Role of the spindle pole body of yeast in mediating assembly of the prospore membrane during meiosis

Meiotic Cytokinesis in Saccharomyces cerevisiae: Spores That Just Need Closure

In the budding yeast Saccharomyces cerevisiae, sporulation occurs during starvation of a diploid cell and results in the formation of four haploid spores forming within the mother cell ascus. Meiosis divides the genetic material that is encapsulated by the prospore membrane that grows to surround the haploid nuclei; this membrane will eventually become the plasma membrane of the haploid spore. Cellularization of the spores occurs when the prospore membrane closes to capture the haploid nucleus along with some cytoplasmic material from the mother cell, and thus, closure of the prospore membrane is the meiotic cytokinetic event. This cytokinetic event involves the removal of the leading-edge protein complex, a complex of proteins that localizes to the leading edge of the growing prospore membrane. The development and closure of the prospore membrane must be coordinated with other meiotic exit events such as spindle disassembly. Timing of the closure of the prospore membrane depends on the meiotic exit pathway, which utilizes Cdc15, a Hippo-like kinase, and Sps1, an STE20 family GCKIII kinase, acting in parallel to the E3 ligase Ama1-APC/C. This review describes the sporulation process and focuses on the development of the prospore membrane and the regulation of prospore membrane closure.

Spo13/MEIKIN ensures a Two-Division meiosis by preventing the activation of APC/CAma1 at meiosis I

Genome haploidization at meiosis depends on two consecutive nuclear divisions, which are controlled by an oscillatory system consisting of Cdk1-cyclin B and the APC/C bound to the Cdc20 activator. How the oscillator generates exactly two divisions has been unclear. We have studied this question in yeast where exit from meiosis involves accumulation of the APC/C activator Ama1 at meiosis II. We show that inactivation of the meiosis I-specific protein Spo13/MEIKIN results in a single-division meiosis due to premature activation of APC/CAma1 . In the wild type, Spo13 bound to the polo-like kinase Cdc5 prevents Ama1 synthesis at meiosis I by stabilizing the translational repressor Rim4. In addition, Cdc5-Spo13 inhibits the activity of Ama1 by converting the B-type cyclin Clb1 from a substrate to an inhibitor of Ama1. Cdc20-dependent degradation of Spo13 at anaphase I unleashes a feedback loop that increases Ama1's synthesis and activity, leading to irreversible exit from meiosis at the second division. Thus, by repressing the exit machinery at meiosis I, Cdc5-Spo13 ensures that cells undergo two divisions to produce haploid gametes.

Recruitment of the lipid kinase Mss4 to the meiotic spindle pole promotes prospore membrane formation in Saccharomyces cerevisiae

Spore formation in the budding yeast, Saccharomyces cerevisiae, involves de novo creation of four prospore membranes, each of which surrounds a haploid nucleus resulting from meiosis. The meiotic outer plaque (MOP) is a meiosis-specific protein complex associated with each meiosis II spindle pole body (SPB). Vesicle fusion on the MOP surface creates an initial prospore membrane anchored to the SPB. Ady4 is a meiosis-specific MOP component that stabilizes the MOP-prospore membrane interaction. We show that Ady4 recruits the lipid kinase, Mss4, to the MOP. MSS4 overexpression suppresses the ady4∆ spore formation defect, suggesting that a specific lipid environment provided by Mss4 promotes maintenance of prospore membrane attachment to MOPs. The meiosis-specific Spo21 protein is an essential structural MOP component. We show that the Spo21 N terminus contains an amphipathic helix that binds to prospore membranes. A mutant in SPO21 that removes positive charges from this helix shares phenotypic similarities to ady4∆. We propose that Mss4 generates negatively charged lipids in prospore membranes that enhance binding by the positively charged N terminus of Spo21, thereby providing a mechanism by which the MOP-prospore membrane interaction is stabilized.

Gametogenesis: Exploring an Endogenous Rejuvenation Program to Understand Cellular Aging and Quality Control

Gametogenesis is a conserved developmental program whereby a diploid progenitor cell differentiates into haploid gametes, the precursors for sexually reproducing organisms. In addition to ploidy reduction and extensive organelle remodeling, gametogenesis naturally rejuvenates the ensuing gametes, leading to resetting of life span. Excitingly, ectopic expression of the gametogenesis-specific transcription factor Ndt80 is sufficient to extend life span in mitotically dividing budding yeast, suggesting that meiotic rejuvenation pathways can be repurposed outside of their natural context. In this review, we highlight recent studies of gametogenesis that provide emerging insight into natural quality control, organelle remodeling, and rejuvenation strategies that exist within a cell. These include selective inheritance, programmed degradation, and de novo synthesis, all of which are governed by the meiotic gene expression program entailing many forms of noncanonical gene regulation. Finally, we highlight critical questions that remain in the field and provide perspective on the implications of gametogenesis research on human health span.

Moonlighting at the Poles: Non-Canonical Functions of Centrosomes

Centrosomes are best known as the microtubule organizing centers (MTOCs) of eukaryotic cells. In addition to their classic role in chromosome segregation, centrosomes play diverse roles unrelated to their MTOC activity during cell proliferation and quiescence. Metazoan centrosomes and their functional doppelgängers from lower eukaryotes, the spindle pole bodies (SPBs), act as important structural platforms that orchestrate signaling events essential for cell cycle progression, cellular responses to DNA damage, sensory reception and cell homeostasis. Here, we provide a critical overview of the unconventional and often overlooked roles of centrosomes/SPBs in the life cycle of eukaryotic cells.

The Spo13/Meikin pathway confines the onset of gamete differentiation to meiosis II in yeast

Sexual reproduction requires genome haploidization by the two divisions of meiosis and a differentiation program to generate gametes. Here, we have investigated how sporulation, the yeast equivalent of gamete differentiation, is coordinated with progression through meiosis. Spore differentiation is initiated at metaphase II when a membrane-nucleating structure, called the meiotic plaque, is assembled at the centrosome. While all components of this structure accumulate already at entry into meiosis I, they cannot assemble because centrosomes are occupied by Spc72, the receptor of the γ-tubulin complex. Spc72 is removed from centrosomes by a pathway that depends on the polo-like kinase Cdc5 and the meiosis-specific kinase Ime2, which is unleashed by the degradation of Spo13/Meikin upon activation of the anaphase-promoting complex at anaphase I. Meiotic plaques are finally assembled upon reactivation of Cdk1 at entry into metaphase II. This unblocking-activation mechanism ensures that only single-copy genomes are packaged into spores and might serve as a paradigm for the regulation of other meiosis II-specific processes.

