Engagement of specific sites in the plastoquinone niche regulates degradation of the D1 protein in photosystem II.

Rapid degradation of the photosystem-II reaction center protein, D1, is driven by visible and ultraviolet irradiance at low photon-flux in the intact plant. We find that all inhibitors of electron flow that replace bound plastoquinone (QB) from its niche on D1 inhibit UV-B-driven D1 degradation, but only some inhibit visible light-driven degradation. Stabilization of the D1 protein by nitrophenol type inhibitors in visible light depends on the dimensions of the side chain at position 6 of the phenyl ring. Likewise, resistance to trypsinization of D1 at Arg-238 and to electron flow inhibitors in D1 mutant V219I (but not A251V, F255Y, S264A, or L275F) are functions of position 6 side chain dimensions in isolated thylakoids. We conclude that the configuration of the QB niche controls D1 protein degradation in intact plants under physiological photon flux. This is true irrespective of the spectral quality of radiation driving degradation. We show that UV-B-driven D1 protein degradation, but not that driven by visible light, requires plastoquinone in the QB niche to proceed. D1 degradation in visible light occurs as long as specific regions at the end of helix D and in the D-de loop of the protein are not engaged. These regions, through substrate (i.e. QB)-mediated stabilization, are proposed to regulate rapid degradation of the D1 protein.