Essential role for Schizosaccharomyces pombe pik1 in septation

Characterization of Pik1 function in fission yeast reveals its conserved role in lipid synthesis and not cytokinesis

Phosphatidylinositol (PI)-4-phosphate (PI4P) is a lipid found at the plasma membrane (PM) and Golgi in cells from yeast to humans. PI4P is generated from PI by PI4-kinases and can be converted into PI-4,5-bisphosphate [PI(4,5)P2]. Schizosaccharomyces pombe have two essential PI4-kinases - Stt4 and Pik1. Stt4 localizes to the PM, and its loss from the PM results in a decrease of PM PI4P and PI(4,5)P2. As a result, cells divide non-medially due to disrupted cytokinetic ring-PM anchoring. However, the localization and function of S. pombe Pik1 has not been thoroughly examined. Here, we found that Pik1 localizes exclusively to the trans-Golgi and is required for Golgi PI4P production. We determined that Ncs1 regulates Pik1, but unlike in other organisms, it is not required for Pik1 Golgi localization. When Pik1 function was disrupted, PM PI4P but not PI(4,5)P2 levels were reduced, a major difference compared with Stt4. We conclude that Stt4 is the chief enzyme responsible for producing the PI4P that generates PI(4,5)P2. Also, that cells with disrupted Pik1 do not divide asymmetrically highlights the specific importance of PM PI(4,5)P2 for cytokinetic ring-PM anchoring.

Characterization of Pik1 function in fission yeast reveals its conserved role in lipid synthesis and not cytokinesis

Phosphatidylinositol (PI)-4-phosphate (PI4P) is a lipid found at the plasma membrane (PM) and Golgi in cells from yeast to humans. PI4P is generated from PI by PI4-kinases and can be converted to PI-4,5-bisphosphate [PI(4,5)P 2 ]. Schizosaccharomyces pombe have 2 essential PI4-kinases: Stt4 and Pik1. Stt4 localizes to the PM and its loss from the PM results in a decrease of PM PI4P and PI(4,5)P 2 . As a result, cells divide non-medially due to disrupted cytokinetic ring-PM anchoring. However, the localization and function of S. pombe Pik1 has not been thoroughly examined. Here, we found that Pik1 localizes exclusively to the trans-Golgi and is required for Golgi PI4P production. We determined that Ncs1 regulates Pik1, but unlike in other organisms, it is not required for Pik1 Golgi localization. When Pik1 function was disrupted, PM PI4P but not PI(4,5)P 2 levels were reduced, a major difference with Stt4. We conclude that Stt4 is the chief enzyme responsible for producing the PI4P that generates PI(4,5)P 2 . Also, that cells with disrupted Pik1 do not divide asymmetrically highlights the specific importance of PM PI(4,5)P 2 for cytokinetic ring-PM anchoring.

Summary statement

Fission yeast Pik1 localizes exclusively to the trans-Golgi independently of Ncs1, where it contributes to PI4P but not PI(4,5)P 2 synthesis. Pik1 does not affect cytokinesis.

Exploring Genetic Interactions with Telomere Protection Gene pot1 in Fission Yeast

The regulation of telomere length has a significant impact on cancer risk and aging in humans. Circular chromosomes are found in humans and are often unstable during mitosis, resulting in genome instability. Some types of cancer have a high frequency of a circular chromosome. Fission yeast is a good model for studying the formation and stability of circular chromosomes as deletion of pot1 (encoding a telomere protection protein) results in rapid telomere degradation and chromosome fusion. Pot1 binds to single-stranded telomere DNA and is conserved from fission yeast to humans. Loss of pot1 leads to viable strains in which all three fission yeast chromosomes become circular. In this review, I will introduce pot1 genetic interactions as these inform on processes such as the degradation of uncapped telomeres, chromosome fusion, and maintenance of circular chromosomes. Therefore, exploring genes that genetically interact with pot1 contributes to finding new genes and/or new functions of genes related to the maintenance of telomeres and/or circular chromosomes.

