Abstract
When spinach thylakoids were subjected to heat stress, the D1 protein of photosystemII was cleaved, and a 23kDa N-terminal fragment was produced. The cleavage, however, was not observed when the thylakoids were treated with 2M-KSCN, indicating that the protease responsible for the heat-induced cleavage of the D1 protein was solubilized by the KSCN-treatment. In the supernatant of KSCN-solubilized thylakoids, protease activity was detected by gelatin activity gel electrophoresis. The protease activity was stimulated by the addition of Zn, and inhibited by EDTA. Western blot analysis, using antibody against FtsH protease showed that the supernatant of KSCN-washed thylakoids contains FtsH proteases. In addition, we identified the homologue to Arabidopsis FtsH2 and 8 by MALDI-TOF mass analysis of the thylakoids. Reconstitution of the FtsH protease with the thylakoids was also carried out. All these results suggest that the FtsH protease is responsible for the proteolysis of the D1 protein under heat stress.
