Ran-binding protein 3 is a cofactor for Crm1-mediated nuclear protein export

Oxidative stress and signaling through EGFR and PKA pathways converge on the nuclear transport factor RanBP1

Nuclear protein trafficking requires the soluble transport factor RanBP1. The subcellular distribution of RanBP1 is dynamic, as the protein shuttles between the nucleus and cytoplasm. To date, the signaling pathways regulating RanBP1 subcellular localization are poorly understood. During interphase, RanBP1 resides mostly in the cytoplasm. We show here that oxidative stress concentrates RanBP1 in the nucleus, and our study defines the underlying mechanisms. Specifically, RanBP1's cysteine residues are not essential for its oxidant-induced relocation. Furthermore, our pharmacological approaches uncover that signaling mediated by epidermal growth factor receptor (EGFR) and protein kinase A (PKA) control RanBP1 localization during stress. In particular, pharmacological inhibitors of EGFR or PKA diminish the oxidant-dependent relocation of RanBP1. Mutant analysis identified serine 60 and tyrosine 103 as regulators of RanBP1 nuclear accumulation during oxidant exposure. Taken together, our results define RanBP1 as a target of oxidative stress and a downstream effector of EGFR and PKA signaling routes. This positions RanBP1 at the intersection of important cellular signaling circuits.

Nuclear transport proteins: structure, function, and disease relevance

Proper subcellular localization is crucial for the functioning of biomacromolecules, including proteins and RNAs. Nuclear transport is a fundamental cellular process that regulates the localization of many macromolecules within the nuclear or cytoplasmic compartments. In humans, approximately 60 proteins are involved in nuclear transport, including nucleoporins that form membrane-embedded nuclear pore complexes, karyopherins that transport cargoes through these complexes, and Ran system proteins that ensure directed and rapid transport. Many of these nuclear transport proteins play additional and essential roles in mitosis, biomolecular condensation, and gene transcription. Dysregulation of nuclear transport is linked to major human diseases such as cancer, neurodegenerative diseases, and viral infections. Selinexor (KPT-330), an inhibitor targeting the nuclear export factor XPO1 (also known as CRM1), was approved in 2019 to treat two types of blood cancers, and dozens of clinical trials of are ongoing. This review summarizes approximately three decades of research data in this field but focuses on the structure and function of individual nuclear transport proteins from recent studies, providing a cutting-edge and holistic view on the role of nuclear transport proteins in health and disease. In-depth knowledge of this rapidly evolving field has the potential to bring new insights into fundamental biology, pathogenic mechanisms, and therapeutic approaches.

Sphingosine-1-phosphate related signalling pathways manipulating virus replication

Viruses can create a unique cellular environment that facilitates replication and transmission. Sphingosine kinases (SphKs) produce sphingosine-1-phosphate (S1P), a bioactive sphingolipid molecule that performs both physiological and pathological effects primarily by activating a subgroup of the endothelial differentiation gene family of G-protein coupled cell surface receptors known as S1P receptors (S1PR1-5). A growing body of evidence indicates that the SphK/S1P axis is crucial for regulating cellular activities in virus infections like respiratory viruses, enteroviruses, hepatitis viruses, herpes viruses, and arboviruses replicate. Depending on the type of virus, pro- or anti-viral activities of the SphK/S1P axis sometimes rely on the host immune system and sometimes directly through intracellular signalling pathways or cell proliferation. Recent research has shown novel roles of S1P and SphK in viral replication. Sphingosine kinase isoforms (SphK1 and SphK2) levels can be manipulated by several viruses to promote the effects that are expected. Regulation of cellular signalling pathways plays a significant role in the mechanism. The purpose of this review is to provide insight of the characters played by the SphK/S1P axis throughout diverse viral infection processes. We then assess potential therapeutic methods that are based on S1P signalling and metabolism during viral infections.

How SARS-CoV-2 and Other Viruses Build an Invasion Route to Hijack the Host Nucleocytoplasmic Trafficking System

The host nucleocytoplasmic trafficking system is often hijacked by viruses to accomplish their replication and to suppress the host immune response. Viruses encode many factors that interact with the host nuclear transport receptors (NTRs) and the nucleoporins of the nuclear pore complex (NPC) to access the host nucleus. In this review, we discuss the viral factors and the host factors involved in the nuclear import and export of viral components. As nucleocytoplasmic shuttling is vital for the replication of many viruses, we also review several drugs that target the host nuclear transport machinery and discuss their feasibility for use in antiviral treatment.

Nucleocytoplasmic Transport: Regulatory Mechanisms and the Implications in Neurodegeneration

Nucleocytoplasmic transport (NCT) across the nuclear envelope is precisely regulated in eukaryotic cells, and it plays critical roles in maintenance of cellular homeostasis. Accumulating evidence has demonstrated that dysregulations of NCT are implicated in aging and age-related neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Alzheimer's disease (AD), and Huntington disease (HD). This is an emerging research field. The molecular mechanisms underlying impaired NCT and the pathogenesis leading to neurodegeneration are not clear. In this review, we comprehensively described the components of NCT machinery, including nuclear envelope (NE), nuclear pore complex (NPC), importins and exportins, RanGTPase and its regulators, and the regulatory mechanisms of nuclear transport of both protein and transcript cargos. Additionally, we discussed the possible molecular mechanisms of impaired NCT underlying aging and neurodegenerative diseases, such as ALS/FTD, HD, and AD.

The Role of Protein Disorder in Nuclear Transport and in Its Subversion by Viruses

The transport of host proteins into and out of the nucleus is key to host function. However, nuclear transport is restricted by nuclear pores that perforate the nuclear envelope. Protein intrinsic disorder is an inherent feature of this selective transport barrier and is also a feature of the nuclear transport receptors that facilitate the active nuclear transport of cargo, and the nuclear transport signals on the cargo itself. Furthermore, intrinsic disorder is an inherent feature of viral proteins and viral strategies to disrupt host nucleocytoplasmic transport to benefit their replication. In this review, we highlight the role that intrinsic disorder plays in the nuclear transport of host and viral proteins. We also describe viral subversion mechanisms of the host nuclear transport machinery in which intrinsic disorder is a feature. Finally, we discuss nuclear import and export as therapeutic targets for viral infectious disease.

The hippo kinase STK38 ensures functionality of XPO1

The proper segregation of basic elements such as the compartmentalization of the genome and the shuttling of macromolecules between the nucleus and the cytoplasm is a crucial mechanism for homeostasis maintenance in eukaryotic cells. XPO1 (Exportin 1) is the major nuclear export receptor and is required for the export of proteins and RNAs out of the nucleus. STK38 (also known as NDR1) is a Hippo pathway serine/threonine kinase with multifarious functions in normal and cancer cells. In this review, we summarize the history of the discovery of the nucleo/cytoplasmic shuttling of proteins and focus on the major actor of nuclear export: XPO1. After describing the molecular events required for XPO1-mediated nuclear export of proteins, we introduce the Hippo pathway STK38 kinase, synthetize its regulation mechanisms as well as its biological functions in both normal and cancer cells, and finally its intersection with XPO1 biology. We discuss the recently identified mechanism of XPO1 activation by phosphorylation of XPO1_S1055 by STK38 and contextualize this finding according to the biological functions previously reported for both XPO1 and STK38, including the second identity of STK38 as an autophagy regulator. Finally, we phrase this newly identified activation mechanism into the general nuclear export machinery and examine the possible outcomes of nuclear export inhibition in cancer treatment.

