Changes in the organization of Photosystem II following light-induced D1-protein degradation

Molecular origins of induction and loss of photoinhibition-related energy dissipation qI

Photosynthesis fuels life on Earth using sunlight as energy source. However, light has a simultaneous detrimental effect on the enzyme triggering photosynthesis and producing oxygen, photosystem II (PSII). Photoinhibition, the light-dependent decrease of PSII activity, results in a major limitation to aquatic and land photosynthesis and occurs upon all environmental stress conditions. In this work, we investigated the molecular origins of photoinhibition focusing on the paradoxical energy dissipation process of unknown nature coinciding with PSII damage. Integrating spectroscopic, biochemical, and computational approaches, we demonstrate that the site of this quenching process is the PSII reaction center. We propose that the formation of quenching and the closure of PSII stem from the same event. We lastly reveal the heterogeneity of PSII upon photoinhibition using structure-function modeling of excitation energy transfer. This work unravels the functional details of the damage-induced energy dissipation at the heart of photosynthesis.

Differential temperature effects on dissipation of excess light energy and energy partitioning in lut2 mutant of Arabidopsis thaliana under photoinhibitory conditions

The high-light-induced alterations in photosynthetic performance of photosystem II (PSII) and photosystem I (PSI) as well as effectiveness of dissipation of excessive absorbed light during illumination for different periods of time at room (22 °C) and low (8-10 °C) temperature of leaves of Arabidopsis thaliana, wt and lut2, were followed with the aim of unraveling the role of lutein in the process of photoinhibition. Photosynthetic parameters of PSII and PSI were determined on whole leaves by PAM fluorometer and oxygen evolving activity-by a Clark-type electrode. In thylakoid membranes, isolated from non-illuminated and illuminated for 4.5 h leaves of wt and lut2 the photochemical activity of PSII and PSI and energy interaction between the main pigment-protein complexes was determined. Results indicate that in non-illuminated leaves of lut2 the maximum rate of oxygen evolution and energy utilization in PSII is lower, excitation pressure of PSII is higher and cyclic electron transport around PSI is faster than in wt leaves. Under high-light illumination, lut2 leaves are more sensitive in respect to PSII performance and the extent of increase of excitation pressure of PSII, Φ_NO, and cyclic electron transport around PSI are higher than in wt leaves, especially when illumination is performed at low temperature. Significant part of the excessive light energy is dissipated via mechanism, not dependent on ∆pH and to functioning of xanthophyll cycle in LHCII, operating more intensively in lut2 leaves.

Manganese Deficiency in Plants: The Impact on Photosystem II

Manganese (Mn) is an essential plant micronutrient with an indispensable function as a catalyst in the oxygen-evolving complex (OEC) of photosystem II (PSII). Even so, Mn deficiency frequently occurs without visual leaf symptoms, thereby masking the distribution and dimension of the problem restricting crop productivity in many places of the world. Hence, timely alleviation of latent Mn deficiency is a challenge in promoting plant growth and quality. We describe here the key mechanisms of Mn deficiency in plants by focusing on the impact of Mn on PSII stability and functionality. We also address the mechanisms underlying the differential tolerance towards Mn deficiency observed among plant genotypes, which enable Mn-efficient plants to grow on marginal land with poor Mn availability.

The Lack of Lutein Accelerates the Extent of Light-induced Bleaching of Photosynthetic Pigments in Thylakoid Membranes of Arabidopsis thaliana

The high light-induced bleaching of photosynthetic pigments and the degradation of proteins of light-harvesting complexes of PSI and PSII were investigated in isolated thylakoid membranes of Arabidopsis thaliana, wt and lutein-deficient mutant lut2, with the aim of unraveling the role of lutein for the degree of bleaching and degradation. By the means of absorption spectroscopy and western blot analysis, we show that the lack of lutein leads to a higher extent of pigment photobleaching and protein degradation in mutant thylakoid membranes in comparison with wt. The highest extent of bleaching is suffered by chlorophyll a and carotenoids, while chlorophyll b is bleached in lut2 thylakoids during long periods at high illumination. The high light-induced degradation of Lhca1, Lhcb2 proteins and PsbS was followed and it is shown that Lhca1 is more damaged than Lhcb2. The degradation of analyzed proteins is more pronounced in lut2 mutant thylakoid membranes. The lack of lutein influences the high light-induced alterations in organization of pigment-protein complexes as revealed by 77 K fluorescence.

