The Saccharomyces cerevisiae LSB6 gene encodes phosphatidylinositol 4-kinase activity

A type II phosphatidylinositol-4-kinase coordinates sorting of cargo polarizing by endocytic recycling

Depending on their phosphorylation status, derivatives of phosphatidylinositol play important roles in vesicle identity, recognition and intracellular trafficking processes. In eukaryotic cells, phosphatidylinositol-4 phosphate pools generated by specific kinases are key determinants of the conventional secretion pathways. Earlier work in yeast has classified phosphatidylinositol-4 kinases in two types, Stt4p and Pik1p belonging to type III and Lsb6p to type II, with distinct cellular localizations and functions. Eurotiomycetes appear to lack Pik1p homologues. In Aspergillus nidulans, unlike homologues in other fungi, AnLsb6 is associated to late Golgi membranes and when heterologously overexpressed, it compensates for the thermosensitive phenotype in a Saccharomyces cerevisiae pik1 mutant, whereas its depletion leads to disorganization of Golgi-associated PHOSBP-labelled membranes, that tend to aggregate dependent on functional Rab5 GTPases. Evidence provided herein, indicates that the single type II phosphatidylinositol-4 kinase AnLsb6 is the main contributor for decorating secretory vesicles with relevant phosphatidylinositol-phosphate species, which navigate essential cargoes following the route of apical polarization via endocytic recycling.

Regulation of phosphoinositide metabolism in Apicomplexan parasites

Phosphoinositides are a biologically essential class of phospholipids that contribute to organelle membrane identity, modulate membrane trafficking pathways, and are central components of major signal transduction pathways that operate on the cytosolic face of intracellular membranes in eukaryotes. Apicomplexans (such as Toxoplasma gondii and Plasmodium spp.) are obligate intracellular parasites that are important causative agents of disease in animals and humans. Recent advances in molecular and cell biology of Apicomplexan parasites reveal important roles for phosphoinositide signaling in key aspects of parasitosis. These include invasion of host cells, intracellular survival and replication, egress from host cells, and extracellular motility. As Apicomplexans have adapted to the organization of essential signaling pathways to accommodate their complex parasitic lifestyle, these organisms offer experimentally tractable systems for studying the evolution, conservation, and repurposing of phosphoinositide signaling. In this review, we describe the regulatory mechanisms that control the spatial and temporal regulation of phosphoinositides in the Apicomplexan parasites Plasmodium and T. gondii. We further discuss the similarities and differences presented by Apicomplexan phosphoinositide signaling relative to how these pathways are regulated in other eukaryotic organisms.

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.

Acetate and hypertonic stress stimulate vacuole membrane fission using distinct mechanisms

Vacuoles in plants and fungi play critical roles in cell metabolism and osmoregulation. To support these functions, vacuoles change their morphology, e.g. they fragment when these organisms are challenged with draught, high salinity or metabolic stress (e.g. acetate accumulation). In turn, morphology reflects an equilibrium between membrane fusion and fission that determines size, shape and copy number. By studying Saccharomyces cerevisiae and its vacuole as models, conserved molecular mechanisms responsible for fusion have been revealed. However, a detailed understanding of vacuole fission and how these opposing processes respond to metabolism or osmoregulation remain elusive. Herein we describe a new fluorometric assay to measure yeast vacuole fission in vitro. For proof–of–concept, we use this assay to confirm that acetate, a metabolic stressor, triggers vacuole fission and show it blocks homotypic vacuole fusion in vitro. Similarly, hypertonic stress induced by sorbitol or glucose caused robust vacuole fission in vitro whilst inhibiting fusion. Using wortmannin to inhibit phosphatidylinositol (PI) -kinases or rGyp1-46 to inactivate Rab–GTPases, we show that acetate stress likely targets PI signaling, whereas osmotic stress affects Rab signaling on vacuole membranes to stimulate fission. This study sets the stage for further investigation into the mechanisms that change vacuole morphology to support cell metabolism and osmoregulation.

Transmembrane phospholipid translocation mediated by Atg9 is involved in autophagosome formation

The mechanism of isolation membrane formation in autophagy is receiving intensive study. We recently found that Atg9 translocates phospholipids across liposomal membranes and proposed that this functionality plays an essential role in the expansion of isolation membranes. The distribution of phosphatidylinositol 3-phosphate in both leaflets of yeast autophagosomal membranes supports this proposal, but if Atg9-mediated lipid transport is crucial, symmetrical distribution in autophagosomes should be found broadly for other phospholipids. To test this idea, we analyzed the distributions of phosphatidylcholine, phosphatidylserine, and phosphatidylinositol 4-phosphate by freeze-fracture electron microscopy. We found that all these phospholipids are distributed with comparable densities in the two leaflets of autophagosomes and autophagic bodies. Moreover, de novo–synthesized phosphatidylcholine is incorporated into autophagosomes preferentially and shows symmetrical distribution in autophagosomes within 30 min after synthesis, whereas this symmetrical distribution is compromised in yeast expressing an Atg9 mutant. These results indicate that transbilayer phospholipid movement that is mediated by Atg9 is involved in the biogenesis of autophagosomes.

Lipid metabolism has been good to me

My career in research has flourished through hard work, supportive mentors, and outstanding mentees and collaborators. The Carman laboratory has contributed to the understanding of lipid metabolism through the isolation and characterization of key lipid biosynthetic enzymes as well as through the identification of the enzyme-encoding genes. Our findings from yeast have proven to be invaluable to understand regulatory mechanisms of human lipid metabolism. Several rewarding aspects of my career have been my service to the Journal of Biological Chemistry as an editorial board member and Associate Editor, the National Institutes of Health as a member of study sections, and national and international scientific meetings as an organizer. I advise early career scientists to not assume anything, acknowledge others’ accomplishments, and pay it forward.

Phosphoregulation of the autophagy machinery by kinases and phosphatases

Eukaryotic cells use post-translational modifications to diversify and dynamically coordinate the function and properties of protein networks within various cellular processes. For example, the process of autophagy strongly depends on the balanced action of kinases and phosphatases. Highly conserved from the budding yeast Saccharomyces cerevisiae to humans, autophagy is a tightly regulated self-degradation process that is crucial for survival, stress adaptation, maintenance of cellular and organismal homeostasis, and cell differentiation and development. Many studies have emphasized the importance of kinases and phosphatases in the regulation of autophagy and identified many of the core autophagy proteins as their direct targets. In this review, we summarize the current knowledge on kinases and phosphatases acting on the core autophagy machinery and discuss the relevance of phosphoregulation for the overall process of autophagy.

