The 14-3-3 proteins Rad24 and Rad25 negatively regulate Byr2 by affecting its localization in Schizosaccharomyces pombe

Negative regulation of APC/C activation by MAPK-mediated attenuation of Cdc20Slp1 under stress

Mitotic anaphase onset is a key cellular process tightly regulated by multiple kinases. The involvement of mitogen-activated protein kinases (MAPKs) in this process has been established in Xenopus egg extracts. However, the detailed regulatory cascade remains elusive, and it is also unknown whether the MAPK-dependent mitotic regulation is evolutionarily conserved in the single-cell eukaryotic organisms such as fission yeast (Schizosaccharomyces pombe). Here, we show that two MAPKs in S. pombe indeed act in concert to restrain anaphase-promoting complex/cyclosome (APC/C) activity upon activation of the spindle assembly checkpoint (SAC). One MAPK, Pmk1, binds to and phosphorylates Slp1Cdc20, the co-activator of APC/C. Phosphorylation of Slp1Cdc20 by Pmk1, but not by Cdk1, promotes its subsequent ubiquitylation and degradation. Intriguingly, Pmk1-mediated phosphorylation event is also required to sustain SAC under environmental stress. Thus, our study establishes a new underlying molecular mechanism of negative regulation of APC/C by MAPK upon stress stimuli, and provides a previously unappreciated framework for regulation of anaphase entry in eukaryotic cells.

Mutational analyses of the interacting domains of Schizosaccharomyces pombe Byr2 with 14-3-3s

The Byr2 kinase of fission yeast Schizosaccharomyces pombe is recruited to the membrane with the assistance of Ras1. Byr2 is also negatively regulated by 14-3-3 proteins encoded by rad24 and rad25. We conducted domain and mutational analysis of Byr2 to determine which region is critical for its binding to 14-3-3 proteins. Rad24 and Rad25 bound to both the Ras interaction domain in the N-terminus and to the C-terminal catalytic domain of Byr2. When amino acid residues S87 and T94 of the Ras-interacting domain of Byr2 were mutated to alanine, Rad24 could no longer bind to Byr2. S402, S566, S650, and S654 mutations in the C-terminal domain of Byr2 also abolished its interaction with Rad24 and Rad25. More than three mutations in the C-terminal domain were required to abolish completely its interaction with 14-3-3 protein, suggesting that multiple residues are involved in this interaction. Expression of the N-terminal domain of Byr2 in wild-type cells lowered the mating ratio, because it likely blocked the interaction of Byr2 with Ste4 and Ras1, whereas expression of the catalytic domain of Byr2 increased the mating ratio as a result of freeing from intramolecular regulation by the N-terminal domain of Byr2. The S87A and T94A mutations of Byr2 increased the mating ratio and attenuated inhibition of Byr2 by Rad24; therefore, these two amino acids are critical for its regulation by Rad24. S566 of Byr2 is critical for activity of Byr2 but not for its interaction with 14-3-3 proteins. In this study, we show that 14-3-3 proteins interact with two separate domains in Byr2 as negative regulators.

Regulation of sexual differentiation initiation in Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe is an excellent model organism to explore cellular events owing to rich tools in genetics, molecular biology, cellular biology, and biochemistry. Schizosaccharomyces pombe proliferates continuously when nutrients are abundant but arrests in G1 phase upon depletion of nutrients such as nitrogen and glucose. When cells of opposite mating types are present, cells conjugate, fuse, undergo meiosis, and finally form 4 spores. This sexual differentiation process in S. pombe has been studied extensively. To execute sexual differentiation, the glucose-sensing cAMP-PKA (cyclic adenosine monophosphate-protein kinase A) pathway, nitrogen-sensing TOR (target of rapamycin) pathway, and SAPK (stress-activating protein kinase) pathway are crucial, and the MAPK (mitogen-activating protein kinase) cascade is essential for pheromone sensing. These signals regulate ste11 at the transcriptional and translational levels, and Ste11 is modified in multiple ways. This review summarizes the initiation of sexual differentiation in S. pombe based on results I have helped to obtain, including the work of many excellent researchers.

A dominant negative 14-3-3 mutant in Schizosaccharomyces pombe distinguishes the binding proteins involved in sexual differentiation and check point

The homothallic fission yeast Schizosaccharomyces pombe undergoes sexual differentiation when starved, but sam (skips the requirement of starvation for mating) mutants such as those carrying mutations in adenylate cyclase (cyr1) or protein kinase A (pka1) mate without starvation. Here, we identified sam3, a dominant negative allele of rad24, encoding one of two 14-3-3 proteins. Genetic mapping and whole-genome sequencing showed that the sam3 mutation comprises a change in nucleotide at position 959 from guanine to adenine, which switches the amino acid at position 185 from glutamic acid to lysine (E185K). We generated the rad24-E185K integrated mutant and its phenotype was similar to that of the sam3 mutant, including calcium sensitivity and UV non-sensitivity, but the phenotype is different from that of the Δrad24 strain. While the UV-sensitive phenotype was observed in the Δrad24 mutant, it was not observed in the sam3 and rad24-E185K mutants. The expression of the rad24-E185K gene in wild type cells induced spore formation in the nutrient rich medium, confirming rad24-E185K is dominant. This dominant effect of rad24-E185K was also observed in Δras1 and Δbyr2 diploid mutants, indicating that rad24-E185K generate stronger phenotype than rad24 null mutants. Ste11, the key transcription factor for sexual differentiation was expressed in sam3 mutants without starvation and it predominantly localized to the nucleus. The Rad24-E185K mutant protein retained its interaction with Check point kinase1 (Chk1), whereas it reduced interaction with Ste11, an RNA binding protein Mei2, and a MAPKKK Byr2, freeing these proteins from negative regulation by Rad24, that account for the sam phenotype and UV non-sensitive phenotype. Glucose depletion in rad24-E185K or Δpka1 Δrad24 double mutation induced haploid meiosis, leading to the formation of spores in haploid. The position of glutamic acid 185 is conserved in all major 14-3-3s; hence, our finding of a dominant negative allele of 14-3-3 is useful for understanding 14-3-3s in other organisms.

Sporulation: A response to starvation in the fission yeast Schizosaccharomyces pombe

The fission yeast Schizosaccharomyces pombe employs two main strategies to adapt to the environment and survive when starved for nutrients. The strategies employ sporulation via sexual differentiation and extension of the chronological lifespan. When a cell is exposed to nutrient starvation in the presence of a cell of the opposite sex, the cells undergo fusion through conjugation and sporulation through meiosis. S. pombe spores are highly resistant to diverse stresses and may survive for a very long time. In this minireview, among the various sexual differentiation processes induced by starvation, we focused on and summarized the findings of the molecular mechanisms of spore formation in fission yeast. Furthermore, comparative measurements of the chronological lifespan of stationary phase cells and G0 cells and the survival period of spore cells revealed that the spore cells survived for a long period, indicating the presence of an effective mechanism for survival. Currently, many molecules involved in sporulation and their functions are being discovered; however, our understanding of these is not complete. Further understanding of spores may not only deepen our comprehension of sexual differentiation but may also provide hints for sustaining life.

