The Non-Ionic Detergent Brij 58 Conserves the Structure of the Tonoplast H+-ATPase ofMesembryanthemum crystallinumL. During Solubilization and Partial Purification

Proteomic analysis of Mesembryanthemum crystallinum leaf microsomal fractions finds an imbalance in V-ATPase stoichiometry during the salt-induced transition from C3 to CAM

The halophyte Mesembryanthemum crystallinum adapts to salt stress by salt uptake and switching from C3 photosynthesis to CAM (crassulacean acid metabolism). An important role in this process is played by transport proteins in the tonoplast of the central vacuole. In the present study we examine dynamic changes in the protein composition during salt-stress adaptation in microsomes from M. crystallinum leaves. Plants challenged with 400 mM NaCl accumulate salt by day 4 of treatment and malic acid only at day 12; a switching to CAM hence follows any initial steps of salt adaptation with a delay. Using a label-free and semiquantitative approach, we identified the most dramatic changes between the proteome of control plants and plants harvested after 12 days of the treatment; the abundance of 14 proteins was significantly affected. The proteomic data revealed that the majority of the subunits of V-ATPase (vacuolar H(+)-ATPase) holoenzyme. The salt treatment somewhat decreased the abundance of all subunits in the short term (4 days). Long-term adaptation, including the switching to CAM, goes together with a strong increase in the representation of all detectable subunits. Because this increase is subunit-specific, with the highest rise occurring for subunits E and c, the data suggest that long-term adaptation to salt stress correlates with a change in V-ATPase subunit stoichiometry and highlight the structural plasticity of this holoenzyme.

Organ-specific responses of vacuolar H-ATPase in the shoots and roots of C halophyte Suaeda salsa to NaCl

Suaeda salsa L. is a halophytic species that is well adapted to high salinity. In order to understand its salt tolerance mechanism, we examined the growth and vacuolar H(+)-ATPase (V-ATPase) response to NaCl within the shoots and roots. The growth of shoots, but not roots, was dramatically stimulated by NaCl. Cl(-) and Na(+) were mainly accumulated in shoots. V-ATPase activity was significantly increased by NaCl in roots and especially in shoots. Interestingly, antisera ATP95 and ATP88b detected three V(1) subunits (66, 55 and 36 KDa) of V-ATPase only in shoots, while an 18 kDa V(0) subunit of V-ATPase was detected by both antisera in shoots and roots. It suggested that the tissue-specific characteristics of V-ATPase were related to the different patterns of growth and ion accumulation in shoots and roots of S. salsa.

The leaf tonoplast V-H(+)-Atpase activity of a C3 halophyte Suaeda salsa is enhanced by salt stress in a Ca-dependent mode

Suaeda salsa seedlings grown in Hoagland nutrient solution were treated with different concentrations of NaCl combined with two levels of Ca2+ (0 and 20 mmol/L) to study the effect of Ca2+ nutrition on the growth and activity of leaf tonoplast V-H(+)-ATPase. Increase of Ca2+ concentration in the solution markedly increased the relative growth quantity of S. salsa seedlings and Ca2+ and K+ concentration in the leaf cell sap under NaCl stress. The leaf V-H(+)-ATPase activity was significantly increased with increasing NaCl concentration under high Ca2+ application (20 mmol/L), but little changed under Ca2+ starvation (0 mmol/L). Western blot analysis showed that the leaf V-H(+)-ATPase of S. salsa was at least composed of A, B, D and c subunits, and their protein amounts were not affected by NaCl treatments under Ca2+ starvation (0 mmol/ L) with an exception of 100 mmol/L NaCl, but increased under high Ca2+ application (20 mmol/L). There was a positive correlation between activity of V-H(+)-ATPase and the protein amounts of the subunits. The results suggest that Ca2+ nutrition played an important role in the salt tolerance of S. salsa, and that enhancement of V-H(+)-ATPase activity under salt stress was Ca2(+)-dependent.

