Thiol modulation of the chloroplast ATP synthase is dependent on the energization of thylakoid membranes

ATP regeneration by ATPases for in vitro biotransformation

Adenosine triphosphate (ATP) regeneration is a significant step in both living cells and in vitro biotransformation (ivBT). Rotary motor ATP synthases (ATPases), which regenerate ATP in living cells, have been widely assembled in biomimetic structures for in vitro ATP synthesis. In this review, we present a comprehensive overview of ATPases, including the working principle, orientation and distribution density properties of ATPases, as well as the assembly strategies and applications of ATPase-based ATP regeneration modules. The original sources of ATPases for in vitro ATP regeneration include chromatophores, chloroplasts, mitochondria, and inverted Escherichia coli (E. coli) vesicles, which are readily accessible but unstable. Although significant advances have been made in the assembly methods for ATPase-artificial membranes in recent decades, it remains challenging to replicate the high density and orientation of ATPases observed in vivo using in vitro assembly methods. The use of bioproton pumps or chemicals for constructing proton motive forces (PMF) enables the versatility and potential of ATPase-based ATP regeneration modules. Additionally, overall robustness can be achieved via membrane component selection, such as polymers offering great mechanical stability, or by constructing a solid supporting matrix through layer-by-layer assembly techniques. Finally, the prospects of ATPase-based ATP regeneration modules can be expected with the technological development of ATPases and artificial membranes.

Divergent Protein Redox Dynamics and Their Relationship with Electron Transport Efficiency during Photosynthesis Induction

Various chloroplast proteins are activated/deactivated during the light/dark cycle via the redox regulation system. Although the photosynthetic electron transport chain provides reducing power to redox-sensitive proteins via the ferredoxin (Fd)/thioredoxin (Trx) pathway for their enzymatic activity control, how the redox states of individual proteins are linked to electron transport efficiency remains uncharacterized. Here we addressed this subject with a focus on the photosynthetic induction phase. We used Arabidopsis plants, in which the amount of Fd-Trx reductase (FTR), a core component in the Fd/Trx pathway, was genetically altered. Several chloroplast proteins showed different redox shift responses toward low- and high-light treatments. The light-dependent reduction of Calvin-Benson cycle enzymes fructose 1,6-bisphosphatase (FBPase) and sedoheptulose 1,7-bisphosphatase (SBPase) was partially impaired in the FTR-knockdown ftrb mutant. Simultaneous analyses of chlorophyll fluorescence and P700 absorbance change indicated that the induction of the electron transport reactions was delayed in the ftrb mutant. FTR overexpression also mildly affected the reduction patterns of FBPase and SBPase under high-light conditions, which were accompanied by the modification of electron transport properties. Accordingly, the redox states of FBPase and SBPase were linearly correlated with electron transport rates. In contrast, ATP synthase was highly reduced even when electron transport reactions were not fully induced. Furthermore, the redox response of proton gradient regulation 5-like photosynthetic phenotype1 (PGRL1; a protein involved in cyclic electron transport) did not correlate with electron transport rates. Our results provide insights into the working dynamics of the redox regulation system and their differential associations with photosynthetic electron transport efficiency.

Overexpression of thioredoxin-like protein ACHT2 leads to negative feedback control of photosynthesis in Arabidopsis thaliana

Thioredoxin (Trx) is a small redox mediator protein involved in the regulation of various chloroplast functions by modulating the redox state of Trx target proteins in ever-changing light environments. Using reducing equivalents produced by the photosynthetic electron transport chain, Trx reduces the disulfide bonds on target proteins and generally turns on their activities. While the details of the protein-reduction mechanism by Trx have been well investigated, the oxidation mechanism that counteracts it has long been unclear. We have recently demonstrated that Trx-like proteins such as Trx-like2 and atypical Cys His-rich Trx (ACHT) can function as protein oxidation factors in chloroplasts. Our latest study on transgenic Arabidopsis plants indicated that the ACHT isoform ACHT2 is involved in regulating the thermal dissipation of light energy. To understand the role of ACHT2 in vivo, we characterized phenotypic changes specifically caused by ACHT2 overexpression in Arabidopsis. ACHT2-overexpressing plants showed growth defects, especially under high light conditions. This growth phenotype was accompanied with the impaired reductive activation of Calvin-Benson cycle enzymes, enhanced thermal dissipation of light energy, and decreased photosystem II activity. Overall, ACHT2 overexpression promoted protein oxidation that led to the inadequate activation of Calvin-Benson cycle enzymes in light and consequently induced negative feedback control of the photosynthetic electron transport chain. This study highlights the importance of the balance between protein reduction and oxidation in chloroplasts for optimal photosynthetic performance and plant growth.

CBSX2 is required for the efficient oxidation of chloroplast redox-regulated enzymes in darkness

Thiol/disulfide-based redox regulation in plant chloroplasts is essential for controlling the activity of target proteins in response to light signals. One of the examples of such a role in chloroplasts is the activity of the chloroplast ATP synthase (CFoCF1), which is regulated by the redox state of the CF1γ subunit and involves two cysteines in its central domain. To investigate the mechanism underlying the oxidation of CF1γ and other chloroplast redox-regulated enzymes in the dark, we characterized the Arabidopsis cbsx2 mutant, which was isolated based on its altered NPQ (non-photochemical quenching) induction upon illumination. Whereas in dark-adapted WT plants CF1γ was completely oxidized, a small amount of CF1γ remained in the reduced state in cbsx2 under the same conditions. In this mutant, reduction of CF1γ was not affected in the light, but its oxidation was less efficient during a transition from light to darkness. The redox states of the Calvin cycle enzymes FBPase and SBPase in cbsx2 were similar to those of CF1γ during light/dark transitions. Affinity purification and subsequent analysis by mass spectrometry showed that the components of the ferredoxin-thioredoxin reductase/thioredoxin (FTR-Trx) and NADPH-dependent thioredoxin reductase (NTRC) systems as well as several 2-Cys peroxiredoxins (Prxs) can be co-purified with CBSX2. In addition to the thioredoxins, yeast two-hybrid analysis showed that CBSX2 also interacts with NTRC. Taken together, our results suggest that CBSX2 participates in the oxidation of the chloroplast redox-regulated enzymes in darkness, probably through regulation of the activity of chloroplast redox systems in vivo.