Cotargeting of XPO1 Enhances the Antileukemic Activity of Midostaurin and Gilteritinib in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a hematopoietic stem-cell-derived leukemia with often successive derived driver mutations. Late onset acquisition of internal tandem duplication in FLT3 (FLT3-ITD) at a high variant allele frequency often contributes to full transformation to a highly proliferative, rapidly progressive disease with poor outcome. The FLT3-ITD mutation is targetable with approved FLT3 small molecule inhibitors, including midostaurin and gilteritinib. However, outside of patients receiving allogeneic transplant, most patients fail to respond or relapse, suggesting alternative approaches of therapy will be required. We employed genome-wide pooled CRISPR knockout screening as a method for large-scale identification of targets whose knockout produces a phenotypic effect that enhances the antitumor properties of FLT3 inhibitors. Among the candidate targets we identified the effect of XPO1 knockout to be synergistic with midostaurin treatment. Next, we validated the genetic finding with pharmacologic combination of the slowly reversible XPO1 inhibitor selinexor with midostaurin and gilteritinib in FLT3-ITD AML cell lines and primary patient samples. Lastly, we demonstrated improved survival with either combination therapy compared to its monotherapy components in an aggressive AML murine model, supporting further evaluation and rapid clinical translation of this combination strategy.

Ran-binding protein 3 is associated with human spermatogenesis and male infertility

Ran-binding protein 3 (RanBP3) is a Ran-interacting protein, which participates in the Ran GTPase system in cancer cell biology. However, the expression pattern and physiological role of RanBP3 remain largely unknown. In this study, we found that RanBP3 was expressed in human testes and localised to spermatogonium and spermatocyte of germ cells. In subcellular structure, its localisation is in the nucleus and cytoplasm. Interestingly, compared with normal groups, RanBP3 expression was lower in groups of patients with Maturation Arrest (MA) and Sertoli cell-only syndrome (SCO) when considered by the Johnson Score. RanBP3 expression in the MA group and SCO groups was dramatically lower than that in the normal control group. Studies have shown that RanBP3, which is one of the helper factors of Ran, is mainly participate in the nucleocytoplasmic transport of cells. RanBP3 helps Ran to achieve some functions such as nucleocytoplasmic transport, spindle assembly during mitosis and nuclear assembly after mitosis. Consequent changes in the expression of RanBP3 may associate with human spermatogenesis disorders and male infertility. The identification and characterisation of RanBP3 enhances our understanding of the molecular mechanisms underpinning its function in human spermatogenesis and male infertility.

Intracellular Ca2+ Homeostasis and Nuclear Export Mediate Exit from Naive Pluripotency

Progression through states of pluripotency is required for cells in early mammalian embryos to transition away from heightened self-renewal and toward competency for lineage specification. Here, we use a CRISPR mutagenesis screen in mouse embryonic stem cells (ESCs) to identify unexpected roles for nuclear export and intracellular Ca2+ homeostasis during the exit out of the naive state of pluripotency. Mutation of a plasma membrane Ca2+ pump encoded by Atp2b1 increased intracellular Ca2+ such that it overcame effects of intracellular Ca2+ reduction, which is required for naive exit. Persistent self-renewal of ESCs was supported both in Atp2b1-/-Tcf7l1-/- double-knockout ESCs passaged in defined media alone (no LIF or inhibitors) and in wild-type cells passaged in media containing only calcitonin and a GSK3 inhibitor. These new findings suggest a central role for intracellular Ca2+ in safeguarding naive pluripotency.

The coming-of-age of nucleocytoplasmic transport in motor neuron disease and neurodegeneration

The nuclear pore is the gatekeeper of nucleocytoplasmic transport and signaling through which a vast flux of information is continuously exchanged between the nuclear and cytoplasmic compartments to maintain cellular homeostasis. A unifying and organizing principle has recently emerged that cements the notion that several forms of amyotrophic lateral sclerosis (ALS), and growing number of other neurodegenerative diseases, co-opt the dysregulation of nucleocytoplasmic transport and that this impairment is a pathogenic driver of neurodegeneration. The understanding of shared pathomechanisms that underpin neurodegenerative diseases with impairments in nucleocytoplasmic transport and how these interface with current concepts of nucleocytoplasmic transport is bound to illuminate this fundamental biological process in a yet more physiological context. Here, I summarize unresolved questions and evidence and extend basic and critical concepts and challenges of nucleocytoplasmic transport and its role in the pathogenesis of neurodegenerative diseases, such as ALS. These principles will help to appreciate the roles of nucleocytoplasmic transport in the pathogenesis of ALS and other neurodegenerative diseases, and generate a framework for new ideas of the susceptibility of motoneurons, and possibly other neurons, to degeneration by dysregulation of nucleocytoplasmic transport.

Alterations of the nuclear transport system in hepatocellular carcinoma - New basis for therapeutic strategies

Hepatocellular carcinoma (HCC) is among the most prevalent human malignancies worldwide with rising incidence in industrialised countries, few therapeutic options and poor prognosis. To expand and improve therapeutic strategies, identification of drug targets involved in several liver cancer-related pathways is crucial. Virtually all signal transduction cascades cross the nuclear envelope and therefore require components of the nuclear transport system (NTS), including nuclear transport receptors (e.g. importins and exportins) and the nuclear pore complex. Accordingly, members of the NTS represent promising targets for therapeutic intervention. Selective inhibitors of nuclear export have already entered clinical trials for various malignancies. Herein, we review the current knowledge regarding alterations of the NTS and their potential for targeted therapy in HCC.

Crystal structure of the Xpo1p nuclear export complex bound to the SxFG/PxFG repeats of the nucleoporin Nup42p

Xpo1p (yeast CRM1) is the major nuclear export receptor that carries a plethora of proteins and ribonucleoproteins from the nucleus to cytoplasm. The passage of the Xpo1p nuclear export complex through nuclear pore complexes (NPCs) is facilitated by interactions with nucleoporins (Nups) containing extensive repeats of phenylalanine-glycine (so-called FG repeats), although the precise role of each Nup in the nuclear export reaction remains incompletely understood. Here we report structural and biochemical characterization of the interactions between the Xpo1p nuclear export complex and the FG repeats of Nup42p, a nucleoporin localized at the cytoplasmic face of yeast NPCs and has characteristic SxFG/PxFG sequence repeat motif. The crystal structure of Xpo1p-PKI-Nup42p-Gsp1p-GTP complex identified three binding sites for the SxFG/PxFG repeats on HEAT repeats 14-20 of Xpo1p. Mutational analyses of Nup42p showed that the conserved serines and prolines in the SxFG/PxFG repeats contribute to Xpo1p-Nup42p binding. Our structural and biochemical data suggest that SxFG/PxFG-Nups such as Nup42p and Nup159p at the cytoplasmic face of NPCs provide high-affinity docking sites for the Xpo1p nuclear export complex in the terminal stage of NPC passage and that subsequent disassembly of the nuclear export complex facilitates recycling of free Xpo1p back to the nucleus.

