Mug27 is a meiosis-specific protein kinase that functions in fission yeast meiosis II and sporulation

Comparative transcriptome analysis revealed candidate genes involved in fruiting body development and sporulation in Ganoderma lucidum

Ganoderma lucidum is an edible mushroom highly regarded in the traditional Chinese medicine. To better understand the molecular mechanisms underlying fruiting body development in G. lucidum, transcriptome analysis based on RNA sequencing was carried out on different developmental stages: mycelium (G1); primordium (G2); young fruiting body (G3); mature fruiting body (G4); fruiting body in post-sporulation stage (G5). In total, 26,137 unigenes with an average length of 1078 bp were de novo assembled. Functional annotation of transcriptomes matched 72.49% of the unigenes to known proteins available in at least one database. Differentially expressed genes (DEGs) were identified between the evaluated stages: 3135 DEGs in G1 versus G2; 120 in G2 versus G3; 3919 in G3 versus G4; and 1012 in G4 versus G5. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of DEGs identified in G1 versus G2 revealed that, in addition to global and overview maps, enriched pathways were related to amino acid metabolism and carbohydrate metabolism. In contrast, DEGs identified in G2 versus G3 were mainly assigned to the category of metabolism of amino acids and their derivatives, comprising mostly upregulated unigenes. In addition, highly expressed unigenes associated with the transition between different developmental stages were identified, including those encoding hydrophobins, cytochrome P450s, extracellular proteases, and several transcription factors. Meanwhile, highly expressed unigenes related to meiosis such as DMC1, MSH4, HOP1, and Mek1 were also analyzed. Our study provides important insights into the molecular mechanisms underlying fruiting body development and sporulation in G. lucidum.

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.

Analysis of the S. pombe Meiotic Proteome Reveals a Switch from Anabolic to Catabolic Processes and Extensive Post-transcriptional Regulation

Meiotic progression in S. pombe is regulated by stage-specific gene expression and translation, changes in RNA stability, expression of anti-sense transcripts, and targeted proteolysis of regulatory proteins. We have used SILAC labeling to examine the relative levels of proteins in diploid S. pombe cells during meiosis. Among the 3,268 proteins quantified at all time points, the levels of 880 proteins changed at least 2-fold; the majority of proteins showed stepwise increases or decreases during the meiotic divisions, while some changed transiently. Overall, we observed reductions in proteins involved in anabolism and increases in proteins involved in catabolism. We also observed increases in the levels of proteins of the ESCRT-III complex and revealed a role for ESCRT-III components in chromosome segregation and spore formation. Correlation with studies of meiotic gene expression and ribosome occupancy reveals that many of the changes in steady-state protein levels are post-transcriptional.

Meiosis-specific localization of the exocytic Rab Ypt2 in fission yeast

Fission yeast Ypt2, an orthologue of the mammalian small GTPase Rab8, is responsible for post-Golgi membrane trafficking. During meiosis, Ypt2 localizes at the spindle pole body (SPB), where it regulates de novo biogenesis of the spore plasma membrane. Recruitment of Ypt2 to the SPB is dependent on its meiosis-specific GDP/GTP exchange factor (GEF), the SPB-resident protein Spo13. Here we have examined the SPB recruitment of Ypt2 by Spo13. The GEF activity of Spo13 was required, but not essential for recruitment. Furthermore, Ypt2 recruitment was regulated in a meiosis-specific manner and partially regulated by the nuclear Dbf2-related (NDR) kinase Sid2, indicating the existence of a novel regulatory mechanism for localization of Rab GTPases during meiosis.

Lipid droplet dynamics during Schizosaccharomyces pombe sporulation and their role in spore survival

Upon nitrogen starvation, the fission yeast Schizosaccharomyces pombe forms dormant spores; however, the mechanisms by which a spore sustains life without access to exogenous nutrients remain unclear. Lipid droplets are reservoirs of neutral lipids that act as important cellular energy resources. Using live-cell imaging analysis, we found that the lipid droplets of mother cells redistribute to their nascent spores. Notably, this process was actin polymerization-dependent and facilitated by the leading edge proteins of the forespore membrane. Spores lacking triacylglycerol synthesis, which is essential for lipid droplet formation, failed to germinate. Our results suggest that the lipid droplets are important for the sustenance of life in spores.