A distinct inner nuclear membrane proteome in Saccharomyces cerevisiae gametes

The inner nuclear membrane (INM) proteome regulates gene expression, chromatin organization, and nuclear transport; however, it is poorly understood how changes in INM protein composition contribute to developmentally regulated processes, such as gametogenesis. We conducted a screen to determine how the INM proteome differs between mitotic cells and gametes. In addition, we used a strategy that allowed us to determine if spores synthesize their INM proteins de novo, rather than inheriting their INM proteins from the parental cell. This screen used a split-GFP complementation system, where we were able to compare the distribution of all C-terminally tagged transmembrane proteins in Saccharomyces cerevisiae in gametes to that of mitotic cells. Gametes contain a distinct INM proteome needed to complete gamete formation, including expression of genes linked to cell wall biosynthesis, lipid biosynthetic and metabolic pathways, protein degradation, and unknown functions. Based on the inheritance pattern, INM components are made de novo in the gametes. Whereas mitotic cells show a strong preference for proteins with small extraluminal domains, gametes do not exhibit this size preference likely due to the changes in the nuclear permeability barrier during gametogenesis. Taken together, our data provide evidence for INM changes during gametogenesis and shed light on mechanisms used to shape the INM proteome of spores.

The ESCRT-III complex is required for nuclear pore complex sequestration and regulates gamete replicative lifespan in budding yeast meiosis

Cellular aging occurs as a cell loses its ability to maintain homeostasis. Aging cells eliminate damaged cellular compartments and other senescence factors via self-renewal. The mechanism that regulates cellular rejuvenation remains to be further elucidated. Using budding yeast gametogenesis as a model, we show here that the endosomal sorting complex required for transport (ESCRT) III regulates nuclear envelope organization. During gametogenesis, the nuclear pore complex (NPC) and other senescence factors are sequestered away from the prospore nuclei. We show that the LEM-domain protein Heh1 (Src1) facilitates the nuclear recruitment of ESCRT-III, which is required for meiotic NPC sequestration and nuclear envelope remodeling. Furthermore, ESCRT-III-mediated nuclear reorganization appears to be critical for gamete rejuvenation, as hindering this process curtails either directly or indirectly the replicative lifespan in gametes. Our findings demonstrate the importance of ESCRT-III in nuclear envelope remodeling and its potential role in eliminating senescence factors during gametogenesis.

Cross-talk between autophagy and sporulation in Saccharomyces cerevisiae

Unicellular organisms, like yeast, have developed mechanisms to overcome environmental stress conditions like nutrient starvation. Autophagy and sporulation are two such mechanisms employed by yeast cells. Autophagy is a well-conserved, catabolic process that degrades excess and unwanted cytoplasmic materials and provides building blocks during starvation conditions. Thus, autophagy maintains cellular homeostasis at basal conditions and acts as a survival mechanism during stress conditions. Sporulation is an essential process that, like autophagy, is triggered due to stress conditions in yeast. It involves the formation of ascospores that protect the yeast cells during extreme conditions and germinate when the conditions are favorable. Studies show that autophagy is required for the sporulation process in yeast. However, the exact mechanism of action is not clear. Furthermore, several of the core autophagy gene knockouts do not sporulate and at what stage of sporulation they are involved is not clear. Besides, many overlapping proteins function in both sporulation and autophagy and it is unclear how the pathway-specific roles of these proteins are determined. All these observations suggest that the two processes cross-talk. Individually, some key features from both the processes remain to be studied with respect to the source of membrane for autophagosomes, prospore membrane (PSM) formation, and closure of the membranes. Therefore, it becomes crucial to study the cross-talk between autophagy and sporulation. In this review, the cross-talk between the two pathways, the common protein machineries have been discussed.

The Smk1 MAPK and Its Activator, Ssp2, Are Required for Late Prospore Membrane Development in Sporulating Saccharomyces cerevisiae

During sporulation in the budding yeast Saccharomyces cerevisiae, proper development of the prospore membrane is necessary for the formation of viable spores. The prospore membrane will eventually become the plasma membrane of the newly formed haploid spore and also serves as the template for the deposition of the spore wall. The prospore membrane is generated de novo during meiosis II and the growing edge of the prospore membrane is associated with the Leading Edge Protein (LEP) complex. We find that the Smk1 MAP kinase, along with its activator Ssp2, transiently localizes with the LEP during late meiosis II. SSP2 is required for the leading edge localization of Smk1; this localization is independent of the activation state of Smk1. Like other LEP components, the localization of Smk1 at the leading edge also depends on Ady3. Although prospore membrane development begins normally in smk1 and ssp2 mutants, late prospore membrane formation is disrupted, with the formation of ectopic membrane compartments. Thus, MAP kinase signaling plays an important role in the formation of the prospore membrane.

Saccharomyces spores are born prepolarized to outgrow away from spore-spore connections and penetrate the ascus wall

How nonspore haploid Saccharomyces cells choose sites of budding and polarize towards pheromone signals in order to mate has been a subject of intense study. Unlike nonspore haploids, sibling spores produced via meiosis and sporulation by a diploid cell are physically interconnected and encased in a sac derived from the old cell wall of the diploid, called the ascus. Nonspore haploids bud adjacent to previous sites of budding, relying on stable cortical landmarks laid down during prior divisions, but because spore membranes are made de novo, it was assumed that, as is known for fission yeast, Saccharomyces spores break symmetry and polarize at random locations. Here, we show that this assumption is incorrect: Saccharomyces cerevisiae spores are born prepolarized to outgrow, prior to budding or mating, away from interspore bridges. Consequently, when spores bud within an intact ascus, their buds locally penetrate the ascus wall, and when they mate, the resulting zygotes adopt a unique morphology reflective of repolarization towards pheromone. Long-lived cortical foci containing the septin Cdc10 mark polarity sites, but the canonical bud site selection programme is dispensable for spore polarity, thus the origin and molecular composition of these landmarks remain unknown. These findings demand further investigation of previously overlooked mechanisms of polarity establishment and local cell wall digestion and highlight how a key step in the Saccharomyces life cycle has been historically neglected.

A Noncanonical Hippo Pathway Regulates Spindle Disassembly and Cytokinesis During Meiosis in Saccharomyces cerevisiae

Meiosis in the budding yeast Saccharomyces cerevisiae is used to create haploid yeast spores from a diploid mother cell. During meiosis II, cytokinesis occurs by closure of the prospore membrane, a membrane that initiates at the spindle pole body and grows to surround each of the haploid meiotic products. Timely prospore membrane closure requires SPS1, which encodes an STE20 family GCKIII kinase. To identify genes that may activate SPS1, we utilized a histone phosphorylation defect of sps1 mutants to screen for genes with a similar phenotype and found that cdc15 shared this phenotype. CDC15 encodes a Hippo-like kinase that is part of the mitotic exit network. We find that Sps1 complexes with Cdc15, that Sps1 phosphorylation requires Cdc15, and that CDC15 is also required for timely prospore membrane closure. We also find that SPS1, like CDC15, is required for meiosis II spindle disassembly and sustained anaphase II release of Cdc14 in meiosis. However, the NDR-kinase complex encoded by DBF2/DBF20MOB1 which functions downstream of CDC15 in mitotic cells, does not appear to play a role in spindle disassembly, timely prospore membrane closure, or sustained anaphase II Cdc14 release. Taken together, our results suggest that the mitotic exit network is rewired for exit from meiosis II, such that SPS1 replaces the NDR-kinase complex downstream of CDC15.