Leptomycin B alters the subcellular distribution of CRM1 (Exportin 1)

CRM1 (chromosome maintenance region 1, Exportin 1) binds to nuclear export signals and is required for nucleocytoplasmic transport of a large variety of proteins and RNP complexes. Leptomycin B (LMB), the first specific inhibitor of CRM1 identified, binds covalently to cysteine 528 in the nuclear export signal binding region of CRM1 leading to the inhibition of protein nuclear export. Although the biochemical mechanisms of action of CRM1 inhibitors such as LMB are well studied, the subcellular effects of inhibition on CRM1 are unknown. We have found that LMB causes CRM1 to redistribute from the nucleus to the cytoplasm in A549 cells. A significant decrease in nuclear CRM1 coupled with an increase in cytoplasmic CRM1 was sustained for up to 4 h, while there was no change in total CRM1 protein in fractionated cells. Cells expressing an LMB insensitive HA-tagged CRM1-C528S protein were unaffected by LMB treatment, whereas HA-tagged wildtype CRM1 redistributed from the nucleus to the cytoplasm with LMB treatment, similar to endogenous CRM1. GFP-tagged CRM1 protein microinjected into the cytoplasm of A549 cells distributed throughout the cell in untreated cells remained primarily cytoplasmic in LMB-treated cells. Upon nuclear microinjection, GFP-CRM1 translocated to and accumulated in the cytoplasm of LMB-treated cells. Thus, LMB binds to CRM1 and causes its redistribution to the cytoplasm by inhibiting its nuclear import. Decreasing the nuclear availability of CRM1 likely contributes to the accumulation of CRM1 cargo proteins in the nucleus, suggesting a new mechanism of action for LMB.

Samp1, a RanGTP binding transmembrane protein in the inner nuclear membrane

Samp1 is a transmembrane protein of the inner nuclear membrane (INM), which interacts with the nuclear lamina and the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex in interphase and during mitosis, it localizes to the mitotic spindle. Samp1 was recently found to coprecipitate a protein complex containing Ran, a GTPase with fundamental regulatory functions both in interphase and in mitosis. To investigate the interaction between Samp1 and Ran in further detail, we have designed and expressed recombinant fusion proteins of the Chaetomium thermophilum homolog of Samp1 (Ct.Samp1) and human Ran. Pulldown experiments show that Samp1 binds directly to Ran and that Samp1 binds better to RanGTP compared to RanGDP. Samp1 also preferred RanGTP over RanGDP in living tsBN2 cells. We also show that the Ran binding domain is located between amino acids 75–135 in the nucleoplasmically exposed N-terminal tail of Samp1. This domain is unique for Samp1, without homology in any other proteins in fungi or metazoa. Samp1 is the first known transmembrane protein that binds to Ran and could provide a unique local binding site for RanGTP in the INM. Samp1 overexpression resulted in increased Ran concentrations in the nuclear periphery supporting this idea.

Importin β1 targeting by hepatitis C virus NS3/4A protein restricts IRF3 and NF-κB signaling of IFNB1 antiviral response

In this study, newly identified host interactors of hepatitis C virus (HCV) proteins were assessed for a role in modulating the innate immune response. The analysis revealed enrichment for components of the nuclear transport machinery and the crucial interaction with NS3/4A protein in suppression of interferon‐β (IFNB1) induction. Using a comprehensive microscopy‐based high‐content screening approach combined to the gene silencing of nuclear transport factors, we showed that NS3/4A‐interacting proteins control the nucleocytoplasmic trafficking of IFN regulatory factor 3 (IRF3) and NF‐κB p65 upon Sendai virus (SeV) infection. Notably, importin β1 (IMPβ1) knockdown—a hub protein highly targeted by several viruses—decreases the nuclear translocation of both transcription factors and prevents IFNB1 and IFIT1 induction, correlating with a rapid increased of viral proteins and virus‐mediated apoptosis. Here we show that NS3/4A triggers the cleavage of IMPβ1 and inhibits nuclear transport to disrupt IFNB1 production. Importantly, mutated IMPβ1 resistant to cleavage completely restores signaling, similar to the treatment with BILN 2061 protease inhibitor, correlating with the disappearance of cleavage products. Overall, the data indicate that HCV NS3/4A targeting of IMPβ1 and related modulators of IRF3 and NF‐κB nuclear transport constitute an important innate immune subversion strategy and inspire new avenues for broad‐spectrum antiviral therapies.

TGF-β Signaling from Receptors to Smads

Transforming growth factor β (TGF-β) and related growth factors are secreted pleiotropic factors that play critical roles in embryogenesis and adult tissue homeostasis by regulating cell proliferation, differentiation, death, and migration. The TGF-β family members signal via heteromeric complexes of type I and type II receptors, which activate members of the Smad family of signal transducers. The main attribute of the TGF-β signaling pathway is context-dependence. Depending on the concentration and type of ligand, target tissue, and developmental stage, TGF-β family members transmit distinct signals. Deregulation of TGF-β signaling contributes to developmental defects and human diseases. More than a decade of studies have revealed the framework by which TGF-βs encode a context-dependent signal, which includes various positive and negative modifiers of the principal elements of the signaling pathway, the receptors, and the Smad proteins. In this review, we first introduce some basic components of the TGF-β signaling pathways and their actions, and then discuss posttranslational modifications and modulatory partners that modify the outcome of the signaling and contribute to its context-dependence, including small noncoding RNAs.

Leukemia-Associated Nup214 Fusion Proteins Disturb the XPO1-Mediated Nuclear-Cytoplasmic Transport Pathway and Thereby the NF-κB Signaling Pathway

Nuclear-cytoplasmic transport through nuclear pore complexes is mediated by nuclear transport receptors. Previous reports have suggested that aberrant nuclear-cytoplasmic transport due to mutations or overexpression of nuclear pore complexes and nuclear transport receptors is closely linked to diseases. Nup214, a component of nuclear pore complexes, has been found as chimeric fusion proteins in leukemia. Among various Nup214 fusion proteins, SET-Nup214 and DEK-Nup214 have been shown to be engaged in tumorigenesis, but their oncogenic mechanisms remain unclear. In this study, we examined the functions of the Nup214 fusion proteins by focusing on their effects on nuclear-cytoplasmic transport. We found that SET-Nup214 and DEK-Nup214 interact with exportin-1 (XPO1)/CRM1 and nuclear RNA export factor 1 (NXF1)/TAP, which mediate leucine-rich nuclear export signal (NES)-dependent protein export and mRNA export, respectively. SET-Nup214 and DEK-Nup214 decreased the XPO1-mediated nuclear export of NES proteins such as cyclin B and proteins involved in the NF-κB signaling pathway by tethering XPO1 onto nuclear dots where Nup214 fusion proteins are localized. We also demonstrated that SET-Nup214 and DEK-Nup214 expression inhibited NF-κB-mediated transcription by abnormal tethering of the complex containing p65 and its inhibitor, IκB, in the nucleus. These results suggest that SET-Nup214 and DEK-Nup214 perturb the regulation of gene expression through alteration of the nuclear-cytoplasmic transport system.

