Unassembled subunits of the photosynthetic oxygen-evolving complex present in the thylakoid lumen are long-lived and assembly-competent

Single-cell microfluidics in combination with chlorophyll a fluorescence measurements to assess the lifetime of the Chlamydomonas PSBO protein

PSBO is an essential subunit of the oxygen-evolving complex and we recently demonstrated that its lifetime depends on environmental conditions in Chlamydomonas reinhardtii. To assess PSBO lifetime with a high time resolution, we employed (1) a microfluidic platform enabling the trapping of single cells and the parallel measurement of photosynthetic activity, and (2) a nitrate-inducible PSBO amiRNA line. Our microfluidic platform allowed the rapid replacement of the nutrient solution necessary for induction. It also enabled the precise monitoring of the decline in the Fv/Fm value, reflecting PSBO loss. We found that in the dark, at medium and high light intensity, the Fv/Fm value decreased with halftimes of about 25, 12.5, and 5 h, respectively. We also observed that photosynthetic activity was better sustained upon carbon limitation. In the absence of acetate, the halftimes of Fv/Fm diminishment doubled to quadrupled compared with the control, acetate-supplied cultures.

The lifetime of the oxygen-evolving complex subunit PSBO depends on light intensity and carbon availability in Chlamydomonas

PSBO is essential for the assembly of the oxygen-evolving complex in plants and green algae. Despite its importance, we lack essential information on its lifetime and how it depends on the environmental conditions. We have generated nitrate-inducible PSBO amiRNA lines in the green alga Chlamydomonas reinhardtii. Transgenic strains grew normally under non-inducing conditions, and their photosynthetic performance was comparable to the control strain. Upon induction of the PSBO amiRNA constructs, cell division halted. In acetate-containing medium, cellular PSBO protein levels decreased by 60% within 24 h in the dark, by 75% in moderate light, and in high light, the protein completely degraded. Consequently, the photosynthetic apparatus became strongly damaged, probably due to 'donor-side-induced photoinhibition', and cellular ultrastructure was also severely affected. However, in the absence of acetate during induction, PSBO was remarkably stable at all light intensities and less substantial changes occurred in photosynthesis. Our results demonstrate that the lifetime of PSBO strongly depends on the light intensity and carbon availability, and thus, on the metabolic status of the cells. We also confirm that PSBO is required for photosystem II stability in C. reinhardtii and demonstrate that its specific loss also entails substantial changes in cell morphology and cell cycle.

Characterization of the Free and Membrane-Associated Fractions of the Thylakoid Lumen Proteome in Arabidopsis thaliana

The thylakoid lumen houses proteins that are vital for photosynthetic electron transport, including water-splitting at photosystem (PS) II and shuttling of electrons from cytochrome b₆f to PSI. Other lumen proteins maintain photosynthetic activity through biogenesis and turnover of PSII complexes. Although all lumen proteins are soluble, these known details have highlighted interactions of some lumen proteins with thylakoid membranes or thylakoid-intrinsic proteins. Meanwhile, the functional details of most lumen proteins, as well as their distribution between the soluble and membrane-associated lumen fractions, remain unknown. The current study isolated the soluble free lumen (FL) and membrane-associated lumen (MAL) fractions from Arabidopsis thaliana, and used gel- and mass spectrometry-based proteomics methods to analyze the contents of each proteome. These results identified 60 lumenal proteins, and clearly distinguished the difference between the FL and MAL proteomes. The most abundant proteins in the FL fraction were involved in PSII assembly and repair, while the MAL proteome was enriched in proteins that support the oxygen-evolving complex (OEC). Novel proteins, including a new PsbP domain-containing isoform, as well as several novel post-translational modifications and N-termini, are reported, and bi-dimensional separation of the lumen proteome identified several protein oligomers in the thylakoid lumen.

Ascorbate inactivates the oxygen-evolving complex in prolonged darkness

Ascorbate (Asc, vitamin C) is an essential metabolite participating in multiple physiological processes of plants, including environmental stress management and development. In this study, we acquired knowledge on the role of Asc in dark-induced leaf senescence using Arabidopsis thaliana as a model organism. One of the earliest effects of prolonged darkness is the inactivation of oxygen-evolving complexes (OEC) as demonstrated here by fast chlorophyll a fluorescence and thermoluminescence measurements. We found that inactivation of OEC due to prolonged darkness was attenuated in the Asc-deficient vtc2-4 mutant. On the other hand, the severe photosynthetic phenotype of a psbo1 knockout mutant, lacking the major extrinsic OEC subunit PSBO1, was further aggravated upon a 24-h dark treatment. The psbr mutant, devoid of the PSBR subunit of OEC, performed only slightly disturbed photosynthetic activity under normal growth conditions, whereas it showed a strongly diminished B thermoluminescence band upon dark treatment. We have also generated a double psbo1 vtc2 mutant, and it showed a slightly milder photosynthetic phenotype than the single psbo1 mutant. Our results, therefore, suggest that Asc leads to the inactivation of OEC in prolonged darkness by over-reducing the Mn-complex that is probably enabled by a dark-induced dissociation of the extrinsic OEC subunits. Our study is an example that Asc may negatively affect certain cellular processes and thus its concentration and localization need to be highly controlled.

