The CLS2 gene encodes a protein with multiple membrane-spanning domains that is important Ca2+ tolerance in yeast

Golgi apparatus-localized CATION CALCIUM EXCHANGER4 promotes osmotolerance of Arabidopsis

Calcium (Ca2+) is a major ion in living organisms, where it acts as a second messenger for various biological phenomena. The Golgi apparatus retains a higher Ca2+ concentration than the cytosol and returns cytosolic Ca2+ to basal levels after transient elevation in response to environmental stimuli such as osmotic stress. However, the Ca2+ transporters localized in the Golgi apparatus of plants have not been clarified. We previously found that a wild-type (WT) salt-tolerant Arabidopsis (Arabidopsis thaliana) accession, Bu-5, showed osmotic tolerance after salt acclimatization, whereas the Col-0 WT did not. Here, we isolated a Bu-5 background mutant gene, acquired osmotolerance-defective 6 (aod6), which reduces tolerance to osmotic, salt, and oxidative stresses, with a smaller plant size than the WT. The causal gene of the aod6 mutant encodes CATION CALCIUM EXCHANGER4 (CCX4). The aod6 mutant was more sensitive than the WT to both deficient and excessive Ca2+. In addition, aod6 accumulated higher Ca2+ than the WT in the shoots, suggesting that Ca2+ homeostasis is disturbed in aod6. CCX4 expression suppressed the Ca2+ hypersensitivity of the csg2 (calcium sensitive growth 2) yeast (Saccharomyces cerevisiae) mutant under excess CaCl2 conditions. We also found that aod6 enhanced MAP kinase 3/6 (MPK3/6)-mediated immune responses under osmotic stress. Subcellular localization analysis of mGFP-CCX4 showed GFP signals adjacent to the trans-Golgi apparatus network and co-localization with Golgi apparatus-localized markers, suggesting that CCX4 localizes in the Golgi apparatus. These results suggest that CCX4 is a Golgi apparatus-localized transporter involved in the Ca2+ response and plays important roles in osmotic tolerance, shoot Ca2+ content, and normal growth of Arabidopsis.

The ER calcium channel Csg2 integrates sphingolipid metabolism with autophagy

Sphingolipids are ubiquitous components of membranes and function as bioactive lipid signaling molecules. Here, through genetic screening and lipidomics analyses, we find that the endoplasmic reticulum (ER) calcium channel Csg2 integrates sphingolipid metabolism with autophagy by regulating ER calcium homeostasis in the yeast Saccharomyces cerevisiae. Csg2 functions as a calcium release channel and maintains calcium homeostasis in the ER, which enables normal functioning of the essential sphingolipid synthase Aur1. Under starvation conditions, deletion of Csg2 causes increases in calcium levels in the ER and then disturbs Aur1 stability, leading to accumulation of the bioactive sphingolipid phytosphingosine, which specifically and completely blocks autophagy and induces loss of starvation resistance in cells. Our findings indicate that calcium homeostasis in the ER mediated by the channel Csg2 translates sphingolipid metabolism into autophagy regulation, further supporting the role of the ER as a signaling hub for calcium homeostasis, sphingolipid metabolism and autophagy.

Systematic analysis of Ca2+ homeostasis in Saccharomyces cerevisiae based on chemical-genetic interaction profiles

We investigated the global landscape of Ca2+ homeostasis in budding yeast based on high-dimensional chemical-genetic interaction profiles. The morphological responses of 62 Ca2+-sensitive (cls) mutants were quantitatively analyzed with the image processing program CalMorph after exposure to a high concentration of Ca2+ After a generalized linear model was applied, an analysis of covariance model was used to detect significant Ca2+-cls interactions. We found that high-dimensional, morphological Ca2+-cls interactions were mixed with positive (86%) and negative (14%) chemical-genetic interactions, whereas one-dimensional fitness Ca2+-cls interactions were all negative in principle. Clustering analysis with the interaction profiles revealed nine distinct gene groups, six of which were functionally associated. In addition, characterization of Ca2+-cls interactions revealed that morphology-based negative interactions are unique signatures of sensitized cellular processes and pathways. Principal component analysis was used to discriminate between suppression and enhancement of the Ca2+-sensitive phenotypes triggered by inactivation of calcineurin, a Ca2+-dependent phosphatase. Finally, similarity of the interaction profiles was used to reveal a connected network among the Ca2+ homeostasis units acting in different cellular compartments. Our analyses of high-dimensional chemical-genetic interaction profiles provide novel insights into the intracellular network of yeast Ca2+ homeostasis.

