An intracellular ATP-dependent calcium pump within the yeast Schizosaccharomyces pombe, encoded by the gene cta3

Exomer Is Part of a Hub Where Polarized Secretion and Ionic Stress Connect

Plasma membrane and membranous organelles contribute to the physiology of the Eukaryotic cell by participating in vesicle trafficking and the maintenance of ion homeostasis. Exomer is a protein complex that facilitates vesicle transport from the trans-Golgi network to the plasma membrane, and its absence leads to the retention of a set of selected cargoes in this organelle. However, this retention does not explain all phenotypes observed in exomer mutants. The Schizosaccharomyces pombe exomer is composed of Cfr1 and Bch1, and cfr1Δ and bch1Δ were sensitive to high concentrations of potassium salts but not sorbitol, which showed sensitivity to ionic but not osmotic stress. Additionally, the activity of the plasma membrane ATPase was higher in exomer mutants than in the wild-type, pointing to membrane hyperpolarization, which caused an increase in intracellular K⁺ content and mild sensitivity to Na⁺, Ca²⁺, and the aminoglycoside antibiotic hygromycin B. Moreover, in response to K⁺ shock, the intracellular Ca²⁺ level of cfr1Δ cells increased significantly more than in the wild-type, likely due to the larger Ca²⁺ spikes in the mutant. Microscopy analyses showed a defective endosomal morphology in the mutants. This was accompanied by an increase in the intracellular pools of the K⁺ exporting P-type ATPase Cta3 and the plasma membrane Transient Receptor Potential (TRP)-like Ca²⁺ channel Pkd2, which were partially diverted from the trans-Golgi network to the prevacuolar endosome. Despite this, most Cta3 and Pkd2 were delivered to the plasma membrane at the cell growing sites, showing that their transport from the trans-Golgi network to the cell surface occurred in the absence of exomer. Nevertheless, shortly after gene expression in the presence of KCl, the polarized distribution of Cta3 and Pkd2 in the plasma membrane was disturbed in the mutants. Finally, the use of fluorescent probes suggested that the distribution and dynamics of association of some lipids to the plasma membrane in the presence of KCl were altered in the mutants. Thus, exomer participation in the response to K⁺ stress was multifaceted. These results supported the notion that exomer plays a general role in protein sorting at the trans-Golgi network and in polarized secretion, which is not always related to a function as a selective cargo adaptor.

Alkali metal cation transport and homeostasis in yeasts

The maintenance of appropriate intracellular concentrations of alkali metal cations, principally K(+) and Na(+), is of utmost importance for living cells, since they determine cell volume, intracellular pH, and potential across the plasma membrane, among other important cellular parameters. Yeasts have developed a number of strategies to adapt to large variations in the concentrations of these cations in the environment, basically by controlling transport processes. Plasma membrane high-affinity K(+) transporters allow intracellular accumulation of this cation even when it is scarce in the environment. Exposure to high concentrations of Na(+) can be tolerated due to the existence of an Na(+), K(+)-ATPase and an Na(+), K(+)/H(+)-antiporter, which contribute to the potassium balance as well. Cations can also be sequestered through various antiporters into intracellular organelles, such as the vacuole. Although some uncertainties still persist, the nature of the major structural components responsible for alkali metal cation fluxes across yeast membranes has been defined within the last 20 years. In contrast, the regulatory components and their interactions are, in many cases, still unclear. Conserved signaling pathways (e.g., calcineurin and HOG) are known to participate in the regulation of influx and efflux processes at the plasma membrane level, even though the molecular details are obscure. Similarly, very little is known about the regulation of organellar transport and homeostasis of alkali metal cations. The aim of this review is to provide a comprehensive and up-to-date vision of the mechanisms responsible for alkali metal cation transport and their regulation in the model yeast Saccharomyces cerevisiae and to establish, when possible, comparisons with other yeasts and higher plants.

