Detection of a 10 kDa breakdown product containing the C-terminus of the D1-protein in photoinhibited wheat leaves suggests an acceptor side mechanism

Dynamic interplay between photodamage and photoprotection in photosystem II

Photoinhibition is the light-induced reduction in photosynthetic efficiency and is usually associated with damage to the D1 photosystem II (PSII) reaction centre protein. This damage must either be repaired, through the PSII repair cycle, or prevented in the first place by nonphotochemical quenching (NPQ). Both NPQ and D1 repair contribute to light tolerance because they ensure the long-term maintenance of the highest quantum yield of PSII. However, the relative contribution of each of these processes is yet to be elucidated. The application of a pulse amplitude modulation fluorescence methodology, called protective NPQ, enabled us to evaluate of the protective effectiveness of the processes. Within this study, the contribution of NPQ and D1 repair to the photoprotective capacity of Arabidopsis thaliana was elucidated by using inhibitors and mutants known to affect each process. We conclude that NPQ contributes a greater amount to the maintenance of a high PSII yield than D1 repair under short periods of illumination. This research further supports the role of protective components of NPQ during light fluctuations and the value of protective NPQ and q_Pd as unambiguous fluorescence parameters, as opposed to q_I and F_v /F_m , for quantifying photoinactivation of reaction centre II and light tolerance of photosynthetic organisms.

Quality control of Photosystem II: the molecular basis for the action of FtsH protease and the dynamics of the thylakoid membranes

The reaction center-binding D1 protein of Photosystem II is damaged by excessive light, which leads to photoinhibition of Photosystem II. The damaged D1 protein is removed immediately by specific proteases, and a metalloprotease FtsH located in the thylakoid membranes is involved in the proteolytic process. According to recent studies on the distribution and organization of the protein complexes/supercomplexes in the thylakoid membranes, the grana of higher plant chloroplasts are crowded with Photosystem II complexes and light-harvesting complexes. For the repair of the photodamaged D1 protein, the majority of the active hexameric FtsH proteases should be localized in close proximity to the Photosystem II complexes. The unstacking of the grana may increase the area of the grana margin and facilitate easier access of the FtsH proteases to the damaged D1 protein. These results suggest that the structural changes of the thylakoid membranes by light stress increase the mobility of the membrane proteins and support the quality control of Photosystem II.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of Arabidopsis thaliana Deg8

Arabidopsis thaliana Deg8, an ATP-independent serine endopeptidase, is involved in the repair of photosystem II (PSII), specifically the degradation of the photo-damaged PSII reaction centre D1 protein. To understand the molecular mechanism underlying the participation of Deg8 in the degradation of the photo-damaged D1 protein, the structure of Deg8 is needed. Until recently, however, no structure of Deg8 had been solved. In this study, Deg8 from A. thaliana was cloned, overexpressed and purified in Escherichia coli. Crystallization was performed at 277 K using tribasic sodium citrate as the precipitant and the crystals diffracted to 2.0 Å resolution, belonging to space group C2 with unit-cell parameters a = 129.5, b = 124.2, c = 93.3 Å, α = γ = 90, β = 132.4°. Assuming one trimer in the asymmetric unit, the Matthews coefficient and the solvent content were calculated to be 2.35 Å(3) Da(-1) and 47.6%, respectively.

Recent advances in understanding the assembly and repair of photosystem II

BACKGROUND

Photosystem II (PSII) is the light-driven water:plastoquinone oxidoreductase of oxygenic photosynthesis and is found in the thylakoid membrane of chloroplasts and cyanobacteria. Considerable attention is focused on how PSII is assembled in vivo and how it is repaired following irreversible damage by visible light (so-called photoinhibition). Understanding these processes might lead to the development of plants with improved growth characteristics especially under conditions of abiotic stress.

SCOPE

Here we summarize recent results on the assembly and repair of PSII in cyanobacteria, which are excellent model organisms to study higher plant photosynthesis.