The yeast sphingolipid signaling landscape

Sphingolipids are recognized as signaling mediators in a growing number of pathways, and represent potential targets to address many diseases. The study of sphingolipid signaling in yeast has created a number of breakthroughs in the field, and has the potential to lead future advances. The aim of this article is to provide an inclusive view of two major frontiers in yeast sphingolipid signaling. In the first section, several key studies in the field of sphingolipidomics are consolidated to create a yeast sphingolipidome that ranks nearly all known sphingolipid species by their level in a resting yeast cell. The second section presents an overview of most known phenotypes identified for sphingolipid gene mutants, presented with the intention of illuminating not yet discovered connections outside and inside of the field.

Profilin is required for Ca2+ homeostasis and Ca2+-modulated bud formation in yeast

A cls5-1 mutant of Saccharomyces cerevisiae is specifically sensitive to high concentrations of Ca2+, with elevated intracellular calcium content and altered cell morphology in the presence of 100 mM Ca2+. To reveal the mechanisms of the Ca2+-sensitive phenotype, we investigated the gene responsible and its interacting network. We demonstrated that CLS5 is identical to PFY1, encoding profilin. Involvement of profilin in the maintenance of intracellular Ca2+ homeostasis was supported by the fact that both exchangeable and non-exchangeable intracellular Ca2+ pools in the cls5-1 mutant are higher than those of the wild-type strain. Several mutations of the genes whose proteins physically interact with profilin resulted in the Ca2+-sensitive phenotype. Examination of the intracellular Ca2+ pools indicated that Bni1p, Bem1p, Rho1p, and Cla4p are also required for the maintenance of Ca2+ homeostasis. Quantitative morphological analysis revealed that the Ca2+-induced morphological changes in cls5-1 cells are similar to bem1 and cls4-1 cells. Common Ca2+-induced morphological changes were an increase in cell size and a decrease of the ratio of budded cells in the population. Since a mutation allele of cls4-1 is located in the CDC24 gene, we suggest that profilin, Bem1p, and Cdc24p are required for Ca2+-modulated bud formation. Thus, profilin is involved in Ca2+ regulation in two ways: the first is Ca2+ homeostasis by coordination with Bni1p, Bem1p, Rho1p, and Cla4p, and the second is the requirement of Ca2+ for bud formation by coordination with Bem1p and Cdc24p.

Diversity of Ca2+-induced morphology revealed by morphological phenotyping of Ca2+-sensitive mutants of Saccharomyces cerevisiae

Yeast cell morphology can be treated as a quantitative trait using the image processing software CalMorph. In the present study, we investigated Ca(2+)-induced morphological changes in Ca(2+)-sensitive (cls) mutants of Saccharomyces cerevisiae, based on the discovery that the characteristic Ca(2+)-induced morphological changes in the Ca(2+)-sensitive mutant zds1 reflect changes in the Ca(2+) signaling-mediated cell cycle control pathway. By applying hierarchical cluster analysis to the quantitative morphological data of 58 cls mutants, 31 of these mutants were classified into seven classes based on morphological similarities. The patterns of morphological change induced by Ca(2+) in one class differed from those of another class. Based on the results obtained using versatile methods for phenotypic analysis, we conclude that a high concentration of Ca(2+) exerts a wide variety of effects on yeast and that there are multiple Ca(2+)-regulatory pathways that are distinct from the Zds1p-related pathway.

Regulation of the transport and protein levels of the inositol phosphorylceramide mannosyltransferases Csg1 and Csh1 by the Ca2+-binding protein Csg2

Complex sphingolipids in yeast are known to function in cellular adaptation to environmental changes. One of the yeast complex sphingolipids, mannosylinositol phosphorylceramide (MIPC), is produced by the redundant inositol phosphorylceramide (IPC) mannosyltransferases Csg1 and Csh1. The Ca2+-binding protein Csg2 can form a complex with either Csg1 or Csh1 and is considered to act as a regulatory subunit. However, the role of Csg2 in MIPC synthesis has remained unclear. In this study, we found that Csg1 and Csh1 are N-glycosylated with core-type and mannan-type structures, respectively. Further identification of the glycosylated residues suggests that both Csg1 and Csh1 exhibit membrane topology with their C termini in the cytosol and their mannosyltransferase domains in the lumen. After complexing with Csg2, both Csg1 and Csh1 function in the Golgi, and then are delivered to the vacuole for degradation. However, uncomplexed Csh1 cannot exit from the endoplasmic reticulum. We also demonstrated that Ca2+ stimulates IPC-to-MIPC conversion, because of a Csg2-dependent increase in Csg1 levels. Thus, Csg2 has several regulatory functions for Csg1 and Csh1, including stability, transport, and gene expression.

