The fission yeast ES2 homologue, Bis1, interacts with the Ish1 stress-responsive nuclear envelope protein

Fission yeast Ish1 and Les1 interact with each other in the lumen of the nuclear envelope

Nuclear envelope (NE) provides a permeable barrier that separates the eukaryotic genome from the cytoplasm. NE is a double membrane composed of inner and outer nuclear membranes. Ish1 is a stress-responsive NE protein in the fission yeast, Schizosaccharomyces pombe. Les1 is another NE protein that shares several similar domains with Ish1, but the relationship between them remains unknown. In this study, using fluorescence and electron microscopy, we found that most regions of these proteins were localized within the NE lumen. We also found that Ish1 interacted with Les1 via its C-terminal region in the NE lumen and that the NE localization of Ish1 depended on the C-terminal region of Les1. Ish1 and Les1 were co-localized at the NE in interphase cells, but when the nucleus divided at the end of mitosis (closed mitosis), they showed distinguishable localization at the midzone membrane domain. These results suggest the regulated interaction between Ish1 and Les1 in the NE lumen, although this interaction does not appear to be essential for cell survival. This article is protected by copyright. All rights reserved.

3,3'-Diindolylmethane induces apoptosis and autophagy in fission yeast

3,3'-Diindolylmethane (DIM) is a compound derived from the digestion of indole-3-carbinol, found in the broccoli family. It induces apoptosis and autophagy in some types of human cancer. DIM extends lifespan in the fission yeast Schizosaccharomyces pombe. The mechanisms by which DIM induces apoptosis and autophagy in humans and expands lifespan in fission yeasts are not fully understood. Here, we show that DIM induces apoptosis and autophagy in log-phase cells, which is dose-dependent in fission yeast. A high concentration of DIM disrupted the nuclear envelope (NE) structure and induced chromosome condensation at an early time point. In contrast, a low concentration of DIM induced autophagy but did not disrupt NE structure. The mutant defective in autophagy was more sensitive to a low concentration of DIM, demonstrating that the autophagic pathway contributes to the survival of cells against DIM. Moreover, our results showed that the lem2 mutant is more sensitive to DIM. NE in the lem2 mutant was disrupted even at the low concentration of DIM. Our results demonstrate that the autophagic pathway and NE integrity are important to maintain viability in the presence of a low concentration of DIM. The mechanism of apoptosis and autophagy induction by DIM might be conserved in fission yeast and humans. Further studies will contribute to the understanding of the mechanism of apoptosis and autophagy by DIM in fission yeast and humans.

Biallelic FRA10AC1 variants cause a neurodevelopmental disorder with growth retardation

The major spliceosome mediates pre-mRNA splicing by recognizing the highly conserved sequences at the 5' and 3' splice sites and the branch point. More than 150 proteins participate in the splicing process and are organized in the spliceosomal A, B, and C complexes. FRA10AC1 is a peripheral protein of the spliceosomal C complex and its ortholog in the green alga facilitates recognition or interaction with splice sites. We identified biallelic pathogenic variants in FRA10AC1 in five individuals from three consanguineous families. The two unrelated patients 1 and 2 with loss-of-function variants showed developmental delay, intellectual disability, and no speech, while three siblings with the c.494_496delAAG (p.Glu165del) variant had borderline to mild intellectual disability. All patients had microcephaly, hypoplasia or agenesis of the corpus callosum, growth retardation, and craniofacial dysmorphism. FRA10AC1 transcripts and proteins were drastically reduced or absent in fibroblasts of patients 1 and 2. In a heterologous expression system, the p. Glu165del variant impacts intrinsic stability of FRA10AC1 but does not affect its nuclear localization. By co-immunoprecipitation, we found ectopically expressed HA-FRA10AC1 in complex with endogenous DGCR14, another component of the spliceosomal C complex, while the splice factors CHERP, NKAP, RED, and SF3B2 could not be co-immunoprecipitated. Using an in vitro splicing reporter assay, we did not obtain evidence for FRA10AC1 deficiency to suppress missplicing events caused by mutations in the highly conserved dinucleotides of 5' and 3' splice sites in an in vitro splicing assay in patient-derived fibroblasts. Our data highlight the importance of specific peripheral spliceosomal C complex proteins for neurodevelopment. It remains possible that FRA10AC1 may have other and/or additional cellular functions, such as coupling of transcription and splicing reactions.

