Synthesis and assembly of thylakoid protein complexes: multiple assembly steps of photosystem II

AtCYP20-2 is an auxiliary protein of the chloroplast NAD(P)H dehydrogenase complex

AtCYP20-2 is one of 16 immunophilins in thylakoid lumen. The presence of the isomerase domain in AtCYP20-2, an enrichment of AtCYP20-2 in the stroma membranes and it's co-migration with NAD(P)H dehydrogenase (NDH) in native gels provide evidence that AtCYP20-2 is an auxiliary protein of NDH. When different NDH mutants were studied, AtCYP20-2 was found to be strongly reduced especially in mutants deficient in the membrane domain of NDH, thus suggesting a role in the assembly of NDH hydrophobic domain. Lack of AtCYP20-2, however, did not lead to severe malfunction of NDH, indicating redundancy in the function of lumenal immunophilins.

A host-selective toxin of Pyrenophora tritici-repentis, Ptr ToxA, induces photosystem changes and reactive oxygen species accumulation in sensitive wheat

Ptr ToxA (ToxA) is a proteinaceous necrotizing host-selective toxin produced by Pyrenophora tritici-repentis, a fungal pathogen of wheat (Triticum aestivum). In this study, we have found that treatment of ToxA-sensitive wheat leaves with ToxA leads to a light-dependent accumulation of reactive oxygen species (ROS) that correlates with the onset of necrosis. Furthermore, the accumulation of ROS and necrosis could be inhibited by the antioxidant N-acetyl cysteine, providing further evidence that ROS production is required for necrosis. Microscopic evaluation of ToxA-treated whole-leaf tissue indicated that ROS accumulation occurs in the chloroplasts. Analysis of total protein extracts from ToxA-treated leaves showed a light-dependent reduction of the chloroplast protein RuBisCo. In addition, Blue native-gel electrophoresis followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis revealed that ToxA induces changes in photosystem I (PSI) and photosystem II (PSII) in the absence of light, and therefore, the absence of ROS. When ToxA-treated leaves were exposed to light, all proteins in both PSI and PSII were extremely reduced. We propose that ToxA induces alterations in PSI and PSII affecting photosynthetic electron transport, which subsequently leads to ROS accumulation and cell death when plants are exposed to light.

Mass spectrometric characterization of membrane integral low molecular weight proteins from photosystem II in barley etioplasts

In Photosystem II (PSII), a high number of plastid encoded and membrane integral low molecular weight proteins smaller than 10 kDa, the proteins PsbE, F, H, I, J, K, L, M, N, Tc, Z and the nuclear encoded PsbW, X, Y1, Y2 proteins have been described. Here we show that all low molecular weight proteins of PSII already accumulate in the etioplast membrane fraction in darkness, whereas PsaI and PsaJ of photosystem I (PSI) represent the only low molecular weight proteins that do not accumulate in darkness. We found by BN-PAGE separation of membrane protein complexes and selective MS that the accumulation of one-helix proteins from PSII is light independent and occurs in etioplasts. In contrast, in chloroplasts isolated from light-grown plants, low molecular weight proteins were found to specifically accumulate in PSI and II complexes. Our results demonstrate how plants grown in darkness prepare for the induction of chlorophyll dependent photosystem assembly upon light perception. We anticipate that our investigation will provide the essential means for the analysis of protein assembly in any membrane utilizing low molecular weight protein subunits.

Chloroplast NADPH-thioredoxin reductase interacts with photoperiodic development in Arabidopsis

Chloroplast NADPH-thioredoxin reductase (NTRC) belongs to the thioredoxin systems that control crucial metabolic and regulatory pathways in plants. Here, by characterization of T-DNA insertion lines of NTRC gene, we uncover a novel connection between chloroplast thiol redox regulation and the control of photoperiodic growth in Arabidopsis (Arabidopsis thaliana). Transcript and metabolite profiling revealed severe developmental and metabolic defects in ntrc plants grown under a short 8-h light period. Besides reduced chlorophyll and anthocyanin contents, ntrc plants showed alterations in the levels of amino acids and auxin. Furthermore, a low carbon assimilation rate of ntrc leaves was associated with enhanced transpiration and photorespiration. All of these characteristics of ntrc were less severe when plants were grown under a long 16-h photoperiod. Transcript profiling revealed that the mutant phenotypes of ntrc were accompanied by differential expression of genes involved in stomatal development, chlorophyll biosynthesis, chloroplast biogenesis, and circadian clock-linked light perception systems in ntrc plants. We propose that NTRC regulates several key processes, including chlorophyll biosynthesis and the shikimate pathway, in chloroplasts. In the absence of NTRC, imbalanced metabolic activities presumably modulate the chloroplast retrograde signals, leading to altered expression of nuclear genes and, ultimately, to the formation of the pleiotrophic phenotypes in ntrc mutant plants.

Cyanobacterial photosystem II at 2.9-A resolution and the role of quinones, lipids, channels and chloride

Photosystem II (PSII) is a large homodimeric protein-cofactor complex located in the photosynthetic thylakoid membrane that acts as light-driven water:plastoquinone oxidoreductase. The crystal structure of PSII from Thermosynechococcus elongatus at 2.9-A resolution allowed the unambiguous assignment of all 20 protein subunits and complete modeling of all 35 chlorophyll a molecules and 12 carotenoid molecules, 25 integral lipids and 1 chloride ion per monomer. The presence of a third plastoquinone Q(C) and a second plastoquinone-transfer channel, which were not observed before, suggests mechanisms for plastoquinol-plastoquinone exchange, and we calculated other possible water or dioxygen and proton channels. Putative oxygen positions obtained from a Xenon derivative indicate a role for lipids in oxygen diffusion to the cytoplasmic side of PSII. The chloride position suggests a role in proton-transfer reactions because it is bound through a putative water molecule to the Mn(4)Ca cluster at a distance of 6.5 A and is close to two possible proton channels.