The Arf1p GTPase-activating protein Glo3p executes its regulatory function through a conserved repeat motif at its C-terminus

ADP-ribosylation factors (Arfs), key regulators of intracellular membrane traffic, are known to exert multiple roles in vesicular transport. We previously isolated eight temperature-sensitive (ts) mutants of the yeast ARF1 gene, which showed allele-specific defects in protein transport, and classified them into three groups of intragenic complementation. In this study, we show that the overexpression of Glo3p, one of the GTPase-activating proteins of Arf1p (ArfGAP), suppresses the ts growth of a particular group of the arf1 mutants (arf1-16 and arf1-17). Other ArfGAPs do not show such a suppression activity. All these ArfGAPs show sequence similarity in the ArfGAP catalytic domain, but are divergent in the rest of molecules. By domain swapping analysis of Glo3p and another ArfGAP, Gcs1p, we have shown that the non-catalytic C-terminal region of Glo3p is required for the suppression of the growth defect in the arf1 ts mutants. Interestingly, Glo3p and its homologues from other eukaryotes harbor a well-conserved repeated ISSxxxFG sequence near the C-terminus, which is not found in Gcs1p and its homologues. We name this region the Glo3 motif and present evidence that the motif is required for the function of Glo3p in vivo.

GPI7 Involved in Glycosylphosphatidylinositol Biosynthesis Is Essential for Yeast Cell Separation

GPI7 is involved in adding ethanolaminephosphate to the second mannose in the biosynthesis of glycosylphosphatidylinositol (GPI) in Saccharomyces cerevisiae. We isolated gpi7 mutants, which have defects in cell separation and a daughter cell-specific growth defect at the non-permissive temperature. WSC1, RHO2, ROM2, GFA1, and CDC5 genes were isolated as multicopy suppressors of gpi7-2 mutant. Multicopy suppressors could suppress the growth defect of gpi7 mutants but not the cell separation defect. Loss of function mutations of genes involved in the Cbk1p-Ace2p pathway, which activates the expression of daughter-specific genes for cell separation after cytokinesis, bypassed the temperature-sensitive growth defect of gpi7 mutants. Furthermore, deletion of EGT2, one of the genes controlled by Ace2p and encoding a GPI-anchored protein required for cell separation, ameliorated the temperature sensitivity of the gpi7 mutant. In this mutant, Egt2p was displaced from the septal region to the cell cortex, indicating that GPI7 plays an important role in cell separation via the GPI-based modification of daughter-specific proteins in S. cerevisiae.

Yeast sphingolipid metabolism: clues and connections

This review of sphingolipid metabolism in the budding yeast Saccharomyces cerevisiae contains information on the enzymes and the genes that encode them, as well as connections to other metabolic pathways. Particular attention is given to yeast homologs, domains, and motifs in the sequence, cellular localization of enzymes, and possible protein-protein interactions. Also included are genetic interactions of special interest that provide clues to the cellular biological roles of particular sphingolipid metabolic pathways and specific sphingolipids.

Csg1p and newly identified Csh1p function in mannosylinositol phosphorylceramide synthesis by interacting with Csg2p

Csg1p and Csg2p have been shown to be involved in the synthesis of mannosylinositol phosphorylceramide (MIPC) from inositol phosphorylceramide. YBR161w, termed CSH1 here, encodes a protein that exhibits a strong similarity to Csg1p. To examine whether Csh1p also functions in MIPC synthesis, we performed a [3H]dihydrosphingosine labeling experiment. Deltacsg1 cells exhibited only a reduction in the synthesis of mannosylated sphingolipids compared with wild-type cells, whereas the Deltacsg1 Deltacsh1 double deletion mutant exhibited a total loss. These results indicated that Csg1p and Csh1p have redundant functions in MIPC synthesis. Analyses using Deltacsg1 and Deltacsh1 cells in the Deltaipt1, Deltasur2, or Deltascs7 genetic background demonstrated that Csh1p has a different substrate specificity from Csg1p. We also revealed that Csg2p interacts with both Csg1p and Csh1p. Deletion of the CSG2 gene reduced the Csg1p activity and abolished the Csh1p activity. These results suggested that two distinct inositol phosphorylceramide mannosyltransferase complexes, Csg1p-Csg2p and Csh1p-Csg2p, exist.