Sclerotinia sclerotiorum Response to Long Exposure to Glucosinolate Hydrolysis Products by Transcriptomic Approach

White mold disease, caused by the necrotrophic fungus Sclerotinia sclerotiorum, affects Brassica crops. Brassica crops produce a broad array of compounds, such as glucosinolates, which contribute to the defense against pathogens. From their hydrolysis, several products arise that have antimicrobial activity (GHPs) whose toxicity is structure dependent. S. sclerotiorum may overcome the toxic effect of moderate GHP concentrations after prolonged exposure to their action. Our objective was to identify the molecular mechanism underlying S. sclerotiorum response to long exposure to two chemically diverse GHPs: aliphatic GHP allyl-isothiocyanate (AITC) and indole GHP indol-3-carbinol (I3C). We found that the transcriptomic response is dependent on the type of GHP and on their initial target, involving cell membranes in the case of AITC or DNA in the case of I3C. Response mechanisms include the reorganization of chromatin, mediated by histone chaperones hip4 and cia1, ribosome synthesis controlled by the kinase-phosphatase pair aps1-ppn1, catabolism of proteins, ergosterol synthesis, and induction of detoxification systems. These mechanisms probably help S. sclerotiorum to grow and survive in an environment where GHPs are constantly produced by Brassica plants upon glucosinolate breakdown. IMPORTANCEBrassica species, including important vegetable crops, such as cabbage, cauliflower, or broccoli, or oil crops, such as rapeseed, produce specific chemical compounds useful to protect them against pests and pathogens. One of the most destructive Brassica diseases in temperate areas around the world is Sclerotinia stem rot, caused by the fungus Sclerotinia sclerotiorum. This is a generalist pathogen that causes disease over more than 400 plant species, being a serious threat to economically important crops worldwide, including potato, bean, soybean, and sunflower, among many others. Understanding the mechanisms utilized by pathogens to overcome specific plant defensive compounds can be useful to increase plant resistance. Our study demonstrated that Sclerotinia shows different adaptation mechanisms, including detoxification systems, to grow and survive when plant protective compounds are present.

Lem2 and Lnp1 maintain the membrane boundary between the nuclear envelope and endoplasmic reticulum

The nuclear envelope (NE) continues to the endoplasmic reticulum (ER). Proper partitioning of NE and ER is crucial for cellular activity, but the key factors maintaining the boundary between NE and ER remain to be elucidated. Here we show that the conserved membrane proteins Lem2 and Lnp1 cooperatively play a crucial role in maintaining the NE-ER membrane boundary in fission yeast Schizosaccharomyces pombe. Cells lacking both Lem2 and Lnp1 caused severe growth defects associated with aberrant expansion of the NE/ER membranes, abnormal leakage of nuclear proteins, and abnormal formation of vacuolar-like structures in the nucleus. Overexpression of the ER membrane protein Apq12 rescued the growth defect associated with membrane disorder caused by the loss of Lem2 and Lnp1. Genetic analysis showed that Apq12 had overlapping functions with Lnp1. We propose that a membrane protein network with Lem2 and Lnp1 acts as a critical factor to maintain the NE-ER boundary.

In the line-up: deleted genes associated with DiGeorge/22q11.2 deletion syndrome: are they all suspects?

BACKGROUND

22q11.2 deletion syndrome (22q11DS), a copy number variation (CNV) disorder, occurs in approximately 1:4000 live births due to a heterozygous microdeletion at position 11.2 (proximal) on the q arm of human chromosome 22 (hChr22) (McDonald-McGinn and Sullivan, Medicine 90:1-18, 2011). This disorder was known as DiGeorge syndrome, Velo-cardio-facial syndrome (VCFS) or conotruncal anomaly face syndrome (CTAF) based upon diagnostic cardiovascular, pharyngeal, and craniofacial anomalies (McDonald-McGinn and Sullivan, Medicine 90:1-18, 2011; Burn et al., J Med Genet 30:822-4, 1993) before this phenotypic spectrum was associated with 22q11.2 CNVs. Subsequently, 22q11.2 deletion emerged as a major genomic lesion associated with vulnerability for several clinically defined behavioral deficits common to a number of neurodevelopmental disorders (Fernandez et al., Principles of Developmental Genetics, 2015; Robin and Shprintzen, J Pediatr 147:90-6, 2005; Schneider et al., Am J Psychiatry 171:627-39, 2014).