Psb28 protein is involved in the biogenesis of the photosystem II inner antenna CP47 (PsbB) in the cyanobacterium Synechocystis sp. PCC 6803

The role of the Psb28 protein in the structure and function of the photosystem II (PSII) complex has been studied in the cyanobacterium Synechocystis sp. PCC 6803. The protein was localized in the membrane fraction and, whereas most of the protein was detected as an unassembled protein, a small portion was found in the PSII core complex lacking the CP43 antenna (RC47). The association of Psb28 with RC47 was further confirmed by preferential isolation of RC47 from the strain containing a histidine-tagged derivative of Psb28 using nickel-affinity chromatography. However, the affinity-purified fraction also contained a small amount of the unassembled PSII inner antenna CP47 bound to Psb28-histidine, indicating a structural relationship between Psb28 and CP47. A psb28 deletion mutant exhibited slower autotrophic growth than wild type, although the absence of Psb28 did not affect the functional properties of PSII. The mutant showed accelerated turnover of the D1 protein, faster PSII repair, and a decrease in the cellular content of PSI. Radioactive labeling revealed a limitation in the synthesis of both CP47 and the PSI subunits PsaA/PsaB in the absence of Psb28. The mutant cells contained a high level of magnesium protoporphyrin IX methylester, a decreased level of protochlorophyllide, and released large quantities of protoporphyrin IX into the medium, indicating inhibition of chlorophyll (Chl) biosynthesis at the cyclization step yielding the isocyclic ring E. Overall, our results show the importance of Psb28 for synthesis of Chls and/or apoproteins of Chl-binding proteins CP47 and PsaA/PsaB.

Localization of 13 one-helix integral membrane proteins in photosystem II subcomplexes

Photosystem II is a multimeric protein complex of the thylakoid membrane in chloroplasts. Approximately half of the at least 26 different integral membrane protein subunits have molecular masses lower than 10 kDa. After one-dimensional (1D) or two-dimensional (2D) polyacrylamide gel electrophoresis (PAGE) separation, followed by enzymatic digestion of detected proteins, hardly any of these low-molecular-weight (LMW) subunits are detectable. Therefore, we developed a method for the analysis of highly hydrophobic LMW proteins. Intact proteins are extracted from acrylamide gels using a mixture of formic acid and organic solvent, precipitated with acetone, and analyzed by "top-down" mass spectrometry (MS). After offline nanoESI (electrospray ionization) MS, all LMW one-helix proteins from photosystem II were detected. In the four detected photosystem II supercomplexes of Nicotiana tabacum wild-type plants, 11 different one-helix proteins were identified as PsbE, -F, -H, -I, -K, -L, -M, -Tc, -W, and two isoforms of PsbX. The proteins PsbJ, -Y1, and -Y2 were localized in the buffer front after blue native (BN) PAGE, indicating their release during solubilization. Assembled PsbW is detected exclusively in supercomplexes, whereas it is absent in photosystem II core complexes, corroborating the protein's function for assembly of the light-harvesting complexes. This approach will substantiate gel-blot immunoanalysis for localization and identification of LMW protein subunits in any membrane protein complex.

Effects of inactivating psbM and psbT on photodamage and assembly of photosystem II in Synechocystis sp. PCC 6803

PsbM and PsbT have been assigned to electron densities on both photosystem II (PSII) monomers at the PSII dimer interface in X-ray crystallographic structures from Thermosynechoccocus elongatus and T. vulcanus. Our results show that removal of either or both proteins from Synechocystis sp. PCC 6803 resulted in photoautotrophic strains but the DeltaPsbM:DeltaPsbT mutant did not form stable dimers. A CP43-less PSII monomer accumulated in both single mutants, although absence of PsbT destabilized PSII to a greater extent than removing PsbM. Additionally, DeltaPsbT cells exhibited slowed electron transfer between the plastoquinone electron acceptors, Q(A) and Q(B); however, S-state cycling in both mutants was similar to wild type. Oxygen evolution in these mutants rapidly inactivated following exposure to high light where recovery required protein synthesis and could proceed in the dark in DeltaPsbM cells but required light in DeltaPsbT cells. Interestingly, the extent of recovery of oxygen-evolving activity was greatest in the DeltaPsbM:DeltaPsbT strain. We also found recovery required Psb27 in DeltaPsbT cells although, under our conditions, the DeltaPsb27 strain remained similar to wild type. In contrast, the DeltaPsbM:DeltaPsb27 mutant could not assemble PSII beyond a CP43-minus intermediate. Our results suggest essential roles for Psb27 in biogenesis in the DeltaPsbM strain and for repair from photodamage in cells lacking PsbT.

Chloroplast-encoded PsbT is required for efficient biogenesis of photosystem II complex in the green alga Chlamydomonas reinhardtii

The small hydrophobic polypeptide PsbT is associated with the photosystem II (PSII) reaction center (D1/D2 heterodimer). Here, we report the effect of the deletion of PsbT on the biogenesis of PSII complex during light-induced greening of y-1 mutants of the green alga Chlamydomonas reinhardtii. The y-1 is unable to synthesize chlorophylls in the dark but do so in the light. The dark-grown y-1 cells accumulated no major PSII proteins but a small amount of PsbT. Upon illumination, PsbT was immediately synthesized while chlorophylls, major PSII proteins, and O(2)-evolving activity increased after a 1-h lag. The y-1 cells without PsbT accumulated chlorophylls and PSI protein at a similar rate, whereas the accumulation of PSII complex was specifically retarded during greening. The absence of PsbT did not affect the synthesis of PSII proteins. These results indicate that PsbT is required for the efficient biogenesis of PSII complex.

