Conformation and activity of chloroplast coupling factor exposed to low chemical potential of water in cells.

(1) Photophosphorylation, Ca2+-ATPase and Mg2+-ATPase activities of isolated chloroplasts were inhibited 55--65% when the chemical potential of water was decreased by dehydrating leaves to water potentials (psi w) of --25 bars before isolation of the plastids. The inhibition could be reversed in vivo by rehydrating the leaves. (2) These losses in activity were reflected in coupling factor (CF1) isolated from the leaves, since CF1 from leaves with low psi w had less Ca2+-ATPase activity than control CF1 and did not recouple phosphorylation in CF1-deficient chloroplasts. In contrast, CF1 from leaves having high psi w only partially recoupled phosphorylation by CF1-deficient chloroplasts from leaves havig low psi w. This indicated that low psi w affected chloroplast membranes as well as CF1 itself. (3) Coupling factor from leaves having low psi w had the same number of subunits, and the same electrophoretic mobility, and could be obtained with the same yields as CF1 from control leaves. However, direct measurements of fluorescence polarization, ultraviolet absorption, and circular dichroism showed that CF1 from leaves having low psi w differed from control CF1. The CF1 from leaves having low psi w also had decreased ability to bind fluorescent nucleotides (epsilon-ATP and epsilon-ADP). (4) Exposure of isolated CF1 to low psi w in vitro by preincubation in sucrose-containing media inhibited the Ca2+-ATPase activity of the protein in subsequent assays without sucrose. Inclusion of 5 or 10 mM Mg2+ in the preincubation medium markedly inhibited Ca2+-ATPase activity. (5) These results show that CF1 undergoes changes in cells which alter its phosphorylating ability. Since low cell psi w changed the spectroscopic properties but not other protein properties of CF1, the changes were most likely caused by altered confurn, photophosphorylation. The inhibition of ATPase activity in CF1 in vitro at low psi w and high ion concentration mimicked the change in activity seen in vivo.     

Effect of redox and chelating agents on the properties of chloroplast ATPase

The effect of redox and chelating reagents on the ATPase and ATP-synthetase activity in chloroplast membranes as well as the ATPase activity of isolated CF1-coupling factor from chloroplasts has been studied. The Mg2+-ATPase in thylakoid membranes and isolated Ca2+-ATPase is stimulated by dithionite. In the presence of reduced glutathione the effect of dithionite is similar to those of prolonged illumination or heating. Dichlorophenolindophenol partially inhibits this activity as well as citrate, tenoyltrifluoroacetone and the excess fo ATP. Photophosphorylation in chloroplast lamellae is inhibited with dithionite. It is suggested that the membrane bound ATPase from chloroplasts may be in two structural states which differ in their enzymic activity and in the coupling to electron transfer in membrane. The transitions between these states can be induced by redox reagents.     

Coupling factor activity of the purified pea mitochondrial F1-ATPase

The pea cotyledon mitochondrial F1-ATPase was released from the submitochondrial particles by a washing procedure using 300 mM sucrose/2 mM Tricine (pH 7.4). The enzyme was purified by DEAE-cellulose chromatography and subsequent sucrose density gradient centrifugation. Using polyacrylamide gel electrophoresis under non-denaturing conditions, the purified protein exhibited a single sharp band with slightly lower mobility than the purified pea chloroplast CF1-ATPase. The molecular weights of pea mitochondrial F1-ATPase and pea chloroplast CF1-ATPase were found to be 409 000 and 378 000, respectively. The purified pea mitochondrial F1-ATPase dissociated into six types of subunits on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Most of these subunits had mobilities different from the subunits of the pea chloroplast CF1-ATPase. The purified mitochondrial F1-ATPase exhibited coupling factor activity. In spite of the observed differences between CF1 and F1, the mitochondrial enzyme stimulated ATP formation in CF1-depleted pea chloroplast membranes. Thus, the mitochondrial F1 was able to substitute functionally for the chloroplast CF1 in reconstituting photophosphorylation.     