Domesticated LTR-Retrotransposon gag-Related Gene (Gagr) as a Member of the Stress Response Network in Drosophila

The most important sources of new components of genomes are transposable elements, which can occupy more than half of the nucleotide sequence of the genome in higher eukaryotes. Among the mobile components of a genome, a special place is occupied by retroelements, which are similar to retroviruses in terms of their mechanisms of integration into a host genome. The process of positive selection of certain sequences of transposable elements and retroviruses in a host genome is commonly called molecular domestication. There are many examples of evolutionary adaptations of gag (retroviral capsid) sequences as new regulatory sequences of different genes in mammals, where domesticated gag genes take part in placenta functioning and embryogenesis, regulation of apoptosis, hematopoiesis, and metabolism. The only gag-related gene has been found in the Drosophila genome—Gagr. According to the large-scale transcriptomic and proteomic analysis data, the Gagr gene in D. melanogaster is a component of the protein complex involved in the stress response. In this work, we consider the evolutionary processes that led to the formation of a new function of the domesticated gag gene and its adaptation to participation in the stress response. We discuss the possible functional role of the Gagr as part of the complex with its partners in Drosophila, and the pathway of evolution of proteins of the complex in eukaryotes to determine the benefit of the domesticated retroelement gag gene.

Interactome of Glyceraldehyde-3-Phosphate Dehydrogenase Points to the Existence of Metabolons in Paracoccidioides lutzii

Paracoccidioides is a dimorphic fungus, the causative agent of paracoccidioidomycosis. The disease is endemic within Latin America and prevalent in Brazil. The treatment is based on azoles, sulfonamides and amphotericin B. The seeking for new treatment approaches is a real necessity for neglected infections. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is an essential glycolytic enzyme, well known for its multitude of functions within cells, therefore categorized as a moonlight protein. To our knowledge, this is the first approach performed on the Paracoccidioides genus regarding the description of PPIs having GAPDH as a target. Here, we show an overview of experimental GAPDH interactome in different phases of Paracoccidioides lutzii and an in silico analysis of 18 proteins partners. GAPDH interacted with 207 proteins in P. lutzii. Several proteins bound to GAPDH in mycelium, transition and yeast phases are common to important pathways such as glycolysis and TCA. We performed a co-immunoprecipitation assay to validate the complex formed by GAPDH with triose phosphate isomerase, enolase, isocitrate lyase and 2-methylcitrate synthase. We found GAPDH participating in complexes with proteins of specific pathways, indicating the existence of a glycolytic and a TCA metabolon in P. lutzii. GAPDH interacted with several proteins that undergoes regulation by nitrosylation. In addition, we modeled the GAPDH 3-D structure, performed molecular dynamics and molecular docking in order to identify the interacting interface between GAPDH and the interacting proteins. Despite the large number of interacting proteins, GAPDH has only four main regions of contact with interacting proteins, reflecting its ancestrality and conservation over evolution.

14-3-3 Proteins: a window for a deeper understanding of fungal metabolism and development

Isoforms of 14-3-3 proteins, similar to their highly conserved homologs in mammals and plants, are both transcriptionally and functionally affected by their extracellular and intracellular environments. These proteins bind to phosphorylated client proteins to modulate their functions in fungi. Since phosphorylation regulates a plethora of different physiological responses in organisms, 14-3-3 proteins play roles in multiple physiological functions, including those controlling metabolisms, cell division, and responses to environmental stimulation. These proteins could also modulate signaling pathways that transduce inputs from the environment and downstream proteins that elicit physiological responses. Increasing evidence supports a prominent role for 14-3-3 proteins in regulating development and metabolism at various levels. In this review, we first provide a brief summary of the molecular structure of 14-3-3 proteins. Second, we discuss the potential roles of 14-3-3 proteins in the regulation of development and metabolism. Third, we review the roles of 14-3-3 proteins in the regulation of their binding partners, including receptors, protein kinases, and some protein kinase substrates. Finally, this review examines recent advances that further elucidate the role of 14-3-3 proteins in signaling transduction in response to environmental stress.

Overexpression of the transcription factor Rst2 in Schizosaccharomyces pombe indicates growth defect, mitotic defects, and microtubule disorder

In Schizosaccharomyces pombe, the transcription factor Rst2 regulates ste11 in meiosis and fbp1 in glucogenesis downstream of the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) pathway. Here, we demonstrate that Rst2 regulates additional cellular events. Overexpressed Rst2 elevated the frequency of oval, bent, branched, septated, and multi-septated cells. Cells showed normal nuclear divisions but exhibited abnormal nuclear organization at low frequency. In oval cells, microtubules were curved but they were rescued by the deletion of mal3. Since growth defect was not rescued by mal3 deletion, we argue that it is regulated independently. Loss of functional Pka1 exaggerated growth defect upon Rst2 overexpression because its downregulation by Pka1 was lost. Overexpression of Rst2 also caused sensitivity to KCl and CaCl₂. These findings suggest that, in addition to meiosis and glucogenesis, Rst2 is involved in cellular events such as regulation of cell growth, cell morphology, mitosis progression, microtubules structure, nuclear structure, and stress response.

DNA polymerase 5 acetylation by Eso1 is essential for Schizosaccharomyces pombe viability

Eco1/Eso1 protein plays an important role in chromosome segregation, DNA repair and gene regulation. Eco1 mutation induces Roberts syndrome clinically and rDNA transcription disorders in vivo. In this study, we examined the role of Eso1 protein binding to polymerase 5 (Pol5) and the acetylation of Pol5 protein in the regulation of Schizosaccharomyces pombe (S. pombe) viability. Immunoprecipitation and mass spectrometry assays identified Eso1 protein binding to Cdc2, Pol5 and Cdc21, as well as other proteins. Pol5 protein specifically bound to Eso1 protein, but not to the Rad30 part or Rad30 part plus the additional zinc finger domain of Eco1 protein. Mass spectrometry data further identified several acetylation or trimethylation modification sites in the lysine residues of the Pol5 protein. However, the mutation of the Pol5 K47 site to arginine was lethal to S. pombe. Eso1 protein was able to acetylate Pol5 protein and mediate S. pombe viability. On the whole, our data indicate that the Eso1 interaction with Pol5 which acetylates Pol5 protein is essential for S. pombe viability.

