Characterization of the Drosophila Rae1 protein as a G1 phase regulator of the cell cycle

SARS-CoV-2 Orf6 is positioned in the nuclear pore complex by Rae1 to inhibit nucleocytoplasmic transport

The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) accessory protein Orf6 works as an interferon antagonist, in part, by inhibiting the nuclear import activated p-STAT1, an activator of interferon-stimulated genes, and the export of the poly(A) RNA. Insight into the transport regulatory function of Orf6 has come from the observation that Orf6 binds to the nuclear pore complex (NPC) components: Rae1 and Nup98. To gain further insight into the mechanism of Orf6-mediated transport inhibition, we examined the role of Rae1 and Nup98. We show that Rae1 alone is not necessary to support p-STAT1 import or nuclear export of poly(A) RNA. Moreover, the loss of Rae1 suppresses the transport inhibitory activity of Orf6. We propose that the Rae1/Nup98 complex strategically positions Orf6 within the NPC where it alters FG-Nup interactions and their ability to support nuclear transport. In addition, we show that Rae1 is required for normal viral protein production during SARS-CoV-2 infection presumably through its role in supporting Orf6 function.

P-MSC-derived extracellular vesicles facilitate diabetic wound healing via miR-145-5p/ CDKN1A-mediated functional improvements of high glucose-induced senescent fibroblasts

Background

Persistent hyperglycaemia in diabetes causes functional abnormalities of human dermal fibroblasts (HDFs), partially leading to delayed skin wound healing. Extracellular vesicles (EVs) containing multiple pro-healing microRNAs (miRNAs) have been shown to exert therapeutic effects on diabetic wound healing. The present study aimed to observe the effects of EVs derived from placental mesenchymal stem cells (P-MSC-EVs) on diabetic wound healing and high glucose (HG)-induced senescent fibroblasts and to explore the underlying mechanisms.

Methods

P-MSC-EVs were isolated by differential ultracentrifugation and locally injected into the full-thickness skin wounds of diabetic mice, to observe the beneficial effects on wound healing in vivo by measuring wound closure rates and histological analysis. Next, a series of assays were conducted to evaluate the effects of low (2.28 x 1010 particles/ml) and high (4.56 x 1010 particles/ml) concentrations of P-MSC-EVs on the senescence, proliferation, migration, and apoptosis of HG-induced senescent HDFs in vitro. Then, miRNA microarrays and real-time quantitative PCR (RT-qPCR) were carried out to detect the differentially expressed miRNAs in HDFs after EVs treatment. Specific RNA inhibitors, miRNA mimics, and small interfering RNA (siRNA) were used to evaluate the role of a candidate miRNA and its target genes in P-MSC-EV-induced improvements in the function of HG-induced senescent HDFs.

Results

Local injection of P-MSC-EVs into diabetic wounds accelerated wound closure and reduced scar widths, with better-organized collagen deposition and decreased p16INK4a expression. In vitro, P-MSC-EVs enhanced the antisenescence, proliferation, migration, and antiapoptotic abilities of HG-induced senescent fibroblasts in a dose-dependent manner. MiR-145-5p was found to be highly enriched in P-MSC-EVs. MiR-145-5p inhibitors effectively attenuated the P-MSC-EV-induced functional improvements of senescent fibroblasts. MiR-145-5p mimics simulated the effects of P-MSC-EVs on functional improvements of fibroblasts by suppressing the expression of cyclin-dependent kinase inhibitor 1A and activating the extracellular signal regulated kinase (Erk)/protein kinase B (Akt) signaling pathway. Furthermore, local application of miR-145-5p agomir mimicked the effects of P-MSC-EVs on wound healing.

Conclusions

These results suggest that P-MSC-EVs accelerate diabetic wound healing by improving the function of senescent fibroblasts through the transfer of miR-145-5p, which targets cyclin-dependent kinase inhibitor 1A to activate the Erk/Akt signaling pathway. P-MSC-EVs are promising therapeutic candidates for diabetic wound treatment.

