Drosophila melanogaster importin alpha1 and alpha3 can replace importin alpha2 during spermatogenesis but not oogenesis

Drosophila Importin Alpha 1 (Dα1) Is Required to Maintain Germline Stem Cells in the Testis Niche

Stem cell maintenance and differentiation can be regulated via the differential activity of transcription factors within stem cells and their progeny. For these factors to be active, they need to be transported from their site of synthesis in the cytoplasm into the nucleus. A tissue-specific requirement for factors involved in nuclear importation is a potential mechanism to regulate stem cell differentiation. We have undertaken a characterization of male sterile importin alpha 1 (Dα1) null alleles in Drosophila and found that Dα1 is required for maintaining germline stem cells (GSCs) in the testis niche. The loss of GSCs can be rescued by ectopic expression of Dα1 within the germline but the animals are still infertile, indicating a second role for Dα1 in spermatogenesis. Expression of a Dα1 dominant negative transgene in GSCs confirmed a functional requirement for Dα1 in GSC maintenance but expression of the transgene in differentiating spermatogonia did not exhibit a phenotype indicating a specific role for Dα1 within GSCs. Our data indicate that Dα1 is utilized as a regulatory protein within GSCs to facilitate nuclear importation of proteins that maintain the stem cell pool.

β-importin Tnpo-SR promotes germline stem cell maintenance and oocyte differentiation in female Drosophila

Germ cell development requires interplay between factors that balance cell fate and division. Early in their development, germ cells in many organisms divide mitotically with incomplete cytokinesis. Key regulatory events then lead to the specification of mature gametes, marked by the switch to a meiotic cell cycle program. Though the regulation of germ cell proliferation and meiosis are well understood, how these events are coordinated during development remains incompletely described. Originally characterized in their role as nucleo-cytoplasmic shuttling proteins, β-importins exhibit diverse functions during male and female gametogenesis. Here, we describe novel, distinct roles for the β-importin, Transportin-Serine/Arginine rich (Tnpo-SR), as a regulator of the mitosis to meiosis transition in the Drosophila ovary. We find that Tnpo-SR is necessary for germline stem cell (GSC) establishment and self-renewal, likely by controlling the response of GSCs to bone morphogenetic proteins. Depletion of Tnpo-SR results in germ cell counting defects and loss of oocyte identity. We show that in the absence of Tnpo-SR, proteins typically suppressed in germ cells when they exit mitosis fail to be down-regulated, and oocyte-specific factors fail to accumulate. Together, these findings provide new insight into the balance between germ cell division and differentiation and identify novel roles for β-importins in germ cell development.

Parallel import mechanisms ensure the robust nuclear localization of actin in Drosophila

Actin, as an ancient and fundamental protein, participates in various cytoplasmic as well as nuclear functions in eukaryotic cells. Based on its manifold tasks in the nucleus, it is a reasonable assumption that the nuclear presence of actin is essential for the cell, and consequently, its nuclear localization is ensured by a robust system. However, today only a single nuclear import and a single nuclear export pathway is known which maintain the dynamic balance between cytoplasmic and nuclear actin pools. In our work, we tested the robustness of the nuclear import of actin, and investigated whether the perturbations of nuclear localization affect the viability of the whole organism. For this aim, we generated a genetic system in Drosophila, in which we rescued the lethal phenotype of the null mutation of the Actin5C gene with transgenes that express different derivatives of actin, including a Nuclear Export Signal (NES)-tagged isoform which ensures forced nuclear export of the protein. We also disrupted the SUMOylation site of actin, suggested earlier to be responsible for nuclear retention, and eliminated the activity of the single nuclear import factor dedicated to actin. We found that, individually, none of the above mentioned manipulations led to a notable reduction in nuclear actin levels and thus, fully rescued lethality. However, the NES tagging of actin, together with the knock out of its importin, significantly reduced the amount of nuclear actin and induced lethality, confirming that the presence of actin in the nucleus is essential, and thereby, over-secured. Supporting this, we identified novel nuclear importins specific to actin, which sheds light on the mechanism behind the robustness of nuclear localization of actin, and supports the idea of essentiality of its nuclear functions.

Nuclear transport genes recurrently duplicate by means of RNA intermediates in Drosophila but not in other insects

Background

The nuclear transport machinery is involved in a well-known male meiotic drive system in Drosophila. Fast gene evolution and gene duplications have been major underlying mechanisms in the evolution of meiotic drive systems, and this might include some nuclear transport genes in Drosophila. So, using a comprehensive, detailed phylogenomic study, we examined 51 insect genomes for the duplication of the same nuclear transport genes.

Results

We find that most of the nuclear transport duplications in Drosophila are of a few classes of nuclear transport genes, RNA mediated and fast evolving. We also retrieve many pseudogenes for the Ran gene. Some of the duplicates are relatively young and likely contributing to the turnover expected for genes under strong but changing selective pressures. These duplications are potentially revealing what features of nuclear transport are under selection. Unlike in flies, we find only a few duplications when we study the Drosophila duplicated nuclear transport genes in dipteran species outside of Drosophila, and none in other insects.

Conclusions

These findings strengthen the hypothesis that nuclear transport gene duplicates in Drosophila evolve either as drivers or suppressors of meiotic drive systems or as other male-specific adaptations circumscribed to flies and involving a handful of nuclear transport functions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08170-4.

Importins: Diverse roles in male fertility

Regulated nucleocytoplasmic transport is central to the changes in gene expression that underpin cellular development and homeostasis, including in the testis, and proteins in the importin family are the predominant facilitators of cargo transport through the nuclear envelope. Reports documenting cell-specific profiles of importin transcripts and proteins during spermatogenesis led us to hypothesize that importins facilitate developmental switches in the testis. More recently, importins have been shown to serve additional functions, both inside and outside the nucleus; these include acting as subcellular scaffolding, mediating cellular stress responses, and controlling transcription. This paper seeks to provide an overview and update on the functions of importin proteins, with a focus on testis development and spermatogenesis. We present an extended survey of importins by combining published single cell RNAseq data with immunohistochemistry on developing and adult mouse testes. This approach reinforces and broadens knowledge of importins in biological processes, including in spermatogenesis and during testis development, revealing additional avenues for impactful investigations.

Identification of Spodoptera frugiperda importin alphas that facilitate the nuclear import of Autographa californica multiple nucleopolyhedrovirus DNA polymerase

Proteins containing nuclear localization signals (NLSs) are actively transported into the nucleus via the classic importin-α/β-mediated pathway, and NLSs are recognized by members of the importin-α family. Most studies of insect importin-αs have focused on Drosophila to date, little is known about the importin-α proteins in Lepidoptera insects. In this study, we identified four putative importin-α homologues, Spodoptera frugiperda importin-α1 (SfIMA1), SfIMA2, SfIMA4 and SfIMA7, from Sf9 cells. Immunofluorescence analysis showed that SfIMA2, SfIMA4 and SfIMA7 localized to the nucleus, while SfIMA1 distributed in cytoplasm. Additionally, SfIMA4 and SfIMA7 were also detected in the nuclear membrane of Sf9 cells. SfIMA1, SfIMA4 and SfIMA7, but not SfIMA2, were found to associate with the C terminus of AcMNPV DNA polymerase (DNApol) that harbours a typical monopartite NLS and a classic bipartite NLS. Further analysis of protein-protein interactions revealed that SfIMA1 specifically recognizes the bipartite NLS, while SfIMA4 and SfIMA7 bind to both monopartite and bipartite NLSs. Together, our results suggested that SfIMA1, SfIMA4 and SfIMA7 play important roles in the nuclear import of AcMNPV DNApol C terminus in Sf9 cells.

