Cytosolic Ca2+ homeostasis is a constitutive function of the V-ATPase in Saccharomyces cerevisiae

Diverse functions and molecular properties emerging for CAX cation/H+ exchangers in plants

Steep concentration gradients of many ions are actively maintained, with lower concentrations typically located in the cytosol, and higher concentrations in organelles and outside the cell. The vacuole is an important storage organelle for many ions. The concentration gradient of cations is established across the plant tonoplast, in part, by high-capacity cation/H+ (CAX) exchange activity. While plants may not be green yeast, analysis of CAX regulation and substrate specificity has been greatly aided by utilizing yeast as an experimental tool. The basic CAX biology in ARABIDOPSIS has immediate relevance toward understanding the functional interplay between diverse transport processes. The long-range applied goals are to identify novel transporters and express them in crop plants in order to "mine" nutrients out of the soil and into plants. In doing so, this could boost the levels of essential nutrients in plants.

The where, when, and how of organelle acidification by the yeast vacuolar H+-ATPase

All eukaryotic cells contain multiple acidic organelles, and V-ATPases are central players in organelle acidification. Not only is the structure of V-ATPases highly conserved among eukaryotes, but there are also many regulatory mechanisms that are similar between fungi and higher eukaryotes. These mechanisms allow cells both to regulate the pHs of different compartments and to respond to changing extracellular conditions. The Saccharomyces cerevisiae V-ATPase has emerged as an important model for V-ATPase structure and function in all eukaryotic cells. This review discusses current knowledge of the structure, function, and regulation of the V-ATPase in S. cerevisiae and also examines the relationship between biosynthesis and transport of V-ATPase and compartment-specific regulation of acidification.

Saccharomyces cerevisiae Lacking Btn1p Modulate Vacuolar ATPase Activity to Regulate pH Imbalance in the Vacuole

The vacuolar H(+)-ATPase (V-ATPase) along with ion channels and transporters maintains vacuolar pH. V-ATPase ATP hydrolysis is coupled with proton transport and establishes an electrochemical gradient between the cytosol and vacuolar lumen for coupled transport of metabolites. Btn1p, the yeast homolog to human CLN3 that is defective in Batten disease, localizes to the vacuole. We previously reported that Btn1p is required for vacuolar pH maintenance and ATP-dependent vacuolar arginine transport. We report that extracellular pH alters both V-ATPase activity and proton transport into the vacuole of wild-type Saccharomyces cerevisiae. V-ATPase activity is modulated through the assembly and disassembly of the V(0) and V(1) V-ATPase subunits located in the vacuolar membrane and on the cytosolic side of the vacuolar membrane, respectively. V-ATPase assembly is increased in yeast cells grown in high extracellular pH. In addition, at elevated extracellular pH, S. cerevisiae lacking BTN1 (btn1-Delta), have decreased V-ATPase activity while proton transport into the vacuole remains similar to that for wild type. Thus, coupling of V-ATPase activity and proton transport in btn1-Delta is altered. We show that down-regulation of V-ATPase activity compensates the vacuolar pH imbalance for btn1-Delta at early growth phases. We therefore propose that Btn1p is required for tight regulation of vacuolar pH to maintain the vacuolar luminal content and optimal activity of this organelle and that disruption in Btn1p function leads to a modulation of V-ATPase activity to maintain cellular pH homeostasis and vacuolar luminal content.

Functional dependence on calcineurin by variants of the Saccharomyces cerevisiae vacuolar Ca2+/H+ exchanger Vcx1p

The Ca(2+)-dependent protein phosphatase calcineurin is an important regulator of ion transporters from many organisms, including the Saccharomyces cerevisiae vacuolar Ca(2+)/H(+) exchanger Vcx1p. In yeast and plants, cation/H(+) exchangers are important in shaping cytosolic Ca(2+) levels involved in signal transduction and providing tolerance to potentially toxic concentrations of cations such as Ca(2+), Mn(2+) and Cd(2+). Previous genetic evidence suggested Vcx1p is negatively regulated by calcineurin. By utilizing direct transport measurements into vacuolar membrane vesicles, we demonstrate that Vcx1p is a low-affinity Ca(2+) transporter and may also function in Cd(2+) transport, but cannot transport Mn(2+). Furthermore, direct Ca(2+) transport by Vcx1p is calcineurin sensitive. Using a yeast growth assay, a mutant allele of VCX1 (VCX1-S204A/L208P), termed VCX1-M1, was previously found to confer strong Mn(2+) tolerance. Here we demonstrate that this Mn(2+) tolerance is independent of the Ca(2+)/Mn(2+)-ATPase Pmr1p and results from Mn(2+)-specific vacuolar transport activity of Vcx1-M1p. This Mn(2+) transport by Vcx1-M1p is calcineurin dependent, although the localization of Vcx1-M1p to the vacuole appears to be calcineurin independent. Additionally, we demonstrate that mutation of L208P alone is enough to confer calcineurin-dependent Mn(2+) tolerance. This study demonstrates that calcineurin can positively regulate the transport of cations by VCX1-M1p.