Analysis of the protein composition of the spindle pole body during sporulation in Ashbya gossypii

The spores of fungi come in a wide variety of forms and sizes, highly adapted to the route of dispersal and to survival under specific environmental conditions. The ascomycete Ashbya gossypii produces needle shaped spores with a length of 30 μm and a diameter of 1 μm. Formation of these spores relies on actin and actin regulatory proteins and is, therefore, distinct from the minor role that actin plays for spore formation in Saccharomyces cerevisiae. Using in vivo FRET-measurements of proteins labeled with fluorescent proteins, we investigate how the formin AgBnr2, a protein that promotes actin polymerization, integrates into the structure of the spindle pole body during sporulation. We also investigate the role of the A. gossypii homologs to the S. cerevisiae meiotic outer plaque proteins Spo74, Mpc54 and Ady4 for sporulation in A. gossypii. We found highest FRET of AgBnr2 with AgSpo74. Further experiments indicated that AgSpo74 is a main factor for targeting AgBnr2 to the spindle pole body. In agreement with these results, the Agspo74 deletion mutant produces no detectable spores, whereas deletion of Agmpc54 only has an effect on spore length and deletion of Agady4 has no detectable sporulation phenotype. Based on this study and in relation to previous results we suggest a model where AgBnr2 resides within an analogous structure to the meiotic outer plaque of S. cerevisiae. There it promotes formation of actin cables important for shaping the needle shaped spore structure.

Meiotic cellular rejuvenation is coupled to nuclear remodeling in budding yeast

Production of healthy gametes in meiosis relies on the quality control and proper distribution of both nuclear and cytoplasmic contents. Meiotic differentiation naturally eliminates age-induced cellular damage by an unknown mechanism. Using time-lapse fluorescence microscopy in budding yeast, we found that nuclear senescence factors - including protein aggregates, extrachromosomal ribosomal DNA circles, and abnormal nucleolar material - are sequestered away from chromosomes during meiosis II and subsequently eliminated. A similar sequestration and elimination process occurs for the core subunits of the nuclear pore complex in both young and aged cells. Nuclear envelope remodeling drives the formation of a membranous compartment containing the sequestered material. Importantly, de novo generation of plasma membrane is required for the sequestration event, preventing the inheritance of long-lived nucleoporins and senescence factors into the newly formed gametes. Our study uncovers a new mechanism of nuclear quality control and provides insight into its function in meiotic cellular rejuvenation.

Developmental regulation of an organelle tether coordinates mitochondrial remodeling in meiosis

Cellular differentiation involves remodeling cellular architecture to transform one cell type to another. By investigating mitochondrial dynamics during meiotic differentiation in budding yeast, we sought to understand how organelle morphogenesis is developmentally controlled in a system where regulators of differentiation and organelle architecture are known, but the interface between them remains unexplored. We analyzed the regulation of mitochondrial detachment from the cell cortex, a known meiotic alteration to mitochondrial morphology. We found that mitochondrial detachment is enabled by the programmed destruction of the mitochondria-endoplasmic reticulum-cortex anchor (MECA), an organelle tether that bridges mitochondria and the plasma membrane. MECA regulation is governed by a meiotic transcription factor, Ndt80, which promotes the activation of a conserved kinase, Ime2. We further present evidence for Ime2-dependent phosphorylation and degradation of MECA in a temporally controlled manner. Our study defines a key mechanism that coordinates mitochondrial morphogenesis with the landmark events of meiosis and demonstrates that cells can developmentally regulate tethering to induce organelle remodeling.

Proteasome Activity Is Influenced by the HECT_2 Protein Ipa1 in Budding Yeast

The ubiquitin-proteasome system (UPS) controls cellular functions by maintenance of a functional proteome and degradation of key regulatory proteins. Central to the UPS is the proteasome that adjusts the abundance of numerous proteins, thereby safeguarding their activity or initiating regulatory events. Here, we demonstrate that the essential Saccharomyces cerevisiae protein Yjr141w/Ipa1 (Important for cleavage and PolyAdenylation) belongs to the HECT_2 (homologous to E6-AP carboxyl terminus_2) family. We found that five cysteine residues within the HECT_2 family signature and the C-terminus are essential for Ipa1 activity. Furthermore, Ipa1 interacts with several ubiquitin-conjugating enzymes in vivo and localizes to the cytosol and nucleus. Importantly, Ipa1 has an impact on proteasome activity, which is indicated by the activation of the Rpn4 regulon as well as by decreased turnover of destabilized proteasome substrates in an IPA1 mutant. These changes in proteasome activity might be connected to reduced maturation or modification of proteasomal core particle proteins. Our results highlight the influence of Ipa1 on the UPS. The conservation within the HECT_2 family and the connection of the human HECT_2 family member to an age-related degeneration disease might suggest that HECT_2 family members share a conserved function linked to proteasome activity.

Dynamic localization of a yeast development-specific PP1 complex during prospore membrane formation is dependent on multiple localization signals and complex formation

Prospore membrane formation of Saccharomyces cerevisiae provides a powerful model for understanding the mechanisms of de novo membrane formation. Protein phosphatase type1, Glc7, and a sporulation-specific targeting subunit, Gip1, show dynamic localization using multiple localization signals and regulate membrane growth during sporulation.

During the developmental process of sporulation in Saccharomyces cerevisiae, membrane structures called prospore membranes are formed de novo, expand, extend, acquire a round shape, and finally become plasma membranes of the spores. GIP1 encodes a regulatory/targeting subunit of protein phosphatase type 1 that is required for sporulation. Gip1 recruits the catalytic subunit Glc7 to septin structures that form along the prospore membrane; however, the molecular basis of its localization and function is not fully understood. Here we show that Gip1 changes its localization dynamically and is required for prospore membrane extension. Gip1 first associates with the spindle pole body as the prospore membrane forms, moves onto the prospore membrane and then to the septins as the membrane extends, distributes around the prospore membrane after closure, and finally translocates into the nucleus in the maturing spore. Deletion and mutation analyses reveal distinct sequences in Gip1 that are required for different localizations and for association with Glc7. Binding to Glc7 is also required for proper localization. Strikingly, localization to the prospore membrane, but not association with septins, is important for Gip1 function. Further, our genetic analysis suggests that a Gip1–Glc7 phosphatase complex regulates prospore membrane extension in parallel to the previously reported Vps13, Spo71, Spo73 pathway.

The Mitotic Exit Network Regulates Spindle Pole Body Selection During Sporulation of Saccharomyces cerevisiae

Age-based inheritance of centrosomes in eukaryotic cells is associated with faithful chromosome distribution in asymmetric cell divisions. During Saccharomyces cerevisiae ascospore formation, such an inheritance mechanism targets the yeast centrosome equivalents, the spindle pole bodies (SPBs) at meiosis II onset. Decreased nutrient availability causes initiation of spore formation at only the younger SPBs and their associated genomes. This mechanism ensures encapsulation of nonsister genomes, which preserves genetic diversity and provides a fitness advantage at the population level. Here, by usage of an enhanced system for sporulation-induced protein depletion, we demonstrate that the core mitotic exit network (MEN) is involved in age-based SPB selection. Moreover, efficient genome inheritance requires Dbf2/20-Mob1 during a late step in spore maturation. We provide evidence that the meiotic functions of the MEN are more complex than previously thought. In contrast to mitosis, completion of the meiotic divisions does not strictly rely on the MEN whereas its activity is required at different time points during spore development. This is reminiscent of vegetative MEN functions in spindle polarity establishment, mitotic exit, and cytokinesis. In summary, our investigation contributes to the understanding of age-based SPB inheritance during sporulation of S. cerevisiae and provides general insights on network plasticity in the context of a specialized developmental program. Moreover, the improved system for a developmental-specific tool to induce protein depletion will be useful in other biological contexts.

