Studies On The Functions Of Chloroplast Deg1 Protease In Arabidopsis Thaliana

Photosystem II is a large protein-pigment assembly that catalyzes the light-dependent oxidation of water to molucular oxygen and plastoquinone reduction in chloroplast. PSII reaction center complex is composed of the D1 and D2 proteins, the a and B subunits of cytochrome b559 and PsbI protein. The D1 and D2 heterodimer binds all the essential redox components of PSII required electrons from the manganese cluster of the water-oxidizing complex to the plastoquinone pool.PSII is very sensitive to environment, while especially under the high light and high temperature stress, blocked electron-transport made lower Fv/Fm of PSII, at the same time degradation of PSII proteins happened, then the broken proteins degraded by proteases so that new proteins can be combined to PSII to make it active, that is the whole process of photo repair under the photo inhibition. The PSII reaction center Dlprotein being the main target for environment stress induced damage among PSII proteins. Damaged D1 proteins are degraded and subsequently replaced with newly synthesized copies. This efficient repair mechanism must be essential to maintain PSII in a functional state. Also other proteins in chloroplast may damaged by photo inhibition, so keeping the active of PSII becomes very important. Although turnover of the D1 protein has been extensivly studied, it was not clear about the mechanism of protease responsible for the degradation of the damaged proteins until now.More than 200 proteases have been identified in chloroplasts. Among them there's a Deg proteases family, which widely destributed in prokaryotes and eukaryotes, are ATP-independent serine endopeptidase with a catalytic domain of trypsin type and up to three PDZ domains. It has been shown to retain either chaperone or protease functions in a temperature-dependent manner. DegP proteases are essential in the control of protein quality in response to various stress conditions. There are 16 Deg-P like proteases in Arabidopsis thaliana, and it is predicated that at least seven proteases are targeted to chloroplasts.Degl, a thylakoid lumen-localized protease, are encoded by nuclear genes. It has both chaperone and protease activities. As we known recombibinant Degl has been shown to be proteolytically active towards thylakoid lumen proteins such as plastocyanin and PsbO of PSII in vitro. It has been shown Degl is involved the assembly of the PSII complex, probably through interaction with the PSII reaction center D2 protein. Also we found that D1 protein degradation fragments of 16-and 5.2-kDa accumulated in the transgenic plants with reduced levels of Degl, It has been supposed that Degl protease involved in cutting DE-Loop of D1. And we found that Degl protease, FtsH protease and Deg2 Protease might have interaction with each other. However whether the lumen-localized proteins are also the substrates of Degl in vivo remained unknown.Here, we used transgenic plants with reduced Degl protein levels(as low as 5%) to examine whether lumen-localized proteins are also in vivo substrates of Degl. This degl mutant expressed high light sensitivity, grown slowly(including partly abortion), draft and early-flowering. Transgenic plants accumulated Degradation products(like 25kDa and 22kDa Degradation products) of the oxygen evolving protein (PsbO), while the levels of full-length PsbO were not affected. PsbO Degradation products were efficiently Degraded by recombinant Degl. These results suggest that Degl is involved in the Degradation of PsbO Degradation fragments, but not in the initial cleavage event.