RESULTS

The mechanistic relationships between heterozygously deleted 22q11.2 genes and 22q11DS phenotypes are still unknown. We assembled a comprehensive "line-up" of the 36 protein coding loci in the 1.5 Mb minimal critical deleted region on hChr22q11.2, plus 20 protein coding loci in the distal 1.5 Mb that defines the 3 Mb typical 22q11DS deletion. We categorized candidates based upon apparent primary cell biological functions. We analyzed 41 of these genes that encode known proteins to determine whether haploinsufficiency of any single 22q11.2 gene-a one gene to one phenotype correspondence due to heterozygous deletion restricted to that locus-versus complex multigenic interactions can account for single or multiple 22q11DS phenotypes.

CONCLUSIONS

Our 22q11.2 functional genomic assessment does not support current theories of single gene haploinsufficiency for one or all 22q11DS phenotypes. Shared molecular functions, convergence on fundamental cell biological processes, and related consequences of individual 22q11.2 genes point to a matrix of multigenic interactions due to diminished 22q11.2 gene dosage. These interactions target fundamental cellular mechanisms essential for development, maturation, or homeostasis at subsets of 22q11DS phenotypic sites.

Identifying RNA splicing factors using IFT genes in Chlamydomonas reinhardtii

Intraflagellar transport moves proteins in and out of flagella/cilia and it is essential for the assembly of these organelles. Using whole-genome sequencing, we identified splice site mutations in two IFT genes, IFT81 (fla9) and IFT121 (ift121-2), which lead to flagellar assembly defects in the unicellular green alga Chlamydomonas reinhardtii. The splicing defects in these ift mutants are partially corrected by mutations in two conserved spliceosome proteins, DGR14 and FRA10. We identified a dgr14 deletion mutant, which suppresses the 3′ splice site mutation in IFT81, and a frameshift mutant of FRA10, which suppresses the 5′ splice site mutation in IFT121. Surprisingly, we found dgr14-1 and fra10 mutations suppress both splice site mutations. We suggest these two proteins are involved in facilitating splice site recognition/interaction; in their absence some splice site mutations are tolerated. Nonsense mutations in SMG1, which is involved in nonsense-mediated decay, lead to accumulation of aberrant transcripts and partial restoration of flagellar assembly in the ift mutants. The high density of introns and the conservation of noncore splicing factors, together with the ease of scoring the ift mutant phenotype, make Chlamydomonas an attractive organism to identify new proteins involved in splicing through suppressor screening.

Mining the effector repertoire of the biotrophic fungal pathogen Ustilago hordei during host and non-host infection

The success of plant-pathogenic fungi mostly relies on their arsenal of virulence factors which are expressed and delivered into the host tissue during colonization. The biotrophic fungal pathogen Ustilago hordei causes covered smut disease on both barley and oat. In this study, we combined cytological, genomics and molecular biological methods to achieve a better understanding of the molecular interactions in the U. hordei-barley pathosystem. Microscopic analysis revealed that U. hordei densely colonizes barley leaves on penetration, in particular the vascular system. Transcriptome analysis of U. hordei at different stages of host infection revealed differential expression of the transcript levels of 273 effector gene candidates. Furthermore, U. hordei transcriptionally activates core effector genes which may suppress even non-host early defence responses. Based on expression profiles and novelty of sequences, knockout studies of 14 effector candidates were performed in U. hordei, which resulted in the identification of four virulence factors required for host colonization. Yeast two-hybrid screening identified potential barley targets for two of the effectors. Overall, this study provides a first systematic analysis of the effector repertoire of U. hordei and identifies four effectors (Uvi1-Uvi4) as virulence factors for the infection of barley.

Characterization of Aspergillus niger Isolated from the International Space Station