Casein Kinase 1 Coordinates Cohesin Cleavage, Gametogenesis, and Exit from M Phase in Meiosis II

Meiosis consists of DNA replication followed by two consecutive nuclear divisions and gametogenesis or spore formation. While meiosis I has been studied extensively, less is known about the regulation of meiosis II. Here we show that Hrr25, the conserved casein kinase 1δ of budding yeast, links three mutually independent key processes of meiosis II. First, Hrr25 induces nuclear division by priming centromeric cohesin for cleavage by separase. Hrr25 simultaneously phosphorylates Rec8, the cleavable subunit of cohesin, and removes from centromeres the cohesin protector composed of shugoshin and the phosphatase PP2A. Second, Hrr25 initiates the sporulation program by inducing the synthesis of membranes that engulf the emerging nuclei at anaphase II. Third, Hrr25 mediates exit from meiosis II by activating pathways that trigger the destruction of M-phase-promoting kinases. Thus, Hrr25 synchronizes formation of the single-copy genome with gamete differentiation and termination of meiosis.

Lipid droplets are central organelles for meiosis II progression during yeast sporulation

During meiosis II, lipids are remodeled and incorporated into a double-membrane structure termed the prospore membrane (PSM), which grows as it sequesters four haploid nuclei to form spores. Cellular organelle lipid droplets are recruited to the PSM, where they play an active role in organizing membrane morphogenesis.

Neutral lipids, predominantly triacylglycerol (TAG) and sterol ester, are stored within the cellular organelles termed lipid droplets (LDs). Although it is believed that the major function of LDs is to supply the cell with energy and membranes, little is known about the cellular events directly involving LDs and their contents. In this study, we provide cytological evidence that LDs form direct contacts with the prospore membrane (PSM) that is synthesized de novo during meiosis II to sequester the dividing nuclei in sporulating yeast. Lipidomic analyses indicate that TAG lipolysis releases free fatty acids at a time that correlates well with meiosis II progression, concomitant with phospholipid remodeling. Mutants lacking TAG or impaired of TAG hydrolysis show spore wall assembly defects, supporting a role for TAG and/or its metabolites in spore wall morphogenesis. Not only does LD integrity influence spore wall assembly, LDs are also essential for other aspects of spore development. Yeast cells lacking LDs are severely defective in PSM growth and organization and display disrupted spindles, producing dead spores or even failing to form spores. Together these results link LD physiology directly to a unique membrane morphogenesis process critical for development.

Timely Closure of the Prospore Membrane Requires SPS1 and SPO77 in Saccharomyces cerevisiae

During sporulation in Saccharomyces cerevisiae, a double lipid bilayer called the prospore membrane is formed de novo, growing around each meiotic nucleus and ultimately closing to create four new cells within the mother cell. Here we show that SPS1, which encodes a kinase belonging to the germinal center kinase III family, is involved in prospore membrane development and is required for prospore membrane closure. We find that SPS1 genetically interacts with SPO77 and see that loss of either gene disrupts prospore membrane closure in a similar fashion. Specifically, cells lacking SPS1 and SPO77 produce hyperelongated prospore membranes from which the leading edge protein complex is not removed from the prospore membrane in a timely fashion. The SPS1/SPO77 pathway is required for the proper phosphorylation and stability of Ssp1, a member of the leading edge protein complex that is removed and degraded when the prospore membrane closes. Genetic dissection of prospore membrane closure finds SPS1 and SPO77 act in parallel to a previously described pathway of prospore membrane closure that involves AMA1, an activator of the meiotic anaphase promoting complex.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

Roles of septins in prospore membrane morphogenesis and spore wall assembly in Saccharomyces cerevisiae

In mitotically dividing cells, septin proteins form cytoskeletal filaments that function in cell morphogenesis and division. Gametogenesis in yeast couples meiosis with a fundamentally different form of cytokinesis involving de novo membrane synthesis. Budding yeast septins are critical for spore membrane extension and wall assembly.

The highly conserved family of septin proteins has important functions in cytokinesis in mitotically proliferating cells. A different form of cytokinesis occurs during gametogenesis in Saccharomyces cerevisiae, in which four haploid meiotic products become encased by prospore membrane (PSMs) and specialized, stress-resistant spore walls. Septins are known to localize in a series of structures near the growing PSM, but previous studies noted only mild sporulation defects upon septin mutation. We report that directed PSM extension fails in many septin-mutant cells, and, for those that do succeed, walls are abnormal, leading to increased susceptibility to heating, freezing, and digestion by the Drosophila gut. Septin mutants mislocalize the leading-edge protein (LEP) complex required for normal PSM and wall biogenesis, and ectopic expression of the LEP protein Ssp1 perturbs mitotic septin localization and function, suggesting a functional interaction. Strikingly, extra copies of septin CDC10 rescue sporulation and LEP localization in cells lacking Sma1, a phospholipase D–associated protein dispensable for initiation of PSM assembly and PSM curvature but required for PSM extension. These findings point to key septin functions in directing efficient membrane and cell wall synthesis during budding yeast gametogenesis.

Photo-sensitive degron variants for tuning protein stability by light

Background

Regulated proteolysis by the proteasome is one of the fundamental mechanisms used in eukaryotic cells to control cellular behavior. Efficient tools to regulate protein stability offer synthetic influence on molecular level on a selected biological process. Optogenetic control of protein stability has been achieved with the photo-sensitive degron (psd) module. This engineered tool consists of the photoreceptor domain light oxygen voltage 2 (LOV2) from Arabidopsis thaliana phototropin1 fused to a sequence that induces direct proteasomal degradation, which was derived from the carboxy-terminal degron of murine ornithine decarboxylase. The abundance of target proteins tagged with the psd module can be regulated by blue light if the degradation tag is exposed to the cytoplasm or the nucleus.

Results

We used the model organism Saccharomyces cerevisiae to generate psd module variants with increased and decreased stabilities in darkness or when exposed to blue light using site-specific and random mutagenesis. The variants were characterized as fusions to fluorescent reporter proteins and showed half-lives between 6 and 75 minutes in cells exposed to blue light and 14 to 187 minutes in darkness. In blue light, ten variants showed accelerated degradation and four variants increased stability compared to the original psd module. Measuring the dark/light ratio of selected constructs in yeast cells showed that two variants were obtained with ratios twice as high as in the wild type psd module. In silico modeling of photoreceptor variant characteristics suggested that for most cases alterations in behavior were induced by changes in the light-response of the LOV2 domain.

Conclusions

In total, the mutational analysis resulted in psd module variants, which provide tuning of protein stability over a broad range by blue light. Two variants showed characteristics that are profoundly improved compared to the original construct. The modular usage of the LOV2 domain in optogenetic tools allows the usage of the mutants in the context of other applications in synthetic and systems biology as well.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-014-0128-9) contains supplementary material, which is available to authorized users.