A comparative genomic approach using mouse and fruit fly data to discover genes involved in testis function in hymenopterans with a focus on Nasonia vitripennis

Background

Spermatogenesis appears to be a relatively well-conserved process even among distantly related animal taxa such as invertebrates and vertebrates. Although Hymenopterans share many characteristics with other organisms, their complex haplodiploid reproduction system is still relatively unknown. However, they serve as a complementary insect model to Drosophila for studying functional male fertility. In this study, we used a comparative method combining taxonomic, phenotypic data and gene expression to identify candidate genes that could play a significant role in spermatogenesis in hymenopterans.

Results

Of the 546 mouse genes predominantly or exclusively expressed in the mouse testes, 36% had at least one ortholog in the fruit fly. Of these genes, 68% had at least one ortholog in one of the six hymenopteran species we examined. Based on their gene expression profiles in fruit fly testes, 71 of these genes were hypothesized to play a marked role in testis function. Forty-three of these 71 genes had an ortholog in at least one of the six hymenopteran species examined, and their enriched GO terms were related to the G2/M transition or to cilium organization, assembly, or movement. Second, of the 379 genes putatively involved in male fertility in Drosophila, 224 had at least one ortholog in each of the six Hymenoptera species. Finally, we showed that 199 of these genes were expressed in early pupal testis in Nasonia vitripennis; 86 exhibited a high level of expression, and 54 displayed modulated expression during meiosis.

Conclusions

In this study combining phylogenetic and experimental approaches, we highlighted genes that may have a major role in gametogenesis in hymenopterans; an essential prerequisite for further research on functional importance of these genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01825-6.

Proteostasis in the Male and Female Germline: A New Outlook on the Maintenance of Reproductive Health

For fully differentiated, long lived cells the maintenance of protein homeostasis (proteostasis) becomes a crucial determinant of cellular function and viability. Neurons are the most well-known example of this phenomenon where the majority of these cells must survive the entire course of life. However, male and female germ cells are also uniquely dependent on the maintenance of proteostasis to achieve successful fertilization. Oocytes, also long-lived cells, are subjected to prolonged periods of arrest and are largely reliant on the translation of stored mRNAs, accumulated during the growth period, to support meiotic maturation and subsequent embryogenesis. Conversely, sperm cells, while relatively ephemeral, are completely reliant on proteostasis due to the absence of both transcription and translation. Despite these remarkable, cell-specific features there has been little focus on understanding protein homeostasis in reproductive cells and how/whether proteostasis is "reset" during embryogenesis. Here, we seek to capture the momentum of this growing field by highlighting novel findings regarding germline proteostasis and how this knowledge can be used to promote reproductive health. In this review we capture proteostasis in the context of both somatic cell and germline aging and discuss the influence of oxidative stress on protein function. In particular, we highlight the contributions of proteostasis changes to oocyte aging and encourage a focus in this area that may complement the extensive analyses of DNA damage and aneuploidy that have long occupied the oocyte aging field. Moreover, we discuss the influence of common non-enzymatic protein modifications on the stability of proteins in the male germline, how these changes affect sperm function, and how they may be prevented to preserve fertility. Through this review we aim to bring to light a new trajectory for our field and highlight the potential to harness the germ cell's natural proteostasis mechanisms to improve reproductive health. This manuscript will be of interest to those in the fields of proteostasis, aging, male and female gamete reproductive biology, embryogenesis, and life course health.

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

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

Characterization of the Importin-β binding domain in nuclear import receptor KPNA7

The KPNA family of mammalian nuclear import receptors are encoded by seven genes that generate isoforms with 42-86% identity. KPNA isoforms have the same protein architecture and share the functional property of nuclear localization signal (NLS) recognition, however, the tissue and developmental expression patterns of these receptors raise the question of whether subtle differences in KPNA isoforms might be important in specific biological contexts. Here, we show that KPNA7, an isoform with expression mostly limited to early development, can bind Importin-β (Imp-β) in the absence of NLS cargo. This result contrasts with Imp-β interactions with other KPNA family members, where affinity is regulated by NLS cargo as part of a cooperative binding mechanism. The Imp-β binding (IBB) domain, which is highly conserved in all KPNA family members, generally serves to occlude the NLS binding groove and maintain the receptor in an auto-inhibited 'closed' state prior to NLS contact. Cooperative binding of NLS cargo and Imp-β to KPNA results in an 'open'state. Characterization of KPNA2-KPNA7 chimeric proteins suggests that features of both the IBB domain and the core structure of the receptor contribute to the extent of IBB domain accessibility for Imp-β binding, which likely reflects an 'open' state. We also provide evidence that KPNA7 maintains an open-state in the nucleus. We speculate that KPNA7 could function within the nucleus by interacting with NLS-containing proteins.

The Wolbachia cytoplasmic incompatibility enzyme CidB targets nuclear import and protamine-histone exchange factors

Intracellular Wolbachia bacteria manipulate arthropod reproduction to promote their own inheritance. The most prevalent mechanism, cytoplasmic incompatibility (CI), traces to a Wolbachia deubiquitylase, CidB, and CidA. CidB has properties of a toxin, while CidA binds CidB and rescues embryonic viability. CidB is also toxic to yeast where we identified both host effects and high-copy suppressors of toxicity. The strongest suppressor was karyopherin-α, a nuclear-import receptor; this required nuclear localization-signal binding. A protein-interaction screen of Drosophila extracts using a substrate-trapping catalytic mutant, CidB*, also identified karyopherin-α; the P32 protamine-histone exchange factor bound as well. When CidB* bound CidA, these host protein interactions disappeared. These associations would place CidB at the zygotic male pronucleus where CI defects first manifest. Overexpression of karyopherin-α, P32, or CidA in female flies suppressed CI. We propose that CidB targets nuclear-protein import and protamine-histone exchange and that CidA rescues embryos by restricting CidB access to its targets.

Maternal Nanos inhibits Importin-α2/Pendulin-dependent nuclear import to prevent somatic gene expression in the Drosophila germline

Repression of somatic gene expression in germline progenitors is one of the critical mechanisms involved in establishing the germ/soma dichotomy. In Drosophila, the maternal Nanos (Nos) and Polar granule component (Pgc) proteins are required for repression of somatic gene expression in the primordial germ cells, or pole cells. Pgc suppresses RNA polymerase II-dependent global transcription in pole cells, but it remains unclear how Nos represses somatic gene expression. Here, we show that Nos represses somatic gene expression by inhibiting translation of maternal importin-α2 (impα2) mRNA. Mis-expression of Impα2 caused aberrant nuclear import of a transcriptional activator, Ftz-F1, which in turn activated a somatic gene, fushi tarazu (ftz), in pole cells when Pgc-dependent transcriptional repression was impaired. Because ftz expression was not fully activated in pole cells in the absence of either Nos or Pgc, we propose that Nos-dependent repression of nuclear import of transcriptional activator(s) and Pgc-dependent suppression of global transcription act as a 'double-lock' mechanism to inhibit somatic gene expression in germline progenitors.