Towards understanding the functional difference between the two PsbO isoforms in Arabidopsis thaliana--insights from phenotypic analyses of psbo knockout mutants

The extrinsic PsbO subunit of the water-oxidizing photosystem II (PSII) complex is represented by two isoforms in Arabidopsis thaliana, namely PsbO1 and PsbO2. Recent analyses of psbo1 and psbo2 knockout mutants have brought insights into their roles in photosynthesis and light stress. Here we analyzed the two psbo mutants in terms of PsbOs expression pattern, organization of PSII complexes and GTPase activity. Both PsbOs are present in wild-type plants, and their expression is mutually controlled in the mutants. Almost all PSII complexes are in the monomeric form not only in the psbo1 but also in the psbo2 mutant grown under high-light conditions. This results either from an enhanced susceptibility of PSII to photoinactivation or from malfunction of the repair cycle. Notably, the psbo1 mutant displays such problems even under growth-light conditions. These results together with the finding that PsbO2 has a threefold higher GTPase activity than PsbO1 have significance for the turnover of the PSII D1 subunit in Arabidopsis.

Spectral properties of the CP43-deletion mutant of Synechocystis sp. PCC 6803

Spectral properties, particularly fluorescence spectra and their time-dependent behavior, were investigated for a mutant of the cyanobacterium Synechocystis sp. PCC 6803 lacking the 43 kDa chlorophyll-protein (CP43, PsbC). Lack of CP43 was confirmed by a size shift of the corresponding gene and by Western blotting. The CP43-deletion mutant grown under heterotrophic conditions accumulated a small amount of photosystem (PS) II, but virtually no PS II fluorescence was observed. A 686-nm fluorescence band was clearly observed by phycocyanin excitation, coming from the terminal pigments of phycobilisomes. In contrast, no PS I fluorescence was detected by phycocyanin excitation when accumulation of PS II components was not proved by a fluorescence excitation spectrum, indicating that energy transfer to PS I chlorophyll a was mediated by PS II chlorophyll a. Direct connection of phycobilisomes with PS I was not suggested. Based on these fluorescence properties, the energy flow in the CP43-deletion mutant cells is discussed.

Directed mutagenesis of the transmembrane domain of the PsbL subunit of photosystem II in Synechocystis sp. PCC 6803

The PsbL protein is one of three low-molecular-weight subunits identified at the monomer-monomer interface of photosystem II (PSII) [Ferreira et al. (2004) Science 303:1831-1838; Loll et al. (2005) Nature 438:1040-1044]. We have employed site-directed mutagenesis to investigate the role of PsbL in Synechocystis sp. PCC 6803 cells. Truncation of the C-terminus by deleting the last four residues (Tyr-Phe-Phe-Asn) prevented association of PsbL with the CP43-less monomeric sub-complex and therefore blocked PSII assembly resulting in an obligate photoheterotrophic strain. Replacement of these residues with Ala created four photoautotrophic mutants. Compared to wild type, the F37A, F38A, and N39A strains had reduced levels of assembled PSII centers and F37A and F38A cells were readily photodamaged. In contrast, Y36A and Y36F mutants were similar to wild type. However, each of these strains had elevated levels of the CP43-less inactive monomeric complex. Mutations targeting a putative hydrogen bond between Arg-16 and sulfoquinovosyldiacylglycerol resulted in mutants that were also highly susceptible to photodamage. Similarly mutations targeting a conserved Tyr residue (Tyr-20) also destabilized PSII under high light and suggest that Tyr-20-lipid interactions or interactions of Tyr-20 with PsbT influence the ability of PSII to recover from photodamage.

Core protein phosphorylation facilitates the repair of photodamaged photosystem II at high light

Phosphorylation of photosystem II (PSII) reaction center protein D1 has been hypothesised to function as a signal for the migration of photodamaged PSII core complex from grana membranes to stroma lamellae for concerted degradation and replacement of the photodamaged D1 protein. Here, by using the mutants with impaired capacity (stn8) or complete lack (stn7 stn8) in phosphorylation of PSII core proteins, the role of phosphorylation in PSII photodamage and repair was investigated. We show that the lack of PSII core protein phosphorylation disturbs the disassembly of PSII supercomplexes at high light, which is a prerequisite for efficient migration of damaged PSII complexes from grana to stroma lamellae for repair. This results in accumulation of photodamaged PSII complexes, which in turn results, upon prolonged exposure to high light (HL), in general oxidative damage of photosynthetic proteins in the thylakoid membrane.

Structure of Photosystems I and II

Photosynthesis is the major process that converts solar energy into chemical energy on Earth. Two and a half billion years ago, the ancestors of cyanobacteria were able to use water as electron source for the photosynthetic process, thereby evolving oxygen and changing the atmosphere of our planet Earth. Two large membrane protein complexes, Photosystems I and II, catalyze the primary step in this energy conversion, the light-induced charge separation across the photosynthetic membrane. This chapter describes and compares the structure of two Photosystems and discusses their function in respect to the mechanism of light harvesting, electron transfer and water splitting.

