The non-classical nuclear import carrier Transportin 1 modulates circadian rhythms through its effect on PER1 nuclear localization

The nuclear transportation of CHRONO regulates the circadian rhythm

The pace of the endogenous circadian clock is important for organisms to maintain homeostasis. CHRONO has been shown to be a core component of the mammalian clock and has recently been implicated to function in several important physiological aspects. To function properly, CHRONO needs to enter the nucleus to repress transcription. We have previously shown that the N terminus of CHRONO is required for its nuclear entry. However, how CHRONO enters the nucleus and regulates the circadian clock remains unknown. Here, we report that a novel nonclassical nuclear localization signal in the N terminus of CHRONO is responsible for its nuclear entry. Multiple nuclear transporters are identified that facilitate the nuclear import of CHRONO. We show that the Arg63 is the critical amino acid of the nuclear localization signal. Using prime editing technology, we precisely edit the Arg63 to Ala at the genomic loci and demonstrate that this mutation prolongs the circadian period, which is similar to knockdown of CHRONO. By using the CHRONO KO and R63A mutant cells, we also investigated the changes in the cytoplasmic/nuclear distribution of BMAL1. We show that BMAL1 localizes more in the cytoplasm in the deficiency of CHRONO nuclear entry. These results provide a model for CHRONO nuclear entry using a network of importins involved in the regulation of the circadian period.

Two Zn2Cys6-type transcription factors respond to aromatic compounds and regulate the expression of laccases in the white-rot fungus Trametes hirsuta

White-rot fungi differentially express laccases when they encounter aromatic compounds. However, the underlying mechanisms are still being explored. Here, proteomics analysis revealed that in addition to increased laccase activity, proteins involved in sphingolipid metabolism and toluene degradation as well as some cytochrome P450s (CYP450s) were differentially expressed and significantly enriched during 48 h of o-toluidine exposure, in Trametes hirsuta AH28-2. Two Zn2Cys6-type transcription factors (TFs), TH8421 and TH4300, were upregulated. Bioinformatics docking and isothermal titration calorimetry assays showed that each of them could bind directly to o-toluidine and another aromatic monomer, guaiacol. Binding to aromatic compounds promoted the formation of TH8421/TH4300 heterodimers. TH8421 and TH4300 silencing in T. hirsuta AH28-2 led to decreased transcriptional levels and activities of LacA and LacB upon o-toluidine and guaiacol exposure. EMSA and ChIP-qPCR analysis further showed that TH8421 and TH4300 bound directly with the promoter regions of lacA and lacB containing CGG or CCG motifs. Furthermore, the two TFs were involved in direct and positive regulation of the transcription of some CYP450s. Together, TH8421 and TH4300, two key regulators found in T. hirsuta AH28-2, function as heterodimers to simultaneously trigger the expression of downstream laccases and intracellular enzymes. Monomeric aromatic compounds act as ligands to promote heterodimer formation and enhance the transcriptional activities of the two TFs.IMPORTANCEWhite-rot fungi differentially express laccase isoenzymes when exposed to aromatic compounds. Clarification of the molecular mechanisms underlying differential laccase expression is essential to elucidate how white-rot fungi respond to the environment. Our study shows that two Zn2Cys6-type transcription factors form heterodimers, interact with the promoters of laccase genes, and positively regulate laccase transcription in Trametes hirsuta AH28-2. Aromatic monomer addition induces faster heterodimer formation and rate of activity. These findings not only identify two new transcription factors involved in fungal laccase transcription but also deepen our understanding of the mechanisms underlying the response to aromatics exposure in white-rot fungi.

Environmental circadian disruption re-writes liver circadian proteomes

Circadian gene expression is fundamental to the establishment and functions of the circadian clock, a cell-autonomous and evolutionary-conserved timing system. Yet, how it is affected by environmental-circadian disruption (ECD) such as shiftwork and jetlag are ill-defined. Here, we provided a comprehensive and comparative description of male liver circadian gene expression, encompassing transcriptomes, whole-cell proteomes and nuclear proteomes, under normal and after ECD conditions. Under both conditions, post-translation, rather than transcription, is the dominant contributor to circadian functional outputs. After ECD, post-transcriptional and post-translational processes are the major contributors to whole-cell or nuclear circadian proteome, respectively. Furthermore, ECD re-writes the rhythmicity of 64% transcriptome, 98% whole-cell proteome and 95% nuclear proteome. The re-writing, which is associated with changes of circadian regulatory cis-elements, RNA-processing and protein localization, diminishes circadian regulation of fat and carbohydrate metabolism and persists after one week of ECD-recovery.