A fast-evolving X-linked duplicate of importin-α2 is overexpressed in sex-ratio drive in Drosophila neotestacea

Selfish genetic elements that manipulate gametogenesis to achieve a transmission advantage are known as meiotic drivers. Sex-ratio X chromosomes (SR) are meiotic drivers that prevent the maturation of Y-bearing sperm in male carriers to result in the production of mainly female progeny. The spread of an SR chromosome can affect host genetic diversity and genome evolution, and can even cause host extinction if it reaches sufficiently high prevalence. Meiotic drivers have evolved independently many times, though only in a few cases is the underlying genetic mechanism known. In this study we use a combination of transcriptomics and population genetics to identify widespread expression differences between the standard (ST) and sex-ratio (SR) X chromosomes of the fly Drosophila neotestacea. We found the X chromosome is enriched for differentially expressed transcripts and that many of these X-linked differentially expressed transcripts had elevated K_a /K_s values between ST and SR, indicative of potential functional differences. We identified a set of candidate transcripts, including a testis-specific, X-linked duplicate of the nuclear transport gene importin-α2 that is overexpressed in SR. We find suggestions of positive selection in the lineage leading to the duplicate and that its molecular evolutionary patterns are consistent with relaxed purifying selection in ST. As these patterns are consistent with involvement in the mechanism of drive in this species, this duplicate is a strong candidate worthy of further functional investigation. Nuclear transport may be a common target for genetic conflict, as the mechanism of the autosomal Segregation Distorter drive system in D. melanogaster involves the same pathway.

Importin α: functions as a nuclear transport factor and beyond

Nucleocytoplasmic transport is an essential process in eukaryotes. The molecular mechanisms underlying nuclear transport that involve the nuclear transport receptor, small GTPase Ran, and the nuclear pore complex are highly conserved from yeast to humans. On the other hand, it has become clear that the nuclear transport system diverged during evolution to achieve various physiological functions in multicellular eukaryotes. In this review, we first summarize the molecular mechanisms of nuclear transport and how these were elucidated. Then, we focus on the diverse functions of importin α, which acts not merely an import factor but also as a multi-functional protein contributing to a variety of cellular functions in higher eukaryotes.

Comparative Proteomic Profiling Reveals Molecular Characteristics Associated with Oogenesis and Oocyte Maturation during Ovarian Development of Bactrocera dorsalis (Hendel)

Time-dependent expression of proteins in ovary is important to understand oogenesis in insects. Here, we profiled the proteomes of developing ovaries from Bactrocera dorsalis (Hendel) to obtain information about ovarian development with particular emphasis on differentially expressed proteins (DEPs) involved in oogenesis. A total of 4838 proteins were identified with an average peptide number of 8.15 and sequence coverage of 20.79%. Quantitative proteomic analysis showed that a total of 612 and 196 proteins were differentially expressed in developing and mature ovaries, respectively. Furthermore, 153, 196 and 59 potential target proteins were highly expressed in early, vitellogenic and mature ovaries and most tested DEPs had the similar trends consistent with the respective transcriptional profiles. These proteins were abundantly expressed in pre-vitellogenic and vitellogenic stages, including tropomyosin, vitellogenin, eukaryotic translation initiation factor, heat shock protein, importin protein, vitelline membrane protein, and chorion protein. Several hormone and signal pathway related proteins were also identified during ovarian development including piRNA, notch, insulin, juvenile, and ecdysone hormone signal pathways. This is the first report of a global ovary proteome of a tephritid fruit fly, and may contribute to understanding the complicate processes of ovarian development and exploring the potentially novel pest control targets.

Distinctive Nuclear Localization Signals in the Oomycete Phytophthora sojae

To date, nuclear localization signals (NLSs) that target proteins to nuclei in oomycetes have not been defined, but have been assumed to be the same as in higher eukaryotes. Here, we use the soybean pathogen Phytophthora sojae as a model to investigate these sequences in oomycetes. By establishing a reliable in vivo NLS assay based on confocal microscopy, we found that many canonical monopartite and bipartite classical NLSs (cNLSs) mediated nuclear import poorly in P. sojae. We found that efficient localization of P. sojae nuclear proteins by cNLSs requires additional basic amino acids at distal sites or collaboration with other NLSs. We found that several representatives of another well-characterized NLS, proline-tyrosine NLS (PY-NLS) also functioned poorly in P. sojae. To characterize PY-NLSs in P. sojae, we experimentally defined the residues required by functional PY-NLSs in three P. sojae nuclear-localized proteins. These results showed that functional P. sojae PY-NLSs include an additional cluster of basic residues for efficient nuclear import. Finally, analysis of several highly conserved P. sojae nuclear proteins including ribosomal proteins and core histones revealed that these proteins exhibit a similar but stronger set of sequence requirements for nuclear targeting compared with their orthologs in mammals or yeast.

Importin α1 Mediates Yorkie Nuclear Import via an N-terminal Non-canonical Nuclear Localization Signal

The Hippo signaling pathway controls organ size by orchestrating cell proliferation and apoptosis. When the Hippo pathway was inactivated, the transcriptional co-activator Yorkie translocates into the nucleus and forms a complex with transcription factor Scalloped to promote the expression of Hippo pathway target genes. Therefore, the nuclear translocation of Yorkie is a critical step in Hippo signaling. Here, we provide evidence that the N-terminal 1-55 amino acids of Yorkie, especially Arg-15, were essential for its nuclear localization. By mass spectrometry and biochemical analyses, we found that Importin α1 can directly interact with the Yorkie N terminus and drive Yorkie into the nucleus. Further experiments show that the upstream component Hippo can inhibit Importin α1-mediated Yorkie nuclear import. Taken together, we identified a potential nuclear localization signal at the N-terminal end of Yorkie as well as a critical role for Importin α1 in Yorkie nuclear import.

Structural Biology and Regulation of Protein Import into the Nucleus

Proteins are translated in the cytoplasm, but many need to access the nucleus to perform their functions. Understanding how these nuclear proteins are transported through the nuclear envelope and how the import processes are regulated is therefore an important aspect of understanding cell function. Structural biology has played a key role in understanding the molecular events during the transport processes and their regulation, including the recognition of nuclear targeting signals by the corresponding receptors. Here, we review the structural basis of the principal nuclear import pathways and the molecular basis of their regulation. The pathways involve transport factors that are members of the β-karyopherin family, which can bind cargo directly (e.g., importin-β, transportin-1, transportin-3, importin-13) or through adaptor proteins (e.g., importin-α, snurportin-1, symportin-1), as well as unrelated transport factors such as Hikeshi, involved in the transport of heat-shock proteins, and NTF2, involved in the transport of RanGDP. Solenoid proteins feature prominently in these pathways. Nuclear transport factors recognize nuclear targeting signals on the cargo proteins, including the classical nuclear localization signals, recognized by the adaptor importin-α, and the PY nuclear localization signals, recognized by transportin-1. Post-translational modifications, particularly phosphorylation, constitute key regulatory mechanisms operating in these pathways.