High cyclic electron transfer via the PGR5 pathway in the absence of photosynthetic control

The light reactions of photosynthesis couple electron and proton transfers across the thylakoid membrane, generating NADPH, and proton motive force (pmf) that powers the endergonic synthesis of ATP by ATP synthase. ATP and NADPH are required for CO2 fixation into carbohydrates by the Calvin-Benson-Bassham cycle. The dominant ΔpH component of the pmf also plays a photoprotective role in regulating photosystem II light harvesting efficiency through nonphotochemical quenching (NPQ) and photosynthetic control via electron transfer from cytochrome b6f (cytb6f) to photosystem I. ΔpH can be adjusted by increasing the proton influx into the thylakoid lumen via upregulation of cyclic electron transfer (CET) or decreasing proton efflux via downregulation of ATP synthase conductivity (gH+). The interplay and relative contributions of these two elements of ΔpH control to photoprotection are not well understood. Here, we showed that an Arabidopsis (Arabidopsis thaliana) ATP synthase mutant hunger for oxygen in photosynthetic transfer reaction 2 (hope2) with 40% higher proton efflux has supercharged CET. Double crosses of hope2 with the CET-deficient proton gradient regulation 5 and ndh-like photosynthetic complex I lines revealed that PROTON GRADIENT REGULATION 5 (PGR5)-dependent CET is the major pathway contributing to higher proton influx. PGR5-dependent CET allowed hope2 to maintain wild-type levels of ΔpH, CO2 fixation and NPQ, however photosynthetic control remained absent and PSI was prone to photoinhibition. Therefore, high CET in the absence of ATP synthase regulation is insufficient for PSI photoprotection.

The ferredoxin/thioredoxin pathway constitutes an indispensable redox-signaling cascade for light-dependent reduction of chloroplast stromal proteins

To ensure efficient photosynthesis, chloroplast proteins need to be flexibly regulated under fluctuating light conditions. Thiol-based redox regulation plays a key role in reductively activating several chloroplast proteins in a light-dependent manner. The ferredoxin (Fd)/thioredoxin (Trx) pathway has long been recognized as the machinery that transfers reducing power generated by photosynthetic electron transport reactions to redox-sensitive target proteins; however, its biological importance remains unclear, because the complete disruption of the Fd/Trx pathway in plants has been unsuccessful to date. Especially, recent identifications of multiple redox-related factors in chloroplasts, as represented by the NADPH-Trx reductase C, have raised a controversial proposal that other redox pathways work redundantly with the Fd/Trx pathway. To address these issues directly, we used CRISPR/Cas9 gene editing to create Arabidopsis mutant plants in which the activity of the Fd/Trx pathway was completely defective. The mutants generated showed severe growth inhibition. Importantly, these mutants almost entirely lost the ability to reduce several redox-sensitive proteins in chloroplast stroma, including four Calvin-Benson cycle enzymes, NADP-malate dehydrogenase, and Rubisco activase, under light conditions. These striking phenotypes were further accompanied by abnormally developed chloroplasts and a drastic decline in photosynthetic efficiency. These results indicate that the Fd/Trx pathway is indispensable for the light-responsive activation of diverse stromal proteins and photoautotrophic growth of plants. Our data also suggest that the ATP synthase is exceptionally reduced by other pathways in a redundant manner. This study provides an important insight into how the chloroplast redox-regulatory system operates in vivo.

Molecular Regulation and Evolution of Redox Homeostasis in Photosynthetic Machinery

The recent advances in plant biology have significantly improved our understanding of reactive oxygen species (ROS) as signaling molecules in the redox regulation of complex cellular processes. In plants, free radicals and non-radicals are prevalent intra- and inter-cellular ROS, catalyzing complex metabolic processes such as photosynthesis. Photosynthesis homeostasis is maintained by thiol-based systems and antioxidative enzymes, which belong to some of the evolutionarily conserved protein families. The molecular and biological functions of redox regulation in photosynthesis are usually to balance the electron transport chain, photosystem II, photosystem I, mesophyll and bundle sheath signaling, and photo-protection regulating plant growth and productivity. Here, we review the recent progress of ROS signaling in photosynthesis. We present a comprehensive comparative bioinformatic analysis of redox regulation in evolutionary distinct photosynthetic cells. Gene expression, phylogenies, sequence alignments, and 3D protein structures in representative algal and plant species revealed conserved key features including functional domains catalyzing oxidation and reduction reactions. We then discuss the antioxidant-related ROS signaling and important pathways for achieving homeostasis of photosynthesis. Finally, we highlight the importance of plant responses to stress cues and genetic manipulation of disturbed redox status for balanced and enhanced photosynthetic efficiency and plant productivity.

Dissipation of the proton electrochemical gradient in chloroplasts promotes the oxidation of ATP synthase by thioredoxin-like proteins

Chloroplast F_oF₁-ATP synthase (CF_oCF₁) uses an electrochemical gradient of protons across the thylakoid membrane (ΔμH⁺) as an energy source in the ATP synthesis reaction. CF_oCF₁ activity is regulated by the redox state of a Cys pair on its central axis, that is, the γ subunit (CF₁-γ). When the ΔμH⁺ is formed by the photosynthetic electron transfer chain under light conditions, CF₁-γ is reduced by thioredoxin (Trx), and the entire CF_oCF₁ enzyme is activated. The redox regulation of CF_oCF₁ is a key mechanism underlying the control of ATP synthesis under light conditions. In contrast, the oxidative deactivation process involving CF_oCF₁ has not been clarified. In the present study, we analyzed the oxidation of CF₁-γ by two physiological oxidants in the chloroplast, namely the proteins Trx-like 2 and atypical Cys-His-rich Trx. Using the thylakoid membrane containing the reduced form of CF_oCF₁, we were able to assess the CF₁-γ oxidation ability of these Trx-like proteins. Our kinetic analysis indicated that these proteins oxidized CF₁-γ with a higher efficiency than that achieved by a chemical oxidant and typical chloroplast Trxs. Additionally, the CF₁-γ oxidation rate due to Trx-like proteins and the affinity between them were changed markedly when ΔμH⁺ formation across the thylakoid membrane was manipulated artificially. Collectively, these results indicate that the formation status of the ΔμH⁺ controls the redox regulation of CF_oCF₁ to prevent energetic disadvantages in plants.