How to analyze protein complexes by 2D blue native SDS-PAGE

Natural compartmentalization makes proteome analysis of the cell, cell organelles and organelle subfractions possible. Protein complexes are the basis for the next level of compartmentalization that can be addressed well with proteomic technology. Protein complexes organize and maintain the cellular and organelle functions on all levels of complexity in time and space. Cell development and division, transcription and translation, respiration and photosynthesis, transport and metabolism can be defined by the activity of protein complexes. Since a large part of the protein complexes of the cell body are inserted in lipid membrane phases, isolation, separation and protein subunit identification were difficult to address. Blue native polyacrylamide gel electrophoresis (BN-PAGE) provides us with the technology for high resolution separation of membrane protein complexes. Here, we show that high resolution separation of protein complexes by BN-PAGE requires the establishment of a detailed solubilisation strategy. We show that BN/SDS-PAGE provides the scientist with a high resolution array of protein subunits which allows analysis of the specific subunit stoichiometry of a protein complex as well as the assembly of protein complexes by standard protein detection methodology like DIGE, gelblot analysis and mass spectrometry. We envision BN-PAGE to precede classical 2D IEF/SDS-analysis for detailed characterization of membrane proteomes.

Modeling of variant copies of subunit D1 in the structure of photosystem II from Thermosynechococcus elongatus

In the cyanobacterium Thermosynechococcus elongatus BP-1, living in hot springs, the light environment directly regulates expression of genes that encode key components of the photosynthetic multi-subunit protein-pigment complex photosystem II (PSII). Light is not only essential as an energy source to power photosynthesis, but leads to formation of aggressive radicals which induce severe damage of protein subunits and organic cofactors. Photosynthetic organisms develop several protection mechanisms against this photo-damage, such as the differential expression of genes coding for the reaction center subunit D1 in PSII. Testing the expression of the three different genes (psbAI, psbAII, psbAIII) coding for D1 in T. elongatus under culture conditions used for preparing the material used in crystallization of PSII showed that under these conditions only subunit PsbA1 is present. However, exposure to high-light intensity induced partial replacement of PsbA1 with PsbA3. Modeling of the variant amino acids of the three different D1 copies in the 3.0 Å resolution crystal structure of PSII revealed that most of them are in the direct vicinity to redox-active cofactors of the electron transfer chain. Possible structural and mechanistic consequences for electron transfer are discussed.

Crystal structure of cyanobacterial photosystem II at 3.0 A resolution: a closer look at the antenna system and the small membrane-intrinsic subunits

Photosystem II (PSII) is a homodimeric protein-cofactor complex embedded in the thylakoid membrane that catalyses light-driven charge separation accompanied by the water splitting reaction during oxygenic photosynthesis. In the first part of this review, we describe the current state of the crystal structure at 3.0 A resolution of cyanobacterial PSII from Thermosynechococcus elongatus [B. Loll et al., Towards complete cofactor arrangement in the 3.0 A resolution structure of photosystem II, Nature 438 (2005) 1040-1044] with emphasis on the core antenna subunits CP43 and CP47 and the small membrane-intrinsic subunits. The second part describes first the general theory of optical spectra and excitation energy transfer and how the parameters of the theory can be obtained from the structural data. Next, structure-function relationships are discussed that were identified from stationary and time-resolved experiments and simulations of optical spectra and energy transfer processes.

Expression of protein complexes and individual proteins upon transition of etioplasts to chloroplasts in pea (Pisum sativum)

The protein complexes of pea (Pisum sativum L.) etioplasts, etio-chloroplasts and chloroplasts were examined using 2D Blue Native/SDS-PAGE. The most prominent protein complexes in etioplasts were the ATPase and the Clp and FtsH protease complexes which probably have a crucial role in the biogenesis of etioplasts and chloroplasts. Also the cytochrome b(6)f (Cyt b(6)f) complex was assembled in the etioplast membrane, as well as Rubisco, at least partially, in the stroma. These complexes are composed of proteins encoded by both the plastid and nuclear genomes, indicating that a functional cross-talk exists between pea etioplasts and the nucleus. In contrast, the proteins and protein complexes that bind chlorophyll, with the PetD subunit and the entire Cyt b(6)f complex as an exception, did not accumulate in etioplasts. Nevertheless, some PSII core components such as PsbE and the luminal oxygen-evolvong complex (OEC) proteins PsbO and PsbP accumulated efficiently in etioplasts. After 6 h de-etiolation, a complete PSII core complex appeared with 40% of the maximal photochemical efficiency, but a fully functional PSII was recorded only after 24 h illumination. Similarly, the core complex of PSI was assembled after 6 h illumination, whereas the PSI-light-harvesting complex I was stably assembled only in chloroplasts illuminated for 24 h. Moreover, a battery of proteins responsible for defense against oxidative stress accumulated particularly in etioplasts, including the stromal and thylakoidal forms of ascorbate peroxidase, glutathione reductase and PsbS.

The Psb27 protein facilitates manganese cluster assembly in photosystem II

Photosystem II (PSII) is a large membrane protein complex that uses light energy to convert water to molecular oxygen. This enzyme undergoes an intricate assembly process to ensure accurate and efficient positioning of its many components. It has been proposed that the Psb27 protein, a lumenal extrinsic subunit, serves as a PSII assembly factor. Using a psb27 genetic deletion strain (Deltapsb27) of the cyanobacterium Synechocystis sp. PCC 6803, we have defined the role of the Psb27 protein in PSII biogenesis. While the Psb27 protein was not essential for photosynthetic activity, various PSII assembly assays revealed that the Deltapsb27 mutant was defective in integration of the Mn(4)Ca(1)Cl(x) cluster, the catalytic core of the oxygen-evolving machinery within the PSII complex. The other lumenal extrinsic proteins (PsbO, PsbU, PsbV, and PsbQ) are key components of the fully assembled PSII complex and are important for the water oxidation reaction, but we propose that the Psb27 protein has a distinct function separate from these subunits. We show that the Psb27 protein facilitates Mn(4)Ca(1)Cl(x) cluster assembly in PSII at least in part by preventing the premature association of the other extrinsic proteins. Thus, we propose an exchange of lumenal subunits and cofactors during PSII assembly, in that the Psb27 protein is replaced by the other extrinsic proteins upon assembly of the Mn(4)Ca(1)Cl(x) cluster. Furthermore, we show that the Psb27 protein provides a selective advantage for cyanobacterial cells under conditions such as nutrient deprivation where Mn(4)Ca(1)Cl(x) cluster assembly efficiency is critical for survival.