RanBP3 Regulates Melanoma Cell Proliferation via Selective Control of Nuclear Export

Chromosome region maintenance 1-mediated nucleocytoplasmic transport has been shown as a potential anticancer target in various malignancies. However, the role of the most characterized chromosome region maintenance 1 cofactor ran binding protein 3 (RanBP3) in cancer cell biology has never been investigated. Utilizing a loss-of-function experimental setting in a vast collection of genetically varied melanoma cell lines, we observed the requirement of RanBP3 in melanoma cell proliferation and survival. Mechanistically, we suggest the reinstatement of transforming growth factor-β (TGF-β)-Smad2/3-p21(Cip1) tumor-suppressor axis as part of the RanBP3 silencing-associated antiproliferative program. Employing extensive nuclear export sequence analyses and immunofluorescence-based protein localization studies, we further present evidence suggesting the requirement of RanBP3 function for the nuclear exit of the weak nuclear export sequence-harboring extracellular signal-regulated kinase protein, although it is dispensable for general CRM1-mediated nuclear export of strong nuclear export sequence-harboring cargoes. Rendering mechanistic support to RanBP3 silencing-mediated apoptosis, consequent to extracellular signal-regulated kinase nuclear entrapment, we observed increased levels of cytoplasmically restricted nonphosphorylated/active proapoptotic Bcl-2-antagonist of cell death (BAD) protein. Last, we present evidence suggesting the frequently activated mitogen-activated protein kinase signaling in melanoma as a potential founding basis for a deregulated post-translational control of RanBP3 activity. Collectively, the presented data suggest RanBP3 as a potential target for therapeutic intervention in human melanoma.

Mechanistic Insights from Structural Analyses of Ran-GTPase-Driven Nuclear Export of Proteins and RNAs

Understanding how macromolecules are rapidly exchanged between the nucleus and the cytoplasm through nuclear pore complexes is a fundamental problem in biology. Exportins are Ran-GTPase-dependent nuclear transport factors that belong to the karyopherin-β family and mediate nuclear export of a plethora of proteins and RNAs, except for bulk mRNA nuclear export. Exportins bind cargo macromolecules in a Ran-GTP-dependent manner in the nucleus, forming exportin-cargo-Ran-GTP complexes (nuclear export complexes). Transient weak interactions between exportins and nucleoporins containing characteristic FG (phenylalanine-glycine) repeat motifs facilitate nuclear pore complex passage of nuclear export complexes. In the cytoplasm, nuclear export complexes are disassembled, thereby releasing the cargo. GTP hydrolysis by Ran promoted in the cytoplasm makes the disassembly reaction virtually irreversible and provides thermodynamic driving force for the overall export reaction. In the past decade, X-ray crystallography of some of the exportins in various functional states coupled with functional analyses, single-particle electron microscopy, molecular dynamics simulations, and small-angle solution X-ray scattering has provided rich insights into the mechanism of cargo binding and release and also begins to elucidate how exportins interact with the FG repeat motifs. The knowledge gained from structural analyses of nuclear export is being translated into development of clinically useful inhibitors of nuclear export to treat human diseases such as cancer and influenza.

Structural and Functional Characterization of CRM1-Nup214 Interactions Reveals Multiple FG-Binding Sites Involved in Nuclear Export

CRM1 is the major nuclear export receptor. During translocation through the nuclear pore, transport complexes transiently interact with phenylalanine-glycine (FG) repeats of multiple nucleoporins. On the cytoplasmic side of the nuclear pore, CRM1 tightly interacts with the nucleoporin Nup214. Here, we present the crystal structure of a 117-amino-acid FG-repeat-containing fragment of Nup214, in complex with CRM1, Snurportin 1, and RanGTP at 2.85 Å resolution. The structure reveals eight binding sites for Nup214 FG motifs on CRM1, with intervening stretches that are loosely attached to the transport receptor. Nup214 binds to N- and C-terminal regions of CRM1, thereby clamping CRM1 in a closed conformation and stabilizing the export complex. The role of conserved hydrophobic pockets for the recognition of FG motifs was analyzed in biochemical and cell-based assays. Comparative studies with RanBP3 and Nup62 shed light on specificities of CRM1-nucleoporin binding, which serves as a paradigm for transport receptor-nucleoporin interactions.

A non-canonical mechanism for Crm1-export cargo complex assembly

The transport receptor Crm1 mediates the export of diverse cargos containing leucine-rich nuclear export signals (NESs) through complex formation with RanGTP. To ensure efficient cargo release in the cytoplasm, NESs have evolved to display low affinity for Crm1. However, mechanisms that overcome low affinity to assemble Crm1-export complexes in the nucleus remain poorly understood. In this study, we reveal a new type of RanGTP-binding protein, Slx9, which facilitates Crm1 recruitment to the 40S pre-ribosome-associated NES-containing adaptor Rio2. In vitro, Slx9 binds Rio2 and RanGTP, forming a complex. This complex directly loads Crm1, unveiling a non-canonical stepwise mechanism to assemble a Crm1-export complex. A mutation in Slx9 that impairs Crm1-export complex assembly inhibits 40S pre-ribosome export. Thus, Slx9 functions as a scaffold to optimally present RanGTP and the NES to Crm1, therefore, triggering 40S pre-ribosome export. This mechanism could represent one solution to the paradox of weak binding events underlying rapid Crm1-mediated export.

Nuclear Export of Smads by RanBP3L Regulates Bone Morphogenetic Protein Signaling and Mesenchymal Stem Cell Differentiation

Bone morphogenetic proteins (BMPs) play vital roles in regulating stem cell maintenance and differentiation. BMPs can induce osteogenesis and inhibit myogenesis of mesenchymal stem cells. Canonical BMP signaling is stringently controlled through reversible phosphorylation and nucleocytoplasmic shuttling of Smad1, Smad5, and Smad8 (Smad1/5/8). However, how the nuclear export of Smad1/5/8 is regulated remains unclear. Here we report that the Ran-binding protein RanBP3L acts as a nuclear export factor for Smad1/5/8. RanBP3L directly recognizes dephosphorylated Smad1/5/8 and mediates their nuclear export in a Ran-dependent manner. Increased expression of RanBP3L blocks BMP-induced osteogenesis of mouse bone marrow-derived mesenchymal stem cells and promotes myogenic induction of C2C12 mouse myoblasts, whereas depletion of RanBP3L expression enhances BMP-dependent stem cell differentiation activity and transcriptional responses. In conclusion, our results demonstrate that RanBP3L, as a nuclear exporter for BMP-specific Smads, plays a critical role in terminating BMP signaling and regulating mesenchymal stem cell differentiation.

Components and regulation of nuclear transport processes

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

RAN-binding protein 9 is involved in alternative splicing and is critical for male germ cell development and male fertility

As a member of the large Ran-binding protein family, Ran-binding protein 9 (RANBP9) has been suggested to play a critical role in diverse cellular functions in somatic cell lineages in vitro, and this is further supported by the neonatal lethality phenotype in Ranbp9 global knockout mice. However, the exact molecular actions of RANBP9 remain largely unknown. By inactivation of Ranbp9 specifically in testicular somatic and spermatogenic cells, we discovered that Ranbp9 was dispensable for Sertoli cell development and functions, but critical for male germ cell development and male fertility. RIP-Seq and proteomic analyses revealed that RANBP9 was associated with multiple key splicing factors and directly targeted >2,300 mRNAs in spermatocytes and round spermatids. Many of the RANBP9 target and non-target mRNAs either displayed aberrant splicing patterns or were dysregulated in the absence of Ranbp9. Our data uncovered a novel role of Ranbp9 in regulating alternative splicing in spermatogenic cells, which is critical for normal spermatogenesis and male fertility.