Insights into the Cyanobacterial Deg/HtrA Proteases

Proteins are the main machinery for all living processes in a cell; they provide structural elements, regulate biochemical reactions as enzymes, and are the interface to the outside as receptors and transporters. Like any other machinery proteins have to be assembled correctly and need maintenance after damage, e.g., caused by changes in environmental conditions, genetic mutations, and limitations in the availability of cofactors. Proteases and chaperones help in repair, assembly, and folding of damaged and misfolded protein complexes cost-effective, with low energy investment compared with neo-synthesis. Despite their importance for viability, the specific biological role of most proteases in vivo is largely unknown. Deg/HtrA proteases, a family of serine-type ATP-independent proteases, have been shown in higher plants to be involved in the degradation of the Photosystem II reaction center protein D1. The objective of this review is to highlight the structure and function of their cyanobacterial orthologs. Homology modeling was used to find specific features of the SynDeg/HtrA proteases of Synechocystis sp. PCC 6803. Based on the available data concerning their location and their physiological substrates we conclude that these Deg proteases not only have important housekeeping and chaperone functions within the cell, but also are needed for remodeling the cell exterior.

The N-terminal sequence of the extrinsic PsbP protein modulates the redox potential of Cyt b559 in photosystem II

The PsbP protein, an extrinsic subunit of photosystem II (PSII) in green plants, is known to induce a conformational change around the catalytic Mn₄CaO₅ cluster securing the binding of Ca²⁺ and Cl^– in PSII. PsbP has multiple interactions with the membrane subunits of PSII, but how these affect the structure and function of PSII requires clarification. Here, we focus on the interactions between the N-terminal residues of PsbP and the α subunit of Cytochrome (Cyt) b₅₅₉ (PsbE). A key observation was that a peptide fragment formed of the first N-terminal 15 residues of PsbP, ‘pN15’, was able to convert Cyt b₅₅₉ into its HP form. Interestingly, addition of pN15 to NaCl-washed PSII membranes decreased PSII’s oxygen-evolving activity, even in the presence of saturating Ca²⁺ and Cl^– ions. In fact, pN15 reversibly inhibited the S₁ to S₂ transition of the OEC in PSII. These data suggest that pN15 can modulate the redox property of Cyt b₅₅₉ involved in the side-electron pathway in PSII. This potential change of Cyt b₅₅₉, in the absence of the C-terminal domain of PsbP, however, would interfere with any electron donation from the Mn₄CaO₅ cluster, leading to the possibility that multiple interactions of PsbP, binding to PSII, have distinct roles in regulating electron transfer within PSII.

Quality Control of Photosystem II: The Mechanisms for Avoidance and Tolerance of Light and Heat Stresses are Closely Linked to Membrane Fluidity of the Thylakoids

When oxygenic photosynthetic organisms are exposed to excessive light and/or heat, Photosystem II is damaged and electron transport is blocked. In these events, reactive oxygen species, endogenous radicals and lipid peroxidation products generated by photochemical reaction and/or heat cause the damage. Regarding light stress, plants first dissipate excessive light energy captured by light-harvesting chlorophyll protein complexes as heat to avoid the hazards, but once light stress is unavoidable, they tolerate the stress by concentrating damage in a particular protein in photosystem II, i.e., the reaction-center binding D1 protein of Photosystem II. The damaged D1 is removed by specific proteases and replaced with a new copy produced through de novo synthesis (reversible photoinhibition). When light intensity becomes extremely high, irreversible aggregation of D1 occurs and thereby D1 turnover is prevented. Once the aggregated products accumulate in Photosystem II complexes, removal of them by proteases is difficult, and irreversible inhibition of Photosystem II takes place (irreversible photoinhibition). Important is that various aspects of both the reversible and irreversible photoinhibition are highly dependent on the membrane fluidity of the thylakoids. Heat stress-induced inactivation of photosystem II is an irreversible process, which may be also affected by the fluidity of the thylakoid membranes. Here I describe why the membrane fluidity is a key to regulate the avoidance and tolerance of Photosystem II on environmental stresses.

Dark-adapted spinach thylakoid protein heterogeneity offers insights into the photosystem II repair cycle

In higher plants, thylakoid membrane protein complexes show lateral heterogeneity in their distribution: photosystem (PS) II complexes are mostly located in grana stacks, whereas PSI and adenosine triphosphate (ATP) synthase are mostly found in the stroma-exposed thylakoids. However, recent research has revealed strong dynamics in distribution of photosystems and their light harvesting antenna along the thylakoid membrane. Here, the dark-adapted spinach (Spinacia oleracea L.) thylakoid network was mechanically fragmented and the composition of distinct PSII-related proteins in various thylakoid subdomains was analyzed in order to get more insights into the composition and localization of various PSII subcomplexes and auxiliary proteins during the PSII repair cycle. Most of the PSII subunits followed rather equal distribution with roughly 70% of the proteins located collectively in the grana thylakoids and grana margins; however, the low molecular mass subunits PsbW and PsbX as well as the PsbS proteins were found to be more exclusively located in grana thylakoids. The auxiliary proteins assisting in repair cycle of PSII were mostly located in stroma-exposed thylakoids, with the exception of THYLAKOID LUMEN PROTEIN OF 18.3 (TLP18.3), which was more evenly distributed between the grana and stroma thylakoids. The TL29 protein was present exclusively in grana thylakoids. Intriguingly, PROTON GRADIENT REGULATION5 (PGR5) was found to be distributed quite evenly between grana and stroma thylakoids, whereas PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1) was highly enriched in the stroma thylakoids and practically missing from the grana cores. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

The PsbP family of proteins

The PsbP family of proteins consists of 11 evolutionarily related thylakoid lumenal components. These include the archetypal PsbP protein, which is an extrinsic subunit of eukaryotic photosystem II, three PsbP-like proteins (CyanoP of the prokaryotic cyanobacteria and green oxyphotobacteria, and the PPL1 and PPL2 proteins found in many eukaryotes), and seven PsbP-domain (PPD) proteins (PPD1-PPD7, most of which are found in the green plant lineage). All of these possess significant sequence and structural homologies while having very diverse functions. While the PsbP protein has been extensively studied and plays a functional role in the optimization of photosynthetic oxygen evolution at physiological calcium and chloride concentrations, the molecular functions of the other family members are poorly understood. Recent investigations have begun to illuminate the roles that these proteins play in membrane protein complex assembly/stability, hormone biosynthesis, and other metabolic processes. In this review we have examined this functional information within the context of recent advances examining the structure of these components.