Structure of Importin-α from a Filamentous Fungus in Complex with a Classical Nuclear Localization Signal

Neurospora crassa is a filamentous fungus that has been extensively studied as a model organism for eukaryotic biology, providing fundamental insights into cellular processes such as cell signaling, growth and differentiation. To advance in the study of this multicellular organism, an understanding of the specific mechanisms for protein transport into the cell nucleus is essential. Importin-α (Imp-α) is the receptor for cargo proteins that contain specific nuclear localization signals (NLSs) that play a key role in the classical nuclear import pathway. Structures of Imp-α from different organisms (yeast, rice, mouse, and human) have been determined, revealing that this receptor possesses a conserved structural scaffold. However, recent studies have demonstrated that the Impα mechanism of action may vary significantly for different organisms or for different isoforms from the same organism. Therefore, structural, functional, and biophysical characterization of different Impα proteins is necessary to understand the selectivity of nuclear transport. Here, we determined the first crystal structure of an Impα from a filamentous fungus which is also the highest resolution Impα structure already solved to date (1.75 Å). In addition, we performed calorimetric analysis to determine the affinity and thermodynamic parameters of the interaction between Imp-α and the classical SV40 NLS peptide. The comparison of these data with previous studies on Impα proteins led us to demonstrate that N. crassa Imp-α possess specific features that are distinct from mammalian Imp-α but exhibit important similarities to rice Imp-α, particularly at the minor NLS binding site.

Tissue, cell type and stage-specific ectopic gene expression and RNAi induction in the Drosophila testis

The Drosophila testis has numerous advantages for the study of basic cellular processes, as production of sperm requires a highly orchestrated and complex combination of morphological changes and developmentally regulated transitions. Experimental genetics using Drosophila melanogaster has advanced dramatically with the advent of systems for ectopic expression of genetic elements in specific cells. However the genetic tools used in Drosophila research have rarely been generated with the testes in mind, and the utility of relatively few systems has been documented for this tissue. Here I will summarize ectopic expression systems that are known to work for the testis, and provide advice for selection of the most appropriate expression system in specific experimental situations.

Nuclear import of SAMHD1 is mediated by a classical karyopherin α/β1 dependent pathway and confers sensitivity to VpxMAC induced ubiquitination and proteasomal degradation

Background

The deoxynucleotide-triphosphate (dNTP) hydrolase sterile alpha motif domain and HD domain 1 (SAMHD1) is a nuclear protein that inhibits HIV-1 infection in myeloid cells as well as quiescent CD4 T-cells, by decreasing the intracellular dNTP concentration below a level that is required for efficient reverse transcription. The Vpx proteins of the SIV_SMM/HIV-2 lineage of lentiviruses bind SAMHD1 and recruit an ubiquitin ligase, leading to polyubiquitination and proteasomal degradation.

Results

Here, we have investigated the importance of nuclear localization for SAMHD1′s antiviral function as well as its sensitivity to the Vpx protein of SIV_MAC. Using GST pull down assays, as well as RNA silencing approaches, we show that SAMHD1 preferentially uses karyopherin α2 (KPNA2) and a classical N-terminal nuclear localization signal (¹⁴KRPR¹⁷) to enter the nucleus. Reduction of karyopherin β1 (KPNB1) or KPNA2 by RNAi also led to cytoplasmic re-distribution of SAMHD1. Using primary human monocyte-derived macrophages (MDM), a cell type in which SAMHD1 is naturally expressed to high levels, we demonstrate that nuclear localization is not required for its antiviral activity. Cytoplasmic SAMHD1 still binds to Vpx_MAC, is efficiently polyubiquitinated, but is not degraded. We also find that Vpx_MAC-induced SAMHD1 degradation was partially reversed by ubiquitin carrying the K48R or K11R substitution mutations, suggesting involvement of K48 and K11 linkages in SAMHD1 polyubiquitination. Using ubiquitin K-R mutants also revealed differences in the ubiquitin linkages between wild type and cytoplasmic forms of SAMHD1, suggesting a potential association with the resistance of cytoplasmic SAMHD1 to Vpx_MAC induced degradation.

Conclusions

Our work extends published observations on SAMHD1 nuclear localization to a natural cell type for HIV-1 infection, identifies KPNA2/KPNB1 as cellular proteins important for SAMHD1 nuclear import, and indicates that components of the nuclear proteasomal degradation machinery are required for SAMHD1 degradation.

Functional insights of nucleocytoplasmic transport in plants

Plant nucleocytoplasmic transport beyond the nuclear envelope is important not only for basic cellular functions but also for growth, development, hormonal signaling, and responses to environmental stimuli. Key components of this transport system include nuclear transport receptors and nucleoporins. The functional and physical interactions between receptors and the nuclear pore in the nuclear membrane are indispensable for nucleocytoplasmic transport. Recently, several groups have reported various plant mutants that are deficient in factors involved in nucleocytoplasmic transport. Here, we summarize the current state of knowledge about nucleocytoplasmic transport in plants, and we review the plant-specific regulation and roles of this process in plants.

Transcriptional activity and nuclear localization of Cabut, the Drosophila ortholog of vertebrate TGF-β-inducible early-response gene (TIEG) proteins

Background

Cabut (Cbt) is a C₂H₂-class zinc finger transcription factor involved in embryonic dorsal closure, epithelial regeneration and other developmental processes in Drosophila melanogaster. Cbt orthologs have been identified in other Drosophila species and insects as well as in vertebrates. Indeed, Cbt is the Drosophila ortholog of the group of vertebrate proteins encoded by the TGF-ß-inducible early-response genes (TIEGs), which belong to Sp1-like/Krüppel-like family of transcription factors. Several functional domains involved in transcriptional control and subcellular localization have been identified in the vertebrate TIEGs. However, little is known of whether these domains and functions are also conserved in the Cbt protein.

Methodology/Principal Findings

To determine the transcriptional regulatory activity of the Drosophila Cbt protein, we performed Gal4-based luciferase assays in S2 cells and showed that Cbt is a transcriptional repressor and able to regulate its own expression. Truncated forms of Cbt were then generated to identify its functional domains. This analysis revealed a sequence similar to the mSin3A-interacting repressor domain found in vertebrate TIEGs, although located in a different part of the Cbt protein. Using β-Galactosidase and eGFP fusion proteins, we also showed that Cbt contains the bipartite nuclear localization signal (NLS) previously identified in TIEG proteins, although it is non-functional in insect cells. Instead, a monopartite NLS, located at the amino terminus of the protein and conserved across insects, is functional in Drosophila S2 and Spodoptera exigua Sec301 cells. Last but not least, genetic interaction and immunohistochemical assays suggested that Cbt nuclear import is mediated by Importin-α2.

Conclusions/Significance

Our results constitute the first characterization of the molecular mechanisms of Cbt-mediated transcriptional control as well as of Cbt nuclear import, and demonstrate the existence of similarities and differences in both aspects of Cbt function between the insect and the vertebrate TIEG proteins.

