Effects of sequential deletions of residues from the N- or C-terminus on the functions of epsilon subunit of the chloroplast ATP synthase

Assembly of F1F0-ATP synthases

F1F0-ATP synthases are multimeric protein complexes and common prerequisites for their correct assembly are (i) provision of subunits in appropriate relative amounts, (ii) coordination of membrane insertion and (iii) avoidance of assembly intermediates that uncouple the proton gradient or wastefully hydrolyse ATP. Accessory factors facilitate these goals and assembly occurs in a modular fashion. Subcomplexes common to bacteria and mitochondria, but in part still elusive in chloroplasts, include a soluble F1 intermediate, a membrane-intrinsic, oligomeric c-ring, and a membrane-embedded subcomplex composed of stator subunits and subunit a. The final assembly step is thought to involve association of the preformed F1-c10-14 with the ab2 module (or the ab8-stator module in mitochondria)--mediated by binding of subunit δ in bacteria or OSCP in mitochondria, respectively. Despite the common evolutionary origin of F1F0-ATP synthases, the set of auxiliary factors required for their assembly in bacteria, mitochondria and chloroplasts shows clear signs of evolutionary divergence. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Effects of manganese deficiency and added cerium on photochemical efficiency of maize chloroplasts

The mechanism of the fact that manganese deprivation and cerium addition affect the photochemical efficiency of plants is unclear. In this study, we investigated the improvement by cerium of the damage of the photochemical function of maize chloroplasts under manganese-deprived stress. Chlorophyll fluorescence induction measurements showed that the ratio of variable to maximum fluorescence (Fv/Fm) underwent great decreases under manganese deficiency, which was attributed to the reduction of intrinsic quantum efficiency of the photosystem II units. The electron flow between the two photosystems, activities of Mg(2+)-ATPase and Ca(2+)-ATPase, and rate of photophosphorylation on the thylakoid membrane of maize chloroplasts were reduced significantly by exposure to manganese deprivation. Furthermore, the inhibition of cyclic photophosphorylation was more severe than non-cyclic photophosphorylation under manganese deficiency. However, added cerium could relieve the inhibition of the photochemical reaction caused by manganese deprivation in maize chloroplasts. It implied that manganese deprivation could disturb photochemical reaction of chloroplasts strongly, which could be improved by cerium addition.

Improvement of cerium of photosynthesis functions of maize under magnesium deficiency

Rare earth elements can promote photosynthesis, but their mechanisms are still poorly understood under magnesium deficiency. The present study was designed to determine the role of cerium in magnesium-deficient maize plants. Maize was cultivated in Hoagland's solution added with cerium with and without adequate quantities of magnesium. Under magnesium-deficient conditions, cerium can prevents inhibition of synthesis of photosynthetic pigment, improves light energy absorption and conversion, oxygen evolution, and the activity of photo-phosphorelation and its coupling factor Ca(2+)-ATPase. These results suggest that cerium could partly substitute magnesium, improving photosynthesis and plant growth.

FTH1 binds to Daxx and inhibits Daxx-mediated cell apoptosis

As a highly conserved nuclear protein, death domain-associated protein (Daxx) plays an important role in transcriptional control, carcinogenesis, and resistance to virus infection and so on. In order to further investigate the mechanism of Daxx, the yeast two-hybrid technique was used to screen the intra-cellular proteins interacting with Daxx. And 13 positive colonies and three proteins interacting with Daxx were obtained. One of the candidate proteins was identified as ferritin, heavy polypeptide 1(FTH1). The interaction between Daxx and FTH1 was further supported by GST pull-down and co-immunoprecipitation respectively. Then Daxx was determined to induce apoptosis and FTH1 can inhibit Daxx-mediated apoptosis. Besides, it is found that Daxx mediated apoptosis through the Fas-Daxx-ASK1-JNK1 signaling pathway, while FTH1 can inhibit the activation of JNK signaling pathway. We present evidence to demonstrate the FTH1 and Daxx are able to participate in apoptosis pathway through JNK signal molecule and FTH1 can inhibit this pathway.