Sustainability of in vitro light-dependent NADPH generation by the thylakoid membrane of Synechocystis sp. PCC6803

Background

NADPH is used as a reductant in various biosynthetic reactions. Cell-free bio-systems have gained considerable attention owing to their high energy utilization and time efficiency. Efforts have been made to continuously supply reducing power to the reaction mixture in a cyclical manner. The thylakoid membrane (TM) is a promising molecular energy generator, producing NADPH under light. Thus, TM sustainability is of major relevance for its in vitro utilization.

Results

Over 70% of TMs prepared from Synechocystis sp. PCC6803 existed in a sealed vesicular structure, with the F₁ complex of ATP synthase facing outward (right-side-out), producing NADPH and ATP under light. The NADPH generation activity of TM increased approximately two-fold with the addition of carbonyl cyanide-p-(trifluoromethoxy) phenylhydrazone (FCCP) or removal of the F₁ complex using EDTA. Thus, the uncoupling of proton translocation from the electron transport chain or proton leakage through the F_o complex resulted in greater NADPH generation. Biosilicified TM retained more than 80% of its NADPH generation activity after a week at 30°C in the dark. However, activity declined sharply to below 30% after two days in light. The introduction of engineered water-forming NADPH oxidase (Nox^m) to keep the electron transport chain of TM working resulted in the improved sustainability of NADPH generation activity in a ratio (Nox^m to TM)-dependent manner, which correlated with the decrease of singlet oxygen generation. Removal of reactive oxygen species (ROS) by catalase further highlighted the sustainable NADPH generation activity of up to 80% in two days under light.

Conclusion

Reducing power generated by light energy has to be consumed for TM sustainability. Otherwise, TM can generate singlet oxygen, causing oxidative damage. Thus, TMs should be kept in the dark when not in use. Although NADPH generation activity by TM can be extended via silica encapsulation, further removal of hydrogen peroxide results in an improvement of TM sustainability. Therefore, as long as ROS formation by TM in light is properly handled, it can be used as a promising source of reducing power for in vitro biochemical reactions.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01825-1.

Verification of the Relationship between Redox Regulation of Thioredoxin Target Proteins and Their Proximity to Thylakoid Membranes

Thioredoxin (Trx) is a key protein of the redox regulation system in chloroplasts, where it modulates various enzyme activities. Upon light irradiation, Trx reduces the disulfide bonds of Trx target proteins (thereby turning on their activities) using reducing equivalents obtained from the photosynthetic electron transport chain. This reduction process involves a differential response, i.e., some Trx target proteins in the stroma respond slowly to the change in redox condition caused by light/dark changes, while the ATP synthase γ subunit (CF₁-γ) located on the surface of thylakoid membrane responds with high sensitivity. The factors that determine this difference in redox kinetics are not yet known, although here, we hypothesize that it is due to each protein’s localization in the chloroplast, i.e., the reducing equivalents generated under light conditions can be transferred more efficiently to the proteins on thylakoid membrane than to stromal proteins. To explore this possibility, we anchored SBPase, one of the stromal Trx target proteins, to the thylakoid membrane in Arabidopsis thaliana. Analyses of the redox behaviors of the anchored and unanchored proteins showed no significant difference in their reduction kinetics, implying that protein sensitivity to redox regulation is determined by other factors.

Live Cell Imaging of ATP Dynamics in Plant Cells

Adenosine triphosphate (ATP) is a central metabolite that functions as the energy currency in a living cell. Therefore, visualizing cellular ATP dynamics provides the fundamental information necessary to understand the molecular events involving life phenomena. Live cell imaging technologies using fluorescence (FL)-based indicators have been developed to analyze the dynamics of various biological processes, such as intracellular ATP synthesis and consumption. However, the application of FL-based indicators to plant cells is limited due to the presence of strong chlorophyll autofluorescence, which drastically worsen the signal-to-noise ratio. The bioluminescent (BL) indicators that do not require excitation light could overcome this problem. In this chapter, we introduce a methodology to analyze ATP dynamics in plant cells using BL ATP indicators.

Oxidative regulation of chloroplast enzymes by thioredoxin and thioredoxin-like proteins in Arabidopsis thaliana

Plants modulate photosynthesis activity in response to the surrounding environment. It is well known that the redox-responsive protein thioredoxin (Trx) activates photosynthesis-related enzymes in the light. However, the factors involved in deactivating them are not well understood. Recent in vitro experiments suggest that several Trx and Trx-like proteins serve as oxidation factors for Trx-targeted proteins; thus, we examined their functions in vivo. Consequently, we found that f-type Trx and two types of Trx-like proteins, Trx-like 2 and atypical Cys His-rich Trx, were involved in oxidative deactivation of photosynthesis-related enzymes (e.g., fructose-1,6-bisphosphatase, Rubisco activase, and the ATP synthase γ-subunit). Thus, this study reveals the functions of oxidation factors in vivo and elucidates the regulation system for photosynthesis in the dark.

Thioredoxin (Trx) is a protein that mediates the reducing power transfer from the photosynthetic electron transport system to target enzymes in chloroplasts and regulates their activities. Redox regulation governed by Trx is a system that is central to the adaptation of various chloroplast functions to the ever-changing light environment. However, the factors involved in the opposite reaction (i.e., the oxidation of various enzymes) have yet to be revealed. Recently, it has been suggested that Trx and Trx-like proteins could oxidize Trx-targeted proteins in vitro. To elucidate the in vivo function of these proteins as oxidation factors, we generated mutant plant lines deficient in Trx or Trx-like proteins and studied how the proteins are involved in oxidative regulation in chloroplasts. We found that f-type Trx and two types of Trx-like proteins, Trx-like 2 and atypical Cys His-rich Trx (ACHT), seemed to serve as oxidation factors for Trx-targeted proteins, such as fructose-1,6-bisphosphatase, Rubisco activase, and the γ-subunit of ATP synthase. In addition, ACHT was found to be involved in regulating nonphotochemical quenching, which is the mechanism underlying the thermal dissipation of excess light energy. Overall, these results indicate that Trx and Trx-like proteins regulate chloroplast functions in concert by controlling the redox state of various photosynthesis-related proteins in vivo.

