Abstract
The reaction center-binding D1 protein of Photosystem II is oxidatively damaged in the presence of light or heat stress. A metalloprotease FtsH has been suggested to be responsible for the degradation of the damaged D1 protein. Here, we analyzed distribution and subunit structures of FtsH proteases in spinach thylakoids. In thylakoids, the FtsH proteases were present as monomers, dimers or hexamers with quantity ratio of 1.5:1:2. Only hexameric FtsH proteases were detected in the Photosystem II membranes. By the Western blot analyses using antibodies against type A subunits of FtsH (FtsH1/5), and type B subunits of FtsH (FtsH2/8), respectively, it is concluded that the FtsH hexamers are formed by association of one type A dimer and four type B subunits. Under light or heat stress, the monomeric and dimeric FtsH in thylakoids disappeared fast, while hexameric FtsH was resistant to these stresses. Our results strongly suggest that the FtsH proteases show a variety of subunit structures in the stroma thylakoids but most active and stress resistant FtsH hexamers are located near Photosystem II complexes in the grana margin to degrade the D1 protein immediately after it is damaged.
