PI：Dr. Zhang Lixin, Institute of Botany, Chinese Academy of Sciences

NSFC Grant No. : 30630007

Photosystem II (PSII) catalyzes light-dependent water oxidation and plastoquinone reduction in chloroplasts. The proper functioning of PSII is dependent on the chaperone and proteases, which serve to maintain quality control over the biogenesis and maintenance processes. Although the structure and function of PSII have been extensively studied, the knowledge on the regulatory mechanism of the assembly and maintenance of PSII is still limited. To elucidate this, the research group led by Zhang Lixin, Research Professor at the Institute of Botany, CAS, have screened mutants with high chlorophyll fluorescence phenotypes and characterized the low psii accumulation2 (lpa2) mutant of Arabidopsis (Ma et al., 2007). Their results show that the newly synthesized PSII proteins are assembled into PSII, but the assembly is less efficient in the mutant. LPA2 encodes an intrinsic membrane protein but is not an integral subunit of PSII. Protein interaction studies revealed the interaction of LPA2 with the PSII core protein CP43, but not with the PSII reaction center proteins D1 or D2. Moreover, direct interactions of LPA2 with Alb3, which is involved in thylakoid membrane biogenesis and cell division, were also detected. Thus, LPA2 appears to form a complex with Alb3, which assists CP43 assembly within PSII.

A distinct feature of PSII is that it is very sensitive to photoinhibition, the PSII reaction center D1 protein being the main target for light-induced damage among PSII proteins. To identify the protease responsible for this process, they used both biochemical and genetic approaches to assess the physiological importance of DEG proteases in chloroplast function (Sun et al., 2007). DEG5 and DEG8 were found to form a hexamer in the thylakoid lumen and that recombinant DEG8 is proteolytically active towards both a model substrate (-casein) and photodamaged D1 protein of PSII, producing 16-kD N-terminal and 18-kD C-terminal fragments. The deg5deg8 double mutant showed increased photosensitivity and reduced rates of D1 degradation compared with single mutants of deg5 and deg8. A 16-kD N-terminal degradation fragment of the D1 protein was detected in WT plants, but not in deg5deg8 mutant after high light treatment. Therefore, their results suggest that DEG5 and DEG8 have a synergistic function in the primary cleavage of the CD loop of the PSII reaction center protein D1.

Ma, J.F., Peng, L.W., Guo, J.K., Lu, Q.T., Lu, C.M., and Zhang, L.X*. LPA2 is required for efficient assembly of photosystem II in Arabidopsis thaliana. Plant Cell 2007, 19: 1980–1993.

Sun, X.W., Peng, L.W., Guo, J.K., Chi, W., Ma, J.F., Lu, C.M., and Zhang, L.X*.  Formation of DEG5 and DEG8 complexes and their involvement in the degradation of photodamaged photosystem II reaction center D1 protein in Arabidopsis thaliana. Plant Cell 2007, 19: 1347–1361.