Arf1 directly recruits the Pik1-Frq1 PI4K complex to regulate the final stages of Golgi maturation

Proper Golgi complex function depends on the activity of Arf1, a GTPase whose effectors assemble and transport outgoing vesicles. Phosphatidylinositol 4-phosphate (PI4P) generated at the Golgi by the conserved PI 4-kinase Pik1 (PI4KIIIβ) is also essential for Golgi function, although its precise roles in vesicle formation are less clear. Arf1 has been reported to regulate PI4P production, but whether Pik1 is a direct Arf1 effector is not established. Using a combination of live-cell time-lapse imaging analyses, acute PI4P depletion experiments, and in vitro protein–protein interaction assays on Golgi-mimetic membranes, we present evidence for a model in which Arf1 initiates the final stages of Golgi maturation by tightly controlling PI4P production through direct recruitment of the Pik1-Frq1 PI4-kinase complex. This PI4P serves as a critical signal for AP-1 and secretory vesicle formation, the final events at maturing Golgi compartments. This work therefore establishes the regulatory and temporal context surrounding Golgi PI4P production and its precise roles in Golgi maturation.

The type II phosphoinositide 4-kinase FgLsb6 is important for the development and virulence of Fusarium graminearum

Fusarium graminearum is the main pathogenic fungus causing Fusarium head blight (FHB), which is a wheat disease with a worldwide prevalence. In eukaryotes, phosphatidylinositol 4-phosphate (PI4P), which participates in many physiological processes, is located primarily in different organelles, including the trans-Golgi network (TGN), plasma membrane and endosomes. Type II phosphatidylinositol 4-kinases (PI4Ks) are involved in regulating the production of PI4P in yeast, plants and mammalian cells. However, the role of these proteins in phytopathogenic fungi is not well understood. In this study, we characterized the type II PI4K protein FgLsb6 in F. graminearum, a homolog of Lsb6 in Saccharomyces cerevisiae. Unlike Lsb6, FgLsb6 localizes to the vacuoles and endosomes. The ΔFglsb6 mutant displayed defects in vegetative growth, deoxynivalenol (DON) production and pathogenicity. Furthermore, the ΔFglsb6 deletion mutant also exhibited increased resistance to osmotic, oxidative and cell wall stresses. Further analyses of the ΔFglsb6 mutant showed that it was defective in the generation of PI4P on endosomes and endocytosis. Collectively, our data suggest that the decreased vegetative growth and pathogenicity of ΔFglsb6 was due to the conservative roles of FgLsb6 in the generation of PI4P on endosomes and endocytosis.

Microautophagy in the yeast vacuole depends on the activities of phosphatidylinositol 4-kinases, Stt4p and Pik1p

Morphologically, the lipophagy in yeast cell mimics microautophagy, which includes a direct amendment of the vacuolar membrane that engulfs lipid droplets (LDs). The molecular mechanism of the membrane modifications that elicits microautophagy still remains elusive. In this study, an analysis of membrane lipid distribution at a nanoscale level showed that PtdIns(4)P is localized in the cytoplasmic leaflet of microautophagic vesicles, which are derived when the vacuole's membrane domains engulfed LDs both in the stationary phase and in acute nitrogen starvation. Furthermore, the PtdIns(4)P-positive raft-like domains engulf LDs through a microautophagic mechanism. When single temperature-conditional mutants of STT4 or PIK1 PtdIns 4-kinases were used, in the vacuole of STT4 and PIK1 mutant cells, microautophagic vesicles drastically decreased at restrictive temperatures, and the labeling density of PtdIns(4)P on the microautophagic vesicles and the sizes of the mutants' microautophagic vesicles also decreased. These results suggest that both Stt4p and Pik1p have important roles in the microautophagy of the vacuole in the stationary phase and under nitrogen starvation conditions.

Essential and distinct roles of phosphatidylinositol 4-kinases, Pik1p and Stt4p, in yeast autophagy

Autophagy is a degradative cellular pathway that protects eukaryotic cells from starvation/stress. Phosphatidylinositol 4-kinases, Pik1p and Stt4p, are indispensable for autophagy in budding yeast, but participation of PtdIns-4 kinases and their product, phosphatidylinositol 4-phosphate [PtdIns(4)P], is not understood. Nanoscale membrane lipid distribution analysis showed PtdIns(4)P is more abundant in yeast autophagosomes in the luminal leaflet than the cytoplasmic leaflet. PtdIns(4)P is confined to the cytoplasmic leaflet of autophagosomal inner and outer membranes in mammalian cells. Using temperature-conditional single PIK1 or STT4 PtdIns 4-kinase mutants, autophagic bodies in the vacuole of PIK1 and STT4 mutant cells dramatically decreased at restrictive temperatures, and the number of autophagosomes in the cytosol of PIK1 mutants cells was also decreased, whereas autophagosome levels of STT4 mutant cells were comparable to that of wild-type and STT4 mutant cells at permissive temperatures. Localization of PtdIns(4)P in the luminal leaflet in the biological membrane is a novel finding, and differences in PtdIns(4)P distribution suggest substantial differences between yeast and mammals. We also demonstrate in this study that Pik1p and Stt4p play essential roles in autophagosome formation and autophagosome-vacuole fusion in yeast cells, respectively.

An equal opportunity collaboration between lipid metabolism and proteins in the control of membrane trafficking in the trans-Golgi and endosomal systems

Recent years have witnessed the evolution of the cell biology of lipids into an extremely active area of investigation. Deciphering the involvement of lipid metabolism and lipid signaling in membrane trafficking pathways defines a major nexus of contemporary experimental activity on this front. Significant effort in that direction is invested in understanding the trans-Golgi network/endosomal system where unambiguous connections between membrane trafficking and inositol lipid and phosphatidylcholine metabolism were first discovered. However, powered by new advances in contemporary cell biology, the march of science is rapidly expanding that window of inquiry to include ever more diverse arms of the lipid metabolome, and to include other compartments of the secretory pathway as well.

Nanoscale domain formation of phosphatidylinositol 4-phosphate in the plasma and vacuolar membranes of living yeast cells

In budding yeast Saccharomyces cerevisiae, PtdIns(4)P serves as an essential signalling molecule in the Golgi complex, endosomal system, and plasma membrane, where it is involved in the control of multiple cellular functions via direct interactions with PtdIns(4)P-binding proteins. To analyse the distribution of PtdIns(4)P in yeast cells at a nanoscale level, we employed an electron microscopy technique that specifically labels PtdIns(4)P on the freeze-fracture replica of the yeast membrane. This method minimizes the possibility of artificial perturbation, because molecules in the membrane are physically immobilised in situ. We observed that PtdIns(4)P is localised on the cytoplasmic leaflet, but not the exoplasmic leaflet, of the plasma membrane, Golgi body, vacuole, and vesicular structure membranes. PtdIns(4)P labelling was not observed in the membrane of the endoplasmic reticulum, and in the outer and inner membranes of the nuclear envelope or mitochondria. PtdIns(4)P forms clusters of <100 nm in diameter in the plasma membrane and vacuolar membrane according to point pattern analysis of immunogold labelling. There are three kinds of compartments in the cytoplasmic leaflet of the plasma membrane. In the present study, we showed that PtdIns(4)P is specifically localised in the flat undifferentiated plasma membrane compartment. In the vacuolar membrane, PtdIns(4)P was concentrated in intramembrane particle (IMP)-deficient raft-like domains, which are tightly bound to lipid droplets, but not surrounding IMP-rich non-raft domains in geometrical IMP-distributed patterns in the stationary phase. This is the first report showing microdomain formations of PtdIns(4)P in the plasma membrane and vacuolar membrane of budding yeast cells at a nanoscale level, which will illuminate the functionality of PtdIns(4)P in each membrane.

