Fig. 3.

Schematic diagram of the events occurring in the grana and stroma thylakoids under excessive illumination. A horizontal view of the stacked thylakoids is shown. PSII/LHCII supercomplexes (light red) are abundant in the grana regions (LHCII complexes are omitted in this figure). Upon excessive illumination, a portion of the PSII proteins including the reaction center-binding D1 protein is phosphorylated (shown as ‘P’), which protects the proteins from photodamage. However, once PSII complexes, which are present as dimers, are damaged by excessive light (shown by ‘X’), they move to the grana margins, are disintegrated into monomers, and the damaged D1 proteins are dephosphorylated by phosphatases (orange) located in the grana margins. The D1 proteins are then degraded by proteases including FtsH (purple) that are also located in the grana margins. A portion of the photodamaged PSII complexes in the grana (dar red) are immobilized by irreversible protein aggregation. To facilitate degradation of the photodamaged D1 protein in the grana margins, partial unstacking of the thylakoids may be necessary, because it may increase the area of the grana margins and also stimulate mobilization of PSII complexes and the proteases responsible for proteolysis.