A chloroplast cyclophilin functions in the assembly and maintenance of photosystem II in Arabidopsis thaliana

Photosynthetic light reactions rely on the proper function of large protein complexes (including photosystems I and II) that reside in the thylakoid membrane. Although their composition, structure, and function are known, the repertoire of assembly and maintenance factors is still being determined. Here we show that an immunophilin of the cyclophilin type, CYP38, plays a critical role in the assembly and maintenance of photosystem II (PSII) supercomplexes (SCs) in Arabidopsis. Mutant plants with the CYP38 gene interrupted by T-DNA insertion showed stunted growth and were hypersensitive to high light. Leaf chlorophyll fluorescence analysis and thylakoid membrane composition indicated that cyp38 mutant plants had defects in PSII SCs. Sucrose supplementation enabled the rescue of the mutant phenotype under low-light conditions, but failed to mitigate hypersensitivity to high-light stress. Protein radiolabeling assays showed that, although individual thylakoid proteins were synthesized equally in mutant and wild type, the assembly of the PSII SC was impaired in the mutant. In addition, the D1 and D2 components of the mutant PSII had a short half-life under high-light stress. The results provide evidence that CYP38 is necessary for the assembly and stabilization of PSII.

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.

Structural and functional self-organization of Photosystem II in grana thylakoids

The biogenesis of the well-ordered macromolecular protein arrangement of photosystem (PS)II and light harvesting complex (LHC)II in grana thylakoid membranes is poorly understood and elusive. In this study we examine the capability of self organization of this arrangement by comparing the PSII distribution and antenna organization in isolated untreated stacked thylakoids with restacked membranes after unstacking. The PS II distribution was deduced from freeze-fracture electron microscopy. Furthermore, changes in the antenna organization and in the oligomerization state of photosystem II were monitored by chlorophyll a fluorescence parameters and size analysis of exoplasmatic fracture face particles. Low-salt induced unstacking leads to a randomization and intermixing of the protein complexes. In contrast, macromolecular PSII arrangement as well as antenna organization in thylakoids after restacking by restoring the original solvent composition is virtually identical to stacked control membranes. This indicates that the supramolecular protein arrangement in grana thylakoids is a self-organized process.

The PsbZ subunit of Photosystem II in Synechocystis sp. PCC 6803 modulates electron flow through the photosynthetic electron transfer chain

The psbZ gene of Synechocystis sp. PCC 6803 encodes the approximately 6.6 kDa photosystem II (PSII) subunit. We here report biophysical, biochemical and in vivo characterization of Synechocystis sp. PCC 6803 mutants lacking psbZ. We show that these mutants are able to perform wild-type levels of light-harvesting, energy transfer, PSII oxygen evolution, state transitions and non-photochemical quenching (NPQ) under standard growth conditions. The mutants grow photoautotrophically; however, their growth rate is clearly retarded under low-light conditions and they are not capable of photomixotrophic growth. Further differences exist in the electron transfer properties between the mutants and wild type. In the absence of PsbZ, electron flow potentially increased through photosystem I (PSI) without a change in the maximum electron transfer capacity of PSII. Further, rereduction of P700(+) is much faster, suggesting faster cyclic electron flow around PSI. This implies a role for PsbZ in the regulation of electron transfer, with implication for photoprotection.

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.

LPA2 is required for efficient assembly of photosystem II in Arabidopsis thaliana

To elucidate the molecular mechanism of photosystem II (PSII) assembly, we characterized the low psii accumulation2 (lpa2) mutant of Arabidopsis thaliana, which is defective in the accumulation of PSII supercomplexes. The levels and processing patterns of the RNAs encoding the PSII subunits are unaltered in the mutant. In vivo protein-labeling experiments showed that the synthesis of CP43 (for chlorophyll a binding protein) was greatly reduced, but CP47, D1, and D2 were synthesized at normal rates in the lpa2-1 mutant. The newly synthesized CP43 was rapidly degraded in lpa2-1, and the turnover rates of D1 and D2 were higher in lpa2-1 than in wild-type plants. The newly synthesized PSII proteins were assembled into PSII complexes, but the assembly of PSII was less efficient in the mutant than in wild-type plants. LPA2 encodes an intrinsic thylakoid membrane protein, which is not an integral subunit of PSII. Yeast two-hybrid assays indicated that LPA2 interacts with the PSII core protein CP43 but not with the PSII reaction center proteins D1 and D2. Moreover, direct interactions of LPA2 with Albino3 (Alb3), which is involved in thylakoid membrane biogenesis and cell division, were also detected. Thus, the results suggest that LPA2, which appears to form a complex with Alb3, is involved in assisting CP43 assembly within PSII.