Seryl-tRNA Synthetase Shows a Noncanonical Activity of Upregulating Laccase Transcription in Trametes hirsuta AH28-2 Exposed to Copper Ion

The function of Seryl-tRNA synthetase in fungi during gene transcription regulation beyond translation has not been reported. Here, we report a seryl-tRNA synthetase, ThserRS, which can negatively regulate laccase lacA transcription in Trametes hirsuta AH28-2 under exposure to copper ion. ThserRS was obtained through yeast one-hybrid screening using a bait sequence of lacA promoter (-502 to -372 bp). ThserRS decreased while lacA increased at the transcription level in T. hirsuta AH28-2 in the first 36 h upon CuSO4 induction. Then, ThserRS was upregulated, and lacA was downregulated. ThserRS overexpression in T. hirsuta AH28-2 resulted in a decrement in lacA transcription and LacA activity. By comparison, ThserRS silencing led to increased LacA transcripts and activity. A minimum of a 32-bp DNA fragment containing two putative xenobiotic response elements could interact with ThserRS, with a dissociation constant of 919.9 nM. ThserRS localized in the cell cytoplasm and nucleus in T. hirsuta AH28-2 and was heterologously expressed in yeast. ThserRS overexpression also enhanced mycelial growth and oxidative stress resistance. The transcriptional level of several intracellular antioxidative enzymes in T. hirsuta AH28-2 was upregulated. Our results demonstrate a noncanonical activity of SerRS that acts as a transcriptional regulation factor to upregulate laccase expression at an early stage after exposure to copper ions. IMPORTANCE Seryl-tRNA synthetase is well known for the attachment of serine to the corresponding cognate tRNA during protein translation. In contrast, its functions beyond translation in microorganisms are underexplored. We performed in vitro and cell experiments to show that the seryl-tRNA synthetase in fungi with no UNE-S domain at the carboxyl terminus can enter the nucleus, directly interact with the promoter of the laccase gene, and negatively regulate the fungal laccase transcription early upon copper ion induction. Our study deepens our understanding of the Seryl-tRNA synthetase noncanonical activities in microorganisms. It also demonstrates a new transcription factor for fungal laccase transcription.

Step in Time: Conservation of Circadian Clock Genes in Animal Evolution

Over the past few decades, the molecular mechanisms responsible for circadian phenotypes of animals have been studied in increasing detail in mammals, some insects, and other invertebrates. Particular circadian proteins and their interactions are shared across evolutionary distant animals, resulting in a hypothesis for the canonical circadian clock of animals. As the number of species for which the circadian clockwork has been described increases, the circadian clock in animals driving cyclical phenotypes becomes less similar. Our focus in this review is to develop and synthesize the current literature to better understand the antiquity and evolution of the animal circadian clockwork. Here, we provide an updated understanding of circadian clock evolution in animals, largely through the lens of conserved genes characterized in the circadian clock identified in bilaterian species. These comparisons reveal extensive variation within the likely composition of the core clock mechanism, including losses of many genes, and that the ancestral clock of animals does not equate to the bilaterian clock. Despite the loss of these core genes, these species retain circadian behaviors and physiology, suggesting novel clocks have evolved repeatedly. Additionally, we highlight highly conserved cellular processes (e.g., cell division, nutrition) that intersect with the circadian clock of some animals. The conservation of these processes throughout the animal tree remains essentially unknown, but understanding their role in the evolution and maintenance of the circadian clock will provide important areas for future study.

