Loss of Vacuolar H+-ATPase Activity in Organelles Signals Ubiquitination and Endocytosis of the Yeast Plasma Membrane Proton pump Pma1p

Yeast mutants lacking the intracellular V-ATPase proton pump (vma mutants) have reduced levels of the Pma1p proton pump at the plasma membrane and increased levels in organelles including the vacuolar lumen. We examined the mechanism and physiological consequences of Pma1p mislocalization. Pma1p is ubiquitinated in vma mutants, and ubiquitination depends on the ubiquitin ligase Rsp5p and the arrestin-related adaptor protein Rim8p. vma mutant strains containing rsp5 or rim8 mutations maintain Pma1p at the plasma membrane, suggesting that ubiquitination is required for Pma1p internalization. Acute inhibition of V-ATPase activity with concanamycin A triggers Pma1p ubiquitination and internalization. In an endocytosis-deficient mutant (end4Δ) Pma1p is ubiquitinated but retained at the plasma membrane during concanamycin A treatment. Consistent with specificity in signaling loss of V-ATPase activity to Pma1p, another plasma membrane transporter, Mup1p, is not internalized in a vma mutant, and loss of the Mup1p adaptor Art1p does not prevent Pma1p internalization in a vma mutant. Very poor growth of vma2 rsp5-1 and vma2 rim8Δ double mutants suggests that Pma1p internalization benefits the vma mutants. We hypothesize that loss of V-ATPase-mediated organelle acidification signals ubiquitination, internalization, and degradation of a portion of Pma1p as a means of balancing overall pH homeostasis.     

The RAVE complex is an isoform-specific V-ATPase assembly factor in yeast

The regulator of ATPase of vacuoles and endosomes (RAVE) complex is implicated in vacuolar H⁺-translocating ATPase (V-ATPase) assembly and activity. In yeast, rav1∆ mutants exhibit a Vma⁻ growth phenotype characteristic of loss of V-ATPase activity only at high temperature. Synthetic genetic analysis identified mutations that exhibit a full, temperature-independent Vma⁻ growth defect when combined with the rav1∆ mutation. These include class E vps mutations, which compromise endosomal sorting. The synthetic Vma⁻ growth defect could not be attributed to loss of vacuolar acidification in the double mutants, as there was no vacuolar acidification in the rav1∆ mutant. The yeast V-ATPase a subunit is present as two isoforms, Stv1p in Golgi and endosomes and Vph1p in vacuoles. Rav1p interacts directly with the N-terminal domain of Vph1p. STV1 overexpression suppressed the growth defects of both rav1∆ and rav1∆vph1∆, and allowed RAVE-independent assembly of active Stv1p-containing V-ATPases in vacuoles. Mutations causing synthetic genetic defects in combination with rav1∆ perturbed the normal localization of Stv1–green fluorescent protein. We propose that RAVE is necessary for assembly of Vph1-containing V-ATPase complexes but not Stv1-containing complexes. Synthetic Vma⁻ phenotypes arise from defects in Vph1p-containing complexes caused by rav1∆, combined with defects in Stv1p-containing V-ATPases caused by the second mutation. Thus RAVE is the first isoform-specific V-ATPase assembly factor.     

Vacuolar H+-ATPase is involved in preventing heavy metal-induced oxidative stress in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, vacuolar H⁺-ATPase (V-ATPase) involved in the regulation of intracellular pH homeostasis has been shown to be important for tolerances to cadmium, cobalt and nickel. However, the molecular mechanism underlying the protective role of V-ATPase against these metals remains unclear. In this study, we show that cadmium, cobalt and nickel disturbed intracellular pH balance by triggering cytosolic acidification and vacuolar alkalinization, likely via their membrane permeabilizing effects. Since V-ATPase plays a crucial role in pumping excessive cytosolic protons into the vacuole, the metal-sensitive phenotypes of the Δvma2 and Δvma3 mutants lacking V-ATPase activity were supposed to result from highly acidified cytosol. However, we found that the metal-sensitive phenotypes of these mutants were caused by increased production of reactive oxygen species, likely as a result of decreased expression and activities of manganese superoxide dismutase and catalase. In addition, the loss of V-ATPase function led to aberrant vacuolar morphology and defective endocytic trafficking. Furthermore, the sensitivities of the Δvma mutants to other chemical compounds (i.e. acetic acid, H₂O₂, menadione, tunicamycin and cycloheximide) were a consequence of increased endogenous oxidative stress. These findings, therefore, suggest the important role of V-ATPase in preventing endogenous oxidative stress induced by metals and other chemical compounds.     