Topology of the redox network during induction of photosynthesis as revealed by time-resolved proteomics in tobacco

Photosynthetically produced electrons provide energy for various metabolic pathways, including carbon reduction. Four Calvin-Benson cycle enzymes and several other plastid proteins are activated in the light by reduction of specific cysteines via thioredoxins, a family of electron transporters operating in redox regulation networks. How does this network link the photosynthetic chain with cellular metabolism? Using a time-resolved redox proteomic method, we have investigated the redox network in vivo during the dark–to–low light transition. We show that redox states of some thioredoxins follow the photosynthetic linear electron transport rate. While some redox targets have kinetics compatible with an equilibrium with one thioredoxin (TRXf), reduction of other proteins shows specific kinetic limitations, allowing fine-tuning of each redox-regulated step of chloroplast metabolism. We identified five new redox-regulated proteins, including proteins involved in Mg²⁺ transport and ¹O₂ signaling. Our results provide a system-level functional view of the photosynthetic redox regulation network.

The phototroph-specific β-hairpin structure of the γ subunit of FoF1-ATP synthase is important for efficient ATP synthesis of cyanobacteria

The F_oF₁ synthase produces ATP from ADP and inorganic phosphate. The γ subunit of F_oF₁ ATP synthase in photosynthetic organisms, which is the rotor subunit of this enzyme, contains a characteristic β-hairpin structure. This structure is formed from an insertion sequence that has been conserved only in phototrophs. Using recombinant subcomplexes, we previously demonstrated that this region plays an essential role in the regulation of ATP hydrolysis activity, thereby functioning in controlling intracellular ATP levels in response to changes in the light environment. However, the role of this region in ATP synthesis has long remained an open question because its analysis requires the preparation of the whole F_oF₁ complex and a transmembrane proton-motive force. In this study, we successfully prepared proteoliposomes containing the entire F_oF₁ ATP synthase from a cyanobacterium, Synechocystis sp. PCC 6803, and measured ATP synthesis/hydrolysis and proton-translocating activities. The relatively simple genetic manipulation of Synechocystis enabled the biochemical investigation of the role of the β-hairpin structure of F_oF₁ ATP synthase and its activities. We further performed physiological analyses of Synechocystis mutant strains lacking the β-hairpin structure, which provided novel insights into the regulatory mechanisms of F_oF₁ ATP synthase in cyanobacteria via the phototroph-specific region of the γ subunit. Our results indicated that this structure critically contributes to ATP synthesis and suppresses ATP hydrolysis.

Disentangling chloroplast ATP synthase regulation by proton motive force and thiol modulation in Arabidopsis leaves

The chloroplast ATP synthase (CF₁F_o) contains a specific feature to the green lineage: a γ-subunit redox domain that contains a cysteine couple which interacts with the torque-transmitting βDELSEED-loop. This thiol modulation equips CF₁F_o with an important environmental fine-tuning mechanism. In vitro, disulfide formation in the γ-redox domain slows down the activity of the CF₁F_o at low transmembrane electrochemical proton gradient ( [Formula: see text] ), which agrees with its proposed role as chock based on recently solved structure. The γ-dithiol formation at the onset of light is crucial to maximize photosynthetic efficiency since it lowers the [Formula: see text] activation level for ATP synthesis in vitro. Here, we validate these findings in vivo by utilizing absorption spectroscopy in Arabidopsis thaliana. To do so, we monitored the [Formula: see text] present in darkness and identified its mitochondrial sources. By following the fate and components of light-induced extra [Formula: see text] , we estimated the ATP lifetime that lasted up to tens of minutes after long illuminations. Based on the relationship between [Formula: see text] and CF₁F_o activity, we conclude that the dithiol configuration in vivo facilitates photosynthesis by driving the same ATP synthesis rate at a significative lower [Formula: see text] than in the γ-disulfide state. The presented in vivo findings are an additional proof of the importance of CF₁F_o thiol modulation, reconciling biochemical in vitro studies and structural insights.

Chloroplast ATP synthase is reduced by both f-type and m-type thioredoxins

The activity of the molecular motor enzyme, chloroplast ATP synthase, is regulated in a redox-dependent manner. The γ subunit, CF₁-γ, is the central shaft of this enzyme complex and possesses the redox-active cysteine pair, which is reduced by thioredoxin (Trx). In light conditions, Trx transfers the reducing equivalent obtained from the photosynthetic electron transfer system to the CF₁-γ. Previous studies showed that the light-dependent reduction of CF₁-γ is more rapid than those of other Trx target proteins in the stroma. Although there are multiple Trx isoforms in chloroplasts, it is not well understood as to which chloroplast Trx isoform primarily contributes to the reduction of CF₁-γ, especially under physiological conditions. We therefore performed direct assessment of the CF₁-γ reduction capacity of each of the Trx isoforms. The kinetic analysis of the reduction process showed no significant difference in the reduction efficiency between two major chloroplast Trxs, namely Trx-f and Trx-m. Based on the thorough analyses of the CF₁-γ redox dynamics in Arabidopsis thaliana Trx mutant plants, we found that lack of Trx-f or Trx-m had no significant impact on the in vivo light-dependent reduction of CF₁-γ. The results showed that CF₁-γ can accept the reducing power from both Trx-f and Trx-m in chloroplasts.

New Light on Chloroplast Redox Regulation: Molecular Mechanism of Protein Thiol Oxidation

Thiol-based redox regulation is a posttranslational protein modification that plays a key role in adjusting chloroplast functions in response to changing light conditions. Redox-sensitive target proteins are reduced upon illumination, which turns on (or off in a certain case) their enzyme activities. A redox cascade via ferredoxin, ferredoxin-thioredoxin reductase, and thioredoxin has been classically recognized as the key system for transmitting the light-induced reductive signal to target proteins. By contrast, the molecular mechanism underlying target protein oxidation, which is observed during light to dark transitions, remains undetermined over the past several decades. Recently, the factors and pathways for protein thiol oxidation in chloroplasts have been reported, finally shedding light on this long-standing issue. We identified thioredoxin-like2 as one of the protein-oxidation factors in chloroplasts. This protein is characterized by its higher redox potential than that of canonical thioredoxin, that is more favorable for target protein oxidation. Furthermore, 2-Cys peroxiredoxin and hydrogen peroxide are also involved in the overall protein-oxidation machinery. Here we summarize the newly uncovered "dark side" of chloroplast redox regulation, giving an insight into how plants rest their photosynthetic activity at night.