Sac1, a lipid phosphatase at the interface of vesicular and nonvesicular transport

The lipid phosphatase Sac1 dephosphorylates phosphatidylinositol 4-phosphate (PI4P), thereby holding levels of this crucial membrane signaling molecule in check. Sac1 regulates multiple cellular processes, including cytoskeletal organization, membrane trafficking and cell signaling. Here, we review the structure and regulation of Sac1, its roles in cell signaling and development and its links to health and disease. Remarkably, many of the diverse roles attributed to Sac1 can be explained by the recent discovery of its requirement at membrane contact sites, where its consumption of PI4P is proposed to drive interorganelle transfer of other cellular lipids, thereby promoting normal lipid homeostasis within cells.

In vitro and in vivo characterization of modulation of the vacuolar cation channel TRPY1 from Saccharomyces cerevisiae

Saccharomyces cerevisiae possesses a transient receptor potential (TRP) channel homolog TRPY1 in its vacuolar membrane, considered to be an ancestral TRP channel. So far, studies have focused on the channel properties of TRPY1, but its regulation and physiologic role remained to be elucidated. Here, we investigated TRPY1 channel function in vitro and in vivo. Patch-clamp recording of TRPY1 in yeast vacuolar membranes showed that Ca²⁺ on the lumen side inhibited TRPY1-mediated channel activity, whereas luminal Zn²⁺ increased the currents. TRPY1 was activated in the presence of a reducing agent, 2-mercaptoethanol. The cysteine at position 624 was identified as the target for this activating action. This activation was independent of the presence of cytosolic Ca²⁺ . The amplitude of TRPY1-mediated current was reduced by addition of phosphatidylinositol 3-phosphate on the cytosolic side but not by phosphatidylinositol (PI) or phosphatidylinositol 3,5-phosphate. Measurement of the transient Ca²⁺ increase in response to hyper-osmotic shock in several yeast mutants defective in different steps of the PI phosphate biogenesis pathway supported this interpretation. Addition of a microtubule inhibitor strongly decreased the transient cytosolic Ca²⁺ increase upon hyper-osmotic shock. Taken together, the data indicate that the vacuolar TRPY1 Ca²⁺ channel mediates the perception of cytosolic signals that were induced by external changes in osmolarity, and participates in the modulation of cytosolic calcium signaling through Ca²⁺ release from the vacuole to maintain intracellular Ca²⁺ homeostasis in yeast.

Nicotiana alata Defensin Chimeras Reveal Differences in the Mechanism of Fungal and Tumor Cell Killing and an Enhanced Antifungal Variant

The plant defensin NaD1 is a potent antifungal molecule that also targets tumor cells with a high efficiency. We examined the features of NaD1 that contribute to these two activities by producing a series of chimeras with NaD2, a defensin that has relatively poor activity against fungi and no activity against tumor cells. All plant defensins have a common tertiary structure known as a cysteine-stabilized α-β motif which consists of an α helix and a triple-stranded β-sheet stabilized by four disulfide bonds. The chimeras were produced by replacing loops 1 to 7, the sequences between each of the conserved cysteine residues on NaD1, with the corresponding loops from NaD2. The loop 5 swap replaced the sequence motif (SKILRR) that mediates tight binding with phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and is essential for the potent cytotoxic effect of NaD1 on tumor cells. Consistent with previous reports, there was a strong correlation between PI(4,5)P2 binding and the tumor cell killing activity of all of the chimeras. However, this correlation did not extend to antifungal activity. Some of the loop swap chimeras were efficient antifungal molecules, even though they bound poorly to PI(4,5)P2, suggesting that additional mechanisms operate against fungal cells. Unexpectedly, the loop 1B swap chimera was 10 times more active than NaD1 against filamentous fungi. This led to the conclusion that defensin loops have evolved as modular components that combine to make antifungal molecules with variable mechanisms of action and that artificial combinations of loops can increase antifungal activity compared to that of the natural variants.

Characterization of the S. cerevisiae inp51 mutant links phosphatidylinositol 4,5-bisphosphate levels with lipid content, membrane fluidity and cold growth

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and its derivatives diphosphoinositol phosphates (DPIPs) play key signaling and regulatory roles. However, a direct function of these molecules in lipid and membrane homeostasis remains obscure. Here, we have studied the cold tolerance phenotype of yeast cells lacking the Inp51-mediated phosphoinositide-5-phosphatase. Genetic and biochemical approaches showed that increased metabolism of PI(4,5)P2 reduces the activity of the Pho85 kinase by increasing the levels of the DPIP isomer 1-IP7. This effect was key in the cold tolerance phenotype. Indeed, pho85 mutant cells grew better than the wild-type at 15 °C, and lack of this kinase abolished the inp51-mediated cold phenotype. Remarkably, reduced Pho85 function by loss of Inp51 affected the activity of the Pho85-regulated target Pah1, the yeast phosphatidate phosphatase. Cells lacking Inp51 showed reduced Pah1 abundance, derepression of an INO1-lacZ reporter, decreased content of triacylglycerides and elevated levels of phosphatidate, hallmarks of the pah1 mutant. However, the inp51 phenotype was not associated to low Pah1 activity since deletion of PAH1 caused cold sensitivity. In addition, the inp51 mutant exhibited features not shared by pah1, including a 40%-reduction in total lipid content and decreased membrane fluidity. These changes may influence the activity of membrane-anchored and/or associated proteins since deletion of INP51 slows down the transit to the vacuole of the fluorescent dye FM4-64. In conclusion, our work supports a model in which changes in the PI(4,5)P2 pool affect the 1-IP7 levels modulating the activity of Pho85, Pah1 and likely additional Pho85-controlled targets, and regulate lipid composition and membrane properties.