Summary: Lipid droplets of yeast mother cells are shown to redistribute to their nascent spores by live-cell imaging analysis, suggesting that the lipid droplets are important for yeast spore survival.

The exocytic Rabs Ypt3 and Ypt2 regulate the early step of biogenesis of the spore plasma membrane in fission yeast

Two Rabs, Ypt3 and Ypt2, regulating the trafficking of Golgi-derived secretory vesicles have key roles in biogenesis of the spore plasma membrane in fission yeast. During sporulation, the Rabs and secretory vesicles relocalize at the meiotic spindle pole body, where spore plasma membrane formation subsequently initiates.

During fission yeast sporulation, a membrane compartment called the forespore membrane (FSM) is newly formed on the spindle pole body (SPB). The FSM expands by membrane vesicle fusion, encapsulates the daughter nucleus resulting from meiosis, and eventually matures into the plasma membrane of the spore. Although many of the genes involved in FSM formation have been identified, its molecular mechanism is not fully understood. Here a genetic screen for sporulation-deficient mutations identified Ypt3, a Rab-family small GTPase known to function in the exocytic pathway. The ypt3-ki8 mutant showed defects in both the initiation of FSM biogenesis and FSM expansion. We also show that a mutation in Ypt2, another Rab protein that may function in the same pathway as Ypt3, compromises the initiation of FSM formation. As meiosis proceeds, both GFP-Ypt3 and GFP-Ypt2 are observed at the SPB and then relocalize to the FSM. Their localizations at the SPB precede FSM formation and depend on the meiotic SPB component Spo13, a putative GDP/GTP exchange factor for Ypt2. Given that Spo13 is essential for initiating FSM formation, these results suggest that two exocytic Rabs, Ypt3 and Ypt2, regulate the initiation of FSM formation on the SPB in concert with Spo13.

Pombe's thirteen - control of fission yeast cell division by the septation initiation network

The septation initiation network (SIN) regulates aspects of cell growth and division in Schizosaccharomyces pombe and is essential for cytokinesis. Insufficient signalling results in improper assembly of the contractile ring and failure of cytokinesis, generating multinucleated cells, whereas too much SIN signalling uncouples cytokinesis from the rest of the cell cycle. SIN signalling is therefore tightly controlled to coordinate cytokinesis with chromosome segregation. Signalling originates from the cytoplasmic face of the spindle pole body (SPB), and asymmetric localisation of some SIN proteins to one of the two SPBs during mitosis is important for regulation of the SIN. Recent studies have identified in vivo substrates of the SIN, which include components involved in mitotic control, those of the contractile ring and elements of the signalling pathway regulating polarised growth. The SIN is also required for spore formation following meiosis. This has provided insights into how the SIN performs its diverse functions in the cell cycle and shed new light on its regulation.

The meiosis-specific nuclear passenger protein is required for proper assembly of forespore membrane in fission yeast

Sporulation, gametogenesis in yeast, consists of meiotic nuclear division and spore morphogenesis. In the fission yeast Schizosaccharomyces pombe, the four haploid nuclei produced after meiosis II are encapsulated by the forespore membrane (FSM), which is newly synthesized from spindle pole bodies (SPBs) in the cytoplasm of the mother cell as spore precursors. Although the coordination between meiosis and FSM assembly is vital for proper sporulation, the underlying mechanism remains unclear. In the present study, we identified a new meiosis-specific protein Npg1, and found that it was involved in the efficient formation of spores and spore viability. The accumulation and organization of the FSM was compromised in npg1-null cells, leading to the error-prone envelopment of nuclei. Npg1 was first seen as internuclear dots and translocated to the SPBs before the FSM assembled. Genetic analysis revealed that Npg1 worked in conjunction with the FSM proteins Spo3 and Meu14. These results suggest a possible signaling link from the nucleus to the meiotic SPBs in order to associate the onset of FSM assembly with meiosis II, which ensures the successful partitioning of gametic nuclei.