Calcium-induced virulence factors associated with the extracellular matrix of mucoid Pseudomonas aeruginosa biofilms

Pseudomonas aeruginosa colonizes the pulmonary tissue of patients with cystic fibrosis (CF), leading to biofilm-associated infections. The pulmonary fluid of CF patients usually contains elevated concentrations of cations and may contain the P. aeruginosa redox-active pigment pyocyanin, which is known to disrupt calcium homeostasis of host cells. Since divalent cations are important bridging ions for bacterial polysaccharides and since they may play regulatory roles in bacterial gene expression, we investigated the effect of calcium ions on the extracellular matrix constituents of P. aeruginosa biofilms. For mucoid strain P. aeruginosa FRD1, calcium addition (1.0 and 10 mM as CaCl(2)) resulted in biofilms that were at least 10-fold thicker than biofilms without added calcium. Scanning confocal laser microscopy showed increased spacing between cells for the thick biofilms, and Fourier transform infrared spectroscopy revealed that the material between cells is primarily alginate. An algD transcriptional reporter demonstrated that calcium addition caused an eightfold increase in alg gene expression in FRD1 biofilms. Calcium addition also resulted in increased amounts of three extracellular proteases (AprA, LasB, and PrpL). Immunoblots of the biofilm extracellular material established that AprA was harbored within the biofilm extracellular matrix. An aprA deletion mutation and a mutation in gene for a putative P. aeruginosa calmodulin-like protein did not significantly affect calcium-induced biofilm structure. Two-dimensional gel electrophoresis showed increased amounts of phenazine biosynthetic proteins in FRD1 biofilms and in calcium-amended planktonic cultures. Spectrochemical analyses showed that the calcium addition causes a three- to fivefold increase in pyocyanin production. These results demonstrate that calcium addition affects the structure and extracellular matrix composition of mucoid P. aeruginosa biofilms, through increased expression and stability of bacterial extracellular products. The calcium-induced extracellular matrix of mucoid P. aeruginosa consists primarily of the virulence factor alginate and also harbors extracellular proteases and perhaps pyocyanin, a biomolecule that may further disrupt cellular calcium levels.

A genomic screen for yeast vacuolar membrane ATPase mutants

V-ATPases acidify multiple organelles, and yeast mutants lacking V-ATPase activity exhibit a distinctive set of growth defects. To better understand the requirements for organelle acidification and the basis of these growth phenotypes, approximately 4700 yeast deletion mutants were screened for growth defects at pH 7.5 in 60 mm CaCl(2). In addition to 13 of 16 mutants lacking known V-ATPase subunits or assembly factors, 50 additional mutants were identified. Sixteen of these also grew poorly in nonfermentable carbon sources, like the known V-ATPase mutants, and were analyzed further. The cwh36Delta mutant exhibited the strongest phenotype; this mutation proved to disrupt a previously uncharacterized V-ATPase subunit. A small subset of the mutations implicated in vacuolar protein sorting, vps34Delta, vps15Delta, vps45Delta, and vps16Delta, caused both Vma- growth phenotypes and lower V-ATPase activity in isolated vacuoles, as did the shp1Delta mutation, implicated in both protein sorting and regulation of the Glc7p protein phosphatase. These proteins may regulate V-ATPase targeting and/or activity. Eight mutants showed a Vma- growth phenotype but no apparent defect in vacuolar acidification. Like V-ATPase-deficient mutants, most of these mutants rely on calcineurin for growth, particularly at high pH. A requirement for constitutive calcineurin activation may be the predominant physiological basis of the Vma- growth phenotype.

