SPO14 separation-of-function mutations define unique roles for phospholipase D in secretion and cellular differentiation in Saccharomyces cerevisiae

Membrane and organelle rearrangement during ascospore formation in budding yeast

SUMMARYIn ascomycete fungi, sexual spores, termed ascospores, are formed after meiosis. Ascospore formation is an unusual cell division in which daughter cells are created within the cytoplasm of the mother cell by de novo generation of membranes that encapsulate each of the haploid chromosome sets created by meiosis. This review describes the molecular events underlying the creation, expansion, and closure of these membranes in the budding yeast, Saccharomyces cerevisiae. Recent advances in our understanding of the regulation of gene expression and the dynamic behavior of different membrane-bound organelles during this process are detailed. While less is known about ascospore formation in other systems, comparison to the distantly related fission yeast suggests that the molecular events will be broadly similar throughout the ascomycetes.

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.

Microbial induced phosphate precipitation accelerate lead mineralization to alleviate nucleotide metabolism inhibition and alter Penicillium oxalicum's adaptive cellular machinery

Microbial-induced phosphate (P) precipitation (MIPP) based on P-solubilizing microorganisms (PSM) is regarded as a promising approach to bioimmobilize environmental lead (Pb). Nevertheless, the underlying changes of Pb²⁺ biotoxicity in PSM during MIPP process were rarely discussed. The current study explored the Pb²⁺ immobilization and metabolic changes in PSM Penicillium oxalicum postexposure to Pb²⁺ and/or tricalcium phosphate (TCP). TCP addition significantly increased soluble P concentrations, accelerated extracellular Pb mineralization, and improved antioxidative enzyme activities in P. oxalicum during MIPP process. Secondary Pb²⁺ biomineralization products were measured as hydroxypyromorphite [Pb₁₀(PO₄)₆(OH)₂]. Using untargeted metabolomic and transcriptomics, we found that Pb²⁺ exposure stimulated the membrane integrity deterioration and nucleotide metabolism obstruction of P. oxalicum. Correspondingly, P. oxalicum could produce higher levels of gamma-aminobutyric acid (GABA) to enhance the adaptive cellular machineries under Pb²⁺ stress. While the MIPP process improved extracellular Pb²⁺ mineralization, consequently alleviating the nucleotide metabolism inhibition and membrane deterioration. Multi-omics results suggested that GABA degradation pathway was stimulated for arginine biosynthesis and TCA cycle after Pb²⁺ mineralization. These results provided new biomolecular information underlying the Pb²⁺ exposure biotoxicities to microorganisms in MIPP before the application of this approach in environmental Pb²⁺ remediation.

Microautophagy Mediates Vacuolar Delivery of Storage Proteins in Maize Aleurone Cells

The molecular machinery orchestrating microautophagy, whereby eukaryotic cells sequester autophagic cargo by direct invagination of the vacuolar/lysosomal membrane, is still largely unknown, especially in plants. Here, we demonstrate microautophagy of storage proteins in the maize aleurone cells of the endosperm and analyzed proteins with potential regulatory roles in this process. Within the cereal endosperm, starchy endosperm cells accumulate storage proteins (mostly prolamins) and starch whereas the peripheral aleurone cells store oils, storage proteins, and specialized metabolites. Although both cell types synthesize prolamins, they employ different pathways for their subcellular trafficking. Starchy endosperm cells accumulate prolamins in protein bodies within the endoplasmic reticulum (ER), whereas aleurone cells deliver prolamins to vacuoles via an autophagic mechanism, which we show is by direct association of ER prolamin bodies with the tonoplast followed by engulfment via microautophagy. To identify candidate proteins regulating this process, we performed RNA-seq transcriptomic comparisons of aleurone and starchy endosperm tissues during seed development and proteomic analysis on tonoplast-enriched fractions of aleurone cells. From these datasets, we identified 10 candidate proteins with potential roles in membrane modification and/or microautophagy, including phospholipase-Dα5 and a possible EUL-like lectin. We found that both proteins increased the frequency of tonoplast invaginations when overexpressed in Arabidopsis leaf protoplasts and are highly enriched at the tonoplast surface surrounding ER protein bodies in maize aleurone cells, thus supporting their potential connections to microautophagy. Collectively, this candidate list now provides useful tools to study microautophagy in plants.

Structural insights into phospholipase D function

Phospholipase D (PLD) and its metabolic active product phosphatidic acid (PA) engage in a wide range of physiopathologic processes in the cell. PLDs have been considered as a potential and promising drug target. Recently, the crystal structures of PLDs in mammalian and plant have been solved at atomic resolution. These achievements allow us to understand the structural differences among different species of PLDs and the functions of their key domains. In this review, we summarize the sequence and structure of different species of PLD isoforms, and discuss the structural mechanisms for PLD interactions with their binding partners and the functions of each key domain in the regulation of PLDs activation and catalytic reaction.

