Molecular cloning and sequencing of cDNAs encoding the proteolipid subunit of the vacuolar H(+)-ATPase from a higher plant

Evaluation of salt tolerance of oat cultivars and the mechanism of adaptation to salinity

Soil salinity is a threat to agricultural production worldwide. Oat (Avena sativa L.) is an irreplaceable crop in areas with fragile ecological conditions. However, there is a lack of research on salt tolerance evaluation of oat germplasm resources. Therefore, the purpose of this work was to evaluate the salt tolerance of oat cultivars and investigate the mechanism of salt-tolerant oat cultivars' adaptation to salinity. Salt tolerance of 100 oat cultivars was evaluated, and then two salt-tolerant cultivars and two salt-sensitive cultivars were used to compare their physiological responses and expression patterns of Na⁺- and K⁺-transport-related genes under salinity. Principal component analysis and membership function analysis had good predictability for salt tolerance evaluation of oat and other crops. The 100 oat cultivars were clustered into three categories, with three salt tolerance levels. Under saline condition, salt-tolerant cultivars maintained higher growth rate, leaf cell membrane integrity, and osmotic adjustment capability via enhancing the activities of antioxidant enzymes and accumulating more osmotic regulators. Furthermore, salt-tolerant cultivars had stronger capability to restrict root Na ⁺ uptake through reducing AsAKT1 and AsHKT2;1 expression, exclude more Na⁺ from root through increasing AsSOS1 expression, compartmentalize more Na ⁺ into root vacuoles through increasing AsNHX1 and AsVATP-P1 expression, and absorb more K⁺ through increasing AsKUP1 expression, compared with salt-sensitive cultivars. The evaluation procedure developed in this work can be applied for screening cereal crop cultivars with higher salt tolerance, and the elucidated mechanism of oat adaptation to salinity lays a foundation for identifying more functional genes related to salt tolerance.

Proteolipid of vacuolar H+-ATPase of Plasmodium falciparum: cDNA cloning, gene organization and complementation of a yeast null mutant

Vacuolar H(+)-ATPase (V-ATPase), an electrogenic proton pump, is highly expressed in Plasmodium falciparum, the human malaria parasite. Although V-ATPase-driven proton transport is involved in various physiological processes in the parasite, the overall features of the V-ATPase of P. falciparum, including the gene organization and biogenesis, are far less known. Here, we report cDNA cloning of proteolipid subunit c of P. falciparum, the smallest and most highly hydrophobic subunit of V-ATPase. RT-PCR analysis as well as Northern blotting indicated expression of the proteolipid gene in the parasite cells. cDNA, which encodes a complete reading frame comprising 165 amino acids, was obtained, and its deduced amino acid sequence exhibits 52 and 57% similarity to the yeast and human counterparts, respectively. Southern blot analysis suggested the presence of a single copy of the proteolipid gene, with 5 exons and 4 introns. Upon transfection of the cDNA into a yeast null mutant, the cells became able to grow at neutral pH, accompanied by vesicular accumulation of quinacrine. In contrast, a mutated proteolipid with replacement of glutamate residue 138 with glutamine did not lead to recovery of the growth ability or vesicular accumulation of quinacrine. These results indicated that the cDNA actually encodes the proteolipid of P. falciparum and that the proteolipid is functional in yeast.

Characterization of the V-type H((+))-ATPase in the resurrection plant Tortula ruralis: accumulation and polysomal recruitment of the proteolipid c subunit in response to salt-stress

Tortula ruralis is an important experimental system for the study of plant vegetative desiccation tolerance. EST gene discovery efforts utilizing desiccated gametophytes have identified a cDNA Vac1 encoding a predicted polypeptide with significant similarity to the vacuolar H(+)-ATPase c subunit. VAC1, the 167 amino acid deduced polypeptide, has a predicted molecular mass of 16.9 kDa, and a predicted pI of 9.7. Phylogenetic analysis demonstrated that previously characterized proteolipid polypeptide sequences could be reproducibly grouped into two major clades and that VAC1 forms a discrete evolutionary group. RNA blot and Western blot hybridizations were used to analyse expression of Vac1 and accumulation of VAC1 in response to (1) desiccation and rehydration, (2) increased NaCl concentration, and (3) NaCl-shock. During a desiccation-rehydration cycle, Vac1 transcripts are expressed in both the total and polysomal RNA fractions in approximately equal amounts, and the steady-state transcript levels are unchanged. However, Vac1 transcript levels increased in response to both elevated NaCl concentration and NaCl-shock. There is a preferential accumulation of Vac1 transcripts within the polysomal RNA fraction in response to salt stress, and these data suggest that T. ruralis possesses a salinity-stress-dependent and desiccation-stress-independent mechanism for post-transcriptional gene control. Using a cotton anti-c subunit polyclonal antibody raised against the C-terminal domain, it was shown that the amount of Tortula 16 kDa proteolipid in the tonoplast protein fraction was unaffected by any stress treatment.