The initial characterization of the Aspergillus niger isolate JSC-093350089, collected from U.S. segment surfaces of the International Space Station (ISS), is reported, along with a comparison to the extensively studied strain ATCC 1015. Whole-genome sequencing of the ISS isolate enabled its phylogenetic placement within the A. niger/welwitschiae/lacticoffeatus clade and revealed that the genome of JSC-093350089 is within the observed genetic variance of other sequenced A. niger strains. The ISS isolate exhibited an increased rate of growth and pigment distribution compared to a terrestrial strain. Analysis of the isolate's proteome revealed significant differences in the molecular phenotype of JSC-093350089, including increased abundance of proteins involved in the A. niger starvation response, oxidative stress resistance, cell wall modulation, and nutrient acquisition. Together, these data reveal the existence of a distinct strain of A. niger on board the ISS and provide insight into the characteristics of melanized fungal species inhabiting spacecraft environments. IMPORTANCE A thorough understanding of how fungi respond and adapt to the various stimuli encountered during spaceflight presents many economic benefits and is imperative for the health of crew. As A. niger is a predominant ISS isolate frequently detected in built environments, studies of A. niger strains inhabiting closed systems may reveal information fundamental to the success of long-duration space missions. This investigation provides valuable insights into the adaptive mechanisms of fungi in extreme environments as well as countermeasures to eradicate unfavorable microbes. Further, it enhances understanding of host-microbe interactions in closed systems, which can help NASA's Human Research Program maintain a habitat healthy for crew during long-term manned space missions.

Shelterin promotes tethering of late replication origins to telomeres for replication-timing control

DNA replication initiates at many discrete loci on eukaryotic chromosomes, and individual replication origins are regulated under a spatiotemporal program. However, the underlying mechanisms of this regulation remain largely unknown. In the fission yeast Schizosaccharomyces pombe, the telomere-binding protein Taz1, ortholog of human TRF1/TRF2, regulates a subset of late replication origins by binding to the telomere-like sequence near the origins. Here, we showed using a lacO/LacI-GFP system that Taz1-dependent late origins were predominantly localized at the nuclear periphery throughout interphase, and were localized adjacent to the telomeres in the G1/S phase. The peripheral localization that depended on the nuclear membrane protein Bqt4 was not necessary for telomeric association and replication-timing control of the replication origins. Interestingly, the shelterin components Rap1 and Poz1 were required for replication-timing control and telomeric association of Taz1-dependent late origins, and this requirement was bypassed by a minishelterin Tpz1-Taz1 fusion protein. Our results suggest that Taz1 suppresses replication initiation through shelterin-mediated telomeric association of the origins at the onset of S phase.

Cell-surface copper transporters and superoxide dismutase 1 are essential for outgrowth during fungal spore germination

During fungal spore germination, a resting spore returns to a conventional mode of cell division and resumes vegetative growth, but the requirements for spore germination are incompletely understood. Here, we show that copper is essential for spore germination in Schizosaccharomyces pombe Germinating spores develop a single germ tube that emerges from the outer spore wall in a process called outgrowth. Under low-copper conditions, the copper transporters Ctr4 and Ctr5 are maximally expressed at the onset of outgrowth. In the case of Ctr6, its expression is broader, taking place before and during outgrowth. Spores lacking Ctr4, Ctr5, and the copper sensor Cuf1 exhibit complete germination arrest at outgrowth. In contrast, ctr6 deletion only partially interferes with formation of outgrowing spores. At outgrowth, Ctr4-GFP and Ctr5-Cherry first co-localize at the spore contour, followed by re-location to a middle peripheral spore region. Subsequently, they move away from the spore body to occupy the periphery of the nascent cell. After breaking of spore dormancy, Ctr6 localizes to the vacuole membranes that are enriched in the spore body relative to the germ tube. Using a copper-binding tracker, results showed that labile copper is preferentially localized to the spore body. Further analysis showed that Ctr4 and Ctr6 are required for copper-dependent activation of the superoxide dismutase 1 (SOD1) during spore germination. This activation is critical because the loss of SOD1 activity blocked spore germination at outgrowth. Taken together, these results indicate that cell-surface copper transporters and SOD1 are required for completion of the spore germination program.

Neurospora chromosomes are organized by blocks of importin alpha-dependent heterochromatin that are largely independent of H3K9me3

Eukaryotic genomes are organized into chromatin domains with three-dimensional arrangements that presumably result from interactions between the chromatin constituents—proteins, DNA, and RNA—within the physical constraints of the nucleus. We used chromosome conformation capture (3C) followed by high-throughput sequencing (Hi-C) with wild-type and mutant strains of Neurospora crassa to gain insight into the role of heterochromatin in the organization and function of the genome. We tested the role of three proteins thought to be important for establishment of heterochromatin, namely, the histone H3 lysine 9 methyltransferase DIM-5, Heterochromatin Protein 1 (HP1), which specifically binds to the product of DIM-5 (trimethylated H3 lysine 9 [H3K9me3]), and DIM-3 (importin alpha), which is involved in DIM-5 localization. The average genome configuration of the wild-type strain revealed strong intra- and inter-chromosomal associations between both constitutive and facultative heterochromatic domains, with the strongest interactions among the centromeres, subtelomeres, and interspersed heterochromatin. Surprisingly, loss of either H3K9me3 or HP1 had only mild effects on heterochromatin compaction, whereas dim-3 caused more drastic changes, specifically decreasing interactions between constitutive heterochromatic domains. Thus, associations between heterochromatic regions are a major component of the chromosome conformation in Neurospora, but two widely studied key heterochromatin proteins are not necessary, implying that undefined protein factors play key roles in maintaining overall chromosome organization.