CONCLUSIONS

Assembly of PSII is highly co-ordinated and proceeds through a number of distinct assembly intermediates. Associated with these assembly complexes are proteins that are not found in the final functional PSII complex. Structural information and possible functions are beginning to emerge for several of these 'assembly' factors, notably Ycf48/Hcf136, Psb27 and Psb28. A number of other auxiliary proteins have been identified that appear to have evolved since the divergence of chloroplasts and cyanobacteria. The repair of PSII involves partial disassembly of the damaged complex, the selective replacement of the damaged sub-unit (predominantly the D1 sub-unit) by a newly synthesized copy, and reassembly. It is likely that chlorophyll released during the repair process is temporarily stored by small CAB-like proteins (SCPs). A model is proposed in which damaged D1 is removed in Synechocystis sp. PCC 6803 by a hetero-oligomeric complex composed of two different types of FtsH sub-unit (FtsH2 and FtsH3), with degradation proceeding from the N-terminus of D1 in a highly processive reaction. It is postulated that a similar mechanism of D1 degradation also operates in chloroplasts. Deg proteases are not required for D1 degradation in Synechocystis 6803 but members of this protease family might play a supplementary role in D1 degradation in chloroplasts under extreme conditions.

The exposed N-terminal tail of the D1 subunit is required for rapid D1 degradation during photosystem II repair in Synechocystis sp PCC 6803

The selective replacement of photodamaged D1 protein within the multisubunit photosystem II (PSII) complex is an important photoprotective mechanism in chloroplasts and cyanobacteria. FtsH proteases are involved at an early stage of D1 degradation, but it remains unclear how the damaged D1 subunit is recognized, degraded, and replaced. To test the role of the N-terminal region of D1 in PSII biogenesis and repair, we have constructed mutants of the cyanobacterium Synechocystis sp PCC 6803 that are truncated at the exposed N terminus. Removal of 5 or 10 residues blocked D1 synthesis, as assessed in radiolabeling experiments, whereas removal of 20 residues restored the ability to assemble oxygen-evolving dimeric PSII complexes but inhibited PSII repair at the level of D1 degradation. Overall, our results identify an important physiological role for the exposed N-terminal tail of D1 at an early step in selective D1 degradation. This finding has important implications for the recognition of damaged D1 and its synchronized replacement by a newly synthesized subunit.

Oxidative modifications of the Photosystem II D1 protein by reactive oxygen species: from isolated protein to cyanobacterial cells

Action of reactive oxygen species (ROS) on the isolated D1 protein, a key component of Photosystem II (PSII) complex, was studied and compared with the effect of high irradiance on this protein in mildly solubilized photosynthetic membranes and cells of the cyanobacterium Synechocystis. Whereas singlet oxygen caused mainly protein modification reflected by shift of its electrophoretic mobility, action of hydrogen peroxide and superoxide resulted in generation of specific fragments. Hydroxyl radicals as the most ROS induced fast disappearance of the protein. The results substantiate the ability of ROS to cause direct scission of the D1 peptide bonds. Similar D1 modification, fragmentation and additionally cross-linking with other PSII subunits were observed during illumination or hydrogen peroxide treatment of mildly solubilized thylakoids. Peroxide-induced fragmentation did not occur in thylakoids of the strain lacking a ligand to the nonheme iron, confirming the role of this prosthetic group in the D1-specific cleavage. The D1 modification, fragmentation and cross-linking were suppressed by ROS scavengers, supporting the direct role of ROS in these phenomena. Identical symptoms of the ROS-induced D1 damage were detected in illuminated cells of Synechocystis mutants with a higher probability of ROS formation, documenting the relevance of the in vitro results for the situation in vivo.