Cloning and expression of cDNAs encoding plant V-ATPase subunits in the corresponding yeast null mutants

Complementation of yeast null mutants is widely used for cloning of homologous genes from heterologous sources. We have used this method to clone the relevant V-ATPase genes from lemon fruit and Arabidopsis thaliana cDNA libraries. The pH levels are very different in the vacuoles of the lemon fruit and the A. thaliana, yet both are the result of the activity of the same enzyme complex, namely the V-ATPase. In order to investigate the mechanism that enables the enzyme to maintain such differences in pH values, we have compared the subunit composition of the V-ATPase complex from both sources. Towards this end, we have constructed a cDNA library from lemon fruit and cloned it into a similar shuttle vector to the one of the A. thaliana cDNA library, which is commercially available. In this work, we report the cloning and expression of VMA10 from both sources, two isoforms of the lemon proteolipid (VMA3) and the lemon homologue of yeast VPH1/STV1 subunit, LEMAC.

Structure, function and regulation of the plant vacuolar H(+)-translocating ATPase

The plant V-ATPase is a primary-active proton pump present at various components of the endomembrane system. It is assembled by different protein subunits which are located in two major domains, the membrane-integral V(o)-domain and the membrane peripheral V(1)-domain. At the plant vacuole the V-ATPase is responsible for energization of transport of ions and metabolites, and thus the V-ATPase is important as a 'house-keeping' and as a stress response enzyme. It has been shown that transcript and protein amount of the V-ATPase are regulated depending on metabolic conditions indicating that the expression of V-ATPase subunit is highly regulated. Moreover, there is increasing evidence that modulation of the holoenzyme structure might influence V-ATPase activity.

Vacuolar H(+)-ATPase is expressed in response to gibberellin during tomato seed germination

Completion of germination (radicle emergence) by gibberellin (GA)-deficient (gib-1) mutant tomato (Lycopersicon esculentum Mill.) seeds is dependent upon exogenous GA, because weakening of the endosperm tissue enclosing the radicle tip requires GA. To investigate genes that may be involved in endosperm weakening or embryo growth, differential cDNA display was used to identify mRNAs differentially expressed in gib-1 seeds imbibed in the presence or absence of GA(4+7). Among these was a GA-responsive mRNA encoding the 16-kD hydrophobic subunit c of the V(0) membrane sector of vacuolar H(+)-translocating ATPases (V-ATPase), which we termed LVA-P1. LVA-P1 mRNA expression in gib-1 seeds was dependent on GA and was particularly abundant in the micropylar region prior to radicle emergence. Both GA dependence and tissue localization of LVA-P1 mRNA expression were confirmed directly in individual gib-1 seeds using tissue printing. LVA-P1 mRNA was also expressed in wild-type seeds during development and germination, independent of exogenous GA. Specific antisera detected protein subunits A and B of the cytoplasmic V(1) sector of the V-ATPase holoenzyme complex in gib-1 seeds only in the presence of GA, and expression was localized to the micropylar region. The results suggest that V-ATPase plays a role in GA-regulated germination of tomato seeds.

Cloning of the V-ATPase subunit G in plant: functional expression and sub-cellular localization

A 13-kDa tobacco plasma membrane protein was isolated from two-dimensional electrophoresis gels. After microsequencing, RT-PCR techniques and cDNA library screening allowed for the cloning of two cDNAs. These cDNAs encoded for the subunit G of the vacuolar H+-ATPase, the first one identified in plants. Analysis of mRNA distribution showed a maximum level in the leaves and in the stem of the apical part of the tobacco plant. Heterologous functional complementation of the yeast mutant (deltavma10::URA3) was achieved with the two cDNAs. After fractionation of microsomal membranes on linear sucrose gradient, Western blots were performed using antibodies against recombinant protein and three peaks were identified: one which comigrated with the tonoplast marker and the others at slightly higher density corresponding to endoplasmic reticulum and to plasma membrane fractions.