Repression of rRNA synthesis due to a secretory defect requires the C-terminal silencing domain of Rap1p in Saccharomyces cerevisiae

A secretory defect causes specific transcriptional repression of both ribosomal protein and ribosomal RNA genes, suggesting the coupling of plasma membrane and ribosome syntheses. We previously reported that the rap1-17 allele, which produced C-terminally truncated Rap1p, derepressed transcription of ribosomal protein genes when the secretory pathway was blocked. In this paper, we demonstrate that the rap1-17 mutation also leads to significant attenuation of transcriptional repression of rRNA genes due to a secretory defect. In contrast, the rap1-2 temperature-sensitive allele containing a unique missense mutation in the middle of the coding sequence has only a weak effect on repression. These results suggest that the C-terminal silencing domain of Rap1p is required for transcriptional repression of rDNA in response to a secretory defect. We also demonstrated that transcriptional regulation of ribosomal protein genes in response to nitrogen limitation was not affected by the rap1-17 allele, suggesting that the mechanism of nitrogen response is distinct from that of the secretory response.

Yeast 1,3-beta-glucan synthase activity is inhibited by phytosphingosine localized to the endoplasmic reticulum

1,3-beta-D-Glucan, a major filamentous component of the cell wall in the budding yeast Saccharomyces cerevisiae, is synthesized by 1,3-beta-glucan synthase (GS). Although a yeast gene whose product is required for GS activity in vitro, GNS1, was isolated and characterized, its role in GS function has remained unknown. In the current study we show that Deltagns1 cells accumulate a non-competitive and non-proteinous inhibitor(s) in the membrane fraction. Investigations of inhibitory activity on GS revealed that the inhibitor(s) is mainly present in the sphingolipid fraction. It is shown that Deltagns1 cells contain phytosphingosine (PHS), an intermediate in the sphingolipid biosynthesis, 30-fold more than wild-type cells do. The membrane fraction isolated from Deltasur2 cells contains an increased amount of dihydrosphingosine (DHS) and also exhibits reduced GS activity. Among constituents of the sphingolipid fraction, PHS and DHS show striking inhibition in a non-competitive manner. The intracellular level of DHS is much lower than that of PHS in wild-type cells, suggesting that PHS is the primary inhibitor of GS in vivo. The localization of PHS to the endoplasmic reticulum in wild-type cells coincides with that of the inhibitor(s) in Deltagns1 cells. Taken together, our results indicate that PHS is a potent inhibitor of yeast GS in vivo.

Yeast Saccharomyces cerevisiae has two cis-prenyltransferases with different properties and localizations. Implication for their distinct physiological roles in dolichol synthesis

BACKGROUND

Dolichol is a family of long-chain polyprenols, which is utilized as a sugar carrier in protein glycosylation in the endoplasmic reticulum (ER). We have identified a key enzyme of the dolichol synthesis, cis-prenyltransferase, as Rer2p from Saccharomyces cerevisiae. We have also isolated a multicopy suppressor of an rer2 mutant and named it SRT1. It encodes a protein similar to Rer2p but its function has not been established.

RESULTS

The cis-prenyltransferase activity of Srt1p has been proved biochemically in the lysate of yeast cells lacking Rer2p. The polyprenol product of Srt1p is longer in chain length than that of Rer2p and is not sufficiently converted to dolichol and dolichyl phosphate, unlike that of Rer2p. The subcellular localization of these two isozymes has been examined by immunofluorescence microscopy and by the use of GFP fusion proteins. Whereas GFP-Rer2p is localized to the continuous ER and some dots associated with the ER, GFP-Srt1p shows only punctate localization patterns. Immunofluorescence double staining with Erg6p, a marker of lipid particles in yeast, indicates that Srt1p is mainly localized to lipid particles (lipid bodies). RER2 is mainly expressed in the early logarithmic phase, while the expression of SRT1 is induced in the stationary phase.

CONCLUSIONS

We have shown that yeast has two active cis-prenyltransferases with different properties. This result implies that the two isozymes have different physiological roles during the life cycle of the yeast.