Saccharomyces cerevisiae Mutants Affected in Vacuole Assembly or Vacuolar H+-ATPase are Hypersensitive to Lead (Pb) Toxicity

Lead is an important environmental pollutant. The role of vacuole, in Pb detoxification, was studied using a vacuolar protein sorting mutant strain (vps16Δ), belonging to class C mutants. Cells disrupted in VPS16 gene, did not display a detectable vacuolar-like structure. Based on the loss of cell proliferation capacity, it was found that cells from vps16Δ mutant exhibited a hypersensitivity to Pb-induced toxicity, compared to wild type (WT) strain. The function of vacuolar H⁺-ATPase (V-ATPase), in Pb detoxification, was evaluated using mutants with structurally normal vacuoles but defective in subunits of catalytic (vma1Δ or vma2Δ) or membrane domain (vph1Δ or vma3Δ) of V-ATPase. All mutants tested, lacking a functional V-ATPase, displayed an increased susceptibility to Pb, comparatively to cells from WT strain. Modification of vacuolar morphology, in Pb-exposed cells, was visualized using a Vma2p-GFP strain. The treatment of yeast cells with Pb originated the fusion of the medium size vacuolar lobes into one enlarged vacuole. In conclusion, it was found that vacuole plays an important role in the detoxification of Pb in Saccharomyces cerevisiae; in addition, a functional V-ATPase was required for Pb compartmentalization.     

Characterization of<Emphasis Type="Italic"> Schizosaccharomyces pombe</Emphasis> 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.Communicated by M. Johnston     

V-ATPase dysfunction suppresses polyphosphate synthesis in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae accumulates the high levels of inorganic polyphosphates (polyPs) performing in the cells numerous functions, including phosphate and energy storage. The effects of vacuolar membrane ATPase (V-ATPase) dysfunction were studied on polyP accumulation under short-term cultivation in the P_i–excess media after P_i starvation. The addition of bafilomycin A1, a specific inhibitor of V-ATPase, to the medium with glucose resulted in strong inhibition of the synthesis of long-chain polyP and in substantial suppression of short-chain polyP. The addition of bafilomycin to the medium with ethanol resulted in decreased accumulation of high-molecular polyP, while the accumulation of low-molecular polyP was not affected. The levels of polyP synthesis in the mutant strain with a deletion in the vma2 gene encoding a V-ATPase subunit were significantly lower than in the parent strain in the media with glucose and with ethanol. The synthesis of the longest chain polyP was not observed in the mutant cells. The synthesis of only the low-polymer acid-soluble polyP fraction occurred in the cells of the mutant strain. However, the level of polyP1 was nearly tenfold lower than compared to the cells of the parent strain. Both bafilomycin A1 and the mutation in vacuolar ATPase subunit vma2 lead to a considerable decrease of cellular polyP accumulation. Thus, the defects in ΔμH⁺ formation on the vacuolar membrane resulted in the decrease of polyP biosynthesis in S. cerevisiae.     

Involvement of MoVMA11, a Putative Vacuolar ATPase c’ Subunit,in Vacuolar Acidification and Infection-Related Morphogenesis of Magnaporthe oryzae