A visual screen of protein localization during sporulation identifies new components of prospore membrane-associated complexes in budding yeast

During ascospore formation in Saccharomyces cerevisiae, the secretory pathway is reorganized to create new intracellular compartments, termed prospore membranes. Prospore membranes engulf the nuclei produced by the meiotic divisions, giving rise to individual spores. The shape and growth of prospore membranes are constrained by cytoskeletal structures, such as septin proteins, that associate with the membranes. Green fluorescent protein (GFP) fusions to various proteins that associate with septins at the bud neck during vegetative growth as well as to proteins encoded by genes that are transcriptionally induced during sporulation were examined for their cellular localization during prospore membrane growth. We report localizations for over 100 different GFP fusions, including over 30 proteins localized to the prospore membrane compartment. In particular, the screen identified IRC10 as a new component of the leading-edge protein complex (LEP), a ring structure localized to the lip of the prospore membrane. Localization of Irc10 to the leading edge is dependent on SSP1, but not ADY3. Loss of IRC10 caused no obvious phenotype, but an ady3 irc10 mutant was completely defective in sporulation and displayed prospore membrane morphologies similar to those of an ssp1 strain. These results reveal the architecture of the LEP and provide insight into the evolution of this membrane-organizing complex.

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.

The fission yeast synaptobrevin ortholog Syb1 plays an important role in forespore membrane formation and spore maturation

Synaptobrevin, also called vesicle-associated membrane protein (VAMP), is a component of the plasma membrane N-methylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, which plays a key role in intracellular membrane fusion. Previous studies have revealed that, similar to synaptobrevin in other organisms, the fission yeast synaptobrevin ortholog Syb1 associates with post-Golgi secretory vesicles and is essential for cytokinesis and cell elongation. Here, we report that Syb1 has a role in sporulation. After nitrogen starvation, green fluorescent protein (GFP)-Syb1 is found in intracellular dots. As meiosis proceeds, GFP-Syb1 accumulates around the nucleus and then localizes at the forespore membrane (FSM). We isolated a syb-S1 mutant, which exhibits a defect in sporulation. In syb1-S1 mutants, the FSM begins to form but fails to develop a normal morphology. Electron microscopy shows that an abnormal spore wall is often formed in syb1-S1 mutant spores. Although most syb1-S1 mutant spores are germinated, they are less tolerant to ethanol than wild-type spores. The syb1-S1 allele carries a missense mutation, resulting in replacement of a conserved cysteine residue adjacent to the transmembrane domain, which reduces the stability and abundance of the Syb1 protein. Taken together, these results indicate that Syb1 plays an important role in both FSM assembly and spore wall formation.

A conserved regulatory mode in exocytic membrane fusion revealed by Mso1p membrane interactions

Sec1/Munc18 family proteins are important components of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex-mediated membrane fusion processes. However, the molecular interactions and the mechanisms involved in Sec1p/Munc18 control and SNARE complex assembly are not well understood. We provide evidence that Mso1p, a Sec1p- and Sec4p-binding protein, interacts with membranes to regulate membrane fusion. We identify two membrane-binding sites on Mso1p. The N-terminal region inserts into the lipid bilayer and appears to interact with the plasma membrane, whereas the C-terminal region of the protein binds phospholipids mainly through electrostatic interactions and may associate with secretory vesicles. The Mso1p membrane interactions are essential for correct subcellular localization of Mso1p-Sec1p complexes and for membrane fusion in Saccharomyces cerevisiae. These characteristics are conserved in the phosphotyrosine-binding (PTB) domain of β-amyloid precursor protein-binding Mint1, the mammalian homologue of Mso1p. Both Mint1 PTB domain and Mso1p induce vesicle aggregation/clustering in vitro, supporting a role in a membrane-associated process. The results identify Mso1p as a novel lipid-interacting protein in the SNARE complex assembly machinery. Furthermore, our data suggest that a general mode of interaction, consisting of a lipid-binding protein, a Rab family GTPase, and a Sec1/Munc18 family protein, is important in all SNARE-mediated membrane fusion events.

Acetate regulation of spore formation is under the control of the Ras/cyclic AMP/protein kinase A pathway and carbon dioxide in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the Ras/cyclic AMP (cAMP)/protein kinase A (PKA) pathway is a nutrient-sensitive signaling cascade that regulates vegetative growth, carbohydrate metabolism, and entry into meiosis. How this pathway controls later steps of meiotic development is largely unknown. Here, we have analyzed the role of the Ras/cAMP/PKA pathway in spore formation by the meiosis-specific manipulation of Ras and PKA or by the disturbance of cAMP production. We found that the regulation of spore formation by acetate takes place after commitment to meiosis and depends on PKA and appropriate A kinase activation by Ras/Cyr1 adenylyl cyclase but not by activation through the Gpa2/Gpr1 branch. We further discovered that spore formation is regulated by carbon dioxide/bicarbonate, and an analysis of mutants defective in acetate transport (ady2Δ) or carbonic anhydrase (nce103Δ) provided evidence that these metabolites are involved in connecting the nutritional state of the meiotic cell to spore number control. Finally, we observed that the potential PKA target Ady1 is required for the proper localization of the meiotic plaque proteins Mpc70 and Spo74 at spindle pole bodies and for the ability of these proteins to initiate spore formation. Overall, our investigation suggests that the Ras/cAMP/PKA pathway plays a crucial role in the regulation of spore formation by acetate and indicates that the control of meiotic development by this signaling cascade takes places at several steps and is more complex than previously anticipated.

SPO71 mediates prospore membrane size and maturation in Saccharomyces cerevisiae

The mechanisms that control the size and shape of membranes are not well understood, despite the importance of these structures in determining organelle and cell morphology. The prospore membrane, a double lipid bilayer that is synthesized de novo during sporulation in S. cerevisiae, grows to surround the four meiotic products. This membrane determines the shape of the newly formed spores and serves as the template for spore wall deposition. Ultimately, the inner leaflet of the prospore membrane will become the new plasma membrane of the cell upon germination. Here we show that Spo71, a pleckstrin homology domain protein whose expression is induced during sporulation, is critical for the appropriate growth of the prospore membrane. Without SPO71, prospore membranes surround the nuclei but are abnormally small, and spore wall deposition is disrupted. Sporulating spo71Δ cells have prospore membranes that properly localize components to their growing leading edges yet cannot properly localize septin structures. We also found that SPO71 genetically interacts with SPO1, a gene with homology to the phospholipase B gene that has been previously implicated in determining the shape of the prospore membrane. Together, these results show that SPO71 plays a critical role in prospore membrane development.