DGCR14 induces Il17a gene expression through the RORγ/BAZ1B/RSKS2 complex

The Dgcr14/Es2 gene is located in a chromosomal region the loss of which has been associated with DiGeorge syndrome, a cause of immunodeficiency, heart defects, and skeletal abnormalities. However, the role of DGCR14 protein remains to be elucidated. Here, I found that DGCR14 protein acts as a coactivator of RORγt in TH17 cells. Biochemical purification of the RORγ coregulator complex allowed me to identify the associated DGCR14 protein by matrix-assisted laser desorption ionization-time of flight mass spectrometry. Overexpression of Dgcr14 mRNA enhanced RORγt-mediated transcriptional activity and facilitated TH17 cell differentiation. Furthermore, knockdown of Dgcr14 reduced Il17a mRNA expression. I also found that DGCR14 associated with ribosomal S6 kinase 2 (RSK2, also called RpS6ka3) and BAZ1B, both of which were recruited to the Il17a promoter during TH17 cell differentiation. Knockdown of Baz1b or RpS6ka3 also reduced Il17a mRNA expression, and Baz1b knockdown increased transcriptional suppressive histone marks (histone H3K9me3) on the Il17a promoter. My findings showed the roles of DGCR14, RSK2, and BAZ1B in the transcriptional regulation of Il17a mRNA during TH17 cell differentiation.

Systematic analyses of rpm-1 suppressors reveal roles for ESS-2 in mRNA splicing in Caenorhabditis elegans

The PHR (Pam/Highwire/RPM-1) family of ubiquitin E3 ligases plays conserved roles in axon patterning and synaptic development. Genetic modifier analysis has greatly aided the discovery of the signal transduction cascades regulated by these proteins. In Caenorhabditis elegans, loss of function in rpm-1 causes axon overgrowth and aberrant presynaptic morphology, yet the mutant animals exhibit little behavioral deficits. Strikingly, rpm-1 mutations strongly synergize with loss of function in the presynaptic active zone assembly factors, syd-1 and syd-2, resulting in severe locomotor deficits. Here, we provide ultrastructural evidence that double mutants, between rpm-1 and syd-1 or syd-2, dramatically impair synapse formation. Taking advantage of the synthetic locomotor defects to select for genetic suppressors, previous studies have identified the DLK-1 MAP kinase cascade negatively regulated by RPM-1. We now report a comprehensive analysis of a large number of suppressor mutations of this screen. Our results highlight the functional specificity of the DLK-1 cascade in synaptogenesis. We also identified two previously uncharacterized genes. One encodes a novel protein, SUPR-1, that acts cell autonomously to antagonize RPM-1. The other affects a conserved protein ESS-2, the homolog of human ES2 or DGCR14. Loss of function in ess-2 suppresses rpm-1 only in the presence of a dlk-1 splice acceptor mutation. We show that ESS-2 acts to promote accurate mRNA splicing when the splice site is compromised. The human DGCR14/ES2 resides in a deleted chromosomal region implicated in DiGeorge syndrome, and its mutation has shown high probability as a risk factor for schizophrenia. Our findings provide the first functional evidence that this family of proteins regulate mRNA splicing in a context-specific manner.

Genetic and physical interaction of Ssp1 CaMKK and Rad24 14-3-3 during low pH and osmotic stress in fission yeast

The Ssp1 calmodulin kinase kinase (CaMKK) is necessary for stress-induced re-organization of the actin cytoskeleton and initiation of growth at the new cell end following division in Schizosaccharomyces pombe. In addition, it regulates AMP-activated kinase and functions in low glucose tolerance. ssp1⁻ cells undergo mitotic delay at elevated temperatures and G₂ arrest in the presence of additional stressors. Following hyperosmotic stress, Ssp1-GFP forms transient foci which accumulate at the cell membrane and form a band around the cell circumference, but not co-localizing with actin patches. Hyperosmolarity-induced localization to the cell membrane occurs concomitantly with a reduction of its interaction with the 14-3-3 protein Rad24, but not Rad25 which remains bound to Ssp1. The loss of rad24 in ssp1⁻ cells reduces the severity of hyperosmotic stress response and relieves mitotic delay. Conversely, overexpression of rad24 exacerbates stress response and concomitant cell elongation. rad24⁻ does not impair stress-induced localization of Ssp1 to the cell membrane, however this response is almost completely absent in cells overexpressing rad24.