Effects of salt and waterlogging stresses and their combination on leaf photosynthesis, chloroplast ATP synthesis, and antioxidant capacity in wheat

The objective of this study was to investigate the effects of salt (ST) and waterlogging (WL) stresses and their combination (SW) on leaf photosynthesis, chloroplast ATP synthesis, and antioxidant capacity in wheat (Triticum aestivum L.). Two winter wheat cultivars, Huaimai 17 and Yangmai 12, differing in their tolerance to ST and WL stresses were used. The plants were grown in pots and were subjected to ST, WL, and SW from 7 days after anthesis (DAA). The WL and SW treatments lasted for 5 days, while the ST treatment was continuously imposed during the grain filling stage. Significant decrease in net photosynthetic rate (PN) of the flag leaf was observed under the ST and SW treatments from 10 DAA in Yangmai 12 and at 18 DAA in both cultivars, which could be stomatal closure related. At 18 DAA, clear reduction in PN under the ST and SW treatments was observed, which was associated with chlorosis, damages to the photosystem II (PSII), enhanced lipid peroxidation, and depressed ATP synthesis in the chloroplasts of the flag leaf. Whereas, WL treatment alone had slightly negative effect on PN, which was mainly attributed to leaf chlorosis and waste in harvested energy by the PSII reaction center dispersed via non-photochemical approaches.

From protons to OXPHOS supercomplexes and Alzheimer's disease: structure-dynamics-function relationships of energy-transducing membranes

By the elucidation of high-resolution structures the view of the bioenergetic processes has become more precise. But in the face of these fundamental advances, many problems are still unresolved. We have examined a variety of aspects of energy-transducing membranes from large protein complexes down to the level of protons and functional relevant picosecond protein dynamics. Based on the central role of the ATP synthase for supplying the biological fuel ATP, one main emphasis was put on this protein complex from both chloroplast and mitochondria. In particular the stoichiometry of protons required for the synthesis of one ATP molecule and the supramolecular organisation of ATP synthases were examined. Since formation of supercomplexes also concerns other complexes of the respiratory chain, our work was directed to unravel this kind of organisation, e.g. of the OXPHOS supercomplex I(1)III(2)IV(1), in terms of structure and function. Not only the large protein complexes or supercomplexes work as key players for biological energy conversion, but also small components as quinones which facilitate the transfer of electrons and protons. Therefore, their location in the membrane profile was determined by neutron diffraction. Physico-chemical features of the path of protons from the generators of the electrochemical gradient to the ATP synthase, as well as of their interaction with the membrane surface, could be elucidated by time-resolved absorption spectroscopy in combination with optical pH indicators. Diseases such as Alzheimer's dementia (AD) are triggered by perturbation of membranes and bioenergetics as demonstrated by our neutron scattering studies.

Effects of lead on activities of photochemical reaction and key enzymes of carbon assimilation in spinach chloroplast

Photosynthesis is one of the most sensitive processes to lead, but the effects of lead on the transformation of light energy of plants are still not clearly understood. In the present paper, spinach was cultivated in the experimental fields and was sprayed with various concentrations of PbCl2 solution. We investigated the effects of lead on the activities of photochemical reaction and the key enzymes of carbon assimilation in spinach chloroplast. The results showed that Pb2+ treatment could significantly inhibit the Hill reaction activity of spinach chloroplast and photophosphorylation, and it had a more conspicuous effect on cyclic photophosphorylation than non-cyclic photophosphorylation. The activities of ATPase on the thylakoid membrane were severely inhibited under Pb2+-treated condition, and Ca2+ ATPase activity was affected more obviously than Mg2+ ATPase activity. Meanwhile, the activities of the key enzymes of carbon assimilation were also significantly reduced by Pb2+, especially Rubisco activase. The reduction of dry weight of spinach caused by Pb2+ was more significant than that of fresh weight. It implied that Pb2+ could disturb light energy transformation of chloroplast.

Regulatory mechanisms of proton-translocating F(O)F (1)-ATP synthase

H(+)-F(O)F(1)-ATP synthase catalyzes synthesis of ATP from ADP and inorganic phosphate using the energy of transmembrane electrochemical potential difference of proton (deltamu(H)(+). The enzyme can also generate this potential difference by working as an ATP-driven proton pump. Several regulatory mechanisms are known to suppress the ATPase activity of F(O)F(1): 1. Non-competitive inhibition by MgADP, a feature shared by F(O)F(1) from bacteria, chloroplasts and mitochondria 2. Inhibition by subunit epsilon in chloroplast and bacterial enzyme 3. Inhibition upon oxidation of two cysteines in subunit gamma in chloroplast F(O)F(1) 4. Inhibition by an additional regulatory protein (IF(1)) in mitochondrial enzyme In this review we summarize the information available on these regulatory mechanisms and discuss possible interplay between them.