The intracellular dissipation of cytosolic calcium following glucose re-addition to carbohydrate depleted Saccharomyces cerevisiae

Glucose re-addition to carbohydrate starved yeast cells leads to a transient elevation of eytosolic calcium (TECC). Concomitantly, a cytosolic proton extrusion occurs through the activation of the vacuolar H(+)-ATPase and the plasma membrane H(+)-ATPases. This study addressed the dissipation of the TECC through intracellular compartmentalization and the possible affects of the H(+)-ATPases on this process. Both the vacuole and the Golgi-ER apparatus were found to play important roles in distributing calcium to internal stores. Additionally, the inhibition of cytosolic proton extrusion augmented cytosolic calcium responses. A model where pH dependent cytosolic calcium buffering plays an important role in the dissipation of the TECC in Saccharomyces cerevisiae is proposed.

Characterization of Schizosaccharomyces pombe mutants defective in vacuolar acidification and protein sorting

The vacuolar H+-ATPases (V-ATPases) are ATP-dependent proton pumps responsible for acidification of intracellular compartments in eukaryotic cells. To investigate the functional roles of the V-ATPase in Schizosaccharomyces pombe, the gene vma1 encoding subunit A or vma3 encoding subunit c was disrupted. Both deletion mutants lost the capacity for vacuolar acidification in vivo, and showed sensitivity to neutral pH or high concentrations of divalent cations including Ca2+. The delivery of FM4-64 to the vacuolar membrane and accumulation of Lucifer Yellow CH were strongly inhibited in the vma1 and vma3 mutants. Moreover, deletion of the S. pombe vma1+ or vma3+ gene resulted in pleiotropic phenotypes consistent with lack of vacuolar acidification, including the missorting of vacuolar carboxypeptidase Y, abnormal vacuole morphology, and mating defects. These findings suggest that V-ATPase is essential for endocytosis, ion and pH homeostasis, and for intracellular targeting of vacuolar proteins and vacuolar biogenesis in S. pombe.

The glycolytic enzyme aldolase mediates assembly, expression, and activity of vacuolar H+-ATPase

Vacuolar H(+)-ATPases (V-ATPases) are a family of highly conserved proton pumps that couple hydrolysis of cytosolic ATP to proton transport out of the cytosol. How ATP is supplied for V-ATPase-mediated hydrolysis and for coupling of proton transport is poorly understood. We have reported that the glycolytic enzyme aldolase physically associates with V-ATPase. Here we show that aldolase interacts with three different subunits of V-ATPase (subunits a, B, and E). The binding sites for the V-ATPase subunits on aldolase appear to be on distinct interfaces of the glycolytic enzyme. Aldolase deletion mutant cells were able to grow in medium buffered at pH 5.5 but not at pH 7.5, displaying a growth phenotype similar to that observed in V-ATPase subunit deletion mutants. Abnormalities in V-ATPase assembly and protein expression observed in aldolase deletion mutant cells could be fully rescued by aldolase complementation. The interaction between aldolase and V-ATPase increased dramatically in the presence of glucose, suggesting that aldolase may act as a glucose sensor for V-ATPase regulation. Taken together, these findings provide functional evidence that the ATP-generating glycolytic pathway is directly coupled to the ATP-hydrolyzing proton pump through physical interaction between aldolase and V-ATPase.

Activation and significance of vacuolar H+-ATPase in Saccharomyces cerevisiae adaptation and resistance to the herbicide 2,4-dichlorophenoxyacetic acid

The stimulation of the activity of the H(+)-ATPase present in the vacuolar membrane (V-ATPase) of Saccharomyces cerevisiae is here described in response to a moderate stress induced by 2,4-dichlorophenoxyacetic acid (2,4-D). This in vivo activation (up to 5-fold) took place essentially during the adaptation period, preceding cell division under herbicide stress, in coordination with a marked activation of plasma membrane H(+)-ATPase (PM-ATPase) (up to 30-fold) and the decrease of intracellular and vacuolar pH values, suggesting that activation may be triggered by acidification. Single deletion of VMA1 and genes encoding other V-ATPase subunits led to a more extended period of adaptation and to slower growth under 2,4-D stress. Results suggest that a functional V-ATPase is required to counteract, more rapidly and efficiently, the dissipation of the physiological H(+)-gradient across vacuolar membrane registered during 2,4-D adaptation.

