Functional complementation of a null mutation of the yeast Saccharomyces cerevisiae plasma membrane H(+)-ATPase by a plant H(+)-ATPase gene

A plant plasma-membrane H+-ATPase promotes yeast TORC1 activation via its carboxy-terminal tail

The Target of Rapamycin Complex 1 (TORC1) involved in coordination of cell growth and metabolism is highly conserved among eukaryotes. Yet the signals and mechanisms controlling its activity differ among taxa, according to their biological specificities. A common feature of fungal and plant cells, distinguishing them from animal cells, is that their plasma membrane contains a highly abundant H+-ATPase which establishes an electrochemical H+ gradient driving active nutrient transport. We have previously reported that in yeast, nutrient-uptake-coupled H+ influx elicits transient TORC1 activation and that the plasma-membrane H+-ATPase Pma1 plays an important role in this activation, involving more than just establishment of the H+ gradient. We show here that the PMA2 H+-ATPase from the plant Nicotiana plumbaginifolia can substitute for Pma1 in yeast, to promote H+-elicited TORC1 activation. This H+-ATPase is highly similar to Pma1 but has a longer carboxy-terminal tail binding 14-3-3 proteins. We report that a C-terminally truncated PMA2, which remains fully active, fails to promote H+-elicited TORC1 activation. Activation is also impaired when binding of PMA2 to 14-3-3 s is hindered. Our results show that at least some plant plasma-membrane H+-ATPases share with yeast Pma1 the ability to promote TORC1 activation in yeast upon H+-coupled nutrient uptake.

The plasma membrane H+ -ATPase, a simple polypeptide with a long history

The plasma membrane H+ -ATPase of fungi and plants is a single polypeptide of fewer than 1,000 residues that extrudes protons from the cell against a large electric and concentration gradient. The minimalist structure of this nanomachine is in stark contrast to that of the large multi-subunit FO F1 ATPase of mitochondria, which is also a proton pump, but under physiological conditions runs in the reverse direction to act as an ATP synthase. The plasma membrane H+ -ATPase is a P-type ATPase, defined by having an obligatory phosphorylated reaction cycle intermediate, like cation pumps of animal membranes, and thus, this pump has a completely different mechanism to that of FO F1 ATPases, which operates by rotary catalysis. The work that led to these insights in plasma membrane H+ -ATPases of fungi and plants has a long history, which is briefly summarized in this review.

A Ma10 gene encoding P-type ATPase is involved in fruit organic acid accumulation in apple

Acidity is one of the main determinants of fruit organoleptic quality. Here, comparative transcriptome analysis was conducted between two cultivars that showed a significant difference in fruit acidity, but contained homozygous non-functional alleles at the major gene Ma1 locus controlling apple fruit acidity. A candidate gene for fruit acidity, designated M10, was identified. The M10 gene encodes a P-type proton pump, P3A -ATPase, which facilitates malate uptake into the vacuole. The Ma10 gene is significantly associated with fruit malate content, accounting for ~7.5% of the observed phenotypic variation in apple germplasm. Subcellular localization assay showed that the Ma10 is targeted to the tonoplast. Overexpression of the Ma10 gene can complement the defect in proton transport of the mutant YAK2 yeast strain and enhance the accumulation of malic acid in apple callus. Moreover, its ectopic expression in tomato induces a decrease in fruit pH. These results suggest that the Ma10 gene has the capacity for proton pumping and plays an important role in fruit vacuolar acidification in apple. Our study provides useful knowledge towards comprehensive understanding of the complex mechanism regulating apple fruit acidity.

Evolutionary and Functional Analysis of a Chara Plasma Membrane H+-ATPase

H⁺-ATPases are the main transporters in plant and fungal plasma membranes (PMs), comparable to the Na⁺/K⁺ ATPases in animal cells. At the molecular level, most studies on the PM H⁺-ATPases have been focused on land plants and fungi (yeast). The research of PM H⁺-ATPases in green algae falls far behind due to the lack of genetic information. Here we studied a potential PM H⁺-ATPase (CHA1) from Chara australis, a species of green algae belonging to the division Charophyta, members of which are considered to be one of the closest ancestors of land plants. The gene encodes a 107 kDa protein with all 6 P-type ATPase-specific motifs and a long, diverse C-terminal domain. A new amino acid sequence motif R*****Q in transmembrane segment 5 was identified among the known PM H⁺-ATPases from Charophyta and Chlorophyta algae, which is different from the typical PM H⁺-ATPases in yeast or land plants. Complementation analysis in yeast showed that CHA1 could successfully reach the PM, and that proton pump activity was obtained when the last 77 up to 87 amino acids of the C-terminal domain were deleted. PM localization was confirmed in Arabidopsis protoplasts; however, deletion of more than 55 amino acids at the N-terminus or more than 98 amino acids at the C-terminus resulted in failure of CHA1 to reach the PM in yeast. These results suggest that an auto-inhibition domain is located in the C-terminal domain, and that CHA1 is likely to have a different regulation mechanism compared to the yeast and land plant PM H⁺-ATPases.

Isolation of native plasma membrane H+-ATPase (Pma1p) in both the active and basal activation states

The yeast plasma membrane H⁺‐ATPase Pma1p is a P‐type ATPase that energizes the yeast plasma membrane. Pma1p exists in two activation states: an autoinhibited basal state and an activated state. Here we show that functional and stable Pma1p can be purified in native form and reconstituted in artificial liposomes without altering its activation state. Acetylated tubulin has previously been reported to maintain Pma1p in the basal state but, as this protein was absent from the purified preparations, it cannot be an essential component of the autoinhibitory mechanism. Purification of and reconstitution of native Pma1p in both activation states opens up for a direct comparison of the transport properties of these states, which allowed us to confirm that the basal state has a low coupling ratio between ATP hydrolysis and protons pumped, whereas the activated state has a high coupling ratio. The ability to prepare native Pma1p in both activation states will facilitate further structural and biochemical studies examining the mechanism by which plasma membrane H⁺‐ATPases are autoinhibited.

Functional characterization of the Ca2+-ATPase SMA1 from Schistosoma mansoni

Schistosoma mansoni is a parasite that causes bilharzia, a neglected tropical disease affecting hundreds of millions of people each year worldwide. In 2012, S. mansoni had been identified as the only invertebrate possessing two SERCA-type Ca2+-ATPases, SMA1 and SMA2. However, our analysis of recent genomic data shows that the presence of two SERCA pumps is rather frequent in parasitic flatworms. To understand the reasons of this redundancy in S. mansoni, we compared SMA1 and SMA2 at different levels. In terms of sequence and organization, the genes SMA1 and SMA2 are similar, suggesting that they might be the result of a duplication event. At the protein level, SMA1 and SMA2 only slightly differ in length and in the sequence of the nucleotide-binding domain. To get functional information on SMA1, we produced it in an active form in Saccharomyces cerevisiae, as previously done for SMA2. Using phosphorylation assays from ATP, we demonstrated that like SMA2, SMA1 bound calcium in a cooperative mode with an apparent affinity in the micromolar range. We also showed that SMA1 and SMA2 had close sensitivities to cyclopiazonic acid but different sensitivities to thapsigargin, two specific inhibitors of SERCA pumps. On the basis of transcriptomic data available in GeneDB, we hypothesize that SMA1 is a housekeeping Ca2+-ATPase, whereas SMA2 might be required in particular striated-like muscles like those present the tail of the cercariae, the infecting form of the parasite.

