Quality control of Photosystem II: cleavage and aggregation of heat-damaged D1 protein in spinach thylakoids

Moderate heat stress (40 degrees C, 30 min) on spinach thylakoids induced cleavage of the D1 protein, producing an N-terminal 23-kDa fragment, a C-terminal 9-kDa fragment, and aggregation of the D1 protein. A homologue of Arabidopsis FtsH2 protease, which is responsible for degradation of the damaged D1 protein, was abundant in the stroma thylakoids. Two processes occurred in the thylakoids in response to heat stress: dephosphorylation of the D1 protein in the stroma thylakoids, and aggregation of the phosphorylated D1 protein in the grana. Heat stress also induced the release of the extrinsic PsbO, P and Q proteins from Photosystem II, which affected D1 degradation and aggregation significantly. The cleavage and aggregation of the D1 protein appear to be two alternative processes influenced by protein phosphorylation/dephosphorylation, distribution of FtsH, and intactness of the thylakoids.     

Quality control of photosystem II. Cleavage of reaction center D1 protein in spinach thylakoids by FtsH protease under moderate heat stress

When spinach thylakoids were subjected to moderate heat stress (40 degrees C for 30 min), oxygen evolution was inhibited, and cleavage of the reaction center-binding protein D1 of photosystem II took place, producing 23-kDa N-terminal fragments. The D1 cleavage was greatly facilitated by the addition of 0.15 mM ZnCl2 and 1 mM ATP and was completely inhibited by 1 mM EDTA, indicating the participation of an ATP-dependent metalloprotease(s) in the D1 cleavage. Herbicides 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, bromoxynil, and ioxynil, all of which bind to the Q(B) site, inhibited the D1 cleavage, suggesting that the DE-loop of the D1 protein is the heat-sensitive cleavage site. We solubilized the protease by treating the thylakoids with 2 M KSCN and detected a protease activity in the supernatant by gelatin activity gel electrophoresis in the 70-80-kDa region. The antibodies against tobacco FtsH and Arabidopsis FtsH2 reacted with a 70-80-kDa band of the KSCN-solubilized fraction, which suggests the presence of FtsH in the fraction. In accordance with this finding, we identified the homolog to Arabidopsis FtsH8 in the 70-80-kDa region by matrix-assisted laser desorption ionization time-of-flight mass analysis of the thylakoids. The KSCN-solubilized fraction was successively reconstituted with thylakoids to show heat-induced cleavage of the D1 protein and production of the D1 fragment. These results strongly suggest that an FtsH protease(s) is involved in the primary cleavage of the D1 protein under moderate heat stress.     

Quality control of photosystem II: FtsH hexamers are localized near photosystem II at grana for the swift repair of damage

The reaction center-binding D1 protein of Photosystem II is oxidatively damaged by excessive visible light or moderate heat stress. The metalloprotease FtsH has been suggested as responsible for the degradation of the D1 protein. We have analyzed the distribution and subunit structures of FtsH in spinach thylakoids and various membrane fractions derived from the thylakoids using clear native polyacrylamide gel electrophoresis and Western blot analysis. FtsH was found not only in the stroma thylakoids but also in the Photosystem II-enriched grana membranes. Monomeric, dimeric, and hexameric FtsH proteases were present as major subunit structures in thylakoids, whereas only hexameric FtsH proteases were detected in Triton X-100-solubilized Photosystem II membranes. Importantly, among the membrane fractions examined, hexameric FtsH proteases were most abundant in the Photosystem II membranes. In accordance with this finding, D1 degradation took place in the Photosystem II membranes under light stress. Sucrose density gradient centrifugation analysis of thylakoids and the Photosystem II membranes solubilized with n-dodecyl-β-d-maltoside and a chemical cross-linking study of thylakoids showed localization of FtsH near the Photosystem II light-harvesting chlorophyll-protein supercomplexes in the grana. These results suggest that part of the FtsH hexamers are juxtapositioned to PSII complexes in the grana in darkness, carrying out immediate degradation of the photodamaged D1 protein under light stress.     