Interactions of rotor subunits in the chloroplast ATP synthase modulated by nucleotides and by Mg2+

ATP synthases - rotary nano machines - consist of two major parts, F(O) and F(1), connected by two stalks: the central and the peripheral stalk. In spinach chloroplasts, the central stalk (subunits gamma, epsilon) forms with the cylinder of subunits III the rotor and transmits proton motive force from F(O) to F(1), inducing conformational changes of the catalytic centers in F(1). The epsilon subunit is an important regulator affecting adjacent subunits as well as the activity of the whole protein complex. Using a combination of chemical cross-linking and mass spectrometry, we monitored interactions of subunit epsilon in spinach chloroplast ATP synthase with III and gamma. Onto identification of interacting residues in subunits epsilon and III, one cross-link defined the distance between epsilon-Cys6 and III-Lys48 to be 9.4 A at minimum. epsilon-Cys6 was competitively cross-linked with subunit gamma. Altered cross-linking yields revealed the impact of nucleotides and Mg(2+) on cross-linking of subunit epsilon. The presence of nucleotides apparently induced a displacement of the N-terminus of subunit epsilon, which separated epsilon-Cys6 from both, III-Lys48 and subunit gamma, and thus decreasing the yield of the cross-linked subunits epsilon and gamma as well as epsilon and III. However, increasing concentrations of the cofactor Mg(2+) favoured cross-linking of epsilon-Cys6 with subunit gamma instead of III-Lys48 indicating an approximation of subunits gamma and epsilon and a separation from III-Lys48.

TRAF3 interacts with Smac/DIABLO and enhances the proapoptotic effect of Smac/DIABLO in cytoplasm

Smac/DIABLO (second mitochondria-derived activator of caspase/direct IAP-binding protein with low PI) is a 29 kDa mitochondrial precursor protein, which is proteolytically processed in mitochondria into a 23 kDa mature protein. It is released from the mitochondrial intermembrane space to cytosol after an apoptotic trigger. Smac/DIABLO acts as a dimer and it contributes to caspase activation by sequestering the inhibitor of apoptosis proteins (IAPs). In order to further investigate the mechanism of Smac/DIABLO action, we used the mature form of Smac/DIABLO as a bait and screened proteins that interact with mature Smac/DIABLO in human liver cDNA library using the yeast two-hybrid system. Forty-two colonies were obtained after 5.8x10(6) colonies were screened by nutrition limitation and X-galactosidase assay. After DNA sequence analysis and homology retrieval, one of the candidate proteins was identified as TRAF domain of the TNF receptor associated factor 3 (TRAF3). The interaction site between TRAF3 and Smac/DIABLO was identified by beta-galactosidase test. The interaction between TRAF3 and Smac/DIABLO via TRAF domain was identified in vivo by co-immunoprecipitation in HepG2 cells, and the direct interaction between TRAF3 and Smac/DIABLO in vitro was identified by GST-pull down assay. Co-expression of TRAF3 and mature Smac/DIABLO in 293 cells could enhance the Smac/DIABLO-mediated apoptosis. These results suggested that TRAF3 interacted with Smac/DIABLO via TRAF domain, leading to an increased proapoptotic effect of Smac/DIABLO in cytoplasm.

Structural Analysis of the Regulatory Dithiol-containing Domain of the Chloroplast ATP Synthase γ Subunit