Regulated nucleocytoplasmic transport during gametogenesis

Gametogenesis is the process by which sperm or ova are produced in the gonads. It is governed by a tightly controlled series of gene expression events, with some common and others distinct for males and females. Nucleocytoplasmic transport is of central importance to the fidelity of gene regulation that is required to achieve the precisely regulated germ cell differentiation essential for fertility. In this review we discuss the physiological importance for gamete formation of the molecules involved in classical nucleocytoplasmic protein transport, including importins/karyopherins, Ran and nucleoporins. To address what functions/factors are conserved or specialized for these developmental processes between species, we compare knowledge from mice, flies and worms. The present analysis provides evidence of the necessity for and specificity of each nuclear transport factor and for nucleoporins during germ cell differentiation. This article is part of a Special Issue entitled: Nuclear Transport and RNA Processing.

Specific Cooperation Between Imp-α2 and Imp-β/Ketel in Spindle Assembly During Drosophila Early Nuclear Divisions

The multifunctional factors Imp-α and Imp-β are involved in nuclear protein import, mitotic spindle dynamics, and nuclear membrane formation. Furthermore, each of the three members of the Imp-α family exerts distinct tasks during development. In Drosophila melanogaster, the imp-α2 gene is critical during oogenesis for ring canal assembly; specific mutations, which allow oogenesis to proceed normally, were found to block early embryonic mitosis. Here, we show that imp-α2 and imp-β genetically interact during early embryonic development, and we characterize the pattern of defects affecting mitosis in embryos laid by heterozygous imp-α2(D14) and imp-β(KetRE34) females. Embryonic development is arrested in these embryos but is unaffected in combinations between imp-β(KetRE34) and null mutations in imp-α1 or imp-α3. Furthermore, the imp-α2(D14)/imp-β(KetRE34) interaction could only be rescued by an imp-α2 transgene, albeit not imp-α1 or imp-α3, showing the exclusive imp-α2 function with imp-β. Use of transgenes carrying modifications in the major Imp-α2 domains showed the critical requirement of the nuclear localization signal binding (NLSB) site in this process. In the mutant embryos, we found metaphase-arrested mitoses made of enlarged spindles, suggesting an unrestrained activity of factors promoting spindle assembly. In accordance with this, we found that Imp-β(KetRE34) and Imp-β(KetD) bind a high level of RanGTP/GDP, and a deletion decreasing RanGTP level suppresses the imp-β(KetRE34) phenotype. These data suggest that a fine balance among Imp-α2, Imp-β, RanGTP, and the NLS cargos is critical for mitotic progression during early embryonic development.

Birth, death, and replacement of karyopherins in Drosophila

Nucleocytoplasmic transport is a broadly conserved process across eukaryotes. Despite its essential function and conserved mechanism, components of the nuclear transport apparatus have been implicated in genetic conflicts in Drosophila, especially in the male germ line. The best understood case is represented by a truncated RanGAP gene duplication that is part of the segregation distorter system in Drosophila melanogaster. Consistent with the hypothesis that the nuclear transport pathway is at the heart of mediating genetic conflicts, both nucleoporins and directionality imposing components of nuclear transport have previously been shown to evolve under positive selection. Here, we present a comprehensive phylogenomic analysis of importins (karyopherins) in Drosophila evolution. Importins are adaptor molecules that physically mediate the transport of cargo molecules and comprise the third component of the nuclear transport apparatus. We find that importins have been repeatedly gained and lost throughout various stages of Drosophila evolution, including two intriguing examples of an apparently coincident loss and gain of nonorthologous and noncanonical importin-α. Although there are a few signatures of episodic positive selection, genetic innovation in importin evolution is more evident in patterns of recurrent gene birth and loss specifically for function in Drosophila testes, which is consistent with their role in supporting host genomes defense against segregation distortion.

Importin alpha2-interacting proteins with nuclear roles during mammalian spermatogenesis

Spermatogenesis, the process of generating haploid sperm capable of fertilizing the female gamete, requires the timely transport into the nucleus of transcription and chromatin-remodeling factors, mediated by members of the importin (IMP) superfamily. Previous IMP expression profiling implies a role for IMPalpha2 in testicular germ cells late in spermatogenesis. To identify interacting proteins of IMPalpha2 that are potential drivers of germ cell development, we performed yeast two-hybrid screening of an adult mouse testis library. IMPalpha2 interactions were verified by coimmunoprecipitation approaches, whereas immunohistochemical staining of testis sections confirmed their coexpression with IMPalpha2 in specific testicular cell types. Key interactors identified were a novel isoform of a cysteine and histidine rich protein (Chrp), a protein inhibitor of activated STAT (PIAS) family member involved in transcriptional regulation and sumoylation, Androgen receptor interacting protein 3 (Arip3), and Homologous protein 2 (Hop2), known to be involved in homologous chromosome pairing and recombination, all of which are highly expressed in the testis and show mRNA expression profiles similar to that of IMPalpha2 throughout testicular development. This is the first study to identify binding partners of IMPalpha2 in the developmental context of germ line development, and we propose that the regulated expression and timely IMPalpha2-mediated nuclear transport of these proteins may coordinate events during spermatogenesis, with IMPalpha2-mediated nuclear localization representing a potentially critical developmental switch in the testis.

Regulation of Nuclear Import During Differentiation; The IMP alpha Gene Family and Spermatogenesis

Access to nuclear genes in eukaryotes is provided by members of the importin (IMP) superfamily of proteins, which are of α- or β-types, the best understood nuclear import pathway being mediated by a heterodimer of an IMP α and IMP β1. IMP α recognises specific targeting signals on cargo proteins, while IMP β1 mediates passage into, and release within, the nucleus by interacting with other components of the transport machinery, including the monomeric guanine nucleotide binding protein Ran. In this manner, hundreds of different proteins can be targeted specifically into the nucleus in a tightly regulated fashion. The IMP α gene family has expanded during evolution, with only a single IMP α (Srp1p) gene in budding yeast, and three (IMP α1, 2/pendulin and 3) and five (IMP α1, -2, -3, -4 and -6) IMP α genes in Drosophila melanogaster and mouse respectively, which fall into three phylogenetically distinct groups. The fact that IMP α3 and IMP α2 are only present in metazoans implies that they emerged during the evolution of multicellular animals to perform specialised roles in particular cells and tissues. This review describes what is known of the IMP α gene family in mouse and in D. melanogaster, including a comparitive examination of their mRNA expression profiles in a highly differentiated tissue, the testis. The clear implication of their highly regulated synthesis during the course of spermatogenesis is that the different IMP αs have distinct expression patterns during cellular differentiation, implying tissue/cell type-specific roles.