Initial steps in the assembly of the vacuole-type H+-ATPase

The plant vacuole is acidified by a complex multimeric enzyme, the vacuole-type H+-ATPase (V-ATPase). The initial association of ATPase subunits on membranes was studied using an in vitro assembly assay. The V-ATPase assembled onto microsomes when V-ATPase subunits were supplied. However, when the A or B subunit or the proteolipid were supplied individually, only the proteolipid associated with membranes. By using poly(A+) RNA depleted in the B subunit and proteolipid subunit mRNA, we demonstrated A subunit association with membranes at substoichiometric amounts of the B subunit or the 16-kD proteolipid. These data suggest that poly(A+) RNA-encoded proteins are required to catalyze the A subunit membrane assembly. Initial events were further studied by in vivo protein labeling. Consistent with a temporal ordering of V-ATPase assembly, membranes contained only the A subunit at early times; at later times both the A and B subunits were found on the membranes. A large-mass ATPase complex was not efficiently formed in the absence of membranes. Together, these data support a model whereby the A subunit is first assembled onto the membrane, followed by the B subunit.

Differential immunological cross-reactions with antisera against the V-ATPase of Kalanchoë daigremontiana reveal structural differences of V-ATPase subunits of different plant species

Two antisera (ATP88 and ATP95) raised against the V-ATPase holoenzyme of Kalanchoë daigremontiana were tested for their cross-reactivity with subunits of V-ATPases from other plant species. V-ATPases from Kalanchoë blossfeldiana, Mesembryanthemum crystallinum, Nicotiana tabacum, Lycopersicon esculentum, Citrus limon, Lemna gibba, Hordeum vulgare and Zea mays were immunoprecipitated with an antiserum against the catalytic V-ATPase subunit A of M. crystallinum. As shown by silver staining and Western blot analysis with ATP88, subunits A, B, C, D and c were present in all immunoprecipitated V-ATPases. In contrast, ATP95 recognized the whole set of subunits only in K. blossfeldiana, M. crystallinum, H. vulgare and Z. mays. This differential cross reactivity of ATP95 indicates the presence of structural differences of certain V-ATPase subunits. Based on the Bafilomycin A1-sensitive ATPase activity of tonoplast enriched vesicles, and on the amount of V-ATPase solubilized and immunoprecipitated, the specific ATP-hydrolysis activity of the V-ATPases under test was determined. The structural differences correlate with the ability of V-ATPases from different species to hydrolyze ATP at one given assay condition for ATP-hydrolysis measurements. Interestingly V-ATPases showing cross-reactivity of subunits A, B, C, D and c with ATP95 showed higher rates of specific ATP hydrolysis compared to V-ATPases containing subunits which were not labeled by ATP95. Thus, V-ATPases with high turnover rates in our assay conditions may show common structural characteristics which separate them from ATPases with low turnover rates.

Salt stress induces an increased expression of V-type H(+)-ATPase in mature sugar beet leaves

In the halotolerant sugar beet co-expression of V-ATPase and a vacuolar Na+/H(+)-antiporter provides a mechanism for vacuolar salt sequestration. To analyze salt-induced changes in the expression of the vacuolar H(+)-ATPase (V-ATPase) a partial cDNA of the proton-channel forming subunit c was cloned by RT-PCR. Southern blot analysis indicated a small gene family. In control plants transcript levels were high in roots and young growing leaves but low in fully expanded leaves. In mature leaves salt exposure (400 mM, 48 h) induced a strong increase in subunit c-mRNA. Transcripts for the catalytic subunit A followed a similar developmental and stress-modulated pattern, indicating a coordinate regulation of transcripts for both V-ATPase subunits. Concomittant with the mRNA increases the amount of V-ATPase protein increased as well.