Effect of Light Acclimation on the Organization of Photosystem II Super- and Sub-Complexes in Arabidopsis thaliana

To survive under highly variable environmental conditions, higher plants have acquired a large variety of acclimation responses. Different strategies are used to cope with changes in light intensity with the common goal of modulating the functional antenna size of Photosystem II (PSII). Here we use a combination of biochemical and biophysical methods to study these changes in response to acclimation to high light (HL). After 2 h of exposure, a decrease in the amount of the large PSII supercomplexes is observed indicating that plants are already acclimating to HL at this stage. It is also shown that in HL the relative amount of antenna proteins decreases but this decrease is far less than the observed decrease of the functional antenna size, suggesting that part of the antenna present in the membranes in HL does not transfer energy efficiently to the reaction center. Finally, we observed LHCII monomers in all conditions. As the solubilization conditions used do not lead to monomerization of purified LHCII trimers, we should conclude that a population of LHCII monomers exists in the membrane. The relative amount of LHCII monomers strongly increases in plants acclimated to HL, while no changes in the trimer to monomer ratio are observed upon short exposure to stress.

Dynamic control of protein diffusion within the granal thylakoid lumen

The machinery that conducts the light-driven reactions of oxygenic photosynthesis is hosted within specialized paired membranes called thylakoids. In higher plants, the thylakoids are segregated into two morphological and functional domains called grana and stroma lamellae. A large fraction of the luminal volume of the granal thylakoids is occupied by the oxygen-evolving complex of photosystem II. Electron microscopy data we obtained on dark- and light-adapted Arabidopsis thylakoids indicate that the granal thylakoid lumen significantly expands in the light. Models generated for the organization of the oxygen-evolving complex within the granal lumen predict that the light-induced expansion greatly alleviates restrictions imposed on protein diffusion in this compartment in the dark. Experiments monitoring the redox kinetics of the luminal electron carrier plastocyanin support this prediction. The impact of the increase in protein mobility within the granal luminal compartment in the light on photosynthetic electron transport rates and processes associated with the repair of photodamaged photosystem II complexes is discussed.

Adaptation of photosynthesis under iron deficiency in maize

This paper explores the effects of high light stress on Fe-deficient plants. Maize (Zea mays) plants were grown under conditions of Fe deficiency and complete nutrition. Attached, intact leaves of Fe-deficient and control plants were used for gas exchange experiments under suboptimal, optimal and photoinhibitory illumination. Isolated chloroplasts were used to study photosynthetic electron transport system, compromised by the induction of Fe deficiency. The reaction centers of PS II (measured as reduction of Q, the primary electron acceptor of P 680) and PS I (measured as oxidation of P 700) were estimated from the amplitude of light induced absorbance change at 320 and 700 nm, respectively. Plants were subjected to photoinhibitory treatment for different time periods and isolated chloroplasts from these plants were used for electron transport studies. Carbon dioxide fixation in control as well as in Fe-deficient plants decreased in response to high light intensities. Total chlorophyll, P 700 and Q content in Fe-deficient chloroplasts decreased, while Chl a/b ratio and Q/P 700 ratio increased. However, electron transport through PS II suffered more after photoinhibitory treatment as compared to electron transport through PS I or whole chain. Electron transfer through PS I+PS II, excluding the water oxidation complex showed a decrease in Fe-deficient plants. However, electron transport through this part of the chain did not suffer much as a result of photoinhibition, suggesting a defect in the oxidising side of PS II.

Parallel changes in non-photochemical quenching properties, photosynthesis and D1 levels at sudden, prolonged irradiance exposures in Ulva fasciata Delile

The photosynthetic response to a sudden and prolonged high irradiance exposure and following recovery at low irradiance were studied with the aim of investigating the ability to withstand and adapt to high irradiance without prior high light adaptation. When thalli of Ulva fasciata, accustomed to a low irradiance (80 micromol photons m(-2) s(-1)), were exposed to a high irradiance (1500 micromol photons m(-2) s(-1)), the D1 protein was rapidly degraded, reaching a steady-state level after 110 min. This was followed by a fast recovery when thalli were transferred to dim light. The overall ability of non-photochemical quenching of chlorophyll fluorescence decreased and levelled off at a sudden and prolonged exposure to high irradiance and followed the same trend as the D1 level with a fast recovery in dim light. Ulva had intrinsic means to acclimate rapidly to high irradiance, when non-photochemical quenching did not operate properly, by maintaining a smaller fraction of high light tolerant PSII assemblages and by maintaining a high non-photochemical quenching capacity of chlorophyll fluorescence in relation to the variable fluorescence. The overall absorption of light (400-700 nm) remained high during the period of high irradiance exposure. When Ulva were deprived of nutrients in the form of PES media the ability of non-photochemical quenching decreased at photoinhibitory conditions. The possible causes for the responses at prolonged irradiance and the mechanisms for the decrease of non-photochemical quenching are discussed, with implications for field measurements.