Human targets of Pseudomonas aeruginosa pyocyanin

Pseudomonas aeruginosa produces copious amounts of the redoxactive tricyclic compound pyocyanin that kills competing microbes and mammalian cells, especially during cystic fibrosis lung infection. Cross-phylum susceptibility to pyocyanin suggests the existence of evolutionarily conserved physiological targets. We screened a Saccharomyces cerevisiae deletion library to identify presumptive pyocyanin targets with the expectation that similar targets would be conserved in humans. Fifty S. cerevisiae targets were provisionally identified, of which 60% have orthologous human counterparts. These targets encompassed major cellular pathways involved in the cell cycle, electron transport and respiration, epidermal cell growth, protein sorting, vesicle transport, and the vacuolar ATPase. Using cultured human lung epithelial cells, we showed that pyocyanin-mediated reactive oxygen intermediates inactivate human vacuolar ATPase, supporting the validity of the yeast screen. We discuss how the inactivation of V-ATPase may negatively impact the lung function of cystic fibrosis patients.

Vacuole membrane fusion: V0 functions after trans-SNARE pairing and is coupled to the Ca2+-releasing channel

Pore models of membrane fusion postulate that cylinders of integral membrane proteins can initiate a fusion pore after conformational rearrangement of pore subunits. In the fusion of yeast vacuoles, V-ATPase V0 sectors, which contain a central cylinder of membrane integral proteolipid subunits, associate to form a transcomplex that might resemble an intermediate postulated in some pore models. We tested the role of V0 sectors in vacuole fusion. V0 functions in fusion and proton translocation could be experimentally separated via the differential effects of mutations and inhibitory antibodies. Inactivation of the V0 subunit Vph1p blocked fusion in the terminal reaction stage that is independent of a proton gradient. Deltavph1 mutants were capable of docking and trans-SNARE pairing and of subsequent release of lumenal Ca2+, but they did not fuse. The Ca2+-releasing channel appears to be tightly coupled to V0 because inactivation of Vph1p by antibodies blocked Ca2+ release. Vph1 deletion on only one fusion partner sufficed to severely reduce fusion activity. The functional requirement for Vph1p correlates to V0 transcomplex formation in that both occur after docking and Ca2+ release. These observations establish V0 as a crucial factor in vacuole fusion acting downstream of trans-SNARE pairing.

Extracellular Ca(2+) sensing contributes to excess Ca(2+) accumulation and vacuolar fragmentation in a pmr1Delta mutant of S. cerevisiae

Previous studies have suggested that yeast strains lacking the Ca(2+)-ATPase Pmr1p are unable to maintain an adequate level of Ca(2+) within the Golgi apparatus. It is thought that this compartmental store depletion induces a signal that causes an increased rate of Ca(2+) uptake and accumulation in a manner similar to the capacitative Ca(2+) entry (CCE) response in non-excitable mammalian cells. To explore this model further, we examined cellular Ca(2+) uptake and accumulation in a pmr1Delta strain grown in the presence of a reduced level of divalent cations. We found that the level of Ca(2+) uptake and accumulation in a pmr1Delta strain increased as the concentration of divalent cations in the growth medium decreased. These results are inconsistent with a model in which cellular Ca(2+) uptake and accumulation are determined solely by the depletion of Ca(2+) in an intracellular compartment. Instead, our results suggest that a second regulatory mechanism couples cellular Ca(2+) uptake to the availability of Ca(2+) in the extracellular environment. Furthermore, we found that various conditions that increase the level of cytosolic Ca(2+) correlate with vacuolar fragmentation in wild-type (WT), pmr1Delta and pmr1Delta/pmc1Delta yeast strains. This suggests that vacuolar fragmentation might function as a normal physiological response to Ca(2+) stress that increases the vacuolar surface/volume ratio, thereby maximizing the sequestration of this important signaling molecule.