The plasma membrane H+-ATPase gene family in Solanum tuberosum L. Role of PHA1 in tuberization

This study presents the characterization of the plasma membrane (PM) H+-ATPases in potato, focusing on their role in stolon and tuber development. Seven PM H+-ATPase genes were identified in the Solanum tuberosum genome, designated PHA1-PHA7. PHA genes show distinct expression patterns in different plant tissues and under different stress treatments. Application of PM H+-ATPase inhibitors arrests stolon growth, promotes tuber induction, and reduces tuber size, indicating that PM H+-ATPases are involved in tuberization, acting at different stages of the process. Transgenic potato plants overexpressing PHA1 were generated (PHA1-OE). At early developmental stages, PHA1-OE stolons elongate faster and show longer epidermal cells than wild-type stolons; this accelerated growth is accompanied by higher cell wall invertase activity, lower starch content, and higher expression of the sucrose-H+ symporter gene StSUT1. PHA1-OE stolons display an increased branching phenotype and develop larger tubers. PHA1-OE plants are taller and also present a highly branched phenotype. These results reveal a prominent role for PHA1 in plant growth and development. Regarding tuberization, PHA1 promotes stolon elongation at early stages, and tuber growth later on. PHA1 is involved in the sucrose-starch metabolism in stolons, possibly providing the driving force for sugar transporters to maintain the apoplastic sucrose transport during elongation.

Purification and biochemical characterization of NpABCG5/NpPDR5, a plant pleiotropic drug resistance transporter expressed in Nicotiana tabacum BY-2 suspension cells

Pleiotropic drug resistance (PDR) transporters belong to the ABCG subfamily of ATP-binding cassette (ABC) transporters and are involved in the transport of various molecules across plasma membranes. During evolution, PDR genes appeared independently in fungi and in plants from a duplication of a half-size ABC gene. The enzymatic properties of purified PDR transporters from yeast have been characterized. This is not the case for any plant PDR transporter, or, incidentally, for any purified plant ABC transporter. Yet, plant PDR transporters play important roles in plant physiology such as hormone signaling or resistance to pathogens or herbivores. Here, we describe the expression, purification, enzymatic characterization and 2D analysis by electron microscopy of NpABCG5/NpPDR5 from Nicotiana plumbaginifolia, which has been shown to be involved in the plant defense against herbivores. We constitutively expressed NpABCG5/NpPDR5, provided with a His-tag in a homologous system: suspension cells from Nicotiana tabacum (Bright Yellow 2 line). NpABCG5/NpPDR5 was targeted to the plasma membrane and was solubilized by dodecyl maltoside and purified by Ni-affinity chromatography. The ATP-hydrolyzing specific activity (27 nmol min-1 mg-1) was stimulated seven-fold in the presence of 0.1% asolectin. Electron microscopy analysis indicated that NpABCG5/NpPDR5 is monomeric and with dimensions shorter than those of known ABC transporters. Enzymatic data (optimal pH and sensitivity to inhibitors) confirmed that plant and fungal PDR transporters have different properties. These data also show that N. tabacum suspension cells are a convenient host for the purification and biochemical characterization of ABC transporters.

Metal Fluoride Inhibition of a P-type H+ Pump: STABILIZATION OF THE PHOSPHOENZYME INTERMEDIATE CONTRIBUTES TO POST-TRANSLATIONAL PUMP ACTIVATION

The plasma membrane H(+)-ATPase is a P-type ATPase responsible for establishing electrochemical gradients across the plasma membrane in fungi and plants. This essential proton pump exists in two activity states: an autoinhibited basal state with a low turnover rate and a low H(+)/ATP coupling ratio and an activated state in which ATP hydrolysis is tightly coupled to proton transport. Here we characterize metal fluorides as inhibitors of the fungal enzyme in both states. In contrast to findings for other P-type ATPases, inhibition of the plasma membrane H(+)-ATPase by metal fluorides was partly reversible, and the stability of the inhibition varied with the activation state. Thus, the stability of the ATPase inhibitor complex decreased significantly when the pump transitioned from the activated to the basal state, particularly when using beryllium fluoride, which mimics the bound phosphate in the E2P conformational state. Taken together, our results indicate that the phosphate bond of the phosphoenzyme intermediate of H(+)-ATPases is labile in the basal state, which may provide an explanation for the low H(+)/ATP coupling ratio of these pumps in the basal state.

Specific Activation of the Plant P-type Plasma Membrane H+-ATPase by Lysophospholipids Depends on the Autoinhibitory N- and C-terminal Domains

Eukaryotic P-type plasma membrane H(+)-ATPases are primary active transport systems that are regulated at the post-translation level by cis-acting autoinhibitory domains, which can be relieved by protein kinase-mediated phosphorylation or binding of specific lipid species. Here we show that lysophospholipids specifically activate a plant plasma membrane H(+)-ATPase (Arabidopsis thaliana AHA2) by a mechanism that involves both cytoplasmic terminal domains of AHA2, whereas they have no effect on the fungal counterpart (Saccharomyces cerevisiae Pma1p). The activation was dependent on the glycerol backbone of the lysophospholipid and increased with acyl chain length, whereas the headgroup had little effect on activation. Activation of the plant pump by lysophospholipids did not involve the penultimate residue, Thr-947, which is known to be phosphorylated as part of a binding site for activating 14-3-3 protein, but was critically dependent on a single autoinhibitory residue (Leu-919) upstream of the C-terminal cytoplasmic domain in AHA2. A corresponding residue is absent in the fungal counterpart. These data indicate that plant plasma membrane H(+)-ATPases evolved as specific receptors for lysophospholipids and support the hypothesis that lysophospholipids are important plant signaling molecules.

Inositol phosphosphingolipid phospholipase C1 regulates plasma membrane ATPase (Pma1) stability in Cryptococcus neoformans

Cryptococcus neoformans is a facultative intracellular pathogen, which can replicate in the acidic environment inside phagolysosomes. Deletion of the enzyme inositol-phosphosphingolipid-phospholipase-C (Isc1) makes C. neoformans hypersensitive to acidic pH likely by inhibiting the function of the proton pump, plasma membrane ATPase (Pma1). In this work, we examined the role of Isc1 on Pma1 transport and oligomerization. Our studies showed that Isc1 deletion did not affect Pma1 synthesis or transport, but significantly inhibited Pma1 oligomerization. Interestingly, Pma1 oligomerization could be restored by supplementing the medium with phytoceramide. These results offer insight into the mechanism of intracellular survival of C. neoformans.

Pump up the volume - a central role for the plasma membrane H(+) pump in pollen germination and tube growth

The plasma membrane H(+) ATPase is a member of the P-ATPase family transporting H(+) from the cytosol to the extracellular space and thus energizing the plasma membrane for the uptake of ions and nutrients. As a housekeeping gene, this protein can be detected in almost every plant cell including the exclusive expression of specific isoforms in pollen grains and tubes where its activity is a prerequisite for successful germination and growth of pollen tubes. This review summarizes the current knowledge on pollen PM H(+) ATPases and hypothesizes a central role for pollen-specific isoforms of this protein in tube growth. External as well as cytosolic signals from signal transduction and metabolic pathways are integrated by the PM H(+) ATPase and directly translated to tube growth rates, allocating the PM H(+) ATPase to an essential node in the signalling network of pollen tubes in their race to the ovule.

Heterologous expression of Candida albicans Pma1p in Saccharomyces cerevisiae

Candida albicans is a major cause of opportunistic and life-threatening systemic fungal infections, especially in the immunocompromised. The plasma membrane proton-pumping ATPase (Pma1p) is an essential enzyme that generates the electrochemical gradient required for cell growth. We expressed C. albicans Pma1p (CaPma1p) in Saccharomyces cerevisiae to facilitate screening for inhibitors. Replacement of S. cerevisiae PMA1 with C. albicans PMA1 gave clones expressing CaPma1p that grew slowly at low pH. CaPma1p was expressed at significantly lower levels and had lower specific activity than the native Pma1p. It also conferred pH sensitivity, hygromycin B resistance, and low levels of glucose-dependent proton pumping. Recombination between CaPMA1 and the homologous nonessential ScPMA2 resulted in chimeric suppressor mutants that expressed functional CaPma1p with improved H(+) -ATPase activity and growth rates at low pH. Molecular models of suppressor mutants identified specific amino acids (between 531 and 595 in CaPma1p) that may affect regulation of the activity of Pma1p oligomers in S. cerevisiae. A modified CaPma1p chimeric construct containing only 5 amino acids from ScPma2p enabled the expression of a fully functional enzyme for drug screens and structural resolution.

