COVER: Schematic view of the repair cycle in Photosystem II (PSII). Light energy constantly damages photosynthetic proteins. In particular, photooxidative damage at PSII, a large pigment-protein complex consisting of more than 20 subunits and cofactors in the thylakoid membrane, is detrimental to plant survival. Photosynthetic organisms have evolved an efficient PSII repair cycle, where photodamage is targeted to reaction center protein D1 and the photodamaged D1 was efficiently degraded. As indicated in this scheme, an efficient PSII repair requires partial PSII disassembly, specific recognition and degradation of photodamaged D1, insertion of newly-synthesized D1, and PSII reassembly. Kato and Sakamoto (pp. 463–469) describe a proposed mechanism of D1 degradation by prokaryotic proteases in the PSII repair. Accumulating evidence implicates that ATP-dependent membrane metalloprotease FtsH plays a central role in D1 degradation, aided by Deg proteases peripherally attached to thylakoid membranes. [See Kato and Sakamoto, p. 463].
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