In one of the most consequential biochemical reactions on earth, a pigment-protein complex known as photosystem II (PSII) splits water using energy from sunlight. PSII thereby extracts electrons and protons from abundant water and liberates molecular oxygen as a byproduct, enabling plants, algae and certain bacteria to harness the potential of two virtually inexhaustible raw materials, sunlight and water, for the synthesis of organic molecules. Oxygenic photosynthesis transformed our planet by oxygenating the air and water. The oxygenic world, however, turned out to be inhospitable for PSII. In the presence of oxygen, PSII suffers photooxidative damage, seriously undermining photosynthetic efficiency and plant productivity. The reaction center protein D1, found buried at the core of PSII, is usually the target of photodamage. A repair process replaces the photodamaged D1 subunit with a newly synthesized copy. An unresolved question is how the large antenna-core supercomplex structures of plant PSII disassemble for repair? Core protein subunits of plant PSII become phosphorylated in light, and these post-translation modifications have been implicated in disassembly. The precise molecular mechanism by which phosphorylation induces plant PSII disassembly, however, remains a major knowledge gap. We propose that strategically-positioned core protein phosphosites release the peripheral antenna complement from the reaction center core and trigger monomerization of the dimeric core. We further suggest that oxidative protein modifications mediate the disassembly of the monomeric cores into two smaller subcomplexes, facilitating the repair of the photodamaged reaction center protein D1. Oxidative protein modifications thus likely disassemble only the damaged monomeric cores, ensuring an economical photosystem disassembly process. The proposed research will test these hypotheses in the model plant Arabidopsis thaliana. Our approach involves the study of plant PSII disassembly in Arabidopsis mutants with hypophosphorylated or hyperphosphorylated cores. In collaboration with the Dhingra laboratory, we have replaced two PSII core phosphorylation sites with alanine in tobacco using chloroplast genetic engineering. We will study PSII disassembly in these tobacco mutants. We will also treat plant PSII with hydrogen peroxide in vitro and in vivo to test the predicted oxidative protein damage-mediated disassembly. We further aim to calculate the interaction energy between PSII antenna and core at precise in vivo phosphorylation stoichiometries. By these analyses, we will obtain a mechanistic understanding of plant PSII disassembly and repair. The studies proposed here will inform our understanding of the light harvesting strategy, self-repair, and molecular assembly of PSII. These studies will thereby illuminate the design of an optimal antenna system and a light tolerant PSII. As global crop productivity is increasingly important to feed a world population that is continuously growing, photosynthesis is a crucial target for crop improvement. The recent successes with improvement of photosynthetic efficiency in crop plants underscores the importance of obtaining fundamental knowledge on the regulatory mechanisms of photosynthetic light use.
