Functional Analysis Of Chloroplast Deg2 Protease In Arabidopsis Thaliana

Photosystem II (PSII) is a large protein-pigment assembly that catalyzes the light-dependent oxidation of water to molecular oxygen in chloroplast. Although light is the substrate in photosynthesis, it can also be harmful. Excess light will lead to the arrest of electron transport within PSII, the reversible inactivation of PSII, and irreversible damage of the PSII reaction center D1 protein. When the rate damage of D1 exceeds the capacity for repair, photoinhibition occurs, which results in a decrease of the maximum efficiency of PSII photochemistry. The high turnover rates of the D1 protein represent an important mechanism to maintain PSII in a functional state.More than 200 proteases have been identified in chloroplasts. However the proteases involved in D1 degradation and possible mechanisms remained largelhy unknown. Deg2, a chloroplast stromal protein of the non-appressed region of thylakoid membrane, has been shown to perform the primary cleavage of the DE loop of the D1 protein in vitro. However, in vivo role of Deg2 remains unresolved.To investigate the role of Deg2 in vivo, here we isolated an Arabidopsis mutant plant lacking deg2 genes to study the function of Deg2. The phenotype of the deg2 mutant under normal growth conditions was similar to wild-type plants. PSII activity, thylakoid protein levels and protein complexes were not altered in the mutant. During the photoinhibitory light treatment, the extent of susceptibility to light-induced PSII photoinhibition in deg2 mutant was similar to that in wild-type plants in the absence or presence of lincomycin. Western blot analysis showed that the rate of the degradation of D1 polypeptide in deg2 mutants was also equivalent to that of in wild-type. Thus, these results suggest that Deg2 is not necessary for D1 degradation and protection of photosystem II from photoinhibition.