Many functions of vacuole depend on the activity of vacuolar ATPase which is essential to maintain an acidic lumen and create the driving forces for massive fluxes of ions and metabolites through vacuolar membrane. In filamentous fungus Magnaporthe oryzae , subcellular colocalization and quinacrine staining suggested that the V₁V₀ domains of V-ATPase were fully assembled and the vacuoles were kept acidic during infection-related developments. Targeted gene disruption of MoVMA11 gene, encoding the putative c’ subunit of V-ATPase, impaired vacuolar acidification and mimicked the phenotypes of yeast V-ATPase mutants in the poor colony morphology, abolished asexual and sexual reproductions, selective carbon source utilization, and increased calcium and heavy metals sensitivities, however, not in the typical pH conditional lethality. Strikingly, aerial hyphae of the MoVMA11 null mutant intertwined with each other to form extremely thick filamentous structures. The results also implicated that MoVMA11 was involved in cell wall integrity and appressorium formation. Abundant non-melanized swollen structures and rare, small appressoria without penetration ability were produced at the hyphal tips of the ΔMovma11 mutant on onion epidermal cells. Finally, the MoVMA11 null mutant lost pathogenicity on both intact and wounded host leaves. Overall, our data indicated that MoVMA11, like other fungal VMA genes, is associated with numerous cellular functions and highlighted that V-ATPase is essential for infection-related morphogenesis and pathogenesis in M . oryzae .     

Vma21p is a yeast membrane protein retained in the endoplasmic reticulum by a di-lysine motif and is required for the assembly of the vacuolar H(+)-ATPase complex.

The yeast vacuolar proton-translocating ATPase (V-ATPase) is a multisubunit complex comprised of peripheral membrane subunits involved in ATP hydrolysis and integral membrane subunits involved in proton pumping. The yeast vma21 mutant was isolated from a screen to identify mutants defective in V-ATPase function. vma21 mutants fail to assemble the V-ATPase complex onto the vacuolar membrane: peripheral subunits accumulate in the cytosol and the 100-kDa integral membrane subunit is rapidly degraded. The product of the VMA21 gene (Vma21p) is an 8.5-kDa integral membrane protein that is not a subunit of the purified V-ATPase complex but instead resides in the endoplasmic reticulum. Vma21p contains a dilysine motif at the carboxy terminus, and mutation of these lysine residues abolishes retention in the endoplasmic reticulum and results in delivery of Vma21p to the vacuole, the default compartment for yeast membrane proteins. Our findings suggest that Vma21p is required for assembly of the integral membrane sector of the V-ATPase in the endoplasmic reticulum and that the unassembled 100-kDa integral membrane subunit present in delta vma21 cells is rapidly degraded by nonvacuolar proteases.     

The Histoplasma capsulatum vacuolar ATPase is required for iron homeostasis, intracellular replication in macrophages and virulence in a murine model of histoplasmosis

Histoplasma capsulatum is a dimorphic fungal pathogen that survives and replicates within macrophages (Mphi). To identify specific genes required for intracellular survival, we utilized Agrobacterium tumefaciens-mediated mutagenesis, and screened for H. capsulatum insertional mutants that were unable to survive in human Mphi. One colony was identified that had an insertion within VMA1, the catalytic subunit A of the vacuolar ATPase (V-ATPase). The vma1 mutant (vma1::HPH) grew normally on iron-replete medium, but not on iron-deficient media. On iron-deficient medium, the growth of the vma1 mutant was restored in the presence of wild-type (WT) H. capsulatum yeasts, or the hydroxamate siderophore, rhodotorulic acid. However, the inability to replicate within Mphi was only partially restored by the addition of exogenous iron. The vma1::HPH mutant also did not grow as a mold at 28 degrees C. Complementation of the mutant (vma/VMA1) restored its ability to replicate in Mphi, grow on iron-poor medium and grow as a mold at 28 degrees C. The vma1::HPH mutant was avirulent in a mouse model of histoplasmosis, whereas the vma1/VMA1 strain was as pathogenic as WT yeasts. These studies demonstrate the importance of V-ATPase function in the pathogenicity of H. capsulatum, in iron homeostasis and in fungal dimorphism.     