Male fertility depends on successful production of functional sperm. Sperm are produced through spermatogenesis, a process of male germ cell proliferation and differentiation in the testis. Most of the genes involved in spermatogenesis are transcribed and processed into multiple isoforms, which are mainly achieved through alternative splicing. The testis-specific transcriptome, characterized by male germ cell-specific alternative splicing patterns, has been shown to be essential for successful spermatogenesis. However, how these male germ cells-specific alternative splicing events are regulated remains largely unknown. Here, we report that RANBP9 is involved in alternative splicing events that are critical for male germ cell development, and dysfunction of RANBP9 leads to disrupted spermatogenesis and compromised male fertility.

Structural insights into how Yrb2p accelerates the assembly of the Xpo1p nuclear export complex

Proteins and ribonucleoproteins containing a nuclear export signal (NES) assemble with the exportin Xpo1p (yeast CRM1) and Gsp1p-GTP (yeast Ran-GTP) in the nucleus and exit through the nuclear pore complex. In the cytoplasm, Yrb1p (yeast RanBP1) displaces NES from Xpo1p. Efficient export of NES-cargoes requires Yrb2p (yeast RanBP3), a primarily nuclear protein containing nucleoporin-like phenylalanine-glycine (FG) repeats and a low-affinity Gsp1p-binding domain (RanBD). Here, we show that Yrb2p strikingly accelerates the association of Gsp1p-GTP and NES to Xpo1p. We have solved the crystal structure of the Xpo1p-Yrb2p-Gsp1p-GTP complex, a key assembly intermediate that can bind cargo rapidly. Although the NES-binding cleft of Xpo1p is closed in this intermediate, our data suggest that preloading of Gsp1p-GTP onto Xpo1p by Yrb2p, conformational flexibility of Xpo1p, and the low affinity of RanBD enable active displacement of Yrb2p RanBD by NES to occur effectively. The structure also reveals the major binding sites for FG repeats on Xpo1p.

Functional networks of nucleocytoplasmic transport-related genes differentiate ischemic and dilated cardiomyopathies. A new therapeutic opportunity

Heart failure provokes alterations in the expression of nucleocytoplasmic transport-related genes. To elucidate the nucleocytoplasmic transport-linked functional network underlying the two major causes of heart failure, ischemic cardiomyopathy (ICM) and dilated cardiomyopathy (DCM), we examined global transcriptome profiles of left ventricular myocardium tissue samples from 31 patients (ICM, n = 10; DCM, n = 13) undergoing heart transplantation and control donors (CNT, n = 8) using RNA-Sequencing and GeneMANIA. Comparative profiling of ICM versus control and DCM versus control showed 1081 and 2440 differentially expressed genes, respectively (>1.29-fold; P<0.05). GeneMANIA revealed differentially regulated functional networks specific to ICM and DCM. In comparison with CNT, differential expression was seen in 9 and 12 nucleocytoplasmic transport-related genes in ICM and DCM groups, respectively. DDX3X, KPNA2, and PTK2B were related to ICM, while SMURF2, NUP153, IPO5, RANBP3, NOXA1, and RHOJ were involved in DCM pathogenesis. Furthermore, the two pathologies shared 6 altered genes: XPO1, ARL4, NFKB2, FHL3, RANBP2, and RHOU showing an identical trend in expression in both ICM and DCM. Notably, the core of the derived functional networks composed of nucleocytoplasmic transport-related genes (XPO1, RANBP2, NUP153, IPO5, KPNA2, and RANBP3) branched into several pathways with downregulated genes. Moreover, we identified genes whose expression levels correlated with left ventricular mass index and left ventricular function parameters in HF patients. Collectively, our study provides a clear distinction between the two pathologies at the transcriptome level and opens up new possibilities to search for appropriate therapeutic targets for ICM and DCM.

Influenza viral manipulation of sphingolipid metabolism and signaling to modulate host defense system

Viruses attempt to create a distinctive cellular environment to favor viral replication and spread. Recent studies uncovered new functions of the sphingolipid signaling/metabolism during pathogenic virus infections. While sphingolipids such as sphingomyelin and ceramide were reported to influence the entry step of several viruses, sphingolipid-metabolizing enzymes could directly alter viral replication processes. Influenza virus was shown to increase the level of sphingosine kinase (SK) 1 to promote virus propagation. The mechanism involves regulation of intracellular signaling pathways, leading to the amplification of influenza viral RNA synthesis and nuclear export of viral ribonucleoprotein (RNP) complex. However, bovine viral diarrhea virus inhibits SK1 to enhance the efficacy of virus replication, demonstrating the presence of virus-specific strategies for modulation of the sphingolipid system. Therefore, investigating the sphingolipid metabolism and signaling in the context of virus replication could help us design innovative therapeutic approaches to improve human health.

Sphingosine kinase 1 serves as a pro-viral factor by regulating viral RNA synthesis and nuclear export of viral ribonucleoprotein complex upon influenza virus infection

Influenza continues to pose a threat to humans by causing significant morbidity and mortality. Thus, it is imperative to investigate mechanisms by which influenza virus manipulates the function of host factors and cellular signal pathways. In this study, we demonstrate that influenza virus increases the expression and activation of sphingosine kinase (SK) 1, which in turn regulates diverse cellular signaling pathways. Inhibition of SK suppressed virus-induced NF-κB activation and markedly reduced the synthesis of viral RNAs and proteins. Further, SK blockade interfered with activation of Ran-binding protein 3 (RanBP3), a cofactor of chromosome region maintenance 1 (CRM1), to inhibit CRM1-mediated nuclear export of the influenza viral ribonucleoprotein complex. In support of this observation, SK inhibition altered the phosphorylation of ERK, p90RSK, and AKT, which is the upstream signal of RanBP3/CRM1 activation. Collectively, these results indicate that SK is a key pro-viral factor regulating multiple cellular signal pathways triggered by influenza virus infection.

Structure of a truncation mutant of the nuclear export factor CRM1 provides insights into the auto-inhibitory role of its C-terminal helix

Chromosome region maintenance 1/exportin1/Xpo1 (CRM1) associates with the GTPase Ran to mediate the nuclear export of proteins bearing a leucine-rich nuclear export signal (NES). CRM1 consists of helical hairpin HEAT repeats and a C-terminal helical extension (C-extension) that inhibits the binding of NES-bearing cargos. We report the crystal structure and small-angle X-ray scattering analysis of a human CRM1 mutant with enhanced NES-binding activity due to deletion of the C-extension. We show that loss of the C-extension leads to a repositioning of CRM1's C-terminal repeats and to a more extended overall conformation. Normal mode analysis predicts reduced rigidity for the deletion mutant, consistent with an observed decrease in thermal stability. Point mutations that destabilize the C-extension shift CRM1 to the more extended conformation, reduce thermal stability, and enhance NES-binding activity. These findings suggest that an important mechanism by which the C-extension regulates CRM1's cargo-binding affinity is by modulating the conformation and flexibility of its HEAT repeats.