Spatial focalization of pheromone/MAPK signaling triggers commitment to cell-cell fusion

Here, Dudin et al. show that cell fusion does not require a dedicated signal but is triggered by spatial focalization of the same pheromone–GPCR–MAPK signaling cascade that drives earlier mating events in Schizosaccharomyces pombe.

Cell fusion is universal in eukaryotes for fertilization and development, but what signals this process is unknown. Here, we show in Schizosaccharomyces pombe that fusion does not require a dedicated signal but is triggered by spatial focalization of the same pheromone–GPCR (G-protein-coupled receptor)–MAPK signaling cascade that drives earlier mating events. Autocrine cells expressing the receptor for their own pheromone trigger fusion attempts independently of cell–cell contact by concentrating pheromone release at the fusion focus, a dynamic actin aster underlying the secretion of cell wall hydrolases. Pheromone receptor and MAPK cascade are similarly enriched at the fusion focus, concomitant with fusion commitment in wild-type mating pairs. This focalization promotes cell fusion by immobilizing the fusion focus, thus driving local cell wall dissolution. We propose that fusion commitment is imposed by a local increase in MAPK concentration at the fusion focus, driven by a positive feedback between fusion focus formation and focalization of pheromone release and perception.

An account of fungal 14-3-3 proteins

14-3-3s are a group of relatively low molecular weight, acidic, dimeric, protein(s) conserved from single-celled yeast to multicellular vertebrates including humans. Despite lacking catalytic activity, these proteins have been shown to be involved in multiple cellular processes. Apart from their role in normal cellular physiology, recently these proteins have been implicated in various medical consequences. In this present review, fungal 14-3-3 protein localization, interactions, transcription, regulation, their role in the diverse cellular process including DNA duplication, cell cycle, protein trafficking or secretion, apoptosis, autophagy, cell viability under stress, gene expression, spindle positioning, role in carbon metabolism have been discussed. In the end, I also highlighted various roles of yeasts 14-3-3 proteins in tabular form. Thus this review with primary emphasis on yeast will help in appreciating the significance of 14-3-3 proteins in cell physiology.

Phosphorylation-dependent inhibition of Cdc42 GEF Gef1 by 14-3-3 protein Rad24 spatially regulates Cdc42 GTPase activity and oscillatory dynamics during cell morphogenesis

The 14-3-3 protein Rad24 modulates the availability of Cdc42 GEF Gef1, spatially regulating Cdc42 activity during cell morphogenesis. Gef1 is sequestered in the cytoplasm upon 14-3-3 interaction, mediated by Orb6 kinase. The resulting competition for Gef1 promotes anticorrelated Cdc42 oscillations at cell tips.

Active Cdc42 GTPase, a key regulator of cell polarity, displays oscillatory dynamics that are anticorrelated at the two cell tips in fission yeast. Anticorrelation suggests competition for active Cdc42 or for its effectors. Here we show how 14-3-3 protein Rad24 associates with Cdc42 guanine exchange factor (GEF) Gef1, limiting Gef1 availability to promote Cdc42 activation. Phosphorylation of Gef1 by conserved NDR kinase Orb6 promotes Gef1 binding to Rad24. Loss of Rad24–Gef1 interaction increases Gef1 protein localization and Cdc42 activation at the cell tips and reduces the anticorrelation of active Cdc42 oscillations. Increased Cdc42 activation promotes precocious bipolar growth activation, bypassing the normal requirement for an intact microtubule cytoskeleton and for microtubule-dependent polarity landmark Tea4-PP1. Further, increased Cdc42 activation by Gef1 widens cell diameter and alters tip curvature, countering the effects of Cdc42 GTPase-activating protein Rga4. The respective levels of Gef1 and Rga4 proteins at the membrane define dynamically the growing area at each cell tip. Our findings show how the 14-3-3 protein Rad24 modulates the availability of Cdc42 GEF Gef1, a homologue of mammalian Cdc42 GEF DNMBP/TUBA, to spatially control Cdc42 GTPase activity and promote cell polarization and cell shape emergence.

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.

Mate and fuse: how yeast cells do it

Many cells are able to orient themselves in a non-uniform environment by responding to localized cues. This leads to a polarized cellular response, where the cell can either grow or move towards the cue source. Fungal haploid cells secrete pheromones to signal mating, and respond by growing a mating projection towards a potential mate. Upon contact of the two partner cells, these fuse to form a diploid zygote. In this review, we present our current knowledge on the processes of mating signalling, pheromone-dependent polarized growth and cell fusion in Saccharomyces cerevisiae and Schizosaccharomyces pombe, two highly divergent ascomycete yeast models. While the global architecture of the mating response is very similar between these two species, they differ significantly both in their mating physiologies and in the molecular connections between pheromone perception and downstream responses. The use of both yeast models helps enlighten both conserved solutions and species-specific adaptations to a general biological problem.

The zinc finger protein Gsf1 regulates Gsf2-dependent flocculation in fission yeast

Fission yeast flocculates nonsexually by induction of the flocculin encoded by gsf2(+) which is controlled by the positive regulator Mbx2. Here, we report a novel gene designated gsf1(+) found to be a negative regulator of nonsexual flocculation. We identified gsf1(+) as a multicopy suppressor of a sam2 mutation, which caused growth sensitivity to Ca(2+) and also found a nonsense mutation in gsf1(+) in a previously isolated gsf1 mutant. The gsf1(+) gene encodes a 547-aa protein containing a Zn(2)-Cys(6) binuclear cluster-type zinc finger motif. The Gsf1 protein localized in the nucleus, consistent with a role as a transcription factor. Deletion of gsf1(+) resulted in nonsexual flocculation inducible by CaCl2 , which was suppressed by the addition of EDTA or galactose. Both gsf2(+) and mbx2(+) were highly expressed in the gsf1 mutant. gsf1∆ gsf2∆ and gsf1∆ mbx2∆ double mutants did not flocculate, suggesting that gsf1(+) is an upstream regulator. In addition, the gsf1 mutant was sensitive to CaCl2 , KCl, HU, and TBZ, consistent with the possibility that gsf1(+) plays a role in functions unrelated to flocculation. Taken together, these results suggest that nonsexual flocculation in fission yeast is negatively controlled by Gsf1, which controls expression of mbx2(+) and gsf2(+) .