Subsequent events to GTP binding by the plant PsbO protein: structural changes, GTP hydrolysis and dissociation from the photosystem II complex

Besides an essential role in optimizing water oxidation in photosystem II (PSII), it has been reported that the spinach PsbO protein binds GTP [C. Spetea, T. Hundal, B. Lundin, M. Heddad, I. Adamska, B. Andersson, Proc. Natl. Acad. Sci. U.S.A. 101 (2004) 1409-1414]. Here we predict four GTP-binding domains in the structure of spinach PsbO, all localized in the beta-barrel domain of the protein, as judged from comparison with the 3D-structure of the cyanobacterial counterpart. These domains are not conserved in the sequences of the cyanobacterial or green algae PsbO proteins. MgGTP induces specific changes in the structure of the PsbO protein in solution, as detected by circular dichroism and intrinsic fluorescence spectroscopy. Spinach PsbO has a low intrinsic GTPase activity, which is enhanced fifteen-fold when the protein is associated with the PSII complex in its dimeric form. GTP stimulates the dissociation of PsbO from PSII under light conditions known to also release Mn(2+) and Ca(2+) ions from the oxygen-evolving complex and to induce degradation of the PSII reaction centre D1 protein. We propose the occurrence in higher plants of a PsbO-mediated GTPase activity associated with PSII, which has consequences for the function of the oxygen-evolving complex and D1 protein turnover.

High light-induced changes of 77 K fluorescence emission of pea thylakoid membranes with altered membrane fluidity

The effect of lipid phase order of isolated thylakoid membranes on fluorescent characteristics of both photosystems during illumination with high light intensity at 22 degrees C and 4 degrees C was investigated. For artificial modification of membrane fluidity two membrane perturbing agents were applied-cholesterol and benzyl alcohol. 77 K fluorescence emission and excitation spectra of control, cholesterol- and benzyl alcohol-treated thylakoid membranes were analysed in order to determine the high light-induced changes of emission bands attributed to different chlorophyll-protein complexes-F 735, emitted by photosystem I-light-harvesting complex I; and F 685 and F 695, emitted by photosystem II-light-harvesting complex II. Analysis of emission bands showed that high light treatment leads to a decrease of the area of band at 695 nm and a concomitant increase of intensity of the band at 735 nm. The involvement of different pigment pools (chlorophyll a and chlorophyll b) in the energy supply of both photosystems before and after photoinhibitory treatment was estimated on the basis of excitation fluorescence spectra. The dependence of the ratios F 735/F 685 and the band areas at 685 and 695 nm on the illumination time was studied at both temperatures. Data presented indicate that cholesterol incorporation stabilized the intersystem structure in respect to light-induced changes of fluorescence emission of PSI and PSII. It was shown that the effect of fluid properties of thylakoid membranes on the 77 K fluorescence characteristics of main pigment protein complexes of pea thyalkoid membranes depends on the temperature during high light treatment.

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.

Photoinhibition and recovery of photosynthesis in leaves of Vitis berlandieri and Vitis rupestris

Photoinhibition of photosynthesis was studied in Vitis berlandieri and Vitis rupestris leaves under controlled conditions (irradiation of detached leaves to about 1900 micromol m(-2) s(-1)). The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll (Chl) fluorescence (Fv/Fm) and electron transport measurements. The potential efficiency of PS2, Fv/Fm declined, Fo increased significantly in leaves of V. berlandieri, while Fo decreased in V. rupestris. In isolated thylakoids, the rate of whole chain and PS2 activity markedly decreased in high light irradiated more in leaves of V. berlandieri than in leaves of V. rupestris. A smaller inhibition of PS1 activity was also observed in both leaves. In the subsequent dark incubation, fast recovery was observed in both leaves and reached maximum PS2 efficiencies similar to those observed in non-photoinhibited leaves. The artificial exogenous electron donors DPC, NH2OH and Mn2+ failed to restore the high light induced loss of PS2 activity in V. berlandieri leaves, while DPC and NH2OH significantly restored in V. rupestris leaves. It is concluded that high light inactivates on the donor side of PS2 and acceptor side of PS2 in V. rupestris and V. berlandieri leaves, respectively. Quantification of the PS2 reaction center protein D1 and 33 kDa protein of water splitting complex following high light exposure of leaves showed pronounced differences between V. berlandieri and V. rupestris leaves. The marked loss of PS2 activity in high light irradiated leaves was due to the marked loss of D1 protein and 33 kDa protein in V. berlandieri and V. rupestris leaves, respectively.