Comparative transcriptomics of infectious spores from the fungal pathogen Histoplasma capsulatum reveals a core set of transcripts that specify infectious and pathogenic states

Histoplasma capsulatum is a fungal pathogen that infects both healthy and immunocompromised hosts. In regions where it is endemic, H. capsulatum grows in the soil and causes respiratory and systemic disease when inhaled by humans. An interesting aspect of H. capsulatum biology is that it adopts specialized developmental programs in response to its environment. In the soil, it grows as filamentous chains of cells (mycelia) that produce asexual spores (conidia). When the soil is disrupted, conidia aerosolize and are inhaled by mammalian hosts. Inside a host, conidia germinate into yeast-form cells that colonize immune cells and cause disease. Despite the ability of conidia to initiate infection and disease, they have not been explored on a molecular level. We developed methods to purify H. capsulatum conidia, and we show here that these cells germinate into filaments at room temperature and into yeast-form cells at 37°C. Conidia internalized by macrophages germinate into the yeast form and proliferate within macrophages, ultimately lysing the host cells. Similarly, infection of mice with purified conidia is sufficient to establish infection and yield viable yeast-form cells in vivo. To characterize conidia on a molecular level, we performed whole-genome expression profiling of conidia, yeast, and mycelia from two highly divergent H. capsulatum strains. In parallel, we used homology and protein domain analysis to manually annotate the predicted genes of both strains. Analyses of the resultant data defined sets of transcripts that reflect the unique molecular states of H. capsulatum conidia, yeast, and mycelia.

Telomere-nuclear envelope dissociation promoted by Rap1 phosphorylation ensures faithful chromosome segregation

Efficient chromosomal movements are important for the fidelity of chromosome segregation during mitosis; however, movements are constrained during interphase by tethering of multiple domains to the nuclear envelope (NE). Higher eukaryotes undergo open mitosis accompanied by NE breakdown, enabling chromosomes to be released from the NE, whereas lower eukaryotes undergo closed mitosis, in which NE breakdown does not occur. Although the chromosomal movements in closed mitosis are thought to be restricted compared to open mitosis, the cells overcome this problem by an unknown mechanism that enables accurate chromosome segregation. Here, we report the spatiotemporal regulation of telomeres in Schizosaccharomyces pombe closed mitosis. We found that the telomeres, tethered to the NE during interphase, are transiently dissociated from the NE during mitosis. This dissociation from the NE is essential for accurate chromosome segregation because forced telomere tethering to the NE causes frequent chromosome loss. The phosphorylation of the telomere protein Rap1 during mitosis, primarily by Cdc2, impedes the interaction between Rap1 and Bqt4, a nuclear membrane protein, thereby inducing telomere dissociation from the NE. We propose that the telomere dissociation from the NE promoted by Rap1 phosphorylation is critical for the fidelity of chromosome segregation in closed mitosis.

The nuclear envelope and its involvement in cellular stress responses

The nuclear envelope is not only important for the structural integrity of the nucleus, but also involved in a number of cellular functions. It has been shown to be important for maintaining and controlling chromatin organization, sequestering transcription factors, replication, transcription and signalling. The nuclear envelope is thus important for development and differentiation, and some of its components are essential for cell viability. Among the many functions which are emerging for the nuclear envelope is its involvement in protecting the cell against different types of cellular stress. In the present paper, we review key findings which describe the roles of nuclear envelope components in responses to common types of stress conditions.