Regulation of Membrane Turnover by Phosphatidic Acid: Cellular Functions and Disease Implications

Phosphatidic acid (PA) is a simple glycerophospholipid with a well-established role as an intermediate in phospholipid biosynthesis. In addition to its role in lipid biosynthesis, PA has been proposed to act as a signaling molecule that modulates several aspects of cell biology including membrane transport. PA can be generated in eukaryotic cells by several enzymes whose activity is regulated in the context of signal transduction and enzymes that can metabolize PA thus terminating its signaling activity have also been described. Further, several studies have identified PA binding proteins and changes in their activity are proposed to be mediators of the signaling activity of this lipid. Together these enzymes and proteins constitute a PA signaling toolkit that mediates the signaling functions of PA in cells. Recently, a number of novel genetic models for the analysis of PA function in vivo and analytical methods to quantify PA levels in cells have been developed and promise to enhance our understanding of PA functions. Studies of several elements of the PA signaling toolkit in a single cell type have been performed and are presented to provide a perspective on our understanding of the biochemical and functional organization of pools of PA in a eukaryotic cell. Finally, we also provide a perspective on the potential role of PA in human disease, synthesizing studies from model organisms, human disease genetics and analysis using recently developed PLD inhibitors.

Functional analysis of mammalian phospholipase D enzymes

Phosphatidylcholine (PC)-specific phospholipase D (PLD) hydrolyzes the phosphodiester bond of the PC to generate phosphatidic acid (PA) and regulates several subcellular functions. Mammalian genomes contain two genes encoding distinct isoforms of PLD in contrast with invertebrate genomes that include a single PLD gene. However, the significance of two genes within a genome encoding the same biochemical activity remains unclear. Recently, loss of function in the only PLD gene in Drosophila was reported to result in reduced PA levels and a PA-dependent collapse of the photoreceptor plasma membrane due to defects in vesicular transport. Phylogenetic analysis reveals that human PLD1 (hPLD1) is evolutionarily closer to dPLD than human PLD2 (hPLD2). In the present study, we expressed hPLD1 and hPLD2 in Drosophila and found that while reconstitution of hPLD1 is able to completely rescue retinal degeneration in a loss of function dPLD mutant, hPLD2 was less effective in its ability to mediate a rescue. Using a newly developed analytical method, we determined the acyl chain composition of PA species produced by each enzyme. While dPLD was able to restore the levels of most PA species in dPLD^3.1 cells, hPLD1 and hPLD2 each were unable to restore the levels of a subset of unique species of PA. Finally, we found that in contrast with hPLD2, dPLD and hPLD1 are uniquely distributed to the subplasma membrane region in photoreceptors. In summary, hPLD1 likely represents the ancestral PLD in mammalian genomes while hPLD2 represents neofunctionalization to generate PA at distinct subcellular membranes.

Phospholipase D activity couples plasma membrane endocytosis with retromer dependent recycling

During illumination, the light-sensitive plasma membrane (rhabdomere) of Drosophila photoreceptors undergoes turnover with consequent changes in size and composition. However, the mechanism by which illumination is coupled to rhabdomere turnover remains unclear. We find that photoreceptors contain a light-dependent phospholipase D (PLD) activity. During illumination, loss of PLD resulted in an enhanced reduction in rhabdomere size, accumulation of Rab7 positive, rhodopsin1-containing vesicles (RLVs) in the cell body and reduced rhodopsin protein. These phenotypes were associated with reduced levels of phosphatidic acid, the product of PLD activity and were rescued by reconstitution with catalytically active PLD. In wild-type photoreceptors, during illumination, enhanced PLD activity was sufficient to clear RLVs from the cell body by a process dependent on Arf1-GTP levels and retromer complex function. Thus, during illumination, PLD activity couples endocytosis of RLVs with their recycling to the plasma membrane thus maintaining plasma membrane size and composition.

DOI:http://dx.doi.org/10.7554/eLife.18515.001

Certain cells in the eye contain a receptor protein known as rhodopsin that enables them to detect light. Rhodopsin is found in distinct patches on the membrane surrounding each of these “photoreceptor” cells and the number of rhodopsin molecules present controls how sensitive the cell is to light. In humans, vitamin A deficiency or genetic defects can decrease the number of rhodopsin molecules on the membrane, leading to difficulty in seeing in dim light.