Potential Role of the Circadian Clock in the Regulation of Cancer Stem Cells and Cancer Therapy

Circadian rhythms, including sleep/wake cycles as well as hormonal, immune, metabolic, and cell proliferation rhythms, are fundamental biological processes driven by a cellular time-keeping system called the circadian clock. Disruptions in these rhythms due to genetic alterations or irregular lifestyles cause fundamental changes in physiology, from metabolism to cellular proliferation and differentiation, resulting in pathological consequences including cancer. Cancer cells are not uniform and static but exist as different subtypes with phenotypic and functional differences in the tumor microenvironment. At the top of the heterogeneous tumor cell hierarchy, cancer stem cells (CSCs), a self-renewing and multi-potent cancer cell type, are most responsible for tumor recurrence and metastasis, chemoresistance, and mortality. Phenotypically, CSCs are associated with the epithelial-mesenchymal transition (EMT), which confers cancer cells with increased motility and invasion ability that is characteristic of malignant and drug-resistant stem cells. Recently, emerging studies of different cancer types, such as glioblastoma, leukemia, prostate cancer, and breast cancer, suggest that the circadian clock plays an important role in the maintenance of CSC/EMT characteristics. In this review, we describe recent discoveries regarding how tumor intrinsic and extrinsic circadian clock-regulating factors affect CSC evolution, highlighting the possibility of developing novel chronotherapeutic strategies that could be used against CSCs to fight cancer.

Cryptochrome proteins regulate the circadian intracellular behavior and localization of PER2 in mouse suprachiasmatic nucleus neurons

The suprachiasmatic nucleus (SCN), the master circadian clock of the mammalian brain, coordinates cellular clocks across the organism to regulate daily rhythms of physiology and behavior. SCN timekeeping pivots around transcriptional/translational feedback loops whereby PERIOD (PER) and CRYPTOCHROME (CRY) proteins associate and enter the nucleus to inhibit their own expression. The individual and interactive behaviors of PER and CRY and the mechanisms that regulate them are poorly understood. We combined fluorescence imaging of endogenous PER2 and viral vector–expressed CRY in SCN slices and show how CRYs, acting via their C terminus, control nuclear localization and mobility of PER2 to dose-dependently initiate SCN timekeeping and control its period. Our results reveal PER and CRY interactions central to the SCN clockwork.

The ∼20,000 cells of the suprachiasmatic nucleus (SCN), the master circadian clock of the mammalian brain, coordinate subordinate cellular clocks across the organism, driving adaptive daily rhythms of physiology and behavior. The canonical model for SCN timekeeping pivots around transcriptional/translational feedback loops (TTFL) whereby PERIOD (PER) and CRYPTOCHROME (CRY) clock proteins associate and translocate to the nucleus to inhibit their own expression. The fundamental individual and interactive behaviors of PER and CRY in the SCN cellular environment and the mechanisms that regulate them are poorly understood. We therefore used confocal imaging to explore the behavior of endogenous PER2 in the SCN of PER2::Venus reporter mice, transduced with viral vectors expressing various forms of CRY1 and CRY2. In contrast to nuclear localization in wild-type SCN, in the absence of CRY proteins, PER2 was predominantly cytoplasmic and more mobile, as measured by fluorescence recovery after photobleaching. Virally expressed CRY1 or CRY2 relocalized PER2 to the nucleus, initiated SCN circadian rhythms, and determined their period. We used translational switching to control CRY1 cellular abundance and found that low levels of CRY1 resulted in minimal relocalization of PER2, but yet, remarkably, were sufficient to initiate and maintain circadian rhythmicity. Importantly, the C-terminal tail was necessary for CRY1 to localize PER2 to the nucleus and to initiate SCN rhythms. In CRY1-null SCN, CRY1Δtail opposed PER2 nuclear localization and correspondingly shortened SCN period. Through manipulation of CRY proteins, we have obtained insights into the spatiotemporal behaviors of PER and CRY sitting at the heart of the TTFL molecular mechanism.

In Vitro Assays for Measuring Intercellular Coupling Among Peripheral Circadian Oscillators

Circadian clocks can be found in nearly all eukaryotic organisms, as well as certain bacterial strains, including commensal microbiota. Exploring intercellular coupling among cell-autonomous circadian oscillators is crucial for understanding how cellular ensembles generate and sustain coherent circadian rhythms on the tissue level, and thus, rhythmic organ functions. Here we describe a protocol for studying intercellular coupling among peripheral circadian oscillators using three-dimensional spheroid cultures in order to measure coupling strength within peripheral clock networks. We use cell spheroids to simulate in vivo tissue integrity, as well as to increase complexity of cell-cell interactions and the abundance of potential coupling factors. Circadian rhythms are monitored using live-cell imaging of spheroids equipped with circadian reporters over several days.

