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A common feature of light stress in plants, algae, and cyanobacteria is the light-induced damage to the photosystem II complex (PSII), which catalyses the photosynthetic oxidation of water to molecular oxygen. A repair cycle operates to replace damaged subunits within PSII, in particular, the D1 reaction centre polypeptide, by newly synthesized copies. As yet the molecular details of this physiologically important process remain obscure. A key aspect of the process that has attracted much attention is the identity of the protease or proteases involved in D1 degradation. The results are summarized here of recent mutagenesis experiments that were designed to assess the functional importance of the DegP/HtrA and FtsH protease families in the cyanobacterium Synechocystis sp. PCC 6803. Based on these results and the analysis of Arabidopsis mutants, a general model for PSII repair is suggested in which FtsH complexes alone are able to degrade damaged D1.
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PMID:FtsH-mediated repair of the photosystem II complex in response to light stress. 1554 96

The cyanobacterium Synechocystis sp. PCC 6803 contains four members of the FtsH protease family. One of these, FtsH (slr0228), has been implicated recently in the repair of photodamaged photosystem II (PSII) complexes. We have demonstrated here, using a combination of blue native PAGE, radiolabeling, and immunoblotting, that FtsH (slr0228) is required for selective replacement of the D1 reaction center subunit in both wild type PSII complexes and in PSII subcomplexes lacking the PSII chlorophyll a-binding subunit CP43. To test whether FtsH (slr0228) has a more general role in protein quality control in vivo, we have studied the synthesis and degradation of PSII subunits in wild type and in defined insertion and missense mutants incapable of proper assembly of the PSII holoenzyme. We discovered that, when the gene encoding FtsH (slr0228) was disrupted in these strains, the overall level of assembly intermediates and unassembled PSII proteins markedly increased. Pulse-chase experiments showed that this was due to reduced rates of degradation in vivo. Importantly, analysis of epitope-tagged and green fluorescent protein-tagged strains revealed that slr0228 was present in the thylakoid and not the cytoplasmic membrane. Overall, our results show that FtsH (slr0228) plays an important role in controlling the removal of PSII subunits from the thylakoid membrane and is not restricted to selective D1 turnover.
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PMID:The FtsH protease slr0228 is important for quality control of photosystem II in the thylakoid membrane of Synechocystis sp. PCC 6803. 1628 65

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.
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PMID: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. 1630 11

Protein quality control and proteolysis are involved in cell maintenance and environmental acclimatization in bacteria and eukaryotes. The AAA protease FtsH2 of the cyanobacterium Synechocystis sp. PCC 6803 was identified during a screening for mutants impaired in osmoregulation. The ftsH2(-) mutant was salt sensitive because of a decreased level of the osmoprotectant glucosylglycerol (GG). In spite of wild type-like transcription of the ggpS gene in ftsH2(-) cells the GgpS protein content increased but only low levels of GgpS activity were observed. Consequently, salt tolerance of the ftsH2(-) mutant decreased while addition of external osmolyte complemented the salt sensitivity. The proteolytic degradation of the GgpS protein by FtsH2 was demonstrated by an in vitro assay using inverted membrane vesicles. The GgpS is part of a GG synthesizing complex, because yeast two-hybrid screens identified a close interaction with the GG-phosphate phosphatase. Besides GgpS as the first soluble substrate of a cyanobacterial FtsH protease, several other putative targets were identified by a proteomic approach. We present a novel molecular explanation for the salt-sensitive phenotype of bacterial ftsH(-) mutants as the result of accumulation of inactive enzymes for compatible solute synthesis, in this case GgpS the key enzyme of GG synthesis.
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PMID:A membrane-bound FtsH protease is involved in osmoregulation in Synechocystis sp. PCC 6803: the compatible solute synthesizing enzyme GgpS is one of the targets for proteolysis. 1711 40

The cells of Synechocystis sp. PCC 6803 were subjected under photoinhibitory irradiation (600 micromolm(-2)s(-1)) at various temperatures (20-40 degrees C) to study in vivo quality control of photosystem II (PSII). The protease biogenesis and its consequences on photosynthetic efficiency (chlorophyll fluorescence ratio Fv/Fm) of the PSII, D1 degradation and repair were monitored during illumination and darkness. The loss in Fv/Fm value and degradation of D1 protein occurred not only under high light exposure, but also continued when the cells were subjected under dark restoration process after high light exposure. No loss in Fv/Fm value or D1 degradation occurred during recovery under growth/low light (30 micromol m(-2) s(-1)). Further, it helped the resynthesis of new D1 protein, essential to sustain quality control of PSII. In vivo triggering of D1 protein required high light exposure to switch-on the protease biosynthesis to maintain protease pool which induced temperature-dependent enzymatic proteolysis of photodamaged D1 protein during photoinhition and dark incubation. Our findings suggested the involvement and overexpression of a membrane-bound FtsH protease during high light exposure which caused degradation of D1 protein, strictly regulated by high temperature (30-40 degrees C). However, lower temperature (20 degrees C) prevented further loss of photoinhibited PSII efficiency in vivo and also retarded temperature-dependent proteolytic process of D1 degradation.
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PMID:In vivo quality control of photosystem II in cyanobacteria Synechocystis sp. PCC 6803: D1 protein degradation and repair under the influence of light, heat and darkness. 1878 73