A phosphorylation in the c-terminal auto-inhibitory domain of the plant plasma membrane H+-ATPase activates the enzyme with no requirement for regulatory 14-3-3 proteins

The plant plasma membrane H(+)-ATPase is regulated by an auto-inhibitory C-terminal domain that can be displaced by phosphorylation of the penultimate residue, a Thr, and the subsequent binding of 14-3-3 proteins. By mass spectrometric analysis of plasma membrane H(+)-ATPase isoform 2 (PMA2) isolated from Nicotiana tabacum plants and suspension cells, we identified a new phosphorylation site, Thr-889, in a region of the C-terminal domain upstream of the 14-3-3 protein binding site. This residue was mutated into aspartate or alanine, and the mutated H(+)-ATPases expressed in the yeast Saccharomyces cerevisiae. Unlike wild-type PMA2, which could replace the yeast H(+)-ATPases, the PMA2-Thr889Ala mutant did not allow yeast growth, whereas the PMA2-Thr889Asp mutant resulted in improved growth and increased H(+)-ATPase activity despite reduced phosphorylation of the PMA2 penultimate residue and reduced 14-3-3 protein binding. To determine whether the regulation taking place at Thr-889 was independent of phosphorylation of the penultimate residue and 14-3-3 protein binding, we examined the effect of combining the PMA2-Thr889Asp mutation with mutations of other residues that impair phosphorylation of the penultimate residue and/or binding of 14-3-3 proteins. The results showed that in yeast, PMA2 Thr-889 phosphorylation could activate H(+)-ATPase if PMA2 was also phosphorylated at its penultimate residue. However, binding of 14-3-3 proteins was not required, although 14-3-3 binding resulted in further activation. These results were confirmed in N. tabacum suspension cells. These data define a new H(+)-ATPase activation mechanism that can take place without 14-3-3 proteins.

A novel mechanism of P-type ATPase autoinhibition involving both termini of the protein

The activity of many P-type ATPases is found to be regulated by interacting proteins or autoinhibitory elements located in N- or C-terminal extensions. An extended C terminus of fungal and plant P-type plasma membrane H(+)-ATPases has long been recognized to be part of a regulatory apparatus involving an autoinhibitory domain. Here we demonstrate that both the N and the C termini of the plant plasma membrane H(+)-ATPase are directly involved in controlling the pump activity state and that N-terminal displacements are coupled to secondary modifications taking place at the C-terminal end. This identifies the first group of P-type ATPases for which both ends of the polypeptide chain constitute regulatory domains, which together contribute to the autoinhibitory apparatus. This suggests an intricate mechanism of cis-regulation with both termini of the protein communicating to obtain the necessary control of the enzyme activity state.

Regulation of the plant plasma membrane H+-ATPase by its C-terminal domain: what do we know for sure?

The plant plasma membrane H(+)-ATPase is kept at a low activity level by its C-terminal domain, the inhibitory function of which is thought to be mediated by two regions (region I and II) interacting with cytoplasmic domains essential for the catalytic cycle. The activity of the enzyme is well known to be regulated by 14-3-3 proteins, the association of which requires phosphorylation of the penultimate H(+)-ATPase residue, but can be abolished by phosphorylation of residues close-by. The current knowledge about H(+)-ATPase regulation is briefly summed up here, combined with data that query some of the above statements. Expression of various C-terminal deletion constructs of PMA2, a H(+)-ATPase isoform from Nicotiana plumbaginifolia, in yeast indicates that three regions, which do not correspond to regions I or II, contribute to autoinhibition. Their individual and combined action can be abolished by (mimicking) phosphorylation of three threonine residues located within or close to these regions. With respect to the wild-type PMA2, mimicking phosphorylation of two of these residues increases enzyme activity. However, constitutive activation of wild-type PMA2 requires 14-3-3 association. Altogether, the data suggest that regulation of the plant H(+)-ATPase occurs in progressive steps, mediated by several protein kinases and phosphatases, thus allowing gradual as well as fine-tuned adjustment of its activity. Moreover, mating-based split ubiquitin assays indicate a complex interplay between the C-terminal domain and the rest of the enzyme. Notably, their tight contact does not seem to be the cause of the inactive state of the enzyme.

Activation of plant plasma membrane H+-ATPase by 14-3-3 proteins is negatively controlled by two phosphorylation sites within the H+-ATPase C-terminal region

The proton pump ATPase (H(+)-ATPase) of the plant plasma membrane is regulated by an autoinhibitory C-terminal domain, which can be displaced by phosphorylation of the penultimate Thr residue and the subsequent binding of 14-3-3 proteins. We performed a mass spectrometric analysis of PMA2 (plasma membrane H(+)-ATPase isoform 2) isolated from Nicotiana tabacum suspension cells and identified two new phosphorylated residues in the enzyme 14-3-3 protein binding site: Thr(931) and Ser(938). When PMA2 was expressed in Saccharomyces cerevisiae, mutagenesis of each of these two residues into Asp prevented growth of a yeast strain devoid of its own H(+)-ATPases. When the Asp mutations were individually introduced in a constitutively activated mutant of PMA2 (E14D), they still allowed yeast growth but at a reduced rate. Purification of His-tagged PMA2 showed that the T931D or S938D mutation prevented 14-3-3 protein binding, although the penultimate Thr(955) was still phosphorylated, indicating that Thr(955) phosphorylation is not sufficient for full enzyme activation. Expression of PMA2 in an N. tabacum cell line also showed an absence of 14-3-3 protein binding resulting from the T931D or S938D mutation. Together, the data show that activation of H(+)-ATPase by the binding of 14-3-3 proteins is negatively controlled by phosphorylation of two residues in the H(+)-ATPase 14-3-3 protein binding site. The data also show that phosphorylation of the penultimate Thr and 14-3-3 binding each contribute in part to H(+)-ATPase activation.

ZMVHA-B1, the gene for subunit B of vacuolar H+-ATPase from the eelgrass Zostera marina L. Is able to replace vma2 in a yeast null mutant

A vacuolar H(+)-ATPase (VHA) gene (ZMVHA-B1) was isolated from an eelgrass (Zostera marina) leaf cDNA library and was characterized to be approximately 1.4 kbp in length and to encode the B subunit protein of VHA comprising 488 amino acids. ZMVHA-B1 was highly expressed in all organs of eelgrass; the expression level was highest in the leaves. On transformation of a yeast vma2 null mutant with ZMVHA-B1, yeast cells became able to grow at pH 7.5, accompanied by the vesicular accumulation of LysoSensor green DND-189. Thus, ZMVHA-B1 expressed in yeast cells produced a functional B subunit that was efficiently incorporated into the VHA complex and eventually restored vacuolar morphology and activity. This success expedites the application of heterologous expression in yeast mutant cells to the screening of eelgrass genes involved in salt-resistance mechanisms, which are to be utilized in improving important crops.

Functional complementation of the yeast P-type H-ATPase, PMA1, by the Pneumocystis carinii P-type H-ATPase, PCA1

The opportunistic fungus Pneumocystis is the etiologic agent of an interstitial plasma cell pneumonia that primarily afflicts immunocompromised individuals. Like other fungi Pneumocystis maintains a H(+) plasma membrane gradient to drive nutrient uptake and regulates intracellular pH by ATP-dependent proton efflux. Previously, we identified a Pneumocystis gene, PCA1, whose predicted protein product was homologous to fungal proton pumps. In this study, we show by functional complementation in a Saccharomyces strain whose endogenous PMA1 proton pump activity is repressed that the Pneumocystis PCA1 encodes a H(+)-ATPase. The properties of PCA1 characterized in this system closely resemble those of yeast PMA1. Yeast expressing PCA1 grow at low pH and are able to acidify the external media. Maximal enzyme activity (V(max)) and efficiency of substrate utilization (K(m)) in plasma membranes were nearly identical for PCA1 and PMA1. PCA1 contains an inhibitory COOH-terminal domain; removal of the final 40 amino acids significantly increased V(max) and growth at pH 6.5. PCA1 activity was inhibited by proton pump inhibitors omeprazole and lansoprazole, but was unaffected by H(+)/K(+)-ATPase inhibitor SCH28080. Thus, H(+) homeostasis in Pneumocystis is likely regulated as in other fungi. This work also establishes a system for screening PCA1 inhibitors to identify new anti-Pneumocystis agents.