Fruit fly eyes also contain rhodopsin. Exposure to normal levels of light triggers parts of the membranes of fly photoreceptor cells to detach and move into the interior of the cell. These internalized pieces of membrane have two possible fates: they can either be destroyed or recycled back to the cell surface. This membrane turnover adjusts the size of the membrane surrounding the cell and the number of rhodopsin molecules in it to regulate the cell’s sensitivity to light. It is crucial that turnover is tightly regulated in order to maintain the integrity of the cell membrane. However, it is not clear how the process is regulated during light exposure.

Thakur et al. set out to address this question in fruit flies. The experiments show that an enzyme called phospholipase D is activated when photoreceptors are exposed to light. Active phospholipase D – which generates a molecule called phosphatidic acid – coordinates the internalization of pieces of membrane with the recycling of rhodopsin back to the cell surface. Thakur et al. generated fly mutants that lacked phospholipase D and in these animals the internalized rhodopsin was not transported back to the cell membrane. This caused the membrane to shrink in size and decreased the number of rhodopsin molecules in it. As a result, the photoreceptor cells became less sensitive to light.

The findings of Thakur et al. show that in response to normal levels of light, phospholipase D balances membrane internalization and recycling to maintain the size and rhodopsin composition of the membrane. Future challenges will be to work out exactly how phospholipase D is activated and how phosphatidic acid tunes membrane internalization and recycling.

DOI:http://dx.doi.org/10.7554/eLife.18515.002

Dissecting protein function: an efficient protocol for identifying separation-of-function mutations that encode structurally stable proteins

Mutations that confer the loss of a single biochemical property (separation-of-function mutations) can often uncover a previously unknown role for a protein in a particular biological process. However, most mutations are identified based on loss-of-function phenotypes, which cannot differentiate between separation-of-function alleles vs. mutations that encode unstable/unfolded proteins. An alternative approach is to use overexpression dominant-negative (ODN) phenotypes to identify mutant proteins that disrupt function in an otherwise wild-type strain when overexpressed. This is based on the assumption that such mutant proteins retain an overall structure that is comparable to that of the wild-type protein and are able to compete with the endogenous protein (Herskowitz 1987). To test this, the in vivo phenotypes of mutations in the Est3 telomerase subunit from Saccharomyces cerevisiae were compared with the in vitro secondary structure of these mutant proteins as analyzed by circular-dichroism spectroscopy, which demonstrates that ODN is a more sensitive assessment of protein stability than the commonly used method of monitoring protein levels from extracts. Reverse mutagenesis of EST3, which targeted different categories of amino acids, also showed that mutating highly conserved charged residues to the oppositely charged amino acid had an increased likelihood of generating a severely defective est3(-) mutation, which nevertheless encoded a structurally stable protein. These results suggest that charge-swap mutagenesis directed at a limited subset of highly conserved charged residues, combined with ODN screening to eliminate partially unfolded proteins, may provide a widely applicable and efficient strategy for generating separation-of-function mutations.

The Sum1/Ndt80 transcriptional switch and commitment to meiosis in Saccharomyces cerevisiae

Cells encounter numerous signals during the development of an organism that induce division, differentiation, and apoptosis. These signals need to be present for defined intervals in order to induce stable changes in the cellular phenotype. The point after which an inducing signal is no longer needed for completion of a differentiation program can be termed the "commitment point." Meiotic development in the yeast Saccharomyces cerevisiae (sporulation) provides a model system to study commitment. Similar to differentiation programs in multicellular organisms, the sporulation program in yeast is regulated by a transcriptional cascade that produces early, middle, and late sets of sporulation-specific transcripts. Although critical meiosis-specific events occur as early genes are expressed, commitment does not take place until middle genes are induced. Middle promoters are activated by the Ndt80 transcription factor, which is produced and activated shortly before most middle genes are expressed. In this article, I discuss the connection between Ndt80 and meiotic commitment. A transcriptional regulatory pathway makes NDT80 transcription contingent on the prior expression of early genes. Once Ndt80 is produced, the recombination (pachytene) checkpoint prevents activation of the Ndt80 protein. Upon activation, Ndt80 triggers a positive autoregulatory loop that leads to the induction of genes that promote exit from prophase, the meiotic divisions, and spore formation. The pathway is controlled by multiple feed-forward loops that give switch-like properties to the commitment transition. The conservation of regulatory components of the meiotic commitment pathway and the recently reported ability of Ndt80 to increase replicative life span are discussed.