The single-copy genepsbScodes for a phylogenetically intriguing 22 kDa polypeptide of photosystem II

Recombinant phages that encode the complete precursor polypeptide for the 22 kDa polypeptide associated with photosystem II have been serologically selected from two lambda gt11 expression libraries made from polyadenylated RNA of spinach seedlings. The cDNAs hybridize to a 1.3 kb RNA species. The precursor protein is comprised of 274 amino acid residues and carries an N-terminal transit peptide of probably 69 amino acid residues. The mature protein exhibits four predicted transmembrane segments and is shown to be an integral component of photosystem II originating in a single-copy gene. The unique characteristics of this protein are: (i) it is the result of a gene-internal duplication of an ancestor with two membrane spans, (ii) a striking resemblance to LHC I/II, CP24/CP29 apoproteins, and ELIPs, although it does not bind chlorophyll and is present in cyanobacteria, and, as these proteins, (iii) it integrates into the membrane with uncleaved routing signals that display remarkable resemblance to patterns found in bipartite transit peptides.

Photosystem II damage and repair cycle in the green alga Dunaliella salina: involvement of a chloroplast-localized HSP70

The involvement of HSP70B in the photosystem II damage and repair process in Dunaliella salina was investigated. A full-length cDNA of the D. salina hsp70B gene was cloned and sequenced. Expression patterns of the hsp70B gene were investigated upon shifting a D. salina culture from low-light to high-light growth conditions, designed to significantly accelerate the rate of PSII photodamage. Northern blot analyses and nuclear run-on transcription assays revealed a significant but transient induction of hsp70B gene transcription, followed by a subsequent increase in HSP70B protein synthesis and accumulation. Mild detergent solubilization of photoinhibited thylakoid membranes, in which photodamaged PSII centers had accumulated, followed by native gel electrophoresis revealed the formation of a 320 kDa protein complex that contained, in addition to the HSP70B, the photodamaged but as yet undegraded D1 protein as well as D2 and CP47. Evidence suggested that the 320 kDa complex is a transiently forming PSII repair intermediate. Denaturing solubilization of the 320 kDa PSII repair intermediate by SDS-urea resulted in cross-linking of its polypeptide constituents, yielding a 160 kDa protein complex. The role of the HSP70B in the repair of photodamaged PSII centers, e.g. in stabilizing the disassembled PSII-core complex and in facilitating the D1 degradation and replacement process, is discussed.

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.

GTP enhances the degradation of the photosystem II D1 protein irrespective of its conformational heterogeneity at the Q(B) site

The light exposure history and/or binding of different herbicides at the Q(B) site may induce heterogeneity of photosystem II acceptor side conformation that affects D1 protein degradation under photoinhibitory conditions. GTP was recently found to stimulate the D1 protein degradation of photoinactivated photosystem II (Spetea, C. , Hundal, T., Lohmann, F., and Andersson, B. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6547-6552). Here we report that GTP enhances the cleavage of the D1 protein D-E loop following exposure of thylakoid membranes to either high light, low light, or repetitive single turnover flashes but not to trypsin. GTP does not stimulate D1 protein degradation in the presence of herbicides known to affect the accessibility of the cleavage site to proteolysis. However, GTP stimulates degradation that can be induced even in darkness in some photosystem II conformers following binding of the PNO8 herbicide (Nakajima, Y., Yoshida, S., Inoue, Y., Yoneyama, K., and Ono, T. (1995) Biochim. Biophys. Acta 1230, 38-44). Both the PNO8- and the light-induced primary cleavage of the D1 protein occur in the grana membrane domains. The subsequent migration of photosytem II containing the D1 protein fragments to the stroma domains for secondary proteolysis is light-activated. We conclude that the GTP effect is not confined to a specific photoinactivation pathway nor to the conformational state of the photosystem II acceptor side. Consequently, GTP does not interact with the site of D1 protein cleavage but rather enhances the activity of the endogenous proteolytic system.