Dissecting a cyanobacterial proteolytic system: efficiency in inducing degradation of the D1 protein of photosystem II in cyanobacteria and plants

A chromatography fraction, prepared from isolated thylakoids of a fatty acid desaturation mutant (Fad6/desA Colon, two colons Km(r)) of the cyanobacterium Synechocystis 6803, could induce an initial cleavage of the D1 protein in Photosystem II (PSII) particles of Synechocystis 6803 mutant and Synechococcus 7002 wild type as well as in supercomplexes of PSII-light harvesting complex II of spinach. Proteolysis was demonstrated both in darkness and in light as a reduction in the amount of full-length D1 protein or as a production of C-terminal initial degradation fragments. In the Synechocystis mutant, the main degradation fragment was a 10-kDa C-terminal one, indicating an initial cleavage occurring in the cytoplasmic DE-loop of the D1 protein. A protein component of 70-90 kDa isolated from the chromatographic fraction was found to be involved in the production of this 10-kDa fragment. In spinach, only traces of the corresponding fragment were detected, whereas a 24-kDa C-terminal fragment accumulated, indicating an initial cleavage in the lumenal AB-loop of the D1 protein. Also in Synechocystis the 24-kDa fragment was detected as a faint band. An antibody raised against the Arabidopsis DegP2 protease recognized a 35-kDa band in the proteolytically active chromatographic fraction, suggesting the existence of a lumenal protease that may be the homologue DegP of Synechocystis. The identity of the other protease cleaving the D1 protein in the DE-loop exposed on the stromal (cytoplasmic) side of the membrane is discussed.

FtsH is involved in the early stages of repair of photosystem II in Synechocystis sp PCC 6803

When plants, algae, and cyanobacteria are exposed to excessive light, especially in combination with other environmental stress conditions such as extreme temperatures, their photosynthetic performance declines. A major cause of this photoinhibition is the light-induced irreversible photodamage to the photosystem II (PSII) complex responsible for photosynthetic oxygen evolution. A repair cycle operates to selectively replace a damaged D1 subunit within PSII with a newly synthesized copy followed by the light-driven reactivation of the complex. Net loss of PSII activity occurs (photoinhibition) when the rate of damage exceeds the rate of repair. The identities of the chaperones and proteases involved in the replacement of D1 in vivo remain uncertain. Here, we show that one of the four members of the FtsH family of proteases (cyanobase designation slr0228) found in the cyanobacterium Synechocystis sp PCC 6803 is important for the repair of PSII and is vital for preventing chronic photoinhibition. Therefore, the ftsH gene family is not functionally redundant with respect to the repair of PSII in this organism. Our data also indicate that FtsH binds directly to PSII, is involved in the early steps of D1 degradation, and is not restricted to the removal of D1 fragments. These results, together with the recent analysis of ftsH mutants of Arabidopsis, highlight the critical role played by FtsH proteases in the removal of damaged D1 from the membrane and the maintenance of PSII activity in vivo.

A critical role for the Var2 FtsH homologue of Arabidopsis thaliana in the photosystem II repair cycle in vivo

Using a var2-2 mutant of Arabidopsis thaliana, which lacks a homologue of the zinc-metalloprotease, FtsH, we demonstrate that this protease is required for the efficient turnover of the D1 polypeptide of photosystem II and protection against photoinhibition in vivo. We show that var2-2 leaves are much more susceptible to light-induced photosystem II photoinhibition than wild-type leaves. Furthermore, the rate of photosystem II photoinhibition in untreated var2-2 leaves is equivalent to that of var2-2 and wild-type leaves, which have been treated with lincomycin, an inhibitor of the photosystem II repair cycle at the level of D1 synthesis. This is in contrast to untreated wild-type leaves, which show a much slower rate of photosystem II photoinhibition due to an efficient photosystem II repair cycle. The recovery of var2-2 leaves from photosystem II photoinhibition is also impaired relative to wild-type. Using Western blot analysis in the presence of lincomycin we show that the D1 polypeptide remains stable in leaves of the var2-2 mutant under photoinhibitory conditions that lead to D1 degradation in wild-type leaves and that the abundance of DegP2 is not affected by the var2-2 mutation. We conclude, therefore, that the Var2 FtsH homologue is required for the cleavage of the D1 polypeptide in vivo. In addition, we identify a conserved lumenal domain in Var2 that is unique to FtsH homologues from oxygenic phototrophs.

The quenching of photosystem II fluorescence does not protect the D1 protein against light induced degradation in thylakoids

In spinach thylakoids, the quenching of the singlet excited state in the photosystem II antenna by m-dinitrobenzene does not change the rate of the light induced degradation of the D1 reaction centre protein and offers only limited protection against photoinhibition itself. These results are discussed in terms of the role of non-photochemical quenching as a photoprotective strategy.