Insights into the SARS-CoV-2 ORF6 Mechanism of Action

ORF6 is responsible for suppressing the immune response of cells infected by the SARS-CoV-2 virus. It is also the most toxic protein of SARS-CoV-2, and its actions are associated with the viral pathogenicity. Here, we study in silico and in vitro the structure of the protein, its interaction with RAE1 and the mechanism of action behind its high toxicity. We show both computationally and experimentally that SARS-CoV-2 ORF6, embedded in the cytoplasmic membranes, binds to RAE1 and sequesters it in the cytoplasm, thus depleting its availability in the nucleus and impairing nucleocytoplasmic mRNA transport. This negatively affects the cellular genome stability by compromising the cell cycle progression into the S-phase and by promoting the accumulation of RNA-DNA hybrids. Understanding the multiple ways in which ORF6 affects DNA replication may also have important implications for elucidating the pathogenicity of SARS-CoV-2 and developing therapeutic strategies to mitigate its deleterious effects on host cells.

The Hippo Pathway Targets Rae1 to Regulate Mitosis and Organ Size and to Feed Back to Regulate Upstream Components Merlin, Hippo, and Warts

Hippo signaling acts as a master regulatory pathway controlling growth, proliferation, and apoptosis and also ensures that variations in proliferation do not alter organ size. How the pathway coordinates restricting proliferation with organ size control remains a major unanswered question. Here we identify Rae1 as a highly-conserved target of the Hippo Pathway integrating proliferation and organ size. Genetic and biochemical studies in Drosophila cells and tissues and in mammalian cells indicate that Hippo signaling promotes Rae1 degradation downstream of Warts/Lats. In proliferating cells, Rae1 loss restricts cyclin B levels and organ size while Rae1 over-expression increases cyclin B levels and organ size, similar to Hippo Pathway over-activation or loss-of-function, respectively. Importantly, Rae1 regulation by the Hippo Pathway is crucial for its regulation of cyclin B and organ size; reducing Rae1 blocks cyclin B accumulation and suppresses overgrowth caused by Hippo Pathway loss. Surprisingly, in addition to suppressing overgrowth, reducing Rae1 also compromises survival of epithelial tissue overgrowing due to loss of Hippo signaling leading to a tissue “synthetic lethality” phenotype. Excitingly, Rae1 plays a highly conserved role to reduce the levels and activity of the Yki/YAP oncogene. Rae1 increases activation of the core kinases Hippo and Warts and plays a post-transcriptional role to increase the protein levels of the Merlin, Hippo, and Warts components of the pathway; therefore, in addition to Rae1 coordinating organ size regulation with proliferative control, we propose that Rae1 also acts in a feedback circuit to regulate pathway homeostasis.

Exquisite control of organ size is critical during animal development and its loss results in pathological conditions. The Hippo Tumor Suppressor Pathway coordinates regulation of proliferation, growth, apoptosis, and autophagy to determine and maintain precise control of organ size. However, the genes responsible for Hippo-mediated regulation of mitosis or coordination of proliferation within organ size control have evaded characterization. Here, we describe Rae1, an essential WD-repeat containing protein, as a new organ size regulator. By genetic analysis, we show that Rae1 acts downstream of the Hippo Pathway to regulate mitotic cyclins and organ size. In contexts where organ size control is lost by compromised Hippo signaling, we show that there is a requirement for Rae1 that is distinct from the requriement for Yki: reducing Yki levels causes suppression of overgrowth, while reducing Rae1 levels dramatically compromises the survival of Hippo-deficient tissue. Lastly, our studies of Rae1 uncovered a potential post-transcriptional feedback loop that reinforces Yorkie-mediated transcriptional feedback for the Hippo Pathway.

