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, i s driven by visible and ultraviolet irradiance at low photon-flux in t he intact plant. We find that all inhibitors of electron flow that rep lace bound plastoquinone (Q(B)) from its niche on D1 inhibit UV-B-driv en D1 degradation, but only some inhibit visible light-driven degradat ion. 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 A 251V, F255Y, S264A, or L275F) are functions of position 6 side chain d imensions in isolated thylakoids. We conclude that the configuration o f the Q(B) niche controls D1 protein degradation in intact plants unde r 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 p lastoquinone in the Q(B) niche to proceed. D1 degradation in visible l ight occurs as long as specific regions at the end of helix D and in t he D-de loop of the protein are not engaged. These regions, through su bstrate (i.e. Q(B))-mediated stabilization, are proposed to regulate r apid degradation of the D1 protein.