A chloroplast DegP2 protease performs the primary cleavage of the photodamaged D1 protein in plant photosystem II

Although light is the ultimate substrate in photosynthesis, it can also be harmful and lead to oxidative damage of the photosynthetic apparatus. The main target for light stress is the central oxygen-evolving photosystem II (PSII) and its D1 reaction centre protein. Degradation of the damaged D1 protein and its rapid replacement by a de novo synthesized copy represent the important repair mechanism of PSII crucial for plant survival under light stress conditions. Here we report the isolation of a single-copy nuclear gene from Arabidopsis thaliana, encoding a protease that performs GTP-dependent primary cleavage of the photodamaged D1 protein and hence catalysing the key step in the repair cycle in plants. This protease, designated DegP2, is a homologue of the prokaryotic Deg/Htr family of serine endopeptidases and is associated with the stromal side of the non-appressed region of the thylakoid membranes. Increased expression of DegP2 under high salt, desiccation and light stress conditions was measured at the protein level.

GTP bound to chloroplast thylakoid membranes is required for light-induced, multienzyme degradation of the photosystem II D1 protein

Even though light is the driving force in photosynthesis, it also can be harmful to plants. The water-splitting photosystem II is the main target for this light stress, leading to inactivation of photosynthetic electron transport and photooxidative damage to its reaction center. The plant survives through an intricate repair mechanism involving proteolytic degradation and replacement of the photodamaged reaction center D1 protein. Based on experiments with isolated chloroplast thylakoid membranes and photosystem II core complexes, we report several aspects concerning the rapid turnover of the D1 protein. (i) The primary cleavage step is a GTP-dependent process, leading to accumulation of a 23-kDa N-terminal fragment. (ii) Proteolysis of the D1 protein is inhibited below basal levels by nonhydrolyzable GTP analogues and apyrase treatment, indicating the existence of endogenous GTP tightly bound to the thylakoid membrane. This possibility was corroborated by binding studies. (iii) The proteolysis of the 23-kDa primary degradation fragment (but not of the D1 protein) is an ATP- and zinc-dependent process. (iv) D1 protein degradation is a multienzyme event involving a strategic (primary) protease and a cleaning-up (secondary) protease. (v) The chloroplast FtsH protease is likely to be involved in the secondary degradation steps. Apart from its significance for understanding the repair of photoinhibition, the discovery of tightly bound GTP should have general implications for other regulatory reactions and signal transduction pathways associated with the photosynthetic membrane.

Relationship between activity, D1 loss, and Mn binding in photoinhibition of photosystem II

Photoinhibition of photosystem II (PSII) activity and loss of the D1 reaction center protein were studied in PSII-enriched membrane fragments in which the water-splitting complex was inhibited by depletion of either calcium or chloride or by removing manganese. The Ca2+-depleted PSII was found to be the least susceptible to inhibition by light as reported previously (Krieger, A., and Rutherford, A. W. (1997) Biochim. Biophys. Acta 1319, 91-98). This different susceptibility to light was not reflected in the extent of D1 protein loss. In Mn-depleted PSII the loss of activity and the loss of the D1 protein were correlated, while in Cl-- and Ca2+-depleted PSII, there was very little loss of the D1 protein. The production of free radicals and singlet oxygen was measured by EPR spin-trapping techniques in the different samples. 1O2 and carbon-centered radicals could be detected after photoinhibition of active PSII, while hydroxyl radical formation dominated in all of the other samples. In addition, photoinhibition of PSII was investigated in which the functional Mn cluster was reconstituted (i. e., photoactivated). As expected this led to a protection against photoinhibition. When the photoactivation procedure was done in the absence of Ca2+ no activity was obtained although a nonfunctional Mn cluster was formed. Despite the lack of activity the binding of Mn partially protected against the loss of D1. These data demonstrate that, during photoinhibition, the extent of D1 loss is neither affected by the water-splitting activity of the sample nor correlated to the kinetics of PSII activity loss. D1 loss seems to be independent of the chemical nature of the reactive oxygen species formed during photoinhibition and seems to occur only in the absence of Mn. It is proposed that Mn binding protects against D1 loss by maintaining a protein structure which is not accessible to cleavage.