The PHR proteins: intracellular signaling hubs in neuronal development and axon degeneration

During development, a coordinated and integrated series of events must be accomplished in order to generate functional neural circuits. Axons must navigate toward target cells, build synaptic connections, and terminate outgrowth. The PHR proteins (consisting of mammalian Phr1/MYCBP2, Drosophila Highwire and C. elegans RPM-1) function in each of these events in development. Here, we review PHR function across species, as well as the myriad of signaling pathways PHR proteins regulate. These findings collectively suggest that the PHR proteins are intracellular signaling hubs, a concept we explore in depth. Consistent with prominent developmental functions, genetic links have begun to emerge between PHR signaling networks and neurodevelopmental disorders, such as autism, schizophrenia and intellectual disability. Finally, we discuss the recent and important finding that PHR proteins regulate axon degeneration, which has further heightened interest in this fascinating group of molecules.

Non-FG mediated transport of the large pre-ribosomal subunit through the nuclear pore complex by the mRNA export factor Gle2

Multiple export receptors passage bound pre-ribosomes through nuclear pore complexes (NPCs) by transiently interacting with the Phe-Gly (FG) meshwork of their transport channels. Here, we reveal how the non-FG interacting yeast mRNA export factor Gly-Leu-FG lethal 2 (Gle2) functions in the export of the large pre-ribosomal subunit (pre-60S). Structure-guided studies uncovered conserved platforms used by Gle2 to export pre-60S: an uncharacterized basic patch required to bind pre-60S, and a second surface that makes non-FG contacts with the nucleoporin Nup116. A basic patch mutant of Gle2 is able to function in mRNA export, but not pre-60S export. Thus, Gle2 provides a distinct interaction platform to transport pre-60S to the cytoplasm. Notably, Gle2’s interaction platforms become crucial for pre-60S export when FG-interacting receptors are either not recruited to pre-60S or are impaired. We propose that large complex cargos rely on non-FG as well as FG-interactions for their efficient translocation through the nuclear pore complex channel.

The LuWD40-1 gene encoding WD repeat protein regulates growth and pollen viability in flax (Linum Usitatissimum L.)

As a crop, flax holds significant commercial value for its omega-3 rich oilseeds and stem fibres. Canada is the largest producer of linseed but there exists scope for significant yield improvements. Implementation of mechanisms such as male sterility can permit the development of hybrids to assist in achieving this goal. Temperature sensitive male sterility has been reported in flax but the leakiness of this system in field conditions limits the production of quality hybrid seeds. Here, we characterized a 2,588 bp transcript differentially expressed in male sterile lines of flax. The twelve intron gene predicted to encode a 368 amino acid protein has five WD40 repeats which, in silico, form a propeller structure with putative nucleic acid and histone binding capabilities. The LuWD40-1 protein localized to the nucleus and its expression increased during the transition and continued through the vegetative stages (seed, etiolated seedling, stem) while the transcript levels declined during reproductive development (ovary, anthers) and embryonic morphogenesis of male fertile plants. Knockout lines for LuWD40-1 in flax failed to develop shoots while overexpression lines showed delayed growth phenotype and were male sterile. The non-viable flowers failed to open and the pollen grains from these flowers were empty. Three independent transgenic lines overexpressing the LuWD40-1 gene had ∼80% non-viable pollen, reduced branching, delayed flowering and maturity compared to male fertile genotypes. The present study provides new insights into a male sterility mechanism present in flax.