Identification of a Ca2+/H+ antiport in the plant chloroplast thylakoid membrane

To assess the availability of Ca2+ in the lumen of the thylakoid membrane that is required to support the assembly of the oxygen-evolving complex of photosystem II, we have investigated the mechanism of 45Ca2+ transport into the lumen of pea (Pisum sativum) thylakoid membranes using silicone-oil centrifugation. Trans-thylakoid Ca2+ transport is dependent on light or, in the dark, on exogenously added ATP. Both light and ATP hydrolysis are coupled to Ca2+ transport through the formation of a transthylakoid pH gradient. The H+-transporting ionophores nigericin/K+ and carbonyl cyanide 3-chlorophenylhydrazone inhibit the transport of Ca2+. Thylakoid membranes are capable of accumulating up to 30 nmol Ca2+ mg-1 chlorophyll from external concentrations of 15 μM over the course of a 15-min reaction. These results are consistent with the presence of an active Ca2+/H+ antiport in the thylakoid membrane. Ca2+ transport across the thylakoid membrane has significant implications for chloroplast and plant Ca2+ homeostasis. We propose a model of chloroplast Ca2+ regulation whereby the activity of the Ca2+/H+ antiporter facilitates the light-dependent uptake of Ca2+ by chloroplasts and reduces stromal Ca2+ levels.

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.

Developmental regulation of the PsbS gene expression in spinach seedlings: the role of phytochrome

The PsbS gene product (PSII-S) which is an integral subunit of photosystem II has recently been reported to be a new type of pigment-binding protein [11]. The chlorophylls of the PSII-S protein exhibit weak excitonic coupling and this protein is stable also in the absence of pigments. Here we investigated the expression of the PsbS gene in etiolated spinach seedlings grown either in complete darkness or exposed to light of various qualities. The results obtained reveal that the PsbS gene expression in etiolated spinach plants is subjected to endogenous control. This developmental control occurs at different levels of gene expression and results in transient accumulation of the PsbS transcripts with progressing etiolation. During the first two days after emerging of the cotyledons from the seed coat the steady-state level of the PsbS transcripts is regulated mainly through increased transcriptional activity of the PsbS gene. Prolonged growth of the seedlings in the dark resulted in additional post-transcriptional control of the PsbS transcript level. Translational activity of PsbS mRNA estimated by an integration of mRNA into polysomal complexes shows that the translation rate of PsbS mRNA is less influenced by seedling age. The maximal rate of translation is reached at the first day after cotyledons emergence but the translational activity of PsbS mRNA remains still high (50% of maximum) in 8-day old etiolated spinach plants. In addition to the light-independent developmental control, the PsbS gene expression is positively regulated by phytochrome in etiolated seedlings exposed to light. Red light, however, negatively influences the abundance of PsbS transcripts at post-transcriptional level. Studies on blue or far-red light effects reveal that the accumulation of PsbS transcripts exhibits the characteristics of very-low-fluence responses of the phytochrome receptor.

Role of the RCII-D1 protein in the reversible association of the oxygen-evolving complex proteins with the lumenal side of photosystem II

The nuclear-encoded proteins of the oxygen-evolving complex (OEC) of photosystem II are bound on the lumenal side of the thylakoid membrane and stabilize the manganese ion cluster forming the photosystem II electron donor side. The OEC proteins are released from their binding site(s) following light-induced degradation of reaction center II (RCII)-D1 protein in Chlamydomonas reinhardtii. The kinetics of OEC proteins release correlates with that of RCII-D1 protein degradation. Only a limited amount of RCII-D2 protein is degraded during the process, and no loss of the core proteins CP43 and CP47 is detected. The release of the OEC proteins is prevented when the photoinactivated RCII-D1 protein degradation is retarded by addition of 3-(3,5-dichlorophenyl)-1,1-dimethylurea or by a high PQH2/PQ ratio prevailing in membranes of the plastocyanin-deficient mutant Ac208. The released proteins are not degraded but persist in the thylakoid lumen for up to 8 h and reassociate with photosystem II when new D1 protein is synthesized in cells exposed to low light, thus allowing recovery of photosystem II function. Reassociation also occurs following D1 protein synthesis in darkness when RCII activity is only partially recovered. These results indicate that (i) the D1 protein participates in the formation of the lumenal OEC proteins binding site(s) and (ii) the photoinactivation of RCII-D1 protein does not alter the conformation of the donor side of photosystem II required for the binding of the OEC proteins.