Degradation of PsbO by the Deg protease HhoA Is thioredoxin dependent

The widely distributed members of the Deg/HtrA protease family play an important role in the proteolysis of misfolded and damaged proteins. Here we show that the Deg protease rHhoA is able to degrade PsbO, the extrinsic protein of the Photosystem II (PSII) oxygen-evolving complex in Synechocystis sp. PCC 6803 and in spinach. PsbO is known to be stable in its oxidized form, but after reduction by thioredoxin it became a substrate for recombinant HhoA (rHhoA). rHhoA cleaved reduced eukaryotic (specifically, spinach) PsbO at defined sites and created distinct PsbO fragments that were not further degraded. As for the corresponding prokaryotic substrate (reduced PsbO of Synechocystis sp. PCC 6803), no PsbO fragments were observed. Assembly to PSII protected PsbO from degradation. For Synechocystis sp. PCC 6803, our results show that HhoA, HhoB, and HtrA are localized in the periplasma and/or at the thylakoid membrane. In agreement with the idea that PsbO could be a physiological substrate for Deg proteases, part of the cellular fraction of the three Deg proteases of Synechocystis sp. PCC 6803 (HhoA, HhoB, and HtrA) was detected in the PSII-enriched membrane fraction.

Chloroplast ultrastructure and thylakoid polypeptide composition are affected by different salt concentrations in the halophytic plant Arthrocnemum macrostachyum

The effect of different external salt concentrations, from 0 mM to 1030 mM NaCl, on photosynthetic complexes and chloroplast ultrastructure in the halophyte Arthrocnemum macrostachyum was studied. Photosystem II, but not Photosystem I or cytochrome b6/f, was affected by salt treatment. We found that the PsbQ protein was never expressed, whereas the amounts of PsbP and PsbO were influenced by salt in a complex way. Analyses of Photosystem II intrinsic proteins showed an uneven degradation of subunits with a loss of about 50% of centres in the 0 mM NaCl treated sample. Also the shape of chloroplasts, as well as the organization of thylakoid membranes were affected by NaCl concentration, with many grana containing few thylakoids at 1030 mM NaCl and thicker grana and numerous swollen thylakoids at 0 mM NaCl. The PsbQ protein was found to be depleted also in thylakoids from other halophytes.

Effect of protein modification by malondialdehyde on the interaction between the oxygen-evolving complex 33 kDa protein and photosystem II core proteins

Previously we observed that the oxygen-evolving complex 33 kDa protein (OEC33) which stabilizes the Mn cluster in photosystem II (PSII), was modified with malondialdehyde (MDA), an end-product of peroxidized polyunsaturated fatty acids, and the modification increased in heat-stressed plants (Yamauchi et al. 2008). In this study, we examined whether the modification of OEC33 with MDA affects its binding to the PSII complex and causes inactivation of the oxygen-evolving complex. Purified OEC33 and PSII membranes that had been removed of extrinsic proteins of the oxygen-evolving complex (PSIIOEE) of spinach (Spinacia oleracea) were separately treated with MDA. The binding was diminished when both OEC33 and PSIIOEE were modified, but when only OEC33 or PSIIOEE was treated, the binding was not impaired. In the experiment using thylakoid membranes, release of OEC33 from PSII and corresponding loss of oxygen-evolving activity were observed when thylakoid membranes were treated with MDA at 40 degrees C but not at 25 degrees C. In spinach leaves treated at 40 degrees C under light, maximal efficiency of PSII photochemistry (F(v)/F(m) ratio of chlorophyll fluorescence) and oxygen-evolving activity decreased. Simultaneously, MDA contents in heat-stressed leaves increased, and OEC33 and PSII core proteins including 47 and 43 kDa chlorophyll-binding proteins were modified with MDA. In contrast, these changes were to a lesser extent at 40 degrees C in the dark. These results suggest that MDA modification of PSII proteins causes release of OEC33 from PSII and it is promoted in heat and oxidative conditions.

The significance of protein maturation by plastidic type I signal peptidase 1 for thylakoid development in Arabidopsis chloroplasts

Thylakoids are the chloroplast internal membrane systems that house light-harvesting and electron transport reactions. Despite the important functions and well-studied constituents of thylakoids, the molecular mechanism of their development remains largely elusive. A recent genetic study has demonstrated that plastidic type I signal peptidase 1 (Plsp1) is vital for proper thylakoid development in Arabidopsis (Arabidopsis thaliana) chloroplasts. Plsp1 was also shown to be necessary for processing of an envelope protein, Toc75, and a thylakoid lumenal protein, OE33; however, the relevance of the protein maturation in both of the two distinct subcompartments for proper chloroplast development remained unknown. Here, we conducted an extensive analysis of the plsp1-null mutant to address the significance of lumenal protein maturation in thylakoid development. Plastids that lack Plsp1 were found to accumulate vesicles of variable sizes in the stroma. Analyses of the mutant plastids revealed that the lack of Plsp1 causes a reduction in accumulation of thylakoid proteins and that Plsp1 is involved in maturation of two additional lumenal proteins, OE23 and plastocyanin. Further immunoblotting and electron microscopy immunolocalization studies showed that OE33 associates with the stromal vesicles of the mutant plastids. Finally, we used a genetic complementation system to demonstrate that accumulation of improperly processed forms of Toc75 in the plastid envelope does not disrupt normal plant development. These results suggest that proper maturation of lumenal proteins may be a key process for correct assembly of thylakoids.

