Abstract
Under light stress conditions, photodamaged D1 protein is digested by proteases bound to thylakoids. The photodamaged D1 also forms cross-linked products with nearby polypeptides, which are digested by a stromal protease(s). Recently, we found that PsbO protein is also photodamaged under excessive illumination. The damage was revealed as a smeared band of the PsbO in SDS/urea-PAGE and 2D gel electrophoresis. The damage is probably caused by hydroxyl radicals generated by the Fenton reaction mediated by Mn released from the oxygen-evolving site. We suggest that the free PsbO that is released during the light stress from PS II binds the free Mn and is damaged by the hydroxyl radicals. Heat stress also damages D1, and the digestion of the protein was stimulated by Zn and suppressed completely by EDTA. These results suggest that FtsH is involved in the digestion of the heat-damaged D1 protein.