Drosophila rae1 is required for male meiosis and spermatogenesis

Previous studies of RAE1, a conserved WD40 protein, in Schizosaccharomyces pombe and mouse revealed a role in mRNA export and cell cycle progression in mitotic cells. Studies of RAE1 in Drosophila showed that the protein localizes to the nuclear envelope and is required for progression through the G1 phase of the cell cycle but not RNA export in tissue culture cells. Drosophila RAE1 also plays an essential developmental role, as it is required for viability and synaptic growth regulation as a component of an E3 ubiquitin ligase complex. Here we describe characterization of a new Drosophila rae1 mutant that is viable but results in male sterility. The mutant showed striking defects in primary spermatocyte nuclear integrity, meiotic chromosome condensation, segregation, and spindle morphology. These defects led to a failure to complete meiosis but allowed several aspects of spermatid differentiation to proceed, including axoneme formation and elongation. A GFP-RAE1 fusion protein that rescued most of the cytological defects showed a dynamic localization to the nuclear envelope, chromatin and other structures depending on the stage of spermatogenesis. A role for RAE1 in male meiosis, as well as mitotic cells, was also indicated by the defects induced by expression of rae1-RNAi. These studies in Drosophila provide the first evidence for an essential meiotic role of RAE1, and thus define RAE1 as a protein required for both meiotic and mitotic cell cycles.

Complexes of vesicular stomatitis virus matrix protein with host Rae1 and Nup98 involved in inhibition of host transcription

Vesicular stomatitis virus (VSV) suppresses antiviral responses in infected cells by inhibiting host gene expression at multiple levels, including transcription, nuclear cytoplasmic transport, and translation. The inhibition of host gene expression is due to the activity of the viral matrix (M) protein. Previous studies have shown that M protein interacts with host proteins Rae1 and Nup98 that have been implicated in regulating nuclear-cytoplasmic transport. However, Rae1 function is not essential for host mRNA transport, raising the question of how interaction of a viral protein with a host protein that is not essential for gene expression causes a global inhibition at multiple levels. We tested the hypothesis that there may be multiple M protein-Rae1 complexes involved in inhibiting host gene expression at multiple levels. Using size exclusion chromatography and sedimentation velocity analysis, it was determined that Rae1 exists in high, intermediate, and low molecular weight complexes. The intermediate molecular weight complexes containing Nup98 interacted most efficiently with M protein. The low molecular weight form also interacted with M protein in cells that overexpress Rae1 or cells in which Nup98 expression was silenced. Silencing Rae1 expression had little if any effect on nuclear accumulation of host mRNA in VSV-infected cells, nor did it affect VSV's ability to inhibit host translation. Instead, silencing Rae1 expression reduced the ability of VSV to inhibit host transcription. M protein interacted efficiently with Rae1-Nup98 complexes associated with the chromatin fraction of host nuclei, consistent with an effect on host transcription. These results support the idea that M protein-Rae1 complexes serve as platforms to promote the interaction of M protein with other factors involved in host transcription. They also support the idea that Rae1-Nup98 complexes play a previously under-appreciated role in regulation of transcription.

All viruses have mechanisms to suppress or evade host antiviral responses. These mechanisms are critical for viral pathogenicity. Vesicular stomatitis virus (VSV) suppresses antiviral responses by global inhibition of host gene expression mediated by the viral matrix (M) protein. M protein interacts with the host protein Rae1 in a complex with the nucleoporin Nup98. It had been thought that interaction of M protein with Rae1 blocks nuclear-cytoplasmic mRNA transport. However, other data show that Rae1 is not essential for mRNA transport. With this discrepancy in mind, we re-examined the interaction of M protein with Rae1 and Nup98 and the level of host gene expression in which they are involved. A key result was that silencing Rae1 expression did not affect host gene expression, but instead increased cellular resistance to inhibition by M protein. Furthermore, silencing Rae1 expression primarily affected the inhibition of host transcription with no significant effect on nuclear accumulation of mRNA. These results support a model in which Rae1 serves as a “platform” to promote interaction of M protein with cellular targets involved in host transcription. This illustrates a general principle that viral proteins can have multiple cellular effects by interacting with host proteins that are themselves multi-functional.