Distinct roles for classical nuclear import receptors in the growth of multinucleated muscle cells

Proper muscle function is dependent on spatial and temporal control of gene expression in myofibers. Myofibers are multinucleated cells that are formed, repaired and maintained by the process of myogenesis in which progenitor myoblasts proliferate, differentiate and fuse. Gene expression is dependent upon proteins that require facilitated nuclear import, however little is known about the regulation of nucleocytoplasmic transport during the formation of myofibers. We analyzed the role of karyopherin alpha (KPNA), a key classical nuclear import receptor, during myogenesis. We established that five karyopherin alpha paralogs are expressed by primary mouse myoblasts in vitro and that their steady-state levels increase in multinucleated myotubes, suggesting a global increase in demand for classical nuclear import during myogenesis. We used siRNA-mediated knockdown to identify paralog-specific roles for KPNA1 and KPNA2 during myogenesis. KPNA1 knockdown increased myoblast proliferation, whereas KPNA2 knockdown decreased proliferation. In contrast, no proliferation defect was observed with KPNA4 knockdown. Only knockdown of KPNA2 decreased myotube growth. These results identify distinct pathways involved in myoblast proliferation and myotube growth that rely on specific nuclear import receptors suggesting that regulation of classical nuclear import pathways likely plays a critical role in controlling gene expression in skeletal muscle.

Identification of a classical bipartite nuclear localization signal in the Drosophila TEA/ATTS protein scalloped

Drosophila melanogaster wing development has been shown to rely on the activity of a complex of two proteins, Scalloped (Sd) and Vestigial (Vg). Within this complex, Sd is known to provide DNA binding though its TEA/ATTS domain, while Vg modulates this binding and provides transcriptional activation through N- and C-terminal activation domains. There is also evidence that Sd is required for the nuclear translocation of Vg. Indeed, a candidate sequence which shows consensus to the bipartite family of nuclear localization signals (NLSs) has been identified within Sd previously, though it is not known if it is functional, or if additional unpredicted signals that mediate nuclear transport exist within the protein. By expressing various enhanced green fluorescent protein (eGFP) tagged constructs within Drosophila S2 cells, we demonstrate that this NLS is indeed functional and necessary for the proper nuclear localization of Sd. Additionally, the region containing the NLS is critical for the wildtype function of ectopically expressed Sd, in the context of wing development. Using site-directed mutagenesis, we have identified a group of five amino acids within this NLS which is critical for its function, as well as another group of two which is of lesser importance. Together with data that suggests that this sequence mediates interactions with Importin-α3, we conclude that the identified NLS is likely a classical bipartite signal. Further dissection of Sd has also revealed that a large portion of the C-terminal domain of the protein is required its proper nuclear localization. Finally, a Leptomycin B (LB) sensitive signal which appears to facilitate nuclear export is identified, raising the possibility that Sd also contains a nuclear export signal (NES).

Targeted disruption of one of the importin α family members leads to female functional incompetence in delivery

Importin α mediates the nuclear import of proteins through nuclear pore complexes in eukaryotic cells, and is common to all eukaryotes. Previous reports identified at least six importin α family genes in mice. Although these isoforms show differential binding to various import cargoes in vitro, the in vivo physiological roles of these mammalian importin α isoforms remain unknown. Here, we generated and examined importin α5 knockout (impα5(-/-)) mice. These mice developed normally, and showed no gross histological abnormalities in most major organs. However, the ovary and uterus of impα5(-/-) female mice exhibited hypoplasia. Furthermore, we found that impα5(-/-) female mice had a 50% decrease in serum progesterone levels and a 57% decrease in progesterone receptor mRNA levels in the ovary. Additionally, impα5(-/-) uteruses that were treated with exogenous gonadotropins displayed hypertrophy, similarly to progesterone receptor-deficient mice. Although these mutant female mice could become pregnant, the total number of pups was significantly decreased, and some of the pups were dead at birth. These results suggest that importin α5 has essential roles in the mammalian female reproductive organs.

CTNNBL1 is a novel nuclear localization sequence-binding protein that recognizes RNA-splicing factors CDC5L and Prp31

Nuclear proteins typically contain short stretches of basic amino acids (nuclear localization sequences; NLSs) that bind karyopherin α family members, directing nuclear import. Here, we identify CTNNBL1 (catenin-β-like 1), an armadillo motif-containing nuclear protein that exhibits no detectable primary sequence homology to karyopherin α, as a novel, selective NLS-binding protein. CTNNBL1 (a single-copy gene conserved from fission yeast to man) was previously found associated with Prp19-containing RNA-splicing complexes as well as with the antibody-diversifying enzyme AID. We find that CTNNBL1 association with the Prp19 complex is mediated by recognition of the NLS of the CDC5L component of the complex and show that CTNNBL1 also interacts with Prp31 (another U4/U6.U5 tri-snRNP-associated splicing factor) through its NLS. As with karyopherin αs, CTNNBL1 binds NLSs via its armadillo (ARM) domain, but displays a separate, more selective NLS binding specificity. Furthermore, the CTNNBL1/AID interaction depends on amino acids forming the AID conformational NLS with CTNNBL1-deficient cells showing a partial defect in AID nuclear accumulation. However, in further contrast to karyopherin αs, the CTNNBL1 N-terminal region itself binds karyopherin αs (rather than karyopherin β), suggesting a function divergent from canonical nuclear transport. Thus, CTNNBL1 is a novel NLS-binding protein, distinct from karyopherin αs, with the results suggesting a possible role in the selective intranuclear targeting or interactions of some splicing-associated complexes.

Regulation of nucleocytoplasmic transport in skeletal muscle

Proper skeletal muscle function is dependent on spatial and temporal control of gene expression in multinucleated myofibers. In addition, satellite cells, which are tissue-specific stem cells that contribute critically to repair and maintenance of skeletal muscle, are also required for normal muscle physiology. Gene expression in both myofibers and satellite cells is dependent upon nuclear proteins that require facilitated nuclear transport. A unique challenge for myofibers is controlling the transcriptional activity of hundreds of nuclei in a common cytoplasm yet achieving nuclear selectivity in transcription at specific locations such as neuromuscular synapses and myotendinous junctions. Nucleocytoplasmic transport of macromolecular cargoes is regulated by a complex interplay among various components of the nuclear transport machinery, namely nuclear pore complexes, nuclear envelope proteins, and various soluble transport receptors. The focus of this review is to highlight what is known about the nuclear transport machinery and its regulation in skeletal muscle and to consider the unique challenges that multinucleated muscle cells as well as satellite cells encounter in regulating nucleocytoplasmic transport during cell differentiation and tissue adaptation. Understanding how regulated nucleocytoplasmic transport controls gene expression in skeletal muscle may lead to further insights into the mechanisms contributing to muscle growth and maintenance throughout the lifespan of an individual.

Drosophila Importin-α2 is involved in synapse, axon and muscle development

Nuclear import is required for communication between the cytoplasm and the nucleus and to enact lasting changes in gene transcription following stimuli. Binding to an Importin-α molecule in the cytoplasm is often required to mediate nuclear entry of a signaling protein. As multiple isoforms of Importin-α exist, some may be responsible for the entry of distinct cargoes rather than general nuclear import. Indeed, in neuronal systems, Importin-α isoforms can mediate very specific processes such as axonal tiling and communication of an injury signal. To study nuclear import during development, we examined the expression and function of Importin-α2 in Drosophila melanogaster. We found that Importin-α2 was expressed in the nervous system where it was required for normal active zone density at the NMJ and axonal commissure formation in the central nervous system. Other aspects of synaptic morphology at the NMJ and the localization of other synaptic markers appeared normal in importin-α2 mutants. Importin-α2 also functioned in development of the body wall musculature. Mutants in importin-α2 exhibited errors in muscle patterning and organization that could be alleviated by restoring muscle expression of Importin-α2. Thus, Importin-α2 is needed for some processes in the development of both the nervous system and the larval musculature.