The cyanobacterium Synechocystis sp. PCC 6803 expresses four different FtsH protease subunits (FtsH1-4) that assemble into specific homo- and heterocomplexes. The FtsH2/FtsH3 complex is involved in photoprotection but the physiological roles of the other complexes, notably the essential FtsH1/FtsH3 complex, remain unclear. Here we show that the FtsH1 and FtsH3 proteases are involved in the acclimation of cells to iron deficiency. A mutant conditionally depleted in FtsH3 was unable to induce normal expression of the IsiA chlorophyll-protein and FutA1 iron transporter upon iron deficiency due to a block in transcription, which is regulated by the Fur transcriptional repressor. Levels of Fur declined in the WT and the FtsH2 null mutant upon iron depletion but not in the FtsH3 downregulated strain. A similar stabilizing effect on Fur was also observed in a mutant conditionally depleted in the FtsH1 subunit. Moreover, a mutant overexpressing FtsH1 showed reduced levels of Fur and enhanced accumulation of both IsiA and FutA1 even under iron sufficiency. Analysis of GFP-tagged derivatives and biochemical fractionation supported a common location for FtsH1 and FtsH3 in the cytoplasmic membrane. Overall we propose that degradation of the Fur repressor mediated by the FtsH1/FtsH3 heterocomplex is critical for acclimation to iron depletion.
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PMID:Two essential FtsH proteases control the level of the Fur repressor during iron deficiency in the cyanobacterium Synechocystis sp. PCC 6803. 2523 20

The oxygen-evolving photosystem II (PSII) complex located in chloroplasts and cyanobacteria is sensitive to light-induced damage(1) that unless repaired causes reduction in photosynthetic capacity and growth. Although a potential target for crop improvement, the mechanism of PSII repair remains unclear. The D1 reaction center protein is the main target for photodamage(2), with repair involving the selective degradation of the damaged protein by FtsH protease(3). How a single damaged PSII subunit is recognized for replacement is unknown. Here, we have tested the dark stability of PSII subunits in strains of the cyanobacterium Synechocystis PCC 6803 blocked at specific stages of assembly. We have found that when D1, which is normally shielded by the CP43 subunit, becomes exposed in a photochemically active PSII complex lacking CP43, it is selectively degraded by FtsH even in the dark. Removal of the CP47 subunit, which increases accessibility of FtsH to the D2 subunit, induced dark degradation of D2 at a faster rate than that of D1. In contrast, CP47 and CP43 are resistant to degradation in the dark. Our results indicate that protease accessibility induced by PSII disassembly is an important determinant in the selection of the D1 and D2 subunits to be degraded by FtsH.
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PMID:Accessibility controls selective degradation of photosystem II subunits by FtsH protease. 2725 13

One strategy for enhancing photosynthesis in crop plants is to improve their ability to repair photosystem II (PSII) in response to irreversible damage by light. Despite the pivotal role of thylakoid-embedded FtsH protease complexes in the selective degradation of PSII subunits during repair, little is known about the factors involved in regulating FtsH expression. Here we show using the cyanobacterium Synechocystis sp. PCC 6803 that the Psb29 subunit, originally identified as a minor component of His-tagged PSII preparations, physically interacts with FtsH complexes in vivo and is required for normal accumulation of the FtsH2/FtsH3 hetero-oligomeric complex involved in PSII repair. We show using X-ray crystallography that Psb29 from Thermosynechococcus elongatus has a unique fold consisting of a helical bundle and an extended C-terminal helix and contains a highly conserved region that might be involved in binding to FtsH. A similar interaction is likely to occur in Arabidopsis chloroplasts between the Psb29 homologue, termed THF1, and the FTSH2/FTSH5 complex. The direct involvement of Psb29/THF1 in FtsH accumulation helps explain why THF1 is a target during the hypersensitive response in plants induced by pathogen infection. Downregulating FtsH function and the PSII repair cycle via THF1 would contribute to the production of reactive oxygen species, the loss of chloroplast function and cell death.This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.
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PMID:Structure of Psb29/Thf1 and its association with the FtsH protease complex involved in photosystem II repair in cyanobacteria. 2880 7

Picocyanobacteria are the numerically dominant photoautotrophs of the oligotrophic regions of Earth's oceans. These organisms are characterized by their small size and highly reduced genomes. Strains partition to different light intensity and nutrient level niches, with differing photosynthetic apparatus stoichiometry, light harvesting machinery and susceptibility to photoinactivation. In this study, we grew three strains of picocyanobacteria: the low light, high nutrient strain Prochlorococcus marinus MIT 9313; the high light, low nutrient Prochlorococcus marinus MED 4; and the high light, high nutrient marine Synechococcus strain WH 8102; under low and high growth light levels. We then performed matched photophysiology, protein and transcript analyses. The strains differ significantly in their rates of Photosystem II repair under high light and in their capacity to remove the PsbA protein as the first step in the Photosystem II repair process. Notably, all strains remove the PsbD subunit at the same rate that they remove PsbA. When grown under low light, MIT 9313 loses active Photosystem II quickly when shifted to high light, but has no measurable capacity to remove PsbA. MED 4 and WH 8102 show less rapid loss of Photosystem II and considerable capacity to remove PsbA. MIT 9313 has less of the FtsH protease thought to be responsible for the removal of PsbA in other cyanobacteria. Furthermore, by transcript analysis the predominant FtsH isoform expressed in MIT 9313 is homologous to the FtsH 4 isoform characterized in the model strain Synechocystis PCC 6803, rather than the FtsH 2 and 3 isoforms thought to be responsible for PsbA degradation. MED 4 on the other hand shows high light inducible expression of the isoforms homologous to FtsH 2 and 3, consistent with its faster rate of PsbA removal. MIT 9313 has adapted to its low light environment by diverting resources away from Photosystem II content and repair.
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PMID:Strain specific differences in rates of Photosystem II repair in picocyanobacteria correlate to differences in FtsH protein levels and isoform expression patterns. 3056 4