Multi-Modal Regulation of Circadian Physiology by Interactive Features of Biological Clocks

The circadian clock is a fundamental biological timing mechanism that generates nearly 24 h rhythms of physiology and behaviors, including sleep/wake cycles, hormone secretion, and metabolism. Evolutionarily, the endogenous clock is thought to confer living organisms, including humans, with survival benefits by adapting internal rhythms to the day and night cycles of the local environment. Mirroring the evolutionary fitness bestowed by the circadian clock, daily mismatches between the internal body clock and environmental cycles, such as irregular work (e.g., night shift work) and life schedules (e.g., jet lag, mistimed eating), have been recognized to increase the risk of cardiac, metabolic, and neurological diseases. Moreover, increasing numbers of studies with cellular and animal models have detected the presence of functional circadian oscillators at multiple levels, ranging from individual neurons and fibroblasts to brain and peripheral organs. These oscillators are tightly coupled to timely modulate cellular and bodily responses to physiological and metabolic cues. In this review, we will discuss the roles of central and peripheral clocks in physiology and diseases, highlighting the dynamic regulatory interactions between circadian timing systems and multiple metabolic factors.

Circadian Rhythms, Disease and Chronotherapy

Circadian clocks are biological timing mechanisms that generate 24-h rhythms of physiology and behavior, exemplified by cycles of sleep/wake, hormone release, and metabolism. The adaptive value of clocks is evident when internal body clocks and daily environmental cycles are mismatched, such as in the case of shift work and jet lag or even mistimed eating, all of which are associated with physiological disruption and disease. Studies with animal and human models have also unraveled an important role of functional circadian clocks in modulating cellular and organismal responses to physiological cues (ex., food intake, exercise), pathological insults (e.g. virus and parasite infections), and medical interventions (e.g. medication). With growing knowledge of the molecular and cellular mechanisms underlying circadian physiology and pathophysiology, it is becoming possible to target circadian rhythms for disease prevention and treatment. In this review, we discuss recent advances in circadian research and the potential for therapeutic applications that take patient circadian rhythms into account in treating disease.

Circadian Organelles: Rhythms at All Scales

Circadian clocks have evolved in most light-sensitive organisms, from unicellular organisms to mammals. Consequently, a myriad of biological functions exhibits circadian rhythmicity, from behavior to physiology, through tissue and cellular functions to subcellular processes. Circadian rhythms in intracellular organelles are an emerging and exciting research arena. We summarize herein the current literature for rhythmicity in major intracellular organelles in mammals. These include changes in the morphology, content, and functions of different intracellular organelles. While these data highlight the presence of rhythmicity in these organelles, a gap remains in our knowledge regarding the underlying molecular mechanisms and their functional significance. Finally, we discuss the importance and challenges faced by spatio-temporal studies on these organelles and speculate on the presence of oscillators in organelles and their potential mode of communication. As circadian biology has been and continues to be studied throughout temporal and spatial axes, circadian organelles appear to be the next frontier.

TNPO2 variants associate with human developmental delays, neurologic deficits, and dysmorphic features and alter TNPO2 activity in Drosophila

Transportin-2 (TNPO2) mediates multiple pathways including non-classical nucleocytoplasmic shuttling of >60 cargoes, such as developmental and neuronal proteins. We identified 15 individuals carrying de novo coding variants in TNPO2 who presented with global developmental delay (GDD), dysmorphic features, ophthalmologic abnormalities, and neurological features. To assess the nature of these variants, functional studies were performed in Drosophila. We found that fly dTnpo (orthologous to TNPO2) is expressed in a subset of neurons. dTnpo is critical for neuronal maintenance and function as downregulating dTnpo in mature neurons using RNAi disrupts neuronal activity and survival. Altering the activity and expression of dTnpo using mutant alleles or RNAi causes developmental defects, including eye and wing deformities and lethality. These effects are dosage dependent as more severe phenotypes are associated with stronger dTnpo loss. Interestingly, similar phenotypes are observed with dTnpo upregulation and ectopic expression of TNPO2, showing that loss and gain of Transportin activity causes developmental defects. Further, proband-associated variants can cause more or less severe developmental abnormalities compared to wild-type TNPO2 when ectopically expressed. The impact of the variants tested seems to correlate with their position within the protein. Specifically, those that fall within the RAN binding domain cause more severe toxicity and those in the acidic loop are less toxic. Variants within the cargo binding domain show tissue-dependent effects. In summary, dTnpo is an essential gene in flies during development and in neurons. Further, proband-associated de novo variants within TNPO2 disrupt the function of the encoded protein. Hence, TNPO2 variants are causative for neurodevelopmental abnormalities.