PI4KIIIα is required for cortical integrity and cell polarity during Drosophila oogenesis

Phosphoinositides regulate myriad cellular processes, acting as potent signaling molecules in conserved signaling pathways and as organelle gatekeepers that recruit effector proteins to membranes. Phosphoinositide-generating enzymes have been studied extensively in yeast and cultured cells, yet their roles in animal development are not well understood. Here, we analyze Drosophila melanogaster phosphatidylinositol 4-kinase IIIα (PI4KIIIα) during oogenesis. We demonstrate that PI4KIIIα is required for production of plasma membrane PtdIns4P and PtdIns(4,5)P2 and is crucial for actin organization, membrane trafficking and cell polarity. Female germ cells mutant for PI4KIIIα exhibit defects in cortical integrity associated with failure to recruit the cytoskeletal-membrane crosslinker Moesin and the exocyst subunit Sec5. These effects reflect a unique requirement for PI4KIIIα, as egg chambers from flies mutant for either of the other Drosophila PI4Ks, fwd or PI4KII, show Golgi but not plasma membrane phenotypes. Thus, PI4KIIIα is a vital regulator of a functionally distinct pool of PtdIns4P that is essential for PtdIns(4,5)P2-dependent processes in Drosophila development.

Cinderella story: PI4P goes from precursor to key signaling molecule

Phosphatidylinositol lipids are signaling molecules involved in nearly all aspects of cellular regulation. Production of phosphatidylinositol 4-phosphate (PI4P) has long been recognized as one of the first steps in generating poly-phosphatidylinositol phosphates involved in actin organization, cell migration, and signal transduction. In addition, progress over the last decade has brought to light independent roles for PI4P in membrane trafficking and lipid homeostasis. Here, we describe recent advances that reveal the breadth of processes regulated by PI4P, the spectrum of PI4P effectors, and the mechanisms of spatiotemporal control that coordinate crosstalk between PI4P and cellular signaling pathways.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

A wheat PI4K gene whose product possesses threonine autophophorylation activity confers tolerance to drought and salt in Arabidopsis

Phosphoinositides are involved in regulation of recruitment and activity of signalling proteins in cell membranes. Phosphatidylinositol (PI) 4-kinases (PI4Ks) generate PI4-phosphate the precursor of regulatory phosphoinositides. No type II PI4K research on the abiotic stress response has previously been reported in plants. A stress-inducible type II PI4K gene, named TaPI4KIIγ, was obtained by de novo transcriptome sequencing of drought-treated wheat (Triticum aestivum). TaPI4KIIγ, localized on the plasma membrane, underwent threonine autophosphorylation, but had no detectable lipid kinase activity. Interaction of TaPI4KIIγ with wheat ubiquitin fusion degradation protein (TaUDF1) indicated that it might be hydrolysed by the proteinase system. Overexpression of TaPI4KIIγ revealed that it could enhance drought and salt stress tolerance during seed germination and seedling growth. A ubdkγ7 mutant, identified as an orthologue of TaPI4KIIγ in Arabidopsis, was sensitive to salt, polyethylene glycol (PEG), and abscisic acid (ABA), and overexpression of TaPI4KIIγ in the ubdkγ7 mutant compensated stress sensitivity. TaPI4KIIγ promoted root growth in Arabidopsis, suggesting that TaPI4KIIγ might enhance stress resistance by improving root growth. Overexpression of TaPI4KIIγ led to an altered expression level of stress-related genes and changes in several physiological traits that made the plants more tolerant to stress. The results provided evidence that overexpression of TaPI4KIIγ could improve drought and salt tolerance.

Phosphatidylinositol 4-kinases are required for autophagic membrane trafficking

Macroautophagy (hereafter autophagy) is a degradative cellular pathway that protects eukaryotic cells from stress, starvation, and microbial infection. This process must be tightly controlled because too little or too much autophagy can be deleterious to cellular physiology. The phosphatidylinositol (PtdIns) 3-kinase Vps34 is a lipid kinase that regulates autophagy, but the role of other PtdIns kinases has not been examined. Here we demonstrate a role for PtdIns 4-kinases and PtdIns4P 5-kinases in selective and nonselective types of autophagy in yeast. The PtdIns 4-kinase Pik1 is involved in Atg9 trafficking through the Golgi and is involved in both nonselective and selective types of autophagy, whereas the PtdIns4P 5-kinase Mss4 is specifically involved in mitophagy but not nonselective autophagy. Our data indicate that phosphoinositide kinases have multiple roles in the regulation of autophagic pathways.

Type II phosphatidylinositol 4-kinase regulates trafficking of secretory granule proteins in Drosophila

Type II phosphatidylinositol 4-kinase (PI4KII) produces the lipid phosphatidylinositol 4-phosphate (PI4P), a key regulator of membrane trafficking. Here, we generated genetic models of the sole Drosophila melanogaster PI4KII gene. A specific requirement for PI4KII emerged in larval salivary glands. In PI4KII mutants, mucin-containing glue granules failed to reach normal size, with glue protein aberrantly accumulating in enlarged Rab7-positive late endosomes. Presence of PI4KII at the Golgi and on dynamic tubular endosomes indicated two distinct foci for its function. First, consistent with the established role of PI4P in the Golgi, PI4KII is required for sorting of glue granule cargo and the granule-associated SNARE Snap24. Second, PI4KII also has an unforeseen function in late endosomes, where it is required for normal retromer dynamics and for formation of tubular endosomes that are likely to be involved in retrieving Snap24 and Lysosomal enzyme receptor protein (Lerp) from late endosomes to the trans-Golgi network. Our genetic analysis of PI4KII in flies thus reveals a novel role for PI4KII in regulating the fidelity of granule protein trafficking in secretory tissues.

Metabolism and regulation of glycerolipids in the yeast Saccharomyces cerevisiae

Due to its genetic tractability and increasing wealth of accessible data, the yeast Saccharomyces cerevisiae is a model system of choice for the study of the genetics, biochemistry, and cell biology of eukaryotic lipid metabolism. Glycerolipids (e.g., phospholipids and triacylglycerol) and their precursors are synthesized and metabolized by enzymes associated with the cytosol and membranous organelles, including endoplasmic reticulum, mitochondria, and lipid droplets. Genetic and biochemical analyses have revealed that glycerolipids play important roles in cell signaling, membrane trafficking, and anchoring of membrane proteins in addition to membrane structure. The expression of glycerolipid enzymes is controlled by a variety of conditions including growth stage and nutrient availability. Much of this regulation occurs at the transcriptional level and involves the Ino2–Ino4 activation complex and the Opi1 repressor, which interacts with Ino2 to attenuate transcriptional activation of UAS_INO-containing glycerolipid biosynthetic genes. Cellular levels of phosphatidic acid, precursor to all membrane phospholipids and the storage lipid triacylglycerol, regulates transcription of UAS_INO-containing genes by tethering Opi1 to the nuclear/endoplasmic reticulum membrane and controlling its translocation into the nucleus, a mechanism largely controlled by inositol availability. The transcriptional activator Zap1 controls the expression of some phospholipid synthesis genes in response to zinc availability. Regulatory mechanisms also include control of catalytic activity of glycerolipid enzymes by water-soluble precursors, products and lipids, and covalent modification of phosphorylation, while in vivo function of some enzymes is governed by their subcellular location. Genome-wide genetic analysis indicates coordinate regulation between glycerolipid metabolism and a broad spectrum of metabolic pathways.