Subunit composition,biosynthesis, and assembly of the yeast vacuolar proton-translocating ATPase

The yeast vacuole is acidified by a vacuolar proton-translocating ATPase (H⁺-ATPase) that closely resembles the vacuolar H⁺-ATPases of other fungi, animals, and plants. The yeast enzyme is purified as a complex of eight subunits, which include both integral and peripheral membrane proteins. The genes for seven of these subunits have been cloned, and mutant strains lacking each of the subunits (vma mutants) have been constructed. Disruption of any of the subunit genes appears to abolish the function of the vacuolar H⁺-ATPase, supporting the subunit composition derived from biochemical studies. Genetic studies of vacuolar acidification have also revealed an additional set of gene products that are required for vacuolar H⁺-ATPase activity, but may not be part of the final enzyme complex. The biosynthesis, assembly, and targeting of the enzyme is being elucidated by biochemical and cell biological studies of thevma mutants. Initial results suggest that the peripheral and integral membrane subunits may be independently assembled.     

The long physiological reach of the yeast vacuolar H+-ATPase

V-ATPases are structurally conserved and functionally versatile proton pumps found in all eukaryotes. The yeast V-ATPase has emerged as a major model system, in part because yeast mutants lacking V-ATPase subunits (vma mutants) are viable and exhibit a distinctive Vma- phenotype. Yeast vma mutants are present in ordered collections of all non-essential yeast deletion mutants, and a number of additional phenotypes of these mutants have emerged in recent years from genomic screens. This review summarizes the many phenotypes that have been associated with vma mutants through genomic screening. The results suggest that V-ATPase activity is important for an unexpectedly wide range of cellular processes. For example, vma mutants are hypersensitive to multiple forms of oxidative stress, suggesting an antioxidant role for the V-ATPase. Consistent with such a role, vma mutants display oxidative protein damage and elevated levels of reactive oxygen species, even in the absence of an exogenous oxidant. This endogenous oxidative stress does not originate at the electron transport chain, and may be extra-mitochondrial, perhaps linked to defective metal ion homeostasis in the absence of a functional V-ATPase. Taken together, genomic data indicate that the physiological reach of the V-ATPase is much longer than anticipated. Further biochemical and genetic dissection is necessary to distinguish those physiological effects arising directly from the enzyme’s core functions in proton pumping and organelle acidification from those that reflect broader requirements for cellular pH homeostasis or alternative functions of V-ATPase subunits.     

Loss of vacuolar proton-translocating ATPase activity in yeast results in chronic oxidative stress

Yeast mutants lacking vacuolar proton-translocating ATPase (V-ATPase) subunits (vma mutants) were sensitive to several different oxidants in a recent genomic screen (Thorpe, G. W., Fong, C. S., Alic, N., Higgins, V. J., and Dawes, I. W. (2004) Proc. Natl. Acad. Sci. U. S. A. 101, 6564-6569). We confirmed that mutants lacking a V(1) subunit (vma2Delta), V(o) subunit, or either of the two V(o) a subunit isoforms are acutely sensitive to H(2)O(2) and more sensitive to menadione and diamide than wild-type cells. The vma2Delta mutant contains elevated levels of reactive oxygen species and high levels of oxidative protein damage even in the absence of an applied oxidant, suggesting an endogenous source of oxidative stress. vma2Delta mutants lacking mitochondrial DNA showed neither improved growth nor decreased sensitivity to peroxide, excluding respiration as the major source of the endogenous reactive oxygen species in the mutant. Double mutants lacking both VMA2 and components of the major cytosolic defense systems exhibited synthetic sensitivity to H(2)O(2). Microarray analysis comparing wild-type and vma2Delta mutant cells grown at pH 5, permissive conditions for the vma2Delta mutant, indicated high level up-regulation of several iron uptake and metabolism genes that are part of the Aft1/Aft2 regulon. TSA2, which encodes an isoform of the cytosolic thioredoxin peroxidase, was strongly induced, but other oxidative stress defense systems were not induced. The results indicate that V-ATPase activity helps to protect cells from endogenous oxidative stress.     