Characterization of Schizosaccharomyces pombe copper transporter proteins in meiotic and sporulating cells

Meiosis requires copper to undertake its program in which haploid gametes are produced from diploid precursor cells. In Schizosaccharomyces pombe, copper is transported by three members of the copper transporter (Ctr) family, namely Ctr4, Ctr5, and Ctr6. Although central for sexual differentiation, very little is known about the expression profile, cellular localization, and physiological contribution of the Ctr proteins during meiosis. Analysis of gene expression of ctr4(+) and ctr5(+) revealed that they are primarily expressed in early meiosis under low copper conditions. In the case of ctr6(+), its expression is broader, being detected throughout the entire meiotic process with an increase during middle- and late-phase meiosis. Whereas the expression of ctr4(+) and ctr5(+) is exclusively dependent on the presence of Cuf1, ctr6(+) gene expression relies on two distinct regulators, Cuf1 and Mei4. Ctr4 and Ctr5 proteins co-localize at the plasma membrane shortly after meiotic induction, whereas Ctr6 is located on the membrane of vacuoles. After meiotic divisions, Ctr4 and Ctr5 disappear from the cell surface, whereas Ctr6 undergoes an intracellular re-location to co-localize with the forespore membrane. Under copper-limiting conditions, disruption of ctr4(+) and ctr6(+) results in altered SOD1 activity, whereas these mutant cells exhibit substantially decreased levels of CAO activity mostly in early- and middle-phase meiosis. Collectively, these results emphasize the notion that Ctr proteins exhibit differential expression, localization, and contribution in delivering copper to SOD1 and Cao1 proteins during meiosis.

Dma1-dependent degradation of Septation Initiation Network proteins during meiosis in Schizosaccharomyces pombe

The Schizosaccharomyces pombe septation initiation network (SIN) is required for cytokinesis during vegetative growth and spore formation during meiosis. Regulation of the SIN during mitosis has been studied extensively, but less is known about its meiotic regulation. Here, we show that several aspects of the SIN regulation differ between mitosis and meiosis. First, the presence of GTP-bound spg1p is not the main determinant of the timing of cdc7p and sid1p association with the SPB during meiosis. Second, the localisation dependencies of SIN proteins differ from those in mitotic cells, suggesting a modified functional organisation of the SIN during meiosis. Third, there is stage-specific degradation of SIN components in meiosis; byr4p is degraded after meiosis I, while the degradation of cdc7p, cdc11p and sid4p occurs after the second meiotic division and depends upon the ubiquitin ligase dma1p. Finally, dma1p-dependent degradation is not restricted to the SIN, for we show that dma1p is needed for the degradation of mcp6p/hrs1p in meiosis I. Together, these data suggest that stage-specific targetted proteolysis will play an important role in regulating meiotic progression.

NDR2-mediated Rabin8 phosphorylation is crucial for ciliogenesis by switching binding specificity from phosphatidylserine to Sec15

Primary cilia are antenna-like sensory organelles protruding from the plasma membrane. Defects in ciliogenesis cause diverse genetic disorders. NDR2 was identified as the causal gene for a canine ciliopathy, early retinal degeneration, but its role in ciliogenesis remains unknown. Ciliary membranes are generated by transport and fusion of Golgi-derived vesicles to the pericentrosome, a process requiring Rab11-mediated recruitment of Rabin8, a GDP-GTP exchange factor (GEF) for Rab8, and subsequent Rab8 activation and Rabin8 binding to Sec15, a component of the exocyst that mediates vesicle tethering. This study shows that NDR2 phosphorylates Rabin8 at Ser-272 and defects in this phosphorylation impair preciliary membrane assembly and ciliogenesis, resulting in accumulation of Rabin8-/Rab11-containing vesicles at the pericentrosome. Rabin8 binds to and colocalizes with GTP-bound Rab11 and phosphatidylserine (PS) on pericentrosomal vesicles. The phospho-mimetic S272E mutation of Rabin8 decreases affinity for PS but increases affinity for Sec15. These results suggest that NDR2-mediated Rabin8 phosphorylation is crucial for ciliogenesis by triggering the switch in binding specificity of Rabin8 from PS to Sec15, thereby promoting local activation of Rab8 and ciliary membrane formation.