Histone chaperone Asf1 plays an essential role in maintaining genomic stability in fission yeast

The histone H3-H4 chaperone Asf1 is involved in chromatin assembly (or disassembly), histone exchange, regulation of transcription, and chromatin silencing in several organisms. To investigate the essential functions of Asf1 in Schizosaccharomyces pombe, asf1-ts mutants were constructed by random mutagenesis using PCR. One mutant (asf1-33(ts)) was mated with mutants in 77 different kinase genes to identify synthetic lethal combinations. The asf1-33 mutant required the DNA damage checkpoint factors Chk1 and Rad3 for its survival at the restrictive temperature. Chk1, but not Cds1, was phosphorylated in the asf1-33 mutant at the restrictive temperature, indicating that the DNA damage checkpoint was activated in the asf1-33 mutant. DNA damage occured in the asf1-33 mutant, with degradation of the chromosomal DNA observed through pulse-field gel electrophoresis and the formation of Rad22 foci. Sensitivity to micrococcal nuclease in the asf1-33 mutant was increased compared to the asf1⁺ strain at the restrictive temperature, suggesting that asf1 mutations also caused a defect in overall chromatin structure. The Asf1-33 mutant protein was mislocalized and incapable of binding histones. Furthermore, histone H3 levels at the centromeric outer repeat region were decreased in the asf1-33 mutant and heterochromatin structure was impaired. Finally, sim3, which encodes a CenH3 histone chaperone, was identified as a strong suppressor of the asf1-33 mutant. Taken together, these results clearly indicate that Asf1 plays an essential role in maintaining genomic stability in S. pombe.

Complex formation, phosphorylation, and localization of protein kinase A of Schizosaccharomyces pombe upon glucose starvation

Nine sam mutants that undergo sexual differentiation without requiring starvation in Schizosaccharomyces pombe were previously isolated. In this study, we identified a nonsense mutation on the pka1 locus in the sam6 mutant. pka1 encodes a catalytic subunit of protein kinase A (PKA). Replacement and overexpression of pka1 suppressed the KCl sensitivity and hyper-mating phenotype of sam6, confirming that sam6 is an allele of pka1. To characterize further the regulation of Pka1, we tested the physical interaction between Pka1 and Cgs1 (a regulatory subunit of PKA). Pka1 and Cgs1 physically interacted under glucose-limited conditions but not under glucose-rich conditions. In addition, the formation of a Pka1-Cgs1 complex was detected under glucose-limited conditions by Blue Native PAGE. Furthermore, the Pka1 protein was found to be phosphorylated under glucose-starved conditions, and at the same time its localization shifted from the nucleus towards the cytoplasm (mainly the vacuoles), suggesting a strong relationship among phosphorylation, complex formation, and the cytoplasmic distribution of Pka1.

Identification of testis 14-3-3 binding proteins by tandem affinity purification

The 14-3-3 family of proteins interacts with various cellular phosphoproteins and regulates multiple cell signaling cascades. Identification of 14-3-3 interactors is important to define 14-3-3 functions in various biological pathways. The binding partners of protein 14-3-3 in testis are not known. The main goal of this study was to identify the 14-3-3 interactome in testis to determine the 14-3-3 regulated cellular processes in testis. We used transgenic mice expressing tandem affinity tagged 14-3-3ζ (TAP-14-3-3ζ) driven by the ubiquitin promoter to isolate 14-3-3 binding proteins. The 14-3-3 complexes in testis were isolated using a two-step tandem affinity purification (TAP) followed by identification with liquid chromatography/tandem mass spectrometry (LC-MS/MS). A total of 135 proteins were found to be associated with 14-3-3 in vivo in testis. Comparison of the testis 14-3-3 proteome with known 14-3-3 binding proteins showed that 71 of the proteins identified in this study are novel 14-3-3 interactors. Eight of these novel 14-3-3 interacting proteins are predominantly expressed in testis. The 14-3-3 interactors predominant in testis are: protein phosphatase1γ2 (PP1γ2), spermatogenesis associated 18 (SPATA18), phosphoglycerate kinase-2 (PGK2), testis specific gene A-2 (TSGA-2), dead box polypeptide 4 (DDX4), piwi homolog 1, protein kinase NYD-SP25 and EAN57. The fact that some of these proteins are indispensable for spermatogenesis suggests that their binding to 14-3-3 may be important for their function in germ cell division and maturation. These findings are discussed in context of the putative functions of 14-3-3 in spermatogenesis.

Nuclear protein quality is regulated by the ubiquitin-proteasome system through the activity of Ubc4 and San1 in fission yeast

Eukaryotic cells monitor and maintain protein quality through a set of protein quality control (PQC) systems whose role is to minimize the harmful effects of the accumulation of aberrant proteins. Although these PQC systems have been extensively studied in the cytoplasm, nuclear PQC systems are not well understood. The present work shows the existence of a nuclear PQC system mediated by the ubiquitin-proteasome system in the fission yeast Schizosaccharomyces pombe. Asf1-30, a mutant form of the histone chaperone Asf1, was used as a model substrate for the study of the nuclear PQC. A temperature-sensitive Asf1-30 protein localized to the nucleus was selectively degraded by the ubiquitin-proteasome system. The Asf1-30 mutant protein was highly ubiquitinated at higher temperatures, and it remained stable in an mts2-1 mutant, which lacks proteasome activity. The E2 enzyme Ubc4 was identified among 11 candidate proteins as the ubiquitin-conjugating enzyme in this system, and San1 was selected among 100 candidates as the ubiquitin ligase (E3) targeting Asf1-30 for degradation. San1, but not other nuclear E3s, showed specificity for the mutant nuclear Asf1-30, but did not show activity against wild-type Asf1. These data clearly showed that the aberrant nuclear protein was degraded by a defined set of E1-E2-E3 enzymes through the ubiquitin-proteasome system. The data also show, for the first time, the presence of a nuclear PQC system in fission yeast.

Fission yeast Myo51 is a meiotic spindle pole body component with discrete roles during cell fusion and spore formation

Class V myosins are dimeric actin-associated motor proteins that deliver cellular cargoes to discrete cellular locations. Fission yeast possess two class V myosins, Myo51 and Myo52. Although Myo52 has been shown to have roles in vacuole distribution, cytokinesis and cell growth, Myo51 has no as yet discernible function in the vegetative life cycle. Here, we uncover distinct functions for this motor protein during mating and meiosis. Not only does Myo51 transiently localise to a foci at the site of cell fusion upon conjugation, but overexpression of the Myo51 globular tail also leads to disruption of cell fusion. Upon completion of meiotic prophase Myo51 localises to the outside of the spindle pole bodies (SPBs), where it remains until completion of meiosis II. Association of Myo51 with SPBs is not dependent upon actin or the septation initiation network (SIN); however, it is dependent on a stable microtubule cytoskeleton and the presence of the Cdc2-CyclinB complex. We observe a rapid and dynamic exchange of Myo51 at the SPB during meiosis I but not meiosis II. Finally, we show that Myo51 has an important role in regulating spore formation upon completion of meiosis.

Ras and Nox: Linked signaling networks?