Functional Implications of Multiple IM30 Oligomeric States

The inner membrane-associated protein of 30 kDa (IM30), also known as the vesicle-inducing protein in plastids 1 (Vipp1), is essential for photo-autotrophic growth of cyanobacteria, algae and higher plants. While its exact function still remains largely elusive, it is commonly accepted that IM30 is crucially involved in thylakoid membrane biogenesis, stabilization and/or maintenance. A characteristic feature of IM30 is its intrinsic propensity to form large homo-oligomeric protein complexes. 15 years ago, it has been reported that these supercomplexes have a ring-shaped structure. However, the in vivo significance of these ring structures is not finally resolved yet and the formation of more complex assemblies has been reported. We here present and discuss research on IM30 conducted within the past 25 years with a special emphasis on the question of why we potentially need IM30 supercomplexes in vivo.

Thioredoxin-like2/2-Cys peroxiredoxin redox cascade acts as oxidative activator of glucose-6-phosphate dehydrogenase in chloroplasts

Thiol-based redox regulation is crucial for adjusting chloroplast functions under fluctuating light environments. We recently discovered that the thioredoxin-like2 (TrxL2)/2-Cys peroxiredoxin (2CP) redox cascade supports oxidative thiol modulation by using hydrogen peroxide (H2O2) as an oxidizing force. This system plays a key role in switching chloroplast metabolism (e.g. Calvin–Benson cycle) during light to dark transitions; however, information on its function is still limited. In this study, we report a novel protein-activation mechanism based on the TrxL2/2CP redox cascade. Glucose-6-phosphate dehydrogenase (G6PDH) catalyzes the first step of the oxidative pentose phosphate pathway (OPPP). Biochemical studies, including redox state determination and measurement of enzyme activity, suggested that the TrxL2/2CP pathway is involved in the oxidative activation of G6PDH. It is thus likely that the TrxL2/2CP redox cascade shifts chloroplast metabolism to night mode by playing a dual role, namely, down-regulation of the Calvin–Benson cycle and up-regulation of OPPP. G6PDH was also directly oxidized and activated by H2O2, particularly when H2O2 concentration was elevated. Therefore, G6PDH is thought to be finely tuned by H2O2 levels in both direct and indirect manners.

The N-terminal region of the ϵ subunit from cyanobacterial ATP synthase alone can inhibit ATPase activity

ATP hydrolysis activity catalyzed by chloroplast and proteobacterial ATP synthase is inhibited by their ϵ subunits. To clarify the function of the ϵ subunit from phototrophs, here we analyzed the ϵ subunit-mediated inhibition (ϵ-inhibition) of cyanobacterial F₁-ATPase, a subcomplex of ATP synthase obtained from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1. We generated three C-terminal α-helix null ϵ-mutants; one lacked the C-terminal α-helices, and in the other two, the C-terminal conformation could be locked by a disulfide bond formed between two α-helices or an α-helix and a β-sandwich structure. All of these ϵ-mutants maintained ATPase-inhibiting competency. We then used single-molecule observation techniques to analyze the rotary motion of F₁-ATPase in the presence of these ϵ-mutants. The stop angular position of the γ subunit in the presence of the ϵ-mutant was identical to that in the presence of the WT ϵ. Using magnetic tweezers, we examined recovery from the inhibited rotation and observed restoration of rotation by 80° forcing of the γ subunit in the case of the ADP-inhibited form, but not when the rotation was inhibited by the ϵ-mutants or by the WT ϵ subunit. These results imply that the C-terminal α-helix domain of the ϵ subunit of cyanobacterial enzyme does not directly inhibit ATP hydrolysis and that its N-terminal domain alone can inhibit the hydrolysis activity. Notably, this property differed from that of the proteobacterial ϵ, which could not tightly inhibit rotation. We conclude that phototrophs and heterotrophs differ in the ϵ subunit-mediated regulation of ATP synthase.

Thioredoxin-like2/2-Cys peroxiredoxin redox cascade supports oxidative thiol modulation in chloroplasts

Thiol-based redox regulation is central to adjusting chloroplast functions under varying light conditions. A redox cascade via the ferredoxin-thioredoxin reductase (FTR)/thioredoxin (Trx) pathway has been well recognized to mediate the light-responsive reductive control of target proteins; however, the molecular basis for reoxidizing its targets in the dark remains unidentified. Here, we report a mechanism of oxidative thiol modulation in chloroplasts. We biochemically characterized a chloroplast stroma-localized atypical Trx from Arabidopsis, designated as Trx-like2 (TrxL2). TrxL2 had redox-active properties with an unusually less negative redox potential. By an affinity chromatography-based method, TrxL2 was shown to interact with a range of chloroplast redox-regulated proteins. The direct discrimination of thiol status indicated that TrxL2 can efficiently oxidize, but not reduce, these proteins. A notable exception was found in 2-Cys peroxiredoxin (2CP); TrxL2 was able to reduce 2CP with high efficiency. We achieved a complete in vitro reconstitution of the TrxL2/2CP redox cascade for oxidizing redox-regulated proteins and draining reducing power to hydrogen peroxide (H₂O₂). We further addressed the physiological relevance of this system by analyzing protein-oxidation dynamics. In Arabidopsis plants, a decreased level of 2CP led to the impairment of the reoxidation of redox-regulated proteins during light-dark transitions. A delayed response of protein reoxidation was concomitant with the prolonged accumulation of reducing power in TrxL2. These results suggest an in vivo function of the TrxL2/2CP redox cascade for driving oxidative thiol modulation in chloroplasts.