Neuronal calcium sensor-1 (Ncs1p) is up-regulated by calcineurin to promote Ca2+ tolerance in fission yeast

Neuronal calcium sensor (NCS) proteins regulate signal transduction and are highly conserved from yeast to humans. NCS homolog in fission yeast (Ncs1p) is essential for cell growth under extreme Ca(2+) conditions. Ncs1p expression increases approximately 100-fold when fission yeast grows in high extracellular Ca(2+) (>0.1 M). Here, we show that Ca(2+)-induced expression of Ncs1p is controlled at the level of transcription. Transcriptional reporter assays show that ncs1 promoter activity increased 30-fold when extracellular Ca(2+) was raised to 0.1 M and was highly Ca(2+)-specific. Ca(2+)-dependent transcription of ncs1 is abolished by the calcineurin inhibitor (FK506) and by knocking out the calcineurin target, prz1. Thus, Ca(2+)-induced expression of Ncs1p is linked to the calcineurin/prz1 stress response. The Ca(2+)-responsive ncs1 promoter region consists of 130 nucleotides directly upstream from the start codon and contains tandem repeats of the sequence, 5'-caact-3', that binds to Prz1p. The Ca(2+)-sensitive ncs1Delta phenotype is rescued by a yam8 null mutation, suggesting a possible interaction between Ncs1p and the Ca(2+) channel, Yam8p. Ca(2+) uptake and Ncs1p binding to yeast membranes are both decreased in yam8Delta, suggesting Ca(2+)-induced binding of Ncs1p to Yam8p results in channel closure. We propose that Ncs1p promotes Ca(2+) tolerance in fission yeast, in part by cytosolic Ca(2+) buffering and perhaps by negatively regulating the Yam8p Ca(2+) channel.

Schizosaccharomyces pombe possesses two plasma membrane alkali metal cation/H antiporters differing in their substrate specificity

The Schizosaccharomyces pombe plasma membrane Na(+)/H(+) antiporter, SpSod2p, has been shown to belong to the subfamily of yeast Na(+)/H(+) antiporters that only recognize Na(+) and Li(+) as substrates. Nevertheless, most of the studied plasma membrane alkali metal cation/H(+) antiporters from other yeasts have broader substrate specificities, exporting K(+) and Rb(+) as well. Such antiporters probably play two roles in the physiology of cells: the elimination of surplus toxic cations, and the regulation of stable intracellular K(+) content, pH and cell volume. The systematic sequencing of the Sch. pombe genome revealed the presence of an as-yet uncharacterized homolog of the Spsod2 gene (designated Spsod22). Spsod22 and Spsod2 were expressed in Saccharomyces cerevisiae cells lacking their own alkali metal cation efflux systems, and the transport properties of both Sch. pombe antiporters were compared to those of the Sac. cerevisiae Nha1 antiporter expressed under the same conditions. Here we show that SpSod22p has broad substrate specificity upon heterologous expression in Sac. cerevisiae cells and contributes to cell tolerance to high external levels of K(+). Thus, the Sch. pombe genome encodes two plasma membrane alkali metal cation/H(+) antiporters that play different roles in the physiology of the yeast.

K+ fluxes in Schizosaccharomyces pombe

All living cells accumulate high concentrations of K+ in order to keep themselves alive. To this end they have developed a great diversity of transporters. The internal level of K+ is the result of the net balance between the activities of the K+ influx and the K+ efflux transporters. Potassium fluxes have been extensively studied and characterized in Saccharomyces cerevisiae. However, this is not the case in the fission yeast and, in addition, the information available indicates that both yeasts present substantial and interesting differences. In this paper we have reviewed and summarized the information on K+ fluxes in Schizosaccharomyces pombe. We have included some unpublished results recently obtained in our laboratory and, in particular, we have highlighted the significant differences found between the well-known yeast S. cerevisiae and the fission yeast Sch. pombe.

Yeast plasma membrane Ena1p ATPase alters alkali-cation homeostasis and confers increased salt tolerance in tobacco cultured cells

In plants, the plasma membrane Na(+)/H(+) antiporter is the only key enzyme that extrudes cytosolic Na(+) and contributes to salt tolerance. But in fungi, the plasma membrane Na(+)/H(+) antiporter and Na(+)-ATPase are known to be key enzymes for salt tolerance. Saccharomyces cerevisiae Ena1p ATPase encoded by the ENA1/PMR2A gene is primarily responsible for Na(+) and Li(+) efflux across the plasma membrane during salt stress and for K(+) efflux at high pH and high K(+). To test if the yeast ATPase would improve salt tolerance in plants, we expressed a triple hemagglutinin (HA)-tagged Ena1p (Ena1p-3HA) in cultured tobacco (Nicotiana tabacum L.) cv Bright Yellow 2 (BY2) cells. The Ena1p-3HA proteins were correctly localized to the plasma membrane of transgenic BY2 cells and conferred increased NaCl and LiCl tolerance to the cells. Under moderate salt stress conditions, the Ena1p-3HA-expressing BY2 clones accumulated lower levels of Na(+) and Li(+) than nonexpressing BY2 clones. Moreover, the Ena1p-3HA expressing BY2 clones accumulated lower levels of K(+) than nonexpressing cells under no-stress conditions. These results suggest that the yeast Ena1p can also function as an alkali-cation (Na(+), Li(+), and K(+)) ATPase and alter alkali-cation homeostasis in plant cells. We conclude that, even with K(+)-ATPase activity, Na(+)-ATPase activity of the yeast Ena1p confers increased salt tolerance to plant cells during salt stress.