The thylakoid FtsH protease plays a role in the light-induced turnover of the photosystem II D1 protein

The photosystem II reaction center D1 protein is known to turn over frequently. This protein is prone to irreversible damage caused by reactive oxygen species that are formed in the light; the damaged, nonfunctional D1 protein is degraded and replaced by a new copy. However, the proteases responsible for D1 protein degradation remain unknown. In this study, we investigate the possible role of the FtsH protease, an ATP-dependent zinc metalloprotease, during this process. The primary light-induced cleavage product of the D1 protein, a 23-kD fragment, was found to be degraded in isolated thylakoids in the dark during a process dependent on ATP hydrolysis and divalent metal ions, suggesting the involvement of FtsH. Purified FtsH degraded the 23-kD D1 fragment present in isolated photosystem II core complexes, as well as that in thylakoid membranes depleted of endogenous FtsH. In this study, we definitively identify the chloroplast protease acting on the D1 protein during its light-induced turnover. Unlike previously identified membrane-bound substrates for FtsH in bacteria and mitochondria, the 23-kD D1 fragment represents a novel class of FtsH substrate-functionally assembled proteins that have undergone irreversible photooxidative damage and cleavage.     

Quality control of Photosystem II under light stress – turnover of aggregates of the D1 protein in vivo

When photodamaged under excessive light, the D1 protein is digested and removed from Photosystem (PS) II to facilitate turnover of the protein. In vitro studies have shown that part of the photodamaged D1 protein forms aggregates with surrounding polypeptides before being digested by a protease(s) in the stroma [Yamamoto Y (2001) Plant Cell Physiol 42: 121–128]. The aim of this study was to examine whether light-induced aggregation of the D1 protein also occurs in vivo. The following results were obtained: (1) PS II activity in spinach leaves was significantly inhibited by weak illumination (light intensity, 20–100 μE m⁻² s⁻¹), as monitored by chlorophyll fluorescence Fv/Fm, when the leaves were kept at higher temperatures (35–40 °C); (2) aggregation of the D1 protein, as well as cleavage of the protein, was detected in thylakoids isolated from spinach leaves that had been subjected to heat/light stress; (3) aggregates of the D1 protein disappeared after incubation of the leaves at 25 °C in the dark or under illumination with weak light. Since it is dependent on the presence of oxygen, aggregation of the D1 protein is probably induced by reactive oxygen species produced in thylakoids upon illumination at elevated temperatures. Consistent with this notion, singlet oxygen production in thylakoid samples under illumination was shown to be stimulated significantly at higher temperatures.     

Quality control of Photosystem II: an FtsH protease plays an essential role in the turnover of the reaction center D1 protein in Synechocystis PCC 6803 under heat stress as well as light stress conditions.

The role of an AAA protease FtsH (slr0228) in the turnover of the D1 protein was studied under moderate heat stress conditions using wild-type cells of the cyanobacterium Synechocystis PCC 6803 and the mutant cells lacking a homologue of FtsH (slr0228). When the growth temperature of the wild-type was shifted from 30 degrees C to 40 degrees C, growth and oxygen-evolving activity were partially inhibited. Under the same heat stress, growth of the mutant was inhibited more significantly (63% inhibition after 5 days heat stress, compared with 26% inhibition with the wild-type cells) and the oxygen-evolving activity was also impaired in parallel. With heat stress at 42 degrees C, the level of the D1 protein of wild type cells was decreased, whereas that in mutant cells was not. The responses of cyanobacterial cells to heat stress observed here are quite similar to those to light stress that were reported previously. From these results, we suggest that the FtsH protease (slr0228) is responsible for both the heat-induced and light-induced degradation of the D1 protein. Notably, the amount of FtsH increased when the wild-type cells were exposed to heat stress or light stress, indicating that the up-regulation of the FtsH protease in the thylakoids is crucial for the cyanobacterial cells to cope with these abiotic stresses.     