Sporulation in the budding yeast Saccharomyces cerevisiae

In response to nitrogen starvation in the presence of a poor carbon source, diploid cells of the yeast Saccharomyces cerevisiae undergo meiosis and package the haploid nuclei produced in meiosis into spores. The formation of spores requires an unusual cell division event in which daughter cells are formed within the cytoplasm of the mother cell. This process involves the de novo generation of two different cellular structures: novel membrane compartments within the cell cytoplasm that give rise to the spore plasma membrane and an extensive spore wall that protects the spore from environmental insults. This article summarizes what is known about the molecular mechanisms controlling spore assembly with particular attention to how constitutive cellular functions are modified to create novel behaviors during this developmental process. Key regulatory points on the sporulation pathway are also discussed as well as the possible role of sporulation in the natural ecology of S. cerevisiae.

The Aurora kinase Ipl1 is necessary for spindle pole body cohesion during budding yeast meiosis

In budding yeast, the microtubule-organizing center is called the spindle pole body (SPB) and shares structural components with the centriole, the central core of the animal centrosome. During meiotic interphase I, the SPB is duplicated when DNA replication takes place. Duplicated SPBs are linked and then separate to form a bipolar spindle required for homolog separation in meiosis I. During interphase II, SPBs are duplicated again, in the absence of DNA replication, to form four SPBs that establish two spindles for sister-chromatid separation in meiosis II. Here, we report that the Aurora kinase Ipl1, which is necessary for sister-chromatid cohesion, is also required for maintenance of a tight association between duplicated SPBs during meiosis, which we term SPB cohesion. Premature loss of cohesion leads to SPB overduplication and the formation of multipolar spindles. By contrast, the Polo-like kinase Cdc5 is necessary for SPB duplication and interacts antagonistically with Ipl1 at the meiotic SPB to ensure proper SPB separation. Our data suggest that Ipl1 coordinates SPB dynamics with the two chromosome segregation cycles during yeast meiosis.

The fission yeast pleckstrin homology domain protein Spo7 is essential for initiation of forespore membrane assembly and spore morphogenesis

Sporulation in fission yeast represents a unique mode of cell division in which a new cell is formed within the cytoplasm of a mother cell. This event is accompanied by formation of the forespore membrane (FSM), which becomes the plasma membrane of spores. At prophase II, the spindle pole body (SPB) forms an outer plaque, from which formation of the FSM is initiated. Several components of the SPB play an indispensable role in SPB modification, and therefore in sporulation. In this paper, we report the identification of a novel SPB component, Spo7, which has a pleckstrin homology (PH) domain. We found that Spo7 was essential for initiation of FSM assembly, but not for SPB modification. Spo7 directly bound to Meu14, a component of the leading edge of the FSM, and was essential for proper localization of Meu14. The PH domain of Spo7 had affinity for phosphatidylinositol 3-phosphate (PI3P). spo7 mutants lacking the PH domain showed aberrant spore morphology, similar to that of meu14 and phosphatidylinositol 3-kinase (pik3) mutants. Our study suggests that Spo7 coordinates formation of the leading edge and initiation of FSM assembly, thereby accomplishing accurate formation of the FSM.

Yeast cell morphology and sexual reproduction--a short overview and some considerations

Over the decades, basic research in life sciences has profited greatly from the study of the small unicellular fungal species Saccharomyces cerevisiae. This yeast turned out to be key for the identification and understanding of molecular mechanisms that underlay the basic functions of all eukaryotic cells. These include, but are not limited to, the regulatory mechanisms behind cellular reproduction (cell cycle control), cellular morphogenesis (cell polarity, cytoskeleton and membrane trafficking) and the management of cellular information (chromosome biology, transcription and translation). Rapid access to genomic information of many yeast species, combined with bioinformatics analyses, provide information on the evolutionary history of yeasts and the molecular ancestry of their constituents. The availability of a comprehensive list of experimental procedures for these organisms presents now a unique opportunity to learn about variations of molecular processes on an evolutionary scale. Yeast cell morphology is another interesting factor, since cellular shapes influence the interactions with the environment on all levels. In this overview article I provide a short summary of the relevant aspects of yeast cell morphology, in particular in relation to one of the most influencing processes, cellular reproduction by mating and meiosis.

A Bnr-like formin links actin to the spindle pole body during sporulation in the filamentous fungus Ashbya gossypii

Formin proteins are nucleators of actin filaments and regulators of the microtubule cytoskeleton. As such, they play important roles in the development of yeast and other fungi. We show here that AgBnr2, a homologue of the ScBnr1 formin from the filamentous fungus Ashbya gossypii, localizes to the spindle pole body (SPB), the fungal analogue of the centrosome of metazoans. This protein plays an important role in the development of the typical needle-shaped spores of A. gossypii, as suggested by several findings. First, downregulation of AgBNR2 causes defects in sporangium formation and a decrease in the total spore number. Second, a fusion of AgBNR2 to GFP that is driven by the native AgBNR2 promoter is only visible in sporangia. Third, AgBnr2 interacts with a AgSpo21, a sporulation-specific component of the SPB. Furthermore, we provide evidence that AgBnr2 might nucleate actin cables, which are connected to SPBs during sporulation. Our findings add to our understanding of fungal sporulation, particularly the formation of spores with a complex, elongated morphology, and provide novel insights into formin function.

Targeted protein depletion in Saccharomyces cerevisiae by activation of a bidirectional degron

Background

Tools for in vivo manipulation of protein abundance or activity are highly beneficial for life science research. Protein stability can be efficiently controlled by conditional degrons, which induce target protein degradation at restrictive conditions.

Results

We used the yeast Saccharomyces cerevisiae for development of a conditional, bidirectional degron to control protein stability, which can be fused to the target protein N-terminally, C-terminally or placed internally. Activation of the degron is achieved by cleavage with the tobacco etch virus (TEV) protease, resulting in quick proteolysis of the target protein. We found similar degradation rates of soluble substrates using destabilization by the N- or C-degron. C-terminal tagging of essential yeast proteins with the bidirectional degron resulted in deletion-like phenotypes at non-permissive conditions. Developmental process-specific mutants were created by N- or C-terminal tagging of essential proteins with the bidirectional degron in combination with sporulation-specific production of the TEV protease.

Conclusions

We developed a system to influence protein abundance and activity genetically, which can be used to create conditional mutants, to regulate the fate of single protein domains or to design artificial regulatory circuits. Thus, this method enhances the toolbox to manipulate proteins in systems biology approaches considerably.

Membrane assembly modulates the stability of the meiotic spindle-pole body

Spore formation in Saccharomyces cerevisiae is driven by de novo assembly of new membranes termed prospore membranes. A vesicle-docking complex called the meiosis II outer plaque (MOP) forms on the cytoplasmic faces of the spindle-pole bodies at the onset of meiosis II and serves as the initiation site for membrane formation. In this study, a fluorescence-recovery assay was used to demonstrate that the dynamics of the MOP proteins change coincident with the coalescence of precursor vesicles into a membrane. Proteins within the MOP exchange freely with a soluble pool prior to membrane assembly, but after membranes are formed they remain stably within the MOP. By contrast, constitutive spindle-pole-body proteins display low exchange in both conditions. The MOP component Ady4p plays a role in maintaining the integrity of the MOP complex, but this role differs depending on whether the MOP is associated with docked vesicles or a fully formed membrane. These results suggest an architectural rearrangement of the MOP coincident with vesicle fusion.

