Degradation of the D1 protein of photosystem II (PSII) reaction center is a pre-requisite for the repair cycle from photoinhibition. Two types of thylakoid proteases, FtsH and Deg, have been demonstrated to participate in this process. However, the location of the proteolytic sites of the lumenal Deg1 protease within its internal sphere raised the question whether the lumenal-exposed regions of D1 are indeed long enough to reach these sites. Implanting these regions into the stable GFP rendered it sensitive to the presence of Deg1 in vitro, demonstrating that the flexible regions of D1 that protrude into the lumen can penetrate through the three side-openings of Deg1 and reach its internal proteolytic sites. This mode of action, facilitating cooperation between proteases on both sides of the thylakoid membranes, should be applicable to the degradation of other integral thylakoid membrane proteins as well.
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