Synthesis and assembly of H+-ATPase complex by isolated "rough" thylakoids

The synthesis and assembly of chloroplast H+-ATPase complex were studied by analyzing the incorporation of [35S]methionine into the constituent subunits with isolated intact chloroplasts and with thylakoid membranes that had been prepared from the chloroplasts so that they would retain ribosomes. The complex was isolated from thylakoids after labeling and identified by immunoprecipitation with an antiserum specific to CF1. The mechanism for the assembly of the complex was demonstrated to be active in the isolated chloroplasts by the following observations: the plastid genome-regulated subunits (alpha, beta, epsilon, I, and III) were labeled by in organello translation and recovered with the complex, and three other subunits (gamma, delta, and II) were labeled when intact chloroplasts were incubated with translation products from polyadenylated RNA. The two largest subunits, alpha and beta, were translated on thylakoid-bound ribosomes when the thylakoid membranes were incubated with soluble factors from Escherichia coli. They were recovered with the H+-ATPase complex, suggesting that they are translated on the bound ribosomes in the chloroplast, and that the isolated membranes retain the ability to assemble a complete complex. Provided that these observations are the result of de novo assembly of the complex, the imported and processed nuclear-coded subunits are presumed to be pooled not in stroma but on the membrane.     

Chromatographic purification of the chloroplast ATP synthase (CF0-CF1) and the role of CF0 subunit IV in proton conduction

Chromatographic procedures were developed to purify chloroplast ATP synthase (CF0-CF1) in large amounts and to resolve subunits from this enzyme. The ATP synthase thus obtained has high ATP-Pi exchange and Mg2(+)-ATPase activities upon incorporation into asolectin liposomes. The purity of this preparation was about 95%. By modifications of this chromatographic procedure, we purified subunit IV-deficient CF0-CF1, subunit IV-deficient CF0, and subunit IV. Both ATP-Pi exchange and Mg2(+)-ATPase activities were impaired by depletion of subunit IV from CF0-CF1. Partial restoration of these activities was obtained by reconstituting subunit IV-deficient CF0-CF1 with subunit IV. The impairment of these activities was likely caused by a loss in proton conductivity of CF0 upon removal of subunit IV. The dicyclohexylcarbodiimide-sensitive Mg2(+)-ATPase of subunit IV-deficient CF0-CF1 was not as sensitive to the depletion of subunit IV as ATP-Pi exchange. Nearly 90% of subunit IV could be removed, but Mg2(+)-ATPase activity was inhibited by only 40-60%. Thus subunit IV of CF0-CF1 may not participate directly in proton transfer but may have a role in organizing and/or stabilizing CF0 structure.     

Detergent activation of the ATPase activity of chloroplast coupling factor 1

The activation of the ATPase activity of coupling factor 1 (CF1) from chloroplasts by several detergents was studied. Further evidence that detergent micelles are important in the activation of Ca2+-ATPase was obtained. Maximal activation of CA2+-ATPase was achieved with short-chain alkyl-beta-D-glucopyranoside (alkylglucosides) detergents. Treatment of CF1 with hexylglucoside or heptylglucoside followed by hydroxylapatite chromatography caused nearly total removal of the epsilon subunit of the enzyme, whereas treatment with decylglucoside caused less ATPase activation and less loss of the epsilon subunit. The ATPase activity of detergent-activated CF1 was inhibited by purified epsilon subunit. Detergents that form small micelles appear to be most effective in removing the epsilon subunit and in activating the Ca2+-ATPase of CF1. When present during assay, the alkylglucosides also induce a Mg2+-ATPase activity in CF1. Octyl- and nonylglucoside are most effective in promoting this reaction. If, however, CF1 deficient in the epsilon subunit was used, even decylglucoside elicited rapid Mg2+-ATPase hydrolysis. It is concluded that removal of the epsilon subunit, although necessary for the expression of Mg2+-ATPase, is not sufficient. The detergents that cause maximal displacement of the epsilon subunit are less effective in inducing Mg2+-ATPase activity. The selective removal of subunits from CF1 by specific detergents points to potential problems with the use of these detergents in the solubilization of oligomeric membrane proteins.     

Added subunit beta of CF1 as well as gamma/delta/epsilon restore photophosphorylation in partially CF1-depleted thylakoids.