Karyopherin alpha7 (KPNA7), a divergent member of the importin alpha family of nuclear import receptors

Background

Classical nuclear localization signal (NLS) dependent nuclear import is carried out by a heterodimer of importin α and importin β. NLS cargo is recognized by importin α, which is bound by importin β. Importin β mediates translocation of the complex through the central channel of the nuclear pore, and upon reaching the nucleus, RanGTP binding to importin β triggers disassembly of the complex. To date, six importin α family members, encoded by separate genes, have been described in humans.

Results

We sequenced and characterized a seventh member of the importin α family of transport factors, karyopherin α 7 (KPNA7), which is most closely related to KPNA2. The domain of KPNA7 that binds Importin β (IBB) is divergent, and shows stronger binding to importin β than the IBB domains from of other importin α family members. With regard to NLS recognition, KPNA7 binds to the retinoblastoma (RB) NLS to a similar degree as KPNA2, but it fails to bind the SV40-NLS and the human nucleoplasmin (NPM) NLS. KPNA7 shows a predominantly nuclear distribution under steady state conditions, which contrasts with KPNA2 which is primarily cytoplasmic.

Conclusion

KPNA7 is a novel importin α family member in humans that belongs to the importin α2 subfamily. KPNA7 shows different subcellular localization and NLS binding characteristics compared to other members of the importin α family. These properties suggest that KPNA7 could be specialized for interactions with select NLS-containing proteins, potentially impacting developmental regulation.

Organization and regulation of nucleocytoplasmic transport

Separation of DNA replication and transcription, which occur in the nucleus, from protein synthesis, which occurs in the cytoplasm, allows a more precise regulation of these processes. Selective exchange of macromolecules between the two compartments is mediated by proteins of the nuclear pore complex (NPC). Receptor proteins of the karyopherin family interact with NPC components and transfer their cargos between the nucleus and cytoplasm. Nucleocytoplasmic transport pathways are regulated at multiple levels by modulating the expression or function of individual cargoes, transport receptors, or the transport channel. The regulatory levels have increasingly broad effects on the transport pathways and affect a wide range of processes from gene expression to development and differentiation.

The nuclear transport machinery in Caenorhabditis elegans: A central role in morphogenesis

Our full understanding of the various roles for the nuclear transport machinery has come from a variety of model organisms including yeast, nematodes, fruit flies and vertebrates. Using the nematode Caenorhabditis elegans, it has been shown that the karyopherin family of nuclear transporters and the components of the Ran cycle have roles not only in nuclear protein transport, but also in mitotic spindle formation and regulation, and in nuclear envelope assembly. These studies have also demonstrated a role for nuclear transport factors in cellular differentiation and development, particularly for the formation of germ cells. This review highlights the small number of studies in C. elegans that have been critical to our understanding of this important cellular process.

Evolution of the metazoan-specific importin alpha gene family

Importin alphas are import receptors for nuclear localization signal-containing proteins. Most animal importin alphas assort into alpha1, alpha2, and alpha3 groups. Studies in Drosophila melanogaster, Caenorhabditis elegans, and mouse suggest that the animal importin alpha gene family evolved from ancestral plant-like genes to serve paralog-specific roles in gametogenesis. To explore this hypothesis we extended the phylogenetic analysis of the importin alpha gene family to nonbilateral animals and investigated whether animal-like genes occur in premetazoan taxa. Maximum likelihood analysis suggests that animal-like importin alpha genes occur in the Choanoflaggelate Monosiga brevicollis and the amoebozoan Dictyostelium; however, both of these results are caused by long-branch attraction effects. The absence of animal-like alpha genes in premetazoan taxa is consistent with the hypothesis that they duplicated and then specialized to function in animal gametogenesis. The gene structures of the importin alphas provide insight into how the animal importin alpha gene family may have evolved from the most likely ancestral gene. Interestingly, animal alpha1s are more similar to plant and fungal alpha1-like sequences than they are to animal alpha2s or alpha3s. We show that animal alpha1 genes share most of their introns with plant alpha1-like genes, and alpha2s and alpha3s share many more intron positions with each other than with the alpha1s. Together, phylogenetics and gene structure analysis suggests a parsimonious path for the evolution of the mammalian importin alpha gene family from an ancestral alpha1-like progenitor. Finally, these results establish a rational basis for a unified nomenclature of the importin alpha gene family.

IMPa-4, an Arabidopsis importin alpha isoform, is preferentially involved in agrobacterium-mediated plant transformation

Successful transformation of plants by Agrobacterium tumefaciens requires that the bacterial T-complex actively escorts T-DNA into the host's nucleus. VirD2 and VirE2 are virulence proteins on the T-complex that have plant-functional nuclear localization signal sequences that may recruit importin alpha proteins of the plant for nuclear import. In this study, we evaluated the involvement of seven of the nine members of the Arabidopsis thaliana importin alpha family in Agrobacterium transformation. Yeast two-hybrid, plant bimolecular fluorescence complementation, and in vitro protein-protein interaction assays demonstrated that all tested Arabidopsis importin alpha members can interact with VirD2 and VirE2. However, only disruption of the importin IMPa-4 inhibited transformation and produced the rat (resistant to Agrobacterium transformation) phenotype. Overexpression of six importin alpha members, including IMPa-4, rescued the rat phenotype in the impa-4 mutant background. Roots of wild-type and impa-4 Arabidopsis plants expressing yellow fluorescent protein-VirD2 displayed nuclear localization of the fusion protein, indicating that nuclear import of VirD2 is not affected in the impa-4 mutant. Somewhat surprisingly, VirE2-yellow fluorescent protein mainly localized to the cytoplasm of both wild-type and impa-4 Arabidopsis cells and to the cytoplasm of wild-type tobacco (Nicotiana tabacum) cells. However, bimolecular fluorescence complementation assays indicated that VirE2 could localize to the nucleus when IMPa-4, but not when IMPa-1, was overexpressed.

Novel expression of importin alpha homologue in marine teleost, Pagrus major

Importin alpha proteins are critical modulators of the classical nuclear protein import pathway. Although the physiological roles of importin alpha have been extensively studied in invertebrates and mammals, very little is known about their counterparts in lower vertebrates. In this study, to elucidate the roles of importin alpha in a teleost species, we isolated and characterized red seabream (Pagrus major) importin alpha cDNA derived from ovary and found changes in the mRNA levels of importin alpha in male and female red seabream during sexual maturation. The 1846-bp cDNA encodes a 520 amino acid protein that includes the importin beta-binding domain, a short acidic domain, and an armadillo (arm) repeat domain. Northern blot analysis and reverse transcription-polymerase chain reaction (RT-PCR) showed transcription of red seabream importin alpha in testis and ovary but not in the other tissues. The importin alpha mRNA levels in males increase in association with testicular development, whereas those in females remain high throughout sexual maturation. These findings suggest that red seabream ovary-derived importin alpha may be controlled in a tissue-specific manner and may perform unique functions in the gonad in addition to its involvement in nuclear transport.