Thioredoxin regulates G6PDH activity by changing redox states of OpcA in the nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120

Glucose 6-phosphate dehydrogenase (G6PDH) catalyzes the first reaction in the oxidative pentose phosphate pathway. In green plant chloroplasts, G6PDH is a unique redox-regulated enzyme, since it is inactivated under the reducing conditions. This regulation is accomplished using a redox-active cysteine pair, which is conserved in plant G6PDH. The inactivation of this enzyme under conditions of light must be beneficial to prevent release of CO2 from the photosynthetic carbon fixation cycle. In the filamentous, heterocyst-forming, nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120 (Anabaena 7120), G6PDH plays a pivotal role in providing reducing power for nitrogenase, and its activity is also reported to be suppressed by reduction, though Anabaena G6PDH does not conserve the critical cysteines for regulation. Based on the thorough analyses of the redox regulation mechanisms of G6PDH from Anabaena 7120 and its activator protein OpcA, we found that m-type thioredoxin regulates G6PDH activity by changing the redox states of OpcA. Mass spectrometric analysis and mutagenesis studies indicate that Cys393 and Cys399 of OpcA are responsible for the redox regulation property of this protein. Moreover, in vivo analyses of the redox states of OpcA showed that more than half of the OpcA is present as an oxidized form, even under conditions of light, when cells are cultured under the nitrogen-fixing conditions. This redox regulation of OpcA might be necessary to provide reducing power for nitrogenase by G6PDH in heterocysts even during the day.

Effects and mechanism of acid rain on plant chloroplast ATP synthase

Acid rain can directly or indirectly affect plant physiological functions, especially photosynthesis. The enzyme ATP synthase is the key in photosynthetic energy conversion, and thus, it affects plant photosynthesis. To clarify the mechanism by which acid rain affects photosynthesis, we studied the effects of acid rain on plant growth, photosynthesis, chloroplast ATP synthase activity and gene expression, chloroplast ultrastructure, intracellular H(+) level, and water content of rice seedlings. Acid rain at pH 4.5 remained the chloroplast structure unchanged but increased the expression of six chloroplast ATP synthase subunits, promoted chloroplast ATP synthase activity, and increased photosynthesis and plant growth. Acid rain at pH 4.0 or less decreased leaf water content, destroyed chloroplast structure, inhibited the expression of six chloroplast ATP synthase subunits, decreased chloroplast ATP synthase activity, and reduced photosynthesis and plant growth. In conclusion, acid rain affected the chloroplast ultrastructure, chloroplast ATPase transcription and activity, and P n by changing the acidity in the cells, and thus influencing the plant growth and development. Finally, the effects of simulated acid rain on the test indices were found to be dose-dependent.

Crosstalk between chloroplast thioredoxin systems in regulation of photosynthesis

Thioredoxins (TRXs) mediate light-dependent activation of primary photosynthetic reactions in plant chloroplasts by reducing disulphide bridges in redox-regulated enzymes. Of the two plastid TRX systems, the ferredoxin-TRX system consists of ferredoxin-thioredoxin reductase (FTR) and multiple TRXs, while the NADPH-dependent thioredoxin reductase (NTRC) contains a complete TRX system in a single polypeptide. Using Arabidopsis plants overexpressing or lacking a functional NTRC, we have investigated the redundancy and interaction between the NTRC and Fd-TRX systems in regulation of photosynthesis in vivo. Overexpression of NTRC raised the CO2 fixation rate and lowered non-photochemical quenching and acceptor side limitation of PSI in low light conditions by enhancing the activation of chloroplast ATP synthase and TRX-regulated enzymes in Calvin-Benson cycle (CBC). Overexpression of NTRC with an inactivated NTR or TRX domain partly recovered the phenotype of knockout plants, suggesting crosstalk between the plastid TRX systems. NTRC interacted in planta with fructose-1,6-bisphosphatase, phosphoribulokinase and CF1 γ subunit of the ATP synthase and with several chloroplast TRXs. These findings indicate that NTRC-mediated regulation of the CBC and ATP synthesis occurs both directly and through interaction with the ferredoxin-TRX system and is crucial when availability of light is limiting photosynthesis.

The effect of medium viscosity on kinetics of ATP hydrolysis by the chloroplast coupling factor CF1

The coupling factor CF1 is a catalytic part of chloroplast ATP synthase which is exposed to stroma whose viscosity is many-fold higher than that of reaction mixtures commonly used to measure kinetics of CF1-catalyzed ATP hydrolysis. This study is focused on the effect of medium viscosity modulated by sucrose or bovine serum albumin (BSA) on kinetics of Ca(2+)- and Mg(2+)-dependent ATP hydrolysis by CF1. These agents were shown to reduce the maximal rate of Ca(2+)-dependent ATPase without changing the apparent Michaelis constant (К m), thus supporting the hypothesis on viscosity dependence of CF1 activity. For the sulfite- and ethanol-stimulated Mg(2+)-dependent reaction, the presence of sucrose increased К m without changing the maximal rate that is many-fold as high as that of Ca(2+)-dependent hydrolysis. The hydrolysis reaction was shown to be stimulated by low concentrations of BSA and inhibited by its higher concentrations, with the increasing maximal reaction rate estimated by extrapolation. Sucrose- or BSA-induced inhibition of the Mg(2+)-dependent ATPase reaction is believed to result from diffusion-caused deceleration, while its BSA-induced stimulation is probably caused by optimization of the enzyme structure. Molecular mechanisms of the inhibitory effect of viscosity are discussed. Taking into account high protein concentrations in the chloroplast stroma, it was suggested that kinetic parameters of ATP hydrolysis, and probably those of ATP synthesis in vivo as well, must be quite different from measurements taken at a viscosity level close to that of water.

Artificial remodelling of alternative electron flow by flavodiiron proteins in Arabidopsis

In photosynthesis, linear electron transport from water to nicotinamide adenine dinucleotide phosphate (NADP(+)) cannot satisfy the ATP/NADPH production stoichiometry required by the Calvin-Benson cycle. Cyclic electron transport (CET) around photosystem I (PSI) and pseudocyclic electron transport (pseudoCET) can produce ATP without the accumulation of NADPH. Flavodiiron proteins (Flv) are the main mediator of pseudoCET in photosynthetic organisms, spanning cyanobacteria to gymnosperms. However, their genes are not conserved in angiosperms. Here we explore the possibility of complementing CET with Flv-dependent pseudoCET in the angiosperm Arabidopsis thaliana. We introduced FlvA and FlvB genes from the moss Physcomitrella patens into both wild-type (WT) Arabidopsis and the proton gradient regulation 5 (pgr5) mutant, which is defective in the main pathway of CET. We measured rates of pseudoCET using membrane inlet mass spectrometry, along with several photosynthetic parameters. Flv expression significantly increased rates of pseudoCET in the mutant plants, particularly at high light intensities, and partially restored the photosynthetic phenotype. In WT plants, Flv did not compete with PGR5-dependent CET during steady-state photosynthesis, but did form a large electron sink in fluctuating light. We conclude that flavodiiron proteins can help to protect the photosystems in Arabidopsis under fluctuating light, even in the presence of CET.