The extrinsic proteins of Photosystem II

Years of genetic, biochemical, and structural work have provided a number of insights into the oxygen evolving complex (OEC) of Photosystem II (PSII) for a variety of photosynthetic organisms. However, questions still remain about the functions and interactions among the various subunits that make up the OEC. After a brief introduction to the individual subunits Psb27, PsbP, PsbQ, PsbR, PsbU, and PsbV, a current picture of the OEC as a whole in cyanobacteria, red algae, green algae, and higher plants will be presented. Additionally, the role that these proteins play in the dynamic life cycle of PSII will be discussed.

The nuclear-encoded factor HCF173 is involved in the initiation of translation of the psbA mRNA in Arabidopsis thaliana

To gain insight into the biogenesis of photosystem II (PSII) and to identify auxiliary factors required for this process, we characterized the mutant hcf173 of Arabidopsis thaliana. The mutant shows a high chlorophyll fluorescence phenotype (hcf) and is severely affected in the accumulation of PSII subunits. In vivo labeling experiments revealed a drastically decreased synthesis of the reaction center protein D1. Polysome association experiments suggest that this is primarily caused by reduced translation initiation of the corresponding psbA mRNA. Comparison of mRNA steady state levels indicated that the psbA mRNA is significantly reduced in hcf173. Furthermore, the determination of the psbA mRNA half-life revealed an impaired RNA stability. The HCF173 gene was identified by map-based cloning, and its identity was confirmed by complementation of the hcf phenotype. HCF173 encodes a protein with weak similarities to the superfamily of the short-chain dehydrogenases/reductases. The protein HCF173 is localized in the chloroplast, where it is mainly associated with the membrane system and is part of a higher molecular weight complex. Affinity chromatography of an HCF173 fusion protein uncovered the psbA mRNA as a component of this complex.

Functional Heterogeneity of Photosystem II in Domain Specific Regions of the Thylakoid Membrane of Spinach (Spinacia oleracea L.)

A mild sonication and phase fractionation method has been used to isolate five regions of the thylakoid membrane in order to characterize the functional lateral heterogeneity of photosynthetic reaction centers and light harvesting complexes. Low-temperature fluorescence and absorbance spectra, absorbance cross-section measurements, and picosecond time-resolved fluorescence decay kinetics were used to determine the relative amounts of photosystem II (PSII) and photosystem I (PSI), to determine the relative PSII antenna size, and to characterize the excited-state dynamics of PSI and PSII in each fraction. Marked progressive increases in the proportion of PSI complexes were observed in the following sequence: grana core (BS), whole grana (B3), margins (MA), stroma lamellae (T3), and purified stromal fraction (Y100). PSII antenna size was drastically reduced in the margins of the grana stack and stroma lamellae fractions as compared to the grana. Picosecond time-resolved fluorescence decay kinetics of PSII were characterized by three exponential decay components in the grana fractions, and were found to have only two decay components with slower lifetimes in the stroma. Results are discussed in the framework of existing models of chloroplast thylakoid membrane lateral heterogeneity and the PSII repair cycle. Kinetic modeling of the PSII fluorescence decay kinetics revealed that PSII populations in the stroma and grana margin fractions possess much slower primary charge separation rates and decreased photosynthetic efficiency when compared to PSII populations in the grana stack.

Late assembly steps and dynamics of the cyanobacterial photosystem I

The dynamics of photosystem I assembly in cyanobacteria have been addressed using in vivo pulse-chase labeling of Synechocystis sp. PCC 6803 proteins in combination with blue native polyacrylamide gel electrophoresis. The analyses indicate the existence of three different monomeric photosystem I complexes and also the high stability of photosystem I trimers. We show that in addition to a complete photosystem I monomer, containing all 11 subunits, we detected a PsaK-less monomer and a short-lived PsaL/PsaK-less complex. The latter two monomers were missing in the ycf37 mutant of Synechocystis sp. PCC 6803 that accumulates also less trimers. Pulse-chase experiments suggest that the three monomeric complexes have different functions in the biogenesis of the trimer. Based on these findings we propose a model where PsaK is incorporated in the latest step of photosystem I assembly. The PsaK-less photosystem I monomer may represent an intermediate complex that is important for the exchange of the two PsaK variants during high light acclimation. Implications of the presented data with respect to Ycf37 function are discussed.

The PsbQ protein defines cyanobacterial Photosystem II complexes with highest activity and stability

Light-induced conversion of water to molecular oxygen by Photosystem II (PSII) is one of the most important enzymatic reactions in the biosphere. PSII is a multisubunit membrane protein complex with numerous associated cofactors, but it continually undergoes assembly and disassembly due to frequent light-mediated damage as a result of its normal function. Thus, at any instant, there is heterogeneity in the subunit compositions of PSII complexes within the cell. In particular, cyanobacterial PSII complexes have five associated extrinsic proteins, PsbO, PsbP, PsbQ, PsbU, and PsbV. However, little is known about the interactions of the more recently identified PsbQ protein with other components in cyanobacterial PSII. Here we show that PSII complexes can be isolated from the cyanobacterium Synechocystis sp. PCC 6803 on the basis of the presence of a polyhistidine-tagged PsbQ protein. Purification of PSII complexes using a tagged extrinsic protein has not been previously described, and this work conclusively demonstrates that PsbQ is present in combination with the PsbO, PsbU, and PsbV proteins in cyanobacterial PSII. Moreover, PsbQ-associated PSII complexes have higher activity and stability relative to those isolated using histidine-tagged CP47, an integral membrane protein. Therefore, we conclude that the presence of PsbQ defines the fully assembled and optimally active form of the enzyme.