Requirement for Ergosterol in V-ATPase Function Underlies Antifungal Activity of Azole Drugs

Ergosterol is an important constituent of fungal membranes. Azoles inhibit ergosterol biosynthesis, although the cellular basis for their antifungal activity is not understood. We used multiple approaches to demonstrate a critical requirement for ergosterol in vacuolar H⁺-ATPase function, which is known to be essential for fungal virulence. Ergosterol biosynthesis mutants of S. cerevisiae failed to acidify the vacuole and exhibited multiple vma ⁻ phenotypes. Extraction of ergosterol from vacuolar membranes also inactivated V-ATPase without disrupting membrane association of its subdomains. In both S. cerevisiae and the fungal pathogen C. albicans, fluconazole impaired vacuolar acidification, whereas concomitant ergosterol feeding restored V-ATPase function and cell growth. Furthermore, fluconazole exacerbated cytosolic Ca²⁺ and H⁺ surges triggered by the antimicrobial agent amiodarone, and impaired Ca²⁺ sequestration in purified vacuolar vesicles. These findings provide a mechanistic basis for the synergy between azoles and amiodarone observed in vitro. Moreover, we show the clinical potential of this synergy in treatment of systemic fungal infections using a murine model of Candidiasis. In summary, we demonstrate a new regulatory component in fungal V-ATPase function, a novel role for ergosterol in vacuolar ion homeostasis, a plausible cellular mechanism for azole toxicity in fungi, and preliminary in vivo evidence for synergism between two antifungal agents. New insights into the cellular basis of azole toxicity in fungi may broaden therapeutic regimens for patient populations afflicted with systemic fungal infections.     

Deletion of Vacuolar Proton-translocating ATPase Voa Isoforms Clarifies the Role of Vacuolar pH as a Determinant of Virulence-associated Traits in Candida albicans

Vacuolar proton-translocating ATPase (V-ATPase) is a central regulator of cellular pH homeostasis, and inactivation of all V-ATPase function has been shown to prevent infectivity in Candida albicans. V-ATPase subunit a of the V_o domain (V_oa) is present as two fungal isoforms: Stv1p (Golgi) and Vph1p (vacuole). To delineate the individual contribution of Stv1p and Vph1p to C. albicans physiology, we created stv1Δ/Δ and vph1Δ/Δ mutants and compared them to the corresponding reintegrant strains (stv1Δ/ΔR and vph1Δ/ΔR). V-ATPase activity, vacuolar physiology, and in vitro virulence-related phenotypes were unaffected in the stv1Δ/Δ mutant. The vph1Δ/Δ mutant exhibited defective V₁V_o assembly and a 90% reduction in concanamycin A-sensitive ATPase activity and proton transport in purified vacuolar membranes, suggesting that the Vph1p isoform is essential for vacuolar V-ATPase activity in C. albicans. The vph1Δ/Δ cells also had abnormal endocytosis and vacuolar morphology and an alkalinized vacuolar lumen (pH_vph1Δ_/Δ = 6.8 versus pH_vph1Δ_/Δ_R = 5.8) in both yeast cells and hyphae. Secreted protease and lipase activities were significantly reduced, and M199-induced filamentation was impaired in the vph1Δ/Δ mutant. However, the vph1Δ/Δ cells remained competent for filamentation induced by Spider media and YPD, 10% FCS, and biofilm formation and macrophage killing were unaffected in vitro. These studies suggest that different virulence mechanisms differentially rely on acidified vacuoles and that the loss of both vacuolar (Vph1p) and non-vacuolar (Stv1p) V-ATPase activity is necessary to affect in vitro virulence-related phenotypes. As a determinant of C. albicans pathogenesis, vacuolar pH alone may prove less critical than originally assumed.     

Diploids heterozygous for a vma13Delta mutation in Saccharomyces cerevisiae highlight the importance of V-ATPase subunit balance in supporting vacuolar acidification and silencing cytosolic V1-ATPase activity

The V-ATPase H subunit (encoded by the VMA13 gene) activates ATP-driven proton pumping in intact V-ATPase complexes and inhibits MgATPase activity in cytosolic V1 sectors (Parra, K. J., Keenan, K. L., and Kane, P. M. (2000) J. Biol. Chem. 275, 21761-21767). Yeast diploids heterozygous for a vma13Delta mutation show the pH- and calcium-dependent conditional lethality characteristic of mutants lacking V-ATPase activity, although they still contain one wild-type copy of VMA13. Vacuolar vesicles from this strain have approximately 50% of the ATPase activity of those from a wild-type diploid but do not support formation of a proton gradient. Compound heterozygotes with a second heterozygous deletion in another V1 subunit gene exhibit improved growth, vacuolar acidification, and ATP-driven proton transport in vacuolar vesicles. In contrast, compound heterozygotes with a second deletion in a Vo subunit grow even more poorly than the vma13Delta heterozygote, have very little vacuolar acidification, and have very low levels of V-ATPase subunits in isolated vacuoles. In addition, cytosolic V1 sectors from this strain and from the strain containing only the heterozygous vma13Delta mutation have elevated MgATPase activity. The results suggest that balancing levels of subunit H with those of other V-ATPase subunits is critical, both for allowing organelle acidification and for preventing unproductive hydrolysis of cytosolic ATP.     