We investigated the ability of subunits beta, gamma, delta, and epsilon of CF1, the F1-ATPase of chloroplasts, to interact with exposed CF0 in EDTA-treated, partially CF1-depleted thylakoid membranes. We measured the ability of subunits beta, gamma, delta, and epsilon to stimulate the rate of photophosphorylation under continuous light and, for subunit beta, also the ability to diminish the proton leakage through exposed CF0 by deceleration of the decay of electrochromic absorption transients under flashing light. The greatest effect was caused by subunit beta, followed by gamma/delta/epsilon. Pairwise combinations of gamma, delta, and epsilon or each of these subunits alone were only marginally effective. Subunit gamma from the thermophilic bacterium PS 3 in combination with chloroplast delta and epsilon was as effective as chloroplast gamma. The finding that the small CF1 subunits in concert and the beta subunit by itself specifically interacted with the exposed proton channel CF0, qualifies the previous concept of subunit delta acting particularly as a plug to the open CF0 channel. The interactions between the channel and the catalytic portion of the enzyme seem to involve most of the small, and at least beta of the large subunits.     

ATP hydrolysis catalyzed by a beta subunit preparation purified from the chloroplast energy transducing complex CF1.CF0

Washing thylakoid membranes with 1 M LiCl causes the release of the beta subunit from the chloroplast energy transducing complex (CF1.CF0) in spinach chloroplasts. This protein purifies by size exclusion chromatography as a 180-kDa aggregate and, thus, is probably composed of a trimer of beta polypeptides. The purified aggregate binds ADP to a high and a low affinity site with dissociation constants of 15 and 202 microM, respectively. Mg2+ is required for ADP to bind to both sites. Manganese binds to the protein in a cooperative manner to at least two sites with high affinity. The beta subunit preparation catalyzes Mg2+-dependent ATP hydrolysis at rates which are comparable to other subunit-deficient CF1 preparations and is increased by treatments known to activate the Mg2+-ATPase activity of CF1. However, Ca2+ is not an effective cofactor for this reaction and treatments which activate the Ca2+-ATPase of CF1 are either ineffective or inhibitory.     

Modifications of the gamma subunit of chloroplast coupling factor 1 alter interactions with the inhibitory epsilon subunit.

The treatment of chloroplast coupling factor 1 (CF1) with dithiothreitol or with trypsin modifies the gamma subunit. Reduction of the gamma subunit disulfide bond in CF1 in solution with dithiothreitol enhances the dissociation of epsilon (Duhe, R. J., and Selman, B. R. (1990) Biochim. Biophys. Acta 1017, 70-78). The Ca(2+)-ATPase activity of either oxidized or reduced CF1 increases as the enzyme is diluted. Added epsilon subunit inhibits the Ca(2+)-ATPase activity of both forms of the diluted CF1, suggesting that epsilon dissociation is the cause of activation by dilution. Half-maximal activation occurred at much higher concentrations of the reduced CF1, indicating that reduction decreases the affinity for epsilon about 20-fold. Immunoblotting techniques show that there is only one epsilon subunit/CF1 in intact chloroplasts, in thylakoid membranes, and in solution. No epsilon is released from CF1 in thylakoids under conditions of ATP synthesis. The gamma subunit of CF1 in illuminated thylakoids is specifically cleaved by trypsin. CF1 purified from thylakoids treated with trypsin in the light is deficient in epsilon subunit, and has a high rate of ATP hydrolysis. Added epsilon neither inhibits the ATPase activity of, nor binds tightly to the cleaved enzyme.     

The special reaction of photophosphorylation using epsilon ADP--a fluorescent analog of ADP

Photophosphorylation of epsilon ADP in a chloroplast synthetase system reconstituted with CF1 or with CF1 modified by covalently bound epsilon ADP has been studied. The reconstitution of EDTA-treated chloroplasts with CF1 restores the photophosphorylating activity to about 90%. When the CF1 modified by covalently bound epsilon ADP is used for reconstitution the photophosphorylating activity of EDTA-treated chloroplasts is restored to 37%. Based on the results of a photochemical study of the chloroplast ATP-synthetase system reconstituted with CF1 with covalently bound epsilon ADP it may be assumed that the substrate, adenine, participates in proton translocation to inorganic phosphate in the active center of the coupling enzyme during photophosphorylation.     

