Determination of the spatial arrangement of polypeptides in photosystem II by photo-and oxygen radical-induced protein cross-linking.

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The initial purpose of the present research was to identify unknown polypeptides that are located closely to the reaction-center binding D1 protein in photosystem II by the use of chemical cross-linking and light-induced cross-linking methods. However, during the course of the study, we found that a metalloprotease FtsH is located near the core complex of photosystem II. We then have focused our efforts to clarify the roles of the FtsH protease in photosystem II. The following results were obtained.1.By chemical and photochemical cross-linking experiments, we found that the FtsH protease reacts with the D1 protein. These data as well as the membrane fractionation data strongly suggest that the FtsH protease can recognize the photodamaged D1 protein at the grana thylakoids.2.By moderate heat treatment (40゜C for 30 min) on spinach thylakoids, the D1 protein was cleaved. We found that the FtsH is responsible for the cleavage of the heat-damaged D1 protein.3.Using a cyanobacterial mutant lacking the FtsH protease, we confirmed that the the heat-or light-damaged D1 protein in the mutant cells is not removed, and thereby the growth and oxygen-evolving activity are significantly inhibited.All these results strongly suggest that the FtsH protease is essential for the quality control of photosystem II under light and heat stresses.