RAE-1, a novel PHR binding protein, is required for axon termination and synapse formation in Caenorhabditis elegans

Previous studies in Caenorhabditis elegans showed that RPM-1 (Regulator of Presynaptic Morphology-1) regulates axon termination and synapse formation. To understand the mechanism of how rpm-1 functions, we have used mass spectrometry to identify RPM-1 binding proteins, and have identified RAE-1 (RNA Export protein-1) as an evolutionarily conserved binding partner. We define a RAE-1 binding region in RPM-1, and show that this binding interaction is conserved and also occurs between Rae1 and the human ortholog of RPM-1 called Pam (protein associated with Myc). rae-1 loss of function causes similar axon and synapse defects, and synergizes genetically with two other RPM-1 binding proteins, GLO-4 and FSN-1. Further, we show that RAE-1 colocalizes with RPM-1 in neurons, and that rae-1 functions downstream of rpm-1. These studies establish a novel postmitotic function for rae-1 in neuronal development.

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.

Drosophila Rae1 controls the abundance of the ubiquitin ligase Highwire in post-mitotic neurons

The evolutionarily conserved Highwire (Hiw)/Drosophila Fsn E3 ubiquitin ligase complex is required for normal synaptic morphology during development and axonal regeneration after injury. However, little is known about the molecular mechanisms that regulate the Hiw E3 ligase complex. Using tandem affinity purification techniques, we identified Drosophila Rae1 as a previously unknown component of the Hiw/Fsn complex. Loss of Rae1 function in neurons results in morphological defects at the neuromuscular junction that are similar to those seen in hiw mutants. We found that Rae1 physically and genetically interacts with Hiw and restrains synaptic terminal growth by regulating the MAP kinase kinase kinase Wallenda. Moreover, we found that the Rae1 is both necessary and sufficient to promote Hiw protein abundance, and it does so by binding to Hiw and protecting Hiw from autophagy-mediated degradation. These results describe a previously unknown mechanism that selectively controls Hiw protein abundance during synaptic development.

A protein network-guided screen for cell cycle regulators in Drosophila

Background

Large-scale RNAi-based screens are playing a critical role in defining sets of genes that regulate specific cellular processes. Numerous screens have been completed and in some cases more than one screen has examined the same cellular process, enabling a direct comparison of the genes identified in separate screens. Surprisingly, the overlap observed between the results of similar screens is low, suggesting that RNAi screens have relatively high levels of false positives, false negatives, or both.

Results

We re-examined genes that were identified in two previous RNAi-based cell cycle screens to identify potential false positives and false negatives. We were able to confirm many of the originally observed phenotypes and to reveal many likely false positives. To identify potential false negatives from the previous screens, we used protein interaction networks to select genes for re-screening. We demonstrate cell cycle phenotypes for a significant number of these genes and show that the protein interaction network is an efficient predictor of new cell cycle regulators. Combining our results with the results of the previous screens identified a group of validated, high-confidence cell cycle/cell survival regulators. Examination of the subset of genes from this group that regulate the G1/S cell cycle transition revealed the presence of multiple members of three structurally related protein complexes: the eukaryotic translation initiation factor 3 (eIF3) complex, the COP9 signalosome, and the proteasome lid. Using a combinatorial RNAi approach, we show that while all three of these complexes are required for Cdk2/Cyclin E activity, the eIF3 complex is specifically required for some other step that limits the G1/S cell cycle transition.

Conclusions

Our results show that false positives and false negatives each play a significant role in the lack of overlap that is observed between similar large-scale RNAi-based screens. Our results also show that protein network data can be used to minimize false negatives and false positives and to more efficiently identify comprehensive sets of regulators for a process. Finally, our data provides a high confidence set of genes that are likely to play key roles in regulating the cell cycle or cell survival.

Proteomic and functional analysis of the mitotic Drosophila centrosome

Regulation of centrosome structure, duplication and segregation is integrated into cellular pathways that control cell cycle progression and growth. As part of these pathways, numerous proteins with well-established non-centrosomal localization and function associate with the centrosome to fulfill regulatory functions. In turn, classical centrosomal components take up functional and structural roles as part of other cellular organelles and compartments. Thus, although a comprehensive inventory of centrosome components is missing, emerging evidence indicates that its molecular composition reflects the complexity of its functions. We analysed the Drosophila embryonic centrosomal proteome using immunoisolation in combination with mass spectrometry. The 251 identified components were functionally characterized by RNA interference. Among those, a core group of 11 proteins was critical for centrosome structure maintenance. Depletion of any of these proteins in Drosophila SL2 cells resulted in centrosome disintegration, revealing a molecular dependency of centrosome structure on components of the protein translation machinery, actin- and RNA-binding proteins. In total, we assigned novel centrosome-related functions to 24 proteins and confirmed 13 of these in human cells.