PHYSIOLOGY OF ION TRANSPORT ACROSS THE TONOPLAST OF HIGHER PLANTS

The vacuole of plant cells plays an important role in the homeostasis of the cell. It is involved in the regulation of cytoplasmic pH, sequestration of toxic ions and xenobiotics, regulation of cell turgor, storage of amino acids, sugars and CO2 in the form of malate, and possibly as a source for elevating cytoplasmic calcium. All these activities are driven by two primary active transport mechanisms present in the vacuolar membrane (tonoplast). These two mechanisms employ high-energy metabolites to pump protons into the vacuole, establishing a proton electrochemical potential that mediates the transport of a diverse range of solutes. Within the past few years, great advances at the molecular and functional levels have been made on the characterization and identification of these mechanisms. The aim of this review is to summarize these studies in the context of the physiology of the plant cell.

Isolation and sequence analysis of a cDNA encoding the c subunit of a vacuolar-type H(+)-ATPase from the CAM plant Kalanchoë daigremontiana

We report the sequence of a cDNA clone encoding the c ("16 kDa') subunit of a vacuolar-type H(+)-ATPase (V-ATPase) from Kalanchoë daigremontiana, a plant in which the cell vacuole plays a pivotal role in crassulacean acid metabolism. The clone, pKVA211, was isolated from a K. daigremontiana leaf cDNA library constructed in lambda ZAP II using a homologous PCR-generated cDNA probe for the V-ATPase c subunit. The KVA211 cDNA was 839 nucleotides long and included a 20 bp poly(A)+ tail together with a complete 495 bp coding region for a polypeptide with a predicted molecular mass of 16659 Da. The deduced amino acid sequence was highly conserved across the wide range of eukaryotes (vertebrates, invertebrates, fungi, plants and protozoa) in which this gene has now been identified. Sequence comparison of several PCR products and genomic Southern analysis indicated that the V-ATPase c subunit in K. daigremontiana is encoded by a small multi-gene family. Steady-state levels of the KVA211 mRNA were much higher in leaves than in roots or flowers, and expression of this transcript in leaves was shown to be strongly light-dependent.

Down-regulation of plant V-type H+ -ATPase genes after light-induced inhibition of growth

Cell extension growth in the mesocotyl tip of dark-grown Zea mays L. seedlings is dependent on vacuole enlargement and massive flux of ER and Golgi vesicles. Water flow into the expanding vacuole is driven by ion accumulation, which in turn is energized by the vacuolar H+-ATPase (V-ATPase). The V-ATPase energizes the secondary ion transport into the expanding vacuole. As light exposure leads to a strong inhibition of extension growth, the effect of light on transcript levels for subunits A and c of the V-ATPase was analyzed. Partial homologous cDNAs for subunit A and two isoforms of subunit c were cloned by RT-PCR. In dark-grown seedlings transcript levels for both subunits were much higher in the growing mesocotyl tip than in the fully differentiated mesocotyl tissue. Only in the tip region did light exposure lead to a strong and coordinate down-regulation of both mRNAs whereas in the differentiated mesocotyl only a slight decrease was observed. The results indicate that expression of the 'housekeeping' V-type H+-ATPase is strongly regulated in response to growth rate.

Antisense RNA inhibition of the putative vacuolar H(+)-ATPase proteolipid of Dictyostelium reduces intracellular Ca2+ transport and cell viability

Transport of Ca2+ via a P-type pump into the contractile vacuole of Dictyostelium discoideum appears to be facilitated by vacuolar proton (V-H+) ATPase activity. To investigate the involvement of the V-H(+)-ATPase in this process using molecular techniques, we cloned a cDNA (vatP) encoding the putative proteolipid subunit of this enzyme. The deduced protein product of this cDNA is composed of 196 amino acids with a calculated M(r) of 20,148 and the primary structure exhibits high amino acid sequence identity with V-H(+)-ATPase proteolipids from other organisms. vatP is a single-copy gene and it produces one approximately 900 nt transcript at relatively constant levels during growth and development. Attempts to disrupt the endogenous gene using vatP cDNA were unsuccessful. But, expression of vatP antisense RNA reduced the levels of vatP message and V-H(+)-ATPase activity by 50% or more. These antisense strains grew and developed slowly, especially under acidic conditions, and the cells seemed to have difficulty forming acidic vesicles. During prolonged cultivation, all of the antisense strains either reverted to a wild-type phenotype or died. Thus in Dictyostelium, unlike yeast, the V-H(+)-ATPase seems to be indispensable for cell viability. When different antisense strains were analyzed for Ca2+ uptake by the contractile vacuole, they all accumulated less Ca2+ than control transformants. These results are consistent with earlier pharmacological studies which suggested that the V-H(+)-ATPase functions in intracellular Ca2+ transport in this organism.