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.

VPS13 regulates membrane morphogenesis during sporulation in Saccharomyces cerevisiae

The hereditary disorders chorea acanthocytosis and Cohen syndrome are caused by mutations in different members of a family of genes that are orthologs of yeast VPS13. In vegetatively growing yeast, VPS13 is involved in the delivery of proteins to the vacuole. During sporulation, VPS13 is important for formation of the prospore membrane that encapsulates the daughter nuclei to give rise to spores. We report that VPS13 is required for multiple aspects of prospore membrane morphogenesis. VPS13 (1) promotes expansion of the prospore membrane through regulation of phosphatidylinositol phosphates, which in turn activate the phospholipase D, Spo14; (2) is required for a late step in cytokinesis that gives rise to spores; and (3) regulates a membrane-bending activity that generates intralumenal vesicles. These results demonstrate that Vps13 plays a broader role in membrane biology than previously known, which could have important implications for the functions of VPS13 orthologs in humans.

Growth and metabolic control of lipid signalling at the Golgi

PtdIns4P is a key regulator of the secretory pathway and plays an essential role in trafficking from the Golgi. Our recent work demonstrated that spatial control of PtdIns4P at the ER (endoplasmic reticulum) and Golgi co-ordinates secretion with cell growth. The central elements of this regulation are specific phosphoinositide 4-kinases and the phosphoinositide phosphatase Sac1. Growth-dependent translocation of Sac1 between the ER and Golgi modulates the levels of PtdIns4P and anterograde traffic at the Golgi. In yeast, this mechanism is largely dependent on the availability of glucose, but our recent results in mammalian cells suggest that Sac1 phosphatases play evolutionarily conserved roles in the growth control of secretion. Sac1 lipid phosphatase plays also an essential role in the spatial control of PtdIns4P at the Golgi complex. A restricted pool of PtdIns4P at the TGN (trans-Golgi network) is required for Golgi integrity and for proper lipid and protein sorting. In mammalian cells, the stress-activated MAPK (mitogen-activated protein kinase) p38 appears to play a critical role in transmitting nutrient signals to the phosphoinositide signalling machinery at the ER and Golgi. These results suggest that temporal and spatial integration of metabolic and lipid signalling networks at the Golgi is required for controlling the secretory pathway.

The phosphatidylinositol 4-kinases: don't call it a comeback

Phosphatidylinositol 4-phosphate (PtdIns4P) is a quantitatively minor membrane phospholipid which is the precursor of PtdIns(4,5)P (2) in the classical agonist-regulated phospholipase C signalling pathway. However, PtdIns4P also governs the recruitment and function of numerous trafficking molecules, principally in the Golgi complex. The majority of phosphoinositides (PIs) phosphorylated at the D4 position of the inositol headgroup are derived from PtdIns4P and play roles in a diverse array of fundamental cellular processes including secretion, cell migration, apoptosis and mitogenesis; therefore, PtdIns4P biosynthesis can be regarded as key point of regulation in many PI-dependent processes.Two structurally distinct sequence families, the type II and type III PtdIns 4-kinases, are responsible for PtdIns4P synthesis in eukaryotic organisms. These important proteins are differentially expressed, localised and regulated by distinct mechanisms, indicating that the enzymes perform non-redundant roles in trafficking and signalling. In recent years, major advances have been made in our understanding of PtdIns4K biology and here we summarise current knowledge of PtdIns4K structure, function and regulation.

Phosphoinositide signaling during membrane transport in Saccharomyces cerevisiae

Phosphatidylinositol (PI) is distinct from other phospholipids, possessing a head group that can be modified by phosphorylation at multiple positions to generate unique signaling molecules collectively known as phosphoinositides. The set of kinases and phosphatases that regulate PI metabolism are conserved throughout eukaryotic evolution, and numerous studies have demonstrated that phosphoinositides regulate a diverse spectrum of cellular processes, including vesicle transport, cell proliferation, and cytoskeleton organization. Over the past two decades, nearly all PI derivatives have been shown to interact directly with cellular proteins to affect their localization and/or activity. Additionally, there is growing evidence, which suggests that phosphoinositides may also affect local membrane topology. Here, we focus on the role of phosphoinositides in membrane trafficking and underscore the significant role that yeast has played in the field.

PI4P and Rab inputs collaborate in myosin-V-dependent transport of secretory compartments in yeast

Cell polarity involves transport of specific membranes and macromolecules at the right time to the right place. In budding yeast, secretory vesicles are transported by the myosin-V Myo2p to sites of cell growth. We show that phosphatidylinositol 4-phosphate (PI4P) is present in late secretory compartments and is critical for their association with, and transport by, Myo2p. Further, the trans-Golgi network Rab Ypt31/32p and secretory vesicle Rab Sec4p each bind directly, but distinctly, to Myo2p, and these interactions are also required for secretory compartment transport. Enhancing the interaction of Myo2p with PI4P bypasses the requirement for interaction with Ypt31/32p and Sec4p. Together with additional genetic data, the results indicate that Rab proteins and PI4P collaborate in the association of secretory compartments with Myo2p. Thus, we show that a coincidence detection mechanism coordinates inputs from PI4P and the appropriate Rab for secretory compartment transport.

Interruption of inositol sphingolipid synthesis triggers Stt4p-dependent protein kinase C signaling

The protein kinase C (PKC)-MAPK signaling cascade is activated and is essential for viability when cells are starved for the phospholipid precursor inositol. In this study, we report that inhibiting inositol-containing sphingolipid metabolism, either by inositol starvation or treatment with agents that block sphingolipid synthesis, triggers PKC signaling independent of sphingoid base accumulation. Under these same growth conditions, a fluorescent biosensor that detects the necessary PKC signaling intermediate, phosphatidylinositol (PI)-4-phosphate (PI4P), is enriched on the plasma membrane. The appearance of the PI4P biosensor on the plasma membrane correlates with PKC activation and requires the PI 4-kinase Stt4p. Like other mutations in the PKC-MAPK pathway, mutants defective in Stt4p and the PI4P 5-kinase Mss4p, which generates phosphatidylinositol 4,5-bisphosphate, exhibit inositol auxotrophy, yet fully derepress INO1, encoding inositol-3-phosphate synthase. These observations suggest that inositol-containing sphingolipid metabolism controls PKC signaling by regulating access of the signaling lipids PI4P and phosphatidylinositol 4,5-bisphosphate to effector proteins on the plasma membrane.