The conserved foot domain of RNA pol II associates with proteins involved in transcriptional initiation and/or early elongation

RNA polymerase (pol) II establishes many protein-protein interactions with transcriptional regulators to coordinate different steps of transcription. Although some of these interactions have been well described, little is known about the existence of RNA pol II regions involved in contact with transcriptional regulators. We hypothesize that conserved regions on the surface of RNA pol II contact transcriptional regulators. We identified such an RNA pol II conserved region that includes the majority of the "foot" domain and identified interactions of this region with Mvp1, a protein required for sorting proteins to the vacuole, and Spo14, a phospholipase D. Deletion of MVP1 and SPO14 affects the transcription of their target genes and increases phosphorylation of Ser5 in the carboxy-terminal domain (CTD). Genetic, phenotypic, and functional analyses point to a role for these proteins in transcriptional initiation and/or early elongation, consistent with their genetic interactions with CEG1, a guanylyltransferase subunit of the Saccharomyces cerevisiae capping enzyme.

Phosphatidylcholine is essential for efficient functioning of the mitochondrial glycerol-3-phosphate dehydrogenase Gut2 inSaccharomyces cerevisiae

Gut2, the mitochondrial glycerol-3-phosphate dehydrogenase, was previously shown to become preferentially labelled with photoactivatable phosphatidylcholine (PC), pointing to a functional relation between these molecules. In the present study we analyzed whether Gut2 functioning depends on the PC content of yeast cells, using PC biosynthetic mutants in which the PC content was lowered. PC depletion was found to reduce growth on glycerol and to increase glycerol excretion, both indicating that PC is needed for optimal Gut2 functioning in vivo. Using several in vitro approaches the nature of the dependence of Gut2 functioning on cellular PC contents was investigated. The results of these experiments suggest that it is unlikely that the effects observed in vivo are due to changes in cellular Gut2 content, in specific activity of Gut2 in isolated mitochondria, or in the membrane association of Gut2, upon lowering the PC level. The in vivo effects are more likely an indirect result of PC depletion-induced changes in the cellular context in which Gut2 functions, that are not manifested in the in vitro systems used.

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.

Rhabdomere biogenesis in Drosophila photoreceptors is acutely sensitive to phosphatidic acid levels

Phosphatidic acid (PA) is postulated to have both structural and signaling functions during membrane dynamics in animal cells. In this study, we show that before a critical time period during rhabdomere biogenesis in Drosophila melanogaster photoreceptors, elevated levels of PA disrupt membrane transport to the apical domain. Lipidomic analysis shows that this effect is associated with an increase in the abundance of a single, relatively minor molecular species of PA. These transport defects are dependent on the activation state of Arf1. Transport defects via PA generated by phospholipase D require the activity of type I phosphatidylinositol (PI) 4 phosphate 5 kinase, are phenocopied by knockdown of PI 4 kinase, and are associated with normal endoplasmic reticulum to Golgi transport. We propose that PA levels are critical for apical membrane transport events required for rhabdomere biogenesis.

Synthesis and function of membrane phosphoinositides in budding yeast, Saccharomyces cerevisiae

It is now well appreciated that derivatives of phosphatidylinositol (PtdIns) are key regulators of many cellular processes in eukaryotes. Of particular interest are phosphoinositides (mono- and polyphosphorylated adducts to the inositol ring in PtdIns), which are located at the cytoplasmic face of cellular membranes. Phosphoinositides serve both a structural and a signaling role via their recruitment of proteins that contain phosphoinositide-binding domains. Phosphoinositides also have a role as precursors of several types of second messengers for certain intracellular signaling pathways. Realization of the importance of phosphoinositides has brought increased attention to characterization of the enzymes that regulate their synthesis, interconversion, and turnover. Here we review the current state of our knowledge about the properties and regulation of the ATP-dependent lipid kinases responsible for synthesis of phosphoinositides and also the additional temporal and spatial controls exerted by the phosphatases and a phospholipase that act on phosphoinositides in yeast.

Genetic evidence for a SPO1-dependent signaling pathway controlling meiotic progression in yeast

The yeast spindle pole body (SPB) plays a unique role in meiosis, initiating both spindle assembly and prospore membrane synthesis. SPO1, induced early in development, encodes a meiosis-specific phospholipase B (PLB) homolog required at three stages of SPB morphogenesis: MI, MII, and spore formation. Here we report in-depth analysis of the SPO1 gene including its transcriptional control by regulators of early gene expression, protein localization to the ER lumen and periplasmic space, and molecular genetic studies of its role in meiosis. Evidence is presented that multiple arrest points in spo1Delta occur independently, demonstrating that Spo1 acts at distinct steps. Loss of Spo1 is suppressed by high-copy glycosylphosphatidylinositol (GPI) proteins, dependent on sequence, timing, and strength of induction in meiosis. Since phosphatidylinositol (PI) serves as both an anchor component and a lipase substrate, we hypothesized that GPI-protein expression might substitute for Spo1 by decreasing levels of its potential substrates, PI and phosphatidylinositol phosphates (PIPs). Partial spo1Delta complementation by PLB3 (encoding a unique PLB capable of cleaving PI) and relatively strong Spo1 binding to PI(4)P derivatives (via a novel N-terminal lysine-rich fragment essential for Spo1 function) are consistent with this view. Epistasis of SPO1 mutations to those in SPO14 (encoding a PLD involved in signaling) and physical interaction of Spo1 with Spo23, a protein regulating PI synthesis required for wild-type sporulation, further support this notion. Taken together these findings implicate PI and/or PIPs in Spo1 function and suggest the existence of a novel Spo1-dependent meiosis-specific signaling pathway required for progression of MI, MII, and spore formation via regulation of the SPB.