In vitro synthesis and assembly of photosystem II core proteins. The D1 protein can be incorporated into photosystem II in isolated chloroplasts and thylakoids

The D1 reaction center protein of the membrane-bound photosystem II complex (PSII) has a much higher turnover rate than the other PSII proteins. Thus, the D1 protein has to be replaced while the other PSII components are not newly synthesized. In this study, this D1 protein replacement into PSII complexes was followed in two in vitro translation systems: isolated chloroplasts and a homologous run-off translation system consisting primarily of isolated thylakoids with attached ribosomes. The incorporation of newly synthesized radiolabeled products into different (sub)complexes was analyzed by sucrose density gradient centrifugation of n-dodecyl beta -D-maltoside-solubilized thylakoid membranes. This analysis allowed us to follow the release of the nascent polypeptide chains from the ribosomes and identification of at least four assembly steps of the PSII complex, as shown below. (i) Both in isolated chloroplasts and in thylakoids, newly synthesized D1 protein is predominantly incorporated into existing PSII subcomplexes, indicating that synthesis and import of nuclear-encoded factors is not needed for D1 protein replacement. (ii) In chloroplasts, D1 protein incorporation into PSII core complexes is more efficient than during translation in isolated thylakoids. In the thylakoid translation system, a large percentage of radiolabeled D1 protein is found in smaller PSII subcomplexes, like PSII reaction center particles, and as unassembled protein in the membrane. This indicates that stromal factors are required in the replacement process of the D1 protein. (iii) Both in isolated chloroplasts and in thylakoids, the other PSII core proteins D2, CP43, and CP47 are also synthesized and released from the membrane-bound ribosomes, but incorporation into PSII complexes occurs to a much smaller extent than the D1 protein. Instead they accumulate predominantly as unassembled proteins in the thylakoid membrane. (iv) In chloroplasts, synthesis of the D1 protein seems to be adjusted according to the possibilities of incorporation into PSII complexes, while synthesis of the D2 protein, CP43, and CP47 is less regulated and their accumulation as unassembled protein in the membrane is abundant.

The PSII-S protein of higher plants: a new type of pigment-binding protein

An intrinsic 22 kDa protein of photosystem II has been shown to possess high sequence homology with the CAB gene products, but differs from these proteins by an additional putative fourth transmembrane helix. This protein, designated PSII-S in accordance with the assignment of the name psbS to its gene, has been isolated by nonionic detergents and preparative isoelectric focusing in this study. The isolated PSII-S protein was shown to bind 5 chlorophyll molecules (a and b) per protein unit and also several different kinds of carotenoids. The room temperature absorption spectrum of the Qy transition of the chlorophylls bound to the isolated protein is characterized by a broad band with a maximum at 671 nm. The 77 K fluorescence spectrum exhibits a peak at 672 nm. A single photon counting technique was applied to resolve the room temperature decay kinetics of the first excited singlet states in the chlorophyll ensemble of the PSII-S protein. The data can be satisfactorily described by triexponential kinetics with lifetimes of tau 1 = 1.8 ns, tau 2 = 4.4 ns, and tau 3 = 6.1 ns and normalized amplitudes of 0.09, 0.60, and 0.31, respectively. Circular dichroism spectra suggest that, in contrast to LHCII, virtually no pigment coupling exists in the PSII-S protein. Two copies of the PSII-S protein were found per PSII in spinach thylakoids. It displays an unusually extreme lateral heterogeneity, since the PSII beta centers located in the stroma exposed thylakoid regions contained only residual amounts of the PSII-S protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of psbO and psbH deletion mutants of Synechocystis PCC 6803 indicates that degradation of D1 protein is regulated by the QB site and dependent on protein synthesis

