Characterization of a plasma membrane H(+)-ATPase from the extremely acidophilic alga Dunaliella acidophila

The role of plasma membrane H(+) -ATPase in jasmonate-induced ion fluxes and stomatal closure in Arabidopsis thaliana

Methyl jasmonate (MeJA) elicits stomatal closure in many plant species. Stomatal closure is accompanied by large ion fluxes across the plasma membrane (PM). Here, we recorded the transmembrane ion fluxes of H(+) , Ca(2+) and K(+) in guard cells of wild-type (Col-0) Arabidopsis, the CORONATINE INSENSITIVE1 (COI1) mutant coi1-1 and the PM H(+) -ATPase mutants aha1-6 and aha1-7, using a non-invasive micro-test technique. We showed that MeJA induced transmembrane H(+) efflux, Ca(2+) influx and K(+) efflux across the PM of Col-0 guard cells. However, this ion transport was abolished in coi1-1 guard cells, suggesting that MeJA-induced transmembrane ion flux requires COI1. Furthermore, the H(+) efflux and Ca(2+) influx in Col-0 guard cells was impaired by vanadate pre-treatment or PM H(+) -ATPase mutation, suggesting that the rapid H(+) efflux mediated by PM H(+) -ATPases could function upstream of the Ca(2+) flux. After the rapid H(+) efflux, the Col-0 guard cells had a longer oscillation period than before MeJA treatment, indicating that the activity of the PM H(+) -ATPase was reduced. Finally, the elevation of cytosolic Ca(2+) concentration and the depolarized PM drive the efflux of K(+) from the cell, resulting in loss of turgor and closure of the stomata.

An energy balance from absorbed photons to new biomass for Chlamydomonas reinhardtii and Chlamydomonas acidophila under neutral and extremely acidic growth conditions

Chlamydomonas is one of the most well-studied photosynthetic organisms that had important biotechnological potential for future bioproductions of biofuels. However, an energy balance from incident photons to the energy stored in the new biomass is still lacking. In this study, we applied a recently developed system to measure the energy balance for steady state growth of Chlamydomonas reinhardtii grown at pH 6.5, and C. acidophila that was grown at pH 6.5 and 2.6. Energy use efficiency was quantified on the basis of light absorption, photosynthetic quantum yield, photosynthetic and respiratory quotient, and electron partitioning into proteins, carbohydrates and lipids. The results showed that lower growth rates of C. acidophila under both pH conditions were not caused by the differences in the photosynthetic quantum yield or in alternative electron cycling, but rather by differences in the efficiency of light absorption and increased dark respiration. Analysis of the macromolecular composition of the cells during the light phase showed that C. acidophila uses biosynthetic electrons preferentially for carbohydrate synthesis but not for synthesis of lipids. This led to a strong diurnal cycle of the C/N ratio and could explain the higher dark respiration of C. acidophila compared with C. reinhardtii.

Life at acidic pH imposes an increased energetic cost for a eukaryotic acidophile

Organisms growing in acidic environments, pH<3, would be expected to possess fundamentally different molecular structures and physiological controls in comparison with similar species restricted to neutral pH. We begin to investigate this premise by determining the magnitude of the transmembrane electrochemical H+ gradient in an acidophilic Chlamydomonas sp. (ATCC PRA-125) isolated from the Rio Tinto, a heavy metal laden, acidic river (pH 1.7-2.5). This acidophile grows most rapidly at pH 2 but is capable of growth over a wide pH range (1.5-7.0), while Chlamydomonas reinhardtii is restricted to growth at pH>or=3 with optimal growth between pH 5.5 and 8.5. With the fluorescent H+ indicator, 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF), we show that the acidophilic Chlamydomonas maintains an average cytosolic pH of 6.6 in culture medium at both pH 2 and pH 7 while Chlamydomonas reinhardtii maintains an average cytosolic pH of 7.1 in pH 7 culture medium. The transmembrane electric potential difference of Chlamydomonas sp., measured using intracellular electrodes at both pH 2 and 7, is close to 0 mV, a rare value for plants, animals and protists. The 40,000-fold difference in [H+] could be the result of either active or passive mechanisms. Evidence for active maintenance was detected by monitoring the rate of ATP consumption. At the peak, cells consume about 7% more ATP per second in medium at pH 2 than at pH 7. This increased rate of consumption is sufficient to account for removal of H+ entering the cytosol across a membrane with relatively high permeability to H+ (7x10(-8) cm s-1). Our results indicate that the small increase in the rate of ATP consumption can account for maintenance of the transmembrane H+ gradient without the imposition of cell surface H+ barriers.

Primary structure of the plasma membrane H(+)-ATPase from the halotolerant alga Dunaliella bioculata

P-type ATPase-specific oligodeoxyribonucleotides were used to obtain a fragment of the H(+)-ATPase of the salt tolerant alga Dunaliella bioculata by polymerase chain reaction (PCR). This fragment served as a probe in screening a cDNA-library from this organism. The complete primary structure of the ATPase protein (DBPMA1) was deduced from sequencing a 4.7 kb cDNA clone. The protein shows highest homology to H(+)-ATPases from higher plants and fungi (43% identity, 67% similarity) but has a higher calculated molecular mass (123 kDa). The latter can be assigned mainly to an additional hydrophilic domain between transmembrane segments VI and VII and to an extended carboxyterminus. These unusual structural features of DBPMA1 are interpreted in terms of providing regulatory sites of the enzyme. Southern blot analysis suggests the presence of only a single copy of the gene in the haploid D. bioculata genome. To investigate the role of the H(+)-ATPase in the adaption of D. bioculata to different external NaCl concentrations, we employed northern blot analyses. The results indicate that the pma1 transcript level of cells growing in salinities between 0.1 and 3 M NaCl is not directly correlated with the external salt concentration.

Inhibition of the plasma-membrane H(+)-ATPase from Dunaliella adidophila by omeprazole

The acid-activated sulfhydryl reagent omeprazole inhibits light-induced H+ secretion at pH 1 in cells of the halotolerant alga Dunaliella acidophila. Plasma-membrane vesicles, prepared from omeprazole-treated cells, have impaired vanadate-sensitive ATPase and ATP-induced H+ uptake activities. Omeprazole inhibits ATPase activity also in isolated plasma-membrane vesicles. The inhibition is enhanced at acidic pH and can be prevented by protonophores indicating that it is promoted by internal acidification of the vesicles. Mercaptoethanol partially reverses omeprazole inhibition. ADP does not afford protection against omeprazole but it does protect against inhibition by N-ethylmaleimide, indicating that these reagents modify different sulfhydryl groups. It is suggested that omeprazole blocks SH groups of the D. acidophila plasma-membrane H(+)-ATPase, which face the outer side of the cell.