Role of FtsH2 in the repair of Photosystem II in mutants of the cyanobacterium Synechocystis PCC 6803 with impaired assembly or stability of the CaMn4 cluster

The FtsH2 protease, encoded by the slr0228 gene, plays a key role in the selective degradation of photodamaged D1 protein during the repair of Photosystem II (PSII) in the cyanobacterium Synechocystis sp. PCC 6803. To test whether additional proteases might be involved in D1 degradation during high rates of photodamage, we have studied the synthesis and degradation of the D1 protein in ΔPsbO and ΔPsbV mutants, in which the CaMn₄ cluster catalyzing oxygen evolution is less stable, and in the D1 processing mutants, D1-S345P and ΔCtpA, which are unable to assemble a functional cluster. All four mutants exhibited a dramatically increased rate of D1 degradation in high light compared to the wild-type. Additional inactivation of the ftsH2 gene slowed the rate of D1 degradation dramatically and increased the level of PSII complexes. We conclude that FtsH2 plays a major role in the degradation of both precursor and mature forms of D1 following donor-side photoinhibition. However, this conclusion concerned only D1 assembled into larger complexes containing at least D2 and CP47. In the ΔpsbEFLJ deletion mutant blocked at an early stage in PSII assembly, unassembled D1 protein was efficiently degraded in the absence of FtsH2 pointing to the involvement of other protease(s). Significantly, the ΔPsbO mutant displayed unusually low levels of cellular chlorophyll at extremely low-light intensities. The possibilities that PSII repair may limit the availability of chlorophyll for the biogenesis of other chlorophyll-binding proteins and that PsbO might have a regulatory role in PSII repair are discussed.     

Glycinebetaine alleviates the inhibitory effect of moderate heat stress on the repair of photosystem II during photoinhibition

Transformation with the bacterial gene codA for choline oxidase allows Synechococcus sp. PCC 7942 cells to accumulate glycinebetaine when choline is supplemented exogenously. First, we observed two types of protective effect of glycinebetaine against heat-induced inactivation of photosystem II (PSII) in darkness; the codA transgene shifted the temperature range of inactivation of the oxygen-evolving complex from 40-52 °C (with half inactivation at 46 °C) to 46-60 °C (with half inactivation at 54 °C) and that of the photochemical reaction center from 44-55 °C (with half inactivation at 51 °C) to 52-63 °C (with half inactivation at 58 °C). However, in light, PSII was more sensitive to heat stress; when moderate heat stress, such as 40 °C, was combined with light stress, PSII was rapidly inactivated, although these stresses, when applied separately, did not inactivate either the oxygen-evolving complex or the photochemical reaction center. Further our studies demonstrated that the moderate heat stress inhibited the repair of PSII during photoinhibition at the site of synthesis de novo of the D1 protein but did not accelerate the photodamage directly. The codA transgene and, thus, the accumulation of glycinebetaine alleviated such an inhibitory effect of moderate heat stress on the repair of PSII by accelerating the synthesis of the D1 protein. We propose a hypothetical scheme for the cyanobacterial photosynthesis that moderate heat stress inhibits the translation machinery and glycinebetaine protects it against the heat-induced inactivation.     

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.     

Parallel expression of alternate forms of psbA2 gene provides evidence for the existence of a targeted D1 repair mechanism in Synechocystis sp. PCC 6803