Calcium Binding Protein Ncs1 Is Calcineurin Regulated in Cryptococcus neoformans and Essential for Cell Division and Virulence

Intracellular calcium (Ca2+) is crucial for signal transduction in Cryptococcus neoformans, the major cause of fatal fungal meningitis. The calcineurin pathway is the only Ca2+-requiring signaling cascade implicated in cryptococcal stress adaptation and virulence, with Ca2+ binding mediated by the EF-hand domains of the Ca2+ sensor protein calmodulin. In this study, we identified the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) as a member of the EF-hand superfamily. We demonstrated that Ncs1 has a role in Ca2+ homeostasis under stress and nonstress conditions, as the ncs1Δ mutant is sensitive to a high Ca2+ concentration and has an elevated basal Ca2+ level. Furthermore, NCS1 expression is induced by Ca2+, with the Ncs1 protein adopting a punctate subcellular distribution. We also demonstrate that, in contrast to the case with Saccharomyces cerevisiae, NCS1 expression in C. neoformans is regulated by the calcineurin pathway via the transcription factor Crz1, as NCS1 expression is reduced by FK506 treatment and CRZ1 deletion. Moreover, the ncs1Δ mutant shares a high temperature and high Ca2+ sensitivity phenotype with the calcineurin and calmodulin mutants (cna1Δ and cam1Δ), and the NCS1 promoter contains two calcineurin/Crz1-dependent response elements (CDRE1). Ncs1 deficiency coincided with reduced growth, characterized by delayed bud emergence and aberrant cell division, and hypovirulence in a mouse infection model. In summary, our data show that Ncs1 has a significant role as a Ca2+ sensor in C. neoformans, working with calcineurin to regulate Ca2+ homeostasis and, consequently, promote fungal growth and virulence.IMPORTANCECryptococcus neoformans is the major cause of fungal meningitis in HIV-infected patients. Several studies have highlighted the important contributions of Ca2+ signaling and homeostasis to the virulence of C. neoformans Here, we identify the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) and demonstrate its role in Ca2+ homeostasis, bud emergence, cell cycle progression, and virulence. We also show that Ncs1 function is regulated by the calcineurin/Crz1 signaling cascade. Our work provides evidence of a link between Ca2+ homeostasis and cell cycle progression in C. neoformans.

Analysis of the contribution of phosphoinositides to medial septation in fission yeast highlights the importance of PI(4,5)P2 for medial contractile ring anchoring

In Schizosaccharomyces pombe, loss of the plasma membrane PI4-kinase scaffold Efr3 leads to sliding of the cytokinetic ring (CR) away from the cell center during anaphase, implicating phosphoinositides (PIPs) in CR anchoring. However, whether other PIP regulators contribute to CR anchoring has not been investigated. Here we report that mutants of other PIP kinases and their regulators divide with off-center septa, similar to efr3∆. Using new biosensors for S. pombe PIPs, we confirm that these mutants have disrupted PIP composition. We extend a previous finding that a mutant known to decrease PI(3,5)P₂ levels indirectly affects CR positioning by increasing vacuole size which disrupts nuclear position at the onset of mitosis. Indeed, we found that other mutants with increased vacuole size also disrupt medial division via this mechanism. Although elevated plasma membrane PI(4,5)P₂ levels do not affect medial cytokinesis, mutants with decreased levels display CR sliding events indicating a specific role for PI(4,5)P₂ in CR anchoring.

Systematic functional analysis of kinases in the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans is the leading cause of death by fungal meningoencephalitis; however, treatment options remain limited. Here we report the construction of 264 signature-tagged gene-deletion strains for 129 putative kinases, and examine their phenotypic traits under 30 distinct in vitro growth conditions and in two different hosts (insect larvae and mice). Clustering analysis of in vitro phenotypic traits indicates that several of these kinases have roles in known signalling pathways, and identifies hitherto uncharacterized signalling cascades. Virulence assays in the insect and mouse models provide evidence of pathogenicity-related roles for 63 kinases involved in the following biological categories: growth and cell cycle, nutrient metabolism, stress response and adaptation, cell signalling, cell polarity and morphology, vacuole trafficking, transfer RNA (tRNA) modification and other functions. Our study provides insights into the pathobiological signalling circuitry of C. neoformans and identifies potential anticryptococcal or antifungal drug targets.

Cryptococcus neoformans is the leading cause of death by fungal meningoencephalitis. Here, the authors study the roles played by 129 putative kinases in the growth and virulence of C. neoformans, identifying potential targets for development of anticryptococcal drugs.

Contractile ring stability in S. pombe depends on F-BAR protein Cdc15p and Bgs1p transport from the Golgi complex

Cdc15p is known to contribute to cytokinesis in fission yeast; however, the protein is not required to assemble the contractile ring of actin and myosin, but it helps to anchor the ring to the plasma membrane. Cdc15p has a lipid-binding F-BAR domain, suggesting that it provides a physical link between the plasma membrane and contractile ring proteins. However, we find that a more important function of Cdc15p during cytokinesis is to help deliver a transmembrane enzyme, Bgs1p (also called Cps1p), from the Golgi apparatus to the plasma membrane, where it appears to anchor the contractile ring. Bgs1p synthesizes the cell wall in the cleavage furrow, but its enzyme activity is not required to anchor the contractile ring. We estimate that ∼ 2,000 Bgs1p molecules are required to anchor the ring. Without Bgs1p anchors, contractile rings slide along the plasma membrane, a phenomenon that depends on an unconventional type II myosin called Myp2p.