Intercellular coupling between peripheral circadian oscillators by TGF-β signaling

Coupling between cell-autonomous circadian oscillators is crucial to prevent desynchronization of cellular networks and disruption of circadian tissue functions. While neuronal oscillators within the mammalian central clock, the suprachiasmatic nucleus, couple intercellularly, coupling among peripheral oscillators is controversial and the molecular mechanisms are unknown. Using two- and three-dimensional mammalian culture models in vitro (mainly human U-2 OS cells) and ex vivo, we show that peripheral oscillators couple via paracrine pathways. We identify transforming growth factor-β (TGF-β) as peripheral coupling factor that mediates paracrine phase adjustment of molecular clocks through transcriptional regulation of core-clock genes. Disruption of TGF-β signaling causes desynchronization of oscillator networks resulting in reduced amplitude and increased sensitivity toward external zeitgebers. Our findings reveal an unknown mechanism for peripheral clock synchrony with implications for rhythmic organ functions and circadian health.

Post-Translational Mechanisms of Plant Circadian Regulation

The molecular components of the circadian system possess the interesting feature of acting together to create a self-sustaining oscillator, while at the same time acting individually, and in complexes, to confer phase-specific circadian control over a wide range of physiological and developmental outputs. This means that many circadian oscillator proteins are simultaneously also part of the circadian output pathway. Most studies have focused on transcriptional control of circadian rhythms, but work in plants and metazoans has shown the importance of post-transcriptional and post-translational processes within the circadian system. Here we highlight recent work describing post-translational mechanisms that impact both the function of the oscillator and the clock-controlled outputs.

Transportin-1: A Nuclear Import Receptor with Moonlighting Functions

Transportin-1 (Trn1), also known as karyopherin-β2 (Kapβ2), is probably the best-characterized nuclear import receptor of the karyopherin-β family after Importin-β, but certain aspects of its functions in cells are still puzzling or are just recently emerging. Since the initial identification of Trn1 as the nuclear import receptor of hnRNP A1 ∼25 years ago, several molecular and structural studies have unveiled and refined our understanding of Trn1-mediated nuclear import. In particular, the understanding at a molecular level of the NLS recognition by Trn1 made a decisive step forward with the identification of a new class of NLSs called PY-NLSs, which constitute the best-characterized substrates of Trn1. Besides PY-NLSs, many Trn1 cargoes harbour NLSs that do not resemble the archetypical PY-NLS, which complicates the global understanding of cargo recognition by Trn1. Although PY-NLS recognition is well established and supported by several structures, the recognition of non-PY-NLSs by Trn1 is far less understood, but recent reports have started to shed light on the recognition of this type of NLSs. Aside from its principal and long-established activity as a nuclear import receptor, Trn1 was shown more recently to moonlight outside nuclear import. Trn1 has for instance been caught in participating in virus uncoating, ciliary transport and in modulating the phase separation properties of aggregation-prone proteins. Here, we focus on the structural and functional aspects of Trn1-mediated nuclear import, as well as on the moonlighting activities of Trn1.

iTRAQ-based comparative proteomic analysis reveals high temperature accelerated leaf senescence of tobacco (Nicotiana tabacum L.) during flue-curing

Tobacco (Nicotiana tabacum) is extensively cultivated all over the world for its economic value. During curing and storage, senescence occurs, which is associated with physiological and biochemical changes in postharvest plant organs. However, the molecular mechanisms involved in accelerated senescence due to high temperatures in tobacco leaves during curing need further elaboration. We studied molecular mechanisms of senescence in tobacco leaves exposed to high temperature during curing (Fresh, 38 °C and 42 °C), revealed by isobaric tags for relative and absolute quantification (iTRAQ) for the proteomic profiles of cultivar Bi'na1. In total, 8903 proteins were identified, and 2034 (1150 up-regulated and 1074 down-regulated) differentially abundant proteins (DAPs) were obtained from tobacco leaf samples. These DAPs were mainly involved in posttranslational modification, protein turnover, energy production and conversion. Sugar- and energy-related metabolic biological processes and pathways might be critical regulators of tobacco leaves exposed to high temperature during senescence. High-temperature stress accelerated tobacco leaf senescence mainly by down-regulating photosynthesis-related pathways and degrading cellular constituents to maintain cell viability and nutrient recycling. Our findings provide a valuable inventory of novel proteins involved in senescence physiology and elucidate the protein regulatory network in postharvest organs exposed to high temperatures during flue-curing.