Acidification of lysosomes and endosomes

Lysosomes, endosomes, and a variety of other intracellular organelles are acidified by a family of unique proton pumps, termed the vacuolar H(+)-ATPases, that are evolutionarily related to bacterial membrane proton pumps and the F1-F0 H(+)-ATPases that catalyze ATP synthesis in mitochondria and chloroplasts. The electrogenic vacuolar H(+)-ATPase is responsible for generating electrical and chemical gradients across organelle membranes with the magnitude of these gradients ultimately determined by both proton pump regulatory mechanisms and, more importantly, associated ion and organic solute transporters located in vesicle membranes. Analogous to Na+, K(+)-ATPase on the cell membrane, the vacuolar proton pump not only acidifies the vesicle interior but provides a potential energy source for driving a variety of coupled transporters, many of them unique to specific organelles. Although the basic mechanism for organelle acidification is now well understood, it is already apparent that there are many differences in both the function of the proton pump and the associated transporters in different organelles and different cell types. These differences and their physiologic and pathophysiologic implications are exciting areas for future investigation.

Several distinct genes encode nearly identical to 16 kDa proteolipids of the vacuolar H(+)-ATPase from Arabidopsis thaliana

To understand the subcellular roles and the regulation of vacuolar H(+)-ATPases, we have begun to identify the genes encoding the major subunits and to determine their patterns of expression in Arabidopsis thaliana. Two distinct cDNAs (AVA-P1 and AVA-P2) and one genomic sequence (AVA-P3) encoding the 16 kDa subunit have been isolated. The 16 kDa proteolipid is a major component of the membrane integral sector that forms the proton conductance pathway and is required for assembly of the V-ATPase complex. Interestingly, the open reading frame of one full-length cDNA (AVA-P1) and a genomic sequence (AVA-P3) encoded an identical polypeptide of 164 amino acids with a molecular mass of 16,570. The deduced amino acid sequences of the two cDNAs were nearly identical (99%) and hydropathy plots suggested a molecule with four membrane-spanning domains characteristic of V-ATPase proteolipids. The three genes differed mainly in their codon usage and in their 3'-untranslated regions. The coding region of the genomic sequence, AVA-P3, was interrupted by two introns located at the codons for Cys-26 and Arg-121. The presence of additional 16 kDa proteolipid genes was suggested from several polymerase chain reaction (PCR)-amplified fragments that differed from one another in the size of the second intron. PCR 1 had an intron of ca. 800 bp and its identity as AVA-P4, a fourth member of the gene family, was confirmed from sequence analyses of an EST cDNA. The mRNAs of three genes (AVA-P1, AVA-P2 and AVA-P3) were detected in Arabidopsis leaf, root, flower and silique; yet expression of AVA-P1 and AVA-P2 was lower in roots. All three genes were expressed in light- or dark-grown seedlings; however mRNA levels of AVA-P2 were enhanced in etiolated plants. Arabidopsis thaliana, therefore, has at least four distinct genes encoding nearly identical 16 kDa proteolipids, and the enhanced expression of AVA-P2 transcript in etiolated seedlings suggests that an increase in V-ATPase could accompany cell expansion.

Codon usage in plant peroxidase genes

Codon preference and asymmetry in usage in the DNA sequences encoding the mature enzyme protein of 24 plant peroxidases from 12 different species were examined. Codon usage in highly conserved/non-conserved areas of the sequences was analysed, as well as possible deficiency/excess in CpG dinucleotides in the pairs of codon positions. Sequence relationships displayed by overall codon usage, dinucleotide frequencies within codons, and amino acid sequences were also studied. The main findings were: (1) Monocots clustered separately from dicots for overall codon usage and dinucleotide frequencies in codon positions 2 and 3, with six and seven clusters respectively discernible among these 24 peroxidase sequences. The monocot/dicot distinction disappeared in the four clusters among the mature protein amino acid sequences. Overall codon usage in sequences from monocotyledon and dicotyledon species differed, the monocots favouring codons with C or G in the third position. (2) Codon usage was biassed in many sequences, asymmetry was particularly noticeable in the monocots. (3) For repeated amino acids within conserved areas, codon preference appeared dependent on the order in which the repeated amino acid occurred, so that its usage of synonymous codons frequently balanced out.