Vesicle docking to the spindle pole body is necessary to recruit the exocyst during membrane formation in Saccharomyces cerevisiae

The meiosis II outer plaque (MOP) acts a vesicle tethering complex that is a site for de novo membrane formation. Novel mutants in a MOP protein reveal that interaction of vesicles with the MOP surface is required to recruit a second tethering complex, the exocyst, to the vesicles, suggesting a mechanism by which the MOP promotes vesicle fusion.

During meiosis II in Saccharomyces cerevisiae, the cytoplasmic face of the spindle pole body, referred to as the meiosis II outer plaque (MOP), is modified in both composition and structure to become the initiation site for de novo formation of a membrane called the prospore membrane. The MOP serves as a docking complex for precursor vesicles that are targeted to its surface. Using fluorescence resonance energy transfer analysis, the orientation of coiled-coil proteins within the MOP has been determined. The N-termini of two proteins, Mpc54p and Spo21p, were oriented toward the outer surface of the structure. Mutations in the N-terminus of Mpc54p resulted in a unique phenotype: precursor vesicles loosely tethered to the MOP but did not contact its surface. Thus, these mpc54 mutants separate the steps of vesicle association and docking. Using these mpc54 mutants, we determined that recruitment of the Rab GTPase Sec4p, as well as the exocyst components Sec3p and Sec8p, to the precursor vesicles requires vesicle docking to the MOP. This suggests that the MOP promotes membrane formation both by localization of precursor vesicles to a particular site and by recruitment of a second tethering complex, the exocyst, that stimulates downstream events of fusion.

A guaninine nucleotide exchange factor is a component of the meiotic spindle pole body in Schizosaccharomyces pombe

Spore morphogenesis in yeast is driven by the formation of membrane compartments that initiate growth at the spindle poles during meiosis II and grow to encapsulate daughter nuclei. Vesicle docking complexes, called meiosis II outer plaques (MOPs), form on each meiosis II spindle pole body (SPB) and serve as sites of membrane nucleation. How the MOP stimulates membrane assembly is not known. Here, we report that SpSpo13, a component of the MOP in Schizosaccharomyces pombe, shares homology with the guanine nucleotide exchange factor (GEF) domain of the Saccharomyces cerevisiae Sec2 protein. ScSec2 acts as a GEF for the small Rab GTPase ScSec4, which regulates vesicle trafficking from the late-Golgi to the plasma membrane. A chimeric protein in which the ScSec2-GEF domain is replaced with SpSpo13 is capable of supporting the growth of a sec2Delta mutant. SpSpo13 binds preferentially to the nucleotide-free form of ScSec4 and facilitates nucleotide exchange in vitro. In vivo, a Spspo13 mutant defective in GEF activity fails to support membrane assembly. In vitro specificity experiments suggest that SpYpt2 is the physiological substrate of SpSpo13. These results demonstrate that stimulation of Rab-GTPase activity is a property of the S. pombe MOP essential for the initiation of membrane formation.

Role of septins in the orientation of forespore membrane extension during sporulation in fission yeast

During yeast sporulation, a forespore membrane (FSM) initiates at each spindle-pole body and extends to form the spore envelope. We used Schizosaccharomyces pombe to investigate the role of septins during this process. During the prior conjugation of haploid cells, the four vegetatively expressed septins (Spn1, Spn2, Spn3, and Spn4) coassemble at the fusion site and are necessary for its normal morphogenesis. Sporulation involves a different set of four septins (Spn2, Spn5, Spn6, and the atypical Spn7) that does not include the core subunits of the vegetative septin complex. The four sporulation septins form a complex in vitro and colocalize interdependently to a ring-shaped structure along each FSM, and septin mutations result in disoriented FSM extension. The septins and the leading-edge proteins appear to function in parallel to orient FSM extension. Spn2 and Spn7 bind to phosphatidylinositol 4-phosphate [PtdIns(4)P] in vitro, and PtdIns(4)P is enriched in the FSMs, suggesting that septins bind to the FSMs via this lipid. Cells expressing a mutant Spn2 protein unable to bind PtdIns(4)P still form extended septin structures, but these structures fail to associate with the FSMs, which are frequently disoriented. Thus, septins appear to form a scaffold that helps to guide the oriented extension of the FSM.

Phosphatidylinositol-4,5-bisphosphate and phospholipase D-generated phosphatidic acid specify SNARE-mediated vesicle fusion for prospore membrane formation

The soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) family of proteins is required for eukaryotic intracellular membrane fusions. Vesicle fusion for formation of the prospore membrane (PSM), a membrane compartment that forms de novo during yeast sporulation, requires SNARE function, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P(2)], and the activity of the phospholipase D (PLD) Spo14p, which generates phosphatidic acid (PA). The SNARE syntaxin Sso1p is essential for PSM production while the functionally redundant homolog in vegetative growth, Sso2p, is not. We demonstrate that Sso1p and Sso2p bind similarly in vitro to PA or phosphoinositide-containing liposomes and that the conserved SNARE (H3) domain largely mediates PA-binding. Both green fluorescent protein-Sso fusion proteins localize to the developing PSM in wild-type cells and to the spindle pole body in spo14Delta cells induced to sporulate. However, the autoregulatory region of Sso1p binds PI(4,5)P(2)-containing liposomes in vitro with a greater ability than the equivalent region of Sso2p. Overexpression of the phosphatidylinositol-4-phosphate 5-kinase MSS4 in sso1Delta cells induced to sporulate stimulates PSM production; PLD activity is not increased under these conditions, indicating that PI(4,5)P(2) has roles in addition to stimulating PLD in PSM formation. These data suggest that PLD-generated PA and PI(4,5)P(2) collaborate at multiple levels to promote SNARE-mediated fusion for PSM formation.

Protein phosphatase type 1-interacting protein Ysw1 is involved in proper septin organization and prospore membrane formation during sporulation

Sporulation of Saccharomyces cerevisiae is a developmental process in which four haploid spores are generated inside a diploid cell. Gip1, a sporulation-specific targeting subunit of protein phosphatase type 1, together with its catalytic subunit, Glc7, colocalizes with septins along the extending prospore membrane and is required for septin organization and spore wall formation. However, the mechanism by which Gip1-Glc7 phosphatase promotes these events is unclear. We show here that Ysw1, a sporulation-specific coiled-coil protein, has a functional relationship to Gip1-Glc7 phosphatase. Overexpression of YSW1 partially suppresses the sporulation defect of a temperature-sensitive allele of gip1. Ysw1 interacts with Gip1 in a two-hybrid assay, and this interaction is required for suppression. Ysw1 tagged with green fluorescent protein colocalizes with septins and Gip1 along the extending prospore membrane during spore formation. Sporulation is partially defective in ysw1Delta mutant, and cytological analysis revealed that septin structures are perturbed and prospore membrane extension is aberrant in ysw1Delta cells. These results suggest that Ysw1 functions with the Gip1-Glc7 phosphatase to promote proper septin organization and prospore membrane formation.