The D1 protein of Photosystem II (PSII) is recognized as the main target of photoinhibitory damage and exhibits a high turnover rate due to its degradation and replacement during the PSII repair cycle. Damaged D1 is replaced by newly synthesized D1 and, although reasonable, there is no direct evidence for selective replacement of damaged D1. Instead, it remains possible that increased turnover of D1 subunits occurs in a non-selective manner due for example, to a general up-regulation of proteolytic activity triggered during damaging environmental conditions, such as high light. To determine if D1 degradation is targeted to damaged D1 or generalized to all D1, we developed a genetic system involving simultaneous dual expression of wild type and mutant versions of D1 protein. Dual D1 strains (nS345P:eWT and nD170A:eWT) expressed a wild type (WT) D1 from ectopic and a damage prone mutant (D1-S345P, D1-D170A) from native locus on the chromosome. Characterization of strains showed that all dual D1 strains restore WT like phenotype with high PSII activity. Higher PSII activity indicates increased population of PSII reaction centers with WT D1. Analysis of steady state levels of D1 in nS345P:eWT by immunoblot showed an accumulation of WT D1 only. But, in vivo pulse labeling confirmed the synthesis of both S345P (exists as iD1) and WT D1 in the dual strain. Expression of nS345P:eWT in FtsH2 knockout background showed accumulation of both iD1 and D1 proteins. This demonstrates that dual D1 strains express both forms of D1, yet only damage prone PSII complexes are selected for repair providing evidence that the D1 degradation process is targeted towards damaged PSII complexes. Since the N-terminus has been previously shown to be important for the degradation of damaged D1, the possibility that the highly conserved cysteine 18 residue situated in the N-terminal domain of D1 is involved in the targeted repair process was tested by examining site directed mutants of this and the other cysteines of the D1 protein. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

Dephosphorylation of photosystem II proteins and phosphorylation of CP29 in barley photosynthetic membranes as a response to water stress

Kinetic studies of protein dephosphorylation in barley thylakoid membranes revealed accelerated dephosphorylation of photosystem II (PSII) proteins, and meanwhile rapidly induced phosphorylation of a light-harvesting complex (LHCII) b4, CP29 under water stress. Inhibition of dephosphorylation aggravates stress damages and hampers photosystem recovery after rewatering. This increased dephosphorylation is catalyzed by both intrinsic and extrinsic membrane protein phosphatase. Water stress did not cause any thylakoid destacking, and the lateral migration from granum membranes to stroma-exposed lamellae was only found to CP29, but not other PSII proteins. Activation of plastid proteases and release of TLP40, an inhibitor of the membrane phosphatases, were also enhanced during water stress. Phosphorylation of CP29 may facilitate disassociation of LHCII from PSII complex, disassembly of the LHCII trimer and its subsequent degradation, while general dephosphorylation of PSII proteins may be involved in repair cycle of PSII proteins and stress-response-signaling.     

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.     

Oxygen reduction in chloroplast thylakoids results in production of hydrogen peroxide inside the membrane

Hydrogen peroxide production in isolated pea thylakoids was studied in the presence of cytochrome c to prevent disproportionation of superoxide radicals outside of the thylakoid membranes. The comparison of cytochrome c reduction with accompanying oxygen uptake revealed that hydrogen peroxide was produced within the thylakoid. The proportion of electrons from water oxidation participating in this hydrogen peroxide production increased with increasing light intensity, and at a light intensity of 630 μmol quanta m^− 2 s^− 1 it reached 60% of all electrons entering the electron transport chain. Neither the presence of a superoxide dismutase inhibitor, potassium cyanide or sodium azide, in the thylakoid suspension, nor unstacking of the thylakoids appreciably affected the partitioning of electrons to hydrogen peroxide production. Also, osmolarity-induced changes in the thylakoid lumen volume, as well as variation of the lumen pH induced by the presence of Gramicidin D, had negligible effects on such partitioning. The flow of electrons participating in lumen hydrogen peroxide production was found to be near 10% of the total electron flow from water. It is concluded that a considerable amount of hydrogen peroxide is generated inside thylakoid membranes, and a possible mechanism, as well as the significance, of this process are discussed.     