Dm nxf1/sbr gene affects the formation of meiotic spindle in female Drosophila melanogaster

The small bristles (sbr) gene of Drosophila melanogaster belongs to the family of nuclear export factor (NXF) genes that participate in mRNA nuclear export. During meiosis, females of Drosophila melanogaster that carry various combinations of mutant alleles of the Dm nxf1/sbr gene exhibit disruption of the division spindle and misalignment of chromosomes at the metaphase plate. Meiosis of sbr ( 5 ) /+ females is characterized by the formation of tripolar spindles during the first cell division. According to the sequencing results, the sbr ( 5 ) (l(1)K4) lethal allele is a deletion of 492 nucleotides. In SBR(5) protein, 57 of the 146 amino acids that have been lost by deletion belong to the NTF2-like domain.

Dual functions of Nicotiana benthamiana Rae1 in interphase and mitosis

Rae1 performs multiple functions in animal systems, acting in interphase as an mRNA export factor and during mitosis as a mitotic checkpoint and spindle assembly regulator. In this study we characterized multiple functions of Rae1 in plants. Virus-induced gene silencing of Nicotiana benthamiana Rae1, NbRae1, which encodes a protein with four WD40 repeats, resulted in growth arrest and abnormal leaf development. NbRae1 was mainly associated with the nuclear envelope during interphase, and NbRae1 deficiency caused accumulation of poly(A) RNA in the nuclei of leaf cells, suggesting defective mRNA export. In the shoot apex, depletion of NbRae1 led to reduced mitotic activities, accompanied by reduced cyclin-dependent kinase (CDK) activity and decreased expression of cyclin B1, CDKB1-1, and histones H3 and H4. The secondary growth of stem vasculature was also inhibited, indicating reduced cambial activities. Differentiated leaf cells of NbRae1-silenced plants exhibited elevated ploidy levels. Immunolabeling in BY-2 cells showed that NbRae1 protein localized to mitotic microtubules and the cell plate-forming zone during mitosis, and recombinant NbRae1 directly bound to microtubules in vitro. Inhibition of NbRae1 expression in BY-2 cells using a beta-estradiol-inducible RNAi system resulted in severe defects in spindle organization and chromosome alignment and segregation, which correlated with delays in cell cycle progression. Together, these results suggest that NbRae1 plays a dual role in mRNA export in interphase and in spindle assembly in mitosis.

hnRNPs Relocalize to the cytoplasm following infection with vesicular stomatitis virus

Vesicular stomatitis virus (VSV) matrix protein inhibits nuclear-cytoplasmic mRNA transport. The goal of this work is to determine whether VSV inhibits the nuclear-cytoplasmic transport of heterogeneous ribonucleoproteins (hnRNPs), which are thought to serve as mRNA export factors. Confocal microscopy experiments showed that hnRNPA1, hnRNPK, and hnRNPC1/C2, but not hnRNPB1 or lamin A/C, are relocalized to the cytoplasm during VSV infection. We determined whether protein import is inhibited by VSV by transfecting cells with a plasmid encoding enhanced green fluorescent protein (EGFP) tagged with either the M9 nuclear localization sequence (NLS) or the classical NLS. These experiments revealed that both the M9 NLS and the classical NLS are functional during VSV infection. These data suggest that the inhibition of protein import is not responsible for hnRNP relocalization during VSV infection but that hnRNP export is enhanced. We found that hnRNPA1 relocalization was significantly reduced following the silencing of the mRNA export factor Rae1, indicating that Rae1 is necessary for hnRNP export. In order to determine the role of hnRNPA1 in VSV infection, we silenced hnRNPA1 in HeLa cells and assayed three aspects of the viral life cycle: host protein synthesis shutoff concurrent with the onset of viral protein synthesis, replication by plaque assay, and cell killing. We observed that host shutoff and replication are unaffected by the reduction in hnRNPA1 but that the rate of VSV-induced apoptosis is slower in cells that have reduced hnRNPA1. These data suggest that VSV promotes hnRNPA1 relocalization in a Rae1-dependent manner for apoptotic signaling.