Functional analysis of the Aspergillus nidulans kinome

The filamentous fungi are an ecologically important group of organisms which also have important industrial applications but devastating effects as pathogens and agents of food spoilage. Protein kinases have been implicated in the regulation of virtually all biological processes but how they regulate filamentous fungal specific processes is not understood. The filamentous fungus Aspergillus nidulans has long been utilized as a powerful molecular genetic system and recent technical advances have made systematic approaches to study large gene sets possible. To enhance A. nidulans functional genomics we have created gene deletion constructs for 9851 genes representing 93.3% of the encoding genome. To illustrate the utility of these constructs, and advance the understanding of fungal kinases, we have systematically generated deletion strains for 128 A. nidulans kinases including expanded groups of 15 histidine kinases, 7 SRPK (serine-arginine protein kinases) kinases and an interesting group of 11 filamentous fungal specific kinases. We defined the terminal phenotype of 23 of the 25 essential kinases by heterokaryon rescue and identified phenotypes for 43 of the 103 non-essential kinases. Uncovered phenotypes ranged from almost no growth for a small number of essential kinases implicated in processes such as ribosomal biosynthesis, to conditional defects in response to cellular stresses. The data provide experimental evidence that previously uncharacterized kinases function in the septation initiation network, the cell wall integrity and the morphogenesis Orb6 kinase signaling pathways, as well as in pathways regulating vesicular trafficking, sexual development and secondary metabolism. Finally, we identify ChkC as a third effector kinase functioning in the cellular response to genotoxic stress. The identification of many previously unknown functions for kinases through the functional analysis of the A. nidulans kinome illustrates the utility of the A. nidulans gene deletion constructs.

Haploinsufficiency of the Sec7 guanine nucleotide exchange factor gea1 impairs septation in fission yeast

Membrane trafficking is essential to eukaryotic life and is controlled by a complex network of proteins that regulate movement of proteins and lipids between organelles. The GBF1/GEA family of Guanine nucleotide Exchange Factors (GEFs) regulates trafficking between the endoplasmic reticulum and Golgi by catalyzing the exchange of GDP for GTP on ADP Ribosylation Factors (Arfs). Activated Arfs recruit coat protein complex 1 (COP-I) to form vesicles that ferry cargo between these organelles. To further explore the function of the GBF1/GEA family, we have characterized a fission yeast mutant lacking one copy of the essential gene gea1 (gea1+/-), the Schizosaccharomyces pombe ortholog of GBF1. The haploinsufficient gea1+/- strain was shown to be sensitive to the GBF1 inhibitor brefeldin A (BFA) and was rescued from BFA sensitivity by gea1p overexpression. No overt defects in localization of arf1p or arf6p were observed in gea1+/- cells, but the fission yeast homolog of the COP-I cargo sac1 was mislocalized, consistent with impaired COP-I trafficking. Although Golgi morphology appeared normal, a slight increase in vacuolar size was observed in the gea1+/- mutant strain. Importantly, gea1+/- cells exhibited dramatic cytokinesis-related defects, including disorganized contractile rings, an increased septation index, and alterations in septum morphology. Septation defects appear to result from altered secretion of enzymes required for septum dynamics, as decreased secretion of eng1p, a β-glucanase required for septum breakdown, was observed in gea1+/- cells, and overexpression of eng1p suppressed the increased septation phenotype. These observations implicate gea1 in regulation of septum breakdown and establish S. pombe as a model system to explore GBF1/GEA function in cytokinesis.