The Arf-GTPase-activating protein Gcs1p is essential for sporulation and regulates the phospholipase D Spo14p

SPO14, encoding the major Saccharomyces cerevisiae phospholipase D (PLD), is essential for sporulation and mediates synthesis of the new membrane that encompasses the haploid nuclei that arise through meiotic divisions. PLD catalyzes the hydrolysis of phosphatidylcholine to phosphatidic acid (PA) and choline. PA stimulates Arf-GTPase-activating proteins (Arf-GAPs), which are involved in membrane trafficking events and actin cytoskeletal function. To determine if Spo14p-generated PA mediates its biological response through Arf-GAPs, we analyzed the sporulation efficiencies of cells deleted for each of the five known and potential yeast Arf-GAPs. Only gcs1delta mutants display a sporulation defect similar to that of spo14 mutants: cells deleted for GCS1 initiate the sporulation program but are defective in synthesis of the prospore membrane. Endosome-to-vacuole transport is also impaired in gcs1delta cells during sporulation. Furthermore, Arf-GAP catalytic activity, but not the pleckstrin homology domain, is required for both prospore membrane formation and endosome-to-vacuole trafficking. An examination of Gcs1p-green fluorescent protein revealed that it is a soluble protein. Interestingly, cells deleted for GCS1 have reduced levels of Spo14p-generated PA. Taken together, these results indicate that GCS1 is essential for sporulation and suggest that GCS1 positively regulates SPO14.

Regulation of phosphatidylcholine homeostasis by Sec14This paper is one of a selection of papers published in this Special Issue, entitled Young Investigator's Forum

Phosphatidylcholine is the major phospholipid in eukaryotic cells and serves as both a permeability barrier as well as a modulator of a plethora of cellular and biological functions. This review touches on the importance of proper regulation of phosphatidylcholine metabolism on health, and discusses how yeast genetics has contributed to furthering our understanding of the precise molecular events regulated by alterations in phosphatidylcholine metabolism. Yeast studies have determined that the phosphatidylcholine and (or) phosphatidylinositol binding protein, Sec14, is a major regulator of phosphatidylcholine homeostasis. Sec14 itself regulates vesicular transport from the Golgi, and the interrelationship between phosphatidylcholine metabolism and membrane movement within the cell is described in detail. The recent convergence of the yeast genetic studies with that of mammalian cell biology in how cells maintain phosphatidylcholine homeostasis is highlighted.

Phospholipase D activity is essential for actin localization and actin-based motility in Dictyostelium

PLD (phospholipase D) activity catalyses the generation of the lipid messenger phosphatidic acid, which has been implicated in a number of cellular processes, particularly the regulation of membrane traffic. In the present study, we report that disruption of PLD signalling causes unexpectedly profound effects on the actin-based motility of Dictyostelium. Cells in which PLD activity is inhibited by butan-1-ol show a complete loss of actin-based structures, accompanied by relocalization of F-actin into small clusters, and eventually the nucleus, without a visible fall in levels of F-actin. Addition of exogenous phosphatidic acid reverses the effects of butan-1-ol, confirming that these effects are caused by inhibition of PLD. Loss of motility correlates with complete inhibition of endocytosis and a reduction in phagocytosis. Inhibition of PLD caused a major decrease in the synthesis of PtdIns(4,5)P2, which could again be reversed by exogenously applied phosphatidic acid. Thus the essential role of PLD signalling in both motility and endocytosis appears to be mediated directly via regulation of PtdIns(4)P kinase activity. This implies that localized PLD-regulated synthesis of PtdIns(4,5)P2 is essential for Dictyostelium actin function.