The RanGTP gradient - a GPS for the mitotic spindle

The GTPase Ran has a key role in nuclear import and export, mitotic spindle assembly and nuclear envelope formation. The cycling of Ran between its GTP- and GDP-bound forms is catalyzed by the chromatin-bound guanine nucleotide exchange factor RCC1 and the cytoplasmic Ran GTPase-activating protein RanGAP. The result is an intracellular concentration gradient of RanGTP that equips eukaryotic cells with a ;genome-positioning system' (GPS). The binding of RanGTP to nuclear transport receptors (NTRs) of the importin beta superfamily mediates the effects of the gradient and generates further downstream gradients, which have been elucidated by fluorescence resonance energy transfer (FRET) imaging and computational modeling. The Ran-dependent GPS spatially directs many functions required for genome segregation by the mitotic spindle during mitosis. Through exportin 1, RanGTP recruits essential centrosome and kinetochore components, whereas the RanGTP-induced release of spindle assembly factors (SAFs) from importins activates SAFs to nucleate, bind and organize nascent spindle microtubules. Although a considerable fraction of cytoplasmic SAFs is active and RanGTP induces only partial further activation near chromatin, bipolar spindle assembly is robustly induced by cooperativity and positive-feedback mechanisms within the network of Ran-activated SAFs. The RanGTP gradient is conserved, although its roles vary among different cell types and species, and much remains to be learned regarding its functions.

Drosophila Naked cuticle (Nkd) engages the nuclear import adaptor Importin-alpha3 to antagonize Wnt/beta-catenin signaling

Precise control of Wnt/beta-catenin signaling is critical for animal development, stem cell renewal, and prevention of disease. In the fruit fly Drosophila melanogaster, the naked cuticle (nkd) gene limits signaling by the Wnt ligand Wingless (Wg) during embryo segmentation. Nkd is an intracellular protein that is composed of separable membrane- and nuclear-localization sequences (NLS) as well as a conserved EF-hand motif that binds the Wnt receptor-associated scaffold protein Dishevelled (Dsh), but the mechanism by which Nkd inhibits Wnt signaling remains a mystery. Here we identify a second NLS in Nkd that is required for full activity and that binds to the canonical nuclear import adaptor Importin-alpha3. The Nkd NLS is similar to the Importin-alpha3-binding NLS in the Drosophila heat-shock transcription factor (dHSF), and each Importin-alpha3-binding NLS required intact basic residues in similar positions for nuclear import and protein function. Our results provide further support for the hypothesis that Nkd inhibits nuclear step(s) in Wnt/beta-catenin signaling and broaden our understanding of signaling pathways that engage the nuclear import machinery.

Drosophila importin alpha1 performs paralog-specific functions essential for gametogenesis

Importin alpha's mediate nuclear transport by linking nuclear localization signal (NLS)-containing proteins to importin beta1. Animal genomes encode three conserved groups of importin alpha's, alpha1's, alpha2's, and alpha3's, each of which are competent to bind classical NLS sequences. Using Drosophila melanogaster we describe the isolation and phenotypic characterization of the first animal importin alpha1 mutant. Animal alpha1's are more similar to ancestral plant and fungal alpha1-like genes than to animal alpha2 and alpha3 genes. Male and female importin alpha1 (Dalpha1) null flies developed normally to adulthood (with a minor wing defect) but were sterile with defects in gametogenesis. The Dalpha1 mutant phenotypes were rescued by Dalpha1 transgenes, but not by Dalpha2 or Dalpha3 transgenes. Genetic interactions between the ectopic expression of Dalpha1 and the karyopherins CAS and importin beta1 suggest that high nuclear levels of Dalpha1 are deleterious. We conclude that Dalpha1 performs paralog-specific activities that are essential for gametogenesis and that regulation of subcellular Dalpha1 localization may affect cell fate decisions. The initial expansion and specialization of the animal importin alpha-gene family may have been driven by the specialized needs of gametogenesis. These results provide a framework for studies of the more complex mammalian importin alpha-gene family.

Subcellular distribution of importins correlates with germ cell maturation

Importin proteins regulate access to the nucleus by recognizing and transporting distinct cargo proteins. Building on studies in Drosophila and Caenorhabditis elegans, we hypothesized that regulated expression and subcellular localization of specific importins may be linked to mammalian gonadal differentiation. We identified distinct developmental and cellular localization patterns for importins beta1, alpha3, alpha4 and RanBP5 (importin beta3) in fetal and postnatal murine testes using Western blotting and immunohistochemistry. Importin beta1 protein is detected in selected germ and somatic cells in fetal gonads, with a striking perinuclear staining evident from embryonic day (E) 14.5 within testicular gonocytes. RanBP5 exhibits age- and gender-specific subcellular localization within fetal gonads. At E12.5, RanBP5 protein is cytoplasmic in gonocytes but predominantly nuclear in oogonia, but by E14.5 RanBP5 appears nuclear in gonocytes and cytoplasmic in oogonia. In postnatal testes, importin alpha3 and alpha4 in spermatocytes, spermatids, and Sertoli cells display cytoplasmic and nuclear localization, respectively.

Stage-specific Importin13 activity influences meiosis of germ cells in the mouse

Importin-mediated transport of cargoes is known to be a key mechanism for nucleo-cytoplasmic trafficking of molecules. Ipo13, which is a member of Importin-beta gene family, encodes two transcripts by utilizing different transcription start sites. In the mouse, the full-length transcript (L-Ipo13) is expressed in the primordial germ cells in the embryo and is later expressed predominantly at the pachytene phase of meiosis in both male and female germ cells. The shorter transcript (TS-Ipo13) is only expressed in the germ cells in the adult testis. Activity of L-Ipo13, but not TS-Ipo13, mediates the nuclear accumulation of ubiquitin-conjugating enzyme 9 (UBC9), a cargo of human IPO13. This finding is consistent with the progressive accumulation of UBC9 in the nucleus of the meiotic germ cells after the onset of L-Ipo13 expression. Following siRNA knockdown of IPO13 activity in the fetal ovary, fewer germ cells are found to progress to the late-pachytene stage of meiosis and nuclear accumulation of UBC9 is reduced. Our findings strongly implicate a stage-specific role of IPO13 in nuclear-cytoplasmic translocation of cargoes that accompanies meiotic differentiation of the mouse germ cells.

The matricellular protein SPARC is internalized in Sertoli, Leydig, and germ cells during testis differentiation

The gene encoding the matricellular protein secreted protein, acidic and rich in cysteine (SPARC) was identified in a screen for genes expressed sex-specifically during mouse gonad development, as being strongly upregulated in the male gonad from very early in testis development. We present here a detailed analysis of SPARC gene and protein expression during testis development, from 11.5 to 15.5 days post coitum (dpc). Section in situ hybridization analysis revealed that SPARC mRNA is expressed by the Sertoli cells in the testis cords and the fetal Leydig cells, found within the interstitial space between the testis cords. Immunodetection with anti-SPARC antibody showed that the protein was located inside the testis cords, within the cytoplasm of Sertoli and germ cells. In the interstitium, SPARC was present intracellularly within the Leydig cells. The internalization of SPARC in Sertoli, Leydig, and germ cells suggests that it plays an intracellular regulatory role in these cell types during fetal testis development.