A COOH-terminal domain regulates the activity of Leishmania proton pumps LDH1A and LDH1B

Leishmania donovani requires actively transporting proton efflux pumps to survive the acidic environment of macrophage phagolysosomal vacuoles and to maintain an electrogenic H(+) gradient for nutrient uptake. The L. donovani genome contains a differentially expressed pair of genes, LDH1A and LDH1B, with homology to yeast H(+)-ATPases that are 98% identical in sequence with amino acid differences concentrated at the COOH-terminus (15 of last 37 differ), a region implicated in regulation of yeast and plant proton pumps. Functional complementation of a Saccharomyces cerevisiae strain deficient in endogenous H(+)-ATPase activity, support of yeast growth at low pH, and ability to acidify media demonstrate that LDH1A and LDH1B encode proton pumps. LDH1A and LDH1B encode a COOH-terminal autoinhibitory domain as COOH-truncated peptides support increased rates of growth in yeast, enhanced media acidification, increased enzyme activity (V(max)) and decreased K(m). This regulatory domain mediates differing function properties; LDH1A, but not LDH1B, supports yeast growth at pH 3 and LDH1A shows a greater ability to acidify media. Deletion of the last eight amino acids from LDH1B permits growth at pH 3 and increases media acidification, swapping of the COOH-tails between LDH1A and LDH1B results in LDH1A (with LDH1B tail) unable to support yeast growth at pH 3 and LDH1B (with LDH1A tail) now able to support growth at pH 3. Replacement of the COOH-terminal eight amino acids of LDH1B with those from LDH1A also confers the ability to support growth at pH 3. The complementation system for the Leishmania proton pumps in yeast described here provides a means to dissect the functional properties of the two isoforms, a convenient supply of protein for structural analysis and a model amenable to screening proton pump inhibitors for potential anti-leishmanial therapeutics.

Heterologous expression of a plant uracil transporter in yeast: Improvement of plasma membrane targeting in mutants of the Rsp5p ubiquitin protein ligase

Plasma membrane proteins involved in transport processes play a crucial role in cell physiology. On account of these properties, these molecules are ideal targets for development of new therapeutic and agronomic agents. However, these proteins are of low abundance, which limits their study. Although yeast seems ideal for expressing heterologous transporters, plasma membrane proteins are often retained in intracellular compartments. We tried to find yeast mutants potentially able to improve functional expression of a whole set of heterologous transporters. We focused on Arabidopsis thaliana ureide transporter 1 (AtUPS1), previously cloned by functional complementation in yeast. Tagged versions of AtUPS1 remain mostly trapped in the endoplasmic reticulum and were able to reach slowly the plasma membrane. In contrast, untagged AtUPS1 is rapidly delivered to plasma membrane, where it remains in stable form. Tagged and untagged versions of AtUPS1 were expressed in cells deficient in the ubiquitin ligase Rsp5p, involved in various stages of the intracellular trafficking of membrane-bound proteins. rsp5 mutants displayed improved steady state amounts of untagged and tagged versions of AtUPS1. rsp5 cells are thus powerful tools to solve the many problems inherent to heterologous expression of membrane proteins in yeast, including ER retention.

Identification of a Nicotiana plumbaginifolia plasma membrane H(+)-ATPase gene expressed in the pollen tube

In Nicotiana plumbaginifolia, plasma membrane H(+)-ATPases (PMAs) are encoded by a gene family of nine members. Here, we report on the characterization of a new isogene, NpPMA5 (belonging to subfamily IV), and the determination of its expression pattern using the beta-glucuronidase (gusA) reporter gene. pNpPMA5-gusA was expressed in cotyledons, in vascular tissues of the stem (mainly in nodal zones), and in the flower and fruit. In the flower, high expression was found in the pollen tube after in vitro or in vivo germination. Northern blotting analysis confirmed that NpPMA5 was expressed in the pollen tube contrary to NpPMA2 (subfamily I) or NpPMA4 (subfamily II), two genes highly expressed in other tissues. The subcellular localization of PM H(+)-ATPase in the pollen tube was analyzed by immunocytodecoration. As expected, this enzyme was localized to the plasma membrane. However, neither the tip nor the base of the pollen tube was labeled, showing an asymmetrical distribution of this enzyme. This observation supports the hypothesis that the PM H(+)-ATPase is involved in creating the pH gradient that is observed along the pollen tube and is implicated in cell elongation. Compared to other plant PM H(+)-ATPases, the C-terminal region of NpPMA5 is shorter by 26 amino acid residues and is modified in the last 6 residues, due to a sequence rearrangement, which was also found in the orthologous gene of Nicotiana glutinosa, a Nicotiana species distant from N. plumbaginifolia and Petunia hybrida and Lycopersicon esculentum, other Solanacae species. This modification alters part of the PM H(+)-ATPase regulatory domain and raises the question whether this isoform is still regulated.

A cloned prokaryotic Cd2+ P-type ATPase increases yeast sensitivity to Cd2+

CadA, the P1-type ATPase involved in Listeria monocytogenes resistance to Cd(2+), was expressed in Saccharomyces cerevisiae and did just the opposite to what was expected, as it strikingly decreased the Cd(2+) tolerance of these cells. Yeast cells expressing the non-functional mutant Asp(398)Ala could grow on selective medium containing up to 100 microM Cd(2+), whereas those expressing the functional protein could not grow in the presence of 1 microM Cd(2+). The CadA-GFP fusion protein was localized in the endoplasmic reticulum membrane, suggesting that yeast hyper-sensitivity was due to Cd(2+) accumulation in the reticulum lumen. CadA is also known to transport Zn(2+), but Zn(2+) did not protect the cells against Cd(2+) poisoning. In the presence of 10 microM Cd(2+), transformed yeasts survived by rapid loss of their expression vector.

Intracellular transport of a heterologous membrane protein, the human transferrin receptor, in Saccharomyces cerevisiae

We have analyzed the intracellular behavior of the human transferrin receptor (TfR) in Saccharomyces cerevisiae. The major part of the heterologously expressed TfR, which has previously been used as a model for heterologous expression of membrane proteins in yeast, is localized in the endoplasmic reticulum (ER) membranes; a minor fraction is present in the plasma membrane (PM). The stability of the TfR depends on vacuolar proteases, implying that it is degraded in the vacuolar compartment. Degradation is further dependent on favorable transport conditions to this compartment. The main bottleneck of transport seems to be the transition from the ER to the PM. The chaperone Cne1p, which is involved in quality control in the ER, plays a role in regulating the amount of heterologous TfR, as deletion of CNE1 leads to significant accumulation of the protein. This is the first demonstration of the involvement of CNE1 in regulating the level of heterologous membrane proteins.