The proteolipid subunit of the Neurospora crassa vacuolar ATPase: isolation of the protein and the vma-3 gene

We have isolated the proteolipid subunit from the vacuolar ATPase of Neurospora crassa, using ion-exchange chromatography. We have also isolated several cDNA clones and the corresponding genomic DNA that encodes this subunit. The derived protein sequence indicates that the polypeptide is composed of 161 amino acid residues with an M(r) of 16,328 kDa. The gene encoding the proteolipid, named vma-3, is unusual in several respects. It contains four introns and, unlike other fungal genes, has non-coding regions that are as large as the coding regions. The 3' untranslated regions of the cDNAs were quite heterogeneous, with polyadenylation sites more than 300 bp apart. Analysis of the mRNA indicates that two size classes of transcripts are produced, differing in the length of the 3' untranslated region. Mapping of the vma-3 gene showed that it is closely linked, but not adjacent to, vma-1, the gene encoding the 67 kDa subunit of the vacuolar ATPase. This raises the possibility that in N. crassa some of the vacuolar ATPase genes may be clustered.

The same 15 kDa proteolipid subunit is a constituent of two different proteins in Torpedo, the acetylcholine releasing protein mediatophore and the vacuolar H+ ATPase

Using the monoclonal antibody 15K1, we have studied, at the cellular and subcellular levels, the distribution of a 15 kDa proteolipid, identified as the subunit of mediatophore, a presynaptic membrane protein able to release acetylcholine when activated by calcium. Aside from the electric lobe, the antigen distribution in the brain of Torpedo paralleled that of the synaptic vesicle antigen SV2 and did not appear to be related to that of acetylcholine and choline acetyltransferase. The 15 kDa proteolipid antigen was therefore present in all nerve endings and not restricted to cholinergic ones. At the ultrastructural level, on cholinergic nerve endings, the antigen was detected associated to synaptic vesicles and, to a lesser extent, to the presynaptic plasma membrane. Indeed, considering the high sequence homology between the mediatophore subunit (Birman et al., 1990) and the proteolipid subunit of the vacuolar type H+ ATPase, a major enzyme constituent of synaptic vesicles, this distribution was not surprising. To determine whether antibody 15K1 recognizes the vacuolar type H+ ATPase, we chose a non neuronal cell type which possesses a high content of this enzyme, the kidney proton secreting epithelial cells. Indeed, antibody 15K1 intensely labelled the apical plasma membrane of mitochondria rich epithelial cells in kidney tubules. A high density of the antigen was also found associated to intracellular membrane structures such as lysosomal multivesicular bodies, both in kidney epithelial cells and in electromotoneurons. The 15 kDa proteolipid antigen was associated with other vacuolar H+ ATPase subunits in kidney membranes which was not the case in presynaptic plasma membranes. This illustrates that the 15 kDa proteolipid antigen is a constituent of two different protein complexes, which exhibit very different functional properties.

The vacuolar H(+)-ATPase of Saccharomyces cerevisiae is required for efficient copper detoxification, mitochondrial function, and iron metabolism

Mutations in the GEF2 gene of the yeast Saccharomyces cerevisiae have pleiotropic effects. The gef2 mutants display a petite phenotype. These cells grow slowly on several different carbon sources utilized exclusively or primarily by respiration. This phenotype is suppressed by adding large amounts of iron to the growth medium. A defect in mitochondrial function may be the cause of the petite phenotype: the rate of oxygen consumption by intact gef2 cells and by mitochondrial fractions isolated from gef2 mutants was reduced 60%-75% relative to wild type. Cytochrome levels were unaffected in gef2 mutants, indicating that heme accumulation is not significantly altered in these strains. The gef2 mutants were also more sensitive than wild type to growth inhibition by several divalent cations including Cu. We found that the cup5 mutation, causing Cu sensitivity, is allelic to gef2 mutations. The GEF2 gene was isolated, sequenced, and found to be identical to VMA3, the gene encoding the vacuolar H(+)-ATPase proteolipid subunit. These genetic and biochemical analyses demonstrate that the vacuolar H(+)-ATPase plays a previously unknown role in Cu detoxification, mitochondrial function, and iron metabolism.