Phosphoinositide phosphatases in cell biology and disease

Phosphoinositides are essential signaling molecules linked to a diverse array of cellular processes in eukaryotic cells. The metabolic interconversions of these phospholipids are subject to exquisite spatial and temporal regulation executed by arrays of phosphatidylinositol (PtdIns) and phosphoinositide-metabolizing enzymes. These include PtdIns- and phosphoinositide-kinases that drive phosphoinositide synthesis, and phospholipases and phosphatases that regulate phosphoinositide degradation. In the past decade, phosphoinositide phosphatases have emerged as topics of particular interest. This interest is driven by the recent appreciation that these enzymes represent primary mechanisms for phosphoinositide degradation, and because of their ever-increasing connections with human diseases. Herein, we review the biochemical properties of six major phosphoinositide phosphatases, the functional involvements of these enzymes in regulating phosphoinositide metabolism, the pathologies that arise from functional derangements of individual phosphatases, and recent ideas concerning the involvements of phosphoinositide phosphatases in membrane traffic control.

At the poles across kingdoms: phosphoinositides and polar tip growth

Phosphoinositides (PIs) are minor, but essential phospholipid constituents of eukaryotic membranes, and are involved in the regulation of various physiological processes. Recent genetic and cell biological advances indicate that PIs play important roles in the control of polar tip growth in plant cells. In root hairs and pollen tubes, PIs control directional membrane trafficking required for the delivery of cell wall material and membrane area to the growing tip. So far, the exact mechanisms by which PIs control polarity and tip growth are unresolved. However, data gained from the analysis of plant, fungal and animal systems implicate PIs in the control of cytoskeletal dynamics, ion channel activity as well as vesicle trafficking. The present review aims at giving an overview of PI roles in eukaryotic cells with a special focus on functions pertaining to the control of cell polarity. Comparative screening of plant and fungal genomes suggests diversification of the PI system with increasing organismic complexity. The evolutionary conservation of the PI system among eukaryotic cells suggests a role for PIs in tip growing cells in models where PIs so far have not been a focus of attention, such as fungal hyphae.

Essential role for Schizosaccharomyces pombe pik1 in septation

Background

Schizosaccharomyces pombe pik1 encodes a phosphatidylinositol 4-kinase, reported to bind Cdc4, but not Cdc4^G107S.

Principal Findings

Gene deletion revealed that pik1 is essential. In cells with pik1 deleted, ectopic expression of a loss-of-function allele, created by fusion to a temperature-sensitive dihydrofolate reductase, allowed normal cell proliferation at 25°C. At 36°C, cells arrested with abnormally thick, misplaced or supernumerary septa, indicating a defect late in septation. In addition to being Golgi associated, ectopically expressed GFP-tagged Pik1 was observed at the medial cell plane late in cytokinesis. New alleles, created by site-directed mutagenesis, were expressed ectopically. Lipid kinase and Cdc4-binding activity assays were performed. Pik1^D709A was kinase-dead, but bound Cdc4. Pik1^R838A did not bind Cdc4, but was an active kinase. Genomic integration of these substitutions in S. pombe and complementation studies in Saccharomyces cerevisiae pik1-101 cells revealed that D709 is essential in both cases while R838 is dispensable. In S. pombe, ectopic expression of pik1 was dominantly lethal; while, pik1^D709A,R838A was innocuous, pik1^R838A was almost innocuous, and pik1^D709A produced partial lethality and septation defects. The pik1 ectopic expression lethal phenotype was suppressed in cdc4^G107S. Thus, D709 is essential for kinase activity and septation.

Conclusions

Pik1 kinase activity is required for septation. The Pik1 R838 residue is required for important protein-protein interactions, possibly with Cdc4.

Loss of phosphatidylinositol 4-kinase 2alpha activity causes late onset degeneration of spinal cord axons

Phosphoinositide (PI) lipids are intracellular membrane signaling intermediates and effectors produced by localized PI kinase and phosphatase activities. Although many signaling roles of PI kinases have been identified in cultured cell lines, transgenic animal studies have produced unexpected insight into the in vivo functions of specific PI 3- and 5-kinases, but no mammalian PI 4-kinase (PI4K) knockout has previously been reported. Prior studies using cultured cells implicated the PI4K2alpha isozyme in diverse functions, including receptor signaling, ion channel regulation, endosomal trafficking, and regulated secretion. We now show that despite these important functions, mice lacking PI4K2alpha kinase activity initially appear normal. However, adult Pi4k2a(GT/GT) animals develop a progressive neurological disease characterized by tremor, limb weakness, urinary incontinence, and premature mortality. Histological analysis of aged Pi4k2a(GT/GT) animals revealed lipofuscin-like deposition and gliosis in the cerebellum, and loss of Purkinje cells. Peripheral nerves are essentially normal, but massive axonal degeneration was found in the spinal cord in both ascending and descending tracts. These results reveal a previously undescribed role for aberrant PI signaling in neurological disease that resembles autosomal recessive hereditary spastic paraplegia.

Quantification of multiple phosphatidylinositol 4-kinase isozyme activities in cell extracts

A wide spectrum of intracellular signaling events mediated by up to seven different phosphorylated forms of phosphatidylinositol (PtdIns) occurs in all eukaryotic cells. The activities of multiple, nondegenerate PI kinases and phosphatases control these signaling events. The PI 4-kinase isozymes account for the major PI kinase activity in many different cell types, and the activity of each isozyme is differentially regulated. The ability to measure and distinguish the activity of individual enzymes is therefore important and forms the subject of the methods in this chapter. We describe the use and application of a versatile radiometric assay to measuring PI 4-kinase activity in a variety of biochemical contexts, from purified enzymes to membrane preparations and permeabilized cells. Until a suitable nonradioactive reagent becomes available, this assay is destined to remain the most widely used method.

The multiple roles of PtdIns(4)P -- not just the precursor of PtdIns(4,5)P2

The phosphoinositides (PIs) are membrane phospholipids that actively operate at membrane-cytosol interfaces through the recruitment of a number of effector proteins. In this context, each of the seven different PI species represents a topological determinant that can establish the nature and the function of the membrane where it is located. Phosphatidylinositol 4-phosphate (PtdIns(4)P) is the most abundant of the monophosphorylated inositol phospholipids in mammalian cells, and it is produced by D-4 phosphorylation of the inositol ring of PtdIns. PtdIns(4)P can be further phosphorylated to PtdIns(4,5)P(2) by PtdIns(4)P 5-kinases and, indeed, PtdIns(4)P has for many years been considered to be just the precursor of PtdIns(4,5)P(2). Over the last decade, however, a large body of evidence has accumulated that shows that PtdIns(4)P is, in its own right, a direct regulator of important cell functions. The subcellular localisation of the PtdIns(4)P effectors initially led to the assumption that the bulk of this lipid is present in the membranes of the Golgi complex. However, the existence and physiological relevance of ;non-Golgi pools' of PtdIns(4)P have now begun to be addressed. The aim of this Commentary is to describe our present knowledge of PtdIns(4)P metabolism and the molecular machineries that are directly regulated by PtdIns(4)P within and outside of the Golgi complex.