Virtual breakdown of the nuclear envelope in fission yeast meiosis

Asymmetric localization of Ran regulators (RanGAP1 and RanGEF/RCC1) produces a gradient of RanGTP across the nuclear envelope. In higher eukaryotes, the nuclear envelope breaks down as the cell enters mitosis (designated "open" mitosis). This nuclear envelope breakdown (NEBD) leads to collapse of the RanGTP gradient and the diffusion of nuclear and cytoplasmic macromolecules in the cell, resulting in irreversible progression of the cell cycle. On the other hand, in many fungi, chromosome segregation takes place without NEBD (designated "closed" mitosis). Here we report that in the fission yeast Schizosaccharomyces pombe, despite the nuclear envelope and the nuclear pore complex remaining intact throughout both the meiotic and mitotic cell cycles, nuclear proteins diffuse into the cytoplasm transiently for a few minutes at the onset of anaphase of meiosis II. We also found that nuclear protein diffusion into the cytoplasm occurred coincidently with nuclear localization of Rna1, an S. pombe RanGAP1 homolog that is usually localized in the cytoplasm. These results suggest that nuclear localization of RanGAP1 and depression of RanGTP activity in the nucleus may be mechanistically tied to meiosis-specific diffusion of nuclear proteins into the cytoplasm. This nucleocytoplasmic shuffling of RanGAP1 and nuclear proteins represents virtual breakdown of the nuclear envelope.

Rad4TopBP1 associates with Srr2, an Spc1 MAPK-regulated protein, in response to environmental stress

Rad4(TopBP1) is a scaffold in a protein complex containing both replication proteins and checkpoint proteins and plays essential roles in both replication and checkpoint responses. We have previously identified four novel fission yeast mutants of rad4+(TopBP1) to explore how Rad4(TopBP1), a single protein, can play multiple roles in genomic integrity maintenance. Among the four novel mutants, rad4-c17(TopBP1) is a thermosensitive mutant. Here, we characterized rad4-c17(TopBP1) and identified a rad4-c17(TopBP1) allele specific suppressor named srr2+ (suppressor of Rad4(TopBP1) R2 domain). srr2+ has previously been identified as an environmental stress-responsive gene (GenBank accession number AL049644.1, locus spcc191.01). srr2+ null cells are sensitive to hydroxyurea (HU) at elevated temperatures. Deletion of srr2+ in rad4-c17(TopBP1) exacerbates the HU sensitivity of the mutant. Overexpression of srr2+ suppresses the rad4-c17(TopBP1) mutant sensitivity to temperature and HU and restores the compromised ability of rad4-c17(TopBP1) to activating Cds1 kinase in response to HU treatment. Furthermore, stress-activated MAPK, Spc1 (also known as StyI or Phh1), induces the expression and phosphorylation of the Srr2 protein. Significantly, environmental stress induces co-precipitation of Srr2 protein with Rad4(TopBP1), and the co-precipitation is compromised in the rad4-c17(TopBP1) mutant. These results have led us to propose a model; Rad4(TopBP1) exists in a large protein complex to coordinate genomic perturbations with checkpoint responses to maintain genomic integrity. In addition, when cells experience environmental stress, Rad4(TopBP1) associates with Srr2, an Spc1 MAPK-responsive protein, to survive the stress, potentially by providing a link of the Spc1 MAPK response to checkpoint responses.

Glutathione, Altruistic Metabolite in Fungi

Glutathione (GSH; gamma-L-glutamyl-L-cysteinyl-glycine), a non-protein thiol with a very low redox potential (E'0 = 240 mV for thiol-disulfide exchange), is present in high concentration up to 10 mM in yeasts and filamentous fungi. GSH is concerned with basic cellular functions as well as the maintenance of mitochondrial structure, membrane integrity, and in cell differentiation and development. GSH plays key roles in the response to several stress situations in fungi. For example, GSH is an important antioxidant molecule, which reacts non-enzymatically with a series of reactive oxygen species. In addition, the response to oxidative stress also involves GSH biosynthesis enzymes, NADPH-dependent GSH-regenerating reductase, glutathione S-transferase along with peroxide-eliminating glutathione peroxidase and glutaredoxins. Some components of the GSH-dependent antioxidative defence system confer resistance against heat shock and osmotic stress. Formation of protein-SSG mixed disulfides results in protection against desiccation-induced oxidative injuries in lichens. Intracellular GSH and GSH-derived phytochelatins hinder the progression of heavy metal-initiated cell injuries by chelating and sequestering the metal ions themselves and/or by eliminating reactive oxygen species. In fungi, GSH is mobilized to ensure cellular maintenance under sulfur or nitrogen starvation. Moreover, adaptation to carbon deprivation stress results in an increased tolerance to oxidative stress, which involves the induction of GSH-dependent elements of the antioxidant defence system. GSH-dependent detoxification processes concern the elimination of toxic endogenous metabolites, such as excess formaldehyde produced during the growth of the methylotrophic yeasts, by formaldehyde dehydrogenase and methylglyoxal, a by-product of glycolysis, by the glyoxalase pathway. Detoxification of xenobiotics, such as halogenated aromatic and alkylating agents, relies on glutathione S-transferases. In yeast, these enzymes may participate in the elimination of toxic intermediates that accumulate in stationary phase and/or act in a similar fashion as heat shock proteins. GSH S-conjugates may also form in a glutathione S-transferases-independent way, e.g. through chemical reaction between GSH and the antifugal agent Thiram. GSH-dependent detoxification of penicillin side-chain precursors was shown in Penicillium sp. GSH controls aging and autolysis in several fungal species, and possesses an anti-apoptotic feature.