Photoinhibition of photosynthesis in vivo results in singlet oxygen production detection via nitroxide-induced fluorescence quenching in broad bean leaves

In plants experiencing environmental stress, the formation of reactive oxygen is often presumed. In this study, singlet oxygen was detected in broad bean (Vicia faba) leaves that were photoinhibited in vivo. Detection was based on the reaction of singlet oxygen with DanePy (dansyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole) yielding a nitroxide radical (DanePyO) which is EPR active and also features lower fluorescence compared to DanePy. The two (fluorescent and spin) sensor fuctions of DanePy are commensurate, which makes detecting singlet oxygen possible with a spectrofluorimeter in samples hard to measure with EPR spectroscopy [Kálai, T., Hideg, E., Vass, I., and Hideg, K. (1998) Free Radical Biol. Med. 24, 649-652]. We found that in leaves saturated with DanePy, the fluorescence of this double sensor was decreased when the leaves were photoinhibited by 1500 micromol m-2 s-1 photosynthetically active radiation. This fluorescence quenching is the first direct experimental evidence that photoinhibition of photosynthesis in vivo is accompanied by 1O2 production and is, at least partly, governed by the process characterized as acceptor side-induced photoinhibition in vitro.

Occupation of the QB-binding pocket by a photosystem II inhibitor triggers dark cleavage of the D1 protein subjected to brief preillumination

The D1 protein of the photosystem (PS) II reaction center turns over very rapidly in a light-dependent manner initiated by its selective and specific cleavage. The cleavage of D1 was studied by using a PS II inhibitor, N-octyl-3-nitro-2,4,6-trihydroxybenzamide (PNO8), as a molecular probe. The following results were obtained. (i) D1 was selectively cleaved into 23-kDa N-terminal and 9-kDa C-terminal fragments in complete darkness by PNO8 at a single site in a D-E loop connecting membrane-spanning helices D and E. (ii) The cleavage was markedly enhanced when PS II was illuminated for a brief period before the addition of PNO8 in darkness. (iii) The effect of preillumination was slowly lost during incubation in the dark, with a decay half-time of approximately 1 h at 25 degrees C. (iv) The light intensity of preillumination required for the cleavage was much lower than that required for O2 evolution. (v) The light-triggered cleavage of D1 was observed in thylakoids, PS II membranes, and PS II core particles, but not in purified PS II reaction centers. More than 60% of D1 was cleaved into the two fragments with no other by-products. (vi) The cleavage reaction revealed a marked pH dependence that was considerably different from that for inhibition of PS II activity. The results are interpreted as indicating that the binding of PNO8 to the QB-binding pocket triggers proteolytic cleavage of D1 that has been previously modified during illumination.

The rate constant of photoinhibition, measured in lincomycin-treated leaves, is directly proportional to light intensity

Pumpkin leaves grown under high light (500-700 micromol of photons m-2.s-1) were illuminated under photon flux densities ranging from 6.5 to 1500 micromol.m-2.s-1 in the presence of lincomycin, an inhibitor of chloroplast protein synthesis. The illumination at all light intensities caused photoinhibition, measured as a decrease in the ratio of variable to maximum fluorescence. Loss of photosystem II (PSII) electron transfer activity correlated with the decrease in the fluorescence ratio. The rate constant of photoinhibition, determined from first-order fits, was directly proportional to photon flux density at all light intensities studied. The fluorescence ratio did not decrease if the leaves were illuminated in low light in the absence of lincomycin or incubated in darkness in the presence of lincomycin. The constancy of the quantum yield of photoinhibition under different photon flux densities strongly suggests that photoinhibition in vivo occurs by one dominant mechanism under all light intensities. This mechanism probably is not the acceptor side mechanism characterized in the anaerobic case in vitro. Furthermore, there was an excellent correlation between the loss of PSII activity and the loss of the D1 protein from thylakoid membranes under low light. At low light, photoinhibition occurs so slowly that inactive PSII centers with the D1 protein waiting to be degraded do not accumulate. The kinetic agreement between D1 protein degradation and the inactivation of PSII indicates that the turnover of the D1 protein depends on photoinhibition under both low and high light.