The TL29 protein is lumen located, associated with PSII and not an ascorbate peroxidase

The TL29 protein is one of the more abundant proteins in the thylakoid lumen of plant chloroplasts. Based on its sequence homology to ascorbate peroxidases, but without any supporting biochemical evidence, TL29 was suggested to be involved in the plant defense system against reactive oxygen species and consequently renamed to APX4. Our in vivo and in vitro analyses failed to show any peroxidase activity associated with TL29; it bound neither heme nor ascorbate. Recombinant overexpressed TL29 had no ascorbate-dependent peroxidase activity, and various mutational analyses aiming to convert TL29 into an ascorbate peroxidase failed. Furthermore, in the thylakoid lumen no such activity could be associated with TL29 and, additionally, TL29 knock-out mutants did not show any decreased peroxidase activity or increased content of radical oxygen species when grown under light stress. Instead we could show that TL29 is a lumen-located component associated with PSII.

Knockdown of the PsbP protein does not prevent assembly of the dimeric PSII core complex but impairs accumulation of photosystem II supercomplexes in tobacco

The PsbP protein is an extrinsic subunit of photosystem II (PSII) specifically found in land plants and green algae. Using PsbP-RNAi tobacco, we have investigated effects of PsbP knockdown on protein supercomplex organization within the thylakoid membranes and photosynthetic properties of PSII. In PsbP-RNAi leaves, PSII dimers binding the extrinsic PsbO protein could be formed, while the light-harvesting complex II (LHCII)-PSII supercomplexes were severely decreased. Furthermore, LHCII and major PSII subunits were significantly dephosphorylated. Electron microscopic analysis showed that thylakoid grana stacking in PsbP-RNAi chloroplast was largely disordered and appeared similar to the stromally-exposed or marginal regions of wild-type thylakoids. Knockdown of PsbP modified both the donor and acceptor sides of PSII; In addition to the lower water-splitting activity, the primary quinone Q(A) in PSII was significantly reduced even when the photosystem I reaction center (P700) was noticeably oxidized, and thermoluminescence studies suggested the stabilization of the charged pair, S(2)/Q(A)(-). These data indicate that assembly and/or maintenance of the functional MnCa cluster is perturbed in absence of PsbP, which impairs accumulation of final active forms of PSII supercomplexes.

Light induced changes in protein expression and uniform regulation of transcription in the thylakoid lumen of Arabidopsis thaliana

In plants oxygenic photosynthesis is performed by large protein complexes found in the thylakoid membranes of chloroplasts. The soluble thylakoid lumen space is a narrow and compressed region within the thylakoid membrane which contains 80–200 proteins. Because the thylakoid lumen proteins are in close proximity to the protein complexes of photosynthesis, it is reasonable to assume that the lumen proteins are highly influenced by the presence of light.

To identify light regulated proteins in the thylakoid lumen of Arabidopsis thaliana we developed a faster thylakoid preparation and combined this with difference gel electrophoresis (DIGE) of dark-adapted and light-adapted lumen proteomes. The DIGE experiments revealed that 19 lumen proteins exhibit increased relative protein levels after eight hour light exposure. Among the proteins showing increased abundance were the PsbP and PsbQ subunits of Photosystem II, major plastocyanin and several other proteins of known or unknown function. In addition, co-expression analysis of publicly available transcriptomic data showed that the co-regulation of lumen protein expression is not limited to light but rather that lumen protein genes exhibit a high uniformity of expression.

The large proportion of thylakoid lumen proteins displaying increased abundance in light-adapted plants, taken together with the observed uniform regulation of transcription, implies that the majority of thylakoid lumen proteins have functions that are related to photosynthetic activity. This is the first time that an analysis of the differences in protein level during a normal day/night cycle has been performed and it shows that even a normal cycle of light significantly influences the thylakoid lumen proteome. In this study we also show for the first time, using co-expression analysis, that the prevalent lumenal chloroplast proteins are very similarly regulated at the level of transcription.

Morphological and molecular characterization of a spontaneously tuberizing potato mutant: an insight into the regulatory mechanisms of tuber induction

BACKGROUND

Tuberization in potato (Solanum tuberosum L.) represents a morphogenetic transition of stolon growth to tuber formation, which is under complex environmental and endogenous regulation. In the present work, we studied the regulatory mechanisms and the role of different morphogenetic factors in a newly isolated potato mutant, which exhibited spontaneous tuberization (ST). The ST mutant was characterized in detail at morphological, physiological and biochemical levels.

RESULTS

Tuberization of the ST mutant grown in the soil was photoperiod-insensitive; predominantly sessile tubers formed directly from axillary buds even under continuous light. Single-node cuttings of the ST mutant cultured in vitro frequently formed tubers or basal tuber-like swellings instead of normal shoots under conditions routinely used for shoot propagation. The tuberization response of ST cuttings under light was dependent on sucrose, the concentration of which had to exceed certain threshold that inversely correlated with irradiance. Gibberellic acid prevented tuberization of ST cuttings, but failed to restore normal shoot phenotype and caused severe malformations. Carbohydrate analysis showed increased levels of both soluble sugars and starch in ST plants, with altered carbohydrate partitioning and metabolism. Comparative proteomic analysis revealed only a few differences between ST- and wild-type plants, primary amongst which seemed to be the absence of an isoform of manganese-stabilizing protein, a key subunit of photosystem II.

CONCLUSION

ST mutant exhibits complex developmental and phenotypic modifications, with features that are typical for plants strongly induced to tuberize. These changes are likely to be related to altered regulation of photosynthesis and carbohydrate metabolism rather than impaired transduction of inhibitory gibberellin or photoperiod-based signals. The effect of gibberellins on tuberization of ST mutant suggests that gibberellins inhibit tuberization downstream of the inductive effects of sucrose and other positive factors.