Trypanosoma cruzi H+-ATPase 1 (TcHA1) and 2 (TcHA2) genes complement yeast mutants defective in H+ pumps and encode plasma membrane P-type H+-ATPases with different enzymatic properties

Previous studies in Trypanosoma cruzi have shown that intracellular pH homeostasis requires ATP and is affected by H(+)-ATPase inhibitors, indicating a major role for ATP-driven proton pumps in intracellular pH control. In the present study, we report the cloning and sequencing of a pair of genes linked in tandem (TcHA1 and TcHA2) in T. cruzi which encode proteins with homology to fungal and plant P-type proton-pumping ATPases. The genes are expressed at the mRNA level in different developmental stages of T. cruzi: TcHA1 is expressed maximally in epimastigotes, whereas TcHA2 is expressed predominantly in trypomastigotes. The proteins predicted from the nucleotide sequence of the genes have 875 and 917 amino acids and molecular masses of 96.3 and 101.2 kDa, respectively. Full-length TcHA1 and an N-terminal truncated version of TcHA2 complemented a Saccharomyces cerevisiae strain deficient in P-type H(+)-ATPase activity, the proteins localized to the yeast plasma membrane, and ATP-driven proton pumping could be detected in proteoliposomes reconstituted from plasma membrane purified from transfected yeast. The reconstituted proton transport activity was reduced by inhibitors of P-type H(+)-ATPases. C-terminal truncation did not affect complementation of mutant yeast, suggesting the lack of C-terminal autoinhibitory domains in these proteins. ATPase activity in plasma membrane from TcHA1- and (N-terminal truncated) TcHA2-transfected yeast was inhibited to different extents by vanadate, whereas the latter yeast strain was more resistant to extremes of pH, suggesting that the native proteins may serve different functions at different stages in the T. cruzi life cycle.

Overexpression of an archaeal protein in yeast: secretion bottleneck at the ER

Archaeal enzymes have great potential for industrial use; however, expressing them in their natural hosts has proven challenging. Growth conditions for many archaea are beyond typical fermentation capabilities, and to compound the problem, archaea generally achieve much lower biomass yields than Escherichia coli or Saccharomyces cerevisiae. To determine whether a eukaryotic host, S. cerevisiae, would be a suitable alternative for archaeal protein production, we examined the expression of the tetrameric beta-glucosidase from the hyperthermophilic archaeon Pyrococcus furiosus. We engineered the beta-glucosidase to facilitate secretion into the culture medium and have demonstrated the beta-glucosidase's secretion and activity. We determined the dependence of beta-glucosidase secretion on gene copy number and obtained a transformant capable of secreting approximately 10 mg/L in batch culture. All transformants retained large intracellular fractions of beta-glucosidase, indicative of an intracellular bottleneck. Cell fractionation by sucrose density centrifugation and immunofluorescence identified the endoplasmic reticulum as the secretion bottleneck. Preliminary evidence indicates that the cause of this bottleneck is misfolding of the monomeric beta-glucosidase, rather than tetrameric association. Expression at moderately elevated temperatures (between 30 and 40 degrees C) improved beta-glucosidase yields, suggesting that higher temperature expression may improve folding and secretion yields.

Characterization of a hyperthermophilic P-type ATPase from Methanococcus jannaschii expressed in yeast

We report on the biochemical and structural properties of a putative P-type H(+)-ATPase, MJ1226p, from the anaerobic hyperthermophilic Archaea Methanococcus jannaschii. An efficient heterologous expression system was developed in Saccharomyces cerevisiae and a four-step purification protocol, using n-dodecyl beta-d-maltoside, led to a homogeneous detergent-solubilized protein fraction with a yield of over 2 mg of protein per liter of culture. The three-dimensional structure of the purified detergent-solubilized protein obtained at 2.4 nm resolution by electron microscopy showed a dimeric organization in which the size and the shape of each monomer was compatible with the reported structures of P-type ATPases. The purified MJ1226p ATPase was inactive at 40 degrees C and was active at elevated temperature reaching high specific activity, up to 180 micromol of P(i) x min(-1) x mg(-1) at 95 degrees C. Maximum ATPase activity was observed at pH 4.2 and required up to 200 mm monovalent salts. The ATPase activity was stable for several days upon storage at 65 degrees C and was highly resistant to urea and guanidine hydrochloride. The protein formed catalytic phosphoenzyme intermediates from MgATP or P(i), a functional characteristic specific of P-type ATPases. The highly purified, homogeneous, stable, and active MJ1226p ATPase provides a new model for further structure-function studies of P-type ATPases.

Quality control in the yeast secretory pathway: a misfolded PMA1 H+-ATPase reveals two checkpoints

The yeast plasma-membrane H(+)-ATPase, encoded by PMA1, is delivered to the cell surface via the secretory pathway and has recently emerged as an excellent system for identifying quality control mechanisms along the pathway. In the present study, we have tracked the biogenesis of Pma1-G381A, a misfolded mutant form of the H(+)-ATPase. Although this mutant ATPase is arrested transiently in the peripheral endoplasmic reticulum, it does not become a substrate for endoplasmic reticulum-associated degradation nor does it appear to stimulate an unfolded protein response. Instead, Pma1-G381A accumulates in Kar2p-containing vesicular-tubular clusters that resemble those previously described in mammalian cells. Like their mammalian counterparts, the yeast vesicular-tubular clusters may correspond to specific exit ports from the endoplasmic reticulum, since Pma1-G381A eventually escapes from them (still in a misfolded, trypsin-sensitive form) to reach the plasma membrane. By comparison with wild-type ATPase, Pma1-G381A spends a short half-life at the plasma membrane before being removed and sent to the vacuole for degradation in a process that requires both End4p and Pep4p. Finally, in a separate set of experiments, Pma1-G381A was found to impose its phenotype on co-expressed wild-type ATPase, transiently retarding the wild-type protein in the ER and later stimulating its degradation in the vacuole. Both effects serve to lower the steady-state amount of wild-type ATPase in the plasma membrane and, thus, can explain the co-dominant genetic behavior of the G381A mutation. Taken together, the results of this study establish Pma1-G381A as a useful new probe for the yeast secretory system.

Mechanism of cell death and disease resistance induction by transgenic expression of bacterio-opsin

One of the earliest signal transduction events that trigger the hypersensitive response (HR) of plants against pathogen attack is thought to be an alteration of proton flux across the plasma membrane (PM). However, no direct genetic evidence for the involvement of PM-localised proton channels or pumps in the induction of this response has been reported. We previously showed that expression of the bacterial proton pump bacterio-opsin (bO) in transgenic plants resulted in the spontaneous activation of the HR. Here we show that the bO protein is likely localised to the PM in transgenic tobacco plants. Furthermore, mutational analysis shows that induction of the HR by bO expression is dependent upon the capability of bO to translocate protons. Although bO functions as a light-driven proton pump in Halobacteria when assembled with retinal, we also show by mutational analysis that this chromophore binding is unnecessary for its in planta activity. Taken together, our results suggest that expression of bO in plants leads to the insertion of a passive proton channel into the PM. The activity of this channel in the PM results in spontaneous activation of cell death and HR-associated phenotypes including enhanced resistance to a broad spectrum of plant pathogens. Our work provides direct molecular evidence to support a working model in which alterations in ionic homeostasis at the level of the PM may work as one of the critical steps in the signalling pathway for the activation of the HR.

Post-translational modification of plant plasma membrane H(+)-ATPase as a requirement for functional complementation of a yeast transport mutant

Many heterologous membrane proteins expressed in the yeast Saccharomyces cerevisiae fail to reach their normal cellular location and instead accumulate in stacked internal membranes. Arabidopsis thaliana plasma membrane H(+)-ATPase isoform 2 (AHA2) is expressed predominantly in yeast internal membranes and fails to complement a yeast strain devoid of its endogenous H(+)-ATPase Pma1. We observed that phosphorylation of AHA2 in the heterologous host and subsequent binding of 14-3-3 protein is crucial for the ability of AHA2 to substitute for Pma1. Thus, mutants of AHA2, complementing pma1, showed increased phosphorylation at the penultimate residue (Thr(947)), which creates a binding site for endogenous 14-3-3 protein. Only a pool of ATPase in the plasma membrane is phosphorylated. Double mutants carrying in addition a T947A substitution lost their ability to complement pma1. However, mutants affected in both autoinhibitory regions of the C-terminal regulatory domain complemented pma1 irrespective of their ability to become phosphorylated at Thr(947). This demonstrates that it is the activity status of the mutant enzyme and neither redirection of trafficking nor 14-3-3 binding per se that determines the ability of H(+)-pumps to rescue pma1.