Both Ras and Nox represent ancient gene families which control a broad range of cellular responses. Both families mediate signals governing motility, differentiation, and proliferation, and both inhabit overlapping subcellular microdomains. Yet little is known of the precise functional relationship between these two ubiquitous families. In this review, we examine the interface where these two large fields meet.

A large complex mediated by Moc1, Moc2 and Cpc2 regulates sexual differentiation in fission yeast

Sexual differentiation in Schizosaccharomyces pombe is triggered by nutrient starvation and is downregulated by cAMP. Screening programs have identified the moc1/sds23, moc2/ded1, moc3 and moc4/zfs1 genes as inducers of sexual differentiation, even in the presence of elevated levels of cAMP. To investigate possible interactions among Moc1, Moc2, Moc3 and Moc4 proteins, we first screened for individual Moc-interacting proteins using the yeast two-hybrid system and verified the interactions with other Moc proteins. Using this screening process, Cpc2 and Rpl32-2 were highlighted as factors involved in interactions with multiple Moc proteins. Cpc2 interacted with Moc1, Moc2 and Moc3, whereas the ribosomal protein Rpl32-2 interacted with all Moc proteins in the two-hybrid system. Physical interactions of Cpc2 with Moc1, Moc2 and Rpl32-2, and of Rpl32-2 with Moc2 were confirmed by coimmunoprecipitation. In addition, using Blue Native/PAGE, we revealed that each Moc protein exists as a large complex. Overexpression of Moc1, Moc2, Moc3, Moc4 and Rpl32-2 resulted in the efficient induction of a key transcription factor Ste11, suggesting that all proteins tested are positive regulators of Ste11. Considering that Moc2/Ded1 is a general translation factor and that Cpc2 associates with many ribosomal proteins, including Rpl32-2, it is possible that a large Moc-mediated complex, detected in this study, may act as a translational regulator involved in the control of sexual differentiation in S. pombe through the induction of Ste11.

High production of sulfide in coenzyme Q deficient fission yeast

We have constructed coenzyme Q deficient fission yeast strains by deletion of ten different genes, all of which are absolutely required for the CoQ10 biosynthesis. We found that sulfide was highly accumulated in all fission yeast CoQ10 deficient mutants. In fission yeast sulfide is required for the synthesis of cysteine and homocysteine which are catalyzed by cysteine synthase (Cys1a) and homocysteine synthase (Met17), respectively. To better understand the relation between sulfide metabolism and coenzyme Q, we expressed cys1a, met17 and hmt2, which encodes sulfide-quinone oxidoreductase, in CoQ10 deficient mutants and other mutants, and measured the level of sulfide. Although expression of cys1a and met17 lowered sulfide production in CoQ10 deficient mutants, hmt2 did not lower the level of sulfide, because Hmt2 requires coenzyme Q for its function. In contrast, expression of hmt2 lowered sulfide production in cys1a and met17 mutants. These and other results indicate that coenzyme Q is important for sulfide oxidation through sulfide-quinone oxidoreductase to detoxify excess sulfide in fission yeast.

Human Raf-1 proteins associate with Rad24 and Cdc25 in cell-cycle checkpoint pathway of fission yeast, Schizosaccharomyces pombe

Raf-1 is a serine/threonine protein kinase that connects cell surface receptor signals to nuclear transcription factors. By screening Schizosaccharomyces pombe (S. pombe) cDNA library, we isolated Rad24, which is a 14-3-3 homolog that is important in the DNA damage checkpoint in S. pombe, as a Raf-1 interacting protein. The interaction found in yeast was confirmed by co-immunoprecipitation. Furthermore, Cdc25, which has been known to bind to Rad24, also associated with Raf-1 and was phosphorylated in vitro by catalytically active Raf-1. However, in the presence of Raf-1, an interaction between Rad24 and Cdc25 was inhibited in triple hybrid assay, indicating that Raf-1 inhibits the interaction between Rad24 and Cdc25. An in vitro competition assay showed that the binding of Cdc25 and of Rad24 to Raf-1 is mutually exclusive. Western blots of whole cell lysates probed with polyclonal antibodies specific for tyrosine-15-phosphorylated Cdc2 showed that overproduction of Rad24 led to the dephosphorylation of tyrosine residue on Cdc2, which is known to be activated through dephosphorylation by Cdc25 phosphatase. Unexpectedly, overexpression of catalytically inactive mutant protein of Raf-1, S624A, also caused tyrosine dephosphorylation of Cdc2. Thus, these data suggest that Raf-1 may interfere with the role of Rad24 by competing with Rad24 for binding to Cdc25 or a direct phosphorylation of Cdc25, bypassing the checkpoint pathway in DNA repair through Cdc25 activation.

In Schizosaccharomyces pombe the 14-3-3 protein Rad24p is involved in negative control of pho1 gene expression

Expression of Schizosaccharomyces pombe pho1-encoded acid phosphatase is transcriptionally regulated by adenine and phosphate. Four genes, anr1-3 and anr5, encode negative regulators of pho1 expression. Apart from being designated as loci, the anr genes have not been further characterized. In this study we provide evidence that a strain carrying the deletion of rad24, a 14-3-3 protein-encoding gene, exhibits an anr mutant like the phenotype (higher phosphatase activity, higher transcript levels of pho1, lower sensitivity to adenine of pho1 expression) and that rad24 is closely linked, probably allelic, to anr5. By sequencing the two exons of the rad24 gene in a strain carrying the mutant allele anr5-13, we found a T/A-to-C/G transition in the 225th codon of its ORF, causing a leucine-to-serine substitution in a highly conserved region of all proteins of the 14-3-3 family. anr2 and anr3 are not allelic to rad24. The mutant alleles of anr2 and anr3 are recessive to their wild-type alleles and do not belong to the same epistasis group as rad24.

Interactions between the RNA interference effector protein Ago1 and 14-3-3 proteins: consequences for cell cycle progression

The Argonaute family member Ago1 is required for formation of pericentric heterochromatin and small interfering RNA (siRNA)-mediated post-transcriptional gene silencing in the fission yeast Schizosaccharomyces pombe. In addition, we have recently demonstrated that Ago1 function is required for enactment of cell cycle checkpoints (Carmichael, J. B., Provost, P., Ekwall, K., and Hobman, T. C. (2004) Mol. Biol. Cell 15, 1425-1435). Here, we provide evidence that the amino terminus of Ago1 binds to proteins that function in cell cycle regulation including 14-3-3 proteins. Interestingly, the amino terminus of human Ago2, the endonuclease that cleaves siRNA-targeted mRNAs, was also demonstrated to bind 14-3-3 proteins. Overexpression of the Ago1 amino terminus in yeast resulted in cell cycle delay at the G(2)/M boundary. Further investigation revealed that nuclear import of the mitosis-inducing phosphatase Cdc25 is inhibited by overexpression of the Ago1 amino terminus. Under these conditions, we found that the cyclin-dependent kinase Cdc2 is constitutively phosphorylated on tyrosine 15, thereby reducing the activity of this kinase, a situation that delays entry into mitosis. We hypothesize that 14-3-3 proteins are required for Argonaute protein functions in cell cycle and/or gene-silencing pathways.

