Figure 7.

Proposed model of cooperative D1 degradation in the PSII repair cycle. Photodamaged PSII generated at all light intensities migrates to stroma thylakoids from grana stacks, and PSII monomerization occurs. In PSII repair, processive D1 degradation is conducted predominantly by FtsH irrespective of the light intensity (fundamental degradation). In contrast, under photoinhibitory conditions, an endopeptidic cleavage by Deg increases the rate of D1 degradation because smaller D1 cleavage fragments facilitate effective degradation (escape pathway). D1 cleavage by Deg most likely occurs in PSII dimer, PSII monomer, and the RC47 complex. Cleaved D1 fragments are subjected to further degradation by FtsH, but other proteases such as SppA and Clp proteases participate in this pathway with a mechanism that remains unclear.