Effect of nano-TiO2 on photochemical reaction of chloroplasts of spinach

The effects of nano-TiO₂ (rutile) on the photochemical reaction of chloroplasts of spinach were studied. The results showed that when spinach was treated with 0.25% nano-TiO₂, the Hill reaction, such as the reduction rate of FeCy, and the rate of evolution oxygen of chloroplasts was accelerated and noncyclic photophosphorylation (nc-PSP) activity of chloroplasts was higher than cyclic photophosphorylation (c-PSP) activity, the chloroplast coupling was improved and activities of Mg²⁺-ATPase and chloroplast coupling factor I (CF₁)-ATPase on the thylakoid membranes were obviously activated. It suggested that photosynthesis promoted by nano-TiO₂ might be related to activation of photochemical reaction of chloroplasts of spinach.     

Sulphydryl groups in photosynthetic energy conservation. V. Localization of the new disulfide bridges formed by o-iodosobenzoate in coupling factor of spinach chloroplasts.

1. Chemical modification by o-iodosobenzoate of soluble chloroplast coupling factor 1 (CF1) during heat activation resulted in inhibition of its Ca-ATPase activity and in the formation of two new intrapeptide disulfide bridges as suggested by: (a) the disappearance of three out of four accessible thiol groups, two from gamma and one from a beta subunit as a consequence of CF1 modification by o-iodosobenzoate; (b) the total free sulphydryl groups of CF1 were reduced from 8 to 4 after modification of CF1 by o-iodosobenzoate. Two groups disappeared from beta and two from gamma subunits; (c) a second heating step of CF1 in the presence of 10 mM dithioerythritol reversed the inhibition of the ATPase and reduced both the newly formed disulfide bridges and those present in native CF1. 2. Modification of chloroplasts in the light with o-iodosobenzoate resulted in the inhibition of photophosphorylation and ATPase. CF1 isolated and purified from these chloroplasts had its Ca-ATPase activity inhibited and two new disulfide bridges. The total number of free sulphydryl groups was reduced from 8 to 4 and three accessible groups disappeared from beta and gamma subunits.     

Induction by different thioredoxins of ATPase activity in coupling factor 1 from spinach chloroplasts

ATPase activity of the coupling factor 1, CF1, isolated from spinach chloroplasts, was enhanced by reduction with dithiothreitol. Reduced thioredoxins from spinach chloroplasts, Escherichia coli and human lymphocytes replaced dithiothreitol as reductant and activator of the ATPase. CF1 must be in an oxidized activated state to be further activated by reduced thioredoxin. This state was obtained either by heating CF1 or removing the inhibitory intrinsic epsilon subunit from CF1. Efficiency and primary structure of the different thioredoxins were compared. The progressive addition of KCl during ATPase activation by reduced thioredoxin increases then decreases this process. We proposed that three basic amino acids corresponding to arginine 73 and lysines 82 and 96 in Escherichia coli thioredoxin play an important role in the anchorage of the thioredoxin to the negatively charged surface of the CF1 and are involved in the dual effect of KCl. The variations in the screening effect of the negative charges of the CF1 surface by K+ ions can indeed explain the changes in the anchorage of these 3 basic amino acids with concomitant variation in ATPase activity. Human thioredoxin must be 10 times more concentrated than Escherichia coli or spinach chloroplast thioredoxin to exhibit the same activation effect on the ATPase. This fact was related to the properties of a sequence equivalent to the part from amino acid 59 to 72 in Escherichia coli thioredoxin. This part which joins the two lobes of the thioredoxin is more hydrophilic and more negatively charged in human thioredoxin than in Escherichia coli or spinach chloroplast thioredoxin. Although ATPase activation was obtained at a very low concentration of the reduced spinach chloroplast thioredoxin, the thioredoxin formed only a loose complex with CF1.     