Efficient protein depletion by genetically controlled deprotection of a dormant N-degron

Methods that allow for the manipulation of genes or their products have been highly fruitful for biomedical research. Here, we describe a method that allows the control of protein abundance by a genetically encoded regulatory system. We developed a dormant N-degron that can be attached to the N-terminus of a protein of interest. Upon expression of a site-specific protease, the dormant N-degron becomes deprotected. The N-degron then targets itself and the attached protein for rapid proteasomal degradation through the N-end rule pathway. We use an optimized tobacco etch virus (TEV) protease variant combined with selective target binding to achieve complete and rapid deprotection of the N-degron-tagged proteins. This method, termed TEV protease induced protein inactivation (TIPI) of TIPI-degron (TDeg) modified target proteins is fast, reversible, and applicable to a broad range of proteins. TIPI of yeast proteins essential for vegetative growth causes phenotypes that are close to deletion mutants. The features of the TIPI system make it a versatile tool to study protein function in eukaryotes and to create new modules for synthetic or systems biology.

Role of spindle asymmetry in cellular dynamics

The mitotic spindle is mostly perceived as a symmetric structure. However, in many cell divisions, the two poles of the spindle organize asters with different dynamics, associate with different biomolecules or subcellular domains, and perform different functions. In this chapter, we describe some of the most prominent examples of spindle asymmetry. These are encountered during cell-cycle progression in budding and fission yeast and during asymmetric cell divisions of stem cells and embryos. We analyze the molecular mechanisms that lead to generation of spindle asymmetry and discuss the importance of spindle-pole differentiation for the correct outcome of cell division.

The anaphase promoting complex targeting subunit Ama1 links meiotic exit to cytokinesis during sporulation in Saccharomyces cerevisiae

Ascospore formation in yeast is accomplished through a cell division in which daughter nuclei are engulfed by newly formed plasma membranes, termed prospore membranes. Closure of the prospore membrane must be coordinated with the end of meiosis II to ensure proper cell division. AMA1 encodes a meiosis-specific activator of the anaphase promoting complex (APC). The activity of APC(Ama1) is inhibited before meiosis II, but the substrates specifically targeted for degradation by Ama1 at the end of meiosis are unknown. We show here that ama1Delta mutants are defective in prospore membrane closure. Ssp1, a protein found at the leading edge of the prospore membrane, is stabilized in ama1Delta mutants. Inactivation of a conditional form of Ssp1 can partially rescue the sporulation defect of the ama1Delta mutant, indicating that an essential function of Ama1 is to lead to the removal of Ssp1. Depletion of Cdc15 causes a defect in meiotic exit. We find that prospore membrane closure is also defective in Cdc15 and that this defect can be overcome by expression of a form of Ama1 in which multiple consensus cyclin-dependent kinase phosphorylation sites have been mutated. These results demonstrate that APC(Ama1) functions to coordinate the exit from meiosis II with cytokinesis.

Septins localize to microtubules during nutritional limitation in Saccharomyces cerevisiae

Background

In Saccharomyces cerevisiae, nutrient limitation stimulates diploid cells to undergo DNA replication and meiosis, followed by the formation of four haploid spores. Septins are a family of proteins that assemble a ring structure at the mother-daughter neck during vegetative growth, where they control cytokinesis. In sporulating cells, the septin ring disassembles and septins relocalize to the prospore membrane.

Results

Here, we demonstrate that nutrient limitation triggers a change in the localization of at least two vegetative septins (Cdc10 and Cdc11) from the bud neck to the microtubules. The association of Cdc10 and Cdc11 with microtubules persists into meiosis, and they are found associated with the meiotic spindle until the end of meiosis II. In addition, the meiosis-specific septin Spr28 displays similar behavior, suggesting that this is a common feature of septins. Septin association to microtubules is a consequence of the nutrient limitation signal, since it is also observed when haploid cells are incubated in sporulation medium and when haploid or diploid cells are grown in medium containing non-fermentable carbon sources. Moreover, during meiosis II, when the nascent prospore membrane is formed, septins moved from the microtubules to this membrane. Proper organization of the septins on the membrane requires the sporulation-specific septins Spr3 and Spr28.

Conclusion

Nutrient limitation in S. cerevisiae triggers the sporulation process, but it also induces the disassembly of the septin bud neck ring and relocalization of the septin subunits to the nucleus. Septins remain associated with microtubules during the meiotic divisions and later, during spore morphogenesis, they are detected associated to the nascent prospore membranes surrounding each nuclear lobe. Septin association to microtubules also occurs during growth in non-fermentable carbon sources.

The SpoMBe pathway drives membrane bending necessary for cytokinesis and spore formation in yeast meiosis

Precise control over organelle shapes is essential for cellular organization and morphogenesis. During yeast meiosis, prospore membranes (PSMs) constitute bell-shaped organelles that enwrap the postmeiotic nuclei leading to the cellularization of the mother cell's cytoplasm and to spore formation. Here, we analysed how the PSMs acquire their curved bell-shaped structure. We discovered that two antagonizing forces ensure PSM shaping and proper closure during cytokinesis. The Ssp1p-containing coat at the leading edge of the PSM generates a pushing force, which is counteracted by a novel pathway, the spore membrane-bending pathway (SpoMBe). Using genetics, we found that Sma2p and Spo1p, a phospholipase, as well as several GPI-anchored proteins belong to the SpoMBe pathway. They exert a force all along the membrane, responsible for membrane bending during PSM biogenesis and for PSM closure during cytokinesis. We showed that the SpoMBe pathway involves asymmetric distribution of Sma2p and does not involve a GPI-protein-containing matrix. Rather, repulsive forces generated by asymmetrically distributed and dynamically moving GPI-proteins are suggested as the membrane-bending principle.

Sorting signals within the Saccharomyces cerevisiae sporulation-specific dityrosine transporter, Dtr1p, C terminus promote Golgi-to-prospore membrane transport

During sporulation in Saccharomyces cerevisiae, the dityrosine transporter Dtr1p, which is required for formation of the outermost layer of the spore wall, is specifically expressed and transported to the prospore membrane, a novel double-lipid-bilayer membrane. Dtr1p consists of 572 amino acids with predicted N- and C-terminal cytoplasmic extensions and 12 transmembrane domains. Dtr1p missing the largest internal cytoplasmic loop was trapped in the endoplasmic reticulum in both mitotically dividing cells and cells induced to sporulate. Deletion of the carboxyl 15 amino acids, but not the N-terminal extension of Dtr1p, resulted in a protein that failed to localize to the prospore membrane and was instead observed in cytoplasmic puncta. The puncta colocalized with a cis-Golgi marker, suggesting that Dtr1p missing the last 15 amino acids was trapped in an early Golgi compartment. Deletion of the C-terminal 10 amino acids resulted in a protein that localized to the prospore membrane with a delay and accumulated in cytoplasmic puncta that partially colocalized with a trans-Golgi marker. Both full-length Dtr1p and Dtr1p missing the last 10 amino acids expressed in vegetative cells localized to the plasma membrane and vacuoles, while Dtr1p deleted for the carboxyl-terminal 15 amino acids was observed only at vacuoles, suggesting that transport to the prospore membrane is mediated by distinct signals from those that specify plasma membrane localization. Transfer-of-function experiments revealed that both the carboxyl transmembrane domain and the C-terminal tail are important for Golgi complex-to-prospore membrane transport.