Phosphorylation of Crm1 by CDK1-cyclin-B promotes Ran-dependent mitotic spindle assembly

Mitotic spindle assembly in animal cells is orchestrated by a chromosome-dependent pathway that directs microtubule stabilization. RanGTP generated at chromosomes releases spindle assembly factors from inhibitory complexes with importins, the nuclear transport factors that facilitate protein import into the nucleus during interphase. In addition, the nuclear export factor Crm1 has been proposed to act as a mitotic effector of RanGTP through the localized assembly of protein complexes on the mitotic spindle, notably at centrosomes and kinetochores. It has been unclear, however, how the functions of nuclear transport factors are controlled during mitosis. Here, we report that human Crm1 is phosphorylated at serine 391 in mitosis by CDK1-cyclin-B (i.e. the CDK1 and cyclin B complex). Expression of Crm1 with serine 391 mutated to either non-phosphorylated or phosphorylation-mimicking residues indicates that phosphorylation directs the localization of Crm1 to the mitotic spindle and facilitates spindle assembly, microtubule stabilization and chromosome alignment. We find that phosphorylation of Crm1 at serine 391 enhances its RanGTP-dependent interaction with RanGAP1-RanBP2 and promotes their recruitment to the mitotic spindle. These results show that phosphorylation of Crm1 controls its molecular interactions, localization and function during mitosis, uncovering a new mechanism for the control of mitotic spindle assembly by CDK1-cyclin-B. We propose that nuclear transport factors are controlled during mitosis through the selection of specific molecular interactions by protein phosphorylation.

The role of Ran-binding protein 3 during influenza A virus replication

Influenza A virus vRNP nuclear export is CRM1-dependent. Ran-binding protein 3 (RanBP3) is a Ran-interacting protein that is best known for its role as a cofactor of CRM1-mediated cargo nuclear export. In this study, we investigated the role of RanBP3 during the influenza A virus life cycle. We found that RanBP3 was phosphorylated at Ser58 in the early and late phases of infection. Knockdown of RanBP3 expression led to vRNP nuclear retention, suggesting that RanBP3 is involved in vRNP nuclear export. Moreover, we demonstrated that the function of RanBP3 during vRNP nuclear export is regulated by phosphorylation at Ser58, and that RanBP3 phosphorylation is modulated by both PI3K/Akt and Ras/ERK/RSK pathways in the late phase of viral infection.

Human CRM1 augments production of infectious human and feline immunodeficiency viruses from murine cells

Productive replication of human immunodeficiency virus type 1 (HIV-1) occurs efficiently only in humans. The posttranscriptional stages of the HIV-1 life cycle proceed poorly in mouse cells, with a resulting defect in viral assembly and release. Previous work has shown that the presence of human chromosome 2 increases HIV-1 production in mouse cells. Recent studies have shown that human chromosome region maintenance 1 (hCRM1) stimulates Gag release from rodent cells. Here we report that expressions of hCRM1 in murine cells resulted in marked increases in the production of infectious HIV-1 and feline immunodeficiency virus (FIV). HIV-1 production was also increased by hSRp40, and a combination of hCRM1 and hSRp40 resulted in a more-than-additive effect on HIV-1 release. In contrast, the overexpression of mouse CRM1 (mCRM1) minimally affected HIV-1 and FIV production and did not antagonize hCRM1. In the presence of hCRM1 there were large increases in the amounts of released capsid, which paralleled the increases in the infectious titers. Consistent with this finding, the ratios of unspliced to spliced HIV-1 mRNAs in mouse cells expressing hCRM1 and SRp40 became similar to those of human cells. Furthermore, imaging of intron-containing FIV RNA showed that hCRM1 increased RNA export to the cytoplasm.By testing chimeras between mCRM1 and hCRM1 and comparing those sequences to feline CRM1, we mapped the functional domain to HEAT (Huntingtin, elongation factor 3, protein phosphatase 2A, and the yeast kinase TOR1) repeats 4A to 9A and a triple point mutant in repeat 9A, which showed a loss of function. Structural analysis suggested that this region of hCRM1 may serve as a binding site for viral or cellular factors to facilitate lentiviral RNA nuclear export.

NEX-TRAP, a novel method for in vivo analysis of nuclear export of proteins

Transport of proteins between cytoplasm and nucleus is mediated by transport factors of the importin α- and β-families and occurs along a gradient of the small GTPase Ran. To date, in vivo analysis as well as prediction of protein nuclear export remain tedious and difficult. We generated a novel bipartite assay called NEX-TRAP (Nuclear EXport Trapped by RAPamycin) for in vivo analysis of protein nuclear export. The assay is based on the rapamycin-induced dimerization of the modules FRB (FK506-rapamycin (FR)-binding domain) and FKBP (FK506-binding protein-12): a potential nuclear export cargo is fused to FRB, to EYFP for direct visualization as well as to an SV40-derived nuclear localization signal (NLS) for constitutive nuclear import. An integral membrane protein that resides at the trans Golgi network (TGN) is fused to a cytoplasmically exposed FKBP and serves as reporter. EYFP-NLS-FRB fusion proteins with export activity accumulate in the nucleus at steady state but continuously shuttle between nucleus and cytoplasm. Rapamycin-induced dimerization of FRB and FKBP at the TGN traps the shuttling protein outside of the nucleus, making nuclear export permanent. Using several example cargoes, we show that the NEX-TRAP is superior to existing assays owing to its ease of use, its sensitivity and accuracy. Analysis of large numbers of export cargoes is facilitated by recombinational cloning. The NEX-TRAP holds the promise of applicability in automated fluorescence imaging for systematic analysis of nuclear export, thereby improving in silico prediction of nuclear export sequences.

Ran and calcineurin can participate collaboratively in the regulation of spermatogenesis in scallop

Calcineurin is a calcium/calmodulin-dependent protein phosphatase that plays important roles in the transduction of calcium signals in a variety of tissues. In addition, calcineurin has been implicated in the process of spermatogenesis. A novel calcineurin-binding protein, CaNBP75, has been identified in scallop testis. The C-terminal region of CaNBP75 is homologous to the C-terminal region of RanBP3, a Ran-binding domain-containing protein. A small G protein Ran has been involved in spermiogenesis by virtue of the fact that its localization in spermatids changes during spermiogenesis. The current study was performed to investigate the functions of Ran and CaNBP75 in the regulation of calcineurin in testis to further understand the basic functions of calcineurin during spermatogenesis. First, cloning and sequencing of a scallop Ran cDNA isolated from testis revealed that scallop Ran is well-conserved at the amino acid level. Secondly, direct binding of Ran to CaNBP75 was demonstrated in an in vitro pull-down assay. Thirdly, analysis of the tissue distribution of Ran, CaNBP75, and calcineurin showed that these proteins are abundantly expressed in testis. Fourthly, comparison of the expression profiles of Ran and CaNBP75 with that of calcineurin in scallop testis during the maturation cycle revealed that Ran and CaNBP75 mRNA levels increase during meiosis and spermiogenesis, similar to calcineurin. Finally, co-immunoprecipitation analysis suggests that Ran, CaNBP75, and calcineurin interact in scallop testis during maturation. These results suggest that Ran, CaNBP75, and calcineurin may act in a coordinated manner to regulate spermatogenesis.