Consequences of loss of Vph1 protein-containing vacuolar ATPases (V-ATPases) for overall cellular pH homeostasis

In yeast cells, subunit a of the vacuolar proton pump (V-ATPase) is encoded by two organelle-specific isoforms, VPH1 and STV1. V-ATPases containing Vph1 and Stv1 localize predominantly to the vacuole and the Golgi apparatus/endosomes, respectively. Ratiometric measurements of vacuolar pH confirm that loss of STV1 has little effect on vacuolar pH. Loss of VPH1 results in vacuolar alkalinization that is even more rapid and pronounced than in vma mutants, which lack all V-ATPase activity. Cytosolic pH responses to glucose addition in the vph1Δ mutant are similar to those in vma mutants. The extended cytosolic acidification in these mutants arises from reduced activity of the plasma membrane proton pump, Pma1p. Pma1p is mislocalized in vma mutants but remains at the plasma membrane in both vph1Δ and stv1Δ mutants, suggesting multiple mechanisms for limiting Pma1 activity when organelle acidification is compromised. pH measurements in early prevacuolar compartments via a pHluorin fusion to the Golgi protein Gef1 demonstrate that pH responses of these compartments parallel cytosolic pH changes. Surprisingly, these compartments remain acidic even in the absence of V-ATPase function, possibly as a result of cytosolic acidification. These results emphasize that loss of a single subunit isoform may have effects far beyond the organelle where it resides.     

Perturbation of the Vacuolar ATPase: A NOVEL CONSEQUENCE OF INOSITOL DEPLETION*

Depletion of inositol has profound effects on cell function and has been implicated in the therapeutic effects of drugs used to treat epilepsy and bipolar disorder. We have previously shown that the anticonvulsant drug valproate (VPA) depletes inositol by inhibiting myo-inositol-3-phosphate synthase, the enzyme that catalyzes the first and rate-limiting step of inositol biosynthesis. To elucidate the cellular consequences of inositol depletion, we screened the yeast deletion collection for VPA-sensitive mutants and identified mutants in vacuolar sorting and the vacuolar ATPase (V-ATPase). Inositol depletion caused by starvation of ino1Δ cells perturbed the vacuolar structure and decreased V-ATPase activity and proton pumping in isolated vacuolar vesicles. VPA compromised the dynamics of phosphatidylinositol 3,5-bisphosphate (PI3,5P₂) and greatly reduced V-ATPase proton transport in inositol-deprived wild-type cells. Osmotic stress, known to increase PI3,5P₂ levels, did not restore PI3,5P₂ homeostasis nor did it induce vacuolar fragmentation in VPA-treated cells, suggesting that perturbation of the V-ATPase is a consequence of altered PI3,5P₂ homeostasis under inositol-limiting conditions. This study is the first to demonstrate that inositol depletion caused by starvation of an inositol synthesis mutant or by the inositol-depleting drug VPA leads to perturbation of the V-ATPase.     