Chloroplastic thioredoxin m functions as a major regulator of Calvin cycle enzymes during photosynthesis in vivo

Thioredoxins (Trxs) regulate the activity of various chloroplastic proteins in a light-dependent manner. Five types of Trxs function in different physiological processes in the chloroplast of Arabidopsis thaliana. Previous in vitro experiments have suggested that the f-type Trx (Trx f) is the main redox regulator of chloroplast enzymes, including Calvin cycle enzymes. To investigate the in vivo contribution of each Trx isoform to the redox regulatory system, we first quantified the protein concentration of each Trx isoform in the chloroplast stroma. The m-type Trx (Trx m), which consists of four isoforms, was the most abundant type. Next, we analyzed several Arabidopsis Trx-m-deficient mutants to elucidate the physiological role of Trx m in vivo. Deficiency of Trx m impaired plant growth and decreased the CO2 assimilation rate. We also determined the redox state of Trx target enzymes to examine their photo-reduction, which is essential for enzyme activation. In the Trx-m-deficient mutants, the reduction level of fructose-1,6-bisphosphatase and sedoheptulose-1,7-bisphosphatase was lower than that in the wild type. Inconsistently with the historical view, our in vivo study suggested that Trx m plays a more important role than Trx f in the activation of Calvin cycle enzymes.

Application of preparative disk gel electrophoresis for antigen purification from inclusion bodies

Specific antibodies are a reliable tool to examine protein expression patterns and to determine the protein localizations within cells. Generally, recombinant proteins are used as antigens for specific antibody production. However, recombinant proteins from mammals and plants are often overexpressed as insoluble inclusion bodies in Escherichia coli. Solubilization of these inclusion bodies is desirable because soluble antigens are more suitable for injection into animals to be immunized. Furthermore, highly purified proteins are also required for specific antibody production. Plastidic acetyl-CoA carboxylase (ACCase: EC 6.4.1.2) from Arabidopsis thaliana, which catalyzes the formation of malonyl-CoA from acetyl-CoA in chloroplasts, formed inclusion bodies when the recombinant protein was overexpressed in E. coli. To obtain the purified protein to use as an antigen, we applied preparative disk gel electrophoresis for protein purification from inclusion bodies. This method is suitable for antigen preparation from inclusion bodies because the purified protein is recovered as a soluble fraction in electrode running buffer containing 0.1% sodium dodecyl sulfate that can be directly injected into immune animals, and it can be used for large-scale antigen preparation (several tens of milligrams).

ATP synthase

Oxygenic photosynthesis is the principal converter of sunlight into chemical energy. Cyanobacteria and plants provide aerobic life with oxygen, food, fuel, fibers, and platform chemicals. Four multisubunit membrane proteins are involved: photosystem I (PSI), photosystem II (PSII), cytochrome b6f (cyt b6f), and ATP synthase (FOF1). ATP synthase is likewise a key enzyme of cell respiration. Over three billion years, the basic machinery of oxygenic photosynthesis and respiration has been perfected to minimize wasteful reactions. The proton-driven ATP synthase is embedded in a proton tight-coupling membrane. It is composed of two rotary motors/generators, FO and F1, which do not slip against each other. The proton-driven FO and the ATP-synthesizing F1 are coupled via elastic torque transmission. Elastic transmission decouples the two motors in kinetic detail but keeps them perfectly coupled in thermodynamic equilibrium and (time-averaged) under steady turnover. Elastic transmission enables operation with different gear ratios in different organisms.

Redox regulation of CF1-ATPase involves interplay between the γ-subunit neck region and the turn region of the βDELSEED-loop

The soluble F1 complex of ATP synthase (FoF1) is capable of ATP hydrolysis, accomplished by the minimum catalytic core subunits α3β3γ. A special feature of cyanobacterial F1 and chloroplast F1 (CF1) is an amino acid sequence inserted in the γ-subunit. The insertion is extended slightly into the CF1 enzyme containing two additional cysteines for regulation of ATPase activity via thiol modulation. This molecular switch was transferred to a chimeric F1 by inserting the cysteine-containing fragment from spinach CF1 into a cyanobacterial γ-subunit [Y. Kim et al., redox regulation of rotation of the cyanobacterial F1-ATPase containing thiol regulation switch, J Biol Chem, 286 (2011) 9071-9078]. Under oxidizing conditions, the obtained F1 tends to lapse into an ADP-inhibited state, a common regulation mechanism to prevent wasteful ATP hydrolysis under unfavorable circumstances. However, the information flow between thiol modulation sites on the γ-subunit and catalytic sites on the β-subunits remains unclear. Here, we clarified a possible interplay for the CF1-ATPase redox regulation between structural elements of the βDELSEED-loop and the γ-subunit neck region, i.e., the most convex part of the α-helical γ-termini. Critical residues were assigned on the β-subunit, which received the conformation change signal produced by disulfide/dithiol formation on the γ-subunit. Mutant response to the ATPase redox regulation ranged from lost to hypersensitive. Furthermore, mutant cross-link experiments and inversion of redox regulation indicated that the γ-redox state might modulate the subunit interface via reorientation of the βDELSEED motif region.