Evolutionary analysis of WD40 super family proteins involved in spindle checkpoint and RNA export: molecular evolution of spindle checkpoint

The spindle checkpoint delays sister chromatid separation until all chromosomes have undergone bipolar spindle attachment. Previous studies have revealed BUB3, as an essential spindle checkpoint protein and its extensive sequence similarity with Rae1 (Gle2), a highly conserved member of WD40 repeat protein family throughout their length which was first shown to be involved in mRNA export. However, the recent discovery of Rae1 as an essential mitotic checkpoint protein, based on the studies from mouse and drosophila, has renewed the interest in its function during cell division. Study of evolution of proteins involved in checkpoint might throw light on evolution of eukaryotic cell cycle regulation. Here we report the evolutionary relationships between these two WD40 repeat family proteins. Amino acid sequences of BUB3 and Rae1 homologs were retrieved from various databases and phylogenetic analysis was performed with the MEGA program. Multiple sequence alignments of these two protein homologues with the ClustalX software revealed specific amino acid signatures corresponding to the protein function and also few amino acids, which are conserved in BUB3 and Rae1 indicating some common overlapping function. Data indicated a common ancestral origin of these two important proteins and further suggest that, BUB3 mediated cell cycle checkpoint might have evolved with compartmentalization of genetic material into the nucleus in eukaryotes.

The cloning and sequencing of a cDNA encoding a WD repeat protein in cotton (Gossypium hirsutum L.)

In this research, one 1156 bp cDNA containing full open reading frame and encoding a novel 24-kDa protein with four tandem WD repeat motifs was cloned from cotton, therefore was named GhWDR and the GenBank accession number is AY870657. By search of GhWDR cDNA and amino acid sequences in the database, we found that GhWDR and OSJNBa0003G23.2 from Oryza sativa show 90% sequence identity and 84% identity to WD-repeat protein from Arabidopsis thaliana, and also has high sequence identity to other WD repeat proteins, most of which are similar to Pop3 from fission yeast (accession number T39922) and Lst8p from Saccharomyces cerevisiae (accession number NP014392). Therefore, we proposed that GhWDR could act in some cellular processes as pop3 or LST8 does. In addition, the expression of GhWDR in various tissues was studied by RT-PCR, and it is expressed in all of the studied tissues, but the level of expression is low in the leaves when compared to that of other tissues.

A Rae1-containing ribonucleoprotein complex is required for mitotic spindle assembly

Centrosome-independent microtubule polymerization around chromosomes has been shown to require a local gradient of RanGTP, which discharges mitotic cargoes from the nuclear import receptor importin beta. Here, we have used an activity-based assay in Xenopus egg extracts to purify the mRNA export protein Rae1 as a spindle assembly factor regulated by this pathway. Rae1 is a microtubule-associated protein that binds directly to importin beta. Depletion of Rae1 from extracts or cells severely inhibits mitotic spindle assembly. A purified Rae1 complex stabilizes microtubules in egg extracts in a RanGTP/importin beta-regulated manner. Interestingly, Rae1 exists in a large ribonucleoprotein complex, which requires RNA for its activity to control microtubule dynamics in vitro. Furthermore, we provide evidence that RNA associates with the mitotic spindle and that it plays a direct, translation-independent role in spindle assembly. Our studies reveal an unexpected function for RNA in spindle morphogenesis.