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

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

Phosphorylation of PSII proteins in maize thylakoids in the presence of Pb ions

Lead is potentially toxic to all organisms including plants. Many physiological studies suggest that plants have developed various mechanisms to contend with heavy metals, however the molecular mechanisms remain unclear. We studied maize plants in which lead was introduced into detached leaves through the transpiration stream. The photochemical efficiency of PSII, measured as an Fv/Fm ratio, in the maize leaves treated with Pb was only 10% lower than in control leaves. The PSII activity was not affected by Pb ions in mesophyll thylakoids, whereas in bundle sheath it was reduced. Protein phosphorylation in mesophyll and bundle sheath thylakoids was analyzed using mass spectrometry and protein blotting before and after lead treatment. Both methods clearly demonstrated increase in phosphorylation of the PSII proteins upon treatment with Pb²⁺, however, the extent of D1, D2 and CP43 phosphorylation in the mesophyll chloroplasts was clearly higher than in bundle sheath cells. We found that in the presence of Pb ions there was no detectable dephosphorylation of the strongly phosphorylated D1 and PsbH proteins of PSII complex in darkness or under far red light. These results suggest that Pb²⁺ stimulates phosphorylation of PSII core proteins, which can affect stability of the PSII complexes and the rate of D1 protein degradation. Increased phosphorylation of the PSII core proteins induced by Pb ions may be a crucial protection mechanism stabilizing optimal composition of the PSII complexes under metal stress conditions. Our results show that acclimation to Pb ions was achieved in both types of maize chloroplasts in the same way. However, these processes are obviously more complex because of different metabolic status in mesophyll and bundle sheath chloroplasts.     

Linking alterations in tau phosphorylation and cleavage during neuronal apoptosis

Neurofibrillary tangles (NFTs) are classic lesions of Alzheimer's disease. NFTs are bundles of abnormally phosphorylated tau, the paired helical filaments. The initiating mechanisms of NFTs and their role in neuronal loss are still unknown. Accumulating evidence supports a role for the activation of proteolytic enzymes, caspases, in neuronal death observed in brains of patients with Alzheimer's disease. Alterations in tau phosphorylation and tau cleavage by caspases have been previously reported in neuronal apoptosis. However, the links between the alterations in tau phosphorylation and its proteolytic cleavage have not yet been documented. Here, we show that, during staurosporine-induced neuronal apoptosis, tau first undergoes transient hyperphosphorylation, which is followed by dephosphorylation and cleavage. This cleavage generated a 10-kDa fragment in addition to the 17- and 50-kDa tau fragments previously reported. Prior tau dephosphorylation by a glycogen synthase kinase-3beta inhibitor, lithium, enhanced tau cleavage and sensitized neurons to staurosporine-induced apoptosis. Caspase inhibition prevented tau cleavage without reversing changes in tau phosphorylation linked to apoptosis. Furthermore, the microtubule depolymerizing agent, colchicine, induced tau dephosphorylation and caspase-independent tau cleavage and degradation. Both phenomena were blocked by inhibiting protein phosphatase 2A (PP2A) by okadaic acid. These experiments indicate that tau dephosphorylation precedes and is required for its cleavage and degradation. We propose that the absence of cleavage and degradation of hyperphosphorylated tau (due to PP2A inhibition) may lead to its accumulation in degenerating neurons. This mechanism may contribute to the aggregation of hyperphosphorylated tau into paired helical filaments in Alzheimer's disease where reduced PP2A activity has been reported.     

Function of two β-carotenes near the D1 and D2 proteins in photosystem II dimers

The antenna proteins in photosystem II (PSII) not only promote energy transfer to the photosynthetic reaction center (RC) but provide also an efficient cation sink to re-reduce chlorophyll a if the electron transfer (ET) from the Mn-cluster is inhibited. Using the newest PSII dimer crystal structure (3.0 Å resolution), in which 11 β-carotene molecules (Car) and 14 lipids are visible in the PSII monomer, we calculated the redox potentials (E_m) of one-electron oxidation for all Car (E_m(Car)) by solving the Poisson-Boltzmann equation. In each PSII monomer, the D1 protein harbors a previously unlocated Car (Car_D1) in van der Waals contact with the chlorin ring of Chl_Z(D1). Each Car_D1 in the PSII dimer complex is located in the interface between the D1 and CP47 subunits, together with another four Car of the other PSII monomer and several lipid molecules. The proximity of Car bridging between Car_D1 and plastoquinone/Q_A may imply a direct charge recombination of Car⁺Q_A⁻. The calculated E_m(Car_D1) and E_m(Chl_Z(D1)) are, respectively, 83 and 126 mV higher than E_m(Car_D2) and E_m(Chl_Z(D2)), which could explain why Car_D2⁺ and Chl_Z(D2)⁺ are observed rather than the corresponding Car_D1⁺ and Chl_Z(D1)⁺.     