Fission yeast homolog of neuronal calcium sensor-1 (Ncs1p) regulates sporulation and confers calcium tolerance

The neuronal calcium sensor (NCS) proteins (e.g. recoverin, neurocalcins, and frequenin) are expressed at highest levels in excitable cells, and some of them regulate desensitization of G protein-coupled receptors. Here we present NMR analysis and genetic functional studies of an NCS homolog in fission yeast (Ncs1p). Ncs1p binds three Ca2+ ions at saturation with an apparent affinity of 2 microm and Hill coefficient of 1.9. Analysis of NMR and fluorescence spectra of Ncs1p revealed significant Ca2+-induced protein conformational changes indicative of a Ca2+-myristoyl switch. The amino-terminal myristoyl group is sequestered inside a hydrophobic cavity of the Ca2+-free protein and becomes solvent-exposed in the Ca2+-bound protein. Subcellular fractionation experiments showed that myristoylation and Ca2+ binding by Ncs1p are essential for its translocation from cytoplasm to membranes. The ncs1 deletion mutant (ncs1Delta) showed two distinct phenotypes: nutrition-insensitive sexual development and a growth defect at high levels of extracellular Ca2+ (0.1 m CaCl(2)). Analysis of Ncs1p mutants lacking myristoylation (Ncs1p(G2A)) or deficient in Ca2+ binding (Ncs1p(E84Q/E120Q/E168Q)) revealed that Ca2+ binding was essential for both phenotypes, while myristoylation was less critical. Exogenous cAMP, a key regulator for sexual development, suppressed conjugation and sporulation of ncs1Delta, suggesting involvement of Ncs1p in the adenylate cyclase pathway turned on by the glucose-sensing G protein-coupled receptor Git3p. Starvation-independent sexual development of ncs1Delta was also complemented by retinal recoverin, which controls Ca2+-regulated desensitization of rhodopsin. In contrast, the Ca2+-intolerance of ncs1Delta was not affected by cAMP or recoverin, suggesting that the two ncs1Delta phenotypes are mechanistically independent. We propose that Schizosaccharomyces pombe Ncs1p negatively regulates sporulation perhaps by controlling Ca2+-dependent desensitization of Git3p.

Role of fission yeast Tup1-like repressors and Prr1 transcription factor in response to salt stress

In Schizosaccharomyces pombe, the Sty1 mitogen-activated protein kinase and the Atf1 transcription factor control transcriptional induction in response to elevated salt concentrations. Herein, we demonstrate that two repressors, Tup11 and Tup12, and the Prr1 transcription factor also function in the response to salt shock. We find that deletion of both tup genes together results in hypersensitivity to elevated cation concentrations (K(+) and Ca(2+)) and we identify cta3(+), which encodes an intracellular cation transporter, as a novel stress gene whose expression is positively controlled by the Sty1 pathway and negatively regulated by Tup repressors. The expression of cta3(+) is maintained at low levels by the Tup repressors, and relief from repression requires the Sty1, Atf1, and Prr1. Prr1 is also required for KCl-mediated induction of several other Sty1-dependent genes such as gpx1(+) and ctt1(+). Surprisingly, the KCl-mediated induction of cta3(+) expression occurs independently of Sty1 in a tup11Delta tup12Delta mutant and so the Tup repressors link induction to the Sty1 pathway. We also report that in contrast to a number of other Sty1- and Atf1-dependent genes, the expression of cta3(+) is induced only by high salt concentrations. However, in the absence of the Tup repressors this specificity is lost and a range of stresses induces cta3(+) expression.