Regulation of activity in vitro and in vivo of three phospholipases B from Saccharomyces cerevisiae

The genome of the yeast, Saccharomyces cerevisiae, contains three highly similar genes coding for phospholipases B/lysophospholipases. These enzymes behave differently with respect to substrate preferences in vitro and relative contributions to phospholipid catabolism in vivo [Merkel, Fido, Mayr, Pruger, Raab, Zandonella, Kohlwein and Paltauf (1999) J. Biol. Chem. 274, 28121-28127]. It is shown in the present study that, in vitro, pH markedly affects the substrate preference of Plb1p and Plb2p, but not of Plb3p. At the pH optimum of 2.5-3.5, the order of substrate preference of Plb1p and Plb2p is PtdSer (phosphatidylserine)>PtdIns>PtdCho (phosphatidylcholine>PtdEtn (phosphatidylethanolamine). At pH values of 5 and above, the substrate preferences change to PtdCho=PtdEtn for Plb1p and PtdSer=PtdEtn for Plb2p. Accordingly, with cultured cells the ratio of PtdIns/PtdCho breakdown, as reflected in the ratio of GroPIns (glycerophosphoinositol)/GroPCho (glycerophosphocholine) released into the culture medium, is inversely related to the pH of the growth medium. This effect is ascribed to the pH response of Plb1p, because Plb2p does not contribute to the degradation of PtdIns and PtdCho in vivo. Bivalent and tervalent cations activate phospholipases B at pH 5.5, but are inhibitory at pH 2.5. Al3+ at a concentration of 20 mM increases Plb1p activity in vitro by 8-fold and leads to a 9-fold increase in GroPCho release by whole cells. In vivo, cycloheximide strongly inhibits the breakdown of PtdIns, and to a lesser extent PtdCho. However, Al3+-stimulated GroPCho release is almost completely inhibited by cycloheximide. Deletion of PLB3 leads to increased sensitivity to toxic Al3+. Addition of SDS or melittin to cultured cells leads to a significant increase in phospholipid degradation, which is insensitive to inhibition by cycloheximide. Deletion mutants defective in the PLB1 gene are significantly more resistant to SDS than are wild-type cells.

End3p-mediated endocytosis is required for spore wall formation in Saccharomyces cerevisiae

During sporulation in Saccharomyces cerevisiae, vesicles transported to the vicinity of spindle pole bodies are fused to each other to generate bilayered prospore membranes (PSMs). PSMs encapsulate the haploid nuclei that arise from the meiotic divisions and serve as platforms for spore wall deposition. Membrane trafficking plays an important role in supplying vesicles for these processes. The endocytosis-deficient mutant, end3Delta, sporulated poorly and the spores produced lost resistance to ether vapor, suggesting that END3-mediated endocytosis is important for sporulation. End3p-GFP localized to cell and spore peripheries in vegetative and sporulating cells and colocalized with actin structures. Correspondingly, the actin cytoskeleton appeared aberrant during sporulation in end3Delta. Analysis of meiosis in end3Delta mutants revealed that the meiotic divisions occurred with wild-type kinetics. Furthermore, PSMs were assembled normally. However, the levels of proteins required for spore wall synthesis and components of the spore wall layers at spores were reduced, indicating that end3Delta mutants are defective in spore wall synthesis. Thus, END3-mediated endocytosis is important for spore wall formation. Additionally, cytological analyses suggest that trafficking between the plasma membrane and PSMs is important earlier during sporulation.

Genetic evidence of a role for membrane lipid composition in the regulation of soluble NEM-sensitive factor receptor function in Saccharomyces cerevisiae

SEC9 and SPO20 encode SNARE proteins related to the mammalian SNAP-25 family. Sec9p associates with the SNAREs Sso1/2p and Snc1/2p to promote the fusion of vesicles with the plasma membrane. Spo20p functions with the same two partner SNAREs to mediate the fusion of vesicles with the prospore membrane during sporogenesis. A chimeric molecule, in which the helices of Sec9p that bind to Sso1/2p and Snc1/2p are replaced with the homologous regions of Spo20p, will not support vesicle fusion in vegetative cells. The phosphatidylinositol-4-phosphate-5-kinase MSS4 was isolated as a high-copy suppressor that permits this chimera to rescue the temperature-sensitive growth of a sec9-4 mutant. Suppression by MSS4 is specific to molecules that contain the Spo20p helical domains. This suppression requires an intact copy of SPO14, encoding phospholipase D. Overexpression of MSS4 leads to a recruitment of the Spo14 protein to the plasma membrane and this may be the basis for MSS4 action. Consistent with this, deletion of KES1, a gene that behaves as a negative regulator of SPO14, also promotes the function of SPO20 in vegetative cells. These results indicate that elevated levels of phosphatidic acid in the membrane may be required specifically for the function of SNARE complexes containing Spo20p.