Sequence of a 17 kDa vacuolar H(+)-ATPase proteolipid subunit from insect midgut and Malpighian tubules

A 0.4 kb polymerase chain reaction (PCR) product obtained from cDNA made from the midgut and Malpighian tubules of fifth instar larvae of Heliothis virescens was used to screen a larval midgut and Malpighian tubules cDNA library. Four clones were obtained, one of 1.9 kb and others of 1.4 kb. The 1.9 kb clone encodes a 17.2 kDa protein which is highly homologous to other vacuolar ATPases proteolipids. Putative N-glycosylation and DCCD binding sites were observed at amino acid residues 83 and 139, respectively.

Evidence for a common structure for a class of membrane channels

Electron microscopic analysis of gap-junction-like structures isolated from an anthropod (Nephrops norvegicus) and composed of a 16-kDa polypeptide, show the functional unit to be a star-shaped hexamer of protein arranged around a central channel which runs perpendicular to the plane of the membrane. Estimations of the molecular volume carried out on an averaged projection are consistent with a subunit mass of 16-18 kDa. Fourier transform infrared spectroscopy indicates a high alpha-helical content for the protein, supporting secondary-structure predictions of four transmembrane alpha helices/monomer. The averaged projection shows a close resemblance to a hexamer of the 16-kDa protein built on the basis of a four alpha-helical bundle [Finbow, M. E., Eliopoulos, E. E., Jackson, P. J., Keen, J. N., Meagher, L., Thompson, P., Jones, P. C. & Findlay, J. B. C. (1992) Protein Eng. 5, 7-15]. The reconstructed image is also similar to that obtained for gap-junction-like channels isolated from a related arthropod [Homarus americanus; Sikerwar, S. S., Downing, K. H. & Glaeser, R. M. (1991) J. Struct. Biol. 106, 255-263] whose protein content was unknown but which we demonstrate may be composed of a related 16-kDa protein. Previous studies have shown a high sequence identity of the Nephrops 16-kDa protein with the 16-kDa proteolipid subunit c of the vascular H(+)-ATPase, both of which in turn bear similarity to the 8-kDa proteolipid subunit of the F1F0-ATP synthase. Expression of cDNA coding for the Nephrops 16-kDa protein in Saccharomyces cerevisiae, in which the endogenous gene coding for the V-ATPase proteolipid has been inactivated, restores V-ATPase activity and cell growth.

Analysis of the gene encoding a 16-kDa proteolipid subunit of the vacuolar H(+)-ATPase from Manduca sexta midgut and tubules

Vacuolar ATPases (V-ATPases), originally characterised as components of endomembranes, have also been implicated in epithelial ion transport, both in vertebrates and in insects. The ATPase is particularly noteworthy in lepidopteran larvae, where it generates large transepithelial potential differences and short-circuit currents across the midgut epithelium. A cDNA library from Manduca sexta larval midguts and Malpighian tubules was screened with a Drosophila melanogaster cDNA encoding the 16-kDa proteolipid subunit of the V-ATPase, and a 1.4-kb cDNA sequenced in its entirety. The sequence contains a long open reading frame, encoding a putative peptide of 156 amino acids (aa) and with an M(r) of 15,967, in close agreement with values previously suggested by sodium dodecyl sulfate-polyacrylamide gels of M. sexta midgut proteins. Correspondence of the deduced aa sequence with those of other species, particularly D. melanogaster, was extremely close. Northern blots of M. sexta midgut mRNA at high stringency revealed two transcripts of 1.4 and 1.9 kb, whereas genomic Southern blots suggest that there is only a single copy of the gene in M. sexta. The possibility that members of the 16-kDa gene family might serve multiple roles in transport and membrane communication is discussed.