Lipids in photosystem II: interactions with protein and cofactors

Photosystem II (PSII) is a homodimeric protein-cofactor complex embedded in the thylakoid membrane that catalyses light-driven charge separation accompanied by the oxidation of water during oxygenic photosynthesis. Biochemical analysis of the lipid content of PSII indicates a number of integral lipids, their composition being similar to the average lipid composition of the thylakoid membrane. The crystal structure of PSII at 3.0 A resolution allowed for the first time the assignment of 14 integral lipids within the protein scaffold, all of them being located at the interface of different protein subunits. The reaction centre subunits D1 and D2 are encircled by a belt of 11 lipids providing a flexible environment for the exchange of D1. Three lipids are located in the dimerization interface and mediate interactions between the PSII monomers. Several lipids are located close to the binding pocket of the mobile plastoquinone Q(B), forming part of a postulated diffusion pathway for plastoquinone. Furthermore two lipids were found, each ligating one antenna chlorophyll a. A detailed analysis of lipid-protein and lipid-cofactor interactions allows to derive some general principles of lipid binding pockets in PSII and to suggest possible functional properties of the various identified lipid molecules.

PsbI affects the stability, function, and phosphorylation patterns of photosystem II assemblies in tobacco

Photosystem II (PSII) core complexes consist of CP47, CP43, D1, D2 proteins and of several low molecular weight integral membrane polypeptides, such as the chloroplast-encoded PsbE, PsbF, and PsbI proteins. To elucidate the function of PsbI in the photosynthetic process as well as in the biogenesis of PSII in higher plants, we generated homoplastomic knock-out plants by replacing most of the tobacco psbI gene with a spectinomycin resistance cartridge. Mutant plants are photoautotrophically viable under green house conditions but sensitive to high light irradiation. Antenna proteins of PSII accumulate to normal amounts, but levels of the PSII core complex are reduced by 50%. Bioenergetic and fluorescence studies uncovered that PsbI is required for the stability but not for the assembly of dimeric PSII and supercomplexes consisting of PSII and the outer antenna (PSII-LHCII). Thermoluminescence emission bands indicate that the presence of PsbI is required for assembly of a fully functional Q(A) binding site. We show that phosphorylation of the reaction center proteins D1 and D2 is light and redox-regulated in the wild type, but phosphorylation is abolished in the mutant, presumably due to structural alterations of PSII when PsbI is deficient. Unlike wild type, phosphorylation of LHCII is strongly increased in the dark due to accumulation of reduced plastoquinone, whereas even upon state II light phosphorylation is decreased in delta psbI. These data attest that phosphorylation of D1/D2, CP43, and LHCII is regulated differently.

Detection and analysis of protein–protein interactions in organellar and prokaryotic proteomes by native gel electrophoresis: (Membrane) protein complexes and supercomplexes

It is an essential and challenging task to unravel protein-protein interactions in their actual in vivo context. Native gel systems provide a separation platform allowing the analysis of protein complexes on a rather proteome-wide scale in a single experiment. This review focus on blue-native (BN)-PAGE as the most versatile and successful gel-based approach to separate soluble and membrane protein complexes of intricate protein mixtures derived from all biological sources. BN-PAGE is a charge-shift method with a running pH of 7.5 relying on the gentle binding of anionic CBB dye to all membrane and many soluble protein complexes, leading to separation of protein species essentially according to their size and superior resolution than other fractionation techniques can offer. The closely related colorless-native (CN)-PAGE, whose applicability is restricted to protein species with intrinsic negative net charge, proved to provide an especially mild separation capable of preserving weak protein-protein interactions better than BN-PAGE. The essential conditions determining the success of detecting protein-protein interactions are the sample preparations, e.g. the efficiency/mildness of the detergent solubilization of membrane protein complexes. A broad overview about the achievements of BN- and CN-PAGE studies to elucidate protein-protein interactions in organelles and prokaryotes is presented, e.g. the mitochondrial protein import machinery and oxidative phosphorylation supercomplexes. In many cases, solubilization with digitonin was demonstrated to facilitate an efficient and particularly gentle extraction of membrane protein complexes prone to dissociation by treatment with other detergents. In general, analyses of protein interactomes should be carried out by both BN- and CN-PAGE.

LOW PSII ACCUMULATION1 is involved in efficient assembly of photosystem II in Arabidopsis thaliana

To gain insight into the processes involved in photosystem II (PSII) biogenesis and maintenance, we characterized the low psii accumulation1 (lpa1) mutant of Arabidopsis thaliana, which generally accumulates lower than wild-type levels of the PSII complex. In vivo protein labeling experiments showed that synthesis of the D1 and D2 proteins was greatly reduced in the lpa1 mutant, while other plastid-encoded proteins were translated at rates similar to the wild type. In addition, turnover rates of the PSII core proteins CP47, CP43, D1, and D2 were higher in lpa1 than in wild-type plants. The newly synthesized PSII proteins were assembled into functional protein complexes, but the assembly was less efficient in the mutant. LPA1 encodes a chloroplast protein that contains two tetratricopeptide repeat domains and is an intrinsic membrane protein but not an integral subunit of PSII. Yeast two-hybrid studies revealed that LPA1 interacts with D1 but not with D2, cytochrome b6, or Alb3. Thus, LPA1 appears to be an integral membrane chaperone that is required for efficient PSII assembly, probably through direct interaction with the PSII reaction center protein D1.