The gamma subunit of the F1 portion of the chloroplast ATP synthase contains a critically placed dithiol that provides a redox switch converting the enzyme from a latent to an active ATPase. The switch prevents depletion of intracellular ATP pools in the dark when photophosphorylation is inactive. The dithiol is located in a special regulatory segment of about 40 amino acids that is absent from the gamma subunits of the eubacterial and mitochondrial enzymes. Site-directed mutagenesis was used to probe the relationship between the structure of the gamma regulatory segment and its function in ATPase regulation via its interaction with the inhibitory epsilon subunit. Mutations were designed using a homology model of the chloroplast gamma subunit based on the analogous structures of the bacterial and mitochondrial homologues. The mutations included (a) substituting both of the disulfide-forming cysteines (Cys199 and Cys205) for alanines, (b) deleting nine residues containing the dithiol, (c) deleting the region distal to the dithiol (residues 224-240), and (d) deleting the entire segment between residues 196 and 241 with the exception of a small spacer element, and (e) deleting pieces from a small loop segment predicted by the model to interact with the dithiol domain. Deletions within the dithiol domain and within parts of the loop segment resulted in loss of redox control of the ATPase activity of the F1 enzyme. Deleting the distal segment, the whole regulatory domain, or parts of the loop segment had the additional effect of reducing the maximum extent of inhibition obtained upon adding the epsilon subunit but did not abolish epsilon binding. The results suggest a mechanism by which the gamma and epsilon subunits interact with each other to induce the latent state of the enzyme.

Coupling proton movement to ATP synthesis in the chloroplast ATP synthase

The chloroplast F(0)F(1)-ATP synthase-ATPase is a tiny rotary motor responsible for coupling ATP synthesis and hydrolysis to the light-driven electrochemical proton gradient. Reversible oxidation/reduction of a dithiol, located within a special regulatory domain of the gamma subunit of the chloroplast F(1) enzyme, switches the enzyme between an inactive and an active state. This regulatory mechanism is unique to the ATP synthases of higher plants and its physiological significance lies in preventing nonproductive depletion of essential ATP pools in the dark. The three-dimensional structure of the chloroplast F(1) gamma subunit has not yet been solved. To examine the mechanism of dithiol regulation, a model of the chloroplast gamma subunit was obtained through segmental homology modeling based on the known structures of the mitochondrial and bacterial gamma subunits, together with de novo construction of the unknown regulatory domain. The model has provided considerable insight into how the dithiol might modulate catalytic function. This has, in turn, suggested a mechanism by which rotation of subunits in F(0), the transmembrane proton channel portion of the enzyme, can be coupled, via the epsilon subunit, to rotation of the gamma subunit of F(1) to achieve the 120 degrees (or 90 degrees +30 degrees) stepping action that is characteristic of F(1) gamma subunit rotation.

The role of subunit epsilon in the catalysis and regulation of FOF1-ATP synthase

The regulation of ATP synthase activity is complex and involves several distinct mechanisms. In bacteria and chloroplasts, subunit epsilon plays an important role in this regulation, (i) affecting the efficiency of coupling, (ii) influencing the catalytic pathway, and (iii) selectively inhibiting ATP hydrolysis activity. Several experimental studies indicate that the regulation is achieved through large conformational transitions of the alpha-helical C-terminal domain of subunit epsilon that occur in response to membrane energization, change in ATP/ADP ratio or addition of inhibitors. This review summarizes the experimental data obtained on different organisms that clarify some basic features as well as some molecular details of this regulatory mechanism. Multiple functions of subunit epsilon, its role in the difference between the catalytic pathways of ATP synthesis and hydrolysis and its influence on the inhibition of ATP hydrolysis by ADP are also discussed.

Substitutions of the conserved Gly47 affect the CF1 inhibitor and proton gate functions of the chloroplast ATP synthase epsilon subunit

The conserved residue Gly47 of the chloroplast ATP synthase beta subunit was substituted with Leu, Arg, Ala and Glu by site-directed mutagenesis. This process generated the mutants epsilon G47L, epsilon G47R, epsilon G47A and epsilon G47E, respectively. All the beta variants showed lower inhibitory effects on the soluble CF1(-epsilon) Ca2+-ATPase compared with wild-type epsilon. In reduced conditions, epsilon G47E and epsilon G47R had a lower inhibitory effect on the oxidized CF1(-epsilon) Ca2+-ATPase compared with wild-type epsilon. In contrast, epsilon G47L and epsilon G47A increased the Ca2+-ATPase activity of soluble oxidized CF1(-epsilon). The replacement of Gly47 significantly impaired the interaction between the subunit epsilon and gamma in an in vitro binding assay? Further study showed that all epsilon variants were more effective in blocking proton leakage from the thylakoid membranes. This enhanced ATP synthesis of the chloroplast and restored ATP synthesis activity of the reconstituted membranes to a level that was more efficient than that achieved by wild-type epsilon. These results indicate that the conserved Gly47 residue of the epsilon subunit is very important for maintaining the structure and function of the epsilon subunit and may affect the interaction between the epsilon subunit, beta subunit of CF1 and subunit III of CFo, thereby regulating the ATP hydrolysis and synthesis, as well as the proton translocation role of the subunit III of CFo.

