Abstract
Photosystem II is vulnerable to light damage. The reaction center-binding D1 protein is impaired during excessive illumination and is degraded and removed from photosystem II. Using isolated spinach thylakoids, we investigated the relationship between light-induced unstacking of thylakoids and damage to the D1 protein. Under light stress, thylakoids were expected to become unstacked so that the photodamaged photosystem II complexes in the grana and the proteases could move on the thylakoids for repair. Excessive light induced irreversible unstacking of thylakoids. By comparing the effects of light stress on stacked and unstacked thylakoids, photoinhibition of photosystem II was found to be more prominent in stacked thylakoids than in unstacked thylakoids. In accordance with this finding, EPR spin trapping measurements demonstrated higher production of hydroxyl radicals in stacked thylakoids than in unstacked thylakoids. We propose that unstacking of thylakoids has a crucial role in avoiding further damage to the D1 protein and facilitating degradation of the photodamaged D1 protein under light stress.
Highlights
Photosystem II is vulnerable to light damage
Grana formation is more prominent in shade leaves than in sun leaves, so it has been suggested that enrichment of the PSII1⁄7LHCII complex in grana is a strategy of plants to collect excitation energy by PSII under weak light [16]
These results suggest that light-induced electron transport, in PSII and in linear electron transport through Photosystem I (PSI) and PSII, is responsible for the light-induced unstacking of thylakoids
Summary
THYLAKOID UNSTACKING IS NECESSARY TO AVOID FURTHER DAMAGE TO THE D1 PROTEIN AND TO FACILITATE D1 DEGRADATION UNDER LIGHT STRESS IN SPINACH THYLAKOIDS*. Thylakoids were expected to become unstacked so that the photodamaged photosystem II complexes in the grana and the proteases could move on the thylakoids for repair. We propose that unstacking of thylakoids has a crucial role in avoiding further damage to the D1 protein and facilitating degradation of the photodamaged D1 protein under light stress. Lateral migration of the damaged PSII complexes from the grana to the membrane regions where the damaged PSII complexes are repaired is important for degradation of the D1 protein Thylakoid unstacking, if it occurs under light stress, should stimulate diffusion of the protein complexes on the thylakoids, thereby stimulating D1 turnover. We strongly suggest that unstacking of the thylakoids caused by light stress is necessary to avoid further photodamage to the D1 protein and to facilitate degradation and removal of the photodamaged D1 protein from PSII complexes
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