Phosphoinositides and cytokinesis: the "PIP" of the iceberg

Phosphoinositides [Phosphatidylinositol (PtdIns), phosphatidylinositol 3-monophosphate (PtdIns3P), phosphatidylinositol 4-monophosphate (PtdIns4P), phosphatidylinositol 5-monophosphate (PtdIns5P), phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2) ), phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2) ), phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2) ), and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3) )] are lowly abundant acidic lipids found at the cytosolic leaflet of the plasma membrane and intracellular membranes. Initially discovered as precursors of second messengers in signal transduction, phosphoinositides are now known to directly or indirectly control key cellular functions, such as cell polarity, cell migration, cell survival, cytoskeletal dynamics, and vesicular traffic. Phosphoinositides actually play a central role at the interface between membranes and cytoskeletons and contribute to the identity of the cellular compartments by recruiting specific proteins. Increasing evidence indicates that several phosphoinositides, particularly PtdIns(4,5)P(2) , are essential for cytokinesis, notably after furrow ingression. The present knowledge about the specific phosphoinositides and phosphoinositide modifying-enzymes involved in cytokinesis will be first presented. The review of the current data will then show that furrow stability and cytokinesis abscission require that both phosphoinositide production and hydrolysis are regulated in space and time. Finally, I will further discuss recent mechanistic insights on how phosphoinositides regulate membrane trafficking and cytoskeletal remodeling for successful furrow ingression and intercellular bridge abscission. This will highlight unanticipated connections between cytokinesis and enzymes implicated in human diseases, such as the Lowe syndrome.

Phosphoinositide function in cytokinesis

In systems as diverse as yeast, slime mold and animal cells, the levels and distribution of phosphatidylinositol phosphates (PIPs) must be strictly regulated for successful cell cleavage. The precise mechanism by which PIPs function in this process remains unknown. Recent experiments are beginning to shed light on the cellular pathways in which PIPs make key contributions during cytokinesis. In particular, PIPs promote proper actin cytoskeletal organization and direct membrane trafficking in dividing cells. Future research will uncover temporal and spatial regulation of the different PIPs, thus elucidating their role in cytoskeletal and membrane events that drive cell cleavage.

Phosphatidylinositol 4-kinases: hostages harnessed to build panviral replication platforms

Several RNA viruses have recently been shown to hijack members of the host phosphatidylinositol (PtdIns) 4-kinase (PI4K) family of enzymes. They use PI4K to generate membranes enriched in phosphatidylinositide 4-phosphate (PtdIns4P or PI4P) lipids, which can be used as replication platforms. Viral replication machinery is assembled on these platforms as a supramolecular complex and PtdIns4P lipids regulate viral RNA synthesis. This article highlights these recent studies on the regulation of viral RNA synthesis by PtdIns4P lipids. It explores the potential mechanisms by which PtdIns4P lipids can contribute to viral replication and discusses the therapeutic potential of developing antiviral molecules that target host PI4Ks as a form of panviral therapy.

A calmodulin-related light chain from fission yeast that functions with myosin-I and PI 4-kinase

Fission yeast myosin-I (Myo1p) not only associates with calmodulin, but also employs a second light chain called Cam2p. cam2Δ cells exhibit defects in cell polarity and growth consistent with a loss of Myo1p function. Loss of Cam2p leads to a reduction in Myo1p levels at endocytic patches and a 50% drop in the rates of Myo1p-driven actin filament motility. Thus, Cam2p plays a significant role in Myo1p function. However, further studies indicated the existence of an additional Cam2p-binding partner. Cam2p was still present at cortical patches in myo1Δ cells (or in myo1-IQ2 mutants, which lack an intact Cam2p-binding motif), whereas a cam2 null (cam2Δ) suppressed cytokinesis defects of an essential light chain (ELC) mutant known to be impaired in binding to PI 4-kinase (Pik1p). Binding studies revealed that Cam2p and the ELC compete for Pik1p. Cortical localization of Cam2p in the myo1Δ background relied on its association with Pik1p, whereas overexpression studies indicated that Cam2p, in turn, contributes to Pik1p function. The fact that the Myo1p-associated defects of a cam2Δ mutant are more potent than those of a myo1-IQ2 mutant suggests that myosin light chains can contribute to actomyosin function both directly and indirectly (via phospholipid synthesis at sites of polarized growth).

Development of resources for the analysis of gene function in Pucciniomycotina red yeasts