The effects of simultaneous RNAi suppression of PsbO and PsbP protein expression in photosystem II of Arabidopsis

Interfering RNA was used to suppress simultaneously the expression of the four genes which encode the PsbO and PsbP proteins of Photosystem II in Arabidopsis (PsbO: At5g66570, At3g50820 and PsbP: At1g06680, At2g30790). A phenotypic series of transgenic plants was obtained that expressed variable amounts of the PsbO proteins and undetectable amounts of the PsbP proteins. Immunological studies indicated that the loss of PsbP expression was correlated with the loss of expression of the PsbQ, D2, and CP47 proteins, while the loss of PsbO expression was correlated with the loss of expression of the D1 and CP43 proteins. Q(A)(-) reoxidation kinetics in the absence of DCMU indicated that the slowing of electron transfer from Q(A)(-) to Q(B) was correlated with the loss of the PsbP protein. Q(A)(-) reoxidation kinetics in the presence of DCMU indicated that charge recombination between Q(A)(-) and donor side components of the photosystem was retarded in all of the mutants. Decreasing amounts of the PsbO protein in the absence of the PsbP component also led to a progressive loss of variable fluorescence yield (F(V)/F(M)). During fluorescence induction, the loss of PsbP was correlated with a more rapid O to J transition and a loss of the J to I transition. These results indicate that the losses of the PsbO and PsbP proteins differentially affect separate protein components and different PS II functions and can do so, apparently, in the same plant.

Structure, function, and evolution of the PsbP protein family in higher plants

The PsbP is a thylakoid lumenal subunit of photosystem II (PSII), which has developed specifically in higher plants and green algae. In higher plants, the molecular function of PsbP has been intensively investigated by release-reconstitution experiments in vitro. Recently, solution of a high-resolution structure of PsbP has enabled investigation of structure-function relationships, and efficient gene-silencing techniques have demonstrated the crucial role of PsbP in PSII activity in vivo. Furthermore, genomic and proteomic studies have shown that PsbP belongs to the divergent PsbP protein family, which consists of about 10 members in model plants such as Arabidopsis and rice. Characterization of the molecular function of PsbP homologs using Arabidopsis mutants suggests that each plays a distinct and important function in maintaining photosynthetic electron transfer. In this review, recent findings regarding the molecular functions of PsbP and other PsbP homologs in higher plants are summarized, and the molecular evolution of these proteins is discussed.

Quality control of photosystem II: impact of light and heat stresses

Photosystem II is vulnerable to various abiotic stresses such as strong visible light and heat. Under both stresses, the damage seems to be triggered by reactive oxygen species, and the most critical damage occurs in the reaction center-binding D1 protein. Recent progress has been made in identifying the protease involved in the degradation of the photo- or heat-damaged D1 protein, the ATP-dependent metalloprotease FtsH. Another important result has been the discovery that the damaged D1 protein aggregates with nearby polypeptides such as the D2 protein and the antenna chlorophyll-binding protein CP43. The degradation and aggregation of the D1 protein occur simultaneously, but the relationship between the two is not known. We suggest that phosphorylation and dephosphorylation of the D1 protein, as well as the binding of the extrinsic PsbO protein to Photosystem II, play regulatory roles in directing the damaged D1 protein to the two alternative pathways.

Hypothesis: the peroxydicarbonic acid cycle in photosynthetic oxygen evolution

Peroxydicarbonic acid (Podca), a proposed intermediate in photosynthetic oxygen evolution, was synthesized electrochemically. Consistent with literature descriptions of this compound, it was shown to be a highly reactive molecule, spontaneously hydrolyzed to H2O2, as well as susceptible to oxidative and reductive decomposition. In the presence of Mn2+ or Co2+, Podca was quickly broken down with release of O2. The liberation of O2, however, was partially suppressed at high O2 concentrations. In the presence of Ca-washed photosystem II-enriched membranes lacking extrinsic proteins, Podca was decomposed with the release of O2, but only under conditions favoring photosynthetic electron flow (light plus a Hill oxidant). A model is proposed that details how peroxydicarbonic acid could act as an oxygen-evolving intermediate. The hypothesis is consistent with the well-established Kok model and with recent findings related to the chemistry of oxygen evolution.

TLP18.3, a novel thylakoid lumen protein regulating photosystem II repair cycle

A proteome analysis of Arabidopsis thaliana thylakoid-associated polysome nascent chain complexes was performed to find novel proteins involved in the biogenesis, maintenance and turnover of thylakoid protein complexes, in particular the PSII (photosystem II) complex, which exhibits a high turnover rate. Four unknown proteins were identified, of which TLP18.3 (thylakoid lumen protein of 18.3 kDa) was selected for further analysis. The Arabidopsis mutants (SALK_109618 and GABI-Kat 459D12) lacking the TLP18.3 protein showed higher susceptibility of PSII to photoinhibition. The increased susceptibility of DeltaTLP18.3 plants to high light probably originates from an inefficient reassembly of PSII monomers into dimers in the grana stacks, as well as from an impaired turnover of the D1 protein in stroma exposed thylakoids. Such dual function of the TLP18.3 protein is in accordance with its even distribution between the grana and stroma thylakoids. Notably, the lack of the TLP18.3 protein does not lead to a severe collapse of the PSII complexes, suggesting a redundancy of proteins assisting these particular repair steps to assure functional PSII. The DeltaTLP18.3 plants showed no clear visual phenotype under standard growth conditions, but when challenged by fluctuating light during growth, the retarded growth of DeltaTLP18.3 plants was evident.