RRS1, a conserved essential gene, encodes a novel regulatory protein required for ribosome biogenesis in Saccharomyces cerevisiae

A secretory defect causes specific and significant transcriptional repression of both ribosomal protein and rRNA genes (K. Mizuta and J. R. Warner, Mol. Cell. Biol. 14:2493-2502, 1994), suggesting the coupling of plasma membrane and ribosome syntheses. In order to elucidate the molecular mechanism of the signaling pathway, we isolated a cold-sensitive mutant with a mutation in a gene termed RRS1 (regulator of ribosome synthesis), which appeared to be defective in the signaling pathway. The rrs1-1 mutation greatly reduced transcriptional repression of both rRNA and ribosomal protein genes that is caused by a secretory defect. RRS1 is a novel, essential gene encoding a nuclear protein of 203 amino acid residues that is conserved in eukaryotes. A conditional rrs1-null mutant was constructed by placing RRS1 under the control of the GAL1 promoter. Rrs1p depletion caused defects in processing of pre-rRNA and assembly of ribosomal subunits.

Brave little yeast, please guide us to thebes: sphingolipid function in S. cerevisiae

Sphingolipids typically cover the exoplasmic leaflet of the plasma membrane of eukaryotic cells. They differ from the more abundant glycerophospholipids in that they contain ceramide instead of diacylglycerol as a hydrophobic anchor. Why did nature choose to invent this complex class of lipids, and why do eukaryotic cells follow elaborate remodelling pathways in order to generate dozens to hundreds of different molecular species of sphingolipid, depending on cell type? Yeast may, once again, serve as a model to dissect sphingolipid function at various levels. Almost the complete pathway for sphingolipid synthesis in yeast has been uncovered during the past two decades. More recently, key enzymes in sphingolipid degradation and signalling have been identified. Together with a wealth of genetic data obtained from the characterization of various suppressor mutants, this information now allows for an unprecedented analysis of sphingolipid function in this organism. This overview summarizes recent data on sphingolipid function in cell signalling, their role in the heat-stress response and Ca(2+) homeostasis, and addresses their function in transport of glycosylphosphatidylinositol-anchored proteins.

Metabolism and selected functions of sphingolipids in the yeast Saccharomyces cerevisiae

Our knowledge of sphingolipid metabolism and function in Saccharomyces cerevisiae is growing rapidly. Here we discuss the current status of sphingolipid metabolism including recent evidence suggesting that exogenous sphingoid long-chain bases must first be phosphorylated and then dephosphorylated before incorporation into ceramide. Phenotypes of strains defective in sphingolipid metabolism are discussed because they provide hints about the undiscovered functions of sphingolipids and are one of the major reasons for studying this model eukaryote. The long-chain base phosphates, dihydrosphingosine-1-phosphate and phytosphingosine-1-phosphate, have been hypothesized to play roles in heat stress resistance, perhaps acting as signaling molecules. We evaluate the data supporting this hypothesis and suggest future experiments needed to verify it. Finally, we discuss recent clues that may help to reveal how sphingolipid synthesis and total cellular sphingolipid content are regulated.

The inactive form of a yeast casein kinase I suppresses the secretory defect of the sec12 mutant. Implication of negative regulation by the Hrr25 kinase in the vesicle budding from the endoplasmic reticulum

Sec12p is the guanine nucleotide exchange factor of Sar1 GTPase and functions at the very upstream in the vesicle budding reactions from the endoplasmic reticulum (ER). We previously identified three yeast loci, RST1, RST2, and RST3, whose mutations suppressed the temperature-sensitive growth of the sec12-4 mutant (Nakano, A. (1996) J. Biochem. (Tokyo) 120, 642-646). In the present study, we cloned the wild-type RST2 gene by complementation of the cold-sensitive phenotype of the rst2-1 mutant. RST2 turned out to be identical to HRR25, a gene encoding a dual-specificity casein kinase I in yeast. The rst2-1 mutation, which is now renamed hrr25-2, was due to the T176I amino acid replacement in the kinase domain. This mutation remedied not only the temperature-sensitive growth but also the defect of ER-to-Golgi protein transport of sec12. Immunoprecipitation of the hemagglutinin-tagged Hrr25-2 protein and a subsequent protein kinase assay showed that the kinase activity of the mutant protein was markedly reduced. The overproduction of another kinase-minus mutant of Hrr25p (Hrr25p K38A) slightly suppressed the growth defect of sec12-4 as well. These observations suggest that the reduction of the kinase activity in the mutant protein is important for the suppression of sec12. We propose that Hrr25p negatively regulates the vesicle budding from the ER.