A plant proton-pumping inorganic pyrophosphatase functionally complements the vacuolar ATPase transport activity and confers bafilomycin resistance in yeast

V-ATPases (vacuolar H+-ATPases) are a specific class of multi-subunit pumps that play an essential role in the generation of proton gradients across eukaryotic endomembranes. Another simpler proton pump that co-localizes with the V-ATPase occurs in plants and many protists: the single-subunit H+-PPase [H+-translocating PPase (inorganic pyrophosphatase)]. Little is known about the relative contribution of these two proteins to the acidification of intracellular compartments. In the present study, we show that the expression of a chimaeric derivative of the Arabidopsis thaliana H+-PPase AVP1, which is preferentially targeted to internal membranes of yeast, alleviates the phenotypes associated with V-ATPase deficiency. Phenotypic complementation was achieved both with a yeast strain with its V-ATPase specifically inhibited by bafilomycin A1 and with a vma1-null mutant lacking a catalytic V-ATPase subunit. Cell staining with vital fluorescent dyes showed that AVP1 recovered vacuole acidification and normalized the endocytic pathway of the vma mutant. Biochemical and immunochemical studies further demonstrated that a significant fraction of heterologous H+-PPase is located at the vacuolar membrane. These results raise the question of the occurrence of distinct proton pumps in certain single-membrane organelles, such as plant vacuoles, by proving yeast V-ATPase activity dispensability and the capability of H+-PPase to generate, by itself, physiologically suitable internal pH gradients. Also, they suggest new ways of engineering macrolide drug tolerance and outline an experimental system for testing alternative roles for fungal and animal V-ATPases, other than the mere acidification of subcellular organelles.     

Characterization of Synthetic-Lethal Mutants Reveals a Role for the Saccharomyces Cerevisiae Guanine-Nucleotide Exchange Factor Cdc24p in Vacuole Function and Na(+) Tolerance

Cdc24p is the guanine-nucleotide exchange factor for the Cdc42p GTPase, which controls cell polarity in Saccharomyces cerevisiae. To identify new genes that may affect cell polarity, we characterized six UV-induced csl (CDC24 synthetic-lethal) mutants that exhibited synthetic-lethality with cdc24-4(ts) at 23°. Five mutants were not complemented by plasmid-borne CDC42, RSR1, BUD5, BEM1, BEM2, BEM3 or CLA4 genes, which are known to play a role in cell polarity. The csl3 mutant displayed phenotypes similar to those observed with calcium-sensitive, Pet(-) vma mutants defective in vacuole function. CSL5 was allelic to VMA5, the vacuolar H(+)-ATPase subunit C, and one third of csl5 cdc24-4(ts) cells were elongated or had misshapen buds. A cdc24-4(ts) Δvma5::LEU2 double mutant did not exhibit synthetic lethality, suggesting that the csl5/vma5 cdc24-4(ts) synthetic-lethality was not simply due to altered vacuole function. The cdc24-4(ts) mutant, like Δvma5::LEU2 and csl3 mutants, was sensitive to high levels of Ca(2+) as well as Na(+) in the growth media, which did not appear to be a result of a fragile cell wall because the phenotypes were not remedied by 1 M sorbitol. Our results indicated that Cdc24p was required in one V-ATPase mutant and another mutant affecting vacuole morphology, and also implicated Cdc24p in Na(+) tolerance.     

Structural and functional separation of the N- and C-terminal domains of the yeast V-ATPase subunit H

The H subunit of the yeast V-ATPase is an extended structure with two relatively independent domains, an N-terminal domain consisting of amino acids 1-348 and a C-terminal domain consisting of amino acids 352-478. We have expressed these two domains independently and together in a yeast strain lacking the H subunit (vma13Delta mutant). The N-terminal domain partially complements the growth defects of the mutant and supports approximately 25% of the wild-type Mg(2+)-dependent ATPase activity in isolated vacuolar vesicles, but surprisingly, this activity is both largely concanamycin-insensitive and uncoupled from proton transport. The C-terminal domain does not complement the growth defects, and supports no ATP hydrolysis or proton transport, even though it is recruited to the vacuolar membrane. Expression of both domains in a vma13Delta strain gives better complementation than either fragment alone and results in higher concanamycin-sensitive ATPase activity and ATP-driven proton pumping than the N-terminal domain alone. Thus, the two domains make complementary contributions to structural and functional coupling of the peripheral V(1) and membrane V(o) sectors of the V-ATPase, but this coupling does not require that they be joined covalently. The N-terminal domain alone is sufficient for activation of ATP hydrolysis in V(1), but the C-terminal domain is essential for proper communication between the V(1) and V(o) sectors.