Expression, assembly and auxiliary functions of photosystem II oxygen-evolving proteins in higher plants

The oxygen-evolving complex (OEC) of higher plant photosystem II (PSII) consists of an inorganic Mn(4)Ca cluster and three nuclear-encoded proteins, PsbO, PsbP and PsbQ. In this review, we focus on the assembly of these OEC proteins, and especially on the role of the small intrinsic PSII proteins and recently found "novel" PSII proteins in the assembly process. The numerous auxiliary functions suggested during the past few years for the OEC proteins will likewise be discussed. For example, besides being a manganese-stabilizing protein, PsbO has been found to bind calcium and GTP and possess a carbonic anhydrase activity. In addition, specific roles have been suggested for the two isoforms of the PsbO protein in Arabidopsis thaliana. PsbP and PsbQ seem to play an additional role in the formation of PSII supercomplexes and in grana stacking, besides their originally recognized role in providing a proper calcium and chloride ion concentration for water splitting.

The PsbP protein is required for photosystem II complex assembly/stability and photoautotrophy in Arabidopsis thaliana

Interfering RNA was used to suppress the expression of the genes At1g06680 and At2g30790 in Arabidopsis thaliana, which encode the PsbP-1 and PsbP-2 proteins, respectively, of photosystem II (PS II). A phenotypic series of transgenic plants was recovered that expressed intermediate and low amounts of PsbP. Chlorophyll fluorescence induction and Q(A)(-) decay kinetics analyses were performed. Decreasing amounts of expressed PsbP protein led to the progressive loss of variable fluorescence and a marked decrease in the fluorescence quantum yield (F(V)/F(M)). This was primarily due to the loss of the J to I transition. Analysis of the fast fluorescence rise kinetics indicated no significant change in the number of PS II(beta) centers present in the mutants. Analysis of Q(A)(-) decay kinetics in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea indicated a defect in electron transfer from Q(A)(-) to Q(B), whereas experiments performed in the presence of this herbicide indicated that charge recombination between Q(A)(-) and the oxygen-evolving complex was seriously retarded in the plants that expressed low amounts of the PsbP protein. These results demonstrate that the amount of functional PS II reaction centers is compromised in the plants that exhibited intermediate and low amounts of the PsbP protein. Plants that lacked detectable PsbP were unable to survive in the absence of sucrose, indicating that the PsbP protein is required for photoautotrophy. Immunological analysis of the PS II protein complement indicated that significant losses of the CP47 and D2 proteins, and intermediate losses of the CP43 and D1 proteins, occurred in the absence of the PsbP protein. This demonstrates that the extrinsic protein PsbP is required for PS II core assembly/stability.

Differing responses of the two forms of photosystem II carbonic anhydrase to chloride, cations, and pH

The effects of Cl(-), Mn(2+), Ca(2+), and pH on extrinsic and intrinsic photosystem II carbonic anhydrase activity were compared. Under the conditions of our in vitro experiments, extrinsic CA activity, located on the OEC33 protein, was optimum at about 30 mM Cl(-), and strongly inhibited above this concentration. This enzyme is activated by Mn(2+) and stimulated somewhat by Ca(2+). The OEC33 showed dehydration activity that is optimum at pH 6 or below. In contrast, intrinsic CA activity found in the PSII complex after removal of extrinsic proteins was stimulated by Cl(-) up to 0.4 M. Ca(2+) appears to be the required cofactor, which implies that the location of the intrinsic CA activity is in the immediate vicinity of the CaMn(4) complex. Up to now, intrinsic CA has shown only hydration activity that is nearly pH independent.

The Prx Q protein ofArabidopsis thalianais a member of the luminal chloroplast proteome

Peroxiredoxins have been discovered in many organisms ranging from eubacteria to mammals, and their known biological functions include both oxidant defense and signal transduction. The genome of Arabidopsis thaliana encodes for ten individual peroxiredoxins, of which four are located in the chloroplast. The best-characterized member of the chloroplast peroxiredoxins is 2-Cys Prx that is associated with the stroma side of the thylakoid membrane and is considered to participate in antioxidant defense and protection of photosynthesis. This study addressed the chloroplast peroxiredoxin Prx Q and showed that its subcellular location is the lumen of the thylakoid membrane. To get insight in the biological function of the Prx Q protein of Arabidopsis, the protein levels of the Prx Q protein in thylakoid membranes were studied under different light conditions and oxidative stress. A T-DNA knockout mutant of Prx Q did not show any visible phenotype and had normal photosynthetic performance with a slightly increased oxygen evolving activity.

PsbR, a missing link in the assembly of the oxygen-evolving complex of plant photosystem II

The oxygen-evolving complex of eukaryotic photosystem II (PSII) consists of three extrinsic nuclear-encoded subunits, PsbO (33 kDa), PsbP (23 kDa), and PsbQ (17 kDa). Additionally, the 10-kDa PsbR protein has been found in plant PSII and anticipated to play a role in water oxidation, yet the physiological significance of PsbR has remained obscure. Using the Arabidopsis psbR mutant, we showed that the light-saturated rate of oxygen evolution is strongly reduced in the absence of PsbR, particularly in low light-grown plants. Lack of PsbR also induced a reduction in the content of both the PsbP and the PsbQ proteins, and a near depletion of these proteins was observed under steady state low light conditions. This regulation occurred post-transcriptionally and likely involves a proteolytic degradation of the PsbP and PsbQ proteins in the absence of an assembly partner, proposed to be the PsbR protein. Stable assembly of PsbR in the PSII core complex was, in turn, shown to require a chloroplast-encoded intrinsic low molecular mass PSII subunit PsbJ. Our results provided evidence that PsbR is an important link in the PSII core complex for stable assembly of the oxygen-evolving complex protein PsbP, whereas the effects on the assembly of PsbQ are probably indirect. The physiological role of the PsbR, PsbP, and PsbQ proteins is discussed in light of their peculiar expression in response to growth light conditions.