The Pucciniomycotina is an important subphylum of basidiomycete fungi but with limited tools to analyze gene functions. Transformation protocols were established for a Sporobolomyces species (strain IAM 13481), the first Pucciniomycotina species with a completed draft genome sequence, to enable assessment of gene function through phenotypic characterization of mutant strains. Transformation markers were the URA3 and URA5 genes that enable selection and counter-selection based on uracil auxotrophy and resistance to 5-fluoroorotic acid. The wild type copies of these genes were cloned into plasmids that were used for transformation of Sporobolomyces sp. by both biolistic and Agrobacterium-mediated approaches. These resources have been deposited to be available from the Fungal Genetics Stock Center. To show that these techniques could be used to elucidate gene functions, the LEU1 gene was targeted for specific homologous replacement, and also demonstrating that this gene is required for the biosynthesis of leucine in basidiomycete fungi. T-DNA insertional mutants were isolated and further characterized, revealing insertions in genes that encode the homologs of Chs7, Erg3, Kre6, Kex1, Pik1, Sad1, Ssu1 and Tlg1. Phenotypic analysis of these mutants reveals both conserved and divergent functions compared with other fungi. Some of these strains exhibit reduced resistance to detergents, the antifungal agent fluconazole or sodium sulfite, or lower recovery from heat stress. While there are current experimental limitations for Sporobolomyces sp. such as the lack of Mendelian genetics for conventional mating, these findings demonstrate the facile nature of at least one Pucciniomycotina species for genetic manipulation and the potential to develop these organisms into new models for understanding gene function and evolution in the fungi.

Structure of a Ca2+-myristoyl switch protein that controls activation of a phosphatidylinositol 4-kinase in fission yeast

Neuronal calcium sensor (NCS) proteins transduce Ca2+ signals and are highly conserved from yeast to humans. We determined NMR structures of the NCS-1 homolog from fission yeast (Ncs1), which activates a phosphatidylinositol 4-kinase. Ncs1 contains an α-NH2-linked myristoyl group on a long N-terminal arm and four EF-hand motifs, three of which bind Ca2+, assembled into a compact structure. In Ca2+-free Ncs1, the N-terminal arm positions the fatty acyl chain inside a cavity near the C terminus. The C14 end of the myristate is surrounded by residues in the protein core, whereas its amide-linked (C1) end is flanked by residues at the protein surface. In Ca2+-bound Ncs1, the myristoyl group is extruded (Ca2+-myristoyl switch), exposing a prominent patch of hydrophobic residues that specifically contact phosphatidylinositol 4-kinase. The location of the buried myristate and structure of Ca2+-free Ncs1 are quite different from those in other NCS proteins. Thus, a unique remodeling of each NCS protein by its myristoyl group, and Ca2+-dependent unmasking of different residues, may explain how each family member recognizes distinct target proteins.

Cooperation between the septins and the actomyosin ring and role of a cell-integrity pathway during cell division in fission yeast

A major question about cytokinesis concerns the role of the septin proteins, which localize to the division site in all animal and fungal cells but are essential for cytokinesis only in some cell types. For example, in Schizosaccharomyces pombe, four septins localize to the division site, but deletion of the four genes produces only a modest delay in cell separation. To ask if the S. pombe septins function redundantly in cytokinesis, we conducted a synthetic-lethal screen in a septin-deficient strain and identified seven mutations. One mutation affects Cdc4, a myosin light chain that is an essential component of the cytokinetic actomyosin ring. Five others cause frequent cell lysis during cell separation and map to two loci. These mutations and their dosage suppressors define a signaling pathway (including Rho1 and a novel arrestin) for repairing cell-wall damage. The seventh mutation affects the poorly understood RNA-binding protein Scw1 and severely delays cell separation when combined either with a septin mutation or with a mutation affecting the septin-interacting, anillin-like protein Mid2, suggesting that Scw1 functions in a pathway parallel to that of the septins. Taken together, our results suggest that the S. pombe septins participate redundantly in one or more pathways that cooperate with the actomyosin ring during cytokinesis and that a septin defect causes septum defects that can be repaired effectively only when the cell-integrity pathway is intact.

Dual roles for the Drosophila PI 4-kinase four wheel drive in localizing Rab11 during cytokinesis

Successful completion of cytokinesis relies on addition of new membrane, and requires the recycling endosome regulator Rab11, which localizes to the midzone. Despite the critical role of Rab11 in this process, little is known about the formation and composition of Rab11-containing organelles. Here, we identify the phosphatidylinositol (PI) 4-kinase III beta four wheel drive (Fwd) as a key regulator of Rab11 during cytokinesis in Drosophila melanogaster spermatocytes. We show Fwd is required for synthesis of PI 4-phosphate (PI4P) on Golgi membranes and for formation of PI4P-containing secretory organelles that localize to the midzone. Fwd binds and colocalizes with Rab11 on Golgi membranes, and is required for localization of Rab11 in dividing cells. A kinase-dead version of Fwd also binds Rab11 and partially restores cytokinesis to fwd mutant flies. Moreover, activated Rab11 partially suppresses loss of fwd. Our data suggest Fwd plays catalytic and noncatalytic roles in regulating Rab11 during cytokinesis.