Electron-Microscopic Demonstration of a “Head and Stalk” Structure of the Leaf Vacuolar ATPase inMesembryanthemum crystallinumL

The V-type H+ ATPase: molecular structure and function, physiological roles and regulation

It was nearly 30 years before the V-type H+ ATPase was admitted to the small circle of bona fide transport ATPases alongside F-type and P-type ATPases. The V-type H+ ATPase is an ATP-driven enzyme that transforms the energy of ATP hydrolysis to electrochemical potential differences of protons across diverse biological membranes via the primary active transport of H+. In turn, the transmembrane electrochemical potential of H+ is used to drive a variety of (i) secondary active transport systems via H+-dependent symporters and antiporters and (ii) channel-mediated transport systems. For example, expression of Cl- channels or transporters next to the V-type H+ ATPase in vacuoles of plants and fungi and in lysosomes of animals brings about the acidification of the endosomal compartment, and the expression of the H+/neurotransmitter antiporter next to the V-type H+ ATPase concentrates neurotransmitters in synaptic vesicles. First found in association with endosomal membranes, the V-type H+ ATPase is now also found in increasing examples of plasma membranes where the proton pump energizes transport across cell membranes and entire epithelia. The molecular details reveal up to 14 protein subunits arranged in (i) a cytoplasmic V1 complex, which mediates the hydrolysis of ATP, and (ii) a membrane-embedded V0 complex, which translocates H+ across the membrane. Clever experiments have revealed the V-type H+ ATPase as a molecular motor akin to F-type ATPases. The hydrolysis of ATP turns a rotor consisting largely of one copy of subunits D and F of the V1 complex and a ring of six or more copies of subunit c of the V0 complex. The rotation of the ring is thought to deliver H+ from the cytoplasmic to the endosomal or extracellular side of the membrane, probably via channels formed by subunit a. The reversible dissociation of V1 and V0 complexes is one mechanism of physiological regulation that appears to be widely conserved from yeast to animal cells. Other mechanisms, such as subunit-subunit interactions or interactions of the V-type H+ ATPase with other proteins that serve physiological regulation, remain to be explored. Some diseases can now be attributed to genetic alterations of specific subunits of the V-type H+ ATPase.

A structural model of the vacuolar ATPase from transmission electron microscopy

Vacuolar ATPases (V-ATPases) are large, membrane bound, multisubunit protein complexes which function as ATP hydrolysis driven proton pumps. V-ATPases and related enzymes are found in the endomembrane system of eukaryotic organsims, the plasma membrane of specialized cells in higher eukaryotes, and the plasma membrane of prokaryotes. The proton pumping action of the vacuolar ATPase is involved in a variety of vital intra- and inter-cellular processes such as receptor mediated endocytosis, protein trafficking, active transport of metabolites, homeostasis and neurotransmitter release. This review summarizes recent progress in the structure determination of the vacuolar ATPase focusing on studies by transmission electron microscopy. A model of the subunit architecture of the vacuolar ATPase is presented which is based on the electron microscopic images and the available information from genetic, biochemical and biophysical experiments.

Phenotypic subunit composition of the tobacco (Nicotiana tabacum L.) vacuolar-type H(+)-translocating ATPase

The model plant tobacco (Nicotiana tabacum L.) was chosen for a survey of the subunit composition of the V-ATPase at the protein level. V-ATPase was purified from tobacco leaf cell tonoplasts by solubilization with the nonionic detergent Triton X-100 and immunoprecipitation. In the purified fraction 12 proteins were present. By matrix-assisted laser-desorption ionization mass spectrometry (MALDI-MS) and amino acid sequencing 11 of these polypeptides could be identified as subunits A, B, C, D, F, G, c, d and three different isoforms of subunit E. The polypeptide which could not be identified by MALDI analysis might represent subunit H. The data presented here, for the first time, enable an unequivocal identification of V-ATPase subunits after gel electrophoresis and open the possibility to assign changes in polypeptide composition to variations in respective V-ATPase subunits occurring as a response to environmental conditions or during plant development.

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.

Subunit C of the vacuolar H+-ATPase of Hordeum vulgare

The molecular cloning of the first subunit C of the plant vacuolar H+-ATPase is reported. Tonoplast vesicles were purified from barley leaves by sucrose gradient centrifugation, and the tonoplast polypeptides were separated by two-dimensional (2-D) gel electrophoresis. Using an anti-ATPase holoenzyme antibody, a polypeptide was recognized in the molecular mass range of 40 kDa with an isoelectric point of about 6.0, and tentatively identified as subunit C. The polypeptide spot was excised from about 50 2-D gels and subjected to endo Lys C proteolysis. Two proteolytic peptides were sequenced and the amino acid sequences were used to design degenerated oligonucleotides, followed by PCR amplification with cDNA template and screening of a cDNA library synthesized from Hordeum vulgare poly A mRNA of epidermis strips. The full length clone of 1.5 kbp contains an open reading frame of 1062 bp encoding a polypeptide of 354 amino acids with a molecular mass of 39,982 Da and an isoelectric point of 6.04. Amino acid identity with sequences of SUC from animals and fungi is in the range of 36.7 to 38.5%. Expression of the cloned gene was demonstrated by Northern blotting and RT-PCR.