Biosynthesis, glycosylation, and enzymatic processing in vivo of human tripeptidyl-peptidase I

Human tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal serine protease that removes tripeptides from the free N termini of small polypeptides and also shows a minor endoprotease activity. Due to various naturally occurring mutations, an inherited deficiency of TPP I activity causes a fatal lysosomal storage disorder, classic late infantile neuronal ceroid lipofuscinosis (CLN2). In the present study, we analyzed biosynthesis, glycosylation, transport, and proteolytic processing of this enzyme in stably transfected Chinese hamster ovary cells as well as maturation of the endocytosed proenzyme in CLN2 lymphoblasts, fibroblasts, and N2a cells. Human TPP I was initially identified as a single precursor polypeptide of approximately 68 kDa, which, within a few hours, was converted to the mature enzyme of approximately 48 kDa. Compounds affecting the pH of intracellular acidic compartments, those interfering with the intracellular vesicular transport as well as inhibition of the fusion between late endosomes and lysosomes by temperature block or 3-methyladenine, hampered the conversion of TPP I proenzyme into the mature form, suggesting that this process takes place in lysosomal compartments. Digestion of immunoprecipitated TPP I proenzyme with both N-glycosidase F and endoglycosidase H as well as treatment of the cells with tunicamycin reduced the molecular mass of TPP I proenzyme by approximately 10 kDa, which indicates that all five potential N-glycosylation sites in TPP I are utilized. Mature TPP I was found to be partially resistant to endo H treatment; thus, some of its N-linked oligosaccharides are of the complex/hybrid type. Analysis of the effect of various classes of protease inhibitors and mutation of the active site Ser(475) on human TPP I maturation in cultured cells demonstrated that although TPP I zymogen is capable of autoactivation in vitro, a serine protease that is sensitive to AEBSF participates in processing of the proenzyme to the mature, active form in vivo.

Ca(2+) efflux in mitochondria from the yeast Endomyces magnusii

Calcium release pathways in Ca(2+)-preloaded mitochondria from the yeast Endomyces magnusii were studied. In the presence of phosphate as a permeant anion, Ca(2+) was released from respiring mitochondria only after massive cation loading at the onset of anaerobiosis. Ca(2+) release was not affected by cyclosporin A, an inhibitor of the mitochondrial permeability transition. Aeration of the mitochondrial suspension inhibited the efflux of Ca(2+) and induced its re-uptake. With acetate as the permeant anion, a spontaneous net Ca(2+) efflux set in after uptake of approximately 150 nmol of Ca(2+)/mg of protein. The rate of this efflux was proportional to the Ca(2+) load and insensitive to aeration, protonophorous uncouplers, and Na(+) ions. Ca(2+) efflux was inhibited by La(3+), Mn(2+), Mg(2+), tetraphenylphosphonium, inorganic phosphate, and nigericin and stimulated by hypotonicity, spermine, and valinomycin in the presence of 4 mm KCl. Atractyloside and t-butyl hydroperoxide were without effect. Ca(2+) efflux was associated with contraction, but not with mitochondrial swelling. We conclude that the permeability transition pore is not involved in Ca(2+) efflux in preloaded E. magnusii mitochondria. The efflux occurs via an Na(+)-independent pathway, in many ways similar to the one in mammalian mitochondria.

The KEX2 gene of Candida glabrata is required for cell surface integrity

Candida glabrata has emerged as one of the most common causes of candidosis. In order to identify factors that are necessary for viability and pathogenicity of this fungal pathogen, we analysed the role of the KEX2 gene, which codes for a regulatory endoproteinase that is known to process certain virulence factors in Candida albicans. The KEX2 gene from C. glabrata was cloned and found to have 51% and 62% identity and high structural similarities to the homologous counterparts in C. albicans and Saccharomyces cerevisiae. KEX2 was expressed at all time points investigated during growth in complex medium. In order to investigate the role of this putative regulatory proteinase, Kex2-deficient mutants were produced. In addition to known kex2 phenotypes, such as pH and calcium hypersensitivity, the mutants grew in cellular aggregates and were found to be hypersensitive to several antifungal drugs that target the cell membrane, including azoles, amorolfine and amphotericin B. Ultrastructural investigation after exposure to low doses of itraconazole showed azole-specific alterations such as enlarged vacuoles and proliferation of the cytoplasmatic membrane in the kex2 mutants, but not in the control strains. In contrast, antifungals such as 5-flucytosine and hydroxypyridones inhibited growth of the kex2 mutants and the control strains to the same extent. In an in vitro model of oral candidosis, kex2 mutants showed reduced tissue damage in the presence of itraconazole compared with the control infections. These data suggest that Kex2 is involved in the processing of proteins that are essential for cell surface integrity of C. glabrata.