The Npr1 kinase controls biosynthetic and endocytic sorting of the yeast Gap1 permease

Membrane trafficking of the general amino acid permease (Gap1) of Saccharomyces cerevisiae is under nitrogen regulation. In cells growing on proline or urea as the sole nitrogen source, newly synthesized Gap1 is delivered to the plasma membrane, where it accumulates. Upon addition of NH(4)(+), a preferential nitrogen source, Gap1 is endocytosed and targeted to the vacuole, where it is degraded. This down-regulation requires ubiquitination of the permease, and this ubiquitination is dependent on the essential Npi1/Rsp5 ubiquitin ligase. In this study, we investigated the role of the Npr1 kinase in the regulation of Gap1 trafficking. We show that Npr1 is required for stabilization of Gap1 at the plasma membrane: when an npr1(ts) mutant growing on proline is shifted to the restrictive temperature, Gap1 down-regulation is triggered, as it is when NH(4)(+) is added to wild-type cells. The fate of newly synthesized Gap1 en route to the plasma membrane is also under Npr1 control: in an npr1Delta mutant, neosynthesized Gap1 is sorted from the Golgi to the vacuole without passing via the plasma membrane. Similar direct sorting of neosynthesized Gap1 to the vacuole was observed in wild-type cells grown on NH(4)(+). Finally, Gap1 is phosphorylated in NPR1 cells, but this phosphorylation is not strictly dependent on Npr1. Our results show that Npr1 kinase plays a central role in the physiological control of Gap1 trafficking and that this control is exerted not only on Gap1 present at the plasma membrane but also on Gap1 late in the secretory pathway. Npr1 belongs to a subgroup of protein kinases, some of which are reported to exert a positive control on the activity of other permeases. We propose that these kinases also function as regulators of permease trafficking.

Binding of regulatory 14-3-3 proteins to the C terminus of the plant plasma membrane H+ -ATPpase involves part of its autoinhibitory region

The plant plasma membrane H+ -ATPase is activated by the binding of 14-3-3 proteins to its extreme C-terminal amino acids (YTV) and phosphorylation of the penultimate threonine (YpTV) is necessary for this interaction in vivo. However, in the presence of the fungal toxin fusicoccin (FC), binding of 14-3-3 proteins occurs independently of phosphorylation but still involves the YTV motif. Since FC exclusively binds to the complex consisting of both 14-3-3 homologs and the C-terminal domain of the H+ -ATPase, the toxin was used as a tool to reveal potential protein-protein interaction sites in the enzyme's C terminus. We performed in vitro interaction studies by applying various C-terminal parts of the H+ -ATPase PMA2 from Nicotiana plumbaginifolia expressed as glutathione S-transferase fusion peptides in E. coli. Interestingly, the PMA2 region encompassing residues 905-922 is implicated in FC-dependent binding of 14-3-3 homologs. Recently, part of this region has been shown to contribute to the autoinhibitory action of the PMA2 C terminus. Site-directed mutagenesis of individual amino acids localized within this region resulted in a drastic decrease in FC-dependent binding of 14-3-3 proteins. Furthermore, by expressing the corresponding mutants of PMA2 in yeast, we observed a reduced capability of the mutant enzymes to functionally replace the endogenous H+ -ATPase. Notably, the decreased activity of the mutant enzymes was accompanied by a weakened binding of yeast 14-3-3 homologs to the plasma membrane of transformed cells. Taken together, our results suggest that a section of the autoinhibitory C-terminal PMA2 region contributes to binding of activatory 14-3-3 proteins in the absence of FC.

Hydrophobic residues within the predicted N-terminal amphiphilic alpha-helix of a plant mitochondrial targeting presequence play a major role in in vivo import

A deletion and mutagenesis study was performed on the mitochondrial presequence of the beta-subunit of the F(1)-ATP synthase from Nicotiana plumbaginifolia linked to the green fluorescent protein (GFP). The various constructs were tested in vivo by transient expression in tobacco protoplasts. GFP distribution in transformed cells was analysed in situ by confocal microscopy, and in vitro in subcellular fractions by Western blotting. Despite its being highly conserved in different species, deletion of the C-terminal region (residues 48-54) of the presequence did not affect mitochondrial import. Deletion of the conserved residues 40-47 and the less conserved intermediate region (residues 18-39) resulted in 60% reduction in GFP import, whereas mutation of conserved residues within these regions had little effect. Further shortening of the presequence progressively reduced import, with the construct retaining the predicted N-terminal amphiphilic alpha-helix (residues 1-12) being unable to mediate mitochondrial import. However, point mutation showed that this last region plays an important role through its basic residues and amphiphilicity, but also through its hydrophobic residues. Replacing Arg4 and Arg5 by alanine residues and shifting the Arg5 and Leu6 (in order to disturb amphiphilicity) resulted in reduction of the presequence import efficiency. The most dramatic effects were seen with single or double mutations of the four Leu residues (positions 5, 6, 10 and 11), which resulted in marked reduction or abolition of GFP import, respectively. We conclude that the N-terminal helical structure of the presequence is necessary but not sufficient for efficient mitochondrial import, and that its hydrophobic residues play an essential role in in vivo mitochondrial targeting.

Helical stalk segments S4 and S5 of the plasma membrane H+-ATPase from Saccharomyces cerevisiae are optimized to impact catalytic site environment

The stalk segments of P-type ion-translocating enzymes are presumed to play important roles in energy coupling. In this work, stalk segments S4 and S5 of the yeast H(+)-ATPase were examined for helical character, optimal length, and segment orientation by a combination of proline substitution, insertion/deletion mutagenesis, and second-site suppressor analyses. The substitution of various residues for helix-disrupting proline in both S4 (L353P,L353G; A354P; and G371P) and S5 (D676P and I684P) resulted in highly defective or inactive enzymes supporting the importance of helical character and/or the maintenance of essential interactions. The contiguous helical nature of transmembrane segment M5 and stalk element S5 was explored and found to be favorable, although not essential. The deletion or addition of one or more amino acids at positions Ala(354) in S4 and Asp(676) in S5, which were intended to either rotate helical faces or extend/reduce the length of helical segments, resulted in enzyme destabilization that abolished most enzyme assembly. Second-site suppressor mutations were obtained to primary site mutations G371A (S4) and D676G (S5) and were analyzed with a molecular structure model of the H(+)-ATPase. Primary site mutations were predicted to alter the site of phosphorylation either directly or indirectly. The suppressor mutations either directly changed packing around the primary site or altered the environment of the site of phosphorylation. Overall, these data support the view that stalk segments S4 and S5 of the H(+)-ATPase are helical elements that are optimized for length and interactions with other stalk elements and can influence the phosphorylation domain.

The two major plant plasma membrane H+-ATPases display different regulatory properties

The major plant plasma membrane H(+)-ATPases fall into two gene categories, subfamilies I and II. However, in many plant tissues, expression of the two subfamilies overlaps, thus precluding individual characterization. Yeast expression of PMA2 and PMA4, representatives of the two plasma membrane H(+)-ATPase subfamilies in Nicotiana plumbaginifolia, has previously shown that (i) the isoforms have distinct enzymatic properties and that (ii) PMA2 is regulated by phosphorylation of its penultimate residue (Thr) and binds regulatory 14-3-3 proteins, resulting in the displacement of the autoinhibitory C-terminal domain. To obtain insights into regulatory differences between the two subfamilies, we have constructed various chimeric proteins in which the 110-residue C-terminal-encoding region of PMA2 was progressively substituted by the corresponding sequence from PMA4. The PMA2 autoinhibitory domain was localized to a region between residues 851 and 915 and could not be substituted by the corresponding region of PMA4. In contrast to PMA2, PMA4 was poorly phosphorylated at its penultimate residue (Thr) and bound 14-3-3 proteins weakly. The only sequence difference around the phosphorylation site is located two residues upstream of the phosphorylated Thr. It is Ser in PMA2 (as in most members of subfamily I) and His in PMA4 (as in most members of subfamily II). Substitution of His by Ser in PMA4 resulted in an enzyme showing increased phosphorylation status, 14-13-3 binding, and ATPase activity, as well as improved yeast growth. The reverse substitution of Ser by His in PMA2 resulted in the failure of this enzyme to complement the absence of yeast H(+)-ATPases. These results show that the two plant H(+)-ATPase subfamilies differ functionally in their regulatory properties.

