Functional Study Of DEG Proteases (DEG2, DEG9), PSRP-4 In Proteins Turnover Of Arabidopsis Chloroplast

Photosynthesis of eukaryote mainly takes place in chloroplast. PhotosystemⅡ(PSII), a large protein-pigment complex located in the thylakoid membrane of chloroplast, catalyzes the light-dependent oxidation of water to oxygen and electron. PSII is very sensitive to high light. During this progress, PSⅡreaction center protein Dl is mainly the damage target. Photodamaged D1 proteins are degraded and subsequently replced by newly synthesized ones. This effective repair mechanism is very important for maintaining a normal functional status of PSⅡ. Numerous researches have proved that there are more than 200 proteases in chloroplasts, but which of them involved in D1 protein degradation is unclear.DEG2, a chloroplast thylakoid membrane protein at stromal side, has been proved to perform the primary cleavage of D1 protein in vitro. However, null mutation of Deg2 has normal phenotype under high light, which indicate that DEG2 is not necessary for D1 degradation and protection of photosystemⅡfrom photoinhibition.Based on protein sequence alignment, DEG2 and DEG9 share highest homolog in all 16 Degs in Arabidopsis. In order to analysis the function of DEG2 and DEG9, we selected deg2deg9 double mutant by cross of deg2 and deg9 mutants. Loss functions of DEG2 and DEG9 led to sensitive to high light of deg2deg9 mutant. Compared with the wild type, Fv/Fms (maximum of PSⅡphotosynthesis efficient rate) was dramatically reduced and the degradation rate of the Dl protein was slower in the deg2deg9 mutant under high light condition. Furthermore, the interaction of DEG2 and DEG9 was proved by BiFC analysis. In summary, our results show that DEG2 and DEG9 proteases play a specific role in the protection of PSII against photodamage by degradation of D1 protein.There were 59 proteins in chloroplast ribosome,6 of them were plastid specific ribosomal proteins (PSRP1 to 6). PSRP-4 is a small basic ribosome protein of chloroplast which shares no similarity to that of E.coli and photobacterrium. In order to investigate the function of PSRP-4, we have created psrp-4 mutant of Arabidopsis by RNAi technology. Our results indicated that the susceptibility to light-induced photosystemⅡ(PSⅡ) photoinhibition of psrp-4 mutant is similar to wild-type plants.