Positive and negative regulation of a SNARE protein by control of intracellular localization

In Saccharomyces cerevisiae, the developmentally regulated Soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) protein Spo20p mediates the fusion of vesicles with the prospore membrane, which is required for the formation of spores. Spo20p is subject to both positive and negative regulation by separate sequences in its aminoterminal domain. We report that the positive activity is conferred by a short, amphipathic helix that is sufficient to confer plasma membrane or prospore membrane localization to green fluorescent protein. In vitro, this helix binds to acidic phospholipids, and mutations that reduce or eliminate phospholipid binding in vitro inactivate Spo20p in vivo. Genetic manipulation of phospholipid pools indicates that the likely in vivo ligand of this domain is phosphatidic acid. The inhibitory activity is a nuclear targeting signal, which confers nuclear localization in vegetative cells and in cells entering meiosis. However, as cells initiate spore formation, fusions containing the inhibitory domain exit the nucleus and localize to the nascent prospore membrane. Thus, the SNARE Spo20p is both positively and negatively regulated by control of its intracellular localization.

Yeast cells provide insight into alpha-synuclein biology and pathobiology

Alpha-synuclein is implicated in several neurodegenerative disorders, such as Parkinson's disease and multiple system atrophy, yet its functions remain obscure. When expressed in yeast, alpha-synuclein associated with the plasma membrane in a highly selective manner, before forming cytoplasmic inclusions through a concentration-dependent, nucleated process. Alpha-synuclein inhibited phospholipase D, induced lipid droplet accumulation, and affected vesicle trafficking. This readily manipulable system provides an opportunity to dissect the molecular pathways underlying normal alpha-synuclein biology and the pathogenic consequences of its misfolding.

Roles of phosphoinositides and of Spo14p (phospholipase D)-generated phosphatidic acid during yeast sporulation

During yeast sporulation, internal membrane synthesis ensures that each haploid nucleus is packaged into a spore. Prospore membrane formation requires Spo14p, a phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]-stimulated phospholipase D (PLD), which hydrolyzes phosphatidylcholine (PtdCho) to phosphatidic acid (PtdOH) and choline. We found that both meiosis and spore formation also require the phosphatidylinositol (PtdIns)/PtdCho transport protein Sec14p. Specific ablation of the PtdIns transport activity of Sec14p was sufficient to impair spore formation but not meiosis. Overexpression of Pik1p, a PtdIns 4-kinase, suppressed the sec14-1 meiosis and spore formation defects; conversely, pik1-ts diploids failed to undergo meiosis and spore formation. The PtdIns(4)P 5-kinase, Mss4p, also is essential for spore formation. Use of phosphoinositide-specific GFP-PH domain reporters confirmed that PtdIns(4,5)P2 is enriched in prospore membranes. sec14, pik1, and mss4 mutants displayed decreased Spo14p PLD activity, whereas absence of Spo14p did not affect phosphoinositide levels in vivo, suggesting that formation of PtdIns(4,5)P2 is important for Spo14p activity. Spo14p-generated PtdOH appears to have an essential role in sporulation, because treatment of cells with 1-butanol, which supports Spo14p-catalyzed PtdCho breakdown but leads to production of Cho and Ptd-butanol, blocks spore formation at concentrations where the inert isomer, 2-butanol, has little effect. Thus, rather than a role for PtdOH in stimulating PtdIns(4,5)P2 formation, our findings indicate that during sporulation, Spo14p-mediated PtdOH production functions downstream of Sec14p-, Pik1p-, and Mss4p-dependent PtdIns(4,5)P2 synthesis.

Cell signaling in yeast sporulation

Gametogenesis is essential for the propagation of all sexually reproducing organisms and consists of halving the chromosome number through meiosis, and the subsequent packaging of the haploid products into gametes. Meiosis and gamete formation must be tightly coupled to ensure the formation of viable progeny; perturbations result in infertility, inviability, and birth defects. In the yeast Saccharomyces cerevisiae, sexual reproduction occurs via sporulation and is similar in many respects to gametogenesis in mammals. An increasing number of conserved signaling molecules have been shown to be essential for yeast sporulation; recent studies reveal molecular insights into how these molecules regulate this intricate differentiation program.

Dual role for phosphoinositides in regulation of yeast and mammalian phospholipase D enzymes

Phospholipase D (PLD) generates lipid signals that coordinate membrane trafficking with cellular signaling. PLD activity in vitro and in vivo is dependent on phosphoinositides with a vicinal 4,5-phosphate pair. Yeast and mammalian PLDs contain an NH2-terminal pleckstrin homology (PH) domain that has been speculated to specify both subcellular localization and regulation of PLD activity through interaction with phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2). We report that mutation of the PH domains of yeast and mammalian PLD enzymes generates catalytically active PI(4,5)P2-regulated enzymes with impaired biological functions. Disruption of the PH domain of mammalian PLD2 results in relocalization of the protein from the PI(4,5)P2-containing plasma membrane to endosomes. As a result of this mislocalization, mutations within the PH domain render the protein unresponsive to activation in vivo. Furthermore, the integrity of the PH domain is vital for yeast PLD function in both meiosis and secretion. Binding of PLD2 to model membranes is enhanced by acidic phospholipids. Studies with PLD2-derived peptides suggest that this binding involves a previously identified polybasic motif that mediates activation of the enzyme by PI(4,5)P2. By comparison, the PLD2 PH domain binds PI(4,5)P2 with lower affinity but sufficient selectivity to function in concert with the polybasic motif to target the protein to PI(4,5)P2-rich membranes. Phosphoinositides therefore have a dual role in PLD regulation: membrane targeting mediated by the PH domain and stimulation of catalysis mediated by the polybasic motif.