Spatial segregation of Ras signaling: new evidence from fission yeast

The Ras GTPases act as binary switches for signal transduction pathways that are important for growth regulation and tumorigenesis. Despite the biochemical simplicity of this switch, Ras proteins control multiple pathways, and the functions of the four mammalian Ras proteins are not overlapping. This raises an important question--how does a Ras protein selectively regulate a particular activity? One recently emerging model suggests that a single Ras protein can control different functions by acting in distinct cellular compartments. A critical test of this model is to identify pathways that are selectively controlled by Ras when it is localized to a particular compartment. A recent study has examined Ras signaling in the fission yeast Schizosaccharomyces pombe, which expresses only one Ras protein that controls two separate evolutionarily conserved pathways. This study demonstrates that whereas Ras localized to the plasma membrane selectively regulates a MAP kinase pathway to mediate mating pheromone signaling, Ras localized to the endomembrane activates a Cdc42 pathway to mediate cell polarity and protein trafficking. This study has provided unambiguous evidence for compartmentalized signaling of Ras.

Compartmentalized signaling of Ras in fission yeast

Compartment-specific Ras signaling is an emerging paradigm that may explain the multiplex outputs from a single GTPase. The fission yeast, Schizosaccharomyces pombe, affords a simple system in which to study Ras signaling because it has a single Ras protein, Ras1, that regulates two distinct pathways: one that controls mating through a Byr2-mitogen-activated protein kinase cascade and one that signals through Scd1-Cdc42 to maintain elongated cell morphology. We generated Ras1 mutants that are restricted to either the endomembrane or the plasma membrane. Protein binding studies showed that each could interact with the effectors of both pathways. However, when examined in ras1 null cells, endomembrane-restricted Ras1 supported morphology but not mating, and, conversely, plasma membrane-restricted Ras1 supported mating but did not signal to Scd1-Cdc42. These observations provide a striking demonstration of compartment-specific Ras signaling and indicate that spatial specificity in the Ras pathway is evolutionarily conserved.

Yeast 14-3-3 proteins

14-3-3 proteins form a family of highly conserved proteins which are present in all eukaryotic organisms investigated, often in multiple isoforms, up to 13 in some plants. They interact with more than 200 different, mostly phosphorylated proteins. The molecular consequences of 14-3-3 binding are diverse: this binding may result in stabilization of the active or inactive phosphorylated form of the protein, to a conformational alteration leading to activation or inhibition, to a different subcellular localization, to the interaction with other proteins or to shielding of binding sites. The binding partners, and hence the 14-3-3 proteins, are involved in almost every cellular process and 14-3-3 proteins have been linked to several diseases, such as cancer, Alzheimer's disease, the neurological Miller-Dieker and spinocerebellar ataxia type 1 diseases and bovine spongiform encephalopathy (BSE). The yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe both have two genes encoding 14-3-3 proteins, BMH1 and BMH2 and rad24 and rad25, respectively. In these yeasts, 14-3-3 proteins are essential in most laboratory strains. As in higher eukaryotes, yeast 14-3-3 proteins bind to numerous proteins involved in a variety of cellular processes. Recent genome-wide studies on yeast strains with impaired 14-3-3 function support the participation of 14-3-3 proteins in numerous yeast cellular processes. Given the high evolutionary conservation of the 14-3-3 proteins, the experimental accessibility and relative simplicity of yeasts make them excellent model organisms for elucidating the function of the 14-3-3 protein family.

zds1, a novel gene encoding an ortholog of Zds1 and Zds2, controls sexual differentiation, cell wall integrity and cell morphology in fission yeast

While screening for genes that reverse the sporulation-deficient phenotype of the ras1delta diploid Schizosaccharomyces pombe strain, we identified zds1. This gene shares sequence homology with the ZDS1 and ZDS2 genes from Saccharomyces cerevisiae, which appear to be involved in multiple cellular events. Expression of Zds1 in ras1delta diploid cells elevated their sporulation rate from 0.3 to 11.2%. Expression of the Zds1 C-terminal region increased the sporulation rate further (to 21.9%) while introduction of the Zds1 N-terminal region had no effect. zds1 expression did not induce sporulation in strains with mutations in genes participating in the downstream MAP kinase cascade. The zds1-disrupted strain is sensitive to CaCl2, and this effect is suppressed by the C-terminal region of Zds1. The growth of the zds1delta strain is markedly inhibited by cold temperatures, while its viability decreased in the stationary phase. Moreover, the zds1delta strain is round in shape and very sensitive to zymolyase, and its cell wall becomes thicker than that of wild type. Thus, zds1 must be required to maintain cell wall integrity. The Zds1-GFP fusion protein localized to the cytosol, the septum, and the cell cortex. Its localization in the septum was dependent on its C-terminal region. Overexpression of the C-terminal region of Zds1 induced multi-septa and abnormal zygotes. We propose that the C-terminal region is the functional domain of Zds1 while the N-terminal region is a negative regulatory region. Thus, Zds1 is involved in multiple cellular events in fission yeast, including sexual differentiation, Ca2+ tolerance, cell wall integrity, viability in the stationary phase, and cell morphology.

Moc3, a novel Zn finger type protein involved in sexual development, ascus formation, and stress response of Schizosaccharomyces pombe

The cAMP pathway in Schizosaccharomyces pombe is the major nutrient sensing pathway to initiate sexual development when opposite mating type cells exist. We identified moc1-moc4 as genes that overcome a partially sterile S. pombe strain due to an elevation of cAMP. When we compared the strength of inducing ability of sexual development in the same S. pombe strain, Moc1 had highest, Moc2 had lowest, and both Moc3 and Moc4 had intermediate effects. Moc1/Sds23 and Moc2/Ded1 are known to be a potential regulator of M-phase progression and an essential RNA helicase, respectively. While Moc4 was found to be identical with a Zn-finger protein Zfs1, Moc3 (SPAC821.07c) was a novel protein containing a Zn-finger (Zn(2)-Cys(6)) motif. Deletion mutant of the moc3 gene was constructed and its disruptant was found to be lower in mating efficiency and formed aberrant asci. In addition, unexpectedly, a moc3 disruptant was sensitive to CaCl(2) and DNA damaging agents such as MMS and UV. Those phenotypes were opposite to the phenotypes observed in a zfs1 disruptant, and quite different from the ones in a moc1 disruptant. Moc3 localized in the nucleus as observed for Zfs1. Moc3 bound with Moc4/Zfs1 weakly in the two hybrid system, but no other combination of Moc(s) bound each other in the same analysis. Thus, Moc3 is not only involved in sexual development, but also in ascus formation and DNA integrity in an independent manner with Moc1 and Moc2 in S. pombe.

