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.
Auditory neuropathy is a hearing disorder characterized by normal outer hair cell function and abnormal neural conduction of the auditory pathway. Aetiology and clinical presentation of congenital or early-onset auditory neuropathy are heterogeneous, and their correlations are not well understood. Genetic backgrounds and associated phenotypes of congenital or early-onset auditory neuropathy were investigated by systematically screening a cohort of 23 patients from unrelated Japanese families. Of the 23 patients, 13 (56.5%) had biallelic mutations in OTOF, whereas little or no association was detected with GJB2 or PJVK, respectively. Nine different mutations of OTOF were detected, and seven of them were novel. p.R1939Q, which was previously reported in one family in the United States, was found in 13 of the 23 patients (56.5%), and a founder effect was determined for this mutation. p.R1939Q homozygotes and compound heterozygotes of p.R1939Q and truncating mutations or a putative splice site mutation presented with stable, and severe-to-profound hearing loss with a flat or gently sloping audiogram, whereas patients who had non-truncating mutations except for p.R1939Q presented with moderate hearing loss with a steeply sloping, gently sloping or flat audiogram, or temperature-sensitive auditory neuropathy. These results support the clinical significance of comprehensive mutation screening for auditory neuropathy.
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.
Moderate heat stress (40°C for 30 min) on spinach thylakoid membranes induced cleavage of the reaction center-binding D1 protein of photosystem II, aggregation of the D1 protein with the neighboring polypeptides D2 and CP43, and release of three extrinsic proteins, PsbO, -P, and -Q. These heat-induced events were suppressed under anaerobic conditions or by the addition of sodium ascorbate, a general scavenger of reactive oxygen species. In accordance with this, singlet oxygen and hydroxyl radicals were detected in spinach photosystem II membranes incubated at 40°C for 30 min with electron paramagnetic resonance spin-trapping spectroscopy. The moderate heat stress also induced significant lipid peroxidation under aerobic conditions. We suggest that the reactive oxygen species are generated by heat-induced inactivation of a water-oxidizing manganese complex and through lipid peroxidation. Although occurring in the dark, the damages caused by the moderate heat stress to photosystem II are quite similar to those induced by excessive illumination where reactive oxygen species are involved. Photosystem II (PS II)3 in higher plants is a multisubunit complex composed of more than 25 proteins and the associated cofactors. Excitation energy captured by the chlorophylls and carotenoids in the light-harvesting chlorophyll protein complexes of PS II is finally transferred to P680, the reaction center chlorophyll of PS II, where charge separation takes place. In particular, PS II performs oxidation of water and reduction of plastoquinone molecules via chlorophyll-mediated photochemical reactions. Although it plays such an important role in the primary photochemical reaction of photosynthesis, PS II is vulnerable to various environmental stresses such as excessive visible light and high temperature.When irradiated with excessive visible light, the D1 protein is oxidatively damaged, and electron transport is inhibited. This process is referred to as photoinhibition of PS II (1-4). The photo-damaged D1 protein is subsequently degraded by specific proteases (5), and the repair of PS II is accomplished by the integration of a newly synthesized D1 protein to the PS II complex (6). Photoinhibition of PS II is caused by either the socalled acceptor-side or donor-side mechanism or both (4, 7). The acceptor-side photoinhibition takes place when the acceptor side of PS II is over-reduced by excessive illumination and the double-reduced Q A molecule is released from its binding site. Reversed electron flow from the primary electron acceptor pheophytin to P680 in the absence of Q A generates the triplet state P680, which reacts with molecular oxygen to form singlet oxygen ( 1 O 2 ). The 1 O 2 eventually damages the nearby polypeptide, the D1 protein. Alternatively, oxygen molecules may be reduced at the acceptor side of PS II to produce superoxide anion radicals (O 2 . ), which are turned into hydrogen peroxide (H 2 O 2 ) and finally hydroxyl radical (HO ⅐ ) through the Fenton reaction (8). It is claimed, however, that the generation of...
When spinach thylakoids were subjected to moderate heat stress (40°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 ZnCl 2 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. Photosystem II (PS II)2 is prone to various environmental stresses, the most prominent being strong visible light. Under light stress conditions, the reaction center-binding D1 protein of PS II is damaged and is promptly replaced by a newly synthesized D1 protein (1-4). This process is referred to as photoinhibition and repair of PS II, and a lot of effort has been made to elucidate the details of the process. Two basic mechanisms have been shown to operate in the photoinhibitory steps (3, 4). In the acceptor side mechanism of photoinhibition, over-reduction of the acceptor side of PS II by excessive light induces charge recombination between the oxidized primary electron donor P680 ϩ and the reduced primary electron acceptor Pheo Ϫ , leading to the formation of triplet state P680. The triplet state P680 subsequently reacts with molecular oxygen to form a highly reactive singlet oxygen, which is very destructive to the D1 protein. The site of damage in the D1 protein is the stroma-exposed DE-loop of the D1 protein. In the donor side mechanism of photoinhibition, the D1 protein is damaged by endogenous cationic radicals such as P680 ϩ and chlorophyll ϩ generated by illumination of PS II that has an impaired oxygen-evolving system. In this case, electron donation from water to the reaction center is inefficient, and photodamage to the D1 protein takes place at the lumen-exposed AB-loop.The photodamaged D1 protein is degraded by proteolytic enzymes and removed from PS II (5). In the acceptor side photoinh...
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