亚硫酸氢钠在低光强下对叶绿体循环光合磷酸化的促进作用

我们在70年代曾发现喷洒低浓度的亚硫酸氢钠能提高多种作物叶片的光合作用速率(沈允钢等1980)。近年来,谭实和沈允钢(1987)观察到喷洒低浓度的亚硫酸氢钠不但能增加作物叶片的光合作用速率,而且同时增加其呼吸作用速率,并初     

Enhancement of adenosine triphosphatase activity of purified chloroplast coupling factor 1 in aqueous organic solvent

The ATPase activity of purified coupling factor 1 (CF1) of spinach chloroplasts [EC 3.6.1.3] was reversibly enhanced in some aqueous organic solvents, notably methanol, ethanol, and acetone. Pretreatment of CF1 with 20% (v/v) methanol did not affect the subsequent activity. The activity depended entirely on the final concentration of methanol in the reaction mixture. In the presence of 20% methanol, the Km of Ca2+-ATPase from ATP was lowered from 0.4 mM to 0.2 mM. Not only Ca2+, but also Cd2+, Mg2+, Mn2+, and Zn2+ supported the ATPase activity at rates of higher than 7 mumol.mg protein-1 . min-1. Co2+, Ni2+, and Pb2+ supported the activity at rates of 0.5-1.0 mumol.mg protein-1 . min-1. The activities supported by the following cations, if any, were less than 0.2 mumol.mg protein-1 . min-1; Ba2+, Cu2+, Fe2+, Hg2+, Sn2+, and Sr2+. The optimum concentration of methanol for Ca2+-ATPase and Mg2+-ATPase activities was about 30% (v/v). The optimum pH values for Ca2+-ATPase and Mg2+-ATPase activities were about 8.0 and 8.8, respectively. The enhancing effect of organic solvents appears to be associated with their relative lipophilic character as defined by the octanol-water partition coefficient. The Ca2+-ATPase activities of th trypsin-activated and the heat-activated CF1 were inhibited and their Mg2+-ATPase activities were enhanced by the presence of methanol in the reaction mixture.     

Inhibition of the ATPase activity of the catalytic portion of ATP synthases by cationic amphiphiles

Melittin, a cationic, amphiphilic polypeptide, has been reported to inhibit the ATPase activity of the catalytic portions of the mitochondrial (MF1) and chloroplast (CF1) ATP synthases. Gledhill and Walker [J.R. Gledhill, J.E. Walker. Inhibition sites in F1-ATPase from bovine heart mitochondria, Biochem. J. 386 (2005) 591-598.] suggested that melittin bound to the same site on MF1 as IF1, the endogenous inhibitor polypeptide. We have studied the inhibition of the ATPase activity of CF1 and of F1 from Escherichia coli (ECF1) by melittin and the cationic detergent, cetyltrimethylammonium bromide (CTAB). The Ca2+- and Mg2+-ATPase activities of CF1 deficient in its inhibitory epsilon subunit (CF1-epsilon) are sensitive to inhibition by melittin and by CTAB. The inhibition of Ca2+-ATPase activity by CTAB is irreversible. The Ca2+-ATPase activity of F1 from E. coli (ECF1) is inhibited by melittin and the detergent, but Mg2+-ATPase activity is much less sensitive to both reagents. The addition of CTAB or melittin to a solution of CF1-epsilon or ECF1 caused a large increase in the fluorescence of the hydrophobic probe, N-phenyl-1-naphthylamine, indicating that the detergent and melittin cause at least partial dissociation of the enzymes. ATP partially protects CF1-epsilon from inhibition by CTAB. We also show that ATP can cause the aggregation of melittin. This result complicates the interpretation of experiments in which ATP is shown to protect enzyme activity from inhibition by melittin. It is concluded that melittin and CTAB cause at least partial dissociation of the alpha/beta heterohexamer.     

Allosteric properties of CF1-ATPase form chloroplasts

The inhibiting effect of ADP and Mg2+ on CF1-ATPase from chloroplasts depending on their concentration, pH and the presence of stimulating agents of various origin was studied. It was shown that the low Mg-dependent activity of the soluble enzyme is due to non-competitive inhibition of the reaction by Mg2+ in the presence of ADP. The CF1-ATPase stimulators lower the inhibiting effect, thus allowing to detect the "true" Mg-dependent activity of the enzyme. The data obtained are indicative of the existence of Mg2+- and ADP-specific sitein the enzyme, which controls its catalytic activity. The properties and possible role of this site in photophosphorylation are discussed.     