Deletion of the PEST-like region of photosystem two modifies the QB-binding pocket but does not prevent rapid turnover of D1

The rapid turn-over of the D1 polypeptide of the photosystem two complex has been suggested to be due to the presence of a "PEST"-like sequence located between putative transmembrane helices IV and V of D1 (Greenberg, B. M., Gaba, V., Mattoo, A. K. and Edelman, M. (1987) EMBO J. 6, 2865-2869). We have tested this hypothesis by constructing a deletion mutant (delta 226-233) of the cyanobacterium Synechocystis sp. PCC 6803 in which residues 226-233 of the D1 polypeptide, containing the PEST-like sequence, have been removed. The resulting mutant, delta PEST, is able to grow photoautotrophically and give light-saturated rates of oxygen at wild type levels. However electron transfer on the acceptor side of the complex is perturbed. Analysis of cells by thermoluminescence and by monitoring the decay in quantum yield of variable fluorescence following saturating flash excitation indicates that Q-B, but not Q-A, is destabilized in this mutant. Electron transfer on the donor side of photosystem two remains largely unchanged in the mutant. Turnover of the D1 polypeptide as examined by pulse-chase experiments using [35S]methionine was enhanced in the delta PEST mutant compared to strain TC31 which is the wild type control. We conclude that the PEST sequence is not absolutely required for turnover of the D1 polypeptide in vivo although deletion of residues 226-233 does have an effect on the redox equilibrium between QA and QB.

Mathematical modelling of photoinhibition and Photosystem II repair cycle. I. Photoinhibition and D1 protein degradation in vitro and in the absence of chloroplast protein synthesis in vivo

The kinetics of photoinhibition of Photosystem II and D1 protein degradation were studied by applying mathematical modelling to new and published data. The word 'photoinhibition' refers here only to such inhibition of PS II activity that requires chloroplast protein synthesis for recovery. It is shown that acceptor-side photoinhibition in vitro as well as in vivo photoinhibition in higher plants and cyanobacteria in the presence of prokaryotic translation inhibitors follow first-order kinetics. Degradation of damaged D1 protein also fits in a first-order reaction equation with respect to the concentration of photoinhibited PS II centres. It is shown that photoprotective lowering of the ratio of variable to maximum fluorescence can be distinguished from the lowering of this ratio associated with photoinhibition.

In vivo and in vitro photoinhibition reactions generate similar degradation fragments of D1 and D2 photosystem-II reaction-centre proteins

Isolation of photosystem-II reaction centres from pea leaves after photoinhibitory treatment at low temperature (0-1 degrees C) has provided evidence for the mechanism of degradation of the D1 protein in vivo. These isolated reaction centres did not appear to be spectrally distinct from preparations obtained from control leaves that had not been photoinhibited. Breakdown fragments of both the D1 and D2 proteins were, however, found in preparations isolated from photoinhibited leaves, and showed similarities with those detected when isolated reaction centres were exposed to acceptor-side photoinhibition. Analyses of the origin of D1 fragments indicated that the primary cleavage site of this protein was between transmembrane helices IV and V indicative of the acceptor-side mechanism for photoinhibition. The origins of other D1 protein fragments indicate that some donor-side photoinhibition may also have occurred in vivo under the conditions employed. We have shown that the spectral and functional integrating of the isolated photosystem II reaction centre complex is resistant to proteolytic cleavage by trypsin. Use of a more non-specific protease (subtilisin), however, caused significant destabilisation of the special pair of chlorophylls constituting the primary electron donor, P680, with a consequential loss of functional activity. Thus, it is possible that specific cleavage of photosystem-II reaction-centre proteins may occur in vivo following photoinhibitory damage without a significant change in structural integrity, a conclusion supported by the finding that photodamaged and normal reaction centres were isolated together.