Control of the mitotic exit network during meiosis

The mitotic exit network (MEN), a pathway essential for vegetative growth, is largely dispensable for the specialized meiotic divisions, contributing only to timely exit from meiosis II. MEN activity is restricted to meiosis II by multiple regulatory mechanisms distinct from those operative in mitosis.

The mitotic exit network (MEN) is an essential GTPase signaling pathway that triggers exit from mitosis in budding yeast. We show here that during meiosis, the MEN is dispensable for exit from meiosis I but contributes to the timely exit from meiosis II. Consistent with a role for the MEN during meiosis II, we find that the signaling pathway is active only during meiosis II. Our analysis further shows that MEN signaling is modulated during meiosis in several key ways. Whereas binding of MEN components to spindle pole bodies (SPBs) is necessary for MEN signaling during mitosis, during meiosis MEN signaling occurs off SPBs and does not require the SPB recruitment factor Nud1. Furthermore, unlike during mitosis, MEN signaling is controlled through the regulated interaction between the MEN kinase Dbf20 and its activating subunit Mob1. Our data lead to the conclusion that a pathway essential for vegetative growth is largely dispensable for the specialized meiotic divisions and provide insights into how cell cycle regulatory pathways are modulated to accommodate different modes of cell division.

A Meiotic Actin Ring (MeiAR) Essential for Proper Sporulation in Fission Yeast

Sporulation is a unique form of cytokinesis that occurs following meiosis II in many yeasts, during which four daughter cells (spores) are generated within a single mother cell. Here we characterize the role of F-actin in the process of sporulation in the fission yeast Schizosaccharomyces pombe. As shown previously, we find that F-actin assembles into 4 ring structures per ascus, referred to as the MeiAR (meiotic actin ring). The actin nucleators Arp2/3 and formin-For3 assemble into ring structures that overlap with Meu14, a protein known to assemble into the so-called leading edge, a ring structure that is known to guide forespore membrane assembly. Interestingly, F-actin makes rings that occupy a larger region behind the leading edge ring. Time-lapse microscopy showed that the MeiAR assembles near the spindle pole bodies and undergoes an expansion in diameter during the early stages of meiosis II, followed by closure in later stages of meiosis II. MeiAR closure completes the process of forespore membrane assembly. Loss of MeiAR leads to excessive assembly of forespore membranes with a deformed appearance. The rate of closure of the MeiAR is dictated by the function of the Septation Initiation Network (SIN). We conclude that the MeiAR ensures proper targeting of the membrane biogenesis machinery to the leading edge, thereby ensuring the formation of spherically shaped spores.

Roles of the DYRK kinase Pom2 in cytokinesis, mitochondrial morphology, and sporulation in fission yeast

Pom2 is predicted to be a dual-specificity tyrosine-phosphorylation regulated kinase (DYRK) related to Pom1 in Schizosaccharomyces pombe. DYRKs share a kinase domain capable of catalyzing autophosphorylation on tyrosine and exogenous phosphorylation on serine/threonine residues. Here we show that Pom2 is functionally different from the well-characterized Pom1, although they share 55% identity in the kinase domain and the Pom2 kinase domain functionally complements that of Pom1. Pom2 localizes to mitochondria throughout the cell cycle and to the contractile ring during late stages of cytokinesis. Overexpression but not deletion of pom2 results in severe defects in cytokinesis, indicating that Pom2 might share an overlapping function with other proteins in regulating cytokinesis. Gain and loss of function analyses reveal that Pom2 is required for maintaining mitochondrial morphology independently of microtubules. Intriguingly, most meiotic pom2Δ cells form aberrant asci with meiotic and/or forespore membrane formation defects. Taken together, Pom2 is a novel DYRK kinase involved in regulating cytokinesis, mitochondrial morphology, meiosis, and sporulation in fission yeast.

