Abstract
The D1 protein of PSII is oxidatively damaged under light or heat stress. A metalloprotease FtsH has been suggested to be responsible for the degradation of the damaged D1 protein. We analyzed distribution of FtsH proteases in spinach thylakoids. In the thylakoids, the FtsH proteases were present as monomers, dimers or hexamers, while only hexameric FtsH proteases were detected in the PSII membranes, which correspond to the grana (Yoshioka et al. J. Biol. Chem. 2010). As previously suggested, the FtsH hexamers are composed of two types of FtsH subunits. By the Western blot analyses using antibodies against the type A (FtsH1/5) and type B (FtsH2/8) subunits, it is concluded that the type A subunits form dimers, while type B subunits were present as monomers in the thylakoids. When the PSII membranes were exposed to excessive light, D1 degradation which was stimulated by zinc ions was observed. This observation suggests that the degradation of the D1 protein takes place in the grana or grana margin. We are now isolating PSII membranes from the light-stressed thylakoids, and performing Western blotting to see whether FtsH proteases accumulate in the grana under the light stress.