Functional analysis of four members of the PsbP family in photosystem II in Nicotiana tabacum using differential RNA interference

Gene redundancy is frequently found in higher plants and complicates genetic analysis. In this study, a method referred to as 'differential RNA interference (dRNAi)' was used to investigate the psbP gene family in Nicotiana tabacum. PsbP is a membrane-extrinsic subunit of PSII and plays important roles in the water splitting reaction. N. tabacum has four psbP isogenes and the function of each isogene has not yet been characterized in vivo. To obtain transgenic tobacco plants with various amounts and compositions of PsbP members, the psbP isogenes were differentially silenced by RNA interference (RNAi) using the 3'-untranslated region (UTR) as a silencing trigger (dRNAi). In addition, the extra psbP genes without the 3'-UTR were complementarily transformed into the above silenced plants, which accumulated PsbP originating from the exogenous gene while differential silencing of the endogenous target was maintained. By using dRNAi and subsequent complementation (substitution) in dRNAi, we clearly demonstrated that, regardless of the of PsbP members that were accumulated, PSII activity was linearly correlated with the total amount of PsbP. Therefore, we concluded that the protein functions of the PsbP members in N. tabacum are equivalent in vivo, whereas full expression of the four isogenes is required for optimum PSII activity. These results demonstrate that the use of dRNAi and subsequent complementation/substitution in dRNAi would provide a new experimental approach for studying the function of multigene families in plants.

Carbonic anhydrase activity of the photosystem II OEC33 protein from pea

The purpose of this study was to identify the location of one of the two sources of carbonic anhydrase (CA) activity associated with the PSII complex in chloroplast membranes. We tested the hypothesis that the extrinsic 33 kDa protein, OEC33, associated with the oxygen-evolving complex (OEC), is one source of CA activity. We found that precursor OEC33 expressed in Escherichia coli exhibits CA activity, but the expressed precursors of OEC24 or OEC17 do not. The CA activity of OEC33 remained after treatment at 90 degrees C for 15 min. Additional biochemical evidence supports the hypothesis. Only those wash treatments that remove the OEC33 from PSII also remove CA activity. Both immunoblot and CA activity show that the CA tracks the OEC33, in parallel, when PSII undergoes washing at different CaCl2 concentrations. The OEC33 protein purified by HiTrap Q anion exchange chromatography has CA activity that is inhibited by an antibody against OEC33. PSII membranes washed with 1 M CaCl2 to remove OEC33 can be reconstituted either with extracted, purified, OEC33 or with the E. coli-expressed precursor OEC33. Reconstitution partially restores both oxygen evolution and CA activity. For maximal CA activity, OEC33 requires manganese as a cofactor.

Large-scale translocation reversal within the thylakoid Tat system in vivo

In vitro import assays have shown that the thylakoid twin-arginine translocase (Tat) system transports folded proteins in a unidirectional manner. Here, we expressed a natural substrate, pre-23K, and a 23K presequence-green fluorescent protein (GFP) chimera in vivo in tobacco protoplasts. Both are imported into chloroplasts, targeted to the thylakoids, and processed to the mature size by the lumen-facing processing peptidase. However, the vast majority of mature GFP and about half of the 23K are then returned to the stroma. Mutations in the twin-arginine motif block thylakoid targeting and maturation, confirming an involvement of the Tat apparatus. Mutation of the processing site yields membrane-associated intermediate-size protein in vivo, indicating a delayed reversal of translocation to the stroma and suggesting a longer lived interaction with the Tat machinery. We conclude that, in vivo, the Tat system can reject substrates at a late stage in translocation and on a very large scale, indicating the influence of factors that are absent in reconstitution assays.

The Manganese-stabilizing Protein Is Required for Photosystem II Assembly/Stability and Photoautotrophy in Higher Plants

Interfering RNA was used to suppress the expression of two genes that encode the manganese-stabilizing protein of photosystem II in Arabidopsis thaliana, MSP-1 (encoded by psbO-1, At5g66570), and MSP-2 (encoded by psbO-2, At3g50820). A phenotypic series of transgenic plants was recovered that expressed high, intermediate, and low amounts of these two manganese-stabilizing proteins. Chlorophyll fluorescence induction and decay analyses were performed. Decreasing amounts of expressed protein led to the progressive loss of variable fluorescence and a marked decrease in the fluorescence quantum yield (F(v)/F(m)) in both the absence and the presence of dichloromethylurea. This result indicated that the amount of functional photosystem II reaction centers was compromised in the plants that exhibited intermediate and low amounts of the manganese-stabilizing proteins. An analysis of the decay of the variable fluorescence in the presence of dichlorophenyldimethylurea indicated that charge recombination between Q ((A-)) and the S(2) state of the oxygen-evolving complex was seriously retarded in the plants that expressed low amounts of the manganese stabilizing proteins. This may have indicated a stabilization of the S(2) state in the absence of the extrinsic component. Immunological analysis of the photosystem II protein complement indicated that significant losses of the CP47, CP43, and D1 proteins occurred upon the loss of the manganese-stabilizing proteins. This indicated that these extrinsic proteins were required for photosystem II core assembly/stability. Additionally, although the quantity of the 24-kDa extrinsic protein was only modestly affected by the loss of the manganese-stabilizing proteins, the 17-kDa extrinsic protein dramatically decreased. The control proteins ribulose bisphosphate carboxylase and cytochrome f were not affected by the loss of the manganese-stabilizing proteins; the photosystem I PsaB protein, however, was significantly reduced in the low expressing transgenic plants. Finally, it was determined that the transgenic plants that expressed low amounts of the manganese-stabilizing proteins could not grow photoautotrophically.