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.

Purification and reconstitution of the vacuolar H+-ATPases from lemon fruits and epicotyls

The vacuolar H+-ATPases (V-ATPases) of lemon fruits and epicotyls were detergent-solubilized, purified by column chromatography, and reconstituted into artificial proteoliposomes. During purification, a vanadate- and nitrate-sensitive ATPase activity, consisting of partially disassembled V-ATPase complexes, was resolved from the V-ATPase peak. ATPase and H+-transport activities of the purified, reconstituted V-ATPases of both fruit and epicotyl exhibited similar inhibitor profiles, except that the fruit V-ATPase retained partial vanadate sensitivity. Since the V-ATPase activity of native fruit tonoplast vesicles is insensitive to inhibitors (Müller, M. L., Irkens-Kiesecker, U., Rubinstein, B., and Taiz, L. (1996) J. Biol. Chem. 271, 1916-1924), membrane lipids or other factors may protect the fruit V-ATPase from inactivation in vivo. A kinetic analysis of H+-pumping and H+-leakage indicated that the reconstituted epicotyl V-ATPase exhibited twice as much intrinsic uncoupling or slip as the reconstituted fruit V-ATPase. Comparison of their subunit compositions by SDS-polyacrylamide gel electrophoresis indicated that the reconstituted fruit V-ATPase is enriched in two polypeptides of 33/34 and 16 kDa. Moreover, the stalks of negatively stained juice sac V-ATPases appeared thicker than those of epicotyl V-ATPases in electron micrographs.

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.

The role of crassulacean acid metabolism (CAM) in the adaptation of plants to salinity

Two case studies are presented illustrating how the behaviour of plants using crassulacean acid metabolism (CAM) provides adaptation to salinity. Perennial cacti having constitutive CAM show adaptation at the whole-plant level, engaging regulation of stomata, internal CO₂ -recycling and root physiology with salt exclusion. They are stress avoiders. Annual plants such as Mesembryanthemum crystallinum, with inducible CAM, are salt includers. They are stress-tolerant and show reactions at an array of levels: (i) regulation of turgor and gas exchange at the whole-plant level; (ii) metabolic adjustments at the cellular level; (iii) adapptive transport proteins at the membrane level and also (iv) at the macromolecular level; and (v) inductive changes at the gene expression level of the enzyme complement for metabolism (in particular involving glycolysis and malic-acid synthesis with phosphoenolpyruvate carboxylase (PEPC) as the key enzyme, and gluconeogenesis (with pyruvate-phosphate dikinase (PPDK) as a key enzyme) and membrane transport (in particular involving the tonoplast ATPase).

Detergent sensitivity of the tonoplast H(+)-ATPase and its purification from Beta vulgaris

Tonoplast membrane fractions were isolated from beetroot (Beta vulgaris) using a refined method of preparation which significantly improved the yield of active tonoplast H(+)-ATPase, and electron microscopy showed these fractions to be a preparation of small vesicles, of diameter 500 nm to 50 nm and a minor fraction consisting of mainly tubular membrane structures of diameter 5 nm and length up to 1 micron. The stability of the tonoplast H(+)-ATPase was assessed in the presence of many biological detergents, using a linked assay. The addition of detergent to tonoplast membranes generally led to an increase in ATPase activity, and activity was maintained in a wide range of both non-ionic and zwitterionic detergents. Using the non-ionic detergent dodecyl maltoside, the tonoplast H(+)-ATPase was partially purified using ion-exchange chromatography on an HPLC system. Very high rates of ATP hydrolysis were recorded in these fractions. The purified membranes behaved as expected in the presence of known activators and inhibitors. An unexpected observation, however, was that low concentrations of vanadate could significantly increase the rate of H(+)-ATPase activity.

Primary structure of V-ATPase subunit B from Manduca sexta midgut

The amino acid sequence of a vacuolar-type ATPase (V-ATPase) subunit B has been deduced from a cDNA clone isolated from a Manduca sexta larval midgut library. The library was screened by hybridization with a labeled cDNA encoding subunit B of Arabidopsis thaliana tonoplast V-ATPase. The M. sexta V-ATPase subunit B consists of 494 amino acids with a calculated M(r) of 54,902. The amino acid sequence deduced for V-ATPase subunit B of M. sexta is between 98% and 76% identical with that of seven other V-ATPase subunits B and greater than 52% identical with three archaebacterial ATPase subunits B.

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.

Chromaffin granule H(+)-ATPase has F1-like structure

Isolated H(+)-ATPase from chromaffin granules was reconstituted into liposomes and the resultant proteoliposomes were further purified by Ficoll density gradient centrifugation. Studies by electron microscopy showed that proteoliposomes had particle structures (average diameter, about 10 nm) on their outer surface. These particles could be removed from the proteoliposomes by cold treatment. Immuno-electron microscopy showed that these particles were recognized by antibodies against the hydrophilic sector of the enzyme. These results indicate that the H(+)-ATPase has a peripheral membrane structure similar to that of F1-ATPase.