PARENT SESSION

Symposium S7C Biosynthesis and assembly: Protein trafficking
Thursday September 2nd, 2004 2:40 PM-4:40 PM Room 510B
Chair: Ken Cline
Co-Chair: Steven Theg

Evidence that D1 processing is required for Mn binding and extrinsic protein assembly into photosystem II. Johnna Roose^*,1, Himadri Pakrasi¹, ¹ Department of Biology, St. Louis, MO, USA

ABSTRACT- During oxygenic photosynthesis, photosystem II (PSII) functions at the beginning of the photosynthetic electron transfer chain and catalyzes the unique reaction of oxidation of water to molecular oxygen. Assembly of this large membrane protein complex is challenging because of its many components including more than 20 protein subunits and a number of associated cofactors (chlorophylls, pheophytins, plastoquinones, manganese, iron, heme, calcium, and chloride). Also, during its normal function, PSII is damaged and frequently turned over. The maturation of the D1 protein, a key component in PSII, is a critical step in PSII biogenesis. The precursor form of D1 (pD1) contains a C-terminal extension, which is removed by the protease CtpA to yield PSII complexes with oxygen evolution activity. To further characterize the temporal position of D1 processing in the PSII assembly pathway, PSII complexes containing only pD1 were isolated from the cyanobacterium Synechocystis 6803, using the ctpAHT3 mutant. No manganese was detected in purified ctpAHT3 PSII, indicating a severely decreased manganese affinity. Also, the extrinsic proteins PsbO, PsbU, PsbV were not present in PSII isolated from this mutant. The extrinsic proteins PsbO and PsbV were detected in membranes from the ctpAHT3 mutant, but the amount of PsbV protein was consistently less in ctpAHT3 membranes compared to control membranes on a per chlorophyll basis. From these results, we conclude that D1 processing is necessary for Mn binding and assembly of the extrinsic proteins into PSII. Additionally, Psb27, a putative lumenal protein, was found to be more abundant in ctpAHT3 PSII than in HT3 PSII, suggesting a possible role of Psb27 in PSII biogenesis. Altogether these data provide new insights into the temporal order of events during the PSII assembly process. Supported by funding from National Science Foundation. J.L.R. was supported by a graduate student fellowship from NSF.

KEY WORDS: D1 processing, extrinsic proteins, photosystem II, manganese