Modulation of sphingolipid metabolism by the phosphatidylinositol-4-phosphate phosphatase Sac1p through regulation of phosphatidylinositol in Saccharomyces cerevisiae

Sphingolipids and phosphoinositides both play signaling roles in Saccharomyces cerevisiae. Although previous data indicate independent functions for these two classes of lipids, recent genetic studies have suggested interactions between phosphatidylinositol (PtdIns) phosphate effectors and sphingolipid biosynthetic enzymes. The present study was undertaken to further define the effects of phosphatidylinositol 4-phosphate (PtdIns(4)P) metabolism on cell sphingolipid metabolism. The data presented indicate that deletion of SAC1, a gene encoding a PtdIns(4)P phosphatase, increased levels of most sphingolipid species, including sphingoid bases, sphingoid base phosphates, and phytoceramide. In contrast, sac1Delta dramatically reduced inositol phosphosphingolipids, which result from the addition of a PtdIns-derived phosphoinositol head group to ceramides through Aur1p. Deletion of SAC1 decreased PtdIns dramatically in both steady-state and pulse labeling studies, suggesting that the observed effects on sphingolipids may result from modulation of the availability of PtdIns as a substrate for Aur1p. Supporting this hypothesis, acute attenuation of PtdIns(4)P production through Stt4p immediately increased PtdIns and subsequently reduced sphingoid bases. This reduction was overcome by the inhibition of Aur1p. Moreover, modulation of sphingoid bases through perturbation of PtdIns(4)P metabolism initiated sphingolipid-dependent biological effects, supporting the biological relevance for this route of regulating sphingolipids. These findings suggest that, in addition to potential signaling effects of PtdInsP effectors on sphingolipid metabolism, PtdIns kinases may exert substantial effects on cell sphingolipid profiles at a metabolic level through modulation of PtdIns available as a substrate for complex sphingolipid synthesis.

Imaging phosphatidylinositol 4-phosphate dynamics in living plant cells

Polyphosphoinositides represent a minor group of phospholipids, accounting for less than 1% of the total. Despite their low abundance, these molecules have been implicated in various signalling and membrane trafficking events. Phosphatidylinositol 4-phosphate (PtdIns4P) is the most abundant polyphosphoinositide. (32)Pi-labelling studies have shown that the turnover of PtdIns4P is rapid, but little is known about where in the cell or plant this occurs. Here, we describe the use of a lipid biosensor that monitors PtdIns4P dynamics in living plant cells. The biosensor consists of a fusion between a fluorescent protein and a lipid-binding domain that specifically binds PtdIns4P, i.e. the pleckstrin homology domain of the human protein phosphatidylinositol-4-phosphate adaptor protein-1 (FAPP1). YFP-PH(FAPP1) was expressed in four plant systems: transiently in cowpea protoplasts, and stably in tobacco BY-2 cells, Medicago truncatula roots and Arabidopsis thaliana seedlings. All systems allowed YFP-PH(FAPP1) expression without detrimental effects. Two distinct fluorescence patterns were observed: labelling of motile punctate structures and the plasma membrane. Co-expression studies with organelle markers revealed strong co-labelling with the Golgi marker STtmd-CFP, but not with the endocytic/pre-vacuolar marker GFP-AtRABF2b. Co-expression with the Ptdins3P biosensor YFP-2 x FYVE revealed totally different localization patterns. During cell division, YFP-PH(FAPP1) showed strong labelling of the cell plate, but PtdIns3P was completely absent from the newly formed cell membrane. In root hairs of M. truncatula and A. thaliana, a clear PtdIns4P gradient was apparent in the plasma membrane, with the highest concentration in the tip. This only occurred in growing root hairs, indicating a role for PtdIns4P in tip growth.

Phosphoinositides in plants: novel functions in membrane trafficking

Tight regulation of membrane trafficking is crucial to the proper maintenance of the endomembrane trafficking system of eukaryotic cells. Distinct organelles must maintain their identities while at the same time continuously accepting, sorting, and exchanging membrane and luminal cargo constituents. Additionally, many of these organelles differentiate specialized subdomains containing distinct sets of lipids and proteins and restrict certain aspects of membrane trafficking to these regions of the organelle. Phosphoinositides (PIs) are a class of membrane lipids that have emerged as key components in some of these membrane trafficking events. The ability of these lipids to be rapidly produced, modified, and hydrolyzed by distinct classes of phosphatidylinositol (PtdIns) kinases, phosphatases, and phospholipases, allows for their use as finely tuned spatial and temporal landmarks for organelle and sub-organelle domains. In this review we will attempt to highlight some of the recent studies of the roles of this class of lipids in plant membrane trafficking, particularly on their important roles in polarized membrane trafficking in plants.

Ypp1/YGR198w plays an essential role in phosphoinositide signalling at the plasma membrane

Phosphoinositide signalling through the eukaryotic plasma membrane makes essential contributions to many processes, including remodelling of the actin cytoskeleton, vesicle trafficking and signalling from the cell surface. A proteome-wide screen performed in Saccharomyces cerevisiae revealed that Ypp1 interacts physically with the plasma-membrane-associated phosphoinositide 4-kinase, Stt4. In the present study, we demonstrate that phenotypes of ypp1 and stt4 conditional mutants are identical, namely osmoremedial temperature sensitivity, hypersensitivity to cell wall destabilizers and defective organization of actin. We go on to show that overexpression of STT4 suppresses the temperature-sensitive growth defect of ypp1 mutants. In contrast, overexpression of genes encoding the other two phosphoinositide 4-kinases in yeast, Pik1 and Lsb6, do not suppress this phenotype. This implies a role for Ypp1 in Stt4-dependent events at the plasma membrane, as opposed to a general role in overall metabolism of phosphatidylinositol 4-phosphate. Use of a pleckstrin homology domain sensor reveals that there are substantially fewer plasma-membrane-associated 4-phosphorylated phosphoinositides in ypp1 mutants in comparison with wild-type cells. Furthermore, in vivo labelling with [3H]inositol indicates a dramatic reduction in the level of phosphatidylinositol 4-phosphate in ypp1 mutants. This is the principal cause of lethality under non-permissive conditions in ypp1 mutants, as limiting the activity of the Sac1 phosphoinositide 4-phosphate phosphatase leads to restoration of viability. Additionally, the endocytic defect associated with elevated levels of PtdIns4P in sac1Δ cells is restored in combination with a ypp1 mutant, consistent with the opposing effects that these two mutations have on levels of this phosphoinositide.