Role of cyclic electron transport around photosystem I in regulating proton motive force

In addition to ∆pH formed across the thylakoid membrane, membrane potential contributes to proton motive force (pmf) in chloroplasts. However, the regulation of photosynthetic electron transport is mediated solely by ∆pH. To assess the contribution of two cyclic electron transport pathways around photosystem I (one depending on PGR5/PGRL1 and one on NDH) to pmf formation, electrochromic shift (ECS) was analyzed in the Arabidopsis pgr5 mutant, NDH-defective mutants (ndhs and crr4-2), and their double mutants (ndhs pgr5 and crr4-2 pgr5). In pgr5, the size of the pmf, as represented by ECSt, was reduced by 30% to 47% compared with that in the wild type (WT). A gH+ parameter, which is considered to represent the activity of ATP synthase, was enhanced at high light intensities. However, gH+ recovered to its low-light levels after 20 min in the dark, implying that the elevation in gH+ is due to the disturbed regulation of ATP synthase rather than to photodamage. After long dark adaptation more than 2 h, gH+ was higher in pgr5 than in the WT. During induction of photosynthesis, gH+ was more rapidly elevated in pgr5 than that in the WT. Both results suggest that ATP synthase is not fully inactivated in the dark in pgr5. In the NDH-deficient mutants, ECSt was slightly but significantly lower than in the WT, whereas gH+ was not affected. In the double mutants, ECSt was even lower than in pgr5. These results suggest that both PGR5/PGRL1- and NDH-dependent pathways contribute to pmf formation, although to different extents. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Metabolic control of redox and redox control of metabolism in plants

SIGNIFICANCE

Reduction-oxidation (Redox) status operates as a major integrator of subcellular and extracellular metabolism and is simultaneously itself regulated by metabolic processes. Redox status not only dominates cellular metabolism due to the prominence of NAD(H) and NADP(H) couples in myriad metabolic reactions but also acts as an effective signal that informs the cell of the prevailing environmental conditions. After relay of this information, the cell is able to appropriately respond via a range of mechanisms, including directly affecting cellular functioning and reprogramming nuclear gene expression.

RECENT ADVANCES

The facile accession of Arabidopsis knockout mutants alongside the adoption of broad-scale post-genomic approaches, which are able to provide transcriptomic-, proteomic-, and metabolomic-level information alongside traditional biochemical and emerging cell biological techniques, has dramatically advanced our understanding of redox status control. This review summarizes redox status control of metabolism and the metabolic control of redox status at both cellular and subcellular levels.

CRITICAL ISSUES

It is becoming apparent that plastid, mitochondria, and peroxisome functions influence a wide range of processes outside of the organelles themselves. While knowledge of the network of metabolic pathways and their intraorganellar redox status regulation has increased in the last years, little is known about the interorganellar redox signals coordinating these networks. A current challenge is, therefore, synthesizing our knowledge and planning experiments that tackle redox status regulation at both inter- and intracellular levels.

FUTURE DIRECTIONS

Emerging tools are enabling ever-increasing spatiotemporal resolution of metabolism and imaging of redox status components. Broader application of these tools will likely greatly enhance our understanding of the interplay of redox status and metabolism as well as elucidating and characterizing signaling features thereof. We propose that such information will enable us to dissect the regulatory hierarchies that mediate the strict coupling of metabolism and redox status which, ultimately, determine plant growth and development.

Prediction of respective contribution of linear electron flow and PGR5-dependent cyclic electron flow to non-photochemical quenching induction

In chloroplasts, regulated formation of the proton gradient across the thylakoid membrane (ΔpH) is important for controlling non-photochemical quenching (NPQ), which is crucial for plants to perform photosynthesis under fluctuating light conditions. The ΔpH is generated by two electron flows: the linear electron flow (LEF) and the cyclic electron flow (CEF). The Arabidopsis CEF mutant, pgr5, showed significantly lower NPQ values than those observed in WT, indicating that ΔpH, generated by the PGR5-dependent CEF, has a crucial role in controlling NPQ. However, the respective significance of LEF and CEF for ΔpH formation is largely unknown. Here we applied computer simulation to reproduce NPQ induction kinetics and estimate the respective contribution of LEF and PGR5-dependent CEF to the dynamics of ΔpH formation. The results indicate that the contribution of CEF to total ΔpH formation for induction of NPQ varies from 60-80%. The simulation also suggested a role of the PGR5-dependent CEF in accelerating electron transfer in the cytochrome b6f complex.

The chloroplast ATP synthase features the characteristic redox regulation machinery

SIGNIFICANCE

Regulation of the activity of the chloroplast ATP synthase is largely accomplished by the chloroplast thioredoxin system, the main redox regulation system in chloroplasts, which is directly coupled to the photosynthetic reaction. We review the current understanding of the redox regulation system of the chloroplast ATP synthase.

RECENT ADVANCES

The thioredoxin-targeted portion of the ATP synthase consists of two cysteines located on the central axis subunit γ. The redox state of these two cysteines is under the influence of chloroplast thioredoxin, which directly controls rotation during catalysis by inducing a conformational change in this subunit. The molecular mechanism of redox regulation of the chloroplast ATP synthase has recently been determined.

CRITICAL ISSUES

Regulation of the activity of the chloroplast ATP synthase is critical in driving efficiency into the ATP synthesis reaction in chloroplasts.

FUTURE DIRECTIONS

The molecular architecture of the chloroplast ATP synthase, which confers redox regulatory properties requires further investigation, in light of the molecular structure of the enzyme complex as well as the physiological significance of the regulation system.

Light- and metabolism-related regulation of the chloroplast ATP synthase has distinct mechanisms and functions

The chloroplast CF0-CF1-ATP synthase (ATP synthase) is activated in the light and inactivated in the dark by thioredoxin-mediated redox modulation of a disulfide bridge on its γ subunit. The activity of the ATP synthase is also fine-tuned during steady-state photosynthesis in response to metabolic changes, e.g. altering CO2 levels to adjust the thylakoid proton gradient and thus the regulation of light harvesting and electron transfer. The mechanism of this fine-tuning is unknown. We test here the possibility that it also involves redox modulation. We found that modifying the Arabidopsis thaliana γ subunit by mutating three highly conserved acidic amino acids, D211V, E212L, and E226L, resulted in a mutant, termed mothra, in which ATP synthase which lacked light-dark regulation had relatively small effects on maximal activity in vivo. In situ equilibrium redox titrations and thiol redox-sensitive labeling studies showed that the γ subunit disulfide/sulfhydryl couple in the modified ATP synthase has a more reducing redox potential and thus remains predominantly oxidized under physiological conditions, implying that the highly conserved acidic residues in the γ subunit influence thiol redox potential. In contrast to its altered light-dark regulation, mothra retained wild-type fine-tuning of ATP synthase activity in response to changes in ambient CO2 concentrations, indicating that the light-dark- and metabolic-related regulation occur through different mechanisms, possibly via small molecule allosteric effectors or covalent modification.