Differential gene expression by Metarhizium anisopliae growing in root exudate and host (Manduca sexta) cuticle or hemolymph reveals mechanisms of physiological adaptation

Like many other fungal pathogens Metarhizium anisopliae is a facultative saprophyte with both soil-dwelling and insect pathogenic life-stages. In addition, as M. anisopliae traverses the cuticle and enters the hemolymph it must adapt to several different host environments. In this study, we used expressed sequence tags and cDNA microarray analyses to demonstrate that physiological adaptation by M. anisopliae to insect cuticle, insect hemolymph, bean root exudate (a model for life in the soil), and nutrient rich Sabouraud dextrose broth (SDB) involves different subsets of genes. Overall, expression patterns in cuticle and hemolymph clustered separately from expression patterns in root exudates and SDB, indicative of critical differences in transcriptional control during pathogenic and saprophytic growth. However, there were differences in gene expression between hemolymph and cuticle and these mostly involved perception mechanisms, carbon metabolism, proteolysis, cell surface properties, and synthesis of toxic metabolites. These differences suggest previously unsuspected stratagems of fungal pathogenicity that can be tested experimentally. Examples include the switch-off of cuticle-degrading proteases and a dramatic cell wall reorganization during growth in hemolymph.

cda1+, encoding chitin deacetylase is required for proper spore formation in Schizosaccharomyces pombe

In Schizosaccharomyces pombe, a major role of chitin is to build up a complete spore. Here, we analyzed the cda1(+) gene (SPAC19G12.03), which encodes a protein homologous to chitin deacetylases, to know whether it is required for spore formation in S. pombe. The homothallic Deltacda1 strain constructed by homologous recombination was found to form a little amount of abnormal spores that contained one, two, or three asci, similar to (but not as strong as) the phenotype observed in a deletion mutant of chs1 encoding chitin synthase 1. This phenotype is reversed by expression of S. cerevisiae chitin deacetylase CDA1 or CDA2, suggesting that cda1 encodes a chitin deacetylase. To support the role of Cda1 in sporulation, the timing of expression of cda1(+) mRNA increased during sporulation process. We also found that the Cda1 protein self-associated when its binding was tested both by two-hybrid system and immunoprecipitation. Thus, these data indicated that cda1(+) is required for proper spore formation in S. pombe.

A novel gene, msa1, inhibits sexual differentiation in Schizosaccharomyces pombe

Sexual differentiation in the fission yeast Schizosaccharomyces pombe is triggered by nutrient starvation or by the presence of mating pheromones. We identified a novel gene, msa1, which encodes a 533-aa putative RNA-binding protein that inhibits sexual differentiation. Disruption of the msa1 gene caused cells to hypersporulate. Intracellular levels of msa1 RNA and Msa1 protein diminished after several hours of nitrogen starvation. Genetic analysis suggested that the function of msa1 is independent of the cAMP pathway and stress-responsive pathway. Deletion of the ras1 gene in diploid cells inhibited sporulation and in haploid cells decreased expression of mating-pheromone-induced genes such as mei2, mam2, ste11, and rep1; simultaneous deletion of msa1 reversed both phenotypes. Overexpression of msa1 decreased activated Ras1(Val17)-induced expression of mam2. Phenotypic hypersporulation was similar between cells with deletion of only rad24 and both msa1 and rad24, but simultaneous deletion of msa1 and msa2/nrd1 additively increased hypersporulation. Therefore, we suggest that the primary function of Msa1 is to negatively regulate sexual differentiation by controlling the expression of Ste11-regulated genes, possibly through the pheromone-signaling pathway.

Unlocking the code of 14-3-3

One of the most striking 'rags to riches' stories in the protein world is that of 14-3-3, originally identified in 1967 as merely an abundant brain protein. The first clues that 14-3-3 would play an important role in cell biology came almost 25 years later when it was found to interact with various proto-oncogene proteins and signaling proteins. The subsequent identification of 14-3-3 as a phosphoserine/phosphothreonine-binding protein firmly established its importance in cell signaling. 14-3-3 family members are found in all eukaryotes - from plants to mammals - and more than 100 binding partners have been identified to date. The targets of 14-3-3 are found in all subcellular compartments and their functional diversity is overwhelming - they include transcription factors, biosynthetic enzymes, cytoskeletal proteins, signaling molecules, apoptosis factors and tumor suppressors. 14-3-3 binding can alter the localization, stability, phosphorylation state, activity and/or molecular interactions of a target protein. Recent studies now indicate that the serine/threonine protein phosphatases PP1 and PP2A are important regulators of 14-3-3 binding interactions, and demonstrate a role for 14-3-3 in controlling the translocation of certain proteins from the cytoplasmic and endoplasmic reticulum to the plasma membrane. New reports also link 14-3-3 to several neoplastic and neurological disorders, where it might contribute to the pathogenesis and progression of these diseases.

SAM domains can utilize similar surfaces for the formation of polymers and closed oligomers

The mitogen-activated protein kinase (MAPK) Byr2 and its activator Ste4 are involved in the mating pheromone response pathway of Schizosaccharomyces pombe and interact via their SAM domains. SAM domains can self-associate to form higher-order structures, including dimers, polymers and closed oligomers. Ste4-SAM is adjacent to a trimeric leucine zipper domain and we have shown previously that the two domains together (Ste4-LZ-SAM) bind to a monomeric Byr2-SAM with high affinity (Kd approximately 20 nM), forming a 3:1 complex. Here, we map the surfaces of Byr2-SAM and Ste4-SAM that is involved the interaction. A set of 38 mutants of Byr2-SAM and 33 mutants of Ste4-SAM were prepared, covering most of the protein surfaces. These mutants were purified and screened for binding, yielding a map of residues that are required for binding and a complementary map of residues that are not required. We find that the interface maps to regions of the SAM domains that are known to be important for the formation of SAM polymers. These results indicate that SAM domains can create a variety of oligomeric architectures utilizing common binding surfaces.