Release and reactive-oxygen-mediated damage of the oxygen-evolving complex subunits of PSII during photoinhibition

Under photoinhibitory illumination of spinach PSII membranes, the oxygen-evolving complex subunits, OEC33, 24 and 18, were released from PSII. The liberated OEC33 and also OEC24 to a lesser extent were subsequently damaged and then exhibited smeared bands in SDS/urea-PAGE. Once deteriorated, OEC33 could not bind to PSII. The effects of scavengers and chelating reagents on the damage indicated that hydroxyl radicals generated from superoxide in the presence of metal ions were responsible for the damage. These results suggest that, like the D1 protein of the PSII reaction center complex, OEC subunits suffer oxidative damage and turnover under illumination.

Proteome map of the chloroplast lumen of Arabidopsis thaliana

The thylakoid membrane of the chloroplast is the center of oxygenic photosynthesis. To better understand the function of the luminal compartment within the thylakoid network, we have carried out a systematic characterization of the luminal thylakoid proteins from the model organism Arabidopsis thaliana. Our data show that the thylakoid lumen has its own specific proteome, of which 36 proteins were identified. Besides a large group of peptidyl-prolyl cis-trans isomerases and proteases, a family of novel PsbP domain proteins was found. An analysis of the luminal signal peptides showed that 19 of 36 luminal precursors were marked by a twin-arginine motif for import via the Tat pathway. To compare the model organism Arabidopsis with another typical higher plant, we investigated the proteome from the thylakoid lumen of spinach and found that the luminal proteins from both plants corresponded well. As a complement to our experimental investigation, we made a theoretical prediction of the luminal proteins from the whole Arabidopsis genome and estimated that the thylakoid lumen of the chloroplast contains approximately 80 proteins.

In vitro reconstitution of insertion and processing of cytochrome f in a homologous chloroplast translation system

Using a homologous chloroplast translation system, we have reconstituted insertion and processing of the chloroplast-encoded thylakoid protein cytochrome f (pCytf). Cross-linking demonstrated that pCytf nascent chains when attached to the 70 S ribosome tightly interact with cpSecA, but this is strictly dependent on thylakoid membranes and a functional signal peptide. This indicates that cpSecA is only operative in pCytf biogenesis when it is bound to the membrane, most likely as part of the Sec translocon. No evidence for interaction between the 54-kDa subunit of the chloroplast signal recognition particle (cpSRP) and the pCytf nascent chain could be detected, suggesting that pCytf, in contrast to the polytopic D1 protein, does not require cpSRP for targeting. Insertion of pCytf occurred only co-translationally, resulting in processing and accumulation of both the processed signal peptide and the mature protein in the thylakoid. This co-translational membrane insertion and processing required a functional signal peptide and was inhibited by azide, demonstrating that cpSecA is essential for translocation of the soluble luminal domain. pCytf also associated post-translationally with thylakoids, but the soluble N-terminal domain could not be translocated into the lumen. This is the first study in which synthesis, targeting, and insertion of a chloroplast-encoded thylakoid membrane protein is reconstituted from exogenous transcripts and using the chloroplast translational machinery.

Quality control of photosystem II

Photosystem II is particularly vulnerable to excess light. When illuminated with strong visible light, the reaction center D1 protein is damaged by reactive oxygen molecules or by endogenous cationic radicals generated by photochemical reactions, which is followed by proteolytic degradation of the damaged D1 protein. Homologs of prokaryotic proteases, such as ClpP, FtsH and DegP, have been identified in chloroplasts, and participation of the thylakoid-bound FtsH in the secondary degradation steps of the photodamaged D1 protein has been suggested. We found that cross-linking of the D1 protein with the D2 protein, the alpha-subunit of cytochrome b(559), and the antenna chlorophyll-binding protein CP43, occurs in parallel with the degradation of the D1 protein during the illumination of intact chloroplasts, thylakoids and photosystem II-enriched membranes. The cross-linked products are then digested by a stromal protease(s). These results indicate that the degradation of the photodamaged D1 protein proceeds through membrane-bound proteases and stromal proteases. Moreover, a 33-kDa subunit of oxygen-evolving complex (OEC), bound to the lumen side of photosystem II, regulates the formation of the cross-linked products of the D1 protein in donor-side photoinhibition of photosystem II. Thus, various proteases and protein components in different compartments in chloroplasts are implicated in the efficient turnover of the D1 protein, thus contributing to the control of the quality of photosystem II under light stress conditions.

The thylakoid lumen of chloroplasts. Isolation and characterization

The chloroplast compartment enclosed by the thylakoid membrane, the "lumen," is poorly characterized. The major aims of this work were to design a procedure for the isolation of the thylakoid lumen which could be generally used to characterize lumenal proteins. The preparation was a stepwise procedure in which thylakoid membranes were isolated from intact chloroplasts. Loosely associated thylakoid surface proteins were removed, and following Yeda press fragmentation the lumenal content was recovered in the supernatant following centrifugation. The purity and yield of lumenal proteins were determined using appropriate marker proteins specific for the different chloroplast compartments. Quantitative immunoblot analyses showed that the recovery of soluble lumenal proteins was 60-65% (as judged by the presence of plastocyanin), whereas contamination with stromal enzymes was less than 1% (ribulose-bisphosphate carboxylase) and negligible for thylakoid integral membrane proteins (D1 protein). Approximately 25 polypeptides were recovered in the lumenal fraction, of which several were identified for the first time. Enzymatic measurements and/or amino-terminal sequencing revealed the presence of proteolytic activities, violaxanthin de-epoxidase, polyphenol oxidase, peroxidase, as well as a novel prolyl cis/trans-isomerase.