Effect of N,N'-dicyclohexylcarbodiimide on acetylcholine release from Torpedo synaptosomes and proteoliposomes reconstituted with the proteolipid mediatophore

The mediatophore is a presynaptic membrane protein that has been shown to translocate acetylcholine (ACh) under calcium stimulation when reconstituted into artificial membranes. The mediatophore subunit, a 15-kDa proteolipid, presents a very high sequence homology with the N,N'-dicyclohexylcarbodiimide (DCCD)-binding proteolipid subunit of the vacuolar-type H(+)-ATPase. This prompted us to study the effect of DCCD, a potent blocker of proton translocation, on calcium-dependent ACh release. The present work shows that DCCD has no effect on ACh translocation either from Torpedo synaptosomes or from proteoliposomes reconstituted with purified mediatophore. However, using [14C]DCCD, we were able to demonstrate that the drug does bind to the 15-kDa proteolipid subunit of the mediatophore. These results suggest that although the 15-kDa proteolipid subunits of the mediatophore and the vacuolar H(+)-ATPase may be identical, different domains of these proteins are involved in proton translocation and calcium-dependent ACh release and that the two proteins have a different membrane organization.

Evolution of structure and function of V-ATPases

Proton pumping ATPases/ATPsynthases are found in all groups of present-day organisms. The structure of V- and F-type ATPases/ATP synthases is very conserved throughout evolution. Sequence analysis shows that the V- and F-type ATPases evolved from the same enzyme already present in the last common ancestor of all known extant life forms. The catalytic and noncatalytic subunits found in the dissociable head groups of the V/F-type ATPases are paralogous subunits, i.e., these two types of subunits evolved from a common ancestral gene. The gene duplication giving rise to these two genes (i.e., encoding the catalytic and noncatalytic subunits) predates the time of the last common ancestor. Mapping of gene duplication events that occurred in the evolution of the proteolipid, the noncatalytic and the catalytic subunits, onto the tree of life leads to a prediction for the likely subunit structure of the encoded ATPases. A correlation between structure and function of V/F-ATPases has been established for present-day organisms. Implications resulting from this correlation for the bioenergetics operative in proto-eukaryotes and in the last common ancestor are presented. The similarities of the V/F-ATPase subunits to an ATPase-like protein that was implicated to play a role in flagellar assembly are evaluated. Different V-ATPase isoforms have been detected in some higher eukaryotes. These data are analyzed with respect to the possible function of the different isoforms (tissue specific, organelle specific) and with respect to the point in their evolution when these gene duplications giving rise to the isoforms had occurred, i.e., how far these isoforms are distributed.

Plant membrane transport

Recent developments in plant membrane transport, particularly concerning the vacuolar and plasma membranes, have increased our understanding of molecular aspects of primary pumps, carrier systems and ion channels.

Vacuolar H(+)-translocating ATPases from plants: structure, function, and isoforms

The vacuolar H(+)-translocating ATPase (V-type ATPase) plays a central role in the growth and development of plant cells. In a mature cell, the vacuole is the largest intracellular compartment, occupying about 90% of the cell volume. The proton electrochemical gradient (acid inside) formed by the vacuolar ATPase provides the primary driving force for the transport of numerous ions and metabolites against their electrochemical gradients. The uptake and release of solutes across the vacuolar membrane is fundamental to many cellular processes, such as osmoregulation, signal transduction, and metabolic regulation. Vacuolar ATPases may also reside on endomembranes, such as Golgi and coated vesicles, and thus may participate in intracellular membrane traffic, sorting, and secretion. Plant vacuolar ATPases are large complexes (400-650 kDa) composed of 7-10 different subunits. The peripheral sector of 5-6 subunits includes the nucleotide-binding catalytic and regulatory subunits of approximately 70 and approximately 60 kDa, respectively. Six copies of the 16-kDa proteolipid together with 1-3 other subunits make up the integral sector that forms the H+ conducting pathway. Isoforms of plant vacuolar ATPases are suggested by the variations in subunit composition observed among and within plant species, and by the presence of a small multigene family encoding the 16-kDa and 70-kDa subunits. Multiple genes may encode isoforms with specific properties required to serve the diverse functions of vacuoles and endomembrane compartments.

Evolution of organellar proton-ATPases

Proton ATPases function in biological energy conversion in every known living cell. Their ubiquity and antiquity make them a prime source for evolutionary studies. There are two related families of H(+)-ATPases; while the family of F-ATPases function in eubacteria chloroplasts and mitochondria, the family of V-ATPases are present in archaebacteria and the vacuolar system of eukaryotic cells. Sequence analysis of several subunits of V- and F-ATPases revealed several of the important steps in their evolution. Moreover, these studies shed light on the evolution of the various organelles of eukaryotes and suggested some events in the evolution of the three kingdoms of eubacteria, archaebacteria and eukaryotes.