A novel role of Dma1 in regulating forespore membrane assembly and sporulation in fission yeast

By characterizing the fission yeast Dma1's function during meiosis, we revealed that Dma1 is required for spore formation, while it is dispensable for fidelity of nuclear divisions. We also found that Dma1 is functionally related to SIN pathway and meiosis-specific kinase Slk1 during sporulation.

In fission yeast Schizosaccharomyces pombe, a diploid mother cell differentiates into an ascus containing four haploid ascospores following meiotic nuclear divisions, through a process called sporulation. Several meiosis-specific proteins of fission yeast have been identified to play essential roles in meiotic progression and sporulation. We report here an unexpected function of mitotic spindle checkpoint protein Dma1 in proper spore formation. Consistent with its function in sporulation, expression of dma1⁺ is up-regulated during meiosis I and II. We showed that Dma1 localizes to the SPB during meiosis and the maintenance of this localization at meiosis II depends on septation initiation network (SIN) scaffold proteins Sid4 and Cdc11. Cells lacking Dma1 display defects associated with sporulation but not nuclear division, leading frequently to formation of asci with fewer spores. Our genetic analyses support the notion that Dma1 functions in parallel with the meiosis-specific Sid2-related protein kinase Slk1/Mug27 and the SIN signaling during sporulation, possibly through regulating proper forespore membrane assembly. Our studies therefore revealed a novel function of Dma1 in regulating sporulation in fission yeast.

The role of Schizosaccharomyces pombe dma1 in spore formation during meiosis

Meiosis is a specialised form of the cell cycle that gives rise to haploid gametes. In Schizosaccharomyces pombe, the products of meiosis are four spores, which are formed by encapsulation of the four meiosis II nuclei within the cytoplasm of the zygote produced by fusion of the mating cells. The S. pombe spindle pole body is remodelled during meiosis II and membrane vesicles are then recruited there to form the forespore membrane, which encapsulates the haploid nucleus to form a prespore. Spore wall material is then deposited, giving rise to the mature spore. The septation initiation network is required to coordinate cytokinesis and mitosis in the vegetative cycle and for spore formation in the meiotic cycle. We have investigated the role of the SIN regulator dma1p in meiosis; we find that although both meiotic divisions occur in the absence of dma1p, asci frequently contain fewer than four spores, which are larger than in wild-type meiosis. Our data indicate that dma1p acts in parallel to the leading-edge proteins and septins to assure proper formation for the forespore membrane. Dma1p also contributes to the temporal regulation of the abundance of the meiosis-specific SIN component mug27p.

The meiosis-specific Sid2p-related protein Slk1p regulates forespore membrane assembly in fission yeast

Cytokinesis in all organisms involves the creation of membranous barriers that demarcate individual daughter cells. In fission yeast, a signaling module termed the septation initiation network (SIN) plays an essential role in the assembly of new membranes and cell wall during cytokinesis. In this study, we have characterized Slk1p, a protein-kinase related to the SIN component Sid2p. Slk1p is expressed specifically during meiosis and localizes to the spindle pole bodies (SPBs) during meiosis I and II in a SIN-dependent manner. Slk1p also localizes to the forespore membrane during sporulation. Cells lacking Slk1p display defects associated with sporulation, leading frequently to the formation of asci with smaller and/or fewer spores. The ability of slk1 Delta cells to sporulate, albeit inefficiently, is fully abolished upon compromise of function of Sid2p, suggesting that Slk1p and Sid2p play overlapping roles in sporulation. Interestingly, increased expression of the syntaxin Psy1p rescues the sporulation defect of sid2-250 slk1 Delta. Thus, it is likely that Slk1p and Sid2p play a role in forespore membrane assembly by facilitating recruitment of components of the secretory apparatus, such as Psy1p, to allow membrane expansion. These studies thereby provide a novel link between the SIN and vesicle trafficking during cytokinesis.