N-terminal deletion of the gamma subunit affects the stabilization and activity of chloroplast ATP synthase

Five truncation mutants of chloroplast ATP synthase gamma subunit from spinach (Spinacia oleracea) lacking 8, 12, 16, 20 or 60 N-terminal amino acids were generated by PCR by a mutagenesis method. The recombinant gamma genes were overexpressed in Escherichia coli and assembled with alphabeta subunits into a native complex. The wild-type (WT) alphabetagamma assembly i.e. alphabetagammaWT exhibited high (Mg2+)-dependent and (Ca2+)-dependent ATP hydrolytic activity. Deletions of eight residues of the gamma subunit N-terminus caused a decrease in rates of ATP hydrolysis to 30% of that of the alphabetaWT assembly. Furthermore, only approximately 6% of ATP hydrolytic activity was retained with the sequential deletions of gamma subunit up to 20 residues compared with the activity of the alphabetaWT assembly. The inhibitory effect of the epsilon subunit on ATP hydrolysis of these alphabetagamma assemblies varied to a large extent. These observations indicate that the N-terminus of the gamma subunit is very important, together with other regions of the gamma subunit, in stabilization of the enzyme complex or during cooperative catalysis. In addition, the in vitro binding assay showed that the gamma subunit N-terminus is not a crucial region in binding of the epsilon subunit.

Effects of site-directed mutation on the function of the chloroplast ATP synthase epsilon subunit

The previous work in our lab showed that the spinach chloroplast ATP synthase epsilon mutant with 3 amino acid residues deleted from the N-terminus had much lower ability to inhibit ATP hydrolysis and block proton leakage in comparison to a mutant with 1 or 2 residues deleted from the N-terminus. The present study aimed at determining whether there is special importance in the structure and function of the N-terminal third residue of the chloroplast epsilon subunit. The leucine residue at the N-terminal third site (Leu3) of the spinach chloroplast epsilon subunit was replaced with Ile, Phe, Thr, Arg, Glu or Pro by site-directed mutagenesis, forming mutants epsilonL3I, epsilonL3F, epsilonL3T, epsilonL3R, epsilonL3E and epsilonL3P, respectively. These epsilon variants all showed lower abilities to inhibit ATP hydrolysis and to block proton leakage, as compared to the wild type epsilon subunit (epsilonWT). The abilities of mutants epsilonL3I and epsilonL3F to restore the ATP synthesis activity of reconstituted membranes were higher than those of epsilonWT, but the abilities of the other epsilon variants were lower than that of epsilonWT. These results indicate that the hydrophobic and neutral characteristics of Leu3 of the chloroplast epsilon subunit are very important for its ability to inhibit ATP hydrolysis and block proton leakage, and for the ATP synthesis ability of ATP synthase.

The N terminus of MinD contains determinants which affect its dynamic localization and enzymatic activity

MinD is involved in regulating the proper placement of the cytokinetic machinery in some bacteria, including Neisseria gonorrhoeae and Escherichia coli. Stimulation of the ATPase activity of MinD by MinE has been proposed to induce dynamic, pole-to-pole oscillations of MinD in E. coli. Here, we investigated the effects of deleting or mutating conserved residues within the N terminus of N. gonorrhoeae MinD (MinD(Ng)) on protein dynamism, localization, and interactions with MinD(Ng) and with MinE(Ng). Deletions or mutations were generated in the first five residues of MinD(Ng), and mutant proteins were evaluated by several functional assays. Truncation or mutation of N-terminal residues disrupted MinD(Ng) interactions with itself and with MinE. Although the majority of green fluorescent protein (GFP)-MinD(Ng) mutants could still oscillate from pole to pole in E. coli, the GFP-MinD(Ng) oscillation cycles were significantly faster and were accompanied by increased cytoplasmic localization. Interestingly, in vitro ATPase assays indicated that MinD(Ng) proteins lacking the first three residues or with an I5E substitution possessed higher MinE(Ng)-independent ATPase activities than the wild-type protein. These results indicate that determinants found within the extreme N terminus of MinD(Ng) are implicated in regulating the enzymatic activity and dynamic localization of the protein.