Disrupting vesicular trafficking at the endosome attenuates transcriptional activation by Gcn4

The late endosome (MVB) plays a key role in coordinating vesicular transport of proteins between the Golgi complex, vacuole/lysosome, and plasma membrane. We found that deleting multiple genes involved in vesicle fusion at the MVB (class C/D vps mutations) impairs transcriptional activation by Gcn4, a global regulator of amino acid biosynthetic genes, by decreasing the ability of chromatin-bound Gcn4 to stimulate preinitiation complex assembly at the promoter. The functions of hybrid activators with Gal4 or VP16 activation domains are diminished in class D mutants as well, suggesting a broader defect in activation. Class E vps mutations, which impair protein sorting at the MVB, also decrease activation by Gcn4, provided they elicit rapid proteolysis of MVB cargo proteins in the aberrant late endosome. By contrast, specifically impairing endocytic trafficking from the plasma membrane, or vesicular transport to the vacuole, has a smaller effect on Gcn4 function. Thus, it appears that decreasing cargo proteins in the MVB through impaired delivery or enhanced degradation, and not merely the failure to transport cargo properly to the vacuole or downregulate plasma membrane proteins by endocytosis, is required to attenuate substantially transcriptional activation by Gcn4.

Characterization of a new family of protein kinases from Arabidopsis containing phosphoinositide 3/4-kinase and ubiquitin-like domains

At least two of the genes predicted to encode type II PI4K (phosphoinositide 4-kinase) in Arabidopsis thaliana (thale cress), namely AtPI4Kγ4 and AtPI4Kγ7, encode enzymes with catalytic properties similar to those of members of the PIKK (phosphoinositide kinase-related kinase) family. AtPI4Kγ4 and AtPI4Kγ7 undergo autophosphorylation and phosphorylate serine/threonine residues of protein substrates, but have no detectable lipid kinase activity. AtPI4Kγ4 and AtPI4Kγ7 are members of a subset of five putative AtPI4Ks that contain N-terminal UBL (ubiquitin-like) domains. In vitro analysis of AtPI4Kγ4 indicates that it interacts directly with, and phosphorylates, two proteins involved in the ubiquitin–proteasome system, namely UFD1 (ubiquitin fusion degradation 1) and RPN10 (regulatory particle non-ATPase 10). On the basis of the present results, we propose that AtPI4Kγ4 and AtPI4Kγ7 should be designated UbDKγ4 and UbDKγ7 (ubiquitin-like domain kinases γ4 and γ7). These UBL-domain-containing AtPI4Ks correspond to a new PIKK subfamily of protein kinases. Furthermore, UFD1 and RPN10 phosphorylation represents an additional mechanism by which their function can be regulated.

Farnesol-mediated inhibition ofCandida albicansyeast growth and rescue by a diacylglycerol analogue

During Candida albicans yeast cell growth to early stationary phase, metabolites accumulate in the medium, including the quorum-sensing molecule farnesol. We found that besides germ tube inhibition, 40 microM farnesol also inhibited C. albicans yeast growth under yeast growth permissive conditions. Consistent with this observation, transcriptional analysis of yeast cells resuspended in fresh medium with 40 microM farnesol revealed that genes involved in hyphal formation, GTPase activation, mitosis and DNA replication were downregulated many-fold. Farnesol-mediated inhibition of yeast growth was dependent on the growth phase of the C. albicans cells. The growth defect was relieved by addition of a diacylglycerol analogue, implicating phosphatidylinositol signalling in the delay. Although diacylglycerol is an activator of protein kinase C (PKC) in mammalian cells, there is some question about activation of fungal PKCs. A mutant strain deleted for PKC1 responded to farnesol and the diacylglycerol analogue similar to wild-type, suggesting that PKC is not the target of the diacylglycerol analogue.

Inactivation of the Phosphoinositide Phosphatases Sac1p and Inp54p Leads to Accumulation of Phosphatidylinositol 4,5-Bisphosphate on Vacuole Membranes and Vacuolar Fusion Defects

Phosphoinositides direct membrane trafficking, facilitating the recruitment of effectors to specific membranes. In yeast phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) isproposed to regulate vacuolar fusion; however, in intact cells this phosphoinositide can only be detected at the plasma membrane. In Saccharomyces cerevisiae the 5-phosphatase, Inp54p, dephosphorylates PtdIns(4,5)P2 forming PtdIns(4)P, a substrate for the phosphatase Sac1p, which hydrolyzes (PtdIns(4)P). We investigated the role these phosphatases in regulating PtdIns(4,5)P2 subcellular distribution. PtdIns(4,5)P2 bioprobes exhibited loss of plasma membrane localization and instead labeled a subset of fragmented vacuoles in Deltasac1 Deltainp54 and sac1ts Deltainp54 mutants. Furthermore, sac1ts Deltainp54 mutants exhibited vacuolar fusion defects, which were rescued by latrunculin A treatment, or by inactivation of Mss4p, a PtdIns(4)P 5-kinase that synthesizes plasma membrane PtdIns(4,5)P2. Under these conditions PtdIns(4,5)P2 was not detected on vacuole membranes, and vacuole morphology was normal, indicating vacuolar PtdIns(4,5)P2 derives from Mss4p-generated plasma membrane PtdIns(4,5)P2. Deltasac1 Deltainp54 mutants exhibited delayed carboxypeptidase Y sorting, cargo-selective secretion defects, and defects in vacuole function. These studies reveal PtdIns(4,5)P2 hydrolysis by lipid phosphatases governs its spatial distribution, and loss of phosphatase activity may result in PtdIns(4,5)P2 accumulation on vacuole membranes leading to vacuolar fragmentation/fusion defects.

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.

Phosphatidylinositol 4-Phosphate Is Required for Translation Initiation in Saccharomyces cerevisiae

The small natural product wortmannin inhibits protein synthesis by modulating several phosphatidylinositol (PI) metabolic pathways. A primary target of wortmannin in yeast is the plasma membrane-associated PI 4-kinase (PI4K) Stt4p, which is required for actin cytoskeleton organization. Here we show that wortmannin treatment or inactivation of Stt4p, but not disorganization of the actin cytoskeleton per se, leads to a rapid attenuation of translation initiation. Interestingly, inactivation of Pik1p, a wortmannin-insensitive, functionally distinct PI4K, implicated in the regulation of Golgi functions and secretion, also results in severe translation initiation defects with a marked increase of the phosphorylation of the translation initiation factor eIF2alpha. Because wortmannin largely phenocopies the effects of rapamycin (e.g. it triggers nuclear accumulation of Gln3p), it likely also inhibits the PI kinase-related, target of rapamycin (TOR) kinases. Importantly, however, neither inactivation of Stt4p nor Pik1p significantly affects TOR-controlled readouts other than translation initiation, indicating that these PI4Ks do not simply function upstream of TOR. Together, our results reveal the existence of a novel translation initiation control mechanism in yeast that is tightly coupled to the synthesis of distinct PI4P pools.