Mating-type locus of Cryptococcus neoformans: a step in the evolution of sex chromosomes

The sexual development and virulence of the fungal pathogen Cryptococcus neoformans is controlled by a bipolar mating system determined by a single locus that exists in two alleles, alpha and a. The alpha and a mating-type alleles from two divergent varieties were cloned and sequenced. The C. neoformans mating-type locus is unique, spans >100 kb, and contains more than 20 genes. MAT-encoded products include homologs of regulators of sexual development in other fungi, pheromone and pheromone receptors, divergent components of a MAP kinase cascade, and other proteins with no obvious function in mating. The alpha and a alleles of the mating-type locus have extensively rearranged during evolution and strain divergence but are stable during genetic crosses and in the population. The C. neoformans mating-type locus is strikingly different from the other known fungal mating-type loci, sharing features with the self-incompatibility systems and sex chromosomes of algae, plants, and animals. Our study establishes a new paradigm for mating-type loci in fungi with implications for the evolution of cell identity and self/nonself recognition.

Phospholipase D in Phytophthora infestans and its role in zoospore encystment

We show that differentiation of zoospores of the late blight pathogen Phytophthora infestans into cysts, a process called encystment, was triggered by both phosphatidic acid (PA) and the G-protein activator mastoparan. Mastoparan induced the accumulation of PA, indicating that encystment by mastoparan most likely acts through PA. Likewise, mechanical agitation of zoospores, which often is used to induce synchronized encystment, resulted in increased levels of PA. The levels of diacylglycerolpyrophosphate (DGPP), the phosphorylation product of PA, increased simultaneously. Also in cysts, sporangiospores, and mycelium, mastoparan induced increases in the levels of PA and DGPP. Using an in vivo assay for phospholipase D (PLD) activity, it was shown that the mastoparan-induced increase in PA was due to a stimulation of the activity of this enzyme. Phospholipase C in combination with diacylglycerol (DAG) kinase activity also can generate PA, but activation of these enzymes by mastoparan was not detected under conditions selected to highlight 32P-PA production via DAG kinase. Primary and secondary butanol, which, like mastoparan, have been reported to activate G-proteins, also stimulated PLD activity, whereas the inactive tertiary isomer did not. Similarly, encystment was induced by n- and sec-butanol but not by tert-butanol. Together, these results show that Phytophthora infestans contains a mastoparan- and butanol-inducible PLD pathway and strongly indicate that PLD is involved in zoospore encystment. The role of G-proteins in this process is discussed.

Differential regulation of Saccharomyces cerevisiae phospholipase D in sporulation and Sec14-independent secretion

Saccharomyces cerevisiae Spo14, a phosphatidylcholine-specific, phosphatidylinositol (4,5) bisphosphate-activated phospholipase D (PLD), is essential for meiosis and spore formation. Spo14 is also required for secretion in the absence of the phosphatidylinositol/phosphatidylcholine transfer protein Sec14 (i.e., Sec14-independent secretion). In sporulating cells Spo14 is phosphorylated and relocalized within the cell. In contrast, Spo14 does not relocalize and is not phosphorylated in Sec14-independent secretion. Analysis of a partially phosphatidylinositol (4,5) bisphosphate-activated Spo14 mutant, spo14(R894G), revealed that Spo14 function in Sec14-independent secretion, unlike the situation in meiosis, requires fully stimulated PLD activity. Consistent with the differential regulation of Spo14 function during sporulation and secretion, we isolated a mutant allele, spo14-S251P, the product of which is improperly phosphorylated and fails to relocalize and rescue the sporulation phenotype of homozygous spo14 diploids, but supports Sec14-independent secretion. Furthermore, we show that the N-terminal domain of Spo14 is both phosphorylated and sufficient for prospore membrane localization during sporulation. These data indicate that Spo14 phosphorylation and relocalization are essential for the process of sporulation, but dispensable for Sec14-independent secretion. Finally, we demonstrate that Spo14 phosphorylation and relocalization are initiated by nitrogen and glucose limitation and occur independently of the process of meiosis.