Human T lymphotropic virus type 1 regulatory and accessory gene transcript expression and export are not rex dependent

Human T lymphotropic virus type 1 (HTLV-1) requires regulated gene expression from unspliced and alternatively spliced transcripts for efficient replication and persistence. HTLV-1 Rex is known to facilitate cytoplasmic export of unspliced, gag/pol and incompletely spliced env mRNAs, but its contribution to the expression of other viral transcripts has not been experimentally assessed. In this study, we utilized HTLV-1 proviral clones, cellular fractionation, and real-time reverse transcriptase PCR to determine the role of Rex on the expression and export of all viral mRNAs. Our results indicate that the steady-state levels of the different viral mRNAs are modulated by Rex, which we attribute to a redistribution of completely spliced mRNAs toward incompletely spliced mRNAs. Furthermore, we confirmed the positive effect of Rex on the unspliced gag/pol mRNA and singly spliced env mRNA, resulting in increased cytoplasmic expression. However, the cytoplasmic export of the alternatively spliced HTLV-1 mRNAs encoding the accessory proteins and the antisense Hbz mRNA are independent of direct Rex regulation. This is consistent with the conclusion that viral mRNAs that contain the cis-acting repressive sequence (CRS) and/or a fully functional splice donor site require a Rex/RxRE interaction for efficient cytoplasmic expression.

Click strategies for single-molecule protein fluorescence

Single-molecule methods have matured into central tools for studies in biology. Foerster resonance energy transfer (FRET) techniques, in particular, have been widely applied to study biomolecular structure and dynamics. The major bottleneck for a facile and general application of these studies arises from the need to label biological samples site-specifically with suitable fluorescent dyes. In this work, we present an optimized strategy combining click chemistry and the genetic encoding of unnatural amino acids (UAAs) to overcome this limitation for proteins. We performed a systematic study with a variety of clickable UAAs and explored their potential for high-resolution single-molecule FRET (smFRET). We determined all parameters that are essential for successful single-molecule studies, such as accessibility of the probes, expression yield of proteins, and quantitative labeling. Our multiparameter fluorescence analysis allowed us to gain new insights into the effects and photophysical properties of fluorescent dyes linked to various UAAs for smFRET measurements. This led us to determine that, from the extended tool set that we now present, genetically encoding propargyllysine has major advantages for state-of-the-art measurements compared to other UAAs. Using this optimized system, we present a biocompatible one-step dual-labeling strategy of the regulatory protein RanBP3 with full labeling position freedom. Our technique allowed us then to determine that the region encompassing two FxFG repeat sequences adopts a disordered but collapsed state. RanBP3 serves here as a prototypical protein that, due to its multiple cysteines, size, and partially disordered structure, is not readily accessible to any of the typical structure determination techniques such as smFRET, NMR, and X-ray crystallography.

Probing of the nuclear import and export signals and subcellular transport mechanism of varicella-zoster virus tegument protein open reading frame 10

Varicella-zoster virus open reading frame 10 (ORF10), a tegument protein present in the virion, is a member of the alphaherpesvirus UL48 gene family that shares considerable amino acid sequence homology with the UL48 prototype, herpes simplex virus type 1 VP16. VP16 serves multiple functions, including transcriptional activation of viral immediate-early genes. Furthermore, VP16 has been shown to be involved in some aspects of virus assembly and/or maturation. However, little is known concerning the function of ORF10. Here, we found that transient expression of ORF10 fused to enhanced yellow fluorescent protein (EYFP) in COS-7 cells showed the predominantly nuclear localization in the absence of other viral proteins. By constructing a series of ORF10 variants fused to EYFP, a bona fide bipartite nuclear localization signal of ORF10 was, for the first time, determined and mapped to amino acids (aa) 302-347. Additionally, the nuclear export signal (NES) was identified and found to be in a leucine-rich region at aa 226-236. Finally, ORF10 was demonstrated to be targeted to the cytoplasm through the functional NES by chromosomal region maintenance 1-dependent pathway, and to the nucleus via Ran and importin β1-dependent pathway that does not require importin α5.

The nucleoporin-like protein NLP1 (hCG1) promotes CRM1-dependent nuclear protein export

Translocation of transport complexes across the nuclear envelope is mediated by nucleoporins, proteins of the nuclear pore complex that contain phenylalanine-glycine (FG) repeats as a characteristic binding motif for transport receptors. CRM1 (exportin 1), the major export receptor, forms trimeric complexes with RanGTP and proteins containing nuclear export sequences (NESs). We analyzed the role of the nucleoporin-like protein 1, NLP1 (also known as hCG1 and NUPL2) in CRM1-dependent nuclear transport. NLP1, which contains many FG repeats, localizes to the nuclear envelope and could also be mobile within the nucleus. It promotes the formation of complexes containing CRM1 and RanGTP, with or without NES-containing cargo proteins, that can be dissociated by RanBP1 and/or the cytoplasmic nucleoporin Nup214. The FG repeats of NLP1 do not play a major role in CRM1 binding. Overexpression of NLP1 promotes CRM1-dependent export of certain cargos, whereas its depletion by small interfering RNAs leads to reduced export rates. Thus, NLP1 functions as an accessory factor in CRM1-dependent nuclear protein export.

Mechanism and regulation of nucleocytoplasmic trafficking of smad

Smad proteins are the intracellular mediators of transforming growth factor β (TGF-β) signaling. Smads function as transcription factors and their activities require carboxyl-terminal phosphorylation by TGF-β receptor kinases which are embedded in the cell membrane. Therefore, the translocation of activated Smads from the cytoplasm into the nucleus is a rate-limiting step in TGF-β signal transduction into the nucleus. On the other hand, the export of Smads out of the nucleus turns off TGF-β effect. Such spatial control of Smad ensures a tight regulation of TGF-β target genes. Several cross-talk pathways have been shown to affect TGF-β signaling by impairing nuclear translocation of Smad, exemplifying the biological importance of the nuclear transport process. Many laboratories have investigated the underlying molecular mechanism of Smad nucleocytoplasmic translocation, combining genetics, biochemistry and sophisticated live cell imaging approaches. The last few years have witnessed the elucidation of several key players in Smad nuclear transport, most importantly the karyopherins that carry Smads across the nuclear envelope and nuclear pore proteins that facilitate the trans-nuclear envelope movement. The foundation is now set to further elucidate how the nuclear transport process is regulated and exploit such knowledge to manipulate TGF-β signaling. In this review we will discuss the current understanding of the molecular machinery responsible for nuclear import and export of Smads.

Fluorescent recovery after photobleaching (FRAP) analysis of nuclear export rates identifies intrinsic features of nucleocytoplasmic transport

A quantitative description of carrier-mediated nuclear export in live cells is presented. To this end, we fused a prototypical leucine-rich nuclear export signal (NES) to GFP as a cargo model and expressed the fluorescent chimera in live CHO-K1 cells. By modeling FRAP data, we calculate the NES affinity for the export machinery and the maximum rate of nuclear export achievable at saturation of endogenous carriers. The measured active-export time through the Nuclear Pore Complex (NPC) is 18 ms, remarkably similar to the previously determined active-import rate. Also, our results reveal that active export/import and active export/passive diffusion fluxes are uncoupled, thus complementing previous reports on active import/passive diffusion uncoupling. These findings suggest differential gating at the NPC level.