Potassium- or sodium-efflux ATPase, a key enzyme in the evolution of fungi

Potassium is the most abundant cation in cells. Therefore, plant-associated fungi and intracellular parasites are permanently or circumstantially exposed to high K(+) and must avoid excessive K(+) accumulation activating K(+) efflux systems. Because high K(+) and high pH are compatible in natural environments, free-living organisms cannot keep a permanent transmembrane DeltapH and cannot rely only on K(+)/H(+) antiporters, as do mitochondria. This study shows that the Schizosaccharomyces pombe CTA3 is a K(+)-efflux ATPase, and that other fungi are furnished with Na(+)-efflux ATPases, which also pump Na(+). All these fungal ATPases, including those pumping only Na(+), form a phylogenetic group, IID or ENA, among P-type ATPases. By searching in databases and partial cloning of ENA genes in species of Zygomycetes and Basidiomycetes, the authors conclude that probably all fungi have these genes. This study indicates that fungal K(+)- or Na(+)-ATPases evolved from an ancestral K(+)-ATPase, through processes of gene duplication. In yeast hemiascomycetes these duplications have occurred recently and produced bifunctional ATPases, whereas in Neurospora, and probably in other euascomycetes, they occurred earlier in evolution and produced specialized ATPases. In Schizosaccharomyces, adaptation to Na(+) did not involve the duplication of the K(+)-ATPase and thus it retains an enzyme which is probably close to the original one. The parasites Leishmania and Trypanosoma have ATPases phylogenetically related to fungal K(+)-ATPases, which are probably functional homologues of the fungal enzymes.

Molecular cloning of the calcium and sodium ATPases in Neurospora crassa

Using PCR, reverse transcription-PCR (RT-PCR) and colony hybridization in a genomic library, we isolated six genes which encode type II P-type ATPases in Neurospora crassa. The six full-length cDNAs were cloned in a yeast expression vector and transformed into Saccharomyces cerevisiae null Ca2+- or Na+-ATPase mutants. Three cDNAs suppressed the defect of the Ca2+ mutant and two of these protected from Mn2+ toxicity. One cDNA suppressed the defect of the Na+ mutant and two cDNAs were not functional in S. cerevisiae. The expression of the transcripts of the six genes in the presence of Ca2+, Na+, high pH or supporting an osmotic shock indicated that, with the exception of one of the Ca2+-ATPases, the main function of the cloned ATPases is the adaptation to stress conditions. The relationship between the cloned fungal Ca2+- and Na+-ATPases and plant type II P-ATPases is discussed.

The cta3+ gene that encodes a cation-transporting P-type ATPase is induced by salt stress under control of the Wis1-Sty1 MAPKK-MAPK cascade in fission yeast

In Schizosaccharomyces pombe, the Wis1-Sty1 MAP (mitogen-activated protein) kinase signaling cascade is known to play a major role in cellular adaptation to adverse external stimuli, including osmotic stress, oxidative stress, nutrient deprivation, DNA-damaging agents, and heat stress. Nonetheless, it is not known whether or not this particular MAPK cascade is also involved in response to the most common stress, salinity. In this study, we provide evidence that the Wis1-Sty1 MAP cascade is implicated in salt stress response through regulating expression of a salinity-inducible gene. The downstream target gene thus identified is the cta3+ gene, which encodes a cation-transporting P-type ATPase. The salt stress-responsive nature of cta3+ expression was characterized extensively. It was found that not only the Sty1 MAP kinase but also the Atf1 transcription factor is crucial for the inducible expression of cta3+. As far as we know, this is the first instance that the stress-activated Wis1-Sty1 MAPK cascade plays a role in salt stress response in S. pombe.

Metabolism of inositol 1,4,5-trisphosphate in Candida albicans: significance as a precursor of inositol polyphosphates and in signal transduction during the dimorphic transition from yeast cells to germ tubes