A novel jmjC domain protein modulates heterochromatization in fission yeast

The heterochromatin domain at the mat locus of Schizosaccharomyces pombe is bounded by the IR-L and IR-R barriers. A genetic screen for mutations that promote silencing beyond IR-L revealed a novel gene named epe1, encoding a conserved nuclear protein with a jmjC domain. Disruption of epe1 promotes continuous spreading of heterochromatin-associated histone modifications and Swi6 binding to chromatin across heterochromatic barriers. It also enhances position effect variegation at heterochromatic domains, suppresses mutations in silencing genes, and stabilizes the repressed epigenetic state at the mat locus. However, it does not enhance silencing establishment. Our analysis suggests that the jmjC domain is essential for Epe1 activity and that Epe1 counteracts transcriptional silencing by negatively affecting heterochromatin stability. Consistent with this proposition, the meiotic stability of established heterochromatin beyond IR-L is diminished by Epe1 activity, and overexpression of Epe1 disrupts heterochromatin through acetylation of H3-K9 and H3-K14 and methylation of H3-K4. Furthermore, overexpression of Epe1 elevates the rate of chromosome loss. We propose that Epe1 helps control chromatin organization by down-regulating the stability of epigenetic marks that govern heterochromatization.

The sal3(+) gene encodes an importin-beta implicated in the nuclear import of Cdc25 in Schizosaccharomyces pombe

In Schizosaccharomyces pombe, the nuclear accumulation of Cdc25 peaks in G2 and is necessary for the proper timing of mitotic entry. Here, we identify the sal3(+) gene product as an importin-beta homolog that participates in the nuclear import of Cdc25. Loss of sal3(+) results in a cell cycle delay, failure to undergo G1 arrest under nitrogen-starvation conditions, and mislocalization of Cdc25 to the cytosol. Fusion of an exogenous classical nuclear localization sequence (cNLS) to Cdc25 restores its nuclear accumulation in a sal3 disruptant and suppresses the sal3 mutant phenotypes. In addition, we show that enhanced nuclear localization of Cdc25 at endogenous levels of expression advances the onset of mitosis. These results demonstrate that the nuclear translocation of Cdc25 is important for the timing of mitotic entry and that Sal3 plays an important role in this process.

The Scw1 RNA-Binding Domain Protein Regulates Septation and Cell-Wall Structure in Fission Yeast

Loss of the nonessential RNA-binding domain protein, Scw1, increases resistance to cell-wall-degrading enzymes in fission yeast. Surprisingly, scw1 null mutations also suppress the lethality of mutations (cdc11-136, cdc7-24, cdc14-118, sid1-239, sid2-250, sid3-106, sid4-A1, and mob1-1) at all levels of the sid pathway. This pathway forms part of the septation initiation network (SIN), which regulates the onset of septum formation and ensures the proper coupling of mitosis to cytokinesis. In contrast, scw1- mutations do not suppress ts alleles of the rng genes, cdc12 or cdc15. These mutations also prevent the formation of a septum and in addition block assembly and/or function of the contractile acto-myosin ring. sid mutants exhibit a hyper-sensitivity to cell-wall-degrading enzymes that is suppressed by loss of Scw1. Furthermore, scw1--mediated rescue of sid mutants is abolished in the presence of calcofluor white, a compound that interferes with cell-wall synthesis. These data suggest that Scw1 acts in opposition to the SIN as a negative regulator of cell-wall/septum deposition. Unlike components of the SIN, Scw1 is predominantly a cytoplasmic protein and is not localized to the spindle pole body.