Sequencing and heterologous expression in Saccharomyces cerevisiae of a Cryptococcus neoformans cDNA encoding a plasma membrane H(+)-ATPase

A cDNA containing an open reading frame encoding a putative plasma membrane H(+)-ATPase in the human pathogenic basidiomycetous yeast Cryptococcus neoformans was cloned and sequenced by means of PCR and cDNA library hybridization. The cloned cDNA is 3475 bp in length, containing a 2994 bp open reading frame encoding a polypeptide of 997 amino acids. As in the case of another basidiomycetous fungus (Uromyces fabae), the deduced amino acid sequence of CnPMA1 was found to be more homologous to those of P-type H(+)-ATPases from higher plants than to those from ascomycetous fungi. In order to prove the sequenced cDNA corresponds to a H(+)-ATPase, it was expressed in Saccharomyces cerevisiae and found to functionally replace its own H(+)-ATPase. Kinetic studies of CnPMA1 compared to ScPMA1 show differences in V(max) values and H(+)-pumping in reconstituted vesicles. The pH optimum and K(m) values are similar in both enzymes.

Lst1p and Sec24p cooperate in sorting of the plasma membrane ATPase into COPII vesicles in Saccharomyces cerevisiae

Formation of ER-derived protein transport vesicles requires three cytosolic components, a small GTPase, Sar1p, and two heterodimeric complexes, Sec23/24p and Sec13/31p, which comprise the COPII coat. We investigated the role of Lst1p, a Sec24p homologue, in cargo recruitment into COPII vesicles in Saccharomyces cerevisiae. A tagged version of Lst1p was purified and eluted as a heterodimer complexed with Sec23p comparable to the Sec23/24p heterodimer. We found that cytosol from an lst1-null strain supported the packaging of alpha-factor precursor into COPII vesicles but was deficient in the packaging of Pma1p, the essential plasma membrane ATPase. Supplementation of mutant cytosol with purified Sec23/Lst1p restored Pma1p packaging into the vesicles. When purified COPII components were used in the vesicle budding reaction, Pma1p packaging was optimal with a mixture of Sec23/24p and Sec23/Lst1p; Sec23/Lst1p did not replace Sec23/24p. Furthermore, Pma1p coimmunoprecipitated with Lst1p and Sec24p from vesicles. Vesicles formed with a mixture of Sec23/Lst1p and Sec23/24p were similar morphologically and in their buoyant density, but larger than normal COPII vesicles (87-nm vs. 75-nm diameter). Immunoelectronmicroscopic and biochemical studies revealed both Sec23/Lst1p and Sec23/24p on the membranes of the same vesicles. These results suggest that Lst1p and Sec24p cooperate in the packaging of Pma1p and support the view that biosynthetic precursors of plasma membrane proteins must be sorted into ER-derived transport vesicles. Sec24p homologues may comprise a more complex coat whose combinatorial subunit composition serves to expand the range of cargo to be packaged into COPII vesicles. By changing the geometry of COPII coat polymerization, Lst1p may allow the transport of bulky cargo molecules, polymers, or particles.

A plant plasma membrane H+-ATPase expressed in yeast is activated by phosphorylation at its penultimate residue and binding of 14-3-3 regulatory proteins in the absence of fusicoccin

The Nicotiana plumbaginifolia plasma membrane H(+)-ATPase isoform PMA2, equipped with a His(6) tag, was expressed in Saccharomyces cerevisiae and purified. Unexpectedly, a fraction of the purified tagged PMA2 associated with the two yeast 14-3-3 regulatory proteins, BMH1 and BMH2. This complex was formed in vivo without treatment with fusicoccin, a fungal toxin known to stabilize the equivalent complex in plants. When gel filtration chromatography was used to separate the free ATPase from the 14-3-3.H(+)-ATPase complex, the complexed ATPase was twice as active as the free form. Trypsin treatment of the complex released a smaller complex, composed of a 14-3-3 dimer and a fragment from the PMA2 C-terminal region. The latter was identified by Edman degradation and mass spectrometry as the PMA2 C-terminal 57 residues, whose penultimate residue (Thr-955) was phosphorylated. In vitro dephosphorylation of this C-terminal fragment prevented binding of 14-3-3 proteins, even in the presence of fusicoccin. Mutation of Thr-955 to alanine, aspartate, or a stop codon prevented PMA2 from complementing the yeast H(+)-ATPase. These mutations were also introduced in an activated PMA2 mutant (Gln-14 --> Asp) characterized by a higher H(+) pumping activity. Each mutation directly modifying Thr-955 prevented 14-3-3 binding, decreased ATPase specific activity, and reduced yeast growth. We conclude that the phosphorylation of Thr-955 is required for 14-3-3 binding and that formation of the complex activates the enzyme.

Membrane hyperpolarization and salt sensitivity induced by deletion of PMP3, a highly conserved small protein of yeast plasma membrane

Yeast plasma membranes contain a small 55 amino acid hydrophobic polypeptide, Pmp3p, which has high sequence similarity to a novel family of plant polypeptides that are overexpressed under high salt concentration or low temperature treatment. The PMP3 gene is not essential under normal growth conditions. However, its deletion increases the plasma membrane potential and confers sensitivity to cytotoxic cations, such as Na(+) and hygromycin B. Interestingly, the disruption of PMP3 exacerbates the NaCl sensitivity phenotype of a mutant strain lacking the Pmr2p/Enap Na(+)-ATPases and the Nha1p Na(+)/H(+) antiporter, and suppresses the potassium dependency of a strain lacking the K(+) transporters, Trk1p and Trk2p. All these phenotypes could be reversed by the addition of high Ca(2+) concentration to the medium. These genetic interactions indicate that the major effect of the PMP3 deletion is a hyperpolarization of the plasma membrane potential that probably promotes a non-specific influx of monovalent cations. Expression of plant RCI2A in yeast could substitute for the loss of Pmp3p, indicating a common role for Pmp3p and the plant homologue.

The plant plasma membrane H(+)-ATPase: structure, function and regulation

The proton-pumping ATPase (H(+)-ATPase) of the plant plasma membrane generates the proton motive force across the plasma membrane that is necessary to activate most of the ion and metabolite transport. In recent years, important progress has been made concerning the identification and organization of H(+)-ATPase genes, their expression, and also the kinetics and regulation of individual H(+)-ATPase isoforms. At the gene level, it is now clear that H(+)-ATPase is encoded by a family of approximately 10 genes. Expression, monitored by in situ techniques, has revealed a specific distribution pattern for each gene; however, this seems to differ between species. In the near future, we can expect regulatory aspects of gene expression to be elucidated. Already the expression of individual plant H(+)-ATPases in yeast has shown them to have distinct enzymatic properties. It has also allowed regulatory aspects of this enzyme to be studied through random and site-directed mutagenesis, notably its carboxy-terminal region. Studies performed with both plant and yeast material have converged towards deciphering the way phosphorylation and binding of regulatory 14-3-3 proteins intervene in the modification of H(+)-ATPase activity. The production of high quantities of individual functional H(+)-ATPases in yeast constitutes an important step towards crystallization studies to derive structural information. Understanding the specific roles of H(+)-ATPase isoforms in whole plant physiology is another challenge that has been approached recently through the phenotypic analysis of the first transgenic plants in which the expression of single H(+)-ATPases has been up- or down-regulated. In conclusion, the progress made recently concerning the H(+)-ATPase family, at both the gene and protein level, has come to a point where we can now expect a more integrated investigation of the expression, function and regulation of individual H(+)-ATPases in the whole plant context.