Concanamycin A, the specific inhibitor of V-ATPases, binds to the V(o) subunit c

Vacuolar-type ATPase (V-ATPase) purified from the midgut of the tobacco hornworm Manduca sexta is inhibited 50% by 10 nm of the plecomacrolide concanamycin A, the specific inhibitor of V-ATPases. To determine the binding site(s) of that antibiotic in the enzyme complex, labeling with the semisynthetic 9-O-[p-(trifluoroethyldiazirinyl)-benzoyl]-21,23-dideoxy-23-[(125)I]iodo-concanolide A (J-concanolide A) was performed, which still inhibits the V-ATPase 50% at a concentration of 15-20 microm. Upon treatment with UV light, a highly reactive carbene is generated from this concanamycin derivative, resulting in the formation of a covalent bond to the enzyme. In addition, the radioactive tracer (125)I makes the detection of the labeled subunit(s) feasible. Treatment of the V(1)/V(o) holoenzyme, the V(o) complex, and the V-ATPase containing goblet cell apical membranes with concanolide resulted in the labeling of only the proteolipid, subunit c, of the proton translocating V(o) complex. Binding of J-concanolide A to subunit c was prevented in a concentration-dependent manner by concanamycin A, indicating that labeling was specific. Binding was also prevented by the plecomacrolides bafilomycin A(1) and B(1), respectively, but not by the benzolactone enamide salicylihalamide, a member of a novel class of V-ATPase inhibitors.

Processing of V-ATPase subunit B of Mesembryanthemum crystallinum L. is mediated in vitro by a protease and/or reactive oxygen species

Soluble proteins were isolated from leaves of the common ice plant Mesembryanthemum crystallinum L. in the CAM state of photosynthesis and tested for protease activity using amino acid-beta-naphthylamide (NA)-derivatives in a search for proteolytic activity responsible for cleavage of the V-ATPase subunit B. This cleavage is suggested to occur at the peptide bond between Met192 and Glu193. At neutral pH Met-NA was one of seven derivatives which were cleaved by proteases present in this fraction. Enzymes exhibiting proteolytic activity were separated from other soluble proteins by Superose 12-size exclusion FPLC. Incubation of partially purified protease with tonoplast-enriched membrane vesicle fractions isolated from M. crystallinum in the C3-state of photosynthesis led to a decrease in subunit B (55 kDa) protein amount and to the formation of the polypeptide Di (32 kDa), which has been previously suggested to represent a fragment of subunit B. Cleavage of subunit B and the appearance of Di also occurred during incubation of tonoplast vesicles in the presence of reactive oxygen species. In addition to Di, the polypeptide Ei (28 kDa) appeared after incubation with protease and/or reactive oxygen species. Taken into account that Di and Ei cross-reacted with an affinity purified antiserum directed against subunit B, Di as well as Ei might represent fragments of subunit B. These results open new perspectives with respect to the regulation of V-ATPase modification and turnover.

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.

Free-flow electrophoresis for fractionation of Arabidopsis thaliana membranes

Highly purified tonoplast and plasma membrane vesicles were isolated from microsomes of Arabidopsis thaliana by preparative free-flow electrophoresis. The most electronegative fractions were identified as tonoplast using nitrate-inhibited Mg2+-ATPase as enzyme marker. The least electronegative fractions were identified as plasma membrane using glucan-synthase II, UDPG: sterol-glucosyl-transferase, and vanadate-inhibited Mg2+-ATPase as enzyme markers. Other membrane markers, latent inosine-5'-diphosphatase (Golgi), NADPH-cytochrome-c reductase (endoplasmic reticulum) and cytochrome-c oxidase (mitochondria) were recovered in the fractions intermediate between tonoplast and plasma membrane. Immunoblot analysis of membrane fractions by antibodies directed against tonoplast and plasma membrane proteins confirmed the nature and the purity of the isolated membranes. The cytoskeletal protein actin, which was also identified by immunoblotting, was found to be specifically attached to the plasma membrane vesicles. The structural and functional integrity of the isolated membranes from Arabidopsis thaliana is discussed in the light of results obtained for the location of receptors and enzymes, or for the determination of ligand binding activity.

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.

The 32 kDa tonoplast polypeptide Di associated with the V-type H+-ATPase of Mesembryanthemum crystallinum L. in the CAM state: A proteolytically processed subunit B?

In the facultative halophyte Mesembryanthemum crystallinum, the salt- or age-induced transition to crassulacean acid metabolism (CAM) leads to the occurrence of a tonoplast-bound 32 kDa polypeptide (Di). The alignment of its N-terminal protein sequence with protein sequences of recently cloned higher plant V-ATPase B-subunits indicates that Di may be derived from subunit B by proteolytic removal of a protein fragment of about 20 kDa from its N-terminus. Furthermore, an antiserum directed against Di cross-reacts with subunit B from Nicotiana tabacum. It inhibits both proton pumping and ATP hydrolysis of the holoenzyme in M. crystallinum. As Di remains firmly attached to the holoenzyme the proteolytic processing may have functional implications.