The yeast RER2 gene, identified by endoplasmic reticulum protein localization mutations, encodes cis-prenyltransferase, a key enzyme in dolichol synthesis

As an approach to understand the molecular mechanisms of endoplasmic reticulum (ER) protein sorting, we have isolated yeast rer mutants that mislocalize a Sec12-Mfalpha1p fusion protein from the ER to later compartments of the secretory pathway (S. Nishikawa and A. Nakano, Proc. Natl. Acad. Sci. USA 90:8179-8183, 1993). The temperature-sensitive rer2 mutant mislocalizes different types of ER membrane proteins, suggesting that RER2 is involved in correct localization of ER proteins in general. The rer2 mutant shows several other characteristic phenotypes: slow growth, defects in N and O glycosylation, sensitivity to hygromycin B, and abnormal accumulation of membranes, including the ER and the Golgi membranes. RER2 and SRT1, a gene whose overexpression suppresses rer2, encode novel proteins similar to each other, and their double disruption is lethal. RER2 homologues are found not only in eukaryotes but also in many prokaryote species and thus constitute a large gene family which has been well conserved during evolution. Taking a hint from the phenotype of newly established mutants of the Rer2p homologue of Escherichia coli, we discovered that the rer2 mutant is deficient in the activity of cis-prenyltransferase, a key enzyme of dolichol synthesis. This and other lines of evidence let us conclude that members of the RER2 family of genes encode cis-prenyltransferase itself. The difference in phenotypes between the rer2 mutant and previously obtained glycosylation mutants suggests a novel, as-yet-unknown role of dolichol.

A single gene product, claudin-1 or -2, reconstitutes tight junction strands and recruits occludin in fibroblasts

Three integral membrane proteins, clau- din-1, -2, and occludin, are known to be components of tight junction (TJ) strands. To examine their ability to form TJ strands, their cDNAs were introduced into mouse L fibroblasts lacking TJs. Immunofluorescence microscopy revealed that both FLAG-tagged claudin-1 and -2 were highly concentrated at cell contact sites as planes through a homophilic interaction. In freeze-fracture replicas of these contact sites, well-developed networks of strands were identified that were similar to TJ strand networks in situ and were specifically labeled with anti-FLAG mAb. In glutaraldehyde-fixed samples, claudin-1-induced strands were largely associated with the protoplasmic (P) face as mostly continuous structures, whereas claudin-2-induced strands were discontinuous at the P face with complementary grooves at the extracellular (E) face which were occupied by chains of particles. Although occludin was also concentrated at cell contact sites as dots through its homophilic interaction, freeze-fracture replicas identified only a small number of short strands that were labeled with anti-occludin mAb. However, when occludin was cotransfected with claudin-1, it was concentrated at cell contact sites as planes to be incorporated into well- developed claudin-1-based strands. These findings suggested that claudin-1 and -2 are mainly responsible for TJ strand formation, and that occludin is an accessory protein in some function of TJ strands.

Phosphatidylinositol-4-phosphate 5-kinase localized on the plasma membrane is essential for yeast cell morphogenesis

Phosphatidylinositol 4,5-biphosphate (PtdIns(4,5)P2), an important element in eukaryotic signal transduction, is synthesized either by phosphatidylinositol-4-phosphate 5-kinase (PtdIns(4)P 5K) from phosphatidylinositol 4-phosphate (PtdIns(4)P) or by phosphatidylinositol-5-phosphate 4-kinase (PtdIns(5)P 4K) from phosphatidylinositol 5-phosphate (PtdIns(5)P). Two Saccharomyces cerevisiae genes, MSS4 and FAB1, are homologous to mammalian PtdIns(4)P 5Ks and PtdIns(5)P 4Ks. We show here that MSS4 is a functional homolog of mammalian PtdIns(4)P 5K but not of PtdIns(5)P 4K in vivo. We constructed a hemagglutinin epitope-tagged form of Mss4p and found that Mss4p has PtdIns(4)P 5K activity. Immunofluorescent and fractionation studies of the epitope-tagged Mss4p suggest that Mss4p is localized on the plasma membrane, whereas Fab1p is reportedly localized on the vacuolar membrane. A temperature-sensitive mss4-1 mutant was isolated, and its phenotypes at restrictive temperatures were found to include increased cell size, round shape, random distribution of actin patches, and delocalized staining of cell wall chitin. Thus, biochemical and genetic analyses on Mss4p indicated that yeast PtdIns(4)P 5K localized on the plasma membrane is required for actin organization.