N-terminal chimeric constructs improve the expression of sarcoplasmic reticulum Ca(2+)-ATPase in yeast

Wild-type and chimeric constructs comprising rabbit sarcoplasmic reticulum (SR) Ca(2+)-ATPase and the N-terminal cytoplasmic portion of yeast plasma membrane H(+)-ATPase were expressed in yeast under control of a heat-shock regulated promoter. The wild-type ATPase was found predominantly in endoplasmic reticulum (ER) membranes. Addition of the first 88 residues of H(+)-ATPase to the Ca(2+)-ATPase N-terminal end promoted a marked shift in the localization of chimeric H(+)/Ca(2+)-ATPase which accumulated in a light membrane fraction associated with yeast smooth ER. Furthermore, there was a three-fold increase in the overall level of expression of chimeric H(+)/Ca(2+)-ATPase. Similar results were obtained for a chimeric Ca(2+)-ATPase containing a hexahistidine sequence added to its N-terminal end. Both H(+)/Ca(2+)-ATPase and 6xHis-Ca(2+)-ATPase were functional as demonstrated by their ability to form a phosphorylated intermediate and undergo fast turnover. Conversely, a replacement chimera in which the N-terminal end of SR Ca(2+)-ATPase was replaced by the corresponding segment of H(+)-ATPase was not stably expressed in yeast membranes. These results indicate that the N-terminal segment of Ca(2+)-ATPase plays an important role in enzyme assembly and contains structural determinants necessary for ER retention of the ATPase.

Expression analysis of two gene subfamilies encoding the plasma membrane H+-ATPase in Nicotiana plumbaginifolia reveals the major transport functions of this enzyme

The plasma membrane H+-ATPase couples ATP hydrolysis to proton transport, thereby establishing the driving force for solute transport across the plasma membrane. In Nicotiana plumbaginifolia, this enzyme is encoded by at least nine pma (plasma membrane H+-ATPase) genes. Four of these are classified into two gene subfamilies, pma1-2-3 and pma4, which are the most highly expressed in plant species. We have isolated genomic clones for pma2 and pma4. Mapping of their transcript 5' end revealed the presence of a long leader that contained small open reading frames, regulatory features typical of other pma genes. The gusA reporter gene was then used to determine the expression of pma2, pma3 and pma4 in N. tabacum. These data, together with those obtained previously for pma1, led to the following conclusions. (i) The four pma-gusA genes were all expressed in root, stem, leaf and flower organs, but each in a cell-type specific manner. Expression in these organs was confirmed at the protein level, using subfamily-specific antibodies. (ii) pma4-gusA was expressed in many cell types and notably in root hair and epidermis, in companion cells, and in guard cells, indicating that in N. plumbaginifolia the same H+-ATPase isoform might be involved in mineral nutrition, phloem loading and control of stomata aperture. (iii) The second gene subfamily is composed, in N. plumbaginifolia, of a single gene (pma4) with a wide expression pattern and, in Arabidopsis thaliana, of three genes (aha1, aha2, aha3), at least two of them having a more restrictive expression pattern. (iv) Some cell types expressed pma2 and pma4 at the same time, which encode H+-ATPases with different enzymatic properties.

LST1 is a SEC24 homologue used for selective export of the plasma membrane ATPase from the endoplasmic reticulum

In Saccharomyces cerevisiae, vesicles that carry proteins from the ER to the Golgi compartment are encapsulated by COPII coat proteins. We identified mutations in ten genes, designated LST (lethal with sec-thirteen), that were lethal in combination with the COPII mutation sec13-1. LST1 showed synthetic-lethal interactions with the complete set of COPII genes, indicating that LST1 encodes a new COPII function. LST1 codes for a protein similar in sequence to the COPII subunit Sec24p. Like Sec24p, Lst1p is a peripheral ER membrane protein that binds to the COPII subunit Sec23p. Chromosomal deletion of LST1 is not lethal, but inhibits transport of the plasma membrane proton-ATPase (Pma1p) to the cell surface, causing poor growth on media of low pH. Localization by both immunofluorescence microscopy and cell fractionation shows that the export of Pma1p from the ER is impaired in lst1Delta mutants. Transport of other proteins from the ER was not affected by lst1Delta, nor was Pma1p transport found to be particularly sensitive to other COPII defects. Together, these findings suggest that a specialized form of the COPII coat subunit, with Lst1p in place of Sec24p, is used for the efficient packaging of Pma1p into vesicles derived from the ER.

The two major types of plant plasma membrane H+-ATPases show different enzymatic properties and confer differential pH sensitivity of yeast growth

The proton-pumping ATPase (H+-ATPase) of the plant plasma membrane is encoded by two major gene subfamilies. To characterize individual H+-ATPases, PMA2, an H+-ATPase isoform of tobacco (Nicotiana plumbaginifolia), was expressed in Saccharomyces cerevisiae and found to functionally replace the yeast H+-ATPase if the external pH was kept above 5.0 (A. de Kerchove d'Exaerde, P. Supply, J.P. Dufour, P. Bogaerts, D. Thinès, A. Goffeau, M. Boutry [1995] J Biol Chem 270: 23828-23837). In the present study we replaced the yeast H+-ATPase with PMA4, an H+-ATPase isoform from the second subfamily. Yeast expressing PMA4 grew at a pH as low as 4.0. This was correlated with a higher acidification of the external medium and an approximately 50% increase of ATPase activity compared with PMA2. Although both PMA2 and PMA4 had a similar pH optimum (6.6-6.8), the profile was different on the alkaline side. At pH 7.2 PMA2 kept more than 80% of the maximal activity, whereas that of PMA4 decreased to less than 40%. Both enzymes were stimulated up to 3-fold by 100 microgram/mL lysophosphatidylcholine, but this stimulation vanished at a higher concentration in PMA4. These data demonstrate functional differences between two plant H+-ATPases expressed in the same heterologous host. Characterization of two PMA4 mutants selected to allow yeast growth at pH 3.0 revealed that mutations within the carboxy-terminal region of PMA4 could still improve the enzyme, resulting in better growth of yeast cells.

A tobacco syntaxin with a role in hormonal control of guard cell ion channels

The plant hormone abscisic acid (ABA) regulates potassium and chloride ion channels at the plasma membrane of guard cells, leading to stomatal closure that reduces transpirational water loss from the leaf. The tobacco Nt-SYR1 gene encodes a syntaxin that is associated with the plasma membrane. Syntaxins and related SNARE proteins aid intracellular vesicle trafficking, fusion, and secretion. Disrupting Nt-Syr1 function by cleavage with Clostridium botulinum type C toxin or competition with a soluble fragment of Nt-Syr1 prevents potassium and chloride ion channel response to ABA in guard cells and implicates Nt-Syr1 in an ABA-signaling cascade.

Single point mutations distributed in 10 soluble and membrane regions of the Nicotiana plumbaginifolia plasma membrane PMA2 H+-ATPase activate the enzyme and modify the structure of the C-terminal region

The Nicotiana plumbaginifolia pma2 (plasma membrane H+-ATPase) gene is capable of functionally replacing the H+-ATPase genes of the yeast Saccharomyces cerevisiae, provided that the external pH is kept above 5.0. Single point mutations within the pma2 gene were previously identified that improved H+-ATPase activity and allowed yeast growth at pH 4.0. The aim of the present study was to identify most of the PMA2 positions, the mutation of which would lead to improved growth and to determine whether all these mutations result in similar enzymatic and structural modifications. We selected additional mutants in total 42 distinct point mutations localized in 30 codons. They were distributed in 10 soluble and membrane regions of the enzyme. Most mutant PMA2 H+-ATPases were characterized by a higher specific activity, lower inhibition by ADP, and lower stimulation by lysophosphatidylcholine than wild-type PMA2. The mutants thus seem to be constitutively activated. Partial tryptic digestion and immunodetection showed that the PMA2 mutants had a conformational change making the C-terminal region more accessible. These data therefore support the hypothesis that point mutations in various H+-ATPase parts displace the inhibitory C-terminal region, resulting in enzyme activation. The high density of mutations within the first half of the C-terminal region suggests that this part is involved in the interaction between the inhibitory C-terminal region and the rest of the enzyme.