The deactivation process of the proton-ATPase in isolated chloroplasts and whole leaves

Using rapid micromethods for chloroplast isolation and ATPase solubilization from preilluminated leaves, the deactivation of the proton-ATPase after different treatments was compared. The rate of decay of the ‘in vivo’ light-activated membrane-bound Mg²⁺-ATPase was highly dependent on temperature. However, the soluble Ca²⁺-ATPase, extracted from the temperature-inactivated membrane-bound ATPase, was active. Coupling factor 1 with a manifest and stable Ca²⁺-ATPase activity was also solubilized from chloroplasts activated by light in whole leaves and deactivated after chloroplast isolation with gramicidin D. Deactivation of the proton-ATPase in isolated chloroplasts was only associated with the dissipation of the proton gradient. Reaction of the accessible sulfhydryl groups of the membrane-bound proton-ATPase with iodoacetamide prevent inactivation of the enzyme by oxidants. However, the iodoacetamide treatment had not effect on the temperature-dependent decay. The rate of deactivation of the proton-ATPase in whole leaves was similar for both membrane-bound and soluble ATPases. Thus, the oxidation process may play an important role in physiological conditions.     

Binding of the chloroplast coupling factor CF to lipid vesicles

The binding of the chloroplast coupling factor CF to lipid vesicles was analyzed by gel filtration. CF can be bound to vesicles made of chloroplast lipids but not of lecithin. The presence in the vesicle walls of a proteolipid subunit of the hydrophobic component of the coupling factor increases the binding of CF. The apparent binding constant and the maximum protein/lipid ratio are calculated. The Ca2+-ATPase activity of bound CF is markedly lower than that of dissolved CF. It is confirmed that the proteolipid is a N,N'-dicyclohexylcarbodiimide sensitive proton channel. The binding of CF on proteolipid vesicles decreases their proton permeability.     

Inhibition of energy-transducing functions of chloroplast membranes by lipophilic iron chelators.

Lipophilic metal chelators inhibit various energy-transducing functions of chloroplasts. The following observations were made 1. Photophosphorylation coupled to any known mode of electron transfer, i.e. whole-chain noncyclic, the partial noncyclic Photosystem I or Photosystem II reactions, or cyclic, is inhibited by several lipophilic chelators, but not by hydrophilic chelators. 2. The light- and dithioerythritol-dependent Mg2+-ATPase was also inhibited by the lipophilic chelators. 3. Electron transport through either partial reaction. Photosystem I or Photosystem II was not inhibited by lipophilic chelators. Whole-chain coupled electron transport was inhibited by bathophenanthroline, and the inhibition was not reversed by uncouplers. The diketone chelators diphenyl propanedione and nonanedione inhibited the coupled, whole-chain electron transport and the inhibition was reversed by uncouplers, a pattern typical of energy transfer inhibitors. The electron transport inhibition site is localized in the region of platoquinone leads to cytochrome f. This inhibition site is consistent with other recent work (Prince et al. (1975) FEBS Lett. 51, 108 and Malkin and Aparicio (1975) Biochem. Biophys. Res. Commun. 63, 1157) showing that a non-heme iron protein is present in chloroplasts having a redox potential near + 290 mV. A likely position for such a component to function in electron transport would be between plastoquinone and cytochrome f. just where our data suggests there to be a functional metalloprotein. 4. Some of the lipophilic chelators induce H+ leakiness in the chloroplast membrane, making interpretation of their phosphorylation inhibition difficult. However, 1-3 mM nonanedione does not induce significant H+ leakiness, while inhibiting ATP formation and the Mg2+-ATPase. Nonanedione, at those concentrations, causes a two- to four-fold increase in the extent of H+ uptake. 5. These results are consistent with, but do not prove, the involvement of a non-heme iron or a metalloprotein in chloroplast energy transduction.