Activation of the pheromone-responsive MAP kinase drives haploid cells to undergo ectopic meiosis with normal telomere clustering and sister chromatid segregation in fission yeast

Meiosis is a process of importance for sexually reproducing eukaryotic organisms. In the fission yeast Schizosaccharomyces pombe, meiosis normally proceeds in a diploid zygote which is produced by conjugation of haploid cells of opposite mating types. We demonstrate that activation of the pheromone-responsive MAPK, Spk1, by the ectopic expression of a constitutively active form of Byr1 (MAPKK for Spk1) induced the cells to undergo meiosis while in the haploid state. Moreover, the induction of meiosis required Mei2 (a key positive regulator of meiosis), but did not require Mei3; Mei3 is normally required to inactivate the Pat1 kinase (a negative regulator of Mei2) thereby allowing Mei2 to drive meiosis. Therefore, expression of a constitutively active form of Byr1 activates Mei2 without the need of Mei3. In cells induced to undergo meiosis by activating the Spk1 MAPK signaling pathway, telomeres clustered at the spindle pole body (SPB) and centromeres detached normally from the SPB during meiotic prophase, and the cells showed the correct segregation of sister chromatids during meiotic divisions. In contrast, in meiosis induced by inactivation of Pat1, sister chromatids segregate precociously during the first meiotic division. Thus, these results suggest that activation of Spk1 drives meiosis in S. pombe.

Comparative analysis of two 14-3-3 homologues and their expression pattern in the root-knot nematode Meloidogyne incognita

14-3-3 proteins are highly conserved ubiquitous proteins found in all eukaryotic organisms. They are involved in various cellular processes including signal transduction, cell-cycle control, apoptosis, stress response and cytoskeleton organisation. We report here the cloning of two genes encoding 14-3-3 isoforms from the plant parasitic root-knot nematode Meloidogyne incognita, together with an analysis of their expression. Both genes were shown to be transcribed in unhatched second stage larvae, infective second stage larvae, adult males and females. The Mi-14-3-3-a gene was shown to be specifically transcribed in the germinal primordium of infective larvae, whereas Mi-14-3-3-b was transcribed in the dorsal oesophageal gland in larvae of this stage. The MI-14-3-3-B protein was identified by mass spectrometry in in vitro-induced stylet secretions from infective larvae. The stability and distribution of MI-14-3-3 proteins in host plant cells was assessed after stable expression of the corresponding genes in tobacco BY2 cells.

Sla1, a Schizosaccharomyces pombe homolog of the human La protein, induces ectopic meiosis when its C terminus is truncated

Sla1 is a Schizosaccharomyces pombe homolog of the human La protein. La proteins are known to be RNA-binding proteins that bear conserved RNA recognition motifs (La and RRMs), but their biological functions still have not been fully resolved. In this study, we show that the S. pombe La homolog (Sla1) is involved in regulating sexual development. Sla1 truncated in the C terminus (Sla1DeltaC) induced ectopic sporulation in the ras1Delta strain and several other sporulation-deficient mutants. The C terminus contains a nuclear localization signal. While full-length Sla1 localizes in the nucleus, Sla1DeltaC is found throughout the cell, suggesting the cytoplasmic localization of Sla1DeltaC is involved in its sporulation-inducing activity. Further deletion analysis of Sla1 indicated that a small region (35 amino acids) that includes a portion of RRM2 is sufficient to induce sporulation. The La motif (RRM1) is not involved in this activity. Strikingly, Sla1DeltaC induced haploid meiosis in a heterothallic strain, similar to the pat1-114 or mei2-SATA mutation. Sla1DeltaC induced sporulation in a mei3 disruptant but not in a mei2 disruptant, indicating that Sla1DeltaC requires Mei2 to induce haploid meiosis. Deletion of the chromosomal sla1 gene lowered the temperature sensitivity of the pat1-114 mutant. Two-hybrid analysis indicated that Pat1 interacts with Sla1DeltaC but not full-length Sla1. Thus, Sla1DeltaC may block Pat1 activity. This block would remove the inhibition on Mei2, which would then drive the cell into haploid meiosis. Finally, Sla1 was degraded prior to the start of meiosis when we monitored Sla1 in cells in which meiosis was synchronously induced. The ability of truncated Sla1 to induce ectopic meiosis represents a very novel function that has hitherto not been suspected for the La family of proteins.

Fission yeast decaprenyl diphosphate synthase consists of Dps1 and the newly characterized Dlp1 protein in a novel heterotetrameric structure

The analysis of the structure and function of long chain-producing polyprenyl diphosphate synthase, which synthesizes the side chain of ubiquinone, has largely focused on the prokaryotic enzymes, and little is known about the eukaryotic counterparts. Here we show that decaprenyl diphosphate synthase from Schizosaccharomyces pombe is comprised of a novel protein named Dlp1 acting in partnership with Dps1. Dps1 is highly homologous to other prenyl diphosphate synthases but Dlp1 shares only weak homology with Dps1. We showed that the two proteins must be present simultaneously in Escherichia coli transformants before ubiquinone-10, which is produced by S. pombe but not by E. coli, is generated. Furthermore, the two proteins were shown to form a heterotetrameric complex. This is unlike the prokaryotic counterparts, which are homodimers. The deletion mutant of dlp1 lacked the enzymatic activity of decaprenyl diphosphate synthase, did not produce ubiquinone-10 and had the typical ubiquinone-deficient S. pombe phenotypes, namely hypersensitivity to hydrogen peroxide, the need for antioxidants for growth on minimal medium and an elevated production of H2S. Both the dps1 (formerly dps) and dlp1 mutants could generate ubiquinone when they were transformed with a bacterial decaprenyl diphosphate synthase, which functions in its host as a homodimer. This indicates that both dps1 and dlp1 are required for the S. pombe enzymatic activity. Thus, decaprenyl diphosphate from a eukaryotic origin has a heterotetrameric structure that is not found in prokaryotes.

Signal pathway integration in the switch from the mitotic cell cycle to meiosis in yeast

Diploid yeast, like most eukaryotes, can undergo meiotic differentiation to form haploid gametes. Meiotic differentiation and cell growth (proliferation) are mutually exclusive programs, and in yeast the switch between growth and meiosis is controlled by nutritional signals. The signaling pathways that mediate nutritional controls on meiotic initiation fall into three broad classes: those that respond to nutrient starvation, those that respond to non-fermentable carbon sources, and those that respond to glucose. At the onset of meiosis, nutritional signaling pathways converge on transcriptional regulation of two genes: IME1, which encodes a transcription factor; and IME2, which encodes a protein kinase. Transcription of IME1 and IME2 trigger initiation of meiosis, and the expression of these two genes is linked with one other, with expression of later meiotic genes and with early meiotic events such as DNA replication. In addition, the signaling pathways that control IME1 and IME2 expression are themselves integrated through a variety of mechanisms. Thus the signal network that controls the switch from growth to meiotic differentiation provides a signaling code that translates different combinations of extracellular signals into appropriate cellular responses.