of the Thyroid Hormone Receptor

The thyroid hormone receptors, TRα1 and TRβ1, are members of the nuclear receptor superfamily that forms one of the most abundant classes of transcription factors in multicellular organisms. Although primarily localized to the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. The fine balance between nuclear import and export of TRs has emerged as a critical control point for modulating thyroid hormone-responsive gene expression. Mutagenesis studies have defined two nuclear localization signal (NLS) motifs that direct nuclear import of TRα1: NLS-1 in the hinge domain and NLS-2 in the N-terminal A/B domain. Three nuclear export signal (NES) motifs reside in the ligand-binding domain. A combined approach of shRNA-mediated knockdown and coimmunoprecipitation assays revealed that nuclear entry of TRα1 is facilitated by importin 7, likely through interactions with NLS-2, and importin β1 and the adapter importin α1 interacting with both NLS-1 and NLS-2. Interestingly, TRβ1 lacks NLS-2 and nuclear import depends solely on the importin α1/β1 heterodimer. Heterokaryon and fluorescence recovery after photobleaching shuttling assays identified multiple exportins that play a role in nuclear export of TRα1, including CRM1 (exportin 1), and exportins 4, 5, and 7. Even single amino acid changes in TRs dramatically alter their intracellular distribution patterns. We conclude that mutations within NLS and NES motifs affect nuclear shuttling activity, and propose that TR mislocalization contributes to the development of some types of cancer and Resistance to Thyroid Hormone syndrome.