Mutants of the cyanobacterium Synechocystis PCC 6803 lacking the psbO or psbH gene are more vulnerable to photoinhibition than the wild type (WT). In the case of the psbO-less mutant, the increased sensitivity to photodamage is also accompanied by accelerated turnover of the D1 protein and a rapid rate of recovery on transfer to non-photoinhibitory conditions. In contrast, in low light the psbH-less mutant has a poor ability to recover after photoinhibition and has a reduced rate of D1 turnover as compared with WT. Since the psbO gene encodes the 33 kDa manganese-stabilizing protein associated with the water-splitting reaction, the increased sensitivity to photoinduced damage is attributed to perturbation of electron transfer processes on the donor side of photosystem II (PSII). In contrast, the absence of H protein, encoded by the psbH gene, affects the acceptor side of PSII with preferential photoinhibitory damage occurring at the QB site. The apparent consequence of this is that the psbH-less mutant, unlike the psbO-less mutant, is not able to regulate the rate of turnover of the D1 protein. In all cases it was shown that chloramphenicol, which blocks protein synthesis, enhances the rate of photoinhibition as judged by a decrease in oxygen evolution but slows down the rate of degradation of D1 protein compared to that observed during normal turnover.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoinactivation of photosystem II induces changes in the photochemical reaction center II abolishing the regulatory role of the QB site in the D1 protein degradation

The effect of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (diuron) binding at the secondary quinone (QB) binding site of reaction center II (RCII), on the high-light-induced degradation of the RCII proteins D1 and D2, and the core proteins CP43 and CP47 was investigated in vivo in Chlamydomonas reinhardtii. The degradation of the RCII-D2 and the CP43 proteins shows a short lag relative to that of the RCII-D1 protein. Diuron retards but does not prevent the degradation of RCII-D1, D2 and CP43 proteins. The degradation of the CP47 protein is not retarded by diuron. The RCII-D1 protein present in cells photoinactivated in the presence of diuron is subsequently degraded in cells transferred to low light or to darkness. The protein can be replaced (turnover) at least partially under both conditions. The RCII-D1 protein is not degraded during photoinactivation of a cytochrome-bf-defective mutant. Degradation occurs however when the cells are returned to low light permitting slow reoxidation of plastoquinol [Zer, H., Prasil, O. & Ohad, I. (1994) J. Biol. Chem. 269, 17,670-17,676]. Addition of diuron does not prevent the degradation of the protein at this stage. Tryptic digestion of the RCII-D1 protein is partially inhibited by diuron in isolated thylakoids [Trebst, A., Depka, B., Kraft, B. & Johanningmeier, U. (1988) Photosynth. Res. 18, 163-177] but not in thylakoids obtained from photoinactivated cells. We conclude that photoinactivation induces a series of sequential changes in RCII exposing the cleavage site of the RCII-D1 protein to degradation and abolishing the regulatory role of the QB site occupancy by plastoquinone or analog ligands on the cleavage process. The degradation of the RCII-D2 and CP43 proteins may be a secondary process following modification and/or loss of the RCII-D1 protein.

Prolonged high light treatment of plant cells results in changes of the amount, the localization and the electrophoretic behavior of several thylakoid membrane proteins

The effect of a 30 h high light treatment on the amount and the localization of thylakoid proteins was analysed in low light grown photoautotrophic cells of Marchantia polymorpha and Chenopodium rubrum. High light treatment resulted in a net loss of D1 protein which was accompanied by comparable losses of other proteins of the PS II core (reaction center with inner antenna). LHC II proteins were not reduced correspondingly, indicating that these complexes are less affected by prolonged high light. High light influenced the distribution of PS II components between the grana and the stroma region of the thylakoid membrane, probably by translocation of the respective PS II proteins. Additionally, modifications of several thylakoid proteins were detected in high light treated cells of C. rubrum. These effects are discussed in relation to photoinhibitory damage and repair processes.

The intrinsic 22 kDa protein is a chlorophyll-binding subunit of photosystem II

The intrinsic 22 kDa polypeptide associated with photosystem II (psbS protein) was found to be able to bind chlorophyll. Extraction of isolated photosystem II membranes with octyl-thioglucopyranoside, followed by repetitive electrophoresis under partially denaturing conditions gave only one green band. It contained both chlorophyll a and chlorophyll b, exhibited an absorption maximum at 674 nm and a 77 K fluorescence peak at 675 nm. The chlorophyll-protein band contained a single polypeptide of 22 kDa. Based on these results and on previous protein sequence comparisons, it is suggested that the psbS protein is a chlorophyll a/b binding polypeptide and should thus be denoted CP22.