Evolution of a species-specific determinant within human CRM1 that regulates the post-transcriptional phases of HIV-1 replication

The human immunodeficiency virus type-1 (HIV-1) Rev protein regulates the nuclear export of intron-containing viral RNAs by recruiting the CRM1 nuclear export receptor. Here, we employed a combination of functional and phylogenetic analyses to identify and characterize a species-specific determinant within human CRM1 (hCRM1) that largely overcomes established defects in murine cells to the post-transcriptional stages of the HIV-1 life cycle. hCRM1 expression in murine cells promotes the cytoplasmic accumulation of intron-containing viral RNAs, resulting in a substantial stimulation of the net production of infectious HIV-1 particles. These stimulatory effects require a novel surface-exposed element within HEAT repeats 9A and 10A, discrete from the binding cleft previously shown to engage Rev's leucine-rich nuclear export signal. Moreover, we show that this element is a unique feature of higher primate CRM1 proteins, and discuss how this sequence has evolved from a non-functional, ancestral sequence.

PPM1A dephosphorylates RanBP3 to enable efficient nuclear export of Smad2 and Smad3

Smad2 and Smad3 (Smad2/3) are essential signal transducers and transcription factors in the canonical transforming growth factor-β (TGF-β) signalling pathway. Active Smad2/3 signalling in the nucleus is terminated by dephosphorylation and subsequent nuclear export of Smad2/3. Here we report that protein phosphatase PPM1A regulates the nuclear export of Smad2/3 through targeting nuclear exporter RanBP3. PPM1A directly interacted with and dephosphorylated RanBP3 at Ser 58 in vitro and in vivo. Consistently, RanBP3 phosphorylation was elevated in PPM1A-null mouse embryonic fibroblasts. Dephosphorylation of RanBP3 at Ser 58 promoted its ability to export Smad2/3 and terminate TGF-β responses. Our findings indicate the critical role of PPM1A in maximizing exporter activity of RanBP3 for efficient termination of canonical TGF-β signalling.

Ran-dependent nuclear export mediators: a structural perspective

Nuclear export is an essential eukaryotic activity. It proceeds through nuclear pore complexes (NPCs) and is mediated by soluble receptors that shuttle between nucleus and cytoplasm. RanGTPase-dependent export mediators (exportins) constitute the largest class of these carriers and are functionally highly versatile. All of these exportins load their substrates in response to RanGTP binding in the nucleus and traverse NPCs as ternary RanGTP-exportin-cargo complexes to the cytoplasm, where GTP hydrolysis leads to export complex disassembly. The different exportins vary greatly in their substrate range. Recent structural studies of both protein- and RNA-specific exporters have illuminated how exportins bind their cargoes, how Ran triggers cargo loading and how export complexes are disassembled in the cytoplasm. Here, we review the current state of knowledge and highlight emerging principles as well as prevailing questions.

Characterization of molecular determinants for nucleocytoplasmic shuttling of PRV UL54

The pseudorabies virus (PRV) early protein UL54 is a homologue of the herpes simplex virus 1 (HSV-1) immediate-early protein ICP27, which is a multifunctional protein and essential for HSV-1 infection. To determine if UL54 might shuttle between the nucleus and cytoplasm, as has been shown for its homologues in human herpesviruses, the molecular determinants for its nucleocytoplasmic shuttling were investigated. Heterokaryon assays demonstrated that UL54 was a nucleocytoplasmic shuttling protein and this property could not be blocked by leptomycin B, an inhibitor of chromosome region maintenance 1 (CRM1). However, TAP/NXF1 promoted the nuclear export of UL54 and interacted with UL54, suggesting that UL54 shuttles between the nucleus and the cytoplasm via a TAP/NXF1, but not CRM1, dependent nuclear export pathway. Furthermore, UL54 was demonstrated to target to the nucleus through a classic Ran-, importin β1- and α5-dependent nuclear import mechanism.

Expression of nucleocytoplasmic transport machinery: clues to regulation of spermatogenic development

Spermatogenesis is one example of a developmental process which requires tight control of gene expression to achieve normal growth and sustain function. This review is based on the principle that events in spermatogenesis are controlled by changes in the distribution of proteins between the nuclear and cytoplasmic compartments. Through analysis of the regulated production of nucleocytoplasmic transport machinery in mammalian spermatogenesis, this review addresses the concept that access to the nucleus is tightly controlled to enable and prevent differentiation. A broad review of nuclear transport components is presented, outlining the different categories of machinery required for import, export and non-nuclear functions. In addition, the complexity of nomenclature is addressed by the provision of a concise yet comprehensive listing of information that will aid in comparative studies of different transport proteins and the genes which encode them. We review a suite of existing transcriptional analyses which identify common and distinct patterns of transport machinery expression, showing how these can be linked with key events in spermatogenic development. The additional importance of this for human fertility is considered, in light of data that identify which importin and nuclear transport machinery components are present in testicular cancer specimens, while also providing an indication of how their presence (and absence) may be considered as potential mediators of oncogenesis. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Insights into the function of the CRM1 cofactor RanBP3 from the structure of its Ran-binding domain

Proteins bearing a leucine-rich nuclear export signal (NES) are exported from the nucleus by the transport factor CRM1, which forms a cooperative ternary complex with the NES-bearing cargo and with the small GTPase Ran. CRM1-mediated export is regulated by RanBP3, a Ran-interacting nuclear protein. Unlike the related proteins RanBP1 and RanBP2, which promote disassembly of the export complex in the cytosol, RanBP3 acts as a CRM1 cofactor, enhancing NES export by stabilizing the export complex in the nucleus. RanBP3 also alters the cargo selectivity of CRM1, promoting recognition of the NES of HIV-1 Rev and of other cargos while deterring recognition of the import adaptor protein Snurportin1. Here we report the crystal structure of the Ran-binding domain (RBD) from RanBP3 and compare it to RBD structures from RanBP1 and RanBP2 in complex with Ran and CRM1. Differences among these structures suggest why RanBP3 binds Ran with unusually low affinity, how RanBP3 modulates the cargo selectivity of CRM1, and why RanBP3 promotes assembly rather than disassembly of the export complex. The comparison of RBD structures thus provides an insight into the functional diversity of Ran-binding proteins.

Nucleo-cytoplasmic transport of proteins and RNA in plants

Transport of macromolecules between the nucleus and the cytoplasm is an essential necessity in eukaryotic cells, since the nuclear envelope separates transcription from translation. In the past few years, an increasing number of components of the plant nuclear transport machinery have been characterised. This progress, although far from being completed, confirmed that the general characteristics of nuclear transport are conserved between plants and other organisms. However, plant-specific components were also identified. Interestingly, several mutants in genes encoding components of the plant nuclear transport machinery were investigated, revealing differential sensitivity of plant-specific pathways to impaired nuclear transport. These findings attracted attention towards plant-specific cargoes that are transported over the nuclear envelope, unravelling connections between nuclear transport and components of signalling and developmental pathways. The current state of research in plants is summarised in comparison to yeast and vertebrate systems, and special emphasis is given to plant nuclear transport mutants.