Dimeric and monomeric organization of photosystem II. Distribution of five distinct complexes in the different domains of the thylakoid membrane

The supramolecular organization of photosystem II (PSII) was characterized in distinct domains of the thylakoid membrane, the grana core, the grana margins, the stroma lamellae, and the so-called Y100 fraction. PSII supercomplexes, PSII core dimers, PSII core monomers, PSII core monomers lacking the CP43 subunit, and PSII reaction centers were resolved and quantified by blue native PAGE, SDS-PAGE for the second dimension, and immunoanalysis of the D1 protein. Dimeric PSII (PSII supercomplexes and PSII core dimers) dominate in the core part of the thylakoid granum, whereas the monomeric PSII prevails in the stroma lamellae. Considerable amounts of PSII monomers lacking the CP43 protein and PSII reaction centers (D1-D2-cytochrome b559 complex) were found in the stroma lamellae. Our quantitative picture of the supramolecular composition of PSII, which is totally different between different domains of the thylakoid membrane, is discussed with respect to the function of PSII in each fraction. Steady state electron transfer, flash-induced fluorescence decay, and EPR analysis revealed that nearly all of the dimeric forms represent oxygen-evolving PSII centers. PSII core monomers were heterogeneous, and a large fraction did not evolve oxygen. PSII monomers without the CP43 protein and PSII reaction centers showed no oxygen-evolving activity.

The properties of the positively charged loop region in PSI-G are essential for its "spontaneous" insertion into thylakoids and rapid assembly into the photosystem I complex

The PSI-G subunit of photosystem I (PSI) is an 11-kDa membrane protein that plays an important role in electron transport between plastocyanin and PSI and is involved in the stability of the PSI complex. Within the complex, the PSI-G subunit is bound to PSI-B and is in contact with Lhca1. PSI-G has two transmembrane spans connected by a positively charged stromal loop. The loop is inaccessible to proteases, indicating a tightly bound location within the PSI complex. Here, we have studied the insertion mechanism and assembly of PSI-G. We show that the protein inserts into thylakoids by a direct or "spontaneous" pathway that does not involve the activities of any known chloroplast protein-targeting machinery. Surprisingly, the positively charged stromal loop region plays a major role in this process. Mutagenesis or deletions within this region almost invariably lead to a marked lowering of insertion efficiency, strongly indicating a critical role for the loop in the organization of the transmembrane regions prior to or during membrane insertion. Finally, we have examined the assembly of newly inserted PSI-G into the PSI complex, since very little is known of the assembly pathway for this large multimeric complex. Interestingly, we find that inserted PSI-G can be found within the full PSI complex within the import assay time frame after insertion into thylakoids, strongly suggesting that PSI-G normally associates at the end of the assembly process. This is consistent with its location on the periphery of the complex.

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.

Psb29, a conserved 22-kD protein, functions in the biogenesis of Photosystem II complexes in Synechocystis and Arabidopsis

Photosystem II (PSII), the enzyme responsible for photosynthetic oxygen evolution, is a rapidly turned over membrane protein complex. However, the factors that regulate biogenesis of PSII are poorly defined. Previous proteomic analysis of the PSII preparations from the cyanobacterium Synechocystis sp PCC 6803 detected a novel protein, Psb29 (Sll1414), homologs of which are found in all cyanobacteria and vascular plants with sequenced genomes. Deletion of psb29 in Synechocystis 6803 results in slower growth rates under high light intensities, increased light sensitivity, and lower PSII efficiency, without affecting the PSII core electron transfer activities. A T-DNA insertion line in the PSB29 gene in Arabidopsis thaliana displays a phenotype similar to that of the Synechocystis mutant. This plant mutant grows slowly and exhibits variegated leaves, and its PSII activity is light sensitive. Low temperature fluorescence emission spectroscopy of both cyanobacterial and plant mutants shows an increase in the proportion of uncoupled proximal antennae in PSII as a function of increasing growth light intensities. The similar phenotypes observed in both plant and cyanobacterial mutants demonstrate that the function of Psb29 has been conserved throughout the evolution of oxygenic photosynthetic organisms and suggest a role for the Psb29 protein in the biogenesis of PSII.

A previously found thylakoid membrane protein of 14 kDa (TMP14) is a novel subunit of plant photosystem I and is designated PSI-P

We show that the thylakoid membrane phosphoprotein TMP14 is a novel subunit of plant photosystem I (PSI). Blue native/SDS-PAGE and sucrose gradient fractionation demonstrated the association of the protein exclusively with PSI. We designate the protein PSI-P. The presence of PSI-P subunit in Arabidopsis mutants lacking other PSI subunits was analyzed and suggested a location in the proximity of PSI-L, -H and -O subunits. The PSI-P protein was not differentially phosphorylated in state 1 and state 2.

Functional flexibility and acclimation of the thylakoid membrane

Light is an elusive substrate for the function of photosynthetic light reactions of photosynthesis in the thylakoid membrane. Therefore structural and functional dynamics, which occur in the timescale from seconds to several days, are required both at low and high light conditions. The best characterized short-time regulation mechanism at low light is a rapid state transition, resulting in higher absorption cross section of PSI at the expense of PSII. If the low light conditions continue, activation of the lhcb-genes and synthesis of the light-harvesting proteins will occur to optimize the functions of PSII and PSI. At high light, the transition to state 2 is completely inhibited, but the feedback de-excitation of absorbed energy as heat, known as the energy-dependent quenching (q(E)), is rapidly set up. It requires, at least, the DeltapH-dependent activation of violaxanthin de-epoxidase and involvement of the PsbS protein. Another crucial mechanism for protection against the high light stress is the PSII repair cycle. Furthermore, the water-water cycle, cyclic electron transfer around PSI and chlororespiration are important means induced under high irradiation, functioning mainly to avoid an excess production of reactive oxygen species.