The metabolism of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] was examined in yeast cells and germ tubes of Candida albicans. Methods have been developed for analysis of the two key metabolic enzymes, Ins(1,4,5)P3, kinase and phosphatase. ATP-dependent Ins(1,4,5)P3 kinase activity was detected predominantly in the soluble fraction of cell extracts and exhibited a Km of approximately 9 microM. The apparent Km of Ins(1,4,5)P3 phosphatase for Ins(1,4,5)P3 was approximately 480 microM. The slow rate of dephosphorylation of Ins(1,4,5_P3 to inositol bisphosphate suggests a lower importance of the phosphatase within cells compared to the kinase. Since both yeast cells and germ tubes of C. albicans rapidly phosphorylated Ins(1,4,5)P3 to inositol tetrakisphosphate and inositol penta/hexakisphosphate, it is suggested that Ins(1,4,5)P3 has an important role as a precursor for production of these compounds. A sustained increase in cellular Ins(1,4,5)P3 levels was observed during germ tube formation and, prior to the onset of germination between 1 and 2 incubation, the Ins(1,4,5)P3 content increased up to eightfold. Transient increases in the level of Ins(1,4,5)P3 were also observed during yeast-like growth of C. albicans. The possible role and relative importance of Ins(1,4,5)P3 as a precursor for inositol polyphosphates and in signal transduction involving Ca2+ release from internal stores is discussed.

Identification and phylogenetic classification of eleven putative P-type calcium transport ATPase genes in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe

Calcium is an essential second messenger in yeast metabolism and physiology. So far, only four genes coding for calcium translocating ATPases had been discovered in yeast. The recent completion of the yeast Saccharomyces cerevisiae genome allowed us to identify six new putative Ca(++)-ATPases encoding genes. Protein sequence homology analysis and phylogenetic classification of all putative Ca(++)-ATPase gene products from the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe reveal three clusters of homologous proteins. Two of them comprises seven proteins which might belong to a new class of P-type ATPases of unknown subcellular location and of unknown physiological function.

A novel primary Ca(2+)-transport system from Saccharomyces cerevisiae

A novel primary Ca(2+)-transport system in membranes from Saccharomyces cerevisiae is described. Ca2+ transport is strictly dependent on the presence of ATP; other nucleotides like GTP, UTP and CTP do not efficiently (< 10% of the rate of ATP) drive uptake. Transport is inhibited by sodium vanadate with an IC50 of 130 microM, but is insensitive to carbonylcyanide p-trifluoromethoxy-phenylhydrazone, valinomycin, gramicidin or calmodulin. Ca2+ accumulates in a free form and can be readily released by the Ca2+ ionophore A-23187 or by osmotic shock. The apparent Km values of transport activity for free Ca2+ was determined to be 0.11 microM and 5 microM for Mg.ATP, respectively. Taken together the results indicate that the Ca2+ transport described here does not belong to the plasma-membrane-type Ca(2+)-ATPase family but rather to the family of endomembrane-type ATPases. Cell-fractionation studies of crude membranes on sucrose gradient centrifugation have shown that the Ca(2+)-transport activity separates from marker enzymes for endoplasmic reticulum, vacuole, or plasma membrane and migrates with GDPase activity, a marker for the yeast Golgi complex.

Inositol 1,4,5-trisphosphate releases Ca2+ from vacuolar membrane vesicles of Saccharomyces cerevisiae

Inositol 1,4,5-trisphosphate (IP3) induces a release of Ca2+ from vacuolar membrane vesicles of Saccharomyces cerevisiae. The amount released is dependent on IP3 concentration (concentration for half maximal effect, Km, apparent = 0.4 microM). Myo-inositol, and inositol 1,4-bisphosphate up to 50 microM have no effect on Ca2+ levels in the vesicles. The IP3-induced Ca2+ release is blocked by dantrolene and 8-(N,N-diethylamino)-octyl 3,4,5-trimethoxybenzoate-HCl (TMB-8), which are known to block Ca2+ release from Ca2+ stores in animal cells. IP3-induced release of Ca2+ also occurs when Ca2+ is accumulated by means of an artificial pH gradient, indicating that the effect of IP3 is not due to an effect on the vacuolar H(+)-ATPase. The IP3-induced Ca2+ release is not accompanied by a change in the pH gradient, which indicates that it is not due to a reversal of the Ca2+/nH+ antiport or to a decrease in delta pH by IP3. The present results suggest that IP3 may act as a second messenger in the mobilization of Ca2+ in yeast cells. As in plant cells, the vacuolar membrane of yeast seems to contain a Ca2+ channel, which can be opened by IP3. In this respect the vacuole could function as an IP3-regulated intracellular Ca2+ store, equivalent to the endoplasmic- and sarcoplasmic reticulum in animal cells, and play a role in Ca(2+)-dependent signal transduction in yeast cells.