Photoinhibition of Photosystem II. Inactivation, protein damage and turnover

Even though light is the source of energy for photosynthesis, it can also be harmful to plants. Light-induced damage is targetted mainly to Photosystem II and leads to inactivation of electron transport and subsequent oxidative damage of the reaction centre, in particular to the D1 protein. Inactivation and protein damage can be induced by two different mechanisms, either from the acceptor side or from donor side of P680. The damaged D1 protein is triggered for degradation and digested by at least one serine-type proteinase that is tightly associated with the Photosystem II complex itself. The damaged Photosystem II complex dissociates from the light-harvesting antenna and migrates from appressed to non-appressed thylakoid regions where a new D1 protein is co-translationally inserted into the partially disassembled Photosystem II complex. D1 protein phosphorylation probably allows for coordinated biodegradation and biosynthesis of the D1 protein. After religation of cofactors and assembly of subunits, the repaired Photosystem II complex can again be found in the appressed membrane regions. Various protective mechanisms and an efficient repair cycle of Photosystem II allow plants to survive light stress.

On the question of the identity of cytochrome b-560 in thylakoid stromal membranes

Stromal membranes enriched in PS I contain a low potential cytochrome with a reduced α-band peak close to 560 nm. The identity of this cytochrome component has been ascribed either to a low potential form of the Photosystem II cytochrome b-559 or to a different cytochrome with a reduced α-band of 560 nm. The half-bandwidth of the 560 nm component in stromal membranes is identical to that of purified cytochrome b-559. Western blots show that the stromal membranes contain an amount of PS II cytochrome b-559 α-subunit that is more than sufficient to account for the cytochrome b-560 detected spectrophotometrically in these membranes. These immunochemical data and the similarity of (i) the spectral peaks, and (ii) the redox properties of low potential PS II cytochrome b-559 and the b-560 component, suggest that the simplest inference is that the cytochrome b-560 protein in stromal membranes is identical to the PS II cytochrome b-559.

Evidence for loss of D1 protein during photoinhibition of Chenopodium rubrum L. culture cells

The effects of high-light stress on chlorophyllfluorescence parameters, D1-protein turnover and the actual level of this protein were analysed in nitrogen-deficient and nitrogen-replete cells of Chenopodium rubrum L. Changes in the number of atrazine-binding sites and in the D1-protein immunoblot signal indicated that a net loss of D1 protein occurred in high light and was partly reversible in low light. Nitrogen deficiency did not exacerbate these changes. The involvement of D1-protein turnover was shown in pulse-chase experiments with [(35)S]-methionine and by the application of a chloroplastic protein-synthesis inhibitor (chloramphenicol). The slowly reversible non-photochemical fluorescence quenching increased pronouncedly when D1 protein was lost at high irradiances, but its increase was only small when a net loss of D1 protein was produced at moderate irradiances by addition of chloramphenicol. The ratio of variable to maximum fluorescence, Fv/Fm, and the number of atrazine-binding sites were correlated but a proportionality between these parameters could not be observed. We conclude from these results that (i) degradation of D1 protein was not always coupled to its resynthesis, (ii) the actual level of D1 protein reflected the balance between degradation and resynthesis of D1 protein and (iii) changes in the level of D1 protein did not depend on a pronounced increase of the slowly reversible non-photochemical quenching.

Breakdown of the photosystem II reaction center D1 protein under photoinhibitory conditions: identification and localization of the C-terminal degradation products

Illumination of a suspension of thylakoids with light at high intensity causes inhibition of the photosystem II electron transport activity and loss from the membrane of the D1 protein of the photosystem II reaction center. Impairment of the electron transport activity and depletion of D1 protein from the thylakoid membrane of pea were investigated with reference to the presence or absence of oxygen in the suspension. The breakdown products of the D1 protein were identified by immunoblotting with anti-D1 polyclonal antibodies which were proven to recognize mainly the C-terminal region of the protein. The results obtained show that (i) the light-induced inactivation of the photosystem II electron transport activity under anaerobic conditions is faster than in the presence of oxygen; (ii) depletion of D1 protein is observed on a longer time scale with respect to loss of electron transport activity and is faster when photoinhibition is performed in the presence of oxygen; (iii) C-terminal fragments of D1 are only observed when photoinhibition is carried out anaerobically and are mainly localized in the stroma-exposed regions; and (iv) the fragments observed after anaerobic photoinhibition are quickly degraded on further illumination of the thylakoid suspension in the presence of oxygen.