TOR2 is part of two related signaling pathways coordinating cell growth in Saccharomyces cerevisiae

The Saccharomyces cerevisiae genes TOR1 and TOR2 encode phosphatidylinositol kinase homologs. TOR2 has two essential functions. One function overlaps with TOR1 and mediates protein synthesis and cell cycle progression. The second essential function of TOR2 is unique to TOR2 and mediates the cell-cycle-dependent organization of the actin cytoskeleton. We have isolated temperature-sensitive mutants that are defective for either one or both of the two TOR2 functions. The three classes of mutants were as follows. Class A mutants, lacking only the TOR2-unique function, are defective in actin cytoskeleton organization and arrest within two to three generations as small-budded cells in the G2/M phase of the cell cycle. Class B mutants, lacking only the TOR-shared function, and class C mutants, lacking both functions, exhibit a rapid loss of protein synthesis and a G1 arrest within one generation. To define further the two functions of TOR2, we isolated multicopy suppressors that rescue the class A or B mutants. Overexpression of MSS4, PKC1, PLC1, RHO2, ROM2, or SUR1 suppressed the growth defect of a class A mutant. Surprisingly, overexpression of PLC1 and MSS4 also suppressed the growth defect of a class B mutant. These genes encode proteins that are involved in phosphoinositide metabolism and signaling. Thus, the two functions (readouts) of TOR2 appear to involve two related signaling pathways controlling cell growth.

Hydroxylation of Saccharomyces cerevisiae ceramides requires Sur2p and Scs7p

The Saccharomyces cerevisiae SCS7 and SUR2 genes are members of a gene family that encodes enzymes that desaturate or hydroxylate lipids. Sur2p is required for the hydroxylation of C-4 of the sphingoid moiety of ceramide, and Scs7p is required for the hydroxylation of the very long chain fatty acid. Neither SCS7 nor SUR2 are essential for growth, and lack of the Scs7p- or Sur2p-dependent hydroxylation does not prevent the synthesis of mannosyldiinositolphosphorylceramide, the mature sphingolipid found in yeast. Deletion of either gene suppresses the Ca2+-sensitive phenotype of csg2Delta mutants, which arises from overaccumulation of inositolphosphorylceramide due to a defect in sphingolipid mannosylation. Characterization of scs7 and sur2 mutants is expected to provide insight into the function of ceramide hydroxylation.

A homolog of mammalian, voltage-gated calcium channels mediates yeast pheromone-stimulated Ca2+ uptake and exacerbates the cdc1(Ts) growth defect

Previous studies attributed the yeast (Saccharomyces cerevisiae) cdc1(Ts) growth defect to loss of an Mn2+-dependent function. In this report we show that cdc1(Ts) temperature-sensitive growth is also associated with an increase in cytosolic Ca2+. We identified two recessive suppressors of the cdc1(Ts) temperature-sensitive growth which block Ca2+ uptake and accumulation, suggesting that cytosolic Ca2+ exacerbates or is responsible for the cdc1(Ts) growth defect. One of the cdc1(Ts) suppressors is identical to a gene, MID1, recently implicated in mating pheromone-stimulated Ca2+ uptake. The gene (CCH1) corresponding to the second suppressor encodes a protein that bears significant sequence similarity to the pore-forming subunit (alpha1) of plasma membrane, voltage-gated Ca2+ channels from higher eukaryotes. Strains lacking Mid1 or Cch1 protein exhibit a defect in pheromone-induced Ca2+ uptake and consequently lose viability upon mating arrest. The mid1delta and cch1delta mutants also display reduced tolerance to monovalent cations such as Li+, suggesting a role for Ca2+ uptake in the calcineurin-dependent ion stress response. Finally, mid1delta cch1delta double mutants are, by both physiological and genetic criteria, identical to single mutants. These and other results suggest Mid1 and Cch1 are components of a yeast Ca2+ channel that may mediate Ca2+ uptake in response to mating pheromone, salt stress, and Mn2+ depletion.

RIC1, a novel gene required for ribosome synthesis in Saccharomyces cerevisiae

We isolated a temperature-sensitive mutant of Saccharomyces cerevisiae in which transcription both of ribosomal protein genes and of ribosomal RNA is defective at the non-permissive temperature. Temperature-sensitivity for growth is recessive and segregates 2:2. The wild type gene, termed RIC1 (for ribosome control) was cloned by complementation of the temperature-sensitive phenotype from a genomic DNA library based on the CEN plasmid. RIC1 encodes a protein of 1056 amino acid (aa) residues including a putative nuclear localization sequence. Data base searches revealed that RIC1 is a novel gene and predicted aa sequence share some sequence similarity with viral transcriptional regulatory proteins.

