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The mechanism of the degradation of psaB gene product,one of the photosynthetic reaction center subunits of Photosystem I,upon photoinhibition
Authors:Sonoike  Kintake  Kamo  Masaharu  Hihara  Yukako  Hiyama  Tetsuo  Enami  Isao
Institution:(1) Department of Biological Sciences, Gruduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113, Japan;(2) Research Institute for Biosciences, Science University of Tokyo, Yamazaki, Noda, 278, Japan;(3) Department of Biochemistry and Molecular Biology, Faculty of Science, Saitama University, Urawa, 338, Japan;(4) Department of Biology, Faculty of Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo, 162, Japan
Abstract:The psaB gene product (PsaB protein), one of the reaction center subunits of Photosystem I (PS I), was specifically degraded by light illumination of spinach thylakoid membranes. The degradation of the protein yielded N-terminal fragments of molecular mass 51 kDa and 45 kDa. The formation of the 51 kDa fragment was i) partially suppressed by the addition of phenylmethylsulfonyl fluoride or 3,4-dichloroisocoumarin, which are inhibitors of serine proteases, and ii) enhanced in the presence of hydrogen peroxide during photoinhibitory treatment, but iii) not detected following hydrogen peroxide treatment in the dark. These results suggest that the hydroxyl radical produced at the reduced iron-sulfur centers in PS I triggers the conformational change of the PS I complex, which allows access of a serine-type protease to PsaB. This results in the formation of the 51 kDa N-terminal fragment, presumably by cleavage on the loop exposed to the stromal side, between putative helices 8 and 9. On the other hand, the formation of the 45 kDa fragment, which was enhanced in the presence of methyl viologen but did not accompany the photoinhibition of PS I, was not affected by the addition of hydrogen peroxide or protease inhibitors. Another fragment of 18 kDa was identified as a C-terminal counterpart of the 45 kDa fragment. N-terminal sequence analysis of the 18 kDa fragment revealed that the cleavage occurred between Ala500 and Val501 on the loop exposed to the lumenal side, between putative helices 7 and 8 of the PsaB protein.
Keywords:hydroxyl radical  light stress  photosynthesis  serine protease  protein turnover  reactive oxygen species
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