Studies On The Molecular Mechanism Of RBD1 Protein Involved In The Formation Of PSII In Arabidopsis

Photosynthesis refers to the process that algae,photosynthetic bacteria and land plants absorb light energy and convert water and carbon dioxide into oxygen and organic matter.Photosynthesis captures light energy and releases oxygen for life on earth.In higher plants,photosynthesis takes place on the thylakoid membrane of chloroplasts.There are many pigment protein complexes related to photosynthesis distributed on the thylakoid membrane.These protein complexes are PSII,Cyt b6 f,PSI and ATP synthase.They cooperate with each other and perform their respective functions during photosynthesis.PSII is a pigment-protein supercomplex with many protein subunits,pigment molecules and cofactors.It plays an important role in photosynthesis to receive light energy,oxidate water into oxygen and transfer electrons.Although we have a certain understanding of the components of PSII,we know little about how it assembles subunits,repairs damage,resists stress and maintains stability.To understand the mechanism of PSII assembly,this project studied the function RBD1 in PSII biogenesis using molecular biology,biochemistry and genetics approaches.RBD1 is a small iron-containing protein.It has a rubredoxin domain and a C-terminal transmembrane helix.Previous studies have shown that the core subunits of PSII in rbd1 mutants are significantly decreased,and PSII activities are reduced in the absence of RBD1.RBD1 proteins may participate in electron transfer or protect the assembling intermediates of PSII together with other protein subunits.However,its detailed function is still unclear.To investigate the function of RBD1,we purchased the rbd1 mutants,and planted them in soil or MS medium.We found that the homozygous rbd1 mutants could not grow normally in soil,but could grow in MS medium under low light conditions.The mutant plants grew slowly,the cotyledons turned yellow,and the plants were obviously smaller than the wild types.The Fv/Fm value and the chlorophyll fluorescence kinetic curve of the mutants showed that the activities of PSII were significantly reduced in the rbd1 mutants.Western blotting in equal amount of thylakoid protein from WT and rbd1 plants showed that accumulation of Cytb6 f and ATP synthase is silightly increased,and the levels of PSII and PSI were decreased,especially PSII.BN-PAGE and subsequent immunoblot analyses showed that D1,D2,CP43 and CP47 subunits of PSII were mainly present in the PSII monomer in the rbd1 plants.Protein labeling and chasing experiments showed that the synthesis of reaction center proteins D1 and D2 of PSII were recuded and formation of PSII complexes from PSII reaction center is impaired.RBD1 proteins were located on the stroma thylakoid membrane.In addition,we showed that the C-terminal of RBD1 protein is bound to the thylakoid membrane,and the N-terminal is exposed to the stroma,indicating that RBD1 protein is a thylakoid protein.Taken together,we proposed that RBD1 protein is involved in the synthesis of PSII core subunits D1 and D2 as well as their assembly into PSII complexes.These results provide new information of the regulation mechanism of PSII biogenesis in plants.