Elevated cytosolic free Ca2+ concentrations and massive Ca2+ accumulation within vacuoles, in yeast mutant lacking PMR1, a homolog of Ca2+ -ATPase

The Ca2+ -ATPase homolog of Saccharomyces cerevisiae, PMR1, cloned by Rudolph et al. (Cell 58 (1989) 133-145) is required for normal Golgi functions. We have investigated the role of Pmr1-protein in maintaining homeostasis of cytosolic free Ca2+ concentration ([Ca2+]i). It was found that exposure to moderately high Ca2+ concentrations led to elevated levels of [Ca2+]i in cells of pmr1 null mutant, in comparison with cells of pmr2 isogenic mutant (defective in cell-membrane Na+ - ATPase) and of an isogenic wild type. In addition, we showed that PMR1 deletion causes massive accumulation of Ca2+ in the vacuoles and affects the rates of Ca2+ influx and efflux.

Yeast Cls2p/Csg2p localized on the endoplasmic reticulum membrane regulates a non-exchangeable intracellular Ca2+ pool cooperatively with calcineurin

Saccharromyces cerevisiae CLS2 gene product (Cls2p) that is localized on the endoplasmic reticulum is important for the regulation of intracellular Ca2+ in a compartment distinct from the vacuole. Using a vma3 mutation that impairs the Ca2+ sequestering activity into the vacuole, we have shown that the cls2 mutation results in 3.4-fold increase in the Ca2+ pool that is not exchangeable with extracellular Ca2+. Accumulation of Ca2+ within the cls2 cells is synergistically elevated by the addition of immunosuppressant, FK506. Moreover, in the vma3 background, toxicity caused by the cls2 mutation is greatly enhanced by FK506. Given that FK506 inhibits the calcineurin activity, Cls2p likely functions in releasing Ca2+ flux from the endoplasmic reticulum, somehow cooperating with calcineurin.

Membrane protein retrieval from the Golgi apparatus to the endoplasmic reticulum (ER): characterization of the RER1 gene product as a component involved in ER localization of Sec12p

Yeast Sec12p, a type II transmembrane glycoprotein, is required for formation of transport vesicles from the endoplasmic reticulum (ER). Biochemical and morphological analyses have suggested that Sec12p is localized to the ER by two mechanisms: static retention in the ER and dynamic retrieval from the early region of the Golgi apparatus. The rer1 mutant we isolated in a previous study mislocalizes the authentic Sec12p to the later compartments of the Golgi. To understand the role of RER1 on Sec12p localization, we cloned the gene and determined its reading frame. RER1 encodes a hydrophobic protein of 188 amino acid residues containing four putative membrane spanning domains. The rer1 null mutant is viable. Even in the rer1 disrupted cells, immunofluorescence of Sec12p stains the ER, implying that the retention system is still operating in the mutant. To determine the subcellular localization of Rer1p, an epitope derived from the influenza hemagglutinin was added to the C-terminus of Rer1p and the cells expressing this tagged but functional protein were observed by immunofluorescence microscopy. The anti-HA monoclonal antibody stains the cells in a punctate pattern that is typical for Golgi proteins and clearly distinct from the ER staining. This punctate staining was in fact exaggerated in the sec7 mutant that accumulates the Golgi membranes at the restrictive temperature. Furthermore, double staining of Rer1p and Ypt1p, a GTPase that is known to reside in the Golgi apparatus, showed good colocalization. Subcellular fractionation experiments indicated that the fractionation pattern of Rer1p was similar to that of an early Golgi protein, Och1p. From these results, we suggest that Rer1p functions in the Golgi membrane to return Sec12p that has escaped from the static retention system of the ER.

Cooperation of calcineurin and vacuolar H(+)-ATPase in intracellular Ca2+ homeostasis of yeast cells

Saccharomyces cerevisiae VMA genes, encoding essential components for the expression of vacuolar membrane H(+)-ATPase activity, are involved in intracellular ionic homeostasis and vacuolar biogenesis. We report here that the immunosuppressants FK506 and cyclosporin A cause general growth inhibition of the vma3 mutant. Upon addition of the drugs, the mutant grew neither in the presence of more than 5 mM Ca2+ nor above pH 6.0. The action of the immunosuppressants is dependent on their binding proteins and ascribable to inhibition of calcineurin activity; a mutation of a calcineurin subunit (cnb1) shows synthetic lethal interaction with the vma mutation. The addition of FK506 decreases the cytosolic free concentration of Ca2+ in the vma3 mutant cells. Consequently, FK506 induces an 8.9-fold elevation of a nonexchangeable Ca2+ pool. These results suggest that calcineurin controls calcium homeostasis by repression of Ca2+ flux into a cellular compartment(s) and that the vacuolar H(+)-ATPase is essential for cell growth cooperating with calcineurin to regulate the cytosolic free concentration of Ca2+.