Circadian Clocks Make Metabolism Run

Most organisms adapt to the 24-h cycle of the Earth's rotation by anticipating the time of the day through light-dark cycles. The internal time-keeping system of the circadian clocks has been developed to ensure this anticipation. The circadian system governs the rhythmicity of nearly all physiological and behavioral processes in mammals. In this review, we summarize current knowledge stemming from rodent and human studies on the tight interconnection between the circadian system and metabolism in the body. In particular, we highlight recent advances emphasizing the roles of the peripheral clocks located in the metabolic organs in regulating glucose, lipid, and protein homeostasis at the organismal and cellular levels. Experimental disruption of circadian system in rodents is associated with various metabolic disturbance phenotypes. Similarly, perturbation of the clockwork in humans is linked to the development of metabolic diseases. We discuss recent studies that reveal roles of the circadian system in the temporal coordination of metabolism under physiological conditions and in the development of human pathologies.

The NRON complex controls circadian clock function through regulated PER and CRY nuclear translocation

Post-translational regulation plays a central role in the circadian clock mechanism. However, nucleocytoplasmic translocation of core clock proteins, a key step in circadian timekeeping, is not fully understood. Earlier we found that the NRON scaffolding complex regulates nuclear translocation of NFAT and its signaling. Here, we show that components of the NRON complex also regulate the circadian clock. In peripheral cell clock models, genetic perturbation of the NRON complex affects PER and CRY protein nuclear translocation, dampens amplitude, and alters period length. Further, we show small molecules targeting the NFAT pathway alter nuclear translocation of PER and CRY proteins and impact circadian rhythms in peripheral cells and tissue explants of the master clock in the suprachiasmatic nucleus. Taken together, these studies highlight a key role for the NRON complex in regulating PER/CRY subcellular localization and circadian timekeeping.

The Circadian Gene Per1 Plays an Important Role in Radiation-Induced Apoptosis and DNA Damage in Glioma

Objective:

Period1 (PER1), a core circadian gene, not only modulates circadian rhythm but may also play an important role in other biological processes, including pathways involved in the proliferation and apoptosis of tumor cells. In this study, we investigated the mechanism by which the downregulated expression of PER1 promotes the apoptosis of wild-type P53 human glioma U343 cells exposed to X-rays.

Methods:

U343 cells were exposed to 6 mV 10 Gy X-ray irradiation after infection with an shRNA lentivirus to reduce the expression of PER1 and were analyzed by SCGE analysis, flow cytometry, qRT-PCR, and western blotting.

Result:

SCGE analysis revealed that compared with the controls, U343 cells expressing low levels of PER1 showed minor DNA damage when exposed to X-ray irradiation (P<0.05), and the flow cytometry assay showed lower death rates (P<0.05). RT-PCR and western blot analysis both revealed decreased expression of CHK2 and P53, which regulate DNA damage and repair via the CHK2-P53 pathway, and decreased expression of C-MYC, which is related to cell apoptosis.

Conclusion:

Our research suggests that PER1 may play an important role in tumor radiotherapy, which is attributable to enhanced chk2-P53 signaling and proapoptotic processes. These findings provide a new target for the clinical treatment of glioma and a reliable basis for postradiation therapy and gene therapy for glioma and other cancers.

Nuclear import pathway key to rescuing dominant progerin phenotypes

In this issue of Science Signaling, Larrieu et al show that an acetyltransferase inhibitor that rescues many dominant nuclear phenotypes caused by progerin, a truncated form of lamin A, does so by releasing the specialized nuclear import receptor TNPO1 from sequestration by microtubules. This release enables TNPO1-dependent import of specific cargoes, including the nuclear pore protein Nup153 and the heterogeneous nuclear ribonucleoprotein hnRNPA1, thus restoring the functionality of nuclear pore complexes, rebalancing the nucleocytoplasmic Ran gradient, and normalizing gene expression.