Entropy-assisted stacking of thylakoid membranes

Chloroplasts in plants and some green algae contain a continuous thylakoid membrane system that is structurally differentiated into stacked granal membranes interconnected by unstacked thylakoids, the stromal lamellae. Experiments were conducted to test the hypothesis that the thermodynamic tendency to increase entropy in chloroplasts contributes to thylakoid stacking to form grana. We show that the addition of bovine serum albumin or dextran, two very different water-soluble macromolecules, to a suspension of envelope-free chloroplasts with initially unstacked thylakoids induced thylakoid stacking. This novel restacking of thylakoids occurred spontaneously, accompanied by lateral segregation of PSII from PSI, thereby mimicking the natural situation. We suggest that such granal formation, induced by the macromolecules, is partly explained as a means of generating more volume for the diffusion of macromolecules in a crowded stromal environment, i.e., greater entropy overall. This mechanism may be relevant in vivo where the stroma has a very high concentration of enzymes of carbon metabolism, and where high metabolic fluxes are required.     

Differential turnover of the photosystem II reaction centre D1 protein in mesophyll and bundle sheath chloroplasts of maize

Photoinhibition is caused by an imbalance between the rates of the damage and repair cycle of photosystem II D1 protein in thylakoid membranes. The PSII repair processes include (i) disassembly of damaged PSII-LHCII supercomplexes and PSII core dimers into monomers, (ii) migration of the PSII monomers to the stroma regions of thylakoid membranes, (iii) dephosphorylation of the CP43, D1 and D2 subunits, (iv) degradation of damaged D1 protein, and (v) co-translational insertion of the newly synthesized D1 polypeptide and reassembly of functional PSII complex. Here, we studied the D1 turnover cycle in maize mesophyll and bundle sheath chloroplasts using a protein synthesis inhibitor, lincomycin. In both types of maize chloroplasts, PSII was found as the PSII-LHCII supercomplex, dimer and monomer. The PSII core and the LHCII proteins were phosphorylated in both types of chloroplasts in a light-dependent manner. The rate constants for photoinhibition measured for lincomycin-treated leaves were comparable to those reported for C3 plants, suggesting that the kinetics of the PSII photodamage is similar in C3 and C4 species. During the photoinhibitory treatment the D1 protein was dephosphorylated in both types of chloroplasts but it was rapidly degraded only in the bundle sheath chloroplasts. In mesophyll chloroplasts, PSII monomers accumulated and little degradation of D1 protein was observed. We postulate that the low content of the Deg1 enzyme observed in mesophyll chloroplasts isolated from moderate light grown maize may retard the D1 repair processes in this type of plastids.     

A new Escherichia coli gene, fdrA, identified by suppression analysis of dominant negative FtsH mutations

An Escherichia coli membrane protein, FtsH, has been implicated in several cellular processes, including integration of membrane proteins, translocation of secreted proteins, and degradation of some unstable proteins. However, how it takes part in such diverse cellular events is largely unknown. We previously isolated dominant negative ftsH mutations and proposed that FtsH functions in association with some other cellular factor(s). To test this proposal we isolated multicopy suppressors of dominant negative ftsH mutations. One of the multicopy suppressor clones contained an N-terminally truncated version of a new gene that was designated fdrA. The FdrA fragment suppressed both of the phenotypes — increased abnormal translocation of a normally cytoplasmic domain of a model membrane protein and retardation of protein export — caused by dominant negative FtsH proteins. The intact fdrA gene (11.9 min on the chromosome) directed the synthesis of a 60 kDa protein in vitro.