D1 protein of photosystem II: The light sensor in chloroplasts |
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Authors: | U Dwivedi R Bhardwaj |
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Institution: | (1) School of Biochemistry, D A University, Khandwa Road, 452 001 Indore, India |
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Abstract: | Light, controls the “blueprint” for chloroplast development, but at high intensities is toxic to the chloroplast. Excessive
light intensities inhibit primarily photosystem II electron transport. This results in generation of toxic singlet oxygen
due to impairment of electron transport on the acceptor side between pheophytin and QB -the secondary electron acceptor. High light stress also impairs electron transport on the donor side of photosystem II generating
highly oxidizing species Z+ and P680+. A conformationsl change in the photosystem II reaction centre protein Dl affecting its QB-binding site is involved in turning the damaged protein into a substrate for proteolysis.
The evidence indicates that the degradation of D1 is an enzymatic process and the protease that degrades D1 protein has been
shown to be a serine protease Although there is evidence to indicate that the chlorophyll a-protein complex CP43 acts as a
serine-type protease degrading Dl, the observed degradation of Dl protein in photosystem II reaction centre particlesin vitro argues against the involvement of CP43 in Dl degradation. Besides the degradation during high light stress of Dl, and to
a lesser extent D2-the other reaction centre protein, CP43 and CP29 have also been shown to undergo degradation.
In an oxygenic environment, Dl is cleaved from its N-and C-termini and the disassembly of the photosystem II complex involves
simultaneous release of manganese and three extrinsic proteins involved in oxygen evolution. It is known that protein with
PEST sequences are subject to degradation; D1 protein contains a PEST sequence adjacent to the site of cleavage on the outer
side of thylakoid membrane between helices IV and V.
The molecular processes of “triggering” of Dl for proteolytic degradation are not clearly understood. The changes in structural
organization of photosystem II due to generation of oxy-radicals and other highly oxidizing species have also not been resolved.
Whether CP43 or a component of the photosystem II reaction centre itself (Dl. D2 or cy1 b559 subunits), which may be responsible
for degradation of Dl, is also subject to light modification to become an active protease, is also not known. The identity
of proteases degrading Dl, LHCII and CP43 and C29 remains to